On the successful fabrication of auxetic polyurethane foams: Materials requirement, processing strategy and conversion mechanism

This study aims to provide fundamental understandings in several issues critical to the fabrication of auxetic polyurethane (PU) foams by thermal compression process: conversion and auxetic structure fixation mechanisms, materials characteristics essential for the successful auxetic conversion, and optimal conditions for auxetic conversion. Three flexible PU foams with similar morphology were selected for the study. First the commonalities as well as the differences between these foams in terms of their chemical composition, microstructure and thermomechanical properties were thoroughly analyzed. This is followed by the auxetic convertibility study of these three foams. Mechanisms for fixing the structure were elucidated and the windows for processing were interpreted in the context of polymer relaxation. Guided by these understandings, a series of auxetic foams were manufactured at a wide range of conditions. The processing conditions agreed very well with those suggested from the mechanistic investigation, validating the proposed auxetic conversion and structure fixation mechanisms. The mechanism was further confirmed by direct observation of the morphology of the fabricated auxetic foams. (C) 2016 Elsevier Ltd. All rights reserved.