Theta Solvent Research Articles

Pathways and populations: stereoelectronic insights into the exocyclic torsion of 5-(hydroxymethyl)tetrahydropyran.

High level ab initio computations in vacuum and with the IEFPCM implicit solvent model are carried out on 5-(hydroxymethyl)tetrahydropyran to investigate the effects of water on the exocyclic torsional surface. Rotamer populations evaluated from the omega(C-C-C-O), theta(C-C-C-O) solvent surface agree almost quantitatively with experimental values for the closely related methyl 4-deoxy-alpha-D-xylohexopyranoside. Potentials of mean force obtained from the two surfaces show substantial solvent stabilization of the TG (omega = 180 +/- 60 degrees) rotamer and the barriers at omega= 120 and 240 degrees but solvent destabilization at the cis barrier (omega = 0 degrees). Natural bond orbital analyses indicate that energetics of these effects are largely explained by overstabilization of the vacuum GT (omega= 60 +/- 60 degrees) and GG (omega = 300 +/- 60 degrees) rotamers. Solvent stabilization of theta conformations provides entropic stabilization.

Shear small-angle light scattering studies of shear-induced concentration fluctuations and steady state viscoelastic properties

We aimed at elucidating the influence of shear-induced structures (shear-enhanced concentration fluctuations and/or shear-induced phase separation), as observed by rheo-optical methods with small-angle light scattering under shear flow (shear-SALS) and shear-microscopy, on viscoelastic properties in semidilute polystyrene (PS) solutions of 6.0 wt % concentration using dioctyl phthalate (DOP) as a Theta solvent and tricresyl phosphate (TCP) as a good solvent. In order to quantify the effects of the shear-induced structures, we conducted a numerical analysis of rheological properties in a homogeneous solution based on the constitutive equation developed by Kaye-Bernstein, Kearsley, and Zapas (K-BKZ). In the low-to-intermediate shear rate gamma region between tau(w) (-1) and tau(e) (-1), where tau(w) and tau(e) are, respectively, terminal relaxation time and the relaxation time for chain stretching, the steady state rheological properties, such as shear stress sigma and the first normal stress difference N(1), for the PS/DOP and PS/TCP solutions are found to be almost same and also well predicted by the K-BKZ equation, in spite of the fact that there is a significant difference in the shear-induced structures as observed by shear-SALS and shear-microscopy. This implies that the contribution of the concentration fluctuations built up by shear flow to the rheological properties seems very small in this gamma region. On the other hand, once gamma exceeds tau(e) (-1), sigma and N(1) for both PS/DOP and PS/TCP start to deviate from the predicted values. Moreover, when gamma further increases and becomes higher than gamma(a,DOP) (sufficiently higher than tau(e) (-1)), above which rheological and scattering anomalies are observed for PS/DOP, sigma and N(1) for PS/DOP and PS/TCP are significantly larger than those predicted by K-BKZ. Particularly, a steep increase of sigma and N(1) for PS/DOP above gamma(a,DOP) is attributed to an excess free energy stored in the system via the deformation of interface of well-defined domains, which are aligned into the stringlike structure developed parallel to the flow axis, and stretching of the chains connecting the domains in the stringlike structures. Thus, we advocate that the effect of shear-induced structures should be well considered on the behavior of sigma and N(1) at the high gamma region above tau(e) (-1) in semidilute polymer solutions.

Stability of Polydimethylsiloxane-Magnetite Nanoparticle Dispersions Against Flocculation: Interparticle Interactions of Polydisperse Materials

The colloidal stability of dispersions comprised of magnetite nanoparticles coated with polydimethylsiloxane (PDMS) oligomers was investigated theoretically and experimentally. Particle-particle interaction potentials in a theta solvent and in a good solvent for the PDMS were predicted by calculating van der Waals, electrostatic, steric, and magnetic forces as functions of interparticle separation distances. A variety of nanoparticle sizes and size distributions were considered. Calculations of the interparticle potential in dilute suspensions indicated that flocculation was likely for the largest 1% of the population of particles. Finally, the rheology of these complexes over time in the absence of a solvent was measured to probe their stabilities against flocculation as neat fluids. An increase in viscosity was observed upon aging, suggesting that some agglomeration occurs with time. However, the effects of aging could be removed by exposing the sample to high shear, indicating that the magnetic fluids were not irreversibly flocculated.

Reduced sample recovery in liquid chromatography at critical adsorption point of high molar mass polystyrene

Low sample recovery may represent an important drawback in liquid chromatography at the critical adsorption point (LC-CAP) if the critical eluent is not carefully fitted to the system. So far, this problem was often overlooked and only few experimental examples can be found in literature. We showed that in the case of polystyrene (PS) in a tetrahydrofuran (THF)/ n-hexane critical mixed eluent, PS with molar masses higher than 100 kg mol −1 were not eluted from a tandem of two columns packed by bare silica gels with 30 nm and 100 nm pore size, respectively. The polymer trapped within the columns was well recovered after injection of a small volume of pure THF as demonstrated using 2D chromatography. We studied PS conformations by means of small angle neutron scattering and found that the THF/ n-hexane critical eluent is in fact a theta solvent for PS. By replacing it by a CH 2Cl 2/ n-hexane critical mixture, which is a good solvent for PS, the limits of reduced sample recovery was displaced towards far higher molar masses. Thus, thermodynamic quality of eluent – theta or good solvent – plays an important role on the phenomenon of sample recovery.

Conformational Behavior of Comb-like Polyelectrolytes in Selective Solvent: Computer Simulation Study

In this work, we present molecular dynamics simulations of comb-like polyelectrolytes in selective solvent. The studied polymers have a neutral backbone and polyelectrolyte side chains. The solvent is poor for the backbone and the theta solvent for the side chains. The polymers are modeled on a coarse-grained level with implicit solvent. The simulations show that the comb-like polyelectrolytes tend to form intramolecular self-organized structures of the pearl necklace type. This type of conformational behavior has been predicted by Borisov and Zhulina (Borisov, O. V.; Zhulina, E. B. Macromolecules 2005, 38, 2506) for neutral comb-like copolymers in selective solvent. The present study shows that comb-like polyelectrolytes in selective solvent exhibit the same type of behavior; however, it can be controlled by one additional parameter, the degree of dissociation of the grafts. The local conformational characteristics are studied using the ensemble-averaged bond angle cosines as functions of monomer position in the chain, which reveal structural details invisible by other means.

Influence of polymer behaviour in organic solution on the production of polylactide nanoparticles by nanoprecipitation

The aim of this study was to define the parameters determining an optimized yield of monodisperse, nanosized particles after nanoprecipitation of a biodegradable polymer, with a view to industrial scale-up the process. Poly( d, l)-lactides (PLAs) from a homologous series of different molar masses were nanoprecipitated at different initial polymer concentrations from two organic solvents, acetone and tetrahydrofuran (THF), into water without surfactant according to a standardized procedure. Quasi-elastic light scattering and gel permeation chromatography with universal detection were used respectively to size the particles and to determine the molar mass distribution of the polymeric chains forming both nanoparticles and bulk aggregates. The intrinsic viscosity of the polymers as a function of molar mass and solvent were determined by kinematic viscosity measurements in organic solutions. High yields of small nanoparticles were obtained with polymers of lower molar mass (22 600 and 32 100 g/mol). For a given polymer concentration in organic solution, the particle diameter was always lower from acetone than from THF. For initial molar masses higher than 32 100 g/mol, only dilute organic solutions gave significant yields of nanoparticles. Furthermore, polymer mass fractionation occurred with increasing initial molar mass and/or concentration: the nanoparticles were formed by polymeric chains of molar masses significantly lower than the average initial one. In general, nanoparticle production was satisfactory when the initial organic solution of polymer was in the dilute rather than the semi-dilute regime. Moreover, acetone, which acted as a theta solvent for PLA, always led to smaller particles and better yields than THF.

Dynamics of end‐grafted polystyrene brushes in theta solvents

AbstractThe dynamics of the concentration fluctuations in end‐grafted polystyrene brushes in a theta solvent (cyclohexane) are probed by evanescent wave dynamic light scattering at different wavevectors q and temperatures. When the solvent quality changes from marginal to poor, the relaxation function C(q, t) exhibits strong effects as compared with the smooth variation of the brush density profile. From a single exponential above 50 °C, C(q, t) becomes a two‐step decay function. The fast decay is still assigned to the cooperative diffusion albeit slower than in the good solvent regime whereas the slow nonexponential and nondiffusive process might relate to microsegragated and/or chain dynamics in the present polydisperse brush. The relaxation function of the present three brushes with different grafting density reveals similarities and disparities between wet brushes and semidilute polymer solutions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3590–3597, 2006

Chain dimension of cyclic polymers in solutions

Telechelic polystyrenes (PSs) were synthesized and cyclized by end-to-end ring closure reaction under extremely diluted condition. Cyclic PSs were isolated from coupling products by GPC fractionation and precipitational fractionation, and 4 cyclic polymers, whose molecular weights Mw are covering the range 20k⩽Mw⩽600k, were prepared. It was confirmed that all cyclic polymers have high purity (over 95%) by HPLC. Their radii of gyration Rg were determined in benzene-d6 (good solvent) and cyclohexane-d12 (theta solvent) by small-angle neutron scattering and light scattering. It was found that Rg of cyclic polymer can be scaled with Mw as Rg∝Mw0.6 in benzene-d6 and as Rg∝Mw0.5 in cyclohexane-d12, both exponents being the same as for linear counterparts. However, experimentally determined contraction ratio, Rg(cyclic)/Rg(linear), is smaller than the theoretical predictions.

Chain contraction at the critical overlap concentration

The sedimentation of dissolved macromolecules is analyzed in both the regions of very dilute solution and in semidilute regime employing the fractal aggregate model of macromolecular coils and blobs. The macromolecules entering the overlapping region are represented by more compact aggregates than that modeling the individual macromolecule of the same mass and fractal dimension. This regularity is confirmed by analyzing the hydrodynamic behavior of several polymer–solvent systems in a wide interval of fractal dimension including the polymers dissolved in both the theta and real solvents. The reduced hydrodynamic volume of macromolecules is about 0.3–0.5 of that of the individual macromolecule. This well corresponds with the literature data obtained by measurement of the reduction of the static correlation length with concentration and those determined by dielectric study of the end-to-end distance.

Vitrification of polymer solutions as a function of solvent quality, analyzed via vapor pressures

Vapor pressures (headspace sampling in combination with gas chromatography) and glass transition temperatures [differential scanning calorimetry (DSC)] have been measured for solutions of polystyrene (PS) in either toluene (TL) (10-70 degrees C) or cyclohexane (CH) (32-60 degrees C) from moderately concentrated solutions up to the pure polymer. As long as the mixtures are liquid, the vapor pressure of TL (good solvent) is considerably lower than that of CH (theta solvent) under other identical conditions. These differences vanish upon the vitrification of the solutions. For TL the isothermal liquid-solid transition induced by an increase of polymer concentration takes place within a finite composition interval at constant vapor pressure; with CH this phenomenon is either absent or too insignificant to be detected. For PS solutions in TL the DSC traces look as usual, whereas these curves may become bimodal for solutions in CH. The implications of the vitrification of the polymer solutions for the determination of Flory-Huggins interaction parameters from vapor pressure data are discussed. A comparison of the results for TL/PS with recently published data on the same system demonstrates that the experimental method employed for the determination of vapor pressures plays an important role at high polymer concentrations and low temperatures.

Characterization of Model Branched Polymers by Multi-Detector SEC in Good and Theta Solvents

ABSTRACT Well-defined linear and branched polystyrenes with complex architectures (regular combs and centipedes) were characterized via on-line size exclusion chromatography (SEC), in the good solvent tetrahydrofuran and the theta solvent trans-decalin, in order to measure their radii of gyration (R g ), hydrodynamic radii (R H ), and intrinsic viscosities. Various measures of the sizes of these macromolecules were plotted as a function of the retention volume (V R ) in order to examine the validity of different universal calibration strategies for SEC. Hydrodynamic-based calibration curves were found to be universal, whereas a plot of log R g versus V R did not yield universal curves in either solvent. These findings are consistent with previously published data for linear and branched polymers in thermodynamically good solvents. In addition, good solvent data were used to derive values of the epsilon parameter relating radius of gyration-based and intrinsic viscosity-based chain shrinkage factors. Values of about 0.9 were found for both regular combs and centipedes.

ゴムの科学技術の展望 ゴムの非アフィン変形と単鎖の変形

Two parts of the physics of rubber elasticity have not been fully verified experimentally to date. The first one is the entropic elasticity of a single polymer chain. The second one is Affine deformation hypothesis, the bridge between molecular bases of rubber elasticity and phenomenological deformation mechanics of polymer chain network. Atomic force microscopy (AFM) was employed to find a cue to experimental evidence for them. Therefore, we did not give much attention to AFM's high-resolution feature, while its nanometer-scale palpatbility was specially utilized instead. The technique, so-called, nanofishing revealed the entropic elasticity of a single polystyrene chain in good, or theta solvent conditions. The entanglement inside a single polymer chain was detected by nanometer-scale stress-relaxation experiment with quick stepwise strain excitation. The breakdown of Affine deformation was also observed in uniaxially elongated natural rubber vulcanizate, which showed a very wide modulus distribution. We concluded tentatively that the plateau region observed in macroscopic stress-strain curve could be attributed to the remaining of the local region that had sufficiently low modulus even after stretched at the elongation ratio of around 4.

Sharp temperature dependency of 2D spreading behavior in amphiphilic block copolymer at the air–water interface

We report herein characteristic spreading behavior of amphiphilic block copolymer, polystyrene- block-poly(4-vinyl pyridine) (PS-P4VP) at the air–water interface. When the polymer was spread from cyclohexane, a theta solvent for PS and a poor solvent for P4VP, onto the air–water interface, an unexpected sharp temperature dependent change was observed in the surface pressure–area curves. In contrast, when non-theta solvents such as chloroform were used, only negligible temperature dependency was observed. In the cyclohexane solution, the aggregate size of the block copolymer strongly depends on the solution temperatures as revealed by dynamic light scattering measurements. The phenomena observed for cyclohexane should involve the coil-globule conformational transition taking place in the theta solvent. To our knowledge this work presents the first clear example that the aggregation state of polymers in solution is directly reflected to the two-dimensional spreading behavior at the air–water interface.

Bending Rigidity and Induced Persistence Length of Molecular Bottle Brushes: A Self-Consistent-Field Theory

The two-gradient version of the Scheutjens−Fleer self-consistent-field (SF-SCF) approach is employed for the analysis of the average conformations of side chains and corresponding contribution to the bending rigidity, or equivalently the induced persistence length, of molecular bottle brushes both under good and theta solvent conditions. This study is targeted to unravel conformational properties of poly(l-lysine)-graft-poly(ethylene glycol) copolymers in dilute aqueous solutions, where variation of temperature changes the solvent strength for poly(ethylene glycol) in a wide range. We focus on molecular brushes with moderate and high grafting density and large degree of polymerization of grafted chains. In this limit the predictions of an analytical mean-field theory for the dependences of the structural properties of the bottle brush on the architectural parameters are well confirmed. Both the induced persistence length and the ratio between the induced persistence length and the cross-sectional thickness of the bottle brush increase with increasing grafting density and/or increasing degree of polymerization of the grafted chains. However, in the range of moderate chain lengths and grafting densities this ratio remains small, which explains why the effects of the induced bending rigidity on the apparent persistence length have not been observed in earlier numerical experiments. We argue that only molecular bottle brushes with densely grafted long chains possess the potential for lyotropic ordering, both in solutions and at interfaces, due to the expected high effective segment asymmetry.

Prediction of coil-stretch hysteresis for dilute polystyrene molecules in extensional flow

The existence of coil-stretch hysteresis in extensional flows of long-chain polymers in dilute solutions has recently been demonstrated using enormously long deoxyribonucleic acid (DNA) molecules [Schroeder et al., Science 301, 1515–1519 (2003)]; however, there is no demonstration of hysteresis for synthetic polymers of more modest molecular size. We here use Brownian dynamics simulations of bead-spring chains to predict that the minimum molecular weight of polystyrene in a dilute theta solvent that can show a “coil-stretch hysteresis” in a uniaxial extensional flow is around 0.5 million Daltons. We find that the threshold value of the ratio of effective drag coefficients in the stretched versus coiled states, ζstretch∕ζcoil, is about 4.5 for the occurrence of hysteresis, close to the value observed for DNA.

Translational Diffusion Coefficient of Polystyrene Polymacromonomers. Dependence on Side-Chain Length

Dynamic light scattering measurements have been made on two series of polymacromonomer samples consisting of polystyrene with 33 styrene side-chain residues (F33) in cyclohexane at 34.5 °C (a theta solvent) and with 65 styrene side-chain residues (F65) in cyclohexane at 34.5 °C and toluene at 15 °C (a good solvent) to determine the translational diffusion coefficient D as functions of the weight-average degree of polymerization of the main chain, Nw, ranging from 15 to 3030 for F33 and from 42 to 1250 for F65. The D data are compared with those previously obtained for a polystyrene polymacromonomer with a shorter side chain length of 15 styrene residues. The hydrodynamic radius RH at fixed Nw is larger for a longer side-chain polymer and in toluene than in cyclohexane (for F65), reflecting the higher backbone stiffness for the longer side chain and in the good solvent. The Nw-dependence of RH and that of the reduced hydrodynamic radius (the ratio of RH to the radius of gyration ‹S2›1/2) for the respective polymacromonomers are fitted by the theoretical curves for the wormlike chain with the model parameters leading to fits of previous data for the intrinsic viscosity ([η]) and ‹S2›, provided that the chain diameters d are taken to be 1.29–1.35 times those from [η]. This discrepancy in d is similar to what was found for thin stiff chains, revealing certain shortcomings of the current hydrodynamic theories. It is pointed out that polystyrene polymacromonomers with large diameters relative to Kuhn’s segment length are hydrodynamically similar to flexible chains for which the available theories for D contain an error of about 15% in the Gaussian-chain limit.

Simulations of sieving characteristics of macromolecules in porous membranes at high concentrations

The effect of concentration on the diffusive hindrance coefficient D eff/ D ∞ and the convective reflection coefficient σ of random-coil polymers in porous membranes were evaluated. Membrane sieving of nonideal flexible polymers at higher concentrations was modeled in molecular simulations. The partition coefficient Φ of flexible chains in equilibrium with channel- or slit-like pores was computed in good and pseudo-ideal (theta) solvents as a function of coil-to-pore size ratio λ = 2 R g/ d. An increase in concentration brings about enhancement of the partitioning and the change of the partitioning curve Φ versus λ from a convex to a concave shape. Enhanced concentration brings about a considerable permeation of polymers into pores smaller than coil size ( λ > 1) and thus a shift of the cut-off separation limit of membranes. The steric retention data from simulations were combined with the hindrance functions from the hydrodynamic theory to evaluate the coefficients of hindered transport at finite concentrations. For a given coil-to pore size ratio λ, the computations predict an increase of hindered diffusion D eff/ D ∞ with concentration ϕ, in harmony with measurements of dextran and polyethyleneoxide. On the other hand, the computed reflection coefficient σ, a measure of hindered convection in ultrafiltration, decreases with ϕ for a given λ, in qualitative accord with ultrafiltration data reported for polystyrene. The reduction of the coefficient σ by concentration is more intense for random-coil solutes than for hard-sphere solutes.

Viscometric Behavior of Polystyrene in Binary Solutions Used as Eluents in Adsorption Chromatography

Calculated and experimentally measured characteristic viscosities [η] of polystyrene in some binary solvents used as eluents in chromatographic experiments in a wide range of mixture compositions were compared. The values of [η] were measured in solvents of varied thermodynamic quality, including the theta solvent and mixtures of solvents of the following compositions: two good solvents; good solvent and theta solvent; good solvent and precipitant; poor solvent and theta solvent. An equation that relates the anomalies observed in the dependences of [η] on the composition of a solvent mixture to parameters of the Flory-Huggins thermodynamic interaction was suggested.

Swelling of a polymer brush by a poor solvent

AbstractThe addition of a small amount of a poor solvent impurity (methanol) to a theta solvent (cyclohexane) is found to cause appreciable swelling (≈30% increase of the average brush height) in a model end‐grafted polystyrene (PS) brush layer. This unusual type of swelling is not observed if octadecyltrichlorosilane (OTS) is first grafted to the portion of the silicon substrate uncovered by the grafting end‐groups of the PS chains. Brush swelling in the absence of OTS surface protection is interpreted as arising from a segregation of methanol to the solid substrate and the resulting modification of the polymer–surface interaction. We also observe that the addition of a small amount of methanol to an adsorbed PS layer exposed to cyclohexane causes rapid film delamination from the silicon substrate. Together these observations imply a strong influence of surface active impurities on the structure and adhesive stability of polymer layers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4126–4131, 2004

Modeling hydrodynamic interaction in Brownian dynamics: Simulations of extensional and shear flows of dilute solutions of high molecular weight polystyrene

The nonlinear transient extensional and steady-state shear rheological properties of dilute polystyrene solutions of molecular weight 3.9 and 10.2 million in a theta solvent are predicted using Brownian dynamics (BD) simulations with the bead-spring model. Full hydrodynamic interaction is incorporated into BD simulations using the Rotne–Prager tensor. The hydrodynamic interaction parameter h* is obtained a priori by matching the drag force from a fully extended bead-spring model in extensional flow with that from Batchelor’s theory for a cylindrical rod [Hsieh et al. (2003)]. The agreement between experimental data [Gupta et al. (2000)] and simulation results for the transient Trouton ratio versus strain is good from low to medium strains. However, the plateaus at high strains predicted by the simulations are higher than measured. We also find that hydrodynamic interaction hinders the unraveling of a polymer chain in strong extensional flow (Wi≫1) due to the hydrodynamic clustering of beads. In steady she...