Polymer Lifetime Experiences

In this chapter, we will explore the effect of various environmental conditions and the polymers chemical composition and structure on its stability. We will examine the mechanisms associated with degradation and will explore several well studied polymeric systems. Finally, ve will discuss the role of additives and their effects on properties. 

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Fatigue resistance of polymers in a stress cracking environment is a complex topic where molecular variables have a strong influence. FCP experiments are a fast and effective method for determining the resistance to FCP in stress cracking environments and hence to predict polymer lifetime. Other mechanical testing methods have also been cited as they are somewhat more common. 

Thus, in order to reproduce the effect of an experimentally existing activation barrier for the scission/recombination process, one may introduce into the MC simulation the notion of frequency , lo, with which, every so many MC steps, an attempt for scission and/or recombination is undertaken. Clearly, as uj is reduced to zero, the average lifetime of the chains, which is proportional by detailed balance to Tbreak) will grow to infinity until the limit of conventional dead polymers is reached. In a computer experiment Lo can be easily controlled and various transport properties such as mean-square displacements (MSQ) and diffusion constants, which essentially depend on Tbreak) can be studied. 

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

In addition to the gas-phase work, we are computing (2) the vibrational spectra and rotational barriers of polymer fragments to help interpret experiments. By achieving a better understanding of polymers and their chemistry, we hope to design longer lifetime and more corrosion resistant polymers. 

The strength of a fibre is not only a function of the test length, but also of the testing time and the temperature. It is shown that the introduction of a fracture criterion, which states that the total shear deformation in a creep experiment is bounded to a maximum value, explains the well-known Coleman relation as well as the relation between creep fracture stress and creep fracture strain. Moreover, it explains why highly oriented fibres have a longer lifetime than less oriented fibres of the same polymer, assuming that all other parameters stay the same. 

The time necessary for removing one monolayer during a SIMS experiment depends not only on the sputter yield, but also on the type of sample under study. We will make an estimate for two extremes. First, the surface of a metal contains about 1015 atoms/cm2. If we use an ion beam with a current density of 1 nA/cm2, then we need some 150 000 s - about 40 h - to remove one monolayer if the sputter yield is 1, and 4 h if the sputter rate is 10. However, if we are working with polymers we need significantly lower ion doses to remove a monolayer. It is believed [4] that one impact of a primary ion affects an area of about 10 nm2, which is equivalent to a circle of about 3.5 nm diameter. Hence if the sample consists, for example, of a monolayer film of polymer material, a dose of 10n ions/cm2 could in principle be sufficient to remove or alter all material on the surface. With a current density of 1 nA this takes about 1500 s or 25 min only. For adsorbates such as CO adsorbed on a metal surface, we estimate that the monolayer lifetime is at least a factor of 10 higher than that for polymer samples. Thus for static SIMS, one needs primary ion current densities on the order of 1 nA/cm2 or less, and one should be able to collect all spectra of one sample within a quarter of an hour. 

This publication is particularly concerned with applications where a numerical prediction of lifetime is attempted. With traditional materials such as brick, which is much the same as it was fifty or a hundred years ago, prediction would be based on experience. For short-term applications of polymers this is increasingly the case. For longer-term applications, however, this is impossible, since fifty years ago many of the plastics which are common nowadays were only emerging from the laboratory, and the stabilisers which prolong their lives so dramatically had not been developed. 

The principal observation, which the laboratory scientist should not forget, was that most lifetime assessment of polymers is based on experience from service. Many of the respondents to the survey report examining parts taken from service at the end of life, or those that failed during warranty. Service experience is the principal source of information for the definition of insured lifetimes for polymer components in the construction industry. 

In summary, practical experience with predicting the hydrolytic degradation of polyethylene terephthalate is an example of the use of Arrhenius extrapolation, a demonstration of the problems encountered when there are changes in the state of the polymer as the temperature is raised, and an example of the large variability in prediction of lifetime due to the logarithmic scale. 

In practice, most lifetime prediction is based on service experience. Depending on the industry concerned, this can take the form of planned examination of components at the end of their service life or be limited to the explanation of warranty returns. Experience with polymers is now sufficiently long for service experience to be a prime source of information for components with lifetimes of up to 35 years. The construction industry provides a good example of systematic listing of component lifetimes, related to minimum quality levels and modified according to the service conditions. The electrical industry applies statistical methods to life components and predict failures. This, however, strays into the general field of engineering component lifetimes. In this book we are concerned with materials rather than components. 

Irradiation of a PMMA film containing ITX results in rapid photoreduction of ITX as is shown in Figure 5. Similar experiments in a deaerated benzene solution containing PMMA and ITX also result in photoreduction of ITX, albeit at much lower rate. The most likely interpretation of these observations is that triplet excited ITX abstracts hydrogen from the polymer to form a ketyl radical and a macro alkyl radical. The higher photoreduction rate in the film may result from deactivation reactions of triplet excited ITX which are faster in solution than in a glassy film. The triplet lifetime for ITX in a PMMA film is 1 3 sec compared with 10-4 sec in a benzene solution (20). In solution triplet-triplet annihilation... 

Thermal stability as measured by these ramped TGA experiments of the sort previously described are not the definitive test of a polymer s utility at elevated temperature. Rather, for a polymer to be useful at elevated temperatures, it must exhibit some significant retention of useful mechanical properties over a predetermined lifetime at the maximum temperature that will be encountered in its final end use application. While many of the bisbenzocyclobutene polymers have been reported in the literature, only a few have been studied in detail with regards to their thermal and mechanical performance at both room and elevated temperatures. Tables 7-10 show some of the preliminary mechanical data as well as some other physical properties of molded samples of polymers derived from amide monomer 32, ester monomer 40, diketone monomer 14 and polysiloxane monomer 13. The use of the term polyamide, ester etc. with these materials is not meant to imply that they are to be regarded as merely modified linear thermoplastics. Rather, these polymers are for the most part highly crosslinked thermosets. 

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