- Email: [email protected]
Some novel layered-silicate nanocomposites based on a biodegradable hydroxybutyrate copolymer
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 3128–3135
www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Some novel layered-silicate nanocomposites based on a biodegradable hydroxybutyrate copolymer
MACROMOLECULAR NANOTECHNOLOGY
Xiujuan Zhang a, Gui Lin a, Reda Abou-Hussein a, Mohammad K. Hassan Isao Noda c, James E. Mark a,* a
a,b
,
Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, OH 45221-0172, United States b Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076, United States c The Procter & Gamble Company, 8611 Beckett Road, West Chester, OH 45069, United States Received 12 March 2007; received in revised form 19 April 2007; accepted 27 April 2007 Available online 22 May 2007
Abstract This paper describes the preparation, characterization, mechanical properties and thermal stability of layered silicate nanocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-co-PHH, known as NodaxTM). The dispersed phases were organically modified montmorillonites (clay 20A and clay 25A), mica, and talc, and they were introduced by solution mixing. Wide-angle X-ray scattering results and transmission electron microscopy (TEM) images confirmed that these two clays were intercalated and finely distributed in the NodaxTM matrix. This type of layered filler led to remarkable improvements in mechanical properties even at very low loadings. Minimizing aggregation was more of a problem in the case of the mica and the talc, at least in this particular matrix. In some cases, these layered fillers slightly decreased the thermal stability of the NodaxTM. 2007 Elsevier Ltd. All rights reserved. Keywords: Hydroxybutyrate copolymers; NodaxTM; Clays; Mica; Talc; Mechanical properties
1. Introduction Biodegradable polymers can be degraded upon disposal in bioactive environments by organisms, such as bacteria, fungi, and algae, or by hydrolysis in buffer solutions or sea water. Biodegradable polymers are attractive substitutes for many synthetic materials because they can alleviate problems associated with solid-waste disposal. Worldwide con-
*
Corresponding author. E-mail address: [email protected] (J.E. Mark).
sumption of such biodegradable polymers has increased from 14 million kg in 1996 to an estimated 68 million kg in 2001. Poly(3-hydroxyalkanoates) (PHA), a class of biodegradable and biocompatible thermoplastics, has attracted much research interest and industrial attention as a substitute for synthetic materials [1–6]. They are produced directly from renewable resources by microbes, which accumulate PHA as carbon and energy storage materials under unbalanced growth conditions. The copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (NodaxTM) is one of the most promising biodegradable semi-crystalline aliphatic polyesters of this
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.04.043
PHA family [7–9]. This is primarily due to the fact that it has a unique combination of properties, such as full anaerobic degradability, moisture resistance, good barrier properties, relatively high shelf stability (under distribution and home-storage conditions), processability on conventional equipment, and easy dyeability [2]. It has potential applications in the manufacture of storage bags, shopping bags, and slip covers, coatings in automobiles and furniture. This copolymer has the advantage of biodegradability, but is somewhat lacking in strength [3,4]. In previous studies, we showed that an orientation technique induced by pre-stretching a sample was a feasible method to give this material improved mechanical properties [10,11]. Incorporation of exfoliated layered silicates, especially organically modified clays [12–15], to give nanocomposites has attracted considerable attention since the earliest work of some Toyota Corporation researchers [16]. Such introduction of layered silicates into PHA copolymers has not been much studied, especially in the case of NodaxTM. The present study is an attempt to further improve the mechanical properties of this important biodegradable polyester, by incorporating organically modified layered silicates by solution intercalation. Although the incorporation of clay particles is emphasized, mica and talc have been studied as well. The focus is on the morphology and dispersion of these layered silicates in NodaxTM, and the effects of these novel nanoparticles on crystallinity, mechanical properties, and biodegradability.
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consisting of 87–89 mol% 3-hydroxybutyrate, and 11–13 mol% 3-hydroxyhexanoate. It had a number-average molecular weight of 500,000– 600,000 g mol 1 and a polydispersity index Mw/ Mn of 1.9–2.1 (based on polystyrene standards). The layered-silicates, modified with quantum ammonium cations, were purchased from Southern Clay Products Inc., Gonzales, Texas (with designations Cloisite 20A and 25A). The chemical structures of the cations used in the modification of 20A and 25A are shown, respectively, in Fig. 2a and b. The average distances between the layers of these clays 20A and 25A were, respectively, about ˚ . Mica (H4Al6K2O24Si6, 325 mesh, 24.2 and 18.6 A water ground) and talc (3MgO Æ 4SiO2 Æ H2O) were the products of Spectrum Quality Products Inc. Garden, CA and the Aldrich Chemical Company Inc., respectively. All organic solvents were highperformance liquid chromatography grades and were purchased from the Aldrich Company. 2.2. Sample preparations NodaxTM/layered silicate nanocomposites were prepared by solution intercalation. First, NodaxTM and 1, 3, 5, 7, 10 and 15 wt% layered silicate (clays 20A, 25A, mica, and talc) were separately dissolved or dispersed in chloroform and the two resulting dispersions stirred for 1 h, followed by ultrasonic treatment for another hour. Finally, the product was dried and thermal compressed in preparation for the characterizations of morphology and properties.
2. Experimental 2.3. Characterization 2.1. Materials The NodaxTM poly(3-hydroxybutyrate-co-3hydroxyhexanoate), whose structure is shown in Fig. 1, was provided by the Procter & Gamble Co., West Chester, Ohio and was used as received. Based on NMR results, it was a random copolymer
H2 C
O CH CH3
H2 C
O C x O
X-ray diffraction (XRD) measurement of neat NodaxTM, the layered silicates, and the corresponding composites films was carried out at room temperature using an X-ray generator (a Siemens D500 diffractometer) (Cu Ka radiation, with ˚ ). The 2h scan range 30 mA, 40 kV, k = 1.5406 A
CH CH2
HT C
HT
y
O
CH2 CH3
Fig. 1. Molecular structure of poly(3-hydroxybutyrate-co-3hydroxyhexanoate).
H3C
N CH3
HT
H3C
N CH3
CH2 CHCH2CH2CH2CH3 CH2CH3
Fig. 2. Chemical structures of the cations (a) and (b) used to modify clay20A and clay25A, respectively, where HT is hydrogenated tallow (65% C18, 30% C16, 5% C1).
MACROMOLECULAR NANOTECHNOLOGY
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
was 2–15, with a step size of 0.05 and a time per step of 1 s. Transmission electron microscopy (TEM) imagines were obtained using a JEM 1230 EX-II instrument (JEOL, Tokyo, Japan) operated at an acceleration voltage of 80 kV. All of the ultrathin sections (less than 200 nm) were microtomed using a Super NOVA 655001 instrument (Leica, Swiss) with a glass knife and were then subjected to TEM observation without staining. The values of the modulus and tensile strength of samples having dimensions of 30 · 5 · 1 mm3 were measured at room temperature using a fully computerized Instron mechanical tester (Model 1122). The initial gauge length was 30 mm and the cross-head speed was 5 mm/min. The tensile properties of greatest interest were the Young’s moduli and tensile strengths at break and values were obtained as an average of at least five measurements.
Thermogravimetric analysis (TGA) was done on a TA Instrument (TGA 2050 thermogravimetric analyzer) with nitrogen as purging gas. Tests were conducted from 40 to 450 C at a heating rate of 20 C/min. 3. Results and discussion 3.1. Morphology Fig. 3 shows the XRD patterns in the range 2h = 2–15 for the neat NodaxTM, clay 20A, clay 25A, mica, and talc, and their corresponding composites with various amounts of the layered silicates. The pattern of the neat NodaxTM is displayed as a baseline to compare the existence of diffraction peaks coming from the layered silicate dispersed in the matrix. The peak of the neat NodaxTM at 13 corresponds to the crystallites in the matrix. There is no peak in the range of 2h = 2–10 in the NodaxTM/clay
5000 4000
Clay20A
4000
Clay25A
15%
15%
10%
Intensity
Intensity
3000
7% 2000
5%
3000
10% 7% 2000
5%
3%
3%
1000
1000
1%
1%
PHB-co -PHH
PHB-co-PHH
0
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2
3
4
5
6
2 Theta, degrees
7
8
9
10
11
12
13
14
15
2 Theta, degrees
6000
6000
5000
5000
4000
Mica
Intensity
Intensity
MACROMOLECULAR NANOTECHNOLOGY
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15%
3000
10% 2000
7% 5%
1000
3% 1%
3000
10%
15% 7% 5% 3%
1000
PHB-co-PHH 3
Talc
2000
4
5
6
1% PHB-co-PHH
0
0 2
4000
7
8
9
10
2 Theta, degrees
11
12
13
14
15
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2 Theta, degrees
Fig. 3. X-ray diffraction patterns for neat NodaxTM, layered-silicate and NodaxTM/layered-silicate nanocomposites with various layered silicate loadings by solution intercalation. (a) clay 20A, (b) clay 25A, (c) mica, and (d) talc.
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
from the crystallites in the NodaxTM matrix also indicates that the finely dispersed clay 20A layered silicate does not strongly affect the crystalline regions during the thermal processing. The same phenomenon occurs in the clay 25A composites. There is no discernible peak from the clay 25A in the composites at the range 2–10 when the content of the layered silicate is less than 7% (Fig. 3b), which means that the clay 25A is almost completely exfoliated. The characteristic peak from clay 25A in the NodaxTM/clay 25A composites becomes weaker when the clay content is over 7%. The plain peak of the NodaxTM/clay 25A composite compared to that of the neat clay 25A indicates an
MACROMOLECULAR NANOTECHNOLOGY
20A composites in which the content of layered silicate is less than 7%. The result indicates that the d-spacings between layers are so large that the fillers formed exfoliated structures. This exfoliation may be caused by the side chains of the matrix dispersing into the spaces among layers to exfoliate the layers (facilitated by the compatibility between the tallow modifier chains of the clay 20A and the NodaxTM matrix). The characteristic peak of the neat clay 20A is still evident in the composites when the content of clay 20A is over 7%, which demonstrates that the clay 20A was not well exfoliated in the matrix when its concentration is relatively high and thus forms only intercalated structures. The presence of the peak
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Fig. 4. Various TEM images of NodaxTM/layered-silicate nanocomposites (a) clay 20A – 3%, (b) clay 25A – 3%, (c) mica – 3%, and (d) talc – 3%.
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X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
NodaxTM indicates the incorporation of layered silicates does not greatly decrease the crystallinity during the processing.
MACROMOLECULAR NANOTECHNOLOGY
increase in d-spacing and more intercalated structures in the polymer matrix. The presence of the special peak of the crystalline regions in the neat
Fig. 5. Effect of concentration on Young’s modulus (a), tensile strength (b), toughness (c, e), elongation at break (d, e) of NodaxTM/layered silicates nanocomposites. -h-: NodaxTM/clay 20A; -s-: NodaxTM/clay 25A; --: NodaxTM/mica; -D-: NodaxTM/talc, -m-: Elongation at break of NodaxTM/talc.
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
Fig. 5 shows the tensile properties obtained at 25 C for the neat NodaxTM and the NodaxTM/layered silicate composites. The properties include (i) Young’s modulus, (ii) tensile strength (defined as the stress at break), (iii) toughness, and (iv) elongation at break. As shown in Fig. 5, both the Young’s modulus and tensile strength of the nanocomposites were greatly improved compared to those for the pure NodaxTM. Of particular interest is the observation that even very small amounts of the organoclays gave significant reinforcement of the NodaxTM. Larger amounts of layered silicate (>5%) did not improve the Young’s modulus and tensile strength much. In fact, the toughness of the composites decreased with increasing filler concentration except in the case of the NodaxTM/talc composites, where toughness was greatly improved at low talc contents, and decreased only slightly with increasing content. The same trend was observed in the elongation at break for these NodaxTM/talc composites. The values for the clay 20A and the clay
3.3. Thermal stabilities The thermal stabilities of the NodaxTM/layered silicate composites series were determined from the thermogravimetric results. Fig. 6 shows an example of a TGA trace obtained for the neat NodaxTM system. The onset and the end set temperature were determined from the intersection of the two tangents, and the peak degradation temperatures were determined from the inflection points on the curves. Table 1 presents values for the (i) onset temperature, (ii) the end set temperature, (iii) the temperature interval DTemp between these two temperatures, (iv) the degradation peak temperature, and (v) the total weight loss at 350 C. The results showed that the onset temperature was slightly decreased with increasing layered silicates concentration in these composites. Compared with that of neat NodaxTM, the end set temperature was greatly improved at concentrations of fillers less than 5%, for all four silicates. Less improvement and even worsening of properties at higher concentrations obviously resulted from changes in the structures of the nanocomposites, specifically from filler exfoliation at low filler contents to less-effective intercalation at the higher contents. Slight decreases in the end set temperature were also observed with increasing concentration of the layered silicates. The interval between degradation onset and end
Onset
100
80
60
40
20
End set 0 250
260
270
280
290
300
310
320
o
Temperature, C
Fig. 6. Determination of onset and end set temperature from the thermogravimetric results on the NodaxTM.
MACROMOLECULAR NANOTECHNOLOGY
3.2. Mechanical properties
25A NodaxTM composites increased slightly at contents below 1%, but decreased after that. The elongation at break of NodaxTM/mica series increased slightly at contents less than 5%, and almost remained constant beyond that amount.
Weight, %
The XRD pattern of the NodaxTM/mica composites (Fig. 3c) showed that the characteristic peak of mica disappeared in the NodaxTM with 1% mica while it is still present in the other composites. The same results were obtained in the NodaxTM/talc composites (Fig. 3d). These results indicate that the mica and talc could be intercalated and even exfoliated and well dispersed during the ultrasonic treatment at low concentration. In addition to XRD, to validate the morphologies of the nanocomposites, the internal nanometer-scale structures were observed by TEM, which provides direct visualization of the morphology, atomic arrangement, spatial phase distribution, and structural features of a selected sample area. Fig. 4 shows the TEM images of the nanocomposites: (a) NodaxTM/clay 20A – 3%; (b) NodaxTM/clay 25A – 3%; (c) NodaxTM/mica – 3% and (d) NodaxTM/talc – 3%, where the dark entities are intercalated silicate layers. From the TEM results in Fig. 4a and d, it is clear that the stacks of clay 20A and clay 25A forming the clay crystallites are now well dispersed in the polymer matrix. TEM results in Fig. 4c and d also indicate that mica and talc could be intercalated by polymer chains into platelet agglomerates although the surface properties are different from those of organically-modified clays.
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X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
Table 1 Thermal stability parameters of various nanocomposite systems as determined from TGA Filler
Onset temp (C)
End set temp (C)
DTemp (C)
Degradation peak (C)
Total wt. loss @ 350 C (%)
0
289.5
305.5
16.0
295.0
99.8
Clay 20A
1 3 5 7 10 15
286.2 278.8 277.0 272.7 247.9 247.9
316.7 308.7 307.4 302.7 292.5 289.2
20.5 29.9 29.6 30.0 44.6 41.3
300.4 294.9 292.0 287.7 272.6 270.4
99.4 97.6 96.2 94.9 90.7 87.5
Clay 25A
1 3 5 7 10 15
285.2 284.3 286.4 281.6 280.7 272.5
311.4 310.0 312.6 304.4 303.8 303.0
26.2 25.7 26.2 22.8 23.1 30.5
299.3 297.9 300.1 294.5 293.0 290.2
98.8 97.3 96.4 94.4 92.3 89.5
Mica
1 3 5 7 10 15
286.1 286.3 287.7 286.5 286.7 282.4
312.1 312.5 314.5 314.3 311.6 309.8
26.0 26.2 26.8 27.8 24.9 27.4
300.5 300.3 300.9 300.9 300.5 297.4
98.7 97.9 96.4 94.1 86.9 88.1
Talc
1 3 5 7 10 15
289.0 286.5 285.4 286.1 285.1 282.4
315.8 314.5 311.4 315.2 312.1 309.8
26.8 28.0 26.0 29.1 26.0 27.4
304.2 299.7 300.1 300.1 300.5 298.9
99.0 96.2 95.3 94.7 92.1 88.9
MACROMOLECULAR NANOTECHNOLOGY
None
Filler loading
set showed a slight broadening for the NodaxTM/ clay 20A series but remained nearly constant around a value of 27 C for the other three series. However, the interval temperatures were increased by about 11 C compared with that for the neat NodaxTM. The total weight losses of the composites at 350 C, were not generally in direct proportion to the amount of layered silicates incorporated. Reductions of weight loss with increases in layered silicate concentration were of course observed, as expected. 4. Conclusions Solution intercalation methods were developed for preparing novel nanocomposites based on the NodaxTM biodegradable polyester and layered silicates (specifically two organically modified clays, mica, and talc). Exfoliated structures of these fillers in the matrixes are produced at low concentrations, but intercalated structures are probably predominant at higher loadings. The major benefits from
these nanoparticles are the increase in the modulus and toughness at surprisingly low concentrations of the layered silicates. The thermal stability of the NodaxTM polymer decreased slightly with the incorporation of fillers but the interval temperature characterizing the degradation was broadened, especially in the case of some of the clay nanocomposites. Overall, clay nanocomposites have better mechanical properties than those formed from mica and talc mainly because the two kinds of clay used here are organically modified ones which have much better compatibility with NodaxTM polymer than do mica and talc. As for thermal stabilities, in some cases, these layered fillers slightly increased the degradability of the NodaxTM in agreement with studies on other reinforced biodegradable polymers [17]. Acknowledgements It is our pleasure to acknowledge financial support provided JEM by the National Science Foundation through Grant DMR-0314760 (Polymer
Program, Division of Materials Research), and to thank the Geology Department of the University of Cincinnati for the very important help they provided in the XRD studies. References [1] Tian G, Wu Q, Sun S, Noda I, Chen G. J Polym Sci. Polym Phys 2002;40:649. [2] Satkowski MM, Melik DH, Autran JP, Green PR, Noda I, Schechtman LA. Biopolymers. Weinheim: Wiley-VCH; 2001. [3] Noda I, Green PR, Satkowski MM, Schechtman LA. Bimacromolecules 2005;6:580. [4] Venkitachalam R, Mark JE, Noda I. J Appl Polym Sci 2005;95:1519. [5] Harrison GM, Melik DH. J Appl Polym Sci 2006;102:1794. [6] Su F, Iwata T, Tanaka F, Doi Y. Macromolecules 2003;36:6401.
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[7] Cakmak M, Ghanem M, Yamamoto T. Polymer 2005;46:3425. [8] Daly PA, Bruce DA, Melik DH, Harrison GM. J Appl Polym Sci 2005;98:66. [9] Choi JS, Park WH. Macromol Symp 2003;197:65. [10] Hassan MK, Abdel-Latif SA, El-Roudi OM, Sharaf MA, Noda I, Mark JE. J Appl Polym Sci 2004;94:2257. [11] Hassan MK, Abou-Hussein R, Zhang X, Mark JE, Noda I. Mol Cryst Liq Cryst 2006;94:2257. [12] Schmidt DF, Clement F, Giannelis EP. Adv Funct Mater 2006;16:417. [13] Koerner H, Misra D, Tan A, Drummy L, Mirau P, Vaia R. Polymer 2006;47:3426. [14] Pinnavaia TJ, Beall GW. Polymer-Clay Nanocomposites. Chichester: John Wiley & Sons; 2000. [15] Okada A, Usuki A. Macro Mater Eng 2006;291:1449. [16] Usuki A, Kawasumi Y, Kojima M, Fukushima Y, Okada A, Kurauchi T, et al. J Mater Res 1993;8:1174. [17] Suprakas SR, Kazunobu Y, Masami O, Kazue U. Nano Letters 2002;2:1093.
MACROMOLECULAR NANOTECHNOLOGY
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
European Polymer Journal 43 (2007) 3128–3135
www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Some novel layered-silicate nanocomposites based on a biodegradable hydroxybutyrate copolymer
MACROMOLECULAR NANOTECHNOLOGY
Xiujuan Zhang a, Gui Lin a, Reda Abou-Hussein a, Mohammad K. Hassan Isao Noda c, James E. Mark a,* a
a,b
,
Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, OH 45221-0172, United States b Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076, United States c The Procter & Gamble Company, 8611 Beckett Road, West Chester, OH 45069, United States Received 12 March 2007; received in revised form 19 April 2007; accepted 27 April 2007 Available online 22 May 2007
Abstract This paper describes the preparation, characterization, mechanical properties and thermal stability of layered silicate nanocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-co-PHH, known as NodaxTM). The dispersed phases were organically modified montmorillonites (clay 20A and clay 25A), mica, and talc, and they were introduced by solution mixing. Wide-angle X-ray scattering results and transmission electron microscopy (TEM) images confirmed that these two clays were intercalated and finely distributed in the NodaxTM matrix. This type of layered filler led to remarkable improvements in mechanical properties even at very low loadings. Minimizing aggregation was more of a problem in the case of the mica and the talc, at least in this particular matrix. In some cases, these layered fillers slightly decreased the thermal stability of the NodaxTM. 2007 Elsevier Ltd. All rights reserved. Keywords: Hydroxybutyrate copolymers; NodaxTM; Clays; Mica; Talc; Mechanical properties
1. Introduction Biodegradable polymers can be degraded upon disposal in bioactive environments by organisms, such as bacteria, fungi, and algae, or by hydrolysis in buffer solutions or sea water. Biodegradable polymers are attractive substitutes for many synthetic materials because they can alleviate problems associated with solid-waste disposal. Worldwide con-
*
Corresponding author. E-mail address: [email protected] (J.E. Mark).
sumption of such biodegradable polymers has increased from 14 million kg in 1996 to an estimated 68 million kg in 2001. Poly(3-hydroxyalkanoates) (PHA), a class of biodegradable and biocompatible thermoplastics, has attracted much research interest and industrial attention as a substitute for synthetic materials [1–6]. They are produced directly from renewable resources by microbes, which accumulate PHA as carbon and energy storage materials under unbalanced growth conditions. The copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (NodaxTM) is one of the most promising biodegradable semi-crystalline aliphatic polyesters of this
0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2007.04.043
PHA family [7–9]. This is primarily due to the fact that it has a unique combination of properties, such as full anaerobic degradability, moisture resistance, good barrier properties, relatively high shelf stability (under distribution and home-storage conditions), processability on conventional equipment, and easy dyeability [2]. It has potential applications in the manufacture of storage bags, shopping bags, and slip covers, coatings in automobiles and furniture. This copolymer has the advantage of biodegradability, but is somewhat lacking in strength [3,4]. In previous studies, we showed that an orientation technique induced by pre-stretching a sample was a feasible method to give this material improved mechanical properties [10,11]. Incorporation of exfoliated layered silicates, especially organically modified clays [12–15], to give nanocomposites has attracted considerable attention since the earliest work of some Toyota Corporation researchers [16]. Such introduction of layered silicates into PHA copolymers has not been much studied, especially in the case of NodaxTM. The present study is an attempt to further improve the mechanical properties of this important biodegradable polyester, by incorporating organically modified layered silicates by solution intercalation. Although the incorporation of clay particles is emphasized, mica and talc have been studied as well. The focus is on the morphology and dispersion of these layered silicates in NodaxTM, and the effects of these novel nanoparticles on crystallinity, mechanical properties, and biodegradability.
3129
consisting of 87–89 mol% 3-hydroxybutyrate, and 11–13 mol% 3-hydroxyhexanoate. It had a number-average molecular weight of 500,000– 600,000 g mol 1 and a polydispersity index Mw/ Mn of 1.9–2.1 (based on polystyrene standards). The layered-silicates, modified with quantum ammonium cations, were purchased from Southern Clay Products Inc., Gonzales, Texas (with designations Cloisite 20A and 25A). The chemical structures of the cations used in the modification of 20A and 25A are shown, respectively, in Fig. 2a and b. The average distances between the layers of these clays 20A and 25A were, respectively, about ˚ . Mica (H4Al6K2O24Si6, 325 mesh, 24.2 and 18.6 A water ground) and talc (3MgO Æ 4SiO2 Æ H2O) were the products of Spectrum Quality Products Inc. Garden, CA and the Aldrich Chemical Company Inc., respectively. All organic solvents were highperformance liquid chromatography grades and were purchased from the Aldrich Company. 2.2. Sample preparations NodaxTM/layered silicate nanocomposites were prepared by solution intercalation. First, NodaxTM and 1, 3, 5, 7, 10 and 15 wt% layered silicate (clays 20A, 25A, mica, and talc) were separately dissolved or dispersed in chloroform and the two resulting dispersions stirred for 1 h, followed by ultrasonic treatment for another hour. Finally, the product was dried and thermal compressed in preparation for the characterizations of morphology and properties.
2. Experimental 2.3. Characterization 2.1. Materials The NodaxTM poly(3-hydroxybutyrate-co-3hydroxyhexanoate), whose structure is shown in Fig. 1, was provided by the Procter & Gamble Co., West Chester, Ohio and was used as received. Based on NMR results, it was a random copolymer
H2 C
O CH CH3
H2 C
O C x O
X-ray diffraction (XRD) measurement of neat NodaxTM, the layered silicates, and the corresponding composites films was carried out at room temperature using an X-ray generator (a Siemens D500 diffractometer) (Cu Ka radiation, with ˚ ). The 2h scan range 30 mA, 40 kV, k = 1.5406 A
CH CH2
HT C
HT
y
O
CH2 CH3
Fig. 1. Molecular structure of poly(3-hydroxybutyrate-co-3hydroxyhexanoate).
H3C
N CH3
HT
H3C
N CH3
CH2 CHCH2CH2CH2CH3 CH2CH3
Fig. 2. Chemical structures of the cations (a) and (b) used to modify clay20A and clay25A, respectively, where HT is hydrogenated tallow (65% C18, 30% C16, 5% C1).
MACROMOLECULAR NANOTECHNOLOGY
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
was 2–15, with a step size of 0.05 and a time per step of 1 s. Transmission electron microscopy (TEM) imagines were obtained using a JEM 1230 EX-II instrument (JEOL, Tokyo, Japan) operated at an acceleration voltage of 80 kV. All of the ultrathin sections (less than 200 nm) were microtomed using a Super NOVA 655001 instrument (Leica, Swiss) with a glass knife and were then subjected to TEM observation without staining. The values of the modulus and tensile strength of samples having dimensions of 30 · 5 · 1 mm3 were measured at room temperature using a fully computerized Instron mechanical tester (Model 1122). The initial gauge length was 30 mm and the cross-head speed was 5 mm/min. The tensile properties of greatest interest were the Young’s moduli and tensile strengths at break and values were obtained as an average of at least five measurements.
Thermogravimetric analysis (TGA) was done on a TA Instrument (TGA 2050 thermogravimetric analyzer) with nitrogen as purging gas. Tests were conducted from 40 to 450 C at a heating rate of 20 C/min. 3. Results and discussion 3.1. Morphology Fig. 3 shows the XRD patterns in the range 2h = 2–15 for the neat NodaxTM, clay 20A, clay 25A, mica, and talc, and their corresponding composites with various amounts of the layered silicates. The pattern of the neat NodaxTM is displayed as a baseline to compare the existence of diffraction peaks coming from the layered silicate dispersed in the matrix. The peak of the neat NodaxTM at 13 corresponds to the crystallites in the matrix. There is no peak in the range of 2h = 2–10 in the NodaxTM/clay
5000 4000
Clay20A
4000
Clay25A
15%
15%
10%
Intensity
Intensity
3000
7% 2000
5%
3000
10% 7% 2000
5%
3%
3%
1000
1000
1%
1%
PHB-co -PHH
PHB-co-PHH
0
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2
3
4
5
6
2 Theta, degrees
7
8
9
10
11
12
13
14
15
2 Theta, degrees
6000
6000
5000
5000
4000
Mica
Intensity
Intensity
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15%
3000
10% 2000
7% 5%
1000
3% 1%
3000
10%
15% 7% 5% 3%
1000
PHB-co-PHH 3
Talc
2000
4
5
6
1% PHB-co-PHH
0
0 2
4000
7
8
9
10
2 Theta, degrees
11
12
13
14
15
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2 Theta, degrees
Fig. 3. X-ray diffraction patterns for neat NodaxTM, layered-silicate and NodaxTM/layered-silicate nanocomposites with various layered silicate loadings by solution intercalation. (a) clay 20A, (b) clay 25A, (c) mica, and (d) talc.
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
from the crystallites in the NodaxTM matrix also indicates that the finely dispersed clay 20A layered silicate does not strongly affect the crystalline regions during the thermal processing. The same phenomenon occurs in the clay 25A composites. There is no discernible peak from the clay 25A in the composites at the range 2–10 when the content of the layered silicate is less than 7% (Fig. 3b), which means that the clay 25A is almost completely exfoliated. The characteristic peak from clay 25A in the NodaxTM/clay 25A composites becomes weaker when the clay content is over 7%. The plain peak of the NodaxTM/clay 25A composite compared to that of the neat clay 25A indicates an
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20A composites in which the content of layered silicate is less than 7%. The result indicates that the d-spacings between layers are so large that the fillers formed exfoliated structures. This exfoliation may be caused by the side chains of the matrix dispersing into the spaces among layers to exfoliate the layers (facilitated by the compatibility between the tallow modifier chains of the clay 20A and the NodaxTM matrix). The characteristic peak of the neat clay 20A is still evident in the composites when the content of clay 20A is over 7%, which demonstrates that the clay 20A was not well exfoliated in the matrix when its concentration is relatively high and thus forms only intercalated structures. The presence of the peak
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Fig. 4. Various TEM images of NodaxTM/layered-silicate nanocomposites (a) clay 20A – 3%, (b) clay 25A – 3%, (c) mica – 3%, and (d) talc – 3%.
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X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
NodaxTM indicates the incorporation of layered silicates does not greatly decrease the crystallinity during the processing.
MACROMOLECULAR NANOTECHNOLOGY
increase in d-spacing and more intercalated structures in the polymer matrix. The presence of the special peak of the crystalline regions in the neat
Fig. 5. Effect of concentration on Young’s modulus (a), tensile strength (b), toughness (c, e), elongation at break (d, e) of NodaxTM/layered silicates nanocomposites. -h-: NodaxTM/clay 20A; -s-: NodaxTM/clay 25A; --: NodaxTM/mica; -D-: NodaxTM/talc, -m-: Elongation at break of NodaxTM/talc.
X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
Fig. 5 shows the tensile properties obtained at 25 C for the neat NodaxTM and the NodaxTM/layered silicate composites. The properties include (i) Young’s modulus, (ii) tensile strength (defined as the stress at break), (iii) toughness, and (iv) elongation at break. As shown in Fig. 5, both the Young’s modulus and tensile strength of the nanocomposites were greatly improved compared to those for the pure NodaxTM. Of particular interest is the observation that even very small amounts of the organoclays gave significant reinforcement of the NodaxTM. Larger amounts of layered silicate (>5%) did not improve the Young’s modulus and tensile strength much. In fact, the toughness of the composites decreased with increasing filler concentration except in the case of the NodaxTM/talc composites, where toughness was greatly improved at low talc contents, and decreased only slightly with increasing content. The same trend was observed in the elongation at break for these NodaxTM/talc composites. The values for the clay 20A and the clay
3.3. Thermal stabilities The thermal stabilities of the NodaxTM/layered silicate composites series were determined from the thermogravimetric results. Fig. 6 shows an example of a TGA trace obtained for the neat NodaxTM system. The onset and the end set temperature were determined from the intersection of the two tangents, and the peak degradation temperatures were determined from the inflection points on the curves. Table 1 presents values for the (i) onset temperature, (ii) the end set temperature, (iii) the temperature interval DTemp between these two temperatures, (iv) the degradation peak temperature, and (v) the total weight loss at 350 C. The results showed that the onset temperature was slightly decreased with increasing layered silicates concentration in these composites. Compared with that of neat NodaxTM, the end set temperature was greatly improved at concentrations of fillers less than 5%, for all four silicates. Less improvement and even worsening of properties at higher concentrations obviously resulted from changes in the structures of the nanocomposites, specifically from filler exfoliation at low filler contents to less-effective intercalation at the higher contents. Slight decreases in the end set temperature were also observed with increasing concentration of the layered silicates. The interval between degradation onset and end
Onset
100
80
60
40
20
End set 0 250
260
270
280
290
300
310
320
o
Temperature, C
Fig. 6. Determination of onset and end set temperature from the thermogravimetric results on the NodaxTM.
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3.2. Mechanical properties
25A NodaxTM composites increased slightly at contents below 1%, but decreased after that. The elongation at break of NodaxTM/mica series increased slightly at contents less than 5%, and almost remained constant beyond that amount.
Weight, %
The XRD pattern of the NodaxTM/mica composites (Fig. 3c) showed that the characteristic peak of mica disappeared in the NodaxTM with 1% mica while it is still present in the other composites. The same results were obtained in the NodaxTM/talc composites (Fig. 3d). These results indicate that the mica and talc could be intercalated and even exfoliated and well dispersed during the ultrasonic treatment at low concentration. In addition to XRD, to validate the morphologies of the nanocomposites, the internal nanometer-scale structures were observed by TEM, which provides direct visualization of the morphology, atomic arrangement, spatial phase distribution, and structural features of a selected sample area. Fig. 4 shows the TEM images of the nanocomposites: (a) NodaxTM/clay 20A – 3%; (b) NodaxTM/clay 25A – 3%; (c) NodaxTM/mica – 3% and (d) NodaxTM/talc – 3%, where the dark entities are intercalated silicate layers. From the TEM results in Fig. 4a and d, it is clear that the stacks of clay 20A and clay 25A forming the clay crystallites are now well dispersed in the polymer matrix. TEM results in Fig. 4c and d also indicate that mica and talc could be intercalated by polymer chains into platelet agglomerates although the surface properties are different from those of organically-modified clays.
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X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135
Table 1 Thermal stability parameters of various nanocomposite systems as determined from TGA Filler
Onset temp (C)
End set temp (C)
DTemp (C)
Degradation peak (C)
Total wt. loss @ 350 C (%)
0
289.5
305.5
16.0
295.0
99.8
Clay 20A
1 3 5 7 10 15
286.2 278.8 277.0 272.7 247.9 247.9
316.7 308.7 307.4 302.7 292.5 289.2
20.5 29.9 29.6 30.0 44.6 41.3
300.4 294.9 292.0 287.7 272.6 270.4
99.4 97.6 96.2 94.9 90.7 87.5
Clay 25A
1 3 5 7 10 15
285.2 284.3 286.4 281.6 280.7 272.5
311.4 310.0 312.6 304.4 303.8 303.0
26.2 25.7 26.2 22.8 23.1 30.5
299.3 297.9 300.1 294.5 293.0 290.2
98.8 97.3 96.4 94.4 92.3 89.5
Mica
1 3 5 7 10 15
286.1 286.3 287.7 286.5 286.7 282.4
312.1 312.5 314.5 314.3 311.6 309.8
26.0 26.2 26.8 27.8 24.9 27.4
300.5 300.3 300.9 300.9 300.5 297.4
98.7 97.9 96.4 94.1 86.9 88.1
Talc
1 3 5 7 10 15
289.0 286.5 285.4 286.1 285.1 282.4
315.8 314.5 311.4 315.2 312.1 309.8
26.8 28.0 26.0 29.1 26.0 27.4
304.2 299.7 300.1 300.1 300.5 298.9
99.0 96.2 95.3 94.7 92.1 88.9
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None
Filler loading
set showed a slight broadening for the NodaxTM/ clay 20A series but remained nearly constant around a value of 27 C for the other three series. However, the interval temperatures were increased by about 11 C compared with that for the neat NodaxTM. The total weight losses of the composites at 350 C, were not generally in direct proportion to the amount of layered silicates incorporated. Reductions of weight loss with increases in layered silicate concentration were of course observed, as expected. 4. Conclusions Solution intercalation methods were developed for preparing novel nanocomposites based on the NodaxTM biodegradable polyester and layered silicates (specifically two organically modified clays, mica, and talc). Exfoliated structures of these fillers in the matrixes are produced at low concentrations, but intercalated structures are probably predominant at higher loadings. The major benefits from
these nanoparticles are the increase in the modulus and toughness at surprisingly low concentrations of the layered silicates. The thermal stability of the NodaxTM polymer decreased slightly with the incorporation of fillers but the interval temperature characterizing the degradation was broadened, especially in the case of some of the clay nanocomposites. Overall, clay nanocomposites have better mechanical properties than those formed from mica and talc mainly because the two kinds of clay used here are organically modified ones which have much better compatibility with NodaxTM polymer than do mica and talc. As for thermal stabilities, in some cases, these layered fillers slightly increased the degradability of the NodaxTM in agreement with studies on other reinforced biodegradable polymers [17]. Acknowledgements It is our pleasure to acknowledge financial support provided JEM by the National Science Foundation through Grant DMR-0314760 (Polymer
Program, Division of Materials Research), and to thank the Geology Department of the University of Cincinnati for the very important help they provided in the XRD studies. References [1] Tian G, Wu Q, Sun S, Noda I, Chen G. J Polym Sci. Polym Phys 2002;40:649. [2] Satkowski MM, Melik DH, Autran JP, Green PR, Noda I, Schechtman LA. Biopolymers. Weinheim: Wiley-VCH; 2001. [3] Noda I, Green PR, Satkowski MM, Schechtman LA. Bimacromolecules 2005;6:580. [4] Venkitachalam R, Mark JE, Noda I. J Appl Polym Sci 2005;95:1519. [5] Harrison GM, Melik DH. J Appl Polym Sci 2006;102:1794. [6] Su F, Iwata T, Tanaka F, Doi Y. Macromolecules 2003;36:6401.
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[7] Cakmak M, Ghanem M, Yamamoto T. Polymer 2005;46:3425. [8] Daly PA, Bruce DA, Melik DH, Harrison GM. J Appl Polym Sci 2005;98:66. [9] Choi JS, Park WH. Macromol Symp 2003;197:65. [10] Hassan MK, Abdel-Latif SA, El-Roudi OM, Sharaf MA, Noda I, Mark JE. J Appl Polym Sci 2004;94:2257. [11] Hassan MK, Abou-Hussein R, Zhang X, Mark JE, Noda I. Mol Cryst Liq Cryst 2006;94:2257. [12] Schmidt DF, Clement F, Giannelis EP. Adv Funct Mater 2006;16:417. [13] Koerner H, Misra D, Tan A, Drummy L, Mirau P, Vaia R. Polymer 2006;47:3426. [14] Pinnavaia TJ, Beall GW. Polymer-Clay Nanocomposites. Chichester: John Wiley & Sons; 2000. [15] Okada A, Usuki A. Macro Mater Eng 2006;291:1449. [16] Usuki A, Kawasumi Y, Kojima M, Fukushima Y, Okada A, Kurauchi T, et al. J Mater Res 1993;8:1174. [17] Suprakas SR, Kazunobu Y, Masami O, Kazue U. Nano Letters 2002;2:1093.
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X. Zhang et al. / European Polymer Journal 43 (2007) 3128–3135