Showing papers in "Macromolecules in 1990"

A new multiplet-cluster model for the morphology of random ionomers

Abstract: The model is based on the existence of multiplets, which reduce the mobility of the polymer chains in their vicinity. Isolated multiplets act as large cross-links, thus increasing the glass transition temperature of the material. As the ion content is increased, the regions of restricted mobility surrounding each multiplet overlap to form larger contiguous regions of restricted mobility. The model is in good agreement with literature experimental results, especially SAXS and dynamic mechanical data

Crystal structure, conformation and morphology of solution-spun poly(L-lactide) fibers

Abstract: Depending on the spinning and drawing conditions, two crystal structures for solution-spun poly(L-lactide) are obtained. The pseudoorthorhombic α structure (a = 10.6, b = 6.1, and c = 28.8 A) contains two chains in the unit cell and is found at relatively low drawing temperatures and/or low hot-draw ratios. At higher drawing temperatures and/or higher hot-draw ratios a second so-called β structure appears. For this structure an orthorhombic unit cell is proposed (a = 10.31, b = 18.21, and c = 9.00 A) containing six chains. The chain conformations of the α and β structure are left-handed 10/3 and 3/1 helices, respectively. Calculations show that both conformations have approximately the same energy. Therefore, the preference for one of the two structures is determined by packing considerations. In fibers containing a mixture of α and β structure, the latter seems to bear most of the load during stress-strain experiments. Meridional small-angle X-ray scattering experiments yield a maximum for fibers containing only α structure pointing to a lamellar folded-chain morphology. The β structure on the other hand seems to correspond to a fibrillar morphology. Differential scanning calorimetry on unconstrained fibers shows that the β structure melts at a lower temperature than the α structure. The large shift of the peak melting temperature to higher temperatures in melting experiments on constrained fibers indicates that both lamellae and fibrils contribute to the strength of the fibers. This points to a considerable amount of interconnections/ entanglements between adjacent lamellae.

Complex formation between poly(ethylene glycol) and α-cyclodextrin

Abstract: La cyclodextrine α forme avec des PEG de masses moleculaires variables des complexes cristallins stables jusqu'a 300°C dans lesquels 2 motifs ethylene oxyde sont lies a une cavite de la cyclodextrine α

Thermal analysis of the volume phase transition with N-isopropylacrylamide gels

Abstract: Etude par DSC de la transition de type thermoretrait de gels aqueux de N-isopropylamide (NIPA) de solutions de poly(NIPA) et de PVME. Comparaison des temperatures de transition des gels et des solutions obtenues par DSC avec les valeurs obtenues par mesure du gonflement des gels et par mesure des points de trouble des solutions. Interpretation des resultats a l'aide du concept d'interaction hydrophobe. Influence d'additifs de faible masse sur les temperatures de transition

Room temperature polyesterification

Abstract: A new room temperature polymerization method has been developed for the synthesis of high molecular weight polyesters directly from carboxylic acids and phenols. The solution polymerization reaction proceeds under mild conditions, near neutral pH, and also avoids the use of preactivated acid derivatives for esterification. The reaction is useful in the preparation of isoregic ordered chains with translational polar symmetry and also in the polymerization of functionalized or chiral monomers. The conditions required for polymerization in the carbodiimide-based reaction included catalysis by the 1:1 molecular complex formed by 4-(dimethylamino)pyridine and p-toluenesulfonic acid. These conditions were established through studies on a model system involving esterification of p-toluic acid and p-cresol. Self-condensation of several hydroxy acid monomers by this reaction has produced routinely good yields of polyesters with molecular weights greater than 15,000. It is believed that the high extents of reaction required for significant degrees of polymerization result from suppression of the side reaction leading to N-acylurea. The utility of this reaction in the formation of polar chains from sensitive monomers is demonstrated hereby the polycondensation of a chiral hydroxy acid.

Polymorphism in melt crystallized syndiotactic polystyrene samples

Abstract: La cristallisation donne les 2 formes cristallines a conformation plane en zig zag. Influence de la vitesse de refroidissement, de la forme cristalline du materiau de depart, de la temperature maximale de l'etat fondu et de son temps de residence a cette temperature sur le polymorphisme

Thermal degradation of microbial copolyesters: poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate)

Abstract: Tous les copolymeres etudies sont instables a l'etat fondu au dessus de 170°C, quelle que soit leur composition; leur masse moleculaire diminue rapidement avec une cinetique qui suit le modele de rupture de chaine aleatoire aux groupes esters. Les resultats suggerent que les copolyesters dont le point de fusion est inferieur a 160°C peuvent etre utilises en moulage par injection

Temperature dependence of the interaction parameter of polystyrene and poly(methyl methacrylate)

Abstract: Mesure du parametre par SANS d'un copolymere bisequence styrene/MMA. La contribution entropique du parametre est beaucoup plus importante que sa contribution enthalpique; il varie avec la temperature absolue T selon la loi χ=(0,028∓0,002)+(3,9∓0,06)/T

Weak polyelectrolytes between two surfaces: adsorption and stabilization

Abstract: Une theorie derivee de la theorie a champ autocoherent de Scheutyens et Fleer est developpee pour etudier l'adsorption d'un polyelectrolyte flexible. L'influence du degre d'ionisation du polymere du pH et de la force ionique de solution est etudiee. L'interaction entre les 2 surfaces est aussi etudiee

Fluorescence studies of aqueous solutions of poly(N-isopropylacrylamide) below and above their LCST

Abstract: Presence dans le spectre du PNIPAM marque par le pyrene a T ambiante d'une emission forte d'excimere attribuee a des agregats de pyrene a l'etat fondamental. A concentration inferieure a 1 ppm, formation de micelles de polymere isole; a concentration plus elevee, l'agregation intra- et interpolymere a lieu. Le chauffage des solutions au-dessus de leur LCST entraine la destruction des agregats de pyrene. Interpretation des resultats par les interactions entre chaines et entre le polymere et l'eau

Configurational characteristics and scaling behavior of starburst molecules: a computational study

Abstract: A kinetic growth model of starburst molecules having trifunctional branch points with flexible spacers of various stens between pairs of branch points is presented. The self avoiding walk algorithm grows all branches of the same generation simutaneously. For each spacer length, the intramolecular density profiles, radius of gyration of the molecule and the center to end distance of the branches are calculated as each generation is grown

Methanol-water as a co-nonsolvent system for poly(N-isopropylacrylamide)

Abstract: Determination du diagramme de phase du polymere dans le solvant mixte en fonction de la proportion en methanol. Jusqu'a 35% en mode de methanol, l'addition de methanol entraine une diminution de la temperature critique LCST du systeme. Entre 35 et 45% de methanol, la temperature critique LCST augmente brusquement jusqu'a 14,5°C. Au-dela, il n'y a pas de LCST detectable, ce qui suggere l'existence d'un point critique

Photoinduced phase transition of gels

Abstract: Equilibrium swelling curves of copolymer gels of N-isopropylacrylamide and a photosensitive leucocyanide, were determined as a function of temperature under UV irradiation. Without UV irradiation the gels underwent a sharp continuous volume change, whereas upon UV irradiation they showed a discontinuous volume phase transition. For appropriate temperatures, the gels discontinuously swelled in response to irradiation of light and shrank when the light was removed

Rheology of Ordered and Disordered Symmetric Poly(Ethylenepropylene)-Poly(ethylethylene) Diblock Copolymers

Abstract: The linear dynamic mechanical properties of nearly symmetric 55:45 PEP-PEE diblock copolymers and the corresponding homopolymers are described. The results show the influence of composition fluctuations on the rheological properties around the microphase separation transition and confirms the first-order nature of this transition for hearly symmetric samples dynamic shear measurements obtained after a temperature quench from just above to just below the microphase separation temperature demonstrate the ability to supercool the disordered material and provide a measure of the subsequent ordering kinetics

Ordered structure in mixtures of a block copolymer and homopolymers. 2. Effects of molecular weights of homopolymers

Abstract: Etude par SAXS de la structure en microdomaines ordonnes observee pour des melanges de copolymere styrene-isoprene avec des polystyrenes (HS) de masse moleculaire telle que la majorite de ce polymere se solubilise dans les microdomaines de polystyrene du copolymere. Les resultats montrent que l'augmentation de la masse moleculaire du polystyrene HS entraine une diminution de sa miscibilite dans ces microdomaines. Interpretation par une perte d'entropie conformationnelle

Two-parameter scaling for polymers in Θ solvents

Abstract: We revive an old conjecture that a fixed number of binary contacts between chains collec- tively make up the topological constraint commonly referred to as an entanglement. This leads us to a general scaling theory for semidilute polymer solutions which involves two length scales, the screening length ( and the tube diameter a. In good solvents these two lengths have the same concentration depen- dence and we recover the de Gennes results. In 9 solvents the two length scales depend on concentration differently. Combining the concentration dependences of these two length scales with concepts from the- ories of rubber elasticity and reptation leads to new predictions for the plateau modulus G - kT/(a2{) - c7/3 and viscosity 9 - M2/3(c/c*)14/3 in 9 solvents, where M is the polymer molecular weight and c* is the overlap concentration. These predictions are found to compare favorably with available experimental data.

Arrested Tumbling in Shearing Flows of Liquid Crystal Polymers

Abstract: The doi equation for the three-dimensional time-dependent orientation-distribution function for rodlike polymers in the nematic liquid crystalline state in the presence of shearing flow is solved by an expansion in spherical harmonic functions. The results confirm calculations by Marrucci and Maffettone, who showed that a regime of steady-state negative first normal stress difference N 1 exists at shear rates high enough to frustrate the tumbling of the average molecular orientation that occurs at lower shear rates

Interactions between surfaces bearing end-adsorbed chains in a good solvent

Abstract: We measure the force-distance profiles between two curved mica sheets immersed in toluene and in xylene, both in the pure solvents and following addition of polystyrene (PS), end-function-alized PS, PS-X(M), and diblock copolymers PS-PEO with a short PEO block. Our results show that PS does not adsorb from the (good solvents) toluene and xylene but that once PS-X or PS-PEO are added to the solution the surface is rapidly covered. The force profiles following surface coverage are monotonically repulsive

Phase Transition of N-Substituted Acrylamide Gels

Abstract: Les polymeres du N-propyl et du N-isopropyl acrylamide en solution aqueuse presentent un comportement de type LCST et les hydrogels correspondants presentent une transition de phase, contrairement au poly(N-cyclopropyl acrylamide)

Characterization of branching architecture through "universal" ratios of polymer solution properties

Abstract: Experimental and Monte Carlo data for the dilute-solution properties of «lightly branched» polymers (stars, combs, rings, ...) are compared with the renormalization group predictions of Douglas and Freed. The comparisons focus on «universal] dimensionless ratios of the mean dimensions of lightly branched polymers, relative to those of linear polymers having the same molecular weight. Dimensionless ratios involving the polymer second virial coefficient, A 2 , are also tabulated and compared with theory

Molecular weight and polydispersity effects at polymer-polymer interfaces

Abstract: We study the interfacial properties of immiscible polymers, going beyond the usual approximation of infinite molecular weight, complete immiscibility. For finite molecular weights, the interfaces are broader and the interfacial tensions smaller than predicted by this approximation. These corrections are entropic effects and are significant for typical experimental stituations: they are in fact inversely proportional to the incompatibility χ AB N, where χ AB is the Flory interaction parameter between unlike (A and B) monomers and N the number of monomers per chain. In polydisperse systems, small chains accumulate slightly at the interface, lowering the interfacial tension

γ-Isotactic polypropylene. A structure with nonparallel chain axes

Abstract: L'analyse par la methode de Rietveld des donnees de diffraction RX obtenues a −120°C du PP isotactique γ aboutit a la proposition d'une nouvelle structure cristalline. Quelques implications de cette structure sur la morphologie cristalline du PP sont presentees

Chemical and electrochemical copolymerization of aniline with alkyl ring-substituted anilines

Abstract: Preparation de copolymeres aniline/o-toluidine et aniline/m-toluidine et etude de leur conductivite electrique en fonction de leur composition determinee par RMN 1 H. Les toluidines sont plus reactives que l'aniline dans la copolymerisation. Les resultats sont en accord avec le mecanisme propose pour la polymerisation de l'aniline et ses derives

Synthesis and characterization of two regiochemically defined poly(dialkylbithiophenes): a comparative study

Abstract: La polymerisation electrolytique des dimethyl- et dihexyl-3,3' bithiophene-2,2' donne respectivement les versions tete-a-tete du poly(methyl-3 et hexyl-3 thiophene). Les proprietes spectroscopiques des polymeres stereoreguliers sont comparees avec celles des polymeres obtenus a partir du methyl- et de l'hexyl-3 thiophene. La spectrometrie RPE montre que les polymeres stereoreguliers comptent tres peu de defauts magnetiques

Reversible gelation of polyoxazoline by means of diels-alder reaction

Abstract: Un hydrogel a base de polyoxazoline est prepare par reaction de Diels-Alder entre un poly(N-acetylethylenimine) (PAEI) modifie par le furane et un PAEI modifie par le maleimide. Le comportement du gonflement de l'hydrogel est etudie en fonction de la teneur en groupements fonctionnels dans le prepolymere. L'interconversion reversible entre l'hydrogel et le polymere lineaire soluble est realisee par un changement de temperature

Statistical thermodynamics of block copolymer adsorption. 1. Formulation of the model and results for the adsorbed layer structure.

Abstract: Une generalisation de la theorie a champ autocoherent de Scheutjens et Fleer est presentee pour decrire l'adsorption de copolymeres sequences dans un melange multicomposants. Des equations donnant les probabilites de conformation, les profils de distribution de segments et l'energie libre sont deduites

Oxyanions catalyze group-transfer polymerization to give living polymers

Abstract: Les oxyanions (carboxylate, phenolate, sulfinate...) dont le pKa dans le DMSO est compris entre 4,5 et 23, presentent une activite catalytique dans la polymerisation du methacrylate de methyle. L'influence des conditions de reaction sur la polydispersite et le caractere vivant des polymeres est etudiee. Les complexes des oxyanions avec leur acide conjugue sont egalement actifs