Microporosity of Bicontinuous Nanoporous Polymeric Materials, Characterized with Restricted Diffusion

The 1H NMR pulsed-gradient spin−echo method has been used to study the self-diffusion D of the mobile fractions in polymerized bicontinuous microemulsions formed with methyl methacrylate and hydroxyethyl methacrylate, cross-linked with ethylene glycol dimethacrylate, and water containing 10% sodium dodecyl sulfate as surfactant. Measurements were made at 50.5 °C over the bicontinuous range (30−96 wt % water) at diffusion times between 8 and 1000 ms. In addition to the signal from the nondiffusing glassy phase, two distinct components are observed differing in D by 2 orders of magnitude, with an intensity ratio monotonic in water content. Both D values depend weakly on diffusion time, but D attributed to free water increases with water content, whereas D attributed to surfactant decreases with water content in the bicontinuous region. This may be due to a size reduction in the surfactant micelles in samples containing less aqueous phase. Assuming free diffusion of water in the open pores, we extract from the data the reduced permeabilities (i.e., detour and tortuosity), discuss the origin of the slow-D component as surfactant undergoing surface-exchange with the network, and derive an upper limit on the pore size on the order of 1 μm near the midpoint of the bicontinuous concentration domain, in keeping with evidence from SEM imaging; a pore size distribution derived from freezing point depression attests to a large aspect ratio of the porosities