Preparation and characterization of N-isopropylacrylamide/acrylic acid copolymer core–shell microgel particles

A New Design of Poly(N-Isopropylacrylamide) Hydrogels Using Biodegradable Poly(Beta-Aminoester) Crosslinkers as Fertilizer Reservoirs for Agricultural Applications

Poly(N-isopropylacrylamide) (P(NIPAAm)) hydrogels were prepared by free-radical polymerization with biodegradable poly (β-amino ester) (PBAE) crosslinkers at 1 wt% and 3 wt% ratio, and compared with conventional N,N'-methylene bisacrylamide (MBA)-crosslinked hydrogel. The influence of the type, molecular weight, and diacrylate/amine ratio of the crosslinker on the crosslink density, compressive strength, and swelling and biodegradation behavior of the hydrogels was investigated. The hydrogels synthesized with lower molecular weight PBAE crosslinkers showed higher crosslinking degrees and compressive strength and lower swelling ratios. To reveal the controlled release behavior of the fertilizer, KNO₃ was used as the model, and its loading and release behavior from these hydrogels was also examined. The N/T5/1 sample with 1.5/1.0 diacrylate/amine molar ratio and 1 wt% PBAE ratio demonstrated the most controlled release of KNO₃ with 66.9% after 18 days in soil. In addition, the hydrogel with the porosity of 71.65% and crosslinking degree of 2.85 × 10^-5 mol cm^-3 showed a swelling ratio of 69.44 g/g, biodegradation rate of 23.9%, and compressive strength of 1.074 MPa. Thus, it can be concluded that the new designed biodegradable P(NIPAAm) hydrogels can be promising materials as nitrate fertilizer reservoirs and also for controlled fertilizer release in soil media for agricultural applications.

Biomimetic Gradient Hydrogel Actuators with Ultrafast Thermo-Responsiveness and High Strength

Most current hydrogel actuators suffer from either poor mechanical properties or limited responsiveness. Also, the widely used thermo-responsive poly-(N-isopropylacrylamide) (PNIPAM) homopolymer hydrogels have a slow response rate. Thus, it remains a challenge to fabricate thermo-responsive hydrogel actuators with both excellent mechanical and responsive properties. Herein, ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogels containing multivalent vinyl functionalized silica nanoparticles (VSNPs) are fabricated. The ultrafast thermo-responsiveness is due to the mobile polymer chains grafted from the surfaces of the VSNPs, which can facilitate hydrophobic aggregation, inducing the phase transition and generating water transport channels for quick water expulsion. In addition, the copolymerization of NIPAM with acrylic acid (AA) decreases the transition temperature of the thermo-responsive PNIPAM-based hydrogels, contributing to ultrafast thermo-responsive shrinking behavior with a large volume change of as high as 72.5%. Moreover, inspired by nature, intelligent hydrogel actuators with gradient structure can be facilely prepared through self-healing between the ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogel layers and high-strength VSNPs-PAA-Fe³⁺ multibond network (MBN) hydrogel layers. The obtained well-integrated gradient hydrogel actuators show ultrafast thermo-responsive performance within only 9 s in 60 °C water, as well as high strength, and can be used for more practical applications as intelligent soft actuators or artificial robots.

Multiresponsive Microgels: Toward an Independent Tuning of Swelling and Surface Properties

Dual-responsive poly-(N-isopropylacrylamide) (PNIPAM) microgels surface-functionalized with polyethylene glycol (PEG) or poly-2-dimethylaminoethyl methacrylate (PDMAEMA) were developed to enable the swelling behavior and surface properties of the microgels to be tuned independently. The thermo-triggered swelling and pH-triggered surface properties of the microgels were investigated in aqueous suspensions using dynamic light scattering and on substrates using the surface forces apparatus. Grafting polymer chains on the microgel surface did not impede the thermo-triggered swelling behavior of the microgels in suspensions and immobilized on substrates. An unprecedented decoupling of the swelling behavior and surface properties could be obtained. More particularly, the thermo-triggered swelling behavior of the PNIPAM underlying microstructure could be tuned below and above the phase transition temperature with no change in the surface potential and adhesion provided by the surface non-responsive PEG.

Triply-responsive OEG-based microgels and hydrogels: regulation of swelling ratio, volume phase transition temperatures and mechanical properties

Facile methods to coordinate swelling ratio, volume-phase transition temperatures and mechanical properties for pH-, thermal-, and cationic-responsive microgels and hydrogels.

Synthesis and Properties of pH-Thermo Dual Responsive Semi-IPN Hydrogels Based on N,N'-Diethylacrylamide and Itaconamic Acid

A series of semi-interpenetrating polymer network (semi-IPN) hydrogels based on N,N'-diethylacrylamide (DEA) and itaconamic acid (IAM) were synthesized by changing the molar ratio of linear copolymer P(DEA-co-IAM) and DEA monomer. Linear copolymer P(DEA-co-IAM) was introduced into a solution of DEA monomer to prepare pH-thermo dual responsive P(DEA-co-IAM)/PDEA semi-IPN hydrogels. The thermal gravimetric analysis (TGA) revealed that the semi-IPN hydrogel has a higher thermal stability than the conventional hydrogel, while the interior morphology by scanning electron microscopy (SEM) showed a porous structure with the pore sizes could be controlled by changing the ratio of linear copolymer in the obtained hydrogels. The oscillatory parallel-plate rheological measurements and compression tests demonstrated a viscoelastic behavior and superior mechanical properties of the semi-IPN hydrogels. Besides, the lower critical solution temperature (LCST) of the linear copolymers increased with the increase of IAM content in the feed, while the semi-IPN hydrogels increased LCSTs with the increase of linear copolymer content introduced. The pH-thermo dual responsive of the hydrogels was investigated using the swelling behavior in various pH and temperature conditions. Finally, the swelling and deswelling rate of the hydrogels were also studied. The results indicated that the pH-thermo dual responsive semi-IPN hydrogels were synthesized successfully and may be a potential material for biomedical, drug delivery or absorption applications. The further applications of semi-IPN hydrogels are being conducted.

Comparing the Relative Interfacial Affinity of Soft Colloids With Different Crosslinking Densities in Pickering Emulsions

Pickering emulsions prepared by various kinds of soft colloids such as the poly(N-isopropylacrylamide) (PNIPAM)-based microgels, have been studied for decades in order to fabricate stimuli-responsive emulsions. It has been generally viewed that the interfacial properties of the microgel monolayers and the emulsion stability are dominated by the softness or deformability of the microgel particles. However, there is still no convenient way to characterize the adsorption/desorption energy of the microgels at the interface although this is an essential topic for microgel-stabilized emulsions. This paper presents a novel method for directly comparing the relative interfacial affinity of microgel particles with comparable size but different crosslinking densities, therefore, different softness at the oil/water interface. Typical micron-sized PNIPAM-based microgels were synthesized and used in this study. With advanced fluorescent labeling techniques, we are capable of distinguishing different kinds of microgels in a Pickering emulsion. During vigorous agitation, particles with higher adsorption energy are more likely to be found at the oil/water interface instead of the loosely adsorbed counterparts. By counting the ratio of interfacial area occupied by two microgels, the interfacial affinity of them can be compared. It is found that interfacial affinity of microgels is not only dependent on the softness but also strongly correlated with the core-shell morphology of the microgels, especially the outmost collapsed polymer layer at the interface. This result is consistent with the interfacial morphology model proposed by other researchers. The understanding of the stabilization of such Pickering emulsions can help us to design and develop responsive Pickering emulsions with better controlled stability.

Temperature-Responsive Polymer Microgel-Gold Nanorods Composite Particles: Physicochemical Characterization and Cytocompatibility

In this paper, we report an easy route for preparing new metal nanorod-polymer composites consisting of gold nanorods, Au NRs, and temperature responsive copolymer "microgel" particles. The microgel particles of ~200 nm in size, which contain carboxylic acid groups, were prepared by surfactant-free emulsion polymerization of a selected mixture made of N-isopropylacylamide and acrylic acid in the presence of a cross-linker N,N'-methylenebisacrylamide. The electrostatic interactions between the cationic cetyltrimethylammonium bromide (CTAB) stabilized Au NRs and anionic microgel particles were expected to occur in order to prepare stable Au NRs-microgel composite particles. The optical and structural characterization of the composite was achieved using UV-Vis spectroscopy, Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM) and dynamic light scattering (DLS). TEM image shows that Au NRs are attached on the surface of the microgel particles. Dynamic light scattering measurements prove that the composite particles are temperature responsive, which means the particles undergo a decrease in size as the temperature increases above its phase transition temperature. In vitro cytotoxicity of the composite materials were tested by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), Lactate dehydrogenase (LDH), and hemolysis assay, which showed non-toxicity (biocompatibility).

Preparation of Uniform-Sized and Dual Stimuli-Responsive Microspheres of Poly(N-Isopropylacrylamide)/Poly(Acrylic acid) with Semi-IPN Structure by One-Step Method

A novel strategy was developed to synthesize uniform semi-interpenetrating polymer network (semi-IPN) microspheres by premix membrane emulsification combined with one-step polymerization. Synthesized poly(acrylic acid) (PAAc) polymer chains were added prior to the inner water phase, which contained N-isopropylacrylamide (NIPAM) monomer, N,N′-methylene bisacrylamide (MBA) cross-linker, and ammonium persulfate (APS) initiator. The mixtures were pressed through a microporous membrane to form a uniform water-in-oil emulsion. By crosslinking the NIPAM in a PAAc-containing solution, microspheres with temperature- and pH-responsive properties were fabricated. The semi-IPN structure and morphology of the microspheres were confirmed by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The average diameter of the obtained microspheres was approximately 6.5 μm, with Span values of less than 1. Stimuli-responsive behaviors of the microspheres were studied by the cloud-point method. The results demonstrated that semi-IPN microspheres could respond independently to both pH and temperature changes. After storing in a PBS solution (pH 7.0) at 4 °C for 6 months, the semi-IPN microspheres remained stable without a change in morphology or particle size. This study demonstrated a promising method for controlling the synthesis of semi-IPN structure microspheres with a uniform size and multiple functionalities.

Soft particles at a fluid interface

Particles added to a fluid interface can be used as a surface stabilizer in the food, oil and cosmetic industries. As an alternative to rigid particles, it is promising to consider highly deformable particles that can adapt their conformation at the interface. In this study we compute the shapes of soft elastic particles using molecular dynamics simulations of a cross-linked polymer gel, complemented by continuum calculations based on linear elasticity. It is shown that the particle shape is not only affected by the Young's modulus of the particle, but also strongly depends on whether the gel is partially or completely wetting the fluid interface. We find that the molecular simulations for the partially wetting case are very accurately described by the continuum theory. By contrast, when the gel is completely wetting the fluid interface the linear theory breaks down and we reveal that molecular details have a strong influence on the equilibrium shape.

Thermo-responsive poly(ethylene-co-vinyl alcohol) based asymmetric membranes

This paper reports surface modification of poly(ethylene-co-vinyl alcohol) microporous membranes, by surface-initiated atom transfer radical polymerization of N-isopropylacrylamide and its subsequent applications in thermo responsive permeation.

Functionalization of single-walled carbon nanotubes with thermo-responsive poly(N-isopropylacrylamide): effect of the polymer architecture

Star-shaped and linear thermo-responsive PNIPAM polymers were compared for the functionalization of SWNTs. The star-shaped polymer gave the SWNTs a higher solubility and more thermo-responsive properties than its linear counterpart.

Enhanced and sustained topical ocular delivery of cyclosporine A in thermosensitive hyaluronic acid-based in situ forming microgels

For nearly a decade, thermoresponsive ophthalmic in situ gels have been recognized as an interesting and promising ocular topical delivery vehicle for lipophilic drugs. In this study, a series of thermosensitive copolymers, hyaluronic acid-g-poly(N-isopropylacrylamide) (HA-g-PNIPAAm), was synthesized, by coupling carboxylic end-capped PNIPAAm to aminated hyaluronic acid through amide bond linkages, and was used as a potential carrier for the topical ocular administration of cyclosporine A (CyA). The lower critical solution temperature of HA-g-PNIPAAm₅₉ in aqueous solutions was measured as 32.7°C, which was not significantly affected by the polymer concentration. Moreover, HA-g-PNIPAAm₅₉ microgels showed a high drug loading efficiency (73.92%) and a controlled release profile that are necessary for biomedical application. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) observations showed that HA-g-PNIPAAm microgels were spherical in shape with homogeneous size. Based on the result of the eye irritation test, the HA-g-PNIPAAm microgels formulation was shown to be safe and nonirritant for rabbit eyes. In addition, HA-g-PNIPAAm microgels achieved significantly higher CyA concentration levels in rabbit corneas (1455.8 ng/g of tissue) than both castor oil formulation and commercial CyA eye drops. Therefore, these newly described thermoresponsive HA-g-PNIPAAm microgels demonstrated attractive properties to serve as pharmaceutical delivery vehicles for a variety of ophthalmic applications.

HUMIDITY SENSOR USING NIPAAm NANOGEL AS SENSING MEDIUM IN SAW DEVICES

In this paper we report poly N-isopropylacrylamide (NIPAAm) nanogel used as sensing medium in Rayleigh wave type surface acoustic wave (SAW) devices to measure relative humidity. NIPAAm is synthesized by single-step surfactant-free emulsion polymerization reaction method. The prepared nanogel particles have dimensions less than 180 nm. Dynamic light scattering (DLS) analysis of these particles shows increase in size of the particles up to 100% when dispersed in water at room temperature. In this work we have developed two types of relative humidity (RH) sensors. In the first type commercial SAW resonators with resonance frequency at 315 MHz from EPCOSTM were used in our experiment. The casings of two resonators were removed carefully and one of them was left as such to serve as the reference device. The other was coated with the NIPAAm nanogel and used as the sensor. In the second type of SAW-based humidity sensor aluminum interdigital transducers (IDT) are fabricated over YZ lithium niobate piezoelectric substrate using metallization and lithography techniques. The fabricated IDTs have the pitch length of 34 μm and a resonance frequency of 51.6 MHz. Response of sensor to RH is measured by measuring the scattering parameters of both the types of sensors and reference devices recorded using the network analyzer.

Synthesis and properties of polyelectrolyte microgel particles

A series of cationic poly(N-isopropylacrylamide/4-vinylpyridine) [poly(NIPAM/4-VP)] polyelectrolyte co-polymer microgels have been prepared by surfactant free emulsion polymerization (SFEP) with varying compositions of 4-VP and NIPAM. The compositions of 4-VP were 15, 25, 35, 45, 55wt.% relative to NIPAM. The temperature and pH responsive swelling-deswelling properties of these microgels have been investigated using dynamic light scattering (DLS) and electrophoretic mobility measurements. DLS results have shown that the particle diameter of the poly(NIPAM/4-VP) microgels decreases with increasing concentration (wt.%) of 4-VP over the 20-60 degrees C temperature range due to the increased amount of hydrophobic group. The particle size of all poly(NIPAM/4-VP) microgel series increases with decreasing pH, as the 4-VP units become more protonated at low pH below the pK(a) (5.39) of the monomer 4-VP. Electrophoretic mobility results have shown that electrophoretic mobility increases as the temperature/pH increases at a constant background ionic strength (1x10(-4)mol dm(-3) NaCl). These results are in good agreement with DLS results. The temperature/pH sensitivity of these microgels depends on the ratio of NIPAM/4-VP concentration in the co-polymer microgel systems. The combined temperature/pH responsiveness of these polyelectrolyte microgels can be used in applications where changes in particle size with small change in pH or temperature is of great consequence.