Abstract
Novel bio-based polyurethane (PU) was synthesized using vanillyl alcohol (VA) and lysine diisocyanate (LDI) by one-step solution polyaddition process with 1,4-butanediol (BD) as a co-monomer. The ratios of VA/BD were varied in the PU synthesis, and the properties of the obtained PUs with different vanillin contents were characterized. The molecular weights of PUs ranged from 32 to 44 kDa, and slight decreases of the molar masses were found as VA contents increased due to lower reactivity of phenolic-OH in VA structure with respect to aliphatic BD. The low VA contents in the PU provided flexible material with high elongation up to 600%, while the PU with high VA contents presented stiff characteristics with high glass transition temperature (Tg) of 70 °C and Young's Modulus of 1.8 GPa. Furthermore, thermo gravimetrical analysis (TGA) revealed lower thermal degradation temperature for the PU with higher VA contents, suggesting weaker thermal stability of the urethane bond connected with VA unit. Pyrolysis-gas chromatography/mass spectrometry and nuclear magnetic resonance were employed for the investigation of thermal degradation manner of the PU, which detected the formation of urea compound during the thermal degradation via scission of the urethane bond at benzyl-side accompanied by the elimination of carbon dioxide. Based on these findings, the effect of vanillin-derived diol structure on thermal properties for the resulted PUs was studied by using modified diols of diphenol (methoxyhydroquinone, MHQ) and stilbene-type bisphenol (vanillin-derived stilbene, VS). TGA, differential scanning calorimetry and wide angle X-ray diffraction results indicate that the modified diphenol and stilbene structure improve thermo-mechanical properties of the obtained PU. The introduction of the modified diols into PU increased thermal degradation temperature up to 220 °C. The VS-based PU showed highest Tg of approximately 120 °C due to the stiff structure and strong interaction of the stilbene.
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