Composition Fluctuation Effects on Dielectric Normal-Mode Relaxation in Diblock Copolymers. 1. Weak Segregation Regime

Abstract

Dielectric relaxation spectroscopy has been used to investigate the normal-mode relaxation in disordered diblock copolymer melts far from the order-to-disorder transition (ODT). The dielectric spectra are analyzed in order to quantitatively obtain the distribution of relaxation times in the disordered diblocks. The width of the relaxation function shows significant broadening relative to the respective homopolymer distributions when the temperature is decreased and/or the molecular weight is increased. The broadening is attributed to composition fluctuation effects on the normal-mode relaxation. These effects are theoretically accounted for by considering both the short-range fluctuations due to chain connectivity and the long-range concentration fluctuations in diblock copolymers due to the proximity to the ODT and their coupling to the individual block segmental dynamics. Theory not only captures the relative features of the distributions but also quantitatively compares very well with the experimental dielectric spectra.

Computer Simulation of Static and Dynamic Behavior of Diblock Copolymer Melts

“…It is known, however, that for the most common diblocks investigated, for example, poly(styrene-b-1,4-isoprene), 21,22 the two components possess local frictions differing by factor of 10 6 . Even in the homogeneous regime, the presence of concentration fluctuations leads to the existence of two different microenvironments with a difference in the segmental times by factor of 10 4 .…”

“…The ordered states are characterized by a long range order exceeding the molecular sizes and various morphologies depending on f. Up to now, most of the scientific interest has been devoted to static properties of such systems. Only recently, the dynamics of diblock copolymers, and the influence of the order-disorder transition on the dynamics have attracted the increased interest of polymer scientists, both experimentally [17][18][19][20][21][22][23][24][25][26] and theoretically. [26][27][28][29][30][31][32] Computer simulations have also provided results [13][14][15][16] concerning mainly static properties of block copolymers.…”

Dielectric Relaxation of Type-A Polymers in Melts and Solutions

“…Thus, as a crude approximation, we may consider that the dielectric mode distribution is the same for the loop and tail and the dielectric intensity of the loop is twice the intensity of the tail (because the loop is composed of two fragments each being equivalent to the tail). This argument can be cast in a relationship between the relaxation functions of the loop and tail: [42] w l (t) L 2w t (t) (48) The unknotted bridge is composed of two half-fragments equivalent to the tails tethered on the opposing S/I interfaces (cf. Figure 32 c).…”

“…Utilizing Equation (48) and (49) in Equation (41) and (44) and (implicitly) assuming that the I blocks form no knot (m k = 0), he obtained a relationship between the dielectric relaxation functions of the SIIS and precursor SI lamellae:…”

Dielectric Relaxation Spectroscopy of Gradient Copolymers and Block Copolymers: Comparison of Breadths in Relaxation Time for Systems with Increasing Interphase

“…23,32,33 In block copolymer systems, DRS has been used primarily to study diblock, multiblock and star copolymers of the styrene-isoprene comonomer system. [34][35][36][37][38][39][40] These include examination of block dynamics near the order-disorder transition [34][35][36] as well as determining interfacial widths in phase-segregated systems where mobile isoprene midblocks are anchored by glassy styrene end-blocks. [37][38][39][40] The DRS technique has also been extended to study the effects of free surfaces and interfacial interactions on the dynamics of confined polymers.…”