Pore formation and evolution mechanism during biaxial stretching of β-iPP used for lithium-ion batteries separator

Themorphological evolution and pore formationmechanismof different kinds of beta-iPP spherulites during biaxial stretching were analyzed. The smaller radial-growth spherulite deforms heterogeneous, causing a faster reduction in beta-crystal content and higher orientation degree during longitudinal stretching, while the bigger spherulite derived from the hedrites deformed more homogeneous due to the more violent lamellae separation, causing a slower beta-crystal content decline and lower orientation degree. However, plentiful coarse fibrils form within beta-spherulite derived from the hedrites during longitudinal stretching due to abundant lamellae paralleled to the loading direction, with more connections between the coarse fibrils, which delayed the micropores formation and caused the inferior pore size distribution in following transverse stretching. While the radial-growth beta-spherulite generates more refined microfibrils and the higher orientation means lesser connection between finer fibrils, making abundant micropores at the beginning of transverse stretching and superior pore size distribution throughout the whole transverse stretching ultimately. (c) 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).