Journal of Colloid and Interface Science 420 (2014) 174–181

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Journal of Colloid and Interface Science

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Adsorption of polyalkyl glycol ethers and triblock nonionic polymerson PET

0021-9797/$ - see front matter � 2014 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcis.2014.01.012

⇑ Corresponding author at: Departments of Forest Biomaterials and Chemical andBiomelecular Engineering, North Carolina State University, Campus Box 8005,Raleigh, NC 27695-8005, USA. Fax: +1 919 515 6302.

E-mail address: ojrojas@ncsu.edu (O.J. Rojas).

Junlong Song a,b,⇑, Wendy E. Krause c, Orlando J. Rojas b,d,⇑a Jiangsu Provincial Key Laboratory of Pulp and Paper Science & Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, PR Chinab Departments of Forest Biomaterials and Chemical and Biomelecular Engineering, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005, USAc Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USAd Department of Forest Products Technology, Faculty of Chemistry and Materials Sciences, Aalto University, P.O. Box 16300, Aalto FIN-00076, Finland

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 September 2013Accepted 9 January 2014Available online 16 January 2014

Keywords:PolyesterNonionic polymeric surfactantsPolyalkyl glycol ethersTriblock nonionic polymersAdsorptionSelf-assemblyHydrophobic forcesPolyesterPET

Surface modification enables fiber lubrication and processing but little is known about the extent anddynamics of adsorption of typical adsorbates applied for such purposes, which often includes water-sol-uble block nonionic amphiphilic polymers. In this work we used quartz crystal microgravimetry (QCM) toinvestigate the adsorption on poly(ethylene terephthalate) (PET) of polyalkyl glycol ethers and triblockmolecules of ethylene oxide and propylene oxide. The adsorption from aqueous solution of the block non-ionic amphiphilic polymers strongly correlated with the self-association driven by the chain length of therespective hydrophobic blocks. This was demonstrated for the different adsorbing polymers using hydro-phobic numbers calculated from simple group contribution methods. Hydrophobic and van der Waalsinteractions explain the affinity of the nonionic polymers with PET, which lead to adsorption isothermsthat follow Langmuir-based and one-step empirical adsorption models.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

Water-soluble copolymers of poly(ethylene oxide)–poly(pro-pylene oxide) (PEO–PPO) belong to an important class of surfac-tants that find widespread application in detergency, dispersionstabilization, foaming, emulsification, lubrication, and formulationof cosmetics, inks, etc. They are also important component in for-mulations for specialized applications in pharmaceutical and bio-medical industries and can be used as templates for thesynthesis of mesoporous materials and nanoparticles [1–14]. Non-ionic block copolymers tend to form micelles in dilute aqueoussolution and adsorb extensively to a large variety of surfaces; thisis mainly due to their amphiphilic nature. Most of the work re-ported in the literature is concerned with highly hydrophobic[5,10,15] or highly hydrophilic surfaces [4,11]. Factors such asthe architecture of the polymer [7], temperature [6,16] and thepresence of electrolytes [17] are reported to influence the adsorp-tion of nonionic polymers. However, little information is available

about their behavior on common textile substrates, such as polyes-ter and nylon, which are of moderate hydrophobicity.

Poly(ethylene terephthalate) (PET) is a thermoplastic polymerof the polyester family which is widely used in synthetic fibers,beverage, food, liquid containers, and medical devices. Le Guernet al. [18] investigated the adsorption of lignosulphonates on PETby the means of X-ray photoelectron spectroscopy, while Li et al.[19] studied the adsorption of proteins on micropatterned flexiblePET surfaces. Recently, we reported on the adsorption of triblockPEO–PPO–PEO polymers on PET and other polymeric surfaces byusing the QCM technique [20], determined their effect on lubrica-tion properties using atomic force microscopy [21] and unveiledthe phase behavior of aqueous EO19PO29EO19 solutions confinedand sheared between hydrophobic and hydrophilic surfaces byusing computational methods [22]. Here we develop the struc-ture–property relations applicable to nonionic polymers that canassist in the design of finishes and new lubricants for PET. The typeof PET surfaces used consisted of ultrathin and flat films depositedon Quartz Crystal Microbalance (QCM) sensors by spin-coating[23]. Two sets of PEO–PPO copolymers, namely, polyalkylene gly-cols (PAGs) and triblock polymers based on ethylene oxide andpropylene oxide PEO–PPO–PEO (commercially known as Pluronics)were investigated with regards to their adsorption behavior on PETsurfaces by using QCM. Thus, in situ and in real time monitoring of

J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181 175

the interactions between series of these nonionic surface-activepolymers was made possible. The adsorbed mass [24] (obtainedfrom changes in vibration frequency) and energy dissipation [25](which is related to the conformation of the adsorbed species)were monitored as a function of time and concentration. The influ-ence of the nature of nonionic polymers (structure, molecularweight, hydrophilic-hydrophobic balance and length of the hydro-phobic segments) was studied in terms of the adsorption onto PET.

2. Materials and methods

All experiments were performed with deionized water from anion-exchange system (Pureflow, Inc.), which was further processedin a Milli-Q� Gradient unit to ensure ultrapure water with resistiv-ity greater than18 MO cm.

2.1. Materials

The nonionic amphiphiles used included a series of polyalkyl-ene glycols (PAGs) from Dow Chemical Co. (Midland, MI), and tri-block polymers based on ethylene oxide and propylene oxide(Pluronics) from BASF Corporation (Florham Park, NJ). Both typesof polymers were selected on the basis of their frequent use inthe formulation of aqueous-based lubricants in textile and otherapplications. The generic chemical structure of the PAGs and thetriblock polymers used are shown in Fig. 1 while the key structuralparameters (molecular weight and segment numbers m and n) arelisted in Table 1. The polymer samples were chosen for their differ-ent monomer composition and molecular mass (Table 1) as theywere available since a systematic variation in the molecularparameters is difficult to achieve.

2.2. Poly(ethylene terephthalate) films

The solid supports for PET used in this investigation were gold-coated QCM resonators (Q-Sense, Sweden) that were cleaned withPiranha solution (H2SO4 (98%): H2O2 (30%)) for one hour and thensubjected to ultra-violet ozone (UVO) radiation for 10 min immedi-ately before spin-coating (3000 rpm spinning rates, 30 s) with PETsolutions in hexafluoroisopropanol. The resultant PET film thick-ness was measured to be in the 33–38 nm range as determinedby the shift in QCM frequency measured in air and verified byellipsometry. The films were rather smooth, 0.3–1.1 nm as evalu-ated by AFM. More details about film preparation and images canbe found in Ref. [23].

RO-[CH2CHO]n[CH2CH2O]m-H

CH3RO-[CH2CHO]n[CH2CH2O]m-H

CH3RO-[CH2CHO]n[CH2CH2O]m-H

CH3RO-[CH2CHO]n[CH2CH2O]m-H

CH3H-[CH2CH2O]m-[CH2CHO]n[CH2CH2O]m-H

CH3H-[CH2CH2O]m-[CH2CHO]n[CH2CH2O]m-H

CH3H-[CH2CH2O]m-[CH2CHO]n[CH2CH2O]m-H

CH3H-[CH2CH2O]m-[CH2CHO]n[CH2CH2O]m-H

CH3

Fig. 1. Chemical structure of polyalkylene glycols (PAGs) (left) and triblockpolymers (right).

Table 1Relevant data and nomenclature of the polyalkylene glycols (PAGs) and triblock (EOnPOmE

Symbola Commercial name MW

RPO10EO13 UCON 50-HB-400 1230RPO13EO17 UCON 50-HB-660 1590RPO33EO44 UCON 50-HB-5100 3930EO19PO29EO19 Pluronic P65 3400EO76PO29EO76 Pluronic F68 8400EO37PO56EO37 Pluronic P105 6500

a R, P and E stand for butyl, propylene oxide and ethylene oxide groups, respectively.b HLB values were calculated by the method described by Guo et al. [26].c Critical Micelle Concentration (CMC) values of triblock polymers are taken from Ref

2.3. Surface tension measurement

A Fisher Surface Tensiomat�, model 21 with a Du Noüy Ringwas employed to measure the surface tension of the polymericsolutions at 25 �C. The maximum pull force exerted on the ringat the air–liquid interface was measured and translated to unitsof surface tension, mN/m.

2.4. QCM-D technique

A QCM-E4 (Q-sense Inc.) was employed to measure the adsorp-tion of the nonionic polymers on PET films. The principle of theQCM technique involves the monitoring of the resonant frequency(f) of gold-coated piezoelectric material (quartz crystal) which de-pends on the total oscillating mass [27–31]. If the film is thin andrigid, the decrease in frequency is proportional to the mass of thefilm, as stated by Sauerbrey equation [24]:

Dm ¼ � cDfn

ð1Þ

where C = 17.7 ng Hz�1 cm�2 for a 5 MHz quartz crystal and n = 1, 3,5, 7 is the overtone number.

Since shifts in frequency can be detected very accurately, theQCM operates as a very sensitive balance, with a sensitivity of0.5 ng/cm2. The adsorbed mass of the polymeric surfactant (assum-ing uniform coverage) was obtained assuming the respective layerdensity. In our investigation, only the third overtone frequency wasmeasured at a constant temperature, 25 �C. The reported adsorp-tion isotherms were obtained as averages from at least three differ-ent adsorption experiments carried out for each condition.

2.5. Contact angle measurements

Spin coated PET surfaces were immersed in 1% aqueous poly-mer solution and allowed to equilibrate overnight. The surfaceswere then rinsed extensively with milli-Q water and then driedvia a nitrogen gas stream directed gently on the surfaces. The sta-tic, initial water contact angle was then measured using a manualRame-Hart goniometer, 1 min after placing a droplet (10 lL) ontothe surface and outlined by an optical magnifier. The tangent lineon the droplet in the three-phase zone was traced with a protractorwithin the optics. Three different locations on each film were eval-uated and at least three replicates of water contact angle (WCA)measurements were carried out for each location. The contributionof roughness or surface features in WCA is expected to be mini-mum. The average values are reported noting that the standarddeviation was of the same order of magnitude as that of the typicalexperimental error in water contact angle measurements for flatfilms.

On) polymers used.

n (PO) m (EO) HLBb CMCc

10 13 11.9213 17 12.033 44 8.5129 19 21.67 38.22 mM29 76 27.97 320.5 mM56 37 17.34 0.461 mM

. [1].

176 J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181

3. Results and discussion

3.1. Adsorption at the air/water interface

Surface tension isotherms were acquired at the air/liquid inter-face of the respective polymer solution (see Fig. 2a and b for PAGand triblock polymers, respectively). Low surface activity was ob-served for the low molecular weight (MW) polymers, such asRP10E13 and RP13E17 (Table 1). High MW nonionic polymers(MW > 3000 Da) such as RP33E44 and all triblock polymers showedtypical surface tension profiles. The surface tension isotherm ofE37P56E37 (Pluronic P105) displayed three regions characterizedby a steep decrease in surface tension at low concentrations fol-lowed by a less marked surface tension reduction and a levelingoff at high concentrations. The shift from the first and second re-gion in the surface tension profiles was observed to occur at acharacteristic concentration (0.7 lM); the shift from the secondand third region at 0.9 mM corresponded to the critical micellar

Fig. 2. Surface tension profiles for (a) block polyalkylene glycols (PAGs) and (b)triblock (EOnPOmEOn) polymers (25 �C).

concentration (CMC). Our observations are in agreement with thereport by Alexandridis et al. [1,32] with a minor difference in thatthe CMC they reported was lower (0.46 mM), which is explained bythe differences in temperature, measuring methods, and thepresence of electrolytes in the system. For other two triblockpolymers, the CMC reported by the same authors [1,32] was 38.2and 320.5 mM, respectively, which are beyond the concentrationrange considered in our study. For the PAGs, the CMC is relativelyhigh, at concentrations beyond the range tested. Moreover, wenote that all the adsorption experiments discussed here wereperformed at sub-micellar concentrations.

At a given temperature, adsorption of the surface active mole-cules at the air/liquid interface is related to the polymer concentra-tion and surface tension according to the Gibbs adsorptionisotherm:

C ¼ � 1RT

@c@ ln c

� �T

ð2Þ

where U is surface excess; R is universal gas constant(8.3145 J mol�1 K�1); T is Kelvin temperature. In our investigation,the temperature was kept at 25 �C (298 K). As the concentration in-creases, the interface becomes saturated and U = Um for maximumpacking at the interface. Thus the cross sectional molecular areaAm at interface can be calculated according to steepest slope:

Am ¼1

Cm � L0ð3Þ

where L0 is the Avogadro constant. Based on the surface tensionmeasurements, the surface excess and the maximum surface ex-cess, as well as the area for each molecule were calculated, Am

(see Table 2).RPO10EO13 and RPO13EO17 displayed relatively low surface

activity, compared to the other amphiphilic polymers. Accordingly,a low maximum adsorption amount Um and a large adsorbedmolecular area were determined in both cases. The polymersassembled at the water/air interface depending on their hydro-philic–hydrophobic balance. In turn, such balance depends on thesize of the respective blocks. Therefore, hydrophobicity/hydrophi-licity and block size are interrelated, as far as their effect in thepacking density at the interface. RPO10EO13 and RPO13EO17 areloosely packed at the interface, as was the case of the other tri-block (EOnPOmEOn) polymeric surfactants. Within the EOnPOmEOn

amphiphilic macromolecule, the P segments are more hydrophobicthan the E segments. EO19PO29EO19 and EO76PO29EO76, possessingthe same hydrophobic segment, exhibited quite different molecu-lar area, due to different hydrophilic segments. This is obviouslythe result of the two longer hydrophobic chains in the case of EO76-

PO29EO76. Both long hydrophobic chains occupy a larger area thanthose for shorter surfactants crowding the surface. For EO37PO56-

EO37, the molecular area measured was 1.1 nm2, in agreement withthe valued reported by Alexandridis et al. (1 nm2) [32]. Overall, theresults highlight the fact that both hydrophobic and hydrophilicsegments are critical in the formation of interfacial layers at theair/water interface.

Table 2The maximum surface excess Um and area per molecule Am calculated from Gibbsadsorption isotherm.

Polymers Um � 107 (mol m�2) Am (nm2)

RPO10EO13 7.0 2.4RPO13EO17 10.0 1.7RPO33EO44 15.8 1.1EO19PO29EO19 13.9 1.2EO76PO29EO76 10.4 1.6EO37PO56EO37 15.6 1.1

J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181 177

3.2. Adsorption at the PET/water interface

Adsorption experiments were performed with the QCM-D asdescribed in the Methods section. Typical frequency and dissipa-tion profiles as a function of adsorption time are shown in Fig. 3(for EO76PO29EO76 at a concentration of 0.0001% (1.2 � 10�4 mM)).Here, the frequency and dissipation values corresponding to threeovertones were recorded simultaneously. Since the third overtonevibration reaches a larger area in the sensor (as compared to thefifth and seventh overtones), we used the third overtone frequencyand dissipation to determine the changes in the adsorbed mass,according to Eq. (1), unless specified otherwise. In a typicalexperiment, water was first injected continuously in the QCMsample loop until stable (frequency and dissipation) baselines wereobtained. Following, nonionic polymer of given concentration wasinjected. As a result a sharp drop in frequency and increase indissipation were observed (Fig. 3). These changes are indicative ofthe adsorption of the polymer on the surface of the PET-coatedsensor.

The adsorption of nonionic polymers was found to be a very fastprocess, as required in practical applications. Around 10 min afterinjection, both adsorption and dissipation curves reached a pla-teau. The frequency and dissipation values shown in Fig. 3 werecollected for additional 10 min after the plateau values. Polymericsurfactants are known for their slow kinetics, and they usually un-dergo various adsorption stages, involving changes in adsorbedstate conformation at different characteristic times. It is likely thatin our experiments thermodynamic equilibrium was not achievedbut a dynamic equilibrium that leads to steady state QCM signals;this is relevant for the purposed use of the surfactants for surfacemodification in fast processes. Thereafter, we refer to equilibriumas the state at which no measurable changes in adsorption was re-corded with time (plateau conditions). Polymer free solution(water) was always injected at the end of the respective experi-ment in order to rinse the system. As a result, a sharp increase infrequency and drop in dissipation were observed. These changesare indicative of the fact that excess, loosely bound molecules aswell as molecules in the bulk solution were removed during rins-ing. After ca. 10 min, a second steady state was observed withhigher frequency and lower dissipation, as compared with the

Fig. 3. Typical QCM Df and DD profiles for nonionic polymers adsorbing PET. In thisexperiment, EO76PO29EO76 (1.2 � 10�4 mM aqueous solution) was injected contin-uously on a QCM sensor coated with PET. The shifts in frequency (Df) (left axis) anddissipation (DD) (right axis) for three overtones (3rd, light-grey, 5th, grey and 7th,black) are presented simultaneously. The flow rate was maintained at 0.1 ml/minand temperature of 25 �C.

initial values. Based on these observations, the adsorbed mass isreferred to as ‘‘reversible’’ (first plateau) and ‘‘irreversible’’ (secondplateau). From the changes in dissipation it can be concluded thatthe polymer adsorbed layer was very thin and ‘‘rigid’’ (the differ-ence of dissipation with and without adsorbed polymer was lessthan 1 � 10�6 dissipation units).

Five polymer concentrations (ranging from 0.0001 to 1 wt%)were run in QCM adsorption experiments (using at least three rep-etitions). The equilibrium adsorption mass before (reversibleadsorption) and after rinsing (irreversible adsorption) were re-corded as a function of polymer solution concentration, as shownin Fig. 4(a) and (b), respectively. In these curves each data pointwas obtained as an average from multiple runs as that illustratedin Fig. 3. The adsorbed amount was calculated from the frequencychange by using the Sauerbrey equation (Eq. (1)). The deviation inadsorption amounts at high concentrations was noted to be largerthan that at low concentrations, which may due to the influence ofadsorbed layer of polymer prior to injection of solutions. The‘‘reversible’’ adsorption, i.e., adsorption before rinsing, changedroughly linearly with polymer concentration. Furthermore, it isnoted that the irreversible adsorption for some copolymersreached saturation at high polymer concentrations in Fig. 4(b).

We note that from the surface tension isotherms a maximumadsorbed mass in the range of 7–16 � 10�7 mol/m2 was measuredat the air/water interface. However, the adsorbed mass on PET be-fore rinsing was larger. This may be taken as indicative that thatloosely bound polymer multilayers are formed (before rinsing).However, after rinsing the adsorbed mass for each nonionic poly-mer (0.5–7 � 10�7 mol/m2) was found to be lower than the maxi-mum adsorption excess obtained from the surface tensionexperiments.

For rigid, ultrathin, and evenly distributed adsorbed layers, theSauerbrey equation (Eq. (1)) describes a simple relationship be-tween the adsorbed mass (C) and the shift of QCM resonance fre-quency (Df). If the adsorbed layer is viscoelastic (a condition thatapplies to many adsorbed polymers, e.g. proteins), a substantialdeviation from the Sauerbrey approximation occurs. This was notour case since the adsorbed layer was thin and the dissipation val-ues were rather small.

DD–Df plots can reveal structural changes in the adsorbinglayer as they occur with time, i.e., the changes in the (DD/Df) slopeprovide information about the kinetic regimes and conformationalchanges [33]. As such, Fig. 5 shows the DD–Df plots for diblock andtriblock polymers. The curves for RPO10EO13 and RPO13EO17 over-lap and exhibit a steep DD/Df slope. This indicates that thesetwo polymers, with the lowest molecular mass, adsorbed as verysoft layers. Furthermore, DD–Df loops are observed in each case,after rinsing. For the other polymers, two different DD–Df slopeswere observed. The first, flat slope indicates an initial adsorptionin the form of a compact layer. As more molecules diffuse to theinterface, further molecular assembly takes place giving rise to asteeper slope, with large dissipation. Since the binding betweenmultiple adsorbing polymer layers is expected to be weak com-pared with the molecules in the close vicinity to the surface, theycan be removed easily by rinsing (loops in the DD–Df profiles). Fur-thermore, it is also observed that the conformation of the adsorbedlayer for high molecular mass polymers is more compact than inthe cases described before.

3.3. One-step adsorption model

Different surfactant adsorption isotherms have been used in theliterature. They are generally divided into three types, namely,Langmuir (or L-type), S-type, and a combination that containstwo plateau regions (LS-type) [34]. For example, L-type isothermshave been reported for triblock polymers of the Pluronic type when

Fig. 4. QCM equilibrium adsorption isotherms (a) before and (b) after rinsing for nonionic polymers adsorbed on PET surfaces. The reported adsorbed mass was calculated asan average from three adsorption experiments and the standard deviation is reported for each condition (note error bars).

Fig. 5. DD–Df curves to indicate conformational changes during polymer adsorp-tion on PET and after rinsing with water: (a) nonionic diblock polyakylene glycolsand (b) triblock EOnPOmEOn polymers on PET.

178 J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181

adsorbed on different solid surfaces [20]. However, in our casemost of the isotherms followed the S- or LS-types. Recently, Guet al. [34] described isotherms of the L-, S-, and LS-types using a‘‘one-step’’ or a ‘‘two-step’’ model. In the one-step model, the sur-factant monomer interacts with the surface active site(s) to form ahemi-micelle:

SiteþMonomer$ Hemi-micelle

The constant at equilibrium adsorption can be written as:

k ¼ chm=csC ð4Þ

where chm, cs and C are the concentrations of the adsorbed hemi-mi-celle, surface site, and surfactant monomer, respectively. The sur-factant concentration can be related to the adsorption density (C)through a mass action law to yield Eq. (5):

CC1 � C

¼ kCn ð5Þ

where C1 is the maximum adsorption density at high solution con-centration. When n = 1, the expression is reduced to that corre-sponding to the Langmuir L-model. Here the Langmuir adsorptionfits a one-step model (aggregation number is 1). Then the standardfree energy for surface aggregation DG0

sa is calculated using Eq. (6):

DG0sa ¼ �

1n

RT ln k ð6Þ

where R is the gas constant and T is the absolute temperature. Thestandard free energy for hemi-micellization can be compared tothat for micellization:

DG0mic ¼ �RT lnðCMCÞ ð7Þ

Our results, fit well the Langmuirian (R2 = 0.90–0.98) as well as theone-step models (R2 > 0.97). Following Eq. (5) we plotted thelog½C=ðC1 � CÞ� against logC for nonionic polymers adsorption be-fore and after rinsing (Fig. 6). Accordingly, the fitting parametersafter minimizing the weighted sum of squared error betweenexperimental data and corresponding isotherm-computed valuesare given in Table 3, which also includes the standard free energyfor surface aggregation (Eq. (6)).

From Table 3, the aggregation number n is found to be less than1, indicating that each adsorbed molecule occupies more than onesite. For the nonionic surfactants with known CMC values, all threetriblock polymers have a lower free energy of hemi-micellizationthan micellization. This hints to the possibility that such surfaceaggregation on the PET surface is energetically more favored thanmicellization in solution.

Fig. 6. Plots of log½C=ðC� CÞ� against logC for nonionic polymers (a) before and (b)after rinsing (see also Eq. (5)).

Table 3One-step fitting parameters obtained after minimizing the weighted sum of squared eadsorption curves of nonionic polymers on PET before and after rinsing.

Nonionic surfactant C1 K

(10�7 mol m�2)

Before rinsingRPO10EO13 30 9.9RPO13EO17 30 21.9RPO33EO44 20 10.1EO19PO29EO19 25 16.9EO76PO29EO76 20 12.2EO37PO56EO37 20 11.3

After rinsingRPO10EO13 1.2 6.3RPO13EO17 2.5 4.1RPO33EO44 4.6 1546.6EO19PO29EO19 17.6 18.7EO76PO29EO76 8.9 49.0EO37PO56EO37 5.1 9.4

J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181 179

3.4. Effect of polymer structure on adsorption

Since no reaction with the surface takes place upon adsorptionand since there are no charged groups in the nonionic surfactantchains, adsorption is expected to obey physical processes andtherefore van de Waals and hydrophobic interactions are likelyto be dominant. The effect of molecular mass of the nonionic poly-mers on the adsorbed amount is apparent from results shown inFig. 4(a). High molecular mass polymers are subject to larger vande Waals attraction to the surface. However, the effect of polymerhydrophilic–lipophilic balance, HLB, is not as easy to elucidate.

The elucidation of hydrophobic forces has been attempted inclassical discussions of surface science [35–39]. The hydrophobicinteraction has been qualitatively described as that produced byhydrophobic moieties that aggregate or cluster. It is responsiblefor the significant work of adhesion between solid hydrophobicsurfaces in water to minimize contact with the surrounding med-ium [40]. The hydrophobic effect depends on the hydrophobicity ofthe adsorbate and surface and it can be divided into short-, med-ium- and long- ranged forces. In our case, the water contact angleof PET is around 65�, a material whereby the hydrophobic forcesmay be of the short-range type.

The HLB values were used as a way to determine the hydropho-bicity (or hydrophilicity) balance of the molecule. However, thegroup contribution and the effective chain length proposed byGuo et al. [26] motivated the use of an alternative way to charac-terize the hydrophobicity of the polymers. Details about the groupcontribution methods can be found in the literature [41–43]. Themain assumption used in such methods is that a mixture doesnot consist of molecules, but functional groups. By using thermo-dynamic fundamentals, it can be shown that the required activitycoefficients can be calculated when only the interaction parame-ters between the functional groups are known. The advantage ofgroup contribution methods is that the number of functionalgroups is much smaller than the number of possible molecules.

In our case, the R and PO groups are relatively more hydropho-bic than the EO groups (see Table 1) and therefore we use adescriptor for these units based on what is known for a straight al-kyl chain. A fully stretched lipophilic chain length is directly pro-portional to the number of –CH2– groups. Here NCH2;eff is given by

NCH2;eff ¼ 0:965NCH2 � 0:178 ð8Þ

and NPO;eff is given by

NPO;eff ¼ 2:057NPO þ 9:06 ð9Þ

rror between experimental data and corresponding isotherm-computed values for

n R2 �DG0sa �DG0

mic

(kJ mol�1) (kJ mol�1)

0.43 0.99 13.20.48 0.99 15.70.36 0.99 160.41 0.98 16.85 7.950.35 0.99 17.8 2.80.26 0.97 22.8 18.7

0.23 1.00 19.40.18 0.99 19.20.64 0.99 28.30.45 0.97 16.10.44 0.98 21.70.14 0.98 39.2

Fig. 7. Plots of (a) fitted parameters aggregation number, n and (b) calculatedstandard free energy for surface aggregation, �DG0

sa before rinsing as a function ofthe polymer hydrophobic number (see Tables 3 and 4).

180 J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181

where NCH2,eff and NPO,eff represent effective chain lengths for –CH2–and PO groups, while NCH2 and NPO represent actual chain lengthsfor –CH2– and PO groups, respectively. The pre-factors 0.475 for –CH2– and 0.15 for PO were used directly from the literature [26].The hydrophobic number of one molecule is thus the product ofthe effective chain length and the group contribution number. Thecalculated hydrophobic numbers for the polymers listed in Table 1are presented in Table 4.

When the aggregation number, n and standard free energy forsurface aggregation, �DG0

sa are plotted against the respectivehydrophobic number it becomes clear that the affinity betweenthe surface and adsorbate strongly depends on the hydrophobicnumber, as observed in Fig. 7. In this figure we used the data ‘‘be-fore rinsing’’. We note that theoretically the Langmuir modelshould be more suitable to fit the adsorption data obtained afterrinsing since most of adsorbed nonionic surfactant molecules areremoved and a monolayer or sub-monolayer would remain. How-ever, the data before rinsing fitted better the adsorption model;thus, we take the model as an empirical approximation to fit thedata understanding that association at the interface produce devi-ations from the assumptions in the model. When the hydrophobicnumber of a nonionic polymer is larger, a lower aggregation num-ber, n and a lower standard free energy for surface aggregationwere observed.

The affinity of nonionic block copolymer strongly depends onthe hydrophobic number, which is an indicator to equivalenthydrophobic length of only hydrophobic segments in a molecularchain. From this point of view, the architecture of the polymer,i.e. di-block or tri-block copolymer does not matter. However thearchitecture of copolymer may affect the conformation of adsorbedlayer since they have one or two chains extended towards the sur-rounding solution. Since it is difficult to draw meaningful conclu-sions based on the obtained data, we discuss in the followingsection more details about the effect of adsorbate molecularweight and hydrophobic–hydrophilic balance.

The hydrophobic number of Guo et al. [26] as used above, incor-porates both the effect of hydrophobicity of the group (pre-factor)and the molar mass (number of hydrophobic –CH2– and POgroups) but does not consider the contribution of the EO (hydro-philic) components. Since the extent of adsorption depends moreprofoundly on the nature of the anchoring groups (the hydropho-bic segment) it is therefore reasonable to expect that the hydro-phobic number relates closely with the surface aggregationnumber and the free energy to form hemi-micelles.

There is no obvious correlation between the fitted parameters,the aggregation number (n), the calculated standard free energyfor surface aggregation ð�DG0

saÞ after rinsing and the hydrophobicnumber of the polymer. This may be explained by the stability ofthe formed hemi-micelles, which associates via interactions withthe substrate, the structure of the hemi-micelles, and even thestructure of the polymers, e.g. the length of hydrophobic andhydrophilic segments and their conformation in solution.

In this discussion, we did not consider the hydration of theadsorbed polymer. The QCM technique usually overestimates the

Table 4Calculated hydrophobic numbers N for the nonionic polymers investigated.

Polymer NCH2,eff NPO,eff Hydrophobic numbera

RPO10EO13 3.68 29.6 6.2RPO13EO17 3.68 35.8 7.2RPO33EO44 3.68 76.9 13.4EO19PO29EO19 68.7 10.4EO76PO29EO76 68.7 10.3EO37PO56EO37 124.3 18.6

a The pre-factors used to calculate the hydrophobic numbers are 0.475 and 0.15for –CH2– and PO, respectively.

adsorbed mass due to the fact that it measures the effective mass,including the contribution of water coupled to the polymer. Forexample, it is expected that the amount water coupled to EO37-

PO56EO37 adsorbed on PET is about 40% [20]. It is further expectedthat the rest of amphiphilic polymers studied here form hydratedadsorbed layers with similar water coupling contribution; whilethe trends discussed before still hold, an accurate assessment ofthe mass would be required to confirm the contribution ofhydration.

3.5. Contact angle and surface assembly

Adsorption of PO10EO13, RPO13EO17 and EO19PO29EO19 on PETdid not produce a significant change in the water contact angle(WCA) of the surface. RPO33EO44 lowered the WCA. In contrast,the WCA in the case of PET treated with EO76PO29EO76 and EO37-

PO56EO37 showed an extensive change (from 63� to 40� and 52�,respectively). The changes in contact angle support the observed

J. Song et al. / Journal of Colloid and Interface Science 420 (2014) 174–181 181

fact that a layer of nonionic amphiphilic molecules forms uponadsorption. As discussed in previous sections, the hydrophobic seg-ments of the polymer form hemi-micelles via the hydrophobic ef-fects and adsorb onto the hydrophobic surface, leaving thehydrophilic segments of the adsorbed polymers extended out intothe aqueous phase as buoy. This is consistent with the assumptionof one-step model. It is expected that a polymer with high adsorp-tion excess and long hydrophilic chains would lower the contactangle more significantly, as can be confirmed from the data.

The WCA results also demonstrate that the adsorbed mass ofEO76PO29EO76 is lower than that of EO37PO56EO37. EO76PO29EO76

has longer hydrophilic chains than EO37PO56EO37, which explainsthe larger WCA change.

The configuration of adsorbed nonionic block polymer layerscould contribute to reduce friction in boundary lubrication. Thiswas confirmed by lateral force microscopy experiments that mea-sured the friction coefficient between the AFM tip and several sur-faces pretreated with the different nonionic block polymers [21]. Itwas found that both the adsorbed amount of block polymers andthe conformation of adsorbed layer were critical in boundarylubrication.

4. Conclusions

The extent of adsorption of nonionic block polymers onto PETand the adsorption dynamics were measured by the QCM tech-nique. The nature of nonionic amphiphilic molecules, such asmolecular weight, HLB and hydrophobic number were considered.It is suggested that van der Waals and hydrophobic interactionsplay important roles in the adsorption behaviors. The hydrophobicsegments in an amphiphilic polymer provide the affinity to anchorthe macromolecules on the surface while a high molecular masscan provide maximum adsorption density. The nature of themolecular assembly was investigated through contact angle mea-surements. The hydrophobic segments in given amphiphilic non-ionic polymer tend to adsorb onto the hydrophobic surface whilethe hydrophilic blocks tend to buoy in the solvent to reduce theinterfacial energy between PET surface (hydrophobic) and themedium (water). Therefore, the adsorbed nonionic polymer canstabilize the surface and provide the basis for characteristic prop-erties in terms of boundary friction and wear.

Acknowledgments

This work was supported by the National Textile Center underthe Grant C05-NS09, and also partially supported by NSFC(31270613), Talents Foundation of Nanjing Forestry University

(163105003), and the Priority Academic Program Development ofJiangsu Higher Education Institutions.

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