Matsen M W
Department of Chemical Engineering, Department of Physics & Astronomy, and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
J Chem Phys. 2022 May 14;156(18):184901. doi: 10.1063/5.0087587.
We examine athermal binary blends composed of conformationally asymmetric polymers of equal molecular volume next to a surface of width ξ. The self-consistent field theory (SCFT) of Gaussian chains predicts that the more compact polymer with the shorter average end-to-end length, R, is entropically favored at the surface. Here, we extend the SCFT to worm-like chains with small persistence lengths, ℓ, relative to their contour lengths, ℓ, for which R≈2ℓℓ. In the limit of ℓ ≪ ξ, we recover the Gaussian-chain prediction where the segregation depends only on the product ℓℓ, but for realistic polymer/air surfaces with ξ ∼ ℓ, the segregation depends separately on the two quantities. Although the surface continues to favor flexible polymers with smaller ℓ and bulky polymers with shorter ℓ, the effect of bulkiness is more pronounced. This imbalance can, under specific conditions, lead to anomalous surface segregation of the more extended polymer. For this to happen, the polymer must be bulkier and stiffer, with a stiffness that is sufficient to produce a larger R yet not so rigid as to reverse the surface affinity that favors bulky polymers.
我们研究了由等分子体积的构象不对称聚合物组成的非热二元共混物，该共混物靠近宽度为ξ的表面。高斯链的自洽场理论（SCFT）预测，平均端到端长度R较短的更致密聚合物在表面上在熵方面更有利。在这里，我们将SCFT扩展到相对于其轮廓长度ℓ具有小的持久长度ℓ的蠕虫状链，对于这种链，R≈2√(ℓℓ)。在ℓ≪ξ的极限情况下，我们恢复了高斯链的预测，其中偏析仅取决于乘积ℓℓ，但对于具有ξ∼ℓ的实际聚合物/空气表面，偏析分别取决于这两个量。尽管表面继续有利于具有较小ℓ的柔性聚合物和具有较短ℓ的 bulky 聚合物，但bulkiness的影响更为明显。在特定条件下，这种不平衡会导致更伸展的聚合物出现异常的表面偏析。要发生这种情况，聚合物必须更 bulky 且更硬，其刚度足以产生更大的R，但又不能太硬以至于逆转有利于 bulky 聚合物的表面亲和力。
