The force-length relations of a set of linear polyethylene fractions and polymers having most probable molecular weight distributions, encompassing a wide molecular weight range, have been investigated. The study of these polymers can serve as models for semicrystalline polymers. The polymers were crystallized in such a manner as to develop as wide a range as possible in the values of the independent structural variables that describe the crystalline state. Several important generalizations can be made from this work. The sharpness of the transition from a brittle to ductile type deformation is established, as is its dependence on molecular weight and certain of the other key structural parameters. It is also quite evident that there is no unique force-length curve for ductile deformations. The character of these curves is very dependent on molecular weight. The deformation process cannot be described in terms of changes in crystallographic features. Neither is there any influence of the supermolecular structure. The ultimate properties are found to depend only on the weight-average molecular weight, indicating the importance of the noncrystalline regions. The initial modulus also depends on the noncrystalline region, albeit in a complex manner. The main variables involved here are the crystallinity level and the interlamellar thickness. In contrast, the yield stress depends on the structure of the crystallite and associated regions. Possible mechanisms for yielding are discussed. Different portions of the stress-strain curves are governed by different structural and molecular features, indicating the complexity of the process.

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It was pointed out by Flory some years ago that for crystalline polymers the boundary between the ordered crystalline region and the disordered liquidlike region cannot be as sharp as in monomeric systems.'s2 The continuity of a long-chain molecule imposes severe constraints on the transition between the perfect order of the crystalline region and the disorder of the isotropic, liquidlike state. A significant proportion of the flux of chains that emanate from the basal plane of the lamellar crystallite needs to be dissipated because of the severe conformational differences between the two states. This results in the return of some chains to the crystallite of origin but not necessarily in juxtaposition. Therefore, a diffuse boundary, or interphase, is formed. Recent theoretical calculations of model systems have given a more quantitative description of the interfacial structure.*a The major factors that determine the structure and extent of the interphase are the free energy of making a fold or bend, the chain density at the crystallite surface, and the ratio of the cross-sectional area of a chain segment in the crystalline and liquidlike state. Kumar and Yoon estimated that for linear polyethylene the interfacial thickness should range from 10 to 30 A.8 A variety of experimental methods, such as broad-line protong and highresolution 13C NMR,l0J1 electron microsc0py,~~J3 smallangle neutron scattering,14 dielectric relaxation,15J6 and Raman s p e c t r o s~o p y ,~~J~ have shown that a significant interfacial region exists in crystalline polymers. The difference in crystallinity levels between density and heat of fusion measurements is directly related to the interfacial ~0 n t e n t . l~ Therefore, a crystalline polymer is comprised of three major structured regions: the ordered crystalline region, the disordered, isotropic, interlamellar region, and the interfacial region. We report here the determination of the thickness of the interfacial region of linear polyethylene from analysis of the Raman internal and longitudinal acoustical (LAM) modes. Experimental SectionTwo series of linear polyethylene samples were used in this work. One was a set of molecular weight fractions and the other possessed most probable molecular weight distributions. The latter set were prepared following the method of Kaminsky et aL20 using (C2Hd2ZrC12 as catalyst. The highest molecular weight fraction, M , = 1.2 X lo6, was prepared by the fractional crystallization from dilute solution of a high molecular weight whole polymer, M, = 8 X 1Og. The other fractions were obtained from the Societe National Elf Aquitaine and ranged in molecular weight from 1.05 X l@ to 9.11 X 105. All the samples were rapidly quenched into a saturated mixture of dry ice and 2-propanol. This crystallization process was deliberately adopted since it yields crystallites having a narrow thickness distribution.21e Fraction 8.08 X lo4 has been used in previous work and was crystallized at 118 "C for 2 and 20 min.23 This crystallization procedure also resulted in a n...

The postmortem remains of sixty-one war victims were excavated from 6 mass graves in Bosnia and Herzegovina one and a half years after interment. Using standard identification methods, including the matching of medical and dental records, the recognition of distinguishing characteristics such as the use of clothing and belongings, and video superimposition, 35 persons were identified. For the remaining 26 persons identification efforts continue. DNA typing was performed at the HLA DQA1 locus and five PM system loci. Results from DNA typing were confirmed by other methods. DNA profiles of family members of 150 missing persons are now being developed using the 6 loci. These DNA profiles will then be compared with those generated from the bone and teeth remains of the unidentified victims.

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