Polymers degradation chain scission

Split polymer molecules (chain scission), causing lower molecular weight, degraded cohesive strength, and low-molecular-weight by-products... 

In mechanical degradation, chain scission does not occur from individual attack on a specific atom of the macromolecular chain, but from the application of a critical stress. The extensional viscosities of high-molecular-weight, synthetic polymers have been studied for the past two decades, in attempts to relate the values with drag reduction behavior. To date the dynamic uniaxial extensional viscosities of aqueous carbohydrate solutions have not been reported. 

The method of differential radiation induced contrast depends on enhancement of contrast in multicomponent polymers where the components have different electron beam-polymer interactions [173]. Contrast has been observed in sections of styrene-acrylonitrile/poly(methyl methacrylate) (SAN/PMMA) polymers where the PMMA exhibits a high rate of mass loss compared to SAN, creating contrast between the phases. It is well known that electron irradiation results in chain scission and crosslinking, loss of mass and crystallinity [75]. Polystyrene, polyacrylonitrile and SAN crosslink and thus are stable in the electron beam whereas polymers exhibiting chain scission, PMMA and poly(vinyl methyl ether), degrade in the beam. It is suggested that experiments be conducted on the homopolymers to determine the expected irradiation damage mechanism in the multi-component system [173]. 

The surrounding atoms affect the ESR spectrum, which enables the structure of the radicalized chain to be assessed. ESR spectroscopy is useful in studies of radical reactions, e.g. polymerization and degradation. Chain scission accompanies mechanical fracture particularly in highly oriented polymers and in thermosets. ESR has provided useful results in this field. The chain dynamics can also be studied by using spin-labelling techniques, e.g. using nitroxide radicals. 

Table 6.1 presents examples of different photoinitiators which initiate the degradation (chain scission and/or crosslinking) of various polymers. 

Nuclear magnetic resonance (NMR) spectroscopy is one of the most useful methods for the study of degradation (chain scission and crosslinking) and photo-oxidative degradation (disappearance and the formation of new groups which contain protons deutrons carbon C isotopes and fluors The theory of NMR spectroscopy of polymers and experimental methods have been described in many excellent monographs [5,6,295,608,1206,1223, 1327,1699,1885,2064,2111,2221]. 

Exposure of poly(alkylene oxide)s to high-energy radiation, such as electrons from electron beam or gamma radiation from cobalt-60 sources, will alter the structure of the polymers. The main structure alterations involved are degradation, chain scission, and crosslinking. The relative importance of these alteration processes depends on a variety of factors, such as physical state (solid or solution state), dosage, oxygen concentration, and water or moisture content. 

One can arrive at an intuitive acceptance of the maxim that ladder polymers should be more thermally stable than single strand polymers by a consideration of the factor that confers their desirable properties on polymers, i.e., long chain structure (or. alternatively, high molecular wei t). Macromolecules lose their desirable characteristics as soon as degradation (chain scission) becomes extensive. If this process can be prevented, or at least, slowed down, stability should be concomitantly enhanced. Any rupture in a single strand chain is effective in... 

The silyl HA-CTA with a greater degree of silylation can be dissolved in hexane and xylenes, while those with smaller degrees of silylation are soluble in acetone, tetrahydrofiiran, and 1,2-dichloroethane. The time and temperature conditions used to make the xylenes-soluble materials did not cause appreciable degradation (chain scission) of the HA. Silyl HA-CTA is stable in air, polymer samples left in ambient air for 1 year maintained their chemical structure [80]. 

Hoffman Degradation. Polyacrylamide reacts with alkaline sodium hypochlorite [7681-52-9], NaOCl, or calcium hypochlorite [7778-54-3], Ca(OCl)2, to form a polymer with primary amine groups (58). Optimum conditions for the reaction include a slight molar excess of sodium hypochlorite, a large excess of sodium hydroxide, and low temperature (59). Cross-linking sometimes occurs if the polymer concentration is high. High temperatures can result in chain scission. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Norrish type I chemistry is claimed to be responsible for about 15% of the chain scission of ethylene—carbon monoxide polymers at room temperature, whereas at 120°C it promotes 59% of the degradation. Norrish I reactions are independent of temperature and oxygen concentration at temperatures above the T of the polymer (50). 

Radiation-induced degradation reactions are in direct opposition to cross-linking or curing processes, in that the average molecular weight of the preformed polymer decreases because of chain scission and without any subsequent... 

Coagents ate often used with peroxides to increase the state of cure. Some coagents, such as polybutadiene or multifimctional methacrylates, are used at high levels to form polymer grafts or interpenetrating networks. Other coagents such as triaHyl cyanurate, triaHyl trimelHtate, and y /i -phenjiene bismaleimide are used at low levels to reduce the tendency of the polymer to degrade by chain scission. 

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