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Degradation Studies of Biodegradable Composites
Published in Arbind Prasad, Ashwani Kumar, Kishor Kumar, Biodegradable Composites for Packaging Applications, 2023
Over the last six decades, several studies have been done in an attempt to better understand how polyolefin degradation occurs. Thermal degradation of polymers is a complex process that involves the simultaneous synthesis and breakdown of hydroperoxides. Degradation is accelerated by oxygen, dampness, and straining, resulting in brittleness, cracking, and fading. Random degradation of polyethylene occurs when a hydrogen atom migrates from one carbon to the next, resulting in additional fragments. The dynamics of PP and PE degradation are crucial because they show how a complicated radical chain reaction works, as well as how waste incineration and other recycling processes work. A sequence of reactions involving chain scission are the effects associated with the degradation of polymers when subjected to elevated temperatures. Based on the fact that PP and PE are mostly utilized for packaging and represent for the bulk of plastic trash in household waste. As previously stated, the chemistry behind the reaction process is influenced by a number of elements. The majority of chain scission reactions follow a multistep free radical pathway that includes (i) initiation, (ii) propagation, and (iii) termination stages.
Constituent Materials
Published in B. T. Åström, Manufacturing of Polymer Composites, 2018
Several properties other than thermal and mechanical may be of importance depending on the intended application, e.g. electrical properties, optical properties, and tolerance to environmental exposure. Degradation of a polymer may occur through unwanted crosslinking (in thermoplastics) or chain scission, where the latter produces two electron-deficient molecules which readily react chemically. Oxidative degradation involves reaction between atmospheric oxygen and the polymer. In radiative degradation and photooxidation (ultraviolet (UV) light), degradation occurs due to radiation or light causing covalent bonds to be broken if subjected to energies in excess of the bond energy. Chemical attack on a polymer may cause swelling or dissolving (the latter for thermoplastics only). In hydrolysis the polymer reacts with water resulting in chain scission. Increased molecular mobility results in a higher rate of diffusion and absorption of chemicals, meaning that polymers are more susceptible to attack above Tg. Predictably, crystal regions and crosslinks reduce this susceptibility. All kinds of degradation are likely to affect polymer properties in an undesirable fashion. Although polymers generally contain antidegradants to slow down degradation, they nevertheless gradually degrade—it is merely a question of whether the degree of degradation becomes significant during the intended life of a component.
Reactions on Polymers
Published in Charles E. Carraher, Carraher's Polymer Chemistry, 2017
Polymers undergo reactions that are analogous to smaller molecules. Variation generally involves the need for reactants to have contact with the active site. This is more difficult for polymers.Polymer degradation typically occurs via random chain scission, depolymerization, or both, resulting in a loss of chain length and properties associated with polymer length.Among the most important reactions on polymers are those that occur with biomacromolecules such as those involved in the transfer of oxygen and the activity of enzymes. Shape, size, and electronic configuration are all essential factors in the transfer of oxygen and activity of enzymes. The transfer of oxygen can be mathematically described using the classical Michaelis–Menten approach. The two major models describing the activity of nonallosteric enzymes such as myoglobin are referred to as the lock-and-key model and the induced-fit model. Hemoglobin is an example of an allosteric enzyme where the two most popular models describing this behavior are the concerted and sequential models.
Radiation Compatibility of Geopolymer, Polymer, and Composite Materials for Use as Inner Shielding in Radioactive Waste Containers—A Simulation-Based Study
Published in Nuclear Technology, 2022
Two basic radiation effects seen in polymers are chain scission (i.e., breakdown of chains into smaller segments, leading to polymer degradation) and cross-linking (i.e., bonding of polymer chains into a three-dimensional network). Exposure of a polymer to ionizing radiation generally results in both of these effects, but one of them usually dominates in any specific polymer. Chain scission reduces the average molecular weight of the polymer whereas cross-linking increases it, and both effects can alter a polymer’s mechanical properties. Other changes that radiation can instigate in polymers include radiolytic gas production, grafting, and coloration. Gas evolution occurs in almost all polymers exposed to radiation, and the gas formed is mostly hydrogen but often also methane, carbon monoxide, or carbon dioxide.18–22
Fluid flow effects on the degradation kinetics of bioresorbable polymers
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2021
Zhitao Liu, Hongbo Zhang, Huanxin Lai
Hydrolysis is known to be the dominant mechanism in hydrolytic degradation. The diffusion and accumulation of water molecules causes hydrolysis of the ester bonds in the polymer matrix. As a result, these chains are split into water soluble shorter chains (oligomers and monomers) characterized by carboxylic ends. Because of the chain scission, the molecular weight of the polymer decreases gradually. The degradation products then diffuse into the surrounding environment, which results in the mass loss of polymers. The diffusion is generally slower in a large-size device, due to the greater diffusion distance (Siepmann et al. 2005; Xu et al. 2017). The slow diffusion may result in accumulation of acidic products inside the polymer matrix, and they accelerate the degradation process. The phenomenon is known as the autocatalysis (Gentile et al. 2014; Laycock et al. 2017).
Thermal behaviour of polystyrene/silica composites
Published in Philosophical Magazine Letters, 2018
Olga V. Alekseeva, Andrew V. Noskov, Sabir S. Guseynov, Alexander V. Agafonov
Table 1 shows the mean values of listed parameters: the mass loss in the first stage and the non-volatile residue found for the PS/silica composite. The onset of degradation, i.e. where the weight loss begins, plays an important role in studying the degradation behaviour of the composites. The degradation of polymer starts with chain scission. It is followed by depolymerisation and formation of the main evolved products (styrene monomer, dimer and trimer) [32]. Figure 2(a) and (b) shows that the degradation rate increases beyond 660 K. This implies almost all polymer chains are degraded abruptly between 660 and 710 K. The temperature corresponding to the DTG peak, , is also an important indicator. At this weight loss, the material does not retain its initial properties. It can be seen for the composite film containing 5 wt% of SiO2, the TG and DTG curves are shifted towards higher temperatures in comparison with pure PS. This indicates an improvement in the thermal stability of the polymer owing to silica loading.