Lab 7
Mechanical Properties of Polymers

Student: Mikel Ricks
Class: ME 3701-1
Instructor: Yang Mu

This report contains experimental data and results regarding the mechanical properties of polymers. These tests were conducted under controlled environments to determine the tensile strength of certain polymers.

Abstract
This lab report was constructed from the results of testing the tensile strength of various polymers. These tests were done to determine the material properties, such as modulus of elasticity and the ultimate strength. The polymers that were tested were an acrylic and a nylon sample. Each specimen was placed in a tensile test machine and placed under load until failure. This is an important test because polymers are very lightweight and strong materials and it allows uses for them to be discovered by knowing their tensile strength. After the testing the data showed that the acrylic sample had a higher ultimate strength than the nylon sample.
Table of Contents Title | Page No. | Introduction | 3 | Experimental Apparatus and Procedure | 4 | Results | 8 | Discussion | 10 | Conclusion | 10 | References | 10 | Homework | 11 |

List of Nomenclature -Engineering Stress -Engineering Strain -Modulus of Elasticity -Ultimate Strength

List of Figures and Tables Title | Page No. | Figure 1—Specimen Load Sample | 4 | Figure 2—Instron Machine | 4 | Figure 3—Nylon Stress Strain Curve | 5 | Figure 4—Acrylic Stress Strain Curve | 5 | Figure 5—Stress Strain Curve of Different Materials | 8 | Table 1—Acrylic Measurements | 4 | Table 2—Nylon Measurements | 4 | Table 3—Acrylic Properties | 6 | Table 4—Nylon Properties | 6 |

Introduction
There are many materials which are used in the world today in which they are specifically designed for a certain task. An example of these materials are called polymers. Polymers are also referred to as plastics that are high molecular weight materials that are synthesized from simple organic compounds. Polymers are characterized by their low density, low strength, and high mechanical damping ability. Polymers do not handle temperature variations very well, so they are widely used at ambient temperatures. Engineering polymers generally fall into one of three important categories: 1. Thermoplastics Polymers (PVC, Polyester, Nylon, ABS, etc.) are generally composed of long coiling chains which are NOT cross-linked. They have generally low-strength, low-modulus polymers which soften when heated and harden when cooled in a reversible manner.

2. Thermosetting Polymers (Epoxies, Phenolics, Polyurethane, etc.) are generally composed of crosslinked space networks. Crosslinks form between chains when the material is cured. Thermosetting polymers become hard after curing (heating facilitates crosslink formation) and do not soften upon reheating. They are comparatively strong and stiff and can’t be recycled.

3. Elastomers (rubbers) (Polybutadiene, Neoprene, Isoprene, etc.) form the third category and include polymers which can forego a significant amount or reversible plastic deformation. The long chained elastomers do not crosslink and generally return to their minimum energy coiled positions (original shape) when unloaded.

A simple technique that is used to relate the properties and behavior of polymers to their reliability and usage is to observe the polymer’s stress strain behavior using a tensile test. This will reveal a lot of details about the polymer that will help classify what usage it will have. The main things that need to be observed when doing the tests are: 4. Creep: It often occurs in polymers where deformation continues to occur under conditions of constant stress.

5. Stress Relaxation: It refers to the decrease in the amount of stress required to maintain a specific strain.

Experimental Apparatus and Procedure
Tensile Test
The tensile tests were conducted using Instron machine which can be seen in figure 1. This machine is rated up to fifty-thousand pounds and uses hydraulics to achieve its force. It is controlled by a computer which can be operated at a safe distance. The computer monitors the data and records it so it can be studied after the test is concluded. Before the tensile tests were conducted measurements of each specimen were taken and an average for each was calculated and can be seen in the results section. After the measurements were taken, the specimens were loaded into the machine one at a time and the tensile test was applied to each until failure. Figure 2 shows what the specimens experienced as the load increased.
Figure 2
Figure 2

Figure 1
Figure 1

Results
Below are the results from the measurements of each specimen: | Acrylic | | Diameter (in) | Table 1
Table 1
Length (in) | | .356 | 2.351 | | .357 | 2.321 | | .357 | 2.311 | | .357 | 2.332 | | .355 | 2.336 | Average | .3564 | 2.3302 |

| Nylon | | Width (in) | Length (in) | Thickness (in) | | .517 | 3.222 | .126 | | .508 | 3.213 | .125 | | .513 | 3.211 | .122 | | .505 | 3.224 | .13 | | .504 | 3.22 | .126 | Average | .5094 | 3.218 | .1258 |
Table 2
Table 2

Below are the graphs of stress vs strain for the two samples:
Strain

Strain

Stress (Pa)

Stress (Pa)

Figure 3
Figure 3

Strain

Strain

Stress (Pa)

Stress (Pa)

Figure 4
Figure 4

Below are the properties of the materials that were calculated based on the data: Acrylic | Table 3

Table 3

Ult. Strength (kPa) | Mod. E (GPa) | .2% (kPa) | 6.6x10^4 | 1.46 | 5.4x10^4 |

| Table 4

Table 4

Ult. Strength (kPa) | Mod. E (GPa) | .2% (kPa) | 5.8x10^4 | .71 | 3.5x10^4 |

Discussion
The results from the tests ultimately show the nylon is tougher than the acrylic. This is apparent since it can deform to a larger extent. The data collected seemed to be very accurate because the acrylic specimen showed a brittle fracture which is a property of a thermosetting polymer. This means the material is not very ductile and will just fracture under such load. The Nylon specimen is able to absorb more of a load by deforming, but the acrylic can absorb a smaller load without absorbing as much. Each has a unique way of handling the load and will benefit in each case.
Conclusion
Based on the results from the lab tests the conclusion can be made that a thermosetting polymer such as acrylic is a brittle material at room temperature and can absorb less energy than a thermoplastic polymer such as nylon which is more ductile.
References
"Polymers." Polymers. N.p., n.d. Web. 31 Mar. 2014.
"Chemical of the Week -- Polymers." Chemical of the Week -- Polymers. N.p., n.d. Web. 31 Mar. 2014.
"Thermoplastic." Thermoplastic. N.p., n.d. Web. 01 Apr. 2014.
"Thermoset - Definition, Properties and Examples of Thermoset." Thermoset. N.p., n.d. Web. 01 Apr. 2014.

Homework 1. Polymers are held together by secondary bonds. Some primary bonds are involved in a class of polymers called thermosets. The bonding between the chains is a weak van der Waals bond. Van der Waals, permanent dipole and hydrogen bonds are secondary bonds. They are 10 to 50 times weaker than the primary bonds of metals and ceramics and account for the mechanical properties and the low melting temperatures of polymers. The polymers we have described are thermoplastics: when they are heated, their thermal energy overcomes the weak bonds between chains; the material becomes progressively softer until it liquefies. The term "thermoplastic" indicates that the materials can be deformed by heating 2. The main differences between thermoplastics, thermosetting, and elastomers is their bonding. Thermoplastics are generally composed of long coiling chains which are not cross linked. This causes a low strength and a material that is softened when heated and hardened when cooled. Thermosetting polymers are composed of crosslinked space networks. Crosslinks form between chains when the material is cured. Heating does not soften thermosetting polymers. Elastomers can undergo a large amount of reversible plastic deformation because of their rubber like structure. They are not cross linked and generally return to their minimum coiled positions.
4. At relatively high temperatures, polymers become liquid like and deform viscously, while at relatively low temperatures, polymers are rigid solids which generally deform elastically. The boundary between these Elastic and Viscous behaviors is known as the Glass Transition Temperature (Tg). The knowledge of Tg is important because the application of the polymer defines the type of the polymer which is required; and that type can be determined using Tg.

7. Thermoplastics are organic materials that melt when heated. They should be differentiated from thermoset materials which cure, or become set, when they are heated. Thermoplastic materials are melt processable, that is they are formed when they are in a melted or viscous phase. This generally means they are heated, formed, then cooled in their final shape.
8. Thermoset materials are those materials that are made by polymers joined together by chemical bonds, acquiring a highly crosslinked polymer structure.The highly crosslinked structure produced by chemical bonds in thermoset materials, is directly responsible for the high mechanical and physical strength compared with thermoplastics or elastomers materials.On the other hand is this highly crosslinked structure which provides a poor elasticity or elongation of this materials.
9. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. At ambient temperatures, rubbers are thus relatively soft and deformable. Their primary uses are for seals, adhesives and molded flexible parts.

The main difference in the stress strain graph between a metal and polymer can be seen below in figure 6. Polymers will have much smaller toughness and ultimate stresses as seen from the figure. Metals seem to lie in between polymers and ceramics for being ductile and brittle which will allow for more toughness.
Figure 6

Figure 6