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The strain hardening modulus, defined as the slope of the increasing stress with strain during large strain uniax- ial plastic deformation, was extracted from a recently proposed constitutive model for the finite nonlinear viscoelastic deforma- tion of polymer glasses, and compared to previously published experimental compressive true stress versus true strain data of glassy crosslinked poly(methyl methacrylate) (PMMA). The model, which treats strain hardening predominantly as a vis- cous process, with only a minor elastic contribution, agrees well with the experimentally observed dependence of the strain hardening modulus on strain rate and crosslink density in PMMA, and, in addition, predicts the well-known decrease of the strain hardening modulus in polymer glasses with temperature. Gen- eral scaling aspects of continuum modeling of strain hardening behavior in polymer materials are also presented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 00: 000-000, 2010

To monitor the mineral surface coverage and energy, the inorganic cations of two clays (montmorillonite) with different surface area/cation have been exchanged by alkylammonium ions, carrying alkyl chains of different number and length. The prepared OMs were free of unreacted organic ions. With increasing length and number of the alkyl chains, an increase in the basal-plane spacing (d-spacing) of the OM was observed. The d-spacing also increased with increasing CEC of the clay (decreasing available area/cation). The OMs were compounded with PP and their effect on the crystallinity and gas-barrier properties of the polymer was investigated. The OM had no influence on the degree of crystallinity of PP under the processing conditions used. Oxygen permeation through the composites decreased, depending on the cross-sectional area of the exchanged organic cation and the CEC of the clay. These parameters control the mineral surface coverage (consequently the surface energy) as well as the tilt angle of the alkyl chains to the mineral surface, and hence the d-spacing. Increasing the length of the alkyl chains and their number per cation enhanced the d-spacing, clay exfoliation, and the gas-barrier properties of the composites. A mixed morphology, consisting of delaminated aluminosilicate layers and OM tactoids of varying thickness was observed but no intercalation took place. The oxygen permeation coefficient of the nanocomposites was found to be a non-linear function of the volume fraction of the inorganic part of the OM.

In this study, spatial orientational distribution functions of labeled chain segments of cross-linked and linear PMMA were obtained by solid-state NMR as a function of finite deformation (far) below and (far) above the glass transition temperature T g . The applied data analysis allows comparison of theoretical predictions and experimental data, both in terms of the orientational probability distributions as a function of two polar angles, as well as in terms of moments of the distribution. Orientation–strain relationships of chain segments agreed above and below T g with predictions from the rubber-elastic affine network model, but suggests a much denser network below T g than given by the cross-link density or the entanglement density in the melt. This suggested network structure is believed to be the generator of segmental orientation during plastic deformation in the glassy state, independent of the range of applied cross-link densities and deformation rates used in this study.

This study is concerned with the finite, large strain deformation behavior of polymeric glasses. True stress–strain curves in uniaxial compression obtained for five different polymeric glasses: polycarbonate, polystyrene, poly(2,6-dimethyl-1,4-phenylene oxide), and linear and cross-linked poly(methylmethacrylate), revealed a strain-hardening response during plastic deformation that is strain-rate dependent and deviates from neo-Hookean behavior. An empirical modification of the so-called compressible Leonov model by a strain dependent activation volume is suggested, which describes the strain-rate dependent large strain behavior of these glassy polymers in good agreement with experimental data.

Nanocomposites of high-density-polyethylene (HDPE) and organically (dimethyldioctadecylammonium) modified montmorillonite (OM) were prepared and the effect of non-ionic surfactants on the OM exfoliation and composite properties (tensile+gas-permeation) was studied. Amphiphilic block and random copolymers of different chemical structures were used as dispersing agents. The presence of copolymers in the composites led to polymer intercalation that increased the d -spacing and facilitated the exfoliation. Consequently, the permeability coefficient (oxygen) of the nanocomposites was decreased and their stiffness increased. End-functionalized oligomers proved to be more efficient in dispersing the OM than copolymers in which the polar units are randomly distributed along the polymer chain. Poly(ethylene- co -vinyl alcohol) increased the d -spacing but did not improve the properties of the composite probably due to ‘bridging’ the silicate layers, which hindered the exfoliation. The OM exfoliation could be enhanced to such an extent that an inclusions' average aspect ratio of 150 was estimated from the oxygen-permeation measurements. With increasing exfoliation, the stiffness, strength and gas-barrier properties of the composites improved significantly. The oxygen permeability of the HDPE nanocomposites was cut to less than half, thus offering a strong barrier to oxygen and humidity useful for food and drug packaging.

The orientational distribution function for polymer chain segments in plain-strain compressed glassy poly(methyl methacrylate) is evaluated, as a function of two polar angles, from a series of one-dimensional deuterium NMR spectra. The experimental data are analyzed using a tailored regulatory approach. For low degrees of orientational order, the choice of the regularization scheme is shown to become critical and two-dimensional Tikhonov-Phillips regularization yielded the best results for the present study.

The sodium ions of clays with different cation exchange capacities (CEC) have been exchanged with alkyl ammonium ions, in which 1–4 octadecyl chains are attached to the nitrogen atom. Due to the different cation cross-sectional area to available area per cation ratio, the resulting organo-montmorillonites (OMs) have different organic surface coverage and alkyl chain packing density. Nanocomposites of these OMs and HDPE were prepared and the influence of the organic monolayer structure on the exfoliation of montmorillonite and the tensile properties of the composites was studied. A high cation cross-sectional area to available area ratio led to complete surface coverage and large d-spacing, favoring the dispersion of the filler. In spite of the identical chemical structure of the polymer and the organic monolayer, complete exfoliation was not attained. Partial exfoliation was achieved without a compatiblizer, which often adversely affect the mechanical properties of the composites. Enhanced exfoliation increased the elastic modulus and yield stress but decreased the yield strain and stress at break of the nanocomposites. Increased filler loading enhanced the elastic modulus but decreased all other tensile properties. The tensile properties were correlated to the volume fraction of the inorganic part of the OMs and not to the total volume of the OM. Fitting the elastic modulus data to the Halpin-Tsai equation showed that the fitting parameter in this equation is not only related to the aspect ratio of the inclusions.

Composites of HDPE and glass or mica particles with different aspect ratios and surface modifications were prepared. The aspect ratio of the mica platelets was evaluated by image analysis of SEM micrographs. Test specimens, in which the platelet faces are oriented either parallel or perpendicular to the specimen flat surface, were prepared and the orientation of the particles was assessed by X-ray diffraction. Oscillatory rheological measurements in the linear viscoelastic regime were carried out. The in-plane and out-of-plane shear behavior was measured and the Halpin–Tsai structural parameter ζ calculated. For in-plane shear, ζ slightly decreased with increasing aspect ratio, whereas for out-of-plane shear, ζ strongly increased. Surface treatment of the mica particles had practically no influence on the results in the investigated loading range (volume fraction ≤7%).

Structure and reactivity often are dependent on the polarity of chemical bonds. This relationship is reflected by atomic charges in classical (semiempirical) atomistic simulations; however, disagreement between atomic charges from accurate experimental investigations, ab initio methods, and semiempirical methods has not been resolved. Our aim is to improve the basic understanding of the polarity of compounds with a view to make force-field parametrizations more consistent and physically realistic. The concept is based on the relationship between the atomization energies of the elements and the possible strength of covalent bonding and the relationship between the ionization energies/electron affinities of the elements and the possible strength of ionic bonding. Both quantities, energetically, are of the same order of magnitude and influence atomic charges in a compound, which we illustrate by trends across the periodic table. The relationship between the pure elements and a given compound is shown in an e...

Epoxy-OM (organo-montmorillonite) nanocomposites have been synthesized, and their permeability to oxygen and water vapor has been measured. The chemical structure of the organic monolayer ionically bonded to the montmorillonite surface has been varied, and its influence on the swelling, intercalation, and exfoliation behavior of the OM has been studied. Exfoliated aluminosilicate layers build a barrier for the permeating gas molecules, while the polymer intercalated tactoids do not contribute much to the permeation barrier performance. The gas permeation through the composites was correlated to the volume fraction of the impermeable inorganic part of the OM. The incorporation of small volume fractions of the platelike nanoparticles in the polymer matrix decreased its permeability coefficient when the interface between the two heterogeneous phases was properly designed. Long alkyl chains enhanced the polymer intercalation but increased the permeability coefficient probably due to phase separation at the in...

By activating Si-H bonds, poly(hydromethylsiloxane) can be covalently bound in a first step to various metal or polymer surfaces. In a second step, unreacted Si-H bonds can be brought to react with organic compounds having adequate functional groups such as double or triple bonds, carbonyl or hydroxyl groups. This scheme is used to bind biorecognition groups to solids. The novel concept is demonstrated by attaching a newly synthesized biotin derivative to Au. It is shown that the immobilized biotin is capable of binding streptavidin.

Calcium carbonate fillers are usually coated with stearic acid to reduce their surface energy and improve their dispersion in polymers. Commercial products are often over-coated and contain an excess of surfactant. It was found that stearic acid linearly increases the modulus and yield stress of LDPE but reduces its tensile strength, yield strain, and ultimate elongation. The influence of surfactant excess on the tensile properties of low-density polyethylene (LDPE)–CaCO 3 composites was investigated. Compounds of LDPE and optimally coated filler or with excess surfactant were prepared and their properties compared. CaCO 3 increased the stiffness and yield stress of the polymer but reduced all its other tensile properties. Over-coating the filler did not lead to linear accumulation of the effects of filler and stearic acid on the polymer matrix. In fact, surfactant excess amplifies the reinforcing effect on the stiffness but reduces all other mechanical properties of the composite. Calcium stearate, which is sometimes used as acid scavenger, lubricant or processing aid, has the same effect on the polymer properties as stearic acid, but to a smaller extent. It is concluded that it is most advantageous to coat the filler with the optimal amount of surfactant necessary to cover its surface with an organic monolayer unless the influence of excessive coating is required for a certain application. Care must also be taken in interpreting some of the published results, where the quality of the filler coating was not investigated.

Adhesive nanocomposites of organically modified montmorillonite (OM) and polyurethane have been synthesized and their permeability to oxygen and water vapor has been measured. The gas permeation through the composites was correlated to the volume fraction of the impermeable inorganic part of the OM. The incorporation of small volume fractions of the platelike nanoparticles in the polymer matrix decreased the gas transmission rate, when the interface between the two heterogeneous phases was properly designed. The oxygen transmission rate decayed asymptotically with increasing aluminosilicate volume fraction and a 30% reduction was achieved at 3 vol %, when the clay was coated with bis(2-hydroxyethyl) hydrogenated tallow ammonium or alkylbenzyldimethylammonium ions. In contrast, coating the clay surface with dimethyl dihydrogenated tallow ammonium ions leads to an increase in the gas transmission rate with augmenting inorganic fraction. This was attributed to a probable change in morphology resulting from phase separation at the interface between the apolar pure hydrocarbon clay coating and the relatively polar PU. The water vapor permeation through the PU nanocomposites was more strongly reduced than oxygen and a 50% reduction was observed at 3 vol % silicate fraction. This was attributed to stronger interactions and hydrogen bonding of the water molecules with the PU matrix as well as to their clustering. Differences in the hydrophobicity of the clay coating influenced the water transmission rate. No spectroscopic evidence could be obtained for a reaction between the hydroxyl groups of the clay organic coating and the isocyanate groups of the prepolymer. A mixed morphology, that is, exfoliated layers and intercalated particles was observed in all composites. WAXRD and TEM gave a qualitative picture of the microstructure of the nanocomposites but no conclusive information. Some of the problems to be solved before a correlation between the nanocomposite properties and their microstructure can be established have been outlined.

Thermogravimetric analysis was used to investigate the surface cleanliness of calcite fillers and to determine the optimal amount of fatty acid needed to coat the particles with an organic monolayer. The use of excess surfactant led to the formation of a bilayer as well as to the presence of free acid molecules. Both species could be detected by TGA. Optimal coating of calcite with stearic acid gave a monolayer of calcium stearate bicarbonate in which one acid molecule is attached to every Ca2+ present on the surface. The alkyl chains in the monolayer are vertically oriented to the surface and are restricted in their motion. 13C NMR and IR spectroscopy revealed that the chain conformation shows a high trans population, which leads to an ordered solidlike phase. Monolayers of saturated fatty acids with long alkyl chains (≥C10) showed similar behavior, while the lower homologues (≥C10) gave monolayers with dynamically disordered phases. The immobilization of oleic acid molecules by tethering them to the cal...

With a view to the miniaturization of ion-selective electrodes (ISEs), thin (10−20 μm) polymer membranes are directly contacted to Au covered with a redox-active, lipophilic self-assembled monolayer (SAM). Several homogeneous and mixed monolayers are characterized by reflection−absorption infrared spectroscopy, ellipsometry, scanning tunneling microscopy, cyclic voltammetry, and contact angle measurements. These Au/thiol surfaces are combined with different K+-selective sensing membranes based on poly(vinyl chloride) (PVC), polyurethane (PUR), or PVC/PUR blends as a matrix and valinomycin as an ionophore. The sensors are investigated with regard to their potential stability in the presence of O2 and redox-active species. The occurrence of potential drifts upon changing the conditioning KCl solution to a NaCl solution is used as an indicator for the formation of an aqueous film between the membrane and Au/SAM. Stable systems are obtained with mixed monolayers (advancing contact angle θa ≈ 83°) and PVC memb...

A PDMS network, synthesized from a vinyl-terminated precursor, was reinforced by plate-like montmorillonite (volclay) particles with different surface cations. The optimal ratio of crosslinker-to-PDMS precursor was ascertained from the mechanical properties of networks prepared with different crosslinker concentrations. The elastic modulus of the polymer was enhanced by the montmorillonite particles. The increase in modulus was higher in the Li– than in the Na–volclay composites. The ultimate strength of the composites was also strongly enhanced by the small platelets, especially in presence of surface Li+. The stronger influence of Li–volclay on the mechanical properties of the composites can be attributed to the partial formation of an intercalated structure, which leads to thinner particles with a high aspect ratio. Both composite strength and modulus were proportional to the filler-volume-fraction, but the increase in strength was limited by rising particle agglomeration at high loading. In contrast to organic-modified montmorillonite, the inorganic surface of volclay catalyzed the thermal degradation of PDMS. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2175–2183, 2002

In the context of atomistic simulations of solids, two situations often occur: that in which disordered structures (e.g. from “amorphous cell” simulations) are deemed to occur with essentially equal likelihood to form a “glass”, and that in which a particular orientational average over one crystal unit cell is desired, e.g. when a fiber modulus is deduced surmising that identical crystallites are oriented in the direction of the fiber axis with a specified direction of the unit cell frame while all orientations in the transverse directions are equally likely (“fiber symmetry”). The common averaging of elastic constants yields inappropriate results. We apply methods introduced by Hill and by Walpole more than three decades ago and show that with these methods, physically reasonable, self-consistent averages for elastic constants can be obtained as well as bounds considerably narrower than the well-known ones after Voigt and Reuss.

We present an approach to estimate the elastic constants of molecules and nanoparticles, based on the analysis of thermal fluctuations from Monte Carlo (MC) or molecular-dynamics (MD) atomistic simulations. The method and the force-field used for these calculations have been tested by the calculation of Young's modulus of a graphite sample along the basal plane; the calculated value was found to be 1.07 TPa, in very good agreement with the experimentally determined one of 1.02 TPa. The results on a carbon-based nanotube indicate that for the longitudinal direction of the particle, the value of the elastic constant is on the order of 400 GPa. The elastic constant of the considered nanotube in the radial direction is significantly lower, the predicted values being in the range 4–7 GPa. The method was also applied to the elastic constants of a type of siloxane-based nanostructure, whose longitudinal elastic constant (30 GPa) is an order of magnitude lower than the corresponding value for the carbon-based nanotube.

Monolayers of dialkyldimethylammonium (C10−C18) cations have been electrostatically bound to the surface of mica platelets. The structure and chain dynamics of these well defined self-assembled alkane monolayers have been probed by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy, nuclear magnetic resonance, and differential scanning calorimetry (DSC). All results indicate that at low temperatures the alkyl chains preferentially assume an all-trans conformation, leading to a highly ordered two-dimensional lattice. With increasing temperature, the all-trans conformation is gradually transformed to a mixture of trans and gauche conformers. Although the translational freedom of the chains is limited by the electrostatic binding of the headgroups to the substrate, the conformational transformation destroys the two-dimensional lattice leading to disordered molecules with a liquidlike conformation. This phase transition manifests itself in IR-absorption frequency and carbon resonance freque...

The hydrolytic degradation of DegraPolTM/btcg multiblock copolymers has been investigated in buffer solutions of different pH at 37 and 70°C. With 1H NMR spectroscopy it was demonstrated that the degradation rate of these copoly(ester urethane)s is predominantly controlled by the number and distribution of easily hydrolyzable glycolyl-glycolate ester bonds in the soft segments of the polymers. The degradation rate is largely independent of the elastic mechanical properties of the block copolymers, which are dominated by the weight fraction of hard segments. The degradation of DegraPolTM/btcg occurs in several steps. At the beginning, the molar mass decreases due to hydrolytic cleavage of the main chain in the soft segments, while the sample mass remains unaltered. In the second stage, the material looses its elasticity and the elongation at break becomes dependent on the decreasing molar mass. While the molar mass continuously decreases, the experimental contact angle in advance of water on polymer films also decreases due to chemical changes of the film surface and increasing surface roughness. The loss of mass starts at the next stage, where the degradation products predominantly come from the soft segment. The molar mass changes only very little. In the fifth stage of degradation, a decrease in sample dimensions can be observed. With prolonged degradation times, the sample mass and the average molar mass achieve nearly constant values. The remaining material then consists essentially of poly[(3-(R)-hydroxybutyrate)-co-(3-(R)-hydroxyvalerate)] hard segments. The time interval to achieve this state can be varied from a few days up to several months.

Colloidal platinum (diameter 1−2 nm) in styrene was prepared in situ by reduction of a platinum(II) compound followed by partial evaporation of styrene. The resulting dispersions were purified from the reaction side-products, and ammonium O,O‘-dialkyldithiophosphates of different chain length or octadecanethiol was added. Polymerization was started with a radical initiator. Several parameters were varied and, particularly in the presence of long-chain dialkyldithiophosphates, self-assembled superstructures of metal colloids appeared at certain conditions in the resulting poly(styrene)-platinum nanocomposites. Unusual hollow shell structures of typical diameters of 50−300 nm were found, which phenomenologically resembled bilayer vesicles in aqueous solutions, although the formation mechanism of the bilayers and the superstructures in the nanocomposites is considered to differ. It is supposed that the formation of the superstructures in the nanocomposites is induced by crystallization of alkyl chains adsorb...

A poly(dimethylsiloxane) (PDMS) network was reinforced with spherical and plate-like particles. The shape, size distribution, aspect ratio, and surface area of the particles were examined by laser diffraction, gas adsorption, cation exchange, and scanning electron microscopy (SEM). The optimal ratio of cross-linker to PDMS precursor was ascertained from the mechanical properties of networks prepared with different cross-linker concentration. Homogeneous distribution of the filler in the polymer matrix as well as complete dispersion (disaggregation) of the particles are prerequisites for studying the influence of other parameters on the mechanical properties of composites. Mica platelets increased the elastic modulus of PDMS dramatically and were much superior to glass spheres with similar diameter. The modulus rose with increasing diameter and aspect ratio as well as with the volume fraction of the particles. The ultimate strength of the composite was also better enhanced by the plate-like particles than by spheres. However, the strength enhancement was less dramatic and the dependence on the particle size was reversed compared to that of the modulus. The tensile strength increased with increasing volume fraction of the filler but the rise was probably limited in the case of large particles by the emerging agglomeration at high loading. The ultimate elongation of the mica composites was comparable to that of glass spheres. The polymer chains did not intercalate the aluminosilicate layers of mica, as expected.

A new algorithm for generating starting polymer structures for molecular simulations (e.g., MD) in dense phase is presented. The algorithm yields structures that fulfill to a large extent rotational isomeric state (RIS) probabilities and avoid atomic overlap. The heuristic search bases on the new parallel-rotation (ParRot) technique. We tested the performance of the algorithm on two polymeric systems: Atomistic polyethylene and polystyrene. The algorithm permits to tackle the problem of packing chains into large boxes of size up to 50 A in a couple of hours on common workstations. Moreover, our packing algorithm is applicable for general polymer systems. The algorithm requires CPU effort scaling with a power 2.8 in the chain length, and with a power 1.5 in the number of chains.

We develop and test a new elementary Monte Carlo move for use in the off-lattice simulation of polymer systems. This novel Parallel-Rotation algorithm (ParRot) permits moving very efficiently torsion angles that are deeply inside long chains in melts. The parallel-rotation move is extremely simple and is also demonstrated to be computationally efficient and appropriate for Monte Carlo simulation. The ParRot move does not affect the orientation of those parts of the chain outside the moving unit. The move consists of a concerted rotation around four adjacent skeletal bonds. No assumption is made concerning the backbone geometry other than that bond lengths and bond angles are held constant during the elementary move. Properly weighted sampling techniques are needed for ensuring detailed balance because the new move involves a correlated change in four degrees of freedom along the chain backbone. The ParRot move is supplemented with the classical Metropolis Monte Carlo, the Continuum-Configurational-Bias, and Reptation techniques in an isothermal–isobaric Monte Carlo simulation of melts of short and long chains. Comparisons are made with the capabilities of other Monte Carlo techniques to move the torsion angles in the middle of the chains. We demonstrate that ParRot constitutes a highly promising Monte Carlo move for the treatment of long polymer chains in the off-lattice simulation of realistic models of dense polymer systems.

Self-assembled monolayers (SAMs) of 11-mercapto-1-undecanol (TOH) and mixed monolayers of TOH and dodecanethiol (TCH3) were transformed to isocyanate-bearing layers by reaction of the hydroxyl groups in the monolayers and mixed layers with 1,4-phenylene diisocyanate (PDI). PDI reacts with only one of its isocyanate groups, yielding a carbamate (urethane) group; conversion of the hydroxyl groups is nearly quantitative, and the attached phenylene isocyanate moiety is oriented perpendicularly to the surface, as indicated by IR reflection measurements at grazing incidence. The free isocyanate groups readily react under mild conditions with a variety of substances, for example, water, alcohols, and amines. Hence, the conversion of surface hydroxyl groups with PDI offers a versatile and straightforward method to modify surfaces and to facilitate further reactions with additional compounds.

The combination of the thermodynamic-integration approach and Widom's particle-insertion method is shown to be appropriate to determine the excess chemical potential of water in dense, amorphous polymer microstructures from MD simulation at an atomistically detailed level. The two-step method is applied to bisphenol-A–polycarbonate (BPA–PC) and polyvinylalcohol (PVA). The results are compared to previously published calculations on water sorption of various polyamides and show the applicability of the two-step method for the calculation of the excess chemical potential of water in a variety of polymer materials to obtain an estimate of their water sorption behavior.

Bone morphogenetic protein (BMP) is known to require a suitable carrier to induce ectopic bone formation in vivo. To evaluate the suitability of DegraPol-foam, a degradable, elastic, and highly porous polyesterurethane foam as carrier for BMP-induced bone formation, a fraction containing all the active BMPs (BMP cocktail) was combined with DegraPol-foam and implanted subcutaneously into rats. DegraPol-BMP scaffolds were found to induce osteogenesis 2 weeks after implantation as evidenced by morphological and biochemical observations. In addition, the osteoblast-compatibility of DegraPol-foam was examined here. In vitro, primary rat osteoblasts and osteoblasts from the human cell line (HFO1) attached and proliferated preferentially on the surface of the DegraPol-foam. Both cell types exhibited relatively high attachment and low doubling time that resulted in a confluent cell multilayer with spindle-shaped morphology on the surface of the foam. Osteoblasts produced high concentrations of collagen type I and osteocalcin, and expressed increasing levels of alkaline phosphatase (ALP) activity. Taken collectively, both osteoblasts from rat tibia and from the human cell line HFO1 showed high cell attachment and growth, and preserved their phenotype. The geometrical structure of DegraPol is a suitable carrier for BMP for the induction of bone formation.

Fibers of 21 rigid-rod aromatic polyamides with different substitution patterns and their aromatic rings, produced by polycondensation of functionalized p-phenylenediamine and functionalized terephthaloyl dichloride and spun from nematic solutions as described in the accompanying paper (B. H. Glomm, M. C. Grob, P. Neuenschwander, and U. W. Suter, Macromol. Chem. Phys.) were characterized by the mechanical properties most relevant for compressive failure. In particular, the torsional moduli G 0 and the axial compressive strength σ C were determined for each fiber sample before and efter employing a post-spinning heat treatment optimized to improve the degrees of orientation and the crystallinity of the fibers. The dependence of the measured values on the structural parameters of the respective polymers was studied, leading to the result that the volume of the side-chains of the studied aramids seems to influence the extent of the mechanical anisotropy of the fibers, probably through an effect on the interchain interactions. The relationship between the torsional modulus and the axial compressive strength was scrutinized in the light of the theoretical approach of DeTeresa, Allen, and Farris, and Allen, which suggests the existence of a proportionality between G 0 and σ C . In general, the results provided by our experiments are consist with this theoretical approach.

Fibers of various rigid-rod, fully aromatic polyamides, produced by polycondensation of differently substituted p-phenylenediamines and terephthaloyldichlorides, were spun from nematic solution and analyzed with wide-angle X-ray diffraction (WAXD). A post-spin heat treatment was employed to improve the degree of orientation and crystallinity of the fibers. The dominating crystal structures were similar to those found in poly(p-phenylene-terephthalamide) (PPTA) and described in the literature, i. e., “Modification I” and “Modification II”. With the exception of only one case, the investigated fibers do not suffer a structural transformation upon heat treatment comparable to that found in PPTA. Steric and electronic effects of the ring substitution lead in most cases to a small disturbance of the solid-state structure in the fibers. Major structural changes caused by ring substitution are rare. Effects of the ring substitution pattern in the chains on the fiber crystal structure, their structural order, and their orientational properties could be observed. While the substitution of the aromatic rings seems to affect the competition between the basic crystal structures “Modification I” and “Modification II” (in analogy to PPTA) and the observed crystal structures agree with the results of detailed atomistic modeling predictions, no simple pattern seems to exist that would help predict, which candidate crystal structure is most stable.

Two different, novel approaches to crosslink fully aromatic, rigid-rod aramid chains were studied. First, the new rigid-rod aramid poly[1,4-phenylen-2,5-bis(prop-2-ynyloxy)-terephthalamide] with an inherent viscosity of η inh = 2.74 dL/g was synthesized by low temperature polycondensation of p-phenylendiamine and 2,5-bis(prop-2-ynyloxy)-terephthaloylchloride. The pendant alkinyl moieties allowed thermally induced crosslinking at temperatures higher than 200°C. No weight loss was found due to this treatment, but curing gave rise to the formation of stable radicals. However, no fiber spinning experiments were carried out using this material due to the insufficient stability of the polymer chains against degradation when being dissolved in sulfuric acid. Furthermore, fibers of a rigid rod polyamide containing pyrimidine moieties, produced by polycondensation of bis-silylated 2,5-diaminoprimidine and terephthaloyl dichloride, were spun from nematic solutions. Fibers were crosslinked by complexation with nickel(II)-ions in the swollen state. Both crosslinked and non-crosslinked, otherwise identically processed samples, were characterized by wide-angle X-ray diffraction (WAXD) measurements and mechanical tests. A post-spin heat treatment was employed to improve the low degrees of orientation and crystallinity that the untreated fibers in general showed. The dominating crystal structures of both fiber samples are similar to Modification II of the well characterized fibers from poly(p-phenylene-terephthalamide) (PPTA). The number and size of the morphological defects in the crosslinked fibers was significantly higher than in the non-crosslinked samples. The influence of the annealing on the mechanical fiber properties and the molecular order in the fibers was investigated. The values of all mechanical parameters were considerably lower in the case of the crosslinked fibers, probably due to the collapse of the entire supramolecular order and fiber morphology.

With the aim to investitgate the influence of the ring size of cyclodextrin molecules in polymeric inclusion compounds (polypseudorotaxanes, pseudopolyrotaxanes) on the behavior of the corresponding supramolecular structures at surfaces, solutions of poly(decamethylenedimethylammonium) (PDDA) alone and in the presence of α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), or both α-CD and β-CD were allowed to stand for various times during which polymeric inclusion compounds formed. The so obtained solutions were subsequently exposed to delaminated muscovite mica, the surface of which was saturated either with lithium or, occasionally, potassium ions (called Li−mica or K−mica, respectively). Li−mica was better suited for quantitative evaluations. Basically, muscovite surfaces shift the equilibrium of the inclusion complexes previously established in solution entirely or almost entirely to the side of the free components upon adsorption. However, the equlibrium was not installed after adsorption in the system PD...

The structure and phase transitions of an alkyl monolayer tethered to a mica surface have been studied by X-ray, infrared (IR) spectroscopy, and differential scanning calorimetry. All results indicate that the alkyl chains attend an all-trans conformation at low temperatures, leading to a two-dimensional crystalline film that undergoes a first-order transition to an isotropic liquid upon heating. Although the molecules are fixed to the surface at one end, which restricts their translational freedom, they undergo a melting process. It seems that the trans-gauche transformation is enough to destroy the two-dimensional lattice. A model in which the molecules assume a tilted upright position to the mica surface can explain the results obtained. To minimize the conformational entropy and maximize the packing density of the molecules, the chains attached to different mica platelets interdigitate to build an organic interlayer. A crystal−crystal transition was also observed, which led to an increase in the thick...

Liquid platinum complexes were obtained with a ligand with alkyl chains that attach to the coordination plane perpendicularly and herewith render a close packing of the molecules difficult for certain chain lengths. Bis(O,O′-dialkyldithiophosphato)platinum(II) complexes, Pt(Cxdtp)2, where the alkyl chains are methyl (x=1), ethyl (x=2), propyl (x=3), butyl (x=4), pentyl (x=5), hexyl (x=6), heptyl (x=7), dodecyl (x=12), and octadecyl (x=18), were investigated. The platinum compounds were synthesized via two routes: reaction of tetrachloroplatinate(II) with ammonium-O,O′-dialkyldithiophosphates or oxidative addition of (O,O′-dialkylthiophosphoryl)disulfides to tris(styrene)platinum(0). The melting temperatures as a function of the alkyl chain length show a minimum for the complex with pentyl groups. The complexes with pentyl and hexyl groups are liquid at room temperature, and the complexes with butyl and heptyl groups melt only slightly above room temperature. Depending on the complex, the low melting temperatures are induced by an unfavorable packing of the molecules in the solid state, reflected in a low melting enthalpy, or melting entropy. The molecular weights of the liquid complexes are between 700 and 800 and, therefore, extraordinarily high for liquid metal compounds. Depending on the alkyl groups, the complexes decompose at 172–241°C. Crystals of bis(O,O′-diethyldithiophosphato)platinum(II), Pt(C2dtp)2, are monoclinic, space group P21/n, a=8.90, b=8.70, c=12.45 A, α=γ=90°, β=100.89°, and Z=4.

Background/Aim: In an attempt to overcome some of the problems encountered with the materials available for liver embolisation, we investigated a novel radiopaque polymer of the polyurethane family (DegraBloc ® ). Methods: Hepatic artery embolisation of one liver lobe using polyurethane was performed in 19 healthy pigs. Microcirculatory changes were assessed by laser Doppler flowmetry. Radiological and pathological examinations of the livers, hearts and lungs removed provided information about the extent and effect of the embolisation. Results: None of the pigs died due to hepatic failure or toxicity of polyurethane. Microcirculation of embolised liver lobes significantly decreased from 106 (±15) perfusion units (PU) to 45 (±6) PU immediately after embolisation and further to 28 (±7) PU before euthanasia. At this time conventional and angiographic X-ray controls demonstrated the radiopaque casts extending up to the peripheral arteries with signs of degradation over time but without formation of collateral vessels. The main pathological findings consisted of destruction of the portal tract structures and also of large areas of liver necrosis. Polyurethane was encountered in arterioles as small as 10–20 μm, but not in liver sinusoids, hearts or lungs. Conclusions: The novel polymer called DegraBloc ® is a biocompatible, slowly degradable, radiopaque embolic agent. The occlusion of the arterial tree up to the smallest arteriolar diameter combined with concomitant portal vein occlusion leads to sharp segmental necrosis in pig livers without formation of significant collaterals and without systemic embolism. In the treatment of liver tumours polyurethane might provide a promising alternative to conventional embolic materials, provided that it is used with care in patients with advanced liver cirrhosis.

We present a combined quantum mechanics/molecular mechanics study of the Shpol’skii system perylene/n-hexane. The system was modeled utilizing a customized pcff-derived force field optimized with a balanced set of optimization criteria based on geometry, vibrational modes, and the energies and forces in an ensemble of molecular geometries. Spectral shifts were calculated perturbatively using the method of Shalev et al. [J. Chem. Phys. 95, 3147 (1991)]. The calculated shifts are within the experimental uncertainty of the observed 0–0 lines and allow an unambiguous assignment of the three most prominent sites. The proposed assignment differs from that of a previous study.

The local conformation and the defect Gibbs energy of stereoregularity defects in isotactic poly(propylene), i-PP, is calculated by means of the thermodynamic-integration approach. For i-PP with high isotacticity, where the atactic inclusions do not interfere with each other, we found changes in the torsion angles next to the defect, which bring the methyl unit to the position it would have in the conformation of the purely isotactic chain. The Gibbs energy of this inclusion is low and enables cocrystallization corresponding to the uniform inclusion model. For sequences of adjacent defects in the polymer chain, the local chain deformation was found to be much more complex, and a considerable defect Gibbs energy remains that makes the inclusion of atactic sequences less likely.

A poly(hydromethylsiloxane) (PHMS) was bound to aluminum, copper and steel surfaces via activation with cis-[PtCl2(PhCH [dbnd] CH2)2] in solution at room temperature. The attached polymer promotes the adhesion to two-component silicone resins where the curing process is based on catalytic hydrosilylation of olefins. In lap-shear or peel tests, cohesive failure was always observed. An example shows that the adhesive joint withstood boiling water for 200 h without considerable loss of adhesive strength. It is suggested that a small fraction of the olefinic component of the resin, e.g., a poly(dimethylsiloxane) containing some olefinic groups, is also connected with the attached PHMS via catalytic hydrosilylation, thus binding the silicone resin to the surface via the PHMS layer.

The synthesis of the covalent fullerene derivatives 2, 3 and 4 with an increasing number (2, 4 and 6) of 4-cyano-4′-oxybiphenyl mesogenic groups is described. The thermotropic behaviour of these compounds was investigated and compared with the liquid crystalline properties of the non-fullerene containing precursors 6 and 9, and the D2h-symmetric tetrakis(methano)fullerene 1 bearing eight long alkyl chains instead of mesogenic groups. Tetrakis(methano)fullerene 1 without and methanofullerene 2 with two mesogenic groups only showed an amorphous phase with glass transition temperatures Tg = 27 and 50 °C, respectively. Samples of hexakis(methano)fullerene 3 with four mesogenic groups obtained by slow crystallisation gave a nematic mesophase on the first heating (Cr 85 N 157 I) that disappeared after isotropisation. No mesogenic behaviour could be recovered for 3 in successive thermal cycles. Unlike 3, the fullerene dendrimer 4 with six 4-cyano-4′-oxybiphenyl groups showed mesogenic properties independent of the thermal history: low intensity first order phase transitions were recorded in the DSC and confirmed by optical microscopy analysis. Owing to the lack of a characteristic texture, the unambiguous identification of the mesophase was not possible. The effect of the introduction of mesogens onto C60 on the stability of the mesophases and on the preservation of liquid crystalline properties is discussed with respect to the number of mesogenic moieties per fullerene sphere.

We present a detailed experimental and modeling investigation of poly(ethylene-4,4'-biphenyl dicarboxylate) (PEBB) and the related random copolymers poly(ethylene-4,4'-biphenyldicarboxylate-co-2,6-naphthalene dicarboxylate) (PEBB/EN). We find that different polymorphs exist for the PEBB homopolymer and suggest a second crystal unit cell, different from that which is discussed in the literature. PEBB/EN copolymers, which are shown here to be random copolymers, are found to be semicrystalline for all naphthanoate concentrations. By combination of DSC, X-ray, and solid-state NMR experiments, uniform comonomer inclusion is proposed for copolymers containing a high amount of EN; for low EN concentrations, these comonomers are rejected from the crystals. By comparison of the comonomer inclusion Gibbs energy, which was estimated from experiments, to those obtained from thermodynamic integration calculations, it was found that PEBB/EN copolymers are able to cocrystallize by aggregation of like and by segregation of unlike repeat units. Once these comonomer inclusions have reached a considerable size, the average defect Gibbs energy reduces to the thermal energy.

The global orientational order that develops in polycarbonate under plastic deformation has been measured quantitatively by CSA and dipolar DECODER experiments. The results are in substantial agreement with the predictions of an affine entanglement network model. Athermal atomistic simulations, on the other hand, tend to overestimate the orientational order. The orientation behavior in glassy polycarbonate seems to be essentially the same as that in the melt.

Cation-bearing peroxide free-radical initiators were attached to mica surfaces via ion exchange and used to polymerize styrene. Polystyrene chains grafted to the mica by growth from the surface resulted. Scanning electron microscopy showed distinct droplets on the surface. The droplets appear, from their size, to each consist of a single polymer chain molecule. The density of the droplets on the surface could be varied by varying the polymerization time. The droplets could be made to coalesce into thin films by increasing the grafting density, by heating, or by solvent treatment. Upon heating, the droplets appeared to spread on the surface at temperatures well below the normal glass transition temperature of polystyrene.

Mica is a popular model substrate for surface studies due to its availability in sheet form and its molecularly flat surface. As mica has ion-exchangeable surface ions, a variety of organic moities bearing cationic groups can readily be attached to the surface via ion exchange at very high surface densities (∼1 per 50 A 2 ). We wished to investigate the preparation of polymers grafted to mica by free radical polymerization initiated from the surface. Mica powders with ultrahigh specific surface areas (∼100 m 2 g −1 ) were treated with aqueous solutions of a peroxide radical initiator bearing one cationic group to exchange the mica surface ions with initiator cations. The resulting materials were used to initiate the polymerization of styrene. Following reaction, it was found that polystyrene chains were bound to the surface. The individual chains could be imaged in the form of ‘molecular droplets’ using scanning electron microscopy (SEM). The observed kinetics of bound chain growth support the view that chain growth proceeds by propagation of surface free radicals into the liquid monomer phase, i.e. ‘growth from the surface’. The results obtained with the monocationic, surface-bound peroxide initiator strongly differ from those obtained with a dicationic surface-bound azo initiator, where growth has been shown to occur ‘to the surface’. At sufficiently high density of attached chains, the droplets overlap and thin films result.

Several poly(siloxane)s and a poly(silazane) were bound to gold surfaces by activation of Si-H bonds with cis-[PtCl(2)(PhCHdbond;CH(2))(2)]. Under the reaction conditions used here, adsorption of the poly(siloxane) is observed exclusively in presence of the platinum compound. The resulting ultrathin layers were studied with IR spectroscopy at grazing incidence reflection, X-ray photoelectron spectroscopy, contact angle measurements, and ellipsometry. The layer thicknesses (2.9-5.2 nm) are in the typical range of polymer monolayers adsorbed on gold. The wetting properties of the modified gold substrates correspond to those of the pure polymers, indicating that the gold is covered to a high extent or completely with polymer. The highest contact angles of water (115 degrees ) were observed with Si-H-terminated poly(dimethylsiloxane) and are among the highest reported for an ultrathin layer on gold. We assume that the activator cleaves Si-H bonds. The polymers might be bound to the surface by Au-Si bonds or via Pt-Si species. Copyright 1999 Academic Press.

Substituted poly(p-phenylene)s were adsorbed from solution onto gold and copper and oligo(p-phenylene)s onto gold. The layers were investigated with IR spectroscopy at grazing incidence reflection, XPS, NEXAFS, ToF-SIMS, surface profilometry, AFM, SEM, optical microscopy, ellipsometry, and contact angle measurements to examine their formation and structure. The structure and the properties of the investigated layers depend not only on the chemical structure of the polymer but also on the type of substrate. On gold, the polymers form layers of 15−25 A in thickness and the oligomers of ca. 5 A in thickness. On copper, “thick” layers of up to 900 A were also observed. The oligomers have a lower affinity to gold than the polymers. Mixed octadecanethiol−polymer layers were prepared by immersion of polymer-coated substrates in an octadecanethiol solution or by exposure of self-assembled monolayers of octadecanethiol to polymer solutions. The structure of the mixed layers depends on the sequence of the exposure ...

Model muscovite with high cation exchange capacity was prepared, the surface of which was saturated with a single species of alkali metal ions. The kinetics and thermodynamics of the exchange reaction of the 1-dodecylpyridinium ion (NDP+) with alkali metal ions onto that substrate were studied. The exchange rate depended on the type of alkali metal ions present on the surface because of their different affinities to mica. However, in all cases the reaction was fast at the beginning and about 50% of the ions were exchanged within one hour; then the reaction rate decreased and equilibrium was only reached after several hours. This was attributed to a rate-determining rearrangement step in which the alkyl chains rearrange to adopt a dense packing. The reactivity of the alkali metal ions was in the order Li+Na+K+Rb+, Cs+, and in the case of K+, Rb+, and Cs+, equilibrium was only reached after 72 h. The lithium and sodium ions were exchanged almost quantitatively until a saturation value was nearly reached, while the K+, Rb+, and Cs+ exchange isotherms were less steep. The equilibrium constants (K) as well as the ion exchange capacity (S) were calculated by least-squares fits. Since K is infinite for quantitative exchange and decreases asymptotically upon deviation from this ideal behavior, the high K values (10) of the NDP+/Li+ and Na+ exchange cannot be accurately determined. K ranges between 1 and 3 for the NDP+/K+, Rb+, and Cs+ exchange. The affinity of NDP+ to muscovite was similar or slightly higher than that of K+, Rb+, and Cs+, but was much higher than that of Li+ and Na+. The presence of oxonium ions in water did not strongly influence the exchange reaction on delaminated mica, as in the case of mica sheets, due to its high CEC. Copyright 1999 Academic Press.

Mica powders with ultrahigh specific surface areas (ca. 100 m2/g) were treated with aqueous solutions of peroxide radical initiators bearing a single cationic group to exchange the mica surface ions with initiator cations. The stability of the initiators, the adsorption isotherms on the mica, and the adsorption equilibrium constants were measured. The resulting materials initiate the polymerization of styrene at various conditions, forming bound polymer chains in amounts up to ca. 30 wt % polymer (ca. 55 vol %) with respect to the mica. The bound chains could be removed from the surface by extraction with good solvents for poly(styrene) only when the solvent contained ions for ion exchange. On the basis of kinetic evidence, the mechanism of grafting appears to be propagation of free radicals from the surface into the monomer solution, i.e., “growth from the surface”. The results differ strongly from those previously obtained with a dicationic surface-bound azo initiator, where grafting was shown to occur ...

The defect Gibbs energy of hydroxyvalerate comonomer inclusions into the crystals made up by random copolymers of poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHB/HV) is calculated by means of the thermodynamic integration approach. The result obtained for a single inclusion is in excellent agreement with those obtained by fitting experimental melting temperature and cocrystal composition data. Lattice model calculations that cover the whole range of copolymer composition were carried out based on calculations of double inclusion, which revealed a decrease of the average defect Gibbs energy in adjacent defects. On decomposing the Gibbs energy, it is found that the configurational entropy contributes the dominant part of the defect Gibbs energy.