Makromol. Chem. 185,1041 -1061 (1984)

1041

Morphology, crystallization, and thermal behaviour of isotactic polypropyleneAow density polyethylene blends Ezio Martuscelli *, Mariano Pracella, Gaetano Della Volpe, Pietro Greco Istituto di Ricerche su Tecnologia dei Polimeri e Reologia del C.N.R. via Toiano 6,Arc0 Felice (NA), Italy (Date of receipt: October 12, 1983)

SUMMARY: The morphology, the crystallization and thermal behaviour of isotactic polypropylene (iPP) in its blends with two different samples of low density polyethylene(LDPE) was investigatedat temperatures high enough to prevent any solidification of LDPE. It is found that pre-existing liquid LDPE domains are incorporated in intra-spherulitic regions during the isothermal crystallization of iPP. The radial growth rate of spherulites is almost unaffected by the LDPE content. The overall rate of crystallization of iPP, on the contrary, is strongly depressed by the addition of LDPE. A depression of the equilibrium melting temperature of iPP, due to kinetic and morphological effects, is also observed. The depression of the overall kinetic rate constant is accounted for by the negative effect (decrease in the number of nuclei) that the addition of LDPE has on the primary nucleation process of iPP.

Introduction In previous papers we described the influence of crystallizationconditions, composition, and molecular mass of components on the morphology, crystallization, and thermal behaviour of binary crystallizable blends containing isotactic polypropylene ( i P ) 1 - 3). In the case of blends obtained by mixing iPP and rubbers such as ethylene-polypropylene random copolymers, ethylene-polypropylene-dienerandom terpolymers, and poly(isobuty1ene) it was found that the presence of a second non-crystallizable component may drastically influence the primary and secondarynucleation process as well as the observed and equilibrium melting temperature, the radial growth rate of spherulites, and the overall rate constant. The type of influence and the entity was found to be strongly dependent on the molecular characteristics of the rubbery component and of the mode and state of dispersion of the dispersed phase in the melt at T, before and during the crystallization process. Phenomena of rejection, occlusion, coalescence, and deformation of pre-existing rubbery domains may also play an important role in determining the final morphology and the radial growth rate of iPP spherulites as some amount of energy must be dissipated by the crystallizing solid to make possible these processes4). In the present paper we report on results of an investigationconcerning the crystallization and thermal behaviour of iPP-based blends containing as second component two samples of low density polyethylene (LDPE) having different molecular mass, branch content, and branch distribution. 0025-116)3/84/$03.00

1042

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

Crystallization was performed at temperatures high enough to prevent any LDPE solidification. Under such conditions the LDPE is present in the melt as spherically shaped separated domains5).

Experimental part a) Materials and characterization The characteristics of the polymers used in the present study are indicated in Tab. 1. The branching characteristics of LDPE samples were quantitatively examined by gated decoupled carbon-I3 NMR. The resonances have been assigned following the indications reported by Axelson, Levy, and Mandelkern6). The carbon-I3 spectra were obtained at 120"C with the spectrometer operating at 50,28 MHz (delay time = 30 s). Sample concentrations were approximately 40 g/100 ml1,2,4-trichlorobenzene.The chemical shifts were referenced internally to the major recurring backbone methylene carbon resonance which was taken as 29,99 ppm from TMS. Spectral widths were f 1000 Hz with 32 K data points. Lock was provided by an internal capillary containing pure benzene-d6. The integrated resonance intensities and the corresponding chemical shifts of LDPE samples are reported in Tab. 2. A detailed inspection of the observed chemical shift data reveals that LDPE samples have both a high overall degree of branching (28,3 for LDPE CF5 and 31,4 for LDPE ZF5) with a rather high concentration of butyl, pentyl, and of still longer ramifications (see resonance at 23,4 and 22,s ppm, respectively). The absence of a resonance between 19 and 20 ppm indicates that no methyl branches are present in the LDPE samples.

Tab. 1. Molecular characteristics, source, and code of the polymers used in the present study Polymer

Source and Melt flow Density trade index; in g/cm3 names 2,16 kg (at 23 "C) ( d 1 0 '1 (at 190OC)

[q]

-

a)

aw

M,

Isotactic RAPRA polypropylene (iPP)

3,9

0,906

-

307 000

Low density polyethylene (LDPE)

Fertene ZF5-I 800 (Montedison)

0,25

0,918

1,00 f1,05

400000+ 500 000

Low density polyethylene (LDPE)

Fertene CF5-2100 (Montedison)

2,O

0,921

0,80+0,85

120000 150 OOO

a)

b,

Intrinsic viscosity obtained in o-dichlorobenzene at 135OC. From 13C gated decoupled NMR analysis.

+

Mn

Branches per 1 OOO carbonsb,

20

14,5

5'6

31,4

4+5

28,3

Morphology, crystallization, and thermal behaviour of. . .

1043

Tab. 2. I3C NMR integrated resonances intensities for low density polyethylene samples LDPE CF5 and LDPE ZF5 (values normalized in %o) Chemical shift in ppm

LDPE CF5

LDPE ZF5

~

39,3 38,O 373 37,O 35,5 34,6 34,4 33,9 33,O 32,60 32,lO 31,O 30,5 29,9 29,5 29,3 29,O 28,l 27,4 27,l 26,9 26,7 25,8 23,9 23,4 22,8 14,l 11,2 10,9 83

Branches per 1 O00 C-atoms

1,37 13,28 3,66 3,20 0,92 2,75 39,39 13,74 1,83 5 $04 6,87 2,29 52,67 717 10,53

10.08 11,91 2,75 4,58 39,39 3,20 5,95

-

2,29 13,74 12,82 23,36

-

2,29 28,3

15,31 3,19 3,19 1,91 3,233 42,11 12,76 6,38 7.65 4,47 52,32 686 12,76 15.95 12,76 2,55 2,55 41,47 3,19 4,47 2,55 18,50

14,04 24P

-

3,83 31,4

b) Preparation of blends Binary blends between iPP and the two types of LDPE were prepared by simultaneous meltmixing in a Brabender-likeapparatus (Rheocord E. C. of Haak inc.) at a temperature of 200 “C, a mixing time of 10 min, and a rotation speed of 32 rpm. Blend compositions range from 100 wt.40 to 40 wt.-070 in iPP. Thin films of blends (about 10 pm in thickness) were obtained by compression moulding of the bulk samples at about 200°C. c) Radial growth rate measurements

The radial growth rates G = dR/dt (R = radius of spherulites, t = time) were calculated by measuring the size of iPP spherulites as function of time during the isothermal crystallization

1044

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

process. An optical polarizing microscope fitted with an automatized hot stage was used. The hot stage could be held at a steady temperature to &0,02 "C by a proportional controller. The following procedure was used: blend films were sandwiched between a microscope slide and a cover glass, heated about 20 "C above the melting point of iPP, and kept at this temperature for 10 min to destroy any trace of crystallinity; the temperature was then rapidly lowered to T, and the blend allowed to crystallize isothermally. The photomicrographs were taken on the growing spherulites in appropriate intervals. The radius R was measured on print and G calculated as the slope of the straight lines obtained by plotting R against time. The observed melting temperatures T; of iPP, crystallized from melt blends, were also measured by using an optical microscope and heating the film from T, to Tk at a rate of 10 Wmin. The temperature at which birefringence disappeared was taken as T; The isothermal crystallization experiments were performed at T, high enough to prevent any LDPE crystallization. Thus, at T, , the iPP spherulites growed in the presence of melt LDPE.

.

d) Overall rate of crystallization measurements

The overall crystallization kinetics of iPP from melt blends was analyzed by differential scanning calorimetry using a Perkin-Elmer DSC-2B appartus. The samples (about 6 L10 mg) were heated, after melting, at about 190°C for 5 min, then isothermally crystallized at various T, , recording the heat of crystallization as function of permanence time. The fraction X, of material crystallized after time t was determined by means of the relation:

where the first integral is the heat generated at time t and the second is the total heat of crystallization for t = 00. The observed melting temperatures T , of the isothermally crystallized blends were measured also by DSC by heating the samples from T, up to T , with heating rates of 10 K/min. The mass crystallinity index X, of the blends and of the IPP phase was calculated at various T, from the ratios between the apparent enthalpies of fusion A H * and the enthalpy of fusion A H of 100% crystalline iPP.

Results and discussion

a) Morphology and spherulite growth rate Optical micrographs of iPP/LDPE films, taken at a crystallization temperature high enough to prevent any LDPE crystallization, are shown in Fig. 1. As it can be seen, during the growth of iPP spherulites the melt LDPE is partially incorporated in intraspherulitic regions by forming distinct domains. The number, the dimensions, and the shape of such domains seem to be dependent on the crystallization temperature and on the composition. In the case of the iPP/LDPE 50/50 blend an interconnected morphology is observed. A grain structure of the melt as well as of spherulites is observed in the phase contrast micrographs of blends. The shape and orientation of LDPE occlusions remain undisturbed after the crystallizing front has passed. That is to say that the iPP spherulite is unable to deform the LDPE droplets or to push them ahead into the interspherulitic regions.

Morphology, crystallization,and thermal behaviour of. . .

1045

Fig. 1 a

Fig. 1 b

Fig. 1 c

Fig. 1 d

Fig. 1. Optical micrographs of thin films of iPP/LDPE CF5 blends with different composition. a) and b) crystallization temp. T, = 127°C; c) T, = 135°C; d) T, = 131 OC

The lamellae, after passing the LDPE, turn and surround it (see Fig. 2). Since the lamellae from the opposite sides of the LDPE occlusions are not coherent they form a boundary beyond each LDPE drop. The same overall morphological features are observed in blends containing LDPE CFS and LDPE ZFS. The details of a morphological analysis of iPP/LDPE blends together with same theoretical considerations have been published in a previous papers). The spherulite radius R increases linearly with time for all Tc’sand compositions investigated. The values of spherulite growth rate G of pure iPP and of iPP/LDPE blends are reported, for each composition, as function of T,, in Tab. 3. As shown by Figs. 3 a and b for a given T,, G is almost independent of blend composition. This result indicates that the spatial hindrance of spherulites growth due to the presence of LDPE occlusions does not cause any slow down in the growth rate, even though drastic changes in the internal structure of the matrix are observed.

b) Overall rates of crystallization Examples of isotherms of crystallization for pure iPP and iPP/LDPE blends are shown in Fig. 4. From such curves the half-time of crystallization to,s,defined as the time taken for half of the crystallinity to develop, was derived.

2,7.10-5 1,s. 10-5 1,2.10-5 7,6 5,4 10-6 3,3 10-6 2.3 *

125,3 127,3 129,3 131,3 133,3 135,3 137,3

-

2,7 10-5 i,8.10-5 1,2. I O - ~ 7,6 * 5,4 * 10-6 3,3 * 10-6 2,3 *

-

iPP (loo wt.-070)

125,3 127,3 129,3 131,3 133,3 135,3 137.3

T, /"C

2,s 10-5 i,9,. 10-5 i,i -10-5 7.6 * 4,7 * 10-6 3,7 * 10-6

-

--

i,7 10-5 1,2 10-5 7,9 10-6 4,9 * 10-6 2.5 *

iPP (90wt.40)

-

i,9 10-5 1,2 10-5 7,9 * 10-6 5,6 3,2 *

iPP-LDPE ZF5 2,s. 10-5 i,6.10-5 i,7.10-5 i,i 10-5 i,3.10-5 7,O * 6,3 * 4,5 * 10-6 4,6 * 3,O * 3,O *

-

iPP (70 wt.-%)

iPP-LDPE CF5

iPP (80 wt.-70)

4,5 * 10-6 3,5 * 10-6

3,3 * 10-6 2,5.

-

-

3,0 10-5 i,6.10-5 i , i .10-5

3,O

4,9.10-6

2,o 10-5 1,2.10-5 7,O *

iPP (50 wt.-%)

2,s. 10-5 i,6.10-5 i,o. 10-5

-

i,7.10-5 1,s 10-5 8,2 * 4,5 * 10-6 2,9 *

iPP (60wt.-70)

4,3 * 10-6 2,9.10-6

2,7.10-5 1,6-10-5 I , I . 10-5

i,9,10-5 9,5 * 10-6 7,O * 4,4 * 10-6 3,O *

iPP (40w t . 4 0 )

Tab. 3. Radial growth rate G of iPP spherulites crystalliied from iPP/LDPE blends at different compositions and temperatures (G is given in cmh)

z

0

c

P

p

"

k?

#w

3

E r.

c)

B

5

m

Morphology, crystallization,and thermal behaviour of. . .

1047 Tc=129'C

iPP/LDPEI50/50)

Fig. 2. Optical micrograph (detail) of an isothermally crystallized thin film of iPP/LDPE CF5 (50/50) blend (T, = 129OC) showing how lamellae surround LDPE occlusions

iPP-LDPE ZF5

12

Fig. 3a

Fig. 3 b

Fig. 3. Spherulite growth rate G as function of crystallization temp. T,for iPP/LDPE blends with different composition. a) iPP/LDPE CF5 blends; b) iPP/LDPE ZF5 blends

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

1048

e l

i P P lOOX

(t-t)/min Fig. 4 a

* ----

(bl

F5 10% ipP 0 % -LDPE C CF510% iPP 9 90%-LDPE

,

50

Fig. 4 b

Fig. 4 c

(t-{)/rnin

Fig. 4. Isotherms of crystallization, at various crystallization temp. T,, for: a) pure iPP; b) iPP/LDPE CF5 (%/lo) blend; c) iPP/LDPE CF5 (80/20) blend; d) iPP/LDPE ZF5 (80120) blend; e) iPP/LDPE ZF5 (70/30)blend

Morphology, crystallization, and thermal behaviour of. . .

1049

/--

id)

LDPE Z F S 220% 0%

I 40

Fig. 4d

(f-fJ/min

L D P E ZFS 302

Fig. 4e

(t-()/rnin

The overall isothermal crystallization of all blends investigated follows the Avrami equation: log[-log(1

1

-

X,)] = -logK, 2,3

+ n.logt

(2)

where X, is the weight fraction of crystallinity at time t; n and K, are the Avrami constants that depend on nucleation, growth rate, and dimensions of the growth process. From plots of log [ - log(1 - X,)] vs. logt the overall kinetic rate constant K, and n may be easily obtained. The kinetic rate constant is related to the half-time of crystallization tOp5 by means of the relation: K

=

ln2/&

1050

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

I

lipp I O O Z

I

f I,

50

I

Fig. 5. Avrami plots; a) pure iPP; b) iPP/LDPE CF5 (80/20)blend; c) iPP/LDPE ZF5 (70/30)blend

log {(t-tJ/min}

Fig. 5c

Examples of Avrami plots are shown in Fig. 5 . The values of K,n, and to,5,for each of the blends investigated, are listed in Tab. 4 as function of crystallization temperature and composition. Plots of tossagainst T,, at constant composition, and versus LDPE content, for a given crystallization temperature, are shown in Figs. 6 and 7, respectively. From the data of Tab. 3 and the trend of the plots of Figs. 6 and 7 it emerges that when iPP/LDPE blends are allowed to

Morphology, crystallization,and thermal behaviour of. . .

1051

Tab. 4. Overall kinetic rate constant K,, Avrami index n, and half-time of crystallization to,5 for iPP/LDPE blends at different composition and crystallization temperature T, T,/'C

K,/s-"

121,9 122,9 123,9 124,9 125,8 126,8 127,8 128,8 129,8 130,8 131,8

i,o .10-4 4,9.10-5 2,2.10-5 i,o .10-5 1,2.10-5 5,5 .10-6 3 3 .10-6 9,5.10-~ 2,2.10-6 7,o. 6,7.10-7

119,8 120,8 121,8 122,8 123,8 124,8 125,8 126,8 127,8 128,8 129,8

iPP 90%/LDPE 5,9 * 10-6 4,7 * 2,7. 4,9.10-7 2,4.10-7 8,7. lo-' i,6.10-7 6,l . lo-' 6,5 . lo-' 7,3 .10-8 6~~10-9

118,8 119,8 120,8 121,8 122,8 123,s 124,8 125,8 126,8 127,8 128,8 129,8 121,8 123,s 125,8 126,8 127,8 128,8 129,8

n

Is

K,/s-"

n

'0,5 Is

120 165 215 279 345 453 576 748 1014 1332 1584

CF5 10% 23 105 2S 129 2,s 151 2,6 222 2,6 300 2,7 393 2,6 402 26 558 2,4 780 1 020 2,3 2,s 1 428

iPP 8O%/LDPE CF5 20% i,3.10-5 2,3 1 , s . lo-' 292 4,4.10-6 23 2,3 3,7 * 10-6 1,7. 2,3 7 , i . 10-7 2,4 ~3.10-7 5,l . lo-' 1,8 * 9,2.10-9

'0.5

2.4 2,s 2,6 2,6

iPP 90%/LDPE ZF5 10%

i,4.10-7 i , o . 10-7

2,6 2,5

393 498

2,7 * lo-' 8,s. 10-9

24 24

731 1 020

iPP 80%/LDPE ZF5 20% 113 131 170 215 270 336 480 660 866 1130

3,6 * 1,3. 2,2.10-6 5,2.10-7 5,i .10-7 6~~10-7 2,1.10-7 1,s. 10-7 5 3 10-8

iPP 7O%/LDPE ZF5 30% 2,l .10-6 2,5 172 5 s . 10-7 25 282 i , 6 . lo-' 2,5 456 7,9.10-8 2,5 600 1 , l . 10-8 2,7 828 1.3. lo-' 25 1 150 4,o. 10-9 26 1610

1052 Tab. 4.

119,8 120,8 121,8 122,8 123,8 124,8 125,8 126,8 127,8 128,8 129,8

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco Continued.

iPP 60%/LDPE CF5 40% 292 1,7 * lo-' i,3.10-5 292 292 4,9 * 10-6 2,4. 22 1,s. 10-6 22 1,6 * 22

141 161 220 288 362 402

7,O

2,4

798

5,4.10-8 7,i .10-9

2,3 2,s

1 156

iPP 60%/LDPE ZF5 40% 4,7.10-6 23 117 8,8 *

24

205

3,3.10-7

25

324

i,i. I O - ~ 3,3 10-8 8,4. I O - ~ 2,8.10-9

23 2,6 2,6 2.7

507 726 997 1 325

1 620

iPP 5O%/LDPE CF5 50% 119,8 120,8 121,8 122,8 123,8 124,8 125,8 126,8 127,8 128,8 118,8 119,8 120,8 121,8 122,8 123,8 124,8

125,8 126,8 127,8 128,8

2,7 * 1,l .10-6

2,4

245

2,4.10-7

2s

384

7,5

2,s

612

2,4 2,4 2s

990 1212 1 422

*

3,9.10-8 2,l * 10-8 1,2.10-8

iPP 40Uo/LDPE CF5 60% 2,l 159 4,l . 23 196 1,2 * 10-6 2,4 258 4,8.10-7 2s 318 2,o. 10-7 2,s 402 9,4 * 10-8 2,s 519 8,9. 23 618 1,2.10-8 2,7 840 7,6 * 10-lo 3.O 1090 5,5.10-'0 2.9 1481

iPP 5O%/LDPE ZF5 50% 2,5 162

1,l .10-6

2,4

259

1,4.10-7 5,l * 2,8. 1,3. 1,2 10-8

2,7 2,6 2,6 2,6

326 530 715 951 1200

f

25

iPP 40%/LDPE ZF5 60%

1,s. 10-5

3,l

*

23

3%

2,8 *

2,7

575

1,6. 5,7. I O - ~ 3,o. 10-9 2,3.10-9

2,6 2,6 2,6 2,6

882 1155 1 476 1719

crystallize at T, high enough to prevent any LDPE crystallization the overall rate of crystallization of the iPP phase (in such conditions the IPP phase grows in the presence of the melt of LDPE) is strongly depressed by the addition of LDPE.

c) Thermal behaviour and crystallinity The observed melting temperature TA of pure iPP and of iPP isothermally crystallized from melt iPP/LDPE blends increases linearly with the crystallization temperature (see Fig. 8).

Morphology, crystallization,and thermal behaviour of. . .

1053

v)

\

".

iPP-LDPE CF5 iP*lO

so

1400-

1000 -

TC/OC

Tc/T

Fig. 6 b

Fig. 6 a

Fig. 6. Variation of the half-time of crystallization to,5with crystallization temperature T, at constant composition for: a) iPP/LDPE CF5 blends; b) iPP/LDPE ZF5 blends

1

00

a0

4

40

wt.-%

Fig. l a

LDPE CF5

I

20

40

wt.-%

-

6(

LDPE Z F 5

Fig. 7 b

Fig. 7. Variation of the half time of crystallization to5 , at given T,,with the LDPE content for: a) iPP/LDPE CF5 blends; b) iPP/LDPE ZF5 bleids

1054

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

I

(a)

iPP 60 ‘L

- LDPE CF5 40

%

T,/OC

Fig. 8a

2 , .E

(b)

iPP 402 -1DPE ZF5 601

T,/OC

Fig. 8 b Fig. 8. Examples of Hoffman plots for iPP/LDPE blends: a) iPP/LDPE CF5 (60/40)blend; b) iPP/LDPE ZF5 (40/60)blend

The experimental data may be fitted by the equation: TA = (l/y) T, + (1 - l/y) T,

(3)

where T, is the equilibrium melting temperature and y is a morphological factor. According to the surface nucleation theory of polymer crystallization’) y is the ratio between the final thickness I and the kinetically determined initial thickness 1: of chain-folded lamellae. It is assumed that at low undercooling the crystal can thicken from 1: to 1’). The application of Eq. (3) to the experimental TA data allows the calculation, for a given composition, of T, and y. The values are given in Tab. 5 .

1055

Morphology, crystallization,and thermal behaviour of. . .

Tab. 5. Equilibrium melting temperature T, and y values for pure iPP and iPP/LDPE blends as obtained from Hoffman plots Wt.-% iPP

T,/'C

100 90 80 70 60 50 40

198 190 189 192 186 186 182

100 90 80 70 60

198 195 193 193 192 190 190

Y iPP/LDPE CF5 291 2,4 2,6 292 2,7 297 29

iPP/LDPE ZF5

50

40

2J 22 22 2,2 2,3 2,4 2,4

For iPP/LDPE blends a depression of the iPP equilibrium melting temperature is observed. The entity of such depression seems to be larger for LDPE CF5 containing blends. This depression in T, is likely to be caused mainly by kinetic and morphological effects. In fact, in case of thermodynamic effects, the LDPE should be able to act as diluent for iPP, but this is in contrast with the observation of LDPE domains preexisting in the melt at and with the invariance of the radial growth rate of spherulites with composition. The overall mass crystallinity index of iPP and iPP/LDPE blends is shown, as function of T, and composition, in Fig. 9. From the trend of the curves it emerges that, for a given composition, X,is almost independent of T, (the values of X,were calculated from DSC melting endotherms by heating the sample from T, to the TA of iPP, while the LDPE is still in the molten state). d) Temperature dependence of G and K In the case of spherulitic growth with chain folded lamellae, where a coherent twodimensional surface secondary nucleation process controls the radial growth of spherulites, G and K are given, according to the kinetic theory7,*),by the following equations: 1 -log n

K,

+ AF*/(2,3 R T,)

= log A, -

4 6, uue T, 2,3 K AH T, AT

~

(4)

1056

-7 a6

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

1

iPP-LDPE CF5

iPP-LDPE Z F 5

Fig. 9. Overall mass crystallinity index X, of iPP and iPP/LDPE blends as function of T, and composition. The measurements were made while the LDPE is still in the molten state

e-,PPwL I

130

125

T,/OC

IPP-LDPE CF5

iPP 100

iPP 90

iPP60

Fig. l o b Fig. 10. Plots of logG + AF*/(2,3 RT') vs. T,/(T,AT) according to Eq. ( 5 ) for: a) iPP and iPP/LDPE CF5 blends; b) iPP and iPP/LDPE ZF5 blends

1057

Morphology, crystallization, and thermal behaviour of. . .

(b)

IPP-LDPE ZF5

Fig. 1 1 . Plots of l/n log& + AF*/(2,3 RT,) vs. T,/(T,AT) according to Eq. (4) for: a) iPP and iPP/LDPE CF5 blends; b) iPP and iPP/LDPE ZF5 blends 102. I m / K - 1 AT T,

Fig. 11 b

log G

4bouue + AF*/(2,3 R T,) = log Go - 2,3 K AH

T, T, AT

(5)

where AF* is the activation free energy for the transport process through the liquidsolid interface and A@* = T, 4b00a,/(AiYAT) is the free energy for the formation of a nucleus of critical size. The term AF* is usually taken from the W i F timetemperature superposition principleg): AF* = AFWLF=

c,Tc C . + T,

-

~

Tg

4 120 T,

51,6

+

T, - Tg

where Tgis the glass transition temperature. In the case of crystallizable blends the spherulite growth rate as well as the overall kinetic rate constant are strongly influenced by the type of phase structure, existing at T, in the melt'*10).

1058

E. Martuscelli, M. Pracella, G. D. Volpe, P . Greco

0

I

I

20

40

I

60

wt.-% LDPE CF5

Fig. 12a

we 1.1 /-

I

I

I

(AT = 65°C): a) iPP/LDPE CF5 blends; b) iPP/LDPE ZF5 blends (SI-unit: 1 erg = lo-' J)

1 logK,, + AF*/(2,3R + AF*/(2,3R T,) and -

Tm T,) against n T, AT are linear (see Figs. 10 and 11). From the slopes of these lines values for A@* and a, are easily derived for each blend composition l l ) . As shown by Figs. 12 and 13 A@* as well as a, decrease with increasing LDPE content. Such effect is more pronounced in the case of LDPE-CFS containing blends. This observation may be accounted for by the fact that LDPE-CFS has a lower molecular mass than LDPE-ZFS.

Plots of IogG

e) Primary nucleation process The number of primary nuclei per unit volume relation 12)

was calculated by means of the

Morphology, crystallization,and thermal behaviour of. . .

I*/

1059

-.

iPP-LDPE ZF5

3L -45

127

1h

1 /

I

131

T,/OC

Fig. 14 Fig. 13. Variation of the surface free energy of folding, a,, of iPP lamellae with the LDPE content Fig. 14. Variation of log nucleation density (log #) with crystallization temperature (T,) for iPP and iPP/LDPE blends

that assumes a spherical growth with instantaneous nucleation. In Eq. (7) G and K,, are both measured at the same T,; p, and pa are the densities of the crystalline and amorphous phase, respectively, and 1 - L(w)is the weight fraction of polymer that is crystalline at t = w . As shown by Figs. 14 and 15 N i s dependent on both T, and blend composition. From Fig. 14 it can be seen that for a given composition 1ogN decreases almost with a linear trend with the increase of T, , while, from Fig. 15, it emerges that at constant T, ,&'decreases monotonically with the LDPE content. The possible explanation is that the heterogeneities constituting athermal nuclei in the iPP matrix are washed out, desactivated, and/or dissolved by LDPE particles during the process of mixing. Such results are in agreement with optical microscopy observations of isothermally crystallized thin films of iPP/LDPE blends showing, at a given T,, a clear decrease of the number of spherulites per unit area with increasing LDPE content in the blends. From the above findings it may be concluded that the depression observed in the values of the overall kinetic rate constant of iPP/LDPE blends is mainly to be ascribed to the negative effect that the addition of LDPE has on the primary nucleation process of iPP.

1060

E. Martuscelli, M. Pracella, G. D. Volpe, P. Greco

‘2

1 A

Tc=125.80C Tc=128.80C

mI

*O

20

40

M)

Fig. 15. Dependence of nucleation density #from LDPE content at constant crystallization temp. T,. a) iPP/LDPE CF5 blends; b) iPP/LDPE ZF5 blends

wt.-Yo LDPE ZF5 Fig. I5 b

This work was partially supported by “Progetto Finalizzato” Chimica Fine of Italian Research National Council. We wish to thank Dr. A. L . Segre for 13CNMR characterization of LDPE samples.

’)

2,

3, 4,

5,

E. Martuscelli, C. Silvestre, G. Abate, Polymer 23, 229 (1982) E. Martuscelli, C. Silvestre, L. Bianchi, Polymer, in press E. Martuscelli, in “Polymer Blends: Processing, Morphology, and Properties”, ed. by Martuscelli, Palumbo, Krizewski, Plenum Press, N. Y. 1981 Z. Bartczak, A. Galeski, E. Martuscelli, Polym. Eng. Sci., submitted A. Galeski, M. Pracella, E. Martuscelli, J. Polym. Sci., Polym. Phys. Ed., in print D. E. Axelson, G. C. Levy, L. Mandelkern, Macromolecules 12, 41 (1979)

Morphology, crystallization, and thermal behaviour of. . . 7, )’ )’

lo)

1061

J. D. Hoffman, J. I. Lauritzen, in “Treatise on Solid State Chemistry”, Vol. 3, ed. by Hannay, Plenum Press, N. Y. 1976, Chap. 7 J. D. Hoffman, SPE Trans. 4, 315 (1964) M. L. Williams, R. F. Landel, J. D. Ferry, J. Am. Chem. SOC.77, 3701 (1955) E. Martuscelli, Polym. Eng. Sci., in press E. Martuscelli, M. Pracella, M. Avella, R. Greco, G. Ragosta, Makromol. Chem. 181,957 (1980)

L. Mandelkern, “Crystallization in Polymers”, McGraw Hill, N. Y. 1964