Spectroscopic and light scattering studies of the conformational (rod-to-coil) transition of poly(diacetylene) in solution

Abstract

A rod-to-coil conformational transition has been demonstrated for polydiacetylene, 4-butoxy-carbonyl-methylurethane (4BCMU) in solution. The transition can be induced by changing either the temperature or the quality of the solvent. The light scattering and spectroscopic data as a function of polymer concentration have shown that the transition is a single chain (intramolecular) phenomenon. However, because of the large end-to-end length (L≂1.2 μm) of the fully extended polymer, the dilute limit is not reached until concentrations below 10−5 g/cm3. At higher concentrations evidence of cluster growth and aggregation are observed prior to gelation which occurs above a critical concentration c0≂5×10−4 g/cm3. This cluster growth occurs as a result of the rod-like conformation of the individual molecules, but it is not the cause of the transition. The large increase in scattering intensity (at fixed polymer concentration) on going from coil to rod follows directly from the change in dielectric constant due to the spectral shift of the π–π* absorption; no significant increase in molecular weight is implied by the data. A theoretical model of the transition has been developed in which the ordered rod-like conformation is the low temperature phase. Conformational kinks (to a coil phase) cost energy through interruption of the π-electron delocalization and through the breaking of H bonds between R groups. Nevertheless, the increase in entropy associated with the many degrees of freedom of the coil-like conformation is sufficient to lead to the observed transition.

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

“…The transitional region, for which curves can be described by fractional superposition of the two limiting spectra corresponding to the red and yellow phases is found in the range 58 T 66 °C. A slightly lower transition onset temperature and a much more gradual color change have been reported in the literature [13]. Such As mentioned already in other papers [4,13] (11) (11) [39] is observed at T = 55.4 °C for q :> 0.11 Å -1.…”

“…A slightly lower transition onset temperature and a much more gradual color change have been reported in the literature [13]. Such As mentioned already in other papers [4,13] (11) (11) [39] is observed at T = 55.4 °C for q :> 0.11 Å -1. A more detailed study of the transition and of the gel structure is in progress [42].…”

“…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.…”