5. Lecture Notes: Introducton To Polymer Chemstry Polymer: A large molecule (macromolecule) bult up by repettve bondng (covalent) of smaller molecules (monomers) Generally not a well defned structure, or molecular weght. Need to use statstcal propertes to descrbe. Polymers are formed by lnkng monomers through chemcal reacton called polymerzaton. You don t end up wth a unque molecule. monomers chan of monomers A (A A A) / Homopolymer: all A dentcal The most produced/used polymers are homopolymers of termnal alkenes. Produced by radcal polymerzaton. CH =CH (CH CH ) ethylene polyethylene H C=C CH COOCH H C-C CH COOCH methylmethacrylate PA
Copolymers: made up of dfferent monomers A + B (A-B) H C=CHCl + H C=CCl vnyl chlorde vnyldene chlorde H C-CH-CH -C Cl Cl Cl poly(vnylchlorde-co-vnyldene chlorde) Saran A-B-A-B-A-B A-A-A-A-B-A-B alternatng copolymer random copolymer Both of these are rare. ost common s a statstcal copolymer, whch has a statstcal dstrbuton of repeat unts. Block copolymers Two long sequences of repeat unts A-A-A-A-A-A-A-B-B-B-B-B-B-B Α Α Α Α Α Α Α Α Α Α AB dblock copolymer AB graft copolymer 5., Introducton to Polymer Chemstry Page
Structural characterstcs Closely related to materal propertes lnear (unnterrupted straght chan) branched (occasonal branches off longer chan) branch pont networked (many nterconnected lnear chans; one gant molecule) crosslnk Stereochemstry of Lnkages R H R HR H R H ISOTACTIC R groups on same sde of backbone R H R H R H R H SYNDIOTACTIC R groups on alternatng sdes of backbone ATACTIC Random (most common) Zegler-Natta catalysts used for so- and syndo- 5., Introducton to Polymer Chemstry Page
Classfcaton of polymers: Polymers (synthetc) 1) Thermoplastcs (plastcs) lnear, some cross-lnkng can be melted and reformed on heatng a) Amorphous no ordered structure b) Sem-crystallne composed of mcroscopc crystalltes domans of crystallne structure. Can be ordered. bers (nylon, polyester) ) Elastomers (rubbers) moderately cross-lnked can be stretched and rapdly recover ther orgnal dmenson ) Thermostats (resns) massvely cross-lnked very rgd; degrade on heatng 4) Dendrmers multply branched multple consecutve (regular) branches Bopolymers polypeptdes protens-amno acd heteropolymer nuclec acds RNA/DNA polysacchardes sugars 5., Introducton to Polymer Chemstry Page 4
Characterzaton 1) How do polymers respond to an appled force? (study of flow and deformaton: rheology) vscoelastc medum An elastc medum s descrbed by Newton s Law: = kx If you apply a force (a stress), the materal dsplaces by an amount x: x= k x x ksmall kbg small k: weak sprng easly dsplaced bg K: stff sprng dffcult to dsplace x Polymers are often non-newtonan or polymers, we apply a stress, and t leads to nternal dstorton stran. σ = m S stran dsplacement stress elastc modulus stran shear small m stretches easly/compresses easly (rubber) large m small stran produced by stress σ (hard plastcs PA) 5., Introducton to Polymer Chemstry Page 5
The elastc modulus m s hghly temperature dependent! Rubber has small m at room temperature ball bounces At low T, m much larger rubber ball n lqud N shatters when bounced hard plastc Also, plastcs heated above room temperature are less stff. TYPICAL PLOT O m(t) log m plastc T melt T degradaton rubber resn T g T Where s room temperature on ths plot? (depends on whether you have a rubber or plastc) The varous temperatures characterze polymers. 5., Introducton to Polymer Chemstry Page 6
) olecular Weght olar ass () : degree of polymerzaton (# of monomer unts) = 0 : molar mass of polymer molecule 0 : molecular weght of monomer Typcally have dstrbuton of masses (all chan lengths aren t equally long) monodsperse equal chan lengths polydsperse unequal lengths purfed protens, dendrners Characterze the polydspersty through ( ): dstrbuton of molar masses. () n v w We can fnd several statstcal ways of descrbng the molar mass. Comparson of these numbers helps descrbe (). A) Number-average molar mass, n n = N N 0 0 d d (frst moment) N : # of molecules wth degree of polymerzaton : molar mass for degree of polymerzaton I 5., Introducton to Polymer Chemstry Page 7
B) ass- or Weght-average molar mass, w = w w w s the weght fracton: the total mass of molecules wth mass dvded by the total mass of all molecules w N = N N w = N d 0 0 d (second moment of..) C) In experment 4, we are studyng vscosty-average molar mass, v ( v) a 0 = 0 1+ a d d Polydspersty We can descrbe the polydspersty through the wdth of the dstrbuton of molar masses. n < v < w w 1 perfectly monodsperse = 1 n 5., Introducton to Polymer Chemstry Page 8
) Chan dmensons Contour length: length along backbone n bonds of length l n l End-to-end dstance: ore common - measure of the coled system r The dstrbuton of r s characterzed by the rms end-to-end dstance r or a freely jonted chan wth n lnks and no restrctons on bond angle: r = nl Radus of gyraton, R g R g s the rms dstance of a chan segment from the center of mass of the polymer. ntrnsc vscosty R g = [ η] r R 6 g Defne a center of mass; then each chan segment has a certan dstance from the center of mass average x to get R g c.o.m. x 1 = R g x 5., Introducton to Polymer Chemstry Page 9