Macromol. Rapid Commun. 20, 505–509 (1999) 505 Zinc ionomer based on sulfonated maleated styrene-ethylene/ butylene-styrene block copolymer Swapan K. Ghosh, Prajna P.De, Dipak Khastgir, Sadhan K. De* Rubber Technology Centre, Indian Institute of Technology, Kharagpur – 721 302, India (Received: January 7, 1999; revised: February 24, 1999) SUMMARY: A new ionomer, containing both carboxylated and sufonated groups, was obtained by sulfonat- ing a maleated styrene-ethylene/butylene-styrene (m-SEBS) block copolymer, followed by its neutralization with zinc acetate. Dielectric thermal analyses show that the incorporation of ionic groups in m-SEBS is responsible for the occurrence of the high temperature ionic relaxation in addition to the relaxations of the base polymer. Formation of the ionomer causes a significant improvement in stress-strain properties com- pared to the corresponding base polymer. Introduction Polymers having relatively small amount of ions distrib- uted randomly along the chains are usually known as ionomers 1–4) . Ionomers display interesting properties which are primarily ascribed to the presence of the ionic moieties. Generally ionomers are prepared by the two fol- lowing convenient routes: (a) direct neutralization of the preformed polymer; and (b) use of substituted monomers in the polymerization process. Ionomers based on carboxylated or sulfonated anions have been studied by several workers 4–9) . But properties of ionomers having both carboxylate and sulfonate anions on the same backbone have not yet been reported. Ther- moplastic block copolymer elastomers like SEBS have a unique set of properties but one of the main drawbacks is their limitation in high temperature applications. The pre- sent investigation aims to develop a high temperature resistant ionomeric thermoplastic elastomer based on SEBS, containing both carboxylate and sulfonate anions. The ionomer was prepared by controlled sulfonation of styrene-ethylene/butylene-styrene block copolymer (m- SEBS), followed by its neutralization with zinc acetate. The low dielectric loss value and the merging of differ- ent transitions in the high temperature region may over- shadow the characteristic ionic relaxation peaks of the ionomer based on heterophasic block copolymer. How- ever, our preliminary experiments reveal that merging of high temperature relaxations does not occur in the dielec- tric spectra showing variation of permittivity (i. e., dielec- tric constant) against temperature. Mechanical performance of the ionomer at high tem- perature has been studied by measurement of stress-strain properties in the temperature region of 25 C to 75 C. Experimental part Materials The starting polymer was maleated styrene-ethylene/butyl- ene-styrene block copolymer (m-SEBS), Kraton FG1901X, from Shell Chemical Company, USA. The polymer has a styrene content of 30 wt.-%, ethylene-butylene content of 70% and a maleic acid/maleic anhydride level of 19.2 meq/ 100 g of polymer. The weight average molecular weight (M — w ) of the polymer was 110000. Sulfonation 70 g of m-SEBS was taken in 700 ml of 1,2-dichloroethane. After complete dissolution the solution was heated to 50 C and purged with nitrogen for 1 h. Then 42 ml of acetyl sul- fate (1 : 4 mixture of concentrated H 2 SO 4 and acetic anhy- dride) was added and stirred for 2 h at this temperature 10) . Next 70 ml of isopropanol was added to arrest the sulfona- tion reaction. The product was then steam stripped, washed with boiling water for 2 h and dried in vacuum for 15 days at 50 C. The product was designated sulfonated m-SEBS, abbreviated as s-m-SEBS. The sulfonation level was deter- mined by titration with NaOH (standardised with p-toluene sulfonic acid) and found to be 43 meq/100 g of polymer. Preparation of ionomer s-m-SEBS was taken in toluene/methanol (90/10 v/v) mix- ture and a clear solution was obtained. Then a stoichiometric amount of an alcoholic (MeOH) solution of zinc acetate was added in order to minimize the possibility that residual zinc acetate would be trapped in the polymer. Sulfonic acid groups are easy to be neutralized because of the low pK a of sulfonic acid. However, this is not necessarily true for car- boxylic acid which has a higher pK a value and is more diffi- Macromol. Rapid Commun. 20, No. 9 WILEY-VCH VerlagGmbH, D-69451 Weinheim 1999 1022-1336/99/0909–0505$17.50+.50/0