NMR Study of the Solvent Dynamics in Solutions of the Thermoresponsive Polymer pNIPAM

“Smart” or stimuli-sensitive polymers are a relatively new class of polymers, which change their properties in response to a change in their environmental conditions. Such environmental changes include the temperature, pH value, pressure or mechanical force, electric or magnetic fields, or the presence of various (combinations of) small molecules. Consequently, these polymers offer versatile options for the design of novel materials and applications, for instance in sensors, for controlled drug release, especially regarding cancer treatment, anti-fouling coatings, tissue engineering, or water purification. Many of these envisioned applications can be ascribed to the fields of biology and medicine, such that it is desirable for a smart polymer to feature a low toxicity and to react to stimuli near physiological conditions. In this regard, poly(N-isopropylacrylamide) (pNIPAM) combines many favourable properties, which, in combination with a straightforward and robust synthesis, has made it the by far most extensively researched representative of the smart polymers. Many potential applications are built around its lower critical solution temperature (LCST) behaviour in water, i.e. pNIPAM shows a conformational transition from an extended coil state at low temperatures to a collapsed globule state when the solution is heated above ~ 32 °C. However, despite decades of research details on the molecular dynamics in pNIPAM solutions remain elusive. This work uses 2H and 17O nuclear magnetic resonance (NMR) relaxometry to investigate changes in the solvent dynamics in aqueous pNIPAM-based systems as the polymer’s response is triggered by temperature or by addition of a second solvent (ethanol). The isotope specificity of NMR, in combination with our choice of isotope labelling of the components, allows for a clear attribution of the detected signals to water or, where present, ethanol – a distinction which is difficult to achieve with other methods. To isolate the influence of pNIPAM from other effects of e.g. the temperature changes themselves, we used polymer-free reference samples. We find that the conformational transition of pNIPAM causes a fraction of the solvent to feature slowed down dynamics. We propose that this fraction represents solvent molecules which experience geometric restrictions inside the pNIPAM globule. However, at least for moderate polymer concentrations, the restricted solvent molecules can quickly escape from the globules, such that on the timescales of conventional fixed-field NMR, their behaviour is averaged with the faster dynamics of the surrounding bulk-like solvent phase. With field-cycling NMR, we investigated the frequency dependence of the spin relaxation and could quantify the respective dynamics of fast and slow solvent. We found that, in our semi-dilute solutions with ca. 4 wt% polymer, the slow fraction makes up only ∼ 0.3 %, however, its dynamics are broadly distributed around an average slowdown of three orders of magnitude. Similar results were obtained when investigating solutions of pNIPAM microgel instead of the linear polymer chains. For the investigation of the behaviour of the solvents in ternary pNIPAM/ethanol/water mixtures, the reference experiments on polymer-free samples were particularly important, since even bulk mixtures of ethanol and water show non-trivial behaviour. We found that reorientational dynamics of ethanol are qualitatively consistent with the concentration dependence of macroscopic properties like the viscosity. Water dynamics, however, decouple from the overall viscosity at high ethanol concentrations, indicating microscopic phase separation even in the absence of pNIPAM. In the ternary mixtures, pNIPAM still shows its temperature response in the low-ethanol regime. In both fixed-field and field-cycling experiments, we observe a similar slowdown of the solvent dynamics as in the binary pNIPAM/water solutions. However, where both solvents are present, ethanol is more strongly affected by pNIPAM’s collapse. Thus, our results for the solvent dynamics support the concept of preferential adsorption of the alcohol at pNIPAM in these systems, which has so far been discussed in literature mostly from a structural perspective. At higher ethanol concentrations, we observed no signs of a marked temperature response of the solvent dynamics. However, comparison to polymer-free samples indicated that, depending on the solvent composition, the presence of pNIPAM affects the solvent viscosity. This may be due to alterations of the microscopic structuring of ethanol and water. In summary, we used NMR as a powerful tool for the component-resolved investigation of the molecular dynamics of solvents in solutions of pNIPAM, contributing to the further advance of the rational design of applications of smart polymers as well as providing data to test theoretical descriptions of such systems.