IntroductionHigh-performance fibers, used in fabricapplications ranging from bulletproofvests to trampolines, must have a sufficientnumber of chemical and physical bondsfor transferring the stress along the fiber.The fibers should possess high stiffnessand strength to limit their deformation.Stiffness is brought about by the degree towhich the chemical bonds are aligned alongthe fiber axis. In fiber-reinforced compos-ites, the fibers are the load-bearing elementin the structure, and they must adhere wellto the matrix material. An ideal reinforcingfiber must have high tensile and compres-sive moduli, high tensile and compressivestrength, high damage tolerance, low spe-cific weight (grams per square meter),good adhesion to the matrix material, andgood temperature resistance. Fibers of sig-nificance with these properties includepolyethylene, aramid, polybenzobisoxa-zole (PBO), M5, and carbon fibers.Since about 1970, spinning high-performance fibers from self-organized,liquid-crystal phases has been pursuedintensely. The para-aramids (Kevlar, Twaron,Technora) are the best known examples.After coagulation, the para-aramid mole-cules are arranged in hydrogen-bondedsheets reminiscent of cellulose I, the work-horse of engineering in living nature thatprovides strength to trees and that is avail-able in a pure form in cotton and linen.Substantially higher tensile performancethan in the para-aramids has been achievedby the manipulation of polymers that showno conformational mobility at all and arecomposed of rigid-rod structures: an ex-ample is PBO fiber, which is now commer-cially available from Toyobo. AlthoughPBO shows impressive tensile properties,PBO-reinforced composites showed com-pressive yielding at unsatisfactorily lowstress and strain. A few years ago, thesynthesis and manipulation of a high-molecular-weight polymer, rigid-rod innature like PBO but also equipped withstrong intermolecular hydrogen bonds,was achieved. Formed from 2,3,5,6-tetraaminopyridine and 2,6-dihydroxy-terephthalic acid, the polymer is routinelycalled M5, or PIPD, which is an abbrevia-tion from its IUPAC polymer name poly{2,6-diimidazo[4,5-b-4’,5’-e]pyridinylene-1,4(2,5-dihydroxy)phenylene}. The crystalstructure features hydrogen bonds in boththe x and the y directions (z being thepolymer main-chain direction). This isreminiscent of cellulose II, the cellulosemodification that one sees in manufac-tured cellulose fibers, the most prominentexample being viscose rayon—that is, cel-lulose regenerated from solution, which isbetter suited for compressively loaded ap-plications such as tire cords than cellulose Ifibers like cotton.Greatly improved synthesis routes haveled to sufficient amounts of the necessarymonomers to enable spinning of M5 fiber.Even though much optimization remainsto be done, promising mechanical-propertyand structure data have been collected onnew M5 fibers that were spun in an im-provised manner in bench-scale work.Scale-up efforts are under way that shouldproduce fiber samples that perform moreclosely to the potential of the system thandid earlier efforts.Mechanical Properties of FibersThe mechanical properties of organicpolymeric fibers are much higher alongthe fiber axis than in the perpendicular di-rection. Because the polymer chains aremany orders of magnitude shorter thanthe fiber, the fiber stress has to be trans-ferred from one chain to an adjacent chainby intermolecular bonds, preferably involv-ing long stretches of parallel polymerchains. The physical intermolecular bonds,however, are much weaker than theircounterpart covalent bonds in the polymerchain.There are mainly two types of physicalbonds: hydrogen bonds and the weakervan der Waals bonds. The van der Waalsbonds are extremely soft and weak, as inthe bonds between the molecules in candlewax. In contrast to polymeric fibers, thebuilding elements in carbon fibers are cross-linked by chemical (covalent) bonding.The molecular structure and the inter-and intramolecular bonding influence themechanical properties of fibers. Modulusdescribes the elastic extensibility of a mate-rial. Thus, it determines the stress requiredto arrive at a certain strain (deformation).The strength of a material refers to the stressat which the material fails or fractures, butits value depends on the test specimen di-mensions (and testing conditions, such asstrain rate). This apparent gage-length de-pendence occurs because of impurities andMRS BULLETIN/AUGUST 2003 579Assessment of NewHigh-PerformanceFibers for AdvancedApplicationsDoetze J. Sikkema, Maurits G. Northolt, andBehnam PourdeyhimiAbstractHigh-performance fibers, used in fabric applications ranging from bulletproof vests totrampolines, must have a sufficient number of chemical and physical bonds fortransferring the stress along the fiber.To limit their deformation, the fibers should possesshigh stiffness and strength. Stiffness is brought about by the degree to which the chemicalbonds are aligned along the fiber axis. In fiber-reinforced composites, the fibers are theload-bearing element in the structure, and they must adhere well to the matrix material.An ideal reinforcing fiber must have high tensile and compressive moduli, high tensile andcompressive strength, high damage tolerance, low specific weight, good adhesion to thematrix materials, and good temperature resistance.This article reviews and compares theproperties and behavior of novel high-performance fiber materials including polyethylene,aramid, polybenzobisoxazole, M5, and carbon fibers.Keywords: advanced composites, advanced fabrics, aramid fibers, carbon fibers,damage tolerance, M5 fibers (PIPD), polybenzobisoxazole (PBO), polyethylene fibers.www.mrs.org/publications/bulletinother flaws present in the structure, leadingto stress concentrations that result in cata-strophic failure of the material. Naturally,the probability of the presence of impuri-ties is higher in a larger test specimen witha concomitantly lower ultimate strength.Figure 1 shows a typical stress–straincurve for a polymer fiber. In this figure,positive values indicate that the material issubjected to tensile forces, while negativevalues indicate that the material is subjectedto compressive forces. The stress range inwhich the fiber behaves as a purely elasticmaterial lies between the compressive yieldstress, or compressive strength (c), and thetensile yield stress (y), which have approxi-mately the same absolute value. Betweenthe yield stress and the