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T. Cavalli, M. M. Unterlass:
"Synthesis and Characterization of Crystalline Shape-Anisometric Polymer Particles as Model Systems for Self-Assembled Materials";
Poster: 11th European NESY Winterschool & Symposium on Neutron and Synchrotron Radiation, Altaussee, Austria; 2019-03-04 - 2019-03-08.

This project deals with the synthesis of shape-anisotropic polymer particles as model systems for self-assembly. To achieve this goal, we profit from the fact that crystalline compounds are of non-spherical shapes. The preparation of arrays of anisotropic crystalline particles in the colloidal domain (1 nm to 1 μm) proceeds by first synthesizing so-called monomer salt (MS) crystals. MS crystals present a set of peculiar characteristics, such as (i) higher stability and storability in comparison to classic polymer precursors; (ii) proximity of reactive functional groups, and (iii) ideal stoichiometry. MS crystalline particles are first obtained by precipitation upon mixing of two aqueous co-monomer solutions, i.e. an acid (pyromellitic acid, PMA) with a base (p-phenylene diamine, PDA). This step is followed by solid-state polymerization (SSP), which bears the crucial characteristic of yielding polymer particles that retain the shape of the initial MS crystals. SSP is a solvent-free technique recently introduced by our group that involves temperature treatment of the MS particles below their melting point.[1] Therefore, by aiming at the control of the habit of MS crystals, one is able generate a plethora of shape-anisotropic particles. To modify the shape of the crystalline MS particles, we employ additives as crystal-habit modifiers during the crystal growth process.[2] The use of additives is a well-known technique in the crystal growth field. By preferentially attaching additives to different facets of the growing crystal, i.e. to facets with different surface-free energies Gi, additives allow for the modification of crystal habit by inhibiting the growth in specific directions.


For the goal of generating self-assembled structures, control over the crystalline structure and size range of the polymeric particles is a desired feature. Crystalline structure determines the type of interactions between exposed facets of different particles. Preliminary results show that MS particles already self-assemble into ordered phases upon nothing but drying - driven, as we believe through simply their anisotropic shapes thriving to pack closely. The size range of the particles also plays an important role. To date, MS particles have been limited to sizes in the ranges of 200-300 μm, a scale far out the colloidal range required for self-assembly.
With this contribution, we present a study on the effect of additives on the crystal growth of [H₂PMA²⁺PDA^2-]. We investigated the effect of different parameters on the shape and size of the final particles, such as co-monomers and additive concentrations. All particles have been characterized by optical microscopy, scanning electron microscopy, wide angle X-ray diffraction and infrared spectroscopy. Our results show that crystal growth of the prototypical [H₂PMA²⁺PDA^2-] system is indeed affected by specific additives. Overall, we aim at the generation of large sets of anisotropic particles with same chemical composition, that will further serve as building blocks towards the generation of novel materials by self-assembly.