Electromagnetic Processing of Polymers: II. Quantitative Investigations of Microwave Processed Thermoplastics (Microwave
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II.
ELECTROMAGNETIC PROCESSING OF POLYMERS: QUANTITATIVE INVESTIGATIONS OF MICROWAVE PROCESSED THERMOPLASTICS (MICROWAVE CALORIMETRY)
M. CHEN*, M. A. ZUMBRUM, J. C. HEDRICK, J. E. MCGRATH AND T. C. WARD** 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: Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, People's Republic of China **To whom correspondence should be addressed.
ABSTRACT The microwave heatability of various thermoplastic polymers was investigated. The concept of microwave calorimetry was proposed to quantitatively illustrate how viscoelastic behavior controlled microwave heatability. Specifically, heating rate as a function of sample temperature revealed a distinct maximum which was identified as the Tg at 2.45 GHz. The critical temperature, •c T , necessary for rapid microwave heating was identified by drawing a tangent to the heating rate curve and extrapolating to a critical value at zero heating rate. In separate experiments, low frequency (100 kHz) dielectric measurements were made which show the frequency dependence of Tg by means of Arrhenius activation energy plots. In general, the larger the activation energy, the closer the critical heating temperature, T , was to the Tg determined by DSC. The smaller the activation energy, •he further dielectric loss shifted with increased frequency so that T was very far from Tg determined by DSC. c
INTRODUCTION Microwave radiation in tuned waveguides has been successfully used to process numerous reactive [1-17] and non-reactive [17-22] polymers. In this paper, a better understanding of microwave processing of non-reactive systems was achieved by using the proposed concept of microwave calorimetry in conjunction with low frequency dielectric measurements. This technique was conceived based on Equation 1 for capacitive heating of non-reactive polymers [23]: dT/dt = K w E 2c'
(T)
tan 6 a (T)
/
p Cv
(I)
where dT/dt - heating rate, K = constant, w = applied frequency, p polymer density, •V C - heat capacity, E = electric field strength and (0' tan 6 a ) = dielectric loss, assuming absence of convection losses or Since the sample heating rate was directly conductive heating. proportional to the dielectric loss, low frequency dielectric analyses were conducted to predict the shift of the dielectric loss into the microwave region. The presence of this relaxation is necessary for the conversion of electrical into thermal energy. The dipolar dielectric loss of polymeric materials depends very strongly on temperature and frequency [24-26]. Schematically, Figure 1 depicts the temperature and frequency dependence of the Tg (a transition) and a lower temperature secondary transition (B transition). As frequency is Mat. Res. Soc. Symp. Proc. Vol. 189. c1991 Materials Research Society
432
increased, the B transition shifts to higher temperature m
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