Poly(L‐lactide)/C60 nanocomposites: Effects of C60 on crystallization of poly(L‐lactide)

The effects of solvent type and concentration of C₆₀ on the crystallization of poly(L ‐lactide) (PLLA) during solvent evaporation, heating from room temperature, and cooling from the melt were investigated by polarized optical microscopy and differential scanning calorimetry. The addition of C₆₀ enhanced the PLLA crystallization during solvent evaporation, during heating of the melt‐quenched films, and during cooling from the melt of As‐cast films, except for heating and cooling of the PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. In the case of solvent evaporation, the difference in crystallinity between the PLLA films with and without C₆₀ became higher for the solvent with a lower boiling point. In the case of heating of melt‐quenched films, the addition of C₆₀ had a small effect on the crystallinity of PLLA, whereas significantly lowered the peak top and ending temperatures of cold crystallization, except for melt‐quenched PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. The crystallinity of PLLA was determined by the solvent type, rather than by the C₆₀ concentration. In the case of cooling from the melt of As‐cast films, the addition of C₆₀ elevated the crystallinity and cold crystallization temperature values of PLLA films, except for PLLA films prepared with dichloromethane. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2167–2176, 2007     

Isothermal crystallization kinetics of poly(3‐hydroxybutyrate)/layered double hydroxide nanocomposites

Poly(3‐hydroxybutyrate) (PHB)/layered double hydroxides (LDHs) nanocomposites were prepared by mixing PHB and poly(ethylene glycol) phosphonates (PEOPAs)‐modified LDH (PMLDH) in chloroform solution. Both X‐ray diffraction data and TEM micrographs of PHB/PMLDH nanocomposites indicate that the PMLDHs are randomly dispersed and exfoliated into the PHB matrix. In this study, the effect of PMLDH on the isothermal crystallization behavior of PHB was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. Isothermal crystallization results of PHB/PMLDH nanocomposites show that the addition of 2 wt % PMLDH into PHB induced more heterogeneous nucleation in the crystallization significantly increasing the crystallization rate and reducing their activation energy. By adding more PMLDH into the PHB probably causes more steric hindrance of the diffusion of PHB, reducing the transportation ability of polymer chains during crystallization, thus increasing the activation energy. The correlation among crystallization kinetics, melting behavior and crystalline structure of PHB/PMLDH nanocomposites can also be discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3337–3347, 2006     

Isothermal crystallization behavior of poly(L‐lactide) in poly(L‐lactide)‐block‐poly(ethylene glycol) diblock copolymers

The crystal unit‐cell structures and the isothermal crystallization kinetics of poly(L ‐lactide) in biodegradable poly(L ‐lactide)‐block‐methoxy poly(ethylene glycol) (PLLA‐b‐MePEG) diblock copolymers have been analyzed by wide‐angle X‐ray diffraction and differential scanning calorimetry. In particular, the effects due to the presence of MePEG that is chemically connected to PLLA as well as the PLLA crystallization temperature T_C are examined. Though we observe no variation of both the PLLA and MePEG crystal unit‐cell structures with the block ratio between PLLA and MePEG and T_C, the isothermal crystallization kinetics of PLLA is greatly influenced by the presence of MePEG that is connected to it. In particular, the equilibrium melting temperature of PLLA, T, significantly decreases in the diblock copolymers. When the T_C is high so that the crystallization is controlled by nucleation, because of the decreasing T and thereafter the nucleation density with decreasing PLLA molecular weight, the crystallinity of PLLA also decreases with a decrease in the PLLA molecular weight. While, for the lower crystallization temperature regime controlled by the growth mechanism, the crystallizability of PLLA in copolymers is greater than that of pure PLLA. This suggests that the activation energy for the PLLA segment diffusing to the crystallization site decreases in the diblocks. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2438–2448, 2006     

Poly(L‐lactide) crystallization induced by multiwall carbon nanotubes at very low loading

Poly(L ‐lactide) (PLLA)/multiwall carbon nanotube (MWNT) composites were prepared by the solvent‐ultrasonic‐casting method. Only very low concentrations of MWNTs (<0.08 wt %) were added in the composites. Isothermal and nonisothermal crystalline measurements were carried out on PLLA/MWNT composites to investigate the effect of MWNTs on PLLA crystalline behavior. DSC results showed that the incorporation of MWNTs significantly shortened the crystalline induction time and increased the final crystallinity of the composite, which indicated MWNTs have a strong nucleation effect on PLLA even at very low concentrations. The nonisothermal crystallization measurements showed that the MWNTs greatly speed up crystallization during cooling. From isothermal crystallization results, both PLLA and PLLA/MWNT composites samples closely followed a relationship based on Lauritzen‐Hoffman theory, with the regime II to III transition shifting to lower temperature with increasing MWNT concentration. A double melting peak appeared in both nonisothermal and isothermal measurements. The conditions under which it appeared were found to closely depend on the regime II‐III transition temperature obtained from Lauritzen‐Hoffman theory. From the magnitude and position of melting peaks, it is proposed that the double melting peak is caused by a disorder‐order crystal phase transition. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2341–2352, 2009     

Poly(L‐lactide)‐Degrading Activity in Various Actinomycetes

The poly(L ‐lactide) (PLA)‐degrading ability of actinomycetes obtained from culture collections was examined by the formation of clear zones on PLA‐emulsified agar plates. Using 41 genera (105 strains) of actinomycetes with phylogenetic affiliations based on 16S rRNA sequences, PLA degraders were found to be limited to members of the family Pseudonocardiaceae and related genera. They included Amycolatopsis, Saccharothrix, Lentzea, Kibdelosporangium, and Streptoalloteichus. A large number of PLA degraders were widely distributed within the genus Saccharothrix. Most strains forming clear zones on PLA‐emulsified agar plates also formed clear zones on silk fibroin agar plates. Saccharothrix species showed an ability to degrade PLA films and assimilate degradation products in liquid cultures. No significant change of the molecular weight and polydispersity (M _w/M _n) of the remaining film fragments was confirmed. After cultivation for two weeks, many irregular holes/pits on the surface of the film due to the colonization of microorganisms were observed by scanning electron microscopy.

Scanning electron micrograph of the surface of PLA film: A. orientalis subsp. orientalis IFO 12362 after 14 d.     

Fabrication of Poly(L‐lactide)‐block‐Poly(ethylene glycol)‐block‐Poly(L‐lactide) Triblock Copolymer Thin Films with Nanochannels: An AFM Study

This paper aims to report the fabrication of biodegradable thin films with micro‐domains of cylindrical nanochannels through the solvent‐induced microphase separation of poly(L ‐lactide)‐block‐poly(ethylene glycol)‐block‐poly(L ‐lactide) (PLA‐b‐PEG‐b‐PLA) triblock copolymers with different block ratios. In our experimental scope, an increase in each of the block lengths of the PLA and PEG blocks led to both a variation in the average number density (146 to 32 per 100 µm²) and the size of the micro‐domains (140 to 427 nm). Analyses by atomic force microscopy (AFM) and fluorescence microscopy indicated that the hydrophilic PEG nanochannels were dispersed in the PLA matrix of the PLA‐b‐PEG‐b‐PLA films. We demonstrated that the micro‐domain morphology could be controlled not only by the block length of PEG, but also by the solvent evaporation conditions.

     

Clay Functionalization and Organization for Delamination of the Silicate Tactoids in Poly(L‐lactide) Matrix

Summary: Three kinds of organoclay, i.e., Cloisite15A® (C15A), Cloisite20A® (C20A), and Cloisite30B® (C30B) were modified with silane to prepare twice functionalized organoclay (TFC). The pristine Cloisite® clays and the TFC were melt compounded with poly(L ‐lactide) (PLLA) and morphology of the composites was observed to elucidate the effects of the d₀₀₁ gallery distance, the epoxy content of TFC, and the compatibility between the organic ammonium surfactant and PLLA on the degree of exfoliation of the clay layers.

Plots of I_TFC/I_Cloisite versus epoxy content for PLLA composites with (a) Cloisite15A® (C15A), (b) Cloisite20A® (C20A), (c) Cloisite30B® (C30B).     

Thermal degradation kinetics of biodegradable poly(3‐hydroxybutyrate)/layered double hydroxide nanocomposites

The thermal degradation behaviors of biodegradable poly(3‐hydroxybutyrate) (PHB) and PHB/poly(ethylene glycol) phosphonates (PEOPAs)‐modified layered double hydroxide (PMLDH) nanocomposites have been investigated using thermogravimetric analysis. Effects of PMLDH contents on the isothermal degradation kinetics of PHB were explored. These experimental results show that the degradation kinetics of PHB/PMLDH nanocomposites is the chain‐scission process of cyclic β‐elimination reaction with the following autocatalytic reactions, which is very similar to that of pure PHB matrix. Further calculated data based on the autocatalytic model can fit very well with the experimental data. The E_a value of PHB/PMLDH nanocomposites is increased as the content of PMLDH increases. This can be attributed to the incorporation of more PMLDH loading to PHB induced a decrease in the degradation rate and an increase in the residual weight for PHB/PMLDH nanocomposites. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1207–1213, 2008     

Synthesis,characterization, effect of architecture on crystallization,and spherulitic growth of poly(L‐lactide)‐b‐poly(ethylene oxide) copolymers with different branch arms

Well‐defined poly(L ‐lactide)‐b‐poly(ethylene oxide) (PLLA‐b‐PEO) copolymers with different branch arms were synthesized via the controlled ring‐opening polymerization of L ‐lactide followed by a coupling reaction with carboxyl‐terminated poly(ethylene oxide) (PEO); these copolymers included both star‐shaped copolymers having four arms (4sPLLA‐b‐PEO) and six arms (6sPLLA‐b‐PEO) and linear analogues having one arm (LPLLA‐b‐PEO) and two arms (2LPLLA‐b‐PEO). The maximal melting point, cold‐crystallization temperature, and degree of crystallinity (X_c) of the poly(L ‐lactide) (PLLA) block within PLLA‐b‐PEO decreased as the branch arm number increased, whereas X_c of the PEO block within the copolymers inversely increased. This was mainly attributed to the relatively decreasing arm length ratio of PLLA to PEO, which resulted in various PLLA crystallization effects restricting the PEO block. These results indicated that both the PLLA and PEO blocks within the block copolymers mutually influenced each other, and the crystallization of both the PLLA and PEO blocks within the PLLA‐b‐PEO copolymers could be adjusted through both the branch arm number and the arm length of each block. Moreover, the spherulitic growth rate (G) decreased as the branch arm number increased: G_{6sPLLA‐b‐PEO} < G_{4sPLLA‐b‐PEO} < G_{2LPLLA‐b‐PEO} < G_{LPLLA‐b‐PEO}. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2034–2044, 2006     

The Role of Functional Group on the Exfoliation of Clay in Poly(L‐lactide)

Summary: A new method was attempted to improve the interaction between poly(L ‐lactide) (PLLA) with a commercially available organoclay, Cloisite 25A (C25A), which was functionalized with (glycidoxypropyl)trimethoxysilane to introduce epoxy groups, and is referred to as twice‐functionalized organoclay (TFC). Tethering PLLA molecules to the epoxy groups on the surface of the TFC was attempted by melt compounding. X‐Ray diffraction and transmission electron microscopy images showed that fully exfoliated PLLA/TFC nanocomposites were prepared successfully.

Transmission electron micrographs of the PLLA composites of the three clays used here.     

Preparation,characterization, structure‐property relationships,and thermal degradation kinetics of poly (lactic acid)/amidated potassium hydrogen phthalate intercalated layered double hydroxides nanocomposites

A novel nucleating agent, amidated potassium hydrogen phthalate intercalated layered double hydroxides (AP‐LDHs) were prepared using an amidation reaction. Through the structural characterization, it was found that AP‐LDHs had been successfully prepared. Meanwhile, the antibacterial activity of AP‐LDHs was studied. In order to improve the performance of poly (lactic acid) (PLA), PLA/AP‐LDHs nanocomposites were prepared by melt blending. Morphological analysis showed that PLA nanocomposites had an exfoliated structure. Mechanical properties test showed that the mechanical properties of PLA nanocomposites were enhanced. And the fracture scanning electron microscope analysis indicated that the PLA/AP‐LDHs nanocomposites exhibited ductile fracture characteristics. Moreover, differential scanning calorimetry and polarized optical microscopy analysis results demonstrated that the crystallization rate, nucleation density, and crystallinity of PLA/AP‐LDHs were improved. Thermogravimetric analysis and thermal degradation kinetics showed that the thermal stability of the PLA nanocomposites was significantly improved.     

Linear and four‐armed poly(l‐lactide)‐block‐poly(d‐lactide) copolymers and their stereocomplexation with poly(lactide)s

Linear and four‐armed poly(l ‐lactide)‐block‐poly(d ‐lactide) (PLLA‐b‐PDLA) block copolymers are synthesized by ring‐opening polymerization of d ‐lactide on the end hydroxyl of linear and four‐armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four‐armed PLLA/PDLA blends. Compared with the four‐armed PLLA‐b‐PDLA copolymer, the similar linear PLLA‐b‐PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g^?1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four‐armed copolymer/linear PLA blends, both sc and hc develop in the four‐armed PLLA‐b‐PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two‐thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1560–1567     

Enhancing crystallization behavior for optimized performances of poly(TMC‐b‐(LLA‐ran‐GA)) by PDLA/PLLA stereocomplex crystallization

The blends of poly(1,3‐trimethylene carbonate‐b‐(l ‐lactide‐ran‐glycolide)) (PTLG) with poly(d ‐lactide) (PDLA) were prepared via solution‐casting method using CH₂Cl₂ as solvent. The poly(l ‐lactide) (PLLA) segments of PTLG with PDLA chain constructed as stereocomplex structures and growth stereocomplex crystals of PLA (sc‐PLA). The effects of sc‐PLA crystals on thermal behavior, mechanical properties, thermal decomposition of the PTLG/PDLA blends were investigated, respectively. The differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results showed that the total crystallinity of the PTLG/PDLA blends was increased with the PDLA content increasing. Heterogeneous nucleation of sc‐PLA crystals induced crystallization of the PLLA segments in PTLG. The crystallization temperature of samples shifted to 107.5°C for the PTLG/PDLA‐20 blends compared with that of the PTLG matrix, and decreased the half‐time of crystallization. The mechanical measurement results indicated that the tensile strength of the PTLG/PDLA blends was improved from 21.1 MPa of the PTLG matrix to 39.5 MPa of PTLG/PDLA‐20 blends. The results of kinetics of thermal decomposition of the PTLG/PDLA blends by TGA showed that the apparent activation energy of the PTLG/PDLA blends was increased from 59.1 to 72.1 kJ/mol with the increasing of the PDLA content from 3 wt% to 20 wt%, which indicated the enhancement of thermal stability of the PTLG/PDLA blends by addition of PDLA. Furthermore, the biocompatibility of the PTLG/PDLA blends cultured with human adipose‐derived stem cells was evaluated by CCK‐8 and live/dead staining. The experiment results proved the PTLG/PDLA blends were a kind of biomaterial with excellent physical performances with very low cytotoxicity.     

Synthesis of poly(L‐lactide)‐grafted pullulan through coupling reaction between amino group end‐capped poly(L‐lactide) and carboxymethyl pullulan and its aggregation behavior in water

Poly(L ‐lactide) (PLLA) with terminal primary amino groups (PLLA‐NH₂) was synthesized and used to construct PLLA‐grafted pullulan (Pul‐g‐PLLA). It consisted of a hydrophilic carboxymethyl Pul (CM‐Pul) main chain and hydrophobic PLLA graft chains that were created through a direct coupling reaction between PLLA‐NH₂ and CM‐Pul using 2‐ethoxy‐1‐(ethoxycarbonyl)‐1,2‐dihydroquinoline as a condensation reagent. Pul‐g‐PLLAs with over 78 wt % sugar unit content were found to form nanometer‐sized aggregates in water. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5482–5487, 2004     

New Preparation of Structurally Symmetric,Biodegradable Poly(L‐lactide) Disulfides and PLLA–Stabilized,Photoluminescent CdSe Quantum Dots

New, biodegradable poly(L ‐lactide) disulfides, PLLA‐SS‐PLLA, were first prepared through the DMAP‐catalyzed ring‐opening polymerization of L ‐lactide with a dihydroxyethyl disulfide initiator, and were further catalytically reduced into thiol‐end‐functionalized poly(L ‐lactide)s, HO‐PLLA‐SH, with a tributyl phosphine catalyst (PBu₃). Employing the HO‐PLLA‐SH as the ligand, new core‐shell CdSe/PLLA quantum dots (QDs) were continuously prepared via a facile ligand‐exchanging process with the CdSe/TOPO QD precursor. The chemical structures, morphologies and solvent solubility of these prepared CdSe/PLLA QDs were investigated by NMR spectroscopy, FTIR spectroscopy, XRD, TEM and excitation under either room light or UV radiation at 365 nm, demonstrating the successful ligand replacement and the new formation of core‐shell CdSe/PLLA QDs (diameter:4.0 ± 0.3 nm). Finally, UV and FL results indicate the two factors of the HO‐PLLA‐SH ligand molecular weight and the ligand/QD precursor feeding weight ratio were important for preparing stable and highly photoluminescent CdSe/PLLA QDs.

     

Highly improved crystallization behavior of poly(L‐lactide) induced by a novel nucleating agent: substituted‐aryl phosphate salts

In this work, a novel nucleating agent (NA) based on substituted‐aryl phosphate salts was introduced into poly(L‐Lactide) (PLLA). The nonisothermal and isothermal crystallization behaviors of nucleated PLLA samples were investigated through differential scanning calorimetry (DSC), wide angle X‐ray diffraction, and polarized optical microscope (POM). Furthermore, the effect of annealing treatment on the cold crystallization behaviors of nucleated samples was also investigated. The results show that the crystallization of PLLA, whether for the melt crystallization (including nonisothermal and isothermal crystallization process) or for the cold crystallization (including the cold crystallization occurring during the DSC heating process and during the annealing process), is greatly dependent upon the content of NA. At relatively lower NA content (≤0.1 wt%), the nucleation effect of NA is inconspicuous, however, at higher NA content (≥0.2 wt%), it exhibits great nucleation effect for the crystallization of PLLA. Further results show that the double endothermic peak of PLLA depends on the temperature applied for the crystallization. Copyright © 2012 John Wiley & Sons, Ltd.     

Nonisothermal crystallization and morphology of poly(butylene succinate)/layered double hydroxide nanocomposites

Biodegradable poly(butylene succinate) (PBS) and layered double hydroxide (LDH) nanocomposites were prepared via melt blending in a twin-screw extruder. The morphology and dispersion of LDH nanoparticles within PBS matrix were characterized by transmission electron microscopy (TEM), which showed that LDH nanoparticles were found to be well distributed at the nanometer level. The nonisothermal crystallization behavior of nanocomposites was extensively studied using differential scanning calorimetry (DSC) technique at various cooling rates. The crystallization rate of PBS was accelerated by the addition of LDH due to its heterogeneous nucleation effect; however, the crystallization mechanism and crystal structure of PBS remained almost unchanged. In kinetics analysis of nonisothermal crystallization, the Ozawa approach failed to describe the crystallization behavior of PBS/LDH nanocomposites, whereas both the modified Avrami model and the Mo method well represented the crystallization behavior of nanocomposites. The effective activation energy was estimated as a function of the relative degree of crystallinity using the isoconversional analysis. The subsequent melting behavior of PBS and PBS/LDH nanocomposites was observed to be dependent on the cooling rate. The POM showed that the small and less perfect crystals were formed in nanocomposites.     

Synthesis and characterization of poly(vinyl alcohol)‐graft‐[poly(D,L‐lactide)/poly(D,L‐lactide‐co‐glycolide)] hydrogels

Poly(D ,L ‐lactide) and poly(D ,L ‐lactide‐co‐glycolide) with various composition and with one methacrylate and one carboxylate end group were synthesized and grafted onto poly(vinyl alcohol) (PVA) via the carboxylate group. The graft copolymers were crosslinked via the methacrylate groups using a free radical initiator. The polymer networks were characterized by means of NMR and studied qualitatively by means of IR spectroscopy. The influence of the glycolide content in the polyester grafts and of the number of ester units in the grafts on thermal properties and swellability were studied as well. The high swellability in water is characteristic of all hydrogels. Differential scanning calorimetry (DSC) showed a single glass transition temperature that occurs in the range between 51 and 69 °C. Thermogravimetric analysis (TGA) of the networks showed the main loss in weight in the temperature range between 290 and 370 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4536–4544, 2007     

Nonisothermal crystallization behavior and crystalline structure of poly(3‐hydroxybutyrate)/layered double hydroxide nanocomposites

X‐ray diffraction method and differential scanning calorimetry analysis have been used to investigate the nonisothermal crystallization of poly(3‐hydroxybutyrate) (PHB)/poly(ethylene glycol) phosphonates (PEOPAs)‐modified layered double hydroxide (PMLDH) nanocomposites. Effects of cooling rates and PMLDH contents on the nonisothermal crystallization behavior of PHB were explored. These results show that the addition of 2 wt % PMLDH into PHB caused heterogeneous nucleation increasing the crystallization rate and reducing the activation energy. By adding PMLDH into the PHB probably hinder the transport ability of the molecule chains and result in a decreasing crystallity of PHB, thus increasing the activation energy. The correlation among melting behavior, apparent crystallite size, and paracrystalline distortion of PHB/PMLDH nanocomposites has been also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 995–1002, 2007     

Comparative study of poly(L‐lactide) nanocomposites with organic montmorillonite and carbon nanotubes

Organic montmorillonite (OMMT) and the one‐dimensional functionalized multiwalled carbon nanotubes (FMWCNTs) were introduced into poly(L ‐lactide) (PLLA) to prepare PLLA/OMMT and PLLA/FMWCNT nanocomposites, respectively. The effects of nanofillers on melt crystallization and cold crystallization of PLLA were comparatively investigated by using polarized optical microcopy, differential scanning calorimetry and wide angle X‐ray diffraction. The results show that FMWCNTs exhibit higher nucleation efficiency for the melt crystallization of PLLA, whereas OMMT is the better one for the cold crystallization of PLLA. Rheological properties show that both OMMT and FMWCNTs at relatively higher concentrations can form the percolated network structure in the PLLA matrix, however, the latter nanocomposites exhibit relatively denser or more compact percolated networks. The difference of the networks between OMMT and FMWCNTs is suggested to be the main reason for the different cold crystallization behaviors observed in the PLLA/OMMT and PLLA/FMWCNT nanocomposites. The dynamic mechanical analysis measurements show that OMMT is the better one to improve the stiffness of the nanocomposites in the present work. The thermogravimetric analysis measurements show that FMWCNTs have higher efficiency in improving the thermal stability of PLLA compared with OMMT. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013