Application of Electron Spin Resonance Techniques to High Polymer Fracture

Spin-Trapping Analysis of the Thermal Degradation Reaction of Polyamide 66

The radical mechanisms of the thermal degradation of polyamide 66 (PA66) occurring under a vacuum at a temperature range between 80 °C and 240 °C (which includes the temperature of practical applications) were investigated using a spin-trapping electron spin resonance (ST-ESR) technique, as well as FTIR, TG-DTA, and GPC methods. No significant weight loss and no sign of thermal degradation are observed at this temperature range under oxygen-free conditions, but a slight production of secondary amine groups is confirmed by FTIR. GPC analysis shows a small degradation by the main chain scission. ST-ESR analysis reveals two intermediate radicals which are produced in the thermal degradation of PA66: (a) a ^●CH₂− radical generated by main chain scission and (b) a −^●CH− radical generated by hydrogen abstraction from the methylene group of the main chain. The ST-ESR result does not directly confirm that a −NH−^●CH− radical is produced, although this reaction has been previously inferred as the initiation reaction of the thermal degradation of PA; however, the presence of −^●CH− radicals strongly suggests the occurrence of this initiation reaction, which takes place on the α-carbon next to the NH group. The ST-ESR analysis reveals very small levels of reaction, which cannot be observed by common analytical methods such as FTIR and NMR.

Mechanical Properties of Polymeric Solids

A short survey is given of the correlation between mechanical properties and molecular model of homopolymer solids in rubbery, glassy and crystalline state. Particular emphasis is given to the plastic deformation of crystalline solids and the mechanical properties of the fibrous structure obtained by such deformation. The main difference between the spherulitic and the fibrous morphology, which explains the enormous difference in elastic modulus and strength, is not the crystallinity, crystal structure and orientation, but the fact that the basic element of the former structure is the extremely thin and easily deformable crystal lamella and, in the latter case, the very long and strong microfibril. The deformation of the fibrous structure is only possible by the sliding motion of microfibrils which is opposed by the autoadhesion forces between adjacent microfibrils over their full length or by fracture of microfibrils; which demands the rupture of the intrafibrillar tie molecules constituting a much larger fraction of the total number of chains of the crystal lattice than in the case of spherulitic morphology.

Scanning electron microscopic study of the hydrolytic degradation of poly(glycolic acid) suture

This article reports the morphological observations on the surface changes of poly-(glycolic acid) sutures which have been exposed to various dosages of gamma irradiation (0, 2.5, 5.0, 10, 20 and 40 Mrad) and duration of immersion (0, 7, 14, 28, 48, 60, and 90 days) in a physiological saline buffer. The most important gross morphological characteristics of PGA suture hydrolytic degradation is the formation of surface cracks on the filaments. The regularity of the surface cracks increased with an increase in the gamma irradiation and the duration of hydrolysis. Surface cracks were not observed in irradiated sutures that had not been subjected to hydrolytic degradation. The arrangement of the surface cracks, their orientation on the filaments, and the direction of crack propagation provide very useful information for depicting the mechanism of hydrolytic degradation in this class of fibrous material. The microfibrillar model of fiber structure has been used as the basis for the proposed degradation mechanism of PGA in vitro. It is believed that hydrolysis occurs initially in the amorphous regions sandwiched between two crystalline zones, as tie-chain segments, free chain ends, and chain folds in these regions degrade into fragments. As degradation proceeds, the size of the fragments reaches the stage at which they can be dissolved into the buffer medium. This dissolution removes the fragments from the amorphous regions, and surface cracks appeared.