Poly-Ester Covalent Adaptable Network (PECAN) is a bio-derivable epoxy/anhydride thermoset resin that is gaining popularity in the renewable polymers community due to its high performance and reduced carbon footprint, but the high heat-energy required to initiate PECAN polymerization is substantial. Dual-Cure (DC) reactions have been recently developed to reduce this energy burden. Here, the heat for PECAN polymerization is obtained through chemical heating from the exothermic free-radical polymerization reaction of a co-polymer like polymethyl methacrylate (PMMA). However, the resulting PECAN/PMMA DC hybrid material reveals a co-polymer architecture with large phase-separated domains, and the associated mechanical properties are disappointing due to weak interactions along the large domain interfaces. Doping of a small amount (~5% weight) of slightly modified methacrylate derivative monomers – referred to as trojan horse methacrylate monomers – results in DC materials with much improved mechanical properties and visibly improved homogeneity, but the length-scales of inter-polymer miscibility is unknown.

Here, we applied Solid-State NMR (ssNMR) techniques on DC blends with varying trojan horse concentrations to better understand the nanometer-scale architectural and morphological effects of trojan-horse incorporation. In particular, we leveraged the sensitivity of proton spin relaxation rates in both the laboratory (¹H T₁, ~50 nm) and rotating frames (¹H T₁_r, ~2-3 nm) to nano-scale separation of domains: evidence of relaxation time averaging signifies co-polymer mixing. We further used through-space ¹³C-¹³C spin-diffusion methods to probe co-polymer mingling on the sub-nanometer scales.

Our results suggest that 3% trojan horse incorporation is sufficient for complete mixing on the ~50 nm scale but is incomplete on the 2-3 nm scale. 4-5% doping is required for homogeneous mixing below 2-3 nm. Increased co-polymer miscibility as indicated by ssNMR correlates with improved DC polymer mechanical performance, suggesting reduced domain sizes and sub-nanometer scale chain entanglement is responsible for the recovered mechanical properties of DC systems.

Solid State NMR reveals nanometer-scale co-polymer miscibility

Michael McGraw

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