Chain unzipping

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

Random chain cleavage followed by chain unzipping is characterized by high monomer yields and a slow decrease in the molecular weight of the polymer, for example, exhibited by PMMA, poly(a-methyl styrene), polystyrene polytetrafluoroethylene. 

The thermal stability of elastomeric polyacetaldehyde could be improved by end-capping the hydroxyl end groups with acetic anhydride in order to prevent chain unzipping. Further stabilization was obtained by adding polyamide as a thermal stabilizer and an aromatic amine antioxidant. 

Effect of Fiber Degradation on the Corrosion Solution. Hydrolysis and oxidation of protein and cellulose have been described in the literature primarily with the focus on degradation in industrial processing conditions. In alkaline conditions, amino acids are released from silk in a chain unzipping mechanism in acidic conditions, the scissions are random (8,9). As the polymer deteriorates, free carboxyl and amine end groups are formed. Tyrosine oxidizes to a quinone this reaction gives aged silk its yellow coloration. Amorphous areas of the fiber are attacked first. 

A program for simulating runs makes it possible to study the expected behavior of single samples and their mixtures for arbitrarily assigned values of the parameters of the kinetic model. The parameters which are primarily responsible for the shapes of the rate vs time degradation curves are k-, the fraction of chains started per sec, and tne fraction of a started chain unzipping per sec. 

The degradation of poly-a-methylstyrene is unaffected by the presence of polystyrene, but depolymerization of the latter polymer can be brought about at temperatures below 300°C by heating in the presence of poly-a-methylstyrene [320], The rate of polystyrene volatilization then varies as an inverse function of the molecular weight of poly-a-methylstyrene. The system is heterogeneous, consisting of micelles of poly-a-methylstyrene embedded in a polystyrene matrix. It has been suggested that the poly-a-methylstyrene chain unzips completely to a monomer radical which diffuses into the polystyrene matrix and attacks a polystyrene molecule. 

Since the infrared spectra of PnBA films weathered for 2000 hours show a loss of similar magnitude in the GH3, GH2 bands at 2900 cm l and in the carbonyl band at 1735 cm l with no concomitant appearance of any new major bands to account for this, it is tempting to suggest that the decrease in these band intensities may be due to chain unzipping with release of monomer. However, the studies of Gupta and Liang suggest that butyl acrylate is not a major product of short term photooxldatlon. 

During the operation of a fuel cell, some hydroxide radicals, OH, will form. They can attack the PEM and cause its gradual degradation. It has been found (Curtin et al. 2004) that the degradation is mainly through the main chain unzipping for a PEM where the main chains ends are -COOH ... 

Figure 2 Thermal degradation mechanism of poly(alkylcyanoacrylate) by alkyl chain unzipping.

Chen and Fuller (2009a) investigated the effect of humidity on the degradation of Nafion membrane and found the main-chain unzipping process to be dominant under high humidity, while a decrease... 

PFS A membranes, as a commercial product for the chlor-alkah industry, have been widely used in PEMFCs and PEMELCs. Even with its highly stable C-E bonds, the PFSA membrane still sustains appreciable degradation, especially under accelerated conditions such as high temperature, high pressure, and low humidity (Healy et al. 2005). There are three potential degradation pathways chain unzipping, chain scission, and side-group attack, and these will be discussed separately next. 

Many polymers synthesized by free-radical polymerization can sustain depolymerization via a chain unzipping mechanism. Thermal degradation of Nafion indicates that chain unzipping is the main mechanism for Nafion degradation (Wilkie et al. 1991). 

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