Synthesis Of Reactive Block Copolymer Elastomer And Its Toughening Modification To Epoxy And Nylon

Epoxy resin and nylon are notch-sensitive polymer structural materials. One of the successful routines to toughen epoxy/nylon is the incorporation of elastomers, which meet certain problems:toughness improvement was restricted by the poor miscibility between traditional elastomers and epoxy/nylon, and the incorporation of elastomers usually led to significant loss in strength. It is important to design and synthesis a novel polymer to toughen epoxy and nylon without loss in modulus.It is generally believed that the reactive modification of initially immiscible elastomers was able to generate nanostructures in epoxy matrix, or improve their dispersity in nylon matrix. And the incorporation of block copolymers instead of pure elastomers would not lead to significant loss in modulus or glass transition temperature of modified epoxy/nylon blends. In this fashion, reactive polymers:poly(styrene-alt-maleic anhydride)-b-polystyrene-b-poly(n-butyl acrylate)-b-polystyrene (RBCP) and reactive block copolymer core-shell particle, were synthesized and applied as toughening agents for epoxy/nylon. The phase morphology and mechanical performance of modified blends were studied. The influence of chain structure of reactive polymers on the inclusion size and fracture toughness of modified blends was investigated. The following are the innovative achievements:1. A series of poly (styrene-alt-maleic anhydride) (SMA) were synthesized via RAFT solution polymerization mediated by 1-phenylethyl phenyldithioacetate (PEPDTA) as chain transfer agent. Then a series of well-controlled RBCPs with different reactive block length were synthesized via soap-less RAFT miniemulsion polymerization mediated by SMA. The molecular weight distributions of the resulted RBCPs were relatively narrow. The ultimate tensile strength and elastic modulus of RBCP increases with reactive block length, and elongation at break decreases with the increase of reactive block length. This could be attributed to the increased phase separation of RBCP containing longer SMA block. The incorporation of reactive block deteriorated the elasticity of triblock copolymer elastomer.2. The morphologies and mechanical properties of RBCP modified epoxy thermosets were studied. The results revealed that covalent bond was formed between RBCP and epoxy matrix. The interfacial adhesion was therefore increased, leading to the increase in toughness without loss in modulus and Tg. By adjusting the content of RBCP in epoxy blends or reactive block length in RBCP, the size of inclusion in matrix was be effectively controlled over a broad range of length scales from nanometers to a few micrometers. The morphology of RBCP in epoxy matrix was formed via two main strategies:one is vesicle morphology at inclusion-matrix interface, the other is lamellar morphology at RBCP rich phase inside inclusion. The increased loading of RBCP in epoxy matrix would lead to the increase in toughness and decrease in glass transition temperature (Tg) of blend. With the increasing of reactive block length in RBCP, the toughness and glass transition temperature (Tg) of RBCP modified epoxy blends simultaneously increased, which benefited from stronger interfacial adhesion and decreased inclusion size. The critical stress intensity factor reached 2.05 MPa-m1/2, which was 2.5 times the value of unmodified epoxy thermoset. The toughening mechanism of RBCP modified epoxy blends was the cavitation of RBCP inclusion induced matrix yielding. On the other hand, RBCP modified epoxy blends became more strain-rate sensitive with the decrease in inclusion size of RBCP in epoxy matrix.3. Poly (styrene-alt-maleic anhydride)-b-polystyrene-b-poly(n-butyl acrylate) triblock copolymer latex particles was prepared via RAFT miniemulsion· polymerization. By core crosslinking with ethylene glycol dimethyl acrylate, a series of well-defined RBCP core-shell particles with reactive shell, glassy mantle, and rubbery core were obtained. DGEBA/DDM epoxy thermoset was modified by RBCP. SMA in RCSP was covalently bonded to epoxy matrix. The core-shell structures of RBCP core-shell particles were maintained in cured thermosets. With the increased interfacial adhesion, epoxy blends were successfully toughened by RBCP core-shell particles with little loss in modulus and Tg. The increased reactive block length in RBCP core-shell particles improved the dispersity of RBCP core-shell particles in epoxy matrix, leading to the increase in fracture toughness, modulus, and glass transition temperature of modified blends.4. RBCP was also been used to toughen nylon-6. The incorporation of reactive block increased the interfacial adhesion between RBCP inclusions and nylon-6 matrix. It is therefore the dispersed phase of RBCP in nylon-6 matrix was all nano inclusions. As the loading of RBCP in nylon-6 increased, or reactive block length in RBCP increased, the notched impact strength of RBCP modified nylon-6 blends increased. The notched impact strength of nylon-6 blends modified by 10 wt% of RBCP was 5.5 times the value of unmodified nylon-6. RBCP modified nylon-6 blends was toughened with little loss in modulus because the "brittle-ductile" transition was achieved at low rubber content. The toughening mechanism of RBCP modified nylon blends was the cavitation of RBCP inclusion induced matrix yielding.This study achieved the control of morphology and toughness of epoxy/nylon blends modified by reactive block copolymer. Epoxy/nylon were successfully toughened without loss in modulus or glass-transition temperature. This study provides practical implementation of reactive modification for polymer structural materials.