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PBSA blend composites
Accepted Manuscript Title: Enzymatic degradation behavior of nanoclay reinforced biodegradable PLA/PBSA blend composites Author: Thomas Malwela Suprakas Sinha Ray PII: DOI: Reference:
S0141-8130(15)00170-1 http://dx.doi.org/doi:10.1016/j.ijbiomac.2015.03.018 BIOMAC 4952
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
31-7-2014 26-2-2015 9-3-2015
Please cite this article as: T. Malwela, S.S. Ray, Enzymatic degradation behavior of nanoclay reinforced biodegradable PLA/PBSA blend composites, International Journal of Biological Macromolecules (2015), http://dx.doi.org/10.1016/j.ijbiomac.2015.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Submitted to International Journal of Biological Macromolecules
Revised version
Enzymatic
degradation
behavior
of
nanoclay
cr
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biodegradable PLA/PBSA blend composites
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Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 *
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial
an
1
Research, Pretoria 0001, South Africa.
Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg,
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2
reinforced
South Africa.
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____________
Ac ce p
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*Corresponding author. Fax: +27128412229; E-mail: [email protected]
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ABSTRACT Films of a biodegradable PLA/PBSA blend and blend-composites containing 2wt% of C20A,
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C30B and MEE were prepared by solvent casting and spin coating. The films were incubated in vials containing Tris-HCl buffer with proteinase-K, and their weight losses were measured
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after enzymatic degradation. The surface morphology before and after degradation tests
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were studied by SEM and in situ AFM. The results showed that neat PLA had a lower percentage weight loss than neat PBSA, whereas blending them resulted in an increased
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weight loss. The incorporation of C20A into the as-prepared blend accelerated the degradation rate, whereas C30B and MEE decelerated the degradation rate. Annealing at
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70°C reduced the degradation rate of the blend, and the presence of nanoclays further reduced the degradation rates. Annealing at 120°C dramatically decelerated the
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degradation of the blend, whereas the incorporation of nanoclays accelerated the
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degradations rates. The enhancement of the degradation rates in the presence of nanoclays
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indicated that the degradation rates were mainly controlled by the PLA matrix. Thin films were also cast onto a silicon substrate using a spin coater, and enzymatic degradation on the completely crystalline surfaces revealed that enzymatic attack occurred by pitting and surface erosion of the thin films.
Keywords: biodegradable blend, organoclay, enzymatic degradation
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1. Introduction The disposal of non-degradable polymers raises environmental concerns, and as result, biodegradable polymers have received significant research interest as suitable
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replacements. Biodegradable polymers have the ability to completely degrade in the presence of microorganisms [1]. This has accelerated the development of naturally
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occurring biodegradable polymers, such as cellulose, starch, chitin, polyhydroxyalkanoates
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and poly(3-hydroxybutyrate), and synthetic polymers, such as polylactide (PLA), poly(butylene succinate) (PBS), poly[(butylene succinate)-co-adipate] (PBSA), and poly(-
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caprolactone) (PCL). Among these biodegradable polymers, PLA has received more attention because it can be derived from renewable resources and has adaptable degradability [2-5].
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It has applications in biomedical technologies, such as bioabsorbable surgical sutures and implants [6], controlled drug-delivery systems [7], scaffolds in tissue engineering [8], etc.,
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and in packaging [9]. Despite PLA being biodegradable, the basic understanding of its
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enzymatic degradation upon blending with another biodegradable polymer, or rather its
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blend composite, still requires better understanding. So far, investigations of the enzymatic degradation of neat PLA [4, 10-19], blends of PLA/PCL [3, 13, 20], PLA/poly(ethylene glycol) (PEG) [21], PLA/poly(vinyl acetate) (PVA) [22, 23], PLA/cellulose [24], PLLA/poly(methyl methacrylate) (PMMA)/poly(ethylene oxide) (PEO) [25] ternary blend and PLA/layeredsilicate nanocomposites [26] have been reported. Most of the enzymatic degradation studies conducted on neat PLA have generally shown that factors such as the stereochemistry, melting temperature, crystallinity, crystal structure, glass transition temperature, molecular weight and molecular weight distribution influence the enzymatic degradation. For example, the study by MacDonald et al. [14] revealed that the degradation rates of PLA decreased with an increase in crystallinity. On the other hand, Tsuji et al. [27] 3 Page 3 of 41
reported that the enzymatic hydrolysis of completely crystallized PLLA films with restricted amorphous regions mainly took place through the chains in the constrained amorphous regions of the spherulites. In a recent in situ AFM study on crystalline PLLA thin films of
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approximately 100 nm, Kikkawa et al. [28] also observed that the degradation of the free amorphous region was faster than that of the restricted regions. Concerning the
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stereoregularity, Reeve et al. [4] investigated the effect of polylactide stereochemistry on
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enzymatic degradability and noticed that proteinase K preferentially degraded (L)-PLA over (D)-PLA. Similar observations by Li et al. [29] illustrated that, indeed, proteinase K
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preferentially degraded L-L ester bonds over the D-D analogue in PLA films.
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The effect on the biodegradation rate of blending PLA with another polymer depends on the biodegradability of the other polymer and the nature of the miscibility. Gajria et al. [22]
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reported that PLA/PVA formed miscible blends and that their biodegradation rates
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decreased upon the addition of PVA to PLA. Similar observations were also reported by
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Mahalik and Madras [23]. In the case of immiscible blends, Nagata et al. [24] found that blends of PLLA/trifluoroacetylated cellulose were not miscible and that their degradation rates increased in blends of 90PLLA/10Cellulose and 75PLLA/25Cellulose when compared to that of the PLLA homopolymer. However, the report of Sheth et al. [21] showed that, for blends of PLA/poly(ethylene glycol) (PEG), the miscibility ranged from miscible to partially miscible, depending on the concentrations. The authors observed that the weight loss in all blends was higher than that of neat PLLA when subjected to enzymatic degradation. This weight loss showed that blending increased the degradation rate over that of PLLA. At a lower content of PEG (30% or lower), degradation primarily occurred through the enzymatic degradation of PLA, whereas above 30%, the dissolution of PEG occurred. Tsuji et al. [13] 4 Page 4 of 41
observed that the enzymatic degradation rate of an immiscible PCL/PLLA blend of particulate PLLA-rich domains was higher than that of neat PLA. Yew et al. [30] reported an enhancement of the biodegradability of a PLA/rice-starch blend when natural rubber
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(ENR50) was added and α-amylase acted as a catalyst.
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For nanocomposites, a recent study of Singh et al. [26] showed that the incorporation of
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organically modified layered silicates, such as Cloisite®30B (C30B) and Cloisite®15A (C15A), into PLA increased the biodegradation rate, and the effect depends on the organic modifier
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used in the clays. Thus, a higher biodegradation rate was found in the PLA/C30B than in the PLA/C15A nanocomposites. The higher degradation rate was attributed to the presence of
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excess hydroxyl groups in the organic modifier of the C30B nanoclays and the better dispersion of the C30B nanoparticles in the PLA matrix than for C15A. In the case of
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PLA/PMMA/PEO ternary blends, Auliawan et al. [25] observed that the addition of
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Cloisite®10A (C10A) and organically modified vermiculite (OMVT) decreased the
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biodegradation rate in the blend. They also noticed that the organic modifier affected the biodegradation differently, whereas C10A had better biodegradability than OMVT. The recent work of Buzarovska et al. [31] reported the decelerated degradation upon TiO2 loading into PLA when using catalysis by the enzyme α-amylase.
However, a literature search shows that studies on the enzymatic degradation of PLA/PBSA blend/clay composites have not yet been reported. The current work will, therefore, present the first elucidation of the effect of enzymatic degradation by proteinase K on a solvent-cast 70PLA/30PBSA blend and its organoclay-modified blend films containing various types of clays. The blend ratio, PLA/PBSA of 70:30 was selected based on the 5 Page 5 of 41
previous work were ratios were varied from 90:10 to 50:50. The blend ratio 70:30 exhibited a uniform dispersed phase (thermodynamically most stable) than others, hence selected for further analysis [32]. The effect on the enzymatic degradation was determined by weight
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loss and the corresponding surface morphologies were examined by scanning electron microscopy (SEM). The enzymatic degradation studies on the completely crystallized thin
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films on the silicon (100) substrate were performed using an atomic force microscope (AFM)
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equipped with a hot-stage scanner. Thin films were crystallized at 120 and 100°C from a
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melt at 190°C.
2. Experimental section
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2.1. Materials
The PLA used in this study was a commercial product with a D-content of 1.1-1.7%, and it
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was supplied by Unitika Co., Ltd., Japan. According to the supplier, it had an average
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molecular weight of Mw = 200 kg/mol, molecular weight distribution = 1.1, density = 1.25
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g/cm3 (ASTM 1238), glass transition temperature of Tg = ~ 60 °C and melting temperature of Tm = ~170°C. On the other hand, the PBSA with the designation BIONOLLE #3001 was obtained from Showa High Polymer (Japan). According to the supplier, it had Mw = 190 kg/mol, molecular weight distribution = 1.4, density = 1.23 g/cm3 (ASTM 1238), Tg = - 43.8°C, and Tm values of 83.1°C (first) and 94.5°C (second). The molar ratio of succinate unit to the adipate unit is w 4:1 and the content of the coupling agent (hexamethylene diisocyanate) unit isw0.5 mol%. The organoclays Cloisite®30B (C30B) and Cloisite®20A were purchased from Southern Products, USA. According to the supplier, the pristine MMT in C30B was modified with 30 wt% of methyl tallow bis(2-hydroxyethyl) quaternary ammonium salt, whereas in C20A, the pristine MMT was modified by dimethyl dehydrogenated tallow 6 Page 6 of 41
quaternary salt. SOMASIF™ MEE clay was purchased from CO-OP chemicals, Japan. According to the supplier, the SOMASIF™ MEE (cation exchange capacity (CEC) = 120 meq/100 g) was modified with approximately 40% of dipolyoxyethylene alkyl (COCO) methyl
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ammonium. Details regarding the clays can be found in Table 1.
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2.2. Preparation of PLA/PBSA blend and clay-containing blend composite thin films
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The PLA and PBSA blend with a 70:30 weight ratio was separately dissolved in chloroform. The two solutions were then mixed and sonicated for 15 min. Next, the solution mixture
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was cast onto a silicon (100) substrate using a spin coater, rotating at 3000 rpm. The thin films were allowed to dry in air prior to the surface analyses. Similarly, the blend composite
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thin films were prepared by separately dissolving 70 wt% PLA and 30 wt% PBSA and dispersing 2 wt% of C20A in chloroform; the two polymer solutions were first mixed and
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then sonicated for 15 min. Later, the C20A dispersant was added and further sonicated for
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15 min. The solution mixture was cast onto a silicon (100) substrate to form a
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70PLA/30PBSA/2C20A (blend/C20A) composite film. The same method was used to prepare the 70PLA/30PBSA/2C30B (blend/C30B) and 70PLA/30PBSA/2MEE (blend/MEE) composite thin films. The solutions remaining after spin coating were cast onto a glass Petri dish and allowed to dry at room temperature. The surfaces exposed to the air during chloroform evaporation were selected for SEM imaging.
2.3. Enzymatic degradation studies: Measurement of the weight loss The dimensions of 1) the as-prepared and annealed at 70°C and 120°C PLA/PBSA blends and 2) the blends containing 2 wt% of C20A, C30B and MEE were maintained to approximately 6.40 mm x 6.30 mm (length x height), and the thickness ranged from 0.27 mm to 0.51 mm. 7 Page 7 of 41
The weights of the samples were measured prior to immersing them in separate vials containing approximately 5 ml of Tris-HCl buffer (pH=8.60) with Proteinase K. The buffer solution was prepared in a 100-ml flask by adding a sodium azide (N3Na) solution with a
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concentration of 1.538 x 10-4 mol/ml and 20 mg of proteinase K (Sigma, lyophilized powder, with ≥ 30 units/mg protein) while Tris-HCl solution with a concentration of 6.345 x 10-5
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mol/ml was added dropwise until pH = 8.60 was achieved. Thus, there was 1 mg of
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proteinase K in 5 ml of Tris-HCl buffer. The enzymatic degradation studies were performed in an incubated Julabo Bath Shaker (SW22) operated at 200 rpm and 37 °C for
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approximately 15 h. The samples were then thoroughly washed with distilled water and dried in a vacuum-sealed desiccator with lumps of silica for approximately 48 h. The weights
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2.4. Characterization
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of the samples after enzymatic degradation were measured.
The XRD measurements were performed using an X’Pert PRO diffractometer (PANalytical, the Netherlands) in reflection mode. The operating voltage and current were 45 kV and 40 mA, respectively. The wavelength of the Ni-filtered Cu Kα radiation was 0.1542 nm. The exposure time and scan speed for the XRD measurements were 19.7 min and 0.036987 °/s, respectively.
The intercalated clay particle dispersion in the blend matrix was directly visualized using high-resolution TEM (HRTEM). Sections (70-80 nm) from all samples were obtained with a Leica (Austria) EM FC6 cryo-ultramicrotome set at -120°C and at a cutting speed of 0.07 8 Page 8 of 41
mm/s using a Diatome 35 diamond knife (Diatome, Switzerland). Calibrated images were captured with a JEOL2100 HRTEM (JEOL, Japan) set at an operating voltage of 200 kV and
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using a Gatan Ultrascan camera and Digital Micrograph software.
The thermal properties of neat PLA, PBSA, the 70PLA/30PBSA blend and blends containing 2
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wt% C30B, C20A and MEE were studied by DSC (TA instruments, model Q 2000, USA) under
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a constant nitrogen flow of 25 mL/min. The sample weights from solvent-cast films, cast onto Petri dishes and dried overnight in air, were maintained between 4.75-5.90 mg for all
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measurements to minimize the possible thermal lag during the scans. The instrument was calibrated using the temperature and heat-of-fusion of an indium standard, and the base
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line was verified according to the TA instrument protocols. To study the crystallization behavior, the samples were heated to 190°C at a heating rate of 10°C/min, held at that
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temperature for 5 min to erase the previous thermal history, and cooled to -65°C and then
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again heated to 190°C at 10°C/min.
A Carl Zeiss (Germany) Auriga SEM was used for imaging the surfaces of the films after hydrolysis. The samples were coated by an Au-Pd alloy, and the instrument was operated at 3 kV.
A multimode AFM (Nano Scope Version (R) IV) using a 0.5-2.0 Ω∙cm phosphorous (n)-doped
Si tip with a curvature radius of less than 10 nm (Veeco Instruments) was used to study the surface morphology of the unmodified and C30B-modified PLA/PBS blend thin films. The RTESPW tip mounted on a 125-µm-long cantilever with a spring constant of 40 N/m was employed for tapping-mode experiments. The samples were imaged using a scan rate of 0.5 Hz, and the tip frequencies were varied from 280-310 kHz. Three different areas on each 9 Page 9 of 41
thin film surface were imaged, thus collecting data for the as-prepared and blend/C30B composite thin films and the crystallized and hydrolyzed thin films. Only representative
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images are reported in this article.
To study the effect of the crystallization temperature and enzymatic degradation on the
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surface morphology, the samples were crystallized at 100°C and 120°C from a melt at 190°C,
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placed in a beaker and hydrolyzed in Tris-HCl buffer (pH=8.26) for 40 min. The hydrolyzed
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thin films were then analyzed by AFM.
3. Results and discussion
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3.1. Thermal analysis and polymer-organoclay interactions
Fig. 1 presents the DSC second-heating thermograms of (a) neat PLA and PBSA and (b) the
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PLA/PBSA blend and blend-containing organoclays. The measured values of the Tg, cold
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crystallization (Tc) and melting (Tm) peak temperatures are summarized in Table 2. Neat PLA
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exhibited Tg at 61.2°C, Tc at 131.8°C and Tm at 166.6°C. In the case of neat PBSA, Tg could not be identified, and its Tm peak appeared at 96.4°C. In the blend, the Tg associated with PLA shifted to 58.0°C, Tc to 105.6°C and Tm slightly shifted to a higher temperature of 168.0 °C. The melting peak associated with the PBSA matrix shifted to a lower temperature of 93.3°C. The shift in Tg to 58.0°C, associated with the PLA matrix, indicates that the polymer chains of both polymers interacted to some degree.
The blend-containing 2 wt% C20A showed Tg for PLA at 57.9°C, Tc at 104.7°C and Tm at 167.8°C. The Tm peak of PBSA shifted to 90.3°C. The Tg, Tc and Tm peaks associated with the PLA matrix did not show a significant change compared to those of the pure blend. 10 Page 10 of 41
However, the shift in the Tm peak of PBSA to a lower value suggests that most of the clay platelets interacted with the PBSA matrix or moved into the PBSA matrix. The lowest melting enthalpy of 21.4 J∙g-1 for PBSA further shows that it formed a less crystalline matrix
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than the other blend composites. Similarly, in the case of 2 wt% MEE, the Tg, Tc and Tm peaks for PLA did not show a noticeable change. However, the shift in the melting peak of PBSA to
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91.6°C indicates that most of the clay platelets migrated into the PBSA matrix. The inclusion
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of 2 wt% C30B slightly altered the Tg, Tc and Tm peaks of PLA compared to those of the blend. The minor change in the melting peak of PBSA to 93.6°C also suggests that most clay
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platelets interacted with the polymer chains of PBSA.
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The melting enthalpy for the PBSA in the blend was 24.6 J∙g-1, compared to 30.9 J∙g-1 for PLA. When C20A was incorporated, the melting enthalpy for PBSA decreased to 21.4 J∙g-1, and
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the melting enthalpy for PLA increased to 34.4 J∙g-1. These observations imply that C20A
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platelets acted as better nucleating agents in the PLA than in the PBSA matrix; hence, PLA
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exhibited better crystallinity than PBSA when compared to the blend. On the other hand, upon inclusion of MEE in the blend, the melting enthalpies associated with the PBSA and PLA increased. However, the melting enthalpy of PLA showed the highest increase of 37.3 J∙g-1 compared to those of the other blend composites. This shows that MEE platelets acted as nucleating agents better in the PLA than in the PBSA matrix. In the presence of C30B, the melting enthalpies of PBSA and PLA increased. However, C30B exhibited the highest increase in the PBSA enthalpy in comparison to those of the other blend composites. This indicates that the C30B platelets acted as better nucleating agents for the PBSA matrix than for the PLA matrix.
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These observations generally show that the C20A and MEE clay platelets enhanced the crystallinity of the PLA over that of the PBSA matrix. The presence of a dehydrogenated part in C20A was attributed to the reduced crystallinity in the PBSA. MEE improved the
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crystallinity of both PLA and PBSA, but the highest improvement was observed for the PLA. This indicates that the ʹCOCOʹ in MEE interacted with both the PLA and PBSA matrices. The
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presence of C30B clay platelets improved the crystallinity of the PBSA over that of the PLA
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matrix. This effect was attributed to the strong interaction between the C=O bonds present in the PLA and PBSA backbones and the hydroxyl group in the gallery of the C30B silicate
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layers. However, PBSA has more C=O bonds than PLA. As a result, C30B strongly interacted
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with PBSA.
To verify the existence of the interactions of the clay platelets with the two polymers, XRD
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measurements were performed on the pure organoclay powders and their blend
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composites. The results in Fig. 2 generally show that C20A had a high level of dispersion in
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the blend, followed by MEE and C30B, respectively. The characteristic peak of C20A at 2θ = 3.84° disappeared when it was added to the blend, the peak for MEE that was originally at 2θ = 4.37° shifted to 2θ = 2.97° after it was incorporated to the blend and the peak for C30B that was originally at 2θ = 4.87° shifted to 2θ = 4.77°. The clay galleries for MEE expanded by 1 nm, and they expanded by 0.04 nm for C30B. The disappearance of the first-order peak for C20A indicates that it had a higher degree of gallery expansion than the other clays, followed by MEE and C30B, respectively. The increase in the interlayer spacing of the clay platelets indicates that the polymeric chains can penetrate into the clay gallery. According to the observations above, more polymer chains penetrated into the C20A clay galleries, followed by MEE and C30B. The degree of intercalation shows the relationship to the 12 Page 12 of 41
original d001-spacing of the organoclays: 2.30, 2.02 and 1.81 nm for C20A, MEE and C30B, respectively. To support the conclusions based on XRD results, TEM analysis of the composites was conducted, and results are presented in Fig. 3. Although it was difficult to
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identify the two phases in the blend composites, the TEM results show a higher degree of intercalation of the polymer chains in the case of the blend/C20A composite. Comparing the
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XRD results to the DSC results, the crystallinity of PBSA decreased with an increased degree
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of dispersion of the organoclays in the blend matrix.
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3.2 Effect of enzymatic degradation on the as-prepared films
Fig. 4 shows SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c)
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PLA/PBSA blend, (d) blend/C20A, (e) blend/C30B and (f) blend/MEE films. These images were taken on the side of the film that was exposed to air during solvent (chloroform)
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evaporation. The surfaces reveal fibrous structures that were induced by beam heat. These
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fibrous structures can be clearly observed for PLA/PBSA blend (Fig. 4(c) and blend/C20A (Fig.
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4(d)) films, respectively. Fine fibrous structures can be seen for blend/C30B (Fig. 4(e)) and blend/MEE (Fig. 4(f)), respectively. However, in a case where the samples were exposed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h, a dramatic change on morphologies was observed. Fig. 5 depicts the SEM surface morphologies of the as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A, (e) blend/C30B and (f) blend/MEE composite films hydrolyzed in Tris-HCl buffer (pH=8.60) for 15 h. These images were taken on the side of the film that was exposed to air during the solvent (chloroform) evaporation. The roughness and pitting on the film surfaces indicate that enzymatic degradation occurred. Figs. 5(a), (d) and (f) show that erosion occurred on some parts of the surface, and Figs. 5(c) and (e) indicate uniform surface erosion. The morphology in Fig. 5(b) does not 13 Page 13 of 41
clearly reveal the effect of enzymatic degradation because the surface morphologies were inadequate in determining the extent of degradation. The initial weight and the weight after enzymatic degradation were measured and recorded in Table 3, and the equation below
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was used to calculate the percentage of weight loss (%Wloss), where Wi is the initial weight
− × 100
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% =
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and Wf is the weight after enzymatic degradation.
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According to the weight-loss measurements, neat PBSA lost more weight than neat PLA. This indicates that neat PBSA has a higher enzymatic degradation rate than neat PLA.
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Blending two polymers (70 wt% PLA and 30 wt% PBSA) resulted in an increase in the weight loss. The presence of PBSA in the PLA matrix enhanced the degradation rate of the blend.
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This improvement can be attributed to the morphology of the blend and the fact that PBSA
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degrades faster than PLA. These two polymers usually produce phase-separated
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morphologies, in which the enzymatic attack is believed to start mainly at the interfaces of the two polymers. In the presence of 2 wt% C20A, a weight loss that was higher than that of the blend and the other blend composites was observed. This suggests that the inclusion of C20A reduced the sizes of the dispersed PBSA phases, therefore maximizing the number of interfaces between the two polymers. Then, these interfaces accelerated the enzymatic attack, resulting in an increased degradation rate over that of the blend. This argument is supported by the XRD measurement in Fig. 2 that showed a high level of dispersion of C20A when it was added to the PLA/PBSA blend, followed by decreased dispersion of MEE and C30B.
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The presence of 2 wt% C30B and MEE also lowered the weight loss compared to that of the blend. Between the two, C30B has a higher weight loss than MEE. In this case, C30B lowered the weight loss, indicating a deceleration in the enzymatic degradation rate in the blend.
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Apart from the reduction of the PBSA phase size, two effects can be attributed to this: (a) that C30B enhanced the crystallinity of PBSA and (b) that it further improved the properties
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of the interfaces.
The lowest weight loss in the presence of MEE in the blend compared to those in the blend
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and blend composites was attributed to the enhanced crystallinity of both the PLA and PBSA matrices. Furthermore, the XRD results also showed that MEE had a better dispersion in the
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blend than C30B. Both the crystallization and the dispersion of the PBSA phase are believed
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to have contributed to the decelerated enzymatic degradation rate.
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Combining these observations with the DSC results in Table 2, it can be observed that PBSA
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had a higher melting enthalpy of 31.8 J∙g-1 in the presence of C30B, followed by MEE with 27.1 J∙g-1 and C20A with 21.4 J∙g-1. The larger the melting-enthalpy value, the more melted the crystal was. This implies that PBSA formed more crystals in the presence of C30B, followed by MEE and C20A, respectively. According to these observations, the enzymatic degradation rates decreased with increased crystallinity. This agrees with the previous studies of MacDonald et al.14 and Kikkawa et al. [28] that illustrated that the degradation of PLA decreased with increased crystallinity.
These observations generally indicate that, on as-prepared films, neat PBSA had a higher degradation rate than neat PLA. Upon blending them, the degradation rate was enhanced. 15 Page 15 of 41
The presence of 2 wt% C20A further enhanced the degradation rate, whereas the presence of 2 wt% C30B and MEE decelerated the degradation rate of the blend. Compared to the DSC measurements, the weight loss increased upon the reduction in crystallinity of PBSA in
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the blend. The findings further showed that the degradation rates of the blend composite depended on the degree of the intercalated clay particle dispersion in the blend matrix and
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the impact the particles have on the crystallinity of both the PLA and PBSA matrices.
3.3. Effect of the annealing temperature on enzymatic degradation
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The films were annealed at 70°C for 1 h, a temperature at which PBSA will fully crystallize. The weight-loss measurements after enzymatic hydrolysis show a decrease when compared
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to the as-prepared films (see Table 4). The inclusions of organoclays further reduced the weight loss. This decrease generally indicates that the enzymatic degradation rate was
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decelerated. The crystalline PBSA phases improved the film’s resilience to enzymatic attack.
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This agrees with the report of Kikkawa et al. [28], which showed that the crystalline sections
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of PLLA thin films degraded more slowly than the amorphous regions. Fig. 6(a) and Fig.6(b) for the PLA/PBSA blend and the blend/C20A composite films, respectively, had similar surface erosion. The blend/C30B and blend/MEE composite films in Fig. 6(c) and (d), respectively, show that enzymatic degradation occurred through pitting.
These observations suggest that the higher degradation rate mainly occurred through surface erosion, whereas the lower degradation happened through pitting. The weight-loss measurements have generally shown that, for films annealed at 70°C, the presence of organoclays decelerated the enzymatic degradation rates when compared to those of the blend. 16 Page 16 of 41
The films were also annealed at 120°C, and the surface morphologies in Fig. 7 indicate that enzymatic degradation occurred through pitting. The blend with the lowest weight loss of 0.09% contains small pores, whereas the blend/C20A composite with the highest weight of
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0.66% contains larger pores (refer to Table 5 and Fig. 7). Annealing at 120°C dramatically decelerated the enzymatic degradation rate compared to those of the as-prepared samples
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and those annealed at 70°C. In this case, the presence of organoclays enhanced the
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degradation rates compared to those for the pure blend. At this temperature, PLA is expected to fully crystallize, whereas PBSA should be in a molten state. However, PBSA
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contributed to the enzymatic degradation rates.
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crystallizes upon cooling to room temperature (25°C). In this case, both polymers
In summary, the enzymatic degradation studies revealed that PBSA mainly controlled the
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degradation rates of the blend and blend composites of the as-prepared films. The presence
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of C20A accelerated the degradation rates, but C30B and MEE decelerated the degradation
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rates. This was attributed to the enhanced crystallinity in the PBSA matrices induced by these clays (C30B and MEE). This effect was clearly observed on the films annealed at 70°C in which the PBSA fully crystallizes and the degradation rates decreased. In the films annealed at 70°C, the presence of organoclays decelerated the enzymatic degradation rate. In this case, the organoclays are believed to further enhance the crystallinity of the PBSA phases. For films annealed at 120°C, the enzymatic degradation rates were decelerated. At this temperature, PLA is expected to fully crystallize but PBSA in a molten state. When cooled to room temperature, PBSA is again expected to crystallize. The addition of organoclays accelerated the degradation rates. In this case, PLA should be more crystalline
17 Page 17 of 41
than PBSA. This effect suggests that PBSA controlled the degradation rates because the presence of organoclays in the PLA matrix improves its enzymatic degradability.
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3.4. Enzymatic degradation of the blend and blend/C30B thin films studied by AFM Based on the DSC measurements, the organoclay (C30B) improved the crystallinity of both
cr
polymers in the blend when compared to the other organoclays (C20A and MEE), and it also
us
gave similar Tg, Tc and Tm peaks for PLA and a similar melting peak for PBSA to those of the blend. For this reason, the blend/C30B composite was selected for enzymatic studies with
an
AFM equipped with a hot-stage scanner. Fig. 8 depicts AFM height images of thin films of the as-prepared blend and blend/C30B (far left), crystallized at 120°C from a melt at 190°C
M
(middle), and crystallized at 120 °C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min (far right). The as-prepared blend in Fig. 8(a) contained a few large,
d
dispersed PBSA phases, which were reduced in the presence of 2 wt% C30B (see Fig. 8(d)).
te
This indicates that the nanoclays improved the compatibility between PLA and PBSA. When
Ac ce p
crystallized at 120°C from the melt, the crystal growth on the blend and blend/C30B appear to be similar. Their corresponding phases in Figs. 9(a) and (e) (5 x 5 µm2), show that the morphology comprised both edge-on (fibrous-sections of the image) and flat-on lamellae (flat sections of the image).
However, the flat-on lamellae in the blend composite tend to form leaf-like structures (see Fig. 9(e)). Fig. 8(c) and (f) for the blend and blend composites, respectively, show that, after immersing the thin film surfaces in Tris-HCl buffer with Proteinase K for 40 min, degradation formed holes on the films surfaces. These holes are uniformly distributed on the thin-film surface. The presence of these pores indicates that enzymatic degradation occurred; 18 Page 18 of 41
however, based on these morphological changes, it is difficult to determine the extent of the degradation. However, the morphology of a phase image for the blend in Fig. 8(c) shows that there was more surface erosion than in the blend containing 2 wt% C30B, Fig. 8(f).
ip t
The surface of the blend thin film was dominated by flat-on lamellae, whereas the blend composite was dominated by edge-on lamellae. This agrees with the weight-loss
cr
measurements that showed that enzymatic degradation of the blend took place upon the
us
addition of C30B.
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Fig. 10 depicts the thin films of the blend and blend composite crystallized at 100°C from a melt at 190°C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min. The
M
height images do not clearly distinguish the difference in the crystalline morphology.
te
(refer to Fig. 11).
d
However, their corresponding phase images taken on magnified areas depict the difference
Ac ce p
The blend morphology in Fig. 11(a) formed both edge-on and flat-on lamellae, and several circular phases attributed to PBSA were observed throughout the film surface. On the other hand, the blend composite was dominated by flat-on lamellae and few circular dark phases (in Fig. 11(b)). After enzymatic degradation, the blend image in Fig. 10(b) indicates that pores were formed during hydrolysis.
These pores are well distributed throughout the film surface. In this case, the blend composite appeared to have a smaller number of pores than the blend (see Fig. 10(d)). The corresponding phase image in Fig. 11(d) taken on a magnified area illustrates the reduction of pores. The phase image in Fig. 11(b) shows more surface erosion than in Fig. 11(d). This 19 Page 19 of 41
observation indicates that, regardless of the change in the crystalline temperature, the presence of nanoclays reduced the degradation through pitting. Because enzymatic degradation occurs faster on amorphous regions than on crystalline regions, this pitting can
ip t
imply that these completely crystallized spherulites contain uniformly distributed
cr
amorphous regions.
us
4. Conclusions
The results presented in this article indicated that the as-prepared, neat PBSA degraded
an
faster than PLA; however, blending two polymers increased the biodegradation rate over their initial individual degradation rates. The increased degradation rate was attributed to
M
the immiscibility of the two polymers that formed phase-separated morphologies that created more exposure for the enzymatic attack. The inclusion of C20A into the blend
d
accelerated the enzymatic degradation rate, but C30B and MEE decelerated the degradation
te
rates. The XRD results showed a high level of intercalation in the presence of C20A,
Ac ce p
suggesting that the numbers of dispersed phases and interfaces increased. This was associated with the enhanced degradation rate. The deceleration in the case of C30B and MEE was attributed to the enhanced crystallinity of the PBSA matrices, as seen from the DSC results. Films annealed at 70 °C showed a decelerated enzymatic degradation rate, and the addition of organoclays further decreased the degradation rates. This shows that the crystalline PBSA matrices improved the resilience to enzymatic attacks. The addition of nanoclays further enhanced the crystallinity. For films that were annealed at 120 °C, the blend showed a dramatic deceleration of the degradation rate. The presence of organoclays improved the degradation rates, which indicates that PLA mainly controlled the degradation rates of the blend. AFM images revealed that thin films of the fully crystalline PLA/PBSA 20 Page 20 of 41
blend and blend/C30B composite showed that enzymatic degradation occurred through pitting and surface erosion. The extent of the degradation rate could not be determined
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with this technique.
Acknowledgments: The authors would like to thank the CSIR, the DST, and the NRF, South
us
cr
Africa for financial support
References
an
[1] A. Torres, S. M. Li, S. Roussos, M. Vert, Poly(lactic acid) degradation in soil or under controlled conditions, J. Appl. Polym. Sci. 62 (1996) 22952302.
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[2] Y. Tokiwa, B. P. Calabia, Biodegradability and biodegradation of poly(lactide), Appl. Microbiology and Biotechnology 72 (2006) 244251.
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[3] L. Liu, S. Li, H. Garreau, M. Vert, Selective enzymatic degradations of poly(l-lactide) and
te
poly(ε-caprolactone) blend films, Biomacromolecules 1 (2000) 350359.
Ac ce p
[4] M. S. Reeve, S. P. McCarthy, M. J. Downey, R. A. Gross, Polylactide stereochemistry: effect on enzymic degradability, Macromolecules 27 (1994) 825831.
[5] S. J. Holland, B. J. Tighe, P. L. Gould, Polymers for biodegradable medical devices. 1. The potential of polyesters as controlled macromolecular release systems, J. Control. Release. 4 (1986) 155180.
[6] M. S. Taylor, A. U. Daniels, K. P. Andriano, J. Heller, Six bioabsorbable polymers: In vitro acute toxicity of accumulated degradation products, J. Appl. Biomat. 5 (1994) 151157. [7] P. McDonald, J. Lyons, L. Geever, C. Higginbotham, In vitro degradation and drug release from polymer blends based on poly(dl-lactide), poly(l-lactide-glycolide) and poly(εcaprolactone), J. Mater. Sci. 45 (2010) 12841292. 21 Page 21 of 41
[8] J. Sarasua, Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications, J. Mater. Sci: Mater. Medicine. 22 (2011) 25132523. [9] R. Auras, B. Harte, S. Selke, An overview of polylactides as packaging materials,
ip t
Macromol. Biosci. 4 (2004) 835864.
[10] H. Tsuji, S. Miyauchi, Enzymatic hydrolysis of poly(lactide)s: effects of molecular
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weight, l-Lactide content, and enantiomeric and diastereoisomeric polymer blending,
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Biomacromolecules 2 (2001) 597604.
[11] H. Tsuji, M. Ogiwara, S. K. Saha, T. Sakaki, Enzymatic, alkaline, and autocatalytic
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degradation of poly(l-lactic acid): effects of biaxial orientation, Biomacromolecules 7 (2005) 380387.
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[12] H. Tsuji, S. Miyauchi, Poly(l-lactide): 7. Enzymatic hydrolysis of free and restricted amorphous regions in poly(l-lactide) films with different crystallinities and a fixed
te
d
crystalline thickness, Polymer 42 (2001) 44634467. [13] H. Tsuji, Y. Kidokoro, M. Mochizuki, Enzymatic degradation of biodegradable polyester
Ac ce p
composites of poly(L-lactic acid) and poly(ε-caprolactone), Macromol. Mater. Eng. 291 (2006) 12451254.
[14] R. T. MacDonald, S. P. McCarthy, R. A. Gross, Enzymatic degradability of poly(lactide): Effects of chain stereochemistry and material crystallinity, Macromolecules 29 (1996) 73567361.
[15] H. Pranamuda, A. Tsuchii, Y. Tokiwa, Poly (L-lactide)-degrading enzyme produced by amycolatopsis sp, Macromol. Biosci. 1 (2001) 2529. [16] S. Li, S. McCarthy, Influence of crystallinity and stereochemistry on the enzymatic degradation of poly(lactide)s, Macromolecules 32 (1999) 44544456.
22 Page 22 of 41
[17] K. Yamashita, Y. Kikkawa, K. Kurokawa, Y. Doi, Enzymatic degradation of poly(l-lactide) film by proteinase K: quartz crystal microbalance and atomic force microscopy study, Biomacromolecules 6 (2005) 850857.
degradation of poly(lactide)s, Macromolecules 32 (1999) 44544456.
ip t
[18] S. Li, S. McCarthy, Influence of crystallinity and stereochemistry on the enzymatic
cr
[19] D. F. Williams, Enzymatic hydrolysis of polylactic acid, Eng. Medicine 10 (1981) 57.
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[20] G. Sivalingam, S. P. Vijayalakshmi, G. Madras, Enzymatic and thermal degradation of poly(ε-caprolactone), poly(D,L-lactide), and their blends, Indust. Eng. Chem. Res. 43
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(2004) 77027709.
[21] M. Sheth, R. A. Kumar, V. Davé, R. A. Gross, S. P. McCarthy, Biodegradable polymer
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blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 14951505.
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[22] A. M. Gajria, V. Davé, R. A. Gross, S. McCarthy, Miscibility and biodegradability of blends of poly(lactic acid) and poly(vinyl acetate), Polymer 37 (1996) 437444.
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[23] J. P. Mahalik, G. Madras, Enzymatic degradation of poly(D,L-lactide) and its blends with poly(vinyl acetate), J. Appl. Polym. Sci. 101 (2006) 675680.
[24] M. Nagata, F. Okano, W. Sakai, N. Tsutsumi, Separation and enzymatic degradation of blend films of poly(L-lactic acid) and cellulose, J. Polym. Sci. Part A: Polym. Chem. 36 (1998) 18611864.
[25] A. Auliawan, E. M. Woo, Crystallization kinetics and degradation of nanocomposites based on ternary blend of poly(L-lactic acid), poly(methyl methacrylate), and poly(ethylene oxide) with two different organoclays, J. Appl. Polym. Sci. 125 (2012) 444458.
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[26] N. K. Singh, Enzymatic degradation of polylactide/layered silicate nanocomposites: Effect of organic modifiers, J. Appl. Polym. Sci. 127 (2013) 24652474. [27] H. Tsuji, S. Miyauchi, Poly(l-lactide): VI Effects of crystallinity on enzymatic hydrolysis of
ip t
poly(l-lactide) without free amorphous region, Polym. Degrad. Stabil. 71 (2001) 415424.
cr
[28] Y. Kikkawa, H. Abe, T. Iwata, Y. Inoue, Y. Doi, Crystallization, stability, and enzymatic
us
degradation of poly(l-lactide) thin film, Biomacromolecules 3 (2002) 5056.
[29] S. Li, A. Girard, H. Garreau, M. Vert, Enzymatic degradation of polylactide
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stereocopolymers with predominant d-lactyl contents, Polym. Degrad. Stabil. 71 (2000) 6167.
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[30] G. H. Yew, A. M. Mohd Yusof, Z. A. Mohd Ishak, U. S. Ishiaku, Water absorption and
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90 (2005) 488500,
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enzymatic degradation of poly(lactic acid)/rice starch composites, Polym. Degrad. Stabil.
[31] A. Buzarovska, A. Grozdanov, Biodegradable poly(L -lactic acid)/TiO2 nanocomposites:
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Thermal properties and degradation, J. Appl. Polym. Sci. 123 (2012) 21872193.
[32] T. Malwela, S. S. Ray, Investigating the crystal growth behavior of the biodegradable polymer blend thin films using in situ atomic force microscopy. Macromol. Mater. Eng. 299 (2014) 689-697.
24 Page 24 of 41
Submitted to International Journal of Biological Macromolecules
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Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Tg
Tm, PBSA
ΔHm,PBSA
ΔHc
ΔHm,PLA
/ °C
/ J.g
/ °C
/ J.g
/ °C
/ J.g-1
Neat PLA
61.2
-
-
131.8
26.2
166.6
32.0
Neat PBSA
-
96.4
40.5
-
-
-
-
Blend
58.0
93.3
24.6
105.6
22.0
168.0
30.9
Blend/2C20A
57.9
90.3
21.4
104.7
14.0
167.8
34.4
Blend/2MEE
57.6
91.6
27.1
104.4
16.3
167.7
37.3
Blend/2C30B
57.6
93.6
31.8
106.8
22.7
168.0
34.7
te
Ac ce p
-1
Tm,PLA
/ °C
d
-1
Tc
M
Sample Name
an
Table 2. Data calculated from the DSC second heating of neat PLA, neat PBSA, the PLA/PBSA blend and various blend/clay composites.
26 Page 25 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
an
Table 3. Neat PLA and PBSA, their blend and blend composite films hydrolyzed in TrisHCl buffer (pH=8.60) with Proteinase K for 15 h. Initial weight /mg 23.97
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.85
Weight loss /% 0.50
Neat PBSA
19.33
19.20
0.67
Blend
23.6
23.43
0.72
Blend/2MEE
18.84
18.74
0.53
Blend/2C30B
18.69
18.58
0.59
25.20
0.75
Ac ce p
te
d
M
Sample Neat PLA
Blend/2C20A
25.39
27 Page 26 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
an
Table 4. The PLA/PBSA blend and blend composite films annealed at 70 °C and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h. Initial weight /mg 23.3
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.23
Weight loss /% 0.30
Blend/2MEE
15.54
15.50
0.26
Blend/2C30B
17.99
17.95
0.22
Blend/2C20A
17.86
17.81
0.28
Ac ce p
te
d
M
Sample Blend
28 Page 27 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Initial weight /mg 23.09
Blend/2MEE
14.74
Blend/2C30B
15.6
Blend/2C20A
15.07
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.07
Weight loss /% 0.09
d
Sample Blend
M
an
Table 5. The neat PLA/PBSA blend and blend composites annealed at 120 °C and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
0.20
15.56
0.26
14.97
0.66
Ac ce p
te
14.71
29 Page 28 of 41
Highlights
an
us
cr
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Films of a biodegradable PLA/PBSA blend and blend-composites were prepared by solvent casting and spin coating First elucidation of the effect of enzymatic degradation by proteinase K using AFM with a hot-stage scanner The degradation rates of the composites increased in the presence of organoclays Results indicated that the degradation rates were mainly controlled by the PLA matrix.
Ac ce p
te
d
M
[33]
42 Page 29 of 41
ip t
Table 1
cr
Submitted to International Journal of Biological Macromolecules
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Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/ poly[(butylene succinate)-co-adipate] blend composites Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
an
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Table 1. Characteristic parameters of the various organoclays. Organic Modifier
Chemical formula
MMT
C20A
Dimethyl dehydrogenated tallow, quaternary ammonium salt
M
Organoclay
CH3
Mica
C30B
methyl tallow bis(2hydroxyethyl) quaternary ammonium salt
MEE
N+
H3C
Particles length/nm
Cation exchange capacity (CEC), mequiv/100g
d-spacing /nm
~ 100 – 150
95
2.42
~100 – 150
90
1.85
~200 – 300
120
2.30
HT
HT
CH2CH2OH H3C
Ac c
MMT
ep te
d
Clay
N+
T
CH2CH2OH
Dipolyoxy ethylene alkyl (COCO) methyl ammonium salt
R(COCO) CH3
N+
(CH2CH2O)xH
(CH2CH2O)yH x+y=2
Page 30 of 41
Figure 1
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Ac
Fig. 1. DSC second heating thermograms of (a) neat PLA and PBSA and (b) PLA/5C30B, PLA/5MEE, PLA/5C20A composites.
Page 31 of 41
Figure 2
Submitted to International Journal of Biological Macromolecules
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Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
cr
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
8000
C20A 70PLA/30PBSA/2C20A
(a)
25000
MEE 70PLA/30PBSA/2MEE
(b)
20000
Intensity / a.u.
M
6000
4000
2000
0 4
ed
Intensity / a.u.
an
us
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
6
8
15000 10000 5000 0
10
2
4
ce pt
2-theta / degrees
6
8
10
2-theta / degrees
10000
C30B 70PLA/30PBSA/2C30B
(c)
Intensity / a.u.
Ac
8000 6000 4000 2000 0 4
6
8
10
2-theta / degrees
Fig. 2. X-ray diffraction patterns of (a) pure C20A powder and the blend/C20A composite, (b) pure MEE powder and the blend/MEE composite and (c) pure C30B powder and the blend/C30B composite.
Page 32 of 41
Figure 3
Submitted to International Journal of Biological Macromolecules
cr
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 *
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
1
an
us
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
(b)
ed
M
(a)
Ac
ce pt
(c)
Fig. 3. TEM images of the (a) blend/C20A composite, (b) blend/MEE composite and (c) blend/C30B composite.
Page 33 of 41
Figure 4
Submitted to International Journal of Biological Macromolecules
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
ip t
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 4. SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A composite, (e) blend/C30B composite and (f) blend/MEE composite films.
Page 34 of 41
Figure 5
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 5. SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A composite, (e) blend/C30B composite and (f) blend/MEE composite films hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 35 of 41
Figure 6
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 6. SEM surface morphologies of (a) PLA/PBSA blend, (b) blend/C20A composite, (c) blend/2C30B composite and (d) blend/MEE composite films annealed at 70 °C for 1 h and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 36 of 41
Figure 7
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 7. SEM surface morphologies of (a) PLA/PBSA blend, (b) blend/C20A composite, (c) blend/C30B composite and (d) blend/MEE composite films annealed at 120 °C for 1 h and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 37 of 41
Figure 8
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 8. AFM 15 x 15 µm2 height images of the (a) as prepared blend thin film and (b) blend thin film crystallized at 120 °C from a melt at 190 °C, (c) blend thin film hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K, (d) as-prepared blend/C30B composite thin film, (e) blend/C30B composite thin film crystallized at 120 °C from a melt at 190 °C and (f) blend/C30B composite thin film hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 40 min.
Page 38 of 41
Figure 9
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 9. AFM 5 x 5 µm2 phase images (magnified corresponding images of Fig. 7) of the (a) as prepared blend thin film and (b) blend thin film crystallized at 120 °C from a melt at 190 °C, (c) blend thin film hydrolyzed in Tris-HCl buffer (pH=8.26), (d) as-prepared blend/C30B composite thin film, (e) blend/C30B Composite thin film crystallized at 120 °C from a melt at 190 °C and (f) blend/C30B composite thin film hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 40 min.
Page 39 of 41
Figure 10
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 10. AFM 15 x 15 µm2 height images of (a) blend crystallized thin films crystallized at 100 °C from melt at 190 °C and (b) hydrolyzed in Tris-HCl buffer (pH=8.26)
with Proteinase K for 40 min, (c) blend/C30B thin films crystallized at 100 °C from melt at 190 °C and (d) hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min.
Page 40 of 41
Figure 11
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
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an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 11. AFM 5 x 5 µm2 phase images (magnified corresponding images of Fig. 9) of (a) blend crystallized thin films crystallized at 100 °C from a melt at 190 °C and (b) hydrolyzed in Tris-HCl buffer (pH=8.26) for 40 min, (c) blend/C30B composite thin films crystallized at 100 °C from a melt at 190 °C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min.
Page 41 of 41
S0141-8130(15)00170-1 http://dx.doi.org/doi:10.1016/j.ijbiomac.2015.03.018 BIOMAC 4952
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
31-7-2014 26-2-2015 9-3-2015
Please cite this article as: T. Malwela, S.S. Ray, Enzymatic degradation behavior of nanoclay reinforced biodegradable PLA/PBSA blend composites, International Journal of Biological Macromolecules (2015), http://dx.doi.org/10.1016/j.ijbiomac.2015.03.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Submitted to International Journal of Biological Macromolecules
Revised version
Enzymatic
degradation
behavior
of
nanoclay
cr
ip t
biodegradable PLA/PBSA blend composites
us
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 *
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial
an
1
Research, Pretoria 0001, South Africa.
Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg,
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2
reinforced
South Africa.
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____________
Ac ce p
te
*Corresponding author. Fax: +27128412229; E-mail: [email protected]
1 Page 1 of 41
ABSTRACT Films of a biodegradable PLA/PBSA blend and blend-composites containing 2wt% of C20A,
ip t
C30B and MEE were prepared by solvent casting and spin coating. The films were incubated in vials containing Tris-HCl buffer with proteinase-K, and their weight losses were measured
cr
after enzymatic degradation. The surface morphology before and after degradation tests
us
were studied by SEM and in situ AFM. The results showed that neat PLA had a lower percentage weight loss than neat PBSA, whereas blending them resulted in an increased
an
weight loss. The incorporation of C20A into the as-prepared blend accelerated the degradation rate, whereas C30B and MEE decelerated the degradation rate. Annealing at
M
70°C reduced the degradation rate of the blend, and the presence of nanoclays further reduced the degradation rates. Annealing at 120°C dramatically decelerated the
d
degradation of the blend, whereas the incorporation of nanoclays accelerated the
te
degradations rates. The enhancement of the degradation rates in the presence of nanoclays
Ac ce p
indicated that the degradation rates were mainly controlled by the PLA matrix. Thin films were also cast onto a silicon substrate using a spin coater, and enzymatic degradation on the completely crystalline surfaces revealed that enzymatic attack occurred by pitting and surface erosion of the thin films.
Keywords: biodegradable blend, organoclay, enzymatic degradation
2 Page 2 of 41
1. Introduction The disposal of non-degradable polymers raises environmental concerns, and as result, biodegradable polymers have received significant research interest as suitable
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replacements. Biodegradable polymers have the ability to completely degrade in the presence of microorganisms [1]. This has accelerated the development of naturally
cr
occurring biodegradable polymers, such as cellulose, starch, chitin, polyhydroxyalkanoates
us
and poly(3-hydroxybutyrate), and synthetic polymers, such as polylactide (PLA), poly(butylene succinate) (PBS), poly[(butylene succinate)-co-adipate] (PBSA), and poly(-
an
caprolactone) (PCL). Among these biodegradable polymers, PLA has received more attention because it can be derived from renewable resources and has adaptable degradability [2-5].
M
It has applications in biomedical technologies, such as bioabsorbable surgical sutures and implants [6], controlled drug-delivery systems [7], scaffolds in tissue engineering [8], etc.,
d
and in packaging [9]. Despite PLA being biodegradable, the basic understanding of its
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enzymatic degradation upon blending with another biodegradable polymer, or rather its
Ac ce p
blend composite, still requires better understanding. So far, investigations of the enzymatic degradation of neat PLA [4, 10-19], blends of PLA/PCL [3, 13, 20], PLA/poly(ethylene glycol) (PEG) [21], PLA/poly(vinyl acetate) (PVA) [22, 23], PLA/cellulose [24], PLLA/poly(methyl methacrylate) (PMMA)/poly(ethylene oxide) (PEO) [25] ternary blend and PLA/layeredsilicate nanocomposites [26] have been reported. Most of the enzymatic degradation studies conducted on neat PLA have generally shown that factors such as the stereochemistry, melting temperature, crystallinity, crystal structure, glass transition temperature, molecular weight and molecular weight distribution influence the enzymatic degradation. For example, the study by MacDonald et al. [14] revealed that the degradation rates of PLA decreased with an increase in crystallinity. On the other hand, Tsuji et al. [27] 3 Page 3 of 41
reported that the enzymatic hydrolysis of completely crystallized PLLA films with restricted amorphous regions mainly took place through the chains in the constrained amorphous regions of the spherulites. In a recent in situ AFM study on crystalline PLLA thin films of
ip t
approximately 100 nm, Kikkawa et al. [28] also observed that the degradation of the free amorphous region was faster than that of the restricted regions. Concerning the
cr
stereoregularity, Reeve et al. [4] investigated the effect of polylactide stereochemistry on
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enzymatic degradability and noticed that proteinase K preferentially degraded (L)-PLA over (D)-PLA. Similar observations by Li et al. [29] illustrated that, indeed, proteinase K
an
preferentially degraded L-L ester bonds over the D-D analogue in PLA films.
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The effect on the biodegradation rate of blending PLA with another polymer depends on the biodegradability of the other polymer and the nature of the miscibility. Gajria et al. [22]
d
reported that PLA/PVA formed miscible blends and that their biodegradation rates
te
decreased upon the addition of PVA to PLA. Similar observations were also reported by
Ac ce p
Mahalik and Madras [23]. In the case of immiscible blends, Nagata et al. [24] found that blends of PLLA/trifluoroacetylated cellulose were not miscible and that their degradation rates increased in blends of 90PLLA/10Cellulose and 75PLLA/25Cellulose when compared to that of the PLLA homopolymer. However, the report of Sheth et al. [21] showed that, for blends of PLA/poly(ethylene glycol) (PEG), the miscibility ranged from miscible to partially miscible, depending on the concentrations. The authors observed that the weight loss in all blends was higher than that of neat PLLA when subjected to enzymatic degradation. This weight loss showed that blending increased the degradation rate over that of PLLA. At a lower content of PEG (30% or lower), degradation primarily occurred through the enzymatic degradation of PLA, whereas above 30%, the dissolution of PEG occurred. Tsuji et al. [13] 4 Page 4 of 41
observed that the enzymatic degradation rate of an immiscible PCL/PLLA blend of particulate PLLA-rich domains was higher than that of neat PLA. Yew et al. [30] reported an enhancement of the biodegradability of a PLA/rice-starch blend when natural rubber
ip t
(ENR50) was added and α-amylase acted as a catalyst.
cr
For nanocomposites, a recent study of Singh et al. [26] showed that the incorporation of
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organically modified layered silicates, such as Cloisite®30B (C30B) and Cloisite®15A (C15A), into PLA increased the biodegradation rate, and the effect depends on the organic modifier
an
used in the clays. Thus, a higher biodegradation rate was found in the PLA/C30B than in the PLA/C15A nanocomposites. The higher degradation rate was attributed to the presence of
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excess hydroxyl groups in the organic modifier of the C30B nanoclays and the better dispersion of the C30B nanoparticles in the PLA matrix than for C15A. In the case of
d
PLA/PMMA/PEO ternary blends, Auliawan et al. [25] observed that the addition of
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Cloisite®10A (C10A) and organically modified vermiculite (OMVT) decreased the
Ac ce p
biodegradation rate in the blend. They also noticed that the organic modifier affected the biodegradation differently, whereas C10A had better biodegradability than OMVT. The recent work of Buzarovska et al. [31] reported the decelerated degradation upon TiO2 loading into PLA when using catalysis by the enzyme α-amylase.
However, a literature search shows that studies on the enzymatic degradation of PLA/PBSA blend/clay composites have not yet been reported. The current work will, therefore, present the first elucidation of the effect of enzymatic degradation by proteinase K on a solvent-cast 70PLA/30PBSA blend and its organoclay-modified blend films containing various types of clays. The blend ratio, PLA/PBSA of 70:30 was selected based on the 5 Page 5 of 41
previous work were ratios were varied from 90:10 to 50:50. The blend ratio 70:30 exhibited a uniform dispersed phase (thermodynamically most stable) than others, hence selected for further analysis [32]. The effect on the enzymatic degradation was determined by weight
ip t
loss and the corresponding surface morphologies were examined by scanning electron microscopy (SEM). The enzymatic degradation studies on the completely crystallized thin
cr
films on the silicon (100) substrate were performed using an atomic force microscope (AFM)
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equipped with a hot-stage scanner. Thin films were crystallized at 120 and 100°C from a
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melt at 190°C.
2. Experimental section
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2.1. Materials
The PLA used in this study was a commercial product with a D-content of 1.1-1.7%, and it
d
was supplied by Unitika Co., Ltd., Japan. According to the supplier, it had an average
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molecular weight of Mw = 200 kg/mol, molecular weight distribution = 1.1, density = 1.25
Ac ce p
g/cm3 (ASTM 1238), glass transition temperature of Tg = ~ 60 °C and melting temperature of Tm = ~170°C. On the other hand, the PBSA with the designation BIONOLLE #3001 was obtained from Showa High Polymer (Japan). According to the supplier, it had Mw = 190 kg/mol, molecular weight distribution = 1.4, density = 1.23 g/cm3 (ASTM 1238), Tg = - 43.8°C, and Tm values of 83.1°C (first) and 94.5°C (second). The molar ratio of succinate unit to the adipate unit is w 4:1 and the content of the coupling agent (hexamethylene diisocyanate) unit isw0.5 mol%. The organoclays Cloisite®30B (C30B) and Cloisite®20A were purchased from Southern Products, USA. According to the supplier, the pristine MMT in C30B was modified with 30 wt% of methyl tallow bis(2-hydroxyethyl) quaternary ammonium salt, whereas in C20A, the pristine MMT was modified by dimethyl dehydrogenated tallow 6 Page 6 of 41
quaternary salt. SOMASIF™ MEE clay was purchased from CO-OP chemicals, Japan. According to the supplier, the SOMASIF™ MEE (cation exchange capacity (CEC) = 120 meq/100 g) was modified with approximately 40% of dipolyoxyethylene alkyl (COCO) methyl
ip t
ammonium. Details regarding the clays can be found in Table 1.
cr
2.2. Preparation of PLA/PBSA blend and clay-containing blend composite thin films
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The PLA and PBSA blend with a 70:30 weight ratio was separately dissolved in chloroform. The two solutions were then mixed and sonicated for 15 min. Next, the solution mixture
an
was cast onto a silicon (100) substrate using a spin coater, rotating at 3000 rpm. The thin films were allowed to dry in air prior to the surface analyses. Similarly, the blend composite
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thin films were prepared by separately dissolving 70 wt% PLA and 30 wt% PBSA and dispersing 2 wt% of C20A in chloroform; the two polymer solutions were first mixed and
d
then sonicated for 15 min. Later, the C20A dispersant was added and further sonicated for
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15 min. The solution mixture was cast onto a silicon (100) substrate to form a
Ac ce p
70PLA/30PBSA/2C20A (blend/C20A) composite film. The same method was used to prepare the 70PLA/30PBSA/2C30B (blend/C30B) and 70PLA/30PBSA/2MEE (blend/MEE) composite thin films. The solutions remaining after spin coating were cast onto a glass Petri dish and allowed to dry at room temperature. The surfaces exposed to the air during chloroform evaporation were selected for SEM imaging.
2.3. Enzymatic degradation studies: Measurement of the weight loss The dimensions of 1) the as-prepared and annealed at 70°C and 120°C PLA/PBSA blends and 2) the blends containing 2 wt% of C20A, C30B and MEE were maintained to approximately 6.40 mm x 6.30 mm (length x height), and the thickness ranged from 0.27 mm to 0.51 mm. 7 Page 7 of 41
The weights of the samples were measured prior to immersing them in separate vials containing approximately 5 ml of Tris-HCl buffer (pH=8.60) with Proteinase K. The buffer solution was prepared in a 100-ml flask by adding a sodium azide (N3Na) solution with a
ip t
concentration of 1.538 x 10-4 mol/ml and 20 mg of proteinase K (Sigma, lyophilized powder, with ≥ 30 units/mg protein) while Tris-HCl solution with a concentration of 6.345 x 10-5
cr
mol/ml was added dropwise until pH = 8.60 was achieved. Thus, there was 1 mg of
us
proteinase K in 5 ml of Tris-HCl buffer. The enzymatic degradation studies were performed in an incubated Julabo Bath Shaker (SW22) operated at 200 rpm and 37 °C for
an
approximately 15 h. The samples were then thoroughly washed with distilled water and dried in a vacuum-sealed desiccator with lumps of silica for approximately 48 h. The weights
Ac ce p
2.4. Characterization
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d
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of the samples after enzymatic degradation were measured.
The XRD measurements were performed using an X’Pert PRO diffractometer (PANalytical, the Netherlands) in reflection mode. The operating voltage and current were 45 kV and 40 mA, respectively. The wavelength of the Ni-filtered Cu Kα radiation was 0.1542 nm. The exposure time and scan speed for the XRD measurements were 19.7 min and 0.036987 °/s, respectively.
The intercalated clay particle dispersion in the blend matrix was directly visualized using high-resolution TEM (HRTEM). Sections (70-80 nm) from all samples were obtained with a Leica (Austria) EM FC6 cryo-ultramicrotome set at -120°C and at a cutting speed of 0.07 8 Page 8 of 41
mm/s using a Diatome 35 diamond knife (Diatome, Switzerland). Calibrated images were captured with a JEOL2100 HRTEM (JEOL, Japan) set at an operating voltage of 200 kV and
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using a Gatan Ultrascan camera and Digital Micrograph software.
The thermal properties of neat PLA, PBSA, the 70PLA/30PBSA blend and blends containing 2
cr
wt% C30B, C20A and MEE were studied by DSC (TA instruments, model Q 2000, USA) under
us
a constant nitrogen flow of 25 mL/min. The sample weights from solvent-cast films, cast onto Petri dishes and dried overnight in air, were maintained between 4.75-5.90 mg for all
an
measurements to minimize the possible thermal lag during the scans. The instrument was calibrated using the temperature and heat-of-fusion of an indium standard, and the base
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line was verified according to the TA instrument protocols. To study the crystallization behavior, the samples were heated to 190°C at a heating rate of 10°C/min, held at that
d
temperature for 5 min to erase the previous thermal history, and cooled to -65°C and then
Ac ce p
te
again heated to 190°C at 10°C/min.
A Carl Zeiss (Germany) Auriga SEM was used for imaging the surfaces of the films after hydrolysis. The samples were coated by an Au-Pd alloy, and the instrument was operated at 3 kV.
A multimode AFM (Nano Scope Version (R) IV) using a 0.5-2.0 Ω∙cm phosphorous (n)-doped
Si tip with a curvature radius of less than 10 nm (Veeco Instruments) was used to study the surface morphology of the unmodified and C30B-modified PLA/PBS blend thin films. The RTESPW tip mounted on a 125-µm-long cantilever with a spring constant of 40 N/m was employed for tapping-mode experiments. The samples were imaged using a scan rate of 0.5 Hz, and the tip frequencies were varied from 280-310 kHz. Three different areas on each 9 Page 9 of 41
thin film surface were imaged, thus collecting data for the as-prepared and blend/C30B composite thin films and the crystallized and hydrolyzed thin films. Only representative
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images are reported in this article.
To study the effect of the crystallization temperature and enzymatic degradation on the
cr
surface morphology, the samples were crystallized at 100°C and 120°C from a melt at 190°C,
us
placed in a beaker and hydrolyzed in Tris-HCl buffer (pH=8.26) for 40 min. The hydrolyzed
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thin films were then analyzed by AFM.
3. Results and discussion
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3.1. Thermal analysis and polymer-organoclay interactions
Fig. 1 presents the DSC second-heating thermograms of (a) neat PLA and PBSA and (b) the
d
PLA/PBSA blend and blend-containing organoclays. The measured values of the Tg, cold
te
crystallization (Tc) and melting (Tm) peak temperatures are summarized in Table 2. Neat PLA
Ac ce p
exhibited Tg at 61.2°C, Tc at 131.8°C and Tm at 166.6°C. In the case of neat PBSA, Tg could not be identified, and its Tm peak appeared at 96.4°C. In the blend, the Tg associated with PLA shifted to 58.0°C, Tc to 105.6°C and Tm slightly shifted to a higher temperature of 168.0 °C. The melting peak associated with the PBSA matrix shifted to a lower temperature of 93.3°C. The shift in Tg to 58.0°C, associated with the PLA matrix, indicates that the polymer chains of both polymers interacted to some degree.
The blend-containing 2 wt% C20A showed Tg for PLA at 57.9°C, Tc at 104.7°C and Tm at 167.8°C. The Tm peak of PBSA shifted to 90.3°C. The Tg, Tc and Tm peaks associated with the PLA matrix did not show a significant change compared to those of the pure blend. 10 Page 10 of 41
However, the shift in the Tm peak of PBSA to a lower value suggests that most of the clay platelets interacted with the PBSA matrix or moved into the PBSA matrix. The lowest melting enthalpy of 21.4 J∙g-1 for PBSA further shows that it formed a less crystalline matrix
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than the other blend composites. Similarly, in the case of 2 wt% MEE, the Tg, Tc and Tm peaks for PLA did not show a noticeable change. However, the shift in the melting peak of PBSA to
cr
91.6°C indicates that most of the clay platelets migrated into the PBSA matrix. The inclusion
us
of 2 wt% C30B slightly altered the Tg, Tc and Tm peaks of PLA compared to those of the blend. The minor change in the melting peak of PBSA to 93.6°C also suggests that most clay
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platelets interacted with the polymer chains of PBSA.
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The melting enthalpy for the PBSA in the blend was 24.6 J∙g-1, compared to 30.9 J∙g-1 for PLA. When C20A was incorporated, the melting enthalpy for PBSA decreased to 21.4 J∙g-1, and
d
the melting enthalpy for PLA increased to 34.4 J∙g-1. These observations imply that C20A
te
platelets acted as better nucleating agents in the PLA than in the PBSA matrix; hence, PLA
Ac ce p
exhibited better crystallinity than PBSA when compared to the blend. On the other hand, upon inclusion of MEE in the blend, the melting enthalpies associated with the PBSA and PLA increased. However, the melting enthalpy of PLA showed the highest increase of 37.3 J∙g-1 compared to those of the other blend composites. This shows that MEE platelets acted as nucleating agents better in the PLA than in the PBSA matrix. In the presence of C30B, the melting enthalpies of PBSA and PLA increased. However, C30B exhibited the highest increase in the PBSA enthalpy in comparison to those of the other blend composites. This indicates that the C30B platelets acted as better nucleating agents for the PBSA matrix than for the PLA matrix.
11 Page 11 of 41
These observations generally show that the C20A and MEE clay platelets enhanced the crystallinity of the PLA over that of the PBSA matrix. The presence of a dehydrogenated part in C20A was attributed to the reduced crystallinity in the PBSA. MEE improved the
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crystallinity of both PLA and PBSA, but the highest improvement was observed for the PLA. This indicates that the ʹCOCOʹ in MEE interacted with both the PLA and PBSA matrices. The
cr
presence of C30B clay platelets improved the crystallinity of the PBSA over that of the PLA
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matrix. This effect was attributed to the strong interaction between the C=O bonds present in the PLA and PBSA backbones and the hydroxyl group in the gallery of the C30B silicate
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layers. However, PBSA has more C=O bonds than PLA. As a result, C30B strongly interacted
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with PBSA.
To verify the existence of the interactions of the clay platelets with the two polymers, XRD
d
measurements were performed on the pure organoclay powders and their blend
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composites. The results in Fig. 2 generally show that C20A had a high level of dispersion in
Ac ce p
the blend, followed by MEE and C30B, respectively. The characteristic peak of C20A at 2θ = 3.84° disappeared when it was added to the blend, the peak for MEE that was originally at 2θ = 4.37° shifted to 2θ = 2.97° after it was incorporated to the blend and the peak for C30B that was originally at 2θ = 4.87° shifted to 2θ = 4.77°. The clay galleries for MEE expanded by 1 nm, and they expanded by 0.04 nm for C30B. The disappearance of the first-order peak for C20A indicates that it had a higher degree of gallery expansion than the other clays, followed by MEE and C30B, respectively. The increase in the interlayer spacing of the clay platelets indicates that the polymeric chains can penetrate into the clay gallery. According to the observations above, more polymer chains penetrated into the C20A clay galleries, followed by MEE and C30B. The degree of intercalation shows the relationship to the 12 Page 12 of 41
original d001-spacing of the organoclays: 2.30, 2.02 and 1.81 nm for C20A, MEE and C30B, respectively. To support the conclusions based on XRD results, TEM analysis of the composites was conducted, and results are presented in Fig. 3. Although it was difficult to
ip t
identify the two phases in the blend composites, the TEM results show a higher degree of intercalation of the polymer chains in the case of the blend/C20A composite. Comparing the
cr
XRD results to the DSC results, the crystallinity of PBSA decreased with an increased degree
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of dispersion of the organoclays in the blend matrix.
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3.2 Effect of enzymatic degradation on the as-prepared films
Fig. 4 shows SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c)
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PLA/PBSA blend, (d) blend/C20A, (e) blend/C30B and (f) blend/MEE films. These images were taken on the side of the film that was exposed to air during solvent (chloroform)
d
evaporation. The surfaces reveal fibrous structures that were induced by beam heat. These
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fibrous structures can be clearly observed for PLA/PBSA blend (Fig. 4(c) and blend/C20A (Fig.
Ac ce p
4(d)) films, respectively. Fine fibrous structures can be seen for blend/C30B (Fig. 4(e)) and blend/MEE (Fig. 4(f)), respectively. However, in a case where the samples were exposed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h, a dramatic change on morphologies was observed. Fig. 5 depicts the SEM surface morphologies of the as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A, (e) blend/C30B and (f) blend/MEE composite films hydrolyzed in Tris-HCl buffer (pH=8.60) for 15 h. These images were taken on the side of the film that was exposed to air during the solvent (chloroform) evaporation. The roughness and pitting on the film surfaces indicate that enzymatic degradation occurred. Figs. 5(a), (d) and (f) show that erosion occurred on some parts of the surface, and Figs. 5(c) and (e) indicate uniform surface erosion. The morphology in Fig. 5(b) does not 13 Page 13 of 41
clearly reveal the effect of enzymatic degradation because the surface morphologies were inadequate in determining the extent of degradation. The initial weight and the weight after enzymatic degradation were measured and recorded in Table 3, and the equation below
ip t
was used to calculate the percentage of weight loss (%Wloss), where Wi is the initial weight
− × 100
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% =
cr
and Wf is the weight after enzymatic degradation.
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According to the weight-loss measurements, neat PBSA lost more weight than neat PLA. This indicates that neat PBSA has a higher enzymatic degradation rate than neat PLA.
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Blending two polymers (70 wt% PLA and 30 wt% PBSA) resulted in an increase in the weight loss. The presence of PBSA in the PLA matrix enhanced the degradation rate of the blend.
d
This improvement can be attributed to the morphology of the blend and the fact that PBSA
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degrades faster than PLA. These two polymers usually produce phase-separated
Ac ce p
morphologies, in which the enzymatic attack is believed to start mainly at the interfaces of the two polymers. In the presence of 2 wt% C20A, a weight loss that was higher than that of the blend and the other blend composites was observed. This suggests that the inclusion of C20A reduced the sizes of the dispersed PBSA phases, therefore maximizing the number of interfaces between the two polymers. Then, these interfaces accelerated the enzymatic attack, resulting in an increased degradation rate over that of the blend. This argument is supported by the XRD measurement in Fig. 2 that showed a high level of dispersion of C20A when it was added to the PLA/PBSA blend, followed by decreased dispersion of MEE and C30B.
14 Page 14 of 41
The presence of 2 wt% C30B and MEE also lowered the weight loss compared to that of the blend. Between the two, C30B has a higher weight loss than MEE. In this case, C30B lowered the weight loss, indicating a deceleration in the enzymatic degradation rate in the blend.
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Apart from the reduction of the PBSA phase size, two effects can be attributed to this: (a) that C30B enhanced the crystallinity of PBSA and (b) that it further improved the properties
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of the interfaces.
The lowest weight loss in the presence of MEE in the blend compared to those in the blend
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and blend composites was attributed to the enhanced crystallinity of both the PLA and PBSA matrices. Furthermore, the XRD results also showed that MEE had a better dispersion in the
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blend than C30B. Both the crystallization and the dispersion of the PBSA phase are believed
d
to have contributed to the decelerated enzymatic degradation rate.
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Combining these observations with the DSC results in Table 2, it can be observed that PBSA
Ac ce p
had a higher melting enthalpy of 31.8 J∙g-1 in the presence of C30B, followed by MEE with 27.1 J∙g-1 and C20A with 21.4 J∙g-1. The larger the melting-enthalpy value, the more melted the crystal was. This implies that PBSA formed more crystals in the presence of C30B, followed by MEE and C20A, respectively. According to these observations, the enzymatic degradation rates decreased with increased crystallinity. This agrees with the previous studies of MacDonald et al.14 and Kikkawa et al. [28] that illustrated that the degradation of PLA decreased with increased crystallinity.
These observations generally indicate that, on as-prepared films, neat PBSA had a higher degradation rate than neat PLA. Upon blending them, the degradation rate was enhanced. 15 Page 15 of 41
The presence of 2 wt% C20A further enhanced the degradation rate, whereas the presence of 2 wt% C30B and MEE decelerated the degradation rate of the blend. Compared to the DSC measurements, the weight loss increased upon the reduction in crystallinity of PBSA in
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the blend. The findings further showed that the degradation rates of the blend composite depended on the degree of the intercalated clay particle dispersion in the blend matrix and
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cr
the impact the particles have on the crystallinity of both the PLA and PBSA matrices.
3.3. Effect of the annealing temperature on enzymatic degradation
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The films were annealed at 70°C for 1 h, a temperature at which PBSA will fully crystallize. The weight-loss measurements after enzymatic hydrolysis show a decrease when compared
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to the as-prepared films (see Table 4). The inclusions of organoclays further reduced the weight loss. This decrease generally indicates that the enzymatic degradation rate was
d
decelerated. The crystalline PBSA phases improved the film’s resilience to enzymatic attack.
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This agrees with the report of Kikkawa et al. [28], which showed that the crystalline sections
Ac ce p
of PLLA thin films degraded more slowly than the amorphous regions. Fig. 6(a) and Fig.6(b) for the PLA/PBSA blend and the blend/C20A composite films, respectively, had similar surface erosion. The blend/C30B and blend/MEE composite films in Fig. 6(c) and (d), respectively, show that enzymatic degradation occurred through pitting.
These observations suggest that the higher degradation rate mainly occurred through surface erosion, whereas the lower degradation happened through pitting. The weight-loss measurements have generally shown that, for films annealed at 70°C, the presence of organoclays decelerated the enzymatic degradation rates when compared to those of the blend. 16 Page 16 of 41
The films were also annealed at 120°C, and the surface morphologies in Fig. 7 indicate that enzymatic degradation occurred through pitting. The blend with the lowest weight loss of 0.09% contains small pores, whereas the blend/C20A composite with the highest weight of
ip t
0.66% contains larger pores (refer to Table 5 and Fig. 7). Annealing at 120°C dramatically decelerated the enzymatic degradation rate compared to those of the as-prepared samples
cr
and those annealed at 70°C. In this case, the presence of organoclays enhanced the
us
degradation rates compared to those for the pure blend. At this temperature, PLA is expected to fully crystallize, whereas PBSA should be in a molten state. However, PBSA
M
contributed to the enzymatic degradation rates.
an
crystallizes upon cooling to room temperature (25°C). In this case, both polymers
In summary, the enzymatic degradation studies revealed that PBSA mainly controlled the
d
degradation rates of the blend and blend composites of the as-prepared films. The presence
te
of C20A accelerated the degradation rates, but C30B and MEE decelerated the degradation
Ac ce p
rates. This was attributed to the enhanced crystallinity in the PBSA matrices induced by these clays (C30B and MEE). This effect was clearly observed on the films annealed at 70°C in which the PBSA fully crystallizes and the degradation rates decreased. In the films annealed at 70°C, the presence of organoclays decelerated the enzymatic degradation rate. In this case, the organoclays are believed to further enhance the crystallinity of the PBSA phases. For films annealed at 120°C, the enzymatic degradation rates were decelerated. At this temperature, PLA is expected to fully crystallize but PBSA in a molten state. When cooled to room temperature, PBSA is again expected to crystallize. The addition of organoclays accelerated the degradation rates. In this case, PLA should be more crystalline
17 Page 17 of 41
than PBSA. This effect suggests that PBSA controlled the degradation rates because the presence of organoclays in the PLA matrix improves its enzymatic degradability.
ip t
3.4. Enzymatic degradation of the blend and blend/C30B thin films studied by AFM Based on the DSC measurements, the organoclay (C30B) improved the crystallinity of both
cr
polymers in the blend when compared to the other organoclays (C20A and MEE), and it also
us
gave similar Tg, Tc and Tm peaks for PLA and a similar melting peak for PBSA to those of the blend. For this reason, the blend/C30B composite was selected for enzymatic studies with
an
AFM equipped with a hot-stage scanner. Fig. 8 depicts AFM height images of thin films of the as-prepared blend and blend/C30B (far left), crystallized at 120°C from a melt at 190°C
M
(middle), and crystallized at 120 °C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min (far right). The as-prepared blend in Fig. 8(a) contained a few large,
d
dispersed PBSA phases, which were reduced in the presence of 2 wt% C30B (see Fig. 8(d)).
te
This indicates that the nanoclays improved the compatibility between PLA and PBSA. When
Ac ce p
crystallized at 120°C from the melt, the crystal growth on the blend and blend/C30B appear to be similar. Their corresponding phases in Figs. 9(a) and (e) (5 x 5 µm2), show that the morphology comprised both edge-on (fibrous-sections of the image) and flat-on lamellae (flat sections of the image).
However, the flat-on lamellae in the blend composite tend to form leaf-like structures (see Fig. 9(e)). Fig. 8(c) and (f) for the blend and blend composites, respectively, show that, after immersing the thin film surfaces in Tris-HCl buffer with Proteinase K for 40 min, degradation formed holes on the films surfaces. These holes are uniformly distributed on the thin-film surface. The presence of these pores indicates that enzymatic degradation occurred; 18 Page 18 of 41
however, based on these morphological changes, it is difficult to determine the extent of the degradation. However, the morphology of a phase image for the blend in Fig. 8(c) shows that there was more surface erosion than in the blend containing 2 wt% C30B, Fig. 8(f).
ip t
The surface of the blend thin film was dominated by flat-on lamellae, whereas the blend composite was dominated by edge-on lamellae. This agrees with the weight-loss
cr
measurements that showed that enzymatic degradation of the blend took place upon the
us
addition of C30B.
an
Fig. 10 depicts the thin films of the blend and blend composite crystallized at 100°C from a melt at 190°C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min. The
M
height images do not clearly distinguish the difference in the crystalline morphology.
te
(refer to Fig. 11).
d
However, their corresponding phase images taken on magnified areas depict the difference
Ac ce p
The blend morphology in Fig. 11(a) formed both edge-on and flat-on lamellae, and several circular phases attributed to PBSA were observed throughout the film surface. On the other hand, the blend composite was dominated by flat-on lamellae and few circular dark phases (in Fig. 11(b)). After enzymatic degradation, the blend image in Fig. 10(b) indicates that pores were formed during hydrolysis.
These pores are well distributed throughout the film surface. In this case, the blend composite appeared to have a smaller number of pores than the blend (see Fig. 10(d)). The corresponding phase image in Fig. 11(d) taken on a magnified area illustrates the reduction of pores. The phase image in Fig. 11(b) shows more surface erosion than in Fig. 11(d). This 19 Page 19 of 41
observation indicates that, regardless of the change in the crystalline temperature, the presence of nanoclays reduced the degradation through pitting. Because enzymatic degradation occurs faster on amorphous regions than on crystalline regions, this pitting can
ip t
imply that these completely crystallized spherulites contain uniformly distributed
cr
amorphous regions.
us
4. Conclusions
The results presented in this article indicated that the as-prepared, neat PBSA degraded
an
faster than PLA; however, blending two polymers increased the biodegradation rate over their initial individual degradation rates. The increased degradation rate was attributed to
M
the immiscibility of the two polymers that formed phase-separated morphologies that created more exposure for the enzymatic attack. The inclusion of C20A into the blend
d
accelerated the enzymatic degradation rate, but C30B and MEE decelerated the degradation
te
rates. The XRD results showed a high level of intercalation in the presence of C20A,
Ac ce p
suggesting that the numbers of dispersed phases and interfaces increased. This was associated with the enhanced degradation rate. The deceleration in the case of C30B and MEE was attributed to the enhanced crystallinity of the PBSA matrices, as seen from the DSC results. Films annealed at 70 °C showed a decelerated enzymatic degradation rate, and the addition of organoclays further decreased the degradation rates. This shows that the crystalline PBSA matrices improved the resilience to enzymatic attacks. The addition of nanoclays further enhanced the crystallinity. For films that were annealed at 120 °C, the blend showed a dramatic deceleration of the degradation rate. The presence of organoclays improved the degradation rates, which indicates that PLA mainly controlled the degradation rates of the blend. AFM images revealed that thin films of the fully crystalline PLA/PBSA 20 Page 20 of 41
blend and blend/C30B composite showed that enzymatic degradation occurred through pitting and surface erosion. The extent of the degradation rate could not be determined
ip t
with this technique.
Acknowledgments: The authors would like to thank the CSIR, the DST, and the NRF, South
us
cr
Africa for financial support
References
an
[1] A. Torres, S. M. Li, S. Roussos, M. Vert, Poly(lactic acid) degradation in soil or under controlled conditions, J. Appl. Polym. Sci. 62 (1996) 22952302.
M
[2] Y. Tokiwa, B. P. Calabia, Biodegradability and biodegradation of poly(lactide), Appl. Microbiology and Biotechnology 72 (2006) 244251.
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[3] L. Liu, S. Li, H. Garreau, M. Vert, Selective enzymatic degradations of poly(l-lactide) and
te
poly(ε-caprolactone) blend films, Biomacromolecules 1 (2000) 350359.
Ac ce p
[4] M. S. Reeve, S. P. McCarthy, M. J. Downey, R. A. Gross, Polylactide stereochemistry: effect on enzymic degradability, Macromolecules 27 (1994) 825831.
[5] S. J. Holland, B. J. Tighe, P. L. Gould, Polymers for biodegradable medical devices. 1. The potential of polyesters as controlled macromolecular release systems, J. Control. Release. 4 (1986) 155180.
[6] M. S. Taylor, A. U. Daniels, K. P. Andriano, J. Heller, Six bioabsorbable polymers: In vitro acute toxicity of accumulated degradation products, J. Appl. Biomat. 5 (1994) 151157. [7] P. McDonald, J. Lyons, L. Geever, C. Higginbotham, In vitro degradation and drug release from polymer blends based on poly(dl-lactide), poly(l-lactide-glycolide) and poly(εcaprolactone), J. Mater. Sci. 45 (2010) 12841292. 21 Page 21 of 41
[8] J. Sarasua, Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications, J. Mater. Sci: Mater. Medicine. 22 (2011) 25132523. [9] R. Auras, B. Harte, S. Selke, An overview of polylactides as packaging materials,
ip t
Macromol. Biosci. 4 (2004) 835864.
[10] H. Tsuji, S. Miyauchi, Enzymatic hydrolysis of poly(lactide)s: effects of molecular
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weight, l-Lactide content, and enantiomeric and diastereoisomeric polymer blending,
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Biomacromolecules 2 (2001) 597604.
[11] H. Tsuji, M. Ogiwara, S. K. Saha, T. Sakaki, Enzymatic, alkaline, and autocatalytic
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degradation of poly(l-lactic acid): effects of biaxial orientation, Biomacromolecules 7 (2005) 380387.
M
[12] H. Tsuji, S. Miyauchi, Poly(l-lactide): 7. Enzymatic hydrolysis of free and restricted amorphous regions in poly(l-lactide) films with different crystallinities and a fixed
te
d
crystalline thickness, Polymer 42 (2001) 44634467. [13] H. Tsuji, Y. Kidokoro, M. Mochizuki, Enzymatic degradation of biodegradable polyester
Ac ce p
composites of poly(L-lactic acid) and poly(ε-caprolactone), Macromol. Mater. Eng. 291 (2006) 12451254.
[14] R. T. MacDonald, S. P. McCarthy, R. A. Gross, Enzymatic degradability of poly(lactide): Effects of chain stereochemistry and material crystallinity, Macromolecules 29 (1996) 73567361.
[15] H. Pranamuda, A. Tsuchii, Y. Tokiwa, Poly (L-lactide)-degrading enzyme produced by amycolatopsis sp, Macromol. Biosci. 1 (2001) 2529. [16] S. Li, S. McCarthy, Influence of crystallinity and stereochemistry on the enzymatic degradation of poly(lactide)s, Macromolecules 32 (1999) 44544456.
22 Page 22 of 41
[17] K. Yamashita, Y. Kikkawa, K. Kurokawa, Y. Doi, Enzymatic degradation of poly(l-lactide) film by proteinase K: quartz crystal microbalance and atomic force microscopy study, Biomacromolecules 6 (2005) 850857.
degradation of poly(lactide)s, Macromolecules 32 (1999) 44544456.
ip t
[18] S. Li, S. McCarthy, Influence of crystallinity and stereochemistry on the enzymatic
cr
[19] D. F. Williams, Enzymatic hydrolysis of polylactic acid, Eng. Medicine 10 (1981) 57.
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[20] G. Sivalingam, S. P. Vijayalakshmi, G. Madras, Enzymatic and thermal degradation of poly(ε-caprolactone), poly(D,L-lactide), and their blends, Indust. Eng. Chem. Res. 43
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(2004) 77027709.
[21] M. Sheth, R. A. Kumar, V. Davé, R. A. Gross, S. P. McCarthy, Biodegradable polymer
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blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 14951505.
te
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[22] A. M. Gajria, V. Davé, R. A. Gross, S. McCarthy, Miscibility and biodegradability of blends of poly(lactic acid) and poly(vinyl acetate), Polymer 37 (1996) 437444.
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[23] J. P. Mahalik, G. Madras, Enzymatic degradation of poly(D,L-lactide) and its blends with poly(vinyl acetate), J. Appl. Polym. Sci. 101 (2006) 675680.
[24] M. Nagata, F. Okano, W. Sakai, N. Tsutsumi, Separation and enzymatic degradation of blend films of poly(L-lactic acid) and cellulose, J. Polym. Sci. Part A: Polym. Chem. 36 (1998) 18611864.
[25] A. Auliawan, E. M. Woo, Crystallization kinetics and degradation of nanocomposites based on ternary blend of poly(L-lactic acid), poly(methyl methacrylate), and poly(ethylene oxide) with two different organoclays, J. Appl. Polym. Sci. 125 (2012) 444458.
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[26] N. K. Singh, Enzymatic degradation of polylactide/layered silicate nanocomposites: Effect of organic modifiers, J. Appl. Polym. Sci. 127 (2013) 24652474. [27] H. Tsuji, S. Miyauchi, Poly(l-lactide): VI Effects of crystallinity on enzymatic hydrolysis of
ip t
poly(l-lactide) without free amorphous region, Polym. Degrad. Stabil. 71 (2001) 415424.
cr
[28] Y. Kikkawa, H. Abe, T. Iwata, Y. Inoue, Y. Doi, Crystallization, stability, and enzymatic
us
degradation of poly(l-lactide) thin film, Biomacromolecules 3 (2002) 5056.
[29] S. Li, A. Girard, H. Garreau, M. Vert, Enzymatic degradation of polylactide
an
stereocopolymers with predominant d-lactyl contents, Polym. Degrad. Stabil. 71 (2000) 6167.
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[30] G. H. Yew, A. M. Mohd Yusof, Z. A. Mohd Ishak, U. S. Ishiaku, Water absorption and
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90 (2005) 488500,
d
enzymatic degradation of poly(lactic acid)/rice starch composites, Polym. Degrad. Stabil.
[31] A. Buzarovska, A. Grozdanov, Biodegradable poly(L -lactic acid)/TiO2 nanocomposites:
Ac ce p
Thermal properties and degradation, J. Appl. Polym. Sci. 123 (2012) 21872193.
[32] T. Malwela, S. S. Ray, Investigating the crystal growth behavior of the biodegradable polymer blend thin films using in situ atomic force microscopy. Macromol. Mater. Eng. 299 (2014) 689-697.
24 Page 24 of 41
Submitted to International Journal of Biological Macromolecules
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Tg
Tm, PBSA
ΔHm,PBSA
ΔHc
ΔHm,PLA
/ °C
/ J.g
/ °C
/ J.g
/ °C
/ J.g-1
Neat PLA
61.2
-
-
131.8
26.2
166.6
32.0
Neat PBSA
-
96.4
40.5
-
-
-
-
Blend
58.0
93.3
24.6
105.6
22.0
168.0
30.9
Blend/2C20A
57.9
90.3
21.4
104.7
14.0
167.8
34.4
Blend/2MEE
57.6
91.6
27.1
104.4
16.3
167.7
37.3
Blend/2C30B
57.6
93.6
31.8
106.8
22.7
168.0
34.7
te
Ac ce p
-1
Tm,PLA
/ °C
d
-1
Tc
M
Sample Name
an
Table 2. Data calculated from the DSC second heating of neat PLA, neat PBSA, the PLA/PBSA blend and various blend/clay composites.
26 Page 25 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
an
Table 3. Neat PLA and PBSA, their blend and blend composite films hydrolyzed in TrisHCl buffer (pH=8.60) with Proteinase K for 15 h. Initial weight /mg 23.97
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.85
Weight loss /% 0.50
Neat PBSA
19.33
19.20
0.67
Blend
23.6
23.43
0.72
Blend/2MEE
18.84
18.74
0.53
Blend/2C30B
18.69
18.58
0.59
25.20
0.75
Ac ce p
te
d
M
Sample Neat PLA
Blend/2C20A
25.39
27 Page 26 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
an
Table 4. The PLA/PBSA blend and blend composite films annealed at 70 °C and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h. Initial weight /mg 23.3
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.23
Weight loss /% 0.30
Blend/2MEE
15.54
15.50
0.26
Blend/2C30B
17.99
17.95
0.22
Blend/2C20A
17.86
17.81
0.28
Ac ce p
te
d
M
Sample Blend
28 Page 27 of 41
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Initial weight /mg 23.09
Blend/2MEE
14.74
Blend/2C30B
15.6
Blend/2C20A
15.07
Weight when hydrolysed in Tris-HCl (pH=8.60) 23.07
Weight loss /% 0.09
d
Sample Blend
M
an
Table 5. The neat PLA/PBSA blend and blend composites annealed at 120 °C and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
0.20
15.56
0.26
14.97
0.66
Ac ce p
te
14.71
29 Page 28 of 41
Highlights
an
us
cr
ip t
Films of a biodegradable PLA/PBSA blend and blend-composites were prepared by solvent casting and spin coating First elucidation of the effect of enzymatic degradation by proteinase K using AFM with a hot-stage scanner The degradation rates of the composites increased in the presence of organoclays Results indicated that the degradation rates were mainly controlled by the PLA matrix.
Ac ce p
te
d
M
[33]
42 Page 29 of 41
ip t
Table 1
cr
Submitted to International Journal of Biological Macromolecules
us
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/ poly[(butylene succinate)-co-adipate] blend composites Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
an
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa.
Table 1. Characteristic parameters of the various organoclays. Organic Modifier
Chemical formula
MMT
C20A
Dimethyl dehydrogenated tallow, quaternary ammonium salt
M
Organoclay
CH3
Mica
C30B
methyl tallow bis(2hydroxyethyl) quaternary ammonium salt
MEE
N+
H3C
Particles length/nm
Cation exchange capacity (CEC), mequiv/100g
d-spacing /nm
~ 100 – 150
95
2.42
~100 – 150
90
1.85
~200 – 300
120
2.30
HT
HT
CH2CH2OH H3C
Ac c
MMT
ep te
d
Clay
N+
T
CH2CH2OH
Dipolyoxy ethylene alkyl (COCO) methyl ammonium salt
R(COCO) CH3
N+
(CH2CH2O)xH
(CH2CH2O)yH x+y=2
Page 30 of 41
Figure 1
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Ac
Fig. 1. DSC second heating thermograms of (a) neat PLA and PBSA and (b) PLA/5C30B, PLA/5MEE, PLA/5C20A composites.
Page 31 of 41
Figure 2
Submitted to International Journal of Biological Macromolecules
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
cr
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
8000
C20A 70PLA/30PBSA/2C20A
(a)
25000
MEE 70PLA/30PBSA/2MEE
(b)
20000
Intensity / a.u.
M
6000
4000
2000
0 4
ed
Intensity / a.u.
an
us
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
6
8
15000 10000 5000 0
10
2
4
ce pt
2-theta / degrees
6
8
10
2-theta / degrees
10000
C30B 70PLA/30PBSA/2C30B
(c)
Intensity / a.u.
Ac
8000 6000 4000 2000 0 4
6
8
10
2-theta / degrees
Fig. 2. X-ray diffraction patterns of (a) pure C20A powder and the blend/C20A composite, (b) pure MEE powder and the blend/MEE composite and (c) pure C30B powder and the blend/C30B composite.
Page 32 of 41
Figure 3
Submitted to International Journal of Biological Macromolecules
cr
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 *
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
1
an
us
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
(b)
ed
M
(a)
Ac
ce pt
(c)
Fig. 3. TEM images of the (a) blend/C20A composite, (b) blend/MEE composite and (c) blend/C30B composite.
Page 33 of 41
Figure 4
Submitted to International Journal of Biological Macromolecules
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
ip t
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 4. SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A composite, (e) blend/C30B composite and (f) blend/MEE composite films.
Page 34 of 41
Figure 5
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 5. SEM surface morphologies of as-prepared (a) neat PLA, (b) neat PBSA, (c) PLA/PBSA blend, (d) blend/C20A composite, (e) blend/C30B composite and (f) blend/MEE composite films hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 35 of 41
Figure 6
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 6. SEM surface morphologies of (a) PLA/PBSA blend, (b) blend/C20A composite, (c) blend/2C30B composite and (d) blend/MEE composite films annealed at 70 °C for 1 h and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 36 of 41
Figure 7
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 7. SEM surface morphologies of (a) PLA/PBSA blend, (b) blend/C20A composite, (c) blend/C30B composite and (d) blend/MEE composite films annealed at 120 °C for 1 h and hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 15 h.
Page 37 of 41
Figure 8
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 8. AFM 15 x 15 µm2 height images of the (a) as prepared blend thin film and (b) blend thin film crystallized at 120 °C from a melt at 190 °C, (c) blend thin film hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K, (d) as-prepared blend/C30B composite thin film, (e) blend/C30B composite thin film crystallized at 120 °C from a melt at 190 °C and (f) blend/C30B composite thin film hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 40 min.
Page 38 of 41
Figure 9
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 9. AFM 5 x 5 µm2 phase images (magnified corresponding images of Fig. 7) of the (a) as prepared blend thin film and (b) blend thin film crystallized at 120 °C from a melt at 190 °C, (c) blend thin film hydrolyzed in Tris-HCl buffer (pH=8.26), (d) as-prepared blend/C30B composite thin film, (e) blend/C30B Composite thin film crystallized at 120 °C from a melt at 190 °C and (f) blend/C30B composite thin film hydrolyzed in Tris-HCl buffer (pH=8.60) with Proteinase K for 40 min.
Page 39 of 41
Figure 10
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 10. AFM 15 x 15 µm2 height images of (a) blend crystallized thin films crystallized at 100 °C from melt at 190 °C and (b) hydrolyzed in Tris-HCl buffer (pH=8.26)
with Proteinase K for 40 min, (c) blend/C30B thin films crystallized at 100 °C from melt at 190 °C and (d) hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min.
Page 40 of 41
Figure 11
Submitted to International Journal of Biological Macromolecules
Thomas Malwela1, 2 and Suprakas Sinha Ray1, 2 * 1
ip t
Enzymatic degradation behavior of nanoclay reinforced biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites
Ac
ce pt
ed
M
an
us
cr
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa. 2 Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South Africa
Fig. 11. AFM 5 x 5 µm2 phase images (magnified corresponding images of Fig. 9) of (a) blend crystallized thin films crystallized at 100 °C from a melt at 190 °C and (b) hydrolyzed in Tris-HCl buffer (pH=8.26) for 40 min, (c) blend/C30B composite thin films crystallized at 100 °C from a melt at 190 °C and hydrolyzed in Tris-HCl buffer (pH=8.26) with Proteinase K for 40 min.
Page 41 of 41