Funding Source: National Science Foundation (NSF)

Description/Abstract

Our research focuses on the dynamics of polymer blend films and polymer blend films containing nanoparticles. Controlling the properties in these systems is greatly complicated by the simultaneous occurrence of phase separation and wetting, especially when the system contains mobile nanoparticles. Our goal is to provide the insights of basic

science on the phase development of the thin films systems. Our studies can provide the template to tailor the desired morphologies, so that the films can be used in tissue scaffolds or polymeric light emitting devices (LEDs).

Polymer blend thin films (~500nm) of poly(methylmethacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) (φ_PMMA=0.5) undergo three distinctive stages of morphology development: early, intermediate and late stages [1]. The early stage (ES) is dominated by hydrodynamic-flow-driven wetting of PMMA to the free surface and substrate has through a 3D bicontinuous morphology (e.g., tubes) of PMMA:SAN interfaces produced by spinodal decomposition, which provides a pathway for the hydrodynamic-flow-driven wetting of PMMA to the free surface and substrate. During the intermediate stage (IS), discrete, PMMA-rich (denoted PMMA) domains in span the SAN-rich (denoted SAN) matrix in the mid-layer (actually tubes bridging two wetting layers of and grow as 2D disks. PMMA)[13], until theThe late state begins when long-range fluctuations parallel to the wetting layer/mid-layer interface rupture the film. The three-staged development was found for films thicker than ~70nm for critical blends. Effects of lateral confinement and surface patterning were also studied [2].

Using surface force microscopy (SFM) to characterize the IS of the non-wetting phase (SAN-rich) structure, we found six distinct morphological mechanisms as film thickness and composition was varied [3]. These mechanisms are distinguished by (A) PMMA domain coarsening, (B) a mixed mechanism of (A) and (B), (C) lateral material flow of a nearly-bicontinuous in-plane morphology, (D) dewetting of the SAN mid-layer from wetting layers, (E) a metastable trilayer structure, and (F) SAN droplets in a PMMA matrix. The regimes are mapped in Figure 1. Especially, regimes A and C exhibited quasi-2D phase separation with discrete and bicontinuous morphological growth, respectively. In regime A (discrete), we found a universal scaling behavior with time and thickness, which are consistent with a coalescent model. On the other hand, universal scaling failed in regime C (bicontinuous), suggesting that the hydrodynamic pumping mechanism is suppressed by confinement. Details are described in ref [4].

Fig 1. Intermediate Stage Phase Map. (White: SAN, black: PMMA (etched)) [3]

We also studied the impact of mobile nanoparticles on the phase separation dynamics using hydrophobized silica nanoparticles (~22nm). Particle motion and distributions are monitored with transmission electron microscopy (TEM), Rutherford backscattering (RBS), and SFM. In TEM picture from an as-cast film (Fig 2a), lateral homogeneous dispersion of nanoparticles (black) is confirmed. Upon annealing, PMMA-rich phase (white) forms domains scattered in continuous SAN-rich phase (gray) in the IS (Fig 2b). Interestingly, nanoparticles seem to partition into PMMA-rich phase, suggesting a preferential wetting. RBS studies reveal that the vertical distributions of nanoparticles with time are consistent with PMMA wetting layer build-up and back-flows (see [1], PRE (2000)). SFM shows local surface modulus change at the partcile-rich domains and time-dependent growth dynamics. In short, TEM, RBS, and SFM data are self-consistent to show that the particles partition into PMMA-rich phase. In addition, we found that the addition of nanoparticles slows down the phase separation dynamics drastically. Details are discussed in ref [5].

Fig 2. (a-b) TEM images from 120nm thick PMMA:SAN (50:50) films containing 5wt% silica nanoparticles (black dots, ~22nm). Particles are homogeneously dispersed after spin coating (a), then partition into PMMA-rich phase (white) rather than SAN-rich phase (gray) upon annealing. Schematic cartoon of the morphology is drawn in (c). Further evidences of partitioning of particles are observed with RBS and SFM. [5]

References

[1] H. Wang and R. J. Composto, “Understanding Morphology Evolution and Roughening in Phase Separating Thin Film Polymer Blends”, Europhys. Lett., 50, 622 (2000); see also: J. Chem. Phys., 113, 10386 (2000); Phys. Rev. E, 61, 1659 (2000); Langmuir, 17, 2857 (2001); Macromolecules, 35, 2799 (2002); Interf. Sci., 11, 237 (2003).

[2] Bi-min Zhang Newby and R. J. Composto, “Influence of Lateral Confinement on Phaer Separation in Thin Film Polymer Blends”, Macromolecules, 33, 3274 (2000); “Phase Morphology Map of Polymer Blend Thin Films Confined to Narrow Strips”, Phys. Rev. Lett., 87, 098302 (2001); see also: Polymer, 42, 9155 (2001).

[3] Hyun-joong Chung, Howard Wang, and Russell J. Composto, “Interface Modulated Pattern Formations in Thin Film Polymer Blends”, Macromolecules, to be submitted.

[4] Hyun-joong Chung and R. J. Composto, “Breakdown of Dynamic Scaling in Thin Film Binary Liquids Undergoing Phase Separation”, Phys. Rev. Lett., 92, 185704 (2004).

[5] Hyun-joong Chung, Andreas Taubert, Ranjan D. Deshmukh, and Russell J. Composto, “Mobile Nanoparticles and Their Effect on Phase Separation Dynamics in Polymer Blend Film”, Europhys. Lett., in press, (2004).