The Effect of Intrachain Cross-Linking on the Thermomechanical Behavior of Bulk Polymers Assembled Solely from Single Chain Polymer Nanoparticles

Abstract

Chemical cross-linking of polymer chains is a powerful means for tailoring the thermomechanical properties of bulk plastics. Nonetheless, upon cross-linking, processability is reduced as the plastic becomes thermoset. Here, molecular dynamics simulations are used to study the effects of intramolecular chemical cross-linking on chain topology and thermomechanical properties of a bulk, thermoplastic polymer. Polyethylene (PE)-like plastics are assembled purely from chains which have undergone a set level of intrachain cross-linking (to form single chain polymer nanoparticles, SCPNs). We have analyzed the chain topology at an equilibrated state in terms of chain unfolding and entanglement by radius of gyration and primitive path, respectively. The extents of both chain unfolding and chain entanglement were found to decrease with increasing intrachain cross-linking ratio. By applying simulated cooling, uniaxial tension, and uniaxial compression, we characterized the thermomechanical properties at the glassy state. The simulated mechanical testing shows that the bulk polymer becomes stiffer, stronger, and more brittle as the intrachain cross-linking ratio is increased. We observe that the failure of the SCPN-based bulk polymers is a consequence of separation between SCPNs. This study successfully elucidates the effect of intramolecular cross-linking on the thermomechanical properties at bulk, as a clear correlation is shown between the amount of covalent intrachain collapse and interchain interactions.

Highly Stretchable Polymers: Mechanical Properties Improvement by Balancing Intra‐ and Intermolecular Interactions

“…The glass transition temperature was measured using differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) to provide some experimental indications on the effect of intramolecular CL on segmental dynamics. In agreement with ours and others previous results, T g increases in PBMA with increasing CL degree (Table ). T g 's determined by MD simulations found the same trend ( Table 2 ).…”

“…For EL, E1 and E2, pronounced plastic deformation accompanied by strain hardening is seen, but decreases with CL degree. Interestingly, this influence of intramolecular CL on PBMA SCPNs was predicted by our previous MD study and was again observed in the current MD study (Figure c) . By addition of intramolecular CLs, all segments are brought closer together, increasing the intramolecular interactions (Figures S30 and 31, SI) and producing more rigid SCPNs, resulting in a stiffer material.…”

“…In contrast, for E3, the recovered and dissipated energy only reduces slightly for repeats, and is a function of maximum strain. At first glance, this experimental result seems contradictory with the MD finding that SCPNs have more relative chain motion (viscous flow) than ML, however as showed in our previous work, the greater the CL degree, the greater the stress for deforming the SCPN a given amount—since this individual SCPN deformation is reversible, this factor has a strong effect on the dissipation to recovery ratio.…”

“…Many studies have characterized SCPN molecular level behavior in solution, in addition to a few melt studies which looked at the interactions between the folded chains . Finally, more recently, we have conducted experimental and theoretical studies to better understand the influence of intramolecular CLs in the solid state, looking at mechanical and thermomechanical behavior of SCPNs assembled into bulk thermoplastic materials . Our initial studies focused on the effects of intramolecular CLs below the glass transition temperature ( Tg ) such as reducing chain entanglement and decreasing ductility.…”

Effect of intramolecular crosslinker properties on the mechanochemical fragmentation of covalently folded polymers

“…In addition, it has been shown that mechanochemistry concentrates in short CLs in thermoset systems, presumably due to their restricted mobility . Our results can be similarly rationalized—shorter intramolecular CLs induce slower segmental dynamics (as seen by the increase in Tg) and more rigid conformation, therefore being more susceptible to mechanical stress compared to CLs made from longer chains, which can “absorb” the strain through unfolding.…”