- Email: [email protected]
Transport phenomena on interfaces in blends of polypropylene and polyethylene
Transport Phenomena on Interfaces in Blends of Polypropylene and Polyethylene M. KRYSZEWSKI, A. GALt~SKI, T. PAKULA, ANn J. GRt~BOWICZ Centre of Molec~tlar and Macromolecular Studies, Polish Academy of Sciences, LSd~ 40, Poland
Received July 18, 1972; accepted November 8, 1972 Mixing or blending of polymers is one of the most important processes in plastics technology. In this way it is possible, as has been found in various investigations (e.g., 1), to obtain materials characterized by properties which are not only highly desirable in many ways, permitting a wide range of applicability of the products, but also significantly different from the components making up the blend. It is generally known that properties of polyblends depend not only on the kind and weight ratio of the blended polymers but also on the phase structure of the blend, the latter being determined by the parameters of the blending process. The phase structure of the blends can be well defined only in two extreme cases: when the polymers are completely miscible and form a single phase and when they are completely immiscible and form a two-phase system, where the phase structure corresponds to the componential structure of the blend. So far, however, the picture of phase structure of the blends of partly miscible pol2Tners is not completely clear. In earlier studies of blend polymer blending in our laboratories, polypropylene (PP) and polyethylene (PE) were selected as a pair of polymers generally held to be incompatible (2). However, in a recent paper concerned with blends of the same polymers, we have suggested the possibility of interdiffusion of the two components in the melt. In other words, these two polymers may in fact be at least partially miscible.
The adoption of the hypothesis of polymer diffusion in the melt enabled us to interpret the phenomena observed during the crystalization of the polymer blends under investigation. The aim of the present study is to examine the phenomena taking place on the border between the components and to furnish confirmation of the previously adopted hypothesis. EXPERIMENTAL All experiments were carried out using specimens of industrial polymers: PP Moplen manufactured by Montecatini, and four BASF 1 polyethylene samples, distinguished by a different number of CH~ -side groups per 1000 C atmos in the main chain. The polymers used are characterized in Table I. Melts of all polymers were pressed at the temperature 20°C above Tm of PP into 0.8-mm films. In order to obtain sharp boundaries between PP and a particular sample of PE, the films were respectively matched, mounted one upon the other, and melted at 180°C, the result being two-layer films 1.6 mm thick. This was followed by cutting the films into cross sections 30 ~m thick. These thin samples were then heated without air at the temperature 5°C above the melting point of PP for different periods of time. After their cooling, interdiffusion of the components was examined microscopically (in 1The authors are indebted to Badische Anilin and Soda Fabrik for PE samples used in these studies. 85
Copyright ~} 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and [nlerface Science, Vol. 44, No. 1, July 1973
86
KRYSZEWSKI E T AL.
FIG. 1. Optical micrograph of the boundary between nonheated (A) PP and PE Lupolen 3010 S, (B) PP and PE Lupolen 6000 L.
polarized light) by observing the area close to the boundary for structural changes depending
on the time the thin sections were heat-treated on the object glass.
Fro. 2. Structural changes observed in the borderline area between P P - P E (1800 S) after the melt has been heat-treated for 3 hr at 180°C (the sample was slowly cooled in the air). lournal of Colloid and Interface Science, VoL 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
1300
1200
1100
1000
9O0
800
70O
87
600
.500
wavenumber
100[I-- 1
4O0 ~.~-
8 W~ELE~ ....... ~
1~t ~ ~
12
14
16
18
~0
~ , =
22..~__
80
60
:, :~i;~ i~
i ::;i!!: -! if:: IIII~!:~!i)! : !i:;:.i: :i i!;i:i~ii!l!!i:!:i!! i i~:::::::::::::::::::::: ~2-;
=
~4
'
"PP,~ I0 o
1300
wavenumber
1200
1100
1000
900
8O0
I
7O0
I
600
[
..,
5O0
400 c.,.,.,.,
..,=,,,,
Fro. 3. Infrared absorption spectra (a) for pure PP and PE (6000 L) samples; (b) for borderline area obtained with a narrow slit, as shown in the insert. (1) Boundary between the polymers, (2) position of the IR spectrophotometer slit. From the surfaces of the previously-heated thin samples, two-stage carbon replicas were obtained : the replicas were then examined with electron microscope. RESULTS Figure 1 shows the borderline between P P and P E 6000 L seen under the polarizing
microscope in a thin section of a two-layer film which has not been heat-treated. I t can be seen that the border between the two polymers is sharp and that, on both sides, morphological structures characteristic of each of these polymers occur. A similar clear-cut boundary was seen in each of the P P - P E samples examined. Journal of Colloid and Interface Science, Vol. 44, No. l, July 1973
88
KRYSZEWSKI E T A L .
FIG. 4. Optical micrograph of the boundary between PP and PE 6000 L after different periods of heating: (A) { hr, (B) i hr, (C) 2.5 hr (fast-cooled samples from 180°C).
3oo[
-
6OOOL
5000K 1/1000C 301OS 1800S
200] 100[ o
I
2
3
/,
t[h]
FIG. 5. The dependence of the extent of PE to PP diffusion on the length of heating. Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
89
Fla. 6. Optical micrograph of the border between PP and PE 6000 L after different lengths of heating : (A) ~ hr, (B) 1 hr, (C) 2.5 hr (samples were quenched in water from a temperature exceeding TroPE = 137°C by 5°C).
Journal o/lColloidand Interface Science, VoL 44, No. 1, July 1973
90
KRYSZEWSKI E T AL.
600
a
~
IBOOS
P p ~ PE 500-
~
3010S
300
200D
6000L
100-
1
2
3
4
t[~h]
FIG. 7. The dependenceof PP to PE diffusionextent on the time of heating (for coolingconditions, see Fig. 6). The samples were then heat-treated at 180°C (i.e., at a temperature exceeding the melting point of PE and PP) for different periods of time: 1/4, 1, 2.5, 4 hr. The next step was to crystallize the melted samples by cooling in air. Specimen micrographs of the heated samples are shown in Fig. 2. Analyzing the pictures, one can state that heat-treatment leads to structural changes on both sides of the boundary, the extent of these changes being dependent on the time of heat-treatment and the kind of polymers in contact. The changes affect first of all the size of the spherulites, and an examination of the cross section of the samples leads to the conclusion that they take place in the whole borderline area, which excludes the possibility of any significant influence on the part of the glass plates between which the sample has been sandwiched. An analysis of the infrared absorption spectrum of the borderline areas obtained by a narrow slit, as shown in Fig. 3b, revealed the presence of PP macromolecules on the PE side and vice versa, which indicates that interdiffusion of macromolecules of both polymers making up the melt takes place. Typical it-spectra of the border llne area on the PP side are shown in Fig. 3. Absorption values in the range of X -- 13.70 #m and X = 13.88 ~m characteristic Journal of Colloid and Interface Science, Vol. 44, No. 1, J u l y 1973
of PE indicate that a small amount (few percent) of PE is present in the studied area. The structural changes observed, must, Lherefore, be attributed to changes in content of both polymers in the areas close to the boundary. The extent of stl'UCtural changes in the borderline areas can thus be regarded as the region of diffusion of macromolecules of one polymer into the other. To facilitate examination of the extent of this diffusion, the samples heated at 180°C were quenched in water at room temperature. Under such conditions of fast cooling, small morphological PP structures, invisible with optical microscope were formed. Quench crystallizing PE, on the other hand, forms under such conditions a well defined spherulitic structure which can be seen in Figs. 4A, B, C for different periods of heat treatment. In the samples obtained in this way, the presence of PE structures can also be detected on the other side of the boundary and the range of spherulite formation determined. Since formation of PE spherulites requires the accumulation of a certain minimum quantity of the polymer, it should be noted that the values obtained for the depth of spherulite formation beyond the interface must be less than the position of the actual diffusion front. How-
TRANSPORT PHENOMENA ON INTERFACES
91
FIG. 8. Electron micrograph of the boundary between PE and~PP spherulites. ever, confronted with the lack of other convenient means for assessing the extent of diffusion, we have adopted the smaller v a l u e - the range of spherulite formation. The extent to which spherulites of different kinds of P E occur in PP, depending on the length of heating, has been determined and the results are presented in Fig. 5. I t can be seem that the front of the diffusion shifts at a rate decreasing with time, reaching considerable distances in relatively short times. The greatest extent of diffusion is observed in the case of linear polyethylene (Lupolen 6000 L : with 2 CH,/1000 C), despite its greater molecular weight, while P E macromolecules characterized by a greater degree of branching diffuse to shorter distances. This suggests that regularity of the P E chain structure, which manifests itself in the number of -- CH~ side groups per 1000 C atoms of the main chain plays a substantial role here. In order to determine the extent of diffusion of P P into PE, each of the previously fast-cooled samples was heated to a temperature exceeding by 5°C the melting point of a particular kind of P E (see Table I),
so that only P E melted, while the previouslyformed invisible P P morphological structure elements remained intacL These partly melLed samples were again cooled by immersion in water at room temperature. Under such cooling conditions, a greater rate of temperature decrease was achieved than in the case of cooling samples from 180°C. Due to this, in the area TABLE I CHARACTERISTICS OF POL~x~[ER SAMPLESa Polymer
CH3/1000
MW
Lupolen 6000 L Lupolen 5000 K Lupolen 3010 S Lupolen 1800 S Moplen
2 10 20 35 500
medium medium low low
Cb
T.~ Density° M I d
(°C) 132 122 115 109 172
0.960 0.949 0.930 0.918
0.41 0.25 14.7 15.0 0.1
Atactic fractions were removed from PP-samples by 24 hr extraction in boiling n-heptane. bThe content of CHa/1000 C was determined on the basis of infrared absorption spectra. o Density was measured in a gradient column. d The melt indexes were determined in a plastometer at 180°C, using a 2.18 kg weight. Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
92
KRYSZEWSKI E T AL.
i
FIO. 9. Change in spherulite structure depending on the distance from the boundary: (A) 0.2 mm, (B) 0.5 ram, (C) 1 ram.
Journal of Colloid and interface Science, Vol. 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
93
7,O i I )
CA*
• •
•
•
*
•
,
•
o
,
,
,
,
o
e
*
•
•
o
o
o
o
o
g
g
!
•
o
•
•
•
•
•
Q
•
Q
•
C8
•
a
t FIG. i0. Diagram of the distribution of PE and PP concentration in the area close to the boundary. where there were PP macromolecules, polyethylene did not form a microscopically detectable supermolecular structure, since, as might be expected, the rate of PE crystallization in this region was slower than in the case of pure PE. The extent of the area in which no PE spherulites were formed was recognized to be the extent of the front of the diffusion PP into PE (Fig. 6). The results of the measurements performed are shown in Fig. 7. These results indicate that the extent of the diffusion of macromolecules PP into PE depend on the kind of PE. It might be suggested that this dependence is due to various viscosity of melts with particular PE samples, which is manifested by the values of melt index (Table I). Figure 7, then, indicates that the extent of PP molecular diffusion decreases with the increase of PE melt viscosity. Of course, it is impossible, on the basis of the measurements made, to quantitatively compare the extent of interdiffusion of PP and PE molecules for particular polymer pairs, because the criteria adopted to assess the extent of the diffusion of different PP and PE molecules were not identical. Replicas of the surfaces of the heat-treated ~nd crystalized samples were examined with electron microscope and confirmed the existence, on both sides of the initial boundary between PP and PE, of morphological structures characteristic
of both polymers. I t is also seen that both the ratio of their number and their dimensions depend on the distance from the boundary. Formation of both types of spherulites in the borderline area suggests Lhat, in the process of cooling, separation of components takes place, making possible independent crystallization of either polymer. The boundary between PE and PP spherulites--1-/~m-wide--does not have a supermolecular crystalline ordering (Fig. 8), while the boundary between spherulites of the same type, i.e., of the same polymer, is clear cut. Changes in the character of spherutire structure depending on the distance from the border, e.g., changes from ringed to fibrillar structures, can also be observed (Fig. 9). DISCUSSION The results presented above clearly point to interdiffusion of PP and PE macromolecules in the melt of the two polymers, and the rate of diffusion in both directions depends on the regularity of PE chain structure. This diffusion is, however, limited, because even considerably prolonged heat treatment of the samples does not lead to total disappearance of the boundary between the polymers. There exist, then, critical concentrations of one polymer in the other, restricting the possibility of complete mixing of the macromolecules even after a sufficiently long period of time has elapsed. Journal of Colloid and Inlerface Science, Vol. 44, No. I, July 1973
94
KRYSZEWSKI E T AL.
c2
c;
Ca* of the component A and (1 - Ca*) of the component B. The following relations may hold between the values Ya, Y~, and X1, X2:
l
X2
Y a = (1 -
xl)ca* + xi(1 -
ca*),
[1]
g~ =
x2)c~*
ca*).
[2]
(1 -
+
x~(1 -
After transformation, the dependence of the content of particular phases in the blend on their composition can be arrived at, 0
X~
0
1
g~
0
FIG. 11. Dependence of the weight content of the phases on blend composition.
Such a situation can be presented on a schematic diagram showing the dependence of PP and PE density on the distance from the boundary between the two polymers after a period of heat treatment (Fig. 10). Values CA* and Ca* represent critical concentrations of PE in PP and PP in PE, respectively. Polymers blended at ratios lower than critical will thus form one homogeneous phase in the melt. Crystallization of such a melt, however, brings about separation of the components which is evidenced by the formation of independent morphological structures of each component. Repeated melting of the sample should again, after adequate time of heating, cause formation of a homogeneous phase. Blending polymers in the ratio exceeding the corresponding critical concentration brings about formation of a two-phase structure, where both phases are made up of two components. Weight ratios of particular phases will, in such a case, be different from the overall weight ratio of the components (the two ratios being identical when the polymers are completely immiscible). L e t Y A and Y~ stand for the overall content (percent) of the component A and B in the polyblend, and X1 and X2 for weight content (percent) of the phase I and II. By phase I is meant the homogeneous blend containing Ca* of the component B and (1 -- Ca*) of the component A, and, respectively, by phase II is understood the homogenous blend containing Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
=
(ga -- CA*)~[-1 -- (Ca* + Ca*)],
x2 = ( r e -
c~*)/D
-
(ca* + ca*)]
[3] r-4]
These relations are shown on a diagram (Fig. 11). It appears from the diagram that, in the case of polyblends containing a small amount of one of the components, the system is a single-phase one, while, in the range of comparable weight of the components making up the blend, it is a two-phase one. The transfer from single-phase to two-phase system following the change in the content of the blend takes place at CA* and (1 -- CB*) content of the component A in the blend. Such change of polyblends from a single-phase to a two-phase system, at given weight ratio of the components, may cause nonmonotonic changes in polymer blend properties, which is often the case. In the case of P P - P E blends, such changes in properties have been found to occur in blends containing 10-20~o of PE in PP and about 10% of PP in PE (3-6). These values may be close to the corresponding critical concentration values for these blends. The purpose of our future investigations will be to determine them in a more precise way. The general model of phase structure of partly miscible polymer blends as outlined requires adequate confirmation on the basis of thermodynamics. REFERENCES 1, See papers published in Appl. Pohtm. Syrup. 15,
(1971). 2. BoHN, L., Kolloid Z. 213, 55 (1966). 3. KRYSZEWSKI,M., PAKULA,T., A N D GREBOWICZ~J., Vysokomol. Sojed. (in press). 4. SLONI~SKI,G. L., J. Polym. Scl. 30, 625 (1958). 5. I~ouE, M., J. Polym. Sci. A 1, 3427 (1963). 6. PLOCt~OeKI,A., Polimery (in Polish) 10, 23 (1965).
Received July 18, 1972; accepted November 8, 1972 Mixing or blending of polymers is one of the most important processes in plastics technology. In this way it is possible, as has been found in various investigations (e.g., 1), to obtain materials characterized by properties which are not only highly desirable in many ways, permitting a wide range of applicability of the products, but also significantly different from the components making up the blend. It is generally known that properties of polyblends depend not only on the kind and weight ratio of the blended polymers but also on the phase structure of the blend, the latter being determined by the parameters of the blending process. The phase structure of the blends can be well defined only in two extreme cases: when the polymers are completely miscible and form a single phase and when they are completely immiscible and form a two-phase system, where the phase structure corresponds to the componential structure of the blend. So far, however, the picture of phase structure of the blends of partly miscible pol2Tners is not completely clear. In earlier studies of blend polymer blending in our laboratories, polypropylene (PP) and polyethylene (PE) were selected as a pair of polymers generally held to be incompatible (2). However, in a recent paper concerned with blends of the same polymers, we have suggested the possibility of interdiffusion of the two components in the melt. In other words, these two polymers may in fact be at least partially miscible.
The adoption of the hypothesis of polymer diffusion in the melt enabled us to interpret the phenomena observed during the crystalization of the polymer blends under investigation. The aim of the present study is to examine the phenomena taking place on the border between the components and to furnish confirmation of the previously adopted hypothesis. EXPERIMENTAL All experiments were carried out using specimens of industrial polymers: PP Moplen manufactured by Montecatini, and four BASF 1 polyethylene samples, distinguished by a different number of CH~ -side groups per 1000 C atmos in the main chain. The polymers used are characterized in Table I. Melts of all polymers were pressed at the temperature 20°C above Tm of PP into 0.8-mm films. In order to obtain sharp boundaries between PP and a particular sample of PE, the films were respectively matched, mounted one upon the other, and melted at 180°C, the result being two-layer films 1.6 mm thick. This was followed by cutting the films into cross sections 30 ~m thick. These thin samples were then heated without air at the temperature 5°C above the melting point of PP for different periods of time. After their cooling, interdiffusion of the components was examined microscopically (in 1The authors are indebted to Badische Anilin and Soda Fabrik for PE samples used in these studies. 85
Copyright ~} 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and [nlerface Science, Vol. 44, No. 1, July 1973
86
KRYSZEWSKI E T AL.
FIG. 1. Optical micrograph of the boundary between nonheated (A) PP and PE Lupolen 3010 S, (B) PP and PE Lupolen 6000 L.
polarized light) by observing the area close to the boundary for structural changes depending
on the time the thin sections were heat-treated on the object glass.
Fro. 2. Structural changes observed in the borderline area between P P - P E (1800 S) after the melt has been heat-treated for 3 hr at 180°C (the sample was slowly cooled in the air). lournal of Colloid and Interface Science, VoL 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
1300
1200
1100
1000
9O0
800
70O
87
600
.500
wavenumber
100[I-- 1
4O0 ~.~-
8 W~ELE~ ....... ~
1~t ~ ~
12
14
16
18
~0
~ , =
22..~__
80
60
:, :~i;~ i~
i ::;i!!: -! if:: IIII~!:~!i)! : !i:;:.i: :i i!;i:i~ii!l!!i:!:i!! i i~:::::::::::::::::::::: ~2-;
=
~4
'
"PP,~ I0 o
1300
wavenumber
1200
1100
1000
900
8O0
I
7O0
I
600
[
..,
5O0
400 c.,.,.,.,
..,=,,,,
Fro. 3. Infrared absorption spectra (a) for pure PP and PE (6000 L) samples; (b) for borderline area obtained with a narrow slit, as shown in the insert. (1) Boundary between the polymers, (2) position of the IR spectrophotometer slit. From the surfaces of the previously-heated thin samples, two-stage carbon replicas were obtained : the replicas were then examined with electron microscope. RESULTS Figure 1 shows the borderline between P P and P E 6000 L seen under the polarizing
microscope in a thin section of a two-layer film which has not been heat-treated. I t can be seen that the border between the two polymers is sharp and that, on both sides, morphological structures characteristic of each of these polymers occur. A similar clear-cut boundary was seen in each of the P P - P E samples examined. Journal of Colloid and Interface Science, Vol. 44, No. l, July 1973
88
KRYSZEWSKI E T A L .
FIG. 4. Optical micrograph of the boundary between PP and PE 6000 L after different periods of heating: (A) { hr, (B) i hr, (C) 2.5 hr (fast-cooled samples from 180°C).
3oo[
-
6OOOL
5000K 1/1000C 301OS 1800S
200] 100[ o
I
2
3
/,
t[h]
FIG. 5. The dependence of the extent of PE to PP diffusion on the length of heating. Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
89
Fla. 6. Optical micrograph of the border between PP and PE 6000 L after different lengths of heating : (A) ~ hr, (B) 1 hr, (C) 2.5 hr (samples were quenched in water from a temperature exceeding TroPE = 137°C by 5°C).
Journal o/lColloidand Interface Science, VoL 44, No. 1, July 1973
90
KRYSZEWSKI E T AL.
600
a
~
IBOOS
P p ~ PE 500-
~
3010S
300
200D
6000L
100-
1
2
3
4
t[~h]
FIG. 7. The dependenceof PP to PE diffusionextent on the time of heating (for coolingconditions, see Fig. 6). The samples were then heat-treated at 180°C (i.e., at a temperature exceeding the melting point of PE and PP) for different periods of time: 1/4, 1, 2.5, 4 hr. The next step was to crystallize the melted samples by cooling in air. Specimen micrographs of the heated samples are shown in Fig. 2. Analyzing the pictures, one can state that heat-treatment leads to structural changes on both sides of the boundary, the extent of these changes being dependent on the time of heat-treatment and the kind of polymers in contact. The changes affect first of all the size of the spherulites, and an examination of the cross section of the samples leads to the conclusion that they take place in the whole borderline area, which excludes the possibility of any significant influence on the part of the glass plates between which the sample has been sandwiched. An analysis of the infrared absorption spectrum of the borderline areas obtained by a narrow slit, as shown in Fig. 3b, revealed the presence of PP macromolecules on the PE side and vice versa, which indicates that interdiffusion of macromolecules of both polymers making up the melt takes place. Typical it-spectra of the border llne area on the PP side are shown in Fig. 3. Absorption values in the range of X -- 13.70 #m and X = 13.88 ~m characteristic Journal of Colloid and Interface Science, Vol. 44, No. 1, J u l y 1973
of PE indicate that a small amount (few percent) of PE is present in the studied area. The structural changes observed, must, Lherefore, be attributed to changes in content of both polymers in the areas close to the boundary. The extent of stl'UCtural changes in the borderline areas can thus be regarded as the region of diffusion of macromolecules of one polymer into the other. To facilitate examination of the extent of this diffusion, the samples heated at 180°C were quenched in water at room temperature. Under such conditions of fast cooling, small morphological PP structures, invisible with optical microscope were formed. Quench crystallizing PE, on the other hand, forms under such conditions a well defined spherulitic structure which can be seen in Figs. 4A, B, C for different periods of heat treatment. In the samples obtained in this way, the presence of PE structures can also be detected on the other side of the boundary and the range of spherulite formation determined. Since formation of PE spherulites requires the accumulation of a certain minimum quantity of the polymer, it should be noted that the values obtained for the depth of spherulite formation beyond the interface must be less than the position of the actual diffusion front. How-
TRANSPORT PHENOMENA ON INTERFACES
91
FIG. 8. Electron micrograph of the boundary between PE and~PP spherulites. ever, confronted with the lack of other convenient means for assessing the extent of diffusion, we have adopted the smaller v a l u e - the range of spherulite formation. The extent to which spherulites of different kinds of P E occur in PP, depending on the length of heating, has been determined and the results are presented in Fig. 5. I t can be seem that the front of the diffusion shifts at a rate decreasing with time, reaching considerable distances in relatively short times. The greatest extent of diffusion is observed in the case of linear polyethylene (Lupolen 6000 L : with 2 CH,/1000 C), despite its greater molecular weight, while P E macromolecules characterized by a greater degree of branching diffuse to shorter distances. This suggests that regularity of the P E chain structure, which manifests itself in the number of -- CH~ side groups per 1000 C atoms of the main chain plays a substantial role here. In order to determine the extent of diffusion of P P into PE, each of the previously fast-cooled samples was heated to a temperature exceeding by 5°C the melting point of a particular kind of P E (see Table I),
so that only P E melted, while the previouslyformed invisible P P morphological structure elements remained intacL These partly melLed samples were again cooled by immersion in water at room temperature. Under such cooling conditions, a greater rate of temperature decrease was achieved than in the case of cooling samples from 180°C. Due to this, in the area TABLE I CHARACTERISTICS OF POL~x~[ER SAMPLESa Polymer
CH3/1000
MW
Lupolen 6000 L Lupolen 5000 K Lupolen 3010 S Lupolen 1800 S Moplen
2 10 20 35 500
medium medium low low
Cb
T.~ Density° M I d
(°C) 132 122 115 109 172
0.960 0.949 0.930 0.918
0.41 0.25 14.7 15.0 0.1
Atactic fractions were removed from PP-samples by 24 hr extraction in boiling n-heptane. bThe content of CHa/1000 C was determined on the basis of infrared absorption spectra. o Density was measured in a gradient column. d The melt indexes were determined in a plastometer at 180°C, using a 2.18 kg weight. Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
92
KRYSZEWSKI E T AL.
i
FIO. 9. Change in spherulite structure depending on the distance from the boundary: (A) 0.2 mm, (B) 0.5 ram, (C) 1 ram.
Journal of Colloid and interface Science, Vol. 44, No. 1, July 1973
TRANSPORT PHENOMENA ON INTERFACES
93
7,O i I )
CA*
• •
•
•
*
•
,
•
o
,
,
,
,
o
e
*
•
•
o
o
o
o
o
g
g
!
•
o
•
•
•
•
•
Q
•
Q
•
C8
•
a
t FIG. i0. Diagram of the distribution of PE and PP concentration in the area close to the boundary. where there were PP macromolecules, polyethylene did not form a microscopically detectable supermolecular structure, since, as might be expected, the rate of PE crystallization in this region was slower than in the case of pure PE. The extent of the area in which no PE spherulites were formed was recognized to be the extent of the front of the diffusion PP into PE (Fig. 6). The results of the measurements performed are shown in Fig. 7. These results indicate that the extent of the diffusion of macromolecules PP into PE depend on the kind of PE. It might be suggested that this dependence is due to various viscosity of melts with particular PE samples, which is manifested by the values of melt index (Table I). Figure 7, then, indicates that the extent of PP molecular diffusion decreases with the increase of PE melt viscosity. Of course, it is impossible, on the basis of the measurements made, to quantitatively compare the extent of interdiffusion of PP and PE molecules for particular polymer pairs, because the criteria adopted to assess the extent of the diffusion of different PP and PE molecules were not identical. Replicas of the surfaces of the heat-treated ~nd crystalized samples were examined with electron microscope and confirmed the existence, on both sides of the initial boundary between PP and PE, of morphological structures characteristic
of both polymers. I t is also seen that both the ratio of their number and their dimensions depend on the distance from the boundary. Formation of both types of spherulites in the borderline area suggests Lhat, in the process of cooling, separation of components takes place, making possible independent crystallization of either polymer. The boundary between PE and PP spherulites--1-/~m-wide--does not have a supermolecular crystalline ordering (Fig. 8), while the boundary between spherulites of the same type, i.e., of the same polymer, is clear cut. Changes in the character of spherutire structure depending on the distance from the border, e.g., changes from ringed to fibrillar structures, can also be observed (Fig. 9). DISCUSSION The results presented above clearly point to interdiffusion of PP and PE macromolecules in the melt of the two polymers, and the rate of diffusion in both directions depends on the regularity of PE chain structure. This diffusion is, however, limited, because even considerably prolonged heat treatment of the samples does not lead to total disappearance of the boundary between the polymers. There exist, then, critical concentrations of one polymer in the other, restricting the possibility of complete mixing of the macromolecules even after a sufficiently long period of time has elapsed. Journal of Colloid and Inlerface Science, Vol. 44, No. I, July 1973
94
KRYSZEWSKI E T AL.
c2
c;
Ca* of the component A and (1 - Ca*) of the component B. The following relations may hold between the values Ya, Y~, and X1, X2:
l
X2
Y a = (1 -
xl)ca* + xi(1 -
ca*),
[1]
g~ =
x2)c~*
ca*).
[2]
(1 -
+
x~(1 -
After transformation, the dependence of the content of particular phases in the blend on their composition can be arrived at, 0
X~
0
1
g~
0
FIG. 11. Dependence of the weight content of the phases on blend composition.
Such a situation can be presented on a schematic diagram showing the dependence of PP and PE density on the distance from the boundary between the two polymers after a period of heat treatment (Fig. 10). Values CA* and Ca* represent critical concentrations of PE in PP and PP in PE, respectively. Polymers blended at ratios lower than critical will thus form one homogeneous phase in the melt. Crystallization of such a melt, however, brings about separation of the components which is evidenced by the formation of independent morphological structures of each component. Repeated melting of the sample should again, after adequate time of heating, cause formation of a homogeneous phase. Blending polymers in the ratio exceeding the corresponding critical concentration brings about formation of a two-phase structure, where both phases are made up of two components. Weight ratios of particular phases will, in such a case, be different from the overall weight ratio of the components (the two ratios being identical when the polymers are completely immiscible). L e t Y A and Y~ stand for the overall content (percent) of the component A and B in the polyblend, and X1 and X2 for weight content (percent) of the phase I and II. By phase I is meant the homogeneous blend containing Ca* of the component B and (1 -- Ca*) of the component A, and, respectively, by phase II is understood the homogenous blend containing Journal of Colloid and Interface Science, Vol. 44, No. 1, July 1973
=
(ga -- CA*)~[-1 -- (Ca* + Ca*)],
x2 = ( r e -
c~*)/D
-
(ca* + ca*)]
[3] r-4]
These relations are shown on a diagram (Fig. 11). It appears from the diagram that, in the case of polyblends containing a small amount of one of the components, the system is a single-phase one, while, in the range of comparable weight of the components making up the blend, it is a two-phase one. The transfer from single-phase to two-phase system following the change in the content of the blend takes place at CA* and (1 -- CB*) content of the component A in the blend. Such change of polyblends from a single-phase to a two-phase system, at given weight ratio of the components, may cause nonmonotonic changes in polymer blend properties, which is often the case. In the case of P P - P E blends, such changes in properties have been found to occur in blends containing 10-20~o of PE in PP and about 10% of PP in PE (3-6). These values may be close to the corresponding critical concentration values for these blends. The purpose of our future investigations will be to determine them in a more precise way. The general model of phase structure of partly miscible polymer blends as outlined requires adequate confirmation on the basis of thermodynamics. REFERENCES 1, See papers published in Appl. Pohtm. Syrup. 15,
(1971). 2. BoHN, L., Kolloid Z. 213, 55 (1966). 3. KRYSZEWSKI,M., PAKULA,T., A N D GREBOWICZ~J., Vysokomol. Sojed. (in press). 4. SLONI~SKI,G. L., J. Polym. Scl. 30, 625 (1958). 5. I~ouE, M., J. Polym. Sci. A 1, 3427 (1963). 6. PLOCt~OeKI,A., Polimery (in Polish) 10, 23 (1965).