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The glass transition temperature (Tg) of a binary miscible mixture of molecular glasses, termed a coamorphous glass, is often synergistically increased over that expected for an athermal mixture due to the strong interactions between the two components. This synergistic interaction is particularly important for the formulation of coamorphous pharmaceuticals since the molecular interactions and resulting Tg strongly impact stability against crystallization, dissolution kinetics, and bioavailability. Current models that describe the composition dependence of Tg for binary systems, including the Gordon-Taylor, Fox, Kwei, and Braun-Kovacs equations, fail to describe the behavior of coamorphous pharmaceuticals using parameters consistent with experimental ΔCP and Δα. Here, we develop a robust thermodynamic approach extending the Couchman and Karasz method through the use of activity coefficient models, including the two-parameter Margules, non-random-two-liquid (NRTL), and three-suffix Redlich-Kister models. We find that the models, using experimental values of ΔCP and fitting parameters related to the binary interactions, successfully describe observed synergistic elevations and inflections in the Tg versus composition response of coamorphous pharmaceuticals. Moreover, the predictions from the NRTL model are improved when the association-NRTL version of that model is used. Results are reported and discussed for four different coamorphous systems: indomethacin-glibenclamide, indomethacin-arginine, acetaminophen-indomethacin, and fenretinide-cholic acid.

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 polys...

The thermal properties of CL-20 explosive in the bulk and confined in controlled pore glass matrices to nanoscale dimensions were studied using dynamic differential scanning calorimetry. The decomposition reaction of the CL-20 was found to be accelerated in 12-nm-diameter pores compared to the bulk CL-20 with the onset of the decomposition occurring 16–24 °C lower and a fourfold to sevenfold larger reaction rate constant. The total heat of decomposition was found to be independent of pore size and heating rate, and the average activation energy for all samples was found to be 160 ± 7 kJ mol−1.

Ten multiwall carbon nanotubes purchased from two companies with diameters ranging from 7 to 100 nm with tapped bulk densities from 57 to 250 kg m−3 and with similar lengths and powder sizes were analyzed by thermogravimetric analysis in dynamic and isothermal mode to investigate the importance of the diameter size and tapped bulk density on the degradation behavior of carbon nanotubes. In the case where mass/heat transfer effects were eliminated, results showed that the oxidation temperature depends on diameter only at sufficiently small diameters. The number of surface defects as measured from the ID/IG ratio obtained from Raman spectroscopy was not correlated with degradation stability. The tapped bulk density, on the other hand, dominated the pattern of degradation at high enough heating rates where mass/heat transfer effects become appreciable. As found in previous work which examined nanotubes with roughly constant diameter, tapped bulk density linearly correlated with the ending oxidation temperature at sufficiently high heating rates.

A series of second-generation dendronized wedge polymers were synthesized by ring-opening metathesis polymerization, and the linear viscoelastic response over a wide range of temperatures was investigated. From 0 to 90 °C the dynamic moduli (G′(ω) and G″(ω)) were determined, and frequency–temperature superposition was used to create master curves that showed behavior from the terminal zone to the glassy regime. An apparent extremely low rubbery plateau of ∼10 kPa was observed in both the dynamic response and in the corresponding van Gurp–Palmen plot. However, further investigation shows that the apparent rubbery plateau is related to the steady-state recoverable compliance, not the onset of entanglements. In addition, these wedge polymers exhibit an extremely low glassy modulus of ∼100 MPa at 0 °C, which is shown to increase at 1 Hz to ∼700 MPa at −80 °C for the wedge polymer 2G-EHW-311. In addition, both small- and wide-angle X-ray scattering patterns were obtained for all of the polymers investigated, and these showed that the polymer molecules adopt an extended cylinder conformation. Furthermore, based on calorimetric measurements, the polymers were found to exhibit two glass transition temperatures, with a 100 K difference between the higher (T_(g,hi) = 26.8 ± 0.7 °C) and lower glass transition temperatures (T_(g,lo) = −76.1 ± 1.1 °C) for the 2G-EHW-311 material. Hence, an intermediate regime extends to well below the T_(g,hi) to T_(g,lo), providing an explanation for the low glassy modulus of ∼100 MPa at 0 °C and its increase to ∼700 MPa when measured at T_(g,hi) – 100 °C and approaching the T_(g,lo).

An efficient method for synthesis of well-defined, well-characterized, tadpole-shaped polystyrene with a single atom junction point that is optimal for the study of dynamics has been developed using anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure. The difunctional macromolecular linking agent, ω-methyldichlorosilylpolystyrene, was formed by reacting sec-butyllithium-initiated poly(styryl)lithium with excess (30×) methyltrichlorosilane to eliminate formation of linear dimer and three-arm star polystyrene. The asymmetric, three-arm, star precursor was formed by linking excess α-4-pentenylpoly(styryl)lithium (α-PSLi) with the macromolecular linking agent, and the excess α-PSLi functionalized with ethylene oxide before termination with methanol to facilitate chromatographic separation. Cyclization of the three-arm, star precursor to form tadpole-shaped polystyrene was effected in methylene chloride at high dilution using the Grubbs first generation catalyst, bis(tricycl...

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.

Enthalpy recovery for a single polystyrene ultrathin film of 20 nm thickness is studied using Flash DSC over an extended time and temperature range. Results are compared to a bulk sample of the same polystyrene using a similar experimental protocol and analysis procedure in an effort to determine the effects of nanoconfinement. Examined is the cooling rate dependence of the glass transition temperature (Tg) of unaged films which informs the initial fictive temperature (Tfo) and thus the jump size (Tfo - Ta) for a given aging temperature (Ta). Isothermal enthalpy recovery is investigated as a function of both Ta for various cooling rates and as a function of jump size at constant Ta. The apparent activation energies at Tg and along the glassy line are determined and compared, as is the enthalpy recovery aging rate. Although the apparent activation energy along the glass line is the same within experimental error as the bulk, the aging rate is found to be slightly faster in the ultrathin film. Increasing the cooling rate prior to aging increases the aging rate. The results are discussed in the context of the two competing factors which influence the aging rate, namely, the driving force and the molecular mobility. The driving force for aging is dictated by the jump size or cooling rate, i.e., the value of Tfo - Ta. The mobility, on the other hand, is dictated by the relaxation time at the aging temperature, which increases during aging from the value on the initial glass line to that at equilibrium. The initial mobility in the glassy state is dictated by the jump size, being related to Tfo - Ta and the temperature dependence of the relaxation time along the glass line, whereas the mobility at equilibrium is dictated by Ta, Tg, and the temperature dependence and breadth of the equilibrium relaxation time.

The phenomenology of the glass transition and the associated behavior in the near liquid and glassy states are detailed, including the cooling rate dependence of the glass transition, Kovacs’ three signatures of structural recovery, and enthalpy overshoots. Dynamics in the liquid regime just above Tg and the associated temperature dependences are also covered since this behavior is important to understanding the glassy dynamics. The current models of structural recovery and their shortcomings are presented. A number of important unanswered questions are discussed, including how the relaxation time in the glassy state depends on structure, the relationship between the evolution of different properties, the resolution of the Kauzmann paradox, and the behavior of the equilibrium relaxation time below Tg. New experimental approaches are needed to make breakthroughs, such as two that are described: one involving 20 Ma amber to test whether the Vogel temperature dependence continues for the equilibrium state be...

Ionic liquids display outstanding properties and have a wide range of potential uses. Their unique ionic and organic nature also identify them as good glass-forming materials. In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed bis[(trifluoromethane)sulfonyl]amide ([NTf2]−) anion are characterized using a commercial rapid scanning nanocalorimetry instrument, the Mettler Toledo Flash DSC. The limiting fictive temperature Tf′, which is equivalent to the glass transition temperature Tg, is measured on heating as a function of cooling rate. The dynamic fragility is obtained for the series of ionic liquids, and using this data along with a compilation of data from the literature for a total of 50 ionic liquids reveals that aromatic structure increases fragility more than expected simply based on the change in Tg. Our results provide an approach to control fragility of ionic liquids through chemical modifications to the side group.

Vacuum pyrolysis deposition (VPD) has been used to create an ultrastable polymer glass having a fictive temperature Tf of as much as 57 K below the nominal glass transition temperature of the thermally rejuvenated polymer. Amorphous fluoropolymer films 300 to 700 nm thick were created by VPD followed by characterization of the thermal response using rapid-scanning chip calorimetry. The deposition was performed for substrates held at temperatures from 30.0 °C (303.2 K) to 116.7 °C (389.9 K) corresponding to approximately 0.75 to 0.97 times the limiting fictive temperature Tf′ ≈ Tg of the same material determined by cooling then heating at 600 K/s. Consistent with literature observations for small molecules that are vapor deposited in similar conditions relative to the material Tg, large enthalpy overshoots are observed, typical of both highly aged and ultrastable glasses. The 57 K reduction in Tf for the VPD polymers is greater than prior reports for physical vapor deposition of small molecules to form ult...

The structural recovery of amorphous glassy materials is nonlinear and nonexponential, and the relaxation process can be described by the phenomenological Tool–Narayanaswamy–Moynihan and Kovacs–Aklonis–Hutchinson–Ramos models. The nonlinearity parameter x in these models can be determined by several methods, including inflectional analysis, time–temperature superposition, and a new modified temperature-jump method, the latter a modification of the two-step Lagasse et al.’s method. The activation energy ∆h/R can also be determined by the first two methods. In this paper, the applicability of these methods for determining x and ∆h/R is analyzed using simulated structural recovery data as a function of aging time, after cooling at high rates such as those obtainable experimentally using Flash DSC. The results indicate that the activation energy obtained by the time–temperature superposition method is slightly better than that estimated by the inflectional analysis method. The results also indicate that the nonlinearity parameter x can be obtained by the inflectional analysis method for β = 1.0 and for low x and high β. On the other hand, the new modified temperature-jump method works for a broader range of x and β.

The use of nanoreactors to confine monomers and synthesize polymers results in changes in the reaction kinetics and polymer properties making nanoconfinement a potential tool for manipulating and engineering polymer properties. In this perspective, we cover conventional nanoconfinement hosts, nanopore-confined free radical, step-growth, and ring-opening polymerizations, and changes in molecular weight, tacticity, glass transition temperature (Tg), thermal stability, and electrical properties. We use examples from research in our laboratory, as well as comparisons of the work in the literature, to illustrate the competing forces that drive these changes, namely molecular layering or orientation at the nanopore surface, decreased molecular and segmental diffusion, and catalytic or inhibitory effects caused by chemical moieties on the native or surface-functionalized nanopore surface. The majority of nanoconfined polymerizations are found to be accelerated, and in the case of free radical polymerizations to generally yield higher molecular weights and higher isotacticity. Tgs for the nanoconfined polymers tend to increase if strong interactions exist between the polymer and the confinement surface, but depressions are observed for confined polycyanurates; the importance of removing unreacted monomer and comparing results to the bulk material of same molecular weight and structure is emphasized. Examples are also provided of enhanced thermal stability and conductivity of polymers synthesized under nanoconfinement.

The structural recovery of a single polystyrene thin film is studied using Flash differential scanning calorimetry (DSC) for aging temperatures ranging from 50.5 to 100.5 °C and for aging times of up to 18,000 s (300 min). A high fictive temperature glass with T f ′ = 117.0 ± 0.5 °C, obtained after cooling at 1000 K/s, is employed in this study. Structural recovery is investigated by monitoring the evolution of the fictive temperature, T f , which initially remains unchanged and then decreases smoothly and approximately linearly with logarithmic aging time. Equilibrium is reached when T f = T a at the highest aging temperatures. The length of the initial plateau is longer for the lower aging temperatures and is related to the increase in the relaxation time with decreasing temperature along the glass line, increasing approximately one decade for every 21 K. This temperature dependence yields a normalized apparent activation energy (E a /R) of approximately 13 kK for the mobility along the glass line. On the other hand, the apparent activation energy for the mobility along the liquid line is 102 kK.

The intrinsic isotherm, asymmetry of approach, and memory effect experiments are performed for a high fictive temperature polystyrene glass in enthalpy space using nanocalorimetry. Using aging at times as short as 0.01 s and relatively high aging temperatures allows the complete evolution of all three of the signatures of structural recovery to be obtained for the first time in enthalpy space. The results from down jump experiments are compared to those obtained at lower temperatures using conventional differential scanning calorimetry (DSC) with respect to the time required to reach equilibrium, the apparent activation energy, and the Tool–Narayanaswamy–Moynihan (TNM) model parameters, and these results are independent of the measurement method being used; on the other hand, the physical aging rate is higher for the high fictive temperature glass.

The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(N – 2)mCN-1C1im][NTf2] with N = 3–7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CNC1im][NTf2] with N = 3–7). For N = 4–7, the density of the branched IL [(N – 2)mCN–1C1im][NTf2] is the same as that of its linear analogue [CNC1im][NTf2] within the standard uncertainty of the measurements. In the case of the N = 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 N, the glass transition temperature Tg, melting temperature Tm, and viscosity η are higher for the branched IL than for the linear IL. [2mC3C1im][NTf2] is an exception in that its Tm is lower than that of [C4C1im][NTf2]. Moreover, the viscosity of [2mC3C1im][NTf2] is anomalously higher than what wou...

The effect of nanoconfinement on the free radical polymerization of ethyl methacrylate (EMA) and n-butyl-methacrylate (BMA) with di-tert-butyl peroxide (DtBP) initiator is investigated over a wide temperature range from 80 to 190 °C using differential scanning calorimetry. The effective rates are similar for the two bulk monomers although the BMA reacts approximately 11% faster at 95 °C. For nanoconfined cases, the initial reaction rate for monomer confined in the pores of controlled pore glass is enhanced, with larger effects observed in native pores compared to pores in which the native silanol was converted to trimethyl silyl. The onset of autoacceleration also occurs earlier under nanoconfinement, with decreases in both the conversion and the time required to reach autoacceleration, x g e l and t g e l , respectively, and larger changes for the native pores. The induction time follows Arrhenius behavior and increases under nanoconfinement. At polymerization temperatures above 160 °C, depropagation becomes important as the ceiling temperature is approached and seems to be more pronounced under nanoconfinement than in the bulk.

The thermal properties of bulk and nanoconfined HMX were studied using differential scanning calorimetry in dynamic scanning mode at rates ranging from 0.3 to 100 °C/min. At the slowest heating rates, decomposition occurs in the solid phase; at intermediate heating rates, it starts in the solid phase, melts, and finishes with a faster rate of reaction in the liquid state; and at the highest heating rates, the decomposition is entirely in the liquid phase. The activation energy decreases with conversion and is highest for 12 nm-diameter pores. The decomposition reaction is accelerated for nanoconfined HMX compared to the bulk with the onset of decomposition decreased by 1–5 °C in 50 nm-diameter pores and by 4–11 °C in 12 nm-diameter pores. The kinetics of decomposition is well described by a first-order autocatalytic reaction model with the reaction rate constants increasing with nanoconfinement. In addition, the reaction rate constant is one order of magnitude higher in the melt state than in the solid state.

The segmental relaxation time data for poly(vinyl acetate), poly(vinyl chloride), and linear and star polystyrene are analyzed using a model-free method to determine how the temperature- and pressure-dependent relaxation times, τ, scale with the relative configurational thermodynamic properties. The model-free method assumes no specific mathematical form, such as reciprocal linearity, and the configurational properties are referred to an isochronal state to eliminate the bias associated with the definition of the ideal glassy state. The scaling ability of a given configurational property is strongly material-dependent with the logarithm of τ scaling better with TSc and Hc for poly(vinyl acetate), with TSc, Hc, and Uc for poly(vinyl chloride), and with TSc, Hc, and Vc for linear and star polystyrene. The choice of the isochronal reference state does not qualitatively affect the results.

The friction and wear behavior of palladium (Pd)-rich amorphous alloy (Pd43Cu27Ni10P20) against 440C stainless steel under ionic liquids as lubricants, i.e., 1-nonyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C9C1im][NTf2]), were investigated using a ball-on-disc reciprocating tribometer at ambient, 100 and 200 &deg, C with different sliding speeds of 3 and 7 mm/s, whose results were compared to those from crystalline Pd samples. The measured coefficient of friction (COF) and wear were affected by both temperature and sliding speed. The COF of crystalline Pd samples dramatically increased when the temperature increased, whereas the COF of the amorphous Pd alloy samples remained low. As the sliding speed increased, the COF of both Pd samples showed decreasing trends. From the analysis of a 3D surface profilometer and scanning electron microscopy (SEM) with electron dispersive spectroscopy (EDS) data, three types of wear (i.e., delamination, adhesive, and abrasive wear) were observed on the crystalline Pd surfaces, whereas the amorphous Pd alloy surfaces produced abrasive wear only. In addition, X-ray photoelectron spectroscopy (XPS) measurements were performed to study the formation of tribofilm. It was found that the chemical reactivity at the contacting interface increased with temperature and sliding contact speed. The ionic liquids (ILs) were effective as lubricants when the applied temperature and sliding speed were 200 &deg, C and 7 mm/s, respectively.

Enthalpy recovery of a single polystyrene thin film is quantified by both experiments using Flash DSC and mode- ling using a new modified TNM model. Experimental data include Kovacs’ three signatures of structural recovery: intrinsic isotherms after temperature down jumps, the asymmetry of approach after temperature down and up jumps of the same size, and the mem- ory effect after a two-step history. A new modified TNM model is proposed to quantitatively fit all three signatures of structural recovery with a single set of model parameters. Here, we eluci- date the detailed derivation of the new modified model and dem- onstrate its applicability to the experimental Flash DSC results

Trimerization of a mixture containing a mono- and difunctional cyanate ester is investigated under the nanoporous confinement of silanized hydrophobic controlled pore glass using differential scanning calorimetry. The trimerization reaction of the nanoconfined monomer mixture is accelerated relative to the bulk by as much as 12 times in 8 nm pores, but this acceleration is less than half that observed for nanoconfinement of the individual monomers. The absolute reaction rate of the monomer mixture lies between those of the individual species, being slower than the monocyanate ester and faster than the dicyanate ester. The results are consistent with the hypothesis that the reaction acceleration is due to monomer ordering or layering at the pore surface, leading to a local concentration of reactive groups higher than in the bulk. In addition to the influence of nanoconfinement on trimerization kinetics, the molecular weight and glass transition temperature (Tg) of the polycyanurate formed in the nanopores ...

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

The effect of nanoconfinement on the equilibrium free radical polymerization of methyl methacrylate (MMA) is investigated using differential scanning calorimetry. The ceiling temperature is shifted to lower temperatures in 13 nm diameter pores, with pore surface chemistry showing no significant effect. The results indicate that the change in the entropy of propagation decreases in nanopores due to confinement effects (i.e. ΔSp,conf is a more negative value than ΔSp,bulk). The change in the entropy of propagation is independent of temperature for the bulk equilibrium polymerization, whereas the change in the entropy of propagation in nanopores becomes less negative and more bulk-like with increasing polymerization temperature presumably due to the lower molecular weight chains produced at high temperature. The data suggest that our system is one of weak confinement with chain entropy scaling with molecular weight to the 1.1 power (i.e. ∼ N1.1).

The fictive temperature (Tf) was defined by Tool in the 1940s as a measure of glassy structure. Tf is generally measured on heating and can be calculated from the enthalpy overshoot in calorimetric studies using a method developed by Moynihan. Prior work has demonstrated that the limiting fictive temperature (Tf′) is similar to Tg (measured on cooling) and depends on the cooling rate in a manner consistent with the Williams–Landel–Ferry (WLF) relationship. Theoretically, the limiting fictive temperature should not depend on heating rate, but this has been experimentally verified only for a very limited range of heating rates. Here, rapid-scanning chip calorimetry and conventional differential scanning calorimetry (DSC) are combined to investigate Tf′ for polystyrene over a broad range of heating rates ranging from 0.017 to 3000 K/s after cooling at different rates. The results show that Tf′ depends on cooling rate following the WLF equation. On the other hand, Tf′ is not a function of heating rate, consistent with theoretical predictions, in spite of the change in the magnitude and placement of the enthalpy overshoot.

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

The kinetics of the glass transition are measured for a single polystyrene ultrathin film of 20 nm thickness using Flash differential scanning calorimetry (DSC). Tg is measured over a range of cooling rates from 0.1 to 1000 K/s and is depressed compared to the bulk. The depression decreases with increasing cooling rate, from 12 K lower than the bulk at 0.1 K/s to no significant change at 1000 K/s. Isothermal enthalpy recovery measurements are performed from 50 to 115 °C, and from these experiments, the temperature dependence of the induction time along the glass line is obtained, as well as the temperature dependence of the time scale required to reach equilibrium, providing a measure of the shortest effective glassy relaxation time and the longest effective equilibrium relaxation time, respectively. The induction time for the ultrathin film is found to be similar to the bulk at all temperatures presumably because the Tg values are the same due to the use of a cooling rate of 1000 K/s prior to the enthalp...

The effect of nanoconfinement on the free radical polymerization of methyl methacrylate (MMA) is investigated using differential scanning calorimetry, gel permeation chromatography, and 1 H nuclear magnetic resonance. Both hydrophobic and hydrophilic 13 nm-diameter controlled pore glasses (CPG) are used for polymerization under nanoconfinement. The number-average and weight-average molecular weights increase under nanoconfinement because the onset of autoacceleration shifts to shorter times, whereas the polydispersity index at full conversion decreases relative to the bulk value. The tacticity changes from syndiotactic-rich triads for the bulk PMMA to a higher percentage of isotactic-rich triads in hydrophilic pores; the data are described by the first-order Markov model. In addition to the changes in molecular weight and tacticity, the glass transition temperature increases for both pore surfaces compared with the bulk, but the increase in hydrophilic pores is more pronounced.

The reaction kinetics of cyclopentadiene (CPD) dimerization is studied using differential scanning calorimetry (DSC). Both dynamic and isothermal reactions are investigated. The experimental data were fit to a second order autocatalyzed reaction, taking into account the effect of elevated pressure on concentration which results from heating the easily volatilized reactant mixture in a sealed DSC pan. The rate constants and associated activation energies are obtained from the kinetic model, as well as from a model-free method. The results from the modeling with and without pressure effects are compared with one another and literature data.

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...

A glass is not in thermodynamic equilibrium below its glass transition temperature (Tg), and consequently, its properties, such as enthalpy, volume, and mechanical properties, evolve toward equilibrium in a process known as structural recovery or physical aging. Several recent studies have suggested that the extrapolated liquid line is not reached even when properties have ceased to evolve. In this work, we present measurements of the enthalpy recovery of polystyrene at an aging temperature 15 °C below the nominal Tg, for aging times up to 1 year. The results indicate that the equilibrium liquid enthalpy line can indeed be reached for aging 15 K below Tg. The results are analyzed in the context of the TNM model of structural recovery.

The kinetics associated with the glass transition is investigated using differential scanning calorimetry (DSC) for germanium selenide glasses with Ge content ranging from 0 to 30 at.% and average coordination numbers ranging from 2.0 to 2.6. As Ge content increases, the glass transition region broadens and the step change in heat capacity at T g decreases. As a result of physical aging, enthalpy overshoots are observed in DSC heating scans and the corresponding change in enthalpy can be calculated as a function of aging time. The change in enthalpy linearly increases with the logarithm of aging time and then levels off at an equilibrium value that increases with decreasing aging temperature. The time required to reach equilibrium increases with decreasing aging temperature and, at a given distance from T g , the time increases with decreasing germanium content. The results indicate that all samples show expected physical aging behavior, and no evidence is found for a Boolchand intermediate phase characterized by high stability and absence of physical aging.

The calorimetric glass transition (Tg) is measured for single polystyrene ultrathin films using a commercial rapid-scanning chip calorimeter as a function of cooling rate and film thickness. Films have been prepared in two ways: spin-cast films placed on a layer of inert oil or grease and films directly spin-cast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show results consistent with those of a bulk sample measured by conventional DSC. On the other hand, the 47 nm thick film on oil and 71 nm thick films both on oil and on grease show a Tg depression which decreases with increasing cooling rate; the magnitude of the Tg depression is similar to results reported in the literature for the most mobile substrate-supported films. For films directly spin-cast onto the sensor, a Tg depression is not observed for 47 and 71 nm thick films but is observed for a 16 nm thick film. These results are also within the range of the data on supported films in the literatur...

Rapid scanning chip calorimetry is a very useful tool for studying the glass transition and the related enthalpy relaxation kinetics. In this chapter, we review the practical aspects of making fictive temperature and enthalpy recovery measurements, including for ultrathin samples. The cooling rate dependence of the glass transition is discussed, as well as the Tg depression for ultrathin polystyrene and polycarbonate samples. The advantages of the short instrument response time and high cooling rates can be particularly exploited for enthalpy recovery measurements, and these are discussed in detail.

The reaction kinetics of an epoxide-functionalized regenerated cellulose membrane with excess aliphatic amine are examined using differential scanning calorimetry. Both isothermal and dynamic experiments are analyzed. The reaction is found to follow first-order autocatalytic kinetics. The activation energy based on five different methods ranges from 77.0 to 79.7 kJ/mol, indicating that the apparent activation energy for the heterogeneous epoxy/aliphatic amine reaction is considerably higher than the values reported in the literature for the homogeneous reaction.

The bulk and shear rheological properties of a sym- metric three-arm star polystyrene were measured using a self- built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure-volume-temperature behavior, the pressure-depend- ent glass transition temperature (Tg), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher Tgs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above Tg obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using TTg scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear poly- styrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecu- lar mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short-time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 1233-1244, 2012

An expression for the relaxation time, τ, which applies both in the liquid and in the glassy state, is proposed and coupled with the kinetic equation for the volume relaxation of the KAHR (Kovacs, Aklonis, Hutchinson, and Ramos) phenomenological theory. The expression for τ reproduces the WLF (Williams, Landel, and Ferry) behavior above the glass transition temperature, contains the thermodynamic scaling proposed by Casalini and Roland, gives finite values of the equilibrium relaxation time for any value of temperature and pressure, and is assembled in way that the relationship between the logarithmic relaxation time and the internal order parameter is strongly non-linear even when the system is very close to the equilibrium. The resulting model contains ten parameters, and five of these are treated as fitting parameters. With one set of model parameters, the model is able to describe quantitatively the isobaric specific volume response on cooling at various rates and as a function of pressure, the overshoot response on heating after cooling at different rates, the pressure dependence of Tg, the asymmetry of response that characterizes volume relaxation after temperature up and down jumps, and the tau-effective paradox and associated expansion gap. The model also reasonably predicts the overshoot response observed after the two-step memory experiment.

Free radical polymerization of methyl methacrylate in nanopores has been shown to result in a decrease in the time for the onset of autoacceleration. In this work, we simplify our previous kinetic model of nanoconfined methyl methacrylate polymerization, which was based on the work of Verros and coworkers, and incorporate diffusion effects into the model using the Doolittle free volume theory. The simplified model well describes the experimental calorimetric conversion versus time data for isothermal bulk methyl methacrylate polymerization, capturing autoacceleration and the dependence of the limiting conversion on temperature. In order to model the reaction in nanopores, we assume that the diffusion coefficient scales with molecular size to the −3 power and with nanopore diameter to the 1.3 power. Experimental calorimetric conversion versus time data for polymerization in hydrophobic nanopores are well captured by the model, including the decrease in the time to reach autoacceleration with decreasing pore size. The scaling assumed is consistent with that predicted using molecular simulations for good solvent conditions by Avramova and Milchev and by Cui, Ding, and Chen. According to the fit of the experimental data, chain diffusivity is 20–50% of the bulk value in 13 nm-diameter pores.

The effects of nanopore confinement on the crystallization and vitrification of a low molecular weight organic material, tris(4-cumylphenol)-1,3,5-triazine, are investigated using differential scanning calorimetry. The material shows cold crystallization and subsequent melting in the bulk state. Under the nanoconfinement of controlled pore glasses (CPG), cold crystallization and melting shift to lower temperatures. Crystallization kinetics are hindered in nanoconfinement, and no crystallization occurs in 13 nm diameter pores over the course of a week. Using a traditional Avrami analysis, the restricted crystallization under nanopore confinement is quantified; for crystallization at 80 °C, the Avrami exponent decreases with decreasing pore size and the overall crystallization rate is approximately 30 times slower for material confined in 50 nm diameter pores than the bulk. When compared at the temperature at which the crystallization rate is a maximum, the Avrami exponent is higher in nanoconfined samples and the crystallization rate is approximately 10 times slower for material confined in 50 nm diameter pores. Under CPG nanoconfinement, the glass transition temperature also decreases and shows two values; interestingly, the T(g) values further decrease with increasing crystallinity.

The effect of nanoconfinement on the rate of isothermal polymerization of methyl methacrylate (MMA) polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG) with pore diameters of 13, 50, and 111 nm are used for the confinement of the reaction. Both hydrophilic and hydrophobic pore surfaces are studied. The effective reaction rate and the apparent activation energy at low conversions, prior to autoacceleration, are unchanged in hydrophobic pores. On the other hand, in hydrophilic pores, the reaction rate increases by as much as a factor of 8 in the smallest 13 nm hydrophilic pores, and the effective activation energy decreases. For both pore surfaces, the time required to reach autoacceleration decreases with decreasing pore size, with the effect much more pronounced in the hydrophilic pores. The results are consistent with a model of nanoconfined free radical polymerization which accounts for suppressed termination due to a decrease in the diffusivity of nanoconfined chains.

Nanoconfinement of methyl methacrylate free radical polymerization is known to impact the molecular weight and molecular weight distribution of the polymer produced, with results in the literature generally indicating an increase in molecular weight and a concomitant decrease in polydispersity index. In the present work, the mathematical model described by Verros et al. (2005) for free radical bulk polymerization of methyl methacrylate is extended to account for polymerization in nanopores. The model of Verros et al. (2005) incorporates diffusion effects and is capable of describing the conversion and the number- and weight-average molecular weights of the resulting poly(methyl methacrylate) as a function of polymerization time and process conditions. The model is extended by incorporating the effect of nanoconfinement on diffusivity using the scaling reported in the literature. The calculations indicate that nanoconfinement will lead to higher molecular weights and lower polydispersity, and the gel effect will occur earlier. The results are compared to experimental work and implications discussed.

The pressure-volume-temperature (PVT) behavior was studied for two polycyanurate networks having different crosslink densities using a pressurizable dilatometer. The sam- ples were studied at temperatures ranging from 60 to 180 � C and at pressures up to 170 MPa to yield PVT data in both rub- bery and glassy states. The Tait equation is found to well describe the isobaric temperature scan and isothermal pres- sure scan data. The thermal expansion coefficients, instantane- ous bulk moduli, and thermal pressure coefficients are extracted from the data and their dependence on crosslink den- sity is examined. The time-dependent viscoelastic bulk modu- lus (K(t)) is also calculated in the vicinity of the a-relaxation from previously published pressure relaxation experimental data, and the strength and shape of the dispersion are found to be independent of crosslink density. The limiting bulk moduli depend strongly on temperature with those of the more loosely crosslinked sample being lower at a given temperature and pressure, although at Tg(P), the limiting mod- uli of the more loosely crosslinked sample are slightly higher than those of the more highly crosslinked sample. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2509-2517, 2010

The viscoelastic properties of poly(α-methyl styrene), its hexamer, and their athermal blends at various concentrations are studied. Master curves for the dynamic shear responses, G′ and G″, are successfully constructed for both the pure materials and the blends, indicating the validity of the time–temperature superposition principle for these systems. The temperature dependence of the shift factor follows the Vogel–Fulcher behavior over the temperature range studied, and the temperature dependence is slightly weaker for the blends. The rubbery plateau modulus scales with the polymer concentration as G N 0 ∝ ϕ 2 . 04 ± 0 . 32 ; the terminal relaxation time scales with the polymer concentration as τ d ∝ ϕ 1 . 35 ± 0 . 33 . The shape of the segmental dispersion appears unchanged by concentration, which differs from our calorimetric studies where mixtures show obviously temperature-broadened glass transitions and depressed enthalpy overshoots. The TNM (Tool-Narayanaswamy-Moynihan) model indicates that the change in the temperature dependence is not sufficient to account for the observed calorimetric broadening. We conclude that the temperature broadening of the glass transition for our blends is not due to a broadening of the dynamic spectrum or to changes in its temperature dependence. The possibility that the broadening is due to changes in the non-linearity parameter x in the TNM model is also considered. While the broadening could be due to a decreasing value of x, we found that this same decrease would lead to increasing enthalpy overshoots on heating, contrary to the experimental observations. The combination of the calorimetric results with the rheological measurements further indicates that the fundamental basis of the TNM-type of model of structural kinetics in glasses is potentially wrong.