Epoxy resin/poly(ε-caprolactone) blends cured with 2,2-bis[4-(4-aminophenoxy)phenyl]propane. II. Studies by Fourier transform infrared and carbon-13 cross-polarization/magic-angle spinning nuclear magnetic resonance spectroscopy

Crystalline thermosetting blends composed of 2,2′-bis[4-(4-aminophenoxy)-phenyl]propane-crosslinked epoxy resin (ER) and poly(ε-caprolactone) (PCL) were investigated by means of Fourier transform infrared (FTIR) spectroscopy and high-resolution solid-state NMR spectroscopy. FTIR investigations indicated that there were specific intermolecular interactions between ER and PCL and that the intermolecular hydrogen-bonding interactions were weaker than the self-association in pure epoxy. The intermolecular hydrogen bonding was considered to be the driving force for the miscibility of the thermosetting blends. For the examination of the miscibility of the thermosetting blends at the molecular level, high-resolution solid-state 13 C cross-polarity/magic-angle spinning (CP-MAS) NMR spectroscopy was employed. The line width of 13 C CP-MAS spectra decreased with increasing PCL contents, and the chemical shift of the carbonyl carbon resonance of PCL shifted to a low field with an increasing epoxy content in the blends. The proton spin-lattice relaxation experiments in the laboratory frame showed that all the blends possessed identical, composition-dependent relaxation times (i.e., the proton spin-lattice relaxation times in the laboratory frame), suggesting that the thermosetting blends were homogeneous on the scale of 20-30 nm in terms of the spin-diffusion mechanism, and this was in a good agreement with the results of differential scanning calorimetry and dynamic mechanical analysis. For the examination of the miscibility of the blends at the molecular level, the behavior of the proton lattice relaxation in the rotating frame was investigated. The homogeneity of the thermosetting blends at the molecular level was quite dependent on the blend composition. The PCL-lean ER/PCL blends (e.g., 70/30) displayed a single homogeneous amorphous phase, and the molecular chains were intimately mixed on the segmental scale. The PCL-rich blends displayed biexponential decay in experiments concerning the proton spin-lattice relaxation times in the rotating frame, which was ascribed to amorphous and crystalline phases. In the amorphous region, the molecular chains of epoxy and PCL were intimately mixed at the molecular level.