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Nanocomposites of PU Polymers with Nano Chitin and Nano Starch
CHAPTER 10
Nanocomposites of PU Polymers with Nano Chitin and Nano Starch Norma-Aurea Rangel-Vazquez Technical Institute of Agusacalientes, Agusacalientes, Mexico
Contents 10.1 Introduction 10.1.1 Polymers 10.1.2 Polysaccharides 10.1.3 Synthetic Polymers 10.1.4 Nanotechnology 10.1.5 Computational Chemistry 10.2 Methodology 10.2.1 Geometry Optimization 10.2.2 Bond Length 10.2.3 FTIR 10.2.4 MESP 10.3 Results and Discussions 10.3.1 PU/Chitin Nanocomposites 10.3.2 PU/Starch Nanocomposites 10.4 Conclusions References
311 311 313 315 316 318 320 320 320 320 321 321 321 328 334 335
10.1 INTRODUCTION 10.1.1 Polymers Polymers are defined as materials that are obtained from the polymerization of monomers in different process conditions [1]. Typically the polymers are grouped into different kinds of families, including: • According to the source. • Natural; those found in nature, such as cellulose, rubber, vegetable resins, etc. • Semisynthetic; obtained through some chemical process in natural materials, some examples are cellulose acetate, celluloid, and ebonite galalith. Polyurethane Polymers: Composites and Nanocomposites DOI: http://dx.doi.org/10.1016/B978-0-12-804065-2.00010-3
Copyright © 2017 Elsevier Inc. All rights reserved.
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•
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Synthetic; those of low-molecular-weight substances, which are the basis for the formation of monomers and carry out the polymerization process, for example, PE, PP, etc. Thus, the major forces existing in polymers include: van der Waals force. These manifest in those molecules with relatively low polarity such as hydrocarbons. The dipole moments that occur in a time generate a portion of the molecule that is highly negative, while the other section is positive, so very weak electrostatic attractions are obtained while the long-chain polymers submitted to these forces are large. Dipole dipole forces. They are more resistant due to the attraction between chains originating mainly in polyesters. Hydrogen bonds. The intermolecular forces have a high resistance, for example, a nylon fiber has greater tensile strength than steel fiber.
10.1.1.1 Homopolymers and Copolymers Homopolymers have only one repeating unit, for example, PE, PP, and PVC. One example of copolymers that contain more than one type of structural unit is acrylonitrile butadiene styrene (ABS). • According behavior with temperature. • Thermoplastics are polymers that are moldable with temperature, urgent debate chains linear or branched mainly, however if the temperature is achieved by decomposition this generates a disadvantage due to the final applications being reduced, on the other hand, they have economic, recyclable, and have good mechanical properties. • Thermosettings are materials that under heat irreversibly harden, and therefore they chemically decompose on melting, because the degree of crosslinking of chains increases in space. However, they possess excellent properties at elevated temperatures, such as heat resistance, chemistry, stiffness, and dimensional stability. They exhibit an opaque or yellow color [2]. • Type polymerization. • Addition. This process involves the use of catalysts to weigh out the reorganization of the CQC double bonds. • Condensation. The combination of monomers for polymer synthesis occurs secondary to formation of low-molecular-weight molecules such as water, HCl, alcohol, etc. This process is used for obtaining polyesetrs, polyamides, polyhydrocarbons, or polysulfides.
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10.1.2 Polysaccharides These are biomolecules that fall between carbohydrates and are formed by the union of a large number of monosaccharides and have diverse functions, particularly energy and structural reserves. Polysaccharides are chains, branched or not, of more than 10 monosaccharides [3,4]. Polysaccharides are polymers whose constituent monomers are monosaccharides, which are joined by glycosidic bonds repetitively. These compounds have a very high-molecular-weight, depending on the number of monosaccharide units or residues involved in their structure. This number is usually an unknown variable within a range, unlike what happens with biopolymers, such as DNA or proteins polypeptides, having in the chain a fixed number of parts, along with a specific sequence [3]. The main functions of polysaccharides in living systems relate to food reserves (vegetable and animal glycogen starches) and structure (cellulose chitin in vegetables and animals) [5]. 10.1.2.1 Starch Starches are polymers, e.g., where a large number of glucose monosaccharides bind. Starches have the general formula (C6H10O5), where the subscript depends on the type of starch formed. For example, glycogen is an animal starch consisting of about 60,000 glucose units. Glycogen is important as a source of energy storage in both the liver and muscles. When an organism needs energy enzymes, degradation occurs to free the glucose units [4]. The starch granule consists of two polysaccharide types: • Amylose (see Fig. 10.1A) consists of long straight chains of glucose and, in practice, there are some similar to amylopectin, once every several hundred molecules, in which substitutions do not alter their
Figure 10.1 Structure of (A) amylose and (B) amylopectin, respectively where carbon, hydrogen, oxygen atoms.
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properties. The chain amylose has a molecular weight of about one million. • Amylopectin (see Fig. 10.1B) constitutes 80 85% starch. Branching takes place with α(1-6) bonds occurring every 24 to 30 glucose units, these well branching generates a molecular weight of between 10 and 500 million. In some starches, such as potato starch, amylopectin also has some phosphate esters [6 8]. Both are composed of glucose units in the case of amylose bound together by bonds 1 4 leading to a linear chain. In the case of amylopectin branches appear due to bonds 1 6 [7]. 10.1.2.2 Chitin Chitin (see Fig. 10.2) represents the second most abundant natural polymer after cellulose. Its structure is the main constituent of the exoskeleton of crustaceans (crab, shrimp, lobster, squid, and shrimp) and insect cuticle, it is also found in algae, marine diatoms, and the fungal cell wall. Chitin is insoluble in aqueous solution, having a linear structure composed of repeating units of N-acetyl-D-glucosamine (GlcNAc) attached by glycosidic bond type β (1-4). The structure of chitin is highly related to the structure of cellulose, as this occurs in the C-2 hydroxyl group (OH). However, chitin has an acetamide group ( NHCOCH3) on the same carbon [9]. The amino groups of the native chitin are an acetylated form, which may be attached to protein chains or free amines, however during the extraction processes using common acids and alkalis
Figure 10.2 Structure of chitin, where, nitrogen atoms.
carbon,
hydrogen,
oxygen,
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inevitable deacetylation occurs. Chitin samples having different amounts of N-acetyl groups which depend on the source and extraction process, will be produced with degrees of acetylation of 95% [10]. Chitin has low toxicity, is inert in the intestinal tract of mammals, biodegradable due to the presence of chitinases, and is widely distributed in nature in bacteria, fungi, plants, and also in the digestive systems of various animals. Chitin was used to prepare affinity chromatography columns, for enzyme immobilization as absorbent compounds, and as contaminants tioulfato silver complexes as pharmaceutical fibers [11,12]. Crustaceans are the largest source of industrial-scale chitin, with production of approximately 2200 tonnes per year [13]. The chitin content in shellfish varies between 2 and 12% of total body mass; the content of chitin, protein, minerals, and carotenoids in the exoskeleton of crustaceans varies depending on the species, part of the body, nutritional status, and the stage of thereproductive cycle. The chitin exoskeleton protein contains about 15 40% chitin, proteins surroundings.
10.1.3 Synthetic Polymers 10.1.3.1 Polyurethane (PU) Polyurethane (Fig. 10.3) is obtained from the synthesis of two components, isocyanate groups and polyols [14,15]. Component A is a mixture of isocyanates, sometimes prepolymers (prestarted), containing NCO groups that may vary from 18 35% by functionality. Some are brown, with high viscosity (3000 5000 cpsBrookfield), and some are almost transparent and fluid. Sometimes they are kept in dry nitrogen. They also have highly appreciated adhesive properties, which also serve as binders for producing polymaterial blocks [16]. The polyol is a carefully formulated, balanced mixture of glycols
Figure 10.3 PU structure where, atoms, respectively.
carbon,
hydrogen,
oxygen,
nitrogen
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(high-molecular-weight alcohols). They are mixed with blowing agents and other additives such as amines, silicones, water, propellant, and organometallic catalysts. The reaction condition will give characteristics to the final foam. The appearance is like viscous honey and it can have a strong ammonia smell [17,18]. The mixture of the two polyol and isocyanate components, which are liquid at room temperature, and is usually performed with a specific machine, producing an exothermic chemical reaction. This chemical reaction is characterized by the formation of bonds between the polyol and isocyanate, obtaining a solid structure, which is very strong and uniform. The heat from the reaction can be used to evaporate a swelling agent filling the cells that form, so that a solid which has a cellular structure, with a much higher volume occupied by the liquid product, is obtained. 10.1.3.2 Properties It has low gas permeability, and excellent chemical resistance and electrical insulation. These properties allow it to enter the market and its production is easy as well as being relatively inexpensive. Its application in the medical industry is under development, particularly its use in the manufacture of patches, dressings, catheters, microcapsules, and applications that take into account the properties of biodegradability and nontoxicity [14,15].
10.1.4 Nanotechnology Nanotechnology is the science involved in the design, production, and employment of structures and objects that have at least one dimension in the range of 100 nm or less. Nanotechnology could have far-reaching implications for society. Today it is already used in sectors such as information and communications. Also, it is used in cosmetics, sunscreens, textiles, coatings, some food and energy technologies, and in certain health products and drugs. Moreover, nanotechnology could help reduce environmental pollution. Current knowledge in nanoscience comes from developments in the fields of chemistry, physics, life sciences, medicine, and engineering. There are several areas where nanotechnology is under development or undergoing practical application. • In materials science, nanoparticles allow the manufacture of products with new mechanical properties, even in terms of surface friction, wear resistance, and adhesion.
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In biology and medicine, nanomaterials are used to improve drug design and targeted delivery. It is also working on the development of nanomaterials for instrumentation and analytical equipment. Consumer products such as cosmetics, sunscreens, fibers, textiles, dyes, and paints already incorporate nanoparticles. In the field of electronic engineering, nanotechnologies are used, for example, in the design of smaller, faster data storage devices with lower power consumption [19].
10.1.4.1 Industrial Applications The growing technological development in the area of nanomaterials has allowed an increase in industrial applications such as films, substrates, and molecular biology. Biotechnology has generated a great expansion toward nonmetallic materials including metal, polymers, and composites. Different areas that have been and will be involved in the nanotechnologies are: medicine, biology, pharmacology, and materials (all applications in the areas of engineering, such as civil and construction, electronics, mechanics, chemistry, and food, among others). Applications include: • Sensors: For detection of specific compounds in different environments, to evaluate the quality of drinks and food products. • High-efficiency photovoltaic systems for converting solar energy. Screens lightes and functional video, based on polymer electronics. • New high-strength materials for aerospace, biomedical, and transport applications. New prostheses and implants for in vivo placement. • Packaging of food products with better features including barriers to penetration of gases indicating the degree of conservation of these materials. • Materials for filtration and catalysis of hydrocarbons and other substances. • Surface coatings with resistance to corrosion, scratching, and wear. • Cutting tools with high tenacity and reduced brittleness. These nanostructured materials can be of two options, (1) top-down, in which the nanostructures are generated on a block of material, and (2) bottom-up, where nanostructured materials are synthesized from nanoparticles. It is noteworthy that the top-down technique is very similar to current production techniques of electronic microprocessors, while the bottom-up technique is based on technology for obtaining
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materials powders, compact objects, or thin layers, with improved properties with respect to the same materials obtained by conventional technologies [20]. 10.1.4.2 Nanocomposites Nanocomposites are materials composed of two or more phases wherein at least one of these phases has one of their three dimensions in the nanoscale. Nowadays, polymer nanocomposites have high expectations due to the excellent mechanical and thermal properties compared to individual polymers. Only in recent years have they increased polymeric nanocomposites interest in different researches with the aim of obtaining highperformance materials [21].
10.1.5 Computational Chemistry Computational chemistry is studying chemical problems at the microscopic level (atomic molecular) using the equations provided by quantum mechanics (to characterize the electronic structure of atoms and molecules) and statistical mechanics (for macroscopic properties from microscopic constituents). Theoretical chemistry analyzes the mathematical description of chemistry. Computational chemistry refers to computational implementation to solve the corresponding equations. The complexity of the equations proposed for resolution requires addressing the study by introducing various approaches and the use of computers [22]. Computational chemistry is a part of science that includes the areas of chemistry, biology, and physics attached to the computer, which allows the investigation of atoms, molecules, and macromolecules using a computer system. This type of analysis is generally performed when the research laboratory is inappropriate, impractical, or impossible, due to the extreme conditions of these experiments (high temperatures, vacuum conditions, etc.) or the high costs generated. Overall, one can say that it is a discipline that encompasses all aspects of research in chemistry benefiting the application of computers. This discipline includes aspects such as molecular modeling, computational methods, computer-aided molecular design, chemical databases, designing organic synthesis, and searching databases or chemical control equipment for chemical analysis. The main goal of computational chemistry is predicting all kinds of molecular properties of chemical systems using physical chemistry, molecular physics, and quantum physics, and employing a variety of constantly developing theoretical techniques [23].
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10.1.5.1 Properties 1. Geometry optimization. Using the Schro¨dinger equation one can predict theoretically the geometric arrangement of atoms in a molecular species, which will reveal the distances between atoms, angles between bonds, etc. This is called geometry optimization. To do it is very tedious and lengthy calculations are required, but today computers solve this problem quickly (e.g., Gaussian, Hyperchem, or Spartan). The energy of a molecule depends on the position of its atoms. It is known that in nature molecules tend to have the lowest possible energy potential. What the algorithms of these programs do is to move atoms and calculate for each position the potential energy in the molecule. If a potential energy lower than that used in the previous step is considered, then the structure will be more realistic and the steps will proceed in the desired direction. When after a number of steps the algorithm does not find a significant reduction in potential energy it terminates the process. It is said to have found a molecular structure that is at a minimum surface energy potential of this molecule. However, this minimum need not be absolute, but can be a local or relative minimum [24]. 2. MESP. The electrostatic potential (MESP) is related to the electron density, which is very useful in determining the sites for nucleophilic attack, electrophilic reactions, and interactions of hydrogen bonds [25]. An electrostatic potential map to visualize how the electrons are distributed in a molecule uses the colors of the rainbow. Red indicates regions where the electron density is higher, while the blue regions indicate a lower electronic population. Also, yellow or green in parts of the structures indicate a homogeneous electronic balance. 3. FTIR. Fourier transform infrared radiation (FTIR) is a method of IR spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passes through. The resulting spectrum shows the molecular absorption and transmission, creating a molecular fingerprint of the sample. Furthermore, it is noteworthy that with the computational chemistry, single FTIR spectra is obtained for various signals in near, medium, and far regions simultaneously.
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10.1.5.2 Monte Carlo Monte Carlo simulation represents a numerical analysis which is based on sequences of random numbers to analyze various values of variables as a particular problem, wherein the variables to be studied are obtained from the probability distributions of each of these variables, using the following steps. • Determine a probability distribution for important variables. • Construct a cumulative probability distribution for each variable. • Establish a range of random numbers for each variable. • Generate the random numbers. • Real simulation of a number of attempts [26].
10.2 METHODOLOGY 10.2.1 Geometry Optimization The determination of individual molecules was performed using HyperChem 8.0.6 Pro software package, using the PM3 semiempirical method, by Compute tool and the option of Geometry Optimization. Polak-Ribiere ˚ -vacuum. algorithm was used with a gradient (RMS) 0.05 kcal mol21 A Once optimized the geometry information of each of the energy and total energy (E), the energy union (EU), and the heat of formation (ΔHf) are obtained.
10.2.2 Bond Length The optimized structural parameters were used in the calculation of a number of vibrational waves with the PM3 method to characterize all stationary and minimum points.
10.2.3 FTIR For the infrared spectrum of individual molecules to determine the wavelengths the following path is followed: the Compute/vibration rotation analysis option is selected and, subsequently, the vibrational spectrum option with which the spectra were analyzed in various vibrations selecting a particular peak often using the Animate command and vibrations, ˚ . When the signal spectrum is selected, the and amplitude frames of 0.5 A corresponding bond type shows its vibrational mode.
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10.2.4 MESP After obtaining energy optimization using the PM3 method, you can draw diagrams in three dimensions using 3D Mapped Isosurface. The Software Hyperchem shows the electrostatic potential as a contour plot when the option is selected in the dialog of the contour plot. The 3D contour plot indicates the site of varying electron density using color differences, so you can witness the areas susceptible to electrophilic attack (red) and areas susceptible to nucleophilic attack (blue) in a standard plot.
10.3 RESULTS AND DISCUSSIONS 10.3.1 PU/Chitin Nanocomposites 10.3.1.1 Geometry Optimization Table 10.1 shows the thermodynamics results for PU, chitin, and PU/chitin, respectively. The ΔG value indicates that PU is crosslinked with chitin by means of which a van der Waals complex is formed by the proton of the alcohol and the lone electron pair of the nitrogen atom in the isocyanate group. In the nanocomposite, the ΔG increases with increasing electronegativity of substituents (see Fig. 10.4). The effect of the substituents in the case of X-NCO isocyanates is reflected in the PU/chitin nanocomposite. This order reflects that the electronegativity increases the acceptor effect. The more electronegative the substituent is, the more the isocyanate molecule attracts the electrons from π, the donor alcohol. In transition state the aromatic delocalization of the phenyl group extends over the isocyanate group, thus increasing the acceptor effect beside the inductive effect [27]. Fig. 10.4 shows the existence of molecular interactions between the two molecules. This study results in the optimized geometry displayed in Fig. 10.4C, pointing out the existence of an CO OH between the CQO (PU) and hydroxyl groups of chitin. 10.3.1.2 Bond Length Molecular geometries can be specified in terms of bond lengths, bond angles, and torsional angles. The carbon carbon single bond length Table 10.1 Thermodynamics dates of PU, chitin, and PU/chitin Property PU Chitin
PU/chitin
ΔG (Kcal mol21)
2268497
26234
210012
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Figure 10.4 Geometry optimization of (A) PU, (B) chitin, and (C) PU/chitin nanocomposite, where: oxygen, hydrogen, carbon, nitrogen atoms, respectively.
has a range of 1.2 at 3.7 A˚ (see Table 10.2), shorter than expected due to resonance. All of the carbons are sp2 hybridized. Bonds in NCO groups are short and they are only slightly dependent on the substituents [27]. The data in Table 10.2 indicate that a CH σ bond is shorter and stronger than a CC σ bond. This is because the s orbital of hydrogen is closer to the nucleus than is the sp3 orbital of carbon. In addition to being shorter, a CH bond is stronger than a CC bond because there is greater electron density in the region of overlap of an sp3 orbital with the s orbital than in the region of overlap of an sp3 orbital with an sp3 orbital. The length and strength of a CH bond depend on the hybridization of the carbon atom to which the hydrogen is attached [28]. By comparing Tables 10.2 and 10.3 decreases were seen in the wavelengths, which were attributed to crosslinking PU with chitin.
Table 10.2 Bond length of PU and chitin Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.2055 1.3444 1.3998 1.3417 3.7391 1.4147 3.5873 1.2943 4.5953 1.3688 1.2401
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3979 1.3604 4.5706 1.3316 1.4154 1.4009 1.3832 1.3714 3.6662 1.1997
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.2374 3.3190 1.3277 3.4833 1.3152 1.3142 5.3147 1.3408 1.2396 1.2102
1.3720 1.3407 3.4031 1.4269 2.8825 1.2292
C27 O61 C3 O31 C4 O33 C5 C6 N36 C84
1.2209 1.3050 3.3043 1.3321 1.4341
C6 O7 O7 C8 C5 N36 C6 O2 C84 C92
3.6425 1.2908 1.4522 3.4248 1.4574
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Chitin
C1 O2 C1 C3 C3 C4 C4 C5 C1 C27 C84 5 O88
Table 10.3 Bond length of PU/chitin nanocomposite Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.1620 1.2204 1.4600 1.5663 1.6514 1.5448 1.6275 1.7154 1.5048 1.4299 1.2295
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3838 1.4110 1.5799 1.5787 1.6216 1.5564 1.5644 1.4066 1.3633 1.2306
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.4140 1.5341 1.7521 1.6231 1.6083 1.5999 1.6099 1.4831 1.2212 1.1620
1.3947 1.5946 1.7655 1.7068 1.5550 1.2297
C27 O61 C3 O31 C4 O33 C5 C6 N36 C84
1.4848 1.4616 1.4513 1.6938 1.5085
C6 O7 O7 C8 C5 N36 C6 O2 C84 C92
1.4663 1.4789 1.5342 1.3529 1.5268
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Chitin
C1 O2 C1 C3 C3 C4 C4 C5 C1 C27 C84 5 O88
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10.3.1.3 Molecular Orbital The electron orbitals surrounding one atom “overlap” with one or several of the orbitals surrounding another atom, forming a bond. The sp3 orbitals have their electronic density extending much further from the central C atom, giving rise to strong bonding interactions at the beginning of the process of bond formation, and stronger bonds in general, since most electron density is now localized between the two atoms (e.g., C and H) engaged in bonding. The new orbitals exist only in molecularly bonded atoms (i.e., not in atomic carbon). The bonds between atoms using either two sp3 orbitals, or two s orbitals, or an s sp3 combination are termed sigma bonds, and are the strongest covalent bonds known [29]. Tables 10.4 10.6 detail the Table 10.4 Molecular orbitals of PU Orbital HOMO
Orbital
Energy (eV)
200 150 120 90 80 60 20 0 220 270
215.015 214.007 213.668 6.8280 237.433 223.024 215.015 29.763 0.3530 4.7140
Table 10.5 Molecular orbitals of chitin Orbital HOMO
Energy (eV)
2200 2150 2120 290 280 260 220 0 20 70
214.866 213.905 212.892 247.047 236.863 222.878 214.866 26.428 0.611 5.006
Orbital
LUMO
Energy (eV)
170 150 120 70 10 0 210 2130 2170
211.071 239.054 224.218 216.138 210.071 28.719 0.1710 0.3970 1.2230
LUMO
Energy (eV)
2170 2150 2120 270 210 0 10 130 170
210.8420 238.652 223.731 215.995 29.8880 22.5326 0.2770 0.9701 1.3240
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Table 10.6 Molecular orbitals of PU/chitin nanocomposite Orbital HOMO Orbital Energy (eV)
110 90 50 0 210 280 2130
214.35011 211.57841 210.12581 3.4581014 9.02684 15.8838 23.05968
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LUMO Energy (eV)
2110 290 250 0 10 80 130
213.8749 210.8426 209.9615 3.140801 8.74380 14.9570 22.7591
Table 10.7 FTIR results of PU Frequency (cm21)
Assignments
435923956 3775, 3449 3639, 331523095 250422359 1964, 1106 1712, 933, 733
CH (CH2) NH stretching CH (CH2), NH stretching CQN, CQO stretching CC, CO CN, CC, CO, CH
molecular orbitals of PU and chitin in addition to the formation of new bonds in the crosslinking PU/chitin. 10.3.1.4 FTIR Table 10.7 shows the FTIR signals of PU where the absorption peaks for the NH group are located at 3775 2 3449 cm21, the isocyanate group (N 5 C 5 O) was assigned to be seen between 2504 2 2359 cm21 which determines that a subsequent reaction will yield isocyanate reactive groups, while asymmetric and symmetric aliphatic chains (CH) are attributed in the regions of 4359 2 3956, 3639, and 3315 2 3095 cm21, respectively [30,31]. Vibration signals for chitin can be seen in Table 10.8 where the stretch methyl group OCH3 was attributed to 4316 4449 cm21, CH2 scissoring was attributed to 4410 and 4316 cm21. The tension band vibration of OH corresponds to 3971, 3861, and 2609 cm21, by means of the primary vibration stress that was located at NH bands 3424 2 3305 cm21 [32], the vibration signal OH 3971, 3861, and 2609 cm21 bands of the amide I and II to 1571, 1349, 1247, 1094 cm21 corresponding to chitin [33]
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were observed, finally CH bonds of symmetric and asymmetric stretching were assigned to 3163 and 3109 cm21. The process of crosslinking reaction of PU/chitin nanocomposite is illustrated in Table 10.9 showing the crosslinking through hydrogen bonds Table 10.8 FTIR results of chitin Frequency (cm21)
Assignments
4449, 4316 4410, 4316 3971, 3861, 2609 3780, 2949, 2713 342423305 3163 3109 2208, 1994 1776 1670, 960 1571, 1349, 1247, 1094 145521349 960
CH stretching (OCH3) CH2 scissoring OH, CH CH NH, CH, OH CH asymmetric stretching CH symmetric stretching CQO stretching CC, CN CC, CO CC, CH, CO, CN, NH OH NH
Table 10.9 FTIR results of PU/chitin nanocomposite Frequency (cm21) Assignments
8656, 6583 832228010, 7476 7119, 3859 6787 6486 6139 6022 5618 468024459, 3966 436924316, 378323444 3418 3305 3168, 2720 313123097, 2949 2506, 2206, 1996, 1571, 1343 2370, 1996, 1446, 1343 1781, 1683, 1100 1571 1247, 959 959, 733 926
CH stretching (PU) CH stretching (chitin) CH, OH (chitin) OH (chitin) CH symmetric stretching (chitin, PU) NH stretching (PU) CH symmetric stretching, NH stretching (PU) CH symmetric stretching (chitin, PU) CH scissoring (PU) CH stretching (PU) CH, OH (chitin) CN, CH, CC (PU), OH (chitin) CC, CN, CH (chitin) CN, NH (PU) CO, CC, CH (chitin) CC, CO, CH (PU) C 5 O, CO (chitin, PU) C 5 O (chitin) CO, CN, CH (PU) CO, CH (chitin) CH (PU)
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between the OH, COOH, and NH2 groups, an increase is observed in the number of bands as well as in current. The emergence of a second signal of a carbonyl group to 1571 cm21 was attributed to chemical crosslinking that occurs between nano-PU and nano chitin, generating carbonyl groups with different chemical environment was noted, with the original having uncrosslinked polymers to 1964 cm21 [30]. The amorphous phase of the carbonyl group associated with hydrogen bonding was observed at 1781 cm21; at 1683 cm21 it was attributed to the carbonyls associated by hydrogen bonds in crystalline phases, while the carbonyls associated by hydrogen bonds in phases highly packaged crystal were observed at 1571 cm21 [34]. Symmetrical and asymmetrical stretching of the aliphatic chain appreciated to 6486, 6022, 5618, 468024459, 436924316, 3966, and 378323444 cm21, while the glycosidic bond of chitin was located 1781, 1683, and 1100 cm21, which suffer an overlap with the carbonyl bond of PU. One located at 926 cm21 corresponding to bending of the CH band (PU) was observed. Also, a peak was observed at 1160 cm21 for glycosidic bond C O C, finally reduced to the corresponding primary amine bands that are between 1100 and 950 cm21 uptake was observed [32]. 10.3.1.5 MESP Fig. 10.5 shows that areas with potential neutral (green) are attributed to the CC bonds both PU as chitin, while electrophilic areas (red) were assigned to CO bonds as well as the carbonyl groups (C 5 O), it shows the chitin to have a more complex structure, presenting more electrophilic areas, besides the nucleophilic potential was located in the hydrogens.
Figure 10.5 MESP of (A) PU and (B) chitin, respectively.
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Figure 10.6 MESP of PU/chitin nanocomposite. Table 10.10 Thermodynamics data of PU, starch, and PU/starch Property PU Starch
PU/starch
ΔG (Kcal mol21)
2242,103
26234
2161,949
Finally, Fig. 10.6 shows that crosslinking of the PU/chitin nanocomposite is performed between CH (nucleophilic areas) with CO bonds and the carbonyl groups (C 5 O) which represent electrophilic groups, so this area reaction mechanism checks the signals of vibration determined by FTIR.
10.3.2 PU/Starch Nanocomposites 10.3.2.1 Geometry Optimization Table 10.10 shows a negative value of the Gibbs free energy of 2242,103 kcal mol21, indicating that there is a spontaneity in the process of crosslinking of PU and starch.
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Figure 10.7 Geometry optimization of (A) PU, (B) starch, and (C) PU/starch nanocomposite, where: oxygen, hydrogen, carbon, nitrogen atoms, respectively.
Fig. 10.7 shows the formation of the intramolecular CaH?π hydrogen bond might, however, have important implications for the aggregation of HDI molecules. The interaction energies estimated for the π π stacked dimers and systems with the CaH?π hydrogen bond are comparable. Molecule molecule interactions driven by intermolecular π π stacking have to compete with intramolecular CaH?π H-bonds. The highest values of the relative energy start at a CaCaCaC dihedral angle value of 0 and are repeated every 90˚ [35]. It was necessary to establish nonbonded interactions that accurately describe hydrogen bonding of the hydrogen atom pendant to the urethane nitrogen atom with the carbonyl oxygen (OD. . .Hn) and urethane nitrogen (N. . .Hn) atoms. We also needed to establish the ability of our nonbonded potential to reproduce strong electrostatic interactions between carbonyl and amine groups [36]. 10.3.2.2 Bond Length In Tables 10.11 and 10.12 bond lengths of PU, starch and PU/starch nanocomposites are illustrated, where the carbon atoms of rings starch
Table 10.11 Bond lengths of PU and starch Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.2055 1.3444 1.3998 1.3417 3.7391 1.4147 3.5873 1.2943 4.5953 1.3688 1.2401
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3979 1.3604 4.5706 1.3316 1.4154 1.4009 1.3832 1.3714 3.6662 1.1997
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.2374 3.3190 1.3277 3.4833 1.3152 1.3142 5.3147 1.3408 1.2396 1.2102
1.4837 1.6303 1.6615 1.5823 1.5571 1.4639 1.4653 1.5538 1.4594
C6 O7 O7 C8 C1 C21 C21 O24 O24 C64 C64 O65 O65 C69 C69 C68
1.4872 1.5062 1.5958 1.4618 1.4580 1.5847 1.5741 1.7087
C68 C67 C66 C66 C67 C68 C69 C76
1.7452 1.8808 1.7022 1.4370 1.4653 1.0629 1.5601 1.4302
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Starch
C1 C1 C3 C4 C5 C6 C3 C4 C5
O2 C3 C4 C5 C6 O2 O34 O36 O37
C67 C66 C64 O79 O78 O80 C76 O92
Table 10.12 Bond lengths of PU/starch nanocomposite Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.1620 1.2203 1.4573 1.6185 1.5518 1.6318 1.5756 1.6358 1.4991 1.4348 1.2317
C11 O12 O12 C13 C13 C14 C14 C15 C15 C16 C16 C17 C17 C18 C18 O19 O19 C46 C46 5 O47
1.3851 1.4106 1.6195 1.6268 1.5799 1.6371 1.6128 1.3758 1.3592 1.2338
C46 N48 N48 C50 C50 C51 C51 C52 C52 C53 C53 C54 C54 C65 C65 N66 N66 5 C67 C67 5 O68
1.4331 1.4703 1.6223 1.6188 1.6525 1.6138 1.6101 1.4833 1.2213 1.6201
1.6421 1.8591 1.8972 1.8370
C6 O7 O7 C8 C1 C21 C21 O24
1.4917 1.4774 1.5543 1.4575
C68 C67 C66 C66
1.5411 1.6071 1.5661 1.5018
PU
O4 5 C1 C1 5 N2 N2 C3 C3 C5 C5 C6 C6 C7 C7 C8 C8 C9 C9 N10 N10 C11 C11 5 O20 Starch
C1 C1 C3 C4
O2 C3 C4 C5
C67 C66 C64 O79
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have sp3 hybridization generating a nonplanar structure shown in settings chair, also crosslinking points are observed in the bonds CO, NH, and CH to form the PU/starch nanocomposite. 10.3.2.3 Molecular Orbital Tables 10.13 and 10.14 show the HOMO and LUMO orbitals starch and PU/starch nanocomposite in which the energy difference between these orbitals is seen and defined as the hardness of a system according to the theory of Pearson; it is noteworthy that the HOMO orbital electron density is localized mainly in the CC bonds. A distortion in the bond lengths due to instability of the molecular structure caused by phonon vibration also occurs, thus making shorter double bonds and single bonds longer as shown in Tables 10.2, 10.3 and 10.11, 10.12, respectively. Thus, the Peierls distortion gives rise to hybrid molecular orbitals, bonding antibonding π π , and corresponding to π valence bands caused by the highest occupied molecular orbital (HOMO) and the band- π caused by the lowest unoccupied molecular orbital (LUMO), respectively. This change in electron density during the transition π π , produces an asymmetrical variation in the dipole moment and a reduction in the binding energy for the transfer of an electron to the antibonding orbital of the binding [37]. Table 10.13 Molecular orbitals of starch Orbital HOMO (energy—eV)
Orbital
LUMO (energy—eV)
200 170 30 20 210 260
2200 2170 230 220 10 60
2.3286 2.5336 25.4968 2.3286 4.0402 13.0311
2.18050 2.46590 27.6601 2.18050 3.98801 12.7710
Table 10.14 Molecular orbitals of PU/starch nanocomposites Orbital HOMO (energy—eV) Orbital
100 80 60 0 220 270
213.4886 213.2508 212.7801 2.42010 18.0268 19.0683
2100 280 260 0 20 70
LUMO (energy—eV)
213.4199 213.2421 212.7134 2.460801 18.17930 19.14970
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Table 10.15 FTIR results for starch Frequency (cm21)
8043 7569, 7029 6814, 3566 3328 3153, 2997 2589 2469, 1482, 1180 1098 958 755
Assignments
6512, 6197 5846, 3972 3566 6197 5846, 5560 5245, 4749,
CH stretching OH stretching
2190
CH asymmetric stretching CH symmetric stretching COC stretching CO, CC CC, CO, CH CO
1682
10.3.2.4 FTIR Table 10.15 shows the FTIR results for starch where the regions of 7029 6814, 6197 5846, 5560 5245, 4749, and 3566 cm 21 were attributed to stretching of free OH bonds and bonds within and intermolecularly present on the anhydroglucose units of amylose and amylopectin, whereas the amylose and amylopectin were located at 1098 and 948 cm 21, respectively [38 40]. The signal at 2997 cm 21 was assigned to the symmetric stretching of the CH2 groups as well as to the second vibration-D-glucopyranose, respectively [41]. The range of 900 1250 cm 21 corresponded to stretching CO, while 1482 and 1180 cm 21 are specific to stretch CO [40]. Table 10.16 shows the characteristic signals for PU/starch nanocomposite where the crosslinking of the NH and OH bonds (PU/starch) is carried out at 8694 cm21, the signals at 7051 and 3567 cm21 are seen to correspond to stretching vibrations of the CH bond of starch and are verified in the fingerprint to 1181 cm21. The CH vibrations of symmetric and asymmetric stretch PU are located at 7069, 5849, 5232, 4746, and 3986 cm21. The urethane group signal appears at a wavelength of 1695 cm21 due to the increase in the number of hydrogen bonds of the C 5 O groups. This result shows that the hydroxyl groups of starch are contributing to the formation of bonds with the urethane group, thus modifying the intermolecular interactions within the polymer structure [42]. The range of 3328 2 3151 cm21 shows absorption bands that are particular to the stretching vibration of hydroxyl groups that form hydrogen bridges that are inter- and intramolecularly present in the starch
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Table 10.16 FTIR results for PU/starch nanocomposites Frequency (cm21) Assignments
8694 8027 7051, 6835 7069 6178, 5849 5561, 5232, 2990, 1695 1181, 1093 754
7581 3567
3567, 3328 3151 3151 4746, 3986 2460, 1481 970
OH, NH (PU, starch) CH (PU) CH stretching (starch) CH (PU, starch) CH asymmetric stretching (PU) OH, CH stretching (starch) CH symmetric stretching (PU) OH stretching (starch) CH scissoring (PU) CH, CO, CC (starch) C 5 O (PU) CC, CH, CO (PU, starch) CN, CC (PU) OH (starch)
molecule. Stretching vibrations of the carbons were observed at 2990 and 2460 cm21. The observed peaks near 1181 and 1093 cm21 correspond to complex bands attributed to stretching vibrations (CO), both hydroxyl and ether bonds and the present to 1481 cm21 are mostly carbon bending methylene and methine present in the starch. The first region (below 754 cm21) exhibits complex vibrational modes due to a glucose ring skeleton. The region II represents the region of the fingerprint of starch and is the area that is closely related to its structure, allowing the identification of most of the structural changes. Next, signal 970 cm21 represents the glycosidic bond stretch α-(1 4) (COC) [43 45]. 10.3.2.5 MESP In Fig. 10.8A and B the MESP of PU and starch, respectively, the nucleophilic areas were observed in the CH bonds, while electrophilic areas corresponded to the CO groups, which are shown in Fig. 10.8C appreciate the crosslinkings between CH, CO, NH, and CQO bonds to obtain the PU/starch nanocomposite, the sp3 hybridizations were also observed in the carbon atoms present in the molecule of the polysaccharide, which was verified with the bands 2990 and 2460 cm21 shown in Table 10.16.
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Figure 10.8 MESP of (A) PU, (B) starch, and (C) PU/starch nanocomposite.
10.4 CONCLUSIONS The molecular modeling by applying quantum mechanics allows one to study and predict the structural properties of the nanocomposites of PU/ chitin and PU/starch, respectively. The free energy spontaneity determined the process of crosslinking of PU, and the lengths of bonds showed a decrease of the double bonds with respect to the single bonds. Signal FTIR indicated the regions encircling, middle, and far from where shifts were observed in several signals also verified that crosslinking is carried out by the CH, NH, CO, and C 5 O, which was seen by
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consulting with electrostatic potential maps that indicated the nucleophilic and electrophilic areas. Finally, with the calculations of molecular orbitals are changes in energy, that is, an overlap of electrons producing new links detected by FTIR signs was observed.
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Nanocomposites of PU Polymers with Nano Chitin and Nano Starch Norma-Aurea Rangel-Vazquez Technical Institute of Agusacalientes, Agusacalientes, Mexico
Contents 10.1 Introduction 10.1.1 Polymers 10.1.2 Polysaccharides 10.1.3 Synthetic Polymers 10.1.4 Nanotechnology 10.1.5 Computational Chemistry 10.2 Methodology 10.2.1 Geometry Optimization 10.2.2 Bond Length 10.2.3 FTIR 10.2.4 MESP 10.3 Results and Discussions 10.3.1 PU/Chitin Nanocomposites 10.3.2 PU/Starch Nanocomposites 10.4 Conclusions References
311 311 313 315 316 318 320 320 320 320 321 321 321 328 334 335
10.1 INTRODUCTION 10.1.1 Polymers Polymers are defined as materials that are obtained from the polymerization of monomers in different process conditions [1]. Typically the polymers are grouped into different kinds of families, including: • According to the source. • Natural; those found in nature, such as cellulose, rubber, vegetable resins, etc. • Semisynthetic; obtained through some chemical process in natural materials, some examples are cellulose acetate, celluloid, and ebonite galalith. Polyurethane Polymers: Composites and Nanocomposites DOI: http://dx.doi.org/10.1016/B978-0-12-804065-2.00010-3
Copyright © 2017 Elsevier Inc. All rights reserved.
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•
•
• •
Synthetic; those of low-molecular-weight substances, which are the basis for the formation of monomers and carry out the polymerization process, for example, PE, PP, etc. Thus, the major forces existing in polymers include: van der Waals force. These manifest in those molecules with relatively low polarity such as hydrocarbons. The dipole moments that occur in a time generate a portion of the molecule that is highly negative, while the other section is positive, so very weak electrostatic attractions are obtained while the long-chain polymers submitted to these forces are large. Dipole dipole forces. They are more resistant due to the attraction between chains originating mainly in polyesters. Hydrogen bonds. The intermolecular forces have a high resistance, for example, a nylon fiber has greater tensile strength than steel fiber.
10.1.1.1 Homopolymers and Copolymers Homopolymers have only one repeating unit, for example, PE, PP, and PVC. One example of copolymers that contain more than one type of structural unit is acrylonitrile butadiene styrene (ABS). • According behavior with temperature. • Thermoplastics are polymers that are moldable with temperature, urgent debate chains linear or branched mainly, however if the temperature is achieved by decomposition this generates a disadvantage due to the final applications being reduced, on the other hand, they have economic, recyclable, and have good mechanical properties. • Thermosettings are materials that under heat irreversibly harden, and therefore they chemically decompose on melting, because the degree of crosslinking of chains increases in space. However, they possess excellent properties at elevated temperatures, such as heat resistance, chemistry, stiffness, and dimensional stability. They exhibit an opaque or yellow color [2]. • Type polymerization. • Addition. This process involves the use of catalysts to weigh out the reorganization of the CQC double bonds. • Condensation. The combination of monomers for polymer synthesis occurs secondary to formation of low-molecular-weight molecules such as water, HCl, alcohol, etc. This process is used for obtaining polyesetrs, polyamides, polyhydrocarbons, or polysulfides.
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10.1.2 Polysaccharides These are biomolecules that fall between carbohydrates and are formed by the union of a large number of monosaccharides and have diverse functions, particularly energy and structural reserves. Polysaccharides are chains, branched or not, of more than 10 monosaccharides [3,4]. Polysaccharides are polymers whose constituent monomers are monosaccharides, which are joined by glycosidic bonds repetitively. These compounds have a very high-molecular-weight, depending on the number of monosaccharide units or residues involved in their structure. This number is usually an unknown variable within a range, unlike what happens with biopolymers, such as DNA or proteins polypeptides, having in the chain a fixed number of parts, along with a specific sequence [3]. The main functions of polysaccharides in living systems relate to food reserves (vegetable and animal glycogen starches) and structure (cellulose chitin in vegetables and animals) [5]. 10.1.2.1 Starch Starches are polymers, e.g., where a large number of glucose monosaccharides bind. Starches have the general formula (C6H10O5), where the subscript depends on the type of starch formed. For example, glycogen is an animal starch consisting of about 60,000 glucose units. Glycogen is important as a source of energy storage in both the liver and muscles. When an organism needs energy enzymes, degradation occurs to free the glucose units [4]. The starch granule consists of two polysaccharide types: • Amylose (see Fig. 10.1A) consists of long straight chains of glucose and, in practice, there are some similar to amylopectin, once every several hundred molecules, in which substitutions do not alter their
Figure 10.1 Structure of (A) amylose and (B) amylopectin, respectively where carbon, hydrogen, oxygen atoms.
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properties. The chain amylose has a molecular weight of about one million. • Amylopectin (see Fig. 10.1B) constitutes 80 85% starch. Branching takes place with α(1-6) bonds occurring every 24 to 30 glucose units, these well branching generates a molecular weight of between 10 and 500 million. In some starches, such as potato starch, amylopectin also has some phosphate esters [6 8]. Both are composed of glucose units in the case of amylose bound together by bonds 1 4 leading to a linear chain. In the case of amylopectin branches appear due to bonds 1 6 [7]. 10.1.2.2 Chitin Chitin (see Fig. 10.2) represents the second most abundant natural polymer after cellulose. Its structure is the main constituent of the exoskeleton of crustaceans (crab, shrimp, lobster, squid, and shrimp) and insect cuticle, it is also found in algae, marine diatoms, and the fungal cell wall. Chitin is insoluble in aqueous solution, having a linear structure composed of repeating units of N-acetyl-D-glucosamine (GlcNAc) attached by glycosidic bond type β (1-4). The structure of chitin is highly related to the structure of cellulose, as this occurs in the C-2 hydroxyl group (OH). However, chitin has an acetamide group ( NHCOCH3) on the same carbon [9]. The amino groups of the native chitin are an acetylated form, which may be attached to protein chains or free amines, however during the extraction processes using common acids and alkalis
Figure 10.2 Structure of chitin, where, nitrogen atoms.
carbon,
hydrogen,
oxygen,
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inevitable deacetylation occurs. Chitin samples having different amounts of N-acetyl groups which depend on the source and extraction process, will be produced with degrees of acetylation of 95% [10]. Chitin has low toxicity, is inert in the intestinal tract of mammals, biodegradable due to the presence of chitinases, and is widely distributed in nature in bacteria, fungi, plants, and also in the digestive systems of various animals. Chitin was used to prepare affinity chromatography columns, for enzyme immobilization as absorbent compounds, and as contaminants tioulfato silver complexes as pharmaceutical fibers [11,12]. Crustaceans are the largest source of industrial-scale chitin, with production of approximately 2200 tonnes per year [13]. The chitin content in shellfish varies between 2 and 12% of total body mass; the content of chitin, protein, minerals, and carotenoids in the exoskeleton of crustaceans varies depending on the species, part of the body, nutritional status, and the stage of thereproductive cycle. The chitin exoskeleton protein contains about 15 40% chitin, proteins surroundings.
10.1.3 Synthetic Polymers 10.1.3.1 Polyurethane (PU) Polyurethane (Fig. 10.3) is obtained from the synthesis of two components, isocyanate groups and polyols [14,15]. Component A is a mixture of isocyanates, sometimes prepolymers (prestarted), containing NCO groups that may vary from 18 35% by functionality. Some are brown, with high viscosity (3000 5000 cpsBrookfield), and some are almost transparent and fluid. Sometimes they are kept in dry nitrogen. They also have highly appreciated adhesive properties, which also serve as binders for producing polymaterial blocks [16]. The polyol is a carefully formulated, balanced mixture of glycols
Figure 10.3 PU structure where, atoms, respectively.
carbon,
hydrogen,
oxygen,
nitrogen
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(high-molecular-weight alcohols). They are mixed with blowing agents and other additives such as amines, silicones, water, propellant, and organometallic catalysts. The reaction condition will give characteristics to the final foam. The appearance is like viscous honey and it can have a strong ammonia smell [17,18]. The mixture of the two polyol and isocyanate components, which are liquid at room temperature, and is usually performed with a specific machine, producing an exothermic chemical reaction. This chemical reaction is characterized by the formation of bonds between the polyol and isocyanate, obtaining a solid structure, which is very strong and uniform. The heat from the reaction can be used to evaporate a swelling agent filling the cells that form, so that a solid which has a cellular structure, with a much higher volume occupied by the liquid product, is obtained. 10.1.3.2 Properties It has low gas permeability, and excellent chemical resistance and electrical insulation. These properties allow it to enter the market and its production is easy as well as being relatively inexpensive. Its application in the medical industry is under development, particularly its use in the manufacture of patches, dressings, catheters, microcapsules, and applications that take into account the properties of biodegradability and nontoxicity [14,15].
10.1.4 Nanotechnology Nanotechnology is the science involved in the design, production, and employment of structures and objects that have at least one dimension in the range of 100 nm or less. Nanotechnology could have far-reaching implications for society. Today it is already used in sectors such as information and communications. Also, it is used in cosmetics, sunscreens, textiles, coatings, some food and energy technologies, and in certain health products and drugs. Moreover, nanotechnology could help reduce environmental pollution. Current knowledge in nanoscience comes from developments in the fields of chemistry, physics, life sciences, medicine, and engineering. There are several areas where nanotechnology is under development or undergoing practical application. • In materials science, nanoparticles allow the manufacture of products with new mechanical properties, even in terms of surface friction, wear resistance, and adhesion.
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• •
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In biology and medicine, nanomaterials are used to improve drug design and targeted delivery. It is also working on the development of nanomaterials for instrumentation and analytical equipment. Consumer products such as cosmetics, sunscreens, fibers, textiles, dyes, and paints already incorporate nanoparticles. In the field of electronic engineering, nanotechnologies are used, for example, in the design of smaller, faster data storage devices with lower power consumption [19].
10.1.4.1 Industrial Applications The growing technological development in the area of nanomaterials has allowed an increase in industrial applications such as films, substrates, and molecular biology. Biotechnology has generated a great expansion toward nonmetallic materials including metal, polymers, and composites. Different areas that have been and will be involved in the nanotechnologies are: medicine, biology, pharmacology, and materials (all applications in the areas of engineering, such as civil and construction, electronics, mechanics, chemistry, and food, among others). Applications include: • Sensors: For detection of specific compounds in different environments, to evaluate the quality of drinks and food products. • High-efficiency photovoltaic systems for converting solar energy. Screens lightes and functional video, based on polymer electronics. • New high-strength materials for aerospace, biomedical, and transport applications. New prostheses and implants for in vivo placement. • Packaging of food products with better features including barriers to penetration of gases indicating the degree of conservation of these materials. • Materials for filtration and catalysis of hydrocarbons and other substances. • Surface coatings with resistance to corrosion, scratching, and wear. • Cutting tools with high tenacity and reduced brittleness. These nanostructured materials can be of two options, (1) top-down, in which the nanostructures are generated on a block of material, and (2) bottom-up, where nanostructured materials are synthesized from nanoparticles. It is noteworthy that the top-down technique is very similar to current production techniques of electronic microprocessors, while the bottom-up technique is based on technology for obtaining
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materials powders, compact objects, or thin layers, with improved properties with respect to the same materials obtained by conventional technologies [20]. 10.1.4.2 Nanocomposites Nanocomposites are materials composed of two or more phases wherein at least one of these phases has one of their three dimensions in the nanoscale. Nowadays, polymer nanocomposites have high expectations due to the excellent mechanical and thermal properties compared to individual polymers. Only in recent years have they increased polymeric nanocomposites interest in different researches with the aim of obtaining highperformance materials [21].
10.1.5 Computational Chemistry Computational chemistry is studying chemical problems at the microscopic level (atomic molecular) using the equations provided by quantum mechanics (to characterize the electronic structure of atoms and molecules) and statistical mechanics (for macroscopic properties from microscopic constituents). Theoretical chemistry analyzes the mathematical description of chemistry. Computational chemistry refers to computational implementation to solve the corresponding equations. The complexity of the equations proposed for resolution requires addressing the study by introducing various approaches and the use of computers [22]. Computational chemistry is a part of science that includes the areas of chemistry, biology, and physics attached to the computer, which allows the investigation of atoms, molecules, and macromolecules using a computer system. This type of analysis is generally performed when the research laboratory is inappropriate, impractical, or impossible, due to the extreme conditions of these experiments (high temperatures, vacuum conditions, etc.) or the high costs generated. Overall, one can say that it is a discipline that encompasses all aspects of research in chemistry benefiting the application of computers. This discipline includes aspects such as molecular modeling, computational methods, computer-aided molecular design, chemical databases, designing organic synthesis, and searching databases or chemical control equipment for chemical analysis. The main goal of computational chemistry is predicting all kinds of molecular properties of chemical systems using physical chemistry, molecular physics, and quantum physics, and employing a variety of constantly developing theoretical techniques [23].
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10.1.5.1 Properties 1. Geometry optimization. Using the Schro¨dinger equation one can predict theoretically the geometric arrangement of atoms in a molecular species, which will reveal the distances between atoms, angles between bonds, etc. This is called geometry optimization. To do it is very tedious and lengthy calculations are required, but today computers solve this problem quickly (e.g., Gaussian, Hyperchem, or Spartan). The energy of a molecule depends on the position of its atoms. It is known that in nature molecules tend to have the lowest possible energy potential. What the algorithms of these programs do is to move atoms and calculate for each position the potential energy in the molecule. If a potential energy lower than that used in the previous step is considered, then the structure will be more realistic and the steps will proceed in the desired direction. When after a number of steps the algorithm does not find a significant reduction in potential energy it terminates the process. It is said to have found a molecular structure that is at a minimum surface energy potential of this molecule. However, this minimum need not be absolute, but can be a local or relative minimum [24]. 2. MESP. The electrostatic potential (MESP) is related to the electron density, which is very useful in determining the sites for nucleophilic attack, electrophilic reactions, and interactions of hydrogen bonds [25]. An electrostatic potential map to visualize how the electrons are distributed in a molecule uses the colors of the rainbow. Red indicates regions where the electron density is higher, while the blue regions indicate a lower electronic population. Also, yellow or green in parts of the structures indicate a homogeneous electronic balance. 3. FTIR. Fourier transform infrared radiation (FTIR) is a method of IR spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared radiation is absorbed by the sample and some of it is passes through. The resulting spectrum shows the molecular absorption and transmission, creating a molecular fingerprint of the sample. Furthermore, it is noteworthy that with the computational chemistry, single FTIR spectra is obtained for various signals in near, medium, and far regions simultaneously.
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10.1.5.2 Monte Carlo Monte Carlo simulation represents a numerical analysis which is based on sequences of random numbers to analyze various values of variables as a particular problem, wherein the variables to be studied are obtained from the probability distributions of each of these variables, using the following steps. • Determine a probability distribution for important variables. • Construct a cumulative probability distribution for each variable. • Establish a range of random numbers for each variable. • Generate the random numbers. • Real simulation of a number of attempts [26].
10.2 METHODOLOGY 10.2.1 Geometry Optimization The determination of individual molecules was performed using HyperChem 8.0.6 Pro software package, using the PM3 semiempirical method, by Compute tool and the option of Geometry Optimization. Polak-Ribiere ˚ -vacuum. algorithm was used with a gradient (RMS) 0.05 kcal mol21 A Once optimized the geometry information of each of the energy and total energy (E), the energy union (EU), and the heat of formation (ΔHf) are obtained.
10.2.2 Bond Length The optimized structural parameters were used in the calculation of a number of vibrational waves with the PM3 method to characterize all stationary and minimum points.
10.2.3 FTIR For the infrared spectrum of individual molecules to determine the wavelengths the following path is followed: the Compute/vibration rotation analysis option is selected and, subsequently, the vibrational spectrum option with which the spectra were analyzed in various vibrations selecting a particular peak often using the Animate command and vibrations, ˚ . When the signal spectrum is selected, the and amplitude frames of 0.5 A corresponding bond type shows its vibrational mode.
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10.2.4 MESP After obtaining energy optimization using the PM3 method, you can draw diagrams in three dimensions using 3D Mapped Isosurface. The Software Hyperchem shows the electrostatic potential as a contour plot when the option is selected in the dialog of the contour plot. The 3D contour plot indicates the site of varying electron density using color differences, so you can witness the areas susceptible to electrophilic attack (red) and areas susceptible to nucleophilic attack (blue) in a standard plot.
10.3 RESULTS AND DISCUSSIONS 10.3.1 PU/Chitin Nanocomposites 10.3.1.1 Geometry Optimization Table 10.1 shows the thermodynamics results for PU, chitin, and PU/chitin, respectively. The ΔG value indicates that PU is crosslinked with chitin by means of which a van der Waals complex is formed by the proton of the alcohol and the lone electron pair of the nitrogen atom in the isocyanate group. In the nanocomposite, the ΔG increases with increasing electronegativity of substituents (see Fig. 10.4). The effect of the substituents in the case of X-NCO isocyanates is reflected in the PU/chitin nanocomposite. This order reflects that the electronegativity increases the acceptor effect. The more electronegative the substituent is, the more the isocyanate molecule attracts the electrons from π, the donor alcohol. In transition state the aromatic delocalization of the phenyl group extends over the isocyanate group, thus increasing the acceptor effect beside the inductive effect [27]. Fig. 10.4 shows the existence of molecular interactions between the two molecules. This study results in the optimized geometry displayed in Fig. 10.4C, pointing out the existence of an CO OH between the CQO (PU) and hydroxyl groups of chitin. 10.3.1.2 Bond Length Molecular geometries can be specified in terms of bond lengths, bond angles, and torsional angles. The carbon carbon single bond length Table 10.1 Thermodynamics dates of PU, chitin, and PU/chitin Property PU Chitin
PU/chitin
ΔG (Kcal mol21)
2268497
26234
210012
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Figure 10.4 Geometry optimization of (A) PU, (B) chitin, and (C) PU/chitin nanocomposite, where: oxygen, hydrogen, carbon, nitrogen atoms, respectively.
has a range of 1.2 at 3.7 A˚ (see Table 10.2), shorter than expected due to resonance. All of the carbons are sp2 hybridized. Bonds in NCO groups are short and they are only slightly dependent on the substituents [27]. The data in Table 10.2 indicate that a CH σ bond is shorter and stronger than a CC σ bond. This is because the s orbital of hydrogen is closer to the nucleus than is the sp3 orbital of carbon. In addition to being shorter, a CH bond is stronger than a CC bond because there is greater electron density in the region of overlap of an sp3 orbital with the s orbital than in the region of overlap of an sp3 orbital with an sp3 orbital. The length and strength of a CH bond depend on the hybridization of the carbon atom to which the hydrogen is attached [28]. By comparing Tables 10.2 and 10.3 decreases were seen in the wavelengths, which were attributed to crosslinking PU with chitin.
Table 10.2 Bond length of PU and chitin Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.2055 1.3444 1.3998 1.3417 3.7391 1.4147 3.5873 1.2943 4.5953 1.3688 1.2401
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3979 1.3604 4.5706 1.3316 1.4154 1.4009 1.3832 1.3714 3.6662 1.1997
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.2374 3.3190 1.3277 3.4833 1.3152 1.3142 5.3147 1.3408 1.2396 1.2102
1.3720 1.3407 3.4031 1.4269 2.8825 1.2292
C27 O61 C3 O31 C4 O33 C5 C6 N36 C84
1.2209 1.3050 3.3043 1.3321 1.4341
C6 O7 O7 C8 C5 N36 C6 O2 C84 C92
3.6425 1.2908 1.4522 3.4248 1.4574
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Chitin
C1 O2 C1 C3 C3 C4 C4 C5 C1 C27 C84 5 O88
Table 10.3 Bond length of PU/chitin nanocomposite Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.1620 1.2204 1.4600 1.5663 1.6514 1.5448 1.6275 1.7154 1.5048 1.4299 1.2295
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3838 1.4110 1.5799 1.5787 1.6216 1.5564 1.5644 1.4066 1.3633 1.2306
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.4140 1.5341 1.7521 1.6231 1.6083 1.5999 1.6099 1.4831 1.2212 1.1620
1.3947 1.5946 1.7655 1.7068 1.5550 1.2297
C27 O61 C3 O31 C4 O33 C5 C6 N36 C84
1.4848 1.4616 1.4513 1.6938 1.5085
C6 O7 O7 C8 C5 N36 C6 O2 C84 C92
1.4663 1.4789 1.5342 1.3529 1.5268
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Chitin
C1 O2 C1 C3 C3 C4 C4 C5 C1 C27 C84 5 O88
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10.3.1.3 Molecular Orbital The electron orbitals surrounding one atom “overlap” with one or several of the orbitals surrounding another atom, forming a bond. The sp3 orbitals have their electronic density extending much further from the central C atom, giving rise to strong bonding interactions at the beginning of the process of bond formation, and stronger bonds in general, since most electron density is now localized between the two atoms (e.g., C and H) engaged in bonding. The new orbitals exist only in molecularly bonded atoms (i.e., not in atomic carbon). The bonds between atoms using either two sp3 orbitals, or two s orbitals, or an s sp3 combination are termed sigma bonds, and are the strongest covalent bonds known [29]. Tables 10.4 10.6 detail the Table 10.4 Molecular orbitals of PU Orbital HOMO
Orbital
Energy (eV)
200 150 120 90 80 60 20 0 220 270
215.015 214.007 213.668 6.8280 237.433 223.024 215.015 29.763 0.3530 4.7140
Table 10.5 Molecular orbitals of chitin Orbital HOMO
Energy (eV)
2200 2150 2120 290 280 260 220 0 20 70
214.866 213.905 212.892 247.047 236.863 222.878 214.866 26.428 0.611 5.006
Orbital
LUMO
Energy (eV)
170 150 120 70 10 0 210 2130 2170
211.071 239.054 224.218 216.138 210.071 28.719 0.1710 0.3970 1.2230
LUMO
Energy (eV)
2170 2150 2120 270 210 0 10 130 170
210.8420 238.652 223.731 215.995 29.8880 22.5326 0.2770 0.9701 1.3240
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Table 10.6 Molecular orbitals of PU/chitin nanocomposite Orbital HOMO Orbital Energy (eV)
110 90 50 0 210 280 2130
214.35011 211.57841 210.12581 3.4581014 9.02684 15.8838 23.05968
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LUMO Energy (eV)
2110 290 250 0 10 80 130
213.8749 210.8426 209.9615 3.140801 8.74380 14.9570 22.7591
Table 10.7 FTIR results of PU Frequency (cm21)
Assignments
435923956 3775, 3449 3639, 331523095 250422359 1964, 1106 1712, 933, 733
CH (CH2) NH stretching CH (CH2), NH stretching CQN, CQO stretching CC, CO CN, CC, CO, CH
molecular orbitals of PU and chitin in addition to the formation of new bonds in the crosslinking PU/chitin. 10.3.1.4 FTIR Table 10.7 shows the FTIR signals of PU where the absorption peaks for the NH group are located at 3775 2 3449 cm21, the isocyanate group (N 5 C 5 O) was assigned to be seen between 2504 2 2359 cm21 which determines that a subsequent reaction will yield isocyanate reactive groups, while asymmetric and symmetric aliphatic chains (CH) are attributed in the regions of 4359 2 3956, 3639, and 3315 2 3095 cm21, respectively [30,31]. Vibration signals for chitin can be seen in Table 10.8 where the stretch methyl group OCH3 was attributed to 4316 4449 cm21, CH2 scissoring was attributed to 4410 and 4316 cm21. The tension band vibration of OH corresponds to 3971, 3861, and 2609 cm21, by means of the primary vibration stress that was located at NH bands 3424 2 3305 cm21 [32], the vibration signal OH 3971, 3861, and 2609 cm21 bands of the amide I and II to 1571, 1349, 1247, 1094 cm21 corresponding to chitin [33]
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were observed, finally CH bonds of symmetric and asymmetric stretching were assigned to 3163 and 3109 cm21. The process of crosslinking reaction of PU/chitin nanocomposite is illustrated in Table 10.9 showing the crosslinking through hydrogen bonds Table 10.8 FTIR results of chitin Frequency (cm21)
Assignments
4449, 4316 4410, 4316 3971, 3861, 2609 3780, 2949, 2713 342423305 3163 3109 2208, 1994 1776 1670, 960 1571, 1349, 1247, 1094 145521349 960
CH stretching (OCH3) CH2 scissoring OH, CH CH NH, CH, OH CH asymmetric stretching CH symmetric stretching CQO stretching CC, CN CC, CO CC, CH, CO, CN, NH OH NH
Table 10.9 FTIR results of PU/chitin nanocomposite Frequency (cm21) Assignments
8656, 6583 832228010, 7476 7119, 3859 6787 6486 6139 6022 5618 468024459, 3966 436924316, 378323444 3418 3305 3168, 2720 313123097, 2949 2506, 2206, 1996, 1571, 1343 2370, 1996, 1446, 1343 1781, 1683, 1100 1571 1247, 959 959, 733 926
CH stretching (PU) CH stretching (chitin) CH, OH (chitin) OH (chitin) CH symmetric stretching (chitin, PU) NH stretching (PU) CH symmetric stretching, NH stretching (PU) CH symmetric stretching (chitin, PU) CH scissoring (PU) CH stretching (PU) CH, OH (chitin) CN, CH, CC (PU), OH (chitin) CC, CN, CH (chitin) CN, NH (PU) CO, CC, CH (chitin) CC, CO, CH (PU) C 5 O, CO (chitin, PU) C 5 O (chitin) CO, CN, CH (PU) CO, CH (chitin) CH (PU)
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between the OH, COOH, and NH2 groups, an increase is observed in the number of bands as well as in current. The emergence of a second signal of a carbonyl group to 1571 cm21 was attributed to chemical crosslinking that occurs between nano-PU and nano chitin, generating carbonyl groups with different chemical environment was noted, with the original having uncrosslinked polymers to 1964 cm21 [30]. The amorphous phase of the carbonyl group associated with hydrogen bonding was observed at 1781 cm21; at 1683 cm21 it was attributed to the carbonyls associated by hydrogen bonds in crystalline phases, while the carbonyls associated by hydrogen bonds in phases highly packaged crystal were observed at 1571 cm21 [34]. Symmetrical and asymmetrical stretching of the aliphatic chain appreciated to 6486, 6022, 5618, 468024459, 436924316, 3966, and 378323444 cm21, while the glycosidic bond of chitin was located 1781, 1683, and 1100 cm21, which suffer an overlap with the carbonyl bond of PU. One located at 926 cm21 corresponding to bending of the CH band (PU) was observed. Also, a peak was observed at 1160 cm21 for glycosidic bond C O C, finally reduced to the corresponding primary amine bands that are between 1100 and 950 cm21 uptake was observed [32]. 10.3.1.5 MESP Fig. 10.5 shows that areas with potential neutral (green) are attributed to the CC bonds both PU as chitin, while electrophilic areas (red) were assigned to CO bonds as well as the carbonyl groups (C 5 O), it shows the chitin to have a more complex structure, presenting more electrophilic areas, besides the nucleophilic potential was located in the hydrogens.
Figure 10.5 MESP of (A) PU and (B) chitin, respectively.
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Figure 10.6 MESP of PU/chitin nanocomposite. Table 10.10 Thermodynamics data of PU, starch, and PU/starch Property PU Starch
PU/starch
ΔG (Kcal mol21)
2242,103
26234
2161,949
Finally, Fig. 10.6 shows that crosslinking of the PU/chitin nanocomposite is performed between CH (nucleophilic areas) with CO bonds and the carbonyl groups (C 5 O) which represent electrophilic groups, so this area reaction mechanism checks the signals of vibration determined by FTIR.
10.3.2 PU/Starch Nanocomposites 10.3.2.1 Geometry Optimization Table 10.10 shows a negative value of the Gibbs free energy of 2242,103 kcal mol21, indicating that there is a spontaneity in the process of crosslinking of PU and starch.
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Figure 10.7 Geometry optimization of (A) PU, (B) starch, and (C) PU/starch nanocomposite, where: oxygen, hydrogen, carbon, nitrogen atoms, respectively.
Fig. 10.7 shows the formation of the intramolecular CaH?π hydrogen bond might, however, have important implications for the aggregation of HDI molecules. The interaction energies estimated for the π π stacked dimers and systems with the CaH?π hydrogen bond are comparable. Molecule molecule interactions driven by intermolecular π π stacking have to compete with intramolecular CaH?π H-bonds. The highest values of the relative energy start at a CaCaCaC dihedral angle value of 0 and are repeated every 90˚ [35]. It was necessary to establish nonbonded interactions that accurately describe hydrogen bonding of the hydrogen atom pendant to the urethane nitrogen atom with the carbonyl oxygen (OD. . .Hn) and urethane nitrogen (N. . .Hn) atoms. We also needed to establish the ability of our nonbonded potential to reproduce strong electrostatic interactions between carbonyl and amine groups [36]. 10.3.2.2 Bond Length In Tables 10.11 and 10.12 bond lengths of PU, starch and PU/starch nanocomposites are illustrated, where the carbon atoms of rings starch
Table 10.11 Bond lengths of PU and starch Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.2055 1.3444 1.3998 1.3417 3.7391 1.4147 3.5873 1.2943 4.5953 1.3688 1.2401
C10 O11 O11 C12 C12 C13 C13 C14 C14 C15 C15 C39 C39 C40 C40 O41 O41 C42 C42 5 O51
1.3979 1.3604 4.5706 1.3316 1.4154 1.4009 1.3832 1.3714 3.6662 1.1997
C42 N43 N43 C44 C44 C45 C45 C46 C46 C53 C53 C54 C54 C55 C55 N56 N56 5 C57 C57 5 O58
1.2374 3.3190 1.3277 3.4833 1.3152 1.3142 5.3147 1.3408 1.2396 1.2102
1.4837 1.6303 1.6615 1.5823 1.5571 1.4639 1.4653 1.5538 1.4594
C6 O7 O7 C8 C1 C21 C21 O24 O24 C64 C64 O65 O65 C69 C69 C68
1.4872 1.5062 1.5958 1.4618 1.4580 1.5847 1.5741 1.7087
C68 C67 C66 C66 C67 C68 C69 C76
1.7452 1.8808 1.7022 1.4370 1.4653 1.0629 1.5601 1.4302
PU
O16 5 C1 C1 5 N2 N2 C3 C3 C4 C4 C5 C5 C6 C6 C7 C7 C8 C8 N9 N9 C10 C10 5 O29 Starch
C1 C1 C3 C4 C5 C6 C3 C4 C5
O2 C3 C4 C5 C6 O2 O34 O36 O37
C67 C66 C64 O79 O78 O80 C76 O92
Table 10.12 Bond lengths of PU/starch nanocomposite Bond
Bond length (Å)
Bond
Bond length (Å)
Bond
Bond length (Å)
1.1620 1.2203 1.4573 1.6185 1.5518 1.6318 1.5756 1.6358 1.4991 1.4348 1.2317
C11 O12 O12 C13 C13 C14 C14 C15 C15 C16 C16 C17 C17 C18 C18 O19 O19 C46 C46 5 O47
1.3851 1.4106 1.6195 1.6268 1.5799 1.6371 1.6128 1.3758 1.3592 1.2338
C46 N48 N48 C50 C50 C51 C51 C52 C52 C53 C53 C54 C54 C65 C65 N66 N66 5 C67 C67 5 O68
1.4331 1.4703 1.6223 1.6188 1.6525 1.6138 1.6101 1.4833 1.2213 1.6201
1.6421 1.8591 1.8972 1.8370
C6 O7 O7 C8 C1 C21 C21 O24
1.4917 1.4774 1.5543 1.4575
C68 C67 C66 C66
1.5411 1.6071 1.5661 1.5018
PU
O4 5 C1 C1 5 N2 N2 C3 C3 C5 C5 C6 C6 C7 C7 C8 C8 C9 C9 N10 N10 C11 C11 5 O20 Starch
C1 C1 C3 C4
O2 C3 C4 C5
C67 C66 C64 O79
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have sp3 hybridization generating a nonplanar structure shown in settings chair, also crosslinking points are observed in the bonds CO, NH, and CH to form the PU/starch nanocomposite. 10.3.2.3 Molecular Orbital Tables 10.13 and 10.14 show the HOMO and LUMO orbitals starch and PU/starch nanocomposite in which the energy difference between these orbitals is seen and defined as the hardness of a system according to the theory of Pearson; it is noteworthy that the HOMO orbital electron density is localized mainly in the CC bonds. A distortion in the bond lengths due to instability of the molecular structure caused by phonon vibration also occurs, thus making shorter double bonds and single bonds longer as shown in Tables 10.2, 10.3 and 10.11, 10.12, respectively. Thus, the Peierls distortion gives rise to hybrid molecular orbitals, bonding antibonding π π , and corresponding to π valence bands caused by the highest occupied molecular orbital (HOMO) and the band- π caused by the lowest unoccupied molecular orbital (LUMO), respectively. This change in electron density during the transition π π , produces an asymmetrical variation in the dipole moment and a reduction in the binding energy for the transfer of an electron to the antibonding orbital of the binding [37]. Table 10.13 Molecular orbitals of starch Orbital HOMO (energy—eV)
Orbital
LUMO (energy—eV)
200 170 30 20 210 260
2200 2170 230 220 10 60
2.3286 2.5336 25.4968 2.3286 4.0402 13.0311
2.18050 2.46590 27.6601 2.18050 3.98801 12.7710
Table 10.14 Molecular orbitals of PU/starch nanocomposites Orbital HOMO (energy—eV) Orbital
100 80 60 0 220 270
213.4886 213.2508 212.7801 2.42010 18.0268 19.0683
2100 280 260 0 20 70
LUMO (energy—eV)
213.4199 213.2421 212.7134 2.460801 18.17930 19.14970
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Table 10.15 FTIR results for starch Frequency (cm21)
8043 7569, 7029 6814, 3566 3328 3153, 2997 2589 2469, 1482, 1180 1098 958 755
Assignments
6512, 6197 5846, 3972 3566 6197 5846, 5560 5245, 4749,
CH stretching OH stretching
2190
CH asymmetric stretching CH symmetric stretching COC stretching CO, CC CC, CO, CH CO
1682
10.3.2.4 FTIR Table 10.15 shows the FTIR results for starch where the regions of 7029 6814, 6197 5846, 5560 5245, 4749, and 3566 cm 21 were attributed to stretching of free OH bonds and bonds within and intermolecularly present on the anhydroglucose units of amylose and amylopectin, whereas the amylose and amylopectin were located at 1098 and 948 cm 21, respectively [38 40]. The signal at 2997 cm 21 was assigned to the symmetric stretching of the CH2 groups as well as to the second vibration-D-glucopyranose, respectively [41]. The range of 900 1250 cm 21 corresponded to stretching CO, while 1482 and 1180 cm 21 are specific to stretch CO [40]. Table 10.16 shows the characteristic signals for PU/starch nanocomposite where the crosslinking of the NH and OH bonds (PU/starch) is carried out at 8694 cm21, the signals at 7051 and 3567 cm21 are seen to correspond to stretching vibrations of the CH bond of starch and are verified in the fingerprint to 1181 cm21. The CH vibrations of symmetric and asymmetric stretch PU are located at 7069, 5849, 5232, 4746, and 3986 cm21. The urethane group signal appears at a wavelength of 1695 cm21 due to the increase in the number of hydrogen bonds of the C 5 O groups. This result shows that the hydroxyl groups of starch are contributing to the formation of bonds with the urethane group, thus modifying the intermolecular interactions within the polymer structure [42]. The range of 3328 2 3151 cm21 shows absorption bands that are particular to the stretching vibration of hydroxyl groups that form hydrogen bridges that are inter- and intramolecularly present in the starch
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Table 10.16 FTIR results for PU/starch nanocomposites Frequency (cm21) Assignments
8694 8027 7051, 6835 7069 6178, 5849 5561, 5232, 2990, 1695 1181, 1093 754
7581 3567
3567, 3328 3151 3151 4746, 3986 2460, 1481 970
OH, NH (PU, starch) CH (PU) CH stretching (starch) CH (PU, starch) CH asymmetric stretching (PU) OH, CH stretching (starch) CH symmetric stretching (PU) OH stretching (starch) CH scissoring (PU) CH, CO, CC (starch) C 5 O (PU) CC, CH, CO (PU, starch) CN, CC (PU) OH (starch)
molecule. Stretching vibrations of the carbons were observed at 2990 and 2460 cm21. The observed peaks near 1181 and 1093 cm21 correspond to complex bands attributed to stretching vibrations (CO), both hydroxyl and ether bonds and the present to 1481 cm21 are mostly carbon bending methylene and methine present in the starch. The first region (below 754 cm21) exhibits complex vibrational modes due to a glucose ring skeleton. The region II represents the region of the fingerprint of starch and is the area that is closely related to its structure, allowing the identification of most of the structural changes. Next, signal 970 cm21 represents the glycosidic bond stretch α-(1 4) (COC) [43 45]. 10.3.2.5 MESP In Fig. 10.8A and B the MESP of PU and starch, respectively, the nucleophilic areas were observed in the CH bonds, while electrophilic areas corresponded to the CO groups, which are shown in Fig. 10.8C appreciate the crosslinkings between CH, CO, NH, and CQO bonds to obtain the PU/starch nanocomposite, the sp3 hybridizations were also observed in the carbon atoms present in the molecule of the polysaccharide, which was verified with the bands 2990 and 2460 cm21 shown in Table 10.16.
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Figure 10.8 MESP of (A) PU, (B) starch, and (C) PU/starch nanocomposite.
10.4 CONCLUSIONS The molecular modeling by applying quantum mechanics allows one to study and predict the structural properties of the nanocomposites of PU/ chitin and PU/starch, respectively. The free energy spontaneity determined the process of crosslinking of PU, and the lengths of bonds showed a decrease of the double bonds with respect to the single bonds. Signal FTIR indicated the regions encircling, middle, and far from where shifts were observed in several signals also verified that crosslinking is carried out by the CH, NH, CO, and C 5 O, which was seen by
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consulting with electrostatic potential maps that indicated the nucleophilic and electrophilic areas. Finally, with the calculations of molecular orbitals are changes in energy, that is, an overlap of electrons producing new links detected by FTIR signs was observed.
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