In chemical terms, polystyrene is a long chain
hydrocarbon wherein alternating carbon centers are attached to phenyl groups (the name given to the aromatic ring benzene).Polystyrene's chemical formulais (C8H8) it contains the chemical elements carbon and hydrogen.
The material's properties are determined by short-range van DerWaals attractions between polymers
chains. Since the molecules are long hydrocarbon chains that consist of
thousands of atoms, the total attractive force between the molecules is large.
When heated (or deformed at a rapid rate, due to a combination of viscoelastic
and thermal insulation properties), the chains are able to take on a higher
degree of conformation and slide past each other. This intermolecular weakness (versus the high intramolecular strength due to the hydrocarbon
backbone) confers flexibility and elasticity. The ability of the system to be
readily deformed above its glass transition temperature allows polystyrene (and
thermoplastic polymers in general) to be readily softened and molded upon
heating.
Extruded polystyrene is about as strong as an unalloyed aluminum, but much more flexible and much lighter (1.05 g/cm3
vs. 2.70 g/cm3 for aluminum).
Polystyrene (PS) a synthetic aromatic polymer made from the monomer styrene. Polystyrene can be solid or foamed. General-purpose
polystyrene is clear, hard, and rather brittle. It is an inexpensive resin per
unit weight. It is a rather poor barrier to oxygen and water vapor and has a
relatively low melting point. Polystyrene is one of the most widely used plastics, the scale of its production being several billion
kilograms per year. Polystyrene can be naturally transparent, but can be colored with colorants.
Uses include protective packaging (such as packing peanuts and CD and DVD cases), containers (such as
"clamshells"), lids, bottles, trays, tumblers, and disposable cutlery.
As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room
temperature but flows if heated above about 100 °C, its glass transition
temperature.
It becomes rigid again when cooled. This temperature behavior is exploited for extrusion (as in Styrofoam) and also for molding and vacuum forming, since it can be cast into molds with fine detail.
Polystyrene is very slow to biodegrade and is therefore a focus of controversy among
environmentalists. It is increasingly abundant as a form of litter in the outdoor environment, particularly along shores and
waterways, especially in its foam form, and also in increasing quantities in
the Pacific Ocean.
Polymerization
Polystyrene results when styrene monomers interconnect. In
the polymerization, the carbon-carbon pi bond (in the vinyl group) is broken and a new
carbon-carbon single (sigma) bond is formed, attaching 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 depolymerize polystyrene. About a
few thousand monomers typically comprise a chain of polystyrene, giving a molecular
weight of
100,000–400,000.
A 3-D model would show that each of
the chiral backbone carbons lies at the center
of a tetrahedron, with its 4 bonds pointing toward the vertices. Consider that the -C-C- bonds
are rotated so that the backbone chain lies entirely in the plane of the
diagram. From this flat schematic, it is not evident which of the phenyl (benzene) groups are angled outward from the plane of the
diagram, and which ones are inward. The isomer where all of the phenyl groups are on the same side is
called isotactic polystyrene,
which is not produced commercially. Atactic polystyrene
The only commercially important form of polystyrene is atactic, in which the phenyl groups
are randomly distributed on both sides of the polymer chain. This random
positioning prevents the chains from aligning with sufficient regularity to
achieve any crystallinity. The plastic has a glass transition
temperature Tg of
~90 °C. Polymerization is initiated with free radicals.
Syndiotactic
polystyrene
Ziegler-Natta
polymerization
can produce an ordered syndiotactic
polystyrene with the phenyl groups positioned on alternating sides of the
hydrocarbon backbone. This form is highly crystalline with a Tm of 270 °C
(518 °F). Syndiotactic polystyrene resin is currently produced under the
trade name XAREC by Idemitsu corporation. Syndiotactic polystyrene is prepared
by combining a metallocene catalyst with a styrene monomer to generate a
polystyrene chain with a syndiotactic structure.
Degradation
Polystyrene is very chemically inert, being resistant to
acids and bases but is easily dissolved by many chlorinated solvents, and many
aromatic hydrocarbon solvents. Because of its resilience and inertness, it is
used to fabricate many objects of commerce. It is attacked by many organic
solvents, which dissolve the polymer. Foamed polystyrene is used for packaging
chemicals.
Like all organic compounds, polystyrene burns to give carbon dioxide and water vapor. Polystyrene, being an aromatic hydrocarbon, typically combusts incompletely as
indicated by the sooty flame.
Biodegradation
Methanogenic consortia have been shown to degrade styrene as sole carbon source (Grbić-Galić et al. 1990). In this
case styrene degraded to a range of organic intermediates and carbon dioxide.
Taking the carbon dioxide figures as a representation of the amount of styrene
that had completely degraded to gas as is of interest here, rates of styrene
degradation ranged from 0.14 to 0.4 a−1. This is an order of
magnitude faster than the most rapid rate of polystyrene degradation identified
(Kaplan et al. 1979, Sielicki et al. 1978). It is consistent with the T2GGM
polystyrene degradation model (Quintessa and Geofirma 2011b), which considers the
rate-limiting step for polystyrene degradation to be the breakup of
polystyrene, rather than the degradation of styrene.
Pseudomonas putida
is capable of converting styrene oil into the biodegradable
plastic PHA.[15][16] This may be of use in the effective recycling of Polystyrene foam, otherwise thought to be
non-biodegradable.
Mealworms have been shown to be able to eat polystyrene and
degrade it within their larval gut.
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