To investigate the changes in structure of cohesional state of poly(lactic acid)s with different chain tacticities

poly(D

L-lactic acid) (PDLLA)

a low optical activity poly (L-lactic acid) (l-PLLA) without crystallinity

and poly(L-lactic acid)(PLLA) were physically aged at O℃

25℃ and 37℃ for up to 6 months．Their glass transition behavior after physical aging was monitored by means of modulated temperature DSC(MDSC)．The results showed that the glass transition behavior of PDLLA and l-PLLA is sensitive to physical aging as indicated by the non-reversing thermal enthalpy (ΔHnon) generated in the physically aged samples．The rates of physical aging were higher in the early stage of aging and slowed down after aging for one week. At a given temperature

the Tg’s of PDLLA and l-PLLA shifted to higher temperatures and the ΔHnon increased with increasing aging time．The increasing rates of Tg and ΔHnon were speeded up as aging temperature raised．It was found that good linear relationships between ΔHnon and logarithm of aging time for PDLLA and l-PLLA existed at different aging temperatures．Linear dependences of the changing rates of ΔHnon as functions of reciprocals of aging temperatures were obtained for both PDLLA and l-PLLA

and the absolute value of the slope for l-PLLA was slightly larger than that of the PDLLA

indicating that macromolecular chains with higher tacticity need higher activation energy to form physical entanglements. No obvious glass transition was observed for PLLA whether it was aged or not due to its high degree of crystallinity．