Rheological behavior of self-assembling PEG-beta-cyclodextrin/PEG-cholesterol hydrogels

The rheological properties of a recently developed self-assembling hydrogel system composed of beta-cyclodextrin (betaCD)- and cholesterol-derivatized 8-arm star-shaped poly(ethylene glycol) (PEG8) were investigated. To understand and predict the gel rheological properties, data fitting with the Maxwell model as well as comparing the system's concentration-dependent behavior with Cates' model for reversibly breaking chains were performed. To investigate the influence of the polymer architecture, networks were also prepared by replacing the cholesterol-derivatized 8-arm star-shaped PEG by linear bifunctional PEG-cholesterol or by using 4-arm instead of 8-arm polymers. Rheological analysis showed that the 8-arm polymer-based mixtures yielded tight viscoelastic networks, but their storage and loss moduli significantly deviated from those predicted by the Maxwell model. The scaling of the plateau moduli, relaxation times, and zero-shear viscosities with concentration for gels composed of 8-arm cholesterol- and betaCD-derivatized PEG followed a power law with exponents higher than predicted by Cates' model. On the other hand, hydrogels in which linear bifunctional PEG-cholesterol was used instead of 8-arm star-shaped PEG-cholesterol or which were based on 4-arm polymers showed a substantially better fit with the Maxwell model and reduced differences between empirical and Cates' theoretical scaling exponents. Rheological analysis also showed that the hydrogels were thermoreversible. At low temperatures, the gels showed viscoelastic behavior due to slow overall relaxation of the polymer chains. At higher temperatures, however, a reduced number of betaCD/cholesterol complexes and concomitant faster chain relaxation processes eventually led to liquid-like behavior. The relationship between temperature and the relaxation time was used to determine an activation energy of 46 kJ/mol for breaking and reptation of the polymers.     

Local Transient Jamming in Stress Relaxation of Bulk Amorphous Polymers

Bulk amorphous polymers become stretched and parallel-aligned under loading stress,and their intermolecular cooperation slows down the subsequent stress relaxation process.By means of dynamic Monte Carlo simulations,we employed the linear viscoelastic Maxwell model for stress relaxation of single polymers and investigated their intermolecular cooperation in the stress relaxation process of stretched and parallel-aligned bulk amorphous polymers.We carried out thermal fluctuation analysis on the reproduced Debye relaxation and Arrhenius fluid behaviors of bulk polymers.We found a transient state with stretch-coil coexistence among polymers in the stress relaxation process.Further structure analysis revealed a scenario of local jamming at the transient state,resulting in an entropy barrier for stretch-coil transition of partial polymers.The microscopic mechanism of intermolecular cooperation appears as unique to polymer stress relaxation,which interprets the hydrodynamic interactions as one of essential factors raising a high viscosity in bulk amorphous polymers.Our simulations set up a platform of molecular modeling in the study of polymer stress relaxation,which brought new insights into polymer dynamics and the related mechanical/rheological properties.     

Proprietes viscoelastiques des reseaux a l'etat caoutchoutique

The dynamic properties and the stress relaxation of vulcanisates of natural rubber, cis-polybutadiene and a butadiene (76,5)-styrene (23,5) copolymer are analysed in order to determine the influence of some structural parameters on the shape and position on the time scales of the rheological functions characterizing their viscoelasticity.The persistence of the energy loss at low frequencies and the corresponding high level of the distribution of relaxation times are all the more marked as these polymer networks are less cross-linked or contain a larger amount of carbon black HAF. Undoubtedly closely related to the “viscoelastic plateau” of linear polymers, this anomaly is interpreted in terms of an extension of the Rouse theory derived for bulk polymers or their concentrated solutions.     

Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

Pressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Relaxation spectrum of a polymer network with included particles: A regular cubic network model with mutual friction

A theory of the viscoelastic properties of crosslinked polymers with included particles is developed. The model of a regular cubic coarse-grain network, which suggests the viscoelastic interaction of the particles with the crosslink sites, is used. The particles are assumed to be close to isotropic, and their mobility is described via the introduction of a friction coefficient that is directly proportional to the particle radii. In the framework of this model, the spectrum of relaxation times of the network with included particles consists of two branches: One corresponds to the local displacements of the particles relative to the crosslink sites; the other describes the large-scale collective mobility of the particles along with the network fragments. At all values of the viscoelastic parameters of the model, the relative width of the relaxation-time spectrum for the network with included particles is higher than that for the initial network without included particles. This theoretical result qualitatively explains the experimental data on the mechanical and dielectric relaxations of crosslinked composites, which verify the broadening of the frequency dependences of the elasticity modulus, loss modulus, and dielectric-loss factor for the filled crosslinked polymers relative to these dependences for the initial (unfilled) polymer networks.     

Rheological study of transient networks with junctions of limited multiplicity

We theoretically study the viscoelastic and thermodynamic properties of transient gels comprised of telechelic associating polymers. We extend classical theories of transient networks so that correlations among polymer chains through the network junctions are taken into account. This extension enables us to investigate how rheological quantities such as elastic modulus, viscosity, and relaxation time are affected by the association equilibrium, and how these quantities are related to the aggregation number (or multiplicity) of the junctions. In this paper, we assume, in the conventional manner, that chains are elastically effective if both their ends are connected with other chains. It is shown that the dynamic shear moduli are well described in terms of the Maxwell model. As a result of the correlation, the reduced moduli (moduli divided by the polymer concentration) increase with the concentration, but become independent of the concentration in the high-concentration range. The fraction of pairwise junctions is larger at lower concentrations, indicating the presence of concatenated chains in the system, which decreases as the concentration increases. This leads to a network relaxation time that increases with the concentration.     

A model for the non-linear viscoelastic response and physical aging in glassy polymers

Constitutive relations are derived for the non-linear viscoelastic behavior of amorphous polymers subjected to physical aging. The model is based on the concept of temporary networks, where a viscoelastic medium is treated as a network of active chains that break and reform due to micro-Brownian motion. With reference to the Adam–Gibbs theory of cooperative relaxation, the breakage and reformation rates are assumed to depend on the current temperature and the configurational entropy, which is determined as a difference between the specific entropies of the equilibrium liquid and glass. Unlike previous studies, the model accounts for the compressibility of polymers below the glass transition temperature. Constitutive equations for viscoelastic media at finite strains are developed using the laws of thermodynamics. For small values of strains, these relationships are simplified and reduced to linear integral equations with some internal time driven by the fictive temperature and the hydrostatic stress (an extension of the KAHR model to non-linear materials). To verify the constitutive model, we determine the adjustable parameters using the data obtained in short-term creep tests and comparing the results of numerical simulation with the observations in long-term tests. Fair agreement is demonstrated for the experimental data of high-density polyethylene and poly(vinyl chloride) with the numerical predictions.     

Dynamics of hydrogen bond complexes in polymer melts

The dynamics of hydrogen bond complex formation between functional groups which are attached to a polymer chain, is studied in the molten state. The concentration of complexes in the thermodynamic equilibrium is distorted by the application of a large oscillatory strain in the nonlinear viscoelastic regime. The relaxation back to the thermodynamic equilibrium is studied as a function of the temperature in the linear viscoelastic regime. From the mechanical response the kinetic analysis can be performed using a modified Doi-Edwards theory. Using the equilibrium constants obtained from IR-spectroscopy, the rate constants for complex formation and decomplexation are obtained. The temperature dependence is equivalent to the temperature dependence of the zero shear viscosity which implies that complex formation is a diffusion-controlled process.     

Primitive chain network simulations for asymmetric star polymers

For branched polymers, the curvilinear motion of the branch point along the backbone is a significant relaxation source but details of this motion have not been well understood. This study conducts multi-chain sliplink simulations to examine effects of the spatial fluctuation and curvilinear hopping of the branch point on the viscoelastic relaxation. The simulation is based on the primitive chain network model that allows the spatial fluctuations of sliplink and branch point and the chain sliding along the backbone according to the subchain tension, chemical potential gradients, drag force against medium, and random force. The sliplinks are created and∕or disrupted through the motion of chain ends. The curvilinear hopping of the branch point along the backbone is allowed to occur when all sliplinks on a branched arm are lost. The simulations considering the fluctuation and the hopping of the branch point described well the viscoelastic data for symmetric and asymmetric star polymers with a parameter set common to the linear polymer. On the other hand, the simulations without the branch point motion predicted unreasonably slow relaxation for asymmetric star polymers. For asymmetric star polymers, further tests with and without the branch point hopping revealed that the hopping is much less important compared to the branch point fluctuation when the lengths of the short and long backbone arms are not very different and the waiting time for the branch point hopping (time for removal of all sliplinks on the short arm) is larger than the backbone relaxation time. Although this waiting time changes with the hopping condition, the above results suggest a significance of the branch point fluctuation in the actual relaxation of branch polymers.     

Full‐chain dynamics of entangled linear and star polymers

The full‐chain dynamics and the linear viscoelastic properties of monodisperse, entangled linear and star polymers are simulated consistently via an equilibrium stochastic algorithm, based on a recently proposed full‐chain reptation theory 1 that is able to treat self‐consistently mechanisms of chain reptation, chain‐length fluctuations, and constraint release. In particular, it is the first time that the full‐chain simulation for star polymers is performed without subjecting to the great simplifications usually made. To facilitate the study on linear viscoelasticity, we employ a constraint release mechanism that resembles the idea of tube dilation, in contrast to the one used earlier in simulating flows, where constraint release was performed in a fashion similar to double reptation. Predictions of the simulation are compared qualitatively and quantitatively with experiments, and excellent agreement is found for all investigated properties, which include the scaling laws for the zero‐shear‐rate viscosity and the steady‐state compliance as well as the stress relaxation and dynamic moduli, for both polymer systems. The simulation for linear polymers indicates that the full‐chain reptation theory considered is able to predict very well the rheology of monodisperse linear polymers under both linear viscoelastic and flow conditions. The simulation for star polymers, on the other hand, strongly implies that double reptation alone is insufficient, and other unexplored mechanisms that may further enhance stress relaxation of the tube segments near the star center seem crucial, in explaining the linear viscoelasticity of star polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 248–261, 2000     

Copper Adsorption on Chitosan-Derived Schiff Bases

The adsorption of Cu(II) ions onto the chitosan derived Schiff bases obtained from the condensation of chitosan with salicyaldehyde (polymer I), 2,4-dihydroxybenzaldehyde (polymer II) and with 4-(diethylamino) salicyaldehyde (polymer III) in aqueous solutions was investigated. Batch adsorption experiments were carried out as a function of contact time, pH, and polymer mass. The amount of metal-ion uptake of the polymers was determined by using atomic absorption spectrometry (AAS) and the highest Cu(II) ions uptake was achieved at pH 7.0 and by using sodium perchlorate as an ionic strength adjuster for polymers I, II, and III. The isothermal behavior and the kinetics of adsorption of Cu(II) ions on these polymers with respect to the initial mass of the polymer and temperature were also investigated; adsorption isothermal equilibrium data could be clearly explained by the Langmuir equation. The experimental data of the adsorption equilibrium from Cu(II) solution correlates well with the Langmuir isotherm equation.     

A Simple and Versatile Method for the Construction of Nearly Ideal Polymer Networks

Polymer networks usually contain numerous inhomogeneities that deteriorate their physical properties and should be eliminated to create reliable, high‐performance materials. A simple method is introduced for the production of nearly ideal networks from various vinyl polymers through controlled polymerization and subsequent crosslinking. Monodisperse star polymers with bromide end groups were synthesized by atom‐transfer radical polymerization and end‐linked with dithiol linkers using thiol–bromide chemistry. This simple procedure formed nearly ideal polymer networks, as revealed from elasticity of the formed gel and model conjugation reactions involving linear polymers. The versatility of this method was demonstrated by preparing networks of common vinyl polymers, including polyacrylates, polymethacrylate, and polystyrene. This method can be used to prepare multiple functional nearly ideal gels and elastomers and to explore fundamental aspects of polymer networks.     

Chelating ion-exchangers containing N-substituted hydroxylamine functional groups: Part I. N-aroylphenylhydroxylamines

Chelating ion exchangers containing N-carbonylphenylhydroxylamine functional groups have been synthesized and their exchange behaviour with copper, cobalt, iron, vanadium and uranium investigated. Of the two polymers described, a linear oxime-carbonyl polymer exhibited chelating capacity as a function of pH analogous to the chelates formed by BPHA. An oxime-carbonyl polymer based on polyethyleneimine had high capacities for the metal ions studied, but the principal mode of reaction was by electron donation from nitrogen atoms. The absence of co-ion in metal ion capacity studies indicates the possibility of formation of 1:2 and 1:3 metal complexes with the resin. Separations of iron(II)-iron(III) and vanadium-iron appear possible.     

多组分高分子体系动态流变学研究

根据动态流变学基本理论介绍了多组分高分子体系动态流变学行为,评述了动态流变学方法在研究高分子共混体系、嵌段共聚物体系、填充高分子体系及溶胶-凝胶体系的形态、结构方面的最新进展,认为动态流变学方法是研究多组分高分子体系形态与结构的一种有效方法.     

Dispersions of polymer ionomers: I

The principal subject discussed in the current paper is the effect of ionic functional groups in polymers on the formation of nontraditional polymer materials, polymer blends or polymer dispersions. Ionomers are polymers that have a small amount of ionic groups distributed along a nonionic hydrocarbon chain. Specific interactions between components in a polymer blend can induce miscibility of two or more otherwise immiscible polymers. Such interactions include hydrogen bonding, ion-dipole interactions, acid-base interactions or transition metal complexation. Ion-containing polymers provide a means of modifying properties of polymer dispersions by controlling molecular structure through the utilization of ionic interactions. Ionomers having a relatively small number of ionic groups distributed usually along nonionic organic backbone chains can agglomerate into the following structures: (1) multiplets, consisting of a small number of tightly packed ion pairs; and (2) ionic clusters, larger aggregates than multiplets. Ionomers exhibit unique solid-state properties as a result of strong associations among ionic groups attached to the polymer chains. An important potential application of ionomers is in the area of thermoplastic elastomers, where the associations constitute thermally reversible cross-links. The ionic (anionic, cationic or polar) groups are spaced more or less randomly along the polymer chain. Because in this type of ionomer an anionic group falls along the interior of the chain, it trails two hydrocarbon chain segments, and these must be accommodated sterically within any domain structure into which the ionic group enters. The primary effects of ionic functionalization of a polymer are to increase the glass transition temperature, the melt viscosity and the characteristic relaxation times. The polymer microstructure is also affected, and it is generally agreed that in most ionomers, microphase-separated, ion-rich aggregates form as a result of strong ion-dipole attractions. As a consequence of this new phase, additional relaxation processes are often observed in the viscoelastic behavior of ionomers. Light functionalization of polymers can increase the glass transition temperature and gives rise to two new features in viscoelastic behavior: (1) a rubbery plateau above T(g) and (2) a second loss process at elevated temperatures. The rubbery plateau was due to the formation of a physical network. The major effect of the ionic aggregate was to increase the longer time relaxation processes. This in turn increases the melt viscosity and is responsible for the network-like behavior of ionomers above the glass transition temperature. Ionomers rich in polar groups can fulfill the criteria for the self-assembly formation. The reported phenomenon of surface micelle formation has been found to be very general for these materials.     

Analyzing polymer crazing as quasifracture

This paper deals with a viscoelastic boundary element method for analyzing a polymer quasifracture usually called a craze in polymers. A time-dependent boundary stiffness is considered on the quasifracture envelope surface. The viscoelastic property of the glassy polymer is represented by a generalized Kelvin model with multiple retardation times. According to the linear viscoelastic correspondence principle, the associated elasticity solution can be solved by applying the general integral boundary element method. Then the viscoelastic solution in the time domain can be obtained by applying a collocation Laplace inversion transformation. Using these methods, the quasifracture problem composed of an isolated craze opening with time-dependent stiffness traction in a stressed rectangular plate is analyzed. The displacement profile and the stress distribution around the craze envelope surface are computed.     

An asymptotic analysis of polymer desorption and skinning

During desorption of penetrant‐saturated polymers, a glassy skin can form at the exposed surface. The associated dynamics are not purely Fickian due to viscoelastic relaxation effects in the polymer. A model is presented which captures these nonlocal effects. The motion of the glass‐rubber interface and the accumulated desorbed flux are calculated. The model also describes trapping skinning, where an increase in the driving force reduces the amount of penetrant released.     

Linear viscoelastic properties of transient networks formed by associating polymers with multiple stickers

We have developed a single-chain theory that describes dynamics of associating polymer chains carrying multiple associative groups (or stickers) in the transient network formed by themselves and studied linear viscoelastic properties of this network. It is shown that if the average number N of stickers associated with the network junction per chain is large, the terminal relaxation time τ(A) that is proportional to τ(X)N(2) appears. The time τ(X) is the interval during which an associated sticker goes back to its equilibrium position by one or more dissociation steps. In this lower frequency regime ω<1/τ(X), the moduli are well described in terms of the Rouse model with the longest relaxation time τ(A). The large value of N is realized for chains carrying many stickers whose rate of association with the network junction is much larger than the dissociation rate. This associative Rouse behavior stems from the association/dissociation processes of stickers and is different from the ordinary Rouse behavior in the higher frequency regime, which is originated from the thermal segmental motion between stickers. If N is not large, the dynamic shear moduli are well described in terms of the Maxwell model characterized by a single relaxation time τ(X) in the moderate and lower frequency regimes. Thus, the transition occurs in the viscoelastic relaxation behavior from the Maxwell-type to the Rouse-type in ω<1/τ(X) as N increases. All these results are obtained under the affine deformation assumption for junction points. We also studied the effect of the junction fluctuations from the affine motion on the plateau modulus by introducing the virtual spring for bound stickers. It is shown that the plateau modulus is not affected by the junction fluctuations.     

Evaluation of synthetic water-soluble metal-binding polymers with ultrafiltration for selective concentration of americium and plutonium

Water-soluble metal-binding polymers in combination with ultrafiltration are shown to be an effective method for selectively removing dilute actimide ions from acidic solutions of high ionic strength. The actinide-binding properties of commercially available water-soluble polymers and several polymers which have been reported in the literature were evaluated. The functional groups incorporated in the polymers were pyrrolidone, amine, oxime, and carboxylic, phosphonic, or sulfonic acid. The polymer containing phosphonic acid groups gave the best results with high distribution coefficients and concentration factors for²⁴¹Am(III) and²³⁸Pu(III)/(IV) at pH 4 to 6 and ionic strengths of 0.1 to 4.     

Linear thermoviscoelasticity and characterization of noncrystalline EPDM rubber networks

An experimental study has been made to specify how the time-temperature superposition and the linear viscoelastic characteristics vary with the degree of crosslinking for a broad class of noncrystalline peroxide-cured EPDM networks. A new, very sensitive method is applied to determine the horizontal and vertical shift functions in an independent way. All uncrosslinked samples are thermoelasticoviscously simple with horizontal shift functions a_T of the WLF type and vertical shift functions almost independent of temperature, in agreement with recent theoretical understanding. Upon crosslinking, these materials become thermoviscoelastically complex networks, but superposition can still be accomplished by assuming different temperature dependences for the relaxational strength and the equilibrium modulus. The a_T functions can be taken independent of the degree of crosslinking. The vertical shift functions b_T for the relaxational strength vary with the degree of crosslinking between theoretical predictions for uncrosslinked and perfectly crosslinked EPDM networks. The equilibrium moduli of the lightly cured networks decreases with increasing temperature, which is ascribed to the presence of interchain associations between ethylene sequences in the trans state. Upon further crosslinking, these effects gradually vanish and eventually the networks can be described as viscoelastically simple with an energy elastic contribution due to the ethylene trans-gauche transitions. The linear viscoelastic characteristics, namely the storage and loss moduli and compliances and phase-angle master curves and the relaxation and retardation spectra are discussed as a function of the degree of crosslinking. A sol/gel analysis and equilibrium swelling measurements complete the experimental characterization of three familes of five EPDM networks each.