Mighty Fine Equine

Polymer Science

Introduction: Polymer Morphology

Two different statements or forms can be identified in a polymer may display the mechanical or thermo-mechanical process that can be associated with solid, ie., The shape of a crystal or the form of a glass. This is not really the case that all polymers are able to crystallize. As a matter of fact, a high degree of molecular symmetry and regularity of the microstructure in the polymer chains are a prerequisite for crystallization to occur. Even in polymers that crystallize in any case, the final degree of crystallinity is mainly developed less than 100%.

Studies of the physical form, arrangement and structure of molecules or molecular aggregates of a material system refers to what is known of its morphology. Polymer morpho-logy covers the study of the arrangement of macromolecules in the crystalline and amorphous regions of overlap and the overall physical consolidation of molecular aggregates.

Once cooled, the molten state, different polymers have different tendencies to crystallize at different rates depending many factors including physical conditions, the chemical nature of repeating units and the polymer as a whole, their molecular symmetry or segmental and structural regularity or irregularity, as described above. bulky groups independent or chain branches in different lengths to hinder molecular packing and therefore the crystallization. The nature of the crystalline state of polymers is not straightforward and should not be confused with the regular geometry of crystals of compounds with low molecular weight such as sodium chloride or benzoic acid. There are polymers, which are large and amorphous, and they have very poor tend to become ordered structures or oriented by cooling to near or even below room temperature. Natural or synthetic rubbers and glassy polymers such as polystyrene, acrylate and methacrylate polymers belong to this class.

In a crystalline polymer, a chain is given polymer or pass through several crystalline and amorphous areas. The areas are composed of crystalline alignment intermolecular and intramolecular or ordered and therefore tight arrangement of molecules or segments chain, and a lack of results in the formation of amorphous zones.

Glass transition and melting transition

On the basis of following the evolution of a parameter of mechanical properties such as shear modulus with changes (increase) temperature of observation systems for polymer material, can easily turn out - (i) glass transition and (ii) the merger of the phenomena of transition more easily from a chart, and may also have a measure of temperature glass transition, Tg and the melting temperature TM.

The glass transition and melting transition may also be observed and verified from a plot of specific volume (VSP) in relation to temperature. Let us examine the various possibilities in this a melt is cooled from position A to a high temperature which corresponds to a relatively high value and Vsp, fig. 1. The path ABDG shows how specific volume drops down as a compound of low molecular weight is frozen. As the melting temperature Tm is at point B, a sharp discontinuity is observed PSV (BD). The slopes of AB and DG provide measurements of coefficients of thermal expansion of liquid and solid, respectively. The coefficient of thermal expansion also suffers a discontinuity at TM.

Fig.1: Schematic putting Identify any changes in the specific volume (VSP)

a polymer with temperature change.

We may, however, start with a polymer melt and to observe changes in volume as described by the path and there ABHI no significant discontinuity in TM. The liquid line AB also gets extended beyond Tm with lowering temperature and is seen to undergo a change of slope at a temperature much lower TG and finally turns into another linear portion (HI) a constant slope much lower. Here, in fact, the slope change occurs over a small temperature range (usually in May between about 5 - 100C) but an extrapolation of the linear two parties can evaluate the right of Tg by this method. The HI area represents the glassy state which means that the glass transition temperature is reached or passed just as we descend in temperature. Transition to the glassy state is also commonly known as vitrification. This region is the BH the existence of a super cooled liquid or rubbery state the relatively poor dimensional stability, even under the influence of a low stress level.

For all polymers, the state vitreous is always achieved finally cooling, regardless of whether the polymer is crystallizable tested or not. Even in situations promoting training crystals, it does not necessarily mean that the crystallization occurs rapidly or completely. He remains in most cases, a significant portion of amorphous regions after the primary crystallization process is completed.

The path in ABCEFG Fig. 1 represents the case of part crystalline, partially amorphous polymer system. The cooling Tm, crystallization begins and the characteristic discontinuity in Vsp becomes evident, although the clarity in which Tm is revealed is not as pronounced for polymers as a compound low molecular weight, which can be appreciated from the curvature of the portion of the BCEF way. For such a system, FG represents the area and the glassy melt BA or liquid zone and the zone is roughly BCEF the rubbery amorphous (super-cooled liquid) zone. Point F, where the slope between segments EF and FG corresponding changes the glass transition point Tg and the polymer in such cases remains large and amorphous. If the crystallization would be held on cooling below Tm, the amorphous content decreases and in this case, the change of slope at Tg may be much smaller and harder to detect.

The path ABJK May appear as a variation of the road here and ABHI, AB describes a liquid, BJ super cooled liquid or rubbery state and JK describes the glassy state. The path quarts Abhi To ABJK under the condition of a higher cooling rate, it is likely that Tg is shifted to a higher temperature (BT?) for a more rapid rate of cooling.

Thus, the temperature response of linear polymers may be considered divided into three clearly distinct segments:

1. Above Tm:

In this segment, the polymer remains as a merger or a liquid whose viscosity depends on the molecular weight and temperature of observation.

2. Between Tm and Tg:

This domain may lie between groups almost 100% crystalline and nearly 100% of the amorphous molecular chain as a function of polymer structure regularity and the experimental conditions. The amorphous part behaves like a super-cooled liquid in this segment. The overall physical behavior of the polymer in the intermediate segment is a bit like rubber.

3. Tg below:

The polymer material considered as a glass is hard and rigid, showing a specified value coefficient of thermal expansion. The glass is closer to a crystalline solid than a liquid into the model behavior in terms of property settings mechanical. Regarding the molecular order, however, the glass looks more like liquid. There is little difference between linear and crosslinked polymer below Tg.

The Place of Tg depends on the cooling rate. The position of Tm is not subject to this variability, but the degree of crystallinity depends on the experimental conditions and the nature of the polymer. If the cooling rate exceeds the rate of crystallization, it may not be observable change in Tm, even for a crystallizable polymer.

The simple device used to monitor changes in volume during cooling or heating is called a dilatometer, with a glass bulb or bulb to the bottom with a narrow bore capillary at the top, as in Fig. 2. A dilatometer may also be used to study the progress of the polymerization time at a given temperature by following the contraction of the liquid monomer system (the polymer being formed having a density greater that the monomer is polymerized). For studies with a polymer is, the polystyrene, the sample is placed in the bulb, which is then filled with an inert liquid, usually mercury and volume changes with changing temperature (or sometimes at a constant temperature for phase change, as Tm) are then recorded as a thermometer. The expansion / contraction of mercury because of the changes temperature is to be properly accounted for in testing for a change in volume of the polymer sample. The experiments must be performed by placing the dilatometer in a waterbath. The sample must be immiscible with the fluid displacement and degreased to eliminate air entrapment. Specific volume - temperature plot for polystyrene showing a marked decrease 95.60C in the slope indicates the glass transition temperature, Fig. 3.

1. Fig.2: An arrangement for dilatometric Fig. 3: Temperature dependence of

measurement of change in volume of a specific volume for polystyrene showing

1. the glass transition temperature, Tg

(Courtesy: Tata McGraw-Hill, New Delhi)

Thus, it is a common experience that raising or lowering temperature, as the application or removal of stress, a great influence on the physical structure and properties of polymers. with change the temperature of a high polymer material passes through two distinct transitions characterized by (i) the melting point or transition of the first order, denoted by TM and (ii) the glass transition or second order transition, denoted Tg

Melting Point or First Order Transition

Fusion of a crystalline solid or a boil fluid is associated with phase change and the participation of latent heat. Many polymers have high enough symmetry of the molecule and / or regularity of structure they crystallize sufficiently to produce a phase transition in solid, liquid, having a crystalline melting point. The merger is strong enough for some polymers such as nylons, while in most other cases, such as various rubbers and polystyrene, etc., change phase occurs over a temperature range. phase transitions of this kind, particularly in materials of low molecular weight, being associated with strong discontinuities in some primary physical properties such as density or volume, V, [V = (? G /? P) T] and entropy, S [- S = (? G /? T) P], which are first derivatives of free energy, commonly called transitions first order. Although there is melting, a genuine first order transition or melting in the ideal of high polymers is often absent or Missing, for the distribution of molecular weight and chain entanglements of molecules to the origin of the complex phenomenon of flow delayed or viscoelasticity.

Glass Transition or Second Order Transition

Glass transition or second order transition is a transition phase and almost all the polymers or materials high polymer is characterized by a specific temperature glass transition (Tg) or the second control point of transition (SOTP) contained well below its (crystalline) melting point, TM.

At Tg, the parameters of thermodynamic properties S, V and H suffer just slope change when a function of temperature, but without showing sharp discontinuities observed in the case of transitions leading order, such as the idealized plot shown in FIG. 4.

Fig. 4: first order transition showing a phase transition idealized (melting or freezing): Trend of change in volume or entropy with rising temperature, showing discontinuity at the transition point. (Courtesy: Tata McGraw-Hill, New Delhi)

The properties that undergo discontinuities at the glass transition temperature are: heat capacity CP [CP = (? H /? T) P], the coefficient of thermal expansion?,

1 1?

? = (? V /? T) P =. ((? G /? P) T) P

V V? T

and the isothermal compressibility K

1 1

K = - (? V /? P) T = - (? 2G /? P 2) T

V V

which such derivatives are second free energy and for this reason that the glass transition temperature, Tg is commonly called the transition temperature of the second order, Fig. 5. Refractive index (R1) also shows an abrupt change occurred at the transition glass (Tg).

Fig.5: Trends of change in (a) the specific volume, (b) compressibility coefficient of thermal expansion (?) Or isothermal (K) and (c) Refractive index (RI) of polymers with glass transition temperature indicating (Courtesy: Tata McGraw-Hill, New Delhi)

The glass transition is a transition phase and, therefore, involves no latent heat. Below this temperature normally rubber - like polymers lose their flexibility and turn rigid, hard and dimensionally stable and they are then considered in a glassy state, while that above this temperature, all normally rigid, inflexible, hard glassy polymers turn soft and flexible, will be subject to cold flow or creep like you and these men in a rubbery state. The difference between the states glassy, rubbery and does not really lies in their geometric structure, but in the state and the degree of molecular motion.

Below the glass transition temperature, Tg, the chain segments or groups that parts of the chain molecular skeleton may undergo some degree of vibration limited: they do not have the energy required for rotation around routes and positions change with respect to segments of the neighboring chains.At or slightly above Tg, defines the rotation, including secondary groups or branch, and it is conceivable that only segments of short-range molecular rather than the entire molecule high polymer turns at this point. much higher coefficient of thermal expansion just above Tg is indicative of many greater degree of freedom of rotation.

At the respective glass transition or second order transition temperatures, different polymers may be viewed to be in a state isovisqueuse, and in fact, BT is a common reference point for polymers diverse in nature, under which all behave as rigid plastic rigid (glassy polymer) and above which they appear tough and rubbery in nature. As we understand, a useful rubber is a polymer having a Tg well below the temperature room, while a useful plastic is one whose Tg is above room temperature. Table 4.1 lists the Tm and Tg values of some common polymers.

Table 1: TM and Tg values of several polymers

Polymer

Repeat Unit

Tm, 0C

Tg, 0C

Polyethylene

- CH2 - CH2 --

137

-115, -60

Polyoxymethylene

- CH2 - O --

181

-85, -50

Polypropylene (isotactic)

- CH2 - CH (CH3) --

176

- 20

Polyisobutylene

- CH2 - C (CH3) 2 --

44

- 73

Polybutadine (1, 4 cis)

- CH2 - CH = CH - CH2 --

2

- 108

Polyisoprene (1, IEC 4), (NR)

- CH2 - C (CH3) = CH - CH2 --

14

- 73

Poly (dimethyl siloxane)

- OSi (CH3) 2 --

- 85

- 123

Poly (acetate vinyl)

- CH2 - CH (OCOCH3) --

---

28

Poly (vinyl chloride)

- CH2 - CH Cl --

212

81

Polystyrene

- CH2 - CH (C6H5) --

240

95

Poly (methyl methacrylate)

- CH2 - C (CH3) (COOCH3) --

200

105

Poly tetrafluoroethylene

- CF2 - CF2 --

327

126

Poly caprolactam (Nylon 6)

- (CH2) 5 CONH --

215

50

Poly (hexamethylene adipamide)

(Nylon 66)

HN-(CH2) 6-NHCO-(CH2) 4CO --

264

53

Poly (ethylene terephthalate)

- O (CH2) 2 - OCO - (C6H4) CO --

254

69

Poly (ethylene adipate)

- O (CH2) 2 - OCO -- (CH2) 4 CO --

50

-70

Molecular weight and molecular weight distribution of tension or external pressure, the incorporation plasticizer, copolymerization, filling or reinforcing fiber and curing are some of the most important factors that influence the transition temperature glassy, melting point or heat - temperature deformation of a polymer matrix. The comparative reduction of TM and TG for polymer modification by external lamination (the Incorporation of plasticizer) and internal lamination (comonomer incorporation) is shown in Fig. 6. In general, incorporation of a comonomer copolymerization IE is more effective than external plasticization lowering the melting point, while the second process (incorporation of external plasticizer) is more effective than the former (copolymerization) lowering the glass transition temperature. crosslinking causes Uprise important Tg, such as cross-links hinder rotation of string, thus requiring a higher temperature for the creation of rotation of the segments between cross-links. Similarly, higher molecular weight, leading to complex, long-range chain entanglements, limited opportunities for segmental rotation and cause an increase in the value of Tg with remarkable leveling off effect for a molecular weight> 105.

Fig. 6: Diagram showing the parcels to the reduction relative Tm and Tg of a polymer incorporating separately (a) an external plasticizer.and (b) a comonomer for copolymerization. (Photo: Tata McGraw-Hill, New Delhi)

Brittle Point

A polymer is also characterized a temperature called point1 brittle or brittle temperature (Tbr), which is near or slightly above its temperature glass transition (Tg) polymers highest. As the temperature of the polymer to the rubbery state is lowered, nature flexible and rubbery properties are gradually lost and the polymer hardens hardens, at an intermediate stage, a temperature known point fragility is reached at or below which the polymer sample is brittle and breaks or fractures to the sudden application of load.

To compare the weak points of various polymers, it is necessary to test under specified conditions, including sample size and specified thickness, degree and rate of cooling, so that the test is empirical. The brittle point corresponds to a temperature at which the time interval of application of the charge is fair or equal to that required for the test to undergo the required deformations. At a lower temperature, the specimen is not able to deform so quickly, and therefore it unable to withstand the load and therefore ruptures above the brittle temperature, time of load application is more than sufficient for the specimen absorb the energy applied to deform and to avoid fracturing or breakage. lower molecular weight limit opportunities molecular interactions at long distance and tangled chains and thus leads to a higher temperature brittle. Changes in Tg and TBR with polymer molecular weight, as shown schematically in Fig. 7, shows clearly that the patterns of change for both parameters are just the opposite. The difference between the two is much narrower in the range of higher molecular weight, but it becomes progressively wider than the molecular weight decreases.

Fig. 7: Typical plots showing the dependence of the brittle temperature (Tbr) and temperature glass transition (Tg) of polymer molecular wieght.

(Courtesy: Tata McGraw-Hill, New Delhi)

Development crystallinity in polymers

Morphological studies of polymers are mainly related to molecular models and physical state of the crystalline regions of polymers crystallize. Amorphous polymers, semi-crystalline and crystalline are clearly known. It is difficult and May is virtually impossible to attain 100% crystallinity in the polymer bulk. It is also difficult, as different microscopic evidence, to achieve solid amorphous polymers completely devoid of any molecular structure or order segmental oriented or crystallinity. A wide range of structures, covering almost total disorder, various types and degrees of order and order almost complete, may describe the state a physical given polymeric system, according to the test environment, the nature of the polymer and the synthetic route, microstructure and stereo - the sequence of repeating units, and the thermomechanical history of the specimen. In addition, data collected for the degree of crystallinity may also vary depending on the testing method used. The degree of crystallinity data in Table 2 must therefore be regarded as approximate.

Polymers with degrees of crystallinity of 50% is generally accepted to be crystalline. The cellulosic (cellulose acetate) and also regenerated cellulose (rayon) used as fibers have a lower degree of crystallinity to that of native cellulose, the fiber core. Molecules predominantly straight-chain polyethylene (HDPE) showed a degree crystallinity is much higher than any other polymer known (even significantly higher than for the low density polyethylene (LDPE). For HDPE, the crystallinity achievable level is near the upper limit (100%). Atactic polymers in general (including methacrylate Methyl styrene bearing bulky side groups), configurations having irregular fail to crystallize significantly in any circumstances.

Table 2: Approximate degree of crystallinity (%) for different polymers.

Polymer

Crystallinity (%)

Polyethylene (LDPE)

60 - 80

Polyethylene (HDPE)

80 to 98

Polypropylene (Fiber)

55 to 60

Nylon 6 (Fiber)

55 to 60

Terylene (polyester fiber)

55 to 60

Cellulose (cotton fiber)

65 to 70

Regenerated cellulose (viscose rayon fibers)

35 to 40

Gutta-percha

50 to 60

Natural rubber (granulated)

20 to 30

Figure 8 gives a complete idea on the crystallization rate (change in volume per hour) at different temperatures selected. For high density polyethylene (HDPE), the temperature is lowered, the changes in volume proportional to the rate of crystallization increases rapidly and well below the actual melting point (1270C), the volume change soon becomes so fast that measures and monitoring become uncertain and difficult if not practically impossible. The obvious consequence of the very high rate of crystallization in polyethylene is that it is virtually impossible to obtain and isolate the polymer in the amorphous state to say the temperature in ambient conditions. sudden cooling or quenching of melting below room temperature results in a material that is still largely crystalline, although one might expect with the probability of a slightly lower degree of crystallinity developed in the otherwise normal cast - cooling. The reason for this state of affairs is that the time required for crystallization is much shorter than the time taken to cool the polymer sample test.

Fig. 8: Plot of volume versus time (min) showing the density of polylethylene development of crystallinity at different temperatures specified.

(Courtesy: Tata McGraw-Hill, New Delhi)

For practical reasons, therefore, the process of crystallization of polymer is very conveniently studied and measured with confidence using a polymer that is big and amorphous, natural rubber is a polymer such. Merit the use of rubber as a material model for the study of polymer crystallization is that the crystallization process is slow to allow measures because of easy manipulation and it takes place in a convenient temperature range.

It is worth mentioning that all the rubbers (especially those copolymers) are not crystallizable. Only those built up of chains characterized by recurring units chemically identical regular, such as natural rubber, 1, 4 cispolyisoprene and certain categories of polychloroprene are capable of crystallization.

Crystallilzation rubber on Cooling

If non-vulcanized natural rubber (NR) is maintained at a fixed low temperature, eg 00C, it is slowly becoming a little stiff and hard, and loses the flexibility and softness proportionately. However, the material still retains a certain degree of flexibility and tenacity. The physical change is also observed associated with some improvement in the density or lowering the volume changes partners are consequences of the slow development of crystallinity in the material.

Crystallization in a liquid low molecular weight by ordinary cooling or below the freezing takes place very rapidly, following loans and fast molecular rearrangement from a disordered state at a very steady state of the package. A molten polymer system is much more complicated due to congestion chain, which restricts the free movement of chain segments, and therefore, hinder and delay the process desired redevelopment on cooling. Rubber - such as polymers, the time scale of crystallization is generally much longer than for liquids of low molecular weight materials.

Fig. 9: The density on the crystallization of natural rubber,

Plot of relative volume versus time (hours) at different temperatures.

(Courtesy: Tata McGraw-Hill, New Delhi)

Trends of change in relative volume of natural rubber (NR) with time due to different crystallization at low temperature are shown in Fig. 9. The crystallinity and the maximum available period of time necessary for this to happen are very dependent on temperature observation6. In each case, the rate of contraction in volume is relatively slow at first, volume contraction (or crystallization) rate shows a tendency to increase with time, passes through an area to a more stable period of time intermediate then eventually fall down, or decays to stable development of maximum achievable degree of crystallinity at a given temperature. Decrease temperature causes the improvement of the equilibrium rate of crystallization of NR to about-250C, where the equilibrium rate versus temperature plot, Fig. 10 passes through a maximum. A further reduction of the crystallization temperature causes a downward trend the equilibrium rate of crystallization as in fig.10. Crystallization is (almost) finished in about five hours at-250C. In natural rubber, the level / degree of crystallinity in the most favorable situation should not exceed 30%.

Fig. 10: Plot showing trend of changes in the equilibrium rate crystallization with the temperature change to natural rubber (Courtesy: Tata McGraw-Hill, New Delhi)

Mechanism of crystallization

As the molten polymer is maintained at a temperature near or slightly above its melting range, the slow initial rate of crystallization construct (delayed crystallization) is related to the initial process of nucleation. Growth of crystallites is subject to the development and existence of a number of growth centers or very small nuclei for the deposition of oriented chain segments. The growth centers are first formed on the extension of cooling or keeping the melt at the temperature specified by the assembly a small number of chain segments in the course of their random movement (Brownian motion micro) in the prevailing situation. However, nucleation is common to all processes that transform an initially homogeneous environment in a heterogeneous system following the filing of a separate phase.

As growth continues and is sustained, the adverse effect of chain entanglement becomes increasingly severe criticism and finally, giving and severe restrictions on the mobility of chain segments and thus make it difficult for them to reach a position for attachment to one any of the crystallites formed. Beyond this stage, the crystallization rate decreases sharply and finally, the process dies.

Low temperature favors nucleation and decrease the heat of the chain segments, it is less likely that once formed the nucleus would disappear again, The net result is an increase in the number of nuclei and an increase in the overall rate of crystallization with progressive lowering of temperature. At temperatures progressively weaker, however, the overall energy system of polymer, including those available to the chain segments tend to get decreased so that the segments appear almost lose much of their mobility and thus their deposition on a core is gradually slowed down much more efficient and it appears a strong tendency to decrease rates of crystallization. For natural rubber, the crystallization process will effectively be frozen below - 500C, Fig. 10.

Stress - Induced Crystallization of Rubber

It is common knowledge and a matter of wide experience who runs a strip of vulcanized rubber, it is to develop a temporary crystallinity the axial orientation of chain molecules in the direction of stretching and the orientational effect disappears instantly on the withdrawal of the pulling force. A strip of raw rubber or unvulcanized also develops crystallinity when subjected to extensions High on the request of a pulling force, but it remains more or less in the extended state (given the lack of cross-links restraint) without significant shrinkage in its original condition upon release of stress. However, when heated with care in step Later, for example by dipping the test strip in a little warm water (temperature> 300 ° C) crystals melt and allow the band return in large part to its unstrained state.

The cross-links in the form of vulcanized rubber reinforcement points and are responsible for the accumulation to retract or strong restoring force that comes to play in breaking stress - induced orientation (or crystalline structure) on the removal of the applied stress. In the unvulcanized system, lack of cross-links allows varying degrees of chain place so little deviation from the chain on low / moderate extensions and whatever the elastic restoring force builds up is insufficient or too inadequate to break the crystalline structure and induce recovery dimensions. temperature rise test strip and 300C or slightly above this level, allows fusion of crystallites orientation axial, causing chain rubber molecules to coil up and test strip to retract its original or near original (random / No-oriented) state.

Fig. 11: Time-dependence of the crystallization induced by stress (intensification) of unvulcanized rubber held in 00C for various specified set extensions, the plot of the density change (%) versus time (min). (Courtesy: Tata McGraw-Hill, New Delhi)

Fig.11shows the time dependence of crystallization unvalcanized rubber at low temperature (by 00C) on the application of different Fixed extensions reveal trends of% change (increase) in density with time extensible application specified. Moderate Extensions produce effects as those observed for the lowering of temperature. For extensions> 100%, however, the rate of crystallization is very high, so that only finals are practically observable.

Melting rubber

1. Beyond this point, add in the temperature gives a linear plot much more in tune with the volume thermal expansion of amorphous rubber.

1. Fig.12 "melting curve" that showed increased Fig. 13: Melting curve showing a plot

Specific Volume (cm3 / g) vs. temperature (0C) volume relative to the elevation of temperature for natural rubber polyethylene.

(Courtesy: Tata McGraw-Hill, New Delhi)

The curve of polyethylene highly crystalline polymer melting characteristic shows a significant variation in volume and temperature of the beginning and end of the fusion process is limited and generally in the range of 100C or more precisely, within a period of 50C. If after the merger of rubber the temperature is lowered again, Fig. 12, the volume contraction linear rubber continuous amorphous at temperatures much lower and melting curve is not traced in the opposite direction simply because measurable recrystallization can not occur in time - duration of the experiment. For the very crystallizable polymer, polyethylene, however, the melting and crystallization / recrystallization process is largely reversible practical meaning and the recrystallization curve is essentially a trace of the melting curve, Fig. 13 from the opposite direction.

For the polymer amorphous natural rubber, while the merger occurs on a wide range of temperature, the onset of melting and the temperature range over which the fusion process is accomplished and completed are also largely dependent on the temperature at which crystallization has been done earlier. Usually, the melting starts at a temperature which is 4-higher than the temperature at which crystallization has been accomplished earlier 60C, Fig. 14.

Fig. 14: Graph showing the dependence of the melting range of natural rubber in the temperature of crystallization, the diagonal line below the melting (zone gray) indicates that the crystallization temperature. (Courtesy: Tata McGraw-Hill, New Delhi)

1. Thus, it is possible to have simultaneous melting or recrystallization in a row and a given piece of rubber as it is slowly heated over the melting (range of the hatched area in Figure 14.) after initial crystallization and then held at a specific temperature in this merger () temperature range.

Polymer Single Crystals

Single crystals of various crystallizable polymers can be easily grown by slow cooling and precipitation from very dilute solutions. They appear as patches or very thin slices, usually diamond-shaped with a spiral pattern of growth and show it - such as training in the area.

The crystals are very small and can be considered by X-ray diffraction However, they can be easily and conveniently studied by electron microscopy. Electron diffraction pattern and electron microscopy reveal some interesting features on polymer single crystals. The thickness of slices is very low (100 - 200 A) compared the usual length of the polymer chain. The diffraction pattern shows with no uncertainty that the chain axis is perpendicular in terms of slices. The structural model of the crystal is well understood on the basis of the theory well known folded chain. This theory provides that a single polymer molecule must bend or fold back and forth many times through the thickness of slices. These channels are easily folded stacked in the crystal lattice with ease. It is widely accepted that the single crystal comprises an array of folded chains packaged individually and in succession between the top and bottom surfaces or planes and edges of boards more and more as schematically Fig. 15.

Fig. 15: folding to yield the polymer chain single crystal (schematic)

This type of structure-oriented crystals or involving all individual polymer molecules discreetly, without the interposition or interference of other molecules is apparently made possible by large distances that exist to separate individual molecules ideally in very dilute solution, Fig. 16. The wide - the distance of separation ensures the elimination practice of chain entanglements. Therefore, when a segment of a polymer molecule is attached to one edge of the thin crystal growing, it faces almost no competition with other molecules far to the occupation of the nearby site adjacent truss. There will be little impediment to the successive occupation of sites immediately adjacent segments of the same molecule by a folding mechanism of the chain which will continue until the entire molecule is prepared and willing and face in the folds.

Fig. 16: Separation between molecules polymer chain (a) very dilute solutions and (b) concentrated solution (schedule). (Courtesy: Tata McGraw-Hill, New Delhi)

Structure polymers in bulk

Crystalline polymers obtained cooling their background also produce electron microscope showing the structure of lamellae crystallites and for providing little direct evidence of the presence of large amorphous regions. An idealized model of the structure lamellae as in Fig. 17 (a) is probably far from the real state of things and it may not be applicable to all types of polymers. Most polymers other than polyethylene (HDPE and LDPE) contain amorphous regions in the proportion of 20 - 50% or more distributed in the material and the crystalline fields. In the structural model for a real system, a provision must be made to accommodate the amorphous material. In a fringe - Or fringed micelle - model crystallite, Fig. 17 (b), disorientation, fractions of amorphous materials are shown interspersed of crystallites distributed randomly positioned. This model explains and shows morphological features characteristics of materials such as rubber and cellulose polymers or some other non-crystalline or semicrystalline with a pattern of isotropic property. For the different polymers intermediate control of crystallinity, the random mixture model fringed micelle and regularly stacked lamellae model may represent a model of global structure. These structural concepts to the share of flaws commonly encountered, such as interlamellar entanglement, molecular loops of various sizes, irregular times and lengths of interconnecting channels passing through different slices.

Fig. 17: Schematic representation of (a) ideal stacking of lamellar crystals (discrete folded chains), (b) fringes - micellar model showing distributed Random amorphous and crystalline areas, and (c) Model interlamellar amorphous. (Courtesy: Tata McGraw-Hill, New Delhi)

A model consisting of a stack of lamellae spacing and connected by amorphous regions may be called the amorphous interlamellar model, Fig. 17 (c). This unique provides the most useful approach to understanding the profile of mechanical properties of polymers in bulk crystallized Moderate to high crystallinity. The different degrees of ductility and cohesive character, they are direct consequences of the existence of interlamellar links. A like piles of bricks without clay and sand - infill mortar cement, stacks of lamellae (crystals) without the existence of interlamellar tie molecules such as those obtained by slow cooling of a very dilute solution, may be relatively fragile and brittle. The molecules attach to reduce vulnerability and infuse ductility and stability.

Spherulites

Characteristic the most distinctive, prominent and common crystalline bulk (melt cooled) polymers is the development of spherulites is mean spherical crystallites. A-zed spherulites is characterized by a symmetrical structure built - up which can result in a growth cooperatives chain segments, called crystallites oriented radially outward from a core group or three-dimensional Fig. 18. Bulk crystallized polymers are, in fact, not simply a series of stacked plates separated and connected by amorphous regions; units lamellae are closely arranged radially in the spherulites. The crystallization process through which the spherulites are formed following steps successive starting with nucleation. The nucleation process may be helped by deliberate addition of a foreign substance, called agent nucleation. The nucleating agents by their presence, reduce the size of the spherulites by increasing the number of nuclei. The growth of large spherulites contributes to an increased fragility.

Fig. 18: state of the spherulites growth for polypropylene [(a) and (b)] and (c) the schematic structure growth of spherulites (radial and branching of lamellae with a magnified section shows folding chain perpendicular to the radius spherulitic). (Courtesy: Tata McGraw-Hill, New Delhi)

It is generally observed that most of polymers continue to densify slowly after growth spherulites is long over. Post - primary crystallization densification occurs in both regions interspherulitic and regions intraspherulitic. The densification due to secondary crystallization up slowly after the primary growth process spherulites leads to thickening of lamellae, as chain segments are gradually removed in amorphous zones. A further consequence of the secondary crystallization is the tendency to increased fragility. All effects on the mechanical and related properties of the polymer are known to be complex and they are largely dependent on many factors including the rate and duration of cooling, annealing, cold - Expandable design - cooling.

Analysis Thermal

The thermal properties of polymers are ideally studied by techniques such as analysis Differential thermal (DTA) and differential scanning calorimetry (DSC). The DTA technique usually allows detection of the thermal response and effects

1. Fig.19: A block diagram of an apparatus for DTA Fig. 20: A typical DTA thermogram showing

thermal variations a crystallizable polymer (scheme)

(Courtesy: Tata McGraw-Hill, New Delhi)

accompany changes in physical or chemical a system of material when it is heated or cooled in a programmed manner through a transition zone, phase change, chemical transformation or decomposition. It enables location and extent of the glass transition temperature, Tg, the crystallization temperature (Tc) the (crystal) Melting Point (TM), and temperatures of thermal oxidative degradation, crosslinking and other reactions. Figures 19 and 20 respectively show a block diagram of a DTA equipment and schematic representation of a DTA thermogram.

In practice, the material sample and a thermally inert reference material placed in the respective cell of DTA are heated programmatically. any significant physical or chemical change in the test material at a specific temperature, which is the characteristic the subjects in study, is generally associated with thermal change leading to a noticeable difference in temperature (? T) between test and reference material held at the temperature of the oven.? T is recorded as a function of temperature T. Because no thermal change / transition in the sample test? T remains almost unchanged (constant). In DTA, the correlation between the two? T and the energy changes during a specific transition or transformation (reaction) is uncertain and unknown making the conversion of peak areas endotherm or exotherm energies also uncertain. However, the DTA technique is applicable to virtually all polymers and materials of many other systems, revealing in most cases, qualitative information on thermal effects giving clear indications of the transition (endothermic or exothermic) temperatures, Fig. 20. The technique is often unsuitable for quantitative measurements of parameters such as heat capacity, heat fusion or heat of crystallization (for crystallizable polymers) or change in specific heat associated with glass transition of amorphous polymers; quantitative measures, however, are not using differential scanning calorimetry (DSC). In DSC, sample testing and documentation reference are heated by separate units for individual adjustments. The power or energy inputs to electrical appliances such Heat is continually monitored and adjusted accordingly to any thermal effect in the test sample in such a way as to maintain both at identical temperatures. The power differential or thermal energy necessary to achieve this state of affairs is recorded as a function temperature programmed system. For the transition involving the latent heat, as for fusion, heat of transition (merger) is determined by integration of the heat () of energy intake over the time interval corresponding to the transition in question.

Different polymers decompose over different ranges of temperature to release a bit volatile and leaving some residue. Thermogravimetric analysis (TGA) is a technique analysis useful for recording a weight loss or weight retained a sample depending on the temperature, which may then be used for understanding the chemical nature of the polymer. Parallel to the analysis of volatiles released and the residue left behind TGA provides information on the thermal stability and decomposition of the material in an inert atmosphere or in air or oxygen and the moisture content and other volatile or plasticizer, ash content and extent of recovery of crosslinked polymers. The test sample is placed in an oven while it is suspended from one arm of a precision balance. The TGA thermograms were obtained by recording changes in the weight of the test sample as it is held at a fixed temperature or as it is heated in a dynamic manner programmed. thermograms TGA of some selected polymers are presented in fig.21.

Fig. 21: TGA thermograms of some selected polymers

(Courtesy: Tata McGraw-Hill, New Delhi)

References

1. Ghosh, P. Polymer Science and Technology - plastic rubbers, blends and composites, 2nd ed., Tata McGraw Hill, New Delhi, 2002.
2. Hiemenz, PC Polymer Chemistry - Basic concepts, Mercel Dekker, New York, 1984.
3. Billmeyer, Jr., FW Text Book of Polymer Science, 3rd ed., Wiley - Interscience, New York, 1984.
4. Schmidt, AX, and Marlies CA, Principles of High Polymer - Theory and Practice, McGraw-Hill, New York, 1948.
5. Mandelkern, L., Crystalization of Polymers, McGraw-Hill, New York, 1964.
6. Wood, LA, Advances in Colloid Science, H. Mark and GS Whitby, Eds. Wiley Interscience, New York 1946, Vol. 2, pp. 57 to 95.
7. Bekkedahl, N. and LA Wood, Eng. Chem. 23 (1941) 381.
8. Geil, PH Polymer Single Crystals, Interscience, New York, 1963.

Selected Readings

1. Maiti, S., Analysis and characterization of polymers, Anusandhan Pub., Midnapore,

2003.

2. Turi, EA Ed, Thermal Characterization of Polymeric Materials, Academic Press,

New York, 1981.

3. Fried, JR Polymer Science and Technology, Prentice - Hall, Englewood Cliffs, 1995.

4. Treloar, LGR, Introduction to Polymer Science, Wykeham Pub., London, 1970.

About the Author

Lec 28 | MIT 3.091 Introduction to Solid State Chemistry