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Titration of tetraethylenepentamine (TEPA) and its phenyl isocyanate reaction products: A model correction factor for determination of TEPA grafted to carbon surfaces
Carbon Vol. 35, No. 3, pp. 333-340, 1997 Copyright 0 1997Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 %17.00 + 0.00
Pergamon
PII: MOOS-6223(96)00160-l
TITRATION OF TETRAETHYLENEPENTAMINE (TEPA) AND ITS PHENYL ISOCYANATE REACTION PRODUCTS: A MODEL CORRECTION FACTOR FOR DETERMINATION OF TEPA GRAFTED TO CARBON SURFACES 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 28 May 1996; accepted in revisedform
11 October 1996)
Abstract-Ex-PAN carbon fibers (high strength, Celion G30-500, 3K-HTA-7C-N501, BASF Structural Materials, Inc.) were oxidized in 70% nitric acid at 115°C for various times and then reacted with tetraethylenepentamine (TEPA) at 190-200°C thereby grafting TEPA to the fiber through amide bonds. To determine the resulting quantity of surface amino groups present, HCI uptake experiments were conducted. However, charge-charge repulsion prevents complete protonation of all the amino groups at suitable pH values for accurate measurements. Therefore, model compounds were prepared by reacting TEPA with phenyl isocyanate (PhNCO). These compounds were used to model the protonation of amine sites in the TEPA grafted onto nitric-acid oxidized carbon fiber surfaces. TEPA and the model compounds were independently titrated with HCl solution to measure the fraction of amino groups in these compounds which are protonated at various pH values. This titration mimics the titration of TEPA amino groups which would be grafted onto oxidized carbon fiber surfaces. Protonation of TEPA was not complete at pH 1, and protonation of surface-bound TEPA was certainly not complete at pH 2.8, the lowest pH for accurate determination of basic functions on carbon fiber surfaces. Therefore, direct measurements of HCI uptake were lower than the true quantity of amino groups present in TEPA molecules. A correction procedure has been established, based on the titration results from the model TEPA/phenyl isocyanate reaction products, to correct the direct HCI uptake measurements on fiberbound TEPA or TEPA in solution. 0 1997 Elsevier Science Ltd. All rights reserved Key Words-A.
Carbon fibers, B. activation, oxidation, C. surface properties.
1. INTRODUCTION
and matrix resins, the interfacial shear strength can be significantly increased. In the preceding paper [ 11, carbon fibers (CF) were first oxidized in concentrated (70% by weight) nitric acid at 115°C to generate acidic carboxyl and phenolic hydroxyl functions on fiber surfaces
In the preceding paper [l] we introduced tetraethylenepentamine (TEPA) functions onto carbon fibers which had been oxidized to varying degrees with nitric acid. Then we determined the amount of amine groups which had been surface-grafted by HCl uptake. However, the method employed required a correction factor. The determination of correction factors for various TEPA derivatives is now described as part of a research effort to investigate carbon fiber/epoxy composite systems containing a polyurethane elastomeric interphase [ l-101. To realize improved chemical bonding between the fiber surfaces and the polyurethane interphases, reactive functional groups have been generated on the fibers. Mixtures of carboxylic acid and phenolic hydroxyl groups followed by subsequently grafted amino groups were introduced onto fiber surfaces [6, II]. Amino groups could be especially desirable on fiber surfaces since they react with isocyanates and with epoxy groups faster than carboxyl or hydroxyl groups [ 11,121. This might lead to improved polyurethane elastomer/fiber and epoxy matrix/fiber adhesion. Recently, Drzal [ 131 has demonstrated that by increasing chemical bonding between carbon fibers
(eqn(1)) [1,6,11]. Keto, quinoid, and ether functions are also formed. The carboxyl and hydroxyl surface functions were important in this research because they could react with isocyanate groups (to bind polyurethane elastomers) as well as with the epoxy groups of matrix resins [ 11,121. Amino groups were also introduced on fibers by the direct reaction of the nitric-acid oxidized fiber surfaces with TEPA at 190-2OD”C (eqn (2)). This process grafts TEPA molecules to the surface via amide bonds. While high surface concentrations of carboxyl and hydroxyl groups might lead to higher chemical bonding densities from fibers to isocyanatederived polyurethane interphases or to epoxy matrix resins, the use of surface amine groups to promote higher fiber-to-elastomer interphase or fiber-tomatrix covalent chemical bond densities was also considered possible, and hence a worthwhile research topic [1,6,11,13]. TEPA has two primary and three secondary amine functions. Its reaction with carboxyl groups should
*Corresponding author. 33
334
C.
U.
PITTMAN.
JK et ul.
HNO, CFI
and
115 oc
0
+
CFt--c<
CFt-OH
OH
H 2 NANnNANnNH H H
OH
1 go-200
2
oc
H
TEPA
+ H
l
H
TEPA
H
-
CFtC/( N
CF
ideally introduce, four amines for every surfacebound carboxyl group consumed. This leveraging factor could be lower if more than one amine function per TEPA forms an amide bond at the surface (e.g. loop formation, see eqn (3) and ref. [I]). Why is a correction procedure necessary to determine surface-grafted TEPA? Accurate surface group quantitative analysis is required to define amine functional group leveraging effects and provide a clearer picture of the surface chemical transformations. Acid/base uptake was used to measure the basic and acidic functions on treated fibers [ 1,6,11]. In practice, fiber samples with grafted amino groups were submerged in aqueous HCl solutions for 3 hours. The pH values of these HCI solutions were measured before and after fiber submersion. The quantity of the surface-bound amino groups was then determined by the moles of HCl removed from solution, which was calculated from the change in pH value of the HCl solution. This method is quite accurate when the spacing between amino groups is adequate. However, the protonation of surface bound TEPA amino groups by HCl was not complete at pH values down to 2.7-2.9, which represent the highest acid concentrations which can be used for accurate HCl uptake experiments. Successive TEPA protonation becomes more difficult due to the build-up of positive charge repulsion. This is obvious upon examining the titration curve for TEPA in HCl or other mineral acids (see Fig. 1 as a typical example). Experiments have shown that the acid strength of an HCl solution with a pH 1 is not high enough to protonate all five amino groups of TEPA. The reason becomes apparent when examining eqn (4).
(3
(31
Similarly, complete protonation of all the amino groups on the fiber-bound TEPA (see eqns (2) and (3)) would require a low pH (high acidity). However, HCl solutions with such low pH values cannot be used for titrations of fiber surfaces. This limitation exists because the quantity of the surface-bound amino groups present on the amount of fibers which can be submerged in any unit volume of HCl is very small (usually 3 5 50 lteq g-‘). Therefore, the accuracy required in the measurement of the change in pH at very low pH values is impossible to achieve. For example. the change in pH that would occur by protonating surface-bound amines in a solution of pH 1 (in which the maximum possible amount of fiber was submerged) is far too small to measure with the requisite accuracy. Therefore, HCl concentrations no higher than those present at pH 2.8 should be used. At this concentration the complete protonation of TEPA (or bound TEPA) cannot be achieved. Therefore, a calibration procedure was required to correct the direct measurements of HCl uptake. Upon grafting TEPA onto a carbon fiber surface, one or two (if a loop structure forms) amino groups react to form amide bonds (eqns (2) and (3)) [ 1,6,11]. When these grafted TEPA functions are used to promote polyurethane elastomer interphase bonding to the fiber. TEPA’s amino groups react with isocyanate compounds in the preresin mixture [6,7.10, Ill. The developing polyurethane elastomer would become bonded to the fiber through urea functions, as shown in eqn (5). As amine functions are converted to urea groups (or amides at the site of bonding to the fiber surface), the pH value required for complete protonation of
Titration
of TEPA and its phenyl isocyanate
reaction
products
335
pH value
,3
TEPA: 1.471 g HCI: 1.009 N
12 -
8-
1st end polnt volume 22.6 mL
765-
At pH = 3, volume 31.8 mL
4-
Calculated end
321 0
10
5
15
20
25
30
35
40
45
50
Volume of HCI (ml) Fig. 1. Titration
H2
(TEPA)
NANANANANH +H+ H H H 2
+H+
-
of tetraethylenepentamine
-
nNmNnNn;H
H2N
-H+
,,’
lH
3
H
H2
-H+ K,
with aqueous
NnNpNnNnNH
(with
H
+nNnN7p+
HsN
-H+ K3
KZ
2
’ HH
H'
_+H+_
H
HCl solution.
H
H/
‘H
H
NH3
(4)
proton
reorganization)
+nN\NPN\+ HsN / \ / lH Hi lH H HH
NH3
K, andK2 >> K.,orK5
CF
j-!-N-km-i-h HHHH
B n,/n,n\,/n
.
OCN--NC0
R
N--C-N-NC0 n H
CF,-C-N
H
’
H
H
H
(5)
OCN-NC0
-N-N-N-NW!-N-NC0 H
Ox;
H
-
etc.
H
H
OCN-hH PhN==C=O
H 2 NnN-inNn H
NH1 H
-
d-!-N-N-NnN-NH HHHH
H
(model for rurfaca
2 bound TWA)
(6)
PhN==c==o -
MN!!-NnN-N-NnNH H H w
2 ’
+
PhNz-N-N-N-N-
q N-CNHPh
HHHHHH
.Hc\NHp”n + etc. PhN=C=O
PhN=C==O c
TRlSVBSTlTUTlON
.
TETRAS”BSTlT”T,clN
C. U. PITTMAN,JR PI 01.
336
the remaining amine sites rises. This occurs because TEPA moieties now have fewer amine functions left, and charge-charge repulsion is reduced when complete protonation occurs. This process was modeled by reacting TEPA with various amounts of phenyl isocyanate [ 111. One or more of TEPA’s amino groups reacted with phenyl isocyanate and urea groups were formed (see eqn (6)). The complete protonation of these partially substituted TEPA model compounds with HCl was studied to define just how they were different to HCl protonation from the original TEPA. By adjusting the TEPA/phenylisocyanate mole ratio, the amine-tourea group ratio in the model compound product mixture was varied. These model compounds were then titrated with aqueous HCI solution to examine their behavior. This paper deals with the preparation of these model compounds and their titration with aqueous HCl. From these results, a correction procedure was established to permit the accurate determination of the correct fiber surface amine stoichiometry which was present after TEPA grafting. Once the procedure was established, all surface-bound amino group determinations on TEPA-treated carbon fibers (conducted by HCl uptake) could be corrected by multiplying the experimental values by a correction factor to obtain the actual values of the Lleq gg’ of amino functions present on the fibers. This procedure was used in the preceding manuscript [ 1] and in other work where TEPA grafting to fiber surfaces has been employed to introduce amino functions [557,9-l I].
2. EXPERIMENTAL All chemicals used in this work were purchased from Aldrich Chemical Company, Inc. The TEPA was reported to be 98% pentamine by Aldrich. Model compounds were prepared by reacting TEPA with phenyl isocyanate (PhNCO). Reactions were performed at PhNCO/TEPA molar ratios of 0, 1. 2, 3, 4 and 5. The reaction product mixture prepared at PhNCO/TEPA = 1 was used to simulate the surface-bound TEPA molecules in which one amino group has reacted with a surface carboxyl group, resulting in an amide group. The individual product mixtures with the PhNCO/TEPA ratios of 2, 3, 4 and 5 simulated the other cases in which more than one of TEPA’s amino groups had reacted with surface carboxyl groups. These mixtures could also be used to simulate fiber-grafted TEPA after reaction of fibers with isocyanates. The analysis of residual TEPA amino groups remaining on carbon fiber surfaces after reaction with isocyanates is a necessary step in defining the chemical reaction efficiency of bound TEPA with developing polyurethane coatings. TEPA (1.511 g) was dissolved into 10 ml of methyl ether ketone (MEK). Then a weighed amount of phenyl isocyanate (1, 2, 3, 4 or 5 moles per mole of
TEPA, respectively) was added. Reactions were carried out at room temperature for two days under nitrogen protection. The temperature was subsequently raised to 70°C for an additional 15 hours to drive the reaction to completion. Water (30 ml) was added prior to titration so that the subsequent titration could be monitored by a pH meter. The reaction product solutions were titrated with a solution made by dissolving concentrated HCl (aqueous) into methyl ethyl ketone (final concentration in HCl was 1.051 M ). These titrations were monitored by a pH meter (Ion Analyzer 250, Corning). The pH values of the product solutions were plotted against the volume of HCl solution consumed.
3. RESULTS
AND DlSCUSSlON
3. I Titration of TEPA, ethylenediamine and diethylenetriamine with HCl As-received tetraethylenepentamine ( 1.471 g) was titrated with aqueous HCl (Fig. 1). The total volume of HCl solution ( 1.009 N) with the required equivalents to protonate all the five amino groups in this product was 1000 x (1.471/189.3) x 0.98 x S/1.009= 37.7 ml, where 189.3 equals the molecular weight of TEPA. At the first end-point (pH 6.5) 22.6ml of HCl was consumed. This 22.6 ml volume equals 3/S of the total volume (37.7 ml) of HCl solution consumed, indicating that three amino groups in each TEPA molecule were preferably protonated. These could be the two terminal groups and the one at the middle of the molecular chain, i.e. the first, third and fifth positions. A sharp end-point corresponding to 37.7 ml was not observed. For reference, two analogs of TEPA, ethylenediamine (EDA) and diethylenetriamine (DETA) were also titrated with aqueous HCl (Fig. 2). Two endpoints were observed for ethylenediamine. The first and second end points, designated as vl (EDA) and v2 (EDA) in Fig. 2, appeared at pH 8.6 and pH 4, respectively. The ratio of HCl volumes consumed at the first and second ethylenediamine end points was 1: 1. Similarly, diethylenetriamine also exhibited two end points, vl (DETA) and v2 (DETA). However, they appear at much lower pH values (pH 7 and pH 2.3, respectively). The ratio of HCl solution volumes consumed at the first and second end-points were 2:1, indicating that two amino groups were protonated at the first end-point. The total HCl volumes consumed at the second end-point for both ethylenediamine and diethylenetriamine equal the stoichiometrically calculated values. These results clearly indicate that (1) both the amino groups in ethylenediamine and all three amino groups in diethylenetriamine can be protonated by a 1 N HCl solution, (2) two amino groups (perhaps the terminal groups) in diethylenetriamine are preferably protonated as compared with the third (middle?) amino group, (3) as more amino groups are present in a molecule, the acidity for complete protonation
Titration
of TEPA and its phenyl isocyanate
reaction
products
337
pH value EDA: 1.0899 DETA: I.3489 TEPA: 1.471 g
8-
8-
0
10
20
30
Volume of HCI (mL) Fig. 2. Titration
of ethylenediamine (EDA), diethylenetriamine (DETA) and tetraethylenepentamine HCI solution. vr and vz stand for the first and second end-points.
with aqueous
shows six titration curves corresponding to pure TEPA and these five model compound mixtures. The volume of HCl (1.051 M) required to protonate all five amino groups in 1.511 g of TEPA (curve 0 in Fig. 3) was:
rises (e.g. lower pH required), and (4) the last amino group is much more difficult to protonate than those previously protonated. The change of end point pH values going from ethylenediamine (pH 4.0) to diethylenetriamine (pH 2.5) strongly suggests that TEPA’s end point would appear at very low pH (i.e. a very high concentration of HCl solution would be required).
V,=(1.511/189.3)xO.98
x5 x 1000/1.051=37.2m1
The equivalent number of protons transferred per mole of the model PhNCO/TEPA compounds can be determined. To obtain the volumes of HCl consumed in each titration experiment, the end-point of the titration curves must be determined. However, the end points of these curves are not easy to define exactly. In practice, the pH of the HCl solutions at acid uptake equilibrium was between 2.8 and 3.2.
3.2 Titration of model TEPA ureas synthesized
in TEPA phenyl isocyanate reactions The five each of the (PhNCO) to 5) were
(TEPA)
model compound mixtures synthesized in reactions of TEPA with phenyl isocyanate (using PhNCO/TEPA mole ratios from 1 individually titrated with HCl. Figure 3
pH value 14
TEPA: 1.511g HCI: 1.051N
10
20
30
40
Volume of HCI (ml) Fig. 3. Titration of tetraethylenepentamine (TEPA) and its reaction products solution of HCl. The numbers by the curves represent the PhNCO/TEPA
with phenyl isocyanate (PhNCO) with MEK mole ratio used to prepare these products.
C. U. PITTMAN,JR et al.
338
Therefore, the end-point for these titration curves was selected to be pH 3 for convenient comparison and data treatment. The derivation of the correction factors mentioned above is listed in Table 1. The values in row A of Table 1 are the average number of amino groups that would exist in each of the product mixtures obtained in the TEPA/PhNCO reactions, assuming that the addition of the amino N-H across the NC0 groups occurred quantitatively (i.e. yield= 100%). Values in row B are volumes (V,) of HCl solution consumed in titration at pH 3. Row C is the ratio of the HCl volume actually consumed (V,j during the titration of each product vs the volume theoretically required (I/,) by the protonation of all five amino groups. Then, the value of 5 x L’,,ivO (i.e. row D) equals the number of amino groups which were actually protonated by HCl. This is the average number of basic groups present in the individual reaction products obtained when TEPA was reacted with PhNCO at each mole ratio. However, for those reaction products in which the PhNCO/TEPA ratio is 3, 4 and 5, the number of amine groups determined (i.e. 2.145, 1.525 and 1.390 in row D) are higher than the stoichiometritally available number of basic groups (i.e., 2.0, 1.0 and 0 in row A). Obviously, this requires that another base was present. The other base is the fraction of phenyl isocyanate which remains unreacted. Not all of the phenyl isocyanate reacted at these three ratios (i.e. PhNCO/TEPA=3, 4 and 5). Thus, each equivalent of isocyanate remaining unreacted will consume an equivalent of HCl during titration. Also, each equivalent of amino groups remaining unreacted with PhNCO will consume one equivalent of HCI. Therefore, the amount of additional HCl consumed should be twice that of the unreacted phenyl isocyanate. The unreacted phenyl isocyanate and unreacted amino groups due to incomplete reaction with phenyl isocyanate are calculated and shown in rows E and F ofTable 1. Therefore, the quantity of amino groups actually titrated by HCl is the difference between the Table 1. Titration
of TEPA
and its reaction
products
quantity in row D minus the amount in row F. The values of these differences are listed in row G. The amino groups actually available for titration are the sum of those stoichiometrically calculated assuming 100% reaction of isocyanate (row A) and the amount in row F. These sums, listed in row H. are the total amino groups actually present in the TEPA/phenyl isocyanate reaction products. The correction factor K may now be simply defined as the ratio of amino groups that were actually present for titration divided by the amino groups that were actually detected (protonated) in that titration. These values are shown in row J of Table 1. 3.3 Correciion fhctor K The values of K (Table 1) show that protonation was complete at pH 3 when TEPA contained one or two underivatized amino groups per molecule (e.g. when these products were generated in reactions with PhNCO/TEPA mole ratios of 4 or 3). When three amino groups are present per TEPA, complete protonation does not occur at pH 3 and a small correction factor ( 1.113) must be applied. With four and five amino groups per TEPA, the required correction factors, as expected, increase to 1.212 and 1.221, respectively. Considering the conjugate acids in this series we see the dissociation constants will decrease in the following order: H,N + -C,H,-N+H,-C,H,-N -N+H,-C,H,-N+H,> H,N r -CzH4-N(R)-CZH4-N
+H2-C,H,
-N(R)-C,H,-N+H,> H,N + -C2H4-N(R)-C2H,-N(R)-C,H, -N(R)-C,H4-N+HS> H(R)N-CzH4-N(R)-C,H,-N
+H,-CZH,
-N(R)-C2HJ-NH(R) where R represents a PhNHCO function. Two key conclusions are obvious. First, the frac-
with various ratios correction factors
of phenyl
isocyanate:
Molar
(A) Basic groups available on TEPA or its reaction product (B) HCl volume consumed, V, (ml) (C) Ratio of VI/V0 (D) Equivalent number of amino groups (E) Unreacted PhNCO (F) Unreacted amino groups due to incomplete reaction with PhNCO (G) Amino groups actually detected (H ) Amino groups actually present (J ) Correction factor K Notes: V, was the volume required to (A) Stoichiometrically available number. D=C x 5. (E) Determined as follows: If G=D-F. (H) Calculated by H=A+F.
+H,-C,H,
calculation
procedures
and
ratio of PhNCOjTEPA
0
1
2
5.0 30.4 0.819 4.095 0 0 4.095 5.0 1.221
4.0 24.5 0.660 3.300 0 0 3.300 4.0 1.212
3.0
2.0
I.0
20.0 0.539 2.695 0 0 2.695 3.0 1.113
I15.9
11.3 0.305 I.525 0.263 0.263 1.262 I.262 1.000
3
0.429 2.145 0.073 0.073 2.072 2.072 1.ooo
4
5 0
10.3 0.278 1.390 0.695 0.695 0.695 0 695
protonate all five amino groups on pure TEPA. L’(, = 37.20 ml. (B) Experimental results from Fig. 3. (C) Calculated by B/Vo/,. (D) Calculated D-A<0 then E=O, else E=(D-A).12. (F) The same as (E). (G) Calculated (J) Calculated by J=H/G.
1.ooo by by
Titration
of TEPA and its phenyl isocyanate
tion of phenyl isocyanate converted to urea groups decreased as the PhNCO/TEPA mole ratio increased. Second, the successive protonation of amino groups on TEPA and its phenyl isocyanate derivatives becomes increasingly more difficult as more amino groups have been protonated. The formation of urea groups at adjacent amine sites slightly reduces the base strength of the remaining amine groups. This effect is far smaller than the reduction in base strength caused by successive protonation of adjacent amine groups. For example, if four amino groups on TEPA have been protonated, then the last amino group requires a very high acidity to be protonated. However, if four amino groups have been converted to urea functions by reaction with phenyl isocyanate, then the last remaining amino group can be protonated easily. The positive charge induced by each successive protonation step increasingly reduces the base strength of remaining amino groups. A plot of the correction factor (K) vs the number of amino groups present in TEPA and its substituted derivatives is shown in Fig. 4. When the number of amino groups in TEPA bound to fiber surfaces is known, K can be obtained directly from this plot. Most fiber-bound TEPA moieties would have either three or four amino groups. Those bound to the surface via a single amide function would have four amine sites, and those bound in a single loop (e.g. two amide bonds to the surface) would have three amine sites (see eqn (3)). Thus, the correction factor to be applied in titrations of TEPA-treated carbon fibers falls in the range 1.11-1.21. The quantity of fiber-bound amino groups directly measured by HCl uptake must, therefore, be multiplied by a correction factor to obtain a more accurate result. In titrations of pure TEPA in solution a correction factor of 1.22 must be multiplied by the equivalents of WC1 uptake. 3.4 Derivation of equations to calculate the fraction of surface-bound TEPA in loops while applying the appropriate correction factor The correction factor K calculated from Table 1 is applicable only when the number of amino groups 1.3
C :orrection factor
1.2
1.1
1
0.9
L
0
,
1
1
2
3
4
Number of amino groups present
Fig. 4. Correction factors for use with HCl uptake measurements of amino groups in tetraethylenepentamine (TEPA) molecules grafted to carbon fiber surfaces.
3
reaction
products
339
in TEPA molecules is known. However, when determining the quantity of amine groups present on TEPA-treated fiber surfaces, one problem remains. The true average number of amine functions present per bound TEPA molecule is unknown because the fraction of bound TEPA existing on the fiber surface as loop structures is not known. Therefore, a general method must be devised to calculate the mole fraction of the bound TEPA which is present as looped structures. The value of K which should be used to correct the titrations depends on the fraction of looped structures, but that fraction is unknown. Surface-group titration experiments, necessary to determine the fraction of loops, require application of the correct K value to provide the requisite accuracy in the surface amine stoichiometry present. Thus, two unknowns exist. When TEPA is tethered to surfaces (through the reaction of surface carboxyl groups with TEPA amine functions to generate amide links) other information becomes available. The quantity of carboxyl functions consumed in the reaction to anchor TEPA is available from NaOH uptake experiments performed both before and after the fiber reaction with TEPA. From such information, equations may be derived to calculate appropriately the fraction of TEPA present as loops. This derivation invokes two assumptions. First, acidic groups on the fiber surface that disappear during the reaction with TEPA have been converted to amide functions (see eqns (2) and (3)). Second, fiber-bound TEPA exists on the surface in two general forms: those bound through a single amide bond and those bound through two amide bonds (e.g. a looped structure). The fraction of double-looped structures is considered negligible. Looped structures have been previously observed when ethylene diamine was electrochemically reacted at carbon surfaces [ 17,181. Two values were used from the data in Table 1 (row D) determined by HCI uptake experiments at pH 3: (I) 3.300 amino groups were protonated when the bound TEPA had four amino groups, and (2) 2.695 amino groups were protonated when the bound TEPA had groups (looped structures). amino three Furthermore, the following symbols were used in the ensuing derivation. A is the quantity of surface carboxyl groups consumed in the reaction with TEPA (determined by NaOH uptake measurements), B is the apparent quantity of surface-bound amino groups introduced by grafting TEPA (this is equal to the quantity of HCl uptake before correction), T is the total quantity of TEPA molecules grafted onto the fiber surface,f, is the mole fraction of the grafted TEPA present in loops, B, is the true quantity of surface-bound amino groups after correction, and K is the correction factor used to multiply the experimentally measured quantity of surface-bound amino groups to obtain the true value. Based on the balance of amino groups before correction, we have:
2.695Tf=+ 3.300T( 1 -fT) =B
(7)
C. U. PITTMAN,JR~~ al.
340
Based on the mass balance correction, we have: 3TfT+4T( Based on the mass consumed, we have:
of amino
1 -fT)= balance
groups
B,=KB of carboxylic
2Tf;+T(l-f;)=A
after (8) groups (9)
Solving eqns (7) and (9) for f+ gives: (3.300-;j (IO) fT= (0.605+
prepared by the reaction of TEPA with phenyl isocyanate, where amine functions were progressively replaced by non-basic urea functions. A method was derived to use the correction-factor plots with other titration data to determine the correct total amine quantity on TEPA-treated fiber surfaces and the fraction of TEPA bonded in loops. Ackno,oledgemmts~-This research was supported in part by the National Science Foundation through Grants No. STI-8902064, EHR-9108767 and OSR-9452857. Support from the State of Mississippi and Mississippi State University is also gratefully acknowledged.
;) REFERENCES
Solving eqns (8) and (9) for K gives:
(11) Equation (10) permits the calculation of the fraction of surface TEPA bound as loop structures, and eqn (11) is used to give the appropriate value of the correction factor K. For example, consider the case where the quantity of carboxyl groups consumed during the TEPA reaction was determined to be 20 peq g-r from NaOH uptake experiments (before and after the TEPA reaction), and where the experimental HCl uptake after TEPA grafting was 50 peq g-‘. In other words, A =20 peq g-’ and B=50 peq g-r. Substitution in eqn ( 10) gives ,fT (loop fraction) =0.258. The correction factor K is then obtained from eqn ( 11), in which case K= 1.19.
5. 6.
7.
8. 4. CONCLUSIONS
When HCI uptake or titration data are used to measure the quantity of amino groups bound to fiber surfaces as grafted TEPA, the direct measurements could be lower than the true values. This will occur when the acidity of an HCl solution is not high enough to protonate all amino groups present in the grafted TEPA molecules. The acidity of the HCl solution used cannot simply be raised. The HCl concentration must be sufficiently low to provide a large enough change in pH during uptake to measure accurately the number of HCl equivalents being removed from solution upon protonation of surface amine groups. The pH of such solutions should not be lower than 2.9 for our samples. Therefore, a correction factor must be used to correct the direct measurements by HCI uptake. A correction procedure has been established in this work based on the HCI titrations of TEPA and model compounds
9.
Pittman, Jr, C. U., He, G. R., Wu, B. and Gardner, S. D., Carhon, 1992, 35, 317. Gardner, S. D., Pittman, Jr. C. U., J. Conzpositr Muter., 1995, 27(8), 830. Low, B. Y., Gardner, S. D., Pittman, Jr, C. U., Hackett, R. M., Composites Eng., 1995, 5(4), 375. Low, B. Y., Anderson, K. L., Vincent, M., Gardner, S. D., Pittman, Jr, C. U., Hackett, R. M., Composites Eng., 1995, 5(4), 437. Gardner, S. D., Singamsetty. C. S. K., Booth, G. L., He, G. R., Pittman, Jr, C. U., Carbon, 1995,33(5), 587. Pittman, Jr, C. U., He, G. R., Wu, B., Wang, L. and Gardner, S. D., Extended Abstracts qf the Fifth Internutional Conference on Composite Interjkces (ICCI-V). Moleculur Interactions and Structure. Giiteborg, Sweden, 1994, p. 19-2. Pittman. Jr, C. U., He, G. R., Wu, B., Wang, L. and Gardner, S. D., in Proceedings of the First Internationul Conference on Composite Engineering (ICCEII ), ed. D. Hui. New Orleans, LA, 1994, pp. 4033404. Wu, Z., Pittman, Jr, C. U., Gardner, S. D., Carhon, 1995, 33(5), 607. Wu, Z., Pittman, Jr, C. U., Gardner, S. D., Carbon, 1995,33(5),
597.
10. Pittman, Jr, C. U., Gardner, S. D., He, G., Wang, L., Wu. Z., Singamsetty, C., Wu, B. and Booth, G.. Mater. Res. Sot. S.ymp. Proc.,
1995, 385, 195S200.
fiber composites with elasto11. He, G. R., Continuous meric interphases covalently bonded to both fiber surfaces and epoxy matrices., Ph.D. dissertation, Mississippi State University, Mississippi State, MS, 1994. 12. Yosomiya, R., Fujisawd, T. and Morimoto. K., Po!vm. Bull., 1984, 12, 523. 13. Drzal. L. T., Sugiura, N. and Hook, D., in Proceedings of the International Conjkrencr on Composite Engineering (ICCE/3), ed. D. Hui, New Orleans, LA, 1996,
pp. 11-12. 14. Krin, J., Sci. Amer., 1996, 275(4), 74. B., Israelachvili, J. N. and Landman, U., IS. Bhushan, Nature. 1995, 374, 607. 16. Persson, B. N. J. and Tosatti, E. (eds), Physics qfSIiding Friction. Kluwer, Dordecht, 1996. Chem., 1984, 13, 270. 17. Murray, R. W., Elrctroanal. 18. Barbier, B., Pinson, .I., Desarmot, G. and Sanchez, M., J. Ektrochem. SC.. 1990, 137(6), 1757.
Pergamon
PII: MOOS-6223(96)00160-l
TITRATION OF TETRAETHYLENEPENTAMINE (TEPA) AND ITS PHENYL ISOCYANATE REACTION PRODUCTS: A MODEL CORRECTION FACTOR FOR DETERMINATION OF TEPA GRAFTED TO CARBON SURFACES 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 28 May 1996; accepted in revisedform
11 October 1996)
Abstract-Ex-PAN carbon fibers (high strength, Celion G30-500, 3K-HTA-7C-N501, BASF Structural Materials, Inc.) were oxidized in 70% nitric acid at 115°C for various times and then reacted with tetraethylenepentamine (TEPA) at 190-200°C thereby grafting TEPA to the fiber through amide bonds. To determine the resulting quantity of surface amino groups present, HCI uptake experiments were conducted. However, charge-charge repulsion prevents complete protonation of all the amino groups at suitable pH values for accurate measurements. Therefore, model compounds were prepared by reacting TEPA with phenyl isocyanate (PhNCO). These compounds were used to model the protonation of amine sites in the TEPA grafted onto nitric-acid oxidized carbon fiber surfaces. TEPA and the model compounds were independently titrated with HCl solution to measure the fraction of amino groups in these compounds which are protonated at various pH values. This titration mimics the titration of TEPA amino groups which would be grafted onto oxidized carbon fiber surfaces. Protonation of TEPA was not complete at pH 1, and protonation of surface-bound TEPA was certainly not complete at pH 2.8, the lowest pH for accurate determination of basic functions on carbon fiber surfaces. Therefore, direct measurements of HCI uptake were lower than the true quantity of amino groups present in TEPA molecules. A correction procedure has been established, based on the titration results from the model TEPA/phenyl isocyanate reaction products, to correct the direct HCI uptake measurements on fiberbound TEPA or TEPA in solution. 0 1997 Elsevier Science Ltd. All rights reserved Key Words-A.
Carbon fibers, B. activation, oxidation, C. surface properties.
1. INTRODUCTION
and matrix resins, the interfacial shear strength can be significantly increased. In the preceding paper [ 11, carbon fibers (CF) were first oxidized in concentrated (70% by weight) nitric acid at 115°C to generate acidic carboxyl and phenolic hydroxyl functions on fiber surfaces
In the preceding paper [l] we introduced tetraethylenepentamine (TEPA) functions onto carbon fibers which had been oxidized to varying degrees with nitric acid. Then we determined the amount of amine groups which had been surface-grafted by HCl uptake. However, the method employed required a correction factor. The determination of correction factors for various TEPA derivatives is now described as part of a research effort to investigate carbon fiber/epoxy composite systems containing a polyurethane elastomeric interphase [ l-101. To realize improved chemical bonding between the fiber surfaces and the polyurethane interphases, reactive functional groups have been generated on the fibers. Mixtures of carboxylic acid and phenolic hydroxyl groups followed by subsequently grafted amino groups were introduced onto fiber surfaces [6, II]. Amino groups could be especially desirable on fiber surfaces since they react with isocyanates and with epoxy groups faster than carboxyl or hydroxyl groups [ 11,121. This might lead to improved polyurethane elastomer/fiber and epoxy matrix/fiber adhesion. Recently, Drzal [ 131 has demonstrated that by increasing chemical bonding between carbon fibers
(eqn(1)) [1,6,11]. Keto, quinoid, and ether functions are also formed. The carboxyl and hydroxyl surface functions were important in this research because they could react with isocyanate groups (to bind polyurethane elastomers) as well as with the epoxy groups of matrix resins [ 11,121. Amino groups were also introduced on fibers by the direct reaction of the nitric-acid oxidized fiber surfaces with TEPA at 190-2OD”C (eqn (2)). This process grafts TEPA molecules to the surface via amide bonds. While high surface concentrations of carboxyl and hydroxyl groups might lead to higher chemical bonding densities from fibers to isocyanatederived polyurethane interphases or to epoxy matrix resins, the use of surface amine groups to promote higher fiber-to-elastomer interphase or fiber-tomatrix covalent chemical bond densities was also considered possible, and hence a worthwhile research topic [1,6,11,13]. TEPA has two primary and three secondary amine functions. Its reaction with carboxyl groups should
*Corresponding author. 33
334
C.
U.
PITTMAN.
JK et ul.
HNO, CFI
and
115 oc
0
+
CFt--c<
CFt-OH
OH
H 2 NANnNANnNH H H
OH
1 go-200
2
oc
H
TEPA
+ H
l
H
TEPA
H
-
CFtC/( N
CF
ideally introduce, four amines for every surfacebound carboxyl group consumed. This leveraging factor could be lower if more than one amine function per TEPA forms an amide bond at the surface (e.g. loop formation, see eqn (3) and ref. [I]). Why is a correction procedure necessary to determine surface-grafted TEPA? Accurate surface group quantitative analysis is required to define amine functional group leveraging effects and provide a clearer picture of the surface chemical transformations. Acid/base uptake was used to measure the basic and acidic functions on treated fibers [ 1,6,11]. In practice, fiber samples with grafted amino groups were submerged in aqueous HCl solutions for 3 hours. The pH values of these HCI solutions were measured before and after fiber submersion. The quantity of the surface-bound amino groups was then determined by the moles of HCl removed from solution, which was calculated from the change in pH value of the HCl solution. This method is quite accurate when the spacing between amino groups is adequate. However, the protonation of surface bound TEPA amino groups by HCl was not complete at pH values down to 2.7-2.9, which represent the highest acid concentrations which can be used for accurate HCl uptake experiments. Successive TEPA protonation becomes more difficult due to the build-up of positive charge repulsion. This is obvious upon examining the titration curve for TEPA in HCl or other mineral acids (see Fig. 1 as a typical example). Experiments have shown that the acid strength of an HCl solution with a pH 1 is not high enough to protonate all five amino groups of TEPA. The reason becomes apparent when examining eqn (4).
(3
(31
Similarly, complete protonation of all the amino groups on the fiber-bound TEPA (see eqns (2) and (3)) would require a low pH (high acidity). However, HCl solutions with such low pH values cannot be used for titrations of fiber surfaces. This limitation exists because the quantity of the surface-bound amino groups present on the amount of fibers which can be submerged in any unit volume of HCl is very small (usually 3 5 50 lteq g-‘). Therefore, the accuracy required in the measurement of the change in pH at very low pH values is impossible to achieve. For example. the change in pH that would occur by protonating surface-bound amines in a solution of pH 1 (in which the maximum possible amount of fiber was submerged) is far too small to measure with the requisite accuracy. Therefore, HCl concentrations no higher than those present at pH 2.8 should be used. At this concentration the complete protonation of TEPA (or bound TEPA) cannot be achieved. Therefore, a calibration procedure was required to correct the direct measurements of HCl uptake. Upon grafting TEPA onto a carbon fiber surface, one or two (if a loop structure forms) amino groups react to form amide bonds (eqns (2) and (3)) [ 1,6,11]. When these grafted TEPA functions are used to promote polyurethane elastomer interphase bonding to the fiber. TEPA’s amino groups react with isocyanate compounds in the preresin mixture [6,7.10, Ill. The developing polyurethane elastomer would become bonded to the fiber through urea functions, as shown in eqn (5). As amine functions are converted to urea groups (or amides at the site of bonding to the fiber surface), the pH value required for complete protonation of
Titration
of TEPA and its phenyl isocyanate
reaction
products
335
pH value
,3
TEPA: 1.471 g HCI: 1.009 N
12 -
8-
1st end polnt volume 22.6 mL
765-
At pH = 3, volume 31.8 mL
4-
Calculated end
321 0
10
5
15
20
25
30
35
40
45
50
Volume of HCI (ml) Fig. 1. Titration
H2
(TEPA)
NANANANANH +H+ H H H 2
+H+
-
of tetraethylenepentamine
-
nNmNnNn;H
H2N
-H+
,,’
lH
3
H
H2
-H+ K,
with aqueous
NnNpNnNnNH
(with
H
+nNnN7p+
HsN
-H+ K3
KZ
2
’ HH
H'
_+H+_
H
HCl solution.
H
H/
‘H
H
NH3
(4)
proton
reorganization)
+nN\NPN\+ HsN / \ / lH Hi lH H HH
NH3
K, andK2 >> K.,orK5
CF
j-!-N-km-i-h HHHH
B n,/n,n\,/n
.
OCN--NC0
R
N--C-N-NC0 n H
CF,-C-N
H
’
H
H
H
(5)
OCN-NC0
-N-N-N-NW!-N-NC0 H
Ox;
H
-
etc.
H
H
OCN-hH PhN==C=O
H 2 NnN-inNn H
NH1 H
-
d-!-N-N-NnN-NH HHHH
H
(model for rurfaca
2 bound TWA)
(6)
PhN==c==o -
MN!!-NnN-N-NnNH H H w
2 ’
+
PhNz-N-N-N-N-
q N-CNHPh
HHHHHH
.Hc\NHp”n + etc. PhN=C=O
PhN=C==O c
TRlSVBSTlTUTlON
.
TETRAS”BSTlT”T,clN
C. U. PITTMAN,JR PI 01.
336
the remaining amine sites rises. This occurs because TEPA moieties now have fewer amine functions left, and charge-charge repulsion is reduced when complete protonation occurs. This process was modeled by reacting TEPA with various amounts of phenyl isocyanate [ 111. One or more of TEPA’s amino groups reacted with phenyl isocyanate and urea groups were formed (see eqn (6)). The complete protonation of these partially substituted TEPA model compounds with HCl was studied to define just how they were different to HCl protonation from the original TEPA. By adjusting the TEPA/phenylisocyanate mole ratio, the amine-tourea group ratio in the model compound product mixture was varied. These model compounds were then titrated with aqueous HCI solution to examine their behavior. This paper deals with the preparation of these model compounds and their titration with aqueous HCl. From these results, a correction procedure was established to permit the accurate determination of the correct fiber surface amine stoichiometry which was present after TEPA grafting. Once the procedure was established, all surface-bound amino group determinations on TEPA-treated carbon fibers (conducted by HCl uptake) could be corrected by multiplying the experimental values by a correction factor to obtain the actual values of the Lleq gg’ of amino functions present on the fibers. This procedure was used in the preceding manuscript [ 1] and in other work where TEPA grafting to fiber surfaces has been employed to introduce amino functions [557,9-l I].
2. EXPERIMENTAL All chemicals used in this work were purchased from Aldrich Chemical Company, Inc. The TEPA was reported to be 98% pentamine by Aldrich. Model compounds were prepared by reacting TEPA with phenyl isocyanate (PhNCO). Reactions were performed at PhNCO/TEPA molar ratios of 0, 1. 2, 3, 4 and 5. The reaction product mixture prepared at PhNCO/TEPA = 1 was used to simulate the surface-bound TEPA molecules in which one amino group has reacted with a surface carboxyl group, resulting in an amide group. The individual product mixtures with the PhNCO/TEPA ratios of 2, 3, 4 and 5 simulated the other cases in which more than one of TEPA’s amino groups had reacted with surface carboxyl groups. These mixtures could also be used to simulate fiber-grafted TEPA after reaction of fibers with isocyanates. The analysis of residual TEPA amino groups remaining on carbon fiber surfaces after reaction with isocyanates is a necessary step in defining the chemical reaction efficiency of bound TEPA with developing polyurethane coatings. TEPA (1.511 g) was dissolved into 10 ml of methyl ether ketone (MEK). Then a weighed amount of phenyl isocyanate (1, 2, 3, 4 or 5 moles per mole of
TEPA, respectively) was added. Reactions were carried out at room temperature for two days under nitrogen protection. The temperature was subsequently raised to 70°C for an additional 15 hours to drive the reaction to completion. Water (30 ml) was added prior to titration so that the subsequent titration could be monitored by a pH meter. The reaction product solutions were titrated with a solution made by dissolving concentrated HCl (aqueous) into methyl ethyl ketone (final concentration in HCl was 1.051 M ). These titrations were monitored by a pH meter (Ion Analyzer 250, Corning). The pH values of the product solutions were plotted against the volume of HCl solution consumed.
3. RESULTS
AND DlSCUSSlON
3. I Titration of TEPA, ethylenediamine and diethylenetriamine with HCl As-received tetraethylenepentamine ( 1.471 g) was titrated with aqueous HCl (Fig. 1). The total volume of HCl solution ( 1.009 N) with the required equivalents to protonate all the five amino groups in this product was 1000 x (1.471/189.3) x 0.98 x S/1.009= 37.7 ml, where 189.3 equals the molecular weight of TEPA. At the first end-point (pH 6.5) 22.6ml of HCl was consumed. This 22.6 ml volume equals 3/S of the total volume (37.7 ml) of HCl solution consumed, indicating that three amino groups in each TEPA molecule were preferably protonated. These could be the two terminal groups and the one at the middle of the molecular chain, i.e. the first, third and fifth positions. A sharp end-point corresponding to 37.7 ml was not observed. For reference, two analogs of TEPA, ethylenediamine (EDA) and diethylenetriamine (DETA) were also titrated with aqueous HCl (Fig. 2). Two endpoints were observed for ethylenediamine. The first and second end points, designated as vl (EDA) and v2 (EDA) in Fig. 2, appeared at pH 8.6 and pH 4, respectively. The ratio of HCl volumes consumed at the first and second ethylenediamine end points was 1: 1. Similarly, diethylenetriamine also exhibited two end points, vl (DETA) and v2 (DETA). However, they appear at much lower pH values (pH 7 and pH 2.3, respectively). The ratio of HCl solution volumes consumed at the first and second end-points were 2:1, indicating that two amino groups were protonated at the first end-point. The total HCl volumes consumed at the second end-point for both ethylenediamine and diethylenetriamine equal the stoichiometrically calculated values. These results clearly indicate that (1) both the amino groups in ethylenediamine and all three amino groups in diethylenetriamine can be protonated by a 1 N HCl solution, (2) two amino groups (perhaps the terminal groups) in diethylenetriamine are preferably protonated as compared with the third (middle?) amino group, (3) as more amino groups are present in a molecule, the acidity for complete protonation
Titration
of TEPA and its phenyl isocyanate
reaction
products
337
pH value EDA: 1.0899 DETA: I.3489 TEPA: 1.471 g
8-
8-
0
10
20
30
Volume of HCI (mL) Fig. 2. Titration
of ethylenediamine (EDA), diethylenetriamine (DETA) and tetraethylenepentamine HCI solution. vr and vz stand for the first and second end-points.
with aqueous
shows six titration curves corresponding to pure TEPA and these five model compound mixtures. The volume of HCl (1.051 M) required to protonate all five amino groups in 1.511 g of TEPA (curve 0 in Fig. 3) was:
rises (e.g. lower pH required), and (4) the last amino group is much more difficult to protonate than those previously protonated. The change of end point pH values going from ethylenediamine (pH 4.0) to diethylenetriamine (pH 2.5) strongly suggests that TEPA’s end point would appear at very low pH (i.e. a very high concentration of HCl solution would be required).
V,=(1.511/189.3)xO.98
x5 x 1000/1.051=37.2m1
The equivalent number of protons transferred per mole of the model PhNCO/TEPA compounds can be determined. To obtain the volumes of HCl consumed in each titration experiment, the end-point of the titration curves must be determined. However, the end points of these curves are not easy to define exactly. In practice, the pH of the HCl solutions at acid uptake equilibrium was between 2.8 and 3.2.
3.2 Titration of model TEPA ureas synthesized
in TEPA phenyl isocyanate reactions The five each of the (PhNCO) to 5) were
(TEPA)
model compound mixtures synthesized in reactions of TEPA with phenyl isocyanate (using PhNCO/TEPA mole ratios from 1 individually titrated with HCl. Figure 3
pH value 14
TEPA: 1.511g HCI: 1.051N
10
20
30
40
Volume of HCI (ml) Fig. 3. Titration of tetraethylenepentamine (TEPA) and its reaction products solution of HCl. The numbers by the curves represent the PhNCO/TEPA
with phenyl isocyanate (PhNCO) with MEK mole ratio used to prepare these products.
C. U. PITTMAN,JR et al.
338
Therefore, the end-point for these titration curves was selected to be pH 3 for convenient comparison and data treatment. The derivation of the correction factors mentioned above is listed in Table 1. The values in row A of Table 1 are the average number of amino groups that would exist in each of the product mixtures obtained in the TEPA/PhNCO reactions, assuming that the addition of the amino N-H across the NC0 groups occurred quantitatively (i.e. yield= 100%). Values in row B are volumes (V,) of HCl solution consumed in titration at pH 3. Row C is the ratio of the HCl volume actually consumed (V,j during the titration of each product vs the volume theoretically required (I/,) by the protonation of all five amino groups. Then, the value of 5 x L’,,ivO (i.e. row D) equals the number of amino groups which were actually protonated by HCl. This is the average number of basic groups present in the individual reaction products obtained when TEPA was reacted with PhNCO at each mole ratio. However, for those reaction products in which the PhNCO/TEPA ratio is 3, 4 and 5, the number of amine groups determined (i.e. 2.145, 1.525 and 1.390 in row D) are higher than the stoichiometritally available number of basic groups (i.e., 2.0, 1.0 and 0 in row A). Obviously, this requires that another base was present. The other base is the fraction of phenyl isocyanate which remains unreacted. Not all of the phenyl isocyanate reacted at these three ratios (i.e. PhNCO/TEPA=3, 4 and 5). Thus, each equivalent of isocyanate remaining unreacted will consume an equivalent of HCl during titration. Also, each equivalent of amino groups remaining unreacted with PhNCO will consume one equivalent of HCI. Therefore, the amount of additional HCl consumed should be twice that of the unreacted phenyl isocyanate. The unreacted phenyl isocyanate and unreacted amino groups due to incomplete reaction with phenyl isocyanate are calculated and shown in rows E and F ofTable 1. Therefore, the quantity of amino groups actually titrated by HCl is the difference between the Table 1. Titration
of TEPA
and its reaction
products
quantity in row D minus the amount in row F. The values of these differences are listed in row G. The amino groups actually available for titration are the sum of those stoichiometrically calculated assuming 100% reaction of isocyanate (row A) and the amount in row F. These sums, listed in row H. are the total amino groups actually present in the TEPA/phenyl isocyanate reaction products. The correction factor K may now be simply defined as the ratio of amino groups that were actually present for titration divided by the amino groups that were actually detected (protonated) in that titration. These values are shown in row J of Table 1. 3.3 Correciion fhctor K The values of K (Table 1) show that protonation was complete at pH 3 when TEPA contained one or two underivatized amino groups per molecule (e.g. when these products were generated in reactions with PhNCO/TEPA mole ratios of 4 or 3). When three amino groups are present per TEPA, complete protonation does not occur at pH 3 and a small correction factor ( 1.113) must be applied. With four and five amino groups per TEPA, the required correction factors, as expected, increase to 1.212 and 1.221, respectively. Considering the conjugate acids in this series we see the dissociation constants will decrease in the following order: H,N + -C,H,-N+H,-C,H,-N -N+H,-C,H,-N+H,> H,N r -CzH4-N(R)-CZH4-N
+H2-C,H,
-N(R)-C,H,-N+H,> H,N + -C2H4-N(R)-C2H,-N(R)-C,H, -N(R)-C,H4-N+HS> H(R)N-CzH4-N(R)-C,H,-N
+H,-CZH,
-N(R)-C2HJ-NH(R) where R represents a PhNHCO function. Two key conclusions are obvious. First, the frac-
with various ratios correction factors
of phenyl
isocyanate:
Molar
(A) Basic groups available on TEPA or its reaction product (B) HCl volume consumed, V, (ml) (C) Ratio of VI/V0 (D) Equivalent number of amino groups (E) Unreacted PhNCO (F) Unreacted amino groups due to incomplete reaction with PhNCO (G) Amino groups actually detected (H ) Amino groups actually present (J ) Correction factor K Notes: V, was the volume required to (A) Stoichiometrically available number. D=C x 5. (E) Determined as follows: If G=D-F. (H) Calculated by H=A+F.
+H,-C,H,
calculation
procedures
and
ratio of PhNCOjTEPA
0
1
2
5.0 30.4 0.819 4.095 0 0 4.095 5.0 1.221
4.0 24.5 0.660 3.300 0 0 3.300 4.0 1.212
3.0
2.0
I.0
20.0 0.539 2.695 0 0 2.695 3.0 1.113
I15.9
11.3 0.305 I.525 0.263 0.263 1.262 I.262 1.000
3
0.429 2.145 0.073 0.073 2.072 2.072 1.ooo
4
5 0
10.3 0.278 1.390 0.695 0.695 0.695 0 695
protonate all five amino groups on pure TEPA. L’(, = 37.20 ml. (B) Experimental results from Fig. 3. (C) Calculated by B/Vo/,. (D) Calculated D-A<0 then E=O, else E=(D-A).12. (F) The same as (E). (G) Calculated (J) Calculated by J=H/G.
1.ooo by by
Titration
of TEPA and its phenyl isocyanate
tion of phenyl isocyanate converted to urea groups decreased as the PhNCO/TEPA mole ratio increased. Second, the successive protonation of amino groups on TEPA and its phenyl isocyanate derivatives becomes increasingly more difficult as more amino groups have been protonated. The formation of urea groups at adjacent amine sites slightly reduces the base strength of the remaining amine groups. This effect is far smaller than the reduction in base strength caused by successive protonation of adjacent amine groups. For example, if four amino groups on TEPA have been protonated, then the last amino group requires a very high acidity to be protonated. However, if four amino groups have been converted to urea functions by reaction with phenyl isocyanate, then the last remaining amino group can be protonated easily. The positive charge induced by each successive protonation step increasingly reduces the base strength of remaining amino groups. A plot of the correction factor (K) vs the number of amino groups present in TEPA and its substituted derivatives is shown in Fig. 4. When the number of amino groups in TEPA bound to fiber surfaces is known, K can be obtained directly from this plot. Most fiber-bound TEPA moieties would have either three or four amino groups. Those bound to the surface via a single amide function would have four amine sites, and those bound in a single loop (e.g. two amide bonds to the surface) would have three amine sites (see eqn (3)). Thus, the correction factor to be applied in titrations of TEPA-treated carbon fibers falls in the range 1.11-1.21. The quantity of fiber-bound amino groups directly measured by HCl uptake must, therefore, be multiplied by a correction factor to obtain a more accurate result. In titrations of pure TEPA in solution a correction factor of 1.22 must be multiplied by the equivalents of WC1 uptake. 3.4 Derivation of equations to calculate the fraction of surface-bound TEPA in loops while applying the appropriate correction factor The correction factor K calculated from Table 1 is applicable only when the number of amino groups 1.3
C :orrection factor
1.2
1.1
1
0.9
L
0
,
1
1
2
3
4
Number of amino groups present
Fig. 4. Correction factors for use with HCl uptake measurements of amino groups in tetraethylenepentamine (TEPA) molecules grafted to carbon fiber surfaces.
3
reaction
products
339
in TEPA molecules is known. However, when determining the quantity of amine groups present on TEPA-treated fiber surfaces, one problem remains. The true average number of amine functions present per bound TEPA molecule is unknown because the fraction of bound TEPA existing on the fiber surface as loop structures is not known. Therefore, a general method must be devised to calculate the mole fraction of the bound TEPA which is present as looped structures. The value of K which should be used to correct the titrations depends on the fraction of looped structures, but that fraction is unknown. Surface-group titration experiments, necessary to determine the fraction of loops, require application of the correct K value to provide the requisite accuracy in the surface amine stoichiometry present. Thus, two unknowns exist. When TEPA is tethered to surfaces (through the reaction of surface carboxyl groups with TEPA amine functions to generate amide links) other information becomes available. The quantity of carboxyl functions consumed in the reaction to anchor TEPA is available from NaOH uptake experiments performed both before and after the fiber reaction with TEPA. From such information, equations may be derived to calculate appropriately the fraction of TEPA present as loops. This derivation invokes two assumptions. First, acidic groups on the fiber surface that disappear during the reaction with TEPA have been converted to amide functions (see eqns (2) and (3)). Second, fiber-bound TEPA exists on the surface in two general forms: those bound through a single amide bond and those bound through two amide bonds (e.g. a looped structure). The fraction of double-looped structures is considered negligible. Looped structures have been previously observed when ethylene diamine was electrochemically reacted at carbon surfaces [ 17,181. Two values were used from the data in Table 1 (row D) determined by HCI uptake experiments at pH 3: (I) 3.300 amino groups were protonated when the bound TEPA had four amino groups, and (2) 2.695 amino groups were protonated when the bound TEPA had groups (looped structures). amino three Furthermore, the following symbols were used in the ensuing derivation. A is the quantity of surface carboxyl groups consumed in the reaction with TEPA (determined by NaOH uptake measurements), B is the apparent quantity of surface-bound amino groups introduced by grafting TEPA (this is equal to the quantity of HCl uptake before correction), T is the total quantity of TEPA molecules grafted onto the fiber surface,f, is the mole fraction of the grafted TEPA present in loops, B, is the true quantity of surface-bound amino groups after correction, and K is the correction factor used to multiply the experimentally measured quantity of surface-bound amino groups to obtain the true value. Based on the balance of amino groups before correction, we have:
2.695Tf=+ 3.300T( 1 -fT) =B
(7)
C. U. PITTMAN,JR~~ al.
340
Based on the mass balance correction, we have: 3TfT+4T( Based on the mass consumed, we have:
of amino
1 -fT)= balance
groups
B,=KB of carboxylic
2Tf;+T(l-f;)=A
after (8) groups (9)
Solving eqns (7) and (9) for f+ gives: (3.300-;j (IO) fT= (0.605+
prepared by the reaction of TEPA with phenyl isocyanate, where amine functions were progressively replaced by non-basic urea functions. A method was derived to use the correction-factor plots with other titration data to determine the correct total amine quantity on TEPA-treated fiber surfaces and the fraction of TEPA bonded in loops. Ackno,oledgemmts~-This research was supported in part by the National Science Foundation through Grants No. STI-8902064, EHR-9108767 and OSR-9452857. Support from the State of Mississippi and Mississippi State University is also gratefully acknowledged.
;) REFERENCES
Solving eqns (8) and (9) for K gives:
(11) Equation (10) permits the calculation of the fraction of surface TEPA bound as loop structures, and eqn (11) is used to give the appropriate value of the correction factor K. For example, consider the case where the quantity of carboxyl groups consumed during the TEPA reaction was determined to be 20 peq g-r from NaOH uptake experiments (before and after the TEPA reaction), and where the experimental HCl uptake after TEPA grafting was 50 peq g-‘. In other words, A =20 peq g-’ and B=50 peq g-r. Substitution in eqn ( 10) gives ,fT (loop fraction) =0.258. The correction factor K is then obtained from eqn ( 11), in which case K= 1.19.
5. 6.
7.
8. 4. CONCLUSIONS
When HCI uptake or titration data are used to measure the quantity of amino groups bound to fiber surfaces as grafted TEPA, the direct measurements could be lower than the true values. This will occur when the acidity of an HCl solution is not high enough to protonate all amino groups present in the grafted TEPA molecules. The acidity of the HCl solution used cannot simply be raised. The HCl concentration must be sufficiently low to provide a large enough change in pH during uptake to measure accurately the number of HCl equivalents being removed from solution upon protonation of surface amine groups. The pH of such solutions should not be lower than 2.9 for our samples. Therefore, a correction factor must be used to correct the direct measurements by HCI uptake. A correction procedure has been established in this work based on the HCI titrations of TEPA and model compounds
9.
Pittman, Jr, C. U., He, G. R., Wu, B. and Gardner, S. D., Carhon, 1992, 35, 317. Gardner, S. D., Pittman, Jr. C. U., J. Conzpositr Muter., 1995, 27(8), 830. Low, B. Y., Gardner, S. D., Pittman, Jr, C. U., Hackett, R. M., Composites Eng., 1995, 5(4), 375. Low, B. Y., Anderson, K. L., Vincent, M., Gardner, S. D., Pittman, Jr, C. U., Hackett, R. M., Composites Eng., 1995, 5(4), 437. Gardner, S. D., Singamsetty. C. S. K., Booth, G. L., He, G. R., Pittman, Jr, C. U., Carbon, 1995,33(5), 587. Pittman, Jr, C. U., He, G. R., Wu, B., Wang, L. and Gardner, S. D., Extended Abstracts qf the Fifth Internutional Conference on Composite Interjkces (ICCI-V). Moleculur Interactions and Structure. Giiteborg, Sweden, 1994, p. 19-2. Pittman. Jr, C. U., He, G. R., Wu, B., Wang, L. and Gardner, S. D., in Proceedings of the First Internationul Conference on Composite Engineering (ICCEII ), ed. D. Hui. New Orleans, LA, 1994, pp. 4033404. Wu, Z., Pittman, Jr, C. U., Gardner, S. D., Carhon, 1995, 33(5), 607. Wu, Z., Pittman, Jr, C. U., Gardner, S. D., Carbon, 1995,33(5),
597.
10. Pittman, Jr, C. U., Gardner, S. D., He, G., Wang, L., Wu. Z., Singamsetty, C., Wu, B. and Booth, G.. Mater. Res. Sot. S.ymp. Proc.,
1995, 385, 195S200.
fiber composites with elasto11. He, G. R., Continuous meric interphases covalently bonded to both fiber surfaces and epoxy matrices., Ph.D. dissertation, Mississippi State University, Mississippi State, MS, 1994. 12. Yosomiya, R., Fujisawd, T. and Morimoto. K., Po!vm. Bull., 1984, 12, 523. 13. Drzal. L. T., Sugiura, N. and Hook, D., in Proceedings of the International Conjkrencr on Composite Engineering (ICCE/3), ed. D. Hui, New Orleans, LA, 1996,
pp. 11-12. 14. Krin, J., Sci. Amer., 1996, 275(4), 74. B., Israelachvili, J. N. and Landman, U., IS. Bhushan, Nature. 1995, 374, 607. 16. Persson, B. N. J. and Tosatti, E. (eds), Physics qfSIiding Friction. Kluwer, Dordecht, 1996. Chem., 1984, 13, 270. 17. Murray, R. W., Elrctroanal. 18. Barbier, B., Pinson, .I., Desarmot, G. and Sanchez, M., J. Ektrochem. SC.. 1990, 137(6), 1757.