New methods have been developed which enable one to monitor in situ the changes in the dielectric and spectroscopic properties of epoxy materials as they undergo cure by microwave as well as by thermal energy. In situ measurements at 2.45 GHz of the complex dielectric constant and of the infrared spectra of a DGEBA/DDS system undergoing isothermal cure at a number of temperatures have been conducted. Results have shown that although microwave radiation accelerates the curing reaction during early stages of the process, the induced rapid cross-linking creates a molecular network which is rigid enough to trap unreacted functional groups, thus actually causing a lower degree of cure. The cross-linking in samples cured by microwave radiation is caused by an accelerated reaction of the secondary amine group whose reactivity becomes similar to that of the primary amine.

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The pure nqr resonances of the la7Ba and lS5Ba nuclei in BaClz.2Hz0 were observed and measured using a noise-controlled superregenerative spectrometer.2 The resonances were observed and measured using a 25-g sample of BaClz*2Hz0 with the two isotopes present in their natural abundance, 11.32% for la7Ba and 6.59% for la6Ba. Frequency measurements were made a t 300°K. The resonances could not be detected at 77°K. Frequency measurements were made by modulating the quench frequency at a rate of 5-10 Hz, thereby allowing the carrier component of the superregenerative oscillator to be audibly identified on a communications receiver. The spectrometer carrier frequency, fs, which is identified with the resonance by virtue of the spectrometer drive being stopped when the recorder pen is on the center peak of the recorder display, mixes with the variable-frequency oscillator (VFO) signal, f l , of the receiver to produce an intermediate frequency (IF) signal of frequency fs -11. This IF in turn combines with the beat frequency oscillator (BFO) signal, fb, in the receiver to produce an audible output. The VFO is then tuned to produce a null in the audible signal. At the null point (fs -f l ) -f b = 0 and the spectrometer frequency is found by measurement of both the BFO and VFO signals. Frequency measurements were made using a Hewlett-Packard 524C electronic counter. The audible null could be ascertained to an accuracy of f100 Hz. The over-all accuracy of the frequency measurements is limited by the procedure of stopping the spectrometer sweep exactly on an absorption-peak maximum.The pure nqr frequencies of la7Ba and la5Ba in BaClz-2Hz0 were found to be 16.303 f 0.005 MHz (signa1:noise ratio 10) and 10.67 f 0.02 MHZ (signal: noise ratio 5), respectively. The spectrum observed for each isotope was clearly indicative of a single resonance. The observation of a single resonance is in agreement with the known crystal structure of BaC12. 2Hz0, where each barium atom in the unit cell is surrounded simultaneously by equivalent distorted tetrahedra of chlorine atoms and distorted tetrahedra of water molecules.~~4 A search down to 5 MHz revealed no additional resonances. The identification of the resonances as being those of barium rather than chlorine was established from the ratio of the frequencies of the two observed resonances, 1.528 f 0.005. This compares favorably with the reported nuclear quadrupole moment ratio, Q(137Ba)/Q(1s6Ba) = 1.537.5 There is Table I: Nqr Parameters for BaClz. 2H20" -Calod valuesBa*+ Ba1.8+ c E x p t l valuesc--Parameter c1-Cl0.0-7 = 0 7 = 0.78

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