Softening Dispersion Research Articles

Viscoelastic Behavior of Amorphous Polymers near the Glass Temperature

AbstractLack of thermorheological simplicity and possible effects of the molecular weight distribution in the softening dispersion of linear amorphous polymers are demonstrated with viscoelastic data on polymethylacrylate PMA and polyvinylacetate PVAc. Superposition of the short time portion of the softening transitions of the retardation spectra of PMA, PVAc, polystyrene, and amorphous polypropylene APP indicates that the local molecular mobility is a constant at Tg within experimental uncertainty. The general features of the retardation spectra of linear amorphous polymers are discussed.

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

AbstractThermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from Tg + 25°C to Tg + 80°C. Between Tg and Tg + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time‐scale shift factors aT different from those of the terminal (rubberlike to steady‐state) dispersion. The aT values calculated from zero‐shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (Tg ≅ 35°C). The aT shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The aT obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some aTs which cannot be represented by the usual Doolittle free‐volume expression.

Thermomechanical trnasitions in polyphosphazenes

AbstractUnoriented and oriented samples of two phosphazene homopolymers with chemical structure [NPX2]n, where X = CF3CH2O and p‐CH3C6H4O, and three unfilled and filled alkoxy‐type polyphosphazene copolymers, plus one filled aryloxy‐type copolymer were examined with a Rheovibron viscoelastometer. Measurements were made from −120°C to temperatures above the T(1) transition of the respective polymers. The effects of orientation and repeated temperature cycling through the T(1) transition temperatures were studied. The overall physical behavior of these polyphosphazenes was dominated by changes that occurred in the primary softening dispersion and the mesomorphic or T(1) transition region. The enthalpy of the T(1) transition and the T(1) temperature were investigated by differential scanning calorimetry (DSC) as a function of temperature cycling in the T(1) region; for instance, the change in enthalpy in the T(1) region is about an order of magnitude larger than it is at Tm for the unfilled polyphosphazene homopolymers. The Tg's of the polyphosophazenes were also determined. Density measurements were made on oriented and unoriented specimens by the flotation method. Although an increase in density (and crystallinity) was observed when specimens were cycled through the T(1) transition, no significant difference in density was found between oriented and unoriented materials. Some discussion of the molecular origin of the T(1) transition is included.

The Temperature Dependence of the Viscoelastic Behavior of Poly(vinyl acetate)

The torsional creep and creep recovery behavior of a high molecular weight (Mw=6.5×105) poly(vinyl acetate) is reported for the temperature range, 37.5°C to 154°C. The results for the recoverable compliance, Jr(t), reveal the usual two dispersions seen between glassy and steady state behavior. The intermediate rubbery plateau compliance, JN, has a value of 2.8×10−7 cm2 dyn−1, reflecting a molecular weight per entangled unit of 8,500. Though the specimen studied was a fraction, the terminal dispersion was extensive. The steady-state recoverable compliance, Je°, was approximately 80 times larger than JN. Because there was considerable information on the terminal dispersion, it was possible to independently determine its temperature dependence, which was found to be indistinguishable from that of the viscous deformation. However, this temperature dependence is significantly different from that of the softening dispersion. The differences observed are in the same sense as that found previously for the softening dispersion and viscous flow dependences of polystyrene.