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

The confinement of polymer chains to spaces smaller than their bulk dimensions is realized in a variety of systems of scientific and commercial interest1-8. Under these circumstances, the conformations9 and dynamics10 of individual polymer chains may differ greatly from their bulk state. Strategies exploiting such confinement effects have been proposed for creating nanolithographic templates1, separating polyelectrolytes by molecular weight2, and controlling the spatial distribution of surface-bound peptides in order to enhance their bioactivity3-5. Conformations of polymer chains confined to two dimensions are predicted to depend strongly on polymer chain concentration. Swollen polymer coils in two dimensions have been modeled as self-avoiding random walks, for which the radius of gyration (Rg) scales with the number of segments (N) as Rg∼Nv where ν=0.75.11 With increasing chain concentration, screening effects cause a reduction in chain dimensions such that for a 2D monodisperse melt, chains exhibit ideal behavior with ν=0.5.9,12-14 These scaling predictions have been verified through Monte Carlo (MC) simulations.14,15 Three-dimensional chains exhibit identical scaling in the melt (ν=0.5), while ν=0.6 in dilute solution.9 There have been few experimental reports of the determination of ν for polymers in 2D confinement.16-23 Jones et al.16,17 performed small-angle neutron scattering studies on thin polystyrene films incorporating a deuterated polystyrene fraction and found that the scattering intensity scaled with wavevector as I(k)∼k−2 for films of sub-Rg thickness, consistent with ideal chain statistics (ν=0.5). Maier and Radler investigated the conformations of micron-length strands of fluorescently labeled DNA confined in 2D via adsorption to mobile cationic lipid bilayers. For isolated chains they found that Rg scaled with number of DNA base pairs as Rg∼N0.79, while for concentrated systems chain collapse was observed, consistent with theoretical predictions, though no scaling exponent was reported.18,19 Wang and Foltz20 conducted atomic force microscopy (AFM) studies on dense 2D films of nanoropes (worm-like micelles) formed from polystyrene-block-polybutadiene copolymers and found that the lateral dimension scaled with contour length as R∼L0.63. For dilute surface concentrations, however, surface tension effects resulted in more collapsed configurations with R∼L0.51. Sukhishvili et al.21 performed fluctuation correlation spectroscopy on isolated fluorescently-labeled polyethylene oxide (PEO) chains adsorbed onto a self-assembled monolayer on silica and obtained a diffusion coefficient scaling of D∼N−3/2, implying Rg∼N−3/4. From Langmuir trough experiments on polymers confined at the air/water interface, Vilanove and Rondelez22 extracted ν values of 0.56 and 0.79 for poly(methyl methacrylate) and poly(vinyl acetate), respectively, using the semidilute scaling relation Π∼c2ν/(2ν–1), where Π and c denote the surface pressure and concentration, respectively. More recently, a similar analysis by Gavranovic et al.23 on poly(tert-butyl methacrylate) yielded ν=0.53. Akin to the surface-confined DNA and PEO systems described above, amphiphilic comb copolymers composed of a hydrophobic poly(methyl methacrylate) (PMMA) backbone and short, hydrophilic PEO side chains, PMMA-g-PEO, have been proposed to self-organize at the polymer/water interface, resulting in the effective confinement of the backbone to two dimensions3,24,25 for chains at the immediate surface of a PMMA-g-PEO film (Figure 1). To directly probe such 2D conformations, in this study combs modified with maleimide groups on the PEO chain ends were blended at 0.5-10 wt% with unmodified PMMA-g-PEO and cast into films of thickness ∼3Rg. 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 trajectories of individual nanoparticle-decorated chains. The distribution of observed 2D chain lengths was compared to the molecular weight distribution obtained by gel permeation chromatography. The scaling exponent ν for the 2D radius of gyration was calculated, and compared to that obtained from Monte Carlo simulation of a 2D melt having a chain length distribution fitted to that of our experimental system. The results suggest that polydispersity significantly influences 2D melt conformations. Figure 1 (a) Schematic illustration of amphiphilic comb copolymers confined in two dimensions at the polymer film/water interface. Nanoparticles attached to side chains of maleimide-modified comb polymers create nanoparticle clusters at the surface. By contrast, ...