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.

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