During the award period, we have made progress in both experimental work and modeling.

Experimental work:

Film preparation

We have developed solution-casting techniques for producing poly(dimethylsiloxane) (PDMS), poly(etherimide) (PEI), and polycarbonate (PC) membranes, and PDMS-silica composite membranes, and characterized these films using SEM and optical microscopy. We found that longer stir times increased overall dispersion of silica nanoparticles, but did not eliminate large agglomerates (see Figure 1).

Figure 1. SEM images of PDMS with 10 wt% amorphous silica nanoparticles stirred for 72 hours (left; cracks due to sample preparation), 12 hours (middle), and 6 hours (right).

We have also initiated a collaboration with Dr. Jeffrey Pyun and Dr. Mary Wirth at the University of Arizona to address the issue of agglomeration.  Drs. Pyun and Wirth are experts in the field of Surface-Initiated Atom Transfer Radical Polymerization (ATRP), a technique in which a polymer matrix is grown from the surface of the particles themselves.  This should eliminate two major issues: poor interfacial adhesion between the particles and the polymer (there will no longer be a gap between the particles and the polymer) and the agglomeration of nanoparticles in films due to colloidal effects (the polymer corona should prevent aggregation).

Stöber process particles

To examine effects of a wide range of impermeable particle sizes, we have begun synthesizing silica particles using the Stöber process. The particles we obtained were not spherical and were highly agglomerated (see Figure 2); we plan to address this by improving ammonia saturation in the solution and improving temperature control.

Figure 2. SEM images of products formed via the Stöber process using 100% butanol (left) and 1:1 methanol:butanol (right) as the solvent.

Permeation experiments

We have constructed and leak-tested an apparatus for testing single gas permeation through polymer films and developed a MATLAB program for determining the pseudo-steady-state region for data analysis and extracting permeability values.  To test the apparatus, we measured the permeability of nitrogen and carbon dioxide in cross-linked PDMS membranes.  The values (7.2E-7 cm²/sec and 7.5E-6 cm²/sec, respectively) were reproducible and lower than those in Polymer Handbook for uncross-linked PDMS. We also examined PDMS/SiO₂ composite films and found that membranes prepared using longer stir times (which correlate to better nanoparticle dispersion) showed progressively higher permeabilities (permeability increased 25%  with a stir time increase from 3 to 72 hours).

Positron-Annihilation Lifetime Spectroscopy (PALS)

To determine the free volume in polymers and polymer composites (an important indicator of transport properties), we will run PALS on the films. This past summer, we traveled several times to the University of California, Riverside, where we worked with Drs. Allen Mills and Maurizio Biasini to design a PALS apparatus for our work.  Although we did set up a preliminary apparatus at UCR, due to the length and number of experiments required, we plan to apply for separate funding to set up our own PALS system at Harvey Mudd.

Modeling:

We have begun molecular modeling of polymers using Materials Studio 4.0.  We are running molecular dynamics and a series of NVT/NPT equilibration steps to obtain the equilibrium structure of each polymer. We can then simulate gas diffusion and determine free volume in the unit cell.  Our goal is to complete molecular modeling of polymer/inorganic nanocomposite and gas transport therein, which to our knowledge has not previously been modeled. Current theories for increased permeability in glassy polymer nanocomposites hinge on interfacial effects and inhibited polymer packing.

Therefore, we are particularly interested in the interface between the polymer and the nanoparticle, as well as the difference in polymer configuration due to the presence of nanoparticles. We are currently setting up a computational cluster to drastically decrease the computational time for each run (currently at several months for one pure polymer chain).

Research Students:

Over the past year, eleven students majoring in biology, chemistry and engineering have participated in the project. The students have been trained on the theory and use of the SEM, learned techniques and developed protocols for solution-casting polymer membranes, synthesized Stöber particles, learned theory and set up a system for PALS, written literature reviews, given technical presentations, attended a conference on plastics, and toured local chemical companies, including Chevron and Amgen.  The students are listed below, with their current status given in parentheses:

• Lupita Bermudez (ACS-PRF SUMR Scholar; junior)

• J.J. Boyles (junior)

• Karen Brown (senior, applying to graduate programs in organic chemistry)

• Rachel Howden (senior, applying to graduate programs in materials science and chemical engineering)
• Susan Kim (senior)
• Michael Kai Mayeda (senior, applying to graduate programs in chemical and mechanical engineering)

• Donna Phu (junior)
• Heather Schalliol (graduated, working at the Agencia Cordoba de Ciencia in Cordoba, Argentina)

• Gena Urowsky (senior)
• Christina Tang (graduated, now in the chemical engineering graduate program at NC State)

• Seanna Vine (junior)