Solvent Effect on Small-Molecule Thin Film Formation Deposited Using the Doctor Blade Technique

Abstract

Small molecule thin films are the core of some organic optoelectronic devices. Their deposition by solution processes is an advantage for device fabrication and can be achieved via spin coating for small areas and slot-die or doctor blade for larger areas. Solution deposition of small molecule thin films is usually processed only with medium polarity solvents. Herein, the use and influence of solvents with several polarities and physicochemical properties to form small-molecule homogeneous thin films via the doctor blade technique over an area of 25 cm2 have been explored. Solvents with different polarity, heptane, chlorobenzene, N,N-dimethylformamide, acetonitrile, and methanol were used along with different deposition temperatures, from room temperature up to near the boiling temperature for each solvent. With heptane and chlorobenzene, smooth films with an average roughness of 3 nm and thickness of 100–120 nm were obtained. The film was homogeneous over the whole substrate for temperatures from room temperature to close to the boiling temperature of both solvents. On the other hand, with dimethylformamide, a film is observed when the deposition is conducted only at room temperature; when the deposition temperature increases, the formation of agglomerates of several sizes from 1 to 5 nm was observed. With acetonitrile, and methanol, no films were formed, and only nanoaggregates were created on the substrate due to the solvent high vapor pressure, and the agglomerate size depends on the deposition temperature. The measure of the contact angle of pure solvent and solutions indicated that wettability helps to film formation over the whole substrate. For heptane and chlorobenzene, a small angle was measured; meanwhile, the contact angle is large in acetonitrile leading to the formation of nanoaggregates. In the case of methanol solution, although it wets very well, no film is deposited because it has high volatility.

Hydrogel Microneedles with Programmed Mesophase Transitions for Controlled Drug Delivery

“…The first step was to demonstrate the fabrication of MN from the dendritic triblock amphiphile. As a starting point, ethanol and chloroform were used to cast the Tri-C9 polymer; however, aggregates were formed instead of a homogeneous MN array (Figure S2); this is attributed to the swift solvent evaporation rate and the fragility of the polymer film, wherein the polymer chains have limited opportunities to entangle, hindering the formation of a stable film layer on the MN mold . Consequently, poly­(vinyl pyrrolidone) (PVP) was incorporated into the polymeric solution in ethanol, due to its film-strengthening ability. , Upon drying, the MN array was detachable, yet the resultant MNs showed poorly defined shape, nonhomogeneous spreading of the polymer, and incomplete filling of the MN mold cavities (Figure S3).…”