Mikrobubbel Tillverkning av konkav-porositet PDMS Pärlor

Summary

Procedures used to generate microstructured concave-porosity polydimethylsiloxane beads are presented. Effects of electrolyte concentration and identity within the aqueous phase are particularly emphasized.

Abstract

Microbubble fabrication (by use of a fine emulsion) provides a means of increasing the surface-area-to-volume (SAV) ratio of polymer materials, which is particularly useful for separations applications. Porous polydimethylsiloxane (PDMS) beads can be produced by heat-curing such an emulsion, allowing the interface between the aqueous and aliphatic phases to mold the morphology of the polymer. In the procedures described here, both polymer and crosslinker (triethoxysilane) are sonicated together in a cold-bath sonicator. Following a period of cross-linking, emulsions are added dropwise to a hot surfactant solution, allowing the aqueous phase of the emulsion to separate, and forming porous polymer beads. We demonstrate that this method can be tuned, and the SAV ratio optimized, by adjusting the electrolyte content of the aqueous phase in the emulsion. Beads produced in this way are imaged with scanning electron microscopy, and representative SAV ratios are determined using Brunauer–Emmett–Teller (BET) analysis. Considerable variability with the electrolyte identity is observed, but the general trend is consistent: there is a maximum in SAV obtained at a specific concentration, after which porosity decreases markedly.

Introduction

Polydimethylsiloxane (PDMS) is one of the most widely used silicone compounds. Its biocompatibility has led to widespread use in implant and other biomedical engineering structures^1,2. It is trivially cross-linked into elastic structures using an organosilyl compound (such as triethoxysilane), a simple and reliable procedure which has made it useful for cast polymer applications where some flexibility is required³. Once cross-linked, PDMS is largely inert, particularly in biological conditions, and is therefore useful for a variety of food and medical applications^4,5. Ease of casting, chemical inertness, and hydrophobicity have made it a natural choice for microfluidic devices^6,7. Its affinity for non-halogenated, non-polar organic compounds has made it a popular stationary phase in separations chemistry^8-10.

Recently, microbubble fabrications have been used to generate porous beads for use as catalyst structural substrates or in chemical separations^11,12. In both applications, ideal materials will have a maximized surface-area-to-volume (SAV) ratio for best efficiency. In a microbubble fabrication process, microstructuring of materials is typically accomplished by isolating the polymer in aliphatic “microbubbles” by emulsification in an aqueous continuous phase. The initial report of microporous PDMS beads produced them by mechanical emulsification of two phases (aliphatic and aqueous)¹³. The stock PDMS liquid (and its cross-linking agent) is dissolved into the aliphatic phase, which is structured into microscopic beads by being forced to cavitate within the (continuous) aqueous phase. The emulsification is stabilized by the addition of a non-ionic surfactant. When the emulsion is added dropwise to a heated bath, solid beads form by agglomeration of the microbubbles into clusters of tiny spheres of cross-linked PDMS. Our goal in this protocol is to modify this procedure to develop beads with an inverted porosity to improve the SAV ratio of the material.

As reported previously, control of the beads can be directed to some extent by the aliphatic:aqueous ratios in the emulsion. However, we have reported recently that addition of platinum(IV) chloride (PtCl₄) inverts the porosity: materials are formed in which the PDMS is riddled with concave pores¹⁴. This indicates that the aqueous layer cavitates inside the aliphatic one, despite having similar aliphatic:aqueous ratios to those published in the original work¹³. The primary advantage of our method is that this concave porosity should naturally result in an increased SAV ratio, and thus, improved efficiency for analytical chemistry applications. While we are continuing to explore the specific effects of the addition of the platinum compound, we show here that the same effect can be accomplished using any aqueously soluble ionic compound, though perhaps to a reduced extent. Because our techniques also differ in some key aspects from what has been previously reported, we present our protocols here as a video to encourage others to extend our methods. Most notably, we use a common bath sonicator of the type used to clean glassware or other equipment, rather than the (considerably more expensive) probe sonicator often used in microbubble fabrication. This adjusted approach to the microbubble fabrication procedure could potentially be extended for the production of large quantities of bulk materials as well, creating porous sheets or slabs which could have applications for biomedical devices, aerospace and automotive industry, or substrates for chemical catalysis. Users seeking to generate high-SAV-ratio, microstructured materials using other similar polymers for such analyses may find that our protocols can be extended to any polymer for which the microbubble emulsion technique can be applied.

Protocol

1. Framställning av emulsion Emulsions Innehåll Mäss en lämplig mängd salt för att producera 10 ml av 0,03-M lösning. För platina (IV) klorid åtgärd 0,101 g, för zink (II) klorid (ZnCl2) åtgärd 0,032 g och natriumklorid (NaCl) åtgärd 0,018 g. I enskilda provrör, upplösa varje salt i 10 ml DI-vatten. Ställ åt sidan för senare användning. Använd en 20-ml, förslutbara glasampuU för innehållet under hela detta förfarande. Tarera en balans i glasfla…

Representative Results

Representativa SEM-bilder av pärlor som följer av emulsioner med olika elektrolytlösningar betingelser visas i figur 1. Figur 1A visar en vulst som liknar de som erhålles genom DuFaud, et al., 13, produceras med användning av våra förfaranden, utan tillsats av någon elektrolyt. Pärlor som visas i figur 1B-D, vilket resulterar i olika morfologier för varje metalljon. För alla bilder som visas, var 300 pl 0,03-M elektrolytlösningar använde…

Discussion

Pärlorna produceras med hjälp av detta protokoll (och genom att justera elektrolytkoncentrationen och identitet) är fundamentalt annorlunda från de som produceras med en låg jonstyrka emulsion, som kan ses genom jämförelse av Figur 1A till andra SEM-bilder i figur 1. Vår första rapport används PtCl ₄ med avsikten att ytterligare katalysera polymerisationen tvärbindning vid den vattenhaltiga-alifatisk gränssnittet ^14. I denna rapport, va…

Materials

Poly(dimethylsiloxane), vinyl terminated	Sigma-Aldrich	68083-19-2
n-Heptane	Sigma-Aldrich	142-82-5	Flammable
Triethoxysilane	Sigma-Aldrich	998-30-1	Flammable, Accutely Toxic
Sorbitan Monoleate (Span-80)	Fluker	1338-43-8
Platinum (IV) Chloride	Sigma-Aldrich	13454-96-1	Accutely Toxic
Zinc (II) Chloride	Sigma-Aldrich	7646-85-7
Sodium Chloride	Sigma-Aldrich	7647-14-5
2.8L Water Bath Sonicator	VWR	97043-964