Chain Microstructure

Chain Microstructure TOPICS COVERED :- • Linear chains,branching • Cross - linking and network formation • Sequence isomerism • Stereoisomerism in vinyl polymers • Diene polymers (structural isomerism) • Copolymers and blends

Philosophical Approach Illustrate the connection between structure and properties right from the beginning. To do this we need to remind ourselves about the nature of crystallinity RANDOM COIL Like “cooked spaghetti” CRYSTALLINE A bit like "uncooked spaghetti ”

The Effect of Crystallinity on Properties We will be asking how crystallinity affects • Strength • Stiffness • Toughness • Barrier Properties • Solubility • Transparency • Thermal Properties • Etc

Linear and Branched Polymers Linear Branched Which of these is more likely to crystallize?

The answer is linear ! Crystallizes more like this than this Various grades of polyethylene are produced commercially and are often referred to as “high density” or “low density”. Which do you think is the high density polyethylene A. The linear, more crystalline stuff ? B. The (somewhat) branched less crystalline stuff ?

The answer is still linear ! Chains that cannot crystallize (e.g., highly branched ones), or even linear chains that are heated above their crystalline melting points, actually look something like cooked spaghetti or random coils. They do not pack as closely together as in the crystalline state.

The Effect of Crystallinity on Properties The type of polyethylene that goes into milk jugs is stronger, stiffer, but more opaque (less optically clear) than the type of polyethylene that is used to make film wrap (greater optical clarity,more flexible, but less strong) . Can you figure out which type of polyethylene is used to make film wrap ? A. High density B. Low density

Property Change with Increasing Degree of Crystallinity Strength Generally increases with degree of crystallinity Stiffness Generally increases with degree of crystallinity Toughness Generally decreases with degree of crystallinity Generally decreases with increasing degree of crystallinity.Semi-crystalline polymers usually appear opaque because of the difference in refractive index of the amorphous and crystalline domains, which leads to scattering. The scattering will depend upon crystallite size. Optical Clarity Small molecules usually cannot penetrate or diffuse through the crystalline domains, hence “barrier properties”, which make a polymer useful for things like food wrap, increase with degree of crystallinity Barrier Properties Similarly, solvent molecules cannot penetrate the crystalline domains, which must be melted before the polymer will dissolve. Solvent resistance increases with degree of crystallinity Solubility

Short Chain Branching Branching and network formation Long Chain Branching Star Polymer

Network Formation Reacting Trifunctional Molecules Reacting Tetrafunctional Molecules What would happen if you reacted bifunctional molecules ?

OH OH OH CH CH CH CH CH 2 2 2 2 2 OH OH HO CH CH CH 2 2 2 O CH 2 OH CH 2 CH CH 2 O 2 OH H C 2 H C 2 CH 2 OH O CH CH 2 2 HO HO CH 2 HO CH HO CH 2 2 HO Network Formation Here is what a small part of a phenolic resin network looks like. We discussed these in previous lectures.

More Network Formation Networks can also be made by taking linear polymer chains and linking them using covalent bonds. We call this cross-linking

- CH2 CH2 - CH2 CH2 - CH2 CH2 - - - - - - - C = C C = C C = C - - - - - - CH3 H CH3 H CH3 H Network formation by cross-linking An example of cross linking is the reaction of natural rubber or poly(isoprene) ; with sulfur (or, as we prefer, sulphur) . The sulfur interconnects the chains by reacting with the double bonds.

Sn Sn Sn Sn Sn Sn Sn Network formation by cross-linking

Networks - Summary We can make networks by; • Linking together small multi-functional monomers • Cross - linking already formed chains Note; you can change properties dramatically by changing the cross-link density Think of the difference between a rubber band and a rubber tire

Isomerism in Polymers Two molecules are said to be isomers if they are made up of the same number and types of atoms, but differ in the arrangement of these atoms. • Sequence isomerism • Stereoisomerism (in vinyl polymers) • Structural isomerism (in diene polymers)

SequenceIsomerism When a monomer unit adds to a growing chain it usually does so in a preferred direction. Polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) are only a few examples of common polymers where addition is almost exclusively what we call head-to-tail. R-CH -CXY* = (TH) 2 R* + CH2=CXY T H R-CXY-CH* = (HT) 2

- - - - - - - - - - - - CH CXY CH CXY CH CXY CH CXY 2 2 2 2 Sequence Isomerism In many common polymers, such as polystyrene, addition occurs almost exclusively in a head-to-tail fashion. Head to Tail Head to Tail Head to Tail (TH) (TH) (TH) (TH) active site part of growing chain monomer about to collide with active site *

- - - - - - CXY CH CH CXY 2 2 Sequence Isomerism Tail to Tail Head to Tail Head to Head In other polymers, particularly those with smaller substituents, head-to-head and tail-to-tail placements can occur. - - - - - - CH CXY CH CXY 2 2 (TH) (TH) (HT) (TH) Example; poly(vinylidene fluoride), -CH2-CF2- . All head-to-tail units Some head-to-head and tail-to-tail units

Stereoisomerism in Vinyl Polymers Polymerization of a vinyl monomer, CH2= CHX, where X may be a halogen, alkyl or other chemical group (anything except hydrogen!) leads to polymers with microstructures that are described in terms of tacticity. meso diad racemic diad

Isotactic Chains Part of an isotactic polypropylene chain The catalysts developed by Ziegler and Natta produced predominantly isotactic polypropylene, but also atactic polypropylene.

Syndiotactic Chains Here are two more polypropylene chains, both shown as if we were looking down from “on top”. One of these consists of units that are all racemic to one another and is called syndiotactic. The other has a random arrangement of units and we call such chains atactic. Which one is the atactic chain , A or B ? A B

Tacticity in Some Commercially Important Polymers Polystyrene - atactic Polypropylene - largely isotactic PVC - largely atactic (Some syndiotactic sequences ?) PMMA -atactic

Structural Isomerism Diene Polymers CH2 = CX - CH = CH2 X = Hwe have butadiene X = CH3we have isoprene X = Clwe have chloroprene Where if But, what is trans-1,4-polybutadiene, cis-1,4-polyisoprene ?

- CH2 H - CH2 CH2 - - - - - C = C C = C - - - CH3 - CH3 CH2 - CH3 H - CH2 = C - CH = CH2 = CH C- CH3 = CH2 CH2 Structural Isomerism cis-1,4 trans-1,4 1 2 3 4 1,2 unit Isoprene 3,4 unit CH3 H - - -CH2 - C - -CH2 - C - - - Microstructure of Poly(isoprene)

- CH2 - - C = C - - CH3 - CH2 - - C = C - - CH3 CH2 - Structural Isomerism What about cis and trans? CH2 - H cis-1,4 H trans-1,4

-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B- A-A-A-A-A-A- A-A-A-A-A-A-A-A- A-A-A-A-A-A-A-A-A-A- Copolymers -A-B-B-B-A-A-B-A-B-A-A-A-B-A-B-B-A-B-B-A-A-A-B- Random Copolymers -A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B- Alternating Copolymers -A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-A-A-A-A-A-A- Block Copolymers Graft Copolymers

Blends Why are polymer blends important ? A route to new materials Miscible Immiscible Single phase Phase separated

CH4 ----------------------------------------- 16 CH3 - CH3 -------------------------------- 30 Gases CH3 - CH2 _ CH3 ------------------------ 44 CH3 - CH2 -CH2 -CH3 --------------- 58 Liquids CH3 - (CH2)6 _ CH3 --------------- 114 "Semi-solid" CH3 - (CH2)30 _ CH3 --------------- 450 Solids CH3 - (CH2)30000 _ CH3 ---------- 420030 Increasing Molecular Weight Molecular Weight Increasing Molecular Weight

Molecular Weight Distributions The problem with describing the molecular weight of synthetic polymers is that there is always a distribution of chain lengths (although certain polymerizations can give very narrow distributions).

Tensile Strength Melt Viscosity Molecular Weight Why is it important? Mol. Wt. Mol. Wt.

Making Plastic Bottles Molecular Weight is very important in processing. You don’t want the viscosity too high, or the polymer will be difficult to process. At the same time, you want a material that is being extruded, for example, to “hold together” until it solidifies. Let’s look at making plastic bottles as an example.

Blow Molding Bottles are made by blow molding. This is a two-step process. In the first step, a preform is made either by extrusion or injection molding. The second step utilizes air pressure to inflate the preform inside a closed, hollow mold. The polymer expands to take the shape of the cooled mold and solidifies while air pressure remains in the part. After cooling, the mold halves open, the part is ejected, and the next preform enters the mold. Several variations of blow molding are used, depending on the type of product being made.

Extrusion Blow Molding Extrusion blow molding utilizes an extruder to produce the preform to be blown. In a typical arrangement for producing juice bottles, the tube is extruded vertically downward to some predefined length, as shown opposite. This tube is commonly called the parison, from the Latin word for wall. When the parison is the proper length, the two open mold halves shuttle to a position surrounding it, as also shown in the movie. With the parison in position, the mold closes and the tube is cut from the extruder. The mold is designed to pinch the tube and form a seal at the bottom, but leave the top open. The mold then shuttles back to the blow position. At this point, a blow pin is inserted into the hole at the top of the parison, pressurizing the parison against the mold walls with air. After sufficient cooling, the blow pin is removed and the part is ejected. While the part was cooling, the next parison was being extruded, so that it is ready to be taken by the mold upon its return.

Extrusion Blow Molding One of the more important things to understand about extrusion blow molding involves the freely suspended, molten parison. In some operations, the parison may be several feet long and suspended for many seconds before the mold captures it. In all cases, the parison is acted upon by gravity, so the polymer being used must therefore have a good melt strength. Melt strength can be defined as a high resistance to (stretching) flow in the absence of shear, and can be thought of in terms of an extrudate that has a consistency that is more like taffy than water. Certain polymers have been synthesized specifically for good parison melt strength. For example, some grades of high molecular weight, high density polyethylene have good melt strength.

Injection Blow Molding Injection blow molding utilizes an injection molder to produce the preform to be blown. In the example we will consider here, a hollow preform resembling a test tube is made by conventional injection molding. The preform is then transferred to a second mold where it is reheated and blown into final shape, as shown in the animation. Injection blow molding has some advantages over extrusion blow molding. First, it is capable of greater dimensional accuracy in the final product because the injection mold can be machined to produce a very accurate preform. Second, an injection molded preform contains no weld lines, unlike an extruded parison that has a pinch-off at the bottom. If a blow molded product is to be pressurized, like a soda bottle, the weakness of a weld line may be unacceptable. Of course, the fact that two costly molds are required for injection blow molding is a significant disadvantage.

Chain Microstructure