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 strain are investigated using molecular dynamics simulations. Two-stage networks are studied in which a network formed in the unstrained state (stage 1) undergoes additional cross-linking in a uniaxially strained state (stage 2). The equilibrium stress is measured before and after removing some or all of the original (stage 1) cross-links. The results are interpreted in terms of a generalized independent network hypothesis. In networks where the first-stage cross-links are subsequently removed, a fraction (quantified by the stress transfer function Φ) of the second-stage cross-links contribute to the effective first-stage cross-link density. The stress transfer functions extracted from the MD simulations of the reacting networks are found to be in very good agreement with the predictions of Flory and Fricker. It was found that the fractional stress reduction upon removal of the first-stage cross-links could be accurately calculated from the slip tube model of Rubinstein and Panyukov modified to use the theoretical transfer functions of Fricker.

Thermoreversible associating polymer networks. I. Interplay of thermodynamics, chemical kinetics, and polymer physics

“…These simulations can be considered to be an extension of Ref. [104], which allowed breaking and formation of interchain bonds only at a few (discrete) strains; here SBs break cage make the non-SB-related viscous stress contribution unacceptably large, while smaller values lead to more sticky bond breaking/formation during deformation than is desirable at the values of h and τ MC considered. Figure 13 shows the stress difference |σ z − (σ x + σ y )/2|.…”

The mechanics of network polymers with thermally reversible linkages.

“…We note that considerable theoretical and computational efforts have explored the concept of stress-free configurations as a means of explaining compression set in elastomers. Tobolsky proposed the Two-Network Hypothesis [8], examined theoretically by Flory [12], and computationally by Rottach and co-worker [28]. This approach has been applied with success to a variety of problems associated with the continuum thermomechanical behavior of temperature sensitive elastomers [31,35] and photomechanically coupled polymers [18].…”

“…One important inadequacy of this rule is that it does not account for the change of the stress-free configuration due to chain scission (assumption 4), which, as pointed out in many theoretical investigations, involves a load transfer from chains as they are scissioned to the surrounding network [28,12]. However, these theoretical treatments consider the special case of just 2 states of strain.…”

Mechanics of soft active materials with phase evolution

“…A plethora of SAM systems and their respective environmental stimuli are at the forefront of materials science research today. Several examples include: thermally activated shape memory polymers (SMP) (Behl and Lendlein, 2007;Liu et al, 2006;Qi et al, 2008); photo activated polymers (Lendlein et al, 2005;Scott et al, 2005aScott et al, ,b, 2006Long et al, 2009); polymers which undergo thermally induced scission and healing (Rottach et al, 2007;Tobolsky, 1960;Wineman and Min, 2003;Wineman and Shaw, 2007); thermally and photo activated liquid crystal elastomers (Corbett and Warner, 2007;Dunn, 2007;Hon et al, 2008); and thermally and/ or chemically activated biological remodeling and growth (Epstein, 2007;Garikipati et al, 2004Garikipati et al, , 2006. Interest in these systems stems from developing applications in a variety of fields from novel drug delivery and surgery devices in biomedicine and bioengineering to self-assembling and remotely deployable structures.…”