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The influence of crosslinking on isothermal crystallization of LDPE filled with silica
Eur. Polym.J. Vol. 28, No. 12, pp. 154%1552, 1992 Printed in Great Britain. All rights reserved
0014-3057/92$5.00+0.00 Copyright © 1992PergamonPress Ltd
THE INFLUENCE OF CROSSLINKING ON ISOTHERMAL CRYSTALLIZATION OF LDPE FILLED WITH SILICA I. JANIGOV,~, I. CHODAK and I. CHORVATH Polymer Institute, Slovak Academy of Sciences, 84236 Bratislava, Czechoslovakia
(Received 7 April 1992) A~traet--The effect of crosslinking on the isothermal crystallization of low density polyethylene filled with three types of silica was investigated at 375 K. The crosslinking initiated by thermal decomposition of peroxide leads to overall retardation of crystallization. The presence of a filler in the polymer matrix results in the two opposing effects of nucleation and retardation. Crosslinking in the presence of a filler leads to the formation of substantial amounts of an immobile phase resulting in rather unexpected effects, explained by combination of nucleation, retardation and pre-ordering of chains. Comparison of the three types of fillers shows that those with large surface area affect the crystallization to the greatest extent.
x(t)
INTRODUCTION
The crystallization of polyethylene has been studied by many authors, the problem being considered from various aspects [1-10]. It was found that crystallization rate parameters are significantly affected by chemical crosslinking [7-10] as well as by the presence of various additives [11]. Crosslinking results in an increase in chain defects in polymer chains and also leads to reduced mobility of segments. The mobility of chains is also affected by the presence of a filler as a result of adsorption or occlusion of polymer chains [11]. Dispersed additives act as nucleating agents [11]. A similar effect can be caused by the presence of crosslinks [12]. The effects of crosslinking and filler on the melting and crystallization of semicrystalline polymers are ambiguous. The influences of crosslinks and the filler acting together are almost impossible to predict and each case has to be considered separately. Crystallization can be studied by several methods which follow changes in properties sensitive to difference in the degree of crystallinity, e.g. specific volume, crystallization enthalpy, birefrigence, [6]. In this paper, calorimetry was used to investigate the isothermal crystallization of low density modified polyethylene. The method has been widely used [4, 5, 7-11, 13]. The results are usually evaluated by the Avrami equation [14]:
x(t)
= 1 - e -k'°
x(t)=Ifo~(dAH/dt)dt]/[fo (dAH/dt)dt]
x(t) = A,/A
2S/12~
(3)
A t corresponding
to the heat generated during crystallization time t and A is related to the overall crystallization heat. The relative crystallization rate is calculated as the slope of the dependence x(t) = f ( t ) . Although this procedure does not indicate the mode of crystallization, it provides a simple procedure for comparison of crystallization rates of materials. Moreover, the crystallization half-time, which is easy to determine, is proportional to the Avrami rate constant according to the simple relation [14]: k = In 2/t~.5.
(4)
In this paper, DSC was used to determine the changes in the parameters of isothermal crystallization of polyethylene caused by the presence of three types of inorganic fillers in a polymeric matrix crosslinked to different degrees by various concentrations of peroxide. The effects of both filler and crosslinking were investigated separately and also when acting simultaneously. EXPERIMENTAL PROCEDURES
Low density polyethylene (LDPE), Bralen RA 2-19 (Slovnaft, Czechoslovakia MF1 = 2.0 __+0.3), was used as a matrix with three types of silica, viz: Ultrasil VN3 (Degussa, surface area 139 mZ/g), Komsil (Kovohute Mni~ek, surface area 21 m2/g) and Siloxid (Tonaso Ne~temice, surface area 72m2/g). 2,5-dimethyl-2,5-di-t-butyl peroxyhexyne-3 (Luperox 130) was used as the initiator of crosslinking. The samples were prepared in the mixing cha~nber of a Brabender plastograph PLO 331 at 155°C during 5 rain at 75 rpm. The mixed samples were compression moulded at 180°C for 20 min; >99% of the peroxide decomposed under these conditions. The gel fraction was determined from the
1547 EPJ
(2)
The integrals are expressed as the areas under the DSC curve, i.e.
(1)
The rate constant k of isothermal crystallization, as well as the Avrami coefficient n indicating the mode of spatial crystal growth, can be calculated using equation (1). The Avrami equation is usually valid for a limited range of the process, the deviation from linearity being obvious with increasing conversion. With increasing crystallization temperature, the range of linearity becomes narrower as demonstrated by Philips and Kao [8] for polyethylene. When the isothermal crystallization is investigated by DSC, the relative amount of crystalline content
developed in time t corresponds to the equation
[14]:
I. JANIC_,OVk et al.
1548
1.0
--
•
•
]
6. A
/A
/
/
,
•
•
•
•
x x
/0
2_/
:
/°
_
×
/x~
×
x
200
x
x x
x .
"
•
•
" . . . t ;~,"
•
,.,
A 0
O
•
/e
/A
,1
ID
0 °
*
*
..
o
o
I
I
400
600
t (sec) Fig. I. A plot of the amount of crystalline portion of PE vs the time of crystallization for samples with
initiator content 0wt% (curve 1), 0.1 (2), 0.3 (3), 0.6 (4), 1.0 (5), 1.5 (6). weight loss resulting from extraction in boiling xylene for 14 hr. The crystallization was investigated using Perkin-Elmer DSC, model 2 with an on-line connection to a Tektronix 31 calculator. Indium and lead were used as temperature standards and indium for the heat of fusion (AHt, = 28.4 J/g). Isothermal crystallization was carried out using 4-18 mg samples. They were heated to 450 K at a heating rate of 160K/min and left for 10min. They were subsequently cooled at the same rate to the crystallization temperature, 375 K. RESULTS AND DISCUSSION
The influence of the initiator of crosslinking on isothermal crystallization of polyethylene The overall relative crystallization rate was evaluated according to equation (2). The dependence x(t) = f ( t ) for PE crosslinked to various degree are shown in Fig. 1. The crystallization rate, re, was calculated from the slope of linear part of the curve as indicated in Fig. 1. From the curve, the crystallization half-time t0.s was determined. Similarly, equation (1) was used to calculate the Avrami parameters k and n. Using the calculated Avrami exponent n, the crystallization half-time 6.s was used to evaluate the rate constant kto5 according to equation (4). Tables 1-4 show that the values of the Avrami coefficient n are influenced very much by the presence of filler and by crosslinking. Therefore, the crystallization rate constants cannot be used for direct comparison of crystallization rates of different
materials since not only the kinetics but also the mode of crystallization are influenced by modification. However, when comparing the Avrami rate constant k and rate constant k,05 calculated according to equation (4), it is obvious that most values differ within acceptable 25%, and the overall tendency of changing of both constants is the same. Lower values of k,o.s are understandable, since k~0s was calculated from t0.5 determined from that part of the curve where the Avrami plot is far from linear in most cases.
From this point of view, it seems to be entirely justified to discuss the effect of modification of LDPE on crystallization rate from the value v¢ determined according to equation (2). Data from the Avrami analysis can extend the considerations in terms of a possible effect on the mechanism of the process. Regarding the data on crosslinked LDPE (Table 1), the decrease of vc with increasing content of the initiator of crosslinking indicates that the increase in degree of crosslinking results in retardation of crystallization. This result matches the literature data [9]. The retardation of isothermal crystallization of crosslinked polyethylene is caused by the changed mobility of polymer chains and by the increase of the number of defects in the polymer chain due to crosslink formation. The rate constants rise with increased degree of crosslinking, an effect which may be ascribed to a nucleation effect of junction points. This change is overcompensated by increased proportion of defects and decreased mobility leading to lowering of the
Table 1. Changes in the overall heat of fusion AH, crystallization rate vc and half-time of crystallization t0.5 as a function of the a m o u n t o f the initiator of crosslinking (I) I (wt%)
AH (J/g)
t0.s (scc)
vc • 102 (scc- I)
n
k • 109 (sec-")
k,0.5.109 (sec -")
0.0 0. l
- 32.2 - 28.8
97.8 179. I
1.07 0.47
4.0 3.7
8.8 7.4
7. I 4.7
O.3 0.6 1.0 1.5
-- 25.9 --26.7 -- 17.6 -- 5.0
253.4 264.9 293.3 459.2
0.31 0.29 0.19 O. 13
3.0 2.7 2.8 2.6
65.5 456.0 96.0 369.0
39, I 301.0 51.0 300.0
Crosslinking and crystallization of L D P E
1549
Table 2. Chang© of AH, vc, t0.5 as a function of the amount of filler (~) in the mixture with LDPE (wt%)
AH
t0. s
v c • 102
(J/g)
(see)
(see-')
n
k ' 109
(see-')
109 (see-")
k,0s"
0
- 32.2
97.8
1.07
4.0
8.8
7. I
Ultrasil
5 I0 20 35
-36.4 - 30.9 - 30.9 -19.6
105.5 116.1 144.8 125.8
0.95 0.84 0.63 0.90
3.6 3.8 3.6 4.5
50.0 13.0 17.0 1.5
48.0 I 1.0 12.0 1.1
Komsil
5
-26.8
95.9
1.16
4.2
5.4
4.9
Siloxid
5 10 20 35
-23.6 -22.1 -22.6 -16.4
145.1 121.7 154.6 145.3
0.63 0.83 0.57 0.63
3.4 3.9 3.6 3.5
74.0 9.4 24,0 32.0
63.0 6.4 17.0 23.0
Note: Other samples with Komsil could not be measured because of crystallization being too rapid. Table 3. Changes of AH, vc, to.s of LDPE crosslinkcd with 0.8 wt% of Luperox as a function of the amount of the filler in the mixture (~) (wt%)
AH (J/g)
to.5 (sec)
n
k • 109 (scc-")
k,0.s • 109 (s~-")
r e ' 102
(sec ~)
0
--
--
0.23*
UItrasil
5 l0 20
-23.4 -18.0 -7.5
318.6 311.9 142.9
0.23 0.25 0.72
2.7 2.8 3.9
356.0 97.0 8.9
232.0 72.0 6.8
Komsil
5 l0 20 35
-20.5 -20.1 - 16.7 - 10.9
350.8 356.0 358.6 330.8
0.22 0.22 0.23 0.24
2.5 2.6 3.1 2.9
681.0 520.0 42.0 57.0
413.0 272.0 20.0 39.0
Siloxid
5 I0
- 11.3 -6.5
176.8 121.1
0.50 0.87
3.1 4.1
97.0 5.2
81.0 3.9
Note: The value was determined by extrapolation from temperature dependence [17]. The heats of fusion for samples with 35 wt% of Ultrasil and 20 and 35 wt% of Siloxid were below the threshold sensitivity of the instrument.
s p a t i a l g r o w t h o f c r y s t a l s as i n d i c a t e d b y d e c r e a s i n g o f n.
The influence of the presence of a filler on isothermal crystallization of PE Table 2 shows the crystallization parameters for t h e P E m i x t u r e s w i t h t h e t h r e e d i f f e r e n t t y p e s o f silica. T h e r a t e o f c r y s t a l l i z a t i o n o f t h e m i x t u r e s filled w i t h U l t r a s i l o r S i i o x i d d e c r e a s e d w i t h i n c r e a s e o f filler content, the only exception being the mixture with 3 5 % filler.
On the other hand, crystallization of the mixtures containing Komsil was accelerated by the presence of t h e filler s o m u c h t h a t it c r y s t a l l i z e d b e f o r e all experimental operations had been completed. The o n s e t o f c r y s t a l l i z a t i o n c o u l d n o t be d e t e c t e d . I n T a b l e 2, t h e o n l y m e a s u r e d v a l u e is t h a t f o r t h e s a m p l e c o n t a i n i n g 5 % w t o f t h e filler. T h e o b s e r v e d phenomena c a n be e x p l a i n e d b y t w o p a r a l l e l p r o c e s s e s w i t h o p p o s i t e effects: (a) T h e a d s o r p t i o n o f t h e p o l y m e r at t h e filler s u r f a c e r e s u l t i n g in a d e c r e a s e o f t h e m o b i l i t y
Table 4. Changes of AH, vc, t0.5 for a mixture of LDPE with 20 wt% of filler dependent on the concentration of the initiator of crosslinking (I) I (wt%)
AH (J/g)
to.5 (see)
vc ' l02 (see- ~)
144.8 195.5 173.2 135.1
1.52
-30.9 -13.4 -11.3 -3.2 . .
0.63 0.43 0.50 0.85 . .
Komsil
0.1 0.3 0.6 1.0 1.5
-28.8 -28.0 -24.7 -8.8 -2.5
212.9 265.4 373.9 353.6 161.5
0.38 0.30 0.20 0.22 0.63
Siloxid
0.0 0.3 0.6 1.0 1.5
-22.6 - 12.2 - 8.7 -7.5 - 1.5
154.6 153.6 142.9 138.9 136.1
0.57 0.60 0.69 0.72 0.83
Ultrasil
0.0 0.1 0.3 0.6 1.01
. .
. .
n
k" 109 (scc-")
kfo5 " 109 (sec ")
3.6 3.1 3.1 4.4
17.0 112.0 157.0 0.6
12.0 79.0 127.0 0.4
2.8 2.8 2.8 3.0 3.6
252.0 262.0 77.0 39.0 11.0
198.0 191.0 36.0 28.0 10.0
3.6 3.6 4.0 4.1 4.2
24.0 18.0 3.0 2.1 2.0
17.0 12.0 2.2 1.5 1.7
. .
. .
Notes: )The measured heat of fusion is below the threshold sensitivity of the instrument, 2Could not be measured because crystallization was too rapid (see also the note to Table 2). The sample with 0,1 wt% of peroxide and 20% of Siloxid was not measured for experimental reasons.
1550
I. JANIGOV~et al. of the chains leading to a decrease of the crystallization rate. This phenomenon prevails when filler with a large surface area (active fillers Ultrasil and Siloxid) is present in the mixture. At the same time, a decrease of the crystalline portion, quantitatively proportional to the heat of fusion, is observed. (b) The filler acts as a nucleating agent and its presence leads to increase of the crystallization rate. This phenomenon obviously also plays a role in the mixtures with Ultrasil or Siloxid but it is compensated and overlapped by the effect of adsorption of the polymer at the filler surface. On the other hand, the effect of adsorption is much smaller with a low-active filler Komsil and the accelerating nucleation effect prevails in this case. (c) A very large part of the polymer interacts closely with the filler at the high filler content. The situation may arise that the mobility of chains is retarded to such an extent that statistical ordering of the chains cannot be reached even in the melt. Some order of the chains is maintained and the ordering of chains to the regular shape of crystallites is much easier, resulting in an increase of the crystallization rate together with further decrease of the heat of fusion. This effect may lead to some kind of mixed type of crystallization as indicated by the high value of the parameter n in a sample containing the largest amount of the most active filler (35% wt Ultrasil).
The influence of the filler on isothermal crystallization at constant content of the initiator of crosslinking When investigating the isothermal crystallization of filled crosslinked polyethylene, a decrease in the mobility of macromolecules (due to crosslinking and adsorption at the filler surface) as well as nucleation (as a result of nucleating action of the filler) could be expected. The results show that the situation is more complicated. At fixed amount of the initiator of crosslinking (0.8 wt%), the half-time of crystallization decreases and the crystallization rate increases with increasing filler content in a polymer matrix (Table 3). The effect is large in samples with both active fillers Ultrasil and Siloxid unlike crosslinked samples with Komsil, where the changes of crystallization parameters with increasing filler content are marginal. Small changes in the samples with Komsil can be explained by considering two opposite effects influencing the rate of crystallization, viz. retardation due to crosslinking, and nucleation caused by the presence of filler. It is possible that both effects are more or less balanced, the effect of adsorption being only marginal for Komsil with small surface area. On the other hand, the adsorption on Ultrasil and Siloxid surface has to be considered as equal if not dominant compared with the other two effects, i.e. nucleation by filler and crosslinking. In the melt the molecules are also affected by adsorption. The filler acts as a sort of crosslink in this case and ordered structure of semicrystalline material is prevented to some extent. Therefore, as mentioned above, partially
pre-ordered structure persists in the melt. If crosslinking proceeds under these conditions in a melt in the presence of a filler, the pre-ordered structure is fixed on the filler surface by crosslinks to the permanent network and a morphology is formed which accelerates crystallization. On the other hand, in the absence of filler, the ordering of macromolecules in the melt is random and fixation of this structure by crosslinking leads to formation of an increased amount of structural defects which retard crystallization. The overall crystallinity portion is reduced by crosslinking in both cases, as expected. Increase of n and parallel drop of k with increasing concentration of active fillers Ultrasil and Siloxid is of interest. Noting the rather small crystallizable portion as indicated by small AH values, inhomogeneous microstructure can be considered with most of the polymer being adsorbed at the filler surface and densely crosslinked. This part of the matrix is virtually amorphous but ordered so that it can act as an effective nucleating agent for the rest of the matrix. The latter is less densely crosslinked so that small crystallites can be formed. Since the whole matrix is pre-ordered in the melt because of simultaneous effects of crosslinking and polymer adsorption as discussed above, a spatially more complicated structure of crystallites is formed resulting in n values being around 4. The effect of the concentration of the initiator of crosslinking at constant filler content The considerations mentioned in the previous paragraph can also be used for explaining the dependences of the crystallization rate on the initiator concentration at constant filler content. Table 4 shows that the rate of crystallization rises with increasing content of active filler, i.e. Ultrasil or Komsil. This result can be explained by fixing of the chains on the filler surface resulting in some preordering of chains even in the melt. This preordering is fixed by crosslinking. On the other hand, in mixtures with Komsil, the preordering is slight because of low interaction between the filler and the polymer. Randomly ordered chains are therefore crosslinked and crosslinks hinder the crystallite growth leading to slower crystallization. Formation of insoluble portion The insoluble portion, determined by extraction in boiling xylene, depends on the degree of crosslinking as well as on the interaction between the polymer and the filler. Figure 2 shows the dependence of the insoluble portion on the filler concentration for two types of silica. It is seen that the insoluble portion is also formed in uncrosslinked mixtures as a result of binding the polymer into a physical network formed by the filler with irreversibly adsorbed polymer. The portion of the bound polymer in the mixture with Ultrasil being higher than that with Komsil corresponds to the activity of the filler expected from the surface area and at the same time it corresponds fully with the idea of lower mobility of the chains in the melt resulting in some ordering of macromolecules during crosslinking. The dependence of the gel formation on the initiator concentration in the composite with 20% of
Crosslinking and crystallization of LDPE 80
~
60
1
"' ~
tr-.-
-
*/
3
o~
0Z~'~4020
0
10
20
30
(wt %)
Fig. 2. The content of the gel formed vs the filler content in mixtures of LDPE with 0.8% of peroxide and Komsil (curve 1), 0.8 wt% of peroxide and Ultrasil (2), Ultrasil (3) and Komsil (4). filler, shown in Fig. 3, confirms that the suggested explanation of experimental results may be correct. No significant interaction between the filler and the polymer is expected in the samples with Komsil and no extensive adsorption of peroxide is anticipated. Crosslinking proceeds throughout the volume of the matrix in this case. On the other hand, some adsorption of peroxide on the filler surface can be expected in the presence of Ultrasil. The crosslinking of ordered regions close to the filler surface could be prevailing. Macromolecules in these ordered regions are bound into the physical network before crosslinking occurs. This effect leads to increased ordering of macromolecules resulting in faster crystallization. Another consequence is the observed decrease in gel content. This decrease is explained by the adsorption of peroxide at the filler surface since radicals formed lead mostly to an increase of the crosslink density of the material near to the polymer surface which is insoluble because bound into the physical network. Only some of the radicals formed by initiator decomposition contribute to the increase in the overall gel content because of reaction with macromolecules in a bulk matrix further from the filler surface. With increased peroxide concentration, more homogeneous distribution of peroxide occurs resulting in increase of the gel content in mixtures with Ultrasil and in higher crystallization rate in mixtures with Komsil. These considerations are also valid for the samples with Siloxid. This filler is rated between Ultrasil and
80
e~
1
-=:-
40
1551
Komsil if surface area is considered. Minor differences in chemical compositions of fillers must also be considered since they may affect the process of crosslinking. Changes in values o f the heat o f fusion No great attention was paid to the changes of the heat of fusion in the discussion above. The measured heat of fusion of LDPE of - 32.2 J/g corresponds to 11.2% crystallinity calculated according to the heat of fusion of a perfect crystal of PE being AHpE = 287 J/g [15]. The proportion of crystallinity corresponds fairly well with published data [16] for the temperature used in our experiments. A modifiction leads to a significant decrease of the crystalline portion, even approaching zero. The decrease is caused by hindering the mobility of macromolecules and by the presence of defects. The action of these two phenomena can be explained by two assumptions. On the one hand, the ordering of macromolecules in a crystal lattice is hindered resulting in a decrease of the overall crystallizable portion. An alternative explanation is based on the lowering of melting temperature caused by the presence of a filler or because of the crosslinking resulting in a decrease of the total crystallizable portion of the polymer. At the given temperature, a lower proportion of the polymer crystallizes since it is relative to the overall crystallizable portion. An unanswered question in this case refers to the effect of the substantial decrease of the crystallizable portion on the overall crystallization rate, since the rate is given as a ratio between the material crystallized in a time t and the overall heat of fusion determined at the given temperature. This effect has not been considered in this paper. CONCLUSIONS
The investigation of isothermal crystallization of filled and crosslinked LDPE showed that the parameters of crystallization are affected significantly by the modification. Chemical crosslinking leads to overall retardation of the crystallization. The presence of a filler in the polymer matrix results in two opposite effects going on in parallel viz. nucleation and retardation. The parallel effect of the filler and crosslinking results in the formation of a substantial amount of immobile phase which significantly retards the mobility of chains. Changes in the parameters of the crystallization are caused by the two opposing processes (nucleation and retardation). Comparison of the three types of filler shows that those with larger surface area affect the crystallization to a greater extent. Formation of an insoluble portion agrees with the conclusions drawn from DSC measurements. REFERENCES
20
0
0.5
1.0
1.5
Peroxide (wt %)
Fig. 3. The gel formed vs the concentration of the initiator Luperox 130 in samples of LDPE with: 20 wt% of Komsil (curve l), 20 wt% of Ultrasil (2), without the filler (3).
1. Y. K. Godovsky and G. L. Slonimsky. J. Polym. Sci. 12, 1053 (1974). 2. W. Banks, M. Gordon, J. R. Roe and A. Sharpies. Polymer 4, 61 (1963). 3. M. R. Kamal and E. Chu. Polym. Engng Sci. 23, 27 (1983). 4. Y. S. Yadav, P. C. Jain and V. S. Nanda. Thermochim. Acta 71, 313 (1983).
1552
I. JA_~C,OV~ et al.
5. J. Varga, A. Solti and J. Menczel. Periodica Polytechnica 22, 297 (1978). 6. A. Sharpies. Introduction to Polymer Crystallization. Edward Arnold, London (1966). 7. Y. H. Kao and P. J. Phillips. Polymer 27, 1669 (1986). 8. P. J. Phillips and Y. H. Kao. Polymer 27, 1679 (1986). 9. R. M. Gohil and P. J. Phillips. Polymer 27, 1687 (1986). I0. R. M. Gohil and P. J. Phillips. Polymer 27, 1696 (1986). I1. B. ~erjal, V. Musil, B. Pregrad and T. Malava[i~. Thermochim. Acta 134, 139 (1988).
12. I. Chod~k, I. Janigovfi and A. Romanov. Makromolek. Chem. 192, 2791 (1991). 13. E. Pedemonte, A. Turturro and G. Scrnino. Thermochim. Acta 137, 115 (1988). 14. P. C. Vilanova and S. M. Ribas. Polymer 26, 423 (1985). 15. E. Jager, J. M~llvr and B. J. Jungniekel. Prog. Coll. Polym. Sci. 71, 145 (1985). 16. D. Dosk~ilovfi, B. Schneider, J. Jake[ and P. Schmidt. 7th Eur. Symp. Polymer Spectroscopy, Dresden (1985). 17. I. Chorvfith. Thesis, Bratislava (1991).
0014-3057/92$5.00+0.00 Copyright © 1992PergamonPress Ltd
THE INFLUENCE OF CROSSLINKING ON ISOTHERMAL CRYSTALLIZATION OF LDPE FILLED WITH SILICA I. JANIGOV,~, I. CHODAK and I. CHORVATH Polymer Institute, Slovak Academy of Sciences, 84236 Bratislava, Czechoslovakia
(Received 7 April 1992) A~traet--The effect of crosslinking on the isothermal crystallization of low density polyethylene filled with three types of silica was investigated at 375 K. The crosslinking initiated by thermal decomposition of peroxide leads to overall retardation of crystallization. The presence of a filler in the polymer matrix results in the two opposing effects of nucleation and retardation. Crosslinking in the presence of a filler leads to the formation of substantial amounts of an immobile phase resulting in rather unexpected effects, explained by combination of nucleation, retardation and pre-ordering of chains. Comparison of the three types of fillers shows that those with large surface area affect the crystallization to the greatest extent.
x(t)
INTRODUCTION
The crystallization of polyethylene has been studied by many authors, the problem being considered from various aspects [1-10]. It was found that crystallization rate parameters are significantly affected by chemical crosslinking [7-10] as well as by the presence of various additives [11]. Crosslinking results in an increase in chain defects in polymer chains and also leads to reduced mobility of segments. The mobility of chains is also affected by the presence of a filler as a result of adsorption or occlusion of polymer chains [11]. Dispersed additives act as nucleating agents [11]. A similar effect can be caused by the presence of crosslinks [12]. The effects of crosslinking and filler on the melting and crystallization of semicrystalline polymers are ambiguous. The influences of crosslinks and the filler acting together are almost impossible to predict and each case has to be considered separately. Crystallization can be studied by several methods which follow changes in properties sensitive to difference in the degree of crystallinity, e.g. specific volume, crystallization enthalpy, birefrigence, [6]. In this paper, calorimetry was used to investigate the isothermal crystallization of low density modified polyethylene. The method has been widely used [4, 5, 7-11, 13]. The results are usually evaluated by the Avrami equation [14]:
x(t)
= 1 - e -k'°
x(t)=Ifo~(dAH/dt)dt]/[fo (dAH/dt)dt]
x(t) = A,/A
2S/12~
(3)
A t corresponding
to the heat generated during crystallization time t and A is related to the overall crystallization heat. The relative crystallization rate is calculated as the slope of the dependence x(t) = f ( t ) . Although this procedure does not indicate the mode of crystallization, it provides a simple procedure for comparison of crystallization rates of materials. Moreover, the crystallization half-time, which is easy to determine, is proportional to the Avrami rate constant according to the simple relation [14]: k = In 2/t~.5.
(4)
In this paper, DSC was used to determine the changes in the parameters of isothermal crystallization of polyethylene caused by the presence of three types of inorganic fillers in a polymeric matrix crosslinked to different degrees by various concentrations of peroxide. The effects of both filler and crosslinking were investigated separately and also when acting simultaneously. EXPERIMENTAL PROCEDURES
Low density polyethylene (LDPE), Bralen RA 2-19 (Slovnaft, Czechoslovakia MF1 = 2.0 __+0.3), was used as a matrix with three types of silica, viz: Ultrasil VN3 (Degussa, surface area 139 mZ/g), Komsil (Kovohute Mni~ek, surface area 21 m2/g) and Siloxid (Tonaso Ne~temice, surface area 72m2/g). 2,5-dimethyl-2,5-di-t-butyl peroxyhexyne-3 (Luperox 130) was used as the initiator of crosslinking. The samples were prepared in the mixing cha~nber of a Brabender plastograph PLO 331 at 155°C during 5 rain at 75 rpm. The mixed samples were compression moulded at 180°C for 20 min; >99% of the peroxide decomposed under these conditions. The gel fraction was determined from the
1547 EPJ
(2)
The integrals are expressed as the areas under the DSC curve, i.e.
(1)
The rate constant k of isothermal crystallization, as well as the Avrami coefficient n indicating the mode of spatial crystal growth, can be calculated using equation (1). The Avrami equation is usually valid for a limited range of the process, the deviation from linearity being obvious with increasing conversion. With increasing crystallization temperature, the range of linearity becomes narrower as demonstrated by Philips and Kao [8] for polyethylene. When the isothermal crystallization is investigated by DSC, the relative amount of crystalline content
developed in time t corresponds to the equation
[14]:
I. JANIC_,OVk et al.
1548
1.0
--
•
•
]
6. A
/A
/
/
,
•
•
•
•
x x
/0
2_/
:
/°
_
×
/x~
×
x
200
x
x x
x .
"
•
•
" . . . t ;~,"
•
,.,
A 0
O
•
/e
/A
,1
ID
0 °
*
*
..
o
o
I
I
400
600
t (sec) Fig. I. A plot of the amount of crystalline portion of PE vs the time of crystallization for samples with
initiator content 0wt% (curve 1), 0.1 (2), 0.3 (3), 0.6 (4), 1.0 (5), 1.5 (6). weight loss resulting from extraction in boiling xylene for 14 hr. The crystallization was investigated using Perkin-Elmer DSC, model 2 with an on-line connection to a Tektronix 31 calculator. Indium and lead were used as temperature standards and indium for the heat of fusion (AHt, = 28.4 J/g). Isothermal crystallization was carried out using 4-18 mg samples. They were heated to 450 K at a heating rate of 160K/min and left for 10min. They were subsequently cooled at the same rate to the crystallization temperature, 375 K. RESULTS AND DISCUSSION
The influence of the initiator of crosslinking on isothermal crystallization of polyethylene The overall relative crystallization rate was evaluated according to equation (2). The dependence x(t) = f ( t ) for PE crosslinked to various degree are shown in Fig. 1. The crystallization rate, re, was calculated from the slope of linear part of the curve as indicated in Fig. 1. From the curve, the crystallization half-time t0.s was determined. Similarly, equation (1) was used to calculate the Avrami parameters k and n. Using the calculated Avrami exponent n, the crystallization half-time 6.s was used to evaluate the rate constant kto5 according to equation (4). Tables 1-4 show that the values of the Avrami coefficient n are influenced very much by the presence of filler and by crosslinking. Therefore, the crystallization rate constants cannot be used for direct comparison of crystallization rates of different
materials since not only the kinetics but also the mode of crystallization are influenced by modification. However, when comparing the Avrami rate constant k and rate constant k,05 calculated according to equation (4), it is obvious that most values differ within acceptable 25%, and the overall tendency of changing of both constants is the same. Lower values of k,o.s are understandable, since k~0s was calculated from t0.5 determined from that part of the curve where the Avrami plot is far from linear in most cases.
From this point of view, it seems to be entirely justified to discuss the effect of modification of LDPE on crystallization rate from the value v¢ determined according to equation (2). Data from the Avrami analysis can extend the considerations in terms of a possible effect on the mechanism of the process. Regarding the data on crosslinked LDPE (Table 1), the decrease of vc with increasing content of the initiator of crosslinking indicates that the increase in degree of crosslinking results in retardation of crystallization. This result matches the literature data [9]. The retardation of isothermal crystallization of crosslinked polyethylene is caused by the changed mobility of polymer chains and by the increase of the number of defects in the polymer chain due to crosslink formation. The rate constants rise with increased degree of crosslinking, an effect which may be ascribed to a nucleation effect of junction points. This change is overcompensated by increased proportion of defects and decreased mobility leading to lowering of the
Table 1. Changes in the overall heat of fusion AH, crystallization rate vc and half-time of crystallization t0.5 as a function of the a m o u n t o f the initiator of crosslinking (I) I (wt%)
AH (J/g)
t0.s (scc)
vc • 102 (scc- I)
n
k • 109 (sec-")
k,0.5.109 (sec -")
0.0 0. l
- 32.2 - 28.8
97.8 179. I
1.07 0.47
4.0 3.7
8.8 7.4
7. I 4.7
O.3 0.6 1.0 1.5
-- 25.9 --26.7 -- 17.6 -- 5.0
253.4 264.9 293.3 459.2
0.31 0.29 0.19 O. 13
3.0 2.7 2.8 2.6
65.5 456.0 96.0 369.0
39, I 301.0 51.0 300.0
Crosslinking and crystallization of L D P E
1549
Table 2. Chang© of AH, vc, t0.5 as a function of the amount of filler (~) in the mixture with LDPE (wt%)
AH
t0. s
v c • 102
(J/g)
(see)
(see-')
n
k ' 109
(see-')
109 (see-")
k,0s"
0
- 32.2
97.8
1.07
4.0
8.8
7. I
Ultrasil
5 I0 20 35
-36.4 - 30.9 - 30.9 -19.6
105.5 116.1 144.8 125.8
0.95 0.84 0.63 0.90
3.6 3.8 3.6 4.5
50.0 13.0 17.0 1.5
48.0 I 1.0 12.0 1.1
Komsil
5
-26.8
95.9
1.16
4.2
5.4
4.9
Siloxid
5 10 20 35
-23.6 -22.1 -22.6 -16.4
145.1 121.7 154.6 145.3
0.63 0.83 0.57 0.63
3.4 3.9 3.6 3.5
74.0 9.4 24,0 32.0
63.0 6.4 17.0 23.0
Note: Other samples with Komsil could not be measured because of crystallization being too rapid. Table 3. Changes of AH, vc, to.s of LDPE crosslinkcd with 0.8 wt% of Luperox as a function of the amount of the filler in the mixture (~) (wt%)
AH (J/g)
to.5 (sec)
n
k • 109 (scc-")
k,0.s • 109 (s~-")
r e ' 102
(sec ~)
0
--
--
0.23*
UItrasil
5 l0 20
-23.4 -18.0 -7.5
318.6 311.9 142.9
0.23 0.25 0.72
2.7 2.8 3.9
356.0 97.0 8.9
232.0 72.0 6.8
Komsil
5 l0 20 35
-20.5 -20.1 - 16.7 - 10.9
350.8 356.0 358.6 330.8
0.22 0.22 0.23 0.24
2.5 2.6 3.1 2.9
681.0 520.0 42.0 57.0
413.0 272.0 20.0 39.0
Siloxid
5 I0
- 11.3 -6.5
176.8 121.1
0.50 0.87
3.1 4.1
97.0 5.2
81.0 3.9
Note: The value was determined by extrapolation from temperature dependence [17]. The heats of fusion for samples with 35 wt% of Ultrasil and 20 and 35 wt% of Siloxid were below the threshold sensitivity of the instrument.
s p a t i a l g r o w t h o f c r y s t a l s as i n d i c a t e d b y d e c r e a s i n g o f n.
The influence of the presence of a filler on isothermal crystallization of PE Table 2 shows the crystallization parameters for t h e P E m i x t u r e s w i t h t h e t h r e e d i f f e r e n t t y p e s o f silica. T h e r a t e o f c r y s t a l l i z a t i o n o f t h e m i x t u r e s filled w i t h U l t r a s i l o r S i i o x i d d e c r e a s e d w i t h i n c r e a s e o f filler content, the only exception being the mixture with 3 5 % filler.
On the other hand, crystallization of the mixtures containing Komsil was accelerated by the presence of t h e filler s o m u c h t h a t it c r y s t a l l i z e d b e f o r e all experimental operations had been completed. The o n s e t o f c r y s t a l l i z a t i o n c o u l d n o t be d e t e c t e d . I n T a b l e 2, t h e o n l y m e a s u r e d v a l u e is t h a t f o r t h e s a m p l e c o n t a i n i n g 5 % w t o f t h e filler. T h e o b s e r v e d phenomena c a n be e x p l a i n e d b y t w o p a r a l l e l p r o c e s s e s w i t h o p p o s i t e effects: (a) T h e a d s o r p t i o n o f t h e p o l y m e r at t h e filler s u r f a c e r e s u l t i n g in a d e c r e a s e o f t h e m o b i l i t y
Table 4. Changes of AH, vc, t0.5 for a mixture of LDPE with 20 wt% of filler dependent on the concentration of the initiator of crosslinking (I) I (wt%)
AH (J/g)
to.5 (see)
vc ' l02 (see- ~)
144.8 195.5 173.2 135.1
1.52
-30.9 -13.4 -11.3 -3.2 . .
0.63 0.43 0.50 0.85 . .
Komsil
0.1 0.3 0.6 1.0 1.5
-28.8 -28.0 -24.7 -8.8 -2.5
212.9 265.4 373.9 353.6 161.5
0.38 0.30 0.20 0.22 0.63
Siloxid
0.0 0.3 0.6 1.0 1.5
-22.6 - 12.2 - 8.7 -7.5 - 1.5
154.6 153.6 142.9 138.9 136.1
0.57 0.60 0.69 0.72 0.83
Ultrasil
0.0 0.1 0.3 0.6 1.01
. .
. .
n
k" 109 (scc-")
kfo5 " 109 (sec ")
3.6 3.1 3.1 4.4
17.0 112.0 157.0 0.6
12.0 79.0 127.0 0.4
2.8 2.8 2.8 3.0 3.6
252.0 262.0 77.0 39.0 11.0
198.0 191.0 36.0 28.0 10.0
3.6 3.6 4.0 4.1 4.2
24.0 18.0 3.0 2.1 2.0
17.0 12.0 2.2 1.5 1.7
. .
. .
Notes: )The measured heat of fusion is below the threshold sensitivity of the instrument, 2Could not be measured because crystallization was too rapid (see also the note to Table 2). The sample with 0,1 wt% of peroxide and 20% of Siloxid was not measured for experimental reasons.
1550
I. JANIGOV~et al. of the chains leading to a decrease of the crystallization rate. This phenomenon prevails when filler with a large surface area (active fillers Ultrasil and Siloxid) is present in the mixture. At the same time, a decrease of the crystalline portion, quantitatively proportional to the heat of fusion, is observed. (b) The filler acts as a nucleating agent and its presence leads to increase of the crystallization rate. This phenomenon obviously also plays a role in the mixtures with Ultrasil or Siloxid but it is compensated and overlapped by the effect of adsorption of the polymer at the filler surface. On the other hand, the effect of adsorption is much smaller with a low-active filler Komsil and the accelerating nucleation effect prevails in this case. (c) A very large part of the polymer interacts closely with the filler at the high filler content. The situation may arise that the mobility of chains is retarded to such an extent that statistical ordering of the chains cannot be reached even in the melt. Some order of the chains is maintained and the ordering of chains to the regular shape of crystallites is much easier, resulting in an increase of the crystallization rate together with further decrease of the heat of fusion. This effect may lead to some kind of mixed type of crystallization as indicated by the high value of the parameter n in a sample containing the largest amount of the most active filler (35% wt Ultrasil).
The influence of the filler on isothermal crystallization at constant content of the initiator of crosslinking When investigating the isothermal crystallization of filled crosslinked polyethylene, a decrease in the mobility of macromolecules (due to crosslinking and adsorption at the filler surface) as well as nucleation (as a result of nucleating action of the filler) could be expected. The results show that the situation is more complicated. At fixed amount of the initiator of crosslinking (0.8 wt%), the half-time of crystallization decreases and the crystallization rate increases with increasing filler content in a polymer matrix (Table 3). The effect is large in samples with both active fillers Ultrasil and Siloxid unlike crosslinked samples with Komsil, where the changes of crystallization parameters with increasing filler content are marginal. Small changes in the samples with Komsil can be explained by considering two opposite effects influencing the rate of crystallization, viz. retardation due to crosslinking, and nucleation caused by the presence of filler. It is possible that both effects are more or less balanced, the effect of adsorption being only marginal for Komsil with small surface area. On the other hand, the adsorption on Ultrasil and Siloxid surface has to be considered as equal if not dominant compared with the other two effects, i.e. nucleation by filler and crosslinking. In the melt the molecules are also affected by adsorption. The filler acts as a sort of crosslink in this case and ordered structure of semicrystalline material is prevented to some extent. Therefore, as mentioned above, partially
pre-ordered structure persists in the melt. If crosslinking proceeds under these conditions in a melt in the presence of a filler, the pre-ordered structure is fixed on the filler surface by crosslinks to the permanent network and a morphology is formed which accelerates crystallization. On the other hand, in the absence of filler, the ordering of macromolecules in the melt is random and fixation of this structure by crosslinking leads to formation of an increased amount of structural defects which retard crystallization. The overall crystallinity portion is reduced by crosslinking in both cases, as expected. Increase of n and parallel drop of k with increasing concentration of active fillers Ultrasil and Siloxid is of interest. Noting the rather small crystallizable portion as indicated by small AH values, inhomogeneous microstructure can be considered with most of the polymer being adsorbed at the filler surface and densely crosslinked. This part of the matrix is virtually amorphous but ordered so that it can act as an effective nucleating agent for the rest of the matrix. The latter is less densely crosslinked so that small crystallites can be formed. Since the whole matrix is pre-ordered in the melt because of simultaneous effects of crosslinking and polymer adsorption as discussed above, a spatially more complicated structure of crystallites is formed resulting in n values being around 4. The effect of the concentration of the initiator of crosslinking at constant filler content The considerations mentioned in the previous paragraph can also be used for explaining the dependences of the crystallization rate on the initiator concentration at constant filler content. Table 4 shows that the rate of crystallization rises with increasing content of active filler, i.e. Ultrasil or Komsil. This result can be explained by fixing of the chains on the filler surface resulting in some preordering of chains even in the melt. This preordering is fixed by crosslinking. On the other hand, in mixtures with Komsil, the preordering is slight because of low interaction between the filler and the polymer. Randomly ordered chains are therefore crosslinked and crosslinks hinder the crystallite growth leading to slower crystallization. Formation of insoluble portion The insoluble portion, determined by extraction in boiling xylene, depends on the degree of crosslinking as well as on the interaction between the polymer and the filler. Figure 2 shows the dependence of the insoluble portion on the filler concentration for two types of silica. It is seen that the insoluble portion is also formed in uncrosslinked mixtures as a result of binding the polymer into a physical network formed by the filler with irreversibly adsorbed polymer. The portion of the bound polymer in the mixture with Ultrasil being higher than that with Komsil corresponds to the activity of the filler expected from the surface area and at the same time it corresponds fully with the idea of lower mobility of the chains in the melt resulting in some ordering of macromolecules during crosslinking. The dependence of the gel formation on the initiator concentration in the composite with 20% of
Crosslinking and crystallization of LDPE 80
~
60
1
"' ~
tr-.-
-
*/
3
o~
0Z~'~4020
0
10
20
30
(wt %)
Fig. 2. The content of the gel formed vs the filler content in mixtures of LDPE with 0.8% of peroxide and Komsil (curve 1), 0.8 wt% of peroxide and Ultrasil (2), Ultrasil (3) and Komsil (4). filler, shown in Fig. 3, confirms that the suggested explanation of experimental results may be correct. No significant interaction between the filler and the polymer is expected in the samples with Komsil and no extensive adsorption of peroxide is anticipated. Crosslinking proceeds throughout the volume of the matrix in this case. On the other hand, some adsorption of peroxide on the filler surface can be expected in the presence of Ultrasil. The crosslinking of ordered regions close to the filler surface could be prevailing. Macromolecules in these ordered regions are bound into the physical network before crosslinking occurs. This effect leads to increased ordering of macromolecules resulting in faster crystallization. Another consequence is the observed decrease in gel content. This decrease is explained by the adsorption of peroxide at the filler surface since radicals formed lead mostly to an increase of the crosslink density of the material near to the polymer surface which is insoluble because bound into the physical network. Only some of the radicals formed by initiator decomposition contribute to the increase in the overall gel content because of reaction with macromolecules in a bulk matrix further from the filler surface. With increased peroxide concentration, more homogeneous distribution of peroxide occurs resulting in increase of the gel content in mixtures with Ultrasil and in higher crystallization rate in mixtures with Komsil. These considerations are also valid for the samples with Siloxid. This filler is rated between Ultrasil and
80
e~
1
-=:-
40
1551
Komsil if surface area is considered. Minor differences in chemical compositions of fillers must also be considered since they may affect the process of crosslinking. Changes in values o f the heat o f fusion No great attention was paid to the changes of the heat of fusion in the discussion above. The measured heat of fusion of LDPE of - 32.2 J/g corresponds to 11.2% crystallinity calculated according to the heat of fusion of a perfect crystal of PE being AHpE = 287 J/g [15]. The proportion of crystallinity corresponds fairly well with published data [16] for the temperature used in our experiments. A modifiction leads to a significant decrease of the crystalline portion, even approaching zero. The decrease is caused by hindering the mobility of macromolecules and by the presence of defects. The action of these two phenomena can be explained by two assumptions. On the one hand, the ordering of macromolecules in a crystal lattice is hindered resulting in a decrease of the overall crystallizable portion. An alternative explanation is based on the lowering of melting temperature caused by the presence of a filler or because of the crosslinking resulting in a decrease of the total crystallizable portion of the polymer. At the given temperature, a lower proportion of the polymer crystallizes since it is relative to the overall crystallizable portion. An unanswered question in this case refers to the effect of the substantial decrease of the crystallizable portion on the overall crystallization rate, since the rate is given as a ratio between the material crystallized in a time t and the overall heat of fusion determined at the given temperature. This effect has not been considered in this paper. CONCLUSIONS
The investigation of isothermal crystallization of filled and crosslinked LDPE showed that the parameters of crystallization are affected significantly by the modification. Chemical crosslinking leads to overall retardation of the crystallization. The presence of a filler in the polymer matrix results in two opposite effects going on in parallel viz. nucleation and retardation. The parallel effect of the filler and crosslinking results in the formation of a substantial amount of immobile phase which significantly retards the mobility of chains. Changes in the parameters of the crystallization are caused by the two opposing processes (nucleation and retardation). Comparison of the three types of filler shows that those with larger surface area affect the crystallization to a greater extent. Formation of an insoluble portion agrees with the conclusions drawn from DSC measurements. REFERENCES
20
0
0.5
1.0
1.5
Peroxide (wt %)
Fig. 3. The gel formed vs the concentration of the initiator Luperox 130 in samples of LDPE with: 20 wt% of Komsil (curve l), 20 wt% of Ultrasil (2), without the filler (3).
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1552
I. JA_~C,OV~ et al.
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