3.2 Thermal Treatment of the Clay
During support of the catalyst, the coordination catalyst must achieve a uniform
distribution within the clay layers so that interlayer polymerization may take place,
leading to clay exfoliation and dispersion into the formed polymer phase. Therefore, the compatibility between the clay surface, organic solvent, catalyst,
cocatalyst, and polymer has a great impact on the effectiveness of in situ polymerization with clay. To achieve acceptable activities for in situ polymerization, the
active sites immobilized on the clay surface should be shielded from side reactions
that may lead to their deactivation. Pristine clays, such as Na
+
-MMT, are hydrophilic and have a relatively high water content (up to 10 wt%), which acts as a
poison for coordination catalysis [28, 29]. Depending on the clay structure, water
may occur in three different forms [30] that can be determined using thermal
gravimetric analysis: physically adsorbed water is generally released below
100
C; bound water is released below 300
C; and structural water is released
around 600
C. Organic modification of clay can decrease the water content considerably [32]; however, water content is still large enough in the form of structural
water to cause significant catalyst deactivation. During thermal treatment, physically adsorbed, bound, and structural water is removed at different temperature
ranges as specified above, which results in changes in the clay porosity and acidity.
In fact, the acidity, porosity, and cation exchange capacity of clays are closely
related to their water content [42].
3.3 Treatment of Clay with Alkylaluminum Compounds
Treatment of clay with alkylaluminoxanes, such as MAO, TMA, TEA, TIBA etc.,
can remove the residual water on the clay surface and protect the upcoming
coordination catalyst from deactivation [31]. The complete removal of water
from clay requires thermal treatment at temperatures of about 400–600
C, which
leads to the partial collapse of the clay structure and CEC reduction. Hence,
treatment with alkylaluminum compounds and thermal treatment at moderate
temperatures is required for further reduction of the adsorption of water. Additionally, treatment with alkylaluminum compounds will reduce the population of
surface hydroxyl groups, which are also responsible for coordination catalyst
deactivation [28].
4 Nanocomposite Structures
Generally in layered silicates, the layer thickness is of the order of 1 nm and has a
very high aspect ratio (e.g. 10–1,000). A few weight percent of layered silicates that
are properly dispersed throughout the polymer matrix thus create much higher
Polyolefin/Layered Silicate Nanocomposites Prepared by In Situ Polymerization
317
During support of the catalyst, the coordination catalyst must achieve a uniform
distribution within the clay layers so that interlayer polymerization may take place,
leading to clay exfoliation and dispersion into the formed polymer phase. Therefore, the compatibility between the clay surface, organic solvent, catalyst,
cocatalyst, and polymer has a great impact on the effectiveness of in situ polymerization with clay. To achieve acceptable activities for in situ polymerization, the
active sites immobilized on the clay surface should be shielded from side reactions
that may lead to their deactivation. Pristine clays, such as Na
+
-MMT, are hydrophilic and have a relatively high water content (up to 10 wt%), which acts as a
poison for coordination catalysis [28, 29]. Depending on the clay structure, water
may occur in three different forms [30] that can be determined using thermal
gravimetric analysis: physically adsorbed water is generally released below
100
C; bound water is released below 300
C; and structural water is released
around 600
C. Organic modification of clay can decrease the water content considerably [32]; however, water content is still large enough in the form of structural
water to cause significant catalyst deactivation. During thermal treatment, physically adsorbed, bound, and structural water is removed at different temperature
ranges as specified above, which results in changes in the clay porosity and acidity.
In fact, the acidity, porosity, and cation exchange capacity of clays are closely
related to their water content [42].
3.3 Treatment of Clay with Alkylaluminum Compounds
Treatment of clay with alkylaluminoxanes, such as MAO, TMA, TEA, TIBA etc.,
can remove the residual water on the clay surface and protect the upcoming
coordination catalyst from deactivation [31]. The complete removal of water
from clay requires thermal treatment at temperatures of about 400–600
C, which
leads to the partial collapse of the clay structure and CEC reduction. Hence,
treatment with alkylaluminum compounds and thermal treatment at moderate
temperatures is required for further reduction of the adsorption of water. Additionally, treatment with alkylaluminum compounds will reduce the population of
surface hydroxyl groups, which are also responsible for coordination catalyst
deactivation [28].
4 Nanocomposite Structures
Generally in layered silicates, the layer thickness is of the order of 1 nm and has a
very high aspect ratio (e.g. 10–1,000). A few weight percent of layered silicates that
are properly dispersed throughout the polymer matrix thus create much higher
Polyolefin/Layered Silicate Nanocomposites Prepared by In Situ Polymerization
317