BME282 Tutorial 4 Solutions To the Problems Assigned Very Useful PDF

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BMESolutions of Tutorial Week 4 / GRADEDGuided Inquiries (see Douglas textbook)A) Start a thread A“Tutorial roles and polymeric chain shapes” Define roles, as in tutorial 2. GI 5.1 Of the three shapes shown in Figure 5.1, which can be recycled? Explain why. Once you break a covalent bond, you can’t ...

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BME282 Solutions of Tutorial Week 4 / GRADED Guided Inquiries (see Douglas textbook) A) Start a thread A “Tutorial roles and polymeric chain shapes” • Define roles, as in tutorial 2. • GI 5.1.8 Of the three shapes shown in Figure 5.1.4, which can be recycled? Explain why. Once you break a covalent bond, you can’t put it back together. Ans: Linear polymers and branched polymers are recyclable because: Repeating units of linear polymers are joined together end to end in single chains. These long chains are flexible and may be thought of as a mass of spaghetti. For linear polymers, there are extensive weaker van der Waals and hydrogen bonding between the long chains. Since these bonding types are broken relatively easily with heat, linear polymers are typically thermoplastic, which makes these polymers can be remolded and recycled. Branched polymers resemble linear polymers. There are the additional shorter chains, which are called branches. They may result from the side reactions that occur during the synthesis of polymers. The short chains do not bridge from one longer backbone to another, therefore, like linear polymers, heat can break the bonds between the branched polymer chains, and most of them are also classified as thermoplastic. Branched polymers can be remolded and recycled. On the other hand, in crosslinked polymers, adjacent linear chains are joined one to another by covalent bonds. This much stronger bond makes most crosslinked polymers thermosetting. These strong crosslinks resist to soften upon heating. Therefore, these polymers cannot be recycled.

• Ans:

GI 5.1.9 Which polymer will more easily form a crystal; linear or branched? Explain why.

For linear polymers, as there are few restrictions to prevent chain alignment, crystallization is easily accomplished. Any side branches interfere with crystallization, such that branched polymers never are highly crystalline; in fact, excessive branching may totally prevent crystallization. So, it can be said that linear polymers are more easily crystalline than branched polymers.

• Ans:

GI 5.1.12 Of the three tacticities, which least easily forms a crystal? Why?

In terms of the tacticity, atactic polymers are least easily crystalline ones. Because the regularity of the

geometry of the side groups of isotactic and syndiotactic polymers makes them easier to put together adjacent chains with each unit contributing to bonding. Therefore, isotactic and syndiotactic polymers crystallize much more easily. On the other hand, because of the randomly placed side groups of atactic polymers, they are difficult to crystallize.

•

CC 5.1.4 Which type of polypropylene would form a crystal more easily; isotactic or atactic?

Ans: As explained in GI 5.1.12, presence of regularly spaced side groups facilitates the crystallization. Therefore, crystallization occurs much easier in isotactic polypropylene than in the atactic polypropylene. Isotactic polypropylene are commonly crystalline polymers; on the other hand, atactic polypropylene is mostly amorphous, i.e. they have no crystal structure.

Figure 5.1.4 Possible molecular shapes of polymers.

B) Start a thread B “Molecular weight influencing material properties” • GI 5.2.4 Calculate the number average molecular weight, weight average molecular weight, and PDI for the given mixture (15 mol% polymer molecules with molecular weight 500,000 g/mol, and 85 mol% polymer molecules with molecular weight 150,000 g/mol) Ans: 𝑀! = #

∑" 𝑁" #𝑀" ∑" 𝑁"

𝑴𝒏 = #

( 15 ∗ 500000 ) + (85 ∗ 150000) (15 + 85)

𝑀$ = #

= 202,500#𝑔/𝑚𝑜𝑙

∑" 𝑤" #𝑀" ∑" 𝑁" #𝑀"% =# ∑ " 𝑤" ∑" 𝑁" #𝑀" 𝑤" = # 𝑁" #𝑀" #

𝑴𝒘 = ##

( 15 ∗ 500000% ) + ( 85 ∗ 150000% ) 𝑔 = 279629.63 ( 15 ∗ 500000 ) + ( 85 ∗ 150000) 𝑚𝑜𝑙

𝑷𝑫𝑰 = #

• Ans:

𝑀$ = 1.381 𝑀!

"! be less than 𝑀 "" ? Explain why or why not. GI 5.2.5 Can 𝑀

It is expected that the number average molecular weight (Mn) is less than the weight average molecular weight (Mw), because Mn considers the number fraction of the chains within each size range. On the other hand, the Mw considers the weight fraction of chains. Hence, irrespective of the distribution of molecules with varying molecular mass, the weight average cannot be lower than the number average.

•

GI 5.2.10 You are working at a plastics company, and your boss wants to modify an existing polymer so it will be stronger and flow more easily for processing. How would you change the molecular weight distribution to meet both of these requirements?

Ans: First, consider a polymer with a large chain length (i.e. higher molecular weight), it is expected that this polymer will have better strength but with high viscosity. On the other hand, a shorter chain length (lower molecular weight) will have better flowability (easier during processing) but less strength. To have a balance of these properties, you can suggest a mix between longer chains and shorter chains.

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GI 5.3.2 You just cut down some bushes and want to stack them all up in a neat pile. What would you do to the bushes so they all stack up neatly?

Ans: You can try to have them around the same size range and align them properly to enable easy stack up. This will reduce the distances between them and probably enable more density in the stack up if closely aligned.

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GI 5.3.4 For each of the following pairs of polymers, predict which one is more crystalline and explain why: Remember, this is all about how easily you can pack the molecules together. a) Polyethylene or polystyrene b) Polyethylene with many long branches or polyethylene with a few short branches c) Atactic polypropylene or isotactic polypropylene d) Polypropylene or poly(ethylene terephthalate) (PET) e) Polyethylene or a random copolymer of 50% ethylene repeat units and 50% propylene repeat units f) PET melted and rapidly cooled to room temperature or PET melted, cooled to 150 °C, held there for 1 hour, and then cooled to room temperature.

Ans: a) Polyethylene has H as side group and can be crystallized more easily than polystyrene (benzene ring as side group) b) Generally branched chained polymers show lower levels of crystallinity because it prevents dense packing of chains. It is expected that polyethylene with short branches will be more crystalline c) Isotactic polypropylene will tend to be more crystalline. Atactic polypropylene is more likely to have an amorphous structure and tends to give poor packing. d) Polypropylene will be more crystalline because of its simpler side group compared to PET e) Polyethylene is expected to be more crystalline than a random copolymer f) PET held at 150 ℃ for 1 h is expected be more crystalline. PET cooled rapidly is not given sufficient time to crystallize hence tend to be amorphous.

C) Start a thread C “Glass transition temperature and melting” • GI 5.4.5 For each of the following, identify which has lower Tm or Tg and explain why: Remember, this is all about how easily it is for the molecules to move past each other. a) Isotactic polypropylene or isotactic polystyrene b) Polypropylene or poly(vinyl chloride) (PVC) c) Polystyrene with Mn of 5,000 g/mol or polystyrene with Mn of 100,000 g/mol. d) Pure PVC or PVC mixed with some solvent. Ans: a) Isotactic polypropylene has lower Tm and Tg, as its structure has smaller side groups, meaning the structure is more balanced compared to polystyrene, making the transition temperatures lower than polystyrene.

b) Polypropylene have the lower Tm value since Poly (vinyl chloride) is a much smaller polymer than polypropylene thus making the chains more mobile as there is more free volume.

c) Polystyrene with Mn of 5,000 g/mol has lower Tm and Tg. Since Mn represents the number average molecular weight, the smaller molecular weight indicates that there are fewer intermolecular forces. Less energy is required to break the bonds.

d) PVC mixed with a solvent has lower Tm and Tg. Since it is increasing the amount of free volume between polymers that also react to heat by moving more. This allows the polymer to slide around easier.

•

SP 5.10 For each of the following pairs of polymers, indicate which has the higher Tg, or if they are the same, or if you can’t tell, and give a brief (one-sentence) explanation as to why: a) Atactic polystyrene of Mn = 5000 g/mol or atactic polystyrene of Mn = 8000 g/mol b) Pure nylon 6,6 or nylon 6,6 with 5 wt% absorbed water c) Polypropylene or polystyrene of the same Mn d) The following two polymers:

Ans: a) 8,000g/mol polystyrene have a higher Tg. Higher Mn means each molecule is heavier. It is heavier because the DP value is higher. The chains would be longer, therefore leading to more difficulty in the chains moving past each other. b) Pure Nylon. It is harder for solid nylon to move past each other, adding water results in hydrogen bonds forming with the polymer chain which increases the distance between chains and therefore increases the free space which lowers the Tg. In other words, the water action is like some lubrication, allowing the polymer chains to slide on each other easier, leading to a lower Tg. c) Polystyrene have a higher Tg, since it has a larger side group which reduces its flexibility and makes it harder to move past other chains of the same material. d) Poly (vinyl alcohol) has a higher Tg as its side group is more polar than the alcohol side group in poly(vinyl methyl ether). Which increase the distance between chains and therefore the free space between them.

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CP 5.12 Can polyethylene exhibit tacticity? If it can, draw structures showing the three types of tacticity for it. If not, explain why.

Ans: Polythene cannot exhibit tacticity because it is reliant on the differing placement of side groups on the polymer chain. Tacticity requires a difference in the placement of side groups along the chain (above or below, randomly, same side or alternating). Therefore, tacticity cannot occur in polythene.

•

CP 5.13 Can nylon 6 exhibit tacticity? If it can, draw structures showing the three types of tacticity for it. If not, explain why.

Ans: This is because the carbonyl group has a single double-bonded oxygen to the central carbon chain atom. Tacticity occurs in polymers that consist of chains of single bonded carbon-carbon in the main chain. This means that there is no variation in the position that the oxygen can be in. Therefore, it cannot exhibit tacticity because there is no variation pattern to repeat.

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CP 5.14 Two common types of glue are epoxy and super glue. Epoxy cannot be dissolved by any solvent, while super glue can be dissolved by acetone. What does this information tell you about the molecular shapes of these two polymers?

Ans: Twaron has stronger intermolecular forces meaning that it requires more energy to pull apart molecules and the bond between molecules is stronger. That is the reason why polyester has a lower melting point but higher flexibility....