(691b) Rheology and Structure in Particle-Containing Polymer Blends

Various types of particles readily adsorb at the interface between two immiscible liquids or at the surface of a liquid.  Such particles bear some similarity to conventional molecular surfactants.  However, unlike surfactants, particles adsorb almost irreversibly at interfaces – a fact that can lead to interesting phenomena such as the stability of non-spherical jammed drops and particle-bridged emulsions. This talk will describe the effects of such interfacially-adsorbed particles on the structure and flow properties of two-phase polymer blends.

Experiments were conducted on blends of polyethylene oxide (PEO) drops dispersed in a polyisobutylene (PIB) matrix with less than 2 vol% of monodisperse silica particles.  The blends were examined by rheological experiments and Scanning Electron Microscopy (SEM). Particles with two different surface chemistries, which correspond to two different contact angles at the PEO/PIB interface, were considered. We find that particles that are preferentially-wetted by the drop phase are able to glue together two drops by bridging across them. The morphology therefore consists of particle-bridged clusters in which the individual drops are non-spherical and resemble polyhedral foam bubbles. Such blends show strong gel-like rheological behavior including a low-frequency plateau in modulus in linear viscoelastic experiments and a yield stress in steady flow experiments.  In contrast, particles that are equally-wetted by both phases are not capable of bridging across drops; in this case, the morphology consists of nearly-spherical drops that are not clustered, and a yield stress is not evident rheologically. In summary, in polymer blends with added particles, the particle wettability has a major effect on the morphology and flow characteristics.