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Introduction to Polymer Science and Technology Behaviour of polymers

5.3.1.2 Composition of the backbone molecular chain

Structure of the repeating unit:

Backbone chains with aliphatic groups,e.g., - CH2- CH2- or the replacement of some of the "C" atoms by "O" (as in ether links) builds flexibilityinto a polymer, because of ease of rotation about these groups, and lowers T and T .

G m

Repeat-unit Polymer Tg, C Tm, C

-CH2-CH2- PE -120 130

CH3

-Si-O- Silicone rubber -123

(polydimethyl siloxane) CH,

Conversely, aromatic back-bone chains(i.e., benzene rings in the chain) cause stiffness,for example:


Introduction to Polymer Science and Technology Behaviour of polymers


epoxy resins
Poly-2,6-dimethyl
phenylene oxide  


PEEK 143 334

PES (an amorphous TP) 230

Repeat-unit lengths and intermolecular forces:

The frequency of repeat-unit links, and the frequency and the type of intermolecular bonding (e.g., Van der Waals forces or H-bonds)influences T and Tm. In nylons, in spite of an aliphatic backbone, intermolecular H-bonds that are much stronger than Van der Waals forces hold neighbouring chains strongly together and also tend to stabilise the crystallites, therefore result in high T and Tm values. The melting temperature of nylons decrease as the frequency of H-bonding decreases, for example, the melting temperature of Nylon 4,6 (295 C) > Nylon 6,6 (265 C) > Nylon 6,10 (223 C) > Nylon 10,10 (203 C) , etc.

5.3.1.3 Side groups


long flexiblealiphatic side groups decrease T and Tm. Rigid groups include aromatic and/or cyclic structures or tertiary

Bulky,inflexible (rigid) side groups increase and Tm by restriction of bond rotation and main chain stiffening, but long flexiblealij isomeric forms.

Repeat-unit PolymerTg,C Tm,C

H

I
-CH2-C- PP -10 170

^ (isotactic)



PS 100 240

(isotactic)


Introduction to Polymer Science and Technology


Behaviour of polymers


 


H I-CH2-C-

I

IH3C-C-CH3

I

CH3


polyvinyl t-butylether




 


polyvinyl iso-butylether


-18



I

2

I

H3C-C-

I


 

 
CH
CH
CH
CH  

polyvinyl n-butyl ether


-52



 


t - tert or tertiary


5.3.1.4 Molecular polarity

The presence ofpolargroupssuchas -Cl, -OH or -CN tends to raise T and Tm more than non-polar groups of equivalent size. The polar interactions restrict main-chain mobility further by promoting additional intermolecular forces.


Introduction to Polymer Science and Technology Behaviour of polymers

Repeat-unit PolymerTg,C Tm,C

H

I
CH2-C- PP -10 170

^ (isotactic)

H

I

-CH2-C- polyvinylalcohol 85 230

IOH

H

I

CH2-C- PVC 87(81) 227(273)

I

H

I
-CH2-C- polyacrylonitrile 104(130) 317

^ (syndiotactic)

5.3.1.5 Backbone molecular symmetry



Symmetry facilitates rotation about the backbone molecular chain and, thus, causes a drop in T and Tm.


Date: 2015-12-11; view: 830


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