We obtained the single-chain polycarbonate sample by a new fast evaporation method and
found that the polycarbonate sample obtained by this method is completely amorphous, while the
polycarbonate sample obtained by other methods all have a certain degree of crystallinity. The glass
transition temperature (T
g) of the sample decreases with the decreasing of concentration when the
concentration of the prepared solution is below the critical value. The critical concentration we obtained
from the T
g dependence of concentration is 0.9% g/mL and is in accord with that obtained by viscometry
and light scattering methods directly from the solution. The structural relaxation behavior is found also
different from that of a normal bulk sample of polycarbonate. The enthalpic peak of the single-chain
sample is lower than that of the bulk one, which corresponds to the lower glass transition temperature.
The peak of the single-chain sample is lower and broader, and the relaxed enthalpy is much lower
compared with that of the bulk sample. These results have been explained in terms of the effect of
entanglement on the mobility of the segments in polymer and the compact conformation in the single-chain sample.
The effect of entanglements on the glass transition and structural relaxation behaviors
has been studied for polystyrene (PS) and phenolphthalein poly(ether sulfone) (PES-C) samples by fast
evaporation of the solution of concentrations varying from above the overlapping concentration to far
below it, and compared to the results we have studied previously in PC. It has been found that for all the
polymers we have studied, in the concentrated solution region, the T
g of the samples obtained from solution
are independent of the change of concentration and are very close to that of normal bulk samples, whereas
in the dilute solution region the T
g of the samples decrease with the logarithm of decreasing concentration.
The critical concentrations that divide the two distinct regions for the three polymers are 0.9% g/mL for
PC, 0.1% g/mL for PS, and 1% g/mL for PES-C. The decrease of T
g of the samples is interpreted by the
decrease of intermolecular entanglements as the isolation of polymer chains, and the entanglement of
polymer chains restrained the mobility of the segments. The structural relaxation behavior of the polymers
is also found to be different from that of normal bulk samples. The enthalpies of single-chain samples
are lower than that of the bulk ones, which correspond to the lower glass transition temperature; the
peaks are lower and broader, and the relaxed enthalpy is much lower as compared to that of bulk samples.
In the three polymers we have studied, the influence of change of entanglements on both the decrease in
glass transition temperature and relaxed enthalpy is the most significant for PS and the least for PES-C. It is indicated that the interactions in the flexible polymers are weak; thus, the restraint of the
entanglements on the mobility of the segments plays a more important role in the flexible polymers, and
the change of entanglement in the flexible polymers has a more significant influence on the physical
properties.
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