Effect of hydrostatic pressure on irreversible thermal transformations in a polymer glass at low temperatures (original) (raw)
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Pressure Effects on Relaxation in a Polymer Glass: A Persistent Spectral Hole Burning Study
Optics and Spectroscopy, 2005
The influence of hydrostatic pressure in the range of some kilobars on low-temperature (T < 20 K) relaxation in a polymer (polystyrene) glass after optical excitation of a probe chromophore in it is studied using two different kinds of spectral hole burning experiments-under isothermal-isobaric and in temperature cycling conditions. In the first case, the temperature dependence of the hole width reflects the dynamics of interaction of the electronic transition in a probe molecule with soft localized vibrational modes and with two-level systems, whereas, in the second case, the observed residual hole broadening after the temperature cycle arises from activated (overbarrier) transitions in almost symmetric double-well soft potentials. It is shown that both these processes are essentially suppressed by the applied hydrostatic pressure (the hole width in the first case and its increment in the second case are both reduced about twofold at 5 kbar). An extension of the soft potential model for glasses is proposed explaining in a coherent manner both effects. Its essential points are the presence in the potential of an extra term linear in pressure and the soft coordinate and an assumption about asymmetric distribution of the cubic anharmonicity parameter ξ in the potential.
Chemical Physics, 1997
We report time-resolved spectral hole-burning experiments on bacteriochlorophyll-a (BChl-a) doped into the glass triethylamine (TEA) at ambient pressure (Δp=0) and at a pressure of Δp=3.4 GPa. We observe a number of remarkable effects: (a) from the change in the temperature dependence of the “effective” optical homogeneous linewidth Γhom′, we conclude that local order is introduced in TEA under high pressure;
Spectral diffusion in glasses under high pressure: A study by time-resolved hole-burning
The Journal of Chemical Physics, 1999
We have studied optical dephasing and spectral diffusion of the S 1 ←S 0 0-0 transition of bacteriochlorophyll-a (BChl-a) in the glass 2-methyltetrahydrofuran ͑MTHF͒ at ambient (⌬p ϭ0) and high pressure (⌬pϭ3.6 GPa) between 1.2 and 4.2 K by time-resolved hole-burning. The ''effective'' homogeneous linewidth ⌫ hom Ј follows a power law dependence on temperature, ⌫ hom Ј ϭ⌫ 0 ЈϩaT 1.3Ϯ0.1 , where ⌫ 0 Јϭ⌫ 0 ϩ⌫ 0 ET ϩ⌫ 0 ET→SD (t d) is the residual linewidth and aϭa PD ϩa SD (t d)ϩa ET→SD (t d) is the coupling constant. The separate contributions to ⌫ 0 Ј and a are the fluorescence decay rate ⌫ 0 ϭ(2 fl) Ϫ1 , the ''downhill'' energy-transfer rate ⌫ 0 ET , the coupling constants due to ''pure'' dephasing a PD and ''normal'' spectral diffusion a SD (t d), and two terms related to ''extra'' spectral diffusion induced by energy transfer, ⌫ 0 ET→SD (t d) and a ET→SD (t d). We have quantitatively analyzed these contributions at ambient and high pressure. The results show that ''normal'' SD, ''extra'' SD, and ET→SD are strongly influenced by pressure. We have interpreted our findings in terms of a change in the number of two-level-systems, the low-frequency modes characteristic for the glassy state.
Spectral diffusion in organic glasses. Temperature dependence of permanent and transient holes
Chemical Physics Letters, 1993
Spectral diffusion of the S, tSo O-O transition (a) of bacteriochlorophyll a (BChl a) as guest in the glasses ethanol (EtOH), triethylamine (TEA), 2-methyltetrahydrofuran (MTHF) and diethylether at low temperature has been studied by transient and permanent hole burning with a single-mode GaAlAs diode laser at = 780 nm. Transient holes decay on a time scale of about 100 ms, determined by the triplet-state lifetime of BChl a. Their widths are compared to those of permanent holes probed on a time
High pressure effects on low temperature relaxation in solids
Journal of Luminescence, 1992
Persistent spectral hole burning (SHB) was used to probe the influence of high hydrostatic pressure (up to 8.4 kbar) on optical relaxation in chlorin (Chl)-doped glassy (polystyrene, PS) and (poly)crystalline (n-octane. C 5 Shpol'skii system) samples at 4.2 K. Pressure induced width reduction of isobarically burnt and measured holes was found, FWHM of holes being for PS:Chl 4.3 and 2.4 0Hz at 1 atm and 5.1 kbar, respectively, and for C5:Chl (15730 cm 1 line) 130 and 45MHz at 1 atm and 5.5 kbar, respectively. The latter value approaches the lifetime limit of 40 MHz. For the glassy matrix the effect of hole narrowing gives evidence of the dominant role of low frequency quasilocal vibrations in determining the hole width. Pressure induced instability of photoproduct was found in C 5: Chl at pressures above 5.5 kbar.
Statistical thermodynamics of the glass transition and the glassy state of polymers
The Journal of Physical Chemistry, 1972
The hole theory of Simha and Somcynsky is applied to an analysis of the liquid-glass boundary and to the equation of state in the region between the glass transition and the 0-relaxation. Two systems already studied experimentally are considered, namely, polystyrene and poly (o-methylstyrene). The liquid-glass boundary relations are investigated under two sets of conditions corresponding to a low-(LPG) and a high-pressure glass (HPG). The former is formed by cooling the liquid a t atmospheric pressure, whereas the latter is obtained by pressurizing the liquid isothermally. The equation of state is analyzed for LPG only. The link between the conventional thermodynamic relations, experiment, and the statistical theory is formed by identifying the vacancy fraction 1y appearing in the latter with the ordering parameter 2 introduced in the thermodynamic theory. For LPG, ye, the value of y along the boundary, is indeed found to be constant for both polymers. For HPG, 1y, is a decreasing function of pressure, as should be expected. The equation dT,/dP = (bT,/W)z + (bT,/bZ)p X dZ/dP is tested by evaluating the product on the right-hand side by a combination of the statistical theory with experiment. An equation of state for LPG is first computed entirely from theory by assuming that a single constant parameter, y = yg, characterizes not only the liquid-glass boundary line, but the glassy region as well. This results in too low a thermal expansivity, as had been noted earlier by Somcynsky and Simha for several other polymers at atmospheric pressure. Hence, within the frame of the hole theory, y cannot remain constant in the glass but is a function of T and P. It differs, of course, from the function derived by maximization of the configurational partition function of the liquid and is obtained here from experiment. Thus, additional constants enter into the equation of state of the glass, which cannot be obtained solely from the properties of the liquid and the liquid-glass boundary line. On approaching this line, however, the above function reduces to a single constant, viz., yg.
Time-dependent heat capacity in the glass transition region
Journal of Polymer Science Part A-2: Polymer Physics, 1971
Timedependent, apparent heat capacities of glucose, poly(viny1 chloride), polystyrene, selenium, poly(methy1 methacrylate), and poly(2,6-diiethyl-1,4-phenylene ether) in the glass transition region were determined by differential thermal analysis. The thermal history was set by linear cooling a t rates between 0.007 and 16OoC/min. Linear heating for analysis was carried out at rates between 0.3 and 6OO0C/min. Average activation energies of 52,81,90,54,77, and 108 kcal/mole, respectively, were evaluated by using the hole theory of glasses previously developed. Within experimental limitations all data could be described quantitatively by the theoretical expressions using only one parameter, the number of frozen-in holes, to describe the thermal history. Experimental and theoretical limitations are discussed. Measurements of heat capacity of amorphous materials in the glass transition region show nonequilibrium effects due mainly to time-dependent configurational rearrangements of the molecules. At temperatures sufficiently below the glass transition T,, the configuration is virtually frozen in, and the heat capacity of the glass behaves like an equilibrium property. Frequently, the heat capacity of glasses is similar to heat capacity of equilibrium crystals of chemically identical structures down to temperatures as low as 50°K. At temperatures sufficiently above T,, the configurational rearrangements are so fast that their time dependence is not measurable, and an equilibrium heat capacity exists for the melt. The heat capacity contribution due to changes in mode of motion (such as vibrations changing to rotation or translation) is much smaller than that due to configurational rearrangements (such as hole formation) in the T, region. This paper will be concerned with the time-dependent apparent heat capacity of six materials in the glass transition region: glucose (CSHIZO~), selenium (Se), poly(viny1 chloride) (PVC), polystyrene (PS), poly(methy1 methacrylate) (PMMA), and poly(2,6-dimethyl-l,4phenyl ether) (PPO). Early observations of heat capacity as a function of time and temperature established that a maximum and a minimum can occur in the transition region of several organic, inorganic, and polymeric Presently
Thermal lens scanning of the glass transition in polymers
Journal of Applied Physics, 2001
In this article we discuss the use of the thermal lens technique for investigating the thermal properties of polymers as a function of temperature. It is also discussed how the experimentally determined thermal lens parameters can be used to locate the glass transition in polymers. The methodology is tested using a solution casted films of poly͑vinyl chloride͒ as a testing sample. A comparison with conventional differential scanning calorimetry data is made. It is proposed that the current transient thermal lens methodology, with minor changes in its experimental configuration, could be adapted to develop a new methodology called differential thermal lens scanning especially designed for the investigation of the phase transitions in polymers. It is shown that this new methodology could be equally used for the measurement of the thermal expansion coefficient, above and below the glass transition.
Journal of Non-Crystalline Solids, 2015
Understanding why the glass transition temperature (T g ) of polymers deviates substantially from the bulk with nanoscale confinement has been a 20-year mystery. Ever since the observation in the mid-1990s that the T g values of amorphous polymer thin films are different from their bulk values, efforts to understand this behavior have intensified, and the topic remains the subject of intense research and debate. This is due to the combined scientific and technological implications of size-dependent glassy properties. Here, we discuss an intriguing aspect of the glassy behavior of confined amorphous polymers. As experimentally assessed, the glass transition is a dynamic event mediated by segmental dynamics. Thus, it seems intuitive to expect that a change in T g due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of T g for confined polymers based on bulk rules. We highlight past and recent findings that have examined the relationship between T g and segmental dynamics of confined polymers. Within this context, the decoupling between these two aspects of the glass transition in confinement is emphasized. We discuss these results within the framework of our current understanding of the glass transition as well as efforts to resolve this decoupling. Finally, the anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces in polymer thin films is discussed.
Searching for the ideal glass transition, we exploit the ability of glassy polymer films to explore low energy states in remarkably short time scales. We use 30 nm thick polystyrene (PS) films, which in the supercooled state basically display the bulk polymer equilibrium thermodynamics and dynamics. We show that in the glassy state, this system exhibits two mechanisms of equilibrium recovery. The faster one, active well below the kinetic glass transition temperature ($T_g$), allows massive enthalpy recovery. This implies that the 'fictive' temperature ($T_f$) reaches values as low as the predicted Kauzmann temperature ($T_K$) for PS. Once the thermodynamic state corresponding to Tf=TKT_f = T_KTf=TK is reached, no further decrease of enthalpy is observed. This is interpreted as a signature of the ideal glass transition.