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

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.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

The dielectric signature of poly(N-isopropylacrylamide) microgels at the volume phase transition: dependence on the crosslinking density

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Temperature sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels are prepared and investigated using dielectric spectroscopy in a frequency range from 10−1 Hz to 106 Hz at temperatures from 15 °C to 50 °C. The microgels were synthesized with different crosslinker molar ratios resulting in microgels with structural differences. From the dielectric response of the pNIPAM microgels the swelling/deswelling behaviour is monitored by both the temperature (T) and the frequency (f) dependence of the conductivity spectra σ*(f, T). The volume phase transition (VPT) at the lower critical solution temperature (LCST) is deduced by a change in the T-dependence of the DC conductivity σ′DC. It can be explained by a decrease in the effective charge mobility and a reduction in the effective charge number contributing to σ′DC at T > LCST. Addressing the f-dependence of the real part of the conductivity σ′, a pronounced frequency dependence at temperatures above the LCST can be observed whereas at temperatures below the LCST the conductivity spectra resemble that of the pure solvent (water) which is frequency independent. The f-dependence of σ′ at T > LCST is assigned to the collapse of the microgel particles. At the interfaces of the collapsed particles charge carriers are blocked and/or entrapped giving rise to Maxwell–Wagner–Sillars (MWS) polarization effects. The dependence of the MWS effect on the crosslinker amount is studied in detail and conclusions concerning the internal structure of the microgels with respect to their crosslinking density are drawn. Moreover the dielectric data are related to dynamic light scattering data. A correlation between the MWS polarization effect and the swelling/deswelling ratio expressed by the hydrodynamic radius Rh at different temperatures is established for the first time

3-bit Resistive RAM Write-Read Scheme Based on Complementary Switching Mechanism

While standard bipolar switching RRAM memory devices can be programmed into different resistance states, the complementary switching mechanism allows for two distinct switching locations that each can be programmed to these resistance states. In this letter, we present a technique to discriminate these switching locations and report on a novel scheme allowing for sub-μs pulse write and read of eight different logic states in Pt/Ta2O5/Ta/Pt devices by using only four different resistive states. Thus, in addition to the multilevel capability of bipolar switching devices, double the information can be stored and read in a single complementary switching device