J. Phys. Chem. 1993,97, 11865-1 1867
11865
Chain Unzipping in Photothermally-Ablated Superheated Polymer Molecules via SIMS Analysis of Ablated and Repolymerized Polymer Surfaces Gerard R. Pinto' Imaging Systems Research Laboratory, E.I. du Pont de Nemours & Company, Inc., Parlin, New Jersey 08859
Kathryn G. Lloyd DuPont Corporate Center for Analytical Scienced Experimental Station, E323, Wilmington, Delaware I9880 Received: August 23, 1993; In Final Form: September 30, 1993"
In this paper, we attempt to elucidate the thermal decomposition mechanism of photothermally-ablated superheated ethyl methacrylate/2-hydroxyethyl methacrylate (EMA/HEMA) copolymer. O n the basis of secondary ion mass spectrometry (SIMS) measurements of ablated material deposited a t a receptor surface, we detect a distinct change in the comonomer composition of repolymerized copolymer, which we attribute to the thermallyactivated unzipping of the polymer chains within the ablation plume, followed by repolymerization of the ejected monomers. Our results, generically applicable to a variety of polymers that pyrolyze by unzipping, support the essential featureof the model proposed by Dlott for poly(methy1 methacrylate) (PMMA), in that the endothermic decomposition of the superheated polymer chains occurs primarily within the ablation plumeof ejected fragments. Thermal decomposition mechanisms appear to play a critical role in the UV laser ablation of organic polymers. These mechanisms have generally been inferred from measurements of volatile product compositions, similar to those observed in classical Moreover, a number of indirect physical studies have indicated that the ablated surface is subject to ultrafast heating following rapid energy relaxation of the initial electronic e~citation.'~ In this instance, notwithstanding the possible simultaneous occurrence of direct photolysis, the polymer chains experience a sudden temperature jump that places them in a metastable, superheated state. Hence, subsequent thermal decomposition is necessarily explosivein nature, which is consistent with the general characteristics of the ablative process. Despite mounting reports implying a substantial photothermal component in UV laser ablati~n,~+lOJ~ there has been no direct evidence supporting the mechanistic details of an actual thermal decomposition reaction. Recently, however, Dlott and co-workers have responded to this research imperative by focusing on purely photothermal laser ablation, using picosecond optical calorimetry to directly measure the temperature of the solid polymer during the ablative pr0cess.l' Above the ablation threshold, polymer decomposition is manifested by an increase in the effective heat capacity, which is obtained by fitting the temperature data to a simple model of the heat absorbed by the polymer, up to the moment of ablation. Then, assuming that similar decomposition chemistry is valid at both low and high heating rates, simple scaling arguments are used to relate the weight fraction decomposed in a conventional TGA experiment to that deduced by optical calorimetry. In this fashion, Dlott determines that the extent of polymer decomposition at the onset of ablation is always less than the amount that would have decomposed at slower heating rates, for the same amount of heat absorbed. Consequently, the ablated or fragmented polymer chains must continue to decompose. Because these experiments were performed on poly(methy1 methacrylate) (PMMA), Dlott naturally proposes a model involving initial chain cleavage followed by chain depolymerization.
* To whom correspondence should be addressed. Present address: Pinto Communications, 84 72nd St., New York, NY 11209. Contribution No. 6668. Abstract published in Advance ACS Abstracts, November 1, 1993.
The long-chain character of particular polymer molecules facilitates monomer production via thermally-activated unzipping reactions, which are the exact reverse of the propagation processes in polymerization; in the case of PMMA, monomer is obtained in quantitative yield. According to Dlott's analysis, the endothermic depolymerization of PMMA must primarily occur within the ablation plume of ejected fragments. The generality of this prediction is essentially due to the fact that the rates of thermal decomposition in polymers are slower than the ultrafast heating rates common to most ablation experiments. Dlott's study raises fundamental questions concerning the behavior of superheated polymers, including the mechanistic nature of thermochemical reactions induced by ultrafast heating, that is, the influence of excess molecular vibrational energy on chemical dynamics.12-17 Furthermore, in the context of ablation studies, if thermallyactivated unzipping chemistry prevails in the metastable state of a superheated polymer, the uniqueness of this mechanism may provide a simple diagnostic for the identification of photothermal factors in general laser ablation experiments. In this Letter, we seek confirmation of the thermally-activated unzipping mechanism of photothermally-ablated polymer molecules. To address this issue, we recognized that the explosivelyejected monomers in the ablation plume could possibly repolymerize on an adjacent surface,'* that is, a surface plane parallel to the starting film. Ablation of selected copolymers, such as ethyl methacrylate/2-hydroxyethyl methacrylate (EMA/HEMA), that are also known to pyrolyze by unzipping,lg would generate two different comonomers. Since the monomer reactivity ratios at the new surface would be quite unpredictable, we would undoubtedly expect a change in comonomer composition for the repolymerized copolymer. The prospect of forming a reconstituted polymer could only be realized if the thermal decomposition mechanism involvedchain unzipping, since selective bond cleavage is needed to retain the molecular integrity of the individual comonomers. Alternatively, nonselective bond scission would preclude unzipping and, therefore, repolymerization, leaving any measurement that was sensitive to the comonomer composition essentially unchanged. EMA/HEMA copolymers, at four selected comonomer compositions, were solvent-cast out of chlorobenzene onto a 0.1-mm polyester substrate. The 1-pm thin films also contained about 10% w/w of a near-IR-absorbing cyanine dye. The latter was
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0022-365419312097-11865%04.00/0 Q 1993 American Chemical Societv
Letters
11866 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 A
B
127
155
C
D
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185
50150 EMNHEMA Donor Film
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Figure 1. Structural assignmentsfor ions observed in the negativeSIMS
spectra of EMA/HEMA copolymers.27Negative ion fragments at m / z 127 and 155 correspond to individual HEMA and EMA monomer units, respectively, whereas fragment ions at m / z 141 and 185 correspond to linked monomer units of the acrylic copolymer.
used as a molecular point heater,'3 absorbingthe 830-nm radiation from a solid-statediode laser. The (400 ns) light pulse was focused through the back of the transparent polyester substrate, such that the very thin polymer coating was almost completely transferred to an adjacent polyester receptor sheet. Although the original film sheet and the receptor were in relatively intimate contact, a nominal air gap existed between them. The combination was placed on a rotating cylindrical drum that also translated along its axis. In this manner, 1-in. solid squares of ablated material could be created from programmed scans of thousands of individually ablated and closely spaced spots, each one a few microns in diameter.20 Cut pieces from these samples were examined by static secondary ion mass spectrometry (SIMS). In this technique, an energetic Ga+,Cs+,or Ar+ ion beam bombards the samplepolymer surface, generating secondary positive and negative cluster ions related to chemical species on the surface. SIMS has emerged as a very valuable technique for the characterization of polymer surface^.^*-^^ Very high surface sensitivity and molecular specificity are inherent attributes of SIMS analysis, the detected fragment ions emanating from a depth of one or two monomer layers. The sensitivity of SIMS to monomer structure has been highlighted in studies involving random copolymer We cite, specifically, the work of Briggs on the EMA/HEMA system.*' Briggs has demonstrated that the relative peak intensities of a number of highly characteristic fragments vary with compositionalchanges, consistent with a random comonomer sequence. For our discussion, the significant negative ion peaks appear at m / z 127, 155,141, and 185, which areassigned to the structures shown in Figure 1. Structures A and B correspond to individual HEMA and EMA monomer units, respectively, whereas fragment ions C and D correspond to linked monomer units of the acrylic copolymer. Most of the SIMS data were acquired with a Perkin-Elmer/ PHI Model 7000 reflector-type time-of-flight (TOF) mass analyzersystem, using an I-keV Cs+ion source and pulsed electron beam charge compensation. Some preliminary measurements were carried out using a Fisons VG Model 1x23s reflector-type TOF analyzer, with a IO-keV Ar+ ion source. All spectra were acquired under static SIMS conditions. Mass resolution M/dM was typically 3000 or better with the PHI instrument. A typical negative ion spectrum from a 50/50 EMA/HEMA unablated (donor) film is shown in Figure 2. Peaks at m / z 127, 141, 155, 185, and 213 are characteristic of this c~polymer.~~JO The rather intense peak at nominal m / z 149, usually associated
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Figure 3. Relative intensity of the m / z 155 EMA peak, normalized to the sum of the areas of the m / z 127 and 155 peaks (Am), as a function of EMA/HEMAcopolymercomposition. Thecalibrationline represents SIMS data from the surface of unablated (donor) films while the curve illustrates comparative SIMS data from the receptor polymer surfaces after ablation.
with silicone contamination by those familiar with organic SIMS, is in fact a manifestation of the near-IR-absorbing dye. This assignment is supported by examinationof the positive SIMS spectrum, which exhibits none of the characteristic dimethylsilicone peaks yet does show intense peaks in the m / z 400-500 range which we again attribute to the dye. Using the m / z 127 and 155 fragment ions as representative of HEMA and EMA monomers, respectively,we first established the negative ion calibration line, as shown in Figure 3. We have plotted the area of the m / z 155 peak, normalized to the sum of the areas of the m / z 127 and 155 peaks (A&, as a function of copolymer composition, essentiallyreproducing Briggs' data. We were unable to reproduce Briggs' copolymer fragment plots (Alss/ A,,, and AldI/Asum)using our donor film SIMS data. This may be due to the presence of the dye (and its variable concentration). We believe that changesin the m / z 155and 127peaks sufficiently support our conclusions. Having established this calibration curve for the unablated films, we then obtained comparativeSIMS spectra for the receptor polymer surfaces of ablated material. The negative ion data from these measurements are included in Figure 3. Relative intensity data from the receptor surfaces clearly show a departure from the donor calibration line, indicating a distinct change in the copolymer composition for the deposited material. (Since the polymer is being transferred from one polyestersurface to another, we do not expect any matrix effects to play a role here, although the effect of thedye and its accompanyingtransfer are unknown). Data from Freon-washed receptor films are also shown. Only in one case, the 901 10 EMA/HEMA starting composition, does the ratio A l 5 J / A s u m fall outside the error bars. This suggests that at very high EMA concentrationslow-molecular-weightoligomers
Letters with high EMA content may be forming, which, when washed off, leave a more EMA-depleted surface. More experiments are needed to test this hypothesis. The fact that allofthespectraalsoretainfeaturescharacteristic of an EMA/HEMA copolymer29JO establishes the premise that the deposited material is still some acrylic copolymer,generically similar to theoriginal film. Because the identitiesof the individual comonomers are implicitly maintained in the new copolymer, the change in composition can only be rationalized by the thermallyactivated unzipping of the polymer chains within the ablation plume, followed by repolymerization of the ejected monomers at the receptor surface. Conversely, SIMS data from the residual surface after ablation generally coincide with the unablated calibration curves, which is consistent with the observation that the explosively-ejected polymer fragments are propelled at supersonic speeds away from the ablated surface.14J5 This is the first study to clearly establish the details of the thermal decomposition mechanism for a laser-ablated polymer. Our results confirm the model proposed by Dlott, who directly measured the temperature of the solid PMMA polymer on the ultrafast time scale of ablation, prompting the deduction that polymer decomposition continues within the ablation plume of superheated fragments. Hence, our results also implicitlyvalidate Dlott’s assumptions, especially the scaling argument connecting the nature of the decomposition process at both low and high heating rates. Of course, Dlott’s reasoning presupposes the absence of any cooling processes experiencedby the ablated plume. In the present experiments, the receptor acts as a heat sink for the hot mixture, possibly limiting the degree of chain unzipping and repolymerization. However, the rather substantial change in the comonomer composition implies that the rate constant for depolymerization is probably fairly large, a direct consequence of the very high temperatures within the ablation plume. In conclusion, we detect by SIMS surface analysis a distinct change in the comonomer composition of repolymerized copolymer, deposited on a receptor surface parallel to the ablated surface, which we interpret as evidence of thermally-activated chain unzippinggoverningthe decompositionof photothermally-ablated superheated copolymer. Our results, generically applicable to a variety of polymers that pyrolyze by unzipping, support the essential feature of the model proposed by Dlott for PMMA, in that the endothermic decomposition of the polymer chains occurs primarily within the ablation plume of ejected fragments. It should be possible to extend our approach to other more general laser ablation studies, perhaps allowing for the identification of photothermal factors even when direct photolysis contributes to the overall set of reactions occurring in ablative decomposition. Finally, our results illustrate an interesting similarity between a conventional thermochemical reaction and that induced by ultrafast heating, notwithstandingthe highly vibrationally excited state of the superheated polymer chains in the latter process.
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11867
Acknowledgment. We acknowledge the contributions of Dr. K. M. Stika and Mr. M. C. Plummer, DuPont Corporate Center for Analytical Sciences (CCAS), for conducting preliminary quadrupoleSIMS measurements and for many engaging discussions during the progress of these experiments. We also thank Ms. W. J. Justison, DuPont CCAS, along with Dr. S.Reichlmaier and Dr. P. McKeown, Perkin-Elmer/Physical Electronics Laboratories, Eden Prairie, MN, who acquired much of the TOFSIMS data. We also express our thanks to Mr. S . Savini of Du Pont Imaging Systems for conducting the laser ablation experiments. Dr. H. A. Yee, also,of Imaging Systems, supervised the coating of the films while Mr. T. Vanderwende of Du Pont Polymers synthesized the EMA/HEMA copolymers. References and Notes (1) Srinivasan, R.; Braren, B.; Dreyfus, R. W.; Hadel, L.; Seeger, D. E. J . Opt. SOC.Am. B 1986, 3, 785. (2) Srinivasan, R.; Braren, B.; Seeger, D. E.; Dreyfus, R. W. Macromolecules 1986, 19, 916. (3) Dyer, P. E.; Oldershaw, G.A.; Sidhu, J. J. Phys. Chem. 1991, 95, 10004. (4) Dijkkamp, D.; Gozdz, A. S.; Venkatesan, T.; Wu, X. D. Phys. Rev. Lett. 1987, 58, 2142. (5) Dyer, P. E.; Sidhu, J. J . Appl. Phys. 1985, 57, 1420.
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