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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, smart, intelligent, or environmentally sensitive polymers respond to changes in external stimuli such as pH, temperature, ionic strength, surfactants, pressure, light, biomolecules, and magnetic field. These materials are developed in various network architectures such as block copolymers, crosslinked hydrogels, nanogels, inter-penetrating networks, and dendrimers. Stimuli-responsive cationic polymers and hydrogels are an interesting class of “smart” materials that respond reversibly to changes in external pH. These materials have the ability to swell extensively in solutions of acidic pH and de-swell or shrink in solutions of alkaline pH. This reversible swelling-shrinking property brought about by changes in external pH conditions makes these materials useful in a wide range of applications such as drug delivery systems and chemical sensors. This article focuses mainly on the properties of these interesting materials and their applications in drug delivery systems.

New multi-stimuli responsive cationic copolymers based on N-acryloyl-N′-ethyl piperazine (AcrNEP) and N-isopropylacrylamide (NIPAM) were prepared by thermal free-radical solution polymerization in dioxane at 75 °C. The chemical composition of the copolymers was determined by 1H NMR spectroscopy and was found that the copolymers were slightly rich in NIPAM content than that of AcrNEP. The reactivity of the two monomers for the copolymerization reaction was evaluated by the extended Kelen-Tudos method. The distribution of monomer sequence in the copolymer chain was estimated using the terminal copolymerization model. The maximum tendency to alternation (∼ 70%) was at 60 mol % of AcrNEP in the monomer feed. The copolymers were readily soluble in water at room temperature at all compositions and exhibited well-defined lower critical solution temperature (LCST) phenomenon. The influence of various stimuli such as pH, temperature, simple inorganic salts, and surfactants on the LCST of the copolymers was studied in detail. Simple inorganic salts such as sodium chloride, sodium bromide, and sodium sulfate showed a salting-out effect while sodium iodide showed a salting-in effect. The salting-out coefficient of the salts were calculated using the Sestchenow method, and the salting trend followed the order SO42− > Cl− > Br− > I−. The divalent salt was more effective in lowering the LCST than the monovalent salts. The cationic surfactant hexadecyl trimethylammonium bromide at concentrations above the critical micelle concentration caused a gradual increase in the LCST of the copolymer solutions. The intrinsic viscosity and light scattering behavior of the copolymers in water and in sodium chloride solutions were studied in detail. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1175–1183

New cationic linear copolymers of N-vinyl caprolactam (VCL) and N-acryloyl-N′-ethyl piperazine (AcrNEP) were synthesized by thermal free-radical solution polymerization in dioxane at 75 °C. The chemical composition of the copolymers was determined by 1H NMR spectroscopy. The copolymers were water-soluble at all composition and exhibited lower critical solution temperature (LCST) behavior. The LCST was greatly influenced by the AcrNEP content, changes in pH, temperature, salt and surfactant concentration of the external medium. The copolymers were rich in AcrNEP content due to its higher reactivity over VCL. The reactivity of AcrNEP and VCL were determined by the extended Kelen–Tüdös method to be r1AcrNEP = 0.41 and r2VCL = 0.13. The distribution of monomer sequence in the polymer chain was estimated using the terminal copolymerization model and the maximum tendency to alternation (70%) was at 45 mol % of AcrNEP in the feed. The effect of polymer concentration in the range 0.02 to 20 wt % on the LCST behavior showed a rather exponential decrease with the effect being more significant in the dilute regime. Simple inorganic salts such as sodium chloride and sodium bromide showed a salting-out effect while sodium iodide showed a salting-in effect of the copolymers in water. The salting-out coefficient of sodium chloride and sodium bromide evaluated by the Sestchenow equation was 1.37 and 1.12 L mol–1, respectively. The salting trend followed the order Cl– > Br– > I–. The intrinsic viscosity behavior and second-virial coefficient of the copolymers in water and in sodium chloride solution was studied in detail. Cross-linked cationic hydrogels of VCL and AcrNEP with a flexible (N,N′-methylenebisacrylamide) and stiff (1,4-diacryloyl piperazine) chemical cross-linker were prepared. The gels were responsive to pH and temperature changes of external medium. The influence of cross-linker type on the properties of the gels in terms of stimulus response, water transport mechanism, diffusion, and adsorption of an anionic dye (Congo red) were studied in detail.

New functional linear copolymers and crosslinked gels based on a disubstituted heterocyclic acrylamide, i.e., N-acryloyl-N′-phenyl piperazine (AcrNPhP) with acrylic acid (AA) and hydroxyethyl methacrylate (HEMA) were prepared by thermal free-radical polymerization. The composition of the linear copolymers was analyzed by infra-red spectral analysis and elemental analysis methods. The reactivity parameters of the monomers were determined by the Finemann-Ross and Kelen–Tudos linearization methods. Using the values obtained from the Kelen–Tudos linearization methods, the reactivity or resonance stabilization (Q) and polarity (e) of AcrNPhP with AA and HEMA were calculated (AcrNPhP–AA system Q = 1.9, e = −0.6, and AcrNPhP–HEMA system Q = 2.5, e = −1.2). Crosslinked copolymer hydrogels of AcrNPhP with AA and HEMA were prepared by bulk and solution polymerization methods. The method of preparation, the type of crosslinker and monomers showed a profound impact on the pH responsive swelling of the gels. The hydrogels were highly responsive to changes in external pH. The gels composed of AcrNPhP and AA swelled more than 1,200 % above the pK a of acrylic acid. Interestingly, no significant swelling (about 1 %) was observed in solution of low pH (acidic) despite the presence of an ionizable tertiary amine function in the polymer network. This is attributed to the presence of a bulky phenyl group at the tertiary amine function of AcrNPhP which sterically hinders the protonation and thereby the final swelling of the gel. The gels composed of AcrNPhP and HEMA did not display a significant pH responsive behavior due to the same effect. The AcrNPhP–HEMA gel-containing 70 % AcrNPhP swelled to only 50 % despite the presence of a large fraction of the ionizable unit.

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.

‘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.

New crosslinked copolymer hydrogels of N -acryloyl- N ′-methyl piperazine and methyl methacrylate in various compositions were prepared in bulk and solution by photo-initiated free-radical polymerization. The gels exhibited both pH and temperature sensitivities. The response to pH was reversible with a response time of 150 min. The water sorption of the gels was investigated gravimetrically and the collective diffusion coefficients were determined according to the Fickian kinetics at 10°C, 25°C, and 50°C. The apparent activation energy was independent of the composition of the gel networks with a value of 6.26 kJ mol −1 corresponding to a typical diffusion process. The temperature dependence of the equilibrium water content was analyzed by the Gibbs–Helmholtz equation. The enthalpy of mixing decreased with an increase in the hydrophilic content of the gel.

Water-soluble copolymers of N-acryloyl-N′-alkylpiperazine (alkyl: methyl, ethyl) with methyl methacrylate were synthesized and the lower critical solution temperatures (LCSTs) of the copolymers which depend on the compositions and pH of the aqueous solutions are described. The effects of cationic surfactants and simple inorganic salts on the LCSTs are also reported. The influences of pH and temperature on the swelling of the ionizable and thermosensitive hydrogels are studied.

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.

Structures of poly(ethylene propylene)–poly(ethylene oxide) (PEP–PEO) block copolymer micelles were determined from small-angle X-ray scattering and static light scattering and compared to predictions from a thermodynamic model. Both the corona block length and the solvent water–ethanol ratio were changed, leading to a thorough test of this model. With increasing ethanol fraction, the PEP core–solvent interfacial tension decreases, and the solvent quality for PEO changes. The weight-average block masses were 5.0 kDa for PEP and 2.8–49 kDa for PEO. For the lowest PEO molar mass and samples in pure water (except for the highest PEO molar mass), the micelles were cylindrical; for other conditions they were spherical. The structural parameters can be reasonably well described by the thermodynamic model by Zhulina et al. [Macromolecules2005, 38 (12), 5330–5351]; however, they have a stronger dependence on solvent composition and PEO molar mass than predicted.

Copolymers of N -acryloyl- N ′-methylpiperazine and methyl methylacrylate were synthesized and characterized. The monomer ratio in the copolymer was analyzed by FTIR spectroscopy. The copolymer was found to form a complex with Hg(II) ions. A chemically modified electrode (CME) fabricated by attaching a copolymer film to a glassy carbon electrode for the detection of Hg(II) ions is described. The polymer swelled in aqueous solution, allowing easy access of Hg(II) ion for binding. Anodic stripping voltammetry was employed for the detection of Hg(II) ions. The effects of pH of the measurement and deposition solutions, deposition time and the Hg(II) concentration were studied. The limit of detection was estimated to be 0.23 μg/ml.