The Preparation Of Monolithic Silica/Polyimide Aerogels And Their Structures And Properties

Aerogels are unique among solid materials.They have extremely low densities(<0.05 g/cm3),low thermal conductivity(<0.02 W/m.K),and a high inner surface area(>1000 m2/g).Because of their fascinating properties the aerogels find potential applications such as super-thermal insulators,catalyst supports and dielectric materials.Nevertheless,despite that great potential,the lack of large-scale commercial aerogel applications is also immediately apparent.The wide disparity between the real and the realizable applications is reconciled in terms of poor mechanical properties(aerogels are fragile materials).Therefore this paper decide to develop a novel route to monolithic aerogels,the main research content in this paper include:Monolithic silica aerogels can easily be prepared by the sol-gel polymerization of methyltrimethoxysilane precursor followed by the drying of the gels at the ambient pressure.Hydrophobic aerogels are generally prepared by the derivatization method.In this study,we use water as only solvent and abandon the tedious and time consuming derivatization method since it involves the solvent exchange repeatedly.An extent of CTAB content in the alcogels plays a crucial role in obtaining the monolithic aerogels,which could be optimized by a proper control over the CTAB/MTMS molar ratio during the sol-gel synthesis.The optimum addition of CTAB was found at 0.02 g(molar ratio CTAB/MTMS=0.015),when monolithic MTMS silica aerogels show extremely low density of 0.079 g/cm3,high porosity of 88.9%.In addition,the aerogels are super hydrophobic with contact angle as high as 166°.We systematically examine the properties of polyimide aerogels,which are obtained via freeze drying of wet-gels synthesized by 4,4’-diaminodiphenyl ether(ODA),3,3’,4,4’-biphenyltetracar-boxylic dianhydride(BPDA),and cross-linking with TAB,using pyridine and acetic anhydride as catalyst and dehydrating agent,respectively.Different densities of polyimide aerogels were synthesized.The densities of polyimide aerogels vary by turning the concentration of cross-linked polyimide in the solutions.It was found that the porous morphology evolves from coarsened structure to continuous structure,showing decreased structural size.With the decrease of density,the compression strength increases and the thermal conductivity decreases.Obtained by experiment,10%sol concentration polyimide aerogels had a lower density(0.115 g/cm3),thermal conductivity(0.0312 W/m.K)and good mechanical properties(7.97 MPa).However,polyimide aerogels are high-hydrophilic.Two different ways,namely,physical mixture and cross-linked ways that are adopted to prepare hybrid aerogel.Firstly,the inorganic phase was incorporated intothe polyimide backbones at a form of micro-scale hydrophobic silica aerogel powders by a simple physical mixture.Secondly,robust monolithic cross-linked polyimide cross-linked silica aerogels were prepared,with a cross-linked silica particle network and surrounded by a polyimide network,which should function as the skeleton to enhance the mechanical properties.Overall,SiO2 aerogel jammed polyimide aerogels have low thermal conductivity(0.0282 W/m.K),and high mechanical strength(11.34 MPa).In addition,incorporation of SiO2 aerogel also enhances the hydrophobicity(water contact angle up to 96°).