Characterizing the ultimate properties of triblock styrene-diene thermoplastic elastomers

Styrene-diene triblock copolymers with polystyrene end blocks and polydiene midblock are an important group of thermoplastic elastomers (TPEs) and have been widely used in footwear, adhesives, automotive and the automotive industry. These materials exhibit a phase separated structure, with the polystyrene end blocks forming glassy domains distributed throughout the polydiene matrix. Styrenic TPEs have excellent mechanical properties comparable to those of vulcanized rubber, but can be processed like thermoplastics.

The purpose of this research is to investigate the phenomena governing the failure process and ultimate properties in block-copolymer TPEs, and to propose a model to predict the theoretical strength of the materials. Styrene diene block copolymers were chosen as the model materials for this investigation since this class of TPE dominates commercial production of TPEs, and have simple, well characterized molecular structure, from which the properties of other block copolymer can be inferred.

To support the development of the tensile strength model, uniaxial tensile test were performed for three commercial styrene-diene TPEs, polystyrene-polyisoprenepolystyrene (SIS) with 18 and 30 wt% polystyrene (PS) from Dexco Polymers and polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) with 30 wt% PS from Kraton Polymers. Repeated loading tests confirmed that SIS with 18 wt% PS exhibited spherical morphology while the other two materials with 30 wt% PS exhibited continuous hard domains, as expected from the phase separation theory for block copolymers. The stress-extension curves for the second loading of SIS and SEBS with 30 wt% PS exhibit extensive stress softening, suggesting that the continuous domains break up progressively during deformation. The investigation on the influence of loading rate on tensile strength show that the tensile strength of SIS increase with increasing loading rate at low rates, eventually reaching a plateau or peak. The tensile strength of SEBS remained relatively constant over the entire loading rate range studied. The ultimate strain was found to be relatively independent of the loading rate.

To investigate the fracture mechanism governing the ultimate properties of styrenediene TPEs, fractographic studies of the specimens broken during tensile testing were undertaken. SIS18 specimens were fond to contain gross processing flaws, as a result of the increased thickness of the specimens prepared for this material. The fracture surfaces of SIS30 specimens exhibited distinct rough and smooth regions, with the roughness of the surface decreasing with increasing strain rate. The fractures surfaces of the SEBS30 specimens indicated that failure in this material initiated at near surface flaws. The fracture surface in the initiation area was relatively smooth, becoming rough with the onset of rapid crack propagation (in a manner more akin to the behavior of glassy polymers than vulcanized elastomers). The appearance of the rapid crack propagation surface in this material was found to be unaffected by the loading rate.

Based on the findings of the tensile test and fractographic investigations, it is apparent that flaws play a significant role in the fracture surface. In order to predict the ultimate strength of styrene-diene TPEs, it will be necessary to formulate models that account for the effects of flaws and the strength of the undamaged material adjacent to the flaws. In order to predict the effect of material parameters on the theoretical strength of undamaged material, a theoretical strength model has been proposed. The model takes into account the chain pullout and chain scission fracture mechanism expected to prevail at the molecular level. The theoretical strength is formulated as the maximum force the material can sustain per unit area based on the force supported by the glassy PS domains and the elastomeric mid-block chain sections intersecting a planar unit area. The strength contributed by the hard domains and elastomer matrix is related by the maximum force a chain can sustain without undergoing chain pullout or chain scission. The influence of degree of phase separation is also incorporated in the model. The model captures most features observed from experiment. Combined with the influence of flaws, the model is able to explain the different tensile strength behavior reported by different groups.