Block copolymers: Copolymerization of ethylene terephthalate and polyoxyethylene glycols
Authors: Coleman D. 
Published: April 2003

The modification of the amorphous/crystalline ratio in polyethylene terephthalate by copolymerization with glycols or dibasic acids usually results in a lowering of the degree of crystallinity and of melting point, and impaired physical properties. A consideration of the molecular constitution of silk fibroin suggests a way of modifying the molecule to produce an increase in the amorphous content of the polymer with retention of the crystallite structure. In the silk fibroin molecule it is significant that two lengthy segments (about 170 A. long) as contrasted with the rest of the molecule carry bulky side chains which prevent their entry into the crystalline regions of the fiber. The fact that these segments are accommodated in the amorphous regions suggested by analogy that one or two linear molecules of 100–200 A. in length might be successfully incorporated in the polyester molecule with a probable improvement in such physical and chemical properties as flexibility and dyeability. The modifying component must have reactive end groups, be thermally and chemically stable under polymer‐making conditions, and should have water‐sensitive groupings. Polyoxyethylene glycols (Carbowaxes) of molecular weight 1000–6000 satisfy these conditions and with polyethylene terephthalate give high melting “block copolymers” that can contain as much as 30% by weight of the modifying component. The dyeability, moisture regain, and flexibility (measured by determination of the second order transition point) are all materially increased, while at the same time the tenacity and extensibility are unaltered. Since the molecular proportion of the modifying component is small (less than 2% of the repeating units are substituted), there is only a small reduction in melting point. This indicates little alternation in the crystallites, a fact confirmed by the substantially unaltered x‐ray photographs. That the copolymers are not blends but are true compounds is shown by the failure to remove the modifier by continuous extraction with solvents and by the incompatibility of polyoxyethylene glycols and molten polyethylene terephthalates. Absence of fragmentation of the modifying components on entering the polymer chain is shown by its almost quantitative recovery from the alkaline hydrolyzate of the copolymer. Despite their excellent physical properties these copolymers are not technically attractive as fibers because of their instability to ultraviolet light (and to strong sunlight). Furthermore, exposure of the dyed fibers to ultraviolet light shows that the rate of fading of dye has been seriously increased by the introduction of the third component into the molecule. Polyoxyethylene glycols may also be incorporated as side chains in the copolymer molecule to give similar high‐melting copolymers with improved dyeability and retention of physical properties. Copolymers of polyoxydecamethylene glycol and polyethylene terephthalate show good physical properties but no improvement in dyeability, probably because the “blocks” introduced are deficient in ether‐oxygen atoms and hence lacking in hydrophilic character. It is suggested that this novel type of copolymer is probably fairly widespread in the field of proteins, and is not confined to silk fibroin. For example, some of the physical properties of collagen (and of gelatin) are best explained by assuming that certain chain segments, because of their amino‐acid composition, have a strong crystallizing tendency.