MENG QH-An electro-active shape memory fibre by incorporating multi-walled carbon nanotubes

MENG QH-An electro-active shape memory fibre by incorporating multi-walled carbon nanotubes
MENG QH-An electro-active shape memory fibre by incorporating multi-walled carbon nanotubes

IOP P UBLISHING S MART M ATERIALS AND S TRUCTURES Smart Mater.Struct.16(2007)830–836doi:10.1088/0964-1726/16/3/032

An electro-active shape memory?bre by incorporating multi-walled carbon nanotubes

Qinghao Meng,Jinlian Hu1and Lapyan Yeung

Institute of Textiles and Clothing,The Hong Kong Polytechnic University,Hung Kom,

Kowloon,Hong Kong,China

E-mail:tchujl@https://www.360docs.net/doc/ad9624094.html,.hk(Jinlian Hu)

Received16January2007,in?nal form28March2007

Published4May2007

Online at https://www.360docs.net/doc/ad9624094.html,/SMS/16/830

Abstract

In this paper,an electro-active shape memory?bre was fabricated

successively by incorporating multi-walled carbon nanotubes(MWNT).The

shape memory polyurethane(SMP–MWNT)composite was prepared by

in situ polymerization and the SMP–MWNT?bre was prepared by melt

spinning.Scanning electron microscopy(SEM)and transmission electron

microscopy(TEM)observations of the morphology revealed that the

MWNTs are axially aligned and homogenously distributed in the SMP

matrix,which is helpful for the?bre’s electrical conductivity improvement

and for the electro-active shape memory effect.At6.0wt%MWNT content,

the prepared shape memory?bre shape recovery ratio was75%and the?xing

ratio was77%.

1.Introduction

Thermally active shape memory polymer belongs to a kind of

functional material that can hold a temporary deformation at

a temperature below the switching temperature and recover

the original shape when it is heated to a temperature above

the switching temperature.Shape memory polyurethane

(SMP)exhibits the shape memory effect,resulting from the

thermodynamic incompatibility between the hard segments

(aromatic diisocyanates)and the soft segments(aliphatic

polyethers or polyesters).The hard segment phase has a

higher thermal transition temperature(T perm).The soft segment

phase has a lower transition temperature,T g or T m,which acts as a switching temperature.When SMP is heated to the

temperature T trans

the SMP is cooled to a temperature below the switching

temperature T low.If it is reheated up to a temperature above T trans,the original shape can be recovered under entropy elasticity[1–8].Some shape memory polymer with good processing properties,thermal stability,and a relatively high molecular weight can be used to prepare shape memory ?bres[9].

1Author to whom any correspondence should be addressed.

Usually,the shape memory effect is induced by directly heating above the switching temperature.Two research groups Cho et al[10]and Hilmar Koerner et al[11]have conducted research on electrical and infrared radiation stimulation of carbon nanotubes incorporated into SMP.The conductive carbon nanotubes were incorporated into a polymer matrix by the solution mixing process[12–17].Research on SMP–MWNT?bre preparation and the electro-active effect has never been reported.In this paper,we incorporated MWNTs into an SMP matrix by in situ polymerization after the MWNTs were treated with a mixture solvent of concentrated sulfuric acid and nitric acid.With the help of mechanical stirring,ultrasonic vibration,melt blending and melt spinning processes,a relatively homogenous and axially aligned distribution of MWNTs in SMP was achieved.The prepared?bres showed an electrically active shape memory effect.

2.Experimental details

2.1.Materials

MWNTs with a purity of>95%,outside diameters of10–20nm,inside diameters of5–10nm,and~50μm in length were supplied by Chengdu Organic Chemicals Co., Ltd,Chinese Academy of Sciences.They were used after being treated by a mixed solvent of70%nitric acid and98%

0964-1726/07/030830+07$30.00?2007IOP Publishing Ltd Printed in the UK830

An electro-active shape memory?bre by incorporating multi-walled carbon

nanotubes Figure1.Schematic for preparing the SMP–MWNT?bre.

sulfuric acid[3](1.0: 2.5by volume).Figure1shows a schematic of SMP–MWNT?bre preparation.The mixture solvent with MWNTs was heated to140?C and kept for10min (with mechanical stirring).Then an ultrasonicator with a power of100W and a nominal frequency of50kHz was used for1h to distribute the MWNTs in the mixed solvent. The treated MWNTs were collected by a glass?lter(pore size=500nm)after being washed repeatedly with suf?cient distilled water until the pH of the dispersion solution reached 7.0.The collected MWNTs were desiccated in a vacuum oven to remove the residual moisture.The MWNTs were stored in desiccators containing phosphorous pentoxide prior to use.

The SMP–MWNT composites were synthesized by in situ polymerization with polycaprolactone diol-4000(PCL)as the soft segment and isophorone disocyanate(IPDI)and molecular extender1,4-butanediol(BDO)as the hard segment.The synthesis process and?bre spinning process are shown in ?gure1.All the glass vessels were cleaned and heated to remove any residual moisture.All the chemicals were de-moisturized before use.A calculated amount of modi?ed MWNT was pre-distributed in IPDI.The SMP was prepared by pre-polymerization.The reaction was protected by pure nitrogen.After2min of molecular extension reaction,the reaction mixture was injected into twin screws for further reaction and to make SMP chips.SMP mono?laments of 210denier were spun by a single screw with pure nitrogen protection.

3.Analysis and testing

3.1.Scanning electron microscopy(SEM)analysis and transmission electron microscopy(TEM)analysis

The mono?lament fracture surface morphology with different MWNT contents were taken using a Leica Stereoscan440 scanning electron microscope operating at20kV.The fracture surface of the?bre was obtained by embedding the?bre in epoxy and then being broken in liquid nitrogen.The sample fracture surface was coated with a thin layer of gold before the measurement.TEM of the composite?bres was obtained using a Phillips CM120.The SMP?bre was microtomed into two parts at90?in the?bre’s axial direction using a diamond knife (Micro Star Co.)and a Reichert-Jung Ultracut microtome(at room temperature).The experiments were carried out at an accelerating voltage of120kV.

3.2.Conductivity

The?bres’resistances were tested using a?bre YG321 resistance tester.The sample length was5cm in length and 210denier in linear density.This was was conducted ten times on every sample to get the average values.

3.3.The electro-active shape memory effect

An adjustable voltage supplied by a voltage regulator was applied to the?bres.Because the resistance of one mono?lament was very high,20mono?laments were used to test the SMP?bres’electro-active shape memory effect. The?bre’s temperature was monitored using an infrared thermoscope.

3.4.Thermo-mechanical cyclic tensile investigations Thermo-mechanical cyclic tensile tests were carried out using a tensile tester(Instron4411).The sample is connected to a210V voltage supplied by a voltage regulator.The shape memory test of SMP consisted of four procedures.The sample was heated up to a temperature above the switching temperature and then stretched to the speci?c strain.The temperature should be less than the highest thermal transition. The sample was cooled to a temperature below the switching temperature under the constant strain to?x the temporary shape.The sample was released and then heated to a

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Q Meng et

al

Figure2.Schematic stress–strain curves of cyclic tensile

investigation.

Figure3.The prepared electro-active shape memory?bre. temperature above switching temperature,and then the second cycle is begun.The obtained stress–strain curves for the cyclic tensile test are shown in?gure2.εm is the maximum strain in the cyclic tensile tests,εu is the strain after unloading at T low andεp(N)is the residual strain after recovery in the N th cycle.

The?xity ratio(R f)and recovery ratio(R r)were calculated according to the following equations[3,6,18]:

R f(N)=εu(N)

R r(N)=[1?εp(N)]?[1?εp(N?1)]×100%

R r.tot=[1?εp(N)]×100%.

4.Result and discussion

4.1.The SEM and TEM morphology of the prepared

SMP–MWNT?bres

Figure3shows the prepared electro-active shape memory ?bres.Figure4is the SEM image of the MWNTs used in the study.The MWNTs are entangled together with long nano-size dimensions.The outer diameter is about10–20nm.Figure5 shows the fracture surfaces of SMP–MWNT?bres,having1.0, 3.0,5.0,and7.0wt%MWNT contents.

It is observed that,in the SMP–MWNT?bre,most MWNT are perpendicular to the?bre’s fracture surface, which suggests that the MWNTs are preferentially aligned in the?bre’s axial direction.The MWNTs are distributed homogenously in the shape memory?bres,especially at low MWNT content at1.0wt%.To prove the alignment distribution of MWNT further,a TEM investigation

was

Figure4.SEM images of MWNT. conducted.Figure6shows TEM images of the MWNT in shape memory?bres of1.0,3.0,5.0,and7.0wt%contents in shape memory polyurethane?bres.It is very obvious that the MWNTs are preferentially aligned in the?bre’s axial direction.

During in situ polymerization,the treated MWNTs were pre-distributed in IPDI using the ultrasonic process.The SMP molecular chains grew on the treated MWNT surface. As a result,the MWNT adhered better to SMP matrix, especially to the hard segments(diisocyanates)by in situ polymerization compared with that obtained solely by melt blending.After melt blending,extrusion,spinneret drawing, and post-drawing during spinning,the MWNTs were self-aligned,in accord with the orientation of SMP molecular chains.It is concluded that the large drawing ratio produced by the spinning process contributes to the axial alignment of MWNT.The high mechanical shear force during blending, extrusion and melt extrusion at the spinneret promotes the even distribution of MWNTs in the SMP?bres.When the MWNT content was3.0wt%,a little aggregation is observed.With an increase in MWNT content,the MWNT distribution becomes inhomogeneous.

The model shown in?gure7is employed to explain the MWNT distribution and the alignment effect induced by the melt extrusion and melt spinning processes.In?gure7, the soft segments of polyester are shown as being coiled or folded in on themselves.The schematic section length of the zig-gag line corresponds to one repeating unit within the polyol.The isocyanate is shown as a rigid circle.The hard segments are rigid and?xed,having a tendency to adhere to each other through strong hydrogen bonding.During in situ polymerization,the treated MWNTs were pre-distributed in the IPDI through the ultrasonic process.The SMP molecular chains grew on the treated MWNT surface.As a result,the MWNT adhered better to the shape memory polyurethane matrix,especially to the hard segments(diisocyanates)by in situ polymerization.When the SMP–MWNT is heated above the T trans,the soft segment phases melt.If they are then stretched,the intact hard segment phase has more interaction with the long nano-size MWNT at this temperature.During the melt blending,extrusion and spinning process,the higher shear force and drawing ratio contribute to the improved homogenous distribution and axial alignment of the MWNT. This alignment effect is more pronounced,as the SMP is composed of soft and hard segments.The hard segment phase

832

An electro-active shape memory ?bre by incorporating multi-walled carbon

nanotubes

(a)

(b)

(c)

(d)

Figure 5.SEM images of the fracture surface of the SMP–MWNT ?bres with (a)1.0wt%,(b)3.0wt%,(c)5.0wt%,(d)7.0wt%MWNT

contents.

(a)

(b) (c)

(d)

Figure 6.TEM images of the SMP–MWNT ?bres with (a)1.0wt%,(b)3.0wt%,(c)5.0wt%,(d)7.0wt%MWNT contents.

is ?xed by hydrogen bonds,which stretches the curled MWNT to align axially.As a result,the MWNT adheres to the hard segment domains effectively due to the nano-size nature after being cooled to ambient temperature.

4.2.Conductivity

As can be seen from ?gure 5,the MWNTs are homogenously and axially aligned in the SMP matrix due to the melt

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Q Meng et

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Figure7.Schematic representation of the MWNT alignment in SMP ?bres after the melt extrusion and spinning

process.

Figure8.Relationship between the resistance of the?bres and the MWNT content.

spinning,extrusion and melt spinning process.This contributes to the outstanding conductivity of the SMP–MWNT?bre. However,during to the aggregation of MWNTs in the SMP matrix and the dif?culty of fabricating SMP–MWNT?bres with an increase in MWNT content,the SMP–MWNT?bre conductivity cannot reach a much higher level.When the MWNTs reach8.0wt%,the SMP–MWNT?bre could not be fabricated,because of the poor rheological property.The 210denier SMP–MWNT?bre’s resistance with an increase in MWNT content is shown in?gure8.At low MWNT content, the SMP–MWNT?bre resistance is very high and decreases rapidly.When MWNT content reaches6.0wt%,the SMP–MWNT?bre resistance decrease becomes slow.When the MWNT content is very low,the tunnel effect is in charge of the high resistance.When the MWNT is so high that they have a short space interval or are contacted,a conductive net is formed in the?bre.The axially aligned and homogenously distributed MWNTs are helpful for the?bre’s electrical conductivity improvement.

4.3.The electro-active shape memory effect

At MWNT low content,the SMP–MWNT?bre’s electrical resistance was so high that no electro-active shape memory effect was observed.On the other hand,when the MWNT content reached7.0wt%,the prepared SMP–MWNT

?bre’s

(a)

(b)

(c)

Figure9.The?bres’electro-active shape memory effect((a)the original length,(b)the length after elongation,(c)the length after recovery).

(This?gure is in colour only in the electronic version)

maximum strain was very low,which was about40%. Large stretching deformation cannot be performed.In the experiment,SMP–MWNT?bres having 6.0wt%MWNT content with90%elongation at breaking were used.Twenty pieces of?bre were employed together to test the electro-active cyclic tensile properties,because a single?bre’s resistance was too high to produce enough heat to heat the sample temperature to above the switching temperature(about42?C)[18,19]. The?bre’s original length was60mm(the original length in ?gure9(a)).They were stretched to90mm(50%strain)in a

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An electro-active shape memory ?bre by incorporating multi-walled carbon nanotubes

Table 1.The detailed results of cyclic tensile investigation of electro-active SMP–MWNT ?bres.Circle no.εp (N )(%)εu [R f (N )](%)Stress at 100%strain (cN /dtex )R r .tot (N )(%)R r (N )(%)10.077.00.132100.00225.081.50.09775.0075.0337.583.00.08062.5083.34

40.0

83.5

0.074

60.00

96.0

Figure 10.The programmed voltage–clamp position and time relations in the cyclic tensile test.

temperature-controllable chamber at 60?C.After being cooled to ambient temperature and getting rid of external stress,the ?bre’s length became 80mm (the length after elongation in ?gure 9(b)).The strain could not be ?xed completely because of the instant elasticity recovery.Then the ?bres were connected to a 210V voltage.After 4min,the ?bres began to shrink.At last,the ?bres recovered to the original length of about 60mm.

4.4.Thermo-mechanical cyclic tensile investigations In thermo-mechanical cyclic tensile testing,the maximum strain was set at 30%and was considered to be 100%deformation.Twenty pieces of ?bre were employed together to test the electro-active cyclic tensile properties.The program-controlled voltage and clamp position are shown in ?gure 10.First,a 210V voltage is supplied to the ?bres for 5min to soften the ?bres and then stretch them to the maximum strain at a speed of 10mm min ?1;the electricity is cut off and the ?bres are kept at the maximum strain for 15min to ?x the temporary elongation;they are unloaded and the clamp is returned to its original position at a speed of 40mm min ?1;a 210V voltage is supplied for 5min to recover the ?bre’s length;and the second cycle is begun.The above processes were repeated four times,during which the relationship between strain and stress was recorded for analysis.

The obtained cyclic strain–stress curves are shown in ?gure 11.It can be seen that the SMP–MWNT ?bres at 1.0and 3.0wt%show no electro-active shape memory effect.The partial recovery,which is very low,can be attributed to the instantaneous elastic recovery and small degree of creeping deformation.The SMP–MWNT ?bre at 6.0wt%MWNT shows obvious shape memory behaviour.The detailed results of the recovery ratio at every cycle and the total recovery ratio,?xing the ratio and maximum stress at every cycle,are tabulated in table 1.The SMP–MWNT ?ber has a low

shape

00.020.040.060.080.10.120.140.16

0.180

20

40

6080

100

120

Strain (%)

S t r e s s (c N /d t e x )

S t r e s s (c N /d t e x

)

00.020.040.060.080.10.120.140.160.180.2

Strain (%)

Strain (%)

S t r e s s (c N /d t e x )

00.020.040.060.080.10.120.140.16Figure 11.Cyclic tensile investigation of strain–stress curves of SMP–MWNT ?bres ((a)1.0wt%MWNT,(b)3.0wt%MWNT,(c)6.0wt%MWNT).

recovery ratio and ?xing ratio.The maximum stresses at every cycle decrease rapidly,especially in the ?rst three cycles.This is because,to obtain higher electricity conductivity,a high content of MWNTs should be added.In the experiment,the MWNT content was 6.0wt%.Some of the MWNTs are aggregated.Though a good MWNT conductive network was formed,the polyurethane matrix continuity was destroyed.However,even though the shape recovery ratio and the ?xing ratio were low,the electro-active shape memory effect was observed.

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Q Meng et al

5.Conclusion

In this paper,an electro-active shape memory?bre was fabricated by incorporating MWNTs.A polyurethane matrix was synthesized by in situ polymerization,and SMP–MWNT ?bre was prepared by melt spinning.SEM and TEM observations of the morphology revealed the axial alignment and homogenous distribution of MWNTs in the matrix.The axially aligned and homogenously distributed MWNTs are helpful for improving the?bre’s electrical conductivity and, as a result,its electrical active shape memory effect.In this experiment,applying a210V voltage to20pieces of210denier mono?laments,a6.0wt%MWNT content SMP–MWNT?bre electro-active shape memory effect was observed.The?bre had a shape recovery ratio of about75% and a?xing ratio of about77%.

Acknowledgment

This work was supported by the project‘High Perfor-mance Advanced Materials for Textile and Apparel(ref. GHS/088/04)’from HongKong Innovation Technology Fund-ing.

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