Rhodamine-conjugated acrylamide polymers exhibiting selective fluorescence enhancement at specific temperature ranges

A simple copolymer, poly(NIPAM-co-RD), consisting of N-isopropylacrylamide (NIPAM) and rhodamine (RD) units, behaves as a fluorescent temperature sensor exhibiting selective fluorescence enhancement at a specific temperature range (25–40 °C) in water. This is driven by a heat-induced phase transition of the polymer from coil to globule. At low temperature, the polymer exists as a polar coil state and shows very weak fluorescence. At >25 °C, the polymer weakly aggregates and forms a less polar domain within the polymer, leading to fluorescence enhancement. However, at >33 °C, strong polymer aggregation leads to a formation of huge polymer particles, which suppresses the incident light absorption by the RD units and shows very weak fluorescence. In the present work, effects of polymer concentration and type of acrylamide unit in the polymer have been investigated. The increase in the polymer concentration in water leads to a formation of less polar domain even at low temperature and, hence, widens the detectable temperature range to lower temperature. Addition of N-n-propylacrylamide (NNPAM) or N-isopropylmethacrylamide (NIPMAM) component to the polymer, which has lower or higher phase transition temperature than that of NIPAM, enables the aggregation temperature of the polymer to shift. This then shifts the detectable temperature region to lower or higher temperature.     

Fast responsive poly(<Emphasis Type="Italic">N,N</Emphasis>-diethylacrylamide) hydrogels with interconnected microspheres and bi-continuous structures

We prepared thermo-responsive polymer hydrogels by γ-ray irradiation of aqueous solutions of N, N-diethylacrylamide at different temperatures below and above its lower critical solution temperature (LCST). Poly(N, N-diethylacrylamide) gel had a transparent and homogeneous structure when the radiation-induced polymerization and crosslinking were carried out below the LCST (25 °C) of the polymer. On the other hand, cloudy and heterogeneous gels were formed at temperatures above the LCST of the polymer (>35 °C). From environmental scanning electron microscopy observations, the gels prepared at 35 and 40 °C were seen to show sponge-like bi-continuous porous structures, while those prepared at 50 °C showed a porous structure consisting of interconnected microspheres. For temperature changes between 10 and 40 °C, gels with porous structures showed rapid volume transitions on a time scale of about a minute, not only for shrinking but also for swelling processes, which is in remarkable contrast to the porous poly(N-isopropylacrylamide) hydrogels.     

Study on novel hydrogels based on thermosensitive PNIPAAm with pH sensitive PDMAEMA grafts

A series of novel hydrogels based on poly(N-isopropylacrylamide) (PNIPAAm) with pendant poly(N-(2-(dimethylamino) ethyl)-methacrylamide) (PDMAEMA) grafts were designed and synthesized. The influence of the pendant PDMAEMA grafts on the properties of the resulted hydrogels was examined in terms of morphology observed by scanning electron microscopy (SEM), thermal property characterized by differential scanning calorimetry (DSC) and shrinking/swelling kinetics upon external temperature changes. In comparison with the conventional PNIPAAm hydrogels, resulting hydrogels presented favorable pH sensitivity as well as improved thermosensitive properties, including enlarged water containing capability at room temperature and faster shrinking/swelling rate upon heating. In addition, fish DNA, used as a model drug, was loaded into the hydrogels, and the controlled release behavior of the drug-loaded hydrogels at different temperatures (22 and 37 °C) was further studied.     

The kinetics of poly(N-isopropylacrylamide) microgel latex formation

Conversion versus time curves were measured for poly(N-isopropylacrylamide) microgel latexes prepared by polymerization in water with sodium dodecyl sulfate, SDS. Polymerization rates increased with temperature with methylenebisacrylamide crosslinking monomer consumed faster thanN-isopropylacrylamide. The particle diameter decreased with increasing concentrations of SDS in the polymerization recipe and there was evidence that the rate of polymerization increased somewhat with SDS concentration. Particle formation occurred by homogeneous nucleation as micelles were absent.Comparison of particle size distributions from dynamic light scattering to those from a centrifugal sizer led to the conclusion that larger particles within a specific latex were less swollen with acetonitrile than were the smaller ones. This was interpreted as evidence for the polymer in larger particles having a higher crosslink density. Particle swelling was estimated from swelling ratios defined as the particle volume at 25 °C divided by the volume at 50 °C. In the absence of crosslinking poly(N-isopropylacrylamide) linear chains would disolve at 25 °C. The swelling results indicated that the average crosslink density in the particles decreased with conversion. This was explained by the observation that the methylenebisacrylamide was consumed more quickly and is typical of crosslinking in emulsion polymerization where polymer particles have high polymer concentrations at their birth.     

Thermoresponsive amphiphilic symmetrical triblock copolymers with a hydrophilic middle block made of poly(N-isopropylacrylamide): synthesis, self-organization, and hydrogel formation

Several series of symmetrical triblock copolymers were synthesized by the reversible addition fragmentation chain transfer method. They consist of a long block of poly(N-isopropylacrylamide) as hydrophilic, thermoresponsive middle block, which is end-capped by two small strongly hydrophobic blocks made from five different vinyl polymers. The association of the amphiphilic polymers was studied in dilute and concentrated aqueous solution. The polymer micelles found at low concentrations form hydrogels at high concentrations, typically above 30–35 wt.%. Hydrogel formation and the thermosensitive rheological behavior were studied exemplarily for copolymers with hydrophobic blocks of polystyrene, poly(2-ethylhexyl acrylate), and poly(n-octadecyl acrylate). All systems exhibited a cloud point around 30 °C. Heating beyond the cloud point initially favors hydrogel formation but continued heating results in macroscopic phase separation. The rheological behavior suggests that the copolymers associate into flower-like micelles, with only a small share of polymers that bridge the micelles and act as physical cross-linkers, even at high concentrations.     

Analysis of photo-induced hydration of a photochromic poly(N-isopropylacrylamide) - Spiropyran copolymer thin layer by quartz crystal microbalance

We investigated hydration and swelling behavior of a solid state photoresponsive copolymer in water by using a quartz crystal microbalance technique with dissipation measurement (QCM-D technique). On the gold film electrode of a quartz resonator, we deposited a thin layer of a pNSp-NIPAAm, which is a poly(N-isopropylacrylamide) (pNIPAAm) polymer partially modified with a photochromic chromophore, 6-nitrospiropyran (NSp). Using QCM-D measurements, we found that at a temperature of 19 °C both water adsorption and changes in the viscoelasticity of the solid pNSp-NIPAAm layer were induced when pNSp-NIPAAm was irradiated by 365 nm ultraviolet light, which triggers the photoisomerization of the NSp chromophore and makes the structure of the chromophore hydrophilic. At temperatures between 25 and 35 °C, this photo-induced hydration was not observed. These observations suggest that the photoisomerization of the NSp chromophores triggered the photo-induced hydration only when pNIPAAm component is sufficiently hydrophilic, at a temperature of 19 °C.     

pH/temperature dual stimuli-responsive microcapsules with interpenetrating polymer network structure

The microcapsules with interpenetrating polymer network (IPN) structure based on crosslinked poly (N-isopropylacrylamide) (PNIPAM) and crosslinked poly (acrylic acid) (PAA) were fabricated in a three-step process. Firstly, silica/PNIPAM core/shell composite particles were synthesized by thermo-initiated seed precipitation polymerization using 3-(trimethoxysilyl)propyl methacrylate modified silica colloidal particles as seeds and N-isopropylacrylamide and N,N′-methylenebisacrylamide (MBA) as monomer and crosslinker, respectively. Secondly, PAA network was incorporated into the shell of the composite particles by redox-initiated polymerization of acrylic acid and MBA entrapped in the PNIPAM network. Finally, the silica core of the composite particles was removed using hydrofluoric acid under certain condition to produce the microcapsules. The chemical compositions, their mass ratio, and particle sizes of the particles formed in each step were determined by Fourier transformation infrared spectroscopy, thermogravimetry, and dynamic laser light scattering (DLLS), respectively. The IPN structure of the microcapsules was identified by transmission electron microscopy (TEM) using uranyl acetate staining method, and their hollow structure was evidenced by TEM and scanning electron microscopy. Their temperature- or pH-dependent hydrodynamic diameters were measured by DLLS, and the results showed that the microcapules had both pH- and temperature-responsive properties, and the temperature-responsive component and the pH-responsive component inside the microcapsule shell had little interference with each other.     

pH- and temperature-dependent release from cationic vesicles coexisting with copolymer of N-isopropylacrylamide and methacrylic acid

Vesicles containing rhodamine B were prepared by evaporation and hydration method using N-[3-(dimethylamino)propyl]-octadecanamide (DMAPODA) and stearic acid (SA). The vesicles were multi-lamellar on optical and electron micrographs. The mean size of vesicle was 807.9 nm and the values markedly increased by the addition of copolymers of N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) (P(NIPAM-co-MAA)), possibly due to electrostatic interactions between the cationic vesicle and the anionic copolymer. The release of rhodamine B from the vesicles for 20 h was 50–60% at neutral pHs and the values increased up to 93.1% when pH decreased to 3. The increased release is possibly because the salt bridge formed between DMAPODA and SA was broken down at the acidic pH, leading to the disintegration of the vesicles. On the other hand, the release was not as sensitive to temperature as it was to pH. The salt bridge seemed to be stable at the temperatures of the release experiments (23 °C, 33 °C and 43 °C). P(NIPAM-co-MAA) was added to the suspension of the vesicle and the release was investigated with varying pHs and temperatures. The copolymer was pH- and temperature-sensitive in terms of the turbidity change of its solution. Nevertheless, the copolymer was found to have little effect on the pH- and temperature-dependent release of the vesicles.     

Molecularly imprinted beads with double thermosensitive gates for selective recognition of proteins

A new approach is reported on the use of poly(N-isopropylacrylamide) (PNIPAM)-coated molecularly imprinted beads (coated MIP beads) for controlling the release of protein. The coated MIP beads were composed of double layers, an internal thermosensitive lysozyme-imprinted layer, and an external PNIPAM layer. The coated MIP beads were prepared by two-step surface-initiated living-radical polymerization (SIP). In this systemic study, the coated MIP beads had good selectivity to the template protein (lysozyme) and temperature stimulus-responsive behavior, both of which were superior to those of MIP beads having a layer of thermosensitive lysozyme-imprinted polymer only. Using the coated MIP beads, reference proteins and the template lysozyme could be released separately at 38 °C and at 23 °C. The corresponding coated non-imprinted beads (coated NIP beads) did not have such double thermosensitive “gates” with specific selectivity for a particular protein. The proposed smart controlled imprinted system for protein is attractive for chemical carriers, drug-delivery system, and sensors.     

Effect of oxyfluorination on electromagnetic interference shielding behavior of MWCNT/PVA/PAAc composite microcapsules

Composite microcapsules of poly(vinyl alcohol)/poly(acrylic acid)/multi-walled carbon nanotubes were prepared and the electromagnetic interference shielding behavior was evaluated for the composite microcapsules. The dispersion and adhesion of multi-walled carbon nanotubes in microcapsules were improved by the surface modification through direct oxyfluorination which introduced polar groups on the multi-walled carbon nanotubes. The composite microcapsules containing the oxyfluorinated multi-walled carbon nanotubes showed significant increases in permittivity, permeability, and electromagnetic interference shielding efficiency. The electromagnetic interference shielding efficiency of composite microcapsule increased up to 51 dB mainly base on the absorption mechanism.     

Preparation and photoactivity of thermosensitive polymer supported metallophthalocyanine

A novel reactive metallophthalocyanine derivative,zinc tetra(2,4-dichloro-1,3,5-triazine)aminophthalo-cyanine(Zn-TDTAPc),was prepared and immobilized on poly(N-isopropylacrylamide)(PNIPAAm) by covalent bonding to obtain a thermosensitive polymer(Zn-TDTAPc-g-PNIPAAm).Compared with zinc tetraaminophthalocyanine(Zn-TAPc),Zn-TDTAPc-g-PNIPAAm exhibits excellent solubility in water and in most organic solvents.Furthermore,it has a special thermosensitive property in water and the lower critical solution temperature(LCST) is 34.1℃.It was found that both dissolved and precipitated Zn-TDTAPc-g-PNIPAAm present high photoactivity evidenced by the experiment of photocatalytic degra-dation of 1,3-diphenylisobenzofuran(DPBF) in the presence of Zn-TDTAPc-g-PNIPAAm.These proper-ties suggest that it can be used potentially in photodynamic therapy(PDT).     

Controlled Release from and Tissue Response to Physically Bonded Hydrogel Nanoparticle Assembly

Our recent work on synthesis and application of thermally gelling nanoparticle dispersions is briefly reviewed here. These nanoparticles consist of interpenetrating polymer networks (IPN) of poly-acrylic acid (PAAc) and poly(N-isopropylacrylamide) (PNIPAM). The aqueous IPN nanoparticle dispersions with polymer concentrations above 2.5 wt % underwent an inverse thermoreversible gelation at about 33 °C. Dextran markers of various molecular weights as model macromolecular pseudodrugs were mixed with the IPN nanoparticle dispersion at room temperature. At body temperature, the dispersion became a gel. The dextran release profiles were then measured using UV-visible spectroscopy. The biocompatibility of this nanoparticle assembly was assessed using an animal implantation model.     

Entrapment of a weak polyanion and H+/Na+ exchange in confined polyelectrolyte microcapsules

An approach for the entrapment of a polyanion by polyelectrolyte microcapsules is reported. It is based on a reversal changing of microcapsule wall permeability from neutral to basic pH. Polyelectrolyte microcapsules were templated on latex (polystyrene) particles by the layer-by-layer adsorption of oppositely charged polymers of sodium poly(styrene sulfonate) and poly(allylamine hydrochloride), followed by core removal using tetrahydrofuran. In alkaline conditions, the microcapsules swell and become permeable for polymers. During encapsulation, the addition of salt ions increases the amount of the polymer encapsulated and contributes to its protonation because of redistribution of H+ ions across a semipermeable microcapsule wall. The redistribution of small ions across the microcapsule wall was tuned by adding salt according to the Donnan equilibrium and was characterized by H+ sensitive dyes.     

Poly(N-isopropylacrylamide). II. Effect of polymer concentration,temperature, and surfactant on the viscosity of aqueous solutions

The effects of polymer concentration, temperature, and surfactant on the rheological properties of poly(N-isopropylacrylamide), poly NIPAM, were studied. Below 28°C the viscosity decreased with increasing temperature according to the Arrhenius expression. However, at 29°C the viscosity increased to a maximum value at 32°C, the lower critical solution temperature (LCST) for aqueous polyNIPAM. Higher temperatures gave a much lower viscosity. This unusual rheological behavior was explained by the phase behavior of the polymer. Sodium dodecyl sulfate (SDS) binding to polyNIPAM increased the cloud point temperature (CPT) and attenuated the unusual rheological behavior of polyNIPAM in water. © 1993 John Wiley & Sons, Inc.     

Adhesion and growth of goat mammary epithelial cells on novel polyelectrolyte complex

Alginate/poly(acryloxyethyl-trimethylammonium chloride-co-2-hydroxyethyl methacrylate) [poly(Q-co-H)] microcapsules were prepared by ionic gelation (Ca²⁺) for adhesion and growth of goat mammary epithelial cell culture. In the procedure of microcapsule formation, alginate was first pumped into a CaCl₂ solution and then transferred into a poly(Q-co-H). The poly(Q-co-H) was prepared by free-radical polymerization in aqueous solution at 60 °C using potassium persulfate as initiator. The microcapsules obtained were sterilized using gamma radiation according to International Standards Organization (ISO)/TR 13409. Scanning electron microscopy studies indicated the high porosity and rough surface marked by large wrinkles of the microcapsule surface, and the diameter of the microcapsule was approximately 497 μm, and diameters ranking 480–515 μm were obtained. Optical michrography shows the epithelial morphology acquired by goat epithelial mammary cells (GMEC) on poly(Q-co-H)/NaAlg microcapsule surface after 8 h of culture.     

Dynamics studies on thermoresponsive poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) hydrogel in tetrahydrofuran/water mixtures

The properties of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel in tetrahydrofuran/H₂O mixtures were studied. Scanning electron microscopic (SEM) images demonstrate that the hydrogel changes from homogeneous to heterogeneous microstructure upon the addition of tetrahydrofuran to water. This heterogeneous PNIPAAm hydrogel in the mixture solvent exhibits a very slow response rate at temperatures above its lower critical solution temperature. The decreased response rate is attributed to the formation of special ternary complexes including the polymer and the two solvents in the tetrahydrofuran/H₂O mixture. Factors controlling the thermoresponse rate are discussed further and several suggestions are provided for designing and developing fast-response PNIPAAm hydrogels in the future.     

Graft copolymers having hydrophobic backbone and hydrophilic branches. XVIII. Poly(styrene) nanospheres with novel thermosensitive poly(N-vinylisobutyramide)s on their surfaces

Poly(styrene) nanospheres having poly(N-vinylisobutyramide)s (PNVIBA)s, which are structurally the same composition as well-known thermosensitive poly(N-isopropylacrylamide)s (PNIPAAm)s and show the thermosensitive property as well, on their surfaces were synthesized by the free radical polymerization of hydrophilic PNVIBA macromonomers and hydrophobic styrene with AIBN as a radical initiator in ethanol as a polar solvent and were characterized in regard to their thermosensitive properties. Both the NVIBA oligomers and PNVIBA macromonomers that we synthesized showed a lower critical solution temperature (LCST) at around 40°C, as was predicted by our previous research. The nanospheres were spherical in form and have a narrow size distribution. Their sizes could be controlled by varying the molecular weight of the macromonomers and the amount of it in feed. The size in the nanosphere became small above the LCST of the corresponding macromonomer, possibly due to thermosensitive shrinking of the PNVIBA on the nanosphere surface, while transmittance of its dispersion did not change at all at studied temperature range. The nanospheres having the PNVIBA on their surfaces, which response sharply to atmospheres such as dispersion temperature, can be significant and useful materials in technological and medical fields. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2581–2587, 1998     

Thermosensitive aggregates self-assembled by an asymmetric block copolymer of dendritic polyether and poly(N-isopropylacrylamide)

An asymmetric linear-dendritic block copolymer of polyether dendrimer and poly(N-isopropylacrylamide) was prepared by an atom transfer radical polymerization method. The self-assembly behavior and thermosensitive property of this copolymer in water were studied by dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence probe spectroscopy. It was found that the thermosensitive phase transition takes place at the temperature of 37.5 °C; simultaneously the spherical aggregates grow into larger entangled nanotubules. The unique temperature-sensitive supramolecular aggregates may make them especially useful as intelligent capsules for drug delivery systems and as chemical sensors.     

Encapsulation of silica nanoparticles by redox-initiated graft polymerization from the surface of silica nanoparticles

This study describes a facile and versatile method for preparing polymer-encapsulated silica particles by ‘grafting from’ polymerization initiated by a redox system comprising ceric ion (Ce⁴⁺) as an oxidant and an organic reductant immobilized on the surface of silica nanoparticles. The silica nanoparticles were firstly modified by 3-aminopropyltriethoxysilane, then reacted with poly(ethylene glycol) acrylate through the Michael addition reaction, so that hydroxyl-terminated poly(ethylene glycol) (PEG) were covalently attached onto the nanoparticle surface and worked as the reductant. Poly(methyl methacrylate) (PMMA), a common hydrophobic polymer, and poly(N-isopropylacrylamide) (PNIPAAm), a thermosensitive polymer, were successfully grafted onto the surface of silica nanoparticles by ‘grafting from’ polymerization initiated by the redox reaction of Ce⁴⁺ with PEG on the silica surface in acid aqueous solutions. The polymer-encapsulated silica nanoparticles (referred to as silica@PMMA and silica@PNIPAAm, respectively) were characterized by infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. On the contrary, graft polymerization did not occur on bare silica nanoparticles. In addition, during polymerization, sediments were observed for PMMA and for PNIPAAm at a polymerization temperature above its low critical solution temperature (LCST). But the silica@PNIPAAm particles obtained at a polymerization temperature below the LCST can suspend stably in water throughout the polymerization process.     

Multicompartmental Microcapsules with Orthogonal Programmable Two‐Way Sequencing of Hydrophobic and Hydrophilic Cargo Release

Multicompartmental responsive microstructures with the capability for the pre‐programmed sequential release of multiple target molecules of opposite solubility (hydrophobic and hydrophilic) in a controlled manner have been fabricated. Star block copolymers with dual‐responsive blocks (temperature for poly(N‐isopropylacrylamide) chains and pH for poly(acrylic acid) and poly(2‐vinylpyridine) arms) and unimolecular micellar structures serve as nanocarriers for hydrophobic molecules in the microcapsule shell. The interior of the microcapsule can be loaded with water‐soluble hydrophilic macromolecules. For these dual‐loaded microcapsules, a programmable and sequential release of hydrophobic and hydrophilic molecules from the shell and core, respectively, can be triggered independently by temperature and pH variations. These stimuli affect the hydrophobicity and chain conformation of the star block copolymers to initiate out‐of‐shell release (elevated temperature), or change the overall star conformation and interlayer interactions to trigger increased permeability of the shell and out‐of‐core release (pH). Reversing stimulus order completely alters the release process.