15 The Application of Chemiluminescence for the Study of 1
Polymer Mechanochemistry
D. L. FANTER and R. L. LEVY McDonnell Douglas Research Laboratories, McDonnell Douglas Corp., St. Louis, MO 63166
Stress is an important parameter in the service environment of loadbearing polymeric materials (1). The accelerating effect of stress on polymer-aging reactions is recognized; however, no experimental methods exist for direct determination of the effect of stress on the rates of real-time aging reactions. Chemiluminescence offers potential for direct determination of the rates of polymer aging reactions. Exoergic chemical reactions that are accompanied by emission of electromagnetic radiation are called chemiluminescent (2). The chemiluminescence is proportional to the reaction rate, and the spectral distribution provides information on the reaction mechanism (2). The chemiluminescence of polymers observed during thermooxidative degradation is potentially a useful method to monitor polymer aging reactions at temperatures corresponding to the real service environment (3,4,5). However, the studies on chemiluminescence of polymers conducted to date (3,4,5) have been performed in the absence of stress. It was, therefore, necessary to design a special chemiluminescence system capable of studying stressed polymers. This paper describes the design of a stress-chemiluminescence system and discusses exploratory studies on stress-enhanced chemiluminescence of epoxies and nylon 66. Experimental A unique chemiluminescence photon-counting system, designed specifically for measurement of stress-enhanced polymer reactions, was used for the studies described here. This system includes a miniature tensile stress device, a controlled environment chamber and a luminescence detection system and associated optics integrated into a single unit. ^This research was conducted under the McDonnell Douglas Independent Research and Development Program. 0-8412-0485-3/79/47-095-211$05.00/0 © 1979 American Chemical Society
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The chemiluminescence instrument, shown schematically i n Figure 1, measures the l i g h t emitted from polymers during exposure to selected combinations of stress, temperature and atmosphere. The system i s designed to measure alternately the l i g h t generated by two different samples; therefore, two sample heaters and dual o p t i c a l systems are required. The sample chamber, i n addition to being l i g h t - t i g h t , i s also vacuum-tight to control the sample atmosphere. Sample temperatures are maintained by heater pads which are spring-loaded to provide uniform contact during tensile stress of the sample. The two heaters are controlled to within ± 1° and are housed i n a ceramic block for thermal i s o l a t i o n . Tensile loads are applied to one polymer sample by a stress apparatus which includes a small, synchronous gearmotor driving a wormgear attached to one of the tensile grips. A force trans ducer i s mounted between a fixed framework and a second tensile grip. The tensile system i s capable of exerting forces up to 800 N, s u f f i c i e n t to fracture the samples used i n these studies. The s i l i c a optical system conducts collected l i g h t through lenses and l i g h t pipes to a scanning mirror. The l i g h t from one of the o p t i c a l channels passes through a f i l t e r wheel and i s pro jected onto the photocathode of the photomultiplier tube (PMT). The PMT (EMI, 9829QA) was selected to maximize quantum efficiency over a broad spectral range. The spectral d i s t r i b u t i o n of the chemiluminescence can be measured using a series of wheel-mounted f i l t e r s inserted i n the o p t i c a l path to determine the intensity of luminescence emitted at wavelengths above the cutoff wavelength of the individual f i l t e r . The f i l t e r s used for this study are colored glass having sharp cut-offs at 300, 400, 500, 600, and 700 nm. The output from the PMT i s processed by an amplifier/discriminator and photon counting system (Princeton Applied Research model 1112 and 1121). Polymer samples were designed s p e c i f i c a l l y for use i n the chemiluminescence instrument. The samples are basically a tensile dogbone shape with c y l i n d r i c a l ends for insertion into the grips of the stress apparatus. Epoxy specimens were prepared from tetraglycidyl 4-4 diaminodiphenylmethane (TGDDM) cured at 170°C with 4-4 diaminodiphenylsulfone (DDS). Tensile samples were pre pared by a special method based on curing the TGDDM-DDS resin mixture i n s i l i c o n e rubber molds. This method casts reproducible epoxy specimens free of contamination.(6) 1
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Results and Discussion Experiments on the stress-chemiluminescence (SCL) behavior of TGDDM/DDS epoxy and nylon 66 specimens were performed under various conditions to determine the potential of the technique to measure stress-induced and stress-accelerated aging processes. Representative results of t y p i c a l SCL experiments with epoxy specimens are shown i n Figures 2 and 3 where emitted l i g h t (photon counts/s) i s plotted as a function of time and stress. In both
15.
FANTER AND LEVY
Polymer Mechanochemistry
Figure 1.
Stress chemiluminescence system
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