An intriguing problem in condensed matter physics is understanding the glass transition, in particular the dynamics in the equilibrium liquid close to vitrification Recent advances have been made by using hydrostatic pressure as an experimental variable These results are reviewed, with an emphasis in the insight provided into the mechanisms underlying the relaxation properties of glass-forming liquids and polymers

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R03/pdf

Supercooled dynamics of glass-forming liquids and polymers under hydrostatic pressure

The relaxation time dispersions of the primary and secondary dielectric relaxations are studied for molecular glass-forming liquids regarding their dependence on structural relaxation time or lack thereof observed as time-temperature superposition TTS , their changes with fragility, and a possible correlation of the values for the and processes. Toward more fragile liquids, the width w of the peak at the glass transition temperature Tg is known to increase significantly. Additionally, we observe that TTS extends over a broader range of peak relaxation times in the case of higher fragility, with the approach to exponential Debye relaxation being delayed to faster dynamics. The width w of the relaxation at Tg is somewhat lower for more fragile liquids and appears correlated with that of the primary relaxation. The extrapolated coincidence of the dispersions of the and processes occurs at the fragility “limit” of m=170, implying that w w for all molecular supercooled liquids. Materials of high fragility are also those with no clear low-frequency cutoff in the relaxation time distribution—i.e., their susceptibilities require Havriliak-Negami instead of Cole-Davidson fits. For sorbitol, the value of w is seen to change steadily through the -merging region, whereas w displays a kink at the crossover temperature.

Primary and secondary relaxation time dispersions in fragile supercooled liquids

Local segmental relaxation properties of poly(methylmethacrylate) (PMMA) of varying molecular weight are measured by dielectric spectroscopy and analyzed in combination with the equation of state obtained from PVT measurements. Significant variations of glass transition temperature and fragility with molecular weight are observed. In accord with the general properties of glass-forming materials, single molecular weight dependent scaling exponent γ is sufficient to define the mean segmental relaxation time τα and its distribution. This exponent can be connected to the Gruneisen parameter and related thermodynamic quantities, thus demonstrating the interrelationship between dynamics and thermodynamics in PMMA. Changes in the relaxation properties (“dynamic crossover”) are observed as a function of both temperature and pressure, with τα serving as the control parameter for the crossover. At longer τα another change in the dynamics is apparent, associated with a decoupling of the local segmental process from ...

/pdf/effect-of-chain-length-on-fragility-and-thermodynamic-o2vbt99add.pdf

Effect of chain length on fragility and thermodynamic scaling of the local segmental dynamics in poly(methylmethacrylate)

Broadband dielectric spectroscopy is used to inves- tigate molecular dynamics and charge transport in three hyper- branched polyester amides with hydroxyl, phenyl, and stearate terminal groups. At higher temperatures, the dielectric spectra are interpreted in terms of hopping conduction in a spatially randomly varying energy landscape, whereas two secondary dipolar relaxations attributed to librations of the terminal and amide groups dominate the low temperature regime. Despite a shift of more than 3 decades in the dc conductivity upon varia- tion of the end groups, the Barton-Nakajima-Namikawa rela- tion is shown to hold. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1651-1657, 2010

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/polb.21966

Secondary relaxations and electrical conductivity in hyperbranched polyester amides

Structure and molecular dynamics of hyperbranched polymeric systems with urethane and urea linkages

Broadband dielectric spectroscopy is employed to investigate the non-Debye relaxation behavior in a dendrimeric alkyd resin. From temperature-dependent measurements at ambient pressure, we found a very broad distribution of relaxation times. This is attributed to the complex geometrical topology of the molecule. However, compression significantly reduces the non-Debye character of the dielectric response; thus, pressure induces dynamic homogeneity in the dendrimeric alkyd resin.

Effect of pressure on dynamic heterogeneity in dendrimeric alkyd resin.

Broadband dielectric spectroscopy was used to study the relaxation dynamics in bis-5-hydroxypentylphthalate (BHPP) under both isobaric and isothermal conditions. The relaxation dynamics exhibit complex behavior, arising from hydrogen bonding in the BHPP. At ambient pressure above the glass transition temperature Tg, the dielectric spectrum shows a broad structural relaxation peak with a prominent excess wing toward higher frequencies. As temperature is decreased below Tg, the excess wing transforms into two distinct peaks, both having Arrhenius behavior with activation energies equal to 58.8 and 32.6kJ∕mol for slower (β) and faster (γ) processes, respectively. Furthermore, the relaxation times for the β process increase with increasing pressure, whereas the faster γ relaxation is practically insensitive to pressure changes. Analysis of the properties of these secondary relaxations suggests that the β peak can be identified as an intermolecular Johari–Goldstein (JG) process. However, its separation in freq...

S. Maślanka

Papers

Effect of pressure on dynamic heterogeneity in dendrimeric alkyd resin.

Primary and secondary relaxations in bis-5-hydroxypentylphthalate