No abstract

Poly(N-isopropylacrylamide) (PNIPAM) is perhaps the most well-known member of the class of responsive polymers. Free PNIPAM chains have a lower critical solution temperature (LCST) in water at about 30 °C. This very sharp transition (about 5 °C) is attributed to alterations in the hydrogen-bonding interactions of the amide groups. Grafted chains of PNIPAM have shown promise for creating responsive surfaces. Conformational changes of the polymer are likely to play a role in some of these applications, in addition to changes in local interactions. In this work we investigated the temperature-dependent conformational changes of grafted PNIPAM chains in D2O over a range of surface density and molecular weight using neutron reflection. The surface density was controlled using mixed self-assembled monolayers. The molecular weight was controlled using atom transfer radical polymerization (ATRP). Grafted layers were synthesized on gold and also on silicon oxide. The largest conformational changes were observed for intermediate grafting densities and high molecular weights. This is explained by a competition between the well-known chain stretching effect of laterally interacting tethered chains and the phenomenological χ(φ) determined empirically for PNIPAM free chains in water. Comparison is made with the recent numerical SCF calculations of Mendez et al.

In situ neutron reflectivity was used to investigate the effects of density fluctuations on the solubility of supercritical carbon dioxide (scCO 2) in polymer thin films. Deuterated polystyrene, deuterated polybutadiene, and the corresponding random copolymer, deuterated styrene-randombutadiene copolymer, as well as deuterated poly(methyl methacrylate) were investigated. Data were obtained as a function of pressure under two isothermal conditions (T) 36 and 50°C). All the polymer films used showed anomalous swelling and CO 2 sorption on the density fluctuation ridge in the P-T phase diagram of CO2. We found that the magnitude of the swelling was a function of the elasticity of the films rather than the bulk solubility of CO2. The enhanced miscibility of the rubber/CO2 systems, which are very poor in bulk, was found to be almost identical to that of the silicon rubber/CO2 mixture, which is one of the highly miscible polymeric materials under moderate CO2 conditions.