Probing the phase transition of aqueous solutions of linear low molecular weight poly(N-isopropylacrylamide) by dielectric spectroscopy

Abstract

Aqueous solutions of linear poly(N-isopropylacrylamide) (pNIPAM) with different polymer concentrations are investigated using dielectric spectroscopy in a frequency range of 10−1 Hz to 106 Hz at temperatures from 15 °C to 50 °C. The phase transition of pNIPAM is monitored by both the temperature (T) and the frequency (f) dependence of the conductivity spectra σ*(f, T). First, the T-dependence of the DC conductivity σ′DC is investigated and the phase transition (“coil-to-globule” transition) at the lower critical solution temperature (LCST) of pNIPAM is deduced by a change in the T-dependence of σ′DC. The observed hysteresis between heating and cooling runs is discussed in detail in dependence on both the polymer concentration and the rate. Second, for the first time a pronounced f-dependence of the real part of conductivity σ′ is observed at temperatures above the LCST whereas at temperatures below the LCST the conductivity spectra are more or less similar to that of water (frequency independent). This f-dependence of σ′ is assigned to the formation of a kind of soft particle (“globular structure”) at the LCST which is more or less impermeable to water and ions therefore giving rise to Maxwell–Wagner–Sillars (MWS) polarization effects (blocking of charges at the soft particle). The dependence on the concentration is studied in detail.