The Study Of Thermo-Sensitive Polymer Based On Single-Molecule Force Spectroscopy

The study of transition mechanism of polymer chains between coil and globule states is very important and helpful for understanding the processes of the folding/unfolding of proteins, the packaging of DNA molecules and the complexation among polymer chains. It is, however, extremely difficult to study the conformational transition of a biological macromolecules (such as, protein and DNA), due to their complicated structures and interactions with environment. Poly(N-isopropylacrylamide) (PNIPAM), with an amide group and relatively simple structure, has a similar structure as that of proteins. PNIPAM can also undergo the coil-to-globule transition under the triggering of temperature and the composition of the solvent in water and the water/methanol mixed solvent, respectively. Therefore, PNIPAM is an ideal candidate for the study of the conformational investigations of the biological macromolecules.To investigate the mechanical property of the polymer chain during the coil-to-globule transition at the single-chain level, it will deepen our understanding and eventually master controls on the transition process. Moreover, it is also essential to explore the mechanical changes of materials triggered by external stimulus. Atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) is a powerful tool to study the mechanics of polymer chains at the single chain level. In this thesis, the principle of AFM-based SMFS is firstly introduced in detail, including the preparation of samples, the criteria for the single chain extension, the analysis of force curve, and some recent progresses in AFM-based SMFS have been reviewed. Then, we investigated the mechanical properties of PNIPAM at different temperatures (water) and different composition (water/methanol mixed solvent), respectively. Based on the mechanical properties of PNIPAM in water and the water/methanol mixed solvent, we designed the molecular motors that are triggered by the changes of the temperature and composition of the solvent. In order to reveal the effects of hydrogen bond donor on the mechanical property of polymer chain, we also investigated the mechanics of poly(N,N-diethylacrylamide) (PDEAM). On the basis of these systemic investigations, the main conclusions can be summarized as following:(1) Below its lower critical solution temperature in water (LCST,-33℃), the single- chain mechanics of PNIPAM is not affected by the temperature. While above its LCST in water, PNIPAM shows a strongly temperature-dependent single-chain mechanics. When the temperature is increased from 31℃ to 35℃, all the normalized force curves obtained at different temperatures can still be superposed in both the low and high force regimes, but show remarkable deviations in the middle force regime. As the temperature is increased, the middle parts of the normalized force curves drop gradually. This is mainly caused by the changes of the hydration and conformation of the polymer chain. As the temperature further increasing from 35℃, the middle parts of the force curves begin to rise gradually. The middle part of the force curve obtained at 40℃ is completely restored and virtually identical to that obtained at room temperature. We attributed the formation of intrachain hydrogen bonds (-CO…HN-) to the rise of the middle parts of the force curves. Comparing with the force curves of polystyrene (PS) obtained in water (a strongly poor solvent for PS), it indicates that water, even at 50℃, is not the strongly poor solvent for PNIPAM.(2) The single-chain mechanics of PNIPAM shows a reentrant variation upon the changes of Xmethanol from 0% to 100%. i) When Xmethanli≤ 10%, the changes of the composition of the mixed solvent have almost no influence on the mechanics of PNIPAM. ii) When Xmethanol= 10% -16%, the changes of the hydration and the conformation of PNIPAM chain lead to the variation of the mechanics of PNIPAM. iii) When Xmethanol= 16%-18%, the formation of intrachain hydrogen bonds (-CO…HN-) leads to the changes of the mechanics of PNIPAM. iv) When Xmethanol> 18%, the main reason for the variation of the mechanics of PNIPAM is the changes of the solvent quality of the mixed solvent for PNIPAM, which is better and better as Xmethanol increasing. Results indicate that methanol is a better solvent for PNIPAM than water, although the chain of PNIPAM shows a coiled state in the two solvents.(3) Comparing with the mechanics of PDEAM, we find that the side groups of polymers have no detectable effects on the inherent elasticity of the single polymer chain. However, PNIPAM shows a temperature-dependent single-chain mechanics in water when the temperature is increased across its LCST, while PDEAM does not. These differences observed in aqueous solution originate from the different structures of the two polymers: presence/absence of the hydrogen bond donors on their side groups.(4) Inspired by the different mechanics of PNIPAM presented in water and water/methanol mixed solvent, we designe the molecular motors that are triggered by thermo in water and changes of the composition of the water/methanol mixed solvent, respectively. We also analyze the difference of these two kinds of molecular motors in aspect of the work.