Using theory to predict reality

NCCR Bio-Inspired Materials researchers at the University of Geneva are working to fill a gap in theoretical knowledge surrounding the force-response of single macromolecules by developing models to predict and explain the results of practical experiments carried out using a variety of techniques.

Scientists have been using atomic force microscopy (AFM), a high-resolution imaging technique, to visualize nanometer-sized objects and even individual molecules since the 1980s. Near the turn of the millennium, researchers discovered a new area of application for atomic force microscopy: single molecule force spectroscopy (SMFS). As its name suggests, SMFS can be used to study the mechanical properties of individual molecules by measuring their response when exposed to mechanical force. While the first experiments were carried out with biological molecules, DNA for example, this type of spectroscopy has more recently also been used to analyze polymers.

NCCR Principal Investigator Professor Michal Borkovec and postdoctoral researcher Dr. Milad Radiom, together with the rest of their group at the University of Geneva, employed SMFS in order to gain a better understanding of the effect of a chemical environment on the mechanical properties of single polymer chains. The scientists also examined chain rupture and conformational transitions within the polymer chain in response to mechanical force, probing on a molecular level how much force is required in order for chemical bond cleavage or other changes to take place. They used the SMFS technique to investigate the force-extension relationships of polystyrene – one of the most widely used plastics – in different organic solvents to determine at which point the molecule chains rupture.

While much practical research has been done on the force responsiveness of single molecules, few theoretical models exist in this field. The NCCR researchers investigated how theoretical work can help to put the results of these force experiments into the contexts of other, more established theories and parameters. The scientists were indeed able to develop a model showing to what extent the chemical environment influences the force-extension behavior of a polymer molecule. The group also interrogated the validity of such formulae. “One really important outcome we have found is that, in fact, these models are actually rather accurate,” Borkovec reveals.

These theoretical studies were inspired by practical experiments, with the goal of helping to understand and interpret their results. A thorough comprehension of the force-extension relationship of single polymer chains is crucial for the field of soft matter, for instance for work on rubber elasticity, the swelling of polymer brushes, muscle action, or the mechanoresponsive polymers made by other NCCR researchers in which so-called mechanophores are included to serve as weak links. According to Borkovec, however, “the theoretical studies create more questions than answers,” opening up new opportunities for further research.

Reference: Radiom, M.; Maroni, P.; Borkovec, M. Influence of solvent quality on the force response of individual poly(styrene) polymer chains, ACS Macro Letters, 2017, 6, 1052.