Studies On Structure And Properties Of Large Diameter PGLA(90/10)Fiber During In Vitro Degradation

Polyglycolide(PGA),polylactide(PLA)and glycolide-lactide copolymer(PGLA)are widely used in biomedical applications due to their excellent biocompatibility and biodegradability properties,such as sutures,bone fixation,artificial skin graft,drug release systems and tissue scaffolds etc.The main advantage of such synthetic biocompatible polymers is that degradation is due to simple hydrolysis of the ester backbone in aqueous solution and that the degradation products can ultimately be metabolized or eliminated by the body.In this study,the PGLA(90/10)crude fiber with the diameter of 0.5mm was prepared using PGLA(90/10)as raw material by melt spinning.Then the drawn fibers obtained have significantly difference in the structure and properties after the heat-stretching treatment with different draw ratio(3,3.5,4,4.5,5,5.5).Finally,the relationship between the structure and the degradation property of the fiber was investigated via in vitro degradation.The PGLA(90/10)fiber with larger diameter(D>0.2mm)described in this paper can be used as braided stent material for the intestine,gallbladder,esophagus and other body cavities.The mechanical properties of the PGLA(90/10)fiber is not superior,its initial modulus and fracture strength are 1.3GPa and 54.7MPa respectively.But the mechanical properties can be greatly improved through heat-stretching treatment,the initial modulus and break strength can reach 12.3GPa and 764.3MPa respectively,to meet the mechanical requirements of braided stent.In addition,based on the cantilever model,the flexural modulus of the fiber with large defrnmation was calculated through a simple mathematical model.The results showed that as the draw ratio increases,the flexural modulus of the fiber becomes high.Changes in themechanical properties of fiber are essentially caused by the changesin the internal aggregation structure of fiber.From the orientation structure point of view,as the draw ratio increases,the sonic orientation factor fs increases linearly,and the fs was increased to 0.78 when the draw ratio is 5.5.But the crystal orientation factor fof 6 samples have little difference,all is about 0.88.The degree of crystallinity(Xc)were obtained for all samples by Differential Scanning Calorimetry(DSC).The results showed that the degree of crystallinity of the PGLA(90/10)fiber increased greatly after processed by heat-stretching,compared with the PGLA(90/10)UDY fiber.As for the amorphous structure,with the increase of draw ratio,the number of random segment gradually reduced.The random segment was transformed into crystalline structure as well as the oriented mesophase.When the draw ratio<4,random segment is mainly transformed into the crystalline structure;while random segment is mainly transformed into the orientation intermediate state when the draw ratio>4.Spherulites are the main crystal structures in the PGLA(90/10)crude fiber,but microfibrillar structures were formed obviously after stretching treatment.Therefore,the aggregationstructure of the fiber was changed after heat-stretching treatment,then causedthe apparent properties of the PGLA(90/10)have changed,such as the mechanical properties are enhanced.Three PGLA(90/10)fiber samples(the draw ratio was 3,4,5 respectively)have been conducted the degradation test in vitro for 35 days.It was found that improve the draw ratio can effectively slow down the loss of apparent properties such as the quality of fiber,intrinsic viscosity and mechanical properties.Also,the aggregation structures of samples were studied,the results shown that(1)in the early days of degradation(from 0 to 6 days)the degree of crystallinity have increased significantly due to cleavage-induced crystallization;(2)in the degradation later(after 12 days),the degree of crystallinity continued to rise,and this is attributed to the loss of mass of the amorphous region;(3)the degradation rate of the crystal region is much slower than the degradation rate of the amorphous region.The degradation rates of the GA component and the LA component in all samples did not have obvious difference,and the GA/LA mole ratios nearly unchanged.The thermal properties of the fiber have been less affected by the change of draw ratio,the variation trend of thermal properties of the 3 samples are same during degradation.In the process of degradation,the most obvious influenced by draw ratio is the fracture mode of the fiber.The PGLA(90/10)fiber processed by heat-stretching with 3 draw ratio is very accord with the microfibrillar structure described by Peterlin et al,fiber broke into slices.And the PGLA(90/10)fiber processed by heat-stretching with 5 draw ratio has shown a typical shear fracture.This is resulted from the existence of the orientation mesophase and plastic deformation of microfibrillar structure.