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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...

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...

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.

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 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 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 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 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

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...

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 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.

Nanoscale constraint is known to have a significant impact on the thermal properties of materials. In this work, differential scanning calorimetry (DSC) is used to investigate the depression in the glass transition temperature ( T g ) and the reactivity of a monofunctional and of a difunctional cyanate ester cured under nanoscale constraint. Both reactants undergo trimerization, but the former forms a small molecular-weight compound, whereas the latter forms a polycyanurate network material. A T g depresssion is observed for both the reactants and their products; the magnitude of the depression seems to be related to the size of the molecule being confined relative to the confinement size. The trimerization reaction is accelerated relative to the bulk when the reactants are confined in nanopores. This is clearly observed by a shift in the reaction exotherms to lower temperatures for dynamic temperature scans. Quantification of the acceleration is accomplished by converting the dynamic temperature scan data to conversion versus time data assuming constant activation energy. The results are consistent with acceleration factors obtained from isothermal cure studies, but the dynamic data is considerably easier to obtain.

The Tg depression and kinetic behavior of stacked polystyrene ultrathin films is investigated by differential scanning calorimetry (DSC) and compared with the behavior of bulk polystyrene. The fictive temperature (Tf) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (Tg). The stacked ultrathin films show enthalpy overshoots in DSC heating scans which are reduced in height but occur over a broader temperature range relative to the bulk response for a given change in fictive temperature. The cooling rate dependence of the limiting fictive temperature, Tf′, is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the Tg depression between the ultrathin film sample and the bulk is inversely related to the cooling rate. We also find that the rate of physical aging of the stacked ultrathin films is comparable with the bulk when aging is performed at the same distance from Tg; however, when conducted at the same aging temperature, the ultrathin film samples show accelerated physical aging, that is, a shorter time is required to reach equilibrium for the thin films due to their depressed Tg values. The smaller distance from Tg also results in a reduced logarithmic aging rate for the thin films compared with the bulk, although this is not indicative of longer relaxation times. The DSC heating curves obtained as a function of cooling rate and aging history are modeled using the Tool-Narayanaswamy-Moynihan model of structural recovery; the stacked ultrathin film samples show lower β values than the bulk, consistent with a broader distribution of relaxation times. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2741–2753, 2008

The structural recovery of a single polystyrene thin film is investigated using nanocalorimetry at aging times as short as 0.01 s, as well as aging at temperatures as high as15 K above the nominal glass transition temperature for high fictive-temperature glasses obtained at high cooling rates. The results indicate that structural recovery progresses as expected when the aging temperature is low compared to the initial fictive temperature; in this case, the fictive temperature evolves smoothly towards the aging temperature at a rate that depends on the aging temperature and initial fictive temperature (i.e., on the cooling rate prior to aging), and at equilibrium, the fictive temperature Tf = Ta. For the case when the aging temperature is much higher than the initial fictive temperature, no relaxation occurs during isothermal aging. For the intermediate case when the aging temperature is in the vicinity of the initial fictive temperature, relaxation occurs both during isothermal aging and during cooling after aging, with the result that the equilibrium fictive temperature is lower than the aging temperature. The use of a new equation for the relaxation time in the framework of the Tool–Narayanaswamy–Moynihan (TNM) model allows description of the data with a single set of model parameters.

The absolute heat capacity and glass transition temperature (Tg) of unsupported ultrathin films were measured with differential scanning calorimetry with the step-scan method in an effort to further examine the thermodynamic behavior of glass-forming materials on the nanoscale. Films were stacked in layers with multiple preparation methods. The absolute heat capacity in both the glass and liquid states decreased with decreasing film thickness, and Tg also decreased with decreasing film thickness. The magnitude of the Tg depression was closer to that observed for films supported on rigid substrates than that observed for freely standing films. The stacked thin films regained bulk behavior after the application of pressure at a high temperature. The effects of various preparation methods were examined, including the use of polyisobutylene as an interleaving layer between the polystyrene films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3518–3527, 2006

A kinetic study of the trimerization of monocyanate ester both in the bulk and in the nanoconfinement of controlled pore glass is performed using differential scanning calorimetry. Both isothermal and dynamic experiments are analyzed. Although the activation energy for the reaction is the same within experimental error for the bulk and nanoconfined samples (approximately 21-23 kcal/mol), the reaction is accelerated under nanoconfinement by approximately 50 times in 13 nm pores compared with bulk.

Flash differential scanning calorimetry was used to study the glass transition temperature Tg of polycarbonate ultrathin films. The investigation was made as a function of film thickness from 22 to 350 nm and over a range of cooling rates from 0.1 to 1000 K/s. Polycarbonate spin cast films were floated on a layer of grease on the calorimetric chip. The results show a greatly reduced glass temperature for the thinnest films relative to the macroscopic value. We also observed that the magnitude of the glass temperature reduction decreases as the cooling rate increases with the highest cooling rates showing little thickness dependence of the Tg. Dynamic fragility and activation energy at Tg were found to decrease with decreasing film thickness. The results are discussed in the context of literature reports for supported and freely standing polycarbonate films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1462–1468