Melting points degradation mechanism

However, these materials present additional characteristics such as stronger mechanical properties e.g. reinforcement of polymer matrices), flame resistance and better thermal properties (higher melting point, degradation and glass transition temperature) than common polymers with no nanofillers. In addition, surface wettability and hydrophobicity are improved, resulting in surfaces more prone to bonding with other polymers and additives. ... 

A very detailed study of thermal PLA degradation was carried out by McNeill and Leiper [17,18]. They used TG, DSC and thermal volatilization analysis (TVA) in combination with H-NMR, IR and MS for product identification. Jamshidi et al. [19] and Zhang et al. [20] discussed inter- and intramolecular transesterfications as causes for tiie reduction of the molar mass when heated above the melting point. Five mechanisms have been postulated (Fig. 8.3). 

PEG can be severely degraded in air. Its melting point and heat of fusion are reduced by as much as 13 °C and 32 kJ kg"1, respectively [81]. The thermal degradation of PEG in air follows a random chain scission oxidation mechanism, and could be suppressed by addition of an antioxidant, 2, 2,-methylene-bis (4-methyl-6-tert-butylphenol) (MBMTBP), due to the reaction of MBMTBP with ROO radicals formed in the propagation step [79]. Low-molecular-weight esters including formic esters are produced as the main products of the thermal degradation of PEG (Scheme 3.17) [80]. 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

Poly(amino acids) are insoluble in common solvents, are difficult to fabricate due to high melting point, and absorb a significant amount of water when their acid content reaches over 50 mol%. To solve these problems, polyesters derived from amino acids and lactic acids [e.g., poly (lactic acid-co-lysine) PLAL] are developed. The PLAL system is further modified by reaction with lysine A-carboxyanhydride derivatives. Another modification of poly(amino acids) includes poly(iminocarbon-ates), which are derived from the polymerization of desaminotyrosyl tyrosine alkyl esters. These polymers are easily processable and can be used as support materials for cell growth due to a high tissue compatibility. Mechanical properties of tyrosine-derived poly(carbonates) are in between those of poly(orthoesters) and poly(lactic acid) or poly(gly-colic acid). The rate of degradation of poly(iminocarbonates) is similar to that of poly (lactic acid). 

Thermosetting adhesives, exhibiting no melting point, consist of highly crosslinked networks of macromolecules. Because of this dense crosslinked structure, they show relatively little creep at elevated temperatures and exhibit relatively little loss of mechanical function when exposed to either elevated temperatures or other degrading environments. Many of these materials are suitable for moderately high-temperature applications. 

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