Melting temperatures of poly(2,6-dimethyl-1,4-phenylene oxide) in methylene chloride solutions: Thermodynamic analysis

Melting temperatures were observed visually for poly-(2,6-dimethyl-1,4-phenylene oxide) in methylene chloride at nine concentrations (polymer weight fractions ranging from 0.0042 to 0.2362). The data were analyzed upon the assumptions that ΔH_u and ΔS_u, the molar heat and entropy of fusion per polymer unit, are constant over the temperature range studied, and that a Flory-Huggins chemical potential expression with a concentration-independent pair interaction parameter, χ₁ = (0.5 + ψ₁) + ψ₁Φ/T, satisfactorily describes the polymer unit activity in the binary solutions. Computation gave ΔH_u = 1404 cal/mole of units (therefore Δh = 11.7 cal/g), ψ₁ = ?0.569₁, and Φ = 342.₄°K. The effect of using various combinations of data points upon the values of these three parameters, as determined by least-squares linear regression treatment of the melting temperature expression, is indicated.     

Structure of the hard segments in trans,trans-diisocyanato dicyclohexylmethane based PU elastomers

A model for the structure of the hard segments and the hydrogen bonded network in the hard domains of segmented polyurethane elastomers with trans,trans-diisocyanato dicyclohexylmethane (tt-HMDI)/1,4-butanediol (BDO) based hard segments is proposed. The structure of the bis(methylurethane) oftt-HMDI (Me-ttHMDI-Me) has been determined by single crystal x-ray diffraction analysis and the conformation and packing of the polyurethane hard segments are constructed by connecting the successive Me-ttHMDI-Me units via –CH₂–CH₂– groups using the principle of isomorphic substitution. The conformation and hydrogen bonds of the monomer units are retained. The resulting polyurethane structure is highly crosslinked by a three-dimensional hydrogen bond network. The special packing principle may explain the high melting point as compared to the well-known structure of 4,4-diisocyanato diphenylmethane (MDI)-BDO hard segments and the differences in the material properties.     

Water vapor adsorption and microporous structure of carbon adsorbents. 17. Analysis of experimental isotherms for water vapor adsorption

Water vapor adsorption for various activated carbons with narrow and wide micropore volume distributions and mesopore surface areas between 40 and 300 m²/g have been investigated. For all the isotherms the point of inflection was determined, which can be taken as the point characterizing the formation of a water adsorption layer on the pore wall surface of carbon adsorbents. To do this the adsorption and desorption branches of the isotherms were approximated according to Weibull's distribution. A good correlation was obtained between values for the water monolayer capacity, calculated from the porous structure parameters of the carbons, and the adsorption values corresponding to the isotherm inflection pointsa _inf. For the group of carbons studied the values of relative pressure at the inflection point of the isotherms fell within the range 0.5–0.72.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 31–34, January, 1991.     

Peptide models XVI. The identification of selected HCO—L—SER—NH2 conformers via a systematic grid search using ab initio potential energy surfaces

Multidimensional conformational analysis (MDCA) predicted the existence of nine stable backbone conformations (α_L, α_D, β_L, γ_L, γ_D, δ_L, δ_D, ϵ_L, and ϵ_D) on the 2D-Ramachandran map, E = E(ϕ, ψ), for a single amino acid diamide (HCONH-CHR-CONH₂). The potential energy hypersurfaces (E = E[ϕ, ψ, χ₁, χ₂]) of For-L-Ser-NH₂ associated with the α_L-, bgr;_L-, γ_L-, δ_L-, and ϵ_L-type stable backbone orientations are investigated in this article. An appropriate number of side-chain rotamers is associated with each of the backbone conformers. In the case of serine, where R = −CH₂OH, the two sidechain torsional angles (χ₁, χ₂) should lead to 3 * 3 = 9 different sidechain orientations according to MDCA. For certain backbone structures, some of the sidechain conformations were nonexistent. © 1996 by John Wiley & Sons, Inc.     

The AMO method at small internuclear distances

The AMO function of the hydrogen molecule ψ = ψ_c + η ψ_i, where ψ_c is the covalent part and ψ_i the ionic part, is investigated for small internuclear distances R. We found η → ?1 as R →,?1 as R → 0, contrary to the intuitively expected limit η → 1. However, near R = 0 an analytical expression of ψ is derived, showing that ψ reduces to the helium ground state as R → 0. We have proved that the empirical concept ?covalent and ionic character”? should be replaced by the symmetry argument in the case of small R.     

Dimensional stability of regenerated cellulose film in relation to orientations of crystalline and noncrystalline phases

The dimensional stability of regenerated cellulose film on swelling with water is discussed in relation to the biaxial orientation of the two kinds of structural units, cellulose II crystallites and noncrystalline chain segments, and their anisotropic swelling (anisotropic absorption of water). Considerable dimensional stability in the plane of the film but enormous instability of thickness on swelling in water of some commercial cellophanes is qualitatively interpreted in terms of the planar orientation of crystal (101) planes along the film surface and the orientation of the noncrystalline chain segments parallel to the film surface. The dimensional changes on swelling from the completely dry state to 10% moisture regain were further interpreted quantitatively in terms of the degrees of biaxial orientation of the two kinds of structural units and their degrees of anisotropic swelling by modifying the Hermans monophase model for crystalline and noncrystalline biphase structures. The following degrees of anisotropic swelling of the structural units were thus obtained: q_{c, [101]} = 0.40%, q_{c, [101 ]} = ?0.33%, and q_a = 2.42%.     

Physisorption of alcohols and water vapour by MCM-41, a model mesoporous adsorbent

Adsorption isotherms of methanol, ethanol, propan-1-ol, butan-1-ol and water vapour have been determined on MCM-41, a model mesoporous adsorbent. The isotherms of the alcohols are all of Type IV, whereas the water isotherm is of Type V in the IUPAC classification. Each adsorption isotherm exhibits a sharp step, indicative of capillary condensation within a narrow distribution of mesopores. The isotherms are reversible in the monolayer-multilayer region, but distinctive hysteresis loops are associated with the condensation-evaporation cycle. The area within the loop is dependent on the adsorptive, increasing in scale from methanol to butan-1-ol and water. It is evident that the large internal surface of MCM-41 is somewhat hydrophobic and that its mesopore structure is remarkably uniform and stable.Nomenclature d _p Pore Diameter/nm - P/P ₀ Pressure/Saturation vapour pressure at the temperature of the isotherm run - T Temperature of isotherm run/K - n _m BET monolayer capacity/mmol g^–1 - n(0.1) Amount adsorbed at relative pressureP/P ₀=0.1/µ mol g^–1 - a _m Cross sectional area of an adsorbed molecule/nm² - V _p Effective pore volume/cm³ g^–1     

NMR Investigations of Temperature-Induced Phase Transition in Aqueous Polymer Solutions

The different dynamics of polymer segments forming phase-separated globular structures in aqueous (D₂O) solutions affects both the shape of NMR spectra and NMR relaxation times of polymer and solvent. Two types of the approach are discussed. The first one is based on the reduction of integrated intensities of polymer NMR lines in high-resolution NMR spectra in the system undergoing the coil-globule phase transition. The fraction p of phase-separated units (units with significantly reduced mobility) and subsequently, e.g., thermodynamic parameters ΔH and ΔS characterizing the coil-globule phase transition can be determined. The second approach is based on measurements of ¹H NMR relaxation times of water (HDO) which provide information on behaviour of water during phase transition. The power of both approaches is demonstrated on results obtained with solutions of several thermoresponsive homopolymers and copolymers.     

Basicity of some aliphatic amines in mixed H2O?CH3CN solvents

The values of pK_a^ms (K_a^ms represents ionization constant of conjugate acid of amine base in mixed water–acetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1–0.4 pK units) with the increase in CH₃CN content from 2 to ∼60% v/v. However, the pK_a^ms values at 70% v/v CH₃CN become nearly equal or slightly larger (by ≤0.7 pK units) than the corresponding pK_a^ms at 2% v/v CH₃CN for all neutral and charged amines. The values of pK_a^ms for phenol increase from 10.17 to 13.38 with the increase in the content of CH₃CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, ρ*, obtained from the plots of pK_a^ms against ∑σ* for primary and secondary amines decrease by ca. 0.8 ρ* units with the increase in the CH₃CN content from 2 to 70% v/v. The values of pK_a^ms show an empirical linear relationship with the corresponding values of pK_a^w (where pK_a^w represents the pK_a obtained in aqueous solvent containing 2% v/v CH₃CN), which allows the estimation of a pK_a in mixed H₂O CH₃CN solvents from that in water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 146–152, 2000     

Outliers detection in the statistical accuracy test of a p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> prediction

The regression diagnostics algorithm REGDIA in S-Plus is introduced to examine the accuracy of pK _a predicted with four programs: PALLAS, MARVIN, PERRIN and SYBYL. On basis of a statistical analysis of residuals, outlier diagnostics are proposed. Residual analysis of the ADSTAT program is based on examining goodness-of-fit via graphical diagnostics of 15 exploratory data analysis plots, such as bar plots, box-and-whisker plots, dot plots, midsum plots, symmetry plots, kurtosis plots, differential quantile plots, quantile-box plots, frequency polygons, histograms, quantile plots, quantile-quantile plots, rankit plots, scatter plots, and autocorrelation plots. Outliers in pK _a relate to molecules which are poorly characterized by the considered pK _a program. Of the seven most efficient diagnostic plots (the Williams graph, Graph of predicted residuals, Pregibon graph, Gray L–R graph, Index graph of Atkinson measure, Index graph of diagonal elements of the hat matrix and Rankit Q–Q graph of jackknife residuals) the Williams graph was selected to give the most reliable detection of outliers. The six statistical characteristics, F_exp,R²,R_P²,MEP,AIC{F_{\rm exp},R^{2},R_{\rm P}^{2},{\it MEP},{\it AIC}}, and s in pK _a units, successfully examine the specimen of 25 acids and bases of a Perrin’s data set classifying four pK _a prediction algorithms. The highest values F_exp,R²,R_P²{F_{\rm exp},R^{2},R_{\rm P}^{2}} and the lowest value of MEP and s and the most negative AIC have been found for PERRIN algorithm of pK _a prediction so this algorithm achieves the best predictive power and the most accurate results. The proposed accuracy test of the REGDIA program can also be extended to test other predicted values, as log P, log D, aqueous solubility or some physicochemical properties.     

On the photoreactivity of diacetylene containing thermoplastic block copolymers

A group of block copolymers containing diacetylenes as chain extenders in their hard segments was prepared, based on urethanes, esters, ureas, and amides as hard segments and polybutadienes, polyethers, polyesters, and polysiloxanes as soft segments. Almost all block copolymers were photoreactive, but there was a wide range of sensitivities. The photoreactivity of the copolymers was found to depend on the reactivity of the monomer unit, on the width of the diacetylene stacks in the hard segments, and on the degree of phase separation in the solid films. To explore the range of monomer reactivities we prepared 15 crystalline monomers. Urethanes were in general the most reactive, and this was attributed in part to the specific effect of hydrogen bonding which brings about a shortening of the C₁ to C₄ distance between diacetylenes tend to reduce the photoreactivity. The behavior of identical diacetylene units in the monomer crystal, in the homopolymer, and in the block copolymer is discussed in this paper.     

Synthesis and DSC study of model hard segments from diphenyl methane diisocyanate and butane diol

Model hard segments having different chain length and chain terminating groups, such as methyl, ethyl, octyl, and octadecyl, have been synthesized from diphenyl-methane diisocyanate (MDI) and butanediol and studied by DSC. In the case of short chain ends (i.e., methyl, ethyl, and octyl), hard segments having up to three MDI units show an increase of melting temperature T_m, while those with octadecyl chain end have T_m increase continuously with the number of MDI units. Enthalpy and entropy decrease with the augmentation of the hard-segment length. Side groups on the chain ends have a great influence on the melting properties.     

Segmented polyurethanes with 4,4′-bis-(6-hydroxyhexoxy)biphenyl as chain extender. Part 2. Synthesis and properties of MDI-polyurethanes in comparison with 2,4-TDI-polyurethanes

Linear segmented polyurethanes based on poly(butylene adipate)s (PBA) of different molecular weight (M_n 2000, 1000, and 600), 4,4′-diphenylmethane diisocyanate (MDI) and the mesogenic diol 4,4′-bis-(6-hydroxyhexoxy)biphenyl (BHHBP) as well as the unsegmented polyurethane consisting of MDI/BHHBP units have been synthesized and characterized by elemental analysis, ¹³C-NMR and SEC. The thermal behavior and the morphology were studied by DSC, polarizing microscopy, and DMA. The properties of the MDI-polyurethanes were discussed in relation to the BHHBP chain extended 2,4-TDI-polyurethanes and common 1,4-butanediol chain-extended MDI products. MDI polyurethanes based on PBA (M_n 2000) exhibit a glass transition temperature T_g of about −40°C independent of the hard segment content up to ∼50% hard segments. At higher hard segment contents increasing T_gs were observed. Polyurethanes, based on the shorter polyester soft segments PBA (M_n 1000 or 600), reveal an increase in the glass transition temperatures with growing hard segment content. The thermal transitions caused by melting of the MDI/BHHBP hard segment domains are found at 50 K higher temperatures in comparison with the analogous TDI products with mesogenic BHHBP/TDI hard segments. Shortening of the PBA chain length causes a shift of the thermal transitions to lower temperatures. Polarizing microscopy experiments indicate that liquid crystalline behavior is influenced by both the content of mesogenic hard segments and the chain length of the polyester. © 1996 John Wiley & Sons, Inc.     

Crystallite orientation in stretched polychloroprene networks. II

The orientation of crystallites grown isothermally in several drawn trans-polychloroprene networks is studied as a function of crystallization temperature t_x, degree of crystallinity ω, and elongation ratio α. The orientation distribution is particularly simple for this polymer since the crystallographic c axis (chain axis) orients preferentially along the stretching direction, while a and b are randomly arranged about c. Hence the parameter cos² χ_c adequately characterizes the distribution, where χ_c is the angle between the c axis and the fiber axis, and the average is taken over all crystallites. A treatment due to Krigbaum and Roe is utilized to obtain values of v (the number of statistical segments comprising the crystallization nucleus of critical size) through comparison of the average orientation of crystallites and amorphous statistical segments. The behavior observed falls into two categories. First, if the initial amorphous network is well oriented, 〈cos² χ_c〉 is independent of crystallinity during both crystallization and melting, and v varies with t_z (or the degree of supercooling) as predicted by nucleation theory. If different networks are to have the same crystallite orientation distribution, they must not only be crystallized at the same supercooling, but must also have the same distribution of amorphous segment orientations. Both the relative elongation and the network crosslink density affect the latter distribution. Next, we consider the second category. If the initial amorphous orientation is poor, 〈cos² χ_c〉 decreases linearly during crystallization and increases along approximately the same path during melting. Further, 〈cos² χ_c〉 for a given t_z yields v values which are too large. These two behaviors can be explained if, in the former case, nucleation involves the best oriented statistical segments of all network chains, while in the latter there is a selection according to the chain displacement vector orientation. Thus, if the amorphous orientation is poor, both the orientation and thermodynamic stability of the crystallites decreases with further crystallization. If this decreased stability is reflected in shorter fold lengths, the reversible variation of long period spacing with temperature reported earlier for an oriented polychloroprene network can also be explained as a preferential melting process.     

Segmented polyurethanes with 4,4′-bis-(6-hydroxyhexoxy) biphenyl as chain extender. I. Synthesis and characterization of 2,4-tdi-polyurethanes

Polyurethanes composed of 2,4-toluene diisocyanate (TDI), poly (butylene adipate) diols (PBA) of different molecular weights, and 4,4′-bis-(6-hydroxyhexoxy) biphenyl (BHHBP) were prepared by a two-step solution polymerization process. The polyurethanes were char-acterized by elemental analysis, NMR, and SEC. The thermal properties were investigated by DSC, DMA, and optical polarizing microscopy. Dependent on the molecular weight of the PBA, a shift in the glass transition temperature T_g of the polyurethanes has been observed by DSC and DMA. Polyurethanes based on poly (butylene adipate)s of M_n ～ 2000 exhibited a T_g nearly independent on the hard-segment content up to 50% LC hard segments, indicating the existence of mainly phase separated soft and hard segments. By shortening the PBA chain length up to 1,000 and further to 600, the T_g of the polyester soft-segment phase increases with growing hard-segment content, a consequence of enhanced interaction between the hard and soft segments. This tendency is observed to the greatest extent at polyurethanes with the shortest, polyester diol and can be interpreted as a partial miscibility or compatibility of hard and soft segments. Although in polyurethanes with PBA 2000 the mesophase can be proven at a hard-segment content of ～ 40%, its appearance in polyure-thances prepared with PBA 1000 or PBA 600 requires a hard-segment content > 60%. © 1995 John Wiley & Sons, Inc.     

Copolymerization effect of m-acetoxybenzoic acid on morphology of poly(oxy-1,4-benzenediylcarbonyl) whiskers

Copolymerizations of p-acetoxybenzoic acid (p-ABA) and m-acetoxybenzoic acid (m-ABA) were carried out in liquid paraffin at 330°C without stirring. Needlelike crystals are obtained when the content of m-ABA in the feed (χ_f) is in the range of 0–30 mol %. These needlelike crystals are inclined to show the fibrillation in both ends and the radial growth nature from the central part as χ_f increased. At higher χ_f than 40 mol %, crystals are not precipitated during polymerization. The content of m-oxybenzoyl units in the obtained whiskers (χ_w) are much lower than χ_f. The details of the polymerization and crystallization processes reveal that cooligomers containing more m-oxybenzoyl units cannot participate in the formation of needlelike crystals due to the higher solubility and the segregation effect of crystallization. The whiskers are predominantly formed by co-oligomers consisting almost entirely of p-oxybenzoyl units. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of cyclic poly(oxypropylene)

Poly(propylene glycol) [α-hydro-ω-hydroxypoly(oxypropylene)] of number-average molar mass M̄_n ≈ 2000 g · mol⁻¹ (PPG2000) was cyclised with high conversion (ca. 75%) by reaction with dichloromethane in the presence of powdered KOH. The cyclic product was separated from chain extended polymer by preparative GPC, giving an overall yield of polymer (M̄_n ≈ 2000 g · mol⁻¹, narrow molar mass distribution) in excess of 50%. Characterisation by analytical GPC and ¹³C NMR spectroscopy confirmed cyclisation. DEPT and ¹H-coupled NMR spectra were used to show that the links in cyclic poly(oxypropylene) were 77% single acetal, 12% double acetal and 11% triple acetal (or higher). This complexity probably results from competitive reaction with water introduced with KOH.     

Calorimetric and Dielectric Study of the Segmented Biodegradable Poly(ester‐urethane)s Based on Bacterial Poly[(R)‐3‐hydroxybutyrate]

Two series of segmented poly(ester‐urethane)s were synthesized from bacterial poly[(R)‐3‐hydroxybutyrate]‐diol (PHB‐diol), as hard segments, and either poly(ε‐caprolactone)‐diol (PCL‐diol) or poly(butylene adipate)‐diol (PBA‐diol), as soft segments, using 1,6‐hexamethylene diisocyanate as a chain extender. The hard‐segment content varied from 0 to 50 wt.‐%. These materials were characterized using ¹H NMR spectroscopy and GPC. The polymers obtained were investigated calorimetrically and dielectrically. DSC showed that the T_g of either the PCL or PBA soft segments are shifted to higher temperatures with increasing PHB hard‐segment content, revealing that either the PCL or PBA are mixed with small amounts of PHB in the amorphous domains. The results also showed that the crystallization of soft or hard segments was physically constrained by the microstructure of the other crystalline phase, which results in a decrease in the degree of crystallinity of either the soft or hard segments upon increase of the other component. The dielectric spectra of poly(ester‐urethane)s, based on PCL and PHB, showed two primary relaxation processes, designated as α_S and α_H, which correspond to glass–rubber transitions of PCL soft and PHB hard segments, respectively. Whereas in the case of other poly(ester‐urethane)s, derived from PBA and PHB, only one relaxation process was observed, which broadens and shifts to higher temperature with increasing PHB hard‐segment content. It was concluded from these results that our investigated materials exhibit micro‐phase separation of the hard and soft segments in the amorphous domains.     

Bemerkung zur Gleichheit der Aufspaltungen ΔI (zwischen den ersten beiden π-Ionisationspotentialen) und ΔE (zwischen den entsprechenden π* ← π Übergangsenergien) des Spiro[4,4]nonatetraens. Vorläufige Mitteilung

For a given molecule M, the difference ΔI between the first two vertical ionization potentials I_v,2 and I_v,2 (from MOs ψ₁ and ψ₂) and ΔE between the corresponding singlet-singlet excitation energies E₁ and E₂ (transitions ψ?₁ ←₁, ψ?₁ ψ₂) are related by ΔE = ΔI- (J_2,?1?J_1,?1) ?2(K_1,?1 ? K_2,?1), using Koopmans approximation. A simple MO model suggests that under certain conditions of symmetry and quasi-alternancy (e. g. in spiro[4,4]nonatetraene 1 ) the bracketed differences between the Coulomb- and exchange-integrals should vanish to first order, thus leading to the simple (almost) equality ΔE = ΔI. It is shown that the results from a photoelectron- and electron-spectroscopic investigation of 1 support this conclusion i.e. ΔI = 1.23 eV, ΔE = 1.19 to 1.23 eV.     

Adsorption of an Organic Compound in Two Different Positions: Simulation Taking into Account All Three Parameters of Intermolecular Interaction

Based on a system of two mixed Frumkin isotherms, the adsorption behavior of an organic substance whose molecules change their orientation in the surface layer from plane to vertical with an increase in the volume concentration is simulated. Equations that determine the boundary at which a single-solution system of isotherms turns into a system with several solutions when all three parameters of intermolecular interaction a ₁₁, a ₁₂, and a ₂₂ are nonzero are derived. The dependence of this boundary on the parameter a ₁₂ is shown to deviate substantially from that in the case of coadsorption of two different substances with equal coefficients of displacement.