Wetting of polymers by their solvents (original) (raw)
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Glass Transition Accelerates the Spreading of Polar Solvents on a Soluble Polymer
Physical Review Letters, 2014
We study the wetting of polymer layers by polar solvents. As previously observed, when a droplet of solvent spreads, both its contact angle and velocity decrease with time as a result of solvent transfers from the droplet to the substrate. We show that, when the polymer is initially glassy, the angle decreases steeply for a given value of the velocity, Ug. We demonstrate that those variations result from a plasticization, i.e. a glass transition, undergone by the polymer layer during spreading, owing to the increase of its solvent content. By analyzing previous predictions on the wetting of rigid and soft viscoelastic substrates, we relate Ug to the viscosity of the polymer gel close to glass transition. Finally, we derive an analytical prediction for Ug, based on existing predictions for the water transfer from the droplet to the substrate. Using polar solvents of different natures, we show that the experimental data compares well to the predicted expression for Ug.
Langmuir, 2013
The wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm ϕ(a w), where ϕ is the volume fraction of water in the polymer and a w is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet ϕ at distances as small as 5 μm, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length ξ below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Sof t Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation−condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation−condensation process at small distances. We demonstrate why it is not modified by the second process.
Wetting Behavior of Polymer Droplets: Effects of Droplet Size and Chain Length
Macromolecules
Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adsorbed at solid surfaces as a function of the number of chains making up the droplets and varying droplet sizes. The wetting behavior is quantified via the ratio of the perpendicular to the parallel component of the effective radii of gyration of the droplets and is analyzed further in terms of the adsorption behavior of the polymer chains and the monomers that constitute the droplets. This analysis is complemented by an account of the shape of the droplets in terms of the principal moments of the radius of gyration tensor. Single-chain droplets are found to lie flatter and wet the substrate more than chemically identical multi-chain droplets, which attain a more globular shape and wet the substrate less. The simulation findings are in good agreement with atomic force microscopy (AFM) experiments. The present investigation illustrates a marked dependence of wetting and adsorption on certain structural arrangements and propose this dependence as a technique through which polymer wetting may be tuned.
The Journal of Chemical Physics, 1999
The behavior of both flexible and semiflexible polymers near a liquid-liquid interface is investigated with the aid of the self-consistent-field theory as developed by Scheutjens and Fleer. A ternary system (A/B N /C) is studied near the wetting transition. In a symmetric system, i.e., AB ϭ BC ϭ, a change in the interaction parameter introduces a wetting transition. The ratio of the interfacial width of the binary A/C system and the coil size of the polymer determines the order of this transition. Beyond a certain chain length N c ͑at fixed ͒ the wetting transition is of first order, whereas it is of second order for NϽN c. The characteristics of the prewetting line, including the prewetting critical point, are discussed in some detail. The nontrivial N-dependence of the position of this critical point is analyzed in terms of a crude thermodynamic model. For a semiflexible polymer an increase of the chain stiffness at a certain value of is sufficient to introduce a wetting transition.
Static wetting behaviour of diblock copolymers
Journal De Physique Ii, 1993
~ Thin liquid films of ordered dihlock copolymers deposited on a solid substrate form a multilayer stacking parallel to the solid surface. A multilayer with a finite extend can he stable, metastable, or unstable, depending on thc relative values of the surface ener&ics of the various interfaces. The spreading parameter and chemical potential of a n-layer are derived, and used for classifying all possible situations. It is shown that only monoand hilayers can be siahle, and that non-wetting multilayers are subjected to a long-time piling up instability, leading in practice to the formation of characteristic riggourat-like Structures.
Wetting of crystalline polymer surfaces: A molecular dynamics simulation
The Journal of Chemical Physics, 1995
Molecular dynamics has been used to study the wetting of model polymer surfaces, the crystal surfaces of polyethylene ͑PE͒, poly͑tetrafluoroethylene͒ ͑PTFE͒, and poly͑ethylene terephthalate͒ ͑PET͒ by water and methylene iodide. In the simulation a liquid droplet is placed on a model surface and constant temperature, rigid body molecular dynamics is carried out while the model surface is kept fixed. A generally defined microscopic contact angle between a liquid droplet and a solid surface is quantitatively calculated from the volume of the droplet and the interfacial area between the droplet and the surface. The simulation results agree with the trend in experimental data for both water and methylene iodide. The shape of the droplets on the surface is analyzed and no obvious anisotropy of the droplets is seen in the surface plane, even though the crystal surfaces are highly oriented. The surface free energies of the model polymer surfaces are estimated from their contact angles with the two different liquid droplets.
Polymer Patterns in Evaporating Droplets on Dissolving Substrates
Langmuir, 2004
Self-organized polymer patterns resulting from the evaporation of an organic solvent drop on a soluble layer of polymer are investigated. The patterns can be modulated by changing the rate of evaporation and also the rate of substrate dissolution controlled by its solubility. Both of these affect the contact zone motion and its instabilities, leading to spatially variable rates of substrate etching and redeposition that result from a complex interplay of several factors such as Rayleigh-Benard cells, thermocapillary flow, solutal Marangoni flow, flow due to differential evaporation, osmotic-pressure-induced flow, and contact-line pinning-depinning events. The most complex novel pattern, observed at relatively low rates of evaporation, medium solubility, and without macroscopic contact-line stick-slip, consists of a regularly undulating ring made up of a bundle of parallel spaghetti-like threads or striations and radially oriented fingerlike ridges. Increased rate of evaporation obliterates the polymer threads, producing more densely packed fingers and widely separated multiple rings due to a frequent macroscopic pinning-depinning of the contact line. Near-equilibrium conditions such as slow evaporation or increased solubility of the substrate engender a wider and less undulating single ring.
Evolution and disappearance of solvent drops on miscible polymer subphases
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018
† Equal contribution their drop shapes obey the Augmented Young-Laplace Equation. Over time, the miscible interface diffuses and the shape of the drop evolves. We place 2-microliter drops of water atop miscible poly(acrylamide) solutions. The drop is completely wetted by the subphase, and then remains detectable beneath the surface for many minutes. An initial effective interfacial tension can be approximated to be on the order of 0.5 mN/m using the capillary number. Water and poly(acrylamide) are completely miscible in all concentrations, and yet, when viewed from the side, the drop maintains a capillary shape. Study of this behavior is important to the understanding of effective interfaces between miscible polymer phases, which are pervasive in nature.
A review of polymer dissolution
Polymer dissolution in solvents is an important area of interest in polymer science and engineering because of its many applications in industry such as microlithography, membrane science, plastics recycling, and drug delivery. Unlike nonpolymeric materials, polymers do not dissolve instantaneously, and the dissolution is controlled by either the disentanglement of the polymer chains or by the diffusion of the chains through a boundary layer adjacent to the polymer-solvent interface. This review provides a general overview of several aspects of the dissolution of amorphous polymers and is divided into four sections which highlight (1) experimentally observed dissolution phenomena and mechanisms reported to this date, (2) solubility behavior of polymers and their solvents, (3) models used to interpret and understand polymer dissolution, and (4) techniques used to characterize the dissolution process. q