Preparation And Characterization Of Polystyrene Functionalized By An Isocyanate-bearing Unsaturated Monomer

Styrene-Ethylene-Butylene-Styrene Copolymer (SEBS) is a new kind of thermoplastic elastomer with excellent performance. This copolymer can not only be used as an elastic material, but also be perfectly used as a toughened agent when mixed with the polar resin. However, the lack of polar groups or reactive groups makes SEBS not compatible with the polar engineering plastic. So compatibilizer is needed to improve their compatibility. So far the most widely used method is to graft monomers that contain anhydride, epoxide, carboxyl or oxazoline in solution or in the melt. In the previous researches, acrylic acid (AA) and glycidyl methacrylate (GMA) were employed as functional monomers and grafted onto SEBS to improve their polarity and reactivity. AA and GMA, however, can easily undergo homopolymerization during radical grafting, and as a result, make it difficult to avoid the unexpected formation of AA or GAM hompolymers. Maleic anhydride (MAH) has been successfully applied in polymer functionalization for many years and is also the main functionalized monomer in SEBS modification. However, the monomer has little tendency to homopolymerize, giving small amounts of the hompolymers. The grafting degree (GD) of MAH is, however, limited to a relatively low level. It would be more attractive to find an unsaturated monomer that has high grafting reactivity with SEBS, a low tendency to homopolymerize, and highly reactive functional groups.This paper reports on an isocyanate-bearing unsaturated monomer, allyl (3-isocyanate-4-tolyl) carbamate (TAI) which is used to satisfy the three criteria mentioned before. For the first time, it was adopted as a functionalizing monomer and radically grafted onto SEBS in solution. As is known to all, isocyanate groups (-NCO) have a much higher potential reactivity with amine and hydroxyl groups, and this is attributed to the strong tendency of -NCO to combine with the active hydrogen of these groups. In addition, the polarity of -NCO is stronger than that of the functional groups listed in the second paragraph. The special structure of isocyanate group provides an easier method for characterization. And the allyl group makes the homopolymerization uneasier because of its resonance effect and steric hindrance. SEBS was then functionalized with TAI, by using dicumyl peroxide (DCP) as an initiator in xylene solution. FT-IR was used to confirm the formation of SEBS-g-TAI. The peak at 2266cm-1, characteristic of–NCO groups in SEBS-g-TAI, revealed an evidence of grafting occurrence. The grafting degree was determined by using both chemical titration and FT-IR. The influences of the monomer concentration and the initiator concentration are discussed, respectively. Grafting degree could be adjusted and the maximum was over 1.5wt% without any gelation. The best cost range of monomer lies in 16-18wt%.The best cost range of the initiator lies in 2-3wt%.Isocyanate group (–NCO) have a much higher potential reactivity, which is attributed to the strong tendency to combine with the active hydrogen of these groups. This sensibility makes store difficult, which causes the unconvenience in the application of this monomer. Therefore, a blocked isocyanate, which is stable at certain temperature and releases dissociative–NCO group at the unblocking temperature which further reacts with other–OH group or–NH2 group to form urethane, is needed to dissolve this problem. In this work, caprolactam was used as the blocking agent and the blocked isocyanates were obtained, their structures were characterized by IR spectrums. The blocking degree was evaluated with chemical titration. The unblocking time was investigated by means of FT-IR and TGA.It was observed that the desirable condition of blocking TAI was with caprolactam in a molar ratio of 1to 1 .1at 80oC for 5 h, and the unblocking temperature was 200 oC.The copolymer of styrene with the blocked isocyanate was synthesized in the solution. The structure of the copolymer was observed by IR spectrum and 1HNMR spectrum. It was proved that there were both PS block polymer and PS–r–PTAI random polymer in the chain. The account of PS block polymer became lager when the temperature went higher. The TG equation was validated through the thermal analysis. It proved the fact that the side group of PTAI has some kind of flexibility which makes the glass transition temperature of this copolymer lower than that of polystyrene. The copolymer of styrene and PTAI was investigated by GPC and it was found that in order to obtain the copolymer of higher molecular weight which has more account of PTAI, the best reaction temperature should be lower than 100℃and the fine concentration of the initiator should be around 0.5wt%.