Overview of Siloxane Polymers - American Chemical Society

Chapter 1

Overview of Siloxane Polymers

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James E . Mark Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, O H 45221-0172 ([email protected], jemcom.crs.uc.edu)

T h i s r e v i e w provides coverage o f a variety o f p o l y s i l o x a n e homopolymers and copolymers, and some related materials. Specific systems include (i) linear siloxane polymers [-SiRR'O-] (with various alkyl and aryl R and R ' side groups), (ii) sesquisiloxane polymers possibly having a ladder structure, (iii) siloxane-silarylene polymers [-Si(CH3)2OSi(CH3)2(C6H4) -] (where the phenylenes are either meta or p a r a ) , ( i v ) s i l a l k y l e n e polymers [ - S i ( C H 3 ) 2 ( C H 2 ) m - ] , (v) polysiloxanes o f exhanced crystallizability through modifications o f chemical and stereochemical structures, (vi) elastomers from water -basedemulsions, and (vii) random and block copolymers, and blends of some o f the above. Topics o f particular importance are preparative techniques, end-linking reactions, and the characterization o f the resulting polymers i n terms o f their structures, flexibilities, transition temperatures, permeabilities, and surface and interfacial properties. Applications of these materials include their uses as high-performance fluids, elastomers, coatings, surface modifiers, separation membranes, soft contact lens, body implants, and controlled-release systems. A l s o of interest is the use o f sol-gel hydrolysis-condensation techniques to convert organosilanes to novel reinforcing fillers within elastomers, and to ceramics modified by the presence o f elastomeric domains for the improvement of impact strengths. m

O f the semi-inorganic polymers, the siloxane or "silicone" polymers have been studied the most, and are also o f the greatest commercial importance {1-13). The present review provides an overview o f some o f these polysiloxanes and related materials, emphasizing their structures, most important and interesting physical properties, and a variety of their applications. Siloxane-Type

Polymers

Preparation. Polymers o f the type [ - S i R R O - ] are generally prepared by a ringopening polymerization of a trimer or tetramer (14-19) where R and R can be alkyl or aryl and x is the degree o f polymerization. In this reaction, macrocyclic species are x

© 2000 American Chemical Society

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

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generally formed to the extent of 10-15 wt %. The lower molecular weight ones are generally stripped from the polymer before it is used in a commercial application. Their presence is also of interest from a more fundamental point of view, in two respects. First, the extent to w h i c h they occur can be used as a measure of chain flexibility (20). Second, the separated species can be used to test theoretical predictions of the differences between otherwise identical cyclic and linear molecules (21). In some cases, an end blocker such as Y R ' S i R 2 0 S i R 2 R Y is used to give reactive - O S i F ^ R Y chain ends (22). Polymerization of non-symmetrical cyclics gives stereochemically variable polymers [-SiRR'O-] analogous to the totally organic v i n y l and vinylidene polymers [ - C R R ' C H ^ - ] . In principle, it should be possible to prepare them in the same stereoregular forms (isotactic and syndiotactic) which have been achieved in the case of some of their organic counterparts. W o r k of this type is showing great promise (2325). Polymerization of mixtures of monomers can of course be used to obtain random copolymers. They are generally highly irregular, but now i n the chemical rather than stereochemical sense. C o r r e s p o n d i n g l y , they generally show little i f any crystallizability. Some topics involving polymerizations and related chemical reactions which were covered at the "Silicones and Silicone Modified Materials" symposium are conversions from s i l i c o n itself to semi-inorganics (contribution by L e w i s ) , ring-opening polymerizations (contributions by Chojnowski, Soum, Kress, Jallouli, and Komuro), atom-transfer radical polymerizations (Matyjaszewski), hydrosilation polymerizations (Kaganove, Tronc, Narayan-Sarathy), polymerizations with controlled stereochemistry (Kawakami), condensation polymerizations (Fu), and polysilane syntheses (Newton) (26). Homopolymers. Flexibility. The most important siloxane polymer is poly(dimethylsiloxane) ( P D M S ) [-Si(CH3)20-] (6,9,11). It is also one of the most flexible chain molecules known, both i n the dynamic sense and in the equilbrium sense (20,27-31). Dynamic flexibility refers to a molecule's ability to change spatial arrangements by rotations around its skeletal bonds. The more flexible a chain is in this sense, the more it can be cooled before the chains lose their flexibility and mobility and become glassy. Chains with high dynamic flexibility thus generally have very low glass transition temperatures T . Since exposing a polymer to a temperature below its T generally causes it to become brittle, low values of T can be very advantageous, particularly i n the case of fluids and elastomers. The T of P D M S , - -125 ° C , is the lowest recorded for any common polymer. T w o reasons for this extraordinary dynamic flexibility are the unusually long S i - 0 skeletal bond, and the fact that the oxygen skeletal atoms are not only unencumbered by side groups, they are as small as an atom can be and still have the multi-valency needed to continue a chain structure. A l s o , the S i - O - S i bond angle of - 1 4 3 ° is much more open than the usual tetrahedral bonding occurring at - 1 1 0 ° . In addition, this bond angle has tremendous deformability. These characteristics also increase the chain's equilibrium flexibility, which is the ability of a chain to be compact when in the form of a random coil. This type o f flexibility can have a profound effect on the melting point T of a polymer. In this case, it is the origin of the very low T (-40 ° C ) of P D M S . g

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In this regard, crystallization is very important i n the case o f elastomers, since crystallites can act as reinforcing agents, particularly i f they are strain induced. For this reason, it is of interest to make siloxane-type backbones with increased stiffness, in an attempt to increase the T of the polymer. Examples of ways to make a polymer more rigid is to combine two chains into a ladder structure, insert rigid units such as j> phenylene groups into the chain backbone, or add bulky side groups to the backbone. Insertion o f a silphenylene group [-Si(CH3)2C6H4~] into the backbone of the P D M S repeat unit yields either the siloxane meta and para silphenylene polymers (3238). The T of the former polymer is increased to -48 ° C , but no crystallinity has been m

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observed to date (36,37). Since the repeat unit is symmetric, it should be possible to induce crystallinity by stretching. A s expected, the g-silphenylene group has a larger rigidifying effect, increasing T to -18 ° C , and giving rise to crystallinity with a T of 148 ° C . The resulting polymer is thus a thermoplastic siloxane. Silarylene polymers having more than one phenylene group in the repeat unit could be of considerable interest because of the various meta, para combinations that could presumably be synthesized. It is intriguing that even some flexible siloxane polymers form mesomorphic (liquid-crystalline) phases (39,40). Unusual polysiloxane structures that were covered at the symposium are stiffened chains (Van Dyke, Zhang, Lauter), cyclics (Semlyen, Dagger), ladders and cages (41) (Crivello, Liehtenham, Feher, Rebrov, Carpenter, Rahimian, Haddad),hyperbranched structures (Moller, Herzig, Muzafarov, Vasilenko), dendrimers (42,43) (Dvornic, Owen, Vasilenko, Sheiko, Rebrov), and sheets and tubes (Kenney, Katsoulis) (26).


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Permeability. Siloxane polymers have much higher permeability to gases than most other elastomeric materials (44). They have therefore long been of interest for use as gas separation membranes, the goal being to vary the basic siloxane structure to improve selectivity without decreasing permeability. Some of the polymers which have been investigated in a major project (45) of this type were: [-Si(CH3)RO-], [-Si(CH )XO~], [-Si(C H )RO-], [-Si(CH )2(CH )m-L [ S i ( C H ) ( C H ) S i ( C H ) 0 - ] , and [ S i ( C H ) 2 ( C H ) S i ( C H ) 2 0 - ] , where R is typically an n - a l k y l group and X is an n-propyl group made polar by substitution of atoms such as CI or N . Unfortunately, structural changes that increase the selectivity are generally found to decrease the permeability, and vice versa. Another type of membrane designed as an artificial skin coating for burns also exploits the high permeability of siloxane polymers (46). The inner layer of the membrane consists primarily o f protein and serves as a template for the regenerative growth of new tissue. The outer layer is a sheet of silicone polymer which not only provides mechanical support, but also permits outward escape of excess moisture while preventing ingress of harmful bacteria. Soft contact lens prepared from P D M S provide a final example. The oxygen required by the eye for its metabolic processes must be obtained by inward diffusion from the air rather than through blood vessels. P D M S is ideal for such lenses (46) because of its high oxygen permeability, but it is too hydrophobic to be adequately wetted by the tears covering the eye. This prevents the lens from feeling right, and can also cause very serious adhesion of the lens to the eye itself. One way to remedy this is to graft a thin layer of a hydrophilic polymer to the inner surface of the lens. Because of the thinness of the coating the high permeability of the P D M S is essentially unaffected. 3

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Some Unusual Properties of Poly(Dimethylsiloxane). A t y p i c a l l y l o w values are exhibited for the characteristic pressure (47) (a corrected internal pressure, which is much used in the study of liquids), the bulk viscosity r|, and the temperature coefficient of T|. A l s o , entropies of dilution and excess volumes on mixing P D M S with solvents are much lower than can be accounted for by the Flory Equation of State Theory (47). Finally, as has already been mentioned, P D M S has a surprisingly high permeability. Although the molecular origin of these unusual properties is still not known definitively, a number o f suggestions have been put forward. One involves low intermolecular interactions, and another the very high rotational and oscillatory freedom of the methyl side groups on the polymer. Still others focus on the chain's very irregular cross section (very large at the substituted S i atom and very small at the unsubstituted O atom (47)), or packing problems associated with the alternating large and more normal bond angles.


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Surface and Interfacial Properties. The polysiloxanes generally have very low surface energies (48,49), and considerable research is underway to measure and control surface and interfacial properties i n general. For example, adding fluorine atoms to the side chains on a polysiloxane backbone should have a marked effect in this regard (50). Unusual properties o f polysiloxanes that were covered at the symposium are solubility parameters (Rigby), photoluminescence (Pernisz), formation of mesophases (51-53) ( G o d o v s k y ) , films (Takahara, Inagaki), and surfaces (Wynne, O w e n , K o w a l e w s k i , Jukarainen) (26). Reactive Homopolymers. Types of Reactions. In the typical ring-opening polymerization, reactive hydroxyl groups are automatically placed at the ends of the chains. Substitution reactions carried out on these chain ends can then be used to convert them into other functional groups, and these functionalized polymers can undergo a variety of subsequent reactions. H y d r o x y 1-terminated chains, for example, can undergo condensation reactions with alkoxysilanes (54). A difunctional alkoxysilane leads to chain extension, and a tri- or tetrafunctional one to network formation. Corresponding addition reactions with d i - or triisocyanates represent other possibilities. Similarly, hydrogen-terminated chains can be reacted with molecules having active hydrogen atoms (54). A pair of v i n y l or other unsaturated groups could also be joined by their direct reactions with free radicals. Similar end groups can be placed on siloxane chains by the use of an end blocker during polymerization (22). Reactive groups such as vinyls can of course be introduced as side chains by random copolymerizations involving, for example, methylvinylsiloxane trimers or tetramers. Topics involving functionalized polymers that were covered at the symposium include fluorosilicones (48,50,55) (Narayan-Saratahy), amino acid functionalizations (Matisons), grafts (Priou), hydrosilation reactions (Hu), chain-end functionalizations (Fu, Brzezinska, Miranda), and siloxanes as branches (Kishimoto) (26). Block Copolymers. One of the most important uses o f end-functionalized polymers is the preparation of block copolymers, in part because of the tendency of such copolymers to undergo phase separation into novel morphologies (22,56). The reactions are identical to the chain extensions already mentioned except that the sequences being joined are chemically different. In the case of the - O S i R R ' Y chain ends, R* is typically ( C H ) - 5 and Y can be N H , O H , C O O H , C H = C H , etc. The siloxane sequences containing these ends have been j o i n e d to other polymeric sequences such as carbonates, ureas, urethanes, amides, and imides. Phase separations, blends, and related subjects (57-64) covered at the symposium involved binodal and spinodal phase separations (Viers), block copolymers (Weber, M c G r a t h , Y i l g o r , Gravier), blends (Talmon, Krenceski, Singh, Y i l g o r , Pearce), and interpenetrating networks (65) (Boileau, Wengrovius) (26). 2

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Elastomeric Networks. The networks formed by reacting functionallyterminated siloxane chains with an end linker of functionality three or greater have been extensively used to study molecular aspects o f rubberlike elasticity (32,66,67). They are "model", "ideal", or "tailor-made" networks in that a great deal is known about their structures by virtue of the very specific chemical reactions used to synthesize them. For example, i n the case of a stoichiometric balance between chain ends and functional groups on the end linker, the critically important molecular weight M between cross links is equal to the molecular weight of the chains prior to their end linking. A l s o , the functionality o f the cross links (number of chains emanating from one of them) is simply the functionality o f the end-linking agent. F i n a l l y , the molecular weight distribution of the network chains is the same as that of the starting polymer, and there should be few i f any dangling-chain irregularities. Since these networks have a known degree of cross linking (as inversely measured by M ) , they can be used to test the molecular theories o f rubberlike elasticity, c

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particularly with regard to the possible effects of inter-chain entanglements (32,66). Intentionally imperfect networks can also be prepared, by unbalancing the stoichiometry, or by using chains with reactive groups at only one of their ends. One o f the most interesting types of model networks is the bimodal, which consists of very short chains intimately end linked with the much longer chains that are representative of elastomeric materials (32,66,68-72). These materials have unusually good elastomeric properties, specifically large values of both the ultimate strength and maximum extensibility. Possibly the short chains contribute primarily to the former, and the long chains primarily to the latter. A l s o , not only do short chains improve the ultimate properties of elastomers, but long chains improve the impact resistance of the much more heavily cross-linked thermosets. Some topics involving additives, curing, and reinforcement of elastomers by fillers that were covered at the symposium are: additives (Perry), curing (Tsiang, Singh, C h u , P r i o u , W u , T a y l o r ) , reinforcement (Osaheni, O k e l , C o s g r o v e , C o h e n - A d d a d , Matejka), and water-based elastomers (Liles, Bowens) (26). Cyclic Trapping. If relatively large P D M S cyclics (21,73) are present when linear P D M S chains are end linked, then some of them w i l l be permanently trapped by one or more network chains threading through them (32,74-76). Interpretation of the fraction trapped as a function o f ring size, using rotational isomeric theory and Monte Carlo simulations, provides very useful information on the spatial configurations of cyclic molecules, and the mobilities of the end-linking chains. Copolymers. Random. These materials may be prepared by the copolymerization of a mixture of monomers rather than the homopolymerization o f a single type of monomer (15). One reason for doing this is to introduce functional species, such as vinyls or hydrogens, along the chain backbone to facilitate cross l i n k i n g . Another is the introduction of sufficient chain irregularity to make the polymer inherently noncrystallizable. Block. A s already mentioned, the sequential coupling of functionally-terminated chains of different chemical structure can be used to make block copolymers, including those i n which one or more of the blocks is a polysiloxane (77-79). If the blocks are relatively long, separation into a two-phase system almost invariably occurs. Frequently, one type of block w i l l be in a continuous phase and the other w i l l be dispersed i n it i n domains having an average size the order o f a few hundred angstroms. Such materials can have unique mechanical properties not available from either species when present simply i n homopolymeric form. Sometimes, similar properties can be obtained by the simple blending of two or more polymers (57). Applications. Medical. There are numerous medical applications of siloxane polymers (46). Prostheses, artificial organs, facial reconstruction, and catheters, for example, take advantage o f the inertness, stability, and pliability of the polysiloxanes. Artificial skin, contact lends, and drug delivery systems utilize their high permeability as well. N o n - M e d i c a l . Illustrative non-medical applications are high-performance elastomers, membranes, electrical insulators, water repellents, anti-foaming agents, mold-release agents, adhesives, protective coatings, release control agents for agricultural chemicals, encapsulation media, and hydraulic, heat-transfer, and dielectric fluids (46). They are based on the same properties of polysiloxanes just mentioned and also their ability to modify surfaces and interfaces (for example as water repellents, anti-foaming agents, and mold-release agents).


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Applications o f both types covered at the symposium included hydrogels ( K u n z l e r ) , encapsulants (Samara), tougheners and impact modifiers (Pollack, Kumudinie), surfactants (Davidson), photopatterning and resists (Nagasaki, Harkness, Babitch), release coatings (Gordon), and anti-fouling coatings (Stein) (26).


Silica-Type

Materials

Sol-Gel Ceramics for In-Situ Precipitations. A relatively new area i n v o l v i n g silicon-containing materials is the hydrolysis and condensation o f alkoxysilanes or silicates to give silica ( S i 0 2 ) (80). The process is complicated, involving polymerization and branching, but the overall reaction results i n production of the desired ceramic-like material. Production o f ceramics by this novel route has a variety of advantages. First, much lower temperatures can be used, and higher-purity products obtained. A l s o , the microstructure of the ceramic can be better controlled, and it is relatively simple to form very thin ceramic coatings. Finally, it is much easier to form ceramics "alloys", using the hydrolysis of a mixture o f organometallics, for example silicates and titanates to give a S i 0 2 - T i 0 alloy. Polymers have now been incorporated i n this technology (9,81-108). For example, the same hydrolyses can be carried out within a polymer to generate particles of the ceramic material, typically with an average size o f a few hundred angstroms (66,106,107). Considerable reinforcement o f elastomers, including P D M S , can be achieved in this way. Because of the nature of this in-situ precipitation, the particles are w e l l dispersed and essentially unagglomerated. The particles are also relatively monodisperse, with almost all of them having diameters i n the range 200 - 300 A . Poly(dimethylsiloxane) has also been reinforced with clay-like materials (109). Reinforced polysiloxane elastomers from water-based emulsions are also of interest i n this regard (110-119). 2

Polymer-Modified Glasses. If the hydrolyses in silane-polymer systems are carried out using relatively large amounts of silane, then the silica generated can become the continuous phase, with the elastomeric polysiloxane dispersed i n it (66,106,107). This approach can be used to obtain relatively tough ceramics of reduced brittleness. Acknowledgements It is a pleasure to acknowledge that the author's work i n some of these areas has been supported by the National Science Foundation, the A i r Force Office o f Scientific Research, and the D o w Corning Corporation. Literature Cited (1) (2) (3) (4) (5) (6)

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