Li Tanaka Equation Research Articles

Effect of Multiwalled Carbon Nanotubes on the Conductivity and Swelling Properties of Porous Polyacrylamide Hydrogels

Multiwalled carbon nanotubes (MWCNTs) were added to polyacrylamide (PAM) hydrogels in different proportions to tune their electrical and mechanical properties. The choice of MWCNTs as a reinforcement is justified by the fact that these are highly conducting, fairly stable and flexible particles. A series of MWCNT/PAM hydrogels were prepared by freezing method. The characteristic absorption peaks at 1480 and 1213 cm−1 in the FTIR spectra reveal that MWCNTs are embedded in the PAM hydrogels. Powder x-ray diffractograms and thermogravimetric analysis (TGA) images show that the MWCNT/PAM hydrogels are crystalline, more thermally stable and have a higher electrical conductivity than a traditional PAM hydrogel. Scanning electron micrographs reveal about reduced pore size, homogeneous and denser texture. The swelling properties of all these hybrid hydrogels were found to be better than those of the parent PAM hydrogel. The Li–Tanaka equation was employed to produce the swelling parameters. The diffusion coefficients (D c ) of PAM hydrogel is 10 times higher than the literature value. 0.8% MWCNTs reinforced PAM hydrogel has excellent τc and electrical conductivity (0.76 mS/cm) with improvements in all properties. Lower D c of 0.8% MWCNTs/PAM hydrogel reveal that extent of crosslinking is much important than density of the system for a better collective diffusion of the respective solvent.

Effect of multiwalled carbon nanotube (MWNT) on the behavior of swelling of polyacrylamide–MWNT composites

The purpose of this paper is to discuss the role of multiwalled carbon nanotube in the swelling of polyacrylamide–multiwalled carbon nanotube composites. Swelling experiments were performed in water at various temperatures by real-time monitoring of the decrease in pyranine (Py) and emission light intensity ( Iem). The Stern–Volmer equation is modified for low-quenching efficiencies to interpret the behavior of pyranine intensity during the swelling of polyacrylamide–multiwalled carbon nanotube composites. The Li–Tanaka equation was used to determine the swelling time constants, τ, and cooperative diffusion coefficients, D, from fluorescence intensity, weight, and volume variations of the composite at various temperatures. It was observed that when τ decreased, naturally D increased by increasing temperatures.

Effect of κ-carrageenan on volume phase transition for polyacrylamide (PAAm) hydrogel using the fluorescence technique

Steady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) gels with various content of κ-carrageenan (κC). Disc shaped composite hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with various amounts κC. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was monitored during in situ swelling processes of composite gels. It was observed that fluorescence intensity values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constants, τ and cooperative diffusion coefficients, D from intensity variations during the swelling processes. It was shown that swelling time constants, τ decreased and diffusion coefficients, D increased as the κC content in the composites are increased.

Effect of LCST on the swelling of PAAm-NIPA copolymers: a fluorescence study

Temperature sensitive copolymers were prepared by free radical crosslinking copolymerization in aqueous solution with different molar percentage of N-isopropylacrylamide (NIPA) and acrylamide (AAm) monomers. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as a crosslinker and an initiator, respectively. The steady-state fluorescence (SSF) technique was used to determine the low critical phase transition temperature (LCST) for PAAm-NIPA copolymers. Swelling experiments were performed in water at various temperatures by real time monitoring of pyranine (Py) fluorescence intensity, I which decreased as swelling proceeded. The Stern–Volmer equation is modified for low quenching efficiencies to interpret the behavior of pyranine intensity during the swelling of PAAm-NIPA copolymers. The Li–Tanaka equation was used to determine the swelling time constants, τ 1 and the cooperative diffusion coefficients, D 0 from fluorescence intensity, weight and volume variations of the copolymers at various temperatures. It was observed that τ 1 first increased up to LCST, and then decreased; naturally D 0 decreased up to LCST and then increased upon increasing temperature. It was understood that (LCST) increases as PAAm contents increase in the PAAm-NIPA copolymers.

Swelling Kinetics of PAAm–κ-Carrageenan Composites: A Fluorescence Technique

The steady-state fluorescence (SSF) technique was introduced for studying swelling of disc-shaped polyacrylamide (PAAm) gels containing various amount of κ−carrageenan (κC). They were prepared by free-radical cross-linking copolymerization. N,N′-methylenebisacrylamide (BIS) and ammonium persulfate (APS) were added as a cross-linker and an initiator, respectively. Composite gels were prepared at 80°C with pyranine as a fluorescence probe. After drying of these gels, swelling kinetics were performed in water at 60°C by real-time monitoring of the pyranine fluorescence intensity, I, which decreased as swelling proceeded. The Li–Tanaka equation was used to determine the swelling time constants, τ 1, and cooperative diffusion coefficients, D 0, from fluorescence intensity, weight, and volume variations of the gels during the swelling processes in all cases. It was observed that τ 1 decreased and D 0 increased as the κC concentrations in the composites were increased indicating that high κC gels swell faster than low κC gels.

The Role of Pyranine in Characterization of PAAm-κC Composites by Using Fluorescence Technique

Polyacrylamide (PAAm) doped by κ-carrageenan (κC) gels were prepared with various amounts of κC varying in the range between 0 wt.% and 3 wt.%. Steady-state fluorescence (SSF) technique was employed for studying sol-gel transition and swelling of PAAm-κC composite gels which were prepared by free-radical crosslinking copolymerization. Pyranine was introduced as a fluorescence probe. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allow one to monitor the sol-gel transition and to test the universality of the sol-gel transition as a function of some kinetic parameters like polymer concentration. Observations around the gel point, t ( c ) for PAAm-κC composite gels showed that the gel fraction exponent β obeyed the percolation result for low κC (<2.0 wt. %) however classical results were produced at higher κC (>2.0 wt.%). On the other hand, fluorescence intensity of pyranine was measured during in situ swelling process at various amounts of κC and it was observed that fluorescence intensity values decreased as swelling is proceeded. Li-Tanaka equation was used to determine the swelling time constants, τ and cooperative diffusion coefficients, D.

A fluorescence study on swelling of hydrogels (PAAm) at various cross‐linker contents

AbstractDisk‐shaped acrylamide (AAm) gels were prepared from AAm with various N,N′‐methylenebisacrylamide (Bis) contents as cross‐linker in the presence of ammonium persulfate as an initiator by free‐radical cross‐linking copolymerization in water. Polyacrylamide (PAAm) gels were dried before using for swelling experiments. Steady‐state fluorescence spectrometer was employed during the swelling of PAAm hydrogels in water. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine from various Bis content gel samples was measured during in situ swelling process. It was observed that fluorescence intensity decreased as swelling has proceeded. Gravimetric and volumetric experiments were also performed. The Li–Tanaka equation was used to determine the swelling time constants, τc, and cooperative diffusion coefficients, Dc, from intensity, weight, and volume variations during the swelling processes. It was observed that swelling time constants, τc, increased and diffusion coefficients, Dc, decreased as the cross‐linker content was increased. © 2010 Wiley Periodicals, Inc. Adv Polym Techn 28:215–223, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20163

Swelling of iota-carrageenan gels prepared with various CaCl2 content: A fluorescence study

AbstractIota carrageenan gels prepared with various CaCl2 content were completely dried and then swelled in water vapor. Steady-state fluorescence (SSF) technique was used to monitor the swelling process of each iota carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. Apparent fluorescence intensity, I increased as swelling time increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which the swelling time constants, τ1 and cooperative diffusion coefficients, Dc were determined. It was observed that Dc increased as the swelling temperature was increased. On the other hand at each temperature, it was seen that Dc decreased as CaCl2 content was increased. Activation energies for swelling were obtained and found to be 60.5, 61.0, 61.5 and 62.8 kJmol-1 for the gels prepared with increasing amount of CaCl2 content.

Swelling activation energy of κ‐carrageenan in its gel state: A fluorescence study

AbstractA steady‐state fluorescence technique was employed to study the swelling of κ‐carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. The fluorescence intensity of pyranine was measured during the insitu swelling process of κ‐carrageenan gels. The fluorescence intensity increased exponentially as the swelling time increased. The increase in the fluorescence intensity was modeled with the Li–Tanaka equation, from which the swelling time constants and cooperative diffusion coefficients were determined. The swelling time constants decreased and the cooperative diffusion coefficients increased as the swelling temperature was increased. The swelling activation energies were measured to be 47.05 kJ/mol. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Study on swelling of hydrogels (PAAm) at various temperatures by using fluorescence technique

Steady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) hydrogels. Disc-shaped gels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with N, N′- methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was measured during in situ swelling process at various temperatures and it was observed that fluorescence intensity values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constants, τc and cooperative diffusion coefficients, Dc from intensity, weight and volume variations during the swelling processes. It is observed that swelling time constants, τc decreased and diffusion coefficients, Dc increased as the swelling temperature is increased. The swelling activation energies, ΔE were measured from the intensity, weight and volume variations and found to be 10.7, 32.2 and 64.1 kJ mol−1, respectively.

Transition between collapsed state phases and the critical swelling of a hydrogen bonding gel: Poly(methacrylic acid-co-dimethyl acrylamide)

Transition between collapsed state phases and discontinuous volume phase transition for a hydrogen bonding gel, poly(methacrylic acid-co-dimethyl acrylamide), were observed by using both the volume measurements and fluorescence intensity of the pyranine fluoroprobe (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt) bonded to the gel by means of electrostatic interactions. In the collapsed state, while there is no appreciable change in the volume of the gel, a considerable variation in the fluorescence intensity occurred around 30 degrees C signaling a second order phase transition between collapsed state phases, from relatively frozen to a fluctuating phase. Our analysis of the data around 30 degrees C indicates that the critical point of gel volume transition belongs to the so-called mean-field universality class, as predicted in Onuki [Phys. Rev. A 38, 2192 (1988)] and by Golubovic and Lubensky [Phys. Rev. Lett. 63, 1082 (1989)]. The relaxation time for the equilibrium swelling critically depends on the temperature and diverges near 60 degrees C, where both fluorescence intensity and the volume of the gel change drastically and indicate the discontinuous volume phase transition. The swelling kinetics of the critical gel during the discontinuous volume phase transition can be modeled best with the first term in the expansion of the Li-Tanaka equation for a long initial period of the swelling time.

Reordering of polystyrene gel due to multiple swelling in organic vapor: Fast transient fluorescence technique study

Reordering of disc-shaped polystyrene (PS) gels due to multiple swelling, under organic vapor was studied by using Fast Transient Fluorescence (FTRF) technique. Disc-shaped polystyrene gels were prepared by free radical copolymerization (FRC) of styrene (S) with ethylene glycol dimethacrylate (EGDM) as a crosslinker. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P doped PS gels under chloroform vapor. After each swelling step, gels were left to dry in an oven at 30 °C for consecutive reswelling experiments. Decay curves of P were measured and pyrene lifetimes, τ, were determined. It was observed that τ values decreased as swelling recycles were repeated. It was observed that after the fifth swelling step, two different regimes appeared in the swelling processes in PS gels. Swelling time constant, τ c, and cooperative diffusion coefficients, D c, were determined by using Li–Tanaka equation for each swelling step in both regimes. It was observed that D c values decreased up to tenth swelling step and then remained unchanged in both regimes.

Fast transient fluorescence method for measuring swelling and drying activation energies of a polystyrene gel

Abstract The time-resolved, fast transient fluorescence (TRF) technique which uses a strobe master system (SMS), was employed for studying swelling and drying of disc-shaped polystyrene gels. Disc-shaped gels were prepared by free-radical crosslinking copolymerization of styrene and ethylene glycol dimethacrylate. Pyrene was introduced as a fluorescence probe during polymerization and lifetimes, τ of pyrene were measured during in situ swelling and drying processes. Chloroform was used as an organic vapor agent to induce gel swelling at various temperatures. It was observed that τ values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constant, τc and cooperative diffusion coefficients, Dc for the swelling processes. It is observed that lifetimes, τ of pyrene increased during drying process and an empirical equation was introduced to determine the desorption coefficient, D for drying at various temperatures. The activation energies, ΔE were measured for the swelling and drying processes and found to be 80.0 and 33.5 kJ mol−1, respectively.

Fast Transient Fluorescence Technique for Studying Homopolymer Mobility in a Swelling Gel

Summary: The mobility of pyrene end-capped polystyrene (Py-PSt) trapped in swollen polystyrene gels (PSt) was investigated. PSt gels were prepared by free radical crosslinking copolymerization (FCC) of styrene (St) and ethylene glycol dimethacrylate (EGDM). The pyrene end-capped polystyrene was produced by atom transfer radical polymerization (ATRP). After drying, these disc-shaped PSt gels were left in a toluene solution of Py-PSt of various molecular weights. During this process, Py-PSt chains were trapped in the gel. These swollen gels were re-dried in air and then immersed in pure toluene solution for monitoring the mobility of Py-PSt chains in and out of the gel. These reswelling experiments were performed at room temperature in real time by monitoring pyrene lifetimes outside (τ1) and inside (τ2) of the PSt gel by using in-situ fast transient fluorescence (FTRF) measurements. It was observed that τ2 values decrease as swelling proceeds; however, τ1 values stay constant during swelling. The Li-Tanaka equation was employed to produce the swelling parameters. The swelling time constant, τc was found to increase as the crosslinker density of the gels and the molecular weight, of Py-PSt chains were increased. It was observed that the collective diffusion coefficient, Dc decreased by increasing the molecular weight by obeying Dc ≈ M−1 law. Position of the PSt gel in the fluorescence quartz cell before (a) and after (b) swelling. I0 and Ip are the excitation and the emission intensities.

Molecular weight effect on swelling and viscous flow of polymeric glass exposed to organic vapor: A steady state fluorescence study

Polymeric glasses formed from linear poly(methyl methacrylate) (PMMA) chains in various molecular weights were exposed to chloroform vapor to study swelling and viscous flow mechanisms. In situ steady state fluorescence (SSF) experiments were performed to monitor the vapor uptake and viscous flow processes. Direct illumination of the disc shape films were performed to excite the pyrene (P) molecules embedded inside the PMMA films. Variation in P intensity, I P, was monitored during swelling and viscous flow of the PMMA material exposed to chloroform vapor. It was observed that PMMA film swells like a crosslinked gel at early times by obeying the Li-Tanaka equation. At later times, i.e.longer than terminal relaxation times, τ t, swollen PMMA material flows linearly with time. Swelling time constant, τ c, terminal relaxation time, τ t, and viscosity η of PMMA films during vapor uptake were measured and found to be strongly correlated with the molecular weight (M) of PMMA. The exponent of M for τ t and η were measured and found to be around 3.

Photon transmission study on swelling of κ-carrageenan gels prepared in various concentrations

κ-Carrageenan gels prepared with various carrageenan concentrations in pure water were completely dried and then swelled in pure water. Photon transmission measurements were performed using a UV-Vis (UVV) spectrometer during the swelling of κ-carrageenan gels. Transmitted photon intensity, I tr, increased exponentially as swelling time is increased for all gel samples. The behaviour of I tr was interpreted by Monte-Carlo Simulation. The increase in I tr was quantified by employing Li-Tanaka equation, from which time constants τ 1 and collective diffusion coefficients, D o were determined for the gels in various carrageenan concentrations. Gravimetric and volumetric measurements were also carried out during swelling of gels. It is observed that gel with high carrageenan content possess more double helices and more lattice dislocations and swell slower than gels with low carrageenan content which may contain less double helices and less lattice imperfections. Increase in I tr was interpreted by the homogeneous distribution of double helices in the carrageenan gel system.

A fluorescence study on dissolution of polymeric glasses prepared in various molecular weights

Poly(methyl methacrylate) (PMMA) discs in various molecular weights, M w , were prepared by free-radical polymerization of methyl methacrylate (MMA). Pyrene (P) was introduced during polymerization as a fluorescence probe to monitor the gelation and dissolution processes in chloroform vapor and solvent, respectively. In-situ steady state fluorescence (SSF) experiments were performed to monitor vapor uptake and chain desorption processes. Direct illumination of PMMA discs were performed to excite the P molecules embedded inside the PMMA glass. Variation in P intensity, I, was monitored during the swelling of the PMMA material exposed to chloroform vapor. It was observed that PMMA film swells like a crosslinked polymeric gel at early times by obeying the Li-Tanaka equation. Swelling time constants, τ c , of PMMA discs were measured and found to have a strong correlation with the molecular weight, of PMMA. In a separate experiment, when the PMMA discs were in chloroform, desorption of PMMA chains from glass discs was monitored by observing the change of pyrene fluorescence intensity. A diffusion model with a moving boundary was employed to quantify the fluorescence data observed from dissolving PMMA discs made at various molecular weights. It was observed that desorption coefficient, D, decreased by increasing M w by obeying the DM w -1 law.

Cation effects on swelling of kappa-carrageenan: a photon transmission study.

Swelling behavior of kappa-carrageenan gels in water and KCl solutions was investigated by photon transmission experiments following the preparation of gels in the presence and absence of externally added K+ ion as a gel promoting agent. Transmitted photon intensity, Itr, increased continuously during swelling depending on the carrageenan and ion content in the gel. This increase in Itr was modeled using the Li-Tanaka equation. Both the experimental work and the model showed that the swelling of low carrageenan and ion content gels took less time than that of high ion content gels. It is confirmed that double helices in a swollen gel move much faster in pure water than in KCl solution during swelling processes. Swelling time constants, tau1, and collective diffusion coefficients, Do, were measured for the gels swollen in water and KCl solutions.

Experimental determination of gelation and swelling parameters using time‐resolved fluorescence technique

AbstractA fast transient fluorescence (FTRF) technique was used to study the sol–gel transition during gelation of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM). Pyrene (Py) was used as a fluorescence probe for the in situ polymerization experiments. Swelling of this cylindrical poly(methyl methacrylate) (PMMA) gel in chloroform was monitored using FTRF technique. Fluorescence lifetimes of Py from its decay traces were measured and used to monitor both the gelation and swelling processes. Stern–Volmer kinetics were employed to determine the quenching rate constant before the onset of gelation. MMA consumption rate was measured during gelation process using the Stern–Volmer model. A Li–Tanaka equation was introduced to determine the cooperative diffusion coefficient Dc, which was found to be around 10−5 cm2 s−1 during the swelling of PMMA gel in chloroform. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 238–245, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10052

Swelling of acrylamide gels made at various onset temperatures: an optical transmission study

AbstractOptical transmission experiments were performed using a UV–visible spectrometer during the swelling of polyacrylamide (PAAm) gels. These gels were prepared from acrylamide with N,N′‐methylenebisacrylamide at various onset temperatures (Ton) by free radical crosslinking copolymerization in water and dried before use for swelling experiments. Transmitted light intensity (Itr) from the gels increased during early stages when PAAm gels were immersed in water at room temperature and then decreased continuously as swelling time was increased. Decrease in Itr was attributed to the increase in the scattered light intensity which may originate from the contrast between ‘frozen blob clusters’ and holes in the swelling gel. Decrease in Itr was modelled using the Li–Tanaka equation from which time constants (τ1) and collective diffusion coefficients (D0) were determined for the PAAm gels prepared at different onset temperatures. τ1 and D0 were found to be dependent on Ton. The correlation between Ton and τ1 and D0 was explained in terms of the size of microgels formed during gelation.© 2003 Society of Chemical Industry