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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
Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
Surface Photografting Polymerization of Methyl Methacrylate in N,N-dimethylformamide on Low Density Polyethylene Film 1099
Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
1100 P. Yang, J. Deng, W. Yang
Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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%.
Surface Photografting Polymerization of Methyl Methacrylate in N,N-dimethylformamide on Low Density Polyethylene Film 1101
Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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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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.
1102 P. Yang, J. Deng, W. Yang
Macromol. Chem. Phys. 2004, 205, 1096–1102 www.mcp-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim