Crystallization and Dissolution of Flow-Induced Precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals ($T_m^0=141.2°\mathrm{C}$) and requires stretch of the longest chains of the MWD. After the application of a shear flow of $120{\mathrm{s}}^{-1}$ for 1 s at $142°\mathrm{C}$, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

Figure 1

Schematic drawing of a shish-kebab crystallite.

Figure 2

SAXS and WAXD data recorded after the application of a shear of $120{\mathrm{s}}^{-1}$ for 1 s at $142°\mathrm{C}$. In the two-dimensional scattering images shown at the top, the flow direction is vertical. For the WAXD intensity, the scattering of the amorphous component was subtracted and only the crystalline peaks are visible.

Figure 3

$I_{\mathrm{eq}}^{\mathrm{SAXS}}$ and $X^{\mathrm{WAXD}}$ as a function of time after step shear at $142°\mathrm{C}$. The initial drop of $I_{\mathrm{eq}}^{\mathrm{SAXS}}$ can be fitted with the memory function of the Doi-Edwards (DE) model, the fit is represented by the continuous line.

Figure 4

Crystallization or dissolution of flow-induced precursors depends on their size. The thick gray line represents the tube containing the chain.