Interaction polymer matrix-platelets

Although the assumption that all apatite platelets are of equal thickness may not be well justified, it provides a useful approximation to the real situation. In fact, such assumption is inevitable for an ensemble technique like NMR. It is anticipated that HARDSHIP may be well suited to determine the relative thickness of apatite crystallites in different bone or dentin samples. We note in passing that C-REDOR, which is a hetero-nuclear recoupling technique with active suppression of homonuclear dipolar interaction,42 43 can be used to probe for the size of nanoparticles embedded in polymer matrix,44 provided that the nanoparticles do not... 

In this study, both the normal mode relaxation of the siloxane network and the MWS processes arising from the interaction of the dispersed nanoclay platelets within the polymer network have been observed. Although it is routine practice to observe the primary alpha relaxation of a polymeric system at temperatures below Tg, in this work it is the MWS processes associated with the clay particles within the polymer matrix that are of interest. Therefore, all BDS analyses were conducted at 40°C over a frequency range of 10 to 6.5x10 Hz. At these temperatures, interfacial polarization effects dominate the dielectric response of the filled systems and although it is possible to resolve a normal mode relaxation of the polymer in the unfilled system (see Figure 2), MWS processes arising from the presence of the nanoclay mask this comparatively weak process. 

In another study by Di et al. [47] revealed that the exfoliation of MMT in PLA matrix has been achieved due to the strong interaction between the MMT and polymer matrix. They also stressed that the high surface area of MMT layers had reduced molecular mobility of PLA leading to unique properties for the PLA/ MMT nanocomposites. The exfoliated MMT platelets act as nucleating agents at... 

As illustrated in Fig. 1, layered silicate composite structures fall into three different classes (a) microcomposites with no interaction between the clay galleries and the polymer, (b) intercalated nanocomposites, where the silicate is well-dispersed in a polymer matrix with polymer chains inserted into the galleries between the parallel, sihcate platelets, and (c) exfohated nano composites with fully separated silicate platelets individually dispersed or delaminated within the polymer matrix [12]. However, these terms describe only ideal cases and most observed morphologies fall between the extremes. A more detailed nomenclature will be presented later in this review. 

The exfoliated state takes place when the dispersion of the sheets of the clay is fully achieved and, therefore, no interaction between sheets occurs (Figure 8.2, right). The ideal exfoliated state is that in which the clay platelets are homogeneously distributed throughout the polymer matrix. Generally speaking, layered nanocomposites never exhibit a pure structure on the contrary, their morphology is a combination of all three possibilities with one of them dominant. 

Due to their rigid nature, clay platelets can function as nucleating agents that are able to modify the ciystallization behavior of the polymer matrix such as PVDF-HFP. Organically modified clay promotes an a- to 3-transformation of the polymer crystals. The degree of transformation depends on the nature of the clay surface modifier and the strength of the interactions between the clay and the polymer. ... 

Nanostructured materials or nanocomposites based on polymers have been an area of intense industrial and academic research over the past one-and-a-half decades [7-12]. In principle, nanocomposites are an extreme case of composite materials in which interface interactions between two phases are maximized. In the literature, the term nanocomposite is generally used for polymers with submicrometer dispersions. In polymer-based nanocomposites, nanometersized particles of inorganic or organic materials are homogeneously dispersed as separate particles in a polymer matrix. This is one way of characterizing this type of material. There is, in fact, a wide variety of nanoparticles and a way to differentiate them and to classify them by the number of nanoscale dimensions they possess. Their shape varies and includes (i) one-dimensional needle- or tube-like structures, for example, inorganic nanotubes, carbon nanotubes, sepio-lites, and so on (ii) two-dimensional platelet structures, for... 

Various compounds are used to modify clays in order to produce organoclays that satisfy the requirements of nanocomposite synthesis. Although many modifiers serve mainly as surfactants that expand the gallery spaces between clay platelets and enhance interaction with the polymer matrix, others play an important role by initiating polymerization... 

Exfoliated Nanocomposites In this case, individual silicate layers are separated in polymer matrix by average distances, totally depend on the OMLS loading. In fully exfoliated structures, on the other hand, individual platelets are well separated and have extremely favorable interactions with the polymer matrix (Luduena, 2007). 

From a thermodynamic viewpoint, both the entropic and enthalpic factors are important in controlling the dispersion of clay layers in a polymer matrix [19-21]. It was reported that the confinement of the polymer chains inside the silicate galleries results in a decrease in the overall entropy of the macromolec-ular chains [10,20] this is, however, compensated by the increase in conformational freedom of the tethered alkyl surfactant chains as the inorganic layers separate due to the less confined environment (Figure 11.2). It was also shown that apolar interactions are generally unfavorable and so in the case of nonpolar polymers, there is no favorable excess enthalpy to promote the dispersion of clay platelets and it is hence necessary to improve the interactions between the polymer and clay so as to become more favorable than the alkylammonium-clay interactions. This can be achieved by functionalization of the polymer matrix or addition of compatibilizers [3,18,22-29]. For polar polymers, an alkyl-ammonium surfactant is adequate to offer sufficient excess enthalpy and promote the formation of exfoliated nanocomposites. 

Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. Positron annihilation lifetime spectroscopy (PALS) is an efficient technique used for the analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, namely, molecular and free-volume theories. According to the free-volume theory, the diffusion is not a thermally activated process as in the molecular model, but it is assumed to be the result of random redistributions of free-volume voids within a polymer matrix. Cohen and Turnbull developed the free-volume models that describe the diffusion process when a molecule moves into a void larger than a critical size, Vc- Voids are formed during the statistical redistribution of free volume within the polymer. It is found that the relative fractional free volume of unfilled polymer decreases on the addition of layered silicates. The decrease is attributed to the interaction between layered silicate and polymer because of the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free-volume concentration or relative fractional free volume [49]. 

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