Structure And Properties Of PLLA/PDLA-PEG-PDLA Blends And Fibers

Poly(lactide)(PLA) has excellent biological compatibility and biodegradable property and it is an environmental friendly material. However, the poor flexibility and slow crystallization rate limit its applications and usually the modifications were conducted to improve its properties. In this dissertation, the triblock copolymers of PDLA-PEG-PDLA were employed to blend with pure PLLA to improve its properties. The effect of the content of PDLA-PEG-PDLA, the PDLA segment length and the PEG segment length in the triblock copolymer on its properties was investigated systematically. The main work and conclusions were summarized as follows:1. The structure and properties of PLLA/D50450 and their fibersPLLA/D50450 blends and their fibers were prepared by melt blending and two-step melt spinning respectively. Their crystallization behavior, thermal properties, orientation and mechanical properties were investigated by differential scanning calorimetry(DSC), thermogravimetry(TG), capillary rheological, scanning electron microscope(SEM), wide-angle X-ray scattering(WAXS) and mechanical tests. The results showed that melting temperature has a significant impact on the crystallization behavior of PLLA/D50450 blends. When the content of D50450 in the blends is 10%, the highest crystallization temperature for ? form crystalline is observed at about 127 oC upon cooling from 230 oC. In addition, PLLA/D50450 blends showed better spinnability than pure PLLA, which was demonstrated by the higher crystallinity and orientation of PLLA/D50450 fibers. When the content of D50450 was 2 wt%, the crystallinity and orientation of the fibers reached up the highest value of 27% and-0.39, respectively. However, the modification of PLLA by blending triblock copolymer has little effect on their thermal stability and mechanical properties.2. Effect of PDLA segment length and PEG segment length of copolymer on the structure and properties of modified PLLAThe effects of different PDLA segment length and PEG segment length on the crystallization of modified PLLA were investigated systematically. The results showed as follows:(1) Blending the D121 and PLLA can significantly improve the elongation at break of modified PLLA at room temperature and changed the brittle to ductile. With the increase of the PDLA segment length(D525), the content of the stereocomplex composite in the blends increased, which resulted in the brittle fracture of modified PLLA when stretching at room temperature;(2) With PEG segment length(D525, D545) increasing, the PLLA/D525 and PLLA/D545 blends showed brittle fracture when stretching at room temperature. In contrast, when the stretching temperature were improved to 50 oC or 80 oC, the elongation at break and breaking stress of both two PLLA blends were higher than that pure PLLA. In addition, the properties of PLLA/D545 were better than PLLA/D525;(3) PLLA / D525 and PLLA / D545 blend fibers were prepared by melt-spinning and post-drawing two-step method. The fiber had only ? crystal and no stereocomplex, and the tensile strength of the fiber was low. The spinning process of PLLA / PDLA-PEG-PDLA blends needs further exploration.