References of "Muller-Buschbaum, P."
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See detailMolecular versus macroscopic perspective on the demixing transition of aqueous PNIPAM solutions by studying the dual character of the refractive index
Philipp, Martine UL; Aleksandrova, Ralitsa UL; Müller, Ulrich UL et al

in Soft Matter (2014), 10(37), 7297-7305

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 ... [more ▼]

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. [less ▲]

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See detailImmense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions
Philipp, Martine UL; Müller, Ulrich UL; Aleksandrova, Ralitsa UL et al

in Soft Matter (2013), 9

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 ... [more ▼]

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. [less ▲]

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See detailKinetic processes at the demixing transition of PNIPAM solutions
Philipp, Martine UL; Müller, Ulrich UL; Jiménez Riobóo et al

in Soft Matter (2013), 9

Kinetic processes, which are joined with mass transport, are studied in the vicinity of the sharp LCST-type demixing transition of semi-dilute aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions ... [more ▼]

Kinetic processes, which are joined with mass transport, are studied in the vicinity of the sharp LCST-type demixing transition of semi-dilute aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions. These processes are slow as compared to the highly cooperative collapse of individual polymer chains. Purely elastic properties, that are particularly sensitive to this phase transition, are addressed depending on the temperature, space and time by Brillouin spectroscopy. Above the demixing temperature Tc, we discriminate between kinetics related to the phase separation into PNIPAM-rich and PNIPAM-poor domains and kinetics connected to the impact of gravitation on the on-going phase separation. Using shallow temperature jumps of 0.3 C, the growth of compact PNIPAM-rich agglomerates with identical gel-like mechanical consistency is provoked independently of temperature and position within the sample above Tc. Astonishingly, the transition temperature does not vary while heating or cooling the solutions across the phase transition, although the elastic properties depend strongly on space and time during the equilibration of PNIPAM concentration gradients following the re-entrance into the lowtemperature phase. [less ▲]

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See detailOn the elastic nature of the demixing transition of aqueous PNIPAM solutions
Phillip, M.; Müller, Ulrich UL; Aleksandrova, Ralitsa UL et al

in Soft Matter (2012), 8

Mechanical instabilities accompanying the demixing transition of semi-dilute aqueous poly(Nisopropylacrylamide) (PNIPAM) solutions are probed for the first time with Brillouin spectroscopy, densitometry ... [more ▼]

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. [less ▲]

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See detailStructural properties of poly(N-isopropyl acrylamide)-based systems
Philipp, Martine UL; Magerl, D.; Aleksandrova, Ralitsa UL et al

Poster (2011, June 08)

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See detailStructural properties of poly(N-isopropyl acrylamide)-based systems
Philipp, Martine UL; Magerl, D.; Aleksandrova, Ralitsa UL et al

Poster (2011, June 01)

Detailed reference viewed: 66 (3 UL)