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Cloud Point Driven Dynamics in Aqueous Solutions of Thermoresponsive Copolymers: Are They Akin to Criticality Driven Solution Dynamics?
journal contribution
posted on 2019-12-11, 20:45 authored by Kajal Kumbhakar, Biswajit Saha, Priyadarsi De, Ranjit BiswasCloud point driven interaction and relaxation dynamics
of aqueous
solutions of amphiphilic thermoresponsive copolymers were explored
through picosecond resolved and steady state fluorescence measurements
employing hydrophilic (coumarin 343, C343) and hydrophobic (coumarin
153, C153) solute probes of comparable sizes. These thermoresponsive
random copolymers, with tunable cloud point temperatures (Tcp’s) between 298 and 323 K, were rationally
designed first and then synthesized via reversible addition–fragmentation
chain transfer (RAFT) copolymerization of methyl methacrylate (MMA)
and poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Subsequently,
copolymers were characterized by NMR spectroscopy and size exclusion
chromatography (SEC). A balance between the hydrophilic (PEGMA) and
the hydrophobic (MMA) content dictates the critical aggregation concentration
(CAC), with CAC ∼ 2–14 mg/L for these copolymers in
aqueous media. No abrupt changes in the steady state spectral features
of both C153 and C343 in the aqueous solutions of these polymers near
but below the cloud point temperatures were observed. Interestingly,
spectral properties of C153 in these solutions show the impact of
hydrophobic/hydrophilic interaction balance but not by those of C343.
More specifically, C153 reported a blue shift (relative to that in
neat water) and heterogeneity in its local environment. This suggested
different locations for the hydrophilic (C343) and the hydrophobic
(C153) probes. In addition, the excited state fluorescence lifetime
(⟨τlife⟩) of C153 increased with the
increase of hydrophobic (MMA) content in these copolymers. However,
C343 reported no such variations, although fluorescence anisotropy
decays for both solutes were significantly slowed down in these aqueous
solutions compared to neat water. Anisotropy decays indicated bimodal
time-dependent friction for these solutes in aqueous solutions of
these copolymers but monomodal in neat water. A linear dependence
of the average rotational relaxation rates (⟨krot⟩ = ⟨τrot⟩–1) of the type ⟨krot⟩ ∝ (|T – Tcp|/Tcp)γ with negative values for the exponent γ was observed for both
solutes. No slowing down of the solute rotation with temperature approaching
the Tcp was detected; rather, rotation
became faster upon increasing the solution temperature, suggesting
domination of the local friction.