Strain hardening in bidisperse polymer glasses: Separating the roles of chain orientation and interchain entanglement

Strengthen oriented poly (L-lactic acid) monofilaments via mechanical training

Polymer materials have found extensive applications in the clinical and medical domains due to their exceptional biocompatibility and biodegradability. Compared to metallic counterparts, polymers, particularly Poly (L-lactic acid) (PLLA), are more suitable for fabricating biodegradable stents. As a viscoelastic material, PLLA monofilaments exhibit a creep phenomenon under sustained tensile stress. This study explores the use of creep to enhance the mechanical attributes of PLLA monofilaments. By subjecting the highly oriented monofilaments to controlled, constant force stretching, we achieved notable improvements in their mechanical characteristics. The results, as confirmed by tensile testing and dynamic mechanical analysis, revealed a remarkable 67 % increase in total elongation and over a 20 % rise in storage modulus post-mechanical training. Further microscopic analyses, including Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), revealed enhanced spacing and cavity formation. These mechanical advancements are attributed to the unraveling and a more orderly arrangement of molecular chains in the amorphous regions. This investigation offers a promising approach for augmenting the mechanical properties of PLLA monofilaments, potentially benefiting their application in biomedical engineering.

Mechanisms of reinforcement in polymer nanocomposites

Qualitatively different stress–strain responses of polymer nanocomposites are shown to result from the dynamical evolution of three principal molecular structural motifs in the polymer–filler network.

A hybrid Brownian dynamics/constitutive model for yielding, aging, and rejuvenation in deforming polymeric glasses

We present a hybrid model for polymeric glasses under deformation that combines a minimal model of segmental dynamics with a beads-and-springs model of a polymer, solved by Brownian dynamics (BD) simulations, whose relaxation is coupled to the segmental dynamics through the drag coefficient of the beads. This coarse-grained model allows simulations that are much faster than molecular dynamics and successfully capture the entire range of mechanical response including yielding, plastic flow, strain-hardening, and incomplete strain recovery. The beads-and-springs model improves upon the dumbbell model for glassy polymers proposed by Fielding et al. (Phys. Rev. Lett., 2012, 108, 048301) by capturing the small elastic recoil seen experimentally without the use of ad hoc adjustments of parameters required in the model of Fielding et al. With appropriate choice of parameters, predictions of creep, recovery, and segmental relaxation are found to be in good agreement with poly(methylmethacrylate) (PMMA) data of Lee et al. (Science, 2009, 323, 231-234). Our model shows dramatic differences in behavior of the segmental relaxation time between extensional creep and steady extension, and between extension and shear. The non-monotonic response of the segmental relaxation time to extensional creep and the small elastic recovery after removal of stress are shown to arise from sub-chains that are trapped between folds, and that become highly oriented and stretched at strains of order unity, connecting the behavior of glassy polymers under creep to that of dilute polymer solutions under fast extensional flows. We are also able to predict the effects of polymer pre-orientation in the parallel or orthogonal direction on the subsequent response to extensional deformation.

Mapping Brittle and Ductile Behaviors of Polymeric Glasses under Large Extension

We have carried out a series of tensile extension tests on the two most common polymer glasses to describe their generic mechanical responses as a function of deformation rate at different temperatures. The essentially defect-free polystyrene and poly(methyl methacrylate) both show remarkable re-entrant failure: being ductile at intermediate rates and showing diminishing toughness at both higher and lower rates. We draw phase diagrams to map out the relationship between brittle-like and yield-like states in terms of temperature, rate, and stress. A coherent understanding of the rich phenomenology requires us to describe in more detail the interplay between the chain network and the primary structure bonded by intersegmental van der Waals forces.

Polystyrene Glasses under Compression: Ductile and Brittle Responses

Polystyrene of different molecular weights and their binary mixtures are studied in terms of their various mechanical responses to uniaxial compression at different temperatures. PS of M_w = 25 kg/mol is completely brittle until it is above its glass transition temperature T_g. In contrast, upon incorporation of a high molecular weight component, PS mixtures turn from barely ductile a few degrees below its T_g to ductile over 40° below T_g. In the upper limit, a PS of M_w = 319 kg/mol yields and undergoes plastic flow, even at T = -70 °C. The observed dependence of mechanical responses on molecular weight and molecular weight distribution can be adequately rationalized by the idea that yielding and plastic compression are caused by chain networking.

Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress-strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

A simple theory for glassy-polymeric mechanical response that accounts for large-scale chain relaxation is presented. It captures the crossover from perfect-plastic response to Gaussian strain hardening as the degree of polymerization N increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain-activated relaxation arising from the need to maintain constant chain contour length reduces the characteristic relaxation time by a factor ~εN during active deformation at strain rate ε. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.