Role of Thickness Confinement on Relaxations of the Fast Component in a Miscible A/B Blend

Spatial compositional heterogeneity strongly influences the dynamics of the A and B components of <i>bulk</i> miscible blends. Its effects are especially apparent in mixtures, such as poly­(vinyl methyl ether) (PVME)/polystyrene (PS), where there exist significant disparities between the component glass transition temperatures (<i>T</i>_gs) and relaxation times. The relaxation processes characterized by distinct temperature dependencies and relaxation rates manifest different local compositional environments for temperatures above and below the glass transition temperature of the miscible blend. This same behavior is shown to exist in miscible PS/PVME films as thin as 100 nm. Moreover, in thin films, the characteristic segmental relaxation times τ of the PVME component of miscible PVME/PS blends confined between aluminum (Al) substrates decrease with increasing molecular weight <i>M</i> of the PS component. These relaxation rates are film thickness dependent, in films up to a few hundred nanometers in thickness. This is in remarkable contrast to homopolymer films, where thickness confinement effects are apparent only on length scales on the order of nanometers. These surprisingly large length scales and <i>M</i> dependence are associated with the preferential interfacial enrichmentwetting layer formationof the PVME component at the external Al interfaces, which alters the local spatial blend composition within the interior of the film. The implications are that the dynamics of miscible thin film blends are dictated in part by component <i>T</i>_g differences, disparities in component relaxation rates, component–substrate interactions, and chain lengths (entropy of mixing)

Enhancing Carrier Mobilities in Organic Thin-Film Transistors Through Morphological Changes at the Semiconductor/Dielectric Interface Using Supercritical Carbon Dioxide Processing

Charge-carrier mobilities in poly­(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs) increase 5-fold when OTFTs composed of P3HT films on trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane (FTS) monolayers supported on SiO₂ dielectric substrates (P3HT/FTS/SiO₂/Si) are subjected to supercritical carbon dioxide (scCO₂) processing. In contrast, carrier mobilities in P3HT/octadecyltrichlorosilane (OTS)/SiO₂ OTFTs processed using scCO₂ are comparable to mobilities measured in as-cast P3HT/OTS/SiO₂/Si devices. Topographical images of the free and buried interfaces of P3HT films reveal that scCO₂ selectively alters the P3HT morphology near the buried P3HT/FTS-SiO₂ interface; identical processing has negligible effects at the P3HT/OTS-SiO₂ interface. A combination of spectroscopic ellipsometry and grazing-incidence X-ray diffraction experiments indicate insignificant change in the orientation distribution of the intermolecular π–π stacking direction of P3HT/FTS with scCO₂ processing. The improved mobilities are instead correlated with enhanced in-plane orientation of the conjugated chain backbone of P3HT after scCO₂ annealing. These findings suggest a strong dependence of polymer processing on the nature of polymer/substrate interface and the important role of backbone orientation toward dictating charge transport of OTFTs

Crystallization Mechanism and Charge Carrier Transport in MAPLE-Deposited Conjugated Polymer Thin Films

Although spin casting and chemical surface reactions are the most common methods used for fabricating functional polymer films onto substrates, they are limited with regard to producing films of certain morphological characteristics on different wetting and nonwetting substrates. The matrix-assisted pulsed laser evaporation (MAPLE) technique offers advantages with regard to producing films of different morphologies on different types of substrates. Here, we provide a quantitative characterization, using X-ray diffraction and optical methods, to elucidate the additive growth mechanism of MAPLE-deposited poly­(3-hexylthiophene) (P3HT) films on substrates that have undergone different surface treatments, enabling them to possess different wettabilities. We show that MAPLE-deposited films are composed of crystalline phases, wherein the overall P3HT aggregate size and crystallite coherence length increase with deposition time. A complete pole figure constructed from X-ray diffraction measurements reveals that in these MAPLE-deposited films, there exist two distinct crystallite populations: (i) highly oriented crystals that grow from the flat dielectric substrate and (ii) misoriented crystals that preferentially grow on top of the existing polymer layers. The growth of the highly oriented crystals is highly sensitive to the chemistry of the substrate, whereas the effect of substrate chemistry on misoriented crystal growth is weaker. The use of a self-assembled monolayer to treat the substrate greatly enhances the population and crystallite coherence length at the buried interfaces, particularly during the early stage of deposition. The evolution of the in-plane carrier mobilities during the course of deposition is consistent with the development of highly oriented crystals at the buried interface, suggesting that this interface plays a key role toward determining carrier transport in organic thin-film transistors