Plastics Handout 1

8/10/2019 Plastics Handout 1

1/29

GENERAL INTRODUCTION TO PLASTICS

INDUSTRY SNAPSHOT

Synthetic plastic was invented late in the eighteenth century and did not reach widespread use in the

United States until the 1900s. Swift advances in chemical and manufacturing technologies during thetwentieth century, however, made plastic one of America's most important manufacturing materials. In2001 the U.S. produced 101.1 million pounds of resins. Most important uses of plastics includepackaging (22.6 million pounds; 29 percent of all thermoplastic resins), building and construction (13.2million pounds; 17 percent), and consumer and institutional uses (11.2 million pounds; 12 percent). Thevalue of shipments in 2001 totaled $45.5 billion. According to The Society of the Plastics Industry, Inc.,plastics products is the fourth largest manufacturing segment in the United States, behind motorvehicles, electronics, and petroleum refining. Approximately 21,000 companies manufacture plasticproducts or plastics raw materials in the United States. Production facilities are predominately inCalifornia and the Midwest (Ohio, Michigan, and Illinois), with the top 10 states accounting for 60percent of all plastics employment..

ORGANIZATION AND STRUCTURE

Plastics provide an important alternative to natural materials for a plethora of applications. One of themost important distinguishing factors between plastic and other materials is plastic's ability to "creep"under load, or gradually stretch or flow when subjected to stress. While metals and ceramics exhibit thisproperty as well, they do so only at much higher temperatures. Plastics also resist erosion and do notrequire a coating to protect them against inorganic acids, bases, and water or salt solutions. Perhaps thegreatest advantage that plastics offer, however, is their ability to be molded into any shape and to beprocessed to exhibit any of a massive number of physical characteristics.

Plastics are giant polymers, or long-chain molecules that contain thousands of repeating molecular units.Most polymers are not used in their natural state, but are instead combined with other materials bymixing or melt-state blending (compounding). When combined with other ingredients called additives,the polymers can be shaped and molded under heat and pressure into a resin. Resin usually takes theform of pellets, flakes, granules, powder, or a syrupy liquid, that is ready to be delivered to a processor.There are two basic kinds of plastics: thermoplastics, which can be re-softened to their original conditionby the application of heat; and thermosets, which cannot be resoftened. The production of thermoplasticresins surpasses the production of thermosetting resins by a ratio of about 8 or 9 to 1.

The physical properties of the final plastic product can be altered at various stages of the polymerizationand production process. The most versatile method of varying properties is by compounding. With thismethod, additivessuch as colorants, flame retardants, heat or light stabilizers, or lubricantsmay beadded to the resin to achieve a desired result. Fillers or reinforcementsuch as glass fibers, particulatematerials, or hollow glass spheresmay instead be added to the resin, as may other polymers, whichform a polymer blend or alloy.


2/29


3/29

About 6.2 billion pounds of polystyrene, a fourth major thermoplastic product, were produced in 1998.This resin is used to make disposable packaging, furniture finishings, and miscellaneous consumerproducts. Other thermoplastics segments include polyamide resins, styrene-butadiene, and somepolyesters.

Thermosets. Thermosets, the other division of the plastics industry, account for about 12 percent ofoutput. Unlike thermoplastics, thermosets harden by chemical reaction, and cannot be melted and shapedafter they are created. Thermosets are also considered the more mature and less dynamic segment of theindustry.

Typical thermosets include phenolics, urea-formaldehyde resins, epoxies, and polyester. Phenolics,which account for over 50 percent of all thermoset production, are used principally for construction

products. Such materials include plywood adhesives, insulation, laminates, moldings, and abrasives.Urea, the second largest segment of the thermoset division, is also used as an adhesive for plywood andparticle board. Other uses of this resin include protective coatings and textile and paper treating andcoating.

Thermoset polyesters are used to create plastics that are reinforced with glass fiber and other materials.They are also used to make various construction supplies such as boat and marine equipment,transportation products, and electronics. Epoxy is primarily used as a protective coating for metal goods,but is also used in multiple construction applications. In 1998, 639 million pounds of epoxy were

produced.

Thermoplastics can be classified as vinyl and non-vinyl plastics based on their mer structure. (Note:Dont be confused with the commercial/industrial use of the word vinyl for PVC). Vinylplastics aregenerally derived from the by-products of crude petroleum and have 2 C with single bonds in thebackbone for the mer as shown below.

Depending on R1, R2, R3, R4, we can have different vinyl plastics. If they are all H, the polymer is PE, If3 are H and one is Cl, we get PVC etc. Non-vinyl plasticsdont have any pattern. They can containany number of C, single or double bonds, O, N, phenyl groups etc.


4/29

NATURE OF BONDING

Plastics generally have covalent bonds within the molecule and secondary bonds between molecules.Depending on the electronegativity difference, the covalent bonds can be classified as a) non-polar(when the difference in electronegativity is < ~0.3-0.4) and b) polar (when the difference in

electronegativity is >0.4). Remember, more the difference in electronegativity between atoms, greater isthe degeree of polarity.

Covalent bonds are also characterized by a bond length, bond angle and a specific bond energy. Theseare listed in the tables given below.

Hydrogen Bonds Bond length (nm) Bond energy (kcal/mol)

OH O 0.27 3 to 6


5/29

OH N 0.28NH O 0.29 4NH N 0.31 3 to 5

OH Cl 0.31

NH F 0.28NH Cl 0.32FH F 0.24 7

When there is a significant difference in electronegativity (>~0.4), then polarity effects becomeimportant. Polarity effects can be important in the backbone or pendant groups or both. Some of thecommon groups that can produce polar and non-polar effects are summarized below.

Mer structures

Polymers are characterized by their mer structures. The mer is the repeat unit within each molecule.Typical mer structures for some common polymers are shown at the end of this document. Based on themer structure, polymers can be classified as vinyl and non-vinyl polymers. The general structure for avinyl polymer is given below:

The mer structure consists of a main chain in which there are only C atoms with single covalent bondsand 4 pendant (or side groups) groups. Depending on the pendant groups R1, R2, R3, and R4 differentpolymers can be obtained. In non-vinyl polymers there is no pattern to the mer structure. The mainchain can consist of a random number of C, or O, or N atoms. It can also contain aromatic rings. Thereare many pendant positions. An example of nylon is shown below.

(CH2)6N N

H H

C

O

(CH2)7 C

O

n

n

Main chain

Pendant Groups


6/29

Depending on the composition of the main chain and pendant groups, polarity effects can be introducedin plastics.

Back Bone

Non-Polar Polar

O, NCH2, CH3-C-CH3,C=CS

Pendant Groups

Non-Polar Polar

F, O, N, ClH, CH3 COOH

OH

Representation of polymer molecules

Polymer molecules can be drawn to show their mer structure by showing the various mers. So show themer and enclose it in parentheses with the number n at the end as shown below. This is the structure for

nylon 6,6.

Very oftenthe parentheses may not be drawn but implied. Further, chemists use an abbreviated way ofdrawing structures as shown below. The same molecule for nylon 6,6 is shown in its abbreviated form.

So the C atoms are not drawn but implied at the junctions. Similarly, the H atoms are not shown. Keepin mind that each C atom should show 4 bonds. Similarly, O should have 2 bonds, N should have 3


7/29

bonds, Cl, F should have 1 bond based on their valence. S is an interesting atom, can have 2 or 6 bonds.When it has 2 bonds, the resulting polymer is called polysulfide, while when it has 6 bonds, it is apolysulfone. You should get used to drawing the structure in the abbreviated form.

Let us take a hypotheticalmer structure for a polymer in its abbreviated form as shown below and

expand it.

N O

OH

Cln (1)

So, when we expand it, at every intersection there is a carbon as shown below:

C

N

C

O

C

C

C

C

OH

Cln (2)

Now remember that each C has to have 4 bonds, N, 3 bonds, O 2 bonds etc. So wherever this is notsatisfied add the hydrogens as shown below:

C

N

C

O

C

C

C

C

OH

Cln

H

H

H

H

H

H

H

H

H

H

H

(3)Now we have the complete structure. Note that in the above structure, the structure is drawn to show thebond angles between C, O and N etc. Remember that these are 3 dimensional molecules. Very often,we write these 3-D molecules in a simplified 2-D form. So for the above structure, we can also writethis as follows:

C

OH

H

N

H

C

H

H

O C

H

H

C

H

H

C C

H

Cl

H

H n


8/29

Also, note that when we expanded the structure in 1, we put the hydrogen H associated with the N atomat the bottom(in 3-D it is projecting out of the paper on one side) . We could as well have put it at thetop(in 3-D projecting inward on the other side) as shown below.

C

N

C

O

C

C

C

C

OH

Cln

H

H

H

H

H

H

H

H

H

H

H

This produces an isomer (i.e. same chemical structure, but different positioning) of the structure shownin 3. More on that in the following sections.

Functionality and Functional groups

Polymers are produced by reacting various chemicals (i.e. monomers) with one another. The number ofsites where the polymerization reaction can take place in a monomer is called a functionality. So if amonomer can react at 2 sites to form the polymer chain, then it has a functionality of 2. In general,many of the monomers have a functionality of 2. If one monomer with a functionality of 2 reactionswith another monomer with a functionality of 2, we produce essentially linear chains in most cases. Butis one monomer with a functionality of 2 reacts with another monomer with a functionality of 3, abranched chain or in some cases, crosslinked chains are produced. If a monomer with a functionality of3 reacts with another with a functionality of 3, we get a network type of structure (i.e. thermoset).

Within the mer structure, there is a sub-group called thefunctional groupthat imparts specific propertiesto the polymer. This functional group controls many of the physical, mechanical, chemical properties of

the polymer Many polymers are identified based on their functional group. So the functional groupshown in the mer structure of a nylon is called an amidefunctional group. So, all nylons arepolyamides.

The major functional groups in most plastics are shown below.

N

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

N

H

C

O

C

H

H

C

H

H

C

H

H

C

H

H

C

O

amide


9/29


10/29

Chain Flexibity

The flexibility of the chain is affected by various groups present in the mer/molecule. Chain flexibilityis important in determining crystallinity, glass transition, strength, diffusion etc. In general, if the chainflexibility increases, Tgdecreases, strength may decrease (not always), crystallinity may be increased.

Groups that increase or decrease flexibility are given below:

Increase decrease

Crosslinking

When the mers are joined together (i.e. polymerized), we can produce many different architectures.Imagine each mer as a Lego block. There are many different ways in which these lego blockscan be

joined into long structures. The most common is the lineararchitecture shown below.

In this case, we can use the spagetti model. Each polymer molecule is like a strand of spaghetti. In thiscase, the bonds between molecules is secondary induced or permanent). So when heat a linear


11/29

architecture above a certain temperature, we break all the secondary bonds or melt the plastic. But if wecontinue to heat, we can break the covalent bonds within the molecule. If this happens, we havedestroyed the plastic. Linear architectures are very good for many applications, but in some cases, theydont exhibit sufficient processability or mechanical properties. So there are other possibilities duringpolymerization. We can produce branched structures as shown below:

This can improve the processability and processing for some polymers. But it can interfere withcrystallinity. If we want to improve the strength and modulus significantly, then we can crosslink themolecules. In this case, the secondary bonds between molecules are replaced by covalent bonds

between molecules as shown below.

However, by crosslinking, we can affect crystallinity, and processing. For example, can you melt acrosslinked plastic? Remember what was meltingbreaking the secondary bonds and getting rid of theentanglements. If the chains are linked with covalent bonds, melting becomes difficult. So, it can affectprocessability significantly. One material that has to be crosslinked before using is rubber. This processcalled vulcanizationis done with S as shown below:

+ 4S


12/29

Vulcanization increases the strength significantly. Typically the crosslinking in most rubbers is belowabout 35%.

Length of the molecule

All covalent bonds have a specific bond length and bond angle. In two dimensions, we show thecovalent bond as a horizontal line for simplicity. But we should be looking at these bonds in 3-Despecially for very, very, very large molecules that we have in plastics. Thus the backbone is

illustrated for the case of Polyethylene which consistsonly of C-C bonds.

So, you can see that because of the bond angle, the effectivebond length, l, is as follows:

l= (C-C bond length) x sin(/2)

where is the bond angle. The total effective length of the molecule,L, (straightened out along onedirection) is:

L = Total # of bonds in the main chain xl

The total # of bonds can be obtained by knowing the degree of polymerization and the mer structure. Inthe case of vinyl polymers,:

Total # of Bonds = DP x 2

The 2 comes because there are 2 carbon atoms in the backbone (or main chain) for each mer.

Similarly you can calculate the effective length for polymers with heteroatoms in the main chain fromthe bond angles and bond lengths given above.

Cohesive Energy Density

Many unique properties of polymers can be attributed to their bonding, covalent bonds in 1 direction(i.e. within the polymer) and secondary bonds in the other two directions (i.e. between molecules).

C

C

C

C

C

C

C

C

C

C

C

C

l

/2


13/29

Remember, it is weakest link in an arrangement that dictates most of the properties of the structure. Inthis case, the weakest link is the secondary bond. So the strength of the secondary bond plays a vital incontrolling the properties of the plastic. A measure of the strength of the secondary bond is given by thecohesive energy density (CED):

l

v

VECED

where Evis the molar energy of vaporization, i.e. how much energy is necessary for the completeseparation of the molecules. Vlis the molar volume.

Why is this important? It is important in determining the interaction between a polymer and a liquid.Will the liquid be a good solvent for the polymer? Can we contain a liquid in a polymer? All thesequestions can be answered by the Solubil ity Parameter, . The solubility parameter is the square root ofCED. The old funny units of solubility parameter, were a Hildebrand (H) which works out to(cal/cm3)1/2(because CED has units of cal/cm3). In SI units, solubility parameter has units of (MPa)1/2.1 H = 2.046 (MPa)1/2

When a solvent is placed in contact with a polymer, several things can happen. Initially there isswelling, then a solvated mass calledgelcan form and finally the polymer molecules are separated (i.e.secondary bonds broken) and the molecules are dispersed in solution (i.e. dissolution). can becalculated for the solvent and the polymer. For any polymer with 2to dissolve in a solvent with 1, thefollowing condition has to be satisfied:

2

21 must be small (comes from thermo)

In other words,for maximum solubility, the s for the polymer and the solvent should match.

for various polymers and solvents has been measured experimentally and hasbeen tabulated (eg. see

polymer handbook). Further, can also be estimated (although not as accurate as measurements) fromthe mer structure by known the constants for the pendant groups or functional groups according to thefollowing equation:

m

G

where is the density of the polymer, G are the group constant and mis the mer weight. (see the databelow for the Gs)


14/29


15/29

We have seen so far that plastics consist of long molecules. Inside the molecule, there are STRONGcovalent bonds. So, the molecule itself is strong and thermally resistant. However there are weaksecondarybonds betweenmolecules. These bonds can broken easily by physical, chemical, mechanicalor biological means. That is why plastics have a low melting point, strength etc. Now if we replacesome of the secondary bonds with covalent bonds ( a process called as crosslinking) what should

happen? The strength, MP etc. go up (more on this later). If it isjustthe covalent bonds and secondarybonds, here is what we can expect in terms of the behavior of a plastic as we increase the size of themolecule (or molecular weight).

runny liquid thick liquid syrupy liquid very viscous liquid

Low Mw very high Mw

We need one more factor to explain the solid like behavior we see with plastics and that isENTANGLEMENT. As the molecules becomes larger and larger, because of the limits imposed by the

covalent bonds, the molecule is kinked and coiled (i.e. it is not straight as a rod of uncooked spaghetti).So they entangle between one another (like in cokked spaghetti). It is this entanglement which enablesstress transfer at the molecular level and imparts the solid like properties. As can be expected, the # ofentanglements increases as the size of the molecule increases. So, the properties of plastics can beexplained based on:

a) Covalent bonds (polar or non-polar) WITHIN the long moleculeb) Secondary bonds (induced or permanent) BETWEEN the long moleculesc) Entanglements between the long molecules.

POLYMERIZATION PROCESSES

There are many polymerization processes. The goal is here is to link various monomer units or portionsof monomer units by covalent bonds to produce a long chain polymer. We will discuss the someimportant polymerization processes used to produce a large fraction of commercial polymers andcopolymers. These are a) addition polymerization (also known as chain growth polymerization) and b)condensation polymerization (also known as step growth polymerization) and c) ring openingpolymerization. There are other polymerization techniques for specialty polymers.

Additi on Polymerization

This mode of polymerization is typically used to produce vinyl polymers. In general, the backbone in

the polymer consists only of C atoms (ignoring end groups). The monomer units are typically joinedtogether in the presence of compounds called initiators. The initiator continually generates growth siteswhere monomer molecules can add onto rapidly. So, there is a sequential addition of monomermolecules to a growing center. The growth centers can be generated by i) compounds that produce freeradicals (called free radical initiators) or by ii) cations (cationic polymerization) or by iii) anions(anionic polymerization) and by iv) coordination polymerization using some catalyst. All of the abovepolymerization processes involve 3 basic steps 1) Initiation 2) Propagation and 3) termination. Theinitiation process involves the development of the active growth center. During propagation, monomer


16/29

units find and rapidly attach onto the end of the growing molecule. When the desired molecular size isachieved, the growth process is stopped during the termination phase by adding certain chemicals and/orcontrolling the pressure, temperature bath composition etc.

Let us start with the simplest and one of the most common addition polymerization techniques using free

radical initiators (FRI). The simplest FRI is H2O2. In this case upon heating, the covalent bond(involving 2 electrons) between O-O is broken as shown below to form 2 free radicals:

H O O H 2 H O

These free radicals are now short of one electron and so they are looking for an electron and arereactive. They can react with the monomer to initiate the polymerization process. The more commonFRI is something called benzoyl peroxide and it breaks down into free radicals upon heating as shownbelow.

The free radical produced can react with a monomer and transfer the free radical end as shown below forethylene (to produce polyethylene):

Another monomer can attach onto this radical (or growth center) and the process continues (i.e.propagation):

C

H

H

C

H

H

Initiates the breakup of the double bond in

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

2

Ethylene monomer


17/29

Once the desired molecular size is achieved, the process can be terminated. There are two ways oftermination. The first is called combination. In this case, two long growing centers can combinetogether to form an even longer chain as shown below.

As you can see from the above reaction, for combination, one molecule of benzoyl peroxide is able toproduce 1 chain (or 1 molecule) of the polymer. The other termination reaction is called asdisproportionation. In this case, there is hydrogen transfer and a creation of a double bond as shownbelow:

In this case, one molecule of benzoyl peroxide is able to produce 2 polymer chains. Note that one halfof benzoyl peroxide is present as an end group in long polymer molecule, while the other half is the endgroup of another molecule.

+

Double bond

formed

+

+

C

H

H

C

H

H

+


18/29

Instead of using free radicals we can use ionic compounds for the polymerization. The example ofcationic polymerization is shown below:

Initiation

+ A+X-

Propagation

Termination

+ HX

The anionic polymerization is similar. In this case, polymerization is initiated by compounds thatrelease anions.

C

H

H

C

H

H

C

H

H

C+X-

H

H

A

n-1

C

H

H

C+X-

H

H

C

H

H

C

H

H

A C

H

H

C

H

H

+

n

C

H

H

C+X-

H

H

C

H

H

C

H

H

A

n

C

H

H

C+X-

H

H

C

H

H

C

H

H

A

n

C

H

H

C

H

H

C

H

H

C

H

H

A


19/29


20/29

Take the example of Polycarbonate. In this case, Bisphenol A (on the left) is reacted with Phosgene.The by product is HCl.

One of the reactantscan also be a gas as shown below. In this case, oxygen is bubbled into amonomer (phenol) (in the presence of CuCl and pyridine) to produce polyphenylene oxide sold underthe trade name Noryl.

+ (n/2) H2On

O2

n CCl Cl

O

+

+ 2nHCl

n

C

CH3

CH3

O O C

O

n

+ C

H

H

C

H

H

O O HH

n

+ 2n CH3OH

n


21/29

Let us take the example of nylons. Nylons can be produced by various polymerization methodologies.Let us consider nylon6,6. This produced by reacting two monomers, hexamethylene diamine (on theleft) and adipic acid:

The numbers represent the number of C atoms in the monomer. Another type of nylon can also beproduced by just one monomer. In this case, we produce nylon 4 (so numbered because there are 4carbon atoms in the mer)

This is a case of self condensation, i.e. only one monomer used. Finally there are also cases where no byproduct is produced during polymerization

So step-growth polymerization is typically used to produce non-vinyl polymers.

Ring Opening Polymerization

This is another technique for producing linear polymers. A cyclic monomer is opened up and theresulting fragments joined together during the polymerization process. In most cases, no by-product isproduced. An example for nylon 6 is given below.

(As a side note, remember the naming scheme for nylons. The number after the nylon tells us about thesynthesis. So for example, nylon 6 was synthesized with one monomer which had 6 carbon atoms.Similarly, for nylon 7, nylon 8, nylon 9, nylon 11 etc. In all these cases, the mer structure looks asfollows for example nylon 12:

H2

C

CH2

CH2

CH2

CN

H2C

O

H

heatN

H2C

H2C

H2C

H2C

H2C C

O

n

H

NH2C

H2C

H2C

H

H

C O

O

Hn NH2C

H2C

H2C C O

O

H

H

H

n

+ (n-1)H2O

n

NH2C

H2C

H2C

H2C

H2C

H2C N

H

HH

CH2C

H2C

H2C

H2C C O

O O

H

NH2C

H2C

H2C

H2C

H2C

H2C N

H

H

H

H C

H2

C

H2

C

H2

C

H2

C C O

O O

OH H+n

+ 2nH2O


22/29

(CH2)11N C

H O

n

Then, we have the nylon 46, nylon 66, nylon 69, nylon 610 etc. All these were synthesized with 2monomers. In this case, the first # tells the # of carbon atoms in the first monomer and the secondnumber gives the # of carbon atoms in the second monomer. In the case of nylon 610, the first monomerhas 6 carbons and the second monomer has 10 carbons). The mer structure looks as follows for thesenylons:

(CH2)6N N

H H

C

O

(CH2)7 C

O

n

Notice how the CH2 appears twice in the mer structure indicating two different monomers. The abovenylon is nylon 69.)

Let us continue with ring opening polymerization. Another example is for polyacetal (orpolyoxymethylene) is given below. This polymer is sold under the commercial name Trioxane.

(Note this polymer can also be synthesized by addition polymerization of formaldehyde as shown

below.

O

H2C

O

CH2

O

CH2

ncatalyst

* CH2 O *n


23/29

C

H

H

O

C

H

H

O

This polymer is sold under the commercial name Delrin. Trioxane and Delrin have very differentproperties eventhough they have the same mer structure)

An example for silicone is given below.

In addition those who took ME 4814, remember that many of the common biopolymers such as PLA,PGA, PCL etc. are obtained by ring opening polymerization. Further, PLA, PGA etc. can also beproduced by condensation polymer, but you know the drawback is that by this method, high molecularweights cannot be achieved easily and so ring opening polymerization is commonly used.

Polymer Modifications

Many polymers cannot be synthesized directly from the monomers. Initially another structure issynthesized and then this polymer is modified to produce the final polymer. A good example is thecommon polymer, PVA or polyvinyl alcohol. It is difficult to find a monomer to synthesize PVA sincevinyl alcohol is unstable. So initially, a polymeric structure known as polyvinylacetate is synthesizedand this polymer is eventually modified to produce PVA as shown below:

C

H

C

OH

H

C CH3

O

Polyvinylacetate PVA

C

H

C

OHH

H

n


24/29

The PVA formed can be further reacted to produce other polymers such as polyvinylbutral orpolyvinylformal (which cannot be formed directly from monomers).Many polymers such as PE, PP, PS etc. are chlorinated (i.e. substituting some of the pendant groupswith Cl) or fluorinated (i.e. substituting some of the pendant groups with F) after synthesis.

Chlorination can increase Tg, reduces flammability,and may change crystallinity. Flourination of somevinyl polymers can enhance chemical inertness and improve solvent barrier properties. Sometimes somepolymers are nitrided (i.e. N is substituted in the pendant group). This nitration can enhance moldingproperties in some polymers. Crosslinking of molecules is another modification performed afterpolymerization. Light crosslinking of vinyl polymers enhances Tg, increases strength, reduces solubilityand ductility.

Extent of reaction for condensation polymerization

The extent of reaction can be simply defined as how far the reaction has progressed. Since the originalmonomers should at least have a functionality of 2, there are at least two functional groups of A OR of B

in each monomer. (See for example the monomer BPA used to make polycarbonate:

There are two functional groups (OH) at the end. So this molecule has a functionality of 2. In a generalform, this molecule has 2Afunctional groups,Arepresenting the OH here. Similarly the secondmonomer should have 2Bgroups.)

IfN0is the total number ofAgroups (or B groups) in the original mixture andNis the number of

unreacted A groups at time t, then the extent of reaction,p, can simply be defined as:

0

0

N

NNp

and this simply represents the fraction of reacted A groups. One can also calculate the number averagedegree of polymerization at any time during the reaction by knowing the extent of reaction at any time:

pDPn

1

1

This above equation is called as Carothers equation.

Extent of reaction during addition polymerization

Here we have just one monomer if we are producing a homopolymer. So we can define the extent ofreaction,x, simply as:

+


25/29

0

0

M

MMx

whereM0is the initial number of monomer molecules andMis the number of remaining monomer

molecules at any time. So herexrepresents the fraction of monomer units that have reacted to formpolymer molecules.

Isomers

Depending on how the mers are joined together, we can have various versions of molecules. Theseversions are called isomers, which are the same chemical structure with slightly different arrangementsas shown below. There are many types of isomers. We will examine some of the common types ofisomers that are observed in many plastics.

In some plastics (eg. vinyl but not limited only to them), we can get the 5 versions as shown below with

PVC as the example. In successive mers, notice the positioning of the pendant group Cl, with respect tothe backbone. In these examples, 6 mers are shown.

a) Syndiotactic Head to tail

b) Isotactic head to tail

c) Syndiotactic head to head


26/29

d) Isotactic head to head

e) Atactic

Note that these are the possibilities. Not all of them can be produced for all the polymers. Amongthese, atacticis the least crystalline,syndiotacticand isotacticcan crystallize more easily (depending onother factors). Among these the head to tail arrangement in each version is more crystalline than head tohead version.

Another type of isomerism is possible in other polymers. See for example the structures shown below.Note the shaded regions for the two isomers of the same molecules

The above structures were for polyisoprene. Cis-polyisoprene is an elastomer, while trans-polyisopreneis a very hard substance known as gutta percha that is used in dental fillings after a root canal surgery.As discussed previously, the transstructures can crystallize much more easily than cisstructuresbecause of steric hindrance effects.

cisTrans


27/29

Below, the two isomers for a polyamide (eg. nylon) are shown. In nylon, the cisversion dominates andthus does not contribute to crystallinity. ( However, remember that because of the highly polar amidegroup, nylon can crystallize well. )

Cis

Trans

In general, the transstructures crystallize much more easily than cisstructures.


28/29

Some Common mer structures for polymers

C

H

H

C

H

H

Polyethylene (PE)

Polyvinylalcohol (PVA)

CO O C

OCH3

CH3

C

H

H

C

H

C N

Polycarbonate Polyacrylonitrile (PAN)

C C

H

H

CH3

C O

O

CH3

C C

H

H

Cl

Cl

C C

F

F

F

F

C C

H

H

H

CH3

C C

H

H

CH3

CH3

C C

H

H

H

PMMA

C C

H

H

H

Cl

PVC

PVDC (polyvinylidene chlori

Teflon (PTFE)

Polypropylene (PP)

Polyisobuty

Polystyrene (PS)

C C

H

H

H

OH


29/29

Documents

Plastics Handout 1