A high-density poly(methyl methacrylate) (PMMA) brush (σ = 0.77 chain/nm 2 ) with a lower molecular weight distribution was prepared onto a silicon wafer by surface-initiated atom transfer radical polymerization. The surface of the PMMA brush chains was characterized upon the process of the environmental change, from air to water, using contact angle measurements in conjunction with sum-frequency generation spectroscopy. The surface structure and properties altered less with the changing environment from air to water for the PMMA brush than for a spin-coated film; that is, the extent of surface reorganization could be suppressed by grafting denselypacked chains onto a substrate. Also, the water penetration into the brush surface was inhibited because of the densely packed chain structure.

Intermolecular crowding of densely tethered polymers promotes chain extension and anisotropy that induces many unique properties. In this study, we used conformation-sensitive infrared spectroscopy to determine that chain extension in a polymer brush is associated with local conformation rearrangements, i.e., contraction of side groups and increased proportion of gauche twists in the backbone, which served to increase molecular disorder at or below the segmental scale. This conformational transition points to a particular molecular mechanism for chain extension in densely tethered polymers, wherein increased local disorder facilitates global chain ordering (i.e., chain extension) and therefore supplements our current understanding of chain orientation at a molecular level.

Deviations of chain conformations from Gaussian statistics induce entropic stress that affects the dynamics of polymer materials. Understanding its role is further complicated by the presence of enthalpic stresses, arising from the concurring change in bond energy and intermolecular interactions. By fabricating oriented poly(methyl methacrylate) brushes free from backbone deformation, we successfully identified the role of entropic stress on the nonisothermal relaxation of chains at both global and local levels. We show that the entropic stress unjams the local backbones and is responsible for the occurrence of a stresssensitive glassy state near T g , where the local backbone motion provokes chain contraction upon heating. Remarkably, the intrachain stress does not affect local secondary relaxations involving side-group rotation nor thermal expansion in the deep glassy state. These results cast light on the dynamics and fundamental molecular mechanics of oriented, stressful polymer glasses, such as textile fibers.