SynopsisAs an aid in assessing the ability of antioxidant additives to persist in polymers and thus remain effective in protecting against oxidation, the solubility and diffusion COefficient of two antioxidants in branched polyethylene have been determined in this work. A method was developed for this purpose by which the diffusion coefficient and solubility could be determined simultaneously. The method consists of analyzing the concentration profile across a stack of polyethylene sheets through which the antioxidant was allowed to diffuse. The concentration of antioxidants in polyethylene was determined by a thermogravimetric technique which relies directly on the ability of the additives to suppress oxidation reaction. The diffusion coefficients determined showed excellent agreement with values in the literature which were obtained by a radiotracer method. The solubility of the antioxidants in three normal hydrocarbon solvents of varying molecular sizes was also determined by a conventional technique at various temperatures and found to correlate well with their solubility in polyethylene determined by the diffusion method. In particular, the dependence of the solubility on the size of solvent molecules and on temperature agrees well with an equation derived on the basis of the regular solution theory of liquid mixtures.
Single crystals of linear polyethylene, prepared from a dilute xylene solution, were annealed below their melting temperature under atmospheric and 6 kbar pressure. In order to preserve the identity of the single crystals, they were suspended in an inert solvent medium, silicone oil and ethanol, during annealing. Examination of the annealed crystals under an electron microscope revealed development of numerous reorganization centers consisting of a central, elongated hole surrounded by a raised edge. Characteristics of these holes, especially their location and orientation, were interpreted in terms of the molecular packing that existed prior to the annealing and the mechanism of molecular reorganization that occurred during the annealing. The effect of high pressure was primarily to flatten out the crystals and to increase the number of reorganization centers, but the height of the raised edges remained about the same irrespective of the applied pressure. The present study also showed examples pointing to the importance of differentiating the annealing behavior of monolayer crystals from that of multilayer crystals.
Fiber spinning experiments were carried out with an α‐methyl styrene/silicone block copolymer under various sets of spinning conditions. The behavior observed was very sensitive to the ambient axial temperature profile employed along the spinline and to the initial melt temperature at the die. By optimizing these parameters, very high draw ratios (>400 to 1) could be achieved. Under less optimum conditions, filament rupture and instabilities such as draw resonance, accompanied by periodic diameter and spinline tension fluctuations, were noted. Tensile stress and axial velocity gradient profiles were obtained along the spinline under a variety of spinning conditions. These profiles, together with an independent: rheological characterization of the polymer, provide insights into the mechanisms giving rise to the various types of behavior observed.
The long term stability of biaxially-oriented poly(viny1idene fluoride) has been examined. In isothermal aging from a few minutes to a Few months, log c vs. time shifts between 6OoC at 88°C are characterized to have an activation energy of 1.2 eV. Data on uniaxial poled commercial film shows agreement with this result. Long term room temperature stability of pyroelectricity over a 10 year span augmented other elevated temperature data in supporting the conclusion that thermally activated processes should not be a factor in device reliability.
Measurements of dynamic shear impedance at five frequencies from 23 to 300 MHz and at 25°C are reported for a monodisperse polystyrene as a function of concentration. The concentrations ranged from 3% to 20% polymer in di-n-butyl phthalate, a near-theta solvent, and covered the region from the start of coil overlap to well beyond entanglement. Results are reported in terms of the in-phase, G′−ν1Gs′, and quadrature, G″−υ1ωηs′, components of the dynamic shear modulus of the polymer, where Gs′ is the solvent contribution to the in-phase modulus, ω is the angular frequency, ηs′ is the dynamic solvent viscosity, and υ1 is the volume fraction of solvent. The use of dynamic values for the solvent follows from observation of relaxation and non-Newtonian behavior in the solvent beyond 100 MHz. The usual reduced variables method, even in this modified form, could not be successfully applied to superimpose data at different concentrations, indicating the need for further modification to account for finite concentration effects. The dynamic solution viscosity η′ is found to increase with increasing concentration at fixed frequency. At any one concentration, it decreases with increasing frequency above 200 MHz. This is in contrast to the nearly constant values attained at lower frequencies at concentrations to 15% polymer; the decrease is too large to be accounted for by only solvent viscosity effects. Results are also reported in terms of the reduced dynamic viscosity (η′ − υ1ηs′) / (η − υ1ηs), where η and ηs are the steady-flow values for the solution and the solvent, respectively. A high-frequency limiting value of the reduced viscosity is found to obtain for the lower concentrations at the three lowest frequencies. However, for the highest concentration the limiting value is believed to occur at frequencies lower than those measured, so that a further decrease is, in fact, being observed above 100 MHz. An estimate of 10−1.04 was obtained for the reduced high-frequency limiting viscosity in the limit of infinite dilution. From an estimate of the number of statistical segments (259) based on intrinsic viscosity measurements and on the results of Thurston, and an estimate of the extent of hydrodynamic interaction per segment, h*, of 0.2, a value of 1.97 was obtained for φ/f, the ratio of the internal to segmental friction coefficients. The concentration dependence of φ was determined assuming little or no change in h* with concentration, and an appropriate dependence of f on concentration and viscosity. It ranged from c0.26 below entanglement to c0.07 above it, approximately the one-fourth and zeroth powers, respectively. The concentration dependence of (η′ − υ1ηs′) increased by about c1 while that of (η − υ1ηs) increased by c2 on going from concentrations below to those above entanglement. Plots of the dynamic impedance against frequency indicate that departure from Newtonian behavior occurs sooner for the quadrature component than for the in-phase part. Solvent relaxation was evaluated in terms of Lamb's semiempirical theory for low-molecular-weight liquids. From a comparison of the observed frequency dependence and the reference plots of Lamb, values for G∞′ and τ, the high-frequency limiting dynamic shear modulus and the average relaxation time, were obtained of 1.97×1010 dyn/cm2 and 8.5×10−12 sec, respectively. The magnitude of the latter is discussed and is shown to be in reasonable agreement with estimates of Pinnow, Candau, and Litovitz for n-alkyl bromides. Based on values of the reduced dynamic viscosity obtained by Ferry, Holmes, Lamb, and Matheson at 73 kHz and upon the present results, it appears that the high-frequency limiting viscosity of polystyrene in a near-theta solvent persists for about 312 decades before it decreases further at a frequency in the vicinity of that for the onset of solvent relaxation. Extension of optical techniques (e.g., Brillouin scattering) to studies of polymer solutions is briefly noted.
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