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					Polymer International                                                                                    Polym Int 51:1111–1116 (2002)
                                                                                                                  DOI: 10.1002/pi.1045

Formation of polyacrylate brushes on silica
J Parvole,* L Billon and JP Montfort
                                       `res, UMR 5067, He
Laboratoire de Physico-Chimie des Polyme                                ´ne
                                                        ´lioparc Pau-Pyre ´es, 2 Av P Angot, 64053 Pau Cedex 09, France

    Abstract: We report the formation of polyacrylate (molten state polymers: Tg < room temperature)
    monolayers attached onto silica surfaces using covalently bonded initiators for radical-chain
    polymerisation. In a first reaction step, the initiator is self-assembled on the surface. In a subsequent
    reaction, the initiator is activated and the polymer formed in situ at the surface of the substrate with
    high grafting density (‘grafting from’ procedure). Moreover, the use of a living free radical process
    permits the molecular weight and polydispersity of the polymer chains to be controlled, and thus the
    polymer monolayer thickness also. We have also made a preliminary qualitative characterisation by
    X-ray spectroscopy (XPS) and Fourier transformed infrared (FTIR) measurements.
    # 2002 Society of Chemical Industry

Keywords: controlled radical polymerisation; polyacrylate brushes; silica

INTRODUCTION                                                          Ruhe.7–9 In this case, a performed functionalised azo
The control of surface properties is central to many                  initiator was synthesised and covalently attached to a
areas of research and in numerous commercially                        variety of solid surfaces. Using normal free radical
important technologies ranging from biotechnology to                  polymerisation conditions, linear chains were then
advanced microelectronics.1–3 One method that has                     grown from the surface to give covalently attached
been employed for controlling surface properties is                   polymer brushes with high graft densities and mol-
the utilisation of polymeric brushes. Polymer brushes                 ecular weights (Scheme 1). The advantages of this
refer to an assembly of polymer chains which are                      approach are that the density of initiator groups at the
tethered by one end to a surface or an interface.3–6                  surface can easily be varied, and functionalised
Tethering is sufficiently dense so that polymer chains
are crowded and forced to stretch away from the
surface or interface to minimise free energy. This
situation, in which polymer chains stretch along the
direction normal to the grafting surface, is quite
different from the typical behaviour of flexible
polymer chains in solution, where chains adopt a
random-walk configuration, and from the conforma-
tion of chains physically adsorbed onto surfaces. A
series of experimental results showed that the
deformation of densely tethered chains affects many
aspects of their behaviour and results in many novel
properties of polymer brushes.4 These polymer
brushes can be accomplished in a number of ways,
including a ‘grafting to’ approach which involves the
condensation of a functionalised polymer with the
reactive surface groups of an appropriate substrate,
but it is limited by the crowding of chains at the
surface, which hinders the diffusion of chain ends to
the surface for further attachment.1
   Another possible route is via the ‘grafting from’                  Scheme 1. Concept for the synthesis of covalently attached polymer
technique. Perhaps the best approach to the synthesis                 monolayers via radical graft polymerisation (grafting from technique) using
of polymer brushes is the strategy of Prucker and                     immobilised initiators.

* Correspondence to: J Parvole, Laboratoire de Physico-Chimie des Polyme `res, UMR 5067, He                 ´ne
                                                                                            ´lioparc Pau-Pyre ´es, 2 Av P Angot, 64053
Pau Cedex 09, France
  Poster presentation – Session B: Paper presented at the Polymers in the Third Millennium Conference, 2–6 September 2001, Montpellier,
(Received 18 September 2001; revised version received 4 April 2002; accepted 29 April 2002)

# 2002 Society of Chemical Industry. Polym Int 0959–8103/2002/$30.00                                                                        1111
J Parvole, L Billon, JP Montfort

Scheme 2. Synthesis of the coupling

polymer chains can readily be prepared. However, the        control agent (the grafting of this coupling agent will
traditional free radical procedures precludes control of    be published in a forthcoming paper). Moreover, we
the polymer monolayer thickness and polymer struc-          realised kinetic studies of the free radical polymerisa-
ture, so that we decided to use controlled radical          tions, with the two initiators, at different temperatures,
polymerisation (CRP).                                       concentrations and with a slight excess of counter-
   Living free radical polymerisations have been a topic    radical in order to adjust these three parameters as a
of considerable interest in recent years and have a         function of molecular weight.
number of advantages over traditional free radical             We report the formation of poly(n-butyl acrylate)
procedures.10–15 The controlled nature of the poly-         (PBA) and poly(ethyl acrylate) (PEA) monolayers
merisation process permits structural characteristics       covalently attached to silica surface by the ‘grafting
(molecular weight, polydispersity, etc) of the polymer      from’ procedure. The coupling agent used (Scheme
brush to be varied readily. Another benefit is the ability   2), consisting of anchoring group, cleavable group and
to prepare block copolymers by sequential activation        initiator (Scheme 1),7 contains two triethoxysilyl
of the dormant chain ends in the presence of different      moieties (anchoring group); Peng et al 16 showed that
monomers.                                                   a bidentate initiator can be attached to the surface,
   In this paper, prior to polymer grafting, we tested      either just on one side or on both ends, depending on
reactions of n-butyl acrylate (nBA) and ethyl acrylate
(EA) with two differents initiators equivalent to
coupling agents; the first one consists of a bimolecular
system, azoisobutyronitrile (AIBN) as initiator and
propylnitroxyl (DEPN or SG1) as control agent (SG1
counter-radical; Scheme 2), and the second one of an
alkoxyamine        (N-tert-butyl-N-1-diethylphosphono-
2,2-dimenthylpropyl-0,1-methoxycarbonylethylhy-             Scheme 3. Monofunctional alkoxyamine MONAMS and SG1 counter-
droxilamine (MONAMS); Scheme 3) as initiator and            radical chemical structure.

1112                                                                                       Polym Int 51:1111–1116 (2002)
                                                                                       Polyacrylate brushes on silica surfaces

the reaction conditions. However, apart from slight          a dichloromethane solution of the product over
changes of the molecular weight of the surface-              anhydrous sodium sulfate; the sodium was washed
attached polymer, it does not strongly influence the          with dichloromethane, and the dichloromethane was
formation of brushes.                                        removed under reduced pressure.

                                                             Immobilisation of the initiator on the silica surface
EXPERIMENTAL SECTION                                         Under an atmosphere of dry nitrogen, a solution of 1 g
Materials                                                    of the coupling agent in 40 ml of toluene was added to
Silica (99.8%, fumed) with a specific surface area of         a suspension of 1 g of silica in 50 ml of toluene. The
255 m2 gÀ1 was dried overnight at 120 °C under               mixture was stirred for 8 h. The modified silica
vacuum. Toluene was distilled under a nitrogen               particles were washed and centrifuged with toluene,
atmosphere from over molten sodium. MONAMS                   ethyl alcohol, acidified (HCl) ethyl alcohol/water (1/1
alkoxyamine and SG1 counter-radical were used as             v/v, pH = 3), ethyl alcohol/water (1/1 v/v), ethyl alcohol
received from ATOFINA. All other solvents and                and diethyl ether. The nanoparticles was dried over-
chemicals were used without further purification.             night in vacuum at room temperature.7

Characterisation                                             Polymerisation experiments
X-ray photoelectron spectroscopy analyses were per-          Into a dry round flask were introduced SG1 (slight
formed with a Surface Science Instrument (SSI)               excess $ 5%), MONAMS or AIBN and the required
spectrometer at room temperature, using a mono-              amount of n-butyl or ethyl acrylate monomer under
chromatic and focused (spot diameter of 600 mm,              inert atmosphere. The solution was then thoroughly
100 W) Al Ka radiation (1486.6 eV) under a residual          degassed before transferring it by cannula in several
pressure of 5 Â 10À8 Pa. The hemispherical analyser          schlenks, and the polymerisation mixture was then
worked in constant pass energy mode, 50 eV for high          heated to the desired temperature (100–120 °C). The
resolution spectra and 150 eV for quantitative analysis.     conversion was determined by gravimetry after pre-
The binding energy scale was calibrated from the             cipitation of the sample by EtOH. The polymer
carbon contamination using the Cls line (284.6 eV) (a        obtained was dried under vacuum for several hours
mean atomic percentage of 8% was determined).                prior to characterisation.
   The Fourier transform infrared spectra were re-
corded using a Bruker IFS 66/S spectrometer at a             Formation of polymer monolayer
resolution of 4 cmÀ1 in absorption mode. One hundred         Under inert atmosphere, 1 g of modified silica particles
to 1000 scans were accumulated.                              was suspended in a mixture of acrylate monomer,
     H NMR spectra were recorded at 400 MHz on a             AIBN ([Acrylate]/2[AIBN] = 400–800) and SG1
Bruker Advanced AM400 spectrometer in CDCl3 and              (slight excess). This mixture was thoroughly degassed
the chemical shifts (d) in ppm are referred to internal      and heated at 120 °C for chosen periods of time (0.5–
trimethylsilane (TMS).                                       10 h). The non-attached polymer was then removed by
   Size exclusion chromatography (SEC) characterisa-         the Prucker and Ruhe procedure7 with toluene
tion was performed using a 2690 Waters System with           (acrylates good solvent). The molecular weight of the
THF as eluent and a calibration curve established with       non-bonded polymer was determined by GPC. In
polystyrene standards was used for the determination         order to get the same information about the grafted
of the polyacrylates molar masses.                           polymer, it was cleaved from the surface of the
                                                             substrate (degrafting procedure).7
Synthesis of azo initiator
The coupling agent (Scheme 2, IV) is obtained in
three reactions step (Scheme 2): formation of the acid       RESULTS AND DISCUSSION
chloride (Scheme 2, II) from the corresponding azo           CRP attempts of acrylate monomers
carboxylic acid (Scheme 2, I), esterification of (II)         Gnanou et al 11–14 worked on a novel acyclic b-
with allyl alcohol7,8 (Scheme 2, III) and hydrosilyla-       phosphonylated nitroxide, N-tert-butyl-N-(-1-diethyl-
tion with triethoxysilane, a less sensitive reactant than    phosphono-2-2-dimethyl)propyl nitroxide, developed
trichlorosilane.                                             by Tordo and called SG1 (trademark of ATOFINA).
                                                             Moreover, three alkoxyamines with different function-
Description of the hydrosilation reaction                    ality (1 to 3) have been synthesised upon coupling SG1
Under dry nitrogen atmosphere, 1 g of the ester (III)        with different alkyl radicals.17,18 These alkoxyamines
and 10 ml of triethoxysilane in 20 ml of dry toluene was     based on SG1 have thus been used to polymerise
added a solution of 30 mg of hexachloroplatinic acid in      styrene and n-butyl acrylate, providing well-defined
6 ml of dimethoxyethane/ethanol (1/1 v/v), and the           linear and star structures.12 In the two cases, kinetic
mixture was stirred 15–20 h. After that time the             studies have been realised, and the rate of polymerisa-
product was dried in vacuum (the silane was removed          tion depended on the concentration of initiator with
by distillation) yielding green oil in quantitative yield.   the bimolecular AIBN/SG1 system only.1,11,17 More-
Residual platinum catalyst was removed by filtration of       over, it is significant that adding a slight excess of

Polym Int 51:1111–1116 (2002)                                                                                           1113
J Parvole, L Billon, JP Montfort

                                                                             polymer brushes polymerising from surface-attached
                                                                             initiator molecules is the control of the graft density of
                                                                             the coupling agents. As reactions in surface-attached
                                                                             monolayers are not easily quantified,19 it is desirable to
                                                                             achieve the immobilisation of the initiator in a one-
                                                                             step procedure.7–9
                                                                                The initiator used in this study consists of an azo
                                                                             group which is structurally similar to AIBN and two
                                                                             trifunctional ethoxy silane head groups, which connect
                                                                             the initiator through silanol moieties to the surface of
Figure 1. Evolution of ln([M]0/[M]) as a function of time for free radical   the substrate. The ester group connecting the initiat-
polymerisation of EA under mediated procedure at 120 °C $5% SG1              ing group and the anchor can act as a ‘break-seal’
excess: ~ [EA]/[MONAMS] = 530; & [EA]/[MONAMS] = 840;                        group. Hence, this ester can be cleaved after comple-
& [EA]/2[AIBN] = 530.
                                                                             tion of the layer formation, and the polymer can be
                                                                             removed from the surface and characterised. Before
DEPN independently to the initiating system slowed                           grafting, the silica was dried at 120 °C under vacuum
the polymerisation, although well-controlled samples                         for at least 12 h. This procedure is probably not
of either polystyrene or polyacrylates can be prepared                       sufficient to remove all the water adsorbed at the silica
via this route.15                                                            surface, but it should be noted that if the amount of
   In order to control accurately the polymer mono-                          residual water changes is avoided, reproducible graft
layer thickness and polymer dimensions and structure,                        densities can be achieved.7
we have applied the CRP in our own experimental                                 The next step is the polymerisation reaction of n-
conditions. Polymerisation of nBA and EA was thus                            butyl acrylate and ethyl acrylate from the surface
effected in bulk at different temperatures (100–                             attached-initiator. As the initiator is bound to the
120 °C). The [monomer] to [initiator] ratio and the                          surface by one end (the second part of the initiator can
nitroxide excess were, respectively, set to 400–800 and                      diffuse into solution) or at both ends, it was necessary
5%. For the nitroxide mediated polymerisation of                             to add predetermined amounts of free initiator
polyacrylates in these conditions, the kinetic studies                       (AIBN) to the reaction mixture to obtain a polymer-
fulfill all the criteria of livingness in the presence of                     isation, in a way similar, for example, to the formation
SG1, as shown by Gnanou and coworkers.11–15                                  of polymer brushes from surface grafted alkoxyamine
   The linear relationships between ln([M]0/[M]) and                         initiators.1 Husseman et al made initial attempts to
time (Fig 1) as well as conversion and molecular                             control polymer growth from the surface bound
weight (Fig 2), indicate a high control of the EA                            initiators under standard living free radical polymer-
polymerisation process. Kinetic studies of n-butyl                           isation, but were unsuccessful due to the extremely
acrylate are not presented in this paper because                             low concentration of initiating sites with respect to the
Gnanou and coworkers11–15 published its polymerisa-                          monomer concentration. Attempts to dilute the
tion under conditions almost similar, without signifi-                        concentration of monomer units in the polymerisation
cant modification of the polymerisation process.                              mixture were also unsuccessful due to the severe
   For the next step, one can thus expect to obtain                          reduction in the rate of polymerisation. As we found, it
suitable molecular weight (10 000–100 000 g molÀ1)                           was necessary to add initiator to the reaction mixture.
and polydispersity (1.1–1.3), within a relatively short                      Therefore, the addition of SG1 creates an overall
reaction time (1–10 h).                                                      concentration of nitroxide in the polymerisation
                                                                             mixture, which controls the chain growth of both the
Formation of polymer brushes                                                 immobilised and soluble initiators. Finally, the PBA-
One of the basic requirements for the synthesis of                           modified silica were carefully extracted several times
                                                                             with toluene and centrifuged, until no precipate
                                                                             formed when the supernatant solution was added
                                                                             dropwise to an excess of methanol. In most cases
                                                                             about 10–15 cycles were enough to remove all non-
                                                                             attached PBA from the substrate.
                                                                                We later performed XPS and FTIR measurements
                                                                             to characterise the initiator and polymer layers
                                                                             qualitatively and to verify that all modification reac-
                                                                             tions follow the scheme described.
                                                                                Figure 3 shows the IR spectra of unmodified silica
                                                                             (a), immobilised initiator (b) and PEA monolayer (c).
                                                                             We can see the absorption bands due to the carbonyl
Figure 2. Variation of Mn and Mw /Mn versus conversion for free radical
polymerisation of EA under mediated procedure at 120 °C with $5% SG1         (1722 cmÀ1), the nitrile bonds (2250 cmÀ1) and the
excess: & [EA]/[MONAMS] = 530, ~ Mw /Mn for MONAMS initiator; & [EA]/        C—H stretching vibrations (2850–2990 cmÀ1) of the
2[AIBN] = 530, ~ Mw /Mn for AIBN initiator.                                  coupling agent. The spectra (c) appears clearly as

1114                                                                                                     Polym Int 51:1111–1116 (2002)
                                                                                                        Polyacrylate brushes on silica surfaces

                                                                              (80 °C) in order to get optimal grafting density of the
                                                                              initiator with the triethoxysilane. At this temperature,
                                                                              a large part of the azo initiator would be deactivated
                                                                              and a solution to the problem would be to use at room
                                                                              temperature an initiator with trichlorosilane function.
                                                                              In a forthcoming paper, we will describe the possibility
                                                                              of grafting a trichlorosilane group as coupling agent
                                                                              and to determine a high grafting density by thermal
                                                                                 Moreover, in order to check the polydispersity and
                                                                              molecular weigth of the grafted chains, we cleaved
Figure 3. FTIR spectra of (a) unmodified silica, (b) silica with immobilised   them from the silica particles by reaction of the ester
azo initiator and (c) immobilised PBA.
                                                                              function with methanol in toluene using p-TsOH as a
                                                                              catalyst. The transesterification reaction leads to a
PEA-modified with the absorption bands due to the                              cleavage of the ester group connecting the polymer to
carbonyl (1737 cmÀ1) and the C—H (2850–                                       the surface (cf Degrafting procedure in Reference 7).
2990 cmÀ1).                                                                   Thus, the macromolecular dimensions of polyacry-
   Figure 4 summarises the results of XPS measure-                            lates covalently bonded to the surface can be evaluated
ments. The spectra of the bare substrate (a) shows                            by GPC and compared to the bulk polymer. For
signals due to the presence of silicon (152 eV, Si(2s);                       example, reaction of grafted silica particles, prepared
103 eV, Si(2p)) and oxygen atoms (533 eV, O(1s)).                             from a 400:1 mixture of acrylate monomers and AIBN
After the immobilisation of the coupling agent (b), two                       with a slight excess of SG1, afforded a linear polymer
new signals appear due to the carbon (285 eV, C(1s))                          with a number average molecular weight of 10 000 g
and nitrogen atoms (400 eV, N(1s)) of the azo                                 molÀ1 and a polydispersity of 1.12 which corresponds
compound. A comparison of the XP spectra of the                               closely to the observed Mn for the bulk polymer
coupling agent monolayer (b) and the graft PBA shows                          (15 000 g molÀ1), while the polydispersity (1.17) is
a strong enhancement of the carbon signal at 285 eV                           equal to that of the polymer grafted.
(C(1s)). In additional, in the (c) spectra the signals of
Si(2p) are attenuated and the C(1s)/O(1s) signals ratio
is clearly enhanced.                                                          CONCLUSIONS
   This preliminary study showed, by FTIR and XPS,                            Preliminary studies by XPS and FTIR, before
the possibility to obtain a polyacrylate-modified silica.                      quantitative results, are in good agreement with the
Given the high residual Si intensity in the coated                            scheme described and demonstrate that self-
surface, the density of initiators must be quite low; the                     assembled monolayers of radical-chain initiators at-
problem is the high temperature used for grafting step                        tached to the surface of a high area silica can be used
                                                                              for the preparation of monolayers consisting of
                                                                              terminally attached polyacrylate chains.
                                                                                 The next step will consist of quantitative studies,
                                                                              especially by DSC7 (grafting density), AFM (mono-
                                                                              layer morphology), Ellipsometry (grafting density,
                                                                              layer thickness: molecular weight/thickness relation-
                                                                              ship) and neutron reflectivity (monomer concentra-
                                                                              tion profile).
                                                                                 Moreover, synthesis of an alkoxyamine, based on
                                                                              SG1 nitroxide, as coupling agent is underway.

                                                                              The authors are pleased to acknowledge B Vuillemin
                                                                              and O Guerret from ATOFINA for discussions and
                                                                              supplying SG1 and MONAMS.

                                                                              1 Husseman M, Malmstrom EE, McNamara M, Mate M,
                                                                                 Mecerreyes D, Benoit D, Hedrick JL, Mansky P, Huang E,
                                                                                 Russell TP and Hawker CJ, Macromolecules 32:1424 (1999).
                                                                              2 Mansky P, Liu Y, Huang E, Russell TP and Hawker CJ, Science
                                                                                 Washington 275:1458 (1997).
Figure 4. XPS of (a) unmodified silica, (b) silica with immobilised azo        3 Halperin A, Tirrell M and Lodge TP, Adv Polym Sci 100:31
initiator and (c) immobilised PBA.                                               (1991).

Polym Int 51:1111–1116 (2002)                                                                                                            1115
J Parvole, L Billon, JP Montfort

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1116                                                                                              Polym Int 51:1111–1116 (2002)

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