Effect of molar mass on enthalpy relaxation and crystal nucleation of poly (L-lactic acid)

The effect of the chain length/molar mass of poly (L-lactic acid) (PLLA) on the formation of homogeneous crystal nuclei below and above the glass transition temperature (T-g) has been analyzed by fast scanning chip calorimetry, employing Tammann's nuclei development method. It has been found that the kinetics of crystal nucleation below T-g is independent on the chain length within the investigated range of molar masses from about 60 to 600 kDa, similar as the kinetics of enthalpy relaxation. In contrast, at temperatures well above T-g there is detected that crystal nucleation is slower in samples of higher molar mass. This observation is confirmed by non-isothermal nucleation experiments which showed that suppression of nucleation occurs on cooling at rates of 200, 100, and 30 K/s in samples with molar masses of around 60, 100, and 600 kDa, respectively. The different effect of molar mass on crystal nucleation in the glass and well above Tg is discussed in terms of the length-scale of required chain-mobility and of the critical size of nuclei.