Siebert Hartmut, Becker Patrick, Quijada-Garrido Isabel, Grabowski David A, Schmidt Claudia
Institut für Makromolekulare Chemie, Albert-Ludwigs-Universität Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany.
Solid State Nucl Magn Reson. 2002 Sep-Nov;22(2-3):311-26. doi: 10.1006/snmr.2002.0074.
The flow behavior of nematic liquid crystalline polysiloxanes of the side-chain type is studied by in situ 2H NMR spectroscopy on samples under shear in a cone-and-plate cell. The director orientation as a function of applied shear rate is determined from the quadrupole splitting of the spectra. The data analysis yields the two Leslie viscosity coefficients alpha2 and alpha3 and the flow-alignment parameter lambda = -(alpha3 + alpha2)/(alpha3 - alpha2). The values of lambda were determined for several homopolymers with only one type of side chain and random copolymers containing two different side chains. The results show that the flow behavior is related to the phase structure of the polymers, which varies with their composition. Only polymers with large amounts of smectic clusters in the nematic state show the tumbling instability (absolute value(lambda) < 1); other polymers are flow aligning (absolute value(lambda) > or = 1). For some polymers, a transition from tumbling at low temperature to flow aligning at high temperatures was observed.
通过在锥板流动池中对受剪切的样品进行原位2H核磁共振光谱研究了侧链型向列型液晶聚硅氧烷的流动行为。根据光谱的四极分裂确定指向矢方向与施加剪切速率的函数关系。数据分析得出两个莱斯利粘度系数α2和α3以及流动取向参数λ = -(α3 + α2)/(α3 - α2)。测定了几种仅含一种侧链的均聚物和含两种不同侧链的无规共聚物的λ值。结果表明，流动行为与聚合物的相结构有关，相结构随其组成而变化。只有在向列态中含有大量近晶簇的聚合物表现出翻滚不稳定性（|λ| < 1）；其他聚合物是流动取向的（|λ|≥1）。对于一些聚合物，观察到从低温下的翻滚到高温下的流动取向的转变。
