The molecular mechanism of constructive remodeling of a mechanically-loaded polymer

Large or repeated mechanical loads usually degrade polymers by accelerating fragmentation of their backbones but rarely, they can cause new backbone bonds to form. When these new bonds form faster than the original bonds break, mechanical degradation may be arrested or reversed in real time. Exploiting such constructive remodeling has proven challenging because we lack an understanding of the competition between bond-forming and bond-breaking reactions in mechanically-stressed polymers. Here we report the molecular mechanism and analysis of constructive remodeling driven by the macroradical products of mechanochemical fragmentation of a hydrocarbon backbone. By studying the changing compositions of a random copolymer of styrene and butadiene sheared at 10 °C in the presence of different additives we developed an approach to characterizing this growth/fracture competition, which is generalizable to other underlying chemistries. Our results demonstrate that constructive remodeling is achievable under practically relevant conditions, requires neither complex chemistries, elaborate macromolecular architectures or free monomers, and is amenable to detailed mechanistic interrogation and simulation. These findings constitute a quantitative framework for systematic studies of polymers capable of autonomously counteracting mechanical degradation at the molecular level.

Photolabile Well-Defined Polystyrene Grafted on Silica Nanoparticle via Nitroxide-Mediated Polymerization (NMP)

Herein, the synthesis of a novel nitroxide-mediated polymerization (NMP) initiator bearing a photolabile ortho-nitrobenzyl (oNB) group allowing surface-initiated NMP preparation of well-defined photoresponsive polystyrene grafted on silica nanoparticles is described. The photocleavable and photoresponsive properties of the prepared materials are demonstrated using small angle X-ray scattering (SAXS) characterization.

Molecular Damage Detection in an Elastomer Nanocomposite with a Coumarin Dimer Mechanophore

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross-linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross-linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle-reinforced elastomer.

Multi-scale modeling of the polymer-filler interaction

We report mesoscopic simulations of the interaction between a silica nanoparticle and cis-1,4-polybutadiene chains with realistic coarse-(CG) grained models. The CG models are obtained with a bottom-up Bayesian method based on trajectory matching of atomistic configurations of the system. We then investigate the structural properties of the interfacial region as a function of the grafting density and polymer chain length. We take advantage of the realistic CG models to explore the dynamics of the nanoparticle over a period of 10 microseconds. We show that the dynamics of the nanoparticle is affected by the grafting density and the polymer chain length of the grafted chains.

Development of Coarse-Grained Models for Polymers by Trajectory Matching

Coarse-grained (CG) models allow for simulating the necessary time and length scales relevant to polymers. However, developing realistic force fields at the CG level is still a challenge because there is no guarantee that the CG model reproduces all the properties of the atomistic model. A recent promising method was proposed for small molecules using statistical trajectory matching. Here, we extend this method to the case of polymeric systems. As the quality of the final model crucially depends on the model design, we study and discuss the effect of the modeling choices on the structure and dynamics of bulk polymers before a quantitative comparison is made between CG methods on different properties and polymers.

Coarse-grained simulations of cis- and trans-polybutadiene: A bottom-up approach

We apply the dissipative particle dynamics strategy proposed by Hijón et al. [Faraday Discuss. 144, 301-322 (2010)] and based on an exact derivation of the generalized Langevin equation to cis- and trans-1,4-polybutadiene. We prove that it is able to reproduce not only the structural but also the dynamical properties of these polymers without any fitting parameter. A systematic study of the effect of the level of coarse-graining is done on cis-1,4-polybutadiene. We show that as the level of coarse-graining increases, the dynamical properties are better and better reproduced while the structural properties deviate more and more from those calculated in molecular dynamics (MD) simulations. We suggest two reasons for this behavior: the Markovian approximation is better satisfied as the level of coarse-graining increases, while the pair-wise approximation neglects important contributions due to the relative orientation of the beads at large levels of coarse-graining. Finally, we highlight a possible limit of the Markovian approximation: the fact that in constrained simulations, in which the centers-of-mass of the beads are kept constant, the bead rotational dynamics become extremely slow.

Interplay between polymer chain conformation and nanoparticle assembly in model industrial silica/rubber nanocomposites

The question of the influence of nanoparticles (NPs) on chain dimensions in polymer nanocomposites (PNCs) has been treated mainly through the fundamental way using theoretical or simulation tools and experiments on well-defined model PNCs. Here we present the first experimental study on the influence of NPs on the polymer chain conformation for PNCs designed to be as close as possible to industrial systems employed in the tire industry. PNCs are silica nanoparticles dispersed in a styrene-butadiene-rubber (SBR) matrix whose NP dispersion can be managed by NP loading with interfacial coatings or coupling additives usually employed in the manufacturing mixing process. We associated specific chain (d) labeling, and the so-called zero average contrast (ZAC) method, with SANS, in situ SANS and SAXS/TEM experiments to extract the polymer chain scattering signal at rest for non-cross linked and under stretching for cross-linked PNCs. NP loading, individual clusters or connected networks, as well as the influence of the type, the quantity of interfacial agent and the influence of the elongation rate have been evaluated on the chain conformation and on its related deformation. We clearly distinguish the situations where the silica is perfectly matched from those with unperfected matching by direct comparison of SANS and SAXS structure factors. Whatever the silica matching situation, the additive type and quantity and the filler content, there is no significant change in the polymer dimension for NP loading up to 15% v/v within a range of 5%. One can see an extra scattering contribution at low Q, as often encountered, enhanced for non-perfect silica matching but also visible for perfect filler matching. This contribution can be qualitatively attributed to specific h or d chain adsorption on the NP surface inside the NP cluster that modifies the average scattering neutron contrast of the silica cluster. Under elongation, NPs act as additional cross-linking junctions preventing chain relaxation and giving a deformation of the chain with the NP closer to a theoretical phantom network prediction than a pure matrix.

Revealing nanocomposite filler structures by swelling and small-angle X-ray scattering

Polymer nanocomposites are used widely, mainly for the industrial application of car tyres. The rheological behavior of such nanocomposites depends in a crucial way on the dispersion of the hard filler particles – typically silica nanoparticles embedded in a soft polymer matrix. It is thus important to assess the filler structure, which may be quite difficult for aggregates of nanoparticles of high polydispersity, and with strong interactions at high loading. This has been achieved recently using a coupled TEM/SAXS structural model describing the filler microstructure of simplified industrial nanocomposites with grafted or ungrafted silica of high structural disorder. Here, we present an original method capable of reducing inter-aggregate interactions by swelling of nanocomposites, diluting the filler to low-volume fractions. Note that this is impossible to reach by solid mixing due to the large differences in viscoelasticity between the composite and the pure polymer. By combining matrix crosslinking, swelling in a good monomer solvent, and post-polymerization of these monomers, it is shown that it is possible to separate the filler into small aggregates. The latter have then been characterized by electron microscopy and small-angle X-ray scattering, confirming the conclusions of the above mentioned TEM-SAXS structural model applied directly to the highly loaded cases.

A high-temperature dielectric process as a probe of large-scale silica filler structure in simplified industrial nanocomposites

The existence of two independent filler-dependent high-temperature Maxwell-Wagner-Sillars (MWS) dielectric processes is demonstrated and characterized in detail in silica-filled styrene-butadiene (SB) industrial nanocomposites of simplified composition using Broadband Dielectric Spectroscopy (BDS). The uncrosslinked samples are made with 140 kg mol(-1) SB-chains, half of which carry a single graftable end-function (50% D3), and Zeosil 1165 MP silica incorporated by solid-phase mixing. While one high-temperature process is known to exist in other systems, the dielectric properties of a new silica-related process - strength, relaxation time, and activation energy - have been evidenced and described as a function of silica volume fraction and temperature. In particular, it is shown that its strength follows a percolation behavior as observed with the ionic conductivity and rheology. Moreover, activation energies show the role of polymer layers separating aggregates even when they are percolated. Apart from simultaneous characterization over a broad frequency range up to local polymer and silanol dynamics, it is believed that such high-temperature BDS-measurements can thus be used to detect reorganizations in structurally-complex silica nanocomposites. Moreover, they should contribute to a better identification of dynamical processes via the described sensitivity to structure in such systems.

Basis sets for transition metals: Optimized outer p functions

Although the (n + 1)p orbital is unoccupied in transition-metal ground-state configurations which are all nd(x) (n + 1)s(y) , these (n + 1)p functions play a crucial role in the structure of transition metal complexes. As we show here, the usual solution, adding one or more diffuse functions, can be insufficient to create an orbital of the correct energy. The major problem appears to be due to the incorrect placement of the (n + 1)p orbital's node. Even splitting the most diffuse component of the np orbital and adding a second diffuse function does not completely solve this problem. Although one can usually solve this deficiency by further uncontracting of the np function, here we offer a set of properly optimized (n + 1)p functions that offer a more compact and satisfactory solution to the proper placements of the node. We show an example of the common deficiencies seen in typical basis sets, including standard basis sets in GAUSSIAN94, and show that the new optimized (n + 1)p function performs well compared to a fully uncontracted basis set. © 1996 by John Wiley & Sons, Inc.

Mechanism of aggregate formation in simplified industrial silica styrene-butadiene nanocomposites: effect of chain mass and grafting on rheology and structure

The formation of aggregates in simplified industrial styrene-butadiene nanocomposites with silica filler has been studied using a recent model based on a combination of electron microscopy, computer simulations, and small-angle X-ray scattering. The influence of the chain mass (40 to 280 kg mol(-1), PI < 1.1), which sets the linear rheology of the samples, was investigated for a low (9.5 vol%) and high (19 vol%) silica volume fraction. 50% of the chains bear a single graftable end-group, and it is shown that the (chain-mass dependent) grafting density is the structure-determining parameter. A model unifying all available data on this system is proposed and used to determine a critical aggregate grafting density. The latter is found to be closely related to the mushroom-to-brush transition of the grafted layer. To our best knowledge, this is the first comprehensive evidence for the control of the complex nanoparticle aggregate structure in nanocomposites of industrial relevance by the physical parameters of the grafted layer.

Studying Twin Samples Provides Evidence for a Unique Structure-Determining Parameter in Simplifed Industrial Nanocomposites

The structure of styrene-butadiene (SB) nanocomposites filled with industrial silica has been analyzed using electron microscopy and small-angle X-ray scattering. The grafting density per unit silica surface ρ_D3 was varied by adding graftable SB molecules. By comparing the filler structures at fixed ρ_D3 (so-called "twins"), a surprising match of the microstructures was evidenced. Mechanical measurements show that ρ_D3 also sets the modulus: it is then possible to tune the terminal relaxation time of nanocomposites via the chain length while leaving the modulus and structure unchanged.

Intensity and duration of corticosterone response to stressful situations in Japanese quail divergently selected for tonic immobility

Two genotypes of Japanese quail have been divergently selected since the 1980s for long (LTI) or short (STI) duration of tonic immobility [Mills, A.D., Faure, J.M., 1991. Divergent selection for duration of tonic immobility and social reinstatement behavior in Japanese quail (Coturnix coturnix japonica) chicks. J. Comp. Psychol. 105(1), 25-38.], an unlearnt catatonic state characteristic of a behavioral fear response ([Jones, R.B., 1986. The tonic immobility reaction of the domestic fowl: a review. World's Poult. Sci. J. 42(1), 82-97.]; [Mills, A.D., Faure, J.M., 1991. Divergent selection for duration of tonic immobility and social reinstatement behavior in Japanese quail (Coturnix coturnix japonica) chicks. J. Comp. Psychol. 105(1), 25-38.]). The results of several behavioral tests conducted in LTI and STI quail have led to the conclusion that LTI quail are more fearful than STI quail [Faure, J.M., Mills, A.D., 1998. Improving the adaptability of animals by selection. In: Grandin, T. (Eds.), Genetics and the behavior of domestic animals. Academic Press, San Diego, pp. 235-264.]). However, few studies to date have focused on the Hypothalamic-Pituitary-Adrenal (HPA) axis response to stressful situations in LTI and STI quail, although the HPA axis is involved in fear responses [Siegel, H.S., 1971. Adrenals, Stress and the Environment. World's Poult. Sci. J. 27, 327-349.]. The corticosterone (CORT) response to various putatively stressful situations was therefore assessed in LTI and STI genotypes of quail in order to investigate their HPA axis reactivity to stress. Repeated induction of TI or 1 min manual restraint induced significant and comparable increases in CORT levels in both genotypes as a TI response. On the other hand, higher CORT responses were found in STI than in LTI quail when the manual restraint period lasted for 2 min or after restraint in a crush cage. Maximum CORT responses and genotype differences were maintained throughout the latter test even when it lasted for 120 min. Investigation of the CORT response to a single TI episode showed that CORT levels at the end of TI were negatively correlated with TI duration. Other experimental contexts consisting of isolation in a familiar or novel environment or the presentation of a novel object induced slight but significant and comparable increases in CORT response in both genotypes, whereas change of cagemates did not. In conclusion, the present findings indicate that differences in HPA axis response are observed between LTI and STI genotypes when quail are submitted to intense stressors, resulting in a high and prolonged CORT response. By contrast, plasma corticosterone concentrations do not differ between STI and LTI quail in response to stressful situations of lower intensity, which evoke responses limited in amplitude and duration. Genetic selection for divergent duration of TI has thus affected the HPA axis response to stress, and LTI and STI quail constitute an interesting model to investigate genetic variability of HPA axis activity in birds. More specifically, these genotypes of quail could be used to investigate the occurrence of functional differences at different levels of the HPA axis in order to explain the present findings.

Characterization of CRF, AVT, and ACTH cDNA and pituitary-adrenal axis function in Japanese quail divergently selected for tonic immobility

Higher corticosterone (CORT) responses to acute stress have previously been reported in quail selected for short (STI) duration of tonic immobility (TI) than for long TI (LTI), although behavioral studies indicated that LTI quail were more fearful. To investigate adrenal and pituitary function in these quail lines and their possible involvement in the differences in hypothalamic-pituitary-adrenal (HPA) axis reactivity, we measured CORT responses to adrenocorticotropin (1-24 ACTH), corticotropin-releasing factor (CRF), and arginine vasotocin (AVT) after characterizing the nucleotide acid sequences of these peptides in quail. Although maximum adrenal responses, assessed by ACTH challenge, were higher in STI quail, adrenal sensitivity was comparable for the two genotypes. It is therefore unlikely that differences in HPA axis reactivity involved the adrenal level. AVT and ACTH induced comparable CORT responses in both genotypes, whereas those induced by CRF were much lower. AVT is thus more potent than CRF in quail, but the respective maximum pituitary capacity of both genotypes to secrete ACTH was similar, and it is doubtful that the AVT pathway is involved in the difference in HPA axis reactivity between genotypes. On the other hand, the higher CORT responses induced by CRF in STI quail suggest that CRF might be involved in the differences in HPA axis reactivity between LTI and STI genotypes.

Daily and photoperiod variations of hypothalamic-pituitary-adrenal axis responsiveness in Japanese quail selected for short or long tonic immobility

The aims of this study were to investigate the existence of a circadian rhythm of basal corticosterone (B) plasma concentrations in male and female Japanese quail lines divergently selected for long (LTI) or short (STI) duration of tonic immobility (TI) and the possible effects of photoperiod length on corticotropic axis reactivity. Significant peaks in B levels were observed throughout the day in 3 out of the 4 groups used in our experiments. However, B levels remained very low for all groups (< 5.0 ng/mL) and there was no consensus between groups. We therefore have no evidence from our results that basal B levels follow a circadian rhythm in adult STI and LTI quail held under a long photoperiod (16L:8D). We also showed that rearing under a long photoperiod (16L:8D) was associated with higher basal B levels and higher B adrenal response capacity to 1-24 adrenocorticotropic hormone (ACTH) injection in the STI and LTI lines compared with a shorter period (8L:16D). Higher hypothalamic-pituitary-adrenal (HPA) axis responsiveness to restraint in a crush cage was also measured in female quail reared under the long photoperiod, and similar responses were measured under both photoperiods in males. This result suggests that the effects of photoperiod length involve both local and more central mechanisms in the control of HPA axis responsiveness according to sex. On the other hand, we showed that the genetic selection program for TI responses induced greater increases in the B level following restraint in STI quail than in LTI quail of both sexes under both photoperiods, but the B adrenal response capacity was similar for both lines and sexes. Although further investigations on both lines regarding adrenal sensitivity are necessary before being able to conclude definitively, our findings strongly suggest that the differences observed in HPA axis responsiveness to restraint between lines are probably not due to differences in adrenal function itself but may involve upstream structures of the HPA axis.

Relationship between hypothalamic-pituitary-adrenal axis responsiveness and age, sexual maturity status, and sex in Japanese quail selected for long or short duration of tonic immobility

The influences of age (4 to 12 wk), sexual maturity status, and sex on hypothalamic-pituitary-adrenal (HPA) axis responsiveness were investigated by measuring changes in peripheral basal levels of corticosterone (B) and responses to 10 min of physical restraint in a crush cage or injection of 1-24 adrenocorticotropic hormone (ACTH) in 2 genotypes of Japanese quail divergently selected for long (LTI) or short (STI) duration of tonic immobility (TI). Although gonad development was more advanced in male STI quail, most birds were still immature at 4 wk of age, but sexual maturity was fully acquired by 6 wk of age in both sexes and genotypes. This change was associated with increases in basal B levels in both genotypes and sexes. On the other hand, HPA axis responsiveness to restraint and adrenal responsiveness to 1-24 ACTH injection remained stable in STI quail, whatever the age. Conversely, significant responses to restraint compared with basal B levels were only observed at 4 and 6 wk of age, and adrenal responsiveness increased with age in LTI quail. Moreover, higher B levels were measured in response to restraint in STI than in LTI quail, whereas similar adrenal responses were measured at 9 and 12 wk of age. We concluded that an increase in basal B levels is associated with the stage of sexual maturity acquired, but it did not affect HPA axis responsiveness or adrenal B response capacity. On the other hand, age affected HPA axis responsiveness and adrenal B response capacity in LTI quail of both sexes but not in STI quail. It is hypothesized that functional HPA axis maturity occurs after 6 wk of age in the LTI genotype, but it is reached by 4 wk of age in the STI genotype. In conclusion, the divergent selection program for TI conducted on quail resulted in changes in HPA responsiveness that probably resulted from differences in development rate and function of the adrenal glands or other upstream structures of the HPA axis.