Star-Branched Block Copolymer Ionomers: Synthesis, Characterization, and Properties

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Polymers and High Performance Materials


A new kind of telechelic ionomer has been developed that incorporates multiple ionic groups, i.e., an ionic block, at the ends of each polymer chain. Each ionic block is envisioned to behave as a single ionic unit, analogous to a single ion pair in a random or conventional telechelic ionomer. The ionomers may be either linear or star-branched, and the average number of ionic groups per chain terminus can be strictly controlled. Thus, three-arm star-branched block copolymer ionomers composed of short ionic outer blocks and elastomeric inner blocks were synthesized as follows: oligostyrene blocks containing an average of four styrene units were initiated with s-BuLi; butadiene was added sequentially to produce block copolymer arms; the living arms were linked with trichloromethylsilane. The polybutadiene blocks, containing approximately 40% 1,2-enchainment, were exhaustively hydrogenated by using H-2 and Wilkinson's catalyst in 2-butanone/hexanes. Exhaustive sulfonation of the oligostyrene outer blocks was performed using acetyl sulfate in homogeneous CH2Cl2/hexanes solutions; neutralization was carried out in THF solutions using stoichiometric quantities of metal or quaternary ammonium hydroxides. Investigation of tensile properties revealed that low molecular weight samples, i.e., 10K < MBAR(n) < 30K, were weak, brittle, gellike materials; increasing the molecular weight to MBAR(n) = 120K yielded a tough elastomer with tensile strength of 7 MPa and elongation of 600% for K counterion. The value of the dynamic storage modulus (1 Hz) was constant from -10 to approximately 125-degrees-C, forming a pronounced rubbery plateau, and gradually decreased over the range 125-225-degrees-C for low molecular weight samples; the tan delta displayed a peak in the range 220-260-degree-C with the maximum shifting to higher temperature as the molecular weight decreased. SAXS performed on samples of molecular weight 10K and 18K revealed a large upturn near zero scattering angle for both samples; no other peaks were produced by the 18K sample, but the 10K sample displayed a weak shoulder consistent with a Bragg spacing of 2 x 10(2) angstrom. It is proposed that the properties of these ionomers are consistent with a morphology consisting of very large, widely spaced multiplets. In this model, particularly for low molecular weight samples, it is probable that a considerable proportion of the molecules incorporate more than one arm into a single multiplet, forming non-load-bearing loops, and for a molecule to span the intermultiplet distance, it would necessarily exist in a highly perturbed, chain-expanded state.

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