Physical properties, crystallization, and spherulite growth of linear and 3-arm poly(L-lactide)s

The physical properties, crystallization, and spherulite growth behavior and mechanism of linear and 3-arm poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] have been investigated using absolute molecular weight as a molecular index. The branching reduces the chain mobility of PLLA and must be excluded from the crystalline regions. The former factor gives the higher glass transition temperature (T-g) and starting temperature for thermal degradation (T-d,T-s) of 3-arm PLLA compared with those of linear PLLA. On the other hand, both the former and the latter factors lead to the higher cold crystallization temperature (T-cc), the longer induction period for spherulite growth (t(i)), the lower melting temperature (T-m), crystallinitiy (X-c) and radius growth rate of the spherulties (G) for the 3-arm PLLA compared with those for the linear PLLA. The G of 3-arm PLLA showed the vague dependence on number-average molecular weight (M-n), probably because the branching effect was balanced with the molecular weight effect. At the M-n exceeding critical values, the linear and 3-arm PLLA crystallize in regime II or regime III kinetics, depending on crystallization temperature (T-c). In contrast, at the M-n below critical values, the linear and 3-arm PLLA crystallize according solely to regime III and regime II kinetics, respectively, for all the T-c.