MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization spectra for interpretation.In addition.the FTIR attenuated total reflection (ATR)and diffuse reflectance techniques may be applied for quality assurance of thermoset composite materials to assess their state of cure;i.e.,residual epoxide concentration.(See Section 5.5.3) Although not as popular as IR,laser Raman spectroscopy complements IR as an identification tech- nique and is relatively simple to apply(Reference 4.4.3(a)).As long as the specimen is stable to the high intensity incident light and does not contain species that fluoresce,little or no sample preparation is nec- essary.Solid specimens need only be cut to fit into the sample holder.Transmission spectra are obtained directly with transparent specimens.For translucent specimens,a hole may be drilled into the specimen for passage of the incident light and a transmission spectra obtained by analyzing light scattered perpen- dicular to the incident beam.The spectrum of a turbid or highly scattering specimen is obtained by ana- lyzing the light reflected from its front surface.Powdered samples are simply tamped into a transparent glass tube and fibers can be oriented in the path of the incident beam for direct analysis. 4.4.4 Chromatographic analysis High performance liquid chromatography(HPLC)is the more versatile and economically viable quality assurance technique for soluble resin materials(References 4.4.4(a)-4.4.4(g)).HPLC involves the liq- uid-phase separation and monitoring of separated resin components.Dilute solutions of resin samples are prepared and injected into a liquid mobile phase which is pumped through column(s)packed with a stationary phase to facilitate separation and then into a detector.The detector monitors concentrations of the separated components,and its signal response,recorded as a function of time after injection,pro- vides a"fingerprint"of the sample's chemical composition.Quantitative information may be obtained if the sample components are known and sufficiently well-resolved,and if standards for the components are available.Size exclusion chromatography(SEC),an HPLC technique,is particularly useful in determining the average molecular weights and molecular weight distributions of thermoplastic resins (Reference 4.4.4(g)).Recent advances have resulted in improved and automated HPLC instrumentation that is rela- tively low cost and simple to operate and maintain. A powerful,but technically more demanding,technique for directly analyzing polymers is pyrolysis GC/MS(gas chromatography/mass spectroscopy).In this case,the sample only needs to be rendered sufficiently small to fit onto the pyrolysis probe.Not only can the polymer type be identified by comparing the resulting spectrum with standards,but volatiles and additives can be identified rapidly and quantita- tively,and polymer branching and crosslink density can sometimes be measured. Other chromatographic and spectroscopic techniques have also been considered (References 4.4.3(a),4.4.4(h)-4.4.4(I)).Gas chromatography (GC),GC head-space analysis,and GC-mass spec- troscopy are useful for analyzing residual solvents and some of the more volatile resin components. Combined thermal analysis-GC-mass spectroscopy can be used to identify volatile reaction products during cure (References 4.4.4(m)and 4.4.4(n)). 4.4.5 Molecular weight and molecular weight distribution analysis Techniques for evaluating polymer molecular weight(MW),molecular weight distribution(MWD),and chain structure are listed in Table 4.4.5.Size-exclusion chromatography(SEC)is the most versatile and widely used method for analyzing polymer MW and MWD.Once the solubility characteristics of a poly- mer are known,a suitable solvent can be selected for dilute solution characterization.THF is most often the solvent of choice for SEC,however,toluene,chloroform,TCB,DMF(or DMP)and m-cresol are also used.If the polymer's Mark-Houwink constants,K and a,in the solvent are known,size-exclusion chro- matography (SEC)can be applied to determine the polymer's average MW and MWD (Reference 4.4.5(a)).If the constants are unknown or the polymer has a complex structure (e.g.,branched,a co- polymer,or mixture of polymers),SEC still may be used to estimate the MWD and other parameters relat- ing to the structure and composition of the polymer.Although SEC indicates the presence of soluble non- polymeric components,high performance liquid chromatography(HPLC)is the better technique for char- acterizing residual monomers,oligomers,and other soluble,low MW sample components. 4-6MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-6 spectra for interpretation. In addition, the FTIR attenuated total reflection (ATR) and diffuse reflectance techniques may be applied for quality assurance of thermoset composite materials to assess their state of cure; i.e., residual epoxide concentration. (See Section 5.5.3) Although not as popular as IR, laser Raman spectroscopy complements IR as an identification tech￾nique and is relatively simple to apply (Reference 4.4.3(a)). As long as the specimen is stable to the high intensity incident light and does not contain species that fluoresce, little or no sample preparation is nec￾essary. Solid specimens need only be cut to fit into the sample holder. Transmission spectra are obtained directly with transparent specimens. For translucent specimens, a hole may be drilled into the specimen for passage of the incident light and a transmission spectra obtained by analyzing light scattered perpen￾dicular to the incident beam. The spectrum of a turbid or highly scattering specimen is obtained by ana￾lyzing the light reflected from its front surface. Powdered samples are simply tamped into a transparent glass tube and fibers can be oriented in the path of the incident beam for direct analysis. 4.4.4 Chromatographic analysis High performance liquid chromatography (HPLC) is the more versatile and economically viable quality assurance technique for soluble resin materials (References 4.4.4(a) - 4.4.4(g)). HPLC involves the liq￾uid-phase separation and monitoring of separated resin components. Dilute solutions of resin samples are prepared and injected into a liquid mobile phase which is pumped through column(s) packed with a stationary phase to facilitate separation and then into a detector. The detector monitors concentrations of the separated components, and its signal response, recorded as a function of time after injection, pro￾vides a "fingerprint" of the sample's chemical composition. Quantitative information may be obtained if the sample components are known and sufficiently well-resolved, and if standards for the components are available. Size exclusion chromatography (SEC), an HPLC technique, is particularly useful in determining the average molecular weights and molecular weight distributions of thermoplastic resins (Reference 4.4.4(g)). Recent advances have resulted in improved and automated HPLC instrumentation that is rela￾tively low cost and simple to operate and maintain. A powerful, but technically more demanding, technique for directly analyzing polymers is pyrolysis GC/MS (gas chromatography/mass spectroscopy). In this case, the sample only needs to be rendered sufficiently small to fit onto the pyrolysis probe. Not only can the polymer type be identified by comparing the resulting spectrum with standards, but volatiles and additives can be identified rapidly and quantita￾tively, and polymer branching and crosslink density can sometimes be measured. Other chromatographic and spectroscopic techniques have also been considered (References 4.4.3(a), 4.4.4(h) - 4.4.4(l)). Gas chromatography (GC), GC head-space analysis, and GC-mass spec￾troscopy are useful for analyzing residual solvents and some of the more volatile resin components. Combined thermal analysis - GC-mass spectroscopy can be used to identify volatile reaction products during cure (References 4.4.4(m) and 4.4.4(n)). 4.4.5 Molecular weight and molecular weight distribution analysis Techniques for evaluating polymer molecular weight (MW), molecular weight distribution (MWD), and chain structure are listed in Table 4.4.5. Size-exclusion chromatography (SEC) is the most versatile and widely used method for analyzing polymer MW and MWD. Once the solubility characteristics of a poly￾mer are known, a suitable solvent can be selected for dilute solution characterization. THF is most often the solvent of choice for SEC, however, toluene, chloroform, TCB, DMF (or DMP) and m-cresol are also used. If the polymer's Mark-Houwink constants, K and a, in the solvent are known, size-exclusion chro￾matography (SEC) can be applied to determine the polymer's average MW and MWD (Reference 4.4.5(a)). If the constants are unknown or the polymer has a complex structure (e.g., branched, a co￾polymer, or mixture of polymers), SEC still may be used to estimate the MWD and other parameters relat￾ing to the structure and composition of the polymer. Although SEC indicates the presence of soluble non￾polymeric components, high performance liquid chromatography (HPLC) is the better technique for char￾acterizing residual monomers, oligomers, and other soluble, low MW sample components