Silicones and Silicone-Modified Materials - American Chemical Society

Chapter 21

Surface Properties of Thin Film Poly(dimethylsiloxane) Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch021

1

1

2

H. She , M. K. Chaudhury , and Michael J. Owen 1

Department of Chemical Engineering and Polymer Interface Center, Lehigh University, Bethlehem, PA 18015 Dow Corning Corporation, Mail #C041D1, P.O. Box 994, Midland, M I 48686-0994 2

Exposure o f a silicon wafer to the vapor o f undecenyltrichlorosilane, Cl Si(CH ) CH=CH , results i n the formation o f a self-assembled monolayer film whose outer surface is composed o f olefin groups. This surface can be further derivatized by reacting it with S i H -functional polydimethylsiloxane (PDMS) v i a platinum-catalyzed hydrosilylation reaction. For PDMS chains anchored at one end, the water contact angle hysteresis (difference between advancing and receding values) decreases as the molecular weight o f the PDMS chains and the thickness o f the layer they produce decreases. A sufficiently thin layer produced by the multiple attachments o f the PDMS chain provides a useful low-hysteresis model PDMS substrate for contact angle and other surface studies. This model system is superior to the more usual systems based on PDMS fluids baked onto glass or metals, cross-linked coatings on paper or plastics, and PDMS elastomer surfaces. 3

2 9

2

The surface applications o f silicone products, particularly those based on polydimethylsiloxane ( P D M S ) , are many and varied (/). Familiar examples include release liners for pressure-sensitive adhesives (PSAs), antifoaming agents, and water-repellent treatments for a wide variety o f substrates. This broad diversity of application is a direct consequence o f the low surface energy o f P D M S which is lower than most other polymers except for those based on aliphatic fluorocarbon moieties. Despite the commercial importance o f this aspect o f the properties o f polydimethylsiloxane, there is no fully satisfactory contact angle characterization o f P D M S yet available. There is no lack o f potential candidates; part o f the difficulty in identifying a definitive study lies i n the breadth o f these diverse wetting

322

© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

323

investigations o f P D M S . However, even within apparently similar systems there is considerable range o f reported data.

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch021

Previous Contact Angle Characterizations of P D M S These contact angle studies o f P D M S fall into three broad classes: (a) P D M S fluids baked or otherwise adsorbed onto solids such as glass or metals, (b) cross-linked P D M S coatings on flexible substrates such as paper and plastics, and (c) P D M S elastomer surfaces. Perhaps the most quoted values for P D M S are those o f Zisman (2), an example of the first class. This was a critical surface tension o f wetting study but Owens and Wendt (3) later used Zisman's data in their geometric mean solid surface tension approach. The Zisman quasi-equilibrium, advancing contact angles (8 ) for the three most commonly reported contact angle test liquids are 101 deg for water, 70 deg for methylene iodide, and 36 deg for rc-hexadecane. Typical examples of the second and third classes are Gordon and Colquhoun's (4) study o f P D M S release liners for P S A s and Chaudhury and Whitesides'(i) characterization o f elastomeric P D M S . a

The range o f the contact angle (8 ) data reported i n these and other studies is considerable; of the order o f 20 deg for each o f the most commonly used liquids, 95-113 deg for water, 66-89 deg for methylene iodide, and 27-49 deg for nhexadecane. Some o f this variability is due to neglect o f the various pitfalls inherent in contact angle characterization, but i n each o f these three classes there are basic reasons specific to each class that explain why rigorous, definitive data are not readily available. P D M S is unique among polymers in maintaining its liquid nature to very high molecular weights. Essentially, these three classes are different strategies for obtaining a sufficiently immobile P D M S surface for contact angle study. Unfortunately, each is deficient i n some fundamental manner. a

Class One. When a P D M S film is adsorbed onto a rigid glass or metal substrate, the maximum hydrophobicity effect is not obtained. Evidently, the high chain flexibility and l o w intermolecular forces between polymer chains in P D M S must enable significant interaction to occur between polar water molecules and the hydroxylated surface. A thermal baking treatment is required to develop the familiar highly water-repellent character. Such increases o f contact angle were first described by Hunter et al.(6) over fifty years ago. For example, when films o f 500 cs P D M S were first formed on glass by dipping in benzene solution, water contact angles between 50 and 60 deg were initially observed, values greater than 100 were only obtained by heating to 200°C. The causes o f this behavior are still not fully understood. Obviously, conformational changes such as an increased number o f polymer/surface adsorption sites due to the removal o f adsorbed water are involved, but so too are possibilities of condensation o f surface hydroxyls and residual silanols in the polymer, as well as decomposition, rearrangement and cross-linking catalyzed by the surface. This


324


can happen at surprisingly low temperatures for certain substrates. For example, Willis (7) has reported significant changes in PDMS film structure at temperatures as low as 90-100°C on oxidized copper. Class Two. The use of cross-linked PDMS coatings on paper and plastic substrates avoids this thermal variation difficulty. There is also the advantage that such materials are readily available as commercial products primarily to provide release liners for the facile delivery of PSA-coated labels, decals and tapes. Two types of well-understood cross-linking technology are utilized; tin-catalyzed systems based on the condensation of silanol and alkoxysilyl functionalities, and platinumcatalyzed systems based on hydrosilylation addition of SiH to vinyl functional siloxanes. However, the practical experience in the surface characterization of these coatings is a large variation in contact angles similar to that experienced with the other classes of PDMS surfaces. The situation is yet more perplexing as x-ray photoelectron spectroscopy (XPS) surface composition studies (8) show similar atomic compositions fully consistent with pure PDMS outer surfaces across the range of contact angle variability. Inherent roughness effects, particularly on the most popular paper substrates, are part of the explanation. The type of coating also has a marked effect. They are typically available in three forms, solvent-based, aqueous emulsions, and as 100% neat, solventless materials. Wilson and Freeman (9) have demonstrated morphological differences by scanning electron microscopy (SEM) between these three types of PDMS coating. Other variables such as crosslink density also affect the surface behavior adding to the generally unsatisfactory nature of such coatings for rigorous contact angle characterization. Class Three. Preformed elastomer surfaces would seem to avoid both the film baking and morphological difficulties of the other two systems if molded against an adequately smooth metal substrate. Unfortunately, the frequent use of release agents is a significant contamination danger with elastomer surfaces. More important is the propensity for PDMS elastomers to swell when contacted with many organic liquids. This results in dynamic contact angle variability that is hard to control. Swelling problems are particularly noticeable with the «-alkanes which are the preferred contact angle test liquids for Zisman critical surface tension of wetting determinations of low energy polymers. Elastomers present other complications. They are usually filled systems and although the solid filler does not generally occupy the outermost layers according to the results of several X-ray photoelectron spectroscopy (XPS) studies, they tend to increase the contact angle hysteresis (difference between advancing and receding contact angles, 8 - 0 ) which further reduces confidence in the data. A recent study (10) on the effect of saline exposure on the surface properties of medical grade silicone elastomers exemplifies the magnitude of this contact angle hysteresis. On both peroxide and hydrosilylation cured silica-filled PDMS elastomers, initial a

r


325

liquid drop advancing contact angles for water ranged from 110 to 115 deg, whereas initial liquid drop receding contact angles ranged from 48 to 64 deg.


Preparation of Silicone Surfaces What appears to be needed is a thin film o f P D M S attached by well-understood, low-temperature chemistry to a very smooth, rigid substrate, thereby avoiding the diverse drawbacks o f the previous three classes o f P D M S . Both freshly cleaved mica which is atomically smooth and cleaned silicon wafers, which typically have a rms roughness in the 0.1 to 0.3 nm range depending on cleaning procedure, would appear to be excellent candidates for the smooth, rigid substrate. Hydrosilylation addition offers the best-understood chemistry approach which can be carried out at temperatures as low as ambient. Greatest control with this chemistry is achieved by forming olefin-terminated, self-assembled monolayers o f alkoxysiloxanes on the rigid supports and then forming thin films o f P D M S by the reaction o f S i H functional P D M S with these ordered terminal olefin groups. Freshly cleaved muscovite mica was used without any other treatment. The silicon wafers (obtained from Silicon Quest International) were cleaned i n hot piranha solution ( H S 0 : H 0 = 7:3, v:v - used with great care!), rinsed i n distilled, de-ionized water, dried under nitrogen and plasma oxidized. This was done in a Harrick plasma Cleaner, M o d e l P D C - 3 2 G , using oxygen as the plasma gas at a power o f 100 W for 45 seconds. This treatment generates a thin layer o f silica whose surface is converted to silanol groups (SiOH) by exposure to the water moisture i n the air. A self-assembled monolayer o f undecenyltrichlorosilane [ C l S i ( C H ) C H = C H ] is then formed on these surfaces by exposing them at reduced pressure to the silane vapor from a solution o f silane i n paraffin o i l , prepared and stored under nitrogen. The adsorbed, self-assembled vinyl-terminated alkylsiloxane monolayer (formed from the hydrolysis of the silane and reaction with itself and the surface silanols) was characterized by water and w-hexadecane contact angles and its thickness determined ellipsometrieally. The data are shown in Table I. The data are in excellent agreement with a direct comparison already in the literature (11). The 1.6 nm thickness o f the monolayer on the silicon wafer corresponds to the length o f the molecule i n the trans-extended configuration. Because o f the close matching o f the refractive index between mica and the monolayer, estimation o f the thickness o f the monolayer on mica by ellipsometry was not possible. X P S was also used to verify the presence o f these monolayers. 2

3

2

9

4

2

2

2

The S i H functional P D M S polymers are grafted onto this surface by platinum catalyzed hydrosilylation, using D C platinum 4 catalyst: SiH + H C=CH(CH ) Si0 2

2

9

3 / 2

~> S i ( C H ) „ S i 0 2

3/2

Two types o f S i H functional polymers were used; linear polymers o f varying chain length that are S i H functional at one end only, the other end being trimethylsilyl, and a polymethylhydrogensiloxane/polydimethylsiloxane ( P M H S / P D M S ) copolymer. These materials were synthesized in-house. Table II shows the molecular weight, polydispersity, and layer thickness determined ellipsometrieally o f the linear monofunctional S i H polymers. The structure o f the In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


326

Table I: Characterization of Undecenylsiloxane Monolayers Substrate

9 H 0(deg) a

Silicon Wafer (Si/Si0 ): This work Wasserman et al. Mica

0 C, H 4 (deg)

2

a

6

3

Thickness (nm)

2

11

34 34 44

101 101 95

1.6 1.6

-

Table II: SiH Functional PDMS Polymers Grafted onto Silicon Wafer (Si/Si0 ) 2

Polymer

M (GPC)

PDMS-A PDMS-B PDMS-C PDMS-D PDMS-E

3882 8520 11080 17670 25840

n

Polydispersity Index (GPC) Thickness (nm) 1.101 1.222 1.166 1.319

5.14 7.78 10.16 11.98

1.223

15.16


327

P M H S / P D M S copolymer is (H C)3Si((OSiHCH3) (OSi{CH3} ) OSi(CH )3. After 3

20

2

145

3

10 days o f grafting time at room temperature, the samples were extracted to remove any ungrafted chains.

Note that the highest molecular weight monofunctional

polymer did not graft effectively at room temperature and the grafting process was aided by heating overnight at 7 5 ° C .


Contact Angle Measurement Technique Contact angles were measured using a modification o f the conventional method on a Rame-Hart instrument (12). The samples o f P D M S films were seated on a moveable stage that could be driven laterally to the drop viewing axis. Small drops o f water (1-2 u L ) were formed from a microsyringe. The conventional method measures the advancing and receding contact angle by adding liquid to the drop or withdrawing it from the drop. In our modified method, the advancing and receding angles are measured simultaneously at the two edges o f the triple-phase contact lines by moving the stage laterally against the static water drops (with the syringe needle still in the drop). The advancing contact angle is measured at the right hand side o f the drop i f the stage is driven from right to left where fresh surface is moving under the drop. The receding angle is observed at the left hand side o f the drop where it is retreating from the sample and exposing solid surface to the air that had been previously covered by liquid. In the experiments reported here a lateral stage speed o f 50 pm/s was used. Most o f our study has focused on the S i H functional polymers grafted to the monolayer on the silicon wafer because the monolayer on mica proved not to be hydrolytically stable. A s shown in Table I the instantaneous advancing contact angle o f water is ca 95 deg but it decreases with time. The receding contact angle o f water is zero on this surface. The stability o f the monolayer increases upon ageing either at room temperature or at elevated temperature. The monolayer formed on the silicon wafer is hydrolytically stable with the water contact angle o f 101 deg being invariant with time. Results and Discussion Table III shows the variation o f advancing and receding water contact angle with molecular weight of the P D M S chains grafted to the monolayer on the silicon wafer. The hysteresis increases as a function o f molecular weight from 10 to 26 deg over the range studied. Clearly, considerable hysteresis is possible with long tethered chains even when they are not part o f an elastomeric network. These results indicate that i n our search for low hysteresis surfaces, the thinner the attached P D M S chain the better. However, for chains attached at only one end a sufficiently thin layer to have acceptably low contact angle hysteresis might have insufficient polymeric nature. Multiple attachments along the chain appeared to be a promising approach to the desired goal so for this reason we examined the P M H S / P D M S copolymer. The P D M S film formed in this manner is ca 1.5 nm thick from ellipsometric measurements. This is still significantly more than the 0.6 nm expected i f the polymer chain were fully extended along the surface but much thinner than the films formed by the single end-attached P D M s polymers (Table 2).


328

Table IJI: Contact Angles of Water on P D M S Films End-Grafted onto Si/Si0


Polymer

e

e (deg) 112.0 115.3 116.5 117.5 117.5

r

a

PDMS-A PDMS-B PDMS-C PDMS-D PDMS-E

2

H

(deg)

9.5 15.8 19.5 23.5 26.0

102.5 99.5 97.0 94.0 91.5

Table IV shows contact angle data for this PDMS film grafted onto silicon wafer. Note that these contact angles were measured in the conventional manner to allow a Zisman critical surface tension of wetting determination (a ) to be made from the advancing w-alkane and paraffin oil data. This is shown in Figure 1, a value of 22.7 mN/m being obtained. This surface exhibited small hysteresis and the a value is gratifyingly close to the contact mechanics (Johnson, Kendall and Roberts approach) value of 22.6 mN/m (13). Note also that paraffin wax has a very similar value of 23 mN/m (3). c

c

Table IV: Contact Angles on P H M S / P D M S Film Grafted onto Si/SiO Liquid

cr (mN/m)

Water Paraffin oil w-hexadecane «-tetradecane #-dodecane fl-decane

72.8 32.4 27.7 26.6 25.4 23.9

6 (deg)

6 (deg)

H

105 48 37 32 26 16

3 8 4 3 3 1

r

a

108 56 41 35 29 17

:

a is the surface tension of the contact angle test liquid, G and 9 are advancing and receding contact angles , respectively a

r

The low hysteresis of the PDMS-grafted silicon wafer indicates no extensive swelling effects. There is no particular driving force for the PDMS molecules to pack into a tightly-packed structure; the grafted molecules should be in a liquid-like state. Interestingly, as the chain length of the hydrocarbon molecules increases so does the slight hysteresis. The overall observation of low contact angle hysteresis for this disordered liquid-like polymer film indicates that its surface properties are rather homogeneous - there are no significant defects of the type that pin the contact line at some solid/liquid/air interfaces.



Figure L Critical surface tension of wetting plot of PMHS/PDMS copolymer grafted on Si wafer


330

The advancing (0 = 40 deg) and receding (0 = 37 deg) contact angles of nhexadecane on the PMHS/PDMS copolymer fdm grafted onto mica are similar to those on the silicon wafer. However, this surface exhibits a high hysteresis in the water contact angle, advancing and receding angles are 106 deg and 53 deg respectively. We believe this high hysteresis to be due to the hydrolytic instability of the anchoring monolayer on mica discussed earlier, although it should be noted that grafting the copolymer to the olefin groups increases the hydrolytic stability compared to the unreacted monolayer. The hydrolytic stability increases either by aging the film at room temperature or by heating at higher temperature. These treatments result in a surface that exhibits lower hysteresis in water contact angles (0 = 103-107 deg, 0 = 93-97 deg). The increased stability is most likely caused by further lateral crosslinking of the silanol groups of the alkylsiloxane monolayer as has been postulated by Kessel and Granick (14).


a

a

r

r

Alternative smooth substrates and coupling chemistries between the polymer and substrate may also prove suitable. In particular SiH functional PDMS of the type we have used are known to react directly with oxide surfaces such as silica (15) although any ordering benefits of the self-assembled monolayer would be lost and the hydrogen produced might also be disruptive. Plasma polymerization of hexamethyldisiloxane (HMDS) has been shown to provide an ultra-thin hydrophobic polysiloxane film on mica (16). This layer is homogeneous and water stable with no detectable contact angle hysteresis, but not resistant to peeling in water. The main drawback to this approach for our present purposes is that plasma polymers are usually highly cross-linked and the HMDS plasma polymer is unlikely to be a good model for the uniquely flexible PDMS chain. Summary Our expectation was that a smooth, rigid substrate coated with an ultra-thin PDMS layer anchored by a well-understood and controllable chemistry would avoid the complications associated with thermally baked and cross-linked elastomer surfaces and produce a useful, low hysteresis model substrate for contact angle and other surface studies. The data reported here confirm the validity of the self-assembling, reactive silane monolayer and SiH functional PDMS approach when silicon wafers are used as the substrate. Mica which is even smoother is not so suitable because the similar system on mica is not as hydrolytically stable on mica as it is on oxidized silicon. For PDMS chains anchored at one end, the hysteresis decreases as the molecular weight of the PDMS chains and the thickness of the layer they produce decreases. The thinnest layer is produced by multiple attachments of the PDMS chain using a PMHS/PDMS copolymer. This approach has produced the anticipated useful, low-hysteresis model PDMS substrate for contact angle and other surface studies. Examples of the use of this approach include an estimation of adhesion hysteresis using rolling contact mechanics (12) and an investigation of the effect of interfacial slippage in viscoelastic adhesion (7 7). Our contact angle values


331


are a little higher than the generally accepted Zisman values (2) and our critical surface tension o f wetting value correspondingly a little lower (22.7 m N / m compared to Zisman's 24 m N / m and to the J K R value o f 22.6 m N / m obtained from direct work o f adhesion measurements). We offer no explanation for this difference; Zisman baked his films o f P D M S onto glass at 300°C which is precisely the temperature indicated by Hunter et al. (6) to achieve maximum hydrophobicity. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Owen, M. J. In Siloxane Polymers; Clarson, S. J.; Semlyen, J. A., Eds.; P T R Prentice H a l l : Englewood Cliffs, N J , 1993; p. 309. Zisman, W. A. In Adhesion and Cohesion; Weiss, P., Ed.; Elsevier Publishing Company: N e w York, NY, 1962; p.201. Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci., 1969, 13, 1741. Gordon, D. J.; Colquhoun, J. A. Adhesives Age, 1976, 19(6), 21. Chaudhury, M. K.; Whitesides, G. M. Langmuir, 1991, 7, 1013. Hunter, M. J.; Gordon, M. S.; Barry, A. J.; Hyde, J. F.; Heidenreich, R. D. Ind. Eng. Chem., 1947, 39, 1389.. W i l l i s , R. Nature, 1969, 221, 1134. Duel, L. A.; Owen, M. J. J. Adhesion, 1983, 16, 49. Wilson, J. E.; Freeman, H. A. TAPPI, 1981, 64(2), 95. Kennan, J. J.; Peters, Y. A.; Swarthout, D. E.; Owen, M. J.; Namkanisorn, A.; Chaudhury, M. K. J. Biomed. Mater. Res., 1997, 36, 487. Wasserman, S. R.; Tao, Y-T.; Whitesides, G. M. Langmuir, 1989, 5, 1074. She, H.; Chaudhury, M. K. submitted to Langmuir. Chaudhury, M. K. J. Adhesion Sci. Technol., 1993, 7, 669. Kessel, C. R.; Granick, S. Langmuir, 1991, 7, 532. Reihs, K.; Aguiar Colom, R.; Gleditzsch, S.; Deimel, M.; Hagenhoff, B.; Benninghoven, A. Appl. Surf. Sci., 1995, 84, 107. Proust, J-E.; Perez, E.; Segui, Y.; Montalan, D. J. Colloid Interface Sci., 1988, 126, 629. Newby, B - m . Z.; Chaudhury, M. K.; Brown, H. R. Science, 1995, 269, 1407.


Silicones and Silicone-Modified Materials - American Chemical Society

Recommend Documents