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

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.