Heat capacities of perfluoropolyethers

The heat capacities at constant pressure of liquid perfluoropolyethers with different chain structures were determined above the glass transition temperature up to 480 K by means of differential scanning calorimetry (DSC). The group contributions of the  O , CF₂ , and  CF(CF₃) were calculated as a function of the temperature. Anomalous behavior of ethereal oxygen in a perfluorinated chain, as previously found for group contributions to the glass transition and to the vaporization energy, was observed also for heat capacity where the oxygen contribution is consistently lower for perfluorinated polyoxides in comparison to the hydrogenated homologous. The jump in c_p at the glass transition follows a regular behavior in the sense that ΔC_p/beadmole is within the average range found by Wunderlich for the majority of polymers. Moreover, data obtained in the present work allow the prediction of c_p of perfluoropolyethers of whatever structure between T_g and 480 K. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2073–2082, 1997     

Twinkling fractal theory of the glass transition

In this paper we propose a solution to an unsolved problem in solid state physics, namely, the nature and structure of the glass transition in amorphous materials. The development of dynamic percolating fractal structures near T_g is the main element of the Twinkling Fractal Theory (TFT) presented herein and the percolating fractal twinkles with a frequency spectrum F(ω) ∼ ω^df–1 exp −|ΔE|/kT as solid and liquid clusters interchange with frequency ω. The Orbach vibrational density of states for a fractal is g(ω) ∼ ω^df–1, where d_f = 4/3 and the temperature dependent activation energy behaves as ΔE ∼ (T² − T). The key concept of the TFT derives from the Boltzmann population of excited states in the anharmonic intermolecular potential between atoms, coupled with percolating solid fractal structures near T_g. The twinkling fractal spectrum F(ω) at T_g predicts the correct dynamic heterogeneity behavior via the spatio-temporal thermal fluctuation autocorrelation relaxation function C(t). This function behaves as C(t) ∼ t^−1/3 (short times), C(t) ∼ t^−4/3 (long times) and C(t) ∼ t⁻² (ω < ω_c), which were found to be in excellent agreement with published nanoscale AFM dielectric force fluctuation experiments on a glassy polymer near T_g. Using the Morse potential, the TFT predicts that T_g = 2D_o/9k, where D_o is the interatomic bonding energy ∼ 2–5 kcal/mol and is comparable to the heat of fusion ΔH_f. Because anharmonicity controls both the thermal expansion coefficient α_L and T_g, the TFT uniquely predicts that α_L×T_g ≈ 0.03, which is found to be universal for a broad range of glassy materials from Pyrex to polymers to glycerol. Below T_g, the glassy structure attains a frustrated nonequilibrium state by getting constrained on the fractal structure and the thermal expansion in the glass is reduced by the percolation threshold p_c as α_g ≈ p_cα_L. The change in heat capacity ΔC_p = C_pL–C_pg at T_g was found to be related to the change in dimensionality from D_f to 3 in the Debye approximation as the ratio C_pL/C_pg = 3/D_f, where D_f is the fractal dimension of the glass. For polymers, the TFT describes the molecular weight dependence of T_g, the role of crosslinks on T_g, the Flory-Fox rule of mixtures and the WLF relation for the time-temperature shift factor a_T, which are traditionally viewed in terms of Free-Volume theory. The TFT offers new insight into the behavior of nano-confined glassy materials and the dynamics of physical aging. It also predicts the relation between the melting point T_m and T_g as T_m/T_g = 1/[1−p_c] ≈ 2. The TFT is universal to all glass forming liquids. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2765–2778, 2008     

Infrared spectroscopic studies of polymer transitions. III. Effects of sodium ion on glass transitions in styrene-based ionomers

The glass transition in styrene-based ionomers was investigated by means of infrared spectroscopy and differential scanning calorimetry (DSC). Transition temperatures were determined from the temperature dependence of the peak absorbances of the 1700 and 1745 cm^?1 bands. These transition temperatures agreed with glass transition temperatures (T_g) determined by DSC. With increasing degree of ionization, T_g and the enthalpy ΔH of the residual intermolecular hydrogen bonding increased. The values of T_g obtained were analyzed by the theory of Fox and Loshaek for the effect of crosslinks. It is concluded that sodium ions probably from ionic domains and act as crosslinks to reinforce the residual hydrogen bonding and may increase T_g. The absorbance at 1560 cm^?1 (ν_COO^?) did not change at T_g. This suggests that the glass transition observed here is not due to the onset of the mobility in ionic domains, as has been proposed for ethylene-based ionomers on the basis of dielectric measurements.     

Modulated differential scanning calorimetry

Modulated-temperature differential scanning calorimetry was used to measure the glass transition temperature,T _g, the heat capacity relaxation in the glassy state and the increment of heat capacity, C_p, in the glass transition region for several polymers. The differential of heat capacity with respect to temperature was used to analyseT _g and C_p simply and accurately. These measurements are not affected by complex thermal histories.     

Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

The ability to prepare high T_g low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (T_g) for several thiol–norbornene materials. Results indicate that T_gs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high T_g materials. In particular, as much as a 70 °C increase in T_g was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007     

Crossover in dynamics of polymeric liquids: Back to Tll?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polystyrene (PS), were analyzed in the broad frequency range at temperatures above the glass transition (T_g ). At high temperatures, the spectra followed the qualitative scenario suggested by mode‐coupling theory (MCT) of the glass transition. The crossover temperature (T_c ) was defined to be approximately 1.35 T_g in PIB and approximately 1.15 T_g in PS. At lower temperatures (T < T_c ), the light scattering spectra deviated strongly from the idealized MCT scenario. Different signs of the dynamic transition around T_c are discussed. The difference between the suggested interpretation and an old idea of the liquid–liquid transition in polymeric liquids is stressed: we describe the transition as purely dynamic in nature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2785–2790, 2000     

Glass‐transition temperature: Conversion relationship in the polycyclotrimerization of 4,4′‐thiodiphenylcyanate

Polycyclotrimerization of 4,4′‐thiodiphenylcyanate was adopted as a model system for general thermosetting polymers for studying the relationship between the glass‐transition temperature (T_g) and conversion (α) during network formation. Existing expressions for T_g‐α relationship were used and compared. The experimental T_g‐α data were well fitted to several one‐parameter equations although the physical significance of parametric values thus obtained could not be unambiguously identified. Among the two‐parameter models, both the Hale–Macosko–Bair equation and the so‐called “original” DiBenedetto equation were well fitted by experimental data (when the mean‐field crosslink density was used), yielding parametric values consistent with the original designated physical meanings within the corresponding theoretical frames. Relationships between the parameters in different theories were also discussed. Incidentally, a discontinuity of ΔC_pT_g at the gel point was observed (i.e., ΔC_pT_g is of different values in the pregel and postgel regimes, respectively). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 726–738, 2000     

Synthesis and characterization of soluble polyimides derived from [1,1′;4′,1″]terphenyl‐2′,5′‐diol and biphenyl‐2,5‐diol

Two series of polyimides based on laterally attached p‐terphenyl and biphenyl groups were synthesized. The solubility and thermal properties were studied using DSC, thermogravimetric analysis, and the solubility test. These polymers exhibited good thermal stability and excellent solubility. The high solubility for both polymer series was attributed to the non‐coplanarity of diamine monomers and the use of fluorinated dianhydride, whereas the slightly better solubility for polymers based on p‐terphenyl was attributed to further weakening of interchain interaction of the polymers. Both polymer series exhibited glass‐transition temperatures (T_g's) in the range of 244–272 °C. The T_g's of polymers containing laterally attached p‐terphenyls were higher than those of their counterparts containing biphenyls by 5–17 °C. This was attributed to the formation of an interdigitated structure that hinders the segmental movement of polymer chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2998–3007, 2001     

Heat capacity of poly(vinyl methyl ether)

The heat capacity of poly(vinyl methyl ether) (PVME) has been measured using adiabatic calorimetry and temperature‐modulated differential scanning calorimetry (TMDSC). The heat capacity of the solid and liquid states of amorphous PVME is reported from 5 to 360 K. The amorphous PVME has a glass transition at 248 K (?25 °C). Below the glass transition, the low‐temperature, experimental heat capacity of solid PVME is linked to the vibrational molecular motion. It can be approximated by a group vibration spectrum and a skeletal vibration spectrum. The skeletal vibrations were described by a general Tarasov equation with three Debye temperatures Θ₁ = 647 K, Θ₂ = Θ₃ = 70 K, and nine skeletal modes. The calculated and experimental heat capacities agree to better than ±1.8% in the temperature range from 5 to 200 K. The experimental heat capacity of the liquid rubbery state of PVME is represented by C_p(liquid) = 72.36 + 0.136 T in J K^?1 mol^?1 and compared to estimated results from contributions of the same constituent groups of other polymers using the Advanced Thermal AnalysiS (ATHAS) Data Bank. The calculated solid and liquid heat capacities serve as baselines for the quantitative thermal analysis of amorphous PVME with different thermal histories. Also, knowing C_p of the solid and liquid, the integral thermodynamic functions of enthalpy, entropy, and free enthalpy of glassy and amorphous PVME are calculated with help of estimated parameters for the crystal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2141–2153, 2005     

Pressure and volume dependence of thermal conductivity and isothermal bulk modulus up to 1 GPa for poly(isobutylene)

The thermal conductivity λ and heat capacity per unit volume ρc_p of poly(isobutylene)s, one 2.8 in weight average molecular weight and one 85 kg mol⁻¹ in viscosity average molecular weight (PIB-2800 and PIB-85000), have been measured in the temperature range 170–450 K at pressures up to 2 GPa using the transient hot-wire method. At 297 K and atmospheric pressure, λ = 0.115 W m⁻¹ K⁻¹ for PIB-2800 and λ = 0.120 W m⁻¹ K⁻¹ for PIB-85000. The bulk modulus B_T has been measured in the temperature range 170–297 K up to 1 GPa. At atmospheric pressure, the room temperature bulk moduli B_T are 2.0 GPa for PIB-2800 and 2.5 GPa for PIB-85000 with dB_T/dp = 10 for both. These data were used to calculate the volume dependence of λ, At room temperature and atmospheric pressure (liquid phase) we find g = 3.4 for PIB-2800 and g = 3.9 for PIB-85000, but g depends strongly on temperature for both molecular weights. The difference in g between the glassy state and liquid phase is small and just outside the inaccuracy of g of about 8%. The best predictions for g are given by the theoretical model of Horrocks and McLaughlin. We have found that PIB exhibits two relaxations, where one is associated with the glass transition. The value for dT_g/dp at atmospheric pressure (for the main glass transition) is about 0.21 K MPa⁻¹ for both molecular weights. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1781–1792, 1998     

Thermal properties of single walled carbon nanotube‐silicone nanocomposites

The effect of single walled carbon nanotube (SWCNT) fillers on the low temperature thermal properties and curing behavior of SWCNT‐silicone nanocomposite are reported for the first time. The SWCNT‐silicone composites were prepared by different mixing procedures and characterized by differential scanning calorimetry (DSC). Solution mix, with the aid of sonication and soaking achieved better dispersion of SWCNTs in the silicone. The adding of SWCNTs in polymer seriously hindered the curing of silicone elastomer. The hindrance increased with increasing concentration of SWCNT and the quality of dispersion. The glass transition temperatures (T_g) of the nanocomposites were found to be independent of the SWCNT addition, although, the steps in the heat capacity (Δc_p) of the glass transition were smaller with increasing SWCNTs concentration. The melt crystallization behavior was strongly dependent on the concentration and dispersion of SWCNT in the polymer. The cooling scan showed that the higher concentration and the better dispersion of SWCNTs in the silicone resulted in higher percentage of melt crystallization of this nanocomposite. The correlation of the change of thermal properties to the dispersion of CNT in polymer may be used to determine the quality of SWCNT dispersion in silicone polymer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1845–1852, 2008     

Dependence of the glass transition temperature of poly (methyl methacrylates) on their tacticity

The specific isobaric heat capacities of poly (methylmethacrylates) (PMMA) having various tacticities were measured by the DSC method within a broad range of temperatures including the glass transition. Glasses with uniform thermal history were used in the measurements and the data were treated by employing a procedure which provided the thermodynamic T_g independent of the experimental conditions. The semiquantitative validity of Boyer's empirical relationT _g ×c_p=const. was confirmed; also it was found that within the limits of experimental accuracy the c_p,g values at 298 K andC _p,l values at 400 K are independent of the tacticity of the sample.Using the data thus measured and linearized equations representing the dependence ofT _g on the content of iso-, syndio- and heterotriads, the T_g values of pure isotactic PMMA and pure syndiotactic PMMA were found to be respectively 315 K and 397 K.Dedicated to Professor Dr. F. H. Müller.     

Calorimetric relaxation and glass transition in poly(propylene glycols) and its monomer

The glass transition temperature T_g of propylene glycol (PG) and poly(propylene glycols) (PPGs) of molecular weight up to 4000 has been measured by differential scanning calorimetry, and the activation energy and change in heat capacity ΔC_p have been determined in the glass transition range. The activation energy increases with an increase in the molecular weight of the polymer, and ΔC_p measured at a fixed heating rate decreases. The increase in T_g with molecular weight is remarkably more rapid for poly(propylene glycols) than for other polymers, and a limiting value of T_g is reached for a chain containing 20 monomer units. These results are discussed in terms of the Fox-Flory and the entropy theories. The calorimetric relaxation times are comparable with the extrapolated dielectric relaxation times. The initial increase of ΔC_p from PG to PPG 200 is attributed to the decrease of H-bonding sites from 12 in 3 monomers to 4 on polymerization to PPG 200 and further decrease with increase in molecular weight to an increasingly large amplitude of the β-process at T < T_g.     

Probe rotation near and below the glass transition temperature: Relationship to viscoelasticity and physical aging

Reorientation times ť_c for two probes in several amorphous polymers near the glass transition temperature T_g are reported. T_g for these polymers ranges from 205 to 459 K. Probe reorientation was measured in the time window from 10⁻² to 10⁴ s with a recently developed photobleaching method. ť_c for a given probe at the T_gs of the different polymers varies more than three decades. Viscoelastic relaxation times characteristic of the Rouse modes of the matrix polymers are closely related to probe rotation times and thus also not constant at T_g. The characteristic length scale of motions responsible for the glass transition varies significantly for the three polymers studied. Preliminary physical aging results indicate that probe reorientation in polystyrene ages slightly faster than the volume.     

Novel photoinitiated cationic copolymerizations of 4-methylene-2-phenyl-1,3-dioxolane

Photoinitiated polymerization of 4-methylene-2-phenyl-1,3-dioxolane ( 1 ) was carried out using either tris (4-methylphenyl) sulfonium hexafluoroantimonate or 4-decyloxyphenyl phenyliodonium hexafluoroantimonate as initiators. ¹H-NMR analyses confirmed exclusive ring-opening while DSC and SEC were used to determine the glass transition temperatures (T_gs) and molecular weights, respectively. Photoinitiated cationic copolymerizations of 1 were investigated with several acyclic and cyclic monomers. Copolymerization of 1 with vinyl ethers and a spiroorthoester resulted in copolymers whose thermal properties were dependent on comonomer ratios. Copolymers of 1 and dihydrofuran or dihydropyran afforded soluble polymers with T_gs significantly higher than the homopolymer of 1 . © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2207–2219, 1997     

Radiothermoluminescence. I. Radiothermoluminescence for investigation of thermal and radiation-induced crosslinking of rubber mixtures

Phase heterogeneity of vulcanizates based on cis-polybutadiene (cis-Europrene) and SBR (Europrene 1500) blends obtained by radiation, thermal, two-stage radiation-thermal, and two-stage thermal-radiation processes was investigated by the radiothermoluminescence method. Unvulcanized blends of these polymers are found to be heterogeneous, exhibiting two glass transition temperatures T_g, which coincide with the values for the initial components. Following vulcanization, the difference between the two T_g values for a polyblend decreases. The decrease is more distinct for polyblends crosslinked at high temperatures. When the vulcanization temperature exceeds 140°C and the crosslink density (or 1/M_c) exceeds 5 × 10^?5–1 × 10^?4 mole/cm³, the glass temperature ranges of the crosslinked blend are practically superimposed. Such crosslinked mixtures should be considered pseudohomogeneous in phase.     

Positronium annihilation in amine-cured epoxy polymers

Positronium annihilation spectroscopy (PAS) has been used to study the microstructural properties of amine-cured epoxy polymers. We have determined the free-volume “hole” sizes in these polymers by comparing the observed ortho-positronium lifetimes with the known lifetime–free volume correlation for low-molecular-weight systems. The free volumes for four epoxies with different crosslink densities are found to vary significantly over the temperature range between ?78° and 250°C. The free-volume holes for these polymers are found to range from 0.025 to 0.220 nm³. Two important transition temperatures were found: one corresponds to the glass transition temperature T_g determined by differential scanning calorimetry (DSC), and the other occurs about 80–130°C below T_g. The sub-T_g transition temperature is interpreted tentatively as being where hole size reaches dimensions adequate for positronium trapping or else the onset temperature for local mode or side-chain motions. These two transition temperatures plus two additional onset temperatures are found to be correlated with crosslink densities calculated from stoichiometry.     

Thermophysical properties of azomethine ether main-chain liquid crystalline polymers at pressures to 200 MPa

This article reports on an experimental investigation of the equation of state and the transition behavior of main-chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure. Experimental techniques used included high-pressure dilatometry (PVT measurements) to 200 MPa, high-pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric-pressure) differential scanning calorimetry (DSC). The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, T_g(P), a first-order transition to a nematic state, T_k-n(P), and a first-order transition to an isotropic melt state T_c(P). The atmospheric pressure values of T_k-n and T_c decreased with increasing number of spacer units and showed a clear odd-even effect. T_g and T_k-n both increased with pressure. The pressure dependence of T_c could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below T_k-n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first-order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two-phase nature of the low-temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd-even effect. © 1994 John Wiley & Sons, Inc.     

Heat capacity of poly(trimethylene terephthalate)

The heat capacity of poly(trimethylene terephthalate) (PTT) has been measured using adiabatic calorimetry, standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 to 570 K. The semicrystalline PTT has a glass transition temperature of 331 K. Between 340 and 480 K, PTT can show exothermic ordering depending on the prior degree of crystallization. The melting endotherm of semicrystalline samples occurs between 480 and 505 K, with a typical onset temperature of 489 K (216°C). The heat of fusion of the semicrystalline samples is about 15 kJ mol⁻¹. For 100% crystalline PTT the heat of fusion is estimated to be 30 ± 2 kJ mol⁻¹. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum and the Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations (θ₁ = 550.5 K and θ₂ = θ₃ = 51 K, N_skeletal = 19). The calculated and experimental heat capacities agree to better than ±3% between 5 and 300 K. The experimental heat capacities of liquid PTT can be expressed by: $ C^L_p(exp) $ = 211.6 + 0.434 T J K⁻¹ mol⁻¹ and compare to ±0.5% with estimates from the ATHAS data bank using contributions of other polymers with the same constituent groups. The glass transition temperature of the completely amorphous polymer is estimated to be 310–315 K with a ΔC_p of about 94 J K⁻¹ mol⁻¹. Knowing C_p of the solid, liquid, and the transition parameters, the thermodynamic functions enthalpy, entropy, and Gibbs function were obtained. With these data one can compute for semicrystalline samples crystallinity changes with temperature, mobile amorphous fractions, and resolve the question of rigid-amorphous fractions.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2499–2511, 1998     

Thermal properties of freezing bound water restrained by sodium lignosulfonate-based polyurethane hydrogels

In order to develop a new functional product from lignin, sodium lignosulfonate (LS)-based polyurethane (LSPU) hydrogels were prepared from LS and hexamethylene diisocyanate (HDI) derivatives in water. Isocyanate/hydroxyl group ratio (NCO/OH ratio) was varied from 0.05 to 0.8 mol mol⁻¹, and water content (W_c = mass of water/mass of dry sample) of the obtained LSPU hydrogels was varied from 0 to 3.0 g g⁻¹. Phase transition behavior of hydrogels with various W_c’s was investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). In DSC heating curve of LSPU hydrogels, glass transition, cold crystallization, melting and liquid crystallization were observed. Cold crystallization, two melting peaks and variation of melting enthalpy indicate that three kinds of water, i.e., non-freezing water, freezing bound water and free water, exist in LSPU hydrogel. Glass transition temperature (T_g) decreased from 230 to 190 K in a W_c range where non-freezing water was formed in the hydrogel. T_g increased when freezing bound water was formed in the system. T_g leveled off in a W_c range where normal ice was formed. The effect of NCO/OH ratio on molecular motion of LSPU hydrogel is examined based on T_g and heat capacity difference at T_g (ΔC_p). Water vaporization curve measured by TG also indicates the presence of bound water which evaporates at a temperature higher than ca. 410 K. By atomic force microscopic observation, the size of molecular bundle of LSPU hydrogel is calculated and compared with that of LS-water system. By cross-linking, the height of molecular bundle decreased from ca. 3–1 nm and lignin molecules extend in a flat structure.