Ureidopyrimidone Telechelic Low Molecular Weights Poly(D,L-Lactic Acid) And Its Regulation To The Thermal Processing Performance Of Poly(D,L-Lactic Acid)

Poly（lactic acid）（PLA）is widely used for bone screws,bone splints and other orthopedic devices due to its good biocompatibility and biodegradability.PLA-based orthopedic devices are often fabricated via high-temperature melt molding（160-200°C）,which may lead to thermal degradation of PLA and decrease the mechanical properties of PLA orthopedic devices.Adding plasticizers is among the most convenient and effective way to reduce the processing temperature of PLA.Unfortunately,traditional plasticizers generally decrease the mechanical properties of PLA as well.Therefore,developing a novel type of plasticizers that can reduce the processing temperature without obviously scarificing the mechanical properties of PLA is of great significance to expand the orthopedic applications of PLA.2-Ureido-4[1H]-pyrimidinone（UPy）can form self-complementary quadruple hydrogen bonds,by which UPy-telechelic oligomers can dimerize to exhibit the properties of high molecular weight polymers.However,the dimerization will be destroyed at a temperature higher than 80°C and the UPy-telechelic oligomers may exhibit a low viscous flow temperature featured by oligomers.Therefore,we propose a hypothesis in this work:UPy-telechelic low-molecular-weight PLA in viscous flow state can plasticize high-molecular-weight PLA（HMW-PLA）due to their similarity and intermiscibility so as to reduce the processing temperature of HMW-PLA;after the thermal processing is finished,with the decrease of temperature,the dimerization of UPy（partially）recovers and UPy-telechelic low-molecular-weight PLA behaves like a high-molecular-weight polymer again,which should mitigate or even eliminate the adverse effects of UPy-telechelic low-molecular-weight PLA on the mechanical properties of HMW-PLA.To test this hypothesis,poly（D,L-lactic acid）（PDLLA）was used as the PLA model material.The synthesis of UPy-telechelic low-molecular-weight PDLLA（UPy-diol）was first optimized,and the obtained UPy-diol was further used as an external plasticizer to examine the effect of UPy-diol on the thermal processing and mechanical properties of PDLLA.The main research contents and conclusions are as follows:（1）PDLLA-based macrodiols（diol）with different molecular weights（5K,6K,7K）were prepared by ring-opening polymerization of D,L-lactide with polyethylene glycol-400（PEG400）as the co-initiator,which then reacted with isocynate-terminated UPy（UPy-NCO）via nucleophilic addition to get UPy-telechelic diol（UPy-diol）.The effects of reaction time and molecular weights of diol on the conversion rate of diol were examined during the synthesis of UPy-diol.The results from fourier transform infrared spectroscopy（FTIR）and proton nuclear magnetic resonance spectroscopy（¹H NMR）revealed that diol can be completely converted into UPy-diol by reacting at 100°C in a nitrogen atmosphere for 18 h using Sn（Oct）₂ as catalyst and toluene as solvent,achieving100%conversion rate of diol.The results from differential scanning calorimetry（DSC）and thermogravimetric analysis（TGA）indicated that the glass transition temperature（T_g）of UPy-diol is significantly higher than that of the corresponding diol,and a lower molecular weight of diol corresponds to more obvious increase in the T_g of UPy-diol.（2）UPy-diol plasticized PDLLA（UPy-diol/PDLLA blends）was prepared by both solution blending and melt blending with the mass ratio of UPy-diol/PDLLA as 20/80.DSC and TGA results indicated that PDLLA is well compatible with UPy-diol and UPy-diol does not significantly change the T_g and thermal stability of PDLLA.Melt mass flow rate（MFR）results verified that the flowability of PDLLA melt can be significantly improved by UPy-diol.The MFR for UPy-diol5K/PDLLA and UPy-diol6K/PDLLA blends（prepared by solution blending）were 7.23 g/10 min and 8.44 g/10 min at 145°C,which were significantly higher than the MFR（5.97 g/10 min）of PDLLA at 190°C.Besides,UPy-diol/PDLLA blends have good injection molding processability at 120-130°C,which is significantly lower than the injection temperature of PDLLA（165°C）.（3）The mechanical tensile test results indicated that UPy-diol/PDLLA blends have the obviously better tensile modulus and the slightly lower ultimate strength than pure PDLLA,and UPy-diol5K/PDLLA blends demonstrated the best tensile mechanical properties.The tensile modulus and ultimate strength of UPy-diol5K/PDLLA blends and PDLLA are 3.94±0.62 GPa&3.09±0.37 GPa,39.40±1.90 MPa&44.04±3.90 MPa,respectively,verifying that UPy-diol as the plasticizer will not significantly change the mechanical properties of PDLLA.（4）The dynamic mechanical properties of UPy-diol/PDLLA blends were tested by using the dynamic thermodynamic analysis（DMA）.The results further proved that UPy-diol has good compatibility with PDLLA.The maximum storage modulus(E’_max)of UPy-diol/PDLLA blends in glassy state and the maximum loss modulus(E’’_max)in rubbery state reached 3507 MPa and 470 MPa,respectively,suggesting that UPy-diol as the plasticizer has no significant effects on the mechanical properties and viscoelasticity of PDLLA(E’_max=2769 MPa,E’’_max=438 MPa).In summary,this work has estabilished an optimized process to achieve a complete conversion of PDLLA macrodiol into UPy-telechelic low-molecular-weight PDLLA and confirmed that UPy-diol can plasticize PDLLA to form UPy-diol/PDLLA blends with both low-temperature processability and good mechanical properties.These findings are of great significance to expand the the application of PLA in the field of orthopedics.