Matte black thermoplastic ("Plexiglas")

Charles Krebs · Post by **Charles Krebs** » Fri Sep 18, 2009 5:40 pm

I was setting up and aligning the BHA scope that I use with a vertical illuminator. Instead of glass slides I have a bunch of 2x3" plastic rectangles that I use to place subjects upon. The plastic I use has a "matte" surface, that to the eye appears very smooth. This became my "test" subject as I set up this microscope in it's new location. Here you see this surface photographed at 50X on sensor (20X objective and 2.5X photo-eyepiece). So as you look at these images on screen you are seeing this surface at about 500X.

The first is "brightfield", the second is reflected DIC (I only have one DIC prism, and that is for the 20X)

Ernst Hippe · Post by **Ernst Hippe** » Fri Sep 18, 2009 11:40 pm

Hallo ,

phantastic structure! Do you have any idea why it appears so regular?

Regards - Ernst Hippe

ChrisR · Post by **ChrisR** » Sat Sep 19, 2009 1:36 am

Fab!
How very perspexing. I mean perplexing. I can tell you as a Metallurgy and Materials Science graduate, that

I haven't a clue!

I can't help being reminded of long-chain molecules with cross-links, but nah, they're way smaller. Surely?? Though some polymers have been grown centimeters long.

Plastic like that may be made by extrusion in a continuous process, quite warm. It's probably made of a mixture of constituents (including plasticizers...), with different freezing points. So I'm assuming the first to freeze had the basis of the interlinked structure which shows, and the in-fill is the stuff which was last to freeze, ie having lowest freezing point. It shrunk on freezing/cooling.

Charles Krebs · Post by **Charles Krebs** » Sat Sep 19, 2009 2:13 am

My guess is that there was a mold made to create the matte surface texture as the sheet is formed. This "master mold" might have been etched chemically or (my suspicion) etched with a laser. (It looks microscopic melted metal welding "beads")

These surface patterns can be made very small. This link is not plexi manufacturing but perhaps something similar and more sophisticated:
http://www.wft.bz/gallery6.htm

ChrisR · Post by **ChrisR** » Sat Sep 19, 2009 2:19 am

It's possible. Could be done with rollers. I don't think the stuff would be cast in individual molds, unless they're very expensive bits of plastic!

Charles Krebs · Post by **Charles Krebs** » Sat Sep 19, 2009 2:39 am

No, pretty inexpensive stuff. I was thinking sheets formed by rollers as well.

rjlittlefield · Post by **rjlittlefield** » Sat Sep 19, 2009 8:46 am

This pattern is really quite bizarre!

Do the "bodies" of the "worms" stick up above the flat surface, or are they indented?

--Rik

ChrisR · Post by **ChrisR** » Sat Sep 19, 2009 8:50 am

Aren't they flattened tops on the highest "domes"?

rjlittlefield · Post by **rjlittlefield** » Sat Sep 19, 2009 9:05 am

I'm presuming these are stacked images. If so, then this is the sort of subject where Helicon's 3D surface display can give a lot of insight, by providing a synthetic view from far off to the side.

--Rik

Charles Krebs · Post by **Charles Krebs** » Sat Sep 19, 2009 11:37 am

The "bodies" of the "worms" (and the "rounded domes" along them) are raised up.

Here's the HF 3D surface:

And if you can do cross-eyed 3D this gives a really nice idea of the surface:
http://www.krebsmicro.com/mat_plex_3D+cross.jpg

rjlittlefield · Post by **rjlittlefield** » Sat Sep 19, 2009 11:58 am

Ah! Could you invert the depth profile and make another stereo pair?

I'm starting to think that we're looking at the "negative" imprint of a chemically etched surface. The ridges and broad depressions that I see here might make good sense in terms of a metal surface that's been eroded at the grain boundaries.

--Rik

Charles Krebs · Post by **Charles Krebs** » Sat Sep 19, 2009 12:28 pm

Here's the inverted... if molded, this is what the mold surface would look like.
Rather interesting!

http://www.krebsmicro.com/acrylic_mold_3D.jpg

BTW... I purchased this at "Tap Plastics". I believe this is the manufacturer for many of their sheet products:
http://www.cyro.com/methacrylates/us/products/

rjlittlefield · Post by **rjlittlefield** » Sat Sep 19, 2009 12:55 pm

Bingo. This looks to me like spherical erosion, starting from sites that occur preferentially along grain boundaries. Maybe somebody more familiar with etching & molding processes can confirm or correct this impression.

This type of 3D surface display is very nice for this type of subject, which really does have a well-defined surface.

But I've been a bit puzzled by Helicon's treatment of aspect ratio. It seems like HF always starts by squashing the surface into a square, no matter that the image was a rectangle to begin with. When I played with it I assumed I was doing something wrong or had overlooked some control, but I notice the same thing here. The image starts off as a 3:2 rectangle, but in what we see here the surface has been squashed left-to-right so the shapes are distorted.

Have you noticed this effect before, or seen any discussion of it?

--Rik