Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion

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Polymer Degradation and Stability 93 (2008) 225e232 www.elsevier.com/locate/polydegstab

Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion Bi-Ru Sheng, Bin Li, Bang-Hu Xie*, Wei Yang, Jian-Min Feng, Ming-Bo Yang College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, People’s Republic of China Received 28 June 2007; received in revised form 23 September 2007; accepted 29 September 2007 Available online 18 October 2007

Abstract The molecular weight of ethylene-block-co-polypropylene (co-PP) was adjusted by reactive extrusion with the incorporation of dicumyl peroxide (DCP), and the effect of molecular weight on the crystallization behavior, crystal morphology, and fracture behavior was investigated. It was found that, with increasing DCP content, the molecular weight (MW) decreased and the polydispersity (Mw/Mn) slightly decreased. After modification, the number of spherulites with obscure boundaries increased, and the size of the spherulites was more even due to increasing amount of grafting and micro-cross-linking structures, generated in co-PP degradation, which were acting as nucleating agents. Evaluated by essential work of fracture method, the specific essential work of fracture, we, was found to be strongly dependent on the molecular weight, especially, on the number average molecular weight (Mn) linearly, while the specific non-essential work of fracture, bwp, was enhanced with decreasing z-average molecular weight (Mz), probably owing to the reduction of ultra-high molecular weight component in degraded co-PP. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Controlled-rheology-polypropylene; Crystallization behavior; Crystallite morphology; Essential work of fracture

1. Introduction Due to the high melt viscosity and elasticity caused by high molecular weight (MW) and broad molecular weight distribution (MWD), the processing conditions and thus, the applications of commercial polypropylene (PP) are restricted. Generally, PP resins with low melt viscosity and molecular weight as well as narrow MWD are demanded in high-speed spinning and injection-molded thin-walled products. Such PP resins can be prepared through improvement in the polymerization technology or reactive process via using original PP with high molecular weight and broad MWD. The former is somewhat difficult to carry out because it requires the addition of chain terminators and transfer agents when using conventional reactors, and generally, such operations decrease the output and often increase the cost. Meanwhile, the latter

* Corresponding author. Tel./fax: þ86 28 8540 5324. E-mail address: [email protected] (B.-H. Xie). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.09.011

technique, such as reactive extrusion, via incorporating peroxides to induce controllable degradation of PP chains, can effectively reduce the molecular weights and narrow the MWD of the virgin resin [1]. Moreover, the method of reactive extrusion possesses many merits, such as simple operation, low cost and high productivity. PP prepared in this way (i.e. controlled-rheology PP, abbreviated as CRPP), with low molecular weights and narrow MWD, has been generally adopted in industry. The studies on the reactive mechanism, the rheological and crystallization behavior, as well as mechanical properties of CRPP have been reported extensively [2e6], whereas the fracture behavior has obtained few attentions [7]. It is noted that the MWD of PP copolymer was narrower with respect to the homopolymer [6], thus a series of CRPPs, with different molecular weights and almost constant MWD, can be prepared using PP copolymer. For this reason, a kind of ethylene-block-co-polypropylene (co-PP) was employed as the base polymer in this paper, and a series of CRPPs with different melt viscosities, MW and MWD were prepared via controlling the addition of peroxide (DCP) in reactive extrusion.

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The dependence of molecular weights, crystallization behavior and crystal morphology of CRPP on DCP concentration was investigated. The fracture behavior of injection-molded CRPP sheets, related with variation of molecular weights and microstructure, was evaluated using the essential work of fracture (EWF) method. The proposal of EWF concept and its development in evaluating fracture toughness of polymer materials provide a new approach to investigate the fracture behavior of polymer materials, especially polymer sheets or films. According to the EWF method [8e11], the total energy, Wf, required to fracture a pre-cracked specimen (the double edge notched tension specimen (DENT) used in this article is shown in Fig. 1) can be partitioned into the essential work of fracture, We, and non-essential or plastic work of fracture, Wp. We is essentially the surface energy to generate new crack surface, and Wp is the volume energy dissipated by plastic deformation in fracture process. Thus the relationship can be written as follows: Wf ¼ We þ Wp ¼ we lt þ bwp l2 t wf ¼

ð1Þ

Wf ¼ we ¼ bwp l lt

ð2Þ

where we and wp are the specific essential work of fracture and specific non-essential work of fracture or specific plastic work, respectively [12e19], l being the ligament length, t the specimen thickness and b the plastic zone shape factor which just relates to the shape of the specimen. Provided we and bwp remain independent of l, a linear relationship can be drawn between wf and l of DENT specimen with various ligament lengths. By extrapolation of this line to zero ligament length, the positive interception at wf-axis ( y-axis) gives we and the slope, bwp.

35mm

Load

2. Experimental part 2.1. Materials and specimen preparation For our work, the granular ethylene-block-co-polypropylene (co-PP K8303) was provided by Beijing Yanshan Petrochemical Co., Ltd (PRC), with 17.8 mol% ethylene determined by Fourier Transformed Infra Red (FTIR) spectroscopy. According to pre-selected mass ratios (Table 1), DCP was first mixed with dried co-PP pellets. Subsequently, the reactive extrusion of the mixtures was performed on an SJ-20AX25 single-screw extruder with the temperature profile of 180e210  C. The extruded co-PP (CRPP) was pelletized and then injected into rectangular sheets of 1.5 mm thickness, using an injection-molding machine (PS40E5ASE, Nissei, Japan) (190e210  C). 2.2. Melt flow rate and gel permeation chromatograph (GPC) The melt flow rates (MFR) of co-PP and CRPP through extrusion were measured according to ASTM 1238 86T at 230  C/2.16 kg. The average molecular weight and molecular weight distribution of injected co-PP and CRPP were measured by gel permeation chromatograph (GPC) at 150  C with trichlorobenzene (TCB) as solvent and polystyrene (PS) for calibration. 2.3. Thermal analysis A NETZSCH differential scanning calorimeter (DSC-204) was used to study the melting and crystallization behavior of CRPP with different DCP contents. The DSC scan was performed in the temperature range of 50e200  C, with both cooling rate and heating rate of 10  C/min in a nitrogen atmosphere. Before the scan, the specimen was heated up to 200  C rapidly and kept for 5 min to erase the thermal history. 2.4. Polarized light microscope (PLM) observations

Clamped zone

Cutting from the injected specimen, a piece of specimen was hot-compressed into thin film at 230  C and then cooled to room temperature with a cooling rate of 5  C/min. During cooling, the evolution of crystal morphology was observed by PLM (LEICA DMLP, Germany).

OPDZ

z

IFPZ

l

2.5. Essential work of fracture test

Clamped zone t

Load W

Fig. 1. Double edge notched tensile specimen (DENT).

The injection-molded sheets, with the size of (Z  W  t) 110  40  1.5 mm (Fig. 1), were cut into DENT specimens according to European Structural Integrity Society (ESIS) proposal [20]. The pre-cracks on both sides of the samples were cut perpendicularly to the tensile direction with a fresh razor blade. The ligament lengths of the samples were confined by ESIS criterion. For each material, 20 specimens with various ligament lengths (5e13 mm) were used. The EWF test was conducted on an Instron (4302) universal test machine

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Table 1 Melt flow rate (MFR), molecular weight (number average, weight average, z average) and polydispersity (Mw/Mn) of co-PP and CRPP DCP amount (wt%)

MFR (g/10 min)

Mn (g/mol)

Mw (g/mol)

Mz (g/mol)

Mw/Mn

0 0.02 0.04 0.06 0.08

1.44 2.50 4.29 8.34 8.40

122 500 117 000 100 200 72 900 86 600

532 500 371 900 309 700 255 100 272 900

2 115 900 1 002 200 819 700 801 600 702 600

4.35 3.18 3.09 3.50 3.15

equipped with a 10 kN load cell at a cross-head speed of 5 mm/min and the test temperature was 23  2  C. 3. Results and discussions 3.1. Molecular weight and its polydispersity (Mw/Mn) The results of MFR, average molecular weights and polydispersity (Mw/Mn, Mw and Mn are weight average molecular weight and number average molecular weight, respectively) of CRPP with different DCP concentrations are listed in Table 1. Generally, the MWD of a polymer could be reflected by polydispersity. As shown in Table 1, the MFR of CRPP increased markedly with increasing DCP concentration accompanying with remarkably decreased average molecular weights of CRPP, while the values of polydispersity (Mw/Mn) of CRPP were all about 25% lower than that of base PP. It revealed that even the polydispersity of base PP was relatively low (Mw/Mn z 4), the ratio of Mw/Mn of CRPP could be further narrowed by introducing DCP. Many studies [1e5,21] have manifested that in the melt state of PP, the radicals generated by the DCP could capture the tertiary hydrogen atoms in the molecular chain of PP, forming tertiary polypropylene macroradicals which were prone to degrade through b scission. Moreover, since the probability of the longer molecular chains (the high molecular weight components) to be attacked by the free radicals from the decomposing DCP was much higher than that of the shorter molecular chains (the low molecular weight components) [1], the long chains were more prone to degrade, resulting in narrow MWD and low polydispersity. When the concentrations of DCP were less than 0.06 wt%, all the three kinds of average molecular weights of CRPP decreased with increasing DCP concentration, and the amplitude of reduction of Mz was more pronounced than that of Mw and Mn, especially Mn. When the DCP concentration kept increasing from 0.06 wt% to 0.08 wt%, Mz decreased further while Mn and Mw increased definitely. As shown in the cumulative weight distribution curves of molecular weight (Fig. 2A), the component of the polymer with both low (for the component with MW < 1  105 g/mol and 5  104 g/mol, the content reduction was 7.5 wt% and 4.1 wt%, respectively, as shown in Fig. 2B) and ultra-high molecular weights (for the component with MW > 1.5  106 g/mol, the reduction was 0.25 wt%. Particularly, for the component with MW > 3.6  106 g/mol, the content decreased from 0.11 wt% to an undetectable value, as shown in Fig. 2C.) decreased definitely. Whereas, the concentration of moderateehigh molecular weight component

(MW z 105e106 g/mol) increased remarkably. The increase of the molecular weight was also observed in isotactic polypropylene (iPP) modified by tert-butyl perbenzoate (TBPB) [21]. Noted that Mz was sensitive to ultra-high molecular weight component, it can be concluded that the long molecular chains of co-PP mainly degraded with increasing amount of DCP, and even at high DCP concentration when a great number of free radicals exist, the same scenario could be observed. Reactions such as biradical coupling might occur among the macroradicals with low molecular weight, resulting in the increase of Mn and Mw which were sensitive to moderateehigh molecular weight component. 3.2. Melting and crystallization behavior The DSC results are listed in Table 2. With the addition of DCP, the melting peak temperatures (Tm) were almost constant, and the melting enthalpy (DHm) of co-PP decreased, while the variations of DHm were insignificant with further increasing amount of DCP. Generally, the crystallinity of a polymer, reflected by melting enthalpy (DHm), was mainly determined by its average molecular weight and regularity of chain structure under a given crystallization condition [22]. The crystallization peak temperature (Tc), listed in Table 2, increased with increasing DCP content, indicating that possibly due to the degradation of co-PP molecular chains (especially the ultra-high molecular weight component) induced by DCP, the enhanced molecular chain segment’s mobility favored the diffusion and arrangement of macromolecules into crystal cell, which resulted in the elevated crystallization rate and higher crystallization temperature. Similar results were also obtained by Hamed et al. when investigating the degradation of iPP [3,23]. Some other researches showed that the branching and micro-cross-linking structures induced by large quantity of DCP were beneficial to the increase of nucleation density [24e26] and were also responsible for the acceleration of crystallization. The width of crystallization peak at half height (DW ) could reflect the distribution of crystallite of various dimensions. A narrow size distribution of crystallite could be inferred according to the gradual decrease of DW, when DCP content is increased as shown in Table 2. The crystal morphology of CRPP, which was rarely reported, was depicted by the PLM (polarized light microscope) pictures shown in Fig. 3. As observed in Fig. 3A, the spherulites of base PP were well developed, and the size of these spherulites was uneven, resulting in broad distribution of the dimension. Moreover, distinct boundaries existed among

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228

A

1.0

DCP:0w% DCP:0.02w% DCP:0.06w% DCP:0.08w%

dw_dlogM

0.8 0.6 0.4 0.2 0.0 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

logMw

C

DCP:0w% DCP:0.02w% DCP:0.06w% DCP:0.08w%

0.3

0.4

DCP:0w% DCP:0.02w% DCP:0.06w% DCP:0.08w%

0.3

dw_dlogM

dw_dlogM

B

0.2

0.1

0.2

0.1

0.0

0.0 3.4

3.6

3.8

4.0

4.2

4.4

4.6

6.0

6.2

6.4

6.6

6.8

7.0

7.2

logMw

logMw

Fig. 2. A: cumulative weight distribution curves of molecular weight; B and C: partial enlarged drawing of cumulative low and ultra-high molecular weight distributions, respectively.

spherulites of base PP. With the increasing addition of DCP, the quantity of spherulites in CRPP (Fig. 3BeE) increased, accompanied with narrower size distribution and obscure boundaries derived from the mutual impenetration of these spherulites. It was speculated that the small amount of CRPP chains with branching and micro-cross-linking structures, which were generated in co-PP degradation, may act as nucleating agents [24e26], contributing to the increase of nucleation density. Since more crystallites grew in a confined space at a higher rate, the crystallites tended to squeeze and penetrate with each other, which resulted in smaller size and narrower dimension distribution of the spherulites. Obviously, the results shown in PLM pictures coincided well with DSC results. 3.3. Fracture behavior The loadedisplacement curves of DENT specimens for coPP and CRPP with the same ligament length are presented in Fig. 4. It was found that the fracture characteristics of this series of DENT specimens varied regularly as DCP content increased, that is, the yield stress gradually decreased and the elongation at break remarkably increased. It demonstrated that the stiffness decreased while the ductility of the specimens was enhanced. The total work of fracture (Wf) was obtained by integrating the loadedisplacement curves. In order to understand the contributions of different stages of the

fracture process to the total energy consumption, by the method of Karger-Kocsis [27], the yielding-related specific work of fracture (Wf,y, required for yielding) as well as necking-related specific work of fracture (Wf,n, consumed by necking and tearing) was determined by partitioning the loadedisplacement curves at the yield point, which was ‘‘marked’’ with a maximum in the corresponding loadedisplacement curves, as shown in Fig. 4. Obviously, the value of Wf is the sum of Wf,y and Wf,n. After dividing Wf, Wf,y and Wf,n by lt, their specific values, i.e. wf, wf,y and wf,n, were obtained, which are listed in Table 3. As DCP content increased, the variation of wf was mostly controlled by wf,n,, reflected from similar increasing trend of wf and wf,n, whereas the value of wf,y varied reversely. For semicrystalline polymers, the deformation behavior before yielding mainly experiences molecular and lamellar orientations and slip [28,29]. It is possible that the decreased molecular weight and smaller spherulites improve the mobility of molecular chain segments and slip tendency of the lamellae, which consequently reduced Table 2 Characteristic data of DSC scanning of co-PP and CRPP DCP amount (wt%)

Tm ( C)

DHm (J/g)

Tc ( C)

DW ( C)

0 0.02 0.04 0.06 0.08

166.4 165.6 165.1 165.4 165.3

100.1 91.8 88.0 95.4 85.7

116.7 117.3 118.0 119.0 119.9

5.4 5.0 4.3 4.0 3.9

B.-R. Sheng et al. / Polymer Degradation and Stability 93 (2008) 225e232

229

Fig. 3. PLM pictures of spherocrystal morphology of co-PP and CRPP. A: base PP, BeE: CRPP prepared by 0.02 wt% DCP, 0.04 wt% DCP, 0.06 wt% DCP, 0.08 wt% DCP, respectively.

the energy necessary for yielding (wf,y). After yielding, the increase of wf,n was possibly attributed to the impenetration among spherulites as shown in Fig. 3BeE,which enhanced the strength at the boundaries among spherulites and dragged more polymer into deformation zone and consequently avoided premature fracture at weak boundaries. According to the digital pictures of fractured DENT specimens (Fig. 5), the DENT specimen of co-PP without necking phenomenon showed a rapid crack propagation to fracture, while the addition of DCP in co-PP effectively slowed down the crack propagation and improved the plastic deformation as proved by more distinct necking zone occurring at higher DCP content. All the results confirmed that the decreases of average molecular weight of co-PP, especially of the ultra-high molecular weight component, as well as the increasing number of

spherulites and diminished spherulite size facilitated the mutual movement of both molecular chains (or chain segments) and the crystallites, which was propitious to enlarged plastic deformation. For co-PP and CRPP, the loadedisplacement curves of the DENT specimens with different ligament lengths exhibited nice self-similarity for each material, and the linear relationship between wf and l was ensured by the linear regression coefficients (R2) of all wfel plots higher than 0.92, thus the requirements of EWF method were satisfied. All the EWF parameters obtained from wfel plots are listed in Table 4. The values of we showed some decrease with the addition of DCP, indicating the weakened crack resistance, i.e. fracture toughness. When DCP content was less than 0.06 wt%, the value of we gradually decreased to 60% of that of base PP.

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230 0.7

DCP:0.04w%

Table 3 Specific total work of fracture (wf) and its yielding-related (wf,y) and neckingrelated (wf,n) terms of co-PP and CRPP determined from the loadedisplacement curves of DENT specimens with ligament length of 11 mm

DCP:0.06w%

DCP amount (wt%)

wf,y (kJ/m2)

wf,n (kJ/m2)

wf (kJ/m2)

0 0.02 0.04 0.06 0.08

46.1 32.8 31.4 30.0 25.6

99.6 130.6 150.5 175.1 215.7

145.7 163.4 181.9 205.2 241.3

DCP:0w% DCP:0.02w%

0.6

DCP:0.08w%

Load (kN)

0.5

0.4

0.3

0.2

0.1

Wf.y

Wf.n

0.0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Displacement (mm) Fig. 4. Loadedisplacement curves of DENT specimens with the same ligament length (l ¼ 11 mm) for co-PP and CRPP.

It could be inferred that the decreases of both average molecular weight and crystallite size could impair the crack resistance of CRPP, which maybe related to the slip resistance of molecular chain segments and entanglement among molecular chains as well as the tie chains between crystallites. For a semicrystalline polymer, the overall crystallinity decreased which might not only increase entanglement and possibly the amount of tie chains but also destroy the order and build-up of lamellae, so there was an optimum value about toughness as a function of crystallinity, whereas with decreasing molecular weight the amount of tie chains might decrease [30]. Tie chains could efficiently transfer stress through lamellae, and the region of lamellar fragmentation before crack was augmented [31], thus the crack resistance could be improved. So here the question was which predominated? It seemed that the influence of decreased average molecular weight was predominant, which facilitated the formation of new surfaces during crack propagation, similar results were reported in literature [26,32]. With further increase of DCP content to 0.08 wt%, the value of we raised to some extent with respect to that of co-PP added with 0.06 wt% DCP. It might be owing to the marked reduction of low molecular weight component and remarkable increase of moderateehigh molecular weight component, which counteracted the negative effect caused by the reduction of ultra-high molecular weight component and led to enhanced crack resistance. At almost constant spherulite size (DCP: 0.06e0.08 wt%), the reduction of low molecular weight

component improved the slip resistance of molecular chain segments, and most importantly, the increase of moderatee high molecular weight component could effectively enhance the molecular chain entanglement. Meanwhile, the tie chains between crystallites were also increased. In our work, the dependence of the values of we on Mn and Mw, both of which were more sensitive to low molecular weight component (than Mz), was worth noting, as shown in Fig. 6. The values of we were higher at higher values of Mn or Mw, and especially, almost a linear relationship between Mn and we existed, which could be described as follows:
we ¼ 2:55  Mn  104  0:74 It could be found that, this series of CRPP with different molecular weights, obtained by reactive extrusion with the addition of DCP, showed good similarity in molecular and structural properties among each other, such as chain branching, narrow and similar molecular weight distributions (Table 1). Thus, compared to the direct application of commercial PP with different average molecular weights, it was more convenient to investigate the relationships between fracture behavior and molecular weight using this reaction-extruded CRPP, owing to little influence of discrepancy in molecular chains and molecular weight distribution. Undoubtedly, it was also our original intention of this work. As shown in Table 4, the bwp values of CRPP obviously rose with increasing DCP amount. When the amount of DCP was up to 0.08 wt%, the bwp value of CRPP jumped to twice that of base PP, indicating that the plastic energy consumption during the fracture increased with the increase of DCP content. A definite dependence of bwp value on Mz which was mainly influenced by ultra-high molecular weight component was also observed (Fig. 7), that is, the value of bwp was higher at smaller Mz. It maybe owing to the strong restrictions imposed by ultra-high molecular weight component on the ductility of co-PP. Thus, the reduction of Mz was beneficial to augment

Fig. 5. Pictures of fractured DENT specimens with the same ligament length (l ¼ 11 mm) for co-PP and CRPP.

B.-R. Sheng et al. / Polymer Degradation and Stability 93 (2008) 225e232 Table 4 EWF parameters determined from wfel plots of co-PP and CRPP 0 31.0 10.4 0.92

0.02 28.6 12.3 0.95

0.04 24.5 14.0 0.97

0.06 18.1 17.9 0.96

22

0.08 21.2 21.0 0.96

plastic deformation as shown in Fig. 5 and simultaneously, to enhance plastic energy consumption. Additionally, the decrease of crystallite size of CRPP with increasing DCP content favored the mutual movement among crystallites and also contributed to more plastic energy consumption. 4. Conclusions

140000 Mn

600000

Mw

100000 400000

Mw

Mn

500000

80000 300000 60000 200000 16

18

18 16 14 12 10

(1) The melt flow rate of CRPP increased with addition of DCP due to the remarkable reduction of the average molecular weight. Moreover, the ultra-high molecular weight component decreased, while the moderateehigh molecular weight component increased markedly. (2) As DCP amount increased, the spherulites with even size distribution and obscure boundaries of CRPP increased due to the degraded co-PP with small amount of branching and micro-cross-linking structure acting as nucleating agent. (3) The fracture energy was mainly consumed in the stage of necking and tearing. The crack resistance first decreased with increasing DCP amount and then recovered when DCP amount reached 0.08 wt%, owing to the increase of moderateehigh molecular weight component which effectively enhanced the molecular chain entanglement, meanwhile, the multiplied tie chains between crystallites/ lamellae counteracted the negative effect caused by the reduction of ultra-high molecular weight component. (4) The plastic energy consumption increased with decreasing ultra-high molecular weight component which imposed strong restrictions on ductility. On the other hand, when DCP amount increased, the crystal size decreased which also facilitated the crystal movements and redounded to more plastic energy consumption.

120000

20

wp (MJ/m3)

DCP amount (wt%) we (kJ/m2) bwp (MJ/m3) R2 (wfel )

231

20

22

24

26

28

30

32

34

we(KJ/m2) Fig. 6. Illustration of relationships between molecular weight (Mn, Mw) and we of co-PP and CRPP.

5.8

5.9

6.0

6.1

6.2

6.3

6.4

logMz Fig. 7. Illustration of relationships between molecular weight Mz and bwp of co-PP and CRPP.

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