5.33 Lecture Notes: Introduction To Polymer Chemistry

Transcription

1 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

2 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

3 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

4 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

5 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

6 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

7 ) 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

8 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

9 ) 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