Heats of solution and dilution for polyethylene oxide in several solvents

The Tian-Calvet microcalorimeter, as modified by Maron and Filisko, has been used to measure the heats of solution and dilution of a sample of polyethylene oxide (PEO) of M₂ = ca. 6000 in chloroform, methylene chloride, and water. By applying the Maron theory to these measurements, it was found from the organic solvent data that the heat of fusion of the PEO sample is 57.1 cal/g polymer, independent of the solution concentration or the solvent used. However, the solution behavior in water was different because PEO in organic solvents exists as a random coil, whereas in water its conformation is helical. By combining the water and organic solvent data, it can be shown that conversion of the random coils to the helix involves the evolution of 39.3 cal of heat per gram of polymer. This heat evolution accounts, in all probability, for the solubility of PEO in water.     

Thermodynamics of poly(α-olefin) solutions

The Maron theory of polymer solutions was used to examine the 30°C vapor pressure data of Tait and Livesey on solutions of polyheptene-1, polydecene-1, polydodecene-1, and polyoctadecene-1 in toluene. For polyheptene-1 and polydecene-1, excellent agreement was obtained between theory and experiment over the entire concentration range studied. However, for the other two polymers, deviations were observed between the calculated and observed vapor pressures at higher solution concentrations. These deviations were used to show the presence of crystallinity in these and to ascertain the amounts. Further, it was shown that only one crystalline form of the polymer was present in the polydodecene-1, whereas in the polyoctadecene-1 two crystalline forms were detected. These observations, confirmed by measurements with a differential thermal analyzer (DTA), indicate that for polydodecene-1 and one of the crystalline forms in polyoctadecene-1 the crystallization involves alignment of both backbone and side chains; however, the lower melting form in polyoctadecene-1 appears to have only the side chains aligned in the crystal lattice.     

Thermodynamics of polystyrene solutions. I. Interaction parameters and polymer order

The Maron theory was used to deduce from mostly osmotic pressure and light-scattering data the interaction parameters as a function of concentration, temperature, and polymer molecular weight for solutions of polystyrene in seven different solvents. These interaction parameters were then utilized to calculate the activity coefficients of the solvent in more concentrated solutions for comparison with experimental values deduced from vapor pressure measurements. When order was absent in the polymers (all cases but two), the agreement between calculated and observed activity coefficients was found to be very good. In the two cases where order was present, the activity coefficients observed in the concentrated solutions were higher than those calculated. These deviations were employed to find the degree of polymer order both in solution and in the original polymers.     

Thermodynamics of the system benzene-carbon tetrachloride

The thermodynamic behavior of benzene-carbon tetrachloride solutions at temperatures between 30 and 70°C and over the entire concentration range has been examined in terms of the Maron theory of nonelectrolyte solutions. The interaction parameter calculated from vapor pressure data has been found to be independent of concentration and to be linearly dependent on temperature. Use of this parameter in the equations given by the theory allows complete correlation of the available heats, entropies, and heat capacities of mixing for this system. For all these properties very good agreement has been obtained between the calculated and observed quantities.     

Effect of polymer order on the vapor pressures of polypropylene solutions

The Maron theory of polymer solutions was used to reexamine the data of Brown et al. and Allen et al. [1,2] on the vapor pressures of polypropylene solutions in diisopropyl and diethyl ketones. The results obtained show that the interaction parameters for these systems depend on concentration and temperature, and that these parameters are able to reproduce the observed data so long as the polymers are amorphous. However, in the presence of polymer order, deviations from theory are observed which lead to 31 and 61% crystallinity for the two polypropylene samples used in these studies.     

The fusion of ethylene oxide polymers

Summary Calorimetric measurements by DSC technique have been made in pure polyethylene glycol (PEG) and oxide (PEO) polymers having a very wide molecular-weight range (from 600 to 4000 000) and in PEO (MW 600 000)-NaSCN complexes. It was found that the melting temperature increases with increasing molecular weight, ranging from 293 K in the polymer with MW=600 to 340 K in that with MW=4000 000. The behaviour of the heats of fusion with increasing molecular weight reflects the trend expected in systems, in which the increase of the main chain length produces a relevant growth of the degree of crystallinity. A distinct maximum of the heat of fusion has been found at about MW=10000, this result being an evidence of the high amount of crystalline regions building up the structure of that system. The dependence of the melting temperature on molecular weight has been nicely accounted for by using the expression of Flory, deduced from the statistical theory of polymers having the most probable molecular-weight distribution. The addition of sodium thiocyanate to PEO modifies the morphology of the host polymer and, for salt concentrations higher than 0.03 molar fraction, gives rise to the formation of a PEO-salt crystalline complex characterized by a high melting temperature. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Amorphous structure heat: Temperature dependence of heats of solution for polystyrene in toluene and ethylbenzene

When a polymer is dissolved in a solvent, the heat measured is a sum of a polymer-solvent interaction term and a term related to the structure that existed in the solid polymer relative to its amorphous liquid state. This latter contribution, termed the “residual” heat, can have an endothermic contribution due to the fusion of crystalline regions and an exothermic contribution due to the disruption of structure in noncrystalline amorphous regions. For atactic polystyrene between 30 and 110°C, it is shown that the “residual” heat is exothermic, decreases linearly with temperature differences below T_g, and extrapolates to zero in the vicinity of T_g. The existence of an exothermic heat above T_g is probably related to a 160°C transition in polystyrene. This “residual” heat was further observed to be independent of the pressure at which the polystyrene was glassified.     

Further investigations of cocondensation as a preparation method for amorphous states in transition metal alloys

Thin films of Pt–Ir, Ag–Ir, Ag–Ni and Ag–Y were cocondensed onto sapphire substrates at room temperature. Their atomic structures were investigated by X-ray diffraction. Although the first three systems have endothermic heats of fusion we did not find any sign of decomposition in the Ir containing cocondensed films. Instead, homogeneous solid solutions with fcc structure were formed. For the Ag–Ni system we found partial decomposition. But even here, the solubilities of one component in the other were strongly enhanced with regard to thermodynamical equilibrium. The X-ray diffraction patterns indicate transitions of the two crystalline fcc phases into amorphous states for Ni-contents of about 70 at %. Homogeneous amorphous solid states can be prepared in the Ag–Y system, which has an exothermic heat of fusion. At higher substrate temperatures amorphous and crystalline phases tend to be in metastable equilibrium. The experimental results we present are very suitable to discuss the importance of special material parameters for the formation of amorphous states in cocondensation experiments.     

Thermodynamic behavior of the system benzene-ethylene dichloride

The Maron theory of nonelectrolyte solutions has been used to analyze the thermodynamic behavior of the system benzene-ethylene dichloride. Excellent agreement with experiment has been obtained for all the thermodynamic properties involved. The analysis also shows that the interaction parameter for this system is small and positive, but it varies sharply with both concentration and temperature. As a result the system is nonideal and complex in behavior, despite the fact that vapor pressure data would tend to suggest near ideality.     

Interaction of “extremely diluted solutions” with aqueous solutions of hydrochloric acid and sodium hydroxide: A calorimetric study at 298 K

The “extremely diluted solutions” (EDS) have revealed a really intriguing behaviour, characterized by multiple independent variables. Because of their behaviour, EDS can be described as far-from-equilibrium systems, capable of auto-organizing as a consequence of little perturbations.

We measured the heats of mixing of basic and acid solutions with such EDS and their electrical conductivity, comparing with the analogous heats of mixing and electrical conductivity of the untreated solvent.

In particular, calorimetric titrations have been performed with NaOH or HCl solutions at various concentrations. Plots of the excess heat as a function of the concentration of titrant reveal differences and similarities between the two. By analysing these plots we were able to formulate new hypotheses about the supramolecular organization of the solvent water, when subject to mechanical perturbations stemming from the EDS preparation protocol, that is comprised of an iterative process of successive dilutions and succussions.     

On the Degree of Crystallinity from DSC in the Case of Multiple Melting of Synthetic Polymers

Considering the well‐known phenomenon of multimelting peak of crystalline polymers, it has been demonstrated that for calculating the degree of crystallinity using the heat of fusion, all the melting peaks observed have to be accounted for regardless of their origin‐recrystallization or presence of different crystalline modifications.     

Nonuniform extension of chain segments of a polymer coil in dilute solution

An isolated, flexible polymer chain in dilute solution assumes a random configuration. In reality such a polymer coil is not completely random, because an excluded volume effect requires a modification from randomness. In addition, the coil placed in a better solvent is more expanded than that in a poor solvent. In other words, the polymer chain segments in a better solvent are more extended, compared to those in a neutral solvent. In the present study we have discovered an indication that the chain segments lying on the periphery of the coil are more extended than those in the interior. This discovery has been made in the course of relating intrinsic viscosity to thermodynamic interaction between polymer and solvent. The resulting relationship provides a means of evaluating the thermodynamic interaction parameter from the measured value of intrinsic viscosity, if the molecular weight of the polymer, its intrinsic viscosity in a neutral solvent, and the degree of excess extension of chain segments are known. The last is the parameter discovered in this work. This observation is independent of particular thermodynamic theory, so long as the experimental data are used in a consistent manner. In this work Maron's theory was used, because it is applicable to infinite dilution as well as concentrated solutions.     

Influence of structure on the dynamic mechanical properties of crystalline polymers

A dynamk mechanical theory is presented whose structural elements correspond to such physically measurable material parameters as the compliance of the phases, the percent crystallinity, and the orientation of the crystalline and noncrystalline regions. The theory is tested by using both structural and dynamk mechanical data from samples of isotactic polyproylene films of varying crystallinity and orientation. For the specific case of isotactic polypropylene the general theory is shown to reduce to a simple two-parameter model. It is further shown that the two-parameter model not only identifies the unique properties of each of the phases separately, but also predicts the dynamic mechanical properties of the polymer over a temperature range of—130°C to 120°C, for samples ranging from unoriented to highly oriented, and varying in crystallinity from 40 to 70%. The model can also predict the dynamic mechanical properties over the same temperature range for strips of these samples cut at varying angles to the machine direction.     

FTIR原位研究PCL薄膜的结晶过程

利用带热台的傅里叶变换红外光谱仪 (FTIR) ,原位跟踪了聚ε 己内酯 (PCL)在硅基片及KBr单晶基片上的结晶和熔融过程。发现在结晶过程中 ,由于结晶后分子间的自由体积缩小 ,分子间相互作用增强而导致了CO峰位的漂移及C—O—C ,C—H结晶峰的出现。并通过结晶峰与非晶峰对比计算出了相对结晶度。对不同浓度的PCL/THF溶液成膜的FTIR测试表明 ,薄膜厚度越小 ,结晶度越低。这可能是由于基板及几何维数的限制导致。另外发现 ,相同条件下 ,PCL在KBr基板上成膜后的结晶度要大于在硅片上成膜的结晶度。     

Prediction of the solid–liquid–liquid equilibria of linear and branched semi-crystalline poly-ethylene in solutions of diphenyl ether by Lattice Cluster Theory

ABSTRACT

Recently, Lattice Cluster Theory has been applied to predict liquid–liquid equilibria and solid–liquid equilibria of low and high molecular weight mixtures taking into account the molecular architecture and the nature of crystallinity of the respective component. Herein, an LCT-based theory is applied to calculate solid–liquid–liquid equilibrium of a polyethylene + diphenyl ether system, depending on branching and degree of crystallinity of the polymeric component. Understanding the role that branching number, branching type and degree of polymer crystallinity play in the behaviour of triple and triple critical points is focused on. Insight is given here into constitution and properties of triple and triple critical points in binary polymer solvent systems depending on the molecular architecture of both components, polymer and solvent respectively, and the semi-crystalline nature of the polymer.     

Drying of solvent-borne coatings with pre-loaded drying gas

To investigate the effect of a methanol pre-loaded gas phase on the drying of toluene-poly(vinyl acetate) (PVAc) solutions, the solvent concentration profiles in thin coatings were measured by means of Inverse-Micro-Raman-Spectroscopy (IMRS). Without pre-loading, the toluene-PVAc solution shows a “diffusion controlled skinning" effect, which leads to a strongly extended drying time. In contact with a methanol pre-loaded gas phase, methanol diffuses into the coating accelerating the evaporation of toluene according to the level of pre-loading. After a certain toluene content is obtained, the residual methanol can be removed easily due to its higher vapor pressure and its better diffusion characteristics in PVAc compared to toluene (no skin formation). The presented measurements demonstrate the possibility to reduce the overall drying time of skin-forming coatings by pre-loading the gas phase with a suitable additional solvent, which is more volatile and has better diffusion characteristics in the polymer than the “trapped" solvent.     

Effect of Nafion dispersion solvent on the interfacial properties between the membrane and the electrode of a polymer electrolyte membrane-based fuel cell

A new Nafion binder solution was prepared using a different organic solvent, dimethylacetamide (DMAc), and applied to a polymer electrolyte membrane-based fuel cell. Wide angle X-ray diffraction (WAXD), electrochemical impedance spectroscopy (EIS), and polarization of the fuel cell were carried out to determine the crystallinity of the Nafion binder film, the cell resistance, and the fuel cell performance. This new Nafion binder film, which was created using a homemade Nafion solution containing DMAc, dissolved slower than a recast Nafion film that was made using a commercial Nafion solution in methanol (2 M). It was found that the slow dissolution of the homemade Nafion binder film was due to a more highly developed crystalline morphology, which can lead to good structural integrity in the catalyst layer for long-term operation of the fuel cell. The micellar structure of Nafion in the commercial Nafion binder solution is broken by new organic solvent, which leads to higher physical chain entanglement between the Nafion membrane and the Nafion binder during preparation of the membrane/electrode assembly (MEA), thereby improving the interfacial stability between the membrane and the electrode and providing long-term stability of the fuel cell.     

Improved Biodegradation and Thermal Properties of Poly(lactic acid)/Layered Silicate Nanocomposites

Poly(lactic acid) (PLA)/layered silicate nanocomposites have successfully been prepared using the solution route. Two types of organically modified nanoclays, namely, MEE and MAE have been used. The nanostructure, as observed from wide angle X-ray diffraction, indicates an intercalated hybrid for both PLA–MEE and PLA–MAE, and depends on the type of organic modifier used. Intercalation is higher in PLA–MEE as compared to PLA–MAE system. Crystallite dimensions of nanoclays and nanocomposites have been calculated from the Scherrer equation. Crystallite size of nanocomposites is higher than that of pure nanoclay, which in turn affects the properties of the nanocomposites. Wide angle X-ray diffraction patterns also suggest that PLA and its nanocomposites are predominantly amorphous before annealing but, after annealing, PLA and its nanocomposites are fairly crystalline. The crystallinity of the nanocomposites has decreased in comparison to neat polymer suggesting some sort of interaction between organically modified nanoclay and polymer. The nanohybrids show significant improvement in the thermal properties of the matrix as compared to pristine polymer. The nature of interaction between nanoparticles and polymer is higher in PLA–MEE against PLA–MAE, as evident from the lower value of the heat of fusion in the case of PLA–MEE. The nanoparticles act as nucleating agent, and thereby, control the spherulite dimension of the matrix. The comparison of biodegradation of PLA and its nanocomposites has been studied in several media. Biodegradability of PLA has significantly been enhanced in the presence of nanoclays which has been explained on the basis of amorphous content in the polymer matrix. Finally, the regulated biodegradation has been discussed.     

Thermal Behaviors of Confined Triphenylmethyl Chloride Crystals in Various Molecular Weight Polystyrene

The thermal behaviors of polystyrene (PS)/triphenylmethyl chloride (TPCM) blends with different polymer molecular weights were investigated through differential scanning colorimetry (DSC). It was shown that when solvent content was lower than a critical composition, there was only a single amorphous phase in the blends. With increasing polymer concentration, both T_g and T_m could be detected in DSC curves, revealing that the blends were heterogeneous. The constant T_g of the amorphous phase indicated that the composition of the amorphous phase in the blends did not depend on the solvent concentration, and the T_m depression with decreasing PS content showed the decrease of TPCM crystallite size owing to geometric constraint by the polymer chains. On the basis of the Flory–Huggins theory, the interaction parameters between PS and TPCM in the blends were obtained; they showed that the PS/TPCM blends were not thermodynamically miscible with low polymer content. The Hoffmann-Weeks equation indicated that the crystals corresponding to the lower melting point were unstable. The unstable crystals in the blends were located in the interfacial regions between the crystalline solvent molecules and the amorphous phase. The heat capacity of the blends confirmed the geometric constraint on the crystallization of TPCM in the blends.     

Sorption of Organic Solvents in Poly(Dimethyl Siloxane) Membrane. II. Effect of Sorption on the Polymer Crystals upon Cooling

Polymer crystalline properties in poly(dimethyl siloxane) (PDMS) film after solvation by various solvents was determined using low temperature differential scanning calorimetry (DSC). At various solvent uptake levels, the crystalline thermal properties of the solvated polymer were modified to different extents as revealed by the shifts in crystalline melting point (T_m) and its enthalpy (ΔH_m). Water uptake in PDMS was very limited (<0.01 g/g) and T_m did not significantly change during the sorption process. For toluene and cyclohexane penetrants, T_m moved toward a much lower temperature depending on the sorption levels. At low solvent uptakes, the T_m values decreased linearly with solvent uptakes due to formation of a miscible phase. Beyond a threshold, the T_m remained stable and an additional penetrant fusion peak appeared, implying the onset of a microphase separation phenomenon. The ΔH_m values for the swollen membranes were decreased, with the exception of the water penetrant. This indicates that a lower percentage of polymer chains were involved in the crystalline domain for swollen PDMS.