Morphological mapping and analysis of poly[styrene-b(ethylene-co-butylene)-b-styrene] and its clay nanocomposites by atomic force microscopy

Atomic force microscopy was successfully applied for comprehensive nanoscale surface and bulk morphological characterization of thermoplastic elastomeric triblock copolymers: poly[styrene-b-(ethylene-(-o-butylene)-b-styrene] (SEBS) having different block lengths and their clay based nanocomposites. Commercially available Cloisite((R))20A and octadecyl (C-18) ammonium ion modified montmorillonite clay (OC) prepared in our laboratory by cation exchange reaction were used. The phase detected images in the tapping mode atomic force microscopy exhibited a well-ordered phase separated morphology consisting of bright nanophasic domains corresponding to hard component and darker domains corresponding to softer rubbery ethylene-cobutylene (PEB) lamella for all the neat triblock copolymers. This lamellar morphology gave a domain width of 19-23 nm for styrenic nanophase and 12-15 nm for ethyleneco-butylene phase of SEBS having end to mid block length ratio of 30:70 and block molecular weights of 8800-9.1,200-8800. On increasing the ratio of block lengths of the polymer matrix and the selectivity of the solvent toward the blocks used for casting, the morphological features of the resultant; films altered along with change in domain thickness. The phase images showed position and distribution of the brightest clay stacks in the dark-bright contrast of the base matrix of the nanocomposite. Exfoliated and intercalated-exfoliated morphology obtained in the case of Cloisite((R))20A and OC-based SEBS nanocomposites, respectively, is further supported by X-ray diffraction and transmission electron microscopy studies. The lamellar thickness of the soft phases widened to 50-75 nm, where the layered clay silicates (40-54 nm in length and 4-17 nm in width) were embedded in the soft rubbery phases in the block copolymeric matrix of the nanocomposite. The marginally thicker width of the hard styrenic phases and slightly shrinked width of the soft rubbery lamella can be observed from the regions where no nanofiller is present. Distinct differences in bulk morphologies of the nanocomposites prepared in the melt and the solution processes were obtained with nanocomposites. The presence of clay particles was evident from the almost zero pull-off and snap-in force in the force-distance analysis of SEBS based nanocomposite. This analysis also revealed stronger tip interaction resulting in highest contact and adhesive forces with the softer PEB region relative to the harder PS region. (c) 2006 Wiley Periodicals, Inc.