Solvent swelling, PDMS

Solvent-assisted Microcontact Moulding. A small amount of solvent is spread on a patterned PDMS stamp and the stamp is placed on a polymer, such as a photoresist. The solvent swells the polymer and causes it to expand to fill the surface relief of the stamp. 

Chemical vapor deposition (CVD) of poly(para-xylylene carboxylic acid pentafluorophenolester-co-para-xylylene] (PPX-PPF) on PDMS can prevent it from solvent swelling [262]. 

To emphasise the role of orientational interactions in the appearance of the doublet in 2H NMR spectra, model PDMS networks have been swollen to various extents with a good solvent of PDMS [53]. Figure 15.10 shows the slopes P=A/(X2-X 1) obtained in a PDMS network swollen with chloroform. The swelling process results in stretching... 

For the sake of comparison, we have measured the swelling kinetics of the unloaded and isotropic mPDMS networks in cyclohexane. Cyclohexane is a good solvent for PDMS. The diameter of the gel beads was monitored over time. Figure 45 shows the time evolution of the bead size during swelling. 

To date, MIP microstructures have been successfully fabricated with dimethyl-formamide (DMF) as a solvent in both an acrylate-based [55] and a polyurethane based [59] imprinting system. While this solvent still swells PDMS, its effect is small. Other imprinting systems that utilize water and alcohols as the solvent may be possible since these solvents do not swell the PDMS stamps. Alcohols are the best as they have alow surface tension and wet the surface of the PDMS well. Water, however, has a high surface tension and does not wet the surface of the hydrophobic PDMS stamps. Possible solutions may be to render the stamp more hydrophilic by O2 plasma or UY irradiation to add a small percentage of alcohol ( 5 %) to the aqueous solution to wet the surface, or to use a small vacuum-assisted pump to fill the channels. 

Since the reduced modulus G refers to the unswollen, unstrained, isotropic network, the question arises, first, whether it should be the same for a given network, regardless of the swelling solvent. It was found that for PDMS networks as well as for polystyrene networks the value of G is not the same for a given gel whatever the swelling solvent may be14,22. This result, which implies that the swelling solvent influ-... 

Although PDMS is swollen by many organic solvents, it is unaffected by water, polar solvents (e.g., ethylene glycol), and perfluorinated compounds [367]. A detailed study of solvent compatibility of PDMS has been reported. It was found that non-swelling solvents include water, nitromethane, DMSO, ethylene glycol, acetonitrile, perfluorotributylamine, perfluorodecalin, and propylene carbonate [251]. The compatibility of PDMS to other organic solvents can be improved by coating the PDMS surface by sodium silicate [261]. 

Fig. 6. Effect of the solvent quality on the PDMS suriace layer thickness and particle-particle interaction force a) PDMS surface layer swelling due to good solvency of the liquid medium b) compression of PDMS layer due to low solvency of the liquid medium.

In Fig. 7.17, the dry modulus of various PDMS networks is plotted as a function of their equilibrium swelling in toluene. A single curve results for both model networks (open symbols) made by end-linking linear chains with two reactive ends and networks with intentionally introduced defects in the form of dangling ends (filled symbols) made by end-linking mixtures of chains with one and two reactive ends. The data are fit to Eq. (7.91) as the solid lines in Fig. 7.17 and their intersection determines the crossover concentration 0 = 0.2, which is typical for good solvents. 

Let us consider the states of a chain made of Nx monomers, in solution successively in its own monomer, in its dimer, in its trimer, etc. In the first case, the repulsive interaction between monomers of the chain produces a gyration swelling 3 G which obeys the laws already observed. In the other extreme case where the test chain finds itself in a melt of other identical chains, the repulsive interaction is screened and the test chain is in a quasi-Brownian state. In intermediate situations the swelling 3E0 of the test chain of Nx monomers varies with the number N of monomers (per chain) of the solvent chains. Observation of the variation law of XG with N clearly reveals the structure of polymer solutions. Such observation requires the labelling of the chain made of iV, monomers therefore it is appropriate to use neutron scattering. Kirste and Lehneh2 made the experiment with blends of polydimethylsiloxane (PDMS)... 

Similar to the above polystyrene-based systems, any dry polymeric network would preserve dimensions of its strands on swelling in a -solvent. Nevertheless, this is not the case. In addition to polystyrene that, according to SANS data presented in Table 1.5, perfecdy follows the above expectation, polydimethylsiLoxane (PDMS) networks have been also examined by this technique. They were prepared by crosshnking dissolved Unear PDMS chains, fitted on both ends with silane functions, with tetraaUylox-yethane... 

A major limitation of this technique, however, is that the cross-linked PDMS stamps tend to swell in many organic solvents that include chloroform, methylene chloride, toluene, and tetrahydrofuran. These solvents have been widely used as porogens in the synthesis of molecularly imprinted polymers. The PDMS stamps swelled dramatically in these solvents and subsequently lifted off from the substrate [55]. Once a stamp has lost the conformal contact with the substrate, the assembly can no longer be used to make polymer structures. 

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