Organic Thin Films - American Chemical Society

Chapter 2

From Monolayers of a Tethered Polymer Melt to Freely Suspended Elastic Membranes

Downloaded by TUFTS UNIV on October 1, 2016 | http://pubs.acs.org Publication Date: August 6, 1998 | doi: 10.1021/bk-1998-0695.ch002

Werner A. Goedel Max-Planck Institut für Kolloid- and Grenzflächenforschung, Haus 9.9, Rudower Chaussee 5, 12489 Berlin, Germany Monolayers at the air/water interface of hydrophobic non glassy polymers with ionic head groups have been used as a model system to investigate thermodynamics of "polymer melt brushes". The polymer chains are tethered to the planar surface but are free to assume a distorted 3-dimensional conformation. The thermodynamic properties are dominated by the loss in entropy due to a stretching of the polymer coils away from the interface upon increasing tethering densities. Monolayers of polyisoprenes wit ionic head groups bearing photoreactive side groups (anthracene) have been cross-linked on the water surface via irradiation with U V light. These 40 nanometer thick stabilized films can be transferred to cover holes in solid substrates of 0.3 mm diameter and form freely suspended elastic membranes.

Block copolymers or (semi) telechelic polymers, in which one of the blocks or a small head group drastically differs from the main polymer chain, can be regarded as large amphiphiles. The head groups can aggregate to form (inverted) micelles (1-3) or can strongly adsorb to interfaces in contact with the bulk or a solution of the polymer (4,5). If polymer chains are bound with a surface active head group to an interface with a lateral spacing considerably less than the dimensions of the undisturbed polymer coil, they form a so called "polymer brush". In this brush, the neighboring chains interact with each other and form a continuous film. In general, one has to distinguish two cases: a solvent free-brush of a polymer melt ("melt brush" or "dense brush", (6-9) Figure la) and a polymer brush in contact with a good solvent ("swollen brush", (10) Figure lb). In the first case there is no solvent to fill voids between polymer segments. Therefore, the balance between strong attraction and hard core repulsion essentially fixes the concentration of segments at a value

10

©1998 American Chemical Society

Frank; Organic Thin Films ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


11

^ Z ^ Î R

JUL

water

o

.

β β

°. ο ·

β

.. .

b

a i r

·\ · ·

β

·

β

β

c polymer Β polymer A polymer Β

Figure 1 : Schematic comparison of (a) monolayers of tethered polymers that are free of solvent, (b) monolayers of tethered polymers swollen with solvent, and (c) one lamella of phase separated block copolymers (adapted from ref. 24)



12 close to the bulk density. In the latter case the interaction is primarily osmotic and the concentration of segments is variable. In both cases, the interaction between neighboring chains leads to a distortion of the coils, which are forced away from the interface. The "brush" is considerably thicker than monolayers of polymers which adsorb with each repeat unit. The "melt brush" has its main importance in the context of phase separated block copolymers and the often complex morphologies of these systems (e.g. lamellar [see Figure lc], hexagonal, cubic.) are a result of the balance between the interfacial tension and the deformation of the closely packed polymer chains (7, 9, 11). While there is considerable interest in these bulk systems, it has been very advantageous to study melt polymer brushes as monolayers at flat surfaces. In such a monolayer it is relatively easy to determine and tune the surface concentration of the head groups and to give the system a preferred orientation in space. In order to study equilibrium thermodynamics of these Monolayers it is necessary, however, to use polymers with low glass transition temperatures - e.g. polydienes (72), polydimethylsiloxanes (13 14) or perfluoropolyethers (75). If polymers like polystyrenes are investigated at temperatures below the glass transition, they form hard films or hard particles (16, 17) and the properties of these systems are not significantly changed by the presence of a surface active head group (18, 19). Monolayers of suitable polymers with surface active head groups are convenient model systems to study the thermodynamics of melt polymer brushes; in addition they have a special advantage: Crosslinking should lead to rubber elastic membranes. If the crosslinking is made on the water surface, transfer via the Langmuir-Blodgett technique (LB-technique) (20) offers the unique opportunity to cover holes in a solid substrate and thus create freely suspended membranes. While Langmuir-Blodgett films of low molecular weight substances and liquid polymers easily rupture during transfer, stable suspended membranes have been achieved using glassy polymers often stabilized in addition through crosslinking (27, 22). For various applications, like membrane separation processes or micromechanics, it will be advantageous to have a choice between rubbery and glassy membranes. Freely suspended membranes can be generated as well via spreading a melt or a viscous solution of a polymer across the opening of a substrate followed by crosslinking. Using this process, however, it usually is difficult to achieve films of uniform thickness. Especially at the rim of the opening, one usually obtains a meniscus of excess material (23). By transferring preformed crosslinked membranes one might overcome this difficulty. The paper presented here is composed of two parts: The first part focuses on using monolayers of polyisoprenes with ionic head groups as model systems for a "melt polymer brush"; the second part investigates the potential to form freely suspended elastic membranes via crosslinking.


13 Monolayers as Model Systems for a "Melt Polymer Brush" Linear polyisoprenes with a sulphonate head group have been prepared via anionic polymerization followed by reaction with propanesultone. The characterization of the polymers is summarized in Table 1 (The polymers are copolymers of 71% 1,4cis- 22% 1,4-trans- and 7% 3,4-isoprenyl repeat units, a simplified structure is in cluded in Figure 2. The synthesis has been reported in ref. 24). The polyisoprenes with sulfonate head groups can be spread onto the water surface. The isotherms of the five polymers investigated are shown in Figure 2. The isotherms significantly expand with increasing molecular weight.


Table 1 : Characterization of the polyisoprenes with sulfonate head groups. number of repeat units, Ν

140

M ( G P C ) [g/mol]

9638

n

M /M w

1.14

n

307

538

675

810

20820

36540

45900

55020

1.04

1.02

1.02

1.02

Thermodynamics. If one takes into account the conformational changes of the polymer chain, the isotherms can be interpreted quantitatively ( a more detailed de scription of the analysis is given in ref. 24) In a simple picture one can assume that all polymer chains are bound to the aqueous phase by the head group, while the end group is at the polymer-air interface (see Figure 3a). The surface pressure is given by the difference between the surface tension of the pure water and the film tension of the covered surface; surface and film tension are given by the first derivative of the Gibbs free energy, G, with respect to the area, A . If the volume of the system does not change, changes in the Gibbs free energy, G, are equal to changes in the Helmholtz free energy F:

(1.) /water/ air -interface

/polymer film

The chain ends are separated by the film thickness, R. Therefore, the chains act like springs and store an elastic free energy (25-27): 3

^elastic ~

1

ΨΪ

2

R

(2.) 2

η = number of polymer chains in the monolayer, = the unperturbed mean square end-to-end distance, is proportional to the chain length. k = Boltzmann constant, Τ = temperature 0

2

0


B

14


I • ' • ' I

100

150

200

250

1 1 1 1

300

ι

350

Α/η [À2]

Figure 2 Isotherms of polyisoprenes with sulphonate headgroups and different chain lengths , Ν = number of repeat units, (averages of at least 5 measure ments), (reproduced from ref. 24)

a

b

Figure 3 Schematic comparison between (a) an uniformly stretched brush, and (b) a non uniformly stretched polymer brush according to the Semenov scenario, (reproduced from ref. 24)


15

If one assumes that the hydrophobic region of the monolayer is free of sol vent and has a constant density (close to the bulk value of the polymer), the total volume of the film is given by the product of film thickness and area covered by the film, A . Thus the film thickness is inverse proportional to area per head group:

R A = n N v

R = N-v-f—]

(3.)

Ν = number of repeat units per chain, ν = volume of a repeat unit. And the elastic free energy is inversely proportional to the square of the area.


3

elastic =

2

3η ·Ν ·ν k T -—j-p-

2

A

B

\

(4.)

to

The first derivative of the elastic energy is given by: 3R,

( ^ V3 -

with

2

3k Tcc N

(5.)

B

2

«=v/j(r ) ^i 0

In the case of a freely jointed chain, the constant α can be interpreted as the cross-sectional area of a chain segment A l l other contributions to the surface pressure that are not due to the stretching of the polymer chain are assumed to be independent of the chain length, N . These contributions here are pointed out only in a general form as a function of the area per molecule f(A/n, not N) : n =f ^ n o t

2

NJ+c-k T-a -N-^ B

(6.)

with : c = 3 Two features of this description are important in the following data treat ment: (i) at a given area per head group, A/n, the "elastic part" of the surface pressure de pends linearly on the chain length; (ii) the "elastic part" of the surface pressure is proportional to the third power of the area per head group. In order to test the linear dependency of the surface pressure on the chain length, Figure 4 shows the surface pressure data as a function of chain length, the


16 third parameter being now the area per head group. The theoretically predicted linear dependency can only be tested in a regime of A/n where more than two isotherms can be measured. In this regime between 170 Â and 250 Â , all points fall onto a straight line in accordance with the predictions. In order to test the second prediction, one might take the slope of the abovementioned representation and thus eliminate the first two terms in eq. (6.): Π, Άν3 (7.) 2

2


If the above equation is rescaled by the "cross sectional area of the chain", a, one obtains the dimensionless representation: ΔΠ α

(A IX*

,

ίΔΠ

α

1 ,

„ , [A 11

The double logarithmic representation of eq. (8.) predicts a straight line, the slope of that line is -3 and the intercept is given by the prefactor c. In this rescaled plot all pairs of polymers should fall on a single line. The plot of the experimental data according to eq. (8.) is shown in Figure 5. The straight lines represent the theo retical predictions; the data are compared to the theoretical prediction without any fitting procedure. In this double logarithmic plot the pressure differences between pairs of polymers actually superimpose into a single line; the slope is close to the predicted value of -3. The assumption that the free ends are located at the "upper" surface of the film seems to be quite artificial. A scenario as depicted in Figure 3b) is more likely. It can be shown, however, that any type of affine deformation will lead to the same power law. The prefactor c, however, may differ from the uniformly stretched brush. For example, the more elaborate Semenov theory (7,8) yields the same power law, but with the prefactor c = π /4 « 2.47. This result reflects the fact that in the Semenov brush the chains are allowed to assume thermodynamic equilibrium, while in simple scenario they are fixed at thermodynamically unfavorable positions. As can be seen from Figure 5, the result of the Semenov theory fits the experiment better than uniformly stretched brush. 2

Beside this general agreement between experiment and theory, the experi mental data in Figure 5 lie upon a slightly bent curve, rather than on a straight line, the deviation being mostly pronounced at lower areas per head group or for polymer chains shorter than 300 repeat units. Earlier experiments indicated that there is no significant elastic contribution for chains shorter than 100 atoms (28). Regardless of the details of the model used in the analysis, any affine deformation of the polymer film should give rise to a straight line with the slope -3 in the double logarithmic rep-


17


Ν « 140

307

538

675

810

Chain Length, Ν (rep. units)

Figure 4 Surface pressure of Polyisopren-S0 films as a function of chain length at con stant area per head group (same data as in Figure 2 for clarity only the data for a limited number of areas are included).(reproduced from ref. 24) 3

Α/η [A2]

-2.1

150

200

250

300

"uniformly stretched brush """Semenov type brush

-2.4

0.1

A.2.7 ^ -3.0

i-3.3 ο

-3.6 0.8

(N

2

Organic Thin Films - American Chemical Society

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