Thor L. Smith

Intrinsic viscosities and other rheological properties of flocculated suspensions of nonmagnetic and magnetic ferric oxides

Abstract The viscosity of flocculated suspensions of rhombohedral and acicular (needle-like) αFe2O3 (nonmagnetic) and acicular γFe2O3 (magnetic) particles in ethylene glycol was measured over broad ranges of concentration at shear rates D from a few sec−1 up to 1340 sec−1. The intrinsic viscosity, [η], derived from the concentration dependence of the viscosity, depends linearly on D−γ, where γ is rm1 2 and rm1 3 for suspensions of the acicular and rhombohedral particles, respectively. Intrinsic viscosities are discussed in terms of φpf = 2.5/[η], where φpf is the volume fraction of particles in a floc. At a D of 5.3 sec−1, [η] is about 30 for suspensions of the nonmagnetic particles and is 80 for the magnetic particles. The intrinsic viscosity at infinite shear rate, which increases from 3.3 to 6.0 with the axis ratio of the particles, indicates that small stable flocs exist. From [η] at 5.3 sec−1 and results from a theoretical treatment of floc growth by Vold, the floc size and number of particles in a floc were estimated and are compared with results from sedimentation rates. The dependence of shear stress on D conforms to the Casson equation, which gives the yield stress and viscosity at infinite shear rate. Data are also presented on xylene suspensions of acicular αFe2O3 particles stabilized with a polymeric dispersant.

Rejuvenation and physical ageing of a polycarbonate film subjected to finite tensile strains

Differential storage and loss tensile moduli, E′ and E″, were determined intermittently at 10 Hz on specimens of an annealed polycarbonate film during stress relaxation at static tensile strains from 1.2 to 6.25% at 50°C. It was found that E′ and 1E″ decrease when a static strain is applied but thereafter they increase progressively with time. These changes, which increase with the applied strain until it becomes 4%, are attributed primarily to a rejuvenation of a specimen (an increase in segmental mobility) followed by physical ageing (a progressive decrease in segmental mobility). Measurements at a static strain of 3% at six temperatures from 30°C to 130°C showed, among other things, that the rates of increase of E′ and decrease of E″ are sensibly independent of temperature up to 110°C.

Effect of tensile deformations on gas transport in glassy polymer films

An exploratory study has been undertaken to determine the effect of simple tensile deformations, applied stepwise, on the permeability and diffusion coefficients (P and D) of gases in glassy polymers. For nitrogen in a polycarbonate film at 73°C, it was found that (1P)dPdϵ ⋍ 13 and (1D)dDdϵ ⋍ 5 to 8, where ϵ is the strain. Also, P and D were found to decrease with time at constant strain and to depend somewhat on strain and thermal histories. A calculation of (1D)dDdϵ, based primarily on considerations of free volume and its strain dependence, gives 11.8 or 8.5, depending on the assumptions made. Progressive stepwise application of strain to a polyimide (Kapton) film at 72° and 125°C caused P and D for carbon dioxide to increase. But above 2% strain at 125°C, they decreased until at 6% strain they became smaller than those for the undeformed specimen, a reflection of a straininduced relaxation process.

Physical ageing of stretched specimens of a polycarbonate film and its temperature dependence

Well annealed specimens of polycarbonate film were first subjected rapidly to a static elongation of 2.6% at 13 temperatures from −110 to 110°C. During stress relaxation at each temperature, the static strain was modulated with a sinusoidal tensile strain of small amplitude (⩽ 0.1%), usually from 0.4 to 25 Hz, to determine the (differential) storage modulus E ′. Such measurements were repeated intermittently at values of the elapsed (ageing) time t e during periods of several hours or more. During such periods, E ′ increased continuously, a reflection of physical ageing, a hardening process. Initially, the storage modulus E′ 0 was significantly higher than after the static elongation was applied; the latter produces de-ageing, a softening process. From the data, the ageing rate d log E ′/d log t e was evaluated and found to be independent of frequency and t e . The ageing rate showed a minimum at about 30°C, a pronounced maximum at –40°C and then became almost zero at –110°C. Also discussed is the temperature dependence of d log E ′d log f , where f is the frequency, and the effect at two temperatures of different values of the static elongation on both the ageing rate and the frequency dependence of E′ . From all data, the ageing-rate parameter μ was also evaluated and discussed.

History- and temperature-dependent yield phenomena of polycarbonate related to its rate of physical ageing

Abstract The yield force (or stress) of polycarbonate in simple extension was studied to determine its dependence on deformation history, including the stretching rate, and temperature. At different temperatures, the stretching of a specimen at a constant rate was interrupted both before yielding and during cold drawing to allow the occurrence of stress relaxation and the accompanying physical ageing (hardening). When a test was resumed, progressive erasure of ageing occurred, leading to a yield force whose value compared to that obtained by continuous stretching can be explained qualitatively in terms of the rate of physical ageing, determined earlier at an extension of 2.6% over a wide temperature range. In addition, the present data support the conclusion reached previously that physical ageing occurs during stress relaxation at a fixed deformation. Also determined were the yield and drawing stresses and the yield strain from −90 to 100°C.

Thermal Expansion And Viscoelastic Properties Of A Semi-Rigid Polyimide

Studies were made of the physical properties of the commercially available polyimide Upilex-SGA, which is prepared from biphenyl dianhydride and p-phenylene diamine. Annealing the Upilex-SGA for 2 hr Linder N 2 at 400°C gave a film that expanded continuously when heated at a fixed rate, in contrast to the as-received film. The linear expansion showed a change of slope at 84°C and also at 295°C, the later being T g . The thermal coefficient of linear expansion at all temperatures was very small, even above 295°C it is 27.8 × 10 −6 . Its stress-strain curve did not exhibit a yield point, even though its ultimate elongation is ˜23%. Similar behavior is shown by the PMDA-ODA polyimide, except its ultimate elongation is ˜70%,. The unusual stress- strain curves exhibited by these polyimides is undoubtedly caused by their liquid-crystalline morphology. The stress-relaxation modulus was measured at 0.5% extension and 12 temperatures from 30 to 330°C. Derived isochrones showed that the 1-s tensile modulus at 20°C is 9.0 GPa, but at 330°C it is 2.0 GPa. Creep curves were also measured at a stress of 30 MPa and at 10 temperatures from 30 to 340° C. Master curves prepared from the relaxation and creep data are discussed briefly and evidence is given which, show that the superposition method is not truly valid for this polyimide, which actually is not surprising.

Time-Dependent Properties of Statically Deformed Glassy Polymers: Dynamic Modulus and Gas Transport

After an amorphous polymer has been cooled from above to substantially below its glass transition temperature, its volume decreases continuously with time under isothermal conditions, as has long been recognized.1 This spontaneous volume contraction is accompanied by a progressive decrease in the mobility of short molecular segments and therefore by an increase in the relaxation times which affect the viscoelastic and other physical properties of the glassy polymer, as discussed by Struik.2,3 This physical aging process can be reversed, at least somewhat, by applying a finite tensile strain, which gives an increase in volume. Immediately thereafter, aging begins anew at an increased rate.