Thermoreversible Gelation in Poly(diphenylsiloxane)/Diphenyl Ether Systems Exhibiting Mesophase Behavior

Abstract

The structural features and the thermodynamics of melting of thermally reversible gels of poly(diphenylsiloxane) (PDPhS) formed from 1.5 to 85 wt % solutions in diphenyl ether (DPhE) have been studied by means of differential scanning calorimetry, X-ray diffraction, and electron scanning microscopy. It was established that gelation occurs as a consequence of the crystallization of PDPhS that leads to a specific spherulitic morphology; i.e., overlapped spherulite-like superstructures, which are composed of lamellar crystallites, form a three-dimensional skeleton filled with the solvent. The phase diagram of the PDPhS/DPhE system was constructed. Its particular feature, resulting from the crystal−mesophase transition in the undiluted PDPhS, is interference between the crystal−isotropic solution and the thermotropic mesophase−isotropic solution equilibrium. The equilibrium temperature T m° and the heat Δ H m° of a virtual crystal−isotropic melt transition in PDPhS in the absence of solvent were estimated (280 °C and 10.34 kJ mol-1) through use of the Flory equation when taking the melting temperatures of the annealed gels as the equilibrium melting points of PDPhS crystals in the presence of DPhE. The isotropization temperature (∼560 °C) of the mesophase, which cannot be measured experimentally because of PDPhS thermal degradation, was assessed by extrapolation of the mesophase−isotropic solution boundary curve on the phase diagram of the PDPhS/DPhE system to the 100% polymer concentration. Phase diagrams expected for the system solvent−crystalline flexible/semirigid-chain polymer, exhibiting a crystal−mesophase transition, were schematically considered and compared to that of the PDPhS/DPhE system. On the basis of this consideration, the conclusion was drawn that the mesophase behavior of PDPhS is connected, at least in part, with a relatively high stiffness of its macromolecules.

Organocatalytic controlled/living ring-opening polymerization of cyclotrisiloxanes initiated by water with strong organic base catalysts

“…The syntheses were successful even though homopolymerization of D(Ph2)3 using water and TMnPG was hard to control because of very low solubility of PDPS in common organic solvents. 54 – 56 The polymerizations of D(Me2)3 were first carried out in CH 2 Cl 2 /THF at 30 °C under the conditions of [D (Me2) ] 0 /[H 2 O] 0 /[TMnPG] 0 = 25/1/0.05 for the former and 25/1/0.25 for the latter. The polymerizations were further continued by adding 25 equiv.…”

Synthesis and characterization of alt‐copoly(carbosiloxane)s containing oligodiphenylsiloxane segments

“…This apparently disrupts the packing ability of dimethylsiloxy units needed for crystal growth 14, 15. Siloxane polymers bearing phenyl groups are also interesting because of their higher thermooxidative stabilities in comparison with those of other siloxanes 14, 16, 17. Despite these attractive properties, poly(diphenylsiloxane) (PDPS) has little utility because it is a rather intractable material.…”

“…At this temperature, the crystalline polymer melts to form a liquid‐crystalline mesophase that does not become isotropic until at least 500 °C. Unfortunately, this is above the onset of thermal decomposition of PDPS and hence is a serious problem for the melt processing of PDPS 16, 17. The critical number‐average molecular weight ( M n ) of PDPS for mesophase formation is approximately 2400 g/mol 18…”

Comparison studies of rheological and thermal behaviors of ionic liquids and nanoparticle ionic liquids

“…The crystals of IL and NIL display tiny and broad melting peaks, up to 25 °C and 35 °C, respectively. Their low crystallinity degree reflecting a slow partial melting-recrystallization mechanism 34 . Note that at Cooling and heating rates: 5 °C/min.…”

“…Their low crystallinity degree reflects a slow partial melting-recrystallization mechanism. 34 Note that at temperatures (T) above 0 1C, NIL exhibits a higher crystallization capacity than IL. Such a peculiar behavior is not observed in NPEP at T 4 0 1C, confirming that the weak anion aggregation and the nanosilica accelerate the PEG crystallization process.…”