Mechanical reinforcement of polymer colloidal crystals by supercritical fluids

Visnja Babacic,Jeena Varghese,Emerson Coy,Eunsoo Kang,Mikolaj Pochylski,Jacek Gapinski,George Fytas,Bartlomiej Graczykowski

doi:10.1016/j.jcis.2020.06.104

Abstract

Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a platform for various applications from assembling photonic and phononic crystals, acoustic metamaterials to coating applications. However, the fragility of these systems limits their application horizon. In this work the uniform mechanical reinforcement and tunability of 3D polystyrene colloidal crystals by means of cold soldering are reported. This structural strengthening is achieved by high pressure gas (N2 or Ar) plasticization at temperatures well below the glass transition. Brillouin light scattering is employed to monitor in-situ the mechanical vibrations of the crystal and thereby determine preferential pressure, temperature and time ranges for soldering, i.e. formation of physical bonding among the nanoparticles while maintaining the shape and translational order. This low-cost method is potentially useful for fabrication and tuning of durable devices including applications in photonics, phononics, acoustic metamaterials, optomechanics, surface coatings and nanolithography.

Highlights

Colloidal crystals (CCs) made of nanoparticles (NPs) are low cost volumetric materials that can exhibit unique properties enabled at nanoscale [1,2,3]
Since self-assembled particles are weakly bonded, fragility of polymer CCs remains a critical issue when it comes to their application [1]
In the discussion we will focus on the (1,1) mode as its spectral position enables direct in-situ sensing of the NP-NP physical bonding tuned by both hydrostatic pressure and temperature treatments

Summary

Introduction

Colloidal crystals (CCs) made of nanoparticles (NPs) are low cost volumetric materials that can exhibit unique properties enabled at nanoscale [1,2,3]. Owing to the broad variety of polymeric materials polymer-based CCs are of interest, especially for coatings and nanolithography [10,11] For such applications the cost effectiveness, superhydrophobicity and easy tunability of NPs size and shape is advantageous [3,12,13,14,15,16]. Thermal treatment just below the glass transition temperature (Tg), is probably one of the most straightforward approaches In this process, the NP-NP contact area increases due to the existence of a thin mobile layer on the NP surface, above the so-called softening temperature (Ts) [25,26,27]. We termed this process cold soldering as it is possible well below Tg of bulk PS

Samples

Nanoparticles vibrational modes

Brillouin light scattering

Results and discussion