Showing papers in "Macromolecules in 1992"

X-ray structural studies of poly(3-alkylthiophenes): an example of an inverse comb

Abstract: X-ray diffraction studies of oriented poly(3-n-alkylthiophene) (P3AT's) solution-cast films show that these samples are characterized by well-organized lamellar structures whereby stacks of planar thiophene main chains are uniformly spaced by the allyl side chains. In spite of considerable hydrocarbon chemical bonding and conformational disorder, a high degree of structural regularity is observed with appreciable three-dimensional ordering of the polymer chains. Analysis of the results leads to suggestive models in which an open packing of the alkyl chains is maintained with considerable side-chain disorder and side-chain mobility

Viscoelastic properties of physically crosslinked networks. 1. Transient network theory

Abstract: A simple model is introduced to describe the dynamics of physically cross-linked networks in which junctions are sufficiently weak to break and recombine in thermal fluctuations. The time-evolution equation under arbitrary macrodeformation is derived for the creation and annihilation of the junctions-and hence for the number of elastically effective polymer chains with a fixed end-to-end vector. We focus our attention specifically on the unentangled networks in which the molecular weight M, between neighboring junctions is smaller than the entanglement molecular weight Me, so that each chain obeys Rouse dynamics modified by sticky trapping centers rather than reptation. Stress-strain relation under shear and elonga- tional deformation is detailed for a model network which is made up of polymers of uniform length with associating functional groups at their chain ends. On longer time scales than the junction breakage time, the total number of effective chains decreases with time-resulting in an intranetwork flow. In most polymer blends or solutions of practical interest, polymer chains carry specific groups interacting with each other by associative forces capable of forming bonds, and hence a description in terms of van der Waals type interaction-or x parameter in lattice-theoretical termi- nology-is insufficient. These forces include hydrogen bonding, ionic association, stereocomplex formation, cross- linking by the crystalline segments, and solvent complex- ation. In most common cases the bonding energy is comparable to the thermal energy, so that the bond formation is reversible by the change in temperature or concentration. Thermodynamics of network formation and multiphase competition were studied by the present authors1t2 to describe phase behavior peculiar to the polymer fluids interacting via specific associating forces. The present paper concerns dynamic properties of the thermoreversible networks (or physical gels) to study enhanced viscoelasticity due to the presence of the temporal junctions. There has been an accumulation of evidences that networks with reversible junctions exhibit highly enhanced viscoelastic properties which are quite different from ordinary polymer melts. Typical examples of the exper- imental studies can be taken from biological macro- molecule~~-~ in which networks are formed by hydrogen bonding. It turned out that a mechanical spectrum of dynamical moduli does not show any softening in low frequencies but is rather insensitive over a wide range of frequencies covering from to lo2 rad/s and also that relaxation of stress does not seem to obey a simple exponential decay but is rather well described by a power law.4 It was also found that the complex viscosity is in most cases larger than the stationary viscosity when compared at the frequency which is the same as the stationary shear rate, thus suggesting a failure of the Cox- Merz rule. Another example in current research is that of ther- moplastic elastmers6s7 in which synthetic polymers are cross-linked by specific associating groups. These include, in addition to the hydrogen bonding, charge (electron) transfer, ion pairing, acid-base interaction, dipole-

Self-assembly in aqueous block copolymer solutions

Abstract: The aggregation of EO 99 -PO 65 -EO 99 (EO and PO being ethylene oxide and propylene oxide, respectively) in water has been studied. By GPC and self-diffusion studies, it was found that micelles are formed in dilute solutions, but only at higher temperatures. The micelles formed have a hydrodynamic radius of approrimately 10 nm over a wide temperature range. The residence time of the polymer molecules in the micelles is extremely long (∼hours). At higher polymer concentrations, a clear isotropic gel is formed at intremediate temperatures

Unique behavior of dendritic macromolecules: intrinsic viscosity of polyether dendrimers

Abstract: Polyether dendrimers C(CH 3 )(PhOCH 2 Ph[([(OCH 2 Ph) 2 ] 2 ) 2 ] 2 ) 3 made by the convergent growth approach are characterized by SEC with coupled molecular weight sensitive detection. SEC chromatograms are as narrow as those of polystyrene with M w /M n ≃1.04 and do not broaden appreciably with increasing dendrimer generation, g. Values of M w obtained from light scattering detection are in agreement with the molecular weights of the structures anticipated from the synthetic strategy. Intrinsic viscosity passes through a characteristic maximum as a function of of g while the refractive index increments increase approximately linearly with dendrimer generation

Correlation of structure, mobility, and morphological information in heterogeneous polymer materials by two-dimensional wideline-separation NMR spectroscopy

Abstract: A novel version of the two-dimensional heteronuclear experiment, achieving 1 H wideline separation (WISE), is presented. It allows for the correlation of mobility and structure in organic solids. Differences of molecular dynamics are probed by 1 H wideline shapes, wich are separated in the second dimension by 13 C chemical shifts. With a mixing time inserted before cross polarization from 1 H to 13 C, 1 H spin diffusion allows one to determine the mobility at interfaces and to mesure domain sizes approximately. The technique is demonstrated on poly(siloxane-b-styrene) and on a polystyrene/poly(methyl vinyl ether) blend

Preparation of π-conjugated poly(thiophene-2,5-diyl), poly(p-phenylene), and related polymers using zerovalent nickel complexes. Linear structure and properties of the π-conjugated polymers

Abstract: Dehalogenation polycondensation of 2,5-dihalothiophene and 1,4-dihalobenzene affords polythiophene (PTh) and poly(p-phenylene), respectively, with linear structure and high crystallinity in high yields. Vacuum deposition of both polymers gives crystalline films with their main chain essentially perpendicular to the surface of the substrates. Doping of PTh with iodine and FeCl 3 gives semiconducting adducts, and the doping-undoping process of PTh with iodine and hydrazine is reversible from the viewpoint of not only chemical structure but also packing structure in the solid state.

Failure mechanisms of polymer interfaces reinforced with block copolymers

Abstract: The fracture toughness (characterized by the critical energy release rate G c ) of interfaces between polystyren (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-b-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization and the areal density of block copolymer chains at the interface on the measured G c and on the fracture mechanisms was investigated quantitatively.

Lewis acid catalyzed polymerization of L-lactide. Kinetics and mechanism of the bulk polymerization

Abstract: The kinetics of the L-lactide bulk polymerization was studied using tin(II) bis(2-ethylhexanoate) and zinc bis(2,2-dimethyl-3,5-heptanedionato-O,O'). Up to 80% conversion, the rate of polymerization using tin(II) bis(2-ethylhexanoate) is higher than that with the zinc-containing catalyst, while at conversions beyond 80%, the latter catalyst has the higher rate of polymerization. Crystallization of the newly formed polymer has an accelerating effect on the polymerization. The difference in the rate of polymerization at high conversions for the two catalysts is caused by a difference in crystallinity of the newly formed polymer. Contaminants in the catalyst and monomer are the true initiators in these L-lactide polymerizations. Initiation as well as polymerization proceeds through a Lewis acid catalyzed transesterification reaction between an activated lactone and a hydroxyl group.

Azo polymers for reversible optical storage. 2. Poly[4'-[[2-(acryloyloxy)ethyl]ethylamino]-2-chloro-4-nitroazobenzene]

Abstract: Writing, reading, erasing and rewriting processes are performed using laser beams on a poly[4'-[[2-(acryloyloxy)ethyl]ethylamino]-4-nitroazobenzene] (pDRlA) film. The writing and erasing are statistical processes and the level of optical anisotropy induced depends on the photon flux directed at the sample. The time evolution of the writing process is investigated. A computer simulation of these processes is performed taking into account a relaxation process in which some of the reoriented trans molecules are randomized by thermal motion.

Effects of protein charge heterogeneity in protein-polyelectrolyte complexation

Abstract: The interaction between globular proteins and synthetic polyelectrolytes was studied in 0.10 M NaCl solutions. pH titrations were used to induce the binding of three monomeric proteins of substantially different isoelectric points (BSA, chicken egg lysozyme and bovine pancreas ribonuclease) to polyelectrolytes with a range of charge densities. THe formation of soluble polyion-protein complexes is followed, upon progressive pH change, by phase separation (complex coacervation). Using known protein titration curves, the net charge on the protein at critical conditions could be calculated

Mutual influence of pH and temperature on the swelling of ionizable and thermosensitive hydrogels

Abstract: Hydrogels prepared from N-isopropylacrylamide, (diethylamino)ethyl methacrylate and butyl methacrylate were prepared and were found to possess unique swelling properties. Gel colllapse at high temperatures (at pH= 7.4) was shown to take place even if more than 20 mol% of the ionizable monomer was included due to the increased neutralization of the ionizable monomer with increasing temperature. The temperature range in which the temperature-induced swelling transition occurs is dictated by the amount of ionizable monomer. The pH-induced swelling transition was shown fto be controlled by temperature

Elastic modulus and equilibrium swelling of poly(dimethylsiloxane) networks

Abstract: Model poly(dimethylsiloxane) (PDMS) networks and imperfect networks confining pendant chains and branched structures are prepared by end-linking mixtures of di- and monofunctional PDMS and a tetrafunctional cross-linker. Values of the equilibrium modulus of the networks, obtained from dynamic mechanical experiments, illustrate the importance of the entanglement contribution to the elastic modulus. The extents of equilibrium swelling are determined for the networks in toluene and benzene. The predictions from the c * theorem do not apply to the model networks due to extensive interspersion of the network chains, which leads to entanglements that are not removed by swelling

Compression-molded polyurethane block copolymers. 1. Microdomain morphology and thermomechanical properties

Abstract: The microdomain structure and thermomechanical properties of a series of comprsssion-molded segmented polyurethane block copolgmers are investigated as a function of hard-segment content. The crystallizable hard segments form from the reaction of 4,4'-diphenglmethane diisocganate (MDI) with 1,4-butanediol (BDO). Soft segments are poly(oxypropylene) end-capped with poly(oxyethylene).

Synthesis of a model 3-miktoarm star terpolymer

Abstract: The synthesis of a well-defined, near monodisperse, star terpolymer with three different arms, i.e., polyisoprene, polystyrene, and polybutadiene, is described. The synthetic approach involves the reaction of polyisoprenyllithium with an excess of methyltrichlorosilane followed, after removal of the excess of methyltrichlorosilane, by a stoichiometric addition (titration) of polystyrillithium and finally by a small excess of polybutadienyllithium. The sequence of the addition of the different living polymers is very critical

Molecular Weight Dependence of Viscoelasticity of Polycaprolactone Critical Gels

Abstract: The molecular weight dependence of critical gel properties was determined for poly(c-capro- lactone) diol end-linked with a three-functional isocyanate. The critical gels exhibit the typical scaling behavior with a power law relaxation spectrum H(X) = Go/r(n)(t/Ap, X > b, where GO = G, was found to be the modulus of the fully cross-linked polymer and XO = vo/G, was found to depend on the viscosity of the difunctional prepolymer, 70. The relaxation exponent, n, decreases with increasing cross-linker concentration (increasing stoichiometric ratio, r = (NCOI/(OHl) and increasing molecular weight of the prepolymer. This suggests that the fractal dimension of the critical gel increases with increasing molecular weight and stoi- chiometric ratio.

Crystallization of block copolymers. 1. Small-angle x-ray scattering study of a .epsilon.-caprolactone-butadiene diblock copolymer

Abstract: It was found that crystallization of the poly(e-caprolactone) (PCL) block brings about a dramatic change in the shape of the SAXS pattern; a diffuse intensity maximum arising from the correlation hole effect of disordered block copolymers could be observed at the melt, while it was replaced by a strong intensity peak at a smaller angle for temperatures below the melting temperature of the PCL block (T m ). It was found that the copolymer which was quenched from the melt into a temperature below both T m and T S (microphase-separation temperature of the copolymer) immediately showed a sharp diffraction due to the microstructure of the copolymer

Development of a General Mathematical Framework for Modeling Diffusion-Controlled Free-Radical Polymerization Reactions

Abstract: A new theoretical framework is proposed for modeling diffusion-controlled free-radical po- lymerization reactions Termination and propagation rate constants as well as initiator efficiency are expressed in terms of a reaction-limited term and a diffusion-limited one The latter is shown to depend on the diffusion coefficient of the corresponding species (ie, polymer, monomer, primary radicals) and an effective reaction radius All parameters appearing in the diffusion-limited part of the kinetic rate constants have a clear physical meaning and can be evaluated in terms of the physical and transport properties of the reacting species It is shown that the proposed approach for modeling diffusion-controlled reactions does not require the introduction of critical break points to mark the onset of various diffusional effects (Le, gel effect, glass effect) The ability of the present model to elucidate the mechanism of diffusion-controlled reactions is demonstrated by analyzing the free-radical polymerizations of styrene and methyl methacrylate initiated by the thermal decomposition of AIBN, AIBME, AVN, and LPO chemical initiators It is shown that, at high conversions, initiator efficiency strongly depends on the size of initiator molecules The present model predictions are in excellent agreement with experimental data on monomer conversion, total radical concentration, and average molecular weights measured in different laboratories by ODriscoll and Huang42 and Zhu et all0

Structure/properties of conjugated conductive polymers. 1. Neutral poly(3-alkythiophene)s

Abstract: Soluble conjugated poly(3-alkythiophene)s (P3ATs) including poly(3-butylthiophene) (P3BT), poly(3-octylthiophene) (P3OT), and poly(3-dodecylthiophene) (P3DDT), were prepared chemically and characterized. It is found that each of the P3ATs consists of an ordered phase and a disordered phase. In the ordered phase, the side chains are nearly fully extended and are packed to give a two-layer structure and one-layer structure, the former being the majority. The coplanar subchains are stacked to form planes, and the space between two successsive stacking planes is filled with aligned side chains

Orientation of cellulose microcrystals by strong magnetic fields

Abstract: It is shown that cellulose microcrystals in aqueous solution can be strongly oriented by strong homogeneous magnetic fields so that their long axes became perpendicular to the field direction. In addition, fthe dried cellulose films displayed a uniplanar-axial orientation

Effect of structural symmetry on gas transport properties of polysulfones

Abstract: The effects of structural symmetry of phenylene linkages and methyl group placement on the properties of polysulfones have been investigated. Polysulfones with unsymmetric structures have lower gas permeability and higher selectivity coefficients than their symmetric counterparts. The polysulfones with m-phenylene linkages or dimethyl substitutions to the bisphenol unit have lower glass transition temperatures, lower fractional free volumes, and higher sub-T g relaxation temperatures. The chains of the unsymmetric polymers are generally more efficiently packed and appear to have greater mobility constraints

Application of molecular simulation to derive phase diagrams of binary mixtures

Abstract: The miscibility behavior of three binary mixtures, solvent with solvent, polymer with solvent, and polymer with polymer, was studied by use of a combination of the Flory-Huggins theory and molecular simulation techniques. Fundamental parameters in the Flory-Huggins theory, including the heat of mixing associated with pairwise interactions (Aw12) and the number of possible interaction partners, i.e., coordination number, z, are calculated from molecular simulations. The pair energies (~11, w22, ~12) are obtained by averaging a large number of confiations generated by a Monte Carlo approach which includes the constraints associated with excluded volume. The temperature dependence of the interaction parameter x is obtained with the formalism developed in this study. In all cases, the calculated upper critical solution temperatures compare favorably with experimental values. This approach provides an opportunity to test the Flory- Huggins theory for a number of model binary systems and to characterize their miscibility behavior. This combined approach also facilitates study of the thermodynamic behavior of a binary mixture without possessing specific knowledge or experimental data of the system under investigation.

Morphological alterations during texture-producing plastic plane strain compression of high-density polyethylene

Abstract: Numerous specimens of a single linear polyethylene sample were deformed by plane strain compression in a channel die to very large plastic strains. The mechanisms of deformation were elucidated by density measurements, polarized light microscopy, TEM and wide-angle and small-angle X-ray diffraction.

Poly(L-lactic acid)/Pluronic blends: characterization of phase separation behavior, degradation, and morphology and use as protein-releasing matrixes

Abstract: It is shown that poly(L-lactic acid) (PLA) can be physically blended with nonionic polymeric surfactants, Pluronics, to form a matrix where the surfactants may be entangled in the amorphous regions of PLA. Upon hydration, the blends acquire an additional liquid crystalline phase that is now embedded between the PLA crystalline phase, producing an overall intact surface morphology. This resulted in drug release profiles with a minimum initial protein burst and a longer time release.

Temperature-dependent micellization in aqueous block copolymer solutions

Abstract: The temperature-dependent micellization of Pluronic F127 (EO 99 -PO 65 -EO 99 , EO and PO being ethylene oxide and propylene oxide, respectively) was investigated experimentally and theoretically. While experiments involved the determination of the temperature dependence of the critical micellization concentration (cmc) and the fraction of polymer molecules in micellar form, the theoretical results were obtained through a self-consistent lattice theory for multicomponent mixtures of copolymers with internal states in heterogeneous systems

Force fields, structures, and properties of poly(vinylidene fluoride) crystals

Abstract: We report two new force fields for molecular dynamica simulations of poly(viny1idene fluoride), PVDF. The first, MSXX, was obtained with Hartree-Fock calculations of 1,1,1,3,3-pentafluorobutane plus experimental frequencies of the form I, crystal. The second, the covalent shell model (MSXXS), waa developed to also account for polarization. These force fields were used to predict structure and properties of nine stable structures of poly(viny1idene fluoride) crystals including the four experimentally observed forma plus a fifth crystal form suggested by Lovinger. In each case we used the force field to establish that the structure is mechanically stable and to predict cell parameters, elastic constants, dielectric constants, and piezoelectric constants.

One-pot synthesis of hyperbranched polyethers

Abstract: 5-(Bromomethyl)-1,3-dihydroxybenzene undergoes self-condensation in the presence of K 2 (CO 3 ) 2 and 18-crown-6 to give polymers with M w exceeding 10 5 . Spectroscopic analysis of the hyperbranched polymers reveals that both O-alkylation and C-alkylation occurs during polymerization. The polymers contain a large number of phenolic hydroxyl groups located both at chain ends and at monomer units involved in irregular growth. The phenolic groups of the polymers can be modified readily and completely by simple acylation, benzylationor silylation

Stereocomplex formation between enantiomeric poly(lactic acid)s. 6. Binary blends from copolymers

Abstract: Binary blend films from copolymers of D- and L-lactic acids were obtained by casting mixed solutions of the two copoly(lactic acid)s (PLA) with different D-contents (X D ) and were studied by DSC. Four case from the combinations of binary (A-B) were selected: mixing the same polymer [X D (A)= X D (B)], cross blending under the condition that X D (A)+X D (B)=1, blending between a D-polymer and other PLAs and blending between a 1:1 DL copolymer (X D = 0.5) and other PLAs. Racemic crystallites were exclusively formed between D-rich PLA and L-rich PLA when they had high optical purities. The melting point and enthalpy of the dracemic crystallites decreased with decrease in the optical purity of D-rich DLA or L-rich PLA, suggesting that a D-unit in the L-rich PLA when they had high optical purities

Preparation and morphology of triblock copolymers of the ABC type

Abstract: The variation of morphology of triblock copolymers of the ABC type, poly(isoprene-b-styrene-b-2-vinylpyridine), with comparition was studied by TEM. Four types of morphology were observed for the microphase-separated structures of the samples whole volume fractions of the middle-block, polymer, polystyrene, range from 0.3 to 0.8, while keeping the volume fractions of the end-block polymers, i.e., polyisoprene and poly(2-vinylpyridine), equal. They are a three phase four-layer lamellar structure, an ordered tricontinuous double-diamond structure, a cylindrical structure and a spherical structure

Deformation of ionic polymer gels by electric fields

Abstract: When an electric field is applied to an ionic gel in a buffer solution, the anode side of the gel shrinks if the gel is contacting with the electrode, while it swells if the gel is placed with sufficient separation from the electrode. The changes of ion concentration profiles under an electric field are calculated, taking account of ion transports and electrochemical reactions. Combining these results with Flory's theory for the swelling of ionic gels, the above phenomena are explained. It is predicted that the swelling behavior is governed by the concentration of the dominant ions and that the swelling speed is proportional to the square of the electric current

Relationship between the glass transition temperature and the interaction parameter of miscible binary polymer blends

Abstract: An equation that relates the gh transition temperature, TE, and a binary interaction parameter, x, for miscible binary polymer blends was derived. The equation including no adjustable parameters was based on a thermodynamic mixing formalism using enthalpy as the thermodynamic parameter. The en- thalpy of mixing was written as a van Laar expression, and the T, was formally treated as a second-order Ehrenfest transition. The equation satisfactorily predicts TK-composition curves for miscible bw polymer blends that exhibit either positive or negative deviation from a linear mixing rule, depending on the strength of the interaction. Good agreement was found between calculated x and values experimentally determined by equilibrium melting point depression and inverse gas chromatography.