The Influence of Block Copolymers on Silica-Filled Polyisoprene
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The Influence of Block Copolymers on Silica-Filled Polyisoprene Daniel Gurovich1, Christopher W. Macosko1, and Matthew Tirrell2 1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A. 2 College of Engineering, University of California, Santa Barbara, CA 93106, U.S.A. ABSTRACT The influence of PDMS-polyisoprene diblock copolymers on the dynamic rheological properties of silica-filled uncured polyisoprene was investigated. Unlike binary silica-polyisoprene systems, the ternary systems in our study showed a decrease in low-amplitude dynamic storage modulus (G’) with room-temperature storage time. This aging behaviour is accompanied by an increase in the concentration PDMS segments at the filler-polymer interface, as measured by solid-state 1H NMR. The decrease in G’ is thus attributed to a progressive disruption of a filler-filler network by selective block copolymer adsorption onto the filler surface. INTRODUCTION The properties of filled polymeric materials depend on the balance between fillerfiller and polymer-filler interactions. The polar surface of silica particles interacts strongly with like polar surfaces, but not with the polyisoprene (PI) matrix [1]. Thus, silica is difficult to disperse in polyisoprene, and the resulting dispersions tend to reagglomerate upon aging. Polydimethyl siloxane (PDMS) strongly adsorbs to silica surfaces via hydrogen bonds [2]. Therefore, a PDMS-PI block copolymer is expected to anchor to a silica surface via its PDMS block and modify both filler-filler and polymerfiller interactions. In the present work we study the properties of mixtures containing precipitated silica, polyisoprene, and PDMS-PI copolymers of different compositions.
MATERIALS AND METHODS The basic experimental system consisted of colloidal precipitated silica (SiO2) dispersed in uncured polyisoprene. The silica (Zeopol® 8745) was obtained from J.M. Huber Corporation (Havre-de-Grace, MD) in the form of compressed pellets on the order of several millimeters in size. The specific surface area of the particles was 165 m2/g, as determined by the BET nitrogen adsorption method. In most samples, an anionically synthesized polyisoprene (LIR-50, from Kuraray Co., Japan) of number average molecular weight 49,000 g/mol was used as a matrix. Two control samples were prepared with an anionically synthesized polyisoprene of number average molecular weight 30,000 g/mol, provided by Adel Halasa of Goodyear Tire and Rubber Co. (Akron, OH). Three anionically synthesized PDMS-PI diblock copolymers were used as polymer-filler compatibilizers. Their characteristics are summarized in Table 1. KK7.3.1
Table 1 Designation of Polymer Total M.W. (g/mol) % PDMS 47k PI 47,000 0 30k PI 30,000 0 42k PDMS-PI 42,000 18 31k PDMS-PI 31,000 37 20k PDMS-PI 20,000 11 The filled polymer samples were prepared by mixing palletized silica with the polymer melt in a batch mixer equipped with roller blade rotors (System 90, Haake Buchler Instruments, Inc., Saddle Brook, NJ). Components were load
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