www mrs org publications bulletin Assessment of New High Performance Fibers for Advanced Applications Doetze J Sikkema Maurits G Northolt and Behnam Pourdeyhimi Abstract High performance fibers used in fabric applications ranging from bulletproof vests to trampolines must have a sufficient number of chemical and physical bonds for transferring the stress along the fiber To limit their deformation the fibers should possess high stiffness and strength Stiffness is brought about by the degree to which the chemical bonds are aligned along the fiber axis In fiber reinforced composites the fibers are the load 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 and compressive strength high damage tolerance low specific weight good adhesion to the matrix materials and good temperature resistance This article reviews and compares the properties 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 Introduction High performance fibers used in fabric applications ranging from bulletproof vests to trampolines must have a sufficient number of chemical and physical bonds for transferring the stress along the fiber The fibers should possess high stiffness and strength to limit their deformation Stiffness is brought about by the degree to which the chemical bonds are aligned along the fiber axis In fiber reinforced composites the fibers are the load 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 and compressive strength high damage tolerance low specific weight grams per square meter good adhesion to the matrix material and good temperature resistance Fibers of significance with these properties include polyethylene aramid polybenzobisoxazole PBO M5 and carbon fibers Since about 1970 spinning highperformance fibers from self organized MRS BULLETIN AUGUST 2003 liquid crystal phases has been pursued intensely The para aramids Kevlar Twaron Technora are the best known examples After coagulation the para aramid molecules are arranged in hydrogen bonded sheets reminiscent of cellulose I the workhorse of engineering in living nature that provides strength to trees and that is available in a pure form in cotton and linen Substantially higher tensile performance than in the para aramids has been achieved by the manipulation of polymers that show no conformational mobility at all and are composed of rigid rod structures an example is PBO fiber which is now commercially available from Toyobo Although PBO shows impressive tensile properties PBO reinforced composites showed compressive yielding at unsatisfactorily low stress and strain A few years ago the synthesis and manipulation of a highmolecular weight polymer rigid rod in nature like PBO but also equipped with strong intermolecular hydrogen bonds was achieved Formed from 2 3 5 6tetraaminopyridine and 2 6 dihydroxyterephthalic acid the polymer is routinely called M5 or PIPD which is an abbreviation from its IUPAC polymer name poly 2 6diimidazo 4 5 b 4 5 e pyridinylene1 4 2 5 dihydroxy phenylene The crystal structure features hydrogen bonds in both the x and the y directions z being the polymer main chain direction This is reminiscent of cellulose II the cellulose modification that one sees in manufactured cellulose fibers the most prominent example being viscose rayon that is cellulose regenerated from solution which is better suited for compressively loaded applications such as tire cords than cellulose I fibers like cotton Greatly improved synthesis routes have led to sufficient amounts of the necessary monomers to enable spinning of M5 fiber Even though much optimization remains to be done promising mechanical property and structure data have been collected on new M5 fibers that were spun in an improvised manner in bench scale work Scale up efforts are under way that should produce fiber samples that perform more closely to the potential of the system than did earlier efforts Mechanical Properties of Fibers The mechanical properties of organic polymeric fibers are much higher along the fiber axis than in the perpendicular direction Because the polymer chains are many orders of magnitude shorter than the fiber the fiber stress has to be transferred from one chain to an adjacent chain by intermolecular bonds preferably involving long stretches of parallel polymer chains The physical intermolecular bonds however are much weaker than their counterpart covalent bonds in the polymer chain There are mainly two types of physical bonds hydrogen bonds and the weaker van der Waals bonds The van der Waals bonds are extremely soft and weak as in the bonds between the molecules in candle wax In contrast to polymeric fibers the building elements in carbon fibers are crosslinked by chemical covalent bonding The molecular structure and the interand intramolecular bonding influence the mechanical properties of fibers Modulus describes the elastic extensibility of a material Thus it determines the stress required to arrive at a certain strain deformation The strength of a material refers to the stress at which the material fails or fractures but its value depends on the test specimen dimensions and testing conditions such as strain rate This apparent gage length dependence occurs because of impurities and 579 Assessment of New High Performance Fibers for Advanced Applications other flaws present in the structure leading to stress concentrations that result in catastrophic failure of the material Naturally the probability of the presence of impurities is higher in a larger test specimen with a concomitantly lower ultimate strength Figure 1 shows a typical stress strain curve for a polymer fiber In this figure positive values indicate that the material is subjected to tensile forces while negative values indicate that the material is subjected to compressive forces The stress range in which the fiber behaves as a purely elastic material lies between the compressive yield stress or compressive strength c and the tensile yield stress y which have approximately the same absolute value Between the yield stress and the