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The enthalpy recovery of polystyrene materials with narrow molecular weight distribution is studied by means of fast calorimetry. Two different molecular weights, Mw, have been investigated, namely 2000K and 212k. Intrinsic isotherm experiments at aging temperatures ranging from above to below the nominal Tg have been reported. The effect of the cooling rate on the glass transition temperature is also exanimated and it is showed that the cooling rate-dependence of the Tg is Vogel-Fulcher and not Arrhenius. The main finding is that the enthalpy recovery kinetics is not affected by the molecular weight showing that the contribution to free volume related due to chain terminals is negligible in the molecular weight range (212k-2000k) analysed in this paper.

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (Tg) on film thickness. Sizeable reduction of Tg was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of Tg on film thickness is consistent with the published data of free-standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n-butyl methacrylate), but with large variation in the change of either the Tg or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub-Rouse modes, and the segmental α-relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

The bulk and shear rheological properties of a sym- metric three-arm star polystyrene were measured using a self- built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure-volume-temperature behavior, the pressure-depend- ent glass transition temperature (Tg), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher Tgs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above Tg obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using TTg scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear poly- styrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecu- lar mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short-time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1233-1244, 2012

An expression for the relaxation time, τ, which applies both in the liquid and in the glassy state, is proposed and coupled with the kinetic equation for the volume relaxation of the KAHR (Kovacs, Aklonis, Hutchinson, and Ramos) phenomenological theory. The expression for τ reproduces the WLF (Williams, Landel, and Ferry) behavior above the glass transition temperature, contains the thermodynamic scaling proposed by Casalini and Roland, gives finite values of the equilibrium relaxation time for any value of temperature and pressure, and is assembled in way that the relationship between the logarithmic relaxation time and the internal order parameter is strongly non-linear even when the system is very close to the equilibrium. The resulting model contains ten parameters, and five of these are treated as fitting parameters. With one set of model parameters, the model is able to describe quantitatively the isobaric specific volume response on cooling at various rates and as a function of pressure, the overshoot response on heating after cooling at different rates, the pressure dependence of Tg, the asymmetry of response that characterizes volume relaxation after temperature up and down jumps, and the tau-effective paradox and associated expansion gap. The model also reasonably predicts the overshoot response observed after the two-step memory experiment.

The principal features of the volumetric as well as the viscoelastic response of mechanically stimulated glasses can be summarized as follows: i) the time-aging time shift factors contract upon increasing the probe stress (i.e. the stress apparently modifies the volume recovery kinetics), ii) the volume recovery baseline remains unaltered (i.e. the underlying structure of the stimulated glass remains unchanged) iii) yielding scales linearly with the logarithmic of the strain rate. Here we present a review of the above features with aid of a series of numerically simulated results concerning the responses of glassy polycarbonate. Simulations are obtained coupling a modified KAHR equation with the constitutive law for linear viscoelasticity within the domain of the reduced time. It will be shown that by using a minimum of experimental (PVT and linear viscoelastic) data inputs even the subtle intricacies can be predicted. Furthermore a new class of results concerning the stress-strain behaviour of glassy polymers is presented that never appeared before.

The structural recovery of a single polystyrene thin film is investigated using nanocalorimetry at aging times as short as 0.01 s, as well as aging at temperatures as high as15 K above the nominal glass transition temperature for high fictive-temperature glasses obtained at high cooling rates. The results indicate that structural recovery progresses as expected when the aging temperature is low compared to the initial fictive temperature; in this case, the fictive temperature evolves smoothly towards the aging temperature at a rate that depends on the aging temperature and initial fictive temperature (i.e., on the cooling rate prior to aging), and at equilibrium, the fictive temperature Tf = Ta. For the case when the aging temperature is much higher than the initial fictive temperature, no relaxation occurs during isothermal aging. For the intermediate case when the aging temperature is in the vicinity of the initial fictive temperature, relaxation occurs both during isothermal aging and during cooling after aging, with the result that the equilibrium fictive temperature is lower than the aging temperature. The use of a new equation for the relaxation time in the framework of the Tool–Narayanaswamy–Moynihan (TNM) model allows description of the data with a single set of model parameters.

The density evolution of a semi-crystalline Poly(lactic acid) under ultra/high pressure histories in the region of alpha relaxation has been studied. It was found that the degree of crystallinity does not change in the temperature and pressure ranges under concern and consequently it is assumed that the volume relaxation kinetics involves only the amorphous part of the polymer. The scaling law for relaxation times, recently proposed by Casalini and Roland, has been utilized in the framework of KAHR (Kovacs Aklonis Hutchinson and Ramos) phenomenological theory in order to model the volume relaxation behavior. With this approach the model contains only two fitting parameters, (namely the relaxation time in the reference state, τg, and the fractional exponent, β, that describes the dispersion of the alpha relaxation) instead of the five parameters of the classical KAHR theory. The model predicts accurately both the density evolution of the PLA during the sterilization and Pasteurization treatments simulated in a pressure dilatometer and the density changes occurring in the PLA films utilized as packaging material in the HP industrial processes.

The scaling law for relaxation times (τ's) recently proposed by Casalini and Roland was used in the framework of Kovacs, Aklonis, Hutchinson, and Ramos phenomenological theory. With this approach, it was shown that both the isobaric and the isothermal glass transitions of polystyrene can be reliably predicted with only two fitting parameters, namely, τ in the reference state and the fractional exponent that describes the dispersion of the α relaxation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

The scaling law for relaxation times, recently proposed by Casalini and Roland, is utilized in the framework of KAHR (Kovacs, Aklonis, Hutchinson, and Ramos) phenomenological theory. With this approach it is shown that the pressure, volume, and temperature (PVT) data obtained on Poly(methyl-methacrylate) (PMMA) can be reliably predicted, in the region of the alpha-relaxation, by using only two fitting parameters, namely: the relaxation time in the reference state, τ g , and the fractional exponent, β, that describes the dispersion of the alpha-relaxation.

Glassy polymers confined at nano‐scale do exhibit anomalous thermo‐viscoelastic behavior in respect to their bulk response. For example, in ultra‐thin polymer films both negative coefficient of thermal expansion (NCTE) and glass transition temperature (Tg) depression are observed, experimentally. Theoretical conjectures have been drawn that qualitatively explain some features of highly confined glassy polymers [1,2,3] yet quantitative prediction of NCTE and Tg depression would allow a reliable design of devices operating at nano‐scale. In this paper the thin films anomalies are shortly reviewed and their explanation is tempted with classical existing theories.

Statistical copolycarbonates containing different bisphenol A/tetramethyl bisphenol A (BPA/TMBPA) molar ratios were synthesized by polycondensation reaction between a mixture of the two bisphenols and phosgene. The reaction conditions were selected for producing polymers having comparable molecular weights (M n ≃ 9000 g/mol) but different composition, and the same composition but different molecular weights. The calorimetric properties of the products were investigated by differential scanning calorimetry (DSC) and dynamical-mechanical thermal analysis (DMTA). In particular, the effect of the copolymers molecular characteristics on the glass transition temperature (Tg) and the related relaxation processes were studied. DMTA was employed for evaluating the effect of macromolecular structure on the γ low temperature relaxation peak.