Direct Observation of Interphase Composition in Block Copolymers

Abstract

We demonstrate the fast, direct, and quantitative observation of temperature-and comonomerinduced changes in the interfacial size and composition in phase-separated styrene-butadiene-styrene block copolymers by double-quantum-filtered proton spin-diffusion NMR experiments performed under high-resolution magic-angle spinning conditions. The experiment is based on the dipolar-mediated diffusion of spin magnetization from the (styrenic) rigid phase through a more mobile interphase of variable size and composition into the soft (polybutadiene) domain. The experiment spectroscopically distinguishes between mobilized styrene segments located in the interphase and those mixed (dissolved) in the bulk of the mobile domain. The results indicate that temperature-induced softening due to mobilization of styrene units at the interface and the tendency to become part of an extended interphase is stronger for systems with a lower segregation strength, having statistically distributed styrene comonomers in the soft domain.

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%.…”

“…5 A partial mobilization and mixing of the PS block into the soft PB block at the interface was evidenced by high-resolution magic-angle spinning (MAS)-NMR measurements over a large temperature range. 9 The volume fraction of PB units in the PB-dominated phase is around 95% at 298 K and 91% at 336 K, with the mobilized PS mainly located at the interface.…”

“…For both samples, the decreasing rigid fraction at higher temperatures can be explained by a progressive mobilization of PS chains flanking the interphase. 9 Note that the phase fractions f g , f i and f m reflect the number of protons in the phases and do not correspond to the weight fractions of the PS and PB components. To obtain proper volume fractions, the fitted fractions have to be corrected on the basis of the known respective proton densities.…”

“…The program generally allows to simulate the diffusion process in two or three dimensions, and in its latest version, it considers explicitly the interphase characterized by an averaged composition (spin density) and SD coefficient. 5,9 RESULTS AND DISCUSSION Comparison of the two-and three-phase model It is well established that block copolymers have a nonnegligible interfacial region. 5,9,10,21 From Figure 5, where SD and SR curves are plotted vs the square root of the diffusion time, a sigmoidal initial rise of the source curve is apparent.…”

“…We have previously studied the domain sizes of the combined rigid and mobile phases of KR by high-field NMR, obtaining d m ¼ 17 nm and d r ¼ 42 nm at 309 K. 9 Under these conditions, T 1 relaxation has virtually no influence and our present approach is nicely confirmed by the results from the DMS fits on the basis of the two-phase model at 315 K, where d m ¼ 17.2 nm and d r ¼ 43 nm at 315 K. For the thickness of the rigid domains, our values at 360 K are about 9% (4 nm) larger as compared with the ones obtained in Saalwächter et al 9 In the work of Mauri et al, 5 the same low-field DQ-and MPfilter sequences, the same MSE-FID analysis method and basically the same simulation program (without full three-phase and three-dataset fitting option) were used. The T 1 relaxation times and lamellar sizes of KR obtained by the new DMS fits in the two-phase model are consistent with the values determined by DQ-SD/SR or MP-SD/SR fits in our previous work.…”