Latex

Polymer latex systems are important basic materials used for the formation of latex films in paints, paper, and textile coatings. These films are formed from colloidal dispersions of spherical polymer particles in three steps, which include the spreading of a colloidal dispersion on a substrate, slow evaporation of solvent which brings individual particles into contact and formation of the compact film by thermal annealing. Different film properties (surface roughness, haze, porosity and tensile strength) are directly related to the composition of latex systems and the film formation process. AFM is of interest for characterization of polymer latexes because of its ability to provide high-resolution imaging of these materials without elaborate sample preparation and pretreatment. In the characterization of latex materials AFM studies are aimed at the visualization of their morphology, packing and monitoring of film formation.

Fig 1. Height images of arrays of acrylic latex particles with diameters of (a) 200nm and (b) 500nm.

Polymer latex samples examined by AFM are commonly prepared by spreading latex dispersions on flat substrates (mica, graphite, microscope glass slides, etc.). Depending on the concentration of latex solution, single layer or multilayer films can be obtained. The AFM image in Fig 1a illustrates different types of periodic latex arrangements (hexagonal close-packed arrays and rectangular patterns) common for thin coatings on flat substrates. Having a narrow size distribution of latex particles is one of the factors leading to crystal-like order in thin films and coatings. Top layers of a thick latex film can have less ordered structures similar to that shown in Fig 1b. In thin layers the latex hexagonal packing is close to the ideal one, (Fig 2a), and precise imaging of such structures required fine tuning of AFM feedback and high symmetry of the AFM tip. The surface area difference calculated from the best AFM images is close to that (67%) expected for the ideal close-packed arrangement of identical spheres.

Fig 2. Height images of polystyrene latex before and after 30hr annealing at 110°C. The contrast covers corrugations in the 0-300nm range.

Studies of latex paints are one of the best-known industrial applications of AFM. The main advantage of AFM for such measurements is related to the high sensitivity of this technique to surface corrugations. AFM provides true 3D profile of the morphology of latex layers that can be used for studies of the relationship between surface roughness and optical properties (gloss) as well as the monitoring of film formation. the glass transition of the latex material and can be monitored by measurements of surface roughness. In the initial stage of film formation, roughness of latex coatings drops substantially as seen from a comparison of the images in Fig. 2a-b. During this process the interparticle distance remains the same throughout annealing, whereas the peak-to-valley value decreases, with a very fast drop at early times and a slow leveling off.

In addition to homogeneous systems, latex films can be prepared as blends consisting of several components. AFM imaging of these materials is analogous to studies of other heterogeneous polymer systems where compositional maps obtained in studies at elevated AFM tip forces reveals distribution of individual components (see Heterogeneous Polymer Systems).

There are several factors to consider when choosing AFM probes for studies of latex systems. The size of the latex particles and their arrangement are two crucial factors. To insure most true topographic profiling in the imaging of ordered arrays of latex particles less than 100nm in diameter, one should use AFM probes with an AFM tip radius less than 10nm. AFM tips with larger radii can be used for imaging less ordered coatings or for studies of larger particles. Another important feature is the stiffness of the particles, which is related to the material's glass transition temperature (T_g). When the T_g of the material is greater than 50-60°C one can use AFM probes with a stiffness of 20-40N/m. For materials with lower T_g, one should preferably use softer AFM probes with a stiffness in the 0.5-5N/m range. Similar AFM probes should be applied for hollow latex particles even made of a high T_g material.