The long-chain dynamics in a model homopolymer blend under strong flow: small-angle neutron scattering and theory
The long-chain dynamics in a model homopolymer blend under strong flow: small-angle neutron scattering and theory
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DOI：
10.1039/b817440g
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发表时间：
2009-01-01
期刊：
影响因子：
3.4
通讯作者：
McLeish, Tom C. B.
中科院分区：
文献类型：
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作者：
Graham, Richard S.;Bent, Julian;McLeish, Tom C. B.
我们使用小角中子散射(SANS)测量来提供模型聚苯乙烯共混物中长链的非线性动力学的详细图像。通过加权减去两个具有不同氚含量的完全相同的混合物的SANS测量，我们分离出了流经4：1收缩-膨胀流动的模型混合物的长链组分的单链形状因子。互补的流动双折射还提供了一种在更精细的长度尺度上测量链变形的方法。此外，流动的Weissenberg数比以前对单分散熔体的研究得到了更高的流动Weissenberg数，导致了所测量的单链结构因子的更大的各向异性。在缝隙内的短暂停留时间意味着链条在进入收缩出口时仍然朝向流动方向，从而导致快速的反向流动。我们将这些数据与非线性管模型的简单推广相比较。我们的模型预测完全是从头开始的，所有模型参数都是根据独立的平衡测量确定的。对于狭缝内的收缩入口和随后的松弛，该模型在整个长度范围内与实验数据显示出很好的一致性。然而，在收缩出口处，在SANS和双折射预测中，理论和实验之间存在明显的不一致，我们将其归因于该区域发生的反转流动。
We use small-angle neutron scattering (SANS) measurements to provide a detailed picture of the non-linear dynamics of the long chains in a model polystyrene blend. By a weighted subtraction of SANS measurements from two otherwise identical blends with different deuteration fractions, we isolate the single-chain form factor of the long-chain component of a model blend flowing through a 4 : 1 contraction-expansion flow. Complementary flow-birefringence also provides a measure of chain deformation on finer length-scales. In addition, higher flow Weissenberg numbers than in previous studies on monodisperse melts were achieved, leading to greater anisotropy in the measured single-chain structure factor. The short residence time inside the slit means that the chains are still oriented in the flow direction as they enter the contraction exit, leading to a rapid reversing flow. We compare these data to a simple generalisation of a non-linear tube model. Our model predictions are entirely ab initio, with all model parameters being determined from independent equilibrium measurements. The model shows very good agreement with the experimental data across the full range of length-scales for the contraction entrance and subsequent relaxation within the slit. However, there is conspicuous disagreement between theory and experiments at the contraction exit, in both the SANS and birefringence predictions, which we attribute to the reversing flow that occurs in this region.