Polymer Reorganization Research Articles

Adhesion and Contact Aging of Acrylic Pressure-Sensitive Adhesives to Swollen Elastomers.

Fluid-infused (or swollen) elastomers are known for their antiadhesive properties. The presence of excess fluid at their surface is the main contributor to limiting contact formation and minimizing adhesion. Despite their potential, the mechanisms for adhesion and contact aging to fluid-infused elastomers are poorly understood beyond contact with a few materials (ice, biofilms, glass). This study reports on adhesion to a model fluid-infused elastomer, poly(dimethylsiloxane) (PDMS), swollen with silicone oil. The effects of oil saturation, contact time, and the opposing surface are investigated. Specifically, adhesion to two different adherents with comparable surface energies but drastically different mechanical properties is investigated: a glass surface and a soft viscoelastic acrylic pressure-sensitive adhesive film (PSA, modulus ∼25 kPa). Adhesion between the PSA and swollen PDMS [with 23% (w/w) silicone oil] retains up to 60% of its value compared to contact with unswollen (dry) PDMS. In contrast, adhesion to glass nearly vanishes in contact with the same swollen elastomer. Adhesion to the PSA also displays stronger contact aging than adhesion to glass. Contact aging with the PSA is comparable for dry and unsaturated PDMS. Moreover, load relaxation when the PSA is in contact with the PDMS does not correlate with contact aging for contact with the dry or unsaturated elastomer, suggesting that contact aging is likely caused by chain interpenetration and polymer reorganization within the contact region. Closer to full saturation of the PDMS with oil, adhesion to the PSA decreases significantly and shows a delay in the onset of contact aging that is weakly correlated to the poroelastic relaxation of the elastomer. Additional confocal imaging suggests that the presence of a layer of fluid trapped at the interface between the two solids could explain the delayed (and limited) contact aging to the oil-saturated PDMS.

Unraveling the Microstructure-Related Device Stability for Polymer Solar Cells Based on Nonfullerene Small-Molecular Acceptors.

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long-term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state-of-the-art polymer donors with a commonly used nonfullerene acceptor (NFA), IT-4F, are investigated. The active-layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion-limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene-based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA-based solar cells, which may be the key tactics to develop next-generation OSCs with high efficiency as well as excellent stability.

Mutations on FtsZ lateral helix H3 that disrupt cell viability hamper reorganization of polymers on lipid surfaces

FtsZ filaments localize at the middle of the bacterial cell and participate in the formation of a contractile ring responsible for cell division. Previous studies demonstrated that the highly conserved negative charge of glutamate 83 and the positive charge of arginine 85 located in the lateral helix H3 bend of Escherichia coli FtsZ are required for in vivo cell division. In order to understand how these lateral mutations impair the formation of a contractile ring,we extend previous in vitro characterization of these mutants in solution to study their behavior on lipid modified surfaces. We study their interaction with ZipAand look at their reorganization on the surface. We found that the dynamic bundling capacity of the mutant proteins is deficient, and this impairment increases the more the composition and spatial arrangement of the reconstituted system resembles the situation inside the cell: mutant proteins completely fail to reorganize to form higher order aggregates when bound to an E.coli lipid surface through oriented ZipA.We conclude that these surface lateral point mutations affect the dynamic reorganization of FtsZ filaments into bundles on the cell membrane, suggesting that this event is relevant for generating force and completing bacterial division.

Fast Water Diffusion and Long-Term Polymer Reorganization during Nafion Membrane Hydration Evidenced by Time-Resolved Small-Angle Neutron Scattering.

We report a small-angle neutron scattering study of liquid water sorption in Nafion membranes. The swelling of hydrophilic domains was measured on the nanoscale by combining in situ time-resolved and long-term static experiments, yielding kinetic curves recorded over an unprecedented time scale, from hundreds of milliseconds to several years. At low water content, typically below 5 water molecules per ionic group, a limited subdiffusive regime was observed and ascribed to nanoconfinement and local interactions between charged species and water molecules. Further ultrafast and thermally activated swelling due to massive liquid water sorption was observed and analyzed by using Fick's equation. The extracted mutual water diffusion coefficients are in good agreement with pulsed field gradient NMR self-diffusion coefficient values, evidencing a water diffusion-driven process due to concentration gradients within the Nafion membrane. Finally, after completion of the ultrafast regime, the kinetic swelling curves exhibit a remarkable long-term behavior scaling as the logarithm of time, showing that the polymer membrane can continuously accommodate additional water molecules upon hydration stress. The present nanoscale kinetics results provide insights into the vapor-versus-liquid sorption mechanisms, the nanostructure of Nafion, and the role of polymer reorganization modes, highlighting that the membrane can never reach a steady state.

The influence of interfaces and water uptake on the dielectric response of epoxy-cubic boron nitride composites

In this study, epoxy-cubic boron nitride composites are fabricated, and their dielectric response is investigated. They exhibit the same trend as epoxy composites reinforced with other filler types. Thus, at low filler concentrations, they exhibit a lower relative permittivity than neat epoxy. As the fill grade further increases, the relative permittivity of the composites increases as well. It is suggested that two competing mechanisms, namely polymer re-organization and water uptake, mainly determine the dielectric response of epoxy composites. The former is related to the decrease while the latter to the increase of the relative permittivity. The experimental results clearly verify that water uptake strongly depends on the filler concentration and not only affects the dielectric response of epoxy composites but also it is one of the main mechanisms which dictate their dielectric behavior.

The role of particle distribution in the dielectric response of epoxy–boron nitride nanocomposites

In this study, three different techniques are used for the production of epoxy-boron nitride nanocomposites in an attempt to evaluate the role of particle dispersion and distribution in the dielectric response of nanocomposites. Although the produced nanocomposites have a different particle distribution which is evaluated with a scanning electron microscopy analysis, they exhibit the same trend and similar dielectric response. The slight differences between the samples are plausibly caused by the different amount of water content rather than the particle distribution. The presence of water inside the samples is evaluated with a thermogravimetric analysis including also an analysis with a mass spectrometer. Finally, it is proposed that the dielectric response of the nanocomposites is mainly dictated by two competing mechanisms, namely polymer re-organization and water uptake, and it is suggested that high levels of reproducibility can be achieved when the particles are effectively dispersed in the polymer matrix regardless the particle distribution.

A unified model for the permittivity and thermal conductivity of epoxy based composites

In this paper, three-dimensional models are presented that were developed for both the relative permittivity and the thermal conductivity of composites based on epoxy in an attempt to understand the way in which modified fillers interact with epoxy. Various ceramics were used as fillers, namely magnesium oxide, aluminium nitride, aluminium oxide, hexagonal boron nitride and cubic boron nitride with an average particle size between 22 and 150 nm. The concept of the models is mainly based on two competing mechanisms, i.e., polymer re-organization and water uptake. These were realized in the models by assuming two interfacial layers around the particles, an inner and an outer layer. The former refers to water uptake and the latter demonstrates the influence of the matrix by the presence of surface modified particles and exhibits different properties than both the filler and polymer matrix. The thickness of the inner layer is constant while the outer layer thickness has a dynamic behaviour which strongly depends on the interparticle distance and thus, on the filler content. The same assumptions were made for both models and an accurate fit was achieved between the experimental data and model results.

Wash–dry cycle induced changes in low‐ordered parts of regenerated cellulosic fibers

AbstractThree regenerated cellulosic fiber types: lyocell, viscose, and modal were subjected to repetitive wet–dry treatments. Simulated treatments showed reorganization of the internal fiber structure which could be determined by accessibility studies. The reduction in liquid water retention capacity was found to be greater for lyocell than that for modal and viscose, sorption of iodine, and water vapor reduced for all studied fibers. The wet–dry treatment did not have influence on chemical reactivity of cellulosic fibers characterized by complexation of iron under highly alkaline conditions. The effect of wet–dry treatment on the tenacity, elongation at break, abrasion resistance, and molecular weight distribution of fibers was also explored in this study. The reduced strength in treated specimens was not accompanied by changes in molecular weight distributions. Based on these results, the changes observed in wet–dry‐treated specimens were observed mainly owing to polymer reorganization in amorphous parts of the fibers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Synthesis and in situ core reorganization of smart polymers

A smart poly(ethylene oxide) polymer containing two thermally reversible groups located at the chain center (F2-PEO) was synthesized by a maleimide–furan (MF) [4 + 2] Diels–Alder (DA) reaction between one bis[(furan-2-yl)methyl] terephthalate (F2) and two maleimide-terminated poly(ethylene glycol) methyl ethers (MI-PEOs). MF adducts underwent retro-DA (rDA) addition at a higher temperature, and the maleimide–anthracene (MA) addition reaction could occur simultaneously. Therefore, when F2-PEO and bis((anthracen-10-yl)methyl) succinate (AN2) were refluxed in toluene for 48 h, the cores of F2-PEOs were replaced with the MA adducts in situ to give new polymers AN2-PEOs. Moreover, when F2-PEOs were mixed with the molecules containing four anthracene units (AN4), the rDA reaction of the MF adducts and the MA addition reactions occurred simultaneously, generating four-arm star PEOs. The core reorganization of polymers has been confirmed by SEC, UV, 1H NMR, and MALDI-TOF mass spectra.

Characterization of Polymer Movement in Fiber Welded Cellulose Composites

Several examples of fiber welding were characterized by visible, fluorescent, and scanning electron microscopies to evaluate polymer reorganization. Distinct morphologies were observed that are process dependent. The characterization of process controlled sample morphology is the basis for understanding observed bulk physical properties such as tensile strength and the modulus.

PH-Induced Antireflection Coatings Derived From Hydrogen-Bonding-Directed Multilayer Films

Hydrogen-bonding-directed layer-by-layer assembled films, based on polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles and poly(4-vinylpyridine) (P4VP), were successfully fabricated in methanol. Varying the PAA content in the PS-b-PAA micelles afforded control over the film growth properties, especially the multilayer film thickness. Interestingly, antireflection films with refractive indices that could be tuned between 1.58 and 1.28 were obtained by treatment with an aqueous HCl solution (pH 2.27), and the transmittance obtained was as high as 98.4%. In acid solution, the pyridine group was protonated, destroying the hydrogen bonding between P4VP and PAA. A concomitant pH-induced polymer reorganization in the multilayers resulted in a porous honeycomb-like texture on the substrate.

Dynamic surface tension studies on poly( N-vinylcaprolactam/ N-vinylpyrrolidone/ N, N-dimethylaminoethyl methacrylate) at the air–liquid interface

The adsorption of poly( N-vinylcaprolactam/ N-vinylpyrrolidone/ N, N-dimethylaminoethyl methacrylate), a commercial product with the trade name of Gaffix ® VC-713 and currently used as a kinetic inhibitor for natural gas hydrates, has been experimentally studied at the air–liquid interface using surface tension measurements. The influence of polymer concentration and temperature on the adsorption of the polymer was analysed. The equilibrium experimental data obtained at both 293 K and 278 K has been fitted using the analogous surface equation of state for the Langmuir isotherm. Dynamic adsorption has been modelled using both diffusion-controlled and asymptotic models. The values of the surface tension and the surface excess at saturation showed that the polymer was very active at the air–liquid interface. Equilibrium surface tension indicated that temperature has a weak influence on the number of moles of Gaffix ® VC-713 adsorbed on the interface. The dynamic adsorption was controlled by diffusion process initially, in which a rapid reduction in surface tension was observed. It was then followed by a slow polymer reorganization process. A strong dependence of the dynamic adsorption on both the temperature and the polymer concentrations was well demonstrated.

Stiff polymer adsorption. Onset to pattern recognition

We present the results of extensive off-lattice Monte-Carlo simulations of a stiff polymer chain adsorbing onto a sticky periodic stripe-like pattern of variable width. We have analyzed, in terms of the chain length and rigidity, the adsorption and the pattern recognition process as a function of the stripe width. We have seen that this process is twofold: (i) the chain adsorbs rather isotropically onto the surface at a characteristic temperature T c and (ii) a further reduction in the temperature is needed for the chain to reorganize and adjust to the specific pattern. Such polymer reorganization has been studied through the evaluation of the chain degree of stretching and asphericity. We have found an optimal stripe width that maximizes the stretching. We have introduced a criteria to estimate the characteristic temperature at which the pattern recognition takes place T r< T c and we have studied its dependence with the chain and surface relevant parameters.

動的共有結合を利用した高分子の再編成

動的共有結合を利用した高分子の再編成

Polymer electrolytes and polyelectrolytes: Monte Carlo simulations of thermal effects on conduction

Monte Carlo calculations were carried out to simulate ion diffusion through polymer matrices. A dynamic bond percolation (DBP) model was employed that includes local harmonic motion of covalently bound anions in polyelectrolyte systems. The temperature dependence of cation diffusion was investigated in polyelectrolytes and polymer–salt complexes for 0–100 °C. Systems in which the rate of polymer reorganization is independent of temperature display Arrhenius behavior both above and below the T g of 35 °C. Systems in which the temperature is coupled to the rate of polymer reorganization display VTF behavior above the T g and near Arrhenius behavior below the T g. In all cases, the temperature is coupled to the rate of successful ion jumps. Temperature and T g seem to have no effect on the ion density at which the cation conductivity reaches a maximum.

Electron transfer in catecholamines - tetrapyridine-iron(III) complex ions anchored to polypeptides

[Fe(tetpy)(OH) 2] + complex ions (tetpy = 2,2′,2′',2‴-tetrapyridyl) anchored to poly(L-glutamate) (FeL) or poly(D-glutamate) (FeD) through the γ-carboxylate groups of the polymers [1] exhibit stereoselective peroxidatic activity with a number of optically active substrates, according to the reaction (pH = 7.0, Tris buffer 0.05 M) [2, 3]: ▪ Progressive binding of complex ions was found to determine a coil-to-α-helix transition in the charged polypeptide matrices, as well as aggregation phenomena with a freezing of iron molecules inbetween helical chains. Under these conditions, electron transfer from catecholamines (L-adrenalin and L-dopa) to the central iron(III) ion, which is rate-determining, proceeds stereoselectively because extensive and possibly specific interactions between substrate molecules and the peptidic residues in the close environment of the active sites different steric constraints for the two diastereomeric precursor complexes. The rate constants of electron transfer processes can be expressed by nuclear and electronic factors which are highly sensitive to the separation of the redox centers [4]. Therefore, even small differences in steric hindrances between the diastereo-isomers would affect differently the optimal mutual orientation of the reacting OH group of the substrates and the peripheral tetrapyridyl ligand of the active sites, whose π-system very likely acts as an electron-transfer agent [5]. Preliminary results on conformational analysis of substrate-catalyst adducts support such conclusions. This is reflected in the rate constants of the electron-transfer step, as illustrated by the Lineweaver-Burk plot of the reaction catalyzed by FeL or FeD systems at a complex to polymer-residue ratio [C]/[P] = 0.20, reported in Fig. 1. From the results, the turnover numbers for the oxidation of L-adrenaline are 0.56±0.07 and 2.04±0.23 min −1, respectively, whereas those for the oxidation of L-dopa are 0.42±0.05 and 1.14±0.12 ▪ min −1 (25.9 °C). On the other hand, the Michaelis constants are K M = (1.30±0.18) × 10 −3 and (1.05±0.15) × 10 −3 M in the former case, and (8.0±1.12) × 10 −4 and (5.50±0.48) × 10 −4 M in the latter. According to Marcus theory [6], the ultimate rate of electron transfer depends on the rate of reorganization of the surrounding medium (precursor and successor complex formation). When the asymmetric polymers play an active role in the reaction, being involved in the formation of the precursor complex the polypeptide itself might experience local conformational changes. Since the time scale of these rearrangements is unknown, it is possible that the polymer reorganization retards the overall rate of the electron transfer process. This could explain the finding that stereoselectivity occurs at the expense of the efficiency of reaction, as shown in Fig. 2. Using catalysts at low [C]/[P] ratio, the active sites are exposed to the bulk solvent and a substrate-coordinated metal chelate forms without any ▪ assistance from the coiled polypeptides [3]. This accounts for the lack of stereoselectivity in the oxidation reaction, which takes place at much higher rate however (Fig. 2).