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Structure Formation in High-Performance PBO FibersS. Ran1, C. Burger1, D. Fang1, D. Cookson2,3, K. Yabuki4, Y. Teramoto4,P.M. Cunniff 5, P.J. Viccaro3, B.S. Hsiao1, and B. Chu1*

1Chemistry Department, SUNY Stony Brook, Stony Brook, NY2Australian Nuclear Science and Technology Organization, Menai NSW, Australia3ChemMat CARS, Advanced Photon Source, Argonne National Laboratory, Argonne, IL4Toyobo Co. Ltd., Research Center, Katata, Ohtsu, Shiga, Japan5U.S. Army, Soldier and Biological Chemical Command, Natick, MA

Poly-p-phenylenebenzobisoxazole (PBO) (Figure1) forms the strongest synthetic polymer fiber knownso far. It provides excellent mechanical properties pairedwith extreme thermal stability, making PBO the opti-mum material for applications like lightweight bulletproofvests and fire-resistant suits.

PBO is spun from a lyotropic melt in polyphosphoricacid (PPA) under stretching by an adjustable spin-drawratio (SDR). PPA is removed by coagulation in a waterbath, resulting in the as-spun (AS) fiber, which is sub-sequently heat-treated to form the final high-modulus(HM) fiber. It is the goal of the present study to under-stand the structural evolution of the PBO fiber through-out different processing stages using synchrotron small-angle and wide-angle x-ray scattering (SAXS/WAXS)techniques.

ExperimentalA spinning apparatus consisting of a capillary rhe-

ometer-like barrel with an upper temperature capabil-ity of 300° C, a vertical precision movement to shiftdifferent fiber regions into the beam, a motor drivenplunger, a take-up wheel with adjustable speed provid-ing defined spin draw ratios, and a coagulation waterbath has been set up at beamline X27C of the NationalSynchrotron Light Source (NSLS) at Brookhaven Na-tional Laboratory (BNL) (Figure 2). Further experimen-tal details were given in [1]. The PBO-PPA solution(dope) has been provided by Toyobo Co. In situ wide-angle x-ray scattering (WAXS) and small-angle x-ray

scattering (SAXS) 2D patterns were recorded using aMAR CCD detector. Furthermore, WAXS and SAXSmeasurements have been performed after extendedcoagulation and after heat treatment in vacuum or un-der nitrogen.

Results and DiscussionFigures 3(a) and 3(b) show in situ WAXS patterns

of the PBO fiber before and after passing through thecoagulation water bath, respectively. The two patternsconsist mainly of meridional streaks (due to the mono-mer period) and of equatorial arcs describing the lat-eral packing order and indicating a high degree of pre-ferred orientation. The absence of off-axis reflectionsindicates a translational disorder in the fiber directionin either case. A close inspection of the two WAXS pat-terns before and after coagulation shows that the equa-torial peak positions and, thus, the corresponding lat-eral packing spacings are different. The pattern beforecoagulation (Figure 3(a)) shows two equatorial maximanot following a classical position ratio (like 1:√3 for 2Dhexagonal) and a weaker intensity for the lower angle

Figure 2. PBO in situ spinning apparatus with MAR CCDdetector at beamline X27C in NSLS, BNL.

Figure 1. Chemical structure of poly-p-phenylenebenzo-bisoxazole (PBO).

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peak. This can be interpreted in terms of a liquid-crys-talline packing of plank-shaped structural units. Thelateral packing of the anisometric cross-sections leadsto two distinct spacings giving rise to the observed equa-torial peaks. This situation is known as “sanidic” liquid-crystalline order. The nature of the plank-shaped struc-tural units is not exactly known; it is believed to be awell-defined solvate complex of PBO and PPA.

For the coagulated PBO fiber, the structures inter-preted from the observed WAXS data range from a

sanidic liquid-crystalline pattern with two equatoriallength scales, where now the single PBO molecule isthe plank-shaped structural unit (Figure 4(a)) to a higherordered structure which, apart from the translationaldisorder, approaches the ideal crystalline PBO struc-ture (Figure 4(b)). A characteristic feature of the sanidicphase, with liquid-crystalline short-range order in thecross-section, is a preferred formation of stacks (Fig-ure 4(a)) with higher intra-stack order enhancing theequatorial peak at higher angles corresponding to the

Figure 4. Structure models illustration different degrees of order in the packing of PBO molecules.

(a) sanidic lyotropic order: formation of stacks (b) 2D ordered cross-section; translational disorder

(a) before coagulation

Figure 3. In situ WAXS patterns of the PBO fiber before and after passing through the coagulationwater bath (fiber axis vertical).

(b) after coagulation

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stacking period, and a less ordered lateral packing ofstacks explaining the weaker intensity of the lower angleequatorial peak. The transition to the higher orderedstructure, forming a true 2D lattice in the cross-section(Figure 4(b)) can be observed by the appearance ofmixed hk0 reflections on the equator, most notably the2-10 for PBO.

Figure 5 shows SAXS patterns of as-spun PBO(fully coagulated but not yet heat-treated), high modu-lus PBO (heat-treated, this is the final form of the high-performance fiber) and PBO fibers which were sub-jected to an extended heat treatment (EHT), degrad-ing the local structure and the mechanical properties.The AS fiber only shows an equatorial streak (Figure5(a)) due to elongated microvoids and/or multiple scat-tering from a fibrillar structure. After heat treatment, aSAXS four-point pattern is observed (Figure 5(b)). Anextended heat treatment is able to enhance this four-point pattern (Figure 5(c)). The corresponding WAXSpattern, as well as the poor mechanical properties ofthe EHT fiber, suggests that it underwent a chemicaldegradation of the structure which is apparently ableto enhance the contrast of the four-point pattern. How-ever, it was not clear so far whether this chemical reac-tion is the original cause of the four-point pattern orwhether it only contrasts a pre-existing structure. Re-cent high-brilliance experiments at the ChemMAT CARSbeamline at the Advance Photon Source, Argonne Na-tional Lab, involving a highly collimated, focused beamprobing PBO single filaments in a full vacuum setup,were able to show the existence of the four-point pat-tern already in the AS fiber, proving that a chemicalreaction during the heat treatment is not the originalcause of the four-point pattern.

The important points to be discussed now are (1)the morphology of the superstructure generating thefour-point pattern and its tilt angle and (2) the nature ofits density contrast. Both questions cannot be unam-biguously answered based on scattering data alone.Nevertheless, it will be interesting to see which modelscan be proposed that are compatible with the observedscattering data and any given additional information.

Many possible structures can generate four-pointscattering patterns but it turns out that the orientationinformation of the WAXS patterns and the exact shapeof the SAXS four-point pattern allow us to rule out mostof them. The present high degree of preferred orienta-tion, assessable from the equatorial WAXS arcs, elimi-nates any model generating the tilt angle by inclinedmicrofibrils; all molecule chains and, thus, any mi-crofibrils must be highly preferentially oriented aboutthe fiber axis. Possible structures fulfilling these condi-tions and still producing a four-point scattering patternare, e.g., the checkerboard and the herringbone pat-tern. A careful inspection of the SAXS four-point pat-

Figure 5. SAXS patterns of as-spun (AS), high-modulus (HM),and extended heat treatment (EHT) PBO fibers, the lattertwo showing a four-point pattern (fiber axis vertical).

(a) as-spun

(b) high modulus

(c) extended heat treatment

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terns shows an inclined elongated appearance of thefour maxima. This reciprocal space feature can be in-terpreted in terms of a corresponding inclination of grainboundaries or other boundaries limiting coherent do-mains in real space. Figure 6(a) shows a simulateddensity pattern consisting of inclined lamellar stacks(producing the tilt angle) confined by differently inclinedgrain boundaries (producing the shape of the maxima).Due to these inclined grain boundaries, a herringbone-like alternating arrangement of lamellar inclinations isnot practical; neighboring lamellar stacks have to havesimilar tilt orientations in order to pack densely. Accord-ingly, the scattering from the pattern in Figure 6(a) onlyshows a tilted two-point pattern (Figure 6(b)) and thecomplete four-point pattern is restored after a cylinder

average (Figure 6(c)). It should be noted that thesetransformations are crude 2D approximations only, validfor a qualitative discussion. A more quantitative descrip-tion would take into account the cylindrical symmetry,involving Fourier-Bessel transforms, which cannot beperformed as Fast Fourier Transforms.

Figure 7 suggests a model, illustrating the com-plete texture information of a PBO fiber, that is com-patible with the experimental SAXS data. It is conceiv-able that these types of inclinations can be generatedby shear forces (due to differing flow velocities as afunction of the fiber radius) during the fiber spinningand stretching. A micro-focus SAXS experiment with aprimary beam smaller than half the fiber diameter couldbe useful to verify this model by resolving the four-pointpattern into individual two-point patterns. Further workalong these lines is in progress.

Figure 6. (a) A simulated 2D density map with inclined lamel-lar stacks and inclined grain boundaries. (b) Squared 2DFourier transform of (a). (c) Mirror or cylindrical average of(b). The various Fourier transformation artifacts in (b) and (c)are expected for this limited model and should not distractfrom its fundamental features.

(b) Two-point pattern

(c) Four-point pattern

(a) density map

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While the observed tilt angle can be readily andsatisfactorily explained by shear deformations, no ob-vious explanation presents itself for the nature of thedensity contrast generating the four-point pattern or forthe reasons of its formation. It is reasonable to assumethat the observed period is the result of a kinetic pro-cess reaching the limits of chain ordering due to de-fects, just like in traditional semi-crystalline polymers.However, due to the lack of conformational degrees offreedom in the PBO molecule itself, excluding any kindof classical chain coiling or entanglement and leavingchain ends as the only significant defects, it must beconcluded that the presence of PPA, not hampered bysuch rigidity constraints, plays an important role in theordering process, and that the superstructure generat-ing the four-point pattern, which is imprinted by the pres-ence of PPA, persists even after its removal in the co-agulation bath.

It is assumed that the crystallization process in thefreshly spun uncoagulated fiber produces inclinedlamellar stacks where one lamella type consists of anordered arrangement of well-defined PBO-PPA solvatestructures and the other lamella type consists of a lessordered PBO-PPA arrangement with interdispersedcoiled and entangled PPA. The removal of PPA resultsin crystalline PBO lamellae and in slightly less orderedlamellae where the PBO molecules are allowed to as-sume very gently twisted conformations to accommo-

date slightly different orientations and/ortranslations between two ordered lamel-lae. The chain conformation inside theless ordered domains does not signifi-cantly reduce the overall preferred orien-tation. All the chain ends and other pos-sible defects accumulate inside theseslightly less ordered lamellae, therebyproducing a small density contrast. Aschematic representation of this modelwith the PBO molecules approximated byribbons is sketched in Figure 8. Note thatthe model as depicted is also in accor-dance with a preferred a-axis orientation.The contrast enhancement upon heattreatment probably consists mainly of amore complete migration of the chainends into the less ordered domains anda perfection of the ordered domains, al-though a beginning chemical reactioncannot be ruled out.

Figure 8. Schematic representation of the inclined densitycontrast between highly ordered regions (dark, top and bot-tom) and slightly less ordered regions (light) containing chainends and gently twisted PBO molecules (model is not toscale).

Figure 7. Schematic model illustrating the spatial arrangement of thevarious inclinations present in the density patterns. The flow direction isassumed to be downwards although this information cannot be deducedfrom a scattering pattern. Note that the PBO molecules are perfectlyaligned in the vertical direction; for an explanation of the density contrastsee Figure 8.

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ConclusionsThe models developed in the present study offer a

consistent explanation for the structure and its forma-tion in PBO fibers in agreement with experimental scat-tering data. If the sketches in Figures 7 and 8 turn outto be a good approximation of the reality, it should beclear that the mechanical properties of PBO fibersstrongly depend on the exact details of these models.The slight entanglement taking place in the less orderedregions should lead to an improved response of thefiber to shear forces, not causing a graphite-like unre-stricted falling apart of the structure but resulting in theshear deformation of coherent parallelogram-shapedregions. This effect, together with the present disorderitself, could also explain how the fiber dissipates en-ergy responding to an impact, a very important prop-erty for the designed applications, where a homoge-neous crystalline structure would be susceptible to fail-ure due to its brittleness.

AcknowledgementsSupport of this work by the U.S. Army Research

Office (DAAD190010419), for the Advanced PolymersBeamline X27C by the U.S. Department of Energy(DOE) (DEFG0299ER45760) and for ChemMat CARSby the National Science Foundation/DOE(CHE9522232, CHE0087817) are gratefully acknowl-edged. Research carried out in part at the NationalSynchrotron Light Source, Brookhaven National Labo-ratory, which is supported by the U.S. Department ofEnergy, Division of Materials Sciences and Division ofChemical Sciences, under Contract No. DE-AC02-98CH10886.

Reference[1] S. Ran, C. Burger, D. Fang et al., “In-situ Synchrotron

WAXD/SAXS Studies of Structural Development duringPBO/PPA Solution Spinning” Macromolecules, 35(2),433-439, 2002.