Goran Ungar

Controlling polymer shape through the self-assembly of dendritic side-groups

The chain conformation of polymers plays an important role in controlling their phase behaviour and associated material properties. In the case of flexible polymers, conformation is controlled by the degree of polymerization (DP), with low-DP polymers having extended polymer chains and high-DP polymers adopting random-coil conformations in solution and the bulk amorphous state, and folded conformations in the crystalline phase. Exceptions to this general rule are polymers that contain structurally rigid building blocks, or that are subjected to directional shear forces during solidification. The backbones of semi-flexible and rigid rod-like polymers, for example, are always extended in liquid crystalline and crystalline phases, and gel-spun flexible polymers form extended-chain crystals. Here we report a general strategy for the rational control of polymer conformation through the self-assembly of quasi-equivalent monodendritic (branched) side-groups attached to flexible backbones. At low DPs, the conical monodendrons assemble to produce a spherical polymer with random-coil backbone conformation. On increasing the DP, the self-assembly pattern of the monodendritic units changes to give cylindrical polymers with extended backbones. This correlation between polymer conformation and DP is opposite to that seen in most synthetic and natural macromolecules. We anticipate that our strategy will provide new approaches for the rational design of organized supramolecular materials in areas such as nanotechnology, functional films and fibres, molecular devices, and membranes, expanding the synthetic and technological uses of dendritic building blocks,.

Supramolecular dendritic liquid quasicrystals

A large number of synthetic and natural compounds self-organize into bulk phases exhibiting periodicities on the 10-8–10-6 metre scale as a consequence of their molecular shape, degree of amphiphilic character and, often, the presence of additional non-covalent interactions. Such phases are found in lyotropic systems (for example, lipid–water, soap–water), in a range of block copolymers and in thermotropic (solvent-free) liquid crystals. The resulting periodicity can be one-dimensional (lamellar phases), two-dimensional (columnar phases) or three dimensional (‘micellar’ or ‘bicontinuous’ phases). All such two- and three-dimensional structures identified to date obey the rules of crystallography and their symmetry can be described, respectively, by one of the 17 plane groups or 230 space groups. The ‘micellar’ phases have crystallographic counterparts in transition-metal alloys, where just one metal atom is equivalent to a 103 - 104-atom micelle. However, some metal alloys are known to defy the rules of crystallography and form so-called quasicrystals, which have rotational symmetry other than the allowed two-, three-, four- or six-fold symmetry. Here we show that such quasiperiodic structures can also exist in the scaled-up micellar phases, representing a new mode of organization in soft matter.

The crystallization of ultralong normal paraffins: the onset of chain folding.

The nature of chain folding in polymers and the determination of the chain length at which folding occurs have been central questions in polymer science. The study of the formation of lamellar polymer crystals through chain folding has received a new impetus as a result of the recent synthesis of normal alkanes of strictly uniform chain lengths up to C390 H782. Chain folding is found in all such paraffins starting with C150H302. As with polyethylenes obtained by conventional polymerization, the fold length in the normal alkanes varies with crystallization temperature, but it is always an integral reciprocal of the full chain length. This behavior indicates that the methyl end groups are located at the lamellar surface and that the fold itself must be sharp and adjacently reentrant.

Electro-functional octupolar π-conjugated columnar liquid crystals.

A series of propeller-shaped π-conjugated molecules based on 2,4,6-tris(thiophene-2-yl)-1,3,5-triazines has been designed and synthesized to obtain ambipolar charge-transporting liquid-crystalline materials. The 3-fold electron-donating aromatic units are attached to the electron-accepting triazine core, which forms electro-functional octupolar π-conjugated structures. These octupolar molecules self-organize into one-dimensional columnar nanostructures and exhibit ambipolar carrier transport behavior, which has been revealed by time-of-flight measurements. In this approach, electron-donor and acceptor electro-active segments are assembled individually in each column to give one-dimensional nanostructured materials with precisely tuned electronic properties. Their desirable electronic structures responsible for both hole and electron conductions have also been examined by cyclic voltammetry and theoretical calculations. The present results provide a new guideline and versatile approach to the design of ambipolar conductive nanostructured liquid-crystalline materials.

Radiation effects in polyethylene and n-alkanes

The effect of high-energy radiation on n-alkanes and polyethylene (PE) are reviewed. The article includes a description of (a) primary radiation events, formation, spatial distribution and decay of free radicals and radical migration mechanisms, (b) formation of cross-links and other stable products, (c) the effects of crystallinity, crystalline modifications and morphology on radiation cross-linking, and (d) radiation-induced changes in the crystal lattice and the destruction of crystallinity at high irradiation doses. An attempt is made to bring together conflicting views on various unsolved problems of the radiation physics and chemistry of these important and much studied substances. Long-chain paraffins are given considerable attention since their radiation behaviour often provides a link between that of PE and its short-chain analogues.

Thermotropic hexagonal phases in polymers: common features and classification☆

Abstract Polymeric mesophases with positional order in two dimensions are considered in relation to the established liquid crystal phases. It is proposed that the mesophase with hexagonal symmetry observed in polymers as flexible as polyethylene and poly(diethylsiloxane) is columnar liquid crystal. The relationship is discussed with columnar phases in non-discotic low molar compounds and, particularly, with polymers like poly(dialkylsilane)s or alkylated polyglutamates. It is argued that the presently considered hexagonal phases are properly classified as liquid crystalline in spite of the absence of both rigid moieties and amphiphilic character from some of the polymers concerned. Since the phases described here have in the past prompted the introduction of ‘condis crystals’ as the third class of mesophases, alongside plastic and liquid crystals, some comments are made about the general classification of mesomorphic states.

Inversion of the temperature dependence of crystallization rates due to onset of chain folding

Crystallization rate experiments performed on the uniform alkanes C246H494 and C198H398, by both differential scanning calorimetry and in situ X-ray diffraction (using a synchrotron source), have revealed that these rates, including both primary nucleation and growth, pass through a minimum with increasing supercooling. The first (expected) increase and subsequent (unsuspected) decrease correspond to extended-chain (E) crystallization, the renewed increase beyond the minimum corresponding to chain-folded crystallization with the fold period l being smaller than L but larger than L2, where L is the extended chain length. The anomalously retarded crystallization with increasing supercooling, new even qualitatively, appears to arise through competition between extended- and folded-chain deposition. The attachment of folded chains evidently involves a much lower free energy barrier than does the attachment of extended chains. Even if the former process cannot lead to growth of stable chain-folded crystals above their melting temperature, it seriously hampers the only productive process, chain-extended crystallization. The observed effects, which have come to light owing to the availability of ultra-long and uniform n-alkanes, help to provide new insight into the primary stages of chain-folded crystallization, with many potential consequences, some of which are discussed.

Time-resolved synchrotron X-ray study of chain-folded crystallization of long paraffins

Abstract The examination of the crystallization of strictly uniform ultra-long n-alkanes was resumed with the view of exploring the onset of chain-folding, as laid out previously in ref. 1. The present study, largely on C 246 H 494 , centred on the initial stages of crystallization in the melt, registered in situ by time-resolved small-angle X-ray scattering using a synchrotron X-ray source. The salient new feature was the identification of transient initial fold lengths which were non-integer fractions (NIF) of the chain length. This NIF structure transforms subsequently into forms with integer fraction (IF) fold lengths. In the present study the latter have the extended chain (E) and once-folded (F2) configurations, while NIF has a fold length between the two. The NIF → IF transformation occurs either by lamellar thickening or thinning, or by both. It was found that the NIF state had a more disordered layer surface as compared to the final E and F2 structures, the latter being the states on which the conclusions in ref. 1 had been drawn, which accordingly should apply to the transformed material. Implications of these and several other findings for some central issues in polymer crystallization are briefly discussed. The existence of an initial NIF phase focusses attention to the importance of the fastest kinetic pathway as the determining factor for chain-folded crystal growth with particular attention to the initial chain deposition probability, a line made accessible by the present alkanes.

Effect of radiation on the crystals of polyethylene and paraffins: 1. Formation of the hexagonal lattice and the destruction of crystallinity in polyethylene

Abstract In this, the first of a series of three preliminary papers on the effect of high dose radiation on the paraffinoid crystal lattice, the destruction of crystallinity in polyethylene under the influence of γ-radiation is followed by X-ray diffraction and calorimetry. The principal new finding is the occurrence of an orthorhombic-hexagonal phase transition on heating subsequent to irradiation at ambient temperature beyond a certain critical dose. This othorhombic-hexagonal transition temperature is an increasing function of crystal thickness, and for all samples decreases with increasing dose. While the lattice parameters are seen to change gradually with dose (as observed at room temperature) the destruction of crystallinity itself occurs rather catastrophically beyond a certain dose. It is envisaged that this collapse of crystallinity is taking place as the temperature of the orthorhombic-hexagonal transition, which decreases steadily with dose, approaches the irradiation temperature. The above scheme is supported by the higher radiation sensitivity of the hexagonal, or nearly hexagonal, phase as found here in paraffins. As a side-line, the present findings lead to a grading of hexagonal phases in paraffinoid substances as obtained along different routes.

Mesogenic Behaviour of some 1,4,8,11,15,18,22,25-Octa-alkylphthalocyanines

Abstract Discotic columnar mesophase behaviour exhibited by ten metal-free and copper 1,4,8,11,15,18,22,25- octa-alkylphthalocyanines has been characterised by optical microscopy, differential scanning calorimetry, x-ray diffraction and electronic absorption spectroscopy. Compounds with side-chains as short as hexyl exhibit one or more mesophases, and a total of three different mesophases are apparent for the series as a whole. The highest temperature mesophase is assigned as Dhd. At lower temperatures, some compounds develop a second mesophase of hexagonal symmetry, whereas others form a mesophase of rectangular symmetry. One example shows all three mesophases.