Structural Design And Shape-Memory Properties Of Intrinsically Toughened Epoxy-Based Networks

Shape-memory epoxy polymers (SMEPs) are an important class of shape-memory polymers and have attracted much attention due to the easy tuning in thermal and thermomechanical properties and the excellent shape-memory properties including high shape fixity, high shape recovery and rapid shape-memory response. However, because of the rigid structure and high crosslink density, traditional epoxy networks are basically brittle in nature-their strain at break is relatively low even above their glass transition temperatures (Tgs). Therefore, it is desirable to improve the toughness of epoxy networks.In this thesis, two diglycidyl ethers of ethoxylated bisphenol-A containing two and six oxyethylene units (DGEBAEO-2 and DGEBAEO-6) and a diglycidyl ether of propoxylated bisphenol-A with two oxypropylene units (DGEBAPO-2) were synthesized by "one pot" method via tetramethylammonium bromide/solid sodium hydroxide catalysis. FTIR, ESI-MS,’H NMR and GPC were used to investigate the effect of synthetic conditions on the quality of DGEBAEO-2. Moreover, FTIR and 1H NMR were used to characterize the chemical structures of these intrinsically toughened epoxy resins.Thereafter, DSC was used to investigate the curing behaviors of epoxy resin systems containing DGEBAEO-2, DGEBAEO-6 or DGEBAPO-2, and following three aspects were considered in this work. First, the curing reaction between diglycidyl ether of bisphenol-A (DGEBA) and triethylenetetramine (TETA) is highly and fast exothermic thus the applications of this traditional DGEBA/TETA system may be limited. In this aspect, DGEBAEO-2, DGEBAEO-6 and DGEBAPO-2 were used to modify the DGEBA/TETA system, and the effect of the amount of these intrinsically toughened epoxy resins on the curing behaviors of the resultant epoxy resin systems was studied. The results showed that as the content of DGEBAEO-2, DGEBAEO-6 or DGEBAPO-2 increased, the reaction rate of the epoxy resin systems, the heat release per unit mass of uncured epoxy resin systems and the Tg of the cured epoxy resins decreased. Additionally, the curing kinetics of three representative epoxy resin systems DGEBA/TETA, DGEBA/DGEBAEO-2/ TETA (the mass ratio of DGEBA to DGEBAEO-2 equaled 1:1) and DGEBA/ DGEBAPO-2/TETA (the mass ratio of DGEBA to DGEBAPO-2 equaled 1:1) were studied by non-isothermal DSC. The results showed that the curing reactions of these epoxy resin systems exhibit the characteristic of autocatalysis, and the curing rates can be well described by Sestak-Berggren model. Second, intrinsically toughened DGEBAEO-2 and extrinsically toughened DGEBA/1,4-butanediol diglycidyl ether (DGEBA/DGEBD, the mass ratio of DGEBA to DGEBD was 8:2) are similar in chemical composition, but when they are fully cured with same curing agent, the resultant networks are quit different in flexibility and crosslink density. In this aspect, the curing kinetics of DGEBA/DGEBD and DGEBAEO-2 using hexahydrophthalic anhydride (HHPA) as curing agent and tris-(dimethylaminomethyl) phenol (DMP-30) as accelerator were comparatively studied by non-isothermal DSC with a model-fitting Malek approach and a model-free advanced isoconversional method of Vyazovkin. The dynamic mechanical properties and thermal stabilities of the cured materials were investigated by DMTA and TGA, respectively. The results showed that Sestak-Berggren model can generally simulate well the reaction rates of these two systems, and the activation energy of DGEBA/DGEBD/ HHPA/DMP-30 (ETRS) at high fractional conversion changed much higher than that of DGEBAEO-2/HHPA/DMP-30 (ITRS). In addition, DMTA indicated that the Tg and storage moduli of cured ETRS were higher than those of cured ITRS, and TGA showed that the thermal stabilities of these two cured systems were similar. Third, DGEBAPO-2 was cured by a flexible curing agent 2-methyl-1,5-pentanediamine (MPDA), and then an intrinsically toughened network for potential shape-memory application was obtained. The curing kinetics was systematically investigated by using non-isothermal and isothermal DSC methods; moreover, the dynamic mechanical property, mechanical property and shape-memory property were respectively investigated by DMA, tensile experiment and quantitative shape-memory evaluation method. The results showed that Sestak-Berggren model and Kamal model were able to well describe the non-isothermal curing reaction rate and isothermal curing reaction rate, respectively. Tensile test at 49℃ indicated that this SMEP combined a relatively large elongation at break with a relatively large tensile stress. Quantitative shape-memory evaluation revealed that this SMEP possessed good shape-memory properties with high shape-memory fixity and high shape-memory recovery.After that, based on the network structural design, we prepared a series of intrinsically toughened amine-cured SMEPs (DGEBAEO-2/Jeffamine D230/ iso-phorone diamine series) and a series of intrinsically toughened anhydride-cured SMEPs (DGEBAEO-2/DGEBAEO-6/HHPA/DMP-30 series). The thermal, thermomechanical, mechanical and shape-memory properties of these SMEPs were systematically investigated by DSC, DMTA, tensile test and "fold-deploy" shape-memory test, respectively. For amine-cured SMEPs, as the content of D230 increased, the Tg, rubber modulus and tensile strength at room temperature (RT) of the SMEPs decreased according to DSC, DMTA and tensile test. On the other hand, tensile test above RT showed that the elongation at break depended heavily on the deformation temperature, and "fold-deploy" experiments showed that these SMEPs exhibited high shape-memory fixity (higher than 95%) and high shape-memory recovery (close to 100%). For anhydride-cured SMEPs, as the concentration of DGEBAEO-6 increased, the Tg and rubber modulus decreased according to DSC and DMTA. Moreover, with increasing the content of DGEBAEO-6, tensile test at RT showed that the tensile stress at yield decreased and tensile test around the onset temperature of the glass transition showed that the elongation at break increased. "Fold-deploy" experiments revealed that these SMEPs had shape fixity ratios higher than 95% and shape recovery ratios closing to 100%.In the end, furan functionalized poly(hydroxyaminoethers) (PHAEs) were first synthesized from DGEBAEO-2, DGEBAEO-6 and furfurylamine, and then a series of recyclable "epoxy/amine" networks with shape memory effect was prepared by Diels-Alder reaction between PHAEs and l,5-bis(maleimido)-2-methylpentane (MPDBMI). Compression molding was used to recycle the MPDBMI/PHAE films and FTIR, DSC and solubility test were used to confirm the Diels-Alder and retro-Diels-Alder reactions between PHAEs and MPDBMI. Furthermore, the mechanical properties and shape memory properties of the original and the recycled MPDBMI/PHAE networks were systematically investigated by tensile test and quantitative shape memory evaluation method, respectively. DSC measurement indicated that the retro-Diels-Alder reaction will occur upon heating, and solubility test showed that the recycled MPDBMI/PHAE films may still be considered as recyclable. Moreover, tensile test at RT disclosed that both the original and the recycled MPDBMI/PHAE networks exhibited tough properties. Quantitative shape memory evaluation revealed excellent shape memory performance of the original and the recycled MPDBMI/PHAE networks.