Infrared sintering and evaporative lithography of textured coatings soft matter
When attempting to make hard polymer coatings from polymer colloids, one is faced with the “film formation dilemma”.
Film formation dilemma
If hard or glassy particles are used, capillary forces cause cracking in the layer during the drying process. Without heating in an oven to raise the temperature above the polymer’s glass transition temperature, the particles cannot coalesce, and so a brittle material results. Heating, on the other hand, increases the energy use of the process. One strategy is to add solvents or other plasticizing molecules to soften the particles, but then the process is less environmentally friendly, because it emits volatile organic compounds (VOC). Instead of hard particles, softer ones can be used, but then the final coating is subject to easy abrasion and scratching.
The Soft Matter Group has proposed a way to solve the dilemma by using heating of hard polymer particles with IR radiation. The particles are softened, allowing them to coalesce to create a uniform coating. An international patent application has been filed by Dr. Argyrios Georgadis and Prof. Joe Keddie: WO 2010/097592, “A method of making a hard latex and a hard latex.” 2 Sept. 2010.
Texture on surfaces has a significant impact on properties, including wetting, friction and wear, drag resistance, and light scattering. There are numerous methods to control the topography at the nanometer length scale (such as by using nanoparticles), but fewer methods to control topography over larger length scales.
The Soft Matter Group has developed a method of colloidal assembly to control the topography of polymer coatings. Their method was inspired by evaporative lithography, in which the transport of nanoparticles in a fluid is controlled by modulation of the evaporation rate with a mask. Particles are carried with the fluid that flows to the fast-evaporating regions. See: D. J. Harris et al., Phys. Rev. Lett. (2007) vol. 98, 148301.
In the method developed at Surrey, called infrared radiation-assisted evaporative lithography (or IRAEL), some regions of a wet colloidal polymer film are selectively heated by shining IR light through a shadow mask. Water in the hotter regions evaporates faster, and a lateral flow is set up to replace the water that is lost. Polymer particle accumulate in the irradiated regions. At the same time, the radiative heating causes the particles to sinter together and coalesce into a hard coating. Dr. Argyrios Georgiadis and Prof. Joe Keddie have applied for an international patent on the method: WO/2011/051648 “A Method of Making a Patterned Latex Film and a Patterned Latex Film”, May 2011.
Royal Society of Chemistry journal
Our work on IR evaporative lithography was highlighted on the inside front cover of the Royal Society of Chemistry journal, Soft Matter. See: Georgiadis, A., Routh, A. F., Murray, M. W., & Keddie, J. L. (2011). Soft Matter, 7(23), 11098-11102.
Overview of the IRAEL process: A colloidal film is placed under a mask. IR radiation heats the wet film in the exposed areas. Particles flow with the water and accumulate under the holes in the mask and sinter as a result of heating. (b) A profilometry image (1.5 cm x 1.5 cm) shows the dome-like features of a patterned polymer film created by IRAEL. Here, P = 1.5 mm, hg = 0.7 mm, and hi = 0.33 mm. Scale bar is 3 mm. (c) Definition of relevant experimental parameters in IRAEL: hg, hi, dh, P, and f.
Atomic force microscopy
Atomic force microscopy can be used to study the sintering of a hard acrylic polymer particles (with a Tg of 38 C). The particles initial have topographic heights of more than 100 nm. After four minutes under a 250 W IR lamp, the particles have flattened and deformed to fill voids between each other. More information can be found in this paper: A. Georgiadis et al., Langmuir (2011) 27, 2176-2180