The location of poly(styrene-b-poly(methyl methacrylate)) (PS-b-PMMA) block copolymers
in PMMA/poly(cyclohexyl methacrylate) (PCHMA) melt mixed blends was determined. Among all these
components, only the PS portion of the block copolymer can be stained by ruthenium tetroxide and appears
dark in transmission electron micrographs. Then the locations of block copolymer in the blends, i.e.,
interfaces and micelles, can be easily identified. Morphology and the aggregation of the PS-b-PMMA
were studied as a function of molecular weight and molecular weight fraction. At low molecular weight
PS-b-PMMA, a PMMA/PS-b-PMMA macrophase formed. At higher M
n, small (<1 μm) minor phase
(PMMA) drops coated with copolymer formed, but these drops also contained micelles even at low PS-b-PMMA concentration. Increasing the molecular weight of the PMMA first caused the drop size to
increase, and then the copolymer micelles to relocated from the PMMA to the PCHMA phase. Increasing
block copolymer concentration caused PMMA drop size to decrease roughly in proportion to interfacial
coverage. The results are discussed qualitatively in terms of Leibler's wet−dry brush theory.
Microstructural transformation of a poly(styrene‐b‐butadiene‐b‐styrene), SBS, triblock copolymer blended with asphalt was studied as the asphalt composition was varied from 0 wt% to 96 wt%. Transmission electron microscopy (TEM), dynamic mechanical spectrometry (DMS), and differential scanning calorimetry (DSC) were used. The blends were made in batch mixers at 200°C, or by solution casting from a nonselective solvent (trichloroethane) at ∼28°C. Asphalt partially solubilizes the polybutadene (PB) midblock of the SBS producing saturated PB microdomains along with macrodomains of asphalt. When the asphalt concentration was varied from 10 to 90 wt%, the asphalt phase separated into a variable number of large domains, while the SBS‐rich regions formed a continuous matrix. Networks of SBS‐rich regions were observed at low magnification; these are referred to as macro‐networks. At higher magnification, networks that are stabilized by polystyrene (PS) microdomains (denoted micro‐networks) are also formed. The presence of a macro‐network is also confirmed by stress relaxation tests. The macro‐network broke down into microgel‐like structures when the asphalt composition exceeded 90 wt%. Examination of the interior of the SBS‐rich regions showed that the shape of the PS microdomains transformed from short cylinders to lamellae, hexagonally perforated lamellae (HPL), back to lamellae, short cylinders, and finally to spheres. DMS and DSC indicate a systematic increase in the PB glass transition temperature (Tg) and negligible change in the Tg of PS as the asphalt content increases. Triblock copolymers that can form a macro‐network at low concentration will be more desirable for highway pavement modification.
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