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Preparation, Characterization, and Properties of Resol-Based Benzoxazine Intermediates and Glass Cloth Reinforced Laminates Based on Their Polymers

A series of multifunctional benzoxazine intermediates (Boz-p-x) with different molecular weights and curing behaviors were synthesized based on resol. Their composition was controlled by adjusting the mole ratio (x) of aniline to phenol; i.e. the content of benzoxazine rings formed was varied. The chemical structures and hydrogen-bonding interaction of the intermediates were confirmed by 1H nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Their curing behaviors were investigated by differential scanning calorimetry (DSC). The results indicated that the residual phenolic hydroxyl groups in the intermediates formed two kinds of hydrogen bonding and acted as a catalyst in the curing process. The more residual phenolic hydroxyl present, the higher the glass transition temperature was and the faster the curing reaction. Further, glass cloth reinforced laminates based on poly(Boz-p-x)s were prepared. The mechanical properties of the laminates were tested by dynamic mechanical analysis (DMA) and flexural measurements. The poly(Boz-p-8) laminate exhibited the highest glass transition temperature (T g) and mechanical properties in the laminates. Its T g was 237°C and its flexural modulus and flexural strength were 26.8 GPa and 597 MPa, respectively. The laminates of poly(Boz-p-6), poly(Boz-p-8), and poly(Boz-p-10) showed good modulus retention at elevated temperature due to more ‒OH⋅⋅⋅N hydrogen-bonding interactions. In addition, poly(Boz-p-x)s exhibited high thermal stability and char yield at 800°C.