Tuning the Hydrophobicity of Poly(N-isopropylacrylamide) by Tacticity: a Molecular Dynamics Simulation Study

The research of stimuli triggered, artificial micro- and nano-devices identified poly(N-isopropylacrylamide), PNIPAM, as good candidate to obtain soft matrices with thermal responsivity [1]. In water environment this synthetic macromolecule displays a transition from a soluble state, stable at temperatures lower than a critical value, referred to as lower critical solution temperature, LCST, to an insoluble state. Such transition is reversible and implies both intramolecular coil-to-globule chain rearrangement and aggregation between chains, finally leading to phase separation. To move the phase boundary of the PNIPAM-water binary system without introducing hetero residues is feasible by modifying the polymer tacticity, indeed experimental findings show that meso dyads rich PNIPAMs are more hydrophobic as compared to the corresponding atactic polymers, the opposite occurring for racemo dyads rich PNIPAMs [2]. Therefore a small and local change of stereochemistry can adapt the thermal switch conditions of PNIPAM based systems to the specific application. This work aims to achieve a molecular interpretation of the tacticity dependent water affinity of PNIPAM, which is still missing. Atomistic detailed models with different tacticities were built and their behaviour in diluted aqueous solution was simulated both below and above the LCST. The space/time window accessible to this kind of simulations, a ten of nanometres/few hundreds of nanoseconds, respectively, allowed to monitor the coil-to-globule transition of PNIPAM chains with a degree of polymerization corresponding to about two Kuhn segments. The temperature influence on polymer conformation, detected in these models, agrees with the available experimental data and the analysis of the PNIPAM hydration highlights a cooperative pattern of hydrogen bonded water molecules, as forecast by the Tanaka’s model [3,4]. The structural alterations of such hydration shell, induced by the different stereochemistry, are the main factor determining the modulation of PNIPAM solubility. Many-core computing allowed for this study and will be further exploited for modelling larger and topologically more complex PNIPAM based systems. Acknowledgments. The CINECA award under the ISCRA initiative is acknowledged for the availability of high performance computing resources. The University of Rome Tor Vergata is acknowledged for funding this research within the project AcouGraph (Consolidate the Foundations 2015). References. [1] Halperin, A.; Kröger, M.; Winnik, F. M. Angew. Chem. Int. Ed. 2015, 54, 15342–15367. [2] Nishi, K.; Hiroi, T.; Hashimoto, K.; Fujii, K.; Han, Y.-S.; Kim, T.-H.; Katsumoto, Y.; Shibayama, M. S. Macromolecules 2013, 46, 6225–6232. [3] Okada, Y.; Tanaka, F. Macromolecules 2005, 38, 4465–4471. [4] Chiessi, E.; Paradossi, G. J. Phys. Chem. B 2016, 120, 3765–3776