A fluorescence study on critical exponents during sol-gel phase transition in complex monomeric systems

Methyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (P_y) was introduced as a fluorescence probe and fluorescence lifetimes from its decay traces were measured during sol-gel phase transitions. The fast transient fluorescence (FTRF) technique was used to study the critical exponents during sol-gel phase transition in free-radical crosslinking copolymerization (FCC). The results were interpreted in the view of percolation theory. The critical exponents of gel fraction, β and weight average degree of polymerization, γ were measured near the point of gel effect and found to be around 0.37 ± 0.015 and 1.69 ± 0.05 in all systems studied respectively.     

Film formation from nano‐sized polystyrene latex particles

This work reports on the steady state fluorescence (SSF) technique for studying film formation from surfactant‐free, nano‐sized polystyrene (PS) latex particles prepared via emulsion polymerization. The latex films were prepared from pyrene (P)‐labeled PS particles at room temperature and annealed at elevated temperatures in 5, 10, 15, 20 and 30 min time intervals above the glass transition temperature (T_g) of PS. During the annealing processes, the transparency of the film was improved considerably. Monomer and excimer fluorescence intensities, I_P and I_E respectively, from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of the latex films was monitored by using photon transmission intensity, I_tr. Void closure and interdiffusion stages were modeled and related activation energies were determined and found to be 10.3 and 50.3 kJ mol⁻¹. Void closure temperatures, T_v, were determined from the minima of I_tr value. Copyright © 2005 John Wiley & Sons, Ltd.     

Vapor-induced film formation from low-Tg particles for different solvent compositions

The photon transmission method was used to study latex film formation from poly(vinyl acetate) (PVAc) particles induced by two different solvents. Films with the same latex content were prepared from PVAc particles and exposed to vapor of ethanol-water and acetone-water mixtures in various compositions. Transmitted photon intensities, Itr, from these films increased with increasing vapor exposure time. The increase in Itr is attributed to the increase in crossing density at the polymer-polymer junction. The Prager-Tirrell model was employed to obtain the back-and-forth frequency, nu, of the reptating polymer chain during film formation induced by solvent vapor. It was observed that the produced nu values increase as the solvent content is increased for both solvents. Abilities of both solvents to form films were interpreted with the solubility parameters of the solvents and the PVAc.     

In situ Fluorescence Study of Swelling,Sorption and Desorption Processes in and out of PAAm Gels

Summary: Small molecule sorption and desorption in and out of polyacrylamide (PAAm) gels were studied at various temperatures. Pyranine (P_y) dissolved in water used as a probe. Fluorescence emission intensity, I_p from P_y was monitored for studying sorption and desorption processes. Scattered light intensities, I_sc from PAAm gel was also monitored to observed structural variations during sorption and desorption process. Li-Tanaka model was applied to produce the swelling time constants, τ_c and sorption coefficients, D_c for the swelling processes. On the other hand, sorption and desorption processes were studied and coefficients were produced by using Fickian model. Related activation energies were also calculated from the corresponding physical processes.     

Temperature Effect on Drying and Swelling of Kappa Carrageenan Gels: A Steady State Fluorescence Study

Summary: A novel technique based on in situ steady state fluorescence (SSF) measurements is introduced for studying drying and swelling of κ- carrageenan (kappa carrageenan) gels at various temperatures. κ- carrageenan gels were completely dried and then swelled in water vapor. Pyranine was embedded in κ- carrageenan and used as a fluorescence probe. Scattered light intensities, I_sc and fluorescence intensities, I were monitored during the drying and swelling of κ- carrageenan gels. It was observed that the fluorescence intensity decreased linearly as drying time was increased. A simple model consisting of Case II diffusion was used to quantify the drying processes of the κ- carrageenan gels. This moving boundary model provided packing constant, k₀. During swelling, fluorescence intensity increased exponentially as time is increased. The increase in I, was modeled using Li-Tanaka equation from which swelling time constants, τ_c and cooperative diffusion coefficients, D_c were determined. It was observed that swelling time constants, τ_c decreased and diffusion coefficients, D_c increased as the swelling temperature was increased. Activation energies for drying and swelling were also obtained and found to be 53.9 and 47.2 kJ mol⁻¹, respectively.     

Mathematical Characterization of Thermo-reversible Phase Transitions of Agarose Gels

The thermal phase transition temperatures of high (HMP) and low melting point (LMP) agarose gels were investigated by using UV–vis spectroscopy techniques. Transmitted light intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol–gel) processes. It was observed that the transition temperatures, T_m, defined as the location of the maximum of the first derivative of the sigmoidal transition paths obtained from the UV–vis technique, slightly increased by increasing the agarose concentration in both the HMP and LMP samples. Here, we express the phase transitions of the agar-water system, as a representative of reversible physical gels, in terms of a modified Susceptible-Infected-Susceptible epidemic model whose solutions are the well-known 5-point sigmoidal curves. The gel point is hard to determine experimentally and various computational techniques are used for its characterization. Based on previous work, we locate the gel point, T₀, of sol-gel and gel-sol transitions in terms of the horizontal shift in the sigmoidal transition curve. For the gel-sol transition (heating), T₀ is greater than T_m, i.e. later in time, and the difference between T₀ and T_m is reduced as the agarose content increases. For the sol-gel transition (cooling), T₀ is again greater than T_m, but it is earlier in time for all agarose contents and moves forward in time and gets closer to T_m as the agarose content increases.     

Effect of Calcium Ion Concentration on Small Molecule Desorption from Alginate Beads

Spherical alginate beads were prepared by ionotropic gelation of sodium alginate through the use of calcium ions. Pyranine (Py) was added to the alginate solution as a small molecule probe for fluorescence studies. Desorption of Py in water from the alginate beads cross-linked with calcium ions was studied by using the steady state fluorescence technique. The fluorescence emission intensity (I) from Py was monitored during the desorption process at 512 nm using the time drive mode of the spectrofluorometer. The increase in I was attributed to Py release from the beads. The Fickian diffusion model was used to calculate the desorption coefficients, D_, which were found to be increased up to 3% (w/v) CaCl₂ concentration in the beads, and then decreased with a further increase of CaCl₂ content. On the other hand, the encapsulation efficiency of Py in the calcium alginate beads presented the reverse behavior compared to D. It was observed that, when the content of CaCl₂ was increased, the incubation time, t_0, for the start of desorption increased.     

Phenolic acid contents, essential oil compositions and antioxidant activities of two varieties of Salvia euphratica from Turkey

This study was designed to evaluate the in vitro antioxidant activities of n-hexane (Hex), dichloromethane (DCM), methanol (MeOH) and essential oils (EO) extracts obtained from Salvia euphratica var. euphratica and Salvia euphratica var. leiocalycina and to determine their essential oil and phenolic acid compositions. The samples were screened for their antioxidant activity by using DPPH and β-carotene/linoleic acid assays. Methanol extracts of both varieties exhibited strong antioxidant activities. Our results showed that rosmarinic acid was dominant phenolic acid of MeOH extracts (39.4 and 55.8?μg?mg?1, respectively). The chemical compositions of essential oils of two varieties were analysed and their main components were determined as eucalyptol (18.4%) and trans-pinocarvyl acetate (24.9%), respectively. It can be said that these varieties could be used as natural antioxidant.     

A photon transmission study for film formation from poly(vinyl acetate) latex particles with different molecular weights

The photon transmission technique was used to monitor the temperature evolution of film formation from poly(vinyl acetate) (PVAc) latex particles with two different molecular weights. Two sets of latex films were prepared below the glass transition temperature (T_g) of PVAc, which are named as low (LM) and high molecular weight (HM) films. These films were annealed at elevated temperatures above the T_g of PVAc for various time intervals. It is observed that transmitted photon intensity (I_tr) from these films increased as the annealing temperature was increased. Onset temperatures (T_H) at given times (τ_H) for starting the optical clarity of LM and HM films were measured and used to calculate the healing activation energies (ΔH) for the PVAc minor chains, and found to be as 28.1 kcal/mol and 27.7 kcal/mol, respectively. The increase in the transmitted photon intensity, I_tr above T_H was attributed to the increase in the number of disappeared interfaces between the deformed latex particles. Prager–Tirrell (PT) model was employed to interpret the increase in the crossing density of chains at the junction surfaces. The interdiffusion (backbone) activation energies (ΔE) were measured and found to be 177.5 kcal/mol and 210.7 kcal/mol for a diffusing PVAc chains across the junction surface of LM and HM latex films, respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2918–2925, 2007     

Fast transient fluorescence technique (FTRF) for studying vapor-induced latex film formation

A fast transient fluorescence (FTRF) technique was used to study latex film formation induced by organic solvent vapor. Seven different films with the same latex content were prepared separately from poly(methyl methacrylate) (PMMA) particles and exposed to vapor of various chloroform-heptane mixtures in seven different experiments. Latex films were prepared from pyrene (Py)-labeled latex particles and fluorescence lifetimes of Py were monitored during vapor-induced film formation. It was observed that pyrene lifetimes decreased as vapor exposure time increased. A Stern-Volmer kinetic analysis was used for low quenching efficiencies to interpret the decrease in pyrene lifetimes. A Prager-Tirrel model was employed to obtain back-and-forth frequencies, nu, of the reptating PMMA chains during latex film formation induced by solvent vapor. nu values were found to be correlated with chloroform content in vapor mixture. It was observed that polymer interdiffusion obeyed a t(1/2) law during film formation. The results of optical transmission experiments were found to support these findings.