Physics of Solid Polymers Seoul National University 1#

Physics of Solid Polymers

Seoul National University 1#

optically active enantiomers(left or right handed) C* = “asymmetric centre”; all 4 substituents different

Chiral aminoacids chirality = no mirror plane

* *

C*

H2N R2

COOH

R1 R1

NH2 C*

HOOC R2

Biopolymers

C C H H

CH₃ H C C H H

CH₃ H

C C H H

CH₃ H

C C H H

CH₃ H

C C H H

CH₃ H

C C H H

CH₃ H Is i-PP chiral?

No, there is a mirror plane through every backbone carbon.

(Difference in distance to the two chain ends does not count in practice.)

Proteins

HOOC C NH2 R1

R2

HOOCC NH2 R1

R2 +

HOOC C NH R₁

R₂ C O

C NH₂ R₁

R₂ amide group

Carboxylic acid + amine Æamide+ H₂O

polypeptide C

R1 R2

CN H

{ }x

O

Proteins

• 30+ aminoacids make up all proteins in nature.

• Proteins are aperiodic, but have the same polypeptide backbone.

• All natural aminoacids have the same handedness.

• Only two basic conformations of protein chain segments:

–α-helix –β-sheet

Conformation of polypeptides

Ω fixed

α-helix 18/5 helix (L. Pauling) Stabilized by intramolecular H-bonding C=O⋅⋅⋅⋅⋅H-N

α-Helix

Physics of Solid Polymers

Seoul National University 2#

β-sheet - planar zigzag

(like polyethylene, except –C(R₁,R₂)-CO-NH- replaces –CH₂-)

β-Sheet

6-strand antiparallel β-sheet

• regular conformations stabilized by H-bonds even though the chains are not crystalline.

(in synthetic polymers regular helix is only possible when stabilized by crystallization)

• A complete protein moleule = “tertiary structure”

– straight sections of α-helix and β-sheet + folds

http://www.avatar.se/molscript/examples/

“β-sandwich” structure “β-barrel”

Physics of Solid Polymers

Seoul National University 3#

DNA double helix

•triplets of paired “bases”

connected by multiple H- bond

•“lock-and-key” (molecular recognition)

•mechanism of DNA replication

Triple helix of collagen

• stiff protein fibres in tendon tissue

Hydrogen bonding in synthetic polymers

N H C O

CH₂CH₂CH₂CH₂CH₂N H C O Nylon-6

HO C O

CH₂CH₂CH₂CH₂CH₂NH₂ HO C O

CH2CH2CH2CH2CH2NH2 +

adipic acid

N H C O

CH2CH2CH2CH2CH2N H C O

N H C O

CH2CH2CH2CH2CH2N H C O

Note: Nylon-2 is a polypeptide

Physics of Solid Polymers

Seoul National University 4#

Kevlar and Nomex

C O N H N C

H N

H C O

O

N C H N

H C O

O

C O N H

Kevlar

Kevlar: 1,4 substitution (para) Nomex: 1,3 substitution (meta) Aromatic polyamides (Aramides)

PIEZOELECTRIC POLYMERS

• Poly(vinylidene fluoride) -CH

₂

-CF

₂

-CH

₂

-CF

₂

-

• - Crystals are non-centrosymmetric.

(lack centre of inversion)

•

Æ

ferroelectric

(spontaneously polarize, have net electric dipole)

•

Æ

piezoelectric

(change polarization under pressure)

Æ

use in computer keyboards

Poly(vinylydene fluoride (PVDF)

P