2 research outputs found

    Cononsolvency Revisited: Solvent Entrapment by <i>N</i>‑Isopropylacrylamide and <i>N</i>,<i>N</i>‑Diethylacrylamide Microgels in Different Water/Methanol Mixtures

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    Aqueous dispersions of homo- and copolymer microgels of <i>N</i>-isopropylacrylamide (NiPAm) and <i><i>N</i></i>,<i><i>N</i></i>-diethylacrylamide (DEAm) with different compositions are temperature-dependently studied by means of proton nuclear magnetic resonance spectroscopy (<sup>1</sup>H NMR) and differential scanning calorimetry (DSC). Furthermore, the effect of varying the solvent composition by adding methanol is investigated. Methanol addition leads to a broadening of the thermally induced volume phase transition in case of NiPAm-containing samples, as confirmed by DSC. At the same time, the width of transition approaches the one of neat PDEAm. Two different solvent species, namely bulk-like and restricted solvent, are observed as separate lines in <sup>1</sup>H NMR experiments when the gels deswell. The restricted nature of the second species is affirmed by pulsed field gradient (PFG) NMR self-diffusion experiments. The temperature <i>T</i><sub>split</sub> from which on the restricted species is found cannot be directly related to the volume phase transition temperature determined by DSC. The difference between <i>T</i><sub>split</sub> and the DSC peak temperature changes depending on the NiPAm-content of the microgel. An increase in the shift difference between the two solvent signals with temperature indicates a continuous change of the restricted solvent environment. At even higher temperature, the shift difference of restricted and bulk solvent approaches asymptotically a constant value. In general, the observed effects of methanol addition are consistent with an increasing complexation of the amide protons of the microgel (originating from the NiPAm units) with methanol. In contrast, poly­(DEAm) does not show any anomaly concerning transition width and <i>T</i><sub>split</sub> upon methanol addition. This is attributed to the lack of amide protons. The results indicate that the presence of cononsolvency can be explained by the presence of the amide proton

    Toward Copolymers with Ideal Thermosensitivity: Solution Properties of Linear, Well-Defined Polymers of <i>N</i>‑Isopropyl Acrylamide and <i>N</i>,<i>N</i>‑Diethyl Acrylamide

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    Statistical copolymers of <i>N</i>-isopropyl acrylamide (NIPAM) and <i>N,N</i>-diethyl acrylamide (DEAAM) show a pronounced synergistic depression in their cloud points, though both homopolymers phase separate at significantly higher temperatures close to 30 °C (e.g., <i>Polymer</i> <b>2009</b>, <i>50</i>, 519). While phase separation occurs at 20 °C for the statistical copolymers, the influence of the monomeric sequential arrangement along the backbone was not addressed so far. Thus, we report on the thermosensitive properties of a diblock copolymer PDEAAM-<i>b</i>-PNIPAM and compare it to the homopolymers, mixtures thereof, and to the statistical copolymer of the same molecular weight. These polymers were prepared by controlled radical polymerization, namely Reversible Addition–Fragmentation Chain Transfer (RAFT). Their solution behavior was mainly studied by infrared spectroscopy (IR) of the amide I′ band and by turbidimetry. IR spectroscopy sees a decreasing hydration with heating even below the cloud point for all polymers. This results finally in phase separation, which induces further spectral changes. Rather unexpectedly, the diblock copolymer shows phase separation at temperatures close to the homopolymers, well above the cloud points of the homopolymer mixtures. In turn, the transition temperature of the homopolymer mixture is reduced compared to its homopolymers, which indicates intermolecular attraction between both partners. This behavior can be explained by taking the block length dependencies of the respective cloud points into account and assuming a rather independent phase behavior of each short block (within the copolymer). Then, the increased inherent cloud point of each “half-length” block (compared to the homopolymers) has a stronger effect than the aggregating tendency inherited by the connectivity of the comonomer units. As a result, IR spectroscopy reveals almost ideal behavior of the diblock copolymer, which can be comprehended as an ideal mixture of the homopolymers, each one contributing to the overall signal by its concentration. Finally, <sup>1</sup>H NMR suggests that intermediate aggregation (as seen by light scattering) is not induced by segregation of just one block, but rather by partial and weak complexation between the two components within the diblock copolymer
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