Chemical modification of carbon fiber surfaces by nitric acid oxidation followed by reaction with tetraethylenepentamine

Carbon Vol. 35, No. 3, pp. 317-331, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223/97 $17.00 + 0.00

Pergamon PII: SOOOS-6223(96)00170-4

CHEMICAL MODIFICATION OF CARBON FIBER SURFACES BY NITRIC ACID OXIDATION FOLLOWED BY REACTION WITH TETRAETHYLENEPENTAMINE C. U. PITTMAN, JR,~,* G.-R. HE,~ B. Wub and S. D. GARDNER~ “Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, U.S.A. bDepartment of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, U.S.A. (Received 2 January 1996; accepted in revised form 11 October 1996) Abstract-Amino groups react rapidly with both isocyanates and epoxides. Thus, to prepare carbon fibers which might exhibit enhanced adhesion to both polyurethanes and epoxy resin matrices, attempts were made to introduce a high surface amine concentration onto high-strength carbon fibers (derived from PAN) by nitric acid oxidation followed by reaction with excess tetraethylenepentamine (TEPA). Fibers were oxidized with concentrated (70%) nitric acid at 115°C (20,40, 60 and 90 minutes) to generate surface acidic functions, primarily carboxyl and phenolic hydroxyl groups. These oxidized fibers were then reacted with TEPA at 190-200°C to introduce surface-bound amino functions onto the surface via amide functions. The amide functions formed at the surface to graft TEPA to the surface. TEPA does not react with hydroxyl functions so hydroxyls remain on the surface. Decarboxylation of surface carboxyl groups was minor during the TEPA grafting. The quantity of surface-bound acidic and basic functions on these modified fiber surfaces was measured by NaOH and HCI uptake experiments. Then, methylene blue and metanil yellow dye adsorption experiments were employed to provide a measure of the surface area and both the surface density and steric availability of surface acidic and basic functions. The dye adsorption gave lower values of acidic and basic functional group surface concentrations than did NaOH/HCl uptake measurements. The number of acidic groups increased continuously with increasing oxidation times but after an initial jump in acidic groups per 100 A’ at the start of the oxidation, the density of the surface acidic was approximately constant. Instead, the surface area increased, which accounted for the increase in total acidic groups. Nitrogen BET measurements were also performed and the BET surface areas were compared to those derived from dye adsorption experiments. Similarly, both HCl and metanil yellow adsorption measurements led to a value of 5.6 basic (amine) groups per 100 A’ after reaction with TEPA. The results were consistent with an increase in surface area during nitric acid oxidation. About 52% of the surface acidic functions, which were generated during nitric acid oxidations of 20-60 minutes, had reacted with TEPA. An average of 2.6 amino groups were introduced for each carboxyl group consumed in the reaction with TEPA. Thus substantial looping of TEPA occurs where more than one amino function of TEPA has formed an amide bond at the surface. The average ratio of the total amino groups introduced to the total amount of acidic groups present after nitric acid oxidation was about 1.35. 0 1997 Elsevier Science Ltd. All rights reserved Key Words-A. area, D. surface

Carbon fibers, properties.

B. oxidation,

C. adsorption,

spectroscopy,

D. surface

ing of this elastomeric interphase to both the fiber surface and the matrix will be a very important consideration. Without substantial chemical bonding to augment adhesion between these two interfaces (e.g. fiber/elastomer and elastomer/matrix), the fiberto-matrix adhesion could be weak. Rupture of either the elastomer/fiber or elastomer/matrix interface could degrade composite properties which would be exhibited if both interfaces were strongly bonded. A research effort has recently been established to investigate composite systems containing such fiber-bound elastomeric interphases [ 14,21,23-251. The type, the quantity and the reactivities of the fiber surface functions should be known in order to (1) permit the controlled introduction of a chemically-bonded elastomeric interphase, or (2) to begin to understand fiber matrix bonding. The type of functional groups which are beneficial to have on fiber surfaces depends on the chemical identity of the functional groups available in the interphase elasto-

1. INTRODUCTION adhesion has been a topic of concern in composites technology for decades [l-3]. Previous studies have attempted to generate strong adhesion between the fiber surface and matrix [4-71. Numerous methods have been developed to improve the fiber surface wettability or to increase the quantity of surface functional groups [5,6]. However, the presence of strong interfacial bonding alone may not always lead to satisfactory composite performance. For example, the presence of a tightly bound but stiff interface can result in high stress concentrations at the interface, which results in low impact strength [ 81. The introduction of a strongly bound, concentric, elastomeric interphase layer between the fiber and matrix might be expected to improve this situation greatly [g-13]. However, the extent of chemical bond-

Interfacial

*Corresponding

C. photoelectron

author. 317

C. U. PITTMAN.JR rtui.

318

mers. Polyurethane elastomers, for example, have been used as fiber coating interphase layers in our laboratory in order to increase the impact strength of composites relative to composites made with uncoated fibers [20,21,23-2.51. Polyurethane resin coatings were prepared using isocyanate compounds. Isocyanates react rapidly with amino groups and carboxyl groups. Isocyanates also react with phenolic hydroxyls. Therefore, it is of interest to introduce large amounts of these chemical functions onto fibers if fiber/polyurethane chemical bonding is to be enhanced. Furthermore, it is necessary to characterize these surfaces. If the quantity of the surface functions on fibers is too small then the accuracy and repeatability in measuring this quantity will be poor. Furthermore, fewer fiber-to interphase bonds will be possible. These considerations led to the application of nitric acid oxidations to introduce a substantial amount of acidic functions (carboxyl and phenolic) onto fibers. In addition to acidic functions, keto lactone and quinoid functions are introduced in lower amounts [5]. Nitric acid oxidations at 115°C have been shown to introduce significant surface acidity to carbon fibers [ 15-251. X-ray photoelectron spectroscopy (XPS), angle-resolved photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS) studies have demonstrated that both carboxyl and phenolic hydroxyl groups contribute to this surface acidity (equation 1) [14,17]. Carboxyl and phenolic hydroxyl functions on carbon fiber surfaces can react with isocyanate groups to bind polyurethane elastomers and with epoxy groups to bind epoxy matrix resins to fiber surfaces [ 16,21L251.

//O HSO;

-

CF

CF

I

c\

OH

_

(I)

\IC,i.! lellrblue

: iCli3,z-Y

OH

It is well known [26] that amino groups react more readily with isocyanates than do phenolic hydroxyls. Therefore it might be advantageous to convert the surface carboxylic functions, generated by nitric acid oxidation, to amide functions using the polyamine tetraethylenepentamine (TEPA). Reacting TEPA with oxidized fibers could introduce several amino functions for each carboxyl group consumed (via amide functions). In this way the total amount of available surface-bound functional groups could be increased. For example, four surface-bound amino groups could ideally be introduced for each surface carboxyl group that reacted with TEPA. This is illustrated in equation (2).

Introducing

occur between fiber and interphase (or fiber and matrix) could lead to better adhesion. For example, Drzal [ 271 recently demonstrated that, by increasing interfacial chemical bonding, the interfacial shear strengths in amine-cured epoxy resin/carbon fiber composites could increase by as much as 40%. This paper describes the results of reacting TEPA with carbon fibers that had been oxidized in nitric acid for varying times at 115°C. Base (NaOH) and acid (HCl) uptake experiments were used to measure the quantity of acidic and basic functions, respectively, generated on the treated fibers. A correction procedure has been established in order to obtain accurate results for the measurement of surface-bound amino groups via HCl uptake experiments [22]. This was necessary because positive charge repulsion decreases the base strength of surface-bound TEPA nitrogen atoms as progressive protonation of adjacent amino groups proceeds. Dye adsorption was employed as an alternative method for the determination of both acidic and basic surface functions. Two dyes, methylene blue (MB) and metanil yellow (MY), were previously used for characterization of carbon black surfaces [2X]. These dyes have also been used by Scala and Brooks [29] to measure the acidic and basic functions, respectively, on treated, high-modulus carbon fibers. The large size of the dye molecules, when combined with NaOH and HCl uptake results, helps to provide information about the surface distribution of acidic functional groups. Both dyes adsorb on carbon surfaces via a n--71 dispersion interaction. In addition, MB+ will adsorb at negatively charged sites, and MY _ will adsorb at positively charged sites due to electrostatic attraction.

more

sites for chemical

bonding

to

The concentrations of the surface functional groups on carbon fibers are reported here as a function of different oxidation times after nitric acid oxidation and then after TEPA treatment. The fraction of surface acidic groups which reacted with TEPA and the distribution density of the surface functions were then derived. Furthermore, the bonding configuration of the surface-bound TEPA molecules was investigated. X-ray photoelectron spectroscopy (XPS) has been extensively used in the characterization of carbon fiber surfaces [30-331. For oxidized carbon fibers, the information obtained from XPS includes the atomic ratio of oxygen to carbon and the atomic ratio of various oxidation states of carbon and oxygen from an analysis of the Cls and 01s photo-

Chemical

modification

electron peaks [34-371. The quantities help in the specific definition of the functional groups present. XPS analysis obtains chemical information within the outermost 100 A of the fiber surfaces. However, interfacial adhesion between the fiber surface and the matrix occurs only on the fiber surfaces. Wet chemical analysis can give the information about the chemical structure on the outer graphitic layer of the fibers, which is accessible to a molecule in solution. Where the surface is grooved, crevassed or pitted to depths of 20-300 A with crevasse openings of 5-150 A, for example, XPS data on the outer carbon layer could still be confused with subsurface species. Therefore, the results from XPS and those from chemical analysis can represent different physical meanings. It may not be meaningful to compare the absolute values from XPS and chemical analysis. However, the combination of XPS and chemical analysis can give valuable information about the fiber surface chemistry. Furthermore, angle-resolved XPS (ARXPS) and ion scattering spectroscopy (ISS) can be employed. In this paper, a comparison was made between the results of base/acid uptake and XPS experiments obtained on the same group of fiber specimens. Detailed XPS, ARXPS and ISS experiments on oxidized carbon fibers have been and will be published elsewhere [ 17,381.

2. EXPERIMENTS

2.1 Materials and carbonjber surface treatments The carbon fibers used in this study were PANbased, unsized Celion G30-500, 3K-HTA-7C-NSOl (BASF Structural Materials Inc.) These fibers were unsized and surface-pretreated by the manufacturer. All chemicals were acquired from Aldrich Chemicals, Inc. The tetraethylenepentamine (TEPA) was of technical grade and based on information provided by Aldrich, its nitrogen content was 36.16% (equivalent to 97.7% of the theoretical content) and the pentamine content was 98%. All other chemicals were of ACS reagent grade. Carbon fibers were oxidized in 70% nitric acid at 115°C. The treatment time was varied from 0 to 90 minutes. The fibers were then rinsed with a slow and continuous flow of distilled water for 10 hours, at which time the effluent wash water had long since reached the same pH value as the distilled water employed (pH = 5.5-6). Portions of both the HNO,-treated and the untreated (as-received) carbon fibers were reacted with TEPA to introduce amino groups onto the fiber surfaces. Fiber specimens (40 g) were placed in a 500 ml flask containing a large excess of TEPA (300 ml), followed by heating at 190°C for 17 hours. After cooling, the fiber specimens were removed and washed with distilled water until long after the wash water attained the same pH value as the distilled water employed in the wash. Experiments conducted

of carbon

fiber surfaces

319

at shorter reaction times confirmed that the TEPA grafting reaction was completed within 2-3 hours.

2.2 Determination of the surface acidic functions The surface acidic functions were determined by both base (NaOH) uptake and methylene blue dye adsorption. 2.2.1 Base uptake. Aqueous NaOH solutions were prepared with concentrations from 1 x 10m3 to 2 x 10e3 M. Distilled water was boiled before it was used to prepare each NaOH solution, thereby removing dissolved carbon dioxide. One gram of carbon fiber was immersed in a freshly prepared NaOH solution (25-50 ml) in a plastic vial for 16620 hours. Then the pH value of the resulting NaOH solution was measured by a pH meter (Ion Analyzer 250, Corning). The concentration of the NaOH solution was calculated from this pH value. The quantities of acidic functions on the fiber surfaces were calculated from the change in NaOH concentration before to after the immersion of carbon fibers. 2.2.2 Methylene blue adsorption. Methylene blue (MB) was selected as the dye for measuring the acidic functions on carbon fiber surfaces, MB solutions were made with a concentration of 20mgll’ (53.5 x 10m6 M) (the formula weight is 373.9 due to the presence of three waters of hydration per MB present in the crystals). Oneml of 1 M NaOH solution was added to 1 1 of the MB solution to adjust the solution pH to about 10.8-11. At this high pH all the acidic groups (phenolic hydroxyl, carboxyl, etc.) present on fiber surfaces were completely deprotonated [26]. Therefore, the uptake of MB will not be complicated by the presence of residual undeprotonated acid sites. The fiber specimen (1 g), after being used in NaOH uptake and drying, was immersed in 50-300 ml of MB+ solution for 16 hours. The volume used depended on the quantity of surface acidic functions present on the specimen as determined previously by base uptake. The absorbance of the MB solution was measured by a spectrophotometer (Spectronic 2lD, Milton Roy Co.) at 620 nm. Distilled water was used as a blank. The surface acidic functions determined by MB adsorption were calculated from the change in dye concentration before and after adsorption by fibers.

2.3 The determination of the surface basic functions The surface basic functions were determined by both acid (HCl) uptake and metanil yellow dye adsorption. 2.3.1 Acid uptake. One gram of carbon fiber in 25-50ml of HCl solution was immersed (1 x 10m3-2 x 10m3 M) for 16 hours in a plastic vial. The pH value of the HCl solution was measured and used to calculate the HCl concentration. The quantity of acidic groups was calculated from the change in

C. U. PITTMAN,JR et ul.

320 HCl concentration the fibers.

which occurred

3. RESULTS AND DISCUSSION

upon immersing

2.3.2 Metanil yellow adsorption. Metanil yellow (MY), an anionic dye, was also used to measure the basic functions on the fiber surfaces. The surface basic functions were first protonated with acid in the HCI uptake measurement prior to MY adsorption. These protonated sites were then able to adsorb the MY anions. The MY solution contained 38 mg of MY per liter (100 x 1O-6 M) with 1 ml of 1 M HCl added. This resulted in a MY solution with pH 3, which was the same pH value as that used in HCl uptake measurements. The amino groups on grafted TEPA molecules may not be completely protonated at this pH value. This fact and its influence on MY- adsorption will be discussed later in this paper. Fiber specimens ( 1 g), after being used in HCl uptake measurement and drying, were immersed in 100-300 ml MY solution for I6 hours. The volume of MY solution used depended on the quantity of basic functions present on the specific fiber groups as determined previously by HCl uptake. The absorbance of the MY solution was measured at 424 nm and the quantity of basic functions present on the fibers was calculated by the change in dye concentration after fiber immersion. Absorbance vs concentration plots for MB and MY solutions were obtained from a series of dye solutions of known concentrations. Approximately straight lines were obtained for both dye solutions within an absorbance range from 0 to 2. The conversion factors relating dye concentrations to absorbance were determined to be 25 x lo-’ and 58.2 x 10e6 M per unit absorbance for the MB and MY solutions, respectively. 2.4 Scanning electronic microscopy The morphology of the carbon fiber surfaces were observed by scanning electronic microscopy (Model Stereoscan S360, Cambridge Instrument, Inc.) at 20 kV without gold plating.

3.1 Acidic groups generated by HNO, oxidation Oxygen-containing functional groups present on carbon fiber surfaces include carboxyl groups (R-COOH ), phenolic groups (R-OH ), quinones and lactones, etc. [ 51. NaOH neutralizes carboxyl groups, phenol hydroxyl groups and opens lactone rings to their corresponding carboxylate salts. Therefore, the NaOH uptake method measures the total amount of carboxyl, hydroxyl and possibly lactone groups available on the surfaces. Figure 1 shows the total acidic functions present on the HNO,-oxidized carbon fibers determined by both NaOH uptake (always conducted at pH 11 or higher) and methylene blue adsorption (also see Table 1). The total amount of surface acidic groups (peq g-’ fiber) is shown as a function of oxidation time in 70% HNO, at 115’C. Both NaOH uptake and MB+ adsorption increased with oxidation time, demonstrating that the quantity of acidic functions on the fiber surface increases as exposure of fibers to HNO, continues. However, at a given level 01 fiber oxidation, only about one third as many acid functions were determined by methylene blue adsorption as by NaOH uptake (Fig. 1). Previous studies have demonstrated that competitive Na+ adsorption is not responsible for the lower dye adsorption values [39]. MB+ adsorption remains unchanged over a broad range of Nat concentrations. Therefore, the low values of MB+ adsorption were due to the large size of the methylene blue cations. One MB+ ion may cover several acidic sites upon adsorption. HCl uptake and metanil yellow (MY -) adsorption were also measured for these nitric-acid oxidized carbon fibers. Both measurements were performed at pH 3. The fibers adsorbed a small quantity of MY at pH 3 (only 2.2 peq gg’ after 60 minutes and 5.5 peq g-i after 90 minutes of oxidation in HNO, (see Fig. 2). Their HCl uptake values were close to zero (Table 1).

2.5 X-ray photoelectron spectroscopJ1 The X-ray photoelectron spectroscopy (XPS) experiments were performed on a Physical Electronics PHI Model 1600 surface analysis system. The XPS spectra were obtained using an achromatic Mg Kr ( 1253.6 eV ) X-ray source operated at 200 W. Survey spectra were collected from 0 to 1100 eV with a pass energy equal to 46.95 eV. High-resolution spectra were acquired over the ranges 0-l 10, 270-550 and 1055-1085 eV with the pass energy adjusted to 23.50 eV. During all XPS experiments, the pressure inside the vacuum system was maintained at approximately 10 9 Torr. ARXPS was performed by aligning the carbon fibers in the plane of the X-ray source and the electron energy analyzer (SCA) and tilting the specimen plane 10, 50 and 90” with respect to the SCA entrance [ 141. Further experimental details are available elsewhere [ 14,17,38].

6. Acidic

SO-

functions

. NaOH

(peq/g)

uptake

. MB adsorption 40 30-

Nitric acid oxidation

time (min.)

Fig. I. Acidic functions on HNO,-oxidized carbon fibers versus oxidation time, determined by both NaOH uptake and methylene blue adsorption.

Chemical

modification

of carbon

fiber surfaces

321

Table 1. NaOH and HCl uptake by HNO,-oxidized carbon fiber” surfaces before and after TEPA treatment HNO, oxidation time (minutes) 0

20

(A) Before TEPA treatment (B) After TEPA treatment (C) Change of NaOH uptake (A)-(B)

3.6 3.3 0.3

13.0 4.8 8.2

(D) Before TEPA treatment (E) After TEPA treatment (F) Change of HCl uptake (E)-(D)

0.2 4 3.8

0.3 15.9 15.6

40 NaOH uptake (peq g-r) 18.1 7.2 10.9 HCl uptake (peq gg’) 0.0 26.6 26.6

60

90

33.9 16.0 17.9

48.8 22.0 24.8

0.4 38.6 38.2

0.1 54.9 54.8

a BASF Celion G30-500 carbon fibers were employed. 70% HNO, was used at 115°C in the oxidation treatments. lye adsorption

(pcq/g)

v MB adsorption A MY adsorption

n MB-MY

-%I

I

20 10 Tota! acidic functions

I

I

30

40

by NaOH uptake

50 (peq/g)

Fig. 2. The adsorption of methylene blue and metanil yellow by the HNO,-oxidized carbon fibers vs the total acidic functions determined by NaOH uptake. Many factors could play a role in the adsorption of anions by carbon surfaces. In the case of colloidal carbon black, the isoelectric point (IEP) and the point of zero charge (PZC) have been shown to be important features of the surface chemistry [40-421. Many chemical and physical phenomena of carbon black are associated with their IEP and PZC [40,43]. Experimentally, the IEP and PZC are characterized by the pH value of the solution (in which carbon particles are suspended) corresponding to a specific equilibrium state. Therefore, the chemical or physical behavior of suspended carbon black is related to the pH value of the solution. Anion adsorption, for example, depends on the pH value of the solution [44]. Recent research indicated that when the pH value of a solution was lower than the PZC of a suspended carbon black, protonation on the carbon basal planes may occur at Lewis base sites in the form of (C,H,O’) [4.5]. These protonated Lewis base sites on carbon black surfaces could then adsorb anions in the solution. Thus it is important to consider such phenomena on carbon fiber surfaces. Many surface chemical properties of carbon fibers have been found to be similar to those of carbon blacks, such as the formation of the same acidic and basic functions on their surfaces [46-481. However, carbon fibers greatly differ from carbon blacks. Their specific surface area is much smaller (about three

orders of magnitude). Their gross sizes are far greater (also about three orders of magnitude) than colloidal particles. The PZC is a point at which the surface acidic (or basic) functional groups no longer contribute to the pH value of the solution. These groups cannot further dissociate at this pH value. This point is equivalent to the pKa of weak electrolyte molecules in aqueous solution. The carboxyl, phenolic hydroxyl and lactone functions present on carbon fibers are immobilized. Therefore, they cannot behave in an equilibrium the same way a soluble small molecule in solution would. Although the pKa data are available for small molecules with carboxyl and phenolic hydroxyl groups, there is no satisfactory way to calculate pH values at dissociation equilibrium for treated carbon fiber surfaces. Many techniques used to measure the PZC of carbon blacks [49] are not applicable to the much larger and heavier carbon fibers. Other potential methods could not be conducted in our laboratory. Therefore, the IEP and PZC values were not measured on the carbon fibers used in this study. One can only estimate that the PZC value of oxidized carbon fibers will be lower than pH 7. Although no useable values of the PZC for these carbon fibers are available, the almost zero HCl uptake (Table 1) strongly suggests that no appreciable protonation occurred on the fibers’ basal planes (i.e. in the form of C,H,O+) at pH 3 (or down to pH ~2.7). MY adsorption occurs electrostatically at positively charged sites, and adsorption also occurs at other locations, generally by z--n dispersion effects. During the HNO, oxidation, no basic surface groups were introduced (see line D, Table 1). Thus, MY adsorption onto HNO,-oxidized fibers cannot occur to a significant extent at protonated surface basic groups (to include C-H+ types of sites). Rather, MY- adsorption onto the oxidized fiber surfaces at pH 3 is mainly due to 7[-rc dispersion forces. A key point is that both methylene blue and metanil yellow may, to a first approximation, be similarly adsorbed via such dispersion forces in the same regions of the surface, depending upon which one is present. Thus, the major difference in their overall adsorption values would arise from MB+ also adsorbing at deprotonated acidic sites (e.g. -COO-, -O-).

322

C. U. PITTMAN.JR et ul.

Methylene blue and metanil yellow molecules have comparable sizes and molecular weights. Their experimentally derived projection areas per molecule from carbon black experiments are 197 A” [28] for methylene blue and 179 A’ [28] for metanil yellow, respectively. The sizes of their major molecular planes derived by molecular modeling are even closer to each other. Their molecular weights are 320 D (MB) and 375 D (MY). Considering the similarity in their size, shape and molecular weight, MY adsorption by the oxidized carbon fibers, to a first approximation, may be regarded as a measure of the amount of adsorption, due to n-rc dispersion, of methylene blue by the same fiber specimen. Scala and Brook [22] used methylene blue and metanil yellow as adsorbates to characterize the surface acidity of nitric-acid oxidized Hitco highmodulus graphite fibers derived from a rayon precursor. Their values of the methylene blue and metanil yellow adsorption are comparable with the values reported herein. They also found that the difference in adsorption between MB+ and MY(i.e. MB+-MY-) increased as the fibers were progressively oxidized, and they used this MB+-MYdifference value to characterize the extent of oxidation on the fiber surfaces. Employing ( 1) the approximation that MY adsorption by oxidized fibers is a measure of the dispersive MB+ adsorption and (2) the NaOH uptake values, we have extended the method of Scala and Brook to study the surface chemistry of nitric-acid oxidized PAN-based carbon fibers. Furthermore, this information was investigated as a possible way to approximate changes in fiber surface area. 3.2 Surface area changes during nitric acid oxidation.. a method based on dye adsorption nitrogen BET determinations

vs

The difference between MB+ adsorption and MYadsorption on the oxidized fibers may be regarded as a reasonable approximation of the net amount of methylene blue that was actually adsorbed by the acidic sites, given the assumption that the physical adsorption of MY- is approximately equal to that of MB+. The total number of acidic sites was given by NaOH uptake. Plotting the (MB’-MY -) adsorption difference against NaOH uptake of the same specimen gives an approximately straight line with a slope of 0.215 (Fig. 2). This slope represents the number of methylene blue cations adsorbed per acidic site present on the surfaces. According to this interpretation, the value of the slope indicates that an average of 0.2 15 methylene blue ions was adsorbed per acidic site. In other words, one methylene blue cation was adsorbed for approximately every 4.65 acidic groups (l/0.215) on the fiber surfaces. Either the MB’ ions cover more than one acid site or some acid sites are present in deep narrow crevasses or within pores which are not accessible to MB’ ions. A projection area of each methylene blue ion of

197 A” [28] was then used to estimate the surface area and surface distribution of acidic groups. In this case, the surface area may be defined as that available to MB+ ions per gram of fibers. It was obtained by multiplying the number of adsorbed MB’ ions times 197 A’. Likewise, a second set of values was generated using a projection area of 148 A”, which was generated by molecular modeling of the MB’ ion where this is the area of the major plane of the molecule. These values are summarized in Table 2. The values of surface area or functional group density obtained in this way are based only on the dye’s size, and are totally independent of surface area and geometry changes accompanying the nitric acid oxidations. However, the calculation is based on the assumption that multiple layers of dye have not adsorbed. The fiber surface area was observed to increase with HNO, oxidation time (see Table 2). The MB’-accessible surface areas increased from 1.4 to 3.6 to 6.8 to 10.0m2 gg’ and finally to 16.7 m2 g-’ going from as-received to 20, 40, 60 and 90 minutes of oxidation, respectively. Thus, the increase in MB’ accessible surface area is approximately proportional to oxidation time over the first 60 minutes of oxidation, and then the rate of surface area increase becomes faster. Using these values of surface area, the surface acid group densities (functions per 100 A’) were determined from the NaOH uptake measurements. The functional group densities obtained in this way (Table 2) were 1.4, 2.2, 1.6, 2.1 and 1.8 acidic functions per 100 A’ for as-received fibers to fibers oxidized for 20, 40, 60 and 90 minutes, respectively. Clearly, after a brief period early in the oxidation where the acidic group surface density increases, the surface density of acidic functions remains approximately constant between 20 and 90 minutes of oxidation in 70% HNO, at 115°C when surface area is defined by MB+ uptake. Thus, the increase in total acidic functions introduced by this oxidation is primarily the result of increasing surface area. These results basically agree with results obtained by other investigators who used nitrogen adsorption (BET) to measure changes in fiber surface areas with nitric acid oxidation [50-551. The BET method, of course, measures the total surface area (by definition) in contrast to the “MB+-accessible surface area”. Nitrogen molecules are much smaller than MB+ ions. Furthermore, at first glance it would seem that nitrogen should be able to reach all surface locations accessible to aqueous NaOH. The nitrogen molecule is smaller than a solvated hydroxide ion, and the high diffusivity of nitrogen should favor its reaching all the available surface area. In order to evaluate independently the MB+ adsorption method of estimating surface area, nitrogen BET measurements were carried out on this series of fibers. These measurements and the surface densities of the acid functional groups determined from BET and NaOH uptake are also summarized in Table 2.

Chemical modification of carbon fiber surfaces Table 2. Measurements of the MB+-accessible oxidation time. Acid functional group densities

323

surface areas and the nitrogen BET surface areas of carbon fiber” vs HNO, are derived from each of these surface area measurements and NaOH uptake HNO, 0

oxidation 40

60

90

6.8 5.0

10.0 8.7

16.7 13.4

1.6 2.2

2.1 2.3

1.8 2.2

0.56 19.4 0.91

0.67 30.5 1.7

6.58 4.5 2.5

20

Surface area accessible to MB’ based on MB+ adsorption for two projection areas 197 AZ/~~+b (m* g-r) 1.4 3.6 148 .&*/MB+” (m’ g-l) 1.3 2.5 Acidic functional group surface density based on the MB+ accessable area and NaOH 197 A*/MB+~ (functions per 100 A’) 1.4 2.2 148 A*/MB+d (functions per 100 AZ) 1.7 3.1 Surface area from nitrogen BET and acidic group surface density using NaOH uptake 0.41 Surface area (m* g ‘) Acidic group density” (functions per 100 A*) 5.3 0.18 0.65 Active surface areaf (based on NaOH uptake) (m’ g- ‘)

time (minutes)

uptake

values

“BASF Celion G30-500 carbon fibers were used. 70% HNO, was used at 115°C in the oxidation treatments. ‘Determined by: surface area (m’ g-r) = MB+ads(peq g-‘) x l(Fmol peq-‘) x 6.023 x 10z3(MB’ mol-‘) x 197(A2 per MB+) x 106(fimol mol-r) 10zO(AZ mm2 ) x 106( pm01 mol - ‘) “As per b, but replace 197(A2 per ion) by 148(A2 per ion). d Acidic functions per 100 A” = (Acidic functions by NaOH uptake)(peq gg’) x 10m6(mol peq-‘) x 6.023 x 10Z3(functions

(MB+-determined

surface area)(m’g~‘)x

mol-‘)

1020(A2m-2)~(100AZ)

The MB+-determined surface areas (rows 1 and 2) were used for the projection areas of 197 and 148 A”, respectively. ‘Acidic functions per 100 A’= (Acidic functions by NaOH uptake)(peq g-l) x 10m6(mo1 neq-‘) x 6.023 x 10z3 (functions mol-‘) (N, BET surface

area)(m’

8-l) x 10Z”(AZ mm’) x (100 A’)

The surface areas measured by nitrogen BET were 0.41 mz g-’ (as-received), 0.56 mz g-’ (40 minutes of oxidation), 0.67 mz g- ’ (60 minutes of oxidation) and 6.58 m2 g-’ (90 minutes of oxidation). The BET measurements of surface area, like those of MB+, showed an initial surface area increase approximately proportional to time through 60 minutes of oxidation which was followed by a rapid increase in surface area by 90 minutes However, the BET measurements were substantially smaller than those obtained by MB+ adsorption, and the magnitude of the difference increases with oxidation time. One contributor to this difference could be that multilayer dye adsorption can occur in some locations. Secondly, at negatively charged sites (-0 or COO -) multilayer adsorption may be more pronounced, which would explain why the magnitude of difference between MB+ adsorption and BET surface areas rises with increasing oxidation. However, it is also possible that exposure of oxidized fiber surfaces to basic aqueous solution causes some opening of cracks, crevasses and pores via strong exothermic solvation forces. Thus, aqueous NaOH and aqueous alkali-MB+ solutions may encounter more surface area beneath the outer perifery than can be accessed by nitrogen in BET measurements. Keep in mind that BET measurements are performed on dry fibers where some of this internal pore structure may be closed off, much like a dry, collapsed sponge. This could also explain the unreasonably high surface acidic group densities obtained using NaOH uptake with BET surface area measurements at 40 and 60 minutes of oxidation. The calculated densities are 19.4 and 30.5 acidic functions per

100 A’. This exceeds the maximum possible density of - 12 per 100 A’, strongly suggesting the surface accessed by aqueous NaOH is greater than that measured by BET. Many previous studies have shown that the surface areas of carbon fibers derived from PAN increase during oxidation. For example, starting from fibers with a BET-measured surface area of 8 mz g-r, surface area increments to 35-38 mz g-’ were obtained after oxidations in 60-70% HNO, [50]. Thornel 25 fibers with an initial surface area of 1 m* g-’ exhibited a six-fold increase to 6 m2 g-’ upon oxidation in 60% HNO, and a 1Zfold increase to 12 m* gg’ in 70% HNO, [ 531. Other BET measurements indicated that PAN-based fibers (untreated) had a surface area of 0.5 m2 g- ’ [54], which is close to that calculated from the smooth cylinder model (0.32 m2 g-’ for typical carbon fibers with a diameter of 7 pm and density of 1.78 g cmM3). Our group measured an increase in the surface area of PAN-based T300 (Amoco) fibers from 0.67 m2 g-’ as-received to 2.0 mz g- ’ after 60 minutes and to 12.4 m2 g- ’ after 90 minutes of oxidation in 70% HNO, at 115°C [ 561. During the first 60 minutes, the increase in surface area was proportional to oxidation time. Thus, over all these studies, from four fold to 18.5-fold increases in fiber surface area have been noted by BET. These values are in accord with the 12-fold increase in MB +-accessible surface area detected here for carbon fibers, going from as-received fibers to fibers subjected to 90 minutes of oxidation. Note that a 16-fold increase in surface area was measured by nitrogen

C. U. PITTMAN, JR

324

BET for the same treatment range (Table 2), although the absolute values were smaller. Extensive XPS and ISS data on carbon fibers as a function of nitric acid oxidation was accumulated. ISS studies (outer atomic layer only) showed that an initial increase in the O/C atomic ratio took place at fiber surfaces exposed to very mild HNO, oxidations, but further O/C ratio changes were minor upon continued oxidation. This is consistent with a constant oxygen-containing functional group density present on fiber surfaces as oxidation continues beyond an initial oxidation stage. Thus, the increase of acidic groups detected with increasing oxidation time must be the result of an increasing surface area. Scanning electron microscopy was unable to discern any differences in the fiber surfaces at magnifications up to 7500 x Therefore, the etching and pitting dimensions on the fiber surfaces must be very small, although these features cause the surface area to increase substantially. One physical picture in accord with these results involves preferential oxidation where the lateral graphitic planes intersect the surface. This is illustrated schematically in Fig. 3. Here, grooves, pores or crevasses will form as oxidation proceeds. Often, intersections with subsurface voids will occur. Oxidation at the basal planes is slower, leading to lower acidic group surface densities on these regions. Thus, as the surface area increases, the total quantity of acidic groups increases but, after a very short initial oxidation period, the overall average number of surface acidic groups per unit area does not change greatly (Fig. 3). The close spacing of some acidic groups (especially those at the oxidized

et ~1.

edges of the lateral planes) assures that one large dye molecule will simultaneously cover several acidic sites. Although it is not well represented in Fig. 3, it seems likely that an interconnected subsurface pore, void and crack structure exists which can partially open or swell in the presence of aqueous base.

3.3 Fruction of acidic groups which react with TEPA Not all of the surface acidic groups can react with TEPA phenolic or other hydroxyl groups cannot react with TEPA because phenolic (or aliphatic) hydroxyl groups do not react with amines under the conditions employed [26]. Carboxyl groups which are accessible can react with TEPA. Since TEPA is a linear molecule, its terminal amino groups should be able to reach almost any surface carboxyl group which can be detected by aqueous NaOH. This occurs via sereptation (the snake-like movement of linear molecules into confined volumes). Sereptation is well known and is the same mechanism by which noctane, for example, can penetrate into small-pore zeolite cavities which isobutane or neopentane cannot get into. This assumes that basic TEPA at 190-195 ‘C is able to open some of the subsurface pore/crack structure which, in the model previously mentioned, is not accessible to nitrogen when the fibers are dry. TEPA may also react with some surface lactone functions, converting them into amides. Carboxyl groups were not independently determined (versus phenolic OH) in this work. However, the quantity of phenolic hydroxyl groups present could be approximated as the total remaining acidic

Initial surface

Crevass or pit Further oxidation >>

Some oxidation beginning to occur In uncovered void Fig. 3. Introduction

of acidic functions during nitric acid oxidation of carbon liber surfaces which causes an increase area. Surface ketone and ether functions are also present.

in surface

Chemical

modification

groups present after reaction with TEPA was completed, assuming that almost all of the carboxyls available to aqueous NaOH should also be available at 190°C to TEPA. The approximate fraction of surface acidic groups which reacted was simply defined as the ratio of the acidic groups which reacted with TEPA molecules vs the total amount of acidic groups present before the reaction with TEPA. The fraction of acidic groups which had reacted (e.g. had been consumed) was determined by measuring the acidic functions on carbon fiber surfaces before and after reacting with TEPA. These measurements are shown in Fig. 4 for NaOH uptake and for methylene blue adsorption experiments. Each type of titration was done before and after TEPA treatment of HNO,-oxidized fibers. The quantity of carboxyl groups consumed during the TEPA reaction is obtained by subtracting the NaOH uptake determined after TEPA treatment from the NaOH uptake determined before TEPA treatment. Of the two possible types of acidic reactive groups present on the surface (-OH, -COOH), only the carboxyl groups can react with TEPA because either phenolic or aliphatic hydroxyl groups are essentially inert toward primary or secondary amino groups. Lewis acid or n-acidic surface sites will not react covalently with TEPA. Any TEPA initially complexed to such surface sites will subsequently be washed away under the vigorous washing conditions employed prior to NaOH uptake experiments. Therefore, the difference in the NaOH uptake values, from before to after TEPA treatment measures the carboxyl groups consumed in the reaction with TEPA (as long as decarboxylation does not compete with amide formation, see below). Carboxyl functions may be consumed in two ways: ( 1) amide formation with TEPA and (2) decarboxylation. Plotting the quantity of acidic functions consumed vs the quantity of surface acidic functions originally present (measured by NaOH uptake before TEPA treatment) gives an approximately straight line with a slope of 0.52 and an intercept of zero

:j&&+ 0

40 20 Nitric acid oxidation

60

80

time (min.)

Fig. 4. NaOH uptake and methylene blue adsorption versus the oxidation time by carbon fibers before and after reaction with TEPA.

of carbon

fiber surfaces

325

(Fig. 5). If one assumes no decarboxylation occurred, then the slope of this line represents the average fraction of acidic groups which reacted with TEPA. It seems likely that at 190°C in the basic TEPA environment that some small amount of decarboxylation could have occurred [57]. Approximately 52% of the total acidic groups, determined by NaOH uptake, were able either to react with TEPA or decarboxylate under experimental conditions. The remaining 48% of the acidic groups (which could not react with TEPA) are most probably phenolic hydroxyl functions (no reaction with TEPA is possible). Therefore, the percentage of acidic groups present after TEPA treatment serves as an approximate measurement of the hydroxyl functions introduced during nitric acid treatment. An important question must now be addressed. Does decarboxylation occur from fiber surfaces during the reaction with TEPA? Decarboxylation is known to occur from fiber surfaces rather rapidly between 300” and 500°C [57]. Its onset begins below 300°C and perhaps it occurs very slowly as low as 200°C. Amide formation is essentially completed in 3 hours or perhaps less. Does decarboxylation compete with amide formation under the reaction conditions employed? A previous set of experiments answer is no. strongly suggests that the HNO,-oxidized fibers were treated with thionyl chloride, which converts carboxyl groups to acid chloride functions quantitatively [23]. After generation of the acid chloride surface groups, TEPA was added at 80°C or below. Acidic chlorides react at mild conditions with amines to generate amide bonds. After this reaction, the fraction of acidic groups which had reacted was determined by NaOH uptake [23]. The amount of TEPA which had been grafted under these mild conditions was found to be almost the same as the amount of TEPA grafted to another aliquot of the same HNO,-oxidized fibers where TEPA was heated with the fibers to 19OG195”C. Thus, decarboxylation was not in serious competition with amide formation in these TEPA grafting reactions. The fraction of acidic groups which reacted with TEPA was found to be higher when the acidic groups were determined by methylene blue adsorption (before and after TEPA treatment) than when NaOH uptake was employed. A plot of the acid functions consumed during TEPA treatment ( y axis) versus the acidic groups present before TEPA treatment is shown in Fig. 6 (for as-received fibers to those oxidized for 90 minutes). The plot gave a straight line with a slope of 0.97. The quantity plotted on the y axis is the MB+ adsorption before TEPA treatment minus the MB+ adsorption after TEPA treatment for each specific level of HNO, oxidation. The assumption made here is that after TEPA has been grafted and the fibers are thoroughly washed, the amount of MB+ which is adsorbed by n--n dispersion forces was not changed. Therefore, the change in MB+ adsorption must correspond to the acidic

C. U. PITTMAN, JR

326

Acidic functions

consumed

rt (11.

(peq/g)

o--.---

G7

0

10

20

30

Total acidic functions

40

50

(peq/g)

NaOH uptake (before TEPA rxn) Fig. 5. Acidic

functions

consumed in the reaction HNO,-oxidized carbon

Acidic functions

with TEPA vs the total acidic fibers, both determined by NaOH

consumed

functions uptake.

originally

present

on the

present

on the

(peq/g)

IO Determined by MB adsorption

h := C:c 12 +3? Ep z

8 6-

I :=

‘;: $2

4 3E ccl

4m 2-

z

i 0

4

2

Total acidic functions

Fig. 6. Acidic

functions

8

6

IO

@q/g)

consumed in the reaction with TEPA vs the total acidic functions originally HNO,-oxidized carbon fibers, both determined by methylene blue adsorption.

(-COOH) groups consumed in the TEPA reaction (e.g. the number converted to amide functions). The MB+ adsorption of the original HNO,-treated fibers minus the MY adsorption of these same fibers is plotted on the x axis. Since the amount of MYadsorbed is equal to that of the MB’ adsorbed via z-z dispersion forces, this difference is equal to the amount of MB+ adsorbed at acidic sites. In other words, this difference is the amount of acid groups which can be detected by MB+ adsorption. This straight-line plot represents the fractional conversion of MB+-detectable acidic groups into amide groups in the reaction with TEPA. The value of 0.97 is good evidence that essentially all of the acidic groups which can be detected by the MB+ adsorption method can, in fact, react with TEPA. Considering the sereptation motion available to TEPA one would suspect that

all COOH

groups detected by MB+ could, indeed, be accessed by TEPA. This slope of -1 also shows that MB+ is not significantly adsorbed at hydroxyl groups as -O- MB+.

3.4 Distribution

of surface-bound

umino groups

Amino groups, introduced upon reacting surface carboxyl groups with TEPA, were determined in two ways: (1) by measuring the difference in HCI uptake before and after rection with TEPA, and (2) by measuring the difference in metanil yellow adsorption onto fiber specimens before and after TEPA treatment (Fig. 7). The HCl uptake values in Fig. 7 represent the best available measure of the total of all amine groups which have been grafted to the fiber surfaces. They were obtained using the correction procedure developed and described elsewhere [ 221.

Chemical modification of carbon fiber surfaces 6OBasic functions (peqlg)

I

n

50

HCI uptake

AMY

after TEPA

adsorption

v MY adsorption

0

0

treatment

after TEPA before

TEPA

treatment treatment

80 60 20 40 Nitric acid oxidation time (min.)

Fig. 7. Basic functions on HNO,-oxidized carbon fibers before and after TEPA treatment vs oxidation time, determined by HCl uptake and metanil yellow dye adsorption.

This procedure compensates for the fact that charge-charge repulsion prevents all amino sites on the grafted TEPA functions from being protonated at pH values of 2.7-3.0. A higher acidity is required to completely protonate all adjacent amino groups of the bound TEPA. HCl uptake and metanil yellow (MY -) adsorption measurements were used to evaluate the surfacebound amino group distribution density for HNO,-oxidized carbon fibers before and after TEPA treatment. The amount of HCl uptake by the HNO,-oxidized fibers before TEPA treatment was negligible (between 0 and 0.4 peq g-l, see Table 1). Thus the quantity of basic groups which could be protonated (or coordinated with H,O+) at pH values as low as 2.8 was negligible. However, the amount of MYwhich was adsorbed by n-n dispersion forces before TEPA treatment was substantial (Fig. 7) and could not be neglected. Since the MYcould not have been adsorbed at positively charged sites (only negligible HCl uptake occurred going from pH 7 to pH 2.7), the MY- which was adsorbed must have been largely located at neutral sites. The PZC value of TEPA-treated carbon fibers was unknown. These fibers contain both acidic (phenolic hydroxyls) functions and basic (amine) functions. Adsorption of MY- at protonated amine sites (e.g. pH of MY - solution was 3) and at non-charged sites (largely on basal planes) most probably occurs. Subtraction of the MY- adsorbed on the oxidized fibers (before TEPA treatment) from the MY adsorbed after the fibers were treated with TEPA should eliminate possible errors caused by extra protonation (or H,O+ uptake) on the basal planes. Therefore, the change in the quantity of MYadsorbed (from before to after TEPA treatment) may be regarded as the quantity of MYactually adsorbed by protonated amino groups. This change in MYadsorption (from before to after TEPA treatment) is plotted vs the total amount of surface

327

amino groups (from HCI uptake) present on fibers in Fig. 8. The HCI uptake after TEPA treatment (Table 1 and Fig. 7) measures the quantity of amino groups introduced onto fiber surfaces. A plot of the changes in MY - adsorption (from before vs after TEPA treatment) vs the total amino groups present on the fibers (Fig. 8) is a straight line with a slope of 0.10. This slope represents the average number of metanil yellow anions adsorbed per basic (amino) group. Thus one metanil yellow ion covered an average of approximately ten amino groups on the carbon fiber surfaces. Since the projection area of metanil yellow ion is 179 A’ according to Graham’s studies of carbon black surfaces [28], the average surface density of the basic groups was approximately 1 group per 17.9A2, or about 5.6 basic groups per 100 A’. This surface density of amino groups was higher than the original surface density of acidic groups (2.4 per 100 A’) because each bound TEPA contains either four or three (for loop bonding) amino groups. Thus, by reacting TEPA with oxidized carbon fiber surfaces, the number of functional groups which might react with isocyanates or epoxides was increased.

3.5 Loop structure for grafted TEPA Four amino groups are introduced for each TEPA which reacts with a surface carboxyl group to form a single amide bond (equation 2). This assumes that no loop structures are formed. A huge molar excess of TEPA (neat TEPA) was used in the fiber reactions to try to minimize loop formation. Thus, the ratio of amino groups introduced to acidic groups consumed was first expected to be 4. However experiments determined that this ratio was less than 4. For example, the NaOH uptake of the fiber specimens which had been oxidized in HNO, for 90 minutes was 48.8 peq g-‘. After reaction with TEPA, its NaOH uptake decreased to 22.0 peq g-l. Therefore, (48.8%22.0=) 26.8 peq g-’ of acidic groups (e.g. carboxy1 groups) reacted with TEPA. However, the

Change

of MY adsorption

&q/g)

‘1:

‘0 Total

I

1

I

I

I

I

lo

20

30

40

50

60

in TEPA

treatment

basic

measured

functions

generated

@q/g)

D BS

by HCI uptake

Fig. 8. The change of metanil yellow adsorption, before and after TEPA treatment, vs the total basic functions originally present.

C. U. PITTMAN, J~t~tul.

328

change of HCl uptake by this specimen, from before to after reaction with TEPA, was only 64.7 peq g-’ (corrected according to the procedure in refs [22,39]). Thus, 64.7 ,ueq g-’ of amino groups were grafted onto the fiber surface, while 26.8 peq g-’ of carboxyl groups were consumed. This gives a ratio of amine added per carboxyl consumed of 2.41 (considerably less than 4). The reason why this ratio is so low can be attributed to TEPA loop formation, which takes place on the fiber surface. The formation of loops by surface-bound TEPA molecules occurs as illustrated in Scheme 1. Once tethered to the surface, the probability of another of that TEPA’s amine groups reacting with a nearby surface carboxyl function can be very high. This is a typical chelating effect. In the above example, 26.8 peq g-l of carboxyl groups were consumed. were lost to decarboxylation and If 3peqg-’ 23.6 peq g- ’ reacted with TEPA, then the amine/carboxyl groups reacted ratio would be 64.1123.6 = 2.14. The fraction of TEPA present in loop structures can be approximated by assuming that decarboxylation was a minor pathway and by employing our previously determined titration studies of the TEPA/phenyl isocyanate reaction products [22,39]. Thus, only 3.30 amino groups can be detected by HCl titration (at pH 3) out of every four amino groups present on a grafted TEPA molecule. Only 2.70 amino groups can be detected (at pH 3) [22] when three amino groups are present on grafted TEPA molecules (e.g. when a loop is present). A mass balance of amino groups, acidic groups and TEPA molecules requires the following equations [22,39]: 3.30

g

-1

0

ff=

(3)

L/7

.fA=

(4)

1+.A .fB=

4i(i T

(5)

l-

Linear

o”

-c

I \

N-f:

-N/

H

V

H

P0

C

.NAl H

Loop

NH

H ,NeN-f

C

N

H

0

Scheme 1.

LY,,

2

where ,f:r is the mole fraction of TEPA molecules present on fiber surfaces as loop structures, fA is the mole fraction of carboxyl groups bonded to looped TEPA molecules, and fB is the mole fraction of surface-bound amino groups which are present on looped TEPA molecules. Here, A is the quantity of acidic (carboxyl) groups that reacted with TEPA, and B is the quantity of surface-bound amino groups that were directly detected by HCl uptake before the requisite correction [20]. The correction factor, K, for HCl titration of TEPA may also be defined as the ratio of the true quantity of surface basic groups to the measured quantity. Here, the K value can be expressed by either jr, or fa,or fB and both A, B values. For example. K may be calculated by

4-f; C-j B 1+.A

K= 4

Then the base capacity uptake may be corrected the true base capacity. amino groups added to grafting vs the carboxyl calculated by:

(6)

directly measured by HCl by multiplying by K to get The overall ratio (CA) of the surface through TEPA groups consumed may be

(7) where B, is the true value of base capacity. That is, B, is equal to the base capacity (B) measured directly and then multiplied by the correction factor K. Table 1 shows the difference in NaOH uptake by HNO,-oxidized carbon fibers from before to after reaction with TEPA. This same difference is shown for HCl uptake. The change of NaOH uptake from before to after reaction with TEPA (row C in Table 1) represents the quantity of acidic groups which reacted with TEPA. The change in HCl uptake from before to after reaction with TEPA (row F of Table 1) represents the quantity of amino groups which were grafted onto the fiber surface. Plotting the quantity of acidic groups that reacted with TEPA (i.e. the change of NaOH uptake, row C in Table 1) vs the quantity of amino groups that were grafted (i.e. the change of HCl uptake, row F in Table 1) yields a straight line with a slope of 0.455 (Fig. 9). This slope is the ratio of acidic groups consumed in the TEPA reaction to the basic groups grafted onto the fiber surfaces. This ratio is the term (A/B) in eqn (7). Using eqns (3))( 7), fr, fA and .fB were calculated to be 0.39, 0.56 and 0.33, respectively. The correction factor, K, was found to be 1.18 from eqn (6). The parameters calculated above provide more insight into TEPA-fiber surface reaction. For example, the average number of amino groups on each surface-bound TEPA molecule may be expressed as 4-,f,. In this series of experiments, this value was 3.61. Since one metanil yellow anion was adsorbed per approximately every 10 amino groups on the

Chemical modification of carbon fiber surfaces Acidic functionsconsumed@et&)

329

4. ARXPS O/C atomic ratio (%)

1 Electron take-off angle Acidic functions: NaOH uptake Basic functions: HCI uptake

‘ioi

: 40

so

40 Nitric acid oxidation

60

60 time (min.)

90

Basic functionsintroduced(p&J

Fig. 9. Acidic functions consumed vs the basic functions introduced onto HNO,-oxidized carbon fiber surfaces during the reaction with TEPA.

fiber surface, each surface-bound TEPA adsorbed an average of 0.36 MYions. Using K=l.lS and (A/B) =0.455 in eqn (7), C, was calculated: C,= 1.18/0.455 =2.59. Thus, 2.59 moles of amino groups were introduced for every mole of acidic groups consumed during the reaction with TEPA. Since only 52% of acidic groups (e.g. the COOH groups) could react with TEPA (Section 3.2, the total quantity of amino groups which could be introduced was 2.59 x 0.52 x (total acidic groups present before the reaction of nitric acid oxidized fibers with TEPA). In other words, for every acidic group detected by NaOH uptake on the oxidized fibers it was possible to add 2.59 x 0.52= 1.35 amino groups. This means that the amine functional group “leveraging factor” per surface acidic function was 1.35 using TEPA. Clearly, one requires either a larger polyamine or one with a different structure to TEPA in order to “leverage” substantially the number of amine functions which can be introduced in this sort of reaction.

4. COMPARISON

BETWEEN

CHEMICAL

ANALYSIS

AND XPS

The ARXPS experiments were performed on the same series of HNO,-oxidized carbon fibers discussed above. The electron take-off angles selected were 10, 50 and 90”. Smaller photoelectron take-off angles enhance the surface sensitivity of XPS. Using a 10” take-off angle probes the outer region of the fiber to a depth of approximately 15 A, while a 90” take-off angle incorporated contributions to a depth of about 100 A. The O/C atomic ratio (for an electron takeoff angle of IO’) increased from 0.24 for the as-received fiber specimen to 0.33 for fibers oxidized in HNO, for 90 minutes at 115°C (Fig. 10). This implies an increase in oxygen content by a factor of 33/24= 1.38. When the electron take-off angle was adjusted to 90”, the overall increase in the O/C atomic ratio was approximately 31/13 =2.38. Thus, the increase in oxidized carbon functions, at or very CARBON 31-j-8

Fig. 10. The HNO,-oxidized

oxygen to carbon atomic ratio on the carbon fiber surfaces vs oxidation time. O/C was detected by XPS.

near the surface, with time was less than that in the region farther below the surface (i.e. the subsurface). However, the definition of “subsurface” in this case could include the outer graphitic layer exposed within crevasses, pits and voids, whereas these are really “surface” groups with respect to titrations and reaction with TEPA. Figure 3 is an idealized illustration of the formation of a crevass during oxidation. HNO, oxidation can dig a crevasse by removing carbon from, primarily, the lateral planes. Pits or voids can be formed when a crevasse forms an intersection with a subsurface void. Functional groups formed within a crevasse, pit or void are really surface groups with respect to titrations if liquid can reach them, but they may be subsurface groups as detected by XPS depending on their depth, topographical features and their relationship to the electron escape depth. In contrast to the ARXPS data, NaOH uptake experiments indicated that the quantity of surface acidic functions on fibers increased by a factor of about 9.4 and 13.5 after fiber oxidation in HNO, for 60 and 90 minutes, respectively (Fig. 1 and Table 1, row 1). Deconvolution of the ARXPS Cls spectra gave the relative amounts of different carbon oxidation states, including hydroxyl, ether, ketone, ester, carboxyl and carbonates. These results, together with ISS experiments appear elsewhere [14]. When compared to NaOH uptake experiments, the high O/C atomic ratios from the XPS data suggest that a fraction of the oxygenated functions are not acidic. The presence of ether and ketone functions most probably account for this difference. Thus the surface might be pictured, in part, as represented in Scheme 2. Furthermore, the O/C atomic ratios from ISS (outer

HOOC

OH

HO

Scheme 2.

0

0

COOH

C. U. PITTMAN, JR ef ul.

330

atomic layer) do not increase substantially after a short initial oxidation. However, the O/C ratios from ARXPS increase more with oxidation time the greater the electron take-off angle becomes (90’ > 50” > loo). Thus, the more deeply XPS probes below the surface, the more the O/C ratios increase with oxidation time. This agrees with the concept that pits and crevasses form by penetrating into the original surface. This increases the surface area. It is this increase in surface area that is mostly responsible for the increase in acidic groups detected by titration. 5. CONCLUSIONS Acidic phenolic and carboxyl groups were introduced onto carbon fiber surfaces by nitric acid oxidation (20, 40, 60 and 90 minutes) at 115°C together with non-acidic ether, ketone and perhaps, lactone functions. The acidic-group surface concentration increased approximately in proportion with the oxidation time. The surface density of these groups, determined from a combination of NaOH uptake and methylene blue adsorption experiments, was about 2.4 acidic groups per 100 A”. About 52% of these surface acidic groups (carboxyl groups) reacted with tetraethylenepentamine (TEPA). About 48% of the acidic functions were phenolic hydroxyls. The surface density of the amino groups bound to fibers after reaction with TEPA was 5.6 groups per 100 A’, according to HCI uptake and metanil yellow adsorption experiments. About 39% of grafted TEPA molecules were present in loop structures, An average of 3.61 amino groups were introduced for each grafted TEPA molecule. An average of 2.59 amino groups were introduced for each acidic group consumed. The average ratio of the total amino groups introduced to the total amount of acidic groups present after nitric acid oxidation was about 1.35. Base/acid uptake can determine the total quantity of acidic/basic functions introduced. Careful consideration was needed to derive correction factors with bound TEPA [29,39]. Combining base/acid (NaOH/HCl) uptake and dye (methylene blue/metaniI yellow) adsorption was found to be a method to probe changes in fiber surface areas and to determine the average surface density (groups per 100A2) of these functional groups, independent of the surface area. The surface continuously increased during the fibers’ nitric acid treatment. Crevasses and pits form. However, the density of acidic groups (groups per 100 A’) does not change appreciably after a brief initial oxidation period. Instead, the generation of the new surface accounts for the large increase in acidic groups (peq of groups per g of fiber) with progressive oxidation. The schematic drawing (Fig. 3) provides a view of this process, but one must keep in mind that far more graphite layers than are shown in Fig. 3 make up the actual regions where lateral planes appear at the surface. Thus, dye molecules have no trouble accessing many pits and

Dimensions which are too small to be observed by our SEM studies (x 7500) are still very large relative to molecular sizes. The results of XPS cannot be simply compared with the chemical analysis. High O/C atomic ratios detected by XPS, when coupled with NaOH uptake experiments. demonstrated that a significant quantity of non-acidic oxygenated functional groups such as ethers and ketones are also present after nitric-acid oxidation of their carbon fibers. BET surface areas are smaller than those deduced by MB+ adsorption. It seems likely that BET measurements show that the surface area of dry fibers after nitric acid oxidation are smaller than the total area which can be accessed by aqueous base solutions. Oxidation induces a subsurface morphology which can be partially swollen by an aqueous base thereby exposing acidic groups to titration on internal surfaces not reached by nitrogen under dry conditions.

crevasses.

Ackno~~lrd~erner~ts~This research was supported in part by the National Science Foundation through Grants No. STI-8902064. EHR-9108767 and OSR-9452857. Suoport from the State of Mississippi and Mississippi ‘State University is also gratefully acknowledged. S.D.G. and C.U.P. wish to acknowledge the National Science Foundation (Grant No. CTS-9212295), Mississippi State University and the Department of Chemical Engineering for funding to purchase the XPS/ISS instrumentation.

REFERENCES I. Fitzer, E. and Weiss, R., in Processing und Lisr c$Curhon Fiher Rrinjtirced Plustics. VDI Verlag. 1981, p. 45. 2. Ishida, H. (ed.), Intwfuces in Polymer, Ceramic and Metal Matrix Composites. Elsevier, New York, NY, 1988. Interphases in Compositr 3. Ishida, H. (ed.), Controlled Materials. Elsevier, New York, NY. 1990. E. P.. Interfhcr in Pol~~mer Mutris Corrz4. Plueddemann, posites. Academic Press, New York, NY, 1974. 5. Donnet, J. B. and Bansal, R. C., Cahon Fibers, 2nd edn. Marcel Dekker. New York, NY. 1990. 6. Hughes, J. D. H., Composite Sci. Technol., 1991, 41, 13. 7. Yosomiya, R., Morimoto, K., Nakajima, A., Ikada, Y. and Suzuki, T., Adhesion and Binding in Composites. Marcel Dekker, New York, NY, 1990. 8. Fitzer, E., Jaeger. H. and Weiss, R., Estrndrd Ahstrucls Progrum-I6ih

Bienrrui

Co~fire~cr

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