[Corribus]

What is the cheapest, commercially-available polymer that I can buy in resin form and is reasonably transparent? Preferably I'm looking for something that's as low melting point as possible AND, an added bonus, reasonably soft/flexible. Being able to melt the polymer and force it into molds or extrude through a die is a must - e.g., with a compression molder. I'm thinking polystyrene may be my best bet - are there any other obvious options I'm missing? I'd really love a polyolefin of some kind with only alkyl substitutions but I think I may be out of luck there.

[curiouscat]

Out of curiosity what's the application?

[TyPie]

I think you're looking for clear silly puddy?  It's easy to work with, low melting point kind of, and transparent. 

Polymer properties are broad.  Have you looked at a polymer database?

[Corribus]

The application is that I want to make a clear film that I can load with either colorants or fluorophores. So far I have used LDPE, but this is semicrystalline and thus very hazy. Light scattering makes it hard to measure the optical properties. The materials are fabricated essentially by melt-mixing and extrusion, so the lower the melting point of the polymer, the better chance my additives have of surviving the fabrication process. Thus while PET is clear, the extremely high melting temperature (and other issues like hydrolysis) make it a very challenging material for my needs.

[Enthalpy]

The choice isn't so broad once you put "transparent"...

Pure polystyrene is quite brittle. Flexible if really thin, never soft. Shock-proofing additives (acrylonitrile and butadiene) lose the transparency.

Pmma and PC are the very transparent polymers (better than glass), but their softening point resembles Pet.

PE, PP are all hazy. PVA less so.

The best extrusion candidate for me is PMP, poly(methylpentene). It's very soft and flexible, much more transparent than other polyolefines, but it resists boiling water for sterilization. Not exactly cheap. You've already seen it at syringes: nicely transparent and shock-proof.
http://en.wikipedia.org/wiki/Polymethylpentene
http://de.wikipedia.org/wiki/Polymethylpenten injection at 270°C, ouch...
links to datasheets there.

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The other option would be - if acceptable for your application - to polymerize or polycondensate in situ. You could mix your optical additives first, then polymerize. MMA does it at room temperature with the UV fraction of fluorescent lamps or sunlight (hence work under red light or filament bulbs, and mind the nasty volatiles). Not the extrusion you planned, a bit brittle, but transparent and cold. MMA sells as a glue for PMMA but is useable alone, in moderate thickness.

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What about laminating your optical additives between sheets of transparent plastic like PC? This must work at a lesser temperature than extrusion. Or include the additive in a much thinner glue layer (MMA or standard glue) between two mechanical sheets?

[Corribus]

I am not very familiar with PMP - thanks for the suggestion. Expensive is ok, if the properties are good. This is a little hotter of a melt temperature than I was hoping for, but I might have to settle for a disadvantage somewhere.

I don't need absolute clarity, but it's got to be better than the LDPE I've been using so far, which is about as clear as mud.

I have considered in situ route, but forming a free-standing film would be challenging. Likewise lamination of a sandwhich layer would be an option but for various reasons I need the additive dispersed throughout the entire volume of the material.

Thanks for the suggestions.