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Temperature effect on kinetics of isothermal crystallization of crosslinked filled LDPE—2. Particulate silica with high surface area as a filler
Eur. fol~m. J. Vol. 31, No. 3, pp. 271-274, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057195 $9.50 + 0.00
Pergamon
TEMPERATURE EFFECT ON KINETICS OF ISOTHERMAL CRYSTALLIZATION OF CROSSLINKED FILLED LDPE-2. PARTICULATE SILICA WITH HIGH SURFACE AREA AS A FILLER I. JANIGOVA P’olymer Institute,
Slovak
Academy
(Received 9 December
and I. CHODAK
of Sciences,
842 36 Bratislava,
1993; accepted 8 March
Slovak
Republic
1994)
Abstract-The kinetics of isothermal crystallization of low density polyethylene (LDPE) filled with silica (Ultrasil) and crosslinked by peroxide was investigated by a DSC method. It is suggested that the addition of high-surface-area silica as a filler results in a significant adsorption of LDPE chains on the filler surface, which plays an important role in the kinetics of isothermal crystallization. The isothermal crystallization of filled and crosslinked LDPE is influenced simultaneously by adsorption, nucleation, retardation and formation of pre-ordered structure.
INTRODUCTION
This paper is a continuation of two previous papers dealing with isothermal crystallization of crosslinked filled polyethyleae [l, 21. Several interesting features of the effect of crosslinking and/or filler presence have been described in these papers. The explanation of the dependences observed with low surface area silica is based on the opposite effects of nucleation and the decrease of the chain mobility due to either the presence of the filler or crosslinking. The hypothesis of preordered structure formation fixed by crosslinking as a permanent network is used to explain the extremes in the dependences of crystallization rate vs filler content or peroxide concentration. In this paper the outlined ideas will be checked for a composite of polyethylene filled with a filler with high surface area. Several possibilities for a calculation of the crystalization rate are described in the literature. Most common is the Avrami equation [3] .rc(t ) = 1 - edkm
(1)
where x (1) is relative crystalline portion formed during time t, k is the crystallization rate constant and n is the Avrami coefficient characterizing the mode of the crystal growth. If the value of Avrami coefficient n is known, the crystallization rate constant k can be also calculated according to a simple equation [4] k = In 2j+
(2)
being the crys.tallization halftime. However, in our previous papers [I, 21 the Avrami analysis was impossible to use. The reason was that if an Avrami coeflicient could not be considered as a constant, a rather small scatter in this parameter leads to a substantial scatter in crystallization rate constants which ,are difficult to compare. Since we were not able to determine the value of n precisely t,,z
enough (scatter around f5% or more was found) another mode of evaluation of the results had to be used. A different calculation of overall crystallization rate is based on the determination of a slope of the dependence [5] x (t ) =f(t ).
(3)
Neither crystallization rate constant is calculated in this case nor a different mode of crystal growth is considered. Strictly, the method should be used only for the comparison of similar materials in a narrow temperature range where no change of the crystallization mechanism can be assumed. However, in these cases the comparison of crystallization rate of different materials is more reliable than if Avrami parameters are used. The modified Arrhenius equation can be used for calculation of a temperature coefficient [6], when the isothermal crystallization is investigated at various temperatures V,=A
exp[-E/R(T,-
T,)]
(4)
where V, is overall relative crystallization rate, A is a constant, E is a temperature coefficient of crystallization, T, is a temperature of the sample being in isotropic state and T, is crystallization temperature. EXPERIMENTAL
PROCEDURES
Low density polyethylene (LDPE) Bralen RA 2-19 (Slovnaft, Slovak Republic, MFI = 2.0 f 0.3) was used as a matrix. In this work silica with a high surface area (Ultrasil VN3, Degussa, surface area 139 ml/g) was used as a filler. The crosslinking of LDPE matrix was carried out by 25dimethyl-2.5di-t-butyl oeroxy hexvne-3 (Luperox 130). The samples were prepaied in- the mixing chamber of a Brabender plastograph PLE 331 at 15S’C during 5 min at 75 rpm. The mixed samples were compression moulded at
1. Janigovi and 1. Chodik
212
180°C for 20min. More than 99% of the peroxide was decomposed under these conditions. The isothermal crystallization was investigated using a Perkin-Elmer DSC-2 with on-line connection to a Tektronix 31 calculator. Indium and lead were used as temperature standards and indium for the heat of fusion (AH,, = 28.4 J/g). The samples (weight418 mg) were heated to 450K at a heating rate 160K/min and kept for IO min. They were subsequently cooled at the same rate to the crystallization temperature. Isothermal crystallization was investigated at three different temperatures for each sample in the temperature range 374-377 K. This temperature range seems to be optimal from an experimental point of view since a certain induction period occurs before the beginning of crystallization and the rate of crystallization is high enough if the sensitivity of the equipment is considered. The measured and calculated values for the isothermal crystallization parameters at 375 K were taken from the paper VI. RESULTS
AND DISCUSSION
The eflect of the filler presence on the parameters of isothermal crystallization of LDPE Isothermal
crystallization
of LDPE filled with var-
ious amounts of Ultrasil was investigated in the temperature range 375-377 K. The parameters determined are given in Table 1. It is seen that the dependence of the crystallization rate V, vs filler content goes through a minimum at all experimental temperatures. At the same time the crystallization halftime has a maximum at the same filler concentration. This course differs substantially from the dependences obtained for the LDPE filled with a filler with low surface area, Komsil [2], since an increase of crystallization rate with increasing filler content was observed for these samples at any experimental temperature. This behaviour confirms the assumption that the filler surface area may have a significant influence on the crystallization of the composite matrix. In the case of the filler with low surface area, a nucleation in a summation of individual effects on the crystallization rate plays the decisive role. The Table I. Changes of the overall beat of fusion AH, half-time of crystallization I, ,2 and crystallization rate V, as a function of the amount of filler (@) in the mixture with LDPE at 375, 376 and 377 K AH
;a)
f, ,>
(see)
(J/s)
v;
102
-32.2 -36.4 -30.9 -30.9 - 19.6
97.8 105.5 116.1 144.8 125.8
The effect ofjller content on isothermal crystallization of LDPE crosslinked with constant amount of initiator
The isothermal crystallization of LDPE crosslinked with 0.8 wt% of peroxide and filled with various amount of the filler was investigated in the temperature range 374-376 K. The measured and calculated parameters are given in Table 2. It is seen that heat of fusion was found to be around zero in samples with high filler concentration at any experimental temperature. Low crystalline portion is caused by the effect of both crosslinking and filler content resulting in lower mobility of segments as well as extensive adsorption of polyethylene chains on the filler surface. The crystallization rate values have been found to rise at a filler content 20 wt%. At the same time the Table 2. Changes of AH, 1, >and l’, of LDPE crosslinked with 0.8 wt% of Luperox as a function of the amount of the filler(@) in the mixture at 374, 375 and 376 K @ (wt%)
(set-‘)
375 K 0 5 IO 20 35
adsorption is much less significant, because of low surface area of the filler. If Ultrasil with a much higher surface area was used, the adsorption of LDPE chains on the filler surface plays a significant role. Therefore, with increasing filler content the diminishing of the crystallization rate is observed. This tendency is changed at the highest filler concentration when the rate is rising compared to the lower filler amount. This phenomenon is hardly explicable by the increased nucleation since efficient formation of nuclei is usually caused by a much lower concentration of nucleation agent. It is reasonable to assume that the affect is a demonstration of decreased mobility of adsorbed polyethylene chains on the filler surface pronounced to such an extent that the crystalline ordered structure is not changed to a fully random one under experimental conditions. This effect is more pronounced in crosslinked samples and will be explained in more detail later in this paper. A significant decrease of the values of overall heat of fusion was observed only at a higher filler content. This observation corresponds with a decrease of the crystalline portion due to increased adsorption of the polymer at filler surface.
I .07 0.95 0.84 0.63 0.90
-36.4 -29.4 -33.4 -26.3 -21.9
0 5 IO 20 35
- 36.4 -34.5 - 34.7 -29.9 -26.2
245.5 156.4 200.6 235.5 208.5
0.34 0.63 0.43 0.36 0.45
447.6 286.6 441.2 453.2 398.4
0.18 0.29 0.17 0.16 0.20
11z (set)
v; IO? (set ’ )
165.8 169.9
0.52 0.52
374 K 5 IO 20* 35*
- 18.4 -14.1
5
-23.4 -18.0 - 7.5
375 K
316 K 0 5 IO 20 35
(G
:z 35t
318.6 311.6 142.9
0.23 0.25 0.72
517.5 573. I 339. I
0.14 0.13 0.26 -
376 K 5 IO
377 K ::+
- 20.0 - 16.5 -10.2
*Could not be measured because the crysvallization was too rapid. tThe measured heat of fusion is below the threshold sensitivity of the instrument.
273
Temperature effect on kinetics crystallization halftime is decreasing. At higher filler content a prevailing nucleation effect can be expected which overcompensates the adsorption effect. Crosslinking in the presence of higher filler amount leads to formation of a pre-ordered structure resulting in a higher rate of crystallization as suggested in our previous paper [2]. When compared to the crosslinked samples filled with low-surface filler Komsil [2], the samples filled with Ultrasil show a higher crystallization rate at all experimental temjperatures. The tendency of the changes in all samples is, however, similar, unlike the samples with Komsil content, where the changes are different at different temperatures. In the latter case with an increase of the crystallization rate at 374 K, no significant change at 375 K and a decrease of 376 K was observed with increasing filler content. This apparent disagreement corresponds well with our assumption regarding the effect of surface area on the crystallization course. No extensive adsorption of polyethylene or peroxide at the filler surface is expected in samples filled with low-surface filler Komsil and the crosslinking proceeds more or less homogeneously in the polyethylene melt. Adsorption of both peroxide and polymer can be assumed to occur if filler with high surface #area is present in the sample. The crosslinking is less homogeneous since higher local peroxide concentration is assumed near the filler surface. These parts are expected to be also more ordered because elf lower mobility of partially adsorbed polyethylene chains on the filler surface. A permanent network is formed in a not-fully random polymer and a pre-ordered structure is fixed. This is supposed to contribute significantly to the increased crystallization rate of LDPE if crosslinked in the presence of the filler. The effect of the crosslinking initiator concentration on the crystallization of LDPEjlled with constant silica content
Isothermal crystallization of LDPE filled with 20 wt% of Ultrasil and crosslinked to various crosslinking degrees was investigated in the temperature range 374-376 K. The crystallization kinetic parameters are given in Table 3. The crystallization rate of the sample crosslinked with 0.1 wt% of peroxide is lower and crystallization halftime higher than that of the uncrosslinked sample filled with 20% of the filler. However, with a further increase of crosslinking degree (i.e. with increasing peroxide concentration) the crystallization parameters indicate an increase of the crystallization rate. A similar course was also observed for the samples filled with low-surface filler [2]; nevertheless, the minimum on the dependence V, vs peroxide concentration was observed at higher initiator amount. This phenomenon fits well with the hypothesis of pre-ordered structure formation in the presence of the filler. In the melt, pre-ordered permanent structure is formed if cross-linking starts before original crystalline morphology is completely randomized. This situation can occur if crosslinking starts soon enough and a certain “critical” crosslinking degree is reached blefore a completely random structure is formed. The process can be influenced either by slowing down the formation of random structure
Table
3. Changes
of LDPE the
with
ofAH,
20 wt%
I,.~ and V, for a mixture of filler
as a function
of
concentrationof theinitiatorof crosslinking (I) at 314, 375 and 376 K
I
AH
(VA%)
II 2
(J/s)
(=)
(s@‘)
v;
102
I .OO
374 K 0. I
-
8.8
104.6
0.3
-
7.6
98.0
1.11
0.6 I .o*
-
2.9
88.8 -
1.49
1.5*
-
-
375 K 0.1
-
195.5
0.43
0.3
-11.3
173.2
0.50
0.6 I .o*
-
135.9
0.85 -
13.4 3.2
-
I .5*
376 K 0. I
-14.7
304.9
0.3
-10.0
304.8
0.6
-
254.6 -
4.8
I .ot
0.26 0.26 0.34
-
-
I 51 *Could
not he measured
tion
was
too
because the crystalliza-
raoid
fusion was below
as well
as the heat
the threshold
sensitivity
of of
the instrument. tThe
measured threshold
heat
sensitivity
of
fusion
is below
the
of the instrument.
after melting the polymer if the adsorption of polyethylene chains at the filler surface is extensive and strong enough to keep the structure together until crosslinking proceeds to a sufficient degree or by speeding up the crosslinking by the increase of the concentration of peroxide. In the case of low-surface filler, the random structure is formed rather quickly because the physical polymer-filler interactions are not dense and strong enough to keep the ordered structure until the crosslinking proceeds to a sufficient degree. Therefore at low peroxide concentration, significant peroxide decomposition occurs only after a random structure is formed and subsequent crosslinking contributes to the decreased mobility of the chains and lower crystallization rate. Only if the peroxide concentration is above a certain level, the crosslinking is fast enough to create sufficient density of crosslinks before a random structure is formed. From that concentration, the rate of crystallization starts to rise with increasing peroxide content. On the other hand, if high-surface filler is present the polymer-filler interactions are much stronger and a longer time is necessary to reach a completely random structure. Therefore, a density of crosslinks sufficient for the formation of a permanent preordered structure is reached at a lower peroxide concentration. A further increase of peroxide concentration leads to an increase of the crystallization rate because the critical crosslinking degree is reached in a shorter time, i.e. a more ordered structure is immobilized by a permanent network. The values of crystallization heat are decreasing with rising peroxide content, i.e. with increasing crosslinking degree the crystalline portion decreases. The reason for this is the same as described in our previous paper [2]. It is seen from Table 3 that at any temperature AH value was below the sensitivity of
I.Janigovi
214
Table 4. Changes of the values of temperature coefficient E calculated from equation (4) correlation coefficient r and the range of crystallizationtemperature in dependence on initiator concentration (I) and filler amount (@)
E ;%,
(C,
0.8 0.8 0.1 0.3 0.6
5 IO 20 35 5 IO 20 20 20
the equipment
(J/mole) -
961.2 1290.9 1091.6 1207.1 1210.7 1300.8 1246.3 1344.3 1381.9
r
Range of T (K)
0.9904 0.9984 0.9982 0.9999 0.9826 0.9974 0.9813 0.9946 0.9959
375-377 375s377 375s377 375-377 374-376 374316 374316 374-376 374-376
if 1% or more peroxide
was present in
the sample. Temperature coeficients of isothermal crystallization
The temperature coefficients as calculated from equation (4) are given in Table 4. Two facts are of interest. First, the scatter of Arrhenius-like plots for the samples filled with Ultrasil is rather small as seen from the correlation coefficients given in the table. Second, the temperature coefficients of the crystallization for almost all samples are within the experimental error of E evaluation. These features are completely different when compared to the same parameters calculated for low-surface filler [2], since in the latter case a distinct dependence of E on the composition of the samples was observed. This might mean that if the effects of nucleation vs adsorption are compared, the nucleation plays only a marginal role in crystallization of samples with high-surface filler unlike if low-surface filler is present where a nucleation effect is important compared to adsorp-
and I. Chodak
tion on the filler surface. The two processes with different thermal coefficients are crystal growth and nucleation. Crystal growth always has a similar thermal effect, no matter if it originates from the random melt or a pre-ordered structure. Since the samples filled with Ultrasil nucleation has only a marginal influence, similar values of thermal coefficients are fully understandable. CONCLUSIONS
The investigation of isothermal crystallization of LDPE filled with high surface area silica showed a significant influence of adsorption on the kinetic parameters. In a summation of two effects acting on behaviour of crystallization (nucleation and adsorption), adsorption of LDPE chain on the filler surface plays a more important role. The filling and crosslinking of LDPE results in a formation of a pre-ordered structure, which affects the parameters of isothermal crystallization together with adsorption, nucleation and retardation. REFERENCES
1. 2. 3. 4. 5.
JanigovB, I. Chodak and I. ChorvBth. Eur. P&n. J. 28, 1547 (1992). I. Janigova and I. Chodik. Eur. Polym. J. 30, 1105-l 110 (1994). Y. K. Godovsky and G. L. Slonimsky. J. Polym. Sci. 12, 1053 (1974). A. Sharples. Introduction to Polymer Crystallization. Arnold, London (1966) P. C. Vilanova and S. M. Ribas. Polymer 26, 423 I.
(1985).
6. J. Rychlji and I. Janigovi. Thermochim. Acra 215, 21 I (1993).
Pergamon
TEMPERATURE EFFECT ON KINETICS OF ISOTHERMAL CRYSTALLIZATION OF CROSSLINKED FILLED LDPE-2. PARTICULATE SILICA WITH HIGH SURFACE AREA AS A FILLER I. JANIGOVA P’olymer Institute,
Slovak
Academy
(Received 9 December
and I. CHODAK
of Sciences,
842 36 Bratislava,
1993; accepted 8 March
Slovak
Republic
1994)
Abstract-The kinetics of isothermal crystallization of low density polyethylene (LDPE) filled with silica (Ultrasil) and crosslinked by peroxide was investigated by a DSC method. It is suggested that the addition of high-surface-area silica as a filler results in a significant adsorption of LDPE chains on the filler surface, which plays an important role in the kinetics of isothermal crystallization. The isothermal crystallization of filled and crosslinked LDPE is influenced simultaneously by adsorption, nucleation, retardation and formation of pre-ordered structure.
INTRODUCTION
This paper is a continuation of two previous papers dealing with isothermal crystallization of crosslinked filled polyethyleae [l, 21. Several interesting features of the effect of crosslinking and/or filler presence have been described in these papers. The explanation of the dependences observed with low surface area silica is based on the opposite effects of nucleation and the decrease of the chain mobility due to either the presence of the filler or crosslinking. The hypothesis of preordered structure formation fixed by crosslinking as a permanent network is used to explain the extremes in the dependences of crystallization rate vs filler content or peroxide concentration. In this paper the outlined ideas will be checked for a composite of polyethylene filled with a filler with high surface area. Several possibilities for a calculation of the crystalization rate are described in the literature. Most common is the Avrami equation [3] .rc(t ) = 1 - edkm
(1)
where x (1) is relative crystalline portion formed during time t, k is the crystallization rate constant and n is the Avrami coefficient characterizing the mode of the crystal growth. If the value of Avrami coefficient n is known, the crystallization rate constant k can be also calculated according to a simple equation [4] k = In 2j+
(2)
being the crys.tallization halftime. However, in our previous papers [I, 21 the Avrami analysis was impossible to use. The reason was that if an Avrami coeflicient could not be considered as a constant, a rather small scatter in this parameter leads to a substantial scatter in crystallization rate constants which ,are difficult to compare. Since we were not able to determine the value of n precisely t,,z
enough (scatter around f5% or more was found) another mode of evaluation of the results had to be used. A different calculation of overall crystallization rate is based on the determination of a slope of the dependence [5] x (t ) =f(t ).
(3)
Neither crystallization rate constant is calculated in this case nor a different mode of crystal growth is considered. Strictly, the method should be used only for the comparison of similar materials in a narrow temperature range where no change of the crystallization mechanism can be assumed. However, in these cases the comparison of crystallization rate of different materials is more reliable than if Avrami parameters are used. The modified Arrhenius equation can be used for calculation of a temperature coefficient [6], when the isothermal crystallization is investigated at various temperatures V,=A
exp[-E/R(T,-
T,)]
(4)
where V, is overall relative crystallization rate, A is a constant, E is a temperature coefficient of crystallization, T, is a temperature of the sample being in isotropic state and T, is crystallization temperature. EXPERIMENTAL
PROCEDURES
Low density polyethylene (LDPE) Bralen RA 2-19 (Slovnaft, Slovak Republic, MFI = 2.0 f 0.3) was used as a matrix. In this work silica with a high surface area (Ultrasil VN3, Degussa, surface area 139 ml/g) was used as a filler. The crosslinking of LDPE matrix was carried out by 25dimethyl-2.5di-t-butyl oeroxy hexvne-3 (Luperox 130). The samples were prepaied in- the mixing chamber of a Brabender plastograph PLE 331 at 15S’C during 5 min at 75 rpm. The mixed samples were compression moulded at
1. Janigovi and 1. Chodik
212
180°C for 20min. More than 99% of the peroxide was decomposed under these conditions. The isothermal crystallization was investigated using a Perkin-Elmer DSC-2 with on-line connection to a Tektronix 31 calculator. Indium and lead were used as temperature standards and indium for the heat of fusion (AH,, = 28.4 J/g). The samples (weight418 mg) were heated to 450K at a heating rate 160K/min and kept for IO min. They were subsequently cooled at the same rate to the crystallization temperature. Isothermal crystallization was investigated at three different temperatures for each sample in the temperature range 374-377 K. This temperature range seems to be optimal from an experimental point of view since a certain induction period occurs before the beginning of crystallization and the rate of crystallization is high enough if the sensitivity of the equipment is considered. The measured and calculated values for the isothermal crystallization parameters at 375 K were taken from the paper VI. RESULTS
AND DISCUSSION
The eflect of the filler presence on the parameters of isothermal crystallization of LDPE Isothermal
crystallization
of LDPE filled with var-
ious amounts of Ultrasil was investigated in the temperature range 375-377 K. The parameters determined are given in Table 1. It is seen that the dependence of the crystallization rate V, vs filler content goes through a minimum at all experimental temperatures. At the same time the crystallization halftime has a maximum at the same filler concentration. This course differs substantially from the dependences obtained for the LDPE filled with a filler with low surface area, Komsil [2], since an increase of crystallization rate with increasing filler content was observed for these samples at any experimental temperature. This behaviour confirms the assumption that the filler surface area may have a significant influence on the crystallization of the composite matrix. In the case of the filler with low surface area, a nucleation in a summation of individual effects on the crystallization rate plays the decisive role. The Table I. Changes of the overall beat of fusion AH, half-time of crystallization I, ,2 and crystallization rate V, as a function of the amount of filler (@) in the mixture with LDPE at 375, 376 and 377 K AH
;a)
f, ,>
(see)
(J/s)
v;
102
-32.2 -36.4 -30.9 -30.9 - 19.6
97.8 105.5 116.1 144.8 125.8
The effect ofjller content on isothermal crystallization of LDPE crosslinked with constant amount of initiator
The isothermal crystallization of LDPE crosslinked with 0.8 wt% of peroxide and filled with various amount of the filler was investigated in the temperature range 374-376 K. The measured and calculated parameters are given in Table 2. It is seen that heat of fusion was found to be around zero in samples with high filler concentration at any experimental temperature. Low crystalline portion is caused by the effect of both crosslinking and filler content resulting in lower mobility of segments as well as extensive adsorption of polyethylene chains on the filler surface. The crystallization rate values have been found to rise at a filler content 20 wt%. At the same time the Table 2. Changes of AH, 1, >and l’, of LDPE crosslinked with 0.8 wt% of Luperox as a function of the amount of the filler(@) in the mixture at 374, 375 and 376 K @ (wt%)
(set-‘)
375 K 0 5 IO 20 35
adsorption is much less significant, because of low surface area of the filler. If Ultrasil with a much higher surface area was used, the adsorption of LDPE chains on the filler surface plays a significant role. Therefore, with increasing filler content the diminishing of the crystallization rate is observed. This tendency is changed at the highest filler concentration when the rate is rising compared to the lower filler amount. This phenomenon is hardly explicable by the increased nucleation since efficient formation of nuclei is usually caused by a much lower concentration of nucleation agent. It is reasonable to assume that the affect is a demonstration of decreased mobility of adsorbed polyethylene chains on the filler surface pronounced to such an extent that the crystalline ordered structure is not changed to a fully random one under experimental conditions. This effect is more pronounced in crosslinked samples and will be explained in more detail later in this paper. A significant decrease of the values of overall heat of fusion was observed only at a higher filler content. This observation corresponds with a decrease of the crystalline portion due to increased adsorption of the polymer at filler surface.
I .07 0.95 0.84 0.63 0.90
-36.4 -29.4 -33.4 -26.3 -21.9
0 5 IO 20 35
- 36.4 -34.5 - 34.7 -29.9 -26.2
245.5 156.4 200.6 235.5 208.5
0.34 0.63 0.43 0.36 0.45
447.6 286.6 441.2 453.2 398.4
0.18 0.29 0.17 0.16 0.20
11z (set)
v; IO? (set ’ )
165.8 169.9
0.52 0.52
374 K 5 IO 20* 35*
- 18.4 -14.1
5
-23.4 -18.0 - 7.5
375 K
316 K 0 5 IO 20 35
(G
:z 35t
318.6 311.6 142.9
0.23 0.25 0.72
517.5 573. I 339. I
0.14 0.13 0.26 -
376 K 5 IO
377 K ::+
- 20.0 - 16.5 -10.2
*Could not be measured because the crysvallization was too rapid. tThe measured heat of fusion is below the threshold sensitivity of the instrument.
273
Temperature effect on kinetics crystallization halftime is decreasing. At higher filler content a prevailing nucleation effect can be expected which overcompensates the adsorption effect. Crosslinking in the presence of higher filler amount leads to formation of a pre-ordered structure resulting in a higher rate of crystallization as suggested in our previous paper [2]. When compared to the crosslinked samples filled with low-surface filler Komsil [2], the samples filled with Ultrasil show a higher crystallization rate at all experimental temjperatures. The tendency of the changes in all samples is, however, similar, unlike the samples with Komsil content, where the changes are different at different temperatures. In the latter case with an increase of the crystallization rate at 374 K, no significant change at 375 K and a decrease of 376 K was observed with increasing filler content. This apparent disagreement corresponds well with our assumption regarding the effect of surface area on the crystallization course. No extensive adsorption of polyethylene or peroxide at the filler surface is expected in samples filled with low-surface filler Komsil and the crosslinking proceeds more or less homogeneously in the polyethylene melt. Adsorption of both peroxide and polymer can be assumed to occur if filler with high surface #area is present in the sample. The crosslinking is less homogeneous since higher local peroxide concentration is assumed near the filler surface. These parts are expected to be also more ordered because elf lower mobility of partially adsorbed polyethylene chains on the filler surface. A permanent network is formed in a not-fully random polymer and a pre-ordered structure is fixed. This is supposed to contribute significantly to the increased crystallization rate of LDPE if crosslinked in the presence of the filler. The effect of the crosslinking initiator concentration on the crystallization of LDPEjlled with constant silica content
Isothermal crystallization of LDPE filled with 20 wt% of Ultrasil and crosslinked to various crosslinking degrees was investigated in the temperature range 374-376 K. The crystallization kinetic parameters are given in Table 3. The crystallization rate of the sample crosslinked with 0.1 wt% of peroxide is lower and crystallization halftime higher than that of the uncrosslinked sample filled with 20% of the filler. However, with a further increase of crosslinking degree (i.e. with increasing peroxide concentration) the crystallization parameters indicate an increase of the crystallization rate. A similar course was also observed for the samples filled with low-surface filler [2]; nevertheless, the minimum on the dependence V, vs peroxide concentration was observed at higher initiator amount. This phenomenon fits well with the hypothesis of pre-ordered structure formation in the presence of the filler. In the melt, pre-ordered permanent structure is formed if cross-linking starts before original crystalline morphology is completely randomized. This situation can occur if crosslinking starts soon enough and a certain “critical” crosslinking degree is reached blefore a completely random structure is formed. The process can be influenced either by slowing down the formation of random structure
Table
3. Changes
of LDPE the
with
ofAH,
20 wt%
I,.~ and V, for a mixture of filler
as a function
of
concentrationof theinitiatorof crosslinking (I) at 314, 375 and 376 K
I
AH
(VA%)
II 2
(J/s)
(=)
(s@‘)
v;
102
I .OO
374 K 0. I
-
8.8
104.6
0.3
-
7.6
98.0
1.11
0.6 I .o*
-
2.9
88.8 -
1.49
1.5*
-
-
375 K 0.1
-
195.5
0.43
0.3
-11.3
173.2
0.50
0.6 I .o*
-
135.9
0.85 -
13.4 3.2
-
I .5*
376 K 0. I
-14.7
304.9
0.3
-10.0
304.8
0.6
-
254.6 -
4.8
I .ot
0.26 0.26 0.34
-
-
I 51 *Could
not he measured
tion
was
too
because the crystalliza-
raoid
fusion was below
as well
as the heat
the threshold
sensitivity
of of
the instrument. tThe
measured threshold
heat
sensitivity
of
fusion
is below
the
of the instrument.
after melting the polymer if the adsorption of polyethylene chains at the filler surface is extensive and strong enough to keep the structure together until crosslinking proceeds to a sufficient degree or by speeding up the crosslinking by the increase of the concentration of peroxide. In the case of low-surface filler, the random structure is formed rather quickly because the physical polymer-filler interactions are not dense and strong enough to keep the ordered structure until the crosslinking proceeds to a sufficient degree. Therefore at low peroxide concentration, significant peroxide decomposition occurs only after a random structure is formed and subsequent crosslinking contributes to the decreased mobility of the chains and lower crystallization rate. Only if the peroxide concentration is above a certain level, the crosslinking is fast enough to create sufficient density of crosslinks before a random structure is formed. From that concentration, the rate of crystallization starts to rise with increasing peroxide content. On the other hand, if high-surface filler is present the polymer-filler interactions are much stronger and a longer time is necessary to reach a completely random structure. Therefore, a density of crosslinks sufficient for the formation of a permanent preordered structure is reached at a lower peroxide concentration. A further increase of peroxide concentration leads to an increase of the crystallization rate because the critical crosslinking degree is reached in a shorter time, i.e. a more ordered structure is immobilized by a permanent network. The values of crystallization heat are decreasing with rising peroxide content, i.e. with increasing crosslinking degree the crystalline portion decreases. The reason for this is the same as described in our previous paper [2]. It is seen from Table 3 that at any temperature AH value was below the sensitivity of
I.Janigovi
214
Table 4. Changes of the values of temperature coefficient E calculated from equation (4) correlation coefficient r and the range of crystallizationtemperature in dependence on initiator concentration (I) and filler amount (@)
E ;%,
(C,
0.8 0.8 0.1 0.3 0.6
5 IO 20 35 5 IO 20 20 20
the equipment
(J/mole) -
961.2 1290.9 1091.6 1207.1 1210.7 1300.8 1246.3 1344.3 1381.9
r
Range of T (K)
0.9904 0.9984 0.9982 0.9999 0.9826 0.9974 0.9813 0.9946 0.9959
375-377 375s377 375s377 375-377 374-376 374316 374316 374-376 374-376
if 1% or more peroxide
was present in
the sample. Temperature coeficients of isothermal crystallization
The temperature coefficients as calculated from equation (4) are given in Table 4. Two facts are of interest. First, the scatter of Arrhenius-like plots for the samples filled with Ultrasil is rather small as seen from the correlation coefficients given in the table. Second, the temperature coefficients of the crystallization for almost all samples are within the experimental error of E evaluation. These features are completely different when compared to the same parameters calculated for low-surface filler [2], since in the latter case a distinct dependence of E on the composition of the samples was observed. This might mean that if the effects of nucleation vs adsorption are compared, the nucleation plays only a marginal role in crystallization of samples with high-surface filler unlike if low-surface filler is present where a nucleation effect is important compared to adsorp-
and I. Chodak
tion on the filler surface. The two processes with different thermal coefficients are crystal growth and nucleation. Crystal growth always has a similar thermal effect, no matter if it originates from the random melt or a pre-ordered structure. Since the samples filled with Ultrasil nucleation has only a marginal influence, similar values of thermal coefficients are fully understandable. CONCLUSIONS
The investigation of isothermal crystallization of LDPE filled with high surface area silica showed a significant influence of adsorption on the kinetic parameters. In a summation of two effects acting on behaviour of crystallization (nucleation and adsorption), adsorption of LDPE chain on the filler surface plays a more important role. The filling and crosslinking of LDPE results in a formation of a pre-ordered structure, which affects the parameters of isothermal crystallization together with adsorption, nucleation and retardation. REFERENCES
1. 2. 3. 4. 5.
JanigovB, I. Chodak and I. ChorvBth. Eur. P&n. J. 28, 1547 (1992). I. Janigova and I. Chodik. Eur. Polym. J. 30, 1105-l 110 (1994). Y. K. Godovsky and G. L. Slonimsky. J. Polym. Sci. 12, 1053 (1974). A. Sharples. Introduction to Polymer Crystallization. Arnold, London (1966) P. C. Vilanova and S. M. Ribas. Polymer 26, 423 I.
(1985).
6. J. Rychlji and I. Janigovi. Thermochim. Acra 215, 21 I (1993).