Conductivity percolation of carbon nanotubes (CNT) in polystyrene (PS) latex film

In this study, the effect of multiwalled carbon nanotubes (MWNT) on film formation behaviour and electrical conductivity properties of polystrene (PS) latex film was investigated by using the photon transmission technique and electrical conductivity measurements. Films were prepared by mixing PS latex with different amounts of MWNTs, varying in the range between 0 and 20 wt%. After drying, MWNT content films were separately annealed above the glass transition temperature (Tg) of PS, ranging from 100 to 270 °C, for 10 min. To monitor film formation behavior of PS–MWNT composites, transmitted light intensity, Itr, was measured after each annealing step. The surface conductivity of annealed films at 170 °C was measured and found to increase dramatically above a certain fraction of MWNT (4 wt%) following the percolation theory. This fraction was defined as the percolation threshold of conductivity, Rc. The conductivity scales with the mass fraction of MWNT as a power law with exponent 2.27, which is extremely close to the value of 2.0 predicted by percolation theory. In addition, the increase in Itr during annealing was explained by void closure and interdiffusion processes. Film formation stages were modeled and the corresponding activation energies were measured.

Void closure and interdiffusion processes during latex film formation from surfactant-free polystyrene particles: a fluorescence study

This study reports a steady state fluorescence (SSF) technique for studying film formation from surfactant-free polystyrene (PS) latex particles. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at elevated temperatures for 5-, 10-, 20-, and 30-min time intervals above the glass transition (T(g)) temperature of polystyrene. During the annealing processes, the transparency of the film changed considerably. Scattered light (I(sc)) and fluorescence intensity (I(0P)) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored using photon transmission intensity, I(tr). Scanning electron microscopy (SEM) was used to detect variation in the physical structure of annealed films. Onset temperature for film formation, T(0), void closure, T(v), and healing temperatures, T(h), were determined and corresponding activation energies were measured. Void closure and interdiffusion stages were modeled and related activation energies were determined.

Swelling of Interpenetrating Networklike Particles in a Soft Polymer Matrix

Fast transient fluorescence technique was used to monitor swelling of hard latex particles in a soft polymer matrix. Various film samples were prepared from pyrene (P) and/or naphthalene (N)-labeled poly(methyl methacrylate) (PMMA) particles in a low-molecular-weight poly(isobutylene) (PIB) matrix. These PMMA particles contain interpenetrating PIB channels. Film samples were annealed at elevated temperatures to promote particle swelling. Fluorescence lifetimes, tau, were measured for each film sample. It was observed that tau values decrease as the PIB content in the matrix is increased. The decrease in tau was explained by the increase in quenching of excited P and N molecules by low-molecular-weight PIB penetrating into the PIB channels in the PMMA particles. A drastic decrease in tau above a certain temperature was attributed to the effect of particle size on the swelling of the latex particles in the PIB matrix. It was observed that small particles (P-labeled) swell at much lower annealing temperatures than large (N-labeled) particles.

Site Percolation Model for Latex Film Formation in Soft Polymer Matrix

The UV-visible (UVV) technique was used to monitor latex film formation in a soft polymer matrix. Various film samples were prepared by increasing the amount of poly(methyl methacrylate) (PMMA) particles in a poly(isobutylene) (PIB) matrix. These samples were then annealed above the glass transition temperature to promote latex film formation. Transmitted photon intensities, Itr, were measured for each film. It is observed that Itr decrease as the latex content is increased, which was explained by the increase in scattered light intensity, Isc. The drastic increase in Isc above a certain latex content is attributed to the site percolation of latex particles in the PIB matrix. The percolation threshold and the critical exponent were measured and found to be 0.3 and 0.4, respectively. The increase in Itr by annealing of film samples above Tg was explained with the void closure process below 0.8 occupation probability. When the film is occupied completely with the latex particles, interdiffusion of polymer chains was observed. Viscous flow and chain diffusion activation energies were determined and found to be 8 and 51 kcal/mol, respectively.