Slide-Set-5-Surface_Properties

Slide-Set-5-Surface_Properties - Structure-Property &...

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Unformatted text preview: Structure-Property & Processing Behavior of Hydrogels Lectures 5: Surface Properties Lectures Dr. Anthony Brennan University of Florida Tel: 352.392.6281 Email: [email protected] Plastics Design EMA 4760 - University of Florida Copyright Protected 1 Lectures 5 Lectures Learning Objectives Learning • Surface Energy – Theory – Planar Surfaces • Young’s Equation • Wetting/Dewetting • Osmotic pressure – Theory – Non-Planar Surfaces • Wenzel Wetting/Dewetting – Cos*θ • Cassie Baxter Wetting/Dewetting • Predicting wetting Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 2 Background • Adhesion – the molecular attraction Adhesion exerted between the surfaces of bodies in contact in • Molecular Interactions – Dispersion – Hydrogen bonding – Polar – Ionic – Acid-base Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 3 Mechanisms of Adhesion • Classical “Wetting Theory” ∆ GSL = γ SL − γ S − γ LV = −WSL Vapor Liquid ∆GSL > 0 • where ∆GSL = Change in Gibbs Free Energy for Spreading = Interfacial Surface Energies γ SL , γ LV γ S = Surface Energy of Solid (vacuum) - WSL = Work of Adhesion Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics Vapor Liquid ∆GSL < 0 4 Assumptions • Vapor molecules are adsorbed by surface γ S = γ SV + π e π e = spreading pressure • Defined as: p π e = γ S − γ SV = RT ∫ Γd ln p P • For high energy liquid on low energy solid γ s − γ SV ≈ 0 V Spontaneous spreading occurs γ LV ≤ γ SV + γ SL Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 5 Young’s Equation γ LV cos θ = γ SV − γ SL Sessile Drop Vapor Liquid • Assumes θ Homologous series of liquids Solid No swelling of substrate Low vapor loss Captive Air ‘Molecularly’ smooth surface Solid No impurities Controlled drop size θ – – – – – – Vapor Liquid Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 6 Determining Solid Surface Energies • Zisman Method – Modification of Young’s Modification Eqn Eqn cos θ = 1 + b(γ LV − γ C ) γ C = Critical Surface Tension – Assumes Assumes homologous series of n-alkanes of – low energy solids – Characteristic of Characteristic liquids not solid liquids • Good and Girifalcos – Paired interactions – sum of series γ12 = γ1 + γ 2 − 2φSL (γ1 / γ 2 )1/ 2 Interfacial Free Energy Interaction parameter • Combine w/ Zisman cos θ = −1 + 2φSL (γ 1 / γ 2 )1/ 2 πe = 0 Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 7 Fowkes Equation of State • For hydrocarbons on high energy solids: γ LV ≈ γ d LV γ S ≈ γ LV + γ SL + π e • Thus γ d s (π = c + 2γ LV ) 4γ LV • Consider: – The conformations of a polymer chain from the The liquid to the solid… liquid Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 8 Polymer Adsorption to a surface Polymer chain in solution – thermodynamic equilibrium Polymer chain on surface – thermodynamic equilibrium Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 9 Polymer Chain Conformations Solvated Tethered Adsorbed Train Loop Bridging Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 10 Polymer Chain Conformations Relating to Pressure • Given: Given: π e = spreading pressure – One can equate to vapor pressure and thus a One relationship could exist for osmotic pressure relationship • Given: Given: π 1 B′V10 = + c + 22 RTc2 M 2M • By inspection one can show: B′V10 1 N Au B= = 2 2 M2 u = volume of a chain N A = Avogadro's number Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 11 Polymer Chain Conformations 2 1 V2 • Examining chemical potential: B = − χ 2 2 V1M • Which by comparison gives: ( ) 2 1 − χ V22 2 u= N AV1 u = excluded volume for a chain χ = Flory - Huggins Interaction Parameter or alternatively: • Thus: 3 4 u = π rg 2 3 γ sd can be evaluated in terms of molar mass and configuration Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 12 Polymer Chain Conformations Polymer Alternative Approach Alternative • Consider: – Reptation Theory – Kramer deGennes Brown • For an adsorbed linear chain: La = thickness ≈ aN 2 5 a = monomer or repeat unit size N = number of repeat units • For a grafted brush copolymer: D ( D) Lad ≈ aN a 2 3 D ~ avg. separation distance of chains Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 13 Background Summary • Adhesion is dependent upon a sum of Adhesion all interfacial free energies • The interfacial free energy should scale The with molar mass with • The interfacial free energy should scale The with chain configuration/conformation with • Polymer–polymer solubility contributes Polymer–polymer to γsd through Fowkes equation of State Plastics Design EMA 4760 - University of Florida Copyright Protected Plastics 14 What role does surface topography have on surface energy? energy? Wenzel Theory Cassie-Baxter Theory “Super” or “Ultra” Properties Plastics Design EMA 4760 - University of Florida Copyright Protected 15 Wenzel Model Wenzel θ∗ Vapor Wenzel Model for 5x5x5 Channels (0°) Solid 1 cos θ * Liquid 0 (180°) -1 cos θ * = r cos θ • Determined Determined empirically empirically • r = roughness factor roughness which is Actual/Planar Area Actual/Planar • Assumes contact line Assumes is unconstrained in the radial direction the -1 (180°) 0 cos θ •Problem – what happens when texture is Problem sufficiently rough to give |cos θ *| > 1? Plastics Design EMA 4760 - University of Florida Copyright Protected 1 (0°) 16 Quéré Model Qu dx Vapor Quere Model for 5x5x5 Channels (0°) • Energy Balance: dF = φ s(γ SL-γ SV)dx dF + (1-φ s) γ LVdx + γ LVcos θ * dx cos cos θ ∗ Solid • (180°) Neglects line tension Neglects (assumes no meniscus between features) features) 1 en ze lR eg im e θ∗ 0 W Liquid -1 -1 (180°) 0 cos θ 1 (0°) Texture can impart ultrahydrophobic (cos θ * = φ scos θ – 1) or ultrahydrophobic cos ultrahydrophilic (cos θ * = φ scos θ + 1 – φ s) behavior ultrahydrophilic cos Plastics Design EMA 4760 - University of Florida Copyright Protected 17 Surface Energy Surface Smooth Surfaces • Based on Young’s Based Equation: Equation: Determined by a Zisman plot: Zisman Plot for PDMS 1.0 y = -0.0221x + 1.4313 R2 = 0.9814 Cos Theta 0.7 0.4 0.1 -0.2 -0.5 0.0 20.0 40.0 60.0 80.0 Surface Energy (mN/m) Plastics Design EMA 4760 - University of Florida Copyright Protected 18 Surface Energy Textured Surfaces Surface • Young’s Equation does not apply • Wenzel’s model assumes linear relationship Wenzel’s between contact angle and surface tension still exists, but slope is different: exists, – cos θ = r (γ SV – γ SL) / γ LV cos – surface energy is different from smooth • Quéré’s model does not follow a linear Quéré’s relationship: relationship: – cos θ = [φ s (γ SV – γ SL) / γ LV] + 1 - φ s cos – absolute surface energy is same as smooth, although in the Wenzel regime the apparent surface energy is different (ie the apparent energy exerted by the surface depends on the fluid in contact with it) Copyright Protected fluid Plastics Design EMA 4760 - University of Florida 19 References References • Introduction to Physical Polymer Science, 4th Edition, Lesley Introduction H. Sperling, Wiley Interscience (2006) ISBN 13-978-0-471H. 70606-9 70606-9 • Principles of Polymer Chemistry, P.J. Flory (1953) Cornell Principles University Press, Inc., New York. University • The Physics of Polymers, Gert Strobl (1996) Springer-Verlag, The New York. New • Some figures were reproduced from referenced literature Plastics Design EMA 4760 - University of Florida Copyright Protected 20 ...
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This note was uploaded on 09/18/2011 for the course EMA 4760 taught by Professor Staff during the Spring '10 term at University of Florida.

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