FRACTAL NATURE OF ONE-STEP HIGHLY BRANCHED RIGID RODLIKE MACROMOLECULES AND THEIR GELLED-NETWORK PROGENIES

Several highly branched polymeric systems were prepared by a one-step polymerization. They are characterized by stiff trifunctional branchpoints connected by rigid rodlike segments. A few systems with flexible segments were prepared for comparison. The systems were studied in their pregel and post-gel states. Small-angle X-ray scattering intensity measurements from bone-dry and concentrated solutions are consistent with the expectations of the polymeric fractal model. Static light scattering combined with photon correlation spectroscopy revealed the polymeric species in the pregel state to be highly branched. End-group titration and segment-tip decoration by iodine clearly indicate the highly branched nature of the pregel as well as postgel systems. The kinetics of particle growth prior to the gel point and solution property characteristics both agree with the fractal model. Scanning electron microscopy of the dried pregel material yielded typical fractal morphology. Porosimetry studies of one dry postgel network supports the fractal concept. When compatible macromolecular fillers were added to the reaction mixture of the one-step systems prior to the gel point, the modulus of the resulting “infinite” network gels was consistently lower than the modulus of the corresponding neat gel. When the rigid networks were prepared in two steps from preexisting high-M chains, their modulus increased upon the addition of the same filler macromolcules. The weakening effect of filler macromolecules was even more dramatic in the case of one-step flexible polymer gels. When short, monodisperse oligomers were added to the one-step rigid networks instead of their long filler analogues, no effect on the modulus was observed. We propose two growth morphologies to explain our observations. In the one-step polymerization, random nucleations form polymer fractals. They cluster together and when a sufficient number of them grow enough, a contiguous network is formed that, when reaching from one end of the sample to the other, is best described as an “infinite” cluster of polymeric fractals. At this point gelation occurs. Further reaction more or less fills the sample volume with additional fractals, which may or may not be covalently attached to the “infinite” network. The filler macromolecules are pushed ahead of the fractal growth fronts until trapped in between. Thus they reduce the concentration of strong bonds between fractals and clusters causing a reduction of the modulus of the ensemble as a whole. The oligomers are shorter than the network's segments, so they can be accommodated within the growing fractal and not affect the modulus of the gelled network. In the case of two-step network formation, the solution is randomly filled with high-M macromolecules and when these cross-link the system rapidly gels with only minor variations in local cross-link concentration. Long filler macromolecules appear not to measurably interfere with the formation of interchain bonds. Hence, the modulus of the network does not decrease. © 1990, American Chemical Society. All rights reserved.