Agriculture Reference
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average size of around 200 nm was homogenously dispersed throughout the NR matrix. The
enhanced thermal stability and mechanical properties of the modified St/NR composite was
mainly due to the improved phase interface interactions between the rubber and starch. The
NR was modified by grafting with the dimethylaminoethyl methacrylate (DMAEMA) to form
a latex with cationic water-soluble polymeric ‗hairs' of the polyDMAEMA, they acted only
as a filler in the starch films, but with the modified NR, the mechanical properties of the films
were significantly altered (Rouilly et al., 2004). The elastic modulus was greatly decreased
and the strain at breaks greatly increased. Freeze-fracture TEM micrographs indicated that
there was a strong interaction between the surface of the modified NRL and starch. The
polyDMAEMA chains were more hydrophilic than the starch, and the addition of the grafted
latex resulted in a 20° drop of the water contact angle of the formed film, and a 25% increase
of the water absorption compared to the native starch; however the unmodified NRL, caused
the opposite effect. A thermoplastic starch/NR polymer blend was obtained using natural
latex and cornstarch with an intensive batch mixer at 1b50 o C, and an NR content that varied
from 2.5 to 20% (Carvalho et al., 2003). There was a reduction in the modulus and the tensile
strength, and the blends became less brittle than the thermoplastic starch alone. Increasing the
plasticizer content made it possible to have a greater amount of rubber present. The influence
of the silica and polymer blend ratio on the swelling ratio of the polymer blend in the
presence of 10% starch in a water medium is shown in Figure 20. It is clear that the swelling
ratio of the polymer blend dramatically decreased with an increasing silica content in the
sample. This was because the chemical network structure formed by the combination between
the SiO 2 with PVA or the St in the polymer blend prohibited the water molecules from
dissolving and improved the water uptake of the film. When the NR was increased from 30%
to 50%, the degree of the swelling ratio of the polymer blend decreased further. An
explanation for this result was that the NR consisting of 1,4- cis -polyisoprene chains exhibited
a hydrophobic behavior to prevent water uptake. Therefore, the swelling ratio of the polymer
blend decreased.
Figure 20. Swelling ratio (%) of the (a) 35:35:30 PVA/St/ENR blend and (b) 25:25:50 PVA/St/ENR
blend with different silica contents in water.
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