It was demonstrated for several organic
cononsolvents added to
aqueous solutions of poly(N-isopropylacrylamide)
that they lead to an increasing solubility of the polymer in the low-temperature/high-pressure
region while reducing the solubility of the polymer and therefore
its cloud temperature at atmospheric pressure. A complete inversion
of the phase-separation behavior regarding both the effects of pressure
and temperature was found by increasing the mole fraction of added
ethanol over the whole cononsolvency range. As a special case, at
a mole fraction of x = 0.23 the system cleared up
independent of temperature at a constant pressure of about 1000 bar.
The measurements were performed in an optical high-pressure cell up
to pressures of 3000 bar and down to temperatures of −20 °C.
The results are discussed in terms of solvent–solvent and solvent–polymer
interactions.
This work presents the synthesis and thermoresponsiveness of a random acrylamide copolymer in alcohol–water mixtures and discusses cononsolvency phenomena.
The combination of reversible addition–fragmentation chain transfer (RAFT) and emulsion polymerization has recently attracted much attention as a synthetic tool for high-molecular-weight block copolymers and their micellar nano-objects. Up to recently, though, the use of thermoresponsive polymers as both macroRAFT agents and latex stabilizers was impossible in aqueous media due to their hydrophobicity at the usually high polymerization temperatures. In this work, we present a straightforward surfactant-free RAFT emulsion polymerization to obtain thermoresponsive styrenic block copolymers with molecular weights of around 100 kDa and their well-defined latexes. The stability of the aqueous latexes is achieved by adding 20 vol % of the cosolvent 1,4-dioxane (DOX), increasing the phase transition temperature (PTT) of the used thermoresponsive poly(N-acryloylpyrrolidine) (PAPy) macroRAFT agents above the polymerization temperature. Furthermore, this cosolvent approach is combined with the use of poly(N,N-dimethylacrylamide)-block-poly(N-acryloylpiperidine-co-N-acryloylpyrrolidine) (PDMA-b-P(APi-co-APy)) as the macroRAFT agent owning a short stabilizing PDMA end block and a widely adjustable PTT of the P(APi-co-APy) block in between 4 and 47 °C. The temperature-induced collapse of the latter under emulsion polymerization conditions leads to the formation of RAFT nanoreactors, which allows for a very fast chain growth of the polystyrene (PS) block. In dynamic light scattering (DLS), as well as cryo-transmission electron microscopy (cryoTEM), moreover, all created latexes indeed reveal a high (temperature) stability and a reversible collapse of the thermoresponsive coronal block upon heating. Hence, this paper pioneers a versatile way towards amphiphilic thermoresponsive high-molecular-weight block copolymers and their nano-objects with tailored corona switchability.
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