Synthesis of tethered polystyrene-block-poly(methyl methacrylate) monolayer on a silicate substrate by sequential carbocationic polymerization and atom transfer radical polymerization [15]

Full text of DOI:10.1021/ja984428y

J. Am. Chem. Soc. 1999, 121, 3557-3558
3557
Scheme 1
Synthesis of Tethered Polystyrene-block-Poly(methyl
methacrylate) Monolayer on a Silicate Substrate by
Sequential Carbocationic Polymerization and Atom
Transfer Radical Polymerization
Bin Zhao and William J. Brittain*
Department of Polymer Science
The UniVersity of Akron
Akron, Ohio 44325-3909
ReceiVed December 28, 1998
An assembly of tethered polymer chains in mutual proximity
are called “polymer brushes”.1-3 The amount of theoretical
work4-7 on polymer brushes equals or exceeds the amount of
experimental information8-10 available concerning properties and
structure. Even less is known about diblock polymer brushes.
Because diblock copolymers have a propensity to self-organize,
we are interested in the behavior of diblock copolymers that have
one chain end confined by covalent attachment to a surface. In
this communication, we describe the synthesis of diblock co-
polymer brushes which exhibit surface reorganization in response
to block-selective solvents.
Fabrication of tethered polymers has involved covalent attach-
ment and polymer physisorption. For polymer absorption, diblock
copolymers are used where one block strongly adsorbs to the
surface.11 A disadvantage of these systems is thermal and
solvolytic instability. Polymers with a covalent bond to the surface
can circumvent this shortcoming. Tethering has been accom-
plished by grafting preformed polymers to tethering sites (“graft-
ing to”)12,13 or by polymerizing from surface-immobilized initia-
tors (“grafting from”). Examples of the latter approach include
styrene polymerization using surface-immobilized azo com-
pounds,14 hyperbranched polymer films,15 methyl methacrylate
(MMA) polymerization using a surface-immobilized initiator for
atom-transfer radical polymerization (ATRP),16 and N-carboxy-
anhydride polymerization by amine-functionalized surfaces.17
Here we report the formation of polystyrene-block-PMMA (PS-
b-PMMA) films by a sequential carbocationic polymerization of
styrene followed by radical polymerization of MMA. To our
knowledge, the formation of block copolymer brushes by the
“grafting from” approach is unprecedented. We have characterized
these films by tensiometry, FTIR, XPS, and ellipsometry.
Scheme 1 depicts the synthetic scheme for tethered PS-b-
PMMA block copolymers. Our approach starts with surface
immobilization of trichlorosilane-1, which is an initiator for
carbocationic polymerization.18,19
Silane-1 was deposited on silicate substrates (either silicon ATR
crystals or silicon semiconductor wafers) using conventional
methods. We chose a deuterated silane so that FTIR could be
used to estimate initiator efficiency via the intensity of νC-D
.
Treatment of substrate-2 with styrene under carbocationic
polymerization conditions20 led to tethered PS. We also monitored
styrene polymerization via film thickness using ellipsometry.
X-ray photoelectron spectroscopy (XPS) results and water contact
angles (θa ) 100 ( 1°, θr ) 83 ( 1°) were consistent with an
overlayer of PS. The PS thickness was controlled by experimental
conditions. Decreasing solvent polarity (by diluting CH2Cl2 with
methylcyclohexane), decreasing [TiCl4], or adding dimethylacet-
amide (a Lewis base) decreased the PS film thickness.21 Solvent
extraction did not diminish the PS intensity in the FTIR spectrum,
which is consistent with a covalently attached polymer.
(1) Milner, S. T. Science 1991, 251, 905-914.
(2) De Gennes, P. G. Macromolecules 1980, 13, 1069-1075.
(3) Halperin, A.; Tirrell, M.; Lodge, T. P. AdV. Polym. Sci. 1992, 100,
31-71.
(4) Milner, S. T.; Witten, T. A.; Cates, M. E. Europhys. Lett. 1988, 5,
413-418. Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988,
21, 2610-2619.
(5) Singh, C.; Balasz, A. C. Macromolecules 1996, 29, 8904-8911.
(6) Grest, G. S.; Murat, M. Macromolecules 1993, 26, 3108-3117.
(7) Fredrickson, G. H.; Ajdari, A.; Leibler, L.; Carton, J.-P. Macromolecules
1992, 25, 2882-2889.
(8) Cho, Y. K.; Dhinojwala, A.; Granick, S. J. Polym. Sci. B, Polym. Phys.
1997, 35, 2961-2968.
(9) Roters, A.; Schimmel, M.; Ru¨he, J.; Johannsmann, D. Langmuir 1998,
14, 3999-4004.
(10) Mansky, P.; Liu, Y.; Huang, E.; Russel, T. P.; Hawker, C. Science
1997, 275, 1458-1460. Husseman, M.; Malmstro¨m, E. E.; McNamara, M.;
Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang,
E.; Russel, T. P.; Hawker, C. J. Macromolecules, in press.
(11) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove,
T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.
(12) Ebata, K.; Furukawa, K.; Matsumoto, N. J. Am. Chem. Soc. 1998,
120, 7367-7368.
(18) The synthesis of silane-1 involved a four-step sequence. Addition of
CH3MgBr to 4-bromoacetophenone produced the corresponding alcohol, which
was treated with iodomethane-d3 in the presence of KOH. The Grignard reagent
was formed by reaction with Mg° and was then reacted with SiCl4 to produce
the desired silane-1. The final product was isolated as a 1:1 complex with
THF.
(19) Faust, R.; Kennedy, J. P. J. Polym. Sci., Part A, Polym. Chem. 1987,
25, 1847-1869. Kennedy, J. P.; Iva´n, B. Designed Polymers by Carbocationic
Macromolecular Engineering, Theory and Practice; Hanser: New York, 1992.
(20) An ATR prism or silicon wafer was placed into a 50 mL Schlenk
tube. Anhydrous CH2Cl2, di-tert-butylpyridine (DtBP), and styrene were added
via syringe. After the solution was cooled to -78 °C, TiCl4 was added. Final
concentrations were as follows: [TiCl4]0 ) 0.0236 M, [DtBP]0 ) 0.012 M,
and [styrene]0 ) 0.75 M. After 50 min, methanol was added and the silicate
sample was extracted with anisole for 24 h.
(13) Tsubokawa, N.; Hosoya, M.; Yanadori, K.; Sone, Y. J. Macromol.
Sci. Chem. 1990, A27, 445-457.
(14) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602-613.
(15) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells,
M. J. Am. Chem. Soc. 1996, 118, 3773-3774.
(16) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macro-
molecules 1998, 31, 5934-5936.
(17) Heise, A.; Menzel, H.; Kim, Y.; Foster, M. D.; Wieringa, R. H.;
Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723-728.
(21) Details of these experimental variables will be published in a full-
length paper.
10.1021/ja984428y CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/25/1999

3558 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Communications to the Editor
Figure 1. Reflectance FTIR spectra of (a) substrate-2 (absorption
intensity is magnified 5×), (b) first carbocationic polymerization of
styrene, and (c) second, sequential carbocationic polymerization of styrene.
The film thickness was determined by ellipsometry. Initiator efficiency
Figure 2. Reflectance FTIR spectra of (a) substrate-2 (absorption
intensity is magnified 5×), (b) subsequent carbocationic polymerization
of styrene; and (c) ATRP of MMA. The film thickness was determined
by ellipsometry.
was determined by the decrease in intensity for νC-D at 2075 cm-1
.
Table 1. Water Contact Angles (deg)
Figure 1 shows the FTIR spectra of substrate-2 and the PS
films formed after carbocationic polymerization. Spectra b and c
in Figure 1 represent tethered PS chains formed by sequential
styrene polymerizations. Initiator efficiency was determined by
monitoring the decrease in intensity for νC-D at 2075 cm-1. While
we assume that styrene polymerization in the second step occurs
from chloro-terminated chain ends,22 the spectra in Figure 1
demonstrate that polymerization also involves unreacted initiator
sites. Additional polymerization in the second treatment with
styrene is supported by an increase in IR intensities and increasing
film thickness.
To create a tethered diblock polymer film that did not also
involve initiation from immobilized silane-1, we used radical
polymerization for the second block. Coca and Matyjaszewski23
prepared block copolymers of styrene and MMA in solution using
carbocationic and ATRP.24 We have applied this synthetic strategy
to prepare surface-immobilized diblock copolymer chains. A PS
film was immersed in a solution of MMA and polymerized using
typical ATRP conditions.25 Evidence for PMMA is based on the
appearance of νCdO at 1732 cm-1 (Figure 2). Based on ellipsom-
etry, the film thickness increases by 9 nm. We observed water
contact angles (θa ) 75 ( 1°, θr ) 60 ( 1°) that correspond to
a PMMA surface.26 Attempted MMA polymerization in the
absence of ATRP catalyst and ligand does not lead to polymer-
ization; therefore, the grafted PMMA does not arise from a
thermal, spontaneous polymerization. Also, we subjected sub-
strate-2 to ATRP conditions and observed no PMMA in the FTIR,
which confirms that the cumyl ether is not an effective initiator
for MMA polymerization. Therefore, we conclude that MMA
polymerization occurred at PS chain ends. The PMMA thickness
was affected by time, temperature, and added free initiator.
The grafting efficiency of PS is equivalent to the initiator
efficiency of 2. However, we do not know the initiation efficiency
sample descriptiona,b
θa
θr
surface-immobilized 1 on Si/SiO2 (2)
PS grafted film (3)
67
100
75
98
75
97
75
52
83
60
78
59
78
60
PS-b-PMMA grafted film (4)
diblock after methylcyclohexane immersionc
diblock after CH2Cl2 immersiond
second methylcyclohexane immersionc
second CH2Cl2 immersiond
a This sample corresponds to Figure 2: PS film thickness ) 26 nm,
PMMA thickness ) 9 nm. b Standard deviation for contact angles e2°.
c 30 min immersion in methylcyclohexane at elevated temperature.d 30
min immersion in CH2Cl2 at room temperature.
of MMA to tethered PS chains. Also, we do not have information
about the molecular weight of the tethered diblock copolymers.
We are currently preparing diblock copolymers on silica gel and
will study the molecular weight of chains after cleavage from
the support.
The tethered diblock copolymer undergoes reversible changes
in water contact angles as the film is treated with different solvents
(see Table 1). Initially, the film exhibits a contact angle
characteristic of PMMA; following treatment with methylcyclo-
hexane (a better solvent for PS than for PMMA), the contact angle
increases to a characteristic value for PS. Subsequent treatment
of the same sample with CH2Cl2 (a good solvent for PMMA and
PS) reverses this change. Consistent with the low PS grafting
efficiency, the tethered chains must be spaced sufficiently far apart
to permit large changes in conformational states such that PS
segments can localize at the surface in response to treatment with
methylcyclohexane.
We have synthesized a tethered block copolymer and have
reported large changes in surface composition that are induced
by solvent treatment. We feel that these films are good examples
of responsive films that possess adaptable adhesion or wettability.
We are currently investigating different chemical compositions
for the tethered diblocks. In addition, we are studying the surface
reorganization phenomena using different surface analytical tools.
(22) Faust, R.; Kennedy, J. P. Polym. Bull. 1988, 19, 21-28.
(23) Coca, S.; Matyjaszewski, K. Macromolecules 1997, 30, 2808-2810.
(24) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-
5615.
(25) An ATR prism or silicon wafer was placed into a 50 mL Schlenk
tube followed by addition of anisole, MMA, and CuBr. The mixture was
degassed by an argon purge and the ligand (N,N,N,N,N-pentamethyldiethyl-
enetriamine) was added via syringe. The final concentrations were as
follows: [MMA]0 ) 3.6 M, [ligand]0 ) 0.0147 M, and [CuBr]0 ) 0.0145 M.
(26) The water contact angles for a PMMA film prepared by spin coating
onto a silicon wafer are θa ) 75 ( 1°, θr ) 60 ( 1°.
Acknowledgment. This work was financially supported by the Army
Research Office (DAAH04-96-1-0018).
JA984428Y