Synthese und Mechanische Eigenschaften von Linearen und Langkettenverzweigten Homopolymertopologien mit Modularen Verknüpfungspunkten
Zusammenfassung der Projektergebnisse
Our molecular investigation into the stability of o-quinodimethane based photo-adducts demonstrates that: (i) Cleavage in the molecular vicinity of the mid chain positioned photoadduct is favoured over cleavage at the chain end; (ii) the thermal stability of the polystyrenes is highly dependent on the chain length on either side of the ligation point and, (iii) critically, we find that the photochemical ligation points themselves are stable when exposed up to 180 °C for up to 8 h, with cleavage occurring at the maleimide rather than the Diels-Alder adduct. Harking back to the importance of having stable ligation points resulting from photochemical reactions, we submit that DA adducts derived from o-methylbenzaldehydes show superior stabilities, yet the -ene component with which they ligate needs to be judiciously selected. Preliminary rheological experiments reveal that in oxidative environments the C-C bond connecting the side chains to the backbone is a distinctive weak link. Therefore, further experiments were performed in vacuum pressing atmosphere to remove residual oxygen trapped in the raw sample material. The importance of sample preparation and measurement environment with protective atmosphere also apply to triazole and tetrazole linkages with heteroatomic nitrogen functional groups. Additionally, we provide a correlation of the molecular architecture of polystyrene (PS) comb homopolymers with their stability to temperature and shear forces on the triazole ligation points joining the branches to the lateral polymer chain. The topology of the comb polymer (both loosely and densely grafted), yet also the number of branches per backbone and the length of the side chains are identified as key parameters for the stability of the triazole ligation point. In a previous study from our group, we found that a linear system containing a triazole linkage (in the absence of Br as an end-chain) is rather stable under thermal treatment and shearing. In the current study, our results highlight the stability of loosely grafted combs up to a limited number of grafted branches. The densely grafted homopolymer PS combs, even though exposed to high mechanical stress, show topological stability under thermal conditions used in rheometry. Once the pervaded volume of the branches overlaps each other inter- and intramolecularly, the system is no longer stable, and the presence of free branches is noticed after thermal and mechanical treatment. As a result, a densely grafted comb is more likely to degrade than a loosely grafted one and the longer the side chains are, the more readily cleavage occurs. In addition, our study demonstrates that the process of debonding is specific to the type of stress applied to the material. Indeed, when the comb polymers underwent thermal treatment, cleavage occurred under inert atmosphere at 150 °C (or 180 °C), including the formation of intermolecular bonding leading to gel formation. Specifically, the triazole rings cleave by (i) opening of the triazole ring with a subsequent cleavage of the C-N bond in the ring and (ii) cleavage of the bond between the carbon atom in para position of the styrene ring and the nitrogen atom of the triazole ring. In contrast, when shear forces are applied at the same temperatures under inert atmosphere, we observe the cleavage of the bond between the carbon atom in para-position of the styrene ring and the nitrogen atom of the triazole ring. However, the elimination of nitrogen results in the formation of a dialkene. We thus submit that the grafting density is a major parameter for controlling the stability of comb polymer structures constructed by triazole linkages – our findings have thus key implications for such polymers to extrusion processing or molding.
Projektbezogene Publikationen (Auswahl)
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“Stability of Diels–Alder Photoadducts in Macromolecules”, Polym. Chem. 2018, 9, 3850- 3854
Petit, C.; Bangert, L. D.; Abbasi, M.; Wilhelm, M.; Goldmann, A. S.; Barner-Kowollik, C.
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Stability of Ligation Points in Macromolecules, Frontiers in Polymer Science 2019, Budapest, HU (best poster award for Dr. Petit)
C. Petit, M. Abbasi, T. Fischer, L.D. Bangert, M. Wilhelm, A.S. Goldmann, C. Barner-Kowollik
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“Comb and Bottlebrush Polymers with Superior Rheological and Mechanical Properties”, Adv. Mater. 2019, 31, 1806484
Abbasi, M.; Faust L.; Wilhelm, M.
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“Comb Polymers with Triazole Linkages under Thermal and Mechanical Stress”, Macromolecules 2019, 52, 420-431
Petit, C.; Abbasi, M.; Fischer, T.; Wilhelm, M.; Goldmann, A. S.; Barner-Kowollik, C.
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“Molecular origin of the foam structure in model linear and comb polystyrenes: I. Cell density”, Polymer 2020, 193, 122351
Abbasi, M.; Faust L.; Wilhelm, M.
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“Molecular origin of the foam structure in model linear and comb polystyrenes: II. Volume expansion ratio”, Polymer 2020, 193, 122354
Abbasi, M.; Faust L.; Wilhelm, M.
