Polymer Science and Technology

Question 1

What is a Lewis acid?

Give an Example of a Lewis acid and a Lewis base.

Answer 1

“A Lewis acid is a compound or ionic species which can accept an electron pair from a compound”

It is a chemical species with an empty orbital, capable of accepting an electron pair from a Lewis base.

An example Lewis acid would be Me3B: This has space for an additional electron pair.

An example Lewis base would be NH3, which has a spare electron pair.

Question 2

What is the most widely used epoxy prepolymer?

Draw a diagram of the molecule.

Answer 2

DGEBA: diglycyl ether of Bisphenol A.

This is a colourless viscous liquid. Addition of further BPA with heat and a catalyst can form a BPA glycidal ether epoxy resin.

Question 3

Draw a diagram of bisphenol A.

Answer 3

BPA:

Question 4

Draw a diagram of bisphenol B.

Answer 4

BPB:

Question 5

Organic Acid + Alcohol = ???

Answer 5

Ester

Question 6

Draw Adipic acid

Answer 6

(CH2)4(COOH)2

Question 7

What is the difference between primary and secondary bonding?

give 3 examples of each type.

Answer 7

Primary bonding uses valence electrons - ionic, covalent and metallic bonding are examples.

Secondary bonding does not use valence electrons. Examples are Van der Waals, hydrogen and dipole bonds. The weakest is van der waals.

Question 8

Describe the difference between a fibre, an elastomer and a plastic? What leads to these differences?

Answer 8

Molecules with regular structure can have very close alignment, which leads to strong secondary bonding (secondary bonding requires close spacing). This results in the formation of a fibre. Fibres are linear polymers with high symmetry and high intermolecular forces, which usually arise from the presence of polar groups. Fibres have high modulus, high tensile strength and moderate elongation (<20%).

At the other end of the spectrum are molecules with irregular structure, resulting in weak intermolecular attractive forces (Van de Waals, dipole, hydrogen bonding). This leads to flexible polymer chains, since they are not locked into position relative to one another. These are called elastomers. chain segments of elastomers can undergo high local mobility, but the gross mobility of the chains is limited, usually due to the introduction of a few cross-links. in the absence of applied stress, elastomers have coiled chains. Therefore, elastomers exhibit high elongation, up to 1000%. They recover rapidly on the removal of the stress. Elastomers have low initial modulus in tension, but stiffen as the chains elongate.

Plastics fall between the extremes represented by elastomers and fibres. Polymers such as polypropylene and polyamides can be used as fibres or as plastics, depending on the choice of processing conditions.

Question 9

Name three different types of polymerization reactions.

Answer 9

Addition, ring-opening, condensation.

Question 10

What is the main difference in thermal behaviour between a thermoset and a thermoplastic?

What are the structural causes of these differences?

Answer 10

A thermoset will degrade with heat, but cannot be melted and reformed. a Thermoplastic will undergo a number of transitions as it is heated: from glassy below Tg, to rubbery, to flow.

thermosets have a network linked by cross links. Thermoplastics have a network formed by secondary bonds. these bonds will reversibly break with the addition of heat energy.

Question 11

Describe bulk polymerization

Answer 11

In bulk polymerization, only the monomer (with maybe the catalyst and initiator, but no solvent) is fed into the reactor. the monomer undergoes polymerization, at the end of which a solid mass is removed as the polymer product. this is used widely in the manufacture of ‘condensation polymers’, where the reactions are only mildly exothermic and viscosity is mostly low, enhancing the mixing, heat transfer and bubble elimination.

Question 12

Describe solution polymerization.

Answer 12

Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and the polymer (product) are soluble.

Question 13

Describe suspension polymerization and emulsion polymerization.

Answer 13

Suspension polymerization refers to the polymerization in an aqueous medium with the monomer as the dispersed phases. the polymer formed will be in a solid dispersed phase.

emulsion polymerization is similar to suspension polymerizatio, but the initiator is located in the aqueous phase.

Question 14

Polymers can be classified according to their end use. describe the end use of the following polymers:

1. Diene polymers.
2. Olefins
3. acrylics.

Answer 14

1. Dienes are used in the rubber industry.
2. Olefins are used in the sheet, film and fibre industries.
3. Acrylics are used in coatings and decorative materials.

Question 15

Carothers suggested two polymerization schemes. What are they, and what additional scheme has since been proposed?

Answer 15

Carothers suggested addition and condensation polymerization. The third scheme is ring opening polymerization.

Question 16

what is another name for chain reaction polymerization

Answer 16

Chain reaction polymerization is also known as ADDITION polymerization.

Question 17

What is another name for STEP-REACTION polymerization

Answer 17

Step-reaction polymerization is also known as condensation polymerization.

Question 18

List the steps involved in chain reaction polymerization:

Answer 18

initiation,
propagation,
termination.

[a fourth step called chain transfer may be involved]

Question 19

What are the two methods of termination in a chain reaction (i.e. addition polymerization)?

Answer 19

The two methods of termination in a chain reaction polymerization are
1. combination (coupling). In this, two radical polymer chains react with the mutual destruction of growth activity
2. disproportionation. A ‘labile species’ like hydrogen is moved from one radical polymer to another. this saturates one and causes the other to become neutral, stopping growth of both.

disproportionation requires the breaking of a bond on the radical polymer, so it dominates at higher temperature. combination (or coupling) dominates at low temperature.

Question 20

What is the primary use of a plasticiser? How do they achieve this?

Answer 20

Plasticisers are used to reduce the glass transition temperature (Tg) to below the expected use temperature. This makes the polymer more flexible over the expected temperature range of use.

Plasticisers work by inserting themselves between the molucular chains, reducing the intermolecular forces.

plasticisers may also depend on polar intermolecular attraction between the plasticizer and the polymer molecules, which effectively nullifies the dipole-dipole interactins between polymer molecules. as a result plasticizatin is difficult to achieve in nonpolar polymers like polyolefins and highly crystalline polymers.

Question 21

What is the difference between an internal and an external plasticizer?

Answer 21

An internal plasticizer is added through copolymerization of the plasticizer monomer during polymerization, so that the plasticizer becomes an integral part of the polymer chain. in this case, the plasticizer would usually be a polymer with a low Tg. for example, vinyl acetate and vinylidene chloride (Cl2C=CH2).

An external plasticizer is one incorporated into the resin as an external additive. Typical low molecular-weight external plasticizers for PVCs are esters.

Question 22

What is E glass?

Answer 22

E-glass is boroaluminosilicate glass. e-glass has low alkali-metal content and contains small percentages of calcia (CaO) and magnesia (MgO).

Question 23

When blending a crystalline and amorphous polymer, what properties would you expect from each phase?

Answer 23

Crystalline: chemical resistance, processing ease, stiffness. however, also poor impact strength and limited dimensional stability.

amorphous: high impact strength and dimensional stability, but poor chemical resistance.

Question 24

Name some crystalline and amorphous polymers:

Answer 24

Crystalline:
Nylon, poly(butylene terephthalate) (PBT), poly (ethelyne terephthalate) (PET)

Non-crystalline:
polycarbonate (PC) and polysulfone.

Question 25

what is nonchain scission?

give an example.

Answer 25

nonchain-scission is a degradation process in polymers. Nonchain-scission reactions resulting from, for eacmple, the application of heat, involve elimination of a small molecule (usually a pendant group) - leaving the backbone essentially unchanged.

an example: vinyl polymers are particularly susceptible to thermal degradation. rigid PVC is impossible to process under commercially acceptable conditions without the use of thermal stabilizers. unstabilized PVC undergoes dehydrochlorination near the melt processing temperature. this involves liberation of HCl and the formation of conjugated double bonds (alternating double bonds). this is known as polyene formation and leads to intense coloration.

a second example of nonchain scission is poly (vinyl acetate) or PVAc. when heater, PVAc can liberate acetic acid, followed by polyene formation.

Question 26

What are Chain Scission reactions?

Answer 26

chain scission reactions occur when the chemical bonds in a polymer backbone are broken by heat, ionizing radiation, mechanical stress and chemical reactions, resulting in the formation of free radicals.

in polyethylene, chain scission occurs through the homolytic cleavage of weak bonds in the polymer chain. this results in a complex mixture of low molecular weight degradation products.

Question 27

what is oxidative degradation?

Answer 27

in the presence of oxygen, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which result in polymer degradation. unsaturated polyolefins are particularly susceptible to oxygenation.

Question 28

Name a typical fire retardant used in polymers.

Answer 28

alumina trihydrate 2Al(OH)3. In addition to providing a non-combustible component, alumina trihydrate also liberates water at high temperature.