Langmuir 2006, 22, 319-324
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Selective Betainization of PS-P4VP and Solution Properties Lixin Song and Yeng Ming Lam* School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore, 639798, Republic of Singapore ReceiVed June 2, 2005. In Final Form: October 21, 2005 The ability to control the structure and function of supramolecular self-assemblies gives rise to many patterning possibilities. Making use of the interactions between the copolymers and solvent provides a neat way of controlling the structures. The pyridine residues of each of the diblock copolymer polystyrene-b-poly(4-vinylpyridine) (PSP4VP) was selectively betainized using 1,3-propane sultone under mild conditions to yield a series of novel betaine diblock copolymers in chloroform and toluene, respectively. The solution properties were studied using dynamic light scattering, static light scattering, and 1H NMR spectroscopy. The morphology of the resulting micellar film was studied using transmission electron microscopy. The size of the micelles formed was found to be strongly dependent on the amount of sultone grafted and the shape of PS-P4VP-sultone micelle changed from spherical to elongated with different PS-P4VP-to-sultone ratio.
Introduction Considerable interest in polybetaines arises because of their technological applications. One very interesting property of polymeric betaine is bio- and hemocompatibility, which is a direct result of their highly hygroscopic nature. Polybetaines, which have both negative and positive charge on every monomer residue, have been used in areas such as production of fungicides, synthesis of fire-resistant polymers, lubricating oil additives, and emulsifying agents. The introduction of the ionic groups onto the polymer chain results in marked changes in its dilute solution properties. The specific behavior of the modified polymers is due in part to interactions among the ionic groups. Short-range interactions sometimes inhibit high conversion in the modification reaction. Vinylpyridine copolymers with ionomeric properties have been studied by Gauthier and Eisenberg.1 In their study, polystyreneb-poly(4-vinylpyridine)-b-polystyrene triblock copolymer was quaternized by methyl iodide. The thermal and dynamic mechanical behavior of styrene-4-vinylpyridinium ABA block ionomers was investigated as a function of ion content and method of preparation. Aggregation was observed in dilute solution, and this is related to the presence of the ionic groups.2,3 Galin and Monroy-Soto have synthesized many polysulfobetaines by either polymerizing various betainized monomers or betainizing precursor tertiary amine polymers via free-radical polymerization.3,4 The relatively low solubility of these betaine monomers in organic solvents and the broad molar mass distributions of the resulting polymers are severe limitations if controlled structure polybetaines are required.5 An alternative route to synthesize polybetaines involves the synthesis of precursor aminopolymers followed by betainization using either 1,4-butane sultone or 1,3propane sultone. Poly(vinylpyridine sulfopropyl betaines) have been synthesized by reacting precursor polymers with 1,3-propane sultone by Cardoso and Manero.6 The solution behavior of polybetaines is often opposite that of polyelectrolytes, exhibiting the so-called antipolyelectrolyte * To whom correspondence should be addressed. Email: ymlam@ ntu.edu.sg. (1) Gauthier, S.; Eisenberg, A. Macromolecules 1987, 20, 760. (2) Salamone, J. C.; Volksen, W.; Olson, A. P.; Israel, S. C. Polymer 1978, 19, 1157. (3) Monroy-Soto, V. M.; Galin, J. C. Polymer 1984, 25, 254. (4) Monroy-Soto, V. M.; Galin, J. C. Polymer 1984, 25, 121. (5) Laschewsky, A.; Zerbe, I. Polymer 1991, 32, 2070. (6) Cardoso, J.; Manero, O. J. Polym. Sci. Part B: Polym. Phys. 1991, 29, 639.
effect.7 Chain expansions occur upon the addition of lowmolecular-weight electrolyte, although this is very much dependent on chemical structure, composition, and solution conditions. Numerous workers have described in detail the aqueous solution properties of polymeric betaines.8 Perhaps the most interesting feature of the aqueous solution properties is the lack of solubility in pure water. However, the solubility of polybetaines in organic solvents is relatively less studied. In the present work, we have for the first time synthesized polystyrene-b-poly(4-vinylpyridine) (PS-P4VP) betaine copolymers using PS-P4VP copolymer as a precursor. The selective betainization of the pyridine unit of this diblock copolymer was carried out in chloroform and toluene by using 1,3-propane sultone (Scheme 1). The solution properties in toluene were studied using dynamic light scattering (DLS), static light scattering (SLS), and 1H NMR spectroscopy. The morphology of the resulting micellar film was investigated using transmission electron microscopy (TEM). These studies confirmed that the sultone-induced micellization led to formation of ribbon-shaped nanostructures. Experimental Section Synthesis of Copolymer Polysulfobetaines. PS-P4VP polymers (MnPS ) 11 800 kg/mol, MnP4VP ) 15 000 kg/mol, Mw/Mn ) 1.04) were obtained from Polymer Source, Inc. The betainization of PSP4VP diblock copolymer was carried out in chloroform at 60 °C by using 1,3-propane sultone. The copolymer concentration was fixed at 4 mg/mL while 1,3-propane sultone was added at different amount to reach the desired ratio. The betainization of copolymer was also carried out in toluene at 60 °C for 24 h. The resulting turbid solution was dropped into methanol to precipitate, and dried. The reaction scheme is as shown in Scheme 1. The betainized copolymer was dissolved again in toluene at different concentrations to form micellar solutions. At high concentrations, the solution remained turbid after long hours of sonification. Micellization of PS-P4VP-Sultone. The micellization of the PS-P4VP-sultone system in chloroform was conducted in this way. PS-P4VP copolymer solution in chloroform-d was allowed to equilibrate for 2 days, after which sultone/CDCl3 solution was added dropwise into each of the copolymer solutions until the designated molar ratio was reached. The solution was kept at 60 °C for 24 h. The concentration of block copolymer in the final solutions (7) Lowe, A. B.; McCormick, C. L. Chem. ReV. 2002, 102, 4177. (8) Lowe, A. B.; McCormck, C. L. In Stimuli-ResponsiVe Water-soluble and Amphiphilic Polymers; McCormick, C. L., Ed.; Advances in Chemistry Series No. 780; American Chemical Society: Washington, DC, 2001; p 1.
10.1021/la0514415 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2005
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Song and Lam
Scheme 1. Synthesis of PS-P4VP-Sultone and Micellization.
is 1 mg/mL, while the molar ratios of pyridine units/sultone in the final solutions are 1.0:0.1, 1.0:0.2, 1.0:0.5, and 1.0:1.0, respectively. 1H NMR. The 1H NMR studies were carried out on a Varian XL-300 NMR instrument operating at 300 MHz and using CDCl3 as solvent. TMS was used as an internal reference. Laser Light Scattering. A Brookhaven BIS200SM laser scattering system equipped with a 522-channel Brookhaven BI9000 digital multiple τ correlator was used to perform both the SLS and the DLS experiments. The light source is a power-adjustable vertically polarized 35 mW argon ion laser with a wavelength of 632.8 nm. The inverse Laplace transform of REPES9 supplied with a GENDIST software package was used to analyze the time correlation function (TCF), and the probability of rejection was set to 0.5. Transmission Electron Microscopy. Droplets of solutions were dropped onto a copper grid with holey carbon film and dried in a vacuum. They were then stained with iodine vapor at 80 °C for an hour in a capped container. TEM was performed on a JEOL 2010 operating at a magnification of 30 000×. The microscope is fitted with a LaB6 filament, and an acceleration voltage of 200 kV was used.
decrease. It is noted that when the molar ratio is high, the P4VP block remains in solution. When this is the case, the pyridine signals are fairly strong, indicating the presence of mobile pyridine units. When the molar ratio is low, peak a disappears, indicating that the mobility of most pyridine units is severely restricted. It is interesting to note that the intensity ratio of b to c becomes close to 3:2, which is the ratio of the number of Hb to Hc in the benzene ring. The contribution from the pyridine ring to the signal c relating the Hc is no longer present. It may be possible that the aggregates formed at low molar ratio (4VP/sultone) are
Results and Discussion Chemical Analysis. The 1H NMR spectrum of PS-P4VPsultone (4VP/sultone ) 1.0:0.1) in CDCl3 was shown in Figure 1. Peaks A, B, and C at δ ) 8.35, 7.10, and 6.50, respectively, represent the styrene and pyridine ring hydrogen, as shown in the Figure 2. Peaks D, E, and F represent the proton from 1,3propane sultone. It is noted that the peak associated with proton D in the sultone, which used to be at δ ) 4.40 before being attached to a pyridine ring,10 is now at δ ) 3.75. This indicates the presence of a chemical bond between these two species. Solution Behavior. 1H NMR. The PS-P4VP-sultone at different 4VP/sultone molar ratios (1.0:0.1, 1.0:0.2, 1.0:0.5, 1.0: 1.0) in chloroform-d were characterized by 1H NMR as shown in Figure 2. Looking at the spectra, one can observe that the signal associated with the pyridine ring hydrogen (Ha) disappears as the molar ratios of 4VP/sultone decreased. This may be related to its micellization behavior. There are two possible ways that the micelles may be formed. First, after grafting of sultone to pyridine, the P4VP block may become insoluble and associate to form the core of the aggregates. Second, the interpolymer complexation may lead to physical cross-linking of the polymer chains due to the Coulombic interaction. In this case, the newly formed complex is insoluble and associates to form the core of the aggregates, driving the micellization. Once micellization occurs, these aggregated pyridine units will lose their mobility; hence, the signals associated with the ring hydrogens disappear in the spectra.11 As the shell of the aggregates is made up of PS chains, their hydrogen signals’ intensities should not change with molar ratio(4VP/sultone). With the addition of 1,3propanesultone, only the relative intensity of peak b, assigned to the hydrogen atoms Hb in the benzene rings, does not change, but those of peaks a and c, which are associated to Ha in the pyridine rings and Hc in both the pyridine and benzene rings, (9) Jakes, J.; Czech. J. Phys. B 1988, 38, 1305. (10) Butun, V. Polymer 2003, 44, 7321. (11) Chen, D. Y.; Peng, H. S.; Jiang, M. Macromolecules 2003, 36, 2576.
Figure 1. 1H NMR spectrum of PS-P4VP-sultone (4VP/sultone ) 1:0.1) in CDCl3.
Figure 2. 1H NMR spectra of PS-P4VP-sultone in CDCl3 at different molar ratio (4VP/sultone) values: A ) 1:0.1, B ) 1:0.2, C ) 1:0.5, D ) 1:1.
SelectiVe Betanization of PS-P4VP and Solution Properties
Langmuir, Vol. 22, No. 1, 2006 321 Table 1. DLS Data of PS-P4VP-Sultone (4VP/Sultone ) 1:1) at Different Concentrations concn (mg/mL)
Rh (nm)
polydispersity
0.1 0.2 0.32 1.0 2.0
84.2 83.0 82.2 82.1 81.7
0.169 0.173 0.169 0.172 0.203
Table 2. Characteristics of Micelles Derived from PS-P4VP-Sultone in Toluenea 4VP/sultone
〈Rg〉 (nm)
〈Rh〉 (nm)
F ) 〈Rg〉/〈Rh〉
Mw (×106)
Mw*
Naggregation
1:0 1:0.1 1:0.2 1:0.5 1:1
12.5 19.1 42.8 54.4 166
22.5 24.8 41.7 50.3 84.5
0.56 0.77 1.02 1.08 1.96
6.807 7.684 17.36 25.8 457
26 800 28 538 30 276 35 490 44 182
254 269 573 727 10 343
a Mw, weight-average molecular weight of aggregate (g/mol). Mw*, weight-average molecular weight of unimer, calculated from (Mn + sultone) × polydispersity (g/mol).
Figure 3. Zimm plot of PS-P4VP-sultone in toluene (4VP/sultone ) 1:1, T ) 298 K) for polymer concentrations ranging from 0.1032 to 1.032 mg/mL.
in fact micelles. A further proof may be derived from light scattering data. Static Light Scattering (SLS). SLS provides information on the time-averaged properties of the system, the weight-averaged molecular weight (Mw), the second virial coefficient (A2). The z-averaged radius of gyration (Rg) could be obtained based on the relationship.12
(
)
q2Rg2 1 KC ) 1+ + 2A2C R θ Mw 3 where K ()4π2n2(dn/dC)2/NAλ4) is an optical constant with NA, n, and λ being Avogadro’s number, the solvent refractive index, and the wavelength of incident light in a vacuum, respectively. C is the polymer concentration in g/mL, and Rθ the excess Rayleigh ratio at scattering angle θ. The scattering vector, q ()4πn sin(θ/2)/λ), is defined as the wave vector difference of the scattered and the incident beams. The refractive index increment of the polymer solutions, dn/dC, was measured using a differential refractometer. Rg2 is the mean square radius of gyration, defined as
Rg2 )
1 m
∫0∞ r2 dm ) m1 ∫0∞ (mV)r2 dV
with m the mass, V the volume of the particle, and r the distance from the center of mass within a particle. In this study, SLS measurements were performed at different measurement angles. A typical Zimm plot was used to analyze the SLS data as shown in Figure 3. From the plot, the apparent Mw and Rg were calculated. The average aggregation number of PS-P4VP-sultone in the solution was calculated from the following equation.
Nagg )
Mw(aggregate)
It was obvious that the measured Mw value is much larger than the molecular weight of individual chain, indicating the existence of larger aggregates in the solutions. The relatively large radii of gyration of PS-P4VP-sultone also suggest the presence of large aggregates or polymer clusters in solution. Dynamic Light Scattering (DLS). Dynamic laser light scattering measures the temporal fluctuations of the scattered light produced by Brownian movement of the scattering particles. This temporal variation of scattered radiation yields the Doppler shift, and the broadening of the central Rayleigh line could be used to determine the dynamic properties of the system. The intensity of the scattered light can be analyzed by proton correlation spectroscopy (PCS).13-15 The intensity-intensity autocorrelation function is expressed as
g2(t) )
〈I(t)I(t + τ)〉 〈I(t)2〉
where I(t) is an average value of the products of the scattered intensity at an arbitrary time, t, and I(t + τ) is the intensity registered at delay time τ. The above expression can be simplified using the Siegert relations
g2(t) ) 1 + β|g1(t)|2 where β is the coherence factor and g1(t) is the field autocorrelation function. The normalized field autocorrelation function is described by the expression
g1(t) )
∫ w(Γ) exp(-Γt) dΓ
where w(Γ) is a continuous distribution function of decay rate Γ, which is the inverse of the decay time τ. If the inverse Laplace transform (ILT) is used to analyze the autocorrelation function, the decay time distribution function w(Γ) can be obtained. For the translational diffusion mode, when the measurement angle, θ, is close to 0, the translational diffusion coefficient, D, is related to the decay rate by the expression:
Mw(unimer)
The results obtained were listed in Table 2. (12) Flory, P. J. In Principles of Polymer Chemistry; Cornell University Press: London, 1953.
(13) Brown, W. Dynamic Light Scattering- the Method and Some Applications; Clarendon Press: Boston, 1993. (14) Chu, B. Laser Light Scattering-Basic Principles and Principle, 2nd ed.; Academic Press: Boston, 1991. (15) Brown, W. Light Scattering-Principles and DeVelopment; University Press: Oxford, 1996.
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D)
Γ q2
The decay rates and the square of the scattering vector, q, exhibit a linear relationship, indicating that the decay is due to the translational diffusion of the aggregates in solution. For the translational diffusion mode of large aggregates, the hydrodynamic radius can be determined form the Stokes-Einstein equation:
Rh )
kT 6πη0D0
where η0 is the viscosity of the solvent, T the absolute temperature, D0 the translational diffusion coefficient at infinite dilution, and k the Boltzmann constant. If the diffusion coefficient in a dilute solution, D, is used instead of D0, the apparent hydrodynamic radius is obtained. DLS measurements were carried out at different angles and different concentrations. At high concentrations, the polymer solutions become cloudy, and no light scattering experiment was performed in this region. At lower concentrations, using the Stokes-Einstein relationship, the hydrodynamic radii were calculated. The apparent hydrodynamic radius (Rh) of PS-P4VPsultone (4VP/sultone )1:1) was obtained for various concentrations as shown in Figure 4a and Table 1. Two representative relationship of relaxation rates Γ and q2 for PS-P4VP-sultone at concentration of (A) 1 and (B) 0.1 mg/mL in toluene at molar ratio 4VP/sultone ) 1:1 are shown in Figure 4b. They exhibit a linear relationship and go approximately through the origin, which confirms that the main peak is due to the translational diffusion. It is obvious that the Rh values do not change significantly with concentrations. The decay time distribution functions of PS-P4VP-sultone in toluene at 2 mg/mL at 298 K are shown in Figure 5a. A(τ)
Song and Lam
is the distribution of relaxation time in GEX(General EXponential) model.16 Looking at Figure 5a, two main relaxation peaks in the time distribution plot were observed. The two main peaks shift to the left when the measurement angles are increased. The peaks are assigned as the fast and the slow decay mode respectively, and they can be represented by two characteristic relaxation times, τf and τs (where τf < τs). τf represents the relaxation time of the fast decay mode (the left peak). τs represents the relaxation time of the slow decay mode (the right peak). Meanwhile, the ratio of the two peaks, Af/As, decreases as the measurement angle decreases. Af/As represents the relative contribution to the intensity from these two components. From the SLS results, the large apparent radius of gyration in the solution implies that large clusters exist in the polymer solution. For the large cluster, Af/As is angle dependent because the smaller measurement angle is sensitive to large particles. The relaxation process caused by the translational diffusion movement of molecules exhibit the following relationship between relaxation time and the scattering vector:17
τ ∝ 1/sin2(θ/2) This relation can be used to interpret the shifting of the peaks to the left when the measurement angle is increased. We define ∆τ as the distance of relaxation time between two peaks at a given measurement angle, i.e., ∆τ ) τs - τf. Since ∆τ ∝ 1/sin2(θ/2) for 0 < θ < 180°, 1/sin2(θ/2) would decrease with increasing angle, θ. The q2 dependence of Γ (Figure 5b) of slow mode exhibits a linear relationship and goes through the origin, which confirm that the main peak is due to the translational diffusion. On the basis of the diffusion coefficients, the apparent hydrodynamic radii of slow modes were also determined from the StokesEinstein equation. The size distribution consists of a large peak
Figure 4. (a) Plot of the hydrodynamic radius, Rh, vs the concentration for PS-P4VP-sultone (4VP/sultone ) 1:1) in toluene. (b) Two representative relationship of relaxation rates Γ and q2 for PS-P4VP-sultone at a concentration of (A) 1 and (B) 0.1 mg/mL in toluene at molar ratio 4VP/sultone ) 1:1 at 298 K.
Figure 5. (a) Decay time distribution function at different angles and (b) relationship of relaxation rates Γ and q2 for 2 mg/mL PSP4VP-sultone in toluene at molar ratio 4VP/sultone ) 1:1 at 298 K.
SelectiVe Betanization of PS-P4VP and Solution Properties
Langmuir, Vol. 22, No. 1, 2006 323
Figure 6. (a) Field autocorrelation function and (b) decay time distribution function of toluene solution of PS-P4VP-sultone at different molar ratios (4VP/sultone) values: A ) 1:0.1, B ) 1:0.2, C ) 1:0.5, D ) 1:1, at 1 mg/mL, at 298 K.
with an average Rh of about 97.9 nm, accompanied by a small narrow peak. Although the q2 dependence of Γ (Figure 5b) of the fast mode also exhibits a linear relationship, it does not go through the origin. No aggregate size was calculated for this mode. The main decay mode in the relaxation time distribution is attributed to interchain associative aggregates (97.9 nm) and other aggregates.18,19 Combined with the result of 1H NMR, it is reasonable to conclude that the aggregates at molar ratio of 4VP/sultone ) 1:1 are in fact micelles with P4VP in the core and PS in the corona, and increasing the amount of sultone facilitates the association of large aggregates. The autocorrelation functions and their respective decay time distribution functions of 0.1 wt% PS-P4VP-sultone at different molar ratio at a measurement angle of 90° are shown in Figure 6a and b. Figure 6a shows that the correlation curve shifted to the right with decreasing 4VP-to-sultone ratio. This means that longer decay time are needed at lower 4VP-to-sultone ratio, indicating the presence of large aggregates. This also can be seen from Figure 6b. The peak shifted to the right and become broader with decreasing 4VP-to-sultone ratio. At high molar ratio of 4VP to sultone, only one peak was observed, indicating one relaxation time of a species. At lower molar ratio, the peak becomes wider and shifts to the right, indicating larger aggregates were formed. However, at even lower molar ratio, two main peaks were recorded, indicating two modes of relaxations. A small fastest peak was also observed in the distribution function, and the origin of this peak is still unclear. However, there is the possibility that this small fastest peak is attributed to the internal mode.21 On the basis of the analysis above, it is evident that, at low molar ratio, large clusters exist in the polymer solution, hence giving rise to a large Rh value. In general, the two radii Rg and Rh differ in value. The prefactor F ) Rg/Rh varies for different particle architectures or geometries and is a valuable parameter for structure estimation. The F parameter was calculated for various macromolecular architectures and is tabulated in Table 2. In general, the F value gets larger when the particles become less compact and more anisometric. The main characteristics of the micelles are listed in Table 2. Table 2 shows the Rh of the micelles of PS-P4VP-sultone at different molar ratios at 1 mg/mL in toluene. The Rh of PSP4VP precursor copolymer is also listed as a comparison. The (16) http://www.fki.uu.se/robert.johnsen/gendist.htm. (17) Dai, S.; Tam, K. C.; Jenkins, R. D. Macromolecules 2000, 33, 404. (18) Wang, X. H.; Goh, S. H.; Lu, Z. H.; Lee, S. Y.; Wu, C. Macromolecules 1999, 32, 2786. (19) Li, C. Z.; Zhang, W. C.; Zhou, P.; Du, F. S.; Li, Z. C.; Li, F. M. Acta Polym. Sin. 2001, 4, 557. (20) Dai, S.; Tam, K. C. Macromolecules 2000, 33, 7021. (21) Huber, K.; Burchard, W.; Fetters, L. J. Macromolecules 1984, 17, 54.
Figure 7. TEM image of copolymer film of (a) PS-P4VP and (b) PS-P4VP-sultone (4VP/sultone ) 1:1).
hydrodynamic radius of the PS-P4VP was observed to be 22.5 nm. After loading of a small amount of 1,3-propane sultone (4VP/sultone ) 1:0.1), Rh was measured to be 24.8 nm. Further increase in the amount of sultone to 1:0.2, 1:0.5 and 1:1, Rh increases to about 41.7, 50.3, and 84.5 nm, respectively. The increase of Rh is explained by incorporation of sultone in the micellar core as shown in Scheme 1. There are two possible explanations to account for this phenomenon. The grafting of sultone to pyridine unit may expand the micellar core, thus increasing the micellar size. The other possible explanation is that the newly formed copolymer may result in aggregation of more polymer chains in toluene. This can be confirmed by using contrast-match small angle neutron scattering(SANS). This may also be confirmed by the increase of F value. The F value of the unmodified PS-P4VP in toluene was calculated to be 0.56, consistent with a globular structure of the micelles, as shown in Figure 7a. Upon addition of sultone at molar ratio of 10:1, F increases to 0.77, still consistent with a spherical structure. Upon further increasing the amount of sultone, the F value increases to 1.02. Such a F value is typical for a much less dense particle structure like a star molecule or a random coil conformation.21,22 When the molar ratio was increased further to 1:1, the F value increased sharply. A value of about 2 was observed. This is typical for nonspherical scattering objects such as rods or elongated species.23,24 However, the F value of elongated structures strongly depends on polydispersity. Looking at transmission electron micrographs, as shown in Figure 7b, the structures formed showed high polydispersity. The unexpected increase in the F values can be explained by a loss of control on the particle formation. Agglomerates which are nonspherical in shape are observed. The question is how can the incorporation (22) Burchard, W. AdV. Polym. Sci. 1999, 143, 111. (23) Schmidt, M. Macromolecules 1984, 17, 553. (24) Muller, A.; Burchard, W. Colloid Polym. Sci. 1995, 273, 866.
324 Langmuir, Vol. 22, No. 1, 2006
Song and Lam Scheme 2. Formation of Ribbon-Shaped Micelles
of sultone into micellar core lead to regular ribbon-shaped nanostructures? It is well known that there are two mechanisms governing the aggregation between particles, namely, diffusion-limited aggregation (DLA) and reaction-limited aggregation (RLA).25,26 A loosely connected cluster with a typical fractal dimension of 1.7-1.8 is obtained for DLA, and a denser cluster with a fractal dimension of 2.0-2.2 is obtained for RLA. Recently, Li and co-workers reported segmented wormlike aggregates resulting from the aggregation between individual micelles formed by ABC miktoarm stars.27 It is stated that, in forming a string, the different cores are able to share their coronas, thus protecting them from the highly unfavorable exposure to water. In addition, it was stated by Ma and co-workers that the sphere-to-rod transition requires sphere (micelle) collisions followed by reorganization into smooth cylindrical rods.28 As indicated in Scheme 2, in the case that a small amount of 4VP was betainized, the PS shell is thick enough and the density of the shells is high, the contact and the subsequent fusion between the cores can be prohibited, whereas in cases that more 4VP was grafted with sultone, the cores expand, resulting in a relatively low density of shells on the cores surface. This fluctuation in (25) Jullien, R.; Kolb, M. J. Phys. A 1984, 17, L639. (26) Cheng, H.; Wu, C.; Winnik, M. A. Macromolecules 2004, 37, 5127. (27) Li, Z. B.; Kesselman, E.; Talmon, Y.; Hillmyer, M.; Lodge, T. P. Science 2004, 306, 98. (28) Ma, Q. G.; Remsen, E. E.; Clark, C. G.; Kowalewski, T.; Wooley, K. L. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5058.
density may lead to the exposure and the coupling of the cores when two core-shell nanospheres approach one another. After a fusion of a core with other cores to become a section of a cylinder, the surface area of the core is considerably decreased. The density of shells on the core significantly increased the densification process. Upon formation of the ribbon-shaped object as shown in Figure 7b, the densification is enough to protect further coupling of the core with another core. It is difficult to differentiate whether the sphere-to-rod morphological transition is caused by alteration in the local Coulombic interactions within the nanostructures, a change in solvation or hydrophilicity of the core layer, or a combination of these factors.
Conclusions PS-P4VP betaine copolymers at different betainization degree were synthesized. Sultone-induced micellization in chloroform was observed by disappearance of pyridine ring hydrogen signal in 1H NMR spectra. The laser light scattering study and TEM show the morphology evolution process of the micelle in toluene, from spherical structure to elongated species and increasing micellar size upon increasing amount of sultone grafted. Additional studies are required to understand the effects of the introduction of sultone to polymer assemblies. Controlling the size and shape of micelles can be achieved by simply adjusting the amount of sultone grafted. This system is believed to have potential as vehicles for nanoreactor applications. LA0514415
