Understanding the monomer deuteration effect on the transition temperature of poly(N-isopropylacrylamide) microgels in H2O

Abstract

Obtaining deuterated micro- and nanogels is essential to characterize their architecture and determine their response to crowding using neutron scattering with contrast variation. Experimental studies have reported that deuterated microgels have a higher volume phase transition temperature (VPTT) than hydrogenated microgels. However, to date, the effect of deuteration on the microgel properties has yet to be fully understood, questioning whether deuterated and hydrogenated microgels have comparable responsiveness to external stimuli, structure, and compressibility. To answer these questions, we both simulate and measure the effect of deuteration on the VPTT of N-isopropylacrylamide (NIPAM)-based microgels. Using quantum mechanical methods and considering only the zero-point energy difference, we study different NIPAM-(H₂O)_n complexes and calculate the changes in the value of the Flory interaction parameter as a consequence of deuteration. The resulting interaction parameter is used in the Flory–Rehner theory to predict microgels’ swelling behavior. Using multi-angle dynamic light scattering, we measure the hydrodynamic radius of non- and deuterated microgels as a function of temperature. Based on our model results, we obtain a VPTT shift of 4.9 K, only due to the zero-point energy difference between complexes containing H₂O and NIPAM/D₇NIPAM, which agrees with the 4.3 K shift of the VPTT observed in the experiments. In the future, our approach may be used to adjust effective parameters in molecular dynamics simulations studying deuterated microgels and providing insights into their properties, including VPTT, swelling ratio, and softness.