Partially Hydrolyzed Poly(2-alkyl/aryl-2-oxazoline)s as Thermal Latent Curing Agents: Effect of Composition and Pendant Groups on Curing Behavior.

Poly(2-alkyl/aryl-2-oxazoline)-polyethylenimine (POZ-PEI) copolymers resulting from the partial hydrolysis of poly(2-alkyl/aryl-2-oxazoline)s (POZs) offer highly tunable properties. The amine groups on the PEI units are suitable for a range of postpolymerization modifications such as ring-opening of epoxides, acylation, and coupling. The reactivity of these amines can be controlled by altering the available structural variables of the copolymer. This makes these copolymers promising candidates as thermal latent curing agents (TLCs) for cross-linking of epoxides. In this paper, a range of POZ homopolymers with different alkyl/aryl pendant groups (ethyl/propyl/pentyl/phenyl) and molar masses (1000, 2000, and 5000 g/mol) were hydrolyzed at different hydrolysis ratios (25%, 50%, and 75%) to synthesize POZ-PEI copolymers. The effects of these parameters on the thermal and structural properties of the copolymers were analyzed using (1)H NMR, FTIR, DSC, and TGA. The POZ-PEI copolymers exhibited lower glass transition temperature (T (g)) and decomposition temperature (T (d)) values in contrast to their precursor homopolymers. TLCs based on the obtained POZ-PEI copolymers were prepared and mixed with bisphenol A diglycidyl ether (DGEBA) to obtain one-component epoxy resins (OCERs). The effect of the mentioned variables on the curing behavior of the prepared OCERs was studied in terms of the enthalpy of curing, left limit temperature, and conversion. POZ-PEI-based TLCs with more hydrophobic side chains, at low hydrolysis ratios and with low molar masses, showed the best latency. PPhOZ-PEI-1 copolymer, with a T (g) of 52 °C was chosen as the optimal TLC providing mainly chemical latency though steric effects and physical latency by remaining solid at room temperature. Isothermal DSC tests were performed at different temperatures to examine the stability of the resulting OCER. The results showed that this sample was stable at 40 °C for 3 h and partially cured at 60 °C. Also, the viscoelastic properties of the chosen OCER were investigated by rheology studies, namely, amplitude, frequency, and temperature sweeps. The linear viscoelastic region of the PPhOZ-PEI-1-DGEBA OCER extended up to 10% shear strain. The lowest viscosity for this OCER was observed at 104 °C, and a crossover point was seen at 118 °C. Lastly, the thermomechanical properties of the cured sample were analyzed using DMA, which showed a tan δ peak at 87.6 °C.