Applied Che때S따，

Vo l. 12, No.2, November 2뼈， 285- 7)엉

Influence of thennal curing OD the properties of electrospuD polyca매osilane fiber

Joon-Pyo Jeun , Eun-Jung Kim, Dong-Kwon Seo, Young-Chang Nho, Phil-Hyun Kang*

Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute

Abstract

The effect of thermal curing on the properties of polycarbosilane fiber has been investigated.

Polycarbosilane fibers were fabricated using electrospinning method. Its structure was characterized by Fourier

transformed infrared spectroscopy (FT-IR). The thermal curing beha、 ior was investigated using FT-IR , gel

permeation chromatography (GPC) and thermal stability was characterized by thermogravimetric analysis

(TGA). TG data indicates that thermal stability was improved with increasing the curing time. The molecular

weight and molecular weight distribution were also increased as the thermal oxidation curing time increase

Introduction

Recently, formation of ceramics by the pyrolysis of organometallic polymers has begun to aUract

considerable attention since silicon carbide (SiC) fibers were synthesized from polycarbosilane (PCS).

Organometallic polymers have several advantages for ceramics. One typical example is shown as follows

Organometallic polymers can be easily formed as fibers or films. Then. by heating polymer fibers, ceramics

may be obtained in the original form. In this case, it is possible to have any desired form of ceramics,

impossible by traditional inorganic chemical processes. SiC fiber has been used as reinforcement in advanced

ceramic matrix composites, which are applied in the fields of aerospace, nuclear, and high-temperature

materials, such as gas-turbine engine. In ceramic fiber/ceramic matrix composite, ceramic fibers are demanded

to be fine and strong as possible as they could do, as finer ceramic fibers are more flexible so that it is easy

to make t“ 0- or three-dimensional fabrics. Moreover, it is considered that finer fiber shows higher tensile

strength.

In this study, we reported on the fabrication of PCS fibers as a precursor for SiC fiber and the effect of

thermal oxidation curing of pes fiber. The change of structure was characterized by FT-IR. And the change

of molecular weight and molecular weight distribution were observed by GPe analysis. The thermal properties

were investigated by TGA instrument

Ex perimental

pes (average molecular weight = 1906 g/mol, density = 1.1 g/cm 3

‘NIPUS, Japan) was used as a

precursor polymer SiC fiber. 1,2-Dichroloethane and toluene were used as a solvent“ ithout any further

purification, which were purchased from Aldrich , USA. PCS solution was prepared by dissolving a measured

amount of pes in 1,2-dichroloethane/toluene (50150 wt%). The concentration of the pes solution was 50

wt%. In the electrospinning process, a high electric potential was applied to a droplet of pes solution at the

tip (ID 0.36 mOl) of a syringe needle. The solution was then ejected through a syringe using a syringe flow

pump at feed rate of 0.003 mlimin. applying a voltage of 15 kV and tip-target distance of 120 mOl. The

285

)oon-Pyo )eun' Eun-)ung Kim' Dong-Kwon sea· Young-Chang Nho' Phil-Hyun Kang286

electronspun pes fiber was heated at 10°C/rain over a range of temperatures (190-200°C) in an air gas flow

of 100 ml!min

FT-IR spectra of the thermal cured pes fibers were analyzed using a Bruker Tensor 37 spectrometer

The surface morphology of pes fibers was observed using SE 1'.1 (J SM-6390, lEOL). Specimens “ ere coated

by gold sputtering for 5 min. The working distance of the SEM was 15 mm and the accelerating power was

20 kY. Thermal stability was investigated with a thermogravimetric analyzer (TGA). TGA was conducted 、，v ith

a TA instrument SOT Q600 at a heat rate of 10°C/min, from 50 to looooe ‘ The TGA samples were cut

into small pieces and were machined using a mechanical grinder to maintain sample weights between 9-11

mg. All TGA tests were run in nitrogen gas. The molecular 까 eight distributions were determ ined by gel

permeation chromatography (GPC) using Agilent 1100 series isocratic pump, PL series injector with 100 ul

loop and PLgel_mixed_B (300x7.5mmx2) columns in series (Polymer Laboratories, Shropshire). THF was used

as column eluent and the flow rate of eluent was I mlimin. The column temperature was controlled at 400e.

Results and discussion

The morphology of the pes fibers prepared by electrospinning method was examined by scanning

electron microscopy (SE1'.1). Fig. I shows SEM image of electrospun pes fiber. It can be seen that the fibers

appear to be uniform , smooth, and straight, with a ca. 18-23 urn outer diamete r.

silicon carbide fibe r. The

chemical structures of polycarbosilane by thermal oxidation curing are speculated upon in Fig. 2 by referring

to the polycarbosilane chemical structure mode l. IR spectra of electrospun PCS mat indicated three main

features centered around 2100 cm-I, 1250 cm'l and 2950 em-I. The strong signals of 2100 em-I were

indicated by Si-H stretching. Two other peaks centered at 1250 cm-I (Si-CH3 stretching) and 2950 em -I (C-H

stretching in Si-CH3 stretching) are attributed to the vibration mode for Si-CH3. IR spectra of pes mat

showed the loss of Si-H band, thus it is presumed that curing process results in the formation of Si-Si band

The curing process is necessaη to convert polycarbosilane fibers to

by thermal oxidation of Si-H, Si-CH3.

’’”3센센젠시낀μμ“시센，‘

이‘

‘‘--，‘””센μElι써‘

--control120 min290 min

.' ‘ 420 min600min

Fig. 2. FT-IR spectra of thermally cured pes

10002500 2000 1500

Wavenumber (cm ’)30003500

Fig. I. SEM image of electrospun PCS fibers

웅용화학， 제 12 권 제 2 호， 2뼈

Influence of thermal curing on the properties of electrospun polycar뼈ilane fiber 287

fibers.

The weight losses for the electrospun pes fibers and thermal cured pes fibers are reported in Fig ‘

3. The weight loss of electrospun pes fibers is veη 10“ up to 280eC. Then it reaches 23% at 800ee and

remains stable above this temperature. The weight loss for the thermal cured PCS fibers for 600 min is very

low up to 6000e and reaches only about 11% at 8000 e. This thermal stability is increased with increasing the

thermal curing time.

Table I. and Fig. 4 show the molecular weight distributions of various thermal curd pes fibers. The

electrospun pes had a 10、、 molecular weight ‘ i.e. Mn=I ,900 g/mol, I\1 w=6,289 g!mol, but aftcr thεrmal curing

for 120 min , Mnand Mw increased to 2‘ 192 and 8,774 gimol , respectively, and for 600 min these reached

2,485 and 20.953 gimol, respectively. The polydispercsity also increased to 4.00 and 8.43. respectively. The

GPC results ‘ which prove the curing of PCS. are in good agreement with FT-IR spectra, ‘.vhich show the

dehydrocoupling of Si-H led to the formation of Si-Si and Si-CH 3 bonds under thermal oxidation

10030。

~ 90se.r:;.2'

응80

m

m

i@1드

@〉--umt@α

--control_._..- 120min- 300min--_.. 420min•••• OCKl min

control

70o 200 400 600 800

Temperature (oC)1000 13 14 15 10 17 18 1Q 20

EhAion time (min)

Fig. 3. TGA diagram of thermally cured pes fibers. Fig. 4. GPe curves of thermally cured pes fibers.

Table I Characteristics of thermal cured pes fibers

Mn l)

GPC results

Mw 2) PDI3)

6,289 3.3 I

8,774 4.00

11 ,548 5.01

14,418 5.90

20,953 8.43

Thermal curing time

(min)

o 1,900

120 2,192

300 2,303

420 2,441

600 2,485

I) The number average molecular weight (gimo l)

~) The weight average molecular weight (g!mol)

3) Pl)ly dispersity index (M 까1M 띠

Applied Chemistry, Vol. 12, No.2, 2α%

288 Joon-Pyo Jeun' Eun-Jung Kim· Dong-Kwon Sea' Young-Chang Nho' Phil-Hyun Kang

Conclusion

The PCS fibers prepared using electrospinning method ‘,vere successfully crosslinked by thermal

oxidation reaction , The electrospun PCS fibers appear to be uniform. smooth with average fiber diameter

range from 18 to 23 um , FT-IR pro、 ed the gradual formation of Si-Si and Si-CH) bonds due to the

dehydrocoupling of Si-H under thermal oxidation. With increasing the thermal curing time. the thermal

stability of the pes fibers was improved ‘ and the molecular weight and molecular weight distribution were

also increased.

Acknowlegement

This present work was supported by the Nuclear R&D program from the Ministry of Education

Science and Technology, Korea

References

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polymer of polycarbosilane and polyvinylsilane, Radiation Physics and Chemistry, Volume 60(4-5) ‘ 483-487.

3. Kentaro Suzuya, Kaoru Shibata, Kiyohito Okamura, Kenji Suzuki, (1992), The polycarbosilane-to-SixC I-x

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Journal of the European Ceramic Society, Volume 12( I), 27-4 1.

5. Siobhiil1 Matthews ‘ Mohan J. Edirisinghe, Michael 1. Folkes, (1999), Effect of pre-pyrolysis heat treatment

on the preparation of silicon carbide from a polycarbosilane precursor, Ceram ics International, Volume

25( I), 49-60

웅용화학， 제 12 권 제 2 호， 2008