Crystal Structure Regulation And Thermal Resistance Modification Of Poly(Lactic Acid) Fibers

With the continuous exploitation and utilization of petroleum resources,many additional products have been derived,such as the discovery and development of synthetic plastics,which have greatly promoted social progress.However,most of the products are non-degradable in the natural environment and are difficult to be recycled,their huge production and consumption not only meet the needs of human beings,but also bring about increasingly serious energy crisis and environmental pollution problems.Therefore,in order to get rid of the problems caused by energy crisis and environmental pollution,it is essential to develop non-petroleum-based and biodegradable polymer materials to replace traditional petroleum-based polymer materials.As a completely biodegradable and environmentally friendly polyester compound,PLA fiber meets the requirements of sustainable development,conforms to the social law of green environmental protection and resource recycling,has broad application prospects,and is favored by consumers.However,PLA fibers also have some problems,especially the modification of their heat resistance needs to be improved.At present,there are many ways to modify the heat-resistant of PLA.Basing on the preservation of its complete biodegradability,combining with the actual production process,the purpose of this paper is to study the effect of external force field on morphological structure and its regulation of PLA materials,so as to achieve the purpose of heat-resistant modification.Firstly,the effect of additive content of HPDLA on the crystallinity and the phase separation in PLLA matrix were discussed.Then,based on the results of phase separation behavior,HPDLA/PLLA?20/80??20%HPDLA?blend sheet was selected as the research object to study the effects of different thermal drawing conditions on the crystallization behavior and mechanical properties of the blend were systematically studied.Finally,the effect of external field force on the internal morphology of the fibers of 20%HPDLA blends during melt spinning was analyzed,and the relationship between the change of structure and properties was systematically correlated.The results are as follows:1.In HPDLA/PLLA blend system,the results of SEM and AFM showed that the addition of HPDLA results in phase separation in the matrix of the blend and HPDLA was uniformly dispersed in PLLA matrix in the form of spherical particles.DSC non-isothermal test results showed that with the increase of HPDLA content,the content of SC crystals in the matrix increased.The results of differential scanning calorimeter?DSC?and polarizing microscope?POM?showed that with the increase of HPDLA content,the content of stereocomplex?SC?crystals in the matrix was increased.At the same time,the addition of HPDLA can increase nucleation sites,shorten semi-crystallization time and decrease spherulite size,thus promoting the crystallization of PLLA matrix.2.In the 20%HPDLA blend sheet thermal drafting system,DSC results showed that a high temperature melting peak appeared at 232?by adjusting the thermal drafting conditions.When the thermal drafting multiple was 8 times and the drafting rate was 10 mm·min^-1,the enthalpy corresponding to the melting peak was the highest.The transformation of spherulites into highly oriented crystals can be observed by SEM as well as the POM.The results of DMA showed that the glass transition temperature of PLLA matrix was improved by hot stretching,for which 20%HPDLA blends was about 5?higher than that of PLLA.After the treatment of hot stretching,the breaking strength and elongation at break of 20%HPDLA samples were higher than those of PLLA.3.In 20%HPDLA composite fiber system,because HPDLA can be separated from PLLA matrix,HPDLA is uniformly dispersed in PLLA matrix with spherical particles,which can be in-situ fibrillation under the action of filament flow field.After thermal stretching treatment,the in-situ nanofibers were obtained with more regular arrangement and smaller diameter.For HPDLA has higher strength and modulus than PLLA,in-situ nanofiber structure can avoid interface damage,so the introduction of in-situ nanofiber structure can greatly improve the mechanical properties of composite fibers.At the same time,the in-situ nanofibers arranged neatly in the composite fibers can play a skeleton role and restrict the de-orientation of the oriented PLLA molecular chains.Therefore,the thermal resistance of the composite fibers can be significantly improved.The shrinkage rates of hot air and boiling water were significantly lower than those of pure PLLA fibers.