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Applied Surface Science 257 (2011) 1952–1959

Contents lists available at ScienceDirect

Applied Surface Science

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nterfacial morphology and friction properties of thin PEO and PEO/PAA blendlms

ianke Gu, Guojian Wang ∗

chool of Materials Science and Engineering, Tongji University, Shanghai 200092, China

r t i c l e i n f o

rticle history:eceived 2 July 2010eceived in revised form4 September 2010ccepted 14 September 2010vailable online 22 September 2010

a b s t r a c t

The scanning force microscope (SFM) was used to investigate morphology of poly(ethylene oxide) (PEO)and poly(acrylic acid) (PAA) blend. The effect of solvent and dewetting in surface structure of PEO filmwas reported. The results manifested that the crystallization of PEO could be suppressed completely inultrathin region via using chloroform as a solvent, and the branched-like crystallization was recoveredafter dewetting. Also, the effect of thickness, the ratio of PEO/PAA and dewetting in surface morphologyof PEO–PAA blend films were investigated. These results showed that the crystallization was highlydependent on the ratio of PEO/PAA and the thickness of blend film. Furthermore, we assembled the

eywords:canning force microscopy (SFM)ateral force microscopy (LFM)orphology

EOEO/PAA layer-by-layer filmpin-casting

PEO/PAA layer-by-layer film by spin-casting method for the first time, which exhibited highly efficiency.As a complementary tool, we also used lateral force microscopy (LFM) to explore surface information ofthese films. The result was indicative of interfacial constraints in ultrathin region, and also was supportedby the results showing the spin-casting PEO/PAA blends rather than heterogeneous mixture.

© 2010 Elsevier B.V. All rights reserved.

rystallization

. Introduction

Poly(ethylene oxide) (PEO) is a key material in solid polymerlectrolytes [1], biomaterial applications [2], drug delivery devices3], and pH–sensitive sensors, etc. Unfortunately, the tendency ofrystallization [4] of PEO limit its application in these fields, whichequires reasonable mechanical integrity and electronic properties.rystallization of PEO is strongly affected by the confinement ofhin (less than 1000 nm) and ultrathin (less than 100 nm) films [5].enerally, observation of crystalline morphology has mainly beenone in PEO ultrathin films, and branched structures due to non-quilibrium crystallization by a diffusion-limited aggregation (DLA)rocess [6,7].

In order to overcome the crystallization behavior, poly(acryliccid) (PAA) was introduced into PEO system, which has been doneo clarify the complexation features and structure [8–10]. The mix-ng of aqueous solutions of poly(acrylic acid) (PAA) and PEO inhe acidic conditions [9] should lead to the formation of com-

lexes due to the hydrogen bonding between carboxylic group fromAA and the ether oxygen in PEO. Unfortunately, the precipita-ion of the hydrogen-bonded complexes controlled geometries and

orphologies badly. Recently, the layer-by-layer (LBL) deposition

∗ Corresponding author.E-mail address: wanggj@tongji.edu.cn (G. Wang).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.09.034

method has been developed for the fabrication of thin polymer film[11]. The LBL deposition method has been employed to produceheterogeneous stacks of thin film batteries and fuel cell mem-branes [12–14] via alternating deposition of PEO and PAA layersfrom aqueous solutions [12], by utilizing complementary hydrogenbond donor and acceptor interactions. Lutkenhaus et al. [15] provedthat the layer-by-layer technique has the ability to create PEO–PAAbulk blend film, which investigated by using differential scanningcalorimetry (DSC) and dynamic mechanical analysis (DMA). How-ever, the dipping LBL method still exhibits low efficiency and lowyield.

LBL spin-casting method can produce homogenous film withspeed and cost efficiency, which can be automated easily [16–18].This method has been shown to be superior than the conventionalsolution-dipping method when assembling ultrathin multilayerfilms based on the LBL method [17]. Also, they have found it pos-sible to deposit the polycations and polyanions that are normallyused to assemble these films such as poly(styrenesulfonate) (PSS),poly-(allylamine hydrochloride) (PAH), and poly(acrylic acid) (PAA)[17].

A wide range of techniques were available to measure the sur-

face structure in polymer blend, including X-ray photoelectronspectroscopy (XPS), scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), etc. In recent years, scanningforce microscopy (SFM) emerged as an important analytical tool forcharacterizing of the structure and the properties of heterogeneous

dx.doi.org/10.1016/j.apsusc.2010.09.034http://www.sciencedirect.com/science/journal/01694332http://www.elsevier.com/locate/apsuscmailto:wanggj@tongji.edu.cndx.doi.org/10.1016/j.apsusc.2010.09.034

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olymer blends. SFM is capable of measuring various geometriesn the sample surfaces [19]. Lateral force microscopy (LFM) was acanning probe microscopy technique that quantified differencesn surface friction characteristics [20]. Applied with contact modeFM, LFM was particularly useful for differentiating components ofeterogeneous surfaces [19,21].

The goal of this paper is to provide direct insight surface infor-ation on thin film and blends via spinning casting method. Firstly,e investigate the surface morphology of PEO–PAA blends and pure

EO, from which the effects of solvent, dewetting, ratio and filmhickness of PEO/PAA on PEO crystallization were revealed. Sec-ndly, in order to get unique blends, spin casting method is used tossemble PEO/PAA LBL films. Furthermore, interfacial properties ofhese films are analyzed by LFM, which provides valuable insightnto the surface composition and growth of LBL films.

. Experimental

.1. Materials and sample preparation

PEO (2,000,000 Mw, Aldrich Chemical Co.), and PAA (90,000 Mw,0 wt% water solution, Carbemer Inc.) were used as received. Theolymers were weighed and dissolved in Millipore Milli-Q filteredater (18 M� cm). The PEO solution was stirred for 24 h to ensure

omplete dissolution.Silicon wafers (thickness of 585–610 �m from Addison Engi-

eering, Inc. San Jose, CA) were used as LBL substrates. The siliconafer substrates were cleaned by ultrasonication (Model 08849,ole-Parmer Instrument Company) in a series of solvents for 15 minach, in the following order: acetone, methanol, and Milli-Q filteredater. Following the water cleansing, the substrates were driednder a jet of desiccated nitrogen gas. Immediately before use, theilicon wafer was cleaned in UV for 20 min.

In a typical spin-assembly, PEO solution was spun casted on alean silicon wafer at 50 Hz (3000 rpm) with a spin coater (P6204,pecialty Coating Systems), followed by spinning casting of PAAolution onto the PEO coated silicon wafer, then another PEO solu-ion onto the sample above. This sequence (referred in the followings a bilayer) after being repeated n times produced a [PEO/PAA]nomposite film. After assembly, all films were dried at 110 ◦C for4 h, which has been shown to effectively remove water from LBLssembled films.

Pure PEO films and PEO–PAA blends were spun casted to dif-erent thicknesses on silicon wafer at 50 Hz (3000 rpm) with a spinoater from a solution of PEO in chloroform and PAA–PEO in Milli-Qater, respectively. Pure PEO films were annealed at 70 ◦C, whileure PAA and PEO–PAA blend films were annealed at 110 ◦C in aacuum oven for over 12 h.

.2. Characterization

Scanning force microscopy (SFM) was employed to evaluate theopological information of the films, respectively. SFM was con-ucted with a scanning probe microscope (Topometrix Explorer,eeco, CA) with contact mode cantilever sensors (PPP-CONT,anosensors, nominal and lateral spring constants of ∼0.2 N/mnd 80 N/m, respectively). Lateral force microscopy (LFM) [22]Topometrix Explorer, Veeco, CA) was used to explore mate-ial phase heterogeneities to identify material components. Prioro friction coefficient measurements, the LFM cantilever was

alibrated according to a well established method [20]. Filmhicknesses were determined via a contact mode operated SFMEasyScan 2, Nanosurf AG, Switzerland) of razor blade scratchedlms, a method that has been verified with profilometry involvinghicker films.

ence 257 (2011) 1952–1959 1953

3. Results and discussion

3.1. Morphology of PEO ultrathin film

The effect of solvent on the morphology of the ultrathin PEOfilms was observed by SFM. Following our samples preparationmethod mentioned above, two distinctive surface structures wereformed due to different solvents, i.e., chloroform and water asshown in the SFM images (Fig. 1(a and b)), respectively. Thebranched-like surface (Fig. 1(a)) was similar to those reported inprevious work [23], utilizing toluene as a solvent, while Milli-Q fil-ter water was used in our system. This suggested that the samediffusion-limited aggregation mechanism [6,7] created a similarpattern. Interestingly, we found that a smooth pattern was formedvia spin-casting using chloroform as the solvent, as demonstratedby the SFM image in Fig. 1(b), which indicated that there was noany crystallization structure in ultrathin region.

The dewetting on the crystalline morphology of the PEO filmswas investigated by contact SFM. The samples were spun via PEO inchloroform solution. For unannealed PEO film, the smooth and inte-grated pattern was observed in ultrathin film with the thicknessesof 8 nm (Fig. 1(b)), while the film with the thickness of 105 nmshowed crack with threadlike on surface structure (Fig. 1(c)). Theeffect of film thickness on the crystallization of the PEO films wasreported by hot stage atomic force microscopy by Schonherr et al.[24], which suggested that the crystallinity was limited by dimen-sional or interfacial constraints. This was in agreement with theirwork in which the degree of crystallinity was shown to decreasesteadily for film thicknesses less than 200 nm [25]. Additionally,a similar mechanism was shown where the crystal-growth ratedecreases as the PEO film thickness is reduced from 200 nm to40 nm [5].

However, the morphology of these samples was changed afterannealing, shown in Fig. 1(d and e). In Fig. 1(d), 8 nm PEO film wasdetached from the substrate surface and a branched-like morphol-ogy was formed, which was in correspondence with previous work[26]. For ultrathin film, increasing crystallization temperaturesleaded to branch growth [26]. Similarly, in Fig. 1(e) (thick-ness = 105 nm), the surface exhibited a banded surface morphologyfor annealed 105 nm PEO film, which was in correspondence withprevious report [27]. Hence, transitions in the degree of crystal-lization dependence on the dewetting could be linked directly tochanges in the morphology.

3.2. Morphology of PEO–PAA blend ultrathin film

The effect of film thickness on the crystalline morphology of thePEO–PAA blend films supported on the silicon surface was shownin (Fig. 2). In Fig. 2(a), 18 nm blend film showed smooth morphol-ogy with a bit of small holes, and the intermediate thickness film(31 nm) exhibited a development of banded morphology (Fig. 2(b)),and the banded structure was most clearly seen in the 80 nm film(Fig. 2(c)).

Fig. 3 indicated a change in morphology with PEO–PAA ratio ata given film thickness in ultrathin films. The structure of ultrathinfilms (

1954 X. Gu, G. Wang / Applied Surface Science 257 (2011) 1952–1959

F ing wa thick

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ig. 1. SFM images of PEO films on silicon deposited via spin-casting before anneals a solvent), (c) 105 nm (chloroform as a solvent), and after annealing with the film

f Fig. 4(a) was 18 nm as described above. Fig. 4(c) was an SFMmage obtained after the sample shown in the Fig. 4(a) was brought

o an annealing temperature of 70 ◦C, which exhibited a dendriticrystallization. As the blend films was dewetting, the branchedtructures (Fig. 4(c)) became much wider than the fractal-like onesormed before annealed (Fig. 4(a)) and resembled the structureeferred to as dense branching morphology that was character-

ith the film thickness of (a) 7 nm (Milli-Q water as a solvent), (b) 8 nm (chloroformness of (e) 8 nm and (f) 105 nm chloroform as a solvent.

ized by a stable circular envelope modulated by leading branch tips[29]. However, the dewetting behavior did not affect the morphol-

ogy of PEO–PAA blend with higher PAA portion. In Fig. 4(b and d),the rich PAA blend film with 16 nm thickness (PAA portion = 60%)maintained a stable smooth surface structure. The increase in PAAcontent after exposure to humidity inferred that PAA remainedunmoved on the silicon surface during PEO dewetting, which might

X. Gu, G. Wang / Applied Surface Science 257 (2011) 1952–1959 1955

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ig. 2. Contact mode SFM images with scan size of 10 �m of PEO–PAA (1:0.75) blendlms on silicon wafer using spin-casting as a function of film thickness (a) 18 nm,b) 31 nm, and (c) 80 nm.

e due to the interaction between PAA and substrate. Also, the sim-

lar phenomena were occurred in PEO and PMMA blend ultrathinlm [30]. These results suggested that dewetting strongly affectedhe morphology of PEO–PAA ultrathin blends with higher portionEO.

Fig. 3. Contact mode SFM images with scan size of 10 �m of PEO–PAA blend ultra-thin films (

1956 X. Gu, G. Wang / Applied Surface Science 257 (2011) 1952–1959

Fig. 4. Contact mode SFM images of PEO–PAA blend films on silicon, as function of PEO/PA

Fig. 5. Thickness growth of PEO/PAA prepared by spinning casting method as func-tion of the number of bilayers, as measured by SFM with contact mode.

A ratio, before annealed (a) 1:0.75, (b) 1:1.5 and after annealed (c) 1:0.75, (d) 1:1.5.

early on (up to about 7 bilayers), and then linear growth from thatpoint on, which was similar to the growth of LBL films using dip-ping method [31]. To quantify the relationship between averagethickness of the PEO/PAA LBL film and number of bilayers, the fitequation was provided based on these data (Fig. 5). In exponen-tial growth region (n < 7), the fit exponential equation could beobtained as shown in Eq. (1)

d = 2.08 × exp(n/1.50) + 2.5 (1)where d is the total thickness and n is the number of bilayers ofthe LBL films. From the equation, the growth of thickness was veryslow, which suggested that the negative effective of the substratewas significant to the growth of first 7 bilayers [16]. However, whenthe number of bilayer is up to 7 (n > 7), the fit linear equation couldbe obtained as Eq. (2):

d = 37.07n − 167.22 (2)

Obviously, Eq. (2) yielded for slope a value of 37, which indicated

that the average of thickness was about 37 nm. This was indicativeof the adsorbed material at 7 bilayers and beyond was at a sufficientdistance from the substrate to diminish the substrate’s effect on theadsorption process [16].

X. Gu, G. Wang / Applied Surface Science 257 (2011) 1952–1959 1957

F icon (d(

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ig. 6. Contact mode SFM images with scan size of 10 �m of PEO/PAA LBL blend on silc) (PEO/PAA)2.5, (d) (PEO/PAA)1, (e) (PEO/PAA)2, and (f) (PEO/PAA)4.

Complementary information relating to the mechanism ofilayer growth was obtained using SFM, which is a useful tool toxamine changes in morphology of PEO/PAA LBL blends. Fig. 6 isset of SFM images obtained from increasing number of bilay-

eposited via spin-casting) with number of layers: (a) (PEO/PAA)0.5, (b) (PEO/PAA)1.5,

ers. Fig. 6(a–c) are the successive PEO/PAA LBL blend SFM imageswith PEO outer layer. As discussed above, we observed that thebranched-like surface at PEO film on silicon substrate (Fig. 6(a)).However, for (PEO/PAA)1.5 and (PEO/PAA)2.5 (Fig. 6(b and c)), their

1958 X. Gu, G. Wang / Applied Surface Sc

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ig. 7. Friction coefficient vs. thickness for pure PEO, pure PAA, PEO–PAA blend,nd PEO/PAA LBL films, (inset) difference in PAA and PEO friction coefficient,� = PAA–PEO calculated from best fit lines.

ranched-like morphology were disappeared gradually ,whichndicated that PAA limited the crystallization formed. And also, this

as indicative of PEO/PAA blend for bulk-like film.Increasing bilayers films were observed for the PEO/PAA blend

lms with PAA outer layer whose topographic images were shownn Fig. 6(d–f). During formation of the films based on the PEO/PAABL film, as in the case of the PEO–PAA blend, the homogeneousmorphous film was taken place at all bilayers. For (PEO/PAA)1ith PAA outer layer of Fig. 6(d), and in contrast to (PEO/PAA)0.5

Fig. 6(a)), the image exhibited uniform smooth surface structurehich the branched-like crystals were disappeared completely. In

ases of Fig. 6(e and f), uniform surface still could be observed atPEO/PAA)2 and (PEO/PAA)4 with PAA outer layer.

.4. Local phase analysis

As shown in the previous work [15], it was precarious to assumehat layer-by-layer process involving hydrogen bonding producesruly laminar phase separated systems. Dubas and Schlenoff [31]eported for factors to control the growth of LBL films, which indi-ated excess polymer was accommodated within several layers,ather than in one layer of loops and tails. To further illuminate thisspect, we locally investigated for each added PEO/PAA stack itshase composition. Thereby we employed a technique, i.e., lateralorce microscopy (LFM) [32,33], known for its phase distinguishingptitude.

All lateral force data revealed a monotonic decrease in thepplied force normalized lateral forces, i.e., the friction coefficients, for (a) both virgin phases (PEO and PAA) and (b) the PEO/PAA

BL stacks with decreasing thickness (Fig. 7). This was in accor-ance with previous polymer thin film studies [15,23,24], for filmhicknesses on the order of 100 nm or less, and is pinpointing totructure heterogeneities [34]. Of particular interest in the studyere, was the relative difference in the friction coefficients betweenEO and PAA, �� = �PAA − �PEO, as compiled in the inset of Fig. 7.irst, it revealed, constantly higher � values for PAA than PEO, i.e.,PAA > �PEO, and second, a non-monotonic behavior of �� below300 nm, indicative of a material specific different impact of inter-

acial constraints on structure properties in PEO and PAA. The twobservations were in accordance with the phase state of the two

aterials. At room temperature, PAA is in a glassy state and far

elow its glass transition (Tg,PAA = 99 ◦C) [15], and PEO is in a rub-ery state far above its glass transition (Tg,PEO = −56 ◦C). For thickerlms (>300 nm), �� saturated to a constant film thickness inde-endent value. The minimum in �� at ∼150 nm was highlighting

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ience 257 (2011) 1952–1959

the range over which interfacial constraints are impacting the rel-ative material property difference of PEO and PAA.

Having established with LFM the phase distinguishing featuresfor the two virgin polymer phases, we turned our attention to theblend and “layered” system. As indicated with the fit lines in Fig. 7,the PEO–PAA blend and (PEO/PAA)n LBL films were enveloped bythe virgin material phases of PEO and PAA. Based on the PEO–PAAblend fit line, the friction value of bulk film was in middle of virginPEO and PAA, which indicated that the blend film was homoge-neous. However, for Ideal PEO/PAA LBL film, one would expect for atrue layer-by-layer assembly process, the top layer to consist of PAAalone, and thus, lateral force values to match the virgin PAA phase.Contrary to this and in correspondence to the non-linear “growth”as addressed in Fig. 7, LBL stacks did contain in plane mixtures ofboth polymer components.

4. Conclusions

In this paper, the morphology of the ultrathin films of the PEOand PEO–PAA blends spun on the silicon substrates was investi-gated. According to the direct insight by SFM, the effect of solvent,thickness, and dewetting was significant to branched-like formedand disappeared. The crystallization of ultrathin PEO film exhibitedsmooth morphology using chloroform as a solvent, but recoveredto crystallization image after dewetting. Also, it suggests that theaddition of PAA is an appropriate way of describing how thesebranched-like crystallizations were suppressed completely. Crys-tallization of ultrathin PEO–PAA blend film disappeared in thepresence of higher amount of PAA (>60%). However, the PEO filmshowed small hole in thin film and banded structure in thick filmwhen amount of PAA was lower than 43%. PAA/PEO LBL blend werefabricated at first time, via alternating spin-casting of PEO and PAAlayers from aqueous solutions. All surfaces demonstrated PEO/PAAblends rather than a heterogeneous mixture based on LFM and SFM.Also, it is indicative of the interfacial constraints in the PEO/PAALBL film.

Acknowledgments

We would like to acknowledge Prof. Rene Overney (Departmentof Chemical Engineering, University of Washington) for experimen-tal guidance throughout this work and Daniel Knorr (Departmentof Chemical Engineering, University of Washington) for assistingwith experiments.

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Interfacial morphology and friction properties of thin PEO and PEO/PAA blend filmsIntroductionExperimentalMaterials and sample preparationCharacterization

Results and discussionMorphology of PEO ultrathin filmMorphology of PEO–PAA blend ultrathin filmMorphology of PEO–PAA LBL film via spin-assemblyLocal phase analysis

ConclusionsAcknowledgmentsReferences