Chapter 14-Polymer Structures.ppt

1、1,ISSUES TO ADDRESS., What are the general structural and chemical characteristics of polymer molecules?, What are some of the common polymeric materials, and how do they differ chemically?, How is the crystalline state in polymers different from that in metals and ceramics ?,Chapter 14: Polymer Str

2、uctures,2,What is a Polymer?,Poly mermany repeat unit,Adapted from Fig. 14.2, Callister & Rethwisch 8e.,repeat unit,repeat unit,repeat unit,3,Ancient Polymers,Originally natural polymers were used Wood Rubber Cotton Wool Leather SilkOldest known uses Rubber balls used by Incas Noah used pitch (a nat

3、ural polymer) for the ark,4,Polymer Composition,Most polymers are hydrocarbons i.e., made up of H and C Saturated hydrocarbons Each carbon singly bonded to four other atoms Example: Ethane, C2H6,5,6,Unsaturated Hydrocarbons,Double & triple bonds somewhat unstable can form new bonds Double bond found

4、 in ethylene or ethene - C2H4Triple bond found in acetylene or ethyne - C2H2,7,Isomerism,Isomerism two compounds with same chemical formula can have quite different structures for example: C8H18 normal-octane2,4-dimethylhexane,8,Polymerization and Polymer Chemistry,Free radical polymerizationInitiat

5、or: example - benzoyl peroxide,9,Chemistry and Structure of Polyethylene,Adapted from Fig. 14.1, Callister & Rethwisch 8e.,Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene,10,Bulk or Commodity Polymers,11,Bulk or Commodity Polymers (cont),12,Bulk or Com

6、modity Polymers (cont),VMSE: Polymer Repeat Unit Structures,13,Manipulate and rotate polymer structures in 3-dimensions,14,MOLECULAR WEIGHT, Molecular weight, M: Mass of a mole of chains.,high M,Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights,15,

7、xi = number fraction of chains in size range i,MOLECULAR WEIGHT DISTRIBUTION,Adapted from Fig. 14.4, Callister & Rethwisch 8e.,wi = weight fraction of chains in size range i,Mi = mean (middle) molecular weight of size range i,16,Molecular Weight Calculation,Example: average mass of a class,What is t

8、he average weight of the students in this class: Based on the number fraction of students in each mass range? Based on the weight fraction of students in each mass range?,17,Molecular Weight Calculation (cont.),Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges g

9、ives the following table:,total number,total weight,Calculate the number and weight fraction of students in each weight range as follows:,For example: for the 81-120 lb range,18,Molecular Weight Calculation (cont.),19,Degree of Polymerization, DP,DP = average number of repeat units per chain,DP = 6,

10、mol. wt of repeat unit i,Chain fraction,20,Adapted from Fig. 14.7, Callister & Rethwisch 8e.,Molecular Structures for Polymers,21,Polymers Molecular Shape,Molecular Shape (or Conformation) chain bending and twisting are possible by rotation of carbon atoms around their chain bonds note: not necessar

11、y to break chain bonds to alter molecular shape,Adapted from Fig. 14.5, Callister & Rethwisch 8e.,22,Chain End-to-End Distance, r,Adapted from Fig. 14.6, Callister & Rethwisch 8e.,23,Molecular Configurations for Polymers,Configurations to change must break bonds Stereoisomerism,E,B,A,D,C,C,D,A,B,E,m

12、irror plane,Stereoisomers are mirror images cant superimpose without breaking a bond,24,Tacticity,Tacticity stereoregularity or spatial arrangement of R units along chain,isotactic all R groups on same side of chain,syndiotactic R groups alternate sides,25,Tacticity (cont.),atactic R groups randomly

13、 positioned,26,cis/trans Isomerism,cis cis-isoprene (natural rubber) H atom and CH3 group on same side of chain,trans trans-isoprene (gutta percha) H atom and CH3 group on opposite sides of chain,VMSE: Stereo and Geometrical Isomers,27,Chapter 7 - 19,Manipulate and rotate polymer structures in 3-dim

14、ensions,28,Copolymers,two or more monomers polymerized together random A and B randomly positioned along chain alternating A and B alternate in polymer chain block large blocks of A units alternate with large blocks of B units graft chains of B units grafted onto A backboneA B ,random,block,graft,Ad

15、apted from Fig. 14.9, Callister & Rethwisch 8e.,alternating,29,Crystallinity in Polymers,Ordered atomic arrangements involving molecular chains Crystal structures in terms of unit cells Example shown polyethylene unit cell,Adapted from Fig. 14.10, Callister & Rethwisch 8e.,30,Polymer Crystallinity,C

16、rystalline regions thin platelets with chain folds at faces Chain folded structure,31,Polymer Crystallinity (cont.),Polymers rarely 100% crystalline Difficult for all regions of all chains to become aligned, Degree of crystallinity expressed as % crystallinity.- Some physical properties depend on %

17、crystallinity.- Heat treating causes crystalline regions to grow and % crystallinity to increase.,Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1

18、965.),crystalline,region,amorphous,region,32,Polymer Single Crystals,Electron micrograph multilayered single crystals (chain-folded layers) of polyethylene Single crystals only for slow and carefully controlled growth rates,Adapted from Fig. 14.11, Callister & Rethwisch 8e.,33,Semicrystalline Polyme

19、rs,Spherulite surface,Adapted from Fig. 14.13, Callister & Rethwisch 8e.,Some semicrystalline polymers form spherulite structures Alternating chain-folded crystallites and amorphous regions Spherulite structure for relatively rapid growth rates,34,Photomicrograph Spherulites in Polyethylene,Adapted from Fig. 14.14, Callister & Rethwisch 8e.,Cross-polarized light used - a maltese cross appears in each spherulite,35,Core Problems:,Self-help Problems:,ANNOUNCEMENTS,Reading:,