Localized Thermoresponsive Behavior in P(NIPAm-co-AAc) Copolymers: Structural Insights From Rheology and Small Angle X-Ray Scattering

Abstract

Stimuli-responsive polymers stand out for their ability to respond to small environmental changes. One of the most representative thermo-sensitive materials is poly(N-isopropyl acrylamide) (PNIPAm), which presents reversible phase transitions close to the human body temperature. However, previous studies observed that the copolymerization of NIPAm with small quantities of different monomers like acrylic acid (AAc) results in copolymers with reduced or lost thermo-responsivity. In this study, thermo-sensitive PNIPAm, pH-sensitive poly(acrylic acid) (PAAc), and various proportions of their copolymers P(NIPAm-co-AAc) were obtained by free radical polymerization and thoroughly characterized. Rheological and structural studies reveal the remaining thermosensitivity of the copolymers manifested at short molecular ranges. These alterations in short-range interactions are observed in all samples containing NIPAm, and they are evidenced by changes in the fractality of their structure and flow index behavior of the Viscosity Ostwald–de Waele Model. Particularly, when the copolymer proportion of NIPAm/AAc is about 40/60, the Beaucage model reveals two structural levels, ~200 and ~10 nm. Furthermore, the model exhibits a thermal response of the lower-size substructures, indicating possible segregation of NIPAm-rich regions from copolymer chains. The evidence found in this work could contribute to the development of nanosystems, in which local thermoresponsive effects are sought, such as for active drug targeting.