pubs.acs.org/Macromolecules Published on Web 07/15/2009 r 2009 American Chemical Society 5684 Macromolecules 2009, 42, 5684–5699 DOI: 10.1021/ma8000725 Gradient Interfaces in SBS and SBS/PS Blends and Their Influence on Morphology Development and Material Properties Yi Thomann,* ,† Ralf Thomann, † Alfred Hasenhindl, † and Rolf M :: ulhaupt* ,†,‡ † Freiburg Materials Research Center and Institute for Macromolecular Chemistry, Stefan-Meier-Str. 21, 79104 Freiburg, Germany, and ‡ Freiburg Institute for Advanced Studies (FRIAS), Albertstrasse 19, D-79104 Freiburg, Germany Barbara Heck Polymer Physics, Albert-Ludwigs-Universit :: at, Hermann-Herder-Str. 3, 79104 Freiburg, Germany Konrad Knoll and Helmut Steininger BASF SE, Polymer Research, D-67056, Ludwigshafen, Germany Kay Saalw :: achter* Institut f :: ur Physik-NMR, Martin-Luther-Universit :: at Halle-Wittenberg, Betty-Heimann-Str. 7, D-06120 Halle, Germany Received January 11, 2008; Revised Manuscript Received June 14, 2009 ABSTRACT: The formation of gradient interfaces between PS- and PB-rich microphases in SBS block copolymers was investigated by means of solid-state NMR and solution NMR as well as TEM, AFM, and SAXS as a function of molecular architecture, comparing linear and star-shaped asymmetric block structures, and gradient as well as random incorporation of styrene comonomer into the PB-rich blocks. Although all studied SBS possess a very similar total styrene content, different morphologies and mechanical properties were found in the extruded SBS/PS blends, whose origin could be related to the formation of a compositional interface gradient. Employing the sensitivity of solid-state NMR for hard (glassy) and soft (rubbery) phases as well as their respective chemical compositions, we found that upon raising the temperature up to the PS glass transition different amounts of polystyrene from the hard PS phase “soften” and integrate into the soft PB-rich phase (“PS softening”). The degree of “PS softening” characterizes the interfacial gradients of SBS block copolymers at elevated temperatures up to the melt. The softened PS was found to partially mix into the soft phase and partially remain at the interface, thus forming different gradient interfaces, depending primarily on the amount of styrene randomly incorporated in the PB mobile blocks and much less on a compositional gradient at the block linkages in SBS chains. In SBS/PS blends, SBS with a substantial “PS softening” effect was found to preferentially form elongated PB lamellar morphologies, which lead to improved mechanical ductility. The purpose of this study was to apply different characteriza- tion methods and correlate their results in order to gain important compositional and morphological information as well as their effects on the SBS/PS blend mechanical properties. Rapid and robust low-cost pulsed solid-state NMR methods were established as versatile analytical tools for application in high-output polymer screening (HOPS) and quality control systems, enabling online monitoring of structure-property correlations as well as product quality of SBS-based materials. 1. Introduction Poly(styrene-block-butadiene-co-styrene-block-styrene) (SBS) are rubbery/glassy block copolymers, which can be synthesized by means of living anionic polymerization. Since their commer- cial introduction in the early 1960s, 1 they have been steadily improved. Nowadays, commercially available products can be divided into two basic types: type I “thermoplastic elastomers” with 60-80 wt % soft phase fraction and type II “transparent ductile thermoplastics” with about 20-30 wt % butadiene fraction. All these SBSs can be fine-tuned by varying the sequential comonomer incorporation into the SBS chains or by coupling these chains into star-shaped molecular architectures to achieve specific properties. 2 Kraton introduced by Shell Oil Company, for example, belongs to type I with polybutadiene as mobile blocks, is of rubber kind, and can therefore be used, e.g., as “soft touch” materials. Styroflex from BASF belongs also to type I but with poly(styrene-co-butadiene) as mobile blocks. Typical features of Styroflex include optical transparency, high toughness, good adhesion, and processability; its property profile can match that of plasticized PVC but affords better yield strength, elongation, and elastic recovery. Type II SBS has high hard phase fraction and stretch strain and can therefore be used as packaging film, beakers, and other thermoformed as well as injection molded parts. However, most of type II SBSs, especially with styrene incorporated into the mobile blocks, are employed as blend components in general purpose polystyrene (GPPS) in the *To whom correspondence should be addressed. E-mail: yi.thomann@ fmf.uni-freiburg.de (Y.T.); rolf.muelhaupt@makro.uni-freiburg.de (R.M.); kay.saalwaechter@physik.uni-halle.de (K.S.). Downloaded by FRIEDRICH-ALTHOFF KONS GERMANY on August 5, 2009 Published on July 15, 2009 on http://pubs.acs.org | doi: 10.1021/ma8000725