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Abstract: Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M n) and d-lactide unit contents (X d ) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37°C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0–32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X d , while the k values did not depend on M n. This means that the effects of X d on the hydrolytic degradation rate of the films are higher than those of M n. In contrast, in the period of 32–60 weeks neither X d nor M n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks. [Copyright &y& Elsevier]

Abstract: 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 10wt%. 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 10wt%, 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 0wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1wt%, the t c(S), t c(1/2), and t c(E) were longer than or similar to those at PDLA content of 0wt%, 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 0.3–3wt%, the t c(S), t c(1/2), and t c(E) were shorter than or similar to those at PDLA content of 0wt% 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 1wt% 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 1wt%. 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. [Copyright &y& Elsevier]

Abstract: 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 gmol−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 gmol−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), G 0(II), and G 0(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. [Copyright &y& Elsevier]