Critical Behavior of the Banded-Unbanded Spherulite Transition in a Mixture of Ethylene Carbonate with Polyacrylonitrile

Banded spherulites appear generically when materials with viscous melts are frozen at high undercoolings. The characteristic striped pattern observed in thin samples is believed to reflect a rotation of crystalline axes that occurs as the front propagates radially away from a nucleation site. Common features include an onset of banding at finite undercooling and a divergence of the wavelength near this critical undercooling. Here, by carefully considering systematic errors, we show that the band spacing diverges with a power-law form showing scaling over nearly two decades. We also observe that the bands disorder as the transition point is approached. The critical exponent is non-classical. One possible explanation is that the transition is actually weakly first order. An analogous situation exists for cholesteric liquid crystals in the vicinity of a cholesteric--smectic-A transition.