A Novel Approach To Fast Crystalline P-oly(Ethylene Terephthalate) By Ion-aggrega-Tion-induced And Heterogeneous Coupled Nucleations

Owing to the inclusion of a rigid phenylene group in its constitutional repeating unit, poly(ethylene terephthalate)(PET) is of slow(homogeneous) nucleation and thus low degree of crystallinity, which results in relatively deteriorated overall mechanical properties as well as poor heat resistance that limit its engineering applications. Currently, there have existed two mechanisms for the modification of PET nucleation:(1) PET chain-segments are adsorbed onto the surfaces of a nucleator to initiate heterogeneous nucleation; and(2) PET chain-segments are induced to nucleate by ionic aggregates. In this thesis, to accommodate the physical essence of Mechanism 2, a new concept of “ion-aggregation induced nucleation” has tentatively been proposed, that is, the polymer chain-segments covalently attached to ion pairs, upon ion aggregation via electrostatic forces, are closely packed in the coronas of the ionic aggregates, which intensely induces the crystalline nucleation of the “crowding” chain segments.In this work, a polyamide(PA) ionomer containing ionic groups in its backbone chains has been added to a PET to produce a multiblock copolymer via exchange reactions: therein, partial PET long-blocks are initiated to heterogeneously nucleate by the nonionic fast-crystalline PA short-blocks; the remaining PET long-blocks are induced to nucleate by the ion aggregation of the ion-pairs at their ends. Due to a coupling of the heterogeneous nucleation and the ion-aggregation induced nucleation, the crystalline-nucleation rate of the PET is improved significantly than if either of the nucleating mechanisms functions independently.A PA-66, a PA-612, and a PA-1212, respectively, were ionized precisely with respect to only 5 mol% of their constitutional repeating units to produce corresponding ionomers(PA-66-I, PA-612-I, and PA-1212-I), which was followed by solution-blending of the PET with the three PA’s and their ionomers, respectively, to prepare modified PETs. It was revealed from differential scanning calorimetry(DSC) that, with respective increases in the contents of PA-66 and PA-66-I(up to 5 wt%), the degrees of melt crystallinity of the modified PETs were both improved monotonously, with the former(i.e., of PET/PA-66) lower than the latter(i.e., of PET/PA-66-I) at the same content, and both higher than the neat PET; these express that both of the PA-66 and the PA-66-I served as a nucleating agent, their optimum contents were both 5 wt%, and, most importantly, the effects of ion-aggregation-induced?heterogeneous coupled nucleations by PA-66-I were stronger than the effect of heterogeneous nucleation by PA-66. A further exploration via DSC disclosed that the degrees of melt crystallinity of the PETs modified by PA-612 and PA-1212, and by their ionomers(i.e., PA-612-I and PA-1212-I), respectively, were much lower than those of the PETs modified by PA-66 and by its ionomer. Hence, there existed an optimum nucleating agent over the types and loading levels of the PA’s and their ionomers investigated, which was the PA-66 ionomer(i.e., PA-66-I) at a content of 5 wt%.Based on the above, the effect of thermal history on the crystallization behavior of the 5 wt%-PA-66-I-modified PET was discussed still by use of DSC: the degrees of crystallinity of the PET, the PET/PA-66(5 wt%), and the PET/PA-66-I(5 wt%) all showed a tendency of decreasing with an extension of their high-temperature(275 oC) annealing time, possibly attributable to the constraint of crystallization due to thermal degradation of the PET or to a decrease in the chain regularity via exchange reactions; in addition, the degrees of crystallinity of the three were all decreased with an increase in the cooling rate, likely as a result of both decreases in temperature and time required for the crystal growth. It was further found by the Mo method(to investigate the non-isothermal crystallization kinetics) that, at the same relative degree of crystallinity, the F(T) values of the PET, the PET/PA-66, and the PET/PA-66-I decreased in turn, indicating again that the coupled nucleations of PET by the PA-66-I were stronger than the heterogeneous nucleation of PET by the PA-66. These superior nucleating effects of the PA-66-I to that of the PA-66 were finally corroborated by the X-ray diffraction(XRD) and polarized optical microscopy(POM) observations.Interestingly, compared with those of the PET base-material(i.e., the as-purchased commercial PET), the overall mechanical properties, as well as the heat resistance, of the melt-processed PET/PA-66 and PET/PA-66-I were both improved to a slight extent; further, it was discovered from DSC that the degree of melt crystallinity of the PET base-material was so high that the PA-66 or PA-66-I melt-blended into it only slightly increased its crystallization rate. This is probably because the nucleator(s), initiator(s) and other impurities preexisting in the PET base-material promoted considerably its heterogeneous nucleation, so that the net nucleating effect of PA-66 or PA-66-I on the PET was not pronounced any more. In contrast, owing to the fact that the aforementioned, laboratory-researched PET, PAs, their ionomers, and modified PETs were all subjected to a dissolution?reprecipitation process to remove any nucleators, the crystallinity of the resulting neat and matrix PETs was sharply reduced compared with that of the PET base-material.