Chemistry Reference
In-Depth Information
characteristics of a polymer during processing
162
can be adjusted by the
addition of a small amount of polymer of either very low or very high mo-
lecular weight. Bimodal polyethylene is an example.
306-308
In the case of
branched polyethylenes, bimodal distributions of molecular weight affect
thermal and crystallization behavior,
309
long-term creep failure,
310
and
fracture.
311
In the case of brushlike structures, it has been of interest to
produce bimodal layers of poly(ethylene glycol) on gaseous colloidal
particles.
312
Another example is rubber-toughened thermoplastics in which an elas-
tomer is dispersed as domains within the thermoplastic matrix to im-
prove its mechanical properties.
313,
314
A bimodal distribution of particle
sizes gives the largest improvements.
315-322
Perhaps the small particles are
most efficient at stopping one type of failure mechanism, and the large
particles another type. There is the possibility that a mixture of two chem-
ically different particles, such as silica (SiO
2
) and titania (TiO
2
),
323
could
have significant advantages in elastomer reinforcement, with one func-
tioning best at low and moderate temperatures and the other at elevated
temperatures. In any case, the preparation of bimodal poly(organosiloxane)
nanoparticles is of interest in its own right.
324
These results can be brought into a broader context through applications
not involving polymer networks. Example are the bimodal assemblies of
diblock copolymer micelles and work on targeted bimodal imaging of pan-
creatic cancer.
325
There has also been considerable interest in preparing
porous materials having bimodal distributions of pore sizes.
326-345
Frame-
works having trimodal pore distributions have also been prepared.
346
Stud-
ies exist on bimodal magnetic-fluorescent nanostructures,
347-349
size
distributions of nanoparticles,
350-352
bimodal arrays of nanoparticles on
substrates,
353
nanocomposite scaffolds,354
354
multimodal nanoparticles from
miniemulsions,
355
proton transport in Nafion,356
356
bimodal acid-base behav-
ior on water-silica interfaces,
357
averaging effects in NMR attenuation in
bimodal poly(methyl methacrylate) solutions,
358
and electrospinning of bi-
modal fiber meshes.
359
Other examples include mesoporous silica
360
and
bulk ultrafine-grained nickel.
361
There have even been “ripening” studies of
bimodally distributed AgCl nanoparticles.
362
7.3.3 Trimodal Networks
Differential scanning calorimetry measurements exist on solvent mole-
cules constrained in the pores of a variety of PDMS elastomers. Some re-
sults on trimodal networks have been reported.
287
The several
