Abstract
The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM) microgel particles suspended in water has been investigated in situ as a function of heating and cooling rate with four optical process analytical technologies (PAT), sensitive to structural changes of the polymer. Photon Density Wave (PDW) spectroscopy, Focused Beam Reflectance Measurements (FBRM), turbidity measurements, and Particle Vision Microscope (PVM) measurements are found to be powerful tools for the monitoring of the temperature-dependent transition of such thermo-responsive polymers. These in-line technologies allow for monitoring of either the reduced scattering coefficient and the absorption coefficient, the chord length distribution, the reflected intensities, or the relative backscatter index via in-process imaging, respectively. Varying heating and cooling rates result in rate-dependent lower critical solution temperatures (LCST), with different impact of cooling and heating. Particularly, the data obtained by PDW spectroscopy can be used to estimate the thermodynamic transition temperature of PNIPAM for infinitesimal heating or cooling rates. In addition, an inverse hysteresis and a reversible building of micrometer-sized agglomerates are observed for the PNIPAM transition process.
Highlights
As a typical member of sensitive or Bsmart^ polymers, poly(N-isopropylacrylamide) (PNIPAM) is one of the most frequently investigated polymers within recent decades because of its distinct thermo-sensitive behavior [1,2,3,4,5,6]
The coil-to-globule transition of poly(Nisopropylacrylamide) (PNIPAM) microgel particles suspended in water has been investigated in situ as a function of heating and cooling rate with four optical process analytical technologies (PAT), sensitive to structural changes of the polymer
The data obtained by Photon Density Wave (PDW) spectroscopy can be used to estimate the thermodynamic transition temperature of PNIPAM for infinitesimal heating or cooling rates
Summary
As a typical member of sensitive or Bsmart^ polymers, poly(N-isopropylacrylamide) (PNIPAM) is one of the most frequently investigated polymers within recent decades because of its distinct thermo-sensitive behavior [1,2,3,4,5,6]. PDW spectroscopy is a PAT to quantitatively and independently characterize the absorption and scattering properties of highly turbid liquid suspensions [53, 54]. In consequence, based on the reduced scattering coefficient, PDW spectroscopy can be used for in-line particle sizing in the size regime of approximately 50 nm to 500 μm [38, 53]. PDW spectroscopy is based on photon transport theory in multiple light scattering systems [56]. It operates in the frequency domain, i.e., the emitted light, creating the PDW inside the multiply light scattering liquid, is intensity modulated with different frequencies. In contrast to many other optical technologies, PDW spectroscopy is capable of quantifying the optical properties of the material under investigation at volume fractions of the disperse phase well above 30 %
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