Aggregation of poly($p$-phenylene terephthalamide) chains: Emergence of fiber defects

Some of the strongest polymer-based fibers are obtained by processing a nematic solution of poly-$p$-phenylene terephthalamide (PPTA) in concentrated acid. Molecular dynamics simulations reveal that the lateral density of PPTA chains in the acid solution is inhomogeneous on scales shorter than the persistence length. Hairpin turns, insertions, and other defects are stabilized by chain-chain contacts. These results suggest a mechanism of fiber assembly through coalescence of nanofibrils, and an explanation for the limited strength of PPTA fibers even at high crystallinity.

Figure 1

(a) Atomic structure of PPTA ($n$ indicates the number of repeat units). (b) Coarse-grained (CG) model of PPTA: “N” and “O” indicate the CG particles of the NH and C=O amide groups, respectively.

Figure 2

Shown are the initial and final snapshots of ${\mathrm{PPTA}}_{12}$ (a)–(c), ${\mathrm{PPTA}}_{12}^z$ (d)–(f), and ${\mathrm{PPTA}}_{18}^z$ (g)–(i) in liquid ${\mathrm{H}}_2{\mathrm{SO}}_4$. In all images, ${\mathrm{H}}_2{\mathrm{SO}}_4$ molecules are hidden for clarity. Snapshots for $n=3$ and 6 are included in Fig. S2 [18].

Figure 3

PPTA chains in ${\mathrm{H}}_2{\mathrm{SO}}_4$ achieve a strong local nematic order, quantified using the following properties: the director $\mathbf{n}$, shown along the final snapshot of ${\mathrm{PPTA}}_{12}^z$ (a); the vector tangent to a PPTA chain's axis $\widehat{t}$ (b), the chain-chain contact vector $\widehat{h}$ (c); probabilities of a $\mathrm{NH}\cdots \mathrm{CO}$ contact (d), and of a $\mathrm{NH}\cdots {\mathrm{H}}_2{\mathrm{SO}}_4$ contact (e); nematic order parameter calculated as in Ref. [28] (f); and mean angle between the vectors $\widehat{h}$ and $\mathbf{n}$ (g).

Figure 4

Aggregates of PPTA chains lack solid-crystalline order. (a) Shown are the relative sizes of the ten largest solid-crystalline clusters in simulations of 2 wt. % PPTA in ${\mathrm{H}}_2{\mathrm{SO}}_4$. Also shown are snapshots of ${\mathrm{PPTA}}_6$ (b), a preassembled ${\mathrm{PPTA}}_{12}$ crystal (c), and liquid-crystalline ${\mathrm{PPTA}}_{12}$ (d); individual clusters are drawn in distinct colors for visualization purposes.

Figure 5

Final simulation snapshots of (a) ${\mathrm{PPTA}}_{12}$, (b) ${\mathrm{PPTA}}_{12}^z$, and (c) ${\mathrm{PPTA}}_{18}^z$: buried ${\mathrm{H}}_2{\mathrm{SO}}_4$ molecules are shown as yellow spheres. (d) Crystallographic structure of $p$-phenylene-dibenzamide (PPDB) and ${\mathrm{H}}_2{\mathrm{SO}}_4$ [32]: the heavy atoms of a $3\times 3\times 3$ supercell are viewed from a direction parallel to the ${\mathrm{H}}_2{\mathrm{SO}}_4$ layers.

Figure 6

Elastic properties of PPTA are affected by more than 1 wt. % of ${\mathrm{H}}_2{\mathrm{SO}}_4$. Each Young's modulus $E$ is shown normalized with respect to its value computed on pristine PPTA.

Figure 7

Trace ${\mathrm{H}}_2{\mathrm{SO}}_4$ defects decrease the shear strength of PPTA crystals. Stress-strain curves are shown from the onset of the linear response ($\gamma \ge {\gamma }_0=8^{\circ }$) to the largest strain simulated. Pristine PPTA crystals are compared to crystals with a point ${\mathrm{H}}_2{\mathrm{SO}}_4$ defect (a) and with interrupted chains (b). (c) Illustrates the failure mode of PPTA crystals under shear: PPTA chains (spheres) are viewed along the fiber axis and colored based on initial position.