Preparation And Performance Of Oxidized Graphene/Benzoxazine Thermal Conductive Polymer Composites

With the increasing power densities of electronic components in modern industrial technologies such as information,communication,aerospace and automobile,the performance,lifetime,and reliability of electronic devices is suffering a huge challenge due to rapid heat accumulation.Therefore,there is an urgent need for highly thermally-conductive materials to remove the heat generated from the electronic device to maintain the operating temperature of the devices.Recently,atomic layer graphene have been one of the research hotspots due to their extreme thermal conductivity（⁵,000 W/mK）through phononic transport,which are used as the fillers in the composites with different matrix materials to produce the record-high enhancement of the effective thermal conductivity at the small filler loading fraction（f≤10vol%）.In this thesis,polybenzoxazine/graphene nanocomposites are designed and prepared to obtain enhanced thermal conductivity by using high performance benzoxazine as a novel type of phenolic resin matrix,and graphene oxide（GO）as thermal conductive filler.Three main aspects are investigated systematically in order to find ways to enhance thermal conductivity of thermosets,including:GO as unexpected accelerators of ring-opening polymerization of benzoxazine;silane-functional benzoxazine using as interfacial modifier;synergic effect on thermal conductivity enhancement of composites generated by GO-Ag nano-heterojunction.Graphene oxide（GO）was prepared by Hummers method.The morphology and structure of GO were characterized by atomic force microscopy（AFM）,Fourier transform infrared spectroscopy（FT-IR）,Raman spectroscopy and transmission electron microscopy（TEM）.Benzoxazine monomer（BA-a）was synthesized by using bisphenol A,paraformaldehyde and aniline via Mannich condensation.PBA-a/GO composites were prepared by solution blending BA-a monomer and various amount of graphite oxide,followed by thermally curing process.The chemical changes,reaction temperature and enthalpy,chemoviscosity as well as modulus of the composites during thermally curing process were studied by FT-IR,differential scanning calorimetry（DSC）and rheological analysis.With the addition of GO,lower curing peak temperature and wider curing range are observed as compared with that of pure BA-a.It was hypothesized that carboxyl groups of GO acted as weak organic acid which accelerated the ring opening polymerization of BA-a monomer.Interestingly,BA-a/GO composite with 1 wt%GO was able to form space-filling elastic network,attributing to the catalytic effect and good dispersion of GO.Thermal conductivity measurements of PBA-a/GO composite with various amount of GO were conducted with the“laser flash”technique.The thermal conductivity was increased from an initial value of 0.27 W/mK to 0.47W/mK as the loading increased from 1 wt%to 6 wt%（enhancement factor up to 176%）.Moreover,thermogravimetric analysis（TGA）revealed that PBA-a/GO composite had relatively higher initial decomposition temperature than that of PBA-a sample,which was due to relatively stronger intermolecular interactions between GO and BA-a.A silane-functional benzoxazine（B-TES）was synthesized by using aminopropyltriethoxysilane,4,4’-dihydroxydiphenylsulfone and paraformaldehyde as precursors via Mannich condensation.The benzoxazine structure was characterized by ¹H-NMR,¹³C-NMR and FTIR.Then benzoxazine/GO composites were prepared by solution blending BA-a monomer and various amount of graphene oxide by using B-TES as silane coupling agent,and the chemical changes,reaction temperature and enthalpy,chemoviscosity as well as modulus of the composites during thermally curing process were also characterized by FT-IR,DSC,and rheological analysis.Thermal conductivity measurements of PBA-a/GO composite with various amount of GO were conducted with the“laser flash”technique.The thermal conductivity was increased from an initial value of 0.29 W/mK to 0.56 W/mK as the loading increased from 1 wt%to 6 wt%（enhancement factor up to 229%）.Thermal boundary resistance（R_b）between the GO and PBA-a matrix was theoretically predicted based on Maxwell-Garnett effective medium approximation（EMA）,and the calculated R_b is decreased from 150×10^-9 m²K/W to 130×10^-9 m²K/W by introducing B-TES.Moreover,thermogravimetric analysis（TGA）revealed that PBA-a/GO/B-TES composite had relatively higher initial decomposition temperature and char yield than that of PBA-a sample.Graphene oxide-supported silver nanoparticles（GQDs–Ag）were prepared by using the GO both as a reducing agent and a template.The morphology and structure of GO-Ag were investigated by UV-vis spectra,X-ray diffraction（XRD）,transmission electron microscope（TEM）and scanning electron microscope（SEM）.PBA-a/GO-Ag composites were prepared by solution blending BA-a monomer and various amount of graphite oxide,followed by thermally curing process.Thermal conductivity measurements of PBA-a/GO-Ag composite with various amount of GO-Ag were conducted with the“laser flash” technique.The thermal conductivity was increased from an initial value of 0.17 W/m K to 0.67 W/m K at the loading 6 wt%（enhancement factor up to 294 %）.