117,755 research outputs found
Hydration water in PNIPAM microgels: a molecular dynamics simulation study at low temperatures
No full textPoly-Nisopropylacrylamide (PNIPAM) is a thermo-responsive polymer that has attracted considerable
attention as a “smart” material with a wide variety of applications, ranging from drug delivery to sensors.
PNIPAM microgels can be synthetized by copolymerizing the NIPAM monomer with bisacrylamide (BIS).
The resulting structure is characterized by cross-linked networks that swell in water at room temperature
giving rise to transparent gels.
Nowadays it is well recognized that water has a strong influence on the structural and dynamical behavior
of molecules and macromolecules. Hydration plays an important role in stabilizing many aqueous systems.
Investigations of the dynamics of the hydrating water molecules are therefore crucial to understand the
phase behavior of polymer aqueous solutions.
In the present work the structural and dynamical properties of PNIPAM microgels and the polymer-
induced water properties variations have been investigated upon cooling by means of molecular dynamics
simulations. The PNIPAM network model was built taking into account the inhomogeneous polymer radial
density within the microgel, on the basis of the value of the PNIPAM repeating units/bis-acrylamide mole
ratio, PNIPAM/BIS, used during the synthesis and of the maximum degree of swelling of such microparticle.
The computational methodology was based on previous atomistic modelling studies that characterized the
PNIPAM behavior in aqueous solution [1,2]. A description of the water and PNIPAM dynamics at low
temperatures will be presented for microgel systems at di↵erent hydration levels and compared to recent
neutron scattering experiments [3]
On the molecular origin of the cooperative coil-to-globule transition of poly(: N-isopropylacrylamide) in water
No full textBy means of atomistic molecular dynamics simulations we investigate the behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures below and above the lower critical solution temperature (LCST), including the undercooled regime. The transition between water soluble and insoluble states at the LCST is described as a cooperative process involving an intramolecular coil-to-globule transition preceding the aggregation of chains and the polymer precipitation. In this work we investigate the molecular origin of such cooperativity and the evolution of the hydration pattern in the undercooled polymer solution. The solution behaviour of an atactic 30-mer at high dilution is studied in the temperature interval from 243 to 323 K with a favourable comparison to available experimental data. In the water soluble states of PNIPAM we detect a correlation between polymer segmental dynamics and diffusion motion of bound water, occurring with the same activation energy. Simulation results show that below the coil-to-globule transition temperature PNIPAM is surrounded by a network of hydrogen bonded water molecules and that the cooperativity arises from the structuring of water clusters in proximity to hydrophobic groups. Differently, the perturbation of the hydrogen bond pattern involving water and amide groups occurs above the transition temperature. Altogether these findings reveal that even above the LCST PNIPAM remains largely hydrated and that the coil-to-globule transition is related with a significant rearrangement of the solvent in the proximity of the surface of the polymer. The comparison between the hydrogen bonding of water in the surrounding of PNIPAM isopropyl groups and in the bulk displays a decreased structuring of solvent at the hydrophobic polymer-water interface across the transition temperature, as expected because of the topological extension along the chain of such interface. No evidence of an upper critical solution temperature behaviour, postulated in theoretical and thermodynamics studies of PNIPAM aqueous solution, is observed in the low temperature domain
The reproductive cycle of the commercial sea urchin Paracentrotus lividus in the Ionian sea.
No full textMolecular insights on poly(N-isopropylacrylamide) coil-to-globule transition induced by pressure
No full textBy using extensive all-atom molecular dynamics simulations of an atactic linear polymer chain, we provide microscopic insights into poly(N-isopropylacrylamide) (PNIPAM) coil-to-globule transition addressing the roles played by both temperature and pressure. We detect a coil-to-globule transition up to large pressures, showing a reentrant behavior of the critical temperature with increasing pressure in agreement with experimental observations. Furthermore, again confirming the experimental findings, we report the existence at high pressures of a new kind of globular state. It is characterized by a more structured hydration shell that is closer to PNIPAM hydrophobic domains, as compared to the globular state observed at atmospheric pressure. Our results highlight that temperature and pressure induce a PNIPAM coil-to-globule transition through different molecular mechanisms, opening the way for a systematic use of both thermodynamic variables to tune the location of the transition and the properties of the associated swollen/collapsed states
3D Printing Technologies in Biosensors Production: Recent Developments
No full textRecent advances in 3D printing technologies and materials have enabled rapid development of innovative sensors for applications in different aspects of human life. Various 3D printing technologies have been adopted to fabricate biosensors or some of their components thanks to the advantages of these methodologies over the traditional ones, such as end-user customization and rapid prototyping. In this review, the works published in the last two years on 3D-printed biosensors are considered and grouped on the basis of the 3D printing technologies applied in different fields of application, highlighting the main analytical parameters. In the first part, 3D methods are discussed, after which the principal achievements and promising aspects obtained with the 3D-printed sensors are reported. An overview of the recent developments on this current topic is provided, as established by the considered works in this multidisciplinary field. Finally, future challenges on the improvement and innovation of the 3D printing technologies utilized for biosensors production are discussed
Phenomenological evidence in favor of a remote seismic coupling for large volcanic eurptions
No full textA correlation has been recently found between large earthquakes and the succeeding largest 9 explosive eruptions of the last century, which has been interpreted as a product of co- and post-seismic stress diffusion. Here, we check the statistical significance of the proposed coupling by using a larger dataset, and investigate the reliability of the causality hypothesis. We find that the volcanoes with VEI ≥ 4 eruptions underwent, in the few decades before the volcanic event, higher seismic stress perturbations due to large earthquakes compared to other volcanic areas. The correlation is statistically significant and it is not explained by a spatio-temporal clustering of eruptions and earthquakes due to tectonic pulses. This implies that the large earthquakes indeed triggered the eruption
Water-polymer coupling induces a dynamical transition in microgels
No full textThe long debated protein dynamical transition was recently found also in nonbiological macromolecules, such as poly- N-isopropylacrylamide (PNIPAM) microgels. Here, by using atomistic molecular dynamics simulations, we report a description of the molecular origin of the dynamical transition in these systems. We show that PNIPAM and water dynamics below the dynamical transition temperature T d are dominated by methyl group rotations and hydrogen bonding, respectively. By comparing with bulk water, we unambiguously identify PNIPAM-water hydrogen bonding as mainly responsible for the occurrence of the transition. The observed phenomenology thus crucially depends on the water-macromolecule coupling, being relevant to a wide class of hydrated systems, independently from the biological function
Molecular description of the coil-to-globule transition of Poly(N-isopropylacrylamide) in water/ethanol mixture at low alcohol concentration
No full textPoly (N-isopropylacrylamide), PNIPAM, is a widely studied polymer, which serves as a key constituent of nanostructured soft materials with responsive properties. Upon increasing temperature the PNIPAM polymer chain undergoes a reversible coil-to-globule transition at Tabout 305 K, which is reflected by a volume phase transition in cross-linked architectures, such as microgels, valuable for many practical applications. The addition of a cosolvent is a simple method to tune the transition temperature according to the specific purpose. In this study, we use atomistic molecular dynamics simulations to explore the solution behavior of a PNIPAM chain in a mixture of water and ethanol, acting as cosolvent, at low alcohol concentration. Our simulations reproduce the occurrence of the coil-to-globule transition of the polymer chain at 289 K, a temperature lower than that measured in water, in full agreement with experimental findings. By monitoring the temperature evolution of structural and dynamical properties of the PNIPAM-water-ethanol ternary system, we detect a localization of ethanol molecules at the polymer interface, mainly due to interactions between isopropyl and ethyl groups. We observe that the transition occurs without a release of adsorbed ethanol molecules, but with a loss of water molecules from the surrounding of PNIPAM hydrophobic moieties that favours the aggregation of ethanol molecules close to the polymer. Our results support the idea that both the decreased chemical potential of water in the bulk of the mixture and the competition between water and ethanol molecules in the interactions with the polymer play a driving role in the transition
Connecting elasticity and effective interactions of neutral microgels: The validity of the hertzian model
No full textAn important open problem in materials science is whether a direct connection exists between single-particle elastic properties and macroscopic bulk behavior. Here, we address this question by focusing on the archetype of soft colloids: thermoresponsive microgels. These colloidal-sized polymer networks are often assumed to interact through a simple Hertzian potential, a classic model in linear elasticity theory. By developing an appropriate methodology that can be generalized to any kind of soft particle, we are able to calculate all the elastic moduli of nonionic microgels across their volume phase transition (VPT). Remarkably, we reproduce many features seen in experiments, including the appearance of a minimum in the Poisson's ratio close to the VPT. By calculating the particle-particle effective interactions and the resulting collective behavior, we find that the Hertzian model works well up to moderate values of the packing fraction
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