Laser light scattering and differential scanning calorimetry measurements have been performed for aqueous solutions of thermosensitive linear copolymers of N-vinylcaprolactam and methacrylic acid of different composition. It was shown that the copolymers undergo a phase transition upon temperature increase in neutral and basic solutions. The enthalpy of the phase transition sharply decreases with the increase of methacrylic acid fraction, as shown by differential scanning calorimetry. The copolymers demonstrate pH-sensitive properties as well: intermacromolecular aggregation takes place in acidic media. FTIR spectroscopy measurements show that the aggregation is due to formation of insoluble macromolecular complexes. It was found that such complexes are also formed in the mixtures of homopolymers of poly(N-vinylcaprolactam) with poly(methacrylic acid). Effect of pH on thermosensitive properties of the copolymers is discussed. In weak acidic media there are narrow pH intervals close to pH of aggregation in which thermosensitive properties of the copolymers diminish considerably and the scattering intensity from the solutions at high temperatures is much less than at basic pH. Also, under these conditions the enthalpy of phase transition decreases significantly. The effect of a similarly charged surfactant (sodium dodecyl sulfate, SDS) on thermosensitive properties of the copolymer containing ∼40% of methacrylic acid is studied at pH 3 and 7. SDS solubilizes the copolymer, which is insoluble at pH 3; in the presence of SDS the copolymer is thermosensitive at both pH 3 and pH 7 at studied SDS concentrations; the transition temperature is higher than that of the copolymer in a surfactant-free solution. The enthalpy of the phase transition increases as SDS concentration in solution is increased.

A microfluidic strategy for the encapsulation and stimulus-responsive release of molecules with distinct polarities from the interior of microgels is reported. The approach relies on (i) the generation of a primary O/W emulsion by the ultrasonication method, (ii) MF emulsification of the primary emulsion, and (iii) photopolymerization of the monomer present in the aqueous phase of the droplets, thereby transforming them into microgels. Non-polar molecules are dissolved in oil droplets embedded in the microgels. Polar molecules are physically associated with the hydrogel network. Upon heating, the microgels contract and release polar and non-polar cargo molecules. The approach paves the way for stimuli-responsive vehicles for multiple drug delivery.

Effects of pressure on the phase separation and structure of weakly charged poly(Nisopropylacrylamide-co-acrylic acid) (PNIPA-AAc) solutions and gels with 7 wt % were investigated by small-angle neutron scattering (SANS). The phase diagrams were determined in the pressuretemperature (P-T) plane for the solution and gel with a light scattering method, which were parabolic functions, having maxima at (P ) 38.1 MPa, T ) 37.1 °C; for the solution) and (121.6 MPa, 51.6 °C; for the gel). SANS experiments were carried out according to the P-T phase diagrams. At the low temperature and pressure region, where the systems were in one phase, the scattering intensity functions, I(q)s, were well described with an Ornstein-Zernike function. However, a scattering maximum appeared in I(q)s at elevated temperatures and pressures. The regions where a scattering maximum was observed at T ) 45 °C were P < 70 MPa and 200 MPa < P for the gel and P < 70 MPa for the solution. This scattering maximum is due to microphase separation as a result of antagonism between the electrostatic and hydrophobic interactions. In the case of the solution, an upturn in I(q) at the low q region was observed, indicating occurrence of macrophase separation in addition to microphase separation. For the polymer gels, on the other hand, such an upturn in I(q) was not observed as a result of pinning effect of crosslinks. The phase behavior and critical phenomena of PNIPA-AAc gels and solutions will be discussed from the viewpoint of pressure effects on the hydrophobic interaction.

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We report a high-throughput study of the kinetics of a multicomponent polymerization reaction in a microfluidic reactor integrated with in situ attenuated total reflection Fourier transform infrared spectroscopy. The technique was used to study the kinetics of an exemplary free-radical polymerization reaction of N-isopropylacrylamide, which was initiated by ammonium persulfate in the presence of the accelerator N,N,N′,N′-tetramethylethylenediamine in water. By monitoring the rate of disappearance of the monomer double bonds, we determined the effects of the concentration of the monomer, initiator, and accelerator on the rate of polymerization and the effect of the pH of the reaction system on the reaction kinetics. This work opens the way for the kinetic studies of complex polymer systems in a microfluidic format.

The ionization effects on the pressure-induced phase transition of weakly charged poly(N-isopropylacrylamide-co-acrylic acid) (PNIPA-AAc) gels have been investigated by small-angle neutron scattering. At low temperature, T, and pressure, P, the structure factor of PNIPA-AAc gels was well represented by a Lorentzian (L) function, which was similar to noncharged PNIPA gels. However, at high Ps, the contribution of inhomogeneities became large and a squared-Lorentzian term had to be added in addition to the L term. At high Ts, on the other hand, a scattering maximum appeared, indicating microphase separation. This scattering maximum was suppressed by increasing P up to P approximately 100 MPa and then reincreased at higher Ps. The following facts were disclosed: (1) The peak position and height were very sensitive to P, which is mainly ascribed to strong pressure dependence of hydrophobic interaction, (2) ionization leads to microphase separation at elevated temperatures, (3) the re-entrant phase behavior is commonly observed in the P-T plane due to the parabolic variation of the polymer-solvent interaction with P, and (4) the pressure and temperature dependence of the structure factor was reproduced with the Rabin-Panyukov theory and was interpreted with a convexity of hydrophobic interaction with respect to pressure.

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