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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.

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 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...

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.

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

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.

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.

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 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.

It was recently proposed (Goussev, O. A., Zeman, K. and Suter, U. W., J. Adhesion 56, 45 (1996)) to characterize the joints between materials directly by the maximum bending moment, M max, borne just prior to delamination (delamination moment). This alternative to the energy-release-rate approach was first introduced for the blister test configuration. Here we extend this idea to a cantilever beam test. We suggest, therefore, to evaluate the bending moment in the cantilever-beam experimental setup with an elastic upper plate through direct measurement of the curvature of the upper plate in the vicinity of the separation line. For the profile measurement and determination of the exact location of the delamination line, the projection-moire technique was employed. The methodology was tested on measurement of adhesion of an epoxy adhesive to steel. It was shown that the value of the maximum bending moment remains approximately constant during the delamination, indicating that this quantity is a phys...

The synthesis and characterization of fluorescence-labelled, phase segregating block polyesterurethanes based on the known biocompatible and degradable materials poly[(R)-3-hydroxybutyric acid] (PHB), poly(e-caprolactone) (PCL), and poly[(e-caprolactone)-co-glycolide] (PCL/PGA) is described. To introduce fluorescence labels in the amorphous domains of the polymers, phenacyl 10,11-dihydroxyundecanoate (PDUA) was synthesized and copolymerized with a PHB diol, a PCL diol, and aliphatic diisocyanates. After cleavage of the protecting group (without degradation of the polymer backbone), the carboxyl group was reacted with a fluorescent amine (dansylcadaverine) in the presence of 4,6-diphenylthieno[3,4-d]-1,3-dioxol-2-one 5,5-dioxide (Steglich reagent). Also a polymer containing 13 C-labelled carboxy groups was synthesized and the labelling reaction monitored by 13 C NMR spectroscopy. To introduce fluorescence labels in the hard segment of the polymers, a new low-molecular-weight diol based on poly[(R)-hydroxybutyric acid] and containing protected carboxy groups was synthesized. After the deprotection, the carboxy groups were reacted with a fluorescent amine (dansylcadaverine) in the presence of an activating system based on dicyclohexylcarbodiimide. The labelled PHB diol was then copolymerized with a poly[(e-caprolactone)-co-glycolide] diol and an aliphatic diisocyanate. The so obtained fluorescence-labelled block polyesterurethanes are promising materials for the investigation of the biocompatibility and degradation of the corresponding, non-labelled materials (DegraPol®) by means of fluorescence-aided microscopic and spectroscopic techniques.

The synthesis and characterization of fluorescence-labelled low-molecular-weight polyesters based on known biocompatible and degradable materials such as poly[(R)-3-hydroxybutyric acid] (PHB) and poly(e-caprolactone-co-glycolide) is described. A telechelic OH-terminated poly[(R)-3-hydroxybutyric acids (M a = 2300) was modified at the end groups through esterification with BOC-glycine. After removal of the BOC protection, the so-ohtained free amino groups were conjugated with dansyl chloride to obtain dansyl-labelled end groups. No degradation of the starting material was observed during the several synthetic steps, while the crystallinity of the material was not significantly affected by the modification of the end groups. Fluorescence-labelled low-molecular-weight poly(e-caprolactone) and poly(e-caprolactone-co-glycolide) were obtained by bulk ring-opening polymerization of the corresponding cyclic monomers (e-caprolactone and diglycolide) with dansylcadaverine. Both random (amorphous) and block (semi-crystalline) copolymers were obtained depending on the presence of the catalyst (Bu 2 SnO). The so ohtained fluorescence-labelled polyesters are promising materials for the investigation of the biocompatibility and degradation of block poly(ester-urethane)s (DegraPol®) with fluorescence-aided microscopic and spectroscopic techniques.

We have investigated the localchain conformation in amorphous bisphenol-A polycarbonate at the carbonate–phenylene fragment by a solid-state nuclear magnetic resonance study. Chemical-shielding anisotropy (CSA) powder patterns have been analysed, and their sensitivity to local conformation is discussed. CSA–dipole tensor correlation experiments on isotopically enriched samples were analysed in terms of dihedral-angle distributions. The comparison with atomistic simulations allowed for a critical test of the glass model and provided information for the improvement and refinement of the model.

We describe the synthesis of micro-phase-segregated block-copolyesterurethanes from telechelic hydroxyterminated poly{[(R)-3-hydroxybutyric acid]-co-[(R)-3-hydroxyvaleric acid]} (PHB-diol) as “hard segments” (i. e., crystallizable chain sections) and hydroxyterminated poly(e-caprolactone)-diethylene glycol-poly(e-caprolactone) (PCL-diol) or telechelic hydroxyterminated poly[(adipic acid)-alt-(1,4-butanediol; diethylene glycol; ethylene glycol)] (Diorez®) as “soft segments”, with 2,2,4-triethylhexamethylene diisocyanate (TMDI) or methyl (S)-2,6-diisocyanatohexanoate (LDI). High molecular weights were obtained with or without catalyst, the properties of the polymers depending only slightly on the presence or absence of the catalyst. The materials thus obtained were investigated also with respect to their mechanical properties and it was found that Young's modulus directly depends on the fraction of crystallizable PHB-diol in the block copolymer while the type of non-crystallizable segment or diisocyanate had only a minor influence: Generally, the tensile strength increases and the elongation at break decreases with increasing content of PHB-diol. Around body temperature, these polymers exhibit only mild changes in their mechanical behavior. The chain length of the non-crystallizable segment indirectly influences the morphology and the mechanical properties of the polymers through changes in the phase-segregation behavior.

We describe the preparation of telechelic OH-terminated poly[(R)-3-hydroxybutyric acid] (PHB) and poly{[(R)-3-hydroxybutyric acid]-co-[(R)-3-hydroxyvaleric acid]} (PHB/HV), on a semi-preparative scale, by a transesterification procedure from the high-molecular weight polymers. The oligomers have well-defined reactive end groups and are well suited for the preparation of high-molecular-weight block copolymers by chain extension.

We propose to characterize joints between materials by the maximum bending moment, M max, borne just prior to delamination. We suggest to evaluate M max in the blister test geometry through direct measurement of the blister curvature in the vicinity of the separation line and employ a scanning capacitance microscope for the blister profiling. The methodology and apparatus were tested on measurements of adhesion of two commercial polymer films to Plexiglas and Teflon.

We examine the vibrational density of states of atomistic models of polypropylene glasses containing a single impurity molecule of s‐tetrazine. We discuss existing methods and develop new ones to achieve significant data reduction and navigate through the complex spectrum of the normal modes of the glass. By calculating the participation ratio, the distribution of the kinetic energy of each mode on the atomic coordinates, and a mode‐proximity index to the solute it is possible to identify impurity‐related, polymer‐related, and mixed modes and assess their relative contributions to the vibrational density of states. Activation of specific modes using molecular dynamics allows the observation of anharmonicities in the doped glass, even at very low temperatures.

An effective‐potential approach is presented for improving sampling efficiency in simulations of atomistically detailed models of dense long‐chain liquids. The motion of atoms on short‐time scales is described as rapid fluctuations about the slowly moving mean conformations of the chain molecules. The distribution of these fluctuations is approximated by that of isotropic elastic motion. The interactions between nonbonded pairs of atoms are preaveraged over this distribution and a much softer, effective interaction is obtained, allowing a correspondingly faster exploration of configuration space. The approximate sampling scheme is then tested on two model systems of united‐atom liquid hydrocarbons—a melt of twenty C24 chains and one of ten C71 chains. In the C24 melt, where a comparison with fully equilibrated samples from rigorous algorithms is possible, the preaveraging is shown to produce an improvement in sampling efficiency of up to an order of magnitude at the expense of only a moderate loss in accuracy. The method is then applied to the C71 melt, where currently available, rigorous sampling algorithms fail to produce thorough equilibration. Also in this case, the preaveraging method proves to significantly enhance the sampling speed while producing satisfactory distributions of observables.

We report on the characterization of the conformational properties of the fully aromatic polyamides poly(2-fluoro-p-phenylene-2-fluoroterephthalamide) (FPFT) in methanesulfonic acid using dynamic and static light scattering and viscosity measurements. The results are in quantitative agreement with a model of polydisperse wormlike chains for the solution conformation of FPFT. A comparison with the results obtained for the unsubstituted polymer poly(p-phenylenterephthalamide) (PPTA) under the same experimental conditions indicates that the introduction of halogen substituents on the aromatic rings has only a weak influence on the chain flexibility.

A comparative study of proton-driven and radio-frequency-driven polarization-transfer processes in NMR of rare spins is presented in view of investigating order in partially disordered solids. The results exemplify the complementary use of the two techniques: proton-driven polarization transfer for recording quasi-equilibrium spectra that characterize long-range order, and radio-frequency-driven transfer for the measurement of geometrical rate factors from which information about local order is obtained. Procedures are described for the evaluation of polarization-transfer data. The techniques are applied to the study of order in amorphous samples of atactic polystyrene and syndiotactic poly(methyl methacrylate). In polystyrene, orientational correlation among the phenyl groups is found within a range of 5 A. In poly(methyl methacrylate), local order is below the detection limit.

From the comparison of experimental low‐pressure pVT data for a short alkane with the results of Monte Carlo simulations in the NpT ensemble of an atomistically detailed model of polymethylene (PM) with explicit hydrogens, we have obtained Lennard‐Jones parameters that allow accurate prediction of pVT behavior for liquid long‐chain alkanes at high pressure. The parameters were obtained from the Slater–Kirkwood formula and fitted to the experimental density of n‐pentane at 0.1 MPa; they faithfully reproduce experimental data for chains up to C23H48 (n‐tricosane) and pressures up to 100 MPa over a wide temperature interval.

2′,5′-Diamino-4-(dimethylamino)-4′-nitrostilbene was polymerized with terephthaloyl dichloride and 2,6-difluoroterephthaloyl dichloride in a low-temperature solution polycondensation to give two novel, fully aromatic rigid-rod polyamides. Nematic solutions of these polymers were processed into fibers and films that were characterized by wide-angle X-ray diffraction measurements. A post-spin annealing process was employed to enhance the chain orientation in the fibers. The dominating crystal structure was found to be similar to “modification II” of the fibers from poly(p-phenyleneterephthalamide), but the fibers do not suffer a comparable structural transformation upon heat treatment. A corona-discharge poling process gave rise to a remarkable gain in average chain orientation in the films. Again the crystal structure was found to be similar to “modification II”. The aramids investigated in this work represent a new approach to the design of liquid-crystalline rigid-rod polymers, where different mechanisms of orientation can be combined. In the nematic phase, the rigid-rod molecules form highly oriented domains that can be oriented using mechanical processes such as shearing. In addition the stilbene units that are fixed in the polymer backbone with their dipole moments oriented transverse to the main chain can be oriented in an electric field. The combination of both orientation mechanisms seems to cause a synergistic effect, probably since each affects different levels of the polymer microstructure in the solid.

We present here a short summary of two rather different, but complementary, simulations of plastic deformation in a flexible chain glassy polymer. In the first, atomistic simulation, a molecular structure model is used in which well‐established force fields between atoms and atom groups on a typical chain polymer are introduced to account for the most relevant molecular degrees of freedom that govern both structural and shear relaxations. The principal result of this simulation which has been described in great detail elsewhere [Mott et al., Philos. Mag. 67, 931–978 (1993a)] is that shear relaxations are in the form of abrupt shear transformations occurring in volume elements of ∼10 nm size and resulting in transformation shear strains of ∼2%. This simulation has established that the chemically specific conformational constraints are internal to the volume elements and that the interaction of the elements with their surroundings is elastic, suggesting that the phenomenon can be meaningfully simulated by a two‐dimensional mesoscopic model. In the second part of the present communication we summarize the most important findings of the extensive two‐dimensional mesoscopic simulation which we [Bulatov and Argon, Model. Sim. Mater. Sci. Eng. 2, 167–184 (1994a); 185–202 (1994b); 203–222 (1994c)] have performed. These findings include the important effects of disorder related misfit stresses in governing both quasihomogeneous flow at elevated temperatures and localized shear flow at lower temperatures. In addition, we demonstrate that these misfit stresses are the key ingredient that governs the initial deformation transients, as well as the well‐known distributed kinetics of the deformation process in amorphous media in general.

We investigate the efficiency of two recently introduced hybrid Monte Carlo algorithms for the off‐lattice simulation of dense polymer melts. Their performance is assessed both with respect to the molecular dynamics method and new single‐chain Monte Carlo techniques proposed recently. Our findings show that the long‐time behavior of both hybrid algorithms offer the possibility of calibration with respect to real time.

Ultrahigh specific surface area muscovite with different ions at the surface (Li+, Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+, Cu2+) was treated with aqueous solutions of low molecular weight crown ethers and polymers with crown ether substituents. The adsorption was assessed by UV analysis of the supernatant solution, and with TGA and IR spectroscopy of the mica solids. In contrast to other layered silicates, the low molecular weight crown ethers show no affinity to any of the muscovite surfaces. The polymers can adsorb, however, depending on the type of surface cation. The results indicate that at least some of the crown ether moieties are complexed to surface cations and that the diameter of the ions at the surface plays an important role in the adsorption process.

We describe a novel hybrid Monte Carlo algorithm for the off-lattice simulation of dense, atomistically detailed polymer systems. Bond lengths and bond angles are constrained to their equilibrium values and a generalized-coordinate description using the torsional degrees of freedom is employed. Simple, decoupled, Cartesian-like equations of motion are obtained by the introduction of fictitious angular momenta variables conjugate to the torsional degrees of freedom. A unique time scale, and hence a mapping onto real time, can be obtained by allocating time-independent, effective moments of inertia to each angular momentum variable.

Fibers of three rigid-rod aromatic copolymers, produced by polycondensation of 2,2,6,6-tetraoxo-1,3,5,7-tetrahydro-2,6-dithia-s-indacene-4,8-diamine (DSDA), p-phenylenediamine and terephthaloyl dichloride, were spun from nematic solutions and characterized by wideangle X-ray-diffraction (WAXD) measurements and mechanical tests. A post-spin heat treatment was employed to improve the low degrees of orientation and crystallinity exhibited generally by the untreated fibers. The dominating crystal structures of the copolymers are similar to “modification I” of the well known fibers from poly(p-phenyleneterephthalamide) (PPTA). Thermally induced crosslinking of the fibers in the solid state was performed simultaneously with post-spin treatment. The influence of the annealing conditions on the mechanical fiber properties and the molecular order in the fibers was also investigated. All mechanical parameters were time-dependent, probably due to the generation of stable radicals during heat treatment. The radicals may give rise to chain scission.

Fibers of three constitutionally isomeric rigid-rod polymers, produced by polycondensation of 2,6-dichloro-p-phenylenediamine and terephthaloyl dichloride, were spun from nematic solutions and 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 showed. The dominating crystal structure of constitutionally ordered head-to-head/tail-to-tail fibers seems to be very similar to “Modification II” of the fibers from poly(p-phenyleneterephthalamide) (PPTA), but the fibers do not suffer a comparable structural transformation upon heat treatment. Influences of the constitutional regularity in the chains on the crystal structure, structural order and mechanical properties of the fibers could be observed. Both the observed crystal structure and the experimentally determined mechanical properties agree well with the results of detailed atomistic modeling predictions.

With 2,2,6,6-tetraoxo-1,3,5,7-tetrahydro-2,6-dithia-s-indacene-4,8-diamine (DSDA) (8), a new monomer for the synthesis of thermally crosslinkable rigid-rod aramids was developed. DSDA was copolymerized with p-phenylenediamine and terephthaloyl dichloride by a low-temperature solution polycondensation to give high-molecular-weight aramids (inherent viscosity ηinh > 2,2 dL/g). The fraction of DSDA could be varied from 0 to 100 mol-% of the diamine used in the condensation. All polymers showed a weight loss due to thermal elimination of SO2. Thermal crosslinking could be effected by curing the samples at high temperatures (280–320°C). The crosslinking reaction was investigated by means of thermogravimetry (TG), thermogravimetry/mass spectrometry (TG-MS), Fourier transform infrared (FT-IR), 13C cross polarization-magic angle spinning nuclear magnetic resonance (13C CP-MAS NMR) and electron paramagnetic resonance (EPR). The activation energy for this reaction was found to be between 150 and 170 kJ/mol. Curing of the polymer samples led to the generation of stable, matrix-isolated radicals.

A detailed static atomistic model of dense, glassy polystyrene is simulated using a well established technique that previously proved successful for simple vinyl polymers. Initial chain conformations that are generated using a Monte Carlo technique including periodic continuation conditions are “relaxed” by potential energy minimization. In total 24 microstructures at densities of 1,07 g/cm3 were obtained with a cube-edge length of 18,65 A. Detailed analysis of the minimized structures indicates that intermolecular packing influences create a large variety of chain conformations different from the purely intramolecular ground states. The systems are amorphous, exhibiting random coil behavior. The described structures have been used for a quasistatic simulation of localized motions. These motions include stepwise rotation and oscillation of the phenyl groups. The frequency distribution for the simulated ring motions covers many orders of magnitude. It is very rare that an energy barrier with a reorientation angle indicating a ring “flip” is overcome. Motions with small reorientation of the phenyl rings, and therefore not leading to a ring “flip”, dominate with an average reorientation angle of 16° (±12°). The intermolecular effects of the analyzed processes were found very important and far-reaching, widely influencing the cooperative motions of molecular groups.

Conformational energies, computed with a forcefield including coulombic interactions and a simple accounting for the effects of solvent, of meso and racemic 2,4-diphenylpentane as model substances of polystyrene have been computed as functions of the skeletal torsion angles and the phenyl torsion angles. The relatively high energies of the ḡ conformations rendered these states negligible and no minimum was found in the meso-tt domain. Three minima for the meso diad (gt, tg, gg) and four minima for the racemo diad (tt, tg, gt, gg) are relevant. A two-state rotational isomeric state model is applicable with states at φt = 5°C and φg = 110° for both meso and racemo diads. Statistical weight matrices have been derived and values predicted for the characteristic ratio, the fractions of 2,4-diphenylpentane and 2,4,6-triphenylheptane at stereo-chemical equilibrium and the vicinal NMR coupling constants are found to be consistent with experimental results.

The recently introduced continuum-configurational-bias method for Monte Carlo simulations is employed for the generation of large samples of many-chain n-alkane systems with chain lengths of 11, 24 and 71 carbon atoms. The simulations are used to investigate the adequacy of representing methylene groups as united-atom Lennard-Jones interaction sites, and to test the configurational-bias approach against traditional random moves and reptation moves with respect to the computational efficiency and numerical stability of the calculated ensemble averages. The results of simulations with constant pressure, temperature, and number of molecules demonstrate that, with an appropriate mixture of different types of Monte Carlo moves, an efficient and stable strategy can be obtained. Adjustment of the Lennard-Jones parameters leads to results that are in good agreement with experimental data for the density of liquid alkanes over a large temperature interval.

A new transition‐state theory model has been proposed that is based on the concept that the dynamics of small molecules dissolved in dense polymers is coupled to the elastic thermal motion of dense polymers but can be treated separately from their structural relaxation. The model has been used to study the dynamics of light gases dissolved in atomistic microstructures of poly(isobutylene) and bisphenol‐A‐polycarbonate. Short‐time scale MD runs have been used to characterize the elastic thermal motion of the host matrix. This information on mobility is then used for a stochastic simulation of solute dynamics up to approximately 10 μs. Three time regimes have been observed: a short‐time, high‐mobility domain, a time domain of anomalous diffusion, and a diffusive regime at long times. From the long‐time data diffusion coefficients for He, H2, Ar, O2, and N2 have been estimated. Comparison with experimental data has resulted in satisfactory agreement indicating that the mechanisms of the motion of small gases...

Transition‐state theory was employed to study the dynamics of light gases dissolved in rigid microstructures of glassy polycarbonate and rubbery polyisobutylene modeled in atomistic detail. The gaseous molecules migrated through the polymer structures in a sequence of ‘‘hops’’ between local minima of the potential energy. The solute dynamics was characterized by three time domains: at short times

A Series of chains of different molecular weight of the fully aromatic polyamide poly(2,5-pyrimidinediylterephthalamide)1 (PPYMT) in the solvents 96 wt.-% sulfuric acid and methanesulfonic acid have been characterized by means of static and dynamic light scattering, dilute solution viscosimetry and measurements of the critical polymer volume fraction for the onset of a nematic phase. The experimental results are in good agreement with a model of stiff worm-like chains for the solution conformation of PPYMT, and the persistence length is estimated to be ca. 150 A (lower limit: 50–100 A).

Nanocomposites of gelatine and gold were prepared by reduction of AuCl4H in a gelatine solution, followed by a spin coating process and a brief washing. The thickness of the films thus obtained was typically 200–400 nm. The surface of the nanocomposites was investigated by scanning electron microscopy (SEM) and surface profilometry. The size of the gold particles is in the range of ca. 2–10 nm, as elucidated by transmission electron microscopy (TEM). Refractive indices were measured by ellipsometry at wavelengths of 632.8 and 1300 nm. The refractive index depends linearly on the volume fraction of gold, at least at 1300 nm. The lowest refractive indices measured are 1.008±0.060 at 1300 nm and 0.963±0.062 at 632.8 nm (limits are 95% confidence interval). These values are, to our best knowledge, the lowest reported for a polymer or polymer composite.