Factors Affecting Solution Spinning

1 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 

2 Composition of the Binary Solvent The composition of the binary solvent system used for making dope solution has been shown to have a great influence on the structure and properties of the fiber. The values of maximum draw ratio and tenacity are found to be influenced by the quality of solvent used for dry spinning. Draw-ability of the fibers increases at an optimum intermolecular entanglement of the polymer chains, which occurs at a particular solvent composition of 0 = 0.6 (40 60 chloroform toluene). 

FIGURE 20.12 Effect of chloroform/toluene ratio in the PLLA My = 9 X 10 ) spinning solution (4% (w/v), 60°C) on the fiber drawability at 204° C and subsequent tensile strength of the filaments. Curve a 0 = 0 (chloroform), curve b 0 = 0.5, curve c O = 0.6, curve d O = 0.7, curve e O = 1 (toluene) (adapted from Ref. 25 with permission from Elsevier Ltd.). 

The ultimate tenacity (Gb) of the fibers prepared shows a pronounced maximum of 2.1 GPa at a toluene volume fraction (0) of 0.6. The drawability and tenacity decrease on either side of the binary solvent composition. 

Optimum entanglement density is obtained at the above solvent composition. Both at lower and higher contents of poor solvent, the entanglement density becomes high and results in poor drawability of the spun filaments. Also in the second case, when poor solvent is in higher concentration, reduction of solvent quality hampers appropriate dissolution of PLLA and may also lead to formation of intramolecular chain entanglements and phase separation. 

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To get a deeper insight into the factors which affect the electronic structure of hemopro-teins, systematic studies on heme model compounds have been undertaken by Scholes and coworkers79 247 255. The model systems used by these authors are protohemin, deuterohemin and ferric tetraphenylporphyrin with a series of axial ligands. All the published ENDOR data on high-spin heme model compounds were obtained in frozen solutions with B0 along gi = 2 which is oriented normal to the porphyrin plane. A 1 1... 

A variety of relaxation time studies have been performed on toluene. The choice of deuterated toluene avoids certain complicating factors which affect proton NMR studies, such as, dipolar or spin-spin couplings. The dominant relaxation mechanism is quadrupolar and the relaxation times are determined by the reorientation of the C-D bond vector. Relaxation times such as T, are sensitive to the motions of the solvent around the larmor frequency, which is on the order of 14 MHz in this study. T2 measurements may probe slower motions if the molecule undergoes slow and/or anisotropic motion. The relaxation time results presented in Figure 3 are significantly shorter than those found in bulk toluene solutions (18.19). In bulk toluene, the T and T2 values are equal above the melting temperature (1.2.). In this polymer system T2 < T indicative of slow and/or anisotropic reorientation. 

Fig. 1 Nanofiber modification strategies. In step 1, many polymer and solution properties will affect the size, stability, and reactivity of nanofihers in the final mat, including multipolymer blending strategies, incorporation of soluble factors/drugs/spheres, and/or loading concentrations. In step 2, during spinning, the ambient conditions and voltage applied must be tailored for each polymer source and can be dictated by the nature of the collector or the number of jets employed simultaneously. In step 5, the as-spun mat can be modified by various crosslinking techniques to stabilize the polymers or, in the case of soluble or carrier polymers, these can be leached out by subsequent liquid washes...

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