Abstract
Molecular motions and microstructure are studied as a function of irradiation damage and temperature in branched polyethylene. The methods of proton magnetic resonance, specific volume and density percent crystallinity, mechanical rigidity and loss, and x‐ray determined crystallinities are employed. The samples were subjected to irradiation doses ranging from 0 to 8.3×1018 nvt in the Brookhaven reactor. Significant changes in the proton resonance line shape at 295°K occur for an irradiation of 0.3×1018 nvt. Second moments and intensity ratios indicate that the first effect of irradiation on the line shape can be interpreted as due to the destruction of crystallinity or order. This effect however is offset at higher doses by the restrictive influence of radiation induced crosslinks on molecular chain motion. Estimates of the percent crosslinking aid in the interpretation of the temperature variation of proton resonance line shapes and allow some estimate to be made of the lengths of chain associated with a given type of molecular motion. Activation energies are calculated from NMR and mechanical loss data for these molecular motions. With increasing temperature the proton resonance and mechanical properties are associated first with rotational oscillations, over energy barriers, of short segments (of the order four methylene groups in length) of a chain (activation energy ∼6 kcal/mole) and also with the γ peak of the mechanical loss data. At temperatures exceeding the ``glass transition temperature'' the molecular movements are also described by neo‐Brownian diffusional motions involving chain lengths of about 10 methylene groups (activation energy ∼12 kcal/mole). These latter ``neo‐Brownian'' motions are associated to some extent with the β peak of the mechanical loss data and probably also involve some branch point motion. The majority of the motion occurs principally in the amorphous regions of the branched polyethylene until temperatures above 290°K are reached and the microstructure begins to change due to the ``melting'' of crystallites. The intensity ratio for the complex line shape exhibited by polyethylene at temperatures greater than 200°K are associated with the high‐frequency rigidity of the sample.