Volume 28, Issue 10, 01 October 1957
Index of content:
- SPECIAL ISSUE ON HIGH‐POLYMER PHYSICS
Nuclear Magnetic Resonance, Radiation Damage, and Rigidity in Branched Polyethylene as a Function of Temperature28(1957); http://dx.doi.org/10.1063/1.1722581View Description Hide Description
Molecular motions and microstructure are studied as a function of irradiation damage and temperature in branched polyethylene. The methods of proton magnetic resonance, specific volume and density percent crystallinity, mechanical rigidity and loss, and x‐ray determined crystallinities are employed. The samples were subjected to irradiation doses ranging from 0 to 8.3×1018 nvt in the Brookhaven reactor. Significant changes in the proton resonance line shape at 295°K occur for an irradiation of 0.3×1018 nvt. Second moments and intensity ratios indicate that the first effect of irradiation on the line shape can be interpreted as due to the destruction of crystallinity or order. This effect however is offset at higher doses by the restrictive influence of radiation induced crosslinks on molecular chain motion. Estimates of the percent crosslinking aid in the interpretation of the temperature variation of proton resonance line shapes and allow some estimate to be made of the lengths of chain associated with a given type of molecular motion. Activation energies are calculated from NMR and mechanical loss data for these molecular motions.
With increasing temperature the proton resonance and mechanical properties are associated first with rotational oscillations, over energy barriers, of short segments (of the order four methylene groups in length) of a chain (activation energy ∼6 kcal/mole) and also with the γ peak of the mechanical loss data. At temperatures exceeding the ``glass transition temperature'' the molecular movements are also described by neo‐Brownian diffusional motions involving chain lengths of about 10 methylene groups (activation energy ∼12 kcal/mole). These latter ``neo‐Brownian'' motions are associated to some extent with the β peak of the mechanical loss data and probably also involve some branch point motion. The majority of the motion occurs principally in the amorphous regions of the branched polyethylene until temperatures above 290°K are reached and the microstructure begins to change due to the ``melting'' of crystallites.
The intensity ratio for the complex line shape exhibited by polyethylene at temperatures greater than 200°K are associated with the high‐frequency rigidity of the sample.
28(1957); http://dx.doi.org/10.1063/1.1722582View Description Hide Description
Nuclear magnetic resonance(NMR) studies of three polyethylenes have been made over the temperature range −190°C to 120°C. Two components of the NMR line are observed—a narrow one identified with the ``amorphous'' regions and a broad one identified with the ``crystalline'' regions. Differences between polyethylenes in the shape of the narrow component are observed and discussed.
The variation of line width of the two components with temperature indicates two motional transitions occurring in the ``amorphous'' regions and one in the ``crystalline'' regions over the temperature range studied. Energies of activation inferred for one of the ``amorphous'' transitions agree well with those determined from melt viscosity studies implying a similarity of molecular processes controlling the two measurements and allowing an interpretation of the one observed by NMR to be made.
The NMR data are used to interpret (in terms of molecular motions) the three dispersion regions observed by dynamic mechanical measurements, with the following results: in the vicinity of −100°C, the amorphous regions undergo a transition involving mainly the onset of rotation of linear segments of the polymer chains. Between −35°C and 0°C (depending on the sample), the motion of amorphous chain segments containing branch points leads to a transition. In the neighborhood of 60–100°C (depending on the sample), there are the beginnings of observable motion of the chain segments within crystallites.
Using a new method of decomposing the NMR line into its two components (based on the experimental line shape at low temperatures), the relative intensities of the two components (hence, the relative amounts of fixed and moving nuclei) are determined. This ratio, previously held to be identical with the ``percentage crystallinity,'' is shown to be inconsistent with this view and to support the molecular interpretation presented.
28(1957); http://dx.doi.org/10.1063/1.1722583View Description Hide Description
Two‐dimensional spherulites in thin films of the 6–6 nylon have been studied by selected‐area electron diffraction, bright‐field and dark‐field electron microscopy. The electron diffraction patterns were found to be comparable to studies using limited area x‐ray diffraction. In this sample they showed that the molecules were arranged in sheets with the hydrogen bonds in the plane of the film. Large field areas were used and as a result no preferred direction of orientation was seen. However, in dark‐field electron microscopy, by using only those Bragg electrons scattered into limited portions of the principle rings, the radical growth and branching of discrete crystalline regions was seen. Bright field electron microscopy showed the surface morphology of these structures.
28(1957); http://dx.doi.org/10.1063/1.1722584View Description Hide Description
The effect of molecular orientation on the stress‐optical coefficient of polystyrene at room temperature (24°C) was measured using polystyrene monofilaments with different degrees of orientation. High degrees of orientation were obtained by cold‐stretching. The optical measurements were made using a polarizing microscope fitted with a Sénarmont compensator. The stress‐optical coefficient shows a strong dependence on molecular orientation, using birefringence as the index of molecular orientation. The stress‐optical coefficient appears to decrease linearly vsbirefringence, from a value of about +10 brewsters at zero birefringence to a value of about +4 brewsters at −0.04 birefringence. The elastic(Young's)modulus was also measured (in tension) as a function of orientation, and was found to increase nonlinearly with orientation (from 4.3×105 psi at zero birefringence to 6.1×105 psi at −0.04). A curve of strain‐optical coefficient vs orientation was obtained by multiplying stress‐optical and modulus values; this decreases nonlinearly and to a somewhat lesser extent than the stress‐optical curve (from about +0.03 at zero birefringence to +0.017 at −0.04). The significance of these constants in photoelastic experiments is discussed.
28(1957); http://dx.doi.org/10.1063/1.1722585View Description Hide Description
The effect of temperature on the stress‐optical coefficient of polystyrene was measured at twelve different temperatures from −195° to +24°C using samples of unoriented polystyrene sheet. The stress‐optical coefficient appears to decrease with temperature from a value of about +17 brewsters at −195°C to a value of about +10 brewsters at room temperature. The elastic (Young's) modulus of polystyrene was also measured as a function of temperature using a flexural technique and was found to decrease linearly with temperature from 6.36×105 psi at −198° to 4.65×105 psi at +24°C. A curve of strain‐optical coefficient vs temperature was obtained by multiplying stress‐optical and modulus values; this curve increases with decreasing temperature from about +0.03 at +24° to +0.073 at −195°C.
28(1957); http://dx.doi.org/10.1063/1.1722586View Description Hide Description
Spherulite growth rates have been measured in two polyamides, 66 and 6 nylon, at temperatures ranging from 38 to 141° below the melting points using a moving photomicrographic technique. At constant temperature, the spherulite radius increases at a constant rate. This isothermal growth rate constant is temperature dependent, having low values just below the melting point, passing through a maximum on further supercooling and decreasing to negligible values at room temperature. These facts indicate that spherulite growth is not a diffusioncontrolled process. A reasonable interpretation of the data has been obtained using the concept of growth by two‐dimensional surface nucleation. A theoretical expression for the temperature dependence of the growth rate based on this concept has given good agreement with the experimentally determined values.
28(1957); http://dx.doi.org/10.1063/1.1722587View Description Hide Description
The applicability of Nutting's equation of the mechanical behavior of some soft plasticized polyvinyl chloride preparations noted by Dyson in creep experiments is further demonstrated by force‐length measurements at constant rate of strain. This suggests new criteria for the characterization of soft plasticized PVC. For example, in a stretching experiment in which the sample is subjected to a strain (L−1) which increases linearly with time (t) at the rate (R), definition of ``modulus'' as the limit at small strains of the ratio of the force (F) to the strain is clearly inappropriate. That is, when which may be zero or infinite and also depends on the strain rate.
Some even more general force‐length‐time relationships for plastic are discussed which have been found useful for predicting the conditions of tensile failure. More complicated relationships of particular interest provide for the possibility of force‐time superposition of creep curves. Published creep data are considered from this point of view.
28(1957); http://dx.doi.org/10.1063/1.1722588View Description Hide Description
Drawn, oriented film of polyethylene terephthalate was studied by means of an x‐ray diffraction technique in which the x‐ray beam was directed in three mutually perpendicular directions: (1) through the film, (2) and (3) in the plane of the film, parallel and normal to the direction of draw. Patterns were obtained in each of these directions for the diffractions occurring at both small and large angles. This combination of patterns provides a picture of the film'spolymer texture with these parameters: size, orientation, and arrangement of crystallites; extent of amorphous material; shape of microvoids. In this film the crystallites have planar orientation and are 45 A wide in the plane. A long period of 125 A exists in the direction of draw and is comprised of 75‐A crystallite length and 50‐A amorphous material. There is evidence that the film is composed of lamellae about 60 A thick; these lamellae are stacked in a staggered arrangement such that the crystallites in one lamella are adjacent to the amorphous regions in the lamella below. The small angle diffraction evidence cannot be reconciled with the ``coiled ribbon'' arrangement recently proposed in the literature to explain off‐meridional diffraction spots.
28(1957); http://dx.doi.org/10.1063/1.1722589View Description Hide Description
The heat of sublimation of linear paraffin chains from the crystalline form to the gas at 0°K is calculated from heats of fusion, heats of vaporization, and heat capacities of solid,liquid, and gaseous hydrocarbons. The heat of sublimation, which is defined as the lattice energy of the crystalline phase of linear polyethylene, is 1.84 kcal/mole of CH2 groups. The heat of fusion of crystalline linear polyethylene is 0.922 kcal/mole of CH2 groups.
Flow Birefringence and Stress. V. Correlation of Recoverable Shear Strains with Other Rheological Properties of Polymer Solutions28(1957); http://dx.doi.org/10.1063/1.1722590View Description Hide Description
The correlation of normal stress with the shear stress and flowbirefringence of flowing solutions has been extended to include the recoverable shear strain s which can be measured directly by a rotational viscometer in ``recoil.'' This has been shown experimentally with solutions of a high‐viscosity nitro‐cellulose in n‐butyl acetate and a viscous polyisobutylene. These results show that the principal axes of the tensors of stress, strain, and optical anistropy coincide, even when no constant shear modulus (shear compliance) exists.
28(1957); http://dx.doi.org/10.1063/1.1722591View Description Hide Description
Measurements of elastic modulus and mechanical loss at audio‐frequencies have been obtained on irradiated and nonirradiated specimens of polyhexamethylene adipamide over a broad temperature range extending from 80°K to 600°K. The irradiations were carried out at the Brookhaven and Penn State Reactors. The internal friction spectrum of unirradiated nylon 6–6 contains at least four peaks corresponding to different mechanisms being invoked as the temperature is raised. With irradiation doses from 0.3×1018 nvt to 5.5×1018 nvt, significant changes in the dispersion regions occur. At temperatures above the main softening region, an increase in modulus with temperature is found. This is evidence of rubber‐like behavior and is accounted for by the introduction of cross‐links resulting from ionization and free radical formation produced by the irradiation. At high irradiation doses the percent cross‐linking appears to become independent of dose. The various effects found are discussed in terms of molecular forces and mobility of chain segments.