Studies On Poly(Ether-ester) Copolymers And Carbon Nanotube Composites

Poly(trimethylene terephthalate)(PTT) is a new semicrystalline polymer and its melt crystallization behavior has been attracted wide attention. So far, there have been extensive reports on melt crystallization of pure PTT. PTT blend, and PTT random copolymer. However, there is so little research about melt crystallization behavior and morphology of PTT copolymer with regular structure. In the present study, we synthesized single and double crystalline poly(trimethylene terephthalate)-polyethylene glycol copolymers (PTGs) by selecting various molecular weight PEG as the second structure unit, according to a traditional two-stage melt transesterification and polycondensation. We investigated their phase morphologies, crystallization behaviors and morphologies. In addition, we also prepared PTG/multiwalled carbon nanotube composites (MWCNTs) by in situ polymerization, and investigated the influence of MWCNTs on crystallization, rheology, mechanical, as well as electrical properties of the composites.The two kinds of single crystalline PTGs were synthesized by melt polycondensation and selecting PEG with Mn,PEG=300～4000g/mol as the second structure unit. The one is that there is an almost identical PEG concentration but different in PEG molecular weight in PTGs. The results of small angle x-ray scattering (SAXS), proton nuclear magnetic resonance (1H NMR), atomic force microscope (AFM), as well as transmission electron microscopy (TEM) showed that the higher the molecular weight of PEG, the longer the average sequence length of PTT blocks, and the more obvious the microphase separation. Consequently, the melt point (Tm) of PTGs increases with increasing PEG molecular weight. Polarized optical microscopy (POM) showed that all PTGs can form ring banded spherulites in a certain temperature range, and the band spacing decreases with increasing PEG molecular weight. The second is that the PEG molecular weight in PTGs is the same but the content is different. The results of1H NMR and differential scanning calorimetry (DSC) suggested that the average sequence length of PTT blocks decreases with increasing PEG content. As a consequence, Tm of PTGs decreased with increasing PEG content and the band spacing of spherulites increasing with increasing PEG content at certain Tc. The nonisothermal crystallization morphology of PTGs was also investigated. And the crystal morphology has a strong dependent on cooling rate (φ). When the φ is equal to or lower than2.5℃/min, the spherulite is ringless. When the φ is equal to or higher than20℃/min, the crystal morphology is composed of non-banded spherulite (central part) and ring-banded texture with decreasing band spacing along the radial growth direction. Moreover, the size of non-banded spherulite (central part) reduces with increasing cooling rates.Double crystalline PTGs were synthesized by using PEG (Mn,PEC=10000g/mol) as the second glycol. The phase structure of PTGs was investigated by TEM, which showed that there was an increasing phase separation as PTT content increases. POM revealed that the band spacing decreased with increasing PTT content when crystallized at an identical Tc. The sequential crystallization behavior of PTGs was confirmed by using in situ wide-angle X-ray diffraction (WAXD). The morphological change was traced by using in situ POM. and the results showed that spherulitic morphology formed by the advance crystallization of PTT chains had no change after PEG crystallization. And only a subtle change of quadrant tones was detected.PTG/MWCNTs composites were successfully prepared via in situ polymerization. TEM observation demonstrated that the dispersion of MWCNTs in matrix was greatly improved after MWCNTs were grafted with PEG The single MWCNTs was evenly dispersed in PTG matrix up to1wt%MWCNTs loading. DSC and POM results confirmed that MWCNTs can act as an effective heterogeneous nucleating agent, and the effects of MWCNTs on crystallization and melting of PEG blocks were more pronounced than on those of PTT blocks. The percolation threshold for the rheology was around0.5wt%and the conductivity was approximately1wt%, respectively.