Thin Films of Poly(isoprene-b-ethylene Oxide) Diblock Copolymers on Mica: An Atomic Force Microscopy Study

Abstract

The structural behavior of three amphiphilic semicrystalline poly(isoprene-b-ethylene oxide) block copolymers (PI-b-PEO) with different PEO volume fraction (f(PEO) = 0.32, 0.49, and 0.66), spin-coated on freshly cleaved mica surfaces from aqueous solutions, was investigated by atomic force microscopy. We focus on the dependence of the resulting thin film nanostructures on the molecular characteristics (f(PEO) and molecular weight) and the adsorbed amount. The nanostructures obtained immediately after spin-coating were robust and remained unchanged after annealing and/or aging. The PEO affinity for the highly hydrophilic mica and the tendency of the hydrophobic and low surface energy PI to dewet and be at the free interface caused the soft PI-b-PEO micelles to collapse leading to the formation of 2D dendritic networks over mica. We show that, for all three polymers, the dendritic monolayer thickness can be predicted by a model consisting of a PEO crystallized layer (directly on top of mica) of the same thickness in all cases and a PI brush layer on top. In thicker areas, polymer material self-assembled into conelike multilamellar bilayers on top of the monolayer and oriented parallel to the substrate for both symmetric and asymmetric diblock copolymers with the lowest f(PEO). We compare the lateral morphology of the films and discuss the thickness heterogeneity, which results from the coupling and competition of crystallization kinetics, phase separation, and wetting/dewetting phenomena highlighting the role of the two blocks to inhibit or enhance certain morphologies. We show that the deviation of the f(PEO) = 0.32 thin film from its bulk phase structure (cylinders in hexagonal lattice) continues for several lamellar bilayers away from the substrate. For the asymmetric PI-b-PEO polymer with the higher PEO volume fraction (f(PEO) = 0.66) and higher APT, laterally extensive stacks of flat-on lamellar crystallites formed on the surface demonstrating the crucial role of the PEO crystallization.

Crystallization of polymer chains confined in nanodomains

“…In fact, it is found from many studies on the crystallization behavior that the crystallinity of block chains confined in nanolamellae increases sigmoidally with a finite induction time, indicating that the crystallization is virtually driven by a combined mechanism of heterogeneous nucleation and subsequent crystal growth. Because a comprehensive understanding of the detailed crystallization mechanism of homopolymers confined in nanolamellae is directly applicable to the surface crystallization of homopolymers [88][89][90][91][92] or crystallization within thin films [93][94][95][96][97][98], it is essential to examine the crystallization of homopolymers (and also block chains) confined in various nanolamellae.…”

Recent progress in block copolymer crystallization

“…AFM and transmission electron microscopy (TEM) images were used to monitor the morphology of BCP thin films (film thickness less than 100 nm) under different crystallization conditions. For systems that showed single crystal type crystallization in the thin film, BCP crystal morphologies were distorted single‐crystalline or dendritic in nature. This showed that for specific isothermal T c s, the typical kinetically controlled single crystal growth transitioned toward diffusion‐limited fractal growth due the amorphous block “impurities.” Selective interaction of one block with the substrate, that is, PEO and mica in poly(isoprene)‐ b ‐PEO (PI‐ b ‐PEO), or the free surface, PLLA in PEO‐ b ‐PLLA, led to changes in the BCP assembly and crystallization.…”

“…As new chemistries are developed, as well as new analytical techniques, stronger foundations in BCP crystallization are needed to evaluate and predict structure-property relationships. This initial section covers some of the primary work in BCP phase behavior and accompanying crystallization analysis including, but not limited to, rAC (T g < T c ) or gAC (T g > T c ) copolymers, [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] CC copolymers, [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] thin films, [25,27,28,46,48,56] confined crystallization, [30][31][32][33][34]…”

“…This showed that for specific isothermal T c s, the typical kinetically controlled single crystal growth transitioned toward diffusion-limited fractal growth due the amorphous block "impurities." Selective interaction of one block with the substrate, that is, PEO and mica in [27] or the free surface, PLLA in PEO-b-PLLA, [47] led to changes in the BCP assembly and crystallization. For a CC PEO-b-PLLA system, [49] aP3HB was amorphous and the blocks were miscible, was compared to 6-arm PLLA.…”

Dewetting and microphase separation in symmetric polystyrene‐block‐polyisoprene diblock copolymer ultrathin films

“…Of particular interest are block copolymers, whose inherent nano‐ and micrometric pattern formations hold significant promise for the fabrication of high‐density data storage and nanolithography . The bulk structure of copolymers is determined by several factors, including overall molecular weight, relative volume fraction of the blocks, degree of segregation of the blocks and, in some cases, crystallization of one of the blocks . In thin films, the affinities of the blocks to each of the interfaces become significant .…”