Nascent polyethylene with high molecular mass (M" = f05, M , = LO6) produced in hydrocarbon medium in presence of TiCL/(C2Hs)2A1Cl/(C6Hs)2Mg catalyst was studied by means of X-ray diffraction, electron and light microscopy, and low-angle light scattering. It consists of porous spherulite-fibril aggregates indicating a not very high degree ofcrystallinity a (a = 0,5). Data, concerning the dependence ofspherulite dimensions, degree of crystallinity and other structure parameters of nascent polyethylene on polymerization time and temperature are discussed. Using the same structure-analytical methods some samples of nascent polyethylene were studied after annealing them in nitrogen atmosphere at different temperatures between 11&250°C with successive cooling to room temperature. Furthermore both the endo-and exothermal transitions occurring in the nascent and annealed structures after heating to 250 "C were studied by means of DTA and DSC. The experimental information leads to the conclusion that nascent polyethylene structures consist of crystalline folded chain lamellae with an increased amount of tie molecules between them as well as of extended chain crystals.

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...

To study the effect of organophilic clay concentration on nonisothermal crystallization, poly(L-lactic acid) (PLLA)/montmorillonite (MMT) nanocomposites were prepared by mixing various amounts of commercial MMT (Cloisite V R 30B) and PLLA. The effect of MMT content on melting behavior and crystal structure of nonisothermal crystallized PLLA/MMT nanocomposites was investigated by differential scanning calorimetry (DSC), small-angle Xray scattering, and wide-angle X-ray diffraction (XRD) analyses. The study was focused on the effect of the filler concentration on thermal and structural properties of the nonisothermally crystallized nanocomposite PLLA/MMT.The results obtained have shown that at filler loadings higher than 3 wt %, intercalation of the clay is observed. At lower clay concentrations (1-3 wt %), exfoliation predominates. DSC and XRD analysis data show that the crystallinity of PLLA/MMT composites increases drastically at high clay loadings (5-9 wt %). In these nanocomposites, PLLA crystallizes nonisothermally in an orthorhombic crystal structure, assigned to the a form of PLLA.

The poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV)/montmorillonite(MMT) nanocomposites were investigated by wide‐angle X‐ray scattering (WAXS). The aim of the investigation was solution intercalation of MMT with PHBV. Beside the usual orthorhombic unit cell, a stable pseudohexagonal β‐structure of PHBV was obtained. Well known β‐structure has one common WAXS reflection (d = 0.480 nm), which corresponds to the mean distance of PHBV chains in the pseudohexagonal structure. The new β‐structure has two diffraction peaks in the WAXS pattern. It is a three‐dimensionally ordered crystalline structure oriented in parallel with the silica layers of MMT. The new polymorphic form is supposed to be growing on the layers of MMT. Its layers serve as primary nucleation centers for epitaxial growth of the β‐structure. After annealing, this polymorphic form of PHBV disappears and it is transformed into the more stable α‐form leading to an enhanced total crystallinity of the polymer comprised in the nanocomposite. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 751–755, 2009