Thermal and Strain-Induced Chain Ordering in Lactic Acid Stereocopolymers: Influence of the Composition in Stereomers

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 T d = 90 °C, i.e., about 20–30 °C above T g depending on the [ d ] value of the copolymer, resulted in the strain-induced α′-crystalline phase. In contrast, for drawing at T d = T g + 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.