Clay intercalation of poly(styrene-ethylene oxide) block copolymers studied by two-dimensional solid-state NMR

The intercalation of poly(styrene-ethylene oxide) block copolymers (PS-b-PEO) into a smectite clay, hectorite, has been studied by multinuclear solid-state nuclear magnetic resonance (NMR). The behaviors of two copolymers with similar PEO block lengths (7 and 8.4 kDa) but different PS block lengths (3.6 vs 30 kDa) were compared. Polymer intercalation is assessed by two-dimensional ¹H-²⁹Si heteronuclear correlation (HETCOR) NMR with spin diffusion and refocused ²⁹Si detection for enhanced sensitivity. Hydroxyl protons in the smectite layers serve as crucial spin diffusion references and ¹H magnetization relay points from the polymer to the ²⁹Si in the silicate. Experiments with CRAMPS evolution, with ¹H spin diffusion, and with detection of the sharp OH proton signal after a ¹H T₂ filter provide excellent sensitivity for spin diffusion studies with mixing-time series. Because of the mobility of PEO, in this homonuclear experiment we can observe PEO-PS and clay-polymer spin diffusion simultaneously. While the PS block is found not to be intercalated in either copolymer, definite proof of PEO intercalation in the sample with the shorter, 3.6 kDa PS block is provided by a ¹H-¹³C HETCOR spectrum. In the PS-rich sample, the amount of intercalated PEO is much smaller, and a significant fraction of PEO is not intercalated. Two-dimensional ¹H-²⁹Si correlation NMR without ¹H homonuclear decoupling shows that intercalated PEO has a clearly reduced mobility, most prominently for the PEO nearest to the silicate surface. A model of the PEO blocks intercalating sideways into 50 nm diameter stacks of hectorite can explain the experimental results.