Phase-separating blends, thermally

The behavior of chemical phase-separated blends in the bulk after thermal quenching into the unstable region of the phase diagram is variable. In the bulk, the concentration fluctuations that govern the phase-separation process are random. As a result, the final morphology consists of mutually interconnected domain structures rich in a given blend component that coarsen slowly with time. 

The animation video clip for the surface of the phase-separated blend samples and another of the inner cavity of the blend sample can analyse the branch structure for all of the inner cavities. The scale width increases as the thermal treatment time increases. The micrometre-scale phase-separated structure of the PS-Br/PMMA blends is shown in Figure 23. It is seen that phase separation advances with the thermal treatment time. The distribution of the branch structure for a bicontinuous structure obtained from 3D NMR images is shown in Figure 23, where the thermal treatment times for the samples are 6 h for A, 8 h for B and 10 h for C at 180 °C. As seen from this figure, the fraction that the bicontinuous structure takes of the three branches, at each junction point, is more than 50%. The average distribution of the branching number at the junction points is almost independent of the thermal treatment time in the present experiments. 

Here excimer fluorescence from phenyl-phenyl interactions in PS is the main experimental observable. This blend was selected because it has been demonstrated by numerous other techniques that miscible one-phase blends may be prepared by solution casting from toluene solvent. [2,3] Moreover, the blend may be forced to phase separate by thermal means, leading to a two phase system. In addition, we will consider results for blends of poly(2-vinyl naphthalene) (P2VN) with low molecular weight poly(cyclohexyl methacrylate) (PCMA) and polystyrene (PS). 

Under the DSC conditions (N, scanning rate = 10°C/min), it is apparent that the decomposition processes are occurring at a much faster rate at or near the temperature at which cure is taking place in all the pure dlcyanate samples. Both BADCy and THIOCy showed small exotherms (onset at 277°C and 226°C and peak at 308°C and 289°C, respectively). Their major decompositions began about 251°C and 246°C, respectively, as observed by TGA. On the contrary, all the 1 1 BCB/dlcyanate blends displayed the expected thermal transitions. Besides initial Tg s (20-28°C) and Tm s (171-183°C), all samples showed small exotherms in their DSC scans with maxima at 147-151°C. This is attributable to the thermally-induced crystallization in the mixtures, which also led to some initial phase separation. The polymerization exotherms are consistent with the typical temperature ranges for the known benzocyclobutene-based systems (onset 229-233°C max. 259-266°C). 

The coupling of the order parameter to the temperature gradient also leads to unexpected excursions along the concentration axis in the case of off-critical mixtures. As a consequence, equilibrium phase diagrams lose their usual meaning in thermal nonequilibrium situations, and even an off-critical blend with a temperature above the binodal can be quenched into phase separation by local heating with a laser beam. 

Hill and Barham [133] showed by transmission electron microscopy that blends of high and low molar mass polyethylene melts were homogeneous with no detectable phase separation. The blends were prepared by solution mixing to obtain an initially homogeneous blend before the thermal treatment in the melt. It should be realised that the mechanical mixing of high and low molar mass linear polyethylenes to obtain a homogeneous melt may require considerable work and time. 

Exclusively mechanically interlocked linear polymer blends, typically, are not thermodynamically phase stable. Given sufficient thermal energy (Tuse>Tg), molecular motion will cause disentanglement of the chains and demixing to occur. To avoid phase separation, crosslinking of one or both components results in the formation of a semi-IPN or full-IPN, respectively. Crosslinking effectively slows or stops polymer molecular diffusion and halts the phase decomposition process. 

The following discussion will be restricted to evolution of phase morphologies preferably in late stages in solutions and blends of immiscible polymers wherein phase separation is initiated by solvent evaporation during casting and thermal agitation, respectively. 

Fig. 17a-c. Phase morphologies resulting from thermally induced phase separation of PPTA/PA-6 blends, a 25/75, b 40/60 c 60/40... 

The method used to provide contrast in transmission electron microscopy was successful in demonstrating the presence of a two-phase structure in homopolymer blends of BR and IR (Figure 2a). The opposite situation, i.e., a clear absence of any phase separation in the block copolymers, also is demonstrated, but much less convincingly by the comparison of Figures 2b and 2c. It is necessary to consider the evidence from all of the mechanical and thermal analysis experiments, along with the evidence from microscopy. 

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