Materials Science
by Curtis Marcott, Michael Lo, Eoghan Dillon, Halil I. Akyildiz and Jesse S. Jur
Depth Profiling Trimethylaluminum-Modified PET Fibers by Nanoscale Infrared Spectroscopy
T
he development of flexible electronic devices for use as biosensors, optics or photovoltaic applications is a promis- ing research area.1
Many common, inexpensive polymers can
be processed in large quantities into flexible materials, but they are not generally usable as electronic devices without modifying their electronic or optical properties. Efforts are being made to 1) develop new polymers that are flexible and 2) modify inexpensive, flexible poly- mers such as poly(ethylene terephthalate) (PET) in order to make them useful for flexible electronic devices.
A variety of techniques have been used to functionalize bulk polymers. This paper examines sequential vapor infiltration (SVI) for the functional- ization of PET with trimethylaluminum (TMA) into a flexible, inexpensive photo-luminescent hybrid material.1
The following discussion will focus
on the SVI modification of ~25-μm-diam round PET fibers and their characterization as a function of depth and the SVI processing conditions using nanoscale infrared spectroscopy. These flexible photoluminescent PET hybrid fiber materials may have potential applications in electronic textile products.
Measurement tools that are capable of chemically characterizing the depth of penetration of precursors into fibers are needed to enhance un- derstanding of these hybrid flexible polymers. The recent combination of atomic force microscopy and infrared spectroscopy (AFM-IR) makes it pos- sible to obtain IR spectra with nanoscale spatial resolution that is nearly two orders-of-magnitude greater than conventional Fourier transform infrared (FTIR) microspectroscopy.2,3
This article illustrates how AFM-IR
spectroscopy can be used to characterize the penetration and subsequent reaction of precursors into fibers with the goals of shortening develop- ment time and achieving optimum processing conditions.
Experimental SVI treatments of PET fibers with 2/1 twill woven fabric construction
were conducted using a custom-designed hot-wall flow tube-type reac- tor.3
Trimethylaluminum and high-purity water were used as obtained. Ultrahigh-purity N2 flow (~300 SCCM) was used for precursor delivery
to the system and purging of the chamber resulting in 1 Torr pressure. A schematic of the SVI process flow is given in Figure 1. Following an initial 5-minute purge, samples were exposed to TMA cycles consisting of 0.5-second dose, 30-second hold and 30-second purge. After the TMA cycles were completed, the reactor chamber was evacuated for 5 minutes before exposure to H2
O cycles to oxidize any unreacted methyl groups
with a sequence of 0.2-second dose, 30-second hold and 30-second purge. PET round fibers were treated with SVI and their mass gain was calculated as a weight percentage using the initial and final weight of the sample. The final mass of the samples was measured after the samples
Figure 1 – Schematic of SVI process for TMA-H2 O precursor pair.
were equilibrated for approximately 2 hours, which was deemed suffi- cient, with negligible moisture gain (0.4%).
TMA-treated round fibers were prepared for AFM-IR analysis by embed- ding the fibrous yarn samples in an epoxy adhesive followed by overnight curing at room temperature. The blocks were then cut to a thickness of 200–300 nm at room temperature using a Leica UC7 Ultracut diamond knife microtome (Leica Microsystems Inc., Buffalo Grove, Ill.). All AFM images and AFM-IR spectra were collected on an Anasys nanoIR (Anasys Instruments, Santa Barbara, Calif.).2
A 450-µm-long contact mode can-
tilever (0.1 N/m) was used. Prior to imaging in a Phenom G1 desktop scanning electron microscope (SEM) (Phenom-World BV, Eindhoven, The Netherlands), the samples were sputter-coated with ~10 nm gold/ palladium thin films. For transmission electron microscopy (TEM) analysis, fibrous yarn samples were embedded into Spurr low-viscosity epoxy resin and then cured overnight at room temperature. Films of ~100 nm were prepared by microtomy. The films were floated from deionized water onto copper TEM grids coated with ultrathin amorphous carbon. TEM micrographs were obtained using a Hitachi HF 2000 Field Emission Gun TEM (Hitachi High Technologies America, Inc., Schaumberg, Ill.) with a 200-kV cold emission source.
Results Figure 2 shows a plot of the mass gain of round PET fibers as a function of
the number of TMA SVI cycles at temperatures between 60 and 150 °C.1 A
decrease in the highest mass gain is observed as the process temperature increases. Mass gain saturation is dramatically delayed at lower SVI pro- cessing temperatures. At 60 °C, a nearly linear increase of mass gain up to 90 SVI cycles is observed with no sign of saturation in mass gain. As the temperature increases, the number of cycles to saturation continues to decrease from 120 to 150 °C. SVI is enabled by the diffusion of precursors and the diffusion is limited by high reaction rates at higher temperature. At low temperature (i.e., 60 °C), the linear growth per TMA cycle suggests that the diffusion of the precursor into the polymer matrix is possible
AMERICAN LABORATORY • 12 • MARCH 2015
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