A series of polylactides (PLA) with different stereo sequences are prepared by the copolymerization of L‐lactide and DL‐lactide. It is confirmed that the glass transition temperature (Tg) of the PLA decreases with decreasing optical purity of the lactate units (%ee) according to the Fox's equation. Analysis of the FT‐IR spectra of these PLA samples reveals that the absorbance at 1 265 cm−1 (δCH + νCOC) decreases with increasing L‐content while the absorbance at 1 210 cm−1 (νasCOC + rasCH3) increases with increasing L‐content. These changes in absorbance are reasonably correlated with the randomness and helical nature of the L‐sequenced segments involved in PLA. Namely, the PLA chains with higher L‐content comprise a higher number of short helical blocks that are made of several L‐lactate units. This difference in helical nature causes the opposite dependences of Tg and density on the L‐content of PLA; i.e., the increased Tg and decreased density with increasing L‐content.FT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.magnified imageFT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.
Melt‐crystallized films of poly(L‐lactic acid) (PLLA) with Mv in the range of 3.8 ∼ 46 × 104 consisting of α‐form crystals were uniaxially drawn by solid‐state coextrusion. The effects of Mv, extrusion draw ratio (EDR), and extrusion temperature (Text) on the crystal/crystal transformation from α‐ to β‐form crystals and the resultant tensile properties of drawn products were studied. The crystal transformation proceeded with EDR and more rapidly for the higher Mv's. Furthermore, the crystal transformation proceeded most rapidly with EDR at a Text around 130 °C, independently of the Mv's. As a result of the optimum combination of processing variables influencing the the crystal transformation (Mv, Text, and drawability), highly oriented films consisting of β‐form crystals alone were obtained by coextrusion of higher Mv samples at Text's slightly below the melting temperature (150 ∼ 170 °C) and at higher EDR's > 11. Both the tensile modulus and strength increased rapidly with EDR. The modulus at a given EDR was slightly higher for the samples with higher Mv's. In contrast, the strength at a given EDR was remarkably higher for the higher M
v's. The highest tensile modulus of 8.0 GPa and strength of 500 MPa were obtained with the sample of the highest Mv of 46 × 104 coextruded at 170 °C to the highest EDR of 14.
Three types of copolymers of poly(L‐lactic acid) (PLLA) were synthesized by direct polycondensation of L‐lactic acid and phenyl‐substituted α‐hydroxy acids (L‐phenyllactic acid and D‐ and L‐mandelic acids). It was found that the glass transition temperature of the copolymers comprising L‐mandelic acid became significantly higher (from 58 to 69 °C) with increasing content of L‐mandelic acid (from 0 to 50 mol‐%) although the Mw decreased (from 87 000 to 4 000 Da). The cast films of the L‐mandelic acid containing copolymers showed improved tensile properties compared with those of the PLLA film. This may be due to a pinning effect of the L‐mandelic acid units on the helix formation of PLLA, although 30% of the units were racemized. The enzymatic degradability of the L‐mandelic acid containing copolymers was much higher than that of PLLA, as analyzed with Proteinase K® originating from Tritirachium album.Synthesis of copolymers of L‐lactic acid and phenyl‐substituted α‐hydroxy acids.magnified imageSynthesis of copolymers of L‐lactic acid and phenyl‐substituted α‐hydroxy acids.
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