xps and atr surface studies of block copolyurethanes based on 1,2-ethylene bis(4-phenyl isocyanate)

Macromol. Rapid Commun.20,497–504 (1999) 497

XPS and ATR surface studies of block copolyurethanes based on1,2-ethylene bis(4-phenyl isocyanate)

Tzong-Liu Wang*, Fang-Jung Huang

Department of Chemical Engineering, National Kaohsiung Institute of Technology, Kaohsiung, Taiwan 807,Republic of China

(Received: March 22, 1999; revised: April 26, 1999)

SUMMARY: The surface properties of two block copolyurethanes based on 1,2-ethylene bis(4-phenyl iso-cyanate) (P2PDI), poly(tetramethylene ether glycol) (PTMG) or poly(propylene glycol) (PPG), and ethylenediamine, were investigated by attenuated total reflectance (ATR) infrared spectroscopy and X-ray photoelec-tron spectroscopy (XPS) analysis. The air-facing surfaces (AFS) of both materials are more abundant in thePPG or PTMG soft segment. More PTMG present on AFS indicates that the PTMG soft segment is morehydrophobic than PPG. Besides, the ATR spectra revealed that more phase mixing occurs in the near-surfaceregion than in the bulk and the domain/matrix interface tends to orient to this region for both samples.

IntroductionPolyurethane block copolymers are two-phase materialswhich have been proposed for various blood-contactingapplications due to their favorable tensile properties, fati-gue properties, and thromboresistance1–7). The surfaceproperties of a blood-contacting biomaterial are believedto be an important determinant of biocompatibility, so thesurface properties of polyurethane biomaterials are ofinterest. Studies of polyurethane biomaterials in vacuousing XPS techniques have shown that the degree of sur-face enriched by the soft segment phase is dependent onthe degree of phase mixing8, 9). The degree of phaseseparation is dependent on such variables as the polaritydifference between the pure hard and soft segment com-ponents, hard/soft segment ratios, and the polyol molecu-lar weight3). Thus, all of those variables may play a rolein surface properties and blood compatibility.

Since it was also indicated that the presence of the addi-tional methylene group in the phenylene-ethylene-pheny-lene (P2P) structure compared with the phenylene-methy-lene-phenylene (P1P) structure allows a planar stacking ofthe aromatic rings and interurethane hydrogen bondingswhich would be perpendicular to the aromatic rings7).Hence, polyurethanes prepared from 1,2-ethylene bis(4-phenyl isocyanate) could be different in mechanical prop-erties, light stability, hydrolytic resistance and surfaceproperties with those prepared from the more commonlyused diisocyanate, methylene bis(4-phenyl isocyanate)(MDI). Some of these studies have been reported else-where10, 11). This paper describes the results of initial sur-face studies on block copolyurethanes based on P2PDI.

Experimental partMaterials

4,49-Ethylene dianiline (EDAN, Tokyo Chemicals) was puri-fied by sublimation under vacuum. Ethyl acetate (EA, Tokyo

Chemicals) was distilled over P2O5 before use. Phosgene(Fluka Chemicals) was dried by passing the gases throughDrierite and 4 Amolecular sieve before use. Ethylenedia-mine (EDA, Tokyo Chemicals), methyl isobutyl ketone(MIBK, Hayashi Chemicals), N,N-dimethylformamide(DMF, Tokyo Chemicals), dimethyl sulfoxide (DMSO,Nacalai Tesque, Inc.), and tetrachloromethane (Union Che-micals) were distilled under reduced pressure. Poly(propy-lene glycol) (PPG, Wako Chemicals) and poly(tetramethy-lene ether glycol) (PTMG, Du Pont, Inc.) with molecularweight (MW) = 1000 were degassed under vacuum (600 Pa)at 558C for 3 h to remove absorbed water.

Synthesis of 1,2-ethylene bis(4-phenyl isocyanate) andpreparation of block copolyurethanes

The detailed synthetic route for P2PDI and the polymeriza-tion procedure for block copolyurethanes have beendescribed elsewhere10, 11) and are shown in Scheme 1. Forconvenience, polymers made from P2PDI-PPG1000-EDAand P2PDI-PTMG1000-EDA are designated as PPG-PU andPTMG-PU, respectively.

Characterization

The water uptake of the copolymer was measured from aweight increase after immersing the specimens in deionizedwater for 48 h at room temperature.

The contact angle measurements were carried out withwater using a contact angle goniometer (Rame-hart, Inc.).The urethane polymer films were solvent cast onto pre-cleaned microscope slides and then vacuum dried for 24 hbefore the contact angle measurements were taken. An aver-age of five measurements was taken for each polymer exam-ined.

Transmission infrared spectra of the thin polymer filmswere obtained using a Bio-Rad FTS 165 Fourier transforminfrared spectrometer. The spectra were obtained over the fre-quency range from 4000 to 400 cm–1 at a resolution of 4 cm–1.

Attenuated total reflectance (ATR) spectroscopy was per-formed on a Bio-Rad FTS 165 infrared spectrometer using a

Macromol. Rapid Commun.20, No. 9 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 1022-1336/99/0909–0497$17.50+.50/0

498 T.-L. Wang,F.-J.Huang

zinc selenide (ZnSe) ATR element (2561062 mm, 458aperture)supportedon a flat plate ATR unit. The spectrawerealsocollectedat 4 cm–1 resolution.The ATR techniqueobtainsspectrafrom thesurfaceof thepolymer, with a pene-tration depth ranging from 1500 A to 6000 A. The ATRspectramaybecomparedto thetransmissionspectrato iden-tify differencesbetweennear-surfaceand bulk morphology.In this study, the peakheightsof severalvibrationsareusedto determine relative changesin the hydrogen bonding,phaseseparation,andchemicalstoichiometryof the polyur-ethanes.For both samples,the experimentswerecarriedouton the air-facing surfaces(AFS). In all cases,the ATR peakheightshavebeencorrectedfor the depthof penetrationofthe IR evanescentwaveby multiplying by the wavenumberof thepeak.

An analysisof the chemicalcompositionof the solid sur-face is importantin understandingthe natureof the interac-tion. This interfaceis formedbetweena materialanda biolo-gical system.XPS is a non-destructiveanalysistechniquewhich yields elementalcompositionand bonding informa-tion on theuppermost10l 100A. XPSsurfaceanalysiswascarried out using a VG InstrumentsX-ray photoelectronspectrometer. Mg Ka radiationwasusedastheX-ray sourceandthe photoelectronpeaks(in the wide-scanspectra)fromthe sampleswerenumericallyfitted usingLorentziancurveswith an integral backgroundsubtractionand analyzedat anangleof 458 to the surface.The adventitiousC 1s signalat284.6eV was used to calibrate the charge-shifted energy

scale.The X-ray spot size was 600nm and the resolutionwas about 0.8eV always, the spectra representoriginalexperimental datarecordedto anaccuracyof 0.2eV. In addi-tion, theC1speaksweredeconvolutedfor chemicalidentifi-cation using 100% Gaussianpeaks.For the convenience ofcomparison, all of thesurfacestudieswerecarriedout on theair-facing surfaces(AFS) of the copoly(ether-urethane-urea)samples.

Resultsand discussion

Attenuatedtotal reflectance(ATR)infraredspectroscopy

Fig. 1 shows both transmissionandATR infraredspectraof PPG-PU, representing the morphology of both bulkand near-surface region. The spectra of both regions aresimilar, showingpeaksat thesamewavenumbers,but dif-fering somewhat in relative peak heights. It is evidentthat less free (non-hydrogen bonded) N1H stretchingband at 3450 cm–1 is presentin the near-surfaceregion.In contrast to that a higher degreeof hydrogen bondedN1H of the urethaneor urea groupsappearsat the air-facing surface. In polyether-polyurethanes,the hydrogenbonding proton acceptors are found to be the carbonyloxygen in the hard segment and the ether oxygen in the

Scheme1: Syntheticroutefor thepreparationof P2Ptypecopolyurethanes

XPSandATR surfacestudiesof block copolyurethanesbasedon ... 499

soft segment.Theratio of thebondedto freeurethanecar-bonyl peakswil l increaseasthehydrogenbonding protonacceptorchangesfrom the soft segmentetheroxygentothe carbonyl oxygen.As seenin the ATR spectrum, thehydrogen-bondedurethanecarbonyls form a shoulder at1704 cm–1. The non-hydrogenbondedor free carbonylspossessthe major absorption at 1727 cm–1. Expansion ofthe urethaneamide I band, comparing the near-surface

region with the bulk region (Fig. 2), helps to identifymore bondedandlessnon-bondedurethane carbonylsarepresent in the near-surface region. This observation isconsistent with the result of less non-bonded N1Hstretchingbandin thenear-surface region.

In Fig. 1 the ratio of the soft segment aliphatic etherstretchingpeak(atca.1100cm–1 for thebulk region) to theC2C stretching peak(atca.1597cm–1 for bothregions)in

Fig. 1. Transmission(bulk region)andATR (near-surfaceregion)infraredspectraof PPG-PU

Fig. 2. ExpandedamideI regionof thetransmissionandATR spectraof PPG-PU


thebenzenering of thehardsegmentwasobserved.Afternormalization andexpansionof thealiphaticetherstretch-ing region(Fig. 3), it is apparently seenthat lesssoft seg-mentsexist in the near-surfaceregion. Such a result issomewhatdifferent from the observation of the XPS stu-diesthatwill bediscussedlater. This observation is duetothe different penetration depth of the surfaceregion stu-died with ATR and XPS. In addition, a shift in theC1O1C stretching vibrationto thelow wavenumber sidefor the ATR spectrumis probablydueto hydrogenbond-

ings of etheroxygens with N1H groups of the hardseg-ment. This resultindicates thatmorephasemixing is pre-sent in the near-surface region. Furthermore, it is worthyto notethatmoreO2C1O symmetricstretching vibration(theshoulder at 1050cm–1) is presentin this region, indi-cating that the domain/matrix interfacetendsto orient tothenear-surfaceregion.

In the caseof PTMG-PU, both transmissionand ATRspectra look similar asseenin Fig. 4. Both spectrashowfree urethane carbonyl absorptions at 1731 cm–1 and

Fig. 3. Expandedaliphatic etherstretching regionof thetransmissionandATR spectraof PPG-PU

Fig. 4. Transmission(bulk region)andATR (near-surfaceregion)infraredspectraof PTMG-PU


bondedurethane carbonyls at 1702cm–1. After expansionof the urethaneamideI region (Fig. 5), it canbe clearlyseenthat lessbondedandnon-bonded urethane carbonylpeaksare presentin the near-surface region. Next, afterexpansionof thealiphaticether stretchingregion(Fig. 6),it canbeidentified thatthedomain/matrix interfaceprefersto orient to thenear-surfaceregion. Moreover, theshift ofthe C1O1C stretching banddown to the low wavenum-bersidealsosuggeststhatphasemixing exists in thenear-

surfaceregion dueto hydrogenbondingsof etheroxygensinteractingwith N1H groupsof thehardsegment.

X-ray photoelectronspectroscopy(XPS)analysis

Initial XPS analysis of both copolyurethanesrevealed awide rangeof surfacestructureswith large variationsintheproportionsof thevariousimportant structuralgroupsat thesurfaceof thematerial.

Fig. 5. ExpandedamideI regionof thetransmissionandATR spectraof PTMG-PU

Fig. 6. Expandedaliphatic etherstretching regionof thetransmissionandATR spectraof PTMG-PU


In the wide-scanspectraof both samples, the O(1s)peakappears at ca. 532.6 eV, corresponding to the poly-etheroxygen,andurethaneandureacarbonyl. The N(1s)peakoccursat ca.400.0 eV andcorresponds to the nitro-gen of the urea and urethane groups. The C(1s) peakispresentat 284.6 eV and is regarded asthe polyether car-bonandthecarbon in thehardsegmentsarisingfrom thediisocyanateandurethaneandureacarbonyls.

In the high-resolution spectraof C(1s), as shown inFig. 7 and8, it is evident that theC(1s)corelevel spectraof bothsamplesconsistof threewell-resolvedpeaks.Themajor peak,referencedto 284.6 eV (abbreviated C1H),is ascribedasunsubstitutedaliphatic carbonin polyetherandaromaticcarbonin thediisocyanate.Thepeakshiftedapproximately 1.5 eV (i. e., at 286.1 eV) toward thehigherbinding energy sideof the main peakcorrespondsto the carbon singly boundto oxygen(C1O). The smallpeak,presentat 288.8 eV, canbe attributed to the carbon

double-bonded (C2Ourethane) to oxygen in the urethanelinkage. In the repeatunit of PTMG (O1CH2CH2CH2-CH2), two centercarbonsshouldappearat 284.6eV whiletwo endcarbonsshouldappearat 286.1 eV.

In order to obtain more quantitative information, theXPS intensity ratios havebeencalculated by comparingthe total areaunder eachpeak and applying Scofield’sintensitycoefficients.

Sinceoxygen is presentin both the soft andhard seg-ments, it cannotbe usedto uniquely identify eithercom-ponent. Conversely, the nitrogenconcentration(from theN(1s) peak) and the ratio betweenthe subpeaksin thehigh-resolution C(1s) regioncanbe usedto delineatethehydrocarbon chain, andthe hardandsoft segment distri-butionsin thesurfacelayer.

Tab.1 summarizesseveral key resultsfrom the wide-scanspectra of theXPSanalysis. In thecase of PPG-PU,a higher percentage of oxygen and lower carbon and

Fig. 7. High-resolutionC(1s)spectrum of PPG-PU

Fig. 8. High-resolutionC(1s)spectrum of PTMG-PU


nitrogenconcentrationsarepresent in the surfaceregionthanin thebulk. ThePTMG-PU shows a similar trendasshownin Tab.1. Hence, surfacecomposition valuesshowa consistent trend for both samples. As the numberofoxygensin the soft segment is much larger than that ofthecarbonyl oxygensin theurethane or ureagroups, it isreasonable to considerthat the O/N ratio represents therelative soft segment concentration.In the case of PPG-PU, the O/N ratio on the air-facing surfaceis larger thanthat calculatedfor the bulk region, indicating a higherconcentration of PPG on the air-facing surface.Hence,the PPG soft segment is the lower surfacefree energycomponent than the hard segmentcomponent. Similarresults were observed for the PTMG basedcopolyur-ethane. In addition,the higherO (surface)/O(bulk) ratiofor PTMG-PU indicatesa lower surface energy for thePTMG soft segment than for the PPGsoft segment. Thelower ratio of the N (surface)/N (bulk) ratio for thePTMGbasedmaterial alsosupportsthis observation.

Theseresults indicate that the surfacefree energy ofeachsegment increasesin the order of PTMG a PPGa

hardsegment(P2PDI-EDA) andtheair-facingsurfacesofblock copolyurethanestendto be morehydrophobicthanonewould predictfrom thecopolymerstructure.

Subsequently, asseenfrom Tab.2, the high-resolutionC(1s) spectrashow higher C1O fractions and lowerC2Ourethanefractionsfor theAFS thanfor thebulk in bothcases.This alsoindicatesthat thesoft segmentsaresegre-gated on the air-facing surface. However, the PTMGbasedcopolyurethaneexhibits a lower C1O (surface)/C1O (bulk) ratio than the PPG basedcopolyurethane.This may result from the fact that at the surfacetwo car-bonsin the PTMG repeatunits arenot bound to oxygen.Conversely, the higher C1H (surface)/C1H (bulk) ratiofor PTMG-PU may arisefrom two centercarbons in thePTMG soft segmentarepresenton theair-facingsurface.Additionally, the XPS composition results of both sam-plesshowa higher percentageof C1O anda lower per-centage C2Ourethanein thesurfaceregionthanin thebulk.Thus, PTMG-PU and PPG-PU should be hydrophobicmaterials. For both samples, lower C2Ourethane fractionsfor the AFS than for the bulk may indicate that the

urethanegroups tendto stick into thebulk region. In con-trast, PTMG-PU hasa higher C2Ourethane fraction at theair-facing surface than PPG-PU. This may be attributedto the fact that more urethane groups at the domain/matrix interfaceof PTMG-PUtendto interfacewith air.

Combining the results above,theconclusion is that thePPGand PTMG soft segmentstend to orient to the air-facing surface and PTMG-PU is a more hydrophobicmaterial than PPG-PU.Theseresultswere strongly sup-ported by water absorption testsandcontact angle meas-urements,which areshown in Tab.3.

ConclusionsIn this study, both ATR spectroscopy andXPSconfirm adeviation betweensurfaceand bulk chemical and mor-phological structures.From XPS studies,it was foundthat the PPGand PTMG soft segmentstended to segre-gateon theuppermostsurface,forming a morehydropho-bic surfacethan one would predict from the copolymerstructure. XPS composition resultsalso showedthat thePTMG soft segment wasmore hydrophobic than that ofthe PPG. In addition, as compared to PPG-PU, higherconcentrationsrelatedto more C2Ourethanegroups in theAFS region for PTMG-PU indicated that the domain/matrix interfaceof the PTMG basedcopolyurethanepre-ferred to interfacewith air. Sincethe uppermost surfaceswere abundantwith the soft segmentsfor both samples,the near-surface region investigated by ATR techniqueexhibited that less soft segmentsresidedin this regionthanin thebulk. Furthermore,ATR spectraof bothmate-rials also showed more phase mixing occurred in this

Tab.1. (1) XPS peakposition (in eV) and (2) percentage(inatom-%) from wide-scanspectra (air-facing surface);(3) bulkfraction(in atom-%)a)

Copolymer C N O

PPG-PU (1) 284.6 400.0 532.6(2) 74.6 3.3 22.1(3) 75.2 5.3 19.5

PTMG-PU (1) 284.6 399.6 532.2(2) 77.5 2.2 20.3(3) 78.3 5.3 16.5

a) Calculatedfrom thechainrepeatingstructure.

Tab.2. (1) XPS peakposition (in eV) and (2) percentage(inatom-%) from high-resolutionC(1s)spectra(air-facingsurface);(3) bulk fraction(in atom-%)a)

Copolymer C1H C1O N1C=O

PPG-PU (1) 284.6 86.2 288.8(2) 51.1 46.9 2.0(3) 56.7 40.9 2.4

PTMG-PU (1) 284.6 286.0 288.8(2) 66.0 31.9 2.1(3) 66.0 31.7 2.3

a) Calculated from thechainrepeatingstructure.

Tab.3. Water absorptionand contactangle measurementsofP2PDIbasedpolyurethanes

Sample Waterabsorptionin %

Watercontactanglein degrees

ginh

dl=g

a�

PPG-PU 4.9 64 0.44PTMG-PU 2.6 74 0.64

a) Inherentviscosity at 308C in N,N-dimethylacetamide.


region due to the shift of the aliphatic ether stretchingregion down to the low wavenumber side, and thedomain/matrix interfacetendedto orient to this region.

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xps and atr surface studies of block copolyurethanes based on 1,2-ethylene bis(4-phenyl isocyanate)

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