Molecular versus macroscopic perspective on the demixing transition of aqueous PNIPAM solutions by studying the dual character of the refractive index

Abstract

The phase separation of aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions is known to strongly affect their volume expansion behaviour and the elastic moduli, as the latter are strongly coupled to the macroscopic order parameter. On the molecular scale, considerable changes in H-bonding and hydrophobic interactions, as well as in the structure govern the demixing process. However, the relationship between the molecular and macroscopic order parameters is unclear for such complex phase-separating solutions. We contribute to the clarification of this problem by relating optical to volumetric properties across the demixing transition of dilute to concentrated aqueous PNIPAM solutions. Far from the demixing temperature, the temperature dependence of the refractive index is predominantly determined by thermal expansion. In the course of phase separation, the refractive index is dominated by the anomalous behaviour of the specific refractivity, which reflects the spatio-temporally averaged changes in molecular interactions and the structural reorganization of the demixing solutions. Moreover, the presence of relaxation processes is studied by the complex expansion coefficient using the novel technique of temperature modulated optical refractometry.

Poly(N‐isopropylacrylamide) Phase Diagrams: Fifty Years of Research

“…Thes econd issue we address concerns optical measurements.Different techniques were utilized to obtain demixing curves:t ransmittance, [16][17][18]20,22,23,[26][27][28] scattering intensity at different angles, [24,26,[28][29][30] and refractometry. [31] Thet ransmittance and scattering intensity measurements rely on the light scattered by polymer aggregates having ar efractive index differing from that of the surrounding solution. In turn, the scattering intensity varies with the wavelength l and with the l-dependent refractive index.…”

“…[31] Thet ransmittance and scattering intensity measurements rely on the light scattered by polymer aggregates having ar efractive index differing from that of the surrounding solution. In turn, the scattering intensity varies with the wavelength l and with the l-dependent refractive index.…”

Laser-driven structural transformations in dextran-graft-PNIPAM copolymer/Au nanoparticles hybrid nanosystem: the role of plasmon heating and attractive optical forces

“…The LCST temperature in studied D-g-PNIPAM/AuNPs nanosystem was determined in our recent work 46 to be 33.5 C. The rate of the refractive index decrease at the increase of the temperature dn/ dT for PNIPAM water solution at the temperatures higher than LCST point was taken from ref. 58 to be of À10 À4 C À1 . Then taking the SPR shi Dl of S-peak for non-aggregated Au NPs (Fig.…”

Poly(N‐isopropylacrylamid)‐Phasendiagramme: 50 Jahre Forschung

“…Um Entmischungskurven zu erhalten, wurden diverse Te chniken eingesetzt:T ransmission, [16][17][18]20,22,23,[26][27][28] Intensität bei verschiedenen Streuwinkeln [24,26,[28][29][30] und Refraktometrie. [31] Die Tr ansmissions-und Intensitätsmessungen machen sich das Streulicht der Polymeraggregate zunutze,d eren Brechungsindex sich von dem der umgebenden Lçsung unterscheidet. Die Streuintensität wiederum variiert mit der Wellenlänge l und mit dem l-abhängigen Brechungsindex.…”