Schneider Florian, Balaceanu Andreea, Feoktystov Artem, Pipich Vitaliy, Wu Yaodong, Allgaier Jürgen, Pyckhout-Hintzen Wim, Pich Andrij, Schneider Gerald J
Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at MLZ, Lichtenbergstraße 1, 85747 Garching, Germany.
Langmuir. 2014 Dec 23;30(50):15317-26. doi: 10.1021/la503830w. Epub 2014 Dec 10.
The combination of a set of complementary techniques allows us to construct an unprecedented and comprehensive picture of the internal structure, temperature dependent swelling behavior, and the dependence of these properties on the cross-linker concentration of microgel particles based on N-vinylcaprolactam (VCL). The microgels were synthesized by precipitation polymerization using different amounts of cross-linking agent. Characterization was performed by small-angle neutron scattering (SANS) using two complementary neutron instruments to cover a uniquely broad Q-range with one probe. Additionally we used dynamic light scattering (DLS), atomic force microscopy (AFM), and differential scanning calorimetry (DSC). Previously obtained nuclear magnetic resonance spectroscopy (NMR) results on the same PVCL particles are utilized to round the picture off. Our study shows that both the particle radius and the cross-link density and therefore also the stiffness of the microgels rises with increasing cross-linker content. Hence, more cross-linker reduces the swelling capability distinctly. These findings are supported by SANS and AFM measurements. Independent DLS experiments also found the increase in particle size but suggest an unchanged cross-link density. The reason for the apparent contradiction is the indirect extraction of the parameters via a model in the evaluation of DLS measurements. The more direct approach in AFM by evaluating the cross section profiles of observed microgel particles gives evidence of significantly softer and more deformable particles at lower cross-linker concentrations and therefore verifies the change in cross-link density. DSC data indicate a minor but unexpected shift of the volume phase transition temperature (VPTT) to higher temperatures and exposes a more heterogeneous internal structure of the microgels with increasing cross-link density. Moreover, a change in the total energy transfer during the VPT gives evidence that the strength of hydrogen bonds is significantly affected by the cross-link density. A strong and reproducible deviation of the material density of the cross-linked microgel polymer chains toward a higher value compared to the respective linear chains has yet to be explained.
一组互补技术的结合使我们能够构建出基于N-乙烯基己内酰胺（VCL）的微凝胶颗粒内部结构、温度依赖性溶胀行为以及这些性质对交联剂浓度依赖性的前所未有的全面图景。通过沉淀聚合使用不同量的交联剂合成了微凝胶。使用两种互补的中子仪器通过小角中子散射（SANS）进行表征，以用一个探测器覆盖独特的宽Q范围。此外，我们还使用了动态光散射（DLS）、原子力显微镜（AFM）和差示扫描量热法（DSC）。利用先前在相同PVCL颗粒上获得的核磁共振光谱（NMR）结果来完善这幅图景。我们的研究表明，随着交联剂含量的增加，微凝胶的颗粒半径、交联密度以及因此的刚度都会增加。因此，更多的交联剂会明显降低溶胀能力。这些发现得到了SANS和AFM测量的支持。独立的DLS实验也发现了颗粒尺寸的增加，但表明交联密度不变。明显矛盾的原因是在DLS测量评估中通过模型间接提取参数。AFM中通过评估观察到的微凝胶颗粒的横截面轮廓的更直接方法表明，在较低交联剂浓度下颗粒明显更软且更易变形，因此验证了交联密度的变化。DSC数据表明体积相变温度（VPTT）向更高温度有轻微但意外地偏移，并揭示了随着交联密度增加微凝胶内部结构更不均匀。此外，VPT期间总能量转移的变化表明氢键强度受交联密度显著影响。与相应的线性链相比，交联微凝胶聚合物链的材料密度向更高值有强烈且可重复的偏差，这一点尚待解释。
