We report the first simulations of
nonsolvent-induced phase separation
(NIPS) that predict membrane microstructures with graded asymmetric
pore size distribution. In NIPS, a polymer solution film is immersed
in a nonsolvent bath, enriching the film in nonsolvent, and leading
to phase separation that forms a solid polymer-rich membrane matrix
and polymer-poor membrane pores. We demonstrate how mass-transfer-induced
spinodal decomposition, thermal fluctuations, and glass-transition
dynamicsimplemented with mobility contrast between the polymer-rich
and polymer-poor phasesare essential to the formation of asymmetric
membrane microstructures. Specifically, we show that the competition
between the propagation of the phase-separation and glass-transition
fronts determines the degree of pore-size asymmetry. We also explore
the sensitivity of these microstructures to the initial film composition,
and compare their formation in 2D and 3D.
We develop a multi-fluid model for a ternary polymer solution using the Rayleighian formalism of Doi and Onuki, and give an efficient pseudo-spectral method for solving both the diffusion and momentum equations that result. Subsequently, we find that the numerical simulation is capable of describing systems at the micron length-scale and easily reaches millisecond time-scales. In addition, we characterize the model thermodynamics and kinetics including the (i) phase behavior, (ii) structure of the interfaces, (iii) mutual diffusion coefficients, (iv) bulk spinodal decomposition kinetics with and without hydrodynamics and (v) spinodal decomposition in the presence of an interface with a non-solvent bath. We obtain good qualitative agreement with the expected thermodynamic and kinetic behavior. We also show that a linear stability analysis of the diffusion equation quantitatively predicts the fastest growing mode obtained from simulation and gives insight into the phase separation process relevant for the evolution of microstructure in phase-separating ternary polymer solutions.
Motivated by the much discussed,
yet unexplained, presence of macrovoids
in polymer membranes, we explore the impact of Marangoni flows in
the process of nonsolvent induced phase separation. Such flows have
been hypothesized to be important to the formation of macrovoids,
but little quantitative evidence has been produced to date. Using
a recently developed multifluid phase field model, we find that roll
cells indicative of a solutal Marangoni instability are manifest during
solvent/nonsolvent exchange across a stable interface. However, these
flows are weak and subsequently do not produce morphological features
that might lead to macrovoid formation. By contrast, initial conditions
that lead to an immediate precipitation of the polymer film coincide
with large Marangoni flows that disturb the interface. The presence
of such flows suggests a new experimental and theoretical direction
in the search for a macrovoid formation mechanism.
We use self-consistent field theory (SCFT) to study the self-assembly of diblock copolymers confined in cylindrical and non-cylindrical prepatterns. This situation arises in contact holes -the hole shrink problem-where the goal is to produce a contact hole with reduced dimensions relative to a guiding prepattern. In this study, we focus on systems with a critical dimensions (CD) ranging from ~50nm to ~100nm leading to the formation of a single PMMA domain in the middle of the holes. We found that different morphologies arise from the self-assembly process and are strongly governed by the prepattern dimensions, wetting conditions, shape of prepatten as well as the polymer molecular weight. We also considered blends of diblock copolymers and homopolymers and determined optimal blending configurations that not only favor the formation of the desired cylindrical morphology but also extend the processing window relative to the pure diblock case in cylindrical confinements.
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