105
Bacterial Cellulose Surface Modifications
In.addition.to.conductivity,.transparency.is.also.a.useful.characteristic.for.
optoelectronic. devices.. In. 2005. Yano. et. al.. discovered. the. potential. of. BC.
nanofibers.as.a.reinforcement.material.for.optical.functional.composites..As.
a.result.of.the.size.effect,.the.composites.were.optically.transparent.even.at.
high. fiber. contents. such. as. 70%.. Moreover,. the. composites. presented. low.
thermal.expansion.and.high.strength,.due.to.BNC.nanofiber.reinforcement,.
without.modifying.the.ease.of.bending.of.pure.polymers..However,.because.
of.the.several.hydroxyl.groups.on.the.cellulose.nanofiber,.the.composite.is.
quite.hygroscopic,.which.may.lead.to.moisture.absorption.that.may.result.in.
sional.stability.against.temperature..For.this.reason,.Nogi.et.al..(2006).prepared. a. nanocomposite. reinforced. with. acetylated. BNC. nanofibers. with. a.
low. degree. of. substitution. (DS),. which. was. determined. to. be. 0.17.. Even. at.
a. low. DS,. acetylation. effectively. reduced. the. hygroscopy. of. BC. nanocomposites,.while.not.changing.transparency,.since.hydrophilic.hydroxyl.groups.
on. the. nanofiber. surface. were. replaced. by. less. hydrophilic. acetyl. groups..
In the.follow-up.of.that.work,.Ifuku.et.al..(2007).studied.the.effect.of.different.
degrees.of.substitution.on.BNC.acetates.by.characterizing.their.hygroscopy.
and.other.properties..The.authors.found.that.acetylation.improved.various.
properties.of.BNC.and.their.composites.until.an.optimum.DS.was.reached,.
after. which. excessive. acetylation. reduced. those. properties.. Acetylation.
decreased.the.refractive.index.of.BNC.and.the.loss.in.regular.transmittance.of.
transparent.nanocomposite.at.580.nm.from.neat.acrylic.resin.was.reduced..In.
addition,.it.effectively.reduced.the.hygroscopy.of.BC.nanocomposites,.water.
absorption.at.20°C.becoming.inferior.to.0.5%,.and.leading.to.a.decrease.in.
thermal.expansion..These.results.demonstrate.that.surface.acetylation.of.BC.
nanofibers.without.collapsing.the.crystalline.structure.may.be.desirable.for.
the.enhancement.of.various.properties.of.optically.transparent.composites.
Hydrophobic Derivatives
Recent. research. has. also. focused. on. increasing. the. hydrophobic. surface.
character.of.BNC.in.order.to.improve.its.barrier.properties,.while.keeping.
its.stability.and.good.mechanical.ability..Acylation.of.cellulose.fibers.with.
fatty.acids.may.result.in.an.attractive.way.to.produce.biodegradable.plastics. ,.
as. well. as. to. reinforce. materials. for. composites. with. common. polymeric.
matrices.. The. reactions. leading. to. these. modifications. can. be. performed.
in. homogeneous. (cellulose. fibers. are. previously. dissolved. in. an. appropriate.medium).or.heterogeneous.(modifications.take.place.in.reaction.media,.
where.the.formed.cellulose.derivatives.are.gradually.dissolved).conditions,.
being.the.supramolecular.structure.of.cellulose.destroyed.and.the.original.
physicomechanical.properties.of.fibers.lost,.in.both.systems..However,.under.
Bacterial Cellulose Surface Modifications
In.addition.to.conductivity,.transparency.is.also.a.useful.characteristic.for.
optoelectronic. devices.. In. 2005. Yano. et. al.. discovered. the. potential. of. BC.
nanofibers.as.a.reinforcement.material.for.optical.functional.composites..As.
a.result.of.the.size.effect,.the.composites.were.optically.transparent.even.at.
high. fiber. contents. such. as. 70%.. Moreover,. the. composites. presented. low.
thermal.expansion.and.high.strength,.due.to.BNC.nanofiber.reinforcement,.
without.modifying.the.ease.of.bending.of.pure.polymers..However,.because.
of.the.several.hydroxyl.groups.on.the.cellulose.nanofiber,.the.composite.is.
quite.hygroscopic,.which.may.lead.to.moisture.absorption.that.may.result.in.
sional.stability.against.temperature..For.this.reason,.Nogi.et.al..(2006).prepared. a. nanocomposite. reinforced. with. acetylated. BNC. nanofibers. with. a.
low. degree. of. substitution. (DS),. which. was. determined. to. be. 0.17.. Even. at.
a. low. DS,. acetylation. effectively. reduced. the. hygroscopy. of. BC. nanocomposites,.while.not.changing.transparency,.since.hydrophilic.hydroxyl.groups.
on. the. nanofiber. surface. were. replaced. by. less. hydrophilic. acetyl. groups..
In the.follow-up.of.that.work,.Ifuku.et.al..(2007).studied.the.effect.of.different.
degrees.of.substitution.on.BNC.acetates.by.characterizing.their.hygroscopy.
and.other.properties..The.authors.found.that.acetylation.improved.various.
properties.of.BNC.and.their.composites.until.an.optimum.DS.was.reached,.
after. which. excessive. acetylation. reduced. those. properties.. Acetylation.
decreased.the.refractive.index.of.BNC.and.the.loss.in.regular.transmittance.of.
transparent.nanocomposite.at.580.nm.from.neat.acrylic.resin.was.reduced..In.
addition,.it.effectively.reduced.the.hygroscopy.of.BC.nanocomposites,.water.
absorption.at.20°C.becoming.inferior.to.0.5%,.and.leading.to.a.decrease.in.
thermal.expansion..These.results.demonstrate.that.surface.acetylation.of.BC.
nanofibers.without.collapsing.the.crystalline.structure.may.be.desirable.for.
the.enhancement.of.various.properties.of.optically.transparent.composites.
Hydrophobic Derivatives
Recent. research. has. also. focused. on. increasing. the. hydrophobic. surface.
character.of.BNC.in.order.to.improve.its.barrier.properties,.while.keeping.
its.stability.and.good.mechanical.ability..Acylation.of.cellulose.fibers.with.
fatty.acids.may.result.in.an.attractive.way.to.produce.biodegradable.plastics. ,.
as. well. as. to. reinforce. materials. for. composites. with. common. polymeric.
matrices.. The. reactions. leading. to. these. modifications. can. be. performed.
in. homogeneous. (cellulose. fibers. are. previously. dissolved. in. an. appropriate.medium).or.heterogeneous.(modifications.take.place.in.reaction.media,.
where.the.formed.cellulose.derivatives.are.gradually.dissolved).conditions,.
being.the.supramolecular.structure.of.cellulose.destroyed.and.the.original.
physicomechanical.properties.of.fibers.lost,.in.both.systems..However,.under.