Degradation semicrystalline

The semicrystalline polyesters of the terephthalate and naphthalate family are resistant to a wide range of chemicals at room temperature, including water, alcohols, ketones, ethers, glycols, chlorinated solvents, aliphatic hydrocarbons, and oils. They are slowly hydrolyzed in boiling water and rapidly degraded in strongly basic or acidic medium. 

Why does polymer degradation typically occur in the amorphous region of semicrystalline polymers rather than in the crystalline regions ... 

Phase-transfer techniques are widely used for the preparation of polymers. For example, potassium fluoride is used to produce poly(etherketone)s under phase-transfer conditions (Scheme 10.18). Use of this reagent allows the chloroaro-matics to be used as starting material as opposed to the more expensive flu-oroaromatics that are usually employed [23]. This method is suitable for the synthesis of high molecular weight semicrystalline poly(ether ketone)s, although the presence of excess potassium fluoride in the reaction mixture can lead to degradation reactions. The use of a phase transfer catalyst can allow the use of water-soluble radical initiators, such as potassium peroxomonosulfate used to promote the free-radical polymerization of acrylonitrile [24],... 

PTT polymer pellets must be dried to a moisture level of <30 ppm, preferably in a close-loop hot air dryer, to avoid hydrolytic degradation during melt processing. Drying is carried out with 130 °C hot air with a dew point of < -40 °C for at least 4 h. Because of the faster crystallization rate, PTT pellets are already semicrystalline after pelletizing, and do not require pre-crystallization prior to drying as with PET. The dried polymer is extruded at 250-270 °C into bulk continuous filaments (BCFs), partially oriented yam (POY), spin-draw yam (SDY) and staple fiber. 

Polymers can exist as liquids, as elastomers or as solids but can be transferred into the gaseous state only under very special conditions as are realized in, for example, MALDI mass spectrometry. This is because their molecular weight is so high that thermal degradation sets in before they start to evaporate. Only a few polymers are technically applied in the liquid state (silicon oils, specidty rubbers) but most polymers are applied either as elastomers, or as rigid amorphous or semicrystalline solids. 

Polyethylene oxide) (PEO) is a semicrystalline water-soluble polymer [64, 65], with a crystallinity that is very sensitive to the thermal history of the sample, making this property interesting as an indicator of degradation. Because it is biodegradable and biocompatible, PEO is a good candidate for environmental and medical applications [66-68]. The mechanisms of thermo- and photo-oxidation of PEO have already been investigated [69, 70] on the basis of IR identification of the oxidation products and are summarized in Scheme 10.1. 

Poly(s-caprolactone) Poly(e-caprolactone) is a semicrystalline polymer synthesized by anionic, cationic, free-radical, or ring-opening polymerization [94]. It is available in a range of molecular weights and degrades by bulk hydrolysis autocatalyzed by the carboxylic acid end groups. The presence of enzymes such as protease, amylase, and pancreatic lipase accelerates polymer degradation [95], The various methods of preparation of poly(e-caprolactone) nanoparticles include emulsion polymerization, interfacial deposition, emulsion-solvent evaporation, desolvation, and dialysis. These methods and various applications are extensively reviewed [94],... 

Chemical breakdown usually involves oxidative chain reactions that cause embrittlement of semicrystalline polymers and discoloration of poly(vinyl chloride) and polymers with aromatic groups. The reactions are complicated by the presence of transient intermediates and by rates that depend on minute concentrations of molecular defects, impurities and additives. They also depend on several important piiysi-cal factors outlined in this brief overview of polyolefin degradation, two of these factors, the transfer of excitation energy and the transport of products and protectants, play a major role in stabilization processes. 

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