British Polymer Journal zyxwvutsrqp 22 zyxwvutsrq (1990) 137-14 zyxwvutsr 1 zyxwvutsr Preferential Plasticization of SBS Triblock Copolymer Sebastiao V. Canevarolo & Luiz H. C. Mattoso Departamento de Engenharia de Materiais, Universidade Federal de Siio Carlos, 13560-SBo Carlos-SP, Brazil (Received 14 November 1988; revised version received 27 February 1989; accepted 22 March 1989) Abstract: Thermoplastic rubbers made of triblock copolymers of styrcne- butadieneestyrene in commercial formulations are commonly used in conjunction with pctroleum extender oils, in order to rcduce the melt viscosity of the rubber during its processing. These mineral oils contain known quantities of paraffinic-naphthenic and aromatic molecules, which in turn are preferably absorbed by the rubbery and glassy phases respectively, affecting selectively the copolymer domain structure and thus the rheological and mechanical properties. Using a plasticizer, the melt viscosity of the system is reduced due to the plasticization of the phases, as expected. The melt rheological transition temperature shown by the unplasticized block copolymer is still present in the oil extended formulations. The flow activation energy is affected by the temperature, type and content of plasticizer. For temperatures lower than the rheological transition temperature the aromatic oil gradually weakens and destroys the polystyrene aggregates and the paraffinic oil induces a greater segregation of the polystyrene aggregates. At temperatures above the rheological transition temperature the behaviour of the two is similar because the flow is homogeneous. Key zyxwvutsrq words: thermoplastic rubber, block copolymer, plasticization, flow activation energy. 1 INTRODUCTION ture, shear stress and, as will be shown in this paper, also on the plasticizer type and its content employed Thermoplastic rubbers show a high melt viscosity in the formulations. during the processing stage and so they are usually processed with plasticizers.' These plasticizers are 2 EXPERIMENTAL mainly extender mineral oils having known quan- tities of paraffinic-naphthenic (saturated) and aro- 2.1 zyxw Materials characterization matic molecules which are preferably absorbed by the rubbery (polybutadiene) and glassy (polysty- The thermoplastic rubber used is a styrene-buta- rene) phases, respectively. Thus, the copolymer diene-styrene triblock copolymer produced by domain structure is selectively affected, which in turn Coperbo (Brazil), Coperflex TR-1040 with 40% influences the final properties of the plasticized styrene content and zyxw M,, = 70 500 g/mol (GW,i@,, = formulations.2 Recently, a rheological transition in 1.44)measured with a size exclusion chromatograph SBS was identified3-6 at which the polystyrene end- (Perkin-Elmer, model 601, UV detector set at blocks can be removed from their base-phase. This 254 nm) using polystyrene as molecular weight allows greater microphase segregation in a static standard. The presence of diblock molecules was condition or a complete mixture of the two phases in not observed (as a second peak or shoulder on the a dynamic condition.' - zyxwv l1 The rheological transition mean peak) and only an insignificant fraction temperature (T,) depends on the copolymer struc- (- 1.5 wt%) of homopolystyrene was found. British Polymer Journal 0007-1641/89/$03~50 zyxwvut 0 1989 Society of Chemical Industry. Printed in Great Britain 137