Polymer crystallization mechanism through a mesomorphic state

The isothermal crystallization from the molten state of poly(butylene terephthalate) is studied by differential scanning calorimetry and small-angle x-ray scattering. The relations among the crystallization temperature, lamella thickness, and melting temperature give two equilibrium transition temperatures. We propose a model of crystalline lamella formation through a mesomorphic state at the growth front by which these equilibrium transition temperatures are interpreted as the transition temperature between the mesophase and the crystal, and the one between the melt and the crystal of infinitely thick lamellae. These results show that the mesomorphic state plays a key role in the ordering process of polymers.

Figure 1

(a) DSC curves of PBT’s crystallized at $T_c$ (vertically shifted) and (b) the relation among $T_m$’s, $T_{\text{exo}}$, and $T_c$. $T_{m1}$, $T_{m2}$, $T_{m3}$, and $T_{\text{exo}}$ are, respectively, indicated by filled thin arrows, filled thick arrows, open thin arrows, and open thick arrows in (a) and by circles, squares, triangles, and diamonds in (b). The dotted line represents $T_m=T_c$, and the thin and thick broken curves represent the fitting curves calculated from Eqs. (7) and (8) and Eqs. (6) and (8), respectively (see text). The insets in (a) are enlarged curves around $T_{m2}$ for $T_c=201{}^{\circ }\mathrm{C}$ (below $208{}^{\circ }\mathrm{C}$) and $212{}^{\circ }\mathrm{C}$ (above $208{}^{\circ }\mathrm{C}$). Error bars in (b) are approximately equal to the symbol size.

Figure 2

DSC curves of PBT crystallized at 186.4${}^{\circ }\mathrm{C}$ and annealed at $T_a$. Inserted temperatures represent $T_a$. The inserted arrow indicates $T_{m2}$ in the PBT crystallized at 186.4${}^{\circ }\mathrm{C}$ without annealing.

Figure 3

(a) The Lorentz-factor corrected SAXS curve, $I{q}^2$, at 212${}^{\circ }\mathrm{C}$ of PBT crystallized at 193${}^{\circ }\mathrm{C}$ and (b) ${\ell }^{-1}$ dependence of $T_{m2}$ and $T_c$. The solid curve in (a) is a fitting curve calculated using the “Gaussian correlation model”[21] (see text). The solid curves and the broken curves in (b) are fitting curves calculated using Eqs. (6, 7, 8, 9). Error bars in (b) are approximately equal to the symbol size.

Figure 4

Schematic diagrams of (a) the relation between $G$’s and $T$, (b) the relation between $\Delta G$’s and $\ell$ during isothermal crystallization at $T_c$ below $T_X$, and (c) the growth front of lamella during crystallization. The temperatures of intersections of $G_L$-$G_C\left(\infty \right)$, $G_L$-$G_M\left(\infty \right)$, and $G_M\left(\infty \right)$-$G_C\left(\infty \right)$ are the equilibrium transition temperatures $T_{LC}^0$, $T_{LM}^0$, and $T_{MC}^0$, respectively.