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Maleic anhydride/styrene melt grafting and crosslinking onto ethylene-octene copolymer.

INTRODUCTION

Polyolefin elastomer (POE) is ethylene-octene copolymer developed using a metallocene catalyst by Dow Chemical. POE is characterized by narrow molecular weight distribution and homogeneous comonomer distribution. It has fast mixing and better dispersion properties compared with conventional ethylene-propylene-diene monomer (EPDM) and ethylene-propylene copolymer (EP) (1), (2). Its applications mainly focus on modifying nonpolar polymers. Recently, POE can also be functionalized and/or graft modified by some unsaturated low molecular compounds containing polar functional groups (3-6). This makes it widely using in modifying polar polymers. Functionalization, especially grafting and crosslinking, is an important way to improve the thermal and chemical resistance of POE. Various monomers have been used for melt grafting of POE. Liao and Wu (7) studied the peroxide crosslinking of POE. Maleic anhydride (MAH) has also received intense attention primarily due to the difficulty in homopolymerization of this monomer during melt grafting (8). Some literatures about the MAH grafting and crosslinking of polyolefins (PO) have been reported (9-11). Analysis of MAH-grafted PO has shown that MAH is usually grafted onto the PO chain ends and that the grafted MAH is present as single MAH or succinic anhydride (12), (13).

Some works have been done on optimizing the reaction conditions and extruder parameters to promote the desired reactions while suppressing the undesired ones (14-16). However, due to the inherent complexity of free radical reactions, it is very difficult to incorporate the desired MAH concentration without extensive side reactions (17). Therefore, the grafting degree of MAH monomer is usually low (18), (19). In recent year, it has been found that the incorporation of styrene (St) as a second monomer in the melt grafting process assisted in reducing the PO chain scission and increasing the grafting degree of glycidyl methacrylate (GMA) and MAH on PO (20-24). The idea of using St as a comonomer originated from a detailed analysis of the mechanism of free radical grafting. To obtain higher grafting degree with reduced degradation of polypropylene (PP), it is essential that the PP macroradicals react with the grafting monomers before they undergo chain scission.

The MAH melt grafting system with St comonomer is actually a multimonomer grafting PO system (24), (25). In the system, the addition of St was shown to be effective in improving the grafting degree. In fact, the limited grafting level and some degradation of PO during the melt grafting of MAH onto PO would result from the low reactivity of MAH toward free radicals and its low solubility in PO melt. From the above discussion, we know that a special interaction occurs in the MAH or GMA melt grafting PO due to the addition of St. Li and Xie (26), (27) have prove that the interaction between MAH and St in the melt grafting process of MAH-St onto PP played an important role. Some studies (4), (7) on the graft and crosslinking of POE have been reported. As far as we know, however, no study on styrene assisted melt grafting of MAH onto ethylene-octene copolymer has so far been reported in the literature. The primary aim of the present work was to provide a detailed investigation on melt grafting of MAH onto POE with St as a comonomer, as well as the influence of several factors on the grafting degree, melt flow index (MFI), dynamic rheological behaviors, and thermal stability of MAH-St grafted POE.

EXPERIMENTAL

Materials

Ethylene-octene copolymer (POE), Enagage 8842, was supplied by Dow Chemical, USA. The MAH and styrene (St) monomer used were commercially available (analytical grade). Dicumyl peroxide (DCP) used as an initiator was obtained from Shanghai CHEMWAY Chemical China.

Preparation of Samples

The melt grafting reactions were carried out in a twins-crew extruder (Type TSE-400-44-22, L/D = 40, China). POE pellets, monomer, and peroxide initiator were dry-blended before being charged into the extruder. The temperatures from hopper to die at six different zones are 165, 170, 180, 190, 200, and 200 [degrees] C, respectively, and the screw speed was fixed at 80 rpm

Measurements and Characterization

FTIR Spectroscopy. Grafted samples for Fourier transform infrared spectroscopy [FTIR] spectra measurements were prepared by making films of the mixture in a hot press. Prior to the FTIR measurements, the films were immersed into excessive acetone for 24 h to remove unreacted grafting monomers and possible MAH/St oligomers. The FTIR spectra were then recorded by using a Nicolet Nexus 670 spectrophotometer.

Determination of the Grafting Degree. The grafted POE samples were dissolved in refluxing xylene at a concentration of 1% (wt/vol), and excess acetone was then added to precipitate them. Then the unreactive MAH and St were removed. The purified polymer was collected and dried in a vacuum oven at 60 [degrees]C for 24 h to a constant weight. The grafting degree was determined by a titration procedure. About 1 g of the purified product was solved in 200 ml toluene at the boiling temperature for 1 h, then 30 ml 0.05 mol/l ethanol solution of NaOH was added and kept refluxing for 2 h. And then back-titration was performed by 0.05 mol/l iso-propylalcohol solution of HCl with phenoplthalein as indicator. The grafting degree (%) was calculated according to the followed equation:

Grafting degree (%)

=([V.sub.NaOH][C.sub.NaOH] - [V.sub.HCL] [C.sub.HCL] X [M.sub.MAH-St]/2 X m X 100% (1)

where m is the purified sample weight, and the value of [M.sub.MAH-St] is 202.21.

Melt Flow Index. The MFI of the grafted POE was determined using XRL-400A instrument. The condition of experiment is at 190 [degrees]C under a load of 2.16 kg. MFI is generally expressed in terms of the weight of extrudate in grams at 10 min intervals.

Gel Content. Weighted samples of small pieces in a copper net were put into boiling toluene for 24 h. The extracted samples were washed using acetone, and then dried to a constant weight in a vacuum oven. Then gel content is expressed in the percentage of the weight remaining.

Rheological Measurements. The samples were compression-molded at 180[degrees]C into disks 25 mm in diameter and about 1.0 mm thick. A Gemini 200 rheometer with parallel-plate geometry was used to measure the dynamic viscoelastic properties of POE and grafted POE. The measurements were performed in a frequency range of 0.025-100 Hz at 190[degrees]C. The measurements were performed in nitrogen atmosphere to avoid premature thermal degradation of the samples. The strain values were kept within the linear region.

Thermogravimetric Analysis. Thermogravimetric analysis (TGA) of samples (about 8 mg) was carried out in nitrogen atmosphere with a flow rate of 50 ml/min in a temperature range from 30 to 600[degrees]C at a scanning rate of 10 [degrees]C/min by a Perkin-Elmer Q 50 thermogravimetric analyzer.

RESULTS AND DISCUSSION

FTIR Spectra Analysis

Effect of St Monomer. FTIR characterization is extensively used to investigate the change of molecule structure of polymer, especially for grafting systems (6), (28). In this article, FTIR analysis is used to confirm that whether MAH and/or St are grafted onto POE matrix. Figure 1 presents the FTIR spectra of neat POE, POE-g-MAH, and POE-g-(MAH-St). Both characteristic peaks of POE at 1465 [cm.sup.-1] (the characteristics of --[CH.sub.3] asymmetric) and 724 [cm.sup.-1] [long ethylene sequence --[([CH.sub.2]).sub.2]--(n [greater than or equal to] 5)] exist in all spectra (29). In the cases of POE-g-MAH and POE-g-(MAH-St), new absorption bands at 1782, 1720, and 1030 [cm.sup.-1] clearly appear. The absorption bands at 1782 and 1720 [cm.subp.-1] can be assigned to the absorption of the carbonyl groups (--C=0) of cyclic anhydride. The peak at 1030 [cm.sup.-1] is attributed to the symmetric ring stretching of anhydride (30). The absorptions at 724 and 673 [cm.sup.-1] can be assigned to the characteristic absorption of the POE skeleton and the grafted St, respectively. These results indicate that MAH and St have been successfully grafted onto the POE.

[FIGURE 1 OMITTED]

Effect of the MAH-St Comonomer Concentration. FTIR spectra of POE after reaction with various amounts of MAH-St (MAH/St = 1/1) and 0.2 phr of DCP are presented in Fig. 2. It can be seen from Fig. 2 that the peaks at 1782, 1720, 1030, and 673 [cm.sup.-1] appear in curves B--E. These data indicate that the MAH-St grafting reaction occurs, and the intensities of the peaks at 1782, 1720, and 673 [cm.sup.-1] increase the amount of MAH St, meaning that more MAH-St grafted on the back-bones of POE. These results indicate that the extent of MAH-St grafting reaction increases as the amount of MAH-St increases. However, excessive MAH-St comonomer decreases the intensities of the above peaks. This possible reason is the monomer cage effects as a result of long molecular chain of the monomers (5). Monomer cage effect is generally proportional to monomer concentration.

[FIGURE 2 OMITTED]

Effect of Peroxide Initiator DCP. Figure 3 shows the FTIR spectra of POE after reaction with various amounts of DCP and 4 wt% of MAH-St (MAH/St = 1/1). It is seen that the intensities of the peaks at 1030 and 673 [cm.sup.-1] increase gradually as the amount of initiator. However, peaks at 1782 and 1720 [cm.sup.-1] appear when DCP content above. 0.1 phr, indicating that grafting reaction taking place in the presence of appropriate peroxide initiator. Moreover, the excessive DCP (> 0.2 phr) makes against the grafting reaction. This indicates that the grafting reaction almost finishes when the content of initiator DCP is 0.2 phr.

[FIGURE 3 OMITTED]

Grafting Degree

Figure 4 shows the effect of St concentration on the grafting degree of the grafted POE when the MAH and DCP contents are fixed at 2 wt% and 0.2 phr based on POE, respectively. It can be seen that the grafting degree of POE is remarkably higher in the presence of St than in the absence of St, and increase with increasing St concentration. It reaches a maximum as the weight ratio of MAH and St is 1:1, and then gradually decreases when the concentration of St is higher than that of MAH. The MAH and St can also copolymerize with each other under peroxide DCP to form a chain of MAH-St copolymer, which reacts with POE macroradicals producing branches by termination between radicals. Therefore, the grafting degree of MAH can be greatly improved by St. However, if excessive St is added to the grafting system, some St monomers react with MAH to from MAH-St copolymer. Other St monomer can preferentially react with POE chain macroradicals to produce relatively stable styryl macroradicals, thus POE further reaction with MAH-St copolymer is very difficult. Therefore, the grafting degree of POE decreases with further increasing St content. Then we can conclude from above discussion that the grafting degree of MAH on POE is higher when the ratio of MH/St is 1:1.

[FIGURE 4 OMITTED]

Figures 5 and 6 display the effect of MAH-St and DCP concentrations on the grafting degree of POE when the MAH/St ration is 1:1 based on POE, respectively. It is noted that the grafting degree of MAH-St grafting POE increases fast with increasing MAH-St concentration when the monomer concentration is relatively low. However, the dependence of grating degree on monomer concentration increases slowly after the monomer concentration is 4 wt%/ 96 wt% POE, and then levels off. With the monomer concentration increasing, the change of monomer to react with the backbone increases, therefore the grafting degree increases slowly. On the other hand, MAH-St also acts as the traps for free radicals, and more MAH-St means less polymer radicals and more homopolymerization of MAH-St if MAH concentration is too high.

[FIGURE 5 OMITTED]

It is clearly found from Fig. 6 that this result for the variety trend of grafting degree is in accordance with what was observed for grafting degree changing in Fig. 5. The result shows that when the concentration of MAH-St is fixed, increasing initiator concentration within some limit is effective for improving the grafting degree of MAH-St. However, more concentration of DCP, more concentration of free radicals is present in the reaction blend, which is hampered to leave the cage effect. Then it is difficult to the reaction between POE molecular and MAH-St because of severe chain crosslinking of the POE backbone.

[FIGURE 6 OMITTED]

Melt Flow Index of the Grafted POE

The effect of MAH-St concentration on MEI values of the MAH-St (MAH/St = 1/1) grafted POE is shown in Fig. 7. It can be observed from Fig. 7 that the MFI values decreased remarkably with increasing the comonomer concentration. The MFI value of pure POE is 0.95. However, the MFI value decreases to 0.26 when the comonomer concentration is only 2 wt%, which is approximately one-third of that of pure POE. Moreover, the MFI value decreases further to 0.1 when the comonomer amount is 6 wt%, and then it almost levels off when the MAH-St concentration is greater than about 6 wt%.

[FIGURE 7 OMITTED]

Figure 8 presents the effect of DCP concentration on MFI of the MaH-St (MAH/St = 1/1) grafted POE. Although MAH-St comonomer concentration is fixed, the MFI value still decreases as the initiator DCP amount increases. As the DCP concentration increases, the MFI value of the grafted POE decreases sharply, and then decreases slowly. The decrease of MFI value is due to crosslinking, which make the molecular chains extended and the decrease of mobility of the chains.

[FIGURE 8 OMITTED]

Gel Content of the MAH-St Grafted POE

The quantity of gel is generally used to evaluate the crosslinking degree of grafting polymers. Figure 9 shows the effect of the initiator DCP content on the gel content of grafted POE. It is obviously found from Fig. 9 that the gel content of MAH-St grafted POE increases gradually with increasing the DCP concentration. However, when the initiator content is over 0.2 phr, the gel content increases slowly, and then levels off. This observation indicates that the grafting degree reaches a limit at the end. Gel content first slightly increases to 5.7% when the amount of DCP is less than 0.15 phr, and rapidly grows up to a high level by increasing the initiator concentration. Because the gels may lead to high viscosity of the solution and embarrass the diffusing of monomers and the dissolution of polymers, the graft reaction will be inhibited when the gel content is too high. So grafting degree levels off with increasing the concentration of DCP when more than 2.0 phr DCP is added. Appropriate product with a grafting degree of 2.41% and gel content of 36.5% is obtained when the amount of DCP is 3.0 phr in this condition.

[FIGURE 9 OMITTED]

Dynamic Rheological Characterization

Figures 10 and 11 show the frequency dependency ([omega]) of storage modulus (G') and loss modulus (G") of POE and grafted POE at 190[degrees]C, respectively. Obviously, for a given temperature, both G' and G" of POE and grafted POE increase with increasing the frequency. It can clearly be seen from Figs. 10 and 11 that the G' and G" values of grafted POE are higher at the low frequency region, while lower at high frequency than those of pure POE, confirming that grafted POE presents the typical properties of highly entangled materials. It can be attributed to crosslinking of the POE chain segment in the presence of MAH-St comonomer. Assuming that G' is related to the stored energy, the above finding indicates that the grafted POE has higher elasticity at low frequencies. It can also be observed that grafted POE presents lower G' values, indicating that it has lower elasticity at higher frequencies. The possible reason is chain scission due to ride reactions. Moreover, the chain entanglements decrease with increasing the frequency. Moreover, there are also some unreacted grafting monomers and possible MAH/St oligomers that have lubricant effects in the grafted POE system. All these lead to the decrease of elastic behavior in high frequencies. The dependence of the loss modulus, G", on the investigated frequencies of pure POE and grafted POE is shown in Fig. 11. As can be seen, for grafted POE, G" values are higher than that of pure POE at low frequencies. Taking into account that the dynamic loss modulus G" represents the amount of dissipated energy, such findings indicate that, with addition of MAH-St, grafted POE with higher dissipated energy are produced.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

In Fig. 12 the dependence of the complex viscosity ([eta] *) on the investigated frequencies of grafted POE is compared with those of the pure POE material at 190[degrees]C. As shown, for the whole range of explored frequency, both pure POE and grafted POE exhibit a decrease in [eta]* values with increasing frequency. It can be seen from Fig. 12 that the difference in [eta] * values of POE and grafted POE is very large, indicating that the MAH-St comonomer has great effect on the [eta]* values of POE at 190[degrees]C. The [eta]* values of grafted POE is higher than that of pure POE at the same frequency in the studied frequency range, reflecting the increase of molecular weight. The increase of viscosity is a direct result of the enlargement of the molecular weight, which implies that the material is crosslinked. Similar trends were observed by Kim and Kim [31], who examined a series of HDPE samples, crosslinked via reactive extrusion. They correlated the increase in complex viscosity, and a decrease in MFI. The behavior found in the POE functionalized is attributed to the formation of a low level of long-chain branching and/or the presence of polar groups inserted on the chain. Long branching in polymers is thought to give a more pseudoplastic or shear-thinning characteristic to the material [32]. And this indicates that the permanent change in the molecular structures of POE during melt reactive extrusion in the presence of the MAH-St monomer.

[FIGURE 12 OMITTED]

The loss factor (tan [delta]) versus [omega] is also plotted in Fig. 13. The tan [delta] is very characteristic of the structure parameters of the material, such as network structures due to crosslinking, the and reinforcement with fine fillers [33]. It is clearly seen that the tan [delta] value for pure POE decreases with the increasing [omega], and then levels off at higher [omega], whereas tan [delta] value for grafted POE almost levels off during whole process of [omega] variety. Moreover, the tan [delta] value of grafted POE is much lower than that of pure POE at low frequency. Consequently, the tan [delta] value is sensitive to POE structure in the grafted polymer. The greater the entanglement and crosslinking of the grafted POE chain segment withstanding dynamic deformation and the smaller the portion being broken down and reformed, the lower the value of tan [delta] This indicates that the development and the strength of the addition of MAH-St comonomer have major effects on tan [delta] value of POE.

[FIGURE 13 OMITTED]

Thermal Stability of the Grafted POE

The TGA curves of pure POE and grafted POE with different amount of MAH-St (MAH/St = 1/1) under a flow of nitrogen atmosphere at a heating rate of 10[degrees]C/min are shown in Fig. 14. It can be seen that the thermal stability of grafted POE is greatly improved compared with neat POE. The degradation temperature of grafted POE is higher than that of the pure POE. These results indicate that the crosslinking and the grafting reactions of POE raise its upper temperature limit of application. Moreover, Fig. 14 also shows that the thermal stability of the grafted POE increases slightly as MAH-St content increases (curves B-D). This result suggests that the amount of grafting monomer has hardly effect on the thermal stability of grafted POE. The data of TGA further confirm the crosslinking reaction occurs, which is consistent with the results of the gel content.

[FIGURE 14 OMITTED]

CONCLUSIONS

In our work, the addition of St as comonomer to the melt grafting system of MAH were grafted onto POE using peroxide in the melt, and the structure of MAH and MAH-St grafted POE was characterized by FTIR, respectively. It has been found that the addition of St to the melt grafting system as a comonomer could significantly enhance MAH grafting degree onto POE. The grafting reactive extent of POE is greatly affected by MAH-St comonomer concentration and DCP concentration. Appropriate product with a grafting degree of 2.41% and gel content of 36.5% is obtained when the amount of DCP is 3.0 phr in this condition. The MFI values of the grafted POE decrease as the comonomer or peroxide initiator concentrations increase, which indicates that some cross-linking occurs during the grafted POE process. The gel content keeps increasing with the increase of DCP concentration, and then levels off. The dynamic rheological behavior of POE can be changed by incorporating MAHSt grafting onto POE matrix. At lower frequencies, G' and G" values of grafted POE are higher than those of the pure POE, whereas G' and G" values of the pure POE are higher than those of grafted POE at higher frequencies. The [eta]* values of grafted POE are higher than those of pure POE during the whole frequency variety. TGA results show that the thermal stability of MAH-St grafted POE is higher than that of pure POE. The improved degradation temperature of grafted POE further confirms the crosslinked chain in the grafted POE.

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Xiaolang Chen (1), (2) Hong Wu (3), Jie Yu (2), Shaoyun Guo (3), Zhu Luo (2)

(1) Key Laboratory of advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 6100 031, China

(2) National Engineering Research Center for Compounding and Modification of Polymeric Materials, Materials Eng and Tech Innovation Center of Guizhou, Guiyang 550014,

(3) The State Key Lab of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China

Correspondence to: Jie Yu; e-mail: yujiegz@126.com

Contract grant sponsor: National 863 Project Foundation of China; contract grant number: 2003AA32X230; contract grant sponsor: Science and Technology Foundation of Guizhou Province; contract grant numbers: (2008) 7001 and 3006; contract grant sponsor: Special Funds for Major State Basic Research Projects of China; contract grant number: 2005CB623800.

DOI 10.1002/pen.21071

Published online in Wiley InterScience (www.interscience.wiley.com).
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Author:Chen, Xiaolang; Wu, Hong; Yu, Jie; Guo, Shaoyun; Luo, Zhu
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:9CHIN
Date:Dec 1, 2008
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