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POLYMER SCIENCE
FUNDAMENTALS OF POLYMER SCIENCE
Basic Concepts
Prof. Premamoy Ghosh Polymer Study Centre “Arghya” 3, kabi Mohitlal Road P.P. Haltu, Kolkata- 700078
(21.09.2006)
CONTENTS
Introduction and Historical Perspective Long Chain Structure and Macromolecular Concept Structural Features of a Polymer or Macromolecule Length to Diameter Ratio Polymer Classifications Structure-Property Relationship Survey of Deformation patterns in the Amorphous State Transitions and Rubbery and Flow Regions Property Demand and Polymer Applications Step-Growth (Condensation) Polymerization Degree of Polymerization Chain-Growth (Addition) Polymerization
Key Words
Polymer, Polymerization, Copolymer, Copolymerization, Step-growth polymerization, Chain-growth polymerization, Macromolecular concept, Structure-property relationship, Rubber, Plastics, Fibre, Deformation behaviour, Glass transition, Melting, Cross linking, Stress-induced orientation; Gelation, Gel point.
Introduction and Historical Perspective Man’s quest for new and improved materials has been expanding with time and it can be said to be unending. The materials that have grown into familiarity and acceptance with the ages since the dawn of man’s existence on the mother planet, the Earth are: pieces of rocks and stones, sand, soil and various ceramic items; nails, horns, hydes, skins and bones of animals; wood, leaves and plant fibres, covering grass and straw, cotton, coir, jute, hemp, hair, wool and many other fibres of plant and animal origin; silk fibre of insect origin; natural adhesive / film – forming gums and resins (e.g. agar, algies, rosin, shellac etc.); fossil fuels, viz., coal, lignite, natural gas and petroleum; glass and quartz, and metals and alloys extracted from minerals and ores. Polymers, as a class of materials with potential for use as rubbers, resins, plastics and composites, and as adhesives, laminates and coatings came nearly in the end of the chain of discoveries and developments of materials. Unfolding of the science of polymers and polymer – based materials really had its beginning and headway in the second and third decade of the twentieth century.
Though introduced very late in the chain of materials, polymers occupy a major place and pivotal position in our materials map today. In application prospects and performance characteristics and in property range and diversity, they offer novelty and versatility that can hardly be matched by any other kind of materials. Polymers have gone deep, and far and wide in moulding the present – day human civilization and culture.
Even though scientists, particularly the chemists used to talk about polymers earlier to the early twentieth century, there remained a lot of confusion over the basic understanding of the structures of polymer molecules. It was a common experience for chemists working with polymers that most such materials were very viscous and tacky under melt or solution conditions. One could readily spread the melt or solution into thin films or draw them into fine filaments. In solution, they were recognized more as colloids or associated molecules. Attempts to find their molecular weights from dilute solutions in suitable solvents by cryoscopy often produced irreproducible, doubtful and uncertain and sometimes very high values. For natural rubber, rayon and cellulose derivatives, molecular weights ranging 45,000 – 50,000 or even higher were measured. Such high molecular weight values implied that the relevant polymer molecules were really very large; but this view – point was not favoured or accepted in view of a total lack of structural concept about such large or big molecules. The chemists continued to favour the concept of large associations of much smaller molecules of short – chain or cyclic structures. A state of growing dilemma and confusion imparted fresh impetus to the thinking about the size, shape, complexity, and behavioral patterns of the molecular systems called polymers.
Long Chain Structure and Macromolecular Concept The realization of the long – chain molecular or the macromolecular concept of polymer molecular systems in the 1920s proved to be a vital turning point. Accumulated confused ideas and uncertainties of the earlier decades became meaningful and they were soon translated into practice through production of a large variety of hitherto unknown structures by polymerization and copolymerization of a host of olefinic, diolefinic and vinylic compounds and combinations thereof and by polycondensation reactions between a large variety of bifunctional or polyfunctional compounds bearing well characterizable chemical functional groups.
Table 1:Some Monomers and Corresponding Polymers and Repeat Units
S.No. Monomer Polymer Repeat Unit
- Ethylene Polyethylene
CH 2 = CH 2 – ( CH 2 – CH 2 ) (^) n – – CH 2 – CH 2 –
- Propylene Polypropylene
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
CH 3 CH 3 CH 3
- Styrene Polystyrene
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
- Vinyl chloride Poly (vinyl chloride)
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
C l C l C l
- Vinylidene chloride Poly (vinylidene chloride)
C l C l C l CH 2 = C – ( CH 2 – C ) (^) n – – CH 2 – C – C l C l C l
- Vinyl acetate Poly (vinyl acetate)
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
OCOCH 3 OCOCH 3 OCOCH 3
- Tetrafluoroethylene Poly (tetrafluoroethylene)
CF 2 = CF 2 – ( CF 2 – CF 2 ) (^) n – – CF 2 – CF 2 –
- Methyl acrylate Poly (methyl acrylate)
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
COOCH 3 COOCH 3 COOCH 3
S.No. Monomer Polymer Repeat Unit
9 Methyl methacrylate Poly (methyl methacrylate)
CH 3 CH 3 CH 3 CH 2 = C – ( CH 2 – C ) (^) n – – CH 2 – C – COOCH 3 COOCH 3 COOCH 3
- Acrylonitrile Polyacrylonitrile
CH 2 = CH – ( CH 2 – CH ) (^) n – – CH 2 – CH –
CN C N CN
- Formaldehyde Polyformaldehyde or
Poly (oxymethylene)
CH 2 = O – ( CH 2 – O ) (^) n – – CH 2 – O –
- Caprolactam Polycaprolactam
(Nylon 6 polyamide)
(CH 2 ) 5 [– C – (CH 2 ) 5 – N –] (^) n – C – (CH 2 ) 5 – N –
O=C NH O H O H
- Butadiene Polybutadiene
CH 2 = CH–CH = CH 2 (– CH 2 –CH = CH–CH 2 –) (^) n –CH 2 –CH=CH–CH 2 –
- Chloroprene Polychloroprene
CH 2 = C–CH = CH 2 (– CH 2 –C = CH–CH 2 –) (^) n –CH 2 –C = CH–CH 2 –
Cl Cl Cl
Polymerization of monomer styrene to polystyrene may be represented by
Heat / light n CH 2 = CH ( CH 2 – CH ) (^) n (1) and/or catalyst (Polymerization) Styrene Polystyrene (monomer) (polymer)
contents of jute, a major bast fibre produced in India, are 11 – 14% and 20 – 23% respectively. Semi-synthetic (modified natural) polymers and synthetic polymers are also called man – made polymers. Synthetic polymers are man – made from the very outset.
Table 2: Classification of Polymers
Basis of Classification (^) Polymer Types Examples
(a) Natural (as available in nature)
Natural rubber, natural silk, cellulose, proteins, starch etc. (b) Semisynthetic (Man – made)
Hydrogenated, halogenated and cyclo (natural) rubber; cellulosics (cellulose esters/ethers), etc.
- Origin
(c) Synthetic (Man – made)
Polyethylene, polypropylene, poly- styrene, polybutadiene, nylon poly- amides, polycarbonates, phenolics, amino resins, epoxy resins etc. (a) Thermoplastics (they soften or melt on heating and harden on cooling over many cycles of heating and cooling and retain solubility and fusibility).
Polyethylene, polypropylene, poly- styrene, nylon polyamides, linear polyester [poly (ethylene terephthalate)] , etc.
- Thermal response
(b) Thermosetting (they usually soften or melt initially on heating, but fast undergo chemical changes to finally turn insoluble and infusible).
Phenolic resin, amino resins, epoxy resins, diene rubbers (vulcanized), unsaturated polyesters.
(a) Chain – growth or addition
Polyethylene and other polyolefins, Polystyrene and related vinyl polymers etc.
- Mode of formation
(b) Step – growth or condensation
Polyesters and polyamides, polycarbonates, phenol (urea, melamine) – formaldehyde resins, epoxy resins etc. (a) Linear (having no branches)
High density polyethylene (HDPE), polyvinyls, bifunctional (polyesters and polyamides) etc. (b) Branched (having branches)
Low density polyethylene (LDPE), higher poly (α-olefins), phenolic resoles and resitols, poly (3-hydroxy alkanoates) etc.
- Line Structure
(c) Cross linked or network (having a complex network structure)
Phenolic C-stage (resite) resin, C- stage amino (urea / melamine-formal- dehyde) resins, cured epoxy resin and unsaturated polyester resin etc.
Basis of Classification (^) Polymer Types Examples
(a) Rubbers (showing long – range elasticity)
Natural rubber, (1, 4 cis poly isoprene) synthetic rubbers (polybuta- diene, SBR, nitrile rubber, polychlo- roprene rubber, polylacrylate rubber, polyurethane rubbers, silicone rubbers etc.) (b) Plastics (shapable under pressure, aided by heat)
Polyethylenes, polypropylene (isotactic), polystyrene, poly (vinyl chloride), nylon polyamides linear aromatic polyesters and polyamides, polycarbonates, acetal resins etc.
- Application and Physical properties
(c) Fibres (available in fibrillar or filamentous form)
Cotton (cellulose), natural silk, artificial silk (rayons), poly (ethylene terephthalate) fibre, nylon polyamide fibres etc. (a) Isotactic (stereoregular) (b) Syndiotactic ( do )
- Tacticity
(c) Atactic (stereo irregular)
Poly (α-olefins) and all vinyl and related polymers.
(a) Crystalline (crystallinity, > 50%)
Polyethylene (HDPE and LDPE), polypropylene (isotactic), stretched nylon polyamides, polyoxymethylene etc. cellulose (cotton) fibre. (b) Semi – crystalline (crystallinity, 30 – 50%)
Polybutene, cellulosics (cellulose esters (rayons) particularly if stretched), Gutta percha (1, 4 trans polyisoprene) etc.
- Crystallinity
(c) Amorphous or non- crystalline (crystallinity < 25%)
Natural rubber and most synthetic rubbers,N-alkylated(>15% alkylation) nylon polyamides, poly (methacry- lates and acrylates) poly (vinyl acetate), polystyrene etc.
(b)Thermal response: A molecule of a linear polymer may be schematically represented by a simple line of finite length (straight or zig – zag, but usually wavy)(fig. 2 (a)). A molecule of a branched polymer may, however, be represented by a line of finite length with short or long finite – length branches attached to some of the repeat units in the chain molecule, fig.2 (b). A cross linked polymer can be represented by a network structure that may be a planar network as in graphite or space network, as in diamond, Fig. 2 (c and d). A linear polymer is best exemplified by high-density polyethylene (HDPE), while a branched polymer is exemplified by low-density polyethylene (LDPE) or poly (α – olefins). Linear and branched polymers are commonly thermoplastics, that are soluble and fusible, while the network resins or polymers are examples of thermosetting resins or polymers that turn ultimately insoluble and infusible.
A third distinguishable long chain sequence called atactic sequence would be given by a random spatial disposition of the substituent – R groups, also exemplified by a random mix of isotactic and syndiotactic sequences, viz. H H H H H R H H H R H R
- C – C – C – C – C – C – C – C – C – C – C – C –
H R H R H H H R H H H H Atactic (Random sequence, ( – d – d – l – d – l – l –) ) Isotactic sequence is distinctive as being very regular, while atactic sequence is one that is highly irregular. Syndiotactic sequence may be viewed as regularly irregular or vice versa. Isotactic and syndiotactic polymers are considered as stereospecific or stereoregular while atactic polymers are viewed as random or stereo–irregular. The overall molecular symmetry and crystallinity are in the order isotactic > syndiotactic >> atactic. Isotactic polymers are generally characterized by high melting temperature ( Tm ) and high mechanical properties with relatively high resistance to solvents and chemicals.
Structure – Property Relationship The wide variety and diversity of natural, semi-synthetic and synthetic polymers known currently exhibit wide diversity of properties or property ranges. Some appear rigid, hard, strong and dimensionally stable, while others appear soft, flexible or largely extensible under the influence of stress. Some show ready solubility and fusibility, while others appear more resistant to heat and solvents and may appear even insoluble and infusible. All such properties may vary from a polymer of one type to a polymer of a different type. They may even vary between samples of the same type of polymer depending on molecular weight and molecular weight distribution (MWD) or on how they were synthesized or thermomechanically treated before being tested. (a) Linearity, Branching and Networking: Depending on the property ranges they exhibit, the polymers are classified as rubbers, plastics and fibres. Factors that influence the polymer properties are: molar cohesion, polarity, molecular weight, crystallinity, overall molecular symmetry (both recurrence symmetry and architectural symmetry), linearity and non – linearity of chain molecules, thermomechanical history of the polymer and temperature of observation. Higher molecular weight permits greater degree of chain entanglements, thus resulting in higher melting temperature ( Tm ) and tensile strength ( T.S .).
Fig. 3: Trend of change in Tm and T.S. with change in molecular weight of polymers (Courtesy: Tata McGraw –Hill, New Delhi )
Small extents of branching make the otherwise equivalent polymer less resistant to solvents, chemicals and heat owing to increased molecular mobility manifested through the branch units or pendent groups. High degrees of branching with enhanced branch lengths and ultimate cross linking render the polymer relatively stiff arising from greater degrees of chain entanglements and ultimately forming giant molecules of a network structure, thereby restraining or eliminating scope for large scale molecular mobility or chain slippage and improving resistances to thermal and mechanical loading and hence dimensional stability. Polymers thereby turn less soluble or difficultly fusible or ultimately insoluble and infusible. Cured phenolic and amino resins, epoxy resin systems and vulcanized rubbers are good examples of cross-linked, network polymers or resins.
On cross-linking, basic structural changes in the polymers are introduced and consequently, basic changes or improvements in properties are invariably achieved. By proper process – design and in tune with the set objectives, different degrees of cross linking or cross link density, high or low may be duly achieved, fig. 4.
Fig. 4: Schematic representation of network or cross-linked structures (a) High cross-link density, and (b) Low cross-link density (Courtesy: Tata McGraw –Hill, New Delhi )
(b) Molar Cohesion, Polarity and Crystallinity : These parameters are interrelated and interdependent. A chain molecule having a strong polar structure for its repeat units exerts strong attractive forces on molecules around it. In non–polar polymer systems, the molar
O H
cohesion would be substantially weak. Between non–polar polyethylene ( – CH 2 – CH 2 – ) (^) n
and polar nylon 6 polyamide [– ( CH 2 ) 5 – C – N – ] (^) n , the methylene links are common,
but in the polyamide, the strongly polar CONH inter-unit linkage gets repeated in the chain molecular structure and finds place between every two successive (CH 2 ) 5 chain units. Assuming degrees of polymerization such that the polyethylene and the polyamide chains are of equal length, the forces of attraction between the polyamide chains will be very much stronger than those between the polyethylene chains. This is actually reflected in much higher rigidity, stiffness, tensile strength, melting temperature and crystallizing tendencies of the polyamide. On top of this, extensive intermolecular hydrogen bond formation takes place in the nylon 6 polyamide, more so, on stretching it from the melt condition as it is cooled, thereby resulting in the ready formation of the polyamide fibre. However, melt – cooled nylon 6 polyamide behaves much as a very good engineering plastic.
groups to ester or ether groups ( – ONO 2 in cellulose nitrate, – OCOCH 3 in cellulose acetate,
- OCH 3 in methyl cellulose, – OCH 2 COOH in carboxymethyl cellulose etc.). The completely modified cellulose, giving a degree of substitution (DS) for each glucose unit in the chain molecules equal to three, would be expected to show better strength characteristics than the incompletely modified derivatives, because on complete modification, the overall molecular symmetry is regained.
(e) Copolymerization and Internal Plasticization : Further, it is interesting to note that the homopolymers of monomer M 1 and of monomer M 2 prepared separately would be characterized by a high order of recurrence symmetry each, but when the two monomers will be copolymerized or polymerized together, the resultant copolymer will bear both M 1 and M 2 repeat units in its chain molecular structure thereby infusing a loss of recurrence symmetry; a copolymer of monomers M 1 and M 2 is commonly viewed as a polymer that is internally plasticized, consequent to having duel repeat units and hence loss of molecular symmetry; a 1 : 1 copolymer will generally have melting point ( Tm ) and tensile strength ( T.S ) that are significantly lower than those of the two respective homopolymers as schematically shown in fig. 5, the curve indicating the trend of change of each property parameter with change in mole ratio of M 1 and M 2 monomers used in copolymerization.
Fig. 5: Trend of change in Tm and T.S. with variation of copolymer composition
O
O
H O
H O
(f) Effect of inclusion of flexible inter-unit linkages and rigid bulky groups: It is
apparently surprising that the melting points of most linear aliphatic polyesters [– ( CH 2 )x –
C – O –] (^) n lie well below that of polyethylene. This can be partly explained considering
that the contributions of the polar (– C – O –) groups in enhancing molar cohesion are far
outweighed by the flexibility effects imparted to the chains through the –O– linkages in the chain
backbone. The increase in chain flexibility and hence lowering in melting point by oxygen
linkages in the main chain is further illustrated by a much lower melting point ( ~ 150^0 C )
for the polyurethane [– ( CH 2 ) 5 – N – C – O –] (^) n than that for the analogous polyamide
[ – (CH 2 ) 5 – N – C – ] (^) n that shows a melting point of 215^0 C. Table 3 shows that – S – and –
SO 2 – interunit linkages respectively infuse flexibility with softness, and rigidity with higher
thermostability to the polymer chains.
Table 3: Effect of Variation of Polymer Structure on Properties and Application Prospects
Polymer Repeat Unit Tg, 0 C Tm, 0 C Remarks
Polyethylene – CH 2 – CH 2 – –115, –60 137 General purpose, low modulus (linear), PE polymer with good degree of toughness; excellent electrical insulator. Polystyrene, – CH 2 – CH – 80 to 100 240 Rigid, brittle transparent polymer;
PS excellent insulator.
Poly(vinyl naph- – CH 2 – CH – 150 360 Highly, rigid, brittle polymer;
thalene) PVN excellent insulator.
Poly(para-phenylene), --- 530 Tough, thermostable, intractable PPP polymer with semi-conducting or even good conducting (when doped) properties with electrical conductivity in the range 10-18^ to 102 (Ω cm)- Poly(tetramethylene – (CH 2 ) 4 – O – – 80 60 Weak, flexible polymer. Oxide), PTMO Poly (phenylene O – 80 300 A good engineering thermoplastic Ether), PPE having high strength & toughness.
terephthalate), [ – OOC – – COO – (CH 2 ) 2 – ]n turns the polymer relatively rigid
and very much higher melting (>250^0 C) and it can be readily melt – drawn into a very useful
fibre, the most common polyester fibre. The aromatic hydrocarbon polymer, poly (para
phenylene), [ – –]n is a difficultly processable, tough, thermostable engineering polymer
of much high performance ( Tm = 530^0 C), while poly (phenylene ether), [– – O –]n
is a more readily processable, tough, high performance polymer having a much lower melting
point (300^0 C); presence of the ether – O – linkage renders the aromatic polymer somewhat more
flexible and easily processable at a lower temperature.
In polymer material systems, intermolecular forces are opposed by thermal agitation. The influence of chain length is mainly extensive, i.e. total (cumulative) force of intermolecular attraction increases with molecular weight owing to availability of greater molecular surface, while the force per unit length or area remains by and large constant. By contrast, polarity is intensive and the more polar the structure of the repeat unit, greater is the attractive force per unit length or area.
It is logically appreciable that, lengthening of the substituent group of the isotactic poly ( α – olefin) homologues causes progressive decrease in the melting point from polypropylene (having one carbon side chain or substituent group) to polyheptene (having five-carbon side chain or substituent group); for still higher isotactic α – olefin polymers, the side chains on the alternate carbon atoms become even longer so as to be able to infuse a developing trend of side chain crystallization; as a consequence, the melting point curve reverses its falling trend and then follows an upward trend, as shown in fig. 6.
Fig. 6: Trend of change of Tm of isotactic poly( α – olefin) (– CH 2 – CH –) (^) n with increase in branch (substituent) unit length.
Further, dependence of physical properties on structural (geometrical) isomerism is distinctively exemplified by the well known examples of 1, 4–cis polyisoprene ( natural rubber ) and 1, 4 – turns poly isoprene ( gutta percha ); the former, being devoid of or much poorer in symmetry and hence in crystallinity, is more flexible and behaves as a rubber, while the latter, having a good degree of symmetry and balanced structure, is dimensionally stable under normal conditions and useful as a good plastic.
(g) Effect of Temperature: Generally speaking, all linear amorphous polymers can behave as Hookian elastic ( glassy ) materials, highly elastic ( rubbery ) substances or viscous melts , according to the prevailing temperature of observation and time – scale of experiments. Different property ranges for the same polymer at different temperatures are related to variation in the physical structures or arrangements of the chain molecules, much as a consequence of different types and degrees of deformation.
Survey of Deformation Patterns in the Amorphous State An idealized plot of log (shear) modulus vs. temperature, as in fig. 7, may now be examined and analyzed. The modulus curve commonly shows a number of transitions each of which is connected with the gradual development of an additional molecular movement.
Fig. 7: Temperature dependence of log (shear modulus) in a polymer system showing molecular mechanism of the deformations taking place at different points and notable transition phenomena (Courtesy: Tata McGraw –Hill, New Delhi )
At a temperature below a specified transition temperature, the molecular processes in question are frozen in. With rise in temperature, as the transition point is approached, an additional molecular movement begins to contribute to the deformation mechanism and hence lowers the resistance to deformation, i.e. the modulus.
At very low temperatures, the only deformation occurring is Hookian elastic deformation, which is time-independent and mechanically reversible. The deformation takes place instantaneously on a specified stress application, and it reverts giving complete dimensional recovery on removal of the applied force or stress. The Hookian deformation is also thermodynamically reversible, as
Fig. 8: Temperature-molecular weight plot (Courtesy: Tata McGraw –Hill, New Delhi )
(a) Rubbers or Elastomers : The diagram (fig. 8) further reveals that while useful plastic properties may be exhibited by polymers of wide molecular weight range (> 1000, low polymer or high polymer), good useful rubbery properties are exhibited by polymers which are essentially high polymeric in nature (molecular weight > 100,000). To reveal rubberiness, the polymers must be far above their Tg , and for practical advantage and rightful use, Tg for elastomers or rubbers should be in the range of – 40 to – 80^0 C. This is commonly attained in systems of low cohesive energy density, Table 4, and poor molecular symmetry, resulting in amorphousness, at least in the unstrained state and permitting adequate freedom of molecular motion, so that deformation of high magnitudes take place rapidly. Even though these requirements and features for rubbers imply high local or segmental mobility, the gross or full – scale mobility as in the flow region must be low. Restricted chain slippage must be assured in view of property demand in the form of prompt dimensional regain or recovery on stress release. This restriction is technically achieved by introduction of widely spaced primary valance cross links in the chain molecular system such that on application of forces of extension, large deformations may take place without rupture of primary bonds of the backbone chains. The molar cohesion for rubbers is in the range of 1.9 – 2.1 kcal/g.mol over segmental length of 5 Å.
Table 4: Cohesive Energy Densities of Some (Linear) Polymers Polymer Cohesive energy density, cal/cm Polyethylene 56 – 64 Polystyrene 75 – 88 Polyisobutylene 56 – 64 Polyisoprene (natural rubber) 60 – 67 Poly (vinyl acetate) 82 – 94 Poly (vinyl chloride) 85 – 95 Poly (methyl methacrylate) 78 – 85 Poly (ethylene terephthalate) 90 – 115 Poly (hexamethylene adipamide), nylon 66 180 – 220 Polyacrylonitrile 150 – 230
(b) Fibres : For a typical fibre characterized by high tensile strength and modulus, the polymer must normally possess a combination of high molecular symmetry and high cohesive energy density manifested through polar structures in the repeat units, fostering high order of permanent crystallinity (in the useful temperature zone) on cold drawing. The crystalline melting point, Tm (without decomposition) must preferably be in the range of 200 – 300^0 C so as to make fabrics made from them suitable for hot – pressing or ironing and for trouble-free spinning into a fibre. Resistance to solvents is an advantage, keeping dry – cleaning in view. Again, the molecular weight should be moderate (25,000 – 30,000) and not too high for synthetic fibre – forming polymers so as to ensure speedy, trouble – free spinning from melts and solutions and at the same time, it should be high enough to ensure full development of tensile strength and related properties. Presence of aromatic rings in chain backbones is sometimes helpful as in poly (ethylene terephthalate) or the PET fibre. Extensive intermolecular hydrogen bond formation is often a key factor in synthetic fibre technology; viscose and acetate rayons and polyacrylonitrile
- based acrylic fibres as well as polyamide and poly (vinyl alcohol) based fibres stand in testimony to this. Molar cohesion for 5 Å chain length segment of fibres is quite high and it ranges 4 – 10 kcal/g.mol in general. Fig. 9 shows the feature of orientation of chain molecules on stretch application.
Fig. 9: Orientation of disorganized chain molecules in axial direction on stretching (a) Unstretched (b) Stretched (c) Plastics: Generally, the properties of plastics are more or less intermediate between those of fibres and elastomers or rubbers, with possible good degree of overlap on either side. Plastics are put to use in a vast range of application areas and hence, a wide range of property combinations is associated with them. It is no wonder then that a wide variety of chemical structures may adequately represent and describe them.
Some polymers having high cohesive energy density, such as the nylon polyamides behave as plastics when simply melt – cooled and used without orientation of the molecules by stretching. Orientation of such polymer molecular systems by stretching or cold – drawing infuses major changes in the structure of their physical agglomerates. Axial orientation of the chain molecules find permanence due to (i) establishment of extensive intermolecular H – bonding (as in the polyamides, the acrylic fibre systems etc.) or (ii) physical interlocking of the oriented chain segments (as in PET polyester) of neighbouring chains. Consequently a high order of crystallization sets in and the relevant polymers then behave as excellent fibres. Molar cohesion for 5 Å chain segment of plastics generally fall between 2.2 and 4.0 kcal/g.mol. Tg for plastics and fibres should be far above the room temperature.
The difference between rubbers, plastics and fibres is not really basic or intrinsic; it is rather a matter of degree. Small or minor variations in chemical structure or physical conditio-ning of the
