| Epoxy polymer exhibits super mechanical properties, excellent geometric and chemical stability, good electrical insulating property, tunable properties and high adhesion strength, etc. Epoxy resins are thus usually employed as the adhesive cements, coating and repairing materials, insulators and matrix resins of the composite materials, etc., in the field of aerospace, marine, automobile, architectures, electronics as well as other areas. Epoxy resins have their uses only after they are cured. Epoxy resins cured by anhydrides usually have better properties, such as, low viscosity, low reaction exothermic, as well as low reaction shrinkage. However, the highly crosslinked structure induced insufficient brittleness dramatically reduces the impact strength and peel strength. Therefore, to improve the toughness of epoxy polymers for wider applications is of great importance. And the strategies for toughening the epoxy resins are always the challenge and hot topic in this field. Various methods have been proved to be effective for toughing the epoxy resins. However, by using these methods, the viscosity of the resulting resins is also dramatically increased at the same time, and the modulus, the strength, and the glass transition temperature (Tg) may suffer a minor reduction as well. Thus, to simultaneously improve the strength, the toughness, and the modulus of the resulting resin is always a bottleneck issue. In this thesis, the correlation between the molecular structure and the properties of the epoxy/anhydride rigid networks has been investigated. Flexible segments have been designed to regulate the structures of the rigid networks. Meanwhile, the influence of the flexible segments has also been studied. A novel method has also been developed to unify the toughened epoxy resins with simultaneously enhanced strength, toughness, and modulus.In this thesis, two flexible segments which are different in the content of ether bonds were incorporated in the epoxy/anhydride rigid networks as the diluents to investigate the changes of thermodynamic and mechanical properties. Meanwhile, tensile experiment, DMA test and isothermal DSC were used to investigate the mechanical properties and curing behaviors of the epoxy resin systems with different amount of ether bonds, and following issues in two aspects were studied in this work. First, the cured epoxy resin system with the flexible diluents which contain more ether bonds shows enhanced toughening effect. The tensile strength, the tensile elongation, and the tensile modulus increase as the ether bonds concentration. However, the Tg decrease as the increase of the ether bonds concentration. Additionally, the isothermal DSC results showed that the reaction rates of these two systems were simulated well by the Kamal model. The activation energy (Ea) of non-autocatalytic reaction increase as the ether bonds concentration. And based on this observation, the effect of the DGEBD on the rheological and mechanical properties of the epoxy resin was further studied. The results showed that by increasing the amount of DGEBD, the operable time was increased. By increasing the mass fractions of DGEBD to 10%, the cured resin exhibits a striking tensile strength of 86.25 MPa. By increasing the mass fractions of DGEBD to 15%, the cured resin exhibits a striking tensile elongation of 5.97%. The general properties and the practical application conditions of the epoxy resins are determined by the curing process. So the effect of the content of DMP-30, EMI, and BDMA on the exotherm of curing reaction and gel time was studied. Meanwhile, Non-isothermal DSC was used to investigate the influence of BDMA (0.2 phr/0.5 phr) and DMP-30 (0.2 phr/0.5 phr) on the curing behaviors of DGEBF/MeHHPA systems, respectively. The results showed that minor changes in the amount of accelerators could significantly affect the exotherm of the curing reaction and gel time of the epoxy/anhydride systems. Additionally, the isothermal DSC results showed that Malek method can be used to calculate the kinetic equations of these four systems and Sestak-Berggren model can generally simulate well for the reaction rates as well. By comparing the parameters (m and n) which represent the contribution of autocatalytic reaction and non-autocatalytic reaction, respectively, we can find that for the identical amount of accelerators, the type of the accelerators have little influence on the curing reaction. While, when the amount of the accelerators increased from 0.2 phr to 0.5 phr, the m increases indicating indicating a growing contribution of the autocatalytic reaction in curing reaction. However, for the whole system, the value of m was far less than that of n, although the amount of accelerators was changed. That is to say, the contribution of non-autocatalytic reaction was much more significant than that of the autocatalytic reaction in the epoxy/anhydride systems. And small changes in the amount of accelerators can significantly affect the non-autocatalytic reaction and thereby the curing reaction.Thereafter, bi-, tri-, and tetra-functional epoxy systems were chosen to design the network structures using HHPA as curing agent and DMP-30 as the accelerant with bifunctional DGEBD as the flexible diluents to further adjust the network structure. The effect of micro structure (packing of molecular chain and cross-link density) on the storage modulus, loss modulus, Tg, and Ea of glass transition was studied. The results of wide-angle XRD show that for systems without bifunctional DGEBD, the average distance between the three cured molecular chains is TGDDM> TGPAP> DGEBF. DMA data shows that in the glassy region, the storage modulus (higher to lower) for these three systems is DGEBF> TGDDM> TGPAP and the loss modulus for these three systems is TGDDM> TGPAP> DGEBF. In the elastomeric region, the loss modulus approach to 0 and the storage modulus from higher to lower for these three systems is TGDDM> TGPAP> DGEBF, suggesting that the functionality has a great impact on the cross-link density of a network. The Tg of these three systems exhibit the same tendency. The results of DMA show that after the bifunctional DGEBD was incorporated in these there resin systems, the flexible segments would improve the storage modulus (in the glassy region) for DGEBF and TGPAP systems. However, the flexible segments reduce the storage modulus (in the glassy region) for TGDDM systems. And the flexible segments reduce the Tg and the cross-link density as well. The variation in DMA data with frequency shows an increase in the of glass transition with the increase in the degree of functionality. When the bifunctional DGEBD was incorporated in these there resin systems, the of glass transition began to reduce for these three systems at the same time.In the end, a new toughening method using the mutually exclusive characteristic between the aliphatic pendant chains and the main chains in epoxy resin was proposed. The covalent bonds between the aliphatic pendant chains and the main chains in epoxy resin limit the movement of the dangling side aliphatic chains existed in synthetic epoxy monomers. So the dangling side aliphatic chains can attract each other and gather together to form the nano-, submicro- and micro-sized microphase separation structure in the end. A series of epoxy resins containing various side aliphatic pendant chains were prepared by the reaction between DGEBF and the single-ended fatty amine. The length and content of the side aliphatic pendant chains can adjust the size of microphase separation structure. So the relationship between the size of microphase separation structure and the toughening result can be studied. ESI-MS, GPC, and the epoxy value test show that the target products were synthesized successfully. The AFM data shows that the microphase separation structures were observed as expected. When the chain length of side aliphatic pendant chains is C4 to C8, sizes ranging from 20 to 50 nm and 100 to 250 nm were observed. In addition, micron-sized microphase separation structures were also found when the chain length of side aliphatic pendant chains is C12 to C18. The content of side aliphatic pendant chains can affect the size and amount of microphase separation structures. The SEM data shows that a large amount of river lines were observed in the fracture surfaces when side aliphatic pendant chains were incorporated in resins. When the chain length of side aliphatic pendant chains is C4 to C8, a large amount of river lines were observed. However, the fracture surfaces were covered with lots of voids and divided by a large amount of block lines, when the chain length of side aliphatic pendant chains is C12 to C18. Meanwhile, a mass of river lines are distributed in these "blocks". After zooming in to observe the river lines, the jagged river line is observed when the chain length of side aliphatic pendant chains is C12 to C18. However, when the chain length of side aliphatic pendant chains is C4 to C8, the river lines only shows a straight feature. The same observation can be achieved when we reduce the content of side aliphatic pendant chains. Mechanical properties of the cured epoxy resin systems show that the incorporation of side aliphatic pendant chains significantly toughen the epoxy resins. Cured epoxy resins show enhanced mechanical properties when the stoichiometric ratio of DGEBF and the single-ended fatty amine is tuned to be 5:1 compared with the stoichiometric ratio is of 10:1. When the stoichiometric ratio of DGEBF and the single-ended fatty amine is 5:1 and the pendant chain length increase to C18, the cured resin exhibits a striking tensile strength of 90.65 MPa, an elongation of 6.1%and a modulus of 2.55 GPa. These properties increase 51.23%,52.90%, and 25.12% compared with pure DGEBF system, respectively. |
PDF Full Text Request