PLLA fibers

Figure 13 shows that the diameter of the electrospun HM-PLLA fibers was not signifieantly changed with varied volmne feed rate. The influenee due to the volrune feed rate also diminished when the polymer concentration is low. 

He LM et al (2010) Synergistic effects of electrospun PLLA fiber dimension and pattern on neonatal mouse cerebellum C17.2 stem cells. Acta Biomater 6(8) 2960-2969... 

Schofer, M.D., et al., 2009. Influence of nanofibres on the growth and osteogenic differentiation of stem cells a comparison of biological collagen nanofibers and synthetic PLLA fibers. Journal of Materials Science-Materials in Medicine 20 (3), 767—774. 

Figure 3.23. Epitaxial crystallization of iPP on the PLLA fiber at 415K. [Adapted, by permission, from Liu, K Jin, M La, R Zhang, J Wang, T Zhang, X, Mater. Lett., 125, 209-12, 2014.]...

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. 

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. 

FIGURE 9.13 First and second DSC scans of as-spun PLLA fibers produced by solution spinning (5% chloroform solution). 

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. 

Because the thermal history the PLLA experiences during the melt spinning process causes a decrease of number-average molecular weight of the polymer, it also leads to lower orientation and crystallinity in PLLA fibers. As a result, the longer the period of applied thermal history, the lower are the tensile strength and modulus of the fibers [22]. 

Since the tenacity of PLLA fibers increases with the draw ratio and the draw ratio is dependent on the solvent composition, the final mechanical properties of the fiber are dependent on the solvent composition that was maintained while preparing the dope. Figure 20.12 depicts the tenacity of PLLA fibers (My = 9 x 10 ) spun from 4% (w/v) polymer solutions in various chloroform/toluene mixtures at 60° C and subsequently hot drawn to different draw ratios (1) at 204°C. 

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... 

FIGURE 20.18 Effect of the viscosity-average molecular weight (My) on the tensile strength of wet spun PLLA fibers. Fibers were drawn at optimum temperatures to maximum draw ratio (adapted from Ref. 1 with permission from Elsevier Ltd.). 

LLA (90 10), which were launched in 1970 and 1974, respectively [14, 15]. PDS sutures were also developed in 1981. Because PLLA fibers degrade very slowly, they are not suitable for sutures. However, in applications that require long retention of the strength, PLLA fibers are the preferred material. These include ligament and tendon reconstruction, and stents for vascular and urological surgery. 

PLLA fibers are also utilized in the form of biodegradable stents in cardiovascular and urological surgery. Some designs of stents are shown in Figure 27.3. 

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