Emulsion Polymerization Of Styrene And Methyl Methacrylate, The Copolymerization Of Styrene And Methyl Methacrylate Their Sulfonation And Adsorption Studies On Each ~ Chemistry Hub

Emulsion Polymerization Of Styrene And Methyl Methacrylate, The Copolymerization Of Styrene And Methyl Methacrylate Their Sulfonation And Adsorption Studies On Each

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Abstract

Poly (styrene), poly (methyl methacrylate) and copolymer of styrene/ methyl methacrylate were prepared by batch emulsifier free emulsion polymerization. Sulfonated poly (styrene) and sulfonated copolymer of styrene/ methyl methacrylate was synthesized by direct sulfonation at 40^oC. These products were identified by using IR spectroscopy, Differential Scanning Calorimetery and from adsorption studies using UV/ VIS spectrophotometer. The samples act as an adsorbent and remove methyl red dye from its aqueous solution as a function of adsorbent dose and initial dye concentration. Sulfonated polymer of styrene and copolymer of styrene/ methyl methacrylate show maximum adsorption due to availability of more adsorption site. The studies show that sulfonated poly (styrene) and sulfonated copolymer of styrene/methyl methacrylate can best be used as an adsorbent for the removal of red dye from its aqueous solution as compared to poly (styrene) poly (methyl methacrylate) and copolymer of styrene/ methyl methacrylate.

Introduction

1.1 Polymers

A polymer ^[1] is a large molecule (macromolecule) composed of repeating structural units connected by covalent chemical bond. Well known example of polymer includes plastics, DNA and proteins. A simple example is polypropylene

1.2 Monomer

A monomer (from Greek mono “one" and meros “part”) is a small molecule that may become chemically bonded to other monomers to form a polymer^[2].

1.3 Types of Monomers

1.3.1 Synthetic monomers

Examples of monomers are hydrocarbons such as alkenes and arene homologous series. Here hydrocarbon monomers such as phenyl ethylene and ethene form polymer used as plastics like polyphenylethene (commonly known as polystyrene) and polyethene (commonly known as polyethylene or polythene). Other commercially important monomers include acrylic monomer such as acrylic acid, methyl methacrylate, and acrylamide ^[2].

1.3.2 Natural Monomers

Amino acids are natural monomers ^[2]and polymerize to form proteins glucose monomers can also polymerize to form starches, amylopectins and glycogens polymers. In this case the polymerization reaction is known as dehydration or condensation reaction (due to formation of water as one of products) where a hydrogen atom and a hydroxyl group are lost to form water and a oxygen molecule bonds between each monomer unite.

1.4 Polymerization

Polymerization is a process of reacting monomer molecule together in a chemical reaction to form three dimensional networks or polymer chains.^[2][3] There are many forms of polymerization and different system exist to categorize them.

The main categories are

· Addition polymerization

· Condensation polymerization

1.4.1 Addition polymerization

Chain-growth polymerization or addition polymerization involves the linking together of molecules incorporating double or triple chemical bonds. These unsaturated monomers (the identical molecules which make up the polymers) have extra internal bonds which are able to break and link up with other monomers to form the repeating chains. Addition polymerization is involved in the manufacture of polymers such as polyethylene, polypropylene polyvinyl chloride (PVC).

1.4.2 Condensation polymerization

Step growth polymers or condensation polymerization are defined as polymers formed by the stepwise reaction between functional groups of monomers. Condensation polymerization yields polymers with repeating units having fewer atoms than the monomers from which they are formed. This reaction generally involves the elimination of small molecules such as H2O or HCI.

1.5 Various Polymerization Techniques

There are different techniques of polymerization as

1. Bulk polymerization

2. Solution polymerization

3. Precipitation polymerization

4. Emulsion polymerization

The polymerization of vinyl monomers can be carried out by the following techniques in bulk, solution, suspension or emulsion. When a monomer is heated either in bulk or solution at higher conversions, the gel effect is observed. This effect is also known as Tromorsdorf effect, after the name of its discoverer.

In the radical chain polymerization of a vinyl monomer one would normally expect the reaction rate to fall with time since the monomer concentration decreases with conversion. However the exact opposite behavior is observed in many cases where the rate of polymerization increases with time. This is called a gel effect. To cite an example, pure methyl methacrylate shows dramatic auto acceleration in polymerization rate.

1.5.1 Bulk Polymerization

When monomer is polymerized in bulk, an initiator which can decompose to give free radicals at a fairly good rate at the rate of polymerization is used. There

The process can be carried out at comparatively low temperature poly (methyl are advantages as well as disadvantages of this process. This method is now used for casting objects to be preserved such as an insect, a flower or any other thing. Methacrylate) sheets and rods are made in this way. However, thermal control is difficult to achieve as also in any further isolation from the monomer.

Styrene can be polymerized by bulk or suspension polymerization. In the bulk polymerization, poly (styrene) is made by pre-polymerization in a mass to a thick consistency. It is fed to a vertical tower, polymerized and excluded at the bottom as a band, ground and packaged.

1.5.2 Solution Polymerization

In solution polymerization, on the other hand improved thermal control is possible but it is difficult to remove solvent. Further chain transfer to solvent may limit the molecular weight of the polymer obtained.

This limits the usefulness of the technique^[4].

1.5.3 Emulsion and Suspension polymerization

Emulsion and suspension polymerization in aqueous media are free from the problem of heat transfer and viscosity.

These two methods have assumed industrial importance. It is possible to get higher molecular weight with emulsion polymerization. In emulsion polymerization the monomer is emulsified in water with an emulsifying agent which may be a soap or detergent, usually a redox system is used. In a typical emulsion system a monomer, water, emulsifier, a chain transfer agent and water soluble initiator are usually used^[9].For suspension polymerization of styrene a reaction kettle with arrangement for heating, cooling, and vigorous stirring is used.

At the end of this type of polymerization poly(styrene) is obtained in the form of beads.

1.6 Emulsion polymerization

Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyl ethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Emulsion polymerization is used to manufacture several commercially important polymers. Many of these polymers are used as solid materials and must be isolated from the aqueous dispersion after polymerization. In other cases the dispersion itself is the end product. A dispersion resulting from emulsion polymerization is often called a latex (especially if derived from a synthetic rubber) or an emulsion. These emulsions find applications in adhesives, paints, paper coating and textile coatings. They are finding increasing acceptance and are preferred over solvent-based products in these applications as a result of their eco-friendly characteristics due to the absence of VOCs (Volatile Organic Compounds) in them.

Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern process control schemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.

Early SBR recipes are examples of true batch processes: all ingredients are added at the same time to the reactor. Semi-batch recipes usually include a programmed feed of monomer to the reactor. This enables a starve-fed reaction to insure a good distribution of monomers into the polymer backbone chain. Continuous processes have been used to manufacture various grades of synthetic rubber^[5].

1.6.1 Surfactant-free emulsion polymerization

The presence of surfactant is disadvantage for certain application of emulsion polymers such as those involving instrument calibration and pore size determination. The presence of adsorbed surfactant give rise to somewhat variable properties since the amount of adsorbed surfactant can vary with the polymerization and application condition. Removal of the surfactant either directly or by desorption, can lead to coagulation or flocculation of the destabilized latex. Surfactant-free emulsion polymerization, involving no added surfactant, is a usual approach to solving this problem. The process uses an initiator yielding initiator radicals that impart surface-active properties to the polymer particles. Persulfate is a useful initiator for this purpose. Latexes prepared by the surfactant-free technique are stabilized by chemically bound sulfate groups of the SO₄^-- initiating species derived from persulfate ion. A characteristic of surfactant-free emulsion polymerization is that the particle no. is generally lower by up to about 2 orders of magnitude compared to the typical emulsion polymerization, typically 10^12 versus 10^14 particles per milliliter. This is a consequence of the lower total particle surface area that can be stabilized by the sulfate groups alone relative to that when added surfactant is present. Another approach to producing latexes with chemically bound surface-active groups is to use a reactive surfactant; a surfactant with polymerizable double bond, such as sodium dodecyl allyl sulfosuccinate. Copolymerization of the reactive surfactant with the monomer of interest binds the surface active groups into the polymer chains^[5].

1.6.2 Advantages of emulsion polymerization

Advantages of emulsion polymerization include.^[5]

1. High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate.

2. The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate.

3. Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight.

4. The final product can be used as is and does not generally need to be altered or processed.

1.6.2 Disadvantages of emulsion polymerization

Disadvantages of emulsion polymerization include^[6]

1. Surfactants and other polymerization adjuvant remain in the polymer or are difficult to remove.

2. For dry (isolated) polymers, water removal is an energy-intensive process.

3. Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transferto polymer.

1.6.3 Initiators

Both thermal and redoxgeneration of free radicals have been used in emulsion polymerization. Per sulfate salts are commonly used in both initiationmodes. The per sulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation. Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agentsuch as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C. Although organic peroxides and hydro peroxides are used in emulsion polymerization, initiators are usually water solubleand partitioninto the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water soluble, but one component can be water-insoluble.

1.6.4 Applications

Polymers produced by emulsion polymerization can be divided into three rough categories.

Synthetic rubber

· Some grades of styrene-butadiene (SBR)

· Some grades of Polybutadiene

· Polychloroprene (Neoprene)

· Nitrile rubber

· Acrylic rubber

· Fluoroelastomer

Plastics

· Some grades of PVC

· Some grades of polystyrene

· Some grades of PMMA

· Acrylonitrile-butadiene-styrene terpolymer (ABS)

· Polyvinylidene fluoride

· PTFE

Dispersions (i.e. polymers sold as aqueous dispersions)

· polyvinyl acetate

· polyvinyl acetate copolymers

· latex acrylic paint

· Styrene-butadiene

· VAE (vinyl acetate - ethylene copolymers)

1.7 Copolymer

A heteropolymer or copolymer is a polymer derived from two (or more) monomer species, as opposed to a homopolymer where only one monomer is used.^[5] Copolymerization refers to methods used to chemically synthesize a copolymer.

Commercially relevant copolymers include ABS plastic, SBR, Nitrile rubber, styrene-acrylonitrile, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate.

Since a copolymer consists of at least two types of constitutional units (not structural units), copolymers can be classified based on how these units are arranged along the chain.^[7] These include:

· Alternating copolymers with regular alternating A and B units

· Periodic copolymers with A and B units arranged in a repeating sequence (e.g. (A-B-A-B- B-A-A-A-A-B-B-B)_n)

· Random copolymers (or Statistical Copolymers) with random sequences of monomer A and B

· Block copolymers comprise two or more homopolymer subunits linked by covalent bonds. The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a junction block. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.

· Graft copolymers are a special type of branched copolymer in which the side chains are structurally distinct from the main chain. However the individual chains of a graft copolymer may be homopolymer or copolymer. Different copolymer sequencing is sufficient to define a structural difference, thus an A-B diblock copolymer with A-B alternating copolymer side chains is properly called a graft copolymer.

Copolymers may also be described in terms of the existence of or arrangement of branches in the polymer structure. Linear copolymers consist of a single main chain whereas branched copolymers consist of a single main chain with one or more polymeric side chains. Other special types of branched copolymers include star copolymers, brush copolymers, and comb copolymers. A terpolymer is a copolymer consisting of three distinct monomers. The term is derived from ter (Latin), meaning thrice, and polymer.

1.8 Styrene-methyl methacrylate copolymers

Emulsifier- free emulsion polymerization, mainly with styrene has been widely used to produce sub micrometer monodisperse polymer microspheres with various functional groups. Many studies have been performed in order to understand the process of such polymerization and to monitor the type, the quantity and the distribution of surface functional groups. Such functional polymer microspheres were used as supporter.

1.8.1 Application of Polymer and Copolymer

Polymer of styrene, MMA and copolymer of (styrene-MMA) has found application as;

· Gel polymer electrolytes are prepared from copolymer (styrene-MMA) which exhibit a different range of mechanical and electrical properties^[9].

· Copolymer (styrene-MMA) is also used in preparation of bone cement which is further used to fix artificial prosthesis to bone structure^[10].

· Modified copolymer of styrene and MMA are used in membrane formation^[11].

1.9 Sulphonation

Sulphonation is the broad sense includes all methods of converting organic compounds to euphonic acid or containing or containing structural group CSO₂OH or in some cases N-SO2-OH or they may be defined as

Any of the several methods by which sulphonic acids are prepared^[12]

1.9.1 Sulphonic acid

Sulphonic acid are the organic acids containing sulphur and having the general formula RSO3H, where“ R” is either alkyl or aryl group ^[13]They are derivatives of sulphonic acids (HOSO2OH) in which an OH has been replaced by a carbon group as shown in the structure below

Most of the sulphonic acid are strongly acidic, water soluble, non volatile and hygroscopic, they do not act as oxidizing agent and are stable than carboxylic acid. Sulphonic acids rarely occur naturally, except Taurine(NH2CH2CSO2H) that occur in bile.

1.9.1.1 Aliphatic sulphonic Acid

Aliphatic sulfonic acid is in which alkyl group is present with sulfonic acid e.g CH₃SO₃H, C₂H₅SO₃H etc.

Aliphatic sulphonic acids are less important than aromatic sulphonic acid. They are prepared by;

· Oxidation of mercaptans, alkyl disulphide, dialkyl thiocyanate or alkyl sulphonic acids.

· Interaction of alkyl iodide and metallic sulphite.

· Action of sulfuryl chloride (SO2Cl2) with saturated hydrocarbon in the presence of light.

· Action of sodium bisulphate upon olefin in presence of oxygen.

1.9.1.2 Aromatic Sulphonic acid

Some typical aromatic sulphonic acid for example:

· Benzene sulfonic acid

· P. Toluene sulfonic acid

· O. Xylene sulfonic acid

· P. Xylene sulfonic acid

Are formed by reacting aromatic compounds with sulphonic acid. This process is called sulphonation ^[14].Many aromatic hydrocarbon have been sulphonated.

Sulphonation of aromatic compounds can be done by no. of reagents.

· Oleum^[15]

· SO3 in organic solvent^[6]

· Sulphuric acid

^·Fuming sulphuric acid^[17]

· Chloro sulphonic acid^[18]

· The dioxane adduct of SO3 or amine adduct of SO3^[19]

Use of these agents can cause problems. Sulphuric acid or fuming sulphuric acid causes the formation of by products. The adduct of SO3 which are less reactive sulphonation agents, although they produce little waste acid. Sulphonation of aromatic compounds is also carried out in ionic liquids^[20].

1.9.2 Methods for Sulphonation

Sulphonation of Grignard reagent and lithium reagent. Nucleophilic substitution with sodium sulphite. Sulphonation of aromatic compounds with sulphur trioxide dioxane complex.Trifluroacetic acid , sulphuric acid and sulphur trioxide in dichloromethane can react with aromatic compounds .An important aromatic sulphonic acid is “poly(Styrene) sulphonic acid ".It is important organic compound^[21-23].

1.9.3 Mechanism Of Sulphonation

Aromatic sulphonation is an organic reaction in which hydrogen atom or an arene is replaced by a sulphonic acid functional group in an electrophilic aromatic substitution.

Reagents used are benzene, H₂SO₄, (fuming H₂SO₄)

Electrophilic specie is SO₃H which can be formed by loss of water from the sulphonic acid . Unlike other electrophilic aromatic substitution reaction such as nitration ,sulphonation is reversible, complex and depends mainly on

1-The nature of electrophilic specie which is derived from sulphuric acid.

2-Reactivity of aromatic system under attack^[24]

Removals of water from the system favor the sulphonation product. Heating of sulphonic acid with aqueous H₂SO₄ can result be the reverse reaction that is desulphonation.

Sulphonation with fuming H₂SO₄ strongly favors the formation of sulphonic acid

1.9.4 Application of Sulphonated polymer and Copolymer

Sulfonated polymer of styrene, methyl methacrylate and sulfonated copolymer of (styrene-methyl methacrylate) has extensive applications as follows;

1-New proton exchange membrane, which is durable, low in cost and high in proton conductivity is prepared from sulphonated copolymer(styrene-MMA).

2-A novel anhydrous proton conducting polymer electrolyte based on polystyrene sulphonic acid was synthesized.

3-The use of hydrated polystyrene sulphonic acid and its composites in H2/O2 fuel cell is also found.

4-Sulphonated poylmer of styrene and MMA has good proton conductivity.mobilization of fine metal particles.

2.0 Adsorption

Adsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the adsorbent's surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process.

Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorptions (characteristic of covalent bonding)^[25].

2.1 Isotherms

Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials. The first mathematical fit to an isotherm was published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous adsorbates,

$\frac{x}{m}=kP^{\frac{1}{n}}$

where

x

is the quantity adsorbed,

m

is the mass of the adsorbent,

P

is the pressure of adsorbate and

k

and

n

are empirical constants for each adsorbent-adsorbate pair at a given temperature..

2.1.1 Langmuir

In 1916, Irving Langmuir published a new model isotherm for gases adsorbed on solids, which retained his name. It is a semi-empirical isotherm derived from a proposed kinetic mechanism. It is based on four assumptions:

The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.
Adsorbed molecules do not interact.
All adsorption occurs through the same mechanism.
At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

Langmuir suggested that adsorption takes place through this mechanism:

$A_{g} + S \rightleftharpoons AS$

Where A is a gas molecule and S is an adsorption site. The direct and inverse rate constants are k and k_-1. If we define surface coverage,

θ

, as the fraction of the adsorption sites occupied, in the equilibrium we have

$\theta=\frac{KP}{1+KP}.$

Where

P

is the partial pressure of the gas or the molar concentration of the solution. For very low pressures $\theta\approx KP$ and for high pressures $\theta\approx1$

2.1.2 BET

Often molecules do form multilayers that are, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid. In 1938 Stephan Brunauer, Paul Emmett, and Edward Teller developed a model isotherm that takes that possibility into account. Their theory is called BET theory, after the initials in their last names.

The derivation of the formula is more complicated than Langmuir's. We obtain as follows:

$\frac{x}{v(1-x)}=\frac{1}{v_\mathrm{mon}c}+\frac{x(c-1)}{v_\mathrm{mon}c}.$

x is the pressure divided by the vapor pressure for the adsorbate at that temperature (usually denoted

P / P 0

), v is the STP volume of adsorbed adsorbate, v_mon is the STP volume of the amount of adsorbate required to form a monolayer and c is the equilibrium constant K we used in Langmuir isotherm multiplied by the vapor pressure of the adsorbate. The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate. The Langmuir isotherm is usually better for chemisorption and the BET isotherm works better for physisorption for non-microporous surfaces^{25].

2.2 Types of adsorbents

Oxygen-containing compounds: Are typically hydrophilic and polar, including materials such as silica gel and zeolite.
Carbon-based compounds: Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite.
Polymer-based compounds: Are polar or non-polar functional groups in a porous polymer matrix.^[25]

2.2.1 Polymeric adsorbent

In the past decades, polymeric adsorbents have been emerging as potential alternative to activated carbon in terms of their vast surface area, perfect mechanical rigidity, adjustable surface chemistry and pore size distribution, and feasible regeneration under mild conditions. Generally, polymeric adsorbents can effectively trap many of the ubiquitous organic pollutants, namely, phenolic compounds, organic acids, aromatic or polyaromatic hydrocarbons, alkanes and their derivatives. Upon regeneration, the adsorbed organic chemicals are desorbed and may be recovered for further use. To further improve adsorption performance of a given polymeric adsorbent toward other pollutants such as highly water-soluble compounds (e.g., sulfonated pollutants) and heavy metal ions, surface modification or functionalization has proved to be an effective approach because the functional groups bound to the polymeric matrixes are expected to provide specific interaction with the target pollutants ^[26].

More recently, polymer/inorganic hybrid adsorbents have emerged as a new class of adsorbent materials for deep removal of trace pollutants from waters. Generally speaking, these hybrid adsorbents can be fabricated by irreversibly dispersing inorganic nanoparticles (e.g., metal oxides, inorganic ion exchangers, zero-valent Fe) within different polymeric supports. One of the basic reasons for designing these new hybrid adsorbents relies on the fact that fine or ultrafine inorganic particles are unusable in fixed beds or any flow-through systems because of excessive pressure drops and poor mechanical strength, though most of them exhibit specific affinity toward target pollutants in waters. For example, metal (hydr)oxides namely Fe(III) ^[27], Mn(IV), and Al(III) ^[27]oxides offer specific adsorption affinity toward charged pollutants like heavy metal ions and phosphate or arsenate. In addition, zero-valence Fe can effectively decontaminate some of the disinfection byproducts (DBPs) in drinking waters. Thus, they have to be impregnated into porous supports of larger particle size to overcome the technical bottleneck, and porous polymeric materials seem more attractive than activated carbon, cellulose, alginate, diatomite, and sand due to their excellent mechanical strength and adjustable surface chemistry. Traditional polymeric adsorbents were first developed in the 1960s ^[28],and they were originally developed for use in gel permeation chromatography, but their outstanding physical properties have made them a very popular material for adsorption and filtration processes. Now they have been widely used in removal of organic pollutants from industrial wastewaters or natural waters.