Modification of Montmorillonite with PEO-b-PS Copolymers
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Modification of Montmorillonite with PEO-b-PS Copolymers Frederick L. Beyer and Nora C. Beck Tan Polymers Research Branch, U.S. Army Research Laboratory Aberdeen Proving Grounds, MD 21005, U.S.A. ABSTRACT Wyoming montmorillonite has been modified with block copolymers of poly(styrene) (PS) and poly(ethylene oxide) (PEO). Using melt intercalation techniques, PEO-b-PS copolymers spanning a range of volume fractions and molecular weights were mixed with montmorillonite, annealed above the Tg of PS, and characterized using X-ray diffraction. Most of the block copolymers intercalated the clay, with several samples exhibiting multiple Bragg reflections from increased gallery spacing. Samples with higher PEO volume fractions were most likely to have intercalation. For a given PS block length, longer PEO blocks were more likely to produce intercalation. In general, the increased gallery spacings were close to those produced by modifying montmorillonite with PEO homopolymer. INTRODUCTION Polymer-layered silicate nanocomposites have been fabricated using a variety of polymer matrices and montmorillonite, a 2:1 layered silicate, as the most common filler [1]. Layered silicate clay mineral fillers are commonly modified with small-molecule surfactants, such as octadecylamine. Such surfactants facilitate intercalation by screening the polymer from hydrophilic silicate layer surfaces and lowering the entropic penalty for intercalation. The effectiveness of such surfactants is limited by their length and by the enthalpic interactions between the surfactant and intercalating polymer or monomer [2-5]. Certain hydrophilic polymers such as PEO have been shown to readily intercalate unmodified layered silicates [6]. Rather than displacing interlayer cations naturally found in layered silicates, PEO is thought to solvate the cations and form a helical structure within the silicate galleries [7]. In this work, the ability of PEO to intercalate layered silicate galleries has been used to intercalate amphiphilic diblock copolymers of PS and PEO. Analogous to the modification of layered silicates by small-molecule surfactants, the polar PEO block is drawn to the silicate layer surface and into the silicate gallery. If successful, this technique would allow the modification of layered silicates to render them more susceptible to intercalation by engineering thermoplastics than currently possible with small-molecule surfactants. EXPERIMENTAL Na-montmorillonite from Crook County, Wyoming, (SWy-2) was obtained from the University of Missouri Source Clays Repository. The pristine clay mineral was cleaned by dispersion in deionized water, centrifuging to separate the dispersed clay from impurities, and then drying in a vacuum oven overnight. The clay was then ground into a coarse powder using a mortar and pestle. Low polydispersity PEO homopolymers and PEO-b-PS diblock copolymers were obtained commercially, spanning a broad range of molecular weight and volume fraction.
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