PDMS, property mechanical

Many of the optofluidic devices are made of a soft elastomer, polydimethylsiloxane (PDMS). In this entry we focus on a specific group of tunable optofluidic devices of which the tuning relies on elasticity of PDMS. The mechanical tuning method is achieved by modifying the geometry of the device upon applying internal or external forces, hence deforming the elastomeric components that directly or indirectly alter the optical properties of the device. 

The nature of the bonding in siloxane molecules has been of longstanding interest. Force fields for calculations of PDMS properties have been revised over the years and are now at an advanced state of develop-ment. 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. ... 

In silicone adhesives used to bond structural glazing assemblies, the silicone network is made of very long PDMS chains and is filled with silica that improves the elastomeric properties of the adhesive. The strength of such an adhesive is strongly enhanced through various mechanisms of energy absorption. 

Plasticizer Trimethylsilyl-endblocked- PDMS Adjustment of mechanical properties such as hardness, viscoelasticity, rheology. 

Despite their many outstanding properties, polydimethylsiloxane (PDMS) rubbers require extremely high molecular weights to develop useful mechanical properties. 

Synthesis of siloxane-urethane copolymers from various hydroxyalkyl-terminated PDMS oligomers and aliphatic diisocyanates, such as tetramethylene- and hexame-thylene diisocyanate and HMDI was reported 333,334). Reactions were conducted either in chloroform or 1,4-dioxane and usually low molecular weight, oily products were obtained. No data were available on the molecular weights or the thermal and mechanical properties of the copolymers obtained. These products were later cross-linked by a peroxide. Resulting materials were characterized by IR spectroscopy and water contact angle measurements for possible use as contact lenses. 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

The polyether-PDMS soft segments in segmented polyurea-urethanes have been synthesized to combine the good mechanical properties of PUs and the excellent blood-contacting properties of PDMS. The most widely recognized material in this category is Avcothane [Arkles BC, Med Device Diagn Ind 3 30(1981)], which is characterized as a block copolymer of aromatic polyether urethane and PDMS. 

Hydroxy-terminated PDMS, however, has disadvantages. The monofunctional ends limit the number of connections between the polymer (or oligomer) molecule and the glass network to two. This limitation raises the possibility that some PDMS molecules are not tied at both ends to the glass network if the polycondensation does not go to completion i.e. there may be "dangling" or loose PDMS chains in the final sol-gel material. This occurance of free ends would indeed be anticipated since the extent of reaction most likely is not 100%. Hence, the physical properties, specifically the mechanical behavior of the overall material, would be expected to suffer as a result of loose PDMS chains in the system. Disregarding this potential problem, the mechanical behavior of the sol-gel hybrids are, ultimately, influenced by the mechanical behavior of the modifying elastomer ... 

Hybrid materials with TEOS and triethoxysilane encapped PTMO have been successfully prepared by the sol-gel process. High transparency is observed for most samples, and many of the mechanical properties are considerably improved relative to the earlier reported TEOS-PDMS systems. 

We have investigated the static and dynamic mechanical properties of networks of different chemical and topological structures ( 19,20). In a previous paper, we reported results obtained on networks with crosslink functionality four (21). In the present study, we investigated the effect of the structure of junctions on the mechanical behaviour of PDMS. Rather uncommon networks with comb-like crosslinks were employed, intending that these would be most challenging to theoretical predictions. 

A representative stress-strain curve of one of the PDMS-CaO-Si02 nano-hybrids is shown in Figure 11.7, in comparison with that reported for human cancellous bone [29]. Unlike the usual brittle ceramics, the nano-hybrid was deformable and showed mechanical properties analogous to those of human cancellous bone. 

In order to obtain a nano-hybrid with higher mechanical properties, PDMS was replaced with poly(tetramethylene oxide) (PTMO) [41]. PTMO-Ti02 nano-hybrids were prepared by a sol-gel method from triethoxysilane functionalized PTMO (Si-PTMO) and tetraisopropyltitanate with weight ratios of 30/70, 40/60 and... 

Chen, Q., Miyata, N., Kokubo, T. and Nakamura, T. (2000) Bioactivity and mechanical properties of PDMS-modified Ca0-Si02-Ti02 hybrids prepared by sol-gel process. Journal of Biomedical Materials Research, 51, 605-611. 

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