Exploring Intermolecular Interactions and Self-Assembly in PEO-b-P(HEMA-co-HEMA-g-PCL) Copolymer–Ionic Liquids Systems Derived from Choline and Imidazolinium Cations

Nuclear Magnetic Resonance (NMR) is a powerful tool for characterizing supramolecular structures. The Diffusion Ordered Diffusion Spectroscopy (DOSY) is a relevant technique to evaluate the dynamics of species that diffuse in solution, furnishing the diffusion coefficient (D) values. Species with high molecular weight are more susceptible to presenting a low diffusion coefficient value due to the large size of the molecule, which results in slow movement in the solvent. On the other hand, small molecules can move freely in the medium, presenting greater D values. This tendency can be changed when species of high and low molecular weight interact and diffuse in solution. The study of intermolecular interaction between ionic liquids (ILs) and copolymers is relevant to understanding molecular compatibility and tuning physicochemical properties to develop functional and sustainable materials.

In this context, the goal of this research is to evaluate the intermolecular interactions and aggregation properties of  PEO-b-P(HEMA-co-HEMA-g-PCL) copolymer and two ionic liquids (ILs), derived from choline ([Cho]) and imidazolinium ([EtMIM]) cation, respectively. The IL anion ([LAU]) is derived from lauric acid obtained from natural resources. To conduct the experiments, the IL samples in the presence and absence of copolymer were evaluated from 1H NMR and DOSY to investigate changes in chemical shifts and diffusion coefficient for IL-copolymer samples in comparison with pure ILs and copolymer species prepared in analogous conditions. All samples were solubilized in a 70:30 solution of acetone-d6 and deuterated water.

From results, it was observed that [Cho][LAU] mixed with the copolymer had a diffusion coefficient of 6.13x10⁻¹¹ m²/s. The copolymer that diffuses in the absence of IL showed a D=1.24x10⁻¹¹ m²/s. The D for [Cho][LAU] was 1.32x10⁻¹⁰ m²/s. The different D values for species that diffuse separated or in the IL-copolymer mixture can evidence the  intermolecular interactions in IL-copolymer systems. For [Cho][LAU]-copolymer mixture, the results suggest that IL monomers and/or micelles entered between the chains of the copolymer structure, increasing the compound flexibility. Thus, a greater mobility was observed when compared to the isolated copolymer.

For the mixture of [EtMIM][LAU] and the copolymer, the diffusion coefficient was 2.88x10⁻¹² m²/s. The copolymer and [EtMIM][LAU] separated presented D = 1.24x10⁻¹¹ m²/s, and D = 5.25x10⁻¹¹ m²/s, respectively. In this case, the [EtMIM][LAU] in the presence of copolymer shows a decrease in mobility, based on D value, which may suggest that the two components diffuse together, with more possibility of interactions than the first system discussed. It is important to mention that we expect the same heat transfer effect for all samples evaluated. Then, this effect was not considered in choosing a special pulse sequence. The study evidence the importance of IL cation to promote intermolecular interaction with the copolymer surface.

REFERENCES

P. Groves, Polym. Chem., 2017, DOI: 10.1039/C7PY01577A.