"On-demand" control of thermoresponsive properties of poly(N-isopropylacrylamide) with cucurbit[8]uril host-guest complexes

The chain end complexation of a functional PNIPAM by a cucurbit[8]uril-viologen complex causes a shift in its lower critical solution temperature (LCST) by over 5 °C. An instantaneous phase change of the thermally responsive polymer beyond its LCST can be induced by addition of the aqueous cucurbituril host-guest complex. Subsequent decomplexation upon addition of a competitive guest releases the PNIPAM terminus and triggers complete reversibility.     

Salt effect on the heat-induced association behavior of gold nanoparticles coated with poly(N-isopropylacrylamide) prepared via reversible addition-fragmentation chain transfer (RAFT) radical polymerization

Poly(N-isopropylacrylamide) (PNIPAM) with a narrow molecular weight distribution was prepared by reversible addition-fragmentation chain transfer (RAFT) radical polymerization. A dithioester group at the chain end of PNIPAM thus prepared was cleaved by treating with 2-ethanolamine to provide thiol-terminated PNIPAM with which gold nanoparticles were coated via reactions of the terminal thiol with gold. The thermoresponsive nature of the maximum wavelength of the surface plasmon band and hydrodynamic radius (Rh) for the PNIPAM-coated gold nanoparticles were found to be sensitively affected by added salt. In pure water, Rh for the PNIPAM-coated gold nanoparticles at 40 degrees C (>lower critical solution temperature (LCST)) was smaller than that at 25 degrees C (相似文献   

PNIPAM chain collapse depends on the molecular weight and grafting density

This study demonstrates that the thermally induced collapse of end-grafted poly(N-isopropylacrylamide) (PNIPAM) above the lower critical solution temperature (LCST) of 32 degrees C depends on the chain grafting density and molecular weight. The polymer was grafted from the surface of a self-assembled monolayer containing the initiator (BrC(CH3)2COO(CH2)11S)2, using surface-initiated atom transfer radical polymerization. Varying the reaction time and monomer concentration controlled the molecular weight, and diluting the initiator in the monolayer altered the grafting density. Surface force measurements of the polymer films showed that the chain collapse above the LCST decreases with decreasing grafting density and molecular weight. At T > LCST, the advancing water contact angle increases sharply on PNIPAM films of high molecular weight and grafting density, but the change is less pronounced with films of low-molecular-weight chains at lower densities. Below the LCST, the force-distance profiles exhibit nonideal polymer behavior and suggest that the brush architecture comprises dilute outer chains and much denser chains adjacent to the surface.     

Nanotube Friendly Poly(N‐isopropylacrylamide)

Poly(N‐ispropylacrylamide) [PNIPAM] is a widely studied polymer for use in biological applications due to its lower critical solution temperature (LCST) being so close to the human body temperature. Unfortunately, attempts to combine carbon nanotubes (CNTs) with PNIPAM have been unsuccessful due to poor interactions between these two materials. In this work, a PNIPAM copolymer with 1 mol‐% pyrene side group [p‐PNIPAM] was used to produce a thermoresponsive polymer capable of stabilizing both single and multi‐walled carbon nanotubes (MWNTs) in water. The presence of pyrene in the polymer chain lowers the LCST less than 4 °C and the interaction with nanotubes does not show any influence on LCST. Moreover, p‐PNIPAM stabilized nanotubes show a temperature‐dependent dispersion in water that allows the level of nanotube exfoliation/bundling to be controlled. Cryo‐TEM images, turbidity, and viscosity of these suspensions were used to characterize these thermoresponsive changes. This ability to manipulate the dispersion state of CNTs in water with p‐PNIPAM will likely benefit many biological applications, such as drug delivery, optical sensors, and hydrogels.

     

The solvent quality of water for poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) in the collapsed state: Implications from single-molecule studies

Both of temperature (in water) and composition (in the water/methanol mixed solvent) can induce the coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM). The atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension (F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic (monotonically increasing force) stretching but not plateau (constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.     

The Solvent Quality of Water for Poly(N-isopropylacrylamide) in the Collapsed State: Implications from Single-molecule Studies

Both of temperature(in water) and composition(in the water/methanol mixed solvent) can induce the coil-toglobule transition of poly(N-isopropylacrylamide)(PNIPAM). The atomic force microscope(AFM) based single molecule force spectroscopy(SMFS) has been exploited to investigate the interactions between the polymer chain and solvent at the single-molecule level. It is found that the single-chain mechanics of PNIPAM show a remarkable dependence on the two external stimuli. A confusing experimental result is that all the force-extension(F-E) curves of unfolding an individual PNIPAM globule present a feature of elastic(monotonically increasing force) stretching but not plateau(constant force) stretching predicted by theory. In this article, we clarify that the presence of the interior solvent molecules in the single-chain globule is the origin of the discrepancy between the F-E curves obtained from theory and experiment. Although both of the external stimuli do tend to lower the solvent quality for PNIPAM, water and the water/methanol mixed solvent will never be the strongly poor solvent for PNIPAM, even at the worst condition.     

End-grafted low-molecular-weight PNIPAM does not collapse above the LCST

The interfacial properties of end-grafted temperature-responsive poly(N-isopropylacryamide) (PNIPAM) were quantified by direct force measurements both above and below the lower critical solution temperature (LCST) of 32 degrees C. The forces were measured between identical, opposing PNIPAM films and between a PNIPAM film and a lipid membrane. At the grafting densities and molecular weights investigated, the polymer extension did not change significantly above the LCST, and the polymers did not adhere. Below the LCST, the force-distance profiles suggest a vertical phase separation, which results in a diluter outer layer and a dense surface proximal layer. At large separations, the force profiles agree qualitatively with simple polymer theory but deviate at small separations. Importantly, at these low grafting densities and molecular weights, the end-grafted PNIPAM does not collapse above the LCST. This finding has direct implications for triggering liposomal drug release with end-grafted PNIPAM, but it increases the temperature range where these short PNIPAM chains function as steric stabilizers.     

DNA mutation analysis based on capillary electrochromatography using colloidal poly(N-isopropylacrylamide) particles as pseudostationary phase

Poly(N-isopropylacrylamide) (PNIPAM) is an interesting class of temperature sensitive, water soluble polymer that has a lower critical solution temperature (LCST) of 32 °C. Above the LCST, PNIPAM gets phase-separated and precipitates out from water. The fascinating temperature-sensitive property of PNIPAM has led to a growing interest in diverse fields of applications. Recently, capillary electrochromatography (CEC) has gained attention due to the wide range of applications based on the use of open tubular capillaries. In this paper, the use of phase-separated PNIPAM as a pseudostationary phase for CEC is demonstrated for the detection of single nucleotide polymorphisms (SNPs). Owing to the dynamic coating, the phase-separated PNIPAM particles did not require any immobilization technique and could exist as a mobile stationary phase in the open tubular capillary. The heteroduplex analyses of mutation samples could be successfully performed based on the phase-separated PNIPAM particles in the constructed CEC system. The CEC system, based on PNIPAM particles capable of having a narrow size distribution, shows great potential as an alternative to conventional DNA mutation systems.     

Interactions between temperature-sensitive hydrogel microspheres and granulocytes

Poly(N-isopropylacrylamide) (PNIPAM) has a low critical solution temperature (LCST) at 32°C in water and the hydrophilicity changes through the LCST. The microspheres whose surface was composed of PNIPAM exhibited phase transition behavior around 32°C. Therefore, the interactions between PNIPAM micropheres and granulocytes depended on the temperature. That is, the oxygen consumption and active oxygen production by cells in contact with PNIPAM-containing microspheres and adhesion of the microspheres to the cell surface were more enhanced above the LCST of PNIPAM than below it, whereas no significant temperature dependence of cell–microspheres interaction was observed in nonthermosensitive microsphere systems. It was suggested that the function of cells could be controlled with temperature using the temperature-sensitive microspheres.     

Hydration and phase separation of temperature-sensitive water-soluble polymers

Collapse of a poly(N-isopropylacrylamide)(PNIPAM) chain upon heating and phase diagrams of aqueous PNIPAM solutions with very flat LCST phase separation line are theoretically studied on the basis of cooperative dehydration(simultaneous dissociation of bound water molecules in a group of correlated sequence),and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods.The transition becomes sharper with the cooperativity parameterσof hydration.Reentrant coil-globule-coil transition in mixed solvent of water and methanol is also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves(LCST cononsolvency) with the mole fraction of methanol due to the competition is calculated and compared with the experimental data.Aqueous solutions of hydophobically-modified PNIPAM carrying short alkyl chains at both chain ends(telechelic PNIPAM) are theoretically and experimentally studied.The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation),and separate from the coil-globule transition line.Associated structures in the solution,such as flower micelles,mesoglobules and higher fractal assembly,are studied by USANS with theoretical modeling of the scattering function.     

The imaging of nano and micro globules of short linear thermo responsive polyacrylamides formed above the lower critical solution temperature

An imaging method has been developed to examine thermo responsive polymer coagulates by optical and electron microscopy. Poly-N-isopropylacrylamide (PNIPAM), poly-N-dimethylacrylamide (PDMAM) and a 1:1 PNIPAM-PDMAM copolymer were encapsulated in a gelatin matrix as coagulates above the lower critical solution temperature (LCST), and subsequently examined by optical and electron microscopy. The linear macromolecules PNIPAM and PDMAM were synthesized by chain transfer polymerization with mercaptopropionic acid (3-MPA) as chain transfer reagent. The resulting polymers have an average molar mass of ∼1800 g/mol and low polydispersity. The LCST of thermo responsive polymers is defined in pure water but can also be stimulated at lower than the phase transition temperature employing electrolytes containing inorganic salts such as (NH₄)₂SO₄. Under such conditions the polymers show the typical thermo responsive phase transfer property in form of a visible clouding point. Gelatin was used to maintain this biphasic state by slowly adding water-softened gelatin sheets at a temperature above the LCST, followed by cooling to 3 °C in order to induce gelation. Examination of the gelatin-coagulate matrices by optical and electronic microscopy showed that PNIPAM and its copolymer (PNIPAM/PDMAM 1:1) are entrapped as globular spheres and clusters of spheres. In comparison pure PDMAM, even if it shows a clouding point, does not form typical LCST coagulates. With PNIPAM and the copolymer, micro globule formation is also possible with slow gelatin formation, without first provoking an LCST. In this particular case, the phase transition, or entropic demixing of the polymers respectively, are induced in this case by water absorption of the gelatin matrix.     

Release property of temperature-sensitive alginate beads containing poly(N-isopropylacrylamide)

The graft copolymer (APN) of alginate and poly(N-isopropylacrylamide) (PNIPAM) were synthesized and APN beads were prepared by dropping the aqueous solution of the copolymer into an aqueous solution of Ca(2+) solution. Alginate chains were employed to play a role in forming beads by electrostatic interactions with a multivalent ion, Ca(2+). Grafted PNIPAM segments were adopted to act as a valve for the pores of the beads, since they exhibit the properties of thermal contraction and expansion. The percent of release of blue dextran from APN beads was higher at 40 degrees C than at 25 degrees C. The difference in the release between two temperatures became more distinguishable when the content of PNIPAM in APN beads is higher. Below lower critical solution temperature (LCST), the expanded PNIPAM would close the pores of the beads, resulting in a lower release rate. Above LCST, the thermally contracted polymer would open the pores, resulting in a higher release rate. The percent of release from APN beads were investigated when the temperature of the release medium is altered. The release rate was relatively low at 25 degrees C. The temperature, however, changed up to 40 degrees C, a marked increase in the release rate was observed. These trends were found to be reproducible when the temperature was repeatedly altered between 25 and 40 degrees C. As a result, a stepwise response to the temperature alteration was obtained.     

Dispersion of polystyrene inside polystyrene‐b‐poly(N‐isopropylacrylamide) micelles in water

The addition of mixture of polystyrene‐b‐poly(N‐isopropylacrylamide) (PS‐b‐PNIPAM) and polystyrene homopolymer (h‐PS) in tetrahydrofuran dropwise into water leads to nanoparticles with a PS core and a thermally sensitive PNIPAM shell. The effects of the ratio of the homopolymer to copolymer and temperature on the formation and stabilization of the dispersion were investigated by using a combination of static and dynamic laser light scattering. PNIPAM shell continuously collapses as temperature increases in the range 20–40 °C. Such formed particles are stable even at temperatures much higher than lower critical solution temperature (LCST ～ 32 °C) of PNIPAM. Our results reveal that the area occupied per hydrophilic PNIPAM chain on the hydrophobic PS core remains nearly a constant regardless of the amount of h‐PS in the polymer mixture. This clearly indicates that the surface area occupied per hydrophilic group is a critical parameter for stabilizing particles dispersed in water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749–755, 2010     

Phase transition behavior of unimolecular micelles with thermoresponsive poly(N-isopropylacrylamide) coronas

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.     

Molecular Dynamics Investigation of the Thermo‐Responsive Polymer Poly(N‐isopropylacrylamide)

Using molecular dynamics simulations with an OPLS force field, the lower critical solution temperature (LCST) of single‐ and multiple‐chain PNIPAM solutions in water is investigated. The sample containing ten polymer chains shows a sudden drop in size and volume at 305 K. Such an effect is absent in the single‐chain system. Large fluctuations of the physical properties of a short single‐chain prevent any clear detection of the LCST for the chosen model system, at least on the time scale of 200 ns. The results provide evidence that a critical number of PNIPAM monomer units must be present in the simulated system before MD simulations are capable to detect conformational changes unambiguously.

     

Biotinylated thermoresponsive core cross-linked nanoparticles via RAFT polymerization and "click" chemistry

A straightforward approach to the synthesis of "clickable" thermoresponsive core cross-linked (CCL) nanoparticles was developed. This approach was based on reversible addition-fragmentation chain transfer (RAFT) radical cross-linking polymerization of styrene and divinylbenzene with azide-functionalized poly(N-isopropylacrylamide) (PNIPAM-N(3)) as macro chain transfer agent in a selective solvent. Spherical nanoparticles with a diameter of 12nm were obtained after 24h polymerization. When the lyophilized CCL nanoparticles were dispersed in THF, spherical nanoparticles were observed, confirming the stability of CCL nanoparticles. The transmission electron microscopy (TEM) studies demonstrated that spherical nanoparticles and wormlike structure coexisted in the aqueous solution. The CCL nanoparticles have a lower critical solution temperature (LCST) at about 29.6°C, a little lower than that of PNIPAM homopolymer. Biotin molecules were conjugated to the surface of CCL nanoparticles via "click" chemistry in aqueous media. After bioconjugation, the LCST shifted to 28.3°C. The bioavailability of biotin to protein avidin was evaluated by a 4'-hydroxyazobenzene-2-carboxylic acid/avidin (HABA/avidin) binding assay and TEM.     

Thermoresponsive Polymer Brush Modulation on the Direction of Motion of Phoretically Driven Janus Micromotors

We report a thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) brush functionalized Janus Au–Pt bimetallic micromotor capable of modulating the direction of motion with the change of the ambient temperature. The PNIPAM@Au–Pt micromotor moved along the Au–Pt direction with a speed of 8.5 μm s^?1 in 1.5 % H₂O₂ at 25 °C (below the lower critical solution temperature (LCST) of PNIPAM), whereas it changed the direction of motion (i.e., along the Pt–Au direction) and the speed decreased to 2.3 μm s^?1 at 35 °C (above LCST). Below LCST, PNIPAM brushes grafted on the Au side were hydrophilic and swelled, which permitted the electron transfer and proton diffusion on the Au side, and thus the motion is regarded as a self‐electrophoretic mechanism. However, PNIPAM brushes above LCST became hydrophobic and collapsed, and thus the driving mechanism switched to the self‐diffusiophoresis like that of Pt‐modified Janus silica motors. These motors could reversibly change the direction of motion with the transition of the hydrophobic and hydrophilic states of the grafted PNIPAM brushes. Such a thermoresponsive polymer brush functionalization method provides a new strategy for engineering the kinematic behavior of phoretically driven micro/nanomotors.     

Synthesis and study of the properties of stereocontrolled poly(N-isopropylacrylamide) gel and its linear homopolymer prepared in the presence of a Y(OTf)3 Lewis acid: effect of the composition of methanol-water mixtures as synthesis media

Poly(N-isopropylacrylamide) (PNIPAM) hydrogels and the corresponding linear homopolymers were synthesized in different methanol-water mixtures (x(m) = 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76, where x(m) is the mole fraction of methanol) in the presence of 0.1 M Y(OTf)(3) Lewis acid. The isotacticity (meso dyad (m), %) and cloud-point temperature of these homopolymers were gradually increased and decreased, respectively, with the increase in the x(m) values of the synthesis solvent mixtures. Moreover, the corresponding linear PNIPAM homopolymers prepared in the absence of Y(OTf)(3) showed an almost constant isotacticity of m = 45% and a cloud-point temperature of 33.0 °C. A SEM study revealed that the resulting hydrogels were highly porous except for the gels prepared at x(m) = 0 and 0.76. The swelling ratios of these hydrogels in water at different temperatures and in different methanol-water mixtures at 20 °C and the deswelling rate and the reswelling rate of these hydrogels were studied. All of these swelling results were compared with that of the corresponding gels prepared in the absence of a Lewis acid (Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Bhimireddi, R.; Rai, R.; Ray, B. J. Appl. Polym. Sci.2012, DOI: 10.1002/app.36318) and explained on the basis of the porosity of the gel, the state of aggregation and isotacticity of the PNIPAM chain segment, and the cononsolvency of the methanol-water mixture toward the PNIPAM chain segment.     

Synthesis and characterization of temperature-sensitive cellulose-graft-poly(N-isopropylacrylamide) copolymers

A new series of cellulose-graft-poly(N-isopropylacrylamide)(cellulose-g-PNIPAM) copolymers were prepared by atom transfer radical polymerization(ATRP) of N-isopropylacrylamide monomers from a cellulose-based macro-initiator, which was homogeneously synthesized in an ionic liquid 1-allyl-3-methylimidazolium chloride(Amim Cl). The composition of cellulose-g-PNIPAM copolymers could be adjusted by altering the feeding ratio and reaction time. The resultant copolymers with relatively high content of PNIPAM segments(molar substitution of PNIPAM ? 18.3) were soluble in water at room temperature. Aqueous solutions of cellulose-g-PNIPAM copolymers exhibited clear temperature-sensitive behavior, and their sol-to-gel phase transition properties were investigated by dynamic light scattering(DLS) and UV measurements. Compared with pure PNIPAM, the cellulose-g-PNIPAM copolymers possessed higher lower critical solution temperatures(LCST) in a range from 36.9 ?C to 40.8 ?C, which are close to normal human body temperature, and could be tuned by adjusting the content of PNIPAM segments in copolymers. Spherical structure of cellulose-g-PNIPAM copolymers formed at temperatures above LCST and its morphology was observed by TEM and SEM. These novel cellulose-g-PNIPAM copolymers may be attractive substrates for some biomedical applications, such as drug release and tissue engineering.     

A ratiometric fluorescent metal ion indicator based on dansyl labeled poly(N-isopropylacrylamide) responds to a quenching metal ion

The fluorescence emission of poly(N-isopropylacrylamide) (PNIPAM) covalently tagged with a 5-(dimethylamino)naphthalene-1-sulfonyl (dansyl) fluorophore and an iminodiacetic acid (IDA) chelator changes with temperature and with Cu(II) complexation. Increasing the temperature above the lower critical solution temperature (LCST) causes the polymer to collapse from a coil to a compact globule. This changes the environment experienced by the fluorophore causing a shift in maximum emission wavelength from 546 to 508 nm and an increase in the ratio of emission intensity at 508 nm to emission intensity at 546 nm from 0.70 to almost 1.40. Metal ions can be sensed by working at a temperature where the uncomplexed polymer is in an expanded state due to the charges on the ligand. Complexation with a metal ion such as Cu(II) neutralizes the charges on the ligand causing the polymer to collapse. At 35 °C, the emission intensity maximum shifted from 535 to 510 nm as Cu(II) concentration was increased and the intensity ratio increased from 0.84 to 1.28. By decoupling complexation from fluorescence, we have prepared a ratiometric fluorescent indicator for a metal ion that normally quenches fluorescence. The affinity for Cu(II) was found to be thermally tunable. The log apparent formation constants for the indicator-Cu(II) complex were estimated as the half way point in the intensity ratio vs. pCu curve. The values were determined to be 4.3 at 35 °C and 3.2 at 34 °C respectively.