Chain Conformations at the Surface of a Polydisperse Amphiphilic Comb Copolymer Film

Comb copolymers comprising a poly(methyl methacrylate) (PMMA) backbone and short, polyethylene oxide (PEO) side chains, PMMA-g-PEO, have been proposed to self-organize at the polymer/water interface, resulting in the quasi-2D confinement of the backbone for chains at the immediate surface of PMMA-g-PEO films (D.J. Irvine et al., Biomacromolecules2001, 2, 85-94). To directly probe such 2D conformations, combs modified with maleimide groups on the PEO chain ends were blended at 0.5-10 wt% into unmodified PMMA-g-PEO (M(n) 142 kg/mol, PDI 3.2, 32 wt% PEO) and cast into films ∼35 nm thick. Films were immersed in aqueous solution to induce orientation of surface molecules, and maleimide-functionalized chains at the film/water interface were labeled with 1.4 nm dia. Au nanoparticles. Transmission electron microscopy (TEM) was then used to trace the 2D trajectories of nanoparticle-decorated chains. The distribution of observed chain lengths was in good agreement with that from gel permeation chromatography. The 2D radius of gyration (R(g)) calculated from the observed conformations scaled with number of backbone segments (N) as R(g)∼N(0.69±0.02). Monte Carlo simulations of a 2D melt of comparable chain length distribution yielded a scaling exponent ν=0.67±0.03, suggesting that the deviation from 2D melt behavior arose from polydispersity.