Nanoscale Phenomena (Part 3 of 4) - Importance of the Interface

In the field of polymer nanocomposites, there is a great deal of interest in nanoparticles because they can directly interact and influence the behavior of individual polymer chains.  The ease of mobility of a polymer chain greatly influences its bulk properties.  Generally, we are forced to change the entire structure of the polymer in order to modify something like its use temperature or mechanical properties.  Using nanoparticles, we can completely change the behavior of a conventional polymer that is easier to process or prepare that so-called "engineering plastics."

In the field of polymer nanocomposites, there is a great deal of interest in nanoparticles because they can directly interact and influence the behavior of individual polymer chains.  The ease of mobility of a polymer chain greatly influences its bulk properties.  Generally, we are forced to change the entire structure of the polymer in order to modify something like its use temperature or mechanical properties.  Using nanoparticles, we can completely change the behavior of a conventional polymer that is easier to process or prepare that so-called "engineering plastics."

The last of the major changes in the way a material interacts with its environment with decreasing size is related to  the interface.  Now this effect does overlap some with the prior two examples, but it is significant enough that it is worth mentioning separately.  First, you need to first understand that in the case of plastics, we are already dealing with molecular-scale chains of particles.  The nature of these chains and how they order and interact determines the properties of the polymer (exciting topic – should put together a short piece on another time when the airplane is throwing less turbulence at me – yes, finally in the air).  The exciting thing about a nano-structured material is that if a particle is small enough, it can start directly interacting with the polymer chains.  For example, the particle might “grab” the end of a polymer chain and make it move a little more slowly than it would otherwise.  In that case, the thermal properties of the material might be improved.  This means that a plastic that used to be limited to room temperature use might suddenly be useful at 120F or 140F, which expands the applications.

As a quick aside, the picture of the two particles with polymer chains was generated using a real “simulation” of what a polymer chain would look like, which was based on a short and effective “random walk” code.  Basically, you start at point zero and take a random step into space.  You then continue until you have traveled some distance from your original point.  By itself, this is called a “drunkard’s walk,” but if with each step you had a friend step in behind you and you were all holding hands, you would have a bigger distance from the start point (because you can’t overlap with your friends).  This is a crude analogy to what happens with an isolated polymer chain by itself.  Each unit of a polymer is joined together by a bond (typically covalent, but rarely, and interestingly, ionic for silicones).  Just like when you are holding hands with your friends, you can only go at certain angles without dislocating your arm.  Thus, the walk isn’t truly random, but rather random within a “cone” in front of you.  The simulation includes that effect.  Then I just repeated it a bunch of times until I found one that looked like it went around a sphere and inserted a sphere using PowerPoint.  A real simulation like this would probably take months of computer time.  My cheating approach just took a few hours.  Work smarter, not harder, right?

Optical micrographs of polypropylene modified with 0.1 wt.% of multi-walled carbon nanotubes in pristine (middle, P-MWCNT) and functionalized (right, F-MWCNT) states.  In the pristine case, black aggregated areas can be observed due to the poor interaction between the nanotubes and the PP chains.  For the functionalized case used in this work, the MWCNTs are completely dispersed and uniformly modify the crystal structure at low loading, with no aggregates or non-uniformity apparent.  Work published in acromolecule [Liu P et al. (2012) Macromolecules 46:463. Link]

Optical micrographs of polypropylene modified with 0.1 wt.% of multi-walled carbon nanotubes in pristine (middle, P-MWCNT) and functionalized (right, F-MWCNT) states.  In the pristine case, black aggregated areas can be observed due to the poor interaction between the nanotubes and the PP chains.  For the functionalized case used in this work, the MWCNTs are completely dispersed and uniformly modify the crystal structure at low loading, with no aggregates or non-uniformity apparent.  Work published in acromolecule [Liu P et al. (2012) Macromolecules 46:463. Link]

The presence of the nanoparticle might also change the way that the material folds and packs, which can significantly alter the mechanical properties.  The exciting part here is that for a conventional filler particle, you needed to add a ton of material to have a big effect on properties – typically around 20% or so of filler by volume, which might be upward of 50% increase in mass for dense particles.  With a nanoparticle, you may have a massive effect with only 1% concentration, or even 0.1 or 0.01%.  This is a big deal when it comes to cost and when it comes to recycling.  In my former research group at Texas A&M University (Prof. Hung-Jue Sue group), we were able to individually disperse carbon nanotubes to the individual particle level and incorporated them into polypropylene (PP), which is a particularly pernicious polymer because it doesn't like to interact with much of anything (PP is a fairly balanced polymer, meaning that there is no significant tendency of electrons to shift to one side of the polymer or another; as a result, it does not have a significant dipole and does not interact with nanoparticles).  In most cases, the nanoparticles may be adequately dispersed in the molten or solution state, but when the PP is cooled or cast into a solid material (i.e., a useful one), they aggregate and are not incorporated in the structure of the polypropylene.  One reason for this is that PP crystallizes rapidly - to put it simply, the individual chains fold into a sheet-like structure that extends in 3 dimensions from a central point.  This radial growth results in a famous "Maltese cross" structure.  When nanoparticles are present that do not interact with the polymer chains, they are rejected during this crystal growth phase and accumulate at the intersections of different crystals.  Not only do we lose the surface area we had been hoping to exploit, but now we actually have weak spots between crystallites, resulting in embrittlement.

Sequence of transmission electron microtomography (TEMT) slices of polypropylene modified with 0.1 wt.% functionalized MWCNTs (functionalized with octadecylamine after nanoplatelet-assisted dispersion using zirconium phosphate).  Slices correspond to reconstructed series of projections in 5 nm increments.  Horizontally aligned MWCNT centered in each image and shaded red in (d) and (g) - PP lamellae shaded blue.  Black arrows show PP lamellae initially positioned above F-MWCNT in (d) are below nanotube in deeper slice - direct evidence that the F-MWCNTs are integrated within the crystalline lamellae.  The MWCNTs may play an additional role in reinforcing adjacent amorphous domains between lamellae.  Work published in acromolecule [Liu P et al. (2012) Macromolecules 46:463. Link]

Sequence of transmission electron microtomography (TEMT) slices of polypropylene modified with 0.1 wt.% functionalized MWCNTs (functionalized with octadecylamine after nanoplatelet-assisted dispersion using zirconium phosphate).  Slices correspond to reconstructed series of projections in 5 nm increments.  Horizontally aligned MWCNT centered in each image and shaded red in (d) and (g) - PP lamellae shaded blue.  Black arrows show PP lamellae initially positioned above F-MWCNT in (d) are below nanotube in deeper slice - direct evidence that the F-MWCNTs are integrated within the crystalline lamellae.  The MWCNTs may play an additional role in reinforcing adjacent amorphous domains between lamellae.  Work published in acromolecule [Liu P et al. (2012) Macromolecules 46:463. Link]

In the prior work, the carbon nanotubes were first disentangled to individual particles using a nanoplatelet-assisted dispersion technique, then treated with a functional group to interact with the polymer chains.  The result was the first ever evidence of polypropylene crystallites directly interacting with carbon nanotubes, as revealed by a wonderful technique called transmission electron micro-tomography (TEMT).  The big deal here is that with only 0.1 wt.% of carbon nanotubes, the mechanical properties of PP were significantly improved without sacrificing fracture toughness.  Without the surface treatment, the pristine multi-walled carbon nanotubes (P-MWCNTs) can be seen to aggregate at the crystal boundaries and do not show the same improvement in performance.

I’ve got a lot more to share on this topic, but the turbulence is nasty so I think I’ll close my eyes for a bit.  For recompense, I’ll include a picture from my dissertation showing a single polymer chain interacting with particles of different sizes.  I repeat from before, science is awesome.