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Systematic molecular dynamics simulations, along with dielectric measurements on poly(vinyl methyl ether)/polystyrene blends, are performed to investigate the topological effect on the effective lo...

Blends of polymer solutions of poly(N-isopropylacrylamide-co-dopamine methacrylamide) (NIDO) and poly(N-isopropylacrylamide-co-4-vinylphenylboronic acid) (NIBA) in aqueous solutions were studied rheologically at pH ≈ 9. The functional catechol group of NIDO and the boronic acid group of NIBA could form coordinative complexes, which are strong, yet could be broken by high deformation, while they could reconfigure quickly within 1 min on a macroscopic scale. The linear and nonlinear behaviors of these materials vary significantly for different component mixing ratios. At a total polymer concentration of 10 w/v %, the 1:1 mixture (wNIBA/wNIDO) of the two polymers yields the strongest gel compared with asymmetric blends. The nonlinearity intensities (In/I1) of the materials increase proportionately with strain γ0, but once a significant nonlinearity is reached, they show lower nonlinearity intensities, which is unlike regular nonassociative single phase polymer melts and solutions. Interaction of polymer NIDO and NIBA and the formed supramolecular gels' properties will be clarified by large amplitude oscillatory shear characterization.Blends of polymer solutions of poly(N-isopropylacrylamide-co-dopamine methacrylamide) (NIDO) and poly(N-isopropylacrylamide-co-4-vinylphenylboronic acid) (NIBA) in aqueous solutions were studied rheologically at pH ≈ 9. The functional catechol group of NIDO and the boronic acid group of NIBA could form coordinative complexes, which are strong, yet could be broken by high deformation, while they could reconfigure quickly within 1 min on a macroscopic scale. The linear and nonlinear behaviors of these materials vary significantly for different component mixing ratios. At a total polymer concentration of 10 w/v %, the 1:1 mixture (wNIBA/wNIDO) of the two polymers yields the strongest gel compared with asymmetric blends. The nonlinearity intensities (In/I1) of the materials increase proportionately with strain γ0, but once a significant nonlinearity is reached, they show lower nonlinearity intensities, which is unlike regular nonassociative single phase polymer melts and solutions. Interaction of polymer NIDO...

Isotactic poly (N-isopropylacrylamides) (PNIPAMs) with a wide range of tacticity exhibit upper critical solution temperature (USCT) type phase transition in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI]). The phase transition temperature becomes higher with increasing isotacticity, since more amide groups are aligned along the same side of backbones to form stronger intramolecular hydrogen bonding. Phase diagrams of highly isotactic PNIPAMs show prominent UCST peaks at low concentrations, similar with the typical polymer solution, which implies the dissolution of isotactic PNIPAMs is dominated by the entropy-driven effect. The increase in phase transition temperature with molecular weight is also from the entropic reason. The phenomena above reveal that the phase transition behavior of PNIPAMs in ionic liquids is highly influenced by tacticity effect.

Atomic force microscopy (AFM) indentation is the most suitable way to characterize micromechanical properties of soft materials such as bio tissues. However, the mechanical data obtained from force-indentation measurement are still not well understood due to complex geometry of the bio tissue, nonlinearity of indentation contact, and constitutive relation of hyperelastic soft material. Poly-N-isopropyl acrylamide (PNIPAM) filled with 5 wt% polystyrene (PS) sphere particles material system can be utilized as a simplified model for mimicking a whole host of soft materials. Finite element model has been constructed to simulate indentation as in AFM experiments using colloidal probes for a parametric study, with the main purpose of understanding the effect of particles on overall behavior of mechanical data and local deformation field under indentation contact. Direct comparison between finite element simulation and indentation data from AFM experiments provides a powerful method to characterize soft materials properties quantitatively, addressing the lack of analytical solutions for hard-soft composites, both biological and synthetic ones. In this framework, quantitative relations are found between the depth, at which the particle was embedded, the particle size and the elastic modulus of the overall composite. Comprehensive characterizations were established to distinguish indentation on a particle residing on top of the hydrogel from a particle embedded inside the hydrogel matrix. Finally, different assumptions of interface friction at the boundary between the particle and the hydrogel have been tested and directly compared with experimental measurements.

Two series of different isotactic poly(N-isopropylacrylamide) of number average molar masses (Mn) ∼ 100 000, 200 000 and narrow molar mass distribution are prepared by reversible addition fragmentation chain transfer polymerization. Their thermoreversible, physical gels are prepared in benzyl alcohol and rheologically characterized to assess the effect of tacticity and molar masses on the rheological properties. In general, higher tacticity, higher molar mass, and higher concentration lead to stiffer gels and a sol-gel transition at a higher temperature. The results are interpreted based on partial phase separation leading to stiff but labile spongelike structures. The tacticity influences the strength of the phase separation tendency. Concentration and molar mass determine how far reaching and how connected these domains are.

Random copolymer gels of N -isopropylacrylamide (NIPAM) and N -ethylacrylamide (NEAM) were synthesized in using different monomer compositions in 1:1 methanol-water mixtures and in pure water. The samples were characterized by cononsolvency study in methanol-water mixtures at various temperatures by swelling ratio measurements. It is observed that with the variation of the temperature and monomer compositions in the synthesis reaction mixture, the cononsolvency properties varied significantly. But the variation in the solvents compositions by keeping monomer compositions fixed does not affect the cononsolvency much. All results can be explained on the basis of the differences in thermoresponsive character or difference in the hydrophilicity of PNIPAM and PNEAM in water, preferential adsorption of the solvents by polymer networks, solvent-solvent interactions, heterogeneity of the polymer networks synthesized at different solvent compositions and at different temperatures, difference in their morphological properties, etc.

The thermoreversible sol-gel transition for an ionic liquid (IL) solution of isotactic-rich poly (N-isopropylacrylamides) (PNIPAMs) is investigated by rheological technique. The meso-diad content of PNIPAMs ranges between 47% and 79%, and molecular weight (Mn) is ~35,000 and ~70,000 g/mol for two series of samples. PNIPAMs are soluble in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([BMIM][TFSI]) at high temperatures but undergo a gelation with decreasing temperatures. The transition temperature determined from G&rsquo, G&rdquo, crossover increases with isotacticity, consistent with the previous cloud-point result at the same scanning rate, indicating imide groups along the same side of backbones are prone to be aggregated for formation of a gel. The transition point based on Winter-Chambon criterion is on average higher than that of the G&rsquo, crossover method and is insensitive to tacticity and molecular weight, since it correlates with percolation of globules rather than the further formation of elastic network (G&rsquo, &gt, ). For the first time, the phase diagram composed of both G&rsquo, crossover points for gelation and cloud points is established in PNIPAM/IL mixtures. For low-Mn PNIPAMs, the crossover-point line intersects the cloud-point line. Hence, from solution to opaque gel, the sample will experience two different transitional phases, either clear gel or opaque sol. A clear gel is formed due to partial phase separation of isotactic segments that could act as junctions of network. However, when the partial phase separation is not faster than the formation of globules, an opaque sol will be formed. For high-Mn PNIPAMs, crossover points are below cloud points at all concentrations, so their gelation only follows the opaque sol route. Such phase diagram is attributed to the poorer solubility of high-Mn polymers for entropic reasons. The phase diagram composed of Winter-Chambon melting points, crossover points for melting, and clear points is similar with the gelation phase diagram, confirming the mechanism above.

For the preparation of thermoresponsive copolymers, for e.g., tissue engineering scaffolds or drug carriers, a precise control of the synthesis parameters to set the lower critical solution temperature (LCST) is required. However, the correlations between molecular parameters and LCST are partially unknown and, furthermore, LCST is defined as an exact temperature, which oversimplifies the real situation. Here, random N-isopropylacrylamide (NIPAM)/dopamine methacrylamide (DMA) copolymers were prepared under a systematical variation of molecular weight and comonomer amount and their LCST in water studied by calorimetry, turbidimetry, and rheology. Structural information was deduced from observed transitions clarifying the contributions of molecular weight, comonomer content, end-group effect or polymerization degree on LCST, which were then statistically modeled. This proved that the LCST can be predicted through molecular structure and conditions of the solutions. While the hydrophobic DMA lowers the LCST especially the onset, polymerization degree has an important but smaller influence over all the whole LCST range.

The thermoreversible sol-gel transition for an ionic liquid (IL) solution of isotactic-rich poly (N-isopropylacrylamides) (PNIPAMs) is investigated by rheological technique. The meso-diad content of PNIPAMs ranges between 47% and 79%, and molecular weight (Mn) is ~35,000 and ~70,000 g/mol for two series of samples. PNIPAMs are soluble in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([BMIM][TFSI]) at high temperatures but undergo a gelation with decreasing temperatures. The transition temperature determined from G’-G” crossover increases with isotacticity, consistent with the previous cloud-point result at the same scanning rate, indicating imide groups along the same side of backbones are prone to be aggregated for formation of a gel. The transition point based on Winter-Chambon criterion is on average higher than that of the G’-G” crossover method and is insensitive to tacticity and molecular weight, since it correlates with percolation of globules rather than the further formation of elastic network (G’ > G”). For the first time, the phase diagram composed of both G’-G” crossover points for gelation and cloud points is established in PNIPAM/IL mixtures. For low-Mn PNIPAMs, the crossover-point line intersects the cloud-point line. Hence, from solution to opaque gel, the sample will experience two different transitional phases, either clear gel or opaque sol. A clear gel is formed due to partial phase separation of isotactic segments that could act as junctions of network. However, when the partial phase separation is not faster than the formation of globules, an opaque sol will be formed. For high-Mn PNIPAMs, crossover points are below cloud points at all concentrations, so their gelation only follows the opaque sol route. Such phase diagram is attributed to the poorer solubility of high-Mn polymers for entropic reasons. The phase diagram composed of Winter-Chambon melting points, crossover points for melting, and clear points is similar with the gelation phase diagram, confirming the mechanism above.

In the present study, rapid and highly efficient solid-state synthesis strategy has been successfully employed for the synthesis of zinc oxide nanoparticles (ZnO NPs). Herein, prepared ZnO NPs are characterized and used for the fabrication of highly sensitive, selective, and accountable gas sensors. Structural elucidation, surface composition, and morphological investigations of as-synthesized ZnO NPs respectively, are carried out using X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction measurement techniques. The gas sensing properties of ZnO NPs studied at various operating temperatures and towards different oxidizing and reducing gases demonstrated the ability of ZnO to detect hazardous chlorine (Cl2) gas (optimum @ 200 °C) with rapid response and recovery times. In addition, ZnO sensor film has detected the low 5 ppm concentration of Cl2 with reasonable response of 199% with outstanding repeatability and stability in response. The variation of ZnO response with respect to different Cl2 concentrations has systematically been investigated and explored. Finally, the plausible sensing mechanism of ZnO with Cl2 gas molecules has been proposed using energy band model diagram.

Random copolymer gels of N-isopropylacrylamide (NIPAM) and N-ethylacrylamide (NEAM) were synthesized using a 1:1 monomer molar ratio in different methanol–water (xm = 0, 0.06, 0.13, 0.21, 0.31 0.43, 0.57, 0.76, where xm = mole fraction of methanol) mixtures. The samples were characterized using different techniques like Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), swelling ratio measurements, deswelling kinetics study, atomic force microscopy (AFM), and rheology. We found that, with the variation of the solvent composition (methanol–water mixtures) the properties of the gels varied significantly. These results can be explained on the basis of the interactions of the two different kinds of monomers with different methanol–water mixtures, their different kinds of thermoresponsiveness and hydrophilicity, and their different cononsolvent properties toward methanol–water mixtures.

Nanocomposite stereoregular hybrid gels of poly(N-isopropylacrylamide) were prepared in 1:1 methanol–water mixture in the presence rare earth yttrium trifluoromethanesulfonate Lewis acid and graphene oxide (GO). The nanocomposite gels were characterized by swelling ratio measurements at different temperatures and different pH, deswelling kinetics, scanning electrone microscopy, FTIR, thermogravimetric analysis, powder XRD, Raman spectroscopy, LCST measurement by DSC etc. Drug release experiments also done by using tramadol hydrochloride as a model drug to check their suitability in application. All results were explained on the basis of porosity, cononsolvency of the synthesis solvents, hydrophilicity, isotacticity, pH effect, interactions between solvents and polymer molecules, interactions between polymer and GO or interactions between GO and solvents etc. © 2016, Springer Science+Business Media New York., KC2014ZDZJ0001A ZDSYS201507141105130 JCYJ20140509172719311 University of Akron 2013-7211-8496-02 National Natural Science Foundation of China: 21574086, We gratefully acknowledge the financial support from TUBITAK, Government of Turkey, through 2216 fellowship program for the foreign citizens and B??lent Ecevit University (#2013-7211-8496-02). The authors also thank the National Science Foundation of China (21574086), Nanshan District Key Lab for Biopolymers and Safety Evaluation (No. KC2014ZDZJ0001A), Shenzhen Sci and Tech research Grant (ZDSYS201507141105130), and Shenzhen City Science and Technology Plan Project (JCYJ20140509172719311) for financial support. We thank Prof. Kevin Cavicchi, University of Akron for comments on this manuscript.

Thermoreversible gelation of three different isotactic linear poly(N-isopropylacrylamide) (PNIPAM)s having meso dyad (m) values 62, 68 and 81% has been observed in benzyl alcohol. All the gels were transparent in nature. SEM image of the dried gels showed fibrillar network morphology. Melting temperature of the gels gradually increased with the increase in the concentration. XRD data of dry polymers and their corresponding dry gels showed shifting in the peak positions. Rheological study showed that stronger gels were formed with increasing isotacticity of PNIPAM while lower isotactic sample exhibited typical polymer melt rheology. The formation of a plunge in the storage modulus as well as in the viscosity plot at the same frequency range indicates the reversible nature of the structure breaking/reformation under frequency sweep. Moreover, the mechanical strength of the gel decreased with increase in temperature. UV-Vis kinetic study also indicated the change in the conformation and aggregation of PNIPAM chains during gelation. Molecular modelling calculation showed that the number of solvent molecules involved in forming gel (polymer-solvent compound) decreased with the increase in the isotacticity of the polymer. Gelation rate of these gels was studied as a function of temperature, concentration and isotacticity using test-tube tilting method. It increased with the increase in the concentration and isoacticity of the polymer, and with the decrease in the temperature. Critical gelation concentration of the gel gradually increased with the decrease in the isotacticity and with the increase in the temperature. All these experimental results indicated that gelation occurs presumably through polymer-solvent compound formation.

Poly(N-isopropylacrylamide)s (PNIPAM) with low polydispersity and different isotacticity have been synthesized successfully in different compositions of environment-friendly and less toxic methanol-water mixtures by RAFT polymerization in the presence of 0.1 M Yttrium trifluoromethanesulfonate [Y(OTf) 3]. The reactions were successful with high yield in the solvent compositions having mole-fraction of methanol (x m) at or above 0.31. Molecular weights were found to be in between 2.78 × 10 4 – 5.7 × 10 4 and polydispersity index (M w/ M n= 1.11 – 1.37) was close to unity. Isotacticity [meso diad (m), %] of the polymers increased from 55 to 86% with the increase in the x m value from 0.31 to 1.0. The lower critical solution temperature (LCST) of the formed PNIPAMs in water decreased with the increase in their isotacticity.

The synthesis of amphiphilic block-copolymers of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-acryloyl- l -phenylalanine methyl ester) (PNALPA) by RAFT polymerization is reported. The polymers were characterized by proton NMR (1H NMR), gel permeation chromatography (GPC), powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) to understand their structure-property relationships. All synthesized polymers show pH triggered release at acidic (pH 5) and biological pH (pH 7.4). Critical micellar concentration also varied systematically depending on the amphiphilic nature of the polymers. Biocompatibilities of the polymers are also determined to check their suitability for biomedical applications.

Here, we report on the effect of functional copolymer poly(N-isopropylacrylamide-co-4-vinyl-phenylboronic acid) (NIBA) on the rheology and network structure formed by sodium alginate (SA) through linear and nonlinear viscoelasticity measurements. The hydrogel moduli at pH 3 increased with increasing NIBA addition, while the yield point decreased. Furthermore, these hydrogels showed strain-softening behavior, weak G″-overshoot marking the onset of nonlinearity, and good self-healing properties after large deformation. The zero-strain nonlinearity parameter (Q0) was found to be more sensitive to NIBA-addition than the linear viscoelastic properties. The blends showed a clear peak in the startup test except for SA alone and the peak intensity increased with increasing NIBA-concentration. Finally, based on all data, gelation mechanism and interaction of SA and NIBA will be clarified.

Some physical properties of high molecular weight linear poly(N-isopropylacrylamide)s (PNIPAM) having different isotacticities (m, meso dyad = 47, 62, 68, 81, and 88 %) have been studied. The solubility of these polymers in different solvents varied with their isotacticity. Thermal degradation of higher isotactic polymer started slightly earlier. Cloud point of these polymers in water gradually decreased with increase in m up to 68 %. Surface tension of these polymers in N,N-dimethylformamide (DMF) was slightly lower than DMF and almost independent of their isotacticity in the temperature range of 20–60 °C. But it decreased linearly with the increase in temperature. Moreover, for all polymers, it decreased very slightly with the increase in their concentrations in the range of 0.001–0.01 % (w/v). Viscosity of these polymers in DMF increased with (i) the increase in the concentration range of 1–5 % (w/v), (ii) the increase in their m values, and (iii) the decrease in the temperature in the range of 20–60 oC.

Nanohybrids of graphene with water soluble polymer were synthesized using ‘grafting from’ method. GO, prepared by modified Hummers’ method, was first reacted with sodium azide. Alkyne-terminated RAFT-CTA was synthesized by reaction of propargyl alcohol and S-1-dodecyl-S’-(α,α‘-dimethyl-α”-acetic acid) trithiocarbonate. RAFT-CTA was grafted onto the GO sheets by facile click-reaction and subsequently, N-isopropylacrylamide (NIPAM) and N-ethyleacrylamide (NEAM) were polymerized on graphene sheets via RAFT polymerization method. The respective copolymers with different ratios were also prepared. The nanohybrids were characterized by FTIR, XRD, TGA, Raman, SEM, and AFM. Both SEM and AFM clearly showed rod-like structures for rGO-PNEAM. XRD showed a small peak at 2θ = 19.21°, corresponding to d-spacing ≈ 4.6 Å. In addition, the nanohybrids showed a very broad temperature range for the LCST in water between ca. 30 and 70 °C.

For the preparation of thermoresponsive copolymers, for e.g., tissue engineering scaffolds or drug carriers, a precise control of the synthesis parameters to set the lower critical solution temperature (LCST) is required. However, the correlations between molecular parameters and LCST are partially unknown and, furthermore, LCST is defined as an exact temperature, which oversimplifies the real situation. Here, random N-isopropylacrylamide (NIPAM)/dopamine methacrylamide (DMA) copolymers were prepared under a systematical variation of molecular weight and comonomer amount and their LCST in water studied by calorimetry, turbidimetry, and rheology. Structural information was deduced from observed transitions clarifying the contributions of molecular weight, comonomer content, end-group effect or polymerization degree on LCST, which were then statistically modeled. This proved that the LCST can be predicted through molecular structure and conditions of the solutions. While the hydrophobic DMA lowers the LCST especially the onset, polymerization degree has an important but smaller influence over all the whole LCST range.

Two series of macroporous poly(N-ethylacrylamide) (PNEAM) gels are synthesized in different composition of methanol-water mixtures (x(m)=0, 0.06, 0.13, 0.21, 0.31, and 0.43; where x(m)=mole fraction of methanol) in presence as well as in the absence of 0.1M Y(OTf)(3) Lewis acid as additive. The gels synthesized in the absence of Lewis acid are atactic and in the presence of the same are isotactic. Synthesis of the corresponding linear PNEAM homopolymers shows that, the isotacticity (meso dyad, %) of the resulted polymers increases for the gels prepared in the presence of Lewis acid (LA) and remains constant for the gel prepared in the absence of LA, respectively, with the increase in the concentration of the synthesis solvent methanol. SEM micrographs reveal that, the hydrogels synthesized in the presence of LA are more porous than the gels prepared in the absence of LA. Swelling ratio of all the hydrogels decreases with the increase in the temperature and LA gels show higher swelling ratio values than non LA gels (NLA). Deswelling rate of the hydrogels prepared in methanol-water mixture in presence of LA is faster than the hydrogels prepared in absence of LA. Moreover, reswelling rate increases with increase in the isotacticity of the PNEAM segment in the gel. All these results have been explained on the basis of the formation of highly porous hydrogels with higher isotactic PNEAM chain segment in the presence of LA in methanol-water mixtures. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41668.

Random copolymer hydrogels of N-isopropylacrylamide and N-ethylacrylamide (NEAM) were prepared using different monomer compositions in 1:1 methanol–water mixtures. The samples were characterized by cononsolvency study in methanol-water mixtures at various temperatures, swelling properties measurements, scanning electron microscopy. With changing ratio of the monomers in the reaction mixture, the thermal, morphological and swelling properties, varied significantly. The change in the properties with monomer composition variation are interpreted based on the different thermoresponsive characteristics and interactions of gels of N-isopropylacrylamide and NEAM (homo- and copolymer gels) in water and different methanol-water mixtures, their variable compositions in the synthesis mixtures, and their morphologies. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45176.

Morphology and nanomechanical properties by atomic force microscopy (AFM) in 1:1 correspondence for the poly(N-isopropylacrylamide) (PNIPAM) hydrogel in a water environment. Measurements were performed in force volume mode at medium resolution (64 × 64) using a 2550 nm spherical tip: morphology (A) and Young’s modulus mechanical map (B). Force (nN) vs indentation (nm) graph (C) that highlights the raw data and Hertz fit. Histogram of Young’s modulus values in log-normal scale with Gaussian distribution fits (D) for the quantitative analysis and error calculation.