A comparative study of thermodynamic, conformational, and structural properties of bottlebrush with star and ring polymer melts

Abstract

Thermodynamic, conformational, and structural properties of bottlebrush polymer melts are investigated with molecular dynamics simulations and compared to linear, regular star, and unknotted ring polymer melts to gauge the influence of molecular topology on polymer melt properties. We focus on the variation of the backbone chain length, the grafting density along the backbone, and the length of the side chains at different temperatures above the melt glass transition temperature. Based on these comparisons, we find that the segmental density, isothermal compressibility, and isobaric thermal expansion of bottlebrush melts are quantitatively similar to unknotted ring polymer melts and star polymer melts having a moderate number ( f = 5 to 6) of arms. These similarities extend to the mass scaling of the chain radius of gyration. Our results together indicate that the configurational properties of bottlebrush polymers in their melt state are more similar to randomly branched polymers than linear polymer chains. We also find that the average shape of bottlebrush polymers having short backbone chains with respect to the side chain length is also rather similar to the unknotted ring and moderately branched star polymers in their melt state. As a general trend, the molecular shape of bottlebrush polymers becomes more spherically symmetric when the length of the side chains has a commensurate length as the backbone chain. Finally, we calculate the partial static structure factor of the backbone segments and we find the emergence of a peak at the length scales that characterizes the average distance between the backbone chains. This peak is absent when we calculate the full static structure factor. We characterize the scaling of this peak with parameters characterizing the bottlebrush molecular architecture to aid in the experimental characterization of these molecules by neutron scattering.

Hidden Hyperuniformity in Soft Polymeric Materials

“…Trends in the peak location and its scaling with molecular parameters is discussed in Ref. [33]. For smaller q-values, there is a steep decrease in S b (q) and in S c (q) as q → 0 and for low q-values S b (q) < S(q), as illustrated in Fig.…”

“…The extrapolated values of S(0) from S(q) calculations are in agreement with the recently developed empirical correlation of volume V (T, P ), see Supplementary Information for more details, allowing the estimation of ρ and κ T , via the Eq. 1, meaning that our systems are equilibrated and includes Refs [10,33]. However, the hyperuniform parameter based on S(q) is larger than the threshold value h > 10 −3 , suggesting that when these materials are considered as a whole then they are not effectively hyperuniform.…”

“…To probe the trends of the structural correlations as q → 0, we prepared polymer melt systems having significantly more interaction centers than our original study (for star polymers from 32 400 to 20 250 000 and for bot- tlebrush polymers from 176 000 to 8 448 000 interaction centers) [33]. The resulting S b (q) curves at T > T g are presented in Fig.…”

“…Additional details about the model are provided at the Supplementary Information, which includes Refs. [31][32][33].…”

“…Recent work by the authors have demonstrated that suppression of density fluctuations in the backbones of bottlebrush polymer melts and in the cores of stars occurs over a wide range of molecular parameters, signifying a rather general trend [33]. Indeed, bottlebrush [20], while for the backbone trends we find (c, δ) = (0.00174, 2.12) and for the core particles of star polymers we find (c, δ) = (0.00033, 1.87).…”