Conformational Transition
of Poly(<i>N</i>‑isopropylacrylamide) Single Chains
in Its Cononsolvency Process:
A Study by Fluorescence Correlation Spectroscopy and Scaling Analysis
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Abstract
Fluorescence correlation spectroscopy (FCS) has been
adopted to
investigate the conformational transition of poly(<i>N</i>-isopropylacrylamide) (PNIPAM) single chains with moderate molecular
weights in the cononsolvency process. A practical approach of performing
accurate FCS measurements
with the presence of the refractive index mismatch was developed.
The practical and reliable FCS calibration facilitates the acquisition
of the hydrodynamic radius (<i>R</i><sub>H</sub>) of PNIPAM
single chains with the change of the water–ethanol composition.
By using the synthesized PNIPAM samples covering a range of degrees
of polymerization (<i>N</i>), the scaling analysis in the
relationship of <i>R</i><sub>H</sub> ∼ <i>N</i><sup>ν</sup> exhibits a progressive, re-entrant change of the
scaling index (ν) between good solvent (0.57) and poor solvent
(∼1/3) condition, which is a reflection of a re-entrant conformational
transition of the polymers. Furthermore, the highly asymmetrical feature
of the cononsolvency process of single PNIPAM chains was unveiled,
which indicates a much stronger effect or interaction of the ethanol
molecules to the PNIPAM chain. Comparisons of the present results
with previous reports provided new information to the mechanism model
of the PNIPAM cononsolvency
