Editing Polystyrene (section)

=== Styrene-butane co-polymers ===
Styrene-butane co-polymers can be produced with a low [[1-Butene|butene]] content. Styrene-butane co-polymers include PS-I and SBC (see below), both co-polymers are [[Impact resistance|impact resistant]]. PS-I is prepared by [[Graft polymerization|graft co-polymerization]], SBC by anionic block co-polymerization, which makes it [[Transparency and translucency|transparent]] in case of appropriate block size.<ref>{{cite web|title=Übersicht Polystyrol auf chemgapedia.de|url=http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/9/mac/werkstoff_polystyren/sorten/sorten.vlu/Page/vsc/de/ch/9/mac/werkstoff_polystyren/sorten/copolymere/uebersicht.vscml.html}}</ref>

If styrene-butane co-polymer has a high butylene content, [[styrene-butadiene rubber]] (SBR) is formed.

The impact strength of styrene-butadiene co-polymers is based on phase separation, polystyrene and poly-butane are not soluble in each other (see [[Flory–Huggins solution theory]]). Co-polymerization creates a boundary layer without complete mixing. The butadiene fractions (the "rubber phase") assemble to form particles embedded in a polystyrene matrix. A decisive factor for the improved impact strength of styrene-butadiene copolymers is their higher absorption capacity for deformation work. Without applied force, the rubber phase initially behaves like a [[Filler (materials)|filler]]. Under tensile stress, [[Crazing|crazes]] (microcracks) are formed, which spread to the rubber particles. The energy of the propagating crack is then transferred to the rubber particles along its path. A large number of cracks give the originally rigid material a laminated structure. The formation of each lamella contributes to the consumption of energy and thus to an increase in elongation at break. Polystyrene homo-polymers deform when a force is applied until they break. Styrene-butane co-polymers do not break at this point, but begin to flow, solidify to tensile strength and only break at much higher elongation.<ref name="Domininghaus-2012">{{Cite book|title=Kunststoffe: Eigenschaften und Anwendungen|last=Domininghaus, Hans.|date=2012|publisher=Springer|others=Elsner, Peter., Eyerer, Peter., Hirth, Thomas.|isbn=9783642161735|edition=8., neu bearbeitete und erweiterte Auflage|location=Heidelberg|oclc=834590709}}</ref>{{rp|426}}

With a high proportion of polybutadiene, the effect of the two phases is reversed. Styrene-butadiene rubber behaves like an elastomer but can be processed like a thermoplastic.

==== Impact-resistant polystyrene (PS-I) ====
PS-I ('''''<u>i</u>'''mpact resistant '''<u>p</u>'''oly'''<u>s</u>'''tyrene'') consists of a continuous polystyrene matrix and a rubber phase dispersed therein. It is produced by polymerization of styrene in the presence of polybutadiene dissolved (in styrene). Polymerization takes place simultaneously in two ways:<ref>{{cite web|title=Schlagzähes PS auf chemgapedia.de|url=http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/10/styrol_polymerisation/schlagzaehes_ps/schlagzaehes_ps.vlu.html}}</ref>

* [[Graft polymerization|Graft copolymerization]]: The growing polystyrene chain reacts with a [[double bond]] of the [[polybutadiene]]. As a result, several polystyrene chains are attached to one polybutadiene. 
** <span style="color:#F46C2C">S</span> represents in the figure the styrene [[repeat unit]]
** <span style="color:#00AAC5">B</span> the butadiene repeat unit. However, the middle block often does not consist of such depicted butane homo-polymer but of a styrene-butadiene co-polymer:

:::<span style="color:#F46C2C">SSSSSS&shy;SSSSSSS&shy;SSSSSS</span><span style="color:#00AAC5">BB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">BB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span>&shy;<span style="color:#F46C2C">S</span><span style="color:#00AAC5">BBBB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span>&shy;<span style="color:#F46C2C">SS</span><span style="color:#00AAC5">BBB</span><span style="color:#F46C2C">S</span><span style="color:#00AAC5">B</span><span style="color:#F46C2C">SSSSSSS&shy;SSSSSSS&shy;SSSSSSSSSSSSSSS</span><span style="color:#F46C2C">S</span>

By using a statistical copolymer at this position, the polymer becomes less susceptible to [[Cross linking|cross-linking]] and [[Melt flow index|flows]] better in the melt. For the production of SBS, the first styrene is homopolymerized via anionic copolymerization. Typically, an organometallic compound such as butyllithium is used as a catalyst. Butadiene is then added and after styrene again its polymerization. The catalyst remains active during the whole process (for which the used chemicals must be of high purity). The [[molecular weight distribution]] of the polymers is very low ([[polydispersity]] in the range of 1.05, the individual chains have thus very similar lengths). The length of the individual blocks can be adjusted by the ratio of catalyst to monomer. The size of the rubber sections, in turn, depends on the block length. The production of small structures (smaller than the wavelength of the light) ensure transparency. In contrast to PS-I, however, the block copolymer does not form any particles but has a lamellar structure.

==== Styrene-butadiene rubber ====
{{main article|Styrene-butadiene}}

Styrene-butadiene rubber (SBR) is produced like PS-I by graft copolymerization, but with a lower styrene content. Styrene-butadiene rubber thus consists of a rubber matrix with a polystyrene phase dispersed therein.<ref name="Pfropfcopolymere">{{cite web|title=PS-Pfropfcopolymere auf chemgapedia.de|url=http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/9/mac/werkstoff_polystyren/sorten/sorten.vlu/Page/vsc/de/ch/9/mac/werkstoff_polystyren/sorten/copolymere/sb/pfropfcopolymere.vscml.html}}</ref> Unlike PS-I and SBC, it is not a [[thermoplastic]], but an [[elastomer]]. Within the rubber phase, the polystyrene phase is assembled into domains. This causes physical cross-linking on a microscopic level. When the material is heated above the glass transition point, the domains disintegrate, the cross-linking is temporarily suspended and the material can be processed like a thermoplastic.<ref>{{cite web|title=styrenic block copolymers – IISRP|url=https://iisrp.com/wp-content/uploads/08SBC16Aug2012.pdf}}</ref>