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The membrane is produced by mixing two polymers
— poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) — that
become miscible at about 220 degree C. While PVDF crystallises during cooling,
PMMA does not and separates out; the PMMA is removed using a solvent. This
property of selective crystallisation and separation is taken advantage of to
produce a nano-porous membrane.
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As the membrane is about 1 mm thick, a combination of fine pores and channels
are produced when the PMMA is removed. The average pore size is 50 nm.The
nano-sized porous structure can prevent bacteria from passing through the pores
as bacteria are typically micron-sized.
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However, the bacteria can form a biofilm on the
structure. As a result, the filter’s efficiency will be reduced within a short
period.
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To prevent this and to kill the bacteria, they
mixed silver, titanium dioxide and carbon
nanotubes to the PVDF-PMMA mixture. Due to polarity and specific
interaction with PVDF, all the three added materials got embedded only on the
PVDF.
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Titanium dioxide also
kills the bacteria. Though its antibacterial property is best in
the presence of UV light, the present study did not use UV light.
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Carbon nanotubes kill
the bacteria through direct physical contact — the roughness of
the nanotubes kills the bacteria.
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A combination of all the three nanoparticles was
superior in killing the pathogenic bacteria E. coli.