Single-molecule Force Spectroscopy Of Several Thermally Sensitive Polymers

Some polymers are responsive to the external stimuli, such as pH, light, temperature and electromagnetic field. LCST (Lower Critical Solution Temperature) is a very important property of thermally sensitive polymers. When the temperature is higher than LCST, thermally sensitive polymer dissolved in water will undergo a phase transition. In the free state, the polymer molecules will form hydrogen bonds with water molecules. Because of the limitations in chain conformation, there would be much more bound water molecules in the free coil state than in the completely stretched state. That is to say, when stretched by an AFM, the polymer chain would lose bound water gradually during the stretching process. Thus, these structured water molecules around the polymer chain are forced to undergo rearrangement upon stretching, which would cost energy. We obtain several conclusions through single-molecule force measurements:1. We find that there is a water rearrangement upon chain stretching. We have calculated the energy for this water rearrangement.2. The stronger hydrophobic side groups will result in the lower LCST.3. There is no clear correlation between LCST and the energy needed for the water rearrangement.4. The inherent stretching elasticity of a single polymer chain with carbon-carbon backbone in nonpolar solvent shows that the side groups have no evident influence on the inherent elasticity.