The evolution of grain size and shape as well as type and frequency of grain boundary structures during thermal annealing of lamellar diblock copolymer microstructures is established using large area image reconstruction and analysis. Grain coarsening is found to proceed via an initial transient stage that is characterized by the rapid relaxation of unstable "frozen-in" defects such as kink boundaries and the subsequent quasi-stationary coarsening that is dominated by the continuous relaxation of low-angle symmetric tilt boundaries. The particular relevance of low-angle symmetric tilt boundaries to grain coarsening is interpreted as the consequence of both the associated decrease of boundary energy as well as the availability of favorable kinetic pathwayssuch as grain boundary splittingto facilitate the coarsening process. The inverse relation between grain boundary energy and frequency suggests that the reduction of boundary energy is a relevant governing parameter for the evolution of grain boundary structuresas it is in inorganic materials. The existence of "inert" boundary types (such as asymmetric tilt and twist) thatwithin the experimental windowdo not participate in the coarsening process is expected to have dominant influence on the final morphology that can be attained by thermal annealing of the microstructure. The reduction of the density of inert boundaries during the film preparation process should therefore provide a strategy for increasing the coarsening kinetics in block copolymer films during thermal annealing and thus a path toward a higher degree of order in block copolymer microstructures.

The (relative) energies of symmetric tilt grain boundaries in a strongly segregated lamellar block copolymer are determined by analysis of the dihedral angles at grain-boundary triple junctions. The analysis reveals two regimes: at low and intermediate misorientations (corresponding to a tilt-angle range 0 85 ) the grain-boundary energy is found to depend on the tilt angle as EðÞ $ x , with 2:5 > x ! 0. At large misorientations the grain-boundary energy is found to be independent (within the experimental uncertainty) of the angle of tilt. The transition between the two scaling regimes is accompanied by the transition of the grain-boundary structure from the chevron to the omega morphology. Grain-boundary energy and frequency are found to be inversely related, thus suggesting boundary energy to be an important parameter during grain coarsening in block-copolymer microstructures, as it is in inorganic polycrystalline microstructures. The ability to self-organize into periodically ordered microdomain morphologies makes block-copolymerbased materials intriguing platforms to facilitate transformative technological breakthroughs in a range of areas, including dynamic photonic sensors, solid state ion conductors, and bulk heterojunction materials for polymer photovoltaics [1][2][3][4][5]. In these applications, nonequilibrium structural defects such as grain boundaries (GB) are expected to play an important role in determining material performance because of their impact on the long-range order and tortuosity of diffusion pathways. The occurrence of GB defects is inherent in quiescent organized blockcopolymer (BCP) microstructures and can be related to the nucleation and growth of ordered grains during the structure evolution process, the interaction of disclinations, or mechanically induced kinking because of inhomogeneous solvent evaporation during the late stages of film formation. The basic phenomenology of GB structures in BCP materials was first discussed by Gido and Thomas who classified and evaluated the various GB types associated with tilt and twist deformations of lamellar BCPs [6][7][8][9]. Subsequent experimental studies revealed the relevance of process parameters (such as the application of shear fields), molecular architecture, and composition on GB formation as well as the implications of GB defects on, for example, the permeability of BCP materials [10][11][12][13][14][15][16][17].Despite the abundance of GB defects and their demonstrated relevance on the physical properties of BCP materials, very little is known about the governing parameters that determine the formation of the various types of GB structures in BCPs or the evolution of GB structures during, for example, thermal annealing. One parameter that is of particular interest is the energy penalty associated with GB formation as it provides insight into both the mechanism and driving force of grain coarsening during annealing. The current understanding of GB energies in BCPs is limited to simulation studies, for example, by Schick and co-worke...