Star polymers with a temperature-dependent valence

Hard patchy particles are colloids decorated with attractive spots (patches) on their surface. Recently, many pivotal theoretical and numerical studies have highlighted the richness of phenomena that these systems exhibit, both from the dynamic and the thermodynamic points of view. Examples of interesting applications relevant to this projects are empty liquids (liquid-like states which do not separate at low temperatures and densities), reentrant gels (systems which are dynamically arrested only for a limited range of temperature) and pinched gas-liquid phase diagrams (materials exhibiting a gas-liquid instability region which shrinks at lower densities as the system is cooled down). Unfortunately, fabricating monodisperse, anisotropic nano-- or micro--sized patchy particles in bulk quantities still eludes modern experimental techniques. In this project we build on the work carried out by the co-applicant and its group and focus on a possible realisation of patchy particles: telechelic star polymers. These are supramolecular objects made of diblock copolymer chains (i.e. chains composed by a hydrophilic part and a hydrophobic part) grafted on a central anchoring point. Such particles, due to the dual nature of their arms, exhibit a hierarchical self- assembling scenario. Indeed, on the single particle level and under the right conditions, each particle self-assembles in a soft patchy particle. The shape and topology of the resulting object depend both on the star polymer properties (number of arms and fraction of hydrophobic monomers in a chain) and on the external conditions (temperature, pressure, solvent properties). On a larger scale, particles can then self-assemble into meso- and macroscopic structures. Since the properties of the final structures depend on the shape of and mutual interactions between the constituent objects, it is vital to investigate the single particle conformation diagram as a function of the number of arms, fraction of hydrophobic monomers and temperature. Indeed, the main aim of this project is to fully characterise and control the self-assembly of telechelic star polymers, their mutual interaction strength and shape and how the latter changes upon varying the external conditions in order to Investigate whether telechelic star polymers conform to theoretical predictions like the shrinking of the gas-liquid phase coexisting region upon decreasing the maximum number of bound neighbours and the existence of pinched phase diagrams. Realise in silico technologically relevant materials like reentrant gels and empty liquids and the establishment of guidelines for their realisation in vitro by experimental groups.

Summary for public relations work The fabrication of versatile building blocks that reliably self-assemble into desired ordered and disordered phases is amongst the hottest topics in contemporary materials science. To this end, microscopic units of varying complexity, aimed at assembling the target phases, have been thought, designed, investigated and built. Such a path usually requires laborious fabrication techniques, especially when specific functionalisation of the building blocks is required. Over the course of the project, we have investigated several realistic systems that, through a delicate balance between energy and entropy, form well-defined micro- and macroscopic aggregates that might be exploited in applications. Some of the results of the project, which have been recently highlighted on the cover of the Nanoscale journal, demonstrate that a particular class of polymeric systems, telechelic star polymers, spontaneously form (self-assemble into) soft objects featuring attractive spots (patches) on the surface. We have performed numerical simulations that show that the properties of these soft patchy particles can be finely tuned by controlling the physical and chemical parameters of the solution. We demonstrate that diverse combinations of the parameters can generate particles with the same number of patches but different mechanical properties. This mechanism could provide a neat way of further fine-tuning the elastic properties of the supramolecular network without changing its topology. We have also looked at the effect of using soft particles (called star polymers) to stabilise crystals made of colloids known to suffer from the so-called polymorphism, where very closely related structures exhibit a similar thermodynamic stability. However, sometimes only one of the multiple quasi-stable structures are interesting for applications. Our work introduced the "structure-directed agent" paradigm to the colloidal world, permitting directed self-assembly by adding co-solutes to the solution, without requiring any modifications to the colloids themselves. Such a paradigm represents a new approach that is currently underappreciated in the field of colloidal self-assembly and could be used to synthesise photonic crystals, which are materials with technologically interesting optical properties. Finally, we have investigated the phase behaviour of systems made entirely of DNA. In fact, the advances in DNA nanotechnology and in the synthesis of DNA-based materials call for the development of numerical and theoretical methods for the evaluation of their macroscopic properties. In our contribution, we developed a theoretical approach to predict the thermodynamic behaviour of particles dubbed DNA nanostars. Our results shed light on the dependence of the phase behavior on temperature and salt concentration, providing guidance for future experimental work.