Amphoteric surface hydrogels derived from hydrogen-bonded multilayers: reversible loading of dyes and macromolecules

We used hydrogen-bonded multilayers of poly(N-vinylpyrrolidone) (PVPON) and poly(methacrylic acid) (PMAA) as precursors for producing surface-bound hydrogels and studied their pH-dependent swelling and protein uptake behavior using in situ attenuated total reflection Fourier transform infrared spectroscopy and in situ ellipsometry. The hydrogels were produced by selective chemical cross-linking between PMAA units using carbodiimide chemistry and ethylenediamine (EDA) as a cross-linking reagent, followed by complete removal of PVPON from the film obtained by exposing the film to pH 7.5. As shown by in situ ellipsometry, hydrogels exhibit distinctive polyampholytic swelling as a function of pH, with minimum swelling at pH 4.2-5.7, and increased film thickness at both lower and higher pH values. Film swelling at lower pH values occurs as a result of the presence of amino groups within the hydrogels, which originate from the one-end attachment of the EDA cross-linker to PMAA chains. The pH-switching of hydrogel swelling was fast and reversible. The degree of hydrogel swelling could be also controlled by varying the time allowed for cross-linking. The produced hydrogels were able to absorb large amounts of dyes and proteins of opposite charge reversibly, in response to pH variations. Finally, we demonstrate that proteins included within the hydrogel can easily be replaced with linear polycations. These surface hydrogels hold promise for bioseparation and controlled delivery applications.     

Investigation of the swelling behavior of hydrogels derived from high-molecular-weight poly(2-ethyl-2-oxazoline)

Thermoresponsive hydrogels are of great importance as smart materials. They are usually composed of cross-linked polymers with a lower critical solution temperature (LCST). Although much is known about networks of poly(N-isopropylacrylamide), all other polymers are somewhat neglected. In this work, the temperature-dependent swelling behavior of differently cross-linked thermoresponsive poly(2-ethyl-2-oxazoline) (PEtOx) hydrogels were investigated with regard to varying parameters of the network composition. It was found that the degrees of swelling of the hydrogels converge for a certain polymer/solvent system at a distinct temperature independent of its degree of cross-linking. Furthermore, this temperature correlates with the LCST of the respective starting PEtOx. Its net chain molecular weight M_c only affects the maximum degree of swelling and thus, the swelling–deswelling rate of the hydrogel. The fundamental structure/property relations found in this study could be useful to predict the behavior of other thermoresponsive hydrogels.     

Swelling and Reswelling Characteristics of Cross-Linked Poly(vinyl alcohol)/Chitosan Hydrogel Film

Poly(vinyl alcohol) (PVA), hydrogel was prepared by using glutaraldehyde as a cross-linking agent. The blend semi-synthetic hydrogel film, consisting of PVA and chitosan, was prepared from a solvent-casting technique and characterized for their intermolecular interactions using infrared method. The swelling and reswelling behaviors, as well as mechanical properties of the synthetic and semi-synthetic gels were examined by weighing and tensile testing, respectively. Cross-linking the two types of polymer with glutaraldehyde produces a film with lower crystallinity and smaller swelling and reswelling degrees, but having improved mechanical properties. Also, the two types of films show a pH-dependent swelling characteristic. It was found that, the reswelling properties of synthetic hydrogels can be improved by blending PVA with certain ratio of natural polymer. This blending film, can be improve sandy soil properties for cultivation, such as, controlled release of water.     

Contact angle hysteresis: study by dynamic cycling contact angle measurements and variable angle spectroscopic ellipsometry on polyimide

The phenomenon of contact angle hysteresis was studied on smooth films of polyimide, a polymer type used in the microelectronic industry, by dynamic cycling contact angle measurements based on axisymmetric drop shape analysis-profile in combination with variable angle spectroscopic ellipsometry (VASE). It was found that both advancing and receding contact angles became smaller with increasing the number of cycles and are, therefore, not a property of the dry solid alone. The changes of the wetting behavior during these dynamic cycling contact angle measurements are attributed mainly to swelling and/or liquid retention. To reveal the water-induced changes of the polymer film, the polyimide surface was studied before and after the contact with a water droplet by VASE. Both the experimental ellipsometric spectrum for Delta and that for Psi as well as the corresponding simulations show characteristic shifts due to the contact with water. The so-called effective medium approximation was applied to recover information about the thickness and effective optical constants of the polymer layer from the ellipsometrically measured values of Delta and Psi. On the basis of these results, the swelling and retention behavior of the polyimide films in contact with water droplets were discussed.     

Electron beam irradiation of poly(vinyl methyl ether) films. 2. Temperature-dependent swelling behavior

Temperature-sensitive hydrogel films were synthesized by electron beam irradiation of poly(vinyl methyl ether) (PVME) on silicon (Si/SiO(2)) substrates and gold (Au) coated glass slides. The temperature-dependent swelling behavior of the films in aqueous solution was characterized by in situ spectroscopic ellipsometry and a combination of surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The results of both techniques are compared. The suitability of both techniques for the characterization of the swelling behavior of thin hydrogel films is demonstrated. The volume swelling degree in the swollen state decreases with increasing radiation dose D. This is explained by the fact that the number of formed polymeric radicals, and hence cross-linking density, increases with D. Above the phase-transition temperature, the swelling degrees were independent of D, slightly above 1. The swelling/deswelling process was fully reversible and is mainly directed perpendicular to the substrate surface. The phase-transition temperature was determined to be T(cr) approximately 33 degrees C. However, T(cr) slightly decreases with increasing D and increasing film thickness d.     

Simultaneous measurement of mechanical and surface properties in thermoresponsive, anchored hydrogel films

Hydrogel films have been used extensively in the preparation of biosensors and biomedical devices. The characteristics of the aqueous interface of the polymer layer are significant for the biosensor or device function; likewise, the changing mechanical properties of thermoresponsive polymers are an important feature that affects the polymer behavior. Atomic force microscopy was used here to characterize both the surface and the mechanical properties of polymeric hydrogel films prepared from a thermoresponsive terpolymer of N-isopropylacrylamide and acrylic acid with benzophenonemethacrylate as a photoreactive cross-linker comonomer. The force-distance curves thus obtained were analyzed to assess both the surface forces and the mechanical response that were associated with the hydrogel. These properties were investigated as a function of temperature, in water and in Tris buffer, for different degrees of polymer cross-linking. For samples in water, the distance over which the surface forces were effective was found to remain constant as the temperature was increased from 26 to 42 °C, even though the mechanical response indicated that the samples had been heated past the lower critical solution temperature, or LCST. The bulk of the polymer becomes less soluble above the LCST, although this does not seem to affect the surface properties. This may be due to the segregation of the acrylic acid-rich polymer segments near the gel surface, which is in agreement with reports for related systems.     

Collapse transition in thin films of poly(methoxydiethylenglycol acrylate)

The thermal behavior of poly(methoxydiethylenglycol acrylate) (PMDEGA) is studied in thin hydrogel films on solid supports and is compared with the behavior in aqueous solution. The PMDEGA hydrogel film thickness is varied from 2 to 422?nm. Initially, these films are homogenous, as measured with optical microscopy, atomic force microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering (GISAXS). However, they tend to de-wet when stored under ambient conditions. Along the surface normal, no long-ranged correlations between substrate and film surface are detected with GISAXS, due to the high mobility of the polymer at room temperature. The swelling of the hydrogel films as a function of the water vapor pressure and the temperature are probed for saturated water vapor pressures between 2,380 and 3,170?Pa. While the swelling capability is found to increase with water vapor pressure, swelling in dependence on the temperature revealed a collapse phase transition of a lower critical solution temperature type. The transition temperature decreases from 40.6?°C to 36.6?°C with increasing film thickness, but is independent of the thickness for very thin films below a thickness of 40?nm. The observed transition temperature range compares well with the cloud points observed in dilute (0.1?wt.%) and semi-dilute (5?wt.%) solution which decrease from 45?°C to 39?°C with increasing concentration.     

Responsive thin hydrogel layers from photo-cross-linkable poly(N-isopropylacrylamide) terpolymers

The structural features and swelling properties of responsive hydrogel films based on poly(N-isopropylacrylamide) copolymers with a photo-cross-linkable benzophenone unit were investigated by surface plasmon resonance, optical waveguide mode spectroscopy, and atomic force microscopy. The temperature-dependent swelling behavior was studied with respect to the chemical composition of the hydrogel polymers containing either sodium methacrylate or methacrylic acid moieties. In the sodium methacrylate system, a refractive index gradient was found that was not present in the free acid gel. This refractive index gradient, perpendicular to the swollen hydrogel film surface, could be analyzed in detail by application of the reversed Wentzel-Kramers-Brillouin (WKB) approximation to the optical data. This novel approach to analyzing thin-film gradients with the WKB method presents a powerful tool for the characterization of inhomogeneous hydrogels, which would otherwise be very difficult to capture experimentally. In AFM images of the hydrogel layers, a macroscopic pore structure was observed that depended on the polymer composition as well as on the swelling history. This pore structure apparently prevents the often-observed skin barrier effect and leads to a quickly responding hydrogel.     

A general method to determine ionization constants of responsive polymer thin films

A general method has been developed to determine the ionization constants of polymer thin films based on the stimuli-responsiveness of the polymer. Robust polymer films were fabricated on silicon wafers and gold slides using perfluorophenyl azide (PFPA) as the coupling agent. The ionization constants were measured by a number of techniques including ellipsometry, dynamic contact angle goniometry, and surface plasmon resonance imaging (SPRi). Using poly(4-vinylpyridine) (P4VP) as the model system, P4VP thin films were fabricated and the ionization constants of the films were measured taking advantage of the pH responsive property of the polymer. The pK(a) determined by ellipsometry, ~4.0, reflects the swelling of the polymer film in response to pH. The pK(a) value calculated from the dynamic contact angle measurements, ~5.0, relies on the change in hydrophilicity/hydrophobicity of the films as the polymer undergoes protonation/deprotonation. The pK(a) value measured by SPRi, ~4.9, monitors in situ the change of refractive index of the polymer thin film as it swells upon protonation. This was the first example where SPRi was used to measure the ionization constants of polymers.     

Thermoresponsive super water absorbent hydrogels prepared by frontal polymerization of N‐isopropyl acrylamide and 3‐sulfopropyl acrylate potassium salt

Super water absorbent polymer hydrogels were synthesized by frontal polymerization. These materials were obtained by copolymerizing N‐isopropyl acrylamide (NIPAAm) and 3‐sulfopropyl acrylate potassium salt (SPAK) in the presence of N,N′‐methylene‐bis‐acrylamide as a crosslinker. It was found that their swelling behavior in water can be easily tuned by using either the appropriate monomer ratio or the amount of the crosslinker used. Namely, the swelling ratio was found to range from about 1000% for the NIPAAm homopolymer in the presence of 5.0 mol % of crosslinker, up to 35,000% for the sample containing 87.5 mol % of SPAK and 1.0 mol % of crosslinker. The affinity toward water was also confirmed by contact angle analysis. Moreover, the obtained hydrogels exhibit a thermoresponsive behavior, with a lower critical solution temperature of about 28–30 °C. This value is close to that of poly(NIPAAm) but with a swelling capability that dramatically increases as the amount of SPAK increases. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Thermo-responsive Hercosett/Poly(N-isopropylacrylamide) films: a new, fast, optically responsive coating

Poly(N-isopropylacrylamide) (PNIPAM) is a common thermo-responsive, water-soluble polymer, while Hercosett is a cationic resin commonly employed in the paper industry. In this paper, Hercosett? and poly(N-isopropylacrylamide) (PNIPAM) nanoparticles were used to prepare composite films that show thermo-responsive behavior and swelling–shrinking properties in water. First, size-controlled PNIPAM hydrogel nanoparticles were synthesized. These were then embedded within a matrix of the cationic resin Kymene 577H by film casting. The distribution of nanoparticles in the resin film was investigated. The thermo-responsive properties of the as-synthesized PNIPAM hydrogel nanoparticles and of the composite films were characterized together with the repeatability of the swelling–shrinking cycles. The presence of nanoparticles endowed the film with highly enhanced water retention (in comparison with resin-only films) and, most importantly, thermo-responsiveness. A very fast optical and morphological response was in fact observed. Due to the dual (optical and morphological) response, this new system is suitable for applications in optical or morphological actuation and gating.     

N-异丙基丙烯酰胺温度敏感性水凝胶相转变动力学研究及其应用

 研究指出表观二级动力学方程可以很好地描述N-异丙基丙烯酰胺水凝胶的溶胀和消溶胀动力学.即溶胀动力学方程为dR/dt=k₁(R_e-R)²,消溶胀动力学方程为-dR/dt=k_c(R-R_e)².把这种水凝胶用于分离高分子水溶液时可引入“单位溶张比分离循环的合理时耗”这样一个参量.它根据溶胀和消溶胀过程中的起始溶胀比、平衡溶胀比、表观溶胀动力学常数和表观消溶胀动力学常数求出.具体公式为△t₁(T_s,T_c)=2/[R_c(T_s)-R₀(T_s)]²k_s(T_s)+15/[R₀(T_c)- R_s(T_c)]²k_c(T_c)^1/2在理想情况下,分离过程的“总合理时耗”与△t_1成正比,比例系数为分离过程中的除水总量与干凝胶用量的比值,即△t_r=W_W/WG·△t₁.当根据二个动力学方程求得的总时耗计算值处于(0.9△t_r,1.1△t_r)范围内时,表明所选干凝胶用量和循环溶胀比区段均合适.     

Investigation of polymer–solvent interactions in poly(styrene sulfonate) thin films

The swelling behavior of acid form poly(styrene sulfonate) (PSS‐H) thin films were investigated using in situ spectroscopic ellipsometry (SE) to probe the polymer–solvent interactions of ion‐containing polymers under interfacial confinement. The interaction parameter (χ), related to the polymer and solvent solubility parameters in the Flory–Huggins theory, describes the polymer‐solvent compatibility. In situ SE was used to measure the degree of polymer swelling in various solvent vapor environments, to determine χ for the solvent‐PSS‐H system. The calculated solubility parameter of 40–44 MPa^1/2 for PSS‐H was determined through measured χ values in water, methanol, and formamide environments at a solvent vapor activity of 0.95. Flory–Huggins theory was applied to describe the thickness‐dependent swelling of PSS‐H and to quantify the water‐PSS‐H interactions. Confinement had a significant influence on polymer swelling at low water vapor activities expressed as an increased χ between the water and polymer with decreasing film thickness. As the volume fraction of water approached ～0.3, the measured χ value was ～0.65, indicating the water interacted with the polymer in a similar manner, regardless of thicknesses. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1365–1372     

Real-time monitoring of polymer swelling on the nanometer scale by atomic force microscopy

The swelling of a polymer surface has been monitored in real time on the nanometer scale by atomic force microscopy (AFM). After modification by oxygen plasma treatment, poly(p-phenylene terephthalamide) (PPTA) displays a characteristic nanostructured surface morphology consisting of high-lying features alternating with topographically depressed areas. Selective swelling of the least cross-linked, depressed areas after the adsorption of ambient water or water from saturated humid atmospheres was observed by tapping mode AFM operated in the attractive interaction regime. The swollen areas could be distinguished from the nonswollen ones by local variations in the sample indentation made by the AFM tip when imaging in the tapping mode repulsive interaction regime. Monitoring the swelling of the plasma-treated polymer surface provided a means to reveal the nanometer-scale heterogeneity that this type of treatment creates on the polymer surface, which is something that would not be possible otherwise. Measurement of AFM tip-sample adhesion forces evidenced rapid water adsorption onto the oxygen plasma-treated surface, supporting the idea of water-induced swelling. This high hydrophilicity was interpreted as arising from the incorporation of polar oxygen functionalities, as demonstrated by X-ray photoelectron spectroscopy (XPS).     

Recognition‐Mediated Hydrogel Swelling Controlled by Interaction with a Negative Thermoresponsive LCST Polymer

Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non‐thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT⁴⁺, swelling occurred as a result of host–guest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter‐ions embedded in the network. The immersion of NaphtGel in a solution of poly(N‐isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT⁴⁺ induced positive thermoresponsive behaviour. The LCST‐induced dethreading of the polymer‐based pseudorotaxane upon heating led to transfer of the CBPQT⁴⁺ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF‐based host–guest complexes in solution and contraction of the hydrogel.     

Radiation synthesis of poly[(dimethylaminoethyl methacrylate)-co-(ethyleneglycol dimethacrylate)] hydrogels and its application as a carrier for anticancer delivery

The use of hydrogels as carriers for anticancer delivery has been a subject of significant recent research. In our recent work, we have shown that diffusion-controlled delivery of flutamide from hydrogels containing poly (dimethylaminoethyl methacrylate (DMAEMA)/ethyleneglycol dimethacrylate (EGDMA)) can be possible and controlled by the three-dimensional structure. Hydrogels based essentially on dimethylaminoethyl methacrylate and different ratios of ethyleneglycol dimethacrylate monomers were synthesized using gamma radiation copolymerization. The influence of copolymer composition and pH value of the surrounding medium on swelling behavior into the glassy polymer were discussed. The results showed that the ratio of EGDMA in the comonomer feeding solution has a great effect on the gel fraction and water content in the final hydrogel. In this regard, it was observed that the increase of EGDMA ratio decreased these properties. The ability of the prepared copolymer to be used as drug carrier for anticancer drug-delivery system was estimated using flutamide as a model drug. In vitro drug-release studies in different buffer solutions show that the basic parameters affecting the drug release behavior of hydrogel are the pH of the solution and DMAEMA content of hydrogel.     

Synthesis of Carboxymethyl Cellulose Based Drug Carrier Hydrogel Using Ionizing Radiation for Possible Use as Site Specific Delivery System

A unique natural polymer based colon specific drug carrier was prepared from carboxymethyl cellulose (CMC) and acrylic acid (AAc) in aqueous solution employing γ‐radiation induced copolymerization and crosslinking. The effect of preparation conditions such as the natural polymer content and irradiation dose on gelation process was investigated. The swelling behavior of the prepared hydrogels was characterized by investigating the time and pH dependent swelling of the (CMC/AAc) hydrogels of different CMC content. The effects of the hydrogel composition and pH of the swelling medium on the swelling indices were estimated. The results show that the increment in the CMC content in the feed solution enhances the gelation process. The results also show the dependence of the swelling indices on both hydrogel composition and pH value of the swelling medium. To evaluate the ability of the prepared hydrogel to be used as a colon‐specific drug carrier, the release profile of theophylline was studied as a function of time at pH 1 and pH 7.     

A molecular thermodynamic model for the swelling of thermo-sensitive hydrogels

A new molecular thermodynamic model for describing the swelling behavior of thermo-sensitive hydrogels was developed. The model consists of two terms. One is the contribution of the mixing of hydrogel network and water, which is dependent on the local polymer concentration and the interaction between polymer segment and solvent. A closed packed lattice model for polymer solution developed by Yang et al. was adopted for this term. The other is the elastic contribution derived from the network elasticity, which is dependent on the cross-linking degree of gel network. The elastic Gibbs energy model based on the Gaussian chain model developed by Flory was adopted. The model equation has two parameters. One is an energy parameter ? reflecting the interaction between water and gel network, the other is a size parameter V^* that represents the cross-linking degree of the hydrogel. When the energy parameter ? is expressed as a quadratic of inverse temperature, this model can describe the swelling equilibrium behavior of neutral thermo-sensitive hydrogels quite well. The influences of model parameters were discussed in details. The experimental swelling curves of two kinds of polyacrylamide-based gels were correlated and good agreement was obtained.     

Synthesis and in situ atomic force microscopy characterization of temperature-responsive hydrogels based on poly(2-(dimethylamino)ethyl methacrylate) prepared by atom transfer radical polymerization

Well-defined copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and benzophenone methacrylate (BPMA) with different compositions were synthesized via atom transfer radical polymerization. The molecular weights of these copolymers were Mn approximately 30 000 g/mol, while the BPMA content varied from 2.5 to 10 mol %. The copolymers with a low content of BPMA (2.5 and 5 mol %) exhibited a sharp thermal transition at 33-36 degrees C in aqueous solution. A hydrogel was immobilized and patterned on a silicon wafer via UV treatment of the spin-coated polymer layer using a photomask technique. The thermoresponsive behavior of the patterned polymer gel was quantitatively investigated by variable temperature in situ contact mode atomic force microscopy, which revealed the presence of two lower critical solution temperature regions. One region was between 25 and 30 degrees C, corresponding to the topmost layer of the hydrogel film, and the other region, around 40 degrees C, corresponded to the bulk of the hydrogel. Concurrent lateral force microscopy measurements revealed that, just above the transition temperature, the bulk region exhibited enhanced friction.