Novel flow apparatus for investigating shear-enhanced crystallization and structure development in semicrystalline polymers

An instrument to study the effects of shearing on the crystallization proce ss in semicrystalline polymers is described. It can impose transient stress es similar to those encountered in polymer processing and provides in situ monitoring of microstructure development during and after cessation of flow . Box-like wall shear stress profiles (rise and fall times under 50 ms with maximum wall shear stress on the order of 0.1 MPa) can be applied for cont rolled durations. A unique feature of our device is that it accommodates a wide variety of real-time probes of structure such as visible and infrared polarimetry and light and x-ray scattering measurements. The design also al lows us to retrieve the sample for ex situ optical and electron microscopy. Data are acquired with millisecond resolution enabling us to record the ex tent of shear deformation of the polymer melt during the pressure pulse. Ou r device works with small sample quantities (as little as 5 g; each experim ent takes similar to 500 mg) as opposed to the kilogram quantities required by previous instruments capable of imposing comparable deformations. This orders-of-magnitude reduction in the sample size allows us to study model p olymers and new developmental resins, both of which are typically available only in gram-scale quantities. The compact design of the shear cell makes it possible to transport it to synchrotron light sources for in situ x-ray scattering studies of the evolution of the crystalline structure. Thus, our device is a valuable new tool that can be used to evaluate the crystalliza tion characteristics of resins with experimental compositions or molecular architectures when subjected to processing-like flow conditions. We demonst rate some of the features of this device by presenting selected results on isotactic polypropylenes. (C) 1999 American Institute of Physics. [S0034-67 48(99)04504-9].