Plasma induced grafting of styrene onto nascent polyethylene powder

Eur. Polym.1. Vol. 31, No. 8. pp. 755-760, 1995 Copyright 0 1995 Elxvia Science Ltd Printed in Great Britain. All rights reserved 0014-3057/95 s9.50 + 0.00

Pergamon

PLASMA INDUCED GRAFTING OF STYRENE ONTO NASCENT POLYETHYLENE POWDER JUN TIAN, XIAO LIN, BAOTONG HUANG and JIPING Changchun

Institute

of Applied

Chemistry, Chinese Academy People’s Republic of China

XU

of Sciences, Changchun 130022,

(Received 2 April 1994; accepted in final form 28 July 1994)

Abstract-In this paper, the graft copolymers of styrene to nascent linear polyethylene reactor powders were prepared through plasma graft polymerization. The grafting reaction was initiated by the alkyl radicals formed on the surface of nascent polyethylene with plasma treatment as indicated by electron spin resonance spectra. In graft copolymerization by alkyl radicals, the grafting yield increased with either the plasma power or the plasma treatment time. Compared with ordinary polyethylene powders, nascent polyethylene reactor powders were found to be more easily plasma-grafted. This has been attributed to the greater sensitivity to irradiation in producing reactive centres under the same conditions. High density polyethylene showed almost the same grafting yield as linear low density polyethylene at 50°C. The surface morphology of nascent polyethylene observed by scanning electron microscope grafting showed that the silk-like tibrils were not destroyed by plasma treatment.

INTRODUCTION

Graft polymerization is a well known method for modification of the chemical and physical properties of polymeric materials. Chemical radiation methods have been used to prepare graft copolymers in the past four decades due to its case of preparation and efficiency compared to conventional chemical method. Radiation grafting can be initiated by Co60 y-rays [I], electron beaming [2], and u.v.-light [3] etc. It is also well known that polymer radicals are formed on the surface of polymeric materials during plasma treatment [4-61. One of the characteristics of plasma treatment is the effective energy transfer to

solid surface to create free radicals on a variety of polymer surfaces [7]. Moreover, some of the radicals on polymer surface produced by plasma treatment can initiate graft polymerization of vinyl monomers [8,9]. Hirotsu et al. reported that water-ethanol permselective membranes were prepared through plasma graft polymerization and plasma graft copolymerization of vinyl monomers onto porous polypropylene films [lo-141. This plasma graft polymerization technique can also be applied to prepare the filling-polymerized membrane with a porous high density polyethylene film and grafted poly(methyl acrylate) used as the substrate [15]. But there is still no report on grafting to polyethylene or polypropylene powders by plasma graft polymerization technique. Many papers have been reported on the radiation

before

and after the

only a little amount of irradiation work has been reported on grafting to polyethylene powder presumably due to the difficulties involved in handling powdered polymers [2], and very little is known on the effects of the crystalline state of polyethylene on grafting. Ranogaj et al. studied the influence of the type of polyethylene on the rate and yield of grafting of styrene by using simultaneous irradiation with y-ray and also pre-irradiation in uucuo [24]. Hoffman et al. studied the direct radiation grafting of styrene on high pressure and low pressure polyethylenes and found that the different polycrystalline natures were one of the most important factors in determining the mechanism and resultant efficiency of grafting [25]. The purpose of this work is to investigate the possibility of preparing graft copolymer of styrene onto nascent polyethylene reactor powder through plasma graft polymerization. The effects of several factors on grafting have been studied, such as plasma power, plasma treatment time, and crystallinity of nascent polyethylene reactor powders. Several analytical methods have been used to investigate various interrelated aspects of both the graft copolymer product and the graft process, including electron spin resonance to measure radical concentration, scanning electron microscopy (SEM) to observe surface morphology and wide angle X-ray diffraction to determine crystalline characteristics. EXPERIMENTAL PROCEDURES Materials

grafting of styrene onto polyethylene by both the direct method and the pre-irradiation method [16-231. Most papers have been concerned with the kinetics of grafting and the effects of several factors such as dose rate, total dose, irradiation time, temperature, film thickness and type and level of unsaturation, etc. on grafting polymerization. However,

Nascent polyethylene (HDPE) reactor powder was prepared by the soluble biseyclopentadienylzirconium dichlo-

ride (Cp,ZrCl,)/methylaluminoxane

(MAO) homogeneous

catalyst system at 25°C with glass reactor vessel equipped with a magnetic stirrer under ethylene pressure of I .5 atm. The polymer was isolated after hydrolysizing the catalyst and dried under vacuum at room temperature (< 25°C). The 755

Jun Tian (‘I al.

756 Table I. ReMonship

between grafting yield and nalure of polyethlene powders’ Nascent polyethylene

Common polyethylene

40.5

5.1

Grafting yield (%I

*Plasma Dower: 100 W: olasma treatment tune. 5 min. Post-graft polymerization condi&on: 24 hr. 50 k

ordinary polyethylene powder (200 mesh-free) was prepared by slow precipitation of the dilute solution of nascent polyethylene (0.1 wt%) from ethanol. Nascent linear low density polyethylene (LLDPE) reactor powder was prepared with ethylene and I-hexene under the same procedure of HDPE. Powdered HDPE (200 mesh-free) was used for copolymerization unless otherwise stated. The RI? of nascent polyethylene determined by viscometry (in decalin, 135‘C) was 23.07 x 10“. Chemical grade styrene was first washed with an aqueous solution of sodium hydroxide and then with water, and dried over calcium chloride. and distilled under reduced pressure with CaH?. Plasma graft poiymeriz2lion Nascent polyethylene reactor powders (0.1 g) was placed in an ampule (40 mm id., 100 mm long) and evacuated to 4 x lo-’ torr and flashed with argon gas, evacuated to I x IO-’ torr again and sealed. Nascent polyethylene powders were treated by using a capacitively coupled RF plasma equipment (13.56 MHz) with external plate electrodes (150 x 100 mm). During plasma treatment. the ampule was rotated. After treatment. 8 mL styrene was injected into the ampule under vacuum, and then the ampule was put into the water bath. The grafting reaction was carried out at 50 C. The grafting products were washed with ethanol to remove the excess monomer and then dried under vacuum for 72 hr at 50°C. The yield of styrene homopolymer, measured by the Soxhlet extraction method (using cyclohexanone as the solvent), was found to be so small that we could assume that the whole increase in weight resulted from grafting. The grafting yield was determined by the percent increase of weight based on the original polyethylene weight.

ture [26] (see Fig. 7). Our recent study on the morphology of nascent PE reactor powders [26-281 indicates that they are composed of identical fibrillar, whose interior consists of macrofibrils made up of highly extended macromolecular chains, which are more sensitive to radiation to produce reactive centres by radiation. Furthermore, the nascent polyethylene powder has a much larger specific surface area than the ordinary polyethylene due to its fibrillar texture. This has been confirmed by specific surface area measurement [29]. Table I shows the grafting yields of the nascent and common polyethylene powder which were plasmatreated and grafted under identical conditions. The much larger grafting yield for the nascent polyethylene powder has_ resulted from the more reactive centres on the polymer surface under the same plasma treatment conditions as indicated by ESR spectra (see Fig. 1). In our view, the amount of reactive centres under the same plasma treatment conditions mainly depends on two factors: polymer surface structure and its specific surface area. The more sensitive to radiation of the polymer surface and the larger specific surface area, the more reactive centres formed which results in a larger grafting yield.

C

Measurement IR spectra were obtained using the Alpha-Centauri Infrared Spectrometer (Mattson Camp.. U.S.A.). The samples for IR measurements were prepared by the KBr technique. Only the wave-number region from 2000 to 400cm- ’ is relevant to our discussion. ESR measurements were carried out using a JES-FE-3AX electron spin resonance spectrometer (JEOL Corp., Japan) at room temperature. The treated polyethylene powders (about 70 mg) were removed under vacuum from the ampule. The samples were collected in standard quartz ESR ampule tubes and sealed. The operation frequency was 9.205 gHz, the magnet modulation 100 kHz and the scanning rate 3367 + 250 g. The microwave power and the gain were kept at 0.4 and 100 mW, respectively. The changes of the surface morphology of the original and the grafted nascent polyethylene were observed by a JEOL (JXA-840) type SEM. The wide angle X-ray diffraction measurements were taken on a Philip PW 1700 X-ray automated powder diffractometer: Cu Ka radiation, voltage 40 kV, current 30 mA. The powders of the original and the grafted nascent polyethylene were pressed into films at room temperature in order to prepare the samples for WAXD measurements. RESULTS

Plasma

grafting

AND DISCUSSION

yield for

nascent

and ordinary

polyethylenes Nascent polyethylene reactor powder with metallocene (IV,)/MAO homogeneous

system exhibits the characteristic

prepared

catalyst horous-fibrillar tex-

Fig. I. ESR spectra of plasma-induced polyethylene radicals (A) Nascent HDPE; (B) nascent LLDPE; and (C) ordinary PE. Plasma power: 100 W; plasma treatment time: 5 min.

Plasma induced grafting of styrene

757

Our study mainly counts on the two intrinsic characteristics of the nascent polyethylene powder. ESR results

Room temperature ESR spectrum of plasma-induced radicals of nascent HDPE, LLDPE and common PE powders is shown in Fig. 1, which clearly shows a sextet spectrum attributed to alkyl radicals. The apparent hypertine splitting value of about 30 g is consistent with the results other authors reported [2,25]. It can be seen that the ESR spectrum of plasma-induced HDPE and LLDPE powders shows little difference in the type of radicals. The ESR result indicates that the same alkyl radicals were produced from C-H bonds in both HDPE, LLDPE and ordinary PE during plasma treatment. The concentration of plasma-induced radicals of polymeric material is effected by the conditions of plasma treatment, such as plasma input power and plasma treatment time [6]. We also found that although the spectral intensity increased with either the plasma treatment time or the plasma input power, but the spectra1 pattern remained nearly unchanged. The details will be reported later in another paper. IR results

IR absorption spectra for the original polyethylene and the grafted polyethylene are shown in Fig. 2. Figure 2(A) shows the typical polyethylene spectrum with adsorption band at 727 cm-’ contributed to the methylene bending and rocking regions. The graft copolymer after extraction by cyclohexanone in the IR spectrum is represented in Fig. 2(B). The new adsorption bands due to the grafted components could be found: 700 and 760 cm-’ which are the characteristic adsorption bands for monosubstituted phenyl group. Therefore it may be concluded that the grafted materials are indeed grafted copolymerization of styrene onto nascent linear polyethylene.

0

I

I

20

40

I 60

Plasma

power(W)

I

I

I

80

100

120

3. Relationship between grafting yield and plasma power for plasma treatment time: 5 min; grafting time: 24 hr; grafting temp.: 50°C. Fig.

Plasma graft polymerization

Plasma graft polymerization is carried out by two successive processes, i.e. the plasma surface activation of the substrate and the graft polymerization of monomers, and the grafting is naturally dependent on the reaction factor derived from the processes [10-131. In the plasma pretreatment, plasma power and plasma treatment time affect the concentration of radicals of the surface polymer. Several factors of post-polymerization such as reaction time, grafting temperature, degree of crystallinity and type and level of unsaturation of the irradiated materials etc. have significant influence on the grafting yield. Effect of plasma pretreatment on grafting yield. The grafting yield is plotted as a function of plasma power in Fig. 3. Plasma treatment was 5 min and grafting time was 24 hr at 50°C. Figure 3 shows that the grafting yield increased with plasma power. The concentration of plasma induced radicals on the surface of polyethylene powders increased with plasma power as indicated by ESR spectra. Therefore, the grafting yield increased with the increasing of plasma power.

50

.’

40

A

55 I! .z x

30

E? ‘c

20

./ . ./

; 10

/ . I

I

I

I

2000

1600

1200

800

1

0

I

I

I

I

I

I

1

2

3

4

5

6

400

Wavenumber (cm-l) Fig. 2. IR spectra of original (A) and grafted polyethylene (W

Treatment

time (min)

Fig. 4. Relationship between grafting yield and plasma treatment time. Plasma power: 100 W; grafting time: 24 hr; grafting temp.: 50°C.

Jun Tim PI al.

758

I0

I J

(1

i __1x I2

Ih

L--J 20

?A

I) j 20

30

40 Reaction

Reaction time (hr)

50 temperature

60

70

80

(“C)

Fig. 5. Relationship between gralting yield and graftmg time. Plasma power: 100 W; plasma treatment time: 5 min: grafting temp.: 50 C

Fig. 6. RelationshIp between grafting yield and grafting temperature. Plasma power: IOOW;plasma treatment time: 5 min: grafting time: 24 hr.

The relationship between the grafting yield and the plasma treatment time is shown in Fig. 4. where plasma power was 100 W and grafting time was 24 hr at 50°C. It can be seen that the grafting yield was proportional to plasma treatment time. The concentration of plasma induced radicals on the surface of polyethylene increased with the increasing of plasma treatment time. This caused the increasing of the grafting yield with the increasing of plasma treatment time. The relationship between the grafting yield and the plasma treatment time in our experiment is different from that reported by Hirotsu [13]. Hirotsu found that the grafting yield increased at first with an increase of the pretreatment period and reached maximum around at 2 min. As the plasma pretreatment was continued further. the grafted amount decreased slightly and leveled off after 4--5 min. This difference may be explained that the ratio of surface:volume of small particles is very much larger than that of the film.

decayed increase decrease

Eflect qf’post -polymerization condition on the grgft ing yield. Figure 5 shows the typical curves plotted

-

degree of grafting vs reaction time for HDPE. where plasma power was 100 W and plasma treatment ttme was 5 min. The graft reaction was carried out at 50 C. The grafting yield increased with reaction time. The reaction temperature is one of the important factors to control the grafting. As shown in Fig. 6. the grafting yield increased at first with temperature and reached a maximum at 50 C. As the reaction temperature was continued further. the grafting yield decreased. This phenomenon was also observed by Hirotsu 1131. It is considered that the fraction of

Table 2. Effect of different type 01 nascent polyethylenea on gratimg yield* RtZiCliOIl temp.

( C)

RUiCtlOn tulle (hr)

LLDPE

23.07

20 50

36 I?

23.7 25.8

HDPE

14.64

20 50

36 I?

33.2 25. I

Samples

M.x

IO a

‘Plasma power. 100 W; plasma treatment time: 5 mm

Graliing yield

(YoI

radicals and the termination rate constant at higher temperature. which causes the in the grafting yield at elevated temperature

1301. ,!$&I of’d#krenr types C$ nuscen~ polyethylenes on gwfiing yield. Powders of HDPE and LLDPE were treated by plasma under the same plasma treatment condition, and reacted with styrene at 20 and 50°C. respectively. The grafting yield of HDPE was higher than that of LLDPE at 20°C (see Table 2). This result can be explained as follows: the grafting reaction is considered to occur in the amorphous regions and on the surface of crystalline domains [23,24, 31-331. The radicals are formed on the surface of both HDPE and LLDPE by plasma treatment. Crosslinking is so little that monomer can diffuse into the radical-containing surface of crystalline regions of polyethylene at this temperature. And the termination rate constant of HDPE is smaller than that of LLDPE [24] due to a greater polymer chain rigidity. although it can cause a lower degree of swelling and a lower gel effect compared to LLDPE. Hence, the grafting yield of HDPE is higher than that of LLDPE at 20°C. However, at 50-C, the grafting yield of HDPE is almost the same as that of LLDPE, this result is in disagreement with those other authors reported [24. 25. 32, 34-361. According to Ishigaki ef nl., the degree of grafting is predominantly proportional to the concentration of trapped radicals regardless of the density of polyethylene. The concentration of radicals formed on the surface of both HDPE and LLDPE by plasma treatment can be estimated from the data of ESR. The ESR results showed that the concentration of plasma induced radicals on the surface of HDPE is almost the same as that of plasma induced LLDPE. Moreover. the grafting yield is not effected by the diffusion rate of monomer at this temperature due to the radicals formed on the shallow surface of both HDPE and LLDPE. On the other hand. the effect of higher crystallinity at this temperature is smaller than at 20-C. Therefore. the grafting yield of HDPE is the same as that of LLDPE. The main difference between our experiments and other authors’ is the different irradiation methods. The radicals are formed not only on the surface of

Plasma induced grafting of styrene

I 10

759

I

I

20

30 29 0

Fig. 8. WAXD patterns of original polyethylene (A) and grafted polyethylene (B).

Fig. 7. SEM of the surface morphology of the original and grafted polyethylene: (A) original nascent linear polyethylene and (B) grafted polyethylene. polyethylene through Co@’y-ray radiation, but also the inside of it; besides, at a given dose the more crystalline polyethylene with the same type and level of unsaturation has a higher free radical content [34]. As the temperature rises at 5O”C, styrene can diffuse into the crystalline regions of the high density polyethylene, so that the grafting yield of HDPE is much higher than that of LDPE. Morphology and crystallinity of original and grafted polyethylene

As previously mentioned, the alkyl radicals are formed on the surface of polyethylene powder through plasma treatment and initiate graft polymerization of styrene. The changes of the morphology of the grafted and the original polyethylene were observed by the use of SEM. The morphology of the original is shown in Fig. 7(A). It can be seen that nascent polyethylene is stacked up by a bundle of silk-like fibre made up of macrofibrils which consist of fibrils [26-281. After grafting, the silk-like fibres

Table 3. Lattice

parameters

were also observed. This result indicated that plasma treatment has a little influence on the surface morphology of nascent polyethylene powders. Figure 7 also shows that the surface of nascent polyethylene is covered by the grafted polystyrene. Figure 8 shows the WAXD pattern of the original and the grafted polyethylene. Both the angles and the half-widths of the (110) and (200) peaks remained nearly unchanged. The crystals of the nascent polystyrene from the results of the same crystal parameters (see Table 3) of the polyethylene before and after grafting polymerization, and it also means that the segments of polystyrene have not entered the crystal lamellae of polyethylene. Moreover, even at high polymerization temperature (SOY& polyethylene crystalline lamellae have scarcely been affected by the graft polymerization forces, and the grafted polystyrene chains only cover the surfaces of the nascent polyethylene reactor powders. CONCLUSION

Graft copolymers of styrene to nascent linear polyethylene reactor powders (HDPE and LLDPE) were prepared through plasma graft polymerization. Compared with ordinary polyethylene powders, it was found that nascent polyethylene reactor powders were more liable to be plasma-grafted, which has been attributed to the greater sensitivity to irradiation in producing reactive centres under the same oper-

and Izw/f,,o values of PE in PE-g-PS Lattice parameters

Samules

Grafting yield (%)

a (A)

b (A)

Imil, 0”

II 40

7.470 7.445 7.431 7.428

4.985 4.985 4.967 4.971

23X/100 22.31100 23.ljlOil 23.2/100

PE PE’ LL-66 LL-72 *Plasma

treated

PE

760

Jun Tian et a/

ation conditions. As indicated by the electron spin resonance spectra, the grafting reaction was initiated by the alkyl radicals formed on the surface of nascent polyethylene with plasma treatment. In graft copolymerization by alkyl radicals, the grafting yield increased with either the plasma power or the plasma treatment time. High density polyethylene showed almost the same grafting yield as linear low density polyethylene at 5O’C. AcknoMled~emetlr-This work was financially by the National Natural Science Foundation (NNSFC).

supported of China

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,