Characterizing polymer macrostructures by identifying and locating microstructures along their chains with the kerr effect

Abstract

In this brief report, we demonstrate that Kerr effect measurements, which determine the excess birefringence contributed by polymer solutes in dilute solutions observed under a strong electric field, are highly sensitive to and capable of determining their microstructures, as well as their locations along the macromolecular backbone. Specifically, using atactic triblock copolymers with the same overall composition of styrene (S) and p‐bromostyrene (pBrS) units, but with two different block arrangements, that is, pBrS90‐b‐S120‐b‐pBrS90 (I) and S60‐b‐pBrS180‐b‐S60 (II), which are indistinguishable by NMR, we detected a dramatic difference in their molar Kerr constants (mK), in agreement with those previously estimated. Although similar in magnitude, their Kerr constants differ in sign, with mK(II) positive and mK(I) negative. In addition, S/pBrS random and gradient copolymers synthesized by reversible addition‐fragmentation chain‐transfer (RAFT) polymerization exhibit a heretofore unexpected enhanced enchainment of racemic (r) pBrS‐pBrS diads. Comparison of their observed and calculated mKs suggests that the gradient S/pBrS copolymers possess an unanticipated additional gradient in stereosequence that parallels their comonomer gradient, that is, as the concentration of pBrs units decreases from one end of the copolymer chain to the other, so does the content of r diads. This conclusion could only be reached by comparison of observed and calculated Kerr effects, which access the global properties of macromolecules, and not NMR, which is only sensitive to local polymer structural environments, but not to their locations on the copolymer chains. Molar Kerr constants are characteristic of entire polymer chains and are highly sensitive to their constituent microstructures and their distribution along the chain. They may be used to both identify constituent microstructures and locate them along the polymer chain, thereby enabling, for the first time, characterization of their complete macrostructures. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Exploring Quality in Gradient Copolymers

“…Therefore, other methods were required to characterize the true quality of individual gradient copolymers. While the Kerr effect has proven to be an effective characterization of gradient architecture, AFM was chosen for this research as it is an excellent method to characterize dimensions of individual molecules on a surface. However, linear molecules are difficult to image due to limited lateral and vertical resolution; therefore, previous reports have taken advantage of the bottlebrush architecture to quantify both dimensions and composition of polymer molecules including the correlation of brush gradient with height profiling …”

Beyond microstructures: Using the Kerr Effect to characterize the macrostructures of synthetic polymers

“…If such an experimental technique exists, it must necessarily be sensitive to the entire structures of macromolecular chains. Here, we suggest that the contributions made by polymer solutes to the electrical birefringence observed for their dilute solutions when subjected to strong electric fields, or their Kerr effects, are experimental probes of complete polymer chains that are sensitive enough to distinguish their macrostructures …”

“…Initially to determine if the Kerr effect may be used to probe/determine polymer macrostructures, we selected and synthesized styrene/ p ‐bromostyrene (S/ p BrS) copolymers with controlled micro‐ and macro‐structures. S/ p BrS copolymers were chosen for the following reasons: (1) The dipole moments of S and p BrS repeat units are quite different; (2) for the same stereosequences, the conformational characteristics, or average randomly coiling conformations, of all S/ p BrS copolymers are essentially identical .…”

“…Consequently, portions of their 13 C‐NMR spectra potentially sensitive to microstructure (i.e., the resonance frequencies of their backbone CH and CH 2 and side chain C 1 carbons) are independent of both their comonomer compositions and sequences, as carefully documented in refs. 4 and 7; and (3) conformations that are dependent solely on copolymer stereosequences, but that are independent of their comonomer compositions and sequences, simplify the calculation of m Ks . The latter two reasons are important, because the m Ks measured for polymers can only be interpreted, in terms of the types, quantities, and the locations of their microstructures, by comparison with the m Ks calculated for the corresponding micro‐ and macrostructural features.…”

“…More recently, the 60% p BrS/40% S triblock copolymers S 60 ‐b‐ p BrS 180 ‐b‐S 60 and p Brs 90 ‐b‐S 120 ‐b‐ p BrS 90 were synthesized and their Kerr constants were observed to have similar magnitudes, but to be, respectively, positive and negative. Also reversible addition‐fragmentation chain transfer (RAFT) controlled radical polymerization of random and gradient S/pBrS were conducted and their Kerr constants were observed.…”