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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 (Δ G ), enthalpy (Δ H ), and entropy (Δ S ) 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 ( M c ), the crosslink density ( ρ c ), and the mesh size ( ξ ) were determined. The M c was large at pH 3.0 due to ionization of polymer and chain expansion. The experimental M c was much larger than the theoretical M c which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 Å in the swollen (ionized) state (pH 3.0), and between 100 and 231 Å 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. [ABSTRACT FROM AUTHOR]

Water-soluble copolymers of N-acryloyl-N'-ethylpiperazine (AcrNEP) with methyl methacrylate (MMA) were synthesized to high conversion by free-radical solution polymerization. The composition of the copolymers was determined using Fourier Transform Infra-red Spectroscopy (FTIR). Copolymers containing AcrNEP content above 44 mol% were readily soluble in water and exhibited the critical solution temperature behavior. The copolymers were strongly responsive to changes in pH of the external medium due to the presence of tertiary amine functions that could be protonated at low pH. The influence of various factors such as copolymer composition, pH, temperature, salt and surfactant concentration on the LCST of the copolymers were systematically studied. The intrinsic viscosity of the copolymers in dimethyl formamide decreased with increase in temperature due to a decrease in thermodynamic affinity between polymer chains and solvent molecules. The viscosity behavior of the copolymers in sodium chloride solution was similar to that of classical polyelectrolytes and hydrophobically modified polyacrylate systems. [ABSTRACT FROM AUTHOR]

The article discusses a study of the structure of polyethylene propylene-b-polyethylene oxide (PEP-PEO) block copolymer micelles in a water-ethanol mixture using small-angle X-ray scattering (SAXS) and static light scattering (SLS). The authors synthesized the copolymers by anionic living polymerization and determined their molecular masses using size-exclusion chromatography and matrix-assisted laser deposition. They found a similar SLS contrasts but a different SAXS contrasts of the PEP and PEO components.

Abstract: ‘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–Tüdös (K–T) giving the results r 1 (AcrNEP)=0.58 and r 2 (MMA)=0.91 by the F–R method and r 1=0.72 and r 2=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 100min 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. [Copyright &y& Elsevier]