Melamine Units Research Articles

PNIPAm-Based pH and Thermoresponsive Copolymer Hydrogel for Hydrophobic and Hydrophilic Drug Delivery.

The regulated and targeted administration of hydrophobic and hydrophilic drugs is both promising and challenging in the field of drug delivery. Developing a hydrogel which is responsive to dual stimuli is considered a promising and exciting research area of study. In this work, melamine functionalized poly-N-isopropyl acrylamide-co-glycidyl methacrylate copolymer has been developed by copolymerizing glycidyl methacrylate (GMA) monomer with N-isopropyl acrylamide (NIPAm) and further functionalized with melamine units (pNIPAm-co-pGMA-Mela). The prepared pNIPAm-co-pGMA-Mela copolymer hydrogel was characterized using various characterization techniques, including 1H NMR, FTIR, SEM, zeta potential, and particle size analysis. A hydrophobic drug (ibuprofen, Ibu) and hydrophilic drug (5-fluorouracil, 5-Fu) were selected as model drugs. Dual pH and temperature stimuli-responsive drug release behavior of the pNIPAm-co-pGMA-Mela hydrogel was evaluated under different pH (pH 7.4 and 4.0) and temperature (25 °C, 37 °C, and 45 °C) conditions. Furthermore, the in vitro biocompatibility of the developed pNIPAm-co-pGMA-Mela copolymer hydrogel was determined on MDA-MB-231 cells. The pH and temperature-responsive drug delivery study results reveal that the pNIPAm-co-pGMA-Mela hydrogel system is responsive to both pH and temperature stimuli and exhibits about ~100% of Ibu and 5-Fu, respectively, released at pH 4.0/45 °C. Moreover, the MTT assay and hemocompatibility analysis results proved that the pNIPAm-co-pGMA-Mela hydrogel system is biocompatible and hemocompatible, suggesting that that it could be used for drug delivery applications. The experimental results suggest that the proposed pNIPAm-co-pGMA-Mela hydrogel system is responsive to dual pH and temperature stimuli, and could be a promising drug carrier system for both hydrophilic and hydrophobic drug delivery applications.

Tunable schiff-based networks with different bonding sites for enhanced photocatalytic activity under visible-light irradiation: The effects of steric hindrance

Organic polymers hold great potential in photocatalysis considering their low cost, structural tailorability, and well-controlled degree of conjugation for efficient electron transfer. Among the polymers, Schiff base networks (SNWs) with high nitrogen content have been noticed. Herein, a series of SNWs is synthesized based on the melamine units and dialdehydes with different bonding sites. The chemical and structural variation caused by steric hindrance as well as the related photoelectric properties of the SNW samples are investigated, along with the application exploration on photocatalytic degradation and energy production. The results demonstrate that only SNW-o based on o-phthalaldehyde responds to visible light, which extends to over 550 nm. SNW-o shows the highest tetracycline degradation rate of 0.02516 min−1, under 60-min visible light irradiation. Moreover, the H2O2 production of SNW-o is 2.14 times higher than that of g-C3N4. The enhanced photocatalytic activity could be ascribed to the enlarged visible light adsorption and intramolecular electron transfer. This study indicates the possibility to regulate the optical and electrical properties of organic photocatalysts on a molecular level, providing an effective strategy for rational supramolecular engineering to the applications of organic materials in photocatalysis.

Structural Analysis of the Poly(thymidine)-Melamine Assembly.

Hydrogen bonding between the DNA nucleobases and small organic molecules, such as melamine, is a new strategy for the design of novel DNA materials. Poly(thymidine) DNA and melamine self-assemble into a duplex structure containing two antiparallel DNA strands hydrogen bonded to central melamine units. In this Article, molecular dynamics simulations rationalize the observed antiparallel duplex structure. Alternative duplex and triplex structures with parallel and antiparallel strand orientations are shown to be unstable because of the increase in unfavorable interactions between the DNA backbones.

Ultrathin porous g-CN nanosheets fabricated by direct calcination of pre-treated melamine for enhanced photocatalytic performance

Abstract

Photoactive carbon nitride from melamine hydrochloride supramolecular assembly

The formation of a supramolecular assembly of melamine and chlorine and its use as graphitic carbon nitride (g-C3N4) precursor is shown. We studied the effect of the HCl on the morphology, chemical and photophysical properties of the starting supramolecular assembly up to the final g- C3N4, calcined at 550°C. We found that the HCl altered the crystal structure and morphology of the starting precursor due to better arrangement of the melamine units. The HCl-g-C3N4 demonstrates higher photoactivity toward the hydrogen production from water compared to the pristine one thanks to higher surface area along with better light harvesting and charge separation properties. This work opens new opportunities for rational design of new photocatalysts for environmental and energy-related applications.

Preparation of Polyimide–Cellulose Composite Using Oligoimide with Ethynyl Terminals

Abstract A soluble imide oligomer (IO) (repeating number; 2 and 4) having N-phenylated melamine units and phenylethynyl terminals was prepared, and the application to polyimide composites with cellulose (CEL) crystals derived from pulp (CELp) or Halocynthia (CELh) with or without a modifier was investigated. The CELh additive up to 0.5 wt % was easily dispersed in IO matrix in solution, and thermostable (glass-transition temperature, ca. 285 °C) transparent tough film (tensile strength, ca. 169 MPa; elongation at break, ca. 8.9%; tensile modulus, ca. 4.6 GPa) was obtained, while the addition of a modifier such as cetyltrimethylammonium bromide and poly(ethylene glycol) derivatives was necessary to obtain a similar tough film for the CELp additive.

Synthesis and 2D self-assembly at the liquid–solid interface of novel H-bonding linear π-conjugated oligomers terminated by uracil and melamine units

2D self-assembly of a novel π-conjugated oligomer terminated by two uracil units is solvent dependent and involves multiple intermolecular H-bonds.

Bifacial Peptide Nucleic Acid Directs Cooperative Folding and Assembly of Binary, Ternary, and Quaternary DNA Complexes

We report herein the structuring of single-stranded thymine-rich DNA sequences into peptide-DNA hairpin triplex structures via designed melamine-thymine nucleobase recognition. Melamine-displaying α-peptides were synthesized with the general form (EM*)n, where M* denotes a lysine residue side chain derivatized with melamine, a bifacial hydrogen bond complement for thymine. We have found that (EM*)n peptides, which we term bifacial peptide nucleic acid (bPNA), function as a noncovalent template for thymine-rich DNA tracts. Unstructured DNA of the general form dTnCmTn are bound to (EM*)n peptides and fold into cooperatively melting 1:1 bPNA-DNA hairpin complexes with dissociation constants in the submicromolar to low nanomolar range for n = 4-10. As the length of the interface (n) is decreased, the melting temperature of the bPNA-DNA complex drops significantly, though Kd increases are less substantial, suggestive of strong enthalpy-entropy compensation. This is borne out by differential scanning calorimetry analysis, which indicates enthalpically driven bPNA-DNA base-stacking that becomes markedly less exothermic as the recognition surface n decreases in size. The recognition interface tolerates a high number of "mismatches" and indicates half-site, or monofacial, recognition between melamine and thymine may occur if only 1 complementary nucleobase is available. Association correlates directly with fractional thymine content, with optimal binding when the number of T-T sites match the number of melamine units. Interestingly, when a DNA host has more T-T sites than melamine sites on bPNA, two or three bPNAs can bind to a single DNA, resulting in ternary and quaternary complexes that have higher thermal stability than the binary (1:1) bPNA-DNA complex, suggestive of cooperative multisite binding. In contrast, when two bPNAs of different lengths bind to the same DNA host, a ternary complex is formed with two melting transitions, corresponding to independent melting of each bPNA component from the complex. These data demonstrate that melamine-displaying bPNA recognize thymine-rich DNA in predictable and multifaceted ways that allow binding affinity, structure stability, and stoichiometry to be tuned through simple bPNA length modification and matching with DNA length. Synthetic bPNA structuring elements may be useful tools for biotechnology.

Surface Tectonics of Nanoporous Networks of Melamine‐Capped Molecular Building Blocks formed through Interface Schiff‐Base Reactions

Control over the assembly of molecules on a surface is of great importance for the fabrication of molecule-based miniature devices. Melamine (MA) and molecules with terminal MA units are promising candidates for supramolecular interfacial packing patterning, owing to their multiple hydrogen-bonding sites. Herein, we report the formation of self-assembled structures of MA-capped molecules through a simple on-surface synthetic route. MA terminal groups were successfully fabricated onto rigid molecular cores with 2-fold and 3-fold symmetry through interfacial Schiff-base reactions between MA and aldehyde groups. Sub-molecular scanning tunneling microscopy (STM) imaging of the resultant adlayer revealed the formation of nanoporous networks. Detailed structural analysis indicated that strong hydrogen-bonding interactions between the MA groups persistently drove the formation of nanoporous networks. Herein, we demonstrate that functional groups with strong hydrogen-bond-formation ability are promising building blocks for the guided assembly of nanoporous networks and other hierarchical 2D assemblies.

Hydrogen Bonded non-covalent synthesis by reaction of porphyrin appended calix[4]arene with 5,5-diethylbarbituric acid in solution

Calix[4]arene, diametrically substituted at the upper rim, with two porphyrin appended melamine units spontaneously form well-defined double rosette in the presence of 5,5-diethylbarbituric acid. This assembly consists of nine components which utilize 36 hydrogen bonds and are quite stable in apolar solvents of up to 10−4 M concentration. The formation of assembly between porphyrin appended calix[4]arene and diethylbarbiturate has been broadly studied by UV–visible, fluorescence and 1H NMR spectroscopic techniques. The stoichiometries of proposed aggregates have been analyzed by facile titration of calixarene and barbital. Furthermore, the MALDI-TOF mass measurement is found fully compatible with the observed stoichiometries.

Supramolecular hydrogen-bonded structures between melamine and N-heterocycles

Melamine (M) forms hydrogen bonded adducts with the N-heterocycles tris(2-pyridyl)-triazine (TPTZ) and tetrakis(4-N-methylpyridinumyl)porphyrin tetratosylate (TMPyP-Tos). The 4:3 adduct 4M·3TPTZ was obtained under solvent-free conditions and the 3:1 adduct 3M·TMPyP-Tos·9H2O in aqueous solution. In both adduct phases N-heterocycles are arranged in π–π stacks and melamine molecules are arranged perpendicular to these stacks. Whereas cohesion between the extended π-systems of N-heterocycles is mainly achieved by dispersive interactions no stacking interactions between melamine molecules occur. Melamine units are orientated in such a way to be located most suitable as donors and acceptors, respectively, for a dense hydrogen bonding network. In 4M·3TPTZ all molecules are involved in hydrogen bonding, whereas in 3M·TMPyP-Tos·9H2O hydrogen bridges are primarily formed between melamine, tosylate and water units and the macrocycle TMPyP is mainly bound by electrostatic interactions.

Synthesis and applications of a novel supramolecular polymer network with multiple H-bonded melamine pendants and uracil crosslinkers

A conjugated main-chain copolymer (PBT) consisting of bithiazole, dithieno[3,2-b:2′,3′-d]pyrroles (DTP), and pendent melamine units was synthesized by Stille polymerization, which can be hydrogen-bonded (H-bonded) with proper molar amounts of bi-functional π-conjugated crosslinker F (i.e., two uracil motifs covalently attached to a fluorene core through triple bonds symmetrically) to develop a novel supramolecular polymer network (PBT/F). The effects of multiple H-bonds on light harvesting capabilities, HOMO levels, and photovoltaic properties of polymer PBT and H-bonded polymer network PBT/F are investigated. The formation of supramolecular polymer network (PBT/F) between PBT and F was confirmed by FTIR and XRD measurements. Because of the stronger light absorption, lower HOMO level, and higher crystallinity of H-bonded polymer network PBT/F, the solar cell device containing PBT/F showed better photovoltaic properties than that containing polymer PBT. The preliminary results show that the solar cell device containing 1:1 weight ratio of PBT/F and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) offers the best power conversion efficiency (PCE) value of 0.86% with a short-circuit current density (Jsc) of 4.97 mA/cm2, an open circuit voltage (Voc) of 0.55 V, and a fill factor (FF) of 31.5%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Hierarchical organization of a robust porphyrin cage self-assembled by hydrogen bonds.

Porphyrins appended with four rigid hydrogen bonding motifs on the meso positions were synthesized and self-assembled into a cofacial cage with four complementary bis(decyl)melamine units in dry solvents. The hydrocarbon chains on the melamine mediate the formation of nanofilms on surfaces as the solvent slowly evaporates.

Assemblies of perylene diimide derivatives with melamine into luminescent hydrogels

We report unique and spontaneous formation of hydrogels of perylene derivatives with melamine. The luminescent gel network is formed by H-type aggregation of the perylene core, supramolecularly cross-linked by melamine units. As a result of controlled aggregation in the extended nanofibers, strong exciton fluorescence emission is observed.

Preparation and characterization of melamine–formaldehyde–polyvinylpyrrolidone polymer resin for better industrial uses over melamine resins

AbstractMelamine–formaldehyde–polyvinylpyrrolidone (MFP) polymer resin was prepared with 1 : 16 : 1 ratios of melamine, formaldehyde (CH2O), and polyvinylpyrrolidone amounts, respectively, by condensation polymerization at 6.9 pH. Structures were determined with IR, 1H‐NMR, and 13C‐NMR spectroscopies. Chemical shifts (δ, ppm) were analyzed with singlet at δ 4.5, duplet from 3.13 to 3.17 and a quartet at 1.5 to 2.2 ppm for methylene (CH2) bridging group, pyrrolidone, and polyvinyl constituents. The 3389.25, 1290.38, and 1655.28 cm−1 stretching frequencies of N, CH and COO groups, respectively, were noted on FTIR spectrum. The CN melamine units reacted with CH2O to adjoin with polyvinylpyrrolidone (PVP). An average viscosity molecular weight ($\bar{M}$v) 57,000 g mol−1 was obtained with Mark–Houwink–Sakurada equation. The chemical shift of N(CH2O)2Cpyrrolidone ring on 13C‐NMR spectra was shifted toward lower magnetic field at 175.18 ppm. The resin was partially miscible with water thereby densities and viscosities of aqueous solutions were measured at 298.15 K temperature. It showed higher densities and viscosities than those of water. The resin developed exceptionally higher adhesive strengthen when its 62.29‐μm uniform thin film was applied on surfaces of wooden strips. The resin showed micellar behavior at about 0.009 g/100 mL aqueous solution. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Selective Self-Organization of Guest Molecules in Self-Assembled Molecular Boxes

This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 1(3).(DEB)6 and 3(3).(DEB)6 for different hydroxyl functionalized anthraquinones 2. These receptors are formed by the self-assembly of three calix[4]arene dimelamine derivative molecules (1 or 3) and six diethylbarbiturate (DEB) molecules to give 1(3).(DEB)6 or 3(3).(DEB)6. Encapsulation of 2 occurs in a highly organized manner; that is, a noncovalent hydrogen-bonded trimer of 2 is formed within the hydrogen-bonded receptors 1(3).(DEB)6 and 3(3).(DEB)6. Both receptors 1(3).(DEB)6 and 3(3).(DEB)6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 2(3) trimer. The receptor selectivity toward different anthraquinone derivatives 2 has been studied using 1H NMR spectroscopy, X-ray crystallography, UV spectroscopy, and isothermal microcalorimetry (ITC). The pi-pi stacking between the electron-deficient center ring of the anthraquinone derivatives 2a-c and 2e-g and the relatively electron-poor melamine units of the receptor is the driving force for the encapsulation of the guest molecules. The selectivity of the hydrogen-bonded host for the anthraquinone derivatives is the result of steric interactions between the guest molecules and the calix[4]arene aromatic rings of the host.

Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes

AbstractCalix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well‐defined box‐like assemblies in the presence of two equivalents of 5,5‐diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10−4M. This paper reports the first X‐ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen‐bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3‐bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen‐bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen‐bonded assembly, but this effect very much depends on the exact positions of the groups.

13C-NMR studies of commercial and partially self-condensed hexakis(methoxymethyl)melamine (HMMM) resins

13C-nuclear magnetic resonance (NMR) spectroscopy was employed to study the linkages in three commercial fully methylocated melamine-formaldehyde (HMMM) resins and their partially self-condensed resins by acid catalysis, in order to probe the structures linking melamine units. Distortionless enhancement by polarization transfer (DEPT) experiments show that the partially self-condensed resins contain both methylene ether and methylene linkages between melamine units. The extent of these linkages were estimated by quantitative 13C-NMR spectroscopy using inverse gated decoupling techniques. The results show that the ratios of methylene ether and methylene linkages to the triazine moiety vary from resin to resin. © 1996 John Wiley & Sons, Inc.