Polymer adsorption
Encyclopedia
Adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

 is the adhesion of ions or molecules onto the surface of another phase. Adsorption may occur via physisorption
Physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption...

 and chemisorption
Chemisorption
Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...

. Ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s and molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s can adsorb to many types of surfaces including polymer surfaces. A polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 is a large molecule composed of repeating subunits bound together by covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

s. The adsorption of ions and molecules to polymer surfaces plays a role in many applications including: biomedical, structural, and coatings.

Polymer versus non-polymer surfaces

Polymer surfaces differ from non-polymer surfaces in that the subunits that make up the surface are covalently bonded to one another. Non-polymer surfaces can be bound by ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...

s, metallic bond
Metallic bond
Metallic bonding is the electrostatic attractive forces between the delocalized electrons, called conduction electrons, gathered in an "electron sea", and the positively charged metal ions...

s or intermolecular forces (IMFs)
Intermolecular force
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...

. In a two component system, non-polymer surfaces form when a positive net amount of energy is required to break self-interactions and form non-self-interactions. Therefore, the energy of mixing (ΔmixG) is positive. This amount of energy, as described by interfacial tension, varies for different combinations of materials. However, with polymer surfaces, the subunits are covalently bonded together and the bulk phase of the solid surface does not allow for surface tension to be measured directly. The intermolecular forces between the large polymer molecules are difficult to calculate and cannot be determined as easily as non-polymer surface molecular interactions. The covalently bonded subunits form a surface with differing properties as compared to non-polymer surfaces. Some examples of polymer surfaces include: polyvinyl chloride (PVC)
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

, nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

, polyethylene (PE)
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

, and polypropylene (PP)
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

. Polymer surfaces have been analyzed using a variety of techniques, including: scanning electron microscopy, scanning tunneling microscopy, and infrared spectroscopy.

Adsorption isotherms

The adsorption process can be characterized by determining what amount of the ions or molecules are adsorbed to the surface. This amount can be determined experimentally by the construction of an adsorption isotherm. An adsorption isotherm is a graph of Γ(P,T) versus partial pressure of the adsorbate(P/P0) for a given constant temperature, where Γ(P,T) is the number of molecules adsorbed per surface area. As the partial pressure of the adsorbate increases, the number of molecules per area also increases.

Contact angle and surface tension

Contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...

, the angle at which a liquid droplet meets at a solid surface, is another way to characterize polymer surfaces. Contact angle (θ) is a measure of the wetting
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

 ability of the liquid on a solid surface. Generally, due to low surface energy, liquids will not wet polymer surfaces and the contact angle will be greater than 90°. The liquid molecules are more attracted to other liquid molecules as compared to the polymer surface. Because the polymer surfaces are solid surfaces, surface tension cannot be measured in a traditional way such as using a Wilhelmy plate
Wilhelmy plate
A Wilhelmy plate is a thin plate that is used to measure equilibrium surface or interfacial tension at an air–liquid or liquid–liquid interface. In this method, the plate is oriented perpendicular to the interface, and the force exerted on it is measured...

. Instead, contact angles can be used to indirectly estimate the surface tension of polymer surfaces. This is accomplished by measuring the contact angles of a series of liquids on a polymer surface. A Fox and Zisman plot of cosθ versus surface tensions of the liquids(γL) gives a straight line which can be extrapolated back to determine the critical surface tension of the solid (γc).



Where:
is the contact angle is the surface tension of the liquid is the critical surface tension of the solid

The variable β was previously determined to be approximately 0.03 to 0.04. While the actual surface tension of the solid polymer surface cannot be determined, the Fox and Zisman plot serves as an estimate. However, this estimate may be skewed if there are significant intermolecular forces between the surface and the liquid. Also, this plot is not applicable for binary mixtures of liquids dropped onto a polymer surface. Some estimated surface tensions of different polymers and the contact angles of different liquids on polymer surfaces is shown below.
{| class="wikitable"

|-
! Polymer!! γc (mN/m)
|-
| Polystyrene (PS) || 40.7
|-
| Polyvinylacetate (PVA) || 36.5
|-
| TeflonTM || 20
|-
| Polymethylacrylic acid (PMAA) || 41
|-
| Polypropylene || 33
|-
| Silicone || 24
|-
| Polyimide || 40
|-
| Nylon 6/6 || 41
|}

Adsorption and charge relationship


Different polymer surfaces have different side chains on their monomers that can become charged due to the adsorption or dissociation of adsorbates. For example, polystyrene sulfonate
Sodium polystyrene sulfonate
Sodium polystyrene sulfonate is a type of polymer and ionomer based on polystyrene. It is the sodium salt of polystyrene sulfonic acid.-Chemical properties:...

 has monomers containing negatively charged side chains which can adsorb positively charged adsorbates. Polystyrene sulfonate will adsorb more negatively charged adsorbate than positively charged. Conversely, for a polymer that contains positively charged side chains, such as poly(diallyldimethylammonium chloride)
PolyDADMAC
Polydiallyldimethylammonium chloride, or shortened polyDADMAC is a homopolymer of diallyldimethylammonium chloride . The molecular weight of polyDADMAC is typically in the range of hundreds of thousands of grams per mole, and even up to a million for some products. PolyDADMAC is usually delivered...

, negatively charged adsorbates will be strongly attracted.

Contact angle

Because the ability of a surface to adsorb molecules onto its surface depends on energies of interaction, thermodynamics of adsorption can be used to understand the driving forces for adsorption. To measure the thermodynamics of polymer surfaces, contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...

s are often used to easily obtain useful information. The thermodynamic description of contact angles of a drop of liquid on a solid surface are derived from the equilibrium formed between the chemical potentials of the solid–liquid, solid–vapor, and liquid–vapor interfaces.
At equilibrium, the contact angle of a liquid drop on a surface does not change. Therefore, the Gibbs free energy is equal to 0:


The chemical potentials of the three interfaces must cancel out, producing Young's equation for the relationship between surface energies and contact angles:


where:is the surface tension of the liquid is the contact angle of the liquid is the surface tension of the solid–vapor interface is the surface tension of the solid–liquid interface is the vapor pressure of the liquid at equilibrium.

However, this equation cannot be used to determine the surface energy of a solid surface by itself. It can be used in conjunction with the following equation to determine the relationship between contact angle and surface energy of the solid, as surface tension ≈ surface energy for a solid:


where is the surface energy of the solid is the surface tension of the liquid. and are the dispersive and polar components of the surface energy of the solid

Using these two equations, the surface energy of a solid can be determined simply by measuring the contact angle of two different liquids of known surface tension on that solid's surface.

Copolymers

For heterogeneous surfaces (consisting of two or more different types of material), the contact angle of a drop of liquid at each point along the three phase contact line with a solid surface is a result of the surface tension of the surface at that point. For example, if the heterogeneous regions of the surface form very large domains , and the drop exists entirely within a homogeneous domain, then it will have a contact angle corresponding to the surface tension of that homogeneous region. Likewise, a drop that straddles two domains of differing surface tensions will have different contact angles along the three phase contact line corresponding to the different surface tensions at each point. However, with sufficiently small domains (such as in those of a block copolymer), the observed surface energy of the surface approaches the weighed average of the surface energies of each of the constituents of the surface:


where:
is the overall surface energy of the polymer is the fraction of the component of the polymer's surface is the surface energy of the component

This occurs because as the size of the homogeneous domains become very small compared to the size of the drop, the differences in contact angles along different homogeneous regions becomes indistinguishable from the average of the contact angles.

The observed contact angle is given by the following formula:


where:is the fraction of component is the contact angle component

If the polymer is made out of only two different monomers, it is possible use the above equation to determine the composition of the polymer simply by measuring the contact angle of of a drop of liquid placed on it:


where:
is the observed contact angle is the area fraction of one component, and the area fraction of the other. and are the contact angles of the first and second components of the polymer.

Surface Coverage

One of the defining features of polymer surfaces and coatings is the chemical regularity of the surface. While many materials can be irregular mixtures of different components, polymer surfaces tend to be chemically uniform, with the same distribution of different functional groups across all areas of the surface. Because of this, adsorption of molecules onto polymer surfaces can be easily modeled by the Langmuir or Frumkin Isotherms. The Langmuir equation states that for the adsorption of a molecule of adsorbate A onto a surface binding site S, a single binding site is used, and each free binding site is equally likely to accept a molecule of adsorbate:



where: is the adsorbate is the surface binding site is the bound adsorbate/binding site pair

The equilibrium constant for this reaction is then defined as:



The equilibrium constant is related to the equilibrium surface coverage , which is given by:



where: is the surface coverage (fraction, 0 is empty, 1 is fully covered) is the adsorption equilibrium constant

Adsorption energy

Because many polymers are composed of primarily of hydrocarbon chains with at most slightly polar functional groups, they tend to have low surface energies and thus adsorb rather poorly. While this can be advantageous for some applications, modification of polymer surfaces is crucial for many other applications in which adhering a substrate to its surface is vital for optimal performance. For example, many applications utilize polymers as structural components, but which degrade rapidly when exposed to weather or other sources of wear. Therefore, coatings must be used which protect the structural layer from damage. However, the poor adhesive properties of nonpolar polymers makes it difficult to adsorb the protective coating onto its surface. These types of problems make the measurement and control of surface energies important to development of useful technologies.

The Gibbs energy of adsorption, , can be determined from the adsorption equilibrium constant:


Because is negative for a spontaneous process and positive for a nonspontaneous process, it can be used to understand the tendency for different compounds to adsorb to a surface. In addition, it can be divided into a combination of two components:


which are the Gibbs energies of physisorption
Physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption...

 and chemisorption
Chemisorption
Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...

, respectively. Many polymer applications, such as those which use polytetrafluoroethylene (PTFE, or Teflon) require the use of a surface with specific physisorption properties toward one type of material, while being firmly adhered in place to a different type of material. Because the physisorption energy is so low for these types of materials, chemisorption is used to form covalent bonds between the polymer coating and the surface of the object (such as a pan) which holds it in place. Because the relative magnitudes of chemisorption processes are generally much greater than magnitudes of physisorption processes, this forms a strong bond between the polymer and the surface it is chemically adhered to, while allowing the polymer to retain its physisorption characteristics toward other materials.

Experimentally, the enthalpy and entropy of adsorption are often used to fine-tune the adsorption properties of a material. The enthalpy of adsorption can determined from constant pressure calorimetry:


where: is the heat exchanged is the integral molar enthalpy of adsorption is the number of moles adsorbed.

From the enthalpy of adsorption, the entropy of adsorption can be calculated:


where: is the integral molar entropy of adsorption is the temperature in Kelvin

Together, these are used to understand the driving forces behind adsorption processes.

Protein-resistant coatings

Protein adsorption influences the interactions that occur at the tissue-implant interface. Protein adsorption can lead to blood clots, the foreign-body response and ultimately the degradation of the device. In order to counter-act the effects of protein adsorption, implants are often coated with a polymer coating to decrease protein adsorption.

Polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...

 (PEG) coatings have been shown to minimize protein adsorption in the body. The PEG coating consists of hydrophilic molecules that are repulsive to protein adsorption. Proteins consist of hydrophobic molecules and charge sites that want to bind to other hydrophobic molecules and oppositely charged sites. By applying a thin monolayer coating of PEG, protein adsorption is prevented at the device site. Furthermore, the device’s resistance to protein adsorption, fibroblast adhesion and bacteria adhesion are increased.

Antithrombogenic coatings

The hemocompatability of a medical device is dependent upon surface charge, energy and topography. Devices that fail to be hemocompatabile run the risk of forming a thrombus, proliferation and compromising the immune system. Polymer coatings are applied to devices to increase their hemocompatability. Chemical cascades lead to the formation of fibrous clots. By choosing to use hydrophilic polymer coatings, protein adsorption decreases and the chance of negative interactions with the blood diminishes as well. One such polymer coating that increases hemocompatability is heparin
Heparin
Heparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...

. Heparin is a polymer coating that interacts with thrombin to prevent coagulation. Heparin has been shown to suppress platelet adhesion, complement activation and protein adsorption.

Advanced polymer composites

Advanced polymer composites
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

 are used in the strengthening and rehabilitation of old structures. These advanced composites can be made using many different methods including prepreg, resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

, infusion
Infusion
An infusion is the outcome of steeping plants with desired chemical compounds or flavors in water or oil.-History:The first recorded use of essential oils was in the 10th or 11th century by the Persian polymath Avicenna, possibly in The Canon of Medicine.-Preparation techniques:An infusion is very...

, filament winding and pultrusion
Pultrusion
Pultrusion is a continuous process for manufacture of composite materials with constant cross-section. Reinforced fibers are pulled through a resin, possibly followed by a separate preforming system, and into a heated die, where the resin undergoes polymerization...

. Advanced polymer composites are used in many airplane structures and their largest market is in aerospace and defense.

Fiber reinforced polymers

Fiber reinforced polymer(FRP) are commonly used by civil engineers in their structures. FRPs respond linear-elastically to axial stress, making them a great material to hold a load. FRPs are usually in a laminate
Laminate
A laminate is a material that can be constructed by uniting two or more layers of material together. The process of creating a laminate is lamination, which in common parlance refers to the placing of something between layers of plastic and gluing them with heat and/or pressure, usually with an...

 formation with each lamina having unidirectional fibers, typically carbon or glass, embedded within a layer of light polymer matrix material. FRPs have great resistance against environmental exposure and great durability.

Polytetrafluoroethylene

Polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....

(PTFE)is a polymer used in many applications including non-stick coatings, beauty products, and lubricants. PTFE is a hydrophobic molecule composed of carbon and fluorine. Carbon-fluorine bonds cause PTFE to be a low-friction material, conducive in high temperature environments and resistant to stress cracking. These properties cause PTFE to be non-reactive and used in a wide array of applications.
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