Surface recovery of acrylic coatings under elevated relative humidity (RH) is observed in real-time using atomic force microscopy. It is found that the extent and rate of recovery are closely related to the glass transition temperature (T-g) of the polymer and RH. The phenomenon is explained by viscoelastic mechanics theory, in which mechanical characteristics such as elastic modulus and viscosity of the polymer have influence over the dynamics as well as the extent of recovery. The experimental data suggests that the polymer undergoes change in viscoelasticity at elevated RH due to water uptake. It is believed that this behavior may provide new insights in research areas such as controlled repair of coatings, and this experimental approach will help us better understand the mechanical performance of coatings under deviated environmental conditions.

The surface chemical composition of water-based coatings prepared from urethane–acrylic hybrids or blends is investigated using infrared photo-induced force microscopy. By monitoring the interaction between the optically driven molecular dipole and its mirror image in a metal-coated tip, detailed information of the chemical composition at the coating surface down to 20 nm local resolution in all three dimensions is obtained. The film matrix of the coatings is determined to be the polyurethane while the polyacrylate particles are dispersed in the film. Phase separation of the two materials is found to be at a much smaller length scale in hybrid coatings compared to that in blends. This is because the acrylic monomers are introduced during the dispersion of the polyurethane in the synthesis of the hybrid particles, in contrast to the simple mixing of the two polymer particles in the case of the blends. Photo-induced force microscopy proved to be a powerful tool in the identification and visualization of different chemical species of coating surfaces at the nanoscale.

The surface topography and mechanical properties of coatings prepared using large particle size polyurethane dispersions (PUD) are investigated using atomic force microscopy (AFM) imaging, AFM-based force measurements, and friction force microscopy. PUD coatings, which are prepared from dispersions containing particles of micron size, have surface roughness of 250–300 nm and waviness of 2.5–3 μm resulting from the particle size. The surface moduli of the PUD coatings are varied by tuning the ratio of hard-to-soft segmentation in the polyurethanes and are found to be between 40 and 100 MPa. The friction coefficient obtained in the study is found to be correlated with both the surface modulus of the coatings and the adhesion between the probe and the samples and is well in line with the perceived feel of an experienced human panel. The data are very well behaved and clearly show the utility of this technique in characterizing these types of surfaces.

The influence of several common cosolvents on the level of film formation and surface mechanical properties of water-based acrylic coatings is investigated by atomic force microscopy (AFM) and AFM based nano-indentation technique. The amount of residual cosolvents in the coatings is determined and it is found that the aforementioned properties of coatings with the exact same polymer composition can be significantly different depending on the amount of residual cosolvents, which is related to the evaporation rate of the solvent as well as the interaction between the solvent, water and the polymer. Butyl glycol, for example, has good interaction with the acrylic polymer and water but due to its fast evaporation, does not improve film formation. Texanol, on the other hand, presents in the film at a large amount but it reduces the surface stiffness of the film and can have a detrimental effect on the surface mechanical properties of the coating. (C) 2016 Elsevier B.V. All rights reserved.

随着聚合物材料化学的飞速发展,设计并实现具有力学响应的聚合物材料变得十分重要.其中,力敏基团的设计合成是力响应材料设计的重点.根据在力的诱导下所发生变化的力敏基团种类的不同,本文主要从共价键和非共价相互作用的变化两个方面进行综述.对共价键变化的讨论将主要集中在作用于力敏基团的力源上,即超声、拉力、压力等;对非共价相互作用的变化将对不同的作用分别进行介绍,包括氢键、π-π相互作用等.通过对力化学机理的深入研究和更多力敏基团的设计,科研工作者期待开发更多元、更实际、多功能的材料,相信力化学的发展会带给聚合物材料新的生命.

Using a combination of ellipsometry and friction force microscopy, we study the reversible swelling, collapse and variation in friction properties of covalently bound poly(N-isopropylacrylamide) (PNIPAM) layers on silicon with different grafting densities in response to exposure to good solvents and co-nonsolvent mixtures. Changes in the thickness and segment density distribution of grafted films are investigated by in situ ellipsometry. Based on quantitative modelling of the ellipsometry spectra, we postulate a structural model, which assumes that collapse takes place in the contacting layer between the brush and the co-nonsolvent and the top-collapsed brushes remain hydrated in the film interior. Using the structural model derived from ellipsometry spectra, we analyse the AFM based friction force microscopy data, which were obtained by silica colloidal probes. Results show a large increase of the friction coefficient of PNIPAM grafts when the grafts swollen by water are brought in contact with co-nonsolvents. For instance, the value of the friction coefficient for a medium density brush in water is four times lower than the value observed in a water-methanol (50% v/v) mixture. This increase of friction is accompanied by an increase in adherence between the PNIPAM chains and the silica colloidal probes, and is a result of chain collapse in the graft when contacted by a co-nonsolvent mixture in agreement with the model postulated on the basis of ellipsometric characterisation. The kinetic behaviour of the collapse is assessed by measuring the temporal variation of friction in situ as a function of elapsed time following contact with the co-nonsolvent as a function of graft density. In conclusion, the effect of co-nonsolvency influenced both the thickness of the PNIPAM brushes and the tribological behavior of the brush surfaces.

Block copolymer vesicles containing enzymes and fluorogenic substrates were immobilized and patterned using soft lithography in combination with electrostatic interactions between the vesicles and the surface. The surface coverage of vesicles was found to depend systematically on pH and ionic strength of the solution.

We assess the elastic properties of PS-b-PAA vesicle membranes under different pH values by AFM force measurements. We find that based on the shell deformation theory, the values of the estimated apparent Young's modulus of the vesicle membranes decrease as the pH of the solution increases. The onset of the decrease of E coincides with the surface pK(a) determined from zeta-potential measurements. This decrease of E at higher pH is attributed to electrostatic repulsion between the deprotonated PAA chains resulting in the thinning of the vesicle membrane.

Collapse of poly(N-isopropylacrylamide) (PNIPAM) brushes in the mixed solvent system (water/methanol 50% v/v) is studied by in-situ atomic-force microscopy (AFM). PNIPAM brushes with three different grafting densities and similar chain lengths are synthesized via surface-initiated atom-transfer radical polymerization. By changing the solvent from water to a water/methanol (50% v/v) mixture, the polymer brushes switch from a swollen to collapsed state. AFM force measurements using a silica colloidal probe attached to the tip are employed to obtain the Young's moduli of the polymer brushes in different solvation states. The collapse dynamics of the brush is followed by monitoring the pull-off force (adherence) in situ. The modulus of the swollen high-density polymer brush is four times lower than that of the same brush in the collapsed state. It is shown that in the case of the high-density polymer brush with a thickness (t(in water) ) of 900 nm, the collapse takes place in a time scale of ~25 s, whereas the collapse occurs faster for the medium-density brush (t(in water) = 630 nm) and much more rapidly for the low-density brush (t(in water) = 80 nm). This difference in the response kinetics is primarily ascribed to the time needed for solvent exchange in the polymer brushes.

Herein the reactivity of the enzyme alpha-chymotrypsin in the confinement of polystyrene-block-poly(acrylic acid) (PS-b-PAA) vesicles was investigated Enzyme and substrate molecules were encapsulated in PS-b-PAA vesicles with internal diameters ranging from 26 nm to 165 nm during the formation of the vesicles While the loading efficiencies of enzyme and substrate molecules were practically identical for vesicles of identical size, they were found to increase with decreasing vesicle size The kinetics of the alpha-chymotrypsin catalyzed hydrolysis of N-succinyl-Ala-Ala-Phe-7-amido-4-methylcoumarin (AMC) was evaluated following the increase of the absorption of the product 7-amino-4-methylcoumarin by UV/Vis spectroscopy The values of the catalytic turnover number obtained for reactions inside vesicles with different sizes showed an increase of up to fourteen times compared to the bulk value with decreasing vesicle volume, while the values of the Michaelis-Menten constant decreased, respectively This increase in reactivity of alpha-chymotrypsin is attributed to the effect of vesicle-wall interactions in the finite encapsulated space, where the reagents could diffuse, leading to enhanced collision frequencies

Temperature-induced vesicle-to-micelle transitions of polystyrene-block-poly acrylic acid (PS(139)-b-PAA(17)) aggregates in tetrahydrofuran (THF)/H(2)O solvent mixtures are studied. For a typical system with an initial concentration of PS(139)-b-PAA(17) of 2 wt% and 50 vol% of H(2)O, the morphology of the aggregates changes from vesicles to micelles upon heating from room temperature to 45 °C. The transition temperature is found to depend on the polymer concentration as well as solvent composition. A higher polymer concentration results in a higher transition temperature. The morphological change is attributed to a change in the solvent-polymer interactions, which results in a reduction in interfacial energy. The corresponding temperature-induced morphological change is employed as a strategy for the reversible release and encapsulation of small molecules. The release of Rhodamine 110 bisamide above the transition temperature is observed as a result of the trypsin-catalyzed hydrolysis of the bisamide into Rhodamine 110. Likewise, the successful encapsulation of Rhodamine 110 below the transition temperature is proven using sodium nitrite as a chemical quencher.

The mechanical properties of polystyrene-block-poly(acrylic acid) (PS-b-PAA) vesicles prepared with different block copolymer chain lengths and block length ratios were studied under ambient conditions using atomic force microscopy (AFM). The deformation δ, as well as the spring constant of the membranes kmem, of individual vesicles with external diameter of ∼150 nm and systematically varied membrane thicknesses between 22 nm and 40 nm were calculated from the captured AFM force-displacement data. The application of the shell deformation model provided estimates of the apparent Young's moduli E of the vesicle membranes. While the values of kmem increased with increasing membrane thickness, the values of E were found to decrease. This observed decrease in E with increasing shell thickness coincides with the reduced degree of chain stretching reported in the literature for longer polymer chains.

The impact of the spatial confinement of polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer (BCP) vesicles on the reactivity of encapsulated bovine pancreas trypsin is studied. Enzymes, as well as small molecules, are encapsulated with loading efficiencies up to 30% in BCP vesicles with variable internal volumes between 0.014 aL (internal vesicle diameter, d(in) = 30 nm) and 8 aL (d(in) = 250 nm), obtained by manipulating the vesicle preparation conditions. The kinetics of the trypsin-catalyzed reaction of a fluorogenic substrate inside and outside the vesicles is quantitatively estimated using fluorescence spectroscopic analyses in conjunction with the use of NaNO(2) as selective quencher for non-encapsulated fluorophores. The values of the catalytic turnover number obtained for reactions in differently sized nanoscale reactors show a significant increase (up to approximately 5x) with decreasing BCP vesicle volume, while the values of the Michaelis-Menten constant decrease. The observed increase in enzyme efficiency by two orders of magnitude compared to bulk solution is attributed to an enhanced rate of enzyme-substrate and molecule-wall collisions inside the nanosized reactors, as predicted in the literature on the basis of Monte Carlo simulations.

Studies of reaction kinetics at the single enzyme level have become possible due to the swift development of single molecule detection techniques. In this tutorial review, which targets both general and expert research chemists in the biochemical or physical (organic) chemistry field, we briefly introduce some of these fluorescence microscopic techniques and discuss selected studies on single molecule enzymology. The nature of the frequently observed fluctuations in the reaction rate, the so-called dynamic disorder, is described based on various available models.