Reactivities of amine functions grafted to carbon fiber surfaces by tetraethylenepentamine. Designing interfacial bonding

Carbon Vol. 35, No. 7, pp. 929-943, 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 $17.00 + 0.00

Pergamon PII: SOOO8-6223(97)00047-X

REACTIVITIES OF AMINE FUNCTIONS GRAFTED CARBON FIBER SURFACES BY TETRAETHYLENEPENTAMINE. DESIGNING INTERFACIAL BONDING C. U. PITTMAN,

JR,~** Z. WV,”

W. LI b and ‘Department

of Chemistry

TO

W. JIANG,~ G.-R. HE,~ B. WU,~ S. D. GARDNER~

and ‘Department of Chemical Engineering, Mississippi, MS 39762, U.S.A.

(Received 2 January 1997; accepted in revisedform

Mississippi

State University,

19 February 1997)

Abstract-PAN-base carbon fibers were oxidized with 70% nitric acid at 115°C to introduce surface carboxyl, hydroxyl and other oxygenated functions. Subsequent reactions of these surfaces at 190-200°C with tetraethylenepentamine (TEPA) generated amide bonds at carboxyl and ester sites thereby grafting TEPA and introducing primary and secondary amine groups onto the fiber surfaces. The reactivity of these surface-bound amine groups was evaluated in reactions with the small model reagents: phenyl isocyanates, 1,6-diisocyanatohexane, acetic anhydride and styrene oxide. Larger model reagents including isocyanate-terminated prepolymers and a series of epoxy resin prepolymers (MW 3781000) were also used. The reactivity of the surface carboxyl and hydroxyl functions (introduced by nitric acid oxidation) was also examined in reactions with phenyl isocyanate. About 30% of these acidic functions were able to react with phenyl isocyanate. After grafting TEPA to the fibers, acetic anhydride and phenyl isocyanate reacted with 60% and 52%, respectively, of the surface-grafted amine groups. The larger isocyanate prepolymers reacted with only 2.5 to 7.8% of the amino groups and three epoxy resins, with molecular weights of 378, 500 and 1000, reacted with 15, 7.8 and 5.3% of the surface amino groups, respectively. The grafting efficiencies to surface amino groups (G,) and the molecular weights of the reagent being grafted fit the relationship G,= KM: where K=3.07 x lo3 and a= -0.848 for isocyanates, whereas K=2.57 x lo4 and a = - 1.25 for the epoxy compounds. 0 1997 Elsevier Science Ltd

elastomers. Polyurethane elastomers were used to form interphase layers in our work [ 11,17,23]. Therefore, amino or carboxyl groups, which readily react with the isocyanate groups in developing polyurethanes, are among the functions of choice to be introduced onto carbon fiber surfaces. To accomplish this task, carbon fibers were oxidized by nitric acid to increase the presence of carboxyl groups [ 11,17231. Then the carboxyl groups were reacted with tetraethylenepentamine (TEPA) to introduce amino groups onto fiber surfaces (eqn ( 1)) [ 11,17,23,24]. The quantities of surface functions introduced and their surface densities and distributions have been studied. When TEPA molecules were grafted onto carbon fiber surfaces, which had previously been oxidized with nitric acid, both looped and single grafted forms were attached (see eqn ( 1)) [ 17,24,25]. The amount of grafted amino functions was estimated by HCl uptake [ 17,24,25].

1. INTRODUCTION Interfacial adhesion has been a topic of concern in composite technology for decades. Previous studies have generated improved adhesion between the fiber surface and matrix [l-4]. Numerous methods have been developed to improve the fiber surface wettability or to increase the quantity of surface functional groups [ 2,3]. However, the presence of strong interfacial bonding alone may not lead to satisfactory composite performance. The presence of a tightly bound but stiff interface can result in high stress concentrations at the interface leading to low impact strength [S-7]. The introduction of a third elastomeric interphase layer, between the fiber and matrix, might improve this situation [8-131. However, the extent of chemical bonding of the elastomeric interphase to the fiber surface and to the matrix might be crucial to enhance adhesion at these two interfaces. A research effort has been established in our laboratory to investigate elastomeric interphases [ll-161, to study the introduction of functional groups onto carbon fiber surfaces [ 17-221, and the efficiency of using these functions in chemical bonding [ 17,191. The selection of the surface functions depends on the functional groups available in the interphase

*Corresponding author. 929

930

C.U.PITTMAN

Since these grafted amino functions are on the fibers’ surfaces, it is not clear what fraction of those present is able to enter into chemical reactions, The density of interfacial chemical bonding (fibers to interphase or fibers to matrix) may play an important role in adhesion and in the mechanical properties of composites. In fact, Drzal and coworkers have recently reported an ingenious series of experiments which dissected the effects of surface roughness from chemical bonding on the interfacial shear stress (ZFSS) in PAN-based (IM6) carbon fiber/epoxy matrix composites [26]. Rather low degrees of chemical bonding at the fiber-matrix interface were shown to increase the IFS'Sby 40%. Thus, elucidation of surface functional group reactivities and the density of chemical bonding at the interface (bonds formed per unit surface area) are fundamental to understanding and improving adhesion. To evaluate the reactivities of the surface amino functions grafted to carbon fibers, phenyl isocyanate, 1,6-diisocyanatohexane, styrene oxide and acetic anhydride were used as small model reagents in reactions with the treated carbon fibers. lsocyanates react with grafted amino groups (eqn (2)) and carboxy1 groups (eqn (3)) to yield urea and amide functions, respectively. Epoxides react with surfacegrafted amines to form b-amino alcohols (eqn (4)) and with both carboxyl and hydroxyl groups to generate fi-hydroxy esters and b-hydroxy ethers (eqn (5)), respectively. Acetic anhydride gives amide functions with amines and esters with hydroxyl groups

(eqn (6)).

etaI 1 c’NnN’ ,? CF

H

TN/ H

2

TN/-l

H

WC,

W

H

+

OH

,o

-mB

CH3 C, 0 0

CF

I : I

l-

%

‘N

nnnn

O Ii

0”

N

I!

$H,

O’C’CH.

O&H,

The basicities of both urea and amide groups are too weak to accept a proton in HCl uptake measurements. Therefore, each equivalent of bound amino groups which reacts with either of these two model reagents reduces the amount of HCl uptake by one equivalent. For this reason the measured loss in amino functions corresponded to the quantity of amide or urea functions generated in reaction with acetic anhydride or phenyl isocyanate, respectively. The surface-grafted amines were also reacted with model isocyanate and epoxy prepolymers to evaluate the fraction which could form chemical bonds to these larger reagent molecules. An understanding of the fraction of surface amine groups which reacts with isocyanate and epoxy compounds versus the size of these reagent molecules is essential to predict the density of covalent bonds formed between aminederivatized carbon fibers and polyurethane interphases or epoxy matrices. Difunctional, isocyanateterminated polytetramethylene ether glycols (PTMEG) and diglycidyl ether ohgomers of 2,2bis(4-hydroxyphenyl)propane (commonly termed bisphenol A) were used as the model prepolymers. Three key variables could affect the efficiency 01 these surface reactions for any given functional group class of reagent: I ) the solvents used, 2) the surface concentrations of reactive functional groups and 3) the molecular weights (size) of the prepolymer reagent molecules. The reactivities of the surface functions were evaluated by determining the fraction of surfacebound amine groups which reacted with each of the model compounds. These fractions were calculated from the consumption of surface amines and from the weight gains of carbon fibers after reactions in various solvents.

2. EXPERIMENTS

O CF I,-“:N/p\N/\N/\NA H

H

H

NH2

H

0 CF

t

?H I-C&CH-Ph

“:pNnNnNA H

H

H &d2

CH-Ph AH OH

0 &

CH2 kH-Ph ‘0’

,---0CH2

?H CH-Ph

(5)

The carbon fibers used include Celion G30-500, 3K-HTA-7C-NSOI, (BASF Structural Materials Inc.) and Thornel T300 (AMOCO Performance Product, Inc.). They were produced from a polyacrylonitrile (PAN) precursor. Each tow contained three thousand filaments and the average filament diameter was 7 Llrn. These fibers were surface-pretreated by the manufacturers but not sized. Phenyl isocyanate (98 + %), acetic anhydride ( ACS 98%), methylene blue, metanil yellow, N,N-dimethylformamide (ACS 99.8%), chlorobenzene (99%), 1.6-diisocyanatohexane (98%), anhydrous ethanol

Amines grafted to carbon fibers and molecular sieves (4A, l/16-inch beads, 8-12 mesh) were obtained from the Aldrich Chemical Company, Inc. Xylene (a mixture of o-, m-, and pxylene, ACS certified), pyridine and sodium hydroxide were purchased from the Fisher Scientific Company. Three isocyanate prepolymers from the AIRTHANE series (Air Products and Chemicals, Inc.) were employed in this study. The backbone polyol in all three of these prepolymers was polytetramethylene ether glycol (PTMEG). Two of them (PET-95A and PET-75D) were terminated with toluene diisocyanate (TDI). The third prepolymer (XAPC-722) was terminated with isophorone diisocyanate. The molecular weights of PET-75D, PET-95A and XAPC-722 were 920, 1350 and 1440, respectively. A series of diglycidyl ethers of bisphenolA type epoxy oligomers (EPON series from the Shell Chemical Company) were used as epoxide model prepolymers. Their molecular weights were 360, 500 and 1000, respectively.

2.2 Solvent purification Xylene, chlorobenzene and NJ-dimethylformamide were used as solvents (in increasing order of polarity) for the reactions between carbon fibers and model compounds. They were dried and distilled prior to use. Chlorobenzene and xylene were dried lithium aluminum hydride by refluxing over (5 g L- ‘) for 1 hour and then were distilled. The first and last 10% of distillate were discarded. These solvents were stored over molecular sieves in capped bottles. The molecular sieves had been activated under vacuum at 400°C for 8 hours. N,N-dimethylformamide (DMF) was stirred with barium oxide powder (25 g - ‘) for 30 minutes. Then DMF was distilled under reduced pressure decanted, ( 110-l 15”C, the first and last 10% of the distillate was discarded) and the distillate was stored over activated molecular sieves in a brown bottle. Pyridine was distilled and stored over sodium hydroxide pellets. 2.3 Treatment fiber surfaces

and chemical analysis of carbon

The carbon fibers were oxidized with nitric acid (70%) at 115°C for specific periods from 20 to 90 minutes. The amount of acidic surface functions generated by these oxidations were in the range of 12 to 47peqg-’ as determined by NaOH uptake [24,25]. The surface areas of the as-received fibers and the fibers after each specific nitric acid oxidation treatment were determined by the nitrogen BET method. A portion of each group of these fibers was treated with tetraethylenepentamine (TEPA) at about 190°C for 4 hours. Detailed analysis by HCl uptake demonstrated that 19 to 65 peq g-’ of amino groups were introduced onto the fibers by these TEPA treatments [24,25]. The surface acidic and basic functions were deter-

931

mined by NaOH and HCl uptake, respectively [24,25]. A pH meter (Ion Analyzer 250, Corning) was employed to monitor the changes of pH values of NaOH or HCl solutions before and after the OH- or H+ uptake by fiber specimens. Aqueous NaOH and HCl (1 x 10d3 to 2 x 10e3M) solutions were used. The quantities of the surface functions were then calculated from the changes in the pH values. In addition to the acid/base uptake, dye adsorption measurements were also employed. Methylene blue (MB+) cations and metanil yellow (MY -) anions were adsorbed from aqueous solution by deprotonated acidic groups (negative sites) and by protonated amino groups (positive sites), respectively. The dye adsorption measurements always gave lower values than those obtained from NaOH/HCI uptake due to the large size of the dye ions.

2.4 Synthesis of a higher molecular weight isocyanate prepolymer Airthane PET-75D (NC0 functionality 2.0, MW 920) (50 g, 54 mmol) was reacted with 1,3-butanediol (4.32 g, 48 mmol) under nitrogen for 2 hours at 75°C with continuous stirring. The reaction mixture was then diluted with DMF (100 mL) and heated to 80-85°C for 2 hours while stirring. The final product was a clear viscous liquid. The molecular weight (4800) was determined by GPC in THF versus narrow molecular weight distribution (from 687 to 24150) polystyrene standards for calibration. A universal calibration was employed using an on-line microviscometer detector in addition to refractive index detection. This prepolymer has been abbreviated as Syn-ISOP in this paper. 2.5 Reactions

isocyanate fibers

of acetic anhydride andphenyl with amino groups grafted to carbon

A large molar excess of either acetic anhydride or phenyl isocyanate was reacted with completely submerged fiber specimens (Celion G30-500, 1 g) at 75°C and 9O”C, respectively. No solvent was used. Threehour reaction periods were used in all experiments. Then the excess reagent was decanted and the residual reagent was then removed by Soxhlet extraction with acetone for 3 hours. 2.6 Reactions of isocyanate-terminated prepolymers with amino groups grafted to carbon jibers Two sets of experiments were designed for the reactions of isocyanate-terminated prepolymers with carbon fibers: (A) Isocyanate-terminated prepolymer PET-95A was reacted with carbon fibers containing various levels of grafted amino groups. Five batches of carbon fibers (Celion G30-500) were used which had been oxidized by nitric acid (70%) for 0, 20, 40, 60 and 90 minutes at 115”C, respectively. Each batch had then been treated with TEPA at about 190°C (4

C. U. PITTMANet ul.

932

specimens were washed with a large excess of acetone twice and then refluxed with acetone for 4 hours.

hours). The amount of surface-grafted amine on these specimens varied from 5 to 65 peq g- ’ ( HCl uptake) or 2.7 to 14 peq g-i (MY - adsorption). The isocyanate-terminated prepolymer, PET-95A (4 g, M W 1350), was dissolved in xylene in five vials (each containing 25 mL of xylene). Then the fiber specimens (1 g each) were placed into these vials and reacted at 80°C for 3 hours, respectively. Then the fibers were washed twice with ethanol (Z 100 mL) followed by refluxing in ~200 mL of anhydrous ethanol (lo-12 hours). The fiber samples were then washed twice with acetone (Z 100 mL) and refluxed with acetone ( ~200 mL) for 4 hours. The ethanol treatment converted all remaining isocyanate groups into urethane functions. Urethane groups will not affect quantitation of the remaining amino groups via HCl uptake. (B) Isocyanate prepolymers with different molecular weights were reacted with carbon fibers (Thornel T300) containing grafted amino groups. These TEPA-treated fibers contained 46 to 53 Lteq gg ’ of amino groups. Three isocyanate-terminated prepolymers, PET-75D (MW 920), PET-95A (M W 1350) and Syn-ISOP (MW 4800) were reacted with these fibers in xylene solution at 80°C for 3 hours. The washing procedures of the fiber specimens after reactions were the same as those described in section (A).

2.1 Reactions of epoxyprepolymers groups grafted to carbon fibers

2.8 Reactions of model isocyanate with HNO,-oxidized carbon$bers

Carbon fibers (Thornel T300) were oxidized in refluxing 70% HNO, for 60 minutes. The amount of surface acidic functions (NaOH uptake) ranged from 62 to 70 peq g- ‘. Oxidized fiber specimens were reacted under nitrogen with 25 g of each isocyanate compound in 25 g of solvent for 10 hours. The reaction temperatures employed were 153°C for DMF, 132’C for chlorobenzene and 140°C for

1. Grafting

Acidic functions Solvents Xylene Chlorobenzene DMF

The (DIH)

see

isocyanate

used

included

diisocyanato-

(MW

168), PET-75D 1444) and Syn-ISOP

(MW

Section

2.4.

After

reacting

for

in anhydrous hour to block the isocyanate groups of the grafted diisocyanate molecules. mens were refluxed with acetone for weight gains and the loss of amino then determined. specimens

were

refluxed

2.9 BET surf&e area measurement fibers

10

(MW

920),

(MW

4800,

hours,

the

ethanol for I at the free ends Then the speci24 hours. Fiber functions were

of carbon

The surface area of carbon fibers was measured by BET nitrogen adsorption at the boiling point of liquid nitrogen using Quantasorb instrumentation (Quantacrome Corporation).

with amino

of diisocyanatohexane

xylene. hexane

XAPC-722

Carbon fiber specimens (Celion G30-500) were used from a single batch which had been oxidized with 70% HNO, at 115°C for 50 minutes followed by TEPA treatment at 190°C overnight. These fibers contained 9.78 peq g- ’ (by metanil yellow adsorption) or 52 peq g-’ (by HCl uptake) of grafted amino groups. Five different epoxy prepolymers (2 g each) were dissolved in xylene ( 10 mL) in five vials, respectively. Phenol (40 mg) was added to each to serve as a catalyst. Fiber specimens (1 g) were immersed in these solutions at 80°C for 19 hours. Then the

Table

compounds

3. RESULTS

The reactivities of surface functions were evaluated by obtaining the fraction of these groups which reacted with small model compounds and prepolymers. The reaction fractions were calculated based on the measured loss of surface functions after the reaction. Acid/base uptake and dye adsorption was employed. The reaction fraction was expressed as the

onto HNO,-oxidized

consumed

carbon

fiber surfaces”

Fiber weight gain

(ncq g-‘)

G,(%)

(%)

40.6 39.0 53.0

66.0 63.4 86.2

0.49 0.53

(/ceq g-Y

22.7 24.8

G,(%)’

36.9 40.3b

a The initial acidic functionality was 61.5 peq g -I. The reaction temperatures were 14O’C, 132’C and 153°C for xylene, chlorobenzene and DMF, respectively. The reaction time was 10 hours for all reactions. b This is determined from the weight gain which was converted to peg based on the molecular weight of the grafted units. ’ G, is the grafting effciency based on weight gain measurements. Molecular weight of diisocyanatohexane= 168. Molecular weight of ethanol =46. After the reaction of grafted diisocyanatohexane with ethanol, the resulting formula weight is 168 +46=214 assuming that no looping has occurred. If ~80% of the grafted molecules were looped then the value of G, would be 48.5%. The average mol. wt. of the grafted unit if 80% of the grafted molecules were looped ~(214 0.2+ 168x0.8)/2= 177. Thus G,= (214/177)(40.3)=48.5.

Amines grafted Table 2. Grafting

of isocyanate

Xylene Chlorobenzene DMF

933

fibers

prepolymer ( XAPC-722a) fiber surface8

Acidic functions Solvents

to carbon

onto HNO,-oxidized

consumed

carbon

Fiber weight gain

(peq g-‘)

G,(B)

(%)

35.6 23.5 43.0

57.9 38.2 69.9

0.35 0.58 0.68

(peq g-r) 2.3 3.9 4.6

G,(s) 3.7 6.3 7.5

aAverage formula weight = 1490. (Molecular weight of XAPC-722 = 1444. Molecular weight of ethanol =46. After reaction the grafted formula weight 1444 +46 = 1490). bThe initial acidic functionality was 61.5 peq g-r. The reaction temperatures were 140°C 132°C and 153°C for xylene, chlorobenzene and DMF, respectively. The reaction time was IO hours for all reactions. ‘G. is the grafting efficiency based on weight gain measurements. The peq g- ’ values were based on the weight gain.

of the loss of a specific functional group to the total quantity of those functions present before the reaction. The weight gains of fiber specimens after reactions were also used to calculate the number of molecules grafted onto fiber surfaces (NJ. The measured weight gain (IV,) and the molecular weight of the molecules (MW) were used: NB = W,/MW. If one surface function was consumed for each molecule grafted, then the grafting efficiency (G,) was calculated from G, = N,/N,, where No is the total number of functional groups which were present on the surface to react with the molecules grafted. Throughout most of this discussion the number of surface acidic functions (carboxyl and hydroxyl groups) is given in peq of functions per gram of carbon fiber. However, from nitrogen BET measurements of the fiber surface areas, the functional groups per 100 A” is also available and is used later in the manuscript. ratio

for reactions in chlorobenzene and DMF gave grafting efficiencies of 37% and 40%, respectively. These values are substantially lower than those obtained by NaOH uptake experiments. This would be expected if substantial loop formation occurred. Once one end of 1,6-diisocyanatohexane became bonded to the surface, rapid cyclization would be expected provided an acidic group was located nearby on the surface (see eqn (7)). Each time loop formation occurred one grafted isocyanate consumes two surface acidic functions. This would cause the loss of surface acidic functions measured by NaOH uptake to be larger than that determined by weight gain. Indeed, most of the grafted diisocyanatohexane molecules must have formed loops on the surface in the reactions carried out in DMF. This is evident by noting the grafting efficiency was 86% based on NaOH uptake and 40% from weight gain measurements (Table 1).

3.1 Effect of solvents on the reaction of acidic groups at carbon$ber surfaces with isocyanates Chlorobenzene, xylene and DMF were used as solvents in reactions of HNO,-oxidized carbon fibers with isocyanates. The isocyanate functions can react with both carboxyl and phenol hydroxyls produced during nitric acid treatment. The HNO,-oxidized fibers contained 61.5 peq g-i of these two acidic groups. A small model compound, 1,6diisocyanatohexane (DIH) and the diisocyanate-terminated prepolymer, XAPC-722 (M W= 1444) were reacted with the fibers. The reactivities of the acidic functions were evaluated by both the loss of the acidic carboxyl and phenolic hydroxyl groups and the weight gains (Tables 1 and 2). The 1,6-diisocyanatohexane grafting efficiencies were between 66 and 86% (based on NaOH determinations of the acidic functions consumed) showing that most of the acidic functions could react with this small but highly reactive aliphatic isocyanate. The highest reaction efficiency was obtained in the most polar solvent, DMF (the dipole moments of xylene, chlorobenzene and DMF are 0.23, 1.75 and 3.86, respectively). Fiber weight gain measurements

(7)

934

C. U. PKTMAN er LII

The wettability of the carbon fiber surfaces decreased after the 1,6-diisocyanatohexane grafting reaction. The fibers became less hydrophilic. Small quantities of both groups of fibers (grafted and ungrafted) were cut into 223 mm lengths and placed into vials containing 4 mL of distilled water and 4 mL of xylene. The vials were then shaken and set aside. Several hours later the ungrafted fibers remained dispersed in the aqueous layer while the 1.6-diisocyanatohexane-grafted fibers remained dispersed in the xylene layer. Clearly, the fiber surfaces became more hydrophobic after the grafting reaction. This raises the possibility that some existing surface acidic functions, which remain on the surface, may not be measured by NaOH uptake due to poor surface wetting by water after DIH grafting. As the size of the grafted molecules became larger it is possible that a large graft may cover other unused surface acidic groups shielding them from reacting with NaOH in uptake experiments. If the NaOH uptake was depressed by such shielding then the apparent consumption of acidic functions during grafting would appear to be greater than the true consumption. The grafting efficiency predicted by the loss of acidic functions would then be higher than that observed by the weight gain measurements. This situation existed for the grafting of XAPC-722 (see Table 2). The weight gain data clearly demonstrated grafting efficiencies of only 448% for this large molecule. However, NaOH uptake measurements indicated the grafting eficiencies were 38 to 70%. If the real grafting efficiencies had been in the 3&70% range, the weight gains would have been much higher in the 3.557% range than the 0.3550.68% actually measured. Thus, it is clear that each grafted molecule of XAPC-722 effectively blocked access of both NaOH or another XAPC-722 molecule to unreacted surface acidic groups. The comparison of XAPC-722 with 1,6-diisocyanatohexane grafting data illustrates that grafting efficiency drops with molecular size of the molecule being grafted.

total COOH and -OH groups generated on carbon fiber surfaces could react with phenyl isocyanate to give grafted urethane or amide functions. This grafting efficiency is less than half that obtained with the more reactive aliphatic isocyanate, I .6-isocyanatohexane (which can also form loops). Methylene blue (MB) adsorption measurements were also performed on HNO,-oxidized fibers both before and after reactions with phenyl isocyanate. Table 3 also lists these amounts of MB+ adsorbed. MB+ is adsorbed at both anionic sites ((O- , COO-) to form salts and at locations elsewhere on the fiber where n-rr-dispersion forces provide the attraction. Also listed in Table 3 are the amounts of metanil yellow (MY) adsorbed on the HNO,oxidized fibers. MY adsorption is essentially only occurring via x-rr-dispersion forces. Therefore. MY adsorption represents an approximate blank of the adsorption of methylene blue on the neutral area of the fiber surfaces (since there are almost no protonated basic groups on fiber surfaces after HNO, oxidation for MY to adsorb to) [24,25]. MB’ adsorbs mostly at acidic sites. Subtracting MY adsorption from MB+ adsorption before reaction with phenyl isocyanate (e.g. Table 3. (C)-( E)). gives the quantity of available acidic groups detectable by (or accessible to) MB+ ions. The change in MB+ adsorption by HNO,-oxidized fibers from before to after reaction with phenyl isocyanate is the quantity given by (C)-(D). Plotting the value 01 (C)-(D) versus the value of (C)-(E) gives a straight line with a slope of 0.41 (Fig. I(b)). This slope indicates that 41% of the acidic groups which could be detected by methylene blue adsorption were utilized to react with phenyl isocyanate. It is rccognized that dye adsorption to determine acidic or basic sites is often controversial. For example MY adsorption could be hindered by negative charge build up at the surface so it is a pH-dependent method. It was used herein on protonated surfaces. Therefore, such repulsion should not be a problem.

3.2 Reactions of phenyl isocyanate groups on carbonjibers surf&m

3.3 Reactions ofpherlyl isocyanute and acetic unhydride with amino groups grafted to curbon fibers

with acidic

Surface carboxyl and phenolic hydroxyl functions, generated by the four different HNO, oxidations, were reacted with phenyl isocyanate (PhNCO) to determine their ability to react. NaOH uptake measurements were performed before and after these reactions to follow the consumption of surface functions (Table 3). Rows A and B in Table 3 represent the acidic functions (determined by NaOH uptake) present on HNO,-oxidized carbon fiber before and after reaction with phenyl isocyanate, respectively. The difference between these two rows, (A)-(B), gives the quantity of acidic groups which reacted with phenyl isocyanate. Plotting this difference value against the total quantity of acid functions originally present (row A) gives a straight line with a slope of 0.30 (Fig. l(a)). Therefore, only about 30% of the

The reactivities of amino groups which had been grafted to fiber surfaces using tetraethylenepentamine (TEPA) were evaluated in reactions with acetic anhydride and phenyl isocyanate. The amino groups present before and after reactions with acetic anhydride (Ac,O) and with phenyl isocyanate (PhNCO) were measured by both HCI uptake and MY adsorption (Table 4). These are given in rows (A) through (C) of Table4. The quantity obtained by subtracting (B) from (A) in Table 4 represents the amount of amino groups which reacted with phenyl isocyanate, determined by HCI uptake. Plotting this difference against the total amount of amino groups present before the reaction (row (A)) gives a straight line with a slope of 0.52 (Fig. 2(a)). Similarly. the

Amines grafted Table 3. Acidic

functions

to carbon

fibers

on the HNO,-oxidized carbon fiber surfaces before and after reaction determined by NaOH uptake and methylene blue adsorption Oxidation

Fiber specimen

935

treatment

(A) Oxidized by HNO, (B) HNO,/PhNCO Rows (A)-(B) Grafting efficiency (%) of PhNCO to acidic surface groups

0

4.5 1.4 3.1 69

time with HNO,

20

40

Acidic functions 12.5 8.7 3.8 30

(C) Oxidised by HNO, (D) HNO,/PhNCOb Rows (C)-(D)

1.34 1.12 0.22

Acidic functions 3.1 2.2 0.9

(E) Oxidised by HNO, Rows (C)-(E)

0.64 0.70

1.16 1.94

determined by NaOH 22.7 15.0 7.7 34

with phenyl

isocyanate”,

(min) 60 uptake (peq g- ‘) 32.6 23.4 9.5 29

by MB+ adsorptionb (peq g-r) 5.6 9.3 4.2 6.8 1.4 2.5 MY- adsorptionb (peq gg’) 2.15 3.00 3.45 6.30

90

47.1 31.7 15.4 33

determined

15.3 11.3 4.0 5.83 9.47

a Carbon fibers were oxidized with 70% HNO, at 115°C for 20 to 90 minutes, then reacted with phenyl isocyanate (PhNCO) at 80°C in xylene for 10 hours. b MB: Methylene blue; MY: Metanil yellow; PhNCO: Phenyl isocyanate. c Grafting efficiency= the percentage of acidic functions on the fiber surfaces (phenolic OH, -COOH) which reacted with

phenyl isocyanate.

(A)-(C) term represents the surface amino groups which reacted with acetic anhydride. Plotting this difference against (A) also gave a straight line with a slope of 0.60 (Fig. 2(b)). Acetic anhydride consumed 60% of the original amino groups. The slopes of these two lines represent the average fraction of surface amino groups which can react with these small model molecules. A little more than one half of the grafted amino groups on these fiber surfaces were consumed. The fraction of the amino groups (detected by metanil yellow (MY -) adsorption) which could react with acetic anhydride or phenyl isocyanate should be greater than 0.60 or 0.52, respectively, since MY has a large size and detects only a fraction of the total amino groups present on the surface. One way to approximate the fraction of MY --detected amino groups which could react with acetic anhydride or phenyl isocyanate is outlined in the experimental results which are summarized in the bottom portion of Table 4. In row D the amount of MY - adsorbed by z-n-dispersion is given because basic sites which can be protonated are not present in detectable amounts on the fiber surfaces. After TEPA treatment, amine groups are introduced which can be protonated and then detected by MY _ adsorption. The amount of MY - adsorbed by rc- -rr-dispersion forces must be subtracted. An assumption was made that TEPA grafting does not change the amount of MY adsorbed by dispersion forces. Thus, the difference term (E)-(D) represents the quantity of amino groups detectable by (i.e. accessible to) MY - ions. Next, the difference term (E)-(F) represents the quantity of amino groups (detectable by MY -) which reacted with phenyl isocyanate. Similarly, the difference term (E)-(G) represents the quantity of amino groups which reacted with acetic anhydride.

Plotting these differences against the quantities of amino groups which were originally present (and accessible) to MY - ions (i.e. (E)-(D)) gives straight lines over the range of oxidation treatments. The slopes of these two lines are 1.08 and 1.28 for the reactions with acetic anhydride and phenyl isocyanate, respectively (Fig. 2(a) and (b)). These results imply that essentially all amino groups detected by MY adsorption were able to react with the model compounds. In principle, however, the maximum reaction fraction should be 1. It is likely that experimental error and non-homogeneity (scattering) in the surface properties resulted in a slope > 1. Surfacebound TEPA molecules have an average of 3.6 amino groups upon correcting for loop formation [24,25]. These amino groups can adsorb an average of only about 0.36 MY ions [24,25]. About one half of these grafted amino groups were able to react with the acetic anhydride and phenyl isocyanate compounds. This means an average of about 1.8 of amino groups per grafted TEPA reacted with the model compounds. 3.4 Reactions of isocyanateprepolymers with carboxyl andphenolic groups present on HNO,-oxidized carbon fiber surfaces and with amino groups on TEPA-treated carbon fiber surfaces The reactivities of surface carboxyl and phenolic groups on carbon fibers were evaluated in reactions with isocyanate prepolymers PET-75D (MW 920), PET-95A (MW 1350) and XAPC-722 (MW 1444). These results were compared to reactions with diisocyanatohexane (MW 168). The results are shown in the top section of Table 5. The reactions were all carried out in refluxing DMF (153°C) for 4 hours with the exception of the reaction with PET-95A

C.

I

10

I

I

I

20

30

40

Total acidic functions

U.

PITTMAN et al.

50

(peqlg)

(a)

0

-I

2

4

a

6

Total acidic functions

10

(peqlg)

(b)

Fig. 1.(a) Acidic functions on HNO,-oxidized carbon fibers consumed in the reaction with phenyl isocyanate versus the total acidic functions initially present on the fiber surfaces. Acidic functions were determined by NaOH uptake. Carbon fibers were treated with 70% HNO, at 115°C for 20 to 60 minutes. Reactions with isocyanate were conducted at 80°C for 10 hours without solvent. (b) Acidic functions on HNO,-oxidized carbon fibers consumed in the reaction with phenyl isocyanate versus the total acidic functions initially present on the fiber surfaces. Acidic functions were determined by methylene blue adsorption. Carbon fibers were treated with 70% HNO, at 115°C for 20 to 60 minutes. Reactions with isocyanate were conducted at 80°C for 10 hours

without

solvent.

which was conducted in xylene at 80°C for 3 hours. The grafting efficiencies were determined by weight gain measurements which gave the moles of reagent grafted. These values were compared to the total amount of acidic functions present on the surfaces (column 5). A rapid decrease in grafting efficiency occurred as molecular weight increased. The grafting efficiency for 1,6-diisocyanatohexane was 48.5% (weight gain) but due to extensive loop formation around 80% of the acidic groups reacted with this aliphatic isocyanate based on NaOH uptake. However, for PET-75D, PET-95A, XPAC-722 and Syn-ISOP the grafting efficiencies were 9.6, 7.9, 7.5 and 2.2%, respectively. The larger the molecule, the

smaller its grafting efficiency became. These larger molecules most likely give much lower loop fractions. Clearly, grafting efficiency rapidly dropped as the molecular size of the molecule to be grafted went up. The reaction of PET-95A with amino surface groups on TEPA-treated fibers was also examined. These results are summarized in Table 6. These fibers originally contained 46 peq g 1of amino groups. The loss of amino groups upon continued reaction with PET-95A was measured by both HCl uptake (see Table 6 and Fig. 3) and MY _ adsorption (Fig. 3). The measured weight gains (Table 6 and Fig. 3) as a function of reaction time were far smaller than those predicted by the loss of amino groups from HCl uptake experiments. Furthermore, the measured weight gains were quite similar to the weight gains calculated from the loss of amino groups measured by MY - adsorption. Clearly, only a portion of amino groups that were lost (not detected by HCI uptake) actually reacted with PET-95A. The rest were covered by the polymer and were no longer accessible to measurement. Based on PET-95A weight gain measurements the efficiency of PET-95A reactions with surface amino groups is low (4.8%) following the trend exhibited with acidic group surface grafting efficiencies when reacted with other higher molecular weight molecules. The weight gains achieved when reacting PET-95A with TEPA-treated fibers stayed constant after one hour (Fig. 3). However, amino groups appear to continue to be lost over 10 hours. MY adsorption measurements give a closer prediction of the surface amino groups which will be accessible to and react with PET-95A. The weight gains resulting from reaction of PET-95A with TEPA-treated carbon fibers with varying amounts of grafted amino groups present are shown in Fig. 4. Carbon fibers containing from 5 to 65 peq g- ’ of amino groups all gained approximately the same amount of weight when reacted in xylene at 80°C for 3 hours with PET-95A. The PET-95A was, indeed, chemically bonded since it could not be removed after extensive washing and extraction with acetone. This result suggests that once about 2.5 to 3.0 peq g-’ of PET-95A has been grafted to the fiber surfaces, the resulting coating effectively masks the remaining surface amino groups. Thus, further diffusion to active amine sites and reaction by other PET-95A molecules is kinetically insignificant. The wettability of carbon fibers by water decreased after grafting prepolymer PET-95A. This increase in hydrophobic surface character was similar to that already discussed when 1,6-diisocyanatohexane was grafted to HNO,-oxidized fiber surfaces. The poorer wettability of the fiber surfaces could have further contributed to the apparent consumption of surface amino groups measured by HCl uptake. Not only does PET-95A cover (shield) other amino groups but it may prevent some surface topologies from being wetted by HCl/H,O.

Amines grafted to carbon fibers

937

Table 4. Amino groups on TEPA”-treated carbon fibersb before and after reaction with phenyl isocyanate” and acetic anhydrided, determined by HCl uptake and metanil yellow adsorption Length Fiber specimen

treatment

of time fibers were oxidised

0

20

with HNO,

(min)

40

60

Amino 4.7 2.3 2.4 2.4 2.3 51 49

(A) HNOJTEPA treated (B) HNO,/TEPA/PhNCO”,’ (C) HNOJTEPA/AC~O~,~ Rows (A)-(B) Rows (A)-(C) Grafting Efficiency of PhNCO (%) Grafting Efficiency of Ac,O (%)

Amino 0.64 2.72 1.05 0.96 2.08 1.65 1.76

(D) HNO, oxidised (E) HNOJTEPA (F) HNO,/TEPA/PhNCOC (G) HNOs/TEPA/Ac,O’ Rows (E)-(D) Rows (E)-(F) Rows (E)-(G)

groups determined by HC1 uptake (peq g- ‘) 18.7 31.3 6.7 17.8 5.8 15.5 12.0 13.5 12.9 15.8 64 43 69 59 groups determined by MY- adsorption” (peq gg’) 1.16 2.15 4.40 6.46 1.34 1.63 1.34 2.01 3.24 4.31 3.06 4.83 3.06 4.45

a TEPA: tetraethylenepentamine; PhNCO: phenyl isocyanate; Ac,O: acetic anhydride; MY: metanil yellow. b Carbon fibers were oxidized with 70% HNO, at 115°C for 20 to 60 minutes, followed by TEPA treatment 3 hours. ’ Reacted with phenyl isocyanate at 90°C for 3 hours without solvent. d Reacted with acetic anhydride at 75°C for 3 hours without solvent.

3.5 Reactions of styrene oxide and epoxy prepolymers with surface amino groups on TEPA-treated carbon$bers The reactions of surface amino groups (on TEPAtreated carbon fibers) with styrene oxide and three epoxy prepolymers of increasing molecular weight (from 378 to 1000) were studied (Table 6). The TEPA-treated fibers initially contained 46 peq g-i of surface amino functions. All the reactions were carried out at 80°C in xylene for 19 hours. Weight gain measurements (Table 6) were used to establish the moles of reactant which had been grafted to the surface and the grafting efficiency. The consumption of amino groups by HCl uptake or MYadsorption cannot be used in these reactions because amino groups are not consumed when they react with epoxy groups. Instead, ring opening occurs to form a more highly substituted amino group with a hydroxyl function on the adjacent carbon (see eqn (8)). 2”amine

1’ amine +

(‘F

2/4R

9

CF N-CM-IX-K

NH E

2”amlne

ot4

NH-CH&HrYVR

NH2

F

PH

(8)

famine

Styrene oxide (MW 120) exhibits a high reaction efficiency (~69%) with surface amino groups. A sharp drop in grafting efficiency to 15% occurs using EPON 828 (MW 378). Even if 100% loop formation took place (highly unlikely) the grafting efficiency would only be ~30%. As the molecular weight of the epoxy reagent increased to 500 (EPON 834) and 1000 (EPON 1001) the grafting efficiencies dropped

45.5 21.0 16.7 24.5 28.8 54 63 3.00 9.08 2.40 3.03 6.08 6.68 6.05

at 190°C for

sharply to 7.8 and 5.3%, respectively. These observations dramatically illustrated the general principle: grafting efficiencies of reagent molecules drop rapidly as the size of the reagent increases. Tsubokawa [ 271 suggested the following equation expressed the relation between the number (G,,) of polymer chains grafted onto carbon black surfaces and the molecular weight (M,,) of those polymers: G,, = KM; where K and a are constants. This equation is employed here to describe the relation between the grafting efficiency (G,) and the molecular weight of the compounds being grafted onto carbon fibers: G, = KM; A plot of log(G,) against log(M,) should be a straight line if this equation is valid. Experimental results of the grafting efficiencies for isocyanate compounds and epoxy oligomers calculated from weight gains are summarized in Table 5. A plot of log(G,) against log(M,,) from the data of Table 5 (isocyanates) and Table 6 (epoxides) is illustrated in Fig. 5. Indeed, approximately linear plots were observed. The constants K and a from these plots are shown below. Polymers

grafted

Isocyanateterminated prepolymers Epoxy oligomers

K

a -0.848

3.07 x 103 2.57 x lo4

- 1.256

938

C. U.

PITTMAN

30

$ E

25 -

20 -

: P

15-

: 5

IO-

Fl u.o

5

ii m 0 0

I

I

10

20 Total

I 30

basic

functions

40

50

(veala)

(a) 7 8

1 2

I

c

I

I

3

4

5

6

Total

basic

functions

(peqlg)

(b) Fig. 2. (a) Basic functions on TEPA-treated carbon fibers consumed in the reaction with phenyl isocyanate and acetic anhydride vers3s the total basic functions initially present on the fiber surfaces. Basic functions were determined by HCI uptake. Carbon fibers were treated with 70% HNO, at 115’C for 20 to 60 minutes followed by TEPA treatment at 190°C for 4 hours. -OReacted with phenyl isocyanate at 90°C for 3 hours without solvent. ---C- - Reacted with acetic anhydride at 75°C for 3 hours without solvent. (b) Basic functions on TEPA-treated carbon fibers consumed in the reaction with phenyl isocyanate and acetic anhydride versus the total basic functions initially present on the fiber surfaces. Basic functions were determined by methylene blue adsorption. Carbon fibers were treated with 70% HNO, at 115°C for 20 to 60 minutes followed by TEPA treatment at 190°C for 4 hours. -3Reacted with phenyl isocyanate at 90°C for 3 hours without solvent. - - -O- - - Reacted with acetic anhydride at 75-C for 3 hours without solvent.

Table 5. Grafting

Isocyanate compounds DIH” PET-75D” PET-95A” XAPC-722” SYN-ISOPb

al

3.6 Changes in surface area andfunctional group surface concentration with progressive nitric acid oxidation

zi 2 2

et

efficiencies

of isocyanate reactivities

Molecular weights, (gmolV’) 168 920 1350 1444 4800

a,

Surface areas of the carbon fibers were determined as a function of the extent of nitric acid oxidation using three different methods. The surface area occupied by acidic groups (active surface area) and the surface area determined by MB+ adsorption (assuming monolayer coverage) are given in Table 7. The surface areas of dry fibers determined by using the nitrogen BET method are summarized in Table 8. There is a continual increase in surface area with increasing oxidation time at 115°C. The surface area increase was approximately proportional to oxidation time over the first hour. After that a rapid increase in surface area occurred. After 105 minutes the surface area had increased 28 fold from 0.67 m2 g-’ (as-received) to 18.8 m2 g--l. These surfaces were analyzed by NaOH uptake. There is an increase in total surface functionality. This is shown in Table 8 column three where the amount of surface acidic groups are expressed as peq of groups per gram of fiber. Next, the “surface concentration” of acidic groups, expressed as the average absolute number of functions per 100 A’ of surface area, was obtained using the BET surface areas. The number of acidic functions increased from 4 per 100 A’ on the as-received fibers to 8.3-10.3 per 100 A’ of surface after 20 to 60 minutes of oxidation. Thus, the surface concentration, to a first approximation, appears to stay constant after a short initial period of oxidation. Any increase in total surface functionality is due to an increase in surface area. After 60 minutes a very sharp increase in the surface area occurs with continued oxidation. While the total amount of surface functions (per gram of fiber) also increases, the rate of surface area increase is even faster. Therefore, the surface concentration of acidic groups appears to drop as oxidations in HNO, at I 15°C proceed longer than one hour. The surface concentration of acidic groups present after 20, 40 and 60 minutes of nitric acid oxidation (8.3 to 10.3 functions per 100 A2) is similar to but less than that obtained after 50 W or 200 W oxygen oxidations where lo-12 functions per plasma 100A2 were introduced [28]. However, the total

prepolymers of different molecular of surface carboxylic and phenolic

weights with acidic surface acid functions

functional

groups;

Weight gains (mg g-l)

Moles of reagent grafted( pm01 gg’)

Acidic groups present on fibers(Ltmol g -I)

Grafting efficiency G (%)

5.3 6.5 3.7 6.8 5.2

29.8 7.1 2.7 4.7 1.1

61.5 70.0 33.9 61.5 51.0

48.5 10.1 7.9 7.6 2.2

a DIH: diisocyanatohexane. Dimethylformamide (DMF) b DMF was used as the solvent at 80°C for 10 hours.

was employed

as the solvent at 153°C for 10 hours.

Amines grafted to carbon fibers

939

Table 6. Grafting efficiencies of styrene oxide, epoxy prepolymers and isocyanate prepolymers onto surface amino functions bound to TEPA-treated carbon fibers originally oxidized by HNO; Molecular weight (g mol-r)

Weight gain (mg g-r)

Styrene oxide EPON 828 EPON 834 EPON 1001

120 378 500 1000

3.8 2.6 1.8 2.5

PET-75D PET-95A SYN-ISOP

920 1350 4800

4.2 3.0 6.4

Epoxide or isocyanate reagent

Amino groups originally present (pm01 g-t)

Moles grafted (pm01 g-r)

Epoxide prepolymer? 31.6 6.9 3.6 2.5 Isocyanate prepolymers’ 4.6 2.2 1.3

Grafting efficiency G (%)

46 46 46 46

68.7 15.0 7.8 5.3

53 46 53

8.7 4.8 2.5

“Carbon fibers were oxidized by 70% HN03 at 115°C for a60 minutes followed by tetraethylenepentamine treatment at 190°C for 4 hours. ‘Reaction performed in xylene at 80°C for 19 hours. cReaction performed in xylene at 80°C for 3 hours.

(TEPA)

25 -

0 Reaction

0 2

4

6

0

10

20

30

40

50

60

70

10

Total amino groups (ueqlg) time of PET-95A with surface amines (h)

Fig. 3. Weight gains on carbon fibers versus time in reaction with isocyanate prepolymer, PET-95A (MW= 1350). Carbon fibers were treated with 70% HNO, at 115°C for 60 minutes followed by TEPA treatment at 190°C for 4 hours. Basic functions 46 peq g ’ measured by HCl uptake. 0 Calculated based on HCl uptake results. 0 Calculated based on metanil yellow adsorption results. a Measured weight gains.

quantity of surface acidic functions introduced per gram of fiber by nitric acid oxidation is substantially higher due to the increase in surface area. In the plasma oxidations fiber surface area remained approximately constant [28]. The value of surface acidic functions or amine groups per 100 A” of fiber surface was obtained by dividing the aqueous NaOH or aqueous HCI uptake measurements by the BETmeasured surface area. However, the implicit assumption is made that the NaOH and HCl solutions access the same surface measured by nitrogen BET measurements. However, this is unlikely to be correct. We have accumulated substantial evidence [ 24,25,29] that aqueous NaOH is able to contact a larger area by strong solvation of oxygenated functions along pores, cracks or fissures. This opens up a further network of surfaces below the nominal fiber surface. This “swelling” action means the functional groups

Fig. 4. Weight gains and amino groups consumed on TEPAtreated carbon fibers in the reactions with isocyanate prepolymer, PET-95A, versus the total amino functions initially present on fiber surfaces, Amino functions were determined by HCI uptake. Carbon fibers were treated with 70% HNOJ at 115°C for 20 to 90 minutes followed by TEPA treatment at 190°C for 20 hours. Reaction with PET-95A employed 4 g PET-95A in 25 mL xylene at 80°C for 3 hours followed by refluxing in ethanol for 20 hours. 0 Amino functions consumed determined by HCl uptake. 0 Calculated weight gain if all the amino groups consumed (according to HCl uptake) reacted with PET-95A. D Measured weight gains. detected by aqueous NaOH occur on a larger surface area than nitrogen BET is able to detect. Apparently, on drying some of these morphological features close, thereby excluding nitrogen during BET measurements. Furthermore, long oxidation periods probably introduce micropores where the BET method will likely give surface areas which are too high. The surface areas measured by MB+ (Table 7) were larger than those measured by BET (Table 8) from 0 through 90 minutes of oxidation. This suggests that either the projection area per MB+ ion used (197 A’) was too large or, more likely, that some multilayer adsorption took place. Methylene blue can be adsorbed in dimeric form in some locations. Since MB+ adsorption measurements are made by

940

C. U.PITTMAN etal.

exposing

solutions of expose more area than the dry fiber surfaces used in nitrogen BET measurements. The differences in carbon fiber surface areas, when measured dry (via BET) versus when submerged in aqueous base, can be substantial for extensively electrochemically oxidized fibers (such studies are currently underway in our laboratory). The grafting efficiency of the reagent molecule decreased as the size of the reagent molecules increased (Table 6). This was observed for both isocyanate and epoxide reagents. These grafting reactions were performed on fibers treated with nitric acid and then coated with TEPA. Since the surface areas of these fibers had been obtained by nitrogen BET measurements and the number of amine functions per 100 A’ of surface was known (e.g. Table 8, for example) it was possible to approximate the number of grafted molecules per 100 A’ versus molecular size. These values are listed in Table 9. These values put into perspective the likely chemical bonding density between epoxy resins and TEPA-treated carbon fibers, Typically between one and two chemical bonds per 100 A’ of fiber surface are likely to form between fiber and matrix using the EPON 828-EPON 834 series of resins.

2.0

MB’,

15

al 0 10 $ A

05

I0.c 2.c 1

25

3.5

30

40

Log Mn

Fig. 5. The grafting efficiencies of isocyanate and epoxy prepolymers on carbon fiber surfaces versus the log of the molecular weights, 0 Epoxy prepolymers reacted with amino groups on TEPA-treated carbon fibers at 80°C for 19 hours in xylene solution (2 g prepolymer in 10 mL xylene). 0 Isocyanate prepolymers reacted with acidic groups on HNO,-treated carbon fibers at 153°C for 10 hours in 50% (wt) DMF solution. C! Isocyanate prepolymers reacted with amino groups on TEPA-treated carbon fibers at 80°C for 3 hours in xylene solution.

Table 7. The quantity

of acidic functions

and calculated

surfaces

it is likely

active surface

to

that

Acid functions on fiber surfaces (by NaOH uptake) Active surface areab (based on NaOH uptake) Amount of methylene blue adsorbed Surface area covered by adsorbed methylene blue”

peq g-l m2 gg’ peq gg’ m’g ’

“Celion G30-500 carbon fibers, oxidized in 70% HN03 at 115’C. bBased on the presence of 1 carbon atom per 8.31 A’ on the lateral

basic

wet surfaces

area on HNO,-oxidized Length

(A) (B) (C) (D)

aqueous

these

carbon

of time (min) carbon oxidized in HNO, at

fibers” fibers were

115’C

0

20

40

60

90

3.6 0.18 1.2 1.43

13 0.65 3.0 3.57

18.0 0.91 5.7 6.78

33.9 1.7 8.4 10.0

48.8 2.5 14 16.7

planes

of graphite

crystallites.

(A)(~eyg-‘)xl(~mol~teq-‘)x6.023x1023(site.~nzol~‘)x8.3l(kZ.site~‘) (B)(rr?g_‘)= 10Z0(;piZm~2)x ‘Based on each methylene (C)(peyg-‘)x (D)(&

blue ion occupying

lO~(~molmol-1) an average

area of 197 A* ( [28]).

l(~mol~eq-‘)x197(~2;on-‘)x6.023x1023(ionmol~’)

g-i)= 1020(.$ mm*) X 10~(/lmolmol~‘)

Table 8. Surface

areas and functional

group

surface concentrations

on carbon

Acidic functions Oxidation time” (mm) 0 20 40 60 90

Surface areaa (BET) (m’g-‘) 0.65 0.69 0.73 1.65 1.93 2.10 12.4 12.4

a After 75 minutes of oxidation area had increased to 18.8 mZ gg’.

NaOH uptake (peq g-‘)

fibers as a function

of nitric acid oxidation

Amine functions

(number/100

A’)

HCl uptake (peq g-‘)

after TEPA treatment

(number/100

4.5

4.0

*****

*****

12.5 22.7 32.6

10.3 8.3 9.7

18.7 31.3 46.0

15.4 11.4 13.7

47.1

2.3

66.7

3.2

the measured

surface

area was 10.3 mZ g-l

and after 105 minutes

time

of oxidation

A’)

the surface

Amines grafted Table 9. The number of reagent molecules grafted to TEPAtreated carbon fibers” per 100 A2 of surface versus the size of the molecule being grafted Reagent grafted

being

Styrene oxide EPON 828 EPON 834 EPON 1001 PET-75Db PET-96A SYN-ISOPb

Mol. wt.

G,(%)

120 378 500 1000 920 1350 4800

68.7 15.0 7.8 5.3 8.7 4.8 2.5

Number grafted

of molecules per 100 A’ 9.41 2.06 1.07 0.73 1.19 0.66 0.34

a The carbon fibers employed were oxidized for 60 minutes in 70% HNO, at 115°C and then treated with TEPA at 190-195°C. HCI uptake experiments showed 46 peq g-’ of amine surface functions were present. With a surface area of 2.01 m*g-r this corresponded to 13.7 amine functions per 100 A’. b These fibers contained 53 peq g-’ of amino groups.

4. SUMMARY

AND DISCUSSION

The ability of amino groups, bound to carbon fiber surfaces, to react with small model compounds and prepolymers of various molecular sizes was evaluated. The results may be summarized as follows. ( 1) The surface-bound amino groups on grafted TEPA moieties react with acetic anhydride and phenyl isocyanate. About 50 to 60% of all the surface amino groups can react with these two model compounds. Amino groups which can be detected by metanil yellow adsorption can all react with the model compounds because the size of these dye ions are larger than those of the model reagents. Therefore, reactive sites which can adsorb the dye ions are also geometrically accessible to the reagents. (2) Chemical bonds between the surface carboxyl and phenolic hydroxyl groups with isocyanate prepolymers were formed. Also surface amino groups could react with isocyanate and epoxy-containing reagent molecules. The measured weight gains provided quantification of the surface stoichiometry achieved. However, not all of these acidic or basic surface groups (which can be detected by NaOH or HCl uptake) are able to react with such small model reagents as acetic anhydride, styrene oxide or 1,6-diisocyanatohexane. Grafting efficiencies for 1,6-isocyanatohexane of almost 50% were achieved on HNO,-oxidized fiber surfaces and significant looping occurred. Phenyl isocyanate, acetic anhydride and styrene oxide grafting efficiencies in excess of 60% were achieved with TEPA-treated fibers. (3) The amount of isocyanate or epoxide prepolymers which were grafted to the surface-bound amino groups usually fell in the range from 1 to 3 mg per gram of fiber. The amount grafted did not tend to increase with longer reaction times or large changes in the surface concentration of amino groups. About 2 to 3peqg-’ of isocyanate or epoxide prepolymers could be found per gram of carbon fibers. Grafting efficiencies dropped steadily as the molecular size of

to carbon

fibers

941

model epoxides and isocyanates increased. On oxidized fiber surfaces the graft efficiencies of isocyanates onto acidic functions dropped from E 10 to ~2% as the molecular weight of the isocyanate increased from 920 to 4800. Similar trends were found on TEPA-treated fibers for isocyanate and epoxy prepolymers. About 2 EPON 828 and about 1 EPON 834 molecules were bonded per 100 A’ of fiber surface to TEPA-treated fiber surfaces. (4) An approximately linear relationship existed between the log of the grafting efficiency and the log of the molecular weight of each type of prepolymer which was grafted. The rapid drop in grafting efficiency with an increase in the molecular weight of the reagent involves stearic masking of reactive surface sites in the vicinity of the surface function which formed the grafting chemical bond. The larger the reagent molecule becomes the greater the area of surface it will cover. The following calculation estimated this steric masking effect. The most readily oxidized carbon atoms are concentrated at the edges of the graphite sheets which terminate at the surface in lateral planes. The active surface area on the fiber surfaces was estimated. The spacing between graphite sheets is 3.39 A and the C-C bond lengths in graphite are 1.415 A [ 301. The average area occupied by one carbon atom on the lateral plane is (1.415) sin(60”)(2)(3.39)=8.31

A”

This area may be regarded as the area for a single active site. Thus, one micro-mole of active sites will occupy 6.023 x lO23 (sites mol-r) 1020[~2 m-’

x 8.31 (A’ sitee’)

x 106 ( pmol mol

‘)I

= 0.050 (m* pm01 - ‘) An estimate of the active surface area was calculated as shown in footnote b of Table 7. This method is totally independent of the actual total surface areas measured by nitrogen BET measurements (Table 8). The surface area covered by an adsorbed MB+ dye ion, was calculated based on an average projection area for the MB’ ion of 197 A’ [31]. These values and the method of calculation are also shown in Table 7. The surface areas shown in rows B and D of Table 7 are not intended to be estimates of the total fiber surface areas as a function of nitric acid treatment time. Instead, the values in row B (Table 7) represent an estimate of the area occupied by acidic functions on the surface after various nitric acid treatments. This value is only a relative approximation since some small fraction of oxidized sites exist on basal planes where the area per site may not be equal to that on a lateral plane. Various topological features may also affect the absolute values. However, it is clear that the surface area occupied by carboxyl and phenolic groups steadily increases with longer exposure to HNO,. These areas also represent size-

942

C. U. PITTMANet ul.

able fractions of the total surface areas determined by nitrogen BET measurements when from 20 to 60 minutes of nitric acid exposure was used (comparing the values in column 2 of Table 8 to those of row 2 Table 7 we see that 57781% of the total surface is covered by acidic functions). As-received fibers appear to have about 27% of the total surface covered by acidic groups. As the total surface area begins to rapidly increase (above 60 minutes of oxidation) the fraction covered by acid groups drops (to ~20% at 90 minutes of oxidation). However, this drop is the likely result of the ability of aqueous NaOH to swell highly oxidized surface volumes of the fiber thereby making accessible more area than nitrogen BET is able to measure on dry fibers. The total surface area measured by MB+ adsorption increased 11.7 fold after 90 minutes of oxidation. This increase parallels the results of nitrogen BET measurements which showed that the surface area of the as-received fibers (0.67m ’ g -‘) increases 18.5-fold to 12.4 m2 g-l after 90 minutes of oxidation in HNO,. In a similar manner the approximate surface area covered by methylene blue increased 11.7 fold after 90 minutes of exposure to HNO, oxidation. Before any HNO, oxidation, the measured BET surface area was 0.67 m2 gg’ which is twice that of a smooth cylinder model (0.325 m2 g-r based on a 7 pm fiber diameter and a density of 1.76 g cm-“). The measured surface areas are much smaller than the approximated area covered by MB+ adsorption at all oxidation levels through 90 minutes (row D, Table 7) where the MB+ ions’ large plane is assumed to he flat on the fiber surface. Thus, multiple MB’ dye layers are probably adsorbed on some portions of the surface. However, it is absolutely clear from MB+ adsorption (row D of Table 7) that progressive HNO, oxidation greatly increases the surface area. Obviously the fiber surfaces are not smooth and they become rougher during HNO,-oxidization (MB+, NaOH and BET measurements all agree). Many active sites must be generated in micro-crevasses, pits, defects etc. upon increasing oxidation time. Some microvoids might also open up to expose basal planes because the fraction of the total surface which is active decreases from ~27% (as received) to - 20% after 90 minutes of HNO,-oxidation based on this dye absorption methodology. A similar figure comes from NaOH adsorption vs nitrogen BET (2.5/12.41 x 100=20%). However, this interpretation is tentative in view of the larger surface that can be contacted by aqueous NaOH versus that detected by nitrogen BET. (5) The size distribution of openings into micro structures on fiber surfaces may play an important role in how higher molecular weight reagents can block access to reactive surface functional groups. Ruland and Perret [32,33] observed that the average diameter of the pores perpendicular to the PAN-base carbon fiber axis varied from 6 to 20 A based on X-ray measurements. Ismail [34] measured the size

distribution of mesopores in PAN-based T300 carbon fibers from nitrogen physisorption isotherms. The following results were obtained [33]: Pore diameter (A) Pore volumes (10-6mLg-1)

2op 50

5op 150

150300

3OOG 500 Total

119

335

331

399

1184

Consider the effect that the isocyanate prepolymer PET-95A might have at a fiber surface. The density of PET-95A is approximately 1 g cmm3. Thus 1 ltmol of PET-95A (MW 1348) will have a specific volume of I348 x 10m6 mL. This value is even greater than the total volume of mesopores (1184 x 10m6 mL gg’) measured by Ismail [34]. The polymer chains are present as swollen loose coils in solution. The hydrodynamic volume of these coils in solution is much greater than their specific volume in the solid state. Therefore, PET-95A could not readily enter such pores and graft to their inside surfaces. Of course this specific pore data for the T300 fibers may not be directly applicable to the fibers used in this work but it is instructive. Also. the mesopore structure which Ismail studied may not include all levels of the surface micro structures. The sizes of crevasses and pits created by the HNO1 oxidations must be considered. Some of these may indeed incorporate prepolymers and others may not. As the size of reagent prepolymer increases it will increasingly be excluded from more pore volume. HNO, oxidation increases both the BET surface area and the ‘*active” surface area [35]. The surface area measured by MB+ adsorption and the “active” surface area (HCl uptake) steadily increased during oxidation. This large increase in the active surface area suggests that the total area of lateral planes present at the surface increased during oxidation through 60 minutes. Micro structures, such as pores, pits, crevasses etc. appear to become more highly developed during continued oxidation. Acknol~l~dgemmts~This research was supported in part by the National Science Foundation through Grant Nos. ST1 8902064. EHR-09108767 and OSR-9452857. Support from the State of Mississippi and Mississippi State University is gratefully acknowledged.

REFERENCES E.P. Pluddemann, Interface in Polymer Matrix Composites, Academic Press, 1974. J.B. Donnet, R.C. Bansal, Cabon Fibers, 2nd ed., Marcel Dekker, New York, 1990. Hughes, J.D.H., Cornpositrs Sci. Techn., 1991, 41, 13. R. Yosomiya, K. Morimoto, A. Nakajima, Y. Ikada, T. Suzuki, Adhesion and Bonding in Composites, Marcel Dekker, New York, 1990. 5. E. Fitzer. H. Jaeger, R. Weiss, Ext. Abstr. Program 16th Bienn. Conf:Carbon, 1983, 471-2. _ R.V. and Jakubowski, J.J., Polym. Eng. 6. Subramanian, Sri., 1978, 18(7), 590.

Amines grafted I. L.T. Drzal, M.J. Rich, J.D. Camping, W. Park, Proc. 35th SPI RE/C Conf., 1980, Sec. 20C. 8. Bell, J.P., Chang, J., Rhee, H.W. and Joseph, R., Polym. Composites, 1987, 8( 1), 46. J.R. and MacKerron, D.H., Europe Poly9. MacCallum, mer .I., 1982, 18, 717. 10. Gerard, J.F., Polymer Eng. Sri., 1988, 28(9), 568. 11. W. Li. C.U. Pittman Jr. SD. Gardner. Proc. Inter. Conf. Comp. Eng., ICCE-3, pp. S41l42, New Orleans, LA, Aug. 20&24,-1995. K.L., Vincent. M., Gardner, 12. Low. B.Y.. Anderson. SD., Pittman, Jr. CU., Hackett, R.M., Comp. Eng., 1995,5(4), 437. S.D., Pittman, Jr. C.U., Chang, T.C., Low, 13. Gardner, B.Y. and Hackett, R.M., Composites, 1995, 26(4), 269. 14. Gardner, S.D., Pittman, Jr. C.U., Hackett, R.M., J. Camp. Muter., 1993, 27(8), 830. 15. Gardner, SD., Pittman, Jr. C.U., Hackett, R.M., Comp. Sci. Technol., 1993, 46, 307. 16. Low, B.Y., Gardner, SD., Pittman, Jr. CU., Hackett, R.M., Camp. Eng., 1995, 5(4), 315. 17. C.U. Pittman Jr, SD. Gardner, G.R. He, L. Wang, Z. Wu, C.K. Singamsetty, B. Wu, G. Booth, Mater. Sci. Sot. Symp. Proc. Vol. 385, Polymer/Inorganic Interfaces II. L.T. Drzal. R.L. Opila. N.A. Peppas, C. .. Schutte (Eds.), 1995, p. 195-200. 18. Wu, Z., Pittman, Jr. C.U., Gardner, SD., Carbon, 1995, 33(5), 597. 19. Wu, Z., Pittman, Jr. CU., Gardner, SD., Carbon, 1996, 34(l), 59. 20. Wu, Z., Pittman, Jr. C.U., Gardner, S.D., Carbon, 1995, 33( 5), 607. SD., Singamsetty, C.K., Booth, G.L., He, 21. Gardner, G.R., Pittman, Jr. C.U., Carbon, 1995, 33(5), 587. 22. Gardner, S.D., Singamsetty, C.K., Wu, Z., Pittman, Jr. C.U., Sur@ce and Interface Analysis, 1996, 24, 311.

to carbon

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23. C.U. Pittman Jr, G.R. He, B. Wu, L. Wang, S.D. Gardner. 5th Inter. Conf. Comn. Interface. ICCI-V. P1992. Goteborg, Sweden, June 20-23, 1994. 24. C.U. Pittman Jr, G.R. He, B. Wu, S.D. Gardner, Chemical modification of carbon fiber surfaces by nitric acid oxidation followed by reaction with tetraethylenepentamine, Carbon, 1997, 35(3), 317. Mississippi State Univer25. G.R. He, Ph.D. dissertation, sity, 1994. 26. L.T. Drzal, N. Sugiura, D. Hook, The Role of Chemical Bonding at the Fiber-Matrix Interface on Adhesion in Carbon Fiber/Thermoset Matrix Composites, Proceedings of the International Conference on Composite Engineering (ICCE/3), D. Hui (Ed.), New Orleans, LA, July 21-26; 1996, pp. 11-12. N.. Kabovashi. K. and Sone. Y.. J. Polvm. 27. Tsubokawa. Sri., Polym. Chem. Edk., 1988, 26, 223. Jr, G.R. He, S.D. Gardner, Oxygen 28. C.U. Pittman plasma and isobutylene plasma treatments of carbon fibers. Determination of surface functionality and effects on composite properties, Carbon, December 1996, (submitted). 29. S.F. Waseem, SD. Gardner, G.R. He, W. Jiang, CU. Pittman Jr, Adhesion and surface analysis of carbon fibers electrochemically oxidized in potassium nitrate, manuscript in preparation. 30. Hoffman. W.P.. Vastola. F.J. and Walker. P.L.. Carbon. 1984, 22, 585. 31. Graham, D., J. Phys. Chem., 1955,50, 896. R. Perret, W. Ruland, Carbon Fibers 32. A. Fourdeux, Their Composites and Application, The Plastic lnstitute, London, 1971, p. 57. 33. T. Perret, W. Ruland, 9th Bienn. Conf. Carbon, Boston, summary paper, 1969, p. 158. 34. Ismail, M.K.I., Carbon, 1987, 25(5), 653. R., Carbon, 1977, IS, 257. 35. Rand, B. and Robinson,