The two-dimensional polymerization mechanism of an anthracene-based monomer: A single-crystal-to-single-crystal transformation investigated via total X-ray scattering.
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Date
2019Type
- Doctoral Thesis
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Abstract
Two-dimensional polymerization is a recent advance in polymer chemistry. Using the anthracene-based trifunctional monomer studied in this thesis as an example, two-dimensional polymerization is achieved by pre-organizing the monomer molecules through crystallization in a layered and reactive packaging. The reactive packing enables pairs of anthracene blades from adjacent molecules to face each other. Exposition of the crystal to intense blue light triggers and drives a thermally reversible photochemical reaction, thereby achieving true long-range ordered, two-dimensional polymerization through the gradual formation of anthracene dimers between individual monomer molecules. Understanding the propagation mechanisms of two-dimensional polymerization and depolymerization of these single-crystal-to-single-crystal transformations is of crucial importance for fundamental and applied re- search. Both processes can be suspended at any point by removing the crystal from the triggering source, resulting in X-ray hard partially polymerized crystal structures. Monomer, polymer and intermediate states were investigated using standard X-ray diffraction methods and by analyzing the diffuse X-ray scattering. The first utilizes Bragg scattering to obtain the aver- age crystal structures, while the second uses the three-dimensional difference pair-distribution function method to study characteristics of the real crystal structures. These experiments showed that polymerization and depolymerization propagate predominantly in a random fashion, with a preference of having anthracene dimers surrounded by pairs of anthracene blades.
The properties of the average structures of the monomer, polymer, and all intermediate states were analyzed as a function of the conversion ratio. All investigated crystal structures are described in space group R3. The axes of the unit-cell show a particularly interesting behavior. Upon complete polymerization, the a- and c-axes have increased by about 0.15Å and 0.30Å compared to the monomer. During the first heat treatment step to induce and advance the depolymerization reaction, the a-axis expands by about 0.05 Å, while the c-axis shrinks substantially by about 1.05 Å. When depolymerization continues, the a-axis decreases, while the c-axis increases, and when complete depolymerization is achieved, they are of almost the same size as in the original monomer crystal.
In order to understand the propagation mechanism, it is necessary to gain knowledge about the changes in the real structure, as this provides, among other things, the spatial distribution of the dimers in the crystal during polymerization. Typically, standard X-ray diffraction experiments are not able to provide such insights. However, results from the reaction kinetics, derived from the detailed analysis of the average structure of multiple intermediate states were utilized to obtain a basic understanding of the polymerization and depolymerization propagation mechanisms. These results are interpreted using the Schmidt rules for photochemistry. They state that the reaction probability between photoreactive units is inversely related to the distance between these units. During polymerization, the distance between the un- reacted anthracene blades increases by approximately 0.2 Å, which leads to the aforementioned random propagation where anthracene dimers prefer to be surrounded by unreacted anthracene pairs. In analogy to polymerization, depolymerization propagates in a similar fashion by anthracene dimers breaking up and thereby isolating dimers from other dimers. The isolation is achieved by local stress reduction through the breaking of bonds, which increases the stability of the dimers in the immediate environment. The result of polymerization and depolymerization were supported by the Avrami formalism and could be modeled with Monte Carlo simulations.
The evaluation of the diffuse scattering with the three-dimensional difference pair-distribution function method provided additional insights into the real structure. Its structural properties can be expressed by correlated atomic dis- placements and correlated atomic substitutions. The first indicate a form of interaction between molecules, while the second describe the distribution of dimers in the crystal at a given conversion ratio. The results from analyzing the diffuse scattering confirmed the findings of the average structure analysis but provided a more detailed view on the distribution of dimers. This showed that a dimer influences the reaction probabilities of anthracene pairs up to its third neighborhood, about 45 Å away, within the same layer. In addition, dimerization within a layer also influences the reaction probabilities of anthracene pairs in adjacent layers, making two-dimensional polymerization a three-dimensional process.
The combined results from the average and real structure analysis also showed the crucial importance of mechanisms that reduce stress during polymer growth and thus promote two-dimensional polymerization. Of particular importance is the mobility and the flexibility of the template and solvent molecules incorporated into the structure. Stress relief is achieved by movements of the template along the c-axis and by realignment of some solvent molecules sandwiched between the layers. The correlations of the displacive disorder between the template and the monomer/polymer moieties increase with increasing conversion ratio, indicating that the template loses its ability to dissipate local stress. In addition, the found polymerization and depolymerization mechanisms do not exhibit pronounced reaction fronts. This is fortunate, because such a front would be a domain boundary that could either drastically reduce the achievable sheet size, deform the crystal lattice in such a way that (de-)polymerization propagation is prevented, or destroy the crystal by stress build-up at the reaction front.
This work presents the first detailed structural investigation of the recently discovered two-dimensional polymerization process in single-crystals. This work also contributes to the further development of the three-dimensional difference pair-distribution function method, which was able to model the complex phase transformation from monomer to polymer. Show more
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https://doi.org/10.3929/ethz-b-000371011Publication status
publishedExternal links
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Contributors
Examiner: Kröger, Martin
Examiner: Weber, Thomas
Examiner: Schlüter, A. Dieter
Examiner: Anastasaki, Athina
Examiner: Wörle, Michael
Publisher
ETH ZurichSubject
2D polymer; Solid-state reactions; Photochemistry; 3D delta PDF; Diffuse scattering; Total scattering methods; Single-crystal-to-single-crystal transformationsOrganisational unit
02646 - Institut für Polymere / Institute of Polymers
Funding
157157 - Local and mesoscale studies of two-dimensional polymerization and depolymerization mechanisms in the single crystal (SNF)
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