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

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 ▲]

From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

in Journal of Physical Chemistry B (2014), 118

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The ... [more ▼]

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H2O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions. [less ▲]

Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

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 ▲]

Kinetic processes at the demixing transition of PNIPAM solutions

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 ▲]

Influence of Nanoparticles on the Coupling Between Optical Dipoles in Epoxy-Silica Nanocomposites During Network Formation

in The Journal of Adhesion (2012), 88(7), 566-588

High-performance refractometry and infrared spectroscopy are combined in order to elucidate the gelation process and the glass transition during the network formation of epoxies and epoxy-based ... [more ▼]

High-performance refractometry and infrared spectroscopy are combined in order to elucidate the gelation process and the glass transition during the network formation of epoxies and epoxy-based nanocomposites. Whereas infrared spectroscopy yields the chemical conversion due to the opening of oxirane rings during the covalent network formation, high-performance refractometry is extremely sensitive to the accompanying changes of the arrangement of the molecular network. In accordance with the Lorentz-Lorenz relationship, the evolution of the refractive index seems to reflect that of the mass density during polymerization of the epoxy-based systems within the limits of a few percent. The slight deviations from the Lorentz-Lorenz relationship, which occur during the gelation of the epoxy-based systems, are attributed to long-ranged dipole-dipole interactions, which respond at optical frequencies. This point of view is supported by the fact that chemically inert silica nanoparticles embedded in the pure epoxy matrix as disturbances for these dipole-dipole interactions are able to diminish or even to suppress totally this excess contribution of the refractive index. [less ▲]

Competition Between Chemical Network Formation and Physical Adsorption and Desorption Processes in a Silica-Filled Silicone Rubber: Calorimetry versus Refractometry Studies

in Journal of Adhesion (2012), 88(7), 649-663

Calorimetry and high-performance refractometry are applied to study the network formation and accompanying morphological changes in reactive polydimethylsiloxane systems filled with silica nanoparticles ... [more ▼]

Calorimetry and high-performance refractometry are applied to study the network formation and accompanying morphological changes in reactive polydimethylsiloxane systems filled with silica nanoparticles. Both methods give insight into the structure formation during the polymerization of silica-filled silicone nanocomposites in an impressively complementary way. It will be shown that the specific heat flow as determined by calorimetry does not allow for estimating the chemical conversion whereas the refractive index does, which results from the different perspectives of both techniques on the relevant polymerization, adsorption, and desorption processes occurring in the nanocomposites. Finally, the interphase formation in the polymeric matrix in the vicinity of the nanoparticles and the network formation within the silicone rubber are demonstrated to be strongly correlated processes. [less ▲]

About hypersonic instabilities on different time scales in PNIPAM hydrogels

Presentation (2011, October 25)

Structural properties of poly(N-isopropyl acrylamide)-based systems

Poster (2011, June 08)

Optical Refraction: an alternative view on the effect of nanoparticles on polymeric network formation and the resulting nanocomposites

Presentation (2011, January 14)

Functional nano fillers in epoxy-dicyandiamide adhesives for prolonged shelf life and efficient cure

in e-Polymers (2011), (10), 1-15

Shelf life at room temperature and curing behaviour at elevated temperature are studied for hot-curing accelerated epoxies (EP, diglycidylether of bisphenol A plus dicyandiamide (Dicy)) by FTIR ... [more ▼]

Shelf life at room temperature and curing behaviour at elevated temperature are studied for hot-curing accelerated epoxies (EP, diglycidylether of bisphenol A plus dicyandiamide (Dicy)) by FTIR-spectroscopy and modulated DSC. The accelerator is added either directly or with nano-zeolite filler to the EP. Due to the immobilization of the accelerator in the pores of the nano-zeolite, the shelf life of this EP is 5 times longer than for the EP containing free accelerator. While the free accelerator acts during the whole heating step to curing temperature, the nano-zeolite does not release the accelerator before ca. 100 °C. As monitored by light microscopy, the released accelerator not only supports the curing but also stimulates the dissolution of the solid Dicy. As the result, network formation at 170 °C finishes within less than 25 minutes for the nano-filled EP. [less ▲]