Effect of degree of saponification on the rheological properties of syndiotactic poly(vinyl alcohol)/ water solution

To identify the effect of degree of saponification (DS) of syndiotactic poly(vinyl alcohol)s (s‐PVA)s having similar tacticity and molecular weight on the rheological properties of s‐PVA/water solution, four kinds of (s‐PVA)s with assigned (DS)s, from 93.1 to 97.5%, were prepared by copolymerization of vinyl pivalate (VPi) and vinyl acetate (VAc), followed by saponifying the corresponding copoly(VPi/VAc). The DS played a significant role in rheological behavior. Over the frequency range of 10^?1 to 10² rad/s s‐PVA with higher DS shows more shear thinning at similar molecular weight and tacticity of polymer, suggesting that PVA molecules are more readily oriented as DS increases. This may provide indirect evidence of the spontaneous in situ orientation of s‐PVA molecules at the late stage of saponification. Yield stress is higher for s‐PVA with higher DS at similar molecular weight and tacticity of s‐PVA. This indicates that more domains with internal order are produced at higher saponification. These facts result from increase in stiffness of s‐PVA molecules with proceeding the saponification reaction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 463–467, 2002     

Gelation of poly(vinyl alcohol) in dimethyl sulfoxide/water solvent

Summary Gelation of poly(vinyl alcohol) (PVA) in dimethyl sulfoxide (DMSO)/water was observed at 23°C with viscometry, spectrophotometry, wide angle X-ray scattering and light scattering. Here transparent gel formation was found to take place prior to being turbid in some cases, whereas the solution became turbid prior to gelation in other cases. Whether transparent gel is formed at first or solution becomes turbid, depends on DMSO composition. PVA solution forms gel in the DMSO composition range from 20 to 80 wt.%. Below the boundary DMSO composition of 60–70 wt.%, gelation takes place at first (i.e. transparent gel is formed) and then becomes turbid eventually, while beyond this DMSO composition the solution becomes turbid and then opaque gel is formed.     

Rheological and rheo-optical properties of high molecular weight syndiotactic and atactic polyvinylalcohol solutions

The rheological and rheo-optical properties of solutions of high molecular weight polyvinylalcohol (PVA) with different syndiotactic diad contents in dimethylsulfoxide (DMSO) were investigated in terms of tacticity, molecular weight, and degree of saponification. Tacticity played a significant role in rheological behavior. Over the frequency range of 10^?1 to 10² rad/s PVA with syndiotactic diad content of ～53% (atactic PVA) exhibited almost Newtonian flow behavior whereas PVA with syndiotactic diad content of ～63% (syndiotactic PVA) exhibited Bingham flow behavior. On the plot of storage modulus (G′) against loss modulus (G″) atactic PVA gave slopes of ～2 while syndiotactic PVA gave slopes of ～1. With syndiotactic PVA, an increase of shear rate notably increased flow birefringence (Δn_f) at shear rates higher than 5 sec^?1. On the other hand, only a slight increase in Δn_f was observed in the case of atactic PVA. The effects of molecular weight and degree of saponification were discussed as well.     

Rheological properties of high molecular weight (HMW) syndiotactic poly(vinyl alcohol) (PVA)/HMW atactic PVA blend solutions

To identify the effect of blend ratios of syndiotacticity‐rich poly(vinyl alcohol) (s‐PVA)/atactic PVA (a‐PVA) having similar number‐average degrees of polymerization (P_n)s of 4000 and degrees of saponification (DS)s of 99.9% on the rheological properties of s‐PVA/a‐PVA/water solutions, water‐soluble s‐PVA and a‐PVA with different syndiotactic diad contents of 58.5 and 54.0%, respectively, were prepared by bulk copolymerization of vinyl pivalate and vinyl acetate (VAc) and solution polymerization of VAc, followed by saponifying the corresponding copoly(vinyl pivalate/vinyl acetate) and poly(vinyl acetate). The blend ratios played a significant role in rheological behavior. Over the frequency range of 10^?1–10² rad/s, s‐PVA/a‐PVA blend solutions with larger s‐PVA content show more shear thinning at similar (P_n)s and (DS)s of polymer, suggesting that PVA molecules are more readily oriented as s‐PVA content increases. Yield stress is higher for s‐PVA/a‐PVA blend solutions with larger s‐PVA content at similar (P_n)s and (DS)s of polymer. This indicates that more domains with internal order are produced at larger s‐PVA content in s‐PVA/a‐PVA blend solutions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3934–3939, 2006     

Rheological properties of water solutions of syndiotactic poly(vinyl alcohol) of different molecular weights

To precisely identify the effect of the molecular weight of syndiotacticity‐rich poly(vinyl alcohol) (s‐PVA) on the rheological properties of s‐PVA/water solutions, we prepared four s‐PVAs with a syndiotactic dyad content of 57%, a degree of saponification (DS) of 99.9%, and number‐average degrees of polymerization (P_n's) of 300, 1300, 2700, and 4000. Through a series of experiments, we found that the molecular weight of poly(vinyl alcohol) had a significant influence on the rheological properties of s‐PVA/water solutions. Over a frequency range of 10^?1 to 10² rad/s, the s‐PVA/water solution with the highest P_n value showed the largest values of the complex viscosity, storage modulus, and loss modulus at similar syndiotacticity and DS values of s‐PVA, and this suggested that the higher P_n was, the stronger the internal ordered structure was in the molecules. All the s‐PVA/water solutions showed shear‐thinning behavior, which implied heterogeneity. In a modified Casson plot, Bingham flow behaviors, which gave rise to non‐zero yield stress, were evident. This suggested that some pseudostructure existed in the s‐PVA/water solutions. The yield stress increased with P_n, and this implied that the pseudostructure was developed as P_n increased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1426–1431, 2004     

Dynamical and structural behavior of hydroxyethylcellulose hydrogels obtained by chemical gelation

Rheological, dynamical, de‐swelling, turbidity, and structural features during gelation with a chemical cross‐linking agent of semi‐dilute aqueous solutions of hydroxyethylcellulose (HEC) were investigated at different cross‐linker concentrations. A higher level of cross‐linker leads to faster gelation and stronger incipient hydrogel. The rheological results favor the percolation model. The cross‐linking reaction proceeds over a long time in the post‐gel region, and this produces gels with a solid‐like response. Dynamic light‐scattering experiments reveal a fast and a slow relaxation mode. The slow relaxation time increases and the sample becomes more heterogeneous in the course of gelation. In the post‐gel region, the gels shrink during a long period of time and this phenomenon is accompanied by strong turbidity enhancement. Small‐angle neutron scattering results from samples quenched at a certain stage in the post‐gel regime disclose growth of heterogeneity in the gel with increasing level of cross‐linker addition. At smaller length scales, no effect of cross‐linker addition could be detected on the structural organization, and scattered intensity in this domain suggests that the HEC chains are locally stretched. Copyright © 2006 Society of Chemical Industry     

Gelation of Plasticized Polyvinyl Chloride Compounds in a Co‐Kneader

The co‐kneader is a proper tool for processing polyvinyl chloride (PVC) compounds. Gelation of PVC is complex because of a narrow processing window. At low temperature, gelation is incomplete and leads to poor mechanical properties, while at high temperature, degradation may occur. The aim of this study is to gain a fundamental understanding of the gelation process of PVC in co‐kneaders. Careful examination of the gelation state of extruded PVC samples was carried out qualitatively by swelling the extruded samples in acetone and quantitatively by differential scanning calorimetry measurements. Gelation in various conditions of temperature and shear was firstly studied in a capillary device that ensures both pre‐shearing and extrusion through a circular die. This provided a reference basis for comparison with gelation state obtained in the co‐kneader. Samples were collected in the co‐kneader by means of opening the barrel. The gelation state was particularly examined in connection with the measured temperature at a fixed position along the screw at which a temperature sensor was positioned. The obtained results showed that the co‐kneader allows fast gelation with a mixing energy value lower than 200 kJ/kg when the temperature is higher than a critical value of 165°C for the studied formulation. J. VINYL ADDIT. TECHNOL., 2019. © 2019 Society of Plastics Engineers J. VINYL ADDIT. TECHNOL., 26:316–324, 2020. © 2019 Society of Plastics Engineers     

PAN／DMSO溶液凝胶化行为的流变学研究

通过动态流变测试,研究PAN／DMSO溶液的热致变凝胶化过程和测定PAN／DMSO溶液的凝胶点温度。主要通过测量动态流变学参数（动态模量G′i和G″ii、损耗角正切tanδ、复数粘度η^＊）来表征PAN溶液的凝胶化行为,分析了含水量对溶液凝胶化行为和凝胶结构的影响。结果发现,虽然PAN溶液的凝胶点温度随着溶液中含水量的增加而升高,但是由松弛指数n和分形维数值df表征的凝胶结构几乎不受溶液中含水量的影响。     

Strongly thixotropic viscosity behavior of dimethylsulfoxide solution of polyrotaxane comprising α-cyclodextrin and low molecular weight poly(ethylene glycol)

A strong thixotropic viscosity behavior was observed when polyrotaxane prepared from α-cyclodextrins (CDs) and poly(ethylene glycol) (PEG) with a molecular weight of 2000 was dissolved in dimethylsulfoxide (DMSO). A 10 wt% solution liquefied by vigorous shaking was rapidly gelated by standing - this sol-gel transition was reversible. The time for recovering the viscosity was dependent on the polyrotaxane concentration, i.e., a 10 wt% solution regelated within 30 s, whereas several hours were required for the gelation of a 2.5 wt% solution. The thixotropic nature of the solution was also confirmed by the clockwise hysteresis curve of the viscosity when the shear rate was increased and decreased. The gel permeation chromatography (GPC) measurement of the polyrotaxane in DMSO exhibited peaks in the high molecular weight region. The peak disappeared after the phenylcarbamoylation of polyrotaxane, suggesting that the peak was due to loose aggregations of polyrotaxane in DMSO. On the other hand, the DMSO solution of polyrotaxane prepared from CD and PEG with a molecular weight of 3350 - whose inclusion ratio (51%) is slightly lower than that of PEG2000 polyrotaxane (72%) - neither demonstrated the abovementioned thixotropic viscosity nor the peak corresponding to the aggregations occurring during the GPC measurement. The thixotropic behavior was speculated to be caused by the combined contribution of intermolecular attractive hydrogen bonding and higher rigidity of polyrotaxane prepared from PEG2000 than that of polyrotaxane prepared from PEG3350, presumably due to the higher inclusion ratio of the former than that of the latter.     

Role of the stereosequences of poly(vinyl alcohol) in the rheological properties of syndiotacticity‐rich poly(vinyl alcohol)/water solutions

Syndiotacticity‐rich poly(vinyl alcohols) (s‐PVAs) with various syndiotactic dyad (S‐dyad) contents were prepared by the copolymerization of vinyl pivalate and vinyl acetate with various monomer feed ratios, which was followed by the saponification of copoly(vinyl pivalate/vinyl acetate) to investigate the effect of the stereosequences of s‐PVA on the rheological properties of s‐PVA/water solutions. Through a series of experiments, we identified that the syndiotacticity had a profound influence on the rheological properties of s‐PVA/water solutions. Over a frequency range of 10^?1 to 10² rad/s, s‐PVAs with higher S‐dyad contents showed larger values of complex viscosity and storage modulus and more shear thinning at similar molecular weights and degrees of saponification of the polymer, suggesting that poly(vinyl alcohol) molecules were stiffer and more readily oriented as syndiotacticity increased. All the yield stresses of s‐PVA represented positive values, and s‐PVAs with higher syndiotacticity showed higher yield stresses. This suggests that as syndiotacticity increased, more pseudostructures were present in s‐PVA/water solutions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1858–1863, 2003     

Effect of molecular weight on the rheological properties of atactic poly(vinyl alcohol)/dimethylsulfoxide/water solution

This article describes the molecular weight effect of atactic poly(vinyl alcohol) (a‐PVA) on the rheological properties of 7.5, 10.0, and 12.5 g/dL solutions of a‐PVA with number‐average degrees of polymerization (P_n) of 4000 and 1700 in dimethylsulfoxide/water mixture. a‐PVA with a P_n of 1700 solutions exhibited almost Newtonian flow behavior, whereas high molecular weight a‐PVA, with a P_n of 4000 solutions, exhibited shear‐thinning behavior. On the plot of storage and loss moduli of a‐PVA with a P_n of 1700 solutions, the dynamic storage modulus of a‐PVA, with a P_n of 1700 solutions, was smaller than the dynamic loss modulus over the frequency range of 10^?1 to 10² rad/s. However, the dynamic storage modulus of a‐PVA, with a P_n of 4000 solutions, was smaller than the dynamic loss modulus in the sol state and, in the postgel state, the dynamic storage modulus became larger than the dynamic loss modulus, indicating the evolution of viscoelastic solid properties. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 41–46, 2004     

A study on environment‐friendly polymer gel for water shut‐off treatments in low‐temperature reservoirs

In this study, the zirconium acetate crosslinked gel systems are studied owing to their environment‐friendly and gelation performance in low‐temperature reservoirs through rheological measurements, environmental scanning electron microscopy, and scanning electron microscopy. The effects of various parameters on the gelation properties, stability, and microstructure in bottle test and porous media were addressed. With the increase of concentrations and temperature, gelation time is reduced and gel strength is increased. In addition, the gel systems show salt tolerance and shearing resistance. The environment‐friendly gel systems have a high stability in both injection water and formation water. A three‐dimensional network structure was formed in the gel and confirmed by environmental scanning electron microscopy. The three‐dimensional gel network was also formed in porous media, which bridges across the pore throats and reduced the water permeability in the formation. This study suggests that environment‐friendly polymer gels can be used for water shut‐off treatments in low‐temperature reservoirs. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40154.     

Rheological characterization of the gel point in polymer‐modified asphalts

In this work, the rheological characterization of the gel point in polymer‐modified asphalts is carried out. The viscoelastic properties of polymer‐modified asphalts, in which the polymer is styrene–ethylene butylene–styrene (SEBS) with grafted maleic anhydride (MAH), were measured as a function of MAH concentration. The crosslinking reaction that leads to gelation is characterized by power‐law frequency‐dependent loss and storage modulus (G″ and G′). The relaxation exponent n (a viscoelastic parameter related to the cluster size of the gel) and gel strength S (related to the mobility on the crosslinked chain segments) were determined. The value of the power‐law exponents depends on the composition of polymer, ranging from 0.30 to 0.56, while the value of the rigidity modulus at the gelation point (S) increases with the amount of reactive groups of the modifier polymer. Both n and S are temperature‐dependent in the blends. The blends containing gels present a coarse morphology, which is related to the rheological properties of the matrix and dispersed phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Study of cryostructuration of polymer systems. XX. Foamed poly(vinyl alcohol) cryogels

Foamed poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze–thaw treatment of whipped PVA water solutions (polymer with MW of 69,000 Da and DD ～99 mol % was used), were obtained and their properties were studied. The rheological characteristics and macrostructure of these gel materials were controlled by the same factors as for the ordinary nonfoamed PVA cryogels (initial polymer concentration and freezing–thawing regimes) and also by the conditions of generation of fluid PVA foams. The study of the kinetics of the freeze–thaw‐induced gel formation of these foams revealed that the temperature dependence of the efficiency of cryotropic gelation showed a maximum at about ?1.5°C. The presence of low molecular weight admixtures in the initial polymer solution appears to be a rather important factor because the admixtures were capable of decreasing the stability of fluid PVA foams and weakening both foamed and nonfoamed cryogel samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1609–1619, 2001     

瓶级聚酯质量对流变及加工性能的影响研究

研究和分析了加工温度、特性黏数、摩尔质量及其分布、间苯二甲酸含量(IPA)、二甘醇(DEG)含量等因素对瓶级聚酯流变性及加工性能的影响。结果表明,在低剪切速率下,温度的升高使熔体的切力变稀现象逐渐减弱,当剪切速率达到8 000 s-1以上时,温度及剪切速率都几乎对熔体的表观黏度没有影响;摩尔质量越高,熔体黏度的切变速率依赖性越大,摩尔质量分布宽有利于成型加工条件的控制;在相同剪切速率下,熔体的剪切黏度随着IPA含量的增大而逐渐降低;DEG含量低的聚合物熔体在低剪切速率下出现非牛顿性流动的现象比DEG含量高的要明显得多。     

Curing of an epoxy resin modified with nanoclay monitored by dielectric spectroscopy and rheological measurements

Dielectric and rheological measurements have been performed in “real‐time” to follow curing reaction on blends of a diglycidyl ether of bisphenol‐A epoxy resin with 4,4‐methylene bis(2,6‐diethylaniline) hardener and different amounts of organically modified nanoclay (Nanofil 919) as modifier. The effect of the modifier and its amount on the curing reaction, as well as that of the curing temperature has been studied. Changes in molecular mobility in the reaction mixture have been investigated by dielectric relaxation spectroscopy. Evolutions of ionic conductivity and main relaxation have been analyzed and vitrification times have been obtained and compared with those obtained by rheological measurements. The relaxational behavior has been analyzed through curing in the frequency domain, being the displacement of the main relaxation indicative of the cure reaction advancement. Gelation and vitrification times for the nanocomposite systems have been found to be lower than for the neat resins, indicating a catalytic effect of the clays on the curing reaction. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5927–5933, 2006     

A rheological study of aqueous separan solutions

The concentration dependence of the rheological response of aqueous Separan solutions was measured. The shear and primary normal stress response in steady shearing flow, and the relaxation of shear stress following cessation of steady shear flow, were measured with four different polymer solution concentrations. Two different nonlinear constitutive equations, Bird-Carreau and Meister, were used to analyse the data. The Bird-Carreau model fit the steady shear data very well and the parameters obtained from steady shear data allow reasonable estimates of the stress relaxation of these systems. The Meister model provided a fit of the transient stress relaxation data and could be used to estimate the steady shear response. No quantitative molecular theory was developed in this work. However, the nature of the concentration dependence of the Bird-Carreau parameters suggests that as the polymer concentration of aqueous Separan solutions decreases from 2.0 to 0.05 weight percent the solutions should be modeled less by an entangled network theory and more as slightly overlapping hydrodynamic units. The concentration where this behavior seems to be changing appears to be about 0.25 percent by weight of polymer.     

Performance of fly ash based polymer gels for water reduction in enhanced oil recovery: Gelation kinetics and dynamic rheological studies

The complexity of well and reservoir conditions demands frequent redesigning of water plugging polymer gels during enhanced oil recovery (EOR). In the present study, we developed coal fly ash (CFA) based gels from polyacrylamide (PAM) polymer and polyethyleneimine (PEI) crosslinker for water control in mature oil fields. The CFA acts as an inorganic additive to fine-tune gelation performance and rheological properties of PAM/PEI gel system. Hence, effects of various CFA (0.5 to 2 wt%), PAM (2 to 8.47 wt%) and PEI (0.3 to 1.04 wt%) concentrations on gelation kinetics and dynamic rheology of pure PAM/PEI gel and PAM/PEI-CFA composite gels were studied at a representative reservoir temperature of 90 oC. Experimental results reveal that gelation time of pure PAM/PEI gel increases with increasing CFA addition. Further observation demonstrates that increasing PAM and PEI concentrations decreases the gelation times of PAM/PEI-CFA composite gels. Gelation time was found to be within 3-120 hours. Understanding the property of reaction order enables better prediction of gelation time. Dynamic rheological data show that viscoelastic moduli (G′ and G″) of various PAM/PEI-CFA composite gels improved better as compared to the pure PAM/PEI gel across the strain-sweep and frequency-sweep tests. SEM analysis of selected samples at 72 hours and 720 hours of gelation activity consolidated gelation kinetics and dynamic rheological results. These polymer gels are excellent candidates for sealing water thief zones in oil and gas reservoirs.     

Influence of shearing and time on the rheological properties of milk chocolate during tempering

The quality of chocolate is highly dependent on proper tempering. During tempering, crystals are formed as the chocolate is subjected to shear. The aim of this work was to evaluate the applicability of traditional rheological techniques to gain a better understanding of the relationship between rheological properties and the degree of shearing and time of tempering. Treatments of two shearing rates (15 and 30 s⁻¹) covering four tempering times (0, 400, 600, and 800 s) were investigated. As samples were cooled from a melt temperature of 50°C, the apparent viscosity followed the Arrhenius equation. An induction time, which was affected by shear rate, was observed during tempering, followed by an increase in apparent viscosity, caused by formation of crystalline structures. Various characteristics of the rewarming curve were attributed to polymorphism. Steady-shear constitutive models assessed after rewarming showed that the yield stress was dependent on shear and tempering time. Collectively, findings showed that traditional rheological methods may be used as tools to investigate crystallization and flow properties of molten chocolate indirectly during tempering.     

Selected properties of pH‐sensitive,biodegradable chitosan–poly(vinyl alcohol) hydrogel

Chitosan and poly(vinyl alcohol) (PVA) were used to form a semi‐interpenetrating polymeric network with glutaraldehyde as the cross‐linker. The molecular weight and degree of deacetylation of the chitosan were 612 kDa and 72 %, respectively. The chemical bonds formed by the cross‐linking reaction and transition of these bonds in different pH media were investigated. The gelation property of the chitosan–PVA pregel solution and mechanical properties of the hydrogel were studied. The FTIR spectra of the hydrogel before and after swelling at pH 3 and pH 7 indicated formation of Schiff's base (C?N) and ? NH₃⁺. They also showed pH‐induced transition of C?N to C? N, and ? NH₃⁺ to ? NH₂, as well as the instability of the Schiff's base. The chitosan is essential for hydrogel formation through Schiff's base reaction between the amino groups of the chitosan and the aldehyde groups of the glutaraldehyde. The addition of PVA improved the mechanical properties of the hydrogel. However, PVA tends to leach out at longer swelling times in the acidic medium due to hydrolysis of the gel networks, Schiff's base. Copyright © 2004 Society of Chemical Industry