PLLA Blends Research Articles

Thermal analysis on phase behavior of poly(l-lactic acid) interacting with aliphatic polyesters

Thermal behavior, miscibility, and crystalline morphology in blends of low-molecular-weight poly(l-lactic acid) (LMw-PLLA) or high-molecular-weight PLLA (HMw-PLLA) with various polyesters such as poly(butylene adipate) (PBA), poly(ethylene adipate) (PEA), poly(trimethylene adipate) (PTA), or poly(ethylene succinate) (PESu), respectively, were explored using differential scanning calorimeter (DSC), and polarized-light optical microscopy (POM). Phase behavior in blends of PLLA with other polyesters has been intriguing and not straight forward. Using a low- and high molecular weight PLLA, this study aimed at mainly using thermal analyses for probing the phase behavior, phase diagrams, and temperature dependence of blends systems composed of PLLA of two different molecular weights (low and high) with a series of aliphatic polyesters of different structures varying in the (CH2/CO) ratio in main chains. The blends of LMw-PLLA/PEA and LMw-PLLA/PTA show miscibility in melt and amorphous glassy states. Meanwhile, the LMw-PLLA/PESu blend is immiscible with an asymmetry-shaped upper critical solution temperature (UCST) at 220–240 °C depending on the blend composition. In contrast to miscibility in LMw-PLLA/PTA and LMw-PLLA/PEA blends, HMw-PLLA with polyesters are mostly immiscible; and HMw-PLLA/PTA blend is the only one showing an asymmetry-shaped UCST phase diagram with clarity points at 195–235 °C (depending on composition). Reversibility of UCST behavior, with no chemical transreactions, in these blends was proven by solvent recasting, gel permeation chromatography, and Fourier transform infrared spectroscopy (FT-IR). Crystalline morphology behavior of the LMw-PLLA/PEA and LMw-PLLA/PTA blends furnishes addition evidence for miscibility in the amorphous phase between LMw-PLLA and PTA or PEA. At lower molecular weights, LMw-PLLA is proven to be miscible with poly(ethylene adipate) (PEA) and poly(trimethylene adipate) (PTA). High-M w PLLA/PTA blend exhibits UCST. The phase behavior is significantly influenced by M W’s of weakly interacting PLLA and PTA.

Morphology and rheology of poly(l-lactide)/polystyrene blends filled with silica nanoparticles

The successful development of co-continuous structure from poly(l-lactide) (PLLA) blends by melt mixing with lower PLLA content is highly desired in preparing macroporous biomaterials. However, the low viscosity of PLLA makes it difficult to prepare co-continuous PLLA blends at low PLLA concentration. In this study, hydrophilic silica nanoparticle is adopted to control the morphology of co-continuous polystyrene (PS)/PLLA blends. The influence of nanoparticle concentration on the co-continuity intervals and rheological properties of PS/PLLA blends are examined. The morphological stability of blends against melt annealing is also determined and discussed with a conceptual coarsening model for co-continuous structure. The results demonstrate that the incorporation of silica nanoparticles into PS/PLLA blends can be used to prepare macroporous PLLA structure with controllable pore size at lower PLLA content.

Carbon nanotubes induced microstructure and mechanical properties changes in cocontinuous poly( L‐lactide)/ethylene‐co‐vinyl acetate blends

AbstractToughening of poly( L‐lactide) (PLLA) by elastomer attracts much attention in recent years; however, it is usually associated with the deterioration of modulus and/or strength, resulting in limitation in many applications of the material. In this work, functionalized multiwalled carbon nanotubes (FMWCNTs) were introduced into ethylene‐co‐vinyl acetate toughened PLLA blends. The effects of FMWCNTs content on crystalline structure of PLLA matrix and the morphology of the blends, as well as the selective distribution of FMWCNTs in the ternary nanocomposites were investigated using differential scanning calorimetry (DSC), wide angle X‐ray diffraction, scanning electron microscope, and transmission electron microscope. The results show that FMWCNTs exhibit excellent nucleation role in improving the cold crystallization behaviors of PLLA during the annealing and/or DSC heating processes. The results of mechanical property measurements demonstrate that the modulus, strength, and ductility of the blends can be further improved simultaneously through introducing FMWCNTs. Copyright © 2011 John Wiley & Sons, Ltd.

Improving the Toughness of Poly (L-lactic acid) by Interpenetrating Network Structure

An interpenetrating polymer network was formed by blending of PLLA and PEG cross-linked β-CD to improve the toughness of PLLA, and the mechanical properties of PLLA composites were studied using tensile test. The PLLA composite samples exhibited a single glass transition and their Tg values decreased with increasing PEG/β-CD content, indicating that PLLA and PEG/β-CD are miscible. The PLLA composite samples aged at room temperature achieved the highest fracture strain (about 235%), about 40 times that of the original PLLA sample, while the Young's modulus was not seriously hampered.

Preparation and Characterization of New Biodegradable Films Made from Poly(L-Lactic Acid) and Chitosan Blends Using a Common Solvent

Poly-(L-lactic acid) (PLLA) has been widely used for various biomedical applications due to its interesting properties such as its mechanical behavior, processability, biocompatibility, and biodegradability. Blending this polymer with chitosan that, besides being biodegradable and hydrophilic, can interact with anionic glycosaminoglycans, proteoglycans, and other negatively charged molecules of the extracellular matrix, could constitute an excellent way to improve the biological performance of PLLA in these kinds of applications. Such blends could also be used in environmental applications. In this work a new and simple method of preparing biodegradable blends of chitosan and PLLA at room temperature was developed. To the best of our knowledge, this is the first time that a common solvent for the two polymers has been used, hexafluor-2-propanol (HFIP), to produce a homogeneous solution containing both PLLA and chitosan. We also anticipate that this solvent can also be used to compatibilize other combinations of natural and synthetic polymers. Membranes were then obtained by solvent casting. Films with different fractions of each component were successfully prepared and didn't show visible phase separation. The prepared films were characterized by differential scanning calorimetry (DSC) in order to analyze the miscibility of the two components as a function of the composition of the film.

Different enantioselectivity of two types of poly(lactic acid) depolymerases toward poly( l-lactic acid) and poly( d-lactic acid)

Poly(lactic acid) (PLA) depolymerases are categorized into protease-type and lipase-type. Protease-types can hydrolyze poly( l-lactic acid) (PLLA) but not poly( d-lactic acid) (PDLA). Lipase-types, including cutinase-like enzyme (CLE) from Cryptococcus sp. strain S-2 preferentially hydrolyze PDLA. Both enzymes degraded not only PLA emulsion but also PLA film, in which amorphous region is preferentially attacked, but crystalline region can be also attacked. Stereocomplex PLA (sc-PLA) formed by 50:50 blending of PLLA and PDLA included no homo crystals, but a tiny homo crystallization peak appeared and crystallinity increased by 5% when attacked by CLE, although no significant change of molecular weight and crystalline size was found. Enantioselective degradation must occur in amorphous region of PLLA/PDLA film and preferentially hydrolyzed PDLA, resulting in a slightly excess amount of PLLA remained, which must be crystallized.

Changes in the crystallinity and mechanical properties of poly(l-lactic acid)/poly(butylene succinate-co-l-lactate) blend with annealing process

Biodegradable polymer blends of poly(l-lactic acid) (PLLA) and poly(butylene succinate-co-l-lactate) (PBSL) at various blending ratios are prepared. The blending of PLLA with PBSL results in an increase in the ductility and thermal stability of the blend. However, flexural strength and modulus, as well as loss modulus, decrease with an increase in PBSL content. Annealing is employed to increase blend crystallinity and subsequently improve the mechanical properties of the PLLA/PBSL blend. The influences of annealing time on the crystal modification, thermal properties, and mechanical properties of the PLLA/PBSL blend are investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and three-point bending test, respectively. Crystalline peaks are found in the XRD patterns of all annealed samples. DSC analysis reveals that the degree of crystallinity is enhanced with an increase in annealing time. The flexural modulus also increases with annealing time due to the change in crystalline phases. However, longer periods of annealing, especially over 20 h, result in thermal degradation and subsequently reduce the modulus value of the PLLA/PBSL blend.

The control of surface segregation of blend films using stereocomplex formation between enantiomeric polylactide chains

AbstractThis study investigated the stereocomplex‐induced surface structure of enantiomeric poly(lactide) (PLA) blend films by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The design of the blend systems was based on the principle of surface segregation of a modified component with a low surface energy, fluorine‐end‐capped PLLA (F‐PLLA). Analysis of F‐PLLA/PDLA and F‐PLLA/PLLA blends, yielded a difference in the ion beam‐induced behavior between fluorine and lactide groups. In addition, a completely phase‐separated sample of PLLA with the fluorine end‐capping agent cast onto the surface was used for a real analysis of peak intensities from the two components, simulating bulk compositions. The two blend systems, uncomplexed (F‐PLLA/PLLA) and complexed (F‐PLLA/PDLA) blends, showed quite different surface structures: namely, a higher fluorine concentration was observed in the uncomplexed blends while the complexed blends showed a wider range of fluorine concentrations, depending on the degree of complexation. This indicates that the complexed blends offer a greater level of control over the surface fluorine concentration because this concentration changes more significantly with the blend composition (or the degree of complexation). Copyright © 2010 John Wiley & Sons, Ltd.

A Unique Meta‐Form Structure in the Stereocomplex of Poly(D‐lactic acid) with Low‐Molecular‐Weight Poly(L‐lactic acid)

AbstractThe crystalline structure and crystallization behavior of PLLA crystals in a 1:1 w/w mixture of low‐MW PLLA with high‐MW PDLA were analyzed using WAXD, DSC, and SAXS. Under cold crystallization, homopolymeric PLLA, appearing as a meta crystal, was discovered in the PDLA/LMW‐PLLA blend. The meta‐ and α′ crystal forms of PLLA were found to form on crystallization at a Tcc of 85–95 °C and the α crystal PLLA formed at 100 ≤ Tcc < 120 °C. The meta‐crystal PLLA may be incorporated in the stereocomplexed PDLA/LMW‐PLLA lamellar region. During heating, the meta‐crystal PLLA first partially melted and then repacked directly into the α crystal PLLA without going through the less‐stable α′ form. magnified image

Novel 3D scaffolds of chitosan–PLLA blends for tissue engineering applications: Preparation and characterization

This work addresses the preparation of 3D porous scaffolds of blends of chitosan and poly( l-lactic acid), CHT and PLLA, using supercritical fluid technology. Supercritical assisted phase-inversion was used to prepare scaffolds for tissue engineering purposes. The physicochemical and biological properties of chitosan make it an excellent material for the preparation of drug delivery systems and for the development of new biomedical applications in many fields from skin to bone or cartilage regeneration. On the other hand, PLLA is a synthetic biodegradable polymer widely used for biomedical applications. Supercritical assisted phase-inversion experiments were carried out in samples with different polymer ratios and different polymer solution concentrations. The effect of CHT:PLLA ratio and polymer concentration and on the morphology and topography of the scaffolds was assessed by SEM and Micro-CT. Infra-red spectroscopic imaging analysis of the scaffolds allowed a better understanding on the distribution of the two polymers within the matrix. This work demonstrates that supercritical fluid technology constitutes a new processing technology, clean and environmentally friendly for the preparation of scaffolds for tissue engineering using these materials.

Toughening of Polylactide with Polymerized Soybean Oil

Polymerized soybean oil (polySOY) and isotactic poly(L-lactide) (PLLA) were melt blended to increase the toughness of PLLA in an all renewable blend. The polySOY samples were prepared by crosslinking soybean oil by the addition of a free radical crosslinking agent or by heating the oil in the presence of oxygen. Soybean oil is relatively nonreactive compared to other vegetable oils, and conjugation of the double bonds within the fatty acid chains of the soybean oil triglyceride prior to crosslinking led to significantly increased reactivity. Poly(isoprene-b-L-lactide) block copolymers were used to compatibilize the blends due to the high degree of immiscibility between PLLA and polySOY. The blending of polySOY and PLLA resulted in significant improvements in the tensile toughness of the blend compared to neat PLLA. The blend morphology was dependent on the polySOY gel fraction or weight-average molar mass; the polySOY characteristics were key indicators of the tensile toughness.

Heterostereocomplexation between Biodegradable and Optically Active Polyesters as a Versatile Preparation Method for Biodegradable Materials

The thermal properties and crystallization of biodegradable and optically active poly[(S)-2-hydroxybutyrate] [P(S-2HB)], poly(l-lactide) (PLLA), poly(d-lactide) (PDLA) and their blends were investigated. The results of differential scanning calorimetry, wide-angle X-ray scattering (WAXS), and polarized optical microscopy first indicated heterostereocomplexation between biodegradable and optically active polyesters having different chemical structures and opposite configurations, that is, P(S-2HB) and PDLA. The melting temperature of the heterostereocomplex was higher than those of pure polymers. Such cocrystallization was not observed for P(S-2HB)/PLLA blends having identical configurations. The WAXS profile of P(S-2HB)/PDLA heterostereocomplex was very similar to those of the PLLA/PDLA and P(S-2HB)/P(R-2HB) homostereocomplexes and each crystalline diffraction peak of the heterostereocomplex was located between those of the homostereocomplexes. The present study strongly suggests that heterostereocomplexation will provide a novel versatile method for preparing biodegradable polyester materials with a wide range of physical properties and biodegradability.

Optimisation of polymer scaffolds for retinal pigment epithelium (RPE) cell transplantation

AimTo evaluate a variety of copolymers as suitable scaffolds to facilitate retinal pigment epithelium (RPE) transplantation.MethodsFive blends of poly(l-lactic acid) (PLLA) with poly(d,l-lactic-glycolic acid) (PLGA) were manufactured by a solid–liquid...

Characterization of the mechanical and thermal properties and morphological behavior of biodegradable poly(L‐lactide)/poly(ε‐caprolactone) and poly(L‐lactide)/poly(butylene succinate‐co‐L‐lactate) polymeric blends

AbstractTwo series of biodegradable polymer blends were prepared from combinations of poly(L‐lactide) (PLLA) with poly(ϵ‐caprolactone) (PCL) and poly(butylene succinate‐co‐L‐lactate) (PBSL) in proportions of 100/0, 90/10, 80/20, and 70/30 (based on the weight percentage). Their mechanical properties were investigated and related to their morphologies. The thermal properties, Fourier transform infrared spectroscopy, and melt flow index analysis of the binary blends and virgin polymers were then evaluated. The addition of PCL and PBSL to PLLA reduced the tensile strength and Young's modulus, whereas the elongation at break and melt flow index increased. The stress–strain curve showed that the blending of PLLA with ductile PCL and PBSL improved the toughness and increased the thermal stability of the blended polymers. A morphological analysis of the PLLA and the PLLA blends revealed that all the PLLA/PCL and PLLA/PBSL blends were immiscible with the PCL and PBSL phases finely dispersed in the PLLA‐rich phase. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Blending Effects on Polymorphic Crystallization of Poly(l-lactide)

The effects of miscible blending on the crystallization kinetics and crystalline structure of poly(l-lactide) (PLLA) were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). It was confirmed that the blends of PLLA and poly(d,l-lactide) (PDLLA) are miscible at all compositions. Due to the dilution effect, the crystallization of PLLA was hindered with the presence of PDLLA. The PLLA/PDLLA blends show the irregular or ring-banded spherulites. The polymorphic behavior of PLLA is influenced by the miscible blending with PDLLA. FTIR and WAXD data indicates that the formation of the α-form crystals of PLLA is favored in the blends. With incorporation of 50 wt % PDLLA, the critical temperature for the formation of α′- and α-form crystals decreases from ∼110 (of neat PLLA) to ∼80 °C. The factors affecting the formation of the PLLA α′- and α-form crystals were discussed from the thermod...

Miscibility and Hydrolytic Behavior of Poly(trimethylene carbonate) and Poly(l-lactide) and Their Blends in Monolayers at the Air/Water Interface

In this study, two biodegradable polymers, poly(trimethylene carbonate) (PTMC) and poly(L-lactide) (PLLA) along with a series of PTMC/PLLA blends, were used as spreading materials to form LB monolayers at the air/water interface to study hydrolytic reaction kinetics of the monolayers with the Langmuir film balance technique. The pi-A isotherms of each homopolymer and their blends showed that blends of PTMC and PLLA were miscible on the neutral subphase (pH 7.4), whereas there was evidence of phase separation on the basic subphase (pH 10.7). The hydrolysis behavior of each homopolymer was investigated at these two different pH conditions. The PTMC monolayer showed faster hydrolysis on the neutral subphase (pH 7.4) than on the basic subphase (pH 10.7). However, in the case of the PLLA monolayer, the hydrolysis on the basic subphase is faster than that on the neutral subphase. On the basis of this result, hydrolysis mechanisms of PTMC and PLLA, considering a general hydrolysis mechanism and their stereo structures, are proposed. The hydrolysis rates of blends of PTMC and PLLA were much faster than that of each homopolymer on the basic subphase (pH 10.7). This result, which can be explained by a "dilution effect", was supported by the structure based mechanism proposed here.

Characterization of a PLLA-Collagen I Blend Nanofiber Scaffold with Respect to Growth and Osteogenic Differentiation of Human Mesenchymal Stem Cells

The aim of this study was to enhance synthetic poly(L-lactic acid) (PLLA) nanofibers by blending with collagen I (COLI) in order to improve their ability to promote growth and osteogenic differentiation of stem cells in vitro. Fiber matrices composed of PLLA and COLI in different ratios were characterized with respect to their morphology, as well as their ability to promote growth of human mesenchymal stem cells (hMSC) over a period of 22 days. Furthermore, the course of differentiation was analyzed by gene expression of alkaline phosphatase (ALP), osteocalcin (OC), and COLI. The PLLA-COLI blend nanofibers presented themselves with a relatively smooth surface. They were more hydrophilic as compared to PLLA nanofibers alone and formed a gel-like structure with a stable nanofiber backbone when incubated in aqueous solutions. We examined nanofibers composed of different PLLA and COLI ratios. A composition of 4:1 ratio of PLLA:COLI showed the best results. When hMSC were cultured on the PLLA-COLI nanofiber blend, growth as well as osteoblast differentiation (determined as gene expression of ALP, OC, and COLI) was enhanced when compared to PLLA nanofibers alone. Therefore, the blending of PLLA with COLI might be a suitable tool to enhance PLLA nanofibers with respect to bone tissue engineering.

Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.

The effect of ionic interaction on the miscibility and crystallization behaviors of poly(ethylene glycol)/poly(L‐lactic acid) blends

AbstractThe effect of end groups (2NH2) of poly(ethylene glycol) (PEG) on the miscibility and crystallization behaviors of binary crystalline blends of PEG/poly(L‐lactic acid) (PLLA) were investigated. The results of conductivity meter and dielectric analyzer (DEA) implied the existence of ions, which could be explained by the amine groups of PEG gaining the protons from the carboxylic acid groups of PLLA. The miscibility of PEG(2NH2)/PLLA blends was the best because of the ionic interaction as compared with PEG(2OH, 1OH‐1CH3, and 2CH3)/PLLA blends. Since the ionic interaction formed only at the chain ends of PEG(2NH2) and PLLA, unlike hydrogen bonds forming at various sites along the chains in the other PEG/PLLA blend systems, the folding of PLLA blended with PEG(2NH2) was affected in a different manner. Thus the fold surface free energy played an important role on the crystallization rate of PLLA for the PEG(2NH2)/PLLA blend system. PLLA had the least fold surface free energy and the fast crystallization rate in the PEG(2NH2)/PLLA blend system, among all the PEG/PLLA systems studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Surface-phase separation of PEO-containing biodegradable PLLA blends and block copolymers

The surface chemistry of two series of poly(ethylene oxide) (PEO)-containing biodegradable poly( l-lactic acid) (PLLA) matrix systems has been investigated using time-of-flight SIMS (ToF-SIMS) and XPS. The two systems are (1) PLLA blend matrices with an amphiphilic Pluronic ® P-104 surfactant, P(EO) 27- b-P(PO) 61- b-P(EO) 27, and (2) PLLA- b-PEO diblock and PLLA- b-PEO- b-PLLA triblock copolymers. The phase separation is analyzed in determining the surface enrichment of the component and chemical composition at the polymer–air interface. The PEO component is surface-dominant in the blend system in contrast to the surface excess of poly(propylene oxide) (PPO) in pure Pluronic ® P-104. The block copolymer system shows the surface enrichment of PLLA component. These results can be explained in terms of the change in surface free energy for the block copolymers and the better miscibility of PLLA and PPO against amphiphilic PEO for the blends, respectively.