Formation of highly oriented crystalline structure for polyoxymethylene fiber enables superior intrinsic thermal conductivity

Abstract

Polyoxymethylene (POM) as an engineering plastic with high performances, has been widely applied, while endowed with superior intrinsic thermal conductivity for POM fibers was conducive to broaden their application in thermal management field, which were usually prepared through melt spinning-hot drawing/heat setting process. In this work, the tension annealing process was further incorporated into fiber production to develop highly orientated crystalline structure of fibers. All fibers have a uniform diameter with axial textures on surface, and the tensile strength/modulus increased remarkably by drawing and annealing process. In melting spinning process, the POM chains pre-oriented under stress, resulting in a slight increase of thermal conductivity. In hot drawing/heat setting process, through stress-induced crystallization of POM fiber, the shish-kebab crystalline structure formed at a low draw ratio (DR), while at high DR, a homogeneous and compact oriented microfibrillar crystalline structure formed. As a result, a significant improvement of crystallinity (Xc: 84.42 %) and orientation degree (fc, fa: 0.94) in crystalline/amorphous region of POM chains contributed to the superior axial thermal conductivity, which increased from 0.286 Wm−1K−1 of bulk POM to 2.131 Wm−1K−1. In annealing process, although Xc increased further to 93.37 % under high temperature, forming a nearly perfect crystal, while the f of annealed fiber remained under tension force but without further increase, the axial thermal conductivity showed no obvious improvement, revealing that orientation of POM chains had a more significant promotion on improving thermal conductivity of POM fibers than crystallization.