Predictions for polymers in oxidative environments

A Nonempirical Model for the Lifetime Prediction of Polymers Exposed in Oxidative Environments... 

Immersions of polymer microtensile specimens in solutions of metal ions or lipid emulsions at elevated temperatures for 16 weeks have been reported [32]. Temperatures of 37, 70, and 90°C are recommended with sampling monthly to establish trends. For pacemaker leads, the solutions should include all the metals found within the device, a base, and an acid. For example, aqueous solutions of 1 M AgNOs or 0.1 M C0CI2 (acetylacetanoate) can assess oxidation. Immersion in 1 N acetic acid. Ringer s solution, and 1.0 N HCl can assess hydrolytic resistance. Immersion in 20% intralipid (soybean) emulsion can assess the propensity to absorb lipids. However, in our experience, none of the above in vitro tests appear to be reliably predictive of performance in pacemaker lead insulation. Why The in vivo environment cannot be duplicated in vitro. For example, the oxidation state of an ion varies as a function of what it is dissolved in. Distilled water containing a metal ion does not represent the environment within a lead. This accelerated test predicts that Ag+ will oxidize and degrade polyether polyurethanes while Co will not. Multiple in vivo studies clearly demonstrate exactly the opposite [14, 33]. Traces of cobalt will degrade the polymer in vivo whereas silver will not. 

For the DTO model we must have an estimate of the torsional vibration frequency and the barrier to internal rotation of the constituent monomers. The DTO model fits the experimental data for bulk polymer if H = 5.4 kcal/mole, vt — 1012 c.p.s., and Zt = 30 which are not unreasonable values. One would expect the barrier height to decrease upon dilution (if it changes at all) as the chain environment loosens up. Assuming that rotation about C—O—C bonds is predominate, we take the experimental values of H = 2.63 kcal/mole, vt = 7.26 x 1012 c.p.s. of Fateley and Miller (14) for dimethyl ether. Eq. (2.8) predicts rSJ° = 0.47 X 10-8 sec at 253° K with Zt = 30. We shall use this as our dilute solution result. [The methyl pendant in polypropylene) oxide will act to increase the barrier height due to steric effects, making this calculated relaxation time somewhat low for this choice of a monomer analog.] Tmax is seen to change only by a factor of 102—103 upon dilution in the DTO model. 

On the other hand, the highest LUMO is that of polyethylene oxide (PEO). PEO and polypropylene oxide (PPO) are very familiar as polymer electrolytes. According to this calculated result PEO and PPO can be stable in an anodic environment but may not be stable in a cathodic environment because the HOMO of both polymers is very high. In addition, PEO and PPO are soluble in organic solvents and cannot be used as a binder. The LUMO of polyethylene (PE) is high and we expect it to be stable in an anodic environment. But it is difficult to dissolve PE in organic solvents. The LUMOs of PVdF and SBR are almost the same as that of PE, and thus these compounds may be used as a binder. So, the prediction based on the theoretical calculations is consistent with the actual choice of binder for both electrodes. 

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