The glass transition and aging dynamics of single and stacked thin films of polystyrene (PS) and poly(2-chlorostyrene) (P2CS) were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The glass transition temperature Tg of as-stacked thin films of PS has a strong depression from that of the bulk samples. However, after annealing at high temperatures above Tg, the stacked thin films exhibit glass transition at a temperature almost equal to the Tg of the bulk system. The dynamics of the α-process of stacked P2CS thin films show a time evolution from single thin film-like dynamics to bulk-like dynamics during the isothermal annealing process. The relaxation rate of the α-process becomes smaller with increase in the annealing time. The time scale for the evolution of the α-dynamics during the annealing process is very long compared with that for the reptation dynamics. At the same time, the temperature dependence of the relaxation time for the α-process changes from Arrhenius-like to Vogel-Fulcher-Tammann dependence with increase of the annealing time. The aging dynamics of P2CS thin films with thickness less than 10 nm were also investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility ε" for P2CS thin films with a thickness of 3.7 nm increased with an increase in isothermal aging time, while this was not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in ε" for the ultrathin films is strongly correlated with the presence of a mobile liquid-like layer within the thin films.
In this paper, we briefly overview first the preceding studies on molecular glasses prepared by vapor deposition on a cold substrate, and then explain our recent studies on such molecular glasses and the supercooled liquids (SCLs) obtained from these glasses through glass transition. Glasses of some of alkylbenzene derivatives prepared at the deposition temperatures (Tds) much lower than the glass transition temperature (Tg) show a large excess volume, and undergo a relaxation accompanied with a volume shrinkage in a narrow temperature region just below Tg when the temperature was raised with a constant rate. These samples also undergo a relaxation in the SCL states from a low-density unstable SCL to another SCL. On the other hand, glasses of the same alkylbenzene derivatives prepared by the deposition at Tds close to Tg show a molar volume smaller than that expected for the equilibrium SCLs of these compounds, and undergo a relaxation accompanied with a volume expansion in a narrow temperature region just below Tg. Some of these samples undergo a relaxation in the SCL states showing a gradual expansion in the volume. We discuss such differences in the behavior of vapor-deposited molecular glasses and SCLs in relation to non-equilibrium and local molecular conformations in the samples.
We have combined our recent calorimetric works on vapor-deposited simple molecular glasses and our previous works on ordinary molecular glasses to investigate the universal thermodynamic feature of molecular glasses. Following a brief review for the studies on glass transitions, we describe how the vapor-deposition method is useful for simple molecular glasses. Then, we mention the structure of the adiabatic calorimeter recently developed for vapor-deposited samples. The heat capacities Cp of vapor-deposited carbon tetrachloride (CCl4), propene (CH2=CHCH3) and propane (CH3CH2CH3) are shown. Low-energy excitations characteristic to glasses were observed in all glassy samples. Glass transitions appeared in propene and propane at 56 K and 45 K, respectively; 45 K is the lowest record of Tgs of molecular glasses. The configurational heat capacity ΔCp and entropy Sc are calculated from the Cp data of ten molecular glasses. The universal feature for the temperature dependence of ΔCp is demonstrated by scaling temperature using the Kauzmann temperature TK. The size z* of cooperatively rearranging region (CRR) is also calculated based on the Adam-Gibbs theory. They are increasing with decreasing temperature and frozen at each Tg. The frozen numbers z*(0) are 3-7 molecules and depend on (Tg-TK)/TK; the larger (Tg-TK)/TK is, the smaller z*(0) becomes. Thus, the present work has demonstrated the universal thermodynamic feature of molecular glasses and indicated the validity of the Adam-Gibbs theory based on the experimental data.
Thermal molecular motion of poly(methyl methacrylate) (PMMA) at various interfaces, gases and liquids, was discussed. The αa- and β-relaxation processes were clearly observed even at the air interface. Both relaxation temperatures at the air interface were lower than the corresponding ones in the bulk. In addition, the extent to which the peak temperature for the surface relaxation processes fell below that of the bulk strongly depended on the stereoregularity of the films. Then, thermal molecular motion of PMMA was analyzed at gaseous carbon dioxide (CO2) interface. It was more enhanced than that at gaseous nitrogen interface because CO2 molecules were sorbed in the interfacial region, which played as plasticizing agents. Finally, mechanical properties of PMMA were also studied at interfaces with liquids such as water, hexane and methanol. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of liquid sorbed into the film. Therefore, it can be claimed that interfacial molecular motion of polymers can be regulated on the basis of the aggregation states.
We review our recent studies on the structural disorder in crystal, polycrystal, and glass in a Lennard-Jones binary mixture in three dimensions using molecular dynamics simulation. At a low temperature, the crossovers among these states occur as the amount of the large particles is varied. We define a disorder variable Dj (t) for each particle j in terms of local bond order parameters based on spherical harmonics (Steinhardt order parameters). Mesoscopic structural heterogeneity in glass is then visualized. In fcc crystal and polycrystal, we show that stacking faults play an important role in plasticity.