Studying the miscibility and thermal behavior of polybenzoxazine/poly(ε-caprolactone) blends using DSC, DMA, and solid state 13C NMR spectroscopy

Polymer blends of polybenzoxazine (PBZ) and poly(ε-caprolactone) (PCL) were prepared by solution blending of PCL and benzoxazine monomer (B-m), followed by thermal curing of B-m. The miscibility and thermal behavior of these PBZ/PCL blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and solid state 13C nuclear magnetic resonance (NMR) spectroscopy. The FTIR spectra indicated that hydrogen bonding interactions occur between the carbonyl groups of PCL and the hydroxyl groups of PBZ upon curing. The DSC results revealed that this PBZ/PCL blend system has a single glass transition temperature over the entire range of compositions that we investigated. The DMA results indicated that the values of Tg of the PBZ/PCL blends were higher than those of the pure polymers. In addition, at higher PCL concentrations we observed two glass transitions for the PBZ/PCL blends: One, for the PCL component, occurred in the low-temperature region and the other, for the PBZ component, in the high-temperature region; this finding indicates that PCL and PBZ are partially miscible in the amorphous phase. The most pronounced effect of the addition of PCL was to broaden the glass transition region, judging from the E″ peaks and the decrease in the value of the loss tangent (tan δ) in the transition region upon increasing the PCL content. We have also studied the 1H spin-lattice relaxation times in the laboratory frame, T1H, and in the rotating frame, T1ρH, as a function of the blend composition. The T1H results are in good agreement with those from the DSC analysis; i.e. the blends are completely homogeneous on the scale of 40-70 nm. The values of T1ρH indicate that the PCL present in the blends exists in both crystalline and amorphous phases; the mobility difference between these PCL phases is clearly visible from the T1ρH data. In addition, the amorphous phase of PCL is not miscible with PBZ; i.e. it is larger than 2-4 nm.