Rheological Characterization of Hydroxypropylcellulose Gels

The present paper describes the rheological properties of hydroxypropylcellulose (HPC) gels formulated in propylene glycol (PG), water, ethanol, and mixtures of these components. The effects of molecular weight, polymer concentration, and solvent composition on the apparent viscosity and flow characteristics have been studied by continuous shear rheometry. The HPC gels are shear thinning and do not exhibit significant yield or hysteresis in their rheograms. The apparent viscosity increases with increasing molecular weight and concentration of the polymer, as expected. Although not so pronounced at lower concentrations (≤ 1.5%), HPC gels tend to become increasingly non-Newtonian with increasing molecular weight at higher polymer concentrations (3%). A mathematical model has been proposed for the prediction of viscosities of HPC gels. There exists a high degree of dependence on molecular interactions between various solvent molecules in the prediction of mixture viscosities in ternary systems. The effects of solvent composition on the viscoelastic behavior of these gels have also been examined by dynamic mechanical analysis. The HPC gels are highly viscoelastic and exhibit greater degrees of elasticity with increased PG content in ternary solvent mixtures with water and ethanol. The study also suggests that dynamic mechanical analysis could prove to be a useful tool in the determination of zero-shear viscosities, viscosities that are representative of most realistic situations.     

Thermal stability and melt rheology of poly(<Emphasis Type="Italic">p</Emphasis>-dioxanone)

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO’s thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples’ viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (M _w = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox–Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.     

Determination of viscosities for alumina–polyethylene blends

The rheology of different proportions of low-density polyethylene (LDPE) and low-density polyethylene wax (LDWAX) without and with alumina was studied and correlated to a model. The viscosities of the polymer blend (LDPE + LDWAX) were studied for various compositions of LDWAX at different temperatures. The Arrhenius plot of logarithmic viscosity and reciprocal temperature for the polymer blends is linear. The enthalpy of viscous flow, determined from the slope of the Arrhenius Plot, varies linearly as a function of the average molecular weight. It is thus possible to predict the viscosity of any intermediate composition of the polymer blend. The viscosities of alumina-polymer blends (AP blend) with 50 vol% of alumina, prepared by solvent method, were also studied as a function of temperature, at a shear rate of 1333.33 s^–1. The AP blends show a lower enthalpy of viscous flow compared to pure polymer blend because of the presence of stearic acid used as surfactant for alumina. The model developed in this study enables the prediction of viscosities of AP blends at any weight average molecular weight (M _w) of the main binder and temperature.     

Rheological characterisation of modified binders at mixing and compaction temperature

The mixing temperature for binders is normally chosen by the pavement engineer based on a specific ‘viscosity’ required during hot mix asphalt production. Majority of the unmodified binders exhibit Newtonian behaviour at the mixing temperature and hence the determination of the same is straight-forward. However, when modified binders are used, experiments using a rotational viscometer indicate that the binder exhibits viscoelastic non-Newtonian fluid characteristic even at very high temperature. Consequently, the ‘viscosity’ varies with time and the location where it is measured, and hence is not a unique property of the material. In this work, a thermodynamically consistent, frame-invariant viscoelastic non-Newtonian fluid model was developed to characterise the rheological properties of the binders tested in a rotational viscometer. In the investigation reported here, two types of modified binders, polymer and crumb rubber, and one unmodified binder were used. These binders were subjected to steady and variable shear rate experiments in a rotational viscometer. The viscoelastic non-Newtonian model developed was able to predict reasonably the response of binders subjected to various protocols. In addition, bituminous mixtures were fabricated at different mixing and compaction temperatures using these binders, and the evolution of volumetric properties was investigated. The experimental investigation on mixtures showed that for identical aggregate gradation, the apparent viscosity of the binders played a critical role on the final volumetric properties obtained.     

Relationseips Between Drug Dissolution Profile and Gelling Agent Viscosity in Tablets Prepared with Hydroxypropylmethylcellulose (HPMC) and Sodium Carboxymethylcellulose (NaCMC) Mixtures

Two varieties of HPMC, two varieties of NaCMC and various HPMC/NaCMC mixtures were characterized with the aim of providing a sound basis for the selection of appropriate mixtures to use as gelling agents in controlled-release tablets for hydrosoluble drugs. For both HPMC and NaCMC, one variety was of high and the other of low nominal viscosity. We also investigated possible relationships between the rheological properties of HPMC/NaCMC mixtures and atenolol release from tablets prepared with such mixtures. The mean molecular weights of each polymer variety were estimated on the basis of determination of their intrinsic viscosities in aqueous dispersions. Rotational viscosimetry of 2% aqueous dispersions of the polymers and polymer mixtures revealed rheological synergism in some mixtures. Drug dissolution trials were carried out in water and 0.1 N HCl. Dissolution medium, gelling agent composition and proportion of gelling agent in the tablet all affected dissolution profiles. Fitting of Korsmeyer et al.'s equation to the data for dissolution in water indicated zero-order dissolution kinetics for all formulations. For tablets prepared with the most viscous HPMC variety, %hour dissolution efficiency was closely correlated with the apparent viscosity (shear rate 0.5 s^-1) of the aqueous dispersion of the polymer mixture used as gelling agent. Assays of tablet erosion rates indicated that the erosion mechanism may contribute to the observed zero-order dissolution kinetics, but that other factors are probably also involved.     

Relationseips Between Drug Dissolution Profile and Gelling Agent Viscosity in Tablets Prepared with Hydroxypropylmethylcellulose (HPMC) and Sodium Carboxymethylcellulose (NaCMC) Mixtures

Abstract

Two varieties of HPMC, two varieties of NaCMC and various HPMC/NaCMC mixtures were characterized with the aim of providing a sound basis for the selection of appropriate mixtures to use as gelling agents in controlled-release tablets for hydrosoluble drugs. For both HPMC and NaCMC, one variety was of high and the other of low nominal viscosity. We also investigated possible relationships between the rheological properties of HPMC/NaCMC mixtures and atenolol release from tablets prepared with such mixtures. The mean molecular weights of each polymer variety were estimated on the basis of determination of their intrinsic viscosities in aqueous dispersions. Rotational viscosimetry of 2% aqueous dispersions of the polymers and polymer mixtures revealed rheological synergism in some mixtures. Drug dissolution trials were carried out in water and 0.1 N HCl. Dissolution medium, gelling agent composition and proportion of gelling agent in the tablet all affected dissolution profiles. Fitting of Korsmeyer et al.'s equation to the data for dissolution in water indicated zero-order dissolution kinetics for all formulations. For tablets prepared with the most viscous HPMC variety, %hour dissolution efficiency was closely correlated with the apparent viscosity (shear rate 0.5 s^?1) of the aqueous dispersion of the polymer mixture used as gelling agent. Assays of tablet erosion rates indicated that the erosion mechanism may contribute to the observed zero-order dissolution kinetics, but that other factors are probably also involved.     

Evaluation of different polyolefins as rheology modifier additives in lubricating grease formulations

The purpose of the present work is to evaluate the effect that different polyolefins, used as additives in small proportions, exert on the rheological properties of standard lithium lubricating greases. Grease formulations containing several polyolefins, differing in nature and molecular weight, were manufactured and rheologically characterized. The influence of the type of polymer, molecular weight, crystallinity degree and vinyl acetate content has been analyzed. Small-amplitude oscillatory shear (SAOS) and viscous flow measurements, as well as calorimetric (DSC) and thermogravimetric (TGA) analysis, were carried out. In general, the addition of polymers such as HDPE, LDPE, LLDPE and PP to lithium lubricating greases significantly increases the values of the rheological parameters analyzed, consistency and mechanical stability. However, the use of polyolefins as rheology modifiers does not significantly affect the friction coefficient determined in a tribological contact. The crystallinity degree, mainly dependent on the nature of the polymer, has been found the most highly influencing parameter on the rheology of the lubricating greases studied. However, the rheological modification exerted by EVA copolymers mainly depends on the vinyl acetate content. Thus, a negative effect in both apparent viscosity and linear viscoelastic functions of greases was obtained when polymer vinyl acetate content is higher than 28% (w/w).     

Effects of pre-imidization on rheological behaviors of polyamic acid solution and thermal mechanical properties of polyimide film: an experiment and molecular dynamics simulation

A series of polyamic acid (PAA) solutions and corresponding polyimide (PI) films with different chemical structures were prepared through a partial pre-imidization process, and the rheological behavior of PAA solutions and thermal mechanical properties of PI films were investigated in detail. The FT-IR spectra indicate that the pre-imidization degree (pre-ID) of PI could be accurately controlled by adjusting the amount of dehydration reagents. The PI films with a certain pre-ID exhibit better mechanical properties and lower coefficient of thermal expansion. The viscosity curves of PAA solution with varying shear rate show that the PAA solutions with high pre-ID have low shear sensitivity, which exhibits a lower viscosity at the low shear rate, but retains almost the same viscosity at high shear rate. Chain conformation variations with pre-IDs and solvent content were investigated by molecular dynamic (MD) simulation. The MD results indicate that a sudden increase in cohesive energy density leads to gel at a certain pre-ID and the mobility of molecular chain decreases greatly with solvent volatilization. It is supposed that the molecular chain orientation induced by the pre-imidization will be preserved in the subsequent thermal imidization process. Therefore, the PI films with a certain pre-IDs exhibit more excellent thermal mechanical properties, especially for the PI with rigid-rod segments.

     

超高分子量聚乙烯稀溶液动力黏度的研究

采用旋转黏度计测定超高分子量聚乙烯(UHMWPE)稀溶液的动力黏度,UHMWPE树脂的浓度、溶解温度、溶剂以及相对分子质量分布对动力黏度具有很大的影响,详细分析了UHMWPE稀溶液的黏度特性,并提供了一种评价UHMWPE分子量分布有效的测试方法.     

Intrinsic viscosity of polymers and biopolymers measured by microchip

Intrinsic viscosity provides insight to molecular structure and interactions in solution. A new microchip method is described for fast and accurate measurements of viscosity and intrinsic viscosity of polymer and biopolymer solutions. Polymer samples are diluted with solvent in the microfluidic chip by imposing pressure gradients across the channel network. The concentration and flow dilutions of the polymer sample are calculated from the fluorescent signals recorded over a range of dilutions. The viscosities at various polymer dilutions are evaluated using mass and momentum balances in the pressure-driven microchannel flow. The technique is particularly important to many chemical, biological, and medical applications where sample is available in very small quantities. The intrinsic viscosity experiments were performed for three classes of polymer solutions: (a) poly(ethylene glycol), polymers with linear hydrocarbon chains; (b) bovine serum albumin, biopolymer chains with hydrophobic and hydrophilic amino acids, and (c) DNA fragments, biological macromolecules with double-stranded polymeric chains. The measured values of intrinsic viscosity agree remarkably well with the available data obtained using different methods. The data exhibit power law behavior for molecular weight as described by the Mark-Houwink-Sakurada equation. Experiments were performed to understand the effect of solvent quality and salt concentration on molecular conformations and the intrinsic viscosity of the polymers. This method offers a new way to study the conformational changes in proteins and DNA solutions in various buffer conditions such as pH, ionic strength, and surfactants. The effects of shear rate in the microchannel and mixing time on the accuracy and limitation of the measurement method are discussed.     

Book Review

Aim: Dextran methacrylate (dex-MA) and concanavalin A (con A)-methacrylamide were photopolymerized to produce covalently cross-linked glucose-sensitive gels for the basis of an implantable closed-loop insulin delivery device. Methods: The viscoelastic properties of these polymerized gels were tested rheologically in the non-destructive oscillatory mode within the linear viscoelastic range at glucose concentrations between 0 and 5% (w/w). Results: For each cross-linked gel, as the glucose concentration was raised, a decrease in storage modulus, loss modulus and complex viscosity (compared at 1 Hz) was observed, indicating that these materials were glucose responsive. The higher molecular weight acrylic-derivatized dextrans [degree of substitution (DS) 3 and 8%] produced higher complex viscosities across the glucose concentration range. Conclusions: These studies coupled with in vitro diffusion experiments show that dex-MA of 70 kDa and DS (3%) was the optimum mass average molar mass to produce gels that show reduced component leach, glucose responsiveness, and insulin transport useful as part of a self-regulating insulin delivery device.     

聚丙烯腈/二甲基亚砜纺丝原液流变性及流变方程拟合

以高黏均分子量(-Mη)聚丙烯腈(PAN)为研究对象,系统研究了黏均分子量(-Mη,以下简称分子量)、温度(T)、固含量(wt)和剪切速率(γ)对聚丙烯腈(PAN)/二甲基亚砜(DMSO)纺丝原液流变性能的影响。结果表明,纺丝原液分子量和剪切速率皆存在一个临界值。在临界值之前,表观黏度随分子量增加迅速增大,随剪切速率的增加迅速降低,在临界值之后,变化趋势均趋于平缓;且临界值不随温度变化而改变;依据试验结果进行表观黏度与四因素线性相关方程拟合,经对比,计算值与实验值相差不大,可以为纺丝原液表观黏度的预测和后续纺丝工艺的改进提供理论参考。     

不同分子量丙烯腈-丙烯酰胺共聚物溶液的流变性研究

将不同分子量的丙烯腈（AN）与丙烯酰胺（AM）的共聚物溶解于二甲亚砜（DMSO）中，获得聚丙烯腈（PAN）溶液。采用旋转粘度计和锥板流变仪，从非牛顿指数n、流动活化能△Eq、结构粘度指数△ηa和弹性模量G等方面比较了不同分子量PAN溶液的流变性能。结果表明，分子量高的PAN溶液的表观粘度对应力变化敏感，分子理低的PAN溶液的表现粘度对温度的变化敏感，无论是从粘性还是从弹性角度看，低分子量PAN溶液的物理稳定性好。     

Prediction of the Thermal Conductivity and Viscosity of Binary and Ternary HFC Refrigerant Mixtures

A recently developed scheme, based on considerations of hard-sphere theory, is used for the simultaneous prediction of the thermal conductivity and the viscosity of binary and ternary HFC refrigerant mixtures, consisting of HFC-32, HFC-125, and HFC-134a. In this prediction scheme, the hypothetical molecular parameters of HFC refrigerant mixtures were assumed to be the molar average of the pure component values. The close agreement between the predicted values and the experimental results of thermal conductivity and viscosity demonstrate the predictive power of this scheme.     

CPE／红泥填充体系流变性能

通过测试一系列不同与配比的ＣＰＥ／红泥填充体系的流变性能，探讨了级分比、温度等对体系流动性的影响，表明该填充体系具有剪切变稀的非牛顿性质；体系的表现切粘度随填充红泥量的增加有不同程度的增大；温度升高，体系的表观切粘度下降，温度对体系流动性的影响随填充红泥量的增加而减少，随切变速率的增大而减少；在低切变速率条件下，红泥粒度对体系的流动性有影响。     

Gelation behaviour of UHMWPE/camphene

The sol–gel transition behaviour of ultra-high molecular weight polyethylene (UHMWPE) in decalin, paraffin oil and camphene has been investigated. UHMWPE solutions exhibit a thermo-reversible gelation, recognized in gels, which is due to crystallization. Viscosity results indicated that polymer molecules disentangled more easily when camphene was used as a solvent. This characteristic implies that camphene is a more desirable solvent for solution processing of UHMWPE. A higher recrystallization temperature, Tc, also occurred at a higher gelation temperature, Tgel. The gel melting temperature, Tm,gel, was also measured and its dependency on polymer concentration was investigated using the Eldridge–Ferry equation. The reciprocal of the absolute gel melting temperature was found to have a linear relationship with the logarithm of polymer concentration     

Solvent control of crack dynamics in a reversible hydrogel

The resistance to fracture of reversible biopolymer hydrogels is an important control factor of the textural characteristics of food gels (such as gummy candies and aspic preparations). It is also critical for their use in tissue engineering, for which mechanical protection of encapsulated components is needed. Its dependence on loading rate and, recently, on the density and strength of crosslinks has been investigated. But, so far, no attention has been paid to solvent or to environment effects. Here we report a systematic study of crack dynamics in gels of gelatin in water/glycerol mixtures. We show in this model system that increasing solvent viscosity slows down cracks; moreover soaking with solvent markedly increases gel fragility; finally tuning the viscosity by adding a miscible liquid affects crack propagation through diffusive invasion of the crack tip vicinity. The results highlight the fact that fracture occurs by viscoplastic chain pull-out. This mechanism, as well as the related phenomenology, should be common to all reversibly crosslinked (physical) gels.     

Liquid Viscosity of Binary Mixtures of Methanol with Ethanol and 1-Propanol from 273.15 to 333.15 K

The liquid viscosities and densities of two binary mixtures of methanol with ethanol and 1-propanol were measured in the temperature range from 273.15 to 333.15 K with a capillary viscometer and a glass pycnometer, respectively. The uncertainties in the measured viscosities were estimated to be smaller than 1.3%. The experimental viscosity values could be fitted to the Mertsch and Wolf equation within 2%.     

High-performance capillary electrophoresis of SDS-protein complexes using UV-transparent polymer networks.

This paper demonstrates the use of UV-transparent replaceable polymer networks for the separation of SDS-protein complexes on the basis of molecular weight. First, the use of linear (i.e. non-cross-linked) polyacrylamide is shown to provide molecular separation of SDS-protein complexes. A study reveals such columns to yield significantly greater lifetime than cross-linked gels because of the flexibility of the noncovalently attached polymer chains. However, column lifetime was still found to be limited (approximately 20-40 injections), and detection at 214 nm was problematical because of the absorbance of polyacrylamide. UV-transparent polymer networks of dextran and PEG were substituted for polyacrylamide with successful molecular weight sieving of SDS-protein complexes at 214 nm. Due to their low to moderate viscosities, these networks could be routinely replaced leading to the possibility of hundreds of injections with a single column. Migration time reproducibilities of 0.5% RSD or less were found with replacement of the network. Using dextran, calibration plots of peak area vs concentration of standard protein were linear over the range of 0.5 microgram/mL up to at least 0.25 mg/mL. Furthermore, plasma samples could be directly utilized because of the strong solvating power of SDS. Rapid separation of protein mixtures are demonstrated with these UV-transparent polymer networks.     

Rheological Behavior for Polymer Melts and Concentrated Solutions Part VII: A Quantitative Verification for the Molecular Theory of Non-linear Viscoelasticity with Entanglement Constraints in Polymer Melts

Based on the molecular theory of non-linear viscoelasticity with constrained entanglements in polymer melts, the material functions in simple shear flow were formulated, the theoretical relations between η(),ψ10()and shear rate(),and topologically constrained dimension number n＇and a were derived. Linear viscoelastic parameters (η0 and G0N)and topologically constrained dimension number (n＇,a and )as a function of the primary molecular weight(Mn),molecular weight between entanglements (Mc) and the entanglement sites sequence distribution in polymer chain were determined. A new method for determination of viscoelastic parameters (η0,ψ10,G0N and J0e),topologically constrained dimension number(n＇,a and v)and molecularweight (Mn, Mc and Me) from the shear flow measurements was proposed.It was used to determine those parameters and structures of HDPE, making a good agreement between these values and those obtained by other methods. The agreement affords a quantitative verification for the molecular theory of nonlinear viscoelasticity with constrained entanglement in polymer melts.