Fabrication and characterization of microgel-impregnated, thermosensitive PNIPAAm hydrogels

Poly(N-isopropylacrylamide)/poly(ethylene glycol) diacrylate (PNIPAAm/PEG-DA) microgels were used as an additive during the polymerization and/or crosslinking of PNIPAAm hydrogels to improve their thermosensitive properties. The influence of this additive on the property of resulting PNIPAAm hydrogels was investigated and characterized. The interior morphology by scanning electron microscopy (SEM) revealed that microgel impregnated PNIPAAm hydrogels have tighter and constrained porous network structures, although large cavities of 30-40 μm in diameter, occupied by the microgels were sporadically distributed in this constrained network. Differential scanning calorimetry (DSC) studies did not show apparent difference in lower critical solution temperature (LCST) between normal and microgel-impregnated PNIPAAm hydrogels. The incorporating of PNIPAAm/PEG-DA microgels, however, significantly improved mechanical properties of modified hydrogels when comparing with a normal PNIPAAm hydrogel, although the tendency was not strictly proportional to the microgel amount. Based on the temperature-induced swelling ratio data as well as response kinetics, microgel-impregnated hydrogels exhibited improved thermosensitive characteristics in terms of higher equilibrium swelling ratio as well as faster response rates and the level of improvement depended on the amount of microgel impregnated.     

Photo cross-linkable poly(N-isopropylacrylamide) copolymers III: micro-fabricated temperature responsive hydrogels

Micro-fabricated temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were produced by photolithographic patterning of photo cross-linkable polymers. These polymers were synthesized by copolymerization of N-isopropylacrylamide (NIPAAm) and 2-(dimethyl maleimido)-N-ethyl-acrylamide (DMIAAm). The patterning process of polymers with 9.2 mol% DMIAAm and film thickness below 5 μm in the dry state was able to depict a lateral resolution of 4 μm with insignificant shape change. In order to increase the adhesion of the swollen hydrogels, and thus, the resolution of a particular pattern, a special adhesion promoter based on a monochlorosilane anchor group and a chromophore head group was synthesized. If a silicon wafer surface was pretreated with the adhesion promoter, the structures were stable and well adhered even at lower cross-linking densities. The hydrogels are suitable as working substances for micro-actuators because of their thermally induced volume changes. The swelling ratio of the pattern at low temperatures increased with a decreased cross-linking density. As expected from the chemical composition of the gels, the phase transition temperature (T_c) decreased with increasing DMIAAm content. The swelling of microstructures in water in comparison to macroscopic objects occured significantly faster. This behavior was attributed to the small gel dimension but it was even more pronounced because of the sponge-like nanostructure of the hydrogels characterized by high-resolution field emission scanning electron microscopy. Suitable applications of these hydrogels are adjusting limbs in fluid micro-systems such as micro-pumps and micro-valves.     

Synthesis and characterization of pH-sensitivity semi-IPN hydrogel based on hydrogen bond between poly(N-vinylpyrrolidone) and poly(acrylic acid)

A novel spherically shaped semi-interpenetrating network (semi-IPN) hydrogel, which is based on hydrogen bond between chemical crosslinked poly(N-vinylpyrrolidone) (PVP) and linear poly(acrylic acid) (PAA), was prepared. The semi-IPN hydrogel was synthesized by three steps: (1) linear PAA with different molecular weights were obtained by a reaction of free radical polymerization used 2,2′-azo-bis-iso-butyronitrile (AIBN) as an initiator; (2) crosslinked PVP bead was obtained by a reaction of N-vinylpyrrolidone with AIBN used as an initiator and N,N′-methylene-bis-acrylamide (NNMBA) used as a crosslinker by the way of suspension polymerization; (3) complexation occurred between suitable amount of aqueous solution of PAA and the porous PVP bead and was stabilized by multiple frost-defrost, from this step the semi-IPN hydrogel was obtained. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) proved the presence of the hydrogen bond in the hydrogel. The swelling behaviour of the hydrogel was studied in buffer solution with different pH and NaCl aqueous solution. The results showed that the semi-IPN hydrogel had excellent pH-sensitivity in the range of pH from 2.25 to 4.00 and the small molecule salt had little influence on the swelling behaviour of the semi-IPN hydrogel over the range of concentration of NaCl aqueous solution investigated. The results were confirmed further by scanning electron microscope (SEM). The mechanism of swelling and deswelling was discussed.     

Small angle neutron scattering studies on structural inhomogeneities in polymer gels: irradiation cross-linked gels vs chemically cross-linked gels

A comparison of network structure in a solvent was made for two types of poly(N-isopropylacrylamide) gels cross-linked by chemical reaction with N,N′-methylenebisacrylamide (BIS) (chemical gels) and by γ-ray irradiation (γ-ray gels). The cross-linking density dependence for these gels was examined by small angle neutron scattering (SANS). The SANS results indicated an increase of frozen inhomogeneities with an introduction of cross-links for both chemical and γ-ray gels. However, it was found that the effect of cross-linking is much stronger in the chemical gels than in the γ-ray gels. The differences in the structure were successfully interpreted by a statistical-mechanical theory of gels proposed by Panyukov-Rabin (Phys. Rep. 269 (1996) 1). The degree of polymerization between cross-links, N, was a decreasing function of cross-linking content for both types of gels, while that for the γ-ray gels was a weak function of irradiation dose. Quantitative analyses on BIS concentration and γ-ray dose dependence led to an experimental evidence of the existence of cross-linking saturation threshold.     

Synthesis and properties of thermosensitive, crown ether incorporated poly(N-isopropylacrylamide) hydrogel

A crown ether derivative (4′-allyldibenzo-18-crown-6, CE) was covalently incorporated into the network of temperature sensitive poly(N-isopropylacrylamide) (PNIPA) hydrogels by copolymerization in a mixed solvent of water and tetrahydrofuran (H₂O/THF). The poly(N-isopropylacrylamide-co-4′-allyldibenzo-18-crown-6) (poly(NIPA-co-CE)) hydrogels exhibited dramatically faster deswelling rates than normal PNIPA hydrogels at a temperature (50 °C) above their lower critical solution temperatures. The effect of the solvent component ratio in the mixed solvent during the copolymerization on the swelling properties of the poly(NIPA-co-CE) hydrogel was investigated. The thermosensitive poly(NIPA-co-CE) hydrogels have potential applications in the extraction of cations and separation of chiral drugs.     

Simple method for preparation of ultra-thin poly(N-isopropylacrylamide) hydrogel layers and characterization of their thermo-responsive properties

A reactive polymer of N-isopropylacrylamide (co-PNIPAAm) was prepared by copolymerization with a vinyl monomer carrying a protected isocyanate group. Thin layers of co-PNIPAAm were formed on a surface carrying hydroxyl groups using a spin-coater and were cross-linked and immobilized on the surface by heating. These layers were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and surface plasmon resonance (SPR). The thickness of the co-PNIPAAm layers could be controlled from 4 to 35 nm by changing the concentration of the polymer solutions and the spin-coating conditions. Very little amount of γ-globulin was adsorbed onto the co-PNIPAAm layers even when thicknesses as low as 4 nm were analyzed. In addition, the swelling ratios of the co-PNIPAAm layers were about 4.6 and 1.3 at 25 °C and at 40 °C, respectively. These results indicate that the co-PNIPAAm can be very useful in preparing thermo-sensitive layers on substrates and thus various biomedical applications may be useful.     

Synthesis and thermo-responsive behavior of fluorescent labeled microgel particles based on poly(N-isopropylacrylamide) and its related polymers

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) as an intramolecular fluorescent probe were prepared by emulsion polymerization. The thermo-responsive behavior of the VDP-labeled PNIPAM microgel particles dispersed in water was studied by turbidimetric and fluorescence analyses. The transition temperature of the VDP-labeled PNIPAM microgel particles in water determined by turbidimetric analysis was ca. 32.5 °C. The wavelength at the maximum fluorescence intensity of the VDP units linked directly to the microgel particles dramatically blue-shifted around the transition temperature. In addition it gradually blue-shifted even below the transition temperature where there was no change observed in the turbidity. These findings suggest that the gradual shrinking of microgel particles occurs with increasing temperature and the subsequent dramatic shrinking results in the increasing in the turbidity. The transition temperatures of VDP-labeled poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) microgel particles determined by turbidimetric analysis were ca. 23 and ca. 42.5 °C, respectively, and their thermo-responsive behavior was similar to that for the VDP-labeled PNIPAM system. In these three systems the microenvironments around the fluorescent probes above the transition temperatures became more hydrophobic than those below the transition temperature, and the estimated values of microenvionmental polarity around the VDP units on their collapsed states were almost the same.     

Novel triblock copolymers synthesized via radical telomerization of N-isopropylacrylamide in the presence of polypseudorotaxanes made from thiolated PEG and α-CDs

A protocol for the preparation of novel triblock copolymers comprising a polyrotaxane center block and outer blocks of poly(N-isopropylacrylamide) (PNIPAAm) as bulky stoppers was developed, in which N-isopropylacrylamide was allowed to telomerize in the presence of polypseudorotaxanes made from the self-assembly of thiol end-capped PEG with a varying amount of α-CDs under UV irradiation in aqueous solution. The molecular structure of the resulting copolymers was characterized in detail by ¹H NMR, FTIR, XRD, TG and DSC analyses. It was demonstrated that the PNIPAAm blocks are successfully attached to the two terminals of the polypseudorotaxanes and each block having the minimum 7 NIPAAm units seems long and bulky enough to efficiently impede the dethreading of α-CDs from the PEG axle to give rise to the triblock polyrotaxane-containing copolymers.     

Synthesis of rapid responsive gels comprising hydrophilic backbone and poly(N-isopropylacrylamide) graft chains by RAFT polymerization and end-linking processes

A thermosensitive poly(N-isopropylacrylamide) (PNIPAM) grafted gel, which comprises hydrophilic backbone and freely mobile PNIPAM graft chains, was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization and end-linking processes. Functional PNIPAM bearing dithiobenzoate end group (-C(S)S-R) was prepared first, and then it was reacted with divinyl compounds to obtain gel. In order to adjust the composition of the gels, two divinyl compounds, N,N-methylenebisacrylamide (BIS) and poly(ethylene glycol) diacrylate (PEGDAC), were used. The cross-linking polymerization mechanism was proposed. The swelling and deswelling kinetics of the hydrogels were measured. The gels exhibit rapid deswelling kinetics. At the same time, they show rapid swelling kinetics within 30 min, whereas a conventional PNIPAM-co-PEG-co-BIS gel with the same feed composition requires more than 10 h to reach swelling equilibrium.     

Effect of the cross-linking degree on the morphology of poly(NIPAAm-co-AAc) hydrogels

Hydrogels of poly(N-isopropylacrylamide) co-polymerized with acrylic acid [P(NIPAAm-co-AAc)] were synthesized with cross-linking degrees of 2-7% using (N,N′-methylenebisacrylamide). SEM micrographs revealed that the morphology of dry hydrogels changes from interconnected spherical pores to channel-like pores, with the change in the cross-linking degrees from 3 to 5%. The change in morphology is associated with a significant change in the swelling ratio. It was found that the diffusion rates and permeabilities of methylene blue (MB) through the hydrogel with channel-like pores are significantly higher if the main axes of the pores are oriented parallel to the flow of MB molecules, than if it is oriented perpendicularly. These results show that different morphologies can be obtained by controlling the cross-linking degree of P(NIPAAm-co-AAc) hydrogels in a narrow range around 5% and by performing the polymerization reaction in moulds placed in horizontal and vertical positions, opening a new perspective for modulating their properties in applications as matrices for controlled release of drugs or as membranes for separation processes.     

Thermoreversible hydrogels VI: Swelling behavior of the (N-isopropylacrylamide-co-diethyl methyl methacryloyloxyethyl ammonium iodide) copolymeric hydrogels in aqueous salt solutions

A series of N-isopropylacrylamide/diethyl methyl methacryloyloxyethyl ammonium iodide (NIPAAm/DEMMAI) copolymeric gels were prepared from blending NIPAAm, cationic monomer DEMMAI, and N,N′-methylene bisacrylamide (NMBA) in various molar ratios in this article. The effects of the amount of the cationic monomer in the copolymeric gels on the swelling behaviors in water and various saline solutions at various temperatures were investigated. Results showed that the swelling ratios of copolymeric gels were significantly larger than those of pure NIPAAm gel, and that the more the DEMMAI content, the higher the gel transition temperature. In the saline solution, results showed that the swelling ratio for pure NIPAAm gel had not changed significantly with an increase of the salt concentration until the salt concentration was larger than 0.1 M. The swelling ratios for the copolymeric gels NIPAAm/DEMMAI were decreased with increasing salt concentration. In various saline solutions, results showed that the anionic effects were greater than cationic effects in the presence of common anion with different cations and common cation with different anions for these hydrogels. Finally, we also tested the reversibility of the NIPAAm/DEMMAI copolymeric gels. The deswelling and reswelling kinetics were dependent on the temperature which was below or above the gel transition temperature. The gel with a small DEMMAI content has a good reversibility.     

A quantitative analyses of the viscometric data of the coil-to-globule and globule-to-coil transition of poly(N-isopropylacrylamide) in water

The coil-to-globule and globule-to-coil transition of poly(N-isopropylacrylamide) in aqueous solution had been studied by heating and cooling the sample solution with conventional viscosity measurement. A single chain collapsed globule solution was prepared firstly by adding sodium n-dodecyl sulfate (SDS) into the polymer solution at room temperature, as the chain collapsed to compact globule at higher temperature and then the SDS was removed by electro-dialysis. The viscosity data were analyzed in a quantitative way, which permitted to elucidate the transition temperature and the amount of the water in the collapsed globule precisely.     

Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature

Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d₀, at 20 °C was found to be strongly dependent on the preparation temperature, T_prep, where d and d₀ are the diameter of gel during observation and preparation, respectively. The value of d/d₀ was about 1.2 for T_prep=20 °C, but steeply increased by approaching the phase separation temperature ≈32.0 °C. Above 32.0 °C, d/d₀ decreases stepwise to 1.46. This upturn in d/d₀ was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T_prep. Though the shrinking half-time, t_1/2, of gel was on the order of 500 min for T_prep≤20 °C, t_1/2 decreased to 2 min for T_prep≥26 °C. Hence, a rapid shrinking was attained by simply increasing T_prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.     

Synthesis of graft polyacrylamide with responsive self-assembling properties in aqueous media

Graft copolymers with water-soluble backbones were prepared using two different routes: grafting onto and grafting through techniques. The corresponding syntheses, leading to ionic or non-ionic copolymers, are described here in detail and exemplified with various primary structures obtained by changing the chemical nature of the side chains or the grafting ratio. Depending on the polyelectrolyte or neutral character of water-soluble backbones, viscosity-concentration dependences were determined at room temperature in dilute and semi-dilute regimes. Upon heating, PNIPA grafts dehydrate and self-aggregate into hydrophobic microdomains which promote the formation of a physical network above 36 °C. The main features of the thermoassociating properties of the copolymers in aqueous solutions are described using different experimental approaches: rheology, calorimetry, NMR spectroscopy and neutron scattering.     

Micellization and phase transition behavior of thermosensitive poly(N-isopropylacrylamide)-poly(?-caprolactone)-poly(N-isopropylacrylamide) triblock copolymers

Thermosensitive triblock copolymers with two hydrophilic poly(N-isopropylacrylamide) blocks flanking a central hydrophobic poly(?-caprolactone) block were synthesized by atom transfer radical polymerization. Core-shell micellization of the triblock copolymers was inferred from the ¹H NMR spectra derived in two different solvent environments (CDCl₃ and D₂O). The micellar characteristics of these amphiphilic triblock copolymers were studied by pyrene fluorescence techniques, dynamic light scattering and transmission electron microscopy. The critical micelle concentrations of the triblock copolymers were in the range of 4-16 mg/L and the partition coefficients were in the range of 3.10 × 10⁴ to 2.46 × 10⁵. The mean diameters of the micelles, measured by light scattering, were between 90 and 120 nm. The temperature sensitivity of the triblock copolymers was demonstrated by the phase transition of a 250 mg/L aqueous polymer solution at the lower critical solution temperature (LCST). The enthalpy of the phase transition was determined by differential scanning calorimetry. PM3 quantum mechanical calculation method was used to understand the intermolecular interactions between the copolymer and the water molecules. A modular approach was used to simulate the phase transition observed at the LCST.     

Synthesis and thermally responsive characteristics of dendritic poly(ether-amide) grafting with PNIPAAm and PEG

A series of thermally responsive dendritic core-shell polymers were prepared based upon dendritic poly(ether-amide) (DPEA), modified with carboxyl end-capped linear poly(N-isopropylacrylamide) (PNIPAAm-COOH) or both PNIPAAm-COOH and carboxyl end-capped methoxy polyethylene glycol (PEG-COOH) in different ratios via an esterification process to obtain DPEA-PNIPAAm or DPEA-PNIPAAm-PEG. Their molecular structures were verified by gel permeation chromatography, and ¹H NMR and FTIR spectroscopy. The temperature-dependent characteristics study has revealed that DPEA-PNIPAAm exhibits a lower critical solution temperature (LCST) of about 34 °C, whereas DPEA-PNIPAAm-PEG polymers with the PNIPAAm/PEG ratio of about 1.0 and 0.4 possess about 36 °C and 39 °C, respectively, compared with 32 °C for homopolymer PNIPAAm. The critical aggregation temperature was investigated using fluorescence excitation spectrum of pyrene as a sequestered guest molecule based upon the sharp increase of the I₃₃₈/I₃₃₃ value.     

Preparation and evaluation of thermo-reversible copolymer hydrogels containing chitosan and hyaluronic acid as injectable cell carriers

Poly(N-isopropylacrylamide) end-capped with a carboxyl group (PNIPAM-COOH) was grafted to chitosan for synthesizing thermo-reversible chitosan-g-poly(N-isopropylacrylamide) (CPN), which was further grafted with hyaluronic acid (HA) to form hyaluronic acid-g-CPN (HA-CPN). PNIPAM-COOH, CPN and HA-CPN formed injectable free-flowing aqueous solutions and exhibited reversible sol-to-gel phase transition (above 5% polymer concentration) at 30 °C. Chemical properties and temperature-dependent physical properties of the polymer hydrogels, such as rheological behavior, phase transition kinetics, and water content were characterized in detail. The mechanical stiffness of hydrogels increased with the presence of chitosan in the copolymer, but decreased after conjugation with HA. Chitosan and HA grafting also endowed higher water content and resistance to volume contraction during phase change of the copolymer solution. In vitro cell culture experiments with chondrocytes and meniscus cells in HA-CPN hydrogel showed beneficial effects on the cell phenotypic morphology, proliferation, and differentiation. Progressive tissue formation was demonstrated by monotonic increases in extracellular matrix contents and mechanical properties.     

Control of the lower critical solution temperature—type cononsolvency properties of poly(N-isopropylacrylamide) in water—dioxane mixtures through copolymerisation with acrylamide

The behaviour of the homopolymers poly(N-isopropylacrylamide) (PNIPAM), polyacrylamide (PAM) and random copolymers of N-isopropylacrylamide (NIPAM) with acrylamide (AM) was studied by turbidimetry and viscometry in mixtures of water with dioxane. It was found that the well-known lower critical solution temperature-type cononsolvency properties of PNIPAM in water-dioxane mixtures, observed in the water-rich region, can be effectively controlled by copolymerisation of NIPAM with AM. Thus, the cononsolvency properties of the copolymers in water-dioxane mixtures are shifted to higher temperatures and restricted within a narrower solvent composition region as the acrylamide content of the copolymers increases. A significant decrease of the reduced viscosity of the systems exhibiting phase separation properties was observed upon heating, indicative of the collapse of the (co)polymer chains as temperature approaches the corresponding cloud point temperature. Furthermore, when temperature is fixed close to the cloud point temperature, the reduced viscosity decreases with increasing the volume fraction of dioxane, φ, as far as the solvent mixtures are rich in water. On the contrary, the reduced viscosity of PNIPAM in dioxane-rich mixtures is found significantly higher, indicative of an expansion of the polymer chain, as compared to the reduced viscosity of this polymer in the two pure solvents.     

Toward an understanding of thermoresponsive transition behavior of hydrophobically modified N-isopropylacrylamide copolymer solution

Poly(N-isopropylacrylamide-co-vinyl laurate)(PNIPAAm-co-VL) copolymers were prepared at various feed ratios via conventional radical random copolymerization. The formation, composition ratios and molecular weight of copolymers were examined. The thermoresponsive behaviors of PNIPAAm and PNIPAAm-co-VL solutions at low and high concentrations were intensively investigated by turbidity measurement, Micro-DSC, temperature-variable state fluorescence, ¹H NMR and dynamic light scattering (DLS). Several important results were obtained that (1) incorporation of PVL results in much lower and broader LCST regions of the copolymer solutions, and facilitates the formation of hydrophobic microdomains far below LCST, causing a pronounced aggregation in solutions (2) temperature-variable ¹H NMR spectra shows that during the phase transition, the ‘penetration’ of PNIPAAm into the hydrophobic core is a process accompanied with a transition of isopropyl from hydration to dehydration as well as a self-aggregation of hydrophobic chains at different temperature stages (3) according to the ¹H NMR spectra of polymer solutions obtained at varied temperatures, the microdomains from hydrophobic VL moieties have a different accessibility for isopropyl groups and the entire chains during phase transition (4) temperature-variable DLS demonstrates that the temperature-induced transition behavior of copolymers is supposedly divided into three stages: pre-LCST aggregation (<20 °C), coil-globule transition at LCST (20-25 °C) and post-LCST aggregation (>25 °C).