TY - JOUR

T1 - Optical Anisotropy of Tethered Chains

AU - Lodge, Timothy P.

AU - Fredrickson, Glenn H.

PY - 1992/10/1

Y1 - 1992/10/1

N2 - The static intrinsic birefringence and dichroism of dense assemblies of tethered chains, or “brushes”, are computed. The classical trajectory analogy is used to calculate the average segmental anisotropy for strongly-stretched chains, as a function of distance from the grafting surface. This result is used to estimate the birefringence and dichroism of planar polymer brushes and strongly-segregated block copolymer lamellae and cylinders. The birefringence, Δn, varies as the ratio L2/M(h2)0, where L is the layer thickness, M the chain length, and (h2)0 the mean-square unperturbed end-to-end distance. Consequently, Δn is predicted to be independent of M for tethered chains where L ~ M; however, for block copolymers in the strong-segregation limit, Δn ~ M−0.67 For lamellar block copolymers, the calculations indicate that the intrinsic birefringence is surprisingly large and, for the most commonly-examined chemical systems, is often comparable to, or even greater than, the estimated form birefringence. This result reflects the orientation of the chains, i.e., the tendency of the block end-to-end vectors to be aligned, more than stretching. Calculations are also performed for labeled sections of a tethered chain, which indicate that it should be possible to assess the validity of the classical trajectory assumption via infrared dichroism measurements on appropriately deuterated samples.

AB - The static intrinsic birefringence and dichroism of dense assemblies of tethered chains, or “brushes”, are computed. The classical trajectory analogy is used to calculate the average segmental anisotropy for strongly-stretched chains, as a function of distance from the grafting surface. This result is used to estimate the birefringence and dichroism of planar polymer brushes and strongly-segregated block copolymer lamellae and cylinders. The birefringence, Δn, varies as the ratio L2/M(h2)0, where L is the layer thickness, M the chain length, and (h2)0 the mean-square unperturbed end-to-end distance. Consequently, Δn is predicted to be independent of M for tethered chains where L ~ M; however, for block copolymers in the strong-segregation limit, Δn ~ M−0.67 For lamellar block copolymers, the calculations indicate that the intrinsic birefringence is surprisingly large and, for the most commonly-examined chemical systems, is often comparable to, or even greater than, the estimated form birefringence. This result reflects the orientation of the chains, i.e., the tendency of the block end-to-end vectors to be aligned, more than stretching. Calculations are also performed for labeled sections of a tethered chain, which indicate that it should be possible to assess the validity of the classical trajectory assumption via infrared dichroism measurements on appropriately deuterated samples.

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U2 - 10.1021/ma00047a013

DO - 10.1021/ma00047a013

M3 - Article

AN - SCOPUS:0026928832

VL - 25

SP - 5643

EP - 5650

JO - Macromolecules

JF - Macromolecules

SN - 0024-9297

IS - 21

ER -