The development of flexible electronic devices for use as biosensors, optics or photovoltaic applications is a promising 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 polymers 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 functionalization 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 understanding of these hybrid flexible polymers. The recent combination of atomic force microscopy and infrared spectroscopy (AFM-IR) makes it possible 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 development time and achieving optimum processing conditions.
Figure 1 – Schematic of SVI process for TMA-H2O precursor pair.
SVI treatments of PET fibers with 2/1 twill woven fabric construction were conducted using a custom-designed hot-wall flow tube-type reactor.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 H2O 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 were equilibrated for approximately 2 hours, which was deemed sufficient, with negligible moisture gain (0.4%).
TMA-treated round fibers were prepared for AFM-IR analysis by embedding 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 cantilever (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.
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 processing 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 until all the available sites (free volume and reactive sites on the backbone of the polymer) are satisfied. However, the fact that the slope of the line at 60 °C is lower compared to the linear parts of the high-temperature samples suggests slower diffusion rates at lower temperatures. As the process temperature is increased, a rapid increase in the mass gain is observed with a corresponding increase in the initial slope. The mass gain saturates quickly, indicating that no precursor is diffused into the samples for subsequent reaction. This deceleration of diffusion is a result of the higher reaction kinetics, which forms a dense hybrid layer close to the surface and prevents further penetration of the precursors.
Figure 2 – Mass gain of round PET fibers as a function of the number of TMA SVI cycles at temperatures between 60 and 150 °C. TMA cycles were conducted with a sequence of 0.5-second dose/30-second hold/30-second purge followed by H2O cycles of approximately half the number of TMA cycles with a sequence of 0.2-second dose/30-second hold/30-second purge. (Figures 2–5 were adapted from Ref. 1.)
SEM images of pristine and TMA SVI-treated PET fibers are shown in the top half of Figure 3. Cracks have developed as a result of the SVI processing. The bottom half of the figure shows TEM images of cross-sections of the TMA SVI-treated PET fibers obtained at 60 and 150 °C.1 The sample with SVI treatment of 60 °C shows no clear evidence of the formation of a hybrid material, in contrast to the sample exposed at 150 °C, which shows distinct hybrid layer formation.
Figure 3 – SEM micrographs of a) untreated and b) 30 TMA cycle SVI- treated round PET fibers showing the change in fiber morphology upon SVI treatment at 60 °C. TEM micrograph of cross-sections of PET round fibers treated with c) 90 TMA SVI cycles at 60 °C and d) 60 TMA SVI cycles at 150 °C.1
AFM-IR analysis was performed to examine the chemical composition through the fiber cross-section in more detail. Figures 4 and 5 show AFM-IR spectra and AFM images of a cross-section of epoxy-embedded PET round fibers treated with 60 TMA SVI cycles at 150 °C and with 90 TMA SVI cycles at 90 °C, respectively. For both samples, spectra are normalized at 2972 cm–1 in the high-wavenumber region and at 1268 cm–1 in the low-wavenumber region. In the figures, three spectra were collected from each point shown in the AFM image. Typical absorption peaks for PET are observed at 3436, 2972, 2908, 1720, 1410, 1340, 1268 and 1104 cm–1 . For the SVI fiber treated at 150 °C (Figure 4), absorption intensities at 3436, 2972, 2908 and 1720 cm–1 are attenuated closer toward the surface, indicating that more of the hybrid material is forming near the surface. The 1020 cm–1 band is likely due to AlOx formation in the fiber as a result of the SVI processing. The sharp decrease in the C=O stretching peak intensity is coincident with the higher topography in the AFM image and the denser surface layer observed in the TEM micrographs (Figure 3).
Figure 4 – AFM-IR spectra and AFM images of a cross-section of epoxy- embedded PET round fibers treated with 60 TMA SVI cycles at 150 °C. Spectra are normalized at 2972 cm–1 in the high-wavenumber region and at 1268 cm–1 in the low-wavenumber region.1
In contrast, the chemical nature near the surface of the SVI fiber treated at 90 °C is vastly different. Additional peaks are observed at 2920 and 2860 cm–1, corresponding to the methyl (CH3) stretching vibrations that resulted from the TMA precursor treatment (Figure 5). These peaks show higher intensity closer to the surface, suggesting that more TMA polymer reactions take place closer to the surface. The low-wavenumber region shows peaks at exactly the same wavenumber positions as the sample that was TMA SVI treated at 150 °C. However, all the spectra now show similar band intensities within 1.5 μm of the surface. Since the CH3 peaks and AlOx peak (1020 cm–1) are observed, this can be attributed to a more homogeneous composition of the sample within the measured section.
Figure 5 – AFM-IR spectra and AFM images of a cross-section of epoxy- embedded PET round fibers treated with 90 TMA SVI cycles at 90 °C. Spectra are normalized at 2968 cm–1 in the high-wavenumber region and at 1268 cm–1 in the low-wavenumber region.1
Nanoscale AFM-IR spectroscopic depth profiling of the outer portion of surface-modified PET fibers has been shown to be useful for chemically characterizing the hybrid layer produced by SVI treatments with a TMA precursor as a function of treatment temperature and number of SVI cycles. When this chemically specific information is combined with mass gain, TEM and SEM images of the same sample, a picture emerges that suggests high-temperature SVI treatments cause a barrier layer to be formed at the surface of the polymer that slows the diffusion of the reactant. More uniform hybrid layers that extend deeper into the fiber are possible when the TMA SVI treatments are performed at lower temperatures.
The ability to perform nanoscale IR spectroscopic measurements can provide insight into the surface-modified polymers. This approach to depth-profiling the surface of SVI-treated fibers at submicrometer-length scales will help enable advances in this emerging field and lead to the development of other inexpensively modified polymers that can be applied to a wide range of constructs.
- Akyildiz, H.I.; Lo, M. et al. Formation of novel photoluminescent hybrid materials by sequential vapor infiltration into polyethylene terephthalate fibers. J. Mater. Res. 2014, 29, 2817–26.
- Dazzi, A.; Prater, C.B. et al. AFM-IR: combining atomic force microscopy and infrared spectroscopy for nanoscale chemical characterization. Appl. Spectrosc. 2012, 66, 1365–84.
- Akyildiz, H.I.; Padbury, R. et al. Temperature and exposure dependence of hybrid organic-inorganic layer formation by sequential vapor infiltration into polymer fibers. Langmuir 2012, 28(44), 15697–704.
Curtis Marcott, Ph.D., is a senior partner, Light Light Solutions, P.O. Box 81486, Athens, Ga. 30608, U.S.A.; tel.: 513-720-0171; e-mail: [email protected]; www.lightlightsolutions.com. Michael Lo and Eoghan Dillon are application scientists with Anasys Instruments, Santa Barbara, Calif., U.S.A. Halil I. Akyildiz is a Ph.D. candidate in the Department of Textile Engineering, Chemistry, and Science at North Carolina State University, Raleigh, N.C., and Jesse S. Jur is an assistant professor in the Department of Textile Engineering, Chemistry, and Science at North Carolina State University, Raleigh, N.C. This research was supported in part by NSF-SBIR grants 0750512 and 0944400.