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When Soft Matter Goes Really Soft – A New Paradigm for Star Polymer Self-Assembly

Periodic Reporting for period 1 - Star Polymers (When Soft Matter Goes Really Soft – A New Paradigm for Star Polymer Self-Assembly)

Reporting period: 2020-01-13 to 2022-01-12

Using colloidal particles as elementary building blocks for the bottom-up fabrication of superstructures is an attractive route toward functional materials. The characteristic colloidal dimension (10 – 1000 nm) render them uniquely suited for this purpose. The particles are sufficiently small for autonomous recruitment and assembly via Brownian or thermal motion, while the presence of the colloidal length scales endows these materials with unique photonic, mechanical, and porous properties. As for atomic and molecular materials, the arrangement of the particles in the assembled state is a key factor determining the material properties. In turn, these particle arrangements are heavily influence by how the individual particles interact with each other and how the particles are being processes in time. During this Marie-Curie action we aimed to increase the fundamental understanding of colloidal assembly by developing a new model system in which the interactions between the particles could be regulated in time. The first steps towards such new time-dependent colloidal model systems were explored during this project and an experimental proof of concept was provided. By regulating how the interactions change in time, different assembly pathway can be generated. Studying how the exact pathway influences the final assembled state might shed light on how kinetically trapped states can be prevented or even used to fabricate materials with different properties based on one single set of building blocks.
The overarching objective of this Marie-Curie action was the development of new colloidal model systems for self-assembly purposes. The central and well-known concept exploited during this project is the so-called depletion force. This attractive force between colloidal particles is generated in the presence of specifically designed polymers and can be used to generate higher-order superstructures. The novel aspect of the performed research is that the polymers are not simply added to the colloidal dispersions, but rather synthetized in situ from their corresponding monomers. By growing the polymers, the attractive depletion force become time-dependent, which was hypothesized to lead to new out-of-equilibrium colloidal structures. Since the assembly of the particles is in principle governed by the polymerization that runs in the background, this system will be referred to as Polymerization-Induced Colloidal Assembly or PICA in short.

The vast majority of the work performed during this action revolved around the experimental realization of colloidal model systems for self-assembly purposes. This was achieved by combining polymer and colloid chemistry in an innovative way by introducing the PICA concept. To verify the feasibility of this new paradigm for colloidal assembly, the synthetic/experimental endeavors can be divided in three distinct elements: (i) development of experimental set-up, (ii) selection and optimization of suitable polymerization procedures and (iii) selection and synthesis of colloidal particles. With these three components in hand, the PICA concept was experimentally verified.

Part of the work performed were disseminated in scientific publications (6) and presentations (5). Most notable examples are listed below:

- B. G. P. van Ravensteijn*, I. K. Voets, Colloids Out of Equilibrium: An Uphill Battle, Phys. Today 2021

- B. G. P. van Ravensteijn*, I. K. Voets*, Nanoparticles Exploring the Macroscopic World. Matter 2021, 2021, 4, 2661.

- B. G. P. van Ravensteijn, P. A. Hage, I. K. Voets, Framed by Depletion. Nature Mater. 2020, 19, 1261.

- B. G. P. van Ravensteijn*, J. R. Magana, I. K. Voets, Manipulating Matter with a Snap of your Fingers: A Touch of Thanos in Colloid Science. Superhero Science and Technology, 2020, 1, 19.

- B. G. P. van Ravensteijn*, I. K. Voets, W. K. Kegel, R. Eelkema, Out-of-equilibrium Colloidal Assembly Driven by Chemical Reaction Networks. Langmuir 2020, 36, 10639.
The progress beyond the state of the art lies in the fact that this new colloidal model system allows for detailed control over the inter-particle interaction in time. While most colloidal systems do not consider temporal changes in the inter-particles interactions, this time-dependency would offer another degree of freedom to influence the properties of the self-assembled state. Being able to obtain multiple different materials by simply changing the assembly conditions would be a great step forward in exploiting colloidal materials. Furthermore, the knowledge obtained from these model systems might have implications for real-life products, e.g. polymer coatings with filler particles. In these types of coatings, the distribution of the filler particles heavily influences coating properties such as mechanical resilience and opacity. The model system developed here can help predicting how the filler particles will assemble during curing and drying and potentially provide insights in defining optimum processing conditions.
Schematic representation of the developed polymerization induced colloidal assembly (PICA) process