Chain confinement drives the mechanical properties of nanoporous polymers

¹ Department of Engineering Mechanics, Chongqing University - Chongqing, 400044, China
² The State Key Laboratory of Mechanical Transmission, Chongqing University - Chongqing, 400044, China
³ Theoretical & Applied Mechanics, Northwestern University - Evanston, IL 60208, USA
⁴ Department of Mechanical Engineering, Northwestern University - Evanton, IL 60208, USA

^(a) stang@cqu.edu.cn (corresponding author)

Received: 29 December 2013
Accepted: 9 April 2014

Abstract

Recent experiments using nanoindentation and buckling-based metrology (Stafford C. M. et al., Nat. Mater., 3 (2004) 545) have shown the elastic modulus of submicron-nano porous polymers to be lower than that predicted by classical homogenization or finite element simulations, especially at high porosities. This letter presents a model that captures the experimentally observed elastic modulus of nanoporous polymers by assuming that polymer chains are less confined in the interfacial layer close to the free surface of voids than in the bulk. The confinement assumption is incorporated into a recently proposed hyperelastic model, wherein low values of confinement parameters are needed to match the observed mechanical moduli of these materials. Evidence from molecular dynamics and physical experiments further supports the conclusion that variable chain confinement at material interfaces drives the mechanical behavior in nanoporous polymers due to the increasing importance of surface effects. The effect of confinement on instability under compression is also demonstrated since instability may be exploited to create porous polymers with tunable acoustic, electronic or optic properties.