Electromagnetic Processing of Polymers: I. Basic Concepts and Molecular Design of the Macromolecules
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I.
ELECTROMAGNETIC PROCESSING OF POLYMERS: BASIC CONCEPTS AND MOLECULAR DESIGN OF THE MACROMOLECULES
J. C. HEDRICK, D. A. LEWIS*, T. C. WARD AND J. E. MCGRATH** Department of Chemistry, NSF Science and Technology Center: High Performance Polymeric Adhesives and Composites, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212 *Current address: IBM Research, T. 218, Yorktown Heights, NY 10598 **To whom correspondence
J.
Watson Research Center;
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0. Box
should be addressed.
ABSTRACT Microwave processing has been utilized to process thermosetting polymeric materials. Specifically, fundamental studies relating epoxy network generation to processing conditions have been investigated in a tunable cylindrical cavity operating at a frequency of 2.45 GHz. These studies demonstrate that fully cured networks can be generated in ten minutes with the retention of good mechanical properties. Furthermore, toughened epoxy systems which utilize carefully designed amine terminated poly(arylene ether sulfone) thermoplastics as reactive oligomers have resulted in novel phase separated morphologies. In fact, it has been demonstrated that the morphology in these multiphase systems can actually be controlled by utilizing microwave processing. Bismaleimide toughened systems, devised by similar strategies, have demonstrated a 10-20 fold reduction in the time required to achieve full cure.
INTRODUCTION The utilization of microwave radiation to process polymeric materials has been demonstrated in our laboratory [1-8] and elsewhere [9-18]. Accelerated reaction rates have been reported in many systems such as epoxy resins [1,5,9-11], imidization reactions [4], crosslinking reactions of terminally functionalized thermoplastics [1,2], and urethane polymerizations [16]. The mechanism of the accelerated reaction rates is uncertain. Microwave radiation (MR) unlike higher energy forms of radiation is not energetic enough to cause ionization or chain scission in polymeric materials; thus, the chemistry which occurs may be identical to that observed during conventional thermal processing. Two proposals have been put forth to explain the enhanced reaction rates. The first attributes the acceleration to the novel mechanism of energy transfer with electromagnetic processing. Typically, polymers are processed in a thermal environment where heat is slowly conducted from the sample boundary to the sample interior. Microwave radiation, alternatively, results in uniform and rapid volumetric heating due to the interaction of the electromagnetic radiation with the permanent dipole moments in the polymer. Thus, polymers can be heated significantly faster while maintaining good thermal uniformity. The second proposal, put forward by Lewis et al. [4], attributes the accelerated reaction rates to localized heating on the atomic level. This postulate was based on kinetic studies of a model solution imidization reaction and predicts a temperature enhancement around the dipole moments which couple with the radiation. The locali
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