Effects of polyol molecular weights on thermally reversible light scattering behaviors of benzoxazine-urethane copolymers

Abstract

The thermally reversible light scattering behaviors of benzoxazine-urethane copolymer was investigated in this work. Benzoxazine monomers (BA-a) were synthesized by the solventless synthesis technique from bisphenol-A, para-formaldehyde, and aniline with the molar ratio of 1:4:2, respectively. Urethane prepolymer was prepared by reacting poly(propylene glycol) (Mw = 1000, 2000, or 3000 g/mol) with toluene diisocyanate (TDI) at the molar ratio of 1:2 under a nitrogen atmosphere. The obtained urethane prepolymer (PU) was labelled as PU1K, PU2K, and PU3K with respect to the molecular weights of polyether polyol employed in the preparation step. Urethane prepolymer and benzoxazine monomers were crosslinked during the thermal curing by the urethane linkages as confirmed by ATR FT-IR spectroscopy and Raman microspectroscopy. The network forming reaction of the BA-a/PU binary mixture occurred simultaneously as a single exothermic peak was observed in the differential scanning calorimetry (DSC) thermograms. The curing temperature of BA-a/PU binary mixtures increased when PU mass fraction increased. The BA-a/PU binary mixtures were completely cured when a step-by-step thermal curing protocols were employed. The BA-a/PU alloys exhibited improved thermal stability due to the presence of BA-a as observed by the thermogravimetric analysis. Furthermore, polybenzoxazine-urethane alloys with PU3K mass fractions of 30%, 40%, and 50% exhibited thermally reversible light scattering phenomena. It was observed that BA-a/PU3K alloy at 30% wt of urethane shifted from the opaque state at room temperature to the transparent state when the temperature of the specimen was raised to 128°C. Transparent PU3K/BA-a alloys became opaque when the specimens were left to cool down to room temperature. BA-a/PU3K alloys were phase-separated from some fractions of PU3K as observed from the dynamic mechanical analysis. BA-a/PU3K alloys in the form of micrometer polymeric particles and dendritic morphology were the light scatters in our system as exhibited in the optical micrographs in transmission and cross-polarized transmission modes. Consequently, BA-a/PU3K alloys developed from this research are a promising candidate for applying in a high thermal sensing application.