Thermal and strain-induced chain ordering in lactic acid stereocopolymers: Influence of the composition in stereomers

cited By 32; International audience; The present work reports on the study of the influence of the composition in stereomer on the mechanical behavior of d- and l-lactic acid stereocopolymers in relation with strain-induced structural evolution. The stress-strain behavior and structural changes upon stretching above the glass transition temperature have been investigated by means of in situ wide-angle X-ray scattering for a series of nearly random stereocopolymers covering a large range of compositions. All members of the family behaved like elastomers in the initial stage of stretching. However, stereocopolymers of low contents in d-units, [d], exhibited a strain-hardening phenomenon starting at strain 150% < ε < 200%, depending on the draw temperature. This strain-hardening decreased with increasing [d] up to the critical concentration [d]* ≈ 8 mol % above which all copolymers displayed an un-cross-linked rubber behavior over the whole strain range. The decrease of the strain-hardening also depended on the draw temperature, yet this sensitivity did not affect the critical concentration [d]* of the position of the transition. The occurrence of the strain-hardening has been correlated with the buildup of a strain-induced ordered phase, the nature of which being directly related to the draw temperature. Drawing at Td = 90 °C, i.e., about 20-30 °C above Tg depending on the [d] value of the copolymer, resulted in the strain-induced α′-crystalline phase. In contrast, for drawing at Td = Tg + 5 °C, the mesomorphic phase was formed. Moreover, it has been shown that the higher [d], the lower the content of ordered phase, irrespective whether it was crystal or mesophase. A parallel is made between the crystallization capabilities of the copolymers from the quiescent melt and under straining. Only stereocopolymers with [d] < [d]* were able to crystallize from the melt, as in the case of strain-induced crystallization. The occurrence of the strain-induced mesophase only in the case of crystallizable copolymers is taken as a hint of an intermediate state of order precursory to the crystalline state. © 2011 American Chemical Society.