Abstract
In this study, ABA triblock copolymers consisting of poly(L-lactide) (PLLA) A block and poly(2-ethyl-2-oxazoline) (PEOz) B block are synthesized. By controlling inlet ratio of L-lactide and 2-ethyl-2-oxazoline, triblock copolymers with various amphiphilicity can be obtained. Afterward, casting polymer films are degraded in the presence of proteinase K. During degradation, the composition and properties of films and degradation residues are analyzed in detail, respectively. The purpose is to investigate the degradation behavior of these polymers thoroughly. In addition, by virtue of unique microphase separation of triblock copolymer and the distinct amphiphilicity between PEOz and PLLA, we expect one of the blocks can be removed quickly by enzymatic degradation, and left the highly-ordered nanostructure.Therefore, on one hand the weight and water absorption change of films are weighed. The rate of weight loss is proportioned to water absorption. The molecular weight and molecular weight distribution are determined by gel permeation chromatography, and it shows degradation prefer lower molecular weight polymers in the beginning. Furthermore, according to 1H nuclear magnetic resonance analysis, the weight loss in early stage is mainly attributed to PEOz, and then PEOz and PLLA are washed away at fixed proportion. And the surface and cross section morphology are observed by scanning electronic microscope. The result shows there are different morphology changes between the film surface that contacts air and that contacts Teflon mold. The air side of films is more degradable than Teflon side, and it would generate lots of porous microspheres on the surface. Last, the increase of crystallinity and recrystallization phenomena that reduce degradation rate are observed by differential scanning calorimetry. On the other hand, the analysis of degradation residues shows it consists of particles dispersed in solution that comprise PEOz and poly(ethyleneimine) (PEI) copolymers and PLLA oligomers and monomers dissolved in solution.Based on the results, a mechanism of enzymatic degradation is proposed that the degradation behavior of PLLA-PEOz-PLLA triblock copolymer displays as two stages. In the first stage, the loss of PEOz-dominant segments forms particles dispersed in solution, and results in porous films. And in the second stage, enzyme permeates films via these pores makes the loss of a large number PLLA of which oligomers and monomers dissolved in solution, and makes films with uniform pores. Therefore, we provide a point of view which takes degradation residues into consideration to evaluate the properties in application and to solve the encountered problems of this biodegradable polymer. Besides, the highly-ordered erosion film may provide a economic and convenient way to produce scaffold in tissue engineering and photonic crystal in drug delivery or other applications.