Palit Swomitra, He Lilin, Hamilton William A, Yethiraj Arun, Yethiraj Anand
Department of Physics and Physical Oceanography, Memorial University, St. John's, Newfoundland A1B3X7, Canada.
Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
Phys Rev Lett. 2017 Mar 3;118(9):097801. doi: 10.1103/PhysRevLett.118.097801.
The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of "crowding" proteins in cells, colloid-polymer mixtures, and nanoparticle "fillers" in polymer solutions and melts. In this Letter, we study the effect of spherical inert nanoparticles (which we refer to as "crowders") on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gradient NMR and small-angle neutron scattering (SANS), respectively. The diffusion coefficients exhibit a plateau below a characteristic polymer concentration, which we identify as the overlap threshold concentration c^{⋆}. Above c^{⋆}, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. The radius of gyration obtained from SANS in the crossover regime changes linearly with an increase in polymer concentration, and must be extrapolated to c^{⋆} in order to obtain the radius of gyration of an individual polymer chain. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. There is significant chain compression, however, when the crowder size is much smaller than the polymer radius gyration.
颗粒对聚合物在溶液中行为的影响在许多重要现象中都很重要，比如细胞中蛋白质“拥挤”效应、胶体 - 聚合物混合物以及聚合物溶液和熔体中的纳米颗粒“填料”。在本信函中，我们分别使用脉冲场梯度核磁共振（pulsed - field - gradient NMR）和小角中子散射（SANS）研究了球形惰性纳米颗粒（我们称之为“拥挤剂”）对溶液中聚合物扩散系数和回转半径的影响。扩散系数在低于特征聚合物浓度时呈现出一个平台期，我们将其确定为重叠阈值浓度(c^{\star})。在(c^{\star})之上，在稀溶液和半稀溶液区域之间的交叉区域，发现（长时间）自扩散系数在所有拥挤剂填充分数下都普遍随聚合物浓度呈指数下降，这与长时间动力学的结构基础一致。在交叉区域通过小角中子散射得到的回转半径随聚合物浓度增加呈线性变化，并且必须外推到(c^{\star})才能得到单个聚合物链的回转半径。当聚合物回转半径和拥挤剂尺寸相当时，聚合物尺寸受拥挤剂存在的影响非常小，这与最近的计算机模拟结果一致。然而，当拥挤剂尺寸远小于聚合物回转半径时，会有显著的链压缩。
