Melt-Spun PLLA

PLLA fibers with molecular weights of 1.8 x 10 and 2.6 X 10, which were spun from the melt, reached maximum tensile properties at a drawing temperature of about 110°C and a draw ratio of 8. This temperature was found to be the optimum drawing temperature. The fiber sample with of 1.8x10 had a tensile strength of 0.5 GPa, an initial modulus of 6 GPa, and an elongation at break of 25%. The sample with Mv of 2.6 X 10 had a tensile strength of 0.48 GPa, an initial 

TABLE 20.6 Extrusion Conditions and Finai Properties of Fibers as Reported in the Literature 

Initial (xlO-3) Extrusion Temperature i°C) Collection Speed (m/min) Nozzle Diameter (mm) As-Spun Fiber CrystalUnity (%) Final (xl0 ) Drawn Fiber Diameter (pm) Fiber Strength Modulus (GPa) References 

FIGURE 20.14 Stress-strain curves of the high-speed spun PLA fibers (adapted from Ref. 3 with permission from John Wiley Sons, Inc.). 

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FIGURE 9.11 Crystallinity of melt-spun PLLA fiber drawn at 160°C (O) and crystallinity of solution-spun PLLA fiber drawn at 190°C ( ) as a function of draw ratio. 

FIGURE 9.12 DSC curves of melt-spun PLLA fibers collected at different rates, as indicated. Reprinted from Ref. 76. Copyright 1997, with permission from Elsevier. 

Data for melt-spun PLLA fiber, d = average fiber diameter (nm). Source Inai et al. 2005b. 

The drawing temperature is selected above the Tg in order to modify the amorphous phase in the rubbery state the initial crystallinity, the nucleation and growth rate according to the crystallization regimes I-III, the polymer chain mobility, and the polymer molecular weight are important factors for optimization of fiber orientation and crystallization. Typically, PLLA melt-spun fibers after drawing at 160°C reach a crystallinity of about 50-70% [24, 80]. 

At the same time, the relatively high molecular weight (between 300 and 900 kDa) inhibits the development of high crystallinity due to the lower polymer chain mobility [78,81, 82]. Figure 9.14 shows the first and second DSC scans of as-spun PLLA fibers produced from dry spinning. Low Tg due to the residual solvent that acts as a plasticizer, and low and Tjn were observed in the first scan. In the second DSC scan after a mass loss of about 15% attributable to solvent remotion, a Tg of 65°C and crystallization and melting peaks at 140 and ISO C, respectively, with a crystallinity of about 25%, were observed. 

Nishimura et al. (2005) produced PLLA fibers using a melt spinning process. PLLA was first dried in vacuum at 70 °C, melted and extraded at a melt temperature of 220 °C in order to avoid thermal degradation. Pellets were extruded with a single-screw extruder, at the end of which a spinning nozzle with 12 holes was placed. Extrusion took place in a water bath kept at 45 °C, and spun fibers were drawn at 98 °C in order to have enough energy to achieve solvent evaporation. The mechanical and other properties of the PLLA fibers were preserved. 

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