Volume 30, Issue 10, 01 October 1959
Index of content:
- SPECIAL ISSUE ON HIGH‐POLYMER PHYSICS
30(1959); http://dx.doi.org/10.1063/1.1734984View Description Hide Description
Molecular motion in several glassy polymers has been examined by proton resonance methods in the temperature range 77–425°K, and has been interpreted in terms of the detailed behavior of molecular chains and substituents. Comparisons are made between transitions seen by proton resonance and glass transitions seen by other methods. Powles' observations on poly(methyl methacrylate) are qualitatively borne out. The influence of thermal history and polymerization conditions is considered. Motion has been examined also in several polymeric methacrylates, including the ethyl, n‐butyl, isobutyl, n‐hexyl, n‐octyl, lauryl. docosyl, and cyclohexyl esters. Some details of molecular configuration are considered for the ethyl ester. In poly(methyl alpha‐chloroacrylate) the low‐temperature values of the second moment are much smaller than expected. These depressed values are ascribed to rotation of all of the methyl groups about the threefold axis.
30(1959); http://dx.doi.org/10.1063/1.1734985View Description Hide Description
A survey has been made of spherulites formed by the isothermal crystallization of polypropylene from the melt in the temperature range 110°–148°C. Four types of spherulite have been distinguished and their structural morphology, optical properties, melting behavior, and growth rates have been examined.
30(1959); http://dx.doi.org/10.1063/1.1734986View Description Hide Description
An attempt is made to account for the observed birefringence of the various types of polypropylene spherulite in terms of preferred crystalline orientation. Evidence is also presented showing that some of these spherulites crystallize with a new metastable crystal structure, not previously described.
Nuclear Magnetic Resonance and Thermal Expansion in Partially Crystalline Polypropylene, Poly(Butene‐1), and Poly(Pentene‐1)30(1959); http://dx.doi.org/10.1063/1.1734987View Description Hide Description
The specific volume of partially crystalline specimens of polypropylene, poly(butene‐1), and poly(pentene‐1) from about 160°K to above the melting point has been studied. In addition, proton spin magnetic resonance of a partially crystalline poly(butene‐1) specimen has been observed over the 77°K to 320°K temperature range. Percent crystallinities as functions of temperature for polypropylene and poly(butene‐1) calculated from the specific volume data are compared. The poly(butene‐1) specimens show higher densities than the polypropylene samples at 170–180°K although the crystalliniites of the former are 25–40% lower than the latter. In addition, the temperature dependence of the specific volume from 160–280°K is greater for poly(butene‐1) than for polypropylene. Somewhat similar trends are noted in a comparison of the NMR(nuclear magnetic resonance) second moment versus temperature curves for these two materials.
30(1959); http://dx.doi.org/10.1063/1.1734988View Description Hide Description
A phenomenological theory of the molecular orientation dependence of mechanical breakdown phenomena is presented for a model linear high‐polymer solid. Taking into account the variation of the volume a general microscopic molecular model is first introduced. After simplification an approximation is considered. For one‐directional orientation the result indicates a rapid increase in the fracture strength as the degree of molecular orientation increases. The maximum fracture strength is found to increase to nearly six times the unoriented strength.
30(1959); http://dx.doi.org/10.1063/1.1734989View Description Hide Description
According to the molecular theory of rubberelasticity, the stress τ is related to the relative length α in simple extension according to τ=kT(ν/V)<α>2(α−1/α2), where (ν/V)<α>2 is a parameter of the network structure. Improved agreement with experiment has been claimed through use of the relation τ=(2C 1+2C 2/α) (α−1/α2) which contains the additional parameter C 2. In order to explore factors affecting this theoretically inexplicable term, stress‐elongation measurements were performed on networks prepared from natural rubber, Butyl rubber, poly‐(dimethylsiloxane), poly‐(ethyl acrylate), and poly‐(methyl methacrylate) under a wide range of experimental conditions. Results may be summarized as follows: (1) the C 2 term offers improved agreement over the extension phase of an elongation cycle, but not for the subsequent retraction phase; (2) apparent values of C 2 (determined on extension) vary widely from one polymer to another, increasing with hysteresis and with proximity to the glass transition temperature; (3) they are insensitive to the degree of cross linking; (4) C 2 is markedly reduced by permanently swelling the network with a diluent, or by swelling and deswelling the sample at each tension; (5) incorporation of diluent prior to establishing a network by cross linking with gamma radiation likewise lowers C 2, the value characteristic of the unswollen polymer being restored upon removal of the diluent. The results demonstrate that the controversial C 2 term arises from the difficulty of attaining elastic equilibrium; under ideal experimental conditions the value of this parameter becomes negligible. An alleged limitation of the theory of rubberelasticity is thus refuted.
30(1959); http://dx.doi.org/10.1063/1.1734990View Description Hide Description
The problem of the helical flow of a general fluid is solved making no special constitutive assumptions other than incompressibility. It is shown that from a knowledge of the rheologists'viscosity function one can calculate the dependence of the velocity field in helical flow on the geometry and applied forces. The complete stress distribution (including normal stresses) may be calculated from a knowledge of three basic material functions which can, in principle, be determined from an experimental study of simple shearing flow. Expressions are derived for the discharge rate and for the angular velocities of the bounding cylinders as functions of the applied forces.
30(1959); http://dx.doi.org/10.1063/1.1734991View Description Hide Description
An improved instrument for constructing particle size distribution diagrams from electron micrographs is described. Its use permits the measurement of large numbers of particles rapidly, and with low observer fatigue. The magnification of the image to be measured may be chosen to make the instrument error less than the resolution of the electron microscope.
The device consists of a measuring unit in the form of a pair of dividers, controlling a servosystem which automatically classifies the particles and plots the histogram as an array of steel balls in a slotted matrix.
Adjustments for zero and span are provided, and the total number of particles measured is recorded.
30(1959); http://dx.doi.org/10.1063/1.1734992View Description Hide Description
Single crystals of polyoxymethylene have been grown from dilute solution. They consist of hexagonal lamellae, often thickened by growth from screw dislocations. The chain molecules are normal to the lamellae which are 100 A thick. Thus, it is suggested that the molecules have a folded conformation. In many of the crystals double Bragg reflections were found in the electron diffraction patterns, resulting from rotation of the lamellae during growth. Dark field electron microscope photographs of these crystals exhibit moiré patterns.