Gradient porosity poly(dicyclopentadiene)
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This article describes the preparation of gradient porosity thermoset polymers. The technique used is based on polymerizing a solution of cross-linkable dicyclopentadiene and 2-propanol. The forming polymer being insoluble in 2-propanol, phase separation occurs. Subsequent drying of the 2-propanol gives porosities up to 80%. An apparatus was built to produce a gradient in 2-propanol concentration in a flask, resulting in polymerized gradient porosity rods. The resulting materials have been characterized by scanning electron microscopy (SEM) and density measurements. A mathematical model which allows prediction of the gradient produced is also presented.
I. INTRODUCTION
Functional graded materials are materials engineered with gradual transitions in microstructure and/or composition. The motivation for producing gradient materials comes from functional performance requirements varying with position within the element.1,2 Graded metals and metal–ceramic composites have been widely studied for extremely demanding structural applications, such as aerospace or internal parts of high performance engines.3 Indeed such materials were mainly designed for increased resistance to thermal stresses. The different methods known to produce gradient materials can be classified in two categories: constructive processes and transport based processes.2 The first type consists in building the gradient, layer by layer, with the appropriate distribution of constituents. The second relies on natural transport phenomena such as diffusion or heat transfer to create the gradient. Gradient porosity materials have been produced mainly using spray coating4 or powder technology.5–7 Other reported techniques include selfpropagating high-temperature synthesis8 or electrochemical modification of porous preforms.9 Functionally gradient composites containing polymers have also been investigated. For example an epoxy/ carbon fiber gradient composite has been produced using a centrifugal method,10 and polyimide/aluminum and polyimide/copper gradient composites have been made by powder technology.11,12 For polymer/polymer systems the most important application is gradient refractive index optical fibers.13 However, very little work has been
a) b)
Address all correspondence to this author. Present address: The Ångstrom Laboratory, Uppsala University, S-75121 Uppsala, Sweden. J. Mater. Res., Vol. 16, No. 7, Jul 2001
published in this area, and attention must be paid to terminology. The expression gradient polymers designates either a gradient in concentration of one monomer along a single chain of copolymer (also more adequately termed gradient copolymers),14 –16 or functionally graded polymeric materials (FGPM). The main techniques for producing FGPM are diffusion polymerization14 –17 and powder technology combined with compression molding.18 The first is limited to thin parts, and the second is difficult to apply to polymers and limited, for compaction reasons, to symmetrical shapes. Graded porosities are nevertheless quite common in polymers, den
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