**6. Polylactic acid**

In our laboratory we have been exploring materials for ion permeable separating membranes showing ease of fabrication (primarily electro-spinning), chemical and mechanical stability, and low cost.

Poly lactic acid, an alpha polyester discovered in 1932 by Carothers in DuPont [10] has been the polymer of choice. There are two main routes for the synthesis of PLA, direct condensation of lactic acid, which produce low molecular weight polymers (2,000-10,000). This procedure can be followed by de-polymerization to form a cyclic dimmer intermediate followed by a ring opening polymerization to produce high molecular weight PLA (>100,000). The process is solvent free which represent a synthetic advantages for the scaling up, minimizing waste disposal. An alternative approach to obtain high molecular weight PLA is using chain coupling agents or azeotropic dehydrative condensation (figure 3) [11].

One of the principal advantages of PLA is that the precursor monomer, lactic acid, can be obtained from renewable sources including the fermentation of molasses, potato starch, or of the dextrose from corn. It can also obtained from petrochemical sources, however, due to the Materials and Processes for Ion Permeable Separating Membranes by Electro-Spinning http://dx.doi.org/10.5772/57094 151

**Figure 3.** Synthetic routes for the production of Poly lactic acid

as to provoke this change in resistance is called the percolation threshold (PT). Although the PT for most polymers is small, for PLA at the highest concentration used in this study, the

Besides polypropylene another common separating membrane is a composite layer of polytetra-fluoro-ethylene (PTFE or Teflon) between nylon or polyester layers. Polypropylene is a low-density linear polymer often used for its reasonable mechanical properties and low cost. The PTFE composite (GoreTexTM) [7] relies on the properties of PTFE whose mechanical properties are defined by the rate of the strain applied during its manufacture. If high enough, it produces billions of slit shaped nanoscopic pores per square inch, allowing the flow of ions but impeding the passage of particles or colloids. The polypropylene membranes are often weaved mechanically, but they can be electro-statically deposited by electro-spinning, a process that will be described shortly. Polypropylene is also notorious for its sensitivity to sunlight, in particular the UV part of the solar spectrum as mentioned above, where carbon bonds in their chain structures are attacked by the UV photons. The ultra-violet rays modify the affected bonds to form highly reactive free radicals, which then further react with atmos‐ pheric oxygen forming carbonyl groups in the main chain. These chemical transformations modify the polymer properties, in particular mechanical properties. The device might lose color and surface cracks will appear often leading to device failure. There are other types of membranes being explored for the particular application of electrochemical super-capacitors. Some work on bipolar membranes constructed using ion-exchange membranes (anions) and a cation exchange solution to form the bipolar structure. This proto-membrane is then coated with a NAFION layer. NAFION has been utilized using other formulations [8]. Sulfonated poly (ether ether ketone, SPEEK) has been used, as a proton conducting polymer membrane

In our laboratory we have been exploring materials for ion permeable separating membranes showing ease of fabrication (primarily electro-spinning), chemical and mechanical stability,

Poly lactic acid, an alpha polyester discovered in 1932 by Carothers in DuPont [10] has been the polymer of choice. There are two main routes for the synthesis of PLA, direct condensation of lactic acid, which produce low molecular weight polymers (2,000-10,000). This procedure can be followed by de-polymerization to form a cyclic dimmer intermediate followed by a ring opening polymerization to produce high molecular weight PLA (>100,000). The process is solvent free which represent a synthetic advantages for the scaling up, minimizing waste disposal. An alternative approach to obtain high molecular weight PLA is using chain coupling

One of the principal advantages of PLA is that the precursor monomer, lactic acid, can be obtained from renewable sources including the fermentation of molasses, potato starch, or of the dextrose from corn. It can also obtained from petrochemical sources, however, due to the

agents or azeotropic dehydrative condensation (figure 3) [11].

membrane resistance exceeded tens of MegOhms.

in similar applications [9].

**6. Polylactic acid**

and low cost.

150 Advances in Nanofibers

optimization of cornstarch bacterial fermentation using a Lactobacillus strain, this is now a days the most often employed method to obtained the monomer. Poly-L-lactic acid (PLLA), the principal product of the reaction under controlled synthetic conditions, is chiral, that is, it rotates the polarization plane of light, that is why the L, for levo-rotation. PLLA has a crystal‐ linity of around 37%, a glass transition temperature between 60-65 °C, a melting temperature between 173-178 °C and a tensile modulus between 2.7-16 GPa. Interestingly, heat resistant PLLA can withstand temperatures of 110 °C (230F) [12]. These relatively high temperatures represent an adequate factor of safety for automotive and other traction applications of supercapacitors.

Besides its synthetic and physical properties advantages, PLLA is a biodegradable and biocompatible polymer approved by the FDA for food packaging and implantable medical devices, allowing it use in EDL applications as for example pacemakers.

There are several examples in the literature of PLA and other polymers [13] been combined with other materials to form composites. In a composite, one mixes (or react) two or more materials (phases) with diverse properties, to combine the effect of those properties in the product material or phase. In our continual study of materials for permeable membranes, we have combined the ease of fabrication by electro-spinning fibers formation of polymeric materials, with the enhanced mechanical and electrical properties of carbonaceous allotropes materials, and some metallic colloids such as silver. Although diamond and graphite are the most commonly mentioned allotropic phases of carbon, there are multiple members in this family, from graphene through various types of carbon nano-tubes and bucky-balls, to carbon onions, they span a gamma of physical and chemical characteristics of substantial relevance to super / pseudo- capacitor technology [14]. More on polymer based composites will be presented latter in the chapter.
