Heated Stage IR Microscopy for Cure Studies of Polyimide Matrix Composites1 S. Gan and J.C. Seferis2 Polymeric Composites Laboratory Department of Chemical Engineering University of Washington Seattle, Washington 98195, USA CONTENTS Page ABSTRACT 120 INTRODUCTION 120 EXPERIMENTAL 120 Materials 122 RESULTS AND DISCUSSION 122 Calibration of the Internal Standard at 1520 cm"1 122 Polyimide Prepreg Cure Monitoring 122 Monitoring Solvent Loss of the Prepreg 127 CONCLUSIONS 127 ACKNOWLEDGEMENTS 128 REFERENCES 128 1 Submitted for publication December 1990. 119 VoL Ζ No. Ζ 1992 Heated Stage IR Microscopy for Cure Studies of Polyimide Matrix Composites ABSTRACT In general, FT-IR can monitor the vibration of atoms and molecules associated with the formation and The use of an Infrared reflectance microscope for dissociation of chemical bonds. Although the use of following the cure of a polyimide prepreg was evaluated. infrared spectroscopy has been well established in cure With this method which incorporates a controlled heated kinetics of polyimides by a number of authors /5,6/, one stage to the IR microscope, the IR reflectance spectra of cannot study composites by this conventional method a prepreg sample can be obtained without any special because the carbon fiber or other reinforcing material preparation. A reasonable reproducibility and signal/ usually masks the IR spectra contributed by the neat noise ratio was obtained by aligning the sample stage resin portion of the composite. As a result, many before initiating the IR scans. The IR reflectance spectra potential applications of infrared analysis to composites of the polyimide prepreg generated by the microscope during cure, such as real-time interactions of fiber and were found to be in agreement with conventional resin matrix during cure, have been impractical or transmission spectra of the neat resin except for impossible. differences in the baseline and the shape of the bands. The kinetics of the curing reaction in polyimide prepreg Diffuse reflectance IR is a powerful method for was elucidated based on interpretations of the generated observing spectra of powdered samples. However, this reflectance spectra. In addition, a comparison of data method suffers when used in studying prepregs /7,8,9/. generated with neat resin and prepreg samples elucidated Some researchers have recently developed a new infrared the effects of carbon fibers on the cure kinetics of the analysis technique for composites using a mid-infrared polyimide matrix. transmitting optical fiber /10/. There are, however, dif- ficulties in this technique. One of them is still the lack of readily available optical fibers that are transparent to INTRODUCTION the IR beam. Also, because the matrix shrinks during cure and the optical fiber may separate from the matrix, The ultimate goal for monitoring the cure of any IR signal contributed by the matrix may be lost. advanced composite materials is to eventually develop closed-loop, on-line process monitoring and control. Recently, a different IR technique which com- Although the potential still exists for this, at present in- bines microscopy with the FT-IR spectrometer has been situ methods such as dielectric and acoustic analysis are applied to composites /11/. Very little or no advanced primarily used for process monitoring and characteriza- sample preparation is required. As a result, the benefit tion /1,2/. Although these methods are quite well of the IR microscope is to monitor the cure of the developed, they only monitor indirectly cure and reaction polymeric matrix in the presence of carbon fiber or other kinetic effects. The lack of detailed chemical kinetic reinforcements focusing on a specific area of the sample. information in process monitoring may be viewed as In this work, specifically, an evaluation of the IR mic- one of the obstacles in developing these techniques to roscope for monitoring cure of a prepreg sample was their full potential. Our approach has been to utilize undertaken. The effects of carbon fiber on a polyimide several techniques to monitor cure and develop a matrix were evaluated by comparing both neat resin and physically realistic kinetic model that incorporates prepreg data. chemical information about the polymeric systems under consideration /3/. Indeed for unreinforced thermoplastic systems, we have used successfully Fourier Transform EXPERIMENTAL Infrared Spectroscopy (FT-IR) to identify the polymer structural features and relate them to the processing A schematic diagram of the IR microscope used conditions employed in their manufacture /4/. is shown in Fig. 1. A 590 Infrared/Optical microscope 120 S. Gan and J.C. Seferis Science and Engineering of Composite Materials MCT DETECTOR EYEPIECE APERTURE LIGHT SOURCE INFARED BEAM SAMPLE Fig. 1: Schematic diagram of the Reflectance Infrared Microscope. (Bruker Analytische Messtechnik GmbH) interfaced with The infrared beam, as well as any residual laser beam, a Nicolet FTIR spectrometer (formally IBM IR 32) was was blocked and the light from the samples passed to the utilized. In addition a LINKAN TH 600 heated stage eyepieces. The desired sample area was identified and was utilized as an integral part of the setup. centered by means of the adjustments of the sample stage. To place the unit in the measurement mode, the The measurement procedure was as follows: The binocular eyepiece assembly was turned clockwise and binocular eyepiece assembly was swiveled counter- the infrared beam passed from the interferometer through clockwise and the visible radiation passed to the sample or onto the sample and into the detector. Using this either in the transmittance or in the reflectance mode. technique, spectra were taken between room temperature Vol. z No. Ζ 1992 Heated Stage IR Microscopy for Cure Studies of Polyimide Matrix Composites and 300 'C, at a heating rate of 1 "C/min. The results The scan region was from 1000 cm"1 to 2000 cm"1. are an average of 512 scans to 4 cm"1 resolution. Polyimide has significant bands within this region. It should be noted that the peak position and relative intensity are the same for both samples but the baseline Materials and the peak shapes are different. Important peaks were seen at 1720, 1662, 1527, 1319, 1290, 1226 and 1089 NR 150 polyimide resin was used as-received cm"1. This good quality, easily interpretable reflectance from E.I. du Pont de Nemours & Co. The solid content spectrum can provide a good base for kinetic studies. was 65%. The solvents for the resin were methanol and N-methylpyrolidone /12/. The amine component (a meta- and para-benzenediamine mixture) was used as- Calibration of the Internal Standard at received from E.I. du Pont de Nemours & Co. Prepreg 1520 cm'1 of the resin with T-300 carbon fiber was prepared using a laboratory scale process which has previously been The observed peak at 1520 cm"1 was assigned as described in detail /13/. Commercially available prepreg the absorption of the aromatic amine and was used as the of this polyimide system, Avimid N, was also tested internal standard. This calibration procedure is called selectively for comparison. "Univariate Standard Addition" /14/. The uncured NR 150 is a mixture of diamine, tetraacid and solvents. Three basic sample configurations were tested: Thus, it is necessary to calibrate the amount of amine in (1) A thin resin layer on a salt plate in conventional the presence of other components. This was accomp- transmission mode, lished by adding several known amounts of amine to (2) A resin layer on a glass plate in reflectance mode, uncured polyimide. The IR spectra were taken and the (3) Prepreg directly placed on the heated stage in peak height at 1520 cm"1 was measured. The calibrated reflectance mode. result is shown in Fig. 3. As can be seen, there is reasonable linearity between peak height and amine amount. It is also possible to find the original amount RESULTS AND DISCUSSION of amine in uncured polyimide by extrapolating the calibration line. The value of the horizontal intercept The IR microscope was used to perform the gave the original amount of amine. The amine content normal angle reflectance IR analysis. The reflectance measured by this technique agreed with the amount reaching the detector was found to be strongly dependent provided by the manufacturer. on the sample configuration which changed with respect to area, temperature, and even time. Because of this, poor reproducibility and low signal/noise ratio were Polyimide Prepreg Cure Monitoring frequently the result. This was overcome using the fol- lowing technique. The intensity of the signal reaching A piece of the prepreg without any sampling the detector had to be maximized before taking IR scans. preparation was placed on the heated stage under the IR In addition, the IR signal was kept at a relatively microscope. The temperature was increased at a rate of 1 constant value during the experiment by aligning the 'C/min. The IR reflectance spectra shown in Fig. 4 sample stage manually. Future studies have the poten- were obtained at 25 *C and 170 "C. Comparison of tial of being computerized with an automated stage. these two spectra demonstrates that the technique can be used to monitor cure of the prepreg. Comparing the Fig. 2 shows spectra obtained with the normal spectrum at 170 °C with the one at 25 'C, new bands - angle both in transmission and reflectance IR analysis. the results of imide ring forming after cure - are clearly 122 S. Gan andJ.C. Seferis Science and Engineering of Composite Materials WAVENUMBER Fig. 2: Comparison between transmission spectrum and reflectance spectrum for NR 150 (uncured polyimide resin). Add Amine (g) Fig. 3: Relative IR absorption at 1520 cm-1 as a function of phenylamine amounts added to NR 150 solution producing an Univariate Addition Standard plot. VoL Ζ No. Ζ 1992 Heated Stage IR Microscopy for Cure Studies of Potyimide Matrix Composites 0) Ο c 03 _Q Ο σ) -Ω < 0 > "-ι—' CD cc 2000 1800 1600 1400 1200 1000 WAVENUMBER Fig.