Molecular weight effects on particle and polymer microstructure in concentrated polymer solutions

摘要

Microstructure of particles and polymers in concentrated polymer solutions are examined. Silica nanoparticles with diameters of 44 nm are suspended in concentrated poly(ethylene glycol) (PEG) solutions working with PEG molecular weights from 300 to 20 000. Using time-domain NMR, we explore the effects of PEG adsorption on the silica particles on the mobility of polymer segments. Our results reveal that polymer mobility varies from the particle surface to the bulk with different relaxation times; polymers are glassy near the particle surface and mobile in the bulk. While polymer segments adsorb to the particle surfaces to form a glassy layer in a molecular weight independent manner, the number of polymer segments with intermediate mobility between that of the glassy state and the bulk increases with molecular weight. Systematic variations in the nanoparticle and polymer microstructure in concentrated polymer solutions are explored using contrast-matching small angle neutron scattering techniques demonstrating that the thickness of adsorption layer of polymer segments increases with increasing PEG molecular weights supporting NMR observations. However, the particle microstructure evolves in a manner suggesting attractive interactions increase with molecular weight. We compare experimental observations and predictions by the polymer reference interaction site model (PRISM) revealing the limits of the theory when applied to the silica-concentrated PEG solution system. While PRISM predicts experimental observations in polymer and particle microstructure quantitatively at low molecular weight, it fails as molecular weight increases, suggesting that polymer configurations are not able to sample equilibrium configurations.