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.

The modulus and internal friction of polytetrafluoroethylene were measured with longitudinal waves at a frequency of 12 Mc between 248° and 548°K and the fluorine magnetic resonance was studied between 77° and 375°K. The samples covered a wide range of crystallinities and included a specimen which had not been sintered (as polymerized material which had not been heated above the melting temperature). The results resolve discrepancies which existed in the literature and introduce new information about the relaxations and first-order transitions in poly tetrafluoroethylene. In the ultrasonic work it is shown that the "19°C" and" 30°C" diffuse first-order crystalline transitions can be studied independently of the crystalline relaxation which occurs at 418°K at 12 Mc. The "19°C" transition is not observed but the "30°C" transition causes an appreciable decrease in the modulus. X-ray data show that this accompanies a decrease in the rotational order of the lattice. This transition is found to occur over a wider temperature range in unsintered polymer (possibly because of a

The proton magnetic resonance in polymethyl acrylate, polymethyl methacrylate, and polyethyl methacrylate has been studied between 77 and 400°K. The second moments of the absorption lines are compared with the values calculated for some of the modes of molecular motion possible in these polymers. This comparison leads to the conclusion that motion of the ester methyl and ethyl groups in the respective polymers occurs below 77°K. and that motion of the main chain methyl groups in polymethyl methacrylate and polyethyl methacrylate occurs above 130°K. This interpretation of the results is shown to be in agreement with previous measurements of the mechanical and dielectric properties of these polymers. The shape of the absorption line observed in polymethyl methacrylate at 77°K. was reproduced by a suitably weighted superposition of the absorption lines of rigid and rotating methyl groups and a rigid methylene group confirming that motion of one methyl group occurs below 77°K. in polymethyl methacrylate.

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