Vapor-Induced Negative Expansion of Porous Cross-Linked Polymers Observed by Optical Resonance

Area of Science:

• Materials Science
• Polymer Chemistry
• Nanotechnology

Background:

• Mesoporous materials offer high surface area for various applications.
• Polymeric microspheres are versatile building blocks in materials science.
• Responsive materials that change size with environmental stimuli are of significant interest.

Purpose of the Study:

• To synthesize and characterize mesoporous cross-linked polymeric microspheres (PMS) that exhibit negative expansion.
• To investigate the mechanism behind the observed shrinkage upon organic vapor absorption.
• To explore the potential of these microspheres in sensing applications.

Main Methods:

• Emulsion polymerization using ethylene glycol dimethacrylate (EGDMA) and (+)-limonene as a template.
• Photocuring with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as an initiator.
• Characterization using Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), N2 sorption isotherms, and confocal microscopy with photoluminescence spectroscopy.

Main Results:

• Synthesized PMS with a BET surface area of 193 m²/g.
• Observed a reversible diameter contraction of -2.2% in PMS upon exposure to n-hexane vapor.
• Demonstrated that vapor condensation induces Laplace pressure, overcoming polymer elasticity and causing shrinkage.
• Utilized Nile Red luminescence and whispering gallery mode (WGM) optical resonance for in-situ monitoring of shrinkage.

Conclusions:

• Mesoporous cross-linked polymeric microspheres can undergo significant negative expansion in response to organic vapors.
• The shrinkage mechanism is attributed to inward Laplace pressure from vapor condensation within mesopores.
• The luminescent and WGM properties of incorporated Nile Red allow for sensitive detection of this dimensional change, paving the way for novel vapor sensors.