No abstract

Optical tweezers are employed to measure the forces of interaction within a single pair of DNAgrafted colloids in dependence of the molecular weight of the DNA-chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short-range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance dependence is analyzed by means of a theoretical treatment based on the compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.PACS numbers: 82.35. Rs, 82.70.Dd, 87.80.Cc Surface treatment of colloidal particles and the ensuing manipulation and control of their interaction properties is a topic of high and lasting interest, on the grounds of both technological relevance and fundamental importance. On the first count, the main issue pertains to the fact that surface treatment is necessary to achieve colloidal stabilization by inducing thereby a repulsive force that acts against the ubiquitous dispersion attractions between the colloids. Charge stabilization and steric stabilization, the latter being caused by grafted polymer chains, are the two most common mechanisms, whereas grafting of polyelectrolyte (PE) chains on a colloid provides a natural combination of both and results to an electrosteric repulsion. On the second count, surface treatment by polymer grafting provides the possibility to tune the effective colloid interaction by 'dressing' the hard sphere potential with a soft tail, whose range, strength and overall functional form can be controlled by changing the properties of the polymer brush, e.g., its grafting density, height or charge. Systems interacting by a combination of a hard sphere potential and a subsequent short-range repulsion show a tremendous variety in their equilibrium [1,2,3,4] and dynamical [5,6,7] properties.Considerable work has been carried out in the study of the so-called osmotic PE-brushes [8, 9, 10], which result for high surface grafting densities and are characterized by the fact that they spherically condense the vast majority of the counterions released by the chains. These, in turn, bring about an entropic effective force between the brushes, which has been quantitatively analyzed for PE-brushes [11] and stars [12]. On the other hand, little is known for the opposite case of low surface grafting density, for which the theoretical considerations that lead to the interaction between osmotic brushes break down. In this Letter, we investigate by a combination of sensitive and accurate experiments and theoretical analysis the effective forces between spherical DNA brushes and establish a novel mechanism of interaction between those, which results from the mutual compression of PEchains of the colloids against the surface of each other. The quantitative characteristics of the resulting forces are vastly different from those between osmotic brushes.The experimental inves...

This work reports a comparative study of the response of poly(2,3-dimethoxybenzyl methacrylate), poly(2,5-dimethoxybenzyl methacrylate), and poly(3,4-dimethoxybenzyl methacrylate) to electrical perturbation fields over wide frequency and temperature windows with the aim of investigating the influence of the location of the dimethoxy substituents in the phenyl moieties on the relaxation behavior of the polymers. The dielectric loss isotherms above T g exhibit a blurred relaxation resulting from the overlapping of secondary relaxations with the glass−rubber or α relaxation. At high temperatures and low frequencies, the α relaxation is hidden by the ionic conductive contribution to the dielectric loss. As usual, the real component of the complex dielectric permittivity in the frequency domain increases with decreasing frequency until a plateau is reached corresponding to the glass−rubber (α) relaxation. However, at high temperatures, the real permittivity starts to increase again with decreasing frequency until a second plateau is reached, a process that presumably reflects a distributed Maxwell−Wagner−Sillars relaxation or α′ absorption. The α and α′ processes appear respectively as asymmetric and symmetric relaxations in the loss electrical modulus isotherms in the frequency domain. To facilitate the deconvolution of the overlapping absorptions, the time retardation spectra of the polymers were computed from the complex dielectric permittivity in the frequency domain using linear programming regularization parameter techniques. The spectra exhibit three secondary absorptions named, in increasing order of time γ′, γ, and β followed by the α relaxation. At long times and well separated from the α absorption the α′ relaxation appears. The replacement of the hydrogen of the phenyl group in position 2 by the oxymethyl moiety enhances the dielectric activity of the poly(dimethoxybenzyl methacrylate)s. The temperature dependence of the relaxation times associated with the different relaxations is studied, and the molecular origin of the secondary relaxations is qualitatively discussed.

The relaxation behavior of poly(3-methylbenzyl methacrylate), poly(3-fluorobenzyl methacrylate), and poly(3-chlorobenzyl methacrylate) was thoroughly studied by broadband dielectric spectroscopy with the aim of investigating the influence of slight differences in chemical structure on the response of polymers to electric perturbation fields. Retardation spectra calculated from dielectric isotherms utilizing linear programming regularization parameter techniques were used to facilitate the deconvolution of strongly overlapped absorptions. Above the glass transition temperature, the spectra of the two halogenated polymers present a secondary γ process well separated from a prominent peak resulting from the overlapping of the α and β relaxations. The spectra of poly(3-methylbenzyl methacrylate) exhibit at long times a well-developed α absorption followed in decreasing order of time by two weak absorptions, named β and γ, whose intensities increase with temperature. The temperature dependence of the distance of the α peak from the β and γ peaks, expressed in terms of log(f max, β/f max, α) and log(f max, γ/f max, α), respectively, is studied. The Williams ansatz and the extended ansatz give a fairly good account of the relaxation behavior of the polymers. The stretch exponent associated with the α relaxation increases with temperature from ca. 0.2 at low temperatures to the vicinity of 0.5 at high temperatures. At low temperatures, the α relaxation is described by a Vogel-type equation, but at high temperature the β and α processes are roughly described by the same Arrhenius equation. In the whole temperature range, the activation energy o the γ relaxation is significantly lower than that of the β absorption. The mechanisms involved in the development of the secondary relaxations are qualitatively discussed.

A nematic comb-shaped copolymer and its nanocomposites containing 0.063−0.54 in vol % of silver nanoparticles were studied by broadband dielectric spectroscopy. The frequency dependence of specific alternating current (ac) conductivity was used to estimate the temperature-frequency intervals of charge transfer by long and short distances, respectively. With increasing the concentration of nanoparticles, specific ac conductivity increases. The concentration dependence of dielectric permittivity suggests that distribution of nanoparticles is homogeneous, and conducting channels are not formed. With increasing the concentration of silver nanoparticles, the glass transition temperature of the nanocomposites, described in terms of the strength/fragility concept, increases, whereas the strength parameter D decreases (i.e., “fragility” increases).

The relaxation behavior of poly(2,3-dichlorobenzyl methacrylate) is studied by broadband dielectric spectroscopy in the frequency range of 10(-1)-10(9) Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called beta process. At temperatures close to Tg, the dynamical alpha relaxation already overlaps with the beta process, the degree of overlapping increasing with temperature. The deconvolution of the alpha and beta relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the beta relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the alpha and beta peaks, expressed as log(fmax;beta/fmax;alpha) where fmax is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the a onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the alpha process, without beta assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds.

Water interacting with a polymer reveals a number of properties very different to bulk water. These interactions lead to the redistribution of hydrogen bonds in water. It results in modification of thermodynamic properties of water and the molecular dynamics of water. That kind of water is particularly well observable at temperatures below the freezing point of water, when the bulk water crystallizes. In this work, we determine the amount of water bound to the polymer and of the so-called pre-melting water in poly(vinyl methyl ether) hydrogels with the use of Raman spectroscopy, dielectric spectroscopy, and calorimetry. This analysis allows us to compare various physical properties of the bulk and the pre-melting water. We also postulate the molecular mechanism responsible for the pre-melting of part of water in poly(vinyl methyl ether) hydrogels. We suggest that above −60 °C, the first segmental motions of the polymer chain are activated, which trigger the process of the pre-melting.

This work describes the calculation of retardation time spectra by minimization of the square of the differences between experimental compliance results and those recalculated from the spectra. Spectra were computed taking analytical complex dielectric results as the basis of the minimization process. Comparison of the spectra computed from both the complex dielectric permittivity and the dielectric loss with those calculated analytically shows that minimization methods based on complex dielectric permittivity data are more accurate than those based only on loss dielectric results.