Solution-Spun PLLA

Solution-spun fibers are also drawn to improve their mechanical properties. The various factors that influence these properties are discussed below. 

FIGURE 20.17 Dependence of the tensile strength on the draw ratio for two samples of PLLA with different molecular weights drawn at indicated temperatures, (a) My = 53 x 10 , 200.5°C , 196.5°C. (b) My = 3.5 x 10 , 180°C , 190°C (adapted from Ref. 1 with permission from Elsevier Ltd.). 

R Matsuoka, S. Miyazaki, N. Matsuo, Poly(lactic acid) fibers with good softness, biodegradability and tensile strength and manufacture thereof, JP 2001098417, 2001, CAN 134 267670. 

NatureWorks Polymer Processing Guides—Crystallizing and Drying of PLA. NatureWorks LLC, Minnetonka, MN, 2007. 

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

Preparation of Dope There are two approaches for making the dope solution first, by the direct dissolution of PLLA in the solvent mixture and second, by first dissolving PLLA in chloroform followed by addition of toluene. However, in the first case, the as-spun filaments obtained have lower tenacity (1.35 GPa) and limited drawability (maximum draw ratio achievable, /L = 8). Apparently, the dissolution of PLLA in the solvent mixture does not take place readily in the first case. Also, prolonged heating of the spinning solution can result in degradation of the polymer. Therefore, the second approach is more desirable. Also, in the second... 

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 solution dry spun at the optimum solvent composition exhibits low elasticity or low extent of extrudate swell at the spinneret. This allows easier extension of the spun fluid into a finer denier during spinning. 

Table 5.3 Heat of fusion of the as-sp m fibers, maximum draw ratio, diameter of drawn fiber and tensile strength of PLLA filament, spun from 4 wt% solutions in chloroform/toluene (40/60) into surroundings at various temperatures [38]...

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