PDLA Chains Research Articles

Formation of stereocomplex in enantiomeric poly(lactide)s via recrystallization of homocrystals: An in-situ X-ray scattering study

The formation of stereocomplex (SC) in enantiomeric poly(lactide)s via recrystallization of homocrystals was investigated by using simultaneous wide-angle and small-angle X-ray scattering. When the symmetric blend of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) was heated from the glassy state at a slow heating rate (5°C/min), predominant homocrystals were formed during heating (cold crystallization). Melting of homocrystals and crystallization of stereocomplex occurred almost simultaneously. In this case, the diffusion of PLLA and PDLA chains played an important role in determining the maximum attainable SC crystallinity. The crystallization behavior during annealing at different temperatures (185°C, 195°C and 225°C) and subsequent cooling was further investigated to understand the effectiveness of annealing on the formation of stereocomplex. The stereocomplex content was unchanged during annealing at 185°C and kept constant during cooling to room temperature. The SC crystallinity increased slightly during annealing at 195°C and subsequently cooling. On the other hand, the content of SC increased during annealing at 225°C and increased remarkably during cooling. A possible mechanism considering the diffusion of PLLA and PDLA chains was proposed.

Forming CNT-guided stereocomplex networks in polylactide-based nanocomposites

Grafting a nanoparticle surface using a polymer similar to the matrix has been widely applied to control the spatial organization of nanoparticles. However, the fabrication of target materials with well-defined nanoparticle arrangement remains fundamentally difficult because of the absence of specific interactions between the matrix and the graft. In this study, the self-networking structure of poly(d-lactide)-grafted carbon nanotubes (CNT-g-PDLA) in poly(l-lactide) (PLLA) matrix was investigated. Specific interactions between enantiomeric pairs not only promoted CNT dispersion, but also contributed to the regular phase-separation-like CNT self-networking. Furthermore, the grafted PDLA chains preferably formed stable stereocomplex crystallites with the PLLA matrix, and the CNT self-networking resulted in the self-assembly of 3D continuous stereocomplex scaffold. It was demonstrated that the CNT-guided stereocomplex network endows polylactide-based nanocomposites with significantly improved mechanical strength, heat-resistance, and electrical conductivity at low CNT concentrations.

Remarkably Enhanced Impact Toughness and Heat Resistance of poly(l-Lactide)/Thermoplastic Polyurethane Blends by Constructing Stereocomplex Crystallites in the Matrix

As an eco-friendly polymer with tremendous potential to replace traditional petroleum-based and nonbiodegradable polymers, the current use of poly(l-lactide) (PLLA) in large-scale commercial applications still faces some barriers mostly associated with its inherent brittleness and poor heat resistance. In this work, we propose a novel and facile strategy to simultaneously address these obstacles by introducing small amounts of poly(d-lactide) (PDLA) into thermoplastic polyurethane (TPU) toughened PLLA blends through melt-blending. The results manifest that the introduced PDLA chains can readily interact with PLLA matrix chains and rapidly cocrystallize into stereocomplex (sc) crystallites capable of acting as an efficient rheology modifier to dramatically improve melt viscoelasticity of the PLLA matrix and subsequently induce the morphological change of the dispersed TPU phase from a typical sea–island structure to a unique networklike structure, thus endowing PLLA/TPU/PDLA blends with remarkably improved...

Molecular Structural Basis for Stereocomplex Formation of Polylactide Enantiomers in Dilute Solution.

Poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) alternatively pack with each other and form stereocomplex crystals (SCs). The crystal habits of SCs formed in the dilute solution highly depend on the molecular weight (⟨Mw⟩). In this study, we investigated chain-folding (CF) structure for 13C labeled PLLA (l-PLLA) chains in SCs with PDLAs that have either high or low ⟨Mw⟩s by employing an advanced Double Quantum (DQ) NMR. It was found that the ensemble average of the successive adjacent re-entry number ⟨n⟩ for the l-PLLA chains drastically change depending on ⟨Mw⟩s of the counter PDLA chains in the SCs. It was concluded that the CF structures of l-PLLA depending on ⟨Mw⟩s of PDLA determine the crystal habits of SCs.

Reinforcing Effects of Poly(D-Lactide)-g-Multiwall Carbon Nanotubes on Polylactide Nanocomposites.

Polylactide (PLA) nanocomposites with multi-walled carbon nanotubes (MWNTs) grafted with poly(L-lactide) or poly(D-lactide) were prepared by solution casting, and their thermal and mechanical properties were evaluated. MWNTs containing hydroxyl groups were treated by ring-opening polymerization of either L-lactide or D-lactide. Fourier transform infrared spectroscopy confirmed that the MWNT surfaces had been modified by the PLLA or PDLA chains. The thermal properties were measured by differential scanning calorimetry and thermogravimetric analysis. The mechanical properties were examined using a universal testing machine. The morphology of the fractured surfaces of the PLA nanocomposites was observed by scanning electron microscopy and transmission electron microscopy. PDLA-g-MWNTs were dispersed more uniformly compared to PLLA-g-MWNTs in the PLA matrix. The incorporation of PDLA-g-MWNTs greatly improved the tensile strength of the nanocomposites regardless of the contents. Thermal analysis revealed different characteristics at specific composites depending on the type of modification.

Effect of chemical and physical branching on rheological behavior of polylactide

This study was aimed at improving the process rheology of polylactide (PLA) melts by means of two strategies. First, PLAs of different branched structures, i.e., star shaped, comblike, and hyper branched, were synthesized and blended with a linear grade analog. Shear and extensional flow rheometry tests were performed on pure materials and their blends to evaluate their rheological properties. It was shown that the presence of branched poly(L-lactide) (PLLA) increased the shear thinning, shear and extensional viscosity, and elastic modulus of linear PLLA at the same time; the star shaped PLLA providing the most significant change. Second, poly(D-lactides) (PDLA) with similar molecular architectures were synthesized to have a double branching effect. In addition to the presence of branched architecture, physical cross-links due to the stereocomplex formation exist between PLLA and PDLA chains. Based on the rheological characterizations in shear and extensional mode, a greater improvement in PLA melt rheological properties was observed for blends containing stereocomplex structure as compared to linear/branched enantiopure blends.

Nano-ordered surface morphologies by stereocomplexation of the enantiomeric polylactide chains: specific interactions of surface-immobilized poly(D-lactide) and poly(ethylene glycol)-poly(L-lactide) block copolymers.

Both AB diblock and ABA triblock copolymers consisting of poly(L-lactide) (PLLA: A) and poly(ethylene glycol) (PEG: B) were deposited on a silicon surface on which poly(D-lactide) (PDLA) had been preimmobilized. The deposit of the diblock copolymer (PLLA-PEG) formed band structures similar to those observed when the same copolymer was directly deposited on the silicon surface. In contrast, the deposit of the triblock copolymer (PLLA-PEG-PLLA) formed many particulates scattering over the surface. When the PLLA-PEG deposit was subjected to water-soaking, the original band morphology was completely replaced by the particulate morphology that was identical to that of the PLLA-PEG-PLLA deposit. Their FT-IR analyses revealed that both copolymers had been bound through the stereocomplex (sc) formation between the preimmobilized PDLA chains and the PLLA blocks of the copolymers. Grazing-incidence small-angle X-ray scattering (GISAXS) also supported these surface morphologies. It was therefore evident that hydrophilic PEG chains can be immobilized on the PDLA-preimmobilized surface by the sc formation.

Four-armed PCL-b-PDLA diblock copolymer: 1. Synthesis, crystallization and degradation

Abstract A series of four-armed poly(e-caprolactone)- b -poly( d -lactic acid) diblock copolymers (4a-PCL- b -PDLA) with well controlled composition were synthesized. The effect of length of PCL and PDLA blocks on the crystallization and enzymatic/alkaline degradation was studied. It was found that the PDLA blocks have great influence on the crystallization of PCL blocks under two different crystallization conditions. The copolymers with shorter PDLA blocks showed faster degradation rate but different spherulite structures after enzymatic degradation or alkaline degradation. The morphological changes of spherulites depended on the PCL chains which were trapped inside or excluded from the spherulites of PDLA when the copolymer crystallized at 80 °C or 30 °C (i.e. above or lower the glass transition temperature of PDLA chains). The spherulite morphology and porous morphology of copolymer films after degradation were well discussed. This work revealed that the crystalline morphology and degradation rate of PCL- b -PDLA films could be well controlled by copolymer composition and confined crystallization of PCL chains within the previously formed lamellar stacks of PDLA.

Precursors in stereo-complex crystals of poly(L-lactic acid)/poly(D-lactic acid) blends under shear flow

To improve the mechanical and the thermal performance of poly(lactic acid) materials, this work focuses on the formation of stereo-complex crystals by blending poly(L-lactic acid) (PLLA) with poly(D-lactic acid) (PDLA). The resulting structure was analyzed using time-resolvedin situX-ray scattering, optical microscopy, differential scanning calorimetry and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow imposed prior to crystallization on higher-order structure formation and acceleration of stereo-complex crystal growth of PLLA and PDLA blends using a wide spatial scale analysis and viscoelastic measurements. Density fluctuations of 100 nm scale were observed prior to nucleation byin situsimultaneous wide- and small-angle X-ray scattering measurements. These density fluctuations grew with time and the intensity increased with increasing shear rate. Furthermore, the results revealed that the PLLA and PDLA chains were only partially interpenetrated; consequently, stereo-complex crystals could grow only in the mixed PLLA/PDLA phase. The correlation length of density fluctuation prior to nucleation was strongly dependent on the mixed phases.

Enhancing the melt stability of polylactide stereocomplexes using a solid-state cross-linking strategy during a melt-blending process

Selective cross-linking of PLLA and PDLA chain couples in the amorphous phase allows for the formation of stereocomplex (sc) crystallites in the continuous melting and recrystallization process to be perfectly reversible.

Effects of interfacial stereocomplexation in cellulose nanocrystal-filled polylactide nanocomposites

Cellulose nanocrystals (CNC), extracted from cellulose fibers by the mean of acid hydrolysis, were subjected to ring opening polymerization of d-Lactide initiated from the hydroxyl groups available at their surface. The resulting CNC-g-PDLA nanohybrids were incorporated in poly(l-lactide) (PLLA) matrix through melt-blending and the obtained nanocomposites were characterized in terms of their thermo-mechanical properties. Surface-induced polylactide (PLA) stereocomplexation resulting from the co-crystallization of grafted PDLA chains and unbounded PLLA ones from the matrix was evidenced. Acquired results and a thorough comparison with their counterparts grafted with PLLA chains reveal the effects of the stereocomplexed PLA structure surrounding CNC. Important alterations of both crystalline structure and its behavior of the final materials were observed originated from an efficient nucleation effect induced by the addition of these nanohybrids. Significant enhancement of the mechanical performances was also obtained most probably brought by the stereocomplexation that stiffens and stabilize further the percolation network.

Rubber toughening of poly(lactic acid): Effect of stereocomplex formation at the rubber‐matrix interface

AbstractIn this study, a series of graft copolymers poly(n‐butyl acrylate)‐g‐poly‐L‐lactide (PBA‐g‐PLLA) and poly(n‐butyl acrylate)‐g‐poly‐D‐lactide (PBA‐g‐PDLA) with various molecular weights and PBA/PLA ratios were prepared from the copolymerization of n‐butyl acrylate and PLA macromers. The PLA macromers were prepared by ring opening polymerization of L‐ or D‐lactide in the presence of 2‐hydroxyethyl acrylate initiator. The synthesized copolymers can be used to toughen brittle PLLA by incorporating them as an elastomeric phase. Interestingly, the much tougher PLLA was obtained when the commercial PLLA was blended with 10 wt % elastomeric graft copolymer containing pendant PDLA chain. It is believed that the formation of stereocomplex between PLLA matrix and PDLA side chain in the elastomer phase differentiates it from the blend of PLLA and PBA‐g‐PLLA, whose side chain has the same configuration as the matrix PLLA. DSC, XRD, and TEM results support this hypothesis. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Molecular Organization of Polylactides Immobilized on a Flat Surface: Observation of Single Crystal Arrays of Homochiral and Stereocomplexed Polylactides

Formation of single crystal arrays of polylactides (PLA) was found on a flat surface immobilized with poly(d-lactide) (PDLA) having different chain lengths. Monoalkoxydimethylsilyl-terminated PDLAs were synthesized by hydrosilylation of (allyloxy)ethyl-terminated PDLAs and covalently immobilized onto silicon surfaces by a silicone coupling reaction in which the immobilization took place homogeneously through a single Si–O–Si bond. Atomic force microscopy (AMF) of the immobilized surfaces revealed that PDLA chains having Mn = 2400–7000 formed many projections (dots) on the surface of the polymer layers, each having a diameter of 30 nm and a height less than 1.0 nm with a narrow size distribution. When these surfaces were treated with free PDLAs and enantiomeric poly(l-lactide)s (PLLA) having similar chain lengths, nano-ordered arrangement of single crystallites arrays of homochiral (hc) and stereocomplexed (sc) PLAs were formed with ordering by the deposition of PDLA and PLLA chains on the surface-immobilized PDLA chains, respectively.

Temperature-Variable FTIR and Solid-State 13C NMR Investigations on Crystalline Structure and Molecular Dynamics of Polymorphic Poly(l-lactide) and Poly(l-lactide)/Poly(d-lactide) Stereocomplex

Crystalline structure and molecular dynamics in alpha and alpha' crystals of poly(L-lactide) (PLLA) and PLLA/poly(Dlactide) (PDLA) stereocomplex (sc) crystals have been investigated by the temperature-variable FTIR and solid-state C-13 CP-MAS NMR spectroscopy. The crystal forms of polylactide (PLA) have different band frequencies, correlation field splittings in FTIR spectra and different line shapes, and resonance splittings in solid-state NMR spectra, which become more distinct with cooling to the cryogenic conditions. The well-resolved splittings in NMR resonances of alpha crystals, attributable to the crystallographically inequivalent sites within crystal unit cell, are considered to be due to the dipolar interactions related to the carbonyl, methyl, and methine groups. The splittings in FTIR bands and NMR resonances are absent in alpha' crystals, indicating the disordered conformation and loose molecular lateral packing within their crystal lattices. The significant FTIR frequency shifts of nu(C=O), nu(CH3), and nu(CH) modes during stereocomplex crystallization of PLLA/PDLA blend and the appearance of spectral splittings at cryogenic conditions suggest the coexistence of weak C-H center dot center dot O=C hydrogen bonds and dipolar interactions between PLLA and PDLA chains in the sc crystals of PLA Below the glass transition temperature (T-g), the spin lattice relaxation times of PLA with different crystalline structures increase in the order of amorphous approximate to alpha' < alpha < sc.

Toughening semicrystalline poly(lactic acid) by morphology alteration

The incorporation of the triblock copolymer (PDLA-PEG-PDLA) into Poly(l-Lactic Acid) (PLLA) has produced a semicrystalline polymer of substantial modulus and strength. These improvements in mechanical properties could potentially increase the utility of this biomass-based polymer. The blended samples have a continuous amorphous phase with crystalline regions being the discontinuous portion. Micro-Raman spectroscopy revealed that a stereocomplex involving PDLA and PLLA chains exists in the crystalline region. It can be concluded that the poly(ethylene glycol), the flexible midblock component, is necessarily dispersed in these crystalline regions. Both morphological features can contribute to the improvement in mechanical properties. Therefore, the successful toughening of PLA may be achieved due to several mechanisms working synergistically.

Crystal and Thermal Properties of PLLA/PDLA Blends Synthesized by Direct Melt Polycondensation

We herein report the effects of the component ratio and method of blending on the synthesis of stereocomplex poly(lactic acid) (SC-PLA) based on poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) prepolymers. PLLA and PDLA were prepared by direct melt polycondensation of lactic acid (DMP). Combined with the dual catalyst system, PLA prepolymers with Mw more than 20,000 were prepared by DMP. PLLA was mixed by powder blending or melt blended with PDLA. It is revealed that melt-point and spherulite growth rate of SC-PLA is strongly dependent on the perfection of SC structure. The melt point of PLA can be increased by nearly 50 °C because of the particular strong intermolecular interaction between PLLA and PDLA chains. Solid-state polycondensation (SSP) is an efficient method to increase the molecular weight of SC-PLA, but it can have a negative effect on the regularity of linear chains of SC-PLA. Thermogravimetry analyzer (TGA) results show that SC structure cannot cause the delay reaction on the thermal degradation of PLA.

Stereo-Complex Crystallization of Poly(lactic acid)s in Block-Copolymer Phase Separation

Block-copolymer containing a poly(L-lactic acid) (PLLA) segment was blended with pure poly(D-lactic acid) (PDLA) chain in chloroform solution and casted into the dried film. This method could form the stereocomplex (Sc) crystal of PLLA/PDLA within the nanometer-sized phase separation self-assembled by block-copolymer. Differential scanning calorimetry measurements of the prepared and annealed films gave the maximum achievable melting temperature of 245 °C, which was highest among those previously reported for PLA Sc-crystals. Surprisingly, all of added PDLA chains were Sc-crystallized, depending on the blend composition with block-copolymer containing PLLA.

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (Mn) of 3.9 × 103, 9.3 × 103, and 1.1 × 104 g mol−1, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (Xc) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle Mn of 9.3 × 103 g mol−1. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in Mn, but seems to be caused by the upper critical Mn values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.

Formation of Crystallosolvates Comprising Nano‐Crystals of Stereocomplex in a Ternary Mixture of Poly(L‐lactide)/Poly(D‐lactide)/Diphenyl Ether

AbstractEqual amounts of poly(L‐lactide) (PLLA) and poly(D‐lactide) (PDLA) were mixed with diphenyl ether (DE) at high temperature in DE‐to‐polymer composition from 0 to 80 wt.‐%. Cooling of the ternary mixtures produced white crystallosolvates without deposition of DE. Various analyses revealed that the crystallosolvates consisted of both homo‐chiral (hc) and stereocomplex (sc) crystals of PLLA and PDLA as well as amorphous domains involving DE. The crystallosolvate obtained at a DE content of 80 wt.‐%, comprised only the sc crystals. Atomic force microscopy (AFM) revealed that many granules having a size of 20 nm in diameter were formed from rod‐like hc crystallites and that the DE was slightly phase‐separated to form nano‐sized reservoirs. The stronger sc interaction between the PLLA and PDLA chains excluded DE from the polymer matrix.magnified image

Novel thymopentin release systems prepared from bioresorbable PLA–PEG–PLA hydrogels

Copolymers were synthesized by ring opening polymerization of l- or d-lactide in the presence of dihydroxyl PEG with molar mass of 6000, 12,000 and 20,000, using zinc lactate as catalyst. Bioresorbable hydrogels were obtained by mixing PLLA–PEG–PLLA and PDLA–PEG–PDLA aqueous solutions due to stereocomplexation between PLLA and PDLA chains. Rheological measurements show that the hydrogels present typical viscoelastic behaviors, although degradation could occur during the gelation process. Thymopentin was taken as a model drug to evaluate the potential of PLA–PEG–PLA hydrogels as carrier of hydrophilic drugs. Various parameters such as copolymer concentration, drug load, copolymer composition and the difference between sol and gel were considered. The release profiles are characterized by an initial burst followed by slower release. Higher copolymer concentration leads to slower release rate and less burst effect due to more compact structure which disfavors drug diffusion. Similarly, higher molar mass of the copolymers disfavors the release of TP5, and hydrogels composed of both PLLA/PEG and PDLA/PEG present slower release rates than single copolymer solutions. In contrast, drug load exhibits little influence on the release profiles due to the high water solubility of TP5. In all cases, nearly 80% of TP5 is released. In vivo studies proved the potential of TP5 containing hydrogels, especially those with a concentration of 25%. Both the CD4 +/CD8 + ratio and the morphology of thymus indicate the immunization efficacy of the TP5 release systems based on PLA/PEG hydrogels.