Freestanding Tough Glassy Membranes Produced by Simple Solvent Casting of Polyrotaxane Derivatives

Mechanically tough glassy membranes with a unique confined main-chain motion are prepared by simple solvent-casting of a series of polyrotaxane derivatives. Polyrotaxanes composed of polyethylene glycol (PEG) and propionylated α-cyclodextrins are thermomoldable and highly soluble in volatile solvents (e.g., > 30 wt % in acetone). Solvent casting instantly produces freestanding transparent films with thicknesses ranging from several tens of micrometers to the submicrometer regime. The threaded rings completely inhibit the crystallization of the threading polymer. Direct mechanical measurement by bulge tests reveals that the membranes are as hard as conventional polymer glasses but extremely extensible and pinhole-free even at submicrometer thickness. The stiffness and extensibility are tunable by manipulating the number of threaded rings in a single threading chain without compromising the high processability and crystallization-inhibitory potency. Because the membrane has neither cross-linking nor additives, it can be easily recycled using the same solution process, thus reproducing the mechanical properties. The high mobility of the confined PEG in the glassy materials is confirmed by viscoelastic analysis. This mobility appears to contribute to both mechanical toughness and the high solubility of CO₂ in the membrane, suggesting its potential utility as a base material for separation membranes.