Polarization rotating plastics

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Jeroen Belleman

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Oct 26, 2020, 9:35:22 AM10/26/20
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While playing with polarizing filters, I found a plastic ruler
that turns out to rotate the polarization angle of the light
passing through it by almost exactly 90 degrees. The ruler is
of a 1mm thick colourless plastic, rather flexible, clearly
not the usual polyethylene or polystyrene. It does not itself
polarize light, it just rotates it. The angle of rotation does
not seem to depend on wavelength. It's made in China.

What plastic could this be?

Thanks,
Jeroen Belleman

Phil Hobbs

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Oct 26, 2020, 10:57:19 AM10/26/20
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There are quite a lot of optically-active plastics. Optical activity is
normally strongly dispersive--what wavelengths did you use?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Jeroen Belleman

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Oct 26, 2020, 11:58:48 AM10/26/20
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On 2020-10-26 15:57, Phil Hobbs wrote:
> On 10/26/20 9:35 AM, Jeroen Belleman wrote:
>> While playing with polarizing filters, I found a plastic ruler that
>> turns out to rotate the polarization angle of the light passing
>> through it by almost exactly 90 degrees. The ruler is of a 1mm
>> thick colourless plastic, rather flexible, clearly not the usual
>> polyethylene or polystyrene. It does not itself polarize light, it
>> just rotates it. The angle of rotation does not seem to depend on
>> wavelength. It's made in China.
>>
>> What plastic could this be?
>
> There are quite a lot of optically-active plastics. Optical activity
> is normally strongly dispersive--what wavelengths did you use?
>
> Cheers
>
> Phil Hobbs
>

Just natural, white light. Some plastics show coloured fringes
when inserted between two polarizing sheets, which is sort-of
what I expected. This Chinese ruler is special: It rotates
the polarization. Inserted between two parallel polarizers, it
has four orientations spaced by 90 degrees where it blocks the
light. But it is not itself polarizing: I see no intensity
variations when looking through it with a single polarizer, nor
when superimposing multiple layers of the same plastic.
There are no colours.

Jeroen Belleman

Joe Gwinn

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Oct 26, 2020, 1:33:52 PM10/26/20
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It's very likely to be polystyrene.

Joe Gwinn

Joe Gwinn

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Oct 26, 2020, 2:18:08 PM10/26/20
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On Mon, 26 Oct 2020 13:33:45 -0400, Joe Gwinn <joeg...@comcast.net>
wrote:
Hmm. There are some kinds of solid plastic that are liquid crystals
(but cooled into rigidity, like glass).

Joe Gwinn

Jeroen Belleman

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Oct 26, 2020, 3:07:20 PM10/26/20
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Polystyrene typically shows coloured fringes, stresses frozen in
the material, does it not? The transparent plastic CD box I have
here, presumably polystyrene, certainly does. The Chinese ruler
is different. There are no visible stress fringes. It just rotates
the polarization by basically the same angle for all colours
everywhere. It doesn't 'feel' like polystyrene either. Too flexible
and rather tough, not brittle.

I'd like to find a clean sheet of the stuff, so that I can make
some more serious measurements. My wife objects to me cutting up
her rulers.

Jeroen Belleman

Joe Gwinn

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Oct 26, 2020, 5:40:43 PM10/26/20
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On Mon, 26 Oct 2020 20:07:16 +0100, Jeroen Belleman
Clear CD boxes are usually polystyrene. and yes that is how that
behaves. So, I agree that polystyrene is ruled out.


> The Chinese ruler
>is different. There are no visible stress fringes. It just rotates
>the polarization by basically the same angle for all colours
>everywhere. It doesn't 'feel' like polystyrene either. Too flexible
>and rather tough, not brittle.

If you dissolve it in acetone, do you get a powder residue?

In other words, is the rotation due to the plastic, or to a mineral
filler (if any)?


>I'd like to find a clean sheet of the stuff, so that I can make
>some more serious measurements. My wife objects to me cutting up
>her rulers.

Liquid crystal polymers (LCPs) are typically very tough. But I'm not
sure any are transparent. Or that one would make cheap rulers from
LCPs.

Wonder if it's polycarbonate, which is transparent like polystyrene.
But I don't recall that it rotates polarization.

Where did this mystery ruler come form?

Joe Gwinn

Jeroen Belleman

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Oct 27, 2020, 5:00:44 AM10/27/20
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It's Chinese, I can't read the brand name. It's a 'comma shaped
French curve ruler'. There are lots of brands that look alike.

Jeroen Belleman

Phil Hobbs

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Oct 27, 2020, 8:24:53 AM10/27/20
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Polystyrene generally has a lot of stress birefringence, which is what
gives the coloured fringes when it's placed between crossed polarizers.

>>
>> Where did this mystery ruler come form?

>
> It's Chinese, I can't read the brand name. It's a 'comma shaped
> French curve ruler'. There are lots of brands that look alike.

To get optical activity, the polymer has to be chiral, like biological
molecules. (A strong sugar solution is optically active as well.)

Jeroen Belleman

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Oct 27, 2020, 8:55:53 AM10/27/20
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I now believe the rulers are some polyester. A sheet of mylar
I tried shows a similar behaviour, though not as pronounced.
(It's much thinner too.) I guess the ruler happens to have
just the right properties to work as a half-wave plate. I'll
try a piece cut from a PET bottle tonight.

A 4mm thick sheet of polycarbonate shows a uniform green or
magenta hue, depending on polarizer orientation. A piece
of thin cellophane varies between yellow and blue.

Jeroen Belleman

Quadibloc

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Nov 3, 2020, 1:05:41 PM11/3/20
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As has been noted, optical activity is quite common in plastics.

If you put a mildly crumpled cellophane wrapper from a pack of cigarettes
between two polarizing filters, you will see beautiful colors.

John Savard

Phil Hobbs

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Nov 3, 2020, 7:43:56 PM11/3/20
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Birefringence is common, optical activity not so much. To get optical
activity, there has to be a net chirality to the polymer, i.e. either
left- or right-handed enantiomers have to predominate. IIUC that always
happens in biological molecules, but doesn't happen by accident in
petrochemicals.

whit3rd

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Nov 10, 2020, 4:14:40 AM11/10/20
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On Monday, October 26, 2020 at 8:58:48 AM UTC-7, Jeroen Belleman wrote:
> On 2020-10-26 15:57, Phil Hobbs wrote:
> > On 10/26/20 9:35 AM, Jeroen Belleman wrote:
> >> While playing with polarizing filters, I found a plastic ruler that
> >> turns out to rotate the polarization angle of the light ...

> > There are quite a lot of optically-active plastics.

> Just natural, white light. Some plastics show coloured fringes
> when inserted between two polarizing sheets, which is sort-of
> what I expected. This Chinese ruler is special: It rotates
> the polarization. Inserted between two parallel polarizers, it
> has four orientations spaced by 90 degrees where it blocks the
> light.

I think that means it's birefringent, i.e. has an orientation (probably
because the polymer was stretched in one direction as the sheet
was rolled out).

When linear polarized light has E-field parallel to the orientation,
the film is N wavelengths thick. When it is perpendicular, the film
is N+1/2 wavelengths thick. There are four inbetween orientations
that correspond to quarter-wave mismatched in two components, that make the
linear polarized light into circular polarized. Circular polarized isn't blocked
by the second linear polarizer.

Inexpensive acetate is the most likely material for a transparent ruler. Two layers
of acetate laminated around a printed film with the markings, perhaps?

Jeroen Belleman

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Nov 10, 2020, 8:25:55 AM11/10/20
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Thanks for your comments. I'll do some more experimenting.

The linearly polarized light is still linearly polarized
after passing through the ruler, because there are still
two orientations where the second polarizer blocks all
light. They're just different orientations.

Jeroen Belleman

Phil Hobbs

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Nov 10, 2020, 12:52:52 PM11/10/20
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Try tipping the ruler and see if that changes.

Jeroen Belleman

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Nov 10, 2020, 2:37:52 PM11/10/20
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Oriented at its darkest, the extinction ratio gets worse
and the residual light passing through takes a deep blue
or brownish hue. There is no perceptible colouration of
the light passing through when angles are adjusted for
maximum transmission

Whit3d is correct that the thing is laminated. Is industrially
produced acetate chiral with one enantiomer dominating?
Probably, I found sources stating it's made from wood pulp.

Thanks,
Jeroen Belleman

Phil Hobbs

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Nov 10, 2020, 2:44:28 PM11/10/20
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Don't think so. Anyway, as he points out, if the effect is
predominantly optical activity, you wouldn't get such pronounced light
and dark fringes.

Some optically active materials, such as sucrose solution, are
isotropic, and some, such as alpha-quartz, aren't.

Tipping it so that the light isn't normally incident will change the
path length.
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