Overall PDMS has a low elastic modulus which enables it to be easily deformed and results in the behavior of a rubber. Linear relationship in Sylgard 184 PDMS between curing temperature and Young's modulus Depending on the application and field, the user is able to tune the properties based on what is demanded. It is widely known to be easily stretched, bent, compressed in all directions. Generally, the cross-linked cured version of PDMS resembles rubber in a solidified form. Specifically, the determination of mechanical properties can be decided before PDMS is cured the uncured version allows the user to capitalize on myriad opportunities for achieving a desirable elastomer. This enables PDMS to become a good substrate that can easily be integrated into a variety of microfluidic and microelectromechanical systems. Since these properties are affected by a variety of factors, this unique polymer is relatively easy to tune. The mechanical properties of PDMS enable this polymer to conform to a diverse variety of surfaces. However, if the same PDMS is poured into a spherical mold and allowed to cure (short flow time), it will bounce like a rubber ball. If some PDMS is left on a surface overnight (long flow time), it will flow to cover the surface and mold to any surface imperfections. However, permanent deformation is rarely seen in PDMS, since it is almost always cured with a cross-linking agent. But the process that is described above is only relevant when cross-linking is present when it is not, the polymer PDMS cannot shift back to the original state even when the load is removed, resulting in a permanent deformation. This time-dependent elastic deformation results from the long-chains of the polymer. When the load itself is removed, the strain is slowly recovered (rather than instantaneously). The loading and unloading of a stress-strain curve for PDMS do not coincide rather, the amount of stress will vary based on the degree of strain, and the general rule is that increasing strain will result in greater stiffness. Viscoelasticity is a form of nonlinear elasticity that is common amongst noncrystalline polymers. However, at short flow times (or low temperatures), it acts like an elastic solid, similar to rubber. PDMS is viscoelastic, meaning that at long flow times (or high temperatures), it acts like a viscous liquid, similar to honey. Such flexible chains become loosely entangled when molecular weight is high, which results in PDMS' unusually high level of viscoelasticity. PDMS molecules have quite flexible polymer backbones (or chains) due to their siloxane linkages, which are analogous to the ether linkages used to impart rubberiness to polyurethanes. The polymer is manufactured in multiple viscosities, ranging from a thin pourable liquid (when n is very low), to a thick rubbery semi-solid (when n is very high). Using this methodology it is possible to synthesize linear block copolymers, heteroarm star-shaped block copolymers and many other macromolecular architectures. Well-defined PDMS with a low polydispersity index and high homogeneity is produced by controlled anionic ring-opening polymerization of hexamethylcyclotrisiloxane. In a similar manner, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain. This can be used to produce hard silicone resins. Under ideal conditions, each molecule of such a compound becomes a branch point. Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. These reactive centers are typically "capped" by reaction with trimethylsilyl chloride:Ģ Si(CH 3) 3Cl + n-2 2 → n-2 2 + 2 HCl Hydrolysis of Si(CH 3) 2Cl 2 generates a polymer that is terminated with silanol groups (– Si(CH 3) 2OH). The acetate is used in consumer applications, such as silicone caulk and adhesives. As a side-effect, the curing process is also much slower in this case. In this case, the polymerization produces acetic acid, which is less chemically aggressive than HCl. For medical and domestic applications, a process was developed in which the chlorine atoms in the silane precursor were replaced with acetate groups. The polymerization reaction evolves hydrochloric acid. N Si(CH 3) 2Cl 2 + ( n + 1) H 2O → HO nH + 2 n HCl Industrial synthesis can begin from dimethyldichlorosilane and water by the following net reaction: The chemical formula for PDMS is CH 3 nSi(CH 3) 3, where n is the number of repeating monomer units. 5 Safety and environmental considerations.4.2 Hydraulic fluids and related applications.
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