Saptarsi Mondal Ultrafast & multidimensional IR spectroscopy
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Vibrational probes & methods

Water structure and free volume in biocompatible polymers

Two connected questions about amorphous polymers: what water does at their surface, and how large the empty spaces inside them actually are — both answered by ultrafast infrared measurement rather than by inference from bulk properties.

Completed Collaborative Korea University

The biocompatibility result is a nice example of a molecular measurement settling a question that bulk characterisation had left open. Contact angle, protein adsorption assays and freezing-point measurements all indicate that PMEA and PPEA behave differently, but none of them says why. Resolving two water populations with different vibrational lifetimes, and showing that the population balance tracks antifouling behaviour, gives the mechanism rather than the correlation.

The free-volume work is the more transferable of the two. The wobbling-in-a-cone model is used throughout polymer and biophysical spectroscopy to convert a rotational anisotropy decay into a cone angle and hence a confining volume. Its standard form assumes the entire decay reflects whole-molecule reorientation. For any probe with a rotatable bond — which is most of them — part of the decay is internal, and attributing that part to confinement makes the inferred free volume too large. Separating the two contributions is a small change to the model with a substantial effect on the number that comes out.

Scientific motivation
Acrylate polymers vary enormously in biocompatibility despite near-identical backbones, and the leading explanation attributes the difference to an intermediate water layer that resists protein adsorption. That explanation had not been tested by a method able to resolve distinct water populations and their dynamics. Separately, free-volume elements govern gas transport and mechanical response in amorphous polymers, but the standard analysis used to measure them makes an assumption that turns out to be wrong.
The research question
Do biocompatible and poorly biocompatible acrylates actually differ in their adsorbed water populations — and can free-volume element size be measured accurately once a probe's internal motion is separated from its whole-molecule motion?
My contribution
I performed the femtosecond mid-infrared pump–probe measurements on the OD stretch of HOD adsorbed to highly biocompatible poly(2-methoxyethyl acrylate) and poorly biocompatible poly(2-phenoxyethyl acrylate), and the analysis distinguishing the water populations. On the free-volume work I contributed the spectroscopic measurement and analysis behind the recast wobbling-in-a-cone treatment.
Results and current status
The two polymers carry two distinct water species with significantly different vibrational lifetimes; PMEA interacts more strongly with water than PPEA through carbonyl–water hydrogen bonding, and the intermediate water lifetime is longer by factors of three and seven in PMEA and bulk water respectively. On free volume, decoupling the internal bond rotational fluctuation of nitrile probes from whole-molecule diffusion — which the standard wobbling-in-a-cone model conflates — gives accurate quantification of static free-volume elements and connects microscopic dynamics to macroscopic polymer properties.
Future direction
The corrected anisotropy treatment applies wherever a probe with internal rotational freedom is used to report on a confining environment, including the nitrile probes used in the interfacial work.
Collaboration With Prof. Kyungwon Kwak and Prof. Minhaeng Cho at the Center for Molecular Spectroscopy and Dynamics, Korea University, and with Sourav Palchowdhury on the free-volume analysis.

Publications from this project

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