Stress-Strain analysis of PE and PS. In this essay, the chemical structure of polyethylene (PE) and polystyrene (PS) will be explained in detail. The polymers will be stretched till they fracture under lab conditions and the result will be analysed.

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STRESS-STRAIN ANALYSIS

Abstract:

In this essay, the chemical structure of polyethylene (PE) and polystyrene (PS) will be explained in detail. The polymers will be stretched till they fracture under lab conditions and the result will be analysed.

Aims:

The aim of this report is to analyse the stress-strain graphs and deduce the mechanical properties of thin and thick polyethylene and polystyrene samples from the data provided.

Background:

Polymers are organic compounds that are made up of smaller repeating molecules known as monomers. These molecules are held together by intermolecular forces. The atoms within a polymer molecule are held together by covalent bonds. Polymers are usually made up of long molecular chains with irregular and entangled coils. The physical properties depend upon the length of the chain of molecules. Polymers are viscoelastic materials since they include characteristics of both elastic materials and viscous materials [Callister].

Polyethylene: Polyethylene is formed when the ethylene gas is reacted under the appropriate conditions [Callister]. This process begins when a reaction takes place between the initiator or catalyst species and the ethylene monomer. This sequential addition of monomers units to the actively growing chain molecule gives a rise to a polymer chain. After the addition of many ethylene monomer units, the final result produced is the polyethylene molecule. The major characteristic of polyethylene is that it is chemically resistant and electrically insulating. Polyethylene is a hard and ductile material. It is typically used in make toys, bottles, ice trays, computer components, furniture packing’s and many more [Callister].

Polystyrene: Polystyrene, commonly known as Styrofoam, is produced from styrene through free radical vinyl polymerisation [PPE]. In polystyrene made products, 95% is air. Polystyrene products can be in the form of foam or solid. Solid polystyrene looks nothing like Styrofoam; it is commonly used in making of video and audio cassettes, yogurt cups and many more.  On the other hand, foamed polystyrene is used to make coffee cups, disposable plates, perishable food containers and many other things. The major characteristics of polystyrene are that they have excellent electrical properties and optical clarity. Previously, chlorofluorocarbons (CFCs) were used in production of polystyrene based products but now a day, making polystyrene based products are environment friendly.

Experimental Procedure.

The six polymer samples which are two thick polyethylene, two thin polyethylene and two polystyrene samples are collected. Each sample is tested one at a time and the dimensional measurements of the samples are recorded by using a micrometer, this includes values of the thickness, width and the length between the two reference points of the polymers. The camera uses the reference points to calculate the final extension of the polyethylene. The polyethylene is then placed between two grips and is extended at a rate of 10mm per minute. Once the polyethylene has reached its maximum extension it under goes the process of necking and breaks at this weak point. Necking only occurs as the polymer goes beyond the elastic point. Polystyrene undergoes the same process using higher rates of extension. The data collected should be used to produce a table and a graph of extension against load. The graph will then display the ultimate tensile strength at the peak. At slower rates the extension of polymers is meant to be greater and the width of the material gradually decreases as the length increases.

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The use of safety goggles is to ensure no excess specimens enter the eye.

Results Table:

The table above shows the experimental value of Young’s Modulus for thin and thick polyethylene and polystyrene. ‘Young’s Modulus is the coefficient of elasticity of a substance, expressing the ratio between a  that acts to change the length of a body and the fractional change in length caused by this ’.

Young’s modulus is calculated my using the graph i.e. find the gradient of the stress-strain cure of the linear elastic region. The data produced by the computer is ...

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