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ولي فعلاً از پلي استايرن البته لاتين اينا رو داشته باشيد خوبه:

16.13 STEREOREGULAR POLYSTYRENE
Polystyrene produced by free-radical polymerisation techniques is part syndiotactic
and part atactic in structure and therefore amorphous. In 1955 NattaI6 and
his co-workers reported the preparation of substantially isotactic polystyrene
using aluminium alkyl-titanium halide catalyst complexes. Similar systems were
also patented by Ziegler17 at about the same time. The use of n-butyl-lithium as
a catalyst has been described. ' * Whereas at room temperature atactic polymers
are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow
molecular weight distribution.
In the crystalline region isotactic polystyrene molecules take a helical form
with three monomer residues per turn and an identity period of 6.65 A. One
hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for
amorphous polymer and is also translucent. The melting point of the polymer is
as high as 230°C. Below the glass transition temperature of 97°C the polymer is
rather brittle.
Because of the high melting point and high molecular weight it is difficult to
process isotactic polystyrenes. Various techniques have been suggested for
injection moulding in the literature but whatever method is employed it is
necessary that the moulding be heated to about 18O"C, either within or outside of
the mould, to allow the material to develop a stable degree of crystallinity.
The brittleness of isotactic polystyrenes has hindered their commercial
development. Quoted Izod impact strengths are only 20% that of conventional
amorphous polymer. Impact strength double that of the amorphous material has,
however, been claimed when isotactic polymer is blended with a synthetic rubber
or a polyolefin.
16.13.1 Syndiotactic Polystyrene
The first production of syndiotactic polystyrene has been credited to research
workers at Idemitsu Kosan in 1985 who used cyclopentadienyl titanium
compounds with methyl aluminoxane as catalyst.
Whereas the isotactic polymer has not been commercialised Dow were
scheduled to bring on stream plant with a nameplate capacity of 37 000 t.p.a. in
1999 to produce a syndiotactic polystyrene under the trade name Questra. The
particular features of this material are:
Tg of about 100°C (similar to that of amorphous polystyrene) and T, of
270°C.
Low density with crystalline and amorphous zones both having densities of
about 1.0Sg/cm3. This is similar to that occurring with poly-4-methyl
pentene-1, discussed in Chapter 11 and with both polymers a consequence of
the spatial requirements in the crystal structure of the substantial side groups.
Processing of Polystyrene 455
An advantage of the matching densities of the two zones or phases is that
there is little warping and generally good dimensional stability.
(c) While the unfilled polymer is somewhat brittle, impact strength is
substantially increased by the use of glass fibres and/or impact modifiers.
(d) While the heat deflection temperatures of unfilled materials are similar to Tgr
that of glass-filled grades approaches T,. This is in line with observation
made with other crystalline thermoplastics as discussed in Chapter 9.
(e) Electrical, chemical and thermal properties and dimensional stability are
similar to those of general purpose ('atactic') polystyrene and thus has some
advantages over more polar crystalline, so-called, engineering plastics such
as the polyamides and linear polyesters.
Units
Some typical properties are given in Table 16.9.
Unfilled 30% glass 30% glass
filled filled and
impact
modified
Table 16.9 Some properties of syndiotactic polystyrene
MPa
MPa
%
"C
in/in
Property
42 121 105
3500 10000 7580
IO 96 117
100 249 232
1 1.5 3.4
1 .os 1.25 1.21
2.6 3.1 3.1
0.0002 0.001 0.001
0.0027 0.003
4.0037 0.004
ASTM
method
Tensile strength
Tensile modulus
Elongation @ break
Notched Izod 23°C D256
Deflection temp. under load @ 1.82MPa
Specific gravity
Dielectric constant
Dissipation factor
Moulding shrinkage
D638
D638
D638
Jlm
D648
D792
D150
D150
D955
Potential applications for glass-filled grades include electronic/electrical
connectors, coil bobbins, relays; automotive lighting and cooling system
components and pump housings and impellers. Unfilled grades are of interest as
capacitor film with a heat resistance that can withstand infra-red reflow soldering
combined with excellent electrical insulation properties little affected by
temperature and frequency. Non-woven fabrics with good heat, moisture and
chemical resistance are of interest for filter media.
There has also been some interest in melt blending with polyamides to increase
the toughness but at some sacrifice to dimensional stability and moisture
resistance.

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