Phase Transitions in Spread Monolayers of Cellulose Ethers

Cellulose ethers containing alkyl side chains of varying length were prepared from (2-hydroxypropyl)cellulose and alkyl bromides. Two different synthetic methods were employed. The average degree of alkyl substitution of the product polymers ranges from 1.8 to 2.8, as determined from 1 H NMR spectroscopy. Surface pressure−area isotherms recorded for spread monolayers depend on temperature, side-chain length, and degree of substitution. Most striking is the temperature dependence. For certain samples, a constant pressure transition is clearly evident in isotherms recorded at higher temperatures. Upon modest cooling, the plateau vanishes, although changes in slope indicate that a phase transition persists. Results are interpreted in terms of partial crystallization in interdigitated side chains. For samples that exhibit a constant pressure transition, the plateau pressure increases with decreasing the subphase temperature. Thermodynamic analysis yields positive entropy and enthalpy changes for the transition, consistent with a transition from a monolayer to a less ordered bilayer. An energy change associated with the transition is calculated as ΔE = 60 ± 15 kJ mol - 1 . LB films transferred to solid substrates were characterized by polarized infrared spectroscopy. These results suggest that the alkyl side chains exist in a predominately all-trans conformation in the LB films.