Unraveling the Linear-to-Bottlebrush Transition by Linear Viscoelastic Response to Increasing Side Chain Length

Abstract

Linear polycyclopentene (PCP) and polynorbornene (PNB) macroinitiators, with a bromoisobutyryloxy pendant on each repeating unit, were prepared by ring-opening metathesis polymerization and grafting-from atom transfer radical polymerization of methyl acrylate (MA) to produce four unique sets of bottlebrush (BB) polymers with identical grafting density (n_g = 4). The four sets, two with PCP and two with PNB, were synthesized to have matched backbone degrees of polymerization that are below (N_bb = 61, 61) and above (N_bb = 394, 337) entanglement molar mass (M_e), which was determined herein as 20 and 28 kg mol^–1, respectively. Within each set, the side chain degree of polymerization (N_sc) was systematically increased from ∼6 to 104, with most focus on N_sc < 60, equating to approximately half the M_e of PMA (M_n/M_e = 0.5). Thermal properties and small amplitude oscillatory shear (SAOS) measurements were investigated for each macroinitiator (N_sc = 0) and BB as a function of increasing N_sc values. The latter resulted in time–temperature superposition (TTS) master plots with excellent overlap that were further investigated to reveal how changes to the entanglement regime result from these systematic increases of side chain length. Both PCP and PNB backbones, when above the linear M_e at N_sc = 0, required only oligomeric side chains (N_sc ≈ 10) for the near-complete loss of an entanglement response within the TTS master curves. At increasing N_sc, the onset of entanglements at higher frequency emerged due to the interdigitation and entanglement of the side chains, consistent with other reports. A critical N_sc of 15–20 was identified, where both the backbone and arm relaxations exhibited the least elastic response, as revealed by van Gurp–Palmen plots of the TTS data. This study provides rational design strategies for the highly researched PNB-based BB systems in addition to a unique comparison to alternative and more flexible PCP-based BBs. It is discovered that each backbone, while having very different glass transition temperatures (105 and 5 °C, respectively), behaved remarkably similar as a function of the hallmark design parameters (n_g, N_bb, and N_sc) for BB systems, suggesting that these parameters and the chemistry of the side chains are more important for overall BB properties in comparison to backbone flexibility.