When crystals flow

Abstract

Semicrystalline polymers are solids that are supposed to flow only above their melting temperature. By using confinement within nanoscopic cylindrical pores, we show that a semicrystalline polymer can flow at temperatures below the melting point with a viscosity intermediate to the melt and crystal states. During this process, the capillary force is strong and drags the polymer chains in the pores without melting the crystal. The unexpected enhancement in flow, while preserving the polymer crystallites, is of importance in the design of polymer processing conditions applicable at low temperatures, e.g., cold drawn polymers such as polytetrafluoroethylene, self-healing, and in nanoconfined donor/acceptor polymers used in organic electronics.

Fig. 1.. Bulk PEO8k thermodynamics, rheology, and morphology.

(A) Differential scanning calorimetry (DSC) heating curve (rate = 10 K/min) showing (apparent) melting at 335 K. The imbibition temperature (T = 330 K) is indicated with an arrow. (B) Storage modulus (green circles), loss modulus (red circles), and viscosity (blue spheres) measured at the imbibition temperature (T = 330 K) measured in rheology. (C) Small-angle x-ray scattering (SAXS) curves of PEO8k obtained at ambient temperature following slow cooling from the melt. The lamellar domain spacing is shown in the inset for different cooling rates, R. a.u., arbitrary units. (D) Spherulitic morphology obtained by polarizing optical microscopy (POM) with crossed polars at ambient temperature following slow cooling from the melt.

Fig. 2.. Imbibition of PEO8k in nanopores revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM).

(A) Left: SEM image from a fractured surface of anodic aluminum oxide (AAO) having nanopores with a diameter of 400 nm infiltrated with PEO8k at 330 K for 28 days. Right: A zoom-in to the blue rectangular dashed area in left. Blue arrows indicate the meniscus. (B) AFM two-dimensional height image corresponding to the cyan rectangular dashed area of (A, right) (C) AFM two-dimensional image to the same area as (B). (D) A zoom-in AFM two-dimensional image to the meniscus region.

Fig. 3.. Nano–infrared (IR) reveals the semicrystalline nature of the polymer in the nanopores.

(A) Schematic arrangement of the sample for the nano-IR measurements. After scanning the edge of the anodic aluminum oxide (AAO) surface, the atomic force microscopy (AFM) tip is positioned on the polymer or on the AAO surface. Then, the wavelength of the IR laser is tuned. The vibrational amplitude of the AFM tip corresponds to the nano-IR response of the selected position, respectively. (B) Topographic image of the AAO nanoporous sample filled by PEO8k. We selected three positions on the PEO (#1, #2, and #3) and three positions on the AAO (#4, #5, and #6), respectively, and recorded IR spectra. (C) Corresponding IR spectra taken at the marked positions in (B). For positions #1 to #3, absorption peaks at 1061, 1108, and 1149 cm⁻¹ are present (in yellow) that are absent for positions #4 to #6. Peaks at 1061, 1108, and 1149 cm⁻¹ are typical for semicrystalline PEO (27). The latter three peaks are also present on PEO films (fig. S1).

Fig. 4.. Molecular mechanism and associated time scales during imbibition of semicrystalline polymers in nanopores.

(A and B) Hierarchical structures and associated kinetics pertinent to semicrystalline polymers: (A) Organization of polymer chains into ordered lamellae involves the movement of segments within the crystalline (d_c) and amorphous (d_α) regions. Four processes are defined as follows: τ_stem represents the diffusion time of PEO chains in the length scale of the crystalline domain (d_c); τ_lc illustrates the growth time of a unit crystal lamellae; <τ_c> depicts the switching time of two helical defect jumps within the crystal; τ_segmental represents the segmental dynamics within the amorphous regions. l represents the intermolecular distance (l = <a + b>/2; a and b are the crystal unit cell parameters along x and y axes, respectively). (B) Imbibition of polymer chains (τ_imbibition) proceeds via adsorption with a characteristic time τ_adsorption. The anchored units on the pore walls are indicated with the red color. (C) Characteristic time scales and their temperature dependence: τ_imbibition (yellow sphere), τ_adsorption (red dashed line), τ_lc (blue spheres), τ_stem (cyan spheres), <τ_c > (orange spheres), and τ_segmental (green crosses). The original data are provided in table S1. (D) The effect of lamellar orientation with respect to the pore axes on the variable penetration lengths. (E) AFM height (left) and phase (right) images reveal well-oriented crystalline (PCL) lamellae inside nanopores (cyan framed regions). Scale bars, 200 nm.