| Polylactic acid(PLA)is one of the most widely studied biobased and biodegradable polymers.There are two main polymerization methods for polylactic acid,one is the ring-opening polymerization of lactide and the other is direct polymerization of lactic acid.Ring-opening polymerization is relatively simple and easy to control while the preparation of lactide is difficult and costly.The problem of direct polymerization is that high molecular weight products can be only obtained through solid-state polymerization,which is complex and time-consuming.Although agricultural and packaging films should be important applications for PLA,its high price at present greatly hinders its large-scale application in this field.Therefore,it is of great significance to develop low-cost PLA films.In view of this,this thesis attempts to prepare modified PLA prepolymer by direct polymerization of lactic acid and unsaturated monomers and to efficiently and rapidly prepare low-cost PLA film via UV copolymerization of the modified prepolymer and polyethylene glycol diacrylate(PEGDA).The research contents of the thesis are as follows:(1)L-lactic acid was polymerized with hydroxyethyl acrylate(HEA),butynediol(BYDO),3-butyneic acid(BYA),itaconic acid(ITA)and butynedioic acid(BYDA),respectively,at 160°C and 200 Pa for 8 hours using methanesulfonic acid(MSA)and stannous acetate(Sn(Ac)2)as catalysts.The corresponding modified PLA prepolymers HO-PLA-V,HO-PLA-OH,HOOC-PLA-Y,HOOC-PLA-COOH(V)and HOOC-PLA-COOH(Y)with different end groups,were obtained.The prepolymers were characterized by FT-IR,~1H NMR and GPC and the results showed that the catalytic activity of MSA/Sn(Ac)2 composite catalyst was higher than that of MSA single catalyst.Each of the five unsaturated monomers was successfully introduced into the corresponding prepolymer molecular chain.The number-average molecular weights(Mns)of the modified prepolymers were ranged from 7.4 kg/mol to 9.4 kg/mol and the Mns of the prepolymers with carboxyl as the main terminal group(HOOC-PLA-Y,HOOC-PLA-COOH(V)and HOOC-PLA-COOH(Y))were generally higher than those of the prepolymers(HO-PLA-V,HO-PLA-OH)with hydroxyl as the main terminal due to the self-catalytic effect of carboxylic acid.(2)In view of the needs of subsequent melt mixing or processing,the thermal properties and pyrolysis behavior of the aforementioned modified prepolymer were systematically studied in this thesis.DSC results revealed that the glass transition temperature Tg,cold crystallization temperature Tc and melting point Tm of carboxyl-terminated prepolymers were higher than those of hydroxyl-terminated prepolymers,which should be attributed to their higher molecular weight.The isothermal pyrolysis behavior of the prepolymers was investigated by TGA at 200?240°C and the pyrolysis products were analyzed by ~1H NMR and GPC.The results indicated that the terminal groups had a significant effect on the isothermal degradation rate of PLA prepolymers.The isothermal degradation rates of the hydroxyl-terminated prepolymers(HO-PLA-OH0.95%/min,HO-PLA-V 0.73%/min)at 200°C were higher than those of the carboxyl-terminated prepolymers(HOOC-PLA-Y 0.41%/min,HOOC-PLA-COOH(V)0.32%/min and HOOC-PLA-COOH(Y)0.29%/min).The molecular weight and its distribution of each prepolymer before and after thermal degradation remained unchanged,and the main pyrolysis product was lactide.Kissinger and Flynn-Wall-Ozawa method were employed to simulate the non-isothermal pyrolysis kinetics of two representative prepolymers and the pyrolysis activation energies were estimated for HOOC-PLA-COOH(V)(154 k J/mol and 129 k J/mol,the former from Kissinger method and the latter Flynn-Wall-Ozawa method,the same as the latter),PLA(134k J/mol and 119 k J/mol)and HO-PLA-OH(130 k J/mol and 116 k J/mol),revealing that the pyrolysis activation energy of carboxyl-terminated prepolymer was greater than that of hydroxyl-terminated prepolymer.Based on the above results,the pyrolysis mechanism of PLA was proposed to be the lactide removal reaction via the backbiting of the hydroxyl terminal.(3)A series of PLA/PEGDA films were prepared by UV irradiation using the modified prepolymer and PEGDA with different molecular weights as raw materials in the presence of benzoin dimethyl ether(651)or benzophenone(BP).FT-IR,gel content,WAXD,DMA and mechanical properties of the films were tested and analyzed.The results showed that 651 had better photoinitiation efficiency under the studied conditions.A single Tg was observed in the DMA curve,which was between the Tgs of PLA and PEG,indicating that the two components were mixed at the molecular level and formed a homogeneous phase.The amount of 651 needed for the film curing could be as low as 0.05 wt%if the UV irradiation time was set to be 1 second while the irradiation time required for the curing could be shortened to be 0.1 second with the 651addition of 0.2 wt%.These results indicated that the present method had the potential to realize the efficient and rapid preparation of PLA/PEGDA films and was expected to be applied in the production of low-cost biodegradable agricultural films in the future. |
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