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New stimuli-responsive cationic hydrogels based on N-acryloyl-N′-propyl piperazine (AcrNPP) were prepared using two types of chemical crosslinkers, viz., N,N-methylene bisacrylamide (Mba) and 1,4-butanediol diacrylate (Bda), respectively. The solubility parameter of the hydrogels was evaluated by equilibrium swelling experiment in a series of twelve solvents with well-distributed range of solubility parameters. The hydrogels exhibited maximum equilibrium swelling ratio in 1-butanol. The experimental swelling data were analyzed by two methods, (1) non-linear curve fitting and (2) least-square regression to determine the solubility parameter of the hydrogels. The solubility parameter determined by the two methods was in the range 11.45–11.85 (cal cm−3)1/2. The solubility parameter of the hydrogel containing the hydrophilic crosslinker (Mba) was higher than the hydrogel with a hydrophobic crosslinker (Bda), in all cases. This showed that the nature of chemical crosslinker influences the swelling and thus the solubility parameter of the hydrogels. The partial solubility parameter of the hydrogels related to the cohesive energy was evaluated by plotting the equilibrium swelling data against dispersion forces, polar forces, and hydrogen bonding. It was observed that both polar forces and hydrogen bonding play an important role in the swelling of these cationic hydrogels. The solubility parameter and the related forces were also estimated theoretically using group contribution methods and compared with the experimental results.

New polyampholyte linear polymers and hydrogels based on N -acryloyl- N ′-ethyl piperazine (AcrNEP) and maleic acid (MaA) were prepared by free-radical solution polymerization. The microstructure of copolymers was statistically estimated by the terminal copolymerization model. A maximum tendency (80%) of alternating sequence of monomers units occurred at about 85 mol % of AcrNEP in the reaction feed. The copolymers at definite monomer compositions (40 mol % of AcrNEP and 59.6 mol % of MaA) exhibited lower critical solution temperature (LCST) behavior in water. The LCST was dependent on monomer composition, pH of external solution, type and concentration of simple salts. The isoelectric points (IEP) of copolymers were found to be influenced by the overall composition of ionic monomer units. The influence of high maleic acid content (14 mol % and 59 mol %) and the type of chemical crosslinker on pH-responsive swelling behavior, water diffusion and dye sorption properties of the hydrogels were studied in detail. The gels exhibited interesting swelling behavior as function of pH (ionic strength = 0.01 M) due to a combination of factors such as pK a of the monomers, charge density, and type of crosslinker. The IEP of the gels were also determined from swelling experiments. The swelling ratio of the gels increased at the IEP with increase in ionic strength of the solution due to anti-polyelectrolyte effect. However below the IEP, the swelling ratio decreased considerably. Dynamic swelling properties of the amphoteric gels in water without added salt were measured. The swelling behavior was found to depend on the ionization state of the monomers, and the swelling followed non-Fickian (anomalous) mechanism. The swelling process of the hydrogels in water followed second-order kinetics. The amount of bound and unbound water in swollen gels was quantified using differential scanning calorimetry. Dye adsorption capacity of the new hydrogels was investigated using a Congo red as a model dye and was in the range 8.37–11.45 mg g −1 . This corresponds to an absorption efficiency of range 68–93%.

Stimuli-responsive cationic hydrogels based on N-acryloyl-N′-propyl piperazine (AcrNPP) crosslinked with N,N′-methylenebisacrylamide (Mba) and 1,4-butanedioldiacrylate (Bda) were prepared by UV light-initiated free-radical polymerization in bulk. The effect of external stimuli and type of crosslinker on the equilibrium swelling behavior and dynamic swelling was investigated in detail in buffer solution of various pH and temperatures. The equilibrium swelling capacity of the gels was large in swelling medium at pH 3.0 than at pH 10.0 due to ionization of polymer network under acidic conditions. With increase in temperature from 25 °C to 45 °C, the gels exhibited negative temperature-responsive (thermo-shrinking/thermophobic) behavior with negative activation energy for diffusion of water. The thermodynamic parameters such as Gibbs' free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for the swelling of gels as function of temperature were negative indicating an exothermic swelling process. Water (media) transport mechanism and diffusion process in thin rectangular gels was studied. At pH 3.0, the diffusion process was non-Fickian (anomalous) while at pH 10.0 it was quasi-Fickian. The transport mechanism was partly influenced by the type of crosslinker in the gel. The dynamic swelling data was analyzed using early-time, late-time and Etters diffusion models. From the equilibrium swelling studies the average molecular weight between crosslinks (Mc), the crosslink density (ρc), and the mesh size (ξ) were determined. The Mc was large at pH 3.0 due to ionization of polymer and chain expansion. The experimental Mc was much larger than the theoretical Mc which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 A in the swollen (ionized) state (pH 3.0), and between 100 and 231 A in the collapsed (non-ionized) state (pH 10.0). The mesh size increased between three to four times during the pH-dependent swelling transition. The state of water in fully swollen hydrogels which influences many important biomaterial properties was determined by differential scanning calorimetry. The bound water content of gels increased linearly with increase in pH of the swelling medium while the unbound water decreased. These hydrogels have potential to be used as controlled drug delivery systems and sorbents for removal of pollutants from water.

Antioxidant is critical for the successful of nerve tissue regeneration, and biomaterials with antioxidant activity might be favorable for peripheral nerve repair. Lignin, a biopolymer from wood with excellent antioxidant properties, is still “unexplored” as biomaterials. To design an antioxidative bioscaffold for nerve regeneration, here we synthesized lignin-polycaprolactone (PCL) copolymers via solvent free ring-opening polymerization (ROP). Then such lignin-PCL copolymers were incorporated with PCL and engineered into nanofibrous scaffolds for supporting the growth of neuron and Schwann cell. Our results showed that the addition of lignin-PCL enhanced the mechanical properties of PCL nanofibers and endowed them with good antioxidant properties (up to 98.3 ± 1.9% free radical inhibition within 4 h). Cell proliferation assay showed that PCL/lignin-PCL nanofibers increased cell viability compared to PCL fibers, especially after an oxidative challenge. Moreover, Schwann cells and dorsal root ganglion (DRG) neurons cultured on the nanofibers to assess their potential for nerve regeneration. These results suggested that nanofibers with lignin copolymers promoted cell proliferation of both BMSCs and Schwann cells, enhanced myelin basic protein expressions of Schwann cells and stimulated neurite outgrowth of DRG neurons. In all, these sustainable, intrinsically antioxidant nanofibers may be a potential candidate for nerve TE applications.

New temperature-responsive hydrogels containing various composition of N-acryloyl morpholine (AM) and N-isopropyl acrylamide (NIPAM) were prepared by free-radical solution polymerization using poly(ethyleneglycol) diacrylate (PEGDA) as the chemical crosslinker. The gels were responsive to changes in external temperature, and exhibited the volume phase transition phenomenon. The gels swelled extensively in water at 23°C, with water-uptake of more than 90% at swelling equilibrium. The influence of molecular heterogeneity of the gels in terms of co-monomer and crosslinker composition on the swelling capacity and equilibrium water uptake was evaluated. The dye adsorption capacity of the gels at room temperature was studied using Congo red as a model dye. The gels displayed significant adsorption capacity of the dye, and the adsorption capacity was found to be dependent on the hydrophilic character of the gel. The characteristic adsorption parameters were determined using the Langmuir isotherm model. The value...

Hydrogels composed of N-isopropyl acrylamide and sodium acrylate were prepared using a new stiff crosslinker 1,4-diacryloyl piperazine by a room-temperature free-radical solution polymerization. The gels swelled extensively in water and the swelling increased with increase in sodium acrylate content. Compared to similar gels with N,N’-methylene bisacrylamide as crosslinker the swelling ratio was lower by 25%. Silver nanoparticles were prepared in the hydrogel matrix by in-situ chemical reduction of silver nitrate with sodium borohydride. The new crosslinker being hydrophobic with less structural flexibility controlled the diffusion of silver and borohydride ions into the gel matrix. Silver nanoparticles were formed only on the periphery of the gel as diffusion into the central matrix was hindered by the nonflexible crosslinker. The gels were responsive to temperature changes and the difference in equilibrium water content from 23 to 55°C was 4.5-fold due to strong hydrophobic interactions. The temperature...

Poly(N-acryloyl-N0-ethyl piperazine-co-N-isopropyla- crylamide) hydrogels were prepared by thermal free-radical copolymerization of N-acryloyl-N 0 -ethyl piperazine (AcrNEP) and N-isopropylacrylamide (NIPAM) in solution using N, N 0 - methylene bisacrylamide as the crosslinking agent. The gels were responsive to changes in external stimuli such as pH and temperature. The pH and temperature responsive character of the gels was greatly dependent on the monomer content, namely AcrNEP and NIPAM, respectively. The gels swelled in acidic (pH 2) and de-swelled in basic (pH 10) solutions with a response time of 60 min. With increase in temperature from 23 to 80 � C the swelling of the gels decreased continuously and this effect was different in acidic and basic solutions. The tem- perature dependence of equilibrium water content of the gels was evaluated by the Gibbs-Helmholtz equation. Detailed anal- ysis of the swelling properties of these new gels in relation to molecular heterogeneity in acidic (pH 2) and basic (pH 10) solu- tions were performed. Water transport property of the gels was studied gravimetrically. In acidic solution, the diffusion process was non-Fickian (anomalous) while in basic solution, the diffusion was quasi-Fickian. The effect was more evident in solution of pH 2 than in pH 10. Various structural parameters of the gels such as number-average molar mass between crosslink (Mc), the crosslink density (qc), and the mesh size (n) were evaluated. The mesh sizes of the hydrogels were between 64 and 783 Ain the swollen state in acidic solution and 20 and 195 Ain the collapsed state in basic solution. The mesh size increased between three to four times during the pH-depend- ent swelling process. The amount of unbound water (free water) and bound water of the gels was also evaluated using differential scanning calorimetry. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 000: 000-000, 2012

New hydrogels based on N-acryloyl-N′-ethylpiperazine (AcrNEP) and N,N-methylene bisacrylamide (MBA) were prepared by thermal initiated solution polymerization. The hydrogels swelled extensively in buffer solutions of low pH due to protonation of the amine functions of the monomers, while the swelling was less significant in buffer solutions of high pH. The increased swelling of the gel in low pH is due to the development and interaction of fixed charges within the gel network. As a result of the electrostatic repulsion between the charges the elastic constraint of the gel is modified which leads to pronounced swelling and hence to high water uptake. Water transport in the hydrogel both in buffer solutions of pH 2.6 and pH 8.4 was non-Fickian due to polymer relaxation (anomalous process). The gels demonstrated good uptake of divalent metal ions such as Ni2+, Co2+, and Zn2+, with high selectivity for Ni2+ ions due to the formation of a more stable ligand-metal complex. The metal uptake capacity increased wi...

Copolymers of N-acryloyl-N′-methylpiperazine (AcrNMP) and 2-hydroxyethyl methacrylate (HEMA) were synthesized by free radical solution polymerization in dioxane at 70 ± 1 °C, using 2,2′-azobisisobutyronitrile (AIBN) as initiator. The copolymer compositions were analyzed by the methods of FTIR spectroscopy and elemental analysis. Both the method of analysis yielded results that agreed reasonably well. The monomer reactivity ratios of the copolymerization were determined by the linearization methods of Finemann–Ross (FR) and Kelen–Tudos (KT). The reactivity parameter results derived using FTIR analysis showed that the copolymerization yielded mainly alternating structure with reactivity ratios, r1(AcrNMP) = 0.263 ± 0.011 and r2(HEMA) = 0.615 ± 0.097 by F–R method and r1 = 0.227 ± 0.074 and r2 = 0.53 ± 0.15 by KT method. Microstructure data calculated by the method of Igarashi also supports the alternating structure (tendency) of the copolymer. Crosslinked polymer gels of this system exhibited remarkably high swelling of more than 500% in water at ambient temperature.

pH responsive copolymer nanogel particles of N‐acryloyl‐N′‐methyl piperazine (AcrNMP) and methyl methacrylate (MMA) were prepared by a suitable oil‐in‐water type microemulsion polymerization. Effect of microemulsion polymerization conditions such as water content, surfactant‐to‐monomer ratio, amount of monomer, and crosslinker content were studied. The nanogel particles showed spherical morphology with narrow size distribution. The nanogels swelled excessively in acidic solutions reaching a diameter of 750 nm and de‐swelled to 350 nm in basic solutions. This pH response to swelling is due to the protonation of N′‐methyl piperazine nitrogen that leads to electrostatic interactions within the nanogel.

Hydrogel nanocomposites containing silver nanoparticles of size 15–21 nm were prepared by diffusion and in-situ chemical reduction in chemically crosslinked polymers based on N-acryloyl-N′-ethyl piperazine (AcrNEP) and N-isopropylacrylamide (NIPAM). The polymer chains of the hydrogel network offered control and stabilization of silver nanoparticles without the need for additional stabilizers. The presence of silver nanoparticles and their size was quantified by UV-Vis absorption spectroscopy and scanning electron microscopy. The nanocomposite hydrogels were responsive to pH and temperature changes of the external environment. The equilibrium weight swelling ratio of the hydrogel nanocomposite was lower in comparison with the precursor hydrogel. Silver nanoparticles present in the nanocomposite offered additional physical crosslinking which influenced media diffusion and penetration velocity. The release of silver nanoparticles from the hydrogel matrix in response to external pH changes was studied. The rate of release of silver nanoparticles was higher in a solution of pH 2.5 due to maximum swelling caused by ionization of the gel network. No significant release of nanoparticles was observed in a solution of pH 7.

‘Stimuli’ responsive copolymers of N-acryloyl-N′-ethyl piperazine (AcrNEP) and methyl methacrylate (MMA) were synthesized by free radical solution polymerization. The copolymers were analyzed as thin films by FTIR spectroscopy. The monomer reactivity ratios were determined by linearization methods of Fineman–Ross (F–R) and Kelen–Tudos (K–T) giving the results r1 (AcrNEP)=0.58 and r2 (MMA)=0.91 by the F–R method and r1=0.72 and r2=1.08 by the K–T method. The latter r values in turn yielded Q=0.59 and e=−0.12 for AcrNEP. Crosslinked copolymer hydrogels of AcrNEP and MMA with various compositions were prepared in bulk and solution by photo-initiated free-radical polymerization. The gels were dual responsive to pH and temperature. The response to pH was reversible with a response time of 100 min with good reversibility and with no loss in swelling capacity. Water sorption of the gels was investigated gravimetrically and the collective diffusion coefficients were determined at 10, 25, and 50 °C. The water sorption of the gels in water was Fickian. The temperature dependence of the equilibrium water content was studied by the Gibbs–Helmholtz equation. The enthalpy of mixing decreased with an increase in the hydrophilic content (AcrNEP) of the gel. Other parameters such as type and amount of crosslinker, preparative conditions, nature of buffers, and salts were found to influence the swelling behavior.

A new water-soluble stimuli-responsive poly(N-acryloyl-N‘-propylpiperazine) (PAcrNPP) was synthesized and characterized. The polymer exhibited a lower critical solution temperature (LCST) in water ...

Copolymers of N -acryloyl- N′ -methylpiperazine (AcrNMP) and methyl methacrylate (MMA) were synthesized in 1,4-dioxane using benzoyl peroxide as initiator at 70 ± 1 °C. The copolymers were analyzed by FTIR spectroscopy. The monomer reactivity ratios were determined by the methods of Fineman-Ross (F-R) and Kelen-Tudos (K-T), yielding the results r 1 (AcrNMP) = 0.552 and r 2 (MMA) = 1.074 by the F-R method and r 1 = 0.466 and r 2 = 0.980 by the K-T method. The latter r values in turn yielded Q = 0.56 and e = −0.49 for AcrNMP.