Single-Chain Conformation of Poly(α-olefins) in Dilute Solutions at the Crossover between Linear and Bottlebrush Architectures

Abstract

The size and conformation of a series of poly­(α-olefin) homopolymers in good solvent conditions were measured by a combination of small-angle neutron scattering (SANS), triple detector size exclusion chromatography (SEC), and MD simulations. The bottlebrush samples were prepared via organometallic coordinative insertion polymerization of 1-alkenes, with carbon numbers ranging from 6 to 18 carbons. A linear polyolefin, that is, polypropylene, is included in this study for comparison. SANS data for all the solutions are well described by the flexible cylinder model, from which the cylinder radius (R) and the Kuhn length (l k) are determined. These two quantities, also calculated by MD simulations, are monotonic increasing functions of the bottlebrush side chain length (N sc). The dependences of the hydrodynamic radius (R h) and the radius of gyration (R g) with the molecular weight (M), measured by SEC and MD simulations, are well described by the power laws, R h ∼ M υh and R g ∼ M υg , respectively. Intriguingly, υh is a nonmonotonic function of N sc, whereas υg increases monotonically with N sc. We postulate that this is due to weakening of the intermolecular hydrodynamic interactions as N sc increases, which also marks a transition from linear-like (for N sc < 9) to brush-like chain architecture (for N sc > 9). Taking the R g data for all the bottlebrush polymers together, they cannot be described by the Kratky–Porod wormlike chain model. We propose an empirical parabolic function of the backbone molecular weight (N bb) and N sc that predicts the R g values for all the bottlebrushes adequately.

Structure–Dilute Solution Property Relationships of Comblike Macromolecules in a Good Solvent

“…To better quantify these results, we extract the scaling exponents and prefactors that characterize eqs 1−3. While many works 4,25 fit independent scaling exponents for different architectures, within uncertainty, we see no change in the scaling exponents in our data as shown in Figures 2A and 2B, suggesting our comblike polymers belong to the same scaling regime of polymer architecture. Moreover, we note that prior experimental works have made similar assumptions.…”

“…ZENO has previously been shown to capture the experimental dilute solution properties. 14,25,43,44 We find that for this solvent−solute combination ZENO overestimates the experimentally measured intrinsic viscosity by ∼30%. Similar overestimation of intrinsic viscosity seems to occur in all molecular dynamics studies attempting to model this solvent−solute pair with ZENO.…”

“…Similar overestimation of intrinsic viscosity seems to occur in all molecular dynamics studies attempting to model this solvent−solute pair with ZENO. 25,32 This deviation could be due to an inaccurate empirical prefactor for this class of shapes; however, we leave the exploration of why this deviation occurs to future work but find that multiplying all our intrinsic viscosity data by a best-fit constant c = 0.68 aligns our simulations with experiments.…”

“…However, this model does not include chemically specific local interactions such as bond angles that prohibit quantitative agreement. More recently, Loṕez-Barroń et al 25 simulate bottlebrush polymers in good solvent using an implicit solvent, united atom model. This model uses united-atom bonds 33 and modifies six nonbonded parameters to match all-atom R g data.…”

Mesoscale Modeling of Agglomeration of Molecular Bottlebrushes: Focus on Conformations and Clustering Criteria

“…High grafting density leads to the pronounced effects of steric repulsions, which results in the stretched conformations of a backbone and side chains compared with the linear chain conformations. The extent of stretching, as well as equilibrium conformation of the MBB, strongly depends on its architecture, chemical nature of the moieties within the backbone and side chains, solvent quality, and concentration of MBBs within the solvent [1,6,15,16]. Relatively high backbone tension can develop depending on bottlebrush conformation [17,18]; this tension can be further amplified upon bottlebrush adsorption onto the interface [19][20][21].…”