Crystal modulus of poly (lactic acid)s, and their stereocomplex

Abstract

Elastic moduli of the crystalline regions (crystal modulus) of poly(lactic acid)s (PLA), poly (l-lactic acid) (PLLA), poly(d-lactic acid) (PDLA) and their stereocomplex (scPLA) in the directions parallel (El) and perpendicular (Et) to the chain axis were measured by X-ray diffraction. The El value of PLLA and PDLA coincided as 14 GPa, whereas the El value of scPLA was 20 GPa at room temperature. These relatively low El values for PLAs and scPLA, compared with other polymers with planar zigzag skeletons in the crystalline regions, were considered to be attributed by helical structure. The difference of the El values between PLAs and scPLA could be explained by the difference of the helix types, that is, 31 helical skeleton of scPLA in the crystalline regions is slightly extended compared with 103 helix of PLAs. In addition, these El values were found to be temperature independent down to cryogenic temperature. The Et values of scPLA were around 4 GPa, which is higher than those (around 3 GPa) of PLLA and PDLA. The higher melting point (∼230 °C) compared with those (∼180 °C) of PLLA and PDLA can be attributed to higher Et value of scPLA, in other words, higher intermolecular interaction of scPLA compared with those of PLLA and PDLA. However, compared with the Et value (4 GPa) of polyethylene where only van der Waals interaction acts, there exist no strong intermolecular interaction for PLAs and scPLA.