Time Domain ¹H NMR, Thermomechanical, and Rheology Multiscale Structural Characterization of Polydimethylsiloxane-Toughened Epoxy Nanocomposites for Liquid Composite Molding

In this study, an applicative-oriented epoxy–amine thermoset was toughened with 1, 3, and 5 wt % of polydimethylsiloxane (PDMS) core–shell-particles (CSP), and the influence of the materials morphology on their functional mechanical properties was investigated for the first time by a multiscale experimental approach. First, rheological measurements showed faster gel behavior in matrices containing PDMS-CSP. Then, 1 wt % PDMS-CSP increased the fracture toughness KIC by more than 100%, as well as giving an increase on the glass transition temperature (Tg) from 90 to 98 °C and Young’s modulus E by 15%. More interestingly, the increase in Tg and E is nonlinear with PDMS-CSP content, with the highest values observed for 1 wt % toughener. DMTA and time domain NMR measurements showed that the matrices cross-link density and network degree of homogeneity do not depend linearly with PDMS-CSP content, and they follow the same trend as for Tg and E. By associating both techniques, a linear relationship was obtained between the network structure probed by NMR and the thermomechanical behavior by DMTA, meaning that the macroscopic properties depend additionally on the network morphology. Finally, by combining all of the aforementioned techniques through a multiscale approach, it was shown that PDMS-CSP acts as a cross-link nucleating agent for the epoxy–amine matrix, inducing higher cross-link densities, thus yielding higher functional mechanical properties. Such an unprecedented synergetic experimental approach associated with a multiscale nanoparticle interaction and monomer cross-linking provides an original and robust manner to determine and deepen the understanding of key structure–macroscopic property relationships for applicative-oriented epoxy thermosets.