An official website of the United States government
Macromolecular architecture is a critical parameter when tuning polymer material properties. Although the implementation of non-linear polymers in different applications has grown over the years, polymer grafted surfaces such as nanoparticles have traditionally been composed of linear thermoplastic polymers, with a limited number of examples demonstrating a diversity in polymer architectures. In an effort to combine polymer architecturally dependent material properties with polymer grafted particles (PGPs), as opposed to conventional methods of tuning polymer grafting parameters such as the number of chains per surface area (i.e., polymer graft density), a series of bottlebrush grafted particles were synthesized using surface-initiated ring-opening metathesis polymerization (SI-ROMP). These bottlebrush PGPs are composed of glassy, semi-crystalline, and elastomeric polymer side chains with controlled backbone degrees of polymerization (Nbb) at relatively constant polymer graft density on the surface of silica particles with diameters equaling approximately 160 or 77 nm. Bottlebrush polymer chain conformations, evaluated by measuring the brush height of surface grafted polymer chains in solution and the melt, undergo drastic changes in macromolecular dimensions in different environments. In solution, brush heights increase linearly as a function of Nbb, consistent with fully stretched chains, which is confirmed using cryogenic transmission electron microscopy (Cryo-TEM). Meanwhile, brush heights are consistently at a minimum in the melt, indicative of chains collapsed on the particle surface. The conformational extremes for grafted bottlebrush polymers are unseen in any linear polymer chain systems, highlighting the effect of macromolecular architecture and surface grafting. Bottlebrush grafted particles are an exciting class of materials where diversifying polymer architectures will expand PGP material design rules that harness macromolecular architecture to dictate properties.
more »
« less
Star-to-Bottlebrush Transition in Extensional and Shear Deformation of Unentangled Polymer Melts
A series of model poly((±)-lactide) (PLA) graft copolymers was synthesized by ring-opening metathesis polymerization and used to probe the star-to-bottlebrush transition in shear and extensional flows. Ten samples with backbone degrees of polymerization 11 ≤ Nbb ≤ 420 were investigated using small-amplitude oscillatory shear (SAOS) and extensional rheometry measurements. Each contained one PLA side chain of length Nsc = 72 per two backbone repeating units on average (graft density of z = 0.5). The star-like to bottlebrush transition was identified at Nbb = 50–69 using SAOS. In extension, melt strain hardening is absent in the star-like melts (Nbb ≤ 50) but is prominent in the bottlebrush limit (Nbb > 69). The onset of melt strain hardening occurs at a time scale equivalent to the Rouse time of the backbone. A molecular interpretation of these results builds upon recent conjectures related to strain-induced increases in interchain friction in bottlebrush polymers. These findings will be useful in designing bottlebrush melts that strain harden, which is critical in various types of processing methods involving extensional flows, including foaming, 3D printing, and film-blowing.
more »
« less
- Award ID(s):
- 1901635
- PAR ID:
- 10402922
- Date Published:
- Journal Name:
- Macromolecules
- ISSN:
- 0024-9297
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Bottlebrush polymers, macromolecules consisting of dense polymer side chains grafted from a central polymer backbone, have unique properties resulting from this well-defined molecular architecture. With the advent of controlled radical polymerization techniques, access to these architectures has become more readily available. However, synthetic challenges remain, including the need for intermediate purification, the use of toxic solvents, and challenges with achieving long bottlebrush architectures due to backbone entanglements. Herein, we report hybrid bonding bottlebrush polymers (systems integrating covalent and noncovalent bonding of structural units) consisting of poly(sodium 4-styrenesulfonate) (p(NaSS)) brushes grafted from a peptide amphiphile (PA) supramolecular polymer backbone. This was achieved using photoinitiated electron/energy transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization in water. The structure of the hybrid bonding bottlebrush architecture was characterized using cryogenic transmission electron microscopy, and its properties were probed using rheological measurements. We observed that hybrid bonding bottlebrush polymers were able to organize into block architectures containing domains with high brush grafting density and others with no observable brushes. This finding is possibly a result of dynamic behavior unique to supramolecular polymer backbones, enabling molecular exchange or translational diffusion of monomers along the length of the assemblies. The hybrid bottlebrush polymers exhibited higher solution viscosity at moderate shear, protected supramolecular polymer backbones from disassembly at high shear, and supported self-healing capabilities, depending on grafting densities. Our results demonstrate an opportunity for novel properties in easily synthesized bottlebrush polymer architectures built with supramolecular polymers that might be useful in biomedical applications or for aqueous lubrication.more » « less
-
Mounted on top of furnaces, laboratory viscometers can be used for the rheological characterization of high temperature melts, such as molten rocks (lava). However, there are no instruments capable of measuring the viscosity of large volumes of high temperature melts outside the laboratory at, for example, active lava flows on volcanoes or at industrial sites. In this article, we describe a new instrument designed to be easy to operate, highly mobile, and capable of measuring the viscosity of high temperature liquids and suspensions (<1350 °C). The device consists of a torque sensor mounted in line with a stainless-steel shear vane that is immersed in the melt and driven by a motor that rotates the shear vane. In addition, a thermocouple placed between the blades of the shear vane measures the temperature of the melt at the measurement location. An onboard microcomputer records torque, rotation rate, and temperature simultaneously and in real time, thus enabling the characterization of the rheological flow curve of the material as a function of temperature and strain rate. The instrument is calibrated using viscosity standards at low temperatures (20–60 °C) and over a wide range of stress (30–3870 Pa), strain rate (0.1–27.9 s−1), and viscosity (10–650 Pa s). High temperature tests were performed in large scale experiments within ∼25 l of lava at temperatures between 1000 and 1350 °C to validate the system’s performance for future use in natural lava flows. This portable field viscometer was primarily designed to measure the viscosity of geological melts at their relevant temperatures and in their natural state on the flanks of volcanoes, but it could also be used for industrial purposes and beyond.more » « less
-
Bottlebrush (BB) elastomers with water-soluble side chains and tissue-mimetic mechanical properties are promising for biomedical applications like tissue implants and drug depots. This work investigates the microstructure and phase transitions of BB elastomers with crystallizable polyethylene oxide (PEO) side chains by real-time synchrotron X-ray scattering. In the melt, the elastomers exhibit the characteristic BB peak corresponding to the backbone-to-backbone correlation. This peak is a distinct feature of BB systems and is observable in small- or medium-angle X-ray scattering curves. In the systems studied, the position of the BB peak ranges from 3.6 to 4.8 nm in BB elastomers. This variation is associated with the degree of polymerization of the polyethylene oxide (PEO) side chains, which ranges from 19 to 40. Upon crystallization of the side chains, the intensity of the peak decays linearly with crystallinity and eventually vanishes due to BB packing disordering within intercrystalline amorphous gaps. This behavior of the bottlebrush peak differs from an earlier study of BBs with poly(ε-caprolactone) side chains, explained by stronger backbone confinement in the case of PEO, a high-crystallinity polymer. Microstructural models based on 1D SAXS correlation function analysis suggest crystalline lamellae of PEO side chains separated by amorphous gaps of monolayer-like BB backbones.more » « less
-
Brush-like graft copolymers (A-g-B), in which linear A-blocks are randomly grafted onto the backbone of a brush-like B-block, exhibit intense strain-stiffening and high mechanical strength on par with load-bearing biological tissues such as skin and blood vessels. To elucidate molecular mechanisms underlying this tissue-mimetic behavior, in situ synchrotron X-ray scattering was measured during uniaxial stretching of bottlebrush- and comb-like graft copolymers with varying densities of poly(dimethyl siloxane) and poly(isobutylene) side chains. In an undeformed state, these copolymers revealed a single interference peak corresponding to the average spacing between the domains of linear A-blocks arranged in a disordered, liquid-like configuration. Under uniaxial stretching, the emergence of a distinct four-spot pattern in the small-angle region indicated the development of long-range order within the material. According to the affine deformation of a cubic lattice, the four-spot pattern’s interference maxima correspond to 110 reflections upon stretching along the [111] axis of the body-centered unit cell. The experimental findings were corroborated by computer simulations of dissipative particle dynamics that confirmed the formation of a bcc domain structure.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.macromol.3c00015
