In the rich and long-standing literature on the flow-induced formation of oriented precursors to polymer crystallization, it is often asserted that the longest, most extended chains are the dominant molecular species in the “shish” of the “shish-kebab” formation. We performed a critical examination of this widely held view, using deuterium labeling to distinguish different chain lengths within an overall distribution. Small-angle neutron-scattering patterns of the differently labeled materials showed that long chains are not overrepresented in the shish relative to their concentration in the material as a whole. We observed that the longest chains play a catalytic role, recruiting other chains adjacent to them into formation of the shish.

Relationship between the structure of injection-molded isotactic polypropylene and its tensile mechanical properties, necking and fracture behaviors in particular, was investigated in terms of micrometer-scale structural inhomogeneity of nanometer- and subnanometer-scale structures. To clarify the micrometer-scale inhomogeneity, we employed scanning microbeam wide-angle X-ray diffraction and small-angle X-ray scattering technique. Four isotactic polypropylene samples were studied, produced using different injection-condition and thermal treatments. The results of scanning microbeam X-ray scattering measurements showed the presence of two types of micrometer-scale structural inhomogeneity in addition to the orientation of molecules: the distribution of polymorphs and of crystalline ordering. The results of scanning microbeam X-ray scattering of deformed sample showed the disappearance of the β-form isotactic polypropylene crystals at the outer regions accompanied by the plastic deformation. It is indicated that the inhomogeneous distribution of crystalline ordering and the existence of different polymorphs are highly related to the tensile mechanical behavior.

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Polypropylene (PP) and its derivatives, poly(5-hexen-1-ol-co-propylene) (PPOH) and poly(1-hexene-co-propylene) (PPH), were comprehensively investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis (DMA), 1H pulsed nuclear magnetic resonance (NMR), infrared (IR), and positron annihilation lifetime (PAL) techniques. The deduced nanoscopic amorphous structure was discussed in comparison with the mechanical properties such as tensile strength. Introducing polar hexenol and nonpolar hexene comonomers into PP chains resulted in a significant reduction of the crystallinity for both the derivatives, which failed to explain the anomalous higher tensile strength of PPOH. The long decay T 2 observation from NMR indicated a lower chain mobility in the amorphous region of PPOH compared to PPH, consistent with the DMA analysis, providing a higher glass transition temperature for the former polymer. The PAL and IR results signified reduced free-volume size along with hydrogen bond formation in the amorphous region of PPOH, illustrating an essential role of the nanoscopic amorphous structure in the superior mechanical strength of PPOH compared to the other polymers.

Supramolecular block copolymers comprising isotactic polypropylene (iPP) and ethylene−propylene random copolymers (EP) with complementary quadruple hydrogen bonding junctions have been prepared by melt-mixing of iPP having a 2-ureido-4[1H]-pyrimidinone (UPy) group (iPP-UPy) and elastic EP bearing a 2,7-diamido-1,8-naphthyridine (Napy) group (EP-Napy). Transmission electron microscope (TEM) analysis of the iPP-UPy/EP-Napy composite showed that the elastic EP domains were well dispersed in the iPP matrix compared with the traditional iPP/EP impact polypropylene copolymer (IPC). The iPP-UPy/EP-Napy hydrogen-bonded pseudo block copolymer effectively acts as a compatibilizer in the IPC and contributes to improved mechanical properties of the resulting iPP/EP composite. There is good correlation between impact strength of the IPC and EP domain size observed by the TEM analysis. The use of the complementary quadruple hydrogen bonding system for blending two immiscible polymers has been shown to result in smaller domain sizes of the EP-phase in the iPP and consequently improved mechanical properties of the supramolecular iPP/EP blends.

COMMUNICATIONStions is significant, and future studies will show whether these findings are limited to the systems here or a general feature for density functional treatments of 5-hexenyl and related radical rearrangements.In conclusion we have been able to draw a clear picture of the lowest energy transition state structures of the cyclizations of 4-penten-I-oxyl radical by ab initio methods. Semiempirical calculations are not suited for our calculations on open-she11 molecules; however, computational studies on radicals 1-5 using UHF/6-31G* and UBPiDZVP methods provide results that are qualitatively and quantitatively in good agreement with the experimental findings. Cornputational MethodsSemiempirical calculations were performed on an SGI IRIS INDIGO R4000/R4400 workstation with the program package VAMP 5.0j5.5 [12]. All other ab initio calculations were performed on a CRAY Y-MP/8-128 computer using the Gaussian 92 program [I31 (Hartree-Fock approach) and DGauss 3.0 [I41 (density functional approach). The UHF/6-31Gt-minimized structures of radicals 1, 3 and 5 were determined by using the default optimization algorithm as implemented in Gaussian 92. The DFT geometries of radicals 1, 3 and 5, were minimized by means of a Newton-Raphson/DFGS optimization technique using a unrestricted nonlocal SCF spin-density approach with the Becke-Perdew functional. A DZVP hasis set was used and the electron density was fitted by means of a triple-zeta A1 set. The transition state geometries were optimized with the optimization algorithm by Baker (UHF) and the standard algorithm (DF) after full calculation of the hessian matrix. Force calculations were applied to characterize minima and transition structures by calculation of their normal vibrations. la,b and 2a,b were derived from mesoporphyrin I1 and etioporphyrin I with enantiotopic faces, respectively, and a stereogenic center at the alkylated nitrogen atom. A---. r h o -I n t F a F n d 1991; 75 No 37124 VCH Verlagsgesellschaft mbH, 0-69451 Wemheim, 1996

Poly(benzyl ether) dendrimers with o‐, m‐, and p‐isomers of dialkoxybenzene at their focal points [o‐, m‐, and p‐(Gn)2Ar], having generation numbers (n) of 0–3, were synthesized. 1H NMR pulse relaxation times (T1) of the exterior MeO groups of o‐ and m‐(Gn)2Ar (n = 0–3) all remained in the range of 0.92–1.43 s. In sharp contrast, an exceptionally short T1 value (0.23 s) was observed for p‐(G3)2Ar. Although their absorption spectral profiles were slightly different from one another, an essential difference was observed for their fluorescence properties. When the generation number was increased, the fluorescence efficiency of o‐(Gn)2Ar increased, but that of p‐(Gn)2Ar decreased, whereas m‐(Gn)2Ar exhibited a relatively small change in the fluorescence efficiency. Fluorescence depolarization studies showed a highly efficient intramolecular energy migration in p‐(G3)2Ar as compared with o‐(G3)2Ar and m‐(G3)2Ar. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3524–3530, 2003

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