Polymer Gels - American Chemical Society

Chapter 8

The Behavior of Amphiphilic Block Copolymers in Water and Their Association Behavior 1

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Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch008

Dirk van Eck , Siegfried Höring , Karsten Busse , and Jörg Kressler 1

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2

Fachbereich C h e m i e a n d Fachbereich Ingenieurwissenschaften, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle, Saale, Germany

Amphiphilic di- and triblock copolymers containing blocks of the sodium salt of methacrylic acid and a methacrylate with a perfluorinated side chain are water soluble when their content of the hydrophobic block is less than 10 mol%. They form micelles in water that can be detected by electron microscopy or dynamic light scattering. The micelles are able to encapsulate colloidal gold particles. At higher polymer concentrations an intermicellar aggregation as a precursor of gel formation occurs as demonstrated by viscosity measurements. This happens at significantly lower concentrations of triblock copolymers in water compared to the respective diblock copolymers.

Hydrophobically modified water soluble polymers (HMWSP) are gaining importance due to their unique rheological and structural properties. The attention has been focused mainly to HMWSP based on random copolymers or water soluble polymers endcapped with hydrophobic groups (1-6). Furthermore, water soluble amphiphilic block copolymers can be used for numerous applications governed by their structure (7,8) e.g. as surfactants (9,10) and drug delivery systems (11,12). In dilute solutions the surface is covered firstly which leads to a decrease of the surface tension. Above a sometimes rather low critical micelle concentration (c.m.c., 13-75) the formation of micelles occurs. With increasing concentration the number of micelles in the solution increases whereas the size or the aggregation number remains mainly constant (16,17) and the surface is not affected (plateau in surface tension). Besides spherical micelles with a core-shell-structure other types like hollow spheres or rodlike

122

© 2003 American Chemical Society In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


123 structures (18,19) may occur. In the case of diblock copolymers the overlap of different micelles at elevated concentrations leads to gelation (20). For A-B-A triblock copolymers in addition bridging by single chains between two micelles may occur which leads to a higher elastic modulus and larger average lifetime (21). Intermicellar associations lead e.g. to a decrease in surface tension (22). Higher concentrated i.e. semidilute solutions of triblock copolymers form network structures as can be seen by rheological measurements (23). For fluorinated units in the hydrophobic blocks the tendency for phase separation is very strong due to the high surface activity (24). In this work, water soluble A-B di- and A-B-A triblock copolymers were synthesized by sequential anionic poly merization of tert-butyl methacrylate (tBMA) and 2-(N-methylperfluorobutanesulfonamido) ethyl methacrylate (FMA) (Figure 1). These block copolymers can be transformed into water soluble species when the FMA content is moderate and when tBMA is transferred into the sodium salt of poly(methacrylic acid). The details are discussed below. The preparation of these block copolymers was extensively investigated by spectroscopic and thermal methods. The block copolymers in the solid state were studied by A F M and SAXS. The solutions of the transformed (amphiphilic) block copolymers in water were characterized by rheology, TEM and dynamic light scattering. Furthermore, the micelle structure of an A-B-A triblock copolymer in water is characterized by SAXS measurements. Finally, the addition of gold colloids was taken into consideration.

Experimental Part Polymerizations. All polymerizations were carried out at -78°C (25). The reaction components were added to a previouslyflamedand argon-purged glass reactor. Di- and triblock copolymers have been synthesized in tetrahydrofuran (THF, Merck) under Ar atmosphere with (l,l-diphenyl-3-methylpentyl) lithium/LiCl and naphthalene potassium as initiator. THF was dried over CaH , distilled from the purple sodium salt of the benzophenone dianion, stored over K/Na alloy, and finally freshly distilled from the alloy. Naphthalene potassium was prepared through the reaction between naphthalene (Fluka) and sodium (Merck) in THF at 0°C (26). l,l-diphenyl-3-methylpentyllithium was synthesized by the reaction of 1,1-diphenylethylene (Merck) with secbutyllithium (Merck; dissolved in cyclohexane) at 25°C just before the polymerization. Terr-butyl methacrylate (tBMA; Polysciences Europe GmbH) was dried and distilled over CaH . Prior to use, it was condensedfrom(C H ) A1 under vacuum according to the standard procedure (27). 2-(Nmethylperfluorobutanesulfamido)ethyl methacrylate (FMA) was kindly supplied by the Bayer AG, recrystallized twice from methanol, and finally sublimated under high vacuum. At first the THF (only in FMA homopolymerization after 2

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In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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5

3


124 preaddition of LiCl) and a solution of the initiator in THF were filled into the reactor through a rubber septum with a stainless steel capillary followed by the dropwise addition of the calculated amount of the first monomer (tBMA). After stirring for 1 h a prechilled THF solution of the second monomer (FMA) was added. The reaction was allowed to last 9 h and then stopped with methanol/water. The polymer was isolated by precipitation in methanol. For the decomposition of thefert-butylester a certain amount of the block copolymer was heated for 2 h at 200°C in vacuum. Some polymers obtained by sequential anionic polymerization are listed in Table I. Size Exclusion Chromatography (SEC) Measurements. For determination of molar mass and molar mass distribution, the samples were prepared by dissolving the purified polymer solid in HPLC grade THF. The SEC was performed in THF at 25°C on a Knauer Chromatograph equipped with styragel columns and refractive index detector at a flow rate of 1.0 ml/min. Poly(methyl methacrylate) standards from Polymer Standard Service were used for calibration. Fluorine-19 and Proton NMR measurements. H - and F-NMR spectra were recorded on a Varian spectrometer operating at 300 MHz ( H) and 282 MHz ( F)inCDCl at25 °C. Thermolyses. The thermal ester splitting of the tBMA units was performed at 200 °C under vacuum. For kinetic studies the reactions were recorded with a FTIR spectrometer (Bruker). Preparation of Aqueous Polymer Solutions. In order to convert the anhydride in sodium methacrylate units the samples of the ester splitting (1- 2 g) were refluxed for 1 h with 1 N NaOH. The solutions were purified by dialysis against deionized water using a dialysis membrane tube (Nadir®, 2.5-3.0 nm) and then filtered with a 0.8 um CA membrane filter. The solid polymer obtained by evaporation of the solvent was dried at 40°C in a vacuum oven and dissolved in deionized water for subsequent measurements. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Measurements were carried out with a MultiMode A F M (Digital Instruments). The tapping mode was applied. The polymer was dissolved in 2,2,3,3-tetrafluoro-1 -propanol or THF and a film was formed by slow evaporation of the solvent at ambient temperature. The samples for T E M were dissolved in water and in some cases colloidal gold was added. The solutions were cast onto copper grids covered with a carbon film. A Zeiss-CEM902 was used with an acceleration voltage of 80 kV. Small Angle X-ray Scattering (SAXS) SAXS measurements were performed in an evacuated Kratky compact camera (Anton Paar K.G.) with an 80 urn entrance slit. Cu K radiation with a wavelength of A,=0.154 nm was used. The scattered intensity, 7, was recorded by a scintillation counter in a step-scanning mode at room temperature. The scattering profiles were corrected for background scattering and desmeared. The cosine Fourier transformation of the scattering curve yields the linear correlation function K(z) defined by (28) 1

19

l

19

3

a

(i)


125

S

X

^ Y° C(CH ) 3

(tBMA)

3


O

^ J v ^ O ^ / ^ S O ^ F g

(FMA)

I

0

Figure 1. Monomers used for sequential anionic polymerization.

Table I. Synthesized di- and triblock copolymers of F M A (A) and tBMA (B) b

Polymer*

M g/mol

M g/mol

M /M„

31Di02

32 600

30 900

1.06

2

3

52Tri07 57Tri07 57Tri08 72Tri26 49Tri38

57 62 62 80 56

51 56 57 72 49

1.10 1.11 1.09 1.12 1.15

7 7 8 26 38

11 11 14 40 54

b

w

n

100 100 400 900 700

700 100 000 400 400

C

FMA mol% vol%

w

a

Sample name: first number is the overall M -value, Di is A-B, Tri is A-B-A, second number means mol% FMA. determined by SEC in THF using PMMA standards, determined by H - , and F-NMR spectroscopy. n

[

19



126 s is the scattering vector (sometimes q=27ts is used). The samples under investigation were 49Tri38 in the solid state and dilute solutions of 57Tri07H in water with concentrations between 0.13 and 1.4 wt%. Dynamic Light Scattering. The experimental equipment consists of a DLSSLS 5000 Laser Light Scattering Goniometer (ALV Langen, Germany), equipped with a 140 mW Nd:YAG laser (diode pumped, frequency doubled; ADLAS Liibeck, Germany) with a wavelength of 532 nm, and an ALV-5000 Multiple Tau digital correlator. The scattered light was detected at an angle of 90° and the measurements were carried out at T=25°C. The correlation functions were analyzed with the CONTIN (29) program. The samples were held in a cylindrical glass vessel. An aqueous solution of 57Tri07H with c=0.26 wt% was filtered through a 5-um Millipore filter. The gold colloid is a 0.013 wt% solution. The dilute samples used for dynamic light scattering are (a) 6.5-10" wt% gold colloid in water, (b) 0.26 wt% of 57Tri07H, and (c) a solution with 4.5-10" wf% gold colloid and 0.18 wt% 57Tri07H. The samples are measured some minutes after preparation. Then sample (c) was put for 30 minutes in an ultrasonic bath and kept for some minutes before measuring again (d). The pair correlation function g (t) obtained by dynamic light scattering is analyzed to get effective diffusion coefficients D of the samples. The hydrodynamic radius of assumed hard spheres is calculated via the StokesEinstein equation r =kT/(67cr]D ^ where k is the Boltzmann constant and 77 is the viscosity of the solvent at temperature T. Rheology and Wilhelmy plate method. The relative viscosities were determined by using a capillary viscometer (capillary diameter 0.6 mm). The measurements were carried out at 25 °C. For surface tension measurements a digital tensiometer K10T of Kruss was used equipped with a platinum plate. 4

4

2

eff

hyd

e

Results and Discussion The synthesis of block copolymers of tBMA and FMA is outlined above. Figure 2 shows the transformation mechanism into water soluble species. A selective two step transformation of the tBMA block into the water soluble poly(sodium methacrylate) was carried out. Firstly, anhydride, isobutene and water is formed at 200°C and then the cleavage of the anhydride is done in IN NaOH under reflux. This procedure does not influence the ester group with the perfluorobutane side chain. The reaction can be followed by FT-IR spectroscopy (30). In order to verify that block copolymers are formed it is necessary to demonstrate a microphase separation of the respective blocks. This can be done e.g. by atomic force microscopy (AFM) as demonstrated in Figure 3. Figure 3a shows the morphology of the triblock copolymer 49Tri38 after slow evaporation of tetrahydrofuran (THF). The phase contrast mode was employed. There is a microphase separation but the phases do not appear very well ordered. Also thermal annealing above the glass transition did not yield a more perfect structure.


127


Sequential block copolymerization ^ ^ ^ ( p m ^

n tBMA + m FMA l.l-D^enyl-3-methy.pentyHithiur^ 1st monomer 2nd monomer THF, -78 °C n tBMA

2m

+

F

M

A

NaP"^ene Potassium

1st monomer 2nd monomer

(

F

H

A

)

^

THF, -78 C

(

f

f

i

M

A

)

|

r

_

f

M

A

1

CHs C H pHs ? \ y =^-^—• ^ C H — C C—CH —

3

200 °C

*CH — 2

_

2

COOC(CH ) 3

tBMA units

Alkaline treatment

3

)

m

triblock copolymer

Thermal treatment ?

M

2

0 ^

\

0

^

3

2

I

C

H

C

I

C=0

^ 0

O

1 N NaOH

CpH

3

-

COO Na Figure 2. Synthesis of amphiphilic di- and triblock copolymers.


+


128

Figure 3. A F M measurements on a film of 49Tri38 a) castfromtetrahydrofuran, b) castfrom2,2,3,3-tetrafluoro-1 -propanol. A better order could be achieved by using 2,2,3,3-tetrafluoro-1 -propanol (TFP) as a solvent (Figure 3b). Obviously this is due to the fact that TFP is a more selective solvent for the blocks with the perfluoro alkyl side chain. Thus, the tBMA block collapses during the evaporation of the solvent and the lamellar order appears more significant. This is afrequentlyobserved phenomenon that using very selective solvents leads to a better order in cast block copolymer films (31). Since the block copolymer 49Tri38 is symmetrically when assuming


129 volumefractionit is also reasonable that a lamellar morphology is formed. The same result was obtained by small angle X-ray scattering (SAXS) as shown in Figure 4.


4000i

0.0

0.1

0.2

0.3

0.4

0.5

-1

s [nm ] Figure 4. Intensity versus s and correlation function of 49Tri38 slowly cooled from the melt obtained by SAXS. 1

The SAXS trace has a first order maximum at s* = 0.041 nm" . Higher order maximums occur at multiples of s*. Thus it is definitively demonstrated that a lamellar order is also achieved in bulk samples which were cooled from the melt. The inset shows the calculated correlation function. The long period is obtained to be 23 nm. After the characterization of the synthesized block copolymers it is now interesting to analyze the water soluble species. It should be remembered that these species were obtained after transformation of the tBMA blocks into the respective sodium salt of methacrylic acid. We use the codes given in Table I but we add an H in order to make clear that the water soluble species are under consideration. The surface activity of the block copolymers in water was studied by surface tension measurements as demonstrated in Figure 5. Figure 5a shows the decrease of the surface tension of water with increasing amount of the diblock copolymer 31Di02H. This is the classical behavior (32,33) and the concentration where the surface tension stays constant with increasing amount of block copolymer in water is called critical micelle concentration. For this block copolymer the c.m.c. value at room temperature is 67 g/L This situation is slightly different for the triblock copolymer 52Tri07H (see Figure 5b). The surface tension decreases at much lower concentrations compared to the diblock


130

a)

b) 70

i«. c o

O 60-

"«66 O

64 O 0

^51CO 4850

cmc

100

CO 0.0


r-

—I

c(g/l]

1.0

c

1

1.5 c[g/I]

1

1

2.0

1

1

2.5

Figure 5. Surface tension of block copolymers in water at room temperature. a)31Di02Hb) 52Tri07H.

copolymer. Then the surface tension decreases further after reaching a critical value c*. If this overlap concentration, which is at 1.3 g/1 for this measurement, coincides with c.m.c. cannot be decided at this point. The low concentration is an indication that the free energy for the formation of micelles and the free energy for the amphiphilic molecules to cover the free surface is in a very similar order of magnitude. This seems to be more relevant to triblock copolymers. For diblock copolymers it is obvious that in a first step the free surface is covered and then the micelles are formed. This size of the micelles could readily be obtained by dynamic light scattering (34). Intensity fraction distributions of hydrodynamic radii obtained by dynamic light scattering for solutions of the polymer 57Tri07H, the gold colloid, and mixtures of them are depicted in Figure 6. The polymer concentration was chosen near c* in order to reduce the concentration of single chains but without having crosslinking effects between different micelles by single triblock molecules. The hydrodynamic radius of gold particles (sample (a)) was determined to r^y =9 nm. The hydrodynamic radius of micelles obtained from a solution with 0.26 wt% 57Tri07H (sample (b)) is much larger with approximately 70 nm. Putting both materials together in one sample without destroying the already formed micellar structure, the intensityfractiondistribution of the hydrodynamic radius shows two peaks close to the radii distributions of the pure components (c). Caused by the fitting procedure the mean value of the peaks has a larger scattering, but two peaks with nearly the same intensity are always distinguished. Treating the sample with ultrasound, the polymer micelles will be affected. The intensity fraction distribution of the hydrodynamic radius obtainedfromsample (d) shows clearly a decrease of larger clusters while the other peak now appears with an increased radius of about 22 nm. The covering of gold by the polymer can also be detected by T E M as shown in Figure 7. d



131

10"

1

10°

1

TO

10

2

10

3

10

4

1 0"

1

10°

10

1

10

2

r [nm]

10

3

10

4

r [nm] 4

Figure 6. The dilute samples used for dynamic light scattering were (a) 6.5-10" wt% gold colloid in water, (b) 0.26 wt% of 57Tri07H, and (c) a solution with 4.5-10 wt% gold colloid and 0.18 wt% 57Tri07H. The samples were measured some minutes after preparation. Then sample (c) was put for 30 minutes in an ultrasonic bath and kept for some minutes before measuring again (d). -4

Figure 7. TEM pictures, the scale bars are 20 nm a) a micelle of 52Tri07H, b) gold colloid (black particles) covered with 52Tri07H.



132 Figure 7a shows a typical micelle. The micelles were transferred from a solution in water onto a carbon film. The samples were not stained for TEM. The dark part is the fluorine containing block due to its larger electron density compared to the other block. When colloidal gold particles are present (see Figure 7b) they are surrounded by the block copolymer after ultrasonic treatment. It is obvious that the hydrophobic block is attached to the gold particles (the intermediate gray value). The effect of covering gold particles by amphiphilic block polymers is well known and is used for the formation of nanoparticles (35). The size of the micelles could also be obtained by SAXS measurements. The background corrected and desmeared intensity distributions are depicted in Figure 8. The intensity is divided by the respective concentration, all curves show the same behavior. Each trace can be approximated by either the OrnsteinZernicke function (eq 3, Figure 9) or a core-shell structure of spheres (eq 4). The correlation length £ and the radii Rj (core) and R (shell) were determined for several concentrations close to c* (Table II). 2

0$, O

• o

• •

0.13 wt% 0.26 wt% 0.7 wt% 1.4 wt% —i 0.5

0.0

.

1 1.0

1

1 1.5

1 2.0

1

[——> 2.5

1

1— 3.0

—I 3.5

1

q [nm ]

Figure 8. The corrected intensity distributions divided by the respective concentration from SAXS measurements (original data in inset, including background) for solutions of 57Tri07H with increasing concentration. i

m = h

1+ f

(3) 2 „2

sin(i?! q)-R q x

qos(R q) y

+

sin(i? q)-R q 2

2

cos(i? q) 2

V


\

2

(4)

133

- 0.7 wt% Ornstein-Zernicke 3

5=10.9 nm


CO c CD

n

i 0.1 1

q [nm' ] Figure 9. Intensity distribution of an aqueous solution with 0.7 wt% 57Tri07H and Ornstein-Zernicke approximation. The assumption of a spherical core-shell behavior cannot be verified unambiguously, even though the obtained radii (Table II) agree with £«R /2 for small Rq (eq 5). The outer radius remains nearly constant at about 18+2 nm. This means that the size of the micelles does not depend on the concentration. The structure of the micelles could be worm-like (Figure 7a), as known from other triblock systems (7, 36). 2

r

I(q)cc

< sm(Rq)-Rqcos(Rq)^

u

f * A

(5)

9

Table II. Size of 57Tri07H micelles in water at 25°C obtained by SAXS using eqs (3) and (4) concentration 0.13 wt% 0.26 wt% 0.7 wt% 1.4 wt%

Ri(nm) 6 6.4 5.7 6.9

R (nm) 15.6 18.2 18.1 17.8 2

4(nm) 7.3 9.2 10.9 9.5

The SAXS measurements were carried out up to the concentration range where intermicellar association takes place. The change in the rheological behavior was also detected by viscosity measurements. Figure 10 shows the reduced viscosity


134 14000

/

n

12000 10000 1

/

1 1

A

/


/

/

/

/

4000 /

4__1

0,5

/

/

/

/

/

/

/

/j

— " A

1

1,0

/

t

/

/

/

1

1

6000

/

/

/

/A

V /

1

8 0 0 0

/

/

/

/

1

'

1,5

c!

1

2,0

•

1

•

2,5

1

3,0

c[g/l] Figure 10. Reduced viscosity of the aqueous solution of 52Tri07H as a function of polymer concentration at 25°C. 77^ of the aqueous solution of 52Tri07H as a function of the polymer concentration. The concentration c* of 1.9 g/1 can be considered as the overlap concentration, i.e. the begin of gel formation. The same effect can be observed at the other polymers (25). Conclusions It is demonstrated that amphiphilic di- and triblock copolymers can be obtained by sequential anionic polymerization of tBMA and FMA. A selective transformation of the tBMA block into the respective Na salt of the methacrylic acid is possible. Various experimental methods indicate the nanophase separation in bulk. Micelles formed in water were observed by T E M and dynamic light scattering. Gold colloids added to the amphiphilic hydrogels are coated with the FMA block. Intermicellar associations as indication for the onset of formation of amphiphilic networks were observed by viscosity measurements. Acknowledgement. The authors thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Dr. Ralf Thomann (Albert-LudwigsUniversitat Freiburg) carried out the T E M measurements. The A F M measurements did Rameshwar Adhikari.


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136 (30) Arndt, K.-F.; Richter, A.; Ludwig, S.; Zimmermann, J.; Kressler, J.; Kuckling, D.; Adler, H.-J. Acta Polym. 1999, 50, 383-390. (31) Hasegawa, H.; Takana, H.; Yamasaki, K.; Hashimoto, T. Macromolecules 1987, 20, 1651. (32) Yu, G.-E.; Deng, Y.; Dalton, S.; Wang, Q.-G.; Attwood, D.; Price, C.; Booth, C. J. Chem. Soc. Faraday Trans. 1992, 88(17), 2537. (33) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (34) Derici, L.; Ledger, S.; Mai, S.-M.; Booth, C.; Hamley, I. W.; Pedersen, J.S. Phys. Chem. Chem. Phys. 1999, 1, 2773-2785. (35) Nanoparticles and nanostructuredfilms;Fendler, J. H., Ed.; WileyVCH: Weinheim 1998. (36) Schillén, K.; Brown, W.; Johnsen, R. M . Macromolecules 1994, 27, 4825.


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