Melting Point Depression of Poly(ethylene oxide) Crystals under Extreme Nanoconfinement

Abstract

Polymer crystallization under nanoscale confinement is governed by finite-size and interfacial effects, yet its behavior at length scales approaching the lamellar thickness remains poorly understood. Here, we investigate the crystallization of poly(ethylene oxide) (PEO) with varying molecular weights confined within silicon nanopores (d ∼ 7–8 nm). Crystallization is strongly suppressed, with the crystallization temperature approaching the homogeneous nucleation regime while deviating from conventional volume scaling. Furthermore, the melting temperature exhibits a pronounced reduction (∼20 K), indicating that lamellar thickness is directly constrained by the geometry. Interestingly, a transition from an extended-chain crystal to a folded-chain crystal occurs in PEO with a molecular weight of 2000 g/mol (PEO2k), while PEO with a molecular weight of 1000 g/mol (PEO1k) maintains an extended-chain crystal, because the contour length of the PEO2k chain exceeds d, whereas that of the PEO1k is comparable to d. These results demonstrate that, when the confinement dimension approaches the lamellar thickness, geometric confinement dictates nucleation and attainable crystal dimensions, whereas interfacial interactions govern crystal orientation, thereby defining the fundamental limits of polymer crystallization under extreme nanoconfinement.