Exfoliated system

Information about surface morphology is also obtained from SEM studies. In general, the intercalated clay layers show an intense peak in the range of 1.5° ° (29 value), whereas exfoliated systems have no distinct peak in that range for their loss of structural integrity shown in the XRD pattern of the nanocomposites. XRD studies indicate that there is no infiu-ence of nanomaterial on poly(e-caprolactone)diol PCL crystallinity in sunflower oil-based hyperbranched polyurethane/silver nanocomposite, but that crystallinity is enhanced in castor oil or Mesua ferrea oil-based hyperbranched polyurethane/MWCNT nanocomposites. ... 

Recently, Moad et al. [288,289] designed and prepared novel copolymer intercalant/dis-persant/exfoliant systems that are effective with unmodified clays at low levels (<20% with respect to clay), can be combined with commercial PP and clay in a conventional melt-mixing process, and do not require the use of additional compatibilizers. PP-clay nanocomposites prepared by direct melt mixing using unmodified MMX clays and a copolymer additive added at a level of only 1 wt.% with respect to PP for 5wt.% clay Authors investigated the following two classes of dispersants (1) polyethylene oxide-based nonionic surfactants... 

There is a general agreement in the literature that exfoliated systems lead to better mechanical properties, particularly higher modulus, than intercalated nanocomposites. Figure 6.5 shows a plot of the function describing the ratio of surface area to volume (AIV) vs aspect ratio for cylindrical particles with a given volume. 

Additionally, the favorable comparison of these theoretical predictions with experimental data (Figure 2A0d) gives some credibility to the conclusions above. In Figure l.lOd we plot experimental water vapor permeabilities of various solvent cast nanocomposite films. The experimental behavior follows closely the theoretical trend and is enclosed between the response of exfoliated systems (especially for low filler loadings) and that of intercalated systems (for moderate and higher loadings). This reflects the same effective filler aspect ratio... 

Nuclear Magnetic Resonance (NMR) Spectroscopy This techni-que85-89 utilizes solid-state NMR to analyze nanoscale dispersion for the overall sample. The iron in the montmorillonite structure facilitates the relaxation of nearby protons, which provides information on the dispersion of the clay in the polymer matrix. In the cases reported, a signal in the polymer is identified and its relaxation time (fy) is measured the relaxation time depends on how close the proton is to a paramagnetic iron atom. On average, the protons of the polymers will be closer to the iron in the clay in a well-exfoliated system and... 

The exfoliated system is almost always quoted as the most desirable, since nanometric dispersion of day platelets maximizes the interfacial region between the filler and the polymer matrix, thus allowing to exploit the excellent mechanical properties of the individual day layers. Moreover, when exfoliation is attained, the number of reinfordng components is dramatically increased, since each day particle contains a very large number of day sheets. To further complicate matters, often a mixed dispersion is observed for day with different populations of tactoids or with partial exfoUation of single layers. Transmission electron microscopy (TEM) and WAXD are by far the most employed characterization techniques that assess the morphology of PLSN. 

In order for nanocomposites to be useful, they must be thermodynamically stable. It is therefore critical to ensure that clay nanoparticles have surfaces that interact with polymer in a way that yields exfoliated structures that do not spontaneously phase separate. Although some intercalated-exfoliated systems may yield useful improvements in properties, the exfoliated state is still the ultimate goal in producing a nanocomposite with the ultimate property enhancements. 

In the previous discussion, a great deal of time has been spent on approaches to surface modification of the clay nanoparticles in order to render the particles to be more compatible with the polymer of interest. This approach mainly concentrates on the enthalpic portion of the Gibb s free energy of intercalation-exfoliation. In order to realize the maximum benefit from a nanocomposite, the exfoliated state is the ultimate goal, since this will present the maximum interfacial interaction between the nanoparticle and the polymer. In reality, a completely exfoliated system probably does not exist, but a Boltzmann distribution of energy states is more likely which invokes some of the entropic terms. In the following... 

The entropic part of the Gibb s free energy in forming intercalates and exfoliates is of critical importance. In the classical treatment of entropy, the entropy of mixing should be positive and contribute to the spontaneous formation of intercalates and exfoliated systems. Countering this increase in entropy is the decrease in entropy experienced by the polymer chain, as it must uncoil in order first, to intercalate and, second, to exfoliate the nanoparticle, yielding the maximum interfacial interaction between the polymer and clay nanoparticles. 

In polymer-clay nanocomposites, to truly reach the ultimate in property improvements requires full exfoliation. A fully exfoliate composite yields the maximum interfacial interaction between the nanoparticle and polymer matrix. In order to produce optimally exfoliated systems requires that direct methods be available to measure the level of exfoliation. The ideal analytical method should be rapid, nondestructive, applicable to many sample matrices, low cost, and should require minimal sample preparation. The only method that fits these criteria is wide-angle X-ray diffraction (WAXD). This method, however, has some major drawbacks that will be discussed in detail in this chapter. 

In the worst case, any discrete peaks may disappear, leaving what appears to be a smooth curve, implying an exfoliated system. In this case, however, it is just a mixture of clay tactoids with a continuously varying... 

Fig. 1 presents the XRD patterns of epoxy/a-ZrP nanocomposites with (a) poor exfoliation, (b) moderate exfoliation, and (c) good exfoliation. All three WAXD patterns show an amorphous epoxy hump at around 18Tof 20. In the poorly exfoliated system (Fig. 1(a)), even though all the peaks from pristine a-ZrP have disappeared, new peaks due to the intercalated a-ZrP layers can be observed. This indicates that intercalated a-ZrP layers are still present in epoxy matrix. WAXD patterns obtained from the moderate and well exfoliated nanocomposites, as shown in (b) and (c), do not show any intercalation peaks, indicating that both of these nanocomposites do not have noticeable intercalated nanoplatelets in the epoxy matrix. However, a minor distinction between WAXD patterns in Fig. 4(b) and Fig. 4(c) can still be found at around 3 5T of 20. TEM investigations were also performed to validate the WAXD observation (to be presented). 

On the other hand, the inclusion of the same amount of a-ZrP nanoplatelets with good exfoliation can significantly improve the tensile modulus by about 50%. Yet, the elongation-at-break is virtually unaffected. The great increase in tensile modulus is surely due to the presence of the well-exfoliated, high aspect ratio a-ZrP nanoplatelets. In the case of the moderately exfoliated system, its tensile properties fall in between the poorly and the fully exfoliated systems. 

Fig. 3 shows DMA spectra of neat epoxy and epoxy nanocomposite systems with both the poorly and the well exfoliated a-ZrP nanoplatelets. Since the DMA of the moderately exfoliated system shows practically the same spectrum as that of the well exfoliated system, it is omitted in Fig. 3. The Tg of the well exfoliated system is found to be 1501C. As discussed in our earlier study [12], the above changes are probably due to the combined effects of (1) the...

The dispersion of nanoclays is known to have a strong role in the barrier properties. Thus, randomly exfoliated systems are expected to yield optimum barrier performance. Crystallinity does also play an important role in promoting barrier properties, however, since crystallinity was not seen to increase significantly in the systems study here, the observed permeability reduction is therefore directly ascribed to the observation of a highly dispersed morphology of the clay in the polymer. 

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