Synthesis, Characterization, and Properties Evaluation of New Degradable Materials
New degradable materials having extremely low (-44°C) and extremely high (163°C) glass transition temperature were synthesized by radical chemistry, using various combinations of vinyl acetate (VAc) and α-methylene-γ-butyrolactone (α-MBL) (present in common tulips) monomers with a cyclic ketene acet...
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|Summary:||New degradable materials having extremely low (-44°C) and extremely high (163°C) glass transition temperature were synthesized by radical chemistry, using various combinations of vinyl acetate (VAc) and α-methylene-γ-butyrolactone (α-MBL) (present in common tulips) monomers with a cyclic ketene acetal, 2-methylene-1,3-dioxepane (MDO). Degradability was catered in polymer chains by ring opening polymerization of the cyclic ketene acetal.
Firstly, synthesis of P(VAc-co-esters) was carried out with an aim to make degradable materials based on MDO and VAc units using radical chemistry. Radical ring-opening copolymerization of 2-methylene-1,3-dioxepane with vinyl acetate in presence of AIBN initiator at 70oC was carried out to achieve the aim. Complete ring opening of MDO was observed during copolymerization which introduced degradable PCL repeat units onto the C-C backbone of poly(vinyl acetate). Microstructure analysis of the copolymers was done using different 1D and 2D NMR techniques. Reactivity ratios were found out by Kelen Tüdos method and were rVAc = 1.53 and rMDO = 0.47 leading to statistical introduction of ester linkages onto the polymer backbone. The materials showed varied glass transition temperatures (from 37 to -44oC) depending upon the amount of ester linkages and very high elongations. Compared to 6.5 x 10-3 GPa modulus and 1007% elongation at break of PVAc, a P(VAc-co-ester) with 5% ester incorporation gave 5.1 x 10-3 GPa modulus and 1093% elongation, while 18% ester incorporation led to 2.9 x 10-3 GPa modulus and 1285% elongation. The hydrolysis products were also tested for cytotoxicity studies in L929 cells and compared with that of known and accepted non-toxic materials like poly(ethyleneimine). The hydrolysed products were non toxic and showed a cell viability > 95%. Keeping in view the combined properties like degradability, non-toxicity and low glass transition temperatures, the resulting materials could therefore be proposed for different applications like degradable gums, coatings etc.
Additionally, a successful effort of synthesizing degradable materials, which could be used for various applications, was carried out. Due to the high softening temperatures (as high as 163°C), these materials were proposed to be used as thermoplastics that would not only be eco-friendly due to their ability to be easily degraded to harmless substances, but would also be useful due to their origin from a plant source. Polymerization of cyclic ketene acetal MDO with a naturally occurring monomer vis-à-vis α-MBL, was carried out to achieve the target. Surprisingly, the two monomers reacted even in absence of any radical initiator at 70°C and above. Complete ring-opening of MDO did not take place, and some ring-retained structures were also present. On investigation, the reaction under consideration was found out to be initiated via charge transfer between nucleophilic double bond of MDO and electrophilic double bond of MBL, and was thereafter proceeded by a mixed mechanism of radical and ionic. The microstructure of the obtained polymers was blocky-random, wherein blocks of MBL and polyester or polyacetal of unequal sizes was obtained. Radical mechanism yielded mostly blocks of MBL and polyesters, whereas ionic mechanism yielded mostly blocks of MBL and polyacetals. It was possible to monitor the mechanism and hence the microstructure of the materials by varying reaction conditions like temperature, monomer feed, and time. Due to the addition of radical initiator, reaction was allowed to undergo mostly via radical mechanism, thereby yielding P(MBL-co-esters).
PMBL being itself degradable due to ester group present in it, not only enhances its degradability due to the incorporation of ester linkages of cyclic ketene acetal units, but also enhances its processability by increasing the softness and thereby decreasing the glass transition temperature from 195°C to as low as -52°C. The copolymer having 47% of MDO units was found to give 9.9% elongation at Fmax and was also found to be elastic, while the sample that constituted 34% MDO gave 5% elongation at Fmax and was found to be brittle. The synthesized materials were also tested for base hydrolysis (5% wt. NaOH in methanol) overnight, and were found to be degradable.
Degradable, eco-friendly, natural source originated, materials were finally synthesized.which could be used for various applications|