(395f) Controlling TiO2 Nanoparticle Distribution within a Coating Film for Surface Mechanical Property Study

Experimental studies have shown that nanocomposite coatings may show superior mechanical and barrier properties and new functionalities, such as self cleaning, self healing, super scratch resistance, super corrosion resistance.  These smart functional coatings should have great application potentials in the automotive, semiconductor, defense, construction, aerospace, and other industries.  In nanopaint design, a certain type of functional or non-functional nanoparticles needs to be dispersed in a coating matrix; a significantly increased surface area of these fillers at nanosize is the main reason behind improved mechanical and barrier properties of nanocoating films.  It is known that layer-by-layer assembly of nanoparticles can be useful in incorporating self-cleaning phenomenon in coating thin films.^[1],[2]  The finding justifies the importance of nanoparticle size and its depth inside coating thin films to obtain superior performance properties.  Hence, understanding and developing a correlation among them is critical to optimize nanocoating’s performance properties.

In developing nanocoatings, the use of water-borne composites such as acrylics is increasing tremendously due to growing environmental concerns with solvent borne coatings.  Some nanoparticles like titanium dioxide (TiO₂) can help in introducing smart functionality into coatings as they behave as oxidizing agents in the matrix. But formulating these nanocoatings with uniform dispersion without aggregation of nanoparticles is still a challenge.

This paper reports formation of nanocoating thin films of polymethyl methacrylate (PMMA) resin with dispersed TiO₂ nanoparticles.  Thin films are prepared by sandwiching a nanoparticle film with known thickness and size of nanoparticles between two polymer nano-layers with varying thicknesses. The multilayer coat is characterized for its surface scratch resistance and hydrophilicity.  A spin coating technique is employed to obtain all nanofilms of polymer and nanoparticles.  A uniform dispersion of nanoparticles in selected solvent chloroform is achieved by modifying the surface of TiO_2.^[3]  The aggregation of particles on applied film is avoided by optimizing the TiO₂ solution concentration and spin coating parameters.  The film thickness and surface smoothness are measured using X-ray diffraction studies of all polymer samples.  A correlation between polymer solution concentration and nano-film thickness is then obtained.  AFM images of TiO₂ films show uniform distribution of particles over the substrate. Various samples with nanoparticle film depth varying from substrate to top of the surface of the coating film are prepared by keeping the total coating film thickness constant at about 200nm. Surface mechanical properties are characterized by nanoindentation using round colloidal probe attached to AFM cantilever.  The mechanical properties are found to be strongly affected by the depth of nanoparticles within the film and a correlation between them is obtained.  The surface hydrophilicity is compared by measuring the change in contact angle of nanocoating films.  The depth at which there is a maximum decrease in the contact angle due to TiO₂ incorporation is recorded.  The experimental results are discussed in terms of betterment of nanocoating film performance properties by controlling the distribution of nanoparticles inside the film. 

References

1. Wu Z, Lee D, Rubner MF, Cohen RE, Structural color in porous, superhydrophilic, and self-cleaning SiO₂/TiO₂ Bragg stacks, Small, 2008, 3(8), 1445-1451.
2. Zhang X, Sato O, Taguchi M, Einaga Y, Murakami T, Fujishima A, Self-Cleaning Particle Coating with Antireflection Properties, Chemistry of Materials, 2005, 17(3), 696-700.
3. Nakayama N, Hayashi T, Preparation of TiO₂ nanoparticles surface-modified by both carboxylic acid and amine: Dispersibility and stabilization in organic solvents, Colloids and Surfaces A, 2008, 317, 543-550.