Toward Elucidating the Role of Number of Oxazine Rings and Intermediates in the Benzoxazine Backbone on Their Thermal Characteristics
2016
N. K., Sini | Endo, Takeshi
We have developed novel di-, tri-, and tetrafunctional benzoxazine monomers (BZ2, BZ3, and BZ4), solely containing benzoxazine moieties arranged one after the other on the backbone, to discover the role of additional oxazine moiety on thermal properties. A tandem reaction was adapted, at first a ring-closure and then subsequent Mannich condensation by reacting 2-((4-hydroxyphenyl)aminomethyl)phenol (HPAMP) and aniline/4-amino-2-((phenylamino)methyl)phenol/4-amino-2-(((4-hydroxy-3-((phenylamino)methyl)phenyl)amino)methyl)phenol in the presence of paraformaldehyde to obtain BZ2/BZ3/BZ4. Tetrafunctional benzoxazine monomers containing an intermediate group (R-BZ4) were also synthesized by reacting HPAMP and p-phenylenediamine/4,4′-diaminodiphenylmethane (PDA/DDM) to understand the differences in thermal properties due to the distance created by those between two benzoxazine moieties. The observations (in DSC and TGA analysis) with increase in number of oxazine rings include (i) reduction in curing temperature from 264 °C (BZ1) to 237 °C (BZ4), however an intermediate distance between BZ4 increased curing temperature to 245 °C (DDM); (ii) slower curing rate, a complete curing was achieved only by heating above 200 °C; (iii) decreased weight loss during ring-opening polymerization, BZ1:90% to BZ4:2.4%; (iv) increase in Tg up to 315 °C; and (v) similar thermal stability for BZ2 to BZ4. Introduction of an intermediate between two oxazine rings in a tetrafunctional benzoxazine (R-BZ4) was found to be the best strategy to obviate weight loss during curing as well as to enhance the thermal degradation temperature of resulting polybenzoxazine (PDDM: Td₁₀ of 395 °C).
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