Controlling Polymer Crystallization Kinetics by Sample History

In this paper systematic changes in the isothermal crystallization kinetics (at T-c = 200 degrees C) of nonequilibrated isotactic polystyrene films, generated through spin coating are reported. Kinetics is tuned by annealing films at T-a = 250 degrees C, i.e., above their nominal melting temperature (T-m(DSC) = 220 degrees C). An exponential decrease in growth rate and a significant decrease of crystal nucleation density by approximately three orders of magnitude are observed by increasing the time (t(a)) provided for equilibration at T-a. The characteristic exponential decay time is approximately four orders of magnitude longer than the equilibrium reptation time at T-a. Interestingly, with increasing t(a), a change in the morphology of lamellar crystals varying from spherulites to hexagons of rather uniform height to finally hexagons exhibiting periodic modulations in height is observed. These observations reveal that understanding and tuning the crystallization kinetics of polymer films under quiescent conditions requires knowledge on how strongly chain conformations deviate from equilibrium.