Indian Journal of Chemical Technology Vol. 8, May 2001, pp. 181- 185

Mechanical and electrical properties of polyimide blend films

Sobhan Niyogi, Sukumar Maiti & Basudam Adhikari*

Materials Science Centre, Indian institute of Technology, Kharagpur 721 302, India

Received 17 November /999; accepted 28 November 2000

The preparation and characterization o f two novel polyimide, one from pyromellitic dianhydride (PMDA)-4,4'­oxydianiline (ODA) and the other from 3,3', 4,4'-benzophenone tetracarboxylic dianhydride (BTDA)-ODA, blend films with nylon 6 were repo rted earlier1.2. PMDA-ODA and nylon 6 blend fi lms have shown up to 14.7 % increase in Young's modulus and 25 % increase in tensile strength at lower elongation over those of control films, whereas the same films have shown around 4.6 % decrease in ultimate tensile strength (except the film PB 2 which shows about I % increase in ultimate tensile strength). BTDA-ODA and nylon 6 blend films have shown 19 % higher ultimate tensile strength as well as 42 % higher Young 's modulus and 47 % higher tensile strength at lower elongation. But in general the dielectric constant of the blend films (both BTDA and PMDA based) measured at 23°, 100° and 200°C and at 1,5 and 10kHz frequency increases with increase in the perce ntage of nylon 6 in the blend and the breakdown voltage decreases with the increase in nylon 6 content in the blend films.

Polyimides are a class of high performance polymers having very good mechanical strength and electrical insulation characteristics, high thermal stabi lity and chemical resistant properties3

.4 . Having these properties polyimides are used in aerospace and microelectronics industries5

•7

• Many new polyimides have been synthesized to overcome their processability problems. Also many blends of polyimides have been prepared8

-12

. But in these blends the reports on mi scibility studies predominate rather than their property evaluation even in the miscible bl ends.

In earlier communications'·2 the preparation and characterization of cost effective blends from two different base polyimides with nylon 6 was reported. In this paper the mechanical and electrical properties of the same blend fi lms are discussed .

Experimental Procedure

Materials The materials, which were used for this study are

as follows: I ,2,4,5-benzene tetracarboxylic dianhydride or commonly known as pyromellitic dianhydride (PMDA), 4,4'-oxydianiline (ODA) { Fluka, Switzerland}, 3,3' ,4,4' -benzophenone tetracarboxylic dianhydride (BTDA) (Gulf Oil Corp. USA), £­caprolactam, magnesium sulphate, calci um ch loride, acetic anhydride, diphosphorus pentoxide, DMF. All

*For correspondence (Fax : 091-03222-83966; E-mai 1: ba @ matsc. !itkgp.ernet. in)

the chemicals were of AR grade and they were purified by following standard methods 13

.

Method of preparation of film At first two different poly(amic acids) (PAA) were

synthesized from PMDA-ODA and BTDA-ODA in DMF following the standard procedure 14

• In this investigation four different compositions of blend films were prepared from each poly (amic acid). In each case of blend preparation a certain amount of the £-caprolactam (depending on the solid PAA in the solution of PAA) and poly(amic acid) solution were weighed accurately in sequence in a weighing bottle. Then they were mixed thoroughly for about 30 min to get a homogeneous mixture. From that homogeneous mixture films were cast on clean glass plates. At first these sticky films were dried under vacuum and then

Scheme I. Scheme for Preparation of Blend Films

Scheme ! -Scheme for preparation of blend films

182 INDIAN J. CHEM . TEC HNOL., MAY 200 1

120 P[lJ .. ......... .

······· ···· ... ··

100 100

80 PB I :. , .

U) : ...

~ 60 : .. ~ :'/ ..

60

40 .•i 40

20

10 15 20 25 30 35 40 45 Strain(%)

Fig. I - Stress versus St rai n Behavior of PMDA based fi lms

160 .-----------------------~D~B~.--~16

140. BB2 DB I

· . . . -·· DBO 14

120 12

- 100 10 c.': 6 80 80 ~

g 60 U) 60

40 40

20 20

0 ¥-----~----~----~----~-----+0 0 4 8 10

Strain(%)

Fi g. 2-Strcss versus Strain Behavior o f BTDA based film s

therma lly cyclodehydrated to get a blend film in each case and the complete scheme for this preparation has been shown in Scheme 1 and the blend formulat ion is presented in the Table 1.

Sample preparation for tensile prope11y measurements Tensile properties were evaluated according to

ASTM D 882-9 1 using an Instron Tensile Tester 4302 (U niversal Tensile Testing Machine) and average of three samples were taken. All the experiments were carried out at a jaw speed of 50 mm/min and film size was I Ox 100 mm. Th ickness of the films (75-1 00 J.lm) were measured at 6-7 places and the average thickness [using a Mitutoyo (Japan) Thickness Gauge] was taken as the film thickness. After thi s all the films were condit ioned in an air circulating oven fo r 48 h at 50±3°C. Before testing the film sample was taken out from oven and allowed to attain room temperatu re by keeping in a desiccator.

Sample preparation for dielectric constant and break­down voltage measurement

In order to measure the capacitance of the films, they were coated with si lver paint in a circular fashion

Table ! -Blend formulat ion

Blend Code BTDA/(PMDA): ODA Wt % of E-caprolactam w.r. L polyimide

BB 0/(PB 0) 1:1 0 % BB 1/(P~ 1) 1:1 11.43 % BB 2/(PB 2) 1: 1 20.52 % BB 3/(PB 3) 1: 1 28.52 % BB 4/(PB 4) 1: 1 34.43 %

PB 0-PB 4 are PMDA-ODA based polyi mi e and BB 0-BB 4 are BTDA-ODA based polyimide

in the following way: A cleaned rectangular aluminium foil of size about 8x4 em was folded to make a square of about 4 em square. The folding li ne was creased properly . A circular hole was made at the middle of the folded aluminium square by punching with a sharp cork borer. The fi lm to be taken for testing was then sandwiched into the fo lded aluminium square to cover the circular hole of the folded square. The film in between the foils was clamped and then colloidal si lver paste was coated on both the surfaces of the film exposed at the circular hole to ensure good contact with the condenser plates. The system was then dried under vacuum, the film was taken out from the folded aluminium square and the sil ver coated circular disc thus obtai ned on the fi lm was used for electrical measurement.

The capacitance and dissipation factor of the fi lm samples were measured using a GR-1620 AP capacitance measuring assembly at th ree frequencies of I ,5 and I 0 kHz at 23, at 100 and at 200°C. From the capacitance (C) and di ssipation factor va lues (D), the dielectric constant (E) and loss fac tor (tan 8) have been calculated using follow ing mathematical relations 15

:

Dielectric constant (E) Self-capacitance (Co) of the fil ms was calculated

from the relation :

Co=AI4xd ... (I)

where A is the area of the sil ver coated circular film in square em and d, its thi ckness in em. As the measure­ments were clone on circular film s in air, Eq. ( I) becomes

C0=0.089xarealthickness (2)

in picofaraday (pf). The dielectric constant (E) of the film under exami nation was then calcul ated using the formul a,

E=C!Co .. .. (3)

where C is the capacitance of the specimen fi lm obtained directly from the measuring instrument.

NIYOGI et a/. : POL YIMIDE BLEND FILMS 183

3.0 r-;:=-;::;n;;-;;-:-::-:-------- ----., ~ (a) PD Series

2. 7 CJ (b) BB Series

2.4 BB I

-;;- 2.1 "-s 1.8 ~

:::: "3 1.5 -g ~ 1.2

BB 3

3.0

2.7

2.4

2.1

1.8

1.5

1.2

0.9

0.6

0.3

0.0

Fig. 3-Young's Mod ulus for (a) PMDA (PB Series) and (b) BTDA (BB Series) based films at 1-4 % elongation

140 ~ (a) PB Series 140 BB4

120 0 (b) BB Series

120

100 100

-;;-"-6 80 80 ~ ~

~ 60 60 til

40 40

20 20

Fig. 4-(a) Stress at 20 % elongati on for PMDA based film (PB Series) (b) Stress at 6 % elongati on for BTDA based film s

Loss Factor (tan 8)-The loss factor, tan 8, was calculated by multiplying di ss ipation facto r (D), obtained directly from the instrument, by the respective frequency (f) in kHz i.e.,

tan D=D xf (4)

Volu111e Resisti vity (R)-As the vo lume resi sti vity of most of the polyimides is very high, their ac vo lume res isti vity value was calculated usi ng the formula 16

log R= 19-2 (E-2) (5)

where R is volume resistivity and E IS dielectric constant.

Breakdown Voltage-Breakdown voltage of all the test films was measured usi ng a Siemens Instrument at 53-55 Hz and increase of vo ltage during measurement was 500 v/s up to the breakdown voltage as per ASTM D- 149-94. Breakdown voltage was directly obtained from the instrument. Average of two specimen data was taken . Flat circular films were

Table 2-Toughness of PMDA and BTDA based poly imide fi lms

Type of Films Toughness Type of Films Toughness (Mpa) (Mpa)

PB 0 30.63 BB 0 8.97 PB I 42.2 1 BB I 7.71 PB 2 35.16 BB 2 6.84 PB 3 40.91 BB 3 4.7 1 PB 4 40.99 884 8. 13

placed in air between 4 mm diameter brass electrodes with 2 em edge radius for the experiment and maximum ambient relative humidity was 89%.

Results and Discussion

Tensile properly measurement Figures 1 and 2 show the stress strain behav iour of

the PMDA and BTDA based blend films . From Fig. I it is observed that the PMDA based bl end films show sli ght decrease in tensil e strength but an increase in elongation at break. But in case of BTDA based blend films tensile strength increases compared to the control film but a decrease in elongation at break is observed.

From Fi gs I and 2 it is found that the tensile strength of the BTDA based films is higher compared to that of PMDA based films , but the elongation at break (EB) which is a very much desired property of thi s type of polyimide films is sufficiently lower for BTDA based films compared to PMDA based film s. In the two systems studied the chemical structures of the dianhydrides are different (Scheme I ). The benzophenone carbonyl in BTDA exerts additional intermolecular interactions, which determines the low elongation at break. Thi s very strong intermolecular interaction in BTDA based syste m studied prevents polymer chain slippage and as a result elongation is lower. This strong intermolecul ar interaction originates from the interaction between the electron rich '0' of ODA moiety and electron deficient benzophenone carbonyl moiety in BTDA 17

. Thi s strong interaction is al so reflected in other properties, mainly high chemical stability of BTDA based film 18

.

But in PMDA based fi lm the above menti oned interaction is absent and as a result molecular chain slippage during tensile testing provides hi gher elongation at break ( -250-500 % higher) compared to BTDA based films. The blend films of PMDA based system show some decrease in tensile strength property but the elongation at break are higher or equivalent to that of the control film . This decrease in tensile strength may be due to the presence of nylon 6

184 INDIAN J. C HEM . TECHNOL. , MAY 2001

4.0.,.-----------------,-4.0

3.9

3.8

;g 3.7 $l

g 3.6 u f 3.5 l)

~ 3.4 Ci

3.3

3.2

PBO PB l PB 2

1- At I kllz andat 2JOC

2 --e- At S kHz and at 2JO C

.l-&- At 10 kHz and at23° C

3.9

3.8

3.7

3.6

3.5

3.4 4- • - Atlk.Hz andat iOOOC

5 - -o- At S kHz and at 1000 C 3.3

6 --a- AtlO kHzand at IQOO C

1 ... 1J. . All kHz andat 2000 C 3 .2

8 ·- -6 ·· At S kHz and at 2000C

9 -- • ·· AI IOkH7. ando! 2000C 3·1

PB3 PB 4

Fig. 5 - Change o f die lectric constant with temperature and fi eld frequency for the PMDA based blend film s

4.0 1- All kHz and at 2JOC

2 --e- At S k: llz and at 230 C

J ---8- At 10kHz and at 230 C

3.8 4- • - At I kll.z andat i OOO C

5 • .() - AI S kH z and at 1000 C

6 -<t - At iOk.Hzandati OOO C

7 ... ,. . At \ kHz andat2000 C

8 -· ·A . At5kHz andat2000 C

4.0

3.8

3.6

3.4

3.2

3.0 '-r-----,-11-----,-----....----.-.J 3.0

BB 0 BB l BB 2 BB 3 BB 4

Fig. 6--Change o f dielectric constan t wi th temperature and fi eld Crcquency for the BTDA based bl end film s

(from E-caprolactam) with lower tensile properties but hi gher elongation at break value. Another facto r which might be influencing the tensile property is the better cyclodehydration (or higher molecular weight) of the base polyimide in presence of the E­

caprolactam at the initial stages of the cyclodehydration. But in case of BTDA based blend films the incorporation of the nylon 6 polymer does not decrease the tensile strength rather slight increase is observed, but a little decrease in elongation at break is also there. So it may be concluded that the presence of the nylon 6 in the system produces a stiffer blend system and this stiffness in structure is responsible for the properties as observed.

From Figs 1 and 2 it is found that the slopes of the stress versus strain curves are steeper for all the blend films (except PB 3, BB 3 and BB 4) and overall the slopes are higher in BTDA based films compared to the PMDA based films. This may be due to various

Table 3-Breakdown voltage of all the films

Type of film Breakdown Type of Breakdown Voltage Film Voltage

(kV/mm) (kV/mm)

PB 0 145 BB 0 136.4 PB I 11 6 BB I 133.3 PB 2 73 BB 2 125 PB 3 67 BB 3 69 PB4 65 BB4 7 1

reasons like favoured orientation or crystallization during preparation of films or the higher interchain attractions 19•

Young's moduli for all the blend films (except BB3 and BB4) at low strain (0.0 1-0.04) are higher compared to the respective contro l films. The Young's modulus is shown in Fig. 3 for PMDA and BTDA based systems.

Again the modulus of the PMDA films at 20 % elongation and modulus of BTDA films at 6 % elongation shows (Fig. 4) that all the bl end films have got higher modulus. The toughness of all the films were calculated from the area under the stress-strain curve and are presented in Table 2.

It is observed from Table 2 that the toughness of the PMDA based blend films is higher as compared to that of the control film, whereas the reverse trend is observed in case of the BTDA based blend films. It is well known that nylon 6 is a tough materi al20

, so when the amount of nylon 6 in the PMDA based blend films increases toughness increases but why the BTDA based blend film s behave in the opposite manner is not very clear. It may be due to the completely different type of interaction of nylon 6 with the base polyimide.

Dielectric constant, dissipation factor and volume resistivity

The dielectric constants of all the PMDA and BTDA based blends and control films and their vanattons with temperature and frequency are presented in Figs 5 and 6 respectively.

It is observed that the dielectric constants decrease with increase in the frequency from 1 to 10kHz. This may be explained in the following manner: at low frequencies the alternating accumulation of charges at the interfaces between different phases of the polymer system takes place but at higher frequencies the accumulation of charges is caused by the polari zati on with dipole orientation21

•

The Fig. 5 shows an overall decreasing trend of E

with increase in frequency as well as temperature.

NIYOGI eta/.: -rOLYIMIDE BLEND FILMS 185

However this trend in E variation is not followed in PB 2 and PB 4 films at 100 °C for some unknown reasons . The BTDA based blend fi lms also behave in a similar fashion as observed for the PMDA based blend except for the film BB 1, which shows lower dielectric constant values compared to the control film (BB 0). The reason behind this type of behaviour is not clear.

Also the hi gher dielectric constant values of the blend films may be due to the lower free volume in these blends as evidenced from higher densit/·2 also attributed higher dielectric constant values to the lower free volume22

.

The dissipation factors vary from 0.0003 to 0.0230 depending on the compos1t1on of the films, temperature and frequency.

Volume Resistivity of all the films at various temperature and frequencies were calculated from Eq. (5) and it was in the order of 10 16.

Breakdown voltage or dielectric strength The breakdown voltage for all the films is

presented in Table 3. From Table 3 it is observed that for the PMDA

based blends the value of breakdown voltage decreases with the decrease in the percentage of base poly imide. This is due to the fact that when the percentage of polyimide decreases and substituted by nylon 6 the energy required for bond rupture and generation of ionic species becomes easier at lower voltages in an aliphatic system like nylon 6. Ultimately these ionic species becomes the carrier of current and rupture of film takes place at a lower voltage. For the BTDA based films also the aforementioned reason is applicable except the film BB 3.

Acknowledgement One of the authors Sobhan Niyogi sincerely thanks

CSIR, New Delhi for awarding research fellowship and M/s. Birla Tyres Ltd., Balasore, India for

providing lnstron facility and Electrical Engineering Department of this Institute for providing electrical properties measurement.

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