Measuring the Micro-Polarity and Hydrogen-Bond Donor/Acceptor Ability of Thermoresponsive N-Isopropylacrylamide/N-tert-Butylacrylamide Copolymer Films Using Solvatochromic Indicators

Abstract

Thin polymer films are important in many areas of biomaterials research, biomedical devices, and biological sensors. The accurate in situ measurement of multiple physicochemical properties of thin polymer films is critical in understanding biocompatibility, polymer function, and performance. In this work we demonstrate a facile spectroscopic methodology for accurately measuring the micro-polarity and hydrogen-bond donor/acceptor ability for a series of relatively hydrophilic thermoresponsive copolymers. The micro-polarity of the N-isopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) co-polymers was evaluated by means of the ET(30), α, β, and π* empirical solvatochromic polarity parameters. The data shows that increasing the NtBA fraction in the dry copolymer film reduces polarity and hydrogen-bonding ability. Within the Kamlet–Taft polarity framework, the NIPAM/NtBA copolymer films are strong hydrogen-bond acceptors, strongly dipolar/polarizable, and rather moderate hydrogen-bond donors. This characterization provides a more comprehensive physicochemical description of polymers, which aids the interpretation of film performance. Comparison of the measured ET(30) values with literature data for other water-soluble polymers show that dry NIPAM/NtBA copolymers are slightly more polar than poly(ethylene oxide), less polar than polyvinyl-alcohol, and approximately the same polarity as poly(N-vinyl-2-pyrrolidone). These findings indicate that this spectroscopic method is a facile, rapid, and nondestructive methodology for measuring polymer properties in situ, suitable for most biomaterials research laboratories.

Polarity Assessment of Thermoresponsive Poly(NIPAM-co-NtBA) Copolymer Films Using Fluorescence Methods

“…Thus it appears that temperature changes in rigid polymer media affects more strongly the I 1 (S 1 ν=0 → S 0 ν=0 ) vibronic band transition than the I 3 (S 1 ν=0 → S 0 ν=1 ) band and this is probably due to differences in vibronic coupling contributions [23]. Analysis of the excitation spectra monitored at the emission of the first vibronic band of pyrene doped polymer films recorded at various temperatures, indicated that there was no excimer formation associated with these mild experimental conditions [32][33][34][35][36][37][38]. Figure 4d shows a plot of the thermally induced changes in the I 1 /I 3 ratio (py parameter) for the different dry poly(NIPAM-co-NtBA) films.…”

“…An increase in temperature from 20 to 40 °C results in small changes in the π * value from 0.9 to 0.88 for pNIPAM and from 0.79 to 0.78 for pNtBA [26,32]. Thus it is clear that, in general, increasing temperature does not significantly affect the polarizability/dipolarity of the NIPAM/NtBA copolymer films.…”

“…However, when we investigated the effect of different excitation wavelengths on the 3-HF probe loaded copolymers, we found that the intensity ratio I N* /I T* ( Figure 7A-C), and the position of the N* band ( Figure 8) varies for every dye. In solvents, this excitation dependence is not observed [32], and so it could be attributed to a classical red-edge effect which is usually observed in rigid media [42][43]. In this case, red-edge excitation will photo-select dielectrically stabilized species, which in turn leads to lower T* emission.…”

Study of Water Adsorption in Poly(N-isopropylacrylamide) Thin Films Using Fluorescence Emission of 3-Hydroxyflavone Probes

“…The degree and rate of water uptake will be largely governed by the polymer composition, and less dense polymers incorporating polar groups that can hydrogen bond strongly with the incoming water will obviously adsorb greater quantities of water. The changes in polarity induced by the adsorbed water, can be sensed using UV-Vis absorption spectroscopy, 17 or by fluorescence spectroscopy if the probes are sensitive to polarity and/or hydrogen bonding effects. 39,40 Fluorescence spectroscopy is a powerful tool for monitoring microscopic changes in polymer systems due to the high sensitivity and selectivity, short response time, and non-destructive nature of the measurement.…”

Fluorescence Analysis of Thermoresponsive Polymers

“…Another important consideration is the fact that these films have large surface area to mass ratios and water uptake is an important factor to consider. This is serious because issues such as device manufacturing, coating stability, device efficacy, and long-term storage are influenced by the physiochemical properties of the polymer [19]. Thus, there is a need for the non-contact, non-destructive, analysis of these types of thermoresponsive polymers in solution and in-situ for fabricated films/ devices.…”