材料物理课件：chapter 4 polymer

1、Chapter 4 Polymer structure,Transmission electron micrograph showing the spherulite（球粒） structure in a natural rubber specimen. Chain-folded lamellar crystallites approximately 10 nm thick extend in radial directions from the center; they appear as white lines in the micrograph. 30,000,Why Study Pol

2、ymer Structures,A relatively large number of chemical and structural characteristics affect the properties and behaviors of polymeric materials. Some of these influences are as follows: 1. Degree of crystallinity of semicrystalline polymers-on density, stiffness, strength, and ductility. 2. Degree o

3、f crosslinking-on the stiffness of rubber-like materials. 3. Polymer chemistry on melting and glass-transition temperatures . Polyethylene(聚乙烯), polyvinylchloride(聚氯乙烯), polytetrafluoroethylene pli,tetr-,flurueili:n (聚四氟乙烯), polypropylene(聚丙烯), and polystyrene(聚苯乙烯,Naturally occurring polymersthose

4、derived from plants and animalshave been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk. Other natural polymers such as proteins(蛋白质), enzymes(酶), starches(淀粉), and cellulose(纤维素) are important in biological and physiological processes in plants and an

5、imals. Many of our useful plastics, rubbers, and fiber materials are synthetic polymers. The synthetics can be produced inexpensively, and their properties may be managed to the degree that many are superior to their natural counterparts. In some applications metal and wood parts have been replaced

6、by plastics, which have satisfactory properties and may be produced at a lower cost. As with metals and ceramics, the properties of polymers are intricately related to the structural elements of the material. This chapter explores molecular and crystal structures of polymers,4.2 HYDROCARBON MOLECULE

7、S,Since most polymers are organic in origin, we briefly review some of the basic concepts relating to the structure of their molecules. First, many organic materials are hydrocarbons; that is, they are composed of hydrogen and carbon. Furthermore, the intramolecular bonds are covalent. Some of the s

8、imple hydrocarbons belong to the paraffin(链烷烃) family; the chainlike paraffin molecules include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). Hydrocarbon compounds with the same composition may have different atomic arrangements, a phenomenon termed isomerism(同质异构,4.3 POLYMER MOL

9、ECULES,The molecules in polymers are gigantic in comparison to the hydrocarbon molecules heretofore discussed; because of their size they are often referred to as macromolecules. For most polymers, these molecules are in the form of long and flexible chains, the backbone of which is a string of carb

10、on atoms; many times each carbon atom singly bonds to two adjacent carbons atoms on either side, represented schematically in two dimensions as follows: These long molecules are composed of structural entities called mer units(单体), which are successively repeated along the chain. The term polymer wa

11、s coined to mean many mers,4.4 THE CHEMISTRY OF POLYMER MOLECULES,If the ethylene gas is subjected catalytically to appropriate conditions of temperature and pressure, it will transform to polyethylene (PE), which is a solid polymeric material. This process begins when an active mer is formed by the

12、 reaction between an initiator or catalyst species (R) and the ethylene mer unit, as follows,The final result, after the addition of many ethylene monomer units, is the polyethylene molecule, a portion of which is shown in Figure 4.1a. This representation is not strictly correct in that the angle be

13、tween the singly bonded carbon atoms is not 180 as shown, but rather close to 109. A more accurate three-dimensional model is one in which the carbon atoms form a zigzag pattern (Figure 4.1b), the C-C bond length being 0.154 nm,If all the hydrogen atoms in polyethylene are replaced by fluorine, the

14、resulting polymer is polytetrafluoroethylene (PTFE); Polyvinyl chloride (PVC), another common polymer, has a structure that is a slight variant of that for polyethylene, in which every fourth hydrogen is replaced with a Cl atom. Furthermore, substitution of the CH3 methyl group for each Cl atom in P

15、VC yields polypropylene (PP). When all the repeating units along a chain are of the same type, the resulting polymer is called a homopolymer(均聚物). Chains may be composed of two or more different mer units, in what are termed copolymers(共聚物,4.5 MOLECULAR WEIGHT,Extremely large molecular weights are t

16、o be found in polymers with very long chains. Ordinarily, an average molecular weight is specified, which may be determined by the measurement of various physical properties such as viscosity(粘度) and osmotic pressure (渗透压). There are two ways of defining average molecular weight,The number-average m

17、olecular weight Mn is obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size range (Figure 4.3a,where Mi represents the mean (middle) molecular weight of size range i, and xi is the fraction of the total number of chains withi

18、n the corresponding size range,A weight-average molecular weight Mw is based on the weight fraction of molecules within the various size ranges (Figure 4.3b,where, again, Mi is the mean molecular weight within a size range, whereas wi denotes the weight fraction of molecules within the same size int

19、erval,An alternate way of expressing average chain size of a polymer is as the degree of polymerization n, which represents the average number of mer units in a chain,4.6 MOLECULAR SHAPE,Polymer chain molecules are not always strictly straight. Single chain bonds are capable of rotation and bending

20、in three dimensions,Polymers consist of large numbers of molecular chains, each of which may bend, coil, and kink in the manner of Figure 4.6. This leads to extensive intertwining and entanglement of neighboring chain molecules. Some of the mechanical and thermal characteristics of polymers are a fu

21、nction of the ability of chain segments to experience rotation in response to applied stresses or thermal vibrations. Rotational flexibility is dependent on mer structure and chemistry,4.7 MOLECULAR STRUCTURE,The physical characteristics of a polymer depend not only on its molecular weight and shape

22、, but also on differences in the structure of the molecular chains. Several molecular structures including linear, branched, crosslinked, and network, in addition to various isomeric configurations can be synthesized by controlled techniques,Linear polymers are those in which the mer units are joine

23、d together end to end in single chains. These long chains are flexible and may be thought of as a mass of spaghetti, as represented schematically in Figure 4.7a, where each circle represents a mer unit. Some of the common polymers that form with linear structures are polyethylene, polyvinyl chloride

24、, polystyrene, polymethyl methacrylate(甲基丙烯酸酯), nylon, and the fluorocarbons,Polymers may be synthesized in which side-branch chains are connected to the main ones, as indicated schematically in Figure 4.7b; these are fittingly called branched polymers,In crosslinked polymers, adjacent linear chains

25、 are joined one to another at various positions by covalent bonds, as represented in Figure 4.7c. Many of the rubber elastic materials are crosslinked; in rubbers, this is called vulcanization(橡胶的硫化,Trifunctional mer units, having three active covalent bonds, form three-dimensional networks (Figure

26、4.7d) and are termed network polymers. Typical Materials: epoxy(环氧树脂) and phenol formaldehyde(苯酚甲醛,4.8 MOLECULAR CONFIGURATIONS,For polymers having more than one side atom or group of atoms bonded to the main chain, the regularity and symmetry of the side group arrangement can significantly influenc

27、e the properties,Head to tail,Head to head,In most polymers, the head-to-tail configuration predominates; often a polar repulsion occurs between R groups for the head-to-head configuration,Stereoisomerism (立体异构,全同立构,间同立构,无规立构,GEOMETRICAL ISOMERISM (几何异构,Other important chain configurations, or geome

28、trical isomers, are possible within mer units having a double bond between chain carbon atoms. Bonded to each of the carbon atoms participating in the double bond is a single side-bonded atom or radical, which may be situated on one side of the chain or its opposite,Conversion of trans to cis, or vi

29、ce versa, is not possible by a simple chain bond rotation because the chain double bond is extremely rigid,4.9 THERMOPLASTIC ANDTHERMOSETTING POLYMERS,The response of a polymer to mechanical forces at elevated temperatures is related to its dominant molecular structure. One classification scheme for

30、 these materials is according to behavior with rising temperature. Thermoplasts (or thermoplastic polymers 热塑性塑料 ) and thermosets (or thermosetting polymers热固性塑料 ) are the two subdivisions,Thermoplasts soften when heated (and eventually liquefy) and harden when cooledprocesses that are totally rever

31、sible and may be repeated. Most linear polymers and those having some branched structures with flexible chains are thermoplastic. These materials are normally fabricated by the simultaneous application of heat and pressure. Thermosetting polymers become permanently hard when heat is applied and do n

32、ot soften upon subsequent heating. Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, and phenolic(酚醛树脂) and some polyester resins(聚酯树脂), are thermosetting,4.10 COPOLYMERS (共聚物,4.11 POLYMER CRYSTALLINITY,Polymer crystallinity is the packing of molecular chains s

33、o as to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex,Molecular substances having small molecules (e.g., water and methane) are normally either totally crystalline (as solids) or totally amorphous (as liquids). As a consequ

34、ence of their size and often complexity, polymer molecules are often only partially crystalline (or semicrystalline), having crystalline regions dispersed within the remaining amorphous material. The density of a crystalline polymer will be greater than an amorphous one of the same material and mole

35、cular weight, since the chains are more closely packed together for the crystalline structure,The degree of crystallinity of a polymer depends on the rate of cooling during solidification as well as on the chain configuration,The molecular chemistry as well as chain configuration also influence the

36、ability of a polymer to crystallize. For linear polymers, crystallization is easily accomplished because there are virtually no restrictions to prevent chain alignment. Any side branches interfere with crystallization, such that branched polymers never are highly crystalline; in fact, excessive bran

37、ching may prevent any crystallization whatsoever. Most network and crosslinked polymers are almost totally amorphous; a few crosslinked polymers are partially crystalline. For copolymers, as a general rule, the more irregular and random the mer arrangements, the greater is the tendency for the devel

38、opment of noncrystallinity. Crystalline polymers are usually stronger and more resistant to dissolution and softening by heat,4.12 POLYMER CRYSTALS,Fringed-micelle model (缨状微束模型) was proposed that a semicrystalline polymer consists of small crystalline regions (crystallites, or micelles), each havin

39、g a precise alignment, which are embedded within the amorphous matrix composed of randomly oriented molecules,chain-folded model (折叠链模型). Each platelet will consist of a number of molecules; however, the average chain length will be much greater than the thickness of the platelet,Summary,Most polyme

40、ric materials are composed of macromolecules, which may be thought of as being composed of mers, smaller structural entities. Molecular weights for high polymers may be in excess of a million. Molecular weight is often expressed in terms of number and weight averages. Several molecular characteristics tha