The crystalline density of polyethylene (PE) is 1000 kg/m3 and the density of am

Question

The crystalline density of polyethylene (PE) is 1000 kg/m3 and the density of amorphous PE is 865 kg/m'. a) Explain why is the density of the amorphous PE lower than that of the crystalline? b) Calculate the mass fraction crystallinity in a sample of linear PE of density 970 kg/m and a sample of branched PE of density 917 kg/m3. Why do the two samples have considerably different densities? Suggest a technique (or techniques) that would allow you to determine whether a sample is semicrystalline of amorphous and explain how would you do the measurement (be brief-no details regarding the instrumentation, detailed data analysis etc. are necessary). c)

Explanation / Answer

a)

Crystalline compounds have ordered molecular chain arrangements, hence they are closely packed and have higher density.

b)

1.

D = 970 kg/m3

Dc = 1000 kg/m3

Da = 865 kg/m3

Let mass fraction of crystallinity = x

1 / D = x / Dc + (1 – x) / Da

1 / 970 = x / 1000 + (1 – x)/ 865

x = 0.8

2.

D = 917 kg/m3

Dc = 1000 kg/m3

Da = 865 kg/m3

Let mass fraction of crystallinity = x

1 / D = x / Dc + (1 – x) / Da

1 / 917 = x / 1000 + (1 – x)/ 865

x = 0.42

Linear PE has higher % crystallinity and has higher density as compared to branched PE.

3.

From Density measurements:

1. Weigh the sample (say = m grams)

2. Put the sample in a volume measuring cylinder. Quickly, find out the displaced volume of water (say V mL).

3. Calculate density = m / V grams/mL

Use the above formula, 1 / D = x / Dc + (1 – x) / Da to find out % crystallinity.

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