Table 11. Solvent Retention Absorbance at 7 . 1 micrometers Absorbance at Cure schedule (N-methyl 9 , 4 micrometers (cumulative) pyrrolidinone) (ethyl alcohol) As received 0.095 0.225 1 Hour at 200 OF (No vacuum“B” Stage) 0.065 0.218 1 Hour at 200 OF (Full vacuum) 0.065 0.218 i Hour at 275 “ F (Full vacuum) 0.067 0.170 21/2Hours at 340 “F (Full vacuum) Trace Trace 2 Hours of post cure at 400 “ F Trace Trace 2 Hours of post cure at 500 “F Not detected Not detected
Advanced Composite Materials. Data relating to the degree of polymerization are presented in Figure 5. Examination of the data points revealed that the bulk of polymerization occurred in the 21/2-hour interval at 340 O F . I n addition, polymerization continued at a nearly constant rate during the post-cure period (400-600 O F ) . The fact that a constant absorbance value was never reached indicated that longer post-cure periods would be required to achieve maximum polymerization. The initial absorbance value can be used to calculate the per cent advancement of as-received resin. In the illustrated example, the initial absorbance value was 0.075 and the maximum value obtained 0.565 (at 600 O F ) . The per cent advancement of the as-received resin consequently was 0.075/0.565 X 100 = 13.3 %; i.e., 13.3 % of the maximum possible polymerization obtainable with this cure schedule had occurred prior to heat treatment. In this case the solvents were present initially (prior to cure) and were volatilized during the cure cycle. Consequently, the absorption bands assignable to the solvents were at maximum values in the as-received material and diminished during the cure cycle. Table I1 shows absorbance values (of the absorption bands assignable t o the solvents) as a function of cure time. From the data it can be seen that all traces of detectable solvents were not removed until after the 500 O F interval was completed in the post-cure cycle. The most rapid rate of solvent removal occurred during the 2l/?hour/340 O F portion of the post-cure cycle. There has been a justifiable reluctance, however, to apply results obtained on thin resin films directly to thicker laminates, The optimum sample thickness for transmission infrared spectrometry is severalfold thinner than the typical
1068
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
laminate; thus, it is not possible to pass radiation directly through a typical laminate. A MIR technique which was evaluated consisted of mounting a half-thickness section of laminate (prepreg) directly on a MIR crystal, (the center of the laminate in contact with the crystal), exposing the coated crystal to the cure cycle under investigation, removing the crystal at various stages in the cure cycle, and scanning the selected spectral region. This mode of analysis yielded unrealistic data, probably because of heat transfer from the crystal t o the surface of the resin being analyzed. The M I R sampling mode could be optimized for characterization of the chemistry of the resin in the center of a laminate by burying a n MIR crystal in the center of a laminate, with one end exposed which could extend outside the curing oven. In situ measurements could then be performed at any stage in the cure cycle without interruption of the cure cycle. An instrument known as an infrared probe ( 5 , 6 ) has recently been introduced and may be suited for laminate curing evaluation. This unit is essentially a n M I R infrared sensing head, detector, and preamplifier assembly which is separated from the main unit. The sensor head (optical element) can be buried or dipped into a sample, with the optical element shielded except for the portion embedded in the sample. Other potential applications include determination of the rate of moisture or solvent penetration into the laminate, detection of degradation due to aging of the resin matrix, and solvent retention characteristics. The mode of analysis would minimize heat transfer into the center of the laminate and would offer the convenience of in situ measurements.
RECEIVED for review September 29, 1971. Accepted December 14,1971. (5) N. J. Harrick, ANAL.CHEM.,36, 188 (1964). (6) Zbid., 43, 1533 (1971).
Correction Indirect Spectrophotometric Determination of Nanomole Quantities of Oxiranes In this paper by H. Edward Mishmash and Clifton E. Meloan [ANAL.CHEM., 44, 835 (1972)l on page 836, column 1, line 14 should read “A range of 0 to 5 pmoles can be determined t o 1 2 0 nmoles.”
