Deswelling Process Research Articles

Effect of preparation temperature on salt-induced deswelling and pattern formation in poly(N-isopropylacrylamide) hydrogels

An ever-increasing demand for novel hydrogels for smarter applications necessitates careful investigation of their swelling–deswelling behavior. In the present work, poly(N-isopropylacrylamide) (PNIPA) hydrogels were prepared at different temperatures. Hydrogels prepared below 25 °C were transparent and homogeneous whereas those prepared above 25 °C were slightly hazy to opaque. Scanning electron microscopy (SEM) studies showed variation in microstructures of the PNIPA hydrogels. In addition to the swelling studies, the deswelling kinetics of the swollen gels were studied in presence of different concentration of additives e.g. NaCl, NaOH at room temperature. The hydrogels prepared at higher temperatures were found to swell more and faster in water. Interestingly, during the salt-induced deswelling process, formation of several patterns was observed. The pattern formation of the PNIPA hydrogels was not only dependent on the concentration of additives in solution, but also on the hydrogel preparation temperature. The microphase separated inhomogeneous nature of the structure for the hydrogels prepared at intermediate temperatures was possibly responsible for the observed pattern formation.

Large-Area Block Copolymer Photonic Gel Films with Solvent-Evaporation-Induced Red- and Blue-Shift Reflective Bands

By choice of large overall molecular weights from 0.5 to 1 M kg/mol, the lamellar microstructures of the high-molecular-weight polystyrene-block-polyisoprene (PS–PI) block copolymers (BCPs) exhibited various long periods and associated different reflected wavelengths. Interestingly, unique solvatochromism-dependent red- and blue-shift reflective bands could be acquired in the PS–PI BCP gel films using a nonselective neutral solvent as an external stimulus. At low polymer concentration, red-shift of the reflectivity was attributed to the increase of BCP long period by the enhancement of the BCP segregation strength, namely, a thermodynamically controlled swelling process. In contrast, at high polymer concentration (ϕp > 0.8), the blue-shifting reflectivity resulted from the decrease of the BCP long period by the collapse of polymer chains, namely, a kinetically controlled deswelling process. Also, well-aligned lamellar microstructures were fabricated using a shear-induced microstructural orientation. This ...

Deswelling of Poly(N-isopropylacrylamide) Derived Hydrogels and Their Nanocomposites with Iron Oxide Nanoparticles As Revealed by X-ray Photon Correlation Spectroscopy

Here we present results on the deswelling behavior of semi-interpenetrating polymer hydrogels of sodium alginate and poly(N-isopropylacrylamide) (PNIPAAm) containing iron oxide nanoparticles by means of X-ray photon correlation spectroscopy (XPCS). The experiments were carried out as a function of temperature starting from temperatures below the low critical solution temperature (LCST) of the materials to temperatures above their LCST. The hydrogels present a hyper-diffusive dynamics of similar nature as that found for the freeze-dried aerogels which has been attributed to their characteristic porous structure. The dynamics of the swollen samples is faster than that of the freeze-dried ones suggesting that water has an effect in accelerating the dynamics of the hydrogels. XPCS has been used to characterize the deswelling process as a function of temperature. Deswelling times for the different samples correlate well with the corresponding relaxation times of the relaxation processes measured by XPCS.

Preparation and characterization of poly(vinyl alcohol) cryogel-silver nanocomposites and evaluation of blood compatibility, cytotoxicity, and antimicrobial behaviors

In this investigation, cryogels composed of poly(vinyl alcohol) (PVA) were prepared by repeated freeze-thaw method. The prepared cryogels served as templates for producing highly stable and uniformly distributed silver nanoparticles via in situ reduction of silver nitrate (AgNO3) using alkaline formaldehyde solution as reducing agent. The structure of the PVA/Ag cryogel nanocomposites was characterized by a Fourier transform infrared and Raman spectroscopy. The morphologies of pure PVA cryogels and PVA/Ag nanocomposites were observed by a scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The SEM analysis suggested that cryogels show a well defined porous morphology whereas TEM micrographs revealed the presence of nearly spherical and well separated Ag nanoparticles with diameter about 100 nm. XRD results showed all relevant Bragg's reflections for crystal structure of silver nanoparticles. The amount of silver in cryogel nanocomposites and thermal stability were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) and thermogravimetric analysis measurements. Mechanical properties of nanocomposites were observed in terms of tensile strength. The antibacterial studies of the synthesized nanosilver containing cryogels showed good antibacterial activity against both gram-negative and gram-positive bacteria. The prepared PVA/Ag nanocomposites were also investigated for swelling and deswelling behaviors. The results reveal that both the swelling and deswelling process depends on the chemical composition of the cryogel silver nanocomposites, number of freeze-Thaw cycles and pH and temperature of the swelling medium. The biocompatibility of the prepared nanocomposites was judged by in vitro methods of percent hemolysis and protein (BSA) adsorption. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers

Photoactive spherical colloids for opal photonic crystals

The synthesis and characterization of submicrometer‐sized polymer particles, functionalized by different techniques with fluorescent dyes and featuring tunable surface charges, are described. Dyes with a polymerizable moiety were incorporated during emulsifier‐free or seeded emulsion polymerization, whereas non‐polymerizable dyes were included into preformed nanoparticles by a swelling and deswelling process. The particle surface charge was controlled through the choice of suitable initiators. All the nanoparticles were successfully used to grow high optical quality opal photonic crystals by the vertical deposition technique. Optical characterization of such photonic crystals pointed out the presence of the optical stop band and the high‐energy van Hove‐like structures, both scaling with the particle diameter according to the scaling laws of photonic crystals and possessing the expected angular dispersion. These results are indicative of the high size uniformity and of the surface quality of the nanospheres independently on the synthetic method adopted. Preliminary data seem to suggest some effect of the opal photonic band structure on the dye fluorescence spectra. POLYM. COMPOS., 34:1443–1450, 2013. © 2013 Society of Plastics Engineers.

Linearly thermoresponsive core–shell microgels: Towards a new class of nanoactuators

In this study we present novel core–shell microgels, with a shell made of poly(N-n-propylacrylamide) (pNNPAM) and a core consisting of poly(N-iso-propylmethacrylamide) (pNIPMAM), exhibiting a unique linear temperature response. The effect is produced by the large LCST gap of 23 °C between the shell- and the core-forming polymer. We demonstrate that the shell exhibits a temperature induced de-swelling process that is almost independent of the swelling properties of the core. Furthermore the active collapse of the shell forces a collapse of the core (which is known as the so-called “corset-effect”). In a region between 25 °C and 41 °C the response of the particles is directly proportional to the temperature. Moreover, the core properties were systematically varied, revealing the possibility to linearly change the magnitude of the linear swelling. Hence, these particles are very promising as piezo-like linear nano-actuators.

Temperature-Sensitive Swelling of Poly(N-isopropylacrylamide) Brushes with Low Molecular Weight and Grafting Density

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) brushes with different molecular weights M(n) and grafting densities σ were prepared by the "grafting-to" method. Changes in their physicochemical properties according to temperature were investigated with the help of in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Brush criteria indicate a transition between a brush conformation below the lower critical solution temperature (LCST) and an intermediate to mushroom conformation above the LCST. By in situ ellipsometry distinct changes in the brush layer parameters (wet thickness, refractive index, buffer content) were observed. A broadening of the temperature region with maximum deswelling occurred with decreasing grafting density. The brush layer properties were independent of the grafting density below the LCST, but showed a virtually monotonic behavior above the LCST. The midtemperature ϑ(half) of the deswelling process increased with increasing grafting density. Thus grafting density-dependent design parameters for such functional films were presented. For the first time, ATR-FTIR spectroscopy was used to monitor segment density and hydrogen bonding changes of these very thin PNIPAAm brushes as a function of temperature based on significant variations of the methyl stretching, Amide I, as well as Amide II bands with respect to intensity and wavenumber position. No dependence on M(n) and σ in the wavenumber shift of these bands above the LCST was found. The temperature profile of these band intensities and thus segment density was found to be rather step-like, exceeding temperatures around the LCST, while the respective profile of their wavenumber positions suggested continuous structural and hydration processes. Remaining buffer amounts and residual intermolecular segment/water interaction in the collapsed brushes above the LCST could be confirmed by both in situ methods.

Conversion of Alcoholic Concentration Variations into Mechanical Force via Core–Shell Capsules

Conversion of chemical signals into mechanical force is very important for implementation of stimuli-responsive hydrogels. We design a core-shell hydrogel capsule that can translate the variations of alcohol concentration into mechanical force. Oil-in-water-in-oil (O/W/O) emulsions are prepared with microfluidic technique and serve as templates for the synthesis of the core-shell capsules. The oil core is ejected from the capsule by the mechanical force generated from the deswelling of the capsule membrane upon increasing the alcohol concentration at a certain temperature below the lower critical solution temperature. The influences of alcohol concentration and temperature on the deswelling process of capsule membranes are investigated systematically. The deswelling rate also plays an important role in the ejection of the oil core. These demonstrations of conversion of alcohol concentration variations into mechanical force provide proof that these core-shell capsules can function as both sensors and actuators of alcohols.

Evaluation of interface quality in organ-cultured lamellar corneal transplants.

BackgroundWith increasing numbers of lamellar keratoplasties, eye banks are challenged to deliver precut lamellar donor tissue. In Europe, the most common technique of corneal storage is organ culture which requires a deswelling process before surgical processing. The aim of this study was to investigate the influence of different deswelling times on the cutting plane quality after microkeratome-assisted lamellar dissection.MethodsEight paired donor corneas (16 specimens) not suitable for transplantation were organ cultured under standard conditions at the Eye Bank of the Ludwig-Maximilians Universität, Munich, Germany. Pairs of corneal buttons were analyzed during the deswelling process in dextrane-containing medium. While one cornea was cut at an early time point during the deswelling process and put back into deswelling medium thereafter, the partner cornea was completely deswollen and dissected after 72 hours. Specimens were then further processed for scanning electron microscopy. Surface quality was assessed both digitally using Scanning Probe Imaging Processing software, and manually by three blinded graders.ResultsThe corneal buttons processed at the beginning of the deswelling process had a smoother surface when compared to the partner cornea that was cut at the end of the deswelling process. In our setting, no relevant difference was detectable between manual and automated microkeratome dissection.ConclusionFor lamellar keratoplasty, organ-cultured corneas should be processed at an early stage during the deswelling process. We interpret the smoother dissection plane during early deswelling as a result of mechanical properties in a highly hydrated cornea.

Photografted temperature-sensitive poly(N-isopropylacrylamide) thin film with a superfast response rate and an interesting transparent–opaque–transparent change in its deswelling process

Fast swelling/deswelling rates with a change of temperature are required for the application of poly(N-isopropylacrylamide) (PNIPAAm)-based materials. This article reports the fabrication of temperature-sensitive PNIPAAm thin films by a UV-induced surface photografting method. The grafted PNIPAAm thin films have high swelling ratios (10–60) and superfast swelling and response rates (<10 s). A fast development and decay of turbidity in the grafted PNIPAAm gel layers at a temperature above the lower critical solution temperature (LCST) is also reported. The excellent swelling and response properties of the PNIPAAm thin films are believed to be due to the small thickness of the grafted layers, the long but less crosslinked polymer chains and their uneven distribution.

Electrochromic performances and photoluminescence characteristics of versatile N-vinylimidazole-based hybrid hydrogels

We report the synthesis and multi-sensitive performances of a novel type of poly(N-vinylimidazole-co-N-methylolacrylamide) (poly(VI-co-NMA)) hydrogels. These hydrogels behave excellent electric-sensitive properties and reversible bending behaviors. The influence of various parameters such as VI concentration, electric voltage, pH, and ionic strength on the electric-sensitive properties was thoroughly investigated. To obtain electrochromic hydrogels, we further prepared colloidal crystal (CC)-loaded hydrogels in which the color of the CC can be tuned by the bending behavior of the hydrogels. Moreover, these as-prepared hydrogels present excellent adsorption performances toward quantum dots, and the photoluminescence (PL) of CdTe-doped poly(VI-co-NMA) hydrogels was investigated. We found that the PL wavelengths of the CdTe-doped hydrogels could be switched reproducibly by altering the process of swelling and deswelling. These multi-functional hydrogels coupling with the characteristics of electrochromism and PL response confer them a variety of potential applications in sensors.

Available Plasma-Ignited Frontal Polymerization Approach toward Facile Fabrication of Functional Polymer Hydrogels

We report a novel approach for quickly fabricating poly(NVP-co-NMA) hydrogels (NVP = 1-vinyl-2-pyrrolidone, NMA = N-methylolacrylamide) by using plasma-ignited frontal polymerization (PIFP). Frontal polymerization was initiated by igniting the upper side of the mixture with plasma. Once initiated, no additional energy was required for the polymerization to occur. We thoroughly investigated the influence of the molar ratios of NVP/NMA on the properties of poly(NVP-co-NMA) hydrogels via this PIFP approach. The swelling rate, swelling behavior, and morphology of polymer hydrogels prepared via PIFP were comparatively investigated on the basis of water contact angle, swelling measurement and scanning electron microscopy (SEM). Results show that the swelling property of poly(NVP-co-NMA) hydrogels depends on NVP/NMA molar ratios, and the swelling capacity of the hydrogels prepared using PIFP approach is superior to that obtained using traditional batch polymerization (BP) method. More interestingly, these hydrogels, able to absorb quantum dots (QDs), exhibit obvious switching effect via altering the process of swelling and deswelling, which can be developed as QD-based moisture sensor.

Swelling and Collapse of an Adsorbed pH-Responsive Film-Forming Microgel Measured by Optical Reflectometry and QCM

The swelling and deswelling of a pH-responsive electrosterically stabilized poly[2-(diethylamino)ethyl methacrylate] microgel adsorbed to silica surfaces have been quantified using the techniques of optical reflectometry (OR) and quartz crystal microbalance (QCM). It is shown that by utilizing and comparing OR measurements performed on wafers with differing oxide layer thicknesses the adsorbed amount and film thickness of the adsorbed microgel in both the swollen and deswollen forms can be determined. Also, the kinetics of the transition can be followed, revealing that collapse is a slower process than swelling, and direct support is provided for the formation of a dense outer layer or skin during collapse that slows the deswelling process. It is shown that the adsorption of this low glass transition temperature film-forming microgel latex is robust to changes in pH after an initial swelling event which is responsible for desorption of a large and variable fraction of the initially adsorbed polymer. Subsequent deswelling and swelling of the adsorbed film indicates that adsorption to a surface greatly hinders the volumetric swelling capacity of the microgel film. In its swollen state the film is only 3-4 times thicker than the collapsed film, whereas for particles in bulk the volume increases by a factor of 20 upon protonation of the tertiary amine residues. QCM results show that even in the collapsed form the film contains a considerable amount of water. Further, the viscoelasticity of the deswollen film is similar to that of the swollen film, suggesting that the degree of cross-linking is the primary determinant of viscoelasticity.

Swelling characteristics of poly(N‐isopropylmethacrylamide‐co‐itaconic acid) gels prepared in various conditions

AbstractThermo‐ and pH‐sensitive gels composed of N‐isopropylmethacrylamide (NIPMA) and itaconic acid (IA) were prepared by free‐radical crosslinking copolymerization in poly(ethylene glycol) (PEG) solution at 22°C and in water at −22°C and at 22°C, which were called as the macroporous gel (MGel), the cryogel (CGel), and the conventional gel (NGel), respectively. The gels were characterized by the equilibrium swelling, interior morphology, and deswelling kinetics. With increasing temperature in water, the gels exhibit reentrant conformational phase transition. During this transition, the gels first deswell in the range of temperature between 5 and 29°C, then drastically reswell if temperature is monotonically increased. The reswelling of the collapsed gels occurs above 45°C. It was shown that the reentrant transition in poly(N‐isopropylmethacrylamide‐co‐itaconic acid) [poly(NIPMA‐co‐IA)] gels requires moderate temperature, so that the hydrophobic interactions between chains dominate the swelling process. Scanning electron micrographs revealed that the interior network structure of MGel is more porous compared with those of CGel and NGel. This more porous structure provided numerous water channels for water diffusion out of the matrix and, therefore, an improved response rate to the external temperature and pH changes during the deswelling process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Optimizing the Deswelling Process of Organ-Cultured Corneas

During deswelling of organ-cultured human corneas, endothelial cell loss occurs. Therefore, it is necessary to minimize the deswelling time and achieving an optimal central corneal thickness (CCT) of approximately 550 microm at the same time. We investigated the minimal deswelling time necessary and analyzed endothelial cell loss. Fifty-eight human corneas were stored between 13 and 81 days in organ culture. CCT was measured by optical coherence tomography. Measurements were performed before preparation, during culturing, before deswelling, and after varying deswelling periods (1-72 hours) using 5% dextran. Additionally, vital staining was performed in 6 human corneas to assess endothelial cell loss between 24 and 30 hours of deswelling. To evaluate absolute cell loss, endothelial cells were counted on human corneal pairs after 24 and 30 hours of deswelling. After organ culture, mean CCT was 1194 microm. After 24 hours of deswelling in dextran-containing medium, mean CCT was 600 microm, whereas after 30 hours, mean CCT was 510 microm and hardly any corneas showed a CCT of more than 550 microm. Almost no further decrease in CCT was observed thereafter. No factors could be identified predicting the necessary deswelling time; however, paired corneas showed significant correlation of deswelling characteristics. We did not see any differences in endothelial cell loss 24 and 30 hours of deswelling or the ratio of living to dead endothelial cell counts. Deswelling for 24 hours does not provide an optimal corneal thickness. Because endothelial cell loss does not increase between 24 and 30 hours of deswelling, a period of 30 hours is more suitable for obtaining sufficient corneal thickness.

Residual H2O2 compromises deswelling function of in vivo rabbit cornea

The purpose of the study was to investigate the effect of H2O2 on corneal swelling, corneal deswelling and to determine the concentration of H2O2 in the aqueous humor. Four types of experiments were performed using the rabbit eye. 1. Central corneal thickness (CCT) measurements after instillation of H2O2 into the cul-de-sac and after wearing H2O2 soaked soft contact lenses (SLC) for 2 h using 60 ppm, 100 ppm and 300 ppm H2O2. 2. Measurement of corneal deswelling induced by wearing SCL and closed eye for 2 h. 3. Determining the H2O2 in the aqueous humor one h after instillation of H2O2 into the cul-de-sac. CCT increases of 7.5% and 10.7% were found after the instillation of 100 ppm and 300 ppm H2O2 into the cul-de-sac. After wearing H2O2 soaked SCLs, CCT increased 4.6% and 5.8%, respectively. The H2O2 in the aqueous humor was 0.087 mM or a 160% increase after using 300 ppm H2O2. The increase in CCT and interference with the deswelling process is interpreted as a toxic effect on the cornea and an interference with the normal dehydration function of the endothelial pump.

Effect of Pore-Forming Agent Type on Swelling Properties of Macroporous Poly(N-[3-(dimethylaminopropyl)]-methacrylamide-co-acrylamide) Hydrogels

Macroporous poly(N-[3-(dimethylaminopropyl)]methacrylamide-co-acrylamide) [P(DMAPMA-co-AAm)] hydrogels were prepared by free-radical crosslinking copolymerization of corresponding monomers in water using two different pore-forming agents such as hydroxypropyl celluose (HPC) and poly(ethylene glycol) (PEG). The effect of these pore-forming agents on the volume phase transition temperature (VPT-T), interior morphology and swelling/deswelling kinetics of the P(DMAPMA-co-AAm) hydrogels was investigated. Scanning electron micrographs revealed that the interior network structure of the hydrogel matrix became more porous due to the presence of HPC or PEG pore-forming agents. The more porous matrix provided numerous water channels for water diffusion in or out of the matrix and, therefore, an improved response rate to the external stimuli. Particularly, due to its unique macroporous structure, the PEG-modified hydrogel showed a tremendously faster response to the external temperature changes during deswelling process and the swelling process at 22°C.

PH-Induced Deswelling Kinetics of Sterically Stabilized Poly(2-vinylpyridine) Microgels Probed by Stopped-Flow Light Scattering

Near-monodisperse, sterically stabilized poly(2-vinylpyridine) (P2VP) microgels were synthesized by emulsion polymerization. These particles exhibited completely reversible pH-responsive swelling/deswelling behavior in aqueous solution. Stopped-flow light scattering was employed to investigate the kinetics of pH-induced deswelling in highly dilute dispersions. Upon a pH jump from 2 to various final solution pH values (>or=5.4), the scattered light intensity of an aqueous dispersion of a 1,960 nm microgel exhibited an abrupt initial increase, followed by a gradual decrease to the final equilibrium value. The whole microgel-to-latex deswelling process occurred over time scales of approximately 0.5-1.0 s, which is much slower than the kinetics for latex-to-microgel swelling. The microgel deswelling kinetics depends on the final pH, with a higher final pH leading to a faster rate of shrinkage. Close inspection of the deswelling kinetics during the early stages (<0.2 s) revealed that initial microgel collapse occurred within approximately 50 ms, with more rapid transitions being observed when higher final pH values were targeted. Addition of external salt significantly accelerates the kinetics of deswelling. Systematic studies of the microgel-to-latex transition for a series of six near-monodisperse P2VP particles (with swollen microgel diameters ranging from 1270 to 4230 nm) has also been investigated. The characteristic deswelling time for initial microgel collapse, tau deswell, correlated fairly well with the initial swollen microgel radius, R, in agreement with the Tanaka equation. Moreover, the collective diffusion coefficient of the gel network, D, calculated from the slope of the tau deswell- R (2) curve, was of the order of 10 (-7) cm (2) s (-1).

A novel sodium carboxymethylcellulose/poly(N‐isopropylacrylamide)/Clay semi‐IPN nanocomposite hydrogel with improved response rate and mechanical properties

AbstractA novel semi‐IPN nanocomposite hydrogel (CMC/PNIPA/Clay hydrogel) based on linear sodium carboxymethylcellulose (CMC) and poly(N‐isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The structure and morphology of these hydrogels were investigated and their swelling and deswelling kinetics were studied in detail. TEM images showed that the clay was substantially exfoliated to form nano‐dimension platelets dispersed homogeneously in the hydrogels and acted as a multifunctional crosslinker. The CMC/PNIPA/Clay hydrogels swell faster than the corresponding PNIPA/Clay hydrogels at pH 7.4, whereas they swell slower than the PNIPA/Clay hydrogels at pH 1.2. The CMC/PNIPA/Clay nanocomposite hydrogels showed much higher deswelling rates, which was ascribed to more passway formed in these hydrogels for water to diffuse in and out. The deswelling process of the hydrogels could be approximately described by the first‐order kinetic equation and the deswelling rate decreased with increasing clay content. The mechanical properties of the CMC/PNIPA/Clay nanocomposite hydrogels were analyzed based on the theory of rubber elasticity. It was found that with increasing clay content, the effective crosslink chain density, ve, increased whereas the molecular weight of the chains between crosslinks Mc decreased. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1546–1555, 2008

Dynamical modeling and experimental evidence on the swelling/deswelling behaviors of pH sensitive hydrogels

A chemo-electro-mechanical fields coupling model was developed based on the Nernst–Planck equation, Donnan theory and elastic mechanics to investigate the swelling/deswelling dynamics of pH sensitive hydrogels. To test the model, chitosan hydrogels with pH sensitivity have been successfully prepared through a radiation crosslinking approach induced by Co-60 gamma ray. The deformation ratio of the hydrogels at radial direction is more than 100% when pH changes from 7 to 9. A finite element simulation was carried out to investigate the stress formation and diffusion inside the hydrogel in its deswelling process. Results showed that the data of simulation fit the experimental results well. Moreover, a stress-diffusion mechanism was proposed to explain the deswelling process.