课后习题答案(英文版)

1.What are linear and branched polymers? Can thermoplastics be branched?

A linear polymer consists of spaghetti-like molecular chains.

In a branched polymer, there are primary polymer chains and secondary offshoots of smaller chains that stem from these main chains.

2.Define

(a) a thermoplastic, (b ) thermosetting plastics,

(c) elastomers, and (d) thermoplastic elastomers.

Thermoplastics are composed of long chains produced by joining together monomers; they typically behave in a plastic, ductile manner. The chains may or may not have branches. Thermosetting polymers are composed of long chains (linear or branched) of molecules that are strongly cross-linked to one another to form three-dimensional network structures. Elastomers These are known as rubbers. They have an elastic deformation >200%. The polymer chains consist of coil-like molecules that can reversibly stretch by applying a force. Thermoplastic elastomers are a special group of polymers. They have the processing ease of thermoplastics and the elastic behavior of elastomers

3.What electrical and optical applications are polymers used for? Explain using examples.

We use polymers in photochromic lenses.

Plastics are often used to make electronic components because of their insulating ability and low dielectric constant.

More recently, significant developments have occurred in the area of flexible electronic devices based on the useful piezoelectricity, semiconductivity, optical and electro-optical properties.

4.What are the major advantages associated with plastics compared to ceramics, glasses, and metallic materials?

Commercial polymers are lightweight, corrosion-resistant material with low strength and stiffness, and they are not suitable for use at high temperatures.

These polymers are relatively inexpensive and are readily formed into a variety of shapes, ranging form plastic bags to mechanical gears to bathtubs.

5.What do the terms condensation polymerization, addition polymerization, initiator mean ? Addition polymerization and condensation polymerization are the two main ways to conduct “polymerization”. The polymers derived from these processes are known as addition and condensation polymers, respectively.

The formation of the most common polymer, polyethylene (PE), from ethylene molecules is an example of addition or chain-growth polymerization.

Ethylene, a gas, is the monomer (single unit). The two carbon atoms are joined by a double covalent bond.

Each carbon atom shares two of its electrons with the second carbon atom, and two hydrogen atoms are bonded to each of the carbon atoms.

In presence of an appropriate combination of heat, pressure, and catalysts, the double bond between the carbon atoms is broken and replaced with a single covalent bond.

The double bond is an unsaturated bond. After changing to a single bond, the carbon atoms are

still joined, but they become active;

other repeat units or mers can be added to produce the polymer chain.

initiator is a kind of chemical required in addition polymerization to initiate monomer to start polymerizing reaction.

In condensation polymerization, a relatively small molecule (such as water, ethanol, methanol etc.) is formed as a result of the polymerization reaction.

6.Explain why low-density polyethylene is good to make grocery bags, however, super high molecular weight polyethylene must be used where strength and very high wear resistance is needed.

In general, for thermoplastics, the higher the average molecular weight the higher will be the melting temperature and the higher will be the Young’s modulus of the polymer.

Degree of Polymerization in general, for a given type of thermoplastic (e.g., polyethylene) the tensile strength, creep resistance, impact toughness, wear resistance, and melting temperature all increase with increasing average molecular weight or degree of polymerization.

7.The molecular weight of polymethyl methacrylate is

250,000g/mol. If all of the polymer chains are the same length,

(a ) calculate the degree of polymerization, and

(b ) the number of chains in 1g of the polymer

The molecular weight of PMMA = ( ) g/mol.

Therefore, the degree of polymerization is:

250000/( ) = ( )MMA molecules per average chain

8.Explain what the following terms mean:

decomposition temperature,

heat distortion temperature,

glass temperature,

and melting temperature.

Why is it that thermoplastics do not have a fixed melting or glass temperature.

At very high temperature, the covalent bonds between the atoms in the linear chain may be destroyed, and the polymer may burn or char.

In thermoplastics decomposition occurs in the liquid state, in thermosets the decomposition occurs in the solid state.

This temperature T d is the degradation (or decomposition) temperature.

Heat distortion temperature under load, which is the temperature at which a given deformation of a beam occurs for a standard load.

A high deflection temperature indicates good resistance to

creep.

Glassy State Below the glass temperature Tg, the linear

amorphous polymer becomes hard, brittle, and glass-like.

This is again not a fixed temperature but a range of

temperatures.

thermoplastics when cooled below the melting temperature,

with the chains becoming closely aligned over appreciable distances.

A sharp increase in the density occurs as the coiled and intertwined chains in the liquid are

rearranged into a more orderly, close-packed structur

9.How do the glass temperatures of polyethylene, polypropylene, and polymethyl

methacrylate compare? Explain their differences, based on the structure of the monomer.

10.What factors influence the crystallinity of polymers? Explain the development and role of

crystallinity in PET and Nylon.

11.For each of the following pairs, recommend the one that will most likely have the better

impact properties at 25℃. Explain each of your choices.

(a) polyethylene versus polystyrene,

(b) low-density polyethylene versus high-density polyethylene, and

(c) polymethyl methacrylate versus polytetrafluoroethylene.

12.

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塑料缩写代号PET HDPE PVC LDPE PP PS Others