Drying of heterogeneous hydrogels formed with various water contents: A photon transmission study

Drying of acrylamide (AAm) gels was monitored by using a photon transmission technique. These hydrogels are formed from AAm and N,N′‐methylenebis(acrylamide) (Bis) with various water contents by the free‐radical crosslinking copolymerization (FCC). The transmitted light intensity, I_tr, increased continuously as AAm gels were dried. The increase in I_tr was attributed to the decrease in the scattered light intensity, I_sc, which might be originated from the contrast between frozen blob clusters and holes in the drying gel. A decrease in I_sc was modeled by using Rayleigh's equation where drying times with various exponents were found to be proportional to the size of the holes. It was observed that the radius of holes, ξ_c, decreased in various powers of the drying time depending on water content used during FCC. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1944–1951, 2001     

Photon transmission technique for monitoring formation and swelling of polyacrylamide gels

The in situ, real-time photon transmission technique was used to monitor the free radical crosslinking copolymerization of acrylamide and N,N′-methylenebisacrylamide (Bis). Gelation experiments were performed with various Bis contents at various wavelengths. It was observed that the transmitted photon intensity, I _tr, decreased dramatically at a certain reaction time, which is attributed to the increase in scattered light intensity, I _sc, during the formation of microgels in the system. The increase in I _scwas modeled using Rayleigh's equation where the reaction time was found to be proportional to the volume of the microgels. The disc-shaped polyacrylamide (PAAm) gels were dried before use during swelling experiments. Transmitted light intensity, I _tr, from the gel increased at initial stages when PAAm gels were immersed in water and then decreased exponentially as the swelling time increased. Decrease in I _trwas attributed to the lattice heterogeneities, which might have originated between microgels and holes in the swelling gel. Decrease in I _trwas modeled using the Li–Tanaka equation from which cooperative diffusion coefficients, D _c, were determined for gels of various Bis contents. It is observed that the D _cvalues increased with the Bis content.     

Two-stage swelling of acrylamide gels: A photon transmission study

Acrylamide gels were prepared from acrylamide (AAm) with various N,N′-methylenebis(acrylamide) (Bis) contents by free-radical crosslinking copolymerization (FCC) in water and dried before used for swelling experiments. Photon transmission measurements were performed using a UV-visible (UVV) spectrometer during the swelling of polyacrylamide gels. Transmitted light intensity I_tr increased linearly at very early times when acrylamide gels were immersed in water, then decreased continuously as swelling time increased. The behavior of I_tr was modeled assuming two-stage swelling mechanisms for the swelling acrylamide gels. The increase in I_tr at early times was quantified using a linear diffusion model and a linear relaxation constant k₀ was measured. The decrease in I_tr was modeled using the Li–Tanaka equation, from which time constants τ₁ and cooperative diffusion coefficients D₀ were determined for acrylamide gels with varying Bis content. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 894–906, 2001     

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

Optical 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 (T_on) by free radical crosslinking copolymerization in water and dried before use for swelling experiments. Transmitted light intensity (I_tr) 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 I_tr 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 I_tr 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 T_on. The correlation between T_on and τ₁ and D₀ was explained in terms of the size of microgels formed during gelation. © 2003 Society of Chemical Industry     

In situ real‐time photon transmission technique for monitoring formation of polyacrylamide gels at various crosslinker contents

The in situ, real‐time, photon transmission technique was used to monitor the free‐radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N′‐methylenebisacrylamide (Bis). Gelation experiments were performed with various Bis contents at various wavelengths. It was observed that the transmitted photon intensity, I_tr, decreased dramatically at a certain reaction time, which is attributed to the increase in scattered light intensity, I_sc, during the formation of microgels in the system. The increase in I_sc was modeled using Rayleigh's equation where the reaction time was found to be proportional to the volume of the microgels. In most cases, the change in I_sc was found to be inversely proportional to the 2.5 power of the wavelength, λ, from which the size of the microgel particles were estimated during the FCC. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 823–830, 2001     

Studies on drying and swelling of PAAm‐NIPA composites in various compositions

The steady‐state fluorescence technique was introduced for studying the drying and swelling of disc‐shaped PAAm‐NIPA composites. Disc‐shaped gels were formed with various acrylamides (AAm) and N‐isopropylacrylamides (NIPA) by free radical crosslinking copolymerization in water. Composites were prepared with pyranine (Py) doped as a fluorescence probe. Scattered light, I_sc, and fluorescence intensities, I, were monitored during drying of these gels. The fluorescence intensity of pyranine increased and decreased as drying and swelling time increased respectively for all samples. The Stern‐Volmer equation combined with moving boundary and Li‐Tanaka models were used to explain the behavior of I during drying and swelling, respectively. It was found that the desorption coefficient, D, increased as NIPA contents were increased for a given temperature during drying. However, the cooperative diffusion coefficient, D₀, increased as NIPA contents were decreased during swelling at a given temperature. Supporting gravimetrical and volumetric experiments were also carried out during drying and swelling of PAAm‐NIPA composites. It was observed that NIPA contents affect the drying and swelling process. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Kinetics models for polyacrylamide-graphene oxide composites as antifoulant

Antifouling paints are used to protect the surface against these organisms such as algae, sea squirts, and barnacles. According to previous research, fish and seaweeds do not adhere to submerged surfaces using wet and soft hydrogels. The aim of this study is to investigate the temperature effect on the performance of antifouling composites to develop new useful antifouling composites for shipping sector. In this swelling experiment, the behavior of hydrogels produced from polyacrylamide (PAAm) and graphene oxide (GO) was investigated at different temperatures. Free-radical cross-linking copolymerization formed composite, using acrylamide, ammonium persulfate, N, N′-methylenebisacrylamide (BIS, Merck), and graphene oxide with various contents. The steady-state fluorescence technique was used for studying the swelling of PAAm-GO composites at various temperatures in pure water. When pyranine fluorescence intensity, I was measured, it decreased until swelling equilibrium was achieved. After the swelling experiment was started, the fluorescence emission (I_em) and scattering light intensities, I_sc from different GO content hydrogels were observed by real-time monitoring at various temperatures. Li-Tanaka and Fickian models were used to determine the diffusion coefficients for the swelling experiments in distilled and Marmara Sea Water for 8 and 50 μl of GO content hydrogels, respectively. According to literature, PAAm is utilized as a surface coating material to reduce biofouling, for this reason, this research will show a way to be able to use PAAm inside antifouling paints material for the marine industry.     

Swelling and drying kinetics of polytetrahydrofuran and polytetrahydrofuran–poly (methyl methacrylate) gels: A photon transmission study

A bifunctional polytetrahydrofuran (PTHF) macromonomer was synthesized by termination of the living polymerization of tetrahydrofuran (THF) initiated by triflic anhydride and the subsequent termination by sodium methacrylate. The PTHF macromonomer thus prepared was polymerized and copolymerized with methyl methacrylate (MMA) by free‐radical polymerization to yield a network and a segmented network of PTHF, both being homogeneous, respectively. These PTHF and PTHF–PMMA gels were used for swelling experiments in chloroform and chloroform vapor. Drying processes were monitored after removing the gels from the solvent and solvent vapor. Photon transmission from PTHF and PTHF–PMMA gels was monitored during swelling and drying processes using a UV‐visible (UVV) spectrophotometer. Transmitted light intensities, I_tr, from these gels increased when they were immersed in chloroform and/or subjected to its vapor. The increase in I_tr was attributed to the homogeneous lattice structure of PTHF and PTHF–PMMA gels which appeared during swelling. The increase in I_tr was modeled using the Li–Tanaka equation from which time constants, τ₁, and cooperative diffusion coefficients, D_C, were determined. A decrease in I_tr after removing choloform and/or its vapor from the cell was observed and attributed to the decrease in homogeneity of lattice structures during drying of the corresponding gels. Time constants, τ2, for the drying processes were also determined. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 632–640, 2003     

In situ photon transmission technique for monitoring phase separation in real time during gelation

Summary In situ, real time photon transmission technique was used to monitor the formation of heterogeneities during the free-radical crosslinking polymerization of acrylamide in the presence of N,N'-methylenebis(acrylamide) (BAAm) as a crosslinker. Two different gelation experiments were performed with various BAAm and water contents separately at room temperature. It was observed that the transmitted photon intensity, I _tr decreases dramatically at a certain time, in both type of experiments during the gel formation process from interconnected microgels. Decrease in I _tr was interpreted in terms of phase separation during gelation. The extent of phase separation was measured depending on the crosslinker content and on the degree of monomer dilution during the gel formation process. Received: 12 April 2000/Revised version: 15 August 2000/Accepted: 5 September 2000     

Effect of spatial gel inhomogeneity on the elastic modulus of strong polyelectrolyte hydrogels

Summary The effect of spatial inhomogeneity on the elastic modulus of ionic poly(acrylamide) (PAAm) hydrogels has been investigated with the static light scattering measurements. The gels were prepared by free-radical crosslinking copolymerization of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) monomers and N,N’-methylenebis(acrylamide) crosslinker. The crosslinker concentration of the hydrogels was fixed in our experiments while the mole fraction of AMPS in the comonomer feed (x_i) was varied over a wide range. Elasticity measurements show that the modulus of elasticity of gels increases with increasing x_i from 0 to 0.2. The excess scattering of gels, that is, the degree of spatial gel inhomogeneity rapidly decreases with increasing x_i and approaches to zero at x_i=0.05. Debye-Bueche analysis of the light scattering data indicates frozen concentration fluctuations in gel; the size of the static structures increases while the extent of concentration fluctuations reduces with increasing x_i. It was shown that the macroscopic elastic properties of PAAm gels are mainly controlled by the microscopic gel structure determined by the scattering measurements.     

Effect of swelling on spatial inhomogeneity in poly(acrylamide) gels formed at various monomer concentrations

The effect of swelling on the spatial inhomogeneity in poly(acrylamide) (PAAm) gels has been investigated with the static light scattering measurements. Four sets of gels were prepared using N,N′-methylenebis(acrylamide) (BAAm) as a crosslinker at various initial monomer concentrations. The crosslinker ratio X (the mole ratio of BAAm to the monomer acrylamide) was fixed at 1/50, 1/61.5, 1/66, and 1/100 in each set of gels. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration (ν_2,cr⁰). ν_2,cr⁰ shifts toward smaller concentrations as X is decreased or, as the gel swells beyond its swelling degree after preparation. Swelling enhances the extent of spatial inhomogeneity in PAAm gels and, this enhancement mainly occurs at low crosslinker ratios. The theoretical prediction of the Panyukov-Rabin theory was found to be in qualitative agreement with the experimental findings. It was also shown that three different effects, namely crosslinker, concentration, and swelling effects determine the extent of inhomogeneities in gels formed at various monomer concentrations.     

Effects of cyclization and pendant vinyl group reactivity on the swelling behavior of polyacrylamide gels

A series of polyacrylamide (PAAm) gels were prepared by free-radical crosslinking copolymerization of acrylamide and N,N′-methylenebis(acrylamide) (BAAm) in water at various crosslinker (BAAm) and chain transfer agent (isopropyl alcohol, IPA) concentrations. It was shown that only 5% of the crosslinker used in the feed forms effective crosslinks in the final hydrogels. At BAAm contents as high as 3 mole%, the equilibrium swelling ratio of the gels in water is independent of the crosslinker content in the feed. This is due to the prevailing multiple crosslinking reactions during the gel formation process. At a fixed crosslinker content, the onset of gelation is shifted towards higher conversions and reaction times as the amount of IPA increases. Addition of IPA in the monomer mixture also increases the equilibrium swelling ratio of PAAm gels. It was shown that the gel crosslinking density increases on rising IPA concentration in the feed due to the increasing rate of intermolecular crosslinking reactions. Received: 30 May 1997/Accepted: 26 June 1997     

Formation and structure of polyacrylamide gels

Acrylamide and N,N′-methylenebis (acrylamide) (AAm–Bis) copolymerization has been in-vestigated in water at a monomer concentration of 1.8 w/v%. Conversion of monomer and pendant vinyl groups was measured as a function of the reaction time up to the onset of macrogelation. Experimental results indicate that 80% of pendant vinyl groups are consumed by cyclization reactions. When the monomer concentration was kept constant at 1.8%, the critical conversion at the gel point shows a minimum at 7.5 mol % Bits. The equilibrium degree of swelling of the polyacrylamide (PAAm) gels is independent of their crosslinker content. Calculation results show that the average reactivity of pendant vinyl groups for intermolecular links decreases as the Bis concentration increases. All these results suggest formation of PAAm microgels prior to the onset of macrogelation. As the reaction proceeds, microgels are connected to a macrogel through their peripheral pendant vinyls and radical ends, whereas those in their interior remain intact. The microgels seem to act as the junction points of the final inhomogeneous networks. © 1996 John Wiley & Sons, Inc.     

Inhomogeneities in poly(acrylamide) gels: position-dependent elastic modulus measurements

Summary The inhomogeneities in poly(acrylamide) (PAAm) gels and the formation mechanism of inhomogeneities were investigated using the results obtained by the mechanical measurements. The gels in the form of rods of 5.6 mm in diameter were prepared at various crosslinker (N,N'-methylene(bis)acrylamide) content. The initial monomer concentration was set to 5.1 w/v %. The results show that the elastic modulus of PAAm gels varies depending on the location, at which the mechanical measurements are carried out. The modulus increases with increasing distance from the bottom of the gel rod. The extent of modulus variation along the gel increases with increasing crosslinker content from 2.6 to 9 % BAAm. Received: 23 January 2001/Revised version: 17 May 2001/Accepted: 17 May 2001     

Swelling behavior of poly(acrylamide)/clay nanocomposite hydrogels in acrylamide aqueous solution

The swelling behaviors of poly(acrylamide) (PAAm)/clay nanocomposite hydrogels (hereinafter abbreviated as NC gels) in acrylamide (AAm) aqueous solution have been investigated. As‐prepared PAAm/clay hydrogels (S‐M gels) were posttreated by immersing them in AAm aqueous solution. It was found that the swelling ratio of the NC gels increased greatly when the concentration of the solution is below a critical concentration (c*), whereas the gels were disintegrated in the solution when the concentration of the solution is above the c*. Some disc‐like particles were found in the AAm solution accompanying with the unusual swelling behaviors. This unusual swelling behavior is resulted from the change of network structure of the NC gels in AAm aqueous solution, which was further convinced by transmission electron microscopy and element analyses. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Correlation between crosslinking efficiency and spatial inhomogeneity in poly(acrylamide) hydrogels

Summary Elasticity and light scattering measurements were carried out on poly(acrylamide) (PAAm) hydrogels prepared from acrylamide (AAm) and N,N’-methylenebisacrylamide (BAAm) monomers under various reaction conditions. Elasticity tests showed that the crosslinking efficiency of BAAm ε_xl, that is the fraction of BAAm forming effective crosslinks decreases as the initial monomer concentration C_o is decreased. At C_o=3%, ε_xl was found to be 10^-2–10^-3, indicating that 99 to 99.9% of BAAm used in the hydrogel preparation are wasted in elastically ineffective links. Debye-Bueche analysis of the light scattering data showed that, irrespective of the gel synthesis conditions, the correlation length ξ, that is, the extension of inhomogeneities in the hydrogels is 10¹ nm. The extent of frozen concentration fluctuations in the hydrogels represented by 〈η²〉 decreases with increasing crosslinking efficiency of BAAm. The combination of the light scattering and the elasticity data of gels shows a direct correlation between the fraction of wasted crosslinker molecules during gelation and the spatial gel inhomogeneity.     

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.     

Preparation and rheological investigation of tough PAAm hydrogel by adding branched polyethyleneimine

In this article, tough hydrogels are prepared by introducing the polyethyleneimine (PEI) with branched structure and a large number of  NH₂ and  NH groups into permanently crosslinked polyacrylamide (PAAm) hydrogels matrix. To investigate the effects of B-PEI and chemical crosslinking agent (Bis) on the strength and toughness of hydrogels, a series of B-PEI/PAAm hydrogels with different mass percentage of Bis and B-PEI were manufactured and the rheological and swelling properties were compared. For all hydrogels, the storage modulus (G′) was much higher than the loss modulus (G″) in the linear viscoelastic region through the whole frequency range. The solid-like behavior and elastic nature (G′ > G″) are attributed to the permanent covalence crosslinking. Therefore, G′ increased when more Bis was added. For the nonlinear oscillatory shear measurement, hydrogels with B-PEI broke at larger γ than the pure PAAm hydrogels, indicating that the toughness of B-PEI/PAAm hydrogels has been improved by introducing B-PEI. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48541.     

Fluorescence study on drying of ι-carrageenan gels at different temperatures prepared with various CaCl<Subscript>2</Subscript> content

The influence of temperature and salt content on drying was investigated by using steady-state fluorescence (SSF) technique. Supporting gravimetric and volumetric measurements were also carried out during drying of gels at various temperatures. ι-Carrageenan gels were prepared with various CaCl₂ content. Pyranine was introduced as a fluorescence probe during gel preparation. Apparent fluorescence intensity, I, was measured during in situ drying process at each temperature and it was observed that fluorescence intensity values decreased for all gel samples. A simple model consisting of Case II diffusion was used to produce the packing constants, k ₀, for helixes. It was observed that k ₀ increased as the drying temperature was increased. On the other hand at each temperature, it was seen that k ₀ decreased as CaCl₂ content was increased. Packing energies for drying processes were obtained from fluorescence, volumetric, and gravimetric measurements separately.     

Temperature effect on the swelling of PAAm‐κ‐carrageenan composites

The steady‐state fluorescence (SSF) technique was used for studying swelling of disc‐shaped polyacrylamide (PAAm)‐κ‐carrageenan (κC) composites which were prepared by free‐radical crosslinking copolymerization at 80°C. Pyranine was introduced as a fluorescence probe during polymerization. Swelling experiments were performed in water at various temperatures by real‐time monitoring of the pyranine (Py) fluorescence intensity, I which decreased as swelling proceeded. Stern–Volmer equation is modified for low quenching efficiencies to interpret the behavior of Py intensity during the swelling of PAAm‐κC composites. The Li‐Tanaka equation was used to determine the swelling time constants, τ₁, and cooperative diffusion coefficients, D₀, from fluorescence intensity, weight, and volume variations of the composites at various temperatures. It was observed that τ₁ first decreased up to 40°C and then increased; naturally, D₀ increased up to 40°C and then decrease for all κC content gels. Swelling activation energies, ΔE, were measured for the swelling composites, which are found to be exothermic and endothermic in between 30–40 and 40–60°C, respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012