Suspension polymerization is used only in free radical type processes. The monomer is mechanically dispersed in a media, usually water. There are cases where an organic media is used in which neither the polymer nor the monomer are soluble in the organic media.

For suspension polymerization, there are two phases, water and organic, and the starting point may be 10 parts of the former, and 1 part of the latter. The initiator used can be water soluble or organic soluble [benzoyl peroxide, AIBN, or (NH4)2(SxO4)y.] Usually the initiator is organic soluble. Poly(vinyl alcohol) dissolved in the aqueous phase is a typical suspending agent.

1. There are two separate phases throughout the whole process.

2. The droplets must be kept far apart.

    3. The rate of suspension polymerization is similar to the rate of bulk polymerization, but the heat transfer is much better. For suspension                 polymerization, initiation, propogation, and termination take place inside the droplet.

• examples include the polymerization of methyl methacrylate, and vinyl chloride.
• The solution polymerization of butadiene, isobutylene and isoprene require a pressure system.
• the media to monomer ratio is 10:1 (vol/vol).
• Particle size is affected by:

• The plant operator must control temperature, and the particle size (of the growing polymer mass in the bubble.) If the particle size gets to large, the particle will absorb too much heat. Particle size may be 0.01 to 0.5 cm, or as low as 1 micron.

A suspension agent is a material that gives a surface activation that keeps droplets from become larger (droplets coming together to form larger droplets is called coalescence.)

Suspension polymerization is similar to bulk polyerization, and it could be considered "bulk polymerization within a droplet." The speed at which the reaction takes place for a given temperature is the same, and just as for bulk polymerization, the kinetics or rates are proportional to monomer concentration.


The properties of the polymer are similar to those of the same polymer made by a bulk polymerization.

Limitations of suspension polymerization-

1. It only applies to free radical process
2. agitation is critical because as the viscosity within the bead rises, the reaction rate increases suddenly ( Tromsdoiff effect.) This leads to a surge in heat generation which does not usually occur in solution or emulsion polymerization.

2. Things to consider for Emulsion polymerization-

1. monomer
2. media (water)
3. soap (also referred to as the emulsifier, or the surfactant)
4. minor amount of mechanical stirring
5. initiator
6. stabilizer (suspension agent)

Difference between between the emulsion and suspension process:

1. For an emulsion polymerization process the small droplets are stabilized by the surfactant.

2. The initiator can be either in the aqueous or the organic phase in emulsion polymerization.

Initially, a small amount of agitation is needed. The surfactant is in the form of micelles. Below is an illustration of a micelle, and for our purposes, this miscelle is in water. The hydrophilic (polar) end groups lineup on the outside and the hydrophobic ends are placed on the inside.

Example:

Example of a molecule that forms a micelle:

CH3 (CH2)16 COO- Na+                                                                     

hydrophobic  hydrophilic

If monomer is added to a micellear dispersion, most of the monomer remains as large droplets, but some of it dissolves in the micelles. Since the monomer droplets are smaller, the micelles present much greater surface area than the monomer droplets. Consequently, when free radicals are generated in the aqueous phase, the micelles capture most of them.

The monomer consumed by the polymerization in the micelle is replaced as more monomer diffuses from the aqueous solution into the micelle. As monomer leaves the aqueous solution, more monomer from monomer drops leaves the monomer droplets and thus maintains a steady concentration of monomer in the aqueous solution.

The first free radical to enter a monomer swollen micelle starts the polymerization. The second free radical to enter the micelle terminates the polymerization. When the third free radical enters the micelle, the process is repeated. As this process repeats, the micelle becomes larger and larger. The micelles are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules around the periphery.

Polymer product is isolated by 'breaking' the latex, usually by the addition of an acid which converts the soap, COO^- Na⁺ to a fatty acid, COOH.

Five types of surfactants There are five categories of wetting agents (i.e., surfactants) used by the coatings industry.

Anionic wetting agents   such as CH₃(CH₂)_nCOO^- NH₄⁺

Cationic wetting agents. Such as CH₃(CH₂)_nNH₃⁺ Cl^-

Electroneutral wetting agents. Such as  CH₃(CH₂)_nNH₃⁺ O^-CO(CH₂)x CH₃

Amphoteric wetting agents. Example:  NH₂CH₂(CH₂)_nCH₂COOH

Nonionic wetting agents. Example:  HO(CH₂CH₂O)_nCH₂(CH₂)_nCH₃

Part Three: Characterization

Chapter 7. Polymer Solutions

Solubility: Most polymers are insoluble in water. Some polymers can be soluble in strong organic solvents. Polymer nonsolubility is an advantage for a finished product. However, it may present a tiresome problem for the engineer who is trying to manufacture a product.

What holds molecules together?

A. Criteria for Polymer Solubility

The solution process. Dissolving a polymer is a slow process that occurs in two stages:

Swelling – the solvent slowly diffuse into the polymer to produce a swollen gel

Disintegration — The gel gradually disintegrates into a true solution. 

Crosslinked polymers do not dissolve, but only swell. The degree of the swelling is mainly determined by the extent of crosslinking. However, this doesn’t mean low solubility imply crosslinking, other features may affect the solubility too, such as high intermolecular forces. Many crystalline polymers, particular nonpolar ones, do not dissolve except at temperature near their crystalline melting points.

The crystallinity melting points of a couple of polymers are listed below

Linear polyethylene, T_m = 135 ^oC

Polytetrafluoroethylene, T_m = 325 ^oC

More polar crystalline polymers, such as 66-nylon, T_m = 265 ^oC, can dissolve at room temperature in solvents that interact strongly with them (such as forming hydrogen bonds).

In general, branched polymers are more readily soluble than their linear counterparts of the same chemical type and molecular weight.

The theory of solubility, based on the thermodynamics of polymer solutions, is highly developed only for linear polymers in the absence of crystallinity.

Solubility of Parameters:

An equation can be conducted as:

d₁ and d₂ refer the solubility parameter of solvent and polymer, respectively.

In general, when d₁ - d₂ > 4, the polymer are expected to be not soluble in this solvent.

Some solubility parameter of solvent are listed below

B. Conformation of Dissolved Polymer Chains.

In solution, a polymer molecule is a randomly coiling mass. The size of the molecular coil is very much influenced by the polymer-solvent interaction forces. In a thermodynamically “good” solvent, where polymer-solvent contacts are highly favored, the coils are relatively extended. In a “poor” solvent, they are relatively contracted.

The random coil arises from the relative freedom of rotation associated with the chain bonds of most polymers and the formidably large number of conformation accessible to the molecules.

Two models

The basis of many single polymer theories stem from Flory's Freely Jointed Chain (FJC) model. This model assumes that chemical bonds are free to rotate and posses a uniform distribution of bond angles. The end-to-end distance or chain vector (r) can be calculated from an Equation.

Freely Rotating Chain, or Worm Like Chain (WLC). This model assumes that the bond angles are fixed at 180-q, but are free to rotate, giving rise to a uniform distribution of dihedral angles. Random conformations were calculated for both models and are shown below.

A:  Freely jointed chain in three dimensions   B: WLC in three dimensions with a fixed bond angle of 109.5 degrees.    C: FJC confined to two dimensions D: WLC confined to two dimensions with a fixed bond angle of 109.5 degrees.

C. Thermodynamic of Polymer Solutions (omit this section)

D. Phase Equilibrium in Polymer Solutions (omit this section)

E. Fractionation of Polymers by Solubility

Bulk fractionation by Nonsolvent Addition

Adding the nonsolvent to a dilute solution to precipitate the polymer

            Column Elution

1. Solvent-Gradient Elution – a series of solvents of gradually increasing solvent power
2. Thermal-Gradient Elution – using temperature gradient from one end to the other of the column, in addition to the gradient of solvent composition. The sample is initially confined to a small zone of the packing at the warm end of the column. Each species undergoes a series of solution and precipitation steps as it progresses down the column.

Effects of Polymer Structure on Solubility-Based Fractionation

Effect of Chemical type

Effect of Chain Branching

Effect of Crystallinity

Howework:

1. List the variables affecting the solubility of polymers.

2. Describe the two stages of the process of dissolving in a polymer and suggest how to speed them up.

4. It is observed that a styrene-butadiene copolymer (d=16.5) is insoluble in pentane (d=14.5) and ethyl acetate (d=18.6), but soluble in a 1:1 mixture of the two, explain.