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Figure 3. Absorption spectra of RD in PMMA at different RH and the emission spectrum of the LED (circles). Figure 2. Effect of “titration” of water on the absorption spectra of RD in acetone. To get an idea of the humidity effect on the absorption spectra of RD in a glassy polymer, “titration” of the RD solution in acetone was performed (Fig. 2). The addition of water resulted in a noteworthy double effect: an exponential decrease of the band area and a nonlinear blue shift of its maximum. The former, hypochromic effect corresponds to the change of the chemical equilibrium between the phenolate and phenolic forms of RD. A specific solvatation—formation of hydrogen bonds between the RD oxygen and the water proton—is responsible for the latter effect, i.e., a hypsochromic shift.

Figure 3 Absorption spectra of RD in PMMA at different RH and the emission spectrum of the LED (circles). Figure 2. Effect of “titration” of water on the absorption spectra of RD in acetone. To get an idea of the humidity effect on the absorption spectra of RD in a glassy polymer, “titration” of the RD solution in acetone was performed (Fig. 2). The addition of water resulted in a noteworthy double effect: an exponential decrease of the band area and a nonlinear blue shift of its maximum. The former, hypochromic effect corresponds to the change of the chemical equilibrium between the phenolate and phenolic forms of RD. A specific solvatation—formation of hydrogen bonds between the RD oxygen and the water proton—is responsible for the latter effect, i.e., a hypsochromic shift.

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The steady-state absorption spectra were measured by a Jasco V-570 spectrophotometer. The piece of solvatochromic film was placed into a quartz cuvette using a stainless steel sample holder for positioning. Some drops of a saturated water solution of Greenspan’s salts were added to the bottom of the closed cuvette to stabilize the relative humidity (RA). We used saturated solutions of LiCl (RH = 12%), MgCl, (RH = 33%), Mg(NO3)2 (RH = 55%) and NaCl (RH = 75%). The cuvette of the spectrophotometer was plugged to the head of a capacity-type RH sensor Hygro-Palm 1 (Rotronic) for monitoring the temperature and RH: Figure 1. Experimental setup. Water sorption kinetics is measured using the transmission of the 667-nm light through a solvatochromic film placed in the temperature controlled vacuum chamber: LED light emitting diode, POF plastic optical fiber, V/, V2 valves, PMT photo multiplier.
The steady-state absorption spectra were measured by a Jasco V-570 spectrophotometer. The piece of solvatochromic film was placed into a quartz cuvette using a stainless steel sample holder for positioning. Some drops of a saturated water solution of Greenspan’s salts were added to the bottom of the closed cuvette to stabilize the relative humidity (RA). We used saturated solutions of LiCl (RH = 12%), MgCl, (RH = 33%), Mg(NO3)2 (RH = 55%) and NaCl (RH = 75%). The cuvette of the spectrophotometer was plugged to the head of a capacity-type RH sensor Hygro-Palm 1 (Rotronic) for monitoring the temperature and RH: Figure 1. Experimental setup. Water sorption kinetics is measured using the transmission of the 667-nm light through a solvatochromic film placed in the temperature controlled vacuum chamber: LED light emitting diode, POF plastic optical fiber, V/, V2 valves, PMT photo multiplier.
Figure 4. Dependence of the absorbance at the wavelength of 669 nm in an RD-PMMA film on the RH at RT ‘he measured RH-induced change of the absorbance at the wavelength of 669 nm is depicted in Fig. 4. A good linear lependence with the RA level sensitivity to within 0.002% was obtained. 3.2. Water sorption kinetics At RH over 10% the Henry’s term dominates in dual-mode sorption model, because the Langmuir-type contribution is already saturated. Henry’s law expresses an ideal dissolution of water in a glassy polymer. RE The water sorption—desorption kinetic curves (Fig. 5) could be well fitted to the Fickian diffusion model with the single diffusion coefficient D using following equation:*°
Figure 4. Dependence of the absorbance at the wavelength of 669 nm in an RD-PMMA film on the RH at RT ‘he measured RH-induced change of the absorbance at the wavelength of 669 nm is depicted in Fig. 4. A good linear lependence with the RA level sensitivity to within 0.002% was obtained. 3.2. Water sorption kinetics At RH over 10% the Henry’s term dominates in dual-mode sorption model, because the Langmuir-type contribution is already saturated. Henry’s law expresses an ideal dissolution of water in a glassy polymer. RE The water sorption—desorption kinetic curves (Fig. 5) could be well fitted to the Fickian diffusion model with the single diffusion coefficient D using following equation:*°
Figure 5. Water sorption—desorption curves at 0 and 30 °C.
Figure 5. Water sorption—desorption curves at 0 and 30 °C.
Figure 7. Van’t Hoff plot of K for water sorption in glassy PMMA. Figure 6. Arrhenius plot of water diffusion coefficient D in glassy PMMA. The data point at 40 °C is underestimated due to limited time resolution of the measurement. The activation energy AE = 53 kJ/mol of the diffusion coefficient obtained from the Arrhenius plot of D agrees well with the literature data.” The temperature variation of K obeys the Van’t Hoff-type dependence that supports the hypothesis that K in Eq. (3) is proportional to the Henry’s law solubility constant kp in Eq. (1). The enthalpy of sorption AHs = -22 kJ/mol is close to the molar enthalpy of water condensation in PMMA.
Figure 7. Van’t Hoff plot of K for water sorption in glassy PMMA. Figure 6. Arrhenius plot of water diffusion coefficient D in glassy PMMA. The data point at 40 °C is underestimated due to limited time resolution of the measurement. The activation energy AE = 53 kJ/mol of the diffusion coefficient obtained from the Arrhenius plot of D agrees well with the literature data.” The temperature variation of K obeys the Van’t Hoff-type dependence that supports the hypothesis that K in Eq. (3) is proportional to the Henry’s law solubility constant kp in Eq. (1). The enthalpy of sorption AHs = -22 kJ/mol is close to the molar enthalpy of water condensation in PMMA.
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