The structure and rheology of polyolefins during crystallization is of critical importance to the polymer processing industry. Optical, thermal, and mechanical properties of polymer products are directly related to the amount of amorphous and crystalline material in the final product, which depends on the temperature and flow history during crystallization. Here we present simultaneous Raman scattering and rheological measurements on crystallizing high density polyethylenes during quiescent and slow flow conditions. Raman spectroscopic measurements during quiescent crystallization allow us to quantify and differentiate three mass fractions of chain conformers: an amorphous fraction, an orthorhombic crystalline fraction, and a fraction of chains that contain many consecutive <i>trans</i> bonds but are not part of the orthorhombic crystal. These non-crystalline consecutive <i>trans</i> (NCCT) conformers are generated as a precursor to crystallinity. Slow steady shear rates (~1 1/s) applied during isothermal crystallization experiments dramatically increase the crystallization rate as well as the amount of NCCT conformers produced. Optical measurements of sheared samples during crystallization reveal the formation of fiber structures that compositionally contain more NCCT conformers than the surrounding melt. To correlate conformational changes with mechanical properties during crystallization we use a novel instrument for simultaneous optical, Raman, and rheological characterization of materials. The increase in the complex shear modulus commonly measured for crystallizing polyethylenes correlates with the growth of chain conformers and the appearance of spherulites within the melt.

Probing polyethylene crystallization via simultaneous Raman spectroscopy and rheology

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.