Controlling Polymer Crystallization Kinetics by Sample History

This paper reports on systematic changes in the isothermal crystallization kinetics (at Tc = 200 °C) of non-equilibrated isotactic polystyrene films, generated through spin-coating. Kinetics is tuned by annealing films at Ta = 250 °C, i.e., above their nominal melting temperature  (TmDSC = 220 °C). An exponential decrease in growth rate and a significant decrease of crystal nucleation density by ca. 3 orders of magnitude is observed by increasing the time (ta) provided for equilibration at Ta. The characteristic exponential decay time is ca. 4 orders of magnitude longer than the equilibrium reptation time at Ta. Interestingly, with increasing ta, 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.