Confined Dynamics in Nanophase-Separated Side Chain Polymers

Abstract

The relaxation behavior of different series of nanophase-separated side chain polymers with long alkyl groups is studied and compared with corresponding results from the literature. Characteristic for these homopolymers is that alkyl groups -belonging to different monomeric units and chains -aggregate to alkyl nanodomains with a typical dimension of about 0.5-2 nm. Relaxation spectroscopy data and calorimetric results show that a polyethylene-like glass transition (α PE ) occurs within these alkyl nanodomains. The α PE process appears at temperatures below the conventional glass transition (α) where the main chains soften. The features of the α PE process in higher poly(n-alkyl methacrylates), poly(n-alkyl acrylates), poly(di-alkyl itaconates), poly(α-n-alkyl β-L-asparates) or hairy rod polyimides are similar [1]. The influence of the alkyl nanodomain size on the α PE process is studied in detail. Common trends have been observed in all homologous series [1]. With increasing length of the alkyl groups and increasing size of the alkyl nanodomains (i) the glass temperature T g (α PE ) increases (cf. full symbols in Fig.1a) and (ii) the α PE process shows a strong-to-fragile transition (Fig.1b). It starts as an Arrhenius-like process in the butyl members with C=4 alkyl carbons per side chain and alkyl nanodomains having a typical dimension of about d≈0.5 nm and approaches a steepness index of m≈50 for dodecyl members with C=12 and d≈2 nm (Fig.1c) [2].These findings are interpreted in the picture of a hindered glass transition and are related to the discussion about dynamic heterogeneities in glass forming liquids. The process observed for the butyl members can be understood as a Johari-Goldstein process in alkyl nanodomains (β PE ). The question whether or not this process survives in larger alkyl nanodomains with a truly cooperative α PE process is discussed [3]. First experiments on quenched side chain polymers seem to indicate that similar localized motions exist also in higher members. The general importance of nanophase separation effects for an understanding of the dynamics of complex materials in nanotechnology and nature will be discussed. References

Quasielastic Neutron Scattering Study on the Dynamics of Poly(alkylene oxide)s

“…It is believed that the key to obtain tailor-made materials is the understanding of the interplay between structure and dynamics in the system. In this direction, a large effort has been devoted in the last years to characterize and to relate the structural and dynamical properties in families of polymers with alkyl side groups, like, e.g., poly­( n -alkyl methacrylates) (PnMAs), − poly­( n -alkyl acrylates), ,, poly­(di- n -alkyl itaconates), − poly­(3-alkyl thiophenes) (P3ATs), , rod polyimides, polyazomethynes, poly­( p -phenylene–terephthalates), , or poly­(α- n -alkyl β- l -aspartates) . The most spectacular effect of varying the microstructure by increasing the side-group length in these systems is a dramatic plasticization effect: a strong decrease of the value of the glass-transition temperature T g .…”

Dynamics in Poly(n-alkyl methacrylates): A Neutron Scattering, Calorimetric, and Dielectric Study

“…The structural and dynamical properties of homopolymers with long alkyl side groups, like the families of poly( n -alkyl methacrylates) (PnMAs), − poly( n -alkyl acrylates) (PnAs), ,, and poly(di- n -alkyl itaconates), − have been extensively studied during the past decades. On the basis of X-ray diffraction, it has been suggested that side groups of different monomeric units tend to aggregate forming self-assembled alkyl nanodomains [called polyethylene (PE)-like nanodomains]. ,, From a dynamical point of view, these materials show a rich variety of relaxational processes, and the existence of two different glass transitions was proposed.…”

Short and Intermediate Range Order in Poly(alkylene oxide)s. A Neutron Diffraction and Molecular Dynamics Simulation Study

“…Over the last years, particular interest has been focused on the structural and dynamical features of the family on poly( n -alkyl methacrylates) (PnMAs) and other polymers containing alkyl side groups. ,− The main reason is the suggested formation of nanodomain structures by self-assembly of the side groups, that are segregated at nanometric scale from the main-chain subsytems. Such a morphology was deduced from the presence in the X-ray diffraction patterns of peaks at relatively low Q -values (“intermediate”-range), with side-group length dependent positions. ,, Later, neutron diffraction combined with isotopic labeling definitely proved such a scenario. , In addition, interesting confinement effects on the dynamics of the side groups have been reported, ,, in principle attributed to confinement effects induced by the surrounding main chains.…”