Differences In Polymer Structure Research Articles

Neutral Ni(II) Catalysts Based on Maple-Lactone Derived [N,O] Ligands for the Polymerization of Ethylene

This paper describes the synthesis of neutral nickel(II) catalysts for the polymerization of ethylene. A series of 2-(arylamino)-5-methylcyclopent-2-en-1-one (aryl = 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, 3,5-dimethylphenyl, and 4-methylphenyl) proligands, derived from the condensation of aryl amines with maple lactone, were synthesized as their keto-enamine tautomers. Deprotonation with NaH gave their respective sodium imino-enolate salts, which underwent salt metathesis with Ni(II) precursors to give square planar [Ni(Ar)(κ2-imino-enolate)(PPh3)] (Ar = Ph, ortho-tolyl) complexes containing 5-membered [N,O] chelates. Precatalysts with N-Dipp (Dipp = 2,6-diisopropylphenyl) substituents were found to exhibit moderate activity in the polymerization of ethylene when using B(C6F5)3 or [Ni(cod)2] (cod = 1,4-cyclooctadiene) as coactivators, producing polymers with a high molecular weight, as determined by GPC. The reaction was found to be highly dependent on temperature, with the catalysts most active at 80 °C. Differences in polymer structure were also found when using B(C6F5)3 or [Ni(cod)2] as the cocatalyst, with [Ni(cod)2] producing a more highly branched polymer. Reducing the steric bulk at the N substituent of the ligand led to inactive species.

Interfacial interaction and joining property of direct injection-molded polymer-metal hybrid structures: A molecular dynamics simulation study

Polymer-metal hybrid structures have been increasingly used to replace metallic components. Direct injection joining is one of the most promising technologies in fabricating polymer-metal hybrid structure. In this study, molecular dynamics method was used to study the interfacial interaction and joining property. Interfacial models with different surface structures on aluminum (Al) substrate were constructed. The influences of surface structure, non-bonded interaction strength, and pull-out force during the separation were systematically explored. Simulation results show that an obvious difference in polymer structure along z-axis is observed during the joining process. Influenced by the interfacial interaction, the atomic density is at its largest near the interface. As the separation continues, growing voids are observed near the interface. Polymer chains close to the Al substrate are greatly stretched, while the chains far away from the interface are relatively steady. Nevertheless, the changes in radius of gyration during the separation are much higher regardless of the substrate structures. The contact area mainly contributes to the interaction energy, whereas the mechanical interlocking formed in the undercut area contributes to the maximum inner force and work of separation. Moreover, the interfacial interaction is also affected by the non-bonded interaction strength and the pull-out force.

Azobenzene‐Incorporated Single‐ and Double‐Stranded Polynorbornenes: Facile Synthesis and Diverse Photoresponsive Property

AbstractDouble‐stranded polymeric ladderphane using azobenzene as linkers and the single‐stranded polynorbornene with azobenzene pendants are synthesized by the ring‐opening metathesis polymerization using Grubbs ruthenium‐based catalyst. Hydrolysis of these as‐prepared polynorbornenes gives the corresponding isotactic single‐stranded polynorbornene having similar polydispersity and degree of polymerization as those of the original polymers. The photosensitive property of double‐stranded ladderphane indicates significant interactions between adjacent chromophores. These single‐ and double‐stranded polynorbornenes incorporating azobenzene chromophores exhibit distinct photoresponsive performance in the dilute solutions. The rate of photoisomerization for double‐stranded ladderphane is found to be from 6 to 9 times slower than that obtained for single‐stranded polymer. This is ascribed to the differences in structure of polymers and the interaction between azobenzene chromophores in the polymers.

Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport.

Single-walled carbon nanotubes (SWCNTs) have potential to act as light-harvesting elements in thin film photovoltaic devices, but performance is in part limited by the efficiency of exciton diffusion processes within the films. Factors contributing to exciton transport can include film morphology encompassing nanotube orientation, connectivity, and interaction geometry. Such factors are often defined by nanotube surface structures that are not yet well understood. Here, we present the results of a combined pump-probe and photoluminescence imaging study of polyfluorene (PFO)-wrapped (6,5) and (7,5) SWCNTs that provide additional insight into the role played by polymer structures in defining exciton transport. Pump-probe measurements suggest exciton transport occurs over larger length scales in films composed of PFO-wrapped (7,5) SWCNTs, compared to those prepared from PFO-bpy-wrapped (6,5) SWCNTs. To explore the role the difference in polymer structure may play as a possible origin of differing transport behaviors, we performed a photoluminescence imaging study of individual polymer-wrapped (6,5) and (7,5) SWCNTs. The PFO-bpy-wrapped (6,5) SWCNTs showed more uniform intensity distributions along their lengths, in contrast to the PFO-wrapped (7,5) SWCNTs, which showed irregular, discontinuous intensity distributions. These differences likely originate from differences in surface coverage and suggest the PFO wrapping on (7,5) nanotubes produces a more open surface structure than is available with the PFO-bpy wrapping of (6,5) nanotubes. The open structure likely leads to improved intertube coupling that enhances exciton transport within the (7,5) films, consistent with the results of our pump-probe measurements.

Fractional separation of polymers in nanochannels: Combined influence of wettability and structure

Trapping macromolecules in nanopits finds diverse applications in polymer separation, filtering biomolecules etc. However, tuning the locomotion of polymers in channels of nanoscopic dimensions is greatly restricted by the comparative advective and diffusive components of velocities. Using the polymer affinity toward the solvent and the wall, and the polymer structure, a mechanism is proposed to induce selective trapping of polymers. Similar to fractional distillation of hydrocarbons based on molecular weight, a technique of fractional segregation, depending on the channel wettability of polymeric chains at different depths in a pit that is located perpendicular to the flow is suggested. Depending on the properties of the polymeric chains and the surface chemistry, the segregation of the polymer at a particular level in the pit can be predicted. This behavior stems from the difference in polymer structure leading to a competition between wettability based enthalpic trapping and structure based entropic trapping. The results of this study suggest a novel way of separating biopolymers based on their structure without relying on the channel geometry. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2118–2125

Understanding the Role of ω-End Groups and Molecular Weight in the Interaction of PNIPAM with Gold Surfaces

Modification of nanoparticle surfaces by adsorption or grafting of polymers allows fine control of hybrid materials properties for diverse applications. To obtain such a control, it is of paramount importance to understand the impact of the polymer structure on the nature and strength of its interaction with the nanoparticle. We investigated here a simple model of hybrid materials made of poly(N-isopropylacrylamide) of different molar masses and end groups interacting with gold surfaces. A series of poly(N-isopropylacrylamide) with number-average molar masses ranging from 3700 to 10000 g·mol–1 were synthesized by reversible addition–fragmentation chain transfer/macromolecular design by interchange of xanthates (RAFT/MADIX). The terminal xanthate group was then reduced into either a thiol or a hydrogen group. Quartz crystal microbalance adsorption/desorption experiments demonstrated that the polymer termini have a strong impact on the mechanism of polymer adsorption on flat gold surfaces. These differences in polymer structure have, in return, a strong influence on the colloidal stability and growth mechanism of nanoparticles when directly synthesized in polymer solution. For those properties, the effect of xanthate group compared very favorably to the conventional thiol moiety. Interestingly, the properties of nanohybrids were poorly affected by the molar mass of the polymer.

Diffusion and viscosity in arabinoxylan solutions: Implications for nutrition

Non-starch polysaccharides such as arabinoxylans have important roles in the human diet, resulting in potential benefits such as increased microbial fermentation, promotion of beneficial microflora, prevention of re-absorption of bile acids leading to lower plasma cholesterol, and retardation of starch digestion. The latter two beneficial effects may arise from viscosity and/or diffusion phenomena in the gastrointestinal tract. To study this, measurements of the viscosity and diffusion coefficients of a polymer probe similar in size to both bile salt micelles and alpha-amylase were carried out for water solutions of three arabinoxylans with differing viscosities. Diffusion coefficients were obtained using fluorescence recovery after photobleaching (FRAP). The concentration dependence of both viscosity and diffusion coefficients followed the usual behaviour of polymers for each of three arabinoxylan samples. However, at a given concentration, the sample with the highest viscosity also had the highest probe diffusion coefficient: the reverse of what would be expected for homogeneous solutions. This apparent anomaly is ascribed to differences in polymer structure between the three samples giving rise to varying levels of local polymer aggregation and consequent microvoids. These differences are verified using characterisation with multiple-detection size-exclusion chromatography. Deviations from simple Stokes–Einstein behaviour are ascribed to the existence of aggregates in solution. The results show that studies of the role of arabinoxylans in human nutrition cannot assume that the diffusion coefficients of species with sizes in the range important for digestive processes in a series of samples will increase with decreasing viscosity at a given concentration: diffusion coefficient and viscosity must be measured independently.

Phase behaviour analysis of solid dispersions of loperamide and two structurally related compounds with the polymers PVP-K30 and PVP-VA64.

The purpose of the present study was to investigate the influence of the structure of a poorly water soluble model drug (loperamide) on the phase behaviour in solid dispersions with PVP-K30. Dispersions with PVP-VA64, a less hydrophilic polymer, were investigated as well in order to study the influence of differences in polymer structure and water content of the samples. The solid dispersions of PVP-K30 or PVP-VA64 with loperamide as well as with two fragments of this molecule were prepared by spray drying. The amount of residual solvents and water was determined with GC and thermogravimetric analysis (TGA). The drug loading of the dispersions was determined using high performance liquid chromatography (HPLC). The solid state properties were evaluated using powder-XRD, IR-spectroscopy and MT-DSC. All mixtures containing loperamide proved to be completely amorphous, whereas the dispersions containing the fragments are only amorphous in case the polymer content is high. The phase diagrams that were constructed clearly show that loperamide exhibits a different behaviour in the solid dispersions than its two building blocks. They also point to the presence of specific intermolecular compound--polymer interactions in the dispersions of one of the fragments with the two polymers. This was confirmed by the IR-results. Despite structural similarities, interactions in dispersions containing loperamide are far less important. In dispersions containing high concentrations of the other fragment, the DSC curves give indications for polymorphism whereas IR and XRD-spectra point towards inclusion of solvent in these samples.

The effects of novel amphipathic block copolymers on stabilization of the rat tear film.

To determine whether various novel amphipathic polymers could be used to stabilize the tear film of the rat. The rheologic properties of these polymers were examined to investigate whether particular structural or physical characteristics improve the stability of the tear film. Amphipathic polymers or particular phospholipids were mixed with a test solution of tears and saline and applied to the clean, dry corneal surface of a rat. The specular reflection of the tear film was observed at high magnification and recorded. For each of the polymers or lipids, the effects on surface regularity and tear break-up time were compared. After the experiments, histologic sections of the tested eyes were prepared and examined for acute cytotoxic effects on the cornea and ocular conjunctiva. Tear film break-up time was markedly affected by differences in polymer structure. Copolymers consisting of separate hydrophobic and hydrophilic regions appeared to be the best stabilizers. No acute cytotoxic effects were observed in histologic sections of corneas to which the polymers had been applied. Amphipathic polymers can be designed to increase tear film stability. Increased tear film stability occurred more readily with copolymers, possibly through their interaction with both lipid and aqueous tear components.

Electrical stability of polyimide siloxane films for interlayer dielectrics in multilevel interconnections

Electrical stability of a polyimide siloxane (PSI) film for ultra-large scale integrated circuit (ULSI) multilevel interconnections is studied. The PSI films, modified by p-aminophenyltrimethoxysilane (APTMS), are designed to have three-dimensional polymer structures through Si–O bonds. It has been revealed that the PSI films are more stable in electrical properties at higher temperatures than 150°C, as compared to the conventional polyimide (PI) films. The electrical conduction mechanism study for the PSI films has revealed that Schottky emission is dominant. Barrier height φ B obtained from the electrical property for the PSI film was 0.460 eV in the temperatures ranging from 25–250°C. On the other hand, barrier height of 0.422 eV at lower temperatures than 150°C and activation energy of 1.09 eV at higher temperatures than 150°C were obtained for the conventional PI film. The difference in polymer structure is very sensitive to the electrical conduction at high temperature, due to sodium ion migration. The ideal band diagrams of metal-insulator-semiconductor (MIS) structures were also discussed. The optical band gaps for PSI and conventional PI films were 3.320 eV and 3.228 eV, respectively. This result suggests that the band gap of PI films can be enlarged by modification with Si–O components. The differential barrier height between the PSI and conventional PI films is 0.038 eV, and is close to the difference in half of optical band gaps (0.046 eV).

TEM studies of resin-based matrix microstructure in carbon/carbon composites

The microstructure of thermoset-resin-derived matrices in unidrectional carbon/carbon (C/C) composites has been investigated by transmission electron microscopy (TEM). A wide range of carbon fibers—derived from PAN, pitch and rayon—were used with two thermoset-resin-matrix precursors: polyarylacetylene (PAA) and phenolic resins. Specimens were examined in both transverse and longitudinal sections using bright-field imaging and selected-area diffraction (SAD). The current studies confirm in much finer detail earlier observations from optical microscopy and scanning electron microscopy (SEM) that matrix orientation is clearly evident at heat treatment temperatures (HTTs) as low as 1100°C and that graphitized lamellar matrix forms at HTTs > ~2400°C and is concentrated in those interfilament regions in which we expect large components of biaxial tensile stress resulting from restraint of pyrolysis shrinkage. The TEM micrographs also establish that most of the fibers and matrices studied are capable of forming intimate fusion bonds following graphitization heat treatment. It is difficult, however, to estimate the extent to which such bonding exists at any particular HTT. We have also made observations of transversely oriented matrix immediately at the fiber surface in some composites. It is suggested that such a microstructure may form as a consequence of movement or deformation of the matrix during the critical carbon-chain-forming process that accompanies matrix pyrolysis. Microporosity at the fiber surface may play a role in this process by providing the opportunity for development of a network of interpenetrating fibrils that effectively bond fiber to matrix. Observations were also made of ripples or striations (produced by ion-milling) that run transverse to the direction of preferred orientation in both fibers and matrices. Possible mechanisms for their formation are discussed. Although two thermosetting resins employed in this study—PAA and phenolic—differ widely in chemical functionalities and structure, their matrix microstructures in the C Cs are largely indistinguishable by the TEM technique employed here, as well as by earlier X-ray diffraction (XRD) results. These observations emphasize the critical role of polymer pyrolysis, in which thermomechanical processes, along with bond breakage and reforming, effectively overcome the initial large differences in polymer structure, leading to the formation of well-graphitized carbons from otherwise nongraphitizing precursors.

Retention Characteristics of Inorganic Salts and Polyethylene Glycols on Polysulfones

Abstract Polysulfone interfacial properties were determined using HPLC. the equilibrium distribution coefficients for a series of inorganic salts as well as a series of poly-ethylene glycols with different molecular weights are presented. the difference in solute-polymer interactions are explained in terms of differences in the polymer structure. Hansen's solubility parameter was used to quantify this difference in polymer structure.

Characterizing the quality of PVC by melt flow testing

AbstractTraditionally, PVC compounds are identified by reference to bulk properties such as hardness, specific gravity, tensile, elongation. While these are useful and descriptive notations, they do not serve effectively to define compound quality. Melt flow is broadly used to great advantage in testing most commerical thermo‐plastics as described in ASTM D‐1238. These procedures utilize the dual advantages that a plastometer is a very expedient instrument to use in a given processing facility, and that a MF test is highly sensitive, capable of detecting relatively small differences in polymer structure and composition. However, PVC's are not considered within D‐1238 for several important reasons. The thermal and rheological instabilities of PVC create special considerations in testing as well as in processing. Furthermore, the polymer is always used in a highly varied compounding‐ingredient‐environment which further complicates the problem.

Interdependence between crystallization and polymerization in diacetylene monolayers

The topochemical polymerization of a diacetylenic lipid with two chains is studied by fluorescence and electron microscopy, electron diffraction, spectroscopic and film balance techniques. It is shown that, dependent on the temperature of pressure-induced crystallization, two different crystallographic structures result. Their crystalline nature is reflected in their morphology as well as in electron diffraction patterns with very sharp regions. The differences in the lattice structure correspond to differences in polymer structure as revealed by distinct absorption and emission spectra and emission intensities that are different by a factor of 100. Polymerization of the low temperature structure proceeds faster than that of the high temperature modification by more than a factor of 5.

Raman spectroscopy of conductive polypyrroles

Recent studies of conductive polymers have focused on the need to produce air-stable processable materials. Since the discovery that conductive polypyrrole films could be prepared electrochemically [1], this polymer has attracted interest because of its much greater stability in air compared with polyacetylene [2]. Recently it has been shown that film morphology and mechanical properties are strongly influenced by the choice of electrolysis conditions. Films with thicknesses of 1 mm or more with properties ranging from rubbery to tough elastomeric can now be obtained with conductivities in the range 20 100 S/cm [3 5]. Factors that influence film properties are solvent, counter-ion and preparation temperature. It is clear that the differences observed at the macroscopic level reflect differences in polymer structure at the microscopic level. The ideal polypyrrole chain structure is a planar array of monomer units joined by ~ (2 5) bonds. This is not the only possible bonding sequence; other links also occur, as shown by the greater crystallinity that results when these links are blocked by the use of fi fl substituted monomers [6]. The elastomeric nature of the polymer films also suggests that they contain crosslinks. It has not yet proved possible to determine directly the concentration of these different defect structures in polypyrrole films by a non-destructive technique. In principle, Raman spectroscopy provides a method for characterization of polypyrrole films, since it has proved to be a powerful method for the study of other conjugated polymers [7]. To date there have been very few reports of Raman spectra for polypyrrole, particularly in the oxidized, conducting state [5]. The broad absorption bands and the rough sample surfaces result in dense black samples, which give weak Raman spectra. We have studied a number of samples prepared using different electrode materials and obtained well-resolved Raman spectra from the surface adjacent to the electrode. The spectra show well-resolved bands in the range 800 to 1600 cm 1. Differences in band positions and frequencies show a strong influence of the electrode material and growth temperature on polymer structure. As yet, a complete assignment of the observed spectra has not been possible. Model calculations and comparison of other polymer properties are being used to aid interpretation.

Synthesis and properties of some alkyds of more complex carboxyl functionality

Syntheses are described where dimer fatty acid (C36), or anhydrides of trimellitic, or pyromellitic, or dodecenylsuccinic acids are substituted in part for components of a model polymer derived from 1 mole each phthalic anhydride and glycerol, and 0.4 mole of linoleic acid. These polymers are compared at several degrees of polyesterification with respect to endgroup analysis, melt and intrinsic viscosity, and fractionation and emulsification behavior in water-solvent mixtures. The data suggest that predictions of the gel point in terms of functionality concepts are of general value only for defining the reaction degrees where insoluble molecules can first appear. The experimental event of gelatification can be delayed to varying degrees by different types and concentrations of inert solvent. The extent of reaction where molecular complexity can give a gellable polymer in the absence of solvency retardation occurs near the theoretical gel point and can be located by inspection of functions relating either melt or intrinsic viscosity to molecular weight. Significant differences in polymer structure have a very small effect on this parameter, nor is there much evidence to show preferential reactivity of similar functional groups associated with different structures. Other data here cited demonstrate some practically useful facts regarding how composition and molecular weight affect solvency behavior; however, no general correlations were found which relate solubility behavior to quantities here miasured.