Temperature-Dependent Conformational Change of PNIPAM Grafted Chains at High Surface Density in Water

Poly(N-isopropylacrylamide) (PNIPAM) exhibits a lower critical solution temperature (LCST) of {approx}30 C in water that is attributed to alterations in the hydrogen-bonding interactions of the amide group. PNIPAM in various forms has been explored for a variety of applications including controlled drug delivery, solute separation, tissue culture substrates, and controlling the adsorption of proteins, blood cells, and bacteria. Grafting PNIPAM onto surfaces is a promising strategy for creating responsive surfaces, since the physical properties of PNIPAM are readily controlled by changing the temperature. Considerable effort has been devoted to studying variations in chain conformations with temperature (T) in PNIPAM-based materials. Kubota et al. studied conformational changes of PNIPAM free chains with temperature for molecular weights ranging from 1.63 x 10{sup 6} to 2.52 x 10{sup 7} g/mol (M{sub w}/M{sub n} > 1.3) in water using laser light scattering. They reported a decrease in the radius of gyration (R{sub g}) as the solution temperature increased above the LCST. The magnitude of the effect was more pronounced with increasing molecular weight, ranging up to a factor of two for the highest molecular weight sample. In a similar study, Wu et al. observed a decrease in R{sub g} of a factor of sevenmore » for a high molecular weight PNIPAM sample with very low polydispersity (M{sub w} = 1.3 x 10{sup 7} g/mol, M{sub w}/M{sub n} < 1.05). Regarding grafted PNIPAM chains, Kidoaki et al. recently employed an iniferter-based graft polymerization method to generate a dense, high molecular weight brush and reported changes in the thickness measured by AFM. The thickness of the grafted layer was obtained from AFM images of the boundary between grafted and nongrafted (ablated by laser light) regions. They found that the swollen film thickness decreased by a factor of {approx}2 with increasing temperature from 25 to 40 C for samples with a range of dry film thickness from 250 to 1500 {angstrom}. More recently, Balamurugan et al. used surface plasmon resonance (SPR) to probe conformational changes in a PNIPAM brush grafted onto a gold layer by atom transfer radical polymerization (ATRP). For a sample with a dry film thickness of 517 {angstrom}, the SPR measurements indicated a significant contraction (extension of the layer with increasing/decreasing) temperature through the transition. Quantification of the change in profile characteristics was not reported, but it was noted that the change in the SPR signal occurred over a much broader range of temperature (15-35 C) than is typical of the transition for free chains in bulk solution. No systematic study of detailed PNIPAM chain conformations has yet been reported as a function of the two critical brush parameters, the surface density and molecular weight. A recent theoretical analysis by Baulin and Halperin has identified the surface density as a critical parameter demarcating different regimes of behavior. This arises from the concentration dependence of the Flory {chi} parameter as obtained from a recent phase behavior study of free chains in solution. Little attention has been paid to the surface density in previous experimental studies of grafted PNIPAM chains. We have begun a systematic study of the temperature-dependent conformational changes of PNIPAM grafted chains in water as a function of surface density and molecular weight using neutron reflection (NR). In previous work, we investigated the conformational changes of PNIPAM chains tethered to silicon oxide using two methods. The first was the 'grafting from' method in which N-isopropylacrylamide monomers were polymerized from the silicon surface with a chain transfer, free-radical technique. In the second method, preformed PNIPAM chains with carboxylic acid end groups associated with terminal hydroxyl groups of a mixed self-assembling monolayer. Detailed concentration profiles of the PNIPAM brushes were determined in D{sub 2}O as a function of temperature and also in d-acetone at room temperature. Profiles were obtained in the two solvents in order to investigate the role of the solvent in mediating interactions. The profiles in D{sub 2}O were bilayers, composed of a very thin layer with higher concentration at the surface and a low concentration layer extending well into the subphase. The very thin, higher concentration surface layer was attributed to attractive segment-surface interactions. The profiles in acetone were smoothly decaying single-layer profiles. The low segment concentration at the surface in acetone indicated that the surface density of these brushes was rather low. The dry film thicknesses were less than 40 {angstrom}, much lower than in the study of Kidoaki et al. On the basis of the molecular weights and dry film thicknesses, the surface density ({sigma}, chains/{angstrom}{sup 2}) ranged from 1 x 10{sup -4} to 2 x 10{sup -4} for those samples. « less