Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

Abstract

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 Grüneisen 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. A IntroductionFor so condensed matter, knowledge about its continuum mechanics plays a decisive role in understanding the molecular cohesion and organization.1-21 When so matter is subjected to a sufficiently small static strain, the mechanical response is controlled by linear elastic, but not viscoelastic, properties.21 If liquids, gels or polymers are excited by a dynamical mechanical probe, their viscoelastic properties become relevant. This relevance increases with the complexity of the molecular structure in terms of inter-and intramolecular degrees of freedom. At sufficiently high probe frequencies, oen lying in the upper MHz or GHz regime, the mechanical relaxation processes are dynamically clamped and the mechanical response of the system is purely elastic again. Hence, for so materials the mechanical response may be of linear elastic nature when probed either statically or at sufficiently high probe frequencies. In such case, it can be described by Hooke's law, using the second order elastic constants (SOECs) provided by the linear elastic stiffness tensor of 4 th order.Of course linear elasticity only describes one aspect of the complex mechanical behaviour of so matter. The regime of linear elastic response is oen soon abandoned for so materials as the stress or strain amplitude is increased in stress-strain or dynamic mechanical experiments (e.g. ref. 4-6, 8 and 11-14). In order to quantify this nonlinear elastic response of the material within the frame of continuum mechanics, Hooke's law must be extended. Usually the consideration of third order elastic constants (TOECs), but no higher order elastic constants, is sufficient to describe the nonlinear elastic response. 21,22Because of the usually...

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

“…1(a), for dilute to semi-dilute PNIPAM solutions with PNIPAM concentrations of maximal 6 mass%, the evolution of the refractive index can be exactly probed until 40 C. Within a temperature range of a few C above T c , a minimum of the refractive index is observed. In agreement with our studies of the longitudinal elastic modulus, [34][35][36] macroscopic order parameter saturates in this temperature range. In the established high-temperature phase, the refractive index shows again its usual, positive temperature dependence.…”

“…The latter effect becomes relevant as soon as the PNIPAM-rich agglomerates have grown larger in diameter than a few 100 nm, so that the solution behaves heterogeneous on the length scale probed by the refractometer. [34][35][36] Using dilatometry, the evolution of the specic volume versus temperature n(T) is probed across the demixing transition for the same dilute to concentrated aqueous PNIPAM solutions. As shown in Fig.…”

“…[20][21][22][23] Upon heating homogeneous PNIPAM solutions across T c , the partially dehydrated macromolecules collapse and gel-like PNIPAM-rich agglomerates grow within the phase-separating solutions. 8,[31][32][33][34][35][36][37] The kinetics related to their growth typically occurs on the time scale of minutes to hours 31,32,35,[38][39][40] and depends much on the polymer concentration of the solution, the applied heating rate and the depth of the temperature jump above T c . 32,35,38,39 The attractive interactions between the emerging PNIPAM-rich clusters, as well as their elasticity, codetermine their nally achieved size and compactness.…”

The Dependence of the Cloud Point, Clearing Point, and Hysteresis of Poly(N-isopropylacrylamide) on Experimental Conditions: The Need for Standardization of Thermoresponsive Transition Determinations

“…A detailed literature evaluation shows that the CSTs, usually but incorrectly claimed as LCSTs, for aqueous solutions of PNIPAAm were reported even as low as 24 °C and as high as around 60–70 °C . As shown by a variety of typical examples in Table S1 (Supporting Information), the thermal transition temperatures of PNIPAAm homopolymers lie between these data, and even a quick look at this dataset induces significant uncertainties due to the reliability of the reported so‐called LCST values of PNIPAAm in the literature, and other thermally responsive polymers as well, on the one hand . On the other hand, it can also be seen in Table S1 (Supporting Information) that a vari...…”

Graphene oxide hydrogel improved sensitivity in one-dimensional photonic crystals for detection of beta-glucan

“…1DPhCs are the simplest photonic crystal structure. [16][17][18][19][20][21][22] To a certain extent, the optical responsive properties of 1DPhCs can be improved when hydrogels are embedded in 1DPhCs. This process is conducted by changing the physical parameters, such as changing the thickness of one of the layers, adding another medium to the structure or removing a layer from the photonic crystals.…”