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

The 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 T_cc of 85–95 °C and the α crystal PLLA formed at 100 ≤ T_cc < 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.

     

Stereocomplex Crystallization of Star‐Shaped 4‐Armed Equimolar Stereo Diblock Poly(lactide)s with Different Molecular Weights: Isothermal Crystallization from the Melt

The isothermal crystallization of star‐shaped four‐armed equimolar stereo diblock poly(lactide) (4‐LD) polymers with different molecular weights is investigated. Solely stereocomplex (SC) crystallites are formed in all equimolar 4‐LD polymers, irrespective of molecular weight and crystallization temperature (T_c). The wide‐angle X‐ray diffractometry, differential scanning calorimetry, and polarized optical microscopy results for crystalline species, crystallinity, and maximum radial growth rate of spherulites values indicate that both branching and diblock architectures disturb the SC crystallization and spherulites growth of equimolar 4‐LD polymers, and the disturbance effect is larger for branching architecture than for diblock architecture. The equilibrium melting temperature (T_m⁰) values are 181.9–266.0 °C, which are comparable with or lower than the value reported earlier (279 °C). The crystallite growth geometries of equimolar of 4‐LD polymers are independent of molecular weight and T_c.

     

Attempts to map the structure and degradation characteristics of aliphatic polyesters derived from lactic and glycolic acids

During the past 5 years, important advances have been accomplished in the understanding of the fate of aliphatic polyesters derived from lactic acid (LA) and glycolic acid (GA) in aqueous media. Hydrolysis of solid LA/GA polymers is now regarded as dependent upon a diffusion-reaction mechanism. Faster central degradation, degradation-induced composition, and morphology changes are three of the most important findings which appeared to be composition-dependent as deduced from the behavior of different LA/GA polymers. An attempt is made to generalize these findings to the whole family and to elaborate a map which could be used to predict degradation characteristics of LA/GA polymers from their initial composition and morphology.     

Biodegradation of PLA/GA polymers: increasing complexity

The degradation of aliphatic polyesters derived from lactic and glycolic acids (PLA/GA) depends on many factors. It has been found recently that the interior of large size devices degrades faster than the outer zone. A qualitative model has been proposed to account for this heterogeneous degradation. It is based on diffusion-reaction phenomena combined with the well-known autocatalytic effect of carboxylic chain ends. This contribution recalls the present understanding of the hydrolytic degradation of PLA/GA polymers and emphasizes its complexity on the basis of the influence of secondary factors such as the presence of a basic load, namely, gentamycin, in poly(lactic acid) matrices, and the presence of long stereoregular sequences in poly( -lactic acid) macromolecules. Biomaterials (1994) 15,1209–1213     

Heterostereocomplex Crystallization and Homocrystallization From the Melt in Blends of Substituted and Unsubstituted Poly(lactide)s

Poly(L ‐2‐hydroxybutyrate) [P(L ‐2HB)] and poly(D ‐lactide) (PDLA) in their blends were phase separated to form P(L ‐2HB)‐ and PDLA‐enriched domains, and heterostereocomplex (HTSC) crystallization occurred at the interface between these domains. In the blends, HTSC crystallites were formed at crystallization temperature (T_c) = 80–160 °C, and their exclusive formation without the formation of other crystalline species was observed at T_c of 150 or 160 °C. The effects of polymer blend ratio on the types of formed crystalline species were very small due to the phase separation of the blends. The presence of a small amount of P(L ‐2HB) decreased the low limit of crystallizable T_c of PDLA homocrystallites from 80 to 60 °C. The equilibrium melting temperature of HTSC crystallites was 257.6 °C.     

Unique Transitions in Morphology and Characteristics of Porous Poly(Lactic Acid) Enantiomers

This study examines unique changes in the morphology and physical properties of monolithic poly(lactic acid) (PLA) enantiomers. It is revealed for the first time that PLA monoliths containing stereocomplex (sc) crystals are successfully produced using enantiomeric poly(L‐lactic acid) (PLLA) and poly(D‐lactic acid) (PDLA) using a simple phase separation process with ternary solvent (1,4‐dioxane/2‐butanone/water). The basic structure of the monoliths is changed drastically from needle‐like to spherical morphology until reaching a PLLA/PDLA ratio to equivalence. The PLA monolith prepared from a stoichiometric amount of PLLA/PDLA has high sc crystallinity without any homochiral crystals, and it shows higher melting temperature (226 °C), surface area (131 m² g^?1), and water contact angle (144.5°) compared with those of neat PLLA (180 °C, 86 m² g^?1, and 137.5°, respectively). Moreover, this sc‐PLA monolith exhibits excellent resistance to several good solvents of PLLA, whereas the pristine PLLA monolith is dissolved instantly in these solvents.     

Synthesis of High‐Molecular‐Weight Poly(L‐lactic acid) by Direct Polycondensation

To prepare poly(lactic acid) (PLA) with improved mechanical properties, high‐molecular‐weight poly(L ‐lactic acid) (PLLA) was synthesized by the direct polycondensation of lactic acid in solution phase. During polymerization, the molecular weight of PLA was influenced by the water content present in the solution; thus, the experimental apparatus was designed to remove the water efficiently and this study was focused on the optimization of the polymerization conditions such as polymerization time, solvent, and kinds of catalyst, etc. Additionally, to search for a good catalyst in the polymerization, the mixed oxide catalysts were synthesized by sol–gel method and it was characterized by XRD and BET. The results show that the M _v of PLA obtained was about 33 000, when 0.2 wt.‐% of SnCl₂ · 2H₂O was used as a catalyst in the polymerization. The DSC study shows that the thermal properties of PLA such as T_g and T_m were influenced by the kind of solvent as well as the molecular weight of PLA.

The effect of various kinds of stannous compounds on the molecular weight of PLA synthesized in solution polymerization with molecular sieve.     

Isothermal Cold‐Crystallization of PLA/PBAT Blends With and Without the Addition of Acetyl Tributyl Citrate

The isothermal cold‐crystallization kinetics of the PLA phase is studied by DSC and the crystallization from the melt by PLOM. Even though the blends exhibit two phases by SEM, several pieces of evidence indicate that partial miscibility may be present in these blends: small changes in both T_g and T_m of the PLA phase; a dependence of the spherulitic growth rate on blend composition and the oclusion of PBAT droplets inside PLA spherulites. Acetyl tributyl citrate is able to plasticize both phases in the blends, but it displays a preference to dissolve within the PBAT rich phase. There is a synergystic effect on the increase in the overall crystallization rate of the PLA rich phase when both ATBC and PBAT are present in the blend.     

Biodegradation of Thermoplastic Starch and its Blends with Poly(lactic acid) and Polyethylene: Influence of Morphology

The room temperature mineralization of thermoplastic starch (TPS) with a high glycerol content and its blends with low‐density polyethylene (LDPE) and polylactic acid (PLA) are examined under controlled degradation conditions. These results are correlated with the morphologies and continuity behavior of the various blend systems. It is found that thermoplastic starch degrades more rapidly than native starch. Lowering the glycerol content in the TPS has virtually no effect on its biodegradation behavior. The only contribution to biodegradation of the TPS blend is from the TPS component. Blending TPS with LDPE and PLA in a co‐continuous morphology at a 50/50 composition provides a significant increase in TPS surface area, which increases the biodegradation rate for the blends as compared to pure TPS. The results indicate a close relationship between morphology, phase continuity, and biodegradation behavior.

     

Stereocomplex Crystallization and Spherulite Growth of Low Molecular Weight Poly(L‐lactide) and Poly(D‐lactide) from the Melt

In isothermal crystallization from the melt, only stereocomplex crystallites as a crystalline species were formed in all the blends at crystallization temperature above 130 °C. The spherulite growth rate and crystallinity values decreased monotonically with deviation of the PDLA content from 50%. Surprisingly, regime analysis revealed that the crystallization mechanism of the blends was independent of PDLA content. In non‐isothermal crystallization of melt‐quenched specimens during heating, the cold crystallization of blends takes place rapidly at a lower temperature compared to that of pure PLLA and PDLA. This is attributable to the rapid stereocomplex crystallization or the nucleating effect of stereocomplex crystallites formed during quenching from the melt or second heating.

     

Polylactide‐Based Nanoparticles with Tailor‐Made Functionalization

Biocompatible polymer nanoparticles are of high interest in chemical, biological, and medical research as potential drug delivery systems. These nanoparticles must be nontoxic and metabolizable. Poly(lactic acid) (PLA) meets these requirements and therefore is widely used for biomedical applications. The miniemulsion/solvent evaporation procedure is a very efficient approach to produce PLA nanoparticles with controlled size and morphology using preformed polymer. In this paper, the synthesis of PLA‐based nanoparticles functionalized with either carboxyl groups or fluorescent molecules is presented. Both functionalities are covalently attached to the particle, thus any physical desorption and leakage of the dye upon storage or experiments performed under physiological conditions, for example during uptake by living cells, can be avoided. The fluorescence properties and stability of the obtained nanoparticles in aqueous solutions with different ionic strength are studied by fluorescence correlation spectroscopy and dynamic light scattering.

     

Polypyrrole-coated electrospun poly(lactic acid) fibrous scaffold: effects of coating on electrical conductivity and neural cell growth

Neuronal activities play critical roles in both neurogenesis and neural regeneration. In that sense, electrically conductive and biocompatible biomaterial scaffolds can be applied in various applications of neural tissue engineering. In this study, we fabricated a novel biomaterial for neural tissue engineering applications by coating electrospun poly(lactic acid) (PLA) nanofibers with a conducting polymer, polypyrole (PPy), via admicellar polymerization. Optimal conditions for polymerization and preparation of PPy-coated electrospun PLA nanofibers were obtained by comparing results from scanning electron microscopy, X-ray photoelectron spectrometer, and surface conductivity tests. In vitro cell culture experiments showed that PPy-coated electrospun PLA fibrous scaffold is not toxic. The scaffold could support attachment and migration of neural progenitor cells. Neurons derived from progenitor exhibited long neurite outgrowth under electrical stimulation. Our study concluded that PPy-coated electrospun PLA fibers had a good biocompatibility with neural progenitor cells and may serve as a promising material for controlling progenitor cell behaviors and enhancing neural repair.     

Stereocomplex Crystallization Behavior and Physical Properties of Linear 1‐Arm, 2‐Arm,and Branched 4‐Arm Poly(L‐lactide)/Poly(D‐lactide) Blends: Effects of Chain Directional Change and Branching

For number‐average molecular weight (M _n) below 1 × 10⁴ g mol^?1, the comparison of cold crystallization temperature and spherulite growth rate and crystallinity of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends exhibits that the effects of chain directional change and branching significantly disturb stereocomplex crystallization. In contrast, the comparison of glass transition and melting temperatures of linear 1‐arm, 2‐arm, and branched 4‐arm poly(L ‐lactide)/poly(D ‐lactide) blends indicates that the effects of chain directional change and branching insignificantly alter and largely increase the segmental mobility of the blends, respectively, and the crystalline thickness of the blends is determined by M _n per one arm not by M _n and is not affected by the molecular architecture.

     

Synthesis and characterization of glycolide,L-lactide,and PDMS-based terpolymers as a support for cell cultures

Poly(lactic acid)/poly(glycolic acid)/poly(dimethylsiloxane) (PLGA/TEGOMER) terpolymers have been synthesized by the ring-opening polymerization of L-lactide and glycolide with α,ω-amine-terminated poly(dimethylsiloxane) prepolymer, using stannous octoate as a catalyst. The resulting terpolymers were characterized by various analytical techniques including size exclusion chromatography, ¹H-nuclear magnetic resonance (¹H-NMR), Fourier transform infrared spectroscopy, and differential scanning calorimetry. The data showed that the terpolymers presented an amorphous structure. The glass transition temperature decreased with increasing TEGOMER unit content. For in vitro degradation studies, porous films were fabricated using a solvent-casting, particulate leaching technique. Degradation of the PLGA/TEGOMER terpolymer was studied in phosphate-buffered saline at pH 7.4 and 37°C. The degradation was followed by intrinsic viscosity, mass loss, and molecular weight measurements, and ¹H-NMR spectroscopy. The mass loss after 55 days was 76% for the PLGA/TEGOMER (71/24/5) sample. Cell growth experiments using Swiss 3T3 fibroblasts demonstrated that PLGA/TEGOMER terpolymer matrices allow the attachment and growth of cells.     

New Complex Crystals of Chiral Poly(L‐lactic acid) and Different Tactic Poly(methyl methacrylate)

Based on evidence from differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier‐transform infrared (FTIR) spectroscopy, a new stereocomplex crystal (DSC T_m = 175 °C, with WAXD 2θ = 10.0° and 12.5°) is proven for the first time between structurally dissimilar chiral poly(L ‐lactic acid) (PLLA) and syndiotactic poly(methyl methacrylate) (sPMMA). There is a strong complexing capacity only between low molecular weight PLLA and sPMMA, in miscible state, at specific weight fractions (70:30). The complexing capacity is more significant when the mixtures are melt‐crystallized at T_c = 110 °C or lower, and the intensity of this complex can be further enhanced if it is annealed between 100 and 160 °C, below its T_m = 175 °C. The new complex crystal can be formed only between PLLA and sPMMA, but not with isotactic or atactic PMMA.