| Polylactic acid(PLA)is a biodegradable thermoplastic polyester prepared by the method of chemical synthesis using green renewable plant resources(corn,potato,etc.)as raw materials.The production process of polylactic acid is environmentally friendly,and its final degradation products are carbon dioxide and water,which will not give rise to issues of environmental pollution.Polylactic acid not only has good mechanical properties and processing properties,but also has excellent optical properties and barrier properties.The good processing properties of polylactic acid enable it to been produced as films,fibers and sheets using a variety of molding techniques and its products are widely applied in the fields of food packaging,film forming,agriculture,forestry,civil engineering and medical supplies.Due to the special properties of ultrafine fiber such as high specific surface area imparted by its unique size effects,ultrafine fibers are widely used in the fields of medical care,high-efficiency protection,fine filtration,automotive industry and agriculture.Porous fiber refers to a type of fiber that exists porous structure on the surface or inside of a single fiber,which has the advantages of high specific surface area and high surface roughness.Combine with advantages of both fibers,the ultra-fine porous fiber has higher specific surface area and porosity,which make it has a broader application prospect.Currently,the methods of ultrafine fiber preparing mainly consist of self-assembly,centrifugal spinning,electrospinning,template synthesis and stretching method.Among them,centrifugal spinning is a method capable of efficiently preparing micro-/nano-ultrafine fibers,which has no polarity requirement for materials or solvents,and has the advantages of high efficiency,low consumption,safety and environmental protection.Based on this,the related research groups at home and abroad have successfully prepared a variety of ultrafine fibers by centrifugal spinning,which have been widely used in the fields of nano-reinforced materials,flexible energy storage materials,biological functional materials,ceramic materials,cell culture and drug transportation.The property of biodegradability of PLA fibers doesn't exist in other synthetic fibers,which is one of the unique advantages of them and makes them have the potential to replace the synthetic fibers(PP,PET etc.)as a new generation of green functional fibers.In this paper,PLA ultrafine fibers were successfully prepared by the method of centrifugal spinning.The laws of influnce of process parameters and different spinning methods on the structure and properties of PLA fibers were studied,and glycerol triacetin(GTA)and polyethylene glycol(PEG)were utilized to modified PLA fibers.The modification effect of addition of two material mentioned above on the structure and properties of polylactic acid fiber has been studied.In the first part of the experiment,the laws of influnce of solvent ratio(dichloromethane(DCM)/ N,N-dimethylformamide(DMF)ratio)and process parameters(needle size,spinner speed,collection distance)on the morphology and diameter of centrifugally spun PLA fibers were studied.The conclusions are drawn that the morphology of the fiber shows a transition from a porous surface to a wrinkled surface when we decrease the proportion of DCM/DMF.When the ratio of DCM/DMF reaches 2.5/1,the degree of wrinkling on the surface of fiber is smallest,and the average diameter of fiber,at this time,is minimized and the uniformity of fiber is relatively good.The surface morphology of PLA fibers prepared only using pure DCM as solvent shows a kind of dense and uniform porous structure.In general,with larger needle size,higher spinner speed and larger collection distance,the fiber with smallest average diameter will be produced.Therefore,DCM/DMF ratio of 2.5/1,needle size of 27 G,spinner speed of 2000 rpm,and collection distance of 12 cm are the optimum spinning parameters obtained of this chapter.In the second part,the differences in structure and properties of PLA fibers prepared by three spinning methods(centrifugal spinning,electrospinning and electro-centrifugal spinning)were investigated.The conclusions are as follows: the surface morphology of centrifugal spinning and electro-centrifugal spinning PLA fibers shows a sort of dense and uniform porous structure,but holes on the surface of electrospun PLA fibers are deeper and have different shapes.As the voltage increase,the average diameter of the electrospun fibers generally does not change much.With the increase of voltage and speed,the average diameter of centrifugal spinning and electro-centrifugal spinning fibers increases.among them,the average diameter of the electrospun fiber is the smallest,followed by the centrifugal fiber,and the electro-centrifugal fiber is the largest.The effects of each spinning method on the internal structure of the fiber are different.Among them,the method of electrospinning destroys the crystal structure of fibers to the greatest extent,followed by the electro-centrifugal spinning,and the centrifugal spinning is the least.The thermal stability of the fibers prepared by the three spinning methods is all weaker than that of the PLA raw materials.among them,the thermal stability of electrostatic centrifugal spinning fibers is the worst.The differences of specific surface area between centrifugal spinning and electro-centrifugal spinning fibers are relatively small.the specific surface area of the electrospun fibers is about twice bigger than that of two fibers mentioned above.The hydrophobicity of fiber membranes prepared by the three spinning methods are all pretty good(CA>120°).Among them,electrospun fibers have the best hydrophobic properties,followed by electro-centrifugal spinning fibers,and centrifugal spinning fibers are the worst.In the third part,the effects of addition of GTA and PEG on the structure and properties of PLA fibers were studied.The conclusion is drawn that the addition of GTA and PEG causes the similar laws of effect on the surface morphology of PLA fibers.With increase of the content of GTA and PEG,the change of the surface morphology of PLA/GTA and PLA/PEG fibers shows a tendency of porous morphology to smooth morphology.To a certain extent,addition of GTA reduces the thermal stability of PLA fibers and also affects the crystal structure of PLA fibers.But the addition of PEG hardly causes effect on the thermal stability and crystal structure of PLA fibers.The hydrophilic properties of both composite fibers are improved compared to PLA fibers and along with the increase of content of GTA and PEG,the hydrophilic properties of the composite fiber get better.Among them,it is worth noting that when the content of GTA reaches 50%,the PLA/GTA fiber membrane appears the phenomenon of super-hydrophilic. |
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