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Thermal behaviour of acrylamide and acrylonitrile grafted nylon 6.

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Die Angewandte Makromolekulare Chemie 28 (1973) 191-205 (Nr. 439)
From the Indian Institute of Technology, Delhi, India
Thermal Behaviour of Acrylamide and Acrylonitrile
Grafted Nylon 6
By D.S. VARMA
and S. RAVISANKAR
(Eingegangen am 28. September 1972)
SUMMARY:
The thermal behaviour of nylon 6, 20 denier, monofilament (semi-dull) and the
graft copolymers of nylon 6 with acrylamide and acrylonitrile was studied using
dynamic thermogravimetry in air a t a heating rate of 6"C/min. up to a temperature
of 550°C. The thermal stabilities of the samples grafted with acrylamide and acrylonitrile to various percentages of grafting were computed from their primary
thermograms by calculating the values of initial decomposition temperature (IDT),
integral procedure decomposition temperature (IPDT) and activation energy (E).
The results show that the thermal stability of the samples grafted with acrylonitrile
increases with increase in graft-on percentage and the thermal stability of the
samples grafted with acrylamide decreases with increase in graft-on percentage.
The reduction is, however, not very significant.
ZUSAMMENFASSUNG :
Das thermische Verhalten von Nylon 6 (20 Denier Monofilamente)und der Pfropfcopolymeren von Nylon 6 mit Acrylamid und Acrylnitril wurde durch dynamische
Thermogravimetrie untersucht. Die thermischen Stabilitaten der gepfropften Proben wurden aus ihren Primarthermogrammen durch Berechnung der Anfangszersetzungstemperatur (IDT), integralen Zersetzungstemperatur (IPDT) und Aktivierungsenergie (E) ermittelt. Die thermische Stabilitat der mit Acrylnitril gepfropften Proben steigt rnit hoherem Pfropfungsgrad an, wiihrend die rnit Acrylamid gepfropften Proben mit steigendem Pfropfungsgrad instabiler werden.
Introduction
The thermal behaviour of polymeric materials over a range of temperature
is a n important aspect and influences their potential utility. SCHWENKER
Jr.
e t al.1 have carried out thermogravimetric analysis (TGA) as well as differential
thermal analysis (DTA) t o investigate the thermal stability of various fibrous
materials.
Modification of nylon 6 by graft copolymerisation has gained considerable
importance and a lot of work has been reported on graft copolymerisation of
nylonz-13. However, so far no work has been reported on the thermal behaviour
of grafted nylon. The thermal behaviour of the polymer is affected by graft
191
D. S. VARMA
and S. RAVISANKAR
copolymerisation, a n d hence a systematic investigation of the thermal behaviour
of nylon grafted with acrylamide and acrylonitrile was carried out and is
reported in this paper.
Grafting on nylon 6 was done by using ceric ion as a n initiator. Thermogravimetric analysis of the grafted samples was carried out by using “Stanton
Thermogravimetric Balance” in air. The thermal stabilities of grafted samples
were compared from their integral procedure decomposition temperature
(IPDT) values as proposed by D O Y L EThe
~ ~ .activation energy of thermal degradation was also calculated t o understand the energetics of the degradation
reaction.
Experimental
M a t e r i a l s U s e d : Semi-dull nylon 6 fibres, 20 denier, soxhlet extracted with
petroleum ether for 8 hrs. to remove the finish applied during processing, were used.
Acrylonitrile and acrylamide crystals were used as monomers. The acrylonitrile
was purified to remove the inhibitor by successive washings with dil. HzSO4, dil.
NazCO3 and with water and then distilled. Acrylamide was recrystallised from
water and used. Baker analyzed reagent grade ceric ammonium sulphate (NH4Ce
[SO414 . 2Hz0) was used as initiator in HzSO4 medium.
Graft Copolymerisation of Nylon 6
Graft copolymerisation was carried out in well stoppered vessels in nitrogen
atmosphere. The nylon samples were swollen in formic acid for lj2 a n hour and then
neutralised with dilute ammonia and then washed with water and dried in air
before grafting. Purified, dried and swollen nylon 6 samples (1 g) were immersed in
a solution of HzS04 (0.1 N to 1 N) and ceric ammonium sulphate (CAS) (0.001 N
to 0.05 N) at a temperature of 60°C. The temperature was maintained at 60°C.
Then the monomer (2 to 15%) was added to the reaction mixture and the contents
of the flask were constantly stirred. An atmosphere of Nz was maintained throughout the course of the reaction. Material to liquor ratio was 1 :50 and the reaction
time was varied from 1 hour to 6 hours. After the required reaction time the nylon
samples were removed, washed thoroughly and then homopolymer was removed.
I n the case of acrylamide grafted samples, the adhering polyacrylamide was removed by boiling in several changes of water, while in acrylonitrile grafted samples
soxhlet extraction with dimethyl formamide (DMF) was carried for 8 hours to
remove the polyacrylonitrile homopolymer. Nylon samples were then dried, conditioned and then weighed. The percentage graft-on was calculated as the percentage
increase of weight over the original weight of the samples.
Dynamic Thermogravimetric Analysis
The thermogravimetric analysis was carried out with “Stanton Model HT-D,
Thermobalance” in air. The samples were cut into small pieces of 1/16’’ approximately and ( 5 0 3 3) mg samples were taken in each case for the thermogravimetric
192
Thermal Behaviour
analysis. The temperature was varied from room temperature t o 500°C in the case
of parent yarn and 550°C in the case of grafted samples and a heating rate of
6 "C/min. was maintained. Primary thermograms were obtained by plotting percent
residual weight against temperature.
Results and Discussions
Optimisation of the Conditions for the Grafting of Acrylumide
and Acrylonitrile on Nylon 6
a) E f f e c t of Acid C o n c e n t r a t i o n o n G r a f t i n g
The effect of acid concentration on the grafting of acrylamide and acrylonitrile onto nylon 6 was studied a t various concentrations of HzS04, between
0.1 N to 1 N, since it has been reported8 that the acid concentration plays an
important role in the case of graft copolymerisation. Below 0.1 N of HzSO4,
ceric hydroxide precipitated out and hence it was not possible to work with
lower concentration of acid. The conditions maintained are given in Table 1
and the results of acid concentration on grafting are shown in Fig. 1. I n acrylonitrile grafting, there was an increase in grafting upto 0.5 N HzS04 and then
a decrease was observed. Similar observations have been reported by RANGA
RAOand K A P U Rwho
~ ~ have studied the grafting of acrylonitrile on cotton.
The reaction of graft copolymerisation and probable homopolymerisation
can be written as follows in analogy to the scheme given by OGIWARAet al.16
for grafting on cellulose :
Initiation :
Ce4+ + A-H
k kd
+ B + A'
ki AM.
A'+M +
~e4+
+M M
+ Ce3+ + H+
+ ~ e 3 ++ H+
Propagation :
AM,
M kXAM,.,
+
+ M k 4' M i + ,
Termination :
AM,+ + Ce4+ 3AMn+I + Ce3+ + H+
' Mn+l + Ce3+ + H+
Mi,, + Ce4+ k2,
A' + Ce4+ 5
+ Ce3f + H+
Mi
AOxidation
product
If all the reactions are irreversible, then the p H should not influence the rate
of polymerisation which is not found to be true. This leads to the conclusion
that the reactions (1) and (3) should be reversible and if this is so, the increase
193
D. S. VARMA
and S. RAVISANKAR
24
2c
1
1E
C
Y
Q
5
14
*
c
b
e
cry lam ide
4
-
C
Fig. 1.
Table 1.
0.4
0.6
0.8
Concentration of sulphuric acid (N)
1
.o
Effect of HzSO4 concentration on grafting of nylon 6.
Conditions for the grafting of acrylamide and acrylonitrile on nylon 6.
Sample wt: 1 gm. Time of swelling: 30 min. Temperature: 60°C
M:L: 1:50.
Conditions maintained
Acrylonitrile
Acrylamide
I
Samde
Description
HzS04
Critical
Conc.
of CAS
B Critical
Conc.
0fHzS04
C Optimum
Monomer
Conc.
D Effect of
Time
194
'
0.2
I
A
0
CAS
Conc.
1N
0.001N
to
0.05 N
0.1 N 0.03 N
to
1.0N
0.5 N 0.03 N
0.5 N 0.03N
Monomer
Time HzSO4 Cone.
(%) (hrs.)
5
2
5
2
2.5
to
15
10
2
1
2
0.5 N 0.001
to
0.050 N
0.1 N 0.005 N
to
1.0 N
0.10N 0.005N
0.10N 0.005N
Mono
mer
~
Time
(%)
(hrs.)
5
2
5
2
2
2
to
15
10
1
2
4
4
6
6
Thermal Behaviour
in H+ concentration should suppress the generation of radicals. This means
that with increased H+ concentration, the graft-on should be reduced which
is found in our case after certain value of acid concentration. Anyhow, depending
on the reactivity of monomers, this value will change and hence we get different
values of optimum H2S04 concentration in both cases.
b) E f f e c t of I n i t i a t o r C o n c e n t r a t i o n
It has been reported8 that there is an optimum concentration of CAS that
gives maximum grafting. The CAS concentration was varied from 0.001 N
to 0.05N. The conditions maintained are given in Table 1 and results are
shown in Figure 2. It is seen that the percentage of grafting increases up to
0.005 N CAS in the case of acrylamide and up to 0.03 N in the case of acrylonitrile samples and then falls. By assuming reversibility of reaction (1) and
(3) and applying steady state conditions, the rate of polymerisation can be
obtained from the above scheme.
But we have to consider two different ranges of concentration of Ce4f.
Case 1 : At low concentration of Ce4+
which shows that with increase in Ce ion concentration the rate of polymerisation should increase which is found to be true in both cases.
Case 11: At higher Concentration of Ce4+ and kt >> k, and ki
It has been already estimated by KATAIet al.17 with ceric ion as initiator
ko
that - = 50, which means, the increase of ceric ion concentration increases
ki
the magnitude of the denominator resulting in reduction of R, which is found
to be true. This indicates that ceric ion a t a critical concentration gives a
maximum graft-on.
c) E f f e c t of Monomer C o n c e n t r a t i o n o n G r a f t i n g
It has been reported18 that the monomer concentration has got direct relation
with the percentage of graft-on. This work was done onto microcrystalline cellulose with acrylamide. I n our investigation, the conditions maintained to
find optimum monomer concentration are given in Table 1 and results are
represented in Fig. 3. It was found that as the monomer concentration was
increased the graft-on also increased.
195
D. S. VARMAand S. RAVISANKAR
28
24
20
4-
a,
c
Q
16
c
0
”
;12
L
a
8
4
0
Fig. 2.
I
I
I
0.04
0.06
Concentration of CAS (N)
0.02
I
0.08
Effect of CAS concentration on grafting of nylon 6.
d) E f f e c t of T i m e on G r a f t i n g
The effect of time on graft copolymerisation was also investigated (Table 1)
and it was found that the time of reaction has got direct relation with increase
in graft-on even though the increase in graft-on a t higher time periods is not
very appreciable (Fig. 4).
ThermogravimetricAnalysis
The thermal behaviour of the grafted nylon samples depends mainly on two
parameters :
I) Percentage graft-on, and
11) Nature of the monomer.
The results of the thermogravimetric analysis of the parent yarn and the
graft copolymers of acrylamide and acrylonitrile on nylon with varying grafton are given in Tables 2 to 4. The primary thermograms obtained from TGA
curves are given in Figures 5 and 6.
The decomposition temperatures, TD, a t different weight losses are given in
Table 2. It is seen in both the graft copolymers, that the decomposition tem-
196
Thermal Behaviour
Concentration of monomer ('lo)
Fig. 3.
Table 2.
Effect of monomer concentration on grafting of nylon 6.
Decomposition temperatures (TD)a t different weight losses.
Sample
Parent
Acrylamide
grafted
Acrylonitrile
grafted
!
Graft-
%
On
TD ("C) Corresponding to the weight loss of
10% 120%
I
30% I400;,
I
50%
I
60%
[
70%
I I
80%
90%
nil
368
386
396
408
416
424
429
434
456
15.8
27.9
51.7
67.5
343
330
266
288
363
348
328
331
380
371
347
350
393
384
364
369
400
391
383
383
409
405
405
396
422
431
457
420
454
469
510
491
494
498
530
519
17.9
20.5
27.8
308
335
346
347
372
372
372
390
388
386
398
400
402
405
408
433
424
427
482
477
484
514
500
500
537
520
525
peratures at low percentages of weight loss is less than t h a t of parent yarn,
indicating less thermal stability at earlier stages. This is found t o be the case
197
D. S. VARMAand S. RAVISANKAR
Acrylamide
T i m e (hours)
Fig. 4. Effect of time on grafting of nylon 6.
Temperature ("C)
Fig. 5.
198
Primary thermograms of parent nylon 6 and nylon 6 grafted with acrylamide. 1 - Parent yarn; 2 - 15,8% graft-on; 3 - 21,9% graft-on; 4 - 51,7%
graft-on; 5 - 67,5% graft-on.
Thermal Behaviour
Table3.
IPDT and IDT values of grafted samples.
1
Sample
Graft-on
I IPDT("C) I
(O/,,)
IDT ("C)
Parent
nil
409
280
Acrylonitrile grafted
Acrylonitrile grafted
Acrylonitrile grafted
17.9
20.5
27.8
417
422
425
270
270
280
Acrylamide grafted
Acrylamide grafted
Acrylamide grafted
Acrylamide grafted
15.8
27.9
51.7
67.5
405
402
402
396
260
260
220
220
Table 4.
Activation energy values of nylon 6 and acrylonitrile and acrylamide
grafted nylon 6.
Sample
Graft-on
Ti
(YO)
( O K )
(OK)
553
533
733
543
733
543
743
533
513
503
743
503
743
773
723
823
723
833
733
833
783
773
733
833
733
823
Parent
Nil
Acrylonitrile Grafted
17.9
20.5
27.8
Acrylamide Grafted
15.8
27.9
51.7
67.5
Tf
K kcal/
mole
54
20
16
26
14
33
15
45
41
28
14
28
11
up to 60% weight loss in the case of acrylamide grafted samples and up to 50%
weight loss in the case of acrylonitrile grafted samples. This may be due to the
fact that acrylamide compared to nylon decomposes a t lower temperatures.
I n the case of acrylonitrile grafted samples, initial loss of HCN and NH3 from
the grafted structure a t low temperatures may account for a higher loss a t
lower temperature. But a t higher weight loss, acrylonitrile grafted samples
show higher decomposition temperatures, for example, a t 70% weight loss the
decomposition temperatures are 482"C, 477°C and 484°C for graft-on of 17.9,
20.5 and 27.8%respectively. At higher percentage of decomposition cyclization
becomes significant and thus the stability increases. The ring structures formed
in acrylonitrile grafts a t higher temperatures19 are highly stable and due to this
stability, the decomposition temperature is higher a t higher percentage of
199
D. S. VARMAand S. RAVISANKAR
weight losses as compared to that of parent yarn where 70% weight loss occurs
a t 429 "C.
Temperature ("C)
Fig. 6.
Primary thermograms of parent nylon 6 and nylon 6 grafted with acrylonitrile. 1 - Parent \yarn; 2 - 17,9% graft-on; 3 - 20,5y0 graft-on; 4 - 27,Syh
graft-on.
It has been reported in the literature that polyacrylonitrile decomposes to
give HCN and NH3 a t lower temperatures, but a t higher temperatures an increase in thermal stability is observed. Studies of polyacrylonitrile pyrolysis in
vacuum a t 250"C, 275"C, 300°C and 325°C indicated a weight loss of 21.5,
34.9, 46.6 and 50.6% respectively223 23. NAGAO
et al.24 have reported considerable evolution of HCN when acrylonitrile is heated a t 200 to 350°C in air or
nitrogen. Thus we can take a specific example of acrylonitrile grafted sample.
If we take the decomposition a t 430"C, it is seen that the residual weight left
of parent yarn is 37%, and it has been reported that for polyacrylonitrile the
residual weight is 30% a t this temperature. So if we consider the 27.8% graft
copolymerised sample, then on calculation the residual weight should have been
35% a t 430 "C, but it is actually 40.2y0,and this indicates the increase in thermal
stability.
The same trend of higher stability a t higher temperatures is observed in
polyacrylamide grafted samples also, but comparatively the temperature of
200
Thermal Behaviour
decomposition is lower than that of acrylonitrile grafted samples. For example,
a t 80% weight loss, the acrylamide grafted samples show decomposition temperatures of 454"C, 469"C, 510°C and 491 "C for graft-on of 15.8,27.9,51.7 and
67.5% respectively, whereas for the same weight loss, the acrylonitrile grafted
samples show decomposition temperatures of 514"C, 500"C, 500°C for graft-on
of 17.9, 20.5 and 27.8% respectively, 80% decomposition in the parent yarn
occurs a t 434°C. This means that the thermal stability in both cases a t higher
temperature is higher than parent yarn. I n acrylonitrile grafted samples, this
is attributed to the ring structure formation as already explained. Cyclization
in the case of acrylamide may also take place and this will confer stability on
the resulting polymer. At lower temperatures, polyacrylamide decomposes to
give water, NH3 etc., but a t higher temperatures a cyclic product similar to
polyacrylonitrile may be formed as a result of the dehydration reaction :
H H H H H H
\c/
H H H H H H
y
\c/
\c/
/ \ / \ / \ /
CH
I
6
/ \
H2N
CH
I
c
I\
CH
I
c
I\
-+
\/
y
/ \ / \ / \ /
CH
CH
CH
I
I
I
0 HzN 0 H2N 0
However, no work has been reported about such ring formation in polyacrylamide, but in similarly constitmuted
aromatic polyhydrazides a cyclization reaction has been reported and this leads to the formation of cyclic 1.3.4-oxadiazoles25.
Since cyclization in the case of polyacrylamide requires the loss of water,
stability is achieved a t higher percentage of weight losses in these samples.
From the primary thermograms it is also quite evident that a t lower temperatures the thermal stability of grafted material is lower than that of parent
yarn whereas the trend is reversed a t higher temperatures, particularly above
420°C in the case of acrylonitrile grafted samples and 430°C in the case of
acrylamide grafted samples.
Table 3 gives the values of IPDT calculated as given by D O Y L Eand
~ ~the
values of IDT of the parent and grafted samples. It is very clear that the IPDT
values increase gradually in the case of acrylonitrile grafted samples and the
increase is in line with the increase in graft-on. It is also to be noted that there
is not much of a difference in IDT value also. This difference in IPDT value
indicates the overall stability increase of the graft copolymer with increase in
graft-on. The increased ring formation a t higher temperatures may be responsible
for this higher stability.
201
D. S. VARMA
and S. RAVISANKAR
I n acrylamide grafted samples, even though there is higher stability a t
higher temperatures, maximum degradation occurs a t considerably lower
temperatures, and the increase in stability a t higher temperature is also not
considerable. The reduction in IPDT proves overall reduction in thermal stability with increase in graft-on. It is to be noted that IDT value also reduced
considerably with increase in graft-on indicating decomposition even a t lower
temperatures. Hence, by looking a t IPDT values, it can be concluded that the
grafts of acrylonitrile on nylon increases overall thermal stability and that of
acrylamide reduces overall stability.
The activation energy has been calculated using the equation given by
DHARWADKAR
and K A R K H A N A W A
: LA~~
lnln(l-a)-l
=
~.E ~. loo
RTi2 (Tf-Ti)
e
+c
where
a = Fraction reacted
E = Activation energy
Ti = Temperature of inception of reaction
Tf = Temperature of completion of reaction
constant
R = gas constant
e = Difference between Tsand temperature under consideration
T, = Temperature at the point of inflection on the thermogram.
c =
The equation is independent of sample size and heating rate.
From the primary thermogram, Ti and Tf were found out. The value of a
was found out a t a temperature difference of 10°C and from this, the value of
In In (1 - a)-1 was computed. The T, value was found as follows:
The rate of decomposition
(
~
)was calculated by finding the weight loss
a t 6°C interval from the primary thermogram (since the heating rate was
(2
S"C/min).Then -)was polotted against (T).The maximum peak in the curve
was taken as T, value. The thermograms were critically analysed to find out
the correct value of T,. (T - T,) values were found for the whole temperature
range. The values of In In ( 1 - a)-l were plotted against (T - T,) and reasonable straight lines were obtained (Fig. 7). If the reaction is single stage then
there will be only one T, as seen in parent yarn and acrylamide grafted samples
of 15.8 and 21.9% graft on. The other samples, i.e. acrylonitrile grafted and
acrylamide grafted samples of 51.7 and 67.5% graft-on, give two stage reactions
giving values of T, a t two different temperatures. I n such cases, values of
(T - T,) were found out a t two different temperature ranges resulting in two
straight lines giving different slopes. Hence activation energy was also calculated separately for two different temperature ranges. The slope of these lines
202
Thermal Behawiour
-7
-50 -30 -10
8 = (T - T,) ("C)
-130 -110 -90 -70
Fig. 7.
10
30
50
A plot of In In (1 -a)-1 versus 0 for parent nylon 6.
were found out which gives In In ( 1 - a)-l/O and then the values of E were
calculated using equation (1). The results are given in Table 4.
E calculated for nylon 6 parent yarn shows a higher value as compared to
that reported by OZAWA27. This variation may be due to a difference in the
molecular weight of the samples.
The activation energy values, obtained with acrylonitrile and acrylamide
grafted samples, cannot be compared, since. in former case, a twostep mechanism
is operating, while in the latter case up to certain graft-on (27.9%)a one step
mechanism and for higher grafted samples a two step mechanism operates.
Similar comparison with parent yarn may also be misleading. Anyhow, the
results of samples with different graft-on of the same monomer can be compared.
It is seen that within the region of 250°C to 450"C, which is the first stage in
all the samples having different graft-on of acrylonitrile, the E value increases
with increase in graft-on. I n the region of 450°C to 550"C, which is the second
stage, the value of E does not show much of a difference. The E values are:
16 kcal/mole, 14 kcal/mole and 15 kcal/mole for samples of 17.9, 20.5 and
27.0% graft-on.
203
D. S. VARMA
and S. RAVISANKAR
The acrylamide samples show a gradual decrease in the E values with increase
in graft-on as seen from Table 4. If the values of E are compared for the various
graft-on of acrylonitrile, a gradual increass in the activation energy in the
first stage of the reaction is obtained with an increase in graft-on. Here activation energy may be used as a criterion for stability. On this basis it may be
concluded that as the graft-on of acrylonitrile increases, the stability of the
products is also increased.
I n the case of acrylamide grafted samples, there is a decrease in E values
with increase in graft-on. This perhaps indicates an overall reduction in stability
with increase in graft-on. The IPDT values also lead to the same conclusion.
However, when one is comparing decomposition temperatures a t various
weight losses above SO%, then it appears that the stability has increased. At
this stage, therefore, it is not possible t o make any decisive comments on the
thermal behaviour of acrylamide grafted samples. For the acrylonitrile grafted
samples, it may be said that the thermal stability has improved.
The authors thank Dr. (Mrs.) I. K. VARMA,Assistant Professor in the
Chemistry Department, I.I.T., Delhi, for many helpful discussions.
3
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and R. H. MARCHESSAULT,
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1
'
2
515.
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6
7
*
(1962) 235.
(1963) 639.
(1963) 615.
15
16
17
18
204
Thermal Behaviour
19
20
21
22
23
24
25
26
27
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