气相生长碳纤维填充聚苯乙烯的熔体动态流变行为

采用修正的两相模型讨论加工条件对不同体积分数(ψ=0～0.05)气相纳米碳纤维(VGCF)填充聚苯乙烯(PS)熔体(200℃)频率(ω)依赖性动态流变的影响.与弱剪切条件(190℃;30 r/min-5 min)下所制备的材料相比,VGCF在强剪切条件(190℃;120 r/min-10 min)下所制备材料中分散均匀,“粒子相”应变放大因子A,(ψ)较低,而松弛指数n较高.将应变(γ)依赖性G″(ω,ψ)应变放大因子A,(ψ,γ)与“粒子相”特征模量G″fψ(,γc),G″f(ψ,γc)引入两相模型,讨论加工条件对γ依赖性非线性流变行为的影响.研究结果表明,两相模型可定量讨论填充熔体动态流变行为.加工条件影响VGCF在基体中的分散性,但不影响“粒子相”弹性贡献R′f(ψ)与n的关系、G′f(ψ,γc)与非线性应变幂律指数x的关系以及G″f(ψ,γc)与非线性应变幂律指数y的关系,说明“粒子相”对填充熔体黏弹性的贡献与其弛豫特性及结构稳定性密切相关.     

泥鳅体表黏液的流变行为

研究了无鳞鱼———泥鳅的体表黏液流变行为,发现黏液的稳态流动存在着3个不同区域:第一区域内,黏度随剪切速率(γ)变化不显著,呈现牛顿流动行为;第二区域内,随γ增大,黏度下降,呈现非牛顿行为;第三区域内,随γ继续增大,黏度又基本保持不变.黏液表观黏度(ηa)与γ的关系可用Carreau模型很好地拟合,其增比黏度(ηsp)与浓度(c)的关系为ηsp∝c1.5,表明黏液处于亚浓缠结区域.在测试频率(ω)范围内,黏液的动态储能模量(G′)大于动态损耗模量(G″),表明与黏性相比较,弹性占优,且G′及G″随ω变化不显著.存在一临界温度(35℃),当低于35℃时,黏液黏度随温度变化不显著,当高于35℃时,黏液变性,表现出不同的流变行为.     

二氧化硅分散体系在应力剪切过程中粘弹性及能耗研究

通过动态应力剪切研究了以乙二醇、丙二醇和丁二醇为分散介质的雾化二氧化硅分散体系的粘弹性以及能耗. 研究发现, 随着应力的增大, 体系都经历了线性粘弹区、剪切变稀区以及剪切增稠区. 在线性粘弹区, 储能模量(G′)、耗能模量(G′′)随着应力(σ)的增大保持不变；在剪切变稀区, G′随着σ的增大而减小, 且乙二醇、丙二醇、丁二醇分散体系的减小幅度依次递减, 而G′′基本保持不变；在剪切增稠区, G′、G′′都随着σ的增大而增大. 在所研究的应力范围内, G′′都大于G′, 体系主要体现粘性, 消耗能量为主. 同时还发现在低剪切应力区, 体系所消耗的能量(Ed)都随着最大应变(γ0)成二次方关系增长, 而在剪切增稠区, 当n=2.79、4.93、4.88时, EG/SiO2、PG/SiO2、BG/SiO2的Ed分别随γ0以指数关系增长.     

低固相含量SiO2分散体系流变性研究

以甘油(GL)和聚乙二醇(PEG)为分散介质, 研究了低体积分数(Φ＜8%)的SiO₂分散体系的稳态和动态的流变性能, 发现体系都具有可逆的剪切变稀和剪切增稠现象. 在剪切速率大于临界剪切速率(γ^_.＞γ_^.c)时, 体系具有明显的剪切增稠现象, 而当γ^_.＜γ_^.c, GL体系只有轻微的剪切变稀现象, PEG体系剪切变稀现象却很明显. 在动态试验中考察了模量以及损耗角正切tg δ随剪切应力(σ)的变化. 在所研究的应力范围内, 体系中耗能模量G"都大于储能模量G', tg δ大于1, 体系主要表现为粘性. 用“粒子簇”生成机理能较好地解释这种剪切变稀和剪切增稠现象, 即剪切变稀是由于连续的空间网络结构被破坏, PEG体系剪切变稀现象较GL体系明显应该与PEG高分子链的松弛以及体系中较少的氢键有关; 剪切增稠是由于粒子簇的生成引起的.     

多壁碳纳米管填充高密度聚乙烯复合材料的导电性和动态流变行为

对多壁碳纳米管/高密度聚乙烯(MWNTs/HDPE)复合材料的导电性和动态流变行为进行了研究.发现复合材料的复数粘度η*随MWNTs含量φ的增大而增大.当φ>3wt%时,η*发生突变,在低ω区域表现为非牛顿流体行为,出现强烈的剪切变稀现象.将其称为流变渗流现象,对应的填料含量即渗流阈值φc.在动态储能模量(G′)、损耗模量(G″)与频率(ω)关系曲线上,随φ增加出现“第二平台”,第二平台的出现表明MWNTs与MWNTs之间、MWNTs与聚合物之间存在相互缠结形成网络的结构.同时发现,在tanδ~ω曲线上的低ω区出现凹谷.认为这是由于MWNTs长链结构在低ω时伸长/收缩,MWNTs与MWNTs相互接触形成了次级网络造成的.经过不同时间热处理后的ω扫描以及动态间扫描的结果证实了这种结构的存在.研究结果表明复合材料的流变渗流阈值与电渗流阈值相一致(均在3%~5%之间),动态流变行为与导电性存在一定的相关性.     

SSBR/SiO_2混炼胶的动态黏弹行为

研究了白炭黑(SiO2)填充溶液聚合丁苯橡胶(SSBR)的动态流变行为,考察了时间-浓度叠加(time-concentration superposition,TCS)原理在粒子填充橡胶中的应用,获得以未填充SSBR为基准的流变叠加曲线.流变叠加曲线在低频下呈现模量平台,复数黏度呈现剪切变稀行为,而损耗因子tanδ在特定频率下出现峰值.基于刚性粒子所导致的应变放大效应以及SiO2粒子间SSBR分子的受限运动,探讨了TCS模量平移因子AG与频率平移因子Aω随SiO2体积分数φ的变化.AG与Aω均为φ的标度函数,但AG～φ关系不符合粒子聚集体团聚(cluster-cluster aggregation,CCA)模型.讨论了偶联剂3-辛酰基硫代-1-丙基三乙氧基硅烷(NXT)对动态流变行为的影响.NXT不影响性叠加曲线的低频平台模量与剪切变稀幂律指数.然而,与不含偶联剂的混炼胶相比,NXT造成SSBR特征弛豫时间缩短,稠度与A增大.     

低固相含量SiO2分散体系流变性研究

以甘油(GL)和聚乙二醇(PEG)为分散介质, 研究了低体积分数(Φ＜8%)的SiO₂分散体系的稳态和动态的流变性能, 发现体系都具有可逆的剪切变稀和剪切增稠现象. 在剪切速率大于临界剪切速率(γ^_.＞γ_^.c)时, 体系具有明显的剪切增稠现象, 而当γ^_.＜γ_^.c, GL体系只有轻微的剪切变稀现象, PEG体系剪切变稀现象却很明显. 在动态试验中考察了模量以及损耗角正切tg δ随剪切应力(σ)的变化. 在所研究的应力范围内, 体系中耗能模量G"都大于储能模量G', tg δ大于1, 体系主要表现为粘性. 用“粒子簇”生成机理能较好地解释这种剪切变稀和剪切增稠现象, 即剪切变稀是由于连续的空间网络结构被破坏, PEG体系剪切变稀现象较GL体系明显应该与PEG高分子链的松弛以及体系中较少的氢键有关; 剪切增稠是由于粒子簇的生成引起的.     

基于遥爪型大分子交联剂的动态化学交联水凝胶的黏弹性质

以壳聚糖为主链,以双端苯醛基聚乙二醇(DF-PEG)为交联剂,以希夫碱为动态交联键制备了动态化学交联水凝胶,通过改变大分子交联剂的分子量和浓度调控凝胶网络结构。以旋转流变仪的动态频率扫描和稳态剪切为主要手段,研究了凝胶结构对凝胶模量、松弛时间、剪切增稠程度的影响。结果表明:DF-PEG的分子量和浓度会影响凝胶网络内弹性活性链的密度从而影响模量、松弛时间,而凝胶剪切增稠程度与弹性活性链密度密切相关。     

SiO2/聚乙二醇非牛顿流体流变性能研究

利用应力控制流变仪考察了SiO₂/聚乙二醇分散体系稳态和动态的流变性能. 实验结果表明, 该体系具有剪切变稀和可逆的剪切增稠现象. 稳态应力实验中, 当应力较小时, 体系具有剪切变稀现象, 而在剪切应力(σ)大于临界剪切应力(σ_c^s, σ_c^s=9.99 Pa)后, 体系粘度急剧增大. 在动态实验中, 剪切应力小于临界剪切应力(σ_c^o, σ_c^o=15.85 Pa)时, 储能模量G′减小, 耗能模量G″与复合粘度η*基本不变, 但σ＞15.85 Pa后, G′、G″及η*同步增大, 且在所研究的应力范围内, G″均大于G′. 同时还考察了测试频率、分散相含量以及分散介质平均分子量的差别对流变性的影响. σ_c^o随测试频率的增大而变大; SiO₂质量分数越大, σ_c^o基本不变, 但增稠现象变得更明显; 与平均分子量小的PEG200体系相比, 平均分子量大的PEG400体系, σ_c^o并未发生改变, 但在增稠之前体系的粘度较低, 增稠之后体系粘度增大的幅度较大.     

硅烷偶联剂对SSBR/SiO_2混炼胶体系动态流变行为的影响

研究了高填充白炭黑(SiO2)补强溶液聚合丁苯橡胶(SSBR)混炼胶体系的动态流变特性,分别考察了在2个混炼温度(120℃、160℃)下添加3种不同偶联剂(TESPT、TESPD、NXT)对其体系中SiO2粒子表面改性的效果.结果表明,在较低混炼温度下添加偶联剂,不利于SiO2粒子表面改性.在较高混炼温度下,偶联剂TESPT、PESPD、NXT均对SiO2粒子表面有改性效果,但同时引起混炼胶不同程度的焦烧,其中以TESPT最明显.随SiO2粒子表面改性程度的提高,出现“Payne效应”的临界应变(γc)值以及低频率(ω)区域lgG′-lgω曲线的斜率值均增大,反映出偶联剂对SiO2与SSBR相互作用的促进以及SiO2粒子分散的改善.     

多壁碳纳米管填充聚苯乙烯熔体的恒温依时性动态模量逾渗行为

在恒温(170～190℃)热处理过程中,多壁碳纳米管(MWCNTs)填充聚苯乙烯(PS)熔体发生动态模量逾渗(DMP)行为,且该行为服从时-温叠加(TTS)原理.MWCNTs体积分数(φ)低于逾渗阈值(φc)时,填充熔体在恒温热处理过程中保持类液行为,热处理对MWCNTs分散状态无显著影响,动态储能模量(G')与动态损耗模量(G″)活化能(EG'与EG″)分别高于、低于PS黏流活化能(Eω),而与PS熔体恒频动态模量活化能相一致;φ>φc时,热处理促成MWCNTs进一步团聚,填充熔体在热处理过程中发生类液-类固转变,EG'与EG″均低于Eω,而动态逾渗时间活化能(Et'与Et″)显著高于Eω且随φ增高而增大.在逾渗转变区,EG'与EG″发生不连续变化.PS/MWCNTs复合体系的恒温依时DMP行为与MWCNTs聚集所导致的三维弹性网络的形成密切相关,PS大分子终端松弛不是决定DMP的关键因素.     

Rheological Study Bringing New Insights into PET/PC Reactive Blends

Summary: Extruded poly(ethylene terephthalate)/polycarbonate (PET/PC) blends, with/without cobalt catalyst and at different polymer ratios, were prepared. Rheological behaviour was discussed in terms of storage (G′) and loss (G″) moduli, loss tangent (Tan δ) and viscosity (η). Both G′ and G″ increased as a function of frequency for all blends. PET was the matrix in the blends with 80 and 50 wt% of polyester but in the PC rich-blend an inversion was observed. In all cases, lower Tan δ values were achieved at high frequencies. The viscosity behaviour showed a catalyst dependency. PET dictated the rheological properties of the blends without catalyst whereas PC governed blends with catalyst. Alcoholysis and acidolysis reactions plus a transesterification reaction occurred on the interface was dependent on the matrix component. These reactions seem to occur at higher extent in blends in which PET is the matrix but the inverse happened in the PC rich-blend.     

Behavior of concentrated polystyrene solutions in large-amplitude oscillating shear fields

The limits of linear viscoelastic behavior of polystyrene solutions have been investigated by subjecting them to large-amplitude oscillatory strains, γ₀. At strains less than one we find that the dynamic storage modulus G′(ω,γ₀) and the dynamic loss modulus G″(ω,γ₀) decrease quadratically with increasing strain. As a measure of the size of the linear viscoelastic region, we have determined the strain at which the moduli have fallen 5% below their zero-strain values. This strain, γNL, is found to be independent of frequency ω at high frequencies but to increase with decreasing frequency at low frequencies. Behavior of this type is in qualitative agreement with the recent constitutive equation developed by Doi and Edwards. More specifically, we find that the rate of decrease of the storage modulus with increasing strain is quite similar to that predicted by their theory, but that the rate of decrease of the loss modulus is much slower. Some possible approaches for improving the agreement with experimental results are suggested. In the course of our work an interesting hydrodynamic instability was observed. The nature of this instability and methods to avoid it are discussed.     

Interfacial rheology of an ultrathin nanocrystalline film formed at the liquid/liquid interface

We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter formed at the toluene-water interface under steady as well as oscillatory shear. Strain amplitude sweep measurements carried out on the film reveal a shear thickening peak in the loss moduli (G") at large amplitudes followed by a power law decay of the storage (G') and loss moduli with exponents in the ratio 2:1. In the frequency sweep measurements at low frequencies, the storage modulus remains nearly independent of the angular frequency, whereas G" reveals a power law dependence with a negative slope, a behavior reminiscent of soft glassy systems. Under steady shear, a finite yield stress is observed in the limit of shear rate .gamma going to zero. However, for .gamma > 1 s-1, the shear stress increases gradually. In addition, a significant deviation from the Cox-Merz rule confirms that the monolayer of Ag nanoparticles at the toluene-water interface forms a soft two-dimensional colloidal glass.     

Preparation and Rheological Characterization of Polymer Nanocomposites Based on Expanded Graphite

A new type of conductive filler, namely expanded graphite (EG), was used to prepare novel nanocomposites. The EG was incorporated into several rather different polymers, specifically polycarbonate (PC), low‐density polyethylene (LDPE), isotactic polypropylene (PP), and polystyrene (PS), using melt mixing in a small‐scale DACA‐Microcompounder. The EG content was varied between 1 and 20 wt%. The rheological properties and morphologies of the nanocomposites were characterized by melt rheology and scanning electron microscopy (SEM), respectively. The melt‐state linear viscoelastic properties were investigated using an ARES rheometer, with the measurements performed in the dynamic mode at various temperatures over a wide range of frequencies. Addition of the EG increased the linear dynamic moduli and melt viscosity of the materials. Up to a certain critical concentration of EG, the materials exhibited a simple liquid‐like behavior. Above this concentration, however, significant changes in the frequency dependences of the moduli and viscosity were observed. In addition, the moduli showed a liquid‐solid transition resulting in a second plateau in the low frequency‐regime, and the complex viscosity revealed shear‐thinning behavior. Specific values of this percolation concentration were found to be at around 4 wt% in the case of PC/EG, 9 wt% for PP/EG and PS/EG, and 12 wt% for PE/EG. This critical concentration was correlated to a network‐like structure formed through interactions between the EG platelets and the polymers. The extent of these complications was found to vary from polymer to polymer, presumably due to different degrees of EG exfoliation and dispersion arising from different EG‐polymer interactions and from variable shearing forces dependent on the polymer viscosities. The formation of network‐like structures is very sensitively displayed using van Gurp‐Palmen plots, which are most suitable for identifying “rheological percolation” in our investigated systems.     

Strain Hardening and Strain Softening of Reversibly Cross-linked Supramolecular Polymer Networks

The large amplitude oscillatory shear behavior of metallo-supramolecular polymer networks formed by adding bis-Pd(II) cross-linkers to poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO) solution is reported. The influence of scanning frequency, dissociation rate of cross-linkers, concentration of cross-linkers, and concentration of PVP solution on the large amplitude oscillatory shear behavior is explored. In semidilute unentangled PVP solutions, above a critical scanning frequency, strain hardening of both storage moduli and loss moduli is observed. In the semidilute entangled regime of PVP solution, however, strain softening is observed for samples with faster cross-linkers (k(d) ～ 1450 s(-1)), whereas strain hardening is observed for samples with slower cross-linkers (k(d) ～ 17 s(-1)). The mechanism of strain hardening is attributed primarily to a strain-induced increase in the number of elastically active chains, with possible contributions from non-Gaussian stretching of polymer chains at strains approaching network fracture. The divergent strain softening of samples with faster cross-linkers in semidilute entangled PVP solutions, relative to the strain hardening of samples with slower cross-linkers, is consistent with observed shear thinning/shear thickening behavior reported previously and is attributed to the fact that the average time that a cross-linker remains detached is too short to permit the local relaxation of polymer chain segments that is necessary for a net conversion of elastically inactive to elastically active cross-linkers. These and other observations paint a picture in which strain softening and shear thinning arise from the same set of molecular mechanisms, conceptually uniting the two nonlinear responses for this system.     

Energy dissipation in concentrated monodisperse colloidal suspensions of silica particles in polyethylene glycol

Highly concentrated colloidal suspensions exhibit a discontinuous shear-thickening behaviour. The transition from a low to a high viscosity state is associated to a large energy dissipation. This effect could find applications in structural damping while the viscosity increase brings added stiffness. In the present work, highly concentrated suspensions of monodisperse spherical silica particles in polyethylene glycol were selected for their strong thickening at low critical shear rates. Their damping properties were characterized by measuring the energy dissipated per cycle at low frequency (below 2 Hz) during oscillatory tests using a rheometer. The influence of parameters such as particle concentration, size and frequency was investigated. Damping was found to overcome that of benchmark elastomeric materials only in high frequencies and high strain domains.     

Development of an energy-based approach for optimized frequency selection for fatigue testing on polymers – Exemplified on polyamide 6

Polymers and composite materials show temperature-dependent material properties. Therefore, the frequency resembles a critical part in fatigue testing, due to its influence on the self-heating of the polymeric material and thereby on the number of cycles to failure. The aim of this paper is the development of a testing method, which allows comparable results with varying frequencies. To minimize the self-heating effect on the fatigue behavior, a model was established for selecting optimized frequencies regarding the load-specific temperature increase of the specimen. A new energy-parameter, the induced energy-rate, was introduced and correlated to the load-specific increase in temperature in multiple and constant amplitude tests at ambient conditions. With this approach, it was possible to determine a threshold value for the newly defined induced energy-rate. A stress-specific model was developed and a limit frequency was calculated. The results were verified in multiple and constant amplitude tests and S/N-curves.     

Measurement of rubber properties for design

The rubber properties required for the design of engineering components are primarily concerned with stiffness, although factors such as strength and ageing resistance must be taken into account in materials selection. Calculation of static behaviour requires a shear modulus at the appropriate strain. In order to allow for the strain dependence of the shear moduli of filled rubbers, it is preferable to measure moduli over a range of strains rather than to rely on a single point measurement.

For the purposes of vibration isolation, the stiffness of the rubber is a function not only of frequency but also of the amplitude of the vibration. To allow for this, the shear moduli of the rubber at low strains, of the order of 1%, are required, over a range of frequencies.