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Rheology and Thermal Properties of Polypropylene Modified by Reactive Extrusion with Dicumyl Peroxide and Trimethylol Propane Triacrylate FENG-HUA SU, HAN-XIONG HUANG Center for Polymer Processing Equipment and Intellectualization, College of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, P. R. China Received: September 23, 2008 Accepted: May 26, 2009

ABSTRACT: Trimethylol propane triacrylate (TMPTA) and dicumyl peroxide (DCP) were used to modify polypropylene (PP) by reactive extrusion in a twin-screw extruder. The effects of TMPTA concentration on oscillatory shear rheology, melt elongational rheology, and thermal properties were comparatively evaluated. Fourier transform infrared spectroscopy indicated that the grafting reaction took place and TMPTA had been grafted onto the PP backbone. Differential scanning calorimetric results showed that the crystallization temperatures of modified PPs were higher than those of the initial and degraded PPs. The rheological characteristics such as higher storage modulus (G ) at low frequency, increased degree of shear thinning, a plateau in tan δ–ω plot, and upturning at high viscosity in the Cole–Cole plots proved that the long-chain

Correspondence to: Feng-Hua Su; e-mail: [email protected]. Contract grant sponsor: Doctoral Program of Higher Education. Contract grant number: 200805611099. Contract grant sponsor: China Postdoctoral Science Foundation. Contract grant number: 20080440749.

Advances in Polymer Technology, Vol. 28, No. 1, 16–25 (2009) 2009 Wiley Periodicals, Inc.

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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION branches have been added to the linear PP molecule. The melt elongational rheology showed that the modified PPs exhibit improved melt strength and increased elongational viscosity in the presence of TMPTA and DCP, which further confirmed the existence of long-chain branching (LCB) in their backbone. According to the analytical results from oscillatory shear rheology and elongational rheology, it can be inferred that the LCB level in modified samples C 2009 Wiley Periodicals, increases with an increase in TMPTA concentration. Inc. Adv Polym Techn 28: 16–25, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20146

KEY WORDS: Polypropylene, Reactive extrusion, Rheology, Thermal properties

Introduction

P

olypropylene (PP) has very attractive solidstate properties such as high modulus and tensile strength, rigidity, excellent heat resistance, and low cost when compared with other thermoplastics. Although PP has many useful advantages, commercial PP when produced with Ziegler–Natta or metallocene catalysts results in highly linear chains and a relatively narrow molecular weight distribution. Its linear structure leads to poor processing ability, such as for extrusion coating, foam extrusion, and film blowing. Branches are introduced into linear PP to produce “high melt strength PP,” which exhibits strain-hardening behavior in the melt state. It is expected that the improvement in the melt strength behavior of PP will substantially contribute to the growth of this polymer in the plastics market. As a result, the preparation and research on high melt strength PP have been very active in the past decades.1−5 The melt strength of PP can be improved by increasing the molecular mass or by introducing long-chain branching (LCB) in the PP backbone. This seems to be the most efficient way. At present, there are several routes to branching in PP intended to improve its melt strength. The direct synthesis of LCB-PP is a very useful method.6−10 But this method is often confined to the laboratory. Electron beam irradiation11,12 and reactive extrusion processing,13−15 which are easily applied in industry, can also be applied to prepare LCB-PP. Electron beam irradiation is carried out on PP in the solid state. However, it is known that the use of highenergy radiation produces branching confined for the most part to the amorphous phase of semicrys-

Advances in Polymer Technology

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talline PP. It is in this region that the segmental mobility and free volume are sufficient for macroradicals that are formed on irradiation to approach one another and form branches.5 The method of reactive extrusion possesses many merits, such as a simple operation, low cost, and high productivity. Lagendijk et al.1 prepared LCB-PP by reaction extrusion in the presence of peroxydicarbonate with various structures. However, PP has a tendency to undergo β-scission due to the nature of PP, which competes with grafting and cross-linking reactions, especially when only peroxide is added in the reaction system. It has been reported that the use of polyfunctional monomers can decrease degradation and improve the degree of branching.2−4,13 The production of LCB in the PP backbone often leads to changes in the rheology and thermal properties of modified PPs. Moreover, rheology has proved to be a reliable method for the verification of the existence of long branches14−17 on the polymeric chain and it is the easiest to implement. Long-chain branching increases the possibility for entanglements in the polymeric melt and, thus, its elasticity. Many researchers have utilized the rheology and crystallization behaviors to prove the existence of long-chain branches in the backbone of modified PPs.3,14−19 Yu and colleagues3 confirmed the branched structure in modified PPs with pentaerythritol triacrylate (PETA) and 2,5-dimethyl-2,5(tertbutylperoxy)hexane peroxide by small-amplitude oscillatory shear experiments. Wang et al.5 analyzed modified PPs containing branched and/or lightly cross-linking chain structures by their differential scanning calorimetric (DSC) behaviors. Besides oscillatory shear rheology, melt elongational rheology is another effective rheological method to confirm the LCB structure in PP backbone. Moreover, the extensibility of polymer melt is

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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION important for polymer processes such as fiber spinning, film blowing, coating, and sheet casting.17,20 Extensibility can be determined by the use of an “extension diagram,” in which the drawdown force needed for elongation of an extruded strand is measured as a function of a slowly increasing drawdown speed.20−22 The main objective of the present research was to prepare LCB-PPs by modifying linear PP with dicumyl peroxide (DCP) and trimethylol propane triacrylate (TMPTA) through reactive extrusion. The oscillatory shear rheology and melt elongational rheology were used to confirm the existence of LCB and to differentiate the LCB level for modified samples with varying TMPTA concentrations in the presence of DCP. Moreover, the melting and crystallization behaviors of initial and modified samples were investigated.

TABLE I Initial and Modified Polypropylenes (PPs) with Different Dicumyl Peroxide (DCP) and Trimethylol Propane Triacrylate (TMPTA) Concentrations Samples

DCP (ppm)

TMPTA (phr)

0 300 300 300 300 300

0 0 1.5 2.0 2.5 3.0

Initial PP (PP0) PP1 PP2 PP3 PP4 PP5

volatilization zone was placed prior to the pumping zone close to the die for the removal of small molecules such as decomposition products of peroxide. Extrudates were cooled in water and then pelletized. The initial and modified PPs with different TMPTA concentrations in the presence of 300 ppm DCP are given in Table I.

Experimental MEASUREMENTS MATERIALS The homopolymer PP powder (T30S) was supplied by Maoming Petrochemical Corporation (Maoming, Guandong, China). The melt-flow index (MFI) was 3.49 g/10 min measured at 2.16 kg and 230◦ C in our experiment. The PP powder was stabilized by the addition of 0.1% Irganox 1010 (Jinhai Albemarle; Ningbo, Zhejiang, China) antioxidant. Dicumyl peroxide was purchased from Lanzhou Auxiliary Agent Plant (Lanzhou, Gansu, China), with half-life time of 10 min at 153◦ C. Trimethylol propane triacrylate was obtained from Yixinghonghui Chemical Co., Ltd. (Yixing, Jiangsu, China). Both DCP and TMPTA were used as received.

SAMPLE PREPARATION To achieve even dispersion of the additives in PP powder, DCP and TMPTA were dissolved in 50 mL of acetone, and then the solution was added to 1000 g of PP powder in an SHR10A high-speed mixer. The mixing of linear PP with antioxidant, peroxide, and TMPTA was performed for 10 min. The modification was carried out in a TE35 corotating twin-screw extruder after mixing. The temperatures of the extruder zones were maintained at 160, 180, 200, 200, 210, and 210◦ C from hopper to die. The throughput and the screw speed were 4.8 kg/h and 60 rpm, respectively. During the extruding process, a de-

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The gel content of reacted PP was determined with a Soxhlet extraction apparatus. The granular reacted PPs obtained from extruder were packed into copper netting and then were extracted in the Soxhlet extraction apparatus with xylene for 24 h at 140◦ C. In general, no gels were observed in the reacted samples. The melt-flow properties of the initial and reacted PPs were measured by an MFI tester CEAST 7072. The measurements were carried out at 230◦ C with a load of 2.16 kg according to the GB3682 standard. Fourier transform infrared (FT-IR) spectra of the purified samples were obtained with a Bruker model Tensor 27 FT-IR spectrometer. The reacted PP was added to xylene and then heated to 140◦ C. The solutions were charged into acetone at room temperature. The unreacted TMPTA monomer and copolymerized TMPTA remained soluble, whereas PP and PP-g-TMPTA precipitated out. Propylene and PP-gTMPTA were separated by filtration and then dried at 80◦ C under vacuum for 48 h. The purified samples were pressed into film and then analyzed with the FT-IR spectrometer. Thermal behaviors of the initial and modified PPs were investigated with a DSC204F1 differential scanning calorimeter from NETZSCH Co., Ltd. (Hanau, Germany). Specimens were heated to 200◦ C at a rate of 10◦ C/min and kept for 3 min to eliminate the thermal history. They were then cooled down to 25◦ C at a rate of 10◦ C/min to measure the crystallization


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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION TABLE II Operating Parameters for the Elongational Experiment (Rheotens) Used to Measure the Melt Strength and Elongational Viscosity Capillary Rheometer Parameter Die geometry Piston

Length = 30 mm Speed = 0.05 mm/s Shear rate = 90 1/s Temperature = 190◦ C Melt time = 10 min

Rheotens 71.97 Parameter Diameter = 1 mm Diameter= 15 mm

temperature. Samples were reheated under the same heating condition to determine the melting temperature. The small-amplitude oscillatory shear rheology was conducted with a Bohlin Gemini 200 rheometer equipped with a parallel-plate fixture (25-mm diameter). Disk-shaped samples were prepared by compression molding to a thickness of 2.0 mm and diameter of 25 mm. The gap between two parallel plates was maintained at 1.75 mm for all these rheological measurements. The measurements were performed as a function of angular frequency (ω) ranging from 0.01 to 100 rad/s at 190◦ C. A fixed strain of 1% was used to ensure that measurements were carried out within the linear viscoelastic range of the materials. The melt extension was measured with a four¨ wheel Gottfert Rheotens 71.97 tester in combination ¨ with a Gottfert Rheograph 25 high-pressure capillary rheometer. The tester was located close to the exit of the capillary. The additional pair of pull-off wheels was integrated to prevent sticking of the elongated polymer strand once it has passed the first pair of pulleys during the test. Table II provides a list of operating parameters used for testing the melt strength and elongational viscosity. The plastic pellets are melted in the heated test cylinder and pressed out of a capillary with a test piston at a constant speed. The melt strand was continuously drawn down at a linear or exponentially accelerating velocity between the two counterrotating wheels of the device mounted on a balanced beam. The drawdown was continuously measured as a function of the angular speed of the wheels as the melt strand was being pulled, and the force and velocity (draw ratio) when the melt strand was broken were deemed as “melt strength” and “drawability” at the tested condition. The obtained force–velocity curves were converted into elongational viscosity– elongation rate curves by fitting the experimental Rheotens data by a Levenberg–Marquart routine and by applying the analytical Wagner model.20−22


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The gap between two wheels = 0.2 mm V0 = 12.0 mm/s Acceleration = 12 mm/s2

The conversion software was provided by Rheotens (Buchen, Germany).

Results and Discussion MELT-FLOW PROPERTIES Figure 1 shows the MFI of the initial and modified PPs. It can be seen that the MFI of PP1 is much higher than that of the initial PP, indicating that the degradation reaction is very severe in the absence of TMPTA. Meanwhile, MFI decreased rapidly as TMPTA was added in the reaction system, which might be attributed to the grafting reaction between PP and TMPTA. The MFI of modified PPs decreases gradually with increasing TMPTA concentration, and TMPTA seems to have no significant effect on MFI at high concentration levels. This might be attributed to increased TMPTA grafting reactions and high consumption of primary peroxide radicals in

FIGURE 1. The effect of the trimethylol propane triacrylate (TMPTA) concentration on melt-flow index of the initial and modified polypropylenes (PPs). DCP, dicumyl peroxide.

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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION TABLE III Differential Scanning calorimetric Results of the Initial and Modified Polypropylenes (PPs) Sample PP0 PP1 PP2 PP3 PP4 PP5

FIGURE 2. Fourier transform infrared spectra (800–2200 cm−1 ) of the initial and modified polypropylenes (PPs). the TMPTA homopolymer. Melt-flow index of the modified PPs with 300 ppm DCP and 2.5 or 3.0 phr TMPTA is slightly higher than that of the initial PP, which might be attributed to a large number of LCBs in their backbone, although the degradation reaction also occurred.

FT-IR SPECTROSCOPY Figure 2 shows the FT-IR spectra of the initial and modified PPs. As shown in the figure, the bands at 1735 cm−1 do not appear in PP1, which is easily understood because no grafting reaction occurred for PP1 in the absence of TMPTA. With the addition of TMPTA, the bands at 1735 cm−1 appear in modified PPs, which can be ascribed to the stretching vibration of the carboxyl group of ester in the TMPTA molecule. The results indicate that TMPTA has been grafted onto the PP backbone during the reactive extrusion process. Moreover, it can be observed that the intensity of the bands at 1735 cm−1 increases with an increase in TMPTA concentration, suggesting that more TMPTA molecules favor the grafting reaction.

DSC ANALYSIS In our research system, the degradation and branching reactions might have taken place simultaneously. In addition, the concentration of TMPTA significantly affects the degree of branching reaction. It is well know that the degradation and branching reactions will result in the change in molecular weight and its distribution, and chain irregularity.

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H m (J/g)

Tm (◦ C)

Tc (◦ C)

107.6 113.6 107.1 95.46 98.39 99.75

164.0 163.4 163.8 163.9 163.7 165.0

113.1 117.7 122.2 123.2 123.7 123.2

All these microstructural changes will affect the thermal properties, especially the crystallization behavior. Table III provides the DSC results for the initial and modified PPs and the corresponding DSC curves are shown in Fig. 3. As shown in Fig. 3a, the melting peak for linear PP (PP0 and PP1) exhibits a small shoulder. However, the existence of grafted chain in the backbone of modified PPs results in a shallower melting peak. The existence of two sharp melting peaks in branched PPs reported by Wang et al.5 could not be observed in our modified PPs, which might be due to the different sensitivity of DSC tester and different testing conditions. As shown in Fig. 3b, only one single crystallization peak is observed for both linear and branched PPs, which agrees well with the observation of Wang et al. Referring to Table III, a higher crystallization temperature (Tc ) can be observed for PP1 than with PP0. The higher the Tc , the faster the crystallization of polymers. The higher crystallization temperature and elevated crystallization rate of PP1 than with the initial PP are due to the degradation of linear PP molecular chain induced by DCP in the absence of TMPTA. The degradation of linear PP results in an enhanced mobility of the molecular chain segment that favors the diffusion and arrangement of macromolecules in the crystal cell, therefore the crystallization rate is elevated and crystallization temperature increases. With the addition of TMPTA, the crystallization temperatures (Tc ) of modified PPs (from PP2 to PP5) are higher than those of the initial PP (PP0) and the degraded PP (PP1). It could be inferred that the branching structure in the backbone of modified PPs is beneficial for the enhancement of crystallization temperature and is also responsible for the acceleration in crystallization. Similar results have been reported by other research.5,23,24 With increasing TMPTA concentration, the melting enthalpy (Hm ) and melting temperature (Tm ) decrease initially, then increase, and finally remain


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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION

FIGURE 4. Storage modulus (G ) versus sweeping frequency (ω) for the initial and modified polypropylenes (PPs) at 190◦ C. Tm and Hm of the modified PPs with increasing TMPTA concentration.

OSCILLATORY SHEAR RHEOLOGY

FIGURE 3. Differential scanning calorimetric curves of the initial and modified polypropylenes (PPs): (a) melting curves; (b) crystallization curves. relatively constant. The complicated changes in Tm and Hm of modified PPs are related to the simultaneously occurring degradation and branching reactions during the reactive extrusion process as DCP and TMPTA are added in the system. First, the degradation reaction will lead to an increase in Hm and a decrease in Tm .5 Second, Hm and Tm should decrease due to the introduction of noncrystallized structural units and chain defects (branching points) into the chains. Third, the polar ester group grafted on the PP backbone should lead to an increase in Hm . Finally, the melting entropy (Sm ) is reduced as a result of chemical branching of the chains, which leads to a decrease in Tm because of the elevation in the temperature (Tm = Hm /Sm ). These four factors affect the changes in Hm and Tm simultaneously, which result in the complicated changes in


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As illustrated above, oscillatory shear rheology is more sensitive to the LCB structure and can be used to determine the existence of the LCB directly.3,4,13,15 The storage modulus (G ) is very sensitive to the LCB structure of modified PPs. The storage modulus plotted as a function of sweeping frequency (ω) of the initial and modified PPs is shown in Fig. 4. It can be seen that PP0 and PP1 exhibit the typical terminal behavior, which is due to their linear chain structures. It is interesting to note that PP2 also exhibits the typical terminal behavior, indicating that the chain scission is dominant when compared with the branching reaction at low concentration of TMPTA. With the further increase in TMPTA concentration, the modified PPs (PP3, PP4, and PP5) are deviated from the terminal behavior. In addition, G increases at low frequency and the terminal slope decreases with increasing TMPTA concentration, suggesting that the branching reaction is dominant for these modified samples. It is well known that the addition of a polyfunctional monomer can stabilize tertiary macroradicals against scission by assisting in the formation of secondary macroradicals that subsequently lead to the branching reaction. This corresponds to the curves of G versus ω of modified PPs with increasing TMPTA concentration. It is well known that LCB structure can increase the degree of shear thinning for polymers. Figure 5

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FIGURE 5. Viscosity (η∗ ) versus sweeping frequency (ω) for the initial and modified polypropylenes (PPs) at 190◦ C.

shows the curves of the complex viscosity (η∗ ) versus ω of the initial and modified PPs. It can be observed that there is a severe decrease in η∗ of PP1 at low frequency and Newtonian zone becomes broader, which is due to the degradation reaction. When TMPTA is added in the system, the complex viscosity curves become higher than that of PP1 at low frequency, which is attributed to the reduced degradation and increased branching reactions. At the same time, the complex viscosity at low frequency increases gradually and shear-thinning starts early at low frequency, which is due to the increased number of LCBs in the backbone of modified PPs with increasing TMPTA concentration. However, the complex viscosity of PP2, PP3, and PP4 is also lower than that of the initial PP (PP0) at low and high frequencies, indicating that the degradation reaction occurs besides the branching reaction. The complex viscosity of PP5 is higher than that of the initial PP (PP0) at low frequency, indicating that a large number of LCBs are produced in its backbone due to the addition of abundant TMPTA in the reaction system. Figure 6 shows the curves of tan δ versus ω of the initial and modified PPs. Yu and colleagues3 investigated the 2,5-dimethyl-2,5(tert-butylperoxy) hexane peroxide–PETA system and found that the plateau of the loss angle becomes more evident as more LCBs are present in the backbone of polymers. The similar result was found in our research system. It can be seen that the curves for PP0 and PP1 rapidly descend with increasing frequency, which is a typical terminal behavior of liquid-like materials. When TMPTA was added in the reaction system,

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FIGURE 6. The curves of tan δ versus ω for the initial and modified polypropylenes (PPs) at 190◦ C.

tan δ decreases rapidly with increasing frequency and exhibits a plateau at a high-frequency region. Moreover, the decrease in tan δ at low frequency is proportional to the TMPTA concentration. As more TMPTA molecules are added in the reaction system, the plateau becomes longer, indicating that a large number of LCBs indeed exist in the backbone of modified PPs. Besides these curves, the Cole–Cole plots (η versus η ) can also be used to illustrate the nonterminal behavior of modified PPs.3,25,26 Figure 7 shows the Cole–Cole plots of the initial and modified PPs. As shown in the figure, the Cole–Cole plots of PP0

FIGURE 7. The Cole–Cole plots of the plain and modified polypropylenes (PPs) at 190◦ C.


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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION and PP1 are close to a semicircle and the higher the molecular weight is, the bigger the radius is. The curves of modified PPs with different concentrations of TMPTA are lower than that of the initial PP and higher than that of PP1, which suggest that the degradation reaction also occurs. However, the curves of modified PPs increase with increasing TMPTA concentration, indicating that the branching reaction becomes dominant in comparison with the degradation reaction. Also, the Cole–Cole plots of PP3, PP4, and PP5 show more evidence of upturning at high viscosity, indicating the existence of a large number of LCBs in their backbone, especially for PP5.

MELT ELONGATIONAL RHEOLOGY As shown before, the existence of LCB can enhance the melt strength of the modified PPs in the melt extensional experiment under the same tested condition. Inversely, the branching level can be indicated with the data of the melt strength under the same tested condition. The melt elongational properties of the initial and modified PPs at 190◦ C as observed by the Rheotens approach are shown in Fig. 8. The “melt strength,” as the maximum force at the breakpoint of the melt strand, and the “drawability,” denoting the maximal elongation at break, can be directly observed from Fig. 8. It can be seen that the melt strength of PP1 is much lower than that of the initial PP, which is due to the degradation reaction in the absence of polyfunctional monomer of

FIGURE 8. The melt elongational properties of the initial and modified polypropylenes (PPs) at 190◦ C as observed by the Rheotens approach (data fitted according to the Wagner model).


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FIGURE 9. Apparent elongational viscosity of the initial and modified polypropylenes (PPs) at 190◦ C with increasing elongational rate (determined from the Rheotens data according to the Wagner model). TMPTA. But the melt strength increases rapidly with the addition of TMPTA in the reaction system. The melt strength of PP can be improved by increasing the molecular mass, broadening the distribution, or introducing branches. Also, the most successful way to improve the melt strength of PP is by the introduction of LCB. In our research system, DCP and TMPTA were used to modify PPs. Therefore, it can be inferred that the improved melt strength of modified PPs in the present system is indeed due to the introduction of LCB structure, and the chemical reaction mechanism is discussed in the subsequent section. Meanwhile, it can be seen that the melt strength of modified PPs increases with increasing TMPTA concentration in the reaction system, which is related to the level of LCB in their backbone. The modified PPs with 2.5 or 3.0 phr TMPTA in the presence of 300 ppm DCP show much higher melt strength than the initial and other modified PPs, which might be attributed to a large number of LCBs in their backbone. Although the melt strength of modified PPs increases rapidly, their drawability decreases rapidly with the addition of TMPTA. The result is related to the elongational viscosity of modified PPs. The apparent elongational viscosity as a function of elongation rate of the initial and modified PPs is shown in Fig. 9. It can be clearly observed that the elongational viscosity of modified samples increases rapidly as the TMPTA is added into the reaction system. Also, consistent with the melt strength, the higher the amount of TMPTA added in the reaction system, the higher is the elongational viscosity of the modified samples. Long-chain branching

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FIGURE 10. The chemical reaction mechanism for the preparation of long-chain branching polypropylene. TMPTA, trimethylol propane triacrylate; DCP, dicumyl peroxide.

increases the possibility for entanglements in the polymeric melt,1,4 which might result in an increase in the elongational viscosity of modified PPs. Moreover, the increased amount of LCBs in the backbone of modified PPs with increasing TMPTA concentration results in the modified samples with higher melt strength and elongational viscosity under the same tested condition. The increase in elongation viscosity might lead to a decrease in drawability under the same tested condition for modified PPs. The elongational data for the initial and modified PPs are in reasonable agreement with such previous findings.

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Discussion Graebling2 found that TMPTA monomer does not exert any influence on the degradation of PP and the presence of sulfide compounds is absolutely necessary to limit the scission of PP and development of the branched structure. However, in our experiment, the formation of branched structures in modified PPs with the addition of TMPTA in the presence of 300 ppm DCP is confirmed by oscillatory shear rheology and melt elongational rheology, although the degradation reaction occurs simultaneously. The


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RHEOLOGY AND THERMAL PROPERTIES OF POLYPROPYLENE MODIFIED BY REACTIVE EXTRUSION oscillatory shear rheological properties of LCB-PP, such as higher G at low frequency, increased degree of shear thinning, plateau in tan δ–ω plot, and upturning at high viscosity in the Cole–Cole plots have been found in our modified PPs. The melt elongational rheology shows that the modified PPs exhibit improved melt strength and increased elongational viscosity in comparison with the initial PP, which further suggests that LCB structure has been produced during the reactive extrusion process. Moreover, the oscillatory rheology and melt elongational rheology of the modified samples indicate that the presence of more TMPTA molecules in the reaction system favors the branching reaction. The chemical reaction mechanism of the reactive extrusion process might be as shown in Fig. 10.

References 1. Lagendijk, R. P.; Hogt, A. H.; Buijtenhuijs, A.; Gotsis, A. D. Polymer 2001, 42, 10035. 2. Graebling, D. Macromolecules 2002, 35, 4602. 3. Tian, J. H.; Yu, W.; Zhou, C. X. Polymer 2006, 47, 7962. ¨ 4. Auhl, D.; Stange, J.; Munstedt, H. Macromolecules 2004, 37, 9465. 5. Wang, X. C.; Tzoganakis, C.; Rempel, G. L. J Appl Polym Sci 1996, 61, 1395. 6. Weng, W.; Hu, W.; Dekmerzian, A. H.; Ruff, C. J. Macromolecules 2002, 35, 3838. 7. Shiono, T.; Azad, S. M.; Ikeda, T. Macromolecules 1999, 32, 5723.

Conclusion LCB-PPs were prepared by modifying linear PP with increasing TMPTA concentration in the presence of 300 ppm DCP. FT-IR measurements directly confirm the grafting reaction and indicate that the higher concentration of TMPTA favors the grafting reaction. Differential scanning calorimetric results show that the crystallization temperatures of the modified PPs are higher than those of the initial and degraded PPs, which might be attributed to the existence of LCB in their backbone. The changes of Tm , Hm , and the level of crystallization of the modified PPs with increasing TMPTA concentration are very complicated, which are due to the complex changes in the microstructure, the size of the crystals, and the melting entropy (Sm ) due to the existence of the branched chain structure. The oscillatory shear rheological features of LCBPP, such as higher G at low frequency, increased degree of shear thinning, a plateau in tan δ–ω plot, and upturning at high viscosity in the Cole–Cole plots have been found in our modified PPs. The result suggests the formation of branched structures in the modified PPs with proper concentration of TMPTA in the presence of 300 ppm DCP. Moreover, the modified PPs exhibit improved melt strength and increased elongational viscosity in comparison with the initial PP, which further suggests that LCB structures have been produced during the reactive extrusion process.


Finally, the test results from oscillatory rheology and melt elongational rheology indicate that more TMPTA molecules in the reaction system favor the branching reaction. A chemical reaction mechanism for the reactive extrusion process is suggested.

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8. Ye, Z.; Alobaidi, F.; Zhu, S. Ind Eng Chem Res 2004, 43, 2860. 9. Bing, L.; Chung, T. C. Macromolecules 1999, 32, 8678. 10. Langston, J. A.; Colby, R. H.; Chung, T. C. M.; Shimizu, F.; Suzuki, T.; Aoki, M. Macromolecules 2007, 40, 2712. 11. Scheve, B. J.; Mayfield, J. W.; Denicola, A. J., Jr. US Patent 4,916,198, 1990. 12. Ali, Z. L.; Youssef, H. A.; Said, H. M.; Saleh, H. H. Adv Polym Technol 2006, 25, 208. 13. Yoshii, F.; Makuuchi, K.; Kikukawa, S. J Appl Polym Sci 1996, 60, 617. 14. Gotsis, A. D.; Zeevenhoven, B. L. F.; Hogt, A. H. Polym Eng Sci 2004, 44, 973. 15. Yamaguchi, M.; Wagner, M. H. Polymer 2006, 47, 3629. 16. Kim, B. K.; Kim, K. J. Adv Polym Technol 1993, 12, 263. 17. Hingmann, R.; Marczinke, B. L. J Rheol 1994, 38, 573. 18. Otaguro, H.; Rogero, S. O.; Yoshiga, A.; Lima, L. F. C. P.; Parra, D. F.; Artel, B. W. H.; Lugão, A. B. Nucl Instrum Methods Phys Res 2007, 265, 232. 19. Spitael, P.; Macosko, C. W. Polym Eng Sci 2004, 44, 2090. 20. Lau, H. C.; Bhattacharya, S. N.; Field, G. J. Polym Eng Sci 1998, 38, 1915. 21. Wagner, M. H.; Collignon, B.; Verbeke, J. Rheol Acta 1996, 35, 117. ¨ 22. Wagner, M. H.; Schulze, V.; Gottfert, A. Polym Eng Sci 1996, 36, 925. 23. Liu, C. S.; Wei, D. F.; Zheng, A. N.; Li, Y.; Xiao, H. N. J Appl Polym Sci 2006, 101, 4114. 24. Nam, G. J.; Yoo, J. H.; Lee, J. W. J Appl Polym Sci 2005, 96, 1793. 25. Vega, J. F.; Santamaria, A. Macromolecules 1998, 31, 3639. 26. Watanabe, H. Prog Polym Sci 1999, 24, 1253.

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