Graphene Oxide Network Formation and Crosslinking in Polybenzoxazines and Poly(R)-3-hydroxybutyrate

Abstract: The search for improvement of the physical properties of two very dissimilar polymeric materials is the aim of this doctoral thesis. The high performance properties of polybenzoxazines are tackled via the preparation of novel amino-functional benzoxazine monomers, as well as by the addition of graphene oxide (GO) for the preparation of polybenzoxazine nanocomposites. The synthesis of the amino-functional benzoxazine monomers is attempted by employing two different approaches. Both amino-monofunctional and bifunctional benzoxazine monomers (P-a- NH2 and P-ddm-NH2) are successfully prepared via deprotection of amine-protected benzoxazines. Tetrachlorophthalimide and trifluoroacetyl are found to be suitable protecting groups. The reactivity of the aminofunctionalized benzoxazine monomers is confirmed through reaction with acid chlorides. Amide-containing benzoxazine model compounds along with polyamides containing benzoxazine moieties in the main chain are prepared accordingly. Thermal properties of the crosslinked poly(amide-benzoxazine)s demonstrate the potentiality of such materials for high performance applications. GO-base polybenzoxazine nanocomposites are also successfully prepared. The focus of this study is placed rather on the degree of dispersability of GO nanoparticles. Rheological analysis of the nanocomposites prepared using two different benzoxazines as matrices, both prior and after polymerization, reveals the importance of the interfacial interactions between GO surface and the polybenzoxazines in order to favor dispersability and thus to modify the physical properties of the nanocomposites. The biobased and biodegradable linear polyester poly(R)-3-hydroxybutyrate (PHB) degrades at temperatures close to its melting temperature (175 oC). The mechanism for the thermal degradation involves the random formation of shorter polymer segments containing crotonyl and carboxyl end groups. Different additives known to react with carboxyl groups and influence the melt stability of well-established polyesters are added to PHB. Their effect on the thermal degradation is analyzed by rheological means and by molar mass measurements. Among the additives employed, multifunctional epoxide and carbodiimide cause minor improvements on the melt rheology. No effect in the rheological behavior is observed when aryl phosphites are employed. Lastly, the use of bifunctional oxazoline and epoxide, and trifunctional aziridine increases the rate of thermal degradation by drastically decreasing the melt stability and thus the molar mass of PHB. GO was also employed as a means to modify the physical properties of PHB. Moderate influence on the thermal properties is observed using DSC and TGA. Although the temperature of decomposition of PHB remains unaltered with the addition of GO, molar mass measurements show an increase of the rate of thermal degradation of PHB in the nanocomposites. The rheological properties, on the other hand, are greatly influenced by the addition of GO nanoparticles. Micromechanical effect and GO network formation are observed to be the cause for such enhancement. In addition, the dynamic properties in the solid state are analyzed according to the modified Halpin-Tsai model for platelet reinforcement. The linear viscoelastic behavior of both GO-based benzoxazine and PHB nanocomposites were analyzed using scaling concepts for fractal networks. In both cases, small percolation volume fractions for the formation of spacefilling network of GO nanoparticles are determined. Finally, the results from the analysis are used to quantify the degree of dispersion and are employed for comparison purposes.