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Phase behavior of blends of linear low density polyethylene and poly(ethene-propene-1-butene)
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 894–902
www.elsevier.com/locate/europolj
Phase behavior of blends of linear low density polyethylene and poly(ethene-propene-1-butene) A.C. Quental, M.I. Felisberti
*
Universidade Estadual de Campinas, UNICAMP, Instituto de Quı´mica, P.O. Box 6154, Campinas, SP, Brazil Received 17 November 2004; accepted 26 November 2004 Available online 26 January 2005
Abstract The aim of this work was the study of blends of linear low density polyethylene (LLDPE) and an ethene-propene-1butene terpolymer (t-PP). Two types of polyethylene were used to prepare the blends: an ethene-co-1-hexene (LLDPE(H)) copolymer and an ethene-co-1-octene (LLDPE(O)) copolymer. These copolymers present similar comonomer contents, molar mass, molar mass distribution and catalyst systems, but differ in their comonomer distribution. The blends were obtained through mechanical mixing using a single screw extruder at different compositions: 20, 40, 50, 60 and 80 wt.% of LLDPE. From DSC measurements two separated melting and crystallization peaks were observed and dynamic mechanical analysis showed two glass transitions indicating that LLDPE/t-PP blends are immiscible in amorphous and crystalline phases in the solid state. X-ray diffraction showed that the unit cell parameters of both polymers in the blends remain unchanged independent of the composition of the blend. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Blends; Thermal properties; Polyolefins
1. Introduction The individual members of the polyolefin family offer a fairly broad spectrum of structures, properties, and applications. This spectrum can be broadened even further by blending individual polyolefins with other polymers. Polyolefin blends have been studied extensively with a view to improve the properties and processability of the polymers involved. They are also of interest in
* Corresponding author. Tel.: +55 19 3788 3130; fax: +55 19 3788 3023. E-mail address: [email protected] (M.I. Felisberti).
recycling plastic waste where polymers of different types are mixed and there is a need to produce materials with acceptable properties. The advantages of the blends include, for example, improvements in impact strength, optical properties, low temperature impact strength, rheological properties and overall mechanical behavior [1,2]. Isotactic polypropylene (iPP) and polyethylene (PE) are the most commonly used polyolefins. Blends of iPP and PE have been regarded as immiscible [1,2]. The thermal and mechanical properties of such immiscible blends depend on not only the proportion of the components but also on the crystallinity of each component [3]. Linear low density polyethylenes (LLDPE) are a class of polyethylenes with linear chains containing only short chain branching due to the insertion of a-olefin
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.11.033
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
units during the copolymerization reaction of ethene with an a-olefin like 1-butene, 1-hexene and 1-octene. Depending on the a-olefin and the catalyst used for the polymerization, LLDPE presents different microstructures with distinct thermal and mechanical properties. LLDPE has acquired great commercial importance mainly in the package industries because of its superior mechanical properties compared to low density polyethylene (LDPE) [4,5]. The terpolymer poly(propene-co-ethene-co-butene-1) (t-PP) is a random terpolymer with low contents of ethene and 1-butene. Like LLDPE, the vinyl groups of comonomers are incorporated into the backbone and the rest of the carbons of the comonomers result in branches. Therefore, the crystalline phase of t-PP is due to the polypropylene fraction. Copolymerization with ethene and 1-butene causes a decrease in the melting and crystallization temperature and crystallinity. Although i-PP and LLDPE are regarded as immiscible, Yamaguchi and co-workers studied the compatibility and the miscibility of isotactic polypropylene (i-PP) and a class of LLDPE with high amounts of a-olefins like 1-hexene and 1-butene. The blends prepared by casting from polymer solutions in xylene at 110 °C showed miscibility which depended largely on the content of aolefin units in LLDPE. LLDPE containing 57.1 mol% of a-olefin and i-PP are miscible in the amorphous phase, whereas i-PP and LLDPE containing 30.0 mol% of a-olefin are immiscible [6,7]. This work aims the study of the phase behavior of the t-PP and LLDPE in the solid state by differential scanning calorimetry (DSC), dynamical-mechanical analysis (DMA) and X-ray diffraction. Two types of LLDPE, with different microstructures synthesized by the same catalyst system but having different types and distributions of comonomers were used.
2. Experimental 2.1. Materials and blend preparations The polymers used in this study were obtained from commercial sources. Some properties are listed in Table 1. Two types of LLDPE synthesized by Ziegler–Natta catalysts using 1-hexene comonomer (LLDPE(H)) and 1-octene comonomer (LLDPE(O)) were blended with the terpolymer poly(ethene-propene-1-butene) (t-PP). The LLDPE(H) and t-PP were supplied by OPP Petrochemical (Brazil) and LLDPE(O) was supplied by Dow Plastics (USA). Blends with different compositions, 80, 60, 50, 40 and 20 wt.% of LLDPE, were prepared in a single screw extruder, with five zones under barrel temperatures of 160 °C, 180 °C, 200 °C, 205 °C and 215 °C from the hopper to die, and a screw speed of 120 rpm. The screw has a Madock mixer and L/D = 32.
2.2.
13
895
C NMR
The 13C NMR spectra were obtained on a Bruker AC/P 300 MHz spectrometer operating at 75.4 MHz. The spectra were recorded with a 12 second delay time, a 90° pulse width and in the complete proton decoupling mode. Analysis were run in 1,2,4-trichlorobenzene at 125 °C in 10 mm quartz tubes for all polymers. 2.3. Size exclusion chromatography (SEC) The average molar mass and molar mass distributions of the polymers were obtained on a Waters 150C gel permeation chromatograph, with Styragel columns HT4, HT5, HT6 operating a 125 °C, using 1,2,4-trichlorobenzene as solvent with a flow rate of 1.0 ml/min using a refractometric detector. Molar mass were obtained using a universal calibration curve derived from standard polystyrene with molar mass ranging from 104 to 107 g/mol. 2.4. Differential scanning calorimetry (DSC) The crystallization and melting behavior of the polymer and their blends was studied by using a TA Instruments 2910 DSC. The following program was used for DSC analysis: the samples were melted at 180 °C, held isothermally for 5 min, then cooled to 0 °C and heated to 180 °C again. The heating and cooling rates were 10 °C/min under a nitrogen atmosphere. The crystallinity were calculated from the ratio between the melting enthalpy (DHm) value from the second scan of the DSC analysis and the melting enthalpy of the 100% crystalline phase (DHo) from the literature. For polyethylenes a DHo value of 289 J/g was used [8,9]. For the terpolymer a DHo value of 209 J/g (polypropylene) was used [10]. 2.4.1. Thermal fractionated crystallization (TFC) The fractionation of the LLDPE in terms of lamella size was performed in the same DSC equipment as described above. The LLDPE were melted at 180 °C, held isothermally for 5 min and then cooled at 10 °C/min to 130 °C and held isothermally for 20 min. Successive isotherms were performed using a step program of 5 °C up to 80 °C. Finally the sample was heated at 10 °C/min to 180 °C and the corresponding DSC curve was recorded. 2.5. X-ray diffraction X-ray diffraction measurements were carried out on a Shimadzu XRD-6000 using CuKa radiation (40 kV, 30 mA and k = 0.154 nm) over a 2h range from 5° to 50° at a scan speed of 2°/min. The samples were compression molded in films with 0.16 mm of thickness in a rectangular stainless steel mold using a heated electric
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
Properties
LLDPE(H)
LLDPE(O)
t-PP
Comonomer
1-hexene
1-octene
Comonomer (wt.%)a Mw (g/mol)b Mn (g/mol)b Mw=Mn Tf (°C)c Tc (°C)c DHc (J/g)c DHf (J/g)c vc (%)c vc (%)d
9.0
10.0
223 000 46 000 5 130 107 117 130 45 44
228 000 44 000 5 127 102 117 130 45 41
Ethene, 1-butene and propene 2.0 ethene 6.0 1-butene 157 000 37 000 4 135 93 71 75 36 41
a
a
Calculated from 13C NMR data at 125 °C in 1,2,4trichlorobenzene. b By GPC at 125 °C in 1,2,4-trichlorobenzene. c By DSC. d By X-ray diffraction.
press under pressure of 1.10 MPa and a temperature of 180 °C for 5 min, followed by cooling at room temperature. 2.6. Dynamic mechanical analysis (DMA) The dynamic mechanical analysis (DMA) was conducted using a TA Instruments DMA 983. The samples were compression molded in sheets with 1.20 mm of thickness in a rectangular stainless steel mold using a heated electric press under a pressure of 1.10 MPa and a temperature of 180 °C for 5 min, followed by cooling in water. The mean dimensions for the sample between the clamps of DMA were 1.20 mm of thickness, 4.6 mm of width and 38.0 mm of length. The analyses were carried out in the temperature scan mode from 150 °C to +150 °C, at frequency of 1 Hz and a heating rate of 2 °C/min.
3. Results and discussion 3.1. Thermal fractionation crystallization (TFC) Information about comonomer distribution is obtained by a specific thermal treatment—thermal fractionation crystallization (TFC) [11]. This method is based on several steps of isothermal crystallization of the polymer on decreasing the temperature from the melt. This process favors the separation of the crystalline material into groups having different lamellae thickness depending on the amount and distribution of the
a-olefin units in the macromolecular chains. The melting endotherm of a fractionated sample is made up of the same number of peaks as the isothermal crystallization steps. Many studies by SEC, 13C NMR, DSC and FTIR analyses in fractionated LLDPE have shown the same characteristics about comonomer distribution. In general, low molar mass chains tend to have more comonomers than high molar mass chains [8,9,11,12]. The high comonomer concentration present in low molar mass chains tends to destroy the crystalline order of the polymer, then these chains will former crystals with thin lamellae at lower crystallization temperatures (Tc), which melt at lower temperature. On the other hand, high molar mass chains with low comonomer concentrations will form crystals with thick lamellae that will melt at higher temperatures (Tm) [13]. The comonomer concentrations of LLDPE(H) and LLDPE(O) are 9 wt.% and 10 wt.%, respectively, as determined by 13C NMR analysis. Fig. 1 shows the DSC melting curves normalized by the sample mass, obtained from the second scan for LLDPE(H) and LLDPE(O) in two situations: LLDPE as received from manufacturer (dashed lines) and the fractionated curves of LLDPE submitted to thermal fractionation crystallization, showing multiple peaks related to the melting of crystal families with different lamellae thickness. Melting curves obtained in a second scan (without fractionated crystallization) show a difference between LLDPE(H) and LLDPE(O), the former shows only one melt peak and the latter shows two peaks. These differences are better understood by TFC. The curves for fractionated samples were divided into two regions: region A reflects melting of thin lamellae formed from chains with low molar mass and high comonomer content; region B reflects melting of thick
A
B LLDPE(H)
exo
Table 1 Typical characteristics of the polymers used in this study
Heat Flow (W/g)
896
LLDPE(O)
40
60
80
100 120 140 o Temperature ( C)
160
180
Fig. 1. DSC curves of LLDPE corresponding to the second scan at 10 °C/min. (a) as received from manufacturer (- - -) and (b) submitted to TFC treatment (—).
3.2. Crystallization and melting behavior Fig. 2(a) and (b) show the crystallization of the LLDPE(H)/t-PP and LLDPE(O)/t-PP blends, respectively, crystallized at a 10 °C/min cooling rate. DSC curves were normalized by mass and shifted to permit better visualization of the transitions. LLDPE(H) shows a crystallization peak with maximum at 107 °C, LLDPE(O) at 102 °C and t-PP at 91 °C. The blends present two crystallization peaks corresponding to different phases. For blends containing 20 wt.% of LLDPE only one crystallization peak is observed. When the LLDPE content increases two peaks, corresponding to the crystallization of both components in the blends, are observed. Blends containing 40–60 wt% of LLDPE show peaks at the same temperatures as the pure polymers. The crystallization temperature of the LLDPE phase is not influenced by the blend compositions as shown in Fig. 3. The t-PP phase crystallizes at temperatures lower than the t-PP for blends containing 80 wt.% of LLDPE.
LLDPE(H)/t-PP 0/100
Heat Flow (W/g)
20/80 40/60 50/50 60/40 80/20 100/0
0
20
40
60
80
100
120
140 160 180
200
o
(a)
exo
Temperature ( C)
LLDPE(O)/t-PP 0/100 20/80
Heat Flow (W/g)
lamellae formed from chains with high molar mass and low comonomer content. The analysis of the relative intensities and position of the peaks in region ‘‘A’’ and region ‘‘B’’ in Fig. 1 allows the comparison of the comonomers distribution in the LLDPE chains. The melting peak in the region B corresponds to the melting of LLDPE chains presenting higher molar mass and lower comonomer content. The crystals families formed by chains with lower molar mass and higher comonomer contents melting in region A. The melting peaks intensity of LLDPE(H) at region B is more intense and shifted to higher temperatures than the corresponding peak of LLDPE(O). This means, the fraction of chains with higher molar mass and containing lower comonomer concentration is higher in LLDPE(H) than in LLDPE(O). Despite the global comonomer in both polyethylene is similar, its distribution is different, the chains of LLDPE(H) with higher molar mass presenting lower comonomer content than LLDPE(O) and this fact is responsible to the higher melting temperature of LLDPE(H). As consequence, LLDPE(H) should present fewer chains containing higher comonomer contents or the comonomer distribution in the polymer chains is more heterogeneous in comparison to LLDPE(O). Despite the same comonomer content in the LLDPE, the comonomer distribution is very different. LLDPE can be described as a mixture of linear chains with high molar mass and branched chains with lower molar mass, but the comonomer distribution in the polymer chains is better in LLDPE(O). These differences in the chain microstructures affect not only the thermal properties but also rheological and mechanical properties and could influence the miscibility with other polymers.
897
exo
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
40/60 50/50 60/40 80/20 100/0
0
(b)
20
40
60
80
100 12 0 140
160
180
200
o
Temperature ( C)
Fig. 2. DSC crystallization curves: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
At this composition the t-PP crystallization peak appears at 66 °C in LLDPE(H) blends and at 76 °C in LLDPE(O) blends on the DSC curves. A similar result was reported by Long et al. for the crystallization temperatures of isotactic polypropylene in i-PP/LLDPE blends [14] and by Puka´nszky et al. for i-PP/EPDM blends [15]. For blends containing less than 80 wt.% of LLDPE, the crystallization of the t-PP phase shifts to higher temperatures than for pure t-PP. The shifting of the crystallization temperature of t-PP was more pronounced in LLDPE(H)/t-PP blends than in LLDPE(O)/t-PP blends, clearly showing the influence of LLDPE on the crystallization of t-PP. The DSC curves showing the melting behavior of the LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 4(a) and (b) respectively. The DSC curves were normalized by mass and shifted for better visualization of the transitions. LLDPE(H) shows a melting peak with minimum at 128 °C, LLDPE(O) shows one at 125 °C with a shoulder at 110 °C due to melting of the different
898
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 120
exo
LLDPE(H)
110
LLDPE(H)/t-PP 0/100 20/80 40/60
100
60/40
LLDPE(O)
Heat Flow (W/g)
Crystallization Temperature (oC)
50/50
90
80
80/20 100/0
70
t-PP
0
20
40
60
80
100 120 140
16 0 180 200
o
(a)
Temperature ( C)
60 LLDPE(O)/t-PP 0/100
0
20
40
60
80
100
% LLDPE
exo
50
20/80 40/60
thickness lamellae (see Fig. 1) and t-PP shows a peak with a minimum at 135 °C. All blends present two melting peaks indicating the presence of two crystalline phases. Fig. 5 shows the melting temperature of both phase as a function of blends composition. The melting temperature of LLDPE phase decreases slightly from the value of the pure polymer. The same is observed of t-PP phase in blends with LLDPE(O).
50/50 60/40
Heat Flow (W/g)
Fig. 3. Crystallization temperatures versus blend compositions for LLDPE/t-PP blends.
80/20 100/0
0
(b)
20
40
60
80
100 120
140 160 180 200
o
Temperature( C)
Fig. 4. DSC melting curves: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
3.3. X-ray diffraction Fig. 6(a) and (b) show the X-ray diffraction patterns of LLDPE(H)/t-PP and LLDPE(O)/t-PP blends, respectively. The unit cell of LLDPE(H) and LLDPE(O) is orthorhombic with characteristic peaks appearing at 2h = 21.57° and 23.89° for LLDPE(H) and 2h = 21.64° and 23.96° for LLDPE(O) corresponding to interference plane (1 1 0) and (2 0 0), respectively. t-PP crystallizes in a monoclinic a form similar to i-PP, with characteristic peaks appearing at 2h = 14.15°, 16.77°, 18.54°, 21.27° and 21.71° that represents the (1 1 0), (0 4 0), (1 3 0), (1 1 1), (1 3 1) and (0 4 1) diffraction planes. The 13C NMR data showed that t-PP presents propene blocks in its chains. The ethene and 1-butene units are distributed randomly in the t-PP chains. This indicates that only polypropylene units crystallize and there are not ethene or 1-butene sequences long enough to crystallize in the random t-PP terpolymer. According to the X-ray diffractograms, Fig. 6(a) and (b), there is no change in the 2h for LLDPE and t-PP independent of the LLDPE used. Table 2 reports the parameters a and b of the crys-
talline unit cell, as determined by X-ray diffraction of pure LLDPE and t-PP and of their blends from the diffractograms. As can be seen, there is no change in the crystal structure with different blend composition. No evidences of a co-crystallization of the blends componentes. 3.4. Dynamic mechanical analysis (DMA) Dynamic mechanical analysis was used to investigate the amorphous phase of the polymers and their blends. The DMA analyses reveal three relaxations prior to melting in polyethylenes, termed as a, b and c transitions [16–18]. The a peaks observed between about +20 °C and +70 °C, is very sensitive to the thermal history and is attributed to the crystalline phase. Slow cooling or quenching of the samples from the molten state significantly affects the position of the a peak and the intensity of the a relaxation normally increases with increasing polymer density. The a relaxation temperature also increases as the branch content decreases. This
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 145
899
LLDPE(H)/t-PP
140
% t-PP
135
Melt Temperature (oC)
0
Intensity (a.u.)
t-PP
20 40 50 60 80 100
130
LLDPE(H) 5 125
10
15
(a) LLDPE(O)
40
60
80
100
% LLDPE Fig. 5. Melt temperatures versus blend compositions for LLDPE/t-PP blends.
behavior is due to different lamellar thickness that are affected by different amounts of branching. b relaxation occurs between about +20 °C and 40 °C, depending on the type of polyethylene and is ascribed to branched structures. Its intensity is thus quite high for LDPE and very small or absent for HDPE. This relaxation results from the motions of chain units in the crystalline-amorphous interfacial regions suggesting that the intensity depends not only on the fraction in the interface but also on the comonomer size and content. The comonomers are excluded from the crystalline phase, generating the interfacial region between amorphous and crystalline phases. The c relaxation of polyethylenes appears from about 125 °C to 110 °C and this range is shifted to lower temperatures with increasing branch content. The intensity of c relaxation tends to decrease with increasing density, which indicates the involvement of an amorphous phase. There are many controversies about the origin of this relaxation. Some authors have assigned glass transition to b or c relaxation. We prefer the glass transition to be associated with c relaxation. The loss modulus curves for the polymers and LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 7(a) and (b) respectively. The same relaxations at the same temperatures as for pure polymer appear in the blends, indicating the immiscibility of the components in the amorphous phase. LLDPE(H) and LLDPE(O) present c relaxations, a peak with a maximum at 125 °C, while b relaxation occurs between 76 °C and 3 °C with
Intensity (a.u.)
115 20
25
30
35
LLDPE(O)/t-PP
120
0
20
2θ (o)
% t-PP 0 20 40 50 60 80 100 5
(b)
10
15
20
25
30
35
2θ (o)
Fig. 6. X-ray diffraction: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
a maximum at 32 °C. t-PP shows a quite intense and wide relaxation corresponding to a glass transition (or c relaxation) from 35 °C to 27 °C, with a maximum at 5 °C. The b relaxation of t-PP appears as a shoulder at 55 °C and the relaxation due to crystalline phase (a) at approximately 80 °C. For the blends, there is an overlapping of the glass transition of the t-PP and b relaxation of the LLDPE. When the amount of t-PP in the blends decreases, the intensities of its glass transitions also decrease, making the visualization of LLDPE b relaxation as a shoulder possible. This situation is quite clear in the blend with 40 % t-PP. Fig. 8(a) and (b) show storage modulus curves for LLDPE(H)/t-PP and LLDPE(O)/t-PP blends. The relaxation of LLDPE and t-PP occurs in a very close temperature range. Because of this, the storage modulus curves for LLDPE and t-PP do not differ significantly. The storage modulus curves of the blends are intermediate between their components. The addition of t-PP causes a increase of the storage modulus at all temperatures analyzed. From analysis of DSC, X-Ray diffraction and DMA it was possible to conclude that the blends are
900
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
Table 2 Crystal unit cell parameters of pure LLDPE(H), LLDPE(O) and t-PP and their blends % t-PP
0 20 40 50 60 80 100
LLDPE(H)/t-PP ˚ LLDPE a/A
˚ t-PP a/A
˚ t-PP b/A
LLDPE(O)/t-PP ˚ LLDPE a/A
˚ t-PP a/A
˚ t-PP b/A
7.45 7.45 7.43 7.42 7.45 7.43 –
– 6.65 6.65 6.65 6.65 6.66 6.66
– 21.11 21.11 21.11 21.13 21.15 21.15
7.43 7.45 7.44 7.42 7.46 7.43 –
– 6.62 6.65 6.61 6.61 6.61 6.60
– 21.13 21.12 21.12 21.10 21.22 21.12
9
10
8
LLDPE(H) / t-PP 0 / 100 20 / 80 40 / 60
E'(Pa)
E'' (MPa)
10
7
10
6
10
50 / 50 5
10
60 / 40 80 / 20 100 / 0
-150
-100
-50
0
50
100
150
200
LLDPE(H)/t-PP
-150
-100
(a)
-50 0 50 o Temperature ( C)
100
150
-50 0 50 o Temperature ( C)
100
150
o
(a)
Temperature ( C) 10
10
9
10
E'' (MPa)
LLDPE(O) / t-PP
-150
(b)
0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
-100
-50
0
50
100
150
200
o
Temperature ( C)
Fig. 7. Loss modulus (E00 ) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
immiscible, and at 20 °C, for example, four phases coexist: amorphous LLDPE, amorphous t-PP, crystalline LLDPE and crystalline t-PP. The damping factor, tan d, of the polymers, LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 9(a) and (b), respectively. The glass transition of the t-PP phase is intense and a very well defined relaxa-
E'(Pa)
8
10
7
10
6
10
LLDPE(O)/t-PP 5
10 -150
(b)
-100
Fig. 8. Storage modulus (E 0 ) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends. t-PP (wt%): 0(j); 20(,); 40(h); 50(r); 60(O); 80(n); 100(d).
tion in the damping curves of the blends containing up to 60 wt.% of LLDPE. Blends containing 80 wt.% of LLDPE present a wide peak corresponding to the overlap of relaxation of both polymers. However, the damping curves of LLDPE(O)/t-PP blends show a second well defined relaxation at 80 °C due the LLDPE(O) crystalline phase. This relaxation is not so evident in LLDPE(H)/t-PP blends because the LLDPE phase
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 0.7 0.6 0.5 tan δ
tan δ
LLDPE(H) / tPP 0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
901
II
0.4 0.3
I
0.2 0.1 0.0
-100
-50
0
50
100
150
200
o
(a)
-150
-100
(a)
-50 0 50 o Temperature ( C)
100
150
Temperature ( C)
tan δ
LLDPE(O) / t-PP 0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
0.35 0.30
II
0.25 tan δ
-150
0.20 0.15
I
0.10 0.05 0.00 -150
(b) -150
(b)
-100
-50
0
50
100
150
-100
-50
0
50
100
150
o
Temperature ( C)
200
o
Temperature ( C)
Fig. 9. Damping factor (tan d) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
crystallizes in thick lamellae as a consequence of the comonomer distribution in the chains, as discussed previously for thermal fractionation crystallization. In order to understand the microstructure effect on the relaxation of the crystalline phase in LLDPE, samples of LLDPE(H) and LLDPE(O) were submitted to different thermal histories. Fig. 10(a) and (b) show curves of tan d for LLDPE(H) and LLDPE(O), respectively, and submitted to different thermal treatments: (I) ‘‘quenching samples’’—the LLDPE was melted at 180 °C for 4 min then cooled in water at 24 °C; (II) ‘‘slow cooling’’—the LLDPE was melted at 180 °C for 4 min then allowed to cool slowly to room temperature. Fast cooling results in a crystalline phase of LLDPE(H) that presents a relaxation at 80 °C, Fig. 10(a) curve (I). In the other hand, slow cooling results in a crystalline phase that does not present a clear relaxation, Fig. 10(a) curve (II). The same behavior was observed in damping curves of LLDPE(H)/t-PP blends because these blends were also slowly cooled after compression molding. The samples inside the mold undergo
Fig. 10. Damping factor (tan d) as a function of temperature for LLDPE with different thermal histories: (I) quenching samples; (II) slow cooling. (a) LLDPE(H) and (b) LLDPE(O).
slow cooling, then the blends present behavior typical of slow cooling samples like curve (I) in Fig. 10(b). However for LLDPE(O) the thermal treatment has a lesser influence on the crystalline phase than for LLDPE(H). Independent of the thermal treatment, LLDPE(O) will form thinner lamellae when compared with LLDPE(H). Thus, in these conditions, a relaxation peak will be associated with LLDPE(O). Although the curves of Fig. 10(b) show a relaxation for the crystalline phase of LLDPE(O), there are differences concerning the intensity and position of this relaxation with thermal treatment.
4. Conclusions Investigation of LLDPE/t-PP blends over the entire composition range revealed that these two polymers are immiscible in both crystalline and amorphous phases. The crystallization and melting behavior of the polymers as well as the unit cell of both crystalline phases do not significantly change.
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At 80 wt.% of LLDPE, the crystallization process of t-PP changes and the crystallization temperature is shifted to lower temperatures. This effect was more pronounced for LLDPE(H)/t-PP blends. The microstructure of LLDPE influences the dynamical mechanical behavior, mainly in the temperature range of the relaxations of the crystalline phase, due to the microstructure of LLDPE, as seen in the loss modulus and damping factors. Acknowledgement
[5]
[6] [7] [8] [9] [10]
The authors are grateful to FAPESP (proc. N° 00/ 10063-0) for financial supports.
[11] [12]
References [1] Deanin RD, Chuang CH. Polyolefin polyblends. In: Vasilean C, Seymour RB, editors. Handbook of polyolefins: synthesis and properties. New York: Marcel Dekker; 1993. p. 779–89. [2] Plochocki AP. Polyolefin blends. In: Paul DR, Newman S, editors. Polymer blends, vol. 2. New York: Academic; 1978. [3] Zhou XQ, Hay JN. Polymer 1993;34(22):4710–6. [4] James DE. Ethylene polymers. In: Mark HF, Bikales NM, Overberger CG, Menges G, editors. Encyclopedia of
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polymer science and engineering, vol. 6. New York: John Wiley and Sons; 1986. p. 429–46. Kaminsky W. Polyolefins. In: Kricheldorf HR, editor. Handbook of polymer synthesis part A. New York: Marcel Dekker; 1992; 1992 [Chapter 1]. Yamaguchi M, Nitta KH, Miyata H, Masuda T. J Appl Polym Sci 1997;63(4):467. p. 467–74. Yamaguchi M, Miyata H, Nitta KH. J Polym Sci Part B: Polym Phys 1997;35(6):953–61. Wilfong DL, Knight GW. J Polym Sci Part B: Polym Phys 1990;28(6):861–70. Karbashewski E, Kale L, Rudin A, Tchir WJ, Cook DG, Pronovost JO. J Appl Polym Sci 1992;44(3):425–34. Brandrup J. In: Immergut EH, editor. Polymer handbook, vol. 27. New York: John Wiley & Sons; 1975. Balbontin G, Camurati I, DallÕOcco T, Zeigler RC. J Mol Catal A: Chem 1995;98(3):123–33. Neves CJ, Monteiro E, Habert AC. J Appl Polym Sci 1993;50(5):817–24. Todo A, Kashiwa N. Macromol Symp 1996;101:301–8. Long Y, Stachurski ZH, Shanks RA. Polym Int 1991;26:143–6. Puka´nszky B, Tu¨do¨s F, Kallo´ A, Bodor G. Polymer 1989;30(8):1399–406. Khanna YP, Turi EA, Taylor TJ, Vickroy VV, Abbot RF. Macromolecules 1985;18(6):1302–9. Brady JM, Thomas EL. J Polym Sci Part B: Polym Phys 1988;26(12):2385–98. Starck P. Eur Polym J 1997;33(3):339–48.
European Polymer Journal 41 (2005) 894–902
www.elsevier.com/locate/europolj
Phase behavior of blends of linear low density polyethylene and poly(ethene-propene-1-butene) A.C. Quental, M.I. Felisberti
*
Universidade Estadual de Campinas, UNICAMP, Instituto de Quı´mica, P.O. Box 6154, Campinas, SP, Brazil Received 17 November 2004; accepted 26 November 2004 Available online 26 January 2005
Abstract The aim of this work was the study of blends of linear low density polyethylene (LLDPE) and an ethene-propene-1butene terpolymer (t-PP). Two types of polyethylene were used to prepare the blends: an ethene-co-1-hexene (LLDPE(H)) copolymer and an ethene-co-1-octene (LLDPE(O)) copolymer. These copolymers present similar comonomer contents, molar mass, molar mass distribution and catalyst systems, but differ in their comonomer distribution. The blends were obtained through mechanical mixing using a single screw extruder at different compositions: 20, 40, 50, 60 and 80 wt.% of LLDPE. From DSC measurements two separated melting and crystallization peaks were observed and dynamic mechanical analysis showed two glass transitions indicating that LLDPE/t-PP blends are immiscible in amorphous and crystalline phases in the solid state. X-ray diffraction showed that the unit cell parameters of both polymers in the blends remain unchanged independent of the composition of the blend. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Blends; Thermal properties; Polyolefins
1. Introduction The individual members of the polyolefin family offer a fairly broad spectrum of structures, properties, and applications. This spectrum can be broadened even further by blending individual polyolefins with other polymers. Polyolefin blends have been studied extensively with a view to improve the properties and processability of the polymers involved. They are also of interest in
* Corresponding author. Tel.: +55 19 3788 3130; fax: +55 19 3788 3023. E-mail address: [email protected] (M.I. Felisberti).
recycling plastic waste where polymers of different types are mixed and there is a need to produce materials with acceptable properties. The advantages of the blends include, for example, improvements in impact strength, optical properties, low temperature impact strength, rheological properties and overall mechanical behavior [1,2]. Isotactic polypropylene (iPP) and polyethylene (PE) are the most commonly used polyolefins. Blends of iPP and PE have been regarded as immiscible [1,2]. The thermal and mechanical properties of such immiscible blends depend on not only the proportion of the components but also on the crystallinity of each component [3]. Linear low density polyethylenes (LLDPE) are a class of polyethylenes with linear chains containing only short chain branching due to the insertion of a-olefin
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.11.033
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
units during the copolymerization reaction of ethene with an a-olefin like 1-butene, 1-hexene and 1-octene. Depending on the a-olefin and the catalyst used for the polymerization, LLDPE presents different microstructures with distinct thermal and mechanical properties. LLDPE has acquired great commercial importance mainly in the package industries because of its superior mechanical properties compared to low density polyethylene (LDPE) [4,5]. The terpolymer poly(propene-co-ethene-co-butene-1) (t-PP) is a random terpolymer with low contents of ethene and 1-butene. Like LLDPE, the vinyl groups of comonomers are incorporated into the backbone and the rest of the carbons of the comonomers result in branches. Therefore, the crystalline phase of t-PP is due to the polypropylene fraction. Copolymerization with ethene and 1-butene causes a decrease in the melting and crystallization temperature and crystallinity. Although i-PP and LLDPE are regarded as immiscible, Yamaguchi and co-workers studied the compatibility and the miscibility of isotactic polypropylene (i-PP) and a class of LLDPE with high amounts of a-olefins like 1-hexene and 1-butene. The blends prepared by casting from polymer solutions in xylene at 110 °C showed miscibility which depended largely on the content of aolefin units in LLDPE. LLDPE containing 57.1 mol% of a-olefin and i-PP are miscible in the amorphous phase, whereas i-PP and LLDPE containing 30.0 mol% of a-olefin are immiscible [6,7]. This work aims the study of the phase behavior of the t-PP and LLDPE in the solid state by differential scanning calorimetry (DSC), dynamical-mechanical analysis (DMA) and X-ray diffraction. Two types of LLDPE, with different microstructures synthesized by the same catalyst system but having different types and distributions of comonomers were used.
2. Experimental 2.1. Materials and blend preparations The polymers used in this study were obtained from commercial sources. Some properties are listed in Table 1. Two types of LLDPE synthesized by Ziegler–Natta catalysts using 1-hexene comonomer (LLDPE(H)) and 1-octene comonomer (LLDPE(O)) were blended with the terpolymer poly(ethene-propene-1-butene) (t-PP). The LLDPE(H) and t-PP were supplied by OPP Petrochemical (Brazil) and LLDPE(O) was supplied by Dow Plastics (USA). Blends with different compositions, 80, 60, 50, 40 and 20 wt.% of LLDPE, were prepared in a single screw extruder, with five zones under barrel temperatures of 160 °C, 180 °C, 200 °C, 205 °C and 215 °C from the hopper to die, and a screw speed of 120 rpm. The screw has a Madock mixer and L/D = 32.
2.2.
13
895
C NMR
The 13C NMR spectra were obtained on a Bruker AC/P 300 MHz spectrometer operating at 75.4 MHz. The spectra were recorded with a 12 second delay time, a 90° pulse width and in the complete proton decoupling mode. Analysis were run in 1,2,4-trichlorobenzene at 125 °C in 10 mm quartz tubes for all polymers. 2.3. Size exclusion chromatography (SEC) The average molar mass and molar mass distributions of the polymers were obtained on a Waters 150C gel permeation chromatograph, with Styragel columns HT4, HT5, HT6 operating a 125 °C, using 1,2,4-trichlorobenzene as solvent with a flow rate of 1.0 ml/min using a refractometric detector. Molar mass were obtained using a universal calibration curve derived from standard polystyrene with molar mass ranging from 104 to 107 g/mol. 2.4. Differential scanning calorimetry (DSC) The crystallization and melting behavior of the polymer and their blends was studied by using a TA Instruments 2910 DSC. The following program was used for DSC analysis: the samples were melted at 180 °C, held isothermally for 5 min, then cooled to 0 °C and heated to 180 °C again. The heating and cooling rates were 10 °C/min under a nitrogen atmosphere. The crystallinity were calculated from the ratio between the melting enthalpy (DHm) value from the second scan of the DSC analysis and the melting enthalpy of the 100% crystalline phase (DHo) from the literature. For polyethylenes a DHo value of 289 J/g was used [8,9]. For the terpolymer a DHo value of 209 J/g (polypropylene) was used [10]. 2.4.1. Thermal fractionated crystallization (TFC) The fractionation of the LLDPE in terms of lamella size was performed in the same DSC equipment as described above. The LLDPE were melted at 180 °C, held isothermally for 5 min and then cooled at 10 °C/min to 130 °C and held isothermally for 20 min. Successive isotherms were performed using a step program of 5 °C up to 80 °C. Finally the sample was heated at 10 °C/min to 180 °C and the corresponding DSC curve was recorded. 2.5. X-ray diffraction X-ray diffraction measurements were carried out on a Shimadzu XRD-6000 using CuKa radiation (40 kV, 30 mA and k = 0.154 nm) over a 2h range from 5° to 50° at a scan speed of 2°/min. The samples were compression molded in films with 0.16 mm of thickness in a rectangular stainless steel mold using a heated electric
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
Properties
LLDPE(H)
LLDPE(O)
t-PP
Comonomer
1-hexene
1-octene
Comonomer (wt.%)a Mw (g/mol)b Mn (g/mol)b Mw=Mn Tf (°C)c Tc (°C)c DHc (J/g)c DHf (J/g)c vc (%)c vc (%)d
9.0
10.0
223 000 46 000 5 130 107 117 130 45 44
228 000 44 000 5 127 102 117 130 45 41
Ethene, 1-butene and propene 2.0 ethene 6.0 1-butene 157 000 37 000 4 135 93 71 75 36 41
a
a
Calculated from 13C NMR data at 125 °C in 1,2,4trichlorobenzene. b By GPC at 125 °C in 1,2,4-trichlorobenzene. c By DSC. d By X-ray diffraction.
press under pressure of 1.10 MPa and a temperature of 180 °C for 5 min, followed by cooling at room temperature. 2.6. Dynamic mechanical analysis (DMA) The dynamic mechanical analysis (DMA) was conducted using a TA Instruments DMA 983. The samples were compression molded in sheets with 1.20 mm of thickness in a rectangular stainless steel mold using a heated electric press under a pressure of 1.10 MPa and a temperature of 180 °C for 5 min, followed by cooling in water. The mean dimensions for the sample between the clamps of DMA were 1.20 mm of thickness, 4.6 mm of width and 38.0 mm of length. The analyses were carried out in the temperature scan mode from 150 °C to +150 °C, at frequency of 1 Hz and a heating rate of 2 °C/min.
3. Results and discussion 3.1. Thermal fractionation crystallization (TFC) Information about comonomer distribution is obtained by a specific thermal treatment—thermal fractionation crystallization (TFC) [11]. This method is based on several steps of isothermal crystallization of the polymer on decreasing the temperature from the melt. This process favors the separation of the crystalline material into groups having different lamellae thickness depending on the amount and distribution of the
a-olefin units in the macromolecular chains. The melting endotherm of a fractionated sample is made up of the same number of peaks as the isothermal crystallization steps. Many studies by SEC, 13C NMR, DSC and FTIR analyses in fractionated LLDPE have shown the same characteristics about comonomer distribution. In general, low molar mass chains tend to have more comonomers than high molar mass chains [8,9,11,12]. The high comonomer concentration present in low molar mass chains tends to destroy the crystalline order of the polymer, then these chains will former crystals with thin lamellae at lower crystallization temperatures (Tc), which melt at lower temperature. On the other hand, high molar mass chains with low comonomer concentrations will form crystals with thick lamellae that will melt at higher temperatures (Tm) [13]. The comonomer concentrations of LLDPE(H) and LLDPE(O) are 9 wt.% and 10 wt.%, respectively, as determined by 13C NMR analysis. Fig. 1 shows the DSC melting curves normalized by the sample mass, obtained from the second scan for LLDPE(H) and LLDPE(O) in two situations: LLDPE as received from manufacturer (dashed lines) and the fractionated curves of LLDPE submitted to thermal fractionation crystallization, showing multiple peaks related to the melting of crystal families with different lamellae thickness. Melting curves obtained in a second scan (without fractionated crystallization) show a difference between LLDPE(H) and LLDPE(O), the former shows only one melt peak and the latter shows two peaks. These differences are better understood by TFC. The curves for fractionated samples were divided into two regions: region A reflects melting of thin lamellae formed from chains with low molar mass and high comonomer content; region B reflects melting of thick
A
B LLDPE(H)
exo
Table 1 Typical characteristics of the polymers used in this study
Heat Flow (W/g)
896
LLDPE(O)
40
60
80
100 120 140 o Temperature ( C)
160
180
Fig. 1. DSC curves of LLDPE corresponding to the second scan at 10 °C/min. (a) as received from manufacturer (- - -) and (b) submitted to TFC treatment (—).
3.2. Crystallization and melting behavior Fig. 2(a) and (b) show the crystallization of the LLDPE(H)/t-PP and LLDPE(O)/t-PP blends, respectively, crystallized at a 10 °C/min cooling rate. DSC curves were normalized by mass and shifted to permit better visualization of the transitions. LLDPE(H) shows a crystallization peak with maximum at 107 °C, LLDPE(O) at 102 °C and t-PP at 91 °C. The blends present two crystallization peaks corresponding to different phases. For blends containing 20 wt.% of LLDPE only one crystallization peak is observed. When the LLDPE content increases two peaks, corresponding to the crystallization of both components in the blends, are observed. Blends containing 40–60 wt% of LLDPE show peaks at the same temperatures as the pure polymers. The crystallization temperature of the LLDPE phase is not influenced by the blend compositions as shown in Fig. 3. The t-PP phase crystallizes at temperatures lower than the t-PP for blends containing 80 wt.% of LLDPE.
LLDPE(H)/t-PP 0/100
Heat Flow (W/g)
20/80 40/60 50/50 60/40 80/20 100/0
0
20
40
60
80
100
120
140 160 180
200
o
(a)
exo
Temperature ( C)
LLDPE(O)/t-PP 0/100 20/80
Heat Flow (W/g)
lamellae formed from chains with high molar mass and low comonomer content. The analysis of the relative intensities and position of the peaks in region ‘‘A’’ and region ‘‘B’’ in Fig. 1 allows the comparison of the comonomers distribution in the LLDPE chains. The melting peak in the region B corresponds to the melting of LLDPE chains presenting higher molar mass and lower comonomer content. The crystals families formed by chains with lower molar mass and higher comonomer contents melting in region A. The melting peaks intensity of LLDPE(H) at region B is more intense and shifted to higher temperatures than the corresponding peak of LLDPE(O). This means, the fraction of chains with higher molar mass and containing lower comonomer concentration is higher in LLDPE(H) than in LLDPE(O). Despite the global comonomer in both polyethylene is similar, its distribution is different, the chains of LLDPE(H) with higher molar mass presenting lower comonomer content than LLDPE(O) and this fact is responsible to the higher melting temperature of LLDPE(H). As consequence, LLDPE(H) should present fewer chains containing higher comonomer contents or the comonomer distribution in the polymer chains is more heterogeneous in comparison to LLDPE(O). Despite the same comonomer content in the LLDPE, the comonomer distribution is very different. LLDPE can be described as a mixture of linear chains with high molar mass and branched chains with lower molar mass, but the comonomer distribution in the polymer chains is better in LLDPE(O). These differences in the chain microstructures affect not only the thermal properties but also rheological and mechanical properties and could influence the miscibility with other polymers.
897
exo
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
40/60 50/50 60/40 80/20 100/0
0
(b)
20
40
60
80
100 12 0 140
160
180
200
o
Temperature ( C)
Fig. 2. DSC crystallization curves: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
At this composition the t-PP crystallization peak appears at 66 °C in LLDPE(H) blends and at 76 °C in LLDPE(O) blends on the DSC curves. A similar result was reported by Long et al. for the crystallization temperatures of isotactic polypropylene in i-PP/LLDPE blends [14] and by Puka´nszky et al. for i-PP/EPDM blends [15]. For blends containing less than 80 wt.% of LLDPE, the crystallization of the t-PP phase shifts to higher temperatures than for pure t-PP. The shifting of the crystallization temperature of t-PP was more pronounced in LLDPE(H)/t-PP blends than in LLDPE(O)/t-PP blends, clearly showing the influence of LLDPE on the crystallization of t-PP. The DSC curves showing the melting behavior of the LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 4(a) and (b) respectively. The DSC curves were normalized by mass and shifted for better visualization of the transitions. LLDPE(H) shows a melting peak with minimum at 128 °C, LLDPE(O) shows one at 125 °C with a shoulder at 110 °C due to melting of the different
898
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 120
exo
LLDPE(H)
110
LLDPE(H)/t-PP 0/100 20/80 40/60
100
60/40
LLDPE(O)
Heat Flow (W/g)
Crystallization Temperature (oC)
50/50
90
80
80/20 100/0
70
t-PP
0
20
40
60
80
100 120 140
16 0 180 200
o
(a)
Temperature ( C)
60 LLDPE(O)/t-PP 0/100
0
20
40
60
80
100
% LLDPE
exo
50
20/80 40/60
thickness lamellae (see Fig. 1) and t-PP shows a peak with a minimum at 135 °C. All blends present two melting peaks indicating the presence of two crystalline phases. Fig. 5 shows the melting temperature of both phase as a function of blends composition. The melting temperature of LLDPE phase decreases slightly from the value of the pure polymer. The same is observed of t-PP phase in blends with LLDPE(O).
50/50 60/40
Heat Flow (W/g)
Fig. 3. Crystallization temperatures versus blend compositions for LLDPE/t-PP blends.
80/20 100/0
0
(b)
20
40
60
80
100 120
140 160 180 200
o
Temperature( C)
Fig. 4. DSC melting curves: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
3.3. X-ray diffraction Fig. 6(a) and (b) show the X-ray diffraction patterns of LLDPE(H)/t-PP and LLDPE(O)/t-PP blends, respectively. The unit cell of LLDPE(H) and LLDPE(O) is orthorhombic with characteristic peaks appearing at 2h = 21.57° and 23.89° for LLDPE(H) and 2h = 21.64° and 23.96° for LLDPE(O) corresponding to interference plane (1 1 0) and (2 0 0), respectively. t-PP crystallizes in a monoclinic a form similar to i-PP, with characteristic peaks appearing at 2h = 14.15°, 16.77°, 18.54°, 21.27° and 21.71° that represents the (1 1 0), (0 4 0), (1 3 0), (1 1 1), (1 3 1) and (0 4 1) diffraction planes. The 13C NMR data showed that t-PP presents propene blocks in its chains. The ethene and 1-butene units are distributed randomly in the t-PP chains. This indicates that only polypropylene units crystallize and there are not ethene or 1-butene sequences long enough to crystallize in the random t-PP terpolymer. According to the X-ray diffractograms, Fig. 6(a) and (b), there is no change in the 2h for LLDPE and t-PP independent of the LLDPE used. Table 2 reports the parameters a and b of the crys-
talline unit cell, as determined by X-ray diffraction of pure LLDPE and t-PP and of their blends from the diffractograms. As can be seen, there is no change in the crystal structure with different blend composition. No evidences of a co-crystallization of the blends componentes. 3.4. Dynamic mechanical analysis (DMA) Dynamic mechanical analysis was used to investigate the amorphous phase of the polymers and their blends. The DMA analyses reveal three relaxations prior to melting in polyethylenes, termed as a, b and c transitions [16–18]. The a peaks observed between about +20 °C and +70 °C, is very sensitive to the thermal history and is attributed to the crystalline phase. Slow cooling or quenching of the samples from the molten state significantly affects the position of the a peak and the intensity of the a relaxation normally increases with increasing polymer density. The a relaxation temperature also increases as the branch content decreases. This
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 145
899
LLDPE(H)/t-PP
140
% t-PP
135
Melt Temperature (oC)
0
Intensity (a.u.)
t-PP
20 40 50 60 80 100
130
LLDPE(H) 5 125
10
15
(a) LLDPE(O)
40
60
80
100
% LLDPE Fig. 5. Melt temperatures versus blend compositions for LLDPE/t-PP blends.
behavior is due to different lamellar thickness that are affected by different amounts of branching. b relaxation occurs between about +20 °C and 40 °C, depending on the type of polyethylene and is ascribed to branched structures. Its intensity is thus quite high for LDPE and very small or absent for HDPE. This relaxation results from the motions of chain units in the crystalline-amorphous interfacial regions suggesting that the intensity depends not only on the fraction in the interface but also on the comonomer size and content. The comonomers are excluded from the crystalline phase, generating the interfacial region between amorphous and crystalline phases. The c relaxation of polyethylenes appears from about 125 °C to 110 °C and this range is shifted to lower temperatures with increasing branch content. The intensity of c relaxation tends to decrease with increasing density, which indicates the involvement of an amorphous phase. There are many controversies about the origin of this relaxation. Some authors have assigned glass transition to b or c relaxation. We prefer the glass transition to be associated with c relaxation. The loss modulus curves for the polymers and LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 7(a) and (b) respectively. The same relaxations at the same temperatures as for pure polymer appear in the blends, indicating the immiscibility of the components in the amorphous phase. LLDPE(H) and LLDPE(O) present c relaxations, a peak with a maximum at 125 °C, while b relaxation occurs between 76 °C and 3 °C with
Intensity (a.u.)
115 20
25
30
35
LLDPE(O)/t-PP
120
0
20
2θ (o)
% t-PP 0 20 40 50 60 80 100 5
(b)
10
15
20
25
30
35
2θ (o)
Fig. 6. X-ray diffraction: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
a maximum at 32 °C. t-PP shows a quite intense and wide relaxation corresponding to a glass transition (or c relaxation) from 35 °C to 27 °C, with a maximum at 5 °C. The b relaxation of t-PP appears as a shoulder at 55 °C and the relaxation due to crystalline phase (a) at approximately 80 °C. For the blends, there is an overlapping of the glass transition of the t-PP and b relaxation of the LLDPE. When the amount of t-PP in the blends decreases, the intensities of its glass transitions also decrease, making the visualization of LLDPE b relaxation as a shoulder possible. This situation is quite clear in the blend with 40 % t-PP. Fig. 8(a) and (b) show storage modulus curves for LLDPE(H)/t-PP and LLDPE(O)/t-PP blends. The relaxation of LLDPE and t-PP occurs in a very close temperature range. Because of this, the storage modulus curves for LLDPE and t-PP do not differ significantly. The storage modulus curves of the blends are intermediate between their components. The addition of t-PP causes a increase of the storage modulus at all temperatures analyzed. From analysis of DSC, X-Ray diffraction and DMA it was possible to conclude that the blends are
900
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902
Table 2 Crystal unit cell parameters of pure LLDPE(H), LLDPE(O) and t-PP and their blends % t-PP
0 20 40 50 60 80 100
LLDPE(H)/t-PP ˚ LLDPE a/A
˚ t-PP a/A
˚ t-PP b/A
LLDPE(O)/t-PP ˚ LLDPE a/A
˚ t-PP a/A
˚ t-PP b/A
7.45 7.45 7.43 7.42 7.45 7.43 –
– 6.65 6.65 6.65 6.65 6.66 6.66
– 21.11 21.11 21.11 21.13 21.15 21.15
7.43 7.45 7.44 7.42 7.46 7.43 –
– 6.62 6.65 6.61 6.61 6.61 6.60
– 21.13 21.12 21.12 21.10 21.22 21.12
9
10
8
LLDPE(H) / t-PP 0 / 100 20 / 80 40 / 60
E'(Pa)
E'' (MPa)
10
7
10
6
10
50 / 50 5
10
60 / 40 80 / 20 100 / 0
-150
-100
-50
0
50
100
150
200
LLDPE(H)/t-PP
-150
-100
(a)
-50 0 50 o Temperature ( C)
100
150
-50 0 50 o Temperature ( C)
100
150
o
(a)
Temperature ( C) 10
10
9
10
E'' (MPa)
LLDPE(O) / t-PP
-150
(b)
0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
-100
-50
0
50
100
150
200
o
Temperature ( C)
Fig. 7. Loss modulus (E00 ) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
immiscible, and at 20 °C, for example, four phases coexist: amorphous LLDPE, amorphous t-PP, crystalline LLDPE and crystalline t-PP. The damping factor, tan d, of the polymers, LLDPE(H)/t-PP and LLDPE(O)/t-PP blends are shown in Fig. 9(a) and (b), respectively. The glass transition of the t-PP phase is intense and a very well defined relaxa-
E'(Pa)
8
10
7
10
6
10
LLDPE(O)/t-PP 5
10 -150
(b)
-100
Fig. 8. Storage modulus (E 0 ) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends. t-PP (wt%): 0(j); 20(,); 40(h); 50(r); 60(O); 80(n); 100(d).
tion in the damping curves of the blends containing up to 60 wt.% of LLDPE. Blends containing 80 wt.% of LLDPE present a wide peak corresponding to the overlap of relaxation of both polymers. However, the damping curves of LLDPE(O)/t-PP blends show a second well defined relaxation at 80 °C due the LLDPE(O) crystalline phase. This relaxation is not so evident in LLDPE(H)/t-PP blends because the LLDPE phase
A.C. Quental, M.I. Felisberti / European Polymer Journal 41 (2005) 894–902 0.7 0.6 0.5 tan δ
tan δ
LLDPE(H) / tPP 0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
901
II
0.4 0.3
I
0.2 0.1 0.0
-100
-50
0
50
100
150
200
o
(a)
-150
-100
(a)
-50 0 50 o Temperature ( C)
100
150
Temperature ( C)
tan δ
LLDPE(O) / t-PP 0 / 100 20 / 80 40 / 60 50 / 50 60 / 40 80 / 20 100 / 0
0.35 0.30
II
0.25 tan δ
-150
0.20 0.15
I
0.10 0.05 0.00 -150
(b) -150
(b)
-100
-50
0
50
100
150
-100
-50
0
50
100
150
o
Temperature ( C)
200
o
Temperature ( C)
Fig. 9. Damping factor (tan d) as a function of temperature: (a) LLDPE(H)/t-PP blends and (b) LLDPE(O)/t-PP blends.
crystallizes in thick lamellae as a consequence of the comonomer distribution in the chains, as discussed previously for thermal fractionation crystallization. In order to understand the microstructure effect on the relaxation of the crystalline phase in LLDPE, samples of LLDPE(H) and LLDPE(O) were submitted to different thermal histories. Fig. 10(a) and (b) show curves of tan d for LLDPE(H) and LLDPE(O), respectively, and submitted to different thermal treatments: (I) ‘‘quenching samples’’—the LLDPE was melted at 180 °C for 4 min then cooled in water at 24 °C; (II) ‘‘slow cooling’’—the LLDPE was melted at 180 °C for 4 min then allowed to cool slowly to room temperature. Fast cooling results in a crystalline phase of LLDPE(H) that presents a relaxation at 80 °C, Fig. 10(a) curve (I). In the other hand, slow cooling results in a crystalline phase that does not present a clear relaxation, Fig. 10(a) curve (II). The same behavior was observed in damping curves of LLDPE(H)/t-PP blends because these blends were also slowly cooled after compression molding. The samples inside the mold undergo
Fig. 10. Damping factor (tan d) as a function of temperature for LLDPE with different thermal histories: (I) quenching samples; (II) slow cooling. (a) LLDPE(H) and (b) LLDPE(O).
slow cooling, then the blends present behavior typical of slow cooling samples like curve (I) in Fig. 10(b). However for LLDPE(O) the thermal treatment has a lesser influence on the crystalline phase than for LLDPE(H). Independent of the thermal treatment, LLDPE(O) will form thinner lamellae when compared with LLDPE(H). Thus, in these conditions, a relaxation peak will be associated with LLDPE(O). Although the curves of Fig. 10(b) show a relaxation for the crystalline phase of LLDPE(O), there are differences concerning the intensity and position of this relaxation with thermal treatment.
4. Conclusions Investigation of LLDPE/t-PP blends over the entire composition range revealed that these two polymers are immiscible in both crystalline and amorphous phases. The crystallization and melting behavior of the polymers as well as the unit cell of both crystalline phases do not significantly change.
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At 80 wt.% of LLDPE, the crystallization process of t-PP changes and the crystallization temperature is shifted to lower temperatures. This effect was more pronounced for LLDPE(H)/t-PP blends. The microstructure of LLDPE influences the dynamical mechanical behavior, mainly in the temperature range of the relaxations of the crystalline phase, due to the microstructure of LLDPE, as seen in the loss modulus and damping factors. Acknowledgement
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The authors are grateful to FAPESP (proc. N° 00/ 10063-0) for financial supports.
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