Structure Development From Simultaneous Phase Separation and Crystallization of Metallocene Polyolefin Blends | NIST

Introduction Olefin polymers are the most widely used materials in the plastics industry today. To optimize their properties and processibility, blending is often used. This is particularly important for the metallocene-catalyst based polyolefins. By altering certain shear-thinning and strain-hardening characteristics, blending can enhance the processibility of the material. This is in addition to any property enhancement or modification already formulated into the blending or alloying process. In the case of polyolefin blends, the mixtures can undergo both liquid-liquid phase separation (LLPS) and crystallization, complicating the blend morphology . This complication can be either advantageous or disadvantageous in term of structural control/property tailoring, depending on whether or not complex structures yield desired properties of the final product. To fully understand and utilize the potential of the complex morphology arising from LLPS and crystallization under non-equilibrium processing conditions such as shear and temperature gradient, we carried out a systematic study involving both multiscale characterization and multi-scale computer modeling on a model polyolefin blend. Comprehensive measurements of the morphology and rheology of the blend as a function of composition, temperature, pressure, shear rate, phase miscibility, crystalline structure, and thermal history are being carried out to form constitutive relationships that can be used for the input of computer modeling for pre-product evaluation, performance prediction and process design etc. In this study, we investigated the dynamics of structure evolution in polyolefin blends undergoing simultaneous LLPS and crystallization or cyclic crystallization and melting (CCM) after LLPS. In the former case, by controlling relative quench depths for LLPS and crystallization, the growth kinetics of the characteristic length shows a crossover of linear dynamics from crystallization to LLPS, separated by a non-linear regime where both ordering processes are important. In the latter, CCM enhances large-scale domain coarsening while introducing fine structures within domains.