No abstract

The influence of solid state drawing of solution-crystallized ultra-high molecular weight polyethylene on the structure has been studied. Results obtained by different wide-angle and small-angle X-ray techniques support a deformation mechanism, which was partially described by Peterlin. An extended mechanism is proposed, which not only explains the orientational effects observed via WAXS and SAXS fairly easily, but can also account for the constancy of the long period, the steady decrease of the meridional SAXS intensity, the increase in crystallinity and the increase in the longitudinal crystallite size as the draw ratio increases (starting from a draw ratio of 10). Furthermore, the observed changes in lateral apparent crystal sizes as well as the stress-strain behaviour during the drawing process can be understood easily in terms of this model.

Melaminium dihydrogenpyrophosphate (bis(2,4,6-triamino-1,3,5-triazinium dihydrogenpyrophosphate, MPy), obtained as a crystalline powder by thermal treatment of melaminium dihydrogenphosphate (MP), is the first intermediate in the synthesis of polymerized melamine phosphates that are used as environmental-friendly flame retardants. The crystal structure of MPy was determined by X-ray powder diffraction and the proposed hydrogen-bonding model was corroborated by solid-state NMR and periodic DFT calculations. A mechanism is proposed for the endothermic dehydration process that takes place in the reaction from MP to MPy, in accordance with energy calculations based on the optimized crystal structures of MPy and MP. A packing analysis of MPy and other melamine-containing compounds revealed some typical melamine packing motifs.

A series of side chain dendritic polymers (SCDP) was produced by copolymerization of dendritic diols of the second, third, and fourth generation with four different semirigid and flexible diisocyanates. The polymerization reactions were conducted both in solution and in bulk. The degree of polymerization and molecular weight distribution of SCDPs were greatly influenced by the flexibility of the diisocyanates and the nature of the reaction medium. The conformation of SCDPs in solution and its dependence on the molecular weight were examined with the aid of intrinsic viscosity ([η]) measurements. According to the results obtained, SCDPs based on the third and fourth generation monodendrons changed in conformation from spherically to cylindrically shaped structures at a certain molecular weight. The supramolecular assembly of dendritic monoand polymers in bulk was investigated by X-ray diffraction measurements. It was demonstrated that these macromolecules were, in principle, able to form body-centered cubic lattices. Some results suggested that the ability to form such a type of higher order structures increased as a result of polymerization of dendritic monomers. These findings were supported by molecular dynamics simulations conducted on dendritic monoand polymers. By means of such calculations, we also studied the effect of the degree of polymerization on the molecular conformation up to a degree of polymerization of seven. We derived the double logarithmic plots of the calculated values of the radius of gyration (R g ) versus molecular weight (R g ∝ M ν ). In a medium consisting of a poor solvent, the data scaled according to ν ) 0.36, which agreed well with some of the theoretical predictions made for the conventional dendrimers (theory: ν ) 0.33). In a good solvent, however, the conformation of SCDPs approached the Gaussian distribution statistics.

We present a detailed experimental and modeling investigation of poly(ethylene-4,4′-biphenyl dicarboxylate) (PEBB) and the related random copolymers poly(ethylene-4,4′-biphenyldicarboxylate-co-2,6-naphthalene dicarboxylate) (PEBB/EN). We find that different polymorphs exist for the PEBB homopolymer and suggest a second crystal unit cell, different from that which is discussed in the literature. PEBB/EN copolymers, which are shown here to be random copolymers, are found to be semicrystalline for all naphthanoate concentrations. By combination of DSC, X-ray, and solid-state NMR experiments, uniform comonomer inclusion is proposed for copolymers containing a high amount of EN; for low EN concentrations, these comonomers are rejected from the crystals. By comparison of the comonomer inclusion Gibbs energy, which was estimated from experiments, to those obtained from thermodynamic integration calculations, it was found that PEBB/EN copolymers are able to cocrystallize by aggregation of like and by segregation of unlike repeat units. Once these comonomer inclusions have reached a considerable size, the average defect Gibbs energy reduces to the thermal energy.