Lubrication by Polyelectrolyte Brushes

We develop a scaling model relating the friction forces between two polyelectrolyte brushes sliding over each other to the separation between grafted surfaces, number of monomers and charges per chain, grafting density of chains, and solvent quality. We demonstrate that the lateral force between brushes increases upon compression, but to a lesser extent than the normal force. The shear stress at larger separations is due to solvent slip layer friction. The thickness of this slip layer sharply decreases at distances on the order of undeformed brush thickness. The corresponding effective viscosity of the layer sharply increases from the solvent viscosity to a much higher value, but this increase is smaller than the jump of the normal force resulting in the drop of the friction coefficient. At stronger compression we predict the second sharp increase of the shear stress corresponding to interpenetration of the chains from the opposite brushes. In this regime the velocity-dependent friction coefficient between two partially interpenetrating polyelectrolyte brushes does not depend on the distance between substrates because both normal and shear forces are reciprocally proportional to the plate separation. Although lateral forces between polyelectrolyte brushes are larger than between bare surfaces, the enhancement of normal forces between opposing polyelectrolyte brushes with respect to normal forces between bare charged surfaces is much stronger resulting in lower friction coefficient. The model quantitatively demonstrates how polyelectrolyte brushes provide more effective lubrication than bare charged surfaces or neutral brushes

Ionic strength dependence of polyelectrolyte brush thickness

Intramolecular repulsion between charged monomers imposes intrinsic tension in polyion, modifies its elasticity, and changes the dependence of polyelectrolyte brush thickness on salt concentration. We demonstrate that intrinsic polyion tension gives rise to a plateau of the brush thickness in the middle of salt dominated regime and to a subsequent steeper brush contraction with increasing salt concentration

Self-Consistent Field Theory of Brushes of Neutral Water-Soluble Polymers

The Self-Consistent Field theory of brushes of neutral water-soluble polymers described by two-state models is formulated in terms of the effective Flory interaction parameter $\chi_{eff}(T,\phi)$ that depends on both temperature, T and the monomer volume fraction, $\phi$. The concentration profiles, distribution of free ends and compression force profiles are obtained in the presence and in the absence of a vertical phase separation. A vertical phase separation within the layer leads to a distinctive compression force profile and a minimum in the plot of the moments of the concentration profile vs. the grafting density. The analysis is applied explicitly to the Karalstrom model. The relevance to brushes of Poly(N-isopropylacrylamide)(PNIPAM) is discussed.Comment: Accepted for publication in the Journal of Chemical Physic

Tension Amplification in Molecular Brushes in Solutions and on Substrates †

Molecular bottle-brushes are highly branched macromolecules with side chains densely grafted to a long polymer backbone. The brush-like architecture allows focusing of the side-chain tension to the backbone and its amplification from the picoNewton to nanoNewton range. The backbone tension depends on the overall molecular conformation and the surrounding environment. Here we study the relation between the tension and conformation of the molecular brushes in solutions, melts, and on substrates. In solutions, we find that the backbone tension in dense brushes with side chains attached to every backbone monomer is on the order of f0N3/8 in athermal solvents, f0N1/3 in θ-solvents, and f0 in poor solvents and melts, where N is the degree of polymerization of side chains, f0≃ kBT/b is the maximum tension in side chains, b is the Kuhn length, kB is Boltzmann constant, and T is absolute temperature. Depending on the side chain length and solvent quality, molecular brushes in solutions develop tension on the order of 10–100 picoNewtons, which is sufficient to break hydrogen bonds. Significant amplification of tension occurs upon adsorption of brushes onto a substrate. On a strongly attractive substrate, maximum tension in the brush backbone is ~ f0N, reaching values on the order of several nanoNewtons which exceed the strength of a typical covalent bond. At low grafting density and high spreading parameter the cross-sectional profile of adsorbed molecular brush is approximately rectangular with thicknes ~bA/S, where A is the Hamaker constant and S is the spreading parameter. At a very high spreading parameter (S > A), the brush thickness saturates at monolayer ~ b. At a low spreading parameter, the cross-sectional profile of adsorbed molecular brush has triangular tent-like shape. In the cross-over between these two opposite cases, covering a wide range of parameter space, the adsorbed molecular brush consists of two layers. Side chains in the lower layer gain surface energy due to the direct interaction with the substrate, while the second layer spreads on the top of the first layer. Scaling theory predicts that this second layer has a triangular cross-section with width R ~ N3/5 and height h ~ N2/5. Using self-consistent field theory we calculate the cap profile y (x) = h (1 − x2/R2)2, where x is the transverse distance from the backbone. The predicted cap shape is in excellent agreement with both computer simulation and experiment

Surface patterning of nanoparticles with polymer patches

Patterning of colloidal particles with chemically or topographically distinct surface domains (patches) has attracted intense research interest1–3. Surface-patterned particles act as colloidal analogues of atoms and molecules4,5, serve as model systems in studies of phase transitions in liquid systems6, behave as ‘colloidal surfactants’7 and function as templates for the synthesis of hybrid particles8. The generation of micrometre- and submicrometre-sized patchy colloids is now efficient9–11, but surface patterning of inorganic colloidal nanoparticles with dimensions of the order of tens of nanometres is uncommon. Such nanoparticles exhibit size- and shape-dependent optical, electronic and magnetic properties, and their assemblies show new collective properties12. At present, nanoparticle patterning is limited to the generation of two-patch nanoparticles13–15, and nanoparticles with surface ripples16 or a ‘raspberry’ surface morphology17. Here we demonstrate nanoparticle surface patterning, which utilizes thermodynamically driven segregation of polymer ligands from a uniform polymer brush into surface-pinned micelles following a change in solvent quality. Patch formation is reversible but can be permanently preserved using a photocrosslinking step. The methodology offers the ability to control the dimensions of patches, their spatial distribution and the number of patches per nanoparticle, in agreement with a theoretical model. The versatility of the strategy is demonstrated by patterning nanoparticles with different dimensions, shapes and compositions, tethered with various types of polymers and subjected to different external stimuli. These patchy nanocolloids have potential applications in fundamental research, the self-assembly of nanomaterials, diagnostics, sensing and colloidal stabilization

The present progressive tense in the self-commentary. Cognitive analysis.

В статье осмысляются видовременная форма Present Progressive Tense в рамках нового, холистического подхода к языку как когнитивной деятельности человека, ориентированной на максимально эффективную адаптацию к среде в ситуации коммуникативного цейтнота. Как следствие, контекст ситуации становится определяющим при изучении языка как системы символов. Источником знания, отражаемого языком, служит наблюдатель, а не говорящий. Поскольку знание, представленное в языке — это опосредованный или непосредственный чувственный опыт человека, то выбор коммуникантом определенной языковой формы обусловлен особенностями его восприятия среды. Показывается, что сокращенная форма Present Progressive Tense в контексте автокомментария отражает внутреннюю и внешнюю среду человека. Внутреннюю, поскольку сокращение глагола be приближает ее к форме простого сказуемого. Внешнюю, т. к. причастие первое смыслового глагола передает восприятие собственных действий как конкретного процесса, развивающегося во . The Present Progressive Tense is analyzed in the frame of the holistic approach to the language as a cognitive activity of a person orienting to the efficient adaptation to the environment in the communicative time trouble. As a result, the context of the situation is crucial to the understanding of language as a system of symbols. The source of knowledge represented in language is not the speaker but the observer. The choice of a certain word by the observer is determined by their perception, because knowledge is the direct or indirect experience of a person. The reduced Present Progressive form in the self-commentary describes both inner and outer niche of the observer. The inner part because the reduction of the verb be makes it closer to the simple predicate. The outer one because the present participle of the main verb portrays perception of one’s own actions as a process

The Present Progressive Passive Tense. Cognitive Examination

В статье форма Present Progressive Passive осмысляется в пределах нового, холистического понимания языка — языка как варианта когнитивного поведения человека, направленного на эффективное приспособление к среде в ситуации коммуникативного цейтнота. Принятие языка как способа адаптации к среде в момент речи ведет к концентрации внимания на контексте ситуации, или речевом контексте, и особенностях восприятия мира человеком. Предполагается, что глагольная форма описывает сопоставление видимой части ситуации, т. е. непосредственно воспринимаемого в акте коммуникации события, с другой частью той же ситуации — событием, которое уже невидимо для наблюдателя. Показано, что эта форма отражает две стороны одной ситуации: непосредственно видимую и результат предшествующего события, который, как и само событие, выступает частью этой ситуации. При этом важно, что наблюдаемая часть идет первой, а результат за ним следует, а не наоборот. Результаты исследования могут применяться в когнитивной лингвистике, когнитивном анализе дискурса, литературной критике . In the article the verbal form Present Progressive Passive is examined within the new, holistic interpretation of language as a cognitive activity of a person aiming at the efficient adjustment to the environment in the time of the communicative time trouble. Understanding of language as the means of the adaptation at the moment of speech leads to focusing of attention on the context of the situation, the speech context, and perception of the reality of the person. It is suggested that the verbal form describes the comparison of the visible part of the situation, i. e. the event being experienced at the moment of speech, with the other part of the same situation that the observer does not see any more. It is shown that this form depicts two parts of the same situation. The part that is being watched and the result of the previous event that as the event itself is only the part of the larger situation. It is sufficient that the part that is being observed goes first and the result follows it, not vice versa. The result can be applied in cognitive linguistics, cognitive analysis of discourse, literary criticism, etc

Performative Verbs in English — The Holistic Approach

В статье перформативные глаголы осмысляются в пределах нового, холистического подхода к языку. Языку как когнитивной активности человека, сфокусированной на максимально эффективном приспособлении к среде в ситуации коммуникативного цейтнота. Поэтому контекст речевой ситуации делается определяющим при трактовке системы языковых символов. Знание, отраженное в языке, всегда исходит от наблюдателя, а не от говорящего. Выбор коммуникантом того или иного слова или формы зависит от его особенностей восприятия среды, так как знание, выраженное в языке, есть непосредственный или опосредованный опыт человека. Думается, что перформативные глаголы в английском языке передают восприятие мира человеком и описание им двух сфер своего опыта в условиях коммуникативного цейтнота. Основным методом исследования стал когнитивный анализ, предполагающий, что выбор говорящим формы глагола зависит от особенностей восприятия им как внешней среды, так и своих внутренних состояний и процессов. Показывается, что данные глаголы в силу того, что представляют собой полноценное действие, а не средство передачи значения, описывают две области опыта наблюдателя: внутреннюю и внешнюю среду. При этом семантика глагола не играет значимой роли. Полученные данные можно использовать не только в теоретических работах по биологии языка, когнитивной грамматике, когнитивному анализу дискурса, когнитивной социолингвистике, литературной критике, теории языковой личности и т. д., но и при обучении практическому английскому . This article interprets performative verbs within a new, holistic approach to the language as a person’s cognitive activity, focused on the most effective adaptation in a communicative situation with limited time. Therefore, the context of the situation becomes decisive when interpreting the system of linguistic symbols. Knowledge, reflected in the language, always comes from the observer, not from the speaker. The communicant’s choice of a particular word or form depends on his or her perception, since the knowledge, expressed in a language, is a person’s direct or indirect experience. It seems that the performative verbs in the English language convey a person’s perception of the world and the description of two areas of their experience in a communicative situation with limited time. The main method of research includes cognitive analysis, suggesting that a speaker’s choice of the verb form depends on his or her perception of both the external environment and the personal internal state and processes. This article shows that these verbs (which constitute a full-fledged action, not a means of transferring meaning) describe two areas of the observer’s experience: the internal and external environment. The semantics of the verb does not play an important role. The obtained data can be used in theoretical works on the biology of language, cognitive grammar, cognitive analysis of discourse, cognitive sociolinguistics, literary criticism, the theory of language personality among others, as well as in teaching English