Pamvouxoglou Andreas, Bogri Panagiota, Nägele Gerhard, Ohno Kohji, Petekidis George
FORTH-IESL and Department of Materials Science and Technology, University of Crete, 71110, Heraklion, Crete, Greece.
Forschungszentrum Jülich GmbH, ICS-3 - Soft Condensed Matter, 52428 Jülich, Germany.
J Chem Phys. 2019 Jul 14;151(2):024901. doi: 10.1063/1.5091845.
We report on a detailed experimental study of the structure and short-time dynamics in fluid-regime suspensions of soft core-shell spherical particles with different molecular weights of the chains forming the soft outer shell, and therefore different degrees of particle softness, using 3D dynamic light scattering (3D-DLS). Owing to the particle softness, the liquid-crystal coexistence regime is found to be broader than that of hard-sphere (HS) suspensions. Static light scattering in the dilute regime yields form factors that can be described using a spherical core-shell model and second virial coefficients A > 0 indicative of purely repulsive interactions. The particle-particle interactions are longer ranged for all considered systems except those of the smaller molecular weight chain grafted particles which show a HS-like behavior. 3D-DLS experiments in the concentrated regime up to the liquid-crystal transition provide the short-time diffusion function, D(q), in a broad range of scattering wavenumbers, q, from which the structural (cage) and short-time self-diffusion coefficients D(q) and D = D(q ≫ q), respectively, are deduced as functions of the effective particle volume fraction, ϕ = c/c, where c is the overlap concentration, calculated using the hydrodynamic particle radius, R. The size of the nearest-neighbor cage of particles is characterized by 2π/q, with D(q) and the static structure factor S(q) attaining at q the smallest and largest values, respectively. Experimental data of D(q) and D are contrasted with analytic theoretical predictions based on a simplifying hydrodynamic radius model where the internal hydrodynamic structure of the core-shell particles is mapped on a single hydrodynamic radius parameter γ = R/R, for constant direct interactions characterized by an (effective) hard-core radius R. The particle softness is reflected, in particular, in the corresponding shape of the static structure factor, while the mean solvent (Darcy) permeability of the particles related to γ is reflected in the dynamic properties only. For grafted particles with longer polymer chains, D(q) and D are indicative of larger permeability values while particles with shorter chains are practically nonpermeable. The particle softness is also evident in the effective random close packing fraction estimated from the extrapolated zero-value limit of the cage diffusion coefficient D(q).
我们利用三维动态光散射（3D-DLS）对具有不同分子量链形成软外壳的软核壳球形颗粒的流体状态悬浮液的结构和短时间动力学进行了详细的实验研究，这些颗粒因此具有不同程度的颗粒柔软度。由于颗粒的柔软性，发现液晶共存区域比硬球（HS）悬浮液的更宽。在稀溶液区域的静态光散射产生的形状因子可以用球形核壳模型来描述，第二维里系数A>0表明存在纯粹的排斥相互作用。除了接枝较小分子量链的颗粒表现出类似硬球的行为外，所有考虑的系统中颗粒间相互作用的范围都更长。在直至液晶转变的浓溶液区域进行的3D-DLS实验提供了在广泛的散射波数q范围内的短时间扩散函数D(q)，从中分别推导出结构（笼）系数和短时间自扩散系数D(q)和D = D(q≫q)，作为有效颗粒体积分数ϕ = c/c的函数，其中c是重叠浓度，使用流体动力学颗粒半径R计算得出。颗粒最近邻笼的大小由2π/q表征，D(q)和静态结构因子S(q)在q处分别达到最小值和最大值。D(q)和D的实验数据与基于简化流体动力学半径模型的解析理论预测进行了对比，在该模型中，核壳颗粒的内部流体动力学结构被映射到单个流体动力学半径参数γ = R/R上，对于以（有效）硬核半径R为特征的恒定直接相互作用。颗粒的柔软性尤其反映在静态结构因子的相应形状中，而与γ相关的颗粒的平均溶剂（达西）渗透率仅反映在动态特性中。对于接枝较长聚合物链的颗粒，D(q)和D表明渗透率值较大，而接枝较短链的颗粒实际上是不可渗透的。颗粒的柔软性在从笼扩散系数D(q)的外推零值极限估计的有效随机密堆积分数中也很明显。
