Volume 29, Issue 10, 01 October 1958
Index of content:
- SPECIAL ISSUE ON HIGH‐POLYMER PHYSICS
29(1958); http://dx.doi.org/10.1063/1.1722954View Description Hide Description
Molecular motion in polypropylene has been studied both by measurements of dynamic mechanical properties and by observations of nuclear magnetic resonance line shapes. The mechanical measurements were made in the audio‐frequency region over the temperature range from 80°K to the melting point. Both completely amorphous and highly crystalline samples of polypropylene were investigated. In the amorphous sample, two transitions were observed, a broad but weak secondary one near 235°K and a relatively sharp primary one occurring near 270°K. For the highly crystalline polypropylene, two transitions centered at 250°K and 300°K, respectively, are associated with increased mobility in the amorphous regions and a third higher temperature transition is associated with crystalline melting. These transitions have also been investigated by observing the changes occurring in NMR line shapes as the temperature is varied.
29(1958); http://dx.doi.org/10.1063/1.1722955View Description Hide Description
Stress‐temperature (constant strain) and length‐temperature (constant stress) measurements have been made on Paracril‐35 (a butadiene‐acrylonitrile copolymer) in the temperature range −80°C to 25°C, which includes the glass‐like, transition, and rubber‐like states for this material (Tg ∼−25°C). Time effects are observed in the transition region, but for temperature gradients used (down to about 1°C in 4 hours) the behavior at low temperatures is unique, indicating the existence of a real transition. This behavior is independent of the strain imposed at room temperature. The shift in transition temperature (in the length‐temperature work) with extension is found to give quantitative support to the theory that the transition occurs at a certain minimum volume. In the rubber‐like region, behavior is represented by the usual equation of state.
29(1958); http://dx.doi.org/10.1063/1.1722956View Description Hide Description
The Doolittle approach to Newtonian viscosity assumes η=A expB/f, where A and B are constants independent of temperature and f, the relative free volume, is (v−v 0)/v 0. Here v is the specific volume and v 0 is the occupied volume. Using machine computations, this equation was applied to published data on polystyrene, replacing v by its equivalent vg +(dv/dT)(T−Tg ), where vg is the specific volume at the glass temperature Tg . The agreement between calculated and observed viscosities is at least as good, and perhaps better, than that afforded by other existing empirical expressions. The constant B is independent of molecular weight and temperature while v 0 depends slightly on molecular weight. A log‐log plot of A against molecular weight M produces two straight lines intersecting at M=35 000. At higher values of M, the slope is 3.4; at lower values, it is slightly greater than one. This treatment separates the melt viscosity of polystyrene into two factors, A, dependent only on structural parameters, and f, which is determined by the specific volume and is a function of temperature.
29(1958); http://dx.doi.org/10.1063/1.1722957View Description Hide Description
A stress‐strain equation is derived for a homogeneous, isotropic material with the assumptions that for any given homogeneous simple tensile strain, the components of the stress tensor are to the first approximation linear, homogeneous functions of the components of the strain tensor and that no volume change occurs during the deformation. Utilizing the dependence of Poisson's ratio upon the extension referred to the initial coordinates, one elastic coefficient, C 12, is found to be sufficient to roughly characterize the first stretch stress‐strain curve. Although experimentally this elastic coefficient is found to be essentially constant for extensions greater than 250% its value increases rapidly as zero extension is approached. This behavior agrees qualitatively with data by Blanchard and Parkinson as to the distribution of secondary bond strengths, wherein they found a large number of relatively low‐energy bonds which would be effective only at small extensions in contributing to modulus reinforcement. Various aspects of stress strain and cure behavior are examined with the derived equation as a basis.
29(1958); http://dx.doi.org/10.1063/1.1722958View Description Hide Description
The infrared spectrum of polyvinyl nitrate has been obtained in the extended region of 70 to 3600 cm−1. Polarized infrared measurements were made on oriented specimens in the range of 350 to 3600 cm−1. Since x‐ray diffraction patterns indicate no crystallinity, the analysis has been based on an assumed atactic configuration. With the help of previous studies of high‐polymer spectra and results obtained on small molecules, it has been possible to make a complete assignment of all of the expected fundamentals. Dichroic measurements have not only permitted a decision on the assignment of the controversial NO2 rocking and bending modes, but suggest certain details concerning the local configuration of the nitrate group. There seems to be evidence that, although the specimen is not crystalline, a significant proportion of the chains have configurations not far from a planar zigzag chain.
29(1958); http://dx.doi.org/10.1063/1.1722959View Description Hide Description
Using a torsion balance immersed in water and natural rubber ring specimens cured with di‐tertiary‐butyl‐peroxide, measurements were made of the isothermal volume dilation of rubber for mean extensions ε=14, 33, and 51%, thus extending the results of Gee, Stern, and Treloar to lower strains. Chain molecular weights Mc =3000, 4400, 5100, and 5500 were employed. The chain molecular weights were determined by swelling in benzene, the uncertainty in each determination being about 10%. Observed fractional volume increases ranged from 3.2×10−5 for ε=14% and Mc =5500 to 14×10−5 for ε=51% and Mc =3000. Using Gee's expression for the volume dilation, but obtaining the slope of the stress‐strain curve from the statistical theory, curves were fitted to the data. The fitting process constituted a determination of Young's modulusE for each rubber specimen. The resulting curves are in good agreement with those of Gee, Stern, and Treloar. Additional determinations of E were made from rough stress‐strain curves and from the swelling data, the internal agreement between the three determinations being fair.
29(1958); http://dx.doi.org/10.1063/1.1722960View Description Hide Description
Samples of a ``high‐density'' polyethylene (ρ=0.964 g/cc) irradiated in a nuclear reactor to thermal neutron doses as high as 2.9×1018 nvt and of ``low‐density'' polyethylene (ρ=0.915 g/cc) subjected to gammaray bombardment from a Co60source up to a dosage of 1×109 rep have been investigated by dynamic mechanical methods from 80°K to 450°K at audio‐frequencies. Degrees of cross‐linking were estimated from the observed changes in resonance frequency at temperatures where the specimens were ``rubber‐like.'' Densities and x‐ray crystallinities at 25°C were also measured.
For the pile‐irradiated ``high‐density'' polyethylene the γ peak found around 165°K shows a marked increase in height and area at dosages above 1018 nvt. The α peak at about 390°K decreases markedly upon receiving a thermal neutron dose of 0.6×1018 nvt. Further changes in this absorption with higher dosages are obscured by the appearance of a β peak around 330°K which is almost absent in the unirradiated material; the appearance of the β region is attributed to the creation of branch points and cross‐link points in the essentially linear polymer by the irradiation.
The irradiated ``low‐density'' specimens exhibit three damping peaks. Over the dosage range studied the γ peak (165°K) increases in height and area, the β peak (270°K) shifts to higher temperatures, and the α peak (360°K) shifts to lower temperatures with increasing dose. The cross‐linking degree is not proportional to dose and to the highest dosages studied, there is very little, if any, reduction in x‐ray crystallinity.
Birefringence Changes during Retraction of Oriented Polystyrene Monofilaments. III. Correlation with Internal Stress Changes29(1958); http://dx.doi.org/10.1063/1.1722961View Description Hide Description
The changes in average birefringence and internal stress during retraction at 85°C have been measured for four different oriented polystyrene monofilaments. The data for the different filaments all appear to fall on a single curve when plotted as internal stress vsbirefringence. Data on internal stress and birefringence obtained for thirteen different oriented filaments (including the four above) in their initial uncontracted state also fall on the same curve. This is in marked contrast with the lack of correlation obtained in Part I where it was attempted to relate birefringence to elongation parameters. Evidently birefringence is related most directly to internal stress rather than elongation, and in fact provides a direct measure of the internal stress. This simple relation seems to be essentially independent of the stress‐time‐temperature history of the samples during orientation. Measurements on an additional filament which had a very nonuniform birefringence distribution (studied also in Part II) indicate that internal stress should be correlated with area average rather than diameter average birefringence, in cases where such a distinction is necessary. The internal stress‐birefringence curve is not linear, contrary to theoretical predictions based on the simple kinetic theory of rubberelasticity. Non‐Gaussian chain statistics may explain this nonlinearity. The present results indicate that birefringence provides a more useful index of molecular orientation than measurements of total amount of retraction.
29(1958); http://dx.doi.org/10.1063/1.1722962View Description Hide Description
Employing concepts first applied to textile fibers by Meyer and Lotmar in calculating the elastic stretch modulus of the crystalline domain of cellulose, the moduli of crystalline polyhexamethylene adipamide (α form), and polyethylene terephthalate have been derived. The treatment employs interatomic force constants for the stretching and bending of primary valence bonds, applied to molecular geometries deduced from the crystal structures of these polymers as determined by Bunn and co‐workers. The modulus of the crystal cell of the first material (nylon 66) is calculated to be 15.7×1011 dyne cm−2, while that of the second is 14.6×1011 dyne cm−2. Supposedly these are approximately the modulus values which fibers having perfectly oriented, efficiently packed molecules would have. In both cases the theoretical value is at least an order of magnitude larger than that found experimentally for actual production samples.
29(1958); http://dx.doi.org/10.1063/1.1722963View Description Hide Description
An apparatus has been constructed to measure the shear modulus and logarithmic decrement of polymers from 4.2 to 100°K. The apparatus consists of a small torsion pendulum suspended on a rigid support at the bottom of a large glass Dewar vessel. After cooling to 4.2°K with liquid helium, the pendulum warms up at an initial rate of 0.1° per minute. Torsional oscillations of the pendulum are excited electrically and recorded photographically. The estimated accuracy of the modulus and logarithmic decrement is ±5.0%.
Measurements of the shear modulus and logarithmic decrement have been made, from 4.2 to 100°K, on a series of tetrafluoroethylene‐hexafluoropropylene copolymers.
29(1958); http://dx.doi.org/10.1063/1.1722964View Description Hide Description
Molecular order and motion in isotactic and atactic polypropylene molecules have been studied by protonmagnetic resonance methods over the temperature range 77–400°K, and by x‐ray diffraction. It is shown that some motion persists at the lowest temperatures, and becomes pronounced in the temperature interval 77–110°K. This behavior is ascribed to motion of the methyl groups about the threefold axis. The resonance undergoes further narrowing near room temperature for the atactic polymer, presumably owing to rotational and translational motions of chain segments. The isotactic compound possesses a narrow resonance superposed on a broad absorption over a wide temperature range. The former, ascribed to the amorphous regions, narrows at higher temperatures than for the atactic polymer, presumably owing to constraints imposed by the crystalline regions. The chains of the isotactic compound are less mobile than in polyethylene, as shown by the resonance studies and by x‐ray diffraction comparisons of the rate of crystallization.
29(1958); http://dx.doi.org/10.1063/1.1722965View Description Hide Description
Measurements of the complex shear compliance (J*=J′−iJ″) of polyvinyl stearate at frequencies from 50 to 5000 cps have resulted in the discovery of several sharp resonances in the compliance as previously described. The effects of temperature and static stress normal to the direction of the dynamic shearing stress have now been studied for this material. The two principal resonances observed at 25°C were located at (1) 180 and (2) 500 cps. At successive temperatures of 30, 36, and 42°C the first resonance remained essentially unchanged, but the second resonance shifted to higher frequencies and decreased in magnitude until it virtually disappeared at 42°C. A return to 25°C and a reapplication of a slight static stress resulted in the return of the second resonance (at 525 cps). At successively increasing static stresses corresponding to compressions up to 1.8% the second resonance now decreased in magnitude and shifted to higher frequencies. The first resonance disappeared at a static compression of 0.8%. Similar effects of static stress were observed at room temperature (∼25°C) on a second set of samples of different dimensions. The reversibility of the effects of static compression were demonstrated by the second set of samples where for example, a resonance originally present at 2.8% static compression disappeared at 3.7% and reappeared when the static compression was reduced to 3.1%. The reversible nature of the effects of static stress seems to preclude the possibility of an explanation of the resonances in terms of changes in dislocation density.