The Synthesis And Crystallization Behavior Of Block Copolymers Containing Double-crystalline Chain Segments

In this paper, block copolymers with double crystallizable segments, polyethylene-b-poly(ethylene oxide) (PE-b-PEO), were synthesized. Crystallization behaviors of PE-b-PEO and another type of block copolymers with double crystallizable segments, poly(adipic anhydride)-poly(2,2-dimethyltrimethylene carbonate) (PAA-PDTC), were studied.Hydroxyl-terminated polybutadiene (HTPB) was prepared by anionic polymerization of butadiene with n-BuLi as initiator and ethylene oxide as terminator. Hydroxyl-terminated polyethylene (HTPE) was obtained by using tosylhydrazide (TSH) to hydrogenate HTPB. PE-b-PEO was synthesized by using terephthaloyldichloride (TPC) as coupling reagent to couple HTPE with PEG. The block copolymers were purified by washing with ethanol and fractionated with hexane. The microstructure, molecular weight and molecular weight distribution of HTPB, HTPE and PE-b-PEO were characterized by H-NMR, FTIR and GPC.The crystallization behaviors of block copolymers with double crystallizable segments, PE-b-PEO, were studied via differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized light microscopy (PLM). In the PE-b-PEO block copolymers, the PE blocks with more branches have lower crystallization and melting temperatures. WAXD results indicated that PE block and PEO block of PE-b-PEO have the same crystal types as the corresponding homopolymers. PLM revealed that existence of PEO block reduces the crystal size of PE block. Comparing crystallization and melting temperatures and enthalpies of PE-b-PEO cooled from different starting temperatures or at different rates, more perfect crystal of PE blocks resulted in lower crystallization and melting temperatures of PEO block in the PE-b-PEO block copolymers with low PEO content, but more perfect crystal of PE blocks resulted in higher crystallization and melting temperatures of PEO block in PE-b-PEO crystallized with high PEO content. The results showed that the previously crystallized PE crystals have two competitive effects, nucleationand confinement, on crystallization of PEO.The crystallization behaviors of another type of block copolymers with double crystallizable segments, PAA-PDTC, were studied via DSC. By comparing crystallization and melting temperatures and enthalpies cooled from different starting temperatures or at different rates, it is observed that the crystallization temperatures and enthalpies of PAA blocks decrease with perfection PDTC crystals. The results showed that the pre-existing PDTC crystals have only confinement effect on crystallization of the PAA block.