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Maleic anhydride-grafted linear low-density polyethylene with low gel content.


Polyethylene (PE) occupies an important position in the commodity thermoplastics market, and is widely used in automobiles, household appliances, construction, and electrical industries. Especially, the development on metallocene technologies as a considerable advance improves its physical properties and extends its applications (1). On the other hand, the functionalized polyethylene via grafting reaction with polar unsaturated monomers in solution, melt, or solid state, has attracted much attention due to its application in polymer blends and composites (2), (3). Among these unsaturated monomers, maleic anhydride (MAH) is widely used. However, the grafting polymerization of MAH onto PE is often accompanied with side reactions, such as crosslinking, which leads to the high content gels (4-6). Many efforts have been made to enhance the grafting degree (GD) and reduce the gel content (GC). Gaylord and coworkers (7-9) proposed that the addition of nitrogen-, phosphorous-, and sulfur-containing organic electron donors could prevent the crosslinking of PE and homopolymerization of MAH. Yao et al. (10) also found that p-benzoquinone, triphenyl phosphate, and tetra-chlomethane are good inhibitors for crosslinking of linear low-density polyethylene (LLDPE) during free radical graft polymerization. However, the above additives simultaneously decrease the GD. Early in 1990, Hegazy et al. (11) used the vinyl comonomer to increase the GD on PE and polytetrafluoroethylene films during radiation-initiated grafting polymerization. White and coworkers also found that adding the vinyl comonomer leads to a higher GD of MAH and less crosslinking of high-density polyethylene (HDPE) at the same time (12-16). Machado et al. (17), (18) studied the effect of PE structure on grafting reaction, and found that the side reactions could be suppressed by adjusting the ratio of ethene/propene or ethane/octene in polyolefins. In other words, the crosslinking reaction primarily occurs in functionalization of polyolefins with low propene or octene content, while the degradation is the main side reaction in functionalization of polyolefins with high propene content. Recently, Razavi et al. (19), (20) studied the parameters affecting the grafting reaction and side reactions involved in the free radical melt grafting of MAH onto LLDPE and HDPE in internal mixer, and found that increasing the rotor speed can enhance the GD and reduce the GC, the chain-branching dominates the side reactions for lower molecular weight polyethylene, whereas the crosslinking is the main side reaction for higher molecular weight polyethylene. In this work, ma-leic anhydride-grafted LLDPE with low GC (LLDPE-g-MAH) was synthesized by the solid-phase grafting polymerization of low molecular weight LLDPE, and its structure and properties were investigated.



Low molecular weight LLDPE granules with melt flow index (MFI) of 49.4 g/10 min (190[degrees]C, 2.16 kg) were purchased from Sabic Petrochem. Co., Saudi Arabia under a trade name of LLMG500026, and were mechanically comminuted to powders with diameter less than 0.9 mm before use. Benzoyl peroxide (BPO) and maleic anhydride (MAH) were supplied by Zhenzhou Shiyan Factory, China. Potassium hydroxide, concentrated hydrochloric-acid, isopropanol, xylene, and ethanol were provided by Wuhan Lianjian Trade Co., China.

Synthesis of LLDPE-g-MAH Copolymers

LLDPE-g-MAH copolymers were synthesized by solid-phase grafting polymerization (21), (22) as follows. The small amount of xylene as interfacial agent was used to etch and swell the surface of LLDPE to provide reaction sites for the graft polymerization. Firstly, LLDPE powders and xylene were mixed in a 10-L stainless steel jacket reaction vessel fitted with a helical scraper-type stainless steel agitator under nitrogen atmosphere. The stirring speed and reaction temperature were set as 60 rpm and 105[degrees]C, respectively. MAH and BPO were divided into four equal parts by weight, and added into the reactor in 15-min intervals. The received LLDPE-g-MAH powders were poured into hot water, washed alternately by xylene and ethanol for three cycles, then dried in a vacuum at 80[degrees]C for 24 h.


The above LLDPE-g-MAH powders were extracted with acetone for 48 h in Soxhlet apparatus to remove the residual MAH and oligomers of MAH, and dried to constant weight under vacuum. Then, the GD of LLDPE-g-MAH was chemically titrated by potassium hydroxide (KOH) solution in isopropyl alcohol as reported in the literature (8), (9), and was calculated by the following equation:

GD = ([N.sub.KOH][V.sub.KOH] - [N.sub.HCl][V.sub.HCl]) x 98.06/2 x 1000 x W x 100% (1)

where [N.sub.KOH] and [V.sub.KOH] are the equivalent concentration (mol/L) and consumed volume (mL) of potassium hydroxide solution in isopropyl alcohol, respectively; [N.sub.HCl]and [V.sub.HCl] are the equivalent concentration (mol/L), and consumed volume (mL) of hydrochloric acid solution in isopropyl alcohol, respectively; W is the weight of LLDPE-g-MAH extracted by acetone.

The GC was measured by weighing. The LLDPE-g-MAH powders extracted by acetone were placed in a pre-weighed stainless steel net and put in 300 mL refluxing xylene for 12 h. The xylene-insoluble gel was dried in a vacuum oven at 60[degrees]C until its weight was constant. GC was calculated by the following equation:

GC = [W.sub.s] - [W.sub.n]/[W.sub.p x 100% (2)

where [W.sub.s], [W.sub.n], and [W.sub.p] represent the total weight of stainless steel net and sample after extraction, the weight of stainless steel net, and the weight of sample before extraction, respectively.

MFI was measured at 190[degrees]C under the weight of a piston weighing, in combination with its plunger, 2.16 kg. The diameter of orifice was 2.095 mm and its length was 8 mm.

Fourier transform infrared (FTIR) spectroscopy measurements were conducted at room temperature in a FTIR EQUINOX55 instrument (Bruker Company, Germany). The samples were pressed into thin films at 180[degrees]C before test.

Differential scanning calorimetry (DSC) analyses were conducted in a Perkin-Elmer DSC-7 instrument at a heating rate of 10[degrees]C/min under dry nitrogen atmosphere. Before the DSC recording, all samples were heated to 200[degrees]C and then kept at this temperature for 5 min to eliminate the influence of their previous thermal histories. They were finally quenched to ambient temperature. For nonisothermal crystallization measurements, samples were heated to 200[degrees]C at a rate of 10[degrees]C/min, kept at 200 C for 3 min, and then cooled to 30[degrees]C at a rate of 10[degrees]C/min.

Wide-angle X-ray diffraction (WAXD) patterns were measured using a Philips X'Pert Pro X-ray diffractometer with Cu K[alpha] radiation ([gamma] = 0.15406 nm) at a generator voltage of 40 kV and a current of 40 mA. The samples were cut from the compression molded plates. All experiments were carried out in the 2[theta] range of 5[degrees] - 50[degrees] at ambient temperature with a scanning speed of 5[degrees]/min and step size of 0.02[degrees].

Polarized optical microscopy (POM) observation was performed in an Olympus BX 51 microscope equipped with a crossed polarizer and a hot stage. The specimen was firstly melted at 200[degrees]C and kept at this temperature for 3 min, and then was cooled down rapidly to the crystallization temperature (about 105[degrees]C) and maintained for the time necessary for crystallization.

Rheological properties were measured with a capillary rheometer (model XLY-1). The capillary die used had a length (L) to diameter (D) ratio of 40/1 and an entrance angle of 180[degrees]. The Rabinowitch and Bagley corrections (23) were applied to all the experimental data.


Formation of LLDPE-g-MAH Copolymers

Table 1 lists the GD, GC, and MFI values of pristine LLDPE and LLDPE-g-MAH copolymers reacted for different grafting polymerization time. Obviously, as the reaction time increases, GD values of LLDPE-g-MAH copolymers increase, and their MFI values decrease. However, the GC values of the different LLDPE-g-MAH copolymers are lower than 3.7%. For example, GD and GC of LLDPE-g-MAH-2 reacted for 4.0 h are 1.3 and 0.8%, respectively, and its MFI decreases to 20.4 g/10 min from 49.4 g/10 min of pristine LLDPE. When the reaction time is 6.0 h, GD, GC, and MFI values of LLDPE-g-MAH-3 are 2.1%, 1.7%, and 10.5 g/10 min, respectively. These indicate that MAH is grafted onto LLDPE chains, and the chain-branching reaction, rather than crosslinking, is the main side reaction in the solid-phase grafting polymerization. Because the chain of this low molecular weight polyethylene is relatively short, the chain transfer and termination by coupling of free radicals may form the complex structure with long branching chains as shown in Fig. 1, which leads to low GC. It is in accordance with the results reported by Razavi et al. (19), and the mechanism will be studied later.
TABLE 1. GD, GC, and MFI values of LLDPE and LLDPE-g-MAH copolymers.

Sample Reaction hour (h) GD (%) GC (%) MFI

Pristine LLDPE 0 0 0 49.4
LLDPE-g-MAH-1 1.5 1.1 0 39.1
LLDPE-g-MAH-2 4.0 1.3 0.8 20.4
LLDPE-g-MAH-3 6.0 2.1 1.7 10.5
LLDPE-g-MAH-4 8.0 2.4 3.7 8.3


Figure 2 presents FTIR spectra of LLDPE and LLDPE-g-MAH copolymers. Compared with the pristine LLDPE, LLDPE-g-MAH has a new peak located at about 1767 [cm.sup.-1], which is associated to the carbonyl groups of MAH. It further confirms that MAH has been grafted onto LLEPE chains.


Thermal Properties

Figures 3 and 4 show DSC heating and cooling curves of LLDPE and LLDPE-g-MAH copolymers, respectively. Table 2 summarizes their onset melting temperature ([T.sub.m], the temperature at the intercept of the tangents at the low temperature side of the endotherm and the baseline), peak melting temperature ([]), and crystallization temperature ([T.sub.c], the temperature at the intercept of the tangents at the high temperature side of the exotherm and the baseline). Apparently, the melting temperatures ([T.sub.m] and []) of LLDPE-g-MAH copolymers are obviously higher than those of pristine LLDPE because the introduction of MAH onto LLDPE chains leads to increasing in molecular polarity, and enhancing the macromolecular interaction. Interestingly, [T.sub.c] values of LLDPE-g-MAH copolymers are varied a little, while [T.sub.c] of LLDPE-grafted acrylic acid is higher 3-4[degrees]C than pristine LLDPE due to the nucleation of polyacrylic acid grafted onto LLDPE chains (24). It is because MAH grafted onto LLDPE is in the form of single succinic anhydride ring as well as short oligomer (25), which has no obvious nucleation to the crystallization of LLDPE chains.



In general, the degree of supercooling, [DELTA]T, which is defined as the difference between [] and [T.sub.c], is often used to characterize the crystallization behavior of polymer melts. A decrease in [DELTA]T indicates the acceleration in the crystallization rate. Table 2 presents the calculated [DELTA]T values. As shown with the increase in [DELTA]T the crystallization rate of LLDPE-g-MAH copolymers is retarded by the free radical grafting of MAH onto LLDPE. It is speculated that the introduction of MAH onto LLDPE and the chain-branching destroy the perfection of LLDPE macro-molecules, subsequently block the folding and packing of polymer chains during melt crystallization.
TABLE 2. Melting and crystallization parameters for LLDPE and
LLDPE-g-MAH copolymers.

Sample [T.sub.m] [] [T.sub.c] [DELTA]T
 ([degrees]C) ([degrees]C) ([degrees]C) ([degrees]C)

Pure LLDPE 110.0 118.9 107.3 11.6
LLDPE-g-MAH-1 114.6 120.2 106.9 13.3
LLDPE-g-MAH-2 115.8 120.7 107.1 13.6
LLDPE-g-MAH-3 115.7 121.2 107.9 13.3
LLDPE-g-MAH-4 115.7 121.5 107.9 13.6

Crystal Structure

Figure 5 depicts POM micrographs of LLDPE and LLDPE-g-MAH copolymers. Obviously, pristine LLDPE shows a well-developed radial spherulitic structure with typical Mailtese cross-pattern (Fig. 5a), and the maximum diameter of the spherulite is up to 80 [micro]m. However, the spherulite ordering of LLDPE-g-MAH-l with GD of 1.1% is lower than that of the pristine LLDPE, and the spherulite size evidently decreases to about 20 [micro]m (Fig. 5b). With further increasing the GD, the regular spherulitization of LLDPE-g-MAH copolymers is restrained. Subsequently, the spherulite sizes of LLDPE-g-MAH copolymers further decrease, and it is difficult to find the Mailtese cross-pattern (Fig. 5c-e).


From the diffractograms in Fig. 6, the pristine LLDPE and LLDPE-g-MAH copolymers show two strong diffraction peaks at about 21.6[degrees] and 24.0[degrees], which correspond to [1, 1,0] and [2, 0, 0] diffraction planes of crystals with a monoclinic configuration, respectively. And the free radical grafting of MAH onto LLDPE does not change the diffraction pattern of LLDPE. To study their crystal structure, peak-fit analyzing software was used to obtain a series of crystal parameters, such as interplanar space (d), half-width ([beta]), apparent crystal size ([L.sub.hkl]), etc. The degree of crystallinity ([X.sub.c]) is calculated by:


[X.sub.c] = 1 - [A.sub.a]/[A.sub.c] + [A.sub.a] (3)

where [A.sub.a] and [A.sub.c] are the areas of the amorphous and the total crystal peaks, respectively. The interplanar spacing (d) and apparent microcrystal size ([L.sub.hkl]) in the direction perpendicular to the (hkl) crystal plane can be determined from Bragg's law and Scherrer's formula (26), respectively.

d = [lambda]/2sin[theta] (4)

[L.sub.hkl] = k' x [lambda]/[[beta].sub.0] x cos[theta] (5)

[[beta].sub.0] = [square root of [[beta].sup.2] - [b.sub.0.sup.2] (6)

where [[beta].sub.0] is the half-width of the reflection corrected for the instrumental broadening according to Eq. 6, [beta] is the half-width of various diffraction peaks, [b.sub.0] is the instrumental broadening factor (0.15[degrees]), [lambda] is the wavelength of radiation used, and k' is the instrument constant (0.9). Table 3 summarizes the crystal structural parameters. Compared with the pristine LLDPE, LLDPE-g-MAH copolymers present little change in the interplanar spacing d values for various peaks. It indicates that the grafting polymerization of MAH on LLDPE occurs predominantly in the amorphous regions since the functional groups in the amorphous regions are more available for reaction than those in the amorphous regions (27).Such behavior has also been observed in the solid-phase chlorination of HDPE (28). With increasing the GD, [X.sub.c] values of LLDPE-g-MAH copolymers decrease significantly due to the more defects from the introduction of MAH onto LLDPE chains and slower crystallization retarded by the chain branching in LLDPE-g-MAH copolymers. Additionally, there is little change in [L.sub.200] values, but [L.sub.110] values tend to decrease with increasing the GD. It indicates that the microcrystal size of LLDPE-g-MAH is smaller than that of the pristine LLDPE, which leads to the formation of finer spherulites in LLDPE-g-MAH since the spherulite is a complex, polycrystalline structure consisting of many microcrystals.
TABLE 3. Crystal structural parameters of LLDPE and LLDPE-g-MAH

Samples Diffraction peak 20 ([degrees]) d (nm)

LLDPE 110 21.6 0.446
 200 24.0 0.403
LLDPE-g-MAH-1 110 21.7 0.445
 200 24.0 0.402
LLDPE-g-MAH-2 110 21.7 0.444
 200 24.1 0.402
LLDPE-g-MAH-3 110 21.6 0.446
 200 24.0 0.404
LLDPE-g-MAH-4 110 21.7 0.444
 200 24.0 0.402

Samples [[beta].sub.0] [L.sub.hkl] (nm) [X.sub.c] (%)

LLDPE 0.43 18.8
 0.65 12.5 51.3
LLDPE-g-MAH-1 0.48 16.7
 0.63 12.9 48.9
LLDPE-g-MAH-2 0.51 16.0
 0.75 10.8 46.4
LLDPE-g-MAH-3 0.50 16.2
 0.69 11.8 43.2
LLDPE-g-MAH-4 0.58 13.9
 0.69 11.8 42.3

Rheological Properties

The rheological properties of polymer materials are important for their processing. Thus, the rheological behaviors of LLDPE and LLDPE-g-MAH copolymers were examined at 200[degrees]C by a capillary rheometer, and the results are shown in Fig. 7. Obviously, the apparent viscosity of LLDPE-g-MAH is higher than that of LLDPE, and increases with the GD due to the branching structure in LLDPE-g-MAH copolymers. In the range of applied shear rates, all of LLDPE and LLDPE-g-MAH copolymers exhibit shear thinning characteristics, and the relationship between their apparent viscosity ([[eta].sub.a]) and shear rate ([gamma]) obeys the power law as the following equation:


[[eta].sub.a] = k[[gamma].sup.n - 1] (7)

where k is constant and n is power law index. Physically, the smaller the n value is, the stronger is the shear-sensitivity. As shown in Table 4, the n value of LLDPE-g-MAH is bigger than that of LLDPE, and increases with the GD of LLDPE-g-MAH copolymers. The results indicate that the shear-sensitivity of LLDPE-g-MAH is weaker than that of LLDPE. Generally, the nonpolar polymer with weaker intermolecular force is more sensitive than polar polymer, and the linear or short-chain branching polymer is also more sensitive than the complex branching polymer due to their different entangled structure. Because of the increased polarity and complex branching structure of LLDPE-g-MAH copolymers, they are difficult to be disentangled under shearing force. The different rheological behaviors of LLDPE and LLDPE-g-MAH copolymers also reflect their different chemical and physical structure due to the grafting polymerization of MAH on LLDPE. Additionally, as shown in Fig. 8, LLDPE and LLDPE-g-MAH copolymers at 180 and 210[degrees]C have the same rheological behaviors as 200[degrees]C. Based on Arrhenius equation, the flow activation energy ([E.sub.[eta]]) values of pristine LLDPE and LLDPE-g-MAH copolymers at shear stress of 73,500 Pa are calculated and listed in Table 5. The [E.sub.[eta]] value of LLDPE-g-MAH is higher than that of LLDPE, and increases with the GD of LLDPE-g-MAH copolymers. In other word, the flowability of LLDPE-g-MAH can be improved by elevating the temperature more efficiently than that of LLDPE.

TABLE 4. Power law index (n) of pristine LLDPE and LLDPE-g-MAH
copolymers at 200 C.

Samples g-MAH-1 g-MAH-2 g-MAH-3 g-MAH-4

n 0.13 0.15 0.22 0.31 0.33

TABLE 5. Flow activation energy of pristine LLDPE and LLDPE-g-MAH
copolymers at shear stress of 73,500 Pa.

Samples g-MAH-1 g-MAH-2 g-MAH-3 g-MAH-4

[E.sub.[eta]] (kJ/mol) 3.63 4.95 5.36 6.16 9.62


Through the solid-phase grafting polymerization of low molecular weight LLDPE with MFI of 49.4 g/10 min, maleic anhydride is successfully grafted onto LLDPE with the GD of 1.1-2.4% and the GC less than 3.7% since the chain-branching reaction is dominant side reaction. The melting temperature of LLDPE-g-MAH copolymer is higher than that of pristine LLDPE due to the increased molecular polarity, but its degree of crystallinity and crystallization rate decrease due to its chain-branching structure. The apparent viscosity of LLDPE-g-MAH is higher than that of pristine LLDPE, and its shear-sensitivity is weakened due to the chain branching and subsequent difficulty in disentanglement under shear force.


As visiting professor of the Centre for Advanced Materials Technology (CAMT), the University of Sydney, XLX thanks Prof. Yiu-Wing Mai's academic and financial supports.


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Correspondence to: Xiaolin Xie; e-mail:

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50573026; contract grant sponsor: Program for New Century Excellent Talents in Universities of China; contract grant number: NCET-05-0640.

DOI 10.1002/pen.21285

Published online in Wiley InterScience (

[C]2009 Society of Plastics Engineers

Le-Ping Huang, (1) Xing-Ping Zhou, (1) Wei Cui, (1) Xiao-Lin Xie, (1) Shen-Yi Tong (2)

(1) Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

(2) Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China
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Author:Huang, Le-Ping; Zhou, Xing-Ping; Cui, Wei; Xie, Xiao-Lin; Tong, Shen-Yi
Publication:Polymer Engineering and Science
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Date:Apr 1, 2009
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