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Metastable Reverse Globular Micelles and Giant Micellar Wires from Block Copolymers.

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[4] M. A. Paver, C. A. Russell, D. S. Wright, Angex. Chem. 1995, 107. 1077;
Angew. Chem. Int. Ed. Engl. 1995. 34. 1545.
I51 X-ray data for 2: C,,,H,,,H,,O,,Sb,,.
F, = 2395.87, space group rhombohedral R5 (no. 148), a = 27.704(8), r = 18.281(6) A, V = 12151(6)A'. 2 = 3.
L = 0.71073 A. p,,,, =1.670 Mgm--'. ~ ( M O =~ 2.035
J
mm-'. Data were COIlected on a Siemens-Stoe AED diffractometer using an oil-coated [lo] crystal
of dimensions 0.3 x 0.2 x 0.2 mm using the S/w method (7.14520545.4"). Of
a total of 5484 reflections collected. 3563 were independent. The structure was
solved by direct methods and refined by full-matrix least-squares on F 2 to final
values of R1 = 0.077 (for 3563 data with F>4uF) and irR2 = 0.231 (all data)
- F ~ ) 2 ] / X w ~ } o . ' , M' =1/
[Rl = ZlFo - F,I/ZJF,I, tvR2 = {[Zw(F
[u2(F2) (XI')'+ y P ] . P = ( F f + 2F:/3)1 [ll]. Largest difference between
peak and hole in the final difference map, 1.965 and - 1.525 e k 3 . Crystallographic data (excluding structure factors) for the structure(s) reported in this
paper have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-179-49. Copies of the data can be
obtained free of charge on application to The Director. CCDC, 12 Union
Road, Cambridge CB2 lEZ, UK (fax: Int. code +(1223) 336-033; e-mail:
teched@chemcrys.cam.ac.uk)
[6] See for example, a) A. J. Blake, C. M. Grant, S. Parsons, J. M. Rawson.
R. E. P. Winnpenny, J. Chem. SOC. Chem. Commun. 1994. 2363; b) A.
Cdneschi, D. Gatteschi, J. Laugier, P. Rey, R. Sessolr. C. Zanchini, J. Am.
Chem. SOC.1988./10,2795; c) A. Miiller, W. Plass. E. Krickmeyer, S. Dillinger,
H. Bogge, A. Armatage. A. Proust, C. Bengholt. U. Bergmann, Angew. Chem.
1994, 106, 897; Angen. Chem. Int. Ed. Engl. 1994. 33, 849; d) A. Miiller. E.
Krickmeyer, J. Meyer. H. Bogge, F. Peters, W Plass, E. Diemann. S. Dillinger,
F. Nonnenbruch, M. Rauderath, C. Menke, ibid. 1995,107,2293 and 1995.34.
2122; e) K. L. Taft, C. D. Delfs, G. C. Papaefthymiou, S. Foner, D. Gatteschi.
S. L. Lippard, J Am Chem. SOC.1994.116,823: f) P. Kliifers. J. Schuhmacher,
Angew. Chem. 1994, 106, 925; Angeu. Chem. I n r . Ed. Engl. 1994. 33. 1863.
[7] a) M. T. Blanda, J. H. Homer, M. Newcomb, J Org Cheni. 1989,54,4626; b)
X. Yang, C. B. Knobler, M. F. Hawthorne, Angen. Chetn. 1991, 103. 1519;
Angew. Chem. Int. Ed. Engl. 1991, 30, 1507.
[8] a) Y. Ni, A. J. Laugh, A. L. Rheingold, 1. Manners, Angen. Chem. 1995, 107.
1079, Angen. Chem. In?. Ed. Engl. 1995.34,998; b) R. T. Oakley. S. J. Rettig,
N. L. Paddock. J. Trotter, J Am. Chem. Sor. 1995, 107,6923.
[9] See for example, a) M. A. Paver, D. Stalke. D. S. Wright, Angew. Chem. 1993,
105, 4 4 5 ; Angew. Chem. In/. Ed. Engl. 1993. 32, 428: b) M. A. Paver. C. A.
Russell. D. Stalke, D. S. Wright, J. Chem. Soc. Chem. Commun. 1993. 1350; c)
A. J. Edwards, M. A Paver, P. R. Raithby, C. A. Russell, D. Stalke. A. Steiner.
D. S. Wright, Inorg. Chem. 1994,33,2370; d) A. J. Edwards. M. A. Paver, P. R.
Raithby. C . A. Russell, D. Stalke. A. Steiner, D. S. Wright, J Chem. Sor.
Dalfon Trans. 1993, 1465.
1101 D. Stalke and T Kottke, J AppL Crmrallogr. 1993, 26, 615.
[Ill G. M. Sheldrick, SHELXL 93, Universiat Gottingen, 1993.
+
length of the constituent blocks. In solution, however, block
copolymers tend to form globular micelles.r2- Their size depends on the solvent and on the absolute and relative length of
the blocks. In quantitative agreement with theoretical predictions, experimental evidence indicates that the cores are not
spherical but have the shape of ellipsoids.[8.9l
Only recently Eisenberg et al. observed for the first time different morphologies of polystyrene(PS)-block-poly(acry1ic acid)
associates in aqueous solution, namely, spheres, rods, lamellae,
An
vesicles, and complex aggregates of reverse micelles.18essential condition for the formation of these structures was that
the PS core block was long relative to the outer poly(acry1ic
acid) block. Dispersion in water was achieved by dissolving the
block copolymer in DMF-rich solvent mixtures, followed by
addition of water and diakysis. Because of the vitrification of the
PS core, the structure can be frozen in a nonequilibrium state.
This work deals with polystyrene-block-poly(2-vinylpyridine)s with a poly(2-vinylpyridine) (P2VP) block that is significantly longer than the PS block. In a dilute solution of toluene
these block copolymers are expected to form reverse globular
micelles, which, however, become unstable as the solvent evaporates, that is, the solution concentration increases. Transmission
electron microscopy (TEM) and scanning force microscopy
(SFM) is employed to study how preservation or transformation of the metastable structures upon formation of thin films
depends on the block lengths and the ionic character of the
core block. In a similar study we recently demonstrated that
intact globular micelles are deposited from a dilute solution
of a symmetrical polystyrene-block-poly(2-vinylpyridine) in
toluene." 16] Transformation of the globular micelles to the
lamellar bulk equilibrium structure could be inhibited by neutralization of the poly(2-vinylpyridine) block with HAuCl,,
which establishes a stabilizing dipole-dipole interaction.
Two polystyrene-block-poly(2-vinylpyridine) samples differing in their absolute molecular weight but having a similar
styrene:P2VP ratio ( R ) were used. Table 1 summarizes the
~
Table 1. Molecular characteristics of the block copolymers.
Metastable Reverse Globular Micelles and
Giant Micellar Wires from Block Copolymers**
Joachim P. Spatz, Stefan MoDmer, and Martin Moiler*
Amphiphilic molecules such as surfactants and lipids associate in aqueous solution to form rather well-defined globular,
rodlike, and platelet micelles, continuous structures, bilayers,
and bilayer vesicles. These can partly transform from one to
another when the solution conditions such as the ionic strength,
the concentration, pH, or temperature are changed."] For block
copolymers (that is, macromolecular amphiphiles) different
structure variations are known in the bulk, depending on the
[*] Prof. Dr. M. Moller, DipLPhys. J. P. Spatz, DipLChem. S. MoDmer
Organische Chemie 111 - Makromolekulare Chemie der Universitat
[**I
Sample
M,(PS) [a1
[gmol-'1
M.(P2VP)
[gmol-'1
PS80-P2VP320
PS180-P2VP700
7.900
19.000
34.900
74.000
PI
M , (total) [cl
[gmol-'1
K I M , [cl
57.600
108.300
1.10
1.19
[a] Molecular weight measured by GPC for the polystyrene block before addition
of the second monomer. [b] Molecular weight calculated from the styrene/2vinylpyridine composition obtained by 'H NMR in relation to the length of the PS
block. [c] Molecular weight measured by GPC for the block copolymer according
to narrow molecular weight polystyrene standards.
molecular characteristics. The polymers have been treated with
different equivalents of HAuCI, per pyridine group in a dilute
solution of toluene. The absolute molecular weight, the R ratio,
are the
and the degree of neutralization L ( L = nHAuCi4/nPZVP)
parameters that effect the kinetic stability of a structure formed
in dilute solution (in this case, globular micelles) when the solvent is evaporated. The rate of evaporation will also influence
the structure transformation, as the stability is only kineticany
Albert-Einstein-Allee 11, D-89069 Ulm (Germany)
controlled. The rate of evaporation will be subject of a subse-
Telefax: Int. code f(731) 502-2883
e-mail: martin.moeller(n'chemie.uni-ulm.de
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 239)
and the Fonds der Chemischen Industrie. We thank the section Elektronenmikroskopie of the University of Ulm for carrying out the electron microscopy
measurements.
quent paper; in the present work it was kept constant for all
investigations.
Figure 1 depicts a TEM and a SFM picture of a thin film of
the PS80-P2VP320 polymer that had been treated with 0.7
equivalents of HAuCI, per pyridine unit ( L = 0.7). Dark areas
1510
0 VCH
Verlagsgesellschufi m b H . 0-694Sl Weinheim. 1996
0570-0833I96l35l3-I510 8 15.00+ ,2510
Angen. Chem. Inl. Ed. EngI. 1996, 35, No. 13/14
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initial step of the transformation from cylinders to wires is
marked by the circle in Figure 2 b. Apparently the transformation occurs on surface contact of the spherical micelles.
Figure 3 shows a scanning force micrograph of the micellar
wires. The image displays the amplitude control signal, emphasizing rather small features in height."" Apparently, the corona
of the cylinders is still corrugated with a periodic structure similar to the dimensions of globular micelles. Comparison of the
contacts between spheres and wires demonstrates that the contour of the latter is less deformed by the contact.
Fig. 1 . Trransmission electron and scanning force micrographs of spherical micelles
formed o f PSXO-P2VP320, R = 0 24, L = 0.7. a ) TEM reveals the core of the micelles (small black dots) b) SFM depicts the outer contour with the corona of the
micelles. which cdiitiot be seen by TEM. a) and h) have the same scale.
in the TEM image represent agglomerations of small gold particles, which form upon irradiation by the electron beam (electron irradiation stimulates reduction of Au"' ions).[12.16] The
arrangement of the gold particles indicates that the globular
micelles were preserved in the solvent-free state. The gold particles mark the micelle core. The outer contours of the micelles
can also be seen in the SFM picture in Figure 1 b. Comparison
of the TEM picture with the S F M picture reveals that the core
remained less affected by the close packing of the micelles within
the thin film. whereas the PS shell was significantly deformed
and approached a honey comb structure as flat contact areas
were formed between the micelles.
Figure 2 shows TEM pictures of a film that was formed under
identical conditions from the same block copolymer, which,
however, had been treated with a smaller amount of HAuC1,
( L = 0.3). In coexistence with the globular micelles a new struc-
Fig. 2 . The TEM images show a) the formation of cylindrical micelles with a n
aspect ratio II of about 2000 and b) cylindrical micelles in direct contact. The circle
marks the initial step of the transformation from cylinders to wires.
ture, giant cylindrical micelles, were formed. Again, the goldloaded core of the micelles is depicted by the black areas in the
TEM pictures. The ratio of the length of the cylinder to its
diameter (the aspect ratio) exceeded values of 2000. While the
diameters of the disks is (32 f 3) nm, the diameter of the stripes
is uniformly (21 3) nm.
Very obviously a number of the spherical micelles coagulated
to form the long isolated cylinders, which are known for low
molecular weight surfactants,['*] but have only been observed
once for block copolymers in solution.[91As shown in Figure 2 b. these micellar wires cross each other without the integrity of the cylinders being affected by the direct contact. The
Fig. 3 . S F M amplitude control signal (length of one side corresponds to 2 prn) of
a monolayer of spherical micelles on top of which cylindrical micelles were formed.
Neither the cylinders nor the globules represent the bulk equilibrium structure for the strongly unsymmetric block copolymers in the solvent-free state.["] Apparently the formation of
the micellar wires is controlled by the decrease in the kinetic
stability of the spherical micelles as the degree of ionization of
the core block is lowered from 0.7 to 0.3. This is consistent with
the observation that no micellar wires were formed when at the
same time the R ratio was increased. At L = 0.3, a PS110P2VP280 ( R = 0.39) did not yield cylindrical structures.
A second, important point is the observed contraction of the
core radius upon formation of the micellar cylinders. This is in
agreement with expectations and can be regarded as the driving
force of the transformation. At the R ratios employed here, the
core blocks have to stretch in order to form globular micelles
with a minimum total interface area, which depends on the size
and the number of m i ~ e l l e s . [ ~'I ~The penalty in entropy for the
stretching is balanced by the reduced contacts between the ionic
and nonpolar units. As the solvent evaporates, the balance is
disturbed and the cylindrical micelles represent a more favorable structure with respect to the free energy. Thus, diminishing
L to 0.3 accelerates the transformation kinetics to the more
Favorable micellar wires.
This is also demonstrated by the comparison of the two block
copolymer samples that have the same composition but different absolute molecular weights (Table 2). The obtained decrease
in the core diameter can be correlated to the increase in kinetic
stability on transforming the globule to cylinders and depends
on the absolute molecular weight. The smaller the ratio D of the
diameter of the spheres to that of the cylindrical rnicelles, the
smaller the free energy gain by the transformation. A higher
molecular weight results in slow relaxation times, which have a
direct influence on the transformation kinetics. This argument is
consistent with the observation that the aspect ratio of the
Table 2. Dimensions of spherical and cylindrical wires.
Dimensions
PS80-b-P2VP330
R = 0.24. L = 0.3
0 , [nmllal
D2 [nml [bl
D It1
32
21
1.52
2~ 103
N
PSI 80-b-P2VP700
R = 0.26. L = 0.3
35
27
1.30
2 x 10,
[a] D , = diameter of the spherical micelle [b] D , = diameter of the micellar wire
[c] D = D,;D,. [d] o = aspect ratio = (micellar length)l(micellar diameter)
molecular wires was reduced considerably as D decreased from
1.52 to 1.30 on changing from PS80-P2VP320 to PS180P2VP700.
Summarizing, we emphasize the following points (Fig. 4): 1)
Coagulation of spherical micelles is explained by the reduced
chain stretching. 2) This results in an increased grafting density
in the corona and a higher shape stability. 3) The ends of the
cylinders are particularly activated because of a reduced grafting density. Points 2) and 3 ) explain the coexistence of spheres
and cylinders and the linear growth of the cylinders.
[lj J. Israelachvili. /titermoleculrrr K! Svrfiice Fnrce.\, 2nd. ed.. Academic Press,
San Diego, 1991.
[2] Z. Gao, A. Eisenberg. Macromolecu1e.s. 1993, 26 7353.
[3] Z. Gao, X. F. Zhong. A. Eisenberg, Macroniu/ecu/rs 1994, 27, 794.
[4] D. Nguyen, S. K. Varshney. C. E. Williams. A. Eisenberg. Macronio1ecule.s
1994, 27, 5086.
[5] D. Nguyen. C. E. Williams, A. Eisenberg, Mtr1.i-oiiio1ecrrle.s1994. 27. 5090.
[6] A. Halperin, Macromo/ecule.s 1990, 23. 2724.
[7] 1.Leibler, H. Orland. J. C. Wheeler, J. Clitm. P h j x 1983. 79. 3550.
[8] Z . Gao, S. K. Varshney, S. Wong. A. Eisenberg. J. C h i i . Pt'ij,.$. 1994.27.7923.
[9] L. Zhang, A. Eisenberg, Science 1995, 268. 1728.
[lo] P.-G. deGennes in SolidSrirre Physics(Ed.: L. Leibert). Academic Press, New
York. 1978; Supplement 14. p. 1.
[I I] J. P.Spatz. A. Roescher, S. Sheiko, G. Krausch, M. Moller, Adis Morw 1995,
7, 731
1996. 29. 3220.
[12] J. P. Spatz. S. Sheiko. M. Moller. Moi~roiiioli~cirl~~s
[I 31 A. Roescher. Dissertation, University of Twente, Holland, 1995
[I41 A. Roescher. M. Moller. Po/wi. M m r . Scr. Eng. 1995, 73. 156
[15] A. Roescher. M. Moller. P o / w Mmr- Sci. Eng. 1995, 72. 283.
[I61 J. P. Spatz. A. Roescher. M. Moller. A A . Marer. 1996, 8. 337
[I71 Digital Instruments. CA. Santa Barbara.
[18] H. Hoffmann. G. Ebert. A n g w . Cliiw. 1988, 100. 933: Angcii.. Climi. I n / E d
D i g / . 1988, 27. 902
1191 F S Bates. Annu. R ~ T P. / g x C/ro,i. 1990, 4 / . 525.
[20] S. MoRmer. Diplomarbeit. Universitgt Ulm, 1996.
[21] M . Moller. R. W. Lenz. Mocroniol. Cliern. 1989. 190, 1153.
[22] J. P. Spatz. S. Sheiko. M Moller. R. W. Winkler, P. Reineker. 0. Marti.
M ~ f i i f ) / l , ~ h ~ i ~1995.
J / o g6.
~ 40.
Synthesis, Structure, and Bonding of a
Mixed-Valent Tetraphosphete**
Walter Frank,* Volker Petry, Elmar Gerwalin, and
Guido J. Reiss
Fig. 4. The formation of micellar cylinders. Globular micelles can only join the ends
of a cylinder. The arrow indicates the direction of growth. However, micellar transformations can be blocked by kinetic stabilization of the structure during the evaporation process.
Investigations of compounds of the general formula P4R4
have so far been concerned exclusively with cyclotetraphosphane (PH), and its derivatives.['] Neither experimental nor
theoretical studies exist that deal with the valence-isomeric
12.5,32b5-tetraphosphete(H,PP), and its derivatives, although
two conceptual approaches are possible to such a mixed-valent
4n-electron four-membered ring system (A): First, it may be
considered an analogue of the diphosphetes B['' and the isovalence-electronic cyclodiphosphazenes C;[31second, it is evidently related to the ph~sphinylidene-o~-phosphoranes[~~
and in
Experimental Procedure
Sample preparation: The polystyrene-hlock-poly(2-vinylpyridine) block copolymers were synthesized by anionic polymerization [20. 211. A solution of block
copolymer (0.5 wt% in toluene) was mixed with different amounts of HAuCI, . H,O, dependingon the molar concentration of pyridinegroups ( L = HAuCI,
P N P ) . The mixture was stirred for at least 24 h to allow complete dissolution of the
gold salt within the cores of the block copolymer micelles. The solutions were
diluted to 1-3 mg block copolymer per milliliter Thin films were prepared by
putting a drop of the solution onto an electron microscopy grid coated with a thin
layer of carbon, which was in direct contact with a soaking tissue to remove the
solution immediately. The film thickness was controlled both in this way and by
varying the concentration
Bright-field TEM images were recorded with a PHILIPS EM400T microscope operated with 80 kV. In order to minimize the destruction of the polymer by the
electron beam. the electron beam intensity was kept as low as possible (second
condenser lens: 50 Pt, lens: 30 Au). Scanning force microscopy with the same samples (grids) that were used for electron microscopy were performed with a
Nanoscope III [I71 operating in the tapping mode [22].
Received: January 25. 1996 [Z 8754 IE]
German version: A n p i . . Clieiii. 1996. 108. 1673-1676
Keywords: block copolymers . micelles
croscopy . thin films
1512
scanning force mi-
mhH, 0-69451 Weinhrbn. 1996
1' VCH Ver/~~sgese//.sc/i~I~~
R
/-\+
=...
/=\+
'.. +
P'
C
R?
R
*.'
+
.....
R ,a..,
\-/'\R
R'
Fi/'\:/'\R
R
P
P
R'
+/-\+...\-/pbR
R
A
B
C
[*I Prof. Dr. W Frank. DiplLChem. V. Petry. DiplLChem. E. Gerwalin.
Dr. G . J. Reiss
Fachbereich Chemie der Universitit
Postfach 3049, D-67653 Kaiserslautern (Germany)
Fax. Int. code +(631)205-2187
e-mail: walter(rr chemie.uni-kl.de
[**I This work was supported by the Fonds der Chemischen lndustrie and the
Bundesland Rheinland-Pfalz (post-graduate grant for V. P. wlth~n the
Graduiertenkolleg "Phosphorchemie als Bindeglied verschiedener chemischer
Disziplinen"). We acknowledge the grant of computing time by the Regionales
Hochschulrechenzentrum Kaiserslautern.
fJ570-0833/96i3513-15IZ B 15.00i .2S!(I
Angvii.. Cliem. Inr. Ed. &g/. 1996, 35. N o . 13/14
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