498 NATURAL AND ARTIFICIAL POLYMERS In spite of the presence of many highly reactive functional groups such as hydroxyls, cellulose is poorly reactive. Interchain molecular interactions (hydro- gen bonds) are strong and ensure the main part of the cohesive properties while preventing the penetration of reagents. The breaking of these interactions is the precondition of any reaction. The ways to achieve such a breaking are given in Section 14.2.2, which deals with cellulose derivatives. Cellulose is not water-soluble but is strongly hydrophilic. This property is responsible for the great comfort exhibited by cellulose-based fibers and by the corresponding fabrics. Under normal conditions of use, cellulose may contain up to 70% of loosely bound water. The partial replacement of polymer–polymer inter- actions by hydrogen bonds between cellulose and water causes a plasticization of the resulting material and thus a lowering of its mechanical characteristics. Whereas the tensile strength of highly crystalline and dry cellulose fibers can reach 700MPa, it can lose up to 30% of its value in wet atmosphere. Still due to the strong cohesion of this material, cellulose is insoluble in most organic solvents. Only some highly polar mixtures such as N,N -dimethylacetamide/ lithium chloride, N -methylmorpholine/water, Cu(OH) 2 /ammonia, trifluoroacetic acid/alkyl chloride, calcium thiocyanate/water, and ammonium thiocyanate/liquid ammonia are solvents of cellulose. In spite of the potential applications of such solutions, they are exploited relatively little due to their high cost. The high degree of crystallinity of cellulose makes difficult the measurement of its glass transition temperature. The latter is located beyond 200 ◦ C but is impossible to measure accurately since cellulose degrades thermally above 180 ◦ C. Obviously, the melting point is not accessible since its value is much higher than the degra- dation temperature. As all polymers that contain oxygen atoms in the main chain, cellulose is sen- sitive to hydrolysis. For example, in acidic medium, a random breaking of the glycosidic oxygen bonds occurs, and species of low degree of polymerization, including glucose, can be obtained from Natural cellulose ( X n ∼ 10 4 ): Technical celluloses (200 < X n < 1000) Hydrocelluloses (30 < X n < 200) Cellodextrins (10 < X n < 30) In addition to acid hydrolysis, cellulose can also undergo both alkaline and enzy- matic degradations. 14.2.1.3. Regenerated Cellulose. A way to solubilize cellulose, other than the direct route presented above, involves the chemical transformation of hydrox- yls followed by the solubilization of the corresponding artificial polymer and the regeneration of the primary polymer. The most important method using this princi- ple consists of treating cellulose with soda to transform a high proportion of OHs into ONa groups. Alkali-cellulose thus obtained is soluble in carbon disulfide and POLYSACCHARIDES AND THEIR DERIVATIVES 499 reacts with this solvent to give cellulose xanthate: Na–O–(C 6 H 9 O 4 )- S=C=S Monomeric unit of cellulose xanthat e S=C S-Na O–(C 6 H 9 O 4 )- Monomeric unit of alkali-cellulose The cellulose is then regenerated as either a fiber or a film by neutralization of the medium with sulfuric acid. This regenerated cellulose is known under the name of viscose rayon. It is utilized for the production of textile fibers which are in great demand and are utilized for the manufacture of hydrophilic films—in particular, in biomedical engineering (e.g., dialysis membranes). These materials have a degree of crystallinity much lower than that of original cellulose, and thus their mechanical characteristics are lesser than those of the original material. They are interesting because they can be processed in the form of films by conventional spinning and extrusion techniques. 14.2.1.4. Domains of Application of Cellulose. Original cellulose is mainly utilized as textile fibers (cotton, flax, hemp, etc.). Their annual production reaches 20 million tons. Extracted from wood (of which it represents ∼50% of the content) by deligni- fication, it becomes the main constituent (∼80%) of paper whatever the method utilized for the treatment of the paper pulp. Cellulose can also be regenerated from solution, the xanthate method being, by far, the most utilized. This regeneration can be made in the form of wires (rayon) used in textile industry (∼2 million tons) or as films for very diverse applications. 14.2.2. Cellulose Derivatives They are artificial polymers that retain the skeleton of the primary cellulose and whose hydroxyl functional groups are transformed under the action of various reagents. The general principles of this chemical modification were presented in Chapter 9. From a general point of view, the properties of these cellulose derivatives are highly affected by the nature of the ester introduced, the degree of polymerization, and, especially, by the residual hydroxyl group content; their total transformation considerably lowers the cohesion of the resulting material and drastically modifies the derived properties. 14.2.2.1. Cellulose Nitrates (CN). They are the source of the oldest thermo- plastics, directly obtained from Nature (see Chapter 1), and were used in first instance to manufacture celluloid (camphor-plasticized cellulose nitrate) and then “artificial” silk as well as supports for photographic films. These applications were given up due to safety considerations but others appeared which still justify their significant production. The nitration of cellulose utilizes an attack of hydroxyls by a nitro-sulfuric mixture to give nitric ester (cell-ONO 2 ) with a maximum degree of substitution 500 NATURAL AND ARTIFICIAL POLYMERS (D.S.) equal to 2.8. The properties of these materials are closely related to their D.S., measured and evaluated by the nitrogen content which is equal to 14.14% by weight for D.S. =3. For example, the cellulose nitrate used to prepare celluloid has a D.S. equal to 1.85, which corresponds to ∼10.8% of nitrogen. The higher the hydrophilicity of cellulose, the lower the D.S. On the other hand, the higher the solubility in usual organic solvents (acetone, esters, etc.), the higher the D.S. This last property is exploited in the manufacture of varnishes for various uses by dissolution in solvent mixtures. An essential characteristic of cellulose nitrates is their capability of undergo- ing thermally breaking to give nitrogen, nitrogen oxides, carbon dioxide, carbon monoxide, and water. This spontaneous reaction requires a high activation energy and is self-catalyzed by the decomposition products. The manufacture of explosives (nitrated cotton) is based on this property. 14.2.2.2. Cellulose Acetates (CA). Acetylation of cellulose is obtained by reaction of the natural polymer with acetic anhydride. The reaction is catalyzed by sulfuric acid. However, to obtain derivatives of high D.S. (>92%), it is advisable to operate in the presence of a diluent. When the diluent is a solvent of cellulose acetate—for instance, acetic acid—the cellulose is gradually swollen by the solvent as substitution proceeds, the latter being catalyzed by mineral acids (Lewis or Brønsted acid). This process is called acetylation in the homogeneous phase. Acetylation in heterogeneous phase (catalyzed by mineral acids) is so called when the diluent is not a solvent of cellulose acetate. Toluene or carbon tetra- chloride are such liquids. Under such conditions the original fibrillary structure is reasonably well-preserved because there is less degradation of the constituting chains. Cellulose acetates with 1.6 < D.S. < 2.0 are soluble in many solvents (ace- tone, esters, chlorinated solvents) and can be plasticized by alkyl phosphates or phthalates to give thermoplastic materials exhibiting a good impact resistance. Cellulose acetate is mainly utilized pure to produce textile fibers exhibiting a medium tensile strength (up to 60 MPa), and providing a great comfort (highly water-absorbent) and aesthetics to the manufactured fabrics. The same principles of manufacture are utilized to prepare cellulose acetate films. In solution, this artificial polymer is utilized in the varnish industry. The annual world production of cellulose acetates with various degree of sub- stitution is in the range of one million tons. 14.2.2.3. Mixed Cellulose Esters. Only those containing acetate units and corresponding to terpolymers of cellulose, cellulose acetate, and a second ester are worthy of interest. Actually, the situation is even more complex since 3 degrees of substitution may be possible, along with 3 sites of substitution with 3 functional groups for each monomeric unit. Mixed esters are obtained by reaction between cellulose ester (cellulose nitrate or acetate) and another acid or an anhydride, generally organic. Cellulose acetonitrates, acetopropionates, and acetobutyrates are utilized as films and varnishes. POLYSACCHARIDES AND THEIR DERIVATIVES 501 14.2.2.4. Cellulose Ethers. They are prepared by reaction of alkyl chlorides or their analogues with alkali-celluloses that are much more reactive substrates than native cellulose itself. The reaction pathway to obtain carboxymethyl cellulose (CMC) is given below as an example: Cell–OH NaOH Cell–ONa Cell–O–CH 2 –COO, Na Cl–CH 2 –COO, Na The resulting product is not only hydrophilic but also totally water-soluble. This property and the capability of this polymer to form aggregates, which increase considerably the viscosity of the corresponding aqueous solutions, result in a wide variety of applications (paper industry, cosmetics, pharmaceutical, food, etc.). Methyl celluloses (MC) are obtained by reaction of methyl chloride with alkali- cellulose and, depending on their D.S., derivatives having different properties of solubility are obtained. With D.S. =1.5, MC are water-soluble; then for higher D.S. they are soluble in organic solvents. Ethyl celluloses (EC) are prepared according to the same method as MC. They are not water-soluble, and their applications are mainly in the field of thermoplastic materials. Hydroxyalkyl ether cellulose [hydroxyethyl- (HEC) and hydroxypropyl cellulose (HPC)] are prepared in a different way. They use the capability of the alkoxide groups of alkali-cellulose to undergo a nucleophilic substitution when reacted with oxiranes: Cell–O − ,Na + + O Cell–O–CH 2 –CH–O − ,Na + CH 3 Sodium salt of hydroxypropylcellulose Cell–O–CH 2 –CH–OH CH 3 HPC Depending upon the nature of alkylene group, the derivatives are more (HEC) or less (HPC) water-soluble. Solubility in organic solvents is reversed. However, in both cases, they are strongly hydrophilic polymers whose applications are mainly in relation with this property. 14.2.3. Starch and Its Derivatives 14.2.3.1. Origin. Starch is the main constituent of certain seeds, certain fruits, and tubers. In seeds and tubers, its content varies from 40% to 70%. It is easy to deduce that its main application is food for humans and animals. For its industrial applications, which correspond to approximately 40 million tons annually, it is extracted from cereal seeds (corns, rice) and from potato tubers. 14.2.3.2. Structure of Starch. Although the general formula of this polysac- charide of vegetable origin is identical to that of cellulose (C 6 H 10 O 5 ) n ,their 502 NATURAL AND ARTIFICIAL POLYMERS physical and physicochemical properties are completely different. The basic con- stitutive unit is a d-glucopyranosyl group, but its configuration is different in starch with respect to that in cellulose: O O CH 2 OH OH OH The repeating unit shown corresponds to one of the two units found in cellulose. Its configuration prevents the optimal development of interchain hydrogen bonds and favors the formation of hydrogen bonds with water which is thus included in the crystal lattice. In addition, it has been shown that starch is, in fact, made up of two families of macromolecular compounds present in variable proportions, depending on the origin: • Amylose, present in minority, which consists of linear chains containing 500–1000 glucopyranosyl groups. • Amylopectin, which is made of branched chains whose monomeric units are of the same type but have irregular units at the branching points. In linear sequences, the monomeric units possess 1,4-links (as in amylose): OH O O CH 2 OH OH OH O O CH 2 OH OH ~~~~~~~~ ~~~~~~~ ~ n Branching can occur from hydroxyl groups: OH O O CH 2 OH OH OH O O CH 2 OH ~~~~~~~~ ~~~~~~~ ~ ~~~~~~~~ OH O O CH 2 OH OH OH O O CH 2 OH OH The molar mass of amylopectin chains can reach 50–70 ×10 6 g·mol −1 . POLYSACCHARIDES AND THEIR DERIVATIVES 503 As concerned the conformational structure, linear sequences are able to crystal- lize in a helical geometry comprising 6 glucopyranyl residues per helix turn (helix 6 1 ) corresponding to a fiber period c =1.069 nm. Helices are assembled by pairs to give double helices (12 monomeric units per turn) that crystallize in the monoclinic system. The degree of crystallinity of starch depends on its origin and varies from 20% to 50%. Amylose is not highly crystalline, and the long linear sequences of amylopectins are responsible for most of the crystallinity. 14.2.3.3. General Characteristics of Starch. The properties of starch are closely related to the existence of hydrogen bonds between the two strands of the double helix forming the crystalline zones. Interchain interactions ensure most of the cohesive properties of the system. However, due to the molecular structure, cer- tain polymer–polymer interactions cannot be established and the cohesive energy density is definitely lower than that of native cellulose. Hydroxyls that do not participate in the cohesion of the system strongly bind to molecules of water, and it is impossible to eliminate the latter without complete destruction of the crystalline lattice. Thus, although starch is insoluble in water at ambient temperature, it swells in hot water without total solubilization. Indeed, amylopectin chains are very long and give entanglements that form physical gels which can be irreversibly deformed under mechanical stress. These gels are thus thermoplastic materials whose temperature of creep under stress can be modulated by varying the water content. A total hydrosolubility is obtained in water containing alkaline metal hydroxide. Due to its lower cohesive energy density, the reactivity of starch is higher than that of cellulose, and similar methods are used to synthesize esters and ethers. 14.2.3.4. Starch Derivatives. The most important ester is starch acetate; it is obtained according to the same method as that leading to cellulose acetate. The gradual substitution of acetate groups for hydroxyls decreases the hydrophilic- ity of modified starch; even at low degrees of modification, the hydrosolubility in hot water disappears and products with D.S. > 1.5 become soluble in organic solvents. Among ethers, only hydroxyalkylethers and alkylammonium ethers are pro- duced in an industrial scale. They are obtained by reaction of their chlorinated derivatives with starch in alkaline medium (alkali-starch). These derivatives have a variable hydrophilic/hydrophobic balance depending on the nature of the ether and the degree of substitution. It is thus possible to adapt their properties to each application. 14.2.3.5. Domains of Application of Starch and its Derivatives. Apart from its direct utilization in food applications, extracted starch is an important industrial product, due to its hydrophilicity, a property that can be used in many respects. Thus, it can serve as viscosifying agent in human or animal food and in the pharmaceutical industry. It is also utilized in the manufacturing processes of 504 NATURAL AND ARTIFICIAL POLYMERS papers and paperboards as additive in concretes or as finishing material in the textile industry. Its capability of being degraded under the effect of biological agents in out-door media is used to induce the biofragmentation of polyolefins or in the industry of packaging (expanded biodegradable starch shaped after plasticization by water). Applications in the chemical industry are numerous, and it can be regarded as the natural polymer that is the most widely utilized by industry as an additive. Concerning starch derivatives, their domains of applications is sensibly the same as those of other natural polymers, but their hydrophilicity could be modulated by the partial chemical modification of hydroxyl groups. Like cellulose derivatives, they can even be used as plastics. 14.3. LIGNIN After cellulose, the most widely found natural polymer of vegetable origin on earth is lignin. Indeed, present to an extent of approximately 20% in the constitution of lignocellulosic materials, it can be estimated that its annual production by Nature is higher than 1 billion tons. Lignin thus generated many studies, but the various problems induced by its utilization are far from being solved. This is due to two main reasons: • First, for its extraction and its subsequent valorization, the three-dimensionality of this polymer requires a partial degradation, which is difficult to control. • Second, the extreme complexity of its molecular structure can be only re- presented by an average composition that can vary with the vegetable species from which it is extracted. A possible molecular structure of an element of the network is represented on the next page. When separated from cellulose by partial degradation during the manufacturing process of paper from wood, lignin is mainly used as fuel in paper industry. Before this ultimate stage, it would be interesting to use it as material, and many attempts were performed in this respect. 14.3.1. Structure of Lignin A three-dimensional polymer consisting mainly of di- and trisubstituted phenyl- propane units can be defined only by its average content of a certain number of molecular functions or groups. Among these various functional groups, hydroxyls and methoxyls are prominent in lignin, and the methoxyl content is generally uti- lized to identify the origin of lignocellulosic materials and the vegetable species from which it emerges. Lignin also contains carbonyls and unsaturations, phenols and carbohydrates which ensure a good compatibility with cellulose in wood. The scheme represented on the next page gives only a rough idea of the structural com- plexity of this polymer. It is important to stress that hydrogen bonds developed LIGNIN 505 with cellulose result in the formation of a composite material with semi-interpene- trated network structure whose excellent properties are well-known. ~~~~ ~~~~ ~~~~ OH OCH 3 O OH HO H 3 CO OOH O- HO H 3 C-O OH HO O OH O OOH O O OH OH OH O O OH OH OH HO OH O CH 3 O H 3 C O HO O O OH OH OH OH O HO OH O-CH 3 O HO O HO H 3 C-O O O-CH 3 HO OH HO O O-CH 3 O O O-CH 3 OH OH H 3 C-O OH OH OCH 3 CO OH CO HO OCH 3 OH O OH H 3 C-O 14.3.2. Extraction of Lignin Due to its cross-linked structure, lignin can be extracted from wood only by breaking up the initial network and a deterioration of its structure. Presently, indus- trial lignins (>50 ×10 6 tons per annum) are species exclusively obtained from a chemical treatment used in the manufacture of paper pulp or cellulose fibers. 506 NATURAL AND ARTIFICIAL POLYMERS Delignification of wood is carried out in either acidic or basic conditions and in the presence of sulfur in various forms. It results primarily from a rupture of –C–O–C–bonds. • Kraft process: the wood shavings are treated at 170 ◦ C under pressure during a few hours in a reactor containing an aqueous solution of soda and sodium sulfide. The resulting hydrolysis allows extraction of a black liquid whose lignin components are recovered by precipitation through modulation of the concentration and the pH. • Lignosulfonate process: the wood is treated by a sulfite (sodium, calcium, ammonium, or magnesium sulfite) which generates SO 2 in situ. The latter reacts with lignin simultaneously and brings about an acid hydrolysis, which induces the degradation of the network and generates highly water-soluble lignosulfonates that can be separated from cellulose. • Although less important, many other processes can be used—in particular, the flash self-hydrolysis that results from the explosion of shavings of wood impregnated with steam under pressure. The product resulting from these extractions appeared as a dark-brown solid whose molar mass and properties depend on the conditions of the network frag- mentation ( M =10 4 to 10 6 g·mol −1 ). 14.3.3. Valorization of Lignin It is mainly used as combustible in paper industry. However, certain more valorizing applications can be found. Lignin is utilized as a filler in blends with certain ther- moplastic polymers (polyolefins, PVC, rubbers, etc.), with the presence of phenol groups ensuring a marked antioxidizing effect. The high percentage of hydroxyls also contributed to use lignins as polymer precursors (macromonomers, function- alized precursors) to give formo-phenolic resins, polyurethanes, or polyesters. 14.4. PROTEIN MATERIALS Without entering into the chemistry of the processes of life, it is worth stressing the importance of certain proteins resulting from either the vegetable or the animal worlds that are used in industry. Textile fibers, wool, and silk are of great and continuing interest, and scientists have copied them in inventing polyamides. 14.4.1. Structure of Proteins It can be considered that these compounds are the products of the polymerization of α-amino-carboxylic acids: H 2 N-*CH-CO 2 H NH-*CH-CO A A PROTEIN MATERIALS 507 The term protein is employed for compounds exhibiting molar mass > ∼10 4 g·mol −1 , and the term polypeptide is reserved for the shorter chains. The *C-marked carbon atom is unsymmetrical (except for A =H) and always has absolute [S] configuration (indicated as L by biochemists). In Nature, the variety of side groups A (20 different A leadingto20differ- ent “residues”) imparts a complexity to the molecular structure of natural proteins, whose extent arises from the combination of 20 different “comonomers” in variable proportions (Table 14.1). The level of structure described as primary corresponds to the distribution of the 20 protein residues along the macromolecular chain. To indicate the arrangement of the various comonomeric units in the polypeptide sequences, it is necessary to give an abbreviation to each residue, corresponding to the first letters of the amino acid. For example, the –Arg–Gly–Asp– sequence is known to exhibit antithrombogenic properties that are used in biomedical engineering. Remark. The increase in both the sensitivity and the precision of the techniques of characterization allows the identification of increasingly long sequences, and it is convenient to indicate each residue by only one letter. Thus, the arginine–glycine–aspartic acid sequence is also indicated by RGD. The average composition of a given protein and the sequential arrangement of the constituting residues depend on its origin i.e. the species from which it results Table 14.1. Designation and structure of the 20 natural protein ‘‘monomers’’ Aspartic acid HOOC–CH 2 –CH(NH 2 )–COOH Glutamic acid HOOC–CH 2 –CH 2 –CH(NH 2 )–COOH Alanine H 2 N–CH(CH 3 )–COOH Arginine H 2 N–C(NH)–NH–(CH 2 ) 3 –CH(NH 2 )–COOH Asparagine H 2 N–CO–CH 2 –CH(NH 2 )–COOH Cysteine HS–CH 2 –CH(NH 2 )–COOH Glutamine H 2 N–CO–CH 2 –CH 2 –CH(NH 2 )–COOH Glycine H 2 N–CH 2 –COOH Histidine Imidazolyl–CH(NH 2 )–COOH Isoleucine CH 3 –CH 2 –CH(CH 3 )–CH(NH 2 )–COOH Leucine (CH 3 ) 2 CH–CH 2 –CH(NH 2 )–COOH Lysine NH 2 –(CH 2 ) 4 –CH(NH 2 )–COOH Methionine CH 3 –S–(CH 2 ) 2 –CH(NH 2 )–COOH Phenylalanine C 6 H 5 –CH 2 –CH(NH 2 )–COOH Proline Pyrrolidyl–COOH Serine HO–CH 2 –CH(NH 2 )–COOH Threonine HO–CH(CH 3 )–CH(NH 2 )–COOH Tryptophane Indolyl–CH 2 –CH(NH 2 )–COOH Tyrosine HO –C 6 H 4 –CH 2 –CH(NH 2 )–COOH Valine (CH 3 ) 2 CH–CH(NH 2 )–COOH [...]... internal and one external) to attain rates of mm triads close to 99 % and activities (see definition in Section 8.8.2) of several hundreds of grams of PP per gram of Ti per h pe MPa Although at present most of the production of iPP uses Ziegler–Natta catalysis, one can expect that metallocene-containing systems will be more used Their high efficiency and the possibilities they offer in the fine control of the... corresponding to R1 and R2 = –H and –CH3 (ethylene, propylene, isobutene) are utilized in the production of polymers to a substantial extent; however, poly(but-1-ene) and poly(4-methylpent-1-ene) have attained the industrial level Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc 513 514 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS 15.1.1... This degree of crystallinity is generally evaluated by the density which varies between 0 .92 and 0 .97 for homopolymers and can be reduced up to 0.88 for linear copolymers (LLDPE stands for “linear” low-density polyethylene) A particular case is that of linear PE with very high molar mass (M w > 3 × 106 g·mol−1 ) whose crystallization can be partially inhibited (d = 0 .94 ) not by the proportion of branches... restrict the extent of transfer and termination reactions The initiator utilized is a complex Lewis acid resulting from the addition of very low quantities of water on AlCl3 Incorporation of isoprene units in the chains is 1–4 (60% trans and 40% cis) The maintainance of the reactor at low temperature ( 95 ◦ C) in spite of the high enthalpy of polymerization is ensured by a circulation of liquid ethylene... discovery of coordination catalysts by Ziegler in 195 3 revolutionized the production of polyethylene Indeed, the catalytic systems based on titanium halides and alkylaluminum offer many advantages relative to the processes (polymerization under moderate pressure) as well as the properties of the resulting polymers The most widely used catalytic systems consist of TiCl4 and Al(C2 H5 )3 , the product of the... solutions and made insoluble in water by formaldehyde treatment LITERATURE K Kamide, Cellulose and Cellulose derivatives, Elsevier, Amsterdam, 2005 J Park, Science and Technology of Rubber, Elsevier, Amsterdam, 2005 Kirk-Othmer (Ed.), Encyclopedia of Chemical Technology, Wiley-Interscience, New York, 199 6 H F Mark, N M Bikales, C G Overberger, and G Menges (Eds.), Encyclopedia of Polymer Science and Technology,... indeed, a certain miscibility of polybutadiene with the polystyrene phase, although weak, appreciably lowers the glass transition temperature of the latter and thus also its cohesion 15.2.1.5 Applications of Polybutadienes The tire industry utilizes large quantity of polybutadiene and derived copolymers However, in connection with the high level of the volume of production of high-impact polystyrene (HIPS)... the manufacture of flexible tubes, and sealing joints such as those required for the formulation of adhesives The annual world production is approximately 500,000 tons VINYL AND RELATED POLYMERS 5 29 15.3 VINYL AND RELATED POLYMERS They correspond to a family of polymers whose main chain consists of sequences of two carbon atoms resulting from the addition on the ethylene double bond: n However, the name... presentation of them 530 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS 15.3.1 Polystyrene and Its Derivatives Acronym: PS n Molecular formula: IUPAC nomenclature: poly(1-phenylethylene) Polystyrene is one of the most common commercial polymers Its basic properties can be modulated by copolymerization and by the varied presentations of its semi-finished products The level of its annual world production and that of. .. for PS and 70 MPa for SAN) As compared to polystyrene, SAN also offers a higher glass transition temperature and a better resistance to solvents—hydrocarbons in particular It is possible to combine the strong density of cohesive energy of SAN and the impact strength of HIPS by copolymerizing styrene and acrylonitrile in the presence of polybutadiene The resulting morphologies are similar to those of HIPS . the industrial level. Organic and Physical Chemistry of Polymers, by Yves Gnanou and Michel Fontanille Copyright  2008 John Wiley & Sons, Inc. 513 514 LINEAR (MONODIMENSIONAL) SYNTHETIC POLYMERS 15.1.1 proportion of branches (or comonomeric units) is high or low. This degree of crystallinity is generally evaluated by the density which varies between 0 .92 and 0 .97 for homopolymers and can be. ratio of comonomer allows the production of PE with density varying from 0. 89 to 0 .95 . The most widely used comonomers are propylene and butene; the degrees of “branching” usually lie between 20 and