Biomedical Engineering Reference
In-Depth Information
14.3.3 Mechanical Properties
The.tuning.of.mechanical.properties.of.nanoibrous.mats.is.essential.and.dependent.upon.
the.application..Evaluation.of.these.properties.can.be.accomplished.by.the.uniaxial.tensile.
test,.AFM,.and.the.nanoindentation.test.
The.uniaxial.tensile.test.utilizes.a.texture.analyzer.suited.with.ixed.grips.lined.with.rub-
ber.on.each.end.(Veleirinho.et.al..2008)..Specimens.to.be.tested.can.be.about.90.mm.long.and.
10.mm.wide.and.are.obtained.perpendicular.to.the.axis.of.the.collector.rotation.(Veleirinho.
et.al..2008)..The.ends.of.the.specimen.are.then.mounted.on.the.grips.using.tape..Sample.
thickness.can.be.determined.in.various.locations.using.a.digital.micrometer..The.stress-
strain.curves.obtained.from.uniaxial.tensile.tests.allow.one.to.determine:.Young's.modulus,.
percentage.elongation.or.ultimate.break.strain,.yield.strength.or.stress.at.maximum.force,.
and.tensile.strength.or.stress.at.break.or.ultimate.tensile.strength.(Veleirinho.et.al..2008).
In.addition.to.imaging.surface.topography,.AFM.is.extremely.useful.in.determining.nano-
iber.mechanical.properties..A.three-point.bend.test.can.be.performed.with.the.AFM.in.con-
tact.mode.(Lee.et.al..2005,.Liao.et.al..2011)..In.order.to.do.so,.a.single.nanoiber.is.suspended.
(over.a.groove.in.a.silicon.wafer,.for.example).with.a.small.delection.applied.to.the.middle.
of.it.with.the.AFM.cantilever.(Tan.and.Lim.2004,.Liao.et.al..2011)..This.test.is.able.to.verify.
the. mechanical. properties. of. a. single. nanoiber. by. observing. the. applied. force. as. a. func-
tion.of.any.displacement.(Lee.et.al..2005,.Liao.et.al..2011)..The.applied.load.is.measured.by.
accumulating.a.plot.of.cantilever.delection.as.a.function.of.sample.position.along.the.Z-axis.
(Lee.et.al..2005)..This.is.also.known.as.a.force.curve..There.are.several.calculations.involved.
in.order.to.determine.the.mechanical.properties,.which.will.not.be.discussed.in.this.chapter.
The.elastic-plastic.nanoindentation.test.is.not.only.a.method.to.determine.mechanical.
properties.of.electrospun.nanoibers,.it.is.also.useful.in.determining.the.accuracy.of.the.
three-point. bend. test. (Li. et. al.. 2003).. Basically,. the. nanoindenter. is. used. in. conjunction.
with. the. AFM.. This. tool. is. able. to. observe. and. record. the. load. and. displacement. of. the.
indenter,.a.three-sided.pyramidal.diamond..The.data.acquired.can.then.be.used.to.calcu-
late.the.hardness.and.elastic.modulus..The.advantages.of.this.test.include:.high.loading,.
more.reined.load.and.displacement.resolution,.better.exploration.of.the.sample,.and.it.is.
possible.to.observe.the.material's.response.in.near.real-time.
14.3.4 Thermal Properties
Thermal.properties.such.as.crystallization.and.melting.processes.can.be.measured.using.
differential.scanning.calorimetry.(DSC).
Preparation.for.DSC.includes.accurately.weighing.electrospun.mats.(of.about.10.mg).into.
sealed.aluminum.pans.(Veleirinho.et.al..2008)..In.order.to.maintain.atmospheric.pressure.
and.to.allow.for.evaporation.of.residual.solvents,.there.should.be.holes.in.the.pan.covers..
Samples.can.then.be.heated.from.30°C.to.300°C,.maintaining.a.heating.rate.of.10°C/min,.
with.constant.low.of.dry.nitrogen..In.order.to.determine.the.percent.crystallinity.(χ
c
),.one.
can.use.Equation.14.3.
(
∆
H
∆
∆
−
H
)
f
c
χ
c
(
% =
)
×
100
.
(14.3)
H
.
f
The.melting.enthalpy.and.enthalpy.of.crystallization.(ΔH
f
.and.ΔH
c
).are.values.obtained.
from. DSC. traces.. The. heat. of. fusion. ∆
H
f
. is. a. property. of. the. completely. crystalline.
substance.
