Increase of refractive index in polymers can be achieved by high polarizability. Low refractive index in polymers is usually linked to fluorination. Does that mean that fluorination reduces polarizability, of is it another effect?
Dear Dr. Patrick Bollgruen,
among different thermoplastic and thermoset polymers the fluorinated polymers are only family with a lower refractive index (RI) (1.3-1.4) than the mean value calculated (1.5). I believe that the chemical composition and structure of the polymer play a pivot role relative to the refractive index value.
For further details and comparison of RI with other polymers, please see at the very interesting web site indicated here: https://omnexus.specialchem.com/tech-library/article/low-ultra-low-refractive-index-polymers
Best regards, Pierluigi Traverso
Patrick Bollgruen You pose a good question. The effect of fluorine with respect to refractive index is seen with inorganic materials too. We have the anti-reflection coating of MgF2 providing the nearest RI (~1.38) to the square root of (RIglass*RIair) for an optimum coating. We have cryolite (1.34) (Eskimos call it 'frost stone') which is close to being index-matched in water (1.33) and thus 'disappears' on immersion in aforesaid liquid. The attached may be of interest. The mineral malladrite, Na2SiF6, has a really low RI of 1.31 or so.
F has a really low k (0.043) atomic refractivity because of the small size of the fluoride anion. See page 31 and page 32 in the classic text by Larsen (1934) attached. The fluoride minerals, though, tend to have rather high densities.
Gladstone-Dale is a good search item for you! This old webinar will only open in Internet Explorer:
Jan 24th, 2006. Optical Properties: the Gladstone-Dale approach
http://www.brainshark.com/brainshark/vu/view.asp?pi=141109&text=M021507
Pierluigi Traverso I'm not doctor (yet!) thank you for the website.
Alan F Rawle thank you for the databases as well. Your response put me on the right track, if you check https://en.wikipedia.org/wiki/Electronegativity, you see that Fluorine has the highest electronegativity, so it pulls everything to itself and is less affected by external polarization than for example phenantrene, which is used to increase refractive index, and doesn't have such electronegative centers.)
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