1. Shaping Porous PDMS Beads with Platinum (IV) Chloride
John R. Bertram and Matthew J. Nee
Department of Chemistry, 1906 College Heights Blvd, Western Kentucky University, Bowling Green, KY 42101
Abstract
Polydimethylsiloxane (PDMS) is an elastic polymer used in a wide variety of applications ranging from pharmaceuticals, to hair conditioner, to analytical techniques such as solid phase micro extraction (SPME). Solid, hydrophobic PDMS beads are made using a mixture of n-heptane, PDMS, and DI-water by a technique called cross-linking. It was observed that the addition of a chloride salt, platinum (IV) chloride, into the synthesis process dramatically altered the surface structure by creating pores and concavities of various sizes. Each pore contributes to an increase in surface area to volume ratio (SAV), which improves techniques such as SPME. Other, much less expensive, chloride salts such as zinc (II) chloride and sodium chloride were introduced to the bead synthesis process. Sodium chloride negatively affected the SAV while zinc (II) chloride raised the SAV to 5 times that of platinum (IV) chloride.
Experimental Methods
As soon as all the drops are added, the contents of the test tube are poured through a filtering apparatus.
The small, congealed precipitates (beads) are then transferred onto a watch glass to dry overnight.
Results
The graphs presented are “plot profiles” processed by ImageJ and composed on MATLab measuring distance (in pixels) versus the gray scale value of the SEM images.
When SEM images were paired with ImageJ it’s possible to see how different concentrations of platinum (IV) chloride, and the alternative addition of zinc (II) chloride and sodium chloride, resulted in a morphology of the beads’ surface. Porosity increased and many concave sites formed attributing to the increase in surface area in the beads containing the zinc and platinum metals.
PDMS
n-Heptane
Varying Amount of Chloride Salt
Triethox-ysilane
Span-80
DI Water
Crosslinking agent is added to the vial and an exothermic reaction is observed.
The vial is then immersed in ice to help keep the solution from overheating and destroying the emulsion.
Solution is then added drop-wise to a test tube with 10mL of a 1g SDS and 750mL solution kept at C.
The beads are then taken to the SEM where images with high contrast are taken to optimize them for ImageJ processing.
All five components are joined in a small vial, capped, then placed into an ice-water bath sonicator for 50 minutes or until an emulsion was visible.
BET Analysis
Introduction
PDMS is a widely used polymer in many biomedical applications because of its low-autofluorescence and viscoelastic properties. It can be molded into microchips that are biocompatible and non-toxic. One such use for these is electrophoresis which is a process that separates proteins from DNA. Another interesting property is its ability to form a hydrophobic, bead-like structure after cross-linking. It was found that the addition of a chloride-based salt into the bead synthesis process greatly improved the surface area to volume ratio of the beads. Zinc (II) chloride had the most dramatic effect, followed by platinum (IV) chloride, where as sodium chloride decreased the SAV.
Sample
Contents
SAV Ratio (cm2/cm3) 1
0.3mL 0.03M PtCl4
1,849 2
0.3mL 0.03M ZnCl2
11,060 3
0.2mL 0.03M PtCl4
756.2 4
0.3mL 0.03M NaCl
298.9
Control
No Salt Added
361.6
ImageJ and SEM Analysis
0 salt added
0.6mL 0.03M platinum (IV) chloride
A BET instrument determines surface area to volume ratio by measuring adsorbtion capacity with nitrogen gas. Sample 2 possessed the highest SAV ratio which contained of 0.03M ZnCl2.
0.2mL 0.03M platinum (IV) chloride
0.75mL 0.03M zinc (II) chloride
Conclusion and Future Studies
Originally it was postulated that the SAV ratio of the concave beads was dependent upon an ionic strength affect, but BET analysis of the ZnCl2 indicates that this is not entirely the case. There is a catalytic affect that invokes porosity in the beads, which explains why, despite an impressive surface profile, the NaCl beads possessed the lowest SAV.
Scaling up this procedure to make films and slabs of high surface area materials for use in methods such as chemical separations is the goal at hand.
0.3mL 0.03M platinum (IV) chloride
0.3mL 0.03M zinc (II) chloride
PDMS Fiber
Solid Phase Micro-Extraction is beneficial to a number of different investigative procedures such as water preservation and forensic toxicology. When the beads are immersed in the liquid or gas in question they extract analytes onto their surface through a process called adsorption. Thus, the more surface area a bead has, the more room it has for analytes increasing their concentration.
0.5mL 0.03M platinum (IV) chloride
0.3mL 0.03M sodium chloride
References
Dufaud, O., Favre, E. and Sadtler, V. (2002), Porous elastomeric beads from crosslinked emulsions. J. Appl. Polym. Sci., 83: 967–971.
Farmer, Brandon C., Mckinley A. Mason, and Matthew J. Nee. "Concave Porosity Non-polar Beads by a Modified Microbubble Fabrication." Materials Letters 98 (2013):
Acknowledgements
This work was funded in part by a WKU Research and Creative Activities Award (WKU RCAP ). Brandon C. Farmer and McKinley A. Mason started this work where specific bead production method is concerned. We are indebted to Dr. John Andersland for his instruction on the SEM and Dr. Yan Cao for his collection of data and operation of the BET analysis.