Rogez D, Francius G, Finkelmann H, Martinoty P
Institut de Mécanique des Fluides et des Solides, UMR 7507, CNRS-ULP, 67000, Strasbourg, France.
Eur Phys J E Soft Matter. 2006 Aug;20(4):369-78. doi: 10.1140/epje/i2005-10132-5. Epub 2006 Aug 9.
We study the mechanical anisotropy of a series of uniaxial side chain nematic elastomers prepared with the same chemical composition but with different preparation protocols. For all the compounds, the experiments performed as a function of temperature show no discontinuity in both G' (//) and G' ( perpendicular) (the labels // and perpendicular stand for the director parallel, respectively perpendicular to the shear displacement) around the nematic-isotropic (N-I) phase transition temperature determined by DSC. They also all show a small decrease in G' (//) starting at temperatures well above this temperature (from approximately 4( degrees ) C to approximately 20( degrees ) C depending on the compound studied) and leading to a small hydrodynamic value of the G' ( perpendicular)/G' (//) ratio. The measurements taken as a function of frequency show that the second plateau in G' (//) and the associated dip in G (//)" expected from dynamic semi-soft elasticity are not observed. These results can be described by the de Gennes model, which predicts small elastic anisotropy in the hydrodynamic and linear regimes. They correspond to the behavior expected for compounds beyond the mechanical critical point, which is consistent with the NMR and specific heat measurements taken on similar compounds. We also show that a reduction in the cross-linking density does not change the non-soft character of the mechanical response. From the measurements taken as a function of frequency at several temperatures we deduce that the time-temperature superposition method does not apply. From these measurements, we also determine the temperature dependence of the longest relaxation time tau(E) of the network for the situations where the director is either parallel or perpendicular to the shear velocity. Finally, we discuss the influence on the measurements of the mechanical constraint associated with the fact that the samples cannot change their shape around the pseudo phase transition, because of their strong adherence on the sample-bearing glass slides.
我们研究了一系列具有相同化学成分但制备方案不同的单轴侧链向列型弹性体的力学各向异性。对于所有化合物，作为温度函数进行的实验表明，在由差示扫描量热法（DSC）测定的向列相 - 各向同性（N - I）相变温度附近，G'（//）和G'（垂直）（标签//和垂直分别表示指向矢平行和垂直于剪切位移方向）均无间断。它们还都显示出，在远高于该温度的温度下（根据所研究的化合物，从约4℃到约20℃），G'（//）开始略有下降，并导致G'（垂直）/G'（//）比值出现较小的流体动力学值。作为频率函数进行的测量表明，未观察到动态半软弹性所预期的G'（//）中的第二个平台以及G（//）"中的相关凹陷。这些结果可以用德热纳模型来描述，该模型预测在流体动力学和线性区域中弹性各向异性较小。它们对应于超出力学临界点的化合物所预期的行为，这与对类似化合物进行的核磁共振（NMR）和比热测量结果一致。我们还表明，交联密度的降低不会改变机械响应的非软特性。从在几个温度下作为频率函数进行的测量中，我们推断时间 - 温度叠加方法不适用。通过这些测量，我们还确定了指向矢平行或垂直于剪切速度的情况下，网络最长弛豫时间τ(E)的温度依赖性。最后，我们讨论了与样品由于强烈粘附在承载样品的载玻片上而在伪相变附近无法改变其形状这一事实相关的机械约束对测量的影响。
