No abstract

Introduction Thermoresponsive polymers are a class of smart materials. The properties of such polymers change significantly in a controlled fashion under the influence of external stimuli 1. To develop synthetic polymer systems that act as protein mimics is a great challenge for polymer scientists. The block design of copolymers is an effective strategy for preparing protein mimics using synthetic polymers 2. An alternative way is to control the monomer sequence, which corresponds to the primary structure of proteins 3. However, the interaction among the monomers often makes it difficult to predict the macroscopic properties of the final products 4, 5. In comparison with monomer sequence control, the block design is advantageous for achieving predictable physicochemical properties of the resultant copolymer and scale-up of the preparation process. Because the individual blocks in the copolymer may retain the characteristics of the homopolymer, the numerous reports on the homopolymers may serve as a block library 6 19. Recently, we investigated the solution properties of amphiphilic alternating multiblock AMB copolymers. The

Poly(ethylene oxide) (PEO)-poly(N-isopropylacrylamide) (PNiPAm) alternating multiblock (AMB) copolymers have been prepared by means of the stereospecific reversible-addition− fragmentation chain transfer polymerization of α,ω-dicarboxy PNiPAm, followed by the amide condensation with α,ω-diamino PEO. The meso diad (m) contents of PNiPAm blocks for the AMB copolymers (AMBm45, AMBm50, and AMBm58) are 45, 50, and 58%, respectively. Dynamic light scattering and small-angle X-ray scattering (SAXS) techniques have revealed that the unimer of AMBm45 in water behaves as an extended coil near room temperature, whereas those of AMBm50 and AMBm58 shrink slightly. As the temperature goes up, a uniform aggregate appears above a certain temperature. The fluorescent probe method has indicated that the hydrophobic core is formed as the temperature increases. According to SAXS and static light scattering measurements, the aggregate possesses a spherical core−shell structure with 10−15 nm radius and consists of 9−14 unimers. These results have suggested that the heat induced aggregate is a multimer micelle with the critical micelle temperature at ∼40 °C. It should be emphasized that the heat-induced aggregate is not a huge droplet arising from the microphase separation, which is often observed for the diblock copolymer having a PNiPAm block. Because an aqueous solution of m-rich PNiPAm has a large hysteresis in the thermal demixing and remixing processes, the association and dissociation of PEO−PNiPAm AMB copolymers also show a hysteresis. In the cooling process, a frozen micelle of AMBm50 and AMBm58 is likely formed. The chain characters of the blocks arising from their primary structures retain well in the solution properties of the AMB copolymers.

Poly(N-isopropylacrylamide) (PNiPAM) compounds with various diad tacticities were prepared, and the molecular interaction properties in a thin film deposited on a gold surface were analyzed using infrared spectroscopy. The intramolecular and intermolecular interactions were found to depend on the tacticity, and only atactic (diad ratio 46 %) PNiPAM exhibits poor molecular interaction even in the bulk sample. On the other hand, the same series of compounds dissolved in an acetone solution were spread on a gold surface to form a thin film. In the dissolution process, the polymer molecules are relaxed via solvation, and they are bound to the gold surface by a molecular interaction to form a submonolayer thin film. In the thin film, the molecular interaction with the gold surface via the N-H group was monitored in the infrared spectra only for a nearly isotactic (m = 90) PNiPAM by an apparent shift of the N-H stretching vibration band. This shift was confirmed by changing the degree of hydrophilicity of the gold surface: a larger shift is found on a gold surface with stronger hydrophilicity. As a result, the conformation of a nearly isotactic molecule is found to be extended by the interaction with the gold surface, which works to immobilize the molecule.