Detailed Studies for Polymer Film
1. Dielectric Surface Control
Physical or chemical properties characterizing a surface of gate dielectric have a huge impact on the electrical properties of organic field-effect transistors. Here, we applied various organic interlayers between an organic semiconductor and a gate dielectric to describe field-effect mobilities being a function of a certain macroscopic parameter associated with the surface energy of gate dielectric. The organic interlayers with various chemical moieties, that is, hydroxyl, methyl, octadecyl, polystyrene, and polymethylmetacrylate, are obtained using diverse organosilane compounds and hydroxyl-end-terminated polymer brushes. Two prototypical vapor-deposited p-type organic small molecules, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene and pentacene, are used as semiconducting layers. We separate the surface energy of the organic interlayers into two terms, that is polar and dispersive terms, and define three parameters consisting of these two terms, so-called surface energy ratio, polar ratio, and polarity. The three parameters are plotted with the field-effect mobilities and it becomes apparent that the field-effect mobility is a function of polar ratio and polarity regardless of the semiconducting material as well as its morphology and crystallinity. In particular, the polarity that is the polar energy term divided by the total surface energy showed a clear exponential relationship, allowing a reliable prediction of field-effect mobilities.
1. Superhydrophobic Surface
A superhydrophobic surface with excellent chemical stability was fabricated using the spraying method, one of the most efficient technologies for producing large-area coatings at low cost. Poly(vinylidene fluoride) (PVDF) was used as a hydrophobic polymer material, and heptadecafluoro-1,1,2,2,-tetra-hydrodecyl)trichlorosilane (FTS), which reacts with moisture during curing, was used to improve the water repellency and durability. Spray coating of PVDF alone yielded PVDF nanostructures described by the Cassie-Baxter model. The water contact angle of a water droplet on this surface, however, was 128°, indicating that the surface was not superhydrophobic. On the other hand, spray-coating a mixed PVDF-FTS solution provided a complex and homogeneous nanostructured surface with excellent water repellency and a contact angle of up to 159°. Immersion of the PVDF-only film for 20 min in N,N-dimethylformamide (DMF), a good solvent for PVDF, led to complete dissolution of the film. By contrast, the PVDF-FTS film maintained its superhydrophobicity with a water contact angle of 151° after 20 min of immersion in DMF, and still exhibited a high contact angle of 142° after 1 h. The PVDF-FTS film developed in the present work should enable the production of large-area superhydrophobic coatings at low cost using a simple spray process. Moreover, the PVDF-FTS film displayed excellent stability against solvents, thus increasing its suitability for robust superhydrophobic applications.
A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from N,N′-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (PDI-CN2), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature one-dimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the PDI-CN2 film. The morphological and crystalline structures of the PDI-CN2 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the PDI-CN2-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm2 V–1 s–1, more than a factor of 2 higher than the mobility of 0.25 cm2 V–1 s–1 submitted to conventional thermal annealing and the mobility of 0.29 cm2 V–1 s–1from the horizontally applied temperature gradient.
2. Organic Semiconductor Structural Control
We developed a stable hydrophilic biocompatible hydrogel-forming coating for polypropylene (PP)-based disposal medical applications. Although PP has a variety of advantages, including good stability and inertness in medical applications, tissue damage and insertion resistance are observed upon insertion of PP-based devices into the human body due to the high hydrophobicity of the PP surface. These issues limit the utility of PP in medical applications. To address these problems, we sought to develop a stable hydrophilic and biocompatible hydrogel-forming layer using polyvinyl pyrrolidone (PVP) combined with a crosslinked polyethyleneglycolacrylate (PEGDA) matrix. Systematic studies of the blended hydrogel-forming PVP:PEGDA were conducted using a variety of blending ratios between the two polymers. The hydrophilicity and water-affinity of the hydrogel-forming layer improved significantly as the PEGDA-to-PVP blending ratio increased. Importantly, the tensile strain at the break point increased by a factor of more than 7, and the strength of adhesion to the PP surface for the 1:1 PVP:PEGDA (PVP(1):PEGDA(1)) blend ratio was 54 times that of the PVP film, determined using tensile strain–stress and peel tests. The water stability of the PVP(1):PEGDA(1) improved significantly. This approach is potentially useful as a biocompatible hydrophilic polymer coating in a variety of low-priced consumable PP commercial medical applications.