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Photo-controlled two-dimensional soft materials from microgel particles at liquid interfaces

Periodic Reporting for period 1 - PhotoSoftMat (Photo-controlled two-dimensional soft materials from microgel particles at liquid interfaces)

Reporting period: 2020-10-01 to 2022-09-30

This MSCA IF broadly covered various topics of relevance for materials science, physical chemistry and soft matter science. In particular, it focussed on studying some fundamental aspects related to the adsorption and interactions between colloidal particles at fluid, air-water or oil-water, interfaces; which are directly related to a variety of materials and processes. Particle adsorption at a fluid interface is responsible for the stabilization of foams and emulsions, or the encapsulation, structuring and manipulation of liquids, and therefore finds numerous applications in oil recovery, food and cosmetic industry, etc. Particle assembly on planar fluid surfaces allows the production of two-dimensional (2D) materials that find uses in catalysis, optics and surface science, to name a few relevant examples.

Currently, alongside with hard, mechanically rigid colloids, soft micro and nanoparticles (microgels) are gaining much attention because of their responses to multiple stimuli and environmental conditions (temperature, pH, concentration, pressure, solvent quality, etc.). This allows tuning their softness, shape and interparticle interactions at will, ultimately producing responsive materials endowed with additional functionalities, including complex phase behaviours and rheological properties stemming from their extent of interpenetration, deformation and compression. Sensitivity to environmental conditions and stimuli is translated to 2D when microgels are adsorbed at fluid interfaces, where they can be used as powerful foams and emulsions stabilizers, or to produce 2D materials of interest for coatings, optics and colloidal lithography, with superior advantages over the use or hard particles.

Overall, this project addressed three fundamental questions in the research field: 1) how does the particle’s internal structure, as designed at the synthesis level, influence its structural and mechanical properties upon interfacial confinement?; 2) which is the exact 3D conformation of a soft object adsorbed at a fluid interface?; and 3) what is the effect of various system parameters (internal polymer density profile, solution temperature and choice of the organic top phase) on the final conformation of an adsorbed microgel? These are at present driving questions in the field, and have been addressed with fundamental contributions stemming from this research project.
During the project, the applicant developed a variety of synthesis protocols in order to precisely control the internal polymer density profile of poly(N-isopropylacrylamide) (pNIPAM) microgels. ‘Standard’ pNIPAM microgels are commonly obtained by free-radical precipitation polymerization in the presence of a crosslinker. Due to the faster reaction rate of most crosslinkers with respect to NIPAM, the microgels develop a morphology characterized by a more cross-linked (denser) core and a less cross-linked corona. Ad-hoc synthesis protocols allowed to obtain a library of soft particles, including “inverse” (more crosslinks in the corona than in the core of the microgels) and hollow microgels. Additionally, reactions in the presence of hard particles (e.g. silica) as seeds for polymerization and growth of a polymeric shell, allowed to produce hard-core soft-shell particles with controlled shell properties (thickness, crosslinking density, etc.)

Successively, the project focused on investigating how the internal polymer density profile influences the single-particle conformation of microgels adsorbed at various oil-water interfaces, and the resulting material properties in 2D assemblies subjected to interfacial compression. An overview of the main achievements is reported in the following section.
The work carried out during the MSCA IF resulted in 3 main scientific achievements.

1) Effect of Internal Architecture on the Assembly of Soft Particles at Fluid Interfaces
In this work, we established clear relations between the microgels internal polymer density profile, and their structural and mechanical properties upon interfacial confinement. This was made possible by the development of protocols to produce core-shell microgels whose soft core can be degraded in a controlled fashion. This strategy allowed us to obtain a series of particles ranging from analogues of standard microgels to completely hollow ones after total core removal. Combined experimental and numerical results from collaborators (group of Dr. Emanuela Zaccarelli, CNR, Roma, Italy) showed that our hollow particles have a thin and deformable shell, leading to a temperature-responsive collapse of the internal cavity and a complete flattening after adsorption at a fluid interface. Mechanical characterization showed that a critical degree of core removal is required to obtain soft disk-like particles at an oil-water interface, which present a distinct response to compression. In particular, at high compression the absence of a core enables the particles to deform orthogonally to the interface and to be continuously compressed without altering the monolayer structure. Overall, these findings showed how fine, single-particle architectural control during synthesis can be engineered to determine the interfacial behavior of microgels, enabling one to link particle conformation with the resulting material properties.

2) Influence of the interfacial tension on the microstructural and mechanical properties of microgels at fluid interfaces
In this work, we investigated how the interfacial tension (γ) of the oil-water interface affects the properties of microgel assemblies by comparing two organic fluid phases, hexane and methyl tert-butyl ether, which have markedly different γ values with water and thus tune the deformation of adsorbed microgels. We rationalized how γ controls the single-particle morphology, which consequently modulates the structural and mechanical response of the monolayers at varying interfacial compression. Specifically, when γ is low, the microgels are less deformed within the interface plane and their polymer networks can rearrange more easily upon lateral compression, leading to softer monolayers. Selecting interfaces with different surface energy offers an additional control to customize the 2D assembly of soft particles, from the fine-tuning of particle size and interparticle spacing to the tailoring of mechanical properties.

3) In-situ imaging of the three-dimensional shape of soft responsive particles at fluid interfaces by atomic force microscopy
In this work, we demonstrated that atomic force microscopy (AFM) can be used for the in-situ reconstruction of the 3D conformation of model pNIPAM microgels adsorbed at an oil-water interface. We imaged the particle topography from both sides of the interface to characterize its in-plane deformation and to visualize the occurrence of asymmetric swelling in the two fluids. Additionally, the technique enabled us to investigate different fluid phases and particle architectures, as well as studying the effect of temperature variations on particle conformation in situ. We envisage that these results open up an exciting range of possibilities to provide microscopic insights on the single-particle behavior of soft objects at fluid interfaces and on the resulting macroscopic material properties.
In situ atomic force microscopy unravels the 3D shape of soft particles at oil-water interfaces
Tuning the internal architecture of soft particles to control their properties in 2D assemblies