1.3　Definition-II

1.3.1　Copolymers

A copolymer is a polymer made from two or more different monomers. Many commercial synthetic polymers are copolymers. It should be noted that the sequence of monomer units along a copolymer chain can vary according to the method and mechanism of synthesis. Three different types of sequencing arrangements are commonly found.

（1）Random copolymers

In random copolymers， no definite sequence of monomer units exists. A copolymer of monomers A and B might be depicted by the arrangement shown in reaction（1.3）. Random copolymers are often formed when olefin-type monomers copolymerized by free-radical-type processes. The properties of random copolymers are usually quite different from those of the related homopolymers.

—A—B—B—B—A—B—A—A—A—B—A—　　（1.3）

（2）Regular copolymers

As the name implies， regular copolymers contain a regular alternating sequence of two monomer units. Olefin polymerization that takes place through ionic-type mechanisms can yield copolymer of this type. Again， the properties of the copolymer usually differ markedly from those of the two related homopolymers.

—A—B—A—B—A—B—A—B—　　（1.4）

（3） Block copolymers

Block copolymers contain a block of one monomer connected to a block of another. Block copolymers usually formed by ionic polymerization process. Unlike other copolymers retain many of the physical characteristics of the homopolymers.

—A—A—A—A—B—B—B—B—　　（1.5）

1.3.2　Terpolymers

A terpolymer contains three different monomer units. There can be regular， random or blocks. For example， EPDM rubber is a typical terpolymer which is randomly copolymerized by ethylene， propylene and third monomer[usually diene， such as 1，4-hexadiene（1，4-HD）， dicyclopentadiene（DCPD） and ethylidenenorbornene（ENB）].

1.3.3　Graft Copolymers

A graft copolymer is usually prepared by linking together two different polymers. For example， a homopolymer derived from monomer A may be induced to react with a homopolymer derived from monomer B to yield the graft copolymer shown in reaction（1.6）. Graft polymers of this type can often be prepared by the gamma or X-irradiation of a mixture of the two homopolymers， or even by mechanical blending of the two homopolymers. Alternatively， a graft copolymer may be prepared by the polymerization of monomer B from initiation sites along the chain of polymer A. Graft copolymers often display properties that are related to those of the two homopolymers.

　　

（1.6）

1.3.4　Thermoplastics

Basically， a thermoplastic is any material that becomes pliable or moldable above a specific temperature and solidifies upon cooling. However， the term is commonly used to describe a substance that passes through a definite sequence of property changes as its temperature is raised.

Most thermoplastics have a high molecular weight. The polymer chains associate through intermolecular forces， which weaken rapidly with increased temperature， yielding a viscous liquid. Thus， thermoplastics may be reshaped by heating and are typically used to produce parts by various polymer processing techniques such as injection molding， compression molding， calendering and extrusion.

Thermoplastics differ from thermosetting polymers， which form irreversible chemical bonds during the curing process. Thermosets do not melt， but decompose and do not reform upon cooling.

In Figure 1.3，the thermoplastic characteristics of an amorphous and a crystalline polymer are compared.

Both amorphous and crystalline thermoplastics are glass at low temperature， and both change from a glass to a rubbery elastomer or flexible plastic as the temperature is raised. This change from glass to elastomer usually takes place over a fairly narrow temperature， and this transition point is known as the glass transition temperature（Tg）.

At temperature aboveTg， amorphous polymers behave in a different manner from crystalline polymers. As the temperature of an amorphous polymer is raised， the rubbery elastomeric phase gradually gives way to a soft， extensible elastomeric phase， finally to a liquid. No sharp transition occurs from one phase to others， and only a gradually change in properties is perceptible.

Crystalline polymers， on the other hand， retain their rubber elastomeric or flexible properties above the glass transition， until the temperature reaches the melting temperature（Tm）. At this point the material liquefies. At the same time， melting is accompanied by a loss of the optical birefringence and crystalline X-ray diffraction effects that are characteristic of the crystalline state.

Above its glass transition temperature and below its melting point， the physical properties of a thermoplastic change drastically without an associated phase change. Some thermoplastics do not fully crystallize below the glass transition temperature， retaining some or all of their amorphous characteristics. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary， as light is scattered strongly by crystallites larger than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure.

1.3.5　Elastomers

An elastomer is a polymer which is in the temperature range between its glass transition temperature and its liquefaction temperature. In practice elastomeric properties become more obvious if the polymer chains are lightly crosslinked. In particular， the liquefaction temperature may be raised by crosslinking， and the polymer may exhibit elastomeric properties over a wider temperature range.

Elastomeric properties appear when the backbone bonds can readily undergo torsional motions to permit uncoiling of the chains when the material is stretched（Figure 1.4）.Crosslinking between the chains prevent the macromelocules from slipping past each other and thus prevent the material from becoming permanently elongated when held under tension. An important question connected with elasticity in this： why do the chains revert to the highly coiled state when the tension on elastomer is released？ The answer lies in the fact that a highly coiled polymer system has a higher degree of disorder， therefore， a higher entropy than a stretched， oriented sample. Thus， the elastic behavior is a direct consequence of the tendency of the system to assume spontaneously a state of maximum entropy. Since free energy， enthalpy， and entropy are related by the usual expression， ΔG=ΔH-TΔS， a stretched rubber band immediately held to the lips is warm， and the same material appears cold immediately after contraction.

1.3.6　Thermosetting Resin

The term thermosetting polymer refers to a range of systems which existinitially as liquids but which， on heating， undergo a reaction to form a solid， highly crosslinked matrix. A typical example is provided by the condensation of methylol melamine to give the hard， tough， crosslinked melamine resin. Partly polymerized systems which are still capable of liquid flow are called prepolymers. Prepolymers are often preferred as starting materials in technology. In practical terms， an uncrosslinked thermoplastic material can be reformed into a different shape by heating，but a thermosetting polymer can not.

1.3.7　Polymer Blends

When two or more polymers are mixed together mechanically， the product is known as a polymer blend. Many polymer blends display properties that are different from those of the individual polymers. Polymer blends can be of two types：（ⅰ） simple mixtures of the polymers， and（ⅱ）genuine block or graft copolymers formed by the physical breaking of bonds， followed by bonding between the different polymeric fragments. The latter type or process can occur when two or more polymers are milled or masticated together. The mechanical shearing can result in the cleavage of bonds followed by cross-recombination.

Words and expressions

macromolecule[,mækrə'mɒlə,kjuːl]：高分子，大分子

synonymous with：与……同义

addition polymer：加成聚合物

condensation polymer：缩合聚合物

conjugated['kɒndʒə,geɪtɪd]：共轭的

ethylene glycol['ɛθə,lin 'ɡlaɪ,kɔl]：乙二醇

polyether ['pɒliː,iːθə]：聚醚，多醚

dehydration [,dihaɪ'dreʃən]：脱水

hydroxymethyl [haɪdrɒksɪ'meθɪl]：羟甲基

esterification [e,sterəfə'keɪʃən]：酯化作用

polyfunctional [pɒlɪ'fʌŋkʃənəl]：多官能的

ethyl benzoate ['ɛθəl 'bɛnzo,et]：苯甲酸乙酯

terephthalic ['teref'θælɪk]：对苯二酸

amino acid：氨基酸

polypeptide [,pɒlɪ'pep,taɪd]：多肽；缩多氨酸

oligomer[ə'lɪgəmə]：低聚物，低聚体

subcategories[sʌb'kætɪgərɪz]：子分类；子范畴

skeletal atom：骨架原子

substituent group：取代基

methyl methacrylate：甲基丙烯酸甲酯

polyacrylonitrile [,pɒlɪ'ækrəloʊ'naɪtrɪl]：聚丙烯腈

light-scattering：光散射

olefin ['oʊləfɪn]：烯烃

hexadiene ['heksədɪrn]：己二烯

dicyclopentadiene [daɪsaɪkləpentə'dɪiːn]：二环戊二烯

ethylidene norbornene：亚乙基降冰片烯

pliable ['plaɪəbəl]：柔韧的； 柔顺的

calendering [kæ'lɪndərɪŋ]：压延

extrusion [ɪk'struʒən]：挤出

birefringence [,baɪrɪ'frɪndʒəns]：双折射

crystallite['krɪstəlaɪt]：微晶

crystalline ['krɪstəlɪn， -,laɪn， -,lin]：结晶的；结晶性

liquefaction [,lɪkwɪ'fækʃən]：液化；溶解

torsional ['tɔːʃənəl]：扭力的，扭转的

uncoiling [ʌn'kɔɪlɪŋ]：伸开，展开

entropy ['ɛntrəpi]：熵

enthalpy [en'θælpɪ]：焓

contraction [kən'trækʃən]：收缩

methylol[miːθɪ'lɒl]：羟甲基

melamine ['meləmiːn]：三聚氰胺

fragment['fræɡmənt]：碎片，片段

masticated ['mæstɪ,keɪtid]：粉碎，塑炼

hemolytic [hiː'mɒlɪtɪk]：溶血的

cleavage ['klivɪdʒ]：分裂；裂开