Mechanical and Thermomechanical Characterization of Glassy Thermoplastics with Intrachain Cross-Links

Nonmonotonic Composition Dependence of Viscosity upon Adding Single-Chain Nanoparticles to Entangled Polymers

Well-characterized single-chain nanoparticles (SCNPs), synthesized from a linear polystyrene precursor through an intramolecular [4 + 4] thermal cycloaddition cross-linking reaction in dilute conditions, were added to entangled polystyrene melts at different concentrations. Starting from the pure linear melt, which is much more viscous than the melt of SCNPs, the zero-shear viscosity increased upon the addition of nanoparticles and reached a maximum before eventually dropping to the value of the SCNP melt. Molecular simulations reveal the origin of this unexpected behavior, which is the interplay of the very different compositional dependences of the dynamics of the two components. The SCNPs become much slower than the linear chains as their concentration decreases because they are threaded by the linear chains, reaching a maximum viscosity which is higher than that of the linear chains at a fraction of about 20%. This behavior is akin to that of single-loop ring polymers when added to linear matrices. This finding provides insights into the design and use of SCNPs as effective entropic viscosity modifiers of polymers and contributes to the discussion of the physics of loopy structures.

Influence of network defects on the conformational structure of nanogel particles: From "closed compact" to "open fractal" nanogel particles

We propose an approach to generate a wide range of randomly branched polymeric structures to gain general insights into how polymer topology encodes a configurational structure in solution. Nanogel particles can take forms ranging from relatively symmetric sponge-like compact structures to relatively anisotropic open fractal structures observed in some nanogel clusters and in some self-associating polymers in solutions, such as aggrecan solutions under physiologically relevant conditions. We hypothesize that this broad "spectrum" of branched polymer structures derives from the degree of regularity of bonding in the network defining these structures. Accordingly, we systematically introduce bonding defects in an initially perfect network having a lattice structure in three and two topological dimensions corresponding to "sponge" and "sheet" structures, respectively. The introduction of bonding defects causes these "closed" and relatively compact nanogel particles to transform near a well-defined bond percolation threshold into "open" fractal objects with the inherent anisotropy of randomly branched polymers. Moreover, with increasing network decimation, the network structure of these polymers acquires other configurational properties similar to those of randomly branched polymers. In particular, the mass scaling of the radius of gyration and its eigenvalues, as well as hydrodynamic radius, intrinsic viscosity, and form factor for scattering, all undergo abrupt changes that accompany these topological transitions. Our findings support the idea that randomly branched polymers can be considered to be equivalent to perforated sheets from a "universality class" standpoint. We utilize our model to gain insight into scattering measurements made on aggrecan solutions.

Supercooled melt structure and dynamics of single-chain nanoparticles: A computer simulation study

By using coarse-grained molecular dynamics simulations, we have investigated the structure and dynamics of supercooled single-chain cross-linked nanoparticle (SCNP) melts having a range of cross-linking degrees ϕ. We find a nearly linear increase in glass-transition temperature (T_g) with increasing ϕ. Correspondingly, we have also experimentally synthesized a series of polystyrene-based SCNPs and have found that the measured T_g estimated from differential scanning calorimetry is qualitatively consistent with the trend predicted by our simulation estimates. Experimentally, an increase in T_g as large as ΔT_g = 61 K for ϕ = 0.36 is found compared with their linear chain counterparts, indicating that the changes in dynamics with cross-links are quite appreciable. We attribute the increase in T_g to the enlarged effective hard-core volume and the corresponding reduction in the free volume of the polymer segments. Topological constraints evidently frustrate the local packing. In addition, the introduction of intra-molecular cross-linking bonds slows down the structural relaxation and simultaneously enhances the local coupling motion on the length scales within SCNPs. Consequently, a more pronounced dynamical heterogeneity (DH) is observed for larger ϕ, as quantified by measuring the dynamical correlation length through the four-point susceptibility parameter, χ₄. The increase in DH is directly related to the enhanced local cooperative motion derived from intra-molecular cross-linking bonds and structural heterogeneity derived from the cross-linking process. These results shed new light on the influence of intra-molecular topological constraints on the segmental dynamics of polymer melts.

Flow Photochemistry for Single-Chain Polymer Nanoparticle Synthesis

Single chain polymer nanoparticles (SCNP) are an attractive polymer architecture that provides functions seen in folded biomacromolecules. The generation of SCNPs, however, is limited by the requirement of a high dilution chemical step, necessitating the use of large reactors to produce processable quantities of material. Herein, the chemical folding of macromolecules into SCNPs is achieved in both batch and flow photochemical processes by the previously described photodimerization of anthracene units in polymethylmethacrylate (100 kDa) under UV irradiation at 366 nm. When employing flow chemistry, the irradiation time is readily controlled by tuning the flow rates, allowing for the precise control over the intramolecular collapse process. The flow system provides a route at least four times more efficient for SCNP formation, reaching higher intramolecular cross-linking ratios five times faster than batch operation.

The Effect of Intramolecular Cross-Linking on Polymer Interactions in Solution

The conformation of a polymer in a solvent is typically defined by the solvent quality, which is a consequence of the solvent and macromolecule's chemistry. Yet, additional factors can affect the polymer conformation, such as non-covalent interactions to surfaces or other macromolecules, affecting the amount of polymer-solvent interactions. Herein, chemically folded polymers with protein-like architectures are studied and compared to their unfolded linear precursor in good solvents using rheology measurements. The current research reveals that permanent folding by intramolecular chemical cross-linking limits the chain mobility and therefore causes a reduction in polymer-solvent interactions, making a good solvent become theta. This change not only affects the "solvent quality" but also leads to a change in particle-particle interactions as a function of concentration. These findings provide crucial insight into the effects of intramolecular cross-links on macromolecule solubility and self-assembly, which are critical for mimicking structurally similar biological materials.