Theta Solvent Research Articles

Conformation of stereoregular PMMA in solutions during the temperature induced conformational transition

The temperature induced conformational transition of stereoregular PMMA in diluted or semi-diluted solution was studied by viscometry, NMR and FT-IR. The conformational energies of polymer chains were calculated from infrared spectral subtraction by following the method described by O’Reilly and Mosher [O’Reilly JM, Mosher RA. Macromolecules, 14 (1981) 602; O’Reilly JM, Teegarden DM, Mosher RA. Macromolecules, 14 (1981) 1693]. The fraction of sequences containing gauche conformation that can be deduced shows a large increase during the conformational transition. This increase in the probability of gauche conformations accounts for an increase in the chain flexibility deduced from the decrease of the calculated characteristic ratio. The increment in the hydrodynamic volume of the chain in chloroform that occurs at the transition temperature can be correlated with an increase in the density of the polymer–solvent specific interactions. The range of temperatures of the conformational transition, namely, 20–40°C, was found to be independent of the quality of the solvent (theta or athermic solvent). However, the cooperativity of the conformational changes σ is shown to depend on the solvent for the i-PMMA. Further, the aggregating character of the solvent (o-dichlorobenzene) shifts the observed transition for the i-PMMA solution towards higher temperatures which is ascribed to the disassociation of the aggregated structures.

Dynamic light scattering studies of atactic and syndiotactic poly(methyl methacrylate)s in dilute theta solution

The present article considers the coil-to-globule transition behavior of atactic and syndiotactic poly(methyl methacrylates), (PMMA) in their theta solvent, n-butyl chloride (nBuCl). Changes in Rh in these polymers with temperature in dilute theta solutions were investigated by dynamic light scattering. The hydrodynamic size of atactic PMMA (a-PMMA-1) in nBuCl (Mw: 2.55 × 106 g/mol) decreases to 61% of that in the unperturbed state at 13.0°C. Atactic PMMA (a-PMMA-2) with higher molecular weight (Mw: 3.3 × 106 g/mol) shows higher contraction in the same theta solvent (αη = Rh(T)/Rh (θ) = 0.44) at a lower temperature, 7.25°C. Although syndiotactic PMMA (s-PMMA) has lower molecular weight than that of atactic samples (Mw: 1.2 × 106), a comparable chain collapse was observed (αη = 0.63) at 9.0°C. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2253–2260, 1999

Kramers potential study of the Rouse-like dynamics of short alkane chains.

In this work we present a Kramers potential study of the orientational dynamics and shear viscosity of short chain alkanes. In this approach the determination of the orientational relaxation time is reduced to the calculation of static moments of single chain conformations. We study a chemically realistic alkane model that asymptotically produces Gaussian chain conformations by means of a Monte Carlo simulation. Our results are applicable to single chain descriptions of polymer melt dynamics and to the intrinsic viscosity of molecules in a Theta solvent. When we map the unknown time unit of our relaxation time result for one particular chain length and temperature to the value obtained for the same parameters from a molecular dynamics simulation of a melt of these chains, we are able to reproduce the experimental data on the chain length dependence of the melt viscosity of alkane chains at this temperature.

Hydrodynamic properties of A8B8 type miktoarm (Vergina) stars

The dilute solution properties of three (PS)8(PI)8 miktoarm (Vergina) stars were investigated by viscometry and dynamic light scattering in toluene and tetrahydrofuran (THF) (common good solvents), cyclohexane at 34.5°C (theta solvent for PS and good for PI) and dioxane at 34°C (theta solvent for PI and good for PS). Experimental intrinsic viscosity [η] and hydrodynamic radii, Rh, values in all solvents were larger for the miktoarm stars in comparison to the calculated ones using a simple model which describes the size of the copolymers as a weighted average of the sizes of the homopolymer stars with the same total molecular weight and number of arms as the copolymer. This expansion is discussed on the basis of the increased number of heterocontacts, the topological constrains imposed by the common junction point in this highly branched miktoarm architecture and the asymmetry in molecular weights of the different kinds of arms. The conformation adopted in dilute solutions can explain, to some extent, the morphological results obtained on the same materials. The ratios of viscometric to hydrodynamic radii are consistent with previous investigations on linear and star polymers and in accord with the hard sphere model. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1329–1335, 1999

On the unperturbed dimensions of perfluoropoly-(oxymethylene-co-oxyethylene)-acetals

Some solution properties of polyacetals containing poly perfluoro (oxymethylene-ran-oxyethylene) macromers (PFPE acetals) were investigated. Of the eight fractions selected dynamic light scattering in 1,1,2 trichlorotrifluoroethane and intrinsic viscosity measurements in four solvents of different thermodynamic quality and polarity, including a theta solvent, were performed. Previous results on chain flexibility of the new polymers could be reanalyzed taking into account non-negligible solvent effects. The characteristic ratio of the new PFPE-acetals was confirmed to be slightly higher than that of the corresponding fully fluorinated polyethers.

Nonequilibrium phase transitions in extreme conditions: effects of shear flow and heat flow

Phase transitions in fluids can be drastically altered by shear flow and heat flow. We briefly discuss the mechanisms of shear effects in three very different fluids. They are near-critical fluids, semidilute polymer solutions in theta solvent, and highly supercooled liquids. As regards heat flow problems, we consider near the superfluid transition, which is extremely sensitive to heat flow and gravity in the vicinity of the -point. In particular, heat flow applied from above and gravity give rise to competing effects, producing unique nonequilibrium states, in which the temperature gradient and the transition temperature gradient under gravity cancel. (i) In a normal-fluid state, the resultant temperature difference can be extremely small and can even be of order 1 nK. (ii) When a superfluid region expands upward into a normal-fluid region, we conjecture that the superfluid velocity approaches a critical velocity, leading to dense generation of vortices whose role is to produce a temperature gradient equal to .

Effect of dilution on a block copolymer in the complex phase window

The phase behavior of a styrene–isoprene (SI) diblock copolymer, with block molecular weights of 1.1 × 104 and 2.1 × 104 g/mol, respectively, is examined in the neutral solvent bis(2-ethylhexyl) phthalate (DOP) and the styrene-selective solvent di-n-butyl phthalate (DBP). DBP is a good solvent for PS, but is near a theta solvent for PI at approximately 90°C. Small-angle X-ray scattering (SAXS), rheology, and static birefringence are used to locate and identify order–order (OOT) and order–disorder transitions (ODT); all three techniques gave consistent results. The neat polymer adopts the gyroid (G) phase at low temperatures, with an OOT to hexagonally-packed cylinders (C) at 185°C, and the ODT at 238°C. Upon dilution with the neutral solvent DOP, the C window is diminished, until for a polymer concentration ϕ = 0.65, a direct G to disorder (D) ODT is observed. These results reflect increased stability of the disordered state, based on the different concentration scalings of the interaction parameter, χ, at the OOT and ODT. The OOT follows the dilution approximation, i.e., χOOT ∼ ϕ−1, but the ODT is found to follow a stronger concentration dependence, i.e., χODT ∼ ϕ−1.4, similar to the scaling of ϕ−1.6 found previously for lamellar SI diblocks in toluene and DOP. Addition of the selective solvent DBP produces dramatic changes in the phase behavior relative to DOP and the melt state; these include transitions to lamellar (L) and perforated layer (PL) structures. The observed phase sequences can be understood in terms of trajectories across the SI melt phase map (temperature vs. composition): addition of a neutral solvent or increasing temperature corresponds to a “vertical” trajectory, whereas adding a selective solvent amounts to a “horizontal” trajectory. When the solvent selectivity depends on temperature, as it does for the SI/DBP system, increasing temperature results in a diagonal trajectory. For both neutral and selective solvents the domain spacing, d*, scales with ϕ and χ as anticipated by self-consistent mean-field theory. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3101–3113, 1998

Dynamic light scattering studies of poly(4-chlorostyrene) and poly(2-chlorostyrene) in theta solvents

Most studies of coil-to-globule transitions of macromolecules have dealt exclusively with the standard polystyrene–cyclohexane system. In this work we aim to extend such investigations to halogenated polystyrenes, namely poly(4-chlorostyrene)(PPCS), poly(2-chlorostyrene)(POCS) and poly(chlorostyrene)(PCS),a mixture of 2- and 4- isomers. Changes in R h of these polymers with temperature in their respective theta solvents were investigated by dynamic light scattering. The hydrodynamic size of PPCS in n-propyl benzene( M w: 2.25×10 6 g mol −1, M w/ M n: 1.16) decreases to 75% of that in the unperturbed state at 22.2°C. POCS ( M w: 10.32×10 6 and M w: 4.30×10 6, M w/ M n: 1.10) in 2-butanone exhibited LCST behaviour and a limited chain collapse was observed with increasing temperature above theta point. The parameters α h= R h/ R h( θ) for POCS with two different molecular weights, (10.32 M and 4.30 M) were found to be 0.67 and 0.77, respectively at 37.0°C. A smooth and continuous contraction was also observed for the mixture of 2- and 4- isomers, PCS ( M w: 2.32×10 6 g mol −1, M w/ M n: 1.06).

AFM observation of single-chain PMMA particles

Atomic force microscopy (AFM) has been applied to observe single-chain polymethylmethacrylate (PMMA) particles that were stored for six months at room temperature after being sprayed from very dilute solutions in good solvents, good/poor mixed solvents, and a theta solvent. Nearly monodisperse PMMA standards of molecular weights ranging from 7.90 × 104 to 1.3 × 106 were used to investigate the effect of molecular weight (MW) on the size of the single-chain PMMA particles. These single-chain particles showed close to spherical shapes. The morphology of single-chain PMMA particles of a given molecular weight has found to be identical in spite of different solvents used for solution spraying. Molecular weight dependence of the particle dimension was also found. The diameters of single-chain PMMA particles after correction of tip-geometry effect were compared with the values estimated from molecular weight and density.

Study of the dissolution process of polystyrene in concentrated cyclohexane solution by MNR relaxation

13C-NMR relaxation times of polystyrene (PS) chains in its theta solvent, cyclohexane, have been measured at different temperatures. It was found that relaxation of carbon nuclei of the side-chain-phenyl groups and those of main chains have remarkably different temperature-dependent relaxation behaviors in the solvent. A two-step model for the dissolution process is proposed. According to the model, swelling of the polymer below θ temperature corresponds mainly to the gradual dispersion of the side-chain phenyl groups; while the complete dissolution above θ temperature corresponds mainly to the gradual dispersion of the main chains at a molecular level. These dispersions reflect the fact that cohesional interaction among side-chain-phenyl rings or main chains are weakened by solvent molecules, which shows the existence of the cohesional entanglements among polymer chains. The results of T1(C) are confirmed by the biexponential dependence of 1H-NMR spin–spin relaxation on temperature.

Dynamic light scattering and rheology of associating sulfonated polystyrene ionomers in non-polar solvents

Dynamic light scattering and reduced viscosity measurements on dilute solutions of sodium sulfonated polystyrene (NaSPS) ionomers of molecular weight 1 × 10 5 g mol −1 and sulfonation levels of 0, 0.62, 0.95 and 1.39 mol% in xylene are described and compared with earlier static light and small-angle neutron scattering studies. The unsulfonated polystyrene and 0.62 mol% ionomer are present in dilute xylene solutions as single expanded chains. The 0.95 mol% ionomer, however, forms an equilibrium between single chains and aggregates of three chains in the concentration range studied (i.e. closed association). The dimensions of both the aggregates and single chains with the latter ionomer are closer to those observed for polystyrene under theta conditions rather than in xylene. With the 1.39 mol% ionomer single chains are found to be in equilibrium with aggregates of all sizes (i.e. open association). The single chains are more compact than expected for polystyrene even in a theta solvent. The aggregates, however, are lower density structures. The viscosities of the ionomer solutions were found to be determined by the total volumes occupied by the single chains and aggregates only in very dilute solutions ( < 0.4 g dl −1). Comparison of the behaviour of these ionomers dissolved in xylene with previous rheological and light scattering studies on different ionomers dissolved in a range of other solvents suggest that the variations in the extent of single-chain collapse and aggregation behaviour with charge level observed in this work could be a common feature of randomly charged ionomers in non-polar solvents. The charge levels at which transitions between different types of behaviour occur are determined primarily by the strengths of the ionic interactions and the dielectric constant of the solvent.

Simulation of the intrinsic viscosity of hyperbranched polymers with varying topology. 1. Dendritic polymers built by sequential addition

Hyperbranched polymers obtained from sequential addition of AB2 monomers to a B3 core are simulated to determine the intrinsic viscosities [η]of hyperbranched polymers with various degrees of branching. A simple hydrocarbon model for the molecules is used together with an RIS Metropolis Monte Carlo procedure which reproduces the experimentally observed properties of linear chains in a theta solvent ([η]αM0.5) and of perfect dendrimers (a peak in the plot of log [η]versus log M). This model, applied to hyperbranched materials, suggests that they should show a similar maximum in [η] at readily achievable branching ratios. It is shown that the degree of branching, despite a recent redefinition, is not an adequate descriptor for the shape of dendritic molecules. The Wiener index of the connectivity tree of a hyperbranched structure, however, shows a near perfect power law correlation with the simulated intrinsic viscosities of molecules with the same molecular weight but different topologies.

Maleic anhydride-co-stearyl methacrylate copolymers. Synthesis, characterization, and some aspects of the solution behavior

Maleic anhydride-stearyl methacrylate copolymer was synthesized by radical polymerization. The copolymer was fractionated and characterized by dynamic and static light scattering (DLS and SLS) experiments in different concentration regimes. Seven fractions of low polydispersity index (<1.18) were studied. A typical behavior corresponding to good and theta (θ) solvents, according to the classical theories for polymers in solution, was found in the dilute regime. The results indicate in a semiquantitative way that this copolymer shows a normal behavior in the dilute regime. The rigidity parameters σ and C∞ show flexibility of the polymer chain, due to the incorporation of the maleic anhydride (MA) units, when the copolymer is compared with those corresponding to other poly(methacrylates) previously reported. In the semidilute regime and depending on the solvent nature, aggregation of the copolymer was detected. The aggregation was analyzed in terms of the values of the autocorrelation function determined by dynamic light scattering.

Apparent hydrodynamic thickness of densely grafted polymer layers in a theta solvent

We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeabil- ity to solvent flow at surface separations less than a ''hydrodynamic thickness.'' The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 247C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copoly- mer. The experimental probe was a surface forces apparatus modified to apply small- amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of solvent alone—the PS chains did not appear to contribute to dissipation over the oscillation frequencies studied. The value of the hydrodynamic thickness (RH) was less than the coil thickness (Lo) measured independently from the onset of surface-surface interactions in the force-distance profile, implying significant penetration of the velocity field into the polymer layer. As the surface-surface separa- tion was reduced from 3Lo to 0.3Lo, the apparent hydrodynamic thickness (R*) de- creased monotonically to values R* ! RH. Physically, this indicates that the ''slip plane'' moved progressively closer to the solid surfaces with decreasing surface-surface separation. This was accompanied by augmentation of the effective viscosity by a factor of up to approximately 5, indicating somewhat diminished permeability of solvent through the overlapping polymer layers. Similar results hold for the flow through surface-anchored polymers in a good solvent. It is interesting to note the strong stretch- ing of densely end-grafted polymers in a theta solvent. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2961-2968, 1997

End-Tethered Polymer Chains under AFM Tips: Compression and Escape in Theta Solvents

We study the problem of an isolated end-grafted polymer chain confined under a finite-sized flat obstacle in a ϑ solvent. For obstacles that are not too large this chain can undergo an escape transition where part of the chain remains trapped and the remainder escapes from under the obstacle. The spinodals of this system are influenced by the presence of a three-body term. We also study the problem of a chain under a tilted obstacle. We show that in this case there are two different escape possibilities. The system can then undergo the following series of transitions C → E → C → E as the chain is compressed, where C = confined and E = escaped.

Equilibrium Swelling and Elastic Modulus of End-linked Poly(dimethylsiloxane) Networks in Theta Solvent

The degree of equilibrium swelling and the dynamic Young's modulus for end-linked poly(dimethylsiloxane) networks in theta solvent are investigated as a function of polymer concentration at preparation. The experimental results agree well with the predictions of modified affine model, while they are not satisfactorily reproduced by the c* theorem.

Dilute Solutions of Diblock Copolymers in a Selective Solvent: I. Evidence for Spherical Star like Micelles

Diblock copolymer polystyrene-polyisoprene PS-PI is studied in dilute solution at room temperature in methylcyclohexane, a good solvent for polyisoprene and a theta solvent for polystyrene at T θ = 70 °C. The molecular mass of the copolymer is 1.1 x 10 6 g/mol and the polystyrene weight fraction is 36%. The diblock copolymer aggregates into star like micelles which are studied by static and quasielastic light scattering. Both techniques lead to a full characterization of the micelles. Well below the overlap concentration, the data are strongly dependent on micelle interactions. The scattered intensity (the diffusion coefficient) presents a maximum (a minimum) at a scattering vector which is concentration dependent. From this dependence we infer that the micelles are in a liquid phase, the mean distance between first neighbor being 400 nm at a concentration C = 10 -3 g/cm 3 . From these measurements we can deduce the mobility per monomer and demonstrate that the micelles move in an effective medium having the macroscopic viscosity of the solution.

Crossover behavior in the viscosity of semiflexible polymers: Intermolecular interactions as a function of concentration and molecular weight

A simple relation is presented for the viscosity of isotropic solutions of flexible, semiflexible and rodlike chains over a range of concentration from dilute solutions to undiluted polymer, chain contour length L and persistence length â. The expression is designed to account for the separate effects of screening of thermodynamic and hydrodynamic interactions, and the onset of intermolecular chain entanglements under Flory theta solvent conditions, or in ‘‘good’’ solvents, for which intramolecular excluded volume is important in dilute solutions. The relation includes a revised (dimensionless) Fox parameter Xc that determines the onset of intermolecular entanglements, where Xc=πNAρLcâ/3ML for semiflexible polymers with â≪L, with ρ the polymer density, ML the mass per unit length of the chain, and Lc the value of L for the onset of entanglement effects in undiluted polymer; Xc≊100 for a number of systems, including rodlike chains.

Properties of aqueous salt solutions of polyvinylpyrrolidone. I. Viscosity characteristics

The properties of dilute solutions of polyvinylpyrrolidone (PVP) in aqueous salt solutions have been studied by measurements of intrinsic viscosity [η] in theta and nontheta solvents. Intrinsic viscosities of the polymer solutions were observed to decrease upon addition of a variety of inorganic salts (phosphates, monohydrogen phosphates, carbonates, sulfates, and dihydrogen phosphates). Values of the Huggins constant, kH, were found to be higher than 0.35 for PVP in pure water at 298 K, whereas in salt solutions they varied from 0.85 to 1.28 in nontheta solvents. Kraemer's constant, kK, was also determined and kH-kK was calculated as 0.78 for PVP in pure water and for aqueous salt solutions of PVP, kH-kK = 0.61 at 298 K. kH values were also higher than the predicted range of 0.5–0.7, and kH-kK values were noted to be 0.73 in theta solvents. © 1996 John Wiley & Sons, Inc.

Quasielastic light scattering of polystyrene in diethyl malonate in semidilute concentration

A detailed light scattering study of polystyrene (PS) in diethyl malonate (DEM) semidilute solution has been carried out. DEM is a theta solvent for PS at 32 °C. Dynamic light scattering measurements carried out at temperatures closed to the theta temperature show only a diffusion mode in the intensity autocorrelation function; no slow modes are found in the semidilute PS/DEM solution with concentration up to more than 18 times the overlap concentration. For this system, the mutual diffusion coefficient is found to be mainly determined by the osmotic modulus, with the solution stress modulus making a negligible contribution. A slowing down of the mutual diffusion coefficient as the temperature approaches the critical temperature (at the critical concentration) is observed. These results show that despite considerable chain entanglements existing in the semidilute/concentrated PS/DEM solution, no viscoelastic slow modes are found. These results are not in agreement with the existing literature data for the same system. The absence of the viscoelastic modes is interpreted as due to the fact that PS and DEM are isopycnic and the coupling of concentration fluctuations and the solution stress modulus is negligible for isopycnic polymer solutions, in accordance with a theory recently developed in this laboratory.