Abstract
Two isomeric molecules, 1,2-cyclohexanediol and isohexide (isosorbide and isomannide) demonstrated potential as rigid building blocks in PLA plasticizer design with tunable material performances. Six plasticizer candidates were synthesized via Fischer esterification in bulk of the rigid isomeric cores and green platform chemicals levulinic acid and valeric acid. The structures were confirmed by 1H NMR and ESI-MS. Based on the calculated Hansen solubility parameters, all the synthesized plasticizer candidates were expected to be miscible with PLA, which was experimentally proven by a significant decrease of glass transition temperature (Tg) and an increase in strain at break. For instance, PLA plasticized with 20 wt% cyclohexanediol levulinate (cis- and trans- mixture) portrayed the lowest Tg of 25 °C and the highest strain at break of 265%, which equals to 44 times of the initial strain at break of neat PLA. Isohexide-based plasticizers with larger rigid cores, isosorbide levulinate and isomannide levulinate enabled superior thermal stability, higher Young’s modulus and stress at break in PLA blends compared with cyclohexanediol-based plasticizers, while retaining high strain at break. Stronger influence of stereoisomerism on plasticization was also observed in PLA blends with isohexide-based plasticizers compared with cyclohexanediol-based plasticizers. Here we present a new pathway to tailor the performance of plasticizer by utilizing isomeric rigid building blocks.
Highlights
The structure of a plasticizer intrinsically determines the plasticizing performance and plasticizer design is a matter of priority
The two families of esters derived from different stereoisomers of isohexide or 1,2-cyclohexanediol and levulinic acid or valeric acid were successfully synthesized through Fischer esterification in the presence of a protonic catalyst, Fig. 1
To verify the chemical structure, the six plasticizer candidates were characterized by 1H and correlation spectroscopy (COSY) Nuclear magnetic resonance (NMR)
Summary
The structure of a plasticizer intrinsically determines the plasticizing performance and plasticizer design is a matter of priority. Isomers are constituted of the same atoms yet with different arrangements of them and can be divided into two groups: constitutional isomers and stereoisomers. The former differs in chemical structure while the latter only vary spatially. A representative example of constitutional isomers in plasticizer industry is the phthalate family. Isomer distribution is generally created in the carbonylation process in which alcohols are synthesized, and after esterification with phthalic acid, pairs of isomeric plasticizers are formed, e.g. di-2ethylhexyl phthalate and diisooctyl phthalate [7]. Since the alkyl chains of the plasticizer interact with the PVC chains, the constitutional isomeric phthalates with various alkyl chains do not provide the same performances in plasticized PVC materials [8]. It has been observed that at a set number of alkyl carbons, when the degree of branching in alkyl chain of plasticizer increased, the plasticizer compatibility increased while the plasticizing efficiency and plasticizer mobility decreased [9]
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