Study On The Construction Of Polynorbornene Crosslinked Network And Its Shape Memory Effect

Polynorbornene(PNB)has a large molecular weight,and a large amount of physical entanglement exists inside it.At the same time,its glass transition temperature(T_g)is about 35?,and its T_g can be regulated by oil-filling.Therefore,PNB has great potential as a shape memory material.In this paper,the cross-linking network were constructed in the PNB matrix by introducing dicumyl oxide(DCP)and zinc methacrylate(ZDMA)respectively at PNB,and the influence of different cross-linking degree on the shape memory performance of PNB was explored.Firstly,by using different content of DCP crosslinking PNB,a covalent cross-lingking network was introduced into PNB to form C-C chemical cross-linking bonds with higher thermal stability,and the influence of different chemical cross-linking degree on PNB was explored.It was found that the chemical cross-linking degree of PNB increased with the increase of DCP content.However,due to the large molecular weight of PNB,the added DCP had little effect on the T_g of PNB.And due to the increase of chemical cross-linking bonds in PNB,the ability of PNB to induce crystallization was destroyed,resulting in a decrease in tensile strength.At three different temperatures of20?,35?and 60?,as the chemical crosslink density increased in PNB,the equilibrium modulus after PNB stress relaxation increased,which could provide driving force for shape recovery.Therefore,the construction of the covalent cross-linking network could improve the shape recovery rate of PNB.And from the shape cycle curves,when the DCP was 1.5 phr,the PNB had the best shape memory recovery rate at 50%and100%strain,and both were as high as 88%,and the PNB had a relatively stable shape fixing rate,basically maintained at around 99.70%.Secondly,this paper also constructed a complex network of covalent cross-linking and ion-crosslinking in the physical entanglement network of PNB by adding different content of ZDMA in the PNB.Under the initiation of DCP,ZDMA could self-polymerize to form nano-aggregates(poly-ZDMA)in the PNB matrix,and could also undergo graft reactiong with double bonds in PNB to form ionic crosslinks.It was found by equilibrium swelling experiment that as the ZDMA content increased,the total crosslink density and ion crosslink density of PNB increased,while the covalent crosslink density showed a weak decrease trend.Combined with X-ray diffraction and Fourier transform infrared spectroscopy,it was known that ZDMA reacted in PNB.At the same time,due to the formation of poly-ZDMA aggregates and ion-crosslinking networks in PNB,they could deform under external force and break before the main chain of PNB,thereby enhancing the tensile strength of PNB.Under the tensile cyclic loading,poly-ZDMA aggregates may be destroyed,and PNB segments were likely to slip on poly-ZDMA aggregates,and ion-crosslinking bonds may also break to cause increased friction dissipation.Therefore,as the amount of ZDMA increased,the hysteresis of the PNB increased.It could be seen from the shape memory cycle curves that the addition of ZDMA had little effect on the shape fixing rate of PNB,and the shape fixing ratio of the crosslinked PNB with different ZDMA content was maintained at about 99.60%.When the content of ZDMA was 3 phr,PNB had the highest shape recovery rate of 92.80%.When the content of ZDMA was further increased,the PNB segment was easily slid on the surface of the poly-ZDMA aggregate due to the formation of more poly-ZDMA aggregates in the PNB.Therefore,the frictional dissipation generated during the deformation process was increased,resulting in a decrease in the shape recovery rate of the PNB.