LCST Of PNIPAM Research Articles

Polymerization-Induced Proteinosome Formation Initiated by Artificial Cells.

Cellular communication is essential for living cells to coordinate the individual cellular responses and make collective behaviors. In the past decade, the communications between artificial cells have aroused great interest due to the potential applications of the structures in bioscience and biotechnology. To mimic the cellular communication, artificial cell assisted synthesis of proteinosomes was studied in this research. Multienzyme proteinosomes with glucose oxidase (GOx) and horseradish peroxidase (HRP) decorated on the membranes were synthesized by the thermally triggered self-assembly approach. Free radicals produced in a cascade reaction taking place on the surfaces of the multienzyme proteinosomes initiated reversible addition-fragmentation chain transfer (RAFT) polymerization of NIPAM at a temperature above LCST of PNIPAM in the presence of bovine serum albumin (BSA) or alcohol dehydrogenase (ADH)/acetaldehyde dehydrogenase (ALDH), and daughter proteinosomes with BSA or ADH/ALDH on the surfaces were fabricated. The structures of the GOx/HRP initiator proteinosomes, and the synthesized daughter proteinosomes were characterized with transmission electron microscopy, atomic force microscopy, fluorescence microscopy, dynamic light scattering, and micro-DSC. Enzyme activity assays demonstrate the high bioactivities of the enzymes on the surfaces of the initiator and the synthesized daughter proteinosomes.

Rigidity Dictates Spontaneous Helix Formation of Thermoresponsive Colloidal Chains in Poor Solvent.

The formation of helical motifs typically requires specific directional interactions. Here, we demonstrate that isotropic interparticle attraction can drive self-assembly of colloidal chains into thermo-reversible helices, for chains with a critical level of backbone rigidity. We prepare thermoresponsive colloidal chains by cross-linking PNIPAM microgel-coated polystyrene colloids ("monomers"), aligned in an AC electric field. We control the chain rigidity by varying cross-linking time. Above the LCST of PNIPAM, there is an effective attraction between monomers so that the colloidal chains are in a bad solvent. On heating, the chains decrease in size. For the most rigid chains, the decrease is modest and is not accompanied by a change in shape. Much less rigid chains form relatively compact structures, resulting in a large increase in the local monomer density. Unusually, chains with intermediate rigidity spontaneously assemble into helical structures. The chain helicity increases with temperature and plateaus above the collapse transition temperature of the microgel particles. We simulate a minimal model that captures the spontaneous emergence of the helical conformations of the polymeric chain and provides insight into this shape transition. Our work suggests that a purely mechanical instability for semiflexible filaments can drive helix formation, without the need to invoke directional interactions.

How does the addition of shape distinct gold nanoparticles influence on the conformational transition of poly(N-isopropylacrylamide)?

HypothesisThe subject of nanomaterials has created immense interest and expectations in the field of science and nanotechnology. Plentiful aspects proposed by gold nanoparticles (AuNPs) and their capability to affect macromolecular transition is the main driving force to execute the current study. A thermo-responsive polymer poly(N-isopropylacrylamide) (pNIPAM) is studied in presence of nanoparticles, particularly gold nanorods and nanospheres to elucidate completely the effect of their shape, surface area and structural morphology on the conformation of pNIPAM. ExperimentsIn this respect, several biophysical techniques such as fluorescence spectroscopy, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) are utilized to examine the interaction of Au nanospheres/nanorods with pNIPAM. For a better understanding of Au nanoparticles morphology, transmission electron microscopy (TEM) is also employed. FindingsIntroducing gold nanoparticles with the polymeric solution promotes the polymer to stay in the coil conformation at a higher temperature than the LCST of aqueous pNIPAM. A shift of 2 and 25.5 °C in the LCST of pNIPAM is observed along with Au nanospheres and nanorods, respectively. The current study provides a better impact in the field of biomedical science specifically drug delivery and tissue engineering as the LCST approaches human body temperature.

Self-limiting aggregation of phospholipid vesicles.

Lipid vesicles are widely used as model systems to study biological membranes. The self-assembly of such vesicles into vesicle pairs provides further opportunity to study interactions between membranes. However, formation of vesicle pairs, while subsequently keeping their colloidal stability intact, is challenging. Here, we report on three strategies that lead to stable finite-sized aggregates of phospholipid vesicles: (i) vesicles containing biotinylated lipids are coupled together with streptavidin, (ii) bridging attraction is exploited by adding cationic polymers (polylysine) to negatively charged vesicles, and (iii) temperature as a control parameter is used for the aggregation of vesicles mixed with a thermo-sensitive surfactant. While each strategy has its own advantages and disadvantages for vesicle pair formation, the latter strategy additionally shows reversible limited aggregation: above the LCST of pNIPAm, vesicle pairs are formed, while below the LCST, single vesicles prevail. Mixing protocols were assessed by dynamic and static light scattering as well as fluorescence correlation spectroscopy to determine under which conditions vesicle pairs dominate the aggregate size distribution. We have strong indications that without subsequent perturbation, the individual vesicles remain intact and no fusion or leakage between vesicles occurs after vesicle pairs have formed.

Structural color and rheology of self-assembled poly(N-isopropylacrylamide-methacrylic acid) microgels in water

Microgels are excellent building blocks for photonic band gap materials (PBG) in water. Aqueous dispersions of temperature and pH responsive microgels composed by isopropylacrylamide (NIPA, 92 mol%) and methacrylic acid (MA, 8 mol%) show structural color in the concentration range of 44–88 mg/mL. At temperatures below the LCST of pNIPAM in water (∼32 °C), the dispersions self-organize as an elastic solid, with concentration-dependent refractive index. At 22 °C the color changes from red to blue when increasing the concentration. This is due to an increase in the elastic modulus. A transition to a liquid phase (characterized by a loss of the elastic modulus and of the structural color) occurs at a temperature just below the LCST. The color is a result of the diffraction of the visible light that do not propagate within the PBG as shown by the confocal reflectivity images of the microgel dispersions.

AuNPs-Based Thermoresponsive Nanoreactor as an Efficient Catalyst for the Reduction of 4-Nitrophenol.

A new AuNPs-based thermosensitive nanoreactor (SiO2@PMBA@Au@PNIPAM) was designed and prepared by stabilizing AuNPs in the layer of poly(N,N’-methylenebisacrylamide) (PMBA) and subsequent wrapping with the temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) layer. The new nanoreactor exhibited high dispersibility and stability in aqueous solution and effectively prevented the aggregation of AuNPs caused by the phase transformation of PNIPAM. The XPS and ATR-FTIR results indicated that AuNPs could be well stabilized by PMBA due to the electron transfer between the N atoms of amide groups in the PMBA and Au atoms of AuNPs. The catalytic activity and thermoresponsive property of the new nanoreactor were invested by the reduction of the environmental pollutant, 4-nitrophenol (4-NP), with NaBH4 as a reductant. It exhibited a higher catalytic activity at 20 °C and 30 °C (below LCST of PNIPAM), but an inhibited catalytic activity at 40 °C (above LCST of PNIPAM). The PNIPAM layer played a switching role in controlling the catalytic rate by altering the reaction temperature. In addition, this nanoreactor showed an easily recyclable property due to the existence of a silica core and also preserved a rather high catalytic efficiency after 16 times of recycling.

Selective Endothelial Cell Adhesion via Mussel-Inspired Hybrid Microfibrous Scaffold

Endothelialization of the polymer substrate is limited by unspecific cell adhesion. Herein, a biodegradable microfibrous scaffold with a reversibly thermoswitchable property was developed to dynamically regulate endothelial progenitor cell adhesion by exposing or concealing cRGD motif to the surface with a thermosensitive moiety (cRGD-PNIPAM) and antifouling moiety linear polyglycerol (LPG). Owing to a reversible αvβ3 integrin–cRGD interaction and ligand presentation, the accelerated endothelial cell adhesion and spreading were achieved. Under the static and dynamic conditions, prestained endothelial cells were quickly attached to the surface at 25 °C via the integrin–cRGD interaction, and the cRGD was the headgroup of the stretched PNIPAM below the LCST of PNIPAM. With the increase of the temperature to 37 °C, a quick detachment of cells from the surface was observed due to the cRGD moiety being shielded by the antifouling LPG layer. As compared to current strategies for endothelialization, for example, ...

Thermal responsiveness and binding affinity of cucurbit[7]uril terminal poly(N-isopropylacrylamide)

The introduction of CB[7] and guests raised the LCST of PNIPAM significantly, and CB[7]-PNIPAM maintained the high binding affinity of CB[7].

Energetics and Mechanism of Conformational Transitions of Protein‐Like NIPAM‐Sodium Styrene Sulfonate Copolymers in Aqueous Solutions

Protein‐like and random NIPAM‐sodium styrene sulfonate copolymers of similar composition have been prepared by radical polymerization in water at temperatures above and below the LCST of PNIPAM, respectively. Thermal transitions of the copolymers in aqueous solutions have been studied by means of dynamic light scattering, viscometry, and high‐sensitivity differential scanning calorimetry. The phase separation or cooperative conformational transitions without phase separation were observed for the random or the protein‐like copolymers, respectively. Transition temperature, enthalpy, and heat capacity increment of the protein‐like copolymer differed insignificantly from those of the random copolymer of similar composition. The transition heat capacity increments of the protein‐like copolymers revealed that only 10–20% of their NIPAM links participate in the formation of a dense water‐free globule core. The coil–globule transitions of the protein‐like copolymers were described by the thermodynamic three‐state model according to the scheme “random coil↔condensed coil↔globule”, which is known to simulate the folding mechanism of globular proteins. image

Self-assembly of multi-responsive poly(N-isopropylacrylamide)-b-poly(N,N-dimethylaminopropylacrylamide) in aqueous media

Stimuli responsive double hydrophilic diblock copolymers poly(N-isopropylacrylamide)-b-poly(N,N-dimethylaminopropylacrylamide) (PNIPAM-b-PDMPA) with constant degree of polymerization (DP) of PNIPAM (=127) and varying DP of PDMPA (=31 and 66) were synthesized by RAFT polymerization, and characterized by NMR, FTIR and GPC. In dilute aqueous solutions, the copolymers remain as unimers in equilibrium with loosely bound aggregates and form aggregates with PNIPAM core in the presence of NaCl or at temperatures above the LCST of PNIPAM. However, at higher temperatures i.e. above cloud point temperature (CPT) of PNIPAM127-b-PDMPA66, core–shell micelles are formed with PNIPAM core. The CPT of both block copolymers decrease in the presence of NaCl and in acidic pH, but increases with increase in DP of PDMPA. The copolymer showed enhanced surface activity in the presence of NaCl. The temperature dependent micellization observed from NMR is supported by turbidity and fluorescence. PDMPA is highly protonated in acidic pH; cryo-TEM and SANS confirm the formation of threadlike micelles in the presence of SDS.

Single chain structure of a poly(N-isopropylacrylamide) surfactant in water.

We present atomistic simulations of a single PNIPAM-alkyl copolymer surfactant in aqueous solution at temperatures below and above the LCST of PNIPAM. We compare properties of the surfactant with pure PNIPAM oligomers of similar lengths, such as the radius of gyration and solvent accessible surface area, to determine the differences in their structures and transition behavior. We also explore changes in polymer-polymer and polymer-water interactions, including hydrogen bond formation. The expected behavior is observed in the pure PNIPAM oligomers, where the backbone folds onto itself above the LCST in order to shield the hydrophobic groups from water. The surfactant, on the other hand, does not show much conformational change as a function of temperature, but instead folds to bring the hydrophobic alkyl tail and PNIPAM headgroup together at all temperatures. The atomic detail available from these simulations offers important insight into understanding how the transition behavior is changed in PNIPAM-based systems.

Preparation and Controlled Drug Release Characteristics of Thermoresponsive PEG/Poly (NIPAM-co-AMPS) Hydrogels

Thermosensitive hydrogels were prepared by free radical polymerization of N-isopropylacrylamide (NIPAM) and 2-acrylamido-2-methylpropanesulphonic acid (AMPS) in presence of polyethylene glycol (PEG) as macroinitiator. The incorporation of functional monomer in the hydrogel was confirmed by infrared spectroscopy. The equilibrium swelling ratio (ESR), deswelling rate, volume transition temperature, and surface morphology of synthesized hydrogels are found to be dependent on concentration of AMPS monomer in the feed composition. The equilibrium swelling ratio (ESR) and water retention properties increase with increasing AMPS concentration in hydrogels and decrease with increasing salt concentration in external medium. The hydrogel with high dose of AMPS has high loading efficiency of theophylline drug, whereas the in-vitro release of the drug is delayed for a prolonged period in swollen gels above LCST of PNIPAM.

Thermo-responsive discotic nematic hydrogels

We synthesized a novel thermo-responsive discotic nematic hydrogel. This liquid crystalline polymer nanocomposite was fabricated using extremely high-aspect-ratio (ξ = diameter/thickness ∼ 103) zirconium phosphate (ZrP) nano-sheets embedded into a polymeric network. These nano-sheets were immersed into a solution of constituent monomers of N-isopropylacrylamide (NIPAm) and acrylamide (AAm) with a subsequent polymerization in situ to obtain a hydrogel embedded with the nematic liquid crystal of the nano-sheets. We analyzed the effect of initiator (APS) and catalyst (TEMED) concentration on the nematic domains at ZrP nano-sheet volume fractions (ϕZrP) near the isotropic–nematic (I–N) transition. The mixing during hydrogel preparation allowed the aligning of the nano-sheets into highly ordered domains, which were eventually disturbed by the effect of APS and TEMED on the polymer matrix. The effect of ZrP nano-sheet concentration and temperature on the nematic hydrogels was also studied. By increasing the temperature above the LCST of PNIPAm, the hydrogels entered into a hydrophilic-to-hydrophobic transition. Interestingly, hydrogels containing ZrP nano-sheets presented increased hydrogel water retention. In addition, we observed that the variation of the nematic ordering (S2) and ϕZrP within the system manipulated the birefringent color patterns of the hydrogels. Finally, a nematic hydrogel placed into a thin-rectangular capillary from 298 K to 333 K presented edge-wrinkling. An isotropic-to-nematic transition guided by the increment in the ZrP nano-sheet nematic ordering was observed at a nano-sheet concentration slightly below the isotropic concentration (ϕI).

<b>pH- and Thermo-Induced Specific Permeability of Chiral Amino Acids through the Peptide Grafted Poly(<i>N</i>-isopropylamide)Network Membrane</b>

In this article we prepared an amphiphilic peptide, (L-Leu-L-Lys)8, grafted poly(N-isopropylacrylamide)network membrane. The graft content of the peptide and the content of the cross-linker were 4% and 16%, respectively. We investigated the permeability of L- and D- phenylalanine through this membrane. The pH- and thermo-induced permeability changes of the network membrane were observed. Under the neutral condition(pH 6.5 and 20℃), the peptide grafted PNIPAm network membrane showed a significant difference between the permeability of L-phenylalanine and D-phenylalanine. The maximum permselectivity(α=2.6)was achieved under this condition. At temperatures above the LCST of PNIPAm, PNIPAm polymers become hydrophobic; hence, the membrane shrunk. And under these conditions the permeable path for L- and D- phenylalanine was formed in the membrane which results in the non-permselectivity through this membrane.

Synthesis, characterization, and temperature-dependent colloidal stability of poly(N-isopropylacrylamide)-grafted polystyrene/poly(styrene-co-4-vinylbenzyl N, N-diethyldithiocarbamate) hairy particles

Poly(N-isopropylacrylamide)-grafted polystyrene/ poly(styrene-co-4-vinylbenzyl N, N-diethyldithiocarbamate) (PNIPAM-grafted PS/P(St-co-VBDC)) hairy particles were synthesized by photo-polymerizing N-isopropylacrylamide monomer in the presence of PS/P(St-co-VBDC) core par- ticles. Here, the VBDC unit, which was incorporated into the surface of core particles by seeded soap-free emulsion copolymerization, acted as a photo-iniferter. By varying the polymerization conditions, a series of hairy particles having different grafting heights and grafting densities was success- fullyobtained.Thehairyparticlesexhibitedwell-definedcore/ shell morphology. PS/P(St-co-VBDC) formed the core which was surrounded by PNIPAM shell. The determination of critical coagulation concentration (CCC) indicated that the hairy particles were stabilized via both electrostatic and steric mechanisms (i.e., electrosteric mechanism) at a temperature lower than LCST of PNIPAM. However, these particles gave muchlowerCCCs whenheatedtothe temperature higherthan LCST, exhibiting temperature-dependent colloidal stability.

Selective removal of 90Sr and 60Co from aqueous solution using N-aza-crown ether functional poly(NIPAM) hydrogels

The ability of temperature and pH responsive cross-linked poly(NIPAM-co-AAc) hydrogel beads containing covalently bound N-aza-crown ethers to selectively complex 90Sr and 60Co under competitive and non-competitive conditions is demonstrated using a combination of autoradiography and scintillation measurements. Due to the presence of copolymerised acrylic acid, which is incorporated to introduce pH responsive behaviour as well as acting as the site of covalent attachment of the N-aza-crown ether, the decrease in particle diameter as a function of increasing temperature on going through the LCST of PNIPAM is reduced in the presence of electrolyte. At pH 2 no complexation of either 90Sr or 60Co was observed. At pH 7 the incorporation of acrylic acid under non-competitive conditions results in the ability to complex ca. 90% 90Sr and 60Co which increases to between 94% and 96% on inclusion of N-aza-crown ether. Under competitive conditions the hydrogels lose the ability to selectively complex 90Sr, although the selectivity for 60Co was enhanced in the presence of N-aza-crown ether. Subsequent recovery of bound 60Co from the hydrogel on lowering to pH 2 was only partially successful, and lead to a decrease in the swelling behaviour on readjustment to pH 7.

Supramolecular Hybrid Hydrogels from Noncovalently Functionalized Graphene with Block Copolymers

In recent years, graphene has become a prevailing topic in the materials community. Numerous applications of graphene have been generated, including gas sensors, photovoltaics, field-effect transistors, etc. In this paper, we demonstrate that graphene sheets could serve as a desirable inorganic constituent in constructing hybrid polymeric hydrogels via supramolecular routes, which currently is not so popular as the other applications. In this paper, graphene oxide (GO) nanosheets were modified by grafting β-cyclodextrins first, leading to chemical converted graphene (CCG), and then noncovalently functionalized by block copolymers AZO-PDMA-b-PNIPAM via inclusion complexation. The resulted hybrid graphene inclusion complex (HGIC) was fully characterized by a combination of techniques including UV–vis spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Then, due to the thermo sensitivity of the PNIPAM block, HGIC solutions exhibited sol–gel transition at elevated temperature. Rheology studies revealed that the gelation of the HGIC suprastructure took place much faster at a temperature close to but lower than the LCST of PNIPAM. It was also found that the gelation temperature increased with the ratio of degree of polymerization of PDMA block to PNIPAM block of HGIC suprastructures. On the basis of the results, a new possible gelation mechanism was proposed. The flexible and ultrathin 2D planar structure of graphene sheets exhibited unprecedented advantage in constructing the 3D network structure of the hydrogels, showing rapid sol–gel transition at elevated temperature.

Poly(N-isopropylacrylamide) grafted into nanopores: Thermo-responsive behaviour in the presence of different salts

Surface initiated polymerization of N(isopropylacrylamide) (NIPAM) was performed by controlled radical polymerization on PET track-etched membranes presenting two different pore diameters (narrow pores: ∼80 nm and large pores: ∼330 nm). The opening and closing characteristics of the resulting PNIPAM-g-PET membranes were investigated by conductometric measurements carried out at different temperatures below and above the LCST of PNIPAM and in the presence of different salts. Depending on the membrane pore size, two types of permeation control mechanisms are observed. In large pore membranes, expanded PNIPAM chains conformations result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, in narrow pore membranes, the expanded PNIPAM brush presents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.

Preparation and characterization of poly (N-isopropylacrylamide)/polyvinylamine core-shell microgels

In this paper, well-defined temperature- and pH-sensitive core-shell microgels were synthesized by graft copolymerization in the absence of surfactant and stabilizer. The microgel particles consisted of poly (N-isopropylacrylamide (NIPAm)) core crosslinked with N, N′-methylene-bisacrylamide (MBA) and polyvinylamine (PVAm) shell. The effect of MBA content and NIPAm/PVAm ratio on microgel size was investigated. SEM showed that the microgels were spherical and had narrow particle-size distribution. TEM images of the microgels clearly displayed well-defined core-shell morphologies. Zeta-potential measurement further elucidated that the microgels possessed positively charged PVAm molecules on the microgel surface. Turbidity measurement and 1H-nuclear magnetic resonance (NMR) experiments indicated that the VPTT of microgels was the same as the LCST of PNIPAm. 1H-NMR experiments also inferred that the methyl proton of N-isopropylacrylamide appeared three peaks and responded to hydrogen-bonding interaction including polymer chain with water molecular, intramolecular interaction and intermolecular interaction, respectively.

A Composite Thermo‐Responsive Membrane System for Improved Controlled‐Release

AbstractA novel composite thermo‐responsive membrane system for improved controlled‐release is successfully developed. The membrane is composed of a porous membrane with grafted poly(N‐isopropylacrylamide) (PNIPAM) gates acting as functional valves, and a cross‐linked PNIPAM hydrogel inside the reservoir acting as the solute carrier. The thermo‐responsive controlled‐release characteristics of the proposed system are studied when the ambient temperature is continuously increased from 20 to 45 °C (across the LCST of PNIPAM) at a constant rate of 1.5 °C/min. The experimental results show that the prepared system exhibits significantly better performance for thermo‐responsive controlled‐release than single‐functional systems currently in existence, due to the cooperative action of the gating membrane and the inner cross‐linked hydrogel. Furthermore, due to the distinctive composite architecture, the proposed system can overcome some inherent disadvantages of current systems, such as the drug security problem of the reservoir‐type systems and the mechanical strength problem of the hydrogel matrix‐type systems. The system proposed in this study provides a new mode for thermo‐responsive controlled‐release.