Mobility of Pressure-Densified and Pressure-Expanded Polystyrene Glasses: Dilatometry and a Test of KAHR Model

The structural recovery of pressure-densified (PDG) and, for the first time, pressure-expanded (PEG) glasses are experimentally investigated using pressurizable dilatometry. Both glasses show early devitrification on heating, indicating that these glasses have more mobility, compared to the conventional isobarically formed glass. The Kovacs–Aklonis–Hutchinson–Ramos (KAHR) model of structural recovery is able to reasonably predict the behavior of the pressure-expanded glass, but the KAHR model fails with the pressure-densified glass. The results suggest two limitations of the model: (i) the structural recovery is assumed to depend on the instantaneous liquid state and (ii) the same relaxation kinetics are assumed for the temperature and pressure perturbations. Modification of the KAHR model, allowing the departure from equilibrium, δ, to initially depend on the liquid state that the glass came from and to evolve toward the state that the glass is going to, improves the ability of the model to predict the early devitrification for the pressure-densified glass. Another modification of the KAHR model, allowing the temperature and pressure perturbations to relax independently of one another, results in effectively capturing the increased thermal expansion coefficient of glass lines during heating, as well as a “memory”-like aging behavior, for the pressure-densified glass

Dodecyl Methacrylate Polymerization under Nanoconfinement: Reactivity and Resulting Properties

The effect of nanoconfinement on the free radical polymerization of dodecyl methacrylate (DMA) with di-tert-butyl peroxide (DtBP) initiator is investigated over a wide temperature range from 110 to 190 °C using differential scanning calorimetry. The reaction shows a distinct induction time, which decreases as temperature increases, with an activation energy that is the same, albeit, of opposite sign, as that for dissociation of the initiator. The rate of reaction increases with increasing temperature and is higher in nanopores than in bulk conditions, with an Arrhenius temperature dependence at temperatures lower than 160 °C and an activation energy that is approximately 10% lower in the nanoconfined cases than for bulk. The higher reaction rate and lower activation energies in the nanopores are presumably due to specific interactions between the monomer and the native silanol groups on the pore surface. The enhancement of the reaction rate is found to be inversely related to the length of the alkyl group and the water contact angle comparerd data for several poly(n-alkyl methacrylate) studied previously. For bulk and nanoconfined DMA polymerizations, the molar mass increases as temperature decreases with a cross-linked product obtained at temperatures below 170 °C. The gel fraction increases as temperature decreases and is nearly 80% at 110 °C. In the nanopores, the molar mass is smaller compared to that in bulk conditions at high temperatures. The results can be described by a simplified recursive model

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

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 <i>n</i>-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: <i>n</i>-alkyl < cycloalkyl < aromatic. The group contribution method is applicable for the density estimation of ionic liquids, and the volume parameters for cyclohexyl, phenyl, and naphthyl groups are reported. The glass transition temperature (<i>T</i>_g) follows the same systematic trend due to substituent flexibility: <i>n</i>-alkyl < cycloalkyl < aromatic. Thermal stability as measured by dynamic thermogravimetric analysis (TGA) is not strongly affected by the substituents on the imidazolium ring; however, slight differences are observed with the higher <i>T</i>_g ionic liquids having lower decomposition temperatures for this series of ionic liquids. On the other hand, the cyclohexylmethyl-substituted ionic liquid exhibits a higher activation energy for degradation than the other ionic liquids based on isothermal TGA, and all ionic liquids studied show significant weight loss at 300 °C indicating that appreciable decomposition can occur at temperatures substantially lower than the onset temperature observed in dynamic TGA scans

Effect of Alkyl Chain Branching on Physicochemical Properties of Imidazolium-Based Ionic Liquids

The branched ionic liquids (ILs) 1-(<i>iso-</i>alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([(<i>N</i> – 2)­mC_<i>N</i>‑1C₁im]­[NTf₂] with <i>N</i> = 3–7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(<i>n</i>-alkyl)-3-methylimidazolium bis­[(trifluoromethane)­sulfonyl]­amide ([C_<i>N</i>C₁im]­[NTf₂] with <i>N</i> = 3–7). For <i>N</i> = 4–7, the density of the branched IL [(<i>N</i> – 2)­mC_<i>N</i>–1C₁im]­[NTf₂] is the same as that of its linear analogue [C_<i>N</i>C₁im]­[NTf₂] within the standard uncertainty of the measurements. In the case of the <i>N</i> = 3 [1mC₂C₁im]­[NTf₂]/[C₃C₁im]­[NTf₂] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given <i>N</i>, the glass transition temperature <i>T</i>_g, melting temperature <i>T</i>_m, and viscosity η are higher for the branched IL than for the linear IL. [2mC₃C₁im]­[NTf₂] is an exception in that its <i>T</i>_m is lower than that of [C₄C₁im]­[NTf₂]. Moreover, the viscosity of [2mC₃C₁im]­[NTf₂] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin–echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes–Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs

Thermal and Rheological Analysis of Polystyrene-Grafted Silica Nanocomposites

Two matrix-free polystyrene-grafted silica nanocomposite samples with graft chain lengths of 35 and 112 kg/mol are characterized by calorimetry and rheometry, and results are compared to neat polystyrenes of comparable molecular weights. The glass transition temperature Tg of the nanocomposites is found to be approximately 1 to 2 K higher than that of the neat materials, whereas the absolute heat capacity is approximately 4–7% lower in the glassy and liquid states. The step change in heat capacity ΔCp at Tg is 15% lower for the nanocomposites, consistent with an immobilized glassy layer of approximately 2 nm. The linear viscoelastic behavior of the nanocomposite samples differs significantly compared to their neat analogs in several ways: first, the G′ versus ω curves shift toward lower frequencies by approximately one decade due to the increase in the glass transition temperature; second, terminal flow behavior is absent; third, the rubbery plateau moduli (GN°) decreases by 7% for the 35 kg/mol grafted particles and increases by approximately two and a half-fold for the 112 kg/mol grafted particles; and fourth, the glassy modulus increases approximately 4% consistent with hydrodynamic reinforcement. On the other hand, the magnitude of the rubbery modulus is attributed to two effects, hydrodynamic reinforcement and a change in the effective entanglement density, which is governed by corona interpenetration coupled with the silica particles acting as physical entanglement points

Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids

In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amide ([(CN/2)2im][NTf2]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([CN–1C1im][NTf2]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an even–odd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation