Article pubs.acs.org/IECR
Use of Fluorescent Probes to Determine Molecular Architecture in Phase Separating Epoxy Systems Kevin B. Davis, Dwaine A. Braasch, Monoj Pramanik, and James W. Rawlins* School of Polymers and High Performance Materials, The University of Southern Mississippi, 118 College Drive No. 5217, Hattiesburg, Mississippi 39406 S Supporting Information *
ABSTRACT: A novel route for morphological characterization of phase separating toughener and cross-linker for epoxy matrices has been investigated using fluorescence spectroscopy. Phase behavior of the individual components is determined by attaching different fluorescent probes to the toughener and cross-linker. In contrast to other techniques such as transmission eletron microscopy (TEM), this method allows resolution between similar components in which selective staining cannot be achieved.
1. INTRODUCTION
2. EXPERIMENTAL SECTION 2.1. Materials. Epon 1004 and Jeffamine D2000 were obtained from Momentive and Huntsman, respectively. Block copolymer tougheners designated T1 was synthesized in the laboratory. Crelan 403 and Jeffamine M2005 were obtained from Bayer Material Science and Huntsman, respectively. Fluorescein isothiocyanate (FITC) and tetramethylrhodamine isothiocyanate (TRITC) were supplied from Thermo Scientific and Life Technologies-Invitrogen, respectively. Methanol, methylene chloride, and N,N-dimethyl formamide (DMF) were purchased from Fisher Scientific. Secondary toughener/ cross-linker designated as UC1 was also synthesized in the laboratory. 2.2. Synthesis of FITC Modified T1. Epon 1004, a DGEBA-based epoxy resin, and Jeffamine D2000, a propylene oxide diamine, from Huntsman were used in the synthesis of block copolymer tougheners designated as T1. Synthesis details are reported elsewhere.23 To create FITC modified T1 (Figure 1a) fluorescein isothiocyanate (FITC, Figure 1c) was chosen as the fluorescent probe for T1. Cost, availability, temperature stability, and functionality were factors in the choice of FITC. T1 was dissolved to 20% solids by weight in DMF and placed in a 3-neck flask. The pH of the solution was brought up by adding 0.7 g sodium borate to the solution (35 mL) followed by FITC addition. FITC was added on a 1:20 mol concentration based on the theoretical hydroxyl groups of T1. Reaction conditions were as follows: 70 °C for 72 h followed by 120 °C for 48 h. The reaction product was dissolved in methanol and precipitated using a saturated NaCl/ water solution. Six washes were performed using this procedure to purify the reaction product. 2.3. Synthesis of TRITC Modified UC1. A secondary toughener/cross-linker designated as UC1 was synthesized from Crelan 403, Jeffamine M2005, and Jeffamine D2000. The
Epoxy resins are typically associated with excellent physical properties such as high glass transition temperatures and good modulus and processability.1−4 However, low fracture toughness, poor service life prediction, and low cyclic fatigue resistance are often cited as deficiencies. Nevertheless, epoxy resins have wide industrial acceptance in many high performance applications.1,2 Overcoming the deficiencies of epoxy resins has been a focus of much research and various methodologies have been developed for improving fracture and fatigue resistance involving specifically engineered materials such as rubbers,5−7 thermoplastics,8−10 block copolymers,11−14 core−shell rubbers,6,15 and nanoparticles.16,17 These materials are typically added during the liquid mixing stage as discrete particles or as soluble materials that phase separate upon cure. As these materials form secondary domains within the parent matrix, their ultimate morphology has become an important element for research. Transmission electron microscopy (TEM) is commonly used to define architectures of this type,6,12,17,18 but it relies on staining to produce sufficient contrast for detecting the various phases. Although TEM offers excellent precision and resolution, staining is, on occasion, not selective, and materials that have similar reactivity to the staining agents are difficult or impossible to resolve. A characterization method that allows improved resolution between similar but phase separated components is therefore highly desirable. Fluorescence spectroscopy has been used extensively to define cell architecture in biological systems and provides excellent specificity with the careful selection of probe type.19−22 In this research, fluorescence spectroscopy coupled with confocal microscopy was employed to define the partitioning of a phase separating toughener (designated as T1) as well as a phase separated cross-linker (designated as UC1) within an epoxy matrix by chemically coupling them with specific fluoroprobes. © 2013 American Chemical Society
Received: Revised: Accepted: Published: 228
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Figure 1. Structures of (a) FITC modified T1 toughener, (b) TRITC modified UC1 cross-linker, (c) FITC, and (d) TRITC.
synthesis details of UC1 are recorded elsewhere.23 As UC1 was used in the same matrix as T1, a differing fluorescent probe was required for characterization. Tetramethylrhodamine isothiocyanate (TRITC, Figure 1d) was chosen as it is commonly used with FITC and can form a fluorescence resonance energy transfer (FRET) pair. To form TRITC modified UC1 (Figure 1b), dry Jeffamine D2000 was first reacted with TRITC in a 3:1 molar ratio in methylene chloride at 50% solids. Reaction time was 24 h at 25 °C followed by 24 h at 35 °C. The excess Jeffamine D2000 was used to ensure that a significant portion of D2000 was singly capped with TRITC. This material was then added to Crelan 403 in methylene chloride (40% solids) and reacted for another 24 h at 35 °C. Jeffamine M2005, a propylene oxide-ethylene oxide monofunctional amine, was then added to the system at 35 °C. Molar ratios were 2.2:1 based on amine functionality of Jeffamine M2005 and free isocyanate of Crelan 403. Reaction was performed at 35 °C for 24 h. Since difunctional amines were found to cross-link with Crelan 403 and gel the reaction, monofunctional M2005 was used as the primary propylene oxide amine. The incorporation levels of D2000/TRITC were very small and did not significantly hinder the reaction. 2.4. Preparation of Epoxy Matrix with FITC tagged T1 and TRITC tagged UC1. To prepare the final epoxy systems, FITC modified T1 was dissolved in Epon 828 at 5% by weight. Additionally, a sample of TRITC modified UC1 was added at 2% by weight. The sample was then heated to 125 °C, and 3,3′diaminodiphenylylsulfone (3,3′-DDS) was added at a stoichio-
metric ratio (based on epoxy to amine hydrogen). Once completely dissolved, the sample was cast on glass microscope slides at approximately 0.3 mm thickness and placed in a 125 °C oven for 1 h. The resulting films were characterized. 2.5. Characterization. Mid-IR spectra were obtained with a Thermo Scientific Nicolet 6700 over a frequency range of 600−4000 cm−1. The instrument was operated at 32 scans with a resolution of 4 cm−1. Near-IR spectra were obtained using a Nicolet 6700 FT-IR Analyzer from Thermo Scientific equipped with an InGaAs detector, CaF2 beam splitter, and a quartzhalogen source. 13C NMR spectra of samples were acquired using a Bruker Avance III NMR Spectrometer operating at a carbon frequency of 150.90 MHz and equipped with a standard 5 mm two channel probe. NMR samples were prepared using deuterated solvent (DMSO-d6). Absorption spectra were recorded in a Tecan Infinite spectrophotometer. Films of Epon 828 and 3,3′-DDS epoxy thermoset (FITC tagged T1 and TRITC tagged UC1) were evaluated for fluorescence using a Zeiss 710 confocal\microscope with a 50× objective.
3. RESULTS AND DISCUSSION Phase separating toughener (T1) and cross-linker (UC1) were synthesized in our laboratory according to a previously reported procedure.23 Chemical structures of T1 and UC1 are provided in Figure S1 (Supporting Information). Synthesis of T1 from a chemical combination of Epon 1004 and Jeffamine D 2000, and as well as UC1 from Jeffamine M2005 and Crelan 403 was verified via near IR and mid-IR spectroscopy, respectively. Near 229
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Figure 2. FTIR of T1, FITC, FITC modified T1 and UC1, TRITC, and TRITC modified UC1.
Figure 3. 13C NMR of reaction product of model compound 1 (Epon 1001) and FITC.
peaks at 4935 and 6541 cm−1 of Jeffamine D2000 were noted with reduced intensity. The disappearance of epoxy peak indicated the reaction between epoxy and amine functional groups forming the T1 product. Figure S3 (Supporting
IR spectra of Epon 1004, Jeffamine D2000, and the resulting product T1 are provided in Figure S2 (Supporting Information). The epoxy peak at 4530 cm−1 in Epon 1004 was not observed in the product spectrum. Additionally, amine 230
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Figure 4. 13C NMR of reaction product of model compound 2 (Jeffamine M2005) and FITC.
ization were attributed to low levels of FITC and TRITC incorporation in both T1 and UC1, respectively. Similar to FTIR, NMR characterization of the compounds proved difficult due to low incorporation levels of FITC. Hanna observed that the reaction between NCS of FITC and secondary hydroxyl groups of phenoxy resins was very difficult to prove because of very low incorporation level of FITC.25 Hanna established the reaction of NCS in FITC with secondary hydroxyl groups using isopropyl alcohol.25 T1 material has both amine and secondary hydroxyl functional groups in its molecule while UC1 has only amine functional groups. The reaction between FITC with either secondary hydroxyl or amine was indirectly proved by conducting the reaction between FITC and model compound 1, DGEBA based material of epoxy equivalent weight of 535 (Epon 1001) which has secondary hydroxyl groups and as well as with model compound 2, polyether amine (Jeffamine M2005) which has primary amine. Reaction of FITC and model compound 1 at a 1:1 ratio (NCS and secondary hydroxyl) was conducted in DMF for 48 h at 70 °C. 13C NMR spectrum of reaction product from Epon 1001 and FITC is displayed in Figure 3. Peaks at 180 and 70.3 ppm associated with the thiourethane bearing carbon and reacted secondary hydroxyl bearing carbon, respectively, were noted in the 13C NMR spectrum of product. The unreacted secondary hydroxyl bearing carbon peak was observed at 67.54 ppm. The presence of thiourethane and
Information) shows mid-IR spectra of Jeffamine M2005, Crelan 403, and their product UC1. Crelan 403 showed characteristic isocyanate (NCO) peak at 2265 cm−1. The NCO peak was not observed in the spectrum of UC1. The data supported the reaction of the isocyanate in Crelan 403 with the NH2 moiety of Jeffamine M2005. As detailed in the Experimental Section, fluorescein isothiocyanate (FITC, Figure 1c) and tetramethylrhodamine isothiocyanate (TRITC, Figure 1d) were attached to T1 and UC1 respectively. The resulting compounds, FITC modified T1 and TRITC modified UC1, are shown in Figure 1a and b, respectively. Mid FTIR spectral overlay of T1, UC1, FITC, FITC modified T1, TRITC, and TRITC modified UC1 are shown in Figure 2. Both FITC and TRITC have the N CS group, while UC1 and T1 have NH2 functional group, which are expected to form thiourea linkage, NC(S) N. NCN and CS generally exhibit peaks at about 1470 and 1414 cm−1 respectively in the mid FTIR.24 Peak(s) at the same wavenumber region for UC1 and T1 material are due to CH2 and CH3 bending. It is thus very difficult to confirm the reaction of NCS functional group with NH2 as nonthiourea materials T1 and UC1 showed peak at 1420− 1490 cm−1. However, NCS peak at 2070 cm−1 was not observed in both FITC tagged T1, and TRITC tagged UC1 spectra, which confirmed and supported the reaction between NCS and NH2. The difficulties in FTIR character231
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reacted secondary hydroxyl bearing carbon peaks in the product attested the occurrence of reaction between NCS of FITC and secondary hydroxyls of model compound 1. The integration ratio of carbons at 70.3 ppm to total secondary hydroxyl carbon was calculated to be 7.3% (ratio of integration of the 70.3 ppm carbons to the integration of secondary hydroxyl carbon at 67.54 ppm plus integration of carbons at 70.3 ppm). This value is related to the extent of reaction of FITC with secondary hydroxyl of model compound 1. The extent of reaction was thus low as secondary hydroxyl to NCS ratio was set at 1:1. 13C NMR spectrum of reaction product of model compound 2 and FITC is shown in Figure 4. Model compound 2 was reacted with FITC at a primary amine to FITC ratio of 2:1. The reaction product is expected to have thiourea linkage. Carbon adjacent to the primary amine of Jeffamine M2005 was found to be shifted which indicated the occurrence of FITC reaction with Jeffamine M2005. From the inset of Figure 4, two peaks in the product spectrum were observed correlating to the reacted and unreacted amine bearing carbons in M2005. The peak at 50.02 ppm is associated with the reacted amine bearing carbon while the peak at 49.82 ppm is related to unreacted amine bearing carbon of Jeffamine M2005. A peak at 180.45 ppm in the product spectrum was also noted which attributed to the thiourea carbons. The integration of these carbon peaks indicated an overestimated conversion (0.58 vs an expected 0.50), but this was close to the complete consumption of FITC by amine functional groups in Jeffamine M2005. Analysis indicated that the reaction of secondary hydroxyl with FITC was limited compared to reaction of primary amine with FITC. This, in turn inferred that NCS of FITC is expected to react preferentially with amine during reaction of T1 with FITC. To determine the amount of incorporated fluorescent probe in the modified compounds, absorbance spectra from 400 to 600 nm were collected using a UV−vis spectrophotometer (Tecan Infinite M1000). Both TRITC and FITC modified compounds were investigated. FITC, FITC modified T1, and T1 were dissolved in DMF, and their absorbance maxima were determined and normalized for concentration (Figure 5A). The ratio of absorbance maxima to pure TRITC was then used to determine the percentage concentration of the sample. For FITC modified T1, this technique gave an incorporation weight of 0.72% which correlates to approximately 15% incorporation efficiency of FITC in the T1 polymer. Absorbance studies were performed in similar fashion for TRITC modified UC1 (Figure 5B). Using reagent ethanol as the solvent, TRITC modified UC1 was compared with pure TRITC and TRITC was determined to be incorporated at 0.016 wt % corresponding to approximately 32% incorporation efficiency. Toughness modifier T1, based on Jeffamine D2000 and Epon 1004 block segments, requires phase separation within the matrix to function as a toughener. The chemical driving force for phase separation (the epoxy phobic character) was provided by the propylene oxide groups14 of Jeffamine D2000. Diglycidyl ether of bisphenol- (DGEBA) block segments of Epon 1004 in T1 behaves as epoxyphilic which provides compatibility with the matrix. T1 has been shown to improve toughness in Epon 828 and 3,3′-DDS systems.23 In previous investigations,23 the T1 toughener was used in conjunction with an uretdione-propylene oxide based toughener-crosslinker designated UC1. UC1 was synthesized from Crelan 403 and Jeffamine M2005 and provided a means of cross-linking by disassociation of the uretdione blocks (Crelan 403).
Figure 5. Absorbance spectra of (A) FITC, FITC modified T1, and T1; (B) TRITC, TRITC modified UC1, and UC1.
Figure 6 displays a light microscopy image of an epoxy matrix toughened with both T1 and UC1. Phase separation of the
Figure 6. Light microscopy images of Epon 282 DDs with 5% FITC modified T1 and 2% TRITC modified UC1 (A) brightfield (B) CDIC.
tougheners can be clearly detected, and the size and shapes correspond well in both brightfield (Figure 6A) and circular differential interference contrast (C-DIC) images (Figure 6B). Although phase domains are clearly defined in Figure 6, their exact chemical nature is not well established and it is not possible to determine which species (T1 or UC1) constitute the two phases. However, the FITC and TRITC modified tougheners can be selectively excited by choosing the proper wavelength of light and the resulting fluorescence images corresponds to the particular excited probe. The fluorescence image in Figure 7A is the result of laser excitation at 488 nm and subsequent FITC fluorescence. This 232
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This approach results in a qualitative analysis of whether probe tethered compounds are in the same domain or not. In cases where diffuse imaging makes domain determination difficult, fluorescence resonance energy transfer (FRET) analysis can be used to gain position information relative to each probe. Probes that form a FRET pair such as FITC and TRITC can transfer energy in a nonradiative manner at very close distances (1−100 Å). The efficiency in energy transfer is inversely proportional to the sixth power of the distance between the FRET pair.22 Just the detection of FRET energy transfer would indicate close proximity of the tagged molecules within the domain. In this case, the domains were easily defined and phase information gained using traditional techniques. Although 250 nm is a standard lower lateral resolution for confocal microscopy, other techniques exist that can increase resolution significantly. Single probe super resolution microscopy encompasses a class of techniques that rely on the imaging of small subsets of the available probes and prediction of their center points. Photoactivated localization microscopy (PALM), fluorescence photoactivated localization microscopy (FPALM), and stochastic optical reconstruction microscopy (STORM) are examples of the technique.26 The application is not simple and requires specialized probes but can increase resolution down to 50 nm and lower. The use of FRET or other super resolution microscopy techniques allows for applications into the nanoscale domain.
Figure 7. (A) Fluorescence image of 488 nm excited Epon 828 DDS with 5% FITC modified T1 and 2% TRITC modified UC1. (B) Fluorescence image of 543 nm excited Epon 828 DDS with 5% FITC modified T1 and 2% TRITC modified UC1. (C) Image overlay of both A and B.
4. CONCLUSION Fluorescence microscopy can therefore be used to characterize domain architecture in toughened epoxy-amine systems. Using this method, contrast between similar materials can be effected by careful choice of fluorescent probe allowing the detection of domain features that would be difficult to resolve through other techniques. Due to the great number of available fluorescent probes, a wide variety of complex systems can be formulated and their phase behavior elucidated. In the case of T1 and UC1, domain architecture was easily defined and partitioning information gained. UC1 appears to not have diffused into the T1 domain but remained in the bulk of the matrix. Although unexpected, such information is valuable and will be used to better understand and design toughening systems with secondary cross-linkers.
image map correlates well with both the brightfield and the CDIC images of Figure 6. Figure 7B shows a 543 nm excitation fluorescence map of TRITC modified UC1. The combined fluorescence map (Figure 7C) shows that FITC modified T1 phase separates into spherical domains while TRITC modified UC1 remains in the bulk of the matrix. UC1’s was intended to migrate into the T1 phase and increase cross-linking and modulus within the T1 domains, however, the fluorescence images suggest that this is not the case. The confocal images suggest that hard−soft segments, i.e., high-low cross-link density domains are formed within the epoxy matrix. This type of architecture, although not functioning as originally conceived, may provide an opportunity to improve interconnectivity of the T1 domains with the epoxy matrix bulk. Such architecture information was unknown before use of the fluorescent tagged materials and can be used to design epoxy systems with improved toughness. Phase behavior can vary from system to system with matrix, temperature, and domain composition as main driving forces. The resolution limit for confocal microscopy depends on the wavelength of light used, the objective, and the refractive index of the immersion media. The most convenient media is usually air with oil being used due to its refractive index. In this study, a traditional technique was used as the phase separated domains were within the 250−300 nm lateral limit for standard confocal fluorescence microscopy. Objects below this size would still be detectable; however, their boundaries would appear diffuse. In such lower limit cases, the use of two probes with differing excitation/emissions wavelengths is advantageous as it allows the probes to be excited and located individually by progressively exciting from higher to lower energy wavelengths.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1 shows chemical structure of T1 and UC1. Figure S2 displays near IR spectra of Epon 1004, Jeffamine D2000, and their product T1. Figure S3 shows mid-IR spectra of Jeffamine M2005, Crelan 403, and their product, UC1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +1-601-266-5880. Tel.: 601-266-4781. E-mail: james.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the Office of Naval Research through grant no. N00014-07-1-1057. The authors also thank Baobin Kang for her initial assistance with 233
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the confocal microscope, which was purchased by the Mississippi INBRE, via funding by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM103476.
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