N-isopropylacrylamide Copolymers Research Articles

Adsorption and desorption of calcium ions by temperature swing with copolymer of thermosensitive and chelating components grafted on porous ethylene vinyl acetate disk

A novel adsorbent composed of a copolymer of a thermosensitive component, N-isopropylacrylamide (NIPAM), and a chelating component, N-(4-vinyl)benzylethylenediamine (VBEDA), was developed, and the adsorption and desorption behaviors of Ca2+ ions were investigated. The copolymer of NIPAM and VBEDA was grafted onto the pore surfaces of a porous ethylene vinyl acetate (EVA) disk by a plasma-initiated polymerization method. The copolymerization ratios of VBEDA were 3 and 6mol%. Although a hydrophilic component, VBEDA, was copolymerized with NIPAM, the transition temperature of the copolymer was almost the same as that of poly(NIPAM). The permeability through the polymer-grafted EVA disk strongly depended on the temperature. It was quite low below the transition temperature of the grafted polymer, and increased drastically when the temperature exceeded the transition temperature. The polymer-grafted EVA disk adsorbed Ca2+ ions at low temperatures and the adsorption amount decreased significantly when the temperature exceeded the transition temperature of the grafted polymer, and repeated adsorption and desorption by temperature swings were possible. Furthermore, it was suggested that one Ca2+ ion was captured by three VBEDA groups, and this multi-point coordination bond was destroyed by heating as a result of shrinkage of the grafted poly(NIPAM-co-VBEDA) chains, leading to desorption of Ca2+ ion.

Halloysite Nanotube Composited Thermo-responsive Hydrogel System for Controlled-release

Halloysite nanotube-composited thermo-responsive hydrogel system has been successfully developed for controlled drug release by copolymerization of N-isopropylacrylamide (NIPAM) with silane-modified halloysite nanotubes (HNT) through thermally initiated free-radical polymerization. With methylene blue as a model drug, thermo-responsive drug release results demonstrate that the drug release from the nanotubes in the composited hydrogel can be well controlled by manipulating the environmental temperature. When the hydrogel network is swollen at temperature below the lower critical solution temperature (LCST), drug releases steadily from lumens of the embedded nanotubes, whereas the drug release stops when hydrogel shrinks at temperature above the LCST. The release of model drug from the HNT-composited hydrogel matches well with its thermo-responsive volume phase transition, and shows characteristics of well controlled release. The design strategy and release results of the proposed novel HNT-composited thermo-responsive hydrogel system provide valuable guidance for designing responsive nanocomposites for controlled-release of active agents.

Photoswitchable Hydrogel Surface Topographies by Polymerisation‐Induced Diffusion

Herein, we describe the preparation of patterned photoresponsive hydrogels by using a facile method. This polymer-network hydrogel coating consists of N-isopropylacrylamide (NIPAAM), cross-linking agent tripropylene glycol diacrylate (TPGDA), and a new photochromic spiropyran monoacrylate. In a pre-study, a linear NIPAAM copolymer (without TPGDA) that contained the spiropyran dye was synthesised, which showed relatively fast photoswitching behaviour. Subsequently, the photopolymerisation of a similar monomer mixture that included TPGDA afforded freestanding hydrogel polymer networks. The light-induced isomerisation of protonated merocyanine into neutral spiropyran under slightly acidic conditions resulted in macroscopic changes in the hydrophilicity of the entire polymer film, that is, shrinkage of the hydrogel. The degree of shrinkage could be controlled by changing the chemical composition of the acrylate mixture. After these pre-studies, a hydrogel film with spatially modulated cross-link density was fabricated through polymerisation-induced diffusion, by using a patterned photomask. The resulting smooth patterned hydrogel coating swelled in slightly acidic media and the swelling was higher in the regions with lower cross-linking densities, thus yielding a corrugated surface. Upon exposure to visible light, the surface topography flattened again, thus showing that a hydrogel coating could be created, the topography of which could be controlled by light irradiation.

Temperature-Responsive Graft Copolymer Hydrogels for Controlled Swelling and Drug Delivery

Temperature-responsive graft copolymers of N-isopropylacrylamide and Jeffamine® M-1000 acrylamide were synthesized to provide controlled swelling without introducing degradable moieties or increasing the LCST above body temperature. Jeffamine® M-1000 caused a small LCST increase (0.24–0.27°C/wt%) and a broader sol-gel transition. Twenty wt% copolymer gels (Mw> 225 kDa) retained their initial volume after 42 days, while homopolymer gels shrank by more than 50%. Copolymer gels eluted <20% of ovalbumin over 6 days whereas homopolymer gels released >90% within 3 h. These results suggest that Jeffamine® M-1000 acrylamide is suitable for inclusion in N-isopropylacrylamide-based biomaterials to control swelling and drug release nearly independently of LCST.

Dry–wet spinning of temperature-sensitive PVDF hollow-fiber membranes at different bore fluid flow rates

In this work, dry–wet spinning was conducted to fabricate poly(vinylidene fluoride)-g-poly(N-isopropylacrylamide) (PVDF-g-PNIPAAm) copolymer hollow-fiber membranes. The effect of the bore fluid flow rate (BFFR) on the structure and performance of each hollow-fiber membrane was investigated by X-ray photoelectron spectroscopy (XPS), field emission scan electronic microscopy (FESEM), pore-size measurements, mechanical property evaluations, and a filtration experiment. It was found that the grafted PNIPAAm chains aggregated on the inner surface of the fiber membrane due to the effect of the bore fluid. When the permeation temperature was increased from 20 to 45 °C, a drastic reduction in the pure-water flux and a remarkable increase in the retention of bovine serum albumin (BSA) were observed at around 32 °C, indicating that the as-prepared fiber membranes exhibit excellent temperature sensitivity. Pore-size measurements confirmed that both the mean pore size and the porosity increase with increasing BFFR, endowing the fiber membranes with high pure-water fluxes and low retentions of BSA.

Thermo-sensitive polyelectrolytes as draw solutions in forward osmosis process

A series of polyelectrolytes were evaluated as draw solutions for the forward osmosis (FO) process. Such polyelectrolytes were synthesized by copolymerization of N-isopropylacrylamide with different amounts of sodium acrylate. These polyelectrolytes were thermo-sensitive and water soluble. Hot ultrafiltration (HUF) operated at 45°C and 2bar was used as a low-energy method to recover the water from the polyelectrolyte draw solutions. The results showed that 4%PNIPAM-SA solution worked best among nine polyelectrolytes in the forward osmosis process and HUF process, and its FO water flux was 0.347LMH while the feed solution was pure water and its water recovery fraction was 65.2%.

Synthesis of thermoresponsive glycopolymers via ATRP of N-isopropylacrylamide and N-allylacrylamide and subsequent thiol–ene reaction

To prepare thermoresponsive glycopolymers, new alkene functional copolymers based on N-isopropylacrylamide were synthesized and functionalized with sugar moieties. The copolymerization of N-isopropylacrylamide with N-allylacrylamide by ATRP led to soluble, well-defined polymers which were reacted with 1-thiosugars via a thiol–ene reaction. In addition to 1H NMR spectroscopy, FT-IR spectroscopy, elemental analysis and size exclusion chromatography, the glycopolymers were characterized by turbidimetry, which revealed sharp and reversible coil-to-globule transitions of the prepared copolymers. Furthermore, lectin binding studies revealed the interaction of the galactosylated polymers to the lectin RCA120 depending on their degree of functionalization. The results demonstrate that the discussed synthetic strategy represents a practical and flexible route towards functional N-isopropylacrylamide copolymers.

Elongational rheology of NIPAM-based hydrogels

Hydrogels of different composition based on the copolymerization of N-isopropyl acrylamide and surfmers of different chemical structure were tested in elongation using Hencky/real definitions for stress, strain, and strain rate, offering a more scientific insight into the effect of deformation on the properties. In a range between $\dot {\varepsilon }=10$ and 0.01 s $^{-1}$ , the material properties are independent of strain rate and show a very clear strain hardening with a “brittle” sudden fracture. The addition of surfmer increases the strain at break $\varepsilon _{\mathrm {H}}^{\max }$ and at the same time leads to a failure of hyperelastic models. The samples can be stretched up to Hencky strains $\varepsilon _{\mathrm {H}}^{\max }$ between 0.6 and 2.5, depending on the molecular structure, yielding linear Young’s moduli E $_{0}$ between 2,700 and 39,000 Pa. The strain-rate independence indicates an ideal rubberlike behavior and fracture in a brittle-like fashion. The resulting stress at break $\sigma _{\textrm max}$ can be correlated with $\varepsilon _{\mathrm {H}}^{\max } $ and $E_{0}$ as well as with the solid molar mass between the cross-linking points $M_{\mathrm {c}}^{\textrm {solids}} $ , derived from $E_{0}$ .

Temperature, pH, and Glucose Responsive Gels via Simple Mixing of Boroxole- and Glyco-Based Polymers.

Statistical copolymers of N-isopropylacrylamide (NIPAAm) and 5-methacrylamido-1,2-benzoxaborole (MAAmBo) have been synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The solution properties of the NIPAAm homopolymers and statistical copolymers were investigated and it was found that, besides temperature and pH, the statistical copolymers were also responsive to the presence of free glucose in solution. Furthermore, responsive hydrogels and nanogels were formed spontaneously by simply mixing the statistical copolymers of P(NIPAAm-st-MAAmBO)s and well-defined glycopolymers. These gels were found to have temperature, pH, and glucose responsive properties.

Infrared spectroscopic study on LCST behavior of poly(N,N-bis(2-methoxyethyl)acrylamide)

The phase separation behavior and solvation of poly(N,N-bis(2-methoxyethyl)acrylamide) (PbMoEA) in water and alcohol/water mixtures were investigated by infrared (IR) spectroscopy. A critical change in IR spectrum informs that PbMoEA undergoes the phase separation at around 54°C in water. The red shift of ν(CH) band and the blue shift of amide carbonyl band during the phase separation indicate that its alkyl and amide groups are partially dehydrated. The transition temperature (Tp) monotonically increases with increasing concentration of methanol, whereas it draws a re-entrant curve in 1-propanol/water mixtures. The blue shift of amide carbonyl band and the red shifts of ν(CH) bands with increasing alcohol concentration indicate decreases in the number of hydrogen bonds to the carbonyl groups and that of hydrophobically hydrating water molecules around the alkyl groups, respectively. The former effect destabilizes the solutions and lowers Tp whereas the latter raises Tp. The dominance of the former lowers the Tp of PbMoEA in 1-propanol/water mixtures at low alcohol concentrations, and the opposite is the case in methanol/water mixtures. The behavior of random copolymers of N-isopropylacrylamide (NiPAm) and N,N-bis(2-methoxyethyl)acrylamide (bMoEA) in methanol/water mixtures indicates that not only the comonomer content but also at least diad monomer sequence is important for Tp, in other words, the interaction between adjacent monomer units affects the Tp of copolymers.

Amphiphilic copolymers of sucrose methacrylate and acrylic monomers: Bio-based materials from renewable resource

Regioselective sucrose 1′-O-methacrylate obtained by transesterification catalyzed by Proteinase-N was copolymerized with hydrophilic N-isopropylacrylamide and hydrophobic methyl methacrylate in different molar ratios by free radical polymerization. The copolymers were characterized by 13C nuclear magnetic resonance spectroscopy, gel permeation chromatography, differential scanning calorimetry and thermogravimetry. Solubility and phase behavior of aqueous solutions were also investigated. The glass transition of the copolymers presents a positive deviation from the values of the homopolymers due to the high density of inter and intramolecular hydrogen bonding. Their solubility is strongly dependent on the composition. Copolymers poor in methyl methacrylate are water soluble, while copolymers richer in methyl methacrylate behaves as hydrogel. These hydrogels are not chemically crosslinked and their form can be design prior swelling by the conventional processing methods, such as solvent casting and extrusion for instance. Copolymers of N-isopropylacrylamide are water soluble and their aqueous solutions present a lower critical solution temperature behavior forming thermoreversible hydrogels.

Thermo- and pH-Responsive Hydrogels Based on N-Isopropylacrylamide and Allylamine Copolymers

The thermo- and pH-responsive hydrogels were synthesized via copolymerization of N-isopropylacrylamide and al-lylamine hydrochloride monomers. The equilibrium swelling of the hydrogels was studied as a function of temperature and pH in aqueous solutions. It was shown that controlled alteration of the hydrogel phase transition temperature can be achieved by changing their composition and pH of the environment. Increase in content of hydrophilic allylamine from 10 to 60 wt% in monomer mixture causes a shift of the phase transition temperature from 35oC to 47oC. Hydrogels with N-isopropylacrylamide/allylamine hydrochloride mass ratio of 3:2 show the highest pH-response. Values of average molecular weight between polymer cross-links, , and Flory parameter, χ, were calculated using temperature dependences of the equilibrium swelling of the synthesized hydrogel.

Synthesis and Catalytic Activity of Silver Nanoparticles Immobilized onto Composite Microspheres

P(NIPAM-co-AA) copolymer microgels with temperature and pH sensitivities were synthesized by copolymerization of N-isopropylacrylamide(NIPAM) and acrylic acid(AA).Ag/P(NIPAM-co-AA) composite microspheres were prepared via in-situ reduction using ethanol as a reducing agent,based upon a polymer microgel template method.The morphology,composition,and catalytic properties of the prepared composite microspheres were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD),and UV-visible(UV-Vis) spectrophotometry.It was demonstrated that the Ag/P(NIPAMco-AA) composite microspheres had an uneven surface structure.The confinement effect of the template microgels significantly improved the dispersion and stability of the loaded silver particles.In addition,the obtained composite materials exhibited good catalytic activity for the reduction of 4-nitrophenol(4-NP).The observed catalytic activity was related to the swelling and shrinking behavior of the microgel network structure.The catalysis of 4-nitrophenol reduction was controlled by adjusting the thermo-sensitivity of the polymer template.

Synthesis and self-assembly behaviors of four-armed amphiphilic polystyrene-b-poly(N-isopropylacrylamide) copolymers

Four-armed amphiphilic block copolymers polystyrene-b-poly(N-isopropyl acrylamide) (PSt-b-PNIPAAM)4 were synthesized by atom transfer radical polymerization (ATRP) in two steps. Star narrow dispersed polystyrene, (PSt-Br)4, with controlled number-average molecular weight was firstly synthesized by ATRP of styrene (St) using pentaerythritol tetrakis (2-bromoisobutyrate) (4Bri-Bu) as four-armed initiator. Then, (PSt-b-PNIPAAM)4 was prepared using (PSt-Br)4 as macroinitiator by ATRP. The structures of (PSt-Br)4 and (PSt-b-PNIPAAM)4 were confirmed by characterization by nuclear magnetic resonance (1H NMR). The apparent viscosity of the four-armed (PSt-b-PNIPAAM)4 was significantly lower than that of the linear PSt-b-PNIPAAM with the same amount of repeat units of PSt and PNIPAAM. The self-assembly behavior of the four-armed amphiphilic block copolymers (PSt-b-PNIPAAM)4 in mixed solution (DMF/H2O) and the lower critical solution temperature (LCST) of the resulting micelles were investigated by scanning electron microscopy (SEM), dynamic light scattering (DLS) and UV-VIS spectroscopy. The results show that the size of the mono-dispersed spherical micelles decreased with the increment of the chain length of PNIPAAM in the block copolymers, while LCST increased.

Temperature-responsive electrospun nanofibers for ‘on–off’ switchable release of dextran

We propose a new type of ‘smart’ nanofiber (NF) with dynamically and reversibly tunable properties for the ‘on–off’ controlled release of the polysaccharide dextran. The fibers are produced by electrospinning copolymers of N-isopropylacrylamide (NIPAAm) and N-hydroxymethylacrylamide (HMAAm). The OH groups of HMAAm are subsequently crosslinked by thermal curing. The copolymers were successfully fabricated into a well-defined nanofibrous structure with a diameter of about 600–700 nm, and the fibers preserved their morphology even after thermal curing. The resulting crosslinked NFs showed rapid and reversible volume changes in aqueous media in response to cycles of temperature alternation. The fibrous morphology was maintained for the crosslinked NFs even after the cycles of temperature alternation, while non-crosslinked NFs collapsed and dispersed quickly in the aqueous solution. Dextran-containing NFs were prepared by electrospinning the copolymers blended with fluorescein isothiocyanate (FITC)-dextran, and the ‘on–off’ switchable release of FITC-dextran from the crosslinked NFs was observed. Almost all the FITC-dextran was released from the NFs after six heating cycles, whereas only a negligible amount of FITC-dextran was evolved during the cooling process. The reported incorporation of smart properties into NFs takes advantage of their extremely large surface area and porosity and is expected to provide a simple platform for on–off drug delivery.

Effect of particle size on the performance of forward osmosis desalination by stimuli-responsive polymer hydrogels as a draw agent

Stimuli-responsive polymer hydrogels have shown great potential for use as a draw agent in forward osmosis desalination. We have previously shown that the alteration of chemical structure of hydrogel network via the incorporation of ionic groups and light-absorbing particles can significantly improve the efficiency of hydrogels used as draw agent in FO desalination. In this study, particle size is demonstrated to be another key parameter in determining the efficacy of these materials as the draw media. Hydrogel copolymers of sodium acrylate and N-isopropylacrylamide of different particle sizes were prepared via free-radical polymerization and tested as draw agents. The results showed that the initial swelling rate of small particles (2–25μm) was much higher than that of large particles (e.g., 500–1000μm), with the diffusion mechanism shifted from non-Fickian to Fickian diffusion as the particle size decreases. To reclaim the fresh water drawn through the membrane by the polymer particles, the gas pressure and heating were applied as the stimuli on the draw agent to release water. Gas pressure was found to be an effective method of regaining the pure water, and is a more readily applicable method industrially than our previous mechanical pressing. It was found that greater liquid water recovery amounts were achieved from large hydrogel particles, whereas more water was recovered from smaller particles under such a thermal stimulus. This is likely due to the fact that hydrogel particles with different sizes showed different interstitial volumes, particle–particle contact areas, particle-membrane contact zones and hydrogel thickness on the membrane surface.

Preparation and application of thermosensitive poly(NIPAM-co-MAH-β-CD)/(TiO2-MWCNTs) composites for photocatalytic degradation of dinitro butyl phenol (DNBP) under visible light irradiation

Based on the unique temperature responsive characters of polymeric material based on N-isopropylacrylamide (NIPAM), a capacity for organic molecular inclusion of maleic anhydride (MAH) modified β-cyclodextrin (β-CD), and the enhancements in photocatalytic activity of TiO2 doped with multi-walled carbon nanotubes (MWCNTs), the novel thermosensitive poly(NIPAM-co-MAH-β-CD)/(TiO2-MWCNTs) composite photocatalysts were prepared by UV light photoinitiating method. Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), BET surface area and UV–Vis diffuse reflectance spectra (UV–Vis/DRS) were used to characterize the structure, morphology and composition of the as-prepared composites. Results showed that TiO2-MWCNTs nanoparticles were embedded evenly within the thermally responsive copolymer of NIPAM and MAH-β-CD. The thermoresponsive property of the synthesized composites was investigated by using swelling ratio measurements. The photocatalytic performances of the thermosensitive composite catalysts were evaluated for the degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) solution under visible light irradiation. The effect of operational parameters, i.e. pH of the solution, catalyst concentration, irradiation time, initial DNBP concentration on the photocatalytic degradation efficiency was explored and the results obtained were fitted with modified Langmuir–Hinshelwood model to investigate the degradation kinetics and discussed in detail. The repeatability of photocatalytic activity was also tested. The thermosensitive composite photocatalysts exhibited easy separation and less deactivation after several runs. A plausible mechanism is proposed for the photocatalytic degradation pathway of DNBP. The results of this study showed the feasible and potential use of the thermosensitive composites in photodegradation of organic pollutants by controlling temperature simply.

Toward Copolymers with Ideal Thermosensitivity: Solution Properties of Linear, Well-Defined Polymers of N-Isopropyl Acrylamide and N,N-Diethyl Acrylamide

Statistical copolymers of N-isopropyl acrylamide (NIPAM) and N,N-diethyl acrylamide (DEAAM) show a pronounced synergistic depression in their cloud points, though both homopolymers phase separate at significantly higher temperatures close to 30 °C (e.g., Polymer2009, 50, 519). While phase separation occurs at 20 °C for the statistical copolymers, the influence of the monomeric sequential arrangement along the backbone was not addressed so far. Thus, we report on the thermosensitive properties of a diblock copolymer PDEAAM-b-PNIPAM and compare it to the homopolymers, mixtures thereof, and to the statistical copolymer of the same molecular weight. These polymers were prepared by controlled radical polymerization, namely Reversible Addition–Fragmentation Chain Transfer (RAFT). Their solution behavior was mainly studied by infrared spectroscopy (IR) of the amide I′ band and by turbidimetry. IR spectroscopy sees a decreasing hydration with heating even below the cloud point for all polymers. This results final...

Low temperature depolymerization from a copper-based aqueous vinyl polymerization system

A low temperature strategy for the aqueous polymerization of N-isopropylacrylamide (NIPAM) has been developed. The radical polymerization system is featured with the exclusive use of CuCl and tris(2-dimethylaminoethyl)amine (Me6TREN), devoid of any externally added organic atom transfer radical polymerization (ATRP) initiator. The polymerization reaction is tolerant of variation on the N-substitution pattern, as demonstrated by the synthesis of a homopolymer of acrylamide (AM) and a copolymer of NIPAM and N,N-dimethylacrylamide (DMAA). Importantly, an intriguing phenomenon of depolymerization has been observed at 0 °C with the addition of either 2,2,6,6-tetramethylpiperidinooxy (TEMPO) or 1,4-benzoquinone (BQ) to the polymerization system. A control experiment carried out with conventional 2,2′-azobisisobutyronitrile (AIBN) radical system indicates that copper is required in the depolymerization process. The depolymerization reaction is therefore proposed to proceed through β-alkyl elimination from CuII coordination center. These observations provide a likely starting point for the establishment of new synthetic paradigms for the precision recycling of vinyl monomers under mild and benign conditions.

Effect of crosslinkers on size and temperature sensitivity of poly(N-isopropylacrylamide) microgels

In this work, we study the effect of crosslinkers on the size and swelling properties of temperature sensitive N-isopropylacrylamide (NIPAAm) microgels produced by dispersion polymerization. The crosslinkers studied were N,N′-methylenebisacrylamide (MBA), ethylene glycol dimethacrylate (EGDMA) an 3,9-divinyl-2,4,8,10-tetra-oxaspiro[5.5] undecane (DVA). The type of crosslinker had a major impact on the size and swelling behavior, although the proportion of crosslinker used in each case was low (maximum 5 mol%). The effect can be related to the hydrophilic/hydrophobic characteristics of the crosslinkers. DVA produces smaller hydrogels with reduced swelling ratio, MBA produces bigger microgels with higher swelling ratio, while EGDMA results in an intermediate behavior. With increasing amount of crosslinker used in the synthesis, the extent of the swelling ratio decreases for DVA and EGDMA crosslinked microgels, while increases for MBA crosslinked microgels. There is also a slight effect on the critical transition temperature (T c) of the microgels from 30 °C (DVA) to 34 °C (MBA) as observed in copolymers of NIPAAm with hydrophilic or hydrophobic comonomers.