CHAPTER10shodhganga.inflibnet.ac.in/bitstream/10603/62297/16/16_chapter 10.pdfCHAPTER10 Chapter 10...

.. , thermoplastic toughened polyimide clay nanocomposites

CHAPTER 10

Chapter 10

Preparation and evaluation of thermo mechanicaland barrier properties of thermoplastic toughenedpolyimide clay nanocomposites

226

.. , thermoplastic toughened polyimide clay nanocomposites

10.1 Introduction

Chapter 10

The thermal stability of both the polymer and the organic-treated layered

silicate has a significant influence on the physical, barrier and flammability

performance of the resultant nanocomposite. The improved physical and material

properties expected from a polymer-clay nanocompositc will not be achieved if the

processing temperature is too high (2:250°C) or the processing temperature has the

long residence time under high shear conditions in an internal mixer. This is

because of the thermal degradation of organic modifiers like alkyl ammonium salts

which have an onset of degradation of around 200°C. Since many of the polymers

used in aerospace applications requires high processing temperatures, the full

potential of nanoclays in improving the mechanical, thermal and barrier properties

of the nanocomposites are not exploited because of the thermal degradation of the

conventional organic modifiers at the high processing temperature of the polymers.

Thus the low thermal stability of the alkyl ammonium salts used as organic

modifier in smectite type clays limits their suitability in polymer clay

nanocomposites where the matrix requires high processing temperatures (250

300°C).

Reports show that thermal stability of the organic modifiers can be

improved by replacing with hetero cyclic compounds like imidazolium and imide

type organic cations and phosphonium salts [1,2]. Liang et al [1] observed a

deintercalation of clay layers as well as reduction in mechanical properties for

polyimide clay nanocomposites having hexadecyl ammonium salts as clay

modifier, where as better mechanical properties and exfoliated morphology was

observed for imide modifiers such as N-[4-(4'- amino phenyl)]phenyl phthalimide

and N-[4-(4' -aminophenoxy)] phenyl phthalimide. It is reported that the

phosphonium exchanged montmorillonite provides better thermal stability than

ammonium exchanged clays [2].

In the present study, we attempt to modify the clay with thermally stable (2:

300°C) organic modifier, tetra phenyl phosphonium salt. The study further extends

for the preparation of polyimide clay nanocomposites using the above modified

clay and compared with polyimide clay nanocomposites derived from quaternary

227

... thermoplastic toughened polyimide clay nanocomposites Chapter 10

alkyl ammonium-modified clay (Cloisite 30B). The present chapter further

investigates the incorporation of a thermoplastic (PES) into the polyimide clay

nanocomposites.

10.2 Experimental

10.2.1 Materials

High purity tetra phenyl phosphonium bromide was used as recieved for

the modification of the clay. Cloisite Na (Southern clay product) sodium ion

modified montmorillonite, dool =I1.7Ao, CEC = 92.6 meq/lOOg clay, specific

gravity = 2.86 g/cc] and Cloisite 30B (Southern clay product) [methyl tallow (bis

2- hydroxy ethyl) quaternary ammonium modified montmorillonite, doOl =18.5Ao,

CEC = 95 meq/1 OOg clay, specific gravity=1.98g/cc] were dried under vacuum at

70°C before use. Dimethyl acetamide (DMAC) was distilled over phosphorus

pentoxide under vacuum and stored over 4A molecular sieves. Polyamic acid in

DMAC dispersion with 12.3 weight percent was obtained from industry and used

as such.

10.2.2 Organic modification of clay

The organophilic clay was prepared by a cationic exchange reaction

between the sodium cations of MMT clay (Cloisite Na) and Tetra phenyl

phosphonium cations of the tetra phenyl phosophonium bromide. The intercalating

agent used for the cation exchange was calculated by the equation 10.1 [3]

CEC X--xW(g)x2=-xlxl000 (10.1)100 M

wwhere CEC, cation exchange capacity

W, Weight of clay (g)

2, the excess amount of the intercalating agent to ensure the fully

intercalation as per CEC

X, amount of intercalating agent used

Mw, molecular weight 0 the intercalating agent

5g of montmorillonite clay having CEC of 92.5 milli equivalents/IOOg was stirred

with 500ml distilled water in a round bottom flask at 80°C for 2 hours by

mechanical stirring. A separate solution containing an excess amount of

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.. , thermoplastic toughened polyimide clay nanocomposites Chapter 10

intercalating agent (3.88g) in 50 ml of distilled water was added slowly to the

montmorrilonite suspension with vigorous stirring. The mixture was stirred for six

hours at 90°C followed by ultrasonication for 1 hour. The modified clay was

recovered from the suspension by filtration and the samples were repeatedly

washed till the absence of bromide ion. The modified clay was dried in vacuum at

90°C overnight.

10.2.3 Preparation of poly (amic acid) - clay dispersion

Poly amic acid clay dispersion was prepared by solution dispersion

technique. The modified clay dispersion was prepared in DMAC solvent by

stirring the mixture for 3 hours at 90°C followed by ultrasonication for 2 hours.

Then the Poly (amic acid) - DMAC dispersion and clay - DMAC dispersion (1, 3,

5 and 8 wt%) were mixed mechanically for six hours followed by 1 hour

ultrasonication at room temperature.

10.2.4 Preparation of polyimide clay film nanocomposites

Thin films were casted from both neat poly (amic acid) solution and poly

(amic acid) -organoclay (I, 3, 5 and 8 wt%) mixtures in DMAC. The solution or

the mixture was poured into clean, dry plate and cured at 60°C for 30 minutes,

110°C for 2 hours, 150°C for I hour and 300°C for 1hour under nitrogen

atmosphere to get polyimide and polyimide clay nanocomposite films.

10.2.5 Characterization

The modified clays were characterized by FTIR, XRD and TGA. The

nanocomposite films were characterized by XRD, TGA, Helium gas permeability

and tensile properties as per the details described in chapter-2.

10.3 Results and Discussion

10.3.1 FTIR spectrum of clays

FTlR spectra of sodium montmorillonite (Cloisite Na), quaternary alkyl

ammonium modified clay (Cloisite 30B) and tetra phenyl phosphonium modified

clay (TPPMC) are shown in Fig 10.1,10.2 &10.3 respectively. The FTIR spectrum

of sodium montmorillonite (Fig 10.1) showed the characteristic peaks at 1040 cm-1,

229

.. , thennoplastic toughened polyimide clay nanocomposites Chapter 10

3628 cm') and 600-400 cm'] due to Si-O stretching vibration, -OH stretching of the

lattice water/AI-O stretching and Si-O bending respectively.

The FTIR spectrum of Cloisite 30B (Fig 10.2) also possess the same peaks

as observed in Cloisite Na along with the additional characteristic peaks of the

quaternary alkyl ammonium modifier at 2927 cm'! , 2853 cm'! and 1470 cm'! due

to -CH3 stretching (asymmetric carbon atom), -CH2 stretching and -NH bending

vibrations respectively.

38.4Cloisite Na

35

30

25

%T 203618

15

10

5

3.0

4000.0 3000 2000

1639

an-I1500 1000

525

470

407.0

Fig 10.1 FTIR spectrum of the unmodified clay (Cloisite Na)41.5

35

30

253632

\I.T20

15

10

5

3.0

4000.0

2927

3000 2000cm-J

lSOO 1000

1046

522

463

500400.0

Fig 10.2 FTIR spectrum of the quaternary alkyl ammonium modified clay(Cloisite 30B)

In addition to the Characteristic peaks of Cloisite Na, the tetra phenyl

phosphonium modified clay (Fig 10.3) showed the characteristic peaks

230

... thennoplastic toughened polyimide clay nanocomposites Chapter 10

corresponding to the aromatic C=C stretching at 1637, skeletal vibrations of the

phenyl nucleus at 1441 cm-I and C-H bending of phenyl ring at 722 em-I. This

confirms the presence of modifier inside the clay.

48.648

47

46

4S

44

43

42%T

41

40

39

38

37

TPPMC

34323621

722 689

527459

36 1043

500400.01000150020003000

3S.0+- ---r- -----, -,-- ...,.- ............,

4000.0cm-I

Fig 10.3 FTIR spectrum ofthe tetra phenyl phosphonium modified clay (TPPMC)

10.3.2 X ray Diffraction of clays

a

18.3 AD

c, , , , , , , , ,0 5 10 15 20 25 30 35

29

Fig 10.4 X ray diffraction patterns of a) Cloisite Na b) TPPMC and c) Cloisite 30B

231


The X -ray diffraction curves of Cloisite Na, Cloisite 30B and TPPMC are

given in Fig 10.4. The dool spacing of Cloisite Na is found to be 12 AO (28 =7.38°).

It can be seen that the doo ! spacing of the Cloisite Na is increased to 18.3 A°

(28 =4.81°), 18.4 A° (28 =4.8°) by the incorporation of quaternary alkyl ammonium

modifier and the tetra phenyl phosphonium modifier respectively confirming the

intercalation of the organic modifier in between the clay layers.

10.3.3 Thermo gravimetric analysis of clays

Thermal stability of the clays was determined by thermo gravimetric

analysis and the thermograms of Cloisite 30B and TPPMC are shown in Fig 10.5.

From the Fig 10.5, it reveals that the onset of degradation for phosphonium

modified clay is observed at 310°C while for the ammonium-modified clay is at

210°C indicating higher thermal stability of the phosphorous compound. The

improved thermal stability of TPPMC is advantageous in using it as nanofiller in

matrices like polyimide where a high processing temperature is required.

2100C

100

95·····r.:;--3100C

90 - Cloisite 30B

~......··TPPMC.. 85.c

Cl'. 80~

75'.

70

65

o 200 400 600

Temperature (DC)

800 1000

Fig 10.5 Thermal stability of the TPPMC and Cloisite 30B

10.3.4 X-ray diffraction of polyimide clay nanocomposites and PES toughenedpolyimide clay nanocomposites

X ray diffraction pattern of polyimide clay nanocomposites and PES

toughened polyimide clay nanocomposites are shown in Fig 10.6. It is observed

232


that the diffraction peak of Cloisite 30B is shifted from 2 8= 4.81 ° (dool=18.3A0) to

28= 2.76°(dool = 31.9 A0) for polyimide clay nanocomposites. Interestingly, all

polyimide clay nanocomposites have shown an additional deintercalation peak

around 28 = 6.94°(dool = 12.7Ao) along with intercalated peak. This is attributed to

the thermal degradation of organic modifier during the imidization at 300°C as the

onset of the thermal degradation of Cloisite 308 is :s 21 O°C (Fig 10.5). As the clay

content increases from 3 to 5 wt%, the deintercalation peak (2 8= 6.9°, doo1 =12.8

A0) of the polyimide clay nanocomposites become more prominent. The d spacing

of the intercalated peak also reduced to 28 = 3.6°(dooJ = 24.8 AO). The addition of5

wt% PES to the polyimide clay nanocomposites having 3 wt% Cloisite 30B

showed d spacing value of 26.1 A° (28=3.4°), which is less than the d spacing value

of polyimide clay nanocomposites without PES. Similar deintercalation (dool =

12.54Ao, 28=7.04°) is also observed in the PES toughened polyimide clay

nanocomposites. As clay concentration increases from 3 to 5 wt% in the PES

toughened polyimide clay system, no appreciable change was observed in the

intercalation peak at 28= 3.5° (dool =25 AO) as well as in deintercalation peak at

28 = 6.9°(dool = 12.8 AO).

The X-ray diffraction curves of the polyimide clay nanocomposites having

thermally stable tetra phenyl phosponium modified clay (TPPMC) are shown in

Fig 10.7. Polyimide clay nanocomposite having 3 wt% TPPMC showed a d

spacing value of 31 A° (28=2.86°) indicating an intercalated morphology.

Interestingly, no deintercalated peak was observed in polyimide-TPPMC system,

indicating that organic modifier i.e., tetra phenyl phosphonium is thermally stable

at the processing temperature of polyimide (300°C). PES toughened polyimide

clay system was also not showing any deintercalated peak. The intercalated peak of

the PES toughened system is observed at 28=3.3°(dool=27 A0). As PES is

incorporated into polyimide clay system the viscosity of the matrix resin increases

and therefore the entry of the matrix resin into clay layers is restricted resulting in

the decrease of d spacing of the intercalation.

233

... thermoplastic toughened polyimide clay nanocomposites

~~

'OA. e12.5A a= Cloisite 30B

d b= Polyimide/3 wt% Cloisite 30B

~8A. c= Polyimide/S wt% Cloisite 30B

~2.8A. C ,d_=_P_O_IY_i_m_id_e_/S_wt_%_P_E_S_/3_wt'l_*,_C_'_Oi_si_te_3_0---,B_e= Polyimide/S wt% PES/S wt% Cloisite 30B

31.9A·

b

~ao 10 20 30 40

29

Chapter 10

Fig 10.6 XRD of the polyimide clay nanocomposites with respect to Cloisite 30B

~-~ .- - -------------,a= Cloisite 30Bb= Polyimide/3 wt% TPPMC

28.0AO c= Polyimide/5 wt% PES/3 wt% TPPMC

c

b

-_",",-i~ ao~~ro20-~3ii-~~

29

Fig 10.7 XRD of the polyimide clay nanocomposites with respect to TPPMC

234


10.3.5 Tensile properties of the polyimide clay nanocomposites

Tensile properties of the poly imide clay nanocomposites are given in Table

10.1. Tensile strength and % elongation at break are decreased with increase in

concentration of Cloisite 30B. This may be due to the agglomeration of

deintercalated clay arising from the thermal degradation of organic modifier,

thereby creating the defects. Similar trend is observed for polyimide clay

nanocomposites by Delozier et al [4] and Agag et al [5]. On the other hand, the

tensile strength of the poly imide-TPPMC nanocomposite is retained. This is

because of the absence of deintercalation. Even though, the addition of PES to the

polyimide marginally improved its tensile strength, it is decreased drastically with

the incorporation of both Cloisite 30B and TPPMC. Among the clays, TPPMC

based polyimide nanocomposite showed better tensile strength.

Tensile modulus is increased linearly as the clay concentration increased. A

71 % increase is observed in tensile modulus for 8 wt% Cloisite 30B-polyimide

nanocomposite compared to pristine polyimide. 3-wt% TPPMC incorporated

polyimide nanocomposite showed 56% improvement in tensile modulus against

the improvement of 20% for Cloisite 30B counterpart. This can also be attributed

to the thermal degradation of organic modifier of the later, which caused the

deintercalation of clay particles as observed in Fig 10.6. PES toughened polyimide

clay nanocomposites also showed similar trend in tensile modulus.

The area under the stress strain curve indic:1tes the toughness of the

materials. Fig 10.8 and Fig 10.9 represents the stress strain curves of polyimide

clay nanocomposites derived from Cloisite 30B and TPPMC respectively. The area

under the stress strain curve for neat polyimide is found to be higher, compared to

filled polyimide indicating higher toughness. As the concentration of Cloisite 30B

increases, the toughness decreases drastically, because of the thermal degradation

of organic modifier, which caused the deintercalation of clay particles, which in

tum decreased the surface area of interaction between clay and polymer. This is

also evident from the decrease in the area of stress strain curve (Fig 10.8). It is

interesting to note that the addition of PES to the polyimide has increased the area

under stress strain curve compared to poly imide clay nanocomposites. This may be

235


due to the plasticization effect of PES polymer. Similar improvement in toughness

was observed by the addition of TPPMC to the polyimide. This improvement is

attributed to the absence of deintercalation due to the high thermal stability of the

organic modifier, which enhances the interfacial interaction between clay particles

and polymer.

Table 10.1 Mechanical and barrier properties of the polyimide claynanocomposites

Composition Tensile Tensile %Elongation Helium Leakstrength Modulus rate(MPa) (OPa) (IO-6m bar

lit/sec/cm2)

Polyimide 104±15 2.5±0.3 31±4 14±2Polyimide/ 1 wt% cloisite 30B 92±8 2.8±0.2 17±7 IO±3Polyimide/ 3 wt% cloisite 30B 78±9 3.0±0.1 7±3 7±2Polyimide/ 5 wt% cloisite 30B 76±8 3.7±0.2 6±3 4.6±1Polyimide/8 wt% cloisite 30B 69±10 4.2±OJ 4±2 2.6±2

Polyimide/ 5 wt% PES 109±16 3.2±0.2 11±4 21±5Polyimide/ 5 wt% PES / 3 wt% 67±10 3.5±OJ 6±3 8.5±2

cloisite 30BPolyimide/ 5 wt% PES / 5 wt% 62±9 4.2±0.3 3±2 5±2

cloisite 30BPolyimide/3 wt% TPPMC 108±15 3.9±0.2 IO±4 1O±3

Polyimide/ 5 wt% PES / 3 wt% 77±10 3.1±0.4 9±4 13.4±3TPPMC

12

10

8Ul

i 6~'iiis::::

~ 4

2

0

......

-PI-------. PI/1 wt"10 Cloisite 30B........... Plt3 wt% Cloisite 30B----- PitS wt% Cloisite 30B--- Plt8 wt% Cloisit~ 30B--- PitS wt"10 PES-- PI/S wt% PESt3 wt% Cloisite 30B-- PitS wt"10 PEStS wt"10 Cloisite 30B

o 5 10 15 20 25

Tensile strain (%)

30 35

Fig 10.8 Stress strain curves of Cloisite 30B incorporated polyimide claynanocomposites

236

.. , thermoplastic toughened polyimide clay nanocomposites Chapter 10

12

10

8IIIIIIQ)

~ 6~'iiic:Q)

4I-

2

0

--PI-------. PII3 wt% TPPMC............ PII5 wt% PES------ PI/5 wt% PES/3 wt% TPPMC

o 5 10 15 20 25 30 35

Tensile strain

Fig 10.9 Stress strain curves of TPPMC incorporated polyimide claynanocomposites

10.3.6 Helium gas permeability of polyimide clay nanocomposites

Helium gas permeability of the polyimide clay nanocomposites is given in

Table 10.1. In the case of Cloisite 30B system, the gas permeability decreases

dramatically with the increase in clay content. The gas permeability was decreased

to 82% by the addition of 8wt% Cloisite 30B to polyimide. The enhanced barrier

properties of polymer nanocomposites compared to conventional composite may

be explained by the labyrinth or tortuous pathway model (Fig10.10). When a film

of polymer nanocomposite is formed, the sheet - like clay layers orient in parallel

with the film surface, As a result, gas molecules have to take a long way around

the impermeable clay layers in polymer nanocomposites than in pristine polymer

matrix when they traverse an equivalent film thickness. Yano et al [6] reported that

the water permeability of the polyimide nanocomposites was reduced by the

incorporation of organoclays having different aspect ratio.

The helium gas permeability was increased to 50% by the addition of 5 wt%

PES to the neat polyimide, indicating the amorphous nature of thermoplastic.

237


However the helium gas permeability is decreased by the addition of Cloisite 30B

to the PES toughened polyimide system. TPPMC modified polyimide system also

showed similar trend in gas permeability as shown by Cloisite 30B counterparts.

Conventional composites .. Tortuous patb" in polymerclay nanocomposites

Fig 10.10 Labyrinth or tortuous pathway model

10.3.7 Differential Scanning Calorimetry

The glass transition temperature (Tg) of the polyimide clay nanocomposites

is determined by using DSC and the values are given in Table 10.2. The Tg of the

polyimide was decreased from 427°C to 403°C by the addition of 5 wt% PES.

Similarly, Tg of the polyimide was decreased to 410°C and 406°C with the addition

of 3 wt% Cloisiite 30B and 3 wt% TPPMC respectively. The decrease in Tg of the

polyimide by the addition of PES,Cloisite 30B and TPPMC may be attributed to

the plasticization effect of PES and organic modifier present in the clay layers.

Among the polyimide clay nanocomposites, the decrease in Tg is more pronounced

in Polyimide- TPPMC system compared to Polyimide-Cloisite 30B due to the

presence of thermally stable (>300°C) organic modifier. Similar trend was

observed in the PES toughened polyimide clay nanocomposites. Liang et al [1]

also observed a decrease in Tg of polyimide clay nanocomposites with highly

thermally stable organoclay. Though the Tg of polyimide decreases considerably

by the addition of PES compared to that of the organoclay addition, the hybrid

system containing both PES and organoclay in polyimide shows much lower in Tg.

This may be due to the combined effect of plasticization of toughened PES as well

as the plasticization effect of organic modifier present in the clay layers.

238


Table 10.2 DSC values of polyimide clay nanocomposites

Composition Tg by DSC (0C)

Polyimide 427

Polyimide/ 5 wfllo PES 403

Polyimide/ 3 wt% c10isite 30B 410

Polyimide/3 wt% TPPMC 406

Polyimide/ 5 wt% PES/ 3 wt% c10isite 30B 395

Polyimide/ 5 wt% PES/ 3 wt% TPPMC 391

10.3.8 Thermal stability ofthe polyimide clay nanocomposites

-PI------ PI/S wt% PES.......... PI/3 wt% Cloisite 30B

---- PI/3 wt% TPPMC-- PI/S wt% PES/3 wt% Cloisite 30B- PI/S wt% PES/3 wt% TPPMC

100

90

;eo800

~.s:Cl

~70

80

500 200 400 600 800 1000

Temperature ( 0C)

Fig 10.11 Thermal stability of the polyimide clay nanocomposites

A typical thermogram of polyimide and polyimide clay nanocomposite in

air is given in Fig 10.11. The onset of thermal degradation is found to decrease

with the addition of PES. On the other hand all polyimide clay nanocomposites

showed an increase in the onset of thermal degradation. This may be due to the

homogeneous dispersion of clay layers all through the matrix and prevents the

permeability of volatile products derived from the degradation of aromatic

polyimide. It is further noticed that nanocomposite film show a delayed

decomposition pattern compared to the pristine polyimide all through the

239

... thennoplastic toughened polyimide clay nanocompo ite hapeer 10

temperature regime which further confinns that silicat layers obstructs the free

passage of volatile gases from the film. A similar trend is observed by Agag et al in

the case ofBPDA/PDA- clay nanocomposites [5].

10.3.9 Optical transparency

Photograph of the polyimide and its nanocomposites were taken to investigate

the optical transparency. The photographs are shown in Fig 10. 12.0ptical transparency

of the nanocomposites is not changed with the addition of 1 wt% clay to the

polyimide. At higher clay concentration, the optical transparency is faded slightly.

However, the optical transparency is retained even for higher clay concentration unlike

conventional polymer microcomposites. The retention of optical transparency is due to

the nanolevel distribution of clay particles. Another reason is that the dimensions of

the clay platelets are less than the wavelength of the light and allows the passage of

light without any hindrance. It is also clear that the darkening of polyimide

nanocomposite films due to the thennal degradation of organic modifier in the case of

Cloisite 30B system is reduced by the use of phosphonium modified clay.

Polyimide

Polyimide / Cloisite 30B (3 wt%)




Polyimide / PES (5 wt%)

-Polyimide / PES (5 wt%) / Cloisite 30B (3 wt%) Polyimide / PES (5 wt%) / Cloisite 30B (5 wt%)

Polyimide / TPPMC (3 wt%) Polyimide / PES (5 wt%) TPPMC (3 wt%)

Fig 10.12 Photographs showing the optical transparency of polyimide - claynanocomposites

240

... thennoplastic toughened polyimide clay nanocomposites

10.4 Conclusions

Chapter 10

Thermally stable (~ 300°C) tetra phenyl phosphonium modified clay was

synthesized and characterized. Polyimide clay nanocomposite films were

processed by the incorporation of alkyl ammonium modified clay (Cloisite 30B)

and tetra phenyl phosphonium modified clay. Mechanical, thermal and barrier

properties of polyimide clay nanocomposites are compared with neatpolyimide

film. Similarly PES toughened polyimide clay nanocomposite films were

processed and their properties are compared with neat polyimide film. It is found

that tetra phenyl phosphonium modified polyimide clay nanocomposites possess

better toughness and mechanical properties compared to the alkyl ammonium

modified polyimide clay nanocomposites due to the higher thermal stability of

former organic modifier. However, no appreciable change in helium gas

permeability is observed between tetra phenyl phosphonium and alkyl ammonium

modified clays. As clay concentration increases the helium gas permeability

decreases irrespective of the clay used. On the other hand PES addition increases

the helium gas permeability. The Tg of the polyimide system decreases by the

addition of PES as well as clay particles, where in more decrease is observed for

tetra phenyl phosohonium modified system. TOA curves reveal marginal

improvement in thermal stability for clay reinforced polyimide clay

nanocomposites. The optical transparency is retained even for higher clay

concentration.

10.5 References

1. Liang Z-M, Yin J, Xu H-J. Polymer 44, 1391 (2003)

2. Xie W, Xie R, Pan W-P, Hunter D, Koene B, Tan L-S, Vaia R. Chern Mater

14, 4837 (2002)

3. Narkhede JS, Shertukde VV, J Polym Mater 25,1-14 (2008)

4. Delozier OM, Orwoll RA, Cahoon JF, Johnston NJ, Smith Jr. JO, Connell

JW. Polymer 43, 813 (2002)

5. Agag T, Koga T, Takeichi T. Polymer 42,3399 (2001)6. Yano K, Usuki A, Okada A. J Polym Sci Part A: Polym Chem 35,2289

(1997)

241

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