NMR observation of the conformations and motions of polymers confined to the narrow channels of their inclusion compounds

Abstract

AbstractWhen polymers are guests in crystalline inclusion compounds (ICs) formed with small‐molecule hosts, they occupy a unique environment. In a cocrystallization process the small‐molecule host forms a crystalline lattice containing long narrow channels where the guest polymer chains are included. Because of the narrow channel diameter and because neighboring channels are separated by walls formed exclusively from the small‐molecule host lattice, the included polymer chains are highly extended and separated from polymer chains in other IC channels. As a consequence, polymer‐IC crystals provide a unique solid state environment for the included polymer chains and serve as models useful for assessing the contributions made by the inherent behavior of individual polymer chains to the properties of ordered, bulk polymers, which can be obscured by pervasive interactions between their tightly packed polymer chains. In this paper we describe the conformations and motions of polymer chains confined to the narrow channels of the following polymer‐ICs: i. polyethylene and trans‐1, 4‐polybutadiene in their ICs with perhydrotriphenylene, ii. polyepsilon caprolactone and its diblock and triblock copolymers with polybutadiene and poly (ethylene oxide) in their ICs with urea, and iii. nylon‐6 in its ICs with alpha‐, beta‐, and gamma‐cyclodextrins. High resolution, solid state NMR serves as both the conformational (C‐13 chemical shifts) and motional (relaxation times and line shapes) probe. Comparison with identical NMR measurements performed on the bulk homo‐ and copolymer samples permits us to draw several conclusions regarding the relationships between the conformations and motions of polymers and their dependence on their ordered solid state environments.