Morphology and Cation Local Structure in a
Blend of Copper-neutralized
Carboxy-terminatedPolybutadiene and
Poly(styrene-co-4-vinylpyridine)
RICHARD A. REGISTER, Department of Chemical Engineering,
University of Wisconsin, Madison, Wisconsin 53706, ASHISH SEN*
and ROBERT A. WEISS, Institute of Materials Science and
Department of Chemical Engineering, University of Connecticut,
Storrs, Connecticut 06268, and CHI LI and
STUART COOPER, Department of Chemical Engineering,
University of Wisconsin, Madison, Wisconsin 53706
*
Synopsis
Carboxy-terminated polybutadiene neutralized with Cu2+ (CuPBD) and its blend with
poly(styrene-co-4-vinylpyridine)(SW)were examined by differential scanning calorimetry (DSC),
transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS),and extended
x-ray absorption fine structure (EXAFS) spectroscopy. The DSC results indicate that in the
blend substantial mixing occurs in the CuPBD-rich phase, although complete miscibility is not
achieved, and the SVP-rich phase remains relatively pure. The TEM micrographs indicate that
the morphology, while irregular, is reasonably described as bicontinuous, with a domain size of
order 100 nm. The SAXS patterns show that the ionic aggregates present in CuPBD are
destroyed upon blending, which is interpreted as being due to steric hindrances between ionic
groups coordinated to vinylpyridine nitrogens. The EXAFS radial structu; e function of the blend
exhibits a marked decrease in the Cu-Cu peak in comparison with CuPBD, indicating a change in
local structure upon blending. The results indicate that some of the SVP is miscible with CuPBD
owing t o complexation between the pendant pyridine groups and the Cu2+ ions, which disrupts
the ionic aggregates. However, the two materials are not fully miscible, leading to a rather coarse
two-phase morphology.
INTRODUCTION
Recently there has been great interest in the preparation of miscible
polymer blends.' In general, however, high-molecular-weight homopolymers
are immiscible, owing to the small entropy gain in mixing long polymer chains
coupled with the usually positive enthalpy .of mixing two unlike substances.
One means to increase the miscibility of a polymer pair is to provide specific
interaction sites on the two homopolymer chains, such as hydrogen bond
donor and acceptor groups. These specific interactions then provide a negative
heat of mixing and increase the miscibility of the homopolymers. An even
stronger interaction may be obtained by employing groups which carry
*Present address: Texaco Research, P.O. Box 509, Beacon, New York 12508.
'Present address: Scientific Research Staff, Ford Motor Company, Dearborn, Michigan 48121.
*To whom correspondence should be addressed.
Journal of Polymer Science: Part B: Polymer Physics, Vol. 27, 1911-1925 (1989)
0 1989 John Wiley & Sons, Inc.
CCC 0887-6266/89/091911-15$04.00
1912
REGISTER ET AL.
positive and negative charges to form a Lewis acid-base pair in the blend.
Blends in which proton transfer occurs from an acidic to a basic moiety, such
as from a sulfonic acid to a basic nitrogen, have been studied by Eisenberg
et al.2-5 More recently, Lundberg et al.6-8 have studied blends of a sulfonated
ethylene-propylene-dene (SEPDM) rubber neutralized with a transition metal
(Zn2') and polystyrene copolymerized with 4-vinylpyridine (SVP). Electron
microscopy revealed distinct SPEDM and SVP domains in the blends, but the
dramatic increase in ultimate extensibility compared with analogous unfunctionalized blends indicated that coordination of the vinylpyridine nitrogens to
the Zn2+ cations provided good adhesion between the domains.
Two monomer units which are often combined are polystyrene and polybutadiene, especially through block or graft copolymerization. A t low butadiene
contents, these materials are similar to high-impact polystyrene, whereas
triblock polymers with low styrene contents are thermoplastic e1astomers.l
The random copolymer of styrene and butadiene has also been used for many
years as a component of the rubber stock in tires. The great range of
properties available in materials based on just these two monomer units
reflects the versatility of combining a monomer which gives a glassy homopolymer with one which gives an elastomeric homopolymer at service
temperatures. Because of the costs involved in synthesizing block or graft
copolymers, blending the homopolymer resins is an attractive route. To
provide good adhesion between the microdomains, a strong specific interaction
is needed, such as transition metal coordination.
The carboxylic acid group has been shown to be too weak to transfer a
proton to 4-vinylpyridine; only hydrogen bonds were formed, which were
insufficient to fully homogenize poly(styrene-co-methacrylic acid) and poly(ethylacrylate-co-4-vinylpyridine).z~3
In this investigation, a sample of carboxy-terminated polybutadiene, neutralized with copper (CuPBD), was
blended with a polystyrene containing a small fraction of 4-vinylpyridine
units (SVP). The Lewis basicity of the vinylpyridine nitrogen atom and the
acidity of the copper cation, as well as its coordination chemistry, suggest the
possibility of complex formation between the functional groups on the two
homopolymers. The materials were studied by differential scanning calorimetry (DSC) to evaluate the degree of miscibility, by transmission electron
microscopy (TEM), and small-angle x-ray scattering (SAXS) to determine the
size scale of phase separation, if any, and by extended x-ray absorption fine
structure (EXAFS) spectroscopy to ascertain whether the nitrogen atoms
coordinate to the copper ions. In the SEPDM/SVP case,6-8 formation of a
1: 1 N : Zn complex was postulated. The particular blend studied here contains 0.82 vinylpyridine nitrogens per carboxylate residue overall, or 1.64
nitrogens per copper ion. If full miscibility is achieved, then, there are
sufficient vinylpyridine nitrogens to coordinate to all the Cu2+ions at a 1: 1
stoichiometry. Some preliminary results on these materials have been presented recently.42
EXPERIMENTAL
Sample Preparation
The acid form of the carboxy-terminated polybutadiene (Scientific Polymer
Products, Inc.) had a number-average molecular weight of 4600 g/mol and a
MORPHOLOGY AND CATION LOCAL STRUCTURE
1913
polydispersity of 1.96 as determined by gel permeation chromatography (GPC).
Titration with alcoholic sodium hydroxide to the phenolphthalein endpoint
indicated a carboxyl group concentration of 0.52 meq/g po1ymer.l6 This
equates to a number-average functionality of 2.39 rather than the expected
value of 2, which suggests that the GPC molecular weights may be high.
Neutralization to the stoichiometric point (based on the titration results) was
accomplished by dissolving the polymer in toluene [20% solution (w/v)] and
refluxing with Cu(CH,COO), . H,O dissolved in 3 mL of water/methanol
(50 :50 v/v). The solvent was then removed under vacuum and the CuPBD
dried at 75-80°C under vacuum for 24 h. The styrene-vinylpyridine copolymer was prepared by free-radical copolymerization following the method of
Lundberg et al.6 and was found to contain 3.42 mol% vinylpyridine by
elemental analysis for nitrogen. The polystyrene-equivalent number-average
molecular weight and polydispersity were found to be 142,000 and 2.04, using
a Waters 510 GPC and tetrahydrofuran solution at 25°C.
The blend was prepared by dissolving the SVP in tetrahydrofuran and the
CuPBD in toluene/tetrahydrofuran (50 :50 v/v) and mixing the two solutions
to give a solute ratio of 56.2:43.8 SVP/CuPBD (w/w). The solution was
refluxed for 24 h. The solvent was then removed under reduced pressure. The
blend was dried under vacuum at 80°C for 24 h and then compression-molded
into a disk at 188OC and 11 MPa and allowed to cool gradually to room
temperature over a period of 2 h. The blend appeared dark brown and rigid,
whereas the pure CuPBD alone was a highly viscous, dark brownish-green
liquid. The materials were stored in a desiccator over CaSO,.
DSC Procedures
DSC thermograms were collected over the range - 120-220°C on a PerkinElmer DSC-I1 interfaced to a TADS data station. The heating rate was
20°C/min; indium and mercury were used as calibration standards. The
sample weights used were 13 k 3 mg. The materials were heated to 220"C,
then quenched at 320°C/min to -12O"C, to eliminate the interference of
water vaporization with the SVP glass transition and spurious transitions
owing to flow of the samples in the DSC pans. The samples were then heated
again under the same conditions to give the traces shown in Figure 1. No
apparent change in the positions or breadths of the transitions occurred owing
to the heating and quenching. Glass transition temperatures are given as the
transition midpoints, whereas breadths were computed using lines tangent to
the curve on either side of the transition and to the point of inflection.
TEM Procedures
Samples of the blend for observation with a high-voltage transmission
electron microscope (TEM) were prepared by the procedure of Li et al.,9
sectioning with a Reichert Ultracut-E microtome equipped with a Reichert
FC-4 cryostage. Freshly prepared glass knives were used in dry-knife sectioning at -80°C. Optical reflection indicated that the sections were approximately 200 nm in thickness. The sections were transferred to 500 mesh Au
grids, rinsed with distilled water, dried under ambient conditions, and stored
under vacuum at room temperature. Because of the low mass density of
copper in the blend (0.71% by weight), no domain structure was observable in
REGISTER ET AL.
1914
-150
-100
-50
0
50
100
150
200
Temperature ("C)
Fig. 1. DSC data for CuPBD (upper trace), SVP (middle trace), and blend (lower trace).
the as-prepared sections. The contrast was enhanced by selectively staining
the PBD-rich domains with dry vapor from OsO, crystals using 2 h of
expure.
The uncoated sections were examined with an AEI EM7 Mk l l High
Voltage Electron Microscope at the NIH Integrated Microscopy Resource at
the University of Wisconsin-Madison,using a 1.0 MeV accelerating potential.
Bright field images were recorded on Kodak SO-163 photographic film,using a
10 pm objective aperture, at a magnification of 40,000X . To minimize the
electron dose, a video camera with digital image enhancement was used for all
sample viewing except photographic exposure. Two seconds of exposure were
used to record the photographic image, at critical focus, resulting in an
electron dosagelo of approximately 3000 electrons/nm2. No visible alterations
in the morphology were observed throughout the process. A complete description of the microscope and video system may be found elsewhere."
SAXS MEASUREMENTS
The small-angle x-ray scattering (SAXS)experiments were performed with
an Elliot GX-21 rotating anode x-ray generator operated with a copper target
at 30 kV accelerating potential and 15 mA emission current. CuK, x-rays were
selected by filtering with nickel foil and by pulse-height analysis at the
detector. An Anton-Paar compact Kratky camera was used to collimate the
x-rays into a line measuring 0.75 cm by 100 pm at the sample. The scattered
x-rays were detected with a TEC 211 linear position sensitive detector,
positioned at a sample-to-detector distance of 60 cm for a q range of 0.16-5.8
nm-' [ q = (4a/X)sin8, where 28 is the scattering angle]. The CuPBD, a
viscous liquid, was contained between polyimide windows. The data were
corrected for detector sensitivity, window and empty beam scattering, and
transmittance. Because of the low scattering power of these samples, a
moderate amount of cubic spline smoothing was first applied to the data,
which were then desmeared by the iterative method of Lake" and placed on
an absolute intensity scale by comparison with a calibrated Lupolen polyethylene standard.12
MORPHOLOGY AND CATION LOCAL STRUCTURE
1915
TABLE 1
DSC Results
Sample
T,("C)
Breadth ("C)
CuPBD
SVP
Blend
- 77
10
7
102
- 52
102
24
10
EXAFS MEASUREMENTS
The transmission EXAFS spectra were collected in the vicinity of the
copper K edge (8978.9 eV in metal) on the C-1 station of the Cornell High
Energy Synchrotron Source (CHESS). The CuPBD sample was placed between polyimide windows to provide support. Data reduction followed a
standard procedure of preedge and postedge background removal, extraction
of the EXAFS oscillations x( k), and Fourier transformation-backtransformation to isolate the EXAFS contribution from a selected region in real space.13-15
Cu(CH,COO,), . H20 (Baker, ACS Standard) was chosen as a model compound for this study, and the specimen was prepared by spreading layers of
powder on pieces of polyimide tape and stacking the pieces to obtain a
reasonably homogeneous absorbance across the film.
RESULTS AND DISCUSSION
DSC Results
The DSC thermograms of the quenched samples are shown in Figure 1, and
the transition temperatures and breadths are listed in Table I. The two blend
components each poersess a single sharp glass transition, at -77°C for the
CuPBD and 102°C for the SVP. The blend possesses a very broad transition
centered at - 52OC as well as a sharp transition at 102OC. This indicates that
full miscibility has not been achieved and that the blend contains essentially
pure SVP-rich domains. On the other hand, the elevation and broadening of
the low-temperature transition indicates that the PBD-rich domains contain a
substantial fraction of SVP. Part of the broadening may also be due to a
broad interfacial zone; 17*18 however, the insensitivity of the high-temperature
transition to blending indicate3 that the composition profiles across the
interface would have to be strongly asymmetric.
One explanation consistent with the DSC results is that, upon blending,
some of the S W chains blend with the CuPBD owing to coordination of the
Cu2+ ions by vinylpyridine nitrogens. This elevates the glass transition
temperature of the PBD-rich phase. However, the full amount of the SVP is
not miscible with the CuPBD, so domains of relatively pure SVP remain in
the blend. The telechelic character of the CuPBD may also allow for the
formation of grafbtype structures, which could have a higher degree of phase
separation than a blend of two random copolymers.
1916
REGISTER E T AL.
To estimate the composition of the PBD-rich phase, we employed the Fox
equation: l 9
where Tg is the glass transition temperature of the PBD-rich phase (221 K),
Tgl and Tg2 are the glass transition temperatures of the pure CuPBD and
SVP, respectively (196 and 375 K),and w, is the weight fraction of SVP in the
mixed phase. This equation may be arranged as
which, with the numbers given above, yields w, = 0.237. Thus, the PBD-rich
phase is roughly 76 :24 CuPBD/SVP by weight, indicating that the mixing of
SVP in CuPBD is indeed substantial.
The domain continuity in multiphase polymers is often determined by the
phases’ volume fractions.’ Denoting the PBD-rich phase “A” and the SVP-rich
phase “B”, the mass fractions of the phases are
mA = ml-
1
1 - w,
--1 - m ,
(3)
where m, is the overall mass fraction of CuPBD in the blend, equal to 0.438
from the blend preparation. This yields mA = 0.574. The pure-component
densities are roughly 0.907 g/cm3 for CuPBD (unneutralized carboxyterminated PBD2’) and 1.05 g/cm3 for SVP (typical value for polystyrene21);
assuming no volume change of mixing, this gives a density for the PBD-rich
phase of 0.937 g/cm3, whereas the SVP-rich phase, being essentially pure, is
assumed to retain its pure-component density. From the density and mA, m B
values, the volume fractions.of the PBD-rich phase is calculated to be
0, = 0.61. For comparison, a 39:61 composition in a styrene-butadiene diblock copolymer gives a morphology consisting of alternating polystyrene and
polybutadiene lamellae.n This suggests that the morphology in the present
case is likely to have cocontinuous A and B phases, although a morphology
consisting of dispersed A (polystyrene-rich) phases in a B (polybutadiene-rich)
matrix is also possible. The composition of the mixed phase determined above
equates to a N :Cu ratio of 0.40 :1, which suggests that not all Cu2+ions can
coordinate to vinylpyridine nitrogens. This will be shown to be consistent
with the EXAFS results discussed below.
TEM Results
A thin section (200 nm) of the blend was examined by high-voltage transmission electron microscopy (TEM), as shown in Figure 2. Roughly circular
light and dark regions of order 100 nm in size can be seen, both of which
appear to poeseas some interconnectivity.The dark regions represent PBD-rich
MORPHOLOGY AND CATION LOCAL STRUCTURE
1917
Fig. 2. Transmission electron micrograph of thin section of blend. The white spot containing
the arrowhead is a hole in the specimen,which aids in focusing. The other light domains are rich
in S W ,whereas the dark domains are rich in CuPBD, which has been selectively stained with
oso,.
areas that have been selectively stained with OsO,, whereas the light regions
represent the SW-rich domains. The SW-rich domains appear uniformly
white, in agreement with the DSC results that indicate that this phase is
virtually pure SW. The remainder of the material exhibits a mottled texture,
which is likely to be due both to d overlapping domains of and to
inhomogeneity in the PBD-rich phase. The unusually broad low-temperature
glass transition observed by DSC for the blend suggests that the PBD-rich
domains span a range of compositions, which appears to be c o b e d by the
REGISTER ET AL.
1918
20.
-CUPBD
______ Blend
n
01
I
E
15.-
A
Y
01
‘
0 10.-
Q
(--‘I
Fig. 3. SAXS patterns for CuPBD (-)
and blend (---).
micrograph in Figure 2. Because the morphology is irregular, it is difficult to
conclude whether both phases are truly continuous, but this description seems
most reasonable.
SAXS Results
SAXS patterns for the CuPBD and the blend are shown in Figure 3. In the
CuPBD, the scattering is due to contrast between ionic aggregates and the
polybutadiene matrix,=^^^ which gives rise to the so-called “ionomer peak”; in
this material, the “peak” is only visible as a shoulder on the low-q upturn in
scattered intensity. The most interesting feature of Figure 3 is the absence of
the ionomer peak in the blend SAXS pattern. This suggests that, upon
blending, the ionic aggregates are destroyed. If coordination between the
vinylpyridine nitrogens and the copper ions is occurring, steric hindrance
between the ionic groups could prevent the formation of ionic aggregates2
This would yield a PBD-rich phase containing no ionic aggregates but with
substantial dissolved SVP,in agreement with the DSC results. In the case of
sulfonated polystyrene ionomers, rheological measurements25have shown that
trialkyl ammonium counterions effectively suppress ionic interactions, presumably due to steric hindrance. Coordination to an entire polymer chain
through a vinylpyridine group should have a similar effect.
The low-q upturns evident in Figure 3 may well have different origins in
the pure CuPBD and the blend. A recent investigation by anomalous smallangle x-ray scattering suggests that, for ionomers, the upturn is due to an
this source of
inhomogenous distribution of dissolved ionic g r o ~ p sAlthough
.~
scattering could also be present in the blend, the coarse morphology shown by
TEM in Figure 2 will also give rise to scattering. Immiscible polyurethanepoly(methylmethacry1ate) blends formed by interstitial polymerization of
methylmethacrylate exhibit TEM
similar to Figure 2. Scattering from these materialsa was found to be well-described by the Debye-
MORPHOLOGY AND CATION LOCAL STRUCTURE
“t
g‘“i.
1919
X
3C =-O
X
X
W
X
N
0 1.5
“ 1
X
K
X
0
0
0
0
0
0
R
0
ua0.5
X
0
W
-0.0
0.0
0.1
0.2
0.3
q2 (nm-2)
0 d
Fig. 4. Debye plots of data in Figure 3, background subtracted, CuPBD ( 0 )and blend ( X ) .
Bueche random two-phase model with correlation lengths of order 100 nm,
in agreement with the sizes measured from the micrographs. The
Debye-Buechez8-29model is described by
where I, is the intensity scattered by a single electron, V is the scattering
volume, Ap is the electron density difference between the phases, f l is the
volume fraction of one of the phases, and c is the correlation length, which
describes the average size of the phases. According to the above equation, a
plot of I - l I 2 versus q 2 (Debye plot) should give a straight line. Debye plots of
the unsmoothed desmeared SAXS data, after background subtraction, are
shown in Figure 4. The background, owing to thermal fluctuations and
wide-angle scattering, was subtracted by fitting the high-q portions of the
data to Porod’s Lawz6 plus a Vonk-like background termz7 of the form
1, = a + bqz. The Debye plots in Figure 4 exhibit substantial curvature, even
a t the lowest attainable values of q, indicating that the Debye-Bueche model
is not applicable here despite the superficial similarity of the CuPBD-SVP
blend to the polyurethane-poly(methylmethacry1ate) blend. Figure 4 also
shows that the upturn is more intense in the blend than in the pure CuPBD.
EXAFS Results
The application of EXAFS to i ~ n o m e r s ~and
~ - the
~ ~ methods of data
analysis have been described previously by several investigators. The absorption modulation which is the EXAFS signal arises because photoelectrons
REGISTER ET AL.
1920
which are ejected by the absorbed x-rays can be backscattered by atoms
coordinated to the absorbing atom; superposition of the outgoing and
backscattered electron waves gives rise to an interference pattern. The
,
k is the photoelectron wavevector, contains
EXAFS signal ~ ( k )where
information on the type of atoms in coordination shell j , the distance R j to
this shell, and the static and vibrational disorder of the shell, measured as the
Debye-Waller factor uj. The number of atoms in shell j is contained in the
amplitude Aj, which is equal to the number of atoms in the shell reduced by a
factor resulting from inelastic scattering. For materials with similar coordination structure, the A j values are proportional to the number of atoms in the
shell.
To convert the EXAFS signal from wavevector to real space, it is Fouriertransformed; the magnitude of the transform is termed the radial structure
function, or RSF. Each nonartifactual peak in the RSF represents a distinct
coordination shell; the peak positions are shifted slightly from the true shell
distances by a phase shift +j that the photoelectron experiences in backscattering. The EXAFS data were analyzed with single-electron single-scattering
theory: l4
A.
+ 4(k)]exp( -2k;uf)
x ( k ) = i ---LF.(k)sin[2kRj
kRg '
where k is the wavevector magnitude defined as k = (27r/h)[2rn( E - Eo)]1/2,
where E is the incident x-ray energy, h is Planck's constant, and rn is the
mass of an electron. E, is approximately equal to the edge energy but is
allowed to vary slightly to provide the best fit to the data and to correct for
any errors in energy calibrati~n.~'The shift is given below by the parameter
AE,, where E , = E, - AE,, and E, is the experimental edge energy. F,(k)
and c#.~~(k)
are the backscattering amplitude and phase-shift functions, respectively, which are characteristic of the types of atoms in shell j and the
absorbing atom. The calculations of Teo and Lee3' were used throughout for
these functions, except for slight modifications to accommodate experimental
data from model compounds studied previously, as discussed by Pan.%
Copper acetate monohydrate (abbreviated CuAc, below) was chosen as a
model compound for this study, based on the previous results of Galland
et al.,33 who found by EXAFS that the coordination structure for CuPBD
prepared by neutralizing with copper isopropoxide under anhydrous conditions strongly resembled that for CuAc,. The crystal structure3' of CuAc,
1 oxygen
shows that the Cu atom is coordinated to 4ooxygenatoms at 1.97
atom a t 2.20 A, and 1 copper atom at 2.64 A.
The EXAFS data for CuAc, are shown in Figure 5. The RSF (Fig. 5b)
shoys several nonartifactual peaks; those which appear at distances less than
1.5 A are artifacts of imperfect background subtraction. Note that the RSF
appears very similar to that dete+ned previously by Galland et al.33 for
CuAc,. The fi9t shell, at about 1.6 A in the RSF, arises from the 4 oxygen
atoms at 1.97 A. The difference in these distance values is due to the phase
shift discussed above. The second peak, near 2.1 A, arises from the Cu atom at
2.64 A. There is some question as to where the oxygen at 2.20 A should
A,
MORPHOLOGY AND CATION LOCAL STRUCTURE
-0.0
1-
2.0 4.0
6.0
80
k
1921
10.0 12.0 14.0
(:-I)
CuAc,. HIO
c
2
0.0
2.0
4.0
6.0
8.0
(1)
Fig. 5. EXAFS data for CIA$. (a) k3x versw k,(b) radial structure function.
Distance
appear, and therefore the structure is rather difficult to model. In principle, it
should be possible to regress three shells simultaneously to describe the
structure; however, the number of degrees of freedom (DF) inherent in the
Fourier transform-backtransform procedure i d 5
DF
2
-(AR)f(Ak),
B
A
where (AR), is the width of the RSF that is backtransformed (1.2 here),
and (Ak), is the width of usable experimental k data (9.0 A-l). There are
therefore approximately seven degrees of freedom. Ten parameters would be
necessary to describe the structure (one Aj, Rj, and uj for each of the three
shells, plus one AEo), which exceeds the number of degrees of freedom. To
obtain a rough estimate of the structural parameters, these two peaks were
modeled as being one shell of 0 and one shell of Cu atoms, which only involves
seven degrees of freedom. The distances listed in Table I1 correspond well to
the crystallographic distances, but the amplitudes must be viewed with some
skepticism because of the “missing oxygen” discussed above. The fitting
results are shown in Figure 6; in Figure 6a, the circles represent the backtransform of the first and second peaks in the RSF, or “filtered” k3x data, whereas
the solid line represents the fit. In Figure 6b, the RSF formed by transforming
the k3x data is shown as the dashed line, whereas the solid line is the
transform of the filtered k3x data. The fits are reasonably good in both cases,
although a high-distance shoulder is predicted in the calculated RSF which is
not observed in the data. This may again be a result of simplifying the
structure in the analysis.
The EXAFS data, k3x versus k , is shown in Figure 7 for CuAc,, CuPBD,
and the blend. The k3x versus k data is similar for all three samples,
supporting the idea that the local structure about the Cu2+ion resembles that
in CuAc,. This similarity is carried over directly into the RSFs, as shown in
Figure 8. Note that the CuPBD RSF strongly resembles that measured by
Galland et al.= for CuPBD prepared under anhydrous conditions. This in
REGISTER ET AL.
1922
TABLE I1
EXAFS Structural Parameters
Shell
number
Sample
CUO
CdC,
CuPBD
Blend
(A
Shell
element
AE,,(eV)
R j (A)
Aj
0
0
29.6
23.2
0
25.1
0
24.6
1.95
1.95
2.63
1.94
2.67
1.95
2.53
1.64
2.58
0.50
2.20
0.36
1.93
0.30
1
1
2
1
2
1
2
cu
cu
cu
uj (A)
Q(W)'
0.063
8.8
5.1b
0.080
0.072
0.062
0.07=
0.063
0.07'
l0.8b
7.6b
aThe quality of fit Q is defined as the square root of the ratio of the s u m of the squares of the
residue to the sum of the squares of the data.
bAgreement factor of a two-shell fit.
'Held fixed during regression.
itself is of some interest, since the neutralization procedures differ widely,
with the material here being exposed to a significant amount of water. Vlaic
et al.= have shown that the neutralization conditions can plan an important
role in deterknining the local structure. In the case considered here, different
neutralization methods appear to give identical local structures.
The structural parameters obtained from the analysis are listed in Table 11.
The consistency in the value of A, between CuAc,, CuPBD, and the blend
indicates that all contain a first coordination shell composed of oxygen atoms
at a distance of 1.95 A. The parameters for the second shell vary somewhat,
and for CuPBD and the blend it was necessary to fix the value of a, to 0.07
(comparable to that in CuAc,) in order for the regression routine to converge.
This problem occurs because the relatively small second peak is in close
proximity in the RSF to a strong Grst peak. Upon blending the CuPBD with
S W ,there appears to be some diminution of the second RSF peak, which for
A
-0.01-
2.0
4.0
6.0
80
k
.P
U
i';l
10.0 12.0 14.0
(k-')
12
8'
c
2
G
o
0.0'
... .--.- _--.. __
I-.
2.0
4.0
Distance
(A)
6.0
8.0
Fig. 6. EXAFS model fitting for &A$. (a) Circles are the backtransform of the first and
second peaks in the RSF, solid line is the fit; (b) dashed line is the FSF, solid line is the fit.
MORPHOLOGY AND CATION LOCAL STRUCTURE
1923
15.0 y
10.0 5.0 X
n
Y
0.0 7
-5.0 - 1 0.0
2.0
6.0
k
10.0
(H-')
14.0
Fig. 7. EXAFS data for CIA$, CuPBD, and the blend, k3x versus K. The curve for CuPBD
has been shifted upward 4 units, while that for the blend has been shifted upward 9.5 units, for
clarity.
the blend appears only as a shoulder. This is reflected in the lower value of A,
for the blend and is consistent with the mixing observed by DSC and SAXS,
which would necessarily involve the disruption of some of the Cu-Cu distances
to provide for the specific interactions between N and Cu2+.The decrease in
R, (Cu-Cu distance) in the blend likely indicates that the second shell peak is
actually quite d,
and the parameters listed contain a significant contribution from the adjacent h t shell. Even though the change in amplitude A, is
within the often-stated 20% error of the e x p e r i ~ n e n t , ~the
. ~ ~qualitative
change in the RSF and the large change in the calculated value of R, strongly
suggest that a structural change has taken place.
C
0
E
0
150
C
3
LL
0
0.0
2.0
4.0
Distance
6.0
(I)
8.0
Fig. 8. EXAFS data for CuA$, CuPBD, and the blend, RSFs. The curve for CuPBD has been
shifted upward 35 units, whereas that for the blend has been shifted upward 70 units, for clarity.
1924
REGISTER ET AL.
The EXAFS data do not directly show the formation of a Cu-N complex in
the blend. A shell of N atoms would be difficult to observe in the blend, for
two reasons. First, as discussed above, the CuAc, structure is rather complicated and actually consists of three shells rather than just the two modeled
here. If a small shell of oxygen atoms can be omitted from the data for the
crystalline compound and a good fit still obtained, then even if some nitrogen
atoms were coordinated to Cu2+ ions in the blend, it is unlikely that they
could be detected based on the quality of fit. Second, since 0 and N differ by
only one atomic number, their backscattering amplitude and phase functions
are quite similar, and it is difficult to distinguish between the two with
EXAFS. Therefore, it is quite possible that a shell of N atoms could be
satisfactorily modeled as a shell of 0 atoms, with slightly different parameters,especially given the additional complications d i s c 4 above. Therefore,
EXAFS cannot be used to definitively conclude that complexation is occurring; however, the data are consistent with this interpretation.
CONCLUSIONS
Samples of CuPBD and its blend with SVP were examined by DSC, TEM,
SAXS, and EXAFS. The TEM and SAXS results indicate that the blend is
not fully miscible and that the morphology consists of irregular domains
roughly 100 nm in size, best described as cocontinuous. The polystyrene phase
remains relatively pure, whereas the elevation and broadening of the PBD
glass transition observed by DSC indicates that substantial mixing is occurring in the PBD phase and interfacial regions. Although an “ionomer peak” is
visible in the SAXS pattern for CuPBD, no such peak can be observed for the
blend, suggesting that upon blending the ionic aggregates are disrupted by
shric interactions between cations coordinated to vinylpyridine nitrogens.
The EXAFS data from the blend indicate that the well-defined Cu-Cu
distance in CuPBD is disordered upon blending, which is also consistent with
the complexation of Cu2+ions by vinylpyridine nitrogen atoms. However, the
large number of shells present makes it impossible to observe a distinct Cu-N
shell in the blend EXAFS data. I t was also found that the coordination
structure of the CuPBD determined by EXAFS was essentially identical to
that for a material prepared by Galland et al. Under anhydrous conditions,
indicating that for this material the neutralization procedure does not have a
strong effect on the local structure.
R. A. R. thanks the staff of the Cornell High Energy Synchrotron Source (CHESS) of the
Wilson Synchrotron Laboratory at Cornell University for the opportunity to carry out the
EXAFS experiments and thanks the Fannie and John Hertz Foundation for financial support
while this work was conducted. C. L. thanks Drs. J. B. Pawley and P. H. Cooke of the NIH
Integrated Microscopy Resource at the University of Wisconsin-Madison for microscope access
and training. The assistance of P. A. Thompson in collecting the EXAFS data and R. R.
Schumacher and R. A. Phillips in acquiring the DSC data is gratefully acknowledged. It is a
pleasure to acknowledgediscusrJionswith Drs. G. Vlaic and C. E. Williams regarding EXAFS data
analysis and the importance of the neutralization procedure. Partial support of this rediearch was
provided by the Polymers Section of the Division of Materials Reaearch of the National Science
Foundation through granta DMR86-03839 (S. L. C.) and DMR-84-07098 (R. A. W.) and by the
Petroleum Reaearch Fund, administered by the American Chemical Society, through grant
19205-AC7(R. A. W.).
MORPHOLOGY AND CATION LOCAL STRUCTURE
1925
References
1. J. A. Manson and L. H. Sperling, Polymer Blends and Composites, Plenum, New York,
1976.
2. A. Eisenberg, P. Smith, and Z.-L. Zhou, Polym. Eng. Sci., 22, 1117 (1982).
3. P. Smith and A. Eisenberg, J. Polym. Sci. Polym. Lett. Ed., 21 223 (1983).
4. Z.-L. Zhou and A. Eisenberg, J. Polym. Sci. Polym. Lett. Ed., 21, 595 (1983).
5. A. Natansohn and A. Eisenberg, Macromolecules,20, 323 (1987).
6. R. D. Lundberg, D. G. Peiffer, and R. R. Phillips. US. Patent 4,480,063, Oct. 30, 1984, to
Exxon Research and Engineering.
7. D. G. Peiffer, I. Duvdevani, P. K. Agarwal, and R. D. Lundberg, J. Polym. Sci. Polym. Lett.
Ed., 24, 581 (1986).
8. P. K . Agarwal, I. Duvdevani, D. G. Peiffer, and R. D. Lundberg, J . Polym. Set. Polym.
Phys. Ed., 26, 839 (1987).
9. C. Li,S. L. Goodman, R. M. Albrecht, and S. L. Cooper, Macromolecules, 21, 2367 (1988).
10. J. B. Pawley, Ultramicroscopy, 13, 387 (1984).
11. J. A. Lake, Actu CrystauOgr., 23, 191 (1967).
12. I. Pilz and 0. Kratky, J. Colloid Interface Sci., 24, 211 (1967).
13. D. E. Sayers, E. A. Stem, and F. W. Lytle, Phys. Rev. Lett., 27, 1024 (1971).
14. E. A. Stem, D. E. Sayers, and F. W. Lytle, Phys. Reo. B, 11,4836 (1975).
15. P. A. Lee, P. H. Citrin, P. Eisenberger, and B. M. Kincaid, Rev. Mod. Phys., 53, 769 (1981).
16. A. Sen, R. A. Weiss, J. Oh, and H. A. Frank, Polym. Prepr. (ACS Diu.Polym. Chem.), 28,
220 (1987).
17. T. Hashimoto, Y. Tsukahara, K. Tachi, and H. Kawai, Macromolecules,16,648 (1983).
18. F. Annighofer and W. Gronski, Colloid Polym. Sci., 261, 15 (1983).
19. T. G. Fox, Bull. Am. Phys. SOC.,1, 123 (1956).
20. Scientific Polymer Products, Ontario, New York, 1987/8 catalog.
21. J. F. Rudd, in Polymer Handbook, 2nd ed., J. Brandrup, and E. H. Immergut, Eds., John
Wiley & Sons, New York, 1975, p. V-59.
22. J. M. G. Cowie, in Developments in Block Copolymers, I. Goodman, Ed., Applied Science,
London, 1982.
23. J. Ledent, F. Fontaine, H. Reynaers, and R. JCCme, Polym. Bull.,14, 461 (1985).
24. C. E. Williams, T. P. Russell, R. JkrCme, and J. Homon, Macromolecules, 19, 2877 (1986).
25. R. A. We&, P. K. Agarwal, and R. D. Lundberg, J. Appl. Polym. Sci., 29,2719 (1984).
26. G. Porod, Kolloid-Z., 124, 83 (1951)
6, 81 (1973).
27. C. G. Vonk, J. Appl. Cry~t.,
28. P. Debye, and A. M. Bueche, J. Appl. Phys., 20, 518 (1949).
29. P. Debye, H. R. Anderson, and H. Brumberger, J. Appl. Phys., 28,679 (1957).
30. Y. S. Ding, S. R. Hubbard, K. 0. Hodgson, R. A. Register, and S. L. Cooper, Macromolecules, 21, 1698 (1988).
31. G. Allen, M. J. Bowden, D. J. Blundell, G. M. Jeffs, J. Vyvoda, and T. White, Polymer, 14,
604 (1973).
32. D. J. Blundell, G. W. Longman, G. D. Wignall, and M. J. Bowden, Polymer, 15, 33 (1974).
33. D. Galland, M. Belakhovsky, F. Medrignac, M. P i n k , G. Vlaic, and R. JCCme, Polymer,
27, 883 (1986).
34. D. J. Yarusso, Y. S. Ding, H.K. Pan, and S. L. Cooper,J.Polym. Sci. Polym. Phys. Ed., 22,
2073 (1984).
35. H. K. Pan, D. J. Yarusso, G. S. Knapp, M. P i n k , A. Meagher, J. M. D. Coey, and S. L.
Cooper, J. Chem.Phys., 79, 4736 (1983).
36. G. Vlaic, C. E. Williams, R. JzMrne, M. R. Tant, and G. L. Wilkes, Polymer, 29,173 (1988).
37. B. K. Teo, and P. A. Lee, J. Am. C h Soc., 101,2815 (1979).
38. H. K. Pan, Ph.D. The& University of Wisconain-Madison, 1983.
39. J. N. Van Niekerk and F. R. L. Sch&g,
Actu CrystaUOgr.., 6, 227 (1953).
40. B. Lengler and P. Eisenberger, Phys. Reo. B, 22,4507 (1980).
41. E. A. Stern and K.Kim, Phys. Reu. B, 23, 781 (1981).
42. A. Sen and R. A. Weise, Am. Chem Soc. Div. Polym. Mat. Sci. Eng. Bepr., 68,981 (1988).
Received September 29,1988
Accepted January 27,1989