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Polymer Comparison

This is the place where you can see the structural differences between NSI's polymers and other, commercial polymers, some of which you may recognize.

On the left is the color key used to characterize the rotational freedom of the backbone bonds of the polymers in the following illustrations.  The colors range from magenta (hot pink) to cyan blue to describe the continuum from a very high degree of rotational freedom to a very low degree of rotational freedom.
Magenta means that the bond is capable of nearly complete (180°) rotation.
Red means that the bond is not quite capable of complete rotation, but can rotate to a very large degree.
Yellow means that the bond is restricted in its rotation to two positions that are 180° apart (for example, palm up versus palm down).  [Sorry about the yellow-on-white difficulty in readability.]
Green means that the bond rotation is restricted to a single favored orientation by at least one feature.
Blue means that the bond rotation is restricted to a single orientation by more than one feature.

So let's look at a common polymer, Nylon, that most of us know from everyday experience (see Figure 1).  Nylon comes in several variations based on differing chain lengths.  But the longer versions have even greater rotational freedom, so this version of Nylon is a good choice for purposes of comparison to the following polymers.

As you can see, all the bonds but one are magenta, which means that the Nylon backbone can rotate freely.  This makes Nylon floppy, like a strand of overcooked spaghetti.  Even the amide bond (yellow) is capable of flipping back and forth between the cis and trans forms.  (Only the trans form is illustrated here.)  Nylon is a good example of a very inexpensive polymer, which is used to make rope, string, fishing line, clear sewing thread, fabrics, washers and drawer slides.

Now, let's compare Nylon to Kevlar (DuPont's brand of para-aramid polymer).  As you can easily see from the dramatic change in the number and color of arrows, Kevlar (Figure 2) is an entirely different kind of polymer than Nylon.  The rings (hexagons, with the three double bonds inside) are rigid.  So the rotational freedom is restricted to the three linkage bonds, which are the same sequence of atoms as found in the linkages of Nylon.

But there are two distinct differences.  One, the central amide bond is green instead of yellow, indicating that there is only one orientation of that bond.  And the adjacent ring-to-carbon bond is yellow instead of magenta, which partially limits rotation about that bond, too.  But the ring-to-nitrogen bond being red instead of magenta is not much of an improvement at all.  Although this bond has structural reasons to want to be yellow, it cannot be yellow due to crowding between the ring and the double-bonded oxygen.  So the ring-to nitrogen bond is twisted instead of lying flat.

Even in DuPont's Nomex (Figure 3, meta-aramid instead of para-aramid) there is still twisting of the ring-to-nitrogen bond.

So even though meta-oriented aramid makes an excellent and superior oven mitt, it is equally as structurally compromised as para-aramid polymer.

One of NSI's key inventions is a solution to this structural conflict.  The steric crowding is changed into a bonding feature by placing nitrogen in the ring at the site of the conflict.  This allows a hydrogen bond to form where—formerly—there was repulsion.  So a nanostructural liability in Kevlar is converted into a nanostructural bonding feature in Stevlar (NSI's analog of Kevlar).  The "kinks" and "twists" in Kevlar that prevent semiconductivity are converted into fully aromatic linkages that facilitate semiconductivity. 

In these illustrations, hydrogen bonds are indicated by dashed lines.  They are weaker bonds than the covalent bonds that are represented by solid lines.  The advantage of hydrogen bonds is that they can be temporarily broken by mechanical forces, thus absorbing impact energies and protecting the backbone bonds.  Then, later, dissipation of the absorbed energy allows the hydrogen bonds to reform, restoring the original nanostructure.  Self-repair capabilities are a novel and inherent property of NSI's polymer nanostructures.

The particular form of chemical notation used in these figures uses lines to indicate bonds, and letters to indicate the non-carbon elements.  Organic chemists (chemists specializing in carbon compounds) prefer it because it "hides" the carbon atoms that tend to be overly plentiful in organic chemicals and that tend to clutter the chemical diagrams.  In other words, every place that two lines come together (forming a vertex) is a carbon atom, which using another form of chemical notation would require a "C" to be placed at that location.  In Figure 4, there are 24 Cs that are not shown, which makes it easier to see the N (nitrogen) and O (oxygen) atoms.

This form of chemical notation also does not illustrate hydrogen (H) atoms that are attached to "hidden" carbon atoms.  In figure 4, there are 12 "invisible" hydrogen (H) atoms, two on each of the nitrogen-containing rings and four on each of the non-nitrogen rings.  So 12 hydrogen (H) atoms and 24 carbon (C) atoms is a total of 36 letters that are not shown.  This "naked backbone" style of notation is commonly used in organic chemistry texts and scientific papers.

Now lets get back to the advancing the cultivation of nanostructure.  In Figure 4, two of the three linkage bonds are restricted from free rotation.  But that leaves one that can rotate.  The next technological advancement is to add another bonding feature to restrict rotation in that final bond.  The end result is a fully vector-directional iminol polymer (see Figure 5).  In other words, there are no longer any yellow, red or magenta arrows in the polymer backbone.  This is what we set out to accomplish.

Up until this point, we have used aramids and iminols as the illustrative polymer system.  But there are other classes of polymer that can be similarly modified to establish full vector directionality.

In Figure 6, Toyobo's Zylon-brand bisoxazole (actually, benzobisoxazole, to be more accurate) polymer is illustrated.  From a rotational-freedom perspective, Zylon has similar degrees of rotational freedom as Stevlar (Figure 4), but with the added benefit of all rotatable bonds being coaxial.  So any bonds that spin do not affect the directionality of the polymer backbone.  It is straight.  Bisoxazoles are a classic example of what are called rigid-rod polymers because of their straightness.

The bisoxazole rigid-rod polymers are closely related to bisimidazole polymers, of which Magellan Systems' M5 polymer (Figure 6a) is probably the highest performing rigid-rod polymer known.  Despite the hydroxy groups that form hydrogen bonds across the polymer linkages, M5 is not vector-directional due to the essential equivalence of the two nitrogen atoms on the five-membered rings.  All it takes is a shift of a proton from one side of the ring to the other and the bonding reverses its directionality.  This cannot happen in the bisoxazole polymers (Figures 6, 7 and 8) due to the fact that the ring is asymmetrical with nitrogen on one side and oxygen on the other.  The hydrogen bond has a strong preference for nitrogen over oxygen (see Figure 7).

The bisoxazole polymers can be made vector-directional by adding hydroxy groups, just like the iminol example in Figure 5.  The result is illustrated in Figure 7 at right. 

There are other symmetries that can be accommodated in bisoxazole polymers relating to the cis or trans positioning of imidazole nitrogens and the cis or trans positioning of the hydroxy groups.  One example of this is provided in Figure 8.  The trans orientation of hydroxy groups or imidazole nitrogen atoms inverts the vector quantities of subsequent monomers.  In figure 8, the use of both monomers in trans orientations switches the orientation of every second monomer back to its original vector.

We will, in the near future, add another, separate web page on the vector-directional properties of individual monomers and their design implications to NSC's tool-kit concept.

NSI's high vision is to market nanostructural tool-kits composed of modular vector-directional parts (monomers) that can be combined into specified sequences to build nanostructured supramolecular assemblies (nanosensors, nanomachines).  This is the promise of a fully functional nanostructural design system, enabled by a fully stocked monomer library.