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The hidden force of the ring

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23.06.2005

It is difficult to imagine modern life without plastics. Look around you, they are everywhere: pens, PC’s, lenses, furniture, etc. They are cheap, long-lasting and light and, moreover, they have good mechanical, thermal and dielectric properties, to such an extent that they have replaced wood, metal or glass in many applications. The polysulphone, phenoxyl or polycarbonate thermoplastics studied in this thesis are highly resistant (the last one being used for car fenders), ductile and flexible.

The quid of the question is that these interesting properties at a macroscopic scale (the ordinary, everyday-life scale) depend on: 1) the structure of the polymer chains and 2) the movements of their component molecules and atoms at a microscopic scale (the scale of atoms). Despite the fact that, on sight, a card (for example) does not “move", the atoms in its interior are continually moving and we would be able to see this if we had a giant magnifying glass. In this study the “glass” used was a technique known as neutron dispersion (NS). By means of NS the relative position of atoms can be known and the movement studied of these small particles, the neutrons, and how they are deviated from their trajectory on passing through the material studied below.

The existence of a direct correlation between the mechanical properties of a thermoplastic and the phenomenon known as Secondary Relaxations has been known for some time amongst the scientific community. Regarding the latter, although it is known that they are linked with the movement of molecules in general, in the majority of cases their exact origin and nature are not known, i.e. exactly how atoms and molecules move and/or the factors that determine that the same molecule in some cases moves and in others does not. In particular, thermoplastics that contain phenyl rings present prominent secondary relaxations and are quite similar amongst each other. Thus, the idea was, through NS techniques, to study the movement of these rings in various thermoplastics (the three previously mentioned), in order to subsequently compare these movements with secondary relaxation phenomena. The phenyl rings are flat and rigid structures (like a coin) that unite the two ends of the principal plastic chain in such a way that the final result is a species of “a necklace of coins”. The peculiarity of the set of materials chosen is that, in each case, the interlinking rings on the chain are separated by different, more or less large and flexible molecular units. That is to say, following on with the metaphor of the “necklace”, different sized and coloured “beads” are inserted between the coin structures.

The results show that the rings carry out oscillation and rotation movements about the axis of the principal chain. It has been shown that the nature of the molecular groups surrounding the ring on the chain (the colour or size of the “beads”) can limit or foment the movement of the same. Thus, for example, the mobility of the rings in polycarbonates is increased by the presence of the adjacent carbonate group. Finally, the comparison of these movements with the secondary relaxations has revealed that the movement of these rings derives from the relaxations in these materials, emphasising the role played by the rings - “the hidden force of the ring”, in their positive properties.

Garazi Andonegi | Source: alphagalileo
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