Polymers Diffusivity Encoded by Stimuli‐Induced Phase Transition: Theory and Application to Poly(N‐Isopropylacrylamide) with Hydrophilic and Hydrophobic End Groups

Abstract

The self‐diffusion of various nano‐objects investigated by high‐resolution nuclear magnetic resonance diffusometry proves to be an efficient method for the characterization of dynamics, aggregation kinetic, and matrix morphology. This study investigates how the two‐state model and Boltzmann function approach can be used for the evaluation of the thermodynamic parameters of temperature‐induced phase transition encoded in polymer diffusivity. The characteristics of the phase transition given by the transition temperature, change of entropy, and width of transition are obtained for poly(N‐isopropylacrylamide) (PNIPAm) linear polymers with hydrophilic and hydrophobic end‐group functionalization. The effect of end groups upon the polymer diffusivity is investigated as a function of molecular weight (M n), from which fractal dimensions and hydrodynamic drag coefficients are obtained. The PNIPAm diffusivity is affected strongly by the end groups, and it is reflected in the hydrodynamic radius dependence upon molecular weight that obeys different power‐law relations. In this study, the synthesis of α‐ω‐heterotelechelic PNIPAm of different molecular weights with a thiol end group and a hydrophilic NIPAm‐like as well as a hydrophobic benzyl end group are described by reversible addition–fragmentation chain‐transfer polymerization.

Diffusion of Gold Nanorods Functionalized with Thermoresponsive Polymer Brushes

“…This can be correlated with the values of hydrodynamic drag coefficients. 35 For instance, the drag coefficient ξ measured by NMR diffusometry have the values of ξ = 12.9 × 10 –11 J s m –2 K –2 and ξ = 191.4 × 10 –11 J s m –2 K –2 for Bn-PNIPAm-SH ( M n = 30 000 g mol –1 ) and NIPAm-PNIPAm-SH ( M n = 45 000 g mol –1 ) brushes, respectively. The grafting density for both PNIPAm types onto AuNRs are in the same range and cannot explain the large differences between the diffusivities of NIPAm-PNIPAm-SH and Bn-PNIPAm-SH brushes (see below).…”

“…The same trend in the evolution of D T with M n was detected for linear, free NIPAm-PNIPAm-SH, and Bn-PNIPAm-SH in the regime of isotropic concentrated solutions. 35 …”

“…To avoid the adverse effect of AuNRs convection, the diffusivity measurements are limited to the expected region of brushes phase transition temperature. 35 …”

“… 30 This interaction is affected by the hydrophilic/hydrophobic character of tethered polymer end-groups and it was detected for linear, free PNIPAm with NIPAm-like and benzyl end groups. 35 Hence, it is expected to be also present for PNIPAm brushes tethered onto AuNRs. In the following, we will assume the end-groups effect to be the same for both translational and rotational diffusivities.…”

“… 32 , 33 The two-state approach was also applied to describe phase transition revealed by self-diffusion coefficients of NIPAm-PNIPAm-SH and Bn-PNIPAm-SH free polymers in aqueous dispersions. 35 …”