| Polylactic acid(PLA)/?-TCP composite with the merits of good biocompatibility and biodegradability has been applied to biomedical materials field.Although PLA/?-TCP composite is a potential biomedical material,but its mechanical properties and degradation characteristics are not ideal,which limits its wide application.Recently,it has become one of the hot spots to improve the mechanical properties and degradation properties of PLA by using ultrafine ?-TCP.When the content of ultrafine ?-TCP is higher in polylactic acid(PLA),it is easy to cause aggregation of ?-TCP and defects in PLA matrix,which resulted in descent of the properties of composites.At present,the main methods of enhancing mechanical strength of PLA are solid state extrusion,injection molding,fibre bundle molding,etc.However,these methods are very harsh on performance of raw materials.Meanwhile,the mechanism of the degradation of the PLA/?-TCP composite should be further studied.To solve the above problems,this paper focuses on the following research.The preparation technology and surface modification of ?-TCP are studied.The effect of ?-TCP content on crystallization behavior of PLA is also investigated.Meanwhile,the effect of equal channel angular pressing(ECAP)on the properties of PLA/?-TCP composites and the degradation mechanism of PLA/?-TCP composites are discussed.?-TCP was obtained by direct precipitation method,using low-price calcium hydroxide(Ca(OH)2)as the Ca-source and phosphoric acid(H3PO4)as the P-source in the presence of polyethylene glycol(PEG).In this preparation process,the effects of reaction temperature(25 ?,45 0C,80 ?)and adding methods of PEG(PEG4000,PEG10000)on the purity and dispersion of P-TCP were mainly discussed.After calcination,the content of ?-TCP phase was the highest when its precursor was prepared under the condition of 25 ?.PEG4000,as dispersant,was added in H3PO4 aqueous solution,which could promote a higher purity of precursor CDHA phase when the CH2CH2O/H+ molar ratio was equal to 1:17.After calcination,the neck joint phenomenon of ?-TCP powder was weakened.The dispersion and homogeneity of ?-TCP powder were all improved.Meanwhile,the size of P-TCP particle with nearly spherical shape was measured in the range of 0.2-0.5 ?m.The effects of four kinds of surface treatment agents(oleic acid,stearic acid,sodium oleic acid and sodium stearate)on the surface properties of ?-TCP powder were studied.In consideration of biocompatibility,the sodium stearate(SS)was utilized to improve the surface properties of P-TCP particles.The modified ?-TCP was added in PLA to prepare the composite materials,which mechanical property and the compatibility of P-TCP powder with PLA matrix were improved.For the non-isothermal melt crystallization at the rate of-10 ?/min,the effect of ?-TCP on the nucleation mechanism and growth mode of PLA were little.During the process of non-isothermal cold crystallization and melting at the rate of 10 ?C/min,when the contents of ?-TCP in PLA were larger than 10 wt.%,the number of nucleation sites were increased in the PLA matrix,and the stability of crystal was better.The isothermal crystallization behavior of PLA/?-TCP composites were investigated at 100 ?.When the contents of P-TCP were more than 5 wt.%in PLA,the crystallization ability of PLA was enhanced.PLA was processed by ECAP one pass at 80 ?,100 ?,120 ? and 140 ?,respectively.When ECAP processing was carried out at 100 ?,the plastic deformation of PLA was more uniform,and its bending strength was higher.Therefore,the effects of different routes and passes on the microhardness,crystal orientation and thermal properties of PLA and PLA/?-TCP composites were investigated at 100 ?.PLA treated by ECAP two passes under A' route(the shear direction of the sample was invariable in continuous process)displayed a higher bending strength.However,a better fracture toughness of PLA appears at three ECAP passes under C'route(the shear direction of the sample was rotated 180° along the loading axis in the continuous process).Compared with PLA,the(200)crystal plane orientation of PLA/20TCP composite was affected by ?-TCP under ECAP one pass.After ECAP processing under different routes and passes,the bending strength,modulus and fracture toughness of PLA/?-TCP composites were all improved.PLA and its composites processed by ECAP one pass were carried out for in vitro degradation in 37 ? phosphate buffer solution(PBS)for 24 weeks.Compared with hot-pressing samples,the water absorption of the ECAP samples were increased,but the weight loss rate were decreased,and the decay rate of bending strength were lower.Meanwhile,the change of pH value of PBS was relatively small.In the process of degradation,no new substance was discovered in the XRD spectra of PLA and its composites,but there was OH peak found on the FT-IR of composites.Along with the degradation,the fiber began to disorient,and the internal structure became loose.The ECAP samples were also degraded in 50 ? PBS and SBF(Simulated Body Fluid)for 12 weeks,respectively.In the two kinds of degradation solution,the changing regularity of water absorption rate weight loss rate and bending strength of all samples were similar,as well as degradation products were the same.The cold crystallization temperature,melting point and the half crystallization temperature of the samples were all decreased. |
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