Composites

A relatively new area that involves silicon-containing materials is the synthesis of “ultrastructure” materials (i.e., materials in which structure can be controlled at the level of 100 Å). An example is the “sol-gel” hydrolysis of alkoxysilanes (organosilicates) to give silica, SiO2. The reaction is complicated, involving polymerization and branching, but the overall reaction may be written . . . Si(OR4 + 2H2O → SiO2 + 4ROH (9.1) . . . where the Si(OR)4 organometallic species is typically tetraethoxysilane such as tetraethylorthosilicate (TEOS, with R being C2H5). In this application, the precursor compound is hydrolyzed and then condensed to yield branched polymers. Eventually a continuous swollen gel is formed. The gel is dried at moderately low temperatures to remove volatile species, and then it is fired into a porous ceramic object that can then be densified and machined into a final ceramic part. The production of ceramics by this novel route triggered interest in the ceramics community because of advantages over the conventional powder-processing approach to ceramics. Advantages include (i) the higher purity of the starting materials, (ii) the relatively low temperatures required, (iii) the possibility of controlling the ultrastructure to reduce the microscopic flaws that lead to failure, (iv) the ease with which ceramic coatings can be formed, and (v) the ease with which ceramic alloys can be prepared (e.g., by hydrolyzing solutions of both silicates and titanates). The sol-gel approach has been used to form ceramic-like phases in a variety of polymers. Poly(dimethylsiloxane) (PDMS) is the most popular. PDMS is relatively weak and stands to benefit most from reinforcement. PDMS is easily absorbs the precursor materials generally used in the solgel process. Nearly monodisperse silica microparticles can be obtained using siloxane elastomer mixtures. In some cases, the PDMS has been part of a copolymer, with ureas, imides, amideimides, and dianilines. In other approaches, the particle surface is modified, for example, with a polysiloxane. Siloxane/silica nanocomposites have also been used as “culture-stone-protective materials.” Sol-gel hydrolysis and condensation can be carried out within a polymeric matrix to generate particles of the ceramic material, typically with an average diameter of a few hundred angstroms.

Applications

Numerous medical applications have been developed for siloxane polymers. Prostheses, artificial organs, objects for facial reconstruction, vitreous substitutes in the eyes, tubing and catheters, for example, take advantage of the inertness, stability, and pliability of polysiloxanes. Artificial skin, contact lenses, and drug delivery systems utilize their high permeability as well. Such biomedical applications have led to extensive biocompatability studies, particularly on the interactions of polysiloxanes with proteins. There has been considerable interest in modifying these materials to improve their suitability for biomedical applications in general. Advances seem to be coming particularly rapidly in the area of high-tech drug-delivery systems. Figure 10.1 shows the range of diameters of Silastic medical-grade siloxane tubing available for medical applications. The smallest tubing has an internal diameter of only 0.012 inches (0.031 cm) and an outer diameter of only 0.025 inches (0.064 cm). Such materials must first be extensively tested (sensitization of skin, tissue cell culture compatibility, implant compatibility). There has been considerable controversy, for example, over the safety of using polysiloxanes in breast implants. The major concern was “bleeding” of low molecular polysiloxanes out of the gels into the chest cavity, followed by transport to other parts of the body. The extent to which “bleeding” occurred and its possible systemic effects on the body were argued vigorously in the media and in the courts, and led to restrictions on the use of polysiloxanes. In the case of controlled drug-delivery systems, the goal is to have the drug released at a relatively constant rate (zero-order kinetics) at a concentration within the therapeutic range. It is obviously important to minimize the amount of time the concentration is in the low, ineffective range, and to eliminate completely the time it is in the high, toxic range (figure 10.2). Figure 10.3 illustrates the use of polysiloxanes in such drug-delivery systems. The goal mentioned is approached by placing the drug inside a siloxane elastomeric capsule, which is then implanted in an appropriate location in the body. The drug within the capsule can be in the free state, in a fluid suspension, or mixed or dissolved into an elastomeric matrix.

Types of Polysiloxanes

The polysiloxane of greatest commercial importance and scientific interest is poly(dimethylsiloxane) (PDMS), [Si(CH3)2 –O –]x, a member of the symmetrical dialkyl polysiloxanes, with repeat unit [SiR2 –O –]x. This polymer is discussed extensively in the following chapters, particularly in chapter 5. Other members of this series are poly(diethylsiloxane) [Si(C2H5) –O–]x, and poly(di-n-propylsiloxane) [SiC3H7)2–O–]x. An example of an aryl member of the symmetrically substituted series is poly(diphenylsiloxane), with repeat unit [Si(C6H5)2–O–]x. This polymer is unusual because of its very high melting point and the mesophase it exhibits. The closely related polymer, poly(phenyl/tolylsiloxane), has also been prepared and studied. The unsymmetrically substituted polysiloxanes have the repeat unit [SiRR’O–]x, and are exemplified by poly(methylphenylsiloxane) [Si(CH3) (C6H5) –O–]xand poly(methylhydrosiloxane) [Si(CH3)(H) –O–]x. In some cases, one of the side chains has been unusually long, for example C6H13, C16H33, and C18H37, including a branched side chain—CH(CH3– (CH2)m–CH3. Another example has methoxy-substituted aromatic fragments as one of the two side chains in the repeat unit. Such chains have stereochemical variability in analogy with the vinyl polymers such as polypropylene [CH(CH3) –CH2–]xand vinylidene polymers such as poly(methyl methacrylate) [C(CH3)(C = OOCH3) –CH2–]xOne can also introduce optically active groups as side chains, the simplest example being the secondary butyl group—CH(CH3)(C2H5). Another example involves redox-active dendritic wedges containing ferrocenyl and carbonylchromium moieties. Other substituents have included phenylethenyl groups, cyclic siloxane groups, and Cr-bound carbazole chromophores. In a reversal of roles, some polymers were prepared to have PDMS side chains on a poly(phenylacetylene) main chain. Siloxane-terminated solubilizing side chains are used to improve the properties of thin-film transistors. Silalkylene polymers have methylene groups replacing the oxygen atoms in the backbone. Poly(dimethylsilmethylene) is an example, [Si(CH3)2–CH2]x. A variation on this theme is to include aryl groups, for example, in poly(dimethyldiphenylsilylenemethylene) [Si(CH3)2CH2Si(C6H5)2]x. Other aryl substituents, specifically tolyl groups, have also been included as side chains. It is also possible to insert a silphenylene group [Si(CH3)2–C6H4–] into the backbone of the polysiloxane repeat unit to give [Si(CH3)2–C6H4– Si(CH3)2O–], in which the phenylene can be para or ortho or meta. A specific example is poly(tetramethyl-p-silphenylene-siloxane).

General Properties

Many of the properties of the polysiloxanes have been tabulated in handbooks of polymer science and engineering. Recent work has included the stretching of polydimethylsiloxane (PDMS) chains, in some cases to their rupture points. The nature of the bonding in siloxane molecules has been of long-standing interest. Force fields for calculations of PDMS properties have been revised over the years and are now at an advanced state of development. Some of the simplest approaches employ the methods of molecular mechanics. Most of the experimental results have been obtained on solutions of polysiloxanes in thermodynamically good solvents. The first member of this series, poly(dimethylsiloxane) (PDMS), [–Si(CH3)2O–]x, has been studied extensively with regard to its configuration-dependent properties. PDMS (figure 2.1) is very similar in structure to the polyphosphate chain in that the successive bond angles are not equal. The Si–O bond length in polysiloxanes is 1.64 Å, and bond angles at the Si and O atoms are 110 and 143°, respectively. This inequality of bond angles causes the all-trans form of the molecule (with rotational angles ϕ = 0°) to form a closed structure after approximately eleven repeat units. The torsional barrier for rotations about the skeletal bonds is very low, which accounts for the high dynamic flexibility and low glasstransition temperature of the PDMS chain. Not surprisingly, low temperature properties are superb. Trans states are of lower energy than gauche states (ϕ = ±120°) in the PDMS chain. This conformational preference may arise from favorable van der Waals interactions between pairs of CH3 groups separated by four bonds in trans states. This factor is apparently more important than favorable coulombic interactions between oppositely charged Si and O atoms separated by three bonds, which are larger in gauche states because of the reduced distance. Comparisons between experimental and theoretical values of various configuration-dependent properties, however, yield a value for this energy difference that is significantly larger than that obtained from the semi-empirical calculations of interactions between nonbonded atoms.

Introduction

Polysiloxanes are unique among inorganic and semi-inorganic polymers; they are also the most studied and the most important with regard to commercial applications. Thus, it’s not surprising that there is an extensive literature describing the synthesis, properties, and applications of these materials, including books, proceedings books, sections of books or encyclopedias, review articles, and historical articles. The purpose of this volume is not to give a comprehensive overview of these polymers but rather to focus on some novel and interesting aspects of polysiloxane science and engineering, including properties, work in progress, and important unsolved problems. The Si-O backbone endows polysiloxanes with a variety of intriguing properties. The strength of the Si-O bond, for example, imparts considerable thermal stability, which is important for high-temperature applications (e.g., as heat-transfer agents and high-performance elastomers). The nature of the bonding and the chemical characteristics of typical side groups impart low surface free energy and therefore desirable surface properties. Polysiloxanes, for example, are used as mold-release agents, waterproofing sprays, and biomedical materials. Structural features of the chains give rise to physical properties that are also of considerable scientific interest. For example, the substituted Si atom and the unsubstituted O atom differ greatly in size, giving the chain a nonuniform cross section. This characteristic affects the way the chains pack in the bulk, amorphous state, which explains the unusual equation-of-state properties (such as compressibility). Also, the bond angles around the O atom are much larger than those around the Si, which makes the planar all trans form of the chain approximate a series of closed polygons, as illustrated in figure 1.1. As a result, siloxane chains exhibit a number of interesting configurational characteristics that impact properties and associated applications. The major categories of homopolymers and copolymers to be discussed are (i) linear siloxane polymers -SiRR’O-] (with various alkyl and aryl R,R’ side groups), (ii) sesquisiloxane polymers possibly having a ladder structure, (iii) siloxane-silarylene polymers [–Si(CH3)2OSi(CH3)2(C6H4)m –] (where the skeletal phenylene units are either meta or para), (iv) silalkylene polymers [–Si(CH3)2(CH2)m–], and (v) random and block copolymers, and blends of some of the above.

Copolymers and Interpenetrating Networks

Random copolymers are prepared by the copolymerization of a mixture of cyclic oligomers. Although the resulting polymer can be quite blocky (figure 8.1), taking the reaction to equilibrium can give a polymer that is essentially random in its chemical sequencing. One reason for preparing copolymers is to introduce functional species, such as hydrogen or vinyl side groups, along the chain backbone to facilitate cross linking. Another reason is the introduction of sufficient chain irregularity to make the polymer inherently noncrystallizable. Specific examples of comonomers include imides, perylenediimide, urethane-ureas, epoxies, other siloxanes, amides, styrene, divinylbenzene, acrylics, silsesquioxanes, polythiophenes, and poly(lactic acid). One novel combination is the preparation of polysiloxanebased episulfide resins. An unusual application is the use of monomethylitaconate- grafted polymethylsiloxane to induce crystal growth of CaCO3. Polysiloxanes containing thermally curable brenzoxazine moieties in the main chain are also in the category. These and other copolymers have been extensively characterized by nuclear magnetic resonance (NMR) spectroscopy. The sequential coupling of functionally terminated chains of different chemical structure can be used to make block copolymers, including those in which one or more of the blocks is a polysiloxane. If the blocks are relatively long, separation into a two-phase system invariably occurs. Frequently, one block will be in a continuous phase and the other will be dispersed in domains having an average size the order of a few hundred angstroms. Such materials can have unique mechanical properties not available from homopolymer species. Sometimes similar properties can be obtained by the simple blending of two or more polymers. Examples of blocks used with polydimethylsiloxane (PDMS) include imides, epoxies, butadienes, ε-caprolactones, amides having trichlorogermyl pendant groups, urethanes, ureas, poly(ethylene glycols), polystyrene, vinyl acetates, acrylates or methacrylates, 2-vinylpyridine, and even other polysiloxanes. Some results have also been reported for polyesters, polyethers, hydroxyethers of bisphenol A, bisphenol A arylene ether sulfones, vinylpyridinebenzoxazines, methyloxazolines, terpyridines, polysulfones, γ-benzyl-Lglutamate, and carboranes. Two other examples are foamed polypropylene and melamine resins. Even ABA, ABC triblock copolymers, and ABCBA pentablock copolymers involving PDMS have been reported.

Elastomeric Networks

Gelation is the cross-linking process that leads to the network structures required for rubberlike elasticity. In some cases, gelation can be reversible. There have been numerous studies involving theory and simulations exploring gelation and the mechanical properties of the resulting networks. Cross linking with free radicals is still quite common. Radiation has often been used to carry out the cross linking, as have new techniques known as “click” chemistry. Hydrosilylation is also popular. Networks have even been designed with movable cross links. Finally, reactive groups can be placed at the chain ends or within the chains themselves. Related studies have involved polydimethylsiloxane (PDMS)-based organogelators, web-to-pillar transitions of gels, and silica aerogels. There has also been interest in polysiloxanes in interpenetrating hydrogels with high oxygen permeabilities and viscoelastic magnetic gels. Organic-inorganic hybrids with relatively low melting temperatures also exist, some of which can be made to be self-healing. Gels are also formed in swelling experiments, which are useful for equilibrium experiments to characterize network structures. One of the recent topics in this area involves stimuli-responsive gels, under the descriptive title of “self-walking gels” “wormlike motion of gels,” and “peristaltic motion of gels.” The earliest studies of networks formed in solution were undertaken to investigate some subtle aspects of the elastic free energy expression— whether or not an additional term in the logarithm of the volume was required. Other studies focused on the properties of networks in general. As can be gathered from chapter 4, it is difficult to obtain information on the topology of a network. Some studies have therefore taken an indirect approach. Networks were prepared in a way as to simplify their topologies, and their properties were measured and interpreted in terms of reduced degrees of network-chain entanglement. The two techniques employed involved separating the chains prior to cross linking by either dissolution or stretching. After cross linking, the solvent is removed or the stretching force is relaxed, and the network is studied (unswollen) with regard to its stress-strain properties, typically in elongation.

Surfaces

Because of the great importance of the surface properties of the polysiloxanes, this topic is treated separately in this chapter. Hydrophobic polysiloxanes having simple aliphatic or aromatic side groups have surfaces that show essentially no attraction to water. In fact, polysiloxanes can serve as water repellants. This property is very useful for applications such as protective coatings on historical monuments and for controlling the surfaces of other polymers, sensors, and quantum dots. Hydrophobic surfaces can be readily regenerated if the surface becomes damaged. Regeneration occurs by rearrangements of the polysiloxane chains so that the hydrophobic methyl groups are once again covering the surface. The flexibility of the siloxane chain backbone facilitates this process. It is also possible to prepare hydrophobic films using methyl-modified siloxane melting gels. Glass surfaces or wool fibers can be coated with polydimethylsiloxane (PDMS) to make them more hydrophobic. In some cases, it is necessary to modify a polysiloxane surface to make it hydrophilic or hydrophobic. Hydrophobization is one aspect of the general topic of modifying and managing the properties of polymer surfaces. An important example involves soft contact lenses that contain PDMS, which is often used because of its very high permeability to oxygen, which is required for metabolic processes within the eye. Such lenses do not feel comfortable however because they do not float properly on the aqueous tears that coat the eye. There are a number of ways to modify the surfaces. There is even a way to make “unreactive” silicones react with inorganic surfaces. In some applications it is useful to have hydrophilicity in the bulk of the polymer instead of just at the surface. One way of doing this is by simultaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains and hydrophobic PDMS chains. Another way is to make a PDMS network with a trifunctional organosilane R’Si(OR) end linker that contains a hydrophilic R’ side chain, such as a polyoxide. Treating only the surfaces is another possibility, for example, by adding hydrophilic brushes by vapor deposition/hydrolysis cycles. Such hydrophilic polysiloxanes can also serve as surfactants.

Preparation, Analysis, and Degradation

Elemental silicon on which the entire technology is based is typically obtained by reduction of the mineral silica with carbon at high temperatures: . . . SiO2 + 2C → Si 2CO (2.1) . . . The silicon is then converted directly to tetrachlorosilane by the reaction . . . Si + 2Cl2 → SiCl4 (2.2) . . Tetrachlorosilane can be used to form an organosilane by the Grignard Reaction . . . SiCl4 + 2 RMgX → R2SiCl2 + 2 MgClX (2.3). . . This relatively complicatreaction has been replaced by the so-called Direct Process or Rochow Process, which starts from elemental silicon as is illustrated by the reaction . . . Si + 2 RCl → R2SiCl2 (2.4) . . . This process also yields RSiCl3 and R3SiCl, which­­ can be removed by distillation. Compounds of formula R2SiCl2 are extremely important as intermediates to a variety of substances having both organic and inorganic character. Hydrolysis gives dihydroxy structures, which can condense to give the basic [–SiR2O–] repeat unit. The nature of the product obtained depends greatly on the reaction conditions. Basic catalysts and higher temperatures favor higher molecular weight linear polymers. Acidic catalysts tend to produce cyclic small molecules or low molecular weight polymers. The hydrolysis approach to polysiloxane synthesis has been largely replaced by ring-opening polymerization of organosilicon cyclic trimers and tetramers, with ionic initiation. These cyclic monomers are produced by the hydrolysis of dimethyldichlorosilane. Under the right conditions, at least 50 wt % of the products are cyclic oligomers. The desired cyclic species are separated from the mixture for use in ring-opening polymerizations such as those described in the following section. In addition, “click” chemistry has been developed for new synthesis techniques in general, and polymerizations in particular. These approaches have been used to prepare polysiloxane elastomers and polydimethylsiloxane (PDMS) copolymers that can function as thermoplastic elastomers. New synthetic strategies for structured silicones, based on B(C6F5)3 have also been developed. Another new approach involves enzymes, such as the lipase enzymatically catalyzed synthesis of silicone aromatic polyesters and silicone aromatic polyamides.

Some Characterization Techniques Useful for Polysiloxanes

The general approach used in choosing a polymer suitable for a particular application is: . . . Polymerization ↔ Structure ↔ Properties ↔ Application . . . For example, if one wants a polymer for fire-resistant fabrics, then a polymer with good high-temperature properties is required, which implies aromatic structures, which suggest condensation polymerizations. More relevant here, however, would be that a polymer remains elastomeric at low temperatures. This requirement evokes a polymer with high flexibility (low glass transition temperature), which indicates use of the polymerization techniques used with the polysiloxanes. An example of a relevant optical property is the birefringence of a deformed polymer network. This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity. Infrared dichroism has also been helpful in this regard. In the case of the crystallizable polysiloxane elastomers, orientation is of critical importance with regard to strain-induced crystallization and the tremendous reinforcement it provides. Segmental orientation has also been characterized by fluorescence polarization, deuterium nuclear magnetic resonance (NMR), and polarized infrared spectroscopy. Infrared spectroscopy has been used to characterize the structures of silica-filled polydimethylsiloxane (PDMS). Other optical and spectroscopic techniques are also important, including positron annihilation lifetime spectroscopy, spectroscopic ellipsometry, confocal Raman spectroscopy, and photoluminescence spectroscopy. Surface-enhanced Raman spectroscopy has been made tunable using gold nanorods and strain control on elastomeric PDMS substrates. A great deal of information is now being obtained on filler dispersion and other aspects of elastomer structure and morphology through the use of scanning probe microscopy, which consists of several approaches. One approach is that of scanning tunneling microscopy (STM), in which an extremely sharp metal tip on a cantilever is passed along the surface while measuring the electric current flowing through quantum mechanical tunneling. Monitoring the current then permits maintaining the probe at a fixed height above the surface. Display of probe height as a function of surface coordinates then gives the desired topographic map. One limitation of this approach is the requirement that the sample be electrically conductive. Atomic force microscopy (AFM), on the other hand, does not require a conducting Surface.