PNIPAM Gels Research Articles

Fully Physically Crosslinked PNIPAM Ionogels with High Mechanical Properties and Temperature‐Managed Adhesion Achieved by H2O/Ionic Liquid Binary Solvents

AbstractPoly(N‐isopropylacrylamide) (PNIPAM) gels change their volume and hydrophilicity in response to temperature stimulus in flexible actuators, flexible sensors, and energy storage management. However, traditional PNIPAM gels exhibit low mechanical properties and poor self‐recovery ability due to weak intermolecular (chain) interactions and necessary covalent crosslinking. Herein, a water/ionic liquid (H2O/IL) binary solvent system that provides additional hydrogen bonding and hydrophobic interactions as well as can significantly increase the solubility (up to 60 wt.%) of NIPAM is developed. The fully physically crosslinked PNIPAM ionogel is prepared by utilizing the H2O/IL binary solvent and exhibited excellent mechanical performances, rapid self‐recovery properties, and strong adhesion strength. In addition, the lower critical solution temperature of the PNIPAM ionogel can also be tuned over a wide range (37‐70 °C) by adjusting the IL ratio in the binary solvent. Furthermore, based on the temperature sensitivity and ionic conductivity of the PNIPAM ionogel, applications of PNIPAM ionogels in smart devices, temperature‐managed adhesion, and flexible sensing are demonstrated, respectively. This work improves various performances of PNIPAM gels by using the H2O/IL binary solvent to adjust non‐covalent interactions and provided new ideas for developing high‐performance temperature‐sensitive gels.

Double-inverse-opal structured films of a hydrogel framework and mobile inorganic particles

Double-inverse-opal (DIO) structured assemblies are three-dimensionally ordered porous materials containing core particles inside. In this work, DIO structured films composed of mobile inorganic colloidal particles within an organic polymer gel framework were developed. As the organic backbone, a poly(N-isopropylacrylamide) gel (PNIPAM gel) was employed to enhance the flexibility and processability of the films. A three-dimensional periodic structure of fluorescent-labelled silica core particles in the film was observed with confocal microscopy. Thermally induced volume transitions of the PNIPAM gels were observed, which showed the potential for stimuli-responsive optical materials. By selective etching of the sacrificial shell layers surrounding the core particles, cavities formed around the cores, and they exhibited random Brownian motion inside these pockets in the film. As a new type of organic-inorganic composite material, the proposed DIO structured films possessed unique characteristics originating from the combination of a flexible backbone and mobile cores, applicable to stimuli-responsive photonic devices, reusable catalytic particle assemblies, etc.

High-transmittance pNIPAm gel smart windows with lower response temperature and stronger solar regulation

Improving the solar regulation and reducing the response temperature of thermochromic smart windows are pivotal importance for energy-saving buildings. However, such two strategies are challenging to be integrated into one window. Herein, a solution for simultaneously regulating the surface tension of the scattering center and the capturing-releasing capacity of pNIPAm for water is implemented, successfully carrying out the enhancement of solar regulation and the reduction of response temperature. The maximum solar shielding and regulation of the modified sandwich structured pNIPAm gel device can reach up to 88.45% and 71.81%, an increase of 52.29% and 73.71% respectively, compared to a pure pNIPAm gel device with the same thickness. Besides, the response temperature is reduced to around 28 ℃, making modified devices have the ability to respond promptly at the comfortable room temperature range boundary (22–28 ℃). Different from flowing, strengthless modified doped microgels, the high-transmittance modified pNIPAm gels maintain the stable patterning performance, and have great potential for further applications. Similar conclusions are obtained using different solvents to reduce the attraction between pNIPAm-water, and the surface tension of water. Therefore, this universal modification framework deserves attention for the future design of energy-saving windows.

Development of a Soft Actuator from Fast Swelling Macroporous PNIPAM Gels for Smart Braille Device Applications in Haptic Technology

The development of a cost-efficient braille device is a crucial challenge in haptic technology to improve the integration of visually impaired people. Exclusion of any group threatens the proper functioning of society. Commercially available braille devices still utilize piezoelectric actuators, which are expensive and bulky. The challenge of a more adapted braille device lies in the integration of a high number of actuators─on a millimeter scale─in order to independently move a matrix of pins acting as tactile cues. Unfortunately, no actuation strategy has been adapted to tackle this challenge. In this study, we develop a soft actuator based on a thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel. We introduce macroporosity to the gel (pores of 10 to 100 μm). It overcomes the diffusion─which is the limiting kinetic factor─and accelerates the gel response time from hours for the bulk gel to seconds for the macroporous gel. We study the properties of porous gels with various porosities. We also compare a mechanically reinforced nanocomposite gel (made of PNIPAM and Laponite clay) to a "classic" gel. As a result, we develop a fast-actuating gel with high cyclic performance. We then develop a single-pin braille setup, where actuation is controlled thanks to a swift temperature control of a macroporous gel cylinder. This new strategy offers a very promising actuation technology. It offers a simple and cost-efficient alternative to the current braille devices.

Comparison of Characteristics of Poly(Nisopropylacrylamide) in Bulk Hydrogel and Ball-milled Microgel Forms

Abstract Poly(N-isopropylacrylamide) hydrogels are a popular temperature sensitive biomaterial. Bulk hydrogels can be quickly and easily processed into microgels using a ball mill. However, there is no information on whether the mechanical milling process affects critical PNIPAM characteristics. In this work, we compare swelling and thermo-responsive properties for a series of PNIPAM gels in bulk and microgel forms.

Conductive and Tough Smart Poly(N‐isopropylacrylamide) Hydrogels Hybridized by Green Deep Eutectic Solvent

AbstractConductive smart hydrogels with several virtues such as similar characters to biological tissues, sensitive response to ambient variations, have shown their excellent talents in the field of flexible electrical sensors, biomedical devices and directional transportation. However, complex preparing approaches or the instable inner structures have not only been time‐consuming, but also broken up the performance and reliability of the smart hydrogel‐based devices. In this work, a facile one‐step method is put forward to synthesize a kind of conductive poly(N‐isopropylacrylamide) (PNIPAM) hydrogel doped by a new green solvent of deep eutectic solvent (DES) containing choline chloride (ChCl) and acrylic acid (AA). Through the copolymerization of AA and NIPAM, the mechanical strength of the DES‐doped PNIPAM hydrogels is drastically improved compared to the pure PNIPAM gels, and some doped hydrogels lost the typical phase transition temperature of PNIPAM. Moreover, due to the ionic property of DES, the hybrid hydrogels also present the thermal‐depending conductivity as well as sensitive deformation response, which can be used as a smart switch in a circuit or a sensing element with environmental response ability. The cost‐effective preparation and the attractive performance of the DES‐doped hydrogels offer a new avenue to construct multi‐functional materials.

Behaviors of 3D-printed objects made of thermo-responsive hydrogels: motion in flow and molecule release ability

Functional and intelligent soft materials are useful for controlled release of medicines and other molecules. Poly(N-isopropylacrylamide) (PNIPAm) gels are well known as thermo-responsive gels with large discontinuous volume change which is commonly called as the volume phase transition. PNIPAm gels are expected to be applied to controlled-releasable material for drug delivery systems. Furthermore, we can fabricate arbitrary shapes made of gels freely by using 3D hydrogel printing techniques. Here we report that the effect of capsule shapes on behaviors of the PNIPAm objects under water flow with Reynolds number 102. In addition, we compared the behaviors of 3D-printed PNIPAm gel objects with 3D-printed polylactic acid (PLA) objects. The velocity at the first period of 2 or 3 s from the beginning of movement is slower than that of second period in the case of PNIPAm objects with ellipsoidal and bullet shapes. In other words, the capsules are accelerated after the period of 2 or 3 s. For the bullet shape objects, the velocity of the PNIPAm gel is approximately three times faster than that of PLA. It is considered that the cause of the difference between the velocities of PLA and PNIPAm gel is the difference of their frictions. The hydrogels possess commonly low friction properties. Finally, it is demonstrated that the 3D-printed PNIPAm capsules can release the solution inside the capsules by shrinking the PNIPAm gels, caused by volume phase transition.

Enrichment of methanol inside pNIPAM gels in the cononsolvency-induced collapse.

Crosslinked poly-N-isopropylacrylamide (pNIPAM) gels adapt to their environment by a unique transition from a flexible, swollen macromolecular network to a collapsed particle. pNIPAM gels are swollen in both, pure water and pure methanol (MeOH). However, a drastic volume loss is observed in mixtures of water and methanol over a wide composition range. This effect is referred to as cononsolvency. Cononsolvency couples the volume phase transition to the transport of the cosolvent into the polymeric network. So far, the mechanisms underlying cononsolvency have not been fully elucidated. To obtain insights on cononsolvency, Raman microspectroscopy was applied to capture spatially resolved spectra distinguishing between the surroundings and the inside of the gel. Here, we used Indirect Hard Modelling (IHM) for the spectral analysis. Mass balancing allowed the calculation of the solvent composition inside the pNIPAM gel. The results show an increased methanol fraction inside the collapsed gel as compared to its surroundings. Furthermore, the sensitivity of the vibrational bands of methanol to its local hydrogen bonding environment allow to derive information about the molecular interactions. The methanol peak shifts measured inside the gel point towards donor-type hydrogen bonds between methanol and the peptide group of pNIPAM in the cononsolvency-induced collapse. The presented data should enhance our understanding of cononsolvency.

The Swelling of Poly(Isopropylacrylamide) Near the θ Temperature: A Comparison between Linear and Cross‐Linked Chains

AbstractThermoresponsive polymeric gels are a class of smart materials with the ability to absorb or release large amounts of solvent when changes in their environment occur. Scaling theories of gel swelling (de Gennes'c∗ theorem and Obukhov's model) predict that the swelling of a gel correlates with that of a linear chain with equal degree of polymerization to that of a network strand. Data on linear and cross‐linked poly(N‐isopropylacrylamide) (PNIPAM) in aqueous solutions of different molar masses under θ temperature and good solvent conditions are examined. The Kuhn length of PNIPAM is evaluated to beLK = 4 ± 1 nm, in strong disagreement with previous estimates by single molecule force spectroscopy. The excluded volume strength (B) varies with the reduced temperature (τ) asB ≃ 4.7τ nm. While the power law exponent for the equilibrium swelling as a function of degree of cross‐linking is close to the scaling predictions, the degree of swelling observed in cross‐linked networks is two to three orders of magnitude larger than that expected from scaling models. The large differences between theoretical predictions and experimental results probably arise from the highly inhomogeneous nature of PNIPAM gels.

Large Swelling Capacities of Crosslinked Poly(N-isopropylacrylamide) Gels in Organic Solvents

ABSTRACTPNIPAM hydrogels are widely studied materials which swell in a great extent in water and water like solvents (e.g. alcohols). The hydrophilic nature of PNIPAM networks is very attractive however it is an important disadvantage at the moment of encapsulating hydrophobic drugs, which minimize their use in other fields. In this work we studied the swelling in different solvent mixtures with water and also in pure nonaqueous solvents, some of them immiscible with water. Accordingly, PNIPAM gels swell strongly in highly polar solvents (e.g. chloroform) but it does not swell in slightly polar solvents (e.g. toluene). The main interaction between the solvent and the polymer chain seems to involve the hydrogen bonding with the amide group, according to the calculated Hansen parameters (δh). It is possible to swell the gel in binary or ternary mixtures containing toluene. In that way, non-polar substances can be loaded inside the gel to change its properties. As a proof of concept, polyaniline (PANI) solubilized in chloroform using camphorsulfonate as solubilizing counterion. The obtained nanocomposites become sensitive to pH changing color and conductivity when exposed to basic or acidic aqueous solutions.

Poly( N-isopropylacrylamide) Cross-Linked Gels as Intrinsic Amphiphilic Materials: Swelling Properties Used to Build Novel Interphases.

The hydrophilic nature of hydrogels allows their swelling in aqueous solutions. In that way, any substance loaded inside the gel is exposed to the aqueous media and could be released if it is soluble in water. However, only substances that are soluble in water can be loaded inside a gel, which can be swelled only in water. In this work, we studied the swelling of poly( N-isopropylacrylamide) (PNIPAM) gels in nonaqueous solvents and their solutions with water. PNIPAM gels swell strongly in highly polar solvents, but they do not swell in slightly polar solvents (e.g., toluene). However, it is possible to swell the gel in mixtures containing toluene. The observed properties of PNIPAM gels allow describing them both as solvogels or amphigels. When the loaded substance is soluble in one solvent (e.g., water) and not in another (e.g., chloroform), the substance is not released but exposed to the new media. As a proof of concept, a colorimetric pH sensor active in CHCl3 and a Cu1+ sensor in water were built. Moreover, using a ternary solution containing toluene linear polystyrene can be loaded inside the gel, making a semi-interpenetrated network. Because PNIPAM swells in water and solvents immiscible in water, a liquid/liquid interphase can be set inside a gel. A near-infrared absorbing dye (soluble in CHCl3) is loaded in only half of a thermoresponsive PNIPAM gel. Upon near-infrared irradiation, only the region where the dye is loaded heats up driving the phase transition of PNIPAM.

Porous thermo-responsive pNIPAM microgels

Thermo-responsive poly(N-isopropylacrylamide) (pNIPAM) gels are very attractive for biological applications due to a phase transition at temperature (32°C) closed to that of biologically relevant. This stimulates the usage of this polymer for cell culture, biosensing, drug delivery, etc. Herein we develop a new simple approach to synthesize porous pNIPAM microgels by hard templating using porous CaCO3 cores. For this purpose, the mesoporous CaCO3 cores have been infiltrated with pNIPAM-4-acryloylbenzophenone (pNIPAM-ABP) followed by chemical crosslinking of the polymer molecules by UV-light and further core removal in HCl. The formed microgels possess reversible temperature response with LCST similar to that of the pure pNIPAM-ABP. Phase transition results in dramatic collapse of the porous microgels with the decrease of their size by up to ten times due to the pore closure. The porous structure is reorganized after the reversible heating–cooling cycle resulting in closure of larger pores and thus more homogeneous polymer distribution. The microgels are both temperature and pH sensitive that makes them attractive for drug delivery applications.

Synthesis of monodisperse composite poly(N-isopropylacrylamide) microgels incorporating dispersive Pt nanoparticles with high contents

A facile synthesis of monodisperse composite poly(N-isopropylacrylamide) (PNIPAM) microgels incorporating dispersive Pt nanoparticles with high contents was proposed using an in situ method in which platinum ions were reduced in the presence of PNIPAM microgels. The high contents of dispersive Pt nanoparticles were achieved in a reduction process where a concentrated Pt ion solution suspending PNIPAM gels was injected into a diluted solution of sodium borohydride (NaBH4). Cross-sectional TEM image and energy dispersive X-ray spectroscopy (EDX) for the composite microgels with a high Pt content showed that the major part of Pt nanoparticles formed in the reduction process was localized in the periphery of the PNIPAM microgels. The amount of Pt nanoparticles incorporated into the composite microgels could be enhanced by a decrease in the number of PNIPAM microgels in the in situ reduction method. A highest incorporation at the Pt content of 46wt% much higher than that previously reported was attained with maintaining their Pt particle sizes less than 5nm when the concentration of PNIPAM gels was adjusted to a low concentration of 7.7×10−2wt% in the reduction process. It could also be confirmed with dynamic light scattering that sizes of the Pt-PNIPAM composite microgels were sharply decreased with an increase in temperature from 25°C to 40°C. In principle, the present method is applicable to the incorporation of other metals than Pt. The applicability was confirmed in additional experiments in which hydrogen tetrachloroaurate (III) was reduced in the presence of PNIPAM microgels.

Hydrophilic–hydrophobic copolymer nano-sized particle gels: Swelling behavior and dependence on crosslinker chain length

Poly N-isopropylacrylamide-co-ethylacrylate [P(NIPAM-co-EA)], Poly N-isopropylacrylamide-co-2-hydroxyethylmethacrylate [P(NIPAM-co-HEMA)], and Poly N-isopropylacrylamide-co-2-hydroxyethylacrylate [P(NIPAM-co-HEA)] nano-sized particle copolymer hydrogels were synthesized to investigate their volume phase transition behavior. Increasing the hydrophobic or hydrophilic monomer content of hydrogels led to uniform changes in transition temperature and swelling ratio. Different statistical thermodynamic models are presented to describe the swelling behavior of copolymer gels. The classical interaction energy parameter in the thermodynamic model, which is not suitable for accurately representing the equilibrium swelling of a copolymer hydrogel, was modified semi-empirically in order to reflect the individual contribution of each hydrogel constituent. In addition, we investigated the volume phase transition of PNIPAM gels as a function of crosslinker chain length. The crosslinkers used in this study were N,N′-methylenebisacrylamide (BIS) and two different molecular weights of Poly(ethyleneglycol) diacrylate (PEGDA, 575 and 700). Interestingly, the use of a long chain crosslinker resulted in a decreased volume change compared to that of a short chain crosslinker without any significant changes in transition temperature. To understand this unexpected result, we used a molecular dynamics simulation for BIS and PEGDA 575 to demonstrate the micro-scale conformation change of crosslinker chain mixed with water or ethanol. The proposed thermodynamic models represent the swelling behavior of the copolymer hydrogels and PNIPAM gels with different length crosslinkers.

Continuous Porous Poly(N-isopropylacrylamide) Gels Prepared from a Bicontinuous Microemulsion

Abstract Continuous porous poly(N-isopropylacrylamide) (pNIPAM) gels were prepared by radical polymerization in a bicontinuous microemulsion (BME) consisting of saline and toluene microphases. The BME gels demonstrated good thermoresponsive water swelling–shrinking performance, exhibiting high speed and large volume change compared with those of homogeneous pNIPAM gels. The BME gels also exhibited swelling with toluene, while the homogeneous pNIPAM gels could not be swollen with toluene.

Improved stimuli-response and mechanical properties of nanostructured poly(N-isopropylacrylamide-co-dimethylsiloxane) hydrogels generated through photopolymerization in lyotropic liquid crystal templates

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are widely studied stimuli-responsive systems due to their significant volume changes at biologically relevant temperatures and a potential wide range of applications including drug delivery, cell cultures, chemical sensors, and desalination. The successful performance of PNIPAM gels often requires a rapid response rate with a significant degree of deswelling when heated above the lower critical solution temperature. However, it is often difficult to design PNIPAM hydrogels with appropriate mechanical strength for the gels to remain functional in a working environment. Herein, lyotropic liquid crystals (LLCs) are utilized to generate a hexagonal nanostructure in PNIPAM hydrogels in order to improve material properties and transport characteristics. Cross-linked methacrylated poly(dimethylsiloxane) (PDMS) was incorporated into PNIPAM gels at varying concentrations through photopolymerization in the hexagonal LLC phase in order to modulate mechanical properties. The hexagonal LLC nanostructure dramatically increases the hydrogel deswelling rate compared to traditional isotropic PNIPAM–PDMS hydrogels of the same chemical composition. Additionally, the ordered LLC structure allows for considerable incorporation of PDMS into the hydrogel without significantly decreasing the water content of the gels. Interestingly, the hexagonal nanostructured hydrogels exhibit similar compressive moduli compared to isotropic hydrogel controls despite having considerably higher water content. These results may be utilized to generate stimuli-sensitive hydrogels with an appropriate rate and degree of deswelling while maintaining necessary mechanical integrity of the gel for use in numerous biomedical and industrial applications.

Study on the novel rare-earth nanocrystals/PNIPAM complex hydrogels prepared by surface-initiated living radical polymerization

A series of novel rare-earth nanocrystals/PNIPAM complex hydrogels were prepared by surface-initiated living radical polymerization with different contents of NaYF4:Eu3+ nanocrystals. The microstructure and performance of the NaYF4:Eu3+ nanocrystals and the complex hydrogels were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC) and photoluminescence (PL). The thermosensitive fluorescence behaviors of the complex hydrogels and the mechanisms were investigated. The results suggested that the nanocrystals were incorporated into PNIPAM gels with covalent bonding and the fluorescence performance of complex hydrogels was influenced greatly by either the ambient temperature or the content of nanocrystals. The PL intensity of the complex hydrogels increased with increasing temperature. The higher content of nanocrystals in the complex hydrogels, the sharper the transitions of volume phase variation around low critical solution temperature (LCST).

A theoretical model for thermal-sensitive microgel with pnipam core and elastic shell

Poly (N-isopropylacrylamide) (PNIPAM) microgels are widely used in drug delivery due to their fast response to temperature. In order to get a better biocompatibility, PNIPAM microgels are typically coated with a layer of biocompatible material, resulting in composite microgels with core-shell structure. In a composite microgel prepared recently, for example, a microsphere of PNIPAM gel is enclosed by a phospholipid membrane, and the composite microgel exhibits a substantial volume transition in response to temperature changes. Here we develop a theoretical model to describe the thermal-responsive behavior of this composite microgel. In particular, we treat the phospholipid membrane as an elastic layer behaving like rubber-like elastomers and adopt the form of the free-energy function for nematic gels (which refer to anther species of thermal-sensitive gels whose behavior has been intensively studied) as that for PNIPAM gels. We show that the thermal-responsive behavior of the composite microgel can be markedly influenced by the membrane. By investigating the state of stress on the interface, we further predict that when the coating membrane is stiff and thin, wrinkles are expected to occur on the outer surface of the composite microgel after the volume transition.

Fractal Structures of the Hydrogels Formed in Situ from Poly(N-isopropylacrylamide) Microgel Dispersions

Dispersions of poly(N-isopropylacrylamide) (PNIPAM) microgel thermally gel in the presence of inorganic salts. The in situ-formed hydrogels, with a network of soft particles, represent a new type of colloidal gels. Here, their fractal structures were determined by rheological measurements, using the models of both Shih et al. and Wu and Morbidelli. According to the definition of Shih et al., the colloidal PNIPAM gels fall into the strong-link regime. Yet the calculated fractal dimension of the floc backbone, x, yielded unrealistic negative values, suggesting this model is inapplicable for the present system. The Wu-Morbidelli model gives physically sounder results. According to this model, the strengths of the inter- and intrafloc links are comparable, and the in situ-formed gels are in the transition regime. The fractal dimension, d(f), of the hydrogel decreases from ∼2.5 to ∼1.8 when the heating temperature increases from 34 to 40 °C. The d(f) values suggest different aggregation mechanisms at different temperatures, that is, a reaction-limited one accompanied by rearrangement at low temperature, a typical reaction-limited one at the intermediate temperature, and a diffusion-limited one at high temperature. With increasing salt concentration, the d(f) of the hydrogel decreases from ∼2.1 to ∼1.7, suggesting the aggregation mechanism changes from reaction-limited to diffusion-limited. The effects of both temperature and salt concentration can be explained by the changes in the interactions among the microgel particles. The thermogellable PNIPAM microgel dispersions may serve as a model system for the study of heat-induced gelation of globular proteins.

Synthesis, characterization, and drug release properties of poly(N‐isopropylacrylamide) gels prepared in methanol–water cononsolvent medium

AbstractPoly(N‐isopropylacrylamide) (PNIPAM) hydrogels were simply prepared by free radical polymerization in different methanol–water mixture. A scanning electron microscopy study revealed that the freeze‐dried hydrogels were macroporous. The swelling ratios in water at 20°C of the resulting hydrogels followed the order: X0.43>X0.21>X0.76 ≈ X0.57>X0.31>X0.13>X0.06>X0, where Xm denotes a gel prepared in a methanol–water mixture with m mole fraction of methanol (xm). Below the lower critical solution temperature, the swelling ratio values of all of the hydrogels gradually decreased with the increase in the temperature. The complete collapse of the PNIPAM chain of all the gels occurred at about 38°C, whereas the same was observed at about 35°C for the conventional gel prepared in water. The swelling ratio values of all the PNIPAM gels in the methanol–water mixtures with different xm values at 20°C passed through a minimum in the cononsolvency zone. The deswelling rates of the hydrogels decreased in the following order: X0.43> X0.31> X0.21> X0.57> X0.76 ≈ X0.13> X0.06> X0. The reswelling rates of these hydrogels decreased in the following order: X0> X0.31> X0.06 ≈ X0.13 > X0.76> X0.57> X0.21> X0.43. The release rates of the Tramadol Hydrochloride drug at 37°C from the drug‐loaded hydrogels were almost same for all of the hydrogels. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012