Meng Dong, Wang Qiang
Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523-1370, USA.
J Chem Phys. 2007 Jun 21;126(23):234902. doi: 10.1063/1.2740633.
We have investigated several effects due to the confinement of polymer melts by impenetrable (hard) surfaces in the self-consistent field calculations. To adequately represent such confinement, the total (normalized) polymer segmental density (volume fraction) is usually constrained to an imposed profile that continuously decreases from 1 in the interior of confined melts to 0 at the surfaces over a short distance. The choice of this profile strongly influences the numerical performance of the self-consistent field calculations. In addition, for diblock copolymers A-B the hard-surface confinement has both energetic and entropic effects: On one hand, the decrease of polymer density from 1 reduces A-B repulsion and favors morphologies with more A-B interfaces near the surfaces. On the other hand, the enrichment of chain ends and depletion of middle segments near the surfaces favor parallel morphologies where chains orient mainly perpendicular to the surfaces. These two effects are comparable in magnitude, and for asymmetric diblock copolymers result in an entropic preference of a neutral surface for the shorter block as proposed previously [Q. Wang et al., Macromolecules 34, 3458 (2001)]. The hard-surface effects are weak in practice and thus manifested only when the surfaces are nearly neutral.
我们在自洽场计算中研究了不可穿透（硬）表面对聚合物熔体的限制所产生的几种效应。为了充分体现这种限制，通常将总（归一化）聚合物链段密度（体积分数）限制在一个规定的分布上，该分布在短距离内从受限熔体内部的1连续降至表面处的0。这种分布的选择对自洽场计算的数值性能有很大影响。此外，对于AB型二嵌段共聚物，硬表面限制同时具有能量和熵效应：一方面，聚合物密度从1降低会减少AB排斥，并有利于在表面附近具有更多AB界面的形态。另一方面，表面附近链端的富集和中间链段的耗尽有利于平行形态，其中链主要垂直于表面取向。这两种效应的大小相当，对于不对称二嵌段共聚物，会导致如先前[Q. Wang等人，《大分子》34, 3458 (2001)]所提出的，较短链段倾向于中性表面的熵偏好。硬表面效应在实际中较弱，因此仅在表面接近中性时才会显现出来。
