Abstract
Summary: Films of poly(L‐lactide) [i.e., poly(L‐lactic acid) (PLLA)] and L‐lactide copolymers with glycolide [P(LLA‐GA)(81/19)], ε‐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 ε‐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.
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