Recently, we demonstrated (Chen et al., <i>Polym. Chem.</i> 2017, 8, 6349) that hydroxyurethane linkages in polyhydroxyurethane (PHU) networks are intrinsically dynamic via transcarbamoylation exchange reactions and reversible cyclic carbonate aminolysis, allowing PHU networks to be reprocessed multiple times with full recovery of cross-link density. Now, we have incorporated this dynamic linkage into three other polymer backbones: polybutadiene (PB), polytetramethylene oxide (PTMO) and polydimethylsiloxane (PDMS). The reprocessability and conformation of resulting networks were evaluated by dynamic mechanical analysis (DMA), uniaxial tensile testing, differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). Within experimental error, both PB and PTMO networks show complete recovery of rubbery plateau modulus and tensile properties after reprocessing, demonstrating the effectiveness of hydroxyurethane dynamic linkages in these systems. In contrast, PDMS networks show some loss in rubbery plateau modulus after each reprocessing step, possibly due to segregation of cross-linkers from PDMS backbones during reprocessing as a result of the thermodynamic incompatibility between the two. SAXS reveals nanophase separation in both PB and PDMS networks, which is absent in the PTMO network. The PTMO network shows cold crystallization behavior with increasing crystallinity when the network is repeatedly remolded, as observed via DMA and DSC. This indicates the occurrence of dynamic chain packing during network rearrangement, leading to a higher fraction of crystallizable domains after each reprocessing step. We have also studied the effect of adding small-molecule chain extenders in PB and PTMO networks.

Reprocessable polymer networks bearing hydroxyurethane dynamic linkages: Effect of backbone structure on reprocessability and network conformation

2019 Spring National Meeting

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