Molecular Dynamics Simulations of Polymer Networks Undergoing Sequential Cross-Linking and Scission Reactions

Abstract

The effects of sequential cross-linking and scission of polymer networks formed in two states of<br/>strain are investigated using molecular dynamics simulations. Two-stage networks are studied in which a network<br/>formed in the unstrained state (stage 1) undergoes additional cross-linking in a uniaxially strained state (stage 2).<br/>The equilibrium stress is measured before and after removing some or all of the original (stage 1) cross-links.<br/>The results are interpreted in terms of a generalized independent network hypothesis. In networks where the<br/>first-stage cross-links are subsequently removed, a fraction (quantified by the stress transfer function &lt;<span class="start-tag">img</span><span class="attribute-name"> src</span>=<span class="attribute-value">"/images/gifchars/Phi.gif" </span><span class="attribute-name">border</span>=<span class="attribute-value">"0"</span>&gt;) of the<br/>second-stage cross-links contribute to the effective first-stage cross-link density. The stress transfer functions<br/>extracted from the MD simulations of the reacting networks are found to be in very good agreement with the<br/>predictions of Flory and Fricker. It was found that the fractional stress reduction upon removal of the first-stage<br/>cross-links could be accurately calculated from the slip tube model of Rubinstein and Panyukov modified to use<br/>the theoretical transfer functions of Fricker.