Selective solvent annealing induced phase separation and dewetting in PMMA/SAN blend ultrathin films

Sequential Selective Solvent On-Film Annealing: Fabrication of Monolayers of Ordered Anisotropic Polymer Particles

Although various strategies have been developed to prepare anisotropic polymeric particles, it remains challenging to fabricate monolayers of anisotropic polymeric particles, which can extend the applications of anisotropic particles. Here, we develop a novel and facile approach to fabricate monolayers of anisotropic polymeric particles. Monolayers of polystyrene (PS) microspheres with a mean diameter of 10 μm are deposited on glass substrates coated with poly(methyl methacrylate) films, followed by sequential selective solvent on-film annealing processes. Monolayers of anisotropic polymeric particles, such as the snowman-like PS particles, are successfully fabricated. Such unique structures possess the long-range ordering of monolayers (the structure factor) and the anisotropic geometry of individual particles (the form factor). The nanomechanical properties of the PS particles are also characterized using atomic force microscopy force volume measurements, showing a decrease in the Young's moduli of the PS particles owing to the looser packing of the polymer chains. This work provides the most facile and versatile strategy by far to fabricate monolayers of ordered anisotropic polymeric particles, which are inaccessible by other traditional means.

Interplay between phase separation and dewetting in PS/PVME thin films: effect of temperature

We studied the effects of temperature on the interplay between dewetting and phase separation at shallow and deep depths at two-phase temperatures in PS/PVME polymer blend thin films. Optical microscopy, AFM measurements, and ellipsometry analysis were performed to investigate the dewetting behavior of the films. At the deep quench depth (phase separation temperature of 115 °C), a two-layer film formed, consisting of a thin PVME layer directly on the surface of a silicon wafer (as the wetting layer) and a bulk layer which was the upper layer. In the bulk layer, the phase separation mechanism was controlled by an apparent nucleation and growth mechanism, which was driven by entropic and anisotropic limitations rather than thermodynamic preferences. After about 106 min of annealing, liquid-liquid dewetting occurred in the interface of the formed layers, triggered by Laplace pressure differences. However, at the shallow quench depth (phase separation temperature of 95 °C), a tri-layered structure formed in the thin films and concentration fluctuations at the interfaces of the formed layers triggered surface fluctuations and instabilities (dewetting phenomenon).

Phase miscibility and dynamic heterogeneity in PMMA/SAN blends through solvent free reactive grafting of SAN on graphene oxide

The spatial distribution of nanoparticles in a particular host polymer matrix can be improved by using brush coated nanoparticles. In this work we have grafted styrene-acrylonitrile (SAN) onto the surface of graphene oxide (GO) and investigated as to how the demixing temperature, morphology and volume cooperativity of PMMA/SAN blends are influenced. Grafting of polymer chains on the surface of nanoparticles usually involves the use of large amounts of solvents, many which are detrimental to the environment besides involving cumbersome processes. SAN-g-GO was prepared by a robust solvent-free strategy wherein the cyano group in SAN was replaced by oxazoline groups during melt mixing in the presence of zinc acetate and ethanol amine. These newly created oxazoline groups reacted with the COOH group of GO under melt extrusion resulting in grafting of SAN on the surface of GO sheets. The effect of SAN-g-GO nanoparticles on the demixing, local segmental motions and morphology evolution for different annealing times was carefully investigated in a classical LCST system, PMMA/SAN blend, using melt rheology, modulated DSC and AFM, respectively. The changes in viscoelastic behavior in the vicinity of demixing are investigated systematically for the control, and blends with GO and SAN-g-GO. Various models were used to gain insight into the spinodal decomposition temperatures of the blends. Interestingly, the demixing temperature determined rheologically and the spinodal decomposition temperature increased significantly in the presence of polymer grafted nanoparticles in comparison to the control and blends with GO. The evolution of the morphology, interfacial driven coarsening as a function of temperature and the localization of nanoparticles were assessed using atomic force microscopy. The cooperatively re-arranging regions estimated from calorimetric measurements begin to suggest enhanced dynamic heterogeneity in the presence of GO and SAN-g-GO in the blends. Taken together, our study reveals that the solvent-free approach of grafting SAN onto GO delays demixing, suppresses coalescence and alters cooperative relaxation in PMMA/SAN blends.

Solvent-vapor-assisted dewetting of prepatterned thin polymer films: control of morphology, order, and pattern miniaturization

Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.