Sintering polymer nano-crystals at high pressure

Abstract

Semicrystalline polymers, such as polyethylene (PE), polyethylene terephthalate (PET) can crystallize in solvent and molten state. Folded-chain crystals with thickness about 10–20 nm are commonly obtained. These folded-chain crystals contain much more defects in the bulk and surface. They are metastable in thermodynamics for their small size in chain-axis direction and folded-chain surface. These problems hinder us to investigate some structures and properties about polymer crystals [1]. After Wunderlich reported that PE extended-chain crystals with thickness up to 3 μm could be formed under high pressure [2], many polymers have been investigated at high pressure. Extended-chain crystals of some polymers, such as polyamide (PA) [3–5], PET [6, 7] and PVDF [8, 9], were obtained through high-pressure crystallization. Because extended-chain crystals have few folded-chain conformation and their thickness is up to several micron meters. These extended-chain crystals are stable in thermodynamics. Up to know, the thickness of reported largest extended-chain crystals is up to 40 μm [10], but the crystallization time was much longer (200 h). This suggests that some new methods should be introduced to grow larger polymer crystals, which are the basis for investigation and application about the structures and properties of polymer crystals. Sintering is a method commonly used in ceramic. Some big crystals are usually formed during sintering. This is induced by abnormal grain growth [11]. The phenomena should be prevented because a few big crystals will destroy the mechanism properties of ceramic. But this method perhaps can be introduced in the growth of polymer crystals. In this work, sintering method was employed to PET nano crystals. Scanning electron microscopy revealed that PET extended-chain crystals with thickness up to 70 μm were formed only sintered for 6 h. This suggested that sintering is a promise method for the growth of large polymer crystals. An un-oriented commercial PET was used as the original material (Yanshan Petrol. Chem. Co. China). The molecular weight, calculated from molecular weight, was about 18 000. The starting material was firstly heated to 568 K for 10 min at normal pressure, and quickly cooled down to 473 K, then the sample was kept at the conditions for 48 h. This process was termed as pre-crystallization. The crystallized PET sample was broken into small granulates (about 0.5 mm) for hydrolysis. The weight ratio of water and PET sample was