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Binary files /dev/null and b/mechanical/images/vimscrot-2021-11-01T12:56:40,483489005+00:00.png differ diff --git a/mechanical/mmme1029_materials.md b/mechanical/mmme1029_materials.md index ea0fe20..4f71009 100755 --- a/mechanical/mmme1029_materials.md +++ b/mechanical/mmme1029_materials.md @@ -261,3 +261,129 @@ $$\rho = \frac m v$$ ### Consolidation Questions 2 > ~~C~~ B + +# Polymers + +## Introduction to Polymers + +There are 3 types of polymers: + +- thermoplastics + + ![](./images/vimscrot-2021-11-01T11:11:38,143311655+00:00.png) + + No cross links between chains. + The lack of cross links allows recycling of polymers by heating it above the glass transition + material, $T_g$, lowering the viscosity. + + An example of thermoplastics is PET, used in water bottles + +- thermosets + + ![](./images/vimscrot-2021-11-01T11:11:59,529214154+00:00.png) + + has lots of cross-links between chains, making it more rigid. + Heating does not lower its viscosity making them much harder/impossible to recycle. + + and example of thermosets is melamine formaldehyde, used on kitchen tabletops + +- elastomers + + ![](./images/vimscrot-2021-11-01T11:12:28,335292407+00:00.png) + + has some cross links and a lot of folding of chains + + Latex is an example of an elastomer + +Polymers are relatively new materials, lightweight, durable, flammable, and degraded by UV light. +They are made of long carbon-carbon chains. + +### Stress-Strain Curve of Polymers + +![](./images/vimscrot-2021-11-01T11:13:39,370133338+00:00.png) + +## Thermoplastics + +The simplest polymer is poly(ethene): + +![](./images/vimscrot-2021-11-01T11:26:51,027062158+00:00.png) + +When 2 polymer chains get close together, Van der Waals (vdw) forces keep them together. +vdw forces are very weak, much weaker than the covalent bonds inside the polymer. + +### Stress Strain Curve + +![](./images/vimscrot-2021-11-01T11:33:17,944832427+00:00.png) + +- During linear deformation, the carbon chains are strethed. +- At yield stress, the carbon chains get untangled and slide past eache other. +- Necking initially allows the chains to slide at lower stress. +- As the chains pull, align, and get closer, the vdw forces get stronger and more stress is required + to fracture. + +### Crystalline and Amorphous/Glassy Solids (Heating and Cooling) + +#### Amorphous Thermoplastics + +- As you heat above $T_g$, the chains get easier to move past each other. +- It is known as an *amorphous supercooled liquid*. +- There is not really a melting point are there are no crystals, but $T_m$ is the point where the + chains are easy to move + +#### Crystalline Polymers + +- The glass transition point does not exist for crystalline polymers +- The solid is difficult to deform below $T_m$ and is not ductile +- Above $T_m$ the chains are very easy to move past each other + +#### Semi-Crystalline + +- Below $T_g$, only local movements in chains are possible, so the material is less ductile. + The solid crystalline regions makes it difficult to move the chains. +- Between $T_g$ and $T_m$, the glassy chains are easier to move but the crystalline regions remain + difficult +- Above $T_m$ the chains easily move past each other + +### Specific Volume vs Temperature + +![](./images/vimscrot-2021-11-01T12:56:40,483489005+00:00.png) + +#### Path ABCD + +- a-b --- Start cooling the true liquid +- b-c --- At the freezing point, $T_m$, the true liquid freezes diretly to a crystalline solid +- c-d --- The crystalline solid cools t room temperature as the temperature is lowered + +#### Path ABEF + +- a-b --- start cooling the true liquid +- b --- at the freezing point nothing freezes +- b-e --- the liquid becomes *supercooled* and contracts and becomes more viscous as the temperature + decreases. + + The supercooled liquid region is between $T_g$ and $T_m$ + + Supercooling requires you to cool the sample quicker than you would for path ABCD + +- e --- $T_g$ is reached and the supercooled liquid sets to a amorphous solid +- e-f --- the amorphous solid cools from room temperature and contract as the temperature is lowered + +## Relative Molar Mass and Degree of Polymerisation + +- Number Average RMM --- $\bar M_n = \sum x_iM_i$ +- Weight Average RMM --- $\bar M_w = \sum w_iM_i$ +- Degree of polymerisation --- $n_n = \frac {\bar M_n} m$ and $n_w = \frac {\bar M_w} m$ + +where + +- $M_i$ is the RMM of the chain +- $x_i$ is the fraction of the polymer that is composed of that chain by number/quantity +- $w_i$ is the fraction of the polymer that is composed of that chain by mass/weight +- $m$ is the RMM of the monomer from which the polymer was made + +## Making Polymers + +There are two ways to make polymers: + +- [Addition Poymerisation](http://www.chemguide.co.uk/14to16/organic/addpolymers.html) +- [Condensation Polymerisation](https://www.chemguide.uk/14to16/organic/condpolymers.html)