Process analytical approaches for the coil-to-globule transition of poly(N-isopropylacrylamide) in a concentrated aqueous suspension

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.

Keywords: Focused Beam Reflectance Measurement; Particle Vision Microscope measurement; Photon Density Wave spectroscopy; Poly(N-isopropylacrylamide); Rate-dependent lower critical solution temperature; Turbidity measurement.

Fig. 1

Reduced scattering coefficient μ _s’ (circle), absorption coefficient μ _a (square), FBRM counts N _{< 50 μm} (triangle), FBRM counts N _50-1000 μm (diamond), relative backscatter index RBI (hexagon), reflected intensity from turbidity I _R (star), temperature ϑ _r (solid line) and temperature difference ϑ _r-ϑ _j (dashed line) as function of time for the PNIPAM suspension (heating and cooling rate of 0.3 K min⁻¹). A-F indicate points in time at which PVM pictures in Fig. 4 were taken

Fig. 2

Reduced scattering coefficient μ _s’ and absorption coefficient μ _a measured by PDW spectroscopy at 515 nm of the PNIPAM suspension (μ _s’ (circle), μ _a (square)) and of the PS suspension (μ _s’ (triangle), μ _a (diamond)) as a function of reactor temperature ϑ _r (heating and cooling rate of 0.2 K min⁻¹, 3 temperature cycles). Dashed line represents the typical literature value for the LCST of PNIPAM

Fig. 3

Relative intensity I _R (star) from turbidity measurements and the relative backscatter index RBI (hexagon) measured by PVM as a function of temperature (heating and cooling rate of 0.3 K min⁻¹, 3 temperature cycles for turbidity measurement, 1 temperature cycle for PVM measurements)

Fig. 4

PVM images at specific temperatures (time points A-F, as indicated in Figs. 1, 3 and 5) of the PNIPAM suspension with a rate of 0.3 K min⁻¹

Fig. 5

FBRM counts N _{< 50 μm} (triangle) and N _50-1000 μm (diamond) as a function of temperature (heating and cooling rate 0.3 K min⁻¹, 1 temperature cycle)

Fig. 6

Reduced scattering coefficient μ _s’ (circle), absorption coefficient μ _a (square), FBRM counts N _{< 50 μm} (triangle), FBRM counts N _50-1000 μm (diamond), relative backscatter index RBI (hexagon), temperature difference ϑ _r-ϑ _j (dashed line) and temperature ϑ _r (line) as function of time for the PNIPAM suspension (heating rate of 0.5 K min⁻¹ in the temperature range from 20 to 30 °C, heating rate of 0.01 K min⁻¹ in the temperature range from 30 to 34 °C, heating rate of 0.5 K min⁻¹ in the range from 34 to 40 °C, cooling rate 0.1 K min⁻¹). Points G-L indicate points in time at which PVM images were taken (cf. Fig. 7)

Fig. 7

PVM images at specific temperatures (G–L, as indicated in Fig. 6) during heating and cooling (heating rate of 0.5 K min⁻¹ in the temperature range from 20 to 30 °C, heating rate of 0.01 K min⁻¹ in the temperature range from 30 to 34 °C, heating rate of 0.5 K min⁻¹ in the range from 34 to 40 °C, cooling rate 0.1 K min⁻¹)

Fig. 8

Reduced scattering coefficient μ _s’ of the PNIPAM suspension as a function of temperature at different heating and cooling rates (3 cycles per rate). Inlay: Temperature region between 31 and 32 °C in detail. Dashed line represents the typical literature value for the LCST of PNIPAM

Fig. 9

Absorption coefficient μ _a of the PNIPAM suspension as a function of temperature at different heating and cooling rates (3 cycles per rate). Dashed line represents the typical literature value for the LCST of PNIPAM

Fig. 10

Relative backscatter index RBI of the PNIPAM suspension as a function of temperature at different heating and cooling rates (1 cycle per rate). Dashed line represents the typical literature value for the LCST of PNIPAM

Fig. 11

Relative intensity at 860 nm I _R of the PNIPAM suspension as a function of temperature at different heating and cooling rates (3 cycles per rate). Dashed line represents the typical literature value for the LCST of PNIPAM

Fig. 12

ϑ ₈₀ as a function of heating (square) and cooling (circle) rate based on the RBI value from PVM measurements (upper graph), the relative intensity I _R from the turbidity probe (middle graph), and the reduced scattering coefficient μ _s’ from PDW spectroscopy (lower graph) for the PNIPAM suspension (3 cycles per rate). Corresponding slopes and intercepts of the linear fits are given in Table 1