ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 10 ppt

562 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS limited. The high mobility of PBT chains and the melting point lower than that of PET make this polymer easier to process than PET; it is mainly processed by injection at a temperature ∼250 ◦ C. PBT is often mixed with short glass fibers, which simultaneously allows an increase of its stress at break (tenacity) and its elastic modulus. Its annual world production is approximately 200,000 tons and its growth rate is high. 15.5.2.3. Poly(bisphenol A carbonate) (PC) O O O n Among the various diols that were tested for polycarbonate synthesis, the one resulting from bisphenol A (BPA) was found to be a material exhibiting interesting mechanical characteristics. Thus, it corroborates the general idea that the intro- duction of bisphenol A groups into a macromolecular chain largely improves the mechanical properties of the resulting material. Among the various methods that can be utilized to produce PC, processing through interfacial polycondensation is widely used. The hydrosoluble precursor is the disodic salt of bisphenol A. It reacts with phosgene (Cl 2 CO) solubilized in a hydrophobic solvent that is generally a chlorinated solvent (CH 2 Cl 2 ,CHCl 3 , C 6 H 5 Cl, etc.): Na + , − O- -O − , Na + + Cl–CO–Cl O O O n 2n NaCl + n Another method for the polycondensation involves the reaction between COCl 2 and bisphenol A in methylene chloride solution in the presence of pyridine to trap the HCl produced. The transesterification with phenyl carbonate is also possible but is more difficult to transpose to an industrial level than the preceding methods. Depending upon the method being used, it is possible to obtain PC whose molar masses vary from 2 ×10 4 to 2 ×10 5 g·mol −1 . However, for injection molding, the best adapted molar masses are in the range 2–3 ×10 4 g·mol −1 . LINEAR CONDENSATION POLYMERS 563 Poly(bisphenol A carbonate) shows a perfectly regular structure. However, it is unable to crystallize spontaneously. Indeed, the rigidity of the chains due to the presence of phenylene rings restricts their possibility of folding up in the molten state. By prolonged annealing, it is, however, possible to crystallize it and then its melting temperature is 260 ◦ C. PC is only used in an amorphous state, thus it gives a completely transparent material whose T g =150 ◦ C. This high temperature causes the great rigidity of the chains. Poly(bisphenol A carbonate) resists the attack of most of the chemicals; it has also a high thermal stability. The mechanical characteristics of PC are good, but its most remarkable specific property is resilience. Its impact strength measured under equivalent conditions is approximately 10 times higher than that of PET, 30 times higher than that of PMMA, and 300 times higher than that of mineral glass. Its applications are thus mainly based on this property and on its transparency: car industry, electrical engineering, packaging, compact disks, and so on. PC is also frequently used in blends with other thermoplastics. Its world production is 2 million tons, and its annual growth rate approaches 10%. 15.5.2.4. Other Aromatic Polyesters. Poly(cyclohexyldimethylene terephtha- late) (PCT) O O O O n As in case of PET, the rate of crystallization of PCT is very slow. Only a long annealing allows a partial crystallization (T m =290 ◦ CandT g =80 ◦ C). Its molecular structure can be considered as a copolymer between cis and trans isomers with respect to cyclohexylene, and the transition temperatures of completely either cis or trans polymers are slightly higher than those given above. PCT is interesting due to its thermal stability, its thermomechanical properties, and its low water absorption. It is mainly used in electronics industry, but it can also be used as structural material in mechanical engineering. Poly(ethylene naphthalene dicarboxylate) (PEN) n O O O O 564 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS It is obtained by polycondensation between ethylene glycol and either naphthalene- 2,6-dicarboxylic acid or its dimethyl ester. Its high thermomechanical characteris- tics and its excellent durability opened markets for it in the fields of electronics, industrial textiles, and food packaging (hot drinks and sparkling beverages). Liquid crystal polyesters (LCP) are generally derived from p-hydroxybenzoic acid: HO OH O O O O n n They are thermotropic liquid crystals obtained from aromatic homo- or copolyesters, which exhibit particularly high thermomechanical characteristics while preserving excellent impact strength up to very low temperatures. Taking into account their high cost, they are utilized only for high-valued applications, particularly in elec- tronics industry. 15.5.3. Aliphatic Polyamides They were the first linear condensation polymers produced on an industrial scale by the end of the 1930s. Contrary to their polyester analogs, they exhibit excellent thermomechanical characteristics. This difference is mainly due to their tendency to form hydrogen bonds that increase their density of cohesive energy. They are indicated by initials PA followed by one (or 2) number(s) referring to the number of carbon atoms in the main chain constituting the monomeric unit. PA followed by one single number (PA-x) results theoretically from the poly- condensation of an α-amino,ω-carboxylic acid: n H 2 N–(CH 2 ) x –COOH −→ [–HN–(CH 2 ) x –CO–] n Actually, these PA are generally prepared by chain polymerization of the corre- sponding lactam. Nevertheless, they are presented here with condensation polymers. Aliphatic PA indicated by two numbers [PA-x, y +2] are generally prepared by polycondensation between a diamine whose number of carbon atoms is given by the first number (x) and a dicarboxylic acid whose structure determines the second number (y +2): H 2 N-(CH 2 ) x -NH 2 + HOOC-(CH 2 ) y -COOH n -[NH-(CH 2 ) x -NH-CO-(CH 2 ) y -CO]- n Remark. In the United States, it is common practice to indicate polyamides by the term “nylon,” which is, in fact, the commercial name of PA produced by DuPont de Nemours Company. LINEAR CONDENSATION POLYMERS 565 Polyamides have certain common chemical, physical, or physicochemical char- acteristics, primarily resulting from their capability of developing hydrogen bonds. So, the zigzag planar structure allows the maximal stabilization of the crystalline state, and the fiber period depends on the value of (x +y). The rate of crystalliza- tion of aliphatic polyamides is high and, although their glass transition temperature is higher than ambient, it is very difficult to quench them in the amorphous state from the molten state by fast cooling. In addition, after quenching, they sponta- neously tend to increase their degree of crystallinity when heated above the ambient temperature. Their density of cohesive energy is high (δ PA-6,6 =28 J 1/2 cm 3/2 ). It is closely dependent on the even or odd number of carbon atoms of the repeating unit. It is easy to explain this phenomenon using the following schemes, which compare the the extent of hydrogen bonds in PA-6 and PA-7. The melting point of PA-7 (223 ◦ C) is logically higher than that of PA-6 (215 ◦ C) in spite of a lower density of cohesive amide functional groups. N N N O H H O O H H N N N O H H O O PA-6 ~~~~ ~~~~ ~~~~ ~~~~ ~~~~ ~~~~ ~~~~ ~~~~ NNN O O O H H H NNN OOO H HH PA-7 By replacing interchain H bonds, the interactions established with water molecules deeply affect number of properties. In fact, water plays the role of a plasticizer that swells the material, diminishes its cohesion, and lowers its T g . This effect is considerable because the density of amide functional groups is high. Thus, from the examination of Table 15.5 related to PA-x, one can see that the capacity of water absorption decreases gradually with the number of methylene groups (x −1) separating two amide functional groups. If melting points are also considered, they decrease with the same structural parameter but inside two series corresponding to an even and an odd number of carbon atoms, respectively. In the same way, 566 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS Table 15.5. Melting temperature and capability of water absorption of polyamides-x, at 100% relative humidity (RH) –[CO–(CH 2 ) x−1 –NH] n – xT m ( ◦ C) % H 2 O 3 340 25 4 260 — 5 258 13.5 6 215 9.5 7 230 5.0 8 200 3.8 9 209 3.4 10 188 2.0 11 190 2.8 12 180 2.7 mechanical characteristics reflect the effect of the density of H bonds inside the material. A similar evolution of the properties can be easily interpreted in the series of PA-x,y. At ambient temperature polyamides are soluble only in highly polar solvents: phenols, formic acid, fluorinated alcohols, and mineral acids. Thus, their struc- tural analysis in solution is difficult, particularly the determination of their molar mass by end group titration. Their solubilization in commonly available solvents (tetrahydrofuran in particular) can be obtained by alkylation of NH groups, which suppresses H bonds and thus lowers the strong cohesion of these materials. All polyamides are suitable for being hydrolyzed in proportion of their water content. Depending on the relative orientation of the dipoles formed by amide functional groups, PA may or may not exhibit piezoelectric properties, i.e., gen- eration of an electric signal under mechanical constraint. PAs that have all their dipoles directed in same direction (PA-7, PA-9, etc.) leads to a marked piezoelec- tric effect whereas the alternation of orientations (in PA-6 for instance) cancels this effect. Many polyamides or copolyamides can be synthesized. Actually, only a small number of them reached a significant industrial development. Polyamides 6 and 6,6 share ∼85% of total annual market of 7.5 million tons (4 million tons for PA-6 and 2.5 million tons for PA-6,6). The three quarters of the production (4 million tons) are used for the manufacture of textile fibers, the rest is utilized as thermoplastic technical polymer. LINEAR CONDENSATION POLYMERS 567 15.5.3.1. Polycaprolactam (PA-6) Molecular structure: n N H O IUPAC designation: poly[imino(1-oxohexamethylene)] Caprolactam monomer, O N H can be prepared either from toluene or from benzene. For example, in the latter case we have OH O NOH NH CO After purification, caprolactam is polymerized by two different methods. The first, which is well known, involves the hydrolysis of caprolactam in order to generate ε-aminocaproic acid whose polycondensation leads to PA-6 having very high molar mass if the free water is eliminated at high temperature by vac- uum pumping. This polymerization “catalyzed” by water is generally carried out continuously. The second method uses the anionic chain polymerization of the heterocy- cle, whose mechanism (complex) was presented in Section 8.6.4. The activated monomer sodium lactamide is used as initiator. This method allows the preparation of statistical copolymers—for example, with lauryllactam, which is the monomer molecule of PA-12. PA-6 crystallizes in the monoclinical system with a fiber period of 1.72 nm corresponding to the chain in total extension. The degree of crystallinity is about 50%; the melting point of PA-6 is 215 ◦ C and its glass transition occurs at 52 ◦ C. Its mechanical characteristics are excellent due to its strong cohesive energy (δ =28 J 1/2 /cm 3/2 ). Its stress at break (σ r ) reaches 80 MPa after chain orientation by drawing, and its initial elastic modulus is equal to 2.8 GPa. Like all other 568 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS polyamides, PA-6 has a remarkable reversible elongation, a property that is used for its applications in textile industry. Strain at the yield point is about 10–15%. In addition to its main application, which is the production of textile fibers, PA-6 is used as structural and technical thermoplastic material in many sectors of the mechanical engineering. Spinning from the melt is used to process PA-6 fibers. The manufacture of various objects can use all common processing techniques of thermoplastics. 15.5.3.2. Polyhexamethyleneadipamide (PA-6,6). Molecular structure: NH NH CO CO n It is obtained by direct polycondensation between adipic acid and hexamethylene- diamine. The following scheme gives one of the main methods of preparation of these two comonomers, which are, moreover, important reaction intermediates: OH OH HOOC-(CH 2 ) 6 -COOH Adipic acid O HOOC-(CH 2 ) 6 -COOH NC-(CH 2 ) 6 -CN H 2 N-(CH 2 ) 6 -NH 2 Adiponitrile Hexamethylene diamin e Initially, both comonomers react in aqueous solution at neutral pH and lead to a salt called “nylon 6,6 salt,” H 3 N (CH 2 ) NH 3 − OOC (CH 2 ) COO − 6 4 ++ which can be obtained in the pure state with a perfect stoichiometry by recrystalliza- tion in water. In addition to the hydrosolubility of this salt, it is much less oxidizable than hexamethylenediamine; thus it does not generate colored by-products that would be inconvenient for most of the applications of the derived materials. For LINEAR CONDENSATION POLYMERS 569 the polymerization, the solution of the salt is concentrated to 80% at 160 ◦ C; then the steam is pumped in order to allow the equilibrium to shift toward amide formation. The desired molar masses ( M n ) are about 20,000 g·mol −1 . PA-6,6 crystallizes in the triclinic system with a fiber period of 1.72 nm. Its degree of crystallinity is about 50%. Its melting point T m is 260 ◦ C and its glass transition temperature is 57 ◦ C, which is slightly higher than that of PA-6. Due to its molecular structure, the development of interchain H bonds is maximal, thus inducing mechanical characteristics slightly higher than those of PA-6. Other physical and physicochemical characteristics are not very different from those of PA-6, and consequently the fields of application are also the same. 15.5.3.3. Other Aliphatic Polyamides. They are mainly polyamides having longer polymethylene sequences. Their marketing was dictated by the need of tech- nical polymers whose mechanical characteristics are less sensitive to the hygrome- try of the ambient conditions than those of PA-6 and PA-6,6. It concerns PA-6,10, which is obtained by the polycondensation between hexamethylenediamine and sebacic acid [HOOC–(CH 2 ) 8 –COOH], PA-11 and PA-12. PA-11 results from the polycondensation of 1-aminoundecanoic acid (molecule whose raw material is castor oil). PA-12 is obtained by polymerization of the corresponding lactam, which itself is obtained from the trimerization of butadiene. These polymers are less cohesive and thus have a glass transition temperature and a melting temperature lower than those of PA-6 and PA-6,6. Their mechan- ical characteristics are also slightly weaker but depend much less on the relative humidity (see Table 15.5). These polymers are not utilized for textile applications. They are used for manufacture of monofilaments (fishing nets, cords for musical instruments, ropes) and also as technical polymers for the surface coating and the molding of various objects that are required to resist moisture. 15.5.4. Aromatic Polyamides (Aramides) As for polyesters, the introduction of an aromatic moiety into polyamide chains considerably changes their physical and physicochemical characteristics. Due to their molecular structure, aromatic polyamides combine together structural regu- larity, stiffness of the chains, and compacity of phenylene rings. This results in a very strong cohesion of the corresponding materials that exhibit exceptionally tough mechanical characteristics. Aramides are prepared by polycondensation using the Schotten–Baumann reac- tion. This reaction, which uses the selected isomers of phthaloyl chloride and phenylenediamine (or its chlorohydrate), is carried out at low temperature (from 0 ◦ Cto−40 ◦ C) in order to avoid side reactions; it is carried out in an amide solution (dimethylacetamide, N -methylpyrrolidone, tetramethylurea, etc.) to which mineral salts are added. 570 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS + H 2 N NH 2 ClCO COCl ~~~~ NH-CO NH CO ~~~~ n n Aramides can also be prepared by interfacial polycondensation but in this case the molar masses obtained are lower than those attained in the solution process. Due to their chain stiffness and their particularly high melting points, aramides cannot be processed by usual techniques applicable to thermoplastics. As in the case of polyacrylonitrile (PAN), they can only be utilized as fibers, which are obtained from the corresponding collodions either by dry spinning or wet spinning processes. In addition to the high level of their mechanical properties (in particular their elastic modulus and stress at break), which are preserved up to temperatures higher than 200 ◦ C, aramide fibers exhibit an excellent durability of their properties even under extreme conditions. They resist very well to most of chemicals except strong acids. Their low combustibility (LOI =28–30) makes them irreplaceable as safety materials. Remark. The combustibility of a polymer is measured by the limit propor- tion of molecular oxygen in a gas mixture O 2 /N 2 for which its combustion is not propagated (LOI, limit oxygen index). Many aramide structures were synthesized and studied. Several of them were pro- duced industrially but at present only two are significantly developed. They are poly(p-phenyleneterephthalamide) and poly(m-phenyleneisophthalamide). Due to different molecular symmetry, these two aramide fibers have different specific physicochemical and application properties. 15.5.4.1. Poly(p-phenyleneterephthalamide) (PPD-T) Molecular structure: ( ) NHNH O O n The molecular symmetry and the rigidity of the chains of this aramide are respon- sible for the mesomorphic structure of this polymer which exhibits a lyotropic character. The trademark of this well-known commercial fiber is Kevlar  .Itis obtained by wet spinning process from a collodion in concentrated sulfuric acid, LINEAR CONDENSATION POLYMERS 571 which induces many problems related to the corrosion of the equipment. The coag- ulation of lyotropic solutions followed by drawing leads to a highly crystalline polymer. The chains crystallize in a monoclinical system with two repeating units per fiber period which corresponds to c =1.28 nm. The estimated melting point of PPD-T is T m =550 ◦ C, but at this temperature the polymer quickly undergoes degradation. The glass transition occurs at about 360 ◦ C. The density of this material is equal to 1.45 and it depends on the degree of drawing. The tenacity (stress at break) of PPD-T can reach 2.8 GPa with an initial elastic modulus equal to 120 GPa. The applications of PPD-T fibers are in the fields where very high mechanical characteristics able to be preserved in a wide range of temperature are required. The fabrics containing this aramide are used in the fields of clothing and of safety cables and industrial fabrics. However, the main application is in the field of composite materials as reinforcement of strongly cohesive matrices (polyepoxy or others). 15.5.4.2. Poly(m-phenyleneisophthalamide) (MPD-I) Molecular structure: NHNH O O n This material is mainly known under the trademark Nomex  .Itwasthefirst aramide produced at the industrial level. As compared to PPD-T, its molecular symmetry is lesser and does not induce the mesomorphic character. Consequently, its solubility is slightly higher, thus allowing the processing of the corresponding fibers by dry spinning from collodions in polar organic solvents. For economic reasons the spinning can be carried out from the solution in which it was prepared. The mechanical characteristics of MPD-I are definitely weaker than those of PPD-T. It can, however, reach a tenacity of 0.6 GPa with an initial elastic modulus of 20 GPa. For this reason, applications are found in the fields where its chemi- cal inertia, self-extinguishability, and excellent behavior at high temperatures are required. In spite of the cost of their development, aramides reached a relatively high level of industrial production, and these materials became essential in many high-value- added applications. 15.5.5. Linear Polyurethanes General molecular formula: –(O–R–O–CO–NH–R  –NH–CO) n – The generic acronym of polyurethanes is PUR, but those that are made of linear chains are often indicated by TPU (thermoplastic polyurethanes). The urethane function, –O–CO–NH–, also called carbamate, is generated by reaction of a hydroxyl group with an isocyanate function (see Section 8.4.3). [...]... UK, 1999 O Olabisi (Ed.), Handbook of Thermoplastics, Marcel Dekker, New York, 1997 A K., Bhowick and H L., Stephens (Ed.), Handbook of Elastomers, Marcel Dekker, New York, 1988 M Lewin and E Pearce (Ed.), Handbook of Fiber Chemistry, Marcel Dekker, New York, 1998 S Fakirov (Ed.), Handbook of Thermoplastic Polyesters, Wiley VCH, Weinheim, 2002 Kirk-Othmer (Ed.), Encyclopedia of Chemical Technology, 4th... generation of the urethane (also called carbamate) functional group serving only for chain extension of diol or diisocyanate prepolymers; the presence of prepolymers of higher valence causes cross-linking The reaction mechanism occurring in the formation of polyurethanes was presented in Chapter 7, and the preparation of prepolymers was presented for linear PUR (Chapter 15) The structure of the prepolymers... polycondensation of o-phthalic anhydride with glycerol, and the resulting network is represented hereafter Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc 583 584 THREE-DIMENSIONAL SYNTHETIC POLYMERS ( ~~~~ O ) O ∼∼∼∼ O O O O ~~~~ ) O O ( ~~~~ O O O O + HO O HO O O HO ~~~~ ( O This polymer may potentially have a high density of cross-links... production of aminoplastic polymers, their annual production being estimated to 1.2 million tons The precursor consists of mono-, di-, and trimethylolurea, OH OH NH O NH2 HO OH NH NH O O NH OH N OH 592 THREE-DIMENSIONAL SYNTHETIC POLYMERS that are water-soluble and are obtained at pH = 8–9 with a molar ratio of formaldehyde/urea equal to ∼6 and at a temperature of about 50◦ C The purpose of a basic... ∼∼∼∼∼∼polyester∼∼∼∼∼∼polyester∼∼∼∼∼∼ PS The structure and thus the mechanical characteristics of these materials can be fine-tuned through two ways: • by variation of the nature and/ or the proportion of the various comonomers entering in the composition of the polyester precursor and/ or • by a possible partial or total substitution of another vinyl monomer (MMA, etc.) for styrene Due to the brittleness of these materials, which... those of the two preceding molecules The reaction mechanisms corresponding to the formation of the precursors and their polymerization were presented in Chapter 7 The precursors are generally prepared in batches from the comonomers in aqueous solution The structure and the properties of the final material are determined by the composition of the reaction medium, its pH, and the time and temperature of. .. oxidation) “Spandex” fibers are never used alone but always blended with some other fibers: cotton, wool, PET, and so on 15.5.6 Other Technical and Specialty Linear Condensation Polymers These polymers correspond to materials whose performances justify their high cost There is a great number of such polymers exhibiting varied structures and widely used due to their specific properties Some are technical polymers. .. of phthalic anhydride as comonomer for the preparation of polyester prepolymer lowers its unsaturation content and reduces the cross-link density of the 586 THREE-DIMENSIONAL SYNTHETIC POLYMERS resulting network In addition, incorporation of phthalic moieties in the network improves its behaviour at high temperatures UP prepolymers possess a molar mass (M n ) ranging between 1800 and 2500 g·mol−1 and. .. poly(propylene oxide), etc.] These polymers are viewed as aliphatic polyethers, thus limiting the possibility of confusion 596 THREE-DIMENSIONAL SYNTHETIC POLYMERS As in the case of the polymers prepared from reactive oligomers (precursors), the properties of polyepoxides are closely related to the nature of the monomer used for the preparation of this precursor In the case of epoxy resins, the potential... obtained by reaction of functional groups carried by the monomeric units of linear chains, are also excluded from this category of polymers 16.1 SATURATED POLYESTERS (ALKYD RESINS) The name “alkyd resins” clearly distinguishes these polymers from thermoplastic polyesters (PET, PBT, PC, etc.) as well as from unsaturated polyesters (UP), which are also part of the family of three-dimensional polymers (see Section . approximately 10 times higher than that of PET, 30 times higher than that of PMMA, and 300 times higher than that of mineral glass. Its applications are thus mainly based on this property and on its. even and an odd number of carbon atoms, respectively. In the same way, 566 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS Table 15.5. Melting temperature and capability of water absorption of polyamides-x,. by the need of tech- nical polymers whose mechanical characteristics are less sensitive to the hygrome- try of the ambient conditions than those of PA-6 and PA-6,6. It concerns PA-6 ,10, which is