Session H8 - Glasses.
MIXED session, Tuesday morning, March 17
402A, Los Angeles Convention Center

[H8.01] Viscoelasticity of Chain-like Inorganic Glasses

A series of oscillatory shear measurements have been conducted on phosphate based inorganic glasses of composition Na0.5Li0.5PO3 in the transition range. These glasses are characterized by a chain-like structure whereby covalent bridges are formed between PO4 tetrahedra. Ionic bonds are formed between the alkali ions and the nonbridging oxygens. A failure of time-temperature superposition was noted. At low frequencies, w, the loss modulus G''exhibits a power law dependence on frequency, where the exponent d increased with temperature from a value of 0.5 to 1.4 for T<Tg,where Tg is the glass transition temperature. It decreased rapidly to a value of 1 for T>Tg. The exponent, c, for the low frequency power law dependence of storage modulus reached a maximum of 1 near Tg and decreased to a value of 0.8 for T>Tg. The temperature dependence of the zero shear viscosity showed no break at Tg.

[H8.02] Influence of physical ageing on the heterogeneity of the amorphous phase of PET.

A comparison of the enthalpic and dielectric relaxations has been performed on samples obtained after various cooling rates. The structure of the glass is characterized by the fictive temperature deduced from the enthalpic measurements. The enthalpic relaxation time dependent of the structure has been evaluated. The dielectric relaxation times distribution has been obtained around the glass transition by the experimental technique of fractional polarisation. The distribution of relaxation times is described by a compensation law. This compensation phenomenon has been described by the structure dependent relaxation time introduced by Feirrera(G.F.L. Ferreira, J. Electrostatics, 36, 323 (1993)). Compensation parameters are not modified by physical ageing but an increase in the maximum value of the activation enthalpy is observed. According to the Hoffman-Williams-Passaglia(J.D. Hoffman, G. Williams, E. Passaglia, J. Polym. Sci., 14, 173 (1966)) model, activation enthalpies reflect the size of the domains where sequences relax cooperatively. The evolution of the size of the domains with physical ageing indicates the evolution of heterogeneity.

[H8.03] The Expansion Gap in Volume Recovery of a PVAc Glass: Kovacs' Data Revisited

In the well known asymmetry of approach experiment, Kovacs showed in 1964

that when logarithm of an instantaneous retardation time, tau-effective, was plotted against the volume departure from equilibrium, the results exhibit a surprising expansion gap. This gap, sometimes also referred to as an a pparent tau-effective paradox, is important for two reasons: to validate

models of structural recovery of glasses; and its existence is questioned

(L.C.E. Struik, Polymer, 38, 4677, 1997). Here we take a new look at the original Kovacs data, some of which was not reported previously, as well as data obtained in Kovacs' group under his supervision at a later date. In all we examined approximately 100 sets of data. We have done a

statistical analysis that accounts for correlation of errors and show convincingly that the expansion gap is real and, as such, can be used as a test of the various available models of structural recovery. This result contradicts the recent analysis by Struik in which he disputes the Kovacs

result but without accounting the correlation of errors in the experiments.

[H8.04] Sub-Glass Temperature Dependence of the Segmental Shift Factor for Polycarbonate: Evidence for Arrhenius Behavior

It is generally thought that the glass transition is marked by a strong temperature dependence of the viscosity or segmental relaxation time. Specifically, it is generally assumed, particularly for fragile glass formers such as polymers, that the viscosity has a Vogel-Fulcher-like or WLF-like temperature dependence that extrapolates to a singularity at the Kauzmann temperature approximately 50 K below the kinetic glass temperature. Here we show results obtained by aging a polycarbonate glass into equilibrium at temperatures from 20 K below the glass transition to about 5 K above it. Our results show that the WLF behavior is not followed and that the segmental shift factors follow a temperature dependence that is consistent with an Arrhenius behavior. Very interestingly, such behavior is predicted by a new model of the glass transition by DiMarzio and Yang [1]. 1. E.A. DiMarzio and A. Yang, J. Res. NIST, 102, 135 (1997).

[H8.05] Influence of conformational local order on high temperature retardation modes of PMMA.

Tacticity is highly responsible for different trends of \alpha and high temperature modes of PMMA. These different behaviors were studied over a series of PMMA with various tacticities at constant molecular weight. The Thermally Stimulated Creep (TSCr) technique, with multifilamented quartz braid acting as support, was used for analyzing retardation modes with a distribution function of retardation times. The low temperature tail of the \alpha retardation mode, at around 50^\circC, has been associated with the presence of smoother isotactic chain segments at their glass transition temperature. At upper temperature, activation enthalpies sharply increase and pass through a maximum at Tg. On the other side of the \alpha peak, a shoulder in activation enthalpies diagram is observed around T/Tg = 1.05 whatever the tacticity is. Moreover cooperativity, in the sense of Starkweather, vanishes at T/Tg = 1.1 for all samples. These two features of enthalpic diagram are coherent with the existence of tacticity dependant retardation modes in the corresponding temperature range. This behavior might be explained by the existence of a physical network due to interactions between conformationaly ordered sequences.

[H8.06] Physical Ageing in Polystyrene Monitored by Probe Rotation

A photobleaching method has been used to measure rubrene and tetracene reorientation during the physical aging of polystyrene at temperatures from Tg - 2K to Tg - 10K. Probe rotation times are completely coupled with the alpha relaxation and ranged from 10 to 10000 s. In down-jump experiments, rotation times increased by up to two orders of magnitude during physical aging. Comparison of up-jump and down-jump experiments revealed the expected assymetry when the temperature jump was greater than 2 K. The time required for probe rotation to reach equilibrium is in good agreement with volume and enthalpy studies. The final rate of approach to equilibrium is different for the two probes. These results suggest that relaxation during physical ageing is spatially heterogeneous and that different observables are sensitive to relaxation on different length scale s.

[H8.07] The Role of Glass Transition Contrast in Miscible Polymer Blend Dynamics

We have studied four miscible polymer blend systems using oscillatory shear at various constant temperatures. We quantify the breakdown of time-temperature superposition in these blends using the temperature dependence of the minimum in the loss tangent at the high frequency end of the rubbery plateau. For systems with a small difference in the glass transition of the two pure components (\Delta T_g), such as SAN/PMMA (\Delta T_g = 20K), time-temperature superposition works reasonably well, and the minimum in the loss tangent barely changes with temperature. As the glass transition contrast increases (\Delta T_g = 63K for PI/PVE, 125K for PS/PVME and 180K for PEO/PMMA), the failure of time-temperature superposition becomes more drastic. The magnitude of the change in the minimum of the loss tangent with temperature increases systematically as the contrast in component glass transitions increases.

[H8.08] Polymer chain dynamics in the glassy and melt states as studied by optical rheometry: Polystyrene

We have used rheooptical methods to study polymer chain dynamics in the glassy and melt states in narrow molecular weight fractions of polystyrene: mechanical shear stress and optical retardance were simultaneously monitored during oscillatory shear. The modified stress optical rule (MSOR) of Osaki and coworkers was used to separate glassy and rubbery contributions to overall shear modulus; master curves of these contributions, and of the total shear modulus, were generated via time temperature superposition. Our estimates for Me, Mc, and h0 are consistent with current literature. The rubbery modes shift more strongly with temperature than the glassy modes, giving rise to thermorheological complexity. The stress optic rule (SOR) is valid in the melt state, and invalid at higher frequencies close to the glass transition. We also discuss two new results from a molecular perspective: (1) The glassy contribution is thermorheologically complex at higher frequencies for all molecular weights. (2) The stress optic ratio (Cr, evaluated as the ratio of birefringence and shear stress at low frequencies in the melt state) decreases with temperature for high molecular weights (M > Mc), as predicted by the Rouse theory, but appears to increase with temperature for low molecular weights (M < Mc).

[H8.09] Free-volume Hole Properties of Gas-Exposed Polymers Probed by Positron Annihilation

Positron annihilation lifetime experiments have been performed in a polysulfone film as a function of temperature and gas exposures. A large variation of free-volume hole size and distribution is observed as a function of gas exposure. Applications of positron annihilation to study membrane systems will be discussed.

[H8.10] Influence of Local Structure on Viscoelasticity and Mositure Transport of Physically Aged Network Glasses

This abstract was not submitted electronically.

Part H of program listing