J. M. Crissman

Internal‐Friction Study of High‐Purity Single Crystals of n Eicosane (C20H42)

Large pure single crystals of n eicosane (C20H42) with [210] growth axes have been prepared by slow crystallizations from the melt. Internal‐friction measurements covering temperatures from 100 to 300 K at ≃ 1 kHz reveal that in the as‐grown single crystals both the α and γ loss peaks characteristic of polycrystalline n alkanes of less purity are absent, from which we conclude that, in polycrystals, these processes are a result of impurities and/or crystal defects. Two very weak loss maxima of obscure origin were observed at about 270 and 287 K for all the crystals studies. In one single crystal progressive stages of light cold work induced a complex spectrum of small loss maxima between 200 and 300 K. These maxima were found to be highly sensitive to aging. We believe that the observed behavior provides evidence for dislocation motion in crystals of this type. No detectable γ peak resulted from the cold work. Data are also included for one single crystal which was irradiated with γ rays.

Dynamic mechanical behavior of polyethylene during creep to failure in uniaxial extension

Dynamic mechanical measurements in torsion superposed on large uniaxial creep strains have been obtained on two types of polyethylene resins. At creep strains of about 10% it is observed that tanδ goes through a minimum and rises sharply thereafter.

Temperature and crosslinking effects on the swelling of poly(isoprene) in Isopiestic conditions

Isopiestic measurements of solvent uptake have been made in the synthetic isoprene/benzene system for both crosslinked and uncrosslinked polymers in order to revisit the question of the swelling activity parameter S. Both the nonzero value of S at zero swelling and the appearance of a peak in S vs. degree of swelling have been observed in some solvent/rubber pairs and we here investigate both the crosslink and temperature dependencies of these phenomena. The data analysis is an extension of prior work from this laboratory using continuum thermodynamics concepts and avoiding molecular models in an attempt to establish the fundamental phenomenology of the process and the validity or lack of validity of the hypothesis that the mixing and elastic contributions to the free energy of networks are separable. We present results from measurements in benzene vapors at temperatures between 10 and 55°C for an uncrosslinked rubber and rubbers crosslinked with 1, 5, 10, and 15 parts per hundred dicumyl peroxide.

Internal friction study of polycrystalline n ‐paraffins

Recent measurements have shown that mechanical relaxation peaks commonly observed in n ‐paraffins are absent in single crystals of n ‐eicosane (C20H42). This work has now been extended to include polycrystalline samples of several highly pure long‐chain hydrocarbons. Internal friction data obtained for a variety of n ‐paraffins, pure or otherwise, have revealed no low‐temperature γ relaxation. Moreover, the α relaxation peak is suppressed completely by purification. In attempts to induce an α or γ loss peak, several dilute mixtures of other n ‐paraffins in pure C20H42 have been examined. Only the addition of odd‐numbered shorter chains induced loss peaks. On the other hand, the presence of a branched impurity, in this case the C20H42 isomers 2‐methylnonadecane and 10‐methylnonadecane, did induce loss peaks, one of which correlates very well to the α process found in as‐received C20H42. It is proposed here that isomer impurities are an important factor in the mechanics of many of the loss processes in long...

X‐ray topographic examination of large paraffin single crystals

The internal microstructure of large melt‐grown paraffin (n‐eicosane, C20H42) single crystals was examined by the Lang x‐ray diffraction projection topographic technique. Crystal selection was facilitated by use of an electro‐optical system which permitted instantaneous display of Laue transmission x‐ray diffraction patterns on a television monitor. The crystals were oriented and topographic diffraction planes were selected by the use of a standard (001) stereographic projection plotted from computed angles between crystallographic planes. A second electro‐optical system which permitted direct viewing of the topographic images was use for rapid alignment of the Lang camera and ensured uniformly exposed topographs. X‐ray topographs were obtained from crystals in the as‐grown, plastically deformed, and γ‐irradiated states. The results indicate that both plastic deformation and γ irradiation caused marked changes in the microstructure of the crystals, and that x‐ray topography can be successfully exploited t...

Multistep stress‐relaxation study of a single crystal of n‐eicosane (C20H42) in torsion

Multistep stress‐relaxation experiments in torsion have been done on a single crystal of the long‐chain hydrocarbon compound n‐eicosane. It was found that at very small strains the behavior approaches that of a linear viscoelastic material, whereas at larger strains the behavior becomes increasingly nonlinear and time dependent. In the region where the stress‐strain behavior becomes nonlinear, it is shown that the second‐step response, in an experiment in which the magnitude of the second step is one half that of the first step, can be described very well using a recent constitutive equation proposed by Zapas.