Techniques for Polymer Modification Behzad Pourabbas Sahand University of Technology Tabriz-Iran pourabas@sut.ac.ir

Syllabuses Surfaces and Interfaces Molecular Interactions Thermodynamics of Surfaces and Interfaces Characterization Methods of Surfaces Reaction On Polymers Polymer Degradation Biological Modification of Polymer Surfaces Plasma Modification of Surfaces Surfactant-Polymer Surfaces Syllabuses

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Surfaces and Interfaces Behzad Pourabbas Sahand University of Technology Tabriz-Iran

God made solids, but surfaces were the work of the devil ------Wolfgang Pauli

Surfaces to Ponder www.stocksurfaces.com. http://strangepaths.com/ http://plus.maths.org http://www.physik.uni-marburg.de Surfaces to Ponder http://www.physics.upenn.edu

Overview Importance of surfaces What is a surface? Surface structure Surface processes Surface interfaces Surfaces in nature Measuring surfaces Modifying surfaces Overview

Importance of Surfaces Materials Touch on Surfaces Catalysts act from surfaces Biological reactions (life) occur on the surfaces On the surfaces: Tribology - friction, lubrication and wear Most metals are weak on the surfaces (corrosion) Importance of Surfaces

Different material create surfaces which are interfaces indeed: Solid / air Solid / liquid Solid / solid Liquid / air Liquid / liquid Liquid / solid Molecules and colloids / particles have surfaces, surface charges, etc. This is what drives proteins to spontaneously fold (surface energy with water) Surfaces Defined

Surfaces and Phases Surface has an Energy: Free energy must be minimized Energy drives most surface reactions Passivation Oxidation Adsorption of hydrocarbon and water Reconstruction and reorientation Surfaces and Phases

Water Phase Diagram http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

CO2 Phase Diagram http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

Heterogeneous Surface Structure Surface formation at different length scales: Diffusion Layers http://www.uni-regensburg.de/Fakultaeten/nat_Fak_I/Mat8/lst/spp/projectSPP1095solidification.html

Real Surfaces Explained Interfaces: Discontinuities Bonds: Dangling bonds, attractive / repulsive forces, unit cell cleavage planes Electron scattering: Surfaces can scatter electrons Failure starts on the surfaces: Cracks have surfaces: cohesive / adhesive failures Real Surfaces Explained

On Very Important surface: Silicon Surface Planes Model of the ideal surface for Si{111}1x1. The open and closed circles represent Si atoms in the first and second layers, respectively. Closed squares are fourth- layer atoms exposed to the surface though the double double-layer mesh. The dashed lines indicated the surface 1x1 unit-cell. http://www.matscieng.sunysb.edu/leed/trunc.html

Silicon Surface viewed by STM Scanning tunnelling microscope image of a Si surface, ~0.3° off (100) orientation showing the type A steps (Si dimers parallel to steps) and type B steps (Si dimers perpendicular to steps). Uppermost part of the surface is at lower right, with downward tilt to upper left. Scale is ~110 nm square (Prof. Max Lagally). http://www.chm.ulaval.ca/chm10139/

Surface Processes Passivation Reconstruction Oxide formation Adventitious carbon Reconstruction Crystalline Polymer orientation Adsorption of gases and water vapor Both can lead to surface passivation Surface Processes

Surface Free Energy Free energy at the surface. The excess energy is called surface free energy and can be quantified as a measurement of energy/area. It is also possible to describe this situation as having a line tension or surface tension which is quantified as a force/length measurement. Surface tension can also be said to be a measurement of the cohesive energy present at an interface. The common units for surface tension are dynes/cm or mN/m. Solids may also have a surface free energy at their interfaces but direct measurement of its value is not possible through techniques used for liquids. Surface Free Energy

Polar liquids, such as water, have strong intermolecular interactions and thus high surface tensions. Any factor which decreases the strength of this interaction will lower surface tension. Thus an increase in the temperature of this system will lower surface tension. Any contamination, especially by surfactants, will lower surface tension. http://www.ksvinc.com/surface_tension.htm Surface Free Energy

The unfavorable contribution to the total (surface) free energy may be minimized in several ways: By reducing the amount of surface area exposed – this is most common / fastest By predominantly exposing surface planes which have a low surface free energy By altering the local surface atomic geometry in a way which reduces the surface free energy Surface Energetics

Surface Tension http://www.sciencekids.co.nz/ http://hyperphysics.phy-astr.gsu.edu/ http://www.sciencekids.co.nz/ Surface Tension

The molecules in a liquid have a certain degree of attraction to each other. The degree of this attraction, also called cohesion, is dependent on the properties of the substance. The interactions of a molecule in the bulk of a liquid are balanced by an equally attractive force in all directions. The molecules on the surface of a liquid experience an imbalance of forces i.e. a molecule at the air/water interface has a larger attraction towards the liquid phase than towards the air or gas phase. Therefore, there will be a net attractive force towards the bulk and the air/water interface will spontaneously minimize its area and contract. Surface Tension http://www.ksvinc.com/LB.htm

The storage of energy at the surface of liquids The storage of energy at the surface of liquids. Surface tension has units of erg cm-2 or dyne cm-1. It arises because atoms on the surface are missing bonds. Energy is released when bonds are formed, so the most stable low energy configuration has the fewest missing bonds. Surface tension therefore tries to minimize the surface area, resulting in liquids forming spherical droplets and allowing insects to walk on the surface without sinking.                                                              Surface Tension http://scienceworld.wolfram.com/physics/SurfaceTension.html

Surface Tension in Action http://www.chem.ufl.edu/~itl/2045/lectures/lec_f.html

Molecular adsorption to Surfaces? There are two principal modes of adsorption of molecules on surfaces: Physical adsorption ( Physisorption ) Chemical adsorption ( Chemisorption ) The basis of distinction is the nature of the bonding between the molecule and the surface. With: Physical adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character. http://www.chem.qmul.ac.uk/surfaces/scc/

Adsorption / Self Assembly Processes on Surfaces Physisorption Physical bonds Chemisorption Chemical bonds Self-Assembled Monolayers (SAMs) Alkane thiols on solid gold surfaces Self assembly of monolayers

Chemi / Physi - Adsorption The graph above shows the PE curves due to physisorption and chemisorption separately - in practice, the PE curve for any real molecule capable of undergoing chemisorption is best described by a combination of the two curves, with a curve crossing at the point at which chemisorption forces begin to dominate over those arising from physisorption alone. The minimum energy pathway obtained by combining the two PE curves is now highlighted in red. Any perturbation of the combined PE curve from the original, separate curves is most likely to be evident close to the highlighted crossing point. http://www.chem.qmul.ac.uk/surfaces/scc/scat2_4.htm

Structure of Polymeric Surfaces AFM of a thin film of a block copolymer - a molecule with a long section that can crystallise (polyethylene oxide), attached to a shorter length of a non-crystallisable material (poly-vinyl pyridine). What you can see is a crystal growing from a screw dislocation. The steps have a thickness of a single molecule folded up a few times. http://www.nanofolio.org/images/gallery01/

Structure of Polymeric Surfaces Atomic force microscopes are ideal for visualizing the surface texture of polymer materials. In comparison to a scanning electron microscope, no coating is required for an AFM. Images A, B, and C are of a soft polymer material and were measured with close contact mode. Field of view: 4.85  µm × 4.85 µm http://www.pacificnanotech.com/polymers_single.html

Polymer Surface Orientation AFM of polymer surface showing molecular orientation. Note the change in scale of the scanning measurement. Polymers can ‘reorient’ over time to reduce surface energy (like a self-assembly process) http://www.msmacrosystem.nl/3Dsurf/Shots/screenShots.htm

Ozone Treated Polypropylene Ozone treated polypropylene showing the affect of energetic oxygen etching of the polymer, and loss of fine structural filaments. AFM images and force measurements show increase in surface energy, as well as an increase in surface ordering of the filaments. http://publish.uwo.ca/~hnie/sc2k.html

~99% of living organisms live in the top 1cm of the ocean Every interface has two surfaces Solid / air Solid / liquid Solid / solid Liquid / air Liquid / liquid Liquid / solid Interesting things happen at interfaces! Like the start of life! ~99% of living organisms live in the top 1cm of the ocean Surface Interfaces

Forces at Interfaces Van Der Val's forces Surface tension Interfacial bonding Hydrophobic / hydrophilic interactions Surface reconstruction / reorientation Driven by, or are part of ‘excess surface free energy’ which must be minimized. Forces at Interfaces

Importance of Interfaces Chemical reactions occur at interfaces Particularly corrosion Scattering energy Electrons Light Phonons An interface is actually two surfaces Importance of Interfaces

Defects at Interfaces Missing atoms Extra atoms Dangling bonds Defects and holes Extra atoms Surface segregation Dangling bonds Disrupted electronic properties Dimensional issues Lattice mismatch / shelves Defects at Interfaces

Cohesive Failure Material A Material B Material fails cohesively within B Material B Cohesive Failure

Adhesive Failure Material A Material fails adhesively between A and B Material B Adhesive Failure

Adhesive Failure (Craze) Schematic representation of the structure at the crack tip in a crazing material are shown at three length scales. It is assumed that only material A crazes. The whole of the craze consists of lain and cross-tie fibrils. Adhesive Failure (Craze) http://www.azom.com/details.asp?ArticleID=2089

Surface Reactions Oxidation Surface diffusion Diffusion and oxidation Adventitious carbon bonding Hydrocarbons from the atmosphere Surface rearrangement Polymers may reorient to minimize energy Surface Reactions

A Typical Surface Hydrocarbons and water rapidly adsorb to a metal or Solid material like silicon or aluminum Oxide layer of about 15 to 20 Angstroms Hydrocarbon layer of about 15 to 20 Angstroms Hydrocarbons and water rapidly adsorb to a metal or Silicon surface. Oxides form to a thickness of about 15 To 20 Angstroms, and hydrocarbons to a similar thickness. This is part of the normal surface passivation process. A Typical Surface

Langmuir-Blodgett Films Definition of LB films History and development Construction with LB films Building simple LB SAMs Nano applications of LB films Surface derivatized nanoparticles Functionalized coatings in LB films Langmuir-Blodgett Films

Langmuir-Blodgett Films A Langmuir-Blodgett film contains of one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. A monolayer is added with each immersion or emersion step, thus films with very accurate thickness can be formed. Langmuir Blodgett films are named after Irving Langmuir and Katherine Blodgett, who invented this technique. An alternative technique of creating single monolayers on surfaces is that of self-assembled monolayers. Retrieved from "http://en.wikipedia.org/wiki/Langmuir-Blodgett_film" Langmuir-Blodgett Films

Langmuir-Blodgett Films Deposition of Langmuir-Blodgett molecular assemblies of lipids on solid substrates. http://www.ksvltd.com/pix/keywords_html_m4b17b42d.jpg http://www.bio21.bas.bg/ibf/PhysChem_dept.html Langmuir-Blodgett Films

Self-assembly is the fundamental principle which generates structural organization on all scales from molecules to galaxies. It is defined as reversible processes in which pre-existing parts or disordered components of a preexisting system form structures of patterns. Self- assembly can be classified as either static or dynamic. http://en.wikipedia.org/wiki/Self-assembly Self Assembly

Molecular Self-Assembly Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly, although in some books and articles the term self-assembly refers only to intermolecular self-assembly. Intramolecular self-assembling molecules are often complex polymers with the ability to assemble from the random coil conformation into a well-defined stable structure (secondary and tertiary structure). An example of intramolecular self-assembly is protein folding. Intermolecular self-assembly is the ability of molecules to form supramolecular assemblies (quarternary structure). A simple example is the formation of a micelle by surfactant molecules in solution. http://en.wikipedia.org/wiki/Self-assembly

Self Assembled Monolayers SAMs – Self Assembled Monolayers Alkane Thiol complexes on gold C10 or longer, functionalized end groups Can build multilayer / complex structures Used for creating biosensors Link bioactive molecules into a scaffold The first cells on earth formed from SAMs

The Self-Assembly Process A schematic of SAM (n-alkanethiol CH3(CH2)nSH molecules) formation on a Au(111) sample. The self-assembly process. An n-alkane thiol is added to an ethanol solution (0.001 M). A gold (111) surface is immersed in the solution and the self-assembled structure rapidly evolves. A properly assembled monolayer on gold (111) typically exhibits a lattice. The Self-Assembly Process

SAM Technology Platform SAM reagents are used for electrochemical, optical and other detection systems. Self-Assembled Monolayers (SAMs) are unidirectional layers formed on a solid surface by spontaneous organization of molecules. Using functionally derivatized C10 monolayer, surfaces can be prepared with active chemistry for binding analytes. http://www.dojindo.com/sam/SAM.html

SAM Surface Derivatization Biomolecules (green) functionalized with biotin groups (red) can be selectively immobilized onto a gold surface using a streptavidin linker (blue) bound to a mixed biotinylated thiol / ethylene glycol thiol self-assembled monolayer. http://www.chm.ulaval.ca/chm10139/peter/figures4.doc

SAMs C10 Imaging with AFM http://sibener-group.uchicago.edu/has/sam2.html

Multilayer LB Film Process Smart Materials for Biosensing Devices – Cell Mimicking Supramolecular Assemblies and Colorimetric Detection of Pathogenic Agents

Surface Contamination All surfaces become contaminated! It is a form of ‘passivation’ Oxidation of metals Adventitious hydrocarbons Chemisorption of ions It can happen very rapidly And be very difficult to remove Surface Contamination

Measuring Surfaces AFM – Atomic Force Microscopy SEM – Scanning Electron Microscopy XPS (ESCA) – X-Ray Photoelectron Spectroscopy AES – Auger Electron Spectroscopy SSIMS – Static Secondary Ion Mass Spectroscopy Laser interferometry / Profilometry Measuring Surfaces

XPS/AES Analysis Volume

Surface Analysis Tools SSX-100 ESCA on the left, Auger Spectrometer on the right

XPS Spectrum of Carbon XPS can determine the types of carbon present by shifts in the binding energy of the C(1s) peak. These data show three primary types of carbon present in PET. These are C- C, C-O, and O-C=O

Surface Treatments Control friction, lubrication, and wear Improve corrosion resistance (passivation) Change physical property, e.g., conductivity, resistivity, and reflection Alter dimension (flatten, smooth, etc.) Vary appearance, e.g., color and roughness Reduce cost (replace bulk material) Surface Treatments

Surface Treatment of NiTi Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray

Surface Treatment of NiTi Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray

Surface Treatment of NiTi XPS spectra of the Ni(2p) and Ti(2p) signals from Nitinol undergoing surface treatments show removal of surface Ni from electropolish, and oxidation of Ni from chemical and plasma etch. Mechanical etch enhances surface Ni. Biomedical Devices and Biomedical Implants – SJSU Guna Selvaduray

Thermal Spray Coating Photomicrographs Plasma Spray Chromium Oxide Coatings Plasma Sprayed Chromium Oxide Coatings with base coatings of Hastelloy C for use in very corrosive environments

Thermal Spray Coating Photomicrographs Plasma Spray Chromium Oxide Coatings Plasma Sprayed Chromium Oxide Coatings with base coatings of Hastelloy C for use in very corrosive environments

Surface Derivatization                                                     A functionalized gold surface contains a polar amino tail, imparting a hydrophilic character compared to the straight chain alkane thiol. This is an example of a SAM http://www.dojindo.com/sam/SAM.html

Snow Cleaning with CO2 http://www.co2clean.com/polymers.html

Surfaces in Nature Cell membranes Skin (ectoderm) Lungs Self-assembled phospholipid bilayers Proteins add functionality to the membrane Skin (ectoderm) Lungs Exchange of O2, CO2, and water vapor Cell surface recognition (m-proteins) Major histocompatibility complex Surfaces in Nature

Molecular Self Assembly 3D diagram of a lipid bilayer membrane - water molecules not represented for clarity http://www.shu.ac.uk/schools/research/mri/model/micelles/micelles.htm Different lipid model top : multi-particles lipid molecule bottom: single-particle lipid molecule

Cell Membranes http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/default.htm

Summary Surfaces are discontinuities Surface area creates energy Dangling bonds lead to passivation Interfaces are critical to ‘bonding’ Surfaces can be modified / derivatized Surfaces are critical to life All important things happen at a surface! Summary

References http://www.eaglabs.com/ http://www.ksvinc.com/LB.htm http://www.dojindo.com/sam/SAM.html http://www.co2clean.com/clnmech.htm http://en.wikipedia.org/wiki/Self-assembly http://www.azom.com/default.asp SJSU Biomedical Materials Program References