No abstract

Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO 2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in "polymer free volume", which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior.

We studied by neutron reflection the architecture of poly(methyl methacrylate) (PMMA) layers adsorbed on hydroxylated quartz from the melt. The samples were prepared by spin-coating about 1 μm thick films of PMMA onto quartz plates, annealing at melt conditions for an extended period, and then leaching away unbound material in good solvent (benzene). Data on dry residual layers indicated a dense PMMA layer the thickness of which gradually increased with annealing time in the melt from an initial minimal value toward a final equilibrium thickness. Evidently, annealing in the melt gradually relaxes the rather flat nonequilibrium structure produced by spin-coating. The thicknesses, h, in a series of dry residual layers annealed for very long times in the melt obey h ∼ N 0.47±.05, where N is the degree of polymerization. This is close to the scaling expected for a reflected random walk (RRW) immobilized by the surface (h ∼ N 1/2). Data on residual adsorbed layers swollen in a good solvent (deuterated benzene) indicate a strongly stretched, brushlike structure with a diffuse segment density profile, φ(z). The segment density decays φ(z) ∼ z -0.77±.03, faster than predicted by RRW statistics and indicates fewer long loops per chain than the RRW model.

The orientation of functional groups, side chains, and repeat units at the surface of a liquid crystal (LC) alignment layer (AL) polyimide was changed by rubbing with a cotton velvet cloth. It was discovered that rubbing induced polar functional groups and repeat units to reorient out-of-the-plane of the surface, and it made nonpolar aliphatic side chains partially reorient inward, toward the bulk of the film. The polar AL surface provided relatively small LC pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles are greatly affected by both electronic interaction and steric repulsion between LC molecules and an alignment layer polyimide surface.