Lower Critical Solution Temperature Of PNIPAM Research Articles

Smart Antifouling and Self‐Cleaning Membrane for Effective Oil/Water Separation

AbstractA novel membrane consisting of cellulose acetate (CA) nanofibers and poly(N‐isopropylacrylamide) (PNIPAM) microparticles is successfully fabricated by simultaneous electrospinning and electrospraying. The CA/PNIPAM membrane has highly effective gravity‐driven separation performances for both oil/water mixtures and oil‐in‐water emulsions. It separates oil droplets from oil‐in‐water mixtures and oil‐water emulsions with rejection rates of 99.83% and 96%, respectively. In order to examine the contribution of PNIPAM particles, the performance of the CA/PNIPAM membrane is compared with the CA membrane. Increased hydrophilicity due to the inclusion of the PNIPAM particles between CA nanofibers results in a higher rejection ratio and superior antifouling performance. While the CA membrane becomes unusable after 10 cycles during the separation of the oil–water emulsion, the CA/PNIPAM membrane is still in good shape after 20 cycles. The self‐cleaning ability of the membrane is examined through the permeation flux below and above the lower critical solution temperature (LCST) of PNIPAM. The reversible thermo‐responsive flux variation proves that the pore sizes increase at temperatures above the LCST of PNIPAM. Moreover, when the lubricating oil–water emulsion is filtered through the membrane, the permeate changes from clear to turbid due to the nonretained oil particles as the temperature passes through the LCST.

Multi-stimuli responsive poly(N-isopropyl-co-tetraphenylethene) acrylamide copolymer mediating AIEgens by controllable tannic acid

Copolymerization of N-isopropylacrylamide (NIPAM) and tetraphenylethene (TPE) acrylamide monomers produced aggregation-induced emission (AIE) active copolymers P1 , P2 , and P3 with relatively lower molecular weights (6000). Like poly-N-isopropylacrylamide (p-NIPAM), an aqueous solution of copolymers ( P1-P3 ) responds to temperature and undergoes a change in fluorescence intensity due to conformational transition (swelling/shrinking) below and above their lower critical solution temperature (LCST), respectively. The LCST of p-NIPAM (32 °C) shifted downward upon the incorporation of TPE in the copolymer chains depending on the concentration of TPE . Compared to P1 , the fluorescence response increased to a five-fold enhancement with increasing concentrations of tannic acid ( TA ) owe to the form of the P1-TA complex. Furthermore, the P1-TA complex showed a surprising change in fluorescence response under the effect of temperature and pH value. The conformational transitions of P1-TA were also studied by dynamic light scattering (DLS). DLS measurements for the P1-TA complex justified its fluorescence response based on particle size at different concentrations and pH value. Such stimuli-responsive AIE-active copolymers find potential applications in drug delivery systems for monitoring loading and release processes by fluorescence. • Compared to P1, the fluorescence intensity of AIE-active P1-TA complex increased to a five-fold enhancement. • A multi-stimuli response on temperature and pH value based on intermolecular hydrogen bonding interaction. • The interpenetrating networks can be formed without using two polymer chains or more.

Biosurfaces Fabricated by Polymerization-Induced Surface Self-Assembly.

Surface biofunctionalization provides an approach to the fabrication of surfaces with improved biological and clinical performances. Biosurfaces have found increasing applications in many areas such as sensing, cell growth, and disease detection. Efficient synthesis of biosurfaces without damages to the structures and functionalities of biomolecules is a great challenge. Polymerization-induced surface self-assembly (PISSA) provides an effective approach to the synthesis of surface nanostructures with different compositions, morphologies, and properties. In this research, application of PISSA in the fabrication of biosurfaces is investigated. Two different reversible addition-fragmentation chain transfer (RAFT) agents, RAFT chain transfer agent (CTA) on silica particles (SiO2-CTA) and CTA on bovine serum albumin (BSA-CTA), were employed in RAFT dispersion polymerization of N-isopropylacrylamide (NIPAM) in water at a temperature above the lower critical solution temperature (LCST) of poly-(isopropylacrylamide) (PNIPAM). After polymerization, PNIPAM layers with BSA on the top surfaces are fabricated on the surfaces of silica particles. Transmission electron microscopy results show that the average PNIPAM layer thickness increases with monomer conversion. Kinetics study indicates that there is a turn point on a plot of ln([M]0/[M]t) versus polymerization time. After the critical point, surface coassembly of PNIPAM brushes and BSA-PNIPAM bioconjugates is performed on the silica particles. The secondary structure and the activity of BSA immobilized on top of the PNIPAM layers are basically kept unchanged in the PISSA process. To prepare permanently immobilized protein surfaces, PNIPAM layers on silica particles are cross-linked. BSA on the top surfaces presents a reversible "on-off" switching property. At a temperature below the LCST of PNIPAM, the activity of the immobilized BSA is retained; however, the BSA activity decreases significantly at a temperature above the LCST because of the hydrophobic interaction between PNIPAM and BSA. Based on this approach, many different biosurfaces can be fabricated and the materials will find applications in many fields, such as enzyme immobilization, drug delivery, and tissue engineering.

Insights on the Lower Critical Solution Temperature Behavior of pNIPAM in an Applied Electric Field

Poly(N-isopropylacrylamide), or pNIPAM, is a free-radical polymer that is commonly studied for uses in surface coatings, tissue engineering, energy storage, biosensing, and more, due to its temperature responsiveness. pNIPAM is known to solubilize at temperatures below its lower critical solution temperature (LCST) and agglomerate above its LCST. This behavior has been shown to be reproducible and reversible. We confirmed this reversibility and the value of the LCST by performing dynamic light scattering (DLS) with a temperature sweep (increase and decrease). However, performing the same experiment under an applied voltage from copper electrodes, we observed a decrease in the LCST of pNIPAM and irreversible aggregation. Here we present preliminary data comparing the LCST behavior of pNIPAM in the presence of applied voltage using copper, aluminum, and carbon electrodes. We present data in support of the hypothesis that a phenomenon is occurring specifically with the use of copper electrodes that is altering pNIPAM LCST behavior.

Shifts of pNIPAM Lower Critical Solution Temperature in an Applied Electric Field

Poly(N-isopropylacrylamide), or pNIPAM, is a free-radical polymer that is commonly studied for uses in surface coatings, tissue engineering, energy storage, biosensing, and more, due to its temperature responsiveness. Below its transition temperature, pNIPAM is dissolved in solution with a small particle size. Above the transition temperature, pNIPAM agglomerates and condenses out of solution. Dynamic Light Scattering (DLS) measures temperature induced agglomeration of pNIPAM by determining the transition temperature, which is typically 30-35oC for pNIPAM. We initially confirmed the lower critical solution temperature (LCST), or the point in which pNIPAM agglomerates, to be thermodynamically reversible and reproducible for both pNIPAM and an electrochemically active poly(N-isopropylacrylamide)-block-poly(ferrocenylmethyl methacrylate) block copolymer, or p(NIPAM-b-FMMA).Applying a potential across the DLS light path, we expected p(NIPAM-b-FMMA) to cause a shift in the transition temperature and agglomeration behavior. However, we unexpectedly observed a shift in the transition behavior of pNIPAM. At an applied electric field of 400 mV/cm and above, the transition temperature of pNIPAM started to shift negative. Further, upon cooling, the pNIPAM never re-dissolves, indicating an irreversible agglomeration in the presence of an applied.We have also started to investigate reasons on why this phenomenon might occur. We will discuss two primary hypotheses, including wetting behavior changes of the pNIPAM and further radicalization of the RAFT agents leading to cross-linking and irreversible behavior.Acknowledgements: The authors would like to acknowledge the financial support from NIH (P20 GM113131), NSF (CBET 1638896), the Collaborative Research Excellence Initiative Pilot Research Program at the University of New Hampshire, and the Hamel Center of Undergraduate Research at the University of New Hampshire. The authors also would like to acknowledge the assistance of Dr. Seitz’s laboratory and Dr. Halpern’s laboratory

Hybrid Complex Coacervate.

Underwater adhesion represents a huge technological challenge as the presence of water compromises the performance of most commercially available adhesives. Inspired by natural organisms, we have designed an adhesive based on complex coacervation, a liquid–liquid phase separation phenomenon. A complex coacervate adhesive is formed by mixing oppositely charged polyelectrolytes bearing pendant thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains. The material fully sets underwater due to a change in the environmental conditions, namely temperature and ionic strength. In this work, we incorporate silica nanoparticles forming a hybrid complex coacervate and investigate the resulting mechanical properties. An enhancement of the mechanical properties is observed below the PNIPAM lower critical solution temperature (LCST): this is due to the formation of PNIPAM–silica junctions, which, after setting, contribute to a moderate increase in the moduli and in the adhesive properties only when applying an ionic strength gradient. By contrast, when raising the temperature above the LCST, the mechanical properties are dominated by the association of PNIPAM chains and the nanofiller incorporation leads to an increased heterogeneity with the formation of fracture planes at the interface between areas of different concentrations of nanoparticles, promoting earlier failure of the network—an unexpected and noteworthy consequence of this hybrid system.

Synthesis of temperature-responsive magnetic mesoporous silica and temperature dependence of its physical properties

In this study, a novel composite (PAMMS) was prepared using poly (N-isopropylacrylamide) (PNIPAM) as a temperature-responsive polymer, magnetite as a magnetic substance, and amino-functionalized mesoporous silica (MS) as an adsorbent. The temperature-dependence of the physical properties such as methyl orange (MO) adsorption, aggregation/sedimentation, and magnetic separation of this composite was investigated. It was found that the composite consisted of 11.3 wt% magnetite, 0.6 mmol/g amino groups, and 4.6 wt% PNIPAM. The MO adsorption behaviour of the PAMMS composite could be well-explained by the Langmuir model and pseudo-second-order rate equation. At 20 °C (below the lower critical solution temperature (LCST) of PNIPAM), the composite showed an MO adsorption amount of 174 μmol/g, which is slightly larger than that (143 μmol/g) at observed 35 °C (above the LCST of PNIPAM). The optical microscopic observations revealed that the size of the PAMMS aggregates at 45 °C (above LCST) was larger than that at room temperature. Hence, the magnetic separation ratio of PAMMS at 45 °C was higher than that at room temperature. The PAMMS composite showed effective MO adsorption below the LCST of PNIPAM and magnetic separation above the LCST of PNIPAM.

Light- and temperature-responsive micellar carriers prepared by spiropyran-initiated atom transfer polymerization: Investigation of photochromism kinetics, responsivities, and controlled release of doxorubicin

Stimuli-responsive block copolymer assemblies are a significant class of smart materials extensively studied for advanced applications such as drug- and gen-delivery systems. Different types of light- and temperature-responsive polymer assemblies containing photochromic spiropyran chain end groups, temperature-responsive poly (N-isopropylacrylamide) (PNIPAM) block, and hydrophobic poly (methyl methacrylate) (PMMA) block were developed. These amphiphilic block copolymers with the ability of self-assembly to micellar structures were synthesized using spiropyran-initiated atom transfer radical polymerization. Regarding the hydrophilic fraction (0.5<fphilic), the amphiphilic block copolymers were self-assembled to spherical micelles in aqueous media as confirmed by TEM images. The size of these polymer assemblies was changed in response to temperature changes and also UV light (365 nm) irradiation. The results of UV–Vis spectroscopy display that photochromic behavior, the kinetics of spiropyran (SP) to merocyanine (MC) and MC to SP isomerizations were significantly affected by the polarity of the polymer blocks in neighboring of the spiropyran chain end group. The lower critical solution temperature (LCST) of PNIPAM shows a shift from 26.5 to 37.5 °C after isomerization of the SP to MC, as a result of increase of its polarity and water-solubility. The doxorubicin (DOX) release can be controlled by pH, temperature, and light by using of multi-responsive block copolymer micelles as smart drug carriers. The release efficiency was reached to 60–85% at pH 5.3, 80–90% at temperatures of above LCST of PNIPAM (40 °C), and also 90–100% under UV irradiation after 48 h. The results display that stimuli-responsive polymer assemblies based on amphiphilic block copolymers containing spiropyran chain end groups are applicable in smart drug-delivery systems, and offer controlling over drug-release by different triggers, such as light irradiation, pH variation, and temperature changes.

Profiling the molecular interactions between a promising thermoresponsive polymer and ionic liquid: A biophysical outlook

Thermoresponsive polymers (TRPs) that undergo significant conformation/aggregation changes in response to changes in temperature have extensive range of potential applications in various fields of science and technology. Among different types of TRPs, poly (N-isopropylacrylamide) (PNIPAM) is a suitable amphiphilic polymer for studying the conformational changes due to its biocompatible behavior with various additives. In order to get deeper insights of the TRPs in the presence of additives, we examined the collapse and aggregation of thermoresponsive polymer (PNIPAM) in aqueous solution containing variable amounts of 1-allyl-3-methylimidazolium bromide ([Amim][Br]). The influence of ionic liquid on the lower critical solution temperature (LCST) of PNIPAM aqueous solution were examined with the aid of comprehensive biophysical techniques such as UV–visible absorption spectroscopy, steady-state fluorescence spectroscopy, thermal fluorescence spectroscopy, viscosity (η) and dynamic light scattering (DLS) measurements. With the addition of 5 mg/mL of [Amim][Br] to the PNIPAM aqueous solution the LCST has been decreased towards lower temperatures. Further with the addition of higher concentration (10 and 15 mg/mL) of [Amim][Br] the decrease in the LCST of PNIPAM has been more pronounced towards lower temperatures. Our experimental results explicitly elucidated that the decrease in the LCST of the PNIPAM with increase in the concentration of [Amim][Br] is due to the substantial variations in the interactions among amide group of polymer, ions of ionic liquid and water molecules. The experimental results of the current investigation can be used for the design of smart polymer-based devices, as its LCST is close to body temperature. The results provide an alternative way to tune the phase transition temperature of the broadly accepted model PNIPAM polymer.

Aggregation behavior of double hydrophilic block copolymers in aqueous media

Linear water-soluble stimuli-responsive double hydrophilic block copolymers, poly(N-isopropylacrylamide)-block-poly(acrylic acid) [abbreviated as PNIPAM-b-PAA (degree of polymerization (DP) of PNIPAM = 149 and DP of PAA = 46 or 85)] were synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization technique and characterized by gel-permeation chromatography (GPC) and nuclear magnetic resonance (1H NMR) spectroscopy. Turbidimetry, spectral, and scattering studies showed that block copolymers remain as molecularly dissolve in water below the lower critical solution temperature (LCST) of PNIPAM block (~32 °C). Turbidimetry results confirm the formation of aggregates with PNIPAM core and PAA as a shell in the presence of NaCl, CaCl2 and at temperatures above the LCST of PNIPAM. The cloud point temperature (TC) of both block copolymers increases with an increase in DP of PAA in neutral and basic pH and decreases at low pH. Small angle neutron scattering (SANS) data confirmed the formation of worm- or thread-like micelles in the presence of a cationic surfactant dodecyltrimethylammonium bromide (DTAB).

Thermally Switchable Liquid Crystals Based on Cellulose Nanocrystals with Patchy Polymer Grafts.

A thermally "switchable" liquid-crystalline (LC) phase is observed in aqueous suspensions of cellulose nanocrystals (CNCs) featuring patchy grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM). "Patchy" polymer decoration of the CNCs is achieved by preferential attachment of an atom transfer radical polymerization (ATRP) initiator to the ends of the rods and subsequent surface-initiated ATRP. The patchy PNIPAM-grafted CNCs display a higher colloidal stability above the lower critical solution temperature (LCST) of PNIPAM than CNCs decorated with PNIPAM in a brush-like manner. A 10 wt% suspension of the "patchy" PNIPAM-modified CNCs displays birefringence at room temperature, indicating the presence of an LC phase. When heated above the LCST of PNIPAM, the birefringence disappears, indicating the transition to an isotropic phase. This switching is reversible and appears to be driven by the collapse of the PNIPAM chains above the LCST, causing a reduction of the rods' packing density and an increase in translational and rotational freedom. Suspensions of the "brush" PNIPAM-modified CNCs display a different behavior. Heating above the LCST causes phase separation, likely because the chain collapse renders the particles more hydrophobic. The thermal switching observed for the "patchy" PNIPAM-modified CNCs is unprecedented and possibly useful for sensing and smart packaging applications.

Temperature-responsive Pickering foams stabilized by poly(N-isopropylacrylamide) nanogels

Poly(N-isopropylacrylamide) (PNIPAM) nanogels were synthesized by emulsion polymerization using sodium dodecyl sulfate (SDS). After removal of SDS by dialysis, the surface tensions of the PNIPAM nanogel aqueous dispersions were measured at various temperatures by the pendant-drop method, and it was found that the surface tensions of the nanogel dispersion below the lower critical solution temperature (LCST) of PNIPAM were much smaller than those of water and comparable to those of the SDS aqueous solution. The stability of the aqueous foams generated by nitrogen bubbling thorough the PNIPAM nanogel dispersion was investigated below and above the LCST of PNIPAM. The foam prepared below the LCST was stable in some degree, whereas almost no foam was formed above the LCST. Moreover, the foam prepared below the LCST was quickly collapsed by changing the temperature above the LCST. This rapid defoaming represents that the surface activity of the PNIPAM nanogel can be switched off by the temperature increase across the LCST.

Waterborne Electrospinning of Poly(N-isopropylacrylamide) by Control of Environmental Parameters.

With increasing toxicity and environmental concerns, electrospinning from water, i.e., waterborne electrospinning, is crucial to further exploit the resulting nanofiber potential. Most water-soluble polymers have the inherent limitation of resulting in water-soluble nanofibers, and a tedious chemical cross-linking step is required to reach stable nanofibers. An interesting alternative route is the use of thermoresponsive polymers, such as poly(N-isopropylacrylamide) (PNIPAM), as they are water-soluble beneath their lower critical solution temperature (LCST) allowing low-temperature electrospinning while the obtained nanofibers are water-stable above the LCST. Moreover, PNIPAM nanofibers show major potential to many application fields, including biomedicine, as they combine the well-known on-off switching behavior of PNIPAM, thanks to its LCST, with the unique properties of nanofibers. In the present work, based on dedicated turbidity and rheological measurements, optimal combinations of polymer concentration, environmental temperature, and relative humidity are identified allowing, for the first time, the production of continuous, bead-free PNIPAM nanofibers electrospun from water. More specifically, PNIPAM gelation was found to occur well below its LCST at higher polymer concentrations leading to a temperature regime where the viscosity significantly increases without compromising the polymer solubility. This opens up the ecological, water-based production of uniform PNIPAM nanofibers that are stable in water at temperatures above PNIPAM's LCST, making them suitable for various applications, including drug delivery and switchable cell culture substrates.

Thermoresponsive Wrinkles on Hydrogels for Soft Actuators

Thermoresponsive Wrinkles on Hydrogels for Soft Actuators

Aqueous Solutions of Poly(ethylene oxide)-Poly(N-isopropylacrylamide): Thermosensitive Behavior and Distinct Multiple Assembly Processes

Detailed phase transition and conformational changes taking place as a function of temperature in poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) semidiluted aqueous solutions are elucidated in the present study. By the use of elaborate vibrational spectroscopy techniques in combination with two-dimensional correlation spectroscopy (2Dcos), three transition regions including respective rich domains (<29 °C), loose aggregations (30-36 °C), and dense sphere micelles (>37 °C) are depicted. In particular, subtle variations of hydrogen bonds are detected even under the lower critical solution temperature (LCST), and the respective rich domain regime is marked with strong participation from hydrogen bonding at different concentrations and compositions. Both the formation of intermolecular hydrogen bonds and the less hydration degrees of PNIPAM segments compared with PNIPAM homopolymer at elevated temperatures verify the evolution of PNIPAM from their own domains to loose aggregations with PEO shells. Dense micelles are formed beyond the LCST of PNIPAM, while the outmost PEOs act as buffer layers and postpone the shrinkage of PNIPAM chains. Due to the existence of a buffer layer, higher phase transition temperatures compared with PNIPAM homopolymer are observed.

The effects of anionic electrolytes and human serum albumin on the LCST of poly(N-isopropylacrylamide)-based temperature-responsive copolymers

Poly(N-isopropylacrylamide) (PNIPAm) is one of the most widely studied temperature-responsive polymers among those that have been applied to biomaterials science and technology. Here, we investigated the importance of interactions between PNIPAm-based copolymers and biological factors. The effects of a series of major anionic electrolytes in biological environments and of human serum albumin (HSA) on the lower critical solution temperature (LCST) of homo-PNIPAm and PNIPAm copolymers were studied, using either a hydrophobic monomer or a cationic monomer. We synthesized P(NIPAm-co-BMA3%) with butyl methacrylate (BMA) as a hydrophobic monomer and P(NIPAm-co-DMAPAm2%) with N,N-dimethylaminopropyl acrylamide (DMAPAm) as a cationic monomer. The LCST of PNIPAm and P(NIPAm-co-DMAPAm2%) decreased with increasing salt concentrations, and the effects of anions on each polymer corresponded to the Hofmeister series. The LCST of P(NIPAm-co-DMAPAm2%) was greatly affected by anionic electrolytes compared with those of homo-PNIPAm and P(NIPAm-co-BMA3%). While the LCST of homo-PNIPAm was not affected by HSA, the LCST of P(NIPAm-co-DMAPAm2%) decreased non-linearly with increasing HSA concentrations. These effects were due to the electrostatic interactions between the positively charged polymer chains and the negatively charged HSA, as well as the stabilization of polymer aggregations with HSA. Under physiological buffer conditions, the LCST of P(NIPAm-co-DMAPAm2%) was not significantly affected by the HSA concentration. These results indicated that depending on the types of copolymers used for biological applications, it is necessary to take into account the effect of biological media while designing polymers.

Micromechanical properties of poly(N-isopropylacrylamide) hydrogel microspheres determined using a simple method

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel microspheres have attracted extensive attention because of their promising diverse biomedical applications. A quantitative understanding of the micromechanical properties of these microspheres is essential for their practical application. Here, we report a simple method for the characterization of the elastic properties of PNIPAM hydrogel microspheres. The results show that PNIPAM hydrogel microspheres exhibit elastic deformation and the obtained force-deformation experimental data fits the Hertz theory well. The moduli of elasticity of the PNIPAM hydrogel microspheres prepared under different conditions were systematically investigated in this work for the first time. The PNIPAM hydrogel microsphere composition significantly affects their micromechanical properties and their temperature sensitivity behavior. PNIPAM hydrogel microspheres with a larger equilibrium volume change have a lower modulus of elasticity. The modulus of elasticity of the PNIPAM hydrogel microspheres at body temperature (37°C, above the lower critical solution temperature (LCST) of PNIPAM) is much higher than that at room temperature (25°C, below the LCST of PNIPAM) because of thermo-induced volume shrinkage and an increase in stiffness. These results provide valuable guidance for the design of smart materials for practical biomedical applications. Moreover, the simple microcompression method presented here also provides a versatile way to investigate the micromechanical properties of microscopic biomedical materials.

Effect of Methanol/Water Mixtures on the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)

ABSTRACTPoly(N-isopropylacrylamide) (PNIPAM) is a thermo-sensitive polymer that exhibits a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, electron paramagnetic resonance (EPR) spectroscopy suggests that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fraction) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 mole fraction of methanol, LCST gets elevated to temperatures above ∼305 K (32°C) and cannot be detected up to 373 K (100 °C). The atomistic origin of this co-solvency effect on the LCST behavior is not completely understood. In the present study, we have used molecular dynamics (MD) simulations to investigate the effect of methanol-water mixtures on conformational transitions and the LCST of PNIPAM. We employ two different force fields i.e. polymer consistent force-field (PCFF) and CHARMM to study solvation dynamics and the PNIPAM LCST phase transition in various methanol-water mixture compositions (0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions). Simulations are conducted at fully atomistic level for three different temperatures (260, 278, and 310 K) and radius of gyration (Rg) of PNIPAM chains was computed for determination of LCST behavior of PNIPAM.

Meso-scale Simulations of Poly(N-isopropylacrylamide) Grafted Architectures

ABSTRACTPoly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its behavior at a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water, and above this temperature, polymer chains collapse and transform into a globule state. The conformational dynamics of single chains of polymer in a solution is known to be different from those of grafted structures that comprise of an ensemble of such single chains. In this study, we have carried out MD simulations of a mesoscopic nanostructure of PNIPAM polymer chains consisting of 60 monomer units grafted onto gold nanoparticles of different diameters, to study the effect of temperature and core particle size on the polymer conformations. Additionally, we have also studied the effect of grafting density on the coil-to-globule transition exhibited by PNIPAM through the LCST. The systems investigated consisted of ∼3 and ∼6 million atoms. Simulations were carried out below and above the LCST of PNIPAM, at 275K and 325K. Simulation trajectories were analyzed for radius of gyration of PNIPAM chains.

Fabrication of temperature- and pH-sensitive liquid-crystal droplets with PNIPAM-b-LCP and SDS coatings by microfluidics.

Dual responsive (temperature and pH) 4-cyano-4'-pentylbiphenyl (5CB) droplets were fabricated by coating with PNIPAM-b-LCP (PNIPAM: poly(N-isopropylacrylamide) and LCP: poly(4-cyanobiphenyl-4'-oxyundecylacrylate)) and sodium dodecyl sulfate (SDS) using a microfluidic method, and were tested for protein detection. The PNIPAM-b-LCP/SDS-functionalized 5CB droplets were effective in detecting proteins in water through a radial-to-bipolar (R-B) orientational change, with detection limits of 0.95, 1.1, 0.12, and 0.07 μM for bovine serum albumin (BSA), lysozyme (LYZ), hemoglobin (Hb), and chymotrypsinogen (ChTg), respectively. The R-B change of the 5CB droplet occurred above the lower critical solution temperature (LCST) of PNIPAM and at pH values below the pI values of the tested proteins, and was reversible by heating/cooling at the LCST of PNIPAM. These sensitive 5CB droplets are simple to prepare, cost-effective, easily detectable, and re-usable (by temperature control) for protein detection. Further, they can be applied to a biosensor after employing the selective units such as aptamers, antibodies, single-stranded DNA, proteins, peptides, and aptamer-binding RNA on the droplet.