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. 4-6
MIL-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 technique 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 necessary. 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 perpendicular to the incident beam. The spectrum of a turbid or highly scattering specimen is obtained by analyzing 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 liquid-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, provides 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 relatively 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 quantitatively, 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 spectroscopy 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 polymer 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 chromatography (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 copolymer, or mixture of polymers), SEC still may be used to estimate the MWD and other parameters relating to the structure and composition of the polymer. Although SEC indicates the presence of soluble nonpolymeric components, high performance liquid chromatography (HPLC) is the better technique for characterizing residual monomers, oligomers, and other soluble, low MW sample components
MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization Light scattering,osmometry,and viscometry are also used to analyze polymer MW.Although seldom applied to synthetic polymers,sedimentation is an excellent technique for characterizing the MW of poly- mers having very large MW.The "special"techniques tend to be somewhat empirical or have limited util- ity and therefore are used less often. New techniques which show great promise for characterizing polymer chain structure also are listed in Table 4.4.5.One of the most promising new techniques is dynamic laser light scattering.Unlike SEC. dynamic light scattering can be applied to any soluble polymer,regardless of temperature or solvent,and does not require polymer standards for calibration.Figure 4.4.5 illustrates the MWD of poly (1,4-phenylenetereph-thalamide)(i.e.,KevlarTM)measured by the laser light scattering (Reference 4.4.5(b). 0 g , g 0 盖 0 ⊙ 7 8 0 8 g 0 10 10 MW FIGURE 4.4.5 Molecular weight distribution(MWD)of KevlarTM in concentrated sulfuric acid,using dynamic laser light scattering. As indicated,the polymer's MWD can be fully characterized using very little sample and a single solu- tion with concentrated sulfuric acid as the solvent. Dilute solution viscometry is a simple technique for determining the limiting viscosity number or intrin- sic viscosity [n]of soluble polymers (Reference 4.4.5(a)).The apparatus is inexpensive and simple to assemble and operate.The [n]of a polymer depends upon its hydrodynamic volume in the solvent and is related to the MW of the polymer. 4.4.6 General scheme for resin material characterization The following questions deserve careful consideration when developing procedures for preparing and characterizing polymer and polymer precursor(thermosetting resins and resin formulations)samples- What are the inherent characteristics of the polymer or prepolymer? Will certain operations cause irreversible changes in the sample? 4-7
MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-7 Light scattering, osmometry, and viscometry are also used to analyze polymer MW. Although seldom applied to synthetic polymers, sedimentation is an excellent technique for characterizing the MW of polymers having very large MW. The "special" techniques tend to be somewhat empirical or have limited utility and therefore are used less often. New techniques which show great promise for characterizing polymer chain structure also are listed in Table 4.4.5. One of the most promising new techniques is dynamic laser light scattering. Unlike SEC, dynamic light scattering can be applied to any soluble polymer, regardless of temperature or solvent, and does not require polymer standards for calibration. Figure 4.4.5 illustrates the MWD of poly (1,4-phenylenetereph-thalamide) (i.e., Kevlar™) measured by the laser light scattering (Reference 4.4.5(b)). FIGURE 4.4.5 Molecular weight distribution (MWD) of KevlarTM in concentrated sulfuric acid, using dynamic laser light scattering. As indicated, the polymer's MWD can be fully characterized using very little sample and a single solution with concentrated sulfuric acid as the solvent. Dilute solution viscometry is a simple technique for determining the limiting viscosity number or intrinsic viscosity [η] of soluble polymers (Reference 4.4.5(a)). The apparatus is inexpensive and simple to assemble and operate. The [η] of a polymer depends upon its hydrodynamic volume in the solvent and is related to the MW of the polymer. 4.4.6 General scheme for resin material characterization The following questions deserve careful consideration when developing procedures for preparing and characterizing polymer and polymer precursor (thermosetting resins and resin formulations) samples - What are the inherent characteristics of the polymer or prepolymer? Will certain operations cause irreversible changes in the sample?
MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization TABLE 4.4.5 Polymer molecular weights,molecular Standard Techniques Parameters Measured Size-Exclusion Chromatography Mol.wgt.averages and MWD,also provides (SEC)informa- tion relating to polymer chain branching,copolymer compo- sition,and polymer shape. Light Scattering(Rayleigh) Weight-average mol.wgt.Mw (g/mol),virial coefficient A2 (mol.cc/g),radius of gyration <R(A),polymer structure, anisotropy,polydispersity. Membrane Osmometry Number-average mol.wgt.M(g/mol),virial coefficient A2 (mol cc/g).Good for polymers with MW's in the range 5000 MW 105,lower MW species must be removed. Vapor Phase Osmometry Same as membrane osmometry except that the technique is best suited for polymers with MW<20,000 g/mol. Viscometry(dilute solution) Viscosity-average mol.wgt.Mn(g/mol)as determined by intrinsic viscosity [n](ml/g)relationship []KM,where K and a are constants. Ultracentrifugation or Sedimentation Sedimentation-diffusion average mol.wgt.Msd as defined by the relationship Md=S /D..Number-and z-average mol. wgt.,Mn and Mz.MWD determined by the relation S=kM2 where k and a are constants.Also provides information on the size and shape of polymer molecules. Special Techniques Parameters Measured Ebulliometry Number-average mol.wgt.M (g/mol)for M<20,000 g/mol. Cryoscopy Number-average mol.wgt.M(g/mol)for M<20,000 g/mol. End Group Analysis Number-average mol.wgt.M(g/mol generally for M< 10,000.Upper limit depends on the sensitivity of the ana- lytical method used. Turbidimetry Weight-average mol.wgt.M(g/mol)and MWD based upon solubility considerations and fractional precipitation of polymers in very dilute solutions 4-8
MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-8 TABLE 4.4.5 Polymer molecular weights, molecular Standard Techniques Parameters Measured Size-Exclusion Chromatography Mol. wgt. averages and MWD, also provides (SEC) information relating to polymer chain branching, copolymer composition, and polymer shape. Light Scattering (Rayleigh) Weight-average mol. wgt. Mw (g/mol), virial coefficient A2 (mol. cc/g2 ), radius of gyration <Rg>z(A), polymer structure, anisotropy, polydispersity. Membrane Osmometry Number-average mol. wgt. Mn (g/mol), virial coefficient A2 (mol cc/g2 ). Good for polymers with MW's in the range 5000 < MW < 106 , lower MW species must be removed. Vapor Phase Osmometry Same as membrane osmometry except that the technique is best suited for polymers with MW < 20,000 g/mol. Viscometry (dilute solution) Viscosity-average mol. wgt. Mη (g/mol) as determined by intrinsic viscosity [η] (ml/g) relationship [η] = KMv where K and a are constants. Ultracentrifugation or Sedimentation Sedimentation-diffusion average mol. wgt. Msd as defined by the relationship Msd = Sw/Dw. Number- and z-average mol. wgt., Mn and Mz. MWD determined by the relation S = kMa where k and a are constants. Also provides information on the size and shape of polymer molecules. Special Techniques Parameters Measured Ebulliometry Number-average mol. wgt. Mn (g/mol) for Mn < 20,000 g/mol. Cryoscopy Number-average mol. wgt. Mn (g/mol) for Mn < 20,000 g/mol. End Group Analysis Number-average mol. wgt. Mn (g/mol generally for Mn < 10,000. Upper limit depends on the sensitivity of the analytical method used. Turbidimetry Weight-average mol. wgt. Mw (g/mol) and MWD based upon solubility considerations and fractional precipitation of polymers in very dilute solutions
MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization weight distribution and chain structure. Principle Liquid chromatography technique.Separates molecules according to their size in so- lution and employs various detectors to monitor concentrations and identify sample components.Requires calibration with standard polymers. Measurement of scattered light intensities from dilute polymer solutions dependent upon solute concentration and scattering angle.Requires solubility,isolation,and in some cases fractionation of polymer molecules. Measurement of pressure differential between dilute polymer solution and solvent separated by a semi-permeable membrane.Colligative property method based upon thermodynamic chemical potential for polymer mixing. Involves isothermal transfer of solvent from a saturated vapor phase to a polymer so- lution and measurement of energy required to maintain thermal equilibrium.A colliga- tive property. Employs capillary or rotational viscometer to measure increase in viscosity of solvent caused by the presence of polymer molecules.Not an absolute method,requires standards. Strong centrifugal field is employed with optical detection to measure sedimentation velocity and diffusion equilibrium coefficients Sw and Dw.Sedimentation transport measurements of dilute polymer solutions corrected for pressure and diffusion pro- vides the sedimentation coefficient S.Permits analysis of gel containing solutions. Principle Measures boiling point elevation by polymer in dilute solution.A colligative property. Measures freezing point depression by polymer in dilute solution.A colligative prop- erty. The number or concentration of polymer chain end groups per weight or concentration of polymer are determined by specific chemical or instrumental techniques. Optical techniques are applied to measure the extent of precipitation as polymer solu- tion is titrated with a non-solvent under isothermal conditions or as the solution pre- pared with a poor solvent is slowly cooled. 4-9
MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-9 weight distribution and chain structure. Principle Liquid chromatography technique. Separates molecules according to their size in solution and employs various detectors to monitor concentrations and identify sample components. Requires calibration with standard polymers. Measurement of scattered light intensities from dilute polymer solutions dependent upon solute concentration and scattering angle. Requires solubility, isolation, and in some cases fractionation of polymer molecules. Measurement of pressure differential between dilute polymer solution and solvent separated by a semi-permeable membrane. Colligative property method based upon thermodynamic chemical potential for polymer mixing. Involves isothermal transfer of solvent from a saturated vapor phase to a polymer solution and measurement of energy required to maintain thermal equilibrium. A colligative property. Employs capillary or rotational viscometer to measure increase in viscosity of solvent caused by the presence of polymer molecules. Not an absolute method, requires standards. Strong centrifugal field is employed with optical detection to measure sedimentation velocity and diffusion equilibrium coefficients Sw and Dw. Sedimentation transport measurements of dilute polymer solutions corrected for pressure and diffusion provides the sedimentation coefficient S. Permits analysis of gel containing solutions. Principle Measures boiling point elevation by polymer in dilute solution. A colligative property. Measures freezing point depression by polymer in dilute solution. A colligative property. The number or concentration of polymer chain end groups per weight or concentration of polymer are determined by specific chemical or instrumental techniques. Optical techniques are applied to measure the extent of precipitation as polymer solution is titrated with a non-solvent under isothermal conditions or as the solution prepared with a poor solvent is slowly cooled
MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization TABLE 4.4.5 Polymer molecular weights,molecular Special Techniques Parameters Measured Chromatographic Fractionation Molecular weight distribution.An absolute MW technique is needed to analyze fractions. Melt Rheometry Weight-average mol.wgt.M (g/mol)and weight-fraction differential molecular weight distribution semi-empirical method. Gel-Sol Analysis of Crosslinked Poly- Gel fraction,Crosslink density mers Swelling Equilibrium Network structure,crosslink density,number-average mol. wgt.of chains between crosslinks M.. Promising Techniques Parameters Measured Laser Light Scattering (quasi-elastic, Same as Rayleigh light scattering plus trans-diffusion coeffi- line-broadening or dynamic) cient,molecular weight distribution,and information relating to gel structure. Field Flow Fractionation(FFF) Mol.wgt.averages and MWD.Requires calibration. Non-Aqueous Reverse-Phase High Mol.wgt.averages and MWD.Requires calibration. Performance Liquid Chromatography HPLC and Thin-Layer Chromatogra- phy TLC Supercritical Fluid Chromatography Mol.wgt.averages and MWD.Requires calibration. (SFC) Neutron Scattering Small Angle Weight-average mol.wgt.Mw(g/mol),Virial coefficient A2 (SANS) (mol-cc/g )Radius of gyration <Rz(A) 4-10
MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-10 TABLE 4.4.5 Polymer molecular weights, molecular Special Techniques Parameters Measured Chromatographic Fractionation Molecular weight distribution. An absolute MW technique is needed to analyze fractions. Melt Rheometry Weight-average mol. wgt. Mw (g/mol) and weight-fraction differential molecular weight distribution semi-empirical method. Gel-Sol Analysis of Crosslinked Polymers Gel fraction, Crosslink density Swelling Equilibrium Network structure, crosslink density, number- average mol. wgt. of chains between crosslinks Mc. Promising Techniques Parameters Measured Laser Light Scattering (quasi-elastic, line-broadening or dynamic) Same as Rayleigh light scattering plus trans-diffusion coefficient, molecular weight distribution, and information relating to gel structure. Field Flow Fractionation (FFF) Mol. wgt. averages and MWD. Requires calibration. Non-Aqueous Reverse-Phase High Performance Liquid Chromatography HPLC and Thin-Layer Chromatography TLC Mol. wgt. averages and MWD. Requires calibration. Supercritical Fluid Chromatography (SFC) Mol. wgt. averages and MWD. Requires calibration. Neutron Scattering Small Angle (SANS) Weight-average mol. wgt. Mw (g/mol), Virial coefficient A2 (mol-cc/g2 ), Radius of gyration <Rg>z (A)