Preparation And Properties Of Block Copolymers And Blends Of Poly(Styrene-acrylonitrile) Via RAFT Emulsion Polymerization

ABS (acrylonitrile-butadiene-styrene) resin as one of top five general plastics, has been widely used due to excellent comprehensive properties like high tensile toughness and impact-resistance from polybutadiene, heat-resistance and chemical stability from polyacrylonitrile, rigidity, glossiness and process ability from polystyrene. Replacing the structural units of butadiene with those of butyl acrylate, ASA (acrylonitrile-styrene- acrylate) resin improves the oxidation stability and solvent- resistance of ABS and becomes a kind of important plastic materials. Both ABS and ASA are made by conventional radical polymerization where the polymer chain microstructures are not well-defined. In the current thesis, the recently-developed RAFT emulsion polymerization using poly(acrylic acid)-b-polystyrene tithiocarbonate (named as AA-b-S-RAFT) as both mediator and surfactant was proposed to make ABS and ASA resins toughened by block copolymers. The major studied topics and the key findings are as follows:1) RAFT ab initio emulsion polymerization of styrene using AA-b-S-RAFT of various structures as both mediator and surfactant was investigated. The particle size and thus particle number of the product latex were more relevant to the length of AA block while the inhibition period was more relevant to the length of S block. A well-controlled polymerization in terms of little coagulum, well predicted molecular weight and low polydispersity (PDI) could only be achieved by using the macroRAFTs of hydrophile-lipophile balance (HLB) values between 13 and 20. With the macroRAFTs of lower HLB values, some coagulum was formed and PDI of the final product was higher. On the other hand, in those cases of the macroRAFTs of higher HLB values, the molecular weight was much higher than the theoretical prediction and PDI was increased in the final product due to a fraction of the AA-b-S-RAFT that was oxidized by KPS. Replacing KPS with V501 as initiator, Mn was in excellent agreement with the theoretical prediction but PDI was higher (>1.4). The final latex particle size could be tuned from 80 nm to 172 nm in diameter simply by changing the structure of AA-b-S-RAFT while the same molecular weight was targeted. The polymerization mediated by AA20-b-S5-RAFT was very successful in terms of short inhibition period, high polymerization rate, predicted molecular weight, narrow molecular weight distribution and little coagulum.2) RAFT ab initio emulsion copolymerization of S and AN using AA2o-b-S5-RAFT as both mediator and surfactant was investigated. Inhibition period did not observe in the copolymerization, and the particle number was insensitive to the RAFT agent concentrations. The copolymerization was well-controlled in terms of linear growth of molecular weight and low PDI. However, the gel effect, which led to the diffusion-controlled RAFT addition reactions in the late stage of the polymerization, played a significant influence on PDI, the degree of which was highly dependent on the targeted molecular weight and monomer compositions. For the copolymerization of S and AN with the azeotropic composition (m(S):m(AN)=3:1), the gel effect could be relieved by increasing the reaction temperature from 70 ℃ to 90 ℃ after nucleation period, resulting in PDI as low as 1.20. However, in the cases of higher AN composition, the gel effect was still pronounced and could not be relieved even increasing the temperature, and a significant amount of coagulum was formed as well.3) SAN matrix materials, SAN/PB block copolymers and SAN/PBA block copolymers were synthesized via RAFT emulsion polymerization, of which ABS and ASA were fabricated. The relationship between polymer chain structure and phase morphology as well as mechanical properties was investigated. The results showed that:a) ABS and ASA with well dispersed rubber particles could be achieved by latex blending. Good dispersion of rubber particles in the matrix was necessary to obtain high toughness. Compared with diblock copolymers, the plastics using triblock copolymers as a rubber component would have longer elongation at break and higher tensile fracture toughness without losing elastic modulus and tensile strength but with decreased izod notched impact strength.b) ASA by latex blending showed better tensile properties (modulus, strength, elongation at break and fracture toughness) while ASA prepared by mechanical blending owned higher izod notched impact strength. Introducing crosslinking structure in PBA block could improve the tensile properties of ASA.c) ASA toughened plastic of triblock copolymer had greater flexural strength, while the flexural modulus of which was as the same as the ones prepared by blending.d) ABS and ASA toughened by block copolymers composed of 60 kg/mol SAN as matrix and 20% rubber content could own good mechanical properties. Specifically, For ABS resin by latex blending with SAN358-b-B4437 diblock copolymer as a rubber component, the elastic modulus, yield tensile strength, ultimate tensile strength, elongation at break, tensile fracture toughness, flexural modulus, flexural strength, izod notched impact strength and vicat softening temperature could reach 2100 MPa, 48 MPa,33 MPa,30%,12.77 MJ-m-3,2270 MPa,77.7 MPa,28.38 KJ-m-2 and 99.5℃ respectively. For ASA plastics prepared by mechanical blending with SAN358-b-CrBA936-b-BA358 triblock copolymer as rubber phase, the elastic modulus, yield tensile strength, ultimate tensile strength, elongation at break, tensile fracture toughness, flexural modulus, flexural strength, izod notched impact strength and vicat softening temperature could reach 2140 MPa、48 MPa、43 MPa、61%、27.34 MJ·m-3、2160 MPa、74.0MPa、7.84 KJ·m-2 and 96.9℃, respectively.