The Conformational Behavior Of Polymer Chains At Interface

This thesis deals with the conformational change of polymer chains at the solid/liquid or air/water interfaces. First, we have investigated the conformation transition of polymer brush and the kinetics of formation of polymer brush at the solid surface, and the self-assembly of polyelectrolyte layer-by-layer deposition. Second, we have studied the conformation and thermosensitivity of the polymers at the air/water interface. Third, we have investigated the mechanism of protein resistance of poly(ethylene oxide) (PEO) and oligo(ethylene oxide) (OEO) chains at the air/water interface. The main results are as follows:1. The temperature induced collapse-to-swelling transition of poly(N-isopropylacrylamide) (PNIPAM) brush grafted on oscillator surface was investigated by using quartz crystal microbalance with dissipation (QCM-D). In the range of 20°C to 38°C, frequency and dissipation gradually change with temperature, indicating that PNIPAM brush gradually collapses. This is in contrast to free PNIPAM chains in solution, which exhibit a sharp collapse transition. The surface restriction, the non-uniformity of polymer brush and the cooperativity between collapse and dehydration transition are thought to be responsible for such a continuous collapse. We have also studied the conformational change of PNIPAM brush in water-methanol mixture. PNIPAM chains swell in either water or methanol. However, as the molar fraction of methanol increases, PNIPAM brush undergoes a swelling-to-collapse-to-swelling transition. The collapse of PNIPAM brush is attributed to the formation of water-methanol complexes which are poor solvents for PNIPAM chains.2. The kinetics of formation of polymer brush of thiol-terminated poly(N-isopropylacrylamide) (HS-PNIPAM) and thiol-terminated poly[(2-dimethylamino)ethyl methacrylate] (HS-PDEM) chains on a gold-coated oscillator surface was investigated by use of QCM-D. In the case of HS-PNIPAM, the frequency and energy dissipation responses revealed that short HS-PNIPAM chains exhibit a three-regime-kinetics of the grafting. In regime I and II, the HS-PNIPAM chains form a pancake and mushroom structure, respectively. In regime III, the chains form brushes. From regime II to regime III, the mushroom-to-brush transition occurs. For the longer HS-PNIPAM chains, due to the strong segment-surface interaction, the segments cannot desorb from the surface, so the chains do not undergo a pancake-to-brush transition. In the case of HS-PDEM, the frequency and energy dissipation responses also reveal a three-regime-kinetics of the grafting. The chains are quickly grafted in regime I forming a random mushroom. In regime II, the grafted chains have a rearrangement to form an ordered mushroom structure. The grafting is accelerated in regime III. As the grafting density increases, the chains form brushes. From regime II to regime III, the mushroom-to-brush transition occurs.3. The effects of temperature, pH and salt concentration on layer-by-layer deposition of polyelectrolyte chains have also been investigated by use of QCM-D. The investigations lead to the following conclusions: (a) The thickness of polyelectrolyte mutilayers increases with temperature. This is because the increasing temperature leads to the increase of penetration between neighboring layers. The hydrophobic effect induced by temperature only facilitates the deposition in the initial several layers. (b) In the case of PSSS/PDEM, from pH = 4 to 7, the amplitudes of oscillation in frequency and dissipation increase with pH, indicating that the increase of penetration between layers, which leads to a thicker film. (c) The thickness of polyelectrolyte mutilayers increases with the concentration of NaCl. In the low salt concentration regime, the deposition of PSSS/PVBTMAC is dominated by the level of surface charge overcompensation. In contrast, in the high salt concentration regime, the deposition is dominated by charge penetration length.4. The conformation and thermosensitivty of two polymers at the air/water interface have been studied by use of Langmuir balance. (a) In the case of PNIPAM, when loops or tails are formed at the interface, PNIPAM chains exhibit thermosensitive properties due to hydration (dehydration) depending on the compression rate. When PNIPAM chains take train conformation at the air/water interface, however, the surface pressure changes are nearly independent of temperature and compression rate because almost all segments of the PNIPAM chains are adsorbed at the interface. (b) In the case of PDEM, it shows the thermosensitivity relates to the hydration (dehydration) of the tertiary amine group. Our results demonstrate that the phase transition of the thermosensitive polymers is probably started by the cooperative hydration (dehydration) and the phase transition at the air/water interface is markedly different from that in aqueous solution.5. Protein adsorption on polystyrene-block-poly(ethylene oxide) (PS₁₀₈-b-PEO₁₁₄) and C₁₆H₃₃(OCH₂CH₂)₁₀OH monolayers was studied by using the Langmuir technique. In the case of PEO monolayer, a protein adsorption minimum is clearly revealed at σ^-1 = 10 nm² for both lysozyme and fibrinogen. Manifested are two grafting density regimes of steric repulsion and compressive attraction between PEO and protein on top of the overall attraction of protein to the air/water interface. The observed protein adsorption minimum coincides with the maximum of the surface segment density at σ^-1 = 10 nm². However, OEO monolayer presents a different scenario, namely, the amount of protein adsorbed decreases monotonically with increasing grafting density, indicating that the OEO chains merely act as steric barrier to protein adsorption onto the air/water interface. In the adsorption of fibrinogen, three distinct kinetic regimes controlled by diffusion, penetration and rearrangement are recognized, whereas only the latter two are made out in the adsorption of lysozyme. This can be ascribed to the relatively larger diffusion coefficient of lysozyme.