Gelation of Rodlike Macromolecules

Abstract

The gel-sol transition for the polydiacetylene 4-butoxycarbonylmethylurethane in toluene is studied. For gelation to occur, the polymer must be in its fully extended rodlike conformation. Specific evidence that the gelation is due to the percolation of rods is presented.PACS numbers: 61.25.Hq, 61.40.Km, 62.10, + sThe properties of rodlike macromolecules in solution have been a subject of interest in recent years. Both the microscopic properties of individual chains and the interactions between chains can lead to novel phenomena. Since such polymers are often conjugated, the rodlike conformation arises primarily from electronic derealization along the polymer backbone. Consequently, changes in conformation are inherently coupled to changes in electronic structure. Moreover, the rodlike conformation can be expected to lead to ordered liquid-crystalline phases, an area of considerable importance.On the other hand, relatively few studies have addressed the effects of interaction between rodlike macromolecules in solution. Doi and Edwards 1 discussed the dynamics of such solutions in the semidilute concentration (c) regime, L~3 « c« (Ld 2 )' 1 , where L and d are the length and diameter of the rod, respectively. The elastic properties of gels of rigid rods were discussed by Doi and Kuzuu 2 for {dL 2 )' 1 <>c« (d 2 L)" 1 . In both theories, the interaction between the rods was assumed to be a hard-core repulsion; the rods cannot pass through each other.Solutions of the polydiacetylene 4-butoxycarbonylmethylurethane (4BCMU) (see Fig. 1) in toluene present an opportunity to study some of these properties, 3 since a rod-to-coil transition has been identified by use of quasielastic light scattering. 4 The rod-to-coil transition has been studied 5 ' 6 as a function of temperature by monitoring the spectroscopy of the 7T-7T* absorption. A theory of this conformation transition has been proposed 5 in which the high-temperature (yellow) phase consists primarily of the cis structure, which is free to coil, whereas the low-temperature (red) phase consists of the trans structure, which is rodlike.In this Letter we present the results of a study of gels of polydiacetylene (4BCMU) in toluene. Our results demonstrate that for gelation to oc-cur, it is essential that the polymer be in its fully extended, rodlike conformation. The shear modulus was measured as a function of temperature and concentration. In addition, the critical concentration, below which no gel forms, was also measured. We find specific evidence that the gel-sol transition for this system is determined by the percolation of rods. This is the first report of detailed studies of the gel-sol transition with rodlike macromolecules.The gels were prepared by dissolving the polydiacetylene (4BCMU) in toluene at T ^ 80 °C. This formed yellow solutions (coils). Upon cooling, the polymer chains undergo the coil-to-rod transition, and the solution becomes a gel.The shear modulus was measured with a mechanical resonance technique. 7 A cylindrical bobbin is suspended coaxially i...

Polymerization of Bolaform Butadiyne 1-Glucosamide in Self-Assembled Nanoscale-Fiber Morphology

“…18 The poly-4BCMU also forms gels in toluene. 21 However, this gelation behavior can be exhibited only by polydiacetylenic BCMUs, not by their monomers.…”

Solutions of polydiacetylenes in good and poor solvents : a light and neutron scattering study

“…On the other hand, it is generally believed that the conjugated backbone is in the trans configuration of the double bonds, but it has been suggested that yellow solutions consist primarily of the cis configuration [9]. Since [10], is related to pure intramolecular coil to rod conformational changes [5,[11][12][13] [14,15]. The second model corresponds to an aggregation of semi-rigid chains, or parts of such chains, leading to clusters [6,16] : this is the fringed micelles model [17].…”

“…For high Mw P-4BCMU samples and polymer concentrations larger than a critical value cG, gel formation occurs. It has been proposed that cG = .0.6 gl-' [13,14] [44]. The visible absorption spectra of dilute toluene solutions of the same P-4BCMU as used in the neutron scattering experiments, and referred to as P-4BCMU-1, are shown in figure 6.…”

“…This color transition corresponds to changes in the electronic structure of the conjugated backbone of the macromolecules. The related structural modifications are not well known [12][13][14][16][17]. For high Mw P-4BCMU samples and polymer concentrations larger than a critical value cG, gel formation occurs.…”

“…It has been suggested that the color transition is a single chain phenomenon corresponding to an intramolecular coil to rod change [5,11,13] ; aggregation as well as gelation then occur as a result of the rodlike conformation of the molecules, but are not the driving force for the transition [14]. Solutions of isolated rod molecules should therefore be observed for low enough polymer concentrations.…”