Poly(l-lactide): XII. Formation, growth, and morphology of crystalline residues as extended-chain crystallites through hydrolysis of poly(l-lactide) films in phosphate-buffered solution

Novel Preparation Method for Poly(l-lactide)-Based Block Copolymers: Extended Chain Crystallites as a Solid-State Macro-Coinitiator

A novel synthetic method for poly(L-lactide) (PLLA)-based diblock copolymers was developed by the use of PLLA extended chain crystallites (or crystalline residues) as a solid-state macro-coinitiator. In this study, we showed one example, i.e., a synthesis of diblock copolymer composed of a crystalline PLLA chain and an amorphous poly(DL-lactide) chain by ring-opening polymerization of DL-lactide initiated with stannous octoate (i.e., tin(II) 2-ethylhexanoate) in the presence of PLLA extended chain crystallites. The PLLA extended chain crystallites were prepared by hydrolytic degradation of crystallized PLLA films at 97 degrees C for 70 h. The chains inside the extended chain crystallites are expected to be protected from transesterfication reaction. Gel permeation chromatography, polarimetry, 1H NMR spectroscopy, wide-angle X-ray scattering, and differential scanning calorimetry revealed that the diblock copolymer poly(L-lactide-block-DL-lactide) was successfully prepared without significant transesterification.

Enzymatic, Alkaline, and Autocatalytic Degradation of Poly(l-lactic acid): Effects of Biaxial Orientation

The hydrolytic degradation of biaxially oriented and de-oriented (melt-crystallized) poly(l-lactic acid) (PLLA) films was investigated in Tris-HCl-buffered solution (pH 8.6) with proteinase K, alkaline solution, and phosphate-buffered solution (pH 7.4) by the use of gravimetry, gel permeation chromatography, differential scanning calorimetry, and scanning electron microscopy. Biaxial orientation disturbed the proteinase K-catalyzed enzymatic degradation of PLLA films and the effects of biaxial orientation overcame those of crystallinity. The former may be due to the fact the enzyme cannot attach to the extended (strained) chains in the amorphous regions of the biaxially oriented PLLA film or cannot catalyze the cleavage of the strained chains. Another probable cause is that the enzyme can act only at the film surface of the biaxially oriented PLLA film, in marked contrast with the case of the de-oriented PLLA films where enzymatic degradation can proceed beneath the spherulitic crystalline residues. The effects of biaxial orientation on the alkaline and autocatalytic degradation of the PLLA films were insignificant for the periods studied here. The crystallinity rather than the biaxial orientation seems to determine the alkaline and autocatalytic degradation rates of the PLLA films. The accumulation of crystalline residues formed as a result of selective cleavage and removal of the amorphous chains was observed for the de-oriented PLLA films, but not for the biaxially oriented PLLA film, when degraded in the presence of proteinase K. This means the facile release of formed crystalline residues from the surface of the biaxially oriented PLLA film during enzymatic degradation, due to the fact that the crystalline regions of the biaxially oriented PLLA film were oriented with their c axis parallel to the film surface.

The effect of crystallinity on the deformation mechanism and bulk mechanical properties of PLLA

Poly (l-lactide) is a widely studied biomaterial, currently approved for use in a range of medical devices, however, most in vitro studies have so far focussed upon either the bulk properties during degradation and/or deformation, or on the microstructure of the unloaded material during degradation. This study aimed to combine these approaches through the technique of simultaneous small-angle X-ray scattering and tensile testing at various stages of degradation up to 8 months, on material with a range of induced microstructures. Results showed that the amorphous material deformed by crazing in the dry, wet and degraded states, however, the mechanism by which the craze developed changed significantly on hydration. Despite this difference, there was little change in the bulk mechanical properties. Crystalline materials deformed through crystal-mediated deformation, with contributions from both cavitation and fibrillated shear, but surprisingly, differences in the length scales within the spherulitic structure caused by annealing at different temperatures had very little effect on the mechanism of deformation, though differences were seen in the bulk properties. Furthermore, hydration had little effect on the crystalline materials, though degradation over 8 months resulted in loss of mechanical properties for samples produced at higher annealing temperatures. In conclusion, the introduction of crystallinity had a huge effect on both bulk and microscopic properties of PLLA, but the spherulitic structure of the crystalline material affected the bulk properties significantly more than it did the micromechanism of deformation.

Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications

Poly(lactide)s [i.e. poly(lactic acid) (PLA)] and lactide copolymers are biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment. They have been used as biomedical materials for tissue regeneration, matrices for drug delivery systems, and alternatives for commercial polymeric materials to reduce the impact on the environment. Since stereocomplexation or stereocomplex formation between enantiomeric PLA, poly(L-lactide) [i.e. poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e. poly(D-lactic acid) (PDLA)] was reported in 1987, numerous studies have been carried out with respect to the formation, structure, properties, degradation, and applications of the PLA stereocomplexes. Stereocomplexation enhances the mechanical properties, the thermal-resistance, and the hydrolysis-resistance of PLA-based materials. These improvements arise from a peculiarly strong interaction between L-lactyl unit sequences and D-lactyl unit sequences, and stereocomplexation opens a new way for the preparation of biomaterials such as hydrogels and particles for drug delivery systems. It was revealed that the crucial parameters affecting stereocomplexation are the mixing ratio and the molecular weight of L-lactyl and D-lactyl unit sequences. On the other hand, PDLA was found to form a stereocomplex with L-configured polypeptides in 2001. This kind of stereocomplexation is called "hetero-stereocomplexation" and differentiated from "homo-stereocomplexation" between L-lactyl and D-lactyl unit sequences. This paper reviews the methods for tracing PLA stereocomplexation, the methods for inducing PLA stereocompelxation, the parameters affecting PLA stereocomplexation, and the structure, properties, degradation, and applications of a variety of stereocomplexed PLA materials.

Alkaline and Enzymatic Degradation ofL-Lactide Copolymers, 1

Films of poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and L-lactide copolymers with glycolide [P(LLA-GA)(81/19)], epsilon-caprolactone [P(LLA-CL)(82/18)], D-lactide [P(LLA-DLA)(95/5), (77/23), and (50/50)] were prepared and a comparative study on the effects of comonomer type and content on alkaline and proteinase K-catalyzed hydrolyses of the films was carried out. The hydrolyzed films were investigated using gravimetry (weight loss and water absorption), differential scanning calorimetry (DSC), polarimetry, and gel permeation chromatography (GPC). To exclude the effects of molecular weight and crystallinity on the hydrolysis, the films were prepared from polymers having similar molecular weights and made amorphous by melt-quenching. It was found that incorporation of hydrophilic glycolide units in L-lactide chains raises the alkaline and enzymatic hydrolyzabilities, whereas incorporation of hydrophobic epsilon-caprolactone units in L-lactide chains reduces the alkaline and enzymatic hydrolyzabilities. On the other hand, incorporation of D-lactide units with the same hydrophilicity of L-lactide units increases the alkaline hydrolyzability but decreases the enzymatic hydrolyzability. The alkaline hydrolyzability of the films of L-lactide copolymers with different kinds of comonomers and P(LLA-DLA) with different D-lactide unit contents can be closely related to their hydrophilicity. On the other hand, the enzymatic hydrolyzability of L-lactide copolymer films with different kinds of comonomers is mainly determined by hydrophilicity, while that of P(LLA-DLA) films is determined by the averaged L-lactyl and D-lactyl unit sequence lengths. The catalytic effect of proteinase K relative to that of alkali on the hydrolysis of P(LLA-DLA)(77/23) and P(LLA-GA)(81/19) films normalized by that of PLLA was lower than unity, whereas the normalized relative catalytic effect of proteinase K on the hydrolysis of P(LLA-CL)(82/18) film was higher than unity, meaning that despite low absolute alkaline and enzymatic hydrolyzability of the P(LLA-CL)(82/18) film, the catalytic effect of proteinase K may be maintained for this copolymer film, probably because of its blocky structure.