Research at high pressure reveals details on phase transition

The high pressure cell with the sample is inserted into a sample chamber at the beam line KWS-3. KWS-3 is a very small angle neutron scattering instrument operated by the Jülich Centre for Neutron Sciences at the Research Neutron Source Heinz Maier-Leibnitz (FRM II). © Marie-Sousai Appavou / Forschungszentrum Jülich

Unlike ice and other inorganic substances, most polymers do not show sharp phase transitions. Instead of melting at a well defined temperature, they gradually soften without significantly changing their microscopic structure. Yet some so-called thermoresponsive polymers show a drastic and discontinuous phase transition with rapid structural changes. An international team of scientists led by Prof. Christine M. Papadakis from TUM’s Physics Department has measured at the MLZ neutron small angle scattering instrument KWS-3, to sheds new light on the physical mechanism behind this peculiar behavior.

Polymers are large molecules, composed of many repeated subunits. Due to their broad range of compositions and hence their different properties, many types of polymers – both natural and synthetic – play important and ubiquitous roles in everyday life.

The phase transition drives a switch

PNIPAM_Figure1 (a-d) Optical microscopy images of a 3 wt% solution of PNIPAM in heavy water at atmospheric pressure in the one-phase state (a) and in the two-phase state (b), and at 800 bars in the one-phase state (c) and in the two-phase state (d). (e,f) Representative SANS data in the two-phase state at atmospheric pressure (e) and at 800 bars (f). - Reprinted with permission from ACS Macro Lett. 6, 1180-1185 (2017). © 2017 American Chemical Society.

(a-d) Optical microscopy images of a 3 wt% solution of PNIPAM in heavy water at atmospheric pressure in the one-phase state (a) and in the two-phase state (b), and at 800 bars in the one-phase state (c) and in the two-phase state (d). (e,f) Representative SANS data in the two-phase state at atmospheric pressure (e) and at 800 bars (f). - Reprinted with permission from ACS Macro Lett. 6, 1180-1185 (2017). © 2017 American Chemical Society.

A special class are so called responsive polymers. These strongly change their properties upon a stimulus, like a small shift in temperature, pressure, pH value or salt concentration. This can be exploited in a number of applications. For instance, gels made from thermoresponsive polymers expand or contract significantly upon a small change of temperature. Therefore, they may serve as switches that open or close valves upon heating or cooling, for example in microfluidic chips. Such gels may also take up, transport and release substances like drugs and are therefore of interest as drug carriers. However, up to now, neither the pathway of the switching process nor the molecular origin of the behavior are well understood.

A solution of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) in water serves as a model system to study these effects. When such a solution is heated above the temperature of the phase boundary the polymers suddenly collapse, release water and form aggregates. However, at ambient pressure the aggregates stop growing at a size of about 100 nm (1/10000 mm or about 1/1000th of the thickness of a hair), and the expected macroscopic phase separation is never completed.

High pressure sheds new light on physics behind the phase transition

PNIPAM_Figure2 (a) SANS data from a 3 wt% PNIPAM solution in heavy water before (black) and after (colors) a jump with a pressure change from 300 to 160 bars, carried out at 35.1 °C. The growth regimes I (blue), II (red) and III (orange) are indicated on the left. (b) Schematic representation of the formation and structural evolution of the mesoglobules during the pressure jump. The black squares indicate similar length scales in each regime. - Reprinted with permission from ACS Macro Lett. 7, 1155-1160 (2018). © 2018 American Chemical Society.

(a) SANS data from a 3 wt% PNIPAM solution in heavy water before (black) and after (colors) a jump with a pressure change from 300 to 160 bars, carried out at 35.1 °C. The growth regimes I (blue), II (red) and III (orange) are indicated on the left. (b) Schematic representation of the formation and structural evolution of the mesoglobules during the pressure jump. The black squares indicate similar length scales in each regime. - Reprinted with permission from ACS Macro Lett. 7, 1155-1160 (2018). © 2018 American Chemical Society.

In order to learn more about this phase transition, a novel approach was required. The international team of scientists around Christine M. Papadakis, professor for soft matter physics at TU Munich, employed a shift in pressure to investigate the transition in more detail, combining the strengths of different experimental techniques like Raman scattering and small angle neutron scattering. This has two distinct advantages to the conventional approach on just probing different temperatures: Changing the pressure affects the hydration behavior and thus the solubility of the polymers, and pressure can be changed very rapidly across the solution, facilitating the characterization of the early stages of aggregate formation.

In order to test the hypothesis, that pressure has an influence on the formation of aggregates, pressures of a few hundred bars were employed. “Combining small angle neutron scattering experiments at TUM with Raman spectroscopy, we were able to show, that pressure indeed weakens the dehydration process of the phase transition. This leads to the aggregates formed at high pressures being significantly larger and having a higher water content”, explains Professor Alfons Schulte, visiting professor from the University of Central Florida, USA.

A further set of neutron scattering experiments were carried out at the Institute Laue-Langevin (ILL) in Grenoble. The temporal resolution of these experiments allowed the researchers to discern three different phases of cluster growth: Within the first second after the pressure jump, nucleation of the clusters takes place, followed by diffusion limited growth. After about 20 s, however, the growth of the globules slows strongly, as a dense shell emerges that hampers the merging of the aggregates.

Combining different experimental methods is the key

KWS-3 is a very small angle neutron scattering instrument at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, running on the focussing mirror principle. The high pressure cell with the sample is inserted into this sample chamber. © W. Schürmann / TUM

Christine Papadakis concludes: “Applying high pressure alters the hydration of responsive polymers, which has a strong effect on the structure in the phase-separated state. Our combination of different experimental methods has given us a comprehensive view of the structure-forming processes. A unique aspect is that time scales ranging from tens of milliseconds to thousands of seconds as well as length scales of around 1 to 100 nm are recorded. Our method creates new opportunities to study the pathways of structural changes in complex systems with unprecedented dynamic range.”

The work was carried out in a close collaboration of the Papadakis group with Prof. Alfons Schulte (University of Central Florida, Orlando, U.S.A.). Mutual research stays were supported by the TUM August-Wilhelm Scheer Guest Professor Program and the Faculty Graduate Center Physics of the TUM Physics Department. Neutron experiments essential for this research were conducted at the Heinz Maier-Leibnitz Zentrum (Garching) and at the Institute Laue-Langevin (Grenoble, France). The SANS High Pressure cell at MLZ is based on the one existing at the Paul Scherrer Institute (Villigen, Switzerland) (see reference in ACS Macro Lett. 6, 1180-1185 (2017)).

Text: Dr. Johannes Wiedersich / TUM