Highly Thermal Conductive Polybenzoxazines Containing Liquid Crystalline Structures

Polybenzoxazine(PBz),a new heat-resistant polymer,is especially suitable for electronic packaging materials due to its high heat-resistance,flame retardance,low dielectric constant,low water absorption,and low cost.However,just like ordinary heat-resistant polymers,PBz has low thermal conductivity,and thus cannot meet the growing demand for heat dissipation of the electronic components.Therefore,there is an urgent need to develop a PBz with high thermal conductivity.In this dissertation,we aim to develop functional PBz with high thermal conductivity.We synthesized two benzoxazine-epoxy interpenetrating polymer networks containing different liquid crystalline phase structures,a cross-linked liquid crystalline PBz,and a linear liquid crystalline PBz.The formation mechanism of liquid crystalline phase and thermal properties were studied in detail.The main contents and results are as follows:Part 1 Synthesis and thermal properties of benzoxazine-epoxy interpenetrating polymer networks containing nematic phase structuresA diamine-based benzoxazine monomer(BZ)and a nematic liquid crystalline epoxy monomer(n-EP)are firstly synthesized.Subsequently,a series of benzoxazine-epoxy interpenetrating polymer networks(n-PBEI)containing nematic liquid crystalline structures are obtained by sequential curing of the n-EP and the BZ in the presence of imidazole.The results show that the preferential curing of n-EP plays a key role in the formation mechanism of nematic phase.Due to the introduction of liquid crystalline structures,the thermal conductivity of n-PBEI increases with increasing content of n-EP.When the content of n-EP is 80 wt%,the thermal conductivity reaches 0.32 W m-1 K-1.Additionally,the heat-resistance of n-PBEI is superior to liquid crystalline epoxy resin.Among them,n-PBEI55 containing equal weight of BZ and n-EP has better comprehensive performance.Its thermal conductivity,glass transition temperature(Tg),and the 10%weight loss temperature(Td10)are 0.28 W m-1 K-1,182 ?,and 360 ?,respectively.By introducing boron nitride(BN)fillers into n-PBEI55,a composite of n-PBEI/BN with the highest thermal conductivity of 3.00 W m-1 K-1 is obtained.n-PBEI/BN40 containing 40 wt%BN has better comprehensive performance.Its thermal conductivity,Tg,and the 10%weight loss temperature(Td10)are 1.08 W m-1 K-1,188?,and 368 ?,respectively.Part 2 Synthesis and thermal properties of benzoxazine-epoxy interpenetrating polymer networks containing smectic phase structuresA smectic liquid crystalline epoxy monomer(s-EP)is firstly synthesized,and then a series of benzoxazine-epoxy interpenetrating polymer networks(s-PBEI)containing smectic liquid crystalline structures are obtained by sequential curing of the s-EP and the BZ in the presence of imidazole.The formation mechanism of smectic phase is"reaction-induced phase separation" caused by the preferential curing of s-EP.Owing to the increment of liquid crystalline structure content,the thermal conductivity of s-PBEI increases with increasing content of s-EP.The thermal conductivity of s-PBEI55 containing equal weight of BZ and s-EP reaches 0.30 W m-1 K-1.Compared to n-PBEI55,the thermal conductivity of s-PBEI55 increases by ca.7.1%due to the highly ordered smectic phase structure.w-PBEI64 containing 60 wt%BZ and 40 wt%s-EP has better comprehensive performance.Its thermal conductivity,Tg,and Td10 are 0.28 W m-1 K-1,216 ?,and 334 ?,respectively,suggesting that s-PBEI64 has high thermal conductivity and high heat-resistance.Part 3 Synthesis and thermal properties of cross-linked liquid crystalline polybenzoxazine bearing cholesterol mesogenA bifunctional benzoxazine monomer(BA-ac)was firstly synthesized using cholesteryl 4-aminobenzoate,bisphenol-A,and paraformaldehyde as raw materials via Mannich condensation reaction.Subsequently,a cross-linked polybenzoxazine[poly(BA-ac)]bearing cholesterol mesogen side groups was obtained through thermally induced ring-opening polymerization of the benzoxazine ring.The chemical structure of BA-ac was characterized by Fourier transform infrared spectroscopy(FTIR),nuclear magnetic resonance spectroscopy(1H NMR and 13C NMR),and electrospray mass spectrometry(ESI-MS).The liquid crystalline behaviors of BA-ac and BA-ac during isothermal curing were investigated by differential scanning calorimetry(DSC),polarizing optical microscope(POM),and small angle X-ray scattering(SAXS).Furthermore,the thermal properties of poly(BA-ac)were investigated by DSC,thermogravimetric analysis(TGA),and laser flash diffusivity apparatus(LFA).The study results show that BA-ac is a monotropic smectic C liquid crystal,and poly(BA-ac)contains a smectic C phase structure.The evolution of the liquid crystalline structure during isothermal curing involves the decrement and disappearance of the crystal phase of BA-ac monomer,and the formation and growth of the liquid crystalline phase of BA-ac oligomer.The formation of the liquid crystalline structure of poly(BA-ac)is mainly caused by the strong ability of cholesterol mesogen to form liquid crystals and its position on the side group of the cross-linked network.Due to the existence of a liquid crystal structure,poly(BA-ac)has high thermal conductivity.Its coefficient of thermal diffusivity is 31%higher than that of a traditional polybenzoxazine poly(BA-a).Poly(BA-ac)also has higher heat-resistance.Its Tg,the 5%weight loss temperature(Td5),and Td10 are 175 ?,306?,and 321?,respectively.Part 4 Synthesis and mesophase formation mechanism of linear liquid crystalline polybenzoxazine having cholesterol mesogenA monofunctional benzoxazine monomer(pC-ac)was firstly synthesized using cholesteryl 4-aminobenzoate,p-cresol,and paraformaldehyde as raw materials via Mannich condensation reaction.Subsequently,a linear polybenzoxazine[poly(pC-ac)]having cholesterol mesogens was obtained through thermally induced ring-opening polymerization of the benzoxazine ring.The chemical structure of pC-ac was characterized by FTIR,1H NMR,13C NMR,and ESI MS.The liquid crystalline behaviors of pC-ac and poly(pC-ac)were investigated by DSC,POM,and SAXS.The formation mechanism of liquid crystalline phase of poly(pC-ac)was studied by POM,FTIR,and gel permeation chromatography(GPC).The results show that pC-ac is not a liquid crystal,and poly(pC-ac)is a linear liquid crystalline polybenzoxazine with a bilayer smectic A phase structure.The mesophase formation of poly(pC-ac)is closely related to the existence of strong short-range interaction between cholesterol mesogen side groups,the absence of-OH…N intramolecular hydrogen bonding,and the presence of a large number of intermolecular hydrogen bonding.Based on FTIR data,we propose that the mesophase formation mechanism of poly(pC-ac)is "restriction of intramolecular hydrogen bonding formation".The formation mechanism of liquid crystalline phase of polybenzoxazine is reported for the first time.Therefore,this work provides a helpful guidance for the design and synthesis of novel liquid crystalline polybenzoxazines in the future.