Well, hopefully I can "clear" up some of this.
I am no chemical (or plastic) engineer. But I did work in injection molding for 17 years.
First of all, not all styrene is the same.
There are differerent grades, and types.
Most stryene is opaque in nature. Meaning, even with no colorant added, you can't see through the parts. It looks almost like it is white, thought it is actually pretty much color neutral. Parts ran with no colorant would appear white, though you would be able to sort of see through it.
Much like this new mug I got for Halloween.
http://www.tylisaari.com/me/halloween/2009/stuff071.JPG
Clear styrene is a special grade of styrene.
All styrene gets it's properties from how it bonds on a molecular level. In this case by lining themselves up end-to-end and forming striations.
This is what makes it stiff, rigid, and allows it to be chemically bonded (you know, melted together with glue) That is also why it is suseptable to breaking and cracking. It has built-in fault lines all through it because the way the material is bonded on a molecular level. It will only flex so much before one of those faults gives out and breaks or cracks.
Clear styrene gets it's clarity by lining those molecules up in even straighter, and more even lines. To allow the light to pass through it without being affected by it.
That is why clear parts are even stiffer then regular parts.
And why they crack and break easier. Less flexibility before one of the faults gives out.
And why they are even more affected by chemicals. Crazing and fogging from glues. Basically those long straight strands of molecules on the surface get bent and contorted. Which in turn causes the light to not pass through them properly.
