Side-chain crystallinity,heat of melting,and thermal transitions in poly[N-(10-n-alkyloxycarbonyl-n-decyl)maleimides] (PEMI)

The heat of melting, the melting temperature T_m, and the sub-T_g transition temperature have been studied from –120°C to above T_m in a series of 11 poly[N-(10-n-alkyloxycarbonyl-n-decyl)]-maleimides (PEMI). Side-chains from ethyl to n-docosyl with n even have been included. The contribution to the heat of melting per methylene group shows that the hexagonal paraffin crystal modification is present in these poly(N-maleimides), in agreement with x-ray data for the same compounds. The enthalpy data show that only a part of the outer methylene groups are present in the crystalline aggregates. Furthermore, DSC traces exhibit a typical distribution of crystallite sizes, which become narrower as the side-chains become longer. The critical chain length needed to form a stable nucleus includes nine methylene groups in the outer part of the n-alkyl side-chain. The influence of the side-chain length and crystallinity on the γ-transition temperature of these polymers was also investigated. In the range where these polymers are essentially amorphous the sub-T_g transition temperature decreases continuously as the number of methylene groups in the side-chain increases. This transition is attributed to internal motion within the external side-group without any interaction with the main chain. This is presumably made possibly by the partial rotation of the oxycarbonyl group. We suggest that this transition is similar to the well known γ transition which has been attributed to various segmental motions in all ethylene copolymers and in all homopolymers containing a determined number of? CH₂? units in the main-chain or in the side-chain. Estimates based on the chemical structure, yield a value for the γ transition of ? CH₂? similar to that measured by other methods in polyethylene and related materials.     

X-ray diffraction study of bulk poly N-substituted maleimides

Summary A series of 7 aliphatic and 6 aromatic poly N-substituted maleimides (PNMI) was synthetized by free radical polymerization. The series was characterized by means of intrinsic viscosity, number average molecular weight and glass transition temperature measurements. The X-ray diffractograms of all samples show two rather broad maxima indicative of non-crystalline structures. The location of the innermost peak strongly depends on the length of the substituted groups and can be associated to intermolecular vectors. The observed linear increase of the spacing associated to the inner maximum with the number of methylene groups of the substituted aliphatic chain favours the idea of predominantly extended lateral conformations. The outer peak due is to intramolecular average distances along the backbone chain and slightly depends on the shape of the substituted groups. This repeat distance fits well with the assumption of a 4/1 helix for the chain conformation. In the aryl series a third maximum appearing as a shoulder on the outer peak suggests the presence of chain sequences coiled in 3/1 helices. The chain bundle dimensions calculated from the broadening of the innermost maximum are of the order of two intermolecular vectors suggesting, hence, the absence of a well defined short range order. A model for the packing of PNMI molecules with lengthy substituted groups is proposed.

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Zusammenfassung Eine Serie von 7 aliphatischen und 6 aromatischen Poly-N-substituierten Maleimiden (PNMI) wurden mit radikalischer Polymerisation synthetisiert. Die Proben wurden mit Hilfe von Messungen der intrinsic-Viskosität, des Molekulargewichts-Zahlenmittels und der Glas-Temperatur charakterisiert. Röntgendiagramme der Proben zeigen zwei ziemlich breite Maxima, entsprechend der nichtkristallinen Struktur. Die Lage des inneren Piks hängt streng von der Länge der substituierten Gruppen ab und kann mit diesbezüglichen zwischenmolekularen Abständen verknüpft werden: Das beobachtete lineare Anwachsen des mit dem inneren Maximum verknüpften Abstandes entspricht nämlich der Anzahl der Methylengruppen in der substituierten aliphatischen Kette und weist darauf hin, daß sich die laterale Konformation vergrößert. Der äußere Pik entspricht mittleren intramolekularen Abständen längs der Kette und hängt nur wenig von den substituierten Gruppen ab. Die Distanz paßt zur Annahme einer 4/1-Helix-Konformation. In der Arylserie erscheint ein 3. Maximum, das auf das Auftreten von 3/1 Helices hinweist. Die berechneten Dimensionen von Kettenbündeln aus der Verbreiterung des inneren Maximums sind von der Größenordnung des doppelten inter-molekularen Abstands und zeigen somit an, daß keine wohldefinierte Nahordnung besteht. Ein Modell für die Packung der PNMI-Moleküle mit substituierten Seitengruppen wird vorgeschlagen.

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Dedicated to Prof. Dr.R. Hosemann on the occasion of his 60th birthday.     

Long-wavelength excitations in a Bose gas at zero temperature

The long-wavelength excitations in a simple model of a dilute Bose gas at zero temperature are investigated from a purely microscopic viewpoint. The role of the interaction and the effects of the condensate are emphasized in a dielectric formulation, in which the response functions are expressed in terms of regular functions that do not involve an isolated single-interaction line nor an isolated single-particle line. Local number conservation is incorporated into the formulation by the generalized Ward identities, which are used to express the regular functions involving the density in terms of regular functions involving the longitudinal current. A perturbation expansion is then developed for the regular functions, producing to a given order in the perturbation expansion an elementary excitation spectrum without a gap and simultaneously response functions that obey local number conservation and related sum rules.Explicit results to the first order beyond the Bogoliubov approximation in a simple one-parameter model are obtained for the elementary excitation spectrum ω_k, the dynamic structure function $\text{S}$(k, ω), the associated structure function $\text{S}$_m(k), and the one-particle spectral function $\text{A}$(k, ω), as functions of the wavevector k and frequency ω. These results display the sharing of the gapless spectrum ω_k by the various response functions and are used to confirm that the sum rules of interest are satisfied. It is shown that ω_k and some of the $\text{S}$_m(k) are not analytic functions of k in the long wavelength limit. The dynamic structure function $\text{S}$(k, ω) can be conveniently separated into three parts: a one-phonon term which exhausts the f sum rule, a backflow term, and a background term. The backflow contribution to the static structure function $\text{S}$₀(k) leads to the breakdown of the one-phonon Feynman relation at order k³. Both $\text{S}$(k, ω) and $\text{A}$(k, ω) display broad backgrounds because of two-phonon excitations. Simple arguments are given to indicate that some of the qualitative features found for various physical quantities in the first-order model calculation might also be found in superfluid helium.     

Studies related to the synthesis of maytansinoids.

Two approaches to the partial synthesis of C₁–C₅ of a bis-nor maytansinoid $\text{1}$ have been investigated starting on the one hand with (S)-(?)-malic acid and on the other, with D-(+)-ribonolactone.