Y. Thomann, R. Thomann, A. Hasenhindl, R. Mülhaupt, B. Heck, K. Knoll, H.
Steininger, K. Saalwächter.
Gradient Interfaces in SBS and SBS/PS
Blends and Their Influence on Morphology Development and Material Properties.
Macromolecules 42, 5684-5699 (2009). DOI 10.1021/ma8000725.
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
characterization 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
NMRmethods 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.