ABSTRACT:Morphology control methods of aromatic polymers in concert with polymerization reaction are described in this review. Many attractive methods have been developed including reaction-induced phase separation of oligomers, epitaxial polymerization, shear-induced chain orientation during gelation, template-based polymerization, suspension polycondensation, solid-state polycondensation and liquid-liquid interfacial precipitation and so on. Among them, the reaction-induced phase separation of oligomers, in which there are two modes of crystallization and liquidliquid phase separation, possess practically high potentiality to create novel polymer materials since it can control not only the morphology but also the molecular chain orientation. Ultimate goal is the embodiment of the nature system in a flask, that is, the control of the materials from primary structures to super structures. The morphology control methods by using polymerization reaction afford the valuable methodology for the three-dimensional structure architecture.[doi:10.1295/polymj.PJ2006083] KEY WORDS Morphology / Phase Separation / Oligomer / Whisker / Microsphere / Epitaxy / Aromatic Polymer / Morphology of polymer materials is of great importance as well as the molecular chain orientation for the creation of new functional and high-performance materials possessing essential properties predicted from their molecular structures. From the both aspects of molecular chain orientation and morphology, one of the ideal materials is a needle-like single crystal of extended molecular chains called polymer whisker. Especially, wide variety of polymeric nanomaterials is required for supporting nanotechnology and the more precise controls are rapidly necessitated in these days. Hence, the demand for the morphology control method increases significantly. Usually, extended molecular chain crystals are difficult to obtain by conventional processing methods and the special processing techniques have been developed thus far.1 There exists another stream for the development of extended molecular chain crystals by means of crystallization during polymerization. The needle-like crystals of both flexible and rigid polymers have been obtained by the solid-state polymerization of the monomers and the polymerization in the solution. Examples of the former polymerization have included oxacyclobutanes such as 1,3,5-trioxane, 2,3 diolefins undergoing four-center type photopolymerization 4,5 and monomers with conjugated triple bonds such as substituted diacetylenes.6,7 On the other hand, the first polymer whisker was obtained by the cationic ring-opening polymerization of 1,3,5-trioxane. [8][9][10] The obtained polyoxymethylene whiskers were 1-2 mm in width and 10-50 mm in length. The molecular chains aligned along the long axis of the whisker. They were brought into practical uses with its unique morphology and good mechanical properties.11,12 However, the polyoxymethylene whisker has a melting temperature at 180 C and the low melting temperature limits the broader c...
Preparation of poly(p-oxycinnamoyl) (POC) microspheres was examined by the reactioninduced phase separation of oligomers during the polycondensation of (E)-4-acetoxycinnamic acid in various solvents. Polymerization in liquid paraffin yielded the microspheres having a smooth surface at 270-330 °C, of which the average diameter was 4.6 µm. In contrast to this, the morphology of the products was drastically changed in Therm S 800 (TS8) and Therm S 900 (TS9) by polymerization temperature. When the polymerization was carried out at 310 °C in TS8 and 300 °C in TS9, the microspheres were obtained, of which the average diameter was 3.9 and 3.3 µm, respectively. The platelike crystals were formed under these temperatures, of which thickness was approximately 0.1 µm. The formation of the microspheres was highly related to the miscibility between the oligomers and the solvents. The average diameter of the microspheres increased with the decrease of the miscibility, and this tendency could be accounted for by the interfacial tension between two immiscible liquid phases. The microspheres were formed through the formation of the microdroplets by the liquid-liquid phase separation of the oligomers and the subsequent solidification of the microdroplets due to the further polymerization in them.
The polymerization of poly(p-oxybenzoyl) (POB) from p-acetoxybenzoic acid was examined in the presence of three kinds of aromatic monofunctional compounds such as p-hexyloxybenzoic acid (HOBA), p-decyloxybenzoic acid (DOBA) and p-octadecyloxybenzoic acid (OOBA) under unstoichiometric polymerization conditions. Polymerizations were carried out in liquid paraffin at 320 °C. POB crystals were obtained by reaction-induced crystallization of oligomers. The obtained POB crystals were found to be consisted of high molecular weight polymers compared with the products obtained by the melt polymerization. Polycondensation proceeded under unstoichiometric condition through following mechanisms; When DPn of oligomers exceeds a critical value, they are crystallized to form the crystals. End-free oligomers are preferentially crystallized due to the lower solubility over end-capped oligomers. End-capped oligomers are also crystallized, and polycondensation proceeds with eliminating end-capping groups of oligomers by trans-esterification just when they are crystallized (mechanism 1). Though a part of the end-capped oligomers are contained in the crystals, end-capping groups are excluded by solid-state polycondensation (mechanism 2). The mechanism 1 is major path in the system of DOBA and OOBA due to the bulkiness of oligomer end groups, and both mechanisms 1 and 2 occur in the system of HOBA.
Composition control of copolyester was examined by using hydrodynamically induced phase separation, during solution polymerization, of p-acetoxybenzoic acid and m-acetoxybenzoic acid (m-ABA). Polymerizations were carried out in liquid paraffin at 1.0% of concentration under shear flow. The polymers were obtained as precipitates formed via phase separation of oligomers and the subsequent polymerization in the precipitates. When the feed ratio of m-ABA (χ f ) was 20 mol %, the molar ratio of m-oxybenzoyl moiety in the precipitated polymers (χ p ) was 5.3, 3.1, and 2.3 mol % at a shear rate (γ) of 0, 147, and 489 s -1 , respectively. The values of χ p were much lower than those of χ f , and shear flow lowered the value of χ p ; thus, controlling shear flow influenced both the reaction rate of the monomers and the miscibility of the oligomers. It especially enhanced the difference on the miscibility between homo-oligomers and co-oligomers. This change in the miscibility by shear flow brought about the more rapid precipitation of homo-oligomers than co-oligomers, leading to the selective formation of poly(p-oxybenzoyl).
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