Gradient Interfaces in SBS and SBS/PS Blends and Their Influence on Morphology Development and Material Properties

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 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 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.

Proton NMR spin-diffusion studies of PS-PB block copolymers at low field: two- vs three-phase model and recalibration of spin-diffusion coefficients

“…For KR, our measured T 2g * (B18 ms at 315 K) agree well with the values determined by Mauri et al 5 Because of the different exponents related to different magnetic field inhomogeneties, our fitted T 2m * and the ones fitted by Mauri et al 5 differ, as is expected when working with samples of different shape on different instruments. The component fractions for KR also match up well with the ones reported in earlier low-field 5,10 and high-field 9 NMR studies, the maximum deviation of our values to the ones in Mauri et al, 5 Saalwächter et al 9 and Thomann et al 10 being about ±2%.…”

“…In most cases, the individual domain sizes for the different samples at the different temperatures determined by SAXS-NMR, which we consider most objective, are only 70-80% of the values obtained from NMR. Similar discrepancies were noted in Saalwächter et al 9 and Thomann et al 10 This could in principle be due to deviations from a pure onedimensional periodic lamellar structure, which could bias both methods in different ways. This is certainly in issue for KR, which was the sample previously investigated, but not for the well-ordered SB sample.…”

“…A faster and more isotropic chain dynamics reduces the residual dipolar couplings, leading to a slower transverse relaxation and a smaller D m , and thus a slower SD process within the mobile phase. We have determined the average residual dipolar coupling of the mobile phase by MP-filtered multiplequantum experiments 10,20 and found that the soft phase of SB is indeed characterized by lower residual dipolar couplings than KR. This is consistent with the T 2m results and confirms that chains grafted on both ends, as is the case in the multiblock structure of KR, have a more network-like and anisotropic behavior than the unilaterally grafted chains in diblock copolymers such as SB.…”

“…This is of course expected considering its narrow polydispersity, as opposed to KR. 10 This means that the assumption of lamellar stacks and with it the model of a purely one-dimensional SD process is better justified for SB.…”

Direct Observation of Interphase Composition in Block Copolymers

“…The approximate molecular weights M n (GPC, PS standard) are 99 and 86 kg/mol, respectively, both with polydispersity PD ≈ 2.1. As addressed in a more detailed study using low-field 1 H NMR and TEM, 19,20 the polymers form disordered lamellar microdomains with a soft-phase thickness of about 13-14 nm at room temperature. NMR spectroscopy.…”