| Ploydicyclopentadiene(PDCPD)has the advantages of low density,high mechanical strength and low thermal conductivity,and has good application prospect in structural materials and thermal insulation.At present,PDCPD aerogel is chemical crosslinked via covalent bonds,and it’s difficult to recycle.This paper presents a method for the preparation of reversible polydicyclopentadiene-based aerogels by introducing noncovalent crosslinking into PDCPD system,which can be recycled repeataly.In addition,it is also subjected to composite modification using a cyclic olefin copolymer(COC).In this paper,a chemically reversible PDCPD-based coordination polymer metallogel was first prepared in combination with linear polydicyclopentadiene(LPDCPD)and crosslinking reaction base on metal coordination,which was used as a precursor of reversible PDCPD aerogels.First,fully soluble LPDCPD was synthesized by using the second generation Grubbs catalyst and the chain transfer agent,1-octene.Subsequently,the 3,6-dipyridyl-1,2,4,5-tetrazine(DPTZ)unit,which possesses coordination abilities with metal ions,was introduced into the LPDCPD chain by the inverse electron demand Diels-Alder reaction(iEDDA).The structure and properties of each step product were characterized by FTIR,NMR,GPC,DSC and TGA.By simply changing the molar ratio of DPTZ to LPDCPD,the coordination sites on DPTZ-LPDCPD can be controlled to manipulate the crosslinking density in subsequent metallogels.A chemically reversible PDCPD-based coordination polymer metallogel was prepared by coordination reaction between DPTZ-LPDCPD macromolecular ligand and metal ion.The effects of crosslinking density,metal ion species,solvent and polymer concentration on the rheological properties of the metallogels were investigated.The results disclose that the gels become more elastic with increasing crosslinking density.The different coordination abilities and forms of metal ion have a great influence on the rheological behaviors of the gels.The compatibility of the solvent with the polymer also has a certain impact on the rheological behaviors.In addition,the strength of the gels increases with the increase of polymer concentration.Finally,the chemical reversibility and reproducibility of the metallogels were also explored.The de-crosslinking of the metallogels was achived by addition of excess ethylenediamine as a competitive ligand.The separated DPTZ-LPDCPD ligand can be re-coordination with metal ions to form the corresponding metallogel.Through solvent replacement,supercritical carbon dioxide dry,the reversible PDCPD-based aerogels were prepared using PDCPD-based metallogels.Besides,COC aerogels were synthesized by vacuum drying using COC gel,and the PDCPD-COC composite aerogels were prepared using physical blends of DPTZ-LPDCPD and COC.The microstructures and pore characteristics of various aerogels were analyzed by SEM and mercury intrusion methods.Simultaneously,their compressive mechanical properties and thermal conductivity were also investigated.The PDCPD-baesd aerogels have micro-sized macroporous structures with a wide pore size distribution and poor mechanical properties.The thermal conductivity of PDCPD-based aerogels is 0.037(W/(m·K)).For the COC aerogels,with the increase of the density,the pore size and porosity decrease,the Young’s modulus increases,the thermal conductivity decreases first and then increases.Unexpectedly,there are many macropores existing in PDCPD-COC composite aerogels,which were caused by phase separation.COC constitutes the skeleton of the aerogel,while the PDCPD is adhered to the COC skeleton or embedded in the skeleton in the form of particles.Futhermore,the effects of PDCPD,COC group distribution and DPTZ content on the performance of aerogel were studied.The results show that the content of PDCPD increases,the pore size,Young’s modulus and thermal conductivity are increased.As to the DPTZ content,when it increases,the pore size and Young’s modulus are improved.The thermal conductivity also increases due to the larger pore size. |
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