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ACCURACY MEASUREMENT OF

GRAFTING DEGREE OF RADIATION-

INDUCED SURFACE GRAFT

POLYMERIZATION BY WEIGHING

Jingxin Lei,* Qiman Li, and Guangjian He

Polymer Research Institute of Sichuan University,

Chengdu 610065, Sichuan, Peoples Republic of China

ABSTRACT

Weighing by an electron analytical balance is one of the simple

and direct methods to measure the grafting degree of a polymer,

usually defined as:

Gm

wa2

wb=wb

100%

1

whereGm,wa, andwbrepresent the grafting degree, the weights of

sample after and before grafting, respectively. However, the

measured grafting degree by weighing is not accurate due to

omission of the weight losses during radiation and surface-graft

copolymerization in the reaction media. In this paper, corona

discharge-induced surface graft copolymerization of acrylic

amide (AAM) onto biaxial oriented polypropylene (BOPP) was

studied. The results show that the weight losses during corona-

discharge treatment and copolymerization in the AAM solution

are quite high compared to the measured grafting degree. The

weight loss increases with treatment time, AAM concentration,

51

Copyright q 2002 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. Fax: (862) 854-02465; E-mail: jxlei@pridns.scu.edu.cn

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reaction time, and temperature. The measured grafting degrees

according to Eq. (1) are all less than zero and decrease with the

reaction time and AAM concentration in the reaction solution,

conflicting with the x-ray photoelectron spectroscopy (XPS) and

infrared (IR) spectra of our previous work (1). After adding the

weight losses, the true grafting degree increases with reaction time

and AAM concentration, in agreement with the rule of common

graft polymerization. The true grafting degree must be described

as:

Gt Gm 1 Gi 1 Gs 2

where Gt, Gm, Gi, and Gs represent the true grafting degree,

measured grafting degree as described in Eq. (1), weight lossduring irradiation, and weight loss in the reaction media in wt%

based on the initial weight, respectively.

Key Words: Grafting degree; Measurement; Radiation-grafted

copolymerization; Weighing

INTRODUCTION

Surface grafting is one of the common methods to improve the surface

properties of polymer materials, e.g., to enhance printing, painting, and adhesive

properties. Glow discharge (2), corona discharge (3,4), ultraviolet light (5,6),

electron beam (7), and g-ray (8,9) radiation-induced surface graft polymeriz-ations are the more favored graft polymerization methods due to their high

efficiency and ease of processing. They introduce active sites (excited molecular,

free radical, peroxide in oxygen-containing atmosphere, etc.) in a polymer to be

used in graft polymerization subsequently. In order to meet the practical

requirements, a suitable grafting degree is the most important parameter for

surface-graft modification. A very low grafting degree cannot be effective

enough for modification of polymer materials, while a very high grafting degree

can decrease some properties and increase the cost. The grafting degree is also the

basis parameter for surface properties and structure analysis. Therefore, an

accurately measured grafting degree may be the most important parameter for

surface-graft modification of polymer materials in theory and practical

applications.

The surface-graft copolymerization has been demonstrated extensively by

various physicochemical methods such as x-ray photoelectron spectroscopy

(XPS) and infrared spectroscopy (IR), but the most simple and direct way to

determine the grafting degree is by weight measurement using an electron

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analytical balance. In this method (3,6 8), the grafting degree is almost defined

as the weight increase of the samples [Eq. (1)]. However, the weight losses during

radiation and copolymerization in the reaction media have been omitted in all the

relevant papers we obtained, which affects the measurement accuracy of the

grafting degree greatly.

It was well known that radiation etches the polymer and reduces its

weight, and when immersed in a certain solution, the additives and the

oligomers in the polymer materials may dissolve, which also leads to the

weight loss of polymer materials. Consequently, the measured grafting degree

as described in Eq. (1) is lower than its actual value. For surface

modification, the absolute amount of grafted monomers is usually quite low

in weight as compared to the bulk weight of samples because the grafting is

restricted only to the surface region. Under some graft conditions, themeasured grafting degree by weighting may be less than zero, but XPS, IR,

or other methods show that the graft copolymerization has taken place. The

true grafting degree is described in Eq. (2).

This article describes the corona discharge-induced graft copolymeriza-

tion of acrylic amide (AAM) onto biaxial oriented polypropylene (BOPP), by

which we intend to demonstrate that the measurement of grafting degree by

weight requires the consideration of weight loss during corona-discharge

pretreatment and polymerization in the graft reaction media.

EXPERIMENTAL

Materials

Biaxial oriented polypropylene film (0.02 mm thickness), a commercial

single layer film, was made through tenter frame process by Foshan East

Packaging Material Co., Foshan, China. Acrylic amide monomer used was of

chemical purity.

Preparation of Graft Sample

The AAM was recrystallized from benzene at 808C, the BOPP film was

coated with benezophenone sensitizer, and grafting was done by the same

procedures as described in the previous study (1). The grafted BOPP films were

washed with hot water at 60708C to remove homopolymers and then dried in a

vacuum oven at 508C for 48 hr. The grafting degree was measured from the

increase in weight of the sample as described in Eq. (1).

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X-Ray Photoelectron Spectroscopy Analysis

The XPS were obtained on a Kratos XSAM 800 spectrometer manufactured

by Kratos Co., Manchester, UK, using a monochromatic Al Ka

photon source.

RESULTS

The data listed in Table 1 show that with the increase in the AAM

concentration in the reaction media, the measured grafting degree increases, butis always less than zero. From Table 2, we can see that the measured grafting

Table 1. Effect of Acrylic Amide (AAM) Concentration on the Grafting Degree of the

Biaxial Oriented Polypropylene (BOPP) Film.

AAM Concentration (%) 5 10 15 20 25 30

Measured grafting degree

(wt%)

22.04 22.1 22.0 21.9 21.6 21.6

Weight loss of untreated BOPP

film during immersion in

AAM solution (wt%)

1.30 1.60 1.70 1.80 2.00 2.20

Weight loss during corona-

discharge treatment (wt%)

2.03 2.03 2.03 2.03 2.03 2.03

True grafting degree (wt%) 1.29 1.53 1.73 1.93 2.43 2.63

Corona-Discharge pretreatment: 2 min; Reaction temperature: 708C; Time: 1.5 hr;

Atmosphere: N2.

Table 2. Effect of Reaction Time on the Grafting Degree of Biaxial Oriented

Polypropylene (BOPP) Film

Reaction Time 0 min 10 min 30 min 1.0 hr 1.5 hr

Measured grafting degree (wt%) 22.03 22.5 22.5 22.52 21.9

Weight loss of untreated BOPP

film during immersion in AAM

solution (wt%)

0 0.70 0.90 1.50 1.80

Weight loss during corona-discharge

treatment (wt%)

2.03 2.03 2.03 2.03 2.03

True grafting degree (wt%) 0.00 0.23 0.43 1.01 1.93

Corona-Discharge pretreatment: 2 min; Reaction temperature: 708C; Acrylic amide

(AAM) concentration: 20%; Atmosphere: N2.

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degree is also less than zero but there is no significant effect on the

copolymerization reaction time. However, in our previous study (1), the XPS and

IR spectra demonstrate that the grafting of AAM onto BOPP has taken place as

expected, and the peel strength of the BOPP film adhered by a commercial 502

adhesive improved quite a lot after grafting. The C1score level scan XPS spectra

Figure 1. X-ray photoelectron spectroscopy of the grafted BOPP films with different

reaction times (corona-discharge pretreatment: 2 min; reaction temperature: 708C, AAM

concentration: 20%, atmosphere: N2) (a) 0 min; (b) 30 min; and (c) 60 min.

Table 3. Weight Loss of the Biaxial Oriented Polypropylene (BOPP) Film During

Corona-Discharge Pretreatment

Treatment Time (min) 0.5 1 2 3 5 10

Weight loss (wt%) 1.20 1.61 2.03 2.12 2.50 4.53

Voltage: 15 kV, at ambient temperature in air.

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of the starting BOPP film and the AAM-grafted BOPP films are shown in Fig. 1.The film surface at the beginning had only a strong alkyl carbon (}C}C})

peak at ,285.0 eV (Fig. 1(a)). The AAM-grafted surface shows one strong peak

at a higher binding energy (Fig. 1(b and c)). The peak of the high binding-energy

region for the grafted surface corresponds to a carbon atom with double bonds to

oxygen and one bond to nitrogen at ,288.0 eV (}(Ov)C}N} from amide

groups). This confirms that the film surface is covered with the grafted AAM. So

we believe that the deviations between the measured grafting degree and the true

grafting degree come from weight losses as discussed above.

From Table 3, we can see that during corona discharge, the weight loss of

BOPP film is very high. However, some of the grafting degrees are still less than

zero after adding the weight loss during radiation (Tables 1 and 2).

The data listed in Tables 1, 2, and 4 show that during immersion in the

AAM solution for copolymerization, the weight loss of the BOPP film increaseswith AAM concentration, reaction time, and temperature, respectively. So weight

losses in the AAM solution at certain conditions also should be added when the

grafting degree is determined.

From Tables 2 and 3, we can see that the true grafting degree, equal to

the measured grafting degree plus weight losses during corona-discharge

treatment and copolymerization in the AAM solution, increases with the

reaction time and the AAM concentration, agreeing with the rule of common

graft polymerization.

CONCLUSIONS

For radiation-induced surface graft copolymerization, the accuracy

measurement of grafting degree by weighing should consider the weight losses

during radiation and surface-graft copolymerization in the reaction media. The

accurate grafting degree must be described as in Eq. (2).

Table 4. Weight Loss of the Biaxial Oriented Polypropylene (BOPP) Film During

Immersion in Acrylic Amide (AAM) Solution at Different Temperatures

Temperature (8C) 30 60 70

Weight loss of untreated BOPP

film during immersion in AAM

solution (wt%)

1.40 1.60 1.80

Time: 1.5 hr; Acrylic amide (AAM) Concentration: 20%; Atmosphere: N2.

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ACKNOWLEDGMENTS

The authors are grateful to the National Nature Science Foundation of

China for supporting this research work (NSFC No. 29804007).

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