Interplay of Substrate Surface Energy and Nanoparticle Concentration in Suppressing Polymer Thin Film Dewetting

It is known that dewetting of a polystyrene (PS) thin film on a silicon substrate gets completely suppressed upon addition of small amount of C₆₀ nanoparticles (NP). The NPs migrate to the film–substrate interface and forms an enriched surface layer of the particles that eventually stabilizes the film by apparent pinning. In this article we quantitatively highlight the unexplored effect of substrate surface energy (γ_S) on the migration of the NPs to the film–substrate interface and their contribution on dewetting suppression. Depending on the relative magnitudes of NP concentration (<i>C</i>_NP) and γ_S, we identify three distinct stability regimes. In regime 1 (<i>C</i>_NP < 0.2%) there is no suppression of dewetting and the final polygonal arrangement of droplets closely resemble dewetted structures in particle free films. However, the size of the polygons becomes smaller in NP containing films when γ_S < γ_C60 (NP surface energy) and larger as γ_S exceeds γ_C60. In regime 2 (0.3% < <i>C</i>_NP < 0.75%) the films dewet partially, and the extent of dewetting is seen to strongly dependent on the relative magnitudes of γ_C60 and γ_S. While dewetting proceeds up to the stage of partial hole growth and coalescence when γ_S < γ_C60, some random isolated holes are seen to form when γ_S > γ_C60. On the basis of direct AFM imaging, we show that in both regimes 1 and 2 the NPs migrate to the substrate–film interface only when γ_S > γ_C60. We show complete suppression of dewetting in regime 3 (<i>C</i>_NP > 1.0%), where the particles are seen to migrate to the substrate for all values of γ_S. The work highlights that entropy driven migration of particles takes place on substrates with any γ_S only above a critical NP concentration (<i>C</i>_NPC) and only on substrates with γ_S > γ_C60 when <i>C</i>_NP < <i>C</i>_NPC. The findings, apart from dewetting suppressing, can guide potential design criteria for applications such as electron extracting layer in organic photovoltaic

Extending Dynamic Range of Block Copolymer Ordering with Rotational Cold Zone Annealing (RCZA) and Ionic Liquids

Scalable and low-cost methods to align and orient block copolymer (BCP) films and membranes are critical for their adaptation for nonlithographic applications. Cold zone annealing (CZA) can align BCP microdomains and is scalable via roll-to-roll (R2R) manufacturing. However, the efficacy of orientation by CZA is strongly dependent on the thermal zone velocity (<i>V</i>_cza). Optimization of this rate can be time-consuming and tedious. To address this shortcoming, we report rotational or radial CZA (RCZA) that provides a combinatorial approach to efficiently determine how linear <i>V</i>_cza rate impacts microdomain orientation. RCZA rapidly identifies the optimal CZA velocities for perpendicular orientation of cylinders in polystyrene-<i>block</i>-poly­(methyl methacrylate) films that previously required tens of measurements [Macromolecules 2012, 45, 7107], demonstrated here with much finer velocity resolution using three overlapping radial regimes. Notably, the efficacy of CZA for perpendicular alignment rapidly decays for <i>V</i>_cza > 10 μm/s. To overcome this limitation, the addition of 2 wt % 1-ethyl-3-methylimidazolium bis­(trifluoro­methyl­sulfonyl)­imide sufficiently alters the surface tension and segmental relaxations via reduced viscosity to increase the processing window for perpendicular cylinders, approximately 75% at <i>V</i>_cza ≈ 330 μm/s, approaching R2R speeds. Further increasing ionic liquid content to 5 wt % leads to mostly parallel orientation due to surface wetting. Ionic liquids can dramatically increase BCP processing speeds for applications, such as membranes, and RCZA can efficiently map out the optimal processing parameters

Polymer Chain Dynamics in Intercalated Poly(ε-caprolactone)/Nanoplatelet Blends

The dynamics of poly­(ε-caprolactone) (PCL) blends with small amounts of Cloisite 30B nanoplatelets were investigated via broadband dielectric relaxation spectroscopy. The terminal relaxation, or normal mode of PCL, was found to be strongly influenced by the presence of the nanoplatelets, exhibiting a relaxation from PCL chains within the Cloisite platelets as well as a bulk-like relaxation. These results, as well as X-ray scattering and rheological results from the literature, help to form a clearer picture of how the structure and dynamics relate to the bulk physical properties in these systems. Cloisite was observed to have minimal impact on both the glassy state dynamics as well as the segmental dynamics, due to the relatively weak interactions between the polymer and the platelets

Dynamic Thermal Field-Induced Gradient Soft-Shear for Highly Oriented Block Copolymer Thin Films

As demand for smaller, more powerful, and energy-efficient devices continues, conventional patterning technologies are pushing up against fundamental limits. Block copolymers (BCPs) are considered prime candidates for a potential solution <i>via</i> directed self-assembly of nanostructures. We introduce here a facile directed self-assembly method to rapidly fabricate unidirectionally aligned BCP nanopatterns at large scale, on rigid or flexible template-free substrates <i>via</i> a thermally induced dynamic gradient soft-shear field. A localized differential thermal expansion at the interface between a BCP film and a confining polydimethylsiloxane (PDMS) layer due to a dynamic thermal field imposes the gradient soft-shear field. PDMS undergoes directional expansion (along the annealing direction) in the heating zone and contracts back in the cooling zone, thus setting up a single cycle of oscillatory shear (maximum lateral shear stress ∼12 × 10⁴ Pa) in the system. We successfully apply this process to create unidirectional alignment of BCP thin films over a wide range of thicknesses (nm to μm) and processing speeds (μm/s to mm/s) using both a flat and patterned PDMS layer. Grazing incidence small-angle X-ray scattering measurements show absolutely no sign of isotropic population and reveal ≥99% aligned orientational order with an angular spread Δθ_fwhm ≤ 5° (full width at half-maximum). This method may pave the way to practical industrial use of hierarchically patterned BCP nanostructures

Transient Interfacial Pattern Formation in Block Copolymer Thin Films via Sequential Thermal and Solvent Immersion Annealing

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under “near-equilibrium” conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling “defect” patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly “defective” film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications

Transient Interfacial Pattern Formation in Block Copolymer Thin Films via Sequential Thermal and Solvent Immersion Annealing

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under “near-equilibrium” conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling “defect” patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly “defective” film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications

Transient Interfacial Pattern Formation in Block Copolymer Thin Films via Sequential Thermal and Solvent Immersion Annealing

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under “near-equilibrium” conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling “defect” patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly “defective” film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications

Self-Cross-Linking of MXene-Intercalated Graphene Oxide Membranes with Antiswelling Properties for Dye and Salt Rejection

Membrane-based water purification is poised to play an important role in tackling the potable water crisis for safe and clean water access for the general population. Several studies have focused on near two-dimensional membranes for this purpose, which is based on an ion rejection technique. However, membrane swelling in these materials has emerged as a significant challenge because it leads to the loss of function. Herein, we report a self-cross-linked MXene-intercalated graphene oxide (GO) membrane that retains ion and dye rejection properties because the physical cross-linking interaction between Ti–O–Ti and neighboring nanosheets effectively suppresses the swelling of the membrane. In addition to the associative Ti–O–Ti bonds, C–O–C, OC–O, and C–OH bonds are also formed, which are important for inhibiting the swelling of the membrane. To ensure the longevity of these membranes in a service context, they were subjected to heat pressurization and subsequent thermal annealing. The membrane subjected to this novel processing history exhibits minimal swelling upon immersion in solutions and retains function, rejecting salt and dyes over a wide range of salt and dye concentrations. Furthermore, these membranes successfully rejected dye and salt over a period of 72 h without a degradation of function, suggesting that these membranes have the requisite durability for water filtration applications

Transient Interfacial Pattern Formation in Block Copolymer Thin Films via Sequential Thermal and Solvent Immersion Annealing

A variety of structures encountered in nature only arise in materials under highly nonequilibrium conditions, suggesting to us that the scope for creating new functional block copolymer (BCP) structures might be significantly enlarged by embracing complex processing histories that allow for the fabrication of structures quite unlike those created under “near-equilibrium” conditions. The present work examines the creation of polymer film structures in which highly nonequilibrium processing conditions allow for the creation of entirely new types of transient BCP morphologies achieved by transitioning between different ordered states. Most previous studies of BCP materials have emphasized ordering them from their disordered state obtained from a solution film casting process, followed by a slow thermal annealing (TA) process at elevated temperatures normally well above room temperature. We have previously shown that achieving the equilibrium TA state can be accelerated by a direct solvent immersion annealing (DIA) preordering step that creates nascent ordered microstructures, followed by TA. In the present work, we examine the reverse nonequilibrium sequential processing in which we first thermally anneal the BCP film to different levels of partial (lamellar) order and then subject it to DIA to swell the lamellae. This sequential processing rapidly leads to a swelling-induced wrinkle pattern that initially grows with immersion time and can be quenched by solvent evaporation into its corresponding glassy state morphology. The article demonstrates the formation of wrinkling “defect” patterns in entangled BCP films by this sequential annealing that does not form under ordinary TA conditions. At long DIA times, these highly “defective” film structures evolve in favor of the equilibrium morphology of parallel lamellae observed with DIA alone. In conjunction with our previous study of sequential DIA + TA, the present TA + DIA study demonstrates that switching the order of these processing methods for block copolymer films gives the same final state morphology in the limit of long time as any one method alone, but with drastically different intermediate transient state morphologies. These transient morphologies could have many applications

Capillary Force Lithography Pattern-Directed Self-Assembly (CFL-PDSA) of Phase-Separating Polymer Blend Thin Films

We report capillary force lithography pattern-directed self-assembly (CFL-PDSA), a facile technique for patterning immiscible polymer blend films of polystyrene (PS)/poly­(methyl methacrylate) (PMMA), resulting in a highly ordered phase-separated morphology. The pattern replication is achieved by capillary force lithography (CFL), by annealing the film beyond the glass transition temperature of both the constituent polymers, while confining it between a patterned cross-linked poly­(dimethyl siloxane) (PDMS) stamp and the silicon substrate. As the pattern replication takes place because of rise of the polymer meniscus along the confining stamp walls, higher affinity of PMMA toward the oxide-coated silicon substrate and of PS toward cross-linked PDMS leads to well-controlled vertically patterned phase separation of the two constituent polymers during thermal annealing. Although a perfect negative replica of the stamp pattern is obtained in all cases, the phase-separated morphology of the films under pattern confinement is strongly influenced by the blend composition and annealing time. The phase-separated domains coarsen with time because of migration of the two components into specific areas, PS into an elevated mesa region and PMMA toward the substrate, because of preferential wetting. We show that a well-controlled, phase-separated morphology is achieved when the blend ratio matches the volume ratio of the elevated region to the base region in the patterned films. The proposed top-down imprint patterning of blends can be easily made roll-to-roll-compatible for industrial adoption