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 1H 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.