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

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 T_m. 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 T_m 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

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

Repeat-unit PolymerT_g,°C T_m,°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

сн₂

I

H3C-C-

I н

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 T_m 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 PolymerT_g,°C T_m,°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 T_m.

Date: 2015-12-11; view: 1053