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Morphology of polyacrylonitrile.

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Die Angewandte Makromotekulare Chemie 25 (1972) 83-96 (Nr. 336)
From the Department of Chemistry, Sardar Pate1 University, Vallabh Vidyanagar,
Gujarat State -India
Morphology of Polyacrylonitrile
(Eingegangen am 3. Dezember 1971)
!rhe present investigation revealed that the features like : single crystals, twin
crystals, hedrites and spherulites could be grown by changing the concentration or
temperature of crystallization of polyacrylonitrile (PAN) from its solution in propylene carbonate and following the technique of film-formation method of crystallization. Different stages of growth have been also reported. The well defined
single crystals of PAN could be grown in the temperature range of 105 to 150°C,
even from the dilute solution. Single crystals of PAN could be also grown from
comentrated solution (> 2.5 yo)a t higher temperature. Formation of spherulites
from sheaf and hedrites has been achieved by increasing the concentration of the
polymer solution. Lamellar thickness is found to increase with crystallization temper.ature.
The potentiality of the film-formation technique is that i t provides an extended
ran,qe of concentration and temperature not only for the kinetic study but also for
the growth of single crystals from polymer solutions.
Durch h d e r u n g von Konzentration oder Temperatur bei der Kristallisation
von Polyacrylnitril aus Losung in Propylen-carbonat durch die Filmbildungsmethode konnen - je nach den Bedingungen - bevorzugt Einkristalle, Zwillingskristalle, Hedrithe und Sphiirolite erhalten werden. Einkristalle entstehen im
Bereicb von 105 bis 150°C aus verdiinnter Losung und aus konzentrierter Losung
(> 2,5 yo) bei hoherer Temperatur. Durch Zunahme der Polymerkonzentration
wircl die Bildung von Garbensphiiroliten und Hedrithen erreicht. Die Lamellendicke wiichst mit der Kristallisationstemperatur an.
Der Vorteil der Filmbildungstechnik liegt darin, da13 sie einen weiten Konzentratiions- und Temperaturbereich anbietet, der nicht nur fur kinetische Untersuchungen, sondern auch fur das Wachstum von Einkristallen aus der Polymerlosung
geeignet ist.
Polyacrylonitrile (PAN) is known to be a very poorly crystalline material.
X-ray diffraction patterns of oriented fibers11 2 show only two diffuse arcs on
equator and none on meridian. However, PAN can be obtained in crystalline
and R. D. PATEL
form, and NATTAet al.3 have demonstrated a hexagonal structure for crystalline syndiotactic polymer. MENCX4 obtained diffraction patterns of rather poor
quality that could not be interpreted in terms of hexagonal structure. Hence
he proposed an orthorhombic unit cell from the observed reflection and crystal
density. HOLLAND
et al.5 were the first to grow single crystals from dilute
solution over a period of several days. The platelets observed were ellipsoidal
in shape with considerable overgrowth. On further investigation63 7 they
concluded that there was a considerable increase in crystal platelet step height
with increase in crystallization temperature and the heighest temperature of
crystallization could be 125°C. Recently, KLEMENT
and G E I L ~have grown
PAN crystals from dilute solution in propylene carbonate a t various temperatures ranging from 69 to 125°C by a technique similar to that described by
et al.5. It has been shown that morphology of full grown crystal
varied with crystallization temperature. They also studied the rate of growth
of crystals a t 100°C. Well defined single crystals could be obtained on glass
slide only a t 100°C after about 23 hrs. If the growth time is increased, considerable overgrowth was observed. The crystals grown a t temperatures higher
or lower than 100°C were found to be of varying morphology. The small angle
X-ray diffraction as well as electron microscopic study of crystals grown above
100 "C showed a small change in lamellar thickness.
The results are not in agreement with those reported by CHIANG et a1.6.
Thus, there are contradictory views regarding the variation of lamellar thickness with crystallization temperature for this polymer.
The aim of the present study is four fold
1. To demonstrate the potentiality of film formation method (FFM)g, for
growing well defined single crystals of PAN.
2. To varify the limiting crystallization temperature, e. g. 125°C suggested by
CHIANGet al.6.
3. To clarify the contradictory results of KLEMENT
and G E I L ~and CHIANG
et al.6 regarding the change in lamellar thickness with crystallization temperature.
4. To study the effect of concentration on growth habit of PAN crystals a t a
particular temperature.
The polymer used in this investigation was prepared by polymerization of
acrylonitrile initiated by ceric ion redox systemlo. The polymer was washed with
water and acetone and further purified by precipitation from its solution in dimethyl
formamide (DMF). The viscosity average molecular weight was found t o be
M, = 6.68 x lo5.
Purified propylene carbonate was used as a solvent of crystallization.
Crystals were grown on glass slides according to the method described by PATEL
and PAT EL^. The glass slide on which the polymer crystals were grown, was placed
in vacuum evaporator, coated with carbon and shadowed with chromium at a
desired angle. Coated slides were kept in moisture for one hour and the replica was
detached from the glass support by floatation on water surface. The film was mounted on support grids by conventional method. The electron microscopy was carried
out on the CARL-ZEISSmodel EF-4 electron optical plant.
The thickness of the platelets were determined directly from electron micrographs by measurements of shadow length and shadow angle. Thickness was measured with an accuracy of & 15 A. I n order to obtain best possible average, large
numbers of individual measurements were made at each crystallization temperatnre.
Isothermal Crystallization of P A N from its Solution in Propylene Carbonate
Figs. 1, 2, 3 and 4 show the single crystals of PAN grown from 0.06y0
solution of PAN in propylene carbonate at 105", 130", 140" and 150°C respectively. Fig. 5 shows the single crystal of PAN grown a t 160°C from 0.25%
solution. As seen from electronmicrographs, single crystals of PAN are elongated ellipses with no overgrowth a s was always observed by previous workers.
Electron diffract,ion study for the same polymer crystal as reported previouslyg, showed a n orthorhombic unit cell dimension. From the results of
present study, it is revealed t h a t there is no considerable change in crystal
mclrphology over the temperature range studied when the crystals are grown
from 0.06y0solution of PAN using FFM. It will be pointed out later on that
one can not only grow PAN crystals above 125°C upto 160°C, but can also
study the rate of crystal growth above 125°C. Thus F F M can be regarded as
Fig. 1
Fig.. 1.
Fig. 2.
Fig. 2
Electron micrograph (enlargement: 12560; crystallization temp. : 105 "C;
conc. of solution : 0.06 yo).
Electron micrograph (enlargement: 7440; cryst. temp. : 130°C; conc. of
solution 0.06 yo).
better method compared to other conventional methods5169 8 according to
which one can grow crystals only over a small range of temperature and that
too with considerable overgrowth.
Fiy. 3.
Fig. 4.
Electron micrograph (enlargement: 12530; cryst. temp.: 140°C; conc. of
solution: 0.06 yo).
Fig. 4. Electron micrograph (enlargement: 11910; cryst. temp.: 150°C; conc. of
solution : 0.06 yo).
Fig. 3.
Fig. 5. Electron micrograph (enlargement : 6 730; cryst. temp. :
160 "C; conc. of solution :
0.25 Yo).
When the growth features are observed in detail, it is also marked that
along with single crystals, various other interesting growth features are found
to grow. Amongst all other growth features the most common one is that of
twinned crystals. Almost in all cases more than 20 yo crystals were found to be
twinned crystals and many of them get twinned via spiral growth mechanism
similar to that observed in case of polyethylene11 (PE). Figs. 6 and 7 respectively show four fold and six fold typical penetration twinned crystals of PAN.
Various types of twinnig along different twinning planes have been observed.
The detail study of which will be published elsewhere.
When the crystals were grown from dilute solution, essentially single platelets were observed, but sometimes formation of spirals (Fig. 6) and terraces
iMorphology of Polyacrylonitrile
Fig. 6.
Fig. 7.
Electron micrograph (enlargement: 16300; cryst. temp.: 130OC; conc. of
solution: 0.06 yo).
Fig. 7 . Electron micrograph (enlargement: 10830; cryst. temp.: 150°C; conc. of
solution: 0.1 Yo)
Fig. 6.
(Fig. 8 a t E) were also observed. Although no evidence indicating overgrowth
formed by spiral growth mechanism has been reported previously. The number
of observations similar to one shown in Fig. 6, directs one to suggest that
twinned crystals get thickened by spiral growth mechanism.
Fig. 8. Electron micrograph (enlargement: 4750; cryst. temp.:
140 "C; conc. of solution :
0.5 yo).
and GEIL~have observed the crystals having streamers tailing off
from the ellipse. Such growth features have not been detected in the present
study. But very similar growth features have been observed in the present
work. Occasionally, crystal as shown in Fig. 9 was observed in which lamellae
radiate equally in all directions out of an ellipse. If the long axis of lamellae
corresponds to 'a' axis of unit cell, it seems from electron micrograph that the
'a' axis of radiating lamellae is aligned a t definite angle of about 60" with 'a'
axis of parent lamella. This suggests that the radiating lamellae may get twinned with parent platelet lamella. It is likely that the streamers observed by
the previous workers8 may be due to the initiation of twinning.
and R. D. PATEL
Fig. 9. Electron micrograph (enlargement: 10690; cryst. temp.:
150°C; conc.
of solution:
0.5 yo).
Crystal Network
Along with single layer crystals sometimes multilayer crystals were also
observed. However, when concentration of polymer was increased, a few single
layer crystals a and large number of multilayer crystals were observed which
clearly suggested that imperfection becomes more pronounced when crystals
were grown from higher concentration. Multilayer structure could be divided
into two parts. One of which could be called terracing which might have formed
by the combination of two spirals of opposite sense and other be called single
spirals. The spirals may have been formed either by joining of two independently growing crystals or due to some dislocation in the crystal itself.
Generally, terracing has been observed on single crystals (Fig. 8). But
whenever twinned crystals are found to form multilayer structure, it is formed
by spiral growth as observed in Fig. 6. It shows two spirals one in each component originating from the same axis.
The second source of spiral growth as mentioned above is the joining of two
independent growing crystals. Such peculiar features are observed and many
times give rise to some fascinating 'Siamese twin' structure. This type of joining
becomes more prominent with increasing the concentration. The crystals join
smoothly along the directions where their crystal structures are matched or
along twinned orientation. But occasionally the join is found t o be mismatched. I n many crystals of PAN, a distinct line of discontinuity is also observed
which can be clearly seen a t A and B in Fig. 8. Fig. 8 is a typical example of
two dimensional net work grown from 0.5 yo solution a t 140 "C. As mentioned
~ quite possible for spirals to be initiated a t
and K E L L E Rit~is
both the ends or a t just one end of the line of join. I n many cases, the spiral
gets separated after further growth. Hence, as a result of additional growth,
many crystals appear as separated parts. This can be visualized from Fig. 8,
in which C and D indicate such developments. Further, it is also observed
(Fig. 8) that several crystals can join together to form a continuous crystal net88
Morphology of Polyacrylonitrile
work. The increase of concentration results into the formation of mult,ilayer
network structure.
Lainellar Thickness
It has been well established for polymer crystals that lamellar thickness is a
strong function of crystallization temperaturel3-16. As pointed out previously,
single crystals can be grown in a wide range of temperature (105 to 160").
Thickness of the crystals grown a t all temperatures were measured from electron micrographs. Fig. 10 shows the variation of lamellar thickness against
crystallization temperature of PAN crystals. For comparison, the small angle
X-ray spacings and lamellar thickness from electron micrographs69 8 are
plotted. As seen from the plot, results of CHIANGet al.6 indicate that the lamellar thickness is a strong function of crystallization temperature in the range of
95 to 125OC for their sample of polymer, while the later study of KLEMENT
and G E I L ~indicates that the crystal thickness remains constant above the
crystallization temperature of 100 "C. Data of the present investigation show
that thickness of lamellae increases with increasing the crystallization temperature but not to extent as reported by CHIANGe t al.6.
Fig. 10. Lamella thickness from electron micrograph (E. M.) and
small angle X-ray spacing
(X-ray) in dependence on
isothermal crystallization
temperature. CHIANG et al.
(E.M.) :-0 -;present work
(E.M.) :---;GEIL(E.M.)
140 130 -
10100 go80-
f /x-x
Different conditions of preparation of PAN have been attributed to the
difference in behaviour of the crystal thickness with crystallization temperature*. However, in the present investigation redox initiation methods was used
to prepare PAN. Yet the results are not in good agreement with the observaand GEIL~.Hence, condition of polymerization cannot be
tions of KLEMENT
regarded as the plausible reason for the difference in behaviour of crystal
and R. D. PATEL
Growth Rate
Single Crystals
Controlled production of crystals is an important factor in the study of
crystalline substances both for understanding crystal growth and for evaluating thermodynamic parameters. Large numbers of observations on the habit of
crystals grown by F F M have the added significance that any randomly
selected crystal has a very high probability of being representative of the
whole preparation. It should also be pointed out a t this stage that they are not
only of the same shape and approximately of the same size, but they are representative of the same stage of growth of that particular conditions. By studying the
crystal growth a t different concentration and temperature, it is noted that
PAN single crystals can be grown in wide range of temperature (105 to 150"C)
from 0.06 yo solution in propylene carbonate using FFM. This is contradictory
with other methods of growth which give crystals of greatly varying size and
shape. Hence, it is concluded that employing FFM, crystals can be grown in
wide range of temperature in contrast to the highest practical crystallization
temperature (125"C) reported by previous workers6. This advantage of growing single crystals in wide range of temperature can be used for studying the
growth rate of single crystals. Therefore crystals were grown from 0.06y0
solution of PAN in propylene carbonate following a different time period of
growth. I n Fig. 11 a typical plot of crystal size in p versus time of crystallization a t the crystallization temperature of 130°C is given. The growth rate,
G, measured from the plot using the slope of the line, is found to be 0.378 p/hr
a t the crystallization temperature of 130"C. The growth rates of single crystals
were also measured in the temperature range of 105 to 150°C. The detailed
study on the growth rate will be published elsewhere.
Egect of Concentration
As mentioned previously, when crystals are grown from dilute solution,
along with single crystals a few collection of lamellae are also observed. This
forms the major habit of crystal growth in case of PAN. As discussed previously spirals, terraces and twinnings are the commonly observed growth
features. I n order to study the effect of concentration higher temperature of
crystallization (150°C) was selected. At this temperature, it is possible to grow
single crystals and various growth stages of hedrite and spherulite, by changing the concentration. This type of study may throw some light on the growth
mechanism of various growth features of PAN. Fig. 12 shows the crystal of
PAN grown from 0.1 yo solution a t 150°C. Along with ellipsoidal crystals,
twinned crystals were also observed ( c . f. Fig. 7). This is a typical electron
micrograph of six-fold penetration twinned crystal. Such structures could be
Morphology of Polyacrylonitrile
Fig. 11. Crystal length in dependency on crystallization time.
considered as an initial growth stage of hedrite. If concentration is increased
to 1 yo, more thickened ellipsoidal and twinned crystals were observed (c. f.
Figs. 13 and 14 respectively). These features could be considered as the intermediate growth stage of hedrite. At this point it should be also noted that
along with many regular six-fold twinned crystals, there has been a tendency
to form rossette shaped twinned crystals as shown in Fig. 15. Further increase
in concentration led to still more thickening of the crystals as seen from Figs.
16 and 17. At 2 yo concentration many crystals were found to be very thick as
shown in Fig. 16. It is very difficult for electron beam to penetrate such crystals, and hence manya time it is difficult to observe distinct lamellae on them.
These growth features are considered as the final growth stage of hedrite. From
the present study it is clear that as the concentration is increased gradually
two types of hedrites, e. g. ellipsoidal and twinned, are commonly observed
among PAN crystals.
Fig. 12
Fig. 13
Electron micrograph (enlargement: 10450; cryst. temp.: 150°C; conc. of
solution: 0.1 yo).
Fig. 13. Electron micrograph (enlargement: 7998; cryst. temp.: 150°C; conc. of
solution: 1 yo).
Fig. 12.
and R. D. PATEL
Fig. 14
Fig. 15
Electron micrograph (enlargement: 8489; cryst. temp.: 150'C; conc. of
solution: 1 yo).
Fig. 15. Electron micrograph (enlargement: 8 0 5 4 ; cryst. temp.: 150OC; conc. of
Fig. 14.
solution: 1 yo).
Fig. I 0
Fig. 17
Electron micrograph (enlargement: 16 140; cryst. temp.: 150°C; conc. of
solution: 2 yo).
Fig. 17. Electron micrograph (enlargement: 8472; cryst. temp.: 15OOC; conc. of
solution: 1.5 yo).
Fig. 16.
The electron micrograph study revealed that hedrites of PAN are composed
of lamellae approximately 130 A thick. Observations on a large number of hedrites show that there is no formation of pit at the centre of PAN hedrites as
is the case with the hedrites either grown from melt or solution of many polymer&. I n case of six-fold twinned hedrites, the screw dislocation is usually
generated from the centre as seen in Figs. 14 and 15. Up t o 2 yoconcentration
of PAN hedrites are found t o grow at 150°C.
Further increase in concentration (2.5%) leads t o formation of sheaves of
PAN (Fig. 18). Such structure can be regarded as the basis t o form spherulite.
The stage of growth shown in Fig. 18 can be considered as an intermediate
growth stage of spherulite. From the same preparation many times irregular
Morphology of Polyacrylonitrile
shaped hedrites as seen in Fig. 19 are also found to grow. From the electron
m:icrograph study it is noted that lamellae are radiating from the centre of
such growth feature. This is possible only if there is a screw dislocation. At
many instances lamellae change their orientation. They do not show essentially planar top feature when they are grown from this concentration.
Fig. 18.
Fig. 19.
Fig. 18. Electron micrograph (enlargement: 10820; cryst. temp.: 150°C; cone. of
solution: 2.5 yo).
Fig. 19. Electron micrograph (enlargement: 8356; cryst. temp.: 150°C; cone. of
solution: 2.5 Yo).
Further developments of various growth features are observed when concentration is increased from 2.5 to 3.2%. The development of final growth
stage of spherulite from a sheaf is shown in Fig. 20. Similarly, developments of
spherulites from hedrite are also observed as shown in Fig. 21 and 22. They
seem to act as heterogeneous nucleus for the development of spherulite.
Fig. 20.
Fig. 21.
Fig. 20. Electron micrograph (enlargement: 12 625; cryst. temp. : 150 "C; cone. of
solution: 3.2 yo).
Fig. 21.
Electron micrograph (enlargement: 16440; cryst. temp.: 150OC; conc. of
solution: 3.2 yo).
Pig. 21 shows the development of spherulite from six-fold twinned ellipsoidal
hedrite (c. f. growth stages Figs. 7 and 14) whereas Fig. 22 shows the spherulite
developed from radiating lamellae getting twinned on the parent ellipsoidal
lamella (c. f. Fig. 9).
Fig. 22.
Electron micrograph (enlargement: 16500;cryst.temp.:
150 "C ; cone. of solution :
3.2 yo).
The electron micrograph study on the PAN crystals grown a t various temperatures and concentrations suggests that single layer crystals of strictly
controlled habit can be obtained by employing the film formation method of
crystallization. Another important significance is that any randomly selected
crystal has a very high probability of being representative of the whole preparation. It should also be noted that the crystals grown a t any condition have
same shape and approximately same size and they represent the same stage of
growth of that particular condition. This advantage of crystal grown by FFM
can be used for understanding crystal growth and effect of various parameters
on growth habit.
Simplest single-layer twins of PAN and other polymers can be grown merely
by changing either concentration or temperature. This is difficult with other
methods. Controlled multilayer habit can also be grown on such twinned crystals, which many times help for understanding the orientation of parent
crystal. Hence it should be mentioned that this method is quite appropriate
to studying the various types of twinned crystals.
If crystals are to be grown from dilute solutions of PAN in propylene carbonate by conventional method it takes several days, but using F F M crystals
can be grown within a considerable short period. Thus, studies on PAN and
other polymers revealed that this method is simpler and more rapid than other
methods employed for crystal growing and results are quite reproducible.
The crystals grown by F F M on glass-slide have been used for studying the
surface topology of various growth features like hedrites and spherulites using
Morphology of Polyaerylonitrile
interferometric study. Since crystals are representative, any change can be
directly related to crystallization conditions prevailing a t that instant. Using
any conventional method it was not possible to study the isothermal crystallization of polymers above turbidity temperature. But F F M successfully helped to grow crystals above turbidity temperature. This extended the range of
studying the effect of temperature on various growth features.
As indicated previously, the habit and size of growth feature can be controlled by this method. I n case of PAN it is also seen that two dimensional
single layer crystals can be grown in a wide range of temperature. Hence the
single crystals grown by this method can be used for kinetic studies.
Studies on the morphology of PAN and other polymers revealed that feature
like single crystals, twin crystals, hedrites and spherulites can be successfully
grown by changing concentration or temperature by this method. Different
sta,ges of growth of all morphological forms obtained, employing the film formation technique enables one (i)to propose a probable growth mechanism for
those forms and (ii)to find out the relationship amongst them.
Previous studies59 69 7 indicated that distinct single crystals of PAN could
not be easily grown in a wide range of temperature from dilute solution. The
highest practical crystallization temperature for this polymer has been reported to be 125°C. Present study revealed that well defined single crystals can
be grown in a wide range of temperature (105 to 160°C) from dilute solution.
It is also possible to extend the range of both the concentration and the temperature for observation of single layer crystal with the present technique of
crystal growing.
Studies on effect of concentration revealed that crystals get thickened by
spiral growth mechanism and ultimately form hedrites. The concentration
range > 2.5 yo yielded spherulites from sheaf and hedrite on crystallization
of PAN. Hence it can be concluded that concentration of polymer has definite
effect on the growth features.
The authors are grateful to Prof. A. R. PATEL,Physics Department, for
providing microscopic facilities, and to Dr. C. K. PATEL,Dr. S. M. PATEL
for their help and valuable suggestions throughout the progress of the work. One of us (R. M. G . ) is grateful to U. G. C. for financial
and C. EYRAUD,
C. R. Acad. Sci. 251
(1960) 2174.
and W. 0. STATTON,
J. Polym. Sci. 55 (1961)
Atti Accad. Naz. Lincei, Cl. Sci. Fis.,
Mat. Natur., Rend. 25 (1958) 3.
Vysokomol. Soedin. 2 (1960) 1635.
J. Polym. Sci. 62 (1962) 145.
and V. F. HOLLAND,
J. Polym. Sci.A 3 (1965) 479.
7 R. CHIANG,J. Polym. Sci. A 3 (1965) 2019.
and P. H. GEIL,J. Polym. Sci. A-2 6 (1968) 1381.
and R. D. PATEL,
J. Polym. Sci. A-2 8 (1970) 47.
J. Polym. Sci. 3 1 (1958) 242.
11 P. H.GEIL, Polymer Single Crystals, Interscience Publishers, a division of
John Wiley and Sons, h e . , New York 1963, p. 150.
and A. KELLER,J. Macromol. Sci. Phys. B 2 (1968) 337.
J. Res.Nat. Bur. Stand. Sect. A 64
(1960) 73.
and J. J. WEEKS,J. Res. Nat. Bur. Stand. Sect. A 66 (1962)
F. P. PRICE,J. Polym. Sci. 42 (1960) 49.
and M. TOSI,Proc. Roy. SOC.(London), Ser. A 263 (1961) 323.
J. Amer. Chem. SOC.84 (1962) 2857.
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