Adhesion studies of latex film surfaces on the meso- and nanoscale

In this paper the effects of surface roughness and annealing temperature (T) of latex coating films on adhesion are discussed for the different stages of the film formation process. The surface free energy of latex films was assessed in terms of practical work of adhesion (W) (or adherence) using a custom-built adhesion-testing device (ATD), atomic force microscopy (AFM), and contact angle measurements. For preannealed latex films surface roughness averages (R_a) were determined from AFM height images and were related to the values of W obtained from ATD measurements at room temperature. The results obtained using these tests exhibiting surface behavior on different length scales indicate a dependence of the measured adhesion on surface roughness and temperature, as well as on the length scale of the measurements.First preannealed samples were studied, which were obtained by heat treatment above the respective glass transition temperatures (T_g). Increasing the temperature of preannealing resulted in a decrease of the adherence observed in ATD experiments at room temperature. However, on the nanoscale, using AFM, no significant variation of the adherence was observed. This observation can be explained by roughness arguments. Preannealing decreases roughness which results in lower adherence values measured by ATD while for essentially single asperity AFM experiments roughness has an insignificant effect. Specimens were also annealed over a constant period of time (90 min) at different temperatures. At the end of the heat treatment, adhesion was measured at the treatment temperature by ATD. The amplified effect of temperature observed in this case on adherence is attributed to the combination of roughness decrease and increasing test temperature. In a third set of experiments completely annealed samples were studied by ATD as well as by AFM as a function of temperature. With increasing T values ATD showed a decrease in adherence, which is attributed to a decreasing surface free energy of the annealed films at elevated T values. AFM, on the other hand, showed an opposite trend which is assigned to increasing penetration of the tip into the tip/wetting polymer samples versus increasing temperature. Finally, annealing isotherms as a function of time were investigated by ATD in situ at different temperatures. This last set of experiments allowed us to optimize annealing time and temperature to achieve complete curing.     

Probing buried carbon nanotubes within polymer-nanotube composite matrices by atomic force microscopy

Multi-walled carbon nanotubes (MW-CNT) inside a polyamide-6 (PA6)-MW-CNT composite were visualized by atomic force microscopy (i) in a field-assisted intermittent contact and (ii) in the tunneling (TUNA) mode. Individual buried MW-CNTs were clearly discerned within the PA6 matrix. An average diameter of 33 ± 5 nm of the MW-CNTs was determined based on field-assisted intermittent contact mode AFM images, which is consistent with the expected size of PA6-coated MW-CNTs. Single well dispersed MW-CNTs that are located in the sub-surface region of the composite were also observed in the TUNA mode. These new AFM approaches circumvent the tedious sample preparation based on ultramicrotoming required for high resolution electron microscopy studies to obtain “in-depth” morphological information and hence are expected to facilitate the analysis of CNT-based and other nanocomposites in the future.     

Vinylimidazole‐based asymmetric ion pair comonomers: Synthesis,polymerization studies and formation of ionically crosslinked PMMA

Vinylimidazole‐based asymmetric ion pair comonomers ( IPC s) which are free from nonpolymerizable counter ions have been synthesized, characterized and polymerized by free radical polymerization (FRP), atom transfer radical polymerization (ATRP), and reversible addition‐fragmentation chain transfer (RAFT) mediated polymerizations in solution and by dispersion polymerization in water. The asymmetric nature of IPC s is due to the fact that cationic component of these IPCs is derived from vinylimidazole (VIm) and anionic component is derived from either styrenesulfonate (SS) or 2‐acrylamido‐2‐methyl‐1‐propanesulfonate. Although under ATRP, conversions are either very low or negligible, FRP and RAFT produces polymers with high to moderate monomer conversions but with different solubility characteristics. This investigation provides insight to the polymerization behavior of each component of the asymmetric IPCs and also its effects on composition and solubility characteristics of the resulting polymers. The IPCs studied here are high temperature ionic liquid and thus the polymers synthesized from these IPCs are highly ionic in nature and possess very strong intermolecular interactions which makes some of these IPC based polymers completely insoluble in organic and aqueous solvents. This highly ionic interaction is exploited to synthesize ionically crosslinked PMMA. MMA on copolymerization with 5–6 mol % of IPC yielded copolymer which is insoluble in common organic solvents like THF, DMF, etc., unlike homo PMMA. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3260–3273     

Observations of crystallization and melting in poly(ethylene oxide)/poly(methyl methacrylate) blends by hot-stage atomic-force microscopy

The binary blend of poly(ethylene oxide)/atactic poly(methyl methacrylate) is examined using hot-stage atomic-force microscopy (AFM) in conjunction with differential scanning calorimetry and optical microscopy. It was found possible to follow in real time the melting process, which reveals itself to be nonuniform. This effect is ascribed to the presence of lamellae having different thicknesses. The crystallization process of poly(ethylene oxide) from the miscible melt is also followed in real time by AFM, affording detailed images of the impingement of adjacent spherulites and direct observation of lamellar growth and subsequent polymer solidification in the interlamellar space.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2643–2651, 1998     

Growth of individual hydrogen-bonded nanostructures on gold monolayers

The growth of individual nanometer-sized (3.4 +/- 1.4 nm) hydrogen bonded assemblies 1(2) x (DEB)6 on gold monolayers was achieved through an exchange reaction between single isolated calix[4]arene dimelamine 2 (1.1 +/- 0.2 nm) embedded in hexanethiol monolayers and double rosette hydrogen bonded assembly 1(3) x (DEB)6 in solution. The growth process was monitored by tapping mode atomic force microscopy (TM-AFM).     

Force spectroscopy of quadruple H-bonded dimers by AFM: dynamic bond rupture and molecular time-temperature superposition

We report on the application of the time-temperature superposition principle to supramolecular bond-rupture forces on the single-molecule level. The construction of force-loading rate master curves using atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) experiments carried out in situ at different temperatures allows one to extend the limited range of the experimentally accessible loading rates and hence to cross from thermodynamic nonequilibrium to quasi-equilibrium states. The approach is demonstrated for quadruple H-bonded ureido-4[1H]-pyrimidinone (UPy) moieties studied by variable-temperature SMFS in organic media. The unbinding forces of single quadruple H-bonding (UPy)2 complexes, which were identified based on a polymeric spacer strategy, were found to depend on the loading rate in the range of 5 nN/s to 500 nN/s at 301 K in hexadecane. By contrast, these rupture forces were independent of the loading rate from 5 to 200 nN/s at 330 K. These results indicate that the unbinding behavior of individual supramolecular complexes can be directly probed under both thermodynamic nonequilibrium and quasi-equilibrium conditions. On the basis of the time-temperature superposition principle, a master curve was constructed for a reference temperature of 301 K, and the crossover force (from loading-rate independent to -dependent regimes) was determined as approximately 145 pN (at a loading rate of approximately 5.6 nN/s). This approach significantly broadens the accessible loading-rate range and hence provides access to fine details of potential energy landscape of supramolecular complexes based on SMFS experiments.     

Polystyrene models. III. modeling backbone/side-group interactions by an ab initio study of 2,4-diphenylpentane

The Structure and the energy of the stable conformations of the two possible stereochemical configurations of 2,4-diphenylpentane are obtained using the ab initio molecular orbital theory. The objective was to mimic the possible structures and determine the corresponding energies of the dyads of syndiotactic and isotactic polystyrene and, consequently, to study the interactions between the phenyl groups. The results of complete geometry optimization showed significant changes in geometrical parameters compared with those expected from the ideal hydrocarbon structure. The steric strain is most pronounced in some of the gauche conformations where large (approximately 40°) distortions of the backbone torsional angles and/or simultaneous phenyl group rotations in the range of 30°–40° away from its global minimum position may occur. In addition to the discussion of the geometrical parameters, the corresponding dipole moments are also calculated and differences related to the various structures and discussed. © 1993 John Wiley & Sons. Inc.