Comb and Bottlebrush Polymers with Superior Rheological and Mechanical Properties

Abstract

combined with the grafting-to method [1,10] results in comb polystyrenes (PSs) with well-entangled monodisperse backbone and side chains; however, branch points along the backbone are randomly distributed. This random distribution of branch points has no distinct effect on the rheological properties of dense comb topologies; however, it will broaden the backbone relaxation time of loose comb topologies even though the molecular weight distribution is narrow enough. [11] A controlled branching point spacing can be achieved through the coupling of living macroanions; [12] however, this method is prone to slightly high molecular weight distribution, Ð > 1.5. Synthesis of welldefined densely grafted bottlebrushes using ROMP and grafting-through method guarantees the presence of one side chain per repeating unit on the backbone. Bottlebrushes with more than one branch per backbone repeating unit can also be synthesized using ROMP and macromonomers with more bulky groups referred as wedge polymers. [13] These bottlebrushes are similar to that of the dendronized polymer with only one layer of grafting. Loosely grafted bottlebrushes with longer distance between the branching points can be achieved using ROMP technique through copolymerization of macromonomer and a diluent molecule, e.g., racemic endo,exo-norbornenyl diesters, with different ratios. However, different reactivity of macromonomer and diluent leads to a gradient of branching point spacing along the backbone, [14] where the conformations of side chains along the backbone are affected by this gradient. [15] It should be mentioned that the bottlebrushes synthesized with ROMP technique have different repeating units in the backbone than the side chains which might cause microphase separation. However this issue could be ignorable in dense bottlebrushes where the volume fraction of backbone is less than 1 wt%. In order to avoid any microphase separation, Sheiko and co-workers [16] synthesized a series of dense combs and loose bottlebrushes homo poly(n-butyl acrylates) (PBA) with similar side chains and systematically varied grafting density through copolymerization of nonfunctional n-butyl acrylates with trimethylsilyl-protected acrylates by ATRP. However, in order to achieve high degree of polymerization (DP) of backbone (entangled system) for dense bottlebrushes, they had to use slightly different backbones, i.e., poly(n-butyl methacrylate). An organometallic coordinative insertion polymerization of α-olefins results in entangled densely grafted bottlebrushes with rod-like side chains where both backbone and side chains have alkane groups. However the dispersity index of such homopolymer bottlebrushes would Comb and bottlebrush polymers present a wide range of rheological and mechanical properties that can be controlled through their molecular characteristics, such as the backbone and side chain lengths as well as the number of branches per molecule or the grafting density. This review investigates the impact of these characteristics specifically on the zero sh...

Small and Medium Amplitude Oscillatory Shear Rheology of Model Branched Polystyrene (PS) Melts

“…Interestingly, η 0 exhibited different behavior in each conformational regime. LC PS behaved like star molecules where η 0 increased exponentially as a function of M w (η 0~e xp (M w )), which is related to the strong frictional effect of the branches on the backbone [22]. A further increase of N br resulted in a continuous decrease of η 0 , reflecting the dilution effect of the branches.…”

“…They also investigated the effect of the number of branches on the linear viscoelastic (zero-shear viscosity) and nonlinear viscoelastic properties (extensional viscosity and strain hardening factor) of the model comb PS set. The results showed that a relative minimum in the zero-shear viscosity and a maximum in strain hardening factor were observed near a transition point from DC to LB, which was associated with the stretched backbone conformation grafted with crowding side chains [21,22]. Up to now, most rheological studies on bottlebrush polymers have focused on linear shear flow [20,23,24] and extensional flow [25,26].…”

3D printing of conjugated polymers

“…If the ink has a relatively high initial viscosity, then the ability of the ink to retain its shape after exiting the nozzle can allow for continuous printing without the need for a curing step between layers. Desired ink rheologies can be achieved through clever molecular design of polymer side chains . Most of the inks currently being developed for DIW systems utilize shear‐thinning dispersions or pastes to achieve high shape retention upon immediate exiting of the nozzle.…”

“…(c,d) Bowl structure with 58° overhang angle successfully printed with conductive PANI‐based ink. (Reproduced from , with permission from IOP Publishing.) [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Desired ink rheologies can be achieved through clever molecular design of polymer side chains. 36 Most of the inks currently being developed for DIW systems utilize shearthinning dispersions or pastes to achieve high shape retention upon immediate exiting of the nozzle. This allows for shorter overall printing times and higher resolution features via careful tuning of ink rheology.…”