Ertan Arda

Electrical and optical percolations of polystyrene latex-multiwalled carbon nanotube composites.

Electrical conductivity and optical transmittance properties of polystyrene (PS)-multiwalled carbon nanotube (MWCNT) composite films were investigated. Composite films were prepared by mixing of various mass fractions of MWCNT in PS-water dispersions. After water evaporates, powder composite films were annealed at 175 degrees C above the glass transition of PS for 20 min. Photon transmission and two point probe resistivity techniques were employed to determine the variations of the optical and the electrical properties of composites. Transmitted light intensity, I(tr) and surface resistivity, R(s) were monitored as a function of MWCNT mass fraction (M). It was observed that, both the surface resistivity and the optical transparency were decreased by increasing the amount of MWCNT added to the polymeric system. Conductivity and optical results were interpreted according to the classical and site percolation theory, respectively. The electrical (sigma) and the optical (op) percolation threshold values and critical exponents were calculated as M(sigma)=1.8 wt.%, M(op)=0-0.13 wt.% and beta(sigma)=2.25, beta(op)=0.32, respectively.

Effect of molecular weight on latex film formation : Photon transmission study

A UV-visible (UVV) technique was used to monitor the evolution of transparency during film formation from hard latex particles. Two different latex films were prepared from particles with high and low molecular weight (HM and LM) poly(methyl methacrylate) (PMMA) separately and annealed at elevated temperatures in various time intervals above the glass transition temperature (Tg). In both films, a continuous increase in the transmitted photon intensity (Itr) was observed above 160°C as the annealing temperature was increased. However, the reflected photon intensity (Irf) first decreased and then increased by showing a minimum in the same temperature range as the annealing temperature was increased. The increase in the transmitted photon intensity (Itr) is attributed to increase in the “crossing density” at the junction surface. The activation energies for back-and-forth motion (ΔEtr) were measured and found to be around 35 and 25 kcal/mol for the reptating polymer chain across the junction surface in the LM and HM films, respectively. The decrease in Irf was explained by the void-closure mechanism, and the increase in the Irf above 160°C was again attributed to the increase in the crossing density at the junction surface. Back-and-forth activation energies (ΔErf) were measured to be around 47 and 18 kcal/mol and the void-closure constants (B) were found to be around 24 × 103 and 12 × 103 K for the LM and HM film samples, respectively.

Photon transmission technique for studying film formation from polystyrene latexes prepared by dispersion polymerization using various steric stabilizers

A photon-transmission method was used to monitor the evolution of transparency during film formation from various polystyrene (PS) particles which were produced using different steric stabilizers, that is, poly( acrylic acid) (PAA), poly(vinyl alcohol) (PVA), and polyvinylpyrrolidone (PVP). The latex films were prepared from PS particles at room temperature and annealed at elevated temperatures in various time intervals above the glass transition (Tg). To simulate the latex film-formation process, a Monte Carlo technique was performed for photon transmission through a rectangular lattice. The number of transmitted (N tr ) photons were calculated as a function of particle-particle interfaces that disappeared. The increase in the transmitted photon intensity (( r ) was attributed to the increase in the number of interfaces that disappeared. The Prager-Tirrell (PT) model was employed to interpret the increase in crossing density at the junction surface. The backbone activation energy (AE) was measured and found to be around 120 kcal mol -1 for a diffusing polymer chain across the junction surface for all PS latex films.

Time and temperature dependence of void closure, healing and interdiffusion during latex film formation

A photon transmission method was used to probe the evolution of transparency during film formation from poly(methyl methacrylate) (PMMA) particles with different molecular weight. The latex films were prepared from low (LM) and high (HM) molecular weighted PMMA particles at room temperature and annealed at elevated temperatures in various time intervals above glass transition (Tg). It was observed that transmitted photon intensities (Itr) from these films increased as the annealing temperature was increased. It is seen from Itr curves that there are two distinct film formation stages, which are named as void closure and interdiffusion processes, respectively. The activation energies for viscous flow and backbone motion were obtained using well-defined models. Viscous flow activation energies (ΔH) were found to be around 150 and 134 kJ/mol for LM and HM films, respectively. Backbone activation energies (ΔEb) were found to increase from 142 to 199 and 59 to 98 kJ/mol in time of annealing for LM and HM films, respectively. Healing points (τH,TH) were determined and using these time–temperature pairs, healing activation energies (ΔEH) were measured and found to be 188 and 117 kJ/mol for LM and HM films, respectively.

Photon transmission method for studying film formation from polstyrene latexes with different molecular weights

A photon-transmission method was used to probe the evolution of transparency during film formation from polystyrene (PS) particles with different molecular weights. The latex films were formed at room temperature from the PS particles having two different average molecular weights and annealed at elevated temperatures in various time intervals above the glass transition (Tg). Onset temperatures (TH) at given times (τH) for the optical clarity of films formed from low (LM) and high molecular (HM) weight PS particles were used to calculate the healing activation energies for the minor chains and found to be 22.0 ± 0.5 and 27.0 ± 0.6 kcal/mol, respectively. The increase in the transmitted photon intensity, Itr, above the TH was attributed to increase in the number of interfaces that disappeared. The Prager–Tirrell (PT) model was employed to interpret the increase in crossing density at the junction surface. The backbone activation energies (ΔE) were measured and found to be 127.8 ± 2.5 kcal/mol for a diffusing polymer chain across the junction surface for LM and HM latex films.

Monovalent and Divalent Cation Effects on Phase Transitions of ι-carrageenan

Phase transitions of ι-carrageenan in a variety of monovalent (Li, Na, K) and divalent (Mg, Ca, Sr) cation solutions were studied during heating and cooling by using photon transmission technique. Photon transmission intensity (I tr ) was monitored against temperature to determine the transition temperatures and activation energies during the heating and cooling processes in the monovalent and divalent systems. Three distinct transition regions were observed during the heating and cooling cycles for the carrageenan-divalent salt system. At the first step of the heating process, dimer groups were transformed into dimers that presented (g-d) transitions, then these dimers were directly converted into a double helix by undergoing a (d-h) transition. In the higher temperature region, a double helix-to-coil (h-c) transition took place. During the cooling process, these transitions are arranged in the order of (c-h), (h-d), and (d-g). The carrageenan-monovalent salt system presented only coil-to-rod-like helix (c-r) and rod-like helix-to-coil (r-c) transitions during the cooling and heating processes, respectively. A hysteresis was observed between (r-c)-(c-r) and (g-d)-(d-g) transitions for the monovalent and divalent cations, respectively.

Molecular recognition during sol–gel and gel–sol transition of kappa–iota carrageenan mixtures

Sol–gel and gel–sol phase transitions of kappa–iota (κ–ι) carrageenan mixtures in two types of salts were studied using photon transmission technique. Here, KCl and CaCl2 were chosen as the proper salts for the carrageenan mixtures. Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of carrageenan mixtures in monovalent and bivalent cation solutions were studied upon heating and cooling by using photon transmission technique. Photon transmission intensity, I tr was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T ch and T nd). Two distinct transition regions were observed in both heating and cooling of the carrageenan-bivalent CaCl2 salt system. At the first step of heating process dimers decompose into double helices by making (d-h) transition. At the high temperature region (h-c) transition takes place. Carrageenan-monovalent KCl salt system presented only (c-h) and (h-c) transitions upon cooling and heating processes. Transition temperatures at the heating and cooling processes were obtained. A hysteresis was observed between (h-c)–(c-h) and (d-h)–(h-d) transitions for the monovalent and bivalent cations respectively.

Thermal Phase Transitions of Agarose in Various Compositions: A Fluorescence Study

The effect of agarose content on thermal phase transitions of the agarose gels was investigated by using Steady State Fluorescence (SSF) method. Scattered light, Isc and fluorescence intensity, Ifl were monitored against temperature during heating and cooling processes to investigate phase transitions. Two regions were observed during the heating and cooling processes. At the high temperature region, double helix to coil (h-c) transition took place. However, during the cooling process coil to double helix (c-h) transitions occurred at low temperature region. Transition energies were determined using the Arrhenius treatment, and found to be strongly correlated with the agarose content in the gel system. Transition temperatures were determined from the derivative of the sigmoidal transition paths and found to be increased by increasing agarose content in both cases.

UV-visible technique for studying powder coatings and their dissolution

UV-Visible (UVV) technique used to monitor powder coating and its dissolution processes from hard latex particles. Three sets of latex coatings were prepared from poly(methyl methacrylate) (PMMA) particles. The first set of coatings was annealed at elevated temperatures in various time intervals during which reflected photon intensity, Irf, was measured. The second set of coatings was annealed at various temperatures in 10 min time intervals during which transmitted intensity, Itr, was measured. Irf first decreased and then increased as the annealing temperature was increased. Decrease in Irf was explained with the void closure mechanism due to viscous flow. Increase in Itr and Irf against time and temperature were attributed to an increase in crossing density at the junction surface. The activation energy of viscous flow, ΔH, was measured and found to be around 8 kcal/mol and the back and forth activation energies (ΔErf and ΔEtr) were measured and found to be around 49 and 53 kcal/mol for a reptating polymer chain across the junction surface. Diffusion of solvent molecules (chloroform) into the annealed latex coatings was followed by desorption of PMMA chains for the third set of films. Desorption of pyrene, P, labeled PMMA chains was monitored in real-time by the absorbance change of pyrene in the polymer-solvent mixture. A diffusion model with a moving boundary was employed to quantify real-time UVV data. Diffusion coefficients of desorbed PMMA chains were measured and found to be between 2 and 0.6 × 10−11 cm2 s−1 in the 100 and 275°C temperature range.

Fluorescence and UV techniques for studying neck growth and equilibration processes during latex film formation

Steady state fluorescence (SSF) and UV visible techniques have been used to study neck growth and equilibration processes during the coalescence of hard latex particles. Latex films were prepared separately by annealing pyrene (P y ) labelled and unlabelled poly(methyl methacrylate) (PMMA) particles above their glass transition temperature. During the annealing processes, the optical clarity of the films increased considerably. Direct fluorescence emission of excited pyrene from labelled latex films was monitored as a function of annealing temperature to detect this change. Void closure temperature (T c ) and time (t c ) were determined at the point where the fluorescence emission intensity became maximal. Below this point, the increase in fluorescence intensity (I op ) against temperature was used to determine the activation energy for viscous flow (ΔH 47 kcal mol -1 ). The decrease in I op above the void closure temperature was used to determine the backbone activation energy (ΔE 44 kcal mol -1 ) for the interdiffusing chains. Unlabelled PMMA particles were used to prepare films for UV-vis measurements. The transmitted photon intensity (I tr ) from these films increased as the annealing temperature was increased. This behaviour was also used to determine the backbone activation energy (ΔE 35 kcal mol -1 ) for the interdiffusing chains.