Morphology-conductivity relationship in crystalline and amorphous sequence-defined peptoid block copolymer electrolytes.

Polymers that dissolve and conduct lithium ions are of great interest in the application of rechargeable lithium batteries. It is generally believed that the transport of ions in these systems is facilitated by rapid segmental motion typically found in rubbery, amorphous polymers. In this paper, we demonstrate that chemically identical ethyleneoxy-containing domains of a block copolymer exhibit comparable conductivities when in an amorphous or a crystalline state. An important feature of this study is the use of sequence-defined block copolypeptoids synthesized by submonomer solid-phase synthesis. Two structurally analogous ethyleneoxy-containing diblock copolypeptoids poly-N-(2-ethyl)hexylglycine-block-poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine (pNeh-b-pNte) and poly-N-decylglycine-block-poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine (pNdc-b-pNte) with 18 monomer units per block were synthesized. Both diblock copolypeptoids have the same conducting block, pNte, but different nonconducting blocks: pNeh, which is amorphous, and pNdc, which is crystalline. Both diblock copolypeptoids self-assemble into a lamellar morphology; however, pNte chains are amorphous in pNeh-b-pNte and crystalline in pNdc-b-pNte. This provides the platform for comparing lithium ion transport in amorphous and crystalline polymer domains that are otherwise similar.

Crystallization in sequence-defined peptoid diblock copolymers induced by microphase separation.

Atomic level synthetic control over a polymer's chemical structure can reveal new insights into the crystallization kinetics of block copolymers. Here, we explore the impact of side chain structure on crystallization behavior, by designing a series of sequence-defined, highly monodisperse peptoid diblock copolymers poly-N-decylglycine-block-poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine (pNdc-b-pNte) with volume fraction of pNte (ϕNte) values ranging from 0.29 to 0.71 and polydispersity indices ≤1.00017. Both monomers have nearly identical molecular volumes, but the pNte block is amorphous while the pNdc block is crystalline. We demonstrate by X-ray scattering and calorimetry that all the block copolypeptoids self-assemble into lamellar microphases and that the self-assembly is driven by crystallization of the pNdc block. Interestingly, the microphase separated pNdc-b-pNte diblock copolymers form two distinct crystalline phases. Crystallization of the normally amorphous pNte chains is induced by the preorganization of the crystalline pNdc chains. We hypothesize that this is due to the similarity of chemical structure of the monomers (both monomers have linear side chains of similar lengths emanating from a polyglycine backbone). The pNte block remains amorphous when the pNdc block is replaced by another crystalline block, poly-N-isoamylglycine, suggesting that a close matching of the lattice spacings is required for induced crystallization. To our knowledge, there are no previous reports of crystallization of a polymer chain induced by microphase separation. These investigations show that polypeptoids provide a unique platform for examining the effect of intertwined roles of side chain organization on the thermodynamic properties of diblock copolymers.

Nanoscale phase separation in sequence-defined peptoid diblock copolymers.

Microphase-separated block copolymer materials have a wide array of potential applications ranging from nanoscale lithography to energy storage. Our understanding of the factors that govern the morphology of these systems is based on comparisons between theory and experiment. The theories generally assume that the chains are perfectly monodisperse; however, typical experimental copolymer preparations have polydispersity indices (PDIs) ranging from 1.01 to 1.10. In contrast, we present a systematic study of the relationship between chemical structure and morphology in the solid state using peptoid diblock copolymers with PDIs of ≤1.00013. A series of comb-like peptoid block copolymers, poly(N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine)-block-poly(N-(2-ethylhexyl)glycine) (pNte-b-pNeh), were obtained by solid-phase synthesis. The number of monomers per chain was held fixed at 36, while the volume fraction of the Nte block (ϕNte) was varied from 0.11 to 0.65. The experimentally determined order-disorder transition temperature exhibited a maximum at ϕNte = 0.24, not ϕNte = 0.5 as expected from theory. All of the ordered phases had a lamellar morphology, even in the case of ϕNte = 0.11. Our results are in qualitative disagreement with all known theories of microphase separation in block copolymers. This raises new questions about the intertwined roles of monomer architecture and polydispersity in the phase behavior of diblock copolymers.