A Raman spectroscopic study of the morphological structure of the polyethylenes

The method described by Strobl and Hagedorn to analyze the Raman spectrum internal modes of semi-crystalline polyethylene has been applied to a set of selected polyethylene samples crystallized under controlled conditions. The crystallite structure can be described in terms of the relative amounts of the crystalline orthorhombic phase, the liquid-like amorphous phase and the interfacial region. The dependence of the level of crystallinity on molecular weight and crystallization conditions is very similar to that found by other methods. However, this method allows for the quantitative determination of the interfacial content which becomes significant for molecular weights greater than about 1×10⁵ for linear polyethylene fractions, and for all the branched samples and copolymers. The degree of crystallinity determined from density measurements is equal to the sum of the crystallinity and interfacial content obtained from the Raman analysis while enthalpy of fusion measurements yield values which are equal to just the crystallinity content. The difference between the level of crystallinity obtained from density and enthalpy of fusion is thus found to be primarily due to interfacial contributions.Dedicated to Prof. Dr. F. H. Müller     

The observation of cis residues in poly(L-proline) in aqueous solution.

Proton Fourier transform NMR spectra at 270 MHz were obtained for three different molecular weight samples of poly(L-proline) in D2O. A weak, but clearly discernable, resonance at 4.3 ppm was observed in each case with an integrated intensity about 2-3% of the Calpha trans proton resonance. Based on the Torchia-Bovey assignment this resonance is attributed to the Calpha cis proton. The presence of cis residues, even in this small concentration, necessitates a reexamination of the conformational properties of this polymer. Definite conclusions cannot be reached from spectra obtained in organic solvents because of the much smaller separation between the cis and trans resonances.     

Crystallization and Melting of Model Polyethylenes with Different Chain Structures

The crystallization and melting of three model polyethylenes of different chain structures have been studied. The polymers studied were a linear copolymer, hydrogenated poly(butadiene); a hydrogenated poly(butadiene)-atactic poly(propylene) diblock copolymer; and a three-arm star hydrogenated poly(butadiene). An important feature of this work was that the crystallizing portions of the copolymers all have the same molecular lengths.It was found that the overall crystallization rate decreases steadily from a linear to a diblock to the star copolymer. The differences in crystallization rates are related primarily to the activation energy for segmental transport. The non-crystallizable structure affects the segmental mobility to different degrees. An estimation of this effect is presented from the analysis of the overall crystallization rates using classical nucleation theory. In spite of thedifferences in their molecular structure, there are no major differences in the supermolecular structure of samples crystallized rapidly or slowly cooled.The melting process followed by DSC of the isothermally crystallized linear and star copolymers shows two endothermic peaks at intermediate undercoolings. The double melting is associated with a partitioning of crystallizable ethylene sequences during crystallization. The longest sequences are preferentially selected in the early stages of the crystallization. Single melting peaks are obtained for high and very low undercoolings for the linear and the star copolymers as well as for the diblock in the whole range of temperatures. The lack of the second, lower melting endotherm in the diblock could be associated with the influence in the crystallization process of the amorphous block in the microphase segregated melt.This revised version was published online in November 2005 with corrections to the Cover Date.     

A Raman-spectroscopic study of solution-crystallized polyethylenes

An analysis of the Raman internal modes of dilute-solution-crystallized homopolymers and co-polymers of ethylene has been made, similar to the work previously reported for the bulk-crystallized polymers. The crystallite structure can be described in terms of the relative amounts of the crystalline orthorhombic phase, the liquidlike amorphous phase, and the interfacial region. These quantities change with the molecular constitution of the chains and the crystallization conditions. The level of crystallinity decreases significantly with increasing counit content as would be expected. In addition, an appreciable interfacial structure develops in copolymers as compared with the homopolymers. A possible relationship between the interfacial content and the relaxation transitions in polyethylene is discussed.     

Multiple melting transitions in natural rubber

Detailed studies of the fusion of natural rubber, following isothermal crystallization, has revealed a hitherto undetected low temperature transition which depends on the crystallization temperature. This transition is shown to be a melting phenomenon which is not caused by any polymorphic structural changes. It can be attributed to the formation of less stable crystallites which develop as the transition progresses.     

Temperature dependence of the bulk crystallization rate of polymers

Data already existing in the literature for the overall crystallization kinetics of a variety of polymers have been analyzed according to different possible nucleation mechanisms. The conclusions reached are similar to those previously deduced from an examination of ata for spherulite growth rates. It is demonstrated that, if allowed a reasonable choice for the equilibrium melting temperature, no unbiased selection of a unique nucleation process can be made. Moreover, a set of universal parameters exists for each of the nucleation and growth processes considered which allows the data for all polymers to be represented by a single straight line. The only quantities that are unique to a given polymer are the equilibriun melting temperature and the activation energy for transport.     

Four-pion final state in pp annihilations at 0.7 to 1.1 GeV/c

Results are reported on the reaction $\text{p}\text{p}\text{→ π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{?}}$ at six lab momenta spanning the region from 0.686 to 1.098 GeV/c. The cross section for this process drops from 5.2 ± 0.4 mb at 0.686 GeV/c to 2.9 ± 0.2 mb at 1.098 GeV/c. Resonance production is determined by means of a model which includes Bose symmetrization and Breit-Wigner amplitudes. The likelihood fit to the resonance channels yields a few per cent ?ππ, approximately 10% in each of the channels fππ, ??, and A₂π, and about 40% ?f production, with errors on the order of 5%. Excellent fits to the mass distributions are obtained. Essentially all production and decay angular distributions are found to be uniform, with only a few small deviations from isotropy reported. The angular correlations between like and unlike charged pion pairs are also found to be fit extremely well by the simple model of resonance production with Bose symmetrization.