Preparation And Properties Of Thermally Induced Multiple Shape Memory Polyimides

Shape memory polymer(SMP),as a functional material,has been widely studied since the 1980 s and has developed rapidly.Until now,it has important application value in aerospace,medical devices,intelligent manufacturing,microelectronics and other fields,and it is also a much-needed intelligent material for the present and the future.Polyimide has been widely concerned by researchers because of its excellent thermal stability,corrosion resistance,radiation resistance,excellent mechanical properties and high glass transition temperature.However,polyimides were not well studied in shape memory as a polymeric material with excellent overall performance.Moreover,the study of the shape memory function of polyimide mainly focuses on the dual-shaped memory effect.There were a few studies on multi-shape memory polyimide,and the multi-shaped memory effect of polyimide was mostly obtained by manufacturing a wider glass transition temperature.The purpose of this paper was to design and synthesize a new type of polyimide by regulating the structure of its molecular chain segment,meanwhile to prepare high performance polyimide with excellent shape memory behavior.First of all,according to the traditional shape memory polymer molecular chain segment combines hardness with softness,a diamine monomer containing multiple methyl structure was designed and synthesized.Then,it was polymerized with a dihydride monomer containing a trifluoromethyl structure,and crosslinked by a multiamino crosslinker with a cage structure,resulting in a series of low crosslink density polyimides.The structure and performance of polyimide containing crosslinking structure were studied,and the results showed that all polyimides with crosslinker had excellent mechanical and thermal properties.With the increase of crosslinker content,the glass transition temperature of polyimide showed the trend of increasing and then decreasing.The shape memory recovery rate was greatly increased after the addition of a certain amount of crosslinker.PI-2 showed great shape memory recovery rate in bending-recoverying test,indicating that the introduction of crosslinking structure improved the shape recovery rate of the material.Further,in order to solve the problem of low fixed rate of pure polyimide in multishape memory.This section designed and synthesized a di-acide monomer containing the imide structure.Then,it was polymerized with a long chain diamine monomer to obtained poly(amide-imide).At the same time,the polymerization of the precursor monomer dihydride with the same kind of long-chain diamine monomer was carried out to obtain polyimide with similar structure.By mixed the two polymers in different ratios,a series of mixed polymer films were obtained and their structure and properties were studied.Both polymers had excellent mechanical and thermal properties,and the five mixed polymer films exhibit two glass transition temperatures.With the increase of polyimide content in the matrix,the temperature strength of the first glass transition of the material decreased gradually,and the temperature strength of the second glass transition gradually increased.The material phase state had undergone the transformation of island structure-double continuous structure-island structure.The composite film with PAI mass fraction of 30% and PI mass fraction of 70% has the best performance,showing good triple shape memory cycle stability.After four cycles,the retention rate and recovery rate are more than 90%.