Zero growth temperature and growth kinetics of crystallizing poly(ε-caprolactone)

Abstract

The lateral growth rate of polymer crystallites rises exponentially with decreasing temperature, thus indicating control by an activation step. Conventional wisdom assumes that the height of the activation barrier diverges at the equilibrium melting point together with the size of a secondary nucleus. Growth rate measurements for spherulites of poly(ε-caprolactone) carried out in a polarizing optical microscope at a series of stepwise increasing temperatures contradict this assumption. The barrier height is inversely proportional to the distance to the ‘zero growth temperature’ T_zg  = 77 °C , which is far below the equilibrium melting point T_f^∞  = 99 °C. The observation lends further support to our view that crystal growth is mediated by a transient mesophase. T_zg is to be identified with the temperature of the hidden transition between the melt and the mesophase.