Seekell Raymond P, Lin Kehua, Zhu Yingxi
LiquiGlide, Inc., 75 Sidney Street, Cambridge, Massachusetts 02139, United States.
Department of Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, Michigan 48202, United States.
Langmuir. 2021 May 4;37(17):5299-5305. doi: 10.1021/acs.langmuir.1c00363. Epub 2021 Apr 22.
Spatial confinement has a great impact on the structures and dynamics of interfacial molecular and polymer liquid films. Most prior research has focused on confined liquids of fixed material compliance and often treated them in approximation to the "hard-sphere" interaction model. In this study, we microscopically investigate the structural dynamics of highly deformable poly(-isopropylacrylamide) (PNIPAM) microgels confined between two solid surfaces in comparison to that of nearly nondeformable microgels of the same chemistry. We observe that the mobility and structural relaxation of highly deformable PNIPAM microgels at an apparent volume fraction, ϕ = 0.49-0.70, show little change with the reduction of gap spacing, in stark contrast to confinement-induced dynamic retardation of "hard-sphere"-like stiff PNIPAM microgels. The critical gap spacing, defined as the onset of confinement effect to deviate from the bulk behavior, is found to be approximately 17-22 particle layers for highly deformable microgels of ϕ = 0.56-0.70, much smaller than that of approximately 40 particle layers or larger for stiff microgels or model "hard-sphere" colloidal liquids of similar ϕ. Additionally, we observe no evident confinement-enhanced structural reorganization of deformable microgels near the confining surfaces when gap spacing approaches the critical gap spacing. Microgel deformation upon strong confinement is attributed to the disrupted confinement-induced ordering of confined microgels. Hence, it is clearly indicated that spatial confinement exhibits a much weaker effect on highly compliant microgel particles than stiff ones, resulting in a significantly less reduction in microgel interfacial dynamics. It therefore gives insights into the molecular design of polymeric thin films of variable compliance to control friction and lubrication.
空间限制对界面分子和聚合物液膜的结构与动力学有很大影响。大多数先前的研究都集中在具有固定材料柔顺性的受限液体上，并且常常将它们近似处理为“硬球”相互作用模型。在本研究中，我们通过显微镜研究了高度可变形的聚（N - 异丙基丙烯酰胺）（PNIPAM）微凝胶在两个固体表面之间的结构动力学，并与相同化学组成的几乎不可变形的微凝胶进行了比较。我们观察到，在表观体积分数ϕ = 0.49 - 0.70时，高度可变形的PNIPAM微凝胶的迁移率和结构弛豫随间隙间距的减小变化很小，这与“硬球”状刚性PNIPAM微凝胶的受限诱导动力学迟缓形成鲜明对比。对于ϕ = 0.56 - 0.70的高度可变形微凝胶，定义为偏离本体行为的受限效应起始点的临界间隙间距约为17 - 22个颗粒层，远小于相同ϕ的刚性微凝胶或模型“硬球”胶体液体的约40个颗粒层或更大的临界间隙间距。此外，当间隙间距接近临界间隙间距时，我们在受限表面附近未观察到可变形微凝胶有明显的受限增强结构重组。强受限下微凝胶的变形归因于受限微凝胶的受限诱导有序性被破坏。因此，清楚地表明空间限制对高度柔顺的微凝胶颗粒的影响比对刚性颗粒的影响弱得多，导致微凝胶界面动力学的降低显著减少。这因此为可变柔顺性聚合物薄膜的分子设计提供了见解，以控制摩擦和润滑。
