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Low surface energy polymers and surface-active block polymers IV: Monolayers at the air-water interface
Thin Solid Films, 190 (1990) 163-173
163
LANGMUIR--BLODGETT AND RELATED FILMS
LOW SURFACE ENERGY POLYMERS AND SURFACE-ACTIVE BLOCK POLYMERS IV: MONOLAYERS AT THE AIR-WATER INTERFACE M. H. LITT, K. S. SHIH, J. B. LANDO AND S. E. RICKERT
Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106 (u.s.A.) (Received September 29, 1989; accepted February 23, 1990)
The spreading of five block copolymers of 2-(p-(t-butyl)phenyl) oxazoline (hydrophobic block) and 2-ethyl oxazoline (hydrophilic block) at the air-water interface was studied using Langmuir-Blodgett film balance techniques. Three regions of compression were found. At low force, the hydrophilic block was being compressed. This continued until the hydrophobic tails contacted each other. The second region was the compression of the hydrophobic tails lying almost fiat on the water surface. As the pressure increased to 40 mN m- 1, the hydrophobic tail rotated so that the side chains were perpendicular to the surface. The area of the hydrophobic block in the most compressed state was about equal to its thickness multiplied by its chain length. As chain length increased, packing efficiency decreased slightly. All compressed phases remained liquid like. The films remained as monolayers up to the highest pressure (about 70mNm-1) permitted by the trough.
1. INTRODUCTION In a series of papers by Litt et al. 1"2 the synthesis and characterization of monodisperse surface-active block co-oligomers with different weight ratios of hydrophobic blocks (derived from 2-(p-t-butylphenyl)-2-oxazoline) and hydrophilic blocks (derived from 2-ethyl-2-oxazoline) have been reported. The effectiveness of these oligomers as surfactants in emulsion polymerization was investigated. It was found that stable latex was obtained in the emulsion polymerization of butyl acrylate (5~ in water). The use of these oligomers as inverse emulsifiers in the emulsion polymerization of styrene was also reported and the various factors that affect the stability of the emulsions were studied. It was found that the cross-sectional area of the surfactant monolayers was 0.77 nm 2 molecule-1 as determined from the average droplet size analysis. This value corresponds quite well to the cross-sectional area (0.88 nm 2 molecule-1) calculated from X-ray data for t-butylphenyl homopolymer 1'3. It was concluded that the surfactant monolayers were highly ordered with their hydrophobic units packed vertically at the interface between the aqueous 0(M~6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
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M.H. LITT et al.
phase and the organic (styrene) phase. The packing of the monolayers of these surfactant oligomers at the air-water interface has not been studied and remains an interesting subject. The objective of this research was to characterize the behavior of these surlhctant monolayers at the air-water interface by obtaining isotherms on a film balance 4. 2. EXPERIMENTALDETAILS
The synthesis procedures for the block co-oligomers were reported elsewhere 2 and will not be discussed here. Figure t shows the chemical structures of the copolymers used in this study. The t-butylphenyl oxazoline unit is hydrophobic. The ethyl oxazoline unit, which is water soluble, is the hydrophilic group on the chain.
Et
I
CO
CO
b
I
- NCH2CH2 NCH CH2 X: Hydrophobic Y: Hydrophilic Fig. 1. Structure of block copolymer components: hydrophobic block, poly (p-(t-butyl)benzoyl ethyleneimine);hydrophylic block, poly(N-propionyl ethyleneimine).
The force-area curves (isotherms) were obtained using a Teflon barrier attached to an inductive linear transducer to measure the differences in surface pressures between a clean water surface and that containing the film. The brass trough was coated with 5 x 10- 3 in of FEP, a copolymer of tetrafluoroethylene with small amounts of a fluorovinyl ether. An IBM PC was interfaced with the film balance for data acquisition and processing. The balance was placed on a vibration isolation table to d a m p environmental disturbances. All the measurements were carried out in a class 10 laminar flow area inside a class 100 clean room. The water used for the subphase was obtained via a multistep purification process. T a p water was first passed through three particle filters and an activated carbon filter; then it went through a water softener. The water then was purified in a reverse osmometry unit. Finally it was distilled in a column. The whole system was purged with nitrogen gas constantly. The water resistivity was at least 17 M I I cm. The p H of the water was 7 after distillation but decreased rapidly to about 5.5 after exposure to air, as a result of the CO2 absorption. The solvent used was H P L C grade chloroform from Aldrich Chemical Company. The solutions were prepared in 10ml volumetric flasks. The concentrations were between 0.95 and 1.11 m g m l - ~ . A fixed amount of solution was spread on the water surface through a digital pipette with a m a x i m u m capacity of 100 ~tl. The water temperatures were controled by circulating thermostatted water underneath the brass trough. The temperature of the water surface was measured by
165
POLYMER MONOLAYERS AT THE A I R - W A T E R INTERFACE
a surface probe and the temperature variation was within 0.1 °C. The compression rates were between 0.79 and 0.99 nm 2 molecule- 1 m i n - 1. The cross-sectional area (limiting area) of the monolayers was determined by extrapolating the most linear region of the pressure rise in the force-area curve to the x axis. The area is expressed in terms of square nanometers per molecule and in this case each molecule is one polymer chain. It was found that an area measurement could be achieved with a precision of 2~o. The precision in surface pressure measurement was limited by the drift in the circuit boards of the control unit for the film balance. However, a variation of less than 5~o was generally obtained. Monolayer properties of polymers are affected by the time the monolayers sit on the water surface after they are spread s-7. This is generally caused by the high viscosities of the polymer films. A change in conformation from a coiled state in solution to an uncoiled monolayer state may require a long time to achieve for high polymer systems. It was found that a dwell time (time between spreading and compressing) o f 10 min was sufficient to obtain reproducible isotherms for these oligomers, probably because they have much lower molecular weights than conventional polymers. 3.
RESULTS AND DISCUSSION
Figure 2 shows the isotherms of the block co-oligomers with hydrophobic unit: hydrophilic unit compositions of 10:20 and 10:30 as well as the triblock with a composition of 10:40:10. The molecular weight of the triblock was taken as 4010, one-half of the true molecular weight of the triblock, because there are two 8O
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Fig. 2. Initial compression curves for monolayers of 10:20, 10:30and 10:40:10oligomers.
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166
M.H. LrrT et al.
h y d r o p h o b i c units p e r block. T h e t e m p e r a t u r e o f the water s u b p h a s e was 18.6 'C. It can be seen t h a t the cross-sectional a r e a for all three o l i g o m e r s is a p p r o x i m a t e l y 1.9nm 2 molecule 1. As can be seen in T a b l e I, the limiting a r e a is 1.84nm z molecule ~ for the 10:20 block, 1.91 nm z molecule ~ for the 10:30 block, a n d 1.93 n m z m o l e c u l e - ~ for the 10:40:10 block. This value is m o r e t h a n d o u b l e that o f the t - b u t y l p h e n y l h o m o p o l y m e r c o p o l y m e r (0.88 nm z) o b t a i n e d f r o m its X - r a y fiber p a t t e r n ~.3. F i g u r e 3 shows the c o m p r e s s i o n - e x p a n s i o n curves o f the 10:30 block. Paths 1 a n d 2 represent the surface film being c o m p r e s s e d at 2.2 cm min l to a pressure o f 25 m N m ~ then e x p a n d e d at the s a m e rate. T h e r e is only a very small a m o u n t o f TABLE I LIMITING SURFACE AREA PER MOLECULE AS A FUNCTION OF HYDROPHOBI(" BLOCK CIIAIN LENGTH AND TEMPERATURE Composition X: Y
Limiting area at 10.2 ' C
Limiting area at 14.1 '("
(nmZmolecule 8:16 10:20 10:30 10:40:10 12:16
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Limiting area at 1 8 . 6 ( "
(nm2molecule
1.59
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(nm2molecule
1.41
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1.34 t.84 1.91 1.93 2.38
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1.2 1.5 1.5 I. 5 1.8
" C a l c u l a t e d from d a t a in refs. 2 a n d 3.
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maximumcompression
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curve of 10:30
167
POLYMER MONOLAYERS AT THE AIR--WATER INTERFACE
hysteresis, suggesting that the molecules did not crystallize. Paths 3 and 4 represent the surface film being compressed to a pressure o f 58 m N m - 1 and then expanded. Again the hysteresis is quite small. One can see that the hysteresis formed by paths 3 and 4 is larger than that for paths I and 2. This is probably due to the pressure which forces the molecules to a more ordered state; some small and imperfect twodimensional crystallites may have been formed at this high pressure (58 m N m - 1). This causes a larger hysteresis. However, it is believed that no large threedimensional crystals were formed after collapse because no hazy films were observed on the water surface as the molecules were compressed to the smallest area permitted by the trough. Typically, hazy films form on the water surface if crystallizable monolayers are compressed beyond their collapse pressures as a result of the formation of three-dimensional bulk crystal phases. All the evidence given above suggests that the hydrophobic chains of the monolayers were not densely packed when compressed and they were not oriented normal to the water surface. The chains probably lie flat on the water surface and remain in a two-dimensional liquid state when they are compressed. If this is true, the cross-sectional area determined from the isotherms should depend on the length of the hydrophobic unit. An oligomer with a composition of 8:16 was synthesized and the isotherms at three different temperatures are shown in Fig. 4. The limiting area was found to be dependent on the temperature o f the subphase. As the temperature o f the subphase was lowered to 10.2 °C, the area increased to 1.59 nm 2 molecule-1. It can be seen 8116 BLOCK COOLIGOMER 8O
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.= =~ ==
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168
M.H. LITTel al.
that the cross-sectional area is approximately 1.34 nm 2 at 18.6 C. It is smaller than that of the oligomers with a longer hydrophobic chain (10 hydrophobic units). The limiting area for another oligomer with a composition of 12:16 was found to be 2.38nm 2 molecule a at 18.6 C. All the results clearly indicate that the area is dependent on the length of the hydrophobic unit and thus the hydrophobic chains are not oriented normal to the water surface. Table I lists the limiting areas for all the oligomers studied at 18.6 C. The areas at lower temperatures for two of the materials (8:16 and 12:16) are also reported. The theoretical calculated areas for such a molecule lying on the surface with the side chains perpendicular to the surface are also given. The experimental values are close to the calculated values, which confirms the hypothesis that the hydrophobic part of the molecule is on edge. This transition is not temperature sensitive and occurs at about the same pressure for all the oligomers studied. It is interesting to note that all the isotherms exhibit two transitions, one at 10-15 mN m - 1 and the other at about 43 mN m 1. Three distinct phases appear on the isotherms for all the oligomers studied. The designations of these three phases (liquid I, liquid II, and solid) are shown in Fig. 4. The lower transition is very similar in shape to the liquid expanded-liquid condensed transitions which commonly occur in fatty acids and phospholipid monolayers 4'8"~. However, the transition pressures are not sensitive to temperature changes for these oligomers, as shown in Fig. 4 for the 8:16 oligomer. It should be pointed out that some materials such as diacetylene compounds and poly(vinyl stearate) can form aggregates of islands on the water surface even in an expanded state ~° 13. However, it is not likely that these oligomers could form islands on spreading because they remain two-dimensional amorphous solids even when they are compressed to a very high pressure. The shape of the lower transition appears to be affected by the length of the hydrophilic blocks. The surface pressure for the 10:30 block is rather high at large area (the monolayer was more expanded) and the lower transition is very broad and ill defined. However, the surface pressures of the 10:20 and 8:16 oligomers are much lower at large area and the lower transition is better defined, especially for the 8:16 block. The isotherm for the 10:40:10 triblock oligomer is almost identical to that of the 10:20 oligomer, probably because the triblock can be effectively treated as a combination of two 10:20 diblocks. This high surface pressure at large area for the 10:30 block is probably caused by the stronger interactions between the longer hydrophilic units in the water subphase. Therefore, packing of the molecules in the liquid I phase appears to be affected by the length of the hydrophilic unit. The liquid II phase has an extrapolated area of between 3.8 and 4.2nm 2 molecule 1 for 10:20, 10:30, and 10:40:10 oligomers, an area of between 2.8 and 2.9nm 2 molecule 1 for the 8:16 block, and an area of 4.3 4.5nm 2 molecule ' for the 12:16 block at 18.6 C. A comparison of the calculated area 2"3 for a hydrophobic block lying flat on the surface gives surface areas per molecule about 1.5 times the experimental values. This implies that the main chain molecules in this phase are lying on the water surface and the side chains (t-butylphenyl) are also lying on the water surface because the amide group on the polymer backbone can interact with the water molecules. However, either they are somewhat tilted or the tbutyl groups overlap. As the area available for the molecules is reduced, the side
POLYMERMONOLAYERSATTHEAIR-WATERINTERFACE
169
chains probably rotate so that they are oriented more or less perpendicular to the water surface in order to accommodate the reduction in area. This accounts for the transition from the liquid II to the solid phase. Although these oligomers are not crystallizable (under the normal conditions, i.e. without annealing) in the bulk state or densely packed in the monolayer state, there may exist some degree of order in the monolayer formed with compression (pressure annealing). This degree of order may be reflected in the force-area curves. The oligomers with 8:16 and 12:16 blocks were chosen for this study because the lengths of the hydrophilic units are the same and yet they represent the limiting cases for the length of the hydrophobic unit. Figure 5 shows the isotherms of 8:16 oligomer at 18.6 °C. These isotherms were obtained from one batch of molecules by compressing from an initial area at 2.2 cm m i n - ~ to the smallest area permitted by the trough, then expanding to the initial area at the same rate. This was repeated three times and the resultant isotherms are shown. It can be seen that the limiting area was reduced on successive compression. The reduction in area was about 5 ~ and 4~(, after the first run and second run respectively. Figure 6 shows the isotherms of 12:16 oligomer at 18.6 °C after successive compressions and expansions. The reduction in area was 10~o and 5~o after the first run and second run respectively. The measurements were very reproducible; the measured limiting areas after three successive compressions at 18.6 °C for these two samples are listed in Table II. The data from two separate experiments are shown. There may be two reasons for this reduction in limiting area with successive runs: (1) the monolayers became more organized as a result of compression, or (2)
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TABLE 11 EFFECT OF C O M P R E S S I O N A~ I ~.(~ C ON FI-]I! LIM1TIN(; SURFA('[! AREA OF SEI.E('TEI) B L O ( ' K C O P O L Y M E R S
Treatment
Limitin~ sur/a('e area/or the./ollowin~ block copolymers. (nm 2 molecule 1) ,~'.-16 olik,oou'r
12:16 oligomer
Experiment I Firsl compression Second compression Third compression
1.34 1.27 1.22
2~38 2.14 2.04
Experiment 2 First compression Second compression Third compression
[.33 1.25 1.20
2.39 2.19 2.08
some of the molecules simply dissolved into the water subphase. In order to see whether the oligomers dissolved easily into the water, the isotherms of 12"16 oligomer were o b t a i n e d at 18.6 ~C for different periods of dwell time. Figure 7 shows the isotherms of this material with 10 a n d 55 min of dwell time. One can see that the limiting areas for these two cases are the same, suggesting that the molecules did n o t dissolve in the water after being on the water surface at a large initial area for 55 rain. It is believed that these oligomers do not dissolve into the water subphase at low surface pressure because the m o l e c u l a r weights are high (relative to those of the
171 12/16
OLIGOHER
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T
=
18.6°C
7o
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!
4
Fig. 7. Effectof annealing time on the compressioncurve of 12:16. usual fatty acid compounds) and the hydrophobic unit contains aromatic rings. However, it should be pointed out that dissolution of monolayers into the water subphase may require an activation energy. Compressing the monolayers may impart enough energy for the monolayers to overcome the barrier s. Figure 8 shows the isotherms of the 12:16 oligomer at 18.6°C under three conditions. Curve a was obtained without any pre-compression. Curve b was obtained after the monolayer had been compressed to 33 m N m - ~ at 2.2 cm min - 1 then expanded at the same rate to the original area. Curve c was obtained after it had been compressed to 47 m N m - 1 at 2.2 cm rain - ~ then expanded at the same rate to the original area. It can be seen that the limiting area was reduced to a larger extent when the monolayer was pre-compressed to 47 m N m - 1 (7Yo compared with 3Yo). All the evidence suggests that this reduction in limiting area is related to the pressure applied to the monolayer film and a certain level of pressure has to be exceeded in order to see this effect. It has been shown that poly(vinyl stearate) monolayer could be pressure annealed to a more densely packed state ~°. The annealing time was found to vary from 1/2 to 3 h, depending on the molecular weight of the polymer. This pressureinduced ordering of the polymeric monolayers may be the main cause for the observed phenomenon. The annealing time employed in these experiments was very short (maximum o f a few minutes) compared with that used for poly(vinyl stearate). This is probably because the molecular weight of the oligomers is very low and the ordering process could be achieved in a short period o f time. Since the creep rates of these oligomers were very high, no attempts were made to pressure anneal the monolayers at a constant pressure for varying amounts of time.
172
M.H. LITT et al.
12116 OLIGOMER I = 18.6°C 70
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o
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NM/MOLECULE
Fig. 8. Effect of cycling to various surface pressures on 12:16:curve a, initial compression curve: curve b. compression curve obtained after compressing to 33 mN m ~and expanding: curve c, compression curve after compressing to 47 mN m- ~and expanding.
Dissolution o f the m o n o l a y e r s might also cause some reduction in the limiting area. However, this effect is believed to be secondary, if there is any. It should be noted that the reduction in limiting area was approximately 15!!,,, for the 12:16 oligomer after three successive runs, whereas it was only 9!!Jofor the 8:16 oligomer under the same conditions. If dissolution occurred, one should expect to see a larger reduction in limiting area for the 8:16 sample because it contains only eight h y d r o p h o b i c units per chain. Therefore, the larger reduction in limiting area for the 12:16 sample was presumably due to an initial poorer packing efficiency for the longer h y d r o p h o b i c chains in this sample. This can be seen by c o m p a r i n g the final limiting area per molecule with the theoretical values given in Table I. The 12:i6 sample was also compressed successively at a temperature of 10.2 ' C . The reduction in area was 6°J~, and 5~o after first compression and second compression respectively. It appeared that pressure-induced ordering was slightly less at this low temperature. This was p r o b a b l y due to the lower mobility o f the m o n o l a y e r s at this temperature. 4. CONCLUSIONS The m o n o l a y e r s o f these block co-oligomers were not densely packed even when they were compressed to the highest pressure (about 70 m N m 1) permitted by the trough. Some small and imperfect two-dimensional crystallites m a y have
POLYMER MONOLAYERSAT THE AIR-WATER INTERFACE
173
been formed during compression to very high pressures and therefore some degree o f hysteresis in the isotherms resulted. The cross-sectional area o f these oligomers determined from the isotherms is dependent on the length o f the h y d r o p h o b i c units, which suggests that the chains are p r o b a b l y lying fiat on the water surface. The packing o f the monolayers in the liquid I phase is affected by the length o f the hydrophilic unit on the chains. The transition from liquid II to solid p r o b a b l y involves the rotation o f the side chains towards the vertical in order to a c c o m m o date the reduction in area available for the monolayers. The limiting area o f the 8:16 and 12:16 oligomers was reduced with each successive compression to the smallest area o f the trough. This could be attributed to pressure-induced ordering o f the monolayers during compression. Pressure annealing was f o u n d to be more effective if the monolayers were compressed to the solid phase on the isotherms. The 12:16 oligomer showed a larger a m o u n t o f reduction in area after successive compression primarily because this sample has longer h y d r o p h o b i c chains, which were initially m o r e poorly packed. REFERENCES 1 M.H. Litt, T.T. ChenandB. R. Hsieh, J. Polym. Sci.,Polym. Chem. Edn.,24(1986)3407.
2 M.H. Litt, B. R. Hsieh, I. M. Krieger, T. T. Chen and H. L. Lu, J. Colloidlnterface Sci., 115 (1987) 312. 3 M.H. Litt, F. Rahl and L. G. Roldan, J. Polym. Sci., Part A2, 7 (1969) 463. 4 G.L. Gaines, Jr.,,Insoluble Monolayers at Liquid-Gas Interfaces, lnterscience, New York, 1966. 5 K.S. Shih, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1988. 6 D.J. Crisp, J. ColloidSci., 1 (1946) 161. 7 S.J. Mumby, J. F. Rabolt and J. D. Swalen, Thin Solid Films, 133 (1985) 161. 8 S.R. Middleton, M. Iwahashi, N. R. Pallas and B. A. Pethica, Proc. R. Soc. London, Ser. A, 396 (1984) 143. 9 N.R. Pallas and B. A. Pethica, Langmuir, 1 (1985) 509. 10 L.C. Uitenham, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1985. 11 D.R. Day, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1980. 12 D.R. Day and J. B. Lando, Macromolecules, 13 (1980) 1478. 13 J.D. Shutt, M.S. Thesis, Case Western Reserve University, Cleveland, OH, 1986.
163
LANGMUIR--BLODGETT AND RELATED FILMS
LOW SURFACE ENERGY POLYMERS AND SURFACE-ACTIVE BLOCK POLYMERS IV: MONOLAYERS AT THE AIR-WATER INTERFACE M. H. LITT, K. S. SHIH, J. B. LANDO AND S. E. RICKERT
Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106 (u.s.A.) (Received September 29, 1989; accepted February 23, 1990)
The spreading of five block copolymers of 2-(p-(t-butyl)phenyl) oxazoline (hydrophobic block) and 2-ethyl oxazoline (hydrophilic block) at the air-water interface was studied using Langmuir-Blodgett film balance techniques. Three regions of compression were found. At low force, the hydrophilic block was being compressed. This continued until the hydrophobic tails contacted each other. The second region was the compression of the hydrophobic tails lying almost fiat on the water surface. As the pressure increased to 40 mN m- 1, the hydrophobic tail rotated so that the side chains were perpendicular to the surface. The area of the hydrophobic block in the most compressed state was about equal to its thickness multiplied by its chain length. As chain length increased, packing efficiency decreased slightly. All compressed phases remained liquid like. The films remained as monolayers up to the highest pressure (about 70mNm-1) permitted by the trough.
1. INTRODUCTION In a series of papers by Litt et al. 1"2 the synthesis and characterization of monodisperse surface-active block co-oligomers with different weight ratios of hydrophobic blocks (derived from 2-(p-t-butylphenyl)-2-oxazoline) and hydrophilic blocks (derived from 2-ethyl-2-oxazoline) have been reported. The effectiveness of these oligomers as surfactants in emulsion polymerization was investigated. It was found that stable latex was obtained in the emulsion polymerization of butyl acrylate (5~ in water). The use of these oligomers as inverse emulsifiers in the emulsion polymerization of styrene was also reported and the various factors that affect the stability of the emulsions were studied. It was found that the cross-sectional area of the surfactant monolayers was 0.77 nm 2 molecule-1 as determined from the average droplet size analysis. This value corresponds quite well to the cross-sectional area (0.88 nm 2 molecule-1) calculated from X-ray data for t-butylphenyl homopolymer 1'3. It was concluded that the surfactant monolayers were highly ordered with their hydrophobic units packed vertically at the interface between the aqueous 0(M~6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
164
M.H. LITT et al.
phase and the organic (styrene) phase. The packing of the monolayers of these surfactant oligomers at the air-water interface has not been studied and remains an interesting subject. The objective of this research was to characterize the behavior of these surlhctant monolayers at the air-water interface by obtaining isotherms on a film balance 4. 2. EXPERIMENTALDETAILS
The synthesis procedures for the block co-oligomers were reported elsewhere 2 and will not be discussed here. Figure t shows the chemical structures of the copolymers used in this study. The t-butylphenyl oxazoline unit is hydrophobic. The ethyl oxazoline unit, which is water soluble, is the hydrophilic group on the chain.
Et
I
CO
CO
b
I
- NCH2CH2 NCH CH2 X: Hydrophobic Y: Hydrophilic Fig. 1. Structure of block copolymer components: hydrophobic block, poly (p-(t-butyl)benzoyl ethyleneimine);hydrophylic block, poly(N-propionyl ethyleneimine).
The force-area curves (isotherms) were obtained using a Teflon barrier attached to an inductive linear transducer to measure the differences in surface pressures between a clean water surface and that containing the film. The brass trough was coated with 5 x 10- 3 in of FEP, a copolymer of tetrafluoroethylene with small amounts of a fluorovinyl ether. An IBM PC was interfaced with the film balance for data acquisition and processing. The balance was placed on a vibration isolation table to d a m p environmental disturbances. All the measurements were carried out in a class 10 laminar flow area inside a class 100 clean room. The water used for the subphase was obtained via a multistep purification process. T a p water was first passed through three particle filters and an activated carbon filter; then it went through a water softener. The water then was purified in a reverse osmometry unit. Finally it was distilled in a column. The whole system was purged with nitrogen gas constantly. The water resistivity was at least 17 M I I cm. The p H of the water was 7 after distillation but decreased rapidly to about 5.5 after exposure to air, as a result of the CO2 absorption. The solvent used was H P L C grade chloroform from Aldrich Chemical Company. The solutions were prepared in 10ml volumetric flasks. The concentrations were between 0.95 and 1.11 m g m l - ~ . A fixed amount of solution was spread on the water surface through a digital pipette with a m a x i m u m capacity of 100 ~tl. The water temperatures were controled by circulating thermostatted water underneath the brass trough. The temperature of the water surface was measured by
165
POLYMER MONOLAYERS AT THE A I R - W A T E R INTERFACE
a surface probe and the temperature variation was within 0.1 °C. The compression rates were between 0.79 and 0.99 nm 2 molecule- 1 m i n - 1. The cross-sectional area (limiting area) of the monolayers was determined by extrapolating the most linear region of the pressure rise in the force-area curve to the x axis. The area is expressed in terms of square nanometers per molecule and in this case each molecule is one polymer chain. It was found that an area measurement could be achieved with a precision of 2~o. The precision in surface pressure measurement was limited by the drift in the circuit boards of the control unit for the film balance. However, a variation of less than 5~o was generally obtained. Monolayer properties of polymers are affected by the time the monolayers sit on the water surface after they are spread s-7. This is generally caused by the high viscosities of the polymer films. A change in conformation from a coiled state in solution to an uncoiled monolayer state may require a long time to achieve for high polymer systems. It was found that a dwell time (time between spreading and compressing) o f 10 min was sufficient to obtain reproducible isotherms for these oligomers, probably because they have much lower molecular weights than conventional polymers. 3.
RESULTS AND DISCUSSION
Figure 2 shows the isotherms of the block co-oligomers with hydrophobic unit: hydrophilic unit compositions of 10:20 and 10:30 as well as the triblock with a composition of 10:40:10. The molecular weight of the triblock was taken as 4010, one-half of the true molecular weight of the triblock, because there are two 8O
I = 18.6°C 70
60 so
z
(n
3o C1..
2O 10 10120"~_~ 0
!
0
2
4
6 AREA,
8
12 ~0.
1
!
!
16
NM/HOLECULE
Fig. 2. Initial compression curves for monolayers of 10:20, 10:30and 10:40:10oligomers.
!
18
|
20
166
M.H. LrrT et al.
h y d r o p h o b i c units p e r block. T h e t e m p e r a t u r e o f the water s u b p h a s e was 18.6 'C. It can be seen t h a t the cross-sectional a r e a for all three o l i g o m e r s is a p p r o x i m a t e l y 1.9nm 2 molecule 1. As can be seen in T a b l e I, the limiting a r e a is 1.84nm z molecule ~ for the 10:20 block, 1.91 nm z molecule ~ for the 10:30 block, a n d 1.93 n m z m o l e c u l e - ~ for the 10:40:10 block. This value is m o r e t h a n d o u b l e that o f the t - b u t y l p h e n y l h o m o p o l y m e r c o p o l y m e r (0.88 nm z) o b t a i n e d f r o m its X - r a y fiber p a t t e r n ~.3. F i g u r e 3 shows the c o m p r e s s i o n - e x p a n s i o n curves o f the 10:30 block. Paths 1 a n d 2 represent the surface film being c o m p r e s s e d at 2.2 cm min l to a pressure o f 25 m N m ~ then e x p a n d e d at the s a m e rate. T h e r e is only a very small a m o u n t o f TABLE I LIMITING SURFACE AREA PER MOLECULE AS A FUNCTION OF HYDROPHOBI(" BLOCK CIIAIN LENGTH AND TEMPERATURE Composition X: Y
Limiting area at 10.2 ' C
Limiting area at 14.1 '("
(nmZmolecule 8:16 10:20 10:30 10:40:10 12:16
t)
Limiting area at 1 8 . 6 ( "
(nm2molecule
1.59
t)
(nm2molecule
1.41
Theoretical Close-packed area ~
~)
(nmZmolecule
1.34 t.84 1.91 1.93 2.38
2.83
~)
1.2 1.5 1.5 I. 5 1.8
" C a l c u l a t e d from d a t a in refs. 2 a n d 3.
10/30
BO
BLOCK COOL1GOMER 18.8" C
70
60
z
50
40
3 30
20
10
0
i
!
i
i
i
i
!
i
i
!
!
!
i
I
i
i
i
I
i
AREA, SQ. NM/MOLECULE Fig. 3. E ~ c t o f c y c l i n g t o t h e oligomer.
maximumcompression
possible o n t h e c o m p r e s s i o n
curve of 10:30
167
POLYMER MONOLAYERS AT THE AIR--WATER INTERFACE
hysteresis, suggesting that the molecules did not crystallize. Paths 3 and 4 represent the surface film being compressed to a pressure o f 58 m N m - 1 and then expanded. Again the hysteresis is quite small. One can see that the hysteresis formed by paths 3 and 4 is larger than that for paths I and 2. This is probably due to the pressure which forces the molecules to a more ordered state; some small and imperfect twodimensional crystallites may have been formed at this high pressure (58 m N m - 1). This causes a larger hysteresis. However, it is believed that no large threedimensional crystals were formed after collapse because no hazy films were observed on the water surface as the molecules were compressed to the smallest area permitted by the trough. Typically, hazy films form on the water surface if crystallizable monolayers are compressed beyond their collapse pressures as a result of the formation of three-dimensional bulk crystal phases. All the evidence given above suggests that the hydrophobic chains of the monolayers were not densely packed when compressed and they were not oriented normal to the water surface. The chains probably lie flat on the water surface and remain in a two-dimensional liquid state when they are compressed. If this is true, the cross-sectional area determined from the isotherms should depend on the length of the hydrophobic unit. An oligomer with a composition of 8:16 was synthesized and the isotherms at three different temperatures are shown in Fig. 4. The limiting area was found to be dependent on the temperature o f the subphase. As the temperature o f the subphase was lowered to 10.2 °C, the area increased to 1.59 nm 2 molecule-1. It can be seen 8116 BLOCK COOLIGOMER 8O
70
.= =~ ==
lo.=r c so
,oM
20
tO 14.2"C~ l i q u l d 0
w
0
w
2
!
!
4
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!
I, !
6
!
I
8
10
'
~
i
12
!
w
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AREA,SQ. NM/MOLECULE
!
T
16
!
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|
20
Fig. 4. Effect oftemperature on thecompression curves of 8:16 polymer. The various ~gions of comp~ssionamla~led.
168
M.H. LITTel al.
that the cross-sectional area is approximately 1.34 nm 2 at 18.6 C. It is smaller than that of the oligomers with a longer hydrophobic chain (10 hydrophobic units). The limiting area for another oligomer with a composition of 12:16 was found to be 2.38nm 2 molecule a at 18.6 C. All the results clearly indicate that the area is dependent on the length of the hydrophobic unit and thus the hydrophobic chains are not oriented normal to the water surface. Table I lists the limiting areas for all the oligomers studied at 18.6 C. The areas at lower temperatures for two of the materials (8:16 and 12:16) are also reported. The theoretical calculated areas for such a molecule lying on the surface with the side chains perpendicular to the surface are also given. The experimental values are close to the calculated values, which confirms the hypothesis that the hydrophobic part of the molecule is on edge. This transition is not temperature sensitive and occurs at about the same pressure for all the oligomers studied. It is interesting to note that all the isotherms exhibit two transitions, one at 10-15 mN m - 1 and the other at about 43 mN m 1. Three distinct phases appear on the isotherms for all the oligomers studied. The designations of these three phases (liquid I, liquid II, and solid) are shown in Fig. 4. The lower transition is very similar in shape to the liquid expanded-liquid condensed transitions which commonly occur in fatty acids and phospholipid monolayers 4'8"~. However, the transition pressures are not sensitive to temperature changes for these oligomers, as shown in Fig. 4 for the 8:16 oligomer. It should be pointed out that some materials such as diacetylene compounds and poly(vinyl stearate) can form aggregates of islands on the water surface even in an expanded state ~° 13. However, it is not likely that these oligomers could form islands on spreading because they remain two-dimensional amorphous solids even when they are compressed to a very high pressure. The shape of the lower transition appears to be affected by the length of the hydrophilic blocks. The surface pressure for the 10:30 block is rather high at large area (the monolayer was more expanded) and the lower transition is very broad and ill defined. However, the surface pressures of the 10:20 and 8:16 oligomers are much lower at large area and the lower transition is better defined, especially for the 8:16 block. The isotherm for the 10:40:10 triblock oligomer is almost identical to that of the 10:20 oligomer, probably because the triblock can be effectively treated as a combination of two 10:20 diblocks. This high surface pressure at large area for the 10:30 block is probably caused by the stronger interactions between the longer hydrophilic units in the water subphase. Therefore, packing of the molecules in the liquid I phase appears to be affected by the length of the hydrophilic unit. The liquid II phase has an extrapolated area of between 3.8 and 4.2nm 2 molecule 1 for 10:20, 10:30, and 10:40:10 oligomers, an area of between 2.8 and 2.9nm 2 molecule 1 for the 8:16 block, and an area of 4.3 4.5nm 2 molecule ' for the 12:16 block at 18.6 C. A comparison of the calculated area 2"3 for a hydrophobic block lying flat on the surface gives surface areas per molecule about 1.5 times the experimental values. This implies that the main chain molecules in this phase are lying on the water surface and the side chains (t-butylphenyl) are also lying on the water surface because the amide group on the polymer backbone can interact with the water molecules. However, either they are somewhat tilted or the tbutyl groups overlap. As the area available for the molecules is reduced, the side
POLYMERMONOLAYERSATTHEAIR-WATERINTERFACE
169
chains probably rotate so that they are oriented more or less perpendicular to the water surface in order to accommodate the reduction in area. This accounts for the transition from the liquid II to the solid phase. Although these oligomers are not crystallizable (under the normal conditions, i.e. without annealing) in the bulk state or densely packed in the monolayer state, there may exist some degree of order in the monolayer formed with compression (pressure annealing). This degree of order may be reflected in the force-area curves. The oligomers with 8:16 and 12:16 blocks were chosen for this study because the lengths of the hydrophilic units are the same and yet they represent the limiting cases for the length of the hydrophobic unit. Figure 5 shows the isotherms of 8:16 oligomer at 18.6 °C. These isotherms were obtained from one batch of molecules by compressing from an initial area at 2.2 cm m i n - ~ to the smallest area permitted by the trough, then expanding to the initial area at the same rate. This was repeated three times and the resultant isotherms are shown. It can be seen that the limiting area was reduced on successive compression. The reduction in area was about 5 ~ and 4~(, after the first run and second run respectively. Figure 6 shows the isotherms of 12:16 oligomer at 18.6 °C after successive compressions and expansions. The reduction in area was 10~o and 5~o after the first run and second run respectively. The measurements were very reproducible; the measured limiting areas after three successive compressions at 18.6 °C for these two samples are listed in Table II. The data from two separate experiments are shown. There may be two reasons for this reduction in limiting area with successive runs: (1) the monolayers became more organized as a result of compression, or (2)
8/16 OLIGOMER aO
T-- 18.6°C
70 " ~ eO 50 Z s-
40 no3 u~ uJ r~ r'l
30
1st compression
~
pression
3rd compression ~
20 10
0
Ii
0
!
2
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|
e
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AREA, SO. NM/MOLECULE
Fig. 5. Effectofcyclingto the maximumcompressionpossibleon the compressioncurvesof 8:16.
170
12116 OLIGOMER = 18.6°C 70
-
-
SO
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~-
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,~
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3rd corn
Or) (/ )
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o
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c u r v e s o f 12:16.
TABLE 11 EFFECT OF C O M P R E S S I O N A~ I ~.(~ C ON FI-]I! LIM1TIN(; SURFA('[! AREA OF SEI.E('TEI) B L O ( ' K C O P O L Y M E R S
Treatment
Limitin~ sur/a('e area/or the./ollowin~ block copolymers. (nm 2 molecule 1) ,~'.-16 olik,oou'r
12:16 oligomer
Experiment I Firsl compression Second compression Third compression
1.34 1.27 1.22
2~38 2.14 2.04
Experiment 2 First compression Second compression Third compression
[.33 1.25 1.20
2.39 2.19 2.08
some of the molecules simply dissolved into the water subphase. In order to see whether the oligomers dissolved easily into the water, the isotherms of 12"16 oligomer were o b t a i n e d at 18.6 ~C for different periods of dwell time. Figure 7 shows the isotherms of this material with 10 a n d 55 min of dwell time. One can see that the limiting areas for these two cases are the same, suggesting that the molecules did n o t dissolve in the water after being on the water surface at a large initial area for 55 rain. It is believed that these oligomers do not dissolve into the water subphase at low surface pressure because the m o l e c u l a r weights are high (relative to those of the
171 12/16
OLIGOHER
6o
T
=
18.6°C
7o
!
0
'
!
;
~
2 4 AREA, SO. NM/MOL~CULE
"'
!
4
Fig. 7. Effectof annealing time on the compressioncurve of 12:16. usual fatty acid compounds) and the hydrophobic unit contains aromatic rings. However, it should be pointed out that dissolution of monolayers into the water subphase may require an activation energy. Compressing the monolayers may impart enough energy for the monolayers to overcome the barrier s. Figure 8 shows the isotherms of the 12:16 oligomer at 18.6°C under three conditions. Curve a was obtained without any pre-compression. Curve b was obtained after the monolayer had been compressed to 33 m N m - ~ at 2.2 cm min - 1 then expanded at the same rate to the original area. Curve c was obtained after it had been compressed to 47 m N m - 1 at 2.2 cm rain - ~ then expanded at the same rate to the original area. It can be seen that the limiting area was reduced to a larger extent when the monolayer was pre-compressed to 47 m N m - 1 (7Yo compared with 3Yo). All the evidence suggests that this reduction in limiting area is related to the pressure applied to the monolayer film and a certain level of pressure has to be exceeded in order to see this effect. It has been shown that poly(vinyl stearate) monolayer could be pressure annealed to a more densely packed state ~°. The annealing time was found to vary from 1/2 to 3 h, depending on the molecular weight of the polymer. This pressureinduced ordering of the polymeric monolayers may be the main cause for the observed phenomenon. The annealing time employed in these experiments was very short (maximum o f a few minutes) compared with that used for poly(vinyl stearate). This is probably because the molecular weight of the oligomers is very low and the ordering process could be achieved in a short period o f time. Since the creep rates of these oligomers were very high, no attempts were made to pressure anneal the monolayers at a constant pressure for varying amounts of time.
172
M.H. LITT et al.
12116 OLIGOMER I = 18.6°C 70
-~.
so
Z
w c,o u)
uJ 0-_
2o 1o
o
!
o
!
!
2 AREA,
!
4 SQ.
!
'
,.
6
NM/MOLECULE
Fig. 8. Effect of cycling to various surface pressures on 12:16:curve a, initial compression curve: curve b. compression curve obtained after compressing to 33 mN m ~and expanding: curve c, compression curve after compressing to 47 mN m- ~and expanding.
Dissolution o f the m o n o l a y e r s might also cause some reduction in the limiting area. However, this effect is believed to be secondary, if there is any. It should be noted that the reduction in limiting area was approximately 15!!,,, for the 12:16 oligomer after three successive runs, whereas it was only 9!!Jofor the 8:16 oligomer under the same conditions. If dissolution occurred, one should expect to see a larger reduction in limiting area for the 8:16 sample because it contains only eight h y d r o p h o b i c units per chain. Therefore, the larger reduction in limiting area for the 12:16 sample was presumably due to an initial poorer packing efficiency for the longer h y d r o p h o b i c chains in this sample. This can be seen by c o m p a r i n g the final limiting area per molecule with the theoretical values given in Table I. The 12:i6 sample was also compressed successively at a temperature of 10.2 ' C . The reduction in area was 6°J~, and 5~o after first compression and second compression respectively. It appeared that pressure-induced ordering was slightly less at this low temperature. This was p r o b a b l y due to the lower mobility o f the m o n o l a y e r s at this temperature. 4. CONCLUSIONS The m o n o l a y e r s o f these block co-oligomers were not densely packed even when they were compressed to the highest pressure (about 70 m N m 1) permitted by the trough. Some small and imperfect two-dimensional crystallites m a y have
POLYMER MONOLAYERSAT THE AIR-WATER INTERFACE
173
been formed during compression to very high pressures and therefore some degree o f hysteresis in the isotherms resulted. The cross-sectional area o f these oligomers determined from the isotherms is dependent on the length o f the h y d r o p h o b i c units, which suggests that the chains are p r o b a b l y lying fiat on the water surface. The packing o f the monolayers in the liquid I phase is affected by the length o f the hydrophilic unit on the chains. The transition from liquid II to solid p r o b a b l y involves the rotation o f the side chains towards the vertical in order to a c c o m m o date the reduction in area available for the monolayers. The limiting area o f the 8:16 and 12:16 oligomers was reduced with each successive compression to the smallest area o f the trough. This could be attributed to pressure-induced ordering o f the monolayers during compression. Pressure annealing was f o u n d to be more effective if the monolayers were compressed to the solid phase on the isotherms. The 12:16 oligomer showed a larger a m o u n t o f reduction in area after successive compression primarily because this sample has longer h y d r o p h o b i c chains, which were initially m o r e poorly packed. REFERENCES 1 M.H. Litt, T.T. ChenandB. R. Hsieh, J. Polym. Sci.,Polym. Chem. Edn.,24(1986)3407.
2 M.H. Litt, B. R. Hsieh, I. M. Krieger, T. T. Chen and H. L. Lu, J. Colloidlnterface Sci., 115 (1987) 312. 3 M.H. Litt, F. Rahl and L. G. Roldan, J. Polym. Sci., Part A2, 7 (1969) 463. 4 G.L. Gaines, Jr.,,Insoluble Monolayers at Liquid-Gas Interfaces, lnterscience, New York, 1966. 5 K.S. Shih, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1988. 6 D.J. Crisp, J. ColloidSci., 1 (1946) 161. 7 S.J. Mumby, J. F. Rabolt and J. D. Swalen, Thin Solid Films, 133 (1985) 161. 8 S.R. Middleton, M. Iwahashi, N. R. Pallas and B. A. Pethica, Proc. R. Soc. London, Ser. A, 396 (1984) 143. 9 N.R. Pallas and B. A. Pethica, Langmuir, 1 (1985) 509. 10 L.C. Uitenham, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1985. 11 D.R. Day, Ph.D. Dissertation, Case Western Reserve University, Cleveland, OH, 1980. 12 D.R. Day and J. B. Lando, Macromolecules, 13 (1980) 1478. 13 J.D. Shutt, M.S. Thesis, Case Western Reserve University, Cleveland, OH, 1986.