Oxidation of stressed polymers as studied by chemiluminescence

Sammanfattning: Since the use of chemiluminescence (CL) is known to be avery sensitive technique for studying oxidation of polymers,its potential as a tool for studying the oxidation of stressedpolymers is thoroughly investigated in this thesis. A number ofearlier reports on stress induced chemiluminescence (SCL) showthat SCL has potential for elucidating polymer stressreactions. However, the conclusions as to the origin of thelight and the effect of stress drawn in these reports are quiteambiguous.The papers summarised in this thesis can be divided into twoparts. Papers I, II, II and IV deal with the construction of aspecially designed SCL instrument and the origin of theluminescence emitted from some polymers during tensile testing.This luminescence was shown to have different sources reflectedin the names:chemiluminescence,thermoluminescence,triboluminescence. It could also be a superposition oftwo or all three phenomena. The chemiluminescence was shown tooriginate mainly from hydroperoxide decomposition due tostress-induced adiabatic heating, which after necking islocalised in the travelling neck shoulders. This CL was thusnot an effect of direct stress. However, some CL was also foundto be due to radical formation at fracture, i.e. directlycaused by stress. The thermoluminescence proved to be a resultof radiation from fluorescent light tubes and was negligible ifthe specimens were kept in the dark priorto a measurement. Thetriboluminescence was found to be emitted from irradiatedspecimens in which stress helped to recombine separatedcharges.Papers V and VI deal with how sub-yield stresses influencethe oxidative degradation of polymers. In order to attainsignificant oxidation within a reasonable time, the process wasaccelerated by an elevated temperature. The materials studiedwere injection moulded parts of polyamide, polypropylene andhigh density polyethylene. Polyamide was also studied as film,in which there is no restricted oxygen diffusion. It was shownthat stress did not change the course of oxidation in any ofthe materials.In addition to these papers, which all deal with stresschemiluminescence, parts of two papers [VII and VIII], whichdiscuss other specific features of the chemiluminescencetechnique are also included in the thesis.Keywords:Stress, chemiluminescence, oxidativedegradation, load, oxidation, thermoluminescence,triboluminescence, polyamide, PA6, PA66, polyethylene, HDPE,polypropylene, PP, hydroperoxides, charge recombination.