33,995 research outputs found
On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water
By 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 PNIPAM water soluble states
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 proximity of the surface of the polymer. The
comparison between the hydrogen bonding of water in the surrounding of PNIPAM
isopropyl groups and in 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
Morphological and Tribological Properties of PMMA/Halloysite Nanocomposites
From an environmental and cost-effective perspective, a number of research challenges can be found for electronics, household, but especially in the automotive polymer parts industry. Reducing synthesis steps, parts coating and painting, or other solvent-assisted processes, have been identified as major constrains for the existing technologies. Therefore, simple polymer processing routes (mixing, extrusion, injection moulding) were used for obtaining PMMA/HNT nanocomposites. By these techniques, an automotive-grade polymethylmethacrylate (PMMA) was modified with halloysite nanotubes (HNT) and an eco-friendly additive N,N′-ethylenebis(stearamide) (EBS) to improve nanomechanical properties involved in scratch resistance, mechanical properties (balance between tensile strength and impact resistance) without diminishing other properties. The relationship between morphological/structural (XRD, TEM, FTIR) and tribological (friction) properties of PMMA nanocomposites were investigated. A synergistic effect was found between HNT and EBS in the PMMA matrix. The synergy was attained by the phase distribution resulted from the selective interaction between partners and favourable processing conditions. Modification of HNT with EBS improved the dispersion of nanoparticles in the polymer matrix by increasing their interfacial compatibility through hydrogen bonding established by amide groups with aluminol groups. The increased interfacial adhesion further improved the nanocomposite scratch resistance. The PMMA/HNT-EBS nanocomposite had a lower coefficient of friction and lower scratch penetration depth than PMMA/HNT nanocomposite.Financial support by the EU Commission through Project H2020-686165-IZADINANO2INDUSTRY
is gratefully acknowledged
Entropy of water and the temperature-induced stiffening of amyloid networks
In water, networks of semi-flexible fibrils of the protein -synuclein
stiffen significantly with increasing temperature. We make plausible that this
reversible stiffening is a result of hydrophobic contacts between the fibrils
that become more prominent with increasing temperature. The good agreement of
our experimentally observed temperature dependence of the storage modulus of
the network with a scaling theory linking network elasticity with reversible
crosslinking enables us to quantify the endothermic binding enthalpy and an
estimate the effective size of hydrophobic patches on the fibril surface.Comment: 13 pages, 6 figure
Study of Hydrophobic Domains in Humic Acids
FyzikálnÄ›-chemická povaha hydrofobnĂch domĂ©n huminovĂ˝ch kyselin byla studována z nÄ›kolika hledisek. K objasnÄ›nĂ vĂ˝znamu fluorescenÄŤnĂch spekter byly vzorky podrobeny sekvenÄŤnĂ frakcionaci, která pomohla k částeÄŤnĂ©mu objasnÄ›nĂ vlivu vodorozpustnĂ˝ch sloĹľek, volnĂ˝ch a vázanĂ˝ch lipidĹŻ na optickĂ© vlastnosti huminovĂ˝ch kyselin. VĂ˝sledky naznaÄŤily, Ĺľe fluorescenÄŤnĂ pĂky tradiÄŤnÄ› pĹ™iĹ™azovanĂ© superpozici jednotlivĂ˝ch struktur jsou spĂše dĹŻsledkem agregaÄŤnĂch vlastnostĂ huminovĂ˝ch molekul tvoĹ™ĂcĂch vlivem hydrofobnĂho efektu zdánlivÄ› vysoce aromatickĂ© struktury. Dále pak bylo zjištÄ›no, Ĺľe na optickĂ˝ch vlastnostech huminovĂ˝ch kyselin majĂ podĂl i molekuly, kterĂ© nemajĂ primárnÄ› fluorofornĂ nebo chromofornĂ vlastnosti. Tento pohled je v souladu s teoriĂ supramolekulárnĂho uspořádánĂ huminovĂ˝ch kyselin. Dále byly studovány agregace, konformaÄŤnĂ chovánĂ a termodynamická stabilita huminovĂ˝ch kyselin pomocĂ metody vysoce rozlišovacĂ ultrazvukovĂ© spektroskopie. Bylo prokázáno, Ĺľe huminovĂ© kyseliny majĂ schopnost agregovat uĹľ od velmi nĂzkĂ˝ch koncentracĂ (The nature of hydrophobic domains in humic acids was studied from different points of view. To shed light on the meaning of fluorescent spectra, the measured samples underwent the sequential extraction which partially revealed the role of water-soluble components, free and bond lipids in optical properties of humic acids. The results indicated that the fluorescence peaks traditionally attributed to the superposition of individual chemical structures are rather a result of aggregation properties of humic molecules and hydrophobic effect driving aromatic molecules together forming aggregates apparently large molecular weight. Further, it seems that there is a significant influence of non-fluorophores and non-chromophores on the optical properties of humic acids. Results are consistent with the theory on supramolecular structure of humic acids. Next, the aggregation, conformational behaviour and thermodynamic stability of humic acids were studied by high resolution ultrasonic spectroscopy. It was demonstrated that humic molecules are able to interact and form aggregates at very low concentration (
Brief overview on bio-based adhesives and sealants
Adhesives and sealants (AS) are materials with excellent properties, versatility, and simple curing mechanisms, being widely used in different areas ranging from the construction to the medical sectors. Due to the fast-growing demand for petroleum-based products and the consequent negative environmental impact, there is an increasing need to develop novel and more sustainable sources to obtain raw materials (monomers). This reality is particularly relevant for AS industries, which are generally dependent on non-sustainable fossil raw materials. In this respect, biopolymers, such as cellulose, starch, lignin, or proteins, emerge as important alternatives. Nevertheless, substantial improvements and developments are still required in order to simplify the synthetic routes, as well as to improve the biopolymer stability and performance of these new bio-based AS formulations. This environmentally friendly strategy will hopefully lead to the future partial or even total replacement of non-renewable petroleum-based feedstock. In this brief overview, the general features of typical AS are reviewed and critically discussed regarding their drawbacks and advantages. Moreover, the challenges faced by novel and more ecological alternatives, in particular lignocellulose-based solutions, are highlighted.Funding Agency
Portuguese Foundation for Science and Technology
PTDC/AGR-TEC/4814/2014;
PTDC/ASP-SIL/30619/2017;
IF/01005/2014.info:eu-repo/semantics/publishedVersio
A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers
We propose and solve a simple model describing secondary structure formation
in random hetero-polymers. It describes monomers with a combination of
one-dimensional short-range interactions (representing steric forces and
hydrogen bonds) and infinite range interactions (representing polarity forces).
We solve our model using a combination of mean field and random field
techniques, leading to phase diagrams exhibiting second-order transitions
between folded, partially folded and unfolded states, including regions where
folding depends on initial conditions. Our theoretical results, which are in
excellent agreement with numerical simulations, lead to an appealing physical
picture of the folding process: the polarity forces drive the transition to a
collapsed state, the steric forces introduce monomer specificity, and the
hydrogen bonds stabilise the conformation by damping the frustration-induced
multiplicity of states.Comment: 24 pages, 14 figure
Compact phases of polymers with hydrogen bonding
We propose an off-lattice model for a self-avoiding homopolymer chain with
two different competing attractive interactions, mimicking the hydrophobic
effect and the hydrogen bond formation respectively. By means of Monte Carlo
simulations, we are able to trace out the complete phase diagram for different
values of the relative strength of the two competing interactions. For strong
enough hydrogen bonding, the ground state is a helical conformation, whereas
with decreasing hydrogen bonding strength, helices get eventually destabilized
at low temperature in favor of more compact conformations resembling
-sheets appearing in native structures of proteins. For weaker hydrogen
bonding helices are not thermodynamically relevant anymore.Comment: 5 pages, 3 figures; revised version published in PR
Functionalization of cotton with poly-NiPAAm/chitosan microgel: Part II. Stimuli-responsive liquid management properties
An innovative strategy for functional finishing of cotton involves application of stimuli-responsive surface modifying system based on temperature- and pH-responsive poly-NiPAAm/chitosan microgel. The stimuli-responsiveness implied to cotton is the consequence of swelling/collapse of the microgel particles incorporated to the fibre surface, which produces an active liquid management system. The performance of functionalized cotton fabric in terms of liquid management properties was assessed by choosing appropriate techniques (water uptake; thin-layer wicking; water retention capacity; and drying capability) and discussion of the results was based on the types of water that are expected to be present in hydrated cotton and stimuli-responsive microgel
Poly(2-oxazolines) in biological and biomedical application contexts.
Polyoxazolines of various architectures and chemical functionalities can be prepared in a living and therefore controlled manner via cationic ring-opening polymerisation. They have found widespread applications, ranging from coatings to pigment dispersants. Furthermore, several polyoxazolines are water-soluble or amphiphilic and relatively non-toxic, which makes them interesting as biomaterials. This paper reviews the development of polyoxazoline-based polymers in biological and biomedical application contexts since the beginning of the millennium. This includes nanoscalar systems such as membranes and nanoparticles, drug and gene delivery applications, as well as stimuli-responsive systems
Hierarchical Self-Assembly of Halogen-Bonded Block Copolymer Complexes into Upright Cylindrical Domains
Self-assembly of block copolymers into well-defined, ordered arrangements of chemically distinct domains is a reliable strategy for preparing tailored nanostructures. Microphase separation results from the system, minimizing repulsive interactions between dissimilar blocks and maximizing attractive interactions between similar blocks. Supramolecular methods have also achieved this separation by introducing small-molecule additives binding specifically to one block by noncovalent interactions. Here, we use halogen bonding as a supramolecular tool that directs the hierarchical self-assembly of low-molecular-weight perfluorinated molecules and diblock copolymers. Microphase separation results in a lamellar-within-cylindrical arrangement and promotes upright cylindrical alignment in films upon rapid casting and without further annealing. Such cylindrical domains with internal lamellar self-assemblies can be cleaved by solvent treatment of bulk films, resulting in separated and segmented cylindrical micelles stabilized by halogen-bond-based supramolecular crosslinks. These features, alongside the reversible nature of halogen bonding, provide a robust modular approach for nanofabricatio
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