Composites Prepared by the Anionic Polymerization of Ethyl 2-Cyanoacrylate within Supercritical Carbon Dioxide-Swollen Poly(tetrafluoroethylene-co-hexafluoropropylene)

8192 Macromolecules 2000, 33, 8192-8199 Composites Prepared by the Anionic Polymerization of Ethyl 2-Cyanoacrylate within Supercritical Carbon Dioxide-Swollen Poly(tetrafluoroethylene-co-hexafluoropropylene) Edward Kung, Alan J. Lesser,* and Thomas J. McCarthy* Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received April 20, 2000; Revised Manuscript Received July 28, 2000 ABSTRACT: Supercritical carbon dioxide (SC CO2) was used as an aid in fabricating polymer/polymer composites. Using a two-stage process, ethyl 2-cyanoacrylate (ECA) monomer was anionically polymerized within poly(tetrafluoroethylene-co-hexafluoropropylene) substrates. The composite fabrication process involved first infusing triphenylphosphine (the initiator) into the substrate using SC CO2. In the second step, monomer was introduced (again using SC CO2) to the substrate. As the monomer absorbed into the initiator-containing substrate, it polymerized. The composite surfaces were characterized using surfaceselective techniques. The mechanical performance of the composites was determined by measuring the adhesive fracture toughness of the composites. The locus of failure of fractured interfaces of composites with epoxy was determined by X-ray photoelectron spectroscopy. Introduction Supercritical carbon dioxide (SC CO2) has been used in the fabrication of polymer-polymer composites. We have demonstrated the viability of infusing monomers into a variety of polymer substrates and subsequently radically polymerizing the monomer within the SC CO2swollen substrates.1,2 Polystyrene/polyethylene composites, in particular, showed very interesting phase morphologies that resulted in enhanced mechanical performance when compared to conventional melt-blended systems.3 Thus far, our research has focused on radical synthesis and bulk characterizations. This paper will discuss research on an anionically polymerized system and the interface properties of the final composites. The polymerization of functional monomers in the surface region of SC CO2-swollen fluoropolymer substrates allows modification of the surface properties of those substrates, producing composite surfaces. Such a technique is an alternative to the rather harsh and hard-to-control techniques usually employed to modify fluoropolymer surfaces. The introduction of nitrile and ester groups can be effected by the polymerization of cyanoacrylate monomers. Polycyanoacrylates are most commonly synthesized by anionic polymerization, although radical polymerization can also be employed.4,5 The anionic polymerization of cyanoacrylates is an extremely fast and robust living polymerization. These traits, which will be discussed below, should allow for facile synthesis of the target surface composites. In a CO2 medium, however, an anionic polymerization may be problematic. CO2 terminates most anionic polymerizations. CO2 is susceptible to nucleophilic attack and will react with anionic initiators or actively propagating carbanions. For most anionic polymerization systems, the resulting carboxylate chain end is normally not reactive enough to continue propagation with monomers or to add to another CO2 molecule (no homopolymerization of CO2 is known to occur).6 Cyanoacrylate monomers, with electron-withdrawing nitrile and ester substituents, are extremely reactive and can be polymerized by very weak nucleophiles (e.g., amines and phosphines). Anionic cyanoacrylate polymerizations are also known to be extremely difficult to terminate. Pepper investigated the stability of (poly)cyanoacrylate anions by calorimetrically measuring the heat evolved with the successive addition of aliquots of monomer after the additions of potential terminating agents to a living reaction mixture.4 It was found that the polymerizations of cyanoacrylates are virtually unaffected by the addition of oxygen, water, and CO2. Only the addition of a strong acid (hydrochloric acid) terminated the polymerization. More recently, Johnston and Pepper studied the stability of polycyanoacrylate anions spectroscopically.7 The ultraviolet spectrum of a living polycyanoacrylate anion possesses an absorption centered at ∼250 nm. It was found that, upon the addition of CO2 to the reaction mixture, the absorption at 250 nm disappeared nearly instantaneously. This result suggests that CO2 reacts with the cyanoacrylate anion to form a different moiety (presumably a carboxylate anion). Johnston and Pepper did not attempt to identify the carboxylate anion but simply stated that the polymer was nearly instantly terminated by CO2. The calorimetric data suggest that despite the existence of the terminal carboxylate anions, implied by the spectroscopic data, cyanoacrylate monomer continues to polymerize. For this to occur, either the carboxylate anion decarboxylates and re-forms the cyanoacrylate anion to continue propagation or the carboxylate anion adds to new monomer to continue propagation. Acetate anions are known to initiate polymerization of cyanoacrylates.8 Scheme 1 shows how either of these two possibilities occur. “I” is a nucleophilic initiator such as an amine or phosphine. If path B occurs, then ester moieties will be incorporated into the backbone of the polymer. This path requires that the kinetics of the cyanoacrylate anion attack on CO2 are competitive with normal homopolymerization (which is known to be very rapid). If the formation of carboxylate anions is competitive, the equilibrium constant (carboxylation-decarboxylation) and the relative rates of paths A and B will dictate which or if both of the two outcomes occur. Running an anionic polymerization of a cyanoacrylate in fluid carbon dioxide should increase the potential for copolymerization. The increased concentration of CO2 10.1021/ma000695o CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/2000 Macromolecules, Vol. 33, No. 22, 2000 Scheme 1 (CO2 now being the solvent) should tend to shift the equilibrium shown in Scheme 1 toward the carboxylate anion. Also, increasing the CO2 concentration will increase the probability of a cyanoacrylate anion finding a CO2 molecule to attack before it encounters another cyanoacrylate monomer. Before we attempted to synthesize polymer composites using cyanoacrylates in SC CO2, we established what product was formed by the anionic polymerization of cyanoacrylates in a CO2 medium. We addressed this issue by examining the polymerization of ethyl 2-cyanoacrylate (ECA) in liquid CO2. Once this issue was resolved, we synthesized composites of poly(ethyl 2-cyanoacylate) (PECA) and poly(tetrafluoroethylene-cohelfluoropropylene) (FEP) and characterized the surfaces of the composites. We modified the technique that had been used for the radical polymerization of styrene within substrates.1-3 Shown in Scheme 2 is the strategy for synthesizing these composites. First, a polymer substrate is soaked in a SC solution of CO2 containing a nucleophilic intiator (pyridine or triphenylphosphine). Then the system is vented, trapping the initiator within the substrate. The substrate is then introduced to a SC solution of CO2 and ECA monomer. As the monomer is absorbed into the substrate and the initiator begins to desorb from the substrate, the two reactants meet within the upper surface regions of the substrate. As the polymerization proceeds, further outward diffusion of the new polymer does not occur. The resulting product possesses a composite surface. Exactly where within the substrate the secondary polymer forms depends on the relative diffusion rates of monomer into the substrate and initiator out of the substrate. Experimental Section Materials. Ethyl 2-cyanoacrylate (ECA) was supplied by Loctite and used without further purification. Poly(tetrafluoroethylene-co-hexafluoropropylene) was supplied by DuPont (Teflon FEP 1000A) as 10 mil (nominally) thick sheets. The FEP was cleaned by sonication in HPLC grade tetrahydrofuran (THF) at 60 °C for 24 h. The two initiators, pyridine (anhydrous, 99.8% pure) and triphenylphosphine (99% pure), were purchased from Aldrich. The pyridine was distilled under dry nitrogen from calcium hydride and stored over molecular sieves. The triphenylphosphine was recrystallized three times from ethanol and dried under vacuum. THF (anhydrous, 99.9% pure) that was used for conventional polymerizations was purchased from Aldrich and distilled as needed under nitrogen from purple sodium benzophenone dianion. Carbon dioxide (Coleman grade, 99.99% pure) was purchased from MerriamGraves and further purified through columns of activated alumina (LaRoche Industries) and a copper catalyst (Engelhard Q-5) to remove water and oxygen, respectively. The CO2 Polymerization of Ethyl 2-Cyanoacrylate 8193 Scheme 2 was compressed using a motorized high-pressure screw pump (Isco model 100DM). Conventional ECA Polymerizations in THF. All glassware was base bathed, acid bathed (2.0 M aqueous sulfuric acid), and then thorough washed with clean water and finally dried at 120 °C. All reagent handling and reactions were conducted under a dry nitrogen atmosphere at 23 °C. Using a plastic syringe, an aliquot of ECA monomer was transferred to a glass reaction vessel containing THF. In a separate vessel, an initiator solution of a predetermined concentration of pyridine in THF was prepared. While the monomer solution was vigorously stirred, a predetermined amount of pyridine/ THF solution was syringed into the reaction flask. The system was allowed to react for 1 h. The product was recovered by precipitation in acidified methanol and subsequent filtration. ECA Polymerizations in Carbon Dioxide. All reactions were carried out in high-pressure stainless steel vessels. All parts of the vessels were washed in clean acetone, acid bathed (2.0 M aqueous sulfuric acid), then thoroughly rinsed in clean water, and finally dried at 120 °C. Two vessels were used per reaction: a larger reaction vessel (22 mL) and a smaller initiator vessel (8.5 mL). All reactions were conducted at 23 °C. Using a plastic syringe, an aliquot of ECA monomer was transferred to the reaction vessel. This vessel was then sealed and filled with liquid CO2 to a pressure of 102 atm. A predetermined amount of pyridine was transferred to the initiator vessel. This vessel was then sealed and filled with liquid CO2 to 204 atm. The concentrations of the ECA/CO2 and pyridine/CO2 solutions were determined gravimetrically. The two vessels were then connected by a 2 in. long stainless steel nipple. Under vigorous agitation using a vortex mixer, the two vessels were opened, allowing some of the contents of the initiator vessel to flow into the reaction vessel as the pressure difference between the two vessels equilibrated. The two vessels were then closed and separated. The reaction vessel was then weighed to determine the mass of pyridine/CO2 solution transferred into the reaction vessel. Polymer Characterization. 13C NMR spectra were recorded in deuterated acetone using a Bruker DPX 300 spectrometer. All chemical shift values are reported in ppm relative to tetramethylsilane. IR spectra were obtained from films solvent-cast on potassium bromide plates using a Bio-Rad FTS 175C spectrometer. The refractive index increment of the PECA in acetone was measured using a Photal RM-102A differential refractometer. Molecular weights of the polymers were measured using gel permeation chromatography (GPC) through Polymer Laboratories PLgel columns with a Wyatt Technology DAWN DSP-F light scattering detector. Acetone was the GPC solvent. Composite Synthesis. All composite syntheses were carried out in 22 mL high-pressure stainless steel vessels. The vessels were prepared in the same manner described above. The composite preparations required two steps, each using two separate 22 mL vessels. All preparations were conducted at 40 °C. 8194 Kung et al. In the first step (step I), the first vessel (vessel I) was charged with a preweighed sample of FEP and an aliquot of initiatorseither pyridine or triphenylphosphine. This vessel was then sealed, tared, and filled with SC CO2 at 40 °C to a pressure of 200 atm. The concentration of the initiator/CO2 solution was determined gravimetrically. Vessel I was then vigorously agitated on a vortex mixer and allowed to soak at 40 °C for 24 h in a heated water bath. At the end of the intiator soak period, the vessel was vented and the FEP specimen removed. The specimen was rinsed throroughly with copious amounts of clean acetone to remove any adsorbed initiator and blown dry with nitrogen. To obtain an approximate mass uptake of the initiator into the FEP, a UV-vis spectrum of the specimen was obtained using a Perkin-Elmer Lambda 2 spectrometer with a virgin FEP specimen as the reference. In the second step (step II), the specimen was placed in the second vessel (vessel II) along with an aliquot of ECA transferred using a plastic syringe. Vessel II was then sealed, tared, and filled with SC CO2 at 40 °C to 200 atm. The concentration of the monomer/CO2 solution was determined gravimetricallly. Vessel II was then vigorously agitated on a vortex mixer and allowed to soak at 40 °C for various reaction times. At the end of the reaction period, the vessel was vented, and any unreacted monomer leaving the vessel during the venting was noted. The specimen was removed and placed in a vial and allowed to sit for 24 h before any analysis was conducted. For comparative purposes, pure PECA specimens were prepared on pieces of clean silicon wafers by a vapor deposition technique developed by Woods et al.9 The silicon wafers were dipped in pyridine and blown dry with nitrogen gas. These substrates were then suspended in a polypropylene container above a pool of ECA monomer that was heated to 40 °C. Within a minute, a relatively uniform layer of PECA forms on the surface of the silicon. Surface Characterization. Surface compositions of the composite specimens were investigated using several surface analytical techniques. Attenuated total reflectance (ATR) IR spectroscopy was performed using a Bio-Rad FTS 175C spectrometer and a 45° germanium ATR element. X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer-Physical Electronics 5100 spectrometer using Mg KR X-rays. All XPS data were collected at takeoff angles of 15° and 75° (between the specimen surface and the detector). Advancing and receding contact angles for both water (deionized to 18 MΩ) and hexadecane (Aldrich, 99% pure) were obtained using a Rame-Hart telescopic goniometer and Gil´ mont syringes. Mechanical Characterization. The mechanical integrity of the composites was determined by measuring the fracture strength in cleavage of adhesive joints between the composites and metal. The adhesive joints were formed by sandwiching composite specimens between two metal strips coated with epoxy to form double-cantilever beam (DCB) specimens. The epoxy adhesive is a consumer grade epoxy from Devcon (2 Ton Clear) that was prepared and applied in accordance with the recommendations of the manufacturer. The metal beams are stainless steel strips with dimensions 0.307 mm × 12.5 mm × 70.5 mm; they were cleaned with acetone prior to applying the epoxy. The DCB specimens were loaded to failure on an Instron 1123 testing machine fitted with an Instron 2525-807 load cell ((100 N) at a 2 mm/min crosshead speed. From the load-displacement curves, the critical loads for failure and the compliances of the specimens were noted. Results and Discussion Polymerization in CO2. The polymerizations conducted in liquid CO2 were precipitation polymerizations. The initial ECA/CO2 solutions, when the systems were observed in a high-pressure view cell, were clear and colorless. The view cell was illuminated to allow observations of the transmitted light. As the polymerizations proceeded, the systems became hazy and changed from Macromolecules, Vol. 33, No. 22, 2000 Figure 1. 13C NMR spectrum of polymer synthesized in liquid CO2 showing clean homopolymerization of ethyl 2-cyanoacrylate. colorless to light yellow and then to dark orange in color. Gradually, the hazy orange solution became dark and colorless and filled with large particles that grew in size until the view cell became completely occluded. The color developments are most likely due to scattering effects caused by small polymer particles suspended in the CO2. The final polymer is a fluffy white product. Copolymerization? The IR spectrum of ECA monomer was compared to those of the products formed by polymerizations in THF and CO2. Both products exhibit the loss of vinyl vibrations at 3128 cm-1 (asymmetric sp2 CH2 stretching) and 1615 cm-1 (CdC stretching). The nitrile vibration at 2246 cm-1 (CtN stretching) is shifted to a slightly higher frequency, and the intensity is decreased due to the loss of conjugation with the vinyl moiety. The product synthesized in THF is expected to be pure homopolymer PECA. By IR, the product formed in CO2 is indistinguishable from the homopolymer prepared in THF. Figure 1 shows the 13C NMR spectrum of the product that was synthesized in CO2. The spectrum matches the spectrum for pure PECA found in the literature with signals at δ 166 (carbonyl carbons), 115 (nitrile carbons), 44 (main chain methylene and quaternary carbons), 64 (side group methylene carbons), and 14 (side group methyl carbons).10 If CO2 were incorporated into the main chain of the polymer to any significant extent, separate signals from main chain carbonyl carbons and shifted signals from main chain methylene and quaternary carbons would be expected. The shifted signals would arise from cyanoacrylate monomer units bonded to electron-withdrawing CO2 monomer units. The extremely strong signals at δ 206 and 30 are due to deuterated acetone. The spectra of products synthesized in THF were identical to that shown in Figure 1. The NMR data concur with the IR results that copolymerization does not occur under the conditions used for synthesis. Molecular Weights. GPC with a light scattering (LS) detector was employed for molecular weight determination. Differential refractometry gave the refractive index increment, dn/dc, for PECA in acetone to be 0.1159 mL/g. Table 1 summarizes the results of the polymerizationssshowing conditions, molecular weights, and yields. Regardless of the solvent employed, unlike truly ideal living anionic polymerizations, the measured molecular weights are much higher than the theoretical values determined by the initial monomer-to-initiator ratio, [M]0/[I]0. Also, the polydispersity ratios, Mw/Mn, are not nearly as narrow as those found in ideal living polymerizations. The results of this investigation agree with the conclusions found by other researchers for Macromolecules, Vol. 33, No. 22, 2000 Polymerization of Ethyl 2-Cyanoacrylate 8195 Table 1. Results of ECA (M) Polymerizations by Pyridine (I) in CO2 and THF at 23 °C 10-3Mnb 10-3Mn yield sol- 103[I]0 no. vent (mol %) [M]0/[I]0a (theor) (GPC/LS) Mw/Mn (%) 1 2 3 4 5 6 7 CO2 CO2 CO2 CO2 CO2 THF THF 36.3 26.7 8.3 1.5 1.2 27.4 8.4 28 38 120 656 830 38 121 3.5 4.7 15.1 82.1 103.9 4.7 15.1 670 770 790 1150 3490 1780 1880 2.0 1.5 1.4 1.7 1.3 1.2 1.6 91.4 85.5 89.8 86.7 80.0 82.8 81.4 a Initial monomer concentration is the same for all reactions, [M]0 ) 1.0 mol %. b Mn(theor) assumes complete initiation, i.e., ) ([M]0/[I]0)(monomer MW). pyridine-initiated polymerizations of cyanoacrylates and indicate slow and incomplete initiator utilization.11-14 Composite Components. The virgin FEP specimens used in this study are translucent in appearance/and possess a density of 2.14 g/mL. XPS analysisof the FEP showed that the starting material is clean and consists of approximately 33% carbon and 66% fluorine (F/C ) 2) as expected; a spectrum of FEP is displayed in Figure 2. The titanium signal may be from some additive(s) in the polymer. Water contact angles for FEP are θA/θR ) 121°/99°; hexadecane contact angles for FEP are θA/θR ) 55°/36°. From XPS analysis, PECA consists of 69% carbon, 20% oxygen, and 11% nitrogen (O/N = 2); a spectrum is displayed in Figure 3. Water contact angles for PECA are θA/θR ) 78°/55°; hexadecane contact angles for PECA are θA/θR ) 3°/0°. For both water and hexadecane, the contact angles are reasonable for a substrate that contains polar nitrile and ester moieties. Composites Using Pyridine as Initiator. Pyridine was used as one of the initiators and was infused into the FEP substrates during step I of the fabrication technique. The UV spectra for FEP specimens soaked in 2.6 wt % and 0.5 wt % solutions of pyridine in CO2 are shown in Figure 4. Using a literature value for the extinction coefficient of pyridine (1995 L mol-1 cm-1 at 250 nm),15 the concentration, c, of pyridine loaded into the FEP specimens can be estimated using the BeerLambert-Bouguet law. The specimens were 0.0232 cm in thickness. The specimen soaked in the 2.6 wt % solution of pyridine/CO2 had a concentration of ap- proximately 0.022 M (or approximately 0.081 wt %), and the specimen soaked in the 0.5 wt % solution had a concentration of approximately 0.0068 M (or approximately 0.014 wt %). The specimens soaked in either 2.6 wt % or 0.5 wt % pyridine/CO2 solutions did not produce composites during step II of the fabrication technique. In both cases, the reaction time was 24 h, and after the reaction period, the specimens were found to be encased in a thick layer of foamed PECA. The PECA foam could be peeled off the FEP substrates, and the underlying substrates did not exhibit any change in mass (i.e., no mass uptake of PECA into the FEP). It was concluded that the diffusion of pyridine out of the substrate is much faster than the diffusion of ECA monomer into the substrate and that polymerization occurred completely outside the substrate. All of the monomer was consumed in the “external” polymerization. Composites Using Triphenylphosphine as Initiator. Triphenylphosphine was also used as an initiator and, like pyridine, was infused into the FEP substrates during step I of the fabrication technique. The UV spectrum for a FEP specimen soaked in a 0.9 wt % solution of triphenylphosphine in CO2 is shown in Figure 5. Using the same method as in the pyridine system, the concentration of triphenylphosphine in FEP after step I was estimated. A literature value for the extinction coefficient of triphenylphosphine is 11 000 L mol-1 cm-1 at 261 nm.16 The concentration of triphenylphosphine in FEP was appoximately 0.00061 M or approximately 0.0075 wt %. Unlike the pyridine system, the triphenylphosphine system produced composites in step II. Composites were synthesized using three reaction times: 6, 24, and 49 h. In all cases, the composite specimens were not encased in PECA. The specimens remained translucent although slightly more turbid than virgin FEP. Also, the specimens gained mass and grew in dimension, both due to the uptake of PECA. The incorporation of PECA in FEP increased with increasing reaction time, as is shown in Table 2. Polymerization of monomer in the fluid phase (outside the substrate) did not occur. Polymerization took place only within the FEP substrates. Figure 2. XPS spectrum of FEP obtained at a 15° takeoff angle between the specimen and the detector. 8196 Kung et al. Macromolecules, Vol. 33, No. 22, 2000 Figure 3. XPS spectrum of PECA obatined at a 15° takeoff angle between the specimen and the detector. Figure 4. UV spectra of FEP specimens soaked in supercritical solutions of pyridine/CO2 at 40 °C and 200 atm for 24 h: (a) soaked in 2.6 wt % pyridine/CO2; (b) soaked in 0.5 wt % pyridine/CO2. Specimens were 0.0232 cm thick. Chemical characterization of the surface of the specimens showed that the materials are composites. Comparisons can be made between pure FEP, pure PECA, and the composites. In all subsequent references, the composites are designated PECA/FEP-X, where X is the reaction time used in step II of the fabrication technique. The ATR IR spectra of virgin FEP, PECA/FEP-24, and PECA/FEP-49 are compared in Figure 6. The XPS spectra for PECA/FEP-24 and PECA/FEP-49 are provided in Figures 7 and 8. The elemental compositions, as determined by XPS, are tabulated in Table 3. The nitrogen, oxygen, and carbon contents increase and the fluorine content decreases with increasing reaction time. The XPS results, however, show a lower content of PECA in the upper 10-50 Å of the specimens compared with the gravimetrically determined compositions shown in Table 2. The hexafluoropropylene comonomer content in FEP is generally 10-12 wt %.17 Assuming that the FEP material is made up of 12 wt % hexafluoropropylene, then the chemical formula for an average FEP repeat unit is C2.075F4.150 with a (repeat unit) molar mass of 103.77 g/mol. The chemical formula for a PECA repeat unit is C6H7NO2 with a molar mass of 125.13 Figure 5. UV spectra of a FEP specimen soaked in a supercritical solution of 0.9 wt % triphenylphosphine/CO2 at 40 °C and 200 atm for 24 h. Specimens were 0.0232 cm thick. Table 2. Mass Uptake and Composition of PECA into FEP as a Function of Reaction Time for the Triphenylphosphine System reaction time (h) mass uptake (%) composition (wt %) 6 24 49 19.4 31.2 69.9 16.3 23.8 41.1 g/mol. Using the nitrogen and fluorine concentrations determined by XPS and the chemical formulas of the repeat units of the component polymers, the mass compositions of the specimens within the sampling depth of the XPS experiments can be calculated. These surface mass compositions are tabulated in Table 4. Contact angle data are tabulated in Table 5. As might be expected, PECA-24 is more like FEP than PECA/ FEP-49. The advancing water contact angle is higher for the composite specimens than pure FEP. This indicates a mixed hydrophobic (FEP) and hydrophilic (PECA) surface. As the water droplet is advanced over hydrophilic domains, the contact line pins causing the contact angle to increase above the pure hydrophobic (FEP) value. The receding water angle also shows pinning and is lower than the pure FEP value, indicat- Macromolecules, Vol. 33, No. 22, 2000 Polymerization of Ethyl 2-Cyanoacrylate 8197 Mechanical Performance. The fracture strength in cleavage of the DCB specimens can be described by the opening mode fracture toughness, Gq. This is a measure of the energy to propagate a crack (create new crack surface area) assuming linear elasticity and is calculated by Gq ) Figure 6. ATR IR spectra comparing a composite synthesized for 49 h (top), a composite synthesized for 24 h (middle), and virgin FEP (bottom). A 45° germanium crystal was used as the ATR element. ing that the composites possess a slightly more hydrophilic surface. The hexadecane results concur with the water results. PECA/FEP-24 specimens were both rinsed briefly in acetone and subjected to an extensive extraction in THF. The extraction involved sonicating a specimen in THF at 60 °C for several days. The specimens were then analyzed gravimetrically and with XPS. By XPS, the surfaces of the specimens showed a significant loss of PECA (Table 6). However, gravimetrically, the specimens lost only a few fractions of a percent in weight. The PECA is washed from the surface of the specimens, but the solvent resistance of FEP prevents the extraction of PECA from within the bulk of the specimens. Certainly, a great deal of the PECA is located below the surface of the specimens. To what depth the PECA is incorporated and how it is distributed are not known. ( ) Pq2 dC(a) 2B da (1) where B is the beam thickness (0.307 mm), Pq is the critical load at failure, and C(a) is the specimen compliance as a function of a, the crack length.18 The use of metal beams in the DCB geometry allows the compliance of the test specimens to be dominated by the compliance of the metal beams. A schematic of the test specimen is shown in Figure 9. The metal beams must be calibrated to obtain their compliance as a function of crack length. DCB test specimens can be simulated by clamping the metal beams together and measuring their compliance at various clamped distances from the loaded ends (thus simulating different crack lengths). From this curve, the compliance as a function of crack length and the derivitave of the compliance with respect to crack length can be determined. This curve can be used to backcalculate the starting crack length of the test specimens once the compliance of the test specimen is measured, because the compliance of the metal beams dominates the compliance of the test specimen. This back calculation requires the use of an iterative numerical method, but that is a trivial problem for modern computers. The specimens behave linear elastically until a critical load is achieved, after which the specimen fails by cleavage between one of the metal beams and the specimen. From the slope of the specimen’s loaddisplacement curve, the compliance of the specimen can be determined, which in turn provides the crack length and the derivative of compliance with respect to crack length. This and the measured critical load allow the calculation of fracture toughness. The locus of failure was determined to differentiate whether cleavage occurs within the specimen, at the specimen-epoxy interface, Figure 7. XPS spectrum of a PECA/FEP composite specimen synthesized for 24 h (PECA/FEP-24) obtained at a 15° takeoff angle between the specimen and the detector. 8198 Kung et al. Macromolecules, Vol. 33, No. 22, 2000 Figure 8. XPS spectrum of a PECA/FEP composite specimen synthesized for 49 h (PECA/FEP-49) obtained at a 15° takeoff angle between the specimen and the detector. Table 3. XPS Atomic Composition Data for FEP/PECA Composite Surfaces sample atomic concentration takeoff angle (deg) C (%) F (%) O (%) N (%) Ti (%) FEP PECA PECA/FEP-24 PECA/FEP-49 15 75 15 75 15 75 15 75 33.5 33.0 69.2 68.8 42.1 37.0 45.2 43.2 65.2 65.7 47.5 56.9 41.6 46.4 1.3 1.3 20.7 20.2 6.7 3.7 9.0 6.7 10.1 11.0 3.3 1.8 4.2 3.7 0.4 0.5 Table 4. Mass Composition of the Surface of PECA/FEP Composites As Determined from XPS Data sample PECA/FEP-24 PECA/FEP-49 takeoff angle (deg) composition as determined from N and F data (wt % PECA) 15 75 25 75 25.8 13.7 33.6 28.5 FEP PECA PECA/FEP-24 PECA/FEP-49 probe fluid water hexadecane water hexadecane water hexadecane water hexadecane treatment rinsed in acetone contact angles θR (deg) θA (deg) 121 55 78 3 122 52 133 46 Table 6. XPS Atomic Composition Data for PECA/FEP-24 Specimens That Were Rinsed in Acetone or Extracted in Tetrahydrofuran none Table 5. Dynamic Contact Angle Data for FEP/PECA Composite Surfaces sample Figure 9. Schematic representation of a double-cantilever beam adhesive test specimen. 99 36 55 0 86 18 83 15 within the epoxy or at the epoxy-metal interface. XPS was employed when the locus of failure was not visually obvious. If failure appeared to occur at the specimenepoxy interface, then both sides of the interface were examined using XPS. The chemical composition of the cleaved surfaces gives an indication of where the failure actually occurred. Table 7 lists the adhesive fracture toughness of various specimens tested, and Table 8 lists the XPS data used for the failure locus determinations. FEP, as is expected, forms a very weak bond with epoxy. XPS reveals that failure occurs cohesively within the FEP extracted in THF atomic concntration takeoff angle (deg) C (%) F (%) O (%) N (%) Ti (%) 15 75 15 75 15 75 42.1 37.0 39.4 37.4 40.9 37.0 47.5 56.9 58.0 61.1 57.2 61.5 6.7 3.7 2.0 0.8 1.2 0.8 3.3 1.8 0.4 0.5 0.7 0.8 0.7 0.8 Table 7. Adhesive Fracture Toughness Data sample adhesive fracture toughness, Gq (kJ/m2) FEP PECA/FEP-24 PECA/FEP-49 0.00051 ( 0.00012 0.00051 ( 0.00011 0.00452 ( 0.00078a a Mixed failure between specimen-epoxy and metal-epoxy interfaces. specimen, as is evidenced by the fluorine content at the surface of the epoxy side. Unfortunately, the PECA/ FEP-24 composites also form very weak bonds to the epoxy. The toughness values for FEP and PECA/FEP24 are identical. The failure locus XPS data show that, like FEP, the PECA/FEP-24 composites also fail cohesively within the specimen. The failure locus data provides a hypothesis for the poor bonding behavior of both samples. It is not the lack of adhesion between FEP and epoxy that causes the poor performance, rather it is FEP’s cohesive weakness that cause easy failure in adhesion tests. The PECA/FEP-24 composite is predominantly FEP in composition and also fails as though Macromolecules, Vol. 33, No. 22, 2000 Polymerization of Ethyl 2-Cyanoacrylate 8199 Table 8. XPS Atomic Composition Data of the Two Sides of Cleaved DCB Test Specimens Where Failure Occurred at the Specimen-Epoxy Interface sample and side virgin cured epoxy FEP specimen side FEP epoxy side PECA/FEP-24 specimen side PECA/FEP-24 epoxy side atomic concentration takeoff angle (deg) C (%) F (%) O (%) N (%) Ti (%) 15 75 15 75 15 75 15 75 15 75 80.0 77.7 35.4 37.7 67.8 71.1 42.6 36.4 70.1 71.4 11.0 12.3 64.4 62.1 18.3 9.1 53.1 61.6 13.9 9.8 9.0 10.0 0.2 11.3 16.2 3.5 1.1 12.5 13.7 2.7 3.6 0.2 0.8 0.9 3.5 5.1 it was pure FEP. Note (Table 8) that no nitrogen is observed in the XPS spectrum of the FEP side of the interface after failure. This indicates failure at FEPrich regions of the composite. This weak cohesive performance of FEP may be a bulk property or evidence of a “weak boundary layer”. In contrast to PECA/FEP-24, PECA/FEP-49 shows an order of magnitude improvement in the adhesion tests. The failure locus was determined by eye. Visually, one could see patches of the specimen where epoxy remained bonded and other areas where the specimen was visible. The PECA/FEP-24 results suggest that adding polar groups to the FEP surface is not necessary to improve adhesive performance. What is needed, is a method of strengthening the cohesive performance. The higher incorporation of PECA in the PECA/FEP-49 specimens may have provided that strengthening. Conclusions The anionic polymerization of ECA in a CO2 medium was attempted and determined to be viable: the anionic polymerization of ECA proceeds effectively to completion in liquid CO2. Although CO2 is susceptible to nucleophilic attack, CO2 is not incorporated into the final polymer product to any measurable extent under the conditions used in this study. Reducing the temperature of the system and diluting the ECA monomer concentration may yield copolymer; however, neither of these experiments were attempted in this work. Generally, the polymerization results in CO2 compare quite well to polymerizations found in the literature with amine initiators in conventional solvents. The high reactivity of cyanoacrylate monomers toward anionic polymerization is derived from the two electron-withdrawing substituents; other monomers with two electron-withdrawing substiuents may anionically polymerize in CO2 as well. Using the composite synthesis method described in Scheme 2, it is possible to produce composites of PECA and FEP. The goal of producing a composite at the surface of the FEP substrate was only partially achieved when triphenylphosphine was used as the initiator. The amount of PECA incorporated into the substrate could easily be varied by controlling the time that the initiator-containing substrate was allowed to soak in the ECA/CO2 solution. With longer soak times, more monomer was absorbed into the substrate and incorporated with the existing living PECA. Surface analysis showed that the surface of the substrate was indeed a mixture of PECA and FEP. However, gravimetric analysis and extraction experiments showed that the majority of the PECA was located below the surface. The ECA monomer diffuses into the substrate faster than triphenylphosphine can diffuse out. The location of the polymerization is thus below the surface. The surface composite fabrication technique investigated here was not optimized. Pyridine diffuses out of the substrate much faster that triphenylphosphine. The experiments with pyridine showed that, with the faster diffusing initiator, the polymerization location was entirely outside the substrate. By selecting an appropriate anionic initiator, one with an appropriate diffusivity under the chosen reaction conditions, the location of the polymerization should be controllable. An initiator that diffuses out of the substrate faster than triphenylphosphine but slower than pyridine should produce a more surface-stratified composite. An investigation of different intiators and different monomers (e.g., butyl 2-cyanoacrylate) would be a logical next step to this research. Still, with enough PECA incorporation, the composites showed an improvement in DCB cleavage tests. DCB cleavage tests are designed to test the integrity of substrate/adhesive joints. The reason for the improved performance of the composites was not due to increased adhesion between the substrate and the test adhesive (epoxy). Adding functional groups to the surface did not improve the performance of the composites. Mechanical integrity in the DCB tests was improved by enhancing of the cohesive strength of the substrate surface. Joints of pure FEP and epoxy are poor, not because of poor adhesion between the two materials: rather, the joints fail within the FEP either due to poor bulk cohesiveness or the existence of a weak surface later. The incorporation of PECA into FEP at high enough concentrations seems to enhance the cohesive strength of the material. Acknowledgment. We thank the Office of Naval Research and the Green Polymer Chemistry Cluster of the Center for UMass-Industry Research on Polymers for financial support. We also thank the NSF-funded UMass Materials Research Science and Engineering Center for use of the central facilities. References and Notes (1) Watkins, J. J.; McCarthy, T. J. Macromolecules 1994, 27, 4845. (2) Watkins, J. J.; McCarthy, T. J. Macromolecules 1995, 28, 4067. (3) Kung, E.; Lesser, A. J.; McCarthy, T. J. Macromolecules 1998, 31, 4160. (4) Pepper, D. C. J. Polym. 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