Indian Journal of Fibre & Textile Research Vol. 24, March 1999, pp. 1-9

Polyacrylonitrile, an unusual linear homopolymer with tWo glass transitions

Z Bashir

BeTIevue, 513 Prasanth Nagar, Uloor, Thiruvananthapurarn 695 0 II ,India

Received 24 October 1997; revised received 8 May 1998; accepted 4 June 1998

The glass transition of polyacrylonitrile (PAN), unlike that of many amorphous and semi-crystalline polymers, ' is a source of considerable puzzlement. By DMA, it has been shown that there are two tan 8 peaks at - 100°C and -150°C in unoriented PAN, while in oriented fibres, there is only a single peak at -100°C. Some authors take the peak at - 100°C while others assign the peak at - 150°C to the glass-rubber transition of PAN. The view taken here is that as the two DMA peaks are of similar intensity, they represent two glass transitions. Hitherto, by DSC, these two glass transitions have not been unequivocally recognised, mainly because the endothermic peaks arising from enthalpy relaxation mask the true natu're of the transitions. In this work, the case for two DSC glass transitions in unoriented PAN is presented. In oriented PAN, however, a single DSC glass transition is observed at - 100°C. Thus, the results from the DSC, which is a less sensitive technique for measuring glass transitions, mirror the DMA findings. Whereas two Tgs can be expected in an immiscible blend of two amorphous polymers, or in a block copolymer with two non-crystallisable units, it is more remarkable 10. find this in a linear homopolymer. The two Tgs in unoriented PAN arise from laterally ordered and amorphous domains, both of which are in a glassy state at ambient. The single transition in oriented PAN is not the conventional glass-rubber transition found in other amorphous or semi-crystalline polymers. It represents a transition from a laterally-ordered glassy state to a more mobile but still laterally-ordered state.

Keywords: Differential scanning calorimetry, Dynamic mechanical analysis, Glass transition, Oriented polymer, Polyacrylonitrile, Unoriented polymer

1 Introduction The thermal behaviour of PAN below 200°C, and

in particular the glass transition temperature, has been an area of confusion. Unlike a truly amorphous polymer such as atactic polystyrene or a semi­crystalline polymer such as polyethylene tereph­thalate (PET), there has been conflicting evidence as well as discordant interpretation of the glass transition in this polymer. This can be guaged by the fact that the values of the Tg of PAN, cited in the literature l

-8

, range from 39°C to 180°C. In the early literature (1940s-1960s), several

methods were employed to estimate the Tg of PAN. Kolb and Izard3 measured the change in specific volume with temperature, of a set of condensation and vinyl polymers. For PAN, they listed a Tg of 8TC. Beevers4 determined the Tg of PAN by plotting the isotropic refractive index against temperature and founa it to lie in the range of 95 .5-1 02.5°C, the value having a dependence on molecular weight. Keavney and Eberlin5 used differential thermal analysis (DTA) to investigate the glass transition of PAN. They established that the Tg rose from 56°C for very low

molecular weight polymer (Mw=1500-2000) and levelled off at about 80°C for Mw values between 75,000-100,000. Keavney and Eberlin recognised that there was something unusual in PAN in that the Tg was not as clear-cut as it was with most polymers5.

Other indirect approaches to find the T .. of PAN have also been employed. Wiley and Brauer6

determined the Tg of a series of butadiene­acrylonitrile copolymers and by a linear extrapolation of the Tg-compostion data, they arrived at a value of 52°C for the Tg of pure polyacrylonitrile. On the other hand, Gerke7 determined the Tg of butadiene­acrylonitrile copolymers and found an extrapolated value of 130°C for pure PAN. Krigbaum and Tokita8

established a Tg of 104°C by linear extrapolation of the measured Tg values of PAN-dimethyl formamide and PAN-gamma butyrolactone solutions. Loshaek and Fox2 used an equation based on the additivity of specific, occupied and free volumes in copolymers to extrapolate the literature data for acrylonitrile­butadiene copolymers and from this they arrived at a figure of 180°C for the Tg of pure PAN. Howard9

measured the Tg of moulded discs of "semi-

2 INDIAN 1. FIBRE TEXT. RES., MARCH 1999

crystalline" PAN by monitoring the linear expansion coefficient and found a value of 87°C. Howard also measured the Tgs of acrylonitrile-vinyl acetate copolymers by the same method, and after analysing the data using the equations derived by Loshaek and Fox2

, concluded that the Tg of completely amorphous pure PAN should be no greater than 110°C.

Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) are the most common methods of measuring the glass transition in polY!llers nowadays. In the 1960-1970"s, several investigators ' O- ' 2 had established by DMA the existence of two major damping peaks at -90-11O°C and 140-170°C in unoriented PAN (the exact temperature depends on factors such as testing frequency and residual solvent content; in this paper, for convenience, the two DMA transitions will be taken as occurring at 100°C and 150°C). Inter­pretation of the two damping peaks has, however, not been unanimous as some authors take the lower, while others take the higher temperature transition as the T/!,.

Likewise, different results have also been obtained fro m the DSC: some investigators 13 found a step-like change at JOO°C and an endothermic peak at 155°C, others 14 report two endothermic peaks at 102°C and 156°C which they thought were second order transi tions, while still others 'S-' 6 report a single Tg near 100°C.

A related controversy is whether PAN can be described by a two-phase model (such as the cryst(jlline-amorphous one) commonly used to visualise semi-crystalline polymers. One view is that of Bohn et at. 17 who were the first to propose that PAN behaves like a single-phase material, with a continuous laterally-ordered structure. The second view opposes this and favours a conventional two­phase model for PAN I8

-1I

. For example, Hinrichsen21

has concluded from small angle X-ray studies on highly oriented fibres that · "polyacrylonitrile, especially in the drawn state, is more exactly described by a two-phase structure than by the particular single-phase structure so far proposed (by Bohn et at. 17)" . A third view proposed by the author is that pAN can show two-phase or single-phase behaviour depending on whether the sample is undrawn or highly drawn22

.

In thi s work, a renewed effort is made to see if the glass transitions in PAN can be observed by DSC and

interpreted. It is shown that an apparently perplexing range of behaviour can be found in the DSC of PAN but this can be rationalised if it is understood that the nature of the specimen (i.e. unoriented or highly oriented) and the thermal history affect the results. It is demonstrated that there is indeed a correlation between the DSC and DMA results.

2 Materials and Methods 2.1 Sample Preparation

Two sets of samples (oriented and unoriented polymers) were prepared. For the unoriented case, reactor powder and cast film, and for the oriented case, highly drawn films were used.

P AN was purchased from Polysciences and was reported to be produced by free radical polymeri­sation of acrylonitrile using azo bis-isobutyronitrile as the initiator. The polymer was obtained as a white powder and was reported to have a Mv of about 100,000. The unoriented cast film was made by preparing a 10% (wtlv) solution of PAN in dimethyl sulphoxide. The solution was poured over a glass plate, and a metal casting bar was pull ed over it to even the thiclmess. The glass plate was left at 25°C for 1 week in an oven equipped with a fan to dry the solvent. Heating was avoided to prevent any thermal annealing. After solidification, the film still contained residual solvent. Hence, it was repeatedly extracted with water for a week to remove the residual DMSO, till residual solvent could not be detected in the infrared spectrum. The film was again dried in the oven at '20°C and then under vacuum (0 .1 mbar) at 20°e. The film specimen was needed for the DMA experiments, and to obtain good results , thorough extraction of residual solvent was necessary. Uniaxially-oriented films were made by drawing the cast fi 1m (x 12) over a hot bar at 120°C22 and used for both DSC and DMA experiments.

2.2 DSC Measurements For measuring the glass transition, a Mettler DSC

30, equipped with liquid nitrogen cooling accessory, was used. Calibration of the temperature response was conducted by recording the melting point onset of indium and benzoic acid at the chosen heating rate for the experiments (20°C/min). The samples were contained in 40 ilL aluminium crucibles with lids and were hermetically sealed. The lid of the crucible was pierced to let the small amount of water in the

BASHIR: POLY ACRYLONITRILE 3

polymer powder to evaporate. Typical sample weights used were 4-5 mg.

The actual heat-cool programmes used for each sample will be cited in the relevant places when the results are discussed.

2.3 DMA Measurements

To compare the DSC results with another independent method of detecting glass transitions, dynamic mechanical analysis (DMA) was peformed on both unoriented and oriented PAN films. For the unoriented case, the undrawn cast film was used. For the oriented case, the uniaxially-drawn film was used. In addition, the dynamic mechanical properties of an unoriented film that had been thermally annealed at 200°C for 30 min were also investigated.

The measurements were made on a Rheometrics Scieotific Dynamic Mechanical Thermal Analyzer. The sample was defom1ed dynamically in a tensile mode. A testing frequency of 1 Hz and a heating rate of 5°C/min were employed. The dynamic mechanical properties of the films were scanned in the interval - 100°C to 200°C.

3 Results and Discussion In order to provide a framework for understanding

the DMA and DSC results, it may be mentioned here that the starting hypothesis of this work is that the unoriented PAN is like a two-phase material, consisting of laterally-ordered and amorphous domains, whereas the oriented polymer behaves like a single-phase, laterally-ordered material. For con­venience, the DMA data are presented first because they are more straightforward and help in the inter­pretation of the more complex DSC results.

3.1 DMA Behaviour of Unoriented PAN-The Case for Two Glass Transitions

Tbe DMA scans of both semi-crystalline and amorpholls polymers may show more than one tan 8 peak . Usually, the strongest (ex) peak is the glass transition , while the other dispersions labelled as f3, y, etc. are much weaker and occur usually at lower temperatures. The f3 transitions are attributed either to motion of side groups or possibly restricted segmental backbone-motion .

With rAN however, when two tan 8 peaks are observed in the DMA (Figs la and Ic) , there are two remarkable features. Firstly, both peaks must be regardcd as having a similar intensity, when it is

I~I 109

(a)

10 108 10

la' 107

lr1' 10'&

-50

1011 1010 (b)

ala" 109

a. " w

0.

.;:; Jt

.!! ~

E'"

Ian 6

, E

0 50 100 ~O 200

0' %2

()'20

()'18

. \6

·10

0 '08

0 ·0&

0 ·04

0·02

0

250

·16 cD

·14 C d

' 12 -QJ

! :::[', "" ,'_----''--_-'--_-L_-,-l-=--_~-~O 02

-150 -tOO -so a 50 tOO 150 · 200

1010

109 0 ·18 (c)

n I .

E' ____ _

tan6~ lOST 10 6

- 150 -100 - SO a so tOO 150 200 250

Tempera lure ,oc

0 ·1& .

1) ' 14

0 ·12

0"10.

Q·oe

. ' . 0 ·06

0 ·04

0 ·02

o

Fig. I- (a) DMA trace of unoriented PAN film showing the two tan 8 peaks consistent with two Tg s

(b) DMA trace of oriented PAN film showing a single tan 6 peak consistent with a single Tg

(c) DMA of unoriented PAN film annealed at 200°C for .30 min ; Ihe effeci of thermal annealing is to cause peak I to dimini sh, suggesting a transformation to a single-phase material

considered that the ex peak is often ten times stronger than Ii f3 transition in a typical amorphous polymer23

such as poly( vinyl chloride). In Fig. I a, peak I is in fact slightly bigger than peak II, but this is partly because the phase associated with it is present in a greater amount; other examples of DMA curves of PAN shown in the literature lO indicate the reverse , with peak II having a slightly higher intensity than

4 INDIAN J. FIBRE TEXT. RES., MARCH 1999

peak I. Hence, we should not regard peaks I and II in Fig. la as a and f3 transitions, similar to those of other polymers, but we should. view them as peaks of similar intensity. SecOlidly, both peaks are inter­mediate in value, lying between the rypical values of the a and f3 transitions of -other polymers. Fig. la shows that the maximum value of tan 8 for PAN is low, typically about 0.20; in contrast, for amorphous polymers such as poly(vinyl . chiorideY3 or polymethacryloni trile24

, the a ' transition typically attains the tan 8 values of - 1.2-1 .4. The fact that both the transitions in Fig. 1 a are of similar intensity means that there is no reason to consider either in preference to the other as the glass transition of P AN- both should be regarded as such.

One may of course wonder at the cause of two glass transitions. Two glass transitions could be expected in a block ·copolymer consisting of two different types of non-crystallisable units, or a blend of two amorphous ·(imd immiscible) polymers, but it is surprising in a linear homopolymer.

Andrews and Kimmel 12 have in the past explicitly proposed that PAN has two glass transitions, based on the DMA behaviour. They describe the solid-state structure of PAN in terms of a "doubly-bonded single phase" . Thus, according to them, the two glass transitions arose from the consecutive loosening of two types of secondary bonding (van der Waals and dipole-dipole association of the nitriles) in a single glassy phase.

In contr~st to Andrews and Kimmel's doubly­bonded single-phase model, Minami et al.IO

•11 adopted

a two-phase model with paracrystalline (i .e. laterally­ordered) and amorphous domains but came to the surprising conclusion that the higher temperature peak (1S0°C) is the glass-rubber transition of the amorphous phase while the lower transition (100°C) was associated with motion in the paracrystalline phase (Fig. 1 ). The reason for assigning the transition at ISO°C to the amorphous phase was due to their experiments which showed that the higher

temperature DMA peak was affected by comonomer content, as well as crosslinking in the comonomer, and the fact that thi s peak vanished on drawing (see Fig. 1 b here, which will be discussed later).

The author is inclined not only to the two-phase model for the unoriented PAN, but al so to Minami e /

al.'s assignment. Further, as stated above, both the DMA relaxation phenomena must be considered as

glass transitions (Minami et al. do not quite state this) . Thus, here it will be proposed that the change at IS0°C observed both in the DMA (and as will be shown in the DSC) is the glass-rubber transition of the amorphous phase, while the change at 100°C is due to the transition of a glassy, laterally-ordered phase to a more mobile, laterally-ordered phase. That is, a two-phase model is proposed here for unoriented PAN, consisting of interconnected laterally-ordered and amorphous domains, both of which are in a glassy state at ambient. As noted from the DMA results, the tan 8 value is small. Hence, the mobility of both phases after the two glass transitions is low and so the material does not become as obviously fluid after the glass transitions as other amorphous polytners such as atactic polystyrene.

3.2 DSC Evidence for two Glass Transitions in Unoriented PAN

The DSC behaviour .of the unoriented specimens (reactor powder) will be discussed first. Fig. 2 shows the typical heating scan when PAN is heated up to 420°C. There is an intensely exothermic degradation reaction, and glass transitions which are weak events cannot be seen on this scale. Due to this reason, the region up to 200°C needed to be studied separately.

Fig. 3 illustrates a sequence of DSC thermo grams of PAN that can be obtained; their complexity suggests that interpretation will not be straight­forward . Fig. 3a shows the first heating scan of the PAN powder in the region of -SO°C to 200°C. There is an endothermic transition with a peak at about 70°C, followed by a glass transition with a conventional sigmoidal profile; these are labelled II and I respec tively in Fig. 3a. The original expectation

* .. c

.2

I ~ "0 "tJ

l l ~

.....t..-...l _ ~l_~---L...-.-.-__ _ L~ ~J._L "-__ . _ ...... - . .. .......... . 1

o . 100 250 3 00

Fig. 2- Heating scan (at 20°C/min ) of a PAN powder between O°C and 420°C. The princi pal feature is the intense exotherm: on this scale, the minor transitions like the Tg cannot be seen

.",'

BASHIR: POLY ACRYLONITRILE 5

f (0) 11 •

-50

'<b) ° .. ..

-50

o

o

II

so 100 150

II

, , ,I , ,I! ! I 1 , , I ! ! ! , , , I I!

50 100 150 200

T ~mp~ralur~ , 'c

Fig.3-Heating scans (at 20°C/min) of PAN (a) First DSC heating scans of PAN reactor polymer.

Transftion labelled" is an endothermic peak masking a TI

, while transition I shows a glass transition with a sigmoidal change in the baseline

(b) Heating scan after annealing at 90°C for 20 min and cooling at 20°C/min to - 100°C

had been that two consecutive step-like changes would be seen, as a result of the two glass transitions indicated by the DMA. A glass transition value cannot be determined when a pronounced endotherm is present (as in transition labelled II in Fig. 3a). The first heating scan in Fig. 3a nevertheless allowed glass transition I to be determined. The glass tran-

sition I temperature (midpoint) and ~Cp values are 140.3°C and 0.22 J/g.K respectively (Fig. 3a). It can be seen that glass transition I corresponds to the higher temperature tan 0 peak in the DMA (Fig. la).

Returning to the endothermic transition II in Fig. 3a, there are two probable causes. Firstly; the free radical polymer powder was highly p.orous and contained residual water that was difficult to remove by drying at moderate temperatures. The use of a pierced DSC crucible can lead to evaporation of water during the heating scan. Even small quantities of water can thus give a broad endotherm because of its high heat of vaporisation and this can mask any glass transition. It must be noted that the peak of the

transition II is actually at 70°C which is lower than the value of the first glaSs transition expected from DMA (100°C). However, the presence of residual moisture in the powder can. also lead to a depression 16

in the glass transition II; it is known that water can bind with nitrile groups2S and this weakens the normally strong dipole-dipole interaction between PAN chains, loosening and making them more mobile. Thus, it is possible that a combination of the depression of the glass transition II due to water and the presence Of the endotherm due to water vaporisation masks . the expected · steP · change . Secondly, quite different from any endothermic peak caused by evaporation of residual water, glass transitions can have an endothermic peak sometimes due to enthalpy relaxation effects. Although the classical DSC profile of a glass transition is a step­like or sigmoidal change in the baseline, endothermic peaks can also occur instead. To understand this, it must be recalled that the DSC profile can reflect the thermal history of the glass26

• Wunderlich26 has shown for atactic polystyrene that when the heating rate is different than the cooling rate used originally to form the glass, an endothermic peak occurs at the Tg• Other thermal-history effects can also produce endothermic peaks at the Tg• For example, it has been shown 27 that an endothermic peak arises when a glass has been aged for a prolonged period below the Tg (physical aging) or if it has been annealed just below the Tg before heating.

In the present case, the thermal history of the glassy state( s) of the PAN reactor powder is somewhat indeterminate. The formation process of the reactor polymer consists of growing chains precipitating out of a liquid medium containing monomer, with the evolution of heat; at the end of the reaction, the polymer powder has to be filtered, washed and be subsequently dried.

Based on the results from tjle DMA, where the first of the two tan 8 peaks is observed at about - 100°C, an~ allowing for some depression due to residual water, it was suspected that the endothermic peak II in Fig. 3a masked a glass transition. To remove the effect of endothermic contributions arising from residual water, different isothermal holding procedures were tried, based on drying the sample by holding it at temperatures near 100°C for short times (the DSC pan had a perforated lid to allow escape of water). The first procedure is described below.

6 INDIAN 1. FIBRE TEXT. RES., MARCH 1999

Heat to 90°C at 80°C/min and hold at 90°C for 20 min, Cool from 90°C to -100°C at 20°C/min, and Heat from -100°C to 260°C at 20°C/min.

The heating scan of PAN following the isothermal hold at 90"C and cooling is shown in Fig. 3b. The only difference that is found is that the broad

, endothemiic peak at 70°C in the as-received polymer (Fig. 3a) has now shifted to form a sharp endothermic 'peak at 100°C (Fig. 3b). While glass transition II still has a peak, glass-transition 1 has the "normal" profile it had originally (Fig. 3). The shift in the peak from 70°C to 100°C for transition II is probably due to the fact that the original powder contained some water that was removed in the !;iample held at 90°C. The endotherm II now has the appearanc'e typically seen in amorphous polymers that have been annealed just below the glass transition2

6-28. Holding at elevated temperatures near the expected glass transition II helps to remove the effect of water but it also has a simultaneous physical aging effect, which may cause the appearance of an endothermic peak at 100°C near this Tg•

Instead of holding the sample near 100°C to remove water, a different procedure was tried next which should have had the simultaneous effect of removing the water as well as forming the glassy states of PAN under a controlled thermal history. A fresh sample ofP AN was heated twice according to the following cycle . Again, the. DSC pan had a perforated lid to allow escape of water.

Heat from . - 100°C to 200°C at 20°C/min (remove residual water and erase thermal history of reactor polymer),

Cool from 200°C to -100°C at 20°C/min (reform glassy components under controlled history), and Heat from -100°C to 200°C at 20°C/min (reheat at same rate as cooling rate).

The first step has the effect of removing water and erasing the 'thermal history as it takes the polymer to a temperature well above that of both glass transitions. The second and third steps follow Wunderlich's procedure with polystyrene by matching the cooling rate of the glass formation with the subsequent heating rate, whereby the endothermic peak of the Tg should disappear6. The initial heating (first step) with the fresh sample of PAN gave a scan as shown already in Fig. 3a. The heating scan after

t 0 .. •

l~ (0)

---4---50 ,

0 .. • (b )

r~ -50

0 (e) ~

"

r~

0

, 0

I ~

50 100 150

II

I I 50 100 150

c ~ ° n

11+1 e 0>

'" '0

! f I I I ! I! ! ! ! , ! ! I ! ',t . I . , . I, . , I

-150 -50 o 50 100 150 20 0

T~mpera tur~ ,"C

Fig. 4--{a) The second heating scan of PAN (after initial heat ing to 200°C and cooling); the first scan is as in Fig. 3 (a)

(b) After cyc ling of PAN three times between -100°C and 200°C, th e fourth heating scan shows two gl a5s transitions with endothermic peaks

(c) Heating scan afte r annealing at 200°C for 90 min; there is now a sing le broad g lass transition at about 90°C, due to the conversion from a two-phase to a single-phase material

the second step is shown in Fig. 4a. It did not show the expected double transitions with two sigmoidal changes in the baseline; instead a weakly exothermic: event (II) , followed by an endothermic: peak (I), was observed. The exothermic event occurred in place of the original endothermic peak II while a new endothermic peak in Fig. 4a appeared at transition K which originally had a conventional glass transition profile (Fig. 3a). Thus, Wunderlich's procedure for removing the endothermic peak masking the glass transi tion I did not appear to work in the present case.

BASHIR: POLY ACRYLONITRILE 7

When the same sample was subjected repeatedly to the above three-step cycle several times, the appearance of the two transitions on each succesive heating scan changed. From the fourth cycle onwards, both transitions showed endothermic peaks. This can be seen in Fig. 4b. The peak temperatures of the two endotherms were about 80°C and ISO°C. It may be seen that these correspond roughly with the two DMA transitions. After about 7 cycles, the heating scan showed that the peak at ISO°C gradually disappears, while the transition at 100°C changed to a "normal" glass transition appearance (i.e. a step-like change but over a very broad temperature range; not shown). The reason for this is that repeated heating to 200°C causes thermal annealing that slowly converts the PAN from a two-phase to a single-phase material ; the single-phase material has a Tg about 100°C. This effect was also probed in a separate experiment which is described next.

In order to follow the hypothesized thermal conversion from a two-phase to a single-phase material , the heating scan after annealing PAN at 200°C for 90 min (i.e. well above the temperatures of both glass transitions) was recorded. The exact procedure is given below:

Heat to 200°C at 80°C/min and isotherm at 200°C for 90 min, Cool from 200°C to -100°C at 20°C/min, and Heat from -100°C to 260°C at 20°C/min

The DSC heating scan after the annealing at 200°C is shown in Fig. 4c. The major point to note is that there is now only a single glass transition whose onset is about 90°C. The transition, however, is very broad and extends over a large temperature range. The interpretation of this is based on the proposal that thermal annealing above both glass transitions leads to the slow conversion from a two-phase to a single­phase material. The amorphous phase has a tendency to order when annealed at 200°C, and this leads to the , peak. at ISO°C diminishing and overlapping with the peak at 100°C; this is the same effect that was observed when a sample was repeatedly cycled more than seven times between -lOO°C and 200°C. Fig. lc shows that a parallel behaviour is seen in the DMA of an unoriented specimen that was thermally annealed at 200°C for 30 min .. It may be seen that the two tan 8 peaks start to merge and form a single broad transition. Two features should be noted from Fig. Ic. Firstly, the peak at ISO°C (attributable to the

amorphous phase) starts to decrease, while the peak l1t 100°C (attributed to the laterally-ordered phase) starts to increase. Secondly, the maximum value of tan 8 has decreased from 0.21 (unannealed, Fig. la) to about 0.14 in the annealed sample (Fig. Ic). This decrease is most probabll9 due to a small degree of nitrile crosslinking/cyclisation that occurs because of thermal degradation at 200°C.

Overall, Figs 3 and 4 show that a puzzling range of thermograms can be obtained with P AN. Any individual thermogram in Figs 3 and 4 would be difficult to interpret on its own, but when taken as a sequence, along with the DMA data, it can be seen that the complexity arises from the fact that enthalpy relaxation effects can accompany both glass transitions. Previous DSC works on PAN have not probed these aspects and in the light of the complex changes shown in Figs 3 and 4, it can be seen why it is difficult to unravel the cause and guess that there might be two glass transitions.

It must be noted that the DSC behaviour reported in this section was found to be repeatable to a high degree with other free-radically polymerised PAN

made with a variety of initiators and under different polymerisation conditions. Further, these results were also observable at a heating rate of 10°C/min except that in the DSC, the signal is always weaker at lower heating rates.

3.3 Single Glass Transition in Oriented PAN by DSC and DMA

The glass transition behaviour of a uniaxiaIly­oriented PAN film was examined. The DMA scan of the uniaxially-oriented PAN film (Fig. Ib) shows that there is now only a single peak at about 100°C, whereas in the original unoriented sample (Fig. la), there were two peaks. Similarly, Minami et a/.30 and Cho et a/. J

' had found that in drawn PAN fibres, the higher of the two tan 8 peaks (ISO°C) disappears, leaving a single peak at 100°C. As with the case of the unoriented film, the maximum value of tan 8 in the oriented film is low (about 0.23, Fig. 1 b). Unlike in the case of the thermally annealed film (Fig. Ic) where a transition to a single phase occurred with a decrease in tan 8 because of crosslinking and cyclisation, with the hot-drawn film, the transition to a single phase occurred without a decrease in tan 8, because the heating period during drawing was a few seconds and the drawing temperature was low

8 INDIAN 1. FIBRE TEXT. RES., MARCH 1999

, o ;

"

~~,~'~,~·LI'~·~'~'~~lu·~,~.~ . . ~~,L'.~'~'~' -150 -50 o SO 100 150 200

Fig. 5-The first DSC heating scan of highly oriented PAN film showing a single Tg at - 100°C

(l20°C) and so there was little chance of nitrile cross­linking.

It was interesting to find out if a parallel behaviour (i.e. a change from two Tgs to a single T J would be seen in the DSC after drawing. Fig. 5 shows the first DSC heating scan of the drawn PAN film. Indeed, now there is only a single prominent glass transition at about 100°C. A similar result was found when highly drawn PAN fibres were used. It is consistent with the DSC thermograms of oriented acrylic fibres shown by others where only a single transition at a similar temperature is observed l6

.

Thus, the DSC results for the oriented PAN correlates with the DMA findings. It also supports the view that whereas unoriented PAN can be represented by a two-phase morphology, the highly oriented PAN behaves like a single-phase material. A similarity in the DMA and DSC behaviour of the oriented and the thermally-annealed but unoriented PAN can also be noticed (Figs I band 5 vs I c and 4c) . It just means that both thermal annealing and drawing have the effect of transforming P AN to a single-phase, laterally-ordered material, although drawing is more effective and quick in converting the polymer to a single-phase material. It may be seen from Figs 4c and 5 that the glass transition in the latter occurs over a naR'ower temperature range.

It is necessary to emphasize that the single glass­transition observed at 100°C in oriented PAN fibres and films is not the familiar glass-rubber transition of an..amorphous phase as found in the oriented fibres of other semi-crystalline polymers. This is a point that IS

not recognised in earlier works ' 5•16

. Instead, it must be considered as a change from a glassy, laterally-

ordered state to a somewhat more mobile, but stiB laterally-ordl:!red phase.

As mentioned earlier for the unoriented PAN, the interpretation in this work differs from that of Andrews and Kimmel 12. These authors proposed two glass transitions but attributed it to a single-phase material with two types of bonding. With their single­phase model, presumably the two types of bonding (van der Waals and dipole-dipole interactions) would exist irrespective of whether the polymer is oriented or not. Hence, in the oriented PAN, according to Andrews and Kimmel's model, one should still expect to find the two glass transitions. However, it has been shown in this section that in oriented PAN, only a single glas3 transition is experimentally observed both by DMA and DSC (Figs I band 5). The current work accounts for this by noting that on drawing, there is a tral1sformati~n to a single, laterally-ordered phase with a reduction of the amorphous phase.

4 Conclusions It is relati vely easy to show by DMA that there are

two tan 8 peaks and hence two major transitions in unoriented PAN. Two equivalent transitions are more difficult to demonstrate by DSC, but their presence is however certain. Residual water and the thermal history of the sample can often lead to endothermic peaks on the first heating scan and this can mask the glass transition(s). In li ght of the DSC results, it is suggested that both the tan 8 peaks in the DMA of unoriented PAN should be regarded as glass transitions.

It is of course remarkable in that there cannot be too ·many linear homopolymers with two glass transitions. In usual semi-crystalline homopolymers, one is accustomed to seeing one glass transition followed by one melting endotherm on heating; in unoriented PAN, we see two glass transitions instead. Certainly, the ex istence of two glass transitions in this polymer would not have been suspected from earlier works based on the extrapolation of Tg data on PAN copolymers or PAN solutions6

-9

• It is proposed that unoriented P AN has a . two-phase morphology consisting of laterally-ordered and amorphous domains. Both phases are in a glassy state at room temperature and this leads to two glass transitions.

On the other hand, thermally-annealed or oriented PAN behaves like a single- phase (laterally-ordered) material. Experimentally, it is found both by DMA

BASHIR: POLY ACRYLONITRILE 9

and DSC that there is only a single glass-transition at 100°C in oriented fibres and films . Industrial acrylic fibres usually contain some comonomer and hence one can expect in these fibres a single (depressed) glass transition between 80°C and 90°C. However, this Tg in the oriented PAN is not the conventional glass-rubber transition of an amorphous phase but represents the transition from an ordered glass to a more mobile but still ordered state. Preliminary hot­stage X-ray work supports this view and will be reported in full later.

Acknowledgement The author is thankful to Dr. R D L Marsh of Rheometrics Scientific for providing help pertaining to the DMA data.

References I Schatzki T F, J Appl Polym Sci, 5 (1961) 5 1. 2 Loshaek S & Fox T G. Bull Am Phys Soc, I (1956) 123. 3 Kolb H J & Izard E F. J Appl Phys, 20 (1949) 564. 4 Beevers R B. J Polym Sci, PI A, 2 (1964) 5257. 5 Keavney J J & Eberlin E C, J Appl Polym Sci, 3 (1960) 47. 6 Wiley R H & Brauer G M, J Polym Sci , 3 (1948) 704. 7 GerkeRH ,JPolymSci, 13(1954)295. 8 Krigbaum W R & Tokita N, J Polym Sci, 43 (1960) 467 . 9 Howard W H, J Appl Polym Sci , 5 (1961) 303 . 10 Minami S, Sato H & Yamada N, Reporls 0 11 progress in

polymer physi('s ill Japall, X (1967) 317.

II Minami S, Yoshihara T & Sato H, Kobunshi Kagaku (Engl Ed), I (1972) 125 .

12 Andrews R 0 & Kimmel R M, J Polym Sci. Polym Lell , 3 (1965) 167.

13 Illers K H, Die Makromol Chem, 124 (1969) 278. 14 Bajaj P & Padmanaban M, Ellro Poly m J , 20 ( 1984) 513. 15 Gupta 0 C, Agrawal J P & Sharma R C, J Appl Polym Sci,

38 ( 1989) 265 . 16 Hori T, Zhang H S, Shimizu T & Zoll inger H, TeXI Res J , 58

(1988) 227. 17 Bohn C R, Schaefgen J R & Statton W 0, J Polym Sci, 55

(1961)531. 18 Andrews R 0, J Polym Sci, Pt C, (14) (1966) 261. 19 Hinrichsen G & Orth H, J Polym Sci. Po~vm Lell, 9 (197 1)

529. 20 Hinrichsen G & Orth H, Kolloid-Z Z Polym , 247 ( 197 1) 844. 2 1 Hinrichsen G, .! I'olym.)·ci. PI C, 38 (1972) 303. 22 Bashir Z, Tipping A & Church S P, Polymllll, 33 ( 1994) 9. 23 Wunderlich B, Th{'/'Illal allalysis (Academic Press, Inc, San

Diego, USA), 1990, 3(i I 24 Bashir Z, .! Mal{'/' Sci Lell , 14 ( 1995) 1105. 25 Bashir Z. Ch urch S P & Waldron D, Polymer, 35 (1994)

%7. 26 Wunder lich B, Therlllal allal),sis (Academic Press, Inc., San

Diego, USA), 1990, 204 . 27 Ali M S & Sheldon R P, .! Af1pl Polym Sci, 14 (1970) 2619 28 Aref-A zar F, Arno ux F Biddlestone & Hay J N, Th ermochim

A('la , 27.'1 (19%) 2 17.

29 Oli ve G H & Oli ve S. Ad,·/Jo!),111 .';ci, 32 (1979) 123 . .'10 Minami S. Yoshlhara T & Sato H, Kobllllshi Kagakll (Ellg!

F:d). 1 ( 1972) 131.

3 1 Cho S H. Park J S, Lee W S & Chung J, PO!YIII Bull, 30 ( 1 ()c)J) 6(>.'1 .