Viscoelastic properties of polyacrylonitrile gels: dependence of sol-gel transition on concentration and aging time

Dynamic viscoelasticity was observed for polyacrylonitrile solution in dimethylacetamide in the process of sol-gel transition. The gel was prepared by the freezing and thawing method, i.e., the solution was frozen at −50°C and then kept at 25°C so as to undergo gelation. The longer the freezing time, the shorter was the gelation time. Also the higher the concentration, the shorter the gelation time. The strength of the network, S and critical exponent n at the transition point were evaluated. The S value increased with increasing concentration and was constant in value in spite of the variation of freezing time when the concentration of the solution was constant.     

Enthalpic effect of polyacrylonitrile on the concentrated solutions in dimethyl sulfoxide: Strong thixotropic behavior and formation of bound solvents

The effect of dipole–dipole interaction by nitrile groups of PAN on the bound state of solvent molecules and the concentrated solution properties in DMSO was investigated. Variation of a solution viscosity exhibited three overlap concentrations, C1*, C2*, and C3*, at 2.7, 8.6, and 16.3 wt%, respectively, representing the transition of concentration regions in the order of dilute, unentangled‐semi dilute, entangled‐semi dilute, and concentrated regions. The two‐dimensional mapping of FT‐IR analysis and dielectric measurement confirmed that the intermolecular interaction of PAN was suddenly enhanced at the C*s, inducing polarization to DMSO. In the ice‐melting process of PAN solutions, two different melting peaks (T_m2 and T_m3) of DMSO newly appeared at each C2* and C3*, suggesting the different types of bound solvents. In the concentrated solutions, the saturated dielectric constant and the strongly delayed evaporation of the solvent even at the boiling point of DMSO along with strong thixotropic behavior were indicative of the stronger confinement state of bound DMSO than in the semidilute solutions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1080–1089     

Rheology and gelation of cellulose/ammonia/ammonium thiocyanate solutions

Liquid crystalline solutions of cellulose in an ammonia/ammonium thiocyanate solvent will form thermoreversible gels at temperatures below 30°C. These gels are of interest both for processing the cellulose/ammonia/ammonium thiocyanate system and because they have an unusual structure, containing neither crystalline nor covalently bonded crosslinks. Although these gels contain neither crystalline nor covalently bonded crosslinks, the dynamic rheological behavior of the system at the gel point was found to be the same as for gels with covalent or crystalline crosslinks with a loss tangent, tan δ, independent of frequency. The kinetics of the gelation process was monitored via dynamic elastic modulus, G′. All samples revealed an exponential increase in G′ with time during gelation, very different from that observed in covalently bonded or crystalline crosslinked systems. Measurements of the loss tangent enabled precise determination of the gelation time for these systems as a function of cellulose concentration and temperature. We found the gel time to be inversely related to cellulose concentration and directly related to temperature. The strong dependence of gel time on these parameters offers a windows of spinnability that can be tailored for processing high modulus cellulose fibers. © 1996 John Wiley & Sons, Inc.     

Chemical and rheological investigations of the sol-gel transition in organically-modified siloxanes

Ormosils are well-known organic-inorganic sol-gel derived materials also called heteropolysiloxanes. This paper presents two basic heteropolysiloxane structures where the organic part is either a short organic chain bridging two silicon atoms for the first material or an organic polymer backbone for the second. Their synthesis is detailed and a variety of experimental techniques (IR, ¹³C and ²⁹Si NMR and CP-MAS NMR, GPC) have been employed to investigate the chemical structure of these new materials. Their mechanical properties, more precisely their viscoelastic behaviour, have been evaluated using dynamic rheological techniques. The storage and loss moduli have been followed during the sol-gel transition at fixed and variable oscillation frequencies. The results have been correlated to the ²⁹Si CP-MAS NMR informations concerning the network polycondensation and compared to a pure inorganic sol-gel material prepared from tetraethoxysilane.     

Effect of processing temperature during spin-on application on the properties of sol-gel silica films

The effect of the temperature (of the substrate and the solution) during film deposition on spin coating process of sol-gel films is discussed. The increase of substrate temperature as well as coating solution liquid temperature leads to formation of thicker films with higher porosity. The temperature dependence of films thickness is mainly determined by the change of solvent vapour pressure with consideration for the change of liquid viscosity.     

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.     

Interfacial tension in a lower critical solution temperature blend: Effect of temperature,blend composition,and deformation of the interphase

Interfacial tension is a very important material parameter in two‐phase polymer blends. It determines the morphology development during processing, which is crucial for the end‐use properties of the material. Although different techniques for interfacial tension measurement give comparable results for immiscible polymers, the determination of the interfacial tension in lower critical solution temperature blends is not straightforward. This is illustrated for poly(α‐methyl styrene acrylonitrile)/poly(methyl methacrylate)(PαMSAN/PMMA), a slightly incompatible polymer pair. Interfacial tension has been measured with three different techniques: small‐amplitude oscillatory shear, recovery after elongation, and elongation of a multilayer sample. The large differences in these results can be attributed to the fact that most experimental techniques determine an apparent value, rather than the thermodynamic equilibrium value, of the interfacial tension. The latter is only obtained if the measurement is performed under quiescent conditions on a system that is composed of the coexisting PαMSAN‐rich and PMMA‐rich phases. The apparent interfacial tension depends on the actual composition of the phases and on the deformation of the interface. An order of magnitude approximation for such effects has been derived from theoretical considerations. Finally, each of these apparent values can be of practical importance. If a blend is prepared by melt mixing of the pure polymers, a high apparent value of interfacial tension should be considered. If, however, a blend is prepared by phase separation of a homogeneous mixture, the thermodynamic value is important. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 679–690, 2002     

A study on the orientation structure and mechanical properties of polyacrylonitrile precursors

The polyacrylonitrile precursors were made through the two‐stage drawing process. The orientation structure was examined through wide‐angle X‐ray diffraction (WAXD). The orientation factors and the modulus were measured through the sound velocity method. The mechanical properties, such as the Young's modulus, the tensile strength and the breaking elongation ratio were obtained by the single fiber tensile test. The results showed that the Young's modulus and the strength of the precursors increased with draw ratio, which is accordant with the enhancement of the micromolecular orientation degree. Therefore the orientation factors obtained from the experiments were compared with the theoretical curves which were predicted through the Crawford and Kolsky's model. The physical meaning of the parameters m and n were analyzed. A good agreement of the orientation factor between the experimental data and the theoretical curve was achieved. Copyright © 2005 John Wiley & Sons, Ltd.     

Modeling of the θ(lower critical solution temperature) in polymer solutions using molecular connectivity indices

In this work, correlations for the estimation of the θ(lower critical solution temperature, LCST) for Polystyrene, Polyethylene, Polypropylene, Polybutene-1, Polypentene-1, Poly(4-methylpentene-1), Polydimethylsiloxane and Polyisobutylene solutions were proposed based on the molecular connectivity index. The correlations give satisfactory estimation of the θ(LCST), with the absolute error smaller than 20 K for most systems studied. Since the new correlations require only molecular connectivity indices of the solvent in the calculations, and the molecular connectivity indices can be calculated easily once the molecular structure of the substance concerned is known, they have better predictive capability and are easier to apply comparing with the existing methods and models.     

NMR and x-ray studies on the morphology of thermoreversible polyacrylonitrile gels

The morphology of thermoreversible polyacrylonitrile–propylene carbonate (PAN-PC) gels was examined using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, “cross-linked” by regions of rigid polymer. The mobile regions of the gels showed solution-type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crysttallites detected previously by x-ray diffraction. Wide-angle x-ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites. © 1993 John Wiley & Sons, Inc.     

Effect of the interfacial interaction on the phase structure and rheological behavior of polypropylene/ethylene– octene copolymer/BaSO4 ternary composites

In polypropylene (PP)/ethylene–octene copolymer (POE)/BaSO₄ ternary composites, two different kinds of phase structures are assumed:(1) POE and BaSO₄ filler are separately dispersed in the PP matrix and (2) POE‐encapsulated filler particles (core–shell structure) are dispersed in the matrix. This depends on the interfacial interaction of the composites. For the design of composites with different interfacial interactions, three routes for the preparation of BaSO₄ master batches were developed. First, a mixture of BaSO₄, POE, and maleic anhydride (MAH) in a certain ratio was extruded in the presence of dicumyl peroxide and then pelletized. In extrusion, MAH‐functionalized POE was in situ formed to enhance the interfacial interaction between POE and BaSO₄. Second, a mixture of POE and BaSO₄ was directly extruded and then pelletized. Third, after BaSO₄ was treated with an organic titanate coupling agent, the treated BaSO₄ and POE were blended in extrusion and then pelletized. Scanning electron microscopy observations showed that the core–shell structure in which POE encapsulates BaSO₄ particles is formed through route 1, whereas POE and BaSO₄ are separately dispersed into the PP matrix through routes 2 and 3. The rheological behavior of PP/POE/BaSO₄ ternary composites was studied with a controlled stress rheometer. The results showed that the interfacial interaction in composites with core–shell morphology is the strongest. Interparticle interactions give rise to the formation of interparticle networks; the stronger the network is, the larger the shear yield stress is and the smaller the thixotropic loop area is. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1804–1812, 2002     

Lower critical solution temperature (LCST) and theta temperature of aqueous solutions of nonionic surface active agents of various polyoxyethylene chain lengths

Cloud points of aqueous solutions of homogeneous poly(oxyethylene)dodecyl ether derivatives (C₁₂(OE)_n: n=2–8) and the apparent theta temperatureT ^ap were determined from the abrupt changes in optical transmittance and the temperature dependence of the second virial coefficient obtained by light scattering measurements. It was found that the lower critical solution temperature (LCST) shifts to a lower temperature and lower concentration as the number of oxyethylene units in a molecule decreases. Because of this behavior of LCST, the modified Flory-Schultz plot of phase separation was applied to the present nonionic surfactant-water system, and its theta temperature obtained. The dependence ofT ^ap on the number of oxyethylene units suggests that the polyoxyethylene chain has different effects on the solubility of C₁₂(OE)_n in water forn less than or equal to 3 from those forn greater than or equal to 4.     

Gelation Studies on a Radical Chain Cross‐Linking Copolymerization Process: Comparison of the Critical Exponents Obtained by Dynamic Light Scattering and Rheology

Summary: The sol–gel transition of a radical chain cross‐linking copolymerization system [N‐vinylcaprolactam/2‐hydroxylethyl methacrylate/allyl methacrylate] has been studied using in situ time‐resolved dynamic light scattering (DLS) and in situ rheology. A critical dynamic behavior was observed near the sol–gel transition, which was characterized by the presence of a power‐law spectra over three decades in the time–intensity correlation function g₂(t) − 1 ∼ t^−μ and over two decades in the oscillatory shear experiment G′(ω) ∼ G″(ω) ∼ ωⁿ. A comparison of the obtained critical exponents μ ≈ 0.62 and n ≈ 0.75 was made. The theory predicts a relationship between these exponents, but up to now no experimental comparison has been done. The experimental results favor the percolation model, with a fractal dimension d_f of the gel clusters of 1.67.

Double‐logarithmic plot of time–intensity correlation functions g₂(t) − 1 versus the delay time t.     

Effect of branching and molecular weight on the viscoelastic properties of poly(butyl acrylate)

A series of poly(butyl acrylate) samples were prepared by emulsion polymerization with a range of molecular weights and degrees of chain branching. Characterization was performed with NMR (giving the fraction of branching, ranging from approximately 0 to 7%), gel permeation chromatography, viscometry, and determination of the gel fraction. The dynamic mechanical response, that is, the frequency dependence of the storage and loss moduli G′(ω) and G″(ω) was measured from 0.02 to 200 Hz. The occurrence of a significant insoluble fraction in the sample meant that full characterization of the molecular weight distribution was not possible, and so an unambiguous separation of the dependencies of the mechanical response on the degree of long‐chain branching (LCB) and short‐chain branching (SCB) and the molecular weight could not be made; however, trends dependent on the molecular weight alone were insufficient to model the results. At high frequencies, all trends in G′(ω) and G″(ω) could be ascribed to molecular weight dependencies; at low frequencies, the effects of both the molecular weight and total degree of branching could be inferred, with more highly branched samples showing lower storage and loss moduli. Although the relative amounts of SCB and LCB could not be determined, no dynamic features attributable to LCB were observed. The low‐frequency trends could be semiquantitatively fitted with reptation and retraction theory if it was assumed that an increased degree of SCB led to an increased tube size. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3335–3349, 2002     

无机电解质及聚合物对Mg-Al混合金属氢氧化物—高岭土分散体系触变性的影响

类水滑石;流变学;无机电解质及聚合物对Mg-Al混合金属氢氧化物—高岭土分散体系触变性的影响     

Effect of the comonomer addition sequence in the synthesis of an acrylonitrile terpolymer on the solution rheology and fiber properties

Polyacrylonitrile terpolymers based on acrylonitrile, acrylic acid, and methyl acrylate were synthesized by reversible addition–fragmentation chain-transfer polymerization with varying the monomer addition sequence during the synthesis. The monomer introduction sequence in the synthesis of a ternary copolymer significantly affects both the rheological properties of solutions and the strength of obtained fibers.     

Interactions of ionic surfactants with gelatin in fluid solutions and the gel state studied by fluorescence techniques,potentiometry, and measurements of viscosity and gel strength

The photobehavior of pyrene (fine structure of the monomeric fluorescence band; excited state lifetimes; excimer/monomeric fluorescence intensity ratios and excited state quenching rates by oxygen) in fluid solutions of gelatin and in the gel state, both in the absence and presence of ionic surfactants, has been examined. Surfactants considered were sodium dodecylsulfate and dodecyltrimethylammonium bromide. Potentiometric measurements performed by using surfactant selective electrodes allowed for the determination of the binding capacity of the surfactants onto gelatin in the gel state. When complemented with viscosity and gel strength measurements, the sesults obtained allow for a discussion of the effects of the surfactant-gelatin interactions on microscopic properties of the solutions or gels and their relation with macroscopic properties.     

Elastic and quasielastic light scattering from mixtures of three polymers and a solvent: The effects of polymethylmethacrylate concentration and molecular weight on the properties of polystyrene/polydimethylsiloxane/toluene solutions

Elastic and quasi-elastic light scattering data obtained from PS/PDMS/PMMA/toluene mixtures are reported. Several systems characterized by different concentrations and different molecular weights of PMMA are considered. For each of these systems, a systematic investigation is performed as a function of the scattering angle and the concentration of the PS/PDMS mixture using both techniques. The PMMA does not contribute directly to the scattered light since its increment of refractive index is zero. However the data show that its presence modifies substantially the apparent interaction parameter of PS and PDMS and the single diffusion coefficient of these polymers in the solution. The variations of these quantities with the concentration and molecular weight of PMMA are obtained. The data are analyzed within a simplified model combining based on the random phase approximation and the Rouse dynamics for ternary polymer mixtures. © 1995 John Wiley & Sons, Inc.     

The influence of shear rate, temperature and chain conformation on the critical concentration c

 When viscometry is used, a crossover phenomenon is observed separating the dilute solutions into extremely dilute solutions and dilute solutions. The critical concentration c ^**, determined from this crossover phenomenon, strongly depends on the shear rate in the solution. At very high values of shear rate the critical concentration c ^** becomes very low and depends only on the contour length of the elongated chains of different polymers. An increase of the temperature induces an increase of c ^** because the relaxation time of the chains decreases. If a polymer adopts a rodlike conformation (in a given solvent at a given temperature) the excluded volume of its chains increases and its critical concentration c ^** decreases. Received: 14 October 1996 Accepted: 3 March 1997