covalently attached graft polymer monolayer on organic polymeric substrate via confined surface...

Covalently Attached Graft Polymer Monolayer onOrganic Polymeric Substrate via Confined SurfaceInhibition Reaction

PENG YANG,1,2 JINGYI XIE,1,2 JING YUAN,1,2 LI ZHANG,1,2* WEINA LIU,1,2{ WANTAI YANG1,2

1Polymer Division, State Key Laboratory of Chemical Resource Engineering, Beijing, 100029, China

2Department of Polymer Science, College of Materials Science and Engineering, Beijing University of Chemical Technology,Beijing, 100029, China

Received 24 May 2006; accepted 28 September 2006DOI: 10.1002/pola.21822Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A grafting technique was proposed for the preparation of polymer mono-layer on polymeric substrate. On the basis of our recent work on polymer-supported in-hibitor (PSI), hydroquinone (HQ) was first implanted onto polypropylene (PP) surfacethrough UV-initiated grafting. The resulting immobilized HQ was used as PSI for thethermal-induced free radical polymerization (FRP) of acrylic acid (AA). The inhibitionmechanism was similar to that of free HQ molecule, that is, polymer chain-carryingradical or peroxy radical could be deactivated by abstracting hydrogen atom fromhydroxyl group of immobilized HQ, and the resulting oxyradical (semiquinone radical)combined with another active chain free radical. According to this mechanism, adevised redox initiator consisting of sodium hydrogen sulfite and ammonium persul-fate was used to initiate FRP of AA in water at low temperature (50 8C). High crystal-line biaxial oriented PP film with HQ immobilized was deliberately laid in this systemas a radical trap to capture poly(acrylic acid) (PAA) short chain radical. Through X-rayphotoelectron spectra (XPS) analysis it was found that the atom ratio of CHQ (carbonin HQ) to CCOOH (carbon in COOH) decreased with prolonging polymerization timeand became stable after about 30 min. The formed PAA short chain on the surfaceshowed a distribution of monolayer, and the saturated thickness was calculated as5–7 A. The degree of polymerization of graft chain in PAA monolayer was estimated as15–20 through three different models. Relating to surface coverage being 100% in idealdensely packed PAA monolayer, real monolayer surface coverage in such reactionsystem was estimated as 12.3–18.5%. This method was expected to give us a generalapproach for constructing kinds of graft polymer monolayer on polymeric substrate,because the involved chemistry was only common inhibition reaction between immobi-lized inhibitor (HQ) and FRP system in solution (herein redox initiating system of AA).We named this grafting chemistry as confined surface inhibition reaction. VVC 2007 Wiley

Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 745–755, 2007

Keywords: functionalization; inhibition; monolayer; polymer; surfaces

INTRODUCTION

Modification of solid surfaces by the chemical orphysical attachment of monolayers have provento be an effective and important method for alter-

*Present address: State Intellectual Property Office ofPeople’s Republic of China.

{Present address: Department of Chemical Engineering,Columbia University, New York, New York 10027-6902.

Correspondence to: W. Yang (E-mail: [email protected]).Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 745–755 (2007)VVC 2007 Wiley Periodicals, Inc.

745

ing surface properties based on molecular level.Monolayers could be directly used with designedmolecular, pack structure and termination groupin theoretical research1–8 and technological appli-cations such as biotechnology,9–13 display,14 nano-science,15–22 lubrication,23 microelectronics,24–26

etc. The methods for constructing such mono-layers mainly involve in four parts: (i) derivativesof fatty acids or bifunctional amphiphiles usingLangmuir-Blodgett (LB)27,28 technique; (ii) self-assembly monolayer (SAM) based on thiol or di-sulfide chemistry on gold surface6–11 (includingother noble metal such as platinum,5,25 silver13,29)and siloxy linkage on glass/silicon1–4,12,30 or metalsurface;31 (iii) electrostatic self-assembly on con-ductive surface;32 (iv) surface coupling betweenunsaturated compounds and Si surface.33 Further,functional monolayer group could initiate chemi-cal grafting,34 coupling,35 or physical adsorption36

of macromolecules to form ultrathin polymer filmor brush structure.

Although these studies mentioned earlier pro-vide excellent models for the preparation of mono-layers, there are only few studies on flexible poly-mer substrates, especially commercial commonplastics. The pioneering work was described bySagiv,37 where octadecyl trichlorosilane was cova-lently bonded onto spin-cast poly(vinyl alcohol).Bohme et al.38 used bola-amphiphile molecules forthe preparation of LB monolayers on the surfaceof spin-cast poly(allylamine hydrochloride) film.Whitesides and coworkers39 systematically inves-tigated the formation of SAMs on oxidized poly(dimethyl siloxane) and polyethylene slab surfacespretreated with oxygen plasma. Their strategyincluded a multistep procedure that consisted ofsurface plasma treatment, the barrier layer for-mation of silicon oxide, and silanization on this sil-icon oxide surface. Ratner and coworkers40 soughtto construct SAM-like structure on a polymericbiomaterial poly(2-hydroxylethyl methacrylate) usinga one-step reaction between polymer surface hy-droxyl group and dodecyl isocyanate. Althoughthe outmost surface showed the unique structureof SAM, the modified surface region was not exactmonolayer.

The surfaces bearing carboxyl groups have re-ceived considerable attention in many technologiessuch as monolayer-initiated surface grafting,24,41

surface patterning,10,42 metal deposition,43 and in-terfacial processes involved in acid–base behavior,colloids, and proteins.44 Accordingly, poly(acrylicacid) (PAA) grafts are deliberately fabricated onpolymer surface via various surface modification

techniques. However, long PAA chains, often withbranch structure, are generated in most of graftingsystems, because of high polymerization activity ofAA monomer, and a possible problem related tothis structure is that well-defined homogeneous orpatterned surface is difficultly obtained.10,42,45 Aswe concerned, if the polymerization degree of AAcould be deliberately suppressed, the polymerproduct would contain short chain with few branchstructure. We recently reported a novel conceptthat the immobilization of an inhibitor (hydroqui-none, HQ) on polymer support (i-HQ), and it wasdemonstrated experimentally that i-HQ showedcertain inhibition ability for conventional free radi-cal polymerization (FRP) of methyl methacrylateand styrene.46 A possible inhibition reaction mech-anism without being demonstrated in detail wasproposed: i-HQ deactivated polymer chain radicalwith short kinetic chain length as its terminalgroup.

The above work shows that i-HQ has a poten-tial to serve as a radical trap for capturing poly-mer chain radical. The surface grafting techniqueby radical trapper has been achieved in severalgroups.47–50 For example, Tsubokawa and cow-orkers found that polycondensed aromatic ringscontained in molecular structure of carbon blackcould trap polymer chain radicals47 obtained bythe dissociation of 2,2,6,6-tetramethyl-1-piperidi-nyloxy-terminated and peroxy-terminated poly-mers, or redox reaction of ceric ions with polymershaving hydroxyl groups.48 Also, they used sur-face radicals produced on c-ray-irradiated silicaas trapping sites to couple polystyrene grafts.49

Bartholome et al. first attached covalently alkoxy-amine functionalities onto silica nanopariclessurfaces via radical trapper, and then used thesealkoxyamine as further initiate sites for nitroxi-de-mediated surface graft polymerization ofstyrene.50 We herein put forward confined surfaceinhibition reaction (CSIR) to fabricate short (pos-sibly also linear) graft polymer monolayer on poly-mer substrate, by using the immobilized inhibitoras a radical trap to capture polymer short chainradical (Scheme 1). In this paper, the inhibitionreaction mechanism of i-HQ was demonstratedfirstly, and then, we devised a model system tofabricate graft polymer monolayer containing veryshort PAA chains (about 15–20 units) on modelpolymer (polypropylene, PP) surface, throughCSIR of i-HQ in redox-initiated polymerizationprocess of low concentration AA aqueous solution.

An urgent problem from this work is the thick-ness measurement of polymer monolayer on poly-

746 YANG ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

mer substrate, because few suitable methods couldbe assigned to polymer surface. Most of methodssuch as ellipsometry, quartz crystal microbalance,and X-ray reflectivity are well founded on the inor-ganic wafer surface. Alternatively, AFM (SPM)may be performed for depth profile analysis forpatterned surface; while for nonpatterned mono-layer surface, this technique needs scratch testfrom an ultrasharp needle to delaminate mono-layer down to the underlayer substrate, and onlyheight variation (layer thickness) in local regioncould be obtained. Angle-resolved X-ray photoelec-tron spectra (AXPS) are often used to measure thethickness from founded formula.51 McCarthy andcoworkers investigated in detail the depth (thick-ness) measurement of modification layer or depo-sition multilayer on polymer substrates.51,52 Anequation was introduced to calculate the thicknessfrom the XPS peak intensity, which was also usedin the thickness (d) estimation of monolayer oninorganic or organic substrate surface:

d ¼ �k lnðN=N0Þ sinh ð1Þ

where N and N0 are the XPS peak intensity ofspecial labeling element seen from the mono-layer-covered and from an original substrate sur-face, this labeling element should be only con-tained in substrate with the absence in mono-layer; d is the thickness of the monolayer (A), k isthe inelastic mean free path of a photoelectronthrough the monolayer (A), and h is the takeoffangle used when making the XPS measurements(between the plane of the surface and the en-trance lens of the detector optics). The inelasticmean free path length k in polymer layer wasabout 19 A, when using AlKa excitation. In oursystem, there was no labeling element being onlycontained in bulk PP substrate, so we tried touse the carbon in HQ as the labeling, that is,

HQ-g-PP film was considered as the \substrate,"which was the platform for PAA monolayerfabrication.

EXPERIMENTAL

Materials

Acetone (AC), HQ, sodium hydrogen sulfite(NaHSO3), and ammonium persulfate (APS)were used as received. AA was purchased fromAtoz Company and purified by distillation underreduced pressure. Commercial casting PP (CPP)films (30 lm thick, the density is about 0.9 g/cm2)and biaxial oriented PP (BOPP) films (19 lmthick) were cut to circle pieces with the diameterbeing 5 cm, and subjected to Soxhlet extractionwith AC and water respectively, for 24 h toexclude impurities and additives before use.

Photoirradiation Procedures for the Preparationof i-HQ on PP (CPP or BOPP) Surface

The apparatus used to perform graft reactionand the setup of the film samples were reportedin detail elsewhere.46 A predetermined amountof AC solution of HQ (10 wt %) was deposited onthe bottom film with a microsyringe. The top filmcovered this solution and the drop of solutionwas spread into an even and very thin liquidlayer under suitable pressure from a quartzplate. Then, the assembly was laid on the holderand was irradiated by a high-pressure mercurylamp (1000 W, the UV intensity at k ¼ 254 nm isabout 8000 lW/cm2) from the topside. After theirradiation for 20 min, the top film (HQ-g-PP)was rinsed by copious AC to remove unreactedHQ absolutely, dried under atmosphere to con-stant weight.

Thermal-Induced Polymerization of AAin the Presence of HQ-g-CPP for InhibitionMechanism Investigation

Thermal-induced Polymerization was performedin the hand-made polymerization bottle. HQ-g-CPP film was placed on the bottom of the bottle,and 20 mL AA aqueous solution (5 wt %) wasadded. After purging nitrogen for 30 min, poly-merization began at 90 8C (water bath). At theend of polymerization, the film (AA-g-CPP) wastaken out, washed by copious water, and driedunder hot atmosphere to constant weight. XPS

Scheme 1. The reaction mechanism of CSIR. [Colorfigure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

COVALENTLY ATTACHED GRAFT POLYMER MONOLAYER 747


was used to investigate the chemical changes onthe surface.

Redox-Initiated Polymerization of AA in thePresence of HQ-g-BOPP for Monolayer Fabrication

Redox-initiating FRP system at low temperaturewas selected. In detail, 0.24 g APS, 0.12 gNaHSO3, and 1 mL AAwere dissolved into 20 mLwater. HQ-g-BOPP film was placed on the bottomof the solution. Such solution was purged bynitrogen for 20 min, and polymerization began at50 8C (water bath) for determined time. At theend of polymerization, the film (mAA-g-BOPP)was taken out, washed by copious water andSoxhlet extracted by hot water for 8 h, driedunder hot atmosphere to constant weight.

Characterization Methods

AXPS were obtained by using PHI-5300/ESCAinstrument and Al/Mg excitation at two angles:158 and 758 (between the plane of the surfaceand the entrance lens of the detector optics). Theobtained binding energy (BE) was referenced tothe main carbon peak set to 285.2 eV. The atomconcentration was given automatically by thesoftware (provided by the manufacturer); in themean time, peak deconvolution was also per-formed by this software, with using Gaussianfunction as peak type. Surface static contactangles were measured with 5 lL of distilled waterdroplet being placed automatically on the modifiedfilm by OCA20 (Dataphysics, Germany). All mea-surements were performed at the room tempera-ture and were the average of at least nine read-ings at different positions across the surface.

RESULTS AND DISCUSSION

The Inhibition Reaction Mechanism of i-HQ

Inhibition reaction of i-HQ was the chemistrybehind the following monolayer fabrication tech-nique, so it was necessary to examine the reactionmechanism. i-HQ (HQ-g-CPP film) was soaked intoAA aqueous solution, and the chemical changes onthe film surface before/after heating the systemfor certain time were firstly investigated by ATR-FTIR, and no observable differences were found.Further, C1s and O1s spectra of HQ-g-CPP filmwere obtained by XPS (Fig. 1). The main peak at285.2 eV from C1s was attributed to C of PP bulk,

while small peak at 287.0 eV belonged to the BEof C in HQ [Fig. 1(a)]. A single peak at about 534.0eV from O1s was considered as the BE of O in HQ[Fig. 1(b)].

After thermal-induced polymerization, C on AA-g-CPP film surface had three chemical states: bulkC in PP chain at 285.2 eV, C in HQ at 287.0 eV,and C in COOH at 289.3 eV [Fig. 2(a)]. Corre-spondingly, O had two chemical states: O in HQat about 533.4 eV and O in COOH at 532.3 eV[Fig. 2(b)]. These results presented that PAA wasincorporated onto HQ-g-PP surface. The existenceof C in HQ showed two possible mechanisms: (1)the remained phenolic hydroxyl in i-HQ inhibitedpolymerization and short AA chain was combinedwith this hydroxyl; (2) surface C��O bond dissoci-ated under heat and surface radical deactivatedgraft polymerization of AA with HQ as the cap-ping agent (semiquinone radical deactivated sur-face graft chain radical). For analyzing real mech-anism, the devised C��O dissociating experimentwas performed, where hydroiodic acid (HI) was

Figure 1. C1s (a) and O1s (b) spectra of HQ-g-CPPfilm.

748 YANG ET AL.


used to react with the grafted surface. C��O bondin organic ether was known to easily dissociateunder the nucleophilic attack of HI (Scheme 2).The grafted film was soaked into HI and the solu-tion was refluxed at 120–130 8C for 4.5 h.53 Afterserious washing, the resulting film was analyzedby XPS. As shown in Figure 3(a), the chemicalstates of C decreased to one kind, that is, except-ing for bulk C peak, C in HQ and C in COOH dis-appeared. This result directly proved that reactionmechanism was that short AA chain was addedonto the end of i-HQ, instead of direct combinationbetween short AA chain and PP surface, becausein the latter, surface C��C bond should not disas-sociate under HI. The corresponding O atom con-tent after HI treating decreased largely [Fig. 3(b)],which reflected that surface COOH and HQ were

eliminated absolutely. The remained O may comefrom certain inevitable oxygen pollution. I atom at619.5 and 631.02 eV was found on the resultingfilm (Fig. 4), which was in accord with the additionof I on the surface in Scheme 2. The whole reac-tion mechanism of i-HQ in monomer was outlinedin Scheme 3: polymer chain-carrying radical orperoxy radical could be deactivated by abstractinghydrogen atom from hydroxyl group of i-HQ, andthe resulting oxyradical (semiquinone radical)combined with another active chain free radical.

The Considerations for Monolayer Fabrication onPolymer Substrate

According to the earlier mechanism, i-HQ reactedwith preliminary active radical, and a short ki-netic chain was added onto the end of i-HQ mole-cule. Enlightened by this work, we proposed astrategy that through selecting polymerizationconditions, this coupled short chain might formmonolayer onto polymer surface. Three factorsshould be considered for forming monolayer onpolymer substrate.39 First, different from inor-ganic wafer, polymer substrate is easy to be swol-len by solvent, and the transportation of reactantsby medium may enlarge the reaction region tosub or interior layer. Second, polymer substratesurface is often made up of crystalline and amor-phous regions and is intrinsically heterogeneous.This nature may result in different reactivity andswelling extent in different surface regions.Third, the macromolecular chain on polymer sur-face has relative structural mobility to that ofinorganic surface, which often facilitates thereconstruction on gentle heating, on mechanicaldeformation, or on contact with organic solvents.This reconstruction may destroy the formedmonolayer structure. As some solutions to thesedrawbacks, Chaudhury, Whitesides and cow-orkers39 used the silicon oxide as barrier layer tosuppress swelling, and Ratner and coworkers40

selected the poor swelling solvent for polymersubstrate to prevent from polymer swelling andoptimize the surface immobilization reaction.Aqueous surface modification of poly(vinylidenefluoride) was investigated by McCarthy and

Scheme 2. The reaction between HI and ether resulting in the dissociation of C��O.

Figure 2. C1s (a) and O1s (b) spectra of AA-g-CPPbefore HI treatment.



Dias.54 According to their explanation, water andpolymer surface in contact with one anotherdefined a sharp interface, and water neither dis-solved nor swelled the polymer. So, the dehydro-fluorination between hydroxide ion in aqueous

phase and polymer surface proceeded very slowly.With additional phase-transfer catalyst, reactioncould be accelerated at two orders of magnitudeand reaction depth was estimated at �10 A orless. Inspired by the above work, we selected highcrystalline BOPP as substrate, and aqueous saltsolution containing redox initiator and AA mono-mer as reaction medium. High crystalline hydro-phobic BOPP plus salt environment could provideultrathin modification layer due to low swellingand wetting from the solvent. Low reaction tem-perature (50 8C) and short reaction time (5–90min) from redox initiator system would suppresspossible swelling and penetration of the formedproduct. According to the above mechanism, theformed short active polymer (PAA) chain in thesolution was instantly added onto i-HQ groups toform the graft layer, and the following experimen-tal results demonstrated that PAA monolayercould be formed on BOPP surface.

The Wettability Evaluation on the Grafted Surface

Static water contact angle was a sensitive indica-tion of the chemical nature of a surface in terms ofthe outermost few tenths of a nanometer. Accord-ingly, as the first characterization for the PAAmonolayer grafts on BOPP surface, the wettabiltyon mPAA-g-BOPP surface were investigated bystatic water contact angle. As shown in Figure 5(upper curve), when PAA chains were graduallyconstructed onto BOPP surface, the contact angledecreased from 988 (blank) to 878 (reacting for30 min), and after that, the contact angle changedslowly with a stable value about 858. The tendencyof contact angle with the reaction time could beexplained well by the following PAA grafting reac-

Scheme 3. The inhibition reaction mechanism ofi-HQ.

Figure 3. C1s (a) and O1s (b) spectra of AA-g-CPPafter HI treatment.

Figure 4. I3d spectrum of AA-g-CPP after HItreatment.

750 YANG ET AL.


tion kinetics (Fig. 6). Low wettability differencebetween original and modified film may be attrib-uted to low surface coverage of PAA monolayerand short polymer chain (see later). Ionization ofsuch monolayer showed lower contact angle (lowercurve) with a stable value about 768. This effect ofionization on contact angle of surface layer con-taining carboxyl groups has been also extensivelyresearched in the literatures.51,55

The Distribution Analysis of PAA Chainsalong the Profile

The distribution of PAA grafts along the profilewas investigated by AXPS (Fig. 6). After thedeconvolution, a typical high-resolution C1s spec-trum of mAA-g-BOPP film was shown in Figure6(a). At both the incidence angles (158 and 758),the atom ratio of CHQ (carbon in HQ) to CCOOH

(carbon in COOH) decreased with reaction time,and arrived at a plateau after about 30 min [Fig.6(b)]. This result showed that the amount of PAAgrafts increased with the reaction time, and satu-rated after 30 min without obvious change. Onthe other hand, it was found that at higher angle,the concentration of CHQ showed an increasecomparing with that at lower angle; on the con-trary, the concentration of CCOOH decreased withincreasing the angle. For example, at 15 min, theconcentration of CHQ increased from 7.87(158) to8.27(758) and reversely, the concentration ofCCOOH decreased from 1.50(158) to 1.21(758).According to the formula: depth ¼ 3k sinh, thesampling depth at 158 and 758 were estimated as10 and 40 A respectively.51 So, the above results

showed that the distribution of HQ on substratesurface was a multilayer with the depth morethan 40 A, while the PAA layer was confinedwithin 40 A. Similar takeoff angle dependence ofthe surface atom concentration on kinds of poly-mer substrates were also described by McCarthyand coworkers51,52

The Thickness Determination of PAA Monolayer

The most important evidence for supportingmonolayer formation was provided through thethickness measurement of the monolayer. In oursystem, we used the carbon in HQ as the label-ing, that is, HQ-g-BOPP film was considered asthe \substrate," which was the platform for PAAmonolayer fabrication. This reference elementamount (atom ratio) in HQ-g-BOPP film could becontrolled exactly and conveniently. With thesame graft amount of HQ on the substrates, theresulting atom concentration of CHQ was onlyattributed to the influence of organic layer thick-

Figure 6. (a) C1s spectrum of mAA-g-BOPP film(incidence angle: 158, reaction time: 30 min). (b) Effectof reaction time on the ratio of CHQ to CCOOH at twotakeoff angles (158 and 758).

Figure 5. Water contact angle on mAA-g-BOPP filmsurface. n: unionized PAA monolayer; l: PAA mono-layer after ionization in 1 wt % NaOH aqueous solu-tion (pH ¼ 13.4) for 10 h.



ness. According to this analysis, we investigatedthe XPS spectra on a series of mAA-g-BOPP filmswith different reaction time, and added the con-centration ratio (peak intensity ratio of CHQ ongrafted surface to CHQ on HQ-g-BOPP surfacebefore grafting) into the eq 1. As we predicted, thethickness increased from 15 min (0.5 A) to 30 min(4.8 A), and after that, the thickness was stable inthe range 5–7 A (Table 1). Obviously, the data at15 min (0.5 A) was not reasonable, because atleast, a carbon–carbon bond length was 1.54 A. Atthe initial stage of the reaction, ultra low numberof PAA grafts distributed sparsely on the HQ-g-BOPP surface. Actually, the substrate mass didnot have obvious change in 10�5 g range and theATR-FTIR spectra of grafted film did not showobvious difference with that of HQ-g-BOPP film.The precondition of eq 1 was that the samplelayer fully covered the underlying surface, sowhen using the data in Table 1 to calculate thethickness, the average layer thickness wasobtained through averaging the distribution ofPAA grafts along the surface. Therefore, unrea-sonable low thickness data was raised when aver-aging the sparse distribution of PAA grafts atthe initial stage of reaction. With the reactionproceeding, the grafting sites and the resultinggraft chains became increasingly dense. Similarto the phenomenon in \Coupling (Grafting To)"technique,35 the barrier effect of the formed graftchains to polymer active chain in solution was in-creasingly enhanced, and after 30 min, this effectwas high enough to suppress the grafting reaction,as a result, the polymer layer thickness stoppedincreasing. This result was in agreement with theplots in Figure 6, that is, with reaction time pro-longing, the amount of HQ groups undertakingthe coupling reaction with PAA chains increasedand became stable after 30 min, the resultingaverage thickness (dry layer) was about 5–7 A.

The Calculation for the Coverage Degreeof Monolayer

Excepting for the thickness, the coverage degreeof a monolayer is also important to its furtherapplication and often used to follow the growthkinetics of a monolayer.18,39,56 In this work, thecoverage degree of PAA monolayer was examinedbased on the following equation:

Surface coverage ¼ real thickness

densely packed

monolayer thickness

� 100%

ð2Þ

Here, we set the surface coverage of the denselypacked PAA monolayer as 100%. For simplifyingthe calculation, such densely PAA monolayer wasviewed approximately as polymer brush struc-ture, so the layer thickness was calculated by theequation: thickness ¼ N � L, where L is the chaincontour length per monomer unit for vinyl poly-mers 2.5 A and N is the degree of polymerization.The degree of polymerization (N) was estimatedby the following theoretical modeling. If we consid-ered the PAA monolayer on PP film surface as amodel system that low molecular polymer chainsconfined on substrate surface, we would estimatethe dimensions of PAA monolayer. PAA chainsconfined (adsorption, deposition, etc.) on substratesurface have been extensively researched.57–60

When PAA chains are confined on the surfaces,the measured layer thickness (may be hydrody-namic values, dry values, etc.) could be comparedwith the radius of gyration (Rg) of a macromole-cule with similar molecular weight in solution.57

The results showed that Rg was greater than thethickness of confined layer. This difference indi-cated some \flattening" of the polymer moleculeswhen they were confined on surfaces.57,58 Thisapparent polymer collapse could be attributed to anumber of factors as follows: the force of the AFMtip during the scan (for AFM measurement), thepartial dehydration of PAA molecule, or a recon-formation on the substrate surface to minimizeelectrostatic repulsion (for ionized PAA) or maxi-mize the hydrophobic forces.58 However, if PAAhad a low molecular weight (for example, Mw

¼ 36,900 was used in ref. 59) and degree of ioniza-tion (low degree complexation and pH), this flat-tening became very weak and the resulting con-fined layer thickness would be approaching to theRg.

59–61 In our system, these conditions were obvi-ously achieved. The other precondition of this

Table 1. The Atom Ratio of Three Kinds of Carbon(CPP, CHQ, and CCOOH) and Thickness Calculation(Takeoff Angle Is 158)

ReactionTime (min) CPP CHQ CCOOH

Thickness(A)

0 (HQ-g-BOPP) 91.23 8.77 – –15 90.63 7.87 1.50 0.530 95.89 3.31 0.79 4.850 96.74 2.76 0.65 5.790 97.54 1.98 0.48 7.3

752 YANG ET AL.


assumption was that the graft density was lowerthan the value above which the graft polymersshould be highly overlapped each other. In fact,this assumption was fully supported in our systemwhere low graft density (or surface coverage) ofPAA monolayer was obtained (see later). Thus, itwas reasonable that the PAA monolayer thicknessin our work was approximately equal to that Rg

of the PAA chain with same molecular weight insolution. According to the founded relationshipbetween Rg and the degree of polymerization N,we could obtain the average segment number ofevery PAA graft chain in monolayer. The difficultyin such calculation was that exact relationshipbetween Rg and N under very low molecularweight was poorly known. Using Monte-Carlomethod, Christos and Carnie62 calculated theshape and distribution functions of partially ion-ized PAA (including unionized) with differentdegree of polymerization (from 10 to 160). Wefound that our value (about 5–7 A) was close to themaximum along special direction in three dimen-sions of the polymer molecule with N being 20.Further, the two founded equations for low-molec-ular-weight PAA from references were used for cal-culation as follow:

Equation A63,64 (using Monte Carlo simula-tions of ionized chains with screened Coulombinteractions):

R2g � N2�0:6A ðthe conditions in ref : 59 : N > 80;

and the degree of ionization is 0ÞEquation B60 (computer simulation using the self-avoiding walk model):

R2g ¼ 1:24 N1:23A ðthe conditions in ref : 60 :

Mw ¼ 36; 900; very low charge density at pH 3ÞThe calculated values for our system using aboveequations were 20 (eq A) and 15 (eq B) respec-tively. We found that three kinds of values fromrefs. 60, 62, 63, and 64 showed good accordance.Therefore, N of graft chain in PAA monolayer wasestimated as 15–20. Adding the value of N into theequation: thickness ¼ N � L, the thickness of suchdensely packed PAA monolayer could be obtainedas about 37.5–50.0 A, so according to eq 2, the sat-urated surface coverage of PAA monolayer onBOPP film was calculated as 12.3–18.5%.

CONCLUSIONS

A strategy was proposed for the preparation ofpolymer monolayer on polymeric substrate,

which was founded on a recent work developedby us. In polymerization system, polymer chain-carrying radical or peroxy radical could be deacti-vated by abstracting hydrogen atom from hy-droxyl group of i-HQ, and the resulting oxyradical(semiquinone radical) combined with anotheractive chain free radical. Inspired by this mecha-nism, devised redox initiator was used to quicklyproduce PAA chain radical in water at low tem-perature (50 8C), and high crystalline BOPP filmwith HQ immobilized was deliberately laid in thissystem to capture such chain radical. ThroughXPS analysis, it was found that the atom ratio ofCHQ to CCOOH decreased with prolonging poly-merization time and became stable after about30 min. The formed PAA short chain on the sur-face showed a monolayer distribution, and thesaturated thickness was calculated as 5–7 A. Thedegree of polymerization (N) of graft chain in PAAmonolayer was estimated as 15–20 through threedifferent models. Relating to surface coveragebeing 100% in ideal densely packed PAA mono-layer, real monolayer surface coverage in suchreaction systemwas estimated as 12.3–18.5%. Theprinciple of thismethodwas only based on commoninhibition reaction of HQ, so it was expected togive a general approach for constructing kinds ofpolymer monolayer on polymeric substrate; on theother hand, through tuning the structure of chainradical before capturing of i-HQ, the formed mono-layer could be controlled. We would like to say thatsome reaction parameters, e.g., monomer concen-tration, initiator concentration, width between topand bottom films, and so on, should put effect onthe molecular weight and graft density of polymermonolayer. The detailed and systematical investi-gation on the effect of reaction parameters shouldbe valuable to control and tune the graft polymerchain, which is our ongoing work that will bereported in the future.

We thank the Major Project (50433040) from NationalNatural Science Foundation of China (NSFC) and theMajor Project (XK100100433) for Polymer Chemistryand Physics Subject Construction from Beijing Muni-cipal Education Commission (BMEC) for financialsupport.

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