PDMS network formation

Figure 2. PDMS network formation the critical reactive-group ratio for gelation, n, versus reactive-group dilution, 1/cm, for the reaction systems of Table 2.

Networks Formed in the Presence of Diluent, s>0.9. A series of six networks were prepared both in bulk and in the presence of oligomeric PDMS (Mn = 1170, no vinyl groups) using as junctions a 0 = 43.9 linear PMHS and as chains a,o>-divinyl PDMS ranging in Mn from 9,320 to 28,600 g mol . The volume fraction of solvent present during network formation, v s, was 0.30 for all six networks and was calculated assuming simple additivity of volumes. The tensile behavior of the networks formed in bulk was measured in bulk, vt = Vf/V = 1. The tensile behavior of the networks formed in solution was measured both on networks with solvent present (vt =1) and on networks from which the oligomeric PDMS had been extracted (vt 1.47). 

PTFE/PDMS can be obtained via a similar process [112]. PTFE chains are trapped in the PDMS network during casting. Jones et al. [113] also patented a process where adipic acid and hexane diamine (or f -caprolactam [114]) were added to a silicone latex leading to the formation of a polyamide/PDMS semi-IPN. 

Abstract This article summarizes a large amount of work carried out in our laboratory on polysiloxane based Interpenetrating Polymer Networks (IPNs). First, a polydimethylsiloxane (PDMS) network has been combined with a cellulose acetate butyrate (CAB) network in order to improve its mechanical properties. Second, a PDMS network was combined with a fluorinated polymer network. Thanks to a perfect control of the respective rates of formation of each network it has been possible to avoid polymer phase separation during the IPN synthesis. Physicochemical analyses of these materials led to classify them as true IPNs according to Sperling s definition. In addition, synergy of the mechanical properties, on the one hand, and of the surface properties, on the other hand, was displayed. 

The curing process takes advantage of the versatile chemical property of silicones. Chemical reactivity is built in the polymer and allows the formation of silicone networks of controlled molecular architectures with specific adhesion properties. The general and inherent molecular properties of the PDMS polymer are conferred to the silicone network. Pure PDMS networks are mechanically weak and do not satisfy the adhesive and cohesive requirements needed for most applications. Incorporation of fillers like silica or calcium carbonate is necessary to reinforce the silicone network (see Composite materials). 

In this type of material, two networks are formed, either simultaneously or sequentially, in such a way as to interpenetrate one another. The networks thus communicate with one another through interchain physical forces and entanglements, rather than through covalent bonds. A particularly simple example is the simultaneous formation of two PDMS networks, one by a condensation end-linking reaction and the other by an addition end-linking reaction, with the two types of chains mixed at the molecular level. - ... 

Serbescu, A. Saalwachter, K., Particle-Induced Network Formation in Linear PDMS Filled with Silica. Polymer 2009, SO, 5434-5442. 

The difference in diffusion behavior of probe polymers has been investigated where the network formation of the host polymer is due to physical entanglement such as in a semidilute solution or covalent cross-linking as in a gel. Aven and Cohen [16] used polystyrene (PS) as a probe polymer. They studied the diffiision coefficient of PS (A/, = 4140,7620, and 14,100) in various concentrations O swollen by tetrahydrofuran (THF) and PDMS M — 26,500) solution using a dynamic light scattering technique. 

Scheme 23.4. Schematic of P VMS-PDMS network system formation where hydride-terminated PDMS is reacted across the PVMS backbone via a platinum catalyst to produce PVMS-PDMS networks. Adapted and reproduced with permission from [47] Copyright 2010 Elsevier.

Recently, it was shown that polydimethylcarbosiloxanes with a small content of side carbonyl groups (PDMS-C) exhibit increasing viscosity and formation of a physical network at elevated temperatures [88,89], This was attributed to a rearrangement of intramolecular hydrogen bonds, which formed between the carboxyls during the synthesis and isolation of the polymers, forming intermolecular hydrogen bonds. 

A model has been developed to describe the penetration of polydimethylsi-loxane (PDMS) into silica agglomerates [120]. The kinetics of this process depend on agglomerate size and porosity, together with fluid viscosity. Shearing experiments demonstrated that rupture and erosion break-up mechanisms occurred, and that agglomerates which were penetrated by polymer were less readily dispersed than dry clusters. This was attributed to the formation of a network between sihca aggregates and penetrated PDMS, which could deform prior to rupture, thereby inhibiting dispersion. 

Whitesides and coworkers combined microfluidic networks with a PDMS platform to create patterned gradients of biomolecules on a surface.121 This method involves a two-step process (1) formation of a gradient of avidin within well-defined patterns by use of microfluidic channels and (2) specific interaction between the avidin gradient pattern and biotin. Such patterns with a density gradient of immobilized biomolecules may find application in studies on cell development and function. 

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