Combinatorial measurements of crystallization growth rate and morphology in thin films of isotactic polystyrene

Combinatorial methods are utilized to study the influence of film thickness (13 nm < h < 120 nm) and crystallization temperature (131 degreesC < T-x < 193 degreesC) on the crystallization of isotactic polystyrene. Films with continuous gradients in film thickness were prepared by flow coating, crystallized on an orthogonal temperature gradient stage, and observed under an optical microscope. Spherulite growth rates were measured as a function of film thickness and temperature, as determined from an optical micrograph library of numerous subregions within the gradient films. We observed that the growth rate had a maximum near 180 degreesC and varied inversely to film thickness, consistent with the results of Sawamura et al. A transition from hexagonal plate to circular growth was observed with increasing undercooling in the thickness range between 23 and 80 nm. Both hexagonal and disordered branched crystalline morphologies formed in ultrathin regions where the film thickness is less than the radius of gyration.