PNIPAM Gels Research Articles

Colloidal crystallization of thermo-sensitive gel spheres of poly (N-isopropyl acrylamide). Influence of gel size

Influence of the gel size on the morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (pNIPAm), was discussed by adding the data of two gel samples of pNIPAm(400–5) and pNIPAm(600–5) of 412 nm (at 25 °C) and 220 nm (at 45 °C) and of 517 nm (at 20 °C) and 294 nm (at 45 °C), respectively. Colloidal single crystals formed, but not so large compared with the giant crystals of small pNIPAm gels reported previously. The suspensions even with ion-exchange resins were turbid and hard to observe the single crystals clearly with the naked eyes as gel size increased. The critical concentration of melting decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. The critical concentration increased as the gel size increased and/or dispersion temperature increased. Density of the gel spheres increased as their size increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Contribution of the electrical double layers on the crystallization increased sharply as temperature increased and gel concentration decreased, respectively. The contribution also increased slightly as sphere size increased, when comparison was made at the same gel concentration in wt.%. The present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed between the water phase and gel spheres, though the gel spheres contain a lot of water molecules at the inner sphere region.

Synthesis and characterization of porous poly(N‐isopropylacrylamide) hydrogels prepared in ethanol–water mixtures

AbstractPoly(N‐isopropylacrylamide) (PNIPAM) hydrogels were prepared by free‐radical polymerization in different ethanol–water mixtures. A scanning electron microscopy study revealed that the resulting hydrogels were macroporous. The swelling ratios of the resultant hydrogels in water at 20°C followed this order: X0.34 ≈ X0.68 > X0.48 > X0.09 > X0.04 > X0, where Xa denotes a gel prepared in an ethanol–water solvent mixture with an ethanol molar fraction of a. Below the lower critical solution temperature, the swelling ratio values of all of the hydrogels gradually decreased with increasing temperature. The complete collapse of the PNIPAM chain of all of these 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 of the PNIPAM gels with different molar fractions of ethanol at 20°C passed through a minimum in the cononsolvency zone. The deswelling rates of the hydrogels decreased in the following order: X0.34 > X0.48 > X0.68 > X 0.09 > X0.04 > X0. The reswelling rates of these hydrogels decreased in the following order: X0 > X0.04 ≈ X0.48 > X0.09 ≈ X0.68 > X 0.34. The freeze‐drying process decreased the swelling ratios but increased the deswelling and reswelling properties of the PNIPAM gels. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Cooperative Hydration Induces Discontinuous Volume Phase Transition of Cross-Linked Poly(N-isopropylacrylamide) Gels in Water

The concept of cooperative dehydration, defined as the simultaneous dissociation of the bound water molecules from the polymer chains in correlated sequences, is applied to study the collapse of cross-linked poly(N-isopropylacrylamide) (PNIPAM) gels in aqueous solutions upon heating above the critical temperature. We examined the applicability of this concept in two situations: (i) free swelling of PNIPAM gels in water (in the absence of applied force), (ii) swelling of PNIPAM gels in water subjected to a uniaxial tension. The transition becomes sharper as the cooperativity parameter σ of hydration decreases (increasing cooperativity), and there is a critical value σc ≃ 0.30 of the cooperativity parameter below which the volume transition is discontinuous. Hence, the real phase transition with thermodynamic discontinuity is shown to be possible for the nonionic polymer gels if the hydration is sufficiently cooperative. The calculations of the swelling ratio and the spinodal lines are compared with the experimental data of PNIPAM gels by Hirotsu for point i and by Suzuki for point ii.

Variation of Elastic Properties of Responsive Polymer Nanotubes

The mechanical properties of thermo-responsive nanoribbons made of poly(N-isopropylacrylamide)-block-poly(ethylene terephthalate) (PNIPAM-b-PET) have been investigated. The nanoribbons are produced by grafting PNIPAM chains from the walls of nanopores of PET track-etched membranes, by dissolving the membranes. The swelling ratio and elastic modulus of the nanoribbons are evaluated as a function of polymer chain length and temperature, from atomic force microscopy (AFM) images and using the thermodynamic theory of swelling. Whereas the elastic properties of nanoribbons are similar to those of PNIPAM gels and brushes when the PNIPAM chain length is low, they increase by 2 orders of magnitude when the chain length becomes similar to the diameter of the nanopores used to synthesize them. This indicates that PNIPAM brushes synthesized in severe conditions of confinement are strongly cross-linked, either due to irreversible trapping of entanglements, or chemical cross-linking occurring in the dense reaction medium. This provides a way to modulate mechanical properties of soft nanostructures, by playing with the degree of confinement of the chains during synthesis.

Adsorption of nile red by poly(N‐isopropylacrylamide) gels in binary water/tetrahydrofuran mixtures

AbstractThe adsorption of Nile Red by poly(N‐isopropylacrylamide) (PNIPAM) gels in binary water/tetrahydrofuran solutions was investigated using absorption spectrophotometry as a function of the volume fraction of THF, φ. Due to the cononsolvency abilities of such binary solvent mixtures, deswelling of the PNIPAM gels is observed in water‐rich mixtures, 0 ≤ φ ≤ 0.6, while the gels reswell for φ > 0.6. The position of the absorption band of Nile Red before and after equilibration with the PNIPAM gels indicates that the composition of the external solvent mixture is not practically influenced by the swelling process. On the other hand, it is found that the gels can effectively adsorb Nile Red in water‐rich mixtures (φ < 0.6), whereas no significant adsorption was observed in tetrahydrofuran‐rich mixtures. In fact, about 80% of the dye is adsorbed by the PNIPAM gels, at φ = 0.2. Under these conditions, the rather shrunk PNIPAM gel offers a more convenient less polar environment for Nile Red than the water‐rich bulk solvent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Carbonated Water Mediated Preparation of Poly(N-isopropylacrylamide) Thermoresponsive Gels and Liquids

Carbonic acid solution was used a medium for the free radical polymerization synthesis of poly( N-isopropylacrylamide) (p-nipam) thermoresponsive polymer as an alternative to conventional inert gas purging. It was found that p-nipam cross-linked gels or linear liquids and p-nipam/dextran magnetite composite gels could be very rapidly prepared and large gels recovered intact from open air vessels. A porogen was necessary for high thermoresponse, and dextran use resulted in microporous composite gels that gave optimal thermal response at weight ratio of p-nipam:dextran of 4:1. Up to 82% weight loss was rapidly obtained upon warming above the lower critical solution temperature. Analysis of products was made by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), and superconducting quantum interference device (SQUID). A simplified and efficient overall method for preparation of biomedical polymers is shown. A wider application of H2CO3 solution as a viable alternative media and to allow open air aqueous polymerizations of water soluble or hydrophilic monomers is indicated.

Polymeric Actuators for Biological Applications

To shed light on the role of cell rheology and mechanotransduction in various physiological and disease states, different techniques of force application, such as optical tweezers and deformable substrates, are employed. In this present paper we describe a new approach for the deformation of cells based on the temperature-sensitive polymer poly(N-isopropylacrylamide), PNIPAM. In response to temperature changes, PNIPAM gels undergo extensive and reversible changes in volume that allow them to be used as actuators for stretching and compressing cells and tissues. Herein we focus mainly on our experience with the deformation of red blood cells as proof of principle, and demonstrate the wealth of possibilities such stimuli-responsive materials may offer as actuators.

The Preparation of Bone-Mimetic Sol-Gel Nanoporous Scaffold of L3-Phase Silica/Hydroxyapatite

This work presented the highly controlled drug delivery system to achieve the suppression of the burst release at initial time and long term release of drug with cancellous bone mimetic nanoporous structures. The materials were prepared by the integration of synthesized inorganic hydroxyapatite (HA) and the hybrid gels of bicontinuous sponge-phased L 3 silicate and thermo-responsive poly(N-isopropylacrylamide) (L 3 -PNIPAm gels). The materials were designed to have the three dimensionally interconnected heterogeneous porosity of macro- and mesoporosity, in which the HA has the macroporosity of 150 μm to be impregnated the drug into the pores and the L 3 -PNIPAm gels have mesoporosity of 5 nm to regulate the temperature sensitive drug-release through the pore channels and polymeric network, respectively. Consequently, these bone-mimetic system gave the highly long term drug release over the 60 days with suppressing the burst release and was able to control the releasing rate per time by the change of the HA and PNIPAm composition ratios.

Thermosensitive Affinity Behavior of Poly(N-isopropylacrylamide) Hydrogels with β-Cyclodextrin Moieties

β-Cyclodextrin-incorporated poly(N-isopropylacrylamide) hydrogels [P(NIPAM-co-CD) gel] were prepared via copolymerization of N-isopropylacrylamide (NIPAM) with mono-(6-N-allylamino-6-deoxy)-β-cyclodextrin (ACD). Using 8-anilino-1-naphthalenesulfonic acid ammonium salt (ANS) as the guest molecule, an experimental study on the temperature dependence of affinity behavior of P(NIPAM-co-CD) gels and PNIPAM gels with ANS was carried out within a wide range of temperatures (21−60 °C). When the temperature increased from below the lower critical solution temperature (LCST) to above the LCST, both P(NIPAM-co-CD) and PNIPAM gels exhibited much more enhanced association constants with ANS than they did at temperatures below the LCST. Moreover, P(NIPAM-co-CD) gels showed greater affinity toward ANS than PNIPAM gels due to the presence of β-cyclodextrin moieties in the polymeric network. Compared with the PNIPAM gel, the P(NIPAM-co-CD) gel exhibited an increased LCST in deionized water and a decreased LCST in dilute aqueous ANS solution.

A novel thermoresponsive hydrogel matrix based on poly(N-ethoxypropylacrylamide)

A new temperature sensitive hydrogel matrix, poly(N-ethoxypropylacrylamide), PNEPAM, was obtained by the bulk polymerization of N-ethoxypropylacrylamide (NEPAM). The monomer, NEPAM was synthesized by the nucleophilic substitution reaction of 3-ethoxypropylamine and acryloyl chloride. The polymerization was performed at +4 °C, by using N,N-methylenebisacrylamide (MBAM) as crosslinker, polyethyleneglycol (PEG) 4000 as diluent, and potassium persulfate (KPS) and tetramethylethylenediamine (TEMED) as the initiator and accelerator, respectively. PNEPAM gel matrices exhibited a thermosensitive bahaviour reasonably similar to poly(N-isopropylacrylamide), PNIPAM gels. The equilibrium swelling ratio at constant temperature increased with increasing initiator concentration and decreasing monomer concentration. The use of PEG 4000 as a diluent in the gel synthesis resulted in a significant enhacement in the thermosensitivity of gel matrix. The equilibrium swelling ratios up to 60 g water/g dry gel were observed in the low-temperature region. The results indicated that PNEPAM gel is a new alternative thermosensitive material to the NIPAM based gels.

Fluorescence study on the mechanism of rapid shrinking of grafted poly( N-isopropylacrylamide) gels and semi-IPN gels

In order to clarify in nanometer scale the mechanism of rapid shrinking of grafted poly( N-isopropylacrylamide) (PNIPAM) hydrogels during temperature jump from room temperature to 40 °C, temperature dependences of dansyl fluorescence were studied for the rapid-shrinking grafted PNIPAM gels in which dansyl groups are attached to graft chains or network main chains. Gradual decrease in dansyl fluorescence peak wavelength, λ f, with the increase in temperature between 17 and 32 °C for grafted chains in the grafted PNIPAM gel from 540 to 500 nm showed the occurrence of coil-globule transition of graft chains during this temperature range. However, λ f of network main chains showed abrupt decrease at 30–34 °C from 540 to 510 nm reproducing the macroscopic volume phase transition of PNIPAM gels. The coil-globule transition and further aggregation of graft chains enhance the formation of heterogeneous structure during the temperature jump, providing water-expelling channels at the surface skin layer of the collapsed PNIPAM gel. The possible importance of large-scale heterogeneity with a much larger size than the mesh size of network on the rapid-shrinking is suggested, because this type of large-scale heterogeneity can be introduced by the network formation in the presence of linear macromonomer. A post-crosslinked semi-IPN gel also showed rapid shrinking and gradual decrease in λ f for the linear polymer chains inside.

Morphology and chain dynamics during collapse transition of PNIPAM gels studied by combined imaging, relaxometry and 129Xe spectroscopy techniques

The temperature-induced shape transition of poly(N-isopropylacrylamide) gels of different cross-link densities was investigated by a combination of NMR techniques allowing the characterization of both the macroscopic collapse as well as the changes on a molecular scale related to the expulsion of water from the gel network. The proton-containing gel phase was visualized by swelling in heavy water, and the volume change was monitored by proton imaging for cross-link densities between 0.5% and 2.5%. Above the transition temperature of 35 degrees C, gel collapse led to a volume change of up to a factor of 17 for the gel of smallest cross-link density. Two spectral lines of 129Xe are found in the gel state and are assigned to the hydrophobic and hydrophilic parts of the network. In the collapsed state, the hydrophobic peak shows a strong shift while the hydrophilic peak disappears. A considerable shortening of both T1 and T2 of the gel protons upon collapse was found at a field of 4.7 T. At lower fields, the effect becomes more pronounced and qualitatively different dispersion behaviors between the swollen and the collapsed states are observed.

Photochromic Cross-Links in Thermoresponsive Hydrogels of Poly(N-isopropylacrylamide): Enthalpic and Entropic Consequences on Swelling Behavior

The preparation and properties of thermosensitive gels of poly(N-isopropylacrylamide) (PNIPAM) cross-linked with azoaromatic chromophores were investigated. Polymer gels were prepared with different concentration of the cross-linking agent, 4,4‘-di[2-methacryloylamide]azobenzene (DMAAB). The gels were optically transparent and possessed a high absorption for ultraviolet light. Characterization of the equilibrium swelling in water revealed that an increase in cross-linking density caused a shift in the volume phase transition to lower temperatures. This shift in the transition temperature (Tc) was interpreted to arise from an increase in the unfavorable interactions between the solvent (water) and the polymer network due to the azoaromatic cross-links. A high cross-linker concentration also resulted in a decrease in the degree of equilibrium swelling due to an increase in the topological constraints of the PNIPAM chains. Below Tc, the shrinking dynamics of the PNIPAM gels with photochromic cross-links followed a Fickian diffusion behavior, and above Tc, the gels collapsed completely within 20 min. During the deswelling, irradiation of the gel and trans−cis photoisomerization of the cross-links were found to increase the expulsion of water from the swollen gels and decrease the water content of the gels by as much as 20−30%. This result contrasts with past studies on PNIPAM gels in which the azochromophore was present in the side chains and irradiation increased the swelling ratio. The largest decrease in water content (ΔRs,max) due to irradiation was observed near Tc, and the magnitude of the change, ΔRs,max, decreased with increase in cross-link density. The changes in swelling due to irradiation were attributed to entropic changes in the network due to irradiation.

Fluorescence label studies of thermo-responsive poly(N-isopropylacrylamide) hydrogels

Temperature-induced phase transitions and microenvironment of poly(N-isopropylacrylamide) (PNIPAM) hydrogels were studied in water using 9-(4-N,N-dimethylaminophenyl)phenanthrene (DP) as an intramolecular fluorescence probe. Fluorescence behavior of the DP-labeled PNIPAM gels depended on the conditions of the gel preparation such as concentrations of monomer and crosslinker. Thermo-responsive behavior of the PNIPAM gel was affected by copolymerization of NIPAM with a hydrophilic monomer N,N-dimethylacrylamide (DMAM) and a hydrophobic monomer methyl methacrylate (MMA). Incorporation of DMAM raised the lower critical solution temperature (LCST) of the PNIPAM gel, and that of MMA lowered it. Obtained results indicate that the NIPAM–DMAM copolymer gels with higher LCST are of a more open, water-swollen nature above their LCST and the NIPAM–MMA copolymer gels with lower LCST are of a less open, water-shrunken nature below their LCST than that of the NIPAM homopolymer gel.

Preparation Temperature Dependence and Effects of Hydrolysis on Static Inhomogeneities of Poly(acrylamide) Gels

The concentration fluctuations of poly(acrylamide) (PAAm) hydrogels prepared at different preparation temperatures, Tprep's, have been studied by dynamic light scattering (DLS). The ensemble average light scattered intensities obtained at the scattering angle of 90°, 〈I〉E, was decomposed into two contributions: one from the static inhomogeneities, 〈IC〉E, and the other from thermal fluctuations, IF. Contrary to the case of poly(N-isopropylacrylamide) (PNIPAm) gel, 〈I〉E was a decreasing function of Tprep. This indicates that these gels have opposite Tprep dependence with respect to the static inhomogeneities. However, the liquidlike properties of these gels, such as IF and the cooperative diffusion coefficient, D, were found to be independent of Tprep, as they should be. It was also found by DLS and swelling experiments that, unlike PNIPAm gels, PAAm gels prepared by redox polymerization were subject to hydrolysis and changed to partially charged gels.

A comparison between the `coil-to-globule' transition of linear chains and the “volume phase transition” of spherical microgels

The coil-to-globule transition of individual poly(N-isopropylacrylamide) (PNIPAM) linear chains and the volume phase transition of spherical PNIPAM microgels were studied by a combination of static and dynamic laser light scattering. The thermodynamically stable collapsed single-chain globule was observed for the first time. The ratio of R g/ R h lower than 0.774, predicted for a uniform sphere, indicates that the coil-to-globule transition is not an `all-or-nothing' process, where R g and R h are the gyration and hydrodynamic radii, respectively. The time scale less than 100 s observed in the coil-to-globule transition is too short to support a suggested high chain-knotting density inside the globule. At the collapsing limit, the single-chain globule and microgel particle still contain 80% and 70% of water respectively in their hydrodynamic volumes. As for the volume phase transition, our results indicated that the volume change of the microgels is practically continuous, in contrast to the discontinuous volume phase transition observed in bulk PNIPAM gels. The discrepancy between a microgel and a bulk gel can be attributed to shear modulus.

Synthesis and Volume Phase Transition of Poly(methacrylic acid-co-N-isopropylacrylamide) Microgel Particles in Water

A series of lightly cross-linked, nearly monodisperse, and random copolymerized microgel particles, based on methacrylic acid (MAA) and N-isopropylacrylamide (NIPAM) of various content, were synthesized in aqueous solution with sodium dodecyl sulfate at pH = 3.1 ± 0.2 and 70 °C. The polymerization kinetics, the colloidal stability, and the size of the microgel particles varied with the comonomer composition. The volume phase transitions of the MAA/NIPAM copolymer microgel particles with different PMAA contents in very dilute aqueous solution (C< 1 × 10-5 g/mL) were investigated by laser light scattering (LLS), as a function of both pH at 25 °C, and temperature at two different pH values of 3.4 and 7.5. To our knowledge, the volume phase transition of such submicron-sized hydrogels sensitive to multiple environmental changes was studied for the first time by LLS. The equilibrium swelling ratio, the critical transition pH, and the critical transition temperature (Tc) were strongly dependent on the comonomer composition. For the pH-sensitive transition, an increase in the PMAA content increased the swelling extent and enhanced the critical transition pH. For the thermal sensitive transition, an introduction of PMAA enabled us to adjust the hydrophobicity of the microgel network chains by varying the solution pH. At low pH, an increase in the highly collapsed hydrophobic PMAA content decreased the Tc, reduced the collapsing extent, and made the transition less sharp. At higher pH (7−10), an increase in the strongly hydrophilic, highly charged PMAA content raised Tc but unexpectedly reduced the collapsing extent and broadened the transition temperature region, which are contrary to the results observed in other weakly charged PNIPAM gels. The possible explanation related to a shell-enriched PMAA with a high charge density and a core-enriched in neutral PNIPAM for the MAA/NIPAM copolymer microgel with low PMAA content is presented.

Light scattering study of spherical poly(N-isopropylacrylamide) microgels

Nearly monodisperse spherical microgel particles of poly(N-isopropylacrylamide) (PNIPAM) were prepared by emulsion polymerization in water. The volume phase transition of the microgel particles was studied by static and dynamic laser light scattering (LLS) in terms of the radius of gyration (Rg ) and hydrodynamic radius (R h). The phase transition temperature of the microgel ∼33.0°C, similar to that of individual PNIPAM chains in water. The temperature independence of R g/Rh(∼0.78 ± 0.03) shows that in both the swollen and collapsed states the microgels are uniform particles behaving as hydrodynamically equivalent spheres. About 94% of the water inside the microgel network is driven out during the phase transition. The volume change of the microgel particles is continuous, in contrast to the discontinuous volume change observed in some bulk PNIPAM gels. The microgel particles are thermodynamically stable even at their collapsing limit. The behavior of microgel particles is compared to that of individual PNIPAM chains.

Volume Phase Transition of Swollen Gels: Discontinuous or Continuous?

Poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical microgels were studied using laser light scattering. The volume change associated with the temperature dependence of the hydrodynamic radii of both the linear chains and the spherical microgels is continuous, in contrast to a discontinuous volume phase transition observed for a bulk PNIPAM gel with an identical composition. A reexamination of the volume phase transition theory shows that the phase transition temperature depends on the length of the subchain between two neighboring cross-linking points inside the gel. A comparison of the results of the linear chains and the spherical microgels suggests that the volume change of a polymer gel should be practically continuous because the inhomogeneity of the subchain length smears the volume phase transition. A possible explanation is also presented to rationalize the discontinuous volume phase transition observed in certain bulk PNIPAM gels, which is based on the bulk shear module of the gel a...