Single Molecule Study of Force-Induced Rotation of Carbon-Carbon Double Bonds in Polymers.
ACS Nano
; 11(1): 194-203, 2017 01 24.
Article
in En
| MEDLINE
| ID: mdl-28114764
ABSTRACT
Carbon-carbon double bonds (CâC) are ubiquitous in natural and synthetic polymers. In bulk studies, due to limited ways to control applied force, they are thought to be mechanically inert and not to contribute to the extensibility of polymers. Here, we report a single molecule force spectroscopy study on a polymer containing CâC bonds using atomic force microscope. Surprisingly, we found that it is possible to directly observe the cis-to-trans isomerization of CâC bonds at the time scale of â¼1 ms at room temperature by applying a tensile force â¼1.7 nN. The reaction proceeds through a diradical intermediate state, as confirmed by both a free radical quenching experiment and quantum chemical modeling. The force-free activation length to convert the cis CâC bonds to the transition state is â¼0.5 Å, indicating that the reaction rate is accelerated by â¼109 times at the transition force. On the basis of the density functional theory optimized structure, we propose that because the pulling direction is not parallel to CâC double bonds in the polymer, stretching the polymer not only provides tension to lower the transition barrier but also provides torsion to facilitate the rotation of cis CâC bonds. This explains the apparently low transition force for such thermally "forbidden" reactions and offers an additional explanation of the "lever-arm effect" of polymer backbones on the activation force for many mechanophores. This work demonstrates the importance of precisely controlling the force direction at the nanoscale to the force-activated reactions and may have many implications on the design of stress-responsive materials.
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Language:
En
Journal:
ACS Nano
Year:
2017
Document type:
Article
Publication country:
EEUU
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ESTADOS UNIDOS
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ESTADOS UNIDOS DA AMERICA
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EUA
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UNITED STATES
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UNITED STATES OF AMERICA
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US
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USA