Effect of the disorder-to-order transition in diblock copolymers on the segmental dynamics: a study using depolarized photon correlation spectroscopy

Abstract

Depolarized light scattering has been applied to study the anisotropy fluctuations in a series of polystyrenelpolyisoprene diblock copolymers. This method is selective for the dynamica of the polystyrene block. The molecular weights of the block copolymers (10500-20800) were chosen such that the microphase separation transition (MST) occurs at temperatures which are experimentally acceeeible with photon correlation spectroscopy. Small-angle X-ray scattering (SAXS) was used to identify the disordered regime, the fluctuationcontrolled transition regime, and the ordered state. The relaxation time spectrum in the disordered state is similar to that of bulk polystyrene. Concentration fluctuations increasingly broaden the spectrum and lead to a bimodal shape. Close to the MST the spectrum becomes dominated by a slow, nearly single exponential contribution. Comparison with the SAXS results suggests that this splitting of the relaxation time spectrum can be related to chain stretching.

Polarized and depolarized dynamic light scattering from a block copolymer melt

“…7 However, they also found a VH mode with similar G, and the internal mode is not VH active. Thus, they attributed both VV and VH modes to a combination of chain stretching and orientation, following an explanation advanced for a similar VH mode reported by Hoffmann et al 1 in PS-PI. In the VH spectrum, one expects to see segmental reorientational dynamics, for those monomers with significant optical anisotropy and slow enough dynamics (i.e., high glass transition temperature) for G to appear in the DLS window.…”

“…Indeed, such a mode is seen for PS-containing polymers. 1,3,7,11 A slower VH mode has also been reported for PS-PB and PS-PI, 1,7 and attributed to chain stretching and orientation, as just discussed. However, a similar VH mode in disordered PS-PMPS and PS-PI melts ( G Ç q 0 ) was assigned to a coupling between orientational and composition fluctuations, due to the growth of large amplitude fluctuations as the ODT is approached.…”

“…Copolymers examined include polystyrene tude is generally far larger than that of any other mode. (PS) -polyisoprene (PI), [1][2][3][4][5][6] PS-polybutadiene (PB), 7 It appears to be similar to the ''ultraslow mode'' obpoly(ethylenepropylene) (PEP) -poly (ethylethylene) served in glass-forming liquids, and attributed to long-(PEE), 8 poly(vinylcyclohexane) (PVCH) -PEE, 8 PVCHrange density fluctuations. 10 Although ubiquitous in polyethylene (PE), 8 PE-PEE, 8 and poly(ethylmethylblock copolymer melts, it is fair to say that its origin is siloxane) (PEMS) -poly(dimethylsiloxane) (PDMS).…”

The bulk dynamics of a compositionally asymmetric diblock copolymer studied using dynamic light scattering

“…As the amplitude of the internal mode is proportional to N 2 (for Gaussian chains), it is usually not observed for block copolymers having low molar masses. Two additional dynamic processes have been observed experimentally in symmetric diblock copolymers: a second diffusive mode, termed X-mode, tentatively attributed to undulations of the lamellar interfaces or to the diffusion of small amounts of homopolymer [11,12], and a q-independent mode giving rise to depolarized scattering as well, attributed to the rotation of stretched chains [13][14][15] or to the reorientational dynamics associated with the largeamplitude composition fluctuations near the ODT [12]. Also, the segmental reorientational dynamics and largescale heterogeneities ("clusters") have been observed (e.g., [15]).…”

Neutron spin echo investigation of the concentration fluctuation dynamics in melts of diblock copolymers

“…16,17 They have investigated the diffusive motions of the block copolymer chains, which occur at length scales larger than the radius of gyration (q Ͻ1/R g ). These experiments were performed using light scattering [18][19][20][21] and photon correlation spectroscopy; 22 most of them were made on block copolymers in solution since melts are generally beyond the scope of dynamic light scattering experiments. Furthermore, such techniques do not allow to observe the single chain dynamics in a melt.…”