The weathering of polymers is reviewed with attention concentrated on the mechanisms of degradation and stabilization, the methods of testing weatherability, and the predictive modelling of weathering behaviour. An introduction to the chemical mechanisms of degradation and stabilization is given and reference made to some of the many reviews available in the literature. Significant emphasis is placed here on engineering aspects, such as the way that weathering influences fracture mechanisms. The difficulties associated with relating accelerated laboratory tests with outdoor service behaviour are discussed. The complexities of the degradation processes limit modelling of the rate of degradation to rather specific systems and the predictions cannot be generalized.

Weathering of polymers: mechanisms of degradation and stabilization, testing strategies and modelling

Cavitation during deformation of semicrystalline polymers

Mechanochemistry of polymers

Electrical aging and relaxation in solid dielectrics are phenomena studied for decades but still largely poorly understood. The lack of understanding, or sometimes the misunderstanding, are partly due to inadequate or deficient theories and partly due to wrong interpretation of theories or concepts developed for other materials than dielectrics. In addition, a serious analysis of some of the empirical and approximate relationships often used to interpret polymer relaxations reveals that they are not only limited and restrictive but also that they are based on very dubious assumptions. Dry or wet electrical aging is much more complex and often includes many phenomena with synergetic effects such as thermal aging, electrochemical reactions, space charge effects, etc. In this lecture, we intend to show that some of the theories often cited in the literature are wrong or inappropriate. We show why they cannot describe the phenomena at play and we present some alternatives for future work in the domains of relaxation and electrical aging. The relations between aging, space charges and polarization of polyethylene (PE) under the influence of high electrical fields (above 20 kV/mm) are discussed in light of our electrical aging model. It is our contention that strong charge injection occurs only after nanocavity formation, i.e. above the critical field. The amorphous phase is then significantly deformed and weak van der Waals attraction bonds are broken, leading to another faster aging regime. The possible relation between the nanocavity formation at moderate fields and bonds breaking at higher fields proposed in our aging model and various polarization measurements is discussed. One objective of this paper is to encourage the development of more complex and complete theories specific to dielectrics. Some experimental work needed to achieve this goal is pointed out.

On the interpretation of some electrical aging and relaxation phenomena in solid dielectrics

Electroluminescence phenomena were observed at the needle tip in polymer samples with a point-plane electrode system under an applied ac voltage at liquid nitrogen temperatures. This luminescence was considered to be caused by the recombination between positive and negative charges injected from the point electrode. The nature and behavior of the charges injected into the polymer became clear through a detailed study of luminescence. A degenerate rate region of polymer was also found at the needle tip after prolonged ac voltage application, which may indicate a depolymerized region caused by the attack of then injected charges. The fact that tree channels started from the degenerate region in the polymer led us to theon conclusion that the injection of charge can be hazardous for polymer solid insulation systems.

The Space Charge Behavior and Luminescence Phenomena in Polymers at 77 K

Submicrocracks, free radicals, and endgroups of scissioned molecules formed in polyethylene, polypropylene, and polycaprolactam under uniaxial tension have been investigated. Measurements were carried out by small-angle x-ray scattering, electron paramagnetic resonance, and infrared spectroscopy. The concentration of submicrocracks is almost the same as that of free radicals but is smaller than the concentration of scissioned macromolecules by approximately three orders of magnitude. The number of scissions per crack proved to be close to the number of macromolecules passing through the cross section of a submicrocrack calculated on the assumption of close packing. It is concluded that submicrocracks in stressed polymers are formed as a result of chain reactions of macromolecular decomposition initiated by the active end primary free radicals.

S. N. Zhurkov

Papers

Mechanism of submicrocrack generation in stressed polymers