The Effect Of Formation And Regulation Of Polylactide Mesophase On The Properties Of PLA Fibers

This dissertation focuses on the molecular origin and controlled formation of PLA mesophase.Moreover,the effect of the PLA mesophase on the ductility and dimensional stability of PLA fibers has also been considered.Firstly,a comparative study of the amorphous PLA fibers and films was conducted to uncover molecular origin and practical merits of the PLA mesophase associated with physical aging.On this basis,the PLA fibers were selected to investigate the effect of isothermal annealing below T_g on the formation of the PLA mesophase.In order to further controlled formation of the PLA mesophase,PLA/poly?glycolic acid??PGA?fibers were used to investigate the effect of the structural evolution of crystalline and amorphous PGA domains on the formation of the PLA mesophase.Finally,the PLA/PGA fibers were selected to systematically consider the synergistic effects of isothermal annealing and PGA content on the formation of the PLA mesophase.Infrared spectroscopy?FTIR?was used to detect the mesophase and associated molecular interactions and conformational rearrangement.Differential scanning calorimetry?DSC?was used to examine the thermal response and stability.Tensile test and dynamic mechanical analysis?DMA?were performed to characterize the macroscopic properties.The main results are as following:1.A comparative study of the oriented amorphous fibers and isotropic films was conducted to uncover the molecular origin and practical merits of the PLA mesophase.The results showed that the mesophase developed readily in the annealed fibers along with enhanced dipole-dipole interactions between the C=O groups.These were realized through the conformational rearrangement?mainly from tt to gt?during which the distance between the C=O groups was reduced to be small enough to allow the generation of the dipole-dipole interactions and the formation of the mesophase.The stretch-induced molecular disentanglement was found necessary to reduce the kinetic barrier for such rearrangement,which was absent for the isotropic films.The annealed fibers showed persistently good ductility and dimensional stability while the films embrittled increasingly with time as a result of the physical aging.Undoubtedly,the molecular disentanglement in the annealed fibers drived the formation of PLA mesophase,rather than physical aging.2.The PLA fibers were selected to investigate the effect of isothermal annealing below T_g on the formation of the PLA mesophase.The results showed that the mesophase developed readily and increased constantly with the annealed temperature,along with the gradually enhanced dipole-dipole interactions.The relative content of the gt conformer also increased in step with that of the mesophase.Moreover,the high temperature endowed the PLA fibers with higher molecular mobility,making the PLA fibers annealed at 51 ^oC contribute to the conformational rearrangement from tg to gt.Whereas the PLA fibers annealed at 37 ^oC were helpful for tt rearrangement into the gt conformers.Furthermore,upon heated above T_g,the mesophase presented in the PLA fibers annealed at 51 ^oC experienced a thermal transition into more cohesional mesophase,which significantly promoted the cold crystallization(T_cc)of PLA and directly formed into?-crystal.The significant formation of highly ordered mesophase slightly reduced the ductility and improved the dimensional stability of the annealed fibers.3.The PLA/PGA fibers annealed at 37 ^oC were used to research the effect of the crystalline PGA domains on the formation of the PLA mesophase.The results showed that the mesophase developed readily and decreased constantly with the PGA content,along with the gradually weakened dipole-dipole interactions.The relative content of the gt conformer also decreased in step with that of the mesophase.However,upon heated above T_g,the enthalpy relaxation due to the physical aging was more remarkable.Further studies showed that the existence of the PGA domains inceraced the PLA entanglement,which restricted the conformational rearrangement and in turn the formation of the PLA mesophase.But it did contribute to the phenomenon of the physical aging.In addition,the decrease of the PLA mesophase and the increase of cohesional entanglement due to physical aging reduced the ductility and improved the dimensional stability of the annealed fibers.4.The PLA/PGA fibers annealed at 37 ^oC were used to discuss the effect of the structural evolution of the amorphous PGA domains on the PLA mesophase formation.The results showed that the mesophase developed readily and increased constantly with the PGA content.However,both the enhanced dipole-dipole interactions and the increase of the gt conformer decreased laterally,which indicated that the local ordering of PLA induced by the gradual crystallization of the PGA domains directly resulted in the formation of the mesophase.Upon heated above T_g,the existence of T_p was the transition to the more cohesional mesophase and the enthalpy relaxation due to the physical aging.The PLA mesophase could not offset the the increase of the cohesional entanglement due to physical aging,which reduced the ductility and improved the dimensional stability of the annealed fibers effectively.5.The PLA/PGA fibers were selected to systematically consider the synergistic effects of the isothermal annealing and the PGA content on the formation of the PLA mesophase.The results showed that the mesophase developed readily and increased constantly with the annealing temperature and the PGA content.Different from the existence of T_p in the fibers annealed at 37 ^oC was the transition to the more cohesional mesophase and the enthalpy relaxation due to the physical aging,the existence of T_p in the fibers annealed at 45 and 51 ^oC was only the transition to the more cohesional mesophase.Moreover,the ductility of the fibers annealed at 37 ^oC decreased with the increase of the PGA content due to formation of the cohesional entanglement caused by physical aging.However,the ductility of the fibers annealed at 45 ^oC slightly decreased with the increase of the PGA content due to formation of the PLA mesophase with more ordered structure.And the formation of PLA mesocrystal in the fibers annealed at 51 ^oC notably reduced the ductility.