Langmuir 1994,10, 1857-1864
1857
End Group Effects on Monolayers of Functionally-Terminated Poly(dimethylsiloxanes) at the Air-Water Interface Thomas J. Lenk, Daniel H. T.Lee,+ and Jeffrey T.Koberstein' Institute of Materials Science and Department of Chemical Engineering, University of Connecticut, 97 North Eagleville Road, Storrs, Connecticut 06269-3136 Received August 9,1993. In Final Form: February 28, 1994@
The formation of Langmuir monolayers from poly(dimethylsi1oxane) oligomers (molecular weights of 900-4000) terminated with methyl, hydroxyl, epoxide, carboxyl, and amine groups is described. The isotherms (except for oligomers with molecular weights below 1500) show the characteristic transitions commonly observed for methyl-terminated PDMS. In addition, the functionally-terminated materials show a transition associated with orientation of the PDMS chains normal to the surface to form a closepacked monolayer. For oligomers with molecular weights below 1000this transition overlaps the standard configurationaltransitions, and the overall shape of the isotherm is determined by the oligomer molecular weight, the functional group, and the nature of the subphase. Most of the materials have cross-sectional areas at collapse of about 100 &/chain, consistent with a structure where the molecules form helices oriented perpendicular to the surface of the subphase. Shorter amine-terminated materials have areas as low as 50-60 Azlchain,consistent with the formation of extended cis-trans caterpillar structures oriented normal to the subphase surface.
Introduction Langmuir-Blodgett (LB) films have traditionally been composed of either low molecular weight amphiphiles or insoluble polymers.lI2 The amphiphiles of interest were initially phospholipids and fatty acids, although this has been extended to a wide variety of derivatized structures, and the films produced by this approach have a thickness proportional to the molecular length. Polymeric systems typically use materials with either a backbone containing both hydrophilic and hydrophobic components, or pendant groups designed to impart amphiphilic character to the chain, and yield films of thickness proportional to the cross-sectional area of the chain. Functionally-terminated oligomers offer a new class of materials combining features of the amphiphilic and polymeric systems. These are polymers of relatively low molecular weight which have hydrophilic functional groups at one or both ends. A wide variety of materials can be used. A lower limit is placed on the molecular weight by the requirement that the material be insoluble even when functionalized. An upper limit on the molecular weight comes from the requirement that the molecular weight be below the entanglement threshold to allowchain separation and orientation, although in practice the molecular weight must below enough so that the entropic loss due to ordering does not overwhelm the binding of the end groups to the surface. Whether or not the backbone itself interacts with the subphase is not so important as long as the interaction of the end group with the subphase is stronger. Some examples of the use of this approach have been the study of hydroxy-terminated poly(dimethylsi1oxane) (PDMS),3
* T o whom correspondence should be addressed. t Current address: Industrial Technology Research Institute, Hsinchu, Taiwan. *Abstract published in Advance ACS Abstracts, April 15,1994. (1)Gaines, G. L.,Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3)Newing, M. J. Trans. Faraday SOC. 1950,46,755-762.
0743-7463/94/2410-1857$04.50/0
trimethylamino-terminated1,4-poly(butadiene),4hydroxyterminated 1,4-poly(isoprene),6and hydroxy- and carboxyterminated perfluoropolyethers.6 In this paper we will present a more comprehensive look at the effect of molecular weight, end group type, and subphase on the Langmuir isotherms of PDMS terminated with epoxy, amine, hydroxyl, and carboxyl groups. Endfunctional PDMS is a unique and interesting material for monolayer studies, since both the backbone units and the end groups can interact with the aqueous subphase. This leads to features in the isotherms typical of both polymeric and small molecule LB layers. The balance between the interactions of the end group and the backbone units with the aqueous subphase controls both the shape of the isotherm and the properties and structure of the monolayer. The surface pressure-area (T-A) isotherms of PDMS have been investigated by many researchers,7-ls but the molecular configurations related to characteristic transitions in the PDMS monolayers are not clear.1214 Banks' and Ellison and Zisman8first reported that PDMS formed insoluble surface films on organic substrates. Banks found that PDMS with an average molecular weight of about 2000 formed stable surface films on such organic liquids as oleic acid, olive oil, triacetin, and ethylene glycol. Ellison and Zisman, using an ethoxy-end-blocked PDMS with a molecular weight of 8250, established that stable mono(4)Christie, P.; Petty, M. C.; Roberta, G. G.; Richards, D. H.; Service, D.; Stewart, M. J. Thin Solid Films 1985,134,75-82. (5)Gentle, I. R.; Saville, P. M.; White, J. W. Langmuir 1993,9,646648. (6)Goedel, W. A.;Xu, C.; Frank, C. W. Langmuir 1993,9,1184-1186. (7)Banks, W. H. Nature 1954,174,365-366. ( 8 ) Ellison, A. H.; Zisman, W. A. J . Phys. Chem. 1986,60,416-421. (9)Fox,H.W.; Solomon, E. M.; Zisman, W. A. J.Phys. Colloid Chem. 1950,54,723-731. (10)Jarvis, N. L. J. Colloid Interface Sci. 1969,29,647-657. (11)Bemett, M.K.;Zisman, W. A. Macromolecules 1971,4(l),47-53. (12)Noll, W.; Steinbach, H.; Sucker, C. J. Polym. Sci., Part C 1971, 34, 123-139. (13)Fox,H.W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39 (ll),1401-1409. (14)Granick, S.; Clarson, S. J.; Formoy, T. R.; Semlyen, J. A. Polymer 1985,26,925-929. (15)Granick, S. Macromolecules 1985,18 (8),1597-1602.
0 1994 American Chemical Society
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1858 Langmuir, Vol. 10, No. 6, 1994
molecular films were formed on tricresyl phosphate and hexadecane. Newing3 investigated the surface properties of hydroxy-terminated silicone compounds and showed that the molecule could be anchored by the terminal OH groups. Fox, Solomon, and Zismangstudied the effect of varying the subphase pH on the rate of hydrolysis of the PDMS monolayer ( n > 14) and established that at 20 "C the rate of hydrolysis of the polysiloxane chain was insignificant. JarvislO studied monolayer films of PDMS on various organic liquids and suggested that monolayer stability and insolubility vary with the molecular weight of the PDMS and the polarity of the subphase liquid. Stable, reversible monomolecular films were formed on all substrates by the higher molecular weight polymers ( n > 14); however, film stability decreased with decreasing polymer molecular weight and decreasing subphase polarity. The configuration of the PDMS films at the organic liquid/air interfaces, as inferred from their studies, appeared to be dependent on the surface composition of the subphase. On tricresyl phosphate and propylene carbonate the high molecular weight polymers adsorbed in a fully extended configuration at large areas per molecule, analogous to their behavior on water. Upon compression, the a-A isotherms suggested the molecules assume a helical configuration. On less polar subphases the higher molecular weight PDMS apparently remained in the helical configuration regardless of the state of compression. Because of the instability and/or solubility of the lower molecular weight PDMS monolayers on less polar subphases, no conclusions were made regarding changes in molecular configuration as a function of film pressure. Bernett and Zisman" studied the behavior of poly(methyl-n-alkylsiloxanes) as monolayers on water. As the malkyl substituent increased in carbon number, values of surface pressure, surface potential, and the perpendicular component of the total electrostatic dipole moment per monomer decreased for a given area per monomer because the polymer molecules approached a more paraffin-like character which decreased their spreading ability. When the n-alkyl substituent was replaced with a large fluoropropyl group, the chains were prevented from forming perfect helices because of the steric hindrance of the bulky groups. Under high compression, the imperfectly formed helices therefore become disoriented, slip over one another, and finally collapse. This is the first of a series of papers which will explore the properties and behavior of monolayers and transferred Langmuir-Blodgett films of functionally-terminated oligomers. In this paper we look at isotherms of functionallyterminated PDMS on an aqueous subphase, particularly their dependence on end group type, molecular weight, and the presence of cadmium in the subphase, and what this implies about the structure of the films themselves. A forthcoming paper will describe film transfer and spectroscopic measurements on transferred monolayers of functionally-terminated PDMS.16
Experimental Section Materials. The materials used in these experiments are described in Table 1. The nomenclature denotes both the functionality and molecular weight of the polymers;e.g., a 1000 molecular weight oligomer with a carboxyl group at each end is designated di-COOH-PDMS-1000. The a,@-functionally-terminated poly(dimethylsi1oxanes)were prepared by Dr. I. Yilgor of the Goldschmidt Chemical Corp. Their synthetic route ensures (16)Lee, D. H. T.; Lenk, T. J.;Knoll,W.; Koberstein, J. T. Manuscript in preparation.
Table 1. Description of the Poly(dimethylsi1oxane) Materials Used in These Experiments molecular
mono/
designation weight di NHz 1130,1970,7800 di di NH2 960,2740 COOH OH epoxy methyl COOH
1000,2020,3400 1000,2000 2000 2000 2940,4100
di di di di mono
functional form (CH2)sNHz (CH&,NHz (CHp)3COOH (CH2)30H
(CH2)3HCOCH2 (CH2)3CH3 (CHZ)~COOH
poly-. dispersity 1.2-1.5 1.2-1.5 1.2-1.5 1.2-1.5 1.2-1.5 1.2-1.5 1.1
(CHd3CH3 a functionality of exactly two and is described elsewhere." The number-average molecular weights of these di-functionallyterminated oligomers (determinedby titration) range from 1000 to 7800 with a polydispersity of about 1.2 to 1.5 (determinedby GPC). The mono-COOH-PDMS oligomers were prepared by Dr. Tezuka Yasuyuki and co-workers of the Technological University of Nagaoka.lE The functionality is near 1.0, but not necessarily exact. Isotherm Measurement, Experiments were conducted on constant-perimetertroughs, a commercial (Joyce-Loebl)trough and a trough with advanced control capabilities constructed at the University of Connectic~t.1~~~ Three subphaseswere used: (1)deionized (DI) water (pH = 5.5), (2) KHCO3-buffered water (pH = 7.2, [KHC03]= 5 X lo"' M), and (3) KHCO3-buffered water containing CdClz (pH = 7.2, [Cdt21= 4 X 1o-L M). The pure water used for the subphasewas obtained from a Millipore Super-Qwater purification system,and typicallyhad a resistivity of 18 MR. All materialswere spread from 0.5-1 mg/mL solutions in HPLC grade chloroform. Typically 10min was allowed before compression of the monolayers to ensure solvent evaporation, and a compression speed of 4 min/min (about 0.05-0.3 (A21 monomer)/min)was used. Surface pressure measurements were made using a filter paper Wilhelmy plate suspended from a Cahn electrobalance.
Results and Discussion Isotherm Shapes. Four basic types of isotherms are observed for end-functionalized PDMS, as illustrated schematically in Figure 1. The numbers associated with each type will be used throughout this paper to refer to isotherm shapes. Isotherms which show an intermediate shape will be described using two numbers; e.g., an isotherm with a shape between that of type 3 and type 4 will be described as type 43 or type 34, with the first number indicating which of the two it most closely resembles. A total of six inflection points or transitions, designated as A-F, are evident in the type 2 isotherms; the other isotherms contain some subset of these (Figure 1). The inflections can be classified into two groups: transitions A-D are associated with interactions between individual monomers and the water subphase. These transitions are always observed in the isotherms for methylterminated PDMS, although all of the assignments are still not clear. Transitions E and F are related to orientational changes involving entire molecules as a result of hydrophilic end groups tethering the molecules to the subphase. Accordingly, only transitions A-D are observed in the type 1 isotherm, that of methyl-terminatedPDMS. For materials in which the end group interacts with the subphase, the molecules can be oriented normal to the surface upon compression and transitions E and F become important as well. The locations of transitions A-D are therefore directly related to an area per monomer and are (17) Yilgor, I.; McGrath, J. E. Adu. Polym. Sci. 1988, 86, 9-11. (18) Kazama, H.; Tezuka, Y.; and Imai, K. Polym. J. 1987, 19 (9), 1091-1100. (19) Lee, D. Ph.D. Dissertation, University of Connecticut, 1990. (20) Mirley, C. L.; Lewis, M. G.; Lee, D. H. T.; Koberstein, J. T. Langmuzr, in press.
End Group Effects on Monolayers of PDMS
Langmuir, Vol. 10, No. 6,1994 1859 Table 2. Summary of Pressure-Area Isotherms for Carboxy-Terminated PDMS on Water. arearatios ~ ( ~ 2 1 p~ monol PF Mu di type B/A C/A D/A molecule) (mN/m) (mN/m) 4100 mono 2 0.84 0.46 0.39 143 10.4 20 2940 mono 2 0.86 0.47 0.39 133 9.1 20 3400 di 2 0.83 0.46 0.38 134 9.3 23 2020 di 23 0.81 0.46 0.43 134 10.2 26 1200 di 32 0.16 n/a n/a 112 12.0 36 lo00 di 34 0.11 n/a n/a 95 13 36 n/a = not applicable.
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Table 3. Effect of End Group on the Pressure-Area Isotherms of Di-End-Functionallzed PDMS. area ratios F(AV PB PF tvDe BIA C/A D/A molecule) (mN/m) Mn B O U P - _ . . , (mN/ml . . ~ , 2000 methyl 1 0.83 0.45 0.40 n/a 8.7 n/a 1970 NH2 32 0.81 0.48 0.41 147 10.9 42 2020 COOH 23 0.81 0.46 0.43 134 10.2 26 2000 OH 2 0.80 0.46 0.39 80 10.3 25 2000 epoxy 32 0.78 n/a nla 102 10.5 22 3 0.78 (0.50) (0.3) 1130 NH2 10.7 42 51 3 0.78 n/a 960 NH2 66 12.2 nla 40 1200 COOH 32 0.16 n/a n/a 12.0 112 36 1000 COOH 34 0.77 n/a nla 95 13 36 1000 OH 34 0.75 n/a n/a 100 13.1 31 4 n/a n/a n/a 1000 epoxy 30 124 n/a a n/a = not applicable. I
2 7 \ Type 1
Area Figure 1. The four basic isotherm types observed with endfunctionalized PDMS. The transitions marked with letters are described further in the text.
essentially independent of molecular weight, while the locations of transitions E and F depend upon the area per molecule (Le., cross-sectional area of the chain backbone) and move to larger values of area per monomer as the number of monomers per molecule decreases. For low molecular weight PDMS, the two groups of transitions overlap and type 3 and type 4 isotherms are observed. This will be discussed in more quantitative terms when molecular weight effects are described, but the distinction between monomeric and molecular transitions will be important throughout this discussion. The determination of molecular areas at collapse is subject to considerable uncertainty in these experiments, and the values provided are only approximate. The accuracy of the values is limited to the accuracy of the determination of the number-average molecular weight (Mn), which is of the order of *5% in the best cases. I t must also be kept in mind that these materials are polydisperse, with some of the molecules having molecular weights well below M,,. This presents a particular problem for the lower molecular weight materials, which exhibit behavior consistent with dissolution of very low molecular weight components, resulting in fewer molecules than expected being present on the surface. Since the position of transitions A and B should be insensitive to molecular weight14 (except for end group effects), an effort to compensate for this material loss was made by scaling the area at collapse (area F in Tables 2-4) to correspond to areas of 19 A2/monomer at A and 15.5 A2/monomer a t B, consistent withFigure 5 and refs 13and 14. The isotherms shown in the figures are the original, unscaled data. PDMS Conformation. Interpretation of the surface pressure-area isotherms necessitates a careful consideration of the conformational characteristics of the PDMS backbone. Several different structures have been suggested for the conformation of linear PDMS chains. On
Table 4. Summary of Langmuir Isotherms for Carboxy-Terminated PDMS on a Cadmium-Containing Subphase, pH 7.2. arearatios ~ ( ~ 2 1 p~ mono/ PF Mu di type B/A C/A D/A molecule) (mN/m) (mN/m) 4100 mono 2 0.83 0.47 0.42 126 10 24 2940 mono 2 0.84 0.45 0.39 115 10 24 3400 di 2 0.83 0.46 0.39 111 9.1 26 2020 di 23 0.83 0.49 n/a 118 10.3 31 1200 di 23 0.19 n/a n/a 11.4 40 110 a n/a = not applicable.
the basis of X-ray diffraction analysis of PDMS crystals, Damaschun proposed a 611 helix with six backbone atoms per unit cell in a tts+s-g+g+configuration (bond rotation angles from trans of about Oo, Oo, 60°, -60°, 120°,and 120°,respectively).21 Flory suggestedthat a more extended helix was consistent with the fiber repeat distance observed by Damaschun,22and such a structure is supported by recent SSi and 13CNMR spectra of crystalline PDMS.23 This helix can be obtained by a rotation of about 35-40° away from trans along each backbone bond. And an extended cis-trans conformation has been suggested for crystalline poly(diethylsiloxane).24 Representations of each of these structures (Damaschun helix, extended helix, and cis-trans conformation) are shown in Figure 2. Endon views, representing the packing in compressed monolayers, are shown in Figure 3. Calculated cross-sectional areas are 96 A2 for the Damaschun helix, 72 A2 for the extended helix, and 52 A2 for the cis-trans configuration. Molecular Weight Effect. The effect of molecular weight can be seen in Figure 4, which shows isotherms of di-carboxy-terminated PDMS of molecular weights ranging from 3400 to 1 W . Some features of the isotherms are (21)Damaschun, V. G. Kolloid-2. 1962,180,65-67. (22)Flory, P. J. Statistical Mechnics of C h i n Molecules; WileyInterscience: New York, 1969;Chapter V. (23)Schilling, F. C.; Gomez, M. A.; Tonelli, A. E. Macromolecules 1991,24,6552-6553. (24)Tsvankin, D. Y.;Papkov, V. S.; Zhukov, V. P.; Godovsky, Y. K.; Svistunov,V. S.;Zhdanov, A. A. J. Polym. Sci.: Polym. Chem. Ed. 1986, 23,1043-1056.
1860 Langmuir,
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Lenk et al.
Area (A*/monomer)
Figure 4. Isotherms of di-carboxy-terminatedPDMS on DI water, pH 5.5 solid line, Mn = 1o00, long dashes, Mn = 2020; short dashes, Mn = 3400.
,
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Figure 2. Side view of PDMS chain structures: (a) extended caterpillar (cis-trans) configuration, (b)extended helix (37O bond rotation), (c) approximate Damaschun helix (tts+s-g+g+).
a
Figure 3. End-on view of PDMS chain structures: (a) extended caterpillar(cis-trans) configuration, (b)extended helix (37" bond rotation), (c) approximate Damaschun helix (tts+s-g+g+).
summarized in Table 2. There is a steady transition from a type 2 to a type 4 isotherm as the molecular weight decreases, due to the increasing overlap of molecular- and monomer-dependent transitions. The pressure in the plateau region (B to C) also increases at lower molecular weights. This is in contrast to the behavior of methyl-
terminated linear PDMS, where no change in plateau pressure with molecular weight was observed. However, a shift to higher pressure was observed for cyclic PDMS as molecular weight decreased.14 By analogy, this might imply that the difunctional PDMS chains are endassociating on the water surface to form loose cyclics or networks. More probably, the higher proportion of end groups present at low molecular weights results in stronger resistance to the transition which occurs at B. For lower molecular weight materials, transition E (molecules orienting upright) also occurs much closer to transition B than for higher molecular weight materials, and some of the shorter molecules (the samples are polydisperse) may already be startingto undergo the transition to an "upright" state near B. This overlap of transitions could also explain what appears to be a slightly smaller area at transition B relative to that for transition A for the lowest molecular weight samples (B/A ratio, Tables 2 and 3). The collapse pressure also increases at lower molecular weight, and the area per molecule at collapse is slightly lower for the 1200 and loo0 molecular weight samples. Since PDMS is not normally crystalline at room temperature (Tm= -40 "C), the forces governing the collapse point can be thought of as a balance between the enthalpy of the head group interaction with the surface and the entropy loss from chain orientation and close packing. Thus, one would expect films of longer chain molecules to collapse more readily. Above a certain limiting chain length (for example, the monocarboxy-PDMSof Table 2) the collapse pressures appear to be little affected by chain length. These areas are considerablyabove those for even the Damaschun helix, implying that for longer chains collapse occurs before the chains become fully extended. Similar behavior is seen for the carboxy materials on a cadmium-containingsubphase (Table 4). Figure 5 illustrates the molecular weight dependence of the area per molecule at various transitions in isotherms of di-amine-terminated PDMS. The area at all transitions except the final collapse is proportional to the molecular weight, indicating monomer-dependenttransitions. The area at collapse is nearly independentof molecular weight, as expected for oriented chains. The values for the first two transitions for the 7800 molecular weight PDMS exceed those obtained by a linear extrapolation of the data for the other materials, perhaps reflecting the decreased influence of the end group solubility and size for the longer chains. The isotherms, except those for materials with molecular weights less than 1200, are reversible and show little hysteresis. Low molecular weight species show a significant decrease in surface pressure when the barrier is
End Group Effects on Monolayers of PDMS
Langmuir, Vol. 10, No. 6, 1994 1861
i
X
Area (A4monomer) Figure 7. Isotherms of di-end-functionalPDMS of approximate molecular weight 2000 on DI water, pH 5.5. End groups are the following: solid line, carboxyl;long dashes, hydroxyl;short dashes, amine; long-short dashes, epoxide.
h
f
1
0
2
3
4
5
6
7
8
Molecular Weight (x
Figure 5. Molecular area at various transitions as a function of molecular weight for di-NHrPDMS monolayers on water.
i 9)
P
2
z
h
Y
1
.
40
20
10
OO
Area (A4monomer) Figure 8. Isotherms of di-end-functionalPDMS of approximate molecular weight lo00 on DI water, pH 5.5. End groups are the following solid line, carboxyl; long dashes, hydroxyl;short dashes, amine; long-short dashes, epoxide.
stopped shortly before the collapse point. Similar observations with methyl-terminated PDMS have been attributed to solvation (solubility) or volatility of low molecular weight species over the duration of the experiment.12 An experiment using time-of-flight secondary ion mass spectroscopy (TOFSIMS) to characterize spin-coated and LB films of a diamino-PDMS showed a large depletion of lower molecular weight material in the LB film as compared to the spin-coated film.26 As discussed earlier, this material was polydisperse, and the material loss was attributed to the dissolution of the very low molecular weight components of the molecular weight distribution. Effect of End Group. The major effect of end group functionality on Langmuir isotherms of end-derivatized PDMS can be seen by comparison of Figure 6 with Figures 7 and 8. The isotherm of PDMS with alkyl end groups (Figure 6) shows one major transition, accompanied by a rise in pressure to about 9 mN/m, and a second small
transition at lower area, characterized by a very small rise in pressure. On the other hand, isotherms of derivatized materials show distinct collapse transitions, analogous to those of classical isotherms of fatty acids and related materiah2 A similar effect was seen by Newing for PDMS with a hydroxyl end gr0up.3 The effects of different types of end groups on the isotherms are illustrated in Figures 7 and 8. In Figure 7, the isotherms for PDMS oligomers with a number-average molecular weight of approximately 2000 and terminated at each end with carboxyl,amine, hydroxyl, or epoxy groups are shown. Isotherms for PDMS of molecular weight approximately 1000 are shown in Figure 8. The major features of the isotherms are summarized in Table 3. Several trends are evident. The first of these is that the collapse pressure increases with the relative end group "strength" that would be inferred from solubilitiesz6and monolayer stability1927of linear alkanes containing similar groups (NHz> COOH > OH > epoxy). The pressure a t transition B increases slightly for the lower molecular weight materials for all end group types. The area at collapse is approximately 100 AYmolecule in most cases, consistent with the formation of either a Damaschun or extended helix. Some deviation from the ideal model is expected due to structural irregularities that will exist at the collapse point, and complete chain extension is possible
(25) Koberstein, J. T.;Lee, D. H. T.; Elman, J. F.; Thompson, P. M.; Yilgor, I. ACS Polym. Prep. 1991, 32 (l),265-266. (26) CRC Handbook of Chemistry and Physrcs, 56th ed.; CRC Preea: Cleveland, 1975.
(27) Ulman,A. Anlntroduction to Ultrathin OrganicFilms; Academic Press: San Diego, 1991. (28) Durig, J. R.; Flanagan, M. J.; Kalasinsky, V. F. J. Chem. Phys. 1977,66,2775-2785.
0
100
200
300
400
500
Area (A2/molecute) Figure 6. Langmuir isotherm of alkyl-tipped PDMS, molecular weight approximately 2000, on DI water, pH 5.5.
1862 Langmuir, Vol. 10, No. 6, 1994
only for very strongly anchored end groups. The 1000 molecular weight diamines have an area at collapseof about 50-60 A2/molecule. This is similar to the values observed by Newing for very low molecular weight hydroxyterminated materials (MW approximately300-500). Since such short chains are incapable of forming a stable helix, an alternative extended caterpillar (cis-trans) conformation was proposed (Figure 2a). Because the amine groups provide a much stronger attachment to the water surface (especially at pH 5.51, a similar structure may be possible for the slightly longer chains in this study (i.e., 1000 MW). For the amine-terminated materials, the molecular area at collapse is also consistent with the cross-sectional area of the caterpillar structure (Figure 3a). It seems probable that the balance of enthalpic (surface anchoring) and entropic (chain disorder) forces may be such for short chains that packing in what would normally be unstable, higher energy structures becomes possible. The area at collapse for di-COOH-PDMS-2020 (transition F in Table 2) is only about 20% greater than that for the 1000 molecular weight material, although the collapse pressure is much lower. The areas at collapse and collapse pressures of the mono-COOH-PDMS (see next section) are also similar to those of the 2000 molecular weight diCOOH-PDMS. This similarity suggests that the carboxyterminated materials form monolayers with one end anchored at the subphase and one end toward the PDMS/ air interface. The higher areas (much greater than even a Damaschun helix) and low collapse pressure for molecular weights greater than 1000 also suggest that the chains are not fully ordered and extended at collapse. The area at collapse for di-NHz-PDMS-1970 is even larger than that for the carboxy-terminated materials, but the collapse pressure is the same as for lower molecular weight amineterminated PDMS. It is unlikely that the same disordered structure would be present in these layers as for those of carboxy-terminated materials which collapse at half the surface pressure. Because of the strong anchoring of the amine groups (as evidenced by the high collapse pressure), and the incongruity of the large area at collapse and the high collapse pressure, it appears that the di-NHz-PDMS1970 may exist as a looped structure with both ends anchored to the subphase. However,the detailed structure of these monolayers is not yet clear. Mono- vs Difunctional Materials. The effect of functionality can be understood by comparison of isotherms of mono- and di-functional materials such as diCOOH-PDMS-3400,mono-COOH-PDMS-2940,and monoCOOH-PDMS-4100. All three of these materials yield type 2 isotherms. The isotherm transition data are summarized in Tables 2 and 4. There is essentially no difference for transitions A-D (monomeric transitions). Collapse pressures are slightly lower for the monofunctional material, probably due to less stabilization by end groups. This would be true whether the difunctional material was looped (both end groups held at the surface) or extended (because adjacent end groups away from the surface could dimerize to stabilize the free chain ends and facilitate packing). The area at collapse is also similar for all of the materials, suggesting that the difunctional material forms monolayers of extended chains (no significant looping). The area at collapse on a cadmiumcontaining subphase (Table 4) supports this hypothesis, as the areas at collapse are nearly the same for all of the mono- and difunctional materials. Effect of Cadmium in the Subphase. The effects of cadmium ions in the subphase for monolayers of carboxyterminated PDMS are shown in Figure 9 and summarized
Lenk et a1. .
~
~
_
.
_
_
_
---1 1
P D M S - 2 0 2 O - C 3 COOH
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,
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in Tables 2 and 4. The presence or absence of cadmium ions has no significant effect on the pressure at transition B or the area ratios for the monomer transitions for any of the materials. There is a small shift of the initial pressure rise (transition A) to lower area (Figure 9) for dicarboxy-PDMS;the shift is negligiblefor the higher MW monocarboxy materials. This may be due to increased bridging or submersion of end groups upon complexation with cadmium ions. However, the collapse pressure increases 3-5 mN/m for all molecular weights used. Since cadmium ions will complex with a large fraction of the end groups at the pH used, this increased resistance to collapse is expected. The area at collapse for all of the materials on a cadmium-containing subphase is similar to that for the 1000 and 1200 molecular weight materials without cadmium present. This area per molecule (around 100-110 A2/molecule)corresponds roughly to the crosssectional area of a Damaschun helix or an extended helix with some structural disorder in the layer (Figure 3). The increase in collapse pressure in the presence of cadmium ions is similar to behavior observed for arachidic acid mono1ayers.l Such films are generally more condensed than fatty acid monolayers on water and exhibit greater cohesion and resistance to shear. This behavior is usually attributed to bridging of head groups by cadmium ions. The propyl spacers between the PDMS chain and the terminal carboxyl group may be long and flexible enough to allow bridging between chains for the COOHPDMS materials, but comparison to the other types of end groups suggest that the observed effects could also be due simply to increased binding of the complexed carboxyl groups to the water surface. Assignment of Transitions. Several papers have attempted to assign the observed T-A isotherm transitions to configurational and conformational changes in the PDMS b a c k b ~ n e . ~ J ~The - ' ~ assignments of previous researchers will be summarized here along with pertinent comments from the results of our studies. In the region before transition A, Fox et al.13 assumed that the molecules were lying flat on the water, with all silicon and oxygen atoms in contact with the water and the methyls pointed up from the interface. From A to B the molecules were thought to be reorienting on the water surface, adopting a zigzag conformation with every other oxygen (or silicon) at the surface. No11 et a1.12 more carefully pointed out that there are two steps in reaching transition A, the first occurring when the surface pressure first departs from zero, and the second (usually at low T ,
_
End Group Effects on Monolayers of PDMS
less than 0.5 mN/m) where the pressure begins to rise as part of the A to B transition. They postulated that the PDMS was oriented on the water surface with the Si-0 bond in a plane normal to the water/air interface, the oxygen on the water side of the interface, and the entire backbone associated with water of hydration. However, this type of planar zigzag structure could be maintained over only a few monomer units, as the difference in Si0-Si and 0-Si-0 bond angles leads to curvature, resulting in formation of a complete loop in only 10 monomer units. It may be possible for distortion in the Si-0-Si bond, which has a low potential barrier to linearization,z8 to accommodate some of the strain, but the proposed structure seems implausible on a large scale. No11 et al. further suggested that the departure from zero pressure occurs when the hydrated molecules come into contact and that the next step involves expulsion of much of the water of hydration. The transition from A to B was attributed to expulsion of the final trapped water of hydration, accompanied by a meshing of the chains “like a zip fastener” through overlap of the methyl groups. However, the close packing of the films which this suggests would seem to make a further change in the film structure with little change in the pressure, which apparently occurs from B to C, very unlikely. From B to C, both sources suggest that the molecules coil into helices. The fact that essentially no pressure rise occurs in this region suggests that the transition undergone by the molecule decreases the area covered by any given monomer without great resistance, and helix formation is consistent with this. A PDMS helix resembles a methylstudded tube (Figure 2), and a chain which was predominantly helical would have only a weak interaction with the subphase and thus could collapse relatively easily. For nonfunctional PDMS the transition from C to D, which is similar to a collapse transition, is accompanied by only a small ( 1mN/m) pressure increase. There is also an area per monomer decrease of about 10% accompanying this transition, perhaps due to compression of the helices before chains begin to slide across one another. The endderivatized molecules consistently have a pressure rise from B to C which is higher than that of the methyl analog, and this difference becomes more pronounced for shorter chains. This could still be consistent with the formation of helices, since those backbone units nearer the end groups would be more restricted and require more pressure to become part of a helix. The transition from C to D is the only point of significant hysteresis in the isotherms of the end-functional materials. It is always present in compression isotherms (provided the molecular weight is high enough for a type 2 isotherm), but if compression proceeds to any significant degree beyond D (but below collapse), the feature is absent in the expansion isotherm. It again appears upon recompression. This is consistent with compressivecollapse of a monolayer of PDMS helices as a step in formation of upright monolayers. Upon expansion, the layers would relax directly to a spread state without going through the strained region from C to D. Little previous reference has been made to transitions E and F. Fox et al. observe a rapid pressure rise and note that the PDMS films have become “solid” at areas around 40 AZ/molecule.13 Although no mention is made of end groups, it is possible that the materials they were working with were unknowingly end-functionalized. Newing, working with hydroxy-terminated PDMS containing 4-6 monomer units (MW = 300-500), noted very distinct collapse transitions at 35 and 60 Az/molecule for monoN
Langmuir, Vol. 10, No. 6,1994 1863
and difunctional molecules, re~pectively.~ These transitions were assigned to collapseof an upright layer of PDMS. The isotherms presented by Newing show that the monofunctional material has what is called a type 3 isotherm in this paper and the difunctional material a type 4 isotherm. The cross-sectional area is compared to that of a planar zigzagconformation ( 28 Az;as mentioned earlier, such a structure could be maintained over only a few monomer units). The values for these very short molecules (and Fox et al.’s result above) suggest that short oligomers do indeed pack as well-ordered chains. The results for amine-terminated materials in this study show that with a sufficiently strong anchor even longer chains will pack in extended ordered structures. For longer chains and weaker anchors, the cross-sectional areas at collapse increase to around 100-110 A2/molecule in most cases, in good agreement with the calculated crosssectional area of 96 A2/moleculefor the Damaschun helixz1 (Figure 3c). However, the collapse pressures are considerably below those for the amine-terminated materials, and it is not clear whether the chains are packing as Damaschun helices or as semiextended structures containing many defects. In general, it appears that the area at collapse is a function of both chain length and end group strength. For short chains, where helices cannot be sustained, or those with very strong end groups, pressures could become large enough to distort the structure toward a lower crosssectional area along the backbone. Formation of a cistrans caterpillar structure would give areas around 50 &/ molecule for chains with one end anchored, and about 100 Az/molecule for close-packed chains with both ends anchored a t the surface. For longer chains or weaker anchors, where helices could be sustained up to the point of collapse, areas would be expected to be 80-100 A2/ molecule at collapse. However, the rotational barrier about the Si-0 bond is estimated to be about RT at normal temperatures,z2and as chain lengths increase beyond the point where the head group anchoring could stabilize the chain against disruptions due to thermal motion, areas at collapse should increase (due to the increased disorder in the chains) and pressures at collapse should decrease (due to the influence of thermal motion in the chains themselves in pulling the head group anchors from the subphase). N
Conclusions End-functional PDMS forms coherent monolayers at the aidwater interface which exhibit both the monomerdependent transitions of nonfunctional PDMS as well as transitions involving orientation of whole chains. The precise shape of the isotherm depends on the oligomer molecular weight, the end group type, and the nature of the subphase. For longer chains the monomeric and molecular transitions can be clearly delineated; as molecular weight decreases the transitions begin to overlap and become difficult to clearly separate. The areas per molecule at collapse are consistent with orientation of the PDMS chains perpendicular to the subphase surface. Shorter chains and those with strong anchors, such as amines, can be compressed into closepacked structures with cross-sectional areas a t collapse consistent with chains in an extended cis-trans structure. Longer chains and more weakly anchored end groups exhibit cross-sectional areas a t collapse of about 100 Az/ molecule. These values are consistent with those estimated for helical structures. As chain lengths increase further, the areas at collapse increase even more and the surface pressures at collapse decrease due to the increased
1864 Langmuir, Vol. 10, No. 6, 1994 disruptive effect of thermal motion in the chains. Even for very long carboxy-terminated chains, at least up to a molecular weight of 4100, adding cadmium ions to the subphase increases the anchoring of the end group and produces a decrease in the cross-sectional area per chain at collapse and an increase in the collapse pressure. From the cross-sectional areas, it appears that most of the difunctional oligomers stand up with one end group at the water surface and one away from the surface. Only di-NHz-PDMS-1970 (a strong anchor on a long chain) shows behavior consistent with formation of loops with both ends at the water surface. This has implications for the deposition mechanism when these films are transferred (Z-type deposition is observed for carboxy-terminated materials). This subject will be explored in more detail in a forthcoming paper.'6 Functionally-terminated oligomers provide an impor-
Lenk et al.
tant new class of materials for Langmuir-Blodgett films. Some examples of these materials have been described previously,G and in this work we demonstrate that the properties of these films can be controlled by the choice of end groups, molecular weight, and subphase. Combined with the myriad possible choices of oligomeric chains, this allows construction of films with control over a wide variety of potentially useful properties.
Acknowledgment. The authors would like to acknowledge helpful discussions with Dr. Thomas P. Russell of the IBM Almaden Research Center and Dr. Wolfgang Knoll of the Max Planck Institute for Polymerforschung. This work was supported in part by the U.S. Army Research Office and by the Connecticut Department of Higher Education under Grant No. 631606.
