Thermo-responsive Behavior Of PNIPAm And Its Applications Studied By Molecular Theory

Poly (N-isopropylacrylamide) (PNIPAm) is one of the most famous temperature-sensitive polymers due to its sharp phase transition at lower critical solution tempera-ture (LCST) in aqueous solution. Namely, at 32℃, its conformation undergoes great change from a hydrophilic coil state to a collapsed hydrophobic globule state, which is attributed to a shift in the distribution of hydrophobic and hydrogen-bond interactions. The applications for such a thermo-responsive polymer are abundant. For example, its homopolymer is especially popular in the application of drug release, peptide separa-tion, and surface modification. Moreover, PNIPAm based copolymers, gels, microgels and surface layers are widely used in drug delivery, nanotechnology, and microflu-idics. In this thesis, we use molecular theory to study the thermo-responsive behav-iors of PNIPAm and its temperature-controlled applications in self-assembly and DNA biosensors.In chapter 1, we briefly introduce the sharp phase transition of PNIPAm in aque-ous solution and the effects of ions, alcohols, ionic liquids on its LCST. Also, the applications of PNIPAm are described and illustrated. We mainly focus on some cur-rent PNIPAm works, such as the latest PNIPAm based copolymers, gels, microgels and surface layers. The possible influential factors and the issues that are still elusive are also given.In chapter 2, the molecular theory adopted in this thesis is introduced. The theory was previously used to study the thermodynamics and structural properties of tethered polymers with the consideration of the conformation, size, and shape of each molecule, and was shown to be in quantitative agreement with simulations and experimental ob-servations. Recently, the theory has been extended to explicitly include hydrogen bonds between polymers and solvent, whereby the polymer solubility depending on tempera-ture was well established.In chapter 3, the effects of temperature, degree of polymerization, and surface coverage on the equilibrium morphology of tethered PNIPAm chains immersed in wa-ter are studied by molecular theory. We employ an empirical parameter that comes from the experiment to describe the interaction between PNIAPm and water. Our re-sults show the volume fraction profiles of PNIAPm are approximately parabolic and extend into water at low temperatures,. In contrast, at high temperatures above LCST, the polymer profiles are collapsed near the surface. Meanwhile, the average height of the polymer undergo a sharp change at LCST that depends on surface coverage and chain length. In addition, the relative magnitude of polymer average height at 20℃ and 40℃ is a nonmonotonic function of surface coverage, with a maximum that shifts to lower surface coverage as the chain length increases in qualitative agreement with ex-periment. At last, we calculate the chemical potential, which suggests that micro-phase separation of PNIAPm appears at high temperatures.In chapter 4, we use the molecular theory for dilute PEO-b-PNIPAm solutions. Although several theoretical works based on the mean-field approximation have been proposed to study the phase behavior of the copolymer solutions, most of these works focused on the structures of copolymer aggregates at fixed temperatures. Changes on the structures aroused by the continuous change of temperature have not been ac-complished. Here, we first take the formation of hydrogen bonds between copolymer monomers and water molecules into account, which enables us to study the impact of temperature on PEO-b-PNIPAm self-assembly effectively by quantitatively describing the different change of water affinities of two blocks. With the increase of tempera-ture, hydrogen bonds between PNIPAm and water break down dramatically, resulting in the hydrophobic character of PNIPAm while PEO remains hydrophilic. Amphiphilic copolymers in the aqueous surrounding can aggregate into various structures:micelle and vesicle. According to the equilibrium criterion of the excess grand potential under the condition of the grand canonical ensemble, we find both structures are stable and can coexist. Theoretically calculated potentials of mean force of aggregates further ver-ify the coexistence of micelle and vesicle, although the LCST of different aggregates is different under this circumstance. Phase diagram as functions of temperature and the weight fraction of PEO (fPEO) is obtained, which shows different regions of micelle, vesicle and their coexistence. It implies the appearance of two types of micelle-vesicle transition:spontaneous and temperature-induced. Since PEO-b-PNIPAm as a thermo-responsive material has a broad range of applications, a systematical investigation of the phase behavior is very useful not only for the scientific interests but also for the practical applications.In chapter 5, we develop a new and general strategy to control the DNA orienta-tion in biosensors. As we know, DNA biosensors hold great application potentials in recognizing small molecules and detecting DNA hybridization. Usually electric field is used to control the orientation of DNA molecules, but the unavailability of electric field in vivo and its confusing electrostatic effects on charged molecules may largely limit its potential applications. Therefore, it is a huge challenge to overcome such defects in bionanotechnology. The main idea here is to copolymerize DNA molecules with re-sponsive polymers that can show swelling/deswelling transitions due to the change of external stimuli, and then graft the copolymers onto an uncharged substrate. In order to highlight the responsive characteristic, we take thermo-responsive polymers as an ex-ample, and reveal multi-responsive behavior and the underlying molecular mechanism of the DNA orientation by combining dissipative particle dynamics simulation and molecular theory. Since swelling/deswelling transitions can be also realized by using other stimuli-responsive (like pH and light) polymers, the present strategy is universal, which can greatly expand the wide use of DNA biosensors for future real applications.In the last chapter, the thesis is summarized, and an outlook for the future works on PNIPAm is described.