ABSTRACT:Three types of semicrystalline ultra high molecular weight polyethylene, obtained with amorphous SiO 2 -supported Ziegler-Natta catalyst systems modified by vanadium and titanium, have been studied by wide angle XRay scattering, dynamic mechanical thermal analysis and microhardness methods. In all the cases a significant amount of monoclinic modification is observed, which is estimated to be of the order of 10%. With increasing the degree of crystallinity and quantity of entanglements, storage modulus and α relaxation increase and γ relaxation decreases. A weak β relaxation occurs only in the sample obtained with vanadium catalytic systems. Microhardness measurements give the possibility of distinguishing the effect of crystallinity from the effect of entanglements: Vickers microhardness is sensitive predominately to crystalline phase, while total microhardness is sensitive also to the structural peculiarities in the amorphous phase.KEY WORDS Ultrahigh Molecular Weight Polyethylene (UHMWPE) / Dynamic Mechanical Thermal Analysis (DMTA) / Wide-Angle X-Ray Diffraction (WAXS) / Microhardness / Ultrahigh Molecular Weight Polyethylene (UHMWPE) (M>10 6 ) possesses excellent mechanical properties and some advantages over conventional polyolefins as a high level of durability, excellent creep characteristics and abrasion resistance. [1][2][3][4][5] For this reason it is used in demanding applications such as artificial hip and knee joints, machine parts, fibres, polymer coating on metal surface, separators, acoustic diaphragms, etc.It is established that this semicrystallyne polymer has a specific morphology. There are many reports concerning the structure of nascent 6 and differently processed UHMWPE samples. 3,[7][8][9] The occurrence of polymorphism 3, 7, 10-12 is characteristic for crystal part, the orthorhombic crystals lattice being the most typical. It was established that the crystal lamellae in both nascent and processed unoriented UHMWPE have similar dimensions, close to 20 nm on average. 6 This thickness could be varied from 10 nm after necking 13 up to 100 nm in highly oriented fibres. 8 There are reported some cases where a part of the typical, stable and prevailing orthorhombic crystals could be transformed into a monoclinic or transient hexagonal phase. 3, 7-11, 14, 15 Thus, monoclinic crystals are observed in highly deformed samples (fibers) 7-10 or in the samples crystallized under stress. 16 Moreover, there are experimental results showing that polyethylene crystals transform from the stable orthorhombic crystal into a transient hexagonal phase, observed when the polymer is subjected to the action of fast electrons 14 or at high pressure and temperature. 3,15 Since the chain mobility is rather high in the hexagonal phase, sintering has been achieved via this transient phase. 3 There were found reports concerning to the study of the amorphous phase by NMR 8 and by positron annihilation lifetime spectroscopy. 17 It was reported a 5% of disordered all-trans interfacial material and/or tie molecu...

ABSTRACT:The correlation between the modulus of elasticity, on the one side, and Vickers microhardness and total microhardness, on the other side, was analyzed in the case of ultrahigh molecular weight polyethylenes. An extension of the application of the power relation between microhardness and the modulus (MH ϭ aE b ) obtained by different methods was suggested. A linear dependence between constants obtained by Vickers and total microhardness measurements and other different techniques of modulus measurement was established, which signifies that both microhardness characteristics change their sensitivity toward the modulus in a similar way.

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