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42355-AC7
The Effect of Compressive Stress on the Photochemical Degradation of Polymers

David R. Tyler, University of Oregon

The goal of this research is to explain why compressive stress affects the rates of polymer photodegradation. While carrying out our initial compression experiments, we found that parameters other than compressive stress were affecting the rates of photodegradation of the polymers. In order to satisfactorily interpret the results of the compressive stress on the degradation rates, we found it was necessary therefore to probe these other experimental parameters before intrepreting the results of compressive stress.

The first problem we encountered was that accurate quantum yield measurements are difficult to obtain for reactions in the solid-state. To overcome this problem, we built a computer-controlled device that simultaneously irradiates and spectroscopically monitors samples in the solid state. The apparatus measures and records the absorbed light intensity as a function of time and also records a reference (unabsorbed) light intensity. A key feature of the apparatus is that the same light beam is used to irradiate and simultaneously monitor the reaction. The polymer films can thus be left in one place, and consequently there are no “positioning errors” associated with these measurements. Another important feature is that a collimated light source (i.e., laser) is used to irradiate the sample, thus reducing errors attributed to inconsistencies in the “area of irradiation” of the sample. The remarkable feature of our computer-controlled device is that it is based on a simple Nd:YAG diode laser pointer. Thus, it is cheap to build and should find extensive use and applications by other research groups. The data obtained with the Nd:YAG diode laser system show far less scatter than data obtained with a high-pressure Hg arc lamp, and consequently the degradation rates and quantum yields obtained with the laser system could be calculated with far greater accuracy.

We also found that precise temperature control of the polymers was necessary in order to obtain good degradation rate data. Consequently, the effect of temperature on the degradation quantum yields of a poly(vinyl chloride) polymer with Cp₂Mo₂(CO)₆ units incorporated into its chains was studied (Cp = cyclopentadienyl). The polymer is photochemically reactive in the absence of oxygen because the CpMo(CO)₃ radicals formed by photolysis of the Mo-Mo bonds react with C-Cl bonds to form CpMo(CO)₃Cl units. Quantum yields as a function of temperature were obtained for this polymer and for two control systems, Cp'₂Mo₂(CO)₆ dispersed in PVC and Cp'₂Mo₂(CO)₆ in hexane/CCl₄ solution (Cp' = C₅H₄CH₃). The quantum yields of the two control systems showed only slight increases with an increase in temperature. For the reaction in hexane/CCl₄, this temperature dependence is attributed to the decrease in viscosity of the solution and the subsequent decrease in the radical-radical recombination efficiency. For the Cp'₂Mo₂(CO)₆ dispersed in PVC, the small temperature dependence is attributed to an increase in free volume as the temperature increases. In contrast to these results, the temperature dependence of the quantum yield of the PVC polymer with Cp₂Mo₂(CO)₆ units along its chains is relatively large. It is proposed that an increase in temperature facilitates the polymer chain relaxation processes (involving recoil and rotation) following photolysis of the Mo-Mo bond. The radical-radical recombination efficiency is subsequently decreased, which leads to a net increase in chain cleavage and degradation efficiency.

The ability to obtain accurate quantum yields with our computer-controlled device that simultaneously irradiates and spectroscopically monitors samples in the solid state, combined with our understanding of thermal effects on degradation rates, means that we can now begin to interpret the results of our compression studies on polymer degradation rates. These studies are underway.

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