Rheology of concentrated isotropic and anisotropic xanthan solutions. 1. A rodlike low molecular weight sample
Rheology of concentrated isotropic and anisotropic xanthan solutions. 1. A rodlike low molecular weight sample
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DOI：
10.1021/ma011526m
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发表时间：
2002-03-12
期刊：
影响因子：
5.5
通讯作者：
Brant, DA
中科院分区：
文献类型：
--
作者：
Lee, HC;Brant, DA
在振荡和稳态剪切实验中研究了棒状黄原胶样品（级分X13 F3，M-w = 1.54 × 10（5）g/mol）在NaCl水溶液（0.01、0.1、1.0 M）中的流变学。聚合物浓度W（重量百分比）跨越各向同性，双相，和完全各向异性制度的溶致液晶相图进行了研究。的流变参数增加的幂律函数的W在各向同性政权，减少与更高的幂律指数在两相政权，并在完全各向异性政权的变化非常缓慢。由于剪切诱导的取向，稳态剪切粘度η对W的曲线中的峰值随着点上的剪切速率（γ）的增加而向较低的W移动。相反，复数粘度η *、储能模量G '和损耗模量G”的曲线图中的峰与频率ω无关，因为在小应变的条件下剪切诱导的取向是最小的。在各向同性、双相和各向异性状态中拟合至η *、G '和G”的幂律曲线分别在W1和Wa处交叉，W1和Wa分别是各向同性和双相之间以及双相和各向异性区域之间的边界，如通过相分离测量所确定的。对于X13 F3，在所研究的任何盐浓度下都没有观察到G'和G”的交叉，并且在所有研究的条件下损耗角正切tanS =（G”/G'）> 1。当绘制适当的简化变量时，参数eta* 和G”符合紧密的主曲线，但eta和G'不符合。Cox-Merz规则是遵守在所有三种盐的条件下研究的所有聚合物浓度在牛顿平台区的约化流动曲线。盐浓度在确定这种离子棒状多糖的W-i和W-a中起重要作用，但它不影响各向同性状态下的牛顿平台粘度。相反，各向异性溶液在不同盐浓度下的粘度随着盐浓度的增加而增加，因为各向异性溶液中的序参数在低盐浓度下比在高盐浓度下大。
The rheology of a rodlike xanthan sample (Fraction X13F3, M-w = 1.54 x 10(5) g/mol) has been studied in aqueous NaCl (0.01, 0.1, 1.0 M) in oscillatory and steady shear experiments. Polymer concentrations W (in weight percent) spanning the isotropic, biphasic, and fully anisotropic regimes of the lyotropic liquid crystalline phase diagram have been investigated. The rheological parameters increase as power law functions of W in the isotropic regime, decrease with higher power law exponents in the biphasic regime, and change very slowly in the fully anisotropic regime. The peak in plot's of steady shear viscosity eta against W shifts toward lower W with increasing shear rate (gamma) over dot,because of the shear-induced orientation. In contrast, peaks in the plots of the complex viscosity eta*, storage modulus G', and loss modulus G" are independent of frequency omega, because shear-induced orientation is minimal under conditions of small strain. Power law curves fit to eta*, G', and G" in the isotropic, bipbasic, and anisotropic regimes cross at W-i and at W-a, the boundaries, respectively, between the isotropic and biphasic and between the biphasic and anisotropic regions as determined by phase separation measurements. No crossover of G' and G" is observed for X13F3 at any salt concentration studied, and the loss tangent tan delta = (G"/G') > 1 under all the conditions investigated. The parameters eta* and G" conform to tight master curves when the appropriate reduced variables are plotted, but eta and G' do not. The Cox-Merz rule is obeyed at all three salt conditions for all polymer concentrations studied in the Newtonian plateau region of the reduced flow curves. The salt concentration plays an important role in determining W-i and W-a for this ionic, rodlike polysaccharide, but it does not affect the Newtonian plateau viscosities in the isotropic regime. In contrast, the viscosities of anisotropic solutions at different salt concentrations increase as the salt concentration increases, because the order parameter in anisotropic solutions is greater at low salt concentration than it is at high salt concentration.