Grace and Company, Cladcsdle, M a r~l a n~ 21029 * This paper was originally published in J. A p p l . Phys., 37, 3385 (1966) and is reproduced here by kind permission of the American Institute of Physics. 14rl EXPERIMENTAL Preparation of Single CrystalsGrex 60-002E (W. R. Grace), a high density polyethylene having 1.4 methyl groups per 1000 carbon atoms, a number-average molecular weight * The accepted terminology of designating the relaxations with Greek letters in order of decreasing temperature is used. Another relaxation, the p relaxation, occurs in branched polyethylene, but this was not observed in the linear polyethylene used in the present work. A general discussion of relaxations in melt-crystallized polyethylene has been given by Boyer.6

The microwave spectrum of ten isotopic species of acetyl chloride are reported. Internal rotation of the methyl group splits some of the rotational transitions into doublets in addition to the chlorine hyperfine structure. Analysis of the internal rotation splitting gives a barrier to internal rotation of 1296±30 cal! mole. From the hyperfine structure, the chlorine nuclear quadrupole coupling constants for CI36 along the C-Cl bond axis and perpendicular to the plane of symmetry of the molecule were determined to be -58.5 and 21.5 Me, respectively. The corresponding values for CP7 were -46.4 and 17.2 Me. These coupling constants have been interpreted to give information about the double bond and ionic character of the C -Cl bond. The follow-ing structural parameters have been determined from the rotational constants of CILCOCI36, CILCOCI37, CI3ILCOCI36, CHzC 13 0CI35, CILC018CI36, and CD z COCp6:

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The experimental techniques used to measure the dynamic shear behaviour of polymers at low frequencies, that is below 10 sec−1, are discussed with particular reference to the torsion pendulum. The various types of relaxations which can occur in amorphous and semicrystalline polymers are considered. Polymethyl methacrylate, polyethyl methacrylate, and polystyrene are examples of amorphous polymers. The effects of crosslinking, intermolecular forces, steric effects, and plasticizers on the main glass transition of other amorphous polymers are also described. Polytetrafluorethylene is an example of a linear, semi‐crystalline polymer. The effect of introducing trifluoromethyl groups along the chain in copolymers of tetrafluoroethylene and hexafluoropropylene is described. Results obtained with polypropylene are also discussed.