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The effect of free space on molecular motion inside the polymer "dead layer" or adsorbed nanolayers on solid surfaces is investigated. Free space is introduced into the nanolayer by choosing a polymer with a relatively big side group, poly

The dynamics of adsorption and desorption of counterions on a polyelectrolyte chain in aqueous solution is studied at a single molecular level. The investigation is through the process of fluoresce...

The physical mechanism of multiple modes in dynamics of polyelectrolyte aqueous solutions has been drawing extensive research attention for decades. This unsolved mystery makes it highly desirable to use new techniques to conduct investigations. In this study, dual-color fluorescence cross-correlation spectroscopy is applied to study the dynamics of individual molecules of a model polyelectrolyte, sodium poly(styrene sulfonate), in aqueous solutions. Anticorrelation in the cross-correlation function is discovered as a result of motion coupling due to interchain electrostatic repulsion. After correction, the self-part of the autocorrelation function is obtained, and the calculated mean square displacement data demonstrate a two-stage diffusion process—a fast one at short time lag and a slow one at long time lag. The two processes are attributed to a faster diffusion inside the cage formed by neighboring chains and a slower diffusion beyond the cage. Effects of the salt level and polyelectrolyte concentrati...

It has long been documented that the reduced viscosity of polyelectrolyte has an anomalous dependence on its concentration, i.e., the Fuoss law. To explore the molecular mechanism, the counterion distribution of sodium polystyrenesulfonate (NaPSS) as a function of concentration is investigated at the single-molecule level. By examination of the fluorescence resonance energy transfer (FRET) between a fluorescence donor on NaPSS chain and an acceptor in the counterions using single-molecule fluorescence spectroscopy, an increase of average counterion–chain distance is discovered upon dilution, indicating the expansion of counterion cloud. By photon counting histogram, an increase of effective charge of the NaPSS chain during dilution is exposed. The variation of these parameters agrees well with that of the reduced viscosity, helping to shed light on the molecular mechanism of the Fuoss law: the expansion of the counterion cloud increases hydrodynamic friction, and the increase of effective charges of NaPSS...

Kinetics of the imidization reaction of poly (amic acid) (PAA) film based on pyromellitic anhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA) was studied by infrared spectroscopy. Attention is paid to the effect of solvent content inside the sample to the reaction kinetics. The results show that the presence of solvent favors the imidization reaction, rendering a much higher reaction kinetics than the case of without solvent. The data indicates that there is almost one NMP solvent molecule lost per imidization reaction. The kinetics of isothermal imidization reaction exhibits huge heterogeneity and is well-fitted by the Kohlraush-Williams-Watts (KWW) equation, from which the characteristic reaction time and the stretching exponent are determined. Both of them decrease with the increasing of the imidization reaction temperature, demonstrating the reaction is retarded with the proceeding of the reaction together with the creation of bigger dynamical heterogeneity.

The re-entrant phase separation inside an evaporating droplet of poly (N-isopropyl acryl amide) (PNIPAM) solution with the binary solvent of ethanol and water is discovered and monitored continuously by optical microscopy. Because of the higher evaporation rate of ethanol than water, the solvent composition is changed by evaporation. As a result, the cononsolvency of PNIPAM makes the polymer precipitate into the solution and redissolve afterwards along the proceeding of the evaporation. Such a simple strategy enables a continuous observation of the re-entrant phase separation due to cononsolvency. Direct optical microscopic observation is made with quantitative description of the process and the details of the flow patterns during the process. The phase separation is found to occur at the air-liquid interface and the precipitates are brought into the droplet by circulations induced by Marangoni flow, providing a new way of mass transportation during phase separation.

Polymers doped with photosensitive molecules/nanoparticles are believed to be promising optical data storage media. In this communication, a simple method is reported to incorporate multilayers of photosensitive molecules with controlled concentration and spatial distribution into a polymer matrix. This simple hot pressing-based method makes it compatible with existing polymer processing techniques. It is demonstrated that this approach is effective to fabricate free-standing and multi-layered optical data storage media with commonly available polymers. The formation of a multilayer architecture within the polymer matrix involves no adhesives or polymers of other kinds, thus eliminating the concern of refractive index-mismatch between adjacent layers and making the whole material macroscopically homogeneous in structure. When polymers with good ductility and high glass transition temperature are used as the matrix, the macroscopically homogeneous structure can help the material to endure deformation and heat without losing the recorded information.

The effect of strengthening of interaction between single fluorescent probes and polymer matrix to the probes' dynamics is investigated using single molecule fluorescence defocus microscopy. By introducing multiple hydroxyl groups to the fluorescent probes (perylene diimide derivatives) which builds up hydrogen bonds between the probe and polymer matrix, the dynamics is discovered to be retarded. This is evidenced by the lowering of the frequency of the vibrational modes in the power spectra of the rotation trajectories of individual fluorescent probes, and also by the decrease in population of rotating probes. The results show that by strengthening the probe-matrix interaction, the local dynamics detected by the probes is equivalent to that detected by a bigger probe, due to the enhanced friction between the probe and the polymer matrix.

This study demonstrates that the conformation transition of single polymer molecules can be adjustable by applying external irradiation. By choosing poly N-isopropylacrylamide as the model system, its coil-to-globule conformation transition temperature is shifted with application of external UV irradiation, due to the photo-isomerization of the azobenzene moiety attached to the chain end. The investigation was conducted at single molecular level by fluorescence correlation spectroscopy, using hydrodynamic radius as the measure of chain dimension. It is discovered that the coil-to-globule transition temperature of the PNIPAM-azo can be controlled up to a difference of 12 °C upon external irradiation, due to the difference in polarity of the azobenzene end group.

A study was made of the anti-polyelectrolyte effect of a model polyzwitterion at the single-chain level and from the point of view of comparison with inorganic salts and organic inner salts. A well-defined polymer of sulfobetaine was synthesized and fluorescently labeled. Fluorescence correlation spectroscopy was adopted to probe diffusion and chain size of polyzwitterion single chains in extremely dilute aqueous solutions over a broad concentration range of added salts. By the introduction of inorganic salts and organic inner salts, the single chains of polyzwitterions expanded with an increase of salt concentration. This observation indicates that the break-up of the dipole-dipole attraction between the zwitterion units is the mechanism of the anti-polyelectrolyte effect of polyzwitterions. The difference in the strength of the anti-polyelectrolyte effect induced by inorganic salts and organic inner salts also indicates the partial contribution from the asymmetric adsorption of ions onto polyzwitterion chains. (c) 2015 Society of Chemical Industry

There has been recognized that there exist three basic relaxation modes in the molecular dynamics of liquid crystalline side chain polymers (SCLCP) – the α-, β- and δ-relaxation process, and the molecular mechanism of the δ-relaxation has been in controversy, i.e. whether it is a “flip-flop” transition or a continuous rotation. The current study provides the experimental evidence of the multiple steps relaxation by time-resolved emission spectroscopy, taking the intrinsic fluorescence from the model mesogen group of 4-cyanobiphenyl. The temperature dependence of the spectra and its dynamics provides new information on the microscopic picture of the δ-relaxation.

A self-healing hydrogel is prepared by crosslinking acrylamide with a host-guest macro-crosslinker assembled from poly(β-cyclodextrin) nanogel and azobenzeneacrylamide. The photoisomerizable azobenzene moiety can change its binding affinity with β-cyclodextrin, therefore the crosslinking density and rheology property of the hydrogel can be tuned with light stimulus. The hydrogel can repair its wound autonomously through the dynamic host-guest interaction. In addition, the wounded hydrogel will lose its ability of self-healing when exposed to ultraviolet light, and the self-healing behavior can be recovered upon the irradiation of visible light. The utilizing of host-guest macro-crosslinking approach manifests the as-prepared hydrogel reversible and light-switchable self-healing property, which would broaden the potential applications of self-healing polymers.

The lateral diffusion of individual polymer chain of polyethylene oxide (PEO) at oil-water interfaces was studied using fluorescence correlation spectroscopy (FCS). The diffusion coefficient of PEO was systematically under variable salt concentration of NaCl in the aqueous phase and it was found that the diffusion coefficient of the adsorbed polymer chain was very much determined by the viscosity of the oil phase. This fact indicates an enhanced hydrophobic interaction of the PEO chain with the oil phase.

Self-assembly is an efficient strategy of constructing microgel-based intelligent materials. However, it remains a challenge to realize the reversible self-assembly of microgels. Herein, a method to guide the self-assembly of soft colloids with light-stimuli is proposed, utilizing the light-responsive host–guest interaction between an azobenzene functionalized nanogel (the guest colloid) and an α-cyclodextrin functionalized microgel (the host colloid). The two colloids can form a stable colloid cluster when the surface of the host colloid is fully packed with the guest colloids. The colloid cluster can disassemble when irradiated with UV light and reassemble when irradiated with visible light. The reversible colloidal self-assembly can be controlled by the interplay between the supramolecular and covalent crosslinking, and can also be adjusted by the addition of competitive host molecules. Besides the light-sensitivity, the colloid cluster inherits the deformability and temperature-sensitivity from its parent colloids. These features are different from the supramolecular self-assembly of hard colloids or macroscopic gels, and manifest the as-prepared colloid cluster potential building blocks of light-responsive materials.