The effects of confinement on glass transition temperature (Tg) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The Tgs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, Tg values can be enhanced or depressed relative to neat, bulk Tg (Tg,bulk) or invariant with nanofiller content. For alumina nanocomposites, Tg increases relative to Tg,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit Tg enhancements as large as 5 K or Tg reductions as large as 17 K relative to Tg,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-001949.pdf

Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging.

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Microstructural and Dynamical Heterogeneities in Ionic Liquids.

Ionic liquids (ILs) have emerged as potential lubricants in 2001. Subsequently, there has been tremendous research interest in ILs from the tribology society since their discovery as novel synthetic lubricating materials. This also expands the research area of ILs. Consistent with the requirement of searching for alternative and eco-friendly lubricants, IL lubrication will experience further development in the coming years. Herein, we review the research progress of IL lubricants. Generally, the tribological properties of IL lubricants as lubricating oils, additives and thin films are reviewed in detail and their lubrication mechanisms discussed. Considering their actual applications, the flexible design of ILs allows the synthesis of task-specific and tribologically interesting ILs to overcome the drawbacks of the application of ILs, such as high cost, poor compatibility with traditional oils, thermal oxidization and corrosion. Nowadays, increasing research is focused on halogen-free ILs, green ILs, synthesis-free ILs and functional ILs. In addition to their macroscopic properties, the nanoscopic performance of ILs on a small scale and in small gaps is also important in revealing their tribological mechanisms. It has been shown that when sliding surfaces are compressed, in comparison with a less polar molecular lubricant, ion pairs resist "squeeze out" due to the strong interaction between the ions of ILs and oppositely charged surfaces, resulting in a film that remains in place at higher shear forces. Thus, the lubricity of ILs can be externally controlled in situ by applying electric potentials. In summary, ILs demonstrate sufficient design versatility as a type of model lubricant for meeting the requirements of mechanical engineering. Accordingly, their perspectives and future development are discussed in this review.

Ionic liquid lubricants: when chemistry meets tribology

Ionic liquids (ILs) have been extensively used in many research and industry fields, including chemical and pharmaceutical applications. Nevertheless, during past years, some works revealed that those green solvents in fact could present certain toxicity levels. This is the reason why some biocompounds from natural sources, such as choline, amino acids, and organic acids, for synthesis of ILs have showed to be alternatives. This means that there is evidence that ILs with low or nontoxic effects could be synthesized, possibly overcoming the major drawback of using them in the food industry. Applications of these third generation ILs, or even the most common ILs, in food processes are scarce and mainly focused on extraction processes and chemical analysis methods. These works have proven that, considering the physical properties of ILs of interest for the food industry such as melting profile, solubility, viscoelasticity, and high biocompatibility, when compared to those commonly used, ILs are alternatives ...

Applications of Ionic Liquids in the Food and Bioproducts Industries

In the present work we report the synthesis and physico-chemical characterization in terms of the viscosity and density of a wide series of cholinium-amino acid based room temperature ionic liquids ([Ch][AA] RTILs). 18 different amino acids were used to obtain 14 room temperature ILs. Among the most common AAs, only valine did not form an RTIL but it is a liquid above 80 °C. With respect to the methods reported in the literature we propose a synthesis based on potentiometric titration which has several advantages such as shorter preparation time, stoichiometry within ±1%, very high yields (close to 100%), high reproducibility, and no use of organic solvents, thus being more environmentally friendly. We tried to prepare dianionic ILs with some AAs with two potentially ionisable groups but in all cases the salts were solids at room temperature. All the ILs were characterized by 1H NMR to confirm the stoichiometry. Physico-chemical properties such as density, viscosity, refractive index and conductivity were measured as a function of temperature and correlated with empirical equations. The values were compared with the data already reported in the literature for some [Ch][AA] ILs. The thermal expansion coefficient αp and the molar volume Vm were also calculated from the experimental density values. Due to the high number of AAs explored and their structural heterogeneity we have been able to find some interesting correlations between the data obtained and the structural features of the AAs in terms of the alkyl chain length, hydrogen bonding ability, stacking and cyclization. Some parameters were also found to be in good agreement with those reported for other ILs. We think that these data can give an important contribution to the understanding of the structure–property relationship of ILs because they focused on the structural effect of the anions, while most data in the literature are focussed on the cations.

Cholinium-amino acid based ionic liquids: a new method of synthesis and physico-chemical characterization

In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed anion, bis[(trifluoromethane)sulfonyl]amide, were investigated. The imidazolium cations included 1-heptyl-3-methylimidazolium, 1-(cyclohexylmethyl)-3-methylimidazolium, 1-benzyl-3-methylimidazolium, 1,3-dibenzylimidazolium, and 1-(2-naphthylmethyl)-3-methylimidazolium. Structure–property relationships were investigated regarding the substituent effects on the imidazolium cation, including n-alkyl versus cycloalkyl and aromatic versus aliphatic, as well as the effects of cation symmetry and larger aromatic polycyclic functionalities. Thermophysical properties investigated include density, thermal transition temperatures, and decomposition temperatures. The densities of the ionic liquids are governed by the substituents on the cation: n-alkyl < cycloalkyl < aromatic. The group contribution method is applicable for the density estimation of ionic liquids, and the volume paramet...

Thermophysical Properties of Imidazolium-Based Ionic Liquids: The Effect of Aliphatic versus Aromatic Functionality

Fragility of ionic liquids measured by Flash differential scanning calorimetry

A series of imidazolium-based ionic liquids with cyclic and aromatic groups and a bis[(trifluoromethane)sulfonyl]amide anion were characterized using dynamic and steady shear rheology in a wide temperature range. The cations investigated include 1-(cyclohexylmethyl)-3-methylimidazolium ([CyhmC1im](+)), 1-benzyl-3-methylimidazolium ([BnzC1im](+)), 1,3-dibenzylimidazolium ([(Bnz)2im](+)), and 1-(2-naphthylmethyl)-3-methylimidazolium ([NapmC1im](+)). Rheological properties are reported from terminal flow to the glassy state. All ionic liquids show very similar flow behavior with a glassy modulus approaching 1 GPa. The temperature dependences of the shift factors used for time-temperature superposition and the viscosity both follow the same VFT or WLF relationship, and the dynamic fragility at Tg ranges from 117 to 130 for the aromatic ionic liquids investigated, considerably more fragile, in the Angell sense, than the aliphatic analogs. Additionally, an anomalous aging effect in the dynamic viscosity response (i.e., a maximum observed at intermediate frequencies) was found using a strain-controlled rheometer but not using a stress-controlled rheometer.

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Rheology of Imidazolium-Based Ionic Liquids with Aromatic Functionality

Bulk and shear rheological studies were performed on a 10 wt % silica nanoparticle-filled polystyrene nanocompo- site. The limiting moduli in glassy and rubbery states are higher for the nanocomposite than for the neat polymer; the increase is consistent with hydrodynamic reinforcement and is slightly higher than the lower bound of the rule of mixtures prediction. All evidence indicates that the presence of nanopar- ticles does not significantly change the polymer dynamics associated with glass transition, except to increase the Tg by 3 K. Comparison of the bulk and shear retardation spectra sug- gests that the underlying mechanisms for both responses are similar at short times and that the long-time chain modes available to the shear are not available to the bulk, consistent with Plazek's earlier findings. In addition, T 2Tg and TV c scal- ing, along with the findings of thermorheological complexity, are discussed. V C 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 621-632

Bulk and shear rheology of silica/polystyrene nanocomposite: Reinforcement and dynamics

The pressure-volume-temperature (PVT) behavior and glass transition behavior of a 10 wt % silica nanoparticle-filled polystyrene (PS) nanocomposite sample are measured using a custom-built pressurizable dilatometer. The PVT data are fitted to the Tait equation in both liquid and glassy states; the coefficient of thermal expansion α, bulk modulus K, and thermal pressure coefficient γ are examined as a function of pressure and compared to the values of neat PS. The glass transition temperature (Tg) is reported as a function of pressure, and the limiting fictive temperature (Tf′) from calorimetric measurements is reported as a function of cooling rate. Comparison with data for neat PS indicates that the nanocomposite has a slightly higher Tg at elevated pressures, higher bulk moduli at all pressures studied, and its relaxation dynamics are more sensitive to volume. The results for the glassy γ values suggest that thermal residual stresses would not be reduced for the nanocomposite sample studied. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1131–1138

Ran Tao

Papers

Thermophysical Properties of Imidazolium-Based Ionic Liquids: The Effect of Aliphatic versus Aromatic Functionality

Fragility of ionic liquids measured by Flash differential scanning calorimetry

Rheology of Imidazolium-Based Ionic Liquids with Aromatic Functionality

Bulk and shear rheology of silica/polystyrene nanocomposite: Reinforcement and dynamics

Pressure‐volume‐temperature and glass transition behavior of silica/polystyrene nanocomposite