Chào các bạn, mình là thành viên mới gia nhập vào câu lạc bộ Hóa Học, rất vui được đóng góp cùng chia sẻ với các bạn những kiến thức tổng hợp trong lĩnh vực Polymer. :cool (
An overview of polymer-layered silicate nanocomposites
Composites that exhibit a change in composition and structure over a nanometer length scale have been shown over the last 10 years to afford remarkable property enhancements relative to conventionally-scaled composites-Schmidt, 1985; Novak, 1993; Mark, 1996… Layered silicates dispersed as a reinforcing phase in an engineering polymer matrix are one of the most important forms of such ‘‘hybrid organic–inorganic nanocomposites’’ _Okada and Usuki, 1995; Giannelis, 1996; Ogawa and Kuroda, 1997… Although the high aspect ratio of silicate nanolayers is ideal for reinforcement, the nanolayers are not easily dispersed in most polymers due to their preferred face-to-face stacking in agglomerated tactoids. Dispersion of the tactoids into discrete monolayers is further hindered by the intrinsic incompatibility of hydrophilic layered silicates and hydrophobic engineering plastics. However, as was first demonstrated by the Toyota group more than 10 years ago_Fukushima and Inagaki, 1987., the replacement of the inorganic exchange cations in the galleries of the native clay by alkylammonium surfactants can compatibilize the surface chemistry of the clay and the hydrophobic polymer matrix. «-Caprolactam was polymerized in the interlayer gallery region of the organoclay to form a true nylon 6–clay nanocomposite_Usuki et al., 1993a,b… At a loading of only 4.2 wt. clay, the modulus doubled, the strength increased more than 50, and the heat distortion temperature increased by 808C compared to the pristine polymer… They also demonstrated that organoclays exfoliated in a nylon 6 polymer matrix greatly improved the dimensional stability, the barrier properties and even the flame retardant properties_Kojima et al., 1993a,b; Gilman et al., 1997… More significantly, these composites have been in use in under-the-hood applications in the automobile industry_Okada and Usuki, 1995… The use of organoclays as precursors to nanocomposite formation has been extended into various polymer systems including epoxys, polyurethanes, polyimides, nitrile rubber, polyesters, polypropylene, polystyrene and polysiloxanes, among others. For true nanocomposites, the clay nanolayers must be uniformly dispersed_exfoliated.in the polymer matrix, as opposed to being aggregated as tactoids or simply intercalated_see Fig. 1… Once nanolayer exfoliation has been achieved, the improvement in properties can be manifested as an increase in tensile properties, as well as enhanced barrier properties, decreased solvent uptake, increased thermal stability and flame retardance _Okada and Usuki, 1995; Giannelis, 1996… The complete dispersion of clay nanolayers in a polymer optimizes the number of available reinforcing elements for carrying an applied load and deflecting cracks. The coupling between the tremendous surface area of the clay _;760 m2rg. and the polymer matrix, facilitates stress transfer to the reinforcement phase, allowing for such tensile and toughening improvements. Conventional polymer–clay composites containing aggregated nanolayer tactoids ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay nanocomposites, such as those that have been achieved for nylon 6 and epoxy systems, have to the contrary shown improvements in all aspects of their mechanical performance. High aspect ratio nanolayers also provide properties that are not possible for larger-scaled composites. The impermeable clay layers mandate a tortuous pathway for a permeant to transverse the nanocomposite_Fig. 2… The enhanced barrier characteristics, chemical resistance, reduced solvent uptake and flame retardance of clay–polymer nanocomposites all benefit from the hindered diffusion pathways through the nanocomposite.
Fig. 1. Schematic illustrations of _A. a conventional; _B. an intercalated; _C. an ordered exfoliated; and _D. a disordered exfoliated polymer–clay nanocomposite. The clay interlayer spacing is fixed in an intercalated nanocomposite. On the other hand, in an exfoliated nanocomposite, the average gallery height is determined by clay silicate loading. The difference between ordered and disordered exfoliated nanocomposites is that the former can be detected by X-ray diffraction and the latter is X-ray amorphous.
Fig. 2. Proposed model for the torturous zigzag diffusion path in an exfoliated polymer–clay nanocomposite when used as a gas barrier_Yano et al., 1993…
II/ Organoclay structures and modeling
The replacement of inorganic exchange cations by organic onium ions on the gallery surfaces of smectite clays not only serves to match the clay surface polarity with the polarity of the polymer, but it also expands the clay galleries. This facilitates the penetration of the gallery space_intercalation.by either the polymer precursors or preformed polymer. Depending on the charge density of clay and the onium ion surfactant, different arrangements of the onium ions are possible. In general, the longer the surfactant chain length, and the higher the charge density of the clay, the further apart the clay layers will be forced. This is expected since both of these parameters contribute to increasing the volume occupied by the intragallery surfactant. Depending on the charge density of the clay, the onium ions may lie parallel to the clay surface as a monolayer, a lateral bilayer, a pseudo-trimolecular layer, or an inclined paraffin structure as illustrated in Fig. 3. At very high charge densities, large surfactant ions can adopt lipid bilayer orientations in the clay galleries. The orientations of onium ion chains in organoclay were initially deduced based on infrared and XRD measurements_Lagaly, 1986… More recent modeling experiments has provided further insights into the packing orientations of the alkyl chains in organically modified layered silicates_Hackett et al., 1998… Molecular dynamics_MD.simulations were used to study molecular properties such as density profiles, normal forces, chain configurations and trans-gauche conformer ratios. For the mono-, bi- and psuedo-trilayers with respective d-spacings of 13.2, 18.0 and 22.7 A° , a disordered liquid-like arrangement of chains was preferred in the gallery. In this disordered arrangement the chains do not remain flat, but instead, overlap and co-mingle with onium ions in opposing layers within the galleries. However, for the trilayer arrangement, the methylene groups are primarily found within a span of two layers and only occasionally do
Fig. 3. Orientations of alkylammonium ions in the galleries of layered silicates with different layer charge densities_Lagaly, 1986…
they continue into the layer opposite to the positive head group. As anticipated, the onium head group is also noted to reside nearer the silicate surface relative to the aliphatic portion of the surfactant. The highest preference conformer is trans over gauche for the maximum surfactant chain length just before the system progresses to the next highest layering pattern. This is expected since the alkyl chains must be optimally packed under such dense surfactant concentrations. The internal gallery pressure determines the d-spacing of an organoclay, which is shown for three different clays with varying surfactant length. The MD simulation experiments have agreed well with experimental XRD data and FTIR spectroscopy for the stacked intragallery alkyl chains, however, the inclined paraffin association (experimentally seen for C15 surfactants with clays of CECs1.2 meq/g and greater) is not addressed and would be a prime target for future modelings.
III/ Organoclay–polymer interactions
If the polarity of the organoclay sufficiently matches the monomer or prepolymer, it will intercalate into the galleries, further spreading clay layers apart. Examples of such behavior have been observed for «-caprolactam _Usuki et al., 1993a., epoxides_Lan and Pinnavaia, 1995. and polyols _Wang and Pinnavaia, 1998a. that intercalate organoclay galleries as unreacted precursors. For long chain onium-exchanged organoclays, the galleries swollen by these precursors show a d-spacing indicative of a paraffin monolayer arrangement _Fig. 5… Even these ideally matched systems, however, do not necessarily form true nanocomposites. Only when the clay layers are forced apart and no longer interact through the onium chains is an ideal nanocomposite formed upon polymerization_see Fig. 1C and D… The complete dispersal, or exfoliation, of the clay nanolayers yields composites with the highest degree of property enhancement. If the layers persist with a repeating layer stacking pattern with the gallery height less than two times the onium ion chain length, then the final product is said to be an intercalated composite which will have regions of very high and very low reinforcer concentration_Fig. 1B… This non-uniform disper- sal of nanolayers limits stress transfer throughout the composite, giving comparatively less than optimal reinforcement. Still poorer, conventionally scaled composites are possible if the clay and polymer exhibit only partial miscibility _Fig. 1A… Here, the clay persists as tactoids of face-to-face stacked agglomerates throughout the polymer matrix. This incomplete dispersal of the reinforcing phase inhibits ideal surface contact between the polymer and clay, creating large regions of pure polymer in the composite.
Fig. 4. Proposed model for the swelling of alkylammonium exchanged clay with a paraffin structure by polymer precursors such as «-caprolactam, epoxide and polyol. However, regardless of the initial charge density of the clay and the orientations of the gallery long chain alkylammonium ions, the gallery height is determined by the vertical orientation of the long chain alkylammonium in the solvated intercalates. Cross-hatched ellipses represent the intercalated polymer precursor species.
Therefore, it is imperative that the surface polarities of polymer and clay be matched in order for the polymer to fully wet and intercalate clay tactoids. The aforementioned nylon 6 and epoxy–clay systems have achieved this exfoliated state and yielded nanocomposites with remarkable properties. Another important feature of these two systems is that protonated alkyl amine cations can catalyze the intragallery polymerization process. This speeds the congested intragallery reaction relative to the bulk polymer, providing a driving force for nanolayer exfoliation in the final composite. Other polymer–clay systems have since been studied with varying degrees of success, as summarized below.