Polymer Degradation and Stabilization

Abstract

IntroductionMost synthetic plastics and natural polymers are principally constituted of the elements C, H, N, and O, all of which are primary components of organic molecules. As with smaller organic molecules, these atoms are attached by relatively weak covalent bonds that are susceptible to chemical attack, particularly to oxidative degradation. During their potential lifetime, engineering plastics are exposed to diverse environmental factors which can act alone or in combination to adversely affect the initial material properties. These changes generally occur over several years, but can be accelerated or retarded in the presence of internal and external elements temperature, and temperature variations. Environmental factors that contribute to degradation include mechanical stresses, temperature variations, humidity, sunlight, oxygen, atmospheric pollutants, and microbial enzymes. Depending on the structural level at which material changes occur, it is usual to distinguish between physical, physicochemical, and chemical degradation.Physical degradation, commonly known as physical aging, results from changes in the thermodynamic state of the system (temperature, pressure, molar volume) during the daily use of the material. During processing, chains may be oriented and the sample inhomogeneously cooled, resulting in internal stress. With time, the chains have the opportunity to relax from their nonequilibrium conformations to a state of lower free energy. Partly immiscible blends, for instance, can remain in a homogeneous metastable state for an extended period of time. The presence of stress or heat can accelerate the phase separation process. The resultant changes in orientation, free volume, internal stress, crystalline content, and

Exhaustive Baeyer–Villiger oxidation: a tailor-made post-polymerization modification to access challenging poly(vinyl acetate) copolymers

“…For example, acid-induced radical generation 13 which is reported to be able to initiate even at low temperatures could cause significant chain scission. In addition, oxidative decomposition 12 instigated by excessive oxidants, or Aldol-type condensations 11 promoted by acid or base could also complicate the proposed PPM. A combination of a literature survey and our preliminary results suggested that a comprehensive evaluation of PMVK under BV conditions was required.…”

“…We were aware that significant challenges might occur during the development. Although small-molecule BV oxidation 10 protocols were rich in literature, multiple side-reaction pathways such as aldol reaction/condensation, 11 undesired oxidation 12 and radical cleavage 13 were known to be associated with BV oxidation. Such side reactions could introduce defects into the polymer backbone and negatively affect the molar mass and dispersity ( Ð ) of the polymers.…”

Molecular Pathways for Polymer Degradation during Conventional Processing, Additive Manufacturing, and Mechanical Recycling

“…Ketones and aldehydes formed by β-scission may react further with hydroperoxides to form other oxidation products. For instance, the formation of peracids proceeds via a multi-step oxidation of aldehydes [65]. Oxygenated functional groups can be formed even in carbon main chain polymers, such as polyolefins [66,67] or styrene-based materials [68].…”

“…Figure 12a displays a degradation initiation with the radical formed reacting with oxygen in Figure 12b. Hydroperoxide is formed by abstracting a hydrogen from a tertiary carbon, which has a lower activation energy than an abstraction reaction on a secondary carbon [65]. This preferred abstraction can be intermolecular (Figure 12c) or intramolecular (Figure 12d).…”

“Exhaustive” Baeyer-Villiger Oxidation of Poly(Methyl Vinyl Ketone) and Its Copolymers

“…For example, acid-induced radical generation 55 which is reported to be able to initiate even at low temperatures could cause significant chain scission. In addition, oxidative decomposition 61,62 instigated by excessive oxidants, or Aldol-type condensations 63−65 promoted by acid or base could also complicate the post-polymerization modification. A combination of a literature survey and our preliminary results suggested that a comprehensive evaluation of PMVK under BV conditions was required.…”