- Email: [email protected]
The supermolecular structure of mixtures of polypropylene with polyethylene
THE SUPERMOLECULAR STRUCTURE OF MIXTURES OF POLYPROPYLENE W I T H POLYETHYLENE* M. KRYSZEWSKI, T. PAKW~A and J. GREMBOWICZ Molecular and Macromolecular Research Centre, Polish Academy of Sciences (Received 5 October 1972)
A study was made of the supermolecular structures formed in two component mixtures of isotaetic poIypropylene with lot, and high density polyethylene relative to the composition of the mixtures and the conditions of heat treatment. The size, shape and degree of order of the supermoleeular structures were determined by the lot- angle light scattering method. The influence of the composition of the mixtures on the processes of melting and crystallization of the individual components was determined. The experimental results point, to the mieroheterogeneous character of the mixtures in the solid state.
_AUTHORS investigating the properties of polymeric materials based on mixtures of two or more polymers have done so mainly by measuring the properties of the samples in relation to the composition of the mixtures [1]. A much smaller number of investigations has been undertaken in connection with the processes of mixing of the polymers and the phase structure of the mixtures. Some papers have appeared relating to mixtures of polypropylene (PP) and polyethylene (PE) dealing mainly with the mechanical properties of these mixtures. However, the data available on the structure of the mixtures is somewhat limited. We therefore investigated PP-PE mixtures in order to determine the effect of their composition, and heat treatment, on the formation and type of supermolecular structural elements appearing in the samples. EXPERIMENTAL
The mixtures under investigation were two component mixtures of isotactic P P (Moplene) with high and low density polyethylene (PEHD, PELD) which were respectively Lupolene 6000L and Lupolene 1800S. The mixtures were prepared by repeated milling with a plastomer, and by repeated compression moulding (190 °) of both components mixed in the predetermined weight ratios. The two methods used for cooling the films were on the one hand rapid cooling, when the fihns with their metallic coatings (0-5 m m thick) were inmmrsed in w~ter at room temperature, and on the other hand slow cooling, when the fihns together with their coatings t-ere kept in air at room temperature. Samples containing from * Vysomokol. soyed. A16: No. 7, 1569-1575, 1974. 1817
1818
M. KRYSZEWSK[ ct al.
10 to 90~o by wt. PE and P P were prepared, as were samples of the pure components. Th(~ P E eontcnts of the samples were measured by infrared spectroscopic analysis. The n u m b e r of CH8 side groups per 1000 main chain C atoms was measured by infrared analysis of the two types of PE. The results obtained were: for P E H D , 0.2 CH3/1000 C, and for P E L D , 35 CHJ1000 C. More complete characterization of the original samples of P P and P E was obtained by determining the melt indices of the polymers in a plastomer at 180 °, with a load of 2-18 kgf. The melt indices for the P E t t D , PELD and P P were 0.41, 15.1 and 0.1 respectively. P P and PE' are both crystallizable polymers, but in the mixtures P E and P P macromolecules are unable to form a joint three-dimensional network in view of the different chemical structures of the chains; the structure of these mixtures is therefore complex, and special methods are required for their investigation. Low angle light scattering was the method mainly used for investigating the melting and crystallization Of components of the mixtures, and likewise for investigation of the emerging supermolecular structures. I n addition, the samples were examined in the interference-polarizing microscope with phase contrast. The apparatus described in [2] was used for photometric analysis of the low angle diffraction patterns.
6
5 4
2
2
I
FxG. 1. Layout of the apparatus (see text). The Stein-Rhodes method [3] was used to determine the sizes of spherulites according to the position of the scattered intensity peaks in the diffraction patterns (Hv) of the samples containing spherulites. The processes of melting and crystallization of the mixtures and the pure components were monitored by recording changes in the intensity of the light scattered by the samples during these processes. Figure 1 shows the layout of the apparatus used for this purpose. The light source was a helium-neon laser (1). The sample was placed in a temperature controlled chamber between two glass plates. The light scattered b y the sample was analysed by means of a rotating cruciform slit (4), and was focussed on the multiplier (tube) (6) b y a condenser (5). Between the sample and the analysing slit is the analyser (3) with the polarization axis oriented perpendicular to the direction of polarization of the incident light. The multiplier current was recorded as a function of sample temperature b y means of a recorder. Figure 2 shows curves of changes in the intensity of the scattered light during the melting and crystallization of the samples plotted for P E H D and P E H D mixed with P P (6 : 4). By analysing these curves one m a y determine for each sample the temperature ranges (denoted by circles) in which melting and crystallization take place, The melting process was observed while the sample temperature was rising at the rate of 4 deg/min, a n d the crystallization process was investigated with a uniform reduction in temperature in a current of air. The shape of the P E t I D - P P curve shows that it is possible to identify the temperature
Supermolecular structure of mixtures of polypropylene with polyethylene
1819
ranges in which melting and crystallization of the individual components of the mixtures take place. By analysing the diffraction patterns with the aid of a cruciform slit one may determine the degree of order of the crystalline structures. The width of the curves in Fig. 2 in the region where the sample is in the crystalline state determines the difference between the total intensity of light scattered at angles of ll = 45 and 90 °. The ratio of this difference to the intensity of light scattered at an angle of lL=90 °, determined by the formula
~.u~90 provides informavion on the degree of order of the crystalline structure of a sample, i.e. the extent to which the latter is incorporated into the regular supermolecular structures [4, 5]. The t/-values were fi)und for all the mixtures in the rang(~' of composition examined.
g.
[
I
fO0
150
I
I
100
f58 T %
b
t
F~c. 2. Characteristic curves of scattered intensity distribution vs. sample temperature for P E (a) and P P (b) during melting (1) and crystallization (2).
To investigate the mixing processes for both types of P E with PP, and the degre(, of their mutual diffusion in a melt, macroscopical investigations of the supermolecular structures formed at the interface between the pure polymers were carried out. This interface was obtained by melting, between glasses, films of the pure polymers in colltact with one another. The samples that had been prepared in this way were then subjected to heating for 5 hr at 200 °, and to crystallization by rapid cooling of the melt. An optical microscope was used for comparative analysis of the supermolecular structures formed in the region of the interface between the polymers in the samples subjected to heating for 5 hr, and in those subjected to crystallization immediately after melting.
1820
M. KRYSZEWSKIet al. DISCUSSION OF RESULTS
Interesting data on the mode of formation of supermolecular structures in polymeric mixtures were obtained by investigation of the crystallization and melting of the individual components of the mixtures. Figure 3 illustrates the relationship between the composition of the mixtures and the temperature ranges in which melting and crystallization of the polymers take place. The results obtained show that the composition of the mixtures has only an insignificant effect on the melting points of the individual components. Irrespective of the
T,° C Cl
b
180I" 140 ~
2
r
18L d
i
C
')N
""
140
2
2 100
20
60
100
PP,%
20
GO
190
:[~'IG. 3. Temperature ranges for the processes of molting (a, c) and crystallization (b, d) in relation to composition of PP P E H D (a, b)and P P - P E L D (c, d) mixtures: 1--temperature range corresponding to PP; 2--ditto, PEI-ID.
composition of the mixture, the melting points of the PE and P P were practically constant. In the case of the P P - P E L D mixtures it was found t h a t the crystallization process was likewise unaffected by the composition of the mixture. Investigations showed that the temperature of the onset of PP crystallization in the mixtures of P P and P E H D fell proportionally with reduction in the P P content.
Supermolccular structure of mixtures of polypropylene with polyethylene
1821
As the PP content rises, a rise in the crystallization temperature of the PE is observed. This is discontinued solely in the region of equal concentrations of the components, which correspond to phase inversion in the mixture of polymers. A study was also made of the supermolecular structures that emerge during crystallization of polymeric mixtures, and the characteristic parameters of these structures were obtained by analysis of the low angle light scattering. For the two types of mixtures different relationships were obtained in regard to the size and shape of supermolecular structures relative to the composition of the mixtures. With mixtures of PP and PELD spherulites appear irrespective of changes in composition, as is shown by the characteristic diffraction patterns (four leaved clover) obtained for these structures. On the other hand the sizes of the spherulites formed in the mixtures differ from those of the spherulites formed in the pure components, and are clearly dependent on the composition of the mixtures. / ~,#rn 2~ a
24
j
/
/
16
/ / / /
8,
t
r
J_
20
1
I
60
_j
j
I
100
20
B
i
{:iO
i
I
700
PP, % FIG, 4. Size of supermolecular structures vs. compositiort of the mixture: a - - P P ~-PELD for the rapidly (1) and slowly (2) cooled samples; b - - P P - ~ P E t t D in rapidly cooled film~.
This is illustrated in Fig. 4 for samples of the P P - P E L D mixtures subjected to slow and rapid cooling conditions. It was found that the mode of cooling influences the size of the structural formations in mixtures where one of the components greatly predominates. In cases where the weight contents of both components are similar, spherulites of practically uniform size are formed, irrespective of the mode of cooling. In the case of the P E - P E H D mixtures the shape of the emerging supermolecular structures depends on the composition of the mixtures. When the samples undergo rapid cooling, a spherulitic structure is formed only when the PE content greatly predominated, and the spherulites are smaller than those in the pure PE (Fig. 4b). The intensity distribution in the diffraction patterns obtained for the samples in which PE greatly predominated agrees with the distribution calculated by Picot, Stein, Motegi and Kawai [6, 7] for a three dimensional
C~
L~
o~
FIG. 5. Light. scattering patterns for the rapidly (a k) and slowly (l--v) cooled samples of P P ]-PEHI): a, 1 - - P E ; b , m - - 1 0 ; c , n - 20;d, o -- 30; e, p - - 40; .f, q 50;.q, r-- 60; h, s - - 70; i, t 80;j, +~--90; k, v--100°,~> PP.
C/2
b~
o
O
t~
O
!
©
7~
¢+
g
c~
+_
4
1824
M. K ~ ¥ s z E w s x I et al.
fan-like model of the scattering structure. The angle of opening of the ikn is reduced as the P P content rises, and for mixtures with a 90~ o P P content the fan shape changes to a rod-like structure. When the mixtures undergo slow cooling the intensity distribution in the diffraction patterns shows that in these samples also, structures corresponding to the fan model emerge. The existence of such structures is conformed by microscopical examination. In mixtures with similar P E H D and P P contents (in the samples subjected to slow and rapid cooling) there are oriented regions that are larger than the supermolecular structures that emerge. This conclusion is based on a comparison of diffraction patterns for these mixtures with the Stein theory [8] which describes the scattering of light by supermolecular structures in an anisotropic medium. Figure 5 shows the diffraction patterns for all compositions of P E H D - P P mixtures with slow or rapid cooling.
J7 /'0-
G
1"6
1'2 O'G -
@
0-8
•
0'2
g'4I
20
I
I
#g
I
t
lgg PP,%
2g
6O
pp,~
log
FIO. 6. Plots of~/vs, composition of mixtures o f P P + P E H D (a) and PP+PELD (b): 1 -rapidly cooled samples; 2--slowly cooled samples. In the case of samples containing spherulites the sizes of the latter were determined b y measurement of the position of scattered intensity maxima in the diffraction patterns. The sizes of the emerging supermolecular structures were estimated from the angular distribution of the scattered light, and by microscopical analysis. Figure 4b gives the sizes obtained in this way for the structures formed in the rapidly cooled samples (dimeters of spherulites and fan-like structures, and lengths of the rod-like structures). The structures formed in the slowly cooled samples are larger than those formed in the rapidly cooled ones, but variations in the size of these structures with the composition of mixtures are similar in either case. Figure 6a illustrates the relationship between the composition of mixtures and the viscosity of melts for the P E H D and P P samples cooled under slow and rapid conditions. The viscosity values fall as the ratio of components in a mixture approaches 1 : 1, and the viscosities of mixtures with l : 1 ratios of the components are very low. In the case of P E L D - P P mixtures the ~/values are highest for pure
S u p e r m o l e c u l a r s t r u c t u r e of m i x t u r e s of p o l y p r o p y l c n e w i t h p o l y e t h y l e n e
1825
PE, and are reduced as the P P content of a mixture rises (Fig. 4b). Microscopical analysis of the supermolecular structures formed at the interface of two layers of the pure polymers showed that in samples crystallized immediately after melting there is a well defined interface between both polymers, and on both sides of this interface there are supermolecular structures characteristic of the pure compo-
Fro'. 7. Microscopical p a t t e r n of t h e P P - - P E H D interface.
nents (Fig. 7). When the samples were kept at an elevated temperature, no change in the P P - P E L D interface was observed. For the PP - P E H D mixture, however, the contact interface became indistinct, and in the region of this interface supermolecular structures characteristic ibr mixtures of these polymers appeared, the changes in question being most characteristic as regards the P P layer, which points to diffusion of the P E H D macromolecules into the molten P P (Fig. 8).
FIG. 8. P P - P E
interftxc(~ a f t e r t h e m e l t h a d bee~l k e p t a t 200 ° for 5 hr.
In view of the foregoing observations it appears that there is a marked difference in the manner of mixing of the components in the two types of mixtures, This follows from the difference in the chain structures of the two types of P E . The P E H D with linear chains without side branches (0.2 CHa groups/1000 C), mechanically mixed with PP, does not form a two phase system in the melt.
1826
M. K~YszEwsxi et al.
The possibility of diffusion of the P E H D molecules into the P P results in the formation of systems with mixing not unlike molecular mixing. The branched P E L D macromolecules (35 CH 3 groups/1000 C) do not diffuse in the melt of the mixture with PP, and the degree of mixing of the components in these mixtures depends solely on mechanical mixing processes. The differences in the degrees of mixing observed for the two types of mixtures would account for the foregoing experimental facts in regard to melting and crystallization. For the P P - P E L D mixtures, which form a two-phase system, it is obvious t h a t the melting and crystallization of the individual components are independent of the composition of the mixtures. As P E H D in mixtures with P P plays the role of a medium giving rise to swelling it facilitates crystallization of the PP, in temperature ranges which are lowered as the P E content of the mixtures rises. The crystallization Of the P P and P E H D components favours phase separation, and for this reason it is likewise observed for these mixtures t h a t the melting points of the components are independent of the amounts of the latter in the mixtures. I n the solid state these mixtures form two phase systems. Translated by R. J. A. HENDRY
REFERENCES
1. Appl. Polymer Symp. 15:1971 2. M. KRYSZEWSKI, A. GALESKI, T. PAKULA and R. SZYLHABEL, Polimery (Polska) 16: 8, 1971 3. R. S. STEIN and M. B. RHODES, J. Appl. Phys. 31: 1873, 1960 4. A. E. N. KFAJZERS, J. J. van AARSTEN and W. PRINS, J. Amer. Chem. Soe. 90: 3107, 1968 5. M. KRYSZEWSKI, A. GALESKI, T. PAKULA, J. GREBOWICZ and P. MILCZAREK, J. Appl. Polymer Sei. 5: 1139, 1971 6. C. PICOT, R. S. STEIN, M. MOTEGI and H. KAWAI, J. Polymer Sei. A-2, 8: 2115, 1970 7. M. MOTEGI, T. OSADA, M. MORITANI and H. KAWAI, Polymer J. 1: 209, 1970 8. C. PICOT and R. S. STEIN, J. Polymer Sei. A-2, 8: 1491, 1970
A study was made of the supermolecular structures formed in two component mixtures of isotaetic poIypropylene with lot, and high density polyethylene relative to the composition of the mixtures and the conditions of heat treatment. The size, shape and degree of order of the supermoleeular structures were determined by the lot- angle light scattering method. The influence of the composition of the mixtures on the processes of melting and crystallization of the individual components was determined. The experimental results point, to the mieroheterogeneous character of the mixtures in the solid state.
_AUTHORS investigating the properties of polymeric materials based on mixtures of two or more polymers have done so mainly by measuring the properties of the samples in relation to the composition of the mixtures [1]. A much smaller number of investigations has been undertaken in connection with the processes of mixing of the polymers and the phase structure of the mixtures. Some papers have appeared relating to mixtures of polypropylene (PP) and polyethylene (PE) dealing mainly with the mechanical properties of these mixtures. However, the data available on the structure of the mixtures is somewhat limited. We therefore investigated PP-PE mixtures in order to determine the effect of their composition, and heat treatment, on the formation and type of supermolecular structural elements appearing in the samples. EXPERIMENTAL
The mixtures under investigation were two component mixtures of isotactic P P (Moplene) with high and low density polyethylene (PEHD, PELD) which were respectively Lupolene 6000L and Lupolene 1800S. The mixtures were prepared by repeated milling with a plastomer, and by repeated compression moulding (190 °) of both components mixed in the predetermined weight ratios. The two methods used for cooling the films were on the one hand rapid cooling, when the fihns with their metallic coatings (0-5 m m thick) were inmmrsed in w~ter at room temperature, and on the other hand slow cooling, when the fihns together with their coatings t-ere kept in air at room temperature. Samples containing from * Vysomokol. soyed. A16: No. 7, 1569-1575, 1974. 1817
1818
M. KRYSZEWSK[ ct al.
10 to 90~o by wt. PE and P P were prepared, as were samples of the pure components. Th(~ P E eontcnts of the samples were measured by infrared spectroscopic analysis. The n u m b e r of CH8 side groups per 1000 main chain C atoms was measured by infrared analysis of the two types of PE. The results obtained were: for P E H D , 0.2 CH3/1000 C, and for P E L D , 35 CHJ1000 C. More complete characterization of the original samples of P P and P E was obtained by determining the melt indices of the polymers in a plastomer at 180 °, with a load of 2-18 kgf. The melt indices for the P E t t D , PELD and P P were 0.41, 15.1 and 0.1 respectively. P P and PE' are both crystallizable polymers, but in the mixtures P E and P P macromolecules are unable to form a joint three-dimensional network in view of the different chemical structures of the chains; the structure of these mixtures is therefore complex, and special methods are required for their investigation. Low angle light scattering was the method mainly used for investigating the melting and crystallization Of components of the mixtures, and likewise for investigation of the emerging supermolecular structures. I n addition, the samples were examined in the interference-polarizing microscope with phase contrast. The apparatus described in [2] was used for photometric analysis of the low angle diffraction patterns.
6
5 4
2
2
I
FxG. 1. Layout of the apparatus (see text). The Stein-Rhodes method [3] was used to determine the sizes of spherulites according to the position of the scattered intensity peaks in the diffraction patterns (Hv) of the samples containing spherulites. The processes of melting and crystallization of the mixtures and the pure components were monitored by recording changes in the intensity of the light scattered by the samples during these processes. Figure 1 shows the layout of the apparatus used for this purpose. The light source was a helium-neon laser (1). The sample was placed in a temperature controlled chamber between two glass plates. The light scattered b y the sample was analysed by means of a rotating cruciform slit (4), and was focussed on the multiplier (tube) (6) b y a condenser (5). Between the sample and the analysing slit is the analyser (3) with the polarization axis oriented perpendicular to the direction of polarization of the incident light. The multiplier current was recorded as a function of sample temperature b y means of a recorder. Figure 2 shows curves of changes in the intensity of the scattered light during the melting and crystallization of the samples plotted for P E H D and P E H D mixed with P P (6 : 4). By analysing these curves one m a y determine for each sample the temperature ranges (denoted by circles) in which melting and crystallization take place, The melting process was observed while the sample temperature was rising at the rate of 4 deg/min, a n d the crystallization process was investigated with a uniform reduction in temperature in a current of air. The shape of the P E t I D - P P curve shows that it is possible to identify the temperature
Supermolecular structure of mixtures of polypropylene with polyethylene
1819
ranges in which melting and crystallization of the individual components of the mixtures take place. By analysing the diffraction patterns with the aid of a cruciform slit one may determine the degree of order of the crystalline structures. The width of the curves in Fig. 2 in the region where the sample is in the crystalline state determines the difference between the total intensity of light scattered at angles of ll = 45 and 90 °. The ratio of this difference to the intensity of light scattered at an angle of lL=90 °, determined by the formula
~.u~90 provides informavion on the degree of order of the crystalline structure of a sample, i.e. the extent to which the latter is incorporated into the regular supermolecular structures [4, 5]. The t/-values were fi)und for all the mixtures in the rang(~' of composition examined.
g.
[
I
fO0
150
I
I
100
f58 T %
b
t
F~c. 2. Characteristic curves of scattered intensity distribution vs. sample temperature for P E (a) and P P (b) during melting (1) and crystallization (2).
To investigate the mixing processes for both types of P E with PP, and the degre(, of their mutual diffusion in a melt, macroscopical investigations of the supermolecular structures formed at the interface between the pure polymers were carried out. This interface was obtained by melting, between glasses, films of the pure polymers in colltact with one another. The samples that had been prepared in this way were then subjected to heating for 5 hr at 200 °, and to crystallization by rapid cooling of the melt. An optical microscope was used for comparative analysis of the supermolecular structures formed in the region of the interface between the polymers in the samples subjected to heating for 5 hr, and in those subjected to crystallization immediately after melting.
1820
M. KRYSZEWSKIet al. DISCUSSION OF RESULTS
Interesting data on the mode of formation of supermolecular structures in polymeric mixtures were obtained by investigation of the crystallization and melting of the individual components of the mixtures. Figure 3 illustrates the relationship between the composition of the mixtures and the temperature ranges in which melting and crystallization of the polymers take place. The results obtained show that the composition of the mixtures has only an insignificant effect on the melting points of the individual components. Irrespective of the
T,° C Cl
b
180I" 140 ~
2
r
18L d
i
C
')N
""
140
2
2 100
20
60
100
PP,%
20
GO
190
:[~'IG. 3. Temperature ranges for the processes of molting (a, c) and crystallization (b, d) in relation to composition of PP P E H D (a, b)and P P - P E L D (c, d) mixtures: 1--temperature range corresponding to PP; 2--ditto, PEI-ID.
composition of the mixture, the melting points of the PE and P P were practically constant. In the case of the P P - P E L D mixtures it was found t h a t the crystallization process was likewise unaffected by the composition of the mixture. Investigations showed that the temperature of the onset of PP crystallization in the mixtures of P P and P E H D fell proportionally with reduction in the P P content.
Supermolccular structure of mixtures of polypropylene with polyethylene
1821
As the PP content rises, a rise in the crystallization temperature of the PE is observed. This is discontinued solely in the region of equal concentrations of the components, which correspond to phase inversion in the mixture of polymers. A study was also made of the supermolecular structures that emerge during crystallization of polymeric mixtures, and the characteristic parameters of these structures were obtained by analysis of the low angle light scattering. For the two types of mixtures different relationships were obtained in regard to the size and shape of supermolecular structures relative to the composition of the mixtures. With mixtures of PP and PELD spherulites appear irrespective of changes in composition, as is shown by the characteristic diffraction patterns (four leaved clover) obtained for these structures. On the other hand the sizes of the spherulites formed in the mixtures differ from those of the spherulites formed in the pure components, and are clearly dependent on the composition of the mixtures. / ~,#rn 2~ a
24
j
/
/
16
/ / / /
8,
t
r
J_
20
1
I
60
_j
j
I
100
20
B
i
{:iO
i
I
700
PP, % FIG, 4. Size of supermolecular structures vs. compositiort of the mixture: a - - P P ~-PELD for the rapidly (1) and slowly (2) cooled samples; b - - P P - ~ P E t t D in rapidly cooled film~.
This is illustrated in Fig. 4 for samples of the P P - P E L D mixtures subjected to slow and rapid cooling conditions. It was found that the mode of cooling influences the size of the structural formations in mixtures where one of the components greatly predominates. In cases where the weight contents of both components are similar, spherulites of practically uniform size are formed, irrespective of the mode of cooling. In the case of the P E - P E H D mixtures the shape of the emerging supermolecular structures depends on the composition of the mixtures. When the samples undergo rapid cooling, a spherulitic structure is formed only when the PE content greatly predominated, and the spherulites are smaller than those in the pure PE (Fig. 4b). The intensity distribution in the diffraction patterns obtained for the samples in which PE greatly predominated agrees with the distribution calculated by Picot, Stein, Motegi and Kawai [6, 7] for a three dimensional
C~
L~
o~
FIG. 5. Light. scattering patterns for the rapidly (a k) and slowly (l--v) cooled samples of P P ]-PEHI): a, 1 - - P E ; b , m - - 1 0 ; c , n - 20;d, o -- 30; e, p - - 40; .f, q 50;.q, r-- 60; h, s - - 70; i, t 80;j, +~--90; k, v--100°,~> PP.
C/2
b~
o
O
t~
O
!
©
7~
¢+
g
c~
+_
4
1824
M. K ~ ¥ s z E w s x I et al.
fan-like model of the scattering structure. The angle of opening of the ikn is reduced as the P P content rises, and for mixtures with a 90~ o P P content the fan shape changes to a rod-like structure. When the mixtures undergo slow cooling the intensity distribution in the diffraction patterns shows that in these samples also, structures corresponding to the fan model emerge. The existence of such structures is conformed by microscopical examination. In mixtures with similar P E H D and P P contents (in the samples subjected to slow and rapid cooling) there are oriented regions that are larger than the supermolecular structures that emerge. This conclusion is based on a comparison of diffraction patterns for these mixtures with the Stein theory [8] which describes the scattering of light by supermolecular structures in an anisotropic medium. Figure 5 shows the diffraction patterns for all compositions of P E H D - P P mixtures with slow or rapid cooling.
J7 /'0-
G
1"6
1'2 O'G -
@
0-8
•
0'2
g'4I
20
I
I
#g
I
t
lgg PP,%
2g
6O
pp,~
log
FIO. 6. Plots of~/vs, composition of mixtures o f P P + P E H D (a) and PP+PELD (b): 1 -rapidly cooled samples; 2--slowly cooled samples. In the case of samples containing spherulites the sizes of the latter were determined b y measurement of the position of scattered intensity maxima in the diffraction patterns. The sizes of the emerging supermolecular structures were estimated from the angular distribution of the scattered light, and by microscopical analysis. Figure 4b gives the sizes obtained in this way for the structures formed in the rapidly cooled samples (dimeters of spherulites and fan-like structures, and lengths of the rod-like structures). The structures formed in the slowly cooled samples are larger than those formed in the rapidly cooled ones, but variations in the size of these structures with the composition of mixtures are similar in either case. Figure 6a illustrates the relationship between the composition of mixtures and the viscosity of melts for the P E H D and P P samples cooled under slow and rapid conditions. The viscosity values fall as the ratio of components in a mixture approaches 1 : 1, and the viscosities of mixtures with l : 1 ratios of the components are very low. In the case of P E L D - P P mixtures the ~/values are highest for pure
S u p e r m o l e c u l a r s t r u c t u r e of m i x t u r e s of p o l y p r o p y l c n e w i t h p o l y e t h y l e n e
1825
PE, and are reduced as the P P content of a mixture rises (Fig. 4b). Microscopical analysis of the supermolecular structures formed at the interface of two layers of the pure polymers showed that in samples crystallized immediately after melting there is a well defined interface between both polymers, and on both sides of this interface there are supermolecular structures characteristic of the pure compo-
Fro'. 7. Microscopical p a t t e r n of t h e P P - - P E H D interface.
nents (Fig. 7). When the samples were kept at an elevated temperature, no change in the P P - P E L D interface was observed. For the PP - P E H D mixture, however, the contact interface became indistinct, and in the region of this interface supermolecular structures characteristic ibr mixtures of these polymers appeared, the changes in question being most characteristic as regards the P P layer, which points to diffusion of the P E H D macromolecules into the molten P P (Fig. 8).
FIG. 8. P P - P E
interftxc(~ a f t e r t h e m e l t h a d bee~l k e p t a t 200 ° for 5 hr.
In view of the foregoing observations it appears that there is a marked difference in the manner of mixing of the components in the two types of mixtures, This follows from the difference in the chain structures of the two types of P E . The P E H D with linear chains without side branches (0.2 CHa groups/1000 C), mechanically mixed with PP, does not form a two phase system in the melt.
1826
M. K~YszEwsxi et al.
The possibility of diffusion of the P E H D molecules into the P P results in the formation of systems with mixing not unlike molecular mixing. The branched P E L D macromolecules (35 CH 3 groups/1000 C) do not diffuse in the melt of the mixture with PP, and the degree of mixing of the components in these mixtures depends solely on mechanical mixing processes. The differences in the degrees of mixing observed for the two types of mixtures would account for the foregoing experimental facts in regard to melting and crystallization. For the P P - P E L D mixtures, which form a two-phase system, it is obvious t h a t the melting and crystallization of the individual components are independent of the composition of the mixtures. As P E H D in mixtures with P P plays the role of a medium giving rise to swelling it facilitates crystallization of the PP, in temperature ranges which are lowered as the P E content of the mixtures rises. The crystallization Of the P P and P E H D components favours phase separation, and for this reason it is likewise observed for these mixtures t h a t the melting points of the components are independent of the amounts of the latter in the mixtures. I n the solid state these mixtures form two phase systems. Translated by R. J. A. HENDRY
REFERENCES
1. Appl. Polymer Symp. 15:1971 2. M. KRYSZEWSKI, A. GALESKI, T. PAKULA and R. SZYLHABEL, Polimery (Polska) 16: 8, 1971 3. R. S. STEIN and M. B. RHODES, J. Appl. Phys. 31: 1873, 1960 4. A. E. N. KFAJZERS, J. J. van AARSTEN and W. PRINS, J. Amer. Chem. Soe. 90: 3107, 1968 5. M. KRYSZEWSKI, A. GALESKI, T. PAKULA, J. GREBOWICZ and P. MILCZAREK, J. Appl. Polymer Sei. 5: 1139, 1971 6. C. PICOT, R. S. STEIN, M. MOTEGI and H. KAWAI, J. Polymer Sei. A-2, 8: 2115, 1970 7. M. MOTEGI, T. OSADA, M. MORITANI and H. KAWAI, Polymer J. 1: 209, 1970 8. C. PICOT and R. S. STEIN, J. Polymer Sei. A-2, 8: 1491, 1970