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Glass Transition (Simple Lab Report) Essays

Abstract The glass transition temperature of polymethylmethacrylate ( PMMA, Perspex) is measured using static and dynamic method while the glass transition temperature of rubber of different size is measured using only dynamic method. The glass transition temperature Tg of Perspex is found to be 90°C using the static method and 160°c using the dynamic method. On the other hand, the Tg of rubber is found to be -30°C. Introduction Perspex is a type of amorphous polymer and has the following repeat unit : If the PMMA is heated , the side group of the polymer will start to rotate.
As the temperature gradually increase and reach a certain temperature , the polymer would lose its stiffness and becomes elastic like a rubber. This temperature is also known as glass transition temperature ,Tg. The Tg is always lower than melting point of the polymer. By heating and cooling the polymer, and observing the changes in their physical properties we could identify Tg. (For static test method of Perspex) Experimental Tg is determine by observing at which temperature does the coefficient of thermal expansion change. This is done by using a mercury dilatometer which contains about 25g of Perspex in the glass bulb.
A thermocouple is placed between the glass bulb and heating coil to measure the temperature. Temperature is recorded everytime the height of Hg column change by 1 cm. When the temperature reach 120°C switch off the heat supply. Continue taking the readings as the temperature falls until 60°C. Results Height of Hg column while heating (cm)| Temperature (°C)| Height of Hg column while cooling (cm)| Temperature (°C)| 19. 1| 60. 9| 35. 4| 118. 9| 20. 1| 65. 6| 34. 4| 107. 4| 21. 1| 68. 7| 33. 4| 104. 2| 22. 1| 73. 9| 32. 4| 101. 5| 23. 1| 77. 7| 31. 4| 99. 1| 24. 1| 81. 6| 30. 4| 96. 7| 25. 1| 90. 0| 29. 4| 93. 9| 26. 1| 94. | 28. 4| 91. 2| 27. 1| 98. 3| 27. 4| 88. 2| 28. 1| 101. 7 | 26. 4| 84. 9| 29. 1| 103. 5| 25. 4| 81. 4| 30. 1| 105. 8| 24. 4| 72. 8| 31. 1| 108. 7| 23. 4| 73. 6| 32. 1| 111. 5| 22. 4| 69. 8| 33. 1| 114. 0| 21. 4| 65. 8| 34. 1| 116. 5| 20. 4| 61. 0| 35. 1| 118. 6| | | Fig. 1 Graph can be found at the end of the report. Discussion The height of Hg column represents the coefficient of thermal expansion . The Tg is able to be found by identifying where the gradient of graph changes. On the graph, there are difference between the Tg of heating and cooling of Perspex. For the heating curve the Tg is found to be 90°C and 73. °C for the cooling curve. This is due to the heat is not being transmitted equally throughout the system, which is cause by the positioning of thermocouple being outside of the bulb. As such to acquire a reliable value of Tg , we should take the average Tg value which is 81. 8°C. (For dynamic test of Perspex) Experimental The Perspex is heated from 100°C to 180°C. At the interval of 20°C drop the ball from 100cm, the rebound height of 3mm ball and 5mm ball is then measured and recorded. Repeat the experiment 3 times to obtain the average value of rebound height to minimise error. Results
Calculate the rate of restitution (e) by using e=(Rebound Height/Drop Height) The coefficient of restitution (e) is a measurement of elasticity of a collision. Thus a change in e means the physical property of Perspex has change and will lead us to finding Tg. Temp °C| Drop Height (cm)| Average rebound height of 3mm ball (cm)| e of 3mm ball| Average rebound height of 5mm ball (cm)| e of 5mm ball| 100| 100| 53. 5| 0. 54| 74. 2| 74. 2| 120| 100| 63. 3| 0. 63| 73. 2| 73. 2| 140| 100| 64. 2| 0. 64| 73. 8| 73. 8| 160| 100| 68. 0| 0. 68| 75. 9| 75. 9| 180| 100| 30. 4| 0. 30| 17. 2| 17. 2| Fig. Graph can be found at the end of the report. (For dynamic test of rubber) Experimental Measure the rebound of 2 rubber squash ball ( one fast and one very slow) which are 80°C, room temperature( 26°C), 0°C, -30°C, -60°C and -90°C respectively which are drop at height of 2m, 2m, 2m, 1m, and 1m. Results Temp °C| Drop height (cm)| Rebound height of 3mm ball (cm)| e of 3mm ball| Rebound height of 5mm ball (cm)| e of 5mm ball| 80±1| 200| 24±1| 0. 12| 37±1| 0. 19| RT 26±1| 200| 9±1| 0. 05| 77±1| 0. 39| 0±1| 200| 78±1| 0. 39| 95±1| 0. 48| -30±1| 100| 85±1| 0. 03| 9±1| 0. 09| -60±1| 100| 47±1| 0. 47| 51±1| 0. 51| 90±1| 100| 29±1| 0. 29| 33±1| 0. 33| Fig. 3 Graph can be found at the end of the report. ( For Perspex and rubber using dynamic method) Discussion When temperature is lower than the Tg of a particular polymer, the chain molecules in the polymer has low energy. Thus it, does not rotate much and remain quite stationary. As a result, when the ball is dropped, most of the energy gained during impact is transferred to kinetic energy, making it rebound higher. However, when the temperature is greater than Tg , the chain molecules in the polymer has high energy. This causes the molecules in the polymer to vibrate excessively.
As such, when the polymer hit the floor most of the energy has already dissipated. Thus, the height of rebound is lower. When the steel balls are dropped onto Perspex, the energy stored in the molecules is converted to kinetic energy of steel balls. E=(1/2)mv2 . As 3mm and 5mm steel balls have different mass, their velocities after rebound varies. Thus, coefficient of restitution is different for both balls. Static method involve very slow rate of testing. Thus, the chain molecules have sufficient time to react accordingly. As for the dynamic method, when the balls are dropped, they only have momentarily contact with the surface.
Thus the Tg measured varies greatly. The speed of ball depends on the Tg of the polymer. By using polymer with different value of Tg we could manipulate their speed. Conclusion By observing the sudden change in gradient of graph from Fig. 1, Fig. 2 and Fig. 3, we could identify the glass transition temperature of the polymer. Finding the Tg of polymer is crucial because at this temperature, thermal expansion, heat capacity, shear modulus, and many other properties changes drastically. Reference www. wikipedia. com Introduction to Polymers 2nd Edition (R. J YOUNG AND P. A. LOVELL) chapter 4. 4 pg 290-299

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