Melt intercalation

Melt intercalation has been a method of choice for the commercial generation of polymer nanocomposites as the technique does not require extensive synthesis and work-up processes. As mentioned earlier, the polymer is melted at high temperature and the melt is then mixed with the filler under shear. The temperature is chosen so as to achieve optimum viscosity in the polymer melt, which is able to withstand shear from the compounder, and also allows good 

This technique is highly suitable for producing thermoplastic polymer nanocomposites in large scale. There are many advantages with direct melt intercalation over solution intercalation. For example, direct melt intercalation is highly specific for the polymer, leading to the formation of new hybrids. In addition, the absence of a solvent makes direct melt intercalation an environmentally sound and economically favorable method for industries [11]. 

In polypropylene-based nanocomposites the dispersion of clay will normally be done at speed varying between 30-80 rpm. The mixing time varies between 5-10 minutes. This helps in dispersing the clay particles and also facilitates the dispersion of clay as intercalated (or) fully dispersed clay platelets. Following this, the mixed blend may be pelletized into fine granules, so that it can be used as raw material for further component processing. Or else it can be injection molded directly to get components of required shape (or) size. 

The setup consists of upstream and downstream stages with the assistance of supercritical fluids (CO ). In upstream stage, polyamide pellets (95 parts) and clay (5 parts) are fed into twin screw extruder. The process parameters include a feed rate - 14 kg/hr, mixing speed of 300 rpm, mixing temperature of 220°C-240°C and the die temperature of 250°C. After processing, samples are pelletized and dried at 80°C overnight. 

In downstream, the pellets are sent to primary extruder. Mixing is done at 20 rpm. The temperatures from feed zone to die head are 180°C, 240°C, 240 C and 260°C. These mixed samples are then sent to a secondary extruder with a screw rotation speed of 90 rpm and the pressure adjustable die temperature is 220°C. The supercritical fluid (CO gas) is compressed at 21 MPa pressure using a positive displacement syringe pump and sent to the secondary extruder. The melt and supercritical fluid are mixed with the help of a distributive-type screw. This mixture is pumped to the pressure adjustable die, where the sample is casted and the supercritical fluid escapes out. A similar fabrication route has been used elsewhere [43,54-55]. 

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SCHEME 2.1 The melt-intercalation method for nanocomposite preparation. 

This is the most widely used naturally occurring rubber. The literature search shows that many research groups have prepared nanocomposites based on this rubber [29-32]. Varghese and Karger-Kocsis have prepared natural rubber (NR)-based nanocomposites by melt-intercalation method, which is very useful for practical application. In their study, they have found increase in stiffness, elongation, mechanical strength, and storage modulus. Various minerals like MMT, bentonite, and hectorite have been used. 

This is a highly polar polymer and crystalline due to the presence of amide linkages. To achieve effective intercalation and exfoliation, the nanoclay has to be modified with some functional polar group. Most commonly, amino acid treatment is done for the nanoclays. Nanocomposites have been prepared using in situ polymerization [85] and melt-intercalation methods [113-117]. Crystallization behavior [118-122], mechanical [123,124], thermal, and barrier properties, and kinetic study [125,126] have been carried out. Nylon-based nanocomposites are now being produced commercially. 

D. Polymer-Clay Nanocomposites Synthesized via Melt Intercalation... 

Nylon-6-clay nanocomposites were also prepared by melt intercalation process [49]. Mechanical and thermal testing revealed that the properties of Nylon-6-clay nanocomposites are superior to Nylon. The tensile strength, flexural strength, and notched Izod impact strength are similar for both melt intercalation and in sim polymerization methods. However, the heat distortion temperature is low (112°C) for melt intercalated Nylon-6-nanocomposite, compared to 152°C for nanocomposite prepared via in situ polymerization [33]. 

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. 

Poly(styrene-fc-butadiene) copolymer-clay nanocomposites were prepared from dioctadecyldimethyl ammonium-exchanged MMT via direct melt intercalation [91]. While the identical mixing of copolymer with pristine montmorillonite showed no intercalation, the organoclay expanded from 41 to 46 A, indicating a monolayer intercalation. The nanocomposites showed an increase in storage modulus with increasing loading. In addition, the Tg for the polystyrene block domain increased with clay content, whereas the polybutadiene block Tg remained nearly constant. 

XRD was used to investigate the spacings of silicate layers of montmorillonite (from 1.9 to 4nm) in PP/montmorillonite (MMT) nanocomposites prepared by in situ graft-intercalation in the presence of acrylamide [331]. Similarly, XRD and TEM were used to study the dispersibility of PP/MMT nanocomposites prepared by melt intercalation using organo-montmorillonite and conventional twin screw extrusion [332]. Various delaminated and intercalated polymer (PA6, PA 12, PS,... 

Since the possibility of direct melt intercalation was first demonstrated [11], melt intercalation has become a method of preparation of the intercalated polymer/ layered silicate nanocomposites (PLSNCs). This process involves annealing, statically or under shear, a mixture of the polymer and organically modified layered fillers (OMLFs) above the softening point of the polymer. During annealing, the polymer chains diffused from the bulk polymer melt into the nano-galleries between the layered fillers. 

In order to understand the thermodynamic issues associated with the nanocomposite formation, Vaia et al. have applied the mean-field statistical lattice model and found that conclusions based on the mean field theory agreed nicely with the experimental results [12,13]. The entropy loss associated with confinement of a polymer melt is not prohibited to nanocomposite formation because an entropy gain associated with the layer separation balances the entropy loss of polymer intercalation, resulting in a net entropy change near to zero. Thus, from the theoretical model, the outcome of nanocomposite formation via polymer melt intercalation depends on energetic factors, which may be determined from the surface energies of the polymer and OMLF. 

R. A. Vaia, E. P. Giannelis, Polymer melt intercalation in organically-modified layered silicates Model predictions and experiment, Macromolecules, vol. 30, pp. 8000-8009, 1997. 

Pantoustier, N., Alexandre, M., Degee, P, Calberg, C., Jerome, R., Henrist, C., et al. (2001). Poly(3-caprolactone) layered silicate nanocomposites effect of clay surface modifiers on the melt intercalation process. e-Polymer, 9, 1-9. 

Choudhury et al. [36] in their work on hydrogenated nitrile butadiene rubber (HNBR)-nanoclay systems showed the thermodynamic aspects of nanocomposite formation using the mean-field-lattice-based description of polymer melt intercalation, which was first proposed by Vaia and Giannelis [37]. Briefly, the free... 

In order to prove the importance of the delaminated layer structure in the direct (melt) intercalation, the intercalation of stearic acid molecules into the clay galleries has been studied by premixing them with organoclay and then successfully incorporating this modified clay into the rubber matrix [59]. A schematic presentation of such modification of clay is given in Fig. 37. 

ABS/clay nanocomposites that are prepared by a direct melt intercalation technique without any conventional flame retardant, show an enhanced formation of char in the course of thermal degradation and thus exhibit an improved thermal stability (74). 

Vaia, R. A., Vasudevan, S., Krawiec, W., Scanlon, L. G., and Giannelis, E. P. New polymer electrolyte nanocomposites Melt intercalation of poly(ethylene oxide) in mica-type silicates, Adv. Mater. (1995), 7, 154-156. 

In the case of mica-type layered silicates it has been recently demonstrated that nanocomposites (both intercalated and delaminated) can be synthesized by direct melt intercalation even with high molecular weight polymers [7-18]. This synthetic method is quite general and is broadly applicable to a range of commodity polymers from essentially non-polar polystyrene, to weakly polar polyethylene terephthalate), to strongly polar nylon. Nanocomposites can, therefore, be processed using currently available techniques such as extrusion, thus lowering the barrier towards commercialization. 

Figure 9 shows a typical temporal series XRD patterns, for a polystyrene Mw= 30,000 (PS30)/octadecyl-ammonium modified fluorohectorite (C18FH) mixture annealed in-situ at 160 °C in vacuum. Details regarding the data collection and analysis are presented in reference [ 12]. The width of the original unintercalated peak and the final intercalated peak appear to be similar, suggesting that the polystyrene melt intercalation does not drastically alter the coherence length or disrupt the layer structure of the silicate crystallites. 

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