surface photografting polymerization of methyl methacrylate in n,n-dimethylformamide on low density...

Surface Photografting Polymerization of Methyl

Methacrylate in N,N-dimethylformamide on Low

Density Polyethylene Film

Peng Yang,1,2 Jianyuan Deng,1,2 Wantai Yang*1,2

1Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Department of Polymer Science,Beijing University of Chemical Technology, Beijing, 100029, ChinaFax: þ86-010-64416338; E-mail: [email protected]

2Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing, China

Received: December 20, 2003; Revised: February 16, 2004; Accepted: March 8, 2004; DOI: 10.1002/macp.200300248

Keywords: DMF-initiated photografting; methyl methacrylate; modification; polyethylene; surfaces

Introduction

Polymer surface modification and functionalisation is of

prime importance in polymer applications from the point of

view of both academies and industries.[1] By various

methods, surface chemical and physical structure are alter-

ed and controlled. In thesemethods, photografting iswidely

known to be useful due to its significant advantages: low

cost of operation, mild reaction conditions, selectivity to

absorb UV light without affecting the bulk polymer and

permanent alteration of surface chemistry.[2] In our labs and

other groups, several aspects of photografting are develop-

ed: kinetics,[2] new technology,[3] mechanism,[4] effects of

various factors,[5] new applications,[6–8] and new photo-

grafting system.[9–12] In this research, the development of

photoinitiator-free grafting system is very important, be-

cause the absence of residual/remanant photoinitiator in

photografting product enhances the stability of graft layer

largely. As some approaches to this, we have developed

auto-initiated photografting system of styrene (St) and

maleic anhydride (MAH).[9,10]

Recently, in the continuing research, it was found acci-

dentally by us that without photoinitiator, some monomers

in N,N-dimethylformamide (DMF) could be photografted

effectively onto polymer substrates. Furthermore, very high

grafting efficiency was achieved in the photografting of

methyl methacrylate (MMA). When some H-abstraction

type (type II) photoinitiators are used in surface photo-

grafting system, MMA has a low polymerization and

grafting reactivity due to its active allylic methyl hydrogen

and the following formation of allylic free radicals with low

initiation reactivity.[13] Excluding the auto-initiated possi-

bility of MMA, we herein found that DMF, as one of

standard organic solvents, induced this photografting.

Summary: It was found that without additional photoini-tiator, methyl methacrylate (MMA) dissolved in commonsolventN,N-dimethylformamide (DMF) could be photograf-ted steadily on low-density polyethylene (LDPE) film surfaceunder UV irradiation. In short irradiation time (4 min) andat room temperature, high grafting efficiency (approaching100%) and remarkable graft polymer amount (grafting per-cent is about 4.6%) was obtained. The possible reactionmechanismwas based on the photosensitivity of DMF, whichinduced this photografting polymerization. This finding isuseful to develop photoinitiator-free grafting or photopoly-merization system. Using SEM, special discrete globularstructure was found on theMMA-grafted LDPE film surface,and a possible model was proposed to interpret it.

Macromol. Chem. Phys. 2004, 205, 1096–1102 DOI: 10.1002/macp.200300248 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1096 Full Paper

In reports about photografting, DMF is often used as a

solvent of polar monomer such as acrylic nitrile (AN),[14]

N-isopropylacrylamide (NIPAAm).[6] Up till now, this phe-

nomenon has not been reported in any work. The funda-

mental finding showed that when DMF was used,

photografting could be achieved steadilywithout additional

photoinitiator, which is very useful to develop photoini-

tiator-freegrafting or photopolymerization system.Further-

more, we found globular protuberances were formed on

the surface of MMA grafted low-density polyethylene

(LDPE) substrate. This finding is expected to instruct

the preparation of functional surface having special surface

topography.

Experimental Part

Materials

Commercial LDPE film (85 mm in thickness) was cut intorectangle samples about 25 cm2 (5� 5 cm2), and then sub-jected to Soxhlet extraction with acetone for 24 h to removeimpurities and additives before use. DMF (AR grade, 99.5%)was purchased fromBeijing Chemical Reagents Company andused without purification.a MMA was obtained from BeijingChemical Reagents Company and purified by distillation toremove inhibitors.

Photografting Procedure

The apparatus used to perform grafting polymerization andthe setup of the film samples has been reported in detailelsewhere.[15] The main polymerization procedure was asfollows. A predetermined amount of mixture of MMA andDMF (the oxygen dissolved in the solution was removed bybubbling nitrogen gas through the solution) was deposited onthe bottom film with a microsyringe. The top film covered thissolution and the drop of solution was spread into an even andvery thin liquid layer under suitable pressure from a quartzplate. This assembly was named by us as ‘‘sandwich’’ struc-ture.[15] Then the assembly was laid on the holder and wasirradiated by UV radiation at room temperature from thetopside (a high-pressure mercury lamp, 1 000 W). The UVintensity at l¼ 254 nm was adjusted by changing the distancefrom the UV lamp to the film. After the irradiation, the twofilms were taken out, separated, dried to constant weight, andthen subjected to Soxhlet extraction with acetone for 8 h toremove homopolymers. Four parameters, grafting percent(GP), grafting efficiency (GE), total monomer conversionpercent (CP) and grafting conversion percent (CG) weredetermined according to the following equations:

GP ¼ ðMG=MSÞ 100% ð1Þ

GE ¼ ðMG=MPÞ 100% ð2Þ

CP ¼ ðMP=MMÞ 100% ð3Þ

CG ¼ ðMG=MMÞ 100% ð4Þ

whereMG is the mass of the grafted polymer,MP is the mass ofthe polymer formed (the total mass of the graft polymer andhomopolymer),MS is themass of blank LDPE substrate,MM isthe mass of the monomer.

Characterization Methods

Fourier Transformed Infrared spectra were recorded on aNicolet Nexus 670 spectrometer with 4 cm�1 resolution, avariable-angle attenuated total reflectance (ATR) accessory(PIKE ATRMax II) was utilized with ZnSe (n¼ 2.43) asinternal reflection element wafer. Gravimetric analysis wasperformed with BP211D electrobalance (Sartorius AG,Germany) with the accuracy being 0.00001g. Scanning elec-tron micrographs (SEM) were obtained with S250HK3(Cambridge) instrument.

Results and Discussion

Firstly, the direct chemical evidence ofMMAgraftedLDPE

sample was provided by ATR-FTIR spectra. As shown

in Figure 1, the absorption band of carbonyl[16] at about

1 730 cm�1 was present in the spectra of MMA grafted

LDPE sample, and the intensity of this band increased with

increasing GP.

Further, some factors affecting this photografting were

investigated. Figure 2 and Figure 3 showed the effects of the

volume ratio of DMF to MMA and irradiation time. At

certain volume ratio, GP increased with prolonging the

irradiation time to about 8min, and then slight decreasewas

observed, e.g., with the ratio being 1:4, GP increased from

1.32% to 4.67% when the irradiation time was elongated

from 2 min to 8 min. GP decreased from 4.67% to 3.53%

when the irradiation timewas prolonged further from 8min

to 16 min. At the initial stage, grafting reaction extent

increased with the increase of irradiation time, which was

reflected by the increase of GP. When irradiation time was

prolonged to certain extent, it was possible that grafted

PMMA chain photo-depolymerized under UV irradiation,b

a High pure DMF by distillation over phosphorus pentoxide wasalso used in same experiments, and similar phenomenon couldbe found with only some decrease of grafting percent andgrafting efficiency. So possible impurities in commercial DMFshould not be considered as initiating species.

b The depolymerization of PMMA under UV irradiation has beenproved and utilized in some fields such as photolithography, thefabrication of template etc. We sandwiched tetrahydrofuransolvent between poly(propylene) and the PMMA-graft-LDPEsample fabricated by this method, and irradiated this assemblyunder UV for 15 min. Gravimetric analysis showed there wasabout 1% weight loss in these irradiated samples.

Surface Photografting Polymerization of Methyl Methacrylate in N,N-dimethylformamide on Low Density Polyethylene Film 1097

Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

which resulted in the slight decrease ofGP.GE has tendency

similar to that ofGP, but reached amaximumvalue at 4min,

e.g. GE increased from 89% to 94% with irradiation time

varying from 2 min to 4 min, and then decreased from 94%

to 78%when irradiation timewas prolonged to 16min. This

meant that although graft polymer did not reach maximum

amount at 4 min, the increase extent of graft polymer was

lower than that of homopolymer when irradiation time was

prolonged further. Some groups have reported that when

photografted acrylic acid (AA) or styrene (St) onto LDPE

film, increasing temperature was favorable to the increase

of GE.[9] In DMF photografting system, very high grafting

efficiency (approaching 100%) and abundant graft polymer

(about 4.5%) were obtained at room temperature.

Keeping the total volume of DMF and MMA unchanged

(5 ml), effect of the volume ratio of DMF: MMA on graft

polymerization was investigated. At certain irradiation

time,GP andGE increased initially with the increase of the

ratio; when the ratiowasmore than 1:4,GP andGE began to

decrease. PMMA andMMA could be co-disssolved well in

DMF, as other experiments in our lab have shown. So this

tendency should not be attributed to heterogeneous PMMA

precipitation. A possible interpretation is as follows: at low

ratio, the amount of monomerwas abundant and the density

Figure 1. The ATR-FTIR spectra on the PMMA-g-LDPE sample: (1) Original LDPE (2)GP¼ 1.2% (irradiation time is 2 min) (3) GP¼ 4.4% (irradiation time is 4 min).

Figure 2. Effects of the volume ratio of DMF: MMA andirradiation time on GP (UV intensity is 5 600 mW/cm2).

Figure 3. Effects of the volume ratio of DMF: MMA andirradiation time on GE (UV intensity is 5 600 mW/cm2).

1098 P. Yang, J. Deng, W. Yang


of surfacemacroradical increasedwith the increase of DMF

amount,which could initiatemoremonomer to graft; higher

amount of DMF dilute the monomer liquid, so low amount

of monomer was graft polymerized, although higher

surface density of macroradical could be obtained.

For investigating the photografting polymerization

course in detail, the two polymerization parameters, CP

and CG, were followed. Figure 4 presents the effect of

irradiation time on CP and CG under certain conditions.

With prolonging irradiation time, CP and CG performed a

noticeable increase; after 8 min, CP and CG began to

decrease slightly. For example, when time was increased

from 2 min to 8 min, CP increased from 12.35 to 40.38%,

CG from 10.58 to 36.72%; when timewas increased from 8

min to 16 min, CP and CG decreased to 36.94 and 28.47%

respectively. From these results, we found explicitly that in

DMF photografting system, the amount of monomer per-

forming graft reaction was so close to that of performing all

polymerization reactions that very high grafting efficiency

was confirmed further.

For investigating the photografting mechanism in this

system, the analogous experiment was performed by apply-

ing pure monomer MMA without DMF under UV

irradiation, but the absorption band of carbonyl at about

1 730 cm�1 was not detected in ATR-FTIR spectra of the

samples which were Soxhlet extracted to remove out

homopolymer absolutely, and no increase in mass of these

samples after irradiation was found. According to above

experimental results, we considered that MMA could not

act as auto-initiator for photografting under UV irradiation,

although self-homopolymerization was possible. Carbonyl

group in DMF may be attributed to this photosensitivity,

since photochemistry of carbonyl group has been well

understood.[17] The photoinitiation mechanism in the

LDPE-AA surface photografting system of some ketones,

which contain similar carbonyl group, was investigated by

Yang and Ranby.[18] Referring to their study, the mechan-

ism of photografting polymerization of MMA in DMF

onto LDPE surface may be briefly outlined as follows

(Scheme 1). UnderUVirradiation, carbonyl absorbs energy

and undertakes dissociation or abstracts hydrogen atom

from LDPE surface. Accordingly, the cleavage produces

two free radicals including ketyl free radical and amine free

radical, the latter could initiate the formation of homo-

polymerMn; the abstraction hydrogen gives a ketyl radical

and a macromolecular free radical which can initiate

graft polymerization. According to the mechanism and the

experimental results above, it is concluded that: 1) reactions

(3) and (4) are themain reactions, since photografting could

effectively and steadily take place, and most of monomer

performing polymerizationwere added to graft chain (CG is

close to CP); 2) in reactions (3) and (4), ki4> ki3, and then

reaction (4) predominantly proceeds, so high extent of

grafting reaction could be achieved, which is reflected by

high GE.

For supporting the above mechanism, same photograft-

ing experiments were performed by using the solution of

MMA in tetrahydrofuran, which only contains methylene

and ether groups. As we expected, no grafting took place on

the sample surface, due to the absence of ester carbonyl

absorption band in ATR-FTIR spectra. Another possible

mechanism is that the growing PMMA abstracts hydrogen

from the methyl groups on the DMF and that this radical

then abstracts hydrogen from the surface. We replaced

DMFwith N,N-dimethylethanolamine (Acros, 99%) which

only contains N,N-dimethyl and alcohol groups. Under

Figure 4. Effect of irradiation time on CP and CG (the volumeratio of DMF to MMA is 1:4, UV intensity is 5 600 mW/cm2).

Scheme 1. The plausible reactionmechanismof photografting polymerization ofMMA inDMF onto LDPE film surface.



same photoreaction conditions, PMMA could not be graf-

ted onto LDPE sample surface, which was proved by ATR-

FTIR.

Further experiments found that besides MMA other

monomers such as vinyl acetate (VAc) andAAalso could be

photografted in DMF under UV irradiation, furthermore,

these monomer could also be photografted onto cast

poly(propylene) (CPP), biaxial oriented poly(propylene)

(BOPP) and poly(ethylene terephthalate) (PET) film. This

showed that when some monomers were dissolved in

DMF, an effective photografting system could be obtained

without additional photoinitiator.

Using ATR-FTIR spectra technique with variable inci-

dence angle, we found that unusual distribution order of

graft polymer existed in the surface of LDPE samples.[19] In

Figure 5, for the same grafted LDPE sample, with incidence

angle increasing, the absorption intensity of ester carbonyl

band decreased. This showed explicitly that more graft

polymer entered the inner surface of LDPE. Similar phe-

nomenon was not found in BOPP and CPP grafted samples.

As shown in Figure 6, for LDPE, the intensity ratio of ester

carbonyl to methylene group decreased with incidence

Figure 5. The ATR-FTIR spectra with variable incidence angles on the PMMA-g-LDPEsample (1) incidence angle¼ 458 (2) incidence angle¼ 558 (3) incidence angle¼ 658.

Figure 6. The profile analysis for grafted PMMAdistribution onthe different polymer samples.

Figure 7. The swelling degree of DMF for different substrate.Measuring method is as follows: soaked polymer substrates intoDMF for a given time, took out them and then removed outabsolutely and quickly DMF absorbed on substrate surface byusing filter paper, following by gravimetric measurement. Dg isdefined as the absorption amount of DMF per square centimeter,Dg¼ (M1�M0)/S, where M1 is the mass of substrate afterswelling by DMF, M0 is the mass of original substrate beforeswelling, S is the surface area of original substrate.



angle increasing, while reverse relationship was found in

BOPPandCPP samples (the intensity ratio of ester carbonyl

to tertiary carbon-hydrogen bending band at 1 157cm�1

was used for these two samples). This presented explicitly

that for LDPE: the deeper the sampling depth is, the more

the graft polymer could be detected; for other substrate: this

order was reversed.

Direct swelling experiment to different polymer sub-

strate by DMFwas performed to interpret the above pheno-

menon. Figure 7 illustrated the swelling extent of different

polymer substrate in DMF. With soaking time prolonging,

swelling degree (Dg) increased steadily and reached a

plateau after certain time. Apparently, eitherDg or swelling

velocity to LDPE was higher largely than PP (BOPP and

CPP). During grafting course, higher swelling degree faci-

litated more MMA into inner surface, and then more graft

chains were formed locally; on the other hand, low swelling

extent to PP favored that the grafting was confined within

outer surface. PP has higher crystallinity than LDPE, which

was possibly attributed to its lower swelling degree.

The surface topography of grafted polymer sample was

investigated by SEM. As shown in Figure 8, globular pro-

tuberances are dispersed evenly on the grafted LDPE sur-

face, while this structure did not appear on grafted

PP surface. This phenomenon was interesting and unusual.

Considering the possible morphology changes during

Figure 8. The globular protuberances on grafted LDPE samples (a) original LDPE film(b) PMMA-grafted LDPE film (GP¼ 1.32%).

Figure 9. The globular protuberances on grafted LDPE samples with different irradiationtime (a) 2 min, GP¼ 1.32% (b) 4 min GP¼ 4.21% (c) 8 min GP¼ 4.87% (d) 16 min,GP¼ 3.5%.



Soxhlet extraction with acetone, we examined the mor-

phology of grafted samples before extraction, but no

observable globular protuberances were found. Apparently,

for crystallinity difference, LDPE chains have more

freedom to move than PP chains, so solvent acetone

induced easily the aggregation of MMA grafted LDPE

region to form globule, while it was difficult to do this in

grafted PP region.

Figure 9 presents the SEM of MMA-grafted LDPE

samples with different irradiation time. We found that with

prolonged irradiation time, the average diameter of globular

protuberances decreased and grafting density increased

with a congregate tendency. Longer irradiation time initia-

xted more DMF to take part in reactions, which resulted in

the increase of grafting density; in the meantime, longer

irradiation time provided DMF with a chance to swell

LDPE to a lager extent, which induced that more MMA

were transported into deeper region from outer surface and

then more PMMA graft polymer were formed locally.c So

the decrease of average diameter actually meant more graft

polymer penetrated into inner surface. As a result, the aver-

age diameter decreased from 500 nm to 170 nm (Figure 9a

to 9c); furthermore, in Figure 9d, no observable globular

structures were seen although GP of this sample reached

about 3.5%. With graft reaction proceeding, grafted area

absorbed more active monomer than other areas due to the

polar similarity between graft PMMA and MMA, which

resulted in the congregating of grafting sites.

Conclusions

Herein we reported that without additional photoinitiator,

MMA dissolved in common solvent DMF could be photo-

grafted steadily onLDPEfilm surface under UVirradiation.

In the devised DMF photografting system, very high graft-

ing efficiency (approaching 100%) and remarkable graft

amount (the maximum of grafting percent is about 4.6%)

were obtained. The grafted LDPE surface had discrete

globular protuberances, while this structure did not appear

on other polymer surface, such as PP and PET, where

common topography of grafted sample was observed.

Acknowledgement: We thankChinese State Outstanding YouthFoundation (20025415) for financial support of this work.

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[2] H. Ma, R. H. Davis, C. N. Bowman,Macromolecules 2000,33, 331.

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[4] W. T. Yang,M. Z. Yin, Y. F. Sun,Chin. J. Polym. Sci. 2000, 5,431.

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c Direct evidencewas provided by following experiment: we pre-soaked the films in the solution of MMA in DMF (v/v¼ 4:1) fordifferent time, following themeasurement of absorption amountaccording to the procedures described in the caption of Figure 7and photografting reaction. The results showed that withsoaking time prolonging, absorption amount increased and alarger GP could be obtained. For instance, when soaking timeprolonged from 5min to 15min, absorption amount varied from0.09 to 0.2 mg/cm2, and the resulting GP increased from 0.06%to 0.10%. Relative to the deposition amount on sample surface,the absorption amount in inner surface is much lower, so theresulting GP is much lower than that obtained by the methoddescribed in Experimental Part.



surface photografting polymerization of methyl methacrylate in n,n-dimethylformamide on low density...

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