Effect of pendant group of polysiloxanes on the thermal and mechanical properties of polybenzoxazine hybrids

Polybenzoxazine (PBa) was successfully hybridized with polysiloxanes by synchronizing two reactions; ring-opening polymerization of benzoxazine (Ba) and sol–gel process of diethoxysilanes. Diethoxydimethylsilane, diethoxymethylphenylsilane, and diethoxydiphenylsilane were used as precursors for the formation of polydimethylsiloxane (PDMS), polymethylphenylsiloxane (PMPS), and polydiphenylsiloxane (PDPS), respectively. The effect of pendant group of polysiloxane on the optical, mechanical, and thermal properties of PBa–polysiloxane hybrids was studied. The progress of sol–gel process was confirmed by IR, 1H NMR and size exclusion chromatography. Opaque PBa–PDMS hybrid films were obtained up to 15 wt% of PDMS content, corresponding to the phase separation with 1–2 μm domain size of PDMS as observed from the scanning electron microscope. Meanwhile, transparent PBa–polysiloxane hybrid films were obtained up to 29 wt% for PMPS and 36 wt% for PDPS, which revealed no apparent domain of PMPS and PDPS, indicating high compatibility of the polysiloxanes with PBa. Dynamic viscoelastic analysis (DMA) of the PBa–PDMS hybrid revealed two glass transition temperatures corresponding to PDMS and PBa components, while the DMA of the PBa–PMPS and PBa–PDPS hybrids revealed only one glass transition temperature. The tensile strength and elongation at break of the hybrid films increased with increasing polysiloxane content. Thermogravimetric analysis revealed high weight residue at 850 °C for PBa–polysiloxanes with phenyl group.