Showing papers in "Macromolecules in 1991"

Poly(styrene-ethylene oxide) block copolymer micelle formation in water: a fluorescence probe study

Abstract: L'etude en fonction de la concentration du copolymere de la variation des proprietes spectroscopiques du pyrene provoquee par son partage entre les phases micellaires et aqueuses permet de determiner les concentrations critiques micellaires et les coefficients de partage

Colloid stabilization with grafted polyelectrolytes

Abstract: The structure of a surface with a fairly dense array of polyelectrolytes end-grafted to it is considered. A simple theory for the scaling properties of the corona thickness, counterion distribution, and disjoining pressure between opposing layers is presented which makes contact with previous numerical studies. It is shown that by distributing the charges on a macromolecules the interlayer force is considerably less sensitive to Debye screening by added electrolytes. The flat layer studies are extended to the case of small (spherical) colloidal particles

Electron diffraction study on the two crystalline phases occurring in native cellulose from an algal cell wall

Abstract: Cellulose from the cell wall of the green alga Microdictyon tenuius exhibits two distinct crystalline phases. The major phase has a one-chain, triclinic (P1) structure with unit cell parameters of a=0.674 nm, b=0.593 nm, c (chain axis)=1.036 nm, α=117°, β=113°, and γ=81°. The crystal unit cell of the minor component has two chains and is monoclinic (P2 1 ), with a=0.801 nm, b=0.817 nm, c (chain axis)=1.036 nm, and γ the monoclinic angle=97.3°. The triclinic phase is metastable, and annealing it in dilute alkali at 260°C converts it into the monoclinic form. The triclinic and the monoclinic structures correspond to the Iα and Iβ spectra, respectively

Dynamics of reversible networks

Abstract: We present a model for dynamics of entangled networks made up of linear chains with many temporary cross-links. At times shorter than the lifetime of a cross-link such networks behave as elastic rubbers (gels). On longer time scales the successive breaking of only a few cross-links allows the chain to diffuse along its confining tube. The motion of a chain in this hindered reptation model is controlled by the concentration and lifetime of tie points. We calculate the self-diffusion coefficient and discuss the stress relaxation in terms of molecular parameters, including the chain length, the number of cross-linking groups per chain, and the lifetime and probability of formation of cross-links. We find good agreement with recent experiments by Stadler et al. on model thermoplastic elastomers.

Combined infrared and electron diffraction study of the polymorphism of native celluloses

Abstract: Cellulose samples of various origins were subjected to heat annealing. The initial samples contained various proportions of Iα (triclinic) and Iβ (monoclinic) phases. They could be classified into two families: a family I where the amount of the phase Iα was either small or below detection and a family II where this phase was the major component. The annealing treatment revealed that the Iα phase was metastable as it could be converted totally to the stable Iβ phase

Cononsolvency in mixed aqueous solutions of poly(N-isopropylacrylamide)

Abstract: The lower critical solution temperature (LCST) of poly(N-isopropylacrylamide)(PNIPAAM), prepared by an aqueous redox polymerization, was studied in the cononsolvent mixture of water and methanol by means of cloud-point and microcalorimetric measurements. The observed solution behavior was quite distinct from that of poly(vinylmethyl ether) in the same binary solvent. Other cononsolvents, including dioxane and tetrahydrofuran, yielded phase diagrams similar to those of PNIPAAM in water-methanol mixtures. Flory-Huggins ternary solution theory can explain the observed results only in terms of a change in the solvent-solvent interaction parameter (χ _(12)) in PNIPAAM solutions. We believe this to be physically implausible in the dilute (0.04%) solutions used in this work. Furthermore, we find in comparing our work with that of Hirotsu (Hirotsu, S. J . Chem. Phys. 1988,88, 427) that the behavior of PNIPAAM in water-methanol mixtures is strikingly insensitive to a 200-fold variation in polymer concentration. We suggest that perturbation of is not the origin of cononsolvency or of the gel collapse transitions reported by Hirotsu and by Tanaka and co-workers (Amiya,T.; Hirokawa, Y.; Hirose, Y.; Li, Y.; Tanaka, T. J . Chem. Phys. 1987, 86,2375)

Synthesis, properties, and biodegradation of poly(1,3-trimethylene carbonate)

Abstract: Poly(1,3-trimethylene carbonate) (PTMC) synthesized by diethylzinc-catalyzed ring-opening polymerization was characterized by its thermal and mechanical properties, permeability, and susceptibility to hydrolytic chain scission in vitro and in vivo. Mark-Houwink coefficients were derived from viscometry and GPC measurements. The rate of hydrolytic chain scission of solid PTMC in phosphate-buffered saline at 37°C, pH 7.4, was approximately 20 times less than that of poly(e-caprolactone); this rate difference was eliminated when the two polymers were compared in solution in aqueous THF. PTMC implanted subcutaneously in rats was degraded by enzymatic degradation process

Macromolecular engineering of polylactones and polylactides. 4. Mechanism and kinetics of lactide homopolymerization by aluminum isopropoxide

Abstract: The ring-opening polymerization of lactides initiated by aluminium isopropoxide in toluene at 70°C proceeds through a «coordination-insertion» mechanism and selective rupture of the acyl oxygen bond of the monomer. A kinetic study has shown that the polymerization is first order in both the monomer and initiatior. The polymerization is typically «living» until a molecular weight of ca. 90000 is reached

Structures of polypeptides from α-amino acids disubstituted at the α-carbon

Abstract: The structural preferences of homopeptides from Aib(α-aminoisobutyric acid), Deg(Cα , α-diethylglycine), Dpg(Cα , α-dipropylglycine) and Ac n c(1-aminocycloalkane-1-carboxylic acid; n=3,5,6) residues, as determined by conformational energy computations, X-ray diffraction analyses, and spectroscopic studies, are reviewed. The results obtained indicate that the 3 10 -helix and the fully extended (C 5 ) conformation are preferentially adopted by long sequences of these Cα , α-disubstituted α-amino acid residues, depending upon bulkiness and nature (whether linear or cyclic) of their side chains

Ordered structure in mixtures of a block copolymer and homopolymers. 1. Solubilization of low molecular weight homopolymers

Abstract: Etude par SAXS de la structure en microdomaines dans des melanges binaires de copolymere bisequence styrene/isoprene avec un polystyrene et dans des melanges ternaires du copolymere avec le polystyrene et le polyisoprene en fonction de la fraction en volume d'isoprene dans les melanges, la masse moleculaire des polymeres et la composition du copolymere etant fixees

Fluorescence studies of hydrophobically modified poly(N-isopropylacrylamides)

Abstract: Preparation et etude des proprietes en solution de copolymeres 100:1 et 200:1 du N-isopropyl acrylamide avec les N-decyl-, N-tetradecyl- et N-stearyl acrylamide et de copolymeres marques par un groupe pyrenyl. Les copolymeres du N-tetradecyl- et du N-stearylacrylamide forment des micelles au-dessous de la temperature critique LCST; au-dessus de la temperature LCST, les micelles sont detruites et les groupes hydrophobes sont distribues aleatoirement parmi les chaines agregees et effondrees de polymere

Coil-globule type transitions in polymers. 1. Collapse of layers of grafted polymer chains

Abstract: The mean-field analytical theory describing the conformational transition related to the collapse of the layers of polymer chains grafted onto the impermeable surfaces of different morphologies (a planar surface, a sphere, and a cylinder) has been developed. The dependence of the character of this transition on the type of the surface morphology is analyzed. Analytical expressions describing the distribution of monomer units density and chain ends along the layer height at arbitrary solvent strength are obtained

Composition dependence of the viscoelasticity of end-linked poly(dimethylsiloxane) at the gel point

Abstract: Precursor composition is shown to have a considerable impact on the viscoelastic properties of a polymer at the gel point. End-linked poly(dimethylsi1oxane) polymers were prepared with various stoi- chiometry, chain length, and concentration. The power law relaxation modulus, G(t) = St-", identifies the gel point, and a small frequency window (2 decades) is sufficient to determine the relaxation exponent, n, and gel strength, S. Both are shown to be strongly composition dependent. The relaxation exponent ranges between 0.19 and 0.92 in this study, which is almost the entire possible range (0 < n < 1). The gel strength ranges over 5 decades. A prepolymer below the entanglement molecular weight produces critical gels with relaxation exponents in the range 0.4-0.7, depending on stoichiometry. A high molecular weight prepoly- mer, above the entanglement molecular weight, produces critical gels with lower n values, in the range of 0.2-0.4, depending on stoichiometry. Addition of a diluent also affects the relaxation exponent, and near the overlap concentration n approaches the limit of 1. In a series of experiments with different stoichiom- etry and concentration, S and n appear to be related by a material characteristic function S = GOA$, where Go and XO are characteristic constants of the precursor material and n depends on composition.

Light-scattering study of the association behavior of styrene-ethylene oxide block copolymers in aqueous solution

Abstract: Des copolymeres bi- et trisequences forment en solution aqueuse des micelles spheriques de 2 types dont les dimensions respectives sont constantes dans le domaine de concentration 2•10 −5 −2•10 −3 g/ml. La fraction en masse des particules les plus grosses decroit quand la concentration augmente. Les petites particules (20 nm de rayon hydrodynamique) sont des micelles regulieres avec un nombre d'association de quelques centaines; les grosses particules (65 nm) sont des amas constitues de dizaines de micelles

Molecular dynamics simulation of a glassy polymer surface

Abstract: The free surface of glassy atactic polypropylene is studied by molecular dynamics computer simulation A model film system exposed to vacuum on both sides is examined at a temperature 22°C below the Tg of the bulk polymer The spatial dependence of the short-time dynamical properties of the model film is determined Conformational transitions are analyzed an average rate for conformational isomerization is calculated by constructing a hazard plot of first passage times

Chemospecific manipulations of a rigid polysaccharide: syntheses of novel chitosan derivatives with excellent solubility in common organic solvents by regioselective chemical modifications

Abstract: Efficient procedures for the preparations of soluble chitosan derivatives have been established on the basis of the regioselective chemical modifications. Selective and quantitative N-phthaloylation of chitosan proceeds smoothly by the reaction of chitosan with phthalic anhydride in N,N-dimethylformamide (DMF) at 130 "C. The resulting phthaloylchitosan exhibits much improved solubility in common organic solvents such as DMF, NJV-dimethylacetamide, dimethyl sulfoxide, and pyridine. The enhanced solubility of new types of chitosan derivatives under homogeneous reaction conditions. Facile conversions of phthaloylchitosan, a key starting material, into several 6-0-substituted derivatives are carried out by the reactions with bulky substituents such as triphenylmethyl (trityl) and @-tolylsulfony1)oxy (tosyloxy) groups. These reactions also proceed in homogeneous solution under mild conditions, and the degrees of substitution are estimated to be 1.0. Subsequent 3-0-acetylation of the secondary hydroxyl groups of 6-0-substituted materials gives rise to regioselectively modified chitosan derivatives showing much better solubility. Specific N-dephthaloylation or 0-detritylation of 6-0-trityl derivatives affords versatile intermediates, which permit the introduction of additional functional groups regioselectively.

Entry of free radicals into latex particles in emulsion polymerization

Abstract: A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.

Swelling of lamellar diblock copolymer by homopolymer: influences of homopolymer concentration and molecular weight

Abstract: The lamellar morphology in a lamellar diblock styrene-isoprene copolymer blended with polystyrenes with various molecular weight (2600-36700) was studied. As the PS concentration increased up to 24 wt %, the PS layer thickness increased, the PI layer thickness decreased and the area per junction increased. Increasing the PS molecular weight at a fixed concentration increased the PS and PI layer thicknesses and decreased the area per junction.

A molecular interpretation of the toughness of glassy polymers

Abstract: A model is proposed that describes the failure of glassy polymers in the crazing regime. This model is based on the realization that the cross-tie fibrils, which are known to exist between primary fibrils in all crazes, can have a profound effect on the failure mechanics of a craze as they can transfer stress between the broken and unbroken fibrils. A very simple model of crack tip stress amplification caused by the cross-ties is shown to work well in the prediction of the fracture toughness of a bulk, high molecular weight, glassy polymer

XPS study of self-doped conducting polyaniline and parent systems

Abstract: The structure and protonation level of sulfonated polyaniline as well as the chemically synthesized emeraldine base and its chloride salt have been investigated. The XPS results indicate that ∼ 50% of the phenyl rings of the sulfonated polyaniline have a hydrogen atom substituted by a -SO 3 H group. Approximately 50% of the nitrogen sites are protonated in this self-doped conducting polymer. The protonation level remains unchanged upon exposure to high vacuum at room temperature. In contrast, emeraldine hydrochloride undergoes partial deprotonation and probably reduction as well under high-vacuum conditions

Cloud-point curves of polymer solutions from thermooptical measurements

Abstract: Thermooptical analysis (TOA) provides a simple, rapid, and reliable experimental method to determine cloud-point curves of binary polymer/solvent systems. Phase diagrams have been obtained for different molecular weights of polystyrene in cyclohexane, methylacetate, ethylacetate, and tert-butylacetate and for poly(ethylene glycol) in water. Polystyrene solutions exhibit both upper and lower critical solution temperatures, while aqueous poly(ethylen glycol) solutions give closed-loop phase diagrams. The TOA apparatus described here uses very small samples (0.02 cm 3 )

Glass transition temperature as a function of conversion in thermosetting polymers

Abstract: A simple equation to model the dependence of the glass transition temperature (T g ) on the extent of reaction in highly cross-linked thermosetting polymers has been derived. The model assumes that the increase in T g is caused by (a) decrease in chain-end concentration, (b) formation of effective cross-links, and (c) further decrease in the configurational entropy due to departure from Gaussian behavior at high cross-link densities