Dispersing hairy nanoparticles in polymer melts

The dispersion of hairy nanoparticles in polymer melts of chemically identical chains was investigated as a function of both molecular weight and volume fraction. Here we provide conclusive evidence that the shape of the phase diagram is determined primarily by the ratio of the chain length of the polymer melt to the chain length of the polymeric shell structure (or hair) of the core/shell nanoparticles, and that the phase behavior of different hairy particles in various polymer melts can be superimposed into one universal graph. Other factors, including the hair density and the particle diameter, are not nearly as significant as the above-noted ratio in this phase separation. In addition, we show that there is a strong connection between the rheological dynamics of the particle-filled system and the thermodynamics of the phase separation behavior. The shear-induced nonlinearity in the particle-filled system appears to display features of a singularity near the phase transition point.     

Small-angle neutron scattering study on block and gradient copolymer aqueous solutions

The small-angle neutron scattering investigation was carried out on semi-dilute aqueous solutions of block and gradient copolymers comprising pEOVE and pMOVE, pEOVE₃₀₀-block-pMOVE₃₀₀ (Block) and p(EOVE-grad-MOVE)₆₀₀ (Grad). Here, pEOVE and pMOVE denote poly(2-ethoxyethyl vinyl ether) and poly(2-methoxyethyl vinyl ether), respectively, and the numbers indicate the degrees of polymerization. The monomer composition in the Grad had a gradient along the polymer chain. For 20.0 wt% solutions, a microphase-separated structure and physical gelation were observed both in Block and in Grad systems. In the case of the Grad system, a gradual microphase separation took place as a function of temperature via a micellization with a small radius of core, characterized by the “reel-in” process, i.e., a winding of polymer chains to the core of a micelle because of the gradient composition. On the other hand, the Block system underwent a stepwise transition with respect to temperature. The relationship between microphase separation and the rheological behavior is explained from the viewpoint of microscopic structure.     

Phase separation in randomly charged polystyrene sulphonate ionomer solutions

The dynamics of randomly charged polystyrene caesium-sulfonate ionomers in semi-dilute solutions were studied using a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS), and bulk rheology. The samples were studied in toluene solutions where the aggregation of the dipolar groups is favoured. Evidence of aggregation in dilute solution is found using DLS and SANS with both the hydrodynamic and static radius of gyration indicating that there is a contraction of the chains due to intra-chain attractive forces. SANS experiments demonstrate the evolution of the aggregates into a network structure as a function of polymer concentration. The association process is caused by the dipolar attraction between the charged groups and introduces two static correlation lengths in the mesh structure of the network; the standard semi-dilute mesh size (ξ=1.12c^{−0.72±0.03}) and an inhomogeneity length (Ξ=24c^0.58±0.05) due to micro-phase separation. The scaling of the amplitudes of the correlation lengths I₁(0)∼c^{−0.33±0.07} and I₂(0)∼c^2.0±0.4 are consistent with good solvent conditions and micro-phase separation, respectively. An imposed shear causes the break up of the micro-phase separated micellar system with a characteristic yield stress for the Bingham step-like shear thinning.     

Self-diffusion in concentrated polystyrene solutions measured by fluorescence recovery after photobleaching

A polystyrene polymer of narrow molecular weight distribution was carboxylated, then reduced, and finally esterified with NBD-aminohexanoic acid [6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminohexanoic acid]. The self-diffusion of the NBD-labelled polystyrene polymer in concentrated solutions of the unlabelled polystyrene polymer was measured by the method of fluorescence recovery after photobleaching over a concentration range from 0.017 g/ml to 0.41 g/ml at room temperature. In the semi-dilute region, the concentration dependence of diffusion coefficient was found to be in agreement with the predictions of scaling concepts.     

Computer simulation of stiff-chain polymers

A review of studies on the computer simulation of the phase behavior of various stiff-chain polymer systems is presented. Methods for calculating phase diagrams of a polymer solution in a computer experiment are discussed, including the methods of extended ensembles, entropic simulation, and the Wang-Landau algorithm to obtain the density-of-states function. The authors’ original results on studying the intramolecular orientational and spatial ordering of monomer units in a single stiff-chain macromolecule in the bulk and near a planar adsorbing surface by means of the Wang-Landau algorithm and using the bond-fluctuation lattice model are presented. Corresponding state diagrams are presented for these two cases. For systems of multiple chains, the phenomenon of nematic liquid-crystalline ordering in semi-dilute solutions in the bulk and in a planar layer is considered, and the phase diagrams for these cases are presented. A survey of the published data on some other promising directions of investigation of stiff-chain polymer systems is presented.     

Molecular thermodynamics approach on phase equilibria of dendritic polymer systems

We suggest a molecular thermodynamic framework to describe the phase behavior of dendritic polymer systems. The proposed model, which is based on the lattice cluster theory, contains correlations of molecular structure and specific interactions such as hydrogen bonding to the phase equilibria of branch-structured polymer systems. We examine liquid-liquid equilibria (LLE) of hyperbranched polymer solutions and vapor-liquid equilibria (VLE) of dendrimer solutions in the viewpoints of effects of a branched structure and specific interaction formations among endgroups of dendritic polymer and solvent molecules. We investigate VLE of dendrimer/solvent (Benzyl Ether Dendrimer/Toluene) systems by the combination of a new lattice-based model and atomistic simulation technique. The interaction energy parameters are obtained by the pairs method [Baschnagel et al., 1991] including Monte Carlo simulation with excluded volume constraint. In the pairs method [Baschnagel et al., 1991], we do not simulate the whole molecule as in molecular dynamics or molecular mechanics, but only monomer segments interacting with solvent molecules. The proposed model shows improvements in prediction for both phase equilibria (VLE and LLE) due to the branched structure and specific interaction due to endgroups at periphery of dendritic polymer molecule. Atomic simulation technique gives good result in prediction without fitting variables. Our results show that the specific interactions between the endgroup and the solvent molecule play an important role in phase behavior of the given systems.     

Characterization of spatial inhomogeneities and dynamic properties of random cross-linked polystyrene networks by dynamic light scattering

The network inhomogeneity and the cooperative motion of the network chains of random cross-linked poly(styrene-co-maleic anhydride) gels were investigated by dynamic light scattering. Measurements were performed for gels in the preparation state as well as in the swelling equilibrium. Network inhomogeneities and cooperative motion were analyzed at varying the cross-linker concentration and the polymer volume fraction. While the cross-linker concentration has only a minor influence on the inhomogeneity and the diffusion constant D_coop, the polymer volume fraction clearly influences both measured properties. The concentration dependence of D_coop can be well described by a power law, as known for semi-dilute polymer solutions. In the preparation state the networks appear homogeneous, exhibiting dynamic contributions to the scattering intensity of 70-90%. Swollen to equilibrium stage, significant heterogeneities emerge, reducing the dynamic contributions to 10-20%.     

Miscibility and rheologically determined phase diagram of poly(ethylene oxide)/poly(<Emphasis Type="Italic">ε</Emphasis>-caprolactone) blends

Poly(ethylene oxide)/poly(ε-caprolactone) (PEO/PCL) blends can be widely used in lithium rechargeable battery area or as medical materials, while the miscibility and phase diagram of the blends are still unclear. The present work attempted to establish the blends’ phase diagram using rheometry and investigated the miscibility. The results showed that a miscibility window of upper critical solution temperature character of the blends is revealed. Meanwhile, the abnormal rheological behavior of PEO at temperatures higher than 130 °C has little influence on the phase diagram determination. Different rheological properties of PEO/PCL blends from those of PEO revealed the existence of interactions between PEO and PCL molecular chains. Whereas shear-induced mixing or shear-induced phase separation might occur in phase diagram determination of PEO/PCL blends using rheometry.     

Assessment of molecular connectedness in semi-dilute polymer solutions by elongational flow

The study of the response of polymer solutions to purely elongational flow-fields, as assessed by birefringence, has been extended to the semi-dilute region. As the concentration was increased the optical effects seen gave direct indication of the onset of network behaviour above a critical strain-rate. The concentration at which such chain interactions first occur was found to be significantly lower than identified by the conventional c1 criterion. At any given concentration a time scale could be identified below which the system responds as a network and above which, as an assembly of isolated chains. This critical disentanglement time decreased with concentration, consistent with the time needed for overlapping chains to diffuse apart. On a time-scale longer than this disentanglement time, the chains display the same coil-stretch transition with increasing strain-rate as in dilute solutions, with allowance for the increased solution viscosity. On this longer time-scale the chains can slip out of each other's environment, in spite of their geometric overlap. Atactic polystyrene and poly(ethylene oxide) were compared, polystyrene showing the greater entanglement effects by the present criterion. Some reference is made to more strongly interacting systems (H bonds, ionic forces) where chains can only extend in a mutually interacting fashion.     

Macromolecular motions and hydrodynamic radius variation in dilute solutions under shear action

Understanding the dynamics of single polymer chains and rheological mechanism in dilute polymer solutions under shear stress is essential for fields such as the petroleum and food industries, biomedical materials and drug delivery. Here we present an experimental method for measuring the viscosity of polymer solutions and studying the variation of single polymer chain conformation and the mechanism of molecular motions according to the relationship between the intrinsic viscosity, [η], and the shear rate. Of striking interest is that we find that [η] changing with the shear rate presents three stages which may explain the nature of the viscoelastic performance of polymer solutions and the isolated molecular motions. The significance of these results is the finding of the polymer chain deformation to match the pore throat which has enormous potential implications in drug delivery, genetics and biomedicine © 2014 Society of Chemical Industry.     

Structure and dynamics of polymeric systems

Two problems that we encounter in the structure formation of polymeric systems are reviewed. One is the dynamics of phase separation of polymer blends and the other is the intramolecular structure formation of associating polymers. In the case of phase separation of polymer blends, we review the model and the simulation method that is suitable for large-scale computer simulations of phase separation of binary fluid mixtures. We also show that simulation results are in quantitative agreement with experimental results of polymer blends. In the case of associating polymers, we treat the intramolecular structure formation in single associating polymers. In order to study the structure formation in polymers with strong attractive interactions, we employ the multicanonical simulation method. We show that a two-step intramolecular conformational transition occurs in periodic associating polymers where associative groups are periodically placed along the chain. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and multiple flower-type micelles are formed via the transition. The number of the associative groups forming a micelle core is limited by the excluded volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition takes place. At this transition point, the two intramolecular micelles merge into one micelle.     

Ultrasonic studies of styrene-butadiene-styrene triblock copolymers

Ultrasonic measurements covering the frequency range 5–1000 MHz are reported on solid styrene-butadiene-styrene triblock copolymers and their solutions in toluene and cyclohexane. In the solid, two distinct relaxation processes were observed, corresponding to the glass transitions of the polystyrene and polybutadiene phases. Two distinct processes were observed also in the swollen solid, the relaxation peaks being shifted to lower temperatures with plasticization of the polymer by the solvent. Comparison of the changes observed with those detected in a similar mixture of the corresponding homopolymers confirms the importance of phase separation in the swollen copolymer. An increase in the high frequency attenuation of semi-dilute solutions can be associated with scattering of the sound wave by micelle structures. The temperature-concentration locus at which this scattering is first observed correlates with other observations of microphase separation in triblock copolymers. At these concentrations, the low frequency relaxation curves deviate from extrapolation from dilute solutions data indicative of significant polymer-polymer entanglement interaction.     

Temperature dependence of chain dimensions

The dimensions of linear atactic polystyrene ($\text{M}\text{?}{}_{\mathrm{w}}\text{= 75 700}$) in cyclohexane have been determined at a series of temperatures using small-angle neutron scattering. Three solutions were examined: dilute (2% polymer), semi-dilute (19% polymer) and concentrated (47% polymer). End-to-end distances obtained from the data were compared with current theories of polymer solutions. For the semi-dilute solution results agreed with scaling law predictions, whereas results from the concentrated solution agreed with the formula obtained by Edwards. Furthermore, the latter results gave a characteristic ratio (C_∞) of 9.5 ± 0.7 for polystyrene.     

Mathematical simulation of wet spinning coagulation process: Dynamic modeling and numerical results

A dynamic model of polymer wet spinning coagulation process is proposed in this article. The model is based on the double diffusion phenomenon, phase separation process, continuity balance, and momentum balance of the entire coagulation process. The uniqueness of the model lies in its dynamic feature. The model can simulate the system's dynamic response to variations in system inputs/parameters. Steady‐state system solutions can also be produced as the long‐time solutions of the dynamic model; a settling time can be observed at the same time. This paper employs a computationally efficient method of lines numerical algorithm for solving the dynamic model. A simulation experiment on a selected non‐solvent‐solvent‐polymer ternary system is carried out to verify the model as well as the numerical method. The dynamic simulation results are analyzed and discussed. At the end of the article, h‐refinement and p‐refinement are used to confirm the spatial convergence of the numerical solutions. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3432–3440, 2016     

Anomalous Viscosity Behavior in Aqueous Solutions of Hyaluronic Acid

Summary Effects of steady shear flows on intermolecular interactions in dilute and semidilute aqueous solutions of hyaluronic acid (HA) are reported. Pronounced shear thinning behavior is observed for solutions of HA at high shear rates, and no hysteresis effects are detected upon the subsequent return to low shear rates. With the aid of the asymmetric flow field-flow fractionation (AFFFF) technique, it is shown that mechanical degradation of the polymer does not take place in these shear viscosity experiments, even at high shear rates. The low shear rate viscosity of a semidilute HA solution decreases by approximately 40% when the temperature is increased from 10 °C to 45 °C. It is shown that when a dilute HA solution is exposed to a low fixed shear rate (0.001 s^-1), a marked viscosification occurs in the course of time and prominent intermolecular complexes are formed. It is argued that shear-induced alignment and stretching of polymer chains promote the evolution of hydrogen-bonded structures, where cooperative zipping of stretched chains generates a network. At a higher constant shear rate (0.1 s^-1), the viscosity decreases as time goes because of the alignment of the polymer chains, but the higher shear flow perturbation prevents the chains in dilute solutions from building up association complexes. The viscosity of an entangled HA solution is not changed in the considered time window at this shear rate, but the network structures breakdown at the highest shear rate (1000 s^-1), and then they are restored upon return to a low shear rate.     

A molecular dynamics study of the effect of ethyl branches on the orthorhombic structure of polyethylene

Molecular dynamics computer simulations are presented for polyethylene crystals containing ethyl branches. The crystals are simulated using an all-atom (explicit hydrogen) molecular mechanics force field. The effect of the branches in expanding the crystalline unit cell is demonstrated for a range of branch densities. We compare the behaviour of two types of model, each consisting of arrays of 48 chains. In the first, the polyethylene chains are effectively infinite in length, by virtue of the periodic boundary conditions, which link the polymer chains across the faces of the simulation box. In the second model, we simulate long n-alkanes. Two different chain lengths are considered, containing 24 or 48 carbon atoms. By examining the individual torsional angles and the setting angles of each segment of each chain, it is possible to demonstrate that branches are incorporated into the unit cell without the introduction of gauche defects in the polymer backbones. The effect of large numbers of branches is to expand the cell to such an extent that a mobile rotator phase is induced i.e. the system forms a dynamic rotationally disordered crystal in which chain sliding occurs readily. Although such high branch densities in the crystalline phase are not accessible experimentally, the prediction of a mesophase is interesting, because it may have implications for crystallisation. For example, the mesophase could occur transiently during crystallisation, as has been suggested for linear chains, and it would fulfil the dual role of allowing the growing crystals to thicken, and providing the branches with the opportunity to diffuse out of the crystal.     

Tailoring the morphology of self-assembled block copolymer hollow fiber membranes

Isoporous asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) hollow fiber membranes were successfully made by a dry-jet wet spinning process. Well-defined nanometer-scale pores around 20–40 nm in diameter were tailored on the top surface of the fiber above a non-ordered macroporous layer by combining block copolymer self-assembly and non-solvent induced phase separation (SNIPS). Uniformity of the surface-assembled pores and fiber cross-section morphology was improved by adjusting the solution concentration, solvent composition as well as some important spinning parameters such as bore fluid flow rate, polymer solution flow rate and air gap distance between the spinneret and the precipitation bath. The formation of the well-organized self-assembled pores is a result of the interplay of fast relaxation of the shear-induced oriented block copolymer chains, the rapid evaporation of the solvent mixture on the outer surface and solvent extraction into the bore liquid on the lumen side, and gravity force during spinning. Structural features of the block copolymer solutions were investigated by small-angle X-ray scattering (SAXS) and rheological properties of the solutions were examined as well. The scattering patterns of the optimal solutions for membrane formation indicate a disordered phase which is very close to the disorder-order transition. The nanostructured surface and cross-section morphology of the membranes were characterized by scanning electron microscopy (SEM). The water flux of the membranes was measured and gas permeation was examined to test the pressure stability of the hollow fibers.     

Conformation of polyacrylamide in aqueous solution with interactive additives and cosolvents

The viscosity of polyacrylamide (PAM) dilute aqueous solutions with NaCl, glucose, and SDS as additives was measured by Ubbelohde viscometry. There was linear relationship between reduced viscosity vs. PAM concentration in aqueous solutions. The Huggins constant k and intrinsic viscosity [η] were used to study the conformation of the polymer chains and the degree of polymer–solvent interaction. In addition, the viscosity of diluted PAM solutions in water with acetone, ethanol, DMF, and ethylene glycol as cosolvent was measured. It was found that the polymer chain conformation contracted as the acetone, ethanol, and DMF cosolvent composition ratio increased, but there was no distinguishing difference between water–ethylene glycol compositions. The solution properties of PAM were used to estimate the swelling properties of PAM gel in the same external conditions, as gel is formed by crosslinking of linear polymer. In good solvent the polymer chain should be expanded, and gel is expected to have large swelling ratio. In water cosolvent systems, when the linear polymer chain underwent coil–globule transition, PAM gel should have volume phase transition under corresponding external conditions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3122–3129, 2003     

Flow-induced chain scission as a physical route to narrowly distributed, high molar mass polymers

We present data showing a substantial narrowing of the polydispersity index (PDI) of high polymers occurring as a consequence of random chain scission events in a transient elongational flow field. In our experiments, semi-dilute aqueous solutions of high-molar mass, polydisperse polymers (PDI>1.4) were injected under pressure through an elongational flow field at the entrance of a capillary tube (i.d. 250 μm). Chain scission events occurring during multiple passes through the capillary entrance cause a marked decrease in PDI, to values as low as 1.12, along with the expected decrease of the average molar mass. The phenomenon appears to be entirely physical and independent of the chemical nature of the polymer, since similar results are obtained with polyacrylamide, polydimethylacrylamide, and poly(ethylene oxide). Statistical modeling of the evolution of the polymer molar mass distribution shows the results to be consistent with the random scission, near the mid-point, of those polymer chains that exceed a certain flow field-dependent critical chain length.     

Diffusion of rigid rodlike polymer in isotropic solutions studied by dissipative particle dynamics simulation

Dissipative particle dynamics (DPD) was employed to simulate the diffusion of rigid rodlike polymers in isotropic solutions. In a dilute solution range, the simulated diffusion behavior is in good agreement with that as described by the Kirkwood theory. In a semi-dilute range, the simulation shows that the DPD model adopting soft repulsive interactions can effectively reproduce the entanglement effect on both rotational and translational diffusions. The rotational diffusion coefficient D_r obeys the asymptotic scaling law D_r ∼ (νL³)⁻² (ν is the number of polymers per volume and L is the polymer length) for the large νL³, which corresponds to formation of a completely enclosed tube in the Doi-Edwards theory. The parallel translational diffusion coefficient D_‖ decreases with ν increase, which can be attributed to the friction effect of surrounding medium. The perpendicular translational diffusion coefficient D_⊥ decays more drastically with ν increase, which is caused by the topological constraint.