Polymer chain conformation in the phase separation process of a binary liquid mixture

A polymer chain conformation change near the critical point of liquid-liquid phase separation was investigated. Poly(N-isopropylacrylamide) labeled with a small amount of carbazolyl group for a fluorophore (P(NIPA-Cz)) was prepared. A ternary system of P(NIPA-Cz)+cyclohexane+methanol was investigated by the fluorescence spectroscopic technique. A mixed solvent of cyclohexane+methanol (CH/MeOH) shows phase separation at the upper critical solution temperature. Light scattering intensity, fluorescence emission intensity and fluorescence anisotropy ratio, as a function of temperature, were measured with quasi statically approaching to the critical demixing point. The fluorescence intensity of the carbazolyl groups attached to the polymer chain decreases with approaching to the critical temperature. This result suggests that the radius of gyration of the polymer decreases upon approaching to the critical demixing point of the solvent. We discuss the collapse and aggregation processes of the polymer based on the fluorescence quenching method. The rotational diffusion coefficient of carbazolyl groups attached to the polymer chain was estimated by the fluorescence depolarization technique. The rotational motion of carbazolyl groups is slowed down upon approaching the critical point.     

Interaction of cylindrical polymer brushes in dilute and semi-dilute solution

We present a systematic study of flexible cylindrical brush-shaped macromolecules in a good solvent by small-angle neutron scattering (SANS), static light scattering (SLS), and by dynamic light scattering (DLS) in dilute and semi-dilute solution. The SLS and SANS data extrapolated to infinite dilution lead to the shape of the polymer that can be modeled in terms of a worm-like chain with a contour length of 380 nm and a persistence length of 17.5 nm. SANS data taken at higher polymer concentration were evaluated by using the polymer reference interaction site model (PRISM). We find that the persistence length reduce from 17.5 nm at infinite dilution to 5.3 nm at the highest concentration (volume fraction 0.038). This is comparable with the decrease of the persistence length in semi-dilute concentration predicted theoretically for polyelectrolytes. This finding reveals a softening of stiffness of the polymer brushes caused by their mutual interaction.     

A Comprehensive Study and Characterization of Colloidal Emeraldine-Base

Soluble emeraldine-base (PANI-EB) was prepared and its structure was characterized using ¹H, ¹³C and 2D (COSY, HMQC and HMBC) NMR. The solubility of the polymer in two solvents (N-methylpyrrolidone (NMP) and dimethyl sulfoxide (DMSO), the structure of the solution (aggregation) and the behavior of polyaniline were studied by viscometry and by dynamic light scattering (DLS) in dilute solution. Viscometric behavior in NMP and Huggins constant determination confirm supramolecular interactions also evidenced by DLS; at end, the molecular weight of the soluble fraction of PANI-EB is determined.     

Entanglement and phase separation in hyperbranched polymer/linear polymer/solvent ternary blends

Hyperbranched polyethylene (HBPE)/linear polystyrene (PS)/chloroform (CF) solution was selected as a model system to investigate the effect of branching structure on entanglement and phase separation behavior in semi-dilute ternary polymer solutions. All the HBPE materials in this work were found to have similar chain architectures and the critical molecular weight was estimated to be 81.2 kDa. The results obtained by elastic light scattering and intrinsic fluorescence methods suggested that all ternary solutions exhibited UCST transition behavior upon cooling. Also, it was found that the increase in the molecular weight of PS led to increase in the phase separation rate, consistent with de Gennes prediction. However, the increase of molecular weight of HBPE did not monotonously reduce the compatibility of polymer components and the phase separation rate in ternary blends is as follows: medium molecular weight HBPE (HBPE-M) > high molecular weight HBPE (HBPE-H) > low molecular weight HBPE (HBPE-L). This abnormal behavior can be explained by the fact that, (i) for HBPE-L, no entanglements between HBPE chains occurred and the branching effect can be ignored, and (ii) for HBPE-M and HBPE-H, entanglement of HBPE chains can be formed, and the dilution of branches on entanglement of backbones should be taken into consideration, that is, the shorter the branches of HBPE, the higher the possibility of interpenetration of HBPE backbones between neighboring molecules and, consequently, the faster aggregation of HBPE during phase separation. Furthermore, a simple model based on decomposition reaction was proposed to quantitatively describe the phase separation kinetics and the apparent activation energies of phase separation were calculated to be −150.3 and −52.3 kJ/mol for HBPE-M/PS/CF and HBPE-H/PS/CF systems, respectively.     

Water effect on the gelation behavior of polyacrylonitrile/dimethyl sulfoxide solution

The gelation behavior of polyacrylonitrile (PAN)/dimethyl sulfoxide (DMSO) solution containing different amounts of water has been investigated using various methods. The ternary phase diagram of PAN/DMSO/water system indicated that water enhanced the temperature at which phase separation of PAN/DMSO solution occurred. Intrinsic viscosities [η] of dilute PAN/DMSO solution and PAN/DMSO/water solution at varied temperatures were measured to examine the influence of water on the phase behavior of PAN/DMSO solution. The presence of water in the solution gave rise to elevated critical temperature T_c. The gelation temperature T_g obtained by measuring the loss tangent tan δ at different oscillation frequencies in a cooling process was found to increase with increased water content in the solution. The critical relaxation exponent n value, however, changed little with varied concentration. During the aging process, the gelation rate of PAN/DMSO solution increases with the water level. The n values of the PAN/DMSO solutions with 2 wt% and 4 wt% water were a little larger than that of the solution without water, which may be explained by the turbid gel resulted from phase separation. The n values obtained in the aging process were larger than those obtained in the cooling process for the same three solutions, ascribed to the weaker gel with less cross-linking points formed in long time. Water led to the formation of denser gel structure. The coarser gel surface can also be attributed to the phase separation promoted by water.     

Matched ring diblock and linear triblock copolymers in dilute solution

A unique diblock copolymer ring and its linear triblock copolymer precursor composed of polystyrene and polydimethylsiloxane have been characterized by static and dynamic light scattering in dilute solution. The measurements were carried out with cyclohexane as the solvent over a temperature range of 12–35°C. Cyclohexane has the useful property that it is nearly isorefractive with the PDMS so that the PDMS block segments are invisible to the light-scattering technique and it is a theta solvent for polystyrene at 34.5°C. The block polymers in this work contain 35.1 wt % of styrene as determined by proton NMR. In the linear triblock polymer, the polystyrene is the center block with PDMS blocks on each side. Static light scattering measurements give 4.31 × 10⁴ for the average molecular weight of the whole polymer. Light scattering also shows that the apparent theta temperature for the linear triblock is shifted by 15°C to a value of 20°C at which point the second virial coefficient drops sharply and phase separation begins to induce aggregation. The diblock ring, however, shows a strongly positive second virial coefficient and no aggregation even at 12°C which is the limit of these experiments. The diffusion coefficients of cyclic diblock (D_c) and linear triblock copolymer (D₁) are measured by dynamic light scattering. The ratio of diffusion coefficients of cyclic and linear copolymers at 14.9°C and 30°C are D_c/D_l = 1.13 and 1.107 respectively. These compare well with prediction of 1.18 for this ratio from consideration of the hydrodynamics of matched linear and cyclic polymer chains. Dynamic light scattering quantitatively confirms that the linear copolymer experiences a solvent quality change near 20°C but the cyclic polymer remains in good solvent over the entire experimental temperature range. © 1993 John Wiley & Sons, Inc.     

Thermodynamic analysis on the cononsolvency of poly (vinyl alcohol) in water–DMSO mixtures through the ternary interaction parameter

Isothermal phase diagrams for the semicrystalline poly (vinyl alcohol) (PVA) in solutions composed of water and dimethylsulfoxide (DMSO) was studied at 25 °C. From the observed phase behavior, PVA was soluble in either water or DMSO individually but crystallization-induced gelation and liquid–liquid demixing were observed in water–DMSO mixtures. Flory–Huggins formalism including three binary interaction parameters and one ternary interaction parameter was used to study the phenomenon of the cononsolvency, i.e. the formation of nonsolvents by mixing two solvents. The equilibrium crystallization line in the DMSO-rich region and the total calculated binodals agreed well with the measured results when a composition-dependent ternary interaction parameter was included into calculations. In contrast, calculations yielded crystallization-induced gelation in the water-rich region, but experiments indicated that PVA remained well dissolved even 1 year after preparation. The discrepancy was explained by the temperature-induced changes in the relative interaction between water and PVA. In addition, the role of the ternary interaction parameter in the cononsolvent ternary polymer systems was discussed. It was found the contribution of the ternary interaction parameter in the cononsolvent system under study is to decline the degree of the cononsolvency. The driving force for cononsolvency is the strong interaction between water and DMSO to form the stable DMSO hydrate to exclude PVA segments in the vicinity of the hydrate.     

Dilute solution properties,chain stiffness,and liquid crystalline properties of cellulose propionate

The solution properties of cellulose derivatives are of interest from both technological and purely scientific aspects. At high concentrations these solutions form liquid crystalline structures. In dilute solution cellulosic chains can be described as semiflexible or wormlike with properties intermediate between random coils and rigid rods. A series of fractions of cellulose propionate have been examined by dilute solution viscometry, static and dynamic light scattering, and polarizing microscopy. Power law exponents are considerably larger than those observed for flexible chains and analysis of the intrinsic viscosity and hydrodynamic radii has yielded chain diameters and Kuhn statistical segment lengths. Corresponding aspect ratios from the hydrodynamic measurements are in good agreement with those obtained from polarizing microscopy, as analyzed in light of Flory's theory. Some aggregation and specific solvent effects have been observed, however separation of these effects has proven to be difficult. Results of these studies are compared to previous work for other cellulose derivatives. ©1995 John Wiley & Sons, Inc.     

Solution behavior of two liquid crystalline polymers of different architectures

Solutions of two different liquid crystalline polymers of high molecular weight are investigated by static and dynamic light scattering (LS), membrane osmometry and size-exclusion chromatography (SEC). Measurements in dilute solution in different solvents showed no specific behavior as formation of aggregates or chain stiffening. Large discrepancies between the LS results and the results from osmometry and SEC show that the latter methods are in the present cases not suitable for molecular weight determination. In semi-dilute solution the osmotic modulus and the time correlation function were studied. Behavior of flexible chains was observed. In one system a slight aggregation of small molecules onto longer chains was found causing less interpenetration of the chains in that solvent. At moderately high concentrations cluster formation was observed from i) a small angle excess scattering, ii) a downturn of the osmotic modulus, and iii) the appearance of a slow motion in the time-correlation function.     

Connecting the wetting and rheological behaviors of poly(dimethylsiloxane)-grafted silica spheres in poly(dimethylsiloxane) melts

Using dynamic light scattering, mechanical rheometry, and visual observation, the static wetting behavior of PDMS-grafted silica spheres (PDMS-g-silica) in PDMS melts is related to their rheology. A phase diagram is mapped out for a constant grafted chain length as a function of grafting density and free polymer chain length. The transition between stable and aggregated regions is determined optically and with dynamic light scattering. It is associated with a first-order wetting transition. In the stable region Newtonian behavior is observed for semidilute suspensions. The hydrodynamic brush thicknesses, deduced from viscosity measurements, correspond closely to values obtained from self-consistent field calculations for the various parameter values. At the transition, the brush collapses suddenly and shear-thinning and thixotropy appear. The rheology indicates a degree of aggregation that increases with increasing length of the free polymer, as suggested by the theory.     

Aggregation behaviour below the gelation threshold for metal-olefin coordination in diene polymers

This study focuses on aggregation below the gelation threshold in ternary solutions containing diene polymers, atactic 1,2-polybutadiene (PBu) or 3,4-polyisoprene (PI), and an inorganic salt, bis(acetonitrile) dichloropalladium(II). Upon mixing, effective coordination crosslinks are formed because the acetonitrile ligands of the palladium salt are displaced by olefinic pendant groups of the polymers. In all cases, the aggregation kinetics correlate with PdCl₂ concentration because the polymer concentration is 100 times greater than the salt concentration. Aggregation is the process that occurs prior to gelation as the transition-metal salt forms a coordination bridge between two different polymer chains. Differential analysis of the initial aggregation rate on the basis of light scattering data at 45° relative to the incident throughput beam reveals that the ‘true’ order of the coordination reaction is close to unity. This suggests that coordination crosslinking is diffusion-controlled in the early stages of aggregation. Half-life analysis of viscous solutions yields an overall reaction order of 1.6 for aggregation of polybutadiene and palladium chloride in tetrahydorfuran (THF). Scaling analysis of the weight-average molecular weight dependence of the root-mean-square size of isolated scattering particles yields an effective exponent of 0.7 for polymer/metal-salt complexes at infinite dilution. This experimental scaling law agrees with literature values for cross-linked polymer networks. In all cases, the size of the aggregates increases at higher PdCl₂ concentration. For polybutadiene/PdCl₂ mixtures in THF or toluene below the gelation threshold, the light-scattering-detected average aggregation number (AN = M_{w, complex}/M_{w, pure polymer} via Zimm-plot intercepts) for low-viscosity solutions is 2, while AN ∼ 9 for viscous THF solutions. In contrast, low aggregation numbers (AN ∼ 2) were calculated for viscous ternary mixtures of 3,4-polyisoprene and PdCl₂ in THF.     

Aggregation behavior and interaction of an amphiphilic drug imipramine hydrochloride with cationic surfactant cetyltrimethylammonium bromide: light scattering studies

The aggregation behavior and interaction of an amphiphilic antidepressant drug imipramine (IMP) hydrochloride with the cationic surfactant cetyltrimethylammonium bromide (CTAB) have been studied using light scattering (both static and dynamic) techniques. Due to rigid tricyclic hydrophobic moiety present in the molecule, the drug shows interesting association behavior. The static light scattering measurements show that the self-association of IMP commenced above a well-defined critical micellar concentration (CMC), which decreases with increasing the mole fraction of the CTAB surfactant. Both the excess Gibbs energy (ΔG(ex)) and the Gibbs energy of micellization (ΔG(M)°) are negative, and decrease with increasing mole fraction of the surfactant. The hydrodynamic diameters (d(h)) of the micellar aggregates were also evaluated using the dynamic light scattering measurements. The data indicate formation of larger aggregates by IMP and CTAB due to mixed micellization and subsequent micellar growth. The results have been analyzed using different models (viz., Clint, Motomura, Rosen, Rubingh, etc.) for mixed micelle formation.     

Polyvinylpyrrolidone Behavior in Water/Ethanol Mixed Solvents: Comparison of Modeling Predictions with Experimental Results

The objective of this work is to study the behavior of a neutral polymer, polyvinylpyrrolidone (PVP), in a mixture of water and ethanol. A comparison of the experimental results with a theoretical model of effective solvent interaction with polymer (ESIP) was made. To do so, dynamic light scattering experiments were used to measure the hydrodynamic radius of PVP (M _w = 3.6 × 10⁵ g·mol^?1) as a function of the ethanol fraction, x _A, in the medium at 25 °C. We show that the polymer adopts an ideal chain–globule–coil conformation transition as the ethanol molar fraction varies. This transition is attributed to the change of the solvent quality which results from water and ethanol complex formation. On the other hand, the ternary PVP/water/ethanol system was described by the ESIP model. From the polymer–effective solvent interaction, the second virial coefficient of the polymer/effective solvent and the preferential adsorption parameter were calculated. The obtained results are in agreement with the reported experiments.     

Effects of polar group saturation on physical gelation of amphiphilic polymer solutions

Monte Carlo simulation on the basis of the comblike coarse grained nonpolar/polar (NP) model has been carried out to study the polar group saturation effect on physical gelation of amphiphilic polymer solutions. The effects of polar group saturation due to hydrogen bonding or ion bridging on the sol-gel phase diagram, microstructure of aggregates, and chain conformation of amphiphilic polymer solutions under four different solvent conditions to either the nonpolar backbone or the polar side chain in amphiphilic polymer chains have been investigated. It is found that an increase of polar group saturation results in a monotonically decreased critical concentration of gelation point, which can be qualitatively supported by the dynamic rheological measurements on pectin aqueous solutions. Furthermore, various solvent conditions to either the backbone or the side chain have significant impact on both chain conformation and microstructure of aggregates. When the solvent is repulsive to the nonpolar backbone but attractive to the polar side chain, the polymer chains are collapsed, and the gelation follows the mechanism of colloidal packing; at the other solvent conditions, the gelation follows the mechanism of random aggregation.     

Phase Behavior of Solutions of Polymers with Multiply Aggregating Groups

We have theoretically investigated phase diagrams of solutions of aggregating polymers. In our model all polymer chains in the solution carry a certain number of aggregating groups (“stickers”), capable of the formation of thermoreversible aggregates, each assumed to consist of exactly m stickers. The treatment of aggregation corresponds to the Flory‐like theory of gelation. We have studied the dependence of the phase diagram on the parameters of aggregation, such as its strength (fraction of monomeric units carrying stickers and aggregation constant) and cooperativity (aggregation number). In the considered system of homopolymer chains the stickers have been found to always promote phase separation. The value of aggregation number m is crucial for the topology of the phase diagram: for m ≥ 5 the triple point may appear in the phase diagram. Conditions on the properties of the system ensuring the appearance of this feature have been obtained. The interrelation between gelation and phase separation for different values of parameters is elucidated.     

Phase behavior and structure of liquid crystalline solutions of cellulose derivatives

Summary Structural and thermodynamic characteristics of liquid-crystalline solutions of four cellulose derivatives in a range of solvents were studied. Basic observations were made on these systems using polarized light microscopy, small angle light scattering, dilute solution and concentrated solution viscosities. The polymers studied include hydroxypropyl cellulose (HPC), cellulose acetate butyrate (CAB), ethyl cellulose (EC), and cellulose triacetate (CT). The formation of the liquid crystalline phase was shown to strongly depend on polymer concentration, solvent type and temperature. The critical volume fraction of polymer required to form the liquid crystal phase varied significantly as the solvent changed. The critical volume fraction decreased with increasing solvent acidity and polymer intrinsic viscosity in a given solvent. The breadth of the two phase region seems to decrease with increasing acidity. The liquid crystalline phase was in most cases determined to be cholesteric. In all cases positively birefringent cellulose derivatives form negative spherulitic domains. In one case, the negativity birefringent system (cellulose triacetate) formed positively birefringent spherulitic liquid crystalline domains. This is interpreted to mean the structure organizes itself by a tangential alignment of polymer chains within the domain. SALS measurements appear to detect domains and in some cases cholesteristic pitch.With 5 figures and 4 tables     

Hydration and phase separation of temperature-sensitive water-soluble polymers

Collapse of a poly(N-isopropylacrylamide)(PNIPAM) chain upon heating and phase diagrams of aqueous PNIPAM solutions with very flat LCST phase separation line are theoretically studied on the basis of cooperative dehydration(simultaneous dissociation of bound water molecules in a group of correlated sequence),and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods.The transition becomes sharper with the cooperativity parameterσof hydration.Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves(LCST cononsolvency) with the mole fraction of methanol due to the competition is calculated and compared with the experimental data.Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends(telechelic PNIPAM) are theoretically and experimentally studied.The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation),and separate from the coil-globule transition line.Associated structures in the solution,such as flower micelles,mesoglobules and higher fractal assembly,are studied by USANS with theoretical modeling of the scattering function.     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

The Soret effect in dilute polymer solutions: influence of chain length, chain stiffness, and solvent quality

Thermal diffusion in dilute polymer solutions is studied by reverse nonequilibrium molecular dynamics. The polymers are represented by a generic bead-spring model. The influence of the solvent quality on the Soret coefficient is investigated. At constant temperature and monomer fraction, a better solvent quality causes a higher affinity for the polymer to the cold region. This may even go to thermal-diffusion-induced phase separation. The sign of the Soret coefficient changes in a symmetric nonideal binary Lennard-Jones solution when the solvent quality switches from good to poor. The known independence of the thermal diffusion coefficients of the molecular weight is reproduced for three groups of polymers with different chain stiffnesses. The thermal diffusion coefficients reach constant values at chain lengths of around two to three times the persistence length. Moreover, rigid polymers have higher Soret coefficients and thermal diffusion coefficients than more flexible polymers.     

用金胺O作荧光探针研究PVA-H_2O-DMSO体系的凝胶过程

<正> 它在可见光区发荧光,其荧光量子效率随环境的粘度不同有较明显的变化。这是由于其分子中的芳香基团与中心碳是单键连接,易于转动。当环境的粘度增加时,这种转动受到阻碍,因而分子吸收激发光能后,损失于这种转动的热能相对减少,荧光量子效率则相对