Jiro Kumaki

We describe the in situ atomic force microscopy (AFM) visualization of the fiber and fractal structure formation of a self-assembled diphenylalanine-based polymer, PAPhePheOH, and corresponding monomer (N-acryloyl-l,l-diphenylalanine, APhePheOH).

The surface chain movements within the microdomains of a polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) and corresponding homopolymer films were observed via in situ real-time atomic force microscopy (AFM) at high temperatures and analyzed quantitatively using particle image velocimetry (PIV). At low temperatures, mobility within the PS microdomains resembled that within the PS homopolymer film, but movements in the PMMA microdomains were notably accelerated compared to the PMMA homopolymer. Conversely, at high temperatures, mobility within both PS and PMMA microdomains was considerably suppressed compared to their respective homopolymer films, likely owing to the fixed linkage of the block chains at the microdomain interface. This combination of real-time AFM observation and PIV analysis is an effective method for quantitatively evaluating surface chain mobility in real space.

Polymer materials are widely used in water or in contact with an aqueous environment. However, evaluating the chain mobility, a crucial parameter, at a polymer-water interface is challenging. In this study, we, for the first time, observed poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(2-methoxyethyl methacrylate) (PMEMA) film surfaces in water via in situ real-time atomic force microscopy (AFM) in tapping mode and quantified the chain mobility. The average displacement between adjacent images (nm/8.75 min) was evaluated using particle image velocimetry. The displacement of PMMA, which has a high bulk glass-transition temperature (T_g) (108 °C) and exhibits limited water absorption, was low both in air (0.54 nm/8.75 min) and water (0.86), while PHEMA, which has a high bulk T_g (99 °C) and exhibits high water absorption, exhibited low mobility in air (0.40) but two orders of magnitude higher mobility in water (60). PMEMA, which has a low bulk T_g (14 °C) and exhibits limited water absorption, already started to move in air (4.5), and its mobility moderately increased in water (20). These behaviors were reasonable, considering the bulk T_g and water absorption characteristics of the polymers. Further, the chain mobility in water was compared with that of dried samples at high temperatures in air. The mobility of PMMA, PHEMA, and PMEMA in water corresponded to that of the dried samples observed in air below the surface T_g (97 °C) for PMMA, at ∼125 °C for PHEMA, and at ∼35 °C for PMEMA. In situ real-time AFM analysis of polymer materials in water is an effective method for evaluating the chain mobility at the polymer/water interface.

Polylactide (PLA) crystallizes to form extendedchain crystals in a Langmuir monolayer because crystallization is accelerated on the water surface. This is a unique situation where chain packing can be analyzed by simply measuring the lamellar thickness. Herein, star-shaped poly(L-lactide)s (PLLAs) with 2-12 arms were synthesized through the polymerization of L-lactide with various polyols as initiators, and their crystallization behavior in a monolayer was studied via atomic force microscopy. The PLLAs comprising 2-4 arms crystallized with all arms aligned in the same direction and being folded at the central polyol unit. Meanwhile, the PLLAs comprising 6 and 12 arms crystallized with both halves of the arms extended from the center to the opposite directions, most likely due to the steric hindrance of the crowded arms. Considering that the PLLAs crystallized from a once-formed condensed amorphous state during compression, they have a strong tendency to crystallize with the arms aligned in the same direction. The crystallization rate of star-shaped PLAs is known to reduce compared with that of a linear PLA even if the number of arms is as few as 2. This should be closely related to the unique crystallization behavior of the star-shaped PLLAs with the arms aligned in the same direction.

The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (T_g) is significantly lower than that in the bulk material. However, the mobility of the chains at the top of the surface has never been directly evaluated. In this study, the movements of the topmost chains of poly(methyl methacrylate) (PMMA) and polystyrene (PS) bulk films were observed in situ at high temperatures with atomic force microscopy in tapping mode. PMMA and PS chains started moving at ∼97 and ∼50 °C, respectively, which were slightly and significantly below the values of their bulk T_g (PMMA, 108 °C; PS, 104 °C), respectively. The activation energies of the apparent diffusion constants of PMMA and PS, derived by particle image velocimetry analysis, were 193 and 151 kJ mol^-1, respectively, and reasonable for the glass transition. Movements of isolated PMMA chains deposited on a PMMA film by the Langmuir-Blodgett technique were also observed and confirmed to be essentially the same as those on the PMMA film surface.

If polymer chains could be deposited on a substrate as a fully extended chain, a procedure known as “molecular combing,” the chain structure could be characterized by atomic force microscopy in more detail than has been possible with the measurements available today. We show here, for the first time, that flexible polymers can be molecularly combed to fully extended chains by the dipping method. We studied the molecular combing of a series of poly(n-alkyl acrylate)s on mica from a chloroform solution by the dipping method and found that poly(n-alkyl acrylate)s with an alkyl group longer than n-octyl can be molecularly combed into straight chains under optimized conditions. With increasing alkyl lengths, the number of chains deposited decreases by four orders of magnitude, and chains become molecularly combed under a wider range of conditions. The length of the molecularly combed chains is ∼80% for poly(n-octyl acrylate) but ∼100% of the all-trans conformation for poly(n-alkyl acrylate)s with an alkyl length longer than n-nonyl.