The response of switchable polymer blends and coatings to temperature variation is important for the development of high-performance materials. Although this has been well studied for bulk materials, a proper understanding at the molecular level, in particular for high stretching forces, is still lacking. Here we investigate the molecular details of the temperature-dependent elastic response of two widely used water-soluble polymers, namely, polyethylene glycol (PEG) and poly(N-isopropylacrylamide) (PNiPAM) with a combined approach using atomic force microscopy (AFM) based single molecule force spectroscopy (SMFS) experiments and molecular dynamics (MD) simulations. SMFS became possible by the covalent attachment of long and defined single polymers featuring a functional end group. Most interestingly, varying the temperature produces contrasting effects for PEG and PNiPAM. Surprising as these results might occur at first sight, they can be understood with the help of MD simulations in explicit water. We find that hydration is widely underestimated for the mechanics of macromolecules and that a polymer chain has competing energetic and entropic elastic components. We propose to use the temperature dependence to quantify the energetic behavior for high stretching forces. This fundamental understanding of temperature-dependent single polymer stretching response might lead to innovations like fast switchable polymer blends and coatings with polymer chains that act antagonistically.
Atomic force microscopy (AFM)-based single molecule force spectroscopy is an ideal tool for investigating the interactions between a single polymer and surfaces. For a true single molecule experiment, covalent attachment of the probe molecule is essential because only then can hundreds of force-extension traces with one and the same single molecule be obtained. Many traces are in turn necessary to prove that a single molecule alone is probed. Additionally, passivation is crucial for preventing unwanted interactions between the single probe molecule and the AFM cantilever tip as well as between the AFM cantilever tip and the underlying surface. The functionalization protocol presented here is reliable and can easily be applied to a variety of polymers. Characteristic single molecule events (i.e., stretches and plateaus) are detected in the forceextension traces. From these events, physical parameters such as stretching force, desorption force and desorption length can be obtained. This is particularly important for the precise investigation of stimuli-responsive systems at the single molecule level. As exemplary systems poly(ethylene glycol) (PEG), poly(N-isopropylacrylamide) (PNiPAM) and polystyrene (PS) are stretched and desorbed from SiO x (for PEG and PNiPAM) and from hydrophobic self-assembled monolayer surfaces (for PS) in aqueous environment.
Atomic force microscopy (AFM)-based single molecule force spectroscopy is an ideal tool for investigating the interactions between a single polymer and surfaces. For a true single molecule experiment, covalent attachment of the probe molecule is essential because only then can hundreds of force-extension traces with one and the same single molecule be obtained. Many traces are in turn necessary to prove that a single molecule alone is probed. Additionally, passivation is crucial for preventing unwanted interactions between the single probe molecule and the AFM cantilever tip as well as between the AFM cantilever tip and the underlying surface. The functionalization protocol presented here is reliable and can easily be applied to a variety of polymers. Characteristic single molecule events (i.e., stretches and plateaus) are detected in the forceextension traces. From these events, physical parameters such as stretching force, desorption force and desorption length can be obtained. This is particularly important for the precise investigation of stimuli-responsive systems at the single molecule level. As exemplary systems poly(ethylene glycol) (PEG), poly(N-isopropylacrylamide) (PNiPAM) and polystyrene (PS) are stretched and desorbed from SiO x (for PEG and PNiPAM) and from hydrophobic self-assembled monolayer surfaces (for PS) in aqueous environment.
In this study, a novel latent ruthenium indenylidene catalyst, bearing morpholine substituted bidentate (N, O) Schiff base is used in nonaqueous emulsion ring‐opening metathesis polymerization (ROMP) reactions for the first time. This novel catalyst has unique features such as latency and high silica gel affinity. The performance of the catalyst is tested on ring‐closing metathesis (RCM), ROMP reactions, and aqueous emulsion ROMP reactions. In addition, nonaqueous (using immiscible binary organic solvents: n‐hexane and dimethylformamide) emulsion ROMP reactions are carried out for the first time using latent ruthenium catalysts. The latency of the catalyst allows us to control the molecular weight of emulsion ROMP polymers by varying HCl/Ru ratio, keeping the nanoparticle sizes stable between 26 and 75 nm in aqueous emulsion and 51 and 208 nm in nonaqueous emulsion ROMP reactions. In addition, high silica gel affinity of the catalyst allows us to synthesize pure RCM products with ruthenium contamination less than 1 ppm.
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