Conformational energy contribution to the heat of solution in polymer–solvent systems. Part I. Theory

The conformational energy contribution (ΔU_conf) to the heat of solution in polymer-solvent systems is presented and discussed in connection with chain conformational properties. In particular, ΔU_conf has been discussed in terms of various possible mechanisms of coil deformation.     

On the heat of solution of polymer with solvent: Polydimethylsiloxane–solvent systems

Heats of mixing and excess volumes at infinite dilution have been obtained at 25°C. for polydimethylsiloxane or its lower oligomers in various solvents by using a twin conduction microcalorimeter and from the pycnometric specific volumes. From those values, excess energies ΔE^M_v at constant volume have been determined. The prediction on intramolecular conformation contributions to the heat of solution as proposed by Bianchi has been evaluated by the values of ΔE^M_v. The heat of solution in the polymer–solvent systems was interpreted by the expression for ΔE^M_v derived from the Van Laar-Hildebrand work on simple liquid mixtures with the solubility parameters of polymers obtained from indirect measurements. The values of conformational intramolecular energy change calculated from dilute solution properties were difficult to rationalize with our results. Our present results suggest that systems in a nonideal state can not be distinguished for certain from those in the ideal state. This conclusion based on apparent values does not deny the possible effect of the conformational energy change.     

A study of the surface analysis of some water‐soluble polymers by inverse gas chromatography

The surface characterization of 2‐(dimethylamino)ethylmethacrylate (DMA) and 2‐(N‐morpholino)ethylmethacrylate (MEMA) homopolymers and DMA–MEMA diblock copolymer was studied using inverse‐gas chromatography (IGC). The analyzed surface properties of (co)polymers were the dispersive component of the surface energy ( ) and the acid–base characters of (co)polymer surfaces. The specific free energy (ΔG^sp), enthalpy (ΔH^sp), and entropy (ΔS^sp) of adsorption of polar probes on (co)polymers were calculated. The values of ΔH^sp were correlated with both the donor and the modified acceptor numbers (AN) of the probes to quantify the acidic K_A and the basic K_D parameters of (co)polymer surfaces. The values obtained for the K_A and K_D parameters indicated basic characters for the surface of (co)polymers. The dispersive component values of the surface energy and the acid–base surface parameters of the DMA–MEMA diblock copolymer surface were found to be between those homopolymers as expected. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of molecular structure on mesomorphism XXI. Monodisperse tetrameric model compounds for liquid crystalline polymers

Abstract

Twinned dimeric mesogens having a rigid-flexible-rigid molecular structure have been shown to be appropriate models for some properties of regularly alternating (rigid-flexible)_n main chain liquid crystalline polymers (lcps). A family of tetrameric monodisperse liquid crystalline model compounds chemically related to known main chain liquid crystalline polymers of the 4-alkoxyphenyl 4′-alkoxy-benzoate type has been synthesized. The tetramers are nematogenic. Alternations in thermodynamic parameters (ΔH, ΔS) for the N-I transition as a function of spacer chain length indicate conformational behaviour of the internal spacers dominates mesophase properties.     

Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.     

Attractive-dominant and repulsive-dominant hydrocarbon reactions

The difference between the expectation values of the total electronic kinetic energy operator (ΔE_K), and the operators accounting for the Coulombic interactions between the electrons and nuclei (ΔV_en), between all pairs of electrons (ΔV_ee), and between all pairs of nuclei (ΔV_nn) for the product and reactant species in a wide variety of hydrocarbon reactions are calculated using single determinant basis set data reported in the literature. Following Allen, their contributions to ΔE_T, the difference between the corresponding total molecular energies and thus the reaction heat, are grouped together as a repulsion energy term, ΔE_rep = ΔE_K + ΔV_ee + ΔV_nn, and an attraction energy term ΔE_attr = ΔV_en. For all but 2 of the 71 individual reactions considered in this paper, the experimental reaction heat at 0°K corrected for zero-point energy contributions, (ΔH)_zpe, is the result of near compensation between far larger ΔE_rep and ΔE_attr terms, in sharp contrast to the much smaller ΔE_rep and ΔE_attr terms which are characteristic of many molecular rotation processes. By matching the sign of (ΔH)_zpe with that of ΔE_rep or ΔE_attr, as the case may be, the reactions are classified as attractive-dominant or repulsive-dominant (46 in the former class and 23 in the latter), a property which is independent of the direction in which the reaction is written. The sign and magnitude of ΔV_ee, ΔV_nn, and ΔV_en and reaction category are discussed in relation to the various kinds of structural change involved in going from reactants to products. For the vast majority of reactions, the numerical relationship ΔV_ee ≈ ΔV_nn has been found to hold to within a few percent.     

Determination of the heat of fusion of isotactic poly(methyl methacrylate)

The heat of fusion (ΔH_u) of isotactic poly(methyl methacrylate) (PMMA) was determined from the combined linear regression analysis of x-ray diffraction data and differential scanning calorimetry. By means of two sample preparation techniques to reduce crystallinity, a thermal pretreatment in which a partial melt and quench cycle was utilized, and a mechanical mixture in which a semicrystalline and amorphous material were blended, a value for ΔH_u = 1200 ± 80 cal/mole (100 g) was found. The present result represents the first measurement of ΔH_u for an acrylic polymer and yields G(-units) = 12 from cryoscopic measurements for the chemical changes caused by high energy irradiation.     

Nematic conformation of oxyethylene spacers involved in main-chain liquid crystals

²HNMR measurements were performed on main-chain dimer and polymer liquid crystals (LC) having oxyethylene (OE) spacers -(OCH₂CH₂)_xO-(x=2, 3). The orientational as well as conformational characteristics of these molecules have been investigated in bulk and in a nematic solution. The OE spacer was found to take spatial arrangements characteristics of the nematic phase. The nematic conformation of the spacer remains nearly invariant over a wide range of temperature and concentration. In these analyses, the ratio of the deuterium quadrupolar splittings Δν/Δν^R (Δν: spacer and Δν^R: mesogenic unit) provided an important information regarding the spatial configuration of molecules in the LC state. The results obtained in this study are consistent with our previous conclusion drawn on a series of main-chain LC oligomers and polymers comprising n-alkane spacers -O(CH₂)_nO- (n=9, 10).     

Equilibrium anionic polymerization of the isopropenyl monomers containing aromatic heterocycles

The anionic polymerization of three monomers, 2-isopropenyl-4,5-dimethyloxazole(I), 2-isopropenylthiazole(II), and 2-isopropenylpyridine(III), was studied in THF. These monomers produced red-colored living polymers on addition of sodium naphthalene or living α-methylstyrene tetramer as an initiator. It was observed that a considerable amount of monomer remained in the respective living polymer–monomer system, indicating that an equilibrium between the polymer and the monomer existed as in the case of α-methylstyrene. At lower temperatures, the conversion of the monomer to the polymer increased. The equilibrium monomer concentrations [M_e] were determined at different temperatures, and the heats (ΔH) and the entropies (ΔS°) of polymerization were obtained by plotting In(1/[M_e]) against 1/T as ΔH = ?9.4, ?6.8, and ?6.2 kcal/mole, ΔS°S = ?22.9, ?16.5, and ?16.6, eu for I, II, and III, respectively.     

Hydrophobic interaction chromatography of proteins: Thermodynamic analysis of conformational changes

For BSA and β-lactoglobulin adsorption to hydrophobic interaction chromatography (HIC) stationary phases leads to conformational changes. In order to study the enthalpy (ΔH_ads), entropy (ΔS_ads), free energy (ΔG_ads) and heat capacity (Δc_p,ads) changes associated with adsorption we evaluated chromatographic data by the non-linear van’t Hoff model. Additionally, we performed isothermal titration calorimetry (ITC) experiments. van’t Hoff analysis revealed that a temperature raise from 278 to 308 K increasingly favoured adsorption seen by a decrease of ΔG_ads from −12.9 to −20.5 kJ/mol for BSA and from −6.6 to −13.2 kJ/mol for β-lactoglobulin. Δc_p,ads values were positive at 1.2 m (NH₄)₂SO₄ and negative at 0.7 m (NH₄)₂SO₄. Positive Δc_p,ads values imply hydration of apolar groups and protein unfolding. These results further corroborate conformational changes upon adsorption and their dependence on mobile phase (NH₄)₂SO₄ concentration. ITC measurements showed that ΔH_ads is dependent on surface coverage already at very low loadings. Discrepancies between ΔH_ads determined by van’t Hoff analysis and ITC were observed. We explain this with protein conformational changes upon adsorption which are not accounted for by van’t Hoff analysis.     

Solubility characteristics of poly(3‐hexylthiophene)

Solubility data for poly(3‐hexylthiophene) (P3HT) in 29 pure solvents are presented and discussed in detail. Functional solubility parameter (FSP) and convex solubility parameter (CSP) computations are performed and the CSP and FSP results are compared to previously reported Hansen solubility parameters (HSPs) and to the parameters calculated using additive functional group contribution methods. The empirical data reveals experimental solubility parameters with substantial polar (δ_P) and hydrogen‐bonding (δ_H) components, which are not intrinsic to the structure of the P3HT polymer. Despite these apparent irregularities, it is shown that the predictor method based on the solubility function, f, does provide a reliable way to quantitatively evaluate the solubility of P3HT in other solvents in terms of a given set of empirical solubility data. The solubility behavior is further investigated using linear solvation energy relationship (LSER) modeling and COSMO‐RS computations of the activity coefficients of P3HT. The LSER model reveals that (1) the cavity term, δ_T, is the dominant factor governing the solubility behavior of P3HT and (2) the solvent characteristics that dictate the structural order (crystallinity) of P3HT aggregates do not similarly influence the overall solubility behavior of the polymer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1075–1087     

Compensation effect in the thermodynamics of conformational equilibria

The solvent dependence of thermodynamic parameters of conformational equilibria in trans-1,2-dichlorocyclohexane and trans-1,2-bromochlorocyclohexane was investigated by infrared absorption spectra. The results obtained show the existence of a compensation effect in the thermodynamics of conformational equilibria: the enthalpy (ΔH₀) and entropy (ΔS₀) differences change in the same direction when going from one solvent to another. A semi-quantitative estimation of the effect is given on the basis of the equations of statistical thermodynamics. It is shown that the temperature dependence of the ΔS₀ value must be taken into account when determining the enthalpy difference of the conformers. This yields the equality of the true and observed ΔH₀ values.     

Thermochemical properties of RE (Gly)4 Im (ClO4) 3 2H2O (RE = La,Pr, Nd,Sm, Eu)

The enthalpies of solution in water for five new light rare earth ternary complexes RE(Gly)₄Im(ClO₄)₃ 2H₂O (RE = La, Pr, Nd, Sm, Eu; Gly‐glycine; Im‐imidazole) were measured by means of a Calvet microcalorimeter. The empirical formula of enthalpy of solution (Δ_solH), relative apparent molar enthalpy (πL_i), relative partial molar enthalpy (L_i) and enthalpy of dilution (Δ_dllH_1,2) were drawn up by the data of enthalpies of solution of these complexes. From three plots of the values of standard enthalpy of solution Δ_sol H?, πL_i, L_i) versus the values of ionic radius (r) of the light rare earth elements, the grouping effect of lanthanide was observed, showing that the coordination bond between rare earth ion and ligand possesses a certain extent of the property of a covalent bond. The standard enthalpies of solution in water of similar complexes, Ce(Gly)₄Im(ClO₄)₃.2H₂O were estimated according to the plot of Δ_solH?, versus r.     

Propriétés thermodynamiques des mélanges binaires. Volume et énergie interne de formation des solutions de l'hexadécane dans les isomères de l'hexane

The volumes of mixing of hexadecane and each of the isomers of hexane have been measured for the equimolar mixtures at 20°C. The results have been used together with previously measured values of ΔH to obtain ΔU_v. A very good correlation is found between the energy of mixing and the properties of the pure alkanes.     

Effect of Preferential Solvation on the Thermodynamic Properties of Antidepressant Drug Trazodone in Aqueous Ethanol: Linear Free‐Energy Relationships

Dissociation constants (pK_a) of trazodone hydrochloride (TZD⋅HCl) in EtOH/H₂O media containing 0, 10, 20, 30, 40, 50, 60, 70, and 80% (v/v) EtOH at 288.15, 298.15, 308.15, and 318.15 K were determined by potentiometric techniques. At any temperature, pK_a decreased as the solvent was enriched with EtOH. The dissociation and transfer thermodynamic parameters were calculated, and the results showed that a non‐spontaneous free‐energy change (Δ_dissG^o>0) and unfavorable enthalpy (Δ_dissH^o>0) and entropy (Δ_dissS^o<0) changes occurred on dissociation of trazodone hydrochloride. The free‐energy change or pK_a varied nonlinearly with the reciprocal dielectric constant, indicating the inadequacy of the electrostatic approach. The dissociation equilibria are discussed on the basis of the standard thermodynamics of transfer, solvent basicity, and solute‐solvent interactions. The values of Δ_transG^o and Δ_transH^o increased negatively with increasing EtOH content, revealing a favorable transfer of trazodone hydrochloride from H₂O to EtOH/H₂O mixtures and preferential solvation of H⁺ and trazodone (TZD). Also, Δ_transS^o values were negative and reached a minimum, in the H₂O‐rich zone that has frequently been related to the initial promotion and subsequent collapse of the lattice structure of water. The pK_a or Δ_dissG^o values correlated well with the Dimroth‐Reichardt polarity parameter E_T(30), indicating that the physicochemical properties of the solute in binary H₂O/organic solvent mixtures are better correlated with a microscopic parameter than the macroscopic one. Also, it is suggested that preferential solvation plays a significant role in influencing the solvent dependence of dissociation of trazodone hydrochloride. The solute‐solvent interactions were clarified on the basis of the linear free‐energy relationships of Kamlet and Taft. The best multiparametric fit to the Kamlet‐Taft equation was evaluated for each thermodynamic parameter. Therefore, these parameters in any EtOH/H₂O mixture up to 80% were accurately derived by means of the obtained equations.     

Monitoring conformational changes of immobilized RNase A and Lysozyme in reductive unfolding by surface plasmon resonance

A labeling-free surface plasmon resonance (SPR) sensor technique was used to monitor the conformational changes of immobilized globular proteins (RNase A and Lysozyme) in chemical unfolding and refolding. The conformational changes of proteins at solid/liquid interface are characterized as two-state transformation (S-shaped) curves through matrix-effect correction and theoretic estimation. By extrapolation with a Santoro-Bolen equation, the SPR results for both reductive immobilized proteins are estimated to 1.9 kcal mole⁻¹ global free energy (ΔG_U) in urea-induced unfolding. But the ΔG_U for RNase A and Lysozyme in GdmCl-induced unfolding are 1.5 and 2.15 kcal mole⁻¹, respectively. The disagreement in free energy is partially accounted for by the differences of intra-molecular interactions and immobilization.     

Stereoregularity of poly(isopropyl acrylate) and its racemization during hydrolysis

The diad tacticity of poly(isopropyl acrylate) was measured from the β-proton absorptions of poly(isopropyl acrylate-α,β-d₂) obtained with a 100 MHz NMR spectrometer, and temperature dependence of the tacticity of the polymers obtained by radical polymerization was determined. Enthalpy and entropy differences between isotactic and syndiotactic addition for poly(isopropyl acrylate) were calculated to give the following values: Δ(ΔS) = 0.7 eu; Δ(ΔH) = 0.51 kcal/mole. In the hydrolysis of poly(isopropyl acrylate-α,β-d₂), it was found that the rate of hydrolysis of poly(isopropyl acrylate) was dependent on the molecular weight rather than on the tacticity. As for the rate of racemization during hydrolysis, the rate for syndiotactic polymer was much faster than that for the isotactic polymer. The exchange reaction of deuterium at α-position with hydrogen occurred in all the polymers during hydrolysis reaction.     

Effects of sizes of nano-copper oxide on the equilibrium constant and thermodynamic properties for the reaction in nanosystem

Thermodynamic properties and equilibrium constant of reaction in nanosystems were analyzed theoretically. The effects of sizes of nano-CuO on thermodynamic properties and equilibrium constant were studied using the reaction of nano-copper oxide and sodium bisulfate as a system. The experimental results indicate that with the sizes of reactant decreasing, the molar Gibbs free energy (Δ_rG_m), the molar enthalpy (Δ_rH_m) and the molar entropy (Δ_rS_m) decrease, but the equilibrium constant (K) increases and there are linear trends between the reciprocal of sizes for nano-CuO and the values of Δ_rG_m, Δ_rH_m, Δ_rS_m and Ln K, which are in agreement with the theoretical analysis.     

Surface activity and thermodynamic properties of water‐soluble polyester surfactants based on 1,3‐dicarboxymethoxybenzene used for enhanced oil recovery

The surface activity and thermodynamic properties for eight low molecular weight nonionic co‐polyester (PE) surfactants have been investigated. Surface and interfacial tensions (IFT) of surfactants in aqueous solutions were measured using the spinning drop technique. From these measurements, the critical micelle concentration (CMC), the surface pressure at CMC (Y_CMC), the maximum surface concentration (Γ_max), the minimum area/molecule at the aqueous solution/air interface (A_min), the effectiveness of surface tension reduction (Π_CMC), the alkane carbon number (n_min) and the IFT at n_min (Y_min) were determined. The thermodynamic parameters of micellization (ΔG_mic, ΔH_mic and ΔS_mic) and of adsorption (ΔG_ad, ΔH_ad and ΔS_ad) for these polymeric surfactants were also calculated. Structural effects on micellization and adsorption are discussed in terms of these parameters. The results show that the ΔG_ad values were more negative than ΔG_mic values for these compounds, so that they favored adsorption before the micellization process. They exhibited IFT in the order of 10⁻³ to 10⁻⁴ mN/m against the thin alkane carbon number range 6–9. This range seemed to be prefered for enhanced oil recovery. Copyright © 2000 John Wiley & Sons, Ltd.