Wettability of Polyhedral Oligomeric Silsesquioxane

Nanostructured Polymer Surfaces

Stefano Turri,* Marinella Levi

Department of Chemistry, Materials and Chemical Engineering ‘‘Giulio Natta’’, Politecnico di Milano, P.zza Leonardo da Vinci 32,20133 Milan, ItalyE-mail: stefano.turri@polimi.it

Received: April 26, 2005; Revised: June 6, 2005; Accepted: June 7, 2005; DOI: 10.1002/marc.200500274

Keywords: contact angle; nanostructures; polyhedral oligomeric silsesquioxanes; polyurethanes; surfaces

Introduction

Polyhedral oligomeric silsesquioxanes (POSS)-based poly-

mer nanocomposites are a new class of nanostructured

materials[1–3] characterized by higher thermal stability,

higher mechanical properties, and better resistance to fire

and atomic oxygen. POSS nanofillers consist of an eight-

cornered substituted cage based on SiO1.5 units. POSS are

available with eight unreactive corner groups (typically

aliphatics, cycloaliphatics, or phenyls), as well as endowed

with one or more functional groups including epoxy,

alcohol, C C double bonds, and many others. In this last

case the nanocomposite material can be realized by nano-

filler chemical grafting onto an existing polymer, or by

copolymerization with other comonomers. In the last eight

years, several new polymer systems were explored includ-

ing styrenics, acrylics, epoxies, polyolefins, polyimides,

and others.[4–6] Among them, POSS-modified elastomeric

polyurethanes were also considered.[7–10] In this commu-

nication we report on the preparation and surface behavior

of a new class of POSS-modified ionomeric polyurethanes

with the aim to investigate the effect of nanostructure forma-

tion on the surface wettability of films applied from aqueous

environment.

Experimental Part

Materials

Anionomeric polyurethanes were synthesized from polytetra-methylene glycol (PTMG, Mn1 000 and 2 000), dimethylolpropionic acid (DMPA), isophorone diisocyanate (IPDI),triethylamine (TEA), and ethylenediamine (EDA). All thesereagents were purchased by Aldrich. TMP-diolisobutyl-POSS(in the following POSS-diol) was from Hybrid Plastics. Itschemical structure is represented in Figure 1.

The reference prepolymer was synthesized by dissolvingPTMG, DMPA, TEA (with COOH/NR3¼ 1 M), IPDI (NCO/

Summary: Some model structures of waterborne polyur-ethane anionomers containing various amounts (ca. 3–20%)of a diol functionalized polyhedral oligomeric silsesquioxane(POSS) nanofiller were prepared. X-ray diffraction showedthe formation of a nanocrystalline structure in all copolymersconsidered. Static contact angle measurements indicated asignificant enhancement of surface hydrophobicity as well asreduction in surface tension components even at the leastPOSS level (3%). Dynamic contact angle cycles allowed theevaluation of the hysteresis, which was found to be large andkinetically increasing in POSS-modified samples. Film topo-graphy was analyzed by AFM, showing a more pronouncedroughness in the nanostructured surface.

The AFM image showing a moderate roughness increase.

Macromol. Rapid Commun. 2005, 26, 1233–1236 DOI: 10.1002/marc.200500274 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 1233

OH¼ 2 M), and the chosen amount of POSS-diol in anhydrousN-methyl pyrrolidone (15% w/w). The prepolymer was thenchain-extended by pouring in cold water containing EDA(NH2/NCO¼ 0.90). Reactions were monitored by chemicaltitration (NCO consumption) and IR spectroscopy. Detailsabout synthetic procedures and molecular characterization ofpolymers will be given in a separate work.[11]

The resulting aqueous dispersions were characterized bysolid content 30� 1% with a white, milky appearance. Poly-mer films about 2–5 mm thick were bar-coated on well-cleanedglass substrates, air-dried for 24 h at ambient temperature andoven-dried at þ50 8C for 1 h, and used for surface character-ization. Thick specimens (ca. 0.5 mm) for X-ray diffraction(XRD) were prepared by casting in PTFE plates, drying atroom temperature for 24 h and then in an air-forced oven atþ50 8C for 72 h.

Characterization

FTIR spectroscopy was carried out with a Nexus Thermo-Nicolet instrument by smearing one or two drops of aqueousdispersion between CaF2 disks. Spectra were recorded with16 scans, resolution 4 cm�1. Wide angle X-ray diffractionexperiments (XRD) were carried out on POSS powder and on500-mm-thick PU films with a Philips PW 1710 diffractometerwith Cu Ka radiation (l¼ 1.54178 A), scanning from 2y¼ 28to 408 with step size 0.02 and time per step 4 s. Static contactangle measurements with bidistilled water and highest puritydiiodomethane were carried out with a Dataphysics OCA 20instrument using 2–3 mL droplets according to the sessile droptechnique. Advancing and receding contact angles with waterwere measured with the same instrument by dispensing 3–5mLdroplets. About 20–30 independent measurements werecarried out, and results expressed as mean value� standard

deviation s. Surface topography was obtained by atomic forcemicroscopy (AFM) in contact mode using silicon tips (scanarea 40� 40 and 10� 10 mm2, contact force 8 nN, frequency1 Hz) with a CP3 Park instrument operating in air.

Results and Discussion

The model ionomeric polyurethanes under investigation

differ from each other in type and amount of soft segment

polyol (PTMG 1 000 or 2 000), as well as in the content of

POSS-diol macromonomer. Compositions are given in

Table 1, along with the results of surface characterization.

As far as morphology is concerned, the XRD patterns of

the cast polyurethane films showed as background two

broad peaks centered to about 188 and 68, which could be

attributed to the amorphous polyether phase and to hard-

soft-type interactions of PU systems.[9,12] Moreover, most

of the POSS-modified samples showed a crystalline peak at

around 2y¼ 88, which can be indexed as the 101 reflection

of the POSS cage.[13] An effect of sample processing on

crystallinity was observed on samples A3 and A6 (lowest

POSS content). In particular, the 101 reflection disappeared

if the cast dispersions were immediately dried in oven at

high temperature and then analyzed. On the other hand,

crystalline peaks were always present in samples contain-

ing 10% of POSS, confirming the findings reported by Fu

et al.[7–9] on segmented elastomeric polyurethanes. As an

example, Figure 2 shows the XRD patterns of A6 sample

after fast and slow processing, in comparison to the parent

POSS diol pattern. The different crystallization behavior

suggests some kinetic limitation in the self-assembling

ability of POSS nanostructures.

Table 1 also reports the results of static contact angles Yversus H2O and CH2I2, along with surface tension gs in its

polar (gsp) and dispersive (gs

d) components calculated accord-

ing to Wu’s harmonic mean method [Equation (1)]:[14]

gs1 ¼ gs þ g1 �4gd

1 � gds

gd1 þ gd

s

� 4gp1 � gp

s

gp1 þ gp

s

gs ¼ gs1 þ g1 � cosY ð1Þ

where gl is the known surface tension of the test liquid and

gsl is the interfacial tension. The incorporation of even a

small amount of POSS macromer strongly enhances the

Figure 1. POSS chemical structure.

Table 1. Composition, static contact angles, and surface tension components gsd and gs

p of polyurethane POSS-modified samples.

Sample PTMG soft phase POSS (w/w) Y static vs. H2O (8� s) Y static vs. CH2I2 (8� s) gsd gs

p

% (Mn) % mN �m�1 mN �m�1

A 55.4 (1 000) 0 80.9� 2.6 44.0� 1.4 34.8 8.7A3 53.1 (1 000) 3.1 99.7� 0.9 64.7� 0.5 26.0 3.3A6 52.2 (1 000) 5.8 104.2� 0.7 62.2� 0.3 28.2 1.2A10 49.1 (1 000) 9.4 101.3� 0.7 60.4� 0.4 28.5 2.1A20 35.6 (1 000) 18.8 103.7� 0.8 62.9� 1.2 27.6 1.5B 65.5 (2 000) 0 84.7� 1.0 46.1� 0.6 34.0 7.2B10 56.1 (2 000) 9.2 101.5� 1.2 64.9� 1.4 26.1 2.6

1234 S. Turri, M. Levi

Macromol. Rapid Commun. 2005, 26, 1233–1236 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

contact angles of the coated surface against both liquids.

The total surface energy of polyurethanes is in any case,

reduced from more than 40 to about 30 mN �m�1. In parti-

cular, the polar component seems very sensitive to the

presence of even few POSS percentages. The result is

achieved very fast in 3% POSS-modified sample, and does

not significantly improve in the 3–20% POSS range.

This behavior was further investigated through measure-

ments of both advancing (Ya) and receding (Yr) contact

angles, as well as by evaluation of their hysteresis

DY¼Ya�Yr during three advancing and receding cycles

(Table 2). As known, hysteresis may depend on a variety of

factors, including surface chemical heterogeneity, rough-

ness, rearrangements of functional groups, changes in

morphology, and so on.[15] In polyphasic systems it is

generally accepted that advancing contact angle is sensitive

to the lower surface tension component, whereas the reced-

ing contact angle is indicative of the higher one. Hysteresis

can be quantitatively correlated to surface interactions

through the definition of the molar free energy of hyste-

resis[16] DGh [Equation (2)]:

DGh ¼ �RT � lnsinYr

sinYa

� �ð2Þ

although not all authors agree with a thermodynamic

treatment of an intrinsically nonequilibrium behavior like

contact angle hysteresis. Moreover, it is known that

hysteresis increases with roughness until the surface

becomes composite, after which it decreases dramatical-

ly.[15]From Table 2 examination, it results that POSS

modification enhances both advancing angles and hyste-

resis. Moreover, the surfaces show a different wettability

kinetic behavior, since the POSS-modified polyurethanes

show a progressive increase of the hysteresis. The kinetic

effects with the wetting cycles are a consequence of modi-

fications occurring at the liquid–solid interface, evolving

toward a more favorable energetic state.[17] This kinetic

effect, although limited in the case under study, may be

attributed to some water absorption by the surface caused by

rearrangements of hydrophilic groups of the polymer (like

the –COOH groups coming from DMPA).

In order to estimate the effect of roughness on surface

behavior, some surfaces (samples A and A10) were examin-

ed by AFM. Both average roughness [Ra ¼ ð1=lÞ �Ð l

0yj jdx]

and root-mean-square roughness [Rq ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1=lÞ �

Ð l0y2dx

q]

were calculated by triplicate experiments, l being the

roughness sampling length and y the height of roughness

trace at a given point from the centerline. AFM topography

images are shown in Figure 3, while numerical results are

Ra¼ 94 A, Rq¼ 132 A for sample A10, and Ra¼ 8.5 A,

Rq¼ 11 A for sample A. The presence of POSS macromers

therefore seems to increase the surface roughness signifi-

cantly, although it is considered by both theoretical and

experimental evidence that Ra< 100 nm generally has a

limited effect on contact angles and hysteresis.[18,19]There-

fore, surface roughness, although significantly raised,

cannot explain the decreased wettability of the POSS

nanostructured polyurethane surfaces. This effect is likely

related to surface-oriented enrichment of POSS structures

bearing low-surface-tension alkyl substituents. Similar

results were found very recently for poly(methyl meth-

acrylate) (PMMA) blended with fluorinated POSS-termi-

nated polymers,[20] and were attributed to coverage of the

outermost layer by POSS heads.

Finally, Table 2 also reports the molar free energies of

hysteresis calculated from Equation (2). It should be under-

lined that common nonpolar polymers like polyolefins

show DGh values <1 kJ �mol�1, and therefore they are of

the order of the strength of dispersive bonds. Higher DGh

values (about 1.4–1.5 kJ �mol�1) were reported for more

Figure 2. Effect of processing conditions on XRD patterns ofsample A6.

Table 2. Advancing contact angles (Ya) versus water, hysteresis, and molar free energies of hysteresis DGh of polyurethane POSS-modified samples.

Sample Ya� s; hysteresis (8) DGh

First cycle Second cycle Third cycle kJ �mol�1

A 77.3� 2.3; 54.8 76.6� 2.3; 55.9 75.3� 2.3; 55.9 2.45A3 95.7� 1.7; 60.0 95.3� 1.6; 61.7 94.2� 1.6; 62.9 1.43A6 100.1� 1.0; 61.8 99.5� 0.9; 63.4 98.4� 0.8; 64.5 1.25A10 96.0� 1.6; 60.8 95.7� 1.4; 62.4 94.7� 1.3; 63.2 1.45B 80.8� 1.2; 48.5 77.8� 0.9; 45.6 76.9� 1.6; 44.8 1.45B10 98.3� 1.1; 61.7 96.5� 1.0; 62.7 95.1� 0.7; 63.4 1.40

Wettability of Polyhedral Oligomeric Silsesquioxane Nanostructured Polymer Surfaces 1235

Macromol. Rapid Commun. 2005, 26, 1233–1236 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

polar structures as polyesters and polyamides.[15] In the

present case the calculated DGh is 2.45 kJ �mol�1 (sample

A) and 1.45 kJ �mol�1 (sample B). The higher free energy

of hysteresis of sample A is likely due to the higher

concentration of polar linkages (urethanes, ureas) corre-

lated to the use of shorter PTMG soft phase. POSS

modification seems efficient in reducing this interaction

term, while the effect is less relevant for the B series based

on PTMG 2 000 polyethers.

Conclusion

A family of POSS-containing ionomeric polyurethanes was

described. It has been shown that the presence of POSS

structures even at very low dosages significantly decreases

the wettability behavior of the polymer surface. This is

likely to be due to surface enrichment and molecular

assembly of nanostructures bearing low-surface-energy

alkyl groups. This result confirms the very recent findings

concerning trifluoromethyl-substituted POSS and the

results open new possibilities for the development of high-

performance nanostructured waterborne coatings.

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Figure 3. AFM topography images of surfaces A (a) and A10 (b).

1236 S. Turri, M. Levi

Macromol. Rapid Commun. 2005, 26, 1233–1236 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim