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Introduction to Polymer Science and Technology Behaviour of polymers5.3.1.2 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 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
Bulky,inflexible (rigid) side groups increase Òê and Tm by restriction of bond rotation and main chain stiffening, but long flexiblealij isomeric forms. Repeat-unit PolymerTg,°C Tm,°C H I ^ (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 ñí2 I H3C-C- I í
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 ^ (isotactic) H I -CH2-C- polyvinylalcohol 85 230 IOH H I CH2-C- PVC 87(81) 227(273) I H I ^ (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: 1211
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