- A large number of paperclips, preferably of different size and colour.
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University of York online course,
Exploring Everyday Chemistry
Polymer structure and propertiesWhen designing a polymer for a particular application, there are a number of factors that affect the physical properties of a polymer, and so need to be considered. For example, the viscosity, strength and toughness of a polymer depends on its molecular weight (this is determined by the number of repeating units in the polymer chain and the molecular weight of the repeating unit). The lower the molecular weight of a polymer, the lower the viscosity and the mechanical properties. On increasing molecular weight, due to increased entanglement of chains, a polymer becomes more viscous and tougher, which makes the processing of the polymer more difficult.Also, polymer chains can attract one another encouraging them to stick together, using secondary bonds or intermolecular forces. Take, for example, the polyamide Kevlar. Its strength comes from strong intermolecular forces between adjacent chains of the Kevlar polymer. One intermolecular force is hydrogen bonding, which describes the electrostatic attraction between some hydrogen atoms in –NH– groups in one chain and the oxygen atoms in –C(=O)– groups in another chain. Additional strength is derived from stacking interactions between the benzene rings in different chains.Another factor that needs to be considered is tacticity. This term is used to describe the way pendent groups on a polymer chain are arranged on a polymer backbone. The tacticity of a polymer is determined by what side of the polymer chain the pendant groups are on (i.e., are all of the groups pointing towards us, or are some pointing away from us). This is important because the relative position of the groups can have dramatic effects on the physical properties of the polymer.Tacticity arises when there are chiral carbons in the polymer chain backbone. For example, this occurs in free radical polymerisation of propene, H2C=CH(CH)3, to form poly(propene), [–CH2–CH(CH)3–]n. When a propene monomer adds to the end of the growing polymer chain, the monomer can either join the pendant CH3 group on the same side as all of the other CH3 groups, or it can join the pendant CH3 group on the side away from the nearest pendant CH3 group. If propene adds to the polymer backbone with the pendant CH3 group on the same side as the previous CH3 group, this is called isotactic. If propene adds so that its CH3 group adds to the opposite side of the previous CH3 group, it is called syndiotactic. If there is no order to the way the CH3 group adds, (completely random) the polymer is said to be atactic (see the pdf in the downloads section below).The tacticity has a dramatic effect on the physical properties of poly(propene). Atactic poly(propene) has little order in the polymer backbone and it is called an amorphous polymer. The polymer chains move across each other when it is pushed or pulled, so it has some flexibility and elasticity. Isotactic poly(propene) has long-range order, which adds mechanical strength and crystallinity. It is called a semicrystalline polymer and it is a stiffer material. The demand for these polymers resulted in the development of a range of catalysts (by Karl Ziegler and Giulio Natta) that can selectively prepare atactic or isotactic poly(propene).Is it possible to illustrate intermolecular forces and/or tacticity in your paperclip model?
Modelling competitionThere will be a prize for the best photo of your model(s) posted on our Padlet site and/or on Twitter or Instagram by 9am on Monday 27 July. The prize will be given to the person whose photo, in the opinion of Andy, is the most striking, detailed, informative and memorable. We will advertise the name of the winner in the following week (in the comments section below) who will be sent a copy of Chemistry3 and a Chemistry@York fidget spinner. (This competition is being run by Andy / University of York and is not affiliated with FutureLearn and any personal details submitted by the learner will only be used for the purpose of sending the prize.)
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Exploring Everyday Chemistry
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