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Showing papers in "Macromolecules in 1975"


Journal ArticleDOI
TL;DR: In this paper, an analytical expression appropriate to the crystalline-amorphous polymer pair is derived from Scott's equation for thermodynamic mixing of two polymers, indicating that the system is compatible in the molten state.
Abstract: Thermal analysis of solution cast mixtures of poly(viny1idene fluoride) (PVF2) and poly(methy1 methacrylate) (PMMA) has been carried out with various programmed temperature profiles. Results from experi- ments conducted in a quasi-equilibrium state show that PVF:! crystallizes in the mixture upon cooling. Further- more, depressions were observed in the melting and the crystallization temperatures. The melting point depression phenomenon is found to be explicable in terms of thermodynamic mixing of a crystalline polymer with an amor- phous polymer. To this end, an analytical expression appropriate to the crystalline-amorphous polymer pair is de- rived from Scott's equation for thermodynamic mixing of two polymers. From this expression the interaction pa- rameter for the polymer pair is found to be -0.295 at 16OoC, indicating that the system is compatible in the molten state. The depression of crystallization temperature which depends strongly on both the composition and the cool- ing rate is attributed to the ability of the PVF2 segments to migrate and to the change of composition in the melt during crystallization. For mixtures with a PVF2 content by weight of less than 0.5, it is possible to suppress the crystal transformation at room temperature with a moderate cooling rate. Finally, it is pointed out that because of the strong kinetic effects of cooling on the thermodynamic state of the mixture, caution should be exercised in the calorimetric study of compatibility of the polymer pair.

1,098 citations








Journal ArticleDOI
TL;DR: Inverse chromatographic procedure has been used to study polymer-polymer miscibility in the molten state as mentioned in this paper, and the results for poly( 6-capro1actone)-poly(viny1 chloride) blends indicate that complementary dissimilarity is the rule of polymer compatibility when specific interacting forces are involved.
Abstract: Reliable methods for identifying soluble polymer pairs have long been limited to the solid state even though the possibility exists that a pair of polymers which is immiscible in the solid is miscible in the melt. Inverse chromatographic procedure has been used to study polymer-polymer miscibility in the molten state. Results for poly( 6-capro1actone)-poly(viny1 chloride) blends indicate that complementary dissimilarity is the rule of polymer compatibility when specific interacting forces are involved. Such forces in PCL-PVC blends are of the same order of magnitude as those between PVC and its usual plasticizers. When these results are combined with earlier treatments of Flory equation of state, it turns out that variation of mixture parameters leads to bimodality of the phase diagram as well as asymmetry of the critical concentration. In the study of polymer compatibility, reliable methods for the identification of soluble polymer pairs are applicable in the solid ~ t a t e ; ~ , ~ ~ those applicable in the melt are generally limited by experimental difficulties. Microscopic methods are applicable only where there are substantial differences in refractive indices and light scattering techniques, including the recently developed pulse-induced critical ~ c a t t e r i n g , ~ are applicable for polymer-solvent systems only. Yet, it is conceivable that two polymers would be miscible in the melt but not in the solid on account of morphological differences, thermodynamic or kinetic changes accompanying crystallization, or vitrification or mere temperature effects on solubility. It is of interest to identify these systems; furthermore, it may be possible to induce miscibility down to the solid state via chemical or physical means. Gas-liquid chromatography (GLC) has received general recognition as an effective simple technique for rapid measurement of polymer-solvent interaction and solvent activity coefficient in molten homopolymers.4-1z I t has also been used in determining such properties10 as the glass transition phenomena and the glassy state, crystallinity, adsorption isotherms, heats of adsorption, surface area, interfacial phenomena, diffusion coefficients, and complex equilibria in solution as well as curing processes in nonvolatile thermoset systems. For these type applications, Guillet has suggested”J2 the name “inverse gas chromatography” based on the fact that conventional usage of GLC determines the property of an unknown sample in the moving phase with a known stationary phase whereas inverse chromatography determines the properties of an unknown stationary phase with the aid of a known vaporizable solute in the moving phase. He considers the latter as a “molecular probe’) experiment where the vaporizable molecules are designated “probe” molecules. In this paper, we describe an inverse chromatographic procedure whereby studies of two homopolymers and their blends, analyzed in terms of Scott’s ternary solution treatment13 of the Flory-Huggins theory,14 yield the polymer-polymer interaction parameter. Relative measures of various contributions such as polar, complexing and noncomplexing interactions are obtained based on the most significant strength of selected probe molecules. Four types of polymer interactions investigated are: (i) proton accepting strength, probed with chloroform and ethanol; (ii) proton donor strength with methyl ethyl ketone and pyridine; (iii) polar strength with acetonitrile and fluorobenzene; (iv) nonpolar strength with hexane and carbon tetrachloride. The division and choice of solutes are based on the relative magnitude of dipole moments, polarizabilities, and hydrogen bonding capabilities. It is recognized, though, that no such clear-cut division exists and tha t association complexes stabilized by electronic and/or electrostatic interactions are possible15 with all the probe molecules chosen. What is proposed is a scale of interaction by which the relative strength of different polymers could be measured. Plausible indices of interaction are the FloryHuggins interaction parameter,14 xil, its counterpart based on conditions of an hypothetical liquid at O’K, xLj*, or the exchange energy parameter of Flory equation of state,1621 XL, . The usual sign convention is assumed; i.e., a large positive value indicates unfavorable interaction, a low value indicates favorable interaction, while a negative value indicates a rather strong specific interaction. A nonpolar probe would generally yield positive quantities representative of noncomplexing contributions whereas specific interacting probes could yield negative values. The total specific interactions of such probes with a stationary solvent comprise54 at least four different kinds: 7~ electrons, dipolar, n electron, proton donor-acceptor interactions. These are, therefore, the basic interpretative concepts that will be applied in the discussion. The above procedure has been applied to poly(viny1 chloride) (PVC), poly(ecapro1actone) (PCL), and their blends. The results indicate that complementary dissimilarity is the rule of polymer miscibility when specific interacting forces are involved. Such forces in PCL-PVC blends are of the same order of magnitude as those between PVC and its usual plasticizers. When these results are combined with McMaster’s applicationzz of Flory equation of state16 z1 to polymer-polymer phase relations, asymmetry of the critical concentration as well as bimodal lower critical solution temperature (lcst) behavior are predicted. Experimental Section Apparatus. A Micro-Tek 2500R gas chrqmatograph equipped with thermal-conductivity detector was used for this study. Temperatures of the injection block, column outlet block, and detector cell were monitored by pyrometer but the column temperature was doubly checked with a Digitec Model 551-4 plug-in platinum resistance thermometer. The average error in column temperature is f0.5’. Flow rate of the helium carrier gas, controlled by a MicroTek regulator valve, is measured by a soap-bubble flowmeter. Column pressure was measured differentially against the atmospheric outlet pressure with a U-tube manometer filled with mercury. The elution profiles were recorded by a Honeywell Electronik 19 chart recorder. Column Preparation. All stationary phases were coated onto Fluoropak-80, 60-80 mesh, by dissolution in appropriate solvent and slow evaporation in a Breeze-Away Packing Dryer (Chemical Research Services, Inc.) which is essentially a fluidized bed. The coated support was packed by a gentle tapping procedure into a 5 ft X 0.25 in. 0.d. stainless steel silanized tubing, the ends of which were loosely plugged with glass wool. The tubing was then coiled to fit the oven chamber. After each experiment, the weight of polymer used was determined by 1-week extraction with refluxing solvent in a Soxhlet extractor equipped with ceramic thimble. Vol. 8, No. 3, May-June 1975 Polymer Compatibility by Gas-Liquid Chromatography 317 Table I Polymer Coating Analysis Table I1 Characteristic Parameters for Solvents and Polymers

201 citations







Journal ArticleDOI
TL;DR: The molecular structure of polyvinyl chloride (PVC) has been examined in this paper, showing that the crystalline regions are not solvated by the plasticizer, and this possible results in complex formation of the type C=O...Cl-C between the carbonyl of the plasticization and the chloride atom of PVC.
Abstract: The molecular structure of plasticized and unplasticized poly(vinyl chloride) (PVC) has been examined. The crystalline vibrational spectrum, obtained for normal commercial PVC, is in agreement with that of extended syndiotactic PVC, prepared from urea complex. The plasticized PVC spectra indicate that the crystalline regions are not solvated by the plasticizer. The plasticizer does solvate amorphous chain segments, and this possible results in complex formation of the type C=O...Cl-C between the carbonyl of the plasticizer and the chloride atom of PVC.




Journal ArticleDOI
TL;DR: Using an improved method for computing conformations of closed rings with symmetry, in conjunction with an improved empirical energy function, the conformational space of Gramicidin S is reexamined and one conformation is judged to be the global minimum-energy conformation.
Abstract: Using an improved method for computing conformations of closed rings with symmetry, in conjunction with an improved empirical energy function, the conformational space of Gramicidin S is reexamined. The search for minimum energy conformations is confined to the subspace containing closed symmetric rings. A large number of initial conformations selected from that subspace is subjected to energy minimization or is eliminated in a sequence of steps designed to locate the global minimum-energy conformation. One conformation having distinctly low energy is found and is judged to be the global minimum-energy conformation. This conformation is of the beta-pleated sheet type and is in complete agreement with experimental data. Similar structures with beta-pleated sheet-type conformations have been proposed previously on the basis of less extensive examiniations of the conformational space; the condition of exact ring closure, and the extensive examination of conformational space, used here, establish this structure on a firm basis.










Journal ArticleDOI
TL;DR: Computed values of chain dimensions based on the single virtual bond scheme are comparable to those calculated previously using a two virtual bond model which permits rotational flexibility in the sugar moieties of the chain.
Abstract: A simplified single virtual bond scheme has been developed for the calculation of mean-square unperturbed dimensions in polynucleotide chains. As a consequence of the structural rigidity of the sugar residues in the chain, it is possible to represent the six chemical bonds comprising the chain backbone repeating unit by a single virtual bond (connecting successive phosphorus atoms). The mutual orientation of a pair of adjoining virtual bonds is determined by the angles of rotation about the phosphodiester bonds adjoining intervening phosphorus atoms and is independent of the orientation of all other virtual bonds in the chain. Computed values of chain dimensions based on the single virtual bond scheme are comparable to those calculated previously using a two virtual bond model which permits rotational flexibility in the sugar moieties of the chain.

Journal ArticleDOI
TL;DR: It has been found that the fraction of glutamic acid residues which are helical in proteins is larger than might be expected from the experimentally determined value of the helical stability constants ofglutamic acid.
Abstract: It has been found that the fraction of glutamic acid residues which are helical in proteins is larger than might be expected from the experimentally determined value of the helical stability constants of glutamic acid. In order to understand this difference, the effect of neighboring charged side chains on the glutamic acid residues in proteins of known structure is examined. It is found that a positively charged side chain four residues away from a glutamic acid greatly enhances its probability to be helical. Similar results are obtained for aspartic acid, lysine, arginine, and histidine.