Rheology of polymer brush under oscillatory shear flow studied by nonequilibrium Monte Carlo simulation

The rheological behaviors of polymer brush under oscillatory shear flow were investigated by nonequilibrium Monte Carlo simulation. The grafted chain under oscillatory shear flow exhibited a waggling behavior like a flower, and the segments were found to have different oscillatory phases along the chain contour. Stress tensor was further obtained based on the statistics of sampled configuration distribution functions. The simulation reproduced the abrupt increase of the first normal stress difference N(1) with the flow velocity over a critical value, as observed in the experiment of Klein et al. [Nature (London) 352, 143 (1991)]. However, our simulation did not reproduce the brush thickening with shear velocity increased, which was suggested to be responsible for the abrupt increase of N(1) in the above-mentioned paper. This simulation demonstrates that the increase of normal stress might be an inherent behavior of polymer brush due to chain deformation under flow.     

The spatial inhomogeneity of rodlike polymeric systems subject to simple shear flow

A novel simulation approach has been developed by the combination of Brownian dynamics and Lebwohl-Lasher nematogen potential, by which we have made preliminary investigation on the spatial inhomogeneity of nematic rodlike polymeric system with planar surface anchoring and subject to simple shear flow. Some rheological behaviors such as the damped oscillation of stresses with time or the negative first normal stress difference at moderate shear rate find good agreement with previous experimental observations reported in literature. The distribution function of rod orientation takes on “breathe-like” time-evolution and is verified not Gaussian-featured. The layered director, order parameter, shear stress and first normal stress difference have also been calculated by statistics, and most importantly, all of these statistic quantities show spatial distribution in different layers and corresponding distribution functions are time-dependent. Therefore, our simulation implies the complicated pattern formation and pattern evolution in this system. It is, hence, very meaningful to further study the spatial inhomogeneity of rodlike polymers under shear flow extensively both in theory and in experiment.     

Rheology of textured rodlike polymeric systems under shear flow as revealed by brownian dynamics simulation

A Brownian dynamics simulation algorithm has been developed in this paper to study the rheology of rodlike polymers under shear flow. Through the introduction of the Lebwohl-Lasher potential and consideration of the nearest-neighbor interaction without mean-field approximation, the spatial inhomogeneity of the director field, stresses as well as the multiple-domain effect are automatically included. By this new algorithm, the transient rheological behaviors of textured and untextured (uniformly aligned) rodlike polymeric systems are investigated. The damped oscillations of stresses are reproduced and the negative first normal stress differences are found at the moderate shear rate, all of which agree with the experimental observations in literature very well. Our simulation also demonstrates that the textured initial states only affect the transient responses of stress etc. at early shear stage and that the multiple domains formed during shear flow started from both textured and untextured initial states are essentially the same. The multiple domains are, therefore, the inherent structures resulting from the instability of shear flow, which are not sensitive to the initial textures, and the so-called multiple domains are conceptually different from the rest textured structures.     

Real-time rheology of shear-induced organoclay dispersion in isotactic polypropylene

In this paper, rheological properties evolution of the simple mixed isotactic polypropylene/organoclay composites, impacted by intermediate- or large-amplitude oscillatory shear fields, was followed by dynamic melt rheometry. The physical meanings of such rheological evolution upon oscillatory shearing, which related closely to the dispersion and intercalation of organoclay in polymer, were discussed deeply. Especially, a structural recovery test was adopted to assess microstructure development induced by large-amplitude oscillatory shear and to better understand the intercalation mechanism. Based on the experimental results, a novel intercalation mechanism that was taken to account for the disentanglement of polymer chains was suggested to describe shear-induced dispersion behaviors of organoclay in polymer matrix.     

Rheological behaviors of a series of hyperbranched polyethers

Hyperbranched polyethers with different structures and molecular weights(MW) were synthesized using the A2+B3 approach by varying monomer ratio, A2 structure, and reaction time. Effects of backbone structure and MW on melt rheological behaviors were investigated by both small amplitude oscillatory shear and steady shear measurements. Master curves were constructed using the time-temperature superposition principle and compared. In the reduced frequency range covered, lg G″~lg(ω·aT) always show a slope of 1.0, suggesting a terminal zone behavior; in contrast, unexpected step changes or plateaus are observed on lg G′ master curves. Effects of backbone structure and MW on master curves were discussed. The Cox-Merz rule was tested at different temperatures and was found to be applicable when flow instability was absent.     

Modeling the phase separation in binary lipid membrane under externally imposed oscillatory shear flow

By adding external velocity terms, the two-dimensional time-dependent Ginzburg-Landau (TDGL) equations are modified. Based on this, the phase separation in binary lipid membrane under externally imposed oscillatory shear flow is numerically modeled employing the Cell Dynamical System (CDS) approach. Considering shear flows with different frequencies and amplitudes, several aspects of such a phase evolving process are studied. Firstly, visualized results are shown via snapshot figures of the membrane shape. And then, the simulated scattering patterns at typical moments are presented. Furthermore, in order to more quantitatively discuss this phase-separation process, the time growth laws of the characteristic domain sizes in both directions parallel and perpendicular to the flow are investigated for each case. Finally, the peculiar rheological properties of such binary lipid membrane system have been discussed, mainly the normal stress difference and the viscoelastic complex shear moduli.     

Rheology of a polymer-based hybrid suspension composed of concentrated poly[(D,L-lactide)-co-glycolide] solution and inorganic salt particles

Suspensions composed of a solution of a biodegradable polyester in a volatile organic solvent, and solid inorganic particles of size in the order of hundreds of microns have been very important in the fabrication of porous scaffolds in the field of tissue engineering. This article reports the basic rheological investigations of this complex fluid type. A Couette geometry covered by silicon oil was found to be an appropriate geometry to retain stability of the rheological measurements. Suspension viscosity increased with the particle volume fraction, and the extent of the increase was much larger than that predicted by the Einstein suspension equation. Both start-up dynamics at the inception of steady shear and relaxation after an abrupt change of oscillatory shear frequency in the suspension showed significantly different behaviors from those in the associated polymer solution. Particle reorganization upon change of rheological state was anticipated.     

Rheological behaviors and molecular motions of semi-diluted Xanthan solutions under shear: Experimental studies

Semi-diluted Xanthan solution has been widely used in various fields, especially in enhancing oil recovery. The oscillatory shear and flow shear behaviors of Xanthan are important to oil flooding. The oscillatory shear relates to molecular motions, while flow shear reflects flowing characterization. In oscillatory shear mode, the storage modulus, loss modulus and tanδ has been measured. Calculating relaxation spectra through storage modulus, we found that the peak of segments’ relaxation heads to smaller relaxation time side. Also, the quantity of relaxation units increases as concentration increases. However, the relaxation time spectra are less affected by salinity. In flow shear mode, the relationship between shear rate and viscosity has been investigated. As concentration or salinity increases, the pseudoplastic of Xanthan solutions becomes more obvious. Furthermore, primary normal stress differences of Xanthan semi-diluted solutions lightly increase at first then sharply decrease as shear rate increases. This abnormal phenomenon may refer to wall slip.     

Oscillatory shear flow of polymeric systems

On the basis of the Leonov viscoelastic constitutive equation, oscillatory shear flow of elastic fluids in the linear and nonlinear regimes has been considered. The Fourier components and associated phase angles of the shear and normal components of the elastic strain tensor have been found as functions of frequency and deformation amplitude in the range usually employed in experiment, and are presented in a form convenient for further rheological applications. In the linear case, the results correspond to many known theories. In the nonlinear case, the theoretical results have been compared with experiments, on different polymeric systems, with very good agreement being obtained for the shear stress in polymeric solutions but only qualitative agreement for the shear stress and first normal-stress difference in polymer melts.     

Transient rheological behavior of lyotropic (acetyl)(ethyl)cellulose/m-cresol solutions

The chiroptical properties and transient rheological behavior of (acetyl)(ethyl)cellulose (AEC) m-cresol liquid crystalline solutions have been investigated. Chiroptical properties were manipulated through (i) increasing degree of acetylation of ethyl cellulose (EC), and (ii) blending AEC with EC. At the same average degree of acetylation (DA), the chiroptical properties of pure AEC were different from those of the EC/AEC mixtures. However, at the same DA, the AEC and mixed AEC/EC solutions showed similar steady state flow and oscillatory behaviors, but the transient behaviors were different. At high flow rates the mixed AEC/EC solutions exhibited double recoil after cessation of steady-state flow, whereas the AEC solutions showed double recoil only in the high DA AEC solutions. All solutions, pure and mixed, had the same stress relaxation behavior. Both pitch and handedness affected the transient behavior. After cessation of high shear rate flow, the rate of modulus evolution decreased with increasing pitch, and was faster in right-handed mesophases than in left-handed ones at a similar pitch.     

Study on Non-Newtonian Behaviors of Lennard-Jones Fluids via Molecular Dynamics Simulations

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheological behaviors of a monoatomic fluid governed by the Lennard-Jones potential. Both steady Couette and oscillatory shear flows are investigated. Shear thinning and normal stress effects are observed in the steady Couette flow simulations. The radial distribution function is calculated at different shear rates to exhibit the change of the microscopic structure of molecules due to shear. We observe that for a larger shear rate the repulsion between molecules is more powerful while the attraction is weaker, and the above phenomena can also be confirmed by the analyses of the potential energy. By applying an oscillatory shear to the system, several findings are worth mentioning here:First, the phase difference between the shear stress and shear rate increases with the frequency. Second, the real part of complex viscosity first increases and then decreases while the imaginary part tends to increase monotonically, which results in the increase of the proportion of the imaginary part to the real part with the increasing frequency. Third, the ratio of the elastic modulus to the viscous modulus also increases with the frequency. These phenomena all indicate the appearance of viscoelasticity and the domination of elasticity over viscosity at high oscillation frequency for Lennard-Jones fluids.     

Dynamics of relaxation of entangled polymers in shear flow

The application of shear flow to entangled polymer melts can strongly modify its rheological and physicochemical behaviors, giving rise to an acceleration of several chemical processes such as diffusion-controlled reactions. In the present work, we investigate the modification of conformational and diffusive properties of an entangled polymer in shear flow by numerical methods. The flow affects both the conformational and diffusive properties of the system, giving rise to a quasinematic ordering of the macromolecules which take prolate spheroid shape with the main axis aligned to the shear direction. The shear flow is found to accelerate the overall diffusion of the chains in all directions at times longer than the polymer relaxation time. The polymer chains display a quite peculiar displacement behavior in direction parallel to the flow. At the same conditions, the linear relation between the diffusion constant in direction perpendicular to the flow and the inverse of the relaxation time, usually adopted in equilibrium regimes, is shown to hold even in the presence of flow.     

Selectivity of shear flow on chains for the degradation reaction of melt polyolefin elastomer with dicumyl peroxide

The degrading behaviors of three kinds of melt polyolefin elastomers (POEs) at presence of dicumyl peroxides (DCPs) in transient shear flow were evaluated at elevated temperature by parallel plate rheometer. The critical shear rates for degradations of all POEs were different from one to another. The rheological and gel permeation chromatograph (GPC) results showed that the degradation was more and more distinct and the duration to achieve one similar level of molecular weight decreased as the shear rate increased. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict probable compositions and chain topologies of the modified products, which were reasonably explained by the suggested mechanism of radical reactions and could confirm that there was a kind of selectivity of shear rate on POE chains for degradation. Higher shear rate can enlarge the differences in mobility of the two scission parts and can further increase the possibility of effective degradation on the original chains.     

铝/镁混合金属氢氧化物正电胶体粒子体系的触变性

采用恒定低剪切速率方法和动态实验方法研究了铝/镁混合金属氢氧化物(MMH)悬浮体的流变性,着重考察了剪切历史和恢复时间等因素对悬浮体触变性的影响,发现MMH粒子深度的增加使得悬浮体从粘性流体变为“类固体”;恒定低剪切速率方法和动态实验方法研究悬浮体的结构恢复过程给出了不同的结果,唯象地解释了MMH悬浮体触变性产生的原因,认为触变性结构是由于粒子间的静电排斥作用而形成的,与粘土悬浮体相比,两者的流变性具有诸多相似之处,只是所带电荷符号相反。     

Self-organization in the flow of complex fluids (colloid and polymer systems). Part 2: Theoretical models

Flow induced transitions in complex fluids are usually accompanied by changes in the internal media structure and the flow symmetry. In this review paper, we discuss the theoretical models and approaches that have been used for the analysis of different types of flow instabilities and flow patterns. The main attention is focused on the basic fluid models which reveal vortex and banding flow structures at high shear rates. The Oldroyd-B fluid is one of such models. The Reynolds and the Weissenberg (or Deborah) numbers are the parameters governing its flow behavior. For this model, the secondary flow patterns arising in viscometric flows of different geometries at the bifurcation point are described. Complex fluids which are able to exist in multiple states can form coexisting bands of different structures with different rheological properties and flowing with different shear rates at the same shear stress. Shear banding is typical for fluids demonstrating non-monotonous flow curves described by such models as the diffusive Johnson-Segalman fluid model, for example. Recent progress in exploring this phenomenon is discussed.     

A microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow

Biological cells in vivo typically reside in a dynamic flowing microenvironment with extensive biomechanical and biochemical cues varying in time and space. These dynamic biomechanical and biochemical signals together act to regulate cellular behaviors and functions. Microfluidic technology is an important experimental platform for mimicking extracellular flowing microenvironment in vitro. However, most existing microfluidic chips for generating dynamic shear stress and biochemical signals require expensive, large peripheral pumps and external control systems, unsuitable for being placed inside cell incubators to conduct cell biology experiments. This study has developed a microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow. Further, based on the lumped-parameter and distributed-parameter models of multiscale fluid dynamics, the oscillatory flow field and the concentration field of biochemical factors has been simulated at the cell culture region within the designed microfluidic chip. Using the constructed experimental system, the feasibility of the designed microfluidic chip has been validated by simulating biochemical factors with red dye. The simulation results demonstrate that dynamic shear stress and biochemical signals with adjustable period and amplitude can be generated at the cell culture chamber within the microfluidic chip. The amplitudes of dynamic shear stress and biochemical signals is proportional to the pressure difference and inversely proportional to the flow resistance, while their periods are correlated positively with the flow capacity and the flow resistance. The experimental results reveal the feasibility of the designed microfluidic chip. Conclusively, the proposed microfluidic generator based on autonomously oscillatory flow can generate dynamic shear stress and biochemical signals without peripheral pumps and external control systems. In addition to reducing the experimental cost, due to the tiny volume, it is beneficial to be integrated into cell incubators for cell biology experiments. Thus, the proposed microfluidic chip provides a novel experimental platform for cell biology investigations.     

组装-聚合法制备聚苯乙烯-聚丙烯酸胶束和凝胶

通过苯乙烯和丙烯酸单体的预组装再聚合的制备方法,在不改变共聚物浓度的前提下制备了共聚物胶束溶液和凝胶,探讨了引发剂(偶氮二异丁腈)浓度对生成的共聚物的聚集体结构以及分子结构的影响.利用核磁共振氢谱、扫描电子显微镜和透射电子显微镜等表征了共聚物的分子结构和聚集行为,此外,借助耗散粒子动力学方法模拟了该体系,辅助实验阐明了不同引发剂浓度下生成的共聚物聚集体结构及相对应的共聚物分子结构,在此基础上,利用动态机械热分析和流变学的表征技术,研究了共聚物胶束溶液和凝胶的流变特性.结果表明,在单体浓度不变的情况下,高引发剂浓度时该体系趋于形成平均嵌段长度较长的两嵌段共聚物,生成稳定的胶束溶液,而低引发剂浓度时趋于形成交替共聚物,得到物理凝胶,耗散粒子动力学模拟得到了与实验一致的结果.流变学表征发现胶束体系和凝胶体系均呈现剪切变稀行为,并确定了凝胶体系的凝胶点及恢复性.     

Aggregation and gelation in hydroxypropylmethyl cellulose aqueous solutions

In this work we present an analysis of the thermal behavior of hydroxypropylmethyl cellulose aqueous solutions, from room temperature to higher temperatures, above gelation. We focus on significant aspects, essentially overlooked in previous work, such as the correlation between polymer hydrophobicity and rheological behavior, and the shear effect on thermal gelation. Micropolarity and aggregation of the polymer chains were monitored by both UV/vis and fluorescence spectroscopic techniques, along with polarized light microscopy. Gel formation upon heating was investigated using rheological experiments, with both large strain (rotational) tests at different shear rates and small strain (oscillatory) tests. The present observations allow us to compose a picture of the evolution of the system upon heating: firstly, polymer reptation increases due to thermal motion, which leads to a weaker network. Secondly, above 55 degrees C, the polymer chains become more hydrophobic and polymer clusters start to form. Finally, the number of physical crosslinks between polymer clusters and the respective lifetimes increase and a three-dimensional network is formed. This network is drastically affected if higher shear rates, at non-Newtonian regimes, are applied to the system.     

STRUCTURING & RHEOLOGY OF MOLTEN POLYMER/CLAY NANOCOMPOSITES

The evolution and the origin of "solid-like state" in molten polymer/clay nanocomposites are studied. Using polypropylene/clay hybrid (PPCH) with sufficient maleic anhydride modified PP (PP-MA) as compatibilizer, well exfoliation yet solid-like state was achieved after annealing in molten state. Comprehensive linear viscoelasticity and non-linear rheological behaviors together with WAXD and TEM are studied on PPCH at various dispersion stages focusing on time,temperature and deformation dependencies of the "solid-like" state in molten nanocomposites. Based on these, it is revealed that the solid-structure is developed gradually along with annealing through the stages of inter-layer expansion by PP-MA,the diffusion and association of exfoliated silicate platelets, the formation of band/chain structure and, finally, a percolated clay associated network, which is responsible for the melt rigidity or solid-like state. The network will be broken down by melt frozen/crystallization and weakened at large shear or strong flow and, even more surprisingly, may be disrupted by using trace amount of silane coupling agent which may block the edge interaction of platelets. The solid-like structure causes characteristic non-linear rheological behaviors, e.g. residual stress after step shear, abnormal huge stress overshoots in step flows and, most remarkably, the negative first normal stress functions in steady shear or step flows. The rheological and structural arguments challenge the existing models of strengthened entangled polymer network by tethered polymer chains connecting clay particles or by chains in confined melts or frictional interaction among tactoids. A scheme of percolated networking of associated clay platelets, which may in band form of edge connecting exfoliated platelets, is suggested to explain previous experimental results.