Cationic Thermoresponsive Poly(<i>N</i>‑vinylcaprolactam) Microgels Synthesized by Emulsion Polymerization Using a Reactive Cationic Macro-RAFT Agent

A series of reactive poly­([2-(acryloyloxy)­ethyl]­trimethyl­ammonium chloride) (P­(AETAC)) cationic polymers with varying degrees of polymerization were synthesized by RAFT/MADIX polymerization and investigated as stabilizers for the emulsion polymerization of <i>N</i>-vinylcaprolactam (PVCL) in the presence of a cross-linker. It was demonstrated that the xanthate chain end of the cationic P­(AETAC-X) polymers played a crucial role to produce stable cationic PVCL-based microgels at higher initial solids content (5–10 wt %) than usually reported for the synthesis of PVCL microgels. The thermoresponsive PVCL microgels with cationic shell undergo a reversible volume shrinkage upon heating in the absence of any hysteresis in accordance with the narrow particle size distribution. The values of the volume phase transition temperature ranged between 28 and 30 °C for the microgels synthesized using 4 and 8 wt % of P­(AETAC-X) based on VCL. The presence of a cationic outer shell onto the microgels was evidenced by the positive values of the electrophoretic mobility. The swelling behavior of the thermoresponsive microgel particles can be tuned by playing on two synthesis variables which are the initial solids content and the content of P­(AETAC-X) macro-RAFT stabilizer. Furthermore, the inner structure of the synthesized microgels was probed by transverse relaxation nuclear magnetic resonance (<i>T</i>₂ NMR) and small-angle neutron scattering (SANS) measurements. The fit of <i>T</i>₂ NMR data confirmed a core–shell morphology with different cross-linking density in PVCL microgels. Through the determination of the network mesh size, SANS was suitable to explain the increase of the values of the PVCL microgel swelling ratios by increasing the initial solids content of their synthesis

Role of Polymer–Particle Adhesion in the Reinforcement of Hybrid Hydrogels

Model hybrid hydrogels reinforced by silica nanoparticles were carefully designed to selectively tune polymer–particle interactions while maintaining similar dispersion states of nanoparticles. This was achieved by changing the nature of the polymer in the gel matrix from poly(dimethylacrylamide) (PDMA), which adsorbs on silica in aqueous suspension, to red poly(acrylamide) (PAAm), which does not, and by polymerizing and cross-linking the gels in situ. The adsorption on silica of both polymers was first quantified by considering linear chains of PDMA or PAAm or random copolymers of both. Changing the pH may also inhibit PDMA adsorption on silica. The properties of gels of PDMA, PAAm, and copolymers were then characterized. The specific effect of polymer adsorption on mechanical properties was demonstrated, with other parameters, specifically the dispersion state of silica, being kept roughly constant. Young’s modulus, or equivalently reinforcement in the linear regime, depends monotonically on the fraction of dimethylacrylamide monomers in the gel. It is enhanced by a factor of 5 due to adsorption only. Adsorption, which is a dynamic process, enhances the dependence of the fracture energy Gc on the draw rate. While Gc is slightly increased at a low draw rate, it is increased by 1 order of magnitude at a high draw rate with respect to the value in the same gel built with nonadsorbing monomers

Polymersomes with PEG Corona: Structural Changes and Controlled Release Induced by Temperature Variation

Thermoresponsive behavior of different kinds of polymersomes was studied using small angle neutron scattering (SANS), transmission electron microscopy (TEM), and proton nuclear magnetic resonance (¹H NMR). The polymersomes were made of block copolymers containing a 2000 Da polyethylene glycol (PEG) as a hydrophilic block and either a liquidlike polymer (e.g., PBA: polybutylacrylate), a solidlike polymer (PS: polystyrene), or a liquid crystalline (LC) polymer as a hydrophobic block. Structural changes in polymersomes are driven in all cases by the critical dehydration temperature of PEG corona, which is closely related to the chemical structure and chain mobility of the hydrophobic block. No structural changes occur upon heating from 25 to 75 °C in the liquidlike polymersomes where the critical dehydration temperature of PEG should be higher than 75 °C. In contrast, glassy PEG-<i>b</i>-PS polymersomes and LC polymersomes show structural changes around 55 °C, which corresponds to the critical dehydration temperature of PEG in those block copolymers. Furthermore, the structural changes depend on the properties of the hydrophobic layer. Glassy PEG-<i>b</i>-PS polymersomes aggregate together above 55 °C, but the bilayer membrane is robust enough to remain intact. This aggregation is reversible, and rather separate polymersomes are recovered upon cooling. However, LC polymersomes display drastic and irreversible structural changes when heated above ∼55 °C. These changes are dependent on the LC structures of the hydrophobic layer. Nematic LC polymersomes turn into thick-walled capsules, whereas smectic LC polymersomes collapse into dense aggregates. As these drastic and irreversible changes decrease or remove the inner compartment volume of the vesicle, LC polymersomes can be used for thermal-responsive controlled release, as shown by a study of calcein release. Finally, toxicity studies proved that LC polymersomes were noncytotoxic and had no effect on cell morphology

Vesicles in ionic liquids

The formation of vesicles from 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) in several room-temperature ionic liquids, namely, 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)), 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF(6)), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf(2)), and N-benzylpyridinium bis(trifluoromethylsulfonyl)imide (BnPyNTf(2)), as well as in a water/BmimBF(4) mixture, was investigated. In pure ionic liquids, observations by staining transmission electron microscopy demonstrated clearly the formation of spherical structures with diameters of 200-400 nm. The morphological characteristics of these vesicles in ionic liquids, in particular, the membrane thicknesses, were first investigated by small-angle neutron scattering measurements. The mean bilayer thickness was found to be similar to 63 +/- 1 angstrom in a deuterated ionic liquid (BnPyNTf(2)-d). This value was similar to that observed in water. The effect of Its on the modification of the phase physical properties of multilamellar vesicles (MLVs) was then investigated by differential scanning calorimetry. In pure IL as in water, DPPC exhibited an endothermic pretransition followed by the main transition. These transition temperatures and the associated enthalpies in ILs were higher than those in water because of a reduction of the electrostatic repulsion between zwitterionic head groups. To better understand the effect of ionic liquid on the formation of multilamellar vesicles, mixtures of BmimBF(4) and water, which are miscible in all proportions, were analyzed (BmimBF(4)/water ratio from 0% to 100%). SANS and DSC experiments demonstrated that the bilayer structure and stability were strongly modified by the IL content. Moreover, matching SANS experiments showed that BmimBF(4) molecules prefer to be located inside the DPPC membrane rather than in water