Showing papers in "Macromolecules in 1999"

Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes

Abstract: Gas separation properties of polymer membrane materials follow distinct tradeoff relations: more permeable polymers are generally less selective and vice versa. Robeson1 identified the best combinations of permeability and selectivity for important binary gas pairs (O2/N2, CO2/CH4, H2/N2, etc.) and represented these permeability/selectivity combinations empirically as αA/B = βA/B , where PA and PB are the permeability coefficients of the more permeable and less permeable gases, respectively, αA/B is selectivity (=PA/PB), and λA/B and βA/B are empirical parameters. This report provides a fundamental theory for this observation. In the theory, λA/B depends only on gas size. βA/B depends on λA/B, gas condensability, and one adjustable parameter.

Controlled Synthesis of Hyperbranched Polyglycerols by Ring-Opening Multibranching Polymerization

Abstract: Glycidol represents a latent cyclic AB2-type monomer that can be polymerized in a ring-opening multibranching polymerization (ROMBP). Hyperbranched aliphatic polyethers with controlled molecular weights and narrow molecular weight distribution have been prepared via anionic polymerization of glycidol with rapid cation-exchange equilibrium. 1,1,1-Tris(hydroxymethyl)propane (TMP) was partially deprotonated (10%) and used as initiator for the anionic polymerization carried out under slow addition conditions to minimize polymerization without initiator as well as cyclization. 13C NMR spectra were employed to assess the degree of branching (DB), which was in the range 0.53−0.59. 13C NMR, MALDI-TOF mass spectrometry, vapor pressure osmometry (VPO), and GPC were used to characterize molecular weights and polydispersities of the polyols formed, showing good agreement. n for the polymers prepared was in the range 15−83 (Mn = 1250−6500), and Mw/Mn obtained was 1.13−1.47, being exceptionally low for hyperbranched...

Factors Controlling the Growth of Polyelectrolyte Multilayers

Abstract: The dependence of polyelectrolyte multilayer thickness on salt concentration, salt type, solvent quality, deposition time, and polymer concentration is evaluated. Polymers are deposited on spinning silicon wafers. For the strong polycation/polyanion pair studied, film thickness is approximately proportional to the number of layers and the salt concentration. The irreversibility of overall molecule adsorption is indicated by the lack of exchange of surface (radiolabeled) for solution polymer. The hydrophobic nature of the driving force for polymer sorption is illustrated by the choice of salt counterion or solvent. Analyzed within the framework of ion exchange, the net energy of ion pair formation is not high, at most a few kT. Salt, competing with polymer segments for the surface, permits localized rearrangements. In the mechanism proposed, excess polymer is accommodated within several layers, rather than in one layer of loops and tails. Steric barriers coupled with slow conformational changes are respons...

Polymers at interfaces : Using atom transfer radical polymerization in the controlled growth of homopolymers and block copolymers from silicon surfaces in the absence of untethered sacrificial initiator

Abstract: The atom transfer radical polymerization (ATRP) of styrene and acrylates from silicon wafers modified with an initiator layer composed of 2-bromoisobutyrate fragments is described. In the presence of the proper ratio of activating and deactivating transition-metal species, controlled radical polymerizations of styrene were observed such that the thickness of the layer consisting of chains grown from the surface increased linearly with the molecular weight of chains polymerized in solution in identical, yet separate, experiments. The layer thickness increased linearly with reaction time for ATRP of styrene and methyl acrylate due to both the extremely low initiator concentration relative to monomer and the low monomer conversion. Further evidence for control was observed by the polymerization of blocks of either methyl or tert-butyl acrylate from the polystyrene layer. Modification of the hydrophilicity of the surface layer was achieved by hydrolysis of the poly(styrene-b-tert-butyl acrylate) to poly(styre...

Controlled Synthesis of Polymer Brushes by “Living” Free Radical Polymerization Techniques

Abstract: The preparation of a wide variety of unique polymer brush structures can be accomplished by “living” free radical polymerization of vinyl monomers from surface-tethered alkoxyamines or from tethered α-halo esters in the presence of (PPh3)2NiBr2. The use of a “living” free radical process permits the molecular weight and polydispersity of the covalently attached polymer chains to be accurately controlled while also allowing the formation of block copolymers by the sequential growth of monomers from the surface. These block and random copolymer brushes have been used to control surface properties.

Interplay of Physical Structure and Photophysics for a Liquid Crystalline Polyfluorene

Abstract: From photophysical evidence, we suggest a structural model based on intrachain ordering that can account for the changes of the absorption spectrum of poly(9,9-dioctylfluorene) (PFO) films under certain physicochemical treatment protocols. We correlate this model to the results of X-ray fiber diffraction experiments.

In situ spectroelectrochemical Raman studies of poly(3,4-ethylenedioxythiophene) (PEDT)

Abstract: The vibrational properties of poly(3,4-ethylenedioxythiophene) (PEDT) have been studied by means of UV−vis−NIR optical absorption spectroscopy and resonance Raman scattering (RRS) spectroscopy with two excitation lines: green (514 nm) and infrared (1064 nm). The two-step oxidative doping process does not induce a drastic change for the Raman bands, but the changes that occur are clearly evidenced. During doping, new bands appear, indicating a modification of the electronic structure of the polymer. Vibrational calculations were carried out using a symmetrized dynamical matrix model and the results were compared with experimental data, especially in the 1200−1600 cm-1 range, where the CαCβ and Cβ−Cβ stretching vibrations are active. It appears that PEDT seems to have an intermediate electronic structure, between the quinoid and benzoid structures.

Thermoreversible Gelation of PEG−PLGA−PEG Triblock Copolymer Aqueous Solutions

Abstract: Aqueous solutions of new biodegradable triblock copolymers, poly(ethylene glycol-b-(dl-lactic acid-co-glycolic acid)-b-ethylene glycol) (PEG−PLGA−PEG), have shown to have sol-to-gel (lower transiti...

Ultrahydrophobic polymer surfaces prepared by simultaneous ablation of polypropylene and sputtering of poly(tetrafluoroethylene) using radio frequency plasma

Abstract: Ultrahydrophobic polypropylene surfaces were prepared by the simultaneous etching of polypropylene and etching/sputtering of poly(tetrafluoroethylene) (PTFE) using inductively coupled radio frequency argon plasma. The semicrystalline polypropylene surface is roughened due to the differential rates at which the crystalline and amorphous regimes ablate and is also fluorinated by the fluorocarbon plasma that results from the ablation/depolymerization of PTFE. The roughness of the polypropylene is controlled by the time of plasma etching. The presence of PTFE increases the rate of polypropylene roughening by reactive ion etching. The resulting roughened and fluorinated polypropylene surfaces were characterized by water contact angle, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy (AFM). Wettability was found to depend on the size scale and topology of the roughness. The most hydrophobic surfaces exhibited advancing and receding water contact angles of θA/θR =...

Living Radical Polymerization with Reversible Addition−Fragmentation Chain Transfer (RAFT Polymerization) Using Dithiocarbamates as Chain Transfer Agents

Abstract: Living radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be achieved with the use of dithiocarbamate derivatives that have the nonbonded electron pair of the nitrogen included as part of an aromatic system. These compounds have been shown to be highly effective in RAFT polymerization of styrene and (meth)acrylate esters to produce polymers of predetermined molecular weight and narrow polydispersity (usually <1.2). By contrast, simple N,N-dialkyl dithiocarbamates (those compounds previously described as “photoiniferters”) are ineffective as RAFT agents. The reasons for the different behavior are discussed.

Preparation, Alignment, and Optical Properties of Soluble Poly(phenylacetylene)-Wrapped Carbon Nanotubes†

Abstract: Carbon nanotube-containing poly(phenylacetylenes) (NT/PPAs) are prepared by in situ polymerizations of phenylacetylene catalyzed by WCl6−Ph4Sn and [Rh(nbd)Cl]2 (nbd = 2,5-norbornadiene) in the pres...

Network Stretching, Slip Processes, and Fragmentation of Crystallites during Uniaxial Drawing of Polyethylene and Related Copolymers. A Comparative Study

Abstract: When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. Relative weights of the different processes change with the crystallinity. The dependencies were analyzed in a comprehensive study on a series of samples encompassing a large range of crystallinities: PE, low-density PE (LDPE), and ethylene−vinylacetate copolymers. The comparison was based on measured true stress−strain curves for constant strain rates. For the samples with higher crystallinity, which show a necking, this was achieved by employing a video-controlled tensile testing machine. Recovery properties of the sample were studied in step-cycle runs, where the load was applied stepwise and interrupted after each step by an unloading−reloading loop. Simultaneous with the mechanical testing, the related textur...

Electrical and Photoinduced Degradation of Polyfluorene Based Films and Light-Emitting Devices

Abstract: Degradation-induced changes in the structural and optical properties of the polyfluorene-based blue emitting films and LEDs are examined using spectroscopic (FTIR, UV−vis, photo- and electroluminescence), analytical (FTIR and ESCA), and scanning probe microscopy techniques. The materials studied are oligomers (DP ∼ 10) of 9,9-di-n-hexylfluorene and its random copolymer with anthracene. In situ FTIR monitoring is used to characterize chemical changes in the active layer of operating LED devices. Two primary mechanisms of degradation are identified. In the first, photooxidation of the polymer matrix leads to the formation of an aromatic ketone, most likely fluorenone at the chain terminating monomer units, which quenches the fluorescence. The second process promotes aggregate formation, which then leads to loss of luminous intensity by exciton transfer and relaxation through excimers.

Packing Length Influence in Linear Polymer Melts on the Entanglement, Critical, and Reptation Molecular Weights

Abstract: We have shown in previous studies that the entanglement molecular weight for a polymer melt, Me, is related by a power law to p, the packing length of the polymer species. We now find that power laws also describe the molecular weights characterizing the melt viscosity, Mc marking the onset of entanglement effects and Mr the crossover to the reptation form. The packing length exponents for Me, Mc, and Mr differ significantly, however. The long-held notion that the ratio Mc/Me has the same value for all species is therefore incorrect. Further, the observed and predicted values of Mr for two species, 1,4-polybutadiene and polyisobutylene, have been found to agree, within the uncertainties, with the projected values. Finally, the variations with packing length are such that all three characteristic molecular weights would appear to converge on the same value near p = 9 A. As yet, no species with such a large packing length has been completely studied rheologically. But the range is not outlandish and is clea...

Preparation of Protein-Resistant Surfaces on Poly(vinylidene fluoride) Membranes via Surface Segregation

Abstract: Self-organizing blends of an amphiphilic comb polymer having a poly(methyl methacrylate) (PMMA) backbone and poly(ethylene oxide) (PEO) side chains in poly(vinylidene fluoride) (PVDF) have been examined as a means to create foul-resistant, self-healing surfaces on polymer membranes. X-ray photoelectron spectroscopy (XPS) analysis of phase inversion membranes prepared from these blends indicates substantial surface segregation of the amphiphilic component, which occurs both during the coagulation step of the phase inversion process and in subsequent annealing of the membranes in water. With annealing, a near-surface coverage of nearly 45 vol % comb polymer is produced on a membrane with a bulk comb concentration of only 3 vol %. Surface enrichment of the hydrophilic comb polymer is shown to impart significant resistance to the adsorption of bovine serum albumin (BSA). XPS analysis of membranes treated with concentrated acid shows that hydrophilic surface layers removed by acid exposure may be regenerated b...

Investigation of electrostatic interactions in polyelectrolyte multilayer films: Binding of anionic fluorescent probes to layers assembled onto colloids.

Abstract: The electrostatic binding of two anionic probe molecules, pyrenetetrasulfonic acid (4-PSA) and 6-carboxyfluorescein (6-CF), to poly(allylamine hydrochloride) (PAH)/poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC)/PSS ultrathin multilayer films assembled onto polystyrene (PS) latex particles has been examined using fluorescence spectroscopy. At the probe concentrations studied, binding was observed only when the outermost layer was oppositely (positively) charged to the probe. The amount of probe bound was found to increase linearly with polyelectrolyte layer number up to about 11 layers (15 nm in thickness for the PAH/PSS films), with saturation binding occurring beyond 11 layers. The binding data reveal that a minimum of about 10−30% of the cationic charges of the polycations in the upper region of the multilayer films (where the probe binds) are not directly electrostatically utilized in the multilayer assembly process through ion-pair binding to oppositely char...

Thermodynamic vs kinetic aspects in the formation and morphological transitions of crew-cut aggregates produced by self-assembly of polystyrene-b-poly(acrylic acid) block copolymers in dilute solution

Abstract: Thermodynamic vs kinetic aspects of the formation of crew-cut aggregates of various structures prepared from polystyrene-b-poly(acrylic acid) diblock copolymers in DMF/water mixtures are explored. In particular, the reversibility of the transitions between the crew-cut aggregates of various morphologies is studied by “jumps” in the polymer or ion concentration at various water contents. The aggregates are prepared by two different methods: micellization is induced either by the addition of water to polymer/DMF solutions or by the direct dissolution of the polymer in DMF/water mixtures. It is shown that as the polymer concentration increases, the morphology of the aggregates changes from spheres to rodlike micelles, to interconnected rods, and then to bilayers. As the water content increases, the boundaries for the formation of the different morphologies move toward lower polymer concentrations. The thermodynamic vs kinetic control of the morphology depends largely on the water content and the method of a...

Synthesis and characterization of injectable poly(N-isopropylacrylamide)-based hydrogels that support tissue formation in vitro

Abstract: Hydrogels that support tissue formation in vitro were developed using poly(N-isopropylacrylamide) [P(NIPAAm)]. Loosely cross-linked P(NIPAAm) and poly(NIPAAm-co-acrylic acid) [P(NIPAAm-co-AAc)] hydrogels were synthesized with N,N‘-methylenebis(acrylamide) cross-linker. At room temperature (RT), the hydrogels were transparent and extremely pliable, while at 37 °C, the matrices became opaque and were significantly more rigid. The P(NIPAAm-co-AAc) hydrogel demonstrated significantly less volume change between RT and 37 °C, contained significantly more water at 37 °C, and had a lower critical solution temperature which was significantly higher, as compared to that of the P(NIPAAm) hydrogel. The hydrogels supported bovine articular chondrocyte viability for at least 28 days of in vitro culture, and cartilage-like tissue was formed in the matrices. These hydrogels can be injected through a small-diameter aperture and offer the benefit of in situ stabilization without the possible deleterious effects of in situ ...

13C NMR Determination of the Composition of Linear Low-Density Polyethylene Obtained with [η3-Methallyl-nickel-diimine]PF6 Complex

Abstract: A complete 13C NMR characterization of two linear low-density polyethylenes (LLDPE), one highly branched and the other lowly branched, is presented. These LLDPE were synthesized with a new catalyst: the [η3-methallyl-nickel-dad]PF6 complex by homopolymerization of ethylene, using different experimental conditions. Methyl, ethyl, propyl, butyl, amyl, and long branches were assigned and quantified.

Synthesis and Characterization of Star Polymers with Varying Arm Number, Length, and Composition from Organic and Hybrid Inorganic/Organic Multifunctional Initiators

Abstract: Multifunctional initiators, derived from cyclotriphosphazenes, cyclosiloxanes, and organic polyols, were used in the synthesis of styrenic and (meth)acrylic star polymers by atom transfer radical polymerization (ATRP). Conditions were identified in each system which provided linear first-order kinetics for polymers with narrow, monomodal molecular weight distributions. Molecular weight measurements relative to linear polystyrene standards showed that the star polymers had lower molecular weights than theoretically predicted. Triple detection SEC measured on poly(n-butyl acrylate) samples demonstrated that the absolute molecular weight matched the theoretical valuethe smaller relative chain length was due to lower hydrodynamic volumes of the star-branched polymers relative to linear analogues. Kinetic arguments were used to demonstrate that each alkyl halide moiety bound to the initiators was participating in ATRP. Well-defined poly(methyl acrylate) stars of molecular weights Mn > 500 000 and low polydispe...

Shear-Enhanced Crystallization in Isotactic Polypropylene. 1. Correspondence between in Situ Rheo-Optics and ex Situ Structure Determination

Abstract: The effects of “short term shearing” on the subsequent crystallization of a polydisperse Ziegler−Natta isotactic polypropylene are observed using in situ optical measurements and ex situ microscopy. Imposition of brief intervals of shear (0.25−20 s, less than a thousandth of the quiescent crystallization time) can reduce the crystallization time by 2 orders of magnitude (e.g., at 141 °C with a wall shear stress of 0.06 MPa). With increasing shearing time, the crystallization time saturates and highly anisotropic growth ensues. This transition to oriented growth correlates with changes in the transient behavior during flow and the semicrystalline morphology observed ex situ. During flow, we observe the generation of long-lived, highly oriented structures (evident in the transient birefringence) under all conditions that induce subsequent growth of highly oriented crystallites. In turn, the development of oriented crystallites observed in situ after cessation of flow correlates with development of a “skin-core” morphology (highly oriented skin on a spherulitic core) observed ex situ. Interestingly, the long-lived structures generated during flow appear at shorter times with increasing temperature (at fixed shear stress), the opposite of the trend one would expect on the basis of the temperature dependence of quiescent crystallization.

Polymerization of Propylene by a New Generation of Iron Catalysts: Mechanisms of Chain Initiation, Propagation, and Termination

Abstract: A new series of iron catalysts bearing tridentate pyridine−bis(imine) ligands are reported for the isospecific polymerization of propylene (PP). The catalytic activities are moderate, with maximum activities exceeding 1600 kg PP/mol Fe h under 1 atm of propylene pressure at −20 °C . End-group analyses of the resultant materials are performed and indicate that the polymerization proceeds via a 2,1 mechanism of propagation leading to the exclusive formation of 1-propenyl end groups following β-H elimination/abstraction and chain transfer. The polymers are highly regioregular, but the regioregularity decreases with decreasing steric bulk of the tridentate ligands. Isotacticities are determined for a number of catalysts bearing ligands with variable ortho substituent patterns and symmetries. In all cases, isotactic material is obtained; the [m]4 methyl pentad content is always in the range of 55−67%. 13C NMR spectroscopy is used to demonstrate that the isotacticity arises from a chain-end control mechanism. T...

Formulation and stability mechanisms of polymerizable miniemulsions

Abstract: Polystyrene latexes in a size range of 30−180 nm can be synthesized by polymerization in miniemulsions where the particle size responds to the amount of the surfactant sodium dodecyl sulfate (SDS) which was varied between 0.5 and 50 wt % with respect to monomer. Turbidity and surface tension measurements were used to characterize the miniemulsification by ultrasonication and the approach to a steady state of droplet size. Since size equilibrium seems to be established by a rate equation of collision-induced droplet fusion and ultrasound fission, miniemulsions realize the minimal droplet size for the distinct amount of surfactant; i.e., they are “critically stabilized” with respect to collisions (τ2 process). Complete stability against Ostwald ripening (τ1 process) is obtained by the addition of a hydrophobe, which was varied over a broad structural range. The efficiency of this hydrophobic agent is given mainly by a very low water solubility. The growth of the critically stabilized miniemulsion droplets i...

Atom Transfer Radical Polymerization of (Meth)acrylamides

Abstract: The polymerization of several (meth)acrylamides [N,N-dimethylacrylamide, N-tert-butylacrylamide, and N-(2-hydroxypropyl)methacrylamide] by atom transfer radical polymerization (ATRP) was attempted. When initiating systems containing ligands commonly used in ATRP, such as linear amines or bipyridines, were employed, very low conversions were noticed in either bulk or solution, after more than 20 h at 90 °C. However, the use of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (Me4Cyclam) as a ligand provided polymers in high yields in a short time, but the polymerization was not controlled. This was attributed to the slow deactivation rate of the catalytic system. Experiments using model compounds for the active chain ends indicated that the low conversion obtained in the polymerization of (meth)acrylamides when linear amines or bipyridines are used as ligands may be explained by a slow activation in conjunction with a fast deactivation. The loss of bromine end groups during the polymerization through...

Thermosensitivity of Aqueous Solutions of Poly(N,N-diethylacrylamide)

Abstract: The thermally induced phase separation of aqueous poly(N,N-diethylacrylamide) (PDEA) was studied by means of differential scanning calorimetry and UV−visible spectrophotometry. The polymer was show...

Thermoreversible Gelation of Aqueous Methylcellulose Solutions

Abstract: The thermoreversible gelation of methylcellulose in aqueous solution has been studied by static and dynamic light scattering (DLS), small-angle neutron scattering (SANS), and rheology. At 20 °C, dilute solution light scattering establishes the molecular weight, second virial coefficient, radius of gyration, and hydrodynamic radius of the polymer. Semidilute solutions exhibit two relaxation modes in DLS, one reflecting cooperative diffusion and the other attributable to pregel clusters. Rheological measurements in this regime also suggest a weak supermolecular association. The gelation of semidilute solutions proceeds in two stages with increasing temperature above 20 °C, consistent with previous reports. The first stage is attributable to clustering of chains, driven by hydrophobic association, and extends up to approximately 50 °C. This process is accompanied by an increase in the low-frequency dynamic elastic modulus, G‘, and an increase in both light and neutron scattered intensity. The DLS properties ...

Employing MALDI-MS on Poly(alkylthiophenes): Analysis of Molecular Weights, Molecular Weight Distributions, End-Group Structures, and End-Group Modifications

Abstract: Regioregular, head-to-tail coupled, poly(3-alkylthiophenes), synthesized by three different methods, were subjected to MALDI-TOF MS analysis. Polymer samples of both high and low polydispersities were examined. Polymer samples of narrow polydispersity were obtained by fractionation of the polymer by Soxhlet extraction with various solvents. Comparisons between the molecular weights calculated by MALDI and GPC of all fractionated polymer samples showed that GPC calculated molecular weights are a factor of 1.2−2.3 times higher than MALDI. The polydispersities calculated by MALDI were identical or slightly lower than those calculated by GPC. Polymer end-group compositions were also analyzed. We found that more than one type of end-group structure could be detected, and these structures are dependent on the synthetic method employed. Chemical modification of the end-group structure was also performed and monitored by MALDI with successful results. We observed that smaller polymer chains were subject to end-gr...

Atom Transfer Polymerization of Methyl Methacrylate Mediated by Alkylpyridylmethanimine Type Ligands, Copper(I) Bromide, and Alkyl Halides in Hydrocarbon Solution

Abstract: Schiff bases of types 1, 2, and 3, easily prepared by the condensation of primary amines with pyridine-2-carboxaldehyde, glyoxal, or 2-acetylpyridine, respectively, are described as ligands for a copper(I) catalyst in the atom transfer polymerization of a range of methacrylates in toluene and xylene solution. Increasing the length of the alkyl group on ligands of type 1 increases the solubility of the catalyst in nonpolar solvents. The rate of polymerization increases on going from R = ethyl to propyl; however, on increasing the length of R further, we see no effect on the rate. The molecular weight distribution is narrow for all ligands where R = n-alkyl, and the number-average molecular weight (Mn) increases linearly with conversion. A decrease in rate and a loss of control are observed when branching is introduced in the α-position of the side chain. The polymerization is approximately first order in initiator, 0.90 ± 0.22, CuBr, 0.90 ± 0.13, and methyl methacrylate, 0.93 ± 0.01. Polymerization with Cu...