Can rheometry measure crystallization kinetics? A comparative study using block copolymers

The isothermal crystallization kinetics and the melting behavior of block copolymers of poly(ethylene oxide) and poly(1,2-butylene oxide) were studied by means of differential scanning calorimetry and rheometry to test the validity of rheological methods. The copolymers had different block lengths (hence different melt structures) and different block architectures (diblock EB and triblock EBE and BEB). For crystallization from disordered and lamellar melts, half-times for crystallization from rheometry were much shorter, and Avrami exponents were higher, than those from calorimetry. For more-highly structured melts (gyroid, hexagonal and cubic spheres), the half-times were comparable but the Avrami exponents from rheometry were still high compared to DSC. The differences between the rates of crystallization from calorimetry and rheometry are an artifact of the rheological measurements, at low crystallite volume fractions the rheology is directly proportional to the degree of crystallinity but at high crystalline volume fractions the proportionality is lost due to the changing connectivity of the crystals. The rates of crystallization ranked in order: lamellar>disordered≈gyroid>hexagonal≫cubic spheres. Other things being equal, the effect of block architecture was insignificant.