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Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers
Polymer Degradation and Stability xxx (2014) 1e6
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers Purba Purnama a, b, Soo Hyun Kim a, c, * a
Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Center for Biomaterials and Supercritical Fluid, Surya University, Tangerang 15810, Banten, Indonesia c KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2013 Received in revised form 12 December 2013 Accepted 6 January 2014 Available online xxx
The advanced bio-based composite of stereocomplex polylactide containing cellulose nanowhiskers was successfully prepared by supercritical fluid technology. This bio-stereocomplexenanocomposite material was produced by stereocomplexation of polylactide-graftcellulose nanowhishkers through supercritical carbon dioxideedichloromethane at 65 C and 350 bar. The bio-stereocomplexenanocomposite polylactide was obtained in high stereocomplex degree (100%). The bio-stereocomplexenanocomposite polylactide exhibits excellent stereocomplex memory which is the main limitation of linear stereocomplex polylactide. It also shows improvement in mechanical properties up to 2.70 GPa (Young’s modulus) and thermal degradation temperature. The combination of stereocomplex and nanocomposite approaches offers simultaneous improvement on physical properties. The bio-stereocomplexenanocomposite polylactide is promising material in the future to replace petroleum-based polymer as ecofriendly materials. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Polylactide Stereocomplex Nanocomposite Stereocomplex memory Eco-friendly polymer
1. Introduction Non-degradable waste is the main problem caused by the use of fossil-based polymers. The high consumption of those traditional polymers in various fields causes tremendous environmental problems. Environmental protection drives attention in order to increase recycling of non-degradable polymer and focus on development of bio-based polymer to replace fossil-based polymers. Polylactide (PLA) is economically bio-based polymer which accomplishes large scale production since 2001 [1,2]. It is potential polymer for replacing fossil-based polymer due to its biodegradability and biocompatibility [3,4]. However, comparing to petroleum-based polymer, PLA exhibits limitation in mechanical and thermal properties. Stereocomplexation [5e7] and nanocomposite [8e10] are proper approaches to improve PLA properties. Stereocomplexation of PLA is an approach to improve PLA physical properties by combining stereospecific PLA (poly (L-lactide) (PLLA) and poly (Dlactide) (PDLA)) [5]. Stereocomplex PLA (s-PLA) can be formed through solution [5,11], melt [12,13], supercritical fluid [7,14], and * Corresponding author. Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5343; fax: þ82 2 958 5308. E-mail address: [email protected] (S.H. Kim).
also microwave irradiation process [15]. In other hand, nanocomposite is an approach to enhance thermal stability, mechanical, and barrier properties of polymer by adding nano-scale inorganic material [8e10,16e18]. PLA nanocomposite can be also generated through solution [19], melt [18], and supercritical fluid [20]. In the nanocomposites fields, the selection of nanoparticle should consider environmental and sustainability aspect for future applications. Cellulose nanowhiskers (CNW) are one of the promising candidates which have excellent mechanical properties [21]. It was generated from bioresources with diameter of whiskers-like regions polysaccharide chain between 5 and 30 nm [22]. In the polymer CNW nanocomposites, the hydrophilic nature of CNW can be solved by surface functionalization [23,24] and polymer grafting [25e27]. The surface-modification of improved the CNW dispersion in organic solvents [23,24]. The silanized CNW enhanced the crystallinity and thermomechanical properties of PLA materials [24]. The grafted PLA onto CNW surface enhanced the compatibility of CNW in the polymer matrix [25]. Furthermore, the partial functionalization of CNW enhanced CNW dispersion in organic solvent and support PLLA polymerization which affect to their properties improvement [27]. Regarding PLA properties enhancement, combination of stereocomplexation and nanocomposite is expected to bring both unique characteristics. In this work, we studied about stereocomplexation of PLA containing CNW through supercritical fluid
0141-3910/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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P. Purnama, S.H. Kim / Polymer Degradation and Stability xxx (2014) 1e6
media and evaluated the properties of generated stereocomplex materials: stereocomplex memory, thermal, and mechanical properties.
dissolved in chloroform and purified by pouring the solution into excess of methanol. Then, the product was dried in oven for 24 h. 2.3. Stereocomplexation of PLAeCNW
2. Experimental sections 2.1. Materials L-Lactide (Biomedichem, Korea, high-purity > 99.0%) and Dlactide (Purac Biochem bv, Netherlands, high-purity > 99.5%) were stored in oven at 55 C for 30 min before used. Stannous octoate (SigmaeAldrich, USA, purity > 95%) was used after purification. Microcrystalline cellulose (SigmaeAldrich, USA, powder, 20 mm), acetic acid (SigmaeAldrich, glacial, 99.85%), hydrochloric acid (SigmaeAldrich, ACS reagent, 37%), carbon dioxide (CO2) (Shin Yang Oxygen Industry Co. Ltd., minimum purity 99.9%), and all solvents (toluene (SigmaeAldrich, HPLC grade), chloroform, methanol (Daejung Chemicals & Metal Co., Ltd., with a purity >99.5%), dichloromethane (JT Baker, HPLC grade)) were used as received.
2.2. Preparation of nanocomposite materials The nanocomposite materials of polylactide-graft-CNW (PLAe CNW) were prepared through in situ bulk polymerization. The acetylated-CNW (a-CNW) was used to produce PLAeCNW. The aCNW was prepared through hydrolysis of microcrystalline cellulose by glacial acetic acid and hydrochloric acid [27]. The freeze-dried aCNW, stannous octoate, and toluene (1 mL) were putted into round bottom flask (ampoule) equipped with magnetic stirrer under nitrogen atmosphere. The mixture was stirred around 5 min. The lactide monomer (D-lactide or L-lactide) was added into the mixture. The monomer to a-CNW composition is 95:5 weight ratio. The monomer to initiator was varied to obtain various level molecular weights of PLA chains. The mixture is then purged with nitrogen gas and vacuum cycle for 3 times and followed by vacuum for 6 h. The ampoule was sealed and submersed in an oil bath at 130 C with continuous stirring for 24 h. The polymer product was
The stereocomplexation of PLAeCNW was proceeded through supercritical CO2edichloromethane (sc-CO2eDCM) as describe in previous work [7]. Briefly, PLLAeCNW and PDLAeCNW with 1:1 weight ratio (1.030 g for each PLLAeCNW or PDLAeCNW) were stereocomplexed in sc-CO2eDCM at 65 C and 350 bar for 5 h in the 50 mL stainless steel high-pressure reactor equipped with magnetic stirring and electrical heating mantle. The reactor was opened immediately after the reaction had finished. The s-PLA was collected and vacuumed at 40 C for 1 night. The s-PLAeCNW1 corresponds to the starting material PLLAeCNW1 and PDLAe CNW1. 2.4. Characterizations The appearance of a-CNW was examined in an atomic force microscopy (AFM). A very dilute suspension was dropped onto fresh mica surface and allowed to dry before analysis. The molecular weight of grafted PLA chains was estimated by evaluation the molecular weight of non-covalently bonded PLA using Gel Permeation Chromatography (GPC) (GPCmax 2001) at 40 C with chloroform as a solvent (flow rate was 1.0 mL/min and polymer concentration was 0.1% (wt/vol)). The non-covalently bonded PLA chain was obtained by stirring the PLAeCNW/chloroform solution for 24 h and centrifugation at 8600 g for 40 min. The acetylation of a-CNW and the graft of PLA chain onto a-CNW were evaluated by Fourier transform infrared (FTIR) spectroscopy, Thermo Mattson model Infinity Gold FT-IR. Thermal properties of nanocomposites were measured by a differential scanning calorimeter (DSC) (Modulated DSC 2910, TA Instrument). The stereocomplexation was examined by DSC and re-confirmed by X-ray diffraction (XRD) (Xray diffractometer Rigaku D/Max-2500 composed of Cu Ka (l ¼ 1.54056 Ǻ, 30 kV, 100 mA) source) instrument. The
Fig. 1. a. AFM image of a-CNW; b. FTIR spectra of a-CNW, PLAeCNW, and PLA.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
P. Purnama, S.H. Kim / Polymer Degradation and Stability xxx (2014) 1e6 Table 1 Summary of the molecular weight of grafted-PLA chains and thermal properties of PLAeCNWs materials. Material
[M]:[I] ratio
Mw (g/mol)
PDI
Tm ( C)a
DH (J/g)b
PDLAeCNW-1 PLLAeCNW-1 PDLAeCNW-2 PLLAeCNW-2 PDLAeCNW-3 PLLAeCNW-3
500 500 1000 1000 2500 2500
33,000 29,000 80,000 66,000 150,000 130,000
1.143 1.172 1.092 1.151 1.589 1.607
174.37 175.39 179.94 179.76 180.42 180.70
49.21 55.66 51.08 48.70 42.79 43.03
[M]:[I] ratio is monomer (lactide) to initiator (stannous octoate) ratio. a Measured by GPC. b Measured by DSC.
stereocomplex memory was evaluated by in situ analysis of High Temperature XRD instrument. 3. Results and discussion The development of fully bio-based materials of PLA and a-CNW is suitable for future materials development which requires sustainability and degradability. As the PLA nanocomposite by grafting PLLA chain into CNW showed properties improvements [27], the stereocomplexation of PLAeCNW nanocomposite is expected bring simultaneous properties improvement from the stereocomplex crystalline structure and the presence of nanoparticle (CNW). In this report, the novel bio-stereocomplexenanocomposite PLA was successfully generated by stereocomplexation of PLAeCNW (PLLAe CNW and PDLAeCNW) through supercritical fluid as media. The generated bio-stereocomplexenanocomposite PLA materials were evaluated to characterize their mechanical and thermal properties, including stereocomplex memory. The a-CNW was selected as a nanoparticle since it was generated from bio-based materials and has partially blocking of CNW surface. Partial surface-modification is required to accommodate the filler compatibility and polymerization initiation [27]. The partially blocking of CNW surface by acetylation was addressed to generate high-molecular weight of grafted PLA chains due to reducing the number of eOH groups. Following the approach in ref 27, a-CNW and various PLAeCNWs were
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successfully obtained [27]. The appearance of a-CNW by AFM analysis was depicted in Fig. 1a. The average size of a-CNW is about 5e20 nm diameter and 200e400 nm length. The degree of acetylation of a-CNW was around 60e70% [27]. The PLAeCNWs were successfully synthesized by grafting PLA chains onto a-CNW surface through in situ bulk polymerization. The molecular weight can be controlled by make variation in monomer to catalyst ratio. The graft structure of PLA chains onto a-CNW particles was evaluated by FTIR as shown in Fig. 1b. The characteristic peak of a-CNW was shown by the presence of peak around 1736 cm1 corresponds to the stretching frequency of the ester carbonyl or acetate. It also shows hydroxyl group band at 3300 cm1. The graft PLA chains onto a-CNW were shown by the presence of intense carbonyl group of grafted PLA chains around 1740 cm1, followed by the disappearance of hydroxyl group (3300 cm1) [28]. The molecular weights of polymer were evaluated from grafted PLA chains through GPC analysis. The molecular weights and thermal characteristics of PLAeCNW materials were summarized on Table 1. The bio-stereocomplexenanocomposite from PLAeCNW (sPLAeCNW) was obtained through sc-CO2eDCM. The sc-CO2eDCM selected as media due to its effectiveness in stereocomplexation compare to others method [7,14]. The nucleation of s-PLA crystallites was driven by CH3∙∙∙O]C interaction between PLLA and PDLA molecules [6]. In the s-PLA crystallites, the molecular motion was reduced by strong bound of the crystal networks. This crystal interaction strongly affects the melting temperature (Tm). Strong bound in crystal networks of PLLA/PDLA fragments brings the Tm of s-PLA crystallite about 50 C higher than homopolymer crystallites [29]. The difference of Tm between s-PLA and homopolymer crystallites can be used to evaluate the formation of s-PLA from PLA mixture through DSC analysis. From DSC thermograms in Fig. 2a, the s-PLAeCNWs exhibit endothermic peak of s-PLA around w230 C without any other peak around w180 C which means that the s-PLAeCNWs were perfectly obtained through sc-CO2e DCM. High s-PLA degree (100%) of s-PLAeCNW was obtained from PLAeCNWs mixture with various molecular weights up to 150,000 g/mol. It also confirmed the previous reports about the effectiveness of sc-CO2eDCM system to produce s-PLA-based materials [7,14,30].
Fig. 2. a. DSC thermogram of the PDLAeCNW3 (homopolymer) and generated s-PLAeCNW materials. Heating rate was fixed at 10 C/min; b. XRD diffractogram of homopolymer PLAeCNW and generated s-PLAeCNW materials.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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Fig. 3. DSC thermogram of s-PLAeCNW1 during first scan and cooling (black line) and s-PLAeCNW1 during second scan (blue line). The heating and cooling rate were 10 C/ min under nitrogen atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In the s-PLA crystal structure, the opposite helical conformations (left- and right-handed) of PLLA and PDLA were packed side by side in parallel fashion and folded to form lamella when s-PLA was formed. Thus, new crystalline structure was formed [29]. Therefore, the s-PLA formation in s-PLAeCNW can be confirmed by XRD analysis because the crystalline structure of s-PLA resulted in different XRD pattern compare to its homopolymers. The diffraction peaks of homopolymer were observed at 2q 16.52 and 19.08 while s-PLA characteristic peaks are 11.86 , 20.56 , and 23.86 of 2q. As depicted in Fig. 2b, the s-PLAeCNW materials show diffraction peak only at 11.86 , 20.56 , and 23.86 of 2q which means, the
s-PLAeCNW materials consist of s-PLA crystal without any homopolymer. As seen in DSC thermograms (Fig. 2a), the s-PLAeCNW1 shows double Tm peaks at 225.09 and 249.01 C which are the characteristics Tm of s-PLA. The s-PLAeCNW3 also shows similar pattern with two overlapped Tm peaks. The presence of double Tm peaks may come from the s-PLA crystallites with different degree of structural ordering [31]. To evaluate this finding, the s-PLAeCNW1 was melted completely and holds at 250 C to remove all thermal memory, and then crystallized during cooling process. As shown in Fig. 3, the cooling process exhibits single crystallization peak at 155.60 C. The second scan of s-PLAeCNW1 shows single peak at 217.37 C which comes from the s-PLA crystallites with perfect degree of structural ordering. The s-PLAeCNW1 was completely melted at 250 C, and re-crystallized with the same structural ordering during cooling process. The DSC thermogram of s-PLAeCNW1 (Fig. 3) also gave another characteristic of s-PLAeCNW materials: stereocomplex memory. Stereocomplex memory is the ability of stereocomplex materials to reform stereocomplex after melted. Stereocomplex memory is main limitation of linear s-PLA due to its unzipped fragment could not re-zip to form s-PLA after melted [30,32]. From the figure, the absence of Tm peak at 180 C means s-PLAe CNW1 was successfully generated from PLLAeCNW1 and PDLAe CNW1. At the molten state, grafted-PLLA and -PDLA fragments are unzipped from s-PLA structure. During cooling process, those unzipped fragments re-contact with their neighbor to form sPLA. The s-PLAeCNW1 exhibits higher recrystallization temperature (155.60 C) compare to its homopolymer (w110 C). It belongs to the recrystallization peak of unzipped fragments from s-PLAeCNW1. The second run of DSC analysis shows single peak above 200 C correspond to the Tm of s-PLAeCNW1. It means the unzipped grafted-PLLA and -PDLA fragments successfully
Fig. 4. The XRD diffractogram of s-PLAeCNW1 (a) and s-PLAeCNW3 (b) during cooling process.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
P. Purnama, S.H. Kim / Polymer Degradation and Stability xxx (2014) 1e6
reformed s-PLAeCNW1. In other word, s-PLAeCNW1 shows excellent stereocomplex memory. Stereocomplex memory is an important property of stereocomplex materials due to the general industrial process is melt process. The stereocomplex memory of s-PLAeCNWs was evaluated further by in situ XRD analysis. The s-PLAeCNW materials were melted at 250 C and followed by cooling to room temperature. Recrystallization process of s-PLAeCNW materials was observed during cooling process. Fig. 4 showed re-crystallization process of s-PLAeCNW1 (Fig. 4a) and s-PLAeCNW3 (Fig. 4b) from molten state. At 250 C, all of s-PLAeCNW materials are completely melted. At 160e180 C, the s-PLAeCNW materials start to re-zip (recrystallize) as s-PLA crystal during cooling process. The s-PLAeCNW1 (Fig. 4a) starts to recrystallize at
Fig. 5. a. The Young’s modulus of PLA, PLLAeCNW3, s-PLA, s-PLAeCNW2 and s-PLAe CNW3; b. The tensile strength and elongation at break of PLA, PLLAeCNW3, s-PLA, sPLAeCNW2 and s-PLAeCNW3; c. TGA traces of a-CNW, s-PLA, s-PLAeCNW1, and sPLAeCNW3 under nitrogen atmosphere. The heating rate was fixed at 10 C/min. The inset shows the first derivative of the weight loss.
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180 C. It may be caused by shorter polymer chains were easily crystallized than longer polymer chain (s-PLAeCNW3). After completely crystallized, the XRD patterns (25 C) have similar pattern with Fig. 2a correspond to the s-PLAeCNW1 and s-PLAe CNW3 are completely re-formed s-PLA crystal after melted. It means that the stereocomplex memory of s-PLAeCNW was improved for low and high molecular weight PLAeCNW materials. The s-PLAeCNW materials show excellent stereocomplex memory compare to linear s-PLA which only remain about 30e 40% of s-PLA crystallites after melted [30]. This excellent stereocomplex memory may be supported by graft structure of supramolecules and the presence of CNW particles. The graft structure of PLAeCNWs has similarity with star-shaped PLA molecules with grafted PLLA and PDLA as chains part and CNW particles as a supramolecule’s core. The star-shaped structure of supramolecule PLAeCNW reduces the chain freedom of grafted PLLA or PDLA and optimizes their neighboring participations in forming s-PLA during cooling process. The presence of a-CNW core with homogeneous distribution in polymer matrix also supports the stereocomplex memory by acting as nucleating agent. So, the grafted structure of supramolecule with a-CNW particle as core improves the stereocomplex memory. The stereocomplexation between PLLA and PDLA has significant effect in the mechanical and thermal properties. The addition of aCNW particles in the stereocomplex matrix is addressed to enhance its properties to higher level. The improvement in mechanical properties can be seen in Fig. 5a and b. As mention in previous report, the s-PLA crystallites increase the tensile strength and Young’s modulus up to 25% higher than neat homopolymers [7]. This improvement came from strong interaction between L-lactide and D-lactide fragments [6,33]. In the polymer nanocomposites, the well-distributed nanoparticles also improved the mechanical properties of polymeric materials [34]. The presence of a-CNW particle as a core in s-PLAeCNW3 enhances the mechanical properties higher than neat s-PLA materials. The Young’s modulus and tensile strength of s-PLAeCNW3 are about 2.70 GPa and 62.96 MPa, respectively. This value represents synergetic effect of strong interaction of molecules in s-PLA crystallites and well distributed CNW particles in stereocomplex matrix. The Young’s modulus (w2.40 GPa) and tensile strength (w48.56 MPa) of s-PLAeCNW2 are lower than s-PLAeCNW3 due to lower molecular weight of the starting materials. The increasing tensile strength and elongation at break show the sufficient bonding between the filler and PLA matrix. Comparing TGA curves of a-CNW, neat s-PLA, and s-PLAeCNW materials, the presence of a-CNW was found to improve the thermal decomposition of stereocomplex materials (Fig. 5b). The s-PLA formation increases the thermal degradation temperature due to strong interaction between L- and D-lactide chains which reduce molecular mobility significantly, then disturbing thermal degradation [6,32]. The maximum weight loss rate temperatures are 327, 349, 355, and 360 C for neat s-PLA, s-PLAeCNW1, s-PLAeCNW3, and a-CNW, respectively. From this data, the thermal degradation properties of s-PLAeCNW1 and s-PLAeCNW3 are higher than neat s-PLA. It means the a-CNW particle can act as a superior insulator and mass transport barrier during thermal degradation process. The maximum weight loss rate temperature of s-PLAeCNW3 is higher than s-PLAeCNW1 due to the difference in molecular weight. The combination of stereocomplex and nanocomposite approaches brings simultaneous improvements in mechanical and thermal properties of polylactide, including stereocomplex memory. The bio-stereocomplexenanocomposite is a potential candidate of the eco-friendly materials in the future due to its sustainability (from renewable resources) and biodegradability.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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4. Conclusions The bio-stereocomplexenanocomposite PLA materials were successfully generated through sc-CO2eDCM at 65 C and 350 bar. The use of a-CNW support to control molecular weights of grafted PLA chains. The s-PLAeCNW materials were easily generated in high degree of s-PLA crystallites (100%). The presence of a-CNW particles and graft structure of supramolecule PLAeCNW support the improvement of the mechanical and thermal properties, including stereocomplex memory. The s-PLAeCNW materials have excellent stereocomplex memory which is able to reform perfect sPLA crystallites after melted. The Young’s modulus of s-PLAeCNW materials was improved up to 2.70 GPa (s-PLAeCNW3) by simultaneous effect from stereocomplexation and nanocomposite (presence of a-CNW particles). The stereocomplex crystallites and a-CNW particle also bring the thermal degradation property of sPLAeCNW materials to higher temperature of the maximum weight loss rate (355 C). This bio-stereocomplexenanocomposite PLA is promising material as eco-friendly materials to replace petroleum-based polymer in the future. Acknowledgment This study was supported by the National Research Foundation of Korea Grant funded by the Korea Government (MEST), NRF2010-C1AAA001-0028939. References [1] Lunt J. Large-scale production, properties and commercial application of polylactic acid polymers. Polym Degrad Stab 1998;59:145e52. [2] Vink ETH, Rabago KR, Glassner DA, Gruber PR. Applications of life cycle assessment to NatureWorksÔ polylactide (PLA) production. Polym Degrad Stab 2003;80:403e19. [3] Garlotta D. A literature review of poly(lactic acid). Polym Environ 2001;9:63e 84. [4] Ikada Y, Tsuji H. Biodegradable polyesters for medical and ecological applications. Macromol Rapid Commun 2000;21:117e32. [5] Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules 1987;20:904e6. [6] Tsuji H. Poly(lactide) stereocomplexes: formation, structure, properties, degradation, and applications. Macromol Biosci 2005;5:569e97. [7] Purnama P, Kim SH. Stereocomplex formation of high-molecular-weight polylactide using supercritical fluid. Macromolecules 2010;43:1137e42. [8] Darder M, Aranda P, Ruiz-Hitzky E. Bionanocomposites: a new concept of ecological, bioinspired, and functional hybrid materials. Adv Mater 2007;19: 1309e19. [9] Darder M, Aranda P, Ferrer ML, Gutierrez MC, del Monte F, Ruiz-Hitzky E. Progress in bionanocomposite and bioinspired foams. Adv Mater 2011;23: 5262e7. [10] Bitinis N, Hernandez M, Verdejo R, Kenny JM, Lopez-Manchado MA. Recent advances in clay/polymer nanocomposites. Adv Mater 2011;23:5229e36. [11] Yamane H, Sasai K. Effect of the addition of poly(D-lactic acid) on thermal property of poly(L-lactic acid). Polymer 2003;44:2569e75.
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Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers Purba Purnama a, b, Soo Hyun Kim a, c, * a
Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Center for Biomaterials and Supercritical Fluid, Surya University, Tangerang 15810, Banten, Indonesia c KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2013 Received in revised form 12 December 2013 Accepted 6 January 2014 Available online xxx
The advanced bio-based composite of stereocomplex polylactide containing cellulose nanowhiskers was successfully prepared by supercritical fluid technology. This bio-stereocomplexenanocomposite material was produced by stereocomplexation of polylactide-graftcellulose nanowhishkers through supercritical carbon dioxideedichloromethane at 65 C and 350 bar. The bio-stereocomplexenanocomposite polylactide was obtained in high stereocomplex degree (100%). The bio-stereocomplexenanocomposite polylactide exhibits excellent stereocomplex memory which is the main limitation of linear stereocomplex polylactide. It also shows improvement in mechanical properties up to 2.70 GPa (Young’s modulus) and thermal degradation temperature. The combination of stereocomplex and nanocomposite approaches offers simultaneous improvement on physical properties. The bio-stereocomplexenanocomposite polylactide is promising material in the future to replace petroleum-based polymer as ecofriendly materials. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Polylactide Stereocomplex Nanocomposite Stereocomplex memory Eco-friendly polymer
1. Introduction Non-degradable waste is the main problem caused by the use of fossil-based polymers. The high consumption of those traditional polymers in various fields causes tremendous environmental problems. Environmental protection drives attention in order to increase recycling of non-degradable polymer and focus on development of bio-based polymer to replace fossil-based polymers. Polylactide (PLA) is economically bio-based polymer which accomplishes large scale production since 2001 [1,2]. It is potential polymer for replacing fossil-based polymer due to its biodegradability and biocompatibility [3,4]. However, comparing to petroleum-based polymer, PLA exhibits limitation in mechanical and thermal properties. Stereocomplexation [5e7] and nanocomposite [8e10] are proper approaches to improve PLA properties. Stereocomplexation of PLA is an approach to improve PLA physical properties by combining stereospecific PLA (poly (L-lactide) (PLLA) and poly (Dlactide) (PDLA)) [5]. Stereocomplex PLA (s-PLA) can be formed through solution [5,11], melt [12,13], supercritical fluid [7,14], and * Corresponding author. Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea. Tel.: þ82 2 958 5343; fax: þ82 2 958 5308. E-mail address: [email protected] (S.H. Kim).
also microwave irradiation process [15]. In other hand, nanocomposite is an approach to enhance thermal stability, mechanical, and barrier properties of polymer by adding nano-scale inorganic material [8e10,16e18]. PLA nanocomposite can be also generated through solution [19], melt [18], and supercritical fluid [20]. In the nanocomposites fields, the selection of nanoparticle should consider environmental and sustainability aspect for future applications. Cellulose nanowhiskers (CNW) are one of the promising candidates which have excellent mechanical properties [21]. It was generated from bioresources with diameter of whiskers-like regions polysaccharide chain between 5 and 30 nm [22]. In the polymer CNW nanocomposites, the hydrophilic nature of CNW can be solved by surface functionalization [23,24] and polymer grafting [25e27]. The surface-modification of improved the CNW dispersion in organic solvents [23,24]. The silanized CNW enhanced the crystallinity and thermomechanical properties of PLA materials [24]. The grafted PLA onto CNW surface enhanced the compatibility of CNW in the polymer matrix [25]. Furthermore, the partial functionalization of CNW enhanced CNW dispersion in organic solvent and support PLLA polymerization which affect to their properties improvement [27]. Regarding PLA properties enhancement, combination of stereocomplexation and nanocomposite is expected to bring both unique characteristics. In this work, we studied about stereocomplexation of PLA containing CNW through supercritical fluid
0141-3910/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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media and evaluated the properties of generated stereocomplex materials: stereocomplex memory, thermal, and mechanical properties.
dissolved in chloroform and purified by pouring the solution into excess of methanol. Then, the product was dried in oven for 24 h. 2.3. Stereocomplexation of PLAeCNW
2. Experimental sections 2.1. Materials L-Lactide (Biomedichem, Korea, high-purity > 99.0%) and Dlactide (Purac Biochem bv, Netherlands, high-purity > 99.5%) were stored in oven at 55 C for 30 min before used. Stannous octoate (SigmaeAldrich, USA, purity > 95%) was used after purification. Microcrystalline cellulose (SigmaeAldrich, USA, powder, 20 mm), acetic acid (SigmaeAldrich, glacial, 99.85%), hydrochloric acid (SigmaeAldrich, ACS reagent, 37%), carbon dioxide (CO2) (Shin Yang Oxygen Industry Co. Ltd., minimum purity 99.9%), and all solvents (toluene (SigmaeAldrich, HPLC grade), chloroform, methanol (Daejung Chemicals & Metal Co., Ltd., with a purity >99.5%), dichloromethane (JT Baker, HPLC grade)) were used as received.
2.2. Preparation of nanocomposite materials The nanocomposite materials of polylactide-graft-CNW (PLAe CNW) were prepared through in situ bulk polymerization. The acetylated-CNW (a-CNW) was used to produce PLAeCNW. The aCNW was prepared through hydrolysis of microcrystalline cellulose by glacial acetic acid and hydrochloric acid [27]. The freeze-dried aCNW, stannous octoate, and toluene (1 mL) were putted into round bottom flask (ampoule) equipped with magnetic stirrer under nitrogen atmosphere. The mixture was stirred around 5 min. The lactide monomer (D-lactide or L-lactide) was added into the mixture. The monomer to a-CNW composition is 95:5 weight ratio. The monomer to initiator was varied to obtain various level molecular weights of PLA chains. The mixture is then purged with nitrogen gas and vacuum cycle for 3 times and followed by vacuum for 6 h. The ampoule was sealed and submersed in an oil bath at 130 C with continuous stirring for 24 h. The polymer product was
The stereocomplexation of PLAeCNW was proceeded through supercritical CO2edichloromethane (sc-CO2eDCM) as describe in previous work [7]. Briefly, PLLAeCNW and PDLAeCNW with 1:1 weight ratio (1.030 g for each PLLAeCNW or PDLAeCNW) were stereocomplexed in sc-CO2eDCM at 65 C and 350 bar for 5 h in the 50 mL stainless steel high-pressure reactor equipped with magnetic stirring and electrical heating mantle. The reactor was opened immediately after the reaction had finished. The s-PLA was collected and vacuumed at 40 C for 1 night. The s-PLAeCNW1 corresponds to the starting material PLLAeCNW1 and PDLAe CNW1. 2.4. Characterizations The appearance of a-CNW was examined in an atomic force microscopy (AFM). A very dilute suspension was dropped onto fresh mica surface and allowed to dry before analysis. The molecular weight of grafted PLA chains was estimated by evaluation the molecular weight of non-covalently bonded PLA using Gel Permeation Chromatography (GPC) (GPCmax 2001) at 40 C with chloroform as a solvent (flow rate was 1.0 mL/min and polymer concentration was 0.1% (wt/vol)). The non-covalently bonded PLA chain was obtained by stirring the PLAeCNW/chloroform solution for 24 h and centrifugation at 8600 g for 40 min. The acetylation of a-CNW and the graft of PLA chain onto a-CNW were evaluated by Fourier transform infrared (FTIR) spectroscopy, Thermo Mattson model Infinity Gold FT-IR. Thermal properties of nanocomposites were measured by a differential scanning calorimeter (DSC) (Modulated DSC 2910, TA Instrument). The stereocomplexation was examined by DSC and re-confirmed by X-ray diffraction (XRD) (Xray diffractometer Rigaku D/Max-2500 composed of Cu Ka (l ¼ 1.54056 Ǻ, 30 kV, 100 mA) source) instrument. The
Fig. 1. a. AFM image of a-CNW; b. FTIR spectra of a-CNW, PLAeCNW, and PLA.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
P. Purnama, S.H. Kim / Polymer Degradation and Stability xxx (2014) 1e6 Table 1 Summary of the molecular weight of grafted-PLA chains and thermal properties of PLAeCNWs materials. Material
[M]:[I] ratio
Mw (g/mol)
PDI
Tm ( C)a
DH (J/g)b
PDLAeCNW-1 PLLAeCNW-1 PDLAeCNW-2 PLLAeCNW-2 PDLAeCNW-3 PLLAeCNW-3
500 500 1000 1000 2500 2500
33,000 29,000 80,000 66,000 150,000 130,000
1.143 1.172 1.092 1.151 1.589 1.607
174.37 175.39 179.94 179.76 180.42 180.70
49.21 55.66 51.08 48.70 42.79 43.03
[M]:[I] ratio is monomer (lactide) to initiator (stannous octoate) ratio. a Measured by GPC. b Measured by DSC.
stereocomplex memory was evaluated by in situ analysis of High Temperature XRD instrument. 3. Results and discussion The development of fully bio-based materials of PLA and a-CNW is suitable for future materials development which requires sustainability and degradability. As the PLA nanocomposite by grafting PLLA chain into CNW showed properties improvements [27], the stereocomplexation of PLAeCNW nanocomposite is expected bring simultaneous properties improvement from the stereocomplex crystalline structure and the presence of nanoparticle (CNW). In this report, the novel bio-stereocomplexenanocomposite PLA was successfully generated by stereocomplexation of PLAeCNW (PLLAe CNW and PDLAeCNW) through supercritical fluid as media. The generated bio-stereocomplexenanocomposite PLA materials were evaluated to characterize their mechanical and thermal properties, including stereocomplex memory. The a-CNW was selected as a nanoparticle since it was generated from bio-based materials and has partially blocking of CNW surface. Partial surface-modification is required to accommodate the filler compatibility and polymerization initiation [27]. The partially blocking of CNW surface by acetylation was addressed to generate high-molecular weight of grafted PLA chains due to reducing the number of eOH groups. Following the approach in ref 27, a-CNW and various PLAeCNWs were
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successfully obtained [27]. The appearance of a-CNW by AFM analysis was depicted in Fig. 1a. The average size of a-CNW is about 5e20 nm diameter and 200e400 nm length. The degree of acetylation of a-CNW was around 60e70% [27]. The PLAeCNWs were successfully synthesized by grafting PLA chains onto a-CNW surface through in situ bulk polymerization. The molecular weight can be controlled by make variation in monomer to catalyst ratio. The graft structure of PLA chains onto a-CNW particles was evaluated by FTIR as shown in Fig. 1b. The characteristic peak of a-CNW was shown by the presence of peak around 1736 cm1 corresponds to the stretching frequency of the ester carbonyl or acetate. It also shows hydroxyl group band at 3300 cm1. The graft PLA chains onto a-CNW were shown by the presence of intense carbonyl group of grafted PLA chains around 1740 cm1, followed by the disappearance of hydroxyl group (3300 cm1) [28]. The molecular weights of polymer were evaluated from grafted PLA chains through GPC analysis. The molecular weights and thermal characteristics of PLAeCNW materials were summarized on Table 1. The bio-stereocomplexenanocomposite from PLAeCNW (sPLAeCNW) was obtained through sc-CO2eDCM. The sc-CO2eDCM selected as media due to its effectiveness in stereocomplexation compare to others method [7,14]. The nucleation of s-PLA crystallites was driven by CH3∙∙∙O]C interaction between PLLA and PDLA molecules [6]. In the s-PLA crystallites, the molecular motion was reduced by strong bound of the crystal networks. This crystal interaction strongly affects the melting temperature (Tm). Strong bound in crystal networks of PLLA/PDLA fragments brings the Tm of s-PLA crystallite about 50 C higher than homopolymer crystallites [29]. The difference of Tm between s-PLA and homopolymer crystallites can be used to evaluate the formation of s-PLA from PLA mixture through DSC analysis. From DSC thermograms in Fig. 2a, the s-PLAeCNWs exhibit endothermic peak of s-PLA around w230 C without any other peak around w180 C which means that the s-PLAeCNWs were perfectly obtained through sc-CO2e DCM. High s-PLA degree (100%) of s-PLAeCNW was obtained from PLAeCNWs mixture with various molecular weights up to 150,000 g/mol. It also confirmed the previous reports about the effectiveness of sc-CO2eDCM system to produce s-PLA-based materials [7,14,30].
Fig. 2. a. DSC thermogram of the PDLAeCNW3 (homopolymer) and generated s-PLAeCNW materials. Heating rate was fixed at 10 C/min; b. XRD diffractogram of homopolymer PLAeCNW and generated s-PLAeCNW materials.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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Fig. 3. DSC thermogram of s-PLAeCNW1 during first scan and cooling (black line) and s-PLAeCNW1 during second scan (blue line). The heating and cooling rate were 10 C/ min under nitrogen atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In the s-PLA crystal structure, the opposite helical conformations (left- and right-handed) of PLLA and PDLA were packed side by side in parallel fashion and folded to form lamella when s-PLA was formed. Thus, new crystalline structure was formed [29]. Therefore, the s-PLA formation in s-PLAeCNW can be confirmed by XRD analysis because the crystalline structure of s-PLA resulted in different XRD pattern compare to its homopolymers. The diffraction peaks of homopolymer were observed at 2q 16.52 and 19.08 while s-PLA characteristic peaks are 11.86 , 20.56 , and 23.86 of 2q. As depicted in Fig. 2b, the s-PLAeCNW materials show diffraction peak only at 11.86 , 20.56 , and 23.86 of 2q which means, the
s-PLAeCNW materials consist of s-PLA crystal without any homopolymer. As seen in DSC thermograms (Fig. 2a), the s-PLAeCNW1 shows double Tm peaks at 225.09 and 249.01 C which are the characteristics Tm of s-PLA. The s-PLAeCNW3 also shows similar pattern with two overlapped Tm peaks. The presence of double Tm peaks may come from the s-PLA crystallites with different degree of structural ordering [31]. To evaluate this finding, the s-PLAeCNW1 was melted completely and holds at 250 C to remove all thermal memory, and then crystallized during cooling process. As shown in Fig. 3, the cooling process exhibits single crystallization peak at 155.60 C. The second scan of s-PLAeCNW1 shows single peak at 217.37 C which comes from the s-PLA crystallites with perfect degree of structural ordering. The s-PLAeCNW1 was completely melted at 250 C, and re-crystallized with the same structural ordering during cooling process. The DSC thermogram of s-PLAeCNW1 (Fig. 3) also gave another characteristic of s-PLAeCNW materials: stereocomplex memory. Stereocomplex memory is the ability of stereocomplex materials to reform stereocomplex after melted. Stereocomplex memory is main limitation of linear s-PLA due to its unzipped fragment could not re-zip to form s-PLA after melted [30,32]. From the figure, the absence of Tm peak at 180 C means s-PLAe CNW1 was successfully generated from PLLAeCNW1 and PDLAe CNW1. At the molten state, grafted-PLLA and -PDLA fragments are unzipped from s-PLA structure. During cooling process, those unzipped fragments re-contact with their neighbor to form sPLA. The s-PLAeCNW1 exhibits higher recrystallization temperature (155.60 C) compare to its homopolymer (w110 C). It belongs to the recrystallization peak of unzipped fragments from s-PLAeCNW1. The second run of DSC analysis shows single peak above 200 C correspond to the Tm of s-PLAeCNW1. It means the unzipped grafted-PLLA and -PDLA fragments successfully
Fig. 4. The XRD diffractogram of s-PLAeCNW1 (a) and s-PLAeCNW3 (b) during cooling process.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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reformed s-PLAeCNW1. In other word, s-PLAeCNW1 shows excellent stereocomplex memory. Stereocomplex memory is an important property of stereocomplex materials due to the general industrial process is melt process. The stereocomplex memory of s-PLAeCNWs was evaluated further by in situ XRD analysis. The s-PLAeCNW materials were melted at 250 C and followed by cooling to room temperature. Recrystallization process of s-PLAeCNW materials was observed during cooling process. Fig. 4 showed re-crystallization process of s-PLAeCNW1 (Fig. 4a) and s-PLAeCNW3 (Fig. 4b) from molten state. At 250 C, all of s-PLAeCNW materials are completely melted. At 160e180 C, the s-PLAeCNW materials start to re-zip (recrystallize) as s-PLA crystal during cooling process. The s-PLAeCNW1 (Fig. 4a) starts to recrystallize at
Fig. 5. a. The Young’s modulus of PLA, PLLAeCNW3, s-PLA, s-PLAeCNW2 and s-PLAe CNW3; b. The tensile strength and elongation at break of PLA, PLLAeCNW3, s-PLA, sPLAeCNW2 and s-PLAeCNW3; c. TGA traces of a-CNW, s-PLA, s-PLAeCNW1, and sPLAeCNW3 under nitrogen atmosphere. The heating rate was fixed at 10 C/min. The inset shows the first derivative of the weight loss.
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180 C. It may be caused by shorter polymer chains were easily crystallized than longer polymer chain (s-PLAeCNW3). After completely crystallized, the XRD patterns (25 C) have similar pattern with Fig. 2a correspond to the s-PLAeCNW1 and s-PLAe CNW3 are completely re-formed s-PLA crystal after melted. It means that the stereocomplex memory of s-PLAeCNW was improved for low and high molecular weight PLAeCNW materials. The s-PLAeCNW materials show excellent stereocomplex memory compare to linear s-PLA which only remain about 30e 40% of s-PLA crystallites after melted [30]. This excellent stereocomplex memory may be supported by graft structure of supramolecules and the presence of CNW particles. The graft structure of PLAeCNWs has similarity with star-shaped PLA molecules with grafted PLLA and PDLA as chains part and CNW particles as a supramolecule’s core. The star-shaped structure of supramolecule PLAeCNW reduces the chain freedom of grafted PLLA or PDLA and optimizes their neighboring participations in forming s-PLA during cooling process. The presence of a-CNW core with homogeneous distribution in polymer matrix also supports the stereocomplex memory by acting as nucleating agent. So, the grafted structure of supramolecule with a-CNW particle as core improves the stereocomplex memory. The stereocomplexation between PLLA and PDLA has significant effect in the mechanical and thermal properties. The addition of aCNW particles in the stereocomplex matrix is addressed to enhance its properties to higher level. The improvement in mechanical properties can be seen in Fig. 5a and b. As mention in previous report, the s-PLA crystallites increase the tensile strength and Young’s modulus up to 25% higher than neat homopolymers [7]. This improvement came from strong interaction between L-lactide and D-lactide fragments [6,33]. In the polymer nanocomposites, the well-distributed nanoparticles also improved the mechanical properties of polymeric materials [34]. The presence of a-CNW particle as a core in s-PLAeCNW3 enhances the mechanical properties higher than neat s-PLA materials. The Young’s modulus and tensile strength of s-PLAeCNW3 are about 2.70 GPa and 62.96 MPa, respectively. This value represents synergetic effect of strong interaction of molecules in s-PLA crystallites and well distributed CNW particles in stereocomplex matrix. The Young’s modulus (w2.40 GPa) and tensile strength (w48.56 MPa) of s-PLAeCNW2 are lower than s-PLAeCNW3 due to lower molecular weight of the starting materials. The increasing tensile strength and elongation at break show the sufficient bonding between the filler and PLA matrix. Comparing TGA curves of a-CNW, neat s-PLA, and s-PLAeCNW materials, the presence of a-CNW was found to improve the thermal decomposition of stereocomplex materials (Fig. 5b). The s-PLA formation increases the thermal degradation temperature due to strong interaction between L- and D-lactide chains which reduce molecular mobility significantly, then disturbing thermal degradation [6,32]. The maximum weight loss rate temperatures are 327, 349, 355, and 360 C for neat s-PLA, s-PLAeCNW1, s-PLAeCNW3, and a-CNW, respectively. From this data, the thermal degradation properties of s-PLAeCNW1 and s-PLAeCNW3 are higher than neat s-PLA. It means the a-CNW particle can act as a superior insulator and mass transport barrier during thermal degradation process. The maximum weight loss rate temperature of s-PLAeCNW3 is higher than s-PLAeCNW1 due to the difference in molecular weight. The combination of stereocomplex and nanocomposite approaches brings simultaneous improvements in mechanical and thermal properties of polylactide, including stereocomplex memory. The bio-stereocomplexenanocomposite is a potential candidate of the eco-friendly materials in the future due to its sustainability (from renewable resources) and biodegradability.
Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004
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4. Conclusions The bio-stereocomplexenanocomposite PLA materials were successfully generated through sc-CO2eDCM at 65 C and 350 bar. The use of a-CNW support to control molecular weights of grafted PLA chains. The s-PLAeCNW materials were easily generated in high degree of s-PLA crystallites (100%). The presence of a-CNW particles and graft structure of supramolecule PLAeCNW support the improvement of the mechanical and thermal properties, including stereocomplex memory. The s-PLAeCNW materials have excellent stereocomplex memory which is able to reform perfect sPLA crystallites after melted. The Young’s modulus of s-PLAeCNW materials was improved up to 2.70 GPa (s-PLAeCNW3) by simultaneous effect from stereocomplexation and nanocomposite (presence of a-CNW particles). The stereocomplex crystallites and a-CNW particle also bring the thermal degradation property of sPLAeCNW materials to higher temperature of the maximum weight loss rate (355 C). This bio-stereocomplexenanocomposite PLA is promising material as eco-friendly materials to replace petroleum-based polymer in the future. Acknowledgment This study was supported by the National Research Foundation of Korea Grant funded by the Korea Government (MEST), NRF2010-C1AAA001-0028939. References [1] Lunt J. Large-scale production, properties and commercial application of polylactic acid polymers. Polym Degrad Stab 1998;59:145e52. [2] Vink ETH, Rabago KR, Glassner DA, Gruber PR. Applications of life cycle assessment to NatureWorksÔ polylactide (PLA) production. Polym Degrad Stab 2003;80:403e19. [3] Garlotta D. A literature review of poly(lactic acid). Polym Environ 2001;9:63e 84. [4] Ikada Y, Tsuji H. Biodegradable polyesters for medical and ecological applications. Macromol Rapid Commun 2000;21:117e32. [5] Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules 1987;20:904e6. [6] Tsuji H. Poly(lactide) stereocomplexes: formation, structure, properties, degradation, and applications. Macromol Biosci 2005;5:569e97. [7] Purnama P, Kim SH. Stereocomplex formation of high-molecular-weight polylactide using supercritical fluid. Macromolecules 2010;43:1137e42. [8] Darder M, Aranda P, Ruiz-Hitzky E. Bionanocomposites: a new concept of ecological, bioinspired, and functional hybrid materials. Adv Mater 2007;19: 1309e19. [9] Darder M, Aranda P, Ferrer ML, Gutierrez MC, del Monte F, Ruiz-Hitzky E. Progress in bionanocomposite and bioinspired foams. Adv Mater 2011;23: 5262e7. [10] Bitinis N, Hernandez M, Verdejo R, Kenny JM, Lopez-Manchado MA. Recent advances in clay/polymer nanocomposites. Adv Mater 2011;23:5229e36. [11] Yamane H, Sasai K. Effect of the addition of poly(D-lactic acid) on thermal property of poly(L-lactic acid). Polymer 2003;44:2569e75.
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Please cite this article in press as: Purnama P, Kim SH, Bio-based composite of stereocomplex polylactide and cellulose nanowhiskers, Polymer Degradation and Stability (2014), http://dx.doi.org/10.1016/j.polymdegradstab.2014.01.004