118
Bacterial Cellulose
to.reach.NOC..It.should.be.noted.that.the.drawability.requires.entanglements.
such.as.cross-linking,.but.they.are.supposed.to.be.minimized..In.the.present.
DMAc/LiCl.system.for.NOC.preparation,.however,.slower.coagulation.using.
water.vapor.yielded.stable.noncrystalline.cellulose..The.key.to.maintaining.
the.amorphous.or.noncrystalline.state.should.be.formation.of.small.amounts.
of.the.3D.cross-linking.network..When.the.cellulose.solution.spread.on.the.
petri.dish.is.exposed.to.a.saturated.water.vapor,.it.turns.out.to.be.a.gel.state.
with.shrinking..The.shrinkage.is.probably.due.to.the.interchain.hydrogen.
bonds.formed.during.the.coagulation.process.of.cellulose.molecules.into.a.
sort. of. network. structures.. After. the. solvent. was. exchanged. into. water,. a.
transparent.water-swollen.fixed.gel.of.cellulose.(WSC).is.obtained..The.WSC.
is. composed. of. approximately. 93. wt%. water. and. 7. wt%. cellulose.. During.
drawing.of.the.WSC,.water.retained.inside.the.WSC.appeared.to.bleed.out.
from. its. surface.. The. exclusion. of. excess. water. from. the. drawn. WSC. gel.
may. be. attributed. to. adjoining. the. cellulose. chains. between. the. junction.
zones,.as.shown.in.Figures 6.3a.and.6.3b..The.junction.zone.in.Figure 6.3a.
represents.the.strongly.hydrogen-bonded.area.where.even.D 2 O.vapor.could.
not. penetrate. (Hishikawa. et. al.. 1999).. The. zone. probably. plays. a. role. as. a.
cross-linking.point. in.the.WSC. fixed. gel. of.an.initial. state.for.NOC..After.
drying. of. the. drawn. WSC. under. a. stressed. state,. it. showed. a. decrease. in.
thickness.of.more.than.80%.and.shrinking.in.width.of.more.than.60%.when.
compared.with.the.starting.sizes.of.the.WSC..The.NOC.films.thus.obtained.
exhibited.almost.perfect.transparency,.as.much.as.the.coagulated.cellulose.
solution.under.water.vapor.atmosphere..This.transparency.suggests.that.the.
NOC.should.consists.of.noncrystalline.regions.
Figure 6.3.also.illustrates.structural.characteristics.of.NOC.in.relation.to.
the. situation. of. OH. groups.. When. NOC. is. prepared. by. uniaxial. stretching.of.WSC,.the.cellulose.molecular.chains.tend.to.be.oriented.toward.the.
stretching.axis,.as.described.already..Further,.the.hydroxymethyl.groups.at.
the.C6 position.of.the.anhydroglucose.unit.are.vertically.stuck.against.the.
surface,. which. indicates. that. the. neighboring. anhydroglucose. ring. planes.
are.facing.each.other..Simultaneously.the.stuck.OH.groups.in.the.individual.molecular.chains.are.also.aligned.like.tracks.along.the.stretching.axis..
In contrast,.the.lateral.order.of.the.OH.groups.among.the.neighboring.chains.
is.not.well.coordinated.because.of.the.slipped.molecular.chain.situation.with.
each.other..Therefore.the.hydrophilic.and.polarized.OH.groups.are.totally.
oriented.as.molecular.tracks.only.in.the.stretching.direction.across.the.entire.
NOC. surface.. Between. the. hydrophilic. molecular. tracks,. the. hydrophobic.
phase.due.to.the.anhydroglucose.plane.also.appeared.alternately,.resulting.
in. both. hydrophilic. and. hydrophobic. tracks. next. to. each. other. across. the.
NOC.surface..The.amphiphilic.molecular.tracks.enhance.the.unique.surface.
properties.of.NOC,.as.described.later.
The.NOC.structure.was.shown.by.the.high-resolution.transmission.electron.microscopy.(TEM).image..The.NOC.template.with.molecular.ordering.
is.shown.in.Figure 6.4.(Kondo.2007;.Kondo.et.al.,.2001)..The.high-resolution.
Bacterial Cellulose
to.reach.NOC..It.should.be.noted.that.the.drawability.requires.entanglements.
such.as.cross-linking,.but.they.are.supposed.to.be.minimized..In.the.present.
DMAc/LiCl.system.for.NOC.preparation,.however,.slower.coagulation.using.
water.vapor.yielded.stable.noncrystalline.cellulose..The.key.to.maintaining.
the.amorphous.or.noncrystalline.state.should.be.formation.of.small.amounts.
of.the.3D.cross-linking.network..When.the.cellulose.solution.spread.on.the.
petri.dish.is.exposed.to.a.saturated.water.vapor,.it.turns.out.to.be.a.gel.state.
with.shrinking..The.shrinkage.is.probably.due.to.the.interchain.hydrogen.
bonds.formed.during.the.coagulation.process.of.cellulose.molecules.into.a.
sort. of. network. structures.. After. the. solvent. was. exchanged. into. water,. a.
transparent.water-swollen.fixed.gel.of.cellulose.(WSC).is.obtained..The.WSC.
is. composed. of. approximately. 93. wt%. water. and. 7. wt%. cellulose.. During.
drawing.of.the.WSC,.water.retained.inside.the.WSC.appeared.to.bleed.out.
from. its. surface.. The. exclusion. of. excess. water. from. the. drawn. WSC. gel.
may. be. attributed. to. adjoining. the. cellulose. chains. between. the. junction.
zones,.as.shown.in.Figures 6.3a.and.6.3b..The.junction.zone.in.Figure 6.3a.
represents.the.strongly.hydrogen-bonded.area.where.even.D 2 O.vapor.could.
not. penetrate. (Hishikawa. et. al.. 1999).. The. zone. probably. plays. a. role. as. a.
cross-linking.point. in.the.WSC. fixed. gel. of.an.initial. state.for.NOC..After.
drying. of. the. drawn. WSC. under. a. stressed. state,. it. showed. a. decrease. in.
thickness.of.more.than.80%.and.shrinking.in.width.of.more.than.60%.when.
compared.with.the.starting.sizes.of.the.WSC..The.NOC.films.thus.obtained.
exhibited.almost.perfect.transparency,.as.much.as.the.coagulated.cellulose.
solution.under.water.vapor.atmosphere..This.transparency.suggests.that.the.
NOC.should.consists.of.noncrystalline.regions.
Figure 6.3.also.illustrates.structural.characteristics.of.NOC.in.relation.to.
the. situation. of. OH. groups.. When. NOC. is. prepared. by. uniaxial. stretching.of.WSC,.the.cellulose.molecular.chains.tend.to.be.oriented.toward.the.
stretching.axis,.as.described.already..Further,.the.hydroxymethyl.groups.at.
the.C6 position.of.the.anhydroglucose.unit.are.vertically.stuck.against.the.
surface,. which. indicates. that. the. neighboring. anhydroglucose. ring. planes.
are.facing.each.other..Simultaneously.the.stuck.OH.groups.in.the.individual.molecular.chains.are.also.aligned.like.tracks.along.the.stretching.axis..
In contrast,.the.lateral.order.of.the.OH.groups.among.the.neighboring.chains.
is.not.well.coordinated.because.of.the.slipped.molecular.chain.situation.with.
each.other..Therefore.the.hydrophilic.and.polarized.OH.groups.are.totally.
oriented.as.molecular.tracks.only.in.the.stretching.direction.across.the.entire.
NOC. surface.. Between. the. hydrophilic. molecular. tracks,. the. hydrophobic.
phase.due.to.the.anhydroglucose.plane.also.appeared.alternately,.resulting.
in. both. hydrophilic. and. hydrophobic. tracks. next. to. each. other. across. the.
NOC.surface..The.amphiphilic.molecular.tracks.enhance.the.unique.surface.
properties.of.NOC,.as.described.later.
The.NOC.structure.was.shown.by.the.high-resolution.transmission.electron.microscopy.(TEM).image..The.NOC.template.with.molecular.ordering.
is.shown.in.Figure 6.4.(Kondo.2007;.Kondo.et.al.,.2001)..The.high-resolution.