Raman spectroscopic observation of conformational changes during thermal exposure and its effects on melting and crystallization behavior of high-density polyethylene
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Abstract
In the present study, we used Raman spectroscopy to reveal conformational changes of high-density polyethylene (HDPE) during long-time thermal exposure in air at 110 °C. In the early stage, we observed an increase in crystallinity complemented by a decrease in the number of non-crystalline short trans sequences owing to structural rearrangement, which is the typical effect of annealing. Deterioration of the mechanical properties, such as a decrease in the strain at break, gradually proceeded, even before chemicrystallization. This will be ascribed to conformational reordering of the crystalline structure, i.e., a gradual increase in crystallinity accompanied by a decrease in both non-crystalline short and long trans sequences. During chemicrystallization, a further increase of crystallinity accompanied by elimination of the non-crystalline short and long trans sequences resulted in embrittlement of the HDPE specimen. Conformational changes in the HDPE specimen after thermal exposure were monitored in situ during heating and cooling. The trans sequence formed during thermal exposure was highly stabilized, resulting in higher crystallinity and trans fractions up to the vicinity of the melting temperature. The detection of non-crystalline short trans sequences can be used to predict the degree of deterioration of thermally exposed HDPE owing to the strong correlation between the decrease in the number of non-crystalline short trans sequences and the strain at break.
期刊介绍：
Polymer Degradation and Stability deals with the degradation reactions and their control which are a major preoccupation of practitioners of the many and diverse aspects of modern polymer technology.
Deteriorative reactions occur during processing, when polymers are subjected to heat, oxygen and mechanical stress, and during the useful life of the materials when oxygen and sunlight are the most important degradative agencies. In more specialised applications, degradation may be induced by high energy radiation, ozone, atmospheric pollutants, mechanical stress, biological action, hydrolysis and many other influences. The mechanisms of these reactions and stabilisation processes must be understood if the technology and application of polymers are to continue to advance. The reporting of investigations of this kind is therefore a major function of this journal.
However there are also new developments in polymer technology in which degradation processes find positive applications. For example, photodegradable plastics are now available, the recycling of polymeric products will become increasingly important, degradation and combustion studies are involved in the definition of the fire hazards which are associated with polymeric materials and the microelectronics industry is vitally dependent upon polymer degradation in the manufacture of its circuitry. Polymer properties may also be improved by processes like curing and grafting, the chemistry of which can be closely related to that which causes physical deterioration in other circumstances.
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