Fabrication of segmented nanofibers by template wetting of multilayered alternating polymer thin films
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- Dougherty, S. & Liang, J. J Nanopart Res (2009) 11: 743. doi:10.1007/s11051-008-9502-0
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Segmented polystyrene (PS) and poly-methyl methacrylate (PMMA) nanofibers were fabricated by wetting nanoporous alumina templates with multilayered polymer thin films. The order and thickness of the polymers within the thin films affected the resulting nanofiber morphology, PS and PMMA segment properties, and created unique core-shell structure in the PMMA segments. The core-shell structure suggests a complex wetting phenomenon. Fabrication of polymer nanostructures by wetting of layered thin films opens the arena of multifunctional, one-dimensional, polymer nanostructures with segments having individual and specific functionalities.
KeywordsHeterogeneousNanofibersPolymersThin filmsWettingCore-shell structureNanomanufacturing
One-dimensional nanostructures offer a wide range of opportunities for a number of scientific fields. The resemblance of nanofibers and nanotubes to pipes, cavities, capsules and wires and the ability to fabricate them from metals, semiconductors, ceramics, and polymers lend them to an endless number of potential applications (Steinhart et al. 2004). The ability to fabricate polymer nanofibers and nanotubes is of particular interest because of the vast array of natural and synthetic polymers available. The utility of polymer nanostructures could be further exemplified with a technique to fabricate heterogeneous nanostructures with discrete and/or alternating segments that provide individual and specific functionalities. This concept has not yet been fully explored for one-dimensional polymer nanostructures. In this communication a variation of the well known template wetting approach is introduced to fabricate alternating, segmented nanofibers of polystyrene (PS) and poly-methylmethacrylate (PMMA) and opens the idea of polymer nanofibers with multiple, discrete, segmented functionalities to the scientific community.
At temperatures much greater than the polymer Tg, the polymer interfacial tension is low and nanotube morphology is favored (Zhang et al. 2006), as illustrated in Fig. 1c and d, which shows a TEM and SEM image of PS nanotubes obtained by wetting an AAO template with a thin film of PS at 210 °C for 10 min. These results indicate that there exists a certain wetting transition temperature for all polymers at which the wetting regime changes from partial to complete wetting (Zhang et al. 2006). In addition to melt temperature, the polymer interfacial tension is also a function of the polymer molecular weight and the interfacial driving force can be further affected by varying the pore diameter of the template (Zhang et al. 2006).
Within the past 15 years a great amount of progress has been made by researchers to better understand the wetting of porous materials and utilize this phenomenon to create a wide array of functional nanomaterials. Steinhart et al. first demonstrated PS, poly-tetrafluoroethylene (PTFE), and PMMA nanotubes by template wetting and introduced the concept of adding functionality by preparing palladium/polymer composites through wetting with a solution of poly-L-lactide (PLLA) and palladium(II) acetate (Steinhart et al. 2002). Since the introduction of template wetting researchers have explored many different variations of this process to produce unique structures. One group fabricated PS nanofibers with partially exposed tips and hydrophilically functionalized them to create hydrophilic-tipped hydrophobic PS nanofibers (Moon and McCarthy 2003), while others have experimented with co-polymer template wetting using PS-b-PMMA (Sun et al. 2005) and poly(vinylidene fluoride-trifluoroethylene) for energy transduction and information recording (Lau et al. 2006). This process can even accommodate biodegradable polymers such as poly(ε-caprolactone) (PCL). PCL nanofibers were fabricated for biomedical applications in which controlled release is desired (Tao and Desai 2007). More recently a bidirectional template wetting (Kriha et al. 2007), also referred to as face-to-face wetting (Kriha et al. 2008), was introduced where both surfaces of the porous template are simultaneously wetted. This variation was used to create one-dimensional nanosturctures with two different materials to produce either a discrete interface or polymer gradient between the two polymers.
In this communication we introduce the idea of using multilayered polymer films to fabricate heterogeneous, segmented nanofibers via nanoporous template wetting. PS and PMMA were chosen as a model system. The thin film properties and interfacial energies associated with PS and PMMA have been extensively studied (Sohn and Yun 2002; Harris et al. 2003; Walheim et al. 1997). This knowledge helps to understand the unique morphology of our heterogeneous nanofibers. By wetting AAO templates with multilayered, alternating PS and PMMA films of different layer thicknesses we were able to fabricate nanofibers with distinct segments of PS and PMMA and achieve very unique core-shell morphology in the PMMA segments.
Spincoating polymer thin films
PS (MW = 2,30,000) and PMMA (MW = 1,20,000) were purchased from Sigma Aldrich. Polymer solutions were made by dissolving PS in toluene and PMMA in 2-Butanone. Polymer solutions were spin coated using a Laurell Technologies model WS-65OS-6NPP/LITE spin coater with speeds from 500 to 8,000 RPM, total spin time of 60 s and acceleration of 10,000 RPM. Films were coated onto 18 mm2 glass coverslides. The thickness of the thin films was measured via UV–vis spectroscopy (Huibers and Shah 1997) using a Genesys 10UV scanning spectrometer.
Nanoporous AAO templates were purchased from SPI supplies. The AAO templates were 60 μm thick and had an average pore size of 200 nm. The AAO templates were placed on top of polymer thin films and the whole stack was heated to 150 °C by hotplate for 48 h. This temperature is 50 °C above the Tg of both PS and PMMA, which have Tgs of approximately 100 °C. After annealing and cooling to room temperature the polymer thin film was removed from the glass coverslip by soaking in DI water. Next the alumina was dissolved in 1 M NaOH, leaving only the polymer nanomaterials. The nanofibers were collected by centrifuge, washed with DI water, and dispersed in ethanol by sonication.
Characterization with TEM
Nanofiber morphology was analyzed using transmission electron microscopy (TEM). Prior to imaging, the nanofiber coated TEM grids were stained using RuO4, a well known gas phase stain which exclusively darkens PS (Sun et al. 2005; Trent et al. 1983). Briefly, RuCl3 · 3H2O (0.2 g) was added to a solution of NaOCl (10 mL) to form RuO4. A small amount of this mixture was placed inside one chamber of a dual chamber glass container. The nanofiber TEM grid was placed in the other chamber and exposed to the volatile RuO4 for approximately 1 min.
Results and discussion
It is shown that the PMMA segments have a PS core. This is understood based upon the behavior of PS and PMMA co-polymers and blends. PMMA is more polar than PS and has been shown to preferentially wet the hydrophilic metal oxides surfaces before PS (Sohn and Yun 2002; Harris et al. 2003). This results in the formation of the PMMA shell and PS core. The formation of core-shell morphology rather than a homogeneous segmented structure is due to a combination of the difference in PMMA and PS film thickness and the competitive interfacial driving forces present at the AAO template/polymer interface.
Heterogeneous, segmented PS and PMMA nanofibers with novel core-shell morphology have been fabricated via template wetting by multilayered, alternating PS and PMMA thin films. These unique structures offer great future opportunities including multifunctional nanofibers that could theoretically be composed of any number of polymers with the desired functionalities and single polymer nanofibers with segments of different diameter by selectively dissolving the polymer shell in the core-shell segments. Our future work will explore the formation and controllability of core-shell segments and will experiment with the formation of segmented polymer nanofibers with both tube and fiber morphology by template wetting of multilayered, alternating polymer films for potential drug delivery and biosensor applications.