Studies on thermal expansion of irradiated polymers from 10K to 340K

Abstract

Thermal expansion is due to anharmonicity in the interatomic potential of a solid. Thermal expansion depends on the types of interaction between the constituent atoms/molecules. It is small for covalently bonded materials and large for Van der Waals bonded materials. In polymers, the molecules are held by covalent bonds along the chain direction and by weak Van der Waals forces perpendicular to the chain direction. Thus, both types of behavior can be expected in polymers.

When polymers are exposed to high-energy radiation, many physical and chemical reactions occur, both temporary and permanent. Crosslinking and degradation are the main processes among all reactions during irradiation.

Polyethylene (PE) is a crosslinking polymer. Upon irradiation, the crosslinking index increases with radiation dose. This can be monitored by measuring viscosity and molecular weight. With increasing dose, viscosity and molecular weight of PE increase. Polyethylene samples were irradiated to doses of 0, 100, 200, 300, 400, and 500 Mrad in air at room temperature using a Cobalt-60 source. It was found that its crystallinity decreases with radiation dose due to its crosslinking property.

The thermal expansion coefficient (?) for PE samples irradiated to 0, 200, and 500 Mrad doses was measured. It was found that ? is invariant with radiation dose below 110 K due to the freezing of the macromolecules at low temperature, and ? decreases with radiation dose above 110 K to 340 K due to crosslinking of PE during irradiation.

In the case of Polypropylene (PP), both crosslinking and degradation processes take place simultaneously during irradiation. At low doses, crosslinking predominates, while at higher doses, degradation becomes significant. Polypropylene samples were irradiated up to 500 Mrad in air at room temperature using a Cobalt-60 source. Its crystallinity decreases with radiation dose due to crosslinking upon irradiation. Thermal expansion coefficient (?) was measured for 0, 250, and 500 Mrad irradiated samples. It was found that ? is invariant with radiation dose below 125 K due to freezing of molecules at low temperature, and ? decreases with radiation dose between 125 K and 340 K due to predominance of crosslinking at higher doses. The invariance of ? between 0 and 250 Mrad is due to the crystallinity effects being compensated by degradation effects.

Polyvinyl Chloride (PVC) undergoes crosslinking when irradiated in vacuum and undergoes chain scission with less crosslinking when irradiated in air. Commercial PVC samples were irradiated in air at room temperature using Cobalt-60 ?-rays. Degradation was confirmed by IR spectra. Thermal expansion coefficient (?) was measured for samples irradiated to 0, 300, and 500 Mrad doses. It was found that ? decreases from 10 K to 110 K and increases from 110 K to 340 K with radiation dose. The decrease in ? with radiation dose between 10 K and 110 K is due to crosslinking, and the increase in ? with radiation dose from 110 K to 340 K is due to degradation of the polymer during irradiation. The anisotropy parameter (??/??) is larger at higher temperature, since ?? has weaker temperature dependence than ??. By analogy, crosslinking has a weaker temperature dependence than degradation, so crosslinking predominates at low temperature while degradation dominates at higher temperatures.

Nylon-6 undergoes both crosslinking and degradation during irradiation, the latter being dominant when irradiation is carried out in air. Degradation is confirmed by depression of its melting point. Thermal expansion coefficient was measured for samples irradiated to 0, 250, and 500 Mrad doses. It was found that the increase in ? with radiation dose is due to degradation of the polymer.