Functional Polyethylene Composites Prepared viaPolymerization of Ethylene With Opal-immobilizedZirconocene Complex

Hao Xu,1,2 Xinwei Wang,2 Cun-Yue Guo,1 Qingshan Li21Beijing National Laboratory for Molecular Sciences, Center for Molecular Sciences, Institute of Chemistry,Chinese Academy of Sciences, Beijing 100080, China

2State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,Qinhuangdao 066004, China

An opal-supported zirconocene complex (OC) was pre-pared and employed to prepare functional polyethylenecomposite for the first time through in situ polymeriza-tion of ethylene. The formation mechanism of anionrelating to opal was explained and the values of anionsreleased from pristine opal and that contained in the as-fabricated polyethylene composites were detected. TheOC exhibited high catalytic activities in ethylene poly-merization with methylaluminoxane (MAO) the cocata-lyst. In addition to higher viscosity-average molecularweights (MZ), experimental results also showed that theresulting polyethylene composites possessed improvedanion-releasing capacity in comparison with pristineopal in addition to increased tensile strength, Young’smodulus, and onset decomposition temperature (Tonset)relative to neat polyethylene due to highly uniform dis-persion of opal in polyethylene matrix. POLYM. COMPOS.,29:380–384, 2008.ª 2008 Society of Plastics Engineers

INTRODUCTION

The past half century has witnessed the most rapid scien-

tific and technological development of polyolefins first pre-

pared in the 1950s with Ziegler-Natta catalytic systems

[1, 2]. Driven by the desire to improve the properties of pol-

yolefinic materials, organometallic or coordinate catalyst

have become one of the hottest research areas. Compared

with traditional Ziegler-Natta catalysts, single-site metallo-

cene catalysts offer unprecedented control over polymer

chain structure and material properties [3]. Despite the

many advantages that homogenous metallocene catalysts

possess, such as high activity and stereoregularity, there are

still some critical problems for their commercial applica-

tions [4, 5] such as poor polymer morphology, low molar

mass polymers, reactor fouling, low bulk density of the

polymer, and the requirement of a large amount of expen-

sive methylaluminoxane (MAO) [6]. An efficient way of

tackling this is to support these homogeneous catalysts on

certain inorganic or organic carriers, such as SiO2, MgCl2,

Al2O3, and polymers, for dropping into slurry and gas-

phase processes and producing polyolefin resins with better

properties and good processability [7].

As an environment-friendly material, opal is a kind ofnon-crystalloid or colloid active inorganic material withSiO2�H2O the major component. It contains water of 1 � 14wt%, small quantities of Fe2O3, Al2O3, Mn, Cu, and otherorganic impurities. Because of the existence of manymicro-holes, opal has specific surface area of 277.3 cm2 g�1

and aperture diameter of 5 � 20 nm. In addition to mois-ture, it also adsorbs chloride, nitrite, and cyanide [8–10].The most significant characteristics of opal lies in its anion-releasing effect, which plays positive roles in air purifica-tion, bacteria inhibition, immunity improvement, etc. Soopal has been finding more and more uses in many fieldsrelating to environment protection and human life, such ashealth care, textile industry, etc. [11].

In this research, we immobilized metallocene catalyst

onto opal powder for the first time and investigated its per-

formance in ethylene polymerization with MAO the co-

catalyst. The as-synthesized polyethylene is expected to

find applications in rag trade, coating industry, and pack-

aging materials owing to opal’s anion releasing effect.

Because opal used herein is as a carrier of the catalyst for

ethylene polymerization, it can disperse uniformly through-

out polyethylene matrix during the polymerization process.

Once this comes true the fabrication methodology can be

extended to the preparation of other functional polymers

including polypropylene, polystyrene, poly(vinyl chloride),

Correspondence to: Dr. Cun-Yue Guo; e-mail: cyguo@iccas.ac.cn

Contract grant sponsor: National Natural Science Foundation of China;

contract grant number: 50573081.

DOI 10.1002/pc.20402

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2008

etc. As green chemistry is becoming a trend in the world

today, anion-releasing functional polymers are sure to find

uses in almost every aspect relating to human activities in

addition to the aforementioned applications and destine to

be prosperous.

EXPERIMENTAL

Materials

All manipulations of air- and/or moisture-sensitive com-

pounds were carried out using drybox procedures or stand-

ard Schlenk techniques. Opal powder was generously

offered by Yanshan University Qicai Healthy Materials

Technology Exploitation (China). Zirconocene dichloride

(Cp2ZrCl2) was synthesized according to the literature [12].

Ethylene of polymerization grade was purchased from Yan-

shan Petro-Chemical (China) and used without further puri-

fication. MAO solution in toluene (1.4 mol L�1) was pur-

chased from Albemarle (USA). Toluene was refluxed over

sodium with benzophenone as the indicator and distilled

under nitrogen atmosphere before use. All other chemicals

were commercially obtained and used as received.

Preparation of the Opal-Supported Cp2ZrCl2 (OC)

Opal of desired amount was dried at 1208C in vacuum

for 5 h. Under argon atmosphere, it was then mixed with

6 mL of MAO at 508C overnight before washed with toluene

to remove un-reacted MAO. Then the MAO-pretreated opal

was reacted with 100 mmol of Cp2ZrCl2 in toluene. After

reacting at 508C for 9 h, the product was washed several

times with toluene until the upper liquid layer turned color-

less and then dried in vacuum at 508C to give light yellow

powder (OC). Spectrophotometric analysis indicated that

Zr content is 15.0 mmol Zr/g catalyst.

Preparation of Polyethylene Composites

To a 250 mL round bottom flask with a magnetic stirring

bar, 50 mL of toluene and required amount of MAO solu-

tion were introduced under atmospheric ethylene pressure.

After the reactants were stirred until the solution was satu-

rated with ethylene, opal-supported Cp2ZrCl2 was trans-

ferred to the flask to initiate the polymerization at predeter-

mined temperature. The reaction was terminated with

10 wt% acidified ethanol after 30 min. The product was fil-

tered, washed with ethanol and water, and dried at 608C in

vacuum to constant weight.

Characterization

The aperture of opal was measured by transmission elec-

tron microscope (TEM) JEM-2010. Chemical component

of the Opal sample are recorded by X-Ray fluorescence

spectroscopy ADVANTPXP-381. The melting temperatures

(Tm) of polyethylene were measured by differential scan-

ning calorimeter (DSC) with a Perkin-Elmer DSC-7 instru-

ment. Ultrahigh purified nitrogen was purged through the

calorimeter. The PE sample (�3 mg) was heated from 50

to 1708C at a rate of 108C min�1, and Tm was recorded for

the second scan. The onset decomposition temperatures

(Tonset) were measured on a Perkin-Elmer 7 series system

TGA apparatus. X-ray powder diffraction data were col-

lected on a Rigaku D/max 2400 diffractometer (Japan)

using Cu Ka radiation of wavelength 0.1541 nm. Diffrac-

tion data were recorded between 1 � 408. Viscosity-averagemolecular weight (MZ) of polyethylene was measured by a

model AVS-300 viscometer (Shott Co) in decahydronaph-

thalene at (135 6 0.1)8C and calculated from the intrinsic

viscosity ([Z]) according to the following equation: [Z]¼ 2.3 � 10�4 M0:82

Z [13]. The Opal and PE samples were

deposited on a sample holder and sputtered with gold. Then

the morphology of those was recorded on a JEOL JSM-

6700F field emission scanning electron microscope (SEM).

The anion content of polyethylene composites was meas-

ured on IC-1000 ion measurement apparatus (Japan) at

258C. The content of zirconium in opal-immobilized

Cp2ZrCl2 was measured by spectrophotometer method of

Arsenanzo III on a 721-s spectrophotometer instrument.

Samples for mechanical property testing were prepared on

CS-183 MINI MAX injection molding machine and carried

out on Instron-1122 device.

RESULTS AND DISCUSSION

Composition of Opal

As described in Table 1, silica is the major component of

opal with the concentration high as 89%. Also contained in

opal are TiO2, Al2O3, etc.

Immobilization of Cp2ZrCl2 on Opal

To make Cp2ZrCl2 effectively supported on opal, MAO

was used in advance to modify opal by reacting with the

TABLE 1. Composition of opal.

Component

Net

intensity

(Kcps)

Concentration

(%)

Normalised

concentration

(%)

SiO2 126.523 46.0 54�89

TiO2 109.346 13.1 0.3�15.4

Al2O3 12.479 4.87 5.72

Fe2O3 25.050 0.93 1.09

MgO 3.934 0.91 1.06

Na2O 1.813 0.82 0.97

CaO 2.675 0.35 0.41

K2O 2.324 0.34 0.40

H2O 5.601 0.56 0.5�2.5

DOI 10.1002/pc POLYMER COMPOSITES—-2008 381

silanol groups lined along the surfaces of silica abundant in

opal. Then Cp2ZrCl2 could be fixed onto opal through the

‘‘bridge’’ of MAO [14].

Formation of Polyethylene Composites

The data of ethylene polymerization under atmospheric

pressure catalyzed by opal-supported Cp2ZrCl2/MAO are

summarized in Table 2.

As other silica-supported zirconocene catalysts [15], the

catalytic activity of OC is somewhat lower than that of

Cp2ZrCl2 (Run 10 vs 1 in Table 2), but high activity as

13.67 � 106 g PE/(mol Zr�h) is reached as the Al/Zr molar

ratio reaches 3000 (Run 3 in Table 2). Reason for this is

that the deactivation reactions in homogeneous catalyst sys-

tem can be inhibited to some extent by the supporting pro-

cess and less cocatalyst is needed for protecting or reviving

the active sites [16]. Additionally, ethylene polymerization

activities increase with both the increment of Al/Zr molar

ratio and polymerization temperature.

Also seen from Table 2 is that the viscosity-average mo-

lecular weight of the polyethylene obtained from OC is

generally higher than that formed with homogeneous cata-

lyst Cp2ZrCl2 because the b-hydrogen transfer is sup-

pressed [17]. Either lowering the polymerization tempera-

ture or decreasing the Al/Zr molar ratio tends to increase

the molecular weight.

The structure of opal changes obviously after the polymer-

ization of ethylene proceeds in it (see Fig. 1). Compared with

pristine opal, the characteristic diffraction peaks at 2y ¼ 6.18and 26.68 disappear completely in the polyethylene composite

and two nascent peaks characteristic of polyethylene crystals

appear at 2y ¼ 21.48 and 23.98, respectively, reflecting that

the ethylene polymerization catalyst is chemically bonded to

opal and modifies its crystal structures.

Morphology of Opal, OC, and Polyethylene Composite

Seen in the SEM images (Fig. 2) is that the quartz of

opal shale is tiny sheet aggregation, which composes a

sphere (Fig. 2a). At higher magnitude the hedgehog-like

spheres (Fig. 2b) are found to consist of numerous rod-like

materials. The outskirt of the sphere often has a circuit cris-

tobalite, its size is 0.1–0.3 mm. Owing to the great amount

of micro-holes, opal used here possesses a specific surface

area of 277.3 cm2 g�1, and an aperture of 5–20 nm with the

moistures adsorption rate of 74.4%.

Quite different from the shape of opal, OC (Fig. 2c)

exhibits no traces of hedgehog morphology of opal. It’s

speculated that the reaction between MAO and the silica

disintegrates the regularity of opal and ensures the immo-

bilization of the active sites. After the polymerization of

ethylene, the components of opal disperse uniformly in pol-

yethylene matrix and not any rod-like materials are found

in the composite (Fig. 2d).

Anion-Releasing Effects of Polyethylene Composites

It’s known that the molecule and atom in air initiate

air ionization in the action of mechanical, light, static,

TABLE 2. Result of ethylene polymerization withOCa.

Run Al/Zr Polym. Temp. Activity Tm DH MZ � 10�4 Anion Opal

mol/mol 8C 106g�(mol Zr�h)�1 8C J/g (g mol�1) (cm�3) wt%

1 1000 70 4.04 134.1 155.1 2.62 2690 3.2

2 2000 70 9.53 131.8 199.5 2.46 1550 1.4

3 3000 70 13.67 132.9 208.3 2.44 1710 1.0

4 1000 50 3.60 134.2 159.3 14.48 4100 3.8

5 2000 50 4.28 133.6 169.4 10.42 2380 3.1

6 3000 50 5.53 134.0 161.1 9.90 1580 2.5

7 1000 30 1.11 134.3 144.7 17.38 1230 12.5

8 2000 30 1.42 134.1 154.3 14.91 2480 9.4

9 3000 30 1.63 134.4 149.1 13.56 1340 8.3

10b 1000 70 7.05 131.2 247.7 1.02 – 0

a Zr, 1 mmol; Polym. Time, 30 min; Ethylene pressure, 0.1 MPa; Toluene, 50 mL.b homogeneous Cp2ZrCl2 was used.

FIG. 1. XRD patterns of (a) opal; (b) polyethylene composite (Run 4 in

Table 2). [Color figure can be viewed in the online issue, which is available

at www.interscience.wiley.com.]

382 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

chemical or biological energy. The outer layer electrons

deviate atomic nucleus; hence the molecule or atom without

electron has positive charge. The electrons departed from

nucleus combine with other molecule or atom to form anion.

Gas molecules, after capturing electron, turn out to be fuga-

cious anionic molecules in air and some ions are neutralized

while others come into being simultaneously. So the con-

tents of positive and negative ions are kept homeostatic.

As observed in the SEM images, there are permanent

spontaneous micro-poles in opal. Different atoms existed in

opal produce micro-electric field. Once water molecules in

air come into the micro-hole of opal, the hydrion and hy-

droxide ion formed combine with electron and hydrogen

dioxide, respectively, with the formation of gaseous

hydrated hydroxide ion, namely anionic water molecule,

and equal amount of hydrogen released to the environment.

(Schemes 1–2).

Because of the characteristic of opal, the resultant poly-

ethylene composite possesses a highest anion-releasing

capacity of 4100 cm�3 (Run 4 in Table 1) in the range

of our experiment investigated—far above the value of

2000 cm�3 of pristine opal. This indicates that the high

homogeneous dispersion of opal components in the polyeth-

ylene composite endows opal with rocket in its anion-

releasing capacity and is of great significance in preparing

functional polyethylene.

As the data in Table 3 indicate [18, 19], most of the poly-

ethylene composites obtained in our experiment reach the

content of anion better than that of a city park and it’s cer-

tain that commodities made from such composites contain-

ing quite small amount of opal are destined to be beneficial

to human health.

FIG. 2. SEM images of (a) opal; (b) opal at higher magnitude; (c) OC; (d) polyethylene composite.

SCHEME 1. Structure of anionic water molecules in air. SCHEME 2. Release of anions from opal.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 383

Physical and Thermal Properties of PolyethyleneComposites

Data in Table 4 show that the mechanical and thermal

properties are improved. The tensile strength, compared

with the value of 23.5 MPa for neat polyethylene (Sample

10), increases by ca. 42% with the increment of Opal and

tends to go up as MMT content increases until a maximum

value of 33.6 MPa is reached as in the case of Sample 6.

Moreover, Young’s modulus of the composites increases

with increasing loading of Opal. The onset decomposition

temperature of polyethylene composites rises by nearly

308C (Sample 7), indicating that highly dispersed Opal fill-

ers play a role in improving thermal stability of the resultant

polyethylene composites similar to that of montmorrillonite

(MMT) layers in the case of PE/MMT nanocomposites [20].

CONCLUSIONS

Functional polyethylene composite capable of releasing

anions can be produced via in situ polymerization of ethyl-

ene with zirconocene complex immobilized on opal. Opal-

supported Cp2ZrCl2 exhibits high ethylene polymerization

activity and produces polyethylene of high molecular

weights with MAO the cocatalyst. The resultant polyethyl-

ene composites possess increased physical and thermal

properties. What is noteworthy is that the as-synthesized

polyethylene composites can build a microclimate better

than a city park by releasing quite high numbers of anion.

It’s reasonable to believe that conventional polyolefin res-

ins, via taking full advantage of the anion-releasing

capacity of opal, are destined to find more and more appli-

cations as high value-added functional polyolefins and in

situ polymerization of olefin will be one of the best ways to

reach such a goal.

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TABLE 3. Relationship between the anion content and human health.

Environments

The content

of anion (cm�3) Functions

Forest. waterfall area 1,00,000–5,00,000 Natural curability

Mountain. seaside 50,000–1,00,000 Sterilization

Environs. field 5,000–50,000 Immunity and

antimicrobial

City park 1,000–2,000 Maintain basic

health

Virescence area 100–200 Edge of the

circadian holdback

Occlusive houses in city 40–50 Maybe place a

premium on headache,

insomnia etc.

Air-conditioned houses 0–25 Maybe place a

premium on

air-condition sickness

TABLE 4. Properties of the resulting polyethylene composites.

Samplea Tensile strength Young’s modulus Tonset Opal

MPa MPa 8C wt%

10 23.5 265 263.2 0

3 28.9 386 388.5 1.0

6 33.6 425 439.6 2.5

9 31.6 459 445.8 8.3

7 27.2 526 448.1 12.5

a The sample number corresponds to that in Table 2.

384 POLYMER COMPOSITES—-2008 DOI 10.1002/pc