No abstract

In this article, Polynorbornene (PNB)/Zinc dimethacrylate (ZDMA)/Dicumyl peroxide (DCP) composites can form dualcrosslinking networks, which contain an ionic crosslinking network, and a part of the C─C covalent crosslinking network. DCP was used to initiate the polymerization of ZDMA to form ionic crosslinking bonds. With the increase of ZDMA, the total crosslink density (V r ) and ionic crosslink density (V r2 ) increased. DCP was consumed in ZDMA polymerization, which made the PNB reduce the covalent crosslinking networks. Leading to the covalent crosslink density (V r1 ) decreased. Compared with covalent crosslinking network, the dual-crosslinking networks stored more energy when deformed, provided better restoring force and a higher shape recovery ratio for materials. When ZDMA exceeded 3.9 wt%, the ZDMA aggregates hindered the movement of molecular chains leading to the shape recovery ratio slightly decreased. When ZDMA was 2.4 wt%, the composite had an optimum shape fixing and shape recovery ratio. This article provided the experimental basis for the research of PNB dual-crosslinking networks, also widened the research of PNB shape memory materials.

In this paper, resorcinol prepolymer (HT1005) in-situ reacted with hexamethoxymethyl melamine (HMMM, formaldehyde donor) to produce cured phenolic resin to modify polynorbornene (PNB) as a shape memory polymer (SMP). A moving die rheometer (MDR) was used to characterize the crosslinking degree of phenolic resin. Fourier transform infrared spectroscopy, differential scanning calorimeter, universal electronic tensile testing machine, dynamic mechanical analysis and X-ray diffraction was used to investigated the mechanical properties and shape memory performance of PNB composites. Infrared results showed that HT1005 reacted with HMMM forming a chemical cross-linking network in PNB. With the increase of HT1005 and HMMM, the phenolic resin network was gradually improved. When stress was applied to the composite, the phenolic resin rigid network first beared part of the external force, which improved the mechanical properties of the composite. When the content of HT1005 was too much, its dispersibility will become poor, and the rigid phenolic resin network will be too dense, which limited the recovery of PNB molecular chains. A small amount of HT1005 will be more uniformly dispersed in the PNB, the strength of the phenolic resin network was moderate, and the composite material had excellent shape memory and mechanical properties.

In this work, tetrakis(dimethyllamino)ethylene (TDAE) plasticized polynorbornene (PNB) was used as the matrix, sulfur (S) and dicumyl peroxide (DCP) were simultaneously used as crosslinking agents to construct dual covalent cross-linking networks in PNB. The effects of different amounts of cross-linkers on the crosslinking degree, mechanical property, glass transition temperature, and PNB shape memory performance were investigated. Two crosslinking mechanisms were examined by Fourier transform infrared spectrometer and Raman spectrometer. The results showed that sulfur-rich cross-linked PNB exhibited a higher crosslinking degree, tensile strength, and slightly higher glass transition temperature than the DCP-rich system. Cross-linked PNB presented better shape memory performance than the uncross-linked one. Sulfur-rich cross-linked PNB showed even better shape memory behavior than the DCP-rich system, both with a shape fixation ratio of over 99% and a shape recovery ratio of over 90%. The reaction mechanism of sulfur and DCP in cross-linking PNB was different. Sulfur reacted with the α-H in PNB to form monosulfide bonds, disulfide bonds, and polysulfide bonds in PNB and the number of polysulfide bonds increased with increased amounts of sulfur. DCP reacted with the double bonds in PNB to form C-C covalent bond crosslinking networks. The crosslinking mechanism revealed that the sulfur-containing cross-linked bonds, especially polysulfide bonds, were more flexible and bore large deformation, which gave the PNB excellent mechanical properties and ensured a higher shape entropy elastic recovery ratio.