Propriétés optiques et volumiques en équilibre et hors-équilibre près de la transition de ségrégation de solutions aqueuses de PNIPAM par réfractométrie modulée en température

Self-assembly is particularly important for soft condensed matter. It may lead to manifold equilibrium or non-equilibrium structures and macroscopic properties of soft materials. The self-organization may be induced by changes in environmental conditions such as temperature, pH and irradiation by light. Among the different polymer families, the stimuli-responsive polymers are particularly sensitive to external stimuli. A structural instability, which is usually related to a demixing phase transition, commonly occurs for stimuli-responsive polymer systems. For such aqueous polymer solutions and hydrogels, a demixing transition of the Lower Critical Solution Temperature (LCST) type often happens, this means that the segregation occurs upon heating. Its macroscopic order parameter is attributed to the difference in mass concentration within the segregated state. Since the variation in mass density is a closely related quantity, the transition is also called a volume phase transition. In case of hydrogels, the order parameter also is the macroscopic deformation. In consequence the related susceptibility is the isothermal compressibility and the demixing transition is denoted as being ferroelastic. In this thesis, basic aspects of the static, kinetic and dynamical nature of the LCSTtype demixing transition are analyzed. Aqueous solutions of the thermo-responsive model homopolymer poly(N-isopropylacrylamide) (PNIPAM) are studied. Part of the phase diagram is probed by varying the polymer concentration in solution and the temperature. In part unconventional experimental techniques were used to access the macroscopic order parameter susceptibilities, like the dynamical volume expansion coefficient, and linear and nonlinear elastic properties at GHz frequencies. The major experimental technique is Temperature Modulated Optical Refractometry (TMOR), which has recently been developed in collaboration with the company Anton Paar (Seelze, Germany) and patented in 2012 by the University of Luxembourg. First studies by Brillouin spectroscopy were carried out on the segregation of aqueous PNIPAM solutions. The linear and in particular nonlinear elastic moduli are much more affected than the thermal expansion coefficient by this phase transition. Furthermore, it could be shown that the specific refractivity is astonishingly sensitive, probably to the changes in molecular interactions, during segregation. This allows for a novel perspective on the microscopic order parameters of the transition. The key investigations of this thesis focus on kinetics and thermo-mechanical relaxation processes related to the segregation, studying optical and mechanical quantities by TMOR. Sinusoidal temperature perturbations were therefore applied around the demixing temperature of different PNIPAM solutions. First direct indications for the location of the binodal and spinodal lines could b

Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

Elastic nonlinearities are particularly relevant for soft materials because of their inherently small linear elasticity. Nonlinear elastic properties may even take over the leading role for the transformation at mechanical instabilities accompanying many phase transitions in soft matter. Because of inherent experimental difficulties, only little is known about third order (nonlinear) elastic constants within liquids, gels and polymers. Here we show that a key concept to access third order elasticity in soft materials is the determination of mode Gr¨uneisen parameters. We report the first direct observation of third order elastic constants across mechanical instabilities accompanying the liquid–liquid demixing transition of semi-dilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions. Immense elastic nonlinearities, leading to a strong strain-softening in the phase-separating PNIPAM solutions, are observed. Molecular mechanisms, which may be responsible for these immense elastic nonlinearities, are discussed. The importance of third order elastic constants in comparison to second order (linear) elastic constants in the demixing PNIPAM solutions evidences the need to focus more on the general role played by nonlinear elasticity at phase transitions within synthetic and biological liquids and gels

On the elastic nature of the demixing transition of aqueous PNIPAM solutions

Mechanical instabilities accompanying the demixing transition of semi-dilute aqueous poly(Nisopropylacrylamide) (PNIPAM) solutions are probed for the first time with Brillouin spectroscopy, densitometry and refractometry. The particular role of the elastic moduli and the mass density at this coil-to-globule transition followed by molecular aggregation is investigated. Even though the demixing transition of PNIPAM solutions is denoted as a volume phase transition, it turns out that this transition is governed by the elastic properties, instead of the volume properties. This is consistent with earlier findings made for the demixing transition in chemically cross-linked PNIPAM hydrogels. Above the demixing temperature, Brillouin spectroscopy discriminates compact PNIPAM-rich agglomerates with sizes larger than 200 nm. Interestingly, these agglomerates possess a sharp distribution of elastic moduli, which can be attributed without any doubt to a material with gel-like mechanical consistency. Thus the phase-separated PNIPAM-rich agglomerates are not in the glassy state

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

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

Phase Instability and Molecular Kinetics Provoked by Repeated Crossing of the Demixing Transition of PNIPAM Solutions

The demixing process of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions can occur either via a nucleation and growth process or via spinodal decomposition. The ensuing self-assembly, leading to heterogeneous morphologies within the PNIPAM solution, is codetermined by kinetic processes caused by molecular transport. By subjecting PNIPAM solutions to cyclic changes in temperature leading to repeated crossing of the demixing transition, we are able to assess the importance of kinetics as well as of overheating and supercooling of the phase transition within the metastable range delimited by the binodal and spinodal lines. First indications about the location of these stability limits for the low- and high-temperature phases, separated by about 1.6 K, could be gained by detailed kinetic studies of the refractive index. These investigations are made possible due to the novel technique of temperature-modulated optical refractometry

Phase instability and molecular kinetics provoked by repeated crossing of the demixing transition of PNIPAM solutions

The demixing process of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions can occur either via a nucleation and growth process or via spinodal decomposition. The ensuing self-assembly, leading to heterogeneous morphologies within the PNIPAM solution, is codetermined by kinetic processes caused by molecular transport. By subjecting PNIPAM solutions to cyclic changes in temperature leading to repeated crossing of the demixing transition, we are able to assess the importance of kinetics as well as of overheating and supercooling of the phase transition within the metastable range delimited by the binodal and spinodal lines. First indications about the location of these stability limits for the low- and high-temperature phases, separated by about 1.6 K, could be gained by detailed kinetic studies of the refractive index. These investigations are made possible due to the novel technique of temperature-modulated optical refractometry