Characterization of Several Polymer Surfaces by Streaming Potential and Wetting Measurements: Some Reflections on Acid–Base Interactions

Journal of Colloid and Interface Science 217, 377–387 (1999) Article ID jcis.1999.6345, available online at http://www.idealibrary.com on

Characterization of Several Polymer Surfaces by Streaming Potential and Wetting Measurements: Some Reflections on Acid–Base Interactions Alexander Bismarck, M. Emin Kumru, and Ju¨rgen Springer 1 Institut fu¨r Technische Chemie, Technische Universita¨t Berlin, Fachgebiet Makromolekulare Chemie, Sekr. TC6, Strasse des 17. Juni 135, 10623 Berlin, Germany Received February 19, 1999; accepted May 24, 1999

Several thermoplastic (technical, engineering, and high-performance) polymers were characterized using contact angle and electrokinetic measurements. From the measured contact angles of various test liquids on polymers, we calculated the solid surface tensions using the different approaches to determine them and compared the results. Zeta (z)-potential measurements gave information about the swelling behavior of the polymers in water, the surface chemistry, and the interactions with dissolved potassium and chloride ions. All investigated polymers displayed an acidic surface character. Comparing the results obtained from the z-potential measurements with the acid-parameter of the surface tension g 1 calculated from the measured “static” contact angles using the van Oss, Good, and Chaudhury approach revealed the same tendency. The correctness of the acid– base approach regarding the “overall” chemical surface character could be shown. However, it seems that the basic parameter g 2 obtained from the acid– base is greatly overestimated. © 1999 Academic Press Key Words: polymers; contact angle; surface tension; zeta potential; acid– base interactions.

INTRODUCTION

Interfacial phenomena are important in almost every industrial process, from heterogeneous catalysis to the manufacturing of composite materials and from environmental protection to medical technology, as well as in technical processes like the fabrication of compound systems (reinforced materials and coated materials). They also affect the adsorption interactions. The knowledge about the presence of reactive functional surface groups is of general interest. Thermoplastic polymers become more and more technically relevant for use as matrix materials in fiber-reinforced composites because of new technological demands such as recycling, biocompatibility, easier handling, and time saving in production processes (1). Since thermoplastic polymers normally do not exhibit reactive funcTo whom correspondence should be addressed. Fax: 1149 30 314 79237; E-mail: [email protected]. 1

tional groups, which are able to form covalent bonds, i.e., to functionalized carbon fibers, attractive interfacial or adhesive interactions between a thermoplastic polymer and carbon fibers can only be achieved by specific interactions (like, for example, hydrogen bonds). Therefore, it is desirable to predict the adhesive behavior between the reinforcing material and the surrounding matrix. From the thermodynamical view of adhesion, it is necessary to know the surface tension of both joint partners (polymers and reinforcing fibers) in order to calculate the appropriate thermodynamical work of adhesion W a. Measuring contact angles is a useful tool to characterize solid surfaces, and these methods (i.e., Wilhelmy plate or sessile drop) are easy to handle and obtain reliable information about the outermost surface layers. Taking the thermodynamical theory of adhesion into consideration, the wetting of the solid by the liquid matrix is improved when the surface tensions of both the reinforcing material g s and the liquid matrix g l (i.e., the polymer melt) are about equal and the interfacial tension g ls is negligible. On the other hand, it was described elsewhere (2) that adhesion becomes maximal when the polar parts of the surface tension between both adherents is equal. z-potential measurements offer a possibility of estimating the state and the amount of dissociable surface functional groups on the investigated solid (3). According to Ha¨ßler and Jacobasch (4) z-potential measurements are an easy and reliable method to estimate adhesive properties between hot melts and the joint partners. With increasing differences between the z-potential plateau values for both components received as result of the pH dependence of the z-potential ( z 5 f(pH)), the measured adhesive strength should also increase. To prove this assumption we studied the adhesive behavior between a thermoplastic matrix-polymer polyamide 12 (PA12) and thermochemically modified carbon fiber surfaces with basic character (5) (H-carbon fibers) (see Fig. 1). Acid– base interactions add to the adhesive strength between suitable modified carbon fibers and the polyamide 12, and improve the properties of the obtained composite material (6).

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FIG. 1. z–pH plot of thermo-treated high tensile (HT) carbon fibers (H-CA*) and polyamide 12 in accordance to the model of Ha¨ßler and Jacobasch (* carbon fibers were high temperature (905°C) treated followed by exposure to oxygen at 242°C).

EXPERIMENTAL

Materials The investigated polymers are thermoplastic standard polymers such as polystyrene with a molecular mass M w 5 198,000 g/mol (synthesized in our laboratory using free-radical polymerization; foils were obtained by hot-pressing the polymer powder at T 5 1608C and 300 bar for 2 min), engineering polymers such as polycarbonate (PC (M w 5 39,300 g/mol determined in tetrahydrofuran (THF) using static light scattering (SLS)), Macrofol ® DE 1-1, Bayer AG, Germany), and polyamide 12 (PA 12 (insoluble in common solvents), Grilamid ® L25, EMS-Chemie AG, Domat, Switzerland) such as high performance polymers, for example, polyetheretherketone (PEEK (insoluble in common solvents), Victrex, Lite K) and polyetherimide (PEI (M w 5 89,100 g/mol determined in dichloromethane using SLS, Ultem, Lite I) (both supplied from Lipp-Terler HIGH PERFORMANCE FILMS, Waidhofen/ Ybbs, Austria). All polymers (except PS) were supplied as foils (thickness 100 mm). All polymers were used as supplied (or synthesized followed by hot-pressing) without further purification (expect drying in a vacuum oven at 75°C and 1 mbar); therefore the results could be affected by surface-specific effects and/or by contaminations (stabilizers, etc.) contained in the polymers.

Contact Angle Measurements to Determine the Solid Surface Tension In order to estimate the solid surface tension of polymers, “static” contact angles of various test liquid droplets on the polymer surfaces were measured by the sessile drop method using the robust-shape-comparison technique (7). This technique is reliable to measure contact angles u . 45° exactly (8, 9). Contact angles smaller than 45° were measured in a tempered room (T 5 20 6 28C) by the known goniometer technique. The sessile drop was formed by depositing the used test liquid from above using a microsyringe on the polymer surface. The surface tensions 2 and their components of the used test liquids taken from the literature are summarized in Tables 1a and 1b. However, as discussed in the literature (10), the van Oss et al. approach is very sensitive to the numerical values of the components and parameters of the liquid surface tension and contact angles. In order to present a “complete” set of solid surface tension data, the surface tension of the investigated polymers were calculated from the measured contact angle values using first the harmonic mean method by Wu (2), second the method suggested by Owens and Wendt (11) as well as Kaebele (12) (since these values are often used to calculate W a between The “overall” surface tension g 1 of the used test liquids were checked using the pendant drop method. 2

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TABLE 1a Polar g lp and Dispersive g ld Components of the Surface Tension of the Used Test Liquids g l Test liquid

g lp [mN/m]

g ld [mN/m]

g l [mN/m]

Water Glycerol Formamide Diiodomethane Ethylene glycol Diethylene glycol DMSO Tricresyl phosphate

51.0 22.8 22.2 6.7 19.0 — 8.7 1.7

21.8 40.6 36.0 44.1 29.3 — 34.9 39.2

72.8 63.4* 58.2 50.8 48.3 45.0 43.6 a 40.9

surface tensions can be obtained by plotting (1 1 cos u) z g l/ 2 z =g ld versus (g lp/g ld) of several test liquids; =g sd resulted from the slope of the straight line and =g sp from the axial section. g s can be calculated as follows:

g s 5 g ds 1 g ps . According to Wu’s harmonic mean method (2) one has to measure the contact angles of two testing liquids with known surface tension components and solve the equation given in Table 2. The surface polarity is given by:

Note. See Refs. (2, 30, 19, 14, 34). a At 23°C.

XP 5

reinforcing materials and the surrounding polymer matrix), and last the method introduced by van Oss, Good, and Chaudhury (13, 14). The final equations to calculate the solid surface tensions are listed in Table 2. The complete theoretical background of these theories, the advantages and drawbacks as well as substantial critics, can be found in the recent literature (15–20). Another empirical method to estimate the critical surface tension g c was developed by Zisman (21, 22). For this the cos u value is plotted versus the surface tension of the test liquid. Extrapolating a straight line through these values to cos u 5 1 yields the surface tension of the liquid which wets the solid surface completely. The surface tension of this theoretical “liquid” equals g c. Following the Zisman approach cosine values of the measured contact angles were plotted versus the surface tension of the test liquid and these values were extrapolated toward cosine 1 (gc5 lim gl) (but not of a homolog serious of liquids,

The critical surface tension g c estimated following the Zisman approach leads to the smallest surface tension values, as expected, since g c equals the surface tension of the liquid which first would spread on the investigated solid surface. All surface tensions determined using the other approaches lead to higher values. The van Oss approach gives the possibility of estimating an acid– base-contribution (originating from donor–acceptor interactions) g ab to the solid surface tension. This approach allows the determination of the attractive interactions which should mainly influence the adhesive properties: the acid– base interactions. On the other hand it is possible to describe the repulsive interactions acting at the interface between adjacent components. These interactions can be described by: ab ab 2 1 1 2 g ab sl 5 g s 1 g l 2 2 Îg s z g l 2 2 Îg s z g l

cosu31

because of complete wetting). The critical surface g c equals the surface tension value of a liquid which will first spread onto the polymer surface. According to Owens and Wendt the components of the solid

g Ps . gs

2 1 1 1 2 Îg 2 s z g l 2 2 Îg s z g l .

The Lifshitz–van der Waals component g LW of the polymer surface tension, reflecting the long-range interactions (includ-

TABLE 1b Surface Tension Parameters for Different Test Liquids According to van Oss, Chaudhury, and Good Test liquid

g l [mN/m]

g lLW [mN/m]

g lab [mN/m]

g l1 [mN/m]

Water Glycerol Formamide Diiodomethane Ethylene glycol DMSO

72.8 64.0 58.0 50.8 48.0 44.0 (42.9)

21.8 34.0 (37.8) 39.0 (31.1) 50.8 29.0 36.0 (29.2)

51.0 30.0 (26.1) 19.0 (26.8) '0 19.0 8.0 (13.8)

25.5 (65.0) 3.92 (4.0) 2.28 (1.3) — 1.92 0.5 (0.2)

g l2 [mN/m] 25.5 (10.0) 57.4 (42.7) 39.6 (143) — 47.0 32.0 (237)

Note. Surface tensions and their parameters (g LW 5 Lifshitz–van der Waals component, g 1 5 electron acceptor or proton donor (acidity parameter) component and g 2 5 electron donor or proton acceptor (basicity parameter) component of the liquid surface tension g l) for different test liquids at 20°C according to van Oss, Chaudhury, and Good (14). The surface tension components, on the scale g l1 5 65.0, g l2 5 10.0 mN/m for water in accordance to Della Volpe and Siboni are in parenthesis.

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TABLE 2 Approaches and Equations Used to Calculate the Solid Surface Tensions from Measured Contact Angles Approach

Equation ~1 1 cos u ! z g l 5 4 z

Wu Owens and Wendt Kaelble

g l z ~1 1 cos u ! 2 Îg

S D S D

5

g dl z g ds g pv z g ps 14z g dl 1g ds g pl 1g ps

Îg ds z

S ÎÎ D g pl

g dl g1 z ~1 1 cos u ! 2 g LW 5 s 4

van Oss, Good, Chaudhury

d l

z g LW 1 ~1 1 cos u ! z g l 5 2 z ( Îg LW s l

1

Îg ds

Îg 1s z g 2l 1 Îg 2s z g 1l )

Note. g l, surface tension of the test liquid; g s, solid surface tension; g d and g are dispersion and polar component, respectively; g LW 5 Lifshitz–van der Waals component; g 1, electron acceptor or proton donor (acidity parameter) component; and g 2, electron donor or proton acceptor (basicity parameter) component of the liquid g l or solid surface tension g s (the subscripts s, l always indicate the solid and the liquid, respectively). p

ing the dispersive interaction, the dipole– dipole interaction, and the dipole–induced dipole interaction, which is overwhelmingly dominated by the dispersion), was calculated from the measured diiodomethane contact angles under the assumption that diiodomethane is a “pure” apolar test liquid. However, the surface tensions calculated using the “dispersive–polar” and “acid– base” approach cannot be used interchangeably since these approaches are based on different physical views. A detailed experimental comparison between these dispersive–polar and acid– base approaches was given in the recent literature by Correia et al. (15).

z-Potential Measurements The z-potentials were determined with the electrokinetic analyzer EKA (Anton Paar KG, Graz, Austria) based on the

streaming potential method (23) for all investigated polymers. Details of this measuring technique can be found in (24, 25). The analyzer was filled with a 10 23 mol/l KCl-electrolyte solution to measure the time dependence of the z-potential. Once in the commercial plate measuring cell, the liquid stream is realized between two equal polymer-foils which were distanced by a PTFE-foil with a defined channel. Before starting the measurements the measuring cell was connected to the analyzer, quickly rinsed with the KCl-electrolyte solution, and degassed. In order to measure the z-potential as a function of the electrolyte concentration the analyzer was filled with distilled water (Millipore, pH 5.6) and the measuring cell was rinsed several times until the conductivity in the measuring system fell below 300 mS/m. First the water value of the z-potential was measured; subsequently the KCl-concentration was raised using a digital burette (Brand, Wertheim, Germany). The system including the measuring cell was rinsed in both measuring directions. The concentration was raised up to 0.01 M KCl. At high electrolyte concentrations the correlation of the measuring values decreases in both directions of flow (dU/dp ' 0). The pH dependence of the z-potential was determined in a 10 23 mol/l KCl-electrolyte solution to keep the ion strengths constant. The pH value was varied in a range of pH 3 to pH 10 by adding drops of 0.1 M HCl or KOH solution. RESULTS AND DISCUSSION

Contact Angle Measurements and Surface Tension The results of the contact angle measurements of various test liquids on polymer surfaces are summarized in Table 3. As can be seen from these results, all investigated polymeric materials are “relatively” hydrophobic. In accordance to the Young equation the smaller the surface tension of the test liquid, the smaller becomes the contact angle measured on the polymer surfaces. Figure 2 exemplarily shows the determination of the

TABLE 3 Contact Angles and Their Cosine Values of Various Test Liquids on the Investigated Polymers Polymer

PS a

Test liquid

u [°]

u [°]

cos u

u [°]

cos u

u [°]

cos u

u [°]

cos u

Water Glycerol Formamide Diiodomethane Ethylene glycol Diethylene glycol DMSO Tricresyl phosphate

85.5 6 3.8 — — 30.2 6 1.8 — — — —

71.5 6 5.7 68.5 6 2.4 53.1 6 3.0 27.6 6 3.7 43.6 6 2.3 — 17.1 6 3.9 13.8 6 4.5

0.316 0.366 0.600 0.885 0.724 — 0.954 0.968

84.2 6 4.7 69.8 6 1.4 56.1 6 2.1 24.2 6 1.2 45.0 6 2.7 — 13.6 6 2.9 12.0 6 1.9

0.100 0.345 0.557 0.912 0.707 — 0.971 0.978

77.1 6 1.2 70.3 6 2.0 50.3 6 4.2 35.6 6 3.1 43.5 6 1.8 — 29.8 6 1.7 17.5 6 2.4

0.223 0.337 0.637 0.812 0.721 — 0.867 0.953

89.1 6 1.6 80.4 6 5.7 72.0 6 2.8 23.2 6 1.8 46.7 6 3.5 35.7 6 2.9 12.7 6 2.8 14.3 6 2.8

0.017 0.166 0.308 0.919 0.685 0.812 0.973 0.967

a

PEEK

PEI

PA12

PS was not characterized in the “acid– base” sense, since it has been studied several times.

PC

CHARACTERIZATION OF SEVERAL POLYMER SURFACES

FIG. 2. of PA12.

381

(a) Zisman plot to estimate the critical surface tension of PA12 and (b) Owens and Wendt plot to determine the surface tension and their components

solid surface tension of PA 12 according to Zisman (Fig. 2a) and Owens and Wendt (Fig. 2b). The contact angles of several test liquids with known surface tensions on various polymers were measured. Calculating the polymer surface tensions (Table 4) using Wu’s harmonic mean equation results in the highest values and the highest surface polarities for most of the cases (expect PC).

All investigated polymers have the same surface tension within the error ranges, but varying surface tension components. Determining surface tensions according to Owens and Wendt give, within the errors, the same overall values of the surface tension, but much lower polar components g p of the surface tension, leading to much smaller surface polarities. The determined surface tensions of the common polymers, PS and PC,

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TABLE 4 Solid Surface Tension of Polymers and Their Components According to Wu (Harmonic Mean), Owens and Wendt (Geometric Mean) and Zisman’s Critical Surface Tension Polymer

Approach

g [mN/m]

g p [mN/m]

g d [mN/m]

XP

PEEK

Wu Owens & Wendt Zisman g c [mN/m] Wu Owens & Wendt Zisman g c [mN/m] Wu Owens & Wendt Zisman g c [mN/m] Wu Owens & Wendt Zisman g c [mN/m] Wu

48.1 6 3.5 42.1

13.1 6 3.0 5.9 40.5 6.5 6 2.3 1.7 40.5 11.0 6 2.0 4.9 39.0 4.2 6 0.7 0.3 6 0.7 38.1 6.4 6 1.8

35.0 6 1.7 36.2

0.37 0.14

40.0 6 2.2 44.5

0.14 0.03

33.0 6 1.7 35.9

0.25 0.14

43.2 6 1.3 44.8 6 0.7

0.09 0.01

38.0 6 1.8

0.14

PEI

PA12

PC

PS

46.5 6 3.2 46.2 44.0 6 2.5 40.7 47.4 6 1.4 45.1 6 1.0 44.4 6 2.6

are in good agreement to published values (2, 11, 26). However, the “relative high” surface polarity of PS might be caused by the thermal strain (thermal oxidation, autoxidation) during the hot-pressing of the raw PS powder. For the determination of the solid surface tension and their components we used the same set of test liquids as proposed by van Oss et al. However, in contrast to the “conventional” (Zisman as well as Owens, Wendt, and Kaelble) dispersive– polar approaches where all measured contact angle values can be used to determine the polymer surface tensions (compare Fig. 2), in the case of the van Oss approach for all investigated polymers and all of the used test liquids unique trend could not be found. In some cases the measured contact angles of the test liquids (see Table 3, test liquids used to determine the surface tension) on the polymer surfaces do not lead to concurrent results. This might be a consequence of the sensitiveness of the van Oss et al. approach to the numerical values of the liquid surface tension components and acid and base parameters (10). However, the determined overall surface tension g, which consists of the sum of the Lifshitz–van der Waals term g LW and the acid and base term g AB(g sAB 5 2=g s2 z g s1), lies for all

investigated polymers in the same range as the surface tensions determined using the dispersive–polar approaches. It can be seen in Table 5 that all investigated polymers seem to exhibit slightly “amphoteric” character. However, the basic parameter g 2 is much bigger compared to the “small” acidic parameter g 1. Indeed one “cannot compare the acid and base components of the same solvent (or substrate), but, eventually, the acid (or base) components of different substrates can be compared” (16). The relative high (“overestimated”) basic g 2 component of the surface tension is caused by the electron ion-pairs of oxygen atoms contained in the polymer (ether, carbonyl, and carbonate functionalities), which are effective Lewis base sites. Comparing the acid parameters g 1 of the polymers gives the following tendency: PC .. PEI . PA12 . PEEK. Therefore, the investigated PC should display the highest acidic surface character. As can be seen, all investigated polymers exhibit different acid– base components g AB of the solid surface tension g due to different chemical structures and the amount of free end-groups of the polymers. Since water is more acidic than basic (27), we also used the scale proposed by Della Volpe and Siboni (16), where water is

TABLE 5 Determined Solid Surface Tension and Their Components of Various Polymers at 20°C According to the van Oss, Good, and Chaudhury Approach on the Scale g 1(H 2O) 5 g 2 (H 2O) 5 25.5 mN/m and in Parentheses on the Scale g 1 (H 2O) 5 65.0 mN/m, g 2 (H 2O) 5 10.0 mN/m (Determined without Ethylene Glycol) Polymer

g LW [mN/m]

g 1 [mN/m]

g 2 [mN/m]

g AB [mN/m]

g [mN/m]

PEEK a PEI b PA12 c PC d

45.2 46.4 41.7 46.8

0.01 (0.01) 0.09 (0.07) 0.02 (0.02) 0.60 (0.27)

7.5 (3.9) 1.0 (0.8) 6.6 (2.6) 4.4 (0.7)

0.6 (0.2) 0.6 (0.2) 0.7 (0.2) 3.3 (0.4)

45.8 (45.4) 47.0 (46.6) 42.4 (41.9) 50.1 (47.2)

Determined without: a glycerol, b formamide, c formamide and ethylene glycol, and d ethylene glycol and DMSO values.

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smaller basic and to slightly higher acidic parameters and therefore to a changed acid– base component g ab and lower “overall” surface tensions.

z-Potential Measurements Since the z-potential of polymers is strongly influenced by the uptake of water and therefore by the swelling of the investigated materials, we measured the time dependence of the z-potential in 10 23 mol/l KCl-electrolyte solution of the “dry” polymers (Figs. 3a and 3b). The decrease of the z-potential with time is caused by the uptake from water, an effect which depends on the hydrophilic character of the solid (28). More precisely, it is due to the swelling of the solid which causes a transfer of the shear plane into the liquid which excludes the diffuse part of the electric double layer from the mechanical or electrical interaction. Another reason is the removal of surface adsorbed soluble components which also causes a decrease of the solid surface potential. As can be seen from Fig. 3 (and Table 6, stressing the main results) the negative z-potential decreases for all investigated polymers from a value z 0 on different time scales asymptotically to a constant but smaller value z `. According to Kanamaru (28) the quotient (z 02z `)/z 0 should correspond to the water uptake at a relative humidity of 100% of the investigated solid. The decrease of the z-potential as a function of time due to the water uptake can be described as 2

dz 5 k z ~ z 2 z `! dt

which leads to FIG. 3. (a) Time dependence of the z-potential ( z 5 f(t)) of PA12 measured in 10 23 mol/l KCl solution and (b) Time dependence of the z-potential ( z 5 f(t)) of PEEK measured in 10 23 mol/l KCl solution.

seen as 6.5 times more acidic than basic in contrast to the assumption made by van Oss et al. where water is as acidic as basic (g 1 5 g 2 5 25.5 mN/m, leading to a g 1 5 65.0 mN/m and g 2 5 10.0 mN/m) for water. Estimating the polymer surface tensions according to Taft’s scale leads to much

2ln

z 2 z` 5 k z t, z0 2 z`

where k is a constant depending on the structure of the investigated solid and is smaller below the glass transition temperature T g than above it (28, 29). All investigated polymers display a different z-potential– time dependence, which should correlate with the different

TABLE 6 Main Results of z-Potential Measurements Polymer

z 0 [mV]

z ` [mV]

(z 02z `)/z 0

k [min 21]

iep

z plateau [mV]

PEEK PEI PA12 PC PS

222 223 217 28 228

216 215 5 23 222

0.27 0.35 1.29 0.63 0.21

4.7 z 10 23 2.7 z 10 23 6.9 z 10 24 9.7 z 10 23 9.8 z 10 22

4.4 4.1 4.2 4.0 4.0

224 223.5 227 26 240.5

Note. z 0 and z ` obtained from z 5 f(t); k, constant of swelling, iep, isoelectric point, and z plateau from z 5 f(pH).

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FIG. 4. pH dependence of the z-potential ( z 5 f(pH)) measured in 10 23 mol/l KCl solution for PEEK, PEI, and PS.

swelling behavior of the materials. Another experimental effect on the functions z 5 f(t) can be seen in Fig. 3b; the influence of the direction of electrolyte flow (we call it the “left–right dependence” of the measured z-potential). Preferred streaming channels must be formed inside the measurement setup depending on the swelling of the polymer under investigation causing the described effect (24). The constant k reflects the velocity of the swelling process of the polymers by water. As can be seen from Table 6, k is the smaller the more water the polymer adsorbs (i.e., the higher the quotient (z 0 2 z `/z 0) is). This is the expected behavior, since an uptake of water changes the structure of the polymer. PS as a hydrophobic polymer will not swell; therefore, the process ends very fast in contrast to PA12 with a very high rate of water uptake and an extremely small constant k. Knowledge of the water uptake (the swelling behavior) of the investigated polymer matrices is also necessary for the construction of composite materials, since the adhesive strength is influenced by adsorption layers (in particular of water) at the common interface between the adhesive and the adherent (30). If one wants to determine the other dependencies of the z-potentials (i.e., z 5 f(pH, c)) one has to make sure that possible superimposed effects due to the swelling of the polymers are excluded. The starting point for these measurements has to be fixed by a long time measurement. The acidity or basicity of solid surfaces can be determined qualitatively by measuring the pH dependence of the z-potential. A plateauarea in the alkaline range for the pH dependence of the z-potential is obtained, if the formation of the electrical double

layer is mainly caused by the dissociation of acidic functional surface groups. If the sign of the z-potential changes in the acidic range, this is first of all due to the repression of the dissociation of the acidic surface groups, and second due to the adsorption of the currently potential determining ions. In the case of alkaline surface groups the analogous tendency of the z-potential occurs. The isoelectric point (iep) is also a measure of the acidity or basicity of a solid surface if the dissociation of surface groups is the dominating mechanism of the formation of the electrical double layer. A low value of the iep indicates a Brønsted acidic character of the solid surface and an alkaline value of the iep signifies dissociable basic surface groups (31). As can be seen from the z-potential–pH-plots (Fig. 4, Table 6), all investigated thermoplastic polymers contain dissociable acidic functional groups due to the manufacturing and further processing (thermal debit or other aging processes) and therefore low iep values and broad plateau areas in the alkaline range. The z 5 f(pH) curves of all polymers are principally analogous. Comparing the z–pH plots of PEEK and PEI these plots are more or less congruent, only the iep values differ. This would mean that PEI should contain a greater amount of acidic dissociable functional groups, which is in contrast to the results obtained by contact angle measurements that indicate a higher surface polarity of PEEK at the same surface tension. There is only one significant difference obvious for the z–pH plot measured for PS compared to those of PEI and PEEK; the iep of the PS is slightly lower but the z plateau value is much more negative. This should indicate a higher hydrophobicity of the PS compared to PEI and PEEK, but on the other hand such a

CHARACTERIZATION OF SEVERAL POLYMER SURFACES

385

FIG. 5. pH dependence of the z-potential ( z 5 f(pH)) measured in 10 23 mol/l KCl solution for to different PA12 samples.

FIG. 6. pH dependence of the z-potential ( z 5 f(pH)) of PA12 measured at varying KCl concentrations.

behavior does not correlate with the measured water contact angles. Figure 5 shows the z-potential–pH dependence of PA12. The expected amphoteric trend was not observed. Since a low iep value was measured the acidic surface character dominates. As can also be seen from this plot, a different sample (i.e., from different manufacturing charges) of the same polymer does not give the same results. The sample quality significantly influences the z-potential–pH function. Both samples of PA12 supplied by the same manufacturer differ in iep values and in the z plateau values. Since these values are a function of the amount of the dissociable functional groups, the PA12 is different in the chemical composition. The z-potential measured as a function of the pH for PC (Fig. 7) shows, unexpectedly (compared to PEI and PEEK), a very small negative z plateau value, while the iep values coincide. The surface reacts weakly acidic. But another mechanism is possible in which the hydroxyl ions at higher pH values saponify the carbonate functionalities in PC. Whichever mechanism is valid, the acidic trend was reproducible. Figures 6 and 7 indicate that different polymer surfaces interact differently with ions dissolved in the electrolyte solution. Measuring the z 5 f(pH) dependence as a function at varying electrolyte (KCl) concentrations allows us to distinguish between specific and nonspecific adsorption of ions. In the case of PA12 (Fig. 6) the iep is a function of the electrolyte concentration whereas for PC (Fig. 7) the iep is independent on the basic concentration. This indicates that the investigated PA12 will preferentially adsorb the dissolved chloride and potassium ions, whereas the PC interact nonpreferentially with these ions. Since in aqueous media the polar interactions mainly comprise the interactions between hydrogen donors and hydrogen

acceptors (between Brønsted acids and bases) (32), it should be possible to compare the results obtained from the measured z-potentials as a function of pH z 5 f(pH) with the calculated acid parameters g 1 (in the Lewis sense: the electron-acceptor parameter of the surface tension) according to the van Oss approach. Comparing the determined “acidic” iep values, as a measure for the amount of Brønsted–acidic surface functional groups as obtained by the g 1 parameters, one will observe the same tendency. However, as shown by the z-potential measurements the overall character of the investigated polymer surfaces is acidic and not basic, as follows from the van Oss approach (the g 2 values are significantly larger compared to the g 1 components). This clearly indicates the high degree of

FIG. 7. pH dependence of the z-potential ( z 5 f(pH)) of PC measured at varying KCl concentrations.

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FIG. 8.

Concentration-dependence of the z-potential ( z 5 f(c)) of PA12, with and without correction in surface conductance (csc).

validity of the method from the acidic parameter’s g 1 point of view. This is hardly the case, however, when both acidic and basic parameters are considered. The concentration dependence of the z-potential gives information about the degree of interactions between the solid surface and the ions of the electrolyte solution, based on specific or electrostatic interactions. For solid materials the z-potential values normally follow a parabolic curve-like trend in accordance to the Stern theory (33). This is caused by the adsorption properties of the solid for potential-determining ions as well as preferentially adsorbed ions. The Stern theory makes no assumptions regarding the sort of charge carriers close to the interface border; it should therefore be valid for solids, where the surface potential is determined through dissociation of surface groups or through the adsorption of ions. The measured concentration dependence of the z-potential for PA12 (without correction in surface conductance) follows the expected parabolic, curve-like trend (Fig. 8). Despite verified, preferential ion adsorption for PA12, it was impossible to obtain reproducible results for the corrected z-potential values. The used measurement setup does not allow a reliable determination of the electrical resistance of the electrolyte solution inside the channel, especially in the case of low KCl concentrations. This resistance value is needed for the correction of the surface conductance according to the Fairbrother and Mastin method. CONCLUSION

All investigated thermoplastic polymers display a relatively hydrophobic surface. First, all applied methods to calculate the

solid surface tensions of the polymers (except the Zisman approach to estimate the critical surface tension g c) provide for the same sample, the same overall solid surface tension g within the error range, but different surface tension components. Second, all investigated polymer samples have within the error range the same overall surface tensions for each method applied, but different surface tension components (g p and g d, respectively, g AB) and therefore different surface polarities. According to Kanamaru it is possible to obtain information about the swelling behavior of polymers in water from the time dependence of the z-potential. All investigated polymers “swell” to a certain degree in water (i.e., the quotient (z 0 2 z `/z 0) is proportional to the water uptake at 100% RH). The investigated polyamide 12 will adsorb water to a very high degree whereas polystyrene will only adsorb low amounts of water. It is also possible to estimate a constant k from the z-potential–time dependence, which reflects the velocity of the water uptake process. As could be seen from the measured pH dependence of the z-potential at different base– electrolyte concentrations the investigated polymers interact differently with the dissolved potassium and chloride ions. In the case of polyamide 12 these ions will be preferentially adsorbed whereas in the case of polycarbonate the ions will be nonspecifically adsorbed. It is also obvious that all polymers show a slightly acidic surface character as can be seen from the overall course of the z 5 f(pH) functions and from the low (acidic) iep. This might be due to thermal stress during the manufacturing and engineering processes.

CHARACTERIZATION OF SEVERAL POLYMER SURFACES

Comparing the results obtained from the z-potential measurements with the acid parameter of the surface tension g 1 calculated from the measured “static” contact angles using the van Oss, Good, and Chaudhury approach, it is remarkable that both give the same tendency. Therefore this acid– base approach seems to give valuable chemical information on the investigated polymers when comparing only one parameter (g 1 or g 2) for different surfaces. However, it is also obvious that the base parameter g 2 is overdetermined. One further advantage of this acid– base approach is that it is possible to estimate repulsive interactions acting across the interface between adjacent components. z-potential and contact angle measurements complement each other and allow the estimation of interactions which might also influence adhesive properties between a reinforcing component (i.e., carbon fibers) and a surrounding matrix material. ACKNOWLEDGMENTS We thank Mrs. H. Oehlert (TU-Berlin) for the synthesis of polystyrene and the polymer characterization, Dipl.-Ing. H. Lipp-Terler (Lipp-Terler High Performance Films, Waidhoffen/Ybbs) for supplying us with PEEK and PEI foils, Mrs. M. Kru¨ger for the determination of the molar weights of the polymers, and Dr. C. Della Volpe for his comments.

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