Modification of cotton-cellulose by preirradiation grafting

Nuclear Instruments and Methods in Physics Research B 236 (2005) 259–265

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Modification of cotton-cellulose by preirradiation grafting

E. Takacs a,*, L. Wojnarovits a, J. Borsa b, J. Papp a, P. Hargittai a, L. Korecz c

a Institute of Isotopes and Surface, Chemical Research Center (CRC), Hungarian Academy of Sciences (HAS),

P.O. Box 77, H-1525 Budapest, Hungaryb Budapest University of Technology and Economics, M}uegyetem rkp. 3, H-1111 Budapest, Hungary

c Institute of Structural Chemistry, Chemical Research Center (CRC), Hungarian Academy of Sciences (HAS),

P.O. Box 17, H-1525 Budapest, Hungary

Available online 19 May 2005

Abstract

Acrylamide (AAm), acylic acid (Aac), 2-hydroxypropil acrylate (HPA), 2-hydroxypropil methacrylate (HPMA) and

N,N 0-methylene bisacrylamide (BAAm) were grafted to cotton-cellulose by the preirradiation method. The samples

were characterized by ESR, degree of polymerization (DP) and tensile strength measurements, by FTIR, gravimetry,

X-ray diffraction and iodine sorption. The grafting yield was strongly monomer dependent: the highest yield was found

for HPMA, for HPA a medium yield, whereas for the others relatively low values were measured. Grafting improved

the swelling of the samples. The highest increase in swelling was observed on samples grafted with Aac and BAAm

showing saturation at about 270% at natural pH. For AAm, HPA and HPMA at low grafting yield an increase in swell-

ing was found and then the swelling decreased with increasing grafting yield. No direct connection was found between

the grafting yield and the swelling properties.

� 2005 Elsevier B.V. All rights reserved.

PACS: 82.50; 61.80; 61.16.B

Keywords: Radiation grafting; Cotton-cellulose; Hydrogel; ESR; X-ray diffraction; Iodine sorption; SEM

1. Introduction

Hydrogels are often produced by irradiation

techniques due to the known advantages: the syn-

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.nimb.2005.03.248

* Corresponding author. Tel.: +36 1 392 2222x3131; fax: +36

1 392 2548.

E-mail address: [email protected] (E. Takacs).

thesis carried out in the absence of catalysts and

initiators moreover polymerization and crosslink-

ing may occur simultaneously. In many applica-

tions it is a disadvantage that the mechanical

strength of hydrogels is rather poor. In order toovercome this difficulty hydrogels are often synthe-

sized by grafting on a surface of a polymer e.g. cel-

lulose with good mechanical stability [1]. Grafting

ed.

mailto:[email protected]

260 E. Takacs et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 259–265

may also be carried out by irradiation techniques.

In the literature there are quite a number of publi-

cations which demonstrate that monomers and

among them also hydrogel forming monomers

can be grafted onto cellulose surface by radiationinitiation. Most of the works published on this field

report on grafting of cellulose with one monomer

e.g. styrene, or N-isopropil acrylamide [2–7].

We have been studying for several years the

radiation degradation and modification of cellu-

R R1

Aac Acrylic acid H OH

HPA Hydroxypropil acrylate H OCH2CH(OH)CH3

HPMA Hydroxypropil methacrylate CH3 OCH2CH(OH)CH3

AAm Acrylamide H NH2

BAAm N,N 0-methylene bisacrylamide H NHCH2NHCOCH@CH2

lose, concentrating mostly on the chemical

changes. To observe these changes we usually ap-

plied higher doses in the range of 50–1000 kGy.

We found oxidation and at the highest doses ap-

plied also changes in the crystalline structure [8–

10]. In this work we prepare hydrogels by grafting

several monomers to cellulose by the preirradia-

tion grafting method and characterize the hydro-gels by a large variety of methods and compare

their properties. Our aim is to prepare grafted cel-

lulose with good hydrogel properties.

2. Experimental

2.1. Materials

Different types of acrylic monomers were in-

volved in the present studies which are known to

form hydrogel by radiation induced polymeriza-

tion [7,11]. We selected an acidic monomer (acrylic

acid), an acrylic acid ester (hydroxypropil acry-

late), a methacrylic acid ester (hydroxypropil

methacrylate) and two acrylamide type mono-mers, one of them was monofunctional (acrylam-

ide itself) and the other was bifunctional (N,

N 0-methylene bisacrylamide). The structures of

the monomers studied and the abbreviations ap-

plied are shown below.

Cotton fabric samples (163 g/m2, Type 405 W,

Test Fabric Inc., USA) were washed first in 1%

acetic acid solution at 50 �C for 5 min, then in dis-

tilled water and the samples were dried. The

monomers were Scientific Polymer Products. The

crystalline monomers (AAm, BAAm) were recrys-

tallized from methanol to remove the stabilizator,

the liquids (Aac, HPA, HPMA) were treatedwith the column supplied together with the mono-

mers. The reagents were of analytical grade. Puri-

fied water was obtained from an ion exchanger

equipment type ELGA Option 4.

2.2. Grafting procedure

Cotton-cellulose fabric samples were irradiatedin air, at room temperature by Co-60 gamma rays

up to 40 kGy dose (dose rate 15 kGy/h). Immedi-

ately after irradiation the samples were immersed

0 10 20 30 40

200

400

600

800

1000

1200

1400

1600

1800

0

100

200

300

400

500

600

DP

Dose, kGy

Tensile strength

DP Ten

sile

str

engt

h, N

Fig. 1. Dose dependence of the degree of polymerization (DP)

and that of the tensile strength (measured on 5 cm wide

samples).

E. Takacs et al. / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 259–265 261

in 5% monomer solutions (water/methanol = 80/

20) at 40 �C for 4 h. N2 bubbling was applied to

deoxygenate the monomer solutions. Bubbling

was started about half hour before grafting and

continued during the whole grafting procedure.Grafted samples were immersed in distilled water

and washed carefully. The remaining monomer

was then removed by extraction in boiling distilled

water for 6 h.

2.3. Characterization of samples

Radicals remaining after irradiation were de-tected by an ESR instrument type JEOL JES-

FE3X. The determination of the degree of

polymerization (DP) was based on the viscosity

measurements of the sample dissolved in cuprie-

thylenediamine (Cuen) solution [12]. The tensile

strength was measured on fabric samples of 5 cm

width using Instron type equipment.

Grafting yield (=100% · (wg � w0)/w0) wasdetermined by weighting the samples before (w0)

and after (wg) grafting. A FTIR instrument Uni-

cam Mattson Research Series 1 with diffuse reflex-

ion detection was also used to follow the grafting.

Swelling (=100% · (ws � wg)/wg) was determined

by weighting the samples after grafting and after

soaking in distilled water (ws) for 1 h.

The accessible surface was determined by iodine

sorption measurement. This method is often used

for characterizations of cellulose accessibility.

Measurements were carried out by dropping

1.2 ml iodine solution on 200 mg cotton fabric

sample. (The iodine stock solution was prepared

by dissolving 40 g KI and 5 g iodine in 50 ml dis-

tilled water.) After 3 min the sample was put into

100 ml saturated Na2SO4 solution at 22.5 �C(thermostated) for 1 h. The amount of iodine ad-

sorbed on cellulose fibre surface was calculated

after titration by Na2S2O3 solution.

The samples used for grafting yield determina-

tion, swelling and iodine sorption measurements

always were in equilibrium with the laboratory

atmosphere therefore contained absorbed water.

This water content was always determined by sep-arate measurement comparing the masses of the

equilibrated sample and heat box dried sample.

The results of these determinations were applied

in grafting, swelling and iodine sorption measure-

ments for correction of the actual mass of samples.

The X-ray diffraction patterns were taken on

samples of 20 · 30 mm using a Philips instrument

equipped with a Bragg–Brentano parafocusinggoniometer, secondary beam graphite monocho-

mator and proportional counter. The scanning

electron microscopic pictures (SEM) were taken

on single fibres coated with Pt layer using JEOL

J SM 5600LV equipment.

3. Results and discussion

3.1. Irradiation of cotton-cellulose

We characterized the irradiated samples by tak-

ing SEM micrographs, measuring the degree of

polymerization and the tensile strength; ESR and

X-ray diffraction measurements were also applied.

As Fig. 1 shows the degree of polymerization de-creases upon irradiation of the samples from an ini-

tial value of 1680 to 480 after 10 kGy irradiation

and to 230 after 40 kGy irradiation. This decrease

of the DP value reflects radiation induced fragmen-

tation of the polysaccharide chain. It is interesting

to note, however, that this fragmentation is not

accompanied by changes in the mechanical proper-

ties of the cotton. Neither the SEMpictures (Fig. 2),nor the tensile strengthmeasurements (Fig. 1) of the

Fig. 2. SEM pictures of untreated cellulose fibres, fibres

irradiated with a dose of 40 kGy and those of irradiated and

grafted.

3200 3300 3400 3500

-0.004

-0.002

0.000

0.002

ES

R s

igna

l, ar

bitr

ary

units

Magnetic field (Gauss)

40 kGy 30 kGy 20 kGy 10 kGy 5 kGy

0 5 10 15 20 25 30 35 40 450

500

1000

1500

2000

2500

Rad

ical

con

cent

ratio

n, a

rbitr

ary

units

Dose, kGy

(A)

(B)

Fig. 3. ESR signal of samples (A) and dose dependence of the

ESR (B) signal 4 h after irradiation.


irradiated samples indicate considerable deteriora-

tion as compared to the unirradiated samples.The samples were irradiated in plastic bags with

large excess of air. Under such conditions, accord-

ing to the literature suggestions, the radicals formed

on the polymer chain are expected to transform to

ROOH and ROOR 0 type hydroperoxides and per-

oxides. However, in our ESR measurements made

4 h after the irradiation we detected radical species

in the samples (Fig. 3(A)). The concentration of

these radials was found to increase with the increas-

ing dose as shown in Fig. 3(B). The signals are

mainly due to peroxy radicals with smaller contri-bution from other radical species. Radical signal

was well detectable even after a month�s storage

at room temperature, however, its intensity de-

creased and we noted some changes in the shape

indicating altered radical structures. The observa-

tion of radicals in the samples after irradiation in

the presence of oxygen indicates that initiation of

polymerization in the grafting experiments mayoccur both, by means of the unreacted radicals

observed in ESR measurements and also by the

thermal decomposition of labile hydroperoxides

and peroxides.

Chemical Research Centre - Laboratory of Diffraction

0

10

20

30

40

50

60

70

80

90

100

Inte

nsi

ty

[Arb

itra

ry u

nit

s]

517.674

108.845

155.906

204.440

253.562

302.979

352.564

2*theta

—— untreated—— AAm—— AAc—— HPA—— HPMA—— NNMBAAm

Fig. 4. X-ray diffraction patterns of untreated (unirradiated,

ungrafted) and grafted samples. Preirradiation dose of the

grafted samples 40 kGy.

0 10 20 30 400

25

50

75

Gra

fting

yie

ld, %

HPMA

BAAm

AAc

HPA

AAm

Gra

fting

yie

ld, %

Dose, kGy

100

200

300

Fig. 5. Grafting yield as a function of preirradiation dose.


Fig. 4 shows the X-ray diffractions patterns of

some of our samples (untreated means unirradi-

ated and ungrafted, the other samples were graftedafter irradiation with 40 kGy). We found no differ-

ences in the diffraction taken on unirradiated sam-

ples and on samples irradiated with the maximum

dose (40 kGy) used in the present experiments. In

another experiment [8] we noted changes in the

crystallinity after irradiation with very high doses

of about 1000 kGy. On the figure the red vertical

lines show places of diffraction maxima belongingto the crystalline phase of cotton-cellulose, and the

blue vertical line shows the center of the diffuse

peak of the amorphous region. In both the irradi-

ated and unirradiated samples the crystalline

content was about 60%.

3.2. Grafting

The grafted samples were characterized by the

grafting yield, by the accessible surface using

iodine sorption measurements, by the crystalline

content as measured by X-ray diffraction, by

SEM pictures and also by FTIR. The grafting

yields as measured by gravimetry increased nearly

linearly with the preirradiation dose (Fig. 5). By

FTIR the absorption peaks at 1740 cm�1 and as-

signed to the stretching vibrations of C@O group

were used to characterize the grafting yield. The

band at 2900 cm�1 assigned to the stretching

vibrations of aliphatic C–H was used as inner stan-

dard. As all the monomers grafted contained C@Ogroup the absorbance at 1740 cm�1 increased with

increasing grafting yield. The results were in good

agreement with those obtained by gravimetry. As

the SEM picture shows (Fig. 2) for hydroxypropil

acrylate (HPA) the grafted layer nearly homoge-

nously covers the surface of the fibres. In the cases

of acrylamide (AAm), acrylic acid (Aac) and N,N 0-

methylene bisacrylamide (BAAM) at the highestpreirradiation dose applied the percentage increase

of the sample mass due to grafting was between

25% and 35%, while for HPA this figure was

�70%. The highest grafting yield of about 270%

was measured for hydroxypropyl methacrylate

(HPMA). This result is surprising, since the meth-

acrylates generally have at least an order of magni-

tude smaller rate coefficients of propagation thanthe acrylates, but the termination rate coefficients

during their polymerizations are practically the

same as for the acrylates. Under grafting condi-

tions termination between two radicals on the

chains attached to the cellulosic fibres is hindered.

Due to the higher stability of the tertiary radicals

in methacrylates, than the secondary radicals in

acrylate polymerization termination by impuritiesis expected to be slower in methacrylates. We guess

that the initiation rate is higher for methacrylates

Fig. 6. Comparison of grafting yield, iodine sorption and

swelling of untreated and grafted samples. Preirradiation dose

30 kGy.

0 50 100

150

200

250

300

Grafting yield, %

BAAmAAc

HPA

AAm

HPMA

Sw

ellin

g, %

0 100 200 300

Grafting yield, %

Fig. 7. Swelling as a function of grafting yield.


than for acrylates again due to radical stability

reasons.The X-ray diffraction method we applied could

not be used for obtaining quantitative results;

however, it gives valuable qualitative information.

Due to grafting the mass of the sample increases

and the percentage of the cellulose fraction and

also the crystalline fraction decreases, at the same

time the rather ill-defined wide reflection of the

amorphous peak increases. On samples graftedby HPMA the amorphous region is very wide

and the crystalline peaks are rather weak. Since

changes in the crystalline region are not expected,

we suppose that grafting modifies not only the

amorphous regions but also the transient region

between the amorphous and crystalline phases.

In all the grafted samples we observed decrease

of the diffraction attributed to the crystalline phaseand strengthening of the amorphous reflexions.

The iodine sorption measurements in Fig. 6

indicate larger accessible surfaces for the grafted

samples than that for the untreated cotton fabric

samples. In our case iodine sorption characterizes

the accessible surface of the cotton and those of

the grafted polymer together. The change in acces-

sible surfaces is the highest for the HPMA andHPA grafted samples. For these samples we found

the largest grafting yields. It seems that iodine can

penetrate to the grafted parts too and iodine sorp-

tion measures both the cellulose accessible surface

and the accessible surface of the grafted part. In

the case of Aac and BAAm grafted samples the

iodine sorption measurements gave very low val-

ues. We think that the grafted part of these sam-

ples is more compact in the case of Aac due to

the H bonding network and in the case of BAAm

due to the crosslinks. For these reasons the huge

iodine molecules cannot penetrate into thenetwork.

3.3. Swelling

The swelling of cellulose in water is limited as a

consequence of the presence of crystalline regions

which restrict the mobility of the polymer chains

[11]. For the untreated cellulose we measured aswelling of 140% (see Fig. 7). The grafting in-

creased the swelling for all monomers used in the

experiments. In the case of Aac, and BAAm the

swelling at lower grafting yield increased with

grafting and saturated at 270%. For HPA and

AAm the swelling percentage at low grafting in-

creased to about 220% and then gradually de-

creased. With HPMA the swelling we measuredwas 180% and again a decrease was found at high-

er grafting percentages. These swelling characteris-

tic should be correlated with the structure, the

pore sizes and also the hydrophilicity of the

grafted layers. The characterization of the struc-

ture and pore size of these samples needs further

investigations.


In the case of BAAm the grafted layer itself

could easily be crosslinked and the distance be-

tween the two double bonds ensures a certain min-

imum pore size resulting in good swelling

properties. In the case of Aac the partial ionizationof the COOH groups and the network formed by

H bonds increases the swelling. The hidrophylicity

of these two monomers is higher that those of the

other monomers. The swelling measured for Aac

grafted samples is in accordance with literature

value obtained at low pH values [11], at high pH

there is an increase in swelling which is attributed

to the complete ionization of the polyacrylic acidchain. The estimated pK of this dissociation is

between pH 4 and pH 5. The pH dependence also

needs further investigations.

In the case of HPMA, HPA and AAm the

decrease in the swelling after the maximum is

probably due to the gradual formation of a closed

compact structure on the surface which hinders the

penetration of water.

4. Conclusions

1. The radiation degradation of cotton-cellulose

starts at very low doses (5–10 kGy) resulting

in decrease of DP. However, the degradation

does not result in a significant change in themechanical properties: irradiation up to

40 kGy dose did not change the tensile strength,

and the SEM pictures did not indicate visually

observable damage of the samples either.

2. Long lived radicals were detected after irradiat-

ing cellulose fabric samples in air. These radi-

cals decayed on several months time scale.

The initiation of the preirradiation graftingprobably takes place by the decomposition of

peroxides and also by the long living radicals.

The grafting yield was strongly monomer

dependent. The highest grafting yield was mea-

sured for HPMA, for HPA we measured a med-

ium yield, whereas for the other monomers we

measured relatively low yields.

3. Grafting results in breakage of secondary Hbonds as indicated by an increased available

surface characterized by iodine sorption. How-

ever, a decreased available surface was mea-

sured on samples grafted by Aac due to

strong H bridges formed between the side

chains and the cellulose chains.

4. The relatively low swelling of cellulose can be

improved by grafting: water uptake increasedfor all of the grafted samples studied. The high-

est increase in swelling was observed with Aac

and BAAm showing saturation at about

270%. For all the others at low grafting yield

an increase in swelling was found and then the

swelling decreased with increasing grafting

yield. It seems that there is no direct connection

between the grafting yield and the swellingproperties.

Acknowledgments

We express our thanks to the Hungarian Sci-

ence Foundation for support (OTKA, No. T 037363, T 034 478 and T 037 294) and to I. Sajo for

the X-ray diffraction measurements.

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Modification of cotton-cellulose by preirradiation grafting

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