In the immersion precipitation process for membrane formation, a polymer casting film is placed in contact with a nonsolvent in a coagulation bath; an essential feature of the membrane formation process is the foray into the metastable region of the ternary phase diagram for the nonsolvent/solvent/polymer system. The primary objective of this article is to trace the origins of such forays. The Maxwell-Stefan diffusion formulation is combined with the Flory-Huggins description of phase equilibrium thermodynamics to set up a model for describing the transient equilibration trajectory that is followed in the polymer casting film. Four different systems are analyzed: water/acetone/CA, water/DMF/PVDF, water/NMP/PSF, and water/NMP/PEI (CA = cellulose acetate; PVDF = poly(vinylidene fluoride); PSF = polysulfone; PEI = polyetherimide, DMF = dimethyl formamide; NMP = N-methyl-2-pyrrolidone). The analysis shows that diffusional forays are mainly engendered due to thermodynamic coupling effects; such effects are quantified by the set of thermodynamic factors , where a i , the activity of species i, is dependent on the volume fractions, ϕ i and ϕ j , of both nonsolvent (i) and solvent (j). In regions close to phase transitions, the off-diagonal elements Γ ij (i ≠ j) are often negative and may attain large magnitudes in relation to the diagonal elements Γ ii . Strong thermodynamic coupling effects cause the transient equilibration trajectories to be strongly curvilinear, causing ingress into the metastable region. If thermodynamic coupling effects are ignored, no such ingress occurs. It is also shown that analogous diffusional forays may lead to emulsion formation in partially miscible liquid mixtures.

中文翻译：

---

在形成聚合物膜的沉浸过程中突出热力学耦合效应。

在用于膜形成的浸没沉淀法中，将聚合物流延膜与非溶剂接触放置在凝结浴中。膜形成过程的基本特征是进入非溶剂/溶剂/聚合物体系的三元相图的亚稳态区域。本文的主要目的是追溯此类尝试的起源。将Maxwell-Stefan扩散公式与相平衡热力学的Flory-Huggins描述相结合，以建立一个模型来描述聚合物流延膜中遵循的瞬态平衡轨迹。分析了四个不同的系统：水/丙酮/ CA，水/ DMF / PVDF，水/ NMP / PSF和水/ NMP / PEI（CA =醋酸纤维素； PVDF =聚偏二氟乙烯； PSF =聚砜； PEI =聚醚酰亚胺 DMF =二甲基甲酰胺；NMP = N-甲基-2-吡咯烷酮）。分析表明，扩散热主要是由于热力学耦合作用引起的。此类影响可以通过一组热力学因子来量化，其中ai是物种i的活性，它取决于非溶剂（i）和溶剂（j）的体积分数ϕ i和ϕ j。在接近相变的区域中，非对角元素Γij（i≠j）通常为负，并且相对于对角元素Γii可能达到较大的幅度。强烈的热力学耦合效应使瞬态平衡轨迹具有强烈的曲线，从而进入亚稳态区域。如果忽略了热力学耦合效应，则不会发生此类侵入。