Polystyrene (PS) that has been exposed to ultraviolet light (UV) undergoes partial dehydrogenation of the alkane polymer backbone which increases its surface energy. Exploiting this photochemistry, we exposed polystyrene films to UV light using a photomask to induce a patterned photochemical reaction producing regions in the film with differing surface energy. Upon heating the solid polymer film with the preprogrammed surface energy pattern to a liquid state, the polymer flows from the low surface energy unexposed regions to high surface energy exposed regions. This flow creates three-dimensional topography by the Marangoni Effect, which describes convective mass transfer due to surface energy gradients. The topographical features can be permanently preserved by quenching the film below its glass to liquid transition temperature. Their shape and organization are only limited by the pattern on the photomask.

The orientation of cylinder-forming poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] was investigated on two sets of polymeric surface treatments: 10 para-substituted polystyrene derivatives with <10 mol % poly(4-vinylbenzyl azide) and a series of poly(styrene-random-4-vinylbenzyl azide) [P(S-r-VBzAz)] copolymers with 5-100 mol % poly(4-vinylbenzyl azide). The copolymers were spin-coated to form thin films and then cross-linked by heating. The resulting films exhibited a range of surface tensions from 21 to 45 dyn/cm. Perpendicular orientation of P(S-b-MMA) cylinders was achieved with poly(p-bromostyrene) and all the [P(S-r-VBzAz)] copolymer surface treatments, most notably the homopolymer of poly(4-vinylbenzyl azide). Films made from these simple copolymers are as effective as random terpolymer alignment layers commonly made from both block monomers and a cross-linkable monomer.

A Marangoni flow is shown to occur when a polymer film possessing a spatially-defined surface energy pattern is heated above its glass transition to the liquid state. This can be harnessed to rapidly manufacture polymer films possessing prescribed height profiles. To quantify and verify this phenomenon, a model is described here which accurately predicts the formation, growth, and eventual dissipation of topographical features. The model predictions, based on numerical solutions of equations governing thin film dynamics with a Marangoni stress, are quantitatively compared to experimental measurements of thin polystyrene films containing photochemically patterned surface energy gradients. Good agreement between the model and the data is achieved at temperatures between 120 and 140 °C for a comprehensive range of heating times using reasonable physical properties as parameter inputs. For example, thickness variations that measure 102% of the starting film thickness are achieved in only 12 minutes of heating at 140 °C, values that are predicted by the model are within 6% and 3 min, respectively. The photochemical pattern that directed this flow possessed only a 0.2 dyne cm(-1) variation in surface tension between exposed and unexposed regions. The physical insights from the validated model suggest promising strategies to maximize the aspect ratio of the topographical features and minimize the processing time necessary to develop them.

Even though the physics of nanoconfined polymers have been extensively studied for years, diffusion of polymer chains along confining interfaces has not been widely studied, likely because there are few experimental techniques available for these measurements. Here a fluorescence recovery after patterned photobleaching (FRAPP) technique is developed using an epifluorescence microscope that allows for direct, in situ, visualization of polymer diffusion over several periods of a photobleached array. This visualization approach is more robust compared to measuring fluorescence intensity alone and also significantly increases the experimental throughput. Using this technique, self-diffusion of poly(isobutyl methacrylate) (PiBMA) was investigated at 80 °C (29 °C above the glass transition temperature, T g) and was found to be film thickness independent down to 30 nm (∼14R g, where R g is the radius of gyration) with a diffusion coefficient well predicted by the Rouse model (1.05 × 10–12 cm2/s). PiBMA is an ideal polymer for this study because it exhibits a film thickness-independent T g down to 15 nm (∼7R g) as measured by spectroscopic ellipsometry. Since the diffusion coefficient of polymers depends strongly on the proximity of diffusion temperature to T g, this attribute allows a straightforward measure of nanoconfined diffusion without superimposed influence from T g nanoconfinement effects.

Introduction. Cubic silsesquioxanes (see Figure 1) are unique molecules that combine three-dimensional cubic symmetry with single nanometer diameters and a core that is the smallest single crystal of silica. Symmetry places a functional group on each vertex in a different octant in Cartesian space providing the opportunity to form covalent bonds accordingly, such that the potential exists to construct materials in 1-, 2-, or 3-dimensions nanometer by nanometer. In principle, this permits manipulation of global properties by tailoring structures at nanometer length scales, allowing the finest control possible. It also provides access to materials with highly reproducible properties and the potential to predict and design them for specific applications. [1][2][3][4][5][6][7][8][9][10] Results and Discussion. We recently began exploring the chemistries and properties of epoxy resins and polyimides made with octaaminophenylsilsesquioxane, [NH 2 PhSiO 1.5 ] 8 , OAPS. [11][12][13][14] In early studies we demonstrated that global silsesquioxane nanocomposite properties can be tailored by controlling the structure of the organic tether linking cube vertices, at nanometer length scales. [15][16][17][18][19] We report here efforts to develop single-phase materials that offer control of the coefficients of thermal expansion (CTE) of silsesquioxane epoxy resins over an order of magnitude. Control of CTE is of considerable importance in multiple materials applications (e.g., coatings that offer resistance to abrasion, corrosion, photooxidation, hydrophobicity, staining, etc.) where the polymer coating is applied to glass, ceramic, or metal substrates with quite dissimilar CTEs. In such instances, thermal cycling often leads to loss of adhesion followed by coating failure via chemical and/or mechanical mechanisms. 20 CTE mismatches are also quite problematic in electronic applications, for example, in interlayer dielectrics and flip-chip underfills. 21 In the latter case, the underfill epoxy must match the CTEs of silicon-based ICs (CTEs of 2-3 µm/°C) with substrates (CTEs of 20-40 µm/°C) to ensure good thermal management. Current epoxy materials require silica fillers to adjust CTEs to g20 µm/°C. Such CTEs are intermediate between substrates and silicon to minimize fatigue at solder joints. These fillers raise resin viscosities to levels near 50 000 mPa‚s, making processing very difficult. Likewise, corrosionresistant epoxy resin coatings on Al alloys for aircraft bodies must minimize environmental corrosion and offer good abrasion resistance and curing at temperatures <50 °C but also have CTEs close to those of the alloys, typically 22-24 µm/°C. Such values were heretofore unknown for simple epoxy systems and especially for primer coats on aircraft fuselages that are typically DGEBA/DDM materials (60-70 µm/°C). 22 Epoxy resin thermosets studied here were produced from a series of epoxys (see Table 1 and Figure 2) formulated using OAPS as the curing agent. The formulations chosen were made according to our original model...

New aminophenylsilsesquioxanes, dodecaaminophenylsilsesquioxane (NH2PhSiO1.5)12 (DAPS), and hexadecaaminooctaphenylsilsesquioxane ((NH2)2PhSiO1.5)8 (HDAPS), were prepared from dodecaphenylsilsesquioxane (PhSiO1.5)12 (DPS), and octaphenylsilsesquioxane (PhSiO1.5)8 (OPS), by nitration in fuming HNO3, followed by reduction with HCO2H/Et3N/Pd/C. The effects of the cage structure and numbers of NH2 groups on the chemical and physical properties of these compounds were compared with octaaminophenylsilsesquioxane (NH2PhSiO1.5)8 (OAPS), and analogous epoxy resin nanocomposites. The cage structure does not affect nitration patterns (primarily meta and para), but affects coefficients of thermal expansion (CTEs), likely because of overall crosslink densities and because of the epoxy tether structures joining cube vertices. CTEs as low as 25 ± 2 ppm°C–1 were found, perhaps the lowest reported for an unfilled epoxy resin. The thermal stabilities of synthesized HDAPS epoxy nanocomposites were 20–30°C lower than those made with OAPS. This result may be a consequence of less-complete curing due to gelation before stoichiometric curing, caused by the much higher crosslink densities in OAPS.

Convection can be harnessed in elegant ways to pattern surfaces, often using uncomplicated equipment and materials, providing an interesting platform for future technological developments in thin film topographic assemblies. This manuscript contains a brief review of thin polymer film patterning methods that rely on directing convection, such as ''coffee ring'' patterning, lithographically induced self-assembly, and electrohydrodynamic patterning. These techniques are described in the context of a recent approach explored in our group for generating topographic patterns by photochemically directing Marangoni flow in thin polymer films with subtle gradients in surface energy. Aspects unique to this process are highlighted so that they may facilitate new developments in manufacturing technologically impactful patterned surfaces. For example, the features produced by photochemically directed Marangoni-driven flow are preprogrammed in the solid state, thermally developed and can be formed in multilayer films.

Coatings and substrates with topographically patterned features will play an important role in efficient technologies for harvesting and transmitting light energy. In order to address these applications, a methodology for prescribing height profiles in polymer films is presented here. This is accomplished by photochemcially patterning a solid-state, sensitized polymer film. After heating the film above its glass transition temperature, melt-state flow is triggered and directed by the chemical pattern. A second light exposure was applied to fully activate a heat-stable photo-crosslinking additive. The features formed here are thermochemically stable and can act as an underlayer in a multilayered film. To exemplify this capability, these films were also used to direct the macroscopic film morphology of a block copolymer overlayer.