Hydrophobic Isopropyl Groups Research Articles

Thermoplastic Polyurethane-poly(N-isopropylacrylamide) Copolymer for Selective Uptake of Alcohol from Aqueous Solution.

Ethanol possesses high economic value, but as an industrial waste, it poses harm to human health and the environment. The paper describes the preparation of a thermoplastic polyurethane-poly(n-isopropylacrylamide) (TPU-PNIPAM) copolymer designed to selectively uptake alcohol in aqueous solution. The material was created by bonding TPU and PNIPAM together through intermolecular hydrogen bonds, enhancing its hydrophobic properties and making it easier to interact with alcohol molecules. As the amount of PNIPAM in TPU increases, the number of hydrophobic isopropyl groups in TPU-PNIPAM also increases, leading to an enhanced selective uptake ability of TPU-PNIPAM for alcohols in aqueous solution. When the temperature reaches 55 °C, the hydrophobic groups in TPU-PNIPAM are more exposed, further enhancing the selective uptake ability of TPU-PNIPAM for alcohols in aqueous solution. TPU-PNIPAM demonstrates selective preferential uptake for various concentrations and types of alcohol in aqueous solutions. The material's selective uptake performance for alcohols increases with their hydrophobicity, so TPU-PNIPAM exhibited the best adsorption performance for a 10 wt% n-propanol solution under the combined effect of steric hindrance. In addition, TPU-PNIPAM exhibited selective adsorption for other organic solvents, which demonstrated the universality of TPU-PNIPAM in removing contaminants from aqueous solutions.

Relationship between temperature-induced changes in internal microscopic structures of poly(N-isopropylacrylamide) microgels and organic dye uptake behavior.

Temperature-induced changes in the internal structures of poly(N-isopropylacrylamide) (pNIPAm) microgels were evaluated by small-angle X-ray scattering (SAXS), and the results were used to explain organic dye uptake by the microgels. The dye uptake experiments were conducted using two organic dyes: cationic rhodamine 6G (R6G) and anionic erythrosine. In the SAXS investigation, the internal structures of the microgels were characterized in terms of the correlation length, ξ, and the distance, d*, which originated from the local packing of the isopropyl groups of two neighboring chains. With increasing temperature up to the volume phase transition temperature (VPTT) of the microgels, the correlation length, ξ, was increased and the distance, d*, was decreased. At the same time, the amounts of the dyes taken up by the pNIPAm microgels were increased, despite a decrease in the volume of the microgels. The results indicated that the pNIPAm chains were closer to each other due to the hydrophobic association of isopropyl groups, which resulted in the growth of the hydrophobic domains. Thus, the hydrophobic interactions between the dyes and pNIPAm were probably accompanied by the domain formation. With a further increase of temperature above the VPTT, the correlation length, ξ, was decreased and then not defined because the Ornstein-Zernike type contribution disappeared, and the distance, d*, was not largely changed. At the same time, the uptake amounts of the dyes per unit volume of the microgels were also not largely changed, which behaved similar to the distance, d*. It was probably due to the fact that the internal structures of the microgels were not largely changed because the isopropyl groups were in contact with each other. The view was supported by the result of the uptake study of the nonthermoresponsive microgels which did not have the hydrophobic isopropyl groups.

Direct osmolyte-macromolecule interactions confer entropic stability to folded states.

Protective osmolytes are chemical compounds that shift the protein folding/unfolding equilibrium toward the folded state under osmotic stresses. The most widely considered protection mechanism assumes that osmolytes are depleted from the protein's first solvation shell, leading to entropic stabilization of the folded state. However, recent theoretical and experimental studies suggest that protective osmolytes may directly interact with the macromolecule. As an exemplary and experimentally well-characterized system, we herein discuss poly(N-isopropylacrylamide) (PNiPAM) in water whose folding/unfolding equilibrium shifts toward the folded state in the presence of urea. On the basis of molecular dynamics simulations of this specific system, we propose a new microscopic mechanism that explains how direct osmolyte-macromolecule interactions confer stability to folded states. We show that urea molecules preferentially accumulate in the first solvation shell of PNiPAM driven by attractive van der Waals dispersion forces with the hydrophobic isopropyl groups, leading to the formation of low entropy urea clouds. These clouds provide an entropic driving force for folding, resulting in preferential urea binding to the folded state and a decrease of the lower folding temperature in agreement with experiment. The simulations further indicate that thermodynamic nonideality of the bulk solvent opposes this driving force and may lead to denaturation, as illustrated by simulations of PNiPAM in aqueous solutions with dimethylurea. The proposed mechanism provides a new angle on relations between the properties of protecting and denaturing osmolytes, salting-in or salting-out effects, and solvent nonidealities.

Effect of crosslinker structure and crosslinker/monomer ratio on network parameters and thermodynamic properties of Poly (N-isopropylacrylamide) hydrogels

The effect of polymer composition and polymerization parameters such as the type of crosslinker and comonomer, crosslinker/monomer ratio and polymerization temperature on the polymer-solvent interactions and mechanical properties of tetraallylammonium bromide (TAB)-crosslinked N-isopropylacrylamide (NIPAAm), N,N’-methylenebisacrylamide (BIS)-crosslinked NIPAAm and TAB-crosslinked NIPAAm - itaconic acid (IA) hydrogels were studied. It was observed that the hydrogels prepared by 0.7 mole / L concentration of NIPAAm and crosslinked with BIS exhibited mechanical weakness and, they were broken even under a pressure, being less than 5.0 N at a swelling temperature of 25 °C while the PNIPAAm hydrogels crosslinked with ionic-octafunctional crosslinker, TAB, had better compression properties. A decrease in the concentration of NIPAAm, increasing TAB content, addition of a hydrophilic/weakly acidic comonomer, IA and an increase in the functionality of crosslinker resulted in the decreasing values of the poymer-solvent interaction parameter, χ at 25 °C. The values of χ , effective crosslinking density (νe) and enthalpy changes during the shrinkage process in the ranges of 33o - 45 °C and 37o- 45 °C indicated the exothermal nature of phase transition and the importance of intermolecular interactions between PNIPAAm chains. The partial molar dilution-enthalpies (ΔH1) and -entropies (ΔS1) that were calculated from the values of enthalpic and entropic components of χ also indicated a similar trend, referring to a decrease in ordered-structuring of water molecules around hydrophobic isopropyl groups with an increase in the swelling temperature.

A Spectroscopic and Computational Study of Propofol Dimers and Their Hydrated Clusters

Propofol (2,6-diisopropylphenol, PPF) homodimers and their complexes with one water molecule are analyzed by means of mass-resolved excitation spectroscopy. Using two-color resonance-enhanced multiphoton ionization (REMPI) the S(1) electronic spectra of these systems are obtained, avoiding fragmentation. Due to the large size of these species, the spectra present a large abundance of lines. Using UV/UV hole-burning spectroscopy, two isomers of PPF(2) are found and the existence of at least three isomers for propofol(2)(H(2)O)(1) (PPF(2)W(1)) is demonstrated. Comparison with the structures calculated at the M06-2X/6-311++G(d,p) and M06-2X/6-31+G(d) levels of theory shows that the main driving forces in PPF(2) are several C-H···π interactions accompanied by dipole-dipole interaction between the OH moieties. On the other hand, there is evidence for the formation of cyclic hydrogen-bond structures in the heterotrimers. A comparison of the results obtained herein with those of similar systems from previously published studies follows.

Interactions of Biological Buffers with Macromolecules: The Ubiquitous “Smart” Polymer PNIPAM and the Biological Buffers MES, MOPS, and MOPSO

Proteins are practically never in a buffer-free solution. We therefore studied the effects of some important biological buffers 2-(N-morpholino)ethanesulfonic acid (MES), 3-(N-morpholino)propanesulfonic acid (MOPS), and 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) on the lower critical solution temperature (LCST) phase transition of poly(N-isopropylacrylamide) (PNIPAM), an isomer of polyleucine, as a model compound for protein. The results from dynamic light scattering (DLS) analysis showed that the LCST of PNIPAM aqueous solutions were decreased significantly with increasing the buffer concentration, due to the presence of the buffers destroying the hydration structures and subsequent aggregation of PNIPAM. Based on density functional theory (DFT) and molecular mechanics calculations, these buffers are highly polar compounds and, therefore, strongly interact with water molecules, causing buffering out of PNIPAM. The buffer’s affinity to water follows the order MOPS > MES > MOPSO. The LCST values determined from DLS follow the order MOPSO > MOPS ≈ MES. The Fourier transform infrared spectra (FTIR) reveal that no direct binding between the buffer and the polymer occurs but interacts with the first hydration shell of the polymer. In comparing with the MOPS buffer, the unexpected effects on the PNIPAM were found in the MOPSO-containing system due to MOPSO has one more OH group. This group interacts with water clusters around the hydrophobic isopropyl groups, which changes the hydration state of the polymer. It also should be noted that the phase transition in the presence of buffers is similar to those observed with kosmotropic salts and sugars (protein stabilizers) in terms of lowering the LCST and the dehydration of PNIPAM groups. Such favoring of the compact globule state observed at high buffer concentrations will provide a protective effect against denaturation of globular proteins. The interactions of the studied buffers with bovine serum albumin (BSA) have been investigated and found that these buffers stabilized BSA, based on DLS, FTIR, and UV–vis absorption studies. The present study will act as a key for the lock of buffer and macromolecule mechanism.

UV resonance Raman determination of molecular mechanism of poly(N-isopropylacrylamide) volume phase transition.

Poly(N-isopropylacrylamide) (PNIPAM) is the premier example of a macromolecule that undergoes a hydrophobic collapse when heated above its lower critical solution temperature (LCST). Here we utilize dynamic light scattering, H-NMR, and steady-state and time-resolved UVRR measurements to determine the molecular mechanism of PNIPAM's hydrophobic collapse. Our steady-state results indicate that in the collapsed state the amide bonds of PNIPAM do not engage in interamide hydrogen bonding, but are hydrogen bonded to water molecules. At low temperatures, the amide bonds of PNIPAM are predominantly fully water hydrogen bonded, whereas, in the collapsed state one of the two normal CO hydrogen bonds is lost. The NH-water hydrogen bonding, however, remains unperturbed by the PNIPAM collapse. Our kinetic results indicate a monoexponential collapse with tau approximately 360 (+/-85) ns. The collapse rate indicates a persistence length of n approximately 10. At lengths shorter than the persistence length the polymer acts as an elastic rod, whereas at lengths longer than the persistence length the polymer backbone conformation forms a random coil. On the basis of these results, we propose the following mechanism for the PNIPAM volume phase transition. At low temperatures PNIPAM adopts an extended, water-exposed conformation that is stabilized by favorable NIPAM-water solvation shell interactions which stabilize large clusters of water molecules. As the temperature increases an increasing entropic penalty occurs for the water molecules situated at the surface of the hydrophobic isopropyl groups. A cooperative transition occurs where hydrophobic collapse minimizes the exposed hydrophobic surface area. The polymer structural change forces the amide carbonyl and N-H to invaginate and the water clusters cease to be stabilized and are expelled. In this compact state, PNIPAM forms small hydrophobic nanopockets where the (i, i + 3) isopropyl groups make hydrophobic contacts. A persistent length of n approximately 10 suggests a cooperative collapse where hydrophobic interactions between adjacent hydrophobic pockets stabilize the collapsed PNIPAM.

Effect of polymer nature on the structure and properties of gel composites with incorporated bentonite particles

Composite gels based on polyacrylamide and poly(N-isopropyl acrylamide) with incorporated sodium bentonite particles are synthesized. It is shown that the presence of hydrophobic isopropyl groups in a polymer molecule promotes the subsequent formation of highly ordered aggregates of clay and cetylpyridinium chloride in a gel composite. An increase in temperature results in the collapse of composite gels based on poly(N-isopropyl acrylamide); however, no marked changes in the structure of lamellar aggregates of clay and surfactant are observed. It is revealed that the gel can stabilize lamellar structures formed in organoclay suspension prior to the incorporation into swollen polymer network.

Synthesis of thermoresponsive polystyrene-based comb-type grafted poly(N-isopropylacrylamide)

Narrowly distributed polystyrene-g-p(N-isopropylacrylamide) (PSt-g-PNIPAM) was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide using the brominated polystyrene as macroinitiator and CuCl combined with hexamethyltriethylenetetramine as catalyst. Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy confirmed the structure of PSt-g-PNIPAM. The gel permeation chromatography (GPC) showed that the graft copolymer had a single distribution peak with molecular weight, M n (g/mol) of 19815 g/mol (using polystyrene as the standard). Differential scanning calorimetry (DSC) revealed that due to both effects of hydrophobic isopropyl groups and hydrogen bonds in the amide group, the glass transition temperature (T g) of PSt-g-PNIPAM enhanced 16.0°C compared to the T g of the polystyrene.

Saccharide Effect on the Deswelling Process of the Thermosensitive Poly(N-isopropylacrylamide) Gel

Abstract The kinetic parameters of deswelling and the change of a lower critical solution temperature (LCST) for poly(N-isopropylacrylamide) gel, poly(NIPAAm) gel, were investigated in the presence of glucose and other sugars as hydrophilic probes. The enthalpy of activation (ΔH≠) and the entropy of activation (ΔS≠) for deswelling were influenced by the concentration of additive sugars. Plots of ΔH≠ against TΔS≠ fell on a straight line, of which the slope was approximately 0.91–0.94. This result shows a compensation effect of the free energy of activation (ΔG≠) concerning the relationship between ΔH≠ and ΔS≠ for deswelling of poly(NIPAAm) gel. The molecular interaction between sugar and water clusters involved with the polymer is responsible for the compensation effect between ΔH≠ and ΔS≠. The equatorial hydroxy groups of sugar molecules interact easily with water clusters around the hydrophobic isopropyl groups (the iceberg) to form hydrogen bonds, which give rise to change the situation of polymer hydration. A decrease of the LCST has close connections with the number of equatorial hydroxy groups of sugars as additives. The LCST decrease in sugar solutions was regarded as the situation change of polymer hydration in the iceberg.

Probing Conformational Isomerizations of Double-Stranded Poly(dA-dT) by a Substitution of Minor Amounts of the Thymine Methyls with Bulky Hydrophobic Isopropyl Groups

We probed conformational polymorphism of a synthetic DNA poly(dA-dT) by introducing various small amounts of bulky spherical hydrophobic isopropyl groups into the polynucleotide primary structure. For this purpose, three mixed copolymers of poly(dA-dT,ip5dU) were synthesized in which 2.6%, 8.6% or 14.2% of the polynucleotide pyrimidine bases had the isopropyl group in position 5. The isopropyls made the formation of both A-form and X-form incomplete, and this effect increased with the increasing isopropyl amount in the polynucleotide. However, the polynucleotide isomerization into the A-form was hindered by the isopropyls while the isomerization into the X-form was rather promoted. This observation indicates that, unlike the A-form, the X-form has the base pairs shifted towards the double helix major groove. Z-form was also promoted by the lowest concentration of the isopropyl groups while the most isopropylated poly(dA-dT) aggregated under the Z-form inducing conditions.

The structure of [D‐Hyi2, L‐Hyi4]meso‐valinomycin revealed by X‐Ray analysis

Direct x-ray analysis has been used to determine the crystal structure of [D-Hyi2, L-Hyi4]meso-valinomycin (cyclo[-D-Val-D-Hyi-L-Val-L-Hyi-(D-Val-L-Hyi-L-Val-D-+ ++Hyi)2-], C60H102N6O18), which crystallized from acetone with two solvent molecules. The crystals are trigonal, space group P32, number of molecules per unit cell Z = 3, cell parameters a = b = 15.2085 (8) A, c = 29.3250 (9) A, gamma = 120 degrees. The standard (R) and weighted (Rw) reliability factors after refinement of the atomic coordinates for C, N, and O atoms in the anisotropic thermal motion approximation, allowing for isotropic H atom contributions, were 0.070 and 0.082, respectively. The molecule adopts a distorted bracelet structure which is stabilized by six N-H ... O = C 4----1 type intramolecular hydrogen bonds. The side chains predominantly occupy external pseudoaxial positions relative to the cylindrical axis of the molecule. In contrast to meso-valinomycin, only four of the six Val carbonyl oxygen atoms are directed inwards to form a coordination centre for the molecule, and the carbonyl oxygen atoms of residues D-Val1 and L-Val3 are twisted outward and point away from the centre of the molecule. Although the analogue has a partially formed ion-binding center, it is inaccessible because the hydrophobic isopropyl groups of the D-Hyi2 and L-Hyi4 residues screen the molecular cavity on both sides.