Study On Preparation And Properties Of Polyurethane Composites Orderly Reinforced By Boron Nitride Nanosheets

Hexagonal boron nitride nanosheets?BNNSs?have high mechanical property and thermal conductivity,which can be comparable to those of graphene.Moreover,BNNSs have higher thermal stability,stronger oxidation resistance,better corrosion resistance and electrical insulation than those of graphene.BNNSs are used as a reinforcement can significantly improve the performance of polymer composites,especially in improving the mechanical properties,thermal conductivity,thermal stability and maintaining the insulation properties of the polymer.However,the high-yield preparation of BNNSs with high quality is still a challenging,which severely limits the research and application of BNNSs.In addition,the processing and application of BNNSs are always limited to the aggregation,the poor dispersion in polymer matrices Therefore,it is of great significance to improve the peeling efficiency of BNNSs and realize its highly ordered arrangement in the resin matrix.This is also a hot field of nanoparticle/polymer composites research.In this paper,the two-dimensional ordered BNNSs/thermoplastic polyurethane elastomer?TPU?composites were prepared based on ordered microstructure ideas,to improve the properties of polymer based composites by efficient preparation of BNNSs,compound magnetic modification,surface covalent modi fication and magnetization orientation.Moreover,the mechanical,thermal and dielectric properties of the as-prepared highly ordered BNNSs/TPU composites were studied.Based on the fact that expanding the interlayer space of layered materials will effectively weaken the interlayer interaction between adjacent layers,this paper proposed a scalable exfoliation process,named as‘low temperature thermal expansion-assisted ultrasonic exfoliation',to prepare high-quality BNNSs.Firstly,the layered hexagonal boron nitride?h-BN?bulk material was oxidized by concentrated alkali solution,and hydroxyl groups were successfully introduced on their surfaces.Then,the instantaneous high pressure generated by the reaction between the-OH group and the H₂ made the h-BN effective thermal expansion,thus weakening its interlayer interaction.Finally,thanks to the effective increase in the spacing between adjacent layers,the h-BN could be stripped into large,thin BNNSs through a brief ultrasonic treatment.The as-produced BNNSs consisted of 3 to 4atomic layers,with an average dimension of up to 1.6?m and high purity and good crystallinity.In addition,a high yield of 26.05%was achieved by this method,which is much higher than the yield of BNNSs obtained by other peeling methods.BNNSs?m BNNSs?modified by Fe₃O₄ nanoparticles were prepared by in situ oxidation of Fe?OH?₂ used H₂O₂,realizing magnetic modification of BNNSs.The effects of different Fe₃O₄ loading on the surface morphology and magnetic property of m BNNSs were investigated.This method was time-consuming and low cost,without heating at high temperatures or the inert gas protection.In addition,the as-obtained Fe₃O₄ nanoparticles had high purity,good crystallinity,uniform particle size distribution,strong magnetic response and large number of-OH groups on their surfaces.Futhermore,the 2,4-tolylene diisocyanate?TDI?was covalently grafted onto the surface of m BNNSs?m BNNS-TDI?by the hydrogen transfer reaction between the hydroxyl and the isocyanate group.TGA analysis showed that the grafting rate of TDI was 8.31%.After the modification with TDI,m BNNS-TDI still exhibited superparamagnetic behavior with a saturation magnetization of 35.25 emug^-1.The force and motion of the magnetic nanosheets in magnetic field were analyzed through the elliptical sheet model.The theoretical magnetic field strength required by the orientation of m BNNS-TDI in the TPU resin was calculated by the Boltzmann statistics.The m BNNS-TDI/TPU composite system was obtained by in-situ polymerization method,and the mixed system was placed in a static magnetic field and rotated to prepare m BNNS-TDI horizontal directionally enhanced TPU based composites?Im BNNS-TDI/TPU?and m BNNSs horizontal directionally enhanced TPU based composites?Im BNNSs/TPU?.The disordered m BNNSs/TPU composites?Rm BNNSs/TPU?were also prepared without magnetic field.SEM and XRD results showed that m BNNSs were poorly dispersed in TPU matrix,and there were many aggregates,which made it difficult to good orientation under magnetic field.The higher the filler content was,the lower the horizontal orientation degree of m BNNSs was.After the modification with TDI,m BNNS-TDI achieved a high horizontal orientation in TPU resin due to a significant increase in dispersibility.The mechanical properties,such as hardness,wear resistance,tensile strength,elongation at break and dynamic mechanical properties,thermal properties,such as T_g,thermal stability and thermal conductivity,and dielectric properties of Rm BNNSs/TPU,Im BNNSs/TPU and Im BNNS-TDI/TPU composites were analyzed.The effects of filler content on the mechanical,thermal and dielectric properties of Im BNNS-TDI/TPU composites were also studied.The experimental results showed that the mechanical and thermal properties of Im BNNS-TDI/TPU composites have been greatly improved by benefiting from the high orientation,uniform dispersion of filler and strong interface force.Moreover,the dielectric constant and loss of Im BNNS-TDI/TPU composites were significantly reduced.