The Notes for
Polymer and Coatings Science-
Chapter One- part one

Historical Introduction and Definitions

What is a polymer?

A polymer is a large molecule comprised of repeating structural units joined by covalently bonds. Poly comes the Greek word for "many" and mer comes from the Greek word for "parts."

We could arbitrarily say a polymer has to have at least 100 repeats for now, but later we will change this definition to account for physical properties of polymers. The repeat unit of a polymer is a group of atoms covalently bonded together in a specific spacial arrangement.

 's Introduction to Polymers.

Introductory Organic texts talk about a homologous series that runs methane, ethane, propane, ..., decane, ..., and so on...
This homologous series can be thought of as two methyl groups, -CH3, tied together by many methylene groups, -CH2-. As you extend this series, you create polyethylene.

CH3 - CH2 - CH2 - CH2 - ... - CH2 - CH2 - CH3

or
CH3  [ - CH2 - ]  - CH3  (a shorthand method)
                n
or finally
[ - CH2 - ] n

n is a subscript to count the number of repeat units. This last shorthand method requires that you remember to add a hydrogen to each end group (CH3 on the end rather than CH2.)

 Flowchart of Polymeric Materials

Inorganic Organic

Historical Development (to be finished at a later date)- The museum is a building ground for the collect of factoids associated with dates.

Polymer Classification (addition vs. condensation)

Polymers can be classified as addition polymers or condensation polymers. This comes from a time when it was simple enough to ask "Did you break a double bond to make your polymer (addition polymerization) or did you eliminate a by-product such as water, methanol, or hydrogen chloride (condensation polymerization)?" There is good reason to segregate polymers into these two classifications because the method of polymerization dictates what problems the engineer will encounter in manufacturing the product. This will be discussed in more detail further on, along with examples of polymers that fit neither classification such as epoxies and polyurethanes.

Polymerization Process (chain vs. step)

A polymerization process can be described as either a chain reaction or a step reaction. This focuses the discussion to some polymers that don't fit into either of the two strict definitions proposed above.

Your discussion of free radical processes in Organic I lecture ties provides ideas needed to understand addition polymerization, and and your coverage of condensation reactions in Organic II lectures ties in to condensation polymerization.

Throughout the notes there are times when a phenomenon may be described, but the notes don't say if it applies to both chain and step polymerization, or just one of the two. I'll try to clear this up.

Chain polymerization polymers

Examples include: Step polymerization polymers:

Examples include: The preparation of polyurethane by the reaction of an isocyanate with an alcohol illustrates a "non-addition" polymerization that does not give off a by product. The hydrogen of the alcohol bonds to the carbon of the isocyanate.



Fundamental Definitions:

 Copolymer- A mixture of two polymers. It may be composed of two bifunctional units and may alternate to give a well-defined recurring unit or the two different monomers may be joined in a random fashion in which no recurring unit can be defined. A copolymer contrasts with a homopolymer.
A copolymerization that results in A-B-A-B-A- etc., is called an alternating copolymer.

A copolymerization where the sequence of A's and B's is random, A-A-B-A-B-B-A-B-A-B-B-B-A etc is an example, is called a random copolymer.

You may wonder at this point, "what if it isn't alternating, but it isn't completely random either?" This would get into the idea of reactivity ratios, which are discussed in the sequel to this class when copolymerization is covered.

A block copolymer is built from first one polymer, and then another, as in A-A-A-A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-B.

A graft copolymer is shown below, where a polymer of 'B' was grafted onto a polymer of 'A'. 
 
-A-A-A-A-A-A-A-A-A-A-
         |
         B
         |
         B
         | 
         B
         |

shows model graphics of random, block, and graft copolymers (type 'copolymers' into the find function when you arrive at the site.)

Linear polymer- a straight chain species, i.e., the units are connected to each other in a chain arrangement. Linear polymer contrasts with branched polymer and crosslinked polymer. Monomer- the building block or structural unit of the polymer. For polyethylene, the building block or structural unit is 
H   H
|   |
C - C -
|   |
H   H
Homopolymer- a polymer containing a single repeat unit. A homopolymer contrasts with a copolymer.

Degree of Polymerization (n)- the number of monomer units that have polymerized together. D.P. values can be as high as 10,000. Please contact me if you are aware of any polymers which can be synthesized with higher D.P. values. Crosslinkage- The formation of crosslinks. Long polymer chains form because each bifunctional monomer unit has two "bonding sites" so it can link to two other monomers. You should be able to see how C=C gives two bonding sites in addition polymerization. Now, if you include monomers which possess three bonding sites (see A* below), then when one of these trifunctional monomers is incorporated into a polymer chain, it has a third site that monomers can attach to: 
- A - A - A* - A - A - A - A -
          |
          A
          |
          A


Monomers which make the top 50 chemicals list
April 10, 1995 Chemical and Engineering News
CHEMICAL RANK production in tons
Ethylene 4th 48,530,000,000
Propylene 7th 28,840,000,000
Styrene 20th 11,270,000,000
Terephthalic Acid 24th 8,640,000,000
Acrylonitrile 39th 3,089,000,000
Vinyl Acetate 40th 3,020,000,000
Adipic Acid 46th 1,800,000,000
Bisphenol A 48th 1,480,000,000

Physical Properties of Polymers

The Physical Properties of a polymer material are largely determined by: A list of physical properties might include: Melting point- the melting point of a polymer does not occur over a sharp temperature range (1- 2 degrees C) as is observed for small organic molecules. If a polymer becomes a melt, there is usually a range of as much as 50 degrees C over which the viscosity of the polymer slowly changes from that of a solid to that of a liquid. Note that for a polymer to melt, a polymer must be a thermoplastic.



Melting pt. vs. # of carbons for straight chain alkane homologs



Boiling point- Polymers never boil.

Boiling pt. vs. # of carbons for straight chain alkane homologs

You may recall from Organic that as the number of carbons in the alkane homologous series increases, the boiling point increases asymptotically. We might chose to define the a polymer as a growing chain of sufficient length such that the mechanical properties are about 80% that of the apparent assomptotic limit. The number 80 is arbitrary. I've seen no reference that set a standard number for this. In the example below, chains of sufficient length to give a minimum performance for some mechanical property in the yellow-green zone are called polymers.



(type "increasing" into the find function when you arrive at the site)

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.

Melt viscosity- ??

Tensile strength- Tensile strength measures how difficult it is to break a substance when stress is applied to pull it apart. Tensile strength generally increases with molecular weight. An Instron grips a sample and pulls it apart. The instron creates a plot of where the y axis is the force exerted between the two grips, and the x axis is the separation distance between the two clamps. Usually, the electronics cause the Instron to increase the distance between the two clamps (pulling) at a constant rate, and the force or "strength" required is measured. This is somewhat analogous to when you were a little kid at the supermarket, and you pulled down on a scale in the Produce section, and watched the scale readout increase.



Tensile strength numbers (psi)- 145 psi = 1 MPa 

polyethylene (low to medium density)      1,000- 2,400
poly(tetrafluoroethylene) (a.k.a. teflon) 3,500
polyethylene (high density)               4,400
poly(dimethylsiloxane)                    5,000
polypropylene                             5,000
poly(vinylidene chloride)                 8,000
polystyrene                               8,000
polyamides                                9,000 to 12,500
polycarbonate                             9,500
polyesters (cast- as opposed to molded) ~10,000
polysulfone                              10,200- 12,000
poly(phenylene oxide)                    10,500
Table from Allcocke and Lampe: edition 1, chapter 21, "The Testing of Polymers" (Excellent reading!)


This is a tensile instrument.
Full picture available-
University of Southern Mississippi Polymer Science Department photo




What holds molecules together?



 Molecular cohesion

The higher the chain interaction, the higher the molecular cohesion. Solubility os determined by intermolecular forces. If you have small forces, then the molecular cohesion energy is low.

Molecular cohesion per 5 angstroms of polymer chain. Values around 5 or higher on the scale below should be considered "high."

polyethylene         1.0 kcal/mole
polyisobutylene      1.1 kcal/mole
rubber               1.3 kcal/mole
poly(vinyl chloride) 2.6 kcal/mole
poly(vinyl acetate)  3.2 kcal/mole
poly(styrene)        4.0 kcal/mole
poly(vinyl alcohol)  4.2 kcal/mole

The notes list a polyamide at 5.8 but don't specify which one.






Last Update- July 8, 1995- wld