An Orientationally Ordered Hierarchical Exfoliated Clay−Block Copolymer Nanocomposite

Abstract

Creation of an orientationally ordered hierarchical nanocomposite can be done by the preferential insertion of individual polymer-covered clay layers into block copolymer (BCP) microdomainsin the present instance, PS-functionalized silicate layers into the polystyrene (PS) domains of a styrene−butadiene−styrene triblock copolymer (SBS). We present here the method required to achieve such an ordered block copolymer−nanoparticle composite and report preliminary mechanical properties. PS is grafted-to or polymerized-from individual clay sheets in order to provide a neutral enthalpic interaction between the PS chains on the clay layers and the PS blocks in the SBS. The molecular weight of the PS chains tethered to the clay was varied, and we show that higher molecular weight is critical in obtaining individual clay sheets dispersed in the SBS matrix. When exfoliated, the clay sheets serve as a template for the lamellar domains. Because of their large areal dimensions and concentration, the clay particles likely form a discotic nematic that is responsible for the observed orientational flipping transition of the BCP layers during the roll-cast process. We also show through experiments and simulations that the mechanical properties of these anisotropic hierarchical nanocomposites are not significantly changed by the insertion of 2 wt % of the clay sheets for deformation parallel to the lamellae.

Hierarchically Ordered Montmorillonite Block Copolymer Brushes

“…50 The most interesting phenomenon they studied was the se- brushes (MMT-PS) and once again found selective segregation into PS domains. 51 Although domain deformation to accommodate MMT-PS particles was much greater with MMT-PS than with Au-PS, the lamellar morphology persisted thanks to the compatibility of MMT plate-like geometry with the lamellar structure. The three dimensional matching of nanoparticle geometry with BCP domain geometry is known as symmetry matching and is an important consideration to develop useful nanocomposites.…”

“…The use of block copolymers in NCPs to date has been largely limited to a supporting matrix for the suspension of homopolymer functionalized inorganic fillers. [50][51][52] The most prevalent method has been the grafting of polystyrene to montmorillonite and solvent blending with a compatible block copolymer; BCP morphology in these cases is nearly always lamellar. The simple blending of clays with block copolymers has not yet yielded any greater degree of control of particle distribution than traditional homopolymer nanocomposites.…”

“…Recently work has focused on confinement effects of BCP assemblies in lithographic patterned channels [123][124][125][126][127] , oil-in-water emulsions, 128,129 and surface grafted nanoparticles. [130][131][132] Each of these phenomena can be viewed independently as specialized applications for self-assembled BCPs or collectively as the tools for hierarchically engineered materials. 133 Of primary concern to this work is the application of BCP selfassembly to nanoparticle alignment; due to the sparse amount of literature about BCP grafted nanoparticles it is 1 Graduate student and Assistant Professor, respectively, Department of Chemical and Biological Engineering, Iowa State University.…”

“…In an effort to produce matched anisotropic nanocomposites Ha et al grafted polystyrene (PS) chains to montmorillonite (MMT) platelets and solution-blended the resulting PS-MMT brushes with lamellar styrenebutadiene-styrene (SBS) triblock copolymer. 130 The dried solution was then roll-cast and demonstrated excellent in-plane orientation of the oblate MMT brushes within the SBS lamellar structure.…”

Preferential Association of Segment Blocks in Polyurethane Nanocomposites

“…Ha et al reported that PS-tethered Cloisite clay particles, which were exfoliated within a styrene-butadiene-styrene (SBS) block copolymer matrix and oriented using a rollcasting technique, templated the SBS morphology 20 . This clay platelet-induced morphology disrupted the alignment of the lamellar microdomains, resulting in minimal improvement in in-plane mechanical properties.…”