Effects of grafting density and molecular weight on the temperature-dependent conformational change of poly(N-isopropylacrylamide) grafted chains in water

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 degrees C. This very sharp transition (about 5 degrees C) is attributed to alterations in the hydrogen-bonding interactions of the an-fide 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 chi(phi) determined empirically for PNIPAM free chains in water. Comparison is made with the recent numerical SCF calculations of Mendez et al.