The effect of cross-linking on the molecular dynamics of the segmental and β Johari–Goldstein processes in polyvinylpyrrolidone-based copolymers

Abstract

Royal Society of ChemistryRedondo Foj, MB.; Sanchis Sánchez, MJ.; Ortiz Serna, MP.; Carsí Rosique, M.; García, JM.; García, FC. (2015). The effect of cross-linking on the molecular dynamics of the segmental and Johari-Goldstein processes in polyvinylpyrrolidone-based copolymers. Soft Matter. 11:7171-7180. doi:10.1039/c5sm00714c. The effect of the cross-link density on the molecular dynamics of copolymers composed of vinylpyrrolidone (VP) and butyl acrylate (BA) was studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). A single glass transition was detected by DSC measurements, showing good copolymer miscibility. The dielectric spectra exhibit conductive processes and three dipolar relaxations labeled as ,  and  in decreasing order of decreasing temperatures. The cross-linker content affects both  and  processes, but the fastest  process is relatively unaffected. An increase of the cross-linking produces a typical effect on the  process dynamics: (i) the glass transition temperature is increased, (ii) the dispersion is broadened (iii) its strength is decreased and (iv) the relaxation times are increased. However, the  process, which is a Johari-Goldstein relaxation possesses typical features of pure Johari-Goldstein relaxation, unexpectedly loses the intermolecular character for the highest cross-linker content. . The basis of the CM is the existence of a temperature-insensitive crossover time, tc, which is about 2 ps for small molecules and polymeric glass formers 32,33 . At times shorter than t , there are basic molecular units that relax independently of each other, whose relaxation function can be described by a simple exponential decay: Effect of cross-linking on the molecular dynamics of the segmental and  Johari-Goldstein processes in Polyvinylpyrrolidone-based copolymersBelénBA (PBA) is hydrophobic and has a glass transition of 219K, while the PVP is an extremely hydrophilic polymer(uncross-linked PVP is water soluble), with glass transition of 448 K. Thus, copolymers comprised of VP and BA can be presented as tractable materials with moderate hydrophilicity and good mechanical and thermal properties. In addition to the copolymers, it is also highly interesting the study of cross-linked polymer networks. The dynamics of cross-linked polymers is influenced by the network composition and the polymer chain architecture. As it is well-known, both the glass-rubber and the subHowever, at times longer than tc, the intermolecular interactions (i.e. cooperativity 34 , which is associated with the dynamic heterogeneity) slow the relaxation. Thus, there is a coupling between the motions of the different basic molecular units. Therefore, the relaxation function assumes the Kohlrausch-Williams-Watts (KWW) equationglass transitions are highly sensitive to changes in the backbone structure, the cross-link density, and the introduction of pendant groups or branches. As a result, the relationship between network structure, chain dynamics and bulk properties of cross-l...