structure and properties of low-isotacticity polypropylene ... · pdf filecore bicomponent...

J Polym Eng 2014; aop

Youhei Kohri * , Tomoaki Takebe , Yutaka Minami , Toshitaka Kanai , Wataru Takarada

and Takeshi Kikutani

Structure and properties of low-isotacticity polypropylene elastomeric fibers prepared by sheath-core bicomponent spinning: effect of localization of high-isotacticity component near the fiber surface

Abstract: Sheath-core type bicomponent melt-spun fibers

were produced by extruding the melts of low-isotacticity

polypropylene (LPP) as the core component and the blend of

LPP and high-isotacticity PP (IPP) as the sheath component.

IPP content in the sheath was changed from 8 wt% to 40

wt% while sheath/core composition was varied from 50/50

to 10/90. Accordingly overall IPP content was kept constant

at 4 wt%. Even though the overall IPP content was intact,

bicomponent fibers with lower contraction ratio after spin-

ning, higher elastic recovery and slightly higher modulus

and strength were obtained by increasing the IPP content

in the sheath and decreasing the sheath layer composition,

i.e., localizing the IPP to the region near the surface in the

fiber cross-section. Structure analysis of the as-spun fibers

suggested the suppression of crystallization of LPP in the

sheath by blending IPP. By contrast, enhancement of molec-

ular orientation and crystallization of the sheath component

were found to occur by localizing the IPP to the region near

the fiber surface. It was speculated that this behavior was

caused by the kinematic mutual interaction of the sheath

and core components in the melt spinning process.

Keywords: bicomponent fiber; elastic recovery; elasto-

meric fiber; isotacticity; polypropylene.

DOI 10.1515/polyeng-2014-0195

Received July 16 , 2014 ; accepted August 21 , 2014

1 Introduction

Polypropylene (PP) has been widely used commercially

for the production of fibers, non-woven fabrics, films,

injection-molded products, etc. For the production of

PP, introduction of metallocene catalysts provided the

technologies for designing new types of polymers [1 – 7] ,

and recently, development of low-isotacticity PP (LPP)

with elastomeric characteristics was achieved. Structure

and properties of such PP have been investigated widely

because of its unique characteristics [8 – 13] . It was reported

that the modification high-order structure and physical

properties, as well as the improvement of processability

of conventional high-isotacticity PP (IPP), was possible by

blending a small amount of elastomeric PP [14 – 17] . By con-

trast, a technology for producing non-woven elastomeric

fabrics has been developed recently by applying the spun-

bonding process to the elastomeric PP [18] . Improvement

of the production of non-woven fabrics achieving a good

balance of processability and elastomeric recoverability of

the product has been attempted by introducing the sheath-

core bicomponent melt spinning process [19 – 22] .

In a previous paper, we investigated the effects of

sheath/core composition on the high-order structure and

mechanical properties of elastomeric bicomponent fibers

consisting of LPP as the core component and the blend of

LPP and IPP as the sheath component, in that the sheath

layer composition and the total IPP content were varied,

while keeping the IPP content in the sheath layer constant

at 10 wt% [23] . Through the analyses of the as-spun bicom-

ponent fibers, it was found that the as-spun fibers exhibit

a relatively large elastic recovery of higher than 85%, and

that the crystallization of IPP was enhanced when the

sheath layer composition was low, while the crystalliza-

tion of LPP was suppressed with the increase in the sheath

layer composition.

*Corresponding author: Youhei Kohri, Performance Materials

Laboratories, Idemitsu Kosan Co., Ltd., Chiba, Japan,

e-mail: [email protected] ; and Organic and Polymeric

Materials, Tokyo Institute of Technology, Tokyo, Japan

Tomoaki Takebe and Yutaka Minami: Performance Materials

Laboratories, Idemitsu Kosan Co., Ltd., Chiba, Japan

Toshitaka Kanai: KT Polymer, Chiba, Japan

Wataru Takarada and Takeshi Kikutani: Organic and Polymeric

Materials, Tokyo Institute of Technology, Tokyo, Japan

Authenticated | [email protected] author's copyDownload Date | 2/25/15 2:16 AM

2 Y. Kohri et al.: Low-isotacticity polypropylene elastomeric fibers prepared by sheath-core spinning

In this research, with the aim of producing LPP fibers

of high elastic recoverability while keeping the overall IPP

content constant, various sheath-core fibers consisting

of LPP as the core component and the blend of LPP and

IPP as the sheath component were produced, and high-

order structure and elastomeric properties of the as-spun

sheath-core bicomponent fibers were investigated.

2 Materials and methods

2.1 Materials

A new fiber and non-woven grade LPP commercialized in

2012 (LPP; L-MODU, S901, Idemitsu Kosan Co., Ltd., Chiba,

Japan) was used in this study. The polymer was produced

through the polymerization process applying a double

cross-linked metallocene catalyst. For the improvement

of the crystallizability of LPP, isotactic PP (IPP, Y6005GM,

Prime Polymer Co., Ltd., Chiba, Japan) was blended to LPP.

Properties of the polymers used in this study are listed in

Table 1 . The melts of both polymers have similar viscos-

ity. LPP has an extremely low melting temperature of 70 ° C

because of its low isotacticity.

2.2 Melt spinning of bicomponent fibers

Sheath-core type bicomponent fibers were produced by

extruding the melts of the blend of LPP and IPP as the

sheath component and LPP as the core component using

two extrusion systems, each of which consisted of a single

screw extruder and a metering pump. The spinneret had

eight spinning holes of 0.5 mm diameter. The extrusion

temperature and throughput rate were set at 220 ° C and

1.0 g/min/hole, respectively. Fiber samples were prepared

at the take-up velocity of 2 km/min. In a previous paper,

sheath IPP content was kept constant at 10 wt% while

sheath layer composition was varied from 0% to 50% [23] .

Accordingly, total IPP content was changed from 0 wt%

to 5 wt%. By contrast, in this series of experiments, IPP

content in the sheath was set from 8 wt% to 40 wt%,

while the sheath/core composition was varied from 50/50

to 10/90. Accordingly, the overall IPP content was kept

constant at 4 wt%. Sheath layer composition, sheath IPP

content and total IPP content are summarized in Table 2 .

Since the as-spun fibers have a relatively low tensile

modulus, the spin-line tension causes the elastic stretch-

ing of the fiber in the spin-line before the fibers are wound

up on the bobbin. Therefore, when the fibers were cut

off from the bobbin after the spinning, contraction of the

fibers to the original length was observed. The contraction

ratio was calculated from the circumference length of the

bobbin, L , and the length of cut-off fiber samples, R , using

the following equation [Eq. (1)]:

-Contraction Ratio (%) 100

L RL

= ×

(1)

2.3 Evaluation of elastic recovery

Elastic recovery measurements were conducted for the

as-spun fibers using a tensile testing machine (Shimadzu

Corporation, Autograph AG-I). The gauge length L 0 was

50 mm, and the cross-head speed for extension and recov-

ery were 150 mm/min. In the measurement, the fibers were

firstly stretched to strain 100%. Secondly, the cross-head

was returned to the initial position. At this moment, the

stress was zero because of the incorporation of permanent

extension after the first stretching. Finally, the cross-head

was moved for the second stretching, and the position L

where the stress started to appear again was measured.

The elastic recovery was calculated by using the following

equation [Eq. (2)]:

Table 1 Properties of low-isotacticity polypropylene (LPP) and

high-isotacticity PP (IPP).

M w M w /M n MFR (g/10 min) T m ( ° C) H (J/g) Xc (%)

LPP 1.2 × 10 5 2.0 60 70 27 13

IPP 1.6 × 10 5 3.7 60 165 90 43

Note: Sheet sample for differential scanning calorimetry (DSC)

analysis was prepared as follows: compression molding of the

polymer at 220 ° C, quenching in water, and conditioning at room

temperature for more than 1 week.

MFR, melt flow rate.

Table 2 Sheath layer composition, sheath high-isotacticity poly-

propylene (IPP) content and overall IPP content of as-spun

IPP/low-isotacticity PP (LPP) bicomponent fibers.

Sheath layer composition (wt%)

Sheath IPP content (wt%)

Overall IPP content (wt%)

10 40 4

20 20 4

30 13 4

40 10 4

50 8 4


Y. Kohri et al.: Low-isotacticity polypropylene elastomeric fibers prepared by sheath-core spinning 3

0

Elastic Recovery (%) 1- 100LL

⎛ ⎞= ×⎜ ⎟⎝ ⎠

(2)

2.4 Tensile test

The load-elongation curves of the as-spun fibers were

obtained using the same tensile testing machine for the

elastic recovery measurement. The gauge length and

tensile speed were 50 mm and 150 mm/min, respectively.

The tensile modulus, the tensile strength and the elonga-

tion at break were obtained averaging at least 10 trials of

the tensile test for each sample.

2.5 Wide angle X-ray diffraction measurement

Wide angle X-ray diffraction (WAXD) patterns of the

as-spun fibers were obtained using an X-ray generator

and a charge-coupled device (CCD) camera (Rigaku Denki,

RTM-18HFVE, CCD Mercury). The generator was oper-

ated at 45 kV and 60 mA, and the WAXD patterns of the

fiber bundle were taken at the exposure condition of five

times/10 s.

2.6 Birefringence measurement

Birefringence of as-spun fibers was analyzed through the

measurements of fiber diameter and optical retardation

using a polarizing optical microscope (Olympus, BH-2)

equipped with a Berek compensator. Since the measure-

ment was performed at the center of the fiber, the obtained

overall birefringence represents the mean value of sheath

and core components averaged along the diameter. Thus,

in comparison with the averaging based on the weight

fractions of both components, there can be a slightly

enhanced contribution of the core component.

2.7 Differential scanning calorimetry measurement

Thermal analysis of the as-spun fibers was performed with

a differential scanning calorimeter (DSC) (Perkin Elmer,

DSC 8500) equipped with a liquid nitrogen cooling system

for the measurements from low temperature. The meas-

urement was conducted from -40 ° C to 220 ° C at a heating

rate of 10 ° C/min.

4014

12

10

8

6

Con

trac

tion

ratio

(%

)

4

2

010 20 30

Sheath layer composition (wt%)40 50

20 13


10 8

Figure 1 Variation of contraction of as-spun high-isotacticity

polypropylene (IPP)/low-isotacticity PP (LPP) bicomponent fibers

after spinning with sheath IPP content and sheath layer composition.

The crystallinity of pure IPP and LPP, X c-IPP

and X c-LPP

,

were calculated by using the following equations [Eqs. (3)

and (4)], where Δ H IPP

and Δ H LPP

are the heat of fusion of

IPP and LPP, and W IPP

and W LPP

are the weight fractions

of the IPP and LPP components. The heat of fusion for PP

with 100% crystallinity, Δ H *, was assumed to be 209 J/g

[24] :

-

1(%) 100

*IPP

c IPPIPP

HXH W

Δ

Δ= × ×

(3)

-

1(%) 100

*LPP

c LPPLPP

HXH W

Δ

Δ= × ×

(4)

3 Results and discussion

3.1 Melt spinning behavior

The contraction ratio of the fiber length after spinning was

plotted against the sheath layer composition in Figure 1 .

IPP content in the sheath layer is also indicated in the

figure. The contraction ratio decreased continuously from

12% to 2% with the decrease of sheath layer composition

from 50% to 10% and the increase of IPP content in the

sheath from 8% to 40%. In other words, a reduction of

the contraction ratio with the constant IPP content was

achieved by localizing the IPP to the region near the fiber

surface in the fiber cross-section.



3.2 Mechanical properties of as-spun fibers

Stress-strain hysteresis curves for the multiple cycles of

stretching and recovery of as-spun fibers with the sheath

IPP content of 40 wt% and 8 wt%, which correspond to

the sheath layer compositions of 10% and 50%, respec-

tively, are compared in Figure 2 . There was a significant

change in the stress-strain behavior between the first and

second stretchings, whereas almost identical behaviors

were observed for the second and the third stretchings.

A certain level of hysteresis was observed between the

stretching and recovery even after the second stretching.

By localizing the IPP to the region near the fiber surface,

the overall stress level during the first stretching increased

significantly, while reduction of the stress level from the

first to the second stretching was more significant.

Variation of elastic recovery after the first stretching

with the increase of sheath layer composition is shown in

Figure 3 . All samples showed a fairly high elastic recovery

of higher than 85%, while there was an improvement of

elastic recovery with the reduction of the sheath layer

composition and the increase of sheath IPP content.

Tensile modulus of the as-spun fibers is plotted against

the sheath layer composition in Figure 4 . The tensile

modulus decreased with the increase in the sheath layer

composition and with the decrease of sheath IPP content.

In a previous paper [23] , it was found that there was a quan-

titative accordance between the variation of contraction

ratio after spinning and the variation of tensile modulus

of the as-spun fibers with the change of sheath layer com-

position. This was because the higher tensile modulus

corresponded to the lower degree of elastic stretching in

the spin-line. In this research, however, the contraction

ratio significantly decreased from 12% to about 1% – 2%

when the sheath layer composition was decreased from

50% to 10%, while the increase in the tensile modulus of

fibers with the increase of sheath IPP content was only

about 25% (from 80 MPa to 100 MPa). The reason for

40120

100

80

60

40

20

Tens

ile m

odul

us (

MP

a)

010 20 30


20 13


10 8

Figure 4 Variation of tensile modulus of as-spun high-isotacticity


with sheath IPP content and sheath layer composition.

40100

95

90

85Ela

stic

rec

over

y (%

)

8010 20 30


20 13


10 8

Figure 3 Variation of elastic recovery of as-spun high-isotacticity



160

140

120

100

80

60

40

20

0

Str

ess

(MP

a)160

140

120

100

80

60

40

20

0

Str

ess

(MP

a)

20 40 60 80 1000

Strain (%)

20 40 60 80 1000

Strain (%)

1st stretching

2nd and 3rdstretching

1st, 2nd and 3rdrecovery

Sheath IPP content: 40 wt% Sheath IPP content: 8 wt%

Figure 2 Stress-strain hysteresis curves for the first, second and third cycles of stretching and recovery of as-spun high-isotacticity

polypropylene (IPP)/low-isotacticity PP (LPP) fibers with the sheath IPP content of 40 wt% and 8 wt%.



40800

700

600

500

400

300

200

100

Tens

ile m

odul

us o

f the

she

ath

com

posi

tion

(MP

a)

010 20 30


20 13


10 8

Figure 5 Variation of the estimated tensile modulus of the sheath

component of as-spun high-isotacticity polypropylene (IPP)/low-

isotacticity PP (LPP) bicomponent fibers with sheath IPP content and

sheath layer composition.

40250

200

150

100

50

Tens

ile s

tren

gth

(MP

a)

010 20 30


20 13


10 8

Figure 6 Variation of tensile strength of as-spun high-isotacticity



40250

200

150

100

50

Str

ain

at b

reak

(%

)

010 20 30


20 13


10 8

Figure 7 Variation of strain at break of as-spun high-isotacticity



such a discrepancy is not clear at the moment. To explain

the extremely low contraction ratio of the fibers with the

sheath layer compositions of 10 wt% and 20 wt% in com-

parison with the tensile moduli of those fibers, progress

of structural change possibly accompanied by the crystal-

lization needs to occur either in the sheath or core compo-

nents after the fiber was wound up on the bobbin.

By contrast, to clarify the origin of the different

characteristics between the fibers with different sheath

layer compositions, the tensile modulus of the sheath

component was estimated based on a simple parallel

model, assuming the constant tensile modulus of 38 MPa

for the core component, which corresponds to that of the

pure LPP single component fiber. The estimated tensile

modulus of the sheath component increased from 113 MPa

to 667 MPa with the decrease of sheath layer composition

as shown in Figure 5 .

Increasing tendency of the tensile modulus with the

decrease of sheath layer composition can be regarded as

a normal behavior, since the IPP content increases at the

same time, however it should be noted that there can be

another mechanism for the increase in the tensile modulus

of sheath component. As described in a previous paper

[25, 26] , when the sheath layer composition is decreased,

its structure development behavior can be altered signifi-

cantly in comparison with its single component spinning,

because of the kinetic mutual interaction of the sheath

and core components in the melt spinning process of

bicomponent fibers. In the bicomponent melt spinning of

the LPP and the blend of IPP and LPP, enhancement of the

structure formation of the blend can be expected because

of its higher crystallizability. Such an effect becomes more

significant for the sheath or core component with lower

composition [23] .

Tenacity and elongation at break of the as-spun fibers

are plotted against sheath layer composition in Figures 6

and 7 . The tensile strength increased, whereas the elonga-

tion at break decreased with the decrease of sheath layer

composition, that is, by localizing the IPP to the region

near the fiber surface.

Summarizing the above, it was found that even though

the total IPP content was kept constant, the contraction

ratio after spinning became lower, and the as-spun fibers

with higher elastic recovery, slightly higher modulus and

strength, and lower elongation were obtained by the

enhanced localization of IPP component in the cross-sec-

tion of LPP fiber.



3.3 Fiber structure

WAXD patterns of the as-spun fibers of various sheath

layer compositions are shown in Figure 8 . All as-spun

fibers showed similar WAXD patterns with highly oriented

α form crystals. Effects of sheath/core composition and

IPP content in the sheath were not clear.

In our previous study on the sheath-core bicompo-

nent spinning of IPP and LPP, the IPP component showed

highly oriented α form crystals, whereas the LPP com-

ponent showed α form crystals of virtually no orienta-

tion [27] . The mechanism for the development of such

structure was speculated as follows. In the bicomponent

spin-line, crystallization of the IPP component proceeded

at a relatively high temperature before the stating of the

crystallization of LPP. This caused the termination of the

spin-line thinning and thus the orientation relaxation of

the LPP component. In this study, however, by changing

the sheath layer component from pure IPP to the blend of

IPP and LPP, both sheath and core components showed

high crystalline orientation.

By contrast, in our previous paper on the sheath-core

bicomponent melt spinning of the blend of IPP and LPP

with the constant IPP content of 10 wt% as the sheath

and the pure LPP as the core [23] , it was speculated for

the mechanism of the formation of highly oriented α form

crystals in the core that even though crystallization of the

IPP component in the blend started to occur at higher

temperatures, deformability of the blend could be main-

tained because the content of IPP was only 10 wt%, and

accordingly orientation of the core component, LPP, could

proceed in the spin-line. In this research, highly oriented

α form crystals were formed in the core even when the IPP

content in the blend was increased up to 40 wt%, sug-

gesting that the deformability of the blend of IPP and LPP

was maintained after starting the crystallization of the

IPP component in the spin-line, even at such a high IPP

content.

Variation of overall birefringence of as-spun fibers

with sheath layer composition and the sheath IPP content

is shown in Figure 9 . All as-spun fibers exhibited similar

levels of birefringence. This is partly because the meas-

ured birefringence in this research represents the aver-

aged birefringence of the sheath and core components.

Considering the above, birefringence of the sheath

component was estimated assuming the birefringence

of the core component LPP to be 9.9 × 10 -3 , which corre-

sponds to the birefringence of the as-spun single compo-

nent LPP fiber prepared at the same take-up velocity. The

detailed calculation method was reported in a previous

paper [23] .

Birefringence of the sheath component estimated

through the method stated above is plotted against the

sheath layer composition and the sheath IPP content as

shown in Figure 10 . When the sheath layer composition

was 50 wt%, no boundary between the sheath and the

core could be observed under the polarizing microscope.

This fact is consistent with the result in Figure 10 that the

sheath and core have a similar level of birefringence at

this composition.

Sheath layercomposition=10%





Figure 8 Wide-angle X-ray diffraction patterns of as-spun high-isotacticity polypropylene (IPP)/low-isotacticity PP (LPP) bicomponent fibers

of five different sheath layer compositions.



By contrast, birefringence of the sheath component

increased with the increase in IPP content in the sheath

layer and also with the decrease in the sheath layer com-

position. Increase of birefringence in the sheath layer with

the increase of IPP content can be expected, however, esti-

mated birefringence of 40 × 10 -3 is significantly higher than

that of single-component IPP fiber prepared at the same

take-up velocity. This result is further evidence for the

enhanced structure formation in the sheath component

when the sheath layer composition was as low as 10%,

which was discussed for the estimated tensile modulus of

the sheath component in Figure 5.

4045

40

35

30

25

20

5

15

10

Bire

frin

genc

e of

she

ath

com

pone

nt×1

000

010 20 30


20 13


10 8

Figure 10 Variation of birefringence of sheath layer of as-spun

high-isotacticity polypropylene (IPP)/low-isotacticity PP (LPP)

bicomponent fibers with sheath IPP content and sheath layer

composition. Birefringence of the sheath layer was estimated

assuming the constant birefringence in the core.

End

othe

rmal

dow

n

Temperature (°C)

-50 50 100 150 200 250

5 wt%

4 wt%

3 wt%

2 wt%

1 wt%

Sheath layercomposition

0

Figure 11 Differential scanning calorimetry (DSC) thermograms of

as-spun high-isotacticity polypropylene (IPP)/low-isotacticity PP

(LPP) bicomponent fibers of six different sheath IPP contents.

4014

13

12

11

9

10

Bire

frin

genc

e ×1

000

810 20 30


20 13


10 8

Figure 9 Variation of overall birefringence of as-spun high-isotacticity

polypropylene (IPP)/low-isotacticity PP (LPP) bicomponent fibers with

sheath IPP content and sheath layer composition measured under

polarizing microscope at the center of individual fiber.

It should be noted that assumption of the birefrin-

gence of the core to be similar to that for the single com-

ponent LPP fiber is reasonable because: (1) if the sheath

composition is only 10%, melt spinning behavior is gov-

erned by the core component; and (2) even if there is an

effect of the kinematic mutual interaction between the

sheath and core components, it leads to the reduction

of the birefringence of the core component, which yields

further increase of the estimated birefringence of the

sheath component.

DSC curves of as-spun fibers are shown in Figure 11 .

Melting peaks of the LPP and IPP components were

detected at 50 ° C – 80 ° C and at around 165 ° C, respectively.

The melting peak of LPP consisted of two peaks, a rela-

tively sharp peak at a lower temperature of around 50 ° C

and a relatively broad peak at a higher temperature of

around 75 ° C. Even though total IPP content was constant,

different behaviors were observed for the melting of LPP

depending on the LPP composition in the sheath. Particu-

larly, with the decrease of sheath layer composition, peak

height of the lower temperature peak tended to decrease

in comparison with the peak area of the higher tempera-

ture peak of 75 ° C.

The melting behavior of the as-spun fibers was ana-

lyzed through DSC. Figure 12 shows the variations of the

melting enthalpies of IPP and LPP as well as the total

melting enthalpy with the changes in the sheath layer

composition and the sheath IPP content. Even though

the total IPP content was kept constant, total melting



enthalpy for IPP increased by localizing the IPP near to

the fiber surface. By contrast, the melting enthalpy for LPP

decreased with the increase of sheath layer composition.

If constant crystallinity of pure LPP in the core is

assumed, this result indicates that the crystallization of

LPP was suppressed by blending IPP. This behavior is

similar to the one observed in a previous paper [23] . By

contrast, a slight increase of the IPP melting peak area

may indicate that a small amount of LPP molecules are

incorporated into the IPP crystals.

3.4 Mechanism of structure formation in sheath-core fibers

Gathering the experimental results stated above, the mech-

anism of fiber structure development in the bicomponent

melt spinning of the LPP and the blend of IPP and LPP

can be summarized as follows. Firstly, when the sheath

layer composition is 50 wt%, the spinning behavior and

structure formation in both sheath and core components

were considered to be similar to those for the single com-

ponent fiber of pure LPP. With the decrease of the sheath

layer composition, crystallinity and molecular orientation

in the sheath were expected to increase because of the

increased content of IPP. In addition, from the view point

of the kinematic mutual interaction of the sheath and

core components in the melt spinning process, structure

development of the sheath was expected to be enhanced

with the decrease in the sheath layer composition. The

reasons for such prediction are that: (1) the kinematic

mutual interaction of the sheath and core components

in the spinning process on the structure development is

more effective to the component with lower composition;

Sheath layer composition (wt%)

0

5

10

15

20

25

30

3540 20 13


Total melting enthalpy

IPP melting enthalpy

LPP melting enthalpy

10 8

10 20 30 40 50

Mel

ting

enth

alpy

H (

J/g)

Figure 12 Variations of heat of fusion of low-isotacticity polypro-

pylene (LPP), high-isotacticity PP (IPP) and total components of

as-spun IPP/LPP bicomponent fibers with sheath IPP content and

sheath layer composition.

and (2) the structure development of the component with

higher IPP content should be enhanced more effectively

because a larger difference in thinning behavior in the

single component melt spinning process in comparison

with that for the pure LPP can be expected because of its

higher crystallizability.

4 Conclusions In the bicomponent melt-spinning of sheath-core fibers,

LPP and the blend of LPP and IPP were used as the core and

sheath components, where IPP content in the sheath was

changed from 8 wt% to 40 wt% and the sheath/core com-

position was varied from 50/50 to 10/90. Even though the

total IPP content was kept constant at 4 wt%, the bicom-

ponent fibers with lower contraction ratio after spinning,

higher elastic recovery and slightly higher modulus and

strength were obtained by localizing IPP to the region near

the surface in the fiber cross-section. Structure analysis of

the as-spun fibers suggested that there was a suppression

of the crystallization of LPP by blending IPP, whereas the

molecular orientation and crystallization of the sheath

component was enhanced by localizing the IPP near to the

fiber surface because of the kinematic mutual interaction

of the sheath and core components in the bicomponent

melt spinning process.

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