6 Non-conventional soft matter

This critical review covers the recent progress in the research of mesogen-jacketed liquid crystalline polymers (MJLCPs), special side-on side-chain liquid crystalline polymers with very short spacers or without spacers. MJLCPs can self-organize into supramolecular columnar phases with the polymer chains aligned parallel to one another or smectic phases with the backbones embedded in the smectic layers. The semi-rigid rod-like MJLCP with a tunable rod shape in both length and diameter provides an excellent building block in designing novel rod–coil liquid crystalline block copolymers which can self-assemble into hierarchical supramolecular nanostructures depending on the competition between liquid crystal formation and microphase separation (229 references).

Mesogen -jacketed liquid crystalline polymers

Thermotropic columnar mesophases were observed with soap molecules a long time ago. Since then, they have been widely investigated thanks to the synthesis of disk-shaped molecules. In such mesophases, the molecules are packed one upon another to form columns. These then organize according to a two-dimensional hexagonal lattice. In recent years, a variety of compounds with quite different molecular architectures which also exhibit columnar mesophases have been reported. A review of the most recent significant systems is presented. It concerns discotic molecules, polycatenar molecules, side-chain polymers without mesogenic moieties, dendrimers, pseudo-polymeric chains of metal soaps, self-assembled systems driven by specific interactions such as hydrogen bonding or dipolar interactions, and polar columnar phases. Typical examples are presented, with emphasis given to the relationships between the columnar structure and the molecular architecture, along with the specific interactions responsible for the establishment of this type of supramolecular organization.

Columnar Order in Thermotropic Mesophases

In this chapter we review flow-induced mesophases in some crystalline polymers with rather differentcharacteristics: polyethylene terephthalate, isotactic polypropylene and polydiethylsiloxane. Becausethese three polymers are representatives of flexible and semi-rigid-chain polymers, the occurrenceof a mesophase suggests a similar behaviour for many other crystalline polymers. The questionmay be not so much whether crystalline polymers show mesophase behaviour, but rather under which conditionsit can be obtained. A flow field turns out to be an effective tool to unravel structural information,even if hidden in quiescent conditions. The emergence of a mesophase affects strongly any subsequentcrystallization. As a result of templating and nucleating effects, a mesophase may acceleratethe crystallization rate, lead to different crystal modifications, change the morphology and guidethe orientation of the crystals. On the basis of mesophase formation new insights into flow-inducedpolymer crystallization emerge, even though the nature of the mesophase ordering is still not fullyunderstood. A re-evaluation of models for the polymer melt (random coil and folded-chain fringed-micellargrains) is proposed to understand the combined experimental observations on mesophase ordering andcrystallization.

Flow-induced mesophases in crystallizable polymers

Thermal properties are among the most characteristic and at the same time the most technologically important properties of polysiloxanes. They embrace a unique combination of pronounced elasticity at unusually low temperatures and high thermal and thermo-oxidative stability at elevated temperatures. These properties are characteristic of polysiloxanes because they directly originate from a specific interplay of some of the most fundamental features inherent to the basic structural building blocks that make up their repeat units, chain segments and entire macromolecules. They are therefore found in more or less all members of this family of polymers. At the same time they are also of outstanding technological importance because not only that they clearly distinguish these unique polymers from their purely organic, -C-C- type counterparts, but they often make polysiloxanes the materials of choice for many applications where performance under extreme service conditions is required and where no other polymer can successfully satisfy the purpose.

Thermal Properties of Polysiloxanes

The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low-temperature modifications (the monoclinic α1-form and presumably the “tetragonal” β1-form) and two high-temperature modifications (the monoclinic α2-form and the “tetragonal” β2-form). In linear PDES the crystal - crystal transitions α1–α2 and β1–β2 occur near 214 and 206 K, respectively. At higher temperatures α2 (280 K) and β2 (290 K) forms transform into the mesomorphic phase αm that gradually melts at 280–300 K giving an amorphous phase. According to x-ray and density data, αm phase is also characterized by monoclinic structure slightly different from hexagonal packing.

Structure and phase transitions in poly(diethylsiloxane)

An unusual transition from the mesopmrphic state to the amorphous state was found on cooling some poly(dialkoxyphosphazene)s. A generalized phase diagram is constructed for these polymers showing the influence of the alkoxy side group length on their glass transition temperatures, crystallization and ability to form mesophases. The competition of interactions of the polar inorganic main chains and the side methylene chains was proposed to be an important factor which controls the phase behaviour of poly(dialkoxyphosphazene)s

Unusual phase behavior of some poly(dialkoxyphosphazenes)

In contrast to the intensity reduction of X-ray reflexes normally observed on increasing the temperature of the polymers examined with an increase in temperature, both the intensity I and the integral intensity P of the main diffraction maximum, characterizing the inter-chain distance in the structure of these polymers, increase. Temperature variations of intensity are fully reversible. An increase in X-ray reflex intensity with an increase in temperature may be the result of thermal expansion, in which electron density of the main part of the macromolecule remains unchanged, but electron density in the boundary zone of intermolecular contacts decreases. These changes increase the intensity of the intermolecular diffraction maximum. A reversible increase in the intensity of diffraction maxima with an increase in temperature may be observed with fair thermal expansion using polymers with a large diameter of macromolecules ( ∼ 10–15 A ).

New type of temperature variation of X-ray diffraction from a number of polymers

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 A, b = 9.96 A, c = 4.96 A, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.

Structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy) phosphazene]

Morphology of macroreticular, isoporous styrene polymers of the type “Styrosorb” with crosslinking degree between 5 and 100% was investigated by scanning and transmission electron microscopy and by small-angle X-ray scattering. After mechanical destruction of polymers prepared by crosslinking a concentrated polystyrene solution, spherical formations of a diameter ⩾ 10 nm are visible, which represent coils of individual polystyrene molecules (or their associates) crosslinked both intra- and intermolecularly. Small-angle X-ray scattering reveals the existence of inhomogeneities of size varying between 9 and 6·5 nm, which correspond to density fluctuations in the dried samples. Polymers obtained by crosslinking a swollen styrene-divinylbenzene copolymer are isomorphous and do not scatter X rays. Properties of the two types of polymers are the same, though.

V. P. Zhukov

Papers

Structure and phase transitions in poly(diethylsiloxane)

Unusual phase behavior of some poly(dialkoxyphosphazenes)

New type of temperature variation of X-ray diffraction from a number of polymers

Structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy) phosphazene]

Morphology of macroreticular isoporous styrene polymers “styrosorb”