Agriculture Reference
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polymer. Some of the common applications for rigid PS are: packaging, containers,
caps, trays, disposable cutlery, tumblers, tubing, and lids; whereas foamed PS are
used for end applications such as insulation board, packaging, fish boxes, decorative
facades, and floating devices.
4.4.1 Structure of Polystyrene
PS is a long-chain hydrocarbon in which alternating carbon centers are attached to
phenyl groups (aromatic benzene rings). Basically, PS contains the elements carbon
and hydrogen. The properties of the polymer are determined by short-range van der
Waals attractions between polymer chains. The molecules are long-hydrocarbon chains
comprising thousands of atoms, so the total attractive force between the molecules
is large. If heated, the chains can take on a higher degree of conformation and slide
past each other. This intermolecular activity confers flexibility and elasticity. The
ability of this system to be deformed readily above its T g allows PS to be softened
and processed upon heating.
4.4.2 Polymerisation
PS is the result of basic styrene monomers interconnecting through polymerisation
(joining of basic units). During polymerisation, the carbon-carbon pi bond in the vinyl
group is broken and a new carbon-carbon single (sigma) bond is formed, leading to
the attachment of another styrene monomer to the chain. The newly formed sigma
bond is much stronger than the pi bond that was broken, thus it is very difficult to
depolymerise PS.
The only commercially important form of PS is atactic (i.e., phenyl groups are
distributed randomly on both sides of the polymer chain). This random positioning
prevents the chains from aligning with sufficient regularity to achieve crystallinity.
If the phenyl groups are positioned on alternating sides of the polymer chain, the PS
produced is syndiotactic . This form of PS is highly crystalline and is, in general, not
produced because the polymerisation process is slow.
Pure PS is brittle but sufficiently hard that a fairly high-performance product can
be made by giving it the properties of a more flexible or stretchable material by
combining it with polybutadiene rubber. These two materials cannot be mixed
because of the amplified effects of intermolecular forces on polymer insolubility.
However, if polybutadiene is added during polymerisation, it can become chemically
bonded to PS, forming a graft copolymer. This process helps to incorporate normal
polybutadiene into the final mix, resulting in high-impact polystyrene and opening
up more possibilities for the use of PS.
 
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