Investigation On The Relationship Between The Intrinsic Structure And Wear Resistance Of Anti-wear Polyethylenes For Artificial Joints

The most commonly used polymeric prosthetic material for artificial joints is ultra-high Molecular Weight Polyethylene(UHMWPE),but the debris generated during the wear process can cause osteolysis and loosening of the prosthesis,limiting the implant prosthesis’ s service life.Petro China Petrochemical Research Institute has developed an Ultra-Low Wear Polyethylene(ULWPE)product on its own in recent years.Previous research has shown that ULWPE has excellent biocompatibility and wear resistance,as well as a high potential for artificial joint applications.However,their friction behaviour and wear resistance mechanisms are not yet fully understood.In this thesis,the molecular chain structure,condensed phase structure,mechanical/physical properties and wear properties of three different molecular weight ULWPEs,two high density polyethylene(HDPE)and two UHMWPEs were systematically characterised and analysed at three scales: micro,meso,and macro scales.The reasons for the excellent wear resistance of ULWPE were elucidated and the conformational relationships between the intrinsic structure and wear resistance of different types of polyethylene were revealed.The main work and conclusions of this paper are as follows:(1)The molecular weights of the three ULWPEs are much smaller than those of UHMWPE,with weight-average molecular weights ranging from 200 to 750 kg/mol;and the molecular weight distribution is narrow,with molecular weight distribution indices ranging from 1.6 to 2.7.The results of FTIR,13C-NMR,dynamic rheology,and SSA tests showed that unlike the short branch structure of HDPE copolymers,the metallocene-catalyzed ULWPE has a long branch structure and the content of long chains increases with the molecular weight of the ULWPE samples.The main topology of ULWPE is probably a "Y" chain structure.(2)Studies of condensed phase structure characterisation and crystallisation behaviour have demonstrated that,whereas the molecular chain structures of different polyethylenes differ,the basic thermal properties do not.The crystalline structure of polyethylene was mainly orthorhombic,with a decreasing trend in crystallinity with increasing molecular weight The presence of long-chain branching(LCB)increased amorphous region entanglement and decreased ULWPE crystallinity.However,the ULWPE molecular chains retained a good overall structure,with an average lamella thickness of 31.61 nm,which was significantly greater than that of HDPE(20.49 nm)and UHMWPE(19.03 nm).The crystallisation rate of the three different molecular weights of ULWPE was faster than that of HDPE and UHMWPE when the same degree of crystallinity was reached in the non-isothermal crystallisation process,again due to the good structural regularity of ULWPE.The activation energy of non-isothermal crystallization increased gradually with increases of the molecular weight of ULWPE,indicating that the ULWPE-700 tends to crystallize more difficultly owing to higher energy barrier.ULWPE also had a significantly higher intermediate phase content than HDPE and a very high entanglement density.(3)Mechanical and physical property characterisation revealed that the tensile properties of polymers was influenced by a combination of molecular chain structure and cohesive structure.The higher the degree of crystallinity,the higher the yield strength and modulus of elasticity exhibited by the polyethylene during the tensile tests.The high entanglement ULWPE-700 and UHMWPE 4150 had significantly lower modulus of elasticity and elongation at break than the other ULWPEs and HDPEs,but showed excellent toughness,exhibited high work of rupture and very high impact strength.Statistical analysis showed that the modulus of elastic modulus E,yield strength σs,elongation at break ε,microhardness and density of the different types of polyethylene in uniaxial tension were all significantly and positively correlated with crystallinity.Meanwhile,the more entanglement in the amorphous zone of the polymer,the higher the corresponding impact strength.(4)The multi-directional pin-on-disc wear test showed that ULWPE exhibited good overall wear resistance and that the wear resistance increased with increasing molecular weight.Under the load of 2 MPa,the wear factors of the different polyethylene types in descending order were: HDPE 4808 > HDPE 5420 > UHMWPE 1050 > ULWPE-300 >UHMWPE 4150 > ULWPE-200 > ULWPE-700.At three different contact pressures of 2,3and 4 MPa,the wear of the different polyethylene types increased with increasing contact pressure.The presence of LCB increased the entanglement in the amorphous zone and increased the ratio of tie-molecules,which enhanced the toughness of ULWPE and therefore possessed excellent wear resistance.ULWPE-700 had the best wear resistance of any polyethylene material and under all loading conditions due to its excellent strength and toughness.(5)Correlation analysis of the intrinsic structural,mechanical,and wear resistance parameters of different polyethylene types revealed that crystallinity and entanglement were the two most important intrinsic structural parameters influencing polymer wear resistance.At a suitable crystallinity(0.5-0.7),increasing the physical entanglement or chemical crosslinking between the molecular chains can optimally match the strength and toughness,maximizing the polymer’s wear resistance.In a word,this thesis first demonstrated the existence of LCB in ULWPE,revealed that the reason for the excellent wear resistance of ULWPE was the combination of excellent strength and toughness,and the micro mechanism of high impact resistance and wear resistance was the increased proportion of tie-molecules and high entanglement in the amorphous regions.Crystallinity and entanglement were found to be the two most core intrinsic structural parameters affecting the wear resistance of polymers.and provided systematic and comprehensive data and a theoretical basis for the subsequent clinical application of ULWPE.