Molecular design of green plasticizers for plastics

There is a growing concern for the toxicity of industrial polymer plasticizers to humans. Especially, phthalate derivatives are very common plasticizers and exhibit toxic effects by degradation. These plasticizers are particularly widely used in the processing of polyvinyl chloride (PVC). Health concerns were raised in the beginning of 1980s. Later then, experiments on rats were shown that phthalate exposure had the environment and health risks, but uncertainty is still carrying on. Even though there has been no certain data relevant to human health because experiments were conducted on animals, phthalate derivatives are banned from using some products such as toys. On the other hand, biobased polyesters have attracted much attention due to their biodegradability and biocompatibility that offer clear advantages for both customers and environment. Polylactic acid (PLA) is an attractive candidate for replacing petrochemical polymers because it is from renewable resources. However, PLA needs to improve some properties by an inclusion of additives to use widely in industrial scale. In our study, various bio-based molecules with potential to replace phthalate derivative plasticizers for PVC and PLA have been modeled by employing atomistic scale simulations. Our approach depends on molecular dynamic simulations to determine polymer/plasticizer interactions and observe plasticization effects via the lowering of glass transition temperature (Tg). Structural stability was evulated by solubility behavior of plasticizers in polymer and to describe Tg values and volumetric behavior of polymer/plasticizer mixtures, pressure-volume-temperature (PVT) relationship was used. Selected bio-based molecules including levulinic acid esters, succinic acid esters, oleate esters, dioctyl adipate and for comparison one of the phthalate derivative and acetyl tributyl citrate were constructed with TRAPPE-UA force field, were optimized and their densities similar to literature were obtained. Then, the atactic PVC homopolymer chains consisting of 150 monomers with equal fractions of meso and racemic diads distributed randomly along the chain were constructed based on the Generalized Amber Force Field (GAFF). Amorphous PLA model was constructed as PLLA consisting of 150 monomers with PLAFF3 force field. Tg and densities of pure PVC and PLA were determined as 344 K and 1.38±0.01 g/cm3 for PVC, 357 K and 1.211±0.01 g/cm3 for PLA. Polymer/plasticizer structures contained 10% to 30% plasticizer by weight to predict the performance of the plasticizers. Biobased derivative esters, including levulinic acid esters and succinic acid, were also constructed with different substituents in the ester groups to observe chain length effect on plasticization. The variation of glass transition temperatures the polymer was investigated and compared as a function of plasticizer type and content by simulating PVT relationship and stability of the polymer/plasticizer mixtures were compared by calculated solubility values. The compatibility of plasticizers was calculated through the cohesive energy densities of the systems and Flory Huggins interaction parameters were obtained by interpolation from energy-volume fraction plots.