Figure 12.7
250
Bacterial Cellulose
(a)
(b)
Figure 12.7
Cross.section.photographs.of.hydrated.BNC.and.BNC/PAni.HCl.composite..In.situ.polymerization.of.Ani.onto.BNC.(a).induces.an.inward.concentration.gradient.of.PAni.(b).on.the.BNC..The.
white.arrows.mark.the.direction.of.the.decreasing.concentration.gradient.of.polymerized.PAni.
solution. from. the. reaction. mixture. later. turned. purple,. and. on. prolonged.
polymerization. it. turned. blackish-brown. due. to. PAni. degradation,. thus.
showing overoxidation.
In.our.work,.BNC.disks.of.2.5.cm.diameter.and.5.mm.thickness.were.used.
for.the.in.situ.polymerization.of.aniline..As.observed.in.Figure 12.7,.given.
the. thickness. of. the. BNC,. PAni. was. not. uniformly. distributed. across. the.
substrate,. the. outer. regions. of. the. disks. having. a. higher. concentration. of.
PAni..It.is.known.that.when.polymerization.occurs.in.the.presence.of.a.high.
surface.area.substrate.(as.is.the.case.of.BNC),.polymerization.occurs.on.the.
substrate’s. surface,. given. the. limited. solubility. of. aniline. in. water. (Qaiser.
et al..2009)..It.is.also.know.that.aniline.forms.aggregates.(with.varying.morphologies.according.to.the.experimental.conditions.used.[Stejskal.et al..2008]).
that.precipitate.from.solution.and/or.aggregate.onto.the.surface.of.a.fibrillar.
template..It.is.possible.that,.with.the.polymerization.of.Ani.onto.the.surface.
of. BNC. fibers,. the. accumulation. of. aggregates. on. the. outer. surface. of. the.
. cellulose.disks.(fibrils).exerted.a.plug.effect.characterized.by.a.reduction.in.
mers.into.the.interior.of.the.nanofibrillar.matrix..The.lower.electrical.conductivity.values.obtained.here.as.compared.to.other.works.may.thus.be.ascribed.
to.the.greater.thickness.of.our.samples..Also,.while.freeze-drying.allows.for.
a.better.preservation.of.the.native.3D.matrix.of.BNC.(Cao.2011;.Clasen.et.al..
2006),.it.lowers.the.number.of.contact.points.between.the.. polymerized.fibers,.
which.in.turn.may.account.for.the.lower.conductivity.values.obtained.here..
A.similar.effect.was.independently.observed.by.Marins.et.al..(2011)..Contrary.
to.our.work,.Hu.et.al..(2011).used.thin.films.(0.03.mm.thickness).and,.and.
this.is.noteworthy,.they.used.oven-dried.BNC.for.the.polymerization..This.
250
Bacterial Cellulose
(a)
(b)
Figure 12.7
Cross.section.photographs.of.hydrated.BNC.and.BNC/PAni.HCl.composite..In.situ.polymerization.of.Ani.onto.BNC.(a).induces.an.inward.concentration.gradient.of.PAni.(b).on.the.BNC..The.
white.arrows.mark.the.direction.of.the.decreasing.concentration.gradient.of.polymerized.PAni.
solution. from. the. reaction. mixture. later. turned. purple,. and. on. prolonged.
polymerization. it. turned. blackish-brown. due. to. PAni. degradation,. thus.
showing overoxidation.
In.our.work,.BNC.disks.of.2.5.cm.diameter.and.5.mm.thickness.were.used.
for.the.in.situ.polymerization.of.aniline..As.observed.in.Figure 12.7,.given.
the. thickness. of. the. BNC,. PAni. was. not. uniformly. distributed. across. the.
substrate,. the. outer. regions. of. the. disks. having. a. higher. concentration. of.
PAni..It.is.known.that.when.polymerization.occurs.in.the.presence.of.a.high.
surface.area.substrate.(as.is.the.case.of.BNC),.polymerization.occurs.on.the.
substrate’s. surface,. given. the. limited. solubility. of. aniline. in. water. (Qaiser.
et al..2009)..It.is.also.know.that.aniline.forms.aggregates.(with.varying.morphologies.according.to.the.experimental.conditions.used.[Stejskal.et al..2008]).
that.precipitate.from.solution.and/or.aggregate.onto.the.surface.of.a.fibrillar.
template..It.is.possible.that,.with.the.polymerization.of.Ani.onto.the.surface.
of. BNC. fibers,. the. accumulation. of. aggregates. on. the. outer. surface. of. the.
. cellulose.disks.(fibrils).exerted.a.plug.effect.characterized.by.a.reduction.in.
mers.into.the.interior.of.the.nanofibrillar.matrix..The.lower.electrical.conductivity.values.obtained.here.as.compared.to.other.works.may.thus.be.ascribed.
to.the.greater.thickness.of.our.samples..Also,.while.freeze-drying.allows.for.
a.better.preservation.of.the.native.3D.matrix.of.BNC.(Cao.2011;.Clasen.et.al..
2006),.it.lowers.the.number.of.contact.points.between.the.. polymerized.fibers,.
which.in.turn.may.account.for.the.lower.conductivity.values.obtained.here..
A.similar.effect.was.independently.observed.by.Marins.et.al..(2011)..Contrary.
to.our.work,.Hu.et.al..(2011).used.thin.films.(0.03.mm.thickness).and,.and.
this.is.noteworthy,.they.used.oven-dried.BNC.for.the.polymerization..This.