Toughening Behavior of Poly(L-Lactic Acid)/Poly(D-Lactic Acid) Asymmetric Blends

The L-configured poly(lactic acid) has exhibited vast appeal in the past decades. However, most previous researches reveal that L-configured poly(lactic acid) exhibits fragile behavior, which limits its applications. Present work clarified that the toughness of poly(lactic acid) depended on the content of crystallites and rigid component incorporated into the specimens. The elongation at break was remarkably high in the L-configured poly(lactic acid)/D-configured poly(lactic acid) with a small amount of D-configured poly(lactic acid) (≤15%). The stretching led to orientation of crystals and amorphous molecular chains and segments. The crystals did not vary, while the amorphous molecular chains transited to mesophase during stretching, and this mesophase formed homocrystallites during heating.     

Physical and morphological characterisation of poly(L-lactide) acid-based electrospun fibrous structures: tunning solution properties

ABSTRACT

Poly(l-lactide) nano/micro fibres were produced using the electrospinning technique. The influence of solvent type and solution concentration on the morphology and structure of electrospun fibres were investigated. For this purpose, the electrospinning was performed using 2, 2, 2-trifluoroethanol (TFE) or dichloromethane (DCM) solvents at solution concentrations in a range of 5–9?wt-%. Morphological studies showed that versus DCM-based fibres with beaded structures and porous surface, when using TFE as a solvent, the uniform bead-free fibres with a smooth surface morphology were produced. Compared to DCM, the fibre diameter decreased using TFE as a solvent with lower vapour pressure while the crystallinity of the fibres increased. By increasing the solution concentration up to 9?wt-%, uniform fibres were produced with growing average diameters. Following these results, we conclude that varying the solution concentration and solvent type can modulate the final properties of electrospun fibres.     

Synthesis and Characterization of Novel Thermotropic Aromatic-Aliphatic Biodegradable Copolyesters Containing D,L-Lactic acid (LA), Poly(butylene terephthalate) (PBT) and Biomesogenic Units

In this study, the copolyesters based on 4-hydroxybenzoic acid (HBA) and vanillic acid (VA), lactic acid (LA) and poly(butylene terephthalate) (PBT) were synthesized via melt polymerization and fully characterized by various measurements. The influences of content of HBA and VA units on thermal behavior, structure and degree of crystallinity of copolyesters were discussed in more detail. It was found that the copolymerization of aliphatic and aromatic units together could make the best use of advantages of the respective polyesters. Moreover, the copolyesters with more than 40 mol% of HBA and VA units could show liquid crystallinity in broad temperature range.     

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

The segmental dynamics of polylactide chains covering the T_g − 30 °C to T_g + 30 °C range was studied in absence and presence of a crystalline phase by dynamic mechanical analysis (DMA) using the framework provided by the WLF theory and the Angell's dynamic fragility concept. An appropriate selection of stereoisomers combined with a thermal conditioning strategy to promote crystallization (above T_g) or relaxation of chains (below T_g) was revealed as an efficient method to tune the ratio of the rigid and mobile amorphous phases in polylactides. A single bulklike mobile amorphous phase was taken for poly(d,l-lactide) (PDLLA). In turn three phases, comprising a mobile amorphous fraction (MAF, X_MA), a rigid amorphous fraction (RAF, X_RA) and a crystalline fraction (X_c) were determined in poly(l-lactide) (PLLA) by modulated differential scanning calorimetry (MDSC) according to a three-phase model. The analysis of results confirms that crystallinity and RAF not only elevate the T_g and the breadth of the glass transition region but also yields an increase in dynamic fragility parameter (m) which entails the existence of a smaller length-scale of cooperativity of polylactide chains in confined environments. Consequently it is proposed that crystallinity is acting in polymeric systems as a topological constraint that, preventing longer range dynamics, provides a faster segmental dynamics by the temperature dependence of relaxation times according to the strong-fragile scheme.     

Biodegradable and conductive hyperbranched terpolymers based on aliphatic polyester,poly(D,L-lactide), and polyaniline used as scaffold in tissue engineering

Designing the novel conductive and biodegradable scaffolds based on star-like hyperbranched terpolymers of aliphatic polyester–poly(D,L-lactide)–polyaniline (S-HAP–PLA–PANI) was the purpose of this research. The electroactivity of tissue engineering scaffold, which in the current work was for the presence of PANI, is an essential factor in its performance because the electrical signals are the pivotal physiological stimuli that control the adhesion and differentiation of various cell types. Star-shaped polymers have attracted conspicuous attention, thanks to their low-cost, well-defined highly functionalized structures and low crystallinity, and they also could be an interesting alternative to the linear analogs for their interaction with surrounding tissue and faster degradation rate. In the present work, after 12 weeks, the mass loses for S-HAP–PLA–PANI sample were calculated to be 45%. Scaffolds based on S-HAP–PLA–PANI/PLA nanofibers having the average diameter of 70–200?nm and electrical conductivity of 0.05 S cm^?1 imitated the natural microenvironment of extra cellular matrix to regulate the cell attachment, proliferation, and differentiation.     

Crystalline,Thermal, and Biodegradable Properties of Poly(L-Lactic Acid)/Poly(D-Lactic Acid)/POSS Melt Blends

A ternary nanocomposite consisting of poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), and epoxy cyclohexyl polyhedral oligomeric silsesquioxane (e-POSS) was prepared by reactive blending method. Scanning electron microscopy revealed that the feeding of three compositions in batches, i.e., PDLA incorporation at different times, was more beneficial for the even dispersion of POSS in matrix. POSS introduction improved the homocrystallinity and stereocomplex of the blends. Rheological properties and heat resistance were enhanced, which indicated potential extensive application of PLLA-based materials. The optimization of degradation stability in saline buffer was attributed to the various hydrophobic properties of blends caused by POSS structure.     

Poly(d,l-lactide)/poly(methyl methacrylate) interpenetrating polymer networks: Synthesis, characterization, and use as precursors to porous polymeric materials

The use of semi-hydrolyzable oligoester-derivatized interpenetrating polymer networks (IPNs) as nanostructured precursors provides a straightforward and versatile approach toward mesoporous networks. Different poly(d,l-lactide) (PLA)/poly(methyl methacrylate) (PMMA)-based IPNs were synthesized by resorting to the so-called in situ sequential method. The PLA sub-network was first generated from a dihydroxy-telechelic PLA oligomer via an end-linking reaction with Desmodur^® RU as a triisocyanate cross-linker. Subsequently, the methacrylic sub-network was created by free-radical copolymerization of methyl methacrylate (MMA) and a dimethacrylate (either bisphenol A dimethacrylate or diurethane dimethacrylate) with varying compositions (initial MMA/dimethacrylate composition ranging from 99/1 to 90/10 mol%). Both cross-linking processes were monitored by real-time infrared spectroscopy. The microphase separation developed in IPN precursors was investigated by differential scanning calorimetry (DSC). Furthermore, the quantitative hydrolysis of the PLA sub-network, under mild basic conditions, afforded porous methacrylic structures with pore sizes ranging from 10 to 100 nm -at most- thus showing the effective role of cross-linked PLA sub-chains as porogen templates. Pore sizes and pore size distributions were determined by scanning electron microscopy (SEM) and thermoporometry via DSC measurements. The mesoporosity of residual networks could be attributed to the good degree of chain interpenetration associated with both sub-networks in IPN precursors, due to their peculiar interlocking framework.     

Assessment of Cytocompatibility of Bulk Modified Poly-L-lactic Acid Biomaterials

The cytocompatibility and hydrophilicity tests were performed by culturing mouse fibroblastic cells on films of poly-L-lactic acid (PLLA), poly(L-lactic-co-glycolide) (PLGA) and poly(L-lactide-co-glycolide)/ bioactive glass (PLGA/BG) or in the presence of extracts from these polymeric materials. The solvent casting method was used to prepare these films. PLLA films were most hydrophobic and PLGA/BG was least hydrophobic. Compared to the other films, PLLA showed the worst results in cytocompatibility. PLGA also showed favorable results for fibroblastic cells viability. PLGA/BG films also demonstrated improved cell compatibility due to the good biocompatibility of the bioactive glass particles. The results of this study indicate the promising biocompatibility of PLGA/BG as biomaterials in medical field.     

Poly(l-lactide) brushes on magnetic multiwalled carbon nanotubes by in-situ ring-opening polymerization

Biodegradable poly(l-lactide) (PLLA) has been covalently grafted onto the surface of magnetic multiwalled carbon nanotubes (m-MWCNTs) by in-situ ring-opening polymerization of lactide. The content of grafting PLLA can be controlled by adjusting the feed ratio of monomer to m-MWCNTs. FT-IR and Raman spectroscopy confirm that PLLA have been covalently attached to the sidewalls of m-MWCNTs. Thermal gravimetric analysis (TGA) indicates that the composites of PLLA grafted m-MWCNTs have a polymer weight percentage of ca. 25.6-33.7 wt%. The scanning eletron microscopy (SEM) and transmission electron microscopy (TEM) are utilized to image the PLLA grafted m-MWCNTs, showing relatively uniform polymer layer coated on the surface of m-MWCNTs. The composites of PLLA grafted m-MWCNTs exhibit superparamagnetic behavior at room temperature and are aligned under a low magnetic field.     

Miscibility and hydrolytic degradation in alkaline solution of poly(l-lactide) and poly(methyl methacrylate) blends

Poly(l-lactide) (PLLA) was melt blended with poly(methyl methacrylate) (PMMA) using a two-roll mill. The miscibility and hydrolytic degradation of the blend films were characterized. It was found that PLLA/PMMA blend has high miscibility in the amorphous state because only single T_g was observed in the DSC and DMA measurements. In alkaline solution, the hydrolytic degradation rate of the blends whose PMMA content is higher than 30 wt% was decelerated while the rate of the blends whose PMMA content is lower than 30 wt% was accelerated. That is, the hydrolytic degradation rate of the blends could be widely controlled by PMMA content in the blend. It was also found that only PLLA was hydrolyzed and eluted into alkaline solution, while PMMA remained during alkaline hydrolysis.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

Controlled crystal nucleation in the melt-crystallization of poly(l-lactide) and poly(l-lactide)/poly(d-lactide) stereocomplex

Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔH_c (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.     

Functionalized micelles from new ABC polyglycidol-poly(ethylene oxide)-poly(d,l-lactide) terpolymers

New ABC type terpolymers of poly(ethoxyethyl glycidyl ether)/poly(ethylene oxide)/poly(d,l-lactide) were obtained by multi-mode anionic polymerization. After successive deprotection of the ethoxyethyl groups from the first block, highly hydroxyl functionalized copolymers of polyglycidol/poly(ethylene oxide)/poly(d,l-lactide) were obtained. These copolymers form elongated ellipsoidal micelles by direct dissolution in water. The micelles consist of a poly(d,l-lactide) core and stabilizing shell of polyglycidol/poly(ethylene oxide). The hydroxyl groups of polyglycidol blocks situated at the micelle surface provide high functionality, which could be engaged in further chemical modification resulting in a potential drug targeting agents. The micellization process of the copolymers in aqueous media was studied by hydrophobic dye solubilization, static and dynamic light scattering, and transmission electron microscopy.     

Melt intercalation of poly(l-lactide) chains into clay galleries

The melt intercalation of poly(l-lactide) (PLLA) chains into silicate galleries has been investigated via a melting process without any shearing force at elevated temperature. Under the melting process, the incorporation of various types of organo-modified montmorillonites into PLLA matrix lead to the increase in the basal spacing of clay particles in different manner without delamination into individual layers. The changes in layer-stacked structures of the clay particles in the PLLA matrix were examined by use of wide-angle X-ray diffraction and transmission electron microscopy. The effects of clay content in PLLA matrix and clay surfactants on the melt intercalation of PLLA were discussed in terms of chain mobility.     

Preparation and characterization of PLLA/P(CL-b-LLA) blends by an in situ ring-opening polymerization

This paper describes a new reactive blending approach to improve the compatibility of poly(l-lactide) (PLLA) and poly(?-caprolacton) (PCL). For this purpose, the ring-opening polymerization of l-lactide (LLA) was carried out in the presence of PCL-OH (OH groups on one end). The P(CL-b-LLA) block copolymers were in situ formed during polymerization, and as a result, PLLA/P(CL-b-LLA) blends were obtained. The characterization of GPC and ¹H NMR verified the synthesis of PLLA/P(CL-b-LLA) blends. Furthermore, the ¹³C NMR spectroscopy showed that no transesterification reaction occurred to a significant extent during LLA polymerization process. The in situ formed P(CL-b-LLA) compatibilizes the phase separated structure of PCL domains in PLLA matrix. The size of PCL domains in PLLA matrix became much smaller than that in a solution blended sample. The average sizes of PCL domains are controllable in the level of sub-micron scale.     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

Molecular modeling simulations and thermodynamic approaches to investigate compatibility/incompatibility of poly(l-lactide) and poly(vinyl alcohol) blends

This paper investigates the molecular modeling simulation approaches for understanding the blend compatibility/incompatibility of poly(l-lactide), PLL and poly(vinyl alcohol), PVA. Blends of PLL/PVA have been widely used in biotechnology as well as membranes in separation science. Realizing their importance, we thought of investigating to verify experimental observations on their compatibility/incompatibility aspects by calculating thermodynamic interactions between PLL and PVA over the entire range of blend compositions. In doing so, Flory-Huggins interaction parameter, χ, was computed for different blends using atomistic simulations to predict blend miscibility. It was found that at 1:9 blend composition of PLL/PVA, miscibility was observed, but increasing immiscibility was prevalent at higher compositions of PLL component. Computed results confirmed the literature findings on differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and mechanical property studies, suggesting the validity of modeling strategies. Plots of Hildebrand solubility parameter, δ, and cohesive energy density, CED, supported these findings. Miscibility of PLL and PVA polymers is attributed to hydrogen-bonding effect. Literature findings have been validated to understand the nature of interactions between different groups of the polymers by computing radial distribution function, RDF, for groups that are tentatively involved in such interactions, leading to miscibility or immiscibility. RDF plot was constructed to identify the exact contribution of particular atoms of polymers to confirm miscibility/immiscibility of blends. Results of this study are correlated well with the reported data. Kinetics of phase separation was examined using density profiles calculated from the MesoDyn approach to examine miscibility/immiscibility aspects of the blends. Computed free energy from the mesoscopic simulation of blends reached equilibrium, particularly when simulation was performed at higher time step, indicating the stability of the blend at certain compositions. X-ray diffraction profiles have been constructed for individual polymers as well as for their blends, which agreed well with the reported data.     

Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.     

Study on crystalline morphology of poly(l-lactide)-poly(ethylene glycol) diblock copolymer

Crystallization behavior, structural development and morphology evolution in a series of diblock copolymers of poly(l-lactide)-block-poly(ethylene glycol) (PLLA-b-PEG) were investigated via differential scanning calorimetry, wide-angle X-ray diffraction, polarized optical microscopy and atomic force microscopy. In these copolymers, both blocks are crystallizable and biocompatible. It was interesting that these PLLA-b-PEG diblock copolymers could form spherulites with banded textures, which was undercooling dependent. Single crystals with an abundance of screw dislocations were also observed via AFM. Such results indicated that these ringed spherulites and single crystals were formed during the crystallization of the PLLA blocks.     

Synthesis and characterization of poly(l-glutamic acid)-block-poly(l-phenylalanine)

Poly(γ-benzyl l-glutamate)-block-poly(l-phenylalanine) was prepared via the ring opening polymerization of γ-benzyl l-glutamate N-carboxyanhydride and l-phenylalanine N-carboxyanhydride using n-butylamine·HCl as an initiator for the living polymerization. Polymerization was confirmed by ¹H-nuclear magnetic resonance spectroscopy and matrix assisted laser desorption ionization time of flight mass spectroscopy. After deprotection, the vesicular nanostructure of poly(l-glutamic acid)-block-poly(l-phenylalanine) particles was examined by transmission electron microscopy and dynamic light scattering. The pH-dependent properties of the nanoparticles were evaluated by means of ζ-potential and transmittance measurements. The results showed that the block copolypeptide could be prepared using simple techniques. Moreover, the easily prepared PGA-PPA block copolypeptide showed pH-dependent properties due to changes in the PGA ionization state as a function of pH; this characteristic could potentially be exploited for drug delivery applications.