 Crude oil extracted from the ground is complex and full of a variety of hydrocarbon structure.  Crude oil refineries sort the different hydrocarbons by size, structure, and weight.  Large hydrocarbons can be broken down into more useful “pieces.”

 All of these are hydrocarbons, bonded together in various structures.  Which of these can you name?

 Top 10 Companies worldwide: Exxon Mobil Wal-Mart Stores Royal Dutch Shell BP General Motors Chevron DaimlerChrysler Toyota Motor Ford Motor ConocoPhillips  How many are oil companies? What do the others sell?

What are “polymers”? A polymer is a large molecule that is created when monomers are joined together. A monomer is a single unit that is used to build a polymer. Polymers may be naturally occurring or man-made (synthetic).

Straight Chained Polymer (made up of one type of monomer) Spaghetti-like structure of straight chain polymers. Coils lead to entanglement (stiffness).

Branched Chain Polymer Branching reduces the density and increases the flexibility of a polymer.

Low density polyethylene has more branches, so it cannot be packed as closely as in linear, high density polyethylene.

Cross Linked Polymers Cross-linking increases stiffness and strength of a polymer.

 Properties are determined by the structure of the molecules and depend on: -type of monomers used -chain length -branching -degree of cross-linking

DIRECTION OF INCREASING STRENGTH

Branched, Cross-linked or Linear?

Polymers are large molecules consisting of chains of small molecules called monomers joined together in a repeating pattern. In the early 1900s, scientists began to understand the makeup of natural polymers and how to make synthetic polymers with properties that complement, or improve on, those of natural materials. One simple synthetic polymer chemists developed is polyethylene. They developed it by repeating units of the monomer ethylene (H 2 C=CH 2 ). Polyethylene is a very large, zigzag- shaped molecule. One small part of a polyethylene chain is shown below.

Chemists and engineers have learned to process and modify molecules of polyethylene in different ways to manufacture common household products with a variety of characteristics. Polyethylene is used to make plastic trash bags, dry cleaning bags, milk jugs and soda bottles. In industry, materials made from polyethylene are tested for what are called “stress-strain behaviors.” stress- strain behaviors include: Tensile strength - the amount of pulling force placed on a material before it breaks Abrasion resistance - toughness of a material against scraping, scuffing or scarring Puncture resistance –ability of a material to keep moving objects from perforating the surface.

Your task You and your lab partners will design an experiment that investigates one stress-strain behaviors of various plastic products made of the synthetic polymer polyethylene. You have been provided with an assortment of plastic products to test. The stress-strain behaviors you will investigate are tensile strength or puncture resistance. Remember the importance of only testing ONE variable at a time, keeping all others constant as much as possible.

Tensile strength The tensile strength of a material measures how much pulling stress the material will endure before failing. This is very important in applications that depend on a polymer's physical strength or durability. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping. In general, tensile strength increases with polymer chain length. Puncture resistance The puncture resistance of a material measures how much force is required for a moving object to break through a material. This is also very important for certain applications such as trash bags –a greater puncture resistance will result in less trash poking through and spilling out on the ground! Puncture resistance also generally increases with greater chain length.

Designing Your Experiment In your own words, state the problem you are going to investigate. Write a hypothesis using an “If… then… because..” statement that describes what you expect to find and why. Include a clear identification of the independent and dependent variables that will be studied. Your experimental design should match the statement of the problem and should be clearly described so someone else could replicate the experiment. Use a diagram if necessary to help explain your design.

Things to consider in your design: 1. How will you measure the amount of stretching the plastic can endure? What will you consider the starting point? What will be the ending point? 2. How can you keep the force of a moving object constant? Is there a natural force you can use that is ALWAYS the same? Remember the importance of only testing ONE variable at a time, keeping all others constant as much as possible. Also remember the importance of making valid conclusions from your data… how many trials will you do?

Polyethylene (PE) used for: flexible bottles, ice trays, plastic bags Some examples of synthetic polymer structures:

Polyvinyl Chloride (PVC) used for: pipes, bottles, CD’s, computer housings

Polypropylene (PP) used for: rope, luggage, carpet, film, polar fleece propylene

Polystyrene (PS) used for: toys, packaging, egg cartons, flotation devices, hot cups

 Gelatin in gummi worms and gummi bears are made from natural polymers!  Bubble gum contains styrene butadiene rubber!  Carbohydrates (starches) and proteins are examples of natural polymers! natural polymers

Polymers at the movies…. Nylon carpet, polyester and acrylic seats, polyester curtains, nylon screen, polyester film strip, waxy polyethylene popcorn tub, starch in popcorn, polystyrene cups, plastic M&M bag, protein in hotdogs, gelatin in gummy bears, paraffin in Junior Mints, sticky stuff on the floor made of soda, butter, Skittles, Milk Duds and more…