The synthesis of nanocomposites via emulsion polymerization was investigated using methyl methacrylate
(MMA) monomer, 10 wt % montmorillonite (MMT) clay, and a zwitterionic surfactant octadecyl dimethyl
betaine (C18DMB). The particle size of the diluted polymer emulsion was about 550 nm, as determined
by light scattering, while the sample without clay had a diameter of about 350 nm. The increase in the
droplet size suggests that clay was present in the emulsion droplets. X-ray diffraction indicated no peak
in the nanocomposites. Transmission electron microscopy showed that emulsion polymerization of MMA
in the presence of C18DMB and MMT formed partially exfoliated nanocomposites. Differential scanning
calorimetry showed an increase of 18 °C in the glass transition temperature (T
g) of the nanocomposites.
A dynamic mechanical thermal analyzer also verified a similar T
g increase, 16 °C, for the partially exfoliated
nanocomposites over poly(methyl methacrylate) (PMMA). Thermogravimetric analysis indicated a 37 °C
increase in the decomposition temperature for a 20 wt % loss. A PMMA nanocomposite with 10 wt %
C18DMB−MMT was also synthesized via in situ polymerization. This nanocomposite was intercalated
and had a T
g 10° lower than the emulsion nanocomposite. The storage modulus of the partially exfoliated
emulsion nanocomposite was superior to the intercalated structure at higher temperatures and to the pure
polymer. The rubbery plateau modulus was over 30 times higher for the emulsion product versus pure
PMMA. The emulsion technique produced nanocomposites of the highest molecular weight with a bimodal
distribution. This reinstates that exfoliated structures have enhanced thermal and mechanical properties
over intercalated hybrids.
Five styrene-butadiene rubber (SBR)/clay nanocomposite or hybrid systems were synthesized via mechanical mixing of SBR using a Brabender mixer and a 2-roll mill in the presence of unmodified sodium montmorillonite (Na-MMT) clay, MMT modified with octadecylamine (C18amine), MMT modified with a zwitterionic surfactant, octadecyldimethyl betaine (C18DMB), and MMT modified with a polymerizable cationic surfactant, vinylbenzyl octadecyldimethyl ammonium chloride (VODAC) or vinylbenzyl dodecyldimethyl ammonium chloride (VDAC). The surfactant chain length and functional groups affected the dispersion of clay nanolayers in the matrix and the overall properties of the nanocomposites. X-ray diffraction (XRD) revealed peaks corresponding to intercalated structures; transmission electron microscopy (TEM) observations agreed well with XRD assessment of the composites. SBR/VODAC-MMT system exhibited the best dispersion among the nanocomposites studied. VODAC-MMT was partially exfoliated in SBR matrix and the average aspect ratio of the nanolayer stacks or aggregates was high (20). Depending on the amount of clay, considerable mechanical reinforcement and gas barrier enhancement were achieved in nanocomposites over pure rubber. Tensile strength in excess of 18MPa was observed in SBR nanocomposites with 30 phr C18 organoclays. The storage modulus at 25C increased by a factor of four by incorporating 10 phr VODAC-MMT in SBR. The most pronounced oxygen barrier enhancement was again observed in SBR/VODAC-MMT nanocomposite with the reduction of permeability by 60% at silicate volume fraction of 0.06. The superior performance of nanocomposites containing VODAC-MMT is attributed to the presence of the vinyl-benzyl group and 18 carbon-atom tail in the surfactant leading to high compatibility with SBR and nano-scale dispersion in the SBR matrix.
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