Crystallization of poly( l ‐lactic acid) in solution with the mosquito‐repellent N , N ‐diethyl‐3‐methylbenzamide

Abstract

Poly(l‐lactic acid) (PLLA) can be used as a carrier for the mosquito repellent N,N‐diethyl‐3‐methylbenzamide (DEET). PLLA dissolves in DEET at elevated temperature and crystallizes on cooling to below the concentration‐dependent equilibrium melting temperature, leading to scaffold formation. In this work, non‐isothermal and isothermal crystallization experiments were performed, using differential scanning calorimetry and polarized‐light optical microscopy. Crystallization of PLLA in solution with DEET is faster than melt‐crystallization of neat PLLA, with the maximum crystallization rate decreasing with decreasing PLLA content in the investigated range from 5 to 50 m% PLLA. The decrease of the maximum crystallization rate with increasing solvent content is due to decreases in both the maximum crystal growth rate and the nuclei density. The observed downward shift of the temperature range of crystallization is caused by the depression of equilibrium melting temperature and a strong decrease of the glass transition temperature, both occurring with increasing solvent concentration. It is assumed that the strong decrease of the glass transition temperature due to the presence of DEET, and the related increase of the mobility of PLLA chains, is the main reason for the increased crystallization rate compared to melt‐crystallization of PLLA. It is demonstrated that a large variety of spherulitic superstructures can be obtained by variation of the crystallization conditions, presumably leading to largely different 3D scaffold structures and DEET‐delivery characteristics.

Melt-Spun Poly(D,L-lactic acid) Monofilaments Containing N,N-Diethyl-3-methylbenzamide as Mosquito Repellent

“…This was a result of the high D-unit content of 11.3% of the present PDLLA as the crystallization of PDLLA with a 4% D-unit content was only suppressed at 3 K·min −1 . In theory, even a higher cooling rate would be required to suppress crystallization of PDLLA in the presence of DEET [ 24 ].…”

“…In contrast, the incorporation of DEET in PLLA has been shown to increase the rate of crystallization [ 23 , 24 ] and the maximum degree of crystallinity of PLLA [ 25 ]. For example, crystallization of pure PLLA could be suppressed at a cooling rate of 5 K·min −1 , but not in a 50 wt.-% DEET solution [ 26 ].…”

Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands

“…Nevertheless, the mismanagement of waste in Africa is already impacting human and environmental health [ 5 ]. Therefore, the present study and other associated investigations [ 6 , 7 , 8 , 9 , 10 , 11 ] opted to investigate poly(lactic acid) as an alternative polymer matrix for repellent incorporation.…”

“…PLA is a fully biodegradable, environmentally friendly polyester derived from renewable resources [ 9 , 12 , 13 , 14 ]. The lactic acid-based repeat unit contains carbon atom that is stereogenic.…”

“…The incorporation of mosquito repellents into PLA was studied previously [ 6 , 7 , 8 , 9 , 10 , 11 ]. Di Lorenzo and Longo [ 19 ] prepared PLLA/DEET blends, containing up to 4.5 wt.% of the repellent, via melt extrusion.…”