Equilibria of extended-chain polymers exhibiting crystalline and liquid-crystalline phases

The phase diagram of poly(p-benzamide) (PBA) in N,N-dimethylacetamide/LiCl solutions was determined for two PBA samples having weight-average molecular weights of about 10,000. The various equilibria were studied using analytical, viscometric, and optical microscope measurements. The phase diagram at 25°C, taking as variables the concentration of polymer (C_p) and LiCl (C_s), involves several equilibria which can be summarized as follows: solid ? isotropic solution when 0.75 ≤ C_s ≤ 2 g/dl, solid ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, isotropic solution ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, and C_p > 6 g/dl, and dilute isotropic solution ? gel when C_s > 4 g/dl. In the C_p range in which the isotropic and anisotropic phases coexist, enrichment of the high-molecular-weight component of the polymer in the anisotropic phase becomes more marked as the volume fraction of the latter phase is decreased. The two PBA samples exhibit noticeable differences in solubility, absolute viscosity, and in their viscosity-concentration behavior. The location of the maximum in the latter dependence does not necessarily coincide with the first appearance of the anisotropic phase. In the absence of a flow field, anisotropic solutions exhibit an irreversible increase in viscosity. Inclusion of the equilibria involving the crystalline state furnishes insight into some of the common observations for extended-chain polymers. A diagram illustrates the superposition of the solubility curves for a crystalline polymer and the liquid-crystal regions. This indicates that, for the high melting crystalline polymers, the crystalline phase should be stable relative to the concentrated anisotropic phase of the wide biphasic region.     

Rheology of hydroxypropylcellulose solutions

The rheology of hydroxypropylcellulose (HPC)—acetic acid solutions was investigated by using a cone-and-plate rheometer and a capillary rheometer, for polymer concentrations ranging from 10 to 80%. Isotropic solutions exhibit a Newtonian plateau followed at higher shear rates by a pseudoplastic zone. The apparent viscosity varies as C^5.2 if concentration C is less than 27% and as C¹³ for 27% < C < 30%. A biphasic interval (isotropic and cholesteric phases) exists between 30 and 35%. A maximum in viscosity is observed at C = 30%, the height of the viscosity peak being a decreasing function of shear rate. Anisotropic solutions are strongly viscoelastic. Both isotropic and anisotropic solutions give results (apparent viscosity, first normal-stress difference, relaxation time, etc.) which are not in good agreement with Doi's theory. This is understandable since the HPC chain cannot be modeled by a rigid rod. Upon heating, anisotropic HPC—acetic acid solutions undergo an anisotropic to isotropic phase transition which is easily detected by a maximum in the temperature dependence of the first normal-stress difference and of the apparent viscosity.     

Concentrated solutions of polydisperse rodlike polymers in the isotropic and lyotropic liquid-crystalline phases. I. The effects of molecular length distribution on phase behavior

The earlier calculations of Moscicki and Williams of the phase behavior of rodlike macromolecules in solution incorporating a Gaussian distribution of rod lengths have been extended to include, in addition to a “basic” Gaussian distribution, a small amount of high-molecular-weight species which are characterized by a δ-function or “box-function” distribution. It is shown that C^*_p, the concentration of polymer at which the biphasic material (isotropic plus anisotropic phase) first appears may be substantially lowered in comparison with that for the system comprising only the basic distribution. The volume fraction Φ_A of anisotropic phase, the compositions of the two phases, and the distribution of species between the phases are calculated for a range of polymer concentration which spans the isotropic, biphasic, and anisotropic phase and, among several features obtained, it is shown that the high-molecular-weight species are essentially responsible for the initial formation of anisotropic phase in the biphasic system. The results of the calculations have important implications for the interpretation of published dielectric relaxation data for polyalkylisocyanates in solution at high concentration and for published shear-flow viscosity data for polyalkylisocyanates and poly(p-benzamide) in solution at high concentration and these are discussed in some detail.     

Chain overlap and entanglements in dilute polymer solutions: Brownian dynamics simulation

We have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrations c < C^*, c^* = critical concentration). The fundamental problem of existence of chain overlaps in dilute solutions is also related to the drag reduction phenomenon (DR). Some experimental results pertinent to DR are explained in terms of entanglements even for solutions at concentrations defined in ppm. We report results of Brownian dynamics simulations of polymer solutions in which the equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements. The results for c = 0.3 c^* demonstrate that both chain entanglements and overlaps take place even in dilute solution. They also confirm predictions from an earlier combinatorial model.     

Rheology of a mixture of liquid-crystal polymers in solution

The rheological behavior of a mixture of two liquid-crystal polymers, hydroxypropyl cellulose and ethyl cellulose, in acetone solution is studied. The total polymer concentration in the solvent is held constant (40%) as the ratio of the two polymers is varied. The mixtures are anisotropic, isotropic, or biphasic (isotropic/anisotropic), depending on the concentration. Curves of viscosity vs shear rate for all the mixtures studied show three regions of viscosity, as described by Onogi and Asada for liquid-crystal polymers. The viscosity as a function of the weight ratio of the two polymers at constant shear rate exhibits deviations from additivity of viscosities of the two components at all concentrations. In mixtures of two polymers in the melt, deviations are also observed; the negative ones are attributed to phase separation and the positive ones to homogenous mixing (comparison with the phase diagram). All the mixtures studied (anisotropic, isotropic, or biphasic), show ranges of shear rates where the first normal-stress difference is negative, as is generally observed for anisotropic liquid-crystal polymers. It is concluded that the isotropic solutions become anisotropic under shear, as they are not far from the critical concentration. © 1994 John Wiley & Sons, Inc.     

Dynamic Light Scattering Characterization of a Water-Soluble Terpolymer with Vinyl Biphenyl in Unsalted and Brine Solutions

The sizes and distributions of polymer chains and associating aggregates, hydrophobic associations, and apparent viscosities were investigated with dynamic light scattering (DLS), fluorescence spectroscopy and viscometry for the hydrophobically associating terpolymer poly[acrylamide(AM)/vinyl biphenyl(VP)/sodium 2-acrylamido-2-methylpropane sulphonate(NaAMPS)] (PAAP) in unsalted and brine solutions. The effects of polymer, NaCl concentration, and temperature on the conformation of polymer chains and the formation of associating structures in aqueous solutions were studied. Intra- and inter-molecular hydrophobic associations were formed and the polymer chains were coiled in unsalted solutions below a critical association concentration (C _p *): 0.05 g·dL^?1, and the apparent viscosity was very low and varied slightly with polymer concentration, but a large number of large aggregates were formed because of the presence of dominant intermolecular associations above C _p *, and the apparent viscosity increased abruptly with increasing polymer concentration. The PAAP chains were comparatively expanded (R _h: 21–92 nm) in brine solutions, resulted from the simultaneous incorporation of the bulky pendant side groups, including the biphenyl and 2-acrylamido-2-methylpropane sulphonate groups. With the addition of NaCl, large associative aggregates in unsalted solutions were destroyed to form comparatively expanded mono-macromolecules and smaller aggregates. However, as the NaCl concentration was increased from to 15 to 80 g·L^?1, more large aggregates were again formed, leading to the obvious salt-thickening behavior. The variation of R _h with temperature proved that intermolecular hydrophobic association is an endothermic process.     

Rheological study of an aromatic polyamide-hydrazide

Rheological and rheo-optical data are reported for the poly(terephthalamide of p-aminobenzhydrazide), X-500, in dimethyl sulfoxide solutions in the concentration range 0.2 to 6.0 g/100 ml. The objective of these measurements is to seek evidence of shear-induced isotropic → nematic phase transition. Three types of measurement, Couette, cone and plate, and capillary rheometer, all indicate that this system exhibits flow instabilities at high shear rates and concentrations. In this region both the viscosity and the primary normal stress difference decrease with time under shear. In the capillary rheometer the apparent viscosity is smaller for shorter capillaries and, for short capillaries, there is a range of shear rates in which plug flow and a coiled extrudate are observed. These instabilities may arise from the existence of a mixture of isotropic and nematic phases. At lower shear rates, where the flow behavior appears normal, the concentration dependence of the flow birefringence increases at a critical polymer concentration C This value is in reasonable accord with the concentration corresponding to the change of slope of logη (measured at low shear rate) vs. logC_p. Both the latter measurements appear to be sensitive to the onset of the phenomenon, which may be due to a shear-induced transition or the rheological effect of chain entanglements.     

Effects of solvent goodness and polymer concentration on rejection of polystyrene from small pores

Solutions of polystyrene of molecular weight 4.5 × 10⁶ and 8.4 × 10⁶ in mixed solvents of carbon tetrachloride/methanol were filtered through track-etched mica membranes at low membrane velocities. The unperturbed hydrodynamic radius of the polymer was always larger than the pore radius. The reflection coefficient σ, defined as the fraction of polymer held back by the membrane, was determined from material balances as a function of solvent flow rate per pore q, volume percent CCl₄ of the solvent, and polymer concentration C₀. In the dilute region (C₀ < C^*) σ was found to depend primarily on q and was essentially independent of chain size (or solvent goodness), molecular weight, and pore radius. In the semidilute region (C₀ > C^*) σ decreased significantly as C₀ was increased.     

CRITICAL CONCENTRATIONS OF α(1→3)-D-GLUCAN FROM LENTINUS EDODES IN NaOH AQUEOUS SOLUTION

Critical concentrations of α-(1→3)-D-glucan L-FV-Ⅱ from Lentinus edodes were studied by viscometry andfluorescence probe techniques. The dependence of the reduced viscosity on concentration of the glucan in 0.5 mol/L NaOHaqueous solutions with or without urea showed two turning points corresponding to the dynamic contact concentration c_s andthe overlap concentration c~* of the polymer. The values of c_s and c~* were found to be 1×10~(-3) g cm~(-3) and 1.1×10~(-2) g cm~(-3),respectively, for L-FV-Ⅱ in 0.5 mol/L NaOH aqueous solutions. The two critical concentrations of L-FV-Ⅱ in 0.5 mol/LNaOH aqueous solutions were also found to be 1.2×10~(-3) g cm~(-3) fbr c_s and 9.2×10~(-3) g cm~(-3) for c~* from the concentrationdependence of phenanthrene fluorescence intensities. The overlap concentration c~* of L-FV-Ⅱ in 0.5 mol/L NaOH aqueoussolutions was lower than that of polystyrene with same molecular weight in benzene, owing to the fact that polysaccharidetends to undergo aggregation caused by intermolecular hydrogen bonding. A normal viscosity behavior of L-FV-Ⅱ in 0.5 mol/L urea/0.5 mol/L NaOH aqueous solutions can still be observed in an extremely low concentration range at 25℃.     

Behavior of γ-ray-irradiated pullulan in aqueous solutions of cationic (cetyltrimethylammonium hydroxide) and anionic (sodium dodecyl sulfate) surfactants

 γ-ray-irradiated pullulan macromolecules acquire properties of an anionic polyelectrolyte and, upon aggregation with the oppositely charged surfactant cetyltrimethylammonium hydroxide, are found to precipitate according to their molecular weight. This provides a convenient means for obtaining polymer fractions with a narrower molecular-weight distribution than those of the original samples. The method can be employed to obtain fractions of radiation-modified pullulan required in the production of a blood-plasma substitute. Anionic properties of γ-ray-irradiated pullulan also manifest themselves in interactions with sodium dodecyl sulfate (SDS) in aqueous solution, which result in a significant change in the viscous behavior of the polysaccharide. Upon an increase in the concentration of γ-ray-irradiated pullulan in an SDS solution, the reduced viscosity of the polymer first increases and, upon reaching a certain concentration, C*, decreases. The C* values were found to be dependent on the molecular weight of the polymer. The phenomena observed are discussed in terms of the general theory of polymer solutions within which C* is treated as a critical concentration at which interpenetration of polymer molecules becomes important. Unperturbed dimensions of γ-ray-irradiated pullulan macromolecules were estimated on the basis of experimental viscosimetric data. Received: 29 March 2001 Accepted: 31 July 2001     

Structural investigations of polymer liquid-crystalline solutions: Aromatic polyamides,hydroxy propyl cellulose,and poly(γ-benzyl-L-glutamate)

A comparative structural investigation of the characteristics of polymer liquid-crystalline solutions including Kevlar® (PPD-T)/sulfuric acid, poly(Cl-p-phenylene terephthalamide) (Cl-PPD-T)/sulfuric acid, poly(γ-benzyl-L-glutamate) (PγBLG)/dioxane, and hydroxypropyl cellulose (HPC)/water was undertaken. Experimental procedures included polarized light microscopy, light scattering, absorption spectra, and x-ray diffraction on solutions at various concentrations and temperatures. Both the two-phase region at the onset of liquid-crystal formation and the wholly anisotropic phase were investigated. Each solution exhibited distinctive characteristics. The PPD-T and Cl-PPD-T solutions were nematic, and the PγBLG and HPC solutions were cholesteric. In the two-phase region the PPD-T, Cl-PPD-T, and PγBLG (but apparently not the HPC) exhibited negatively birefringent spherulites and aggregates of spherulites. The HPC solutions only exhibited spherulitic structures in the single-phase anisotropic system. The structures and orientations in the anisotropic phase for the various polymer solutions is considered. The helicoidal structural characteristics of the PγBLG and HPC solutions are contrasted.     

A 13C NMR study of mesomorphic solutions of poly(p-phenylene terephthalamide)

Solutions of poly(p-phenylene terephthalamide) in fuming sulfuric acid were characterized by ¹³C NMR spectroscopy and solution viscosity measurements over the 2–28% w/w concentration range. The spectra showed the presence of two distinct amide carbonyl resonances at low concentration, tentatively assigned to cis and trans conformations. As the concentration increased, additional carbonyl lines were observed along with significant broadening. Peak area measurements showed that only the polymer molecules in the isotropic environments contributed to the ¹³C NMR spectra and a considerable amount of the polymer remained in the isotropic phase at concentrations previously considered to consist of polymer in highly anisotropic regions. Spin-lattice relaxation times were measured at six concentrations using the inversion recovery method. The aromatic carbons relaxed at a much faster rate (ca. 0.10 s) than the carbonyls (ca. 0.45 s), but the relaxation rates for both carbons were essentially constant over the concentration range, indicating that the observed isotropic phase is not affected by changes in the macroscopic solution behavior so as to alter spin-lattice relaxation mechanisms.     

Neutral polymer slow mode and its rheological correlate

Quasi‐elastic light scattering spectroscopy intensity–intensity autocorrelation functions [S(k,t)] and static light scattering intensities of 1 MDa hydroxypropylcellulose in aqueous solutions were measured. With increasing polymer concentration, over a narrow concentration range, S(k,t) gained a slow relaxation. The transition concentration for the appearance of the slow mode (c^t) was also the transition concentration for the solution‐like/melt‐like rheological transition (c⁺) at which the solution shear viscosity [η_p(c)] passed over from a stretched exponential to a power‐law concentration dependence. To a good approximation, we found c^t[η] ≈ c⁺[η] ≈ 4, [η] being the intrinsic viscosity. The appearance of the slow mode did not change the light scattering intensity (I): from a concentration lower than c^t to a concentration greater than c^t, I/c fell uniformly with increasing concentration. The slow mode thus did not arise from the formation of compact aggregates of polymer chains. If the polymer slow mode arose from long‐lived structures that were not concentration fluctuations, the structures involved much of the dissolved polymer. At 25 °C, the mean relaxation rate of the slow mode approximately matched the relaxation rate for the diffusion of 0.2‐μm‐diameter optical probes observed with the same scattering vector. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 323–333, 2005     

Structural Changes in Block Copolymer Solutions under Shear Flow as Determined by Non‐Equilibrium Molecular Dynamics

Summary: A non‐equilibrium molecular dynamics computer simulation on microsegregated solutions of symmetrical diblock copolymers is reported. As the polymer concentration increases, the system undergoes phase transitions in the following order: body centered cubic (BCC) micelles, hexagonal (HEX) cylinders, gyroid (GYR) bicontinuous networks and lamellae (L), which are the same morphology reported for block copolymer melts. Structural classification is based on the patterns of the anisotropic static structure factor and characteristic 3‐dimensional images. The systems in the BCC micellar (ρσ³ = 0.3) and HEX cylindrical (ρσ³ = 0.4) phases were then subjected to a steady planar shear flow. In weak shear flow, the segregated domains in both systems tend to rearrange into sliding parallel close‐packed layers with their normal in the direction of the shear gradient. At higher shear rates, both systems adopt a perpendicular lamellar structure with the normal along the neutral direction. A further increase in the shear rate results in a decrease in lamellar spacing without any further structural transitions. Two critical shear rate values that correspond to the demarcation of different structural behaviors were found.

Shear‐induced BCC‐LAM phase transition.     

Non-Newtonian flow curves including lower and upper Newtonian regions for dilute and moderately concentrated polystyrene solutions

The non-Newtonian viscosity in steady flow was measured for solutions of polystyrene (M?_w/M?_n = 1.1) in diethyl phthalate at 30.0°C. In the moderately concentrated solutions, from 6.03 × 10^?2 to 5.62 × 10^?1g/cm³, the viscosity data modified by frictional parameters fit the Graessley theoretical curve for a narrow distribution polymer. The dilute solutions, from 3.26 × 10^?3 to 1.57 × 10^?2 g/cm³, were nonentangled systems whose non-Newtonian properties could be explained by the excluded volume effect as proposed by Fixman. On the basis of the non-Newtonian data, it was concluded that the solution of 3.30 × 10^?2 g/cm³ was a lower critical entanglement concentration, which was distinguished from the usual higher critical concentration for entanglement. This lower critical concentration was also found in the concentration dependence of the activation energy of flow and the absorbance at 310 nm.     

Induced changes of polymer organization in a gel of poly(vinyl alcohol) crosslinked by Congo red

Different gel microstructures are induced at variable poly(vinyl alcohol) (PVA) and Congo red concentrations, as revealed by ultrarapid freezing and a replica technique for transmission electron microscopy. The polymer microstructures observed include random coils, rigid polymer rods, and long fibers. The development of the different polymer conformations is proposed to be dependent on the degree of intramolecular and intermolecular crosslinking and on the electrostatic interactions of the Congo red ions. The rigid‐rod conformation appears to be the most energetically stable form; it is disrupted by electrostatic effects around the polymer overlap concentration (C*_PVA). We propose that the gel microstructure influences the physical properties of the gel. Gels possessing the rigid‐rod microstructure have increased Young's storage modulus values. Two possible mechanisms of gelation are suggested. The first describes a one‐stage reaction when the polymer concentration approximates C*_PVA, where polymers in an extended random‐coil conformation undergo intermolecular crosslinking without any microstructural changes. The second describes a two‐stage reaction when the polymer concentration is less than or greater than C*, where a disorder–order transition results in the formation of rigid polymer rods and fibers followed by the formation of a macromolecular network. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1471–1483, 2001     

Rheological properties of internal viscosity models with stress symmetry

Internal viscosity models (IVM) for dilute-solution polymer dynamics differ in how they define the deformational force F ^d which includes φ, the IV coefficient, and in how they treat polymer rotational velocity Ω. Here, the handling of angular momentum is shown to be crucial. A torque balance in simple shear flow at shear rate G leads to stress symmetry and specification of Ω(G) which differs greatly from the conventional Ω = G/2. This determines the G dependence of viscosity η and normal stress coefficient ζ. There are also implications of a transition in rotational behavior as φ approaches a critical value. Predictions of η(G), ζ(G), and η^*(ω) are presented for two versions of F^d : one derived recently by the authors and one being most commonly used at present. Limiting cases for high and low φ, and for high and low G and ω, are discussed. Some differences exist between predictions of the two F^d models, but these are surprisingly minor.     

Electro-optic response of an antiferroelectric liquid crystal in submicron cells

The dynamics of chiral smectic phases of antiferroelectric liquid crystal MHPOBC in a confined geometry has been analysed. Using an electro-optic response technique, the temperature dependences of the relaxation rates and electro-optic strengths of the elementary excitations in thin, planar aligned, wedge-type cells of thickness from 0.3 to 4 μm have been measured and compared with those for a 50 μm hometropically aligned cell. The effects of the confined geometry are the following. (i) The smectic C^* _γ phase does not exist in planar aligned cells with thickness less than 4 μm. Instead of this phase, we have observed the coexistence of the ferroelectric smectic C^* phase and the antiferroelectric smectic C^* _A phase over a very wide temperature range. (ii) The smectic C^* _α phase is stable at all measured thicknesses down to 0.3 μm. (iii) We have observed a decrease of the smectic A-smectic C^* _α phase transition temperature, proportional to the inverse of the cell thickness. (iv) Additional, thickness-independent phase modes have been observed above some critical value of the measuring electric field in all tilted phases.     

Rheological and microcalorimetric studies of a model alkali‐soluble associative polymer (HASE) in nonionic surfactant solutions

Rheological experiments were carried out on a 1 wt % hydrophobically modified alkali‐soluble emulsion (HASE) solutions at pH ∼ 9 in the presence of nonionic polyoxyethylene ether type surfactant (C₁₂EO₂₃). The low shear viscosity and dynamic moduli increases at c > cmc until they reach a maximum at a critical concentration, c^m of approximately 1 mM (∼17 times the cmc of free surfactant) and then decrease. The dominant mechanism at cmc < c < c^m is an increase in the number of intermolecular hydrophobic junctions and a strengthening of the overall associative network structure. Above c^m, the disruption of the associative network causes a reduction in the number of junctions and strength of the overall network structure. The influence of C₁₂EO₂₃ on HASE before cmc could not be detected macroscopically by the rheological technique. However, isothermal titration calorimetry enables the determination of complex binding of surfactant to the polymer. Isothermal titration of C₁₂EO₂₃ into 0.1 wt % HASE indicates that the C₁₂EO₂₃ aggregation in water and 0.1 wt % HASE polymer solutions is entropically driven. A reduction in the critical aggregation concentration (cac) confirms the existence of polymer–surfactant interactions. The hydrophobic micellar junctions cause a decrease in the ΔH and ΔS of aggregation of the nonionic surfactant. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2019–2032, 2000     

Dilute solutions of poly(vinyl alcohol) derived from vinyl trifluoroacetate

The dilute-solution behavior of poly(vinyl alcohol) (PVA_VTFA), derived from vinyl trifluoroacetate, in water-dimethylsulfoxide (DMSO) mixtures was investigated. With solvent mixtures ranging from 10 to 20 vol % DMSO, the relation between the reduced viscosity η_sp/C and the polymer concentration C was linear for polymer concentrations above 0.2 g/dL, whereas in solutions in mixed solvents of other compositions the dependence was linear for polymer concentrations above 0.1 g/dL. The relation between the intrinsic viscosity [η] obtained for aqueous solutions of PVA_VTFA and the molecular weight M estimated from viscosity measurements in solutions of poly(vinyl acetate) (PVA_VTFA), obtained by acetylation of PVA_VTFA, was given by [η] = 7.34 × 10^?4 M^0.63. The value of [η] was greatest for the solvent mixture with 10 vol % DMSO and smallest for about 50 vol % DMSO, and Huggins constants k were smallest and greatest for these two cases, respectively. The turbidity of the solutions of low-molecular-weight PVA_VTFA, was higher than that of high-molecular-weight PVA_VTFA up to 30 vol % DMSO, and the reverse relation held for 40-70 vol % DMSO.