Sunday, May 19, 2013

Creation of triblock copolymer thin films by combining vapor annealing with a raster spray

-->
Adapted from "Spatial and Orientation Control of Cylindrical Nanostructures in ABA Triblock Copolymer Thin Films by Raster Solvent Vapor Annealing", ACSNano,
Jonathan E. Seppala, Ronald L. Lewis, III, and Thomas H. Epps, VOL. 6 NO. 11 98559862 2012)




This nanosciece paper describes a novel approach to annealing polymeric thin films, particularly block copolymer thin films.  Self-assembly is a significant phenomenon in these materials because they open the floodgates for designer nanoscale materials for nanoporous membranes, lithographic masks, and nanopatterning/templating applications (the last two have huge implications for the electronics industry).  These three nanotechnologies often exploit morphologies often found in AB diblock and ABA triblock copolymers (spheres, gyroid, and lamellae) because the thermodynamics of bulk self-assembly is relatively well established.    The morphology of bulk block copolymers is influenced by three major factors: the degree of polymerization (N), the interaction parameter (c), and the volume fraction of the blocks (f). 
With thin films, surface energy becomes an additional factor; it can be exploited by thermal annealing to facilitate copolymer self-assembly via the bestowment of mobility to amorphous regions that are trapped upon casting.  However, thermal annealing is limited to copolymer systems where the components have similar γ‘s and are thermally insensitive.  Another technique is solvent vapor annealing (SVA), which grants mobility by effectively reducing the Tg of the copolymers.  It is a powerful technique, but is limited to small quantities that are typical of research labs. 
The objective is to devise a faster method for large-scale production of block copolymer (BCP) nanotechnologies.  The method must enable control over morphology and orientation of BCP thin films.  The authors propose raster solvent vapor annealing (RSVA): solvent vapor from a bubbler system is directed onto a BCP surface, which then creates a SVA zone.  The zone is modified/expanded by a motorized stage moving in a raster fashion.  

RSVA was performed with a THF-rich vapor stream in single or multiple passes over a 100 nm thick poly(styrene-b-isoprene-b-styrene) (SIS) film, with domains of 29 nm.  The RSVA speeds ranged from 500 µm/s to 3 µm/s.   The RSVA process swelled the films due to hydrolysis, so the film thickness was measured by spectral reflectometry.  The swelling increased the thickness to 160 nm, and eventually dried down to nearly the original thickness, although some samples were reported to have residual solvent. 
Several approaches to RSVA were performed.  One was single-pass, where the stage moved under the nozzle once, with only the speed varying. The as-cast film had a lamellar structure consisting of cylinders oriented parallel to the substrate, but with minimal long-range order.   Varying the speed affected the ordering of the lamellar cylinders.  The slower the speed, the longer the order-range; at 10 µm/s, the cylinders have mostly perpendicular orientation.  The 10 µm/s speed corresponds to an annealing time of 50 s.  The cylinders even looked slightly swollen, which was confirmed by azimuthally integrated 1D profiles from FFTs of the AFM images.    Even reducing the nozzle diameter still induced the ^cylinders, although at lower speeds. 
           


The post-RSVA morphology is an imbroglio of competing forces. The lower g for polyisoprene (32.0 mJ/m2 vs. 40.7 mJ/m2), the majority block, enables wetting of both the free and substrate surfaces, which leads to the propensity for the cylinders to possess parallel orientation.  High RSVA speeds do not change the orientation (see 2a–2c) because the surface energy difference was too large for entropy to take effect.  The slow raster allowed enough solvation to lower the differences in g, which lets a) entropic effects to manifest, and to compensate for the stretching experienced by the cylinders during swelling and deswelling. 
              Briefly, Seppela et al tried two more approaches.  One is multiple passes under the nozzle.  Retracing the RSVA pathways altered the cylindrical orientation toward perpendicularity.  The other is a crossed-path approach; two orthogonal passes cross each other, and the result is a domain dominated by perpendicular cylinders.  Both approaches are supported by crisp AFM phase images. 
            This is a wonderfully written paper, but it helps that I have significant background in polymer chemistry.  Note that only THF was used in the vapor stream; it’s natural to ask if this approach has been done with other solvents, and other BCPs.  dTHF = 18.1 (MPa)1/2, dpolyisoprene = 16.2 (MPa)1/2, dpolystyrene =  18.6 (MPa)1/2, so utilizing similar solubilities is a probable reason for the RSVA setup described in the paper.  One should also ask if this technique can be done for other self-assembling polymeric thin films. Seppela et al noted that this annealing method can be altered according to slit geometries, solvent quality, and substrate temperature–indications of much promise for RSVA.  

Friday, June 29, 2012

Confined Crystallization of Polyethylene Oxide in Nanolayer Assemblies


We live in an era of increasing reliance on the very small to satisfy humanity’s endless needs and desires for new technologies.  Nanotechnology manifests itself in numerous scientific fields, and polymer chemistry is no exception.  Polymers are generally amorphous, but polymer crystallinity can be observed if the conditions are right.  Semi-crystalline polymer chains (possesses crystalline and amorphous phases) such as polyethylene and nylon are often used as barrier films in food, medicine, and electronics industries.  A barrier is considered highly efficient if small gas molecules are relegated to permeating through only the amorphous regions of the chains (crystalline regions are impenetrable).   Efficiency can be fine-tuned by varying the polymer-film processing conditions to suit the desired amount of crystallinity and chain orientation.  Polymer films can now be made thin enough to effectively confine the crystallization process to 2D; this leads to surprising results. 


Conventionally, confined polymer chains crystallize into lamellae with thicknesses of ~10-20 nm with spherelitic morphology.   However, this convention is skirted at the nanoscale, as isotropic growth is severely hampered to the point of producing lamellar crystal orientation.  This orientation is usually perpendicular to the layer (edge-on), but parallel orientations have been reported several times in the literature; mechanisms for orientation determination remain mysterious for the time being. 
Normally, researchers prepare 2D crystallization of polymers via solution processes such as spin-coating or Langmuir-Blodgett (LB) techniques, but these are limited by the solvent requirement and the small quantity of material fabricated.  LB techniques enable layered nm morphologies due to microphase separation of dissimilar block copolymers within the thin films.  Alas, block copolymers are notoriously difficult to synthesize and align with respect to the direction of the thin films. 

Enter a new technique known as layer-multiplying extrusion.  It uses forced assembly to create alternating layers of two polymers that number up to the 100,000s.  Almost any melt-processable polymer can be formulated into kilometers of nanolayered films with thicknesses of ~10 nm.  With less material comes an explosion of new previously unknown properties (“less is more”). 
The materials used in this study are polyethylene oxide (PEO, also known as polyethylene glycol), which has the following structure:

                                                   HO-CH2-(CH2-O-CH2-)n-CH2-OH
The other is ethylene-co-acrylic acid (EAA), a copolymer with much lower crystallinity than PEO:  
Films with 33, 257, and 1025 alternating EAA and PEO layers were extruded, with various thicknesses and composition ratios, including (EAA/PEO vol/vol) 50/50, 70/30, 80/20, and 90/10.  The nominal PEO layer varied from 3.6 µm to 8 nm. 

The films were subjected to oxygen permeability tests with respect to to layer thickness.  The results are shown below:

Fig. 1 The effect of layer thickness on oxygen permeability. (A) Oxygen permeability of films with equal volume fractions of EAA and PEO. The dashed line indicates P// calculated from Eq. 1. (B) Oxygen permeability of the PEO layers from films of varying composition calculated from Eq. 2. The dashed line indicates PPEO. The open symbol is for a film with PEO layer breakup. The solid lines are drawn to guide the eyes. 
The plots show a significant decrease in O2 permeability.  Gas permeability for layered assemblies is modeled by the following equation. 
     (1)
where ��PEO is the volume fraction of PEO and PPEO and PEAA are the permeabilities of PEO and EAA, respectively.  Upon plugging determined values of PPEO and PEAA from literature into Eq. (1), the result did not agree with the findings reported in the plot above. Eq. (1) predicts increasing permeability with respect to decreasing PEO thickness, but the data show the opposite trend. Eq. (1) was then modified to account for the apparent sensitivity to PPEO due to the far lesser permeability of PEO; it still did not agree with the plotted data with the exception of thicker PEO layers as indicated by the dashed line.  Clearly, the PEO nanolayers possess some previously unknown crystalline morphology that bestowed them with staggeringly low permeability.  However, differential scanning calorimetry revealed that the PEO and EAA layers (even the very thin ones) share the same melting enthalpy and melting temperature as the control films; this means that the changes in crystalline morphology granting the PEO nanolayers low permeability was not accompanied by changes in crystallinity nor lamellar thickness. 


Upon examination by AFM, the authors found that the thin 20 nm PEO layers exhibited single lamellae that extended beyond the field of the AFM image.  The single lamellae are said to be very large single crystals.  Reducing the PEO layer thickness to 8 nm then induces breakage, thereby increasing the permeability.  Fig. 2 below shows the AFM image of the 20 nm PEO layer, and an accompanying schematic showing a gas diffusion pathway through the layered assembly.

Fig. 2  AFM phase images of partial cross sections of the layered EAA/PEO films. The PEO layer has substantially higher crystallinity than the EAA layers and hence appears bright in the AFM images. (A) A low-resolution image of an EEA/PEO film with 50/50 composition, 33 alternating layers, and nominal PEO layer thickness of 3.6 mm. (B) A higher-resolution image showing the spherulitic morphology of the 3.6-mm-thick PEO layer. (C) A low-resolution image of an EAA/PEO film with 70/30 composition, 1025 alternating layers and nominal PEO layer thickness of 110 nm. (D) A higher-resolution image of the 110-nm-thick PEO layers showing the oriented stacks of PEO lamellae. (E) A high-resolution image of an EAA/PEO film with 90/10 composition, 1025 alternating layers, and nominal PEO layer thickness of 20 nm showing that the PEO layers crystallized as single, extremely large lamellae. (F) A schematic showing the gas diffusion pathway through the layered assembly with 20-nm- thick PEO layers. The arrows identify the EAA layers and PEO layers. 
The lamellar crystalline region is considered impermeable, with the lamellar fold surfaces constituting the permeable amorphous regions.  As seen in Fig. 2, the gas pathways depend on the frequency of defects such as lamellar edges.  The permeability is now expressed by



   (2)
 where α is the aspect ratio of the impermeable platelets (length/width), and �� is the volume fraction of impermeable platelets; the platelets are orientated perpendicular to the flux.  For the thinnest PEO layers, the aspect ratio was as high as 120, which meant the lamellae extended up to 2 µm for the 20 nm thick layers.   Gradually thickening the PEO layer relaxed the restrictions on 3D growth, which returned the morphology to spherelitic.   The results were further confirmed by small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS). 

This work is a major breakthrough in polymeric applications for nanotechnology because it shows experiment trumping theory, and possibly describes a major advance for gas-barrier films.  Its importance is amply demonstrated by the 51 citations it has generated since its publication in 2009.  Science Magazine accepted the paper because of its reliance on well-established analytical techniques (AFM, differential scanning calorimetry, SAXS, WAXS), and, more importantly, because of its broad significance in the field of nanoscience. 

This significance is underscored by the novel utilization of a relatively new technique–coextrusion–on readily available polymers to engineer nanolayered polymeric formations in sufficient amounts to allow for probing links between the confined crystalline morphology and the properties exhibited.  This opens up new possibilities for packaging methods, i.e., incorporating polymer nanolayers into common polymeric films for less cost, thereby reducing the environmental and energy consequences.  

Wednesday, June 20, 2012

Laser Ablation: Discussion & Conclusion

Fig. 11 from the last entry does not bode well for laser ablation as a profiling technique for CARCs (chemical agent resistant coatings).  Why?  Because it didn't resolve the UV-damanged region in the topcoat.  At least this was a feasibility study, so its purpose was fulfilled, but a better alternative to ATR-mode FTIR depth profiling still awaits discovery.  Fig. 6 from the last entry shows remarkable resilience from the signature peaks after ablation.  That should mean then ablation shouldn't be a factor when one investigates the coating after QUV exposure (accelerated weatherization under controlled conditions).  Fig. 1 shows why.

Fig. 1 FTIR spectra of major organic and inorganic bands for baseline sample (1), 15-mm-deep transmission-mode spectrum in UV-aged sample (2) and 15-mm-deep ablation window in UV-aged sample (3). 
There shouldn't be discernible differences for spectra (2) and (3), but difference is obvious for the carbonyl peak on the left.  Considering that most CARCs (to my limited knowledge) have polyurethane binders, this can be considered a death blow to the possibility of laser ablation being used as a depth-profiling technique for CARC (chemical agent resistant coating) films after long-term exposure to the elements.  The authors speculate that the ablation process creates ether groups (C–O–C), which overlap with carbonyl groups.

In addition, the amide peaks seen in the spectra of the aged samples likely stem from other functional groups that overlap; they might result from a complex interaction between the aging and ablation processes.  There's still the chance that the original amide II group had reformed after the ablation, which explains the awful ablation profile in Fig. 10(b) of the last blog entry.  This reformation effect was seen in previous studies involving UV-induced cross linking between proteins and DNA with little disruption to the bulk protein chemistry.  This is important considering the chemical similarities between peptide bonds (–CO–NH–) in protein and urethane bonds (–O–CO–NH–) within polyurethane.  

The greater activity within the carbonyl region of Fig. 1 above is perhaps caused by a carboxyl group  (–CO–OH) rather than carbonyl or even ether.  If so, there should be larger peaks in the –OH stretching region (~3300 cm-1), but not so large that it surprises the aged-but-unblated sample.  Alas, Fig. 2 below shows this is not the case.

Fig. 2  FTIR spectra showing (OeH) and (CH2) bands for baseline sample (1), 15-mm-deep transmission-mode spectrum in UV-aged sample (2) and 15-mm-deep ablation window in UV-aged sample (3). 
(3) lies between (2) in the CH2 stretching region (2937 cm-1), but not in the -OH area (3364 cm-1).  They attribute this to another unforeseen reaction with the ablation process.  Nevertheless, it's clear from here that femtosecond laser ablation is unreliable as a depth-profile technique for aged CARC films.

Wednesday, May 23, 2012

Laser Ablation: Results

The figure below shows the ablation window depth as a function of stage scan speed.  The removal rate shows exponential decay, but the obvious fact points towed lower removal rates at lower energies; what's important is that the user can still control how stuff is removed by laser.
 Fig.1 Calibration profiles for three different energy levels at several scanning speeds (dwell times). Note absence of ablation window at 35 mJ energy level regardless of scan speed. 
Fig. 1 shown below, shows signs of a minimum window depth feasible within any pulse energy given the maximum scanning speed available. The most important curve is the seemingly flat 35 µJ pulse energy profile, (1 µJ = 10-6J) which stays flat no matter the scan speed, as seen above.  The other two energies (167 and 73 µJ) show clear decay slopes, which can be interpreted as progressive erosion by sample speed reduction (going backwards on the x-axis in Fig. 1 above).  The calibration profiles on the left conform to this finding.


 Fig.2 SEM micrograph showing surface detail of baseline (unablated) coating sample.    
Figs 2-4 are scanning electron microscope (SEM) micrographs, AKA, closeups, of the coating after different laser energies.  Fig. 2 is unabated; Fig. 3 depicts the coating after the 35 µJ laser; Fig. 4 is after 167 µJ.  The coating itself is a TiO2 pigment (small white particles) embedded within the polyurethane binder (uniform dark background); the large spheres and blocky objects are siliceous and talc fillers, respectively.  
Fig. 3 SEM micrograph showing surface detail of coating sample after 35 mJ ablation, 10K mm/s scan speed. Note selective ablation of TiO2 pigment (evidenced by presence of ovaloid cavities) with no apparent disruption to surrounding organic binder. 
Fig.4 SEM micrograph showing surface detail of coating sample after 167 mJ ablation, 10K mm/s scan speed.    
 The laser darkens the pigment to various degrees, even for the 35 µJ laser, which did not ablate the coating.  The 167 µJ scan destroyed most of the pigment and considerably altered the binder morphology to the point of creating a large crater on the surface.  Fig. 5 is a cross-section of the coating and shows a sharp distinction between the ablated and non-ablated regions.
Fig. 5 SEM micrograph of paint cross-section showing transition between unablated and ablated surfaces. Boxed areas are shown in detail in the lower two images. On the left is detail from the unablated surface showing uniform distribution of pigment (small white particles). The right image shows detail from the ablated surface. A 5–8 µm pigment-depleted layer is indicated by the red line. 
It's clear from Fig. 5 that the ablated region has lost much of the pigment and differs significantly in morphology.  TiO2 is a semiconductor with a relatively small band gap which makes it more susceptible to ablation than the surrounding polyurethane binder.

It's going to get a little technical from here on out, but this is where it gets quantitative.  Fig. 6 shows the transmission spectra of a baseline (pristine) sample plus spectra from all three ablation energies (35, 73, 173 µJ).
Fig. 6. Transmission-mode spectrum of unaged baseline and ATR-mode spectra for 35, 73 and 167 µJ ablation. The ester and urethane peaks experienced little change from the ablation process.
The peaks contained within the box on the left did not change much after ablation; this is to be expected since polyurethane binder is mostly what's left in the ablated regions.  There's a large gain in the peak on the right (indicates a gain in quantity for that functional group), but that's a region of considerable overlap between the C–O–C ether and O–Si–O; the authors guess it to be Si–O, since there's a lot of new SiO2 filler exposed.  


Now I compare samples that differ by the amount of simulated weather that they've been exposed to.  Below is Fig. 7.
Fig. 7 ATR-FTIR spectra for baseline, 6-week and 18-week QUV exposed samples of coating. Near-extinction of amide II peak (1523 cm-1) is noted, evidence of photooxidation of urethane groups in the organic binder of the coating. (b) OH absorption (3364 cm-1) is seen to broaden substantially, whereas hydrocarbon peaks (2937, 2863 cm-1) simultaneously decrease, as a result of QUV exposure. 
Keep in mind that cm-1 (wavenumbers) is a unit of frequency just like Hz (s-1); in fact it's proportional by Eνc/λ for the infrared radiation (IR).  You could see from Fig. 7 that the IR is compared for the baseline, 6-weeks QUV exposed, and 18-weeks QUV exposed.  The authors emphasize the complete extinction of the so-called amide II functional group, which means drastic decay for the polyurethane binder.  In part (b) of Fig. 7, the OH/NH band broadens, perhaps due to carboxyl group formation.  Simultaneously,the QUV process reduces the hydrocarbon peaks in the region close to 3000 cm-1.

Fig. 8 presents depth profiles of the ablated window for the baseline and 18-week QUV samples.
 Fig. 9 (a) Ablation window depths for 140 µJ pulse energy, baseline and 18-week QUV samples. (b) Ablation window depths for 80 µJ pulse energy, baseline and 18-week QUV samples. At all scan speed/ pulse energy combinations material removal rate for UV-aged sample exceeds baseline sample 
The depth windows show a greater rate of material removal for the QUV-aged sample than for the baseline; probably due to the weathered urethane binder becoming more broken down.

Fig. 10 shows FTIR depth-profile measurements.  This is the nitty-gritty of the study, as it shows why this method falls short.  
Fig. 11 a) Amide II/C]O ratio for baseline vs. UV-aged coating samples using cross-section transmission-mode FTIR. UV-zone extending 30 mm into topcoat is readily apparent. (b) Amide II/ C]O ratio for baseline vs. UV-aged coating samples using femtosec- ond ablation-assisted depth profiling. UV-damaged zone not apparent. 
Fig. 11 a) repeats a result from an older study where they used transmission-mode FTIR on 3 µm thick cross-sections of the same two samples.  It's mentioned here for the sake of comparison.  Peak ratios are used here to eliminate fluctuating values due to varying sample thicknesses.  Part (a) shows a clear change in the ratios as the depth is increased, meaning the near-surface region is more weather-damaged than the deeper regions in the bulk.  That is not the case for part (b); if the ablation method did its job, it would show a damage gradient just like in part (a).  The next blog will discuss why this is so.  
   

Thursday, March 22, 2012

Laser Ablation: Experimental

The experiment went through two phases.
Phase 1: physical and spectroscopic characterization of the ablation process performed at three different pulse-energy levels and five separate dwell times for determination of the proper process parameters.  
Phase 2: applying these parameters for optimal depth profiling of the coating as seen in the real world.  

The general approach of femtosecond ablation profiling generally meant they ablated a series of craters, AKA"windows", at increasingly greater depths into the sample; they (Keene et al) made the key assumption that the insanely fast ablation process won't appreciably modify the material to the point of changing its spectrum.  The window base makes it relatively convenient to acquire a vibrational spectrum (maybe a mass spectrum too).  Fig. 1 is a schematic comparing the cross-section transmission-mode approach with the ablation method discussed in this entry.  Infrared (IR) data is acquired by micro-ATR at the window base.  The window bases had 3mm × 3mm to allow for the capture of at least 4 IR spectra/window.  Note also that all ablations were done in standard atmosphere.  
 Schematic depicting cross-sectional transmission-mode FTIR approach to depth profiling vs. (b) femtosecond ablation-assisted ATR- Mode FTIR depth profiling. 
They used a chirped-pulse amplified multi pass Ti:Sapphire laser with a repetition rate of 1 kHz, wavelength centered at 780 nm (nanometers) and a pulse duration of about 30 femtoseconds.  An aluminum alloy applied with a composite navy coating system of thickness ~100 µm (100 micrometers, topcoat and primer) was used to optimize the ablation parameters.

Parameter Optimization
Three rows of five 3mm × 3mm windows were generated, the windows in each row being ablated at a different stage scanning speed and each row ablated at a decreasingly lower pulse-energy level.  They used five linear scanning speeds (10K, 8K, 6K, 4K& 2K µm/s); the slower you scan, the longer the laser ablates the surface.  Incident energy levels of 167, 73, & 35 µJ plus a baseline (unweathered) sample were used.

The researchers scanned the ablation windows to characterize the crater topography and depth at each fluence/scan speed.  Fluence refers to the number of particles flowing into an area/unit of time.  Each ablation window was raster-scanned with data collected every 10 µm to generate a dense matrix of data corresponding to the topology of the ablation window.  The values/window were then averaged together to give the average ablation depth at the given scan speed/energy combination.

Remember when I mentioned the assumption regarding surface integrity following ablation?  That was put to the test, first with more IR scans, then Raman spectroscopy, finally with scanning electron microscopy.  In the case of IR spectra, five spectra were collected from the base of each of the three rows via micro-ATR mode.  Those five spectra were then averaged to generate a prototypical amalgamation spectrum to minimize regional variations in chemistry.   The collected spectra from all three windows were compared to collected spectra from unablated regions on the sample in the same way.  
2nd set of data comes from dispersive Raman Spectroscopy.  Similar to IR, they collected raman spectra from the base of the three ablated windows at each energy level (167, 73, & 35 µJ) to see if ablation had induced formation of organic peroxide or derivative species (R–OH) due to energetic interaction with atmospheric oxygen.  

Lastly, SEM was used to further characterize the effects of über-fast ablation on the composite coating surface.  They selected the top and bottom energy levels (167 & 35 µJ) for best comparisons of appearance and elemental composition with the baseline (unspoiled) sample.  Samples were sputtered with gold particles to encourage charge compensation during analysis.  SEM images were collected with 5000×, 10,000×, and 50,000× magnifications.  The samples were immersed in liquid N2 to obtain cross-sections.  The now-separated coatings were embedded in histological wax (the same kind used to embed tissue), then microtomed to a thickness of 30 µm.  Then the samples were place in vacuum-compatable copper tape for SEM analysis.

Spectroscopic depth-profile collection


For proper evaluation of the ablation technique as depicted in Fig. 1b) above, two separate samples were chosen.  Both had been analyzed by transmission-mode FTIR and are of the same military coating system.    One sample was the baseline (unaged) sample while the other underwent an 18-week QUV exposure protocol in an army research lab.   QUV is a brand for accelerated weather testing, and is inpart  summarized by this YouTube video from the company.
For 18 weeks, the sample was subjected to a varying spectral power distribution of noon summer daylight (295-450 nm) in conjunction with a constant temperature of 60°C.

Collection of laser profiles was enabled by discrete ablation depths to allow a spectroscopic sampling at regular intervals into the topcoat (see previous entry for schematic of typical vehicle coating).  Ablation was needed at both the near surface and the window, so they had to ablate two columns of windows at two completely different energy levels: 140 µJ/pulse (column of six windows) and 80 µJ/pulse (five window column).  Both columns used an identical scan speed.  This procedure was done for the aged & unaged samples.  IR spectra were collected at the base of each window via micro-ATR FTIR (link found in an earlier paragraph).

Next entry will cover the results.  Expect lots of Ifrared spectra.  Hopefully I can explain their significance at a level appropriate for this blog.  Cheers.  

Sunday, March 11, 2012

Femtosecond laser ablation

My research will be on organic coatings.  This is the first such post.

As commonly as coatings are used, their weathering-induce degradation is complex and thus remains poorly understood, as coatings can fail in a variety of ways (I think that's what they mean by 'failure modes').  Barrier properties of the coating system can be compromised, along with its mechanical properties.  Proper evaluation of durability and longevity of modern composite coating systems requires characterization of weather/aging phenomena as a function of their spatial distributions.  In the case(s) of homogenous organic coatings, or for coatings with low pigment-to-volume concentrations (PVC), this is typically performed by spectroscopic analysis to cross-sectional samples or by a form of confocal spectroscopy.  Those techniques are fine and dandy if the coating cross-sections are transparent.  If opaque, better get creative, which a team of researchers from SUNY Stony Brook did so.

Background
UV-induced aging of polymeric coatings significantly changes the chemical and morphological structure within the organic matrix of the coating.  Many coatings are based on aromatic ring structures, and are prone to UV absorption and yellowing.  Even aliphatic binders are prone to UV-induced weathering, although to a lesser degree.  Either way, our understanding of these physiochemical changes remain murky; we can't predict if a polymer chain within the coating will cross-link or undergo scission.  
Degradation of composite coatings are even harder to understand; these are coatings that contain a disproportionate amount of inorganic components.  The mechanism depends on both the base chemistry of the organic binder, and on the characteristics of the inorganic additives contained within the coating.  Below is a schematic of such a coating system.  

Most of my research may focus on coatings for military vehicles, so this should be a good model to use in this article.
The closer you get to the surface coating, the more degradation–in other words, weathering/aging also depends on depth.  
Thus, it's best to design a method that yields spectral data and data of the spatial distribution throughout the coating system.  We may a few such depth-profiling techniques already: confocal focusing of an energy beam probe (Confocal Raman Spectroscopy), Secondary Ion Mass Spectroscopy (SIMS), and transmission-mode Fourier Transform Infrared Spectroscopy (transmission-mode FTIR), and UV-Visble spectroscopy.  CRS requires transparent samples, thus out of the question; SIMS won't work for dielectric coatings (practically all organic coatings).  
Transmission-mode FTIR and UV-Visible spectroscopy work well, but they require intact cross-sections with dimensions too small for practical preparation.  A simpler preparation involves microtoming at very small angles to the surface of the coating–effectively extending the depth of the sample to a degree that enables usage of attenuated total reflectance FTIR, but that opens up another can of worms: interlayer mixing due to the microtome blade.  Materials that are too hard and incohesive to microtome successfully are equally vexed when a spectroscopic profile is demanded.  To meet this demand, Keenes et al devised a novel use of a femtosecond (10-15 sec) pulsed laser as an enabler for depth-profiling coatings of this type.
This isn't the first time they used lasers for ablation; previous time scales include nanoseconds (10-9 sec) and picoseconds (10-12 sec).  Laser ablation isn't entirely understood, but it's believed to be the result of a coulombic explosion, in which near-instantaneous ionization of atoms via bright laser beam excites electrons to the point of escaping from their host atoms.  The atoms have become so ionized as to be repulsed by the neighboring positive charges on the irradiated material–repulsions with pressures 10 million times that of standard atmospheric pressure.  This results in a direct solid-to-vapor transition characterized by a violent release of ions from the surface.
Now, people have used ultra-fast laser ablation before, but in the context of mass spectroscopy.  Keenes et al extend this technique to infrared spectroscopy, which is the kind of data that explains the depth profile of the coating.  They hope no post-ablation thermal disruptions will alter the coating composition.
Why do they care about what a coating looks like underneath its surface?  Well, when your client is the U.S. military, you have many reason$ to care.  The scientific reasons will be explained in the next paragraph.  The coatings in question are commonly used on fixed wing/rotary aircraft and naval vessels.  Femtosecond laser ablation is supposed to reveal the underlying layers of the coating to be analyzed ATR-mode FTIR to investigate the depth to which photooxidation of the organic binder has occurred.  A binder is a resin used to keep particles together and supply mechanical strength or to ensure uniform consistency, solidification, or adhesion to a surface coating.
The military coating studied here is a chemical agent resistant coating (CARC) comprising of low-gloss automotive-grade aliphatic polyurethane highly loaded with a pigmentation/filler package.  It seems that this coating in question in part stems from a 1970s patent on tri-functional isocyanate cross linking agents (a bit too technical to delve into for a blog entry).  The main pigment in this coating is a nanoscale spherical titanium dioxide that bestows a light gray color to the coating for optical camouflage.  Diatomaceous earth is added to the coating to lower the gloss (high gloss is bad for camo). SEM micrographs of the coating in question is provided below.
 SEM micrograph (top) of coating cross-section. High volumetric proportion of pigmentation/inorganic fillers is evident from the three EDS elemental maps of Ti (pigment), Si (pigment C flattening agent) and Ca (flattening agent). 
In addition to optical/infrared camouflage, the military uses coatings for protection of metal substrate from corrosion/weathering and resistance to perforation by chemical decontamination agents.  Uncle Sam also demands that its colors don't wash or fade out and it can maintain its mechanical integrity according to appropriate specifications.  This coating has seen extensive action in weathering studies by Keene et al, so it serves as a useful model for this ablation technique to investigate.  Experimental Results and discussion will be detailed in the next entry.  

It's nice to be back!  

Monday, March 14, 2011

Nanocars!

Researchers at the Rice University embarked on a 'fantastic voyage' toward miniaturizing the beloved car to a scale that would be considered science fiction 20 years ago.  In 1998, principal investigator James Tour, professor of organic chemistry at Rice, began this journey.  It took his group eight years to synthesize a functional nanocar.  The goal was to create a nanomachine capable of converting energy-inputs (such as heat or electric fields) into controlled motion on a surface and transport cargo from one place to another on a surface.  Future nanomachine development depends on this because the bottom-up constructions may occur on the surface of semiconductors, metals, or biological and artificial membranes.  Thus, acquring better data on the chemistry and physics on nano-transporters on surfaces is critical; the creation of nanocars is an initial, but significant, step in this scientific frontier.  Nanocars allow us find out basic rules for their design and operation. 

The basic design of a nanocar is much like a real car: it has a  chassis and four axles, except that it's made out of organic groups with pivoting suspension and the axles freely rotate.  The wheels are buckyballs, The entire car measures just 3-4 nm across, just wider than a strand of DNA. 

Though laborious measurements on the STM, the researchers proved that the cars rolled on the surface rather than slide across as previously predicted.  Of course, they had to raise the temperature of the gold surface to 200 ºC, but they were still able to determine that it was easier to move the car forward or backward rather than sideways.