28 3 Structure Morphology Flow of Polymer (a) (b) (c) 6+ 6+ CH2 CO M2+ 8+H C02 CH CH Fig.3.1 Intermolecular forces in polar polymers:a Dipole-dipole interaction in polyester. b Hydrogen bonding in polyamide.c lonic bonding in ionomer in the configurations of successive stereo centers that determines the order of the polymer chain,such as(-CH2-C*HR-)where is stereo center.Figure 3.2 shows the different examples of tacticity.Atactic is that the R group on successive stereo centers are randomly distributed on the two sides of the planar zig-zag polymer chain and thus the polymer chain does not have order.Isotactic is that the stereo center in each repeating unit in the polymer chain has the same configuration.All the R groups are located on one side of the plane of the carbon-carbon polymer chain either all above or all below the plane of the chain.Syndiotactic is that the stereo HH H R H H H R Fig.3.2 Different polymer structures from a monosubstituted ethylene,-(CH2CHR),Atactic (top).Isotactic (center),Syndiotactic (bottom)[1]
in the configurations of successive stereo centers that determines the order of the polymer chain, such as (–CH2–C*HR–)n where * is stereo center. Figure 3.2 shows the different examples of tacticity. Atactic is that the R group on successive stereo centers are randomly distributed on the two sides of the planar zig–zag polymer chain and thus the polymer chain does not have order. Isotactic is that the stereo center in each repeating unit in the polymer chain has the same configuration. All the R groups are located on one side of the plane of the carbon–carbon polymer chain either all above or all below the plane of the chain. Syndiotactic is that the stereo C O O R C O O R δ− δ− δ+ δ+ C N O R δ− δ+ H R N δ− C δ+ H O CH2 CH CO2 - CH2 CH CO2 - M2+ (a) (b) (c) Fig. 3.1 Intermolecular forces in polar polymers: a Dipole–dipole interaction in polyester. b Hydrogen bonding in polyamide. c Ionic bonding in ionomer C C H H H R C C C C H R H H H H H R C C C C H R H H H H H R C C C H H R R H H C C H R H H C C C C H R H R H H H H C C C C H R H R H H H H C C C R H R H H H C C H H H R C C C C H H H R H R H H C C C C H H H R H R H H C C C H H R R H H Fig. 3.2 Different polymer structures from a monosubstituted ethylene, –(CH2CHR)n–, Atactic (top), Isotactic (center), Syndiotactic (bottom) [1] 28 3 Structure Morphology Flow of Polymer
3.1 Chemical and Molecular Structure of Polymer 29 Fig.3.3 Configurations of mm,mr,rr of poly(methyl methacrylate) 72122 H center alternates from one repeating unit to the next with the R groups located alternately on the opposite sides of the plane of the polymer chain. In addition to poly(alpha)olefin,polystyrene,poly(methyl methacrylate)and 1,2 addition of polybutadiene can also exhibit either isotactic or syndiotactic structure.Whether a polymer is isotactic or syndiotactic usually determines its crystal structure,and the assignment of an all-isotactic or all-syndiotactic structure to a polymer can be made from its crystal structure.NMR spectroscopy is a powerful technique to determine the stereospecificity of polymer(the principle of NMR spectroscopy will be discussed in Chap.5).It allows the determination of the stereoregular configuration of successive monomers in sequences.For example, the NMR spectrum of methylene protons in poly(methyl methacrylate)allows one to distinguish between and determine the relative numbers of sequences of two monomer units (dyads)that have syndiotactic (racemic,r)and isotactic (meso,m) symmetry.The alpha-methyl proton resonance allows estimation of the numbers of 3-monomer sequences (triads)with configurations mm,mr,and rr.Figure 3.3 shows the configurations of mm,mr,rr of poly(methyl methacrylate). The synthesized poly(3-(n-hexyl)thiophene)(P3HT)can have three configura- tions:head-to-head,tail-to-tail and head-to-tail.Only the head-to-tail configuration shows high crystallinity due to the presence of even spacing among the monomers in the head-to-tail configuration as shown in Fig.3.4.The stereospecificity of P3HT can also be determined by the NMR as shown in Fig.3.5.The chemical shift of methylene proton of hexyl side chain of thiophene is different resulted from their position,so it can be easily identified and calculated the stereospeci- ficity of the polymer. Many polymers are capable of rotating the plane of polarization of light and are optical active such as poly(L-propylene oxide).The optical activity of low molecular weight compound is associated with the presence of asymmetric carbon
center alternates from one repeating unit to the next with the R groups located alternately on the opposite sides of the plane of the polymer chain. In addition to poly(alpha) olefin, polystyrene, poly(methyl methacrylate) and 1,2 addition of polybutadiene can also exhibit either isotactic or syndiotactic structure. Whether a polymer is isotactic or syndiotactic usually determines its crystal structure, and the assignment of an all-isotactic or all-syndiotactic structure to a polymer can be made from its crystal structure. NMR spectroscopy is a powerful technique to determine the stereospecificity of polymer (the principle of NMR spectroscopy will be discussed in Chap. 5). It allows the determination of the stereoregular configuration of successive monomers in sequences. For example, the NMR spectrum of methylene protons in poly(methyl methacrylate) allows one to distinguish between and determine the relative numbers of sequences of two monomer units (dyads) that have syndiotactic (racemic, r) and isotactic (meso, m) symmetry. The alpha-methyl proton resonance allows estimation of the numbers of 3-monomer sequences (triads) with configurations mm, mr, and rr. Figure 3.3 shows the configurations of mm, mr, rr of poly(methyl methacrylate). The synthesized poly(3-(n-hexyl)thiophene) (P3HT) can have three configurations: head-to-head, tail-to-tail and head-to-tail. Only the head-to-tail configuration shows high crystallinity due to the presence of even spacing among the monomers in the head-to-tail configuration as shown in Fig. 3.4. The stereospecificity of P3HT can also be determined by the NMR as shown in Fig. 3.5. The chemical shift of methylene proton of hexyl side chain of thiophene is different resulted from their position, so it can be easily identified and calculated the stereospeci- ficity of the polymer. Many polymers are capable of rotating the plane of polarization of light and are optical active such as poly(L-propylene oxide). The optical activity of low molecular weight compound is associated with the presence of asymmetric carbon C C H H H R C C H H H R C C H H H R C C H H H R C C H H R H C C H H H R C C H H H R C C H H H R C C H H R H mm rr mr Fig. 3.3 Configurations of mm, mr, rr of poly(methyl methacrylate) 3.1 Chemical and Molecular Structure of Polymer 29
30 3 Structure Morphology Flow of Polymer C6H13 CeH13 Head-to-tail tail-to-tail head-to-head Fig.3.4 Three configurations of poly(3-(n-hexyl)thiophene) The regioregularity of P3HT is around 93.2%. 8器 留面果不话男分号将清高58合 NiNini b" n 6 2.07 b+b=2.07+0.15 =0.932 b b 8 点怎 075 70 656055504540 3530 2520 1510050 fl (ppm) Fig.3.5 Stereospecificity determination of poly(3-(n-hexyl)thiophene)by NMR atoms.However,it is not universally true in polymers.The every second chain of substituted vinyl polymer is theoretically asymmetric,yet such polymers are not usually optically active,even when isotactic or syndiotactic,because of intramo- lecular compensation. 十o-GH-cHh CHa poly(L-propylene oxide)
atoms. However, it is not universally true in polymers. The every second chain of substituted vinyl polymer is theoretically asymmetric, yet such polymers are not usually optically active, even when isotactic or syndiotactic, because of intramolecular compensation. O CH CH3 CH2 n poly(L-propylene oxide) S C6H13 S C6H13 S S C6H13 C6H13 S C6H13 S C6H13 Head-to-tail tail-to-tail head-to-head Fig. 3.4 Three configurations of poly(3-(n-hexyl) thiophene) b b b’ b’ a a S S S n The regioregularity of P3HT is around 93.2%. Fig. 3.5 Stereospecificity determination of poly(3-(n-hexyl) thiophene) by NMR 30 3 Structure Morphology Flow of Polymer
3.2 Crystal Structure of Homopolymer 31 3.2 Crystal Structure of Homopolymer The X-ray patterns of most crystalline polymers show both sharp features asso- ciated with ordered regions and diffuse features with molecularly disordered regions.Therefore both crystalline structure and amorphous structure coexist in the crystalline polymer.Additional evidence indicates that the density of crystalline polymer is in between the theoretical calculated value of complete crystalline polymer and amorphous polymer as shown in Table 3.1. The crystallinity of polymers is closely related to the chemically and geomet- rically regular structure of polymer chain.However,atactic polymer can form crystalline as long as the size of repeating unit can fit into the crystal lattices despite of stereochemical irregularity.Polyethylene can exhibit highly ordered arrangement with all of the carbon atoms in one plane when the C-C bonds form a zig-zag.Figure 3.6a shows these zig-zag sections of chains which easily pack together closely to form orthorhombic crystalline.Single crystal of linear poly- ethylene has been fabricated from a solution in perchloroethylene as shown in Fig.3.6b.Irregularities such as branching in the polyethylene or copolymerization with other different structure monomer will reduce crystallinity. The crystal structure of poly(vinyl alcohol)is similar to that of polyethylene, since the CH(OH)group is small enough to fit into the polyethylene structure in place of a CH2 group.The unit cell is monoclinic.Pairs of chains are linked together by hydrogen bonds and then linked into sheets,as long as the stereo- chemical irregularity allows.Fully extended planar zig-zag is 3.3 kJ/mole of molecular dynamic energy less than that of the gauche.Thus,the zig-zag con- formation is favored to form crystalline structure unless substituents on the chains cause steric hindrance.Syndiotactic polymers such as poly(vinyl chloride), poly(1,2-butadiene),most polyamides and cellulose exhibit the similar crystalline structure.In most aliphatic polyesters and in poly(ethylene terephthalate),the polymer chains are shortened by rotation about the C-O bonds to allow close packing.As a result,the main chains are no longer planar.However the tere- phthalate unit in poly(ethylene terephthalate)remains planar as required by res- onance.That results in higher packaging and thus higher crystallinity for poly(ethylene terephthalate)as compared with aliphatic polyesters. Table 3.1 Densities of polymers in bulk,crystalline and amorphous states [2] Polymer Density (kg dm3) Typical bulk Crystalline Amorphous Polyethylene(LDPE) 0.91-0.93 Polyethylene (LLDPE) 0.93-0.94 1.00 0.86 Polyethylene(HDPE) Polyethylene(VLDPE) 0.95-0.96 0.90-0.91 Poly(ethylene terephthalate) 1.41 1.46 1.34 Poly(tetrafluoro ethylene) 2.19 2.30 2.00
3.2 Crystal Structure of Homopolymer The X-ray patterns of most crystalline polymers show both sharp features associated with ordered regions and diffuse features with molecularly disordered regions. Therefore both crystalline structure and amorphous structure coexist in the crystalline polymer. Additional evidence indicates that the density of crystalline polymer is in between the theoretical calculated value of complete crystalline polymer and amorphous polymer as shown in Table 3.1. The crystallinity of polymers is closely related to the chemically and geometrically regular structure of polymer chain. However, atactic polymer can form crystalline as long as the size of repeating unit can fit into the crystal lattices despite of stereochemical irregularity. Polyethylene can exhibit highly ordered arrangement with all of the carbon atoms in one plane when the C–C bonds form a zig–zag. Figure 3.6a shows these zig–zag sections of chains which easily pack together closely to form orthorhombic crystalline. Single crystal of linear polyethylene has been fabricated from a solution in perchloroethylene as shown in Fig. 3.6b. Irregularities such as branching in the polyethylene or copolymerization with other different structure monomer will reduce crystallinity. The crystal structure of poly(vinyl alcohol) is similar to that of polyethylene, since the CH(OH) group is small enough to fit into the polyethylene structure in place of a CH2 group. The unit cell is monoclinic. Pairs of chains are linked together by hydrogen bonds and then linked into sheets, as long as the stereochemical irregularity allows. Fully extended planar zig–zag is 3.3 kJ/mole of molecular dynamic energy less than that of the gauche. Thus, the zig-zag conformation is favored to form crystalline structure unless substituents on the chains cause steric hindrance. Syndiotactic polymers such as poly(vinyl chloride), poly(1,2-butadiene), most polyamides and cellulose exhibit the similar crystalline structure. In most aliphatic polyesters and in poly(ethylene terephthalate), the polymer chains are shortened by rotation about the C–O bonds to allow close packing. As a result, the main chains are no longer planar. However the terephthalate unit in poly(ethylene terephthalate) remains planar as required by resonance. That results in higher packaging and thus higher crystallinity for poly(ethylene terephthalate) as compared with aliphatic polyesters. Table 3.1 Densities of polymers in bulk, crystalline and amorphous states [2] Polymer Density (kg dm-3 ) Typical bulk Crystalline Amorphous Polyethylene (LDPE) 0.91-0.93 0.93-0.94 0.95-0.96 0.90-0.91 Polyethylene (LLDPE) 1.00 0.86 Polyethylene (HDPE) Polyethylene (VLDPE) Poly(ethylene terephthalate) 1.41 1.46 1.34 Poly(tetrafluoro ethylene) 2.19 2.30 2.00 3.2 Crystal Structure of Homopolymer 31
32 3 Structure Morphology Flow of Polymer (b) Fig.3.6 a The arrangement of C-H in the crystallites of polyethylene.Reproduced by permission of the Royal Society of Chemistry [3].b Single crystal polyethylene shows platelet like structure.Reproduced by permission of Journal of Applied Physics [4] terephthalate The substituted vinyl polymer-(CH2-CHR)-with large bulky R group usually is in amorphous structure through free radical polymerization.For example. poly(methyl methacrylate),polyacrylonitrile,rubber and polystyrene.Amorphous polymers do not scatter light,so they are transparent in visible light.Thus,they can be used as light weight glass such as poly(methyl methacrylate)with a trade name of Flexglas. The large size substituted polymer can be organized in isotactic structure through coordination polymerization.For example,poly(methyl methacrylate), polystyrene can crystallize with a helical conformation in which alternate chain bonds take trans and gauche positions.For the gauche position,the rotation is always in the direction that relieves steric hindrance by placing R and H groups in juxtaposition,generating either a left-hand or a right-hand helix as shown in Fig.3.7.If the side group is not too bulky,the helix has exactly three units per turn and the arrangement is similar to that in Fig.3.7a.This form has been found in isotactic polypropylene,poly(1-butene),polystyrene.More bulky side groups require more space,resulting in the formation of looser helices as shown in Fig.3.7b-d.Isotactic poly(methyl methacrylate)forms a helix with five units in two turns,while polyisobutylene forms a helix with eight units in five turns.The poly(tetrafluoro ethylene)contains larger size of F atom than that of H atom.Two helical conformations are existed.They are twist ribbons in which the fully
C C O O O O terephthalate The substituted vinyl polymer –(CH2–CHR)– with large bulky R group usually is in amorphous structure through free radical polymerization. For example, poly(methyl methacrylate), polyacrylonitrile, rubber and polystyrene. Amorphous polymers do not scatter light, so they are transparent in visible light. Thus, they can be used as light weight glass such as poly(methyl methacrylate) with a trade name of Flexglas. The large size substituted polymer can be organized in isotactic structure through coordination polymerization. For example, poly(methyl methacrylate), polystyrene can crystallize with a helical conformation in which alternate chain bonds take trans and gauche positions. For the gauche position, the rotation is always in the direction that relieves steric hindrance by placing R and H groups in juxtaposition, generating either a left-hand or a right-hand helix as shown in Fig. 3.7. If the side group is not too bulky, the helix has exactly three units per turn and the arrangement is similar to that in Fig. 3.7a. This form has been found in isotactic polypropylene, poly(1-butene), polystyrene. More bulky side groups require more space, resulting in the formation of looser helices as shown in Fig. 3.7b–d. Isotactic poly(methyl methacrylate) forms a helix with five units in two turns, while polyisobutylene forms a helix with eight units in five turns. The poly(tetrafluoro ethylene) contains larger size of F atom than that of H atom. Two helical conformations are existed. They are twist ribbons in which the fully Fig. 3.6 a The arrangement of C–H in the crystallites of polyethylene. Reproduced by permission of the Royal Society of Chemistry [3]. b Single crystal polyethylene shows platelet like structure. Reproduced by permission of Journal of Applied Physics [4] 32 3 Structure Morphology Flow of Polymer