Siloxane Type Research Articles

Cyclic Versus Linear Siloxane Liquid Crystals: Phase Behavior and X-ray Diffraction Results

Abstract The phase behavior of cyclic and linear polysiloxanes containing biphenyl-4-allyloxybenzoate and cholesteryl-4-allyloxybenzoate mesogens is reported. The presence of the cholesterol-based mesogen was observed to induce layered packing structures for both linear and cyclic siloxane backbones. Differences in the mesophase behavior are discussed with respect to composition and type of siloxane. Small differences in the thermal transitions of cyclic and linear compounds containing the same composition of mesogens were observed. X-ray diffraction results are used to confirm the effect of composition on previously proposed packing models. Differences in the reflection spacings and intensity are discussed with regard to composition, temperature, and type of siloxane. Orientational differences between backbone type observed in aligned fibers are also discussed.

Synthesis and Properties of Copolymers of Diphenylsiloxane with Other Organosiloxanes

ABSTRACTWe describe the synthesis, characterization and properties of various types of siloxane polymers containing diphenylsiloxane (P) as a component. The polymer types include di-and tri-block copolymers with dimethylsiloxane (M) as the second component, and random and statistical copolymers with dimethylsiloxane or methylphenylsiloxane (P/M) as the second component. Such copolymers combine siloxane units whose polymers have very different properties. The polydiphenylsiloxane chain is rigid and inflexible, and the polymer is a highly crystalline solid with a liquid crystalline or condis crystalline state and a very high melting (clearing) temperature. In contrast, the polydimethylsiloxane or polymethylphenylsiloxane chains are very flexible and the polymers have very low glass transition temperatures.Polymers of controlled molecular composition, size and architecture were prepared by anionic polymerization of the ‘cyclic trimers’, using lithium-based initiators.The physical properties of the copolymers vary dramatically with composition and architecture. Two types of ‘random’ copolymers can be prepared. In one type, siloxane units of a given type are randomly placed in the chain in groups of three, i.e., the minimum sequence length of a given siloxane type is three siloxane units. In the other type of random copolymer, individual siloxane units are randomly distributed so that the minimum sequence length is a single siloxane unit. The properties of the two types are quite different, showing that subtle changes in sequence distribution can have major effects on physical properties. At molar ratios near 1/1 and with molecular weights of ∼105, the first type of ‘random’ copolymer is an elastic solid with appreciable mechanical properties, whereas the latter type is a sticky gum.Diblock copolymer (P-M) with dimethylsiloxane as the major component are paste-like, whereas the triblock (P-M-P) and star-block copolymers of the same composition are tough elastomers. The block copolymers are molecular composites, in which the polydiphenylsiloxane component separates into crystalline microphases with very uniform fibrillar or lamellar morphologies, and with widths or thicknesses comparable to the length of the polydiphenylsiloxane block, i.e, typically of the order of 100 Å.

Modified GC‐phases for the enantiomeric separation of optically active compounds: Crosslinking experiments with polymeric chiral compounds

AbstractCrosslinking experiments for immobilization of the well‐known chiral stationary phases XE 60‐L‐valine‐(S)‐ and XE 60‐L‐valine‐(R)‐α‐phenylethylamide within capillary columns have been successfully carried out. Crosslinking was also obtained with mixtures of the chiral compound acrylyl‐L‐valine‐(S)‐α‐phenylethylamide and the polyethylene glycol Carbowax 20M. Immobilization of 75% to 90% was achieved by crosslinking the polymeric chiral phases, but sometimes up to 100% of these phases remained within the columns after the usual subsequent solvent treatment. The crosslinking of Carbowax 20M with acrylyl‐L‐valine‐(S)‐α‐phenylethylamide led to a chiral polymer of a polarity different from that of the common siloxane type chiral phases.

The characterization of acid-set silica hydrosols, hydrogels, and dried gel

Acid-set silica hydrosol, hydrogel, and the dried desiccant silica gel have been characterized by various types of measurements and discriminating tests. Acidic silica hydrosols have a molecular weight of about 2,000 at formation and reach about 2,500,000 at the point of gelation. Molecular growth of silicic acid in this hydrosol was found to be linear. Cross-linking of the polysilicic acid chains occurs in the hydrogel state, and the number of cross-links between chains have been estimated. Silicic acid bonding in acidic hydrosols is believed to be by hydrogen bonding between the silanol groups through water. The siloxane bonding in hydrogels aged beyond syneresis was determined by a depolymerization rate study; a low degree of siloxane type bonding was found. To account for the remaining bonding in the hydrogel, a high degree of hydrogen bonding is suspected. The reasons for hydrogen bonding are given. Most of the silanol condensation must occur during the drying of this type silica gel.