Nanocomposites intercalated

Intercalated nanocomposites When the polymer chains intercalate into the clay gallery gaps but are unable to break down the layered structure, they are called intercalated nanocomposites (Figure 2.6a). 

Akelah andMoet [62] followed a modified approach to prepare PS-clay intercalated nanocomposites using a solvent to facilitate intercalation. MMT was ion exchanged using a polymerizable surfactant, vinylbenzyltrimethylammonium chloride. Acetonitrile was found to be the most effective solvent, producing a d-spacing of 2.45 mn versus 2.22 and... 

Silanol-terminated PDMS and hexadecyltrimethylammonium-exchanged clay were used to prepare PDMS-clay nanocomposites via melt intercalation [90]. The melt intercalation nanocomposites did not achieve as high a reinforcement as the aerosilica silicone hybrid, but the nanocomposite formed from solution had a nearly identical reinforcing effect on tensile strength as the aerosilica composite. 

Now it is necessary to understand the meaning of the interlayer expansion in the intercalated nanocomposites. As discussed before, we have to take the interdigitated layer structure into consideration. This structure may suggest that a different... 

In subsequent discussion, we will demonstrate the use and interpretation of some of these techniques. Figure 2a shows typical XRD traces of nanocomposite systems of styrene butadiene rubber (SBR) containing unmodified and modified nanoclay, describing an exfoliated and intercalated nanocomposite [5]. photographs of these systems are also given in the same figure (Fig. 2b, c). In the present case, the information obtained from both the techniques is complimentary. 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

In the work of Wilkie et al.,55,56 oligomers of styrene, vinylbenzyl chloride, and diphenyl vinyl-benzylphosphate and diphenyl vinylphenylphosphate (DPVPP) have been prepared and reacted with an amine and then ion-exchanged onto clay. The resulting modified DPVPP clays have been melted blended with polystyrene and the flammability was evaluated. XRD and TEM observations proved the existence of intercalated nanocomposite structures. Cone calorimeter tests have shown a substantial reduction in the PHRR of about 70% in comparison with pure PS. According to the authors, this reduction was higher than the maximum reduction usually obtained with PS nanocomposites. Other vinylphosphate modified clay nanocomposites were also elaborated. The reduction in PHRR was greater with higher phosphorus content than for DPVPP. Consequently, the reduction in PHRR seemed attributed to both the presence of the clay and to the presence of phosphorus. 

Fig. 18. The steady-shear rheological behavior for a series of intercalated nanocomposites of poly(dimethyl0 95-diphenyl0 05siloxane) with layered silicate (dimethyl ditallow montmo-rillonite) at 25 °C. The silicate loading is varied and are noted in the legend. From Ref. [5].

On a global scale, the linear viscoelastic behavior of the polymer chains in the nanocomposites, as detected by conventional rheometry, is dramatically altered when the chains are tethered to the surface of the silicate or are in close proximity to the silicate layers as in intercalated nanocomposites. Some of these systems show close analogies to other intrinsically anisotropic materials such as block copolymers and smectic liquid crystalline polymers and provide model systems to understand the dynamics of polymer brushes. Finally, the polymer melt-brushes exhibit intriguing non-linear viscoelastic behavior, which shows strainhardening with a characteric critical strain amplitude that is only a function of the interlayer distance. These results provide complementary information to that obtained for solution brushes using the SFA, and are attributed to chain stretching associated with the space-filling requirements of a melt brush. 

Solution blending was found to produce a mixed immiscible intercalated nanocomposite with the clay causing a change in the degradation path (94). In situ polymerisation of polymer layered silicate nanocomposites has been investigated (36). 

An equivalent surface area of 460 m g was determined from the monolayer volume, Vj. The value obtained for the dimensionless energetic constant, C=260, was characteristic of a microporous material. Although the BET surface area may not be a physically precise quantity due to the fact that the nitrogen molecule does not exhibit the same cross-sectional area in a microporous environment as on a flat surface, the BET value is useful for comparisons of relative porosities among a related class of adsorbents. For instance, smectite clays pillared by metal oxide aggregates typically exhibit BET surface areas in the range 150 - 400 m /g. Thus, the TSLS complex is among the more porous intercalated nanocomposites derived from smectite clays. 

Fig. 1 The states of clay dispersion in polymer matrices (A) exfoliated or delaminated nanocomposites (B) intercalated nanocomposites and (C) conventional blends.

The viscosity of the nanocomposites is enhanced considerably at low shear rates and increases mono-tonically with silicate loading (at a given shear rate). Furthermore, the intercalated nanocomposites display a shear-thinning behavior at low shear rates, whereas... 

Intercalated composites in an intercalated composite the insertion of polymer into the clay structure occurs in a crystallographically regular fashion, regardless of the clay to polymer ratio. An intercalated nanocomposite is normally interlayered by only a few molecular layers of polymer and properties of the composite typically resemble those of ceramic materials. 

Exfoliated composites in an exfoliated nanocomposite, the individual clay layers are separated in a continuous polymer matrix by average distances that depend on loading. Usually, the clay content of an exfoliated composite is much lower than that of an intercalated nanocomposite. 

Exfoliation of MMT in the polymer matrix has been shown to occur through a process called slurry compounding, in which the MMT swells in water and then is mixed with Nylon-6 under extrusion [86]. Other techniques include the addition of MMT to water and the subsequent replacement of water by alcohol or acetone with further addition of a silane agent to modify the clay and make it compatible with the polymer matrix [87-89]. Intercalated nanocomposites were also obtained by slurry compounding introducing an epoxy monomer in the hydrated MMT galleries [90]. 

Hoffmann et al. [2000] demonstrated that the low-frequency modulus of exfoU-ated PS-based nanocomposites was higher than for intercalated nanocomposites. This conclusion was confirmed by Mohanty and Nayak [2007], who studied the effect of the MMT exfoliation in PA-6-based CPNCs. The large increase in contact surface between the two phases resulted in improved mechanical properties. The high aspect ratio, p = 200 to 1000, the high tensile modulus of the inorganic filler E 170 GPa), and the large specific surface area (Asp 150 m /g) all play a role in the confinement of the polymer chain—hence in mobility under stress [Yasmin et al., 2006 Utracki, 2009],... 

Several authors used the continuum mechanics for modeling conventional polymer composites as well as PNC. Ren and Krishnamoorti [2003] used a K-BKZ integral constitutive model to predict the steady-state shear behavior of a series of intercalated nanocomposites containing an organo-MMT and a disordered styrene-isoprene diblock copolymer. The model predicts the low-y shear stress properties calculated from the experimental linear stress relaxation and the relaxation-based damping behavior. However, as it does not take into account the effect of clay platelet orientation, it is unable to predict the shear stress behavior at intermediate y and the normal stress behavior at all y and clay contents. 

Okamoto et al. [2000, 2001a] investigated the dispersed structures in PNC with PMMA or its copolymers (MMA with polar monomers) as the matrix. The PNC was prepared by in situ polymerization with 10 wt% of organically modified smectic clay, obtaining intercalated nanocomposites. The storage tensile modulus E and tan 5 of PMMA-clay and PMMA-intercalant were similar. However, when copolymers were used as the matrix, the E of PNC increased over the entire temperature range, but tan 5 peaks shifted to lower T. 

Ren, J., and Krishnamoorti, R., Nonhnear viscoelastic properties of layered-sihcate-based intercalated nanocomposites. Macromolecules, 36, 4443 451 (2003). 

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