Hydrogen Bonding in a Reversible Comb Polymer Architecture: A Microscopic and Macroscopic Investigation

Abstract

Abstract:In this work an investigation of the hydrogen bonding mechanism in a transiently branched comb-like polymer system in the melt is reported. The system under investigation consists of a polybutylene oxide (PBO)-based backbone, randomly functionalized with thymine (thy) groups, in combination with shorter PBO graft chains, end-functionalized with diaminotriazine (DAT) groups. The functional groups are able to associate through hydrogen bonding. The hetero-complementary association of these groups leads to the formation of a transiently branched comb-like polymer system. Since recently virtually exclusive heterocomplementary association could be observed in the supramolecular association of telechelically-modified oligomeric PEG chains, here we aim to extend the supramolecular assembly mechanism towards branched structures. The present work combines Small Angle Neutron Scattering (SANS) experiments on a selectively labeled system with macroscopic dynamics measured in linear rheology response. The association of thy-and DAT-modified components was studied as a function of temperature and composition. The scattering function reveals the formation of a block copolymer and can be exclusively attributed to heterocomplementary association of the hydrogen bonding groups. Scattering functions of nonfunctionalized blends are also reported as references and evidence the change in the microstructure induced by the hetero-complementary association. All scattering profiles were described by means of the Random Phase Approximation (RPA) formalism from which the average aggregation number i.e. comb arm functionality and the equilibrium association constant could be determined directly in the melt state as a function of temperature. On the other hand, rheological measurements were performed in the melt state to study the influence of the reversible bonds on the macroscopic dynamics of the polymer system. The rheology data are in good agreement with the SANS results and confirm the transient comb-like branched architecture. The supramolecular association exhibits characteristic bonding times of the groups in the order of 1 s at -25 °C and therefore makes the thy-DAT pair an ideal candidate for the development of responsive materials that combine permanent and transient linkages for novel applications and self-healing properties.

Supramolecular Dimerization in a Polymer Melt from Small-Angle X-ray Scattering and Rheology: A Miscible Model System

“…The basic and average segment length for thy-DAT PBO, l st = 0.72 nm, is in fair agreement with the reference [41] using SANS. Its size is somewhat larger here due to the approximately 4-fold higher contribution of supramolecular groups in the ratio 3:70 (n B :n A ), whereas it was about 10:1000 monomers in the transient thy/DAT comb case of [41]. The thy-thy association, on the other hand, leads to an larger or stiffer average step size of about~0.9 nm.…”

“…The curves were shifted by means of the TTS procedure towards the reference temperature T0 and agree with a WLF behavior. This is corroborated by the analysis in [40,41] in the temperature range T < T0. There, deviations from the WLF form were observed only for T >> T0 as the result of the opening of H-bonds in mixtures of open and closed states.…”

“…The former structural SAXS study of the thy-DAT association already indicated from extrapolation that the degree of H-bonding would be virtually complete at the reference temperature T0 of −25 • C, at which all former work on PBO was more or less accidently mastered to [40][41][42]. The dimerization should have a clear effect on the dynamics of the hydrogen-bonded complex chains and the relaxation time spectrum.…”

“…Based on the experimental miscibility of thy/DAT and PBO deduced from the former studies [40][41][42], we propose in the following an N-star-diblock copolymer approach, which should be general to capture any signature of supramolecular star-like aggregates with a H-bonding core and with N arms attached to it. The calculation follows the same routes as in [41,47] and is therefore not detailed further here. It is based on a general random phase approach (RPA) for scattering [46,48] and is applicable to almost any particular architecture.…”

“…The Flory-Huggins parameter χ between thy/DAT and PEO derived from them was therefore negligible. In a more recent work, the apparently strongly preferred heterocomplementary thy-DAT association was applied to the formation of transient comb polymers based on PBO likewise directly in the bulk [40,41]. The study was highlighted in [17].…”