Direct Patterning of Conductive Polymer Domains for Photovoltaic Devices

We report a simple approach to control the morphology of polymer/fullerene solar cells based on electron-beam patterning of polymer semiconductors. This process generates conductive nanostructures or microstructures through an in situ cross-linking reaction, where the size, shape, and density of polymer domains are all tunable parameters. Cross-linked polymer structures are resistant to heat and solvents, so they can be incorporated into devices that require thermal annealing or solution-based processing. We demonstrate this method by building “gradient” and nanostructured poly­(3-hexylthiophene)/fullerene solar cells. The power-conversion efficiency of these model devices improves with increasing interfacial area. The flexible methodology can be used to study the effects of active layer design on optoelectronic function

Measuring the Structure of Epitaxially Assembled Block Copolymer Domains with Soft X-ray Diffraction

The size, shape, and interface structure of poly(styrene-b-methyl methacrylate) block copolymer domains assembled on an epitaxial template are characterized with soft X-ray diffraction. The domain size and shape are deformed when the dimensions of the epitaxial template are incommensurate with the equilibrium dimensions of the block copolymer, producing sidewall angles in the range of 1−2° (±0.2°). The average width of the copolymer interface is (4.9 ± 0.1) nm. Comparison with mean-field theoretic predictions for the structure of block copolymer interfaces suggests a low-frequency variance in the copolymer interface position of 1.2 nm2, or a low-frequency line-edge roughness of approximately 3 nm

Interfacial Effects in Conductivity Measurements of Block Copolymer Electrolytes

The ionic conductivity in lamellar block copolymer electrolytes is often anisotropic, where the in-plane conductivity exceeds the through-plane conductivity by up to an order of magnitude. In a prior work, we showed significant anisotropy in the ionic conductivity of a lamellar block copolymer based on polystyrene (PS) and a polymer ionic liquid (PIL), and we proposed that the through-film ionic conductivity was depressed by layering of lamellar domains near the electrode surface. In the present work, we first tested that conclusion by measuring the through-plane ionic conductivity of two model PIL-based systems having controlled interfacial profiles using impedance spectroscopy. The measurements were not sensitive to changes in interfacial composition or structure, so anisotropy in the ionic conductivity of PS-block-PIL materials must arise from an in-plane enhancement rather than a through-plane depression. We then examined the origin of this in-plane enhancement with a series of PS-block-PIL materials, a P(S-r-IL) copolymer, and a PIL homopolymer, where impedance spectra were acquired with a top-contact electrode configuration. These studies show that enhanced in-plane ionic conductivities are correlated with the formation of an IL-rich wetting layer at the free surface, which presumably provides a low-resistance path for ion transport between the electrodes. Importantly, the enhanced in-plane ionic conductivities in these PS-block-PIL materials are consistent with simple geometric arguments based on properties of the PIL, while the through-plane values are an order of magnitude lower. Consequently, it is critical to understand how surface and bulk effects contribute to impedance spectroscopy measurements when developing structure–conductivity relations in this class of materials

Structure-Property Relations of Triblock Copolymer Thermoplastics with Interaction-Tuned Polymer Additives

Block copolymer (BCP) thermoplastics are used in a wide range of commercial products. It is well known that the mechanical performance of these materials depends on the BCP architecture and composition, and the introduction of non-covalent interactions via comonomers can be used to tune key properties. However, tailoring the mechanics of BCPs by blending with polymeric additives is rarely explored, as most BCP/polymer blends have limited miscibility. Here, we examine the structure, mechanics, and thermal stability of a commodity thermoplastic, poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS), with polymeric additives of either polystyrene (PS) or poly(methyl methacrylate-co-cyclohexyl methacrylate) (PrC, 70 mol % cyclohexyl methacrylate). PS and PrC are athermal and enthalpically compatible additives, respectively, for the PS end-blocks in SEBS. The SEBS/PS blends have a narrow miscibility window with respect to PS molecular weight and loading, where either an ordered lamellar morphology or a disordered morphology is observed. In contrast, the attractive interaction between PrC and PS end-blocks leads to complete miscibility of SEBS/PrC blends across the full range of PrC molecular weights (up to 63.8 kg/mol) and loadings (up to 40 vol %) that were studied, where an ordered lamellar morphology with continuity in the rubber phase was generally observed. Consequently, the PrC additives can increase the modulus and yield stress, as well as delay the onset of strain hardening, without loss of toughness. Additionally, PrC additives can elevate the glass transition temperature of the PS blocks and maintain a high modulus at elevated operating temperatures, expanding the service window for SEBS. The principles established in this research could be translated to other types of styrenic BCP thermoplastics

Reaction Kinetics in Acid-Catalyzed Deprotection of Polymer Films

A quantitative description of kinetics in acid-catalyzed polymer deprotection reactions requires proper identification of the controlling mechanisms. We examined the acid-catalyzed deprotection of a glassy poly­(4-hydroxystyrene-co-tert-butyl acrylate) resin using infrared absorbance spectroscopy and stochastic simulations. We interpret experimental data with a model that explicitly accounts for acid transport, where heterogeneities at local length scales are introduced through a nonexponential distribution of waiting times between successive hopping events. A subdiffusive behavior with long-tail kinetics predicts key attributes of the observed deprotection rates, such as a fast initial deprotection, slow conversion at long times, and a nonlinear dependence on acid loading. Most importantly, only two parameters are introduced to offer a near-quantitative description of deprotection levels at low acid loadings and short times. The model is extended to high acid loadings and long times by incorporating a simple acid depletion model based on mutual encounters. Our study suggests that macroscopic deprotection rates are controlled by acid transport in the glassy deprotected polymer, which presents with a strongly non-Fickian behavior

Unidirectional Alignment of Block Copolymer Films Induced by Expansion of a Permeable Elastomer during Solvent Vapor Annealing

One challenge associated with the utilization of block copolymers in nanotechnology is the difficulties associated with alignment and orientation of the self-assembled nanostructure on macroscopic length scales. Here we demonstrate a simple method to generate unidirectional alignment of the cylindrical domains of polystyrene-block-polyisoprene-block-polystyrene, SIS, based on a modification of the commonly utilized solvent vapor annealing (SVA) process. In this modification, cross-linked poly­(dimethylsiloxane) (PDMS) is physically adhered to the SIS film during SVA; differential swelling of the PDMS and SIS produces a shear force to align the ordered domains of SIS in the areas covered by PDMS. This method is termed solvent vapor annealing with soft shear (SVA-SS). The alignment direction can be readily controlled by the shape and placement of the PDMS with the alignment angle equal to the diagonal across the rectangular PDMS pad due to a propagating deswelling front from directional drying of the PDMS by a dry air stream. Herman’s (second order) orientational parameter, S, can quantify the quality of the alignment over large areas with S > 0.94 obtainable using SVA-SS

Control of Ordering and Structure in Soft Templated Mesoporous Carbon Films by Use of Selective Solvent Additives

The structure of ordered mesoporous carbons fabricated using poly­(styrene-block-<i>N</i>,<i>N</i>,-dimethyl-<i>n</i>-octadecylamine <i>p</i>-styrenesulfonate) (PS-<i>b</i>-PSS-DMODA) as the template and phenolic resin (resol) as the carbon source can be easily manipulated by inclusion of low concentrations of low volatility selective solvents in the casting solution. Casting from neat methyl ethyl ketone yields a disordered structure even upon thermal annealing. However, addition of both dioctyl phthalate (DOP, PS selective) and dimethyl sulfoxide (DMSO, resol and PSS-DMODA selective) at modest concentrations to this casting solution provides sufficient mobility to produce highly ordered films with cylindrical mesopores. The DOP acts to swell the hydrophobic domain and can more than double the mesopore size, while the DMSO acts to swell the resol phase. Moreover, the surface area of the mesoporous carbons increases significantly as the meosopore size increases. This is a result of the decrease in wall thickness, which can be ascertained by the constant <i>d</i>-spacing of the mesostructure as the pore size increases. This behavior is counter to the typical effect of pore swelling agents that increase the pore size and decrease the surface area. Moreover, with only 4 wt % DOP/DMSO in the solution (20 wt % relative to solids), the scattering profiles exhibit many orders of diffraction, even upon carbonization, which is not typically observed for soft templated films. Variation in the concentration of DOP and DMSO during casting enables facile tuning of the structure of mesoporous carbon films