Michael J. Fasolka

Generating Thickness Gradients of Thin Polymer Films via Flow Coating

Thickness is a governing factor in the behavior of films and coatings. To enable the high-throughput analysis of this parameter in polymer systems, we detail the design and operation of a “flow coater” device for fabricating continuous libraries of polymer film thickness over tailored ranges. Focusing on the production of model polystyrene film libraries, we thoroughly outline the performance of flow coating by varying critical factors including device geometry, device motion, and polymer solution parameters.

Folding kinetics of proteins and copolymers

We model the kinetic processes by which globular proteins and other heteropolymers fold to compact states. We perform Monte Carlo dynamics simulations on short self‐avoiding copolymer chains on two‐dimensional square lattices. The driving force for collapse is the aversion of nonpolar monomers for water. The chain monomers are of two types, H and P; favorable interactions occur among HH contacts. One respect in which this study differs from previous Monte Carlo folding studies is that the chains are sufficiently short that: (i) we can know unequivocally which conformations are at global minima (the ‘‘native’’ states) and which are at local minima of free energy, (ii) we can explore ‘‘pathway space’’ densely to determine the relative probabilities of all the possible pathways, and thus we establish that the model is ergodic and gives the equilibrium distribution in the long‐time limit. We find that any individual molecule passes through a wide range of conformational states, often many times. Nevertheless,...

Quantifying residual stress in nanoscale thin polymer films via surface wrinkling.

Residual stress, a pervasive consequence of solid materials processing, is stress that remains in a material after external forces have been removed. In polymeric materials, residual stress results from processes, such as film formation, that force and then trap polymer chains into nonequilibrium stressed conformations. In solvent-cast films, which are central to a wide range of technologies, residual stress can cause detrimental effects, including microscopic defect formation and macroscopic dimensional changes. Since residual stress is difficult to measure accurately, particularly in nanoscale thin polymer films, it remains a challenge to understand and control. We present here a quantitative method of assessing residual stress in polymer thin films by monitoring the onset of strain-induced wrinkling instabilities. Using this approach, we show that thin (>100 nm) polystyrene films prepared via spin-coating possess residual stresses of approximately 30 MPa, close to the crazing and yield stress. In contrast to conventional stress measurement techniques such as wafer curvature, our technique has the resolution to measure residual stress in films as thin as 25 nm. Furthermore, we measure the dissipation of residual stress through two relaxation mechanisms: thermal annealing and plasticizer addition. In quantifying the amount of residual stress in these films, we find that the residual stress gradually decreases with increasing annealing time and plasticizer amounts. Our robust and simple route to measure residual stress adds a key component to the understanding of polymer thin film behavior and will enable identification of more effective processing routes that mitigate the detrimental effects of residual stress.

Gradient Solvent Vapor Annealing of Block Copolymer Thin Films Using a Microfluidic Mixing Device

Solvent vapor annealing (SVA) with solvent mixtures is a promising approach for controlling block copolymer thin film self-assembly. In this work, we present the design and fabrication of a solvent-resistant microfluidic mixing device to produce discrete SVA gradients in solvent composition and/or total solvent concentration. Using this device, we identified solvent composition dependent morphology transformations in poly(styrene-b-isoprene-b-styrene) films. This device enables faster and more robust exploration of SVA parameter space, providing insight into self-assembly phenomena.

Double textured cylindrical block copolymer domains via directional solidification on a topographically patterned substrate

Directional solidification of cylinder forming block copolymer films confined between a directionally crystallizing solvent (benzoic acid) and a topographically patterned silicon substrate imparts a particular orientation to the block copolymer microdomains that is dependent of the solidification direction and the local film thickness. The substrate features (30 nm high, 2μm wide square mesas on a 4μm sq lattice) shape the film morphology by periodically modulating the local film thickness. Thicker regions between substrate features (plateaus) exhibit in-plane cylinders aligned in the crystallization direction and thinner regions over the substrate features (mesas) display vertically aligned cylindrical domains. This approach is a simple and general technique for engineering an intended domain orientation in specific areas of a block copolymer film. Development of this method for nanolithographic applications is demonstrated through oxygen plasma reactive ion etching of the patterned cylindrical domains.

Thermal enhancement of AFM phase contrast for imaging diblock copolymer thin film morphology.

A simple and effective means of increasing the morphological detail in AFM phase micrographs of microphase separated block copolymer films is presented. Effective AFM phase imaging of microphase separated systems hinges upon the existence of appropriate contrast mechanisms such as differences in elasticity between the microphase separated domains. For some systems, AFM phase imaging at room temperature results in low contrast images due to a paucity of differential mechanical behavior between the microphase domains, e.g. at room temperature both species are glassy. Through the use of a heating apparatus custom-designed for AFM, an elastic contrast mechanism can be created in some systems by raising the specimen to a temperature between the glass transitions of the constituent polymer species. This serves to preferentially soften one species with respect to the other, thus enhancing the phase contrast mechanism, which results in micrographs with superior detail. This simple technique is demonstrated using films of a series of polystyrene-b-poly(n-alkyl methacrylate) diblock copolymers and both commercial and custom-built heating stages. By choosing appropriate measurement temperatures, AFM phase contrast could be greatly enhanced, or indeed created, when compared to room temperature images of these specimens. For these materials, contrast enhancement required that the sample be heated roughly 20 degrees C above the glass transition of the lower-Tg species.

Effects of Humidity and Sample Surface Free Energy on AFM Probe−Sample Interactions and Lateral Force Microscopy Image Contrast

Contrast between hydrophilic and hydrophobic domains and probe-sample adhesion forces as a function of relative humidity (RH) and sample surface free energy have been investigated using hydrophilic and hydrophobic atomic force microscopy (AFM) probes. For hydrophobic probes, the adhesion force is low, and the AFM image contrast between hydrophilic and hydrophobic domains is poor over the 0-93% RH. For hydrophilic probes, the image contrast between the hydrophilic and hydrophobic domains is poor at low RH but improved at high RH. This image contrast change is related to adhesion force differences between the two domains. In turn, the enhanced adhesion and image contrasts at elevated RH are attributed to capillary forces, which are large over the hydrophilic domains but greatly diminished over the hydrophobic domains. The adhesion force increases slightly with sample surface free energy at low RH, but increases rapidly with increasing sample surface free energy at high RH. The results indicate that for AFM in air, tailoring the RH of the probe-sample environment and utilizing a hydrophilic probe can enhance imaging of materials chemical heterogeneity with nanoscale spatial resolution.

A microfluidic platform for integrated synthesis and dynamic light scattering measurement of block copolymer micelles

Microfluidic devices were developed that integrate the synthesis of well defined block copolymers and dynamic light scattering (DLS) measurement of their micelle formation. These metal devices were designed to operate in contact with organic solvents and elevated temperatures for long periods, and thus were capable of continuous in-channel atom transfer radical polymerization (ATRP) of styrene and (meth)acrylate homopolymers and block copolymers. These devices were equipped with a miniaturized fiber optic DLS probe that included several technology improvements, including a measurement volume of only 4 microlitres, simple alignment, and reduced multiple scattering. To demonstrate the integrated measurement, poly(methyl methacrylate-b-lauryl methacrylate) and poly(methyl methacrylate-b-octadecyl methacrylate) block copolymers were processed on the device with a selective solvent, dodecane, to induce micelle formation. The in situ DLS measurements yielded the size and aggregation behavior of the micelles. For example, the block copolymer solutions formed discrete micelles (D(H) approximately = 25 nm) when the corona block was sufficiently long (f(MMA) < 0.51), but the micelles aggregated when this block was short. This study demonstrates the utility of these new devices for screening the solution behavior of custom synthesized polymeric surfactants and additives.

Combinatorial screening of the effect of temperature on the microstructure and mobility of a high performance polythiophene semiconductor

Using a gradient combinatorial approach, the authors report the effects of temperature on the microstructure and hole mobility of poly(2,5-bis(3-dodecylthiophen-2yl)thieno[3,2-b]thiophene) thin films for application in organic field-effect transistors. The gradient heating revealed a detailed dependence on thermal history. Optimal heat treatment achieved mobilities as high as 0.3cm2V−1s−1. Mobility enhancement coincides with an increase in crystal domain size and orientation, all of which occur abruptly at a temperature closely corresponding to a bulk liquid crystal phase transition.Using a gradient combinatorial approach, the authors report the effects of temperature on the microstructure and hole mobility of poly(2,5-bis(3-dodecylthiophen-2yl)thieno[3,2-b]thiophene) thin films for application in organic field-effect transistors. The gradient heating revealed a detailed dependence on thermal history. Optimal heat treatment achieved mobilities as high as 0.3cm2V−1s−1. Mobility enhancement coincides with an increase in crystal domain size and orientation, all of which occur abruptly at a temperature closely corresponding to a bulk liquid crystal phase transition.

Investigation of thermally responsive block copolymer thin film morphologies using gradients.

We report the use of a gradient library approach to characterize the structure and behavior of thin films of a thermally responsive block copolymer (BCP), poly(styrene-b-tert-butyl acrylate) (PS-b-PtBA), which exhibits chemical deprotection and morphological changes above a thermal threshold. Continuous gradients in temperature and film thickness, as well as discrete substrate chemistry conditions, were used to examine trends in deprotection, nanoscale morphology, and chemical structure. Thermal gradient annealing permitted the extraction of transformation rate constants (k(t)) for the completion of thermal deprotection and rearrangement of the film morphology from a single BCP library on hydroxyl and alkyl surfaces, respectively. The transformation rate constants ranged from 1.45 × 10(-4) s(-1) to 5.02 × 10(-5) s(-1) for temperatures between 185 and 140 °C for hydroxyl surfaces. For the same temperature range, the alkyl surfaces yielded k(t) values ranging from 4.76 × 10(-5) s(-1) to 5.73 × 10(-6) s(-1), an order of magnitude slower compared to hydroxyl surfaces. Activation energies of the thermal deprotection and film transformation on these surfaces were also extrapolated from linear fits to Arrhenius behavior. Moreover, we noted a morphology shift and orientation transformation from parallel lamellae to perpendicular cylinders at the free surface because of changes in volume fraction and surface energetics of the initially symmetric BCP. Using gradient techniques, we are able to correlate morphological and chemical structure changes in a rapid fashion, determine kinetics of transitions, and demonstrate the effect of surface chemistry on the deprotection reaction in thermally responsive BCP thin films.