Interphase Composition Profile in SB/SBS Block Copolymers, Measured with Electron Microscopy, and Microstructural Implications

The behavior of microphase-separated block copolymers has major contributions from the mixed interphase existing between the two homogeneous phases. However, relatively little has been known about the interphase itself and, in particular, its composition profile has never been measured directly. Here, we have used quantitative transmission electron microscopy to acquire the shape of the volume-fraction composition profile across this interphase. Measurements were made on four styrene (S)/butadiene (B) block copolymers having structure SBS (three) and SB (one), with molecular weights 1.0-2.3 X lo5 g/mol and overall volume compositions of 0.24-0.38 styrene. Results, averaged over the four samples, depict an asymmetric interphase profile, being rich in styrene (54 vol %) in agreement with new differential scanning calorimetry data and with dynamic mechanical testing studies of others. This profile is then used in an equilibrium thermodynamic theory (which extends earlier work by adding an enthalpic "Debye" term that accounts for molecular interactions beyond nearest neighbors) to predict various microstructural and thermal properties of the bulk polymer. Excellent agreement with reported data is obtained for the predicted interphase thickness and interphase volume fraction and for predictions of the repeat distance for lamellar SB copolymers. Introduction and molecular architecture. The result of this microphase separation is the formation of either a cocontinuous comseparation *der conditions encountered posite of lamellar elements or a dispersed phase of domains in pradice) of composition, distributed in a continuous matrix (Figure 1). Since these microphase-separated block copolymers exhibit thermal and mechanical properties unlike those of either homopolymer or a random copolymer of similar composition, Block copolymers are known to undergo microphase weight, temperature, *To whom correspondence should be addressed. 0024-9297/88/2221-1377$01.50/0