In Situ Real-Time Atomic Force Microscopy Observations of Chain Mobility at Polymer/Water Interfaces of Poly(methyl methacrylate), Poly(2-hydroxyethyl methacrylate), and Poly(2-methoxyethyl methacrylate) Films in Water.

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 (Tg) (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 Tg (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 Tg (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 Tg 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 Tg (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.

In Situ Real-Time Atomic Force Microscopy Observation of the Surface Mobility on Each Domain of a Polystyrene-b-poly(methyl methacrylate) Film at High Temperatures.

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.

Crystallization of Star-Shaped Poly(l-lactide)s with Arm Chains Aligned in the Same Direction in Two-Dimensional Crystals in a Langmuir Monolayer.

Polylactide (PLA) crystallizes to form extended-chain 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.

Chain Movements at the Topmost Surface of Poly(methyl methacrylate) and Polystyrene Films Directly Evaluated by In Situ High-Temperature Atomic Force Microscopy.

The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (Tg) 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 Tg (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.

Molecular Combing of Various Poly(n-Alkyl Acrylate) Chains on Mica by the Dipping Method.

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.

In situ AFM Observation of the Movements of Isolated Isotactic Poly(methyl methacrylate) Chains in a Precursor Film of an Oligo(methyl methacrylate) Droplet Spreading on Mica.

Atomic force microscopy (AFM) is a powerful tool to observe polymer chains at the molecular level. In this study, we show that the movements of isolated linear polymer chains in a precursor film of a droplet of an oligomer spreading on a substrate could be visualized in situ at the molecular level by AFM for the first time. The system was an isotactic poly(methyl methacrylate) (it-PMMA) solubilized in an oligo(MMA) matrix (it-PMMA/oligo(MMA) = 1/10,000 w/w) spreading on mica under high humidity. Because of the limited resolution of the AFM instrument, condensed linear polymer chains could not be visualized, but a small amount of it-PMMA chains that were solubilized as isolated chains in the oligo(MMA) matrix could be visualized in the precursor film, the contrast of which came from a large difference in glass transition temperature (Tg) of it-PMMA and oligo(MMA). The it-PMMA chains in the precursor film spread in the radial direction of the droplet with vigorously changing chain conformations. The spreading rate of it-PMMA chains under 72% relative humidity was ∼1/30 of the spreading rate of the oligo(MMA) matrix, which was estimated based on the decrease in the volume of the macroscopic droplet. The spreading of the it-PMMA chains and droplet strongly depended on humidity and was suppressed with the decrease in humidity, most likely because of the increase in friction with the substrate. The difference in the spreading rate of it-PMMA and oligo(MMA) further increased under low humidity. The dynamic molecular information of a precursor film by AFM should help to elucidate the wetting dynamics on a substrate.

Preparation of a Si(111) Atomically Flat Substrate via Wet Etching and Evaluation as an AFM Substrate for Observations of Isolated Chains, Crystals, and Crystallization of Isotactic Poly(methyl methacrylate) at the Molecular Level.

To observe a polymer chain deposited on a substrate by atomic force microscopy (AFM) at the molecular level, the substrate should be atomically flat and stable under laboratory conditions and adsorb polymer chains firmly. Therefore, substrates used under laboratory conditions are practically limited to mica, highly ordered pyrolytic graphite, and atomically stepped sapphire, and polymers observed by AFM at the molecular level are also limited. A silicon wafer is frequently used as a substrate for AFM observation for somewhat macroscopic observations, but the surface of the silicon wafer is too rough to observe polymer chains deposited on it at the molecular level. In this study, we prepared an atomically stepped Si(111) substrate via wet etching in NH4F and evaluated it as an AFM substrate. The Si(111) substrate was stable as an AFM substrate, and isolated poly(methyl methacrylate) (it-PMMA) chains and a crystalline monolayer deposited on the substrate were observed by AFM at the molecular level. An it-PMMA amorphous monolayer deposited on mica crystallized under high humidity, but that on the Si(111) substrate did not because of the difference in the surface nature and the crystal structure of the substrates. The Si(111) substrate was hydrophobic, and the it-PMMA monolayers could be deposited as a multilayer, which could not be formed on hydrophilic mica. The crystallization behavior of an it-PMMA amorphous multilayer and an amorphous/crystalline mixed multilayer on the Si(111) substrate was also evaluated.

Self-Assembly of Linear and Cyclic Polylactide Stereoblock Copolymers with a Parallel and Antiparallel Chain Arrangement Distinguishing Their Directions on a Water Surface.

The self-assembly of molecules into a well-ordered structure is one of the most important processes in fabricating sophisticated materials. Here, we show that polymer chains can be self-assembled, distinguishing their direction (parallel or antiparallel), and could be a new useful scaffold for self-assembly in a controlled direction. The system that was used was a stereocomplex (SC) formation of linear and cyclic polylactide (PLA) stereoblock copolymers with a parallel and antiparallel chain arrangement in a Langmuir monolayer. The linear and cyclic stereoblock copolymers with a parallel arrangement formed a well-ordered lamellar SC in the first and second layers upon compression, but the linear and cyclic stereoblock copolymers with an antiparallel arrangement did not form a first-layer lamella and instead formed only the second-layer lamella. These results were only rationally explained by assuming that the enantiomeric PLA chains selectively assembled in a parallel direction, not in an antiparallel direction, in the SC. A simple polymer chain could be self-assembled, distinguishing the direction without a specific interaction group in it.

Hierarchical amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s composed of chiral and achiral phenylacetylenes in dilute solution, liquid crystal, and two-dimensional crystal.

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.

Gemini Thermotropic Smectic Liquid Crystals for Two-Dimensional Nanostructured Water-Treatment Membranes.

We have developed a two-dimensional (2D) liquid-crystalline (LC) nanostructured water-treatment membrane showing high virus rejection ability (over 99.99997% for bacteriophage Qß) and improved water permeation. Polymerizable gemini amphiphiles have been designed and synthesized. They have H-shaped gemini-type structures of thermotropic smectic liquid crystals composed of cationic imidazolium moieties. One of the gemini amphiphiles shows a smectic A phase with an interdigitated bilayer structure. A cross-linked self-standing 2D nanostructured polymer film has been obtained by in situ photopolymerization of the gemini amphiphile in the smectic phase. The length of linkers in gemini amphiphiles affects the formation of LC phases. The 2D nanostructured membrane also showed selective salt rejection.

Visualization of polymer chain conformations in amorphous polyisocyanide Langmuir-Blodgett films by atomic force microscopy.

Polymer Langmuir monolayers are an ideal model for two-dimensional (2D) polymer chains, but our understanding of them is still limited. Using atomic force microscopy, we have for the first time successfully visualized the polymer chain packings in amorphous polyisocyanide monolayers deposited on mica. The long polymer chains, which were partially forced to form hairpin-like conformations, were sophisticatedly packed in the 2D film without any chain stacking. The trend of the persistent lengths of the polymers in the 2D films fairly corresponded to those of 3D chains in solution. The molecular-level information provided by direct observations such as these will improve our understanding of polymers in 2D space.

Separation of C70 over C60 and selective extraction and resolution of higher fullerenes by syndiotactic helical poly(methyl methacrylate).

A one-handed helical polymer, syndiotactic poly(methyl methacrylate) (st-PMMA), recognizes the size and chirality of higher fullerenes through an induced-fit mechanism and can selectively extract enantiomers of the higher fullerenes, such as C(76), C(80), C(84), C(86), C(88,) C(90), C(92), C(94), and C(96). This discovery will generate a practical and valuable method for selectively extracting the elusive higher fullerenes and their enantiomers and opens the way to developing novel carbon cage materials with optical activities.

Strong compression rate dependence of phase separation and stereocomplexation between isotactic and syndiotactic poly(methyl methacrylate)s in a Langmuir monolayer observed by atomic force microscopy.

The stereocomplex formation between isotactic and syndiotactic poly(methyl methacrylate) (it-PMMA, st-PMMA) in a Langmuir monolayer was studied by surface pressure-area isotherms and atomic force microscopy (AFM). We found that the stereocomplex formation was highly sensitive to the compression rate of the monolayer. At a normal compression rate of 0.5 mm/s provided by the moving barrier, the blend monolayer formed a clear phase separation of the it- and st-PMMA domains at 1 mN/m. Further compression to 15 mN/m resulted in a limited degree of stereocomplexation, mainly at the interface between the two domains. However, at a 1/50 slower compression rate of 0.01 mm/s, the blend did not form a clear phase separation at 1 mN/m and quantitatively formed a stereocomplex at 15 mN/m. This apparent immiscibility observed at the faster compression rate was found to be kinetically induced as a result of the rapid compression of the phase-separated mixture at the dilute state because it-PMMA and st-PMMA form expanded and condensed monolayer, respectively. On the other hand, at the slower compression rate, the blend formed a thermodynamically miscible phase, and as a result, the stereocomplex was quantitatively formed. This apparent phase separation of a mixed monolayer composed of an expanded and a condensed monolayer should be a common phenomenon for similar systems and might have caused misjudgment of the miscibility in such cases. The compression rate dependence should be carefully evaluated in order to determine the precise miscibility of blended monolayers in similar systems.

Condensed desmin and actin cytoskeletal communication in lipid droplets.

The interplay between intermediate filaments (IFs) and other cytoskeletal components is important for the integrity and motility of cells. The impact of IF assembly on other components and cell morphology is not yet fully understood. Therefore, we examined the effects of combined desmin and actin assembly on cytoskeletal network arrangement in artificial cell-sized droplets. Fluorescently labeled desmin, with or without actin, was enclosed in droplets prepared with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) using the water-in-oil method. Protein networks were observed using fluorescence microscopy in the presence of 150 mM KCl, 20 mM imidazole-HCl (pH 7.4), 2 mM MgCl2 , and 1 mM adenosine 5'-triphosphate for both desmin and actin assembly. As desmin alone can assemble into filaments within seconds, desmin networks mainly localizing at the inner margins of the droplets were observed within 10 min after assembly initiation. Subsequently, deformations of droplets appeared. Furthermore, a portion of droplets formed desmin-rich protrusions of several micrometers. Notably, actin alone rarely formed protrusions under the same conditions. When 1,2-dioleoyl-sn-glycero-3-phosphocholine was used instead of DOPE, protrusions became less frequent. The combination of desmin and actin increased the number of deformed droplets in which the proteins were considerably colocalized. The assembly process of desmin facilitated colocalization. Atomic force microscopy failed to reveal interactions between the two filament types. These results suggest that the mechanical properties of desmin networks may influence the behavior of actin networks, as well as membrane morphology, possibly reflecting the mechanical function of desmin filaments in muscle cells.

Evaluation of Ring Expansion-Controlled Radical Polymerization System by AFM Observation.

We here present a direct link between the reaction mechanisms for the ring-expansion "vinyl" polymerization system and atomic force microscopy (AFM) observations. The brush-modification clearly discriminates the desired cyclic species with the contour lengths (Lc) of 28-132 nm and molar masses (MAFM) of 60.2-283 kg mol-1 from the other linear ones. The 293 polymer blushes observed in a 1.0 µm × 1.0 µm AFM image are individually characterized, eventually providing clear answers about the mechanisms of this rare polymerization system, which include ring-expansion vinyl polymerizations to generate cyclic polymers, fusions of the generated cycles to form multimers, and their scission to form linear or ring-opened species. The relationship between the molecular chain lengths and the cyclic versus linear morphologies is highlighted.

Helix-sense-controlled synthesis of optically active poly(methyl methacrylate) stereocomplexes.

Optically active poly(methyl methacrylate) (PMMA) stereocomplexes were prepared through the helix-sense-controlled supramolecular inclusion of an isotactic (it) PMMA within the helical cavity of an optically active, fullerene-encapsulated syndiotactic (st) PMMA with a macromolecular helicity memory. The observed and calculated vibrational circular dichroism spectra revealed that the it-PMMA replaced the encapsulated fullerenes to fold into a double-stranded helix with the same handedness as that of the st-PMMA single helix through the formation of a topological triple-stranded helix.

Molecular weight recognition in the multiple-stranded helix of a synthetic polymer without specific monomer-monomer interaction.

Stereoregular isotactic and syndiotactic poly(methyl methacrylate)s (it- and st-PMMAs) are known to form a multiple-stranded complementary helix, so-called stereocomplex (SC) through van der Waals interactions, which is a rare example of helical supramolecular structures formed by a commodity polymer. In this study, we prepared SCs by using uniform it- and st-PMMAs and those with a narrow molecular weight distribution having different molecular weights and investigated their structures in detail using high-resolution atomic force microscopy as a function of the molecular weight and molecular weight distribution of the component PMMAs. We found that complementary it- and st-PMMAs with the longer molecular length determine the total length of the SC, and molecules of the shorter component associate until they fill up or cover the longer component. These observations support a supramolecular triple-stranded helical structure of the SCs composed of a double-stranded helix of two intertwined it-PMMA chains included in a single helix of st-PMMA, and this triple-stranded helix model of the SCs appears to be applicable to the it- and st-PMMAs having a wide range of molecular weights we employed in this study. In homogeneous double-stranded helices of it-PMMA, it has been found that, in mixtures of two it-PMMAs with different molecular weights, chains of the same molecular weight selectively form a double-stranded it-PMMA helix, or recognize the molecular weights of each other ("molecular sorting"). We thus demonstrate that molecular weight recognition is possible, without any specific interaction between monomer units, through the formation of a topological multiple-stranded helical structure based upon van der Waals interaction.

Double-stranded helical polymers consisting of complementary homopolymers.

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.

Two- and three-dimensional smectic ordering of single-handed helical polymers.

Rodlike polymers with precisely defined architectures are ideal building blocks for self-assembled structures leading to novel nanometer-scale devices. We found that the living polymerization of a single isocyanide enantiomer bearing an l-alanine pendant with a long n-decyl chain simultaneously produced diastereomeric right- and left-handed helices with different molecular weights and narrow molecular weight distributions. Each single-handed, rodlike helical polymer with a controlled length and handedness isolated by a facile solvent fractionation method with acetone self-assembled to form well-defined two- and three-dimensional smectic ordering on the nanometer scale on a substrate and in a liquid crystalline state as evidenced by direct atomic force microscopic observations and X-ray diffraction measurements, respectively.

Molecular Combing of a Flexible Polymer Chain by Simple Spin-Casting.

If polymer chains could be fixed on a substrate as a fully elongated chain, a procedure known as "molecular combing", the chain structure could be analyzed more precisely than has been possible with the characterization techniques available today. Although the molecular combing of a rigid biomolecule, DNA, has been attained for the mapping of genetic information, that of flexible chains has never been achieved as yet. We show here that poly(n-nonyl acrylate) (PNA) can be molecularly combed on mica by a simple spin-casting method, and that the chain lengths were in good agreement with that of the all-trans conformation. One of the key factors for successful molecular combing was found to be the weak adsorption of PNA on mica, indicating that flexible polymers may be molecularly combed by adjusting their affinity to the substrate. The molecular combing of polymer chains may open a new way not only to characterize the chain structures more precisely but also to fabricate new nanomaterials based on polymers.