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polymer science

Introduction: Polymer Morphology

Two different states or forms can be identified in a polymer that can display or thermo mechanical properties that may be associated with solids, ie., the shape of a crystal or the form of a glass. It is not really the case that all polymers are able to crystallize. As a matter in fact, a high degree of molecular symmetry and microstructural regularity within the polymer chains is a prerequisite for crystallization to occur. Even in polymers that are crystallized in any case, the ultimate degree of crystallinity developed is mainly less than 100%.

Studies of the physical form, available and structure of molecules or molecular aggregates of a material refers to what is known as morphology. Polymer morphology covers the study of the arrangement of the macromolecules in the crystalline and amorphous regions and overlapping physical grouping general molecular aggregates.

When cooled from the liquid state, different polymers show different tendencies to crystallize at different rates depending on many factors, including conditions in physical, chemical nature of the repeat units and polymer as a whole, its symmetry segmental molecular and structural regularity or irregularity, referred to above. independent groups large or branches of different chain lengths hinder molecular packing and therefore crystallization. The nature of the crystalline state of polymers is not simple and should not be confused with the regular geometry of crystals of low molecular weight compounds such as sodium chloride or benzoic acid. There are polymers that are large and amorphous, and very poor tend to become ordered structures or oriented to cool to room temperature near or even below. natural or synthetic rubbers and glassy polymers such as polystyrene, acrylate and methacrylate polymers belong to this class.

In a crystalline polymer, a polymer chain since there or passed through several crystalline and amorphous areas. Areas crystal alignment are formed by intermolecular and intramolecular and orderly and therefore close together arrangement of molecules or segments of the chain, and lack results in the formation of amorphous zones.

Glass transition and melting transition

On the basis of tracking changes in a parameter of the mechanical properties such as shear modulus to changes (increase) in the temperature of the observation polymer material systems can be easily observed on – (i) glass transition and (ii) the merger phenomena transition more easily from a graphical representation, and may also have a measure of the glass transition temperature, Tg and the melting temperature, Tm.

The glass transition and melting transition can also be observed and checked in a parcel Specific Volume (VSP) versus temperature. Consider the various possibilities as a melt is cooled from position A to a high temperature corresponding to a relatively high value VSP well, fig. 1. ABDG The path shows how the specific volume drops down like a low molecular weight compound freezes. As the melting temperature Tm is reached at point B, a sharp discontinuity is observed in VSP (DB). The slopes and the Directorate AB General measures to give coefficients of thermal expansion of liquid and solid, respectively. The coefficient of thermal expansion also undergoes a discontinuity in MT.

Figure 1: Diagram highlighting the potential changes in specific volume (PSV)

of a polymer with the change temperature.

We, however, can start with a molten polymer material in A and observe the volume changes as described by the ABHI and there is no way notable discontinuity in metric tonnes. The liquid line AB is extended beyond Tm, with decreasing temperature and is undergoing a change in slope at a much lower temperature, Tg, and finally becomes a different linear portion (HI) of a much lower slope constant. Here, in fact, the slope change occurs in a small temperature range (which can range typically around 5 – 100C), but extrapolation of two linear parts allows assessment Tg right by this method. The HI area represents the glassy state that occurs as the glass transition temperature is reached or has just crossed when going down the temperature. transition to the glassy state is also commonly known as vitrification. The region represents the BH existence of a super-cooled liquid or paste state of relatively poor dimensional stability, even under the influence of a low voltage.

For all polymers, the crystalline state always reaches finally cooling, regardless of the polymer being tested is crystallizable or not. Even situations that favor the formation of crystals, it does not necessarily mean that the crystallization occurs rapidly or completely. It remains in most cases a significant part of the amorphous zones after the primary crystallization process is complete.

The ABCEFG path in Fig. 1 represents For some crystalline, partly amorphous polymer system. The cooling of Tm, crystallization starts and the discontinuity feature VSP is evident despite the clarity in which Tm is revealed is not as pronounced for polymers as for a compound of low molecular weight and this can be seen from the curvature of the part of the route BCEF. For this system, glassy FG represents the area and a BA from the merger or liquid zone and the zone BCEF is liquid cooled and much of the amorphous rubber (super) area. Point F, where the slope between the segments EF and FG changes corresponds to the glass transition point, Tg, and the polymer in this case remains, with large and amorphous. If the partial crystallization would occur in the cooling below Tm, the amorphous content decreases and in that case, the change in the slope at Tg can be much smaller and harder to detect.

The ABJK path may appear as a variant of the path ABHI and here, AB describes the liquid, super cooled liquid BJ or state of glue and JK describes the glassy state. The path of change under the condition ABJK ABHI greater cooling rate is likely to Tg also moved to a higher temperature (Tg?) for a faster cooling rate.

Thus, the temperature response of linear polymers can be seen as divided into three segments clearly different

1. Above Tm:

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

2. Between Tm and Tg:

This domain may vary among groups close to 100% crystalline and almost 100% of the amorphous molecular chain of the polymer based structural regularity and experimental conditions. The amorphous part behaves like a super cooled liquid in this segment. The general behavior of polymer physics in the intermediate segment is very similar to a paste.

3. Below Tg:

The polymer material is seen as a tough and rigid glass, showing a coefficient of thermal expansion specified. The glass is closer to a solid to a liquid crystalline pattern of behavior in terms parameters of mechanical properties. In terms of molecular order, however, glass is more like liquids. There is little difference between cross-linked polymer and linear below Tg.

The location of Tg depends on the cooling rate. The location of MT 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 is higher than the rate crystallization, there may be no observable change in MT, even for a crystallizable polymer.

The simple device that tracks volume changes by cooling or heating is called a dilatometer, with a glass ampoule or blisters on the bottom fitted with a narrow gap in the capillary top, as in Fig. 2. A dilatometer can also be used to study the progress of the reaction with time at a given temperature for the next contraction volume of liquid monomer system (the polymer is formed with a higher density than the monomer is polymerized). In studies with a polymer that, 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 temperature constant for a phase change, as in t) are then recorded as a thermometer. The expansion / contraction of mercury due to temperature change must be duly considered during the testing of a change in volume of the polymer sample. The experiments should be carried out by placing the dilatometer in the bathroom thermostat. The sample must be immiscible with the fluid movement and degreasing to remove trapped air. Specific volume – temperature plot for polystyrene shows a clear change in the slope at 95.60C, indicating the glass transition temperature, fig. 3.

1. Figure 2: An agreement to dilatometric Fig. 3: Temperature dependence of

measurement of the change in the volume of a specific volume of polystyrene indicates

1. glass transition temperature, Tg.

(Courtesy: Tata McGraw-Hill, New Delhi)

Therefore, it is an experience common increase or decrease in temperature and the application or removal of stress, greatly influences the physical structure and properties of polymers. With the change of the temperature of a high polymer material passes through two different transitions characterized by (mp) oi first order transition, denoted by Mt and (ii) the transition from glass or second order phase transition, denoted by Tg.

Point Melting or Transition First Order

Fusion of a crystalline solid or a liquid is boiling associated with the change of latent heat and participation. Many high polymers possess sufficient molecular symmetry and exhibit / or structural regularity that crystallize enough to produce a phase transition solid-liquid, a crystalline melting point. The merger is very strong for some polymers such as nylon, while than in most other cases of different rubber and polystyrene, etc., the phase change takes place through a range of temperature. phase transitions of this type, especially in low molecular weight materials, is 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 the first derivatives of the free energy are commonly referred to transitions in the first place. Although we observe the casting of a true first order transition or fusion ideal high frequency polymer absent or missing, in view of the molecular weight distribution and entanglements of the molecules in the chain that leads to the complex phenomenon of delayed or viscoelastic flow.

Glass transition or second order phase transition

glass transition or second order phase transition is a phase transition and almost all high-polymer or polymer material is characterized by a transition temperature specific glass (Tg) or transition point for second (SOTP), which appears below their (crystal) melting point, Tm.

At Tg, the thermodynamic property parameters S, V and H are only subject to changes in the slope when plotted against temperature, however, showing strong discontinuity as shown in the case of first order transitions, as the plot is shown in the idealized figure. 4.

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

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

January 1?

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

V V? T

isothermal compressibility K

January 1

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

V V

which are the second derivatives of the free energy and for this reason that the glass transition temperature, Tg is commonly known as the second transition temperature, fig. 5. Refractive index (R1) also shows an abrupt change in point glass transition (Tg).

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

The glass transition is a phase transition and therefore, it is no latent heat. Below this temperature normal Rubber – Polymers and lose flexibility and turn rigid, durable and dimensionally stable and are then seen in a glassy state, while above this temperature, polymers usually all stiff, stiff, hard turn glassy smooth and flexible, to be subject to cold flow or creep and as perhaps in a state of rubber. The difference between the rubber and glassy states actually lies not in its geometrical structure, but in the state and the degree of molecular motion.

Below the glass transition temperature, Tg, the chain of segments or groups, as part of the molecular chain backbone, may be limited rates of vibration, but have not the energy required to rotate about the bonds and change of position on chains.At segments of the neighbors or slightly above Tg, provides rotation, in particular side groups or branch units, and may only short-range molecular segments instead of the molecule polymer high all would be at this point. The much higher coefficient of thermal expansion beyond Tg is indicative of a greater degree of freedom both rotation.

In the respective glass transition temperatures of transition or second order, different polymers can be seen to be in a state isoviscous, and, indeed, Tg is a common reference point for polymers of various kinds, under which all behave as rigid plastics, solid (crystalline polymer) and above which are leather and rubber in nature. As we understand, a useful rubber is a polymer has a Tg well below room temperature, while a useful plastic is one whose Tg is well above room temperature. Table 4.1 lists the Tm and Tg values of some common polymers.

Table 1: Tm and Tg values of polymers Miscellaneous

Polymer

Repeat Unit

T, 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, 4 cis), (NR)

– CH2 – C (CH3) = CH – CH2 –

14

– 73

Poly (dimethyl siloxane)

– OSI (CH3) 2 –

– 85

– 123

Poly (vinyl acetate)

– CH2 – CH (OCOCH3) –

—

28

Cop (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-NCOA-(CH2) 4CO –

264

53

Poly (ethylene terephthalate)

– O (CH2) 2 – OCO – (C6H4) CO –

254

69

Poly (ethylene adipate)

– O (CH2) 2 – UCO – (CH2) 4 CO –

50

-70

Molecular weight and molecular weight distribution, stress or external pressures, the incorporation plasticizer, copolymerization, fillers or fiber reinforcements and cross linking are some of the most important factors that influence the transition temperature glass, melting point or heat – the temperature of deformation of a polymer matrix. The comparative reduction of Tm and Tg of change plasticization of polymers by external (incorporating plasticizer) and internal lamination (comonomer incorporation) shown in fig. 6. In general, copolymerization comonomer incorporation is, is more effective than external plasticizing point reduction fusion, while the latter process (the addition of external plasticizer) is more effective than the former (copolymerization) in reducing the temperature glass transition. crosslinking causes significant UpRise Tg, as cross-links hinder rotation of the elements of the chain, which requires a higher temperature for the start of the rotation of segments between crosslinks. Similarly, high molecular weight, resulting in complex, long-chain tangle-wide, reduce the potential for segmental rotation and this causes an increase in the value of Tg with a remarkable effect intermittent deadlock over molecular weight 105.

Fig. 6: Schematic showing the plots relative decline in Tm and Tg of a polymer incorporating separate (A) an external plasticizer.and (B) a comonomer for copolymerization. (Courtesy: Tata McGraw-Hill, New Delhi)

The weak point

A polymer is also characterized by a temperature known as brittle or brittle temperature pt.1 (TBR), which is close to or slightly higher than its glass transition temperature (Tg) for polymers higher. When the temperature of the polymer in its rubber floor, flexibility and elastic properties was lost and the polymer stiffens and hardens, at an intermediate stage, called point temperature fragile is achieved at or below which the polymer sample becomes brittle and breaks or fractures in the sudden application of load.

For comparison weak points of different polymers, it is necessary to test under special conditions, including the size and thickness of the sample is specified, the degree and speed cooling, so the proof is empirical in nature. The fragile point corresponds to a temperature at which the time interval of application of the load matches only equal to or necessary for the sample to undergo the necessary deformation. At a lower temperature, the sample can not deform as rapidly, and therefore can not bear the burden and therefore breaks, above the temperature fragile, the time of application of the load is more than enough for the specimen to absorb energy applied and deform to fracture or break away. Low molecular weight limits the possibility of long-range molecular interactions and the entanglements of the chain and therefore leads to a higher temperature brittleness. Changes in Tg and Tbr polymer molecular weight, as shown schematically in Fig. 7 shows clear that the trends of change for the two parameters are just the opposite. The difference is much narrower in the higher molecular weight range, but becomes progressively larger with decreasing the molecular weight.

Fig. 7: Typical plots showing the weak temperature dependence (TBR) and glass transition temperature (Tg) in the polymer molecular Presentation.

(Courtesy: Tata McGraw-Hill, New Delhi)

Development of crystallinity in polymers

Morphological studies of polymers are mainly related to molecular patterns and the physical state of crystallizable polymer crystalline regions. Amorphous, semi-crystalline and crystalline polymers are known prominent. It is difficult and may be practically impossible to reach 100% crystallinity in polymers in bulk. It is also difficult according to different microscopic tests to obtain amorphous solid polymers completely devoid of any molecular or segmental order, or crystallinity oriented structures. A wide range of structures, ranging near total disarray, the different types and degrees of order and order almost complete, they can describe the physical state of a given polymer system, depending on the test environment the nature of the polymer and its synthesis route, microstructure and music – the sequence of repeating units, and the thermomechanical history of the sample test. Moreover, data collected for the degree of crystallinity may also vary depending on the method of test used. The degree of crystallinity data shown in Table 2 should therefore be taken as approximate.

The polymers that show degrees of crystallinity> 50% are commonly recognized as crystal. The cellulose (Cellulose acetate) and regenerated cellulose (viscose) fibers are used as degrees of crystallinity lower than that of native cellulose, the fiber base. Chain molecules predominantly linear high density polyethylene (HDPE) have a degree of crystallinity is much higher than any other polymer known (even quite higher than that of low density polyethylene (LDPE). For high density polyethylene, crystallinity possible grade is close to the upper limit (100%). atactic polymers in general (including methyl methacrylate and styrene having bulky side groups), with irregular configurations significantly not crystallize under any circumstances.

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

Polymer

Crystallinity (%)

Polyethylene (LDPE)

60-80

Polyethylene (HDPE)

80-98

Polypropylene (GRP)

55-60

Nylon 6 (fiber)

55-60

Terylene (polyester fiber)

55-60

The cellulose (cotton fiber)

65-70

regenerated cellulose fiber (viscose rayon)

35-40

Gutta Percha

50-60

Natural Rubber (crystallized)

20-30

Figure 8 gives a complete picture of the crystallization rate (Change in volume over time) at different selected temperatures. For high density polyethylene (HDPE), when the temperature decreases, the volume of proportional changes crystallization rates are rising rapidly and well below the actual melting point (1270C), the volume change soon becomes so fast that measurements and observations is uncertain and difficult if not 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 at room temperature ie under ambient conditions. sudden cooling or quenching of the melt below room temperature results in a material which is still largely crystalline, although as expected with the probability of a somewhat lesser degree of crystallinity than that developed the normal melting – refrigeration. The reason for this state of affairs is that the time required for crystallization is much shorter than the time required for cooling the polymer sample test.

Fig. 8: Plot of relative volume with time (min) showing the densification of polylethylene in the development of crystallinity at different temperatures specified.

(Courtesy: Tata McGraw-Hill, New Delhi)

For practical reasons, therefore, the process of polymer crystallization is very conveniently studied and measured with confidence with a polymer that is usually amorphous, natural rubber is a polymer of such. The merit of using a rubber material as a model for the study of polymer crystallization is that the crystallization process is slow to allow measurements due to easy handling and takes place at a convenient temperature range.

It is noteworthy that all the rubber (in particular, copolymers) are not crystallizable. Only built characterized by chains chemically identical repeating units and regular, such as natural rubber, 1, 4 and certain grades of polychloroprene cispolyisoprene are capable of crystallization.

Fitting rubber Crystallilzation

If unvulcanized natural rubber (NR) is still downward fixed temperature, eg 00C, it slowly gets a little stiff and hard, and loses its flexibility and softness of proportion. However, the material still retains some degree of flexibility and hardness. The physical change is also observed associated with some improvement in the density or decrease in volume associated changes are the result of a slow development of crystallinity in the material.

Crystallization in a liquid stream of low molecular weight in the cooling or below the freezing point takes place very quickly, as a result of smart and quick rearrangement of a disordered state at a very fair state of packaging. A molten polymer system is, however, much more complicated due to chain entanglements, the restriction of the free mobility of chain segments, and thus impede and delay rearrangement process desired cooling. By gum – as polymers, the crystallization time scale is usually much longer than for liquids low molecular weight materials.

Fig. 9: The densification of the crystallization of natural rubber

plot based on volume time (hours) at different temperatures.

(Courtesy: Tata McGraw-Hill, New Delhi)

Trends of change in the relative volume of natural rubber (NR) over time due to crystallization at different low temperature is shown in Fig. 9. The maximum possible crystallinity and the time needed for this to happen are very dependent observation6 temperature. In each case, the volume contraction rate is relatively slow in the beginning, the volume contraction (or crystallization) sample rate an increasing trend over time, undergoes a constant at upper intermediate time period, and finally unfolds, it disintegrates or stabilizes giving maximum development possible degrees of crystallinity at a given temperature. Lowering the temperature causes improvement in the rate constant of crystallization of the NR-250C to about where the rate constant vs temperature plot, fig. 10 passes through a maximum. Further reduction in the crystallization temperature causes a downward trend in the rate of crystallization constant as in fig.10. The crystallization is (almost) completed in approximately five hours at-250C. In natural rubber, grade / level of crystallinity in the most favorable does not exceed 30%.

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

Crystallization mechanism

As the molten polymer remains at a temperature near or slightly above its melting point, the initial slow accumulation rate of crystallization (crystallization Late) is linked with the initial process of nucleation. The crystal growth depends on the development and existence of a number of very small growth centers or cores deposition direction of the chain segments. Growth centers of their formation process of enlargement of refrigeration or preservation of the melt at the temperature specified by the assembly of a small number of chain segments in the course of their random motion (micro-Brownian motion) in the prevailing situation. Nucleation is, however, common to all processes that turn a homogeneous medium initially in a heterogeneous system following the deposition stage separately.

As the growth is sustained and continued, the adverse effect of entanglements in the chain is increasingly serious and ultimately critical, so provide severe restrictions on the mobility of chain segments and therefore making it difficult for them to reach a position of attachment to any of the crystals formed. Beyond this stage, the crystallization rate decreases significantly and, finally, the process shuts down.

Low temperature favors nucleation thermal energy and lower segments of the chain makes it less likely that once formed a core disappear once again, the net result is an increase in the number nuclei and an increase in the overall rate of crystallization with the gradual decrease in temperature. At increasingly lower temperatures, however, the total energy polymer system including the provision of the chain segments tend to fall while the segments almost seem to lose much of their mobility and therefore their deposit in a core group hampered progressively more efficient and there is a marked trend in dropout rates of crystallization. For natural rubber, the crystallization process was effectively frozen below – 500C, fig. 10.

Stress – induced crystallization of rubber

It is common knowledge and a matter of experience that the stretching of a strip of vulcanized rubber, a storm does develop crystallinity the axial orientation of the molecules in the chain along the direction of stretching and that the effect disappears instantly on the orientational withdrawal of the stretching force. A strip of raw or unvulcanized rubber also develops crystallinity when subjected to high extensions in the application of a stretching force, but remains more or less in the extended state (in view of the absence of cross-links contain) without significant retraction to their original state to release stress. However, when heated carefully in the later stage, as by dipping the test strip slightly warm water (temperature> 300C) the crystals melt and allow the strip to a large reverse As a state without tension.

Cross-links in vulcanized rubber points act as reinforcement and are responsible for the accumulation of shrinkage strong or the restoration of force comes into play to break the stress – induced by the orientation (or crystalline structure) on the withdrawal of the applied voltage. In unvulcanized system, the absence of links between allowing various degrees of unwinding chain if the chain's sliding low / moderate extensions, and anyone it accumulates elastic restoring force is too insufficient or inadequate to break the crystalline structure and induce dimensional recovery. Increased temperature the test strip to 300C or slightly above this level, allows the merging of the crystallites oriented axially, causing the rubber chain molecules to coil and the test strip to retract his initial or near initial (random / Unoriented) of the state.

Fig. 11: time dependence of induced crystallization stress (densification) of unvulcanized rubber held at 00C for different fixed orders indicated extensions, plot of density change (%) vs time (min). (Courtesy: Tata McGraw-Hill, New Delhi)

Fig.11shows time dependence unvalcanized crystallization of rubber at low temperature (00C here) on the application of different fixed extensions revealing the trends of change% (increase) in density with time of application specified range. Moderate extensions produce observed effects to lower the temperature. For extensions> 100%, however, crystallization rates are very high, so that only the later stages are almost observables.

The merger of rubber

1. Beyond this point, further improvement in temperature gives a linear plot in line with the volume thermal expansion of amorphous gum.

1. Fig.12 curve 'fusion' showing increase in the figure. 13: Melting curve showing a graph

specific volume (cm3 / g) vs. temperature (0C) depending on volume temperature for natural rubber increased polyethylene.

(Courtesy: Tata McGraw-Hill, New Delhi)

The melting curve of polyethylene polymer highly crystalline typically shows a strong variation in the volume and temperature of the beginning and end of the melting process is generally confined well within the range of 100C or more precisely, in a span of 50 º C. If after melting the rubber, the temperature is lowered again, fig. 12, the linear volume contraction gum amorphous is much lower temperatures and the melting curve did not turn in the opposite direction, simply because there is no measurable recrystallization in time – space of experience. For highly crystallizable polymer, polyethylene, however, the fusion and crystallization processes are generally recrystallization reversible in a practical sense and recrystallization curve is above all a return of the melting curve, fig. 13 from the opposite direction.

For polymer amorphous, natural rubber, while fusion occurs in an extended range of temperature, the onset of fusion and the temperature range over which the fusion process conducted and concluded also largely depends on the temperature at which crystallization took place earlier. Usually, the melting starts at a temperature that is 4-60C higher than the temperature at which crystallization took place earlier, fig. 14.

Fig. 14: Plot showing dependence of fusion range of natural rubber in the crystallization temperature, the diagonal line below the melting range (shaded area) indicating the crystallization temperature. (Courtesy: Tata McGraw-Hill, New Delhi)

1. Thus, it is possible to have simultaneous and consecutive melting and recrystallization of a particular piece rubber is heated slowly through the melting range (shaded area in Fig. 14) after the initial crystallization and has remained at a specific temperature where the merger () temperature range.

Polymer single crystals

loose crystals of different crystallizable polymers can easily be grown by slow cooling and precipitation from very dilute solutions. Appear as thin plates or well plates, generally diamond-shaped spiral of growth and showing step – training in the area.

Individual crystals are very small in size and can not be examined by X-ray diffraction However, they can be easily and conveniently studied by electron microscopy. Electronic diffraction pattern and electron micrographs reveal some interesting features on polymer crystals. The thickness of the layers is very small (100-200 A) in comparison with the length of polymer chain usual. The diffraction pattern reveals no uncertainty that the axis of the chain is perpendicular to the plane of the sheet. The single crystal structural pattern is well understood on the basis of the theory known folded chain well. This theory that provides a single polymer molecule must be bent or forward and backward times the number of times through the thickness of the plates. These chains are easily folded stacked in the crystal lattice easily. It is widely believed that the single crystal comprises an array of strings packed folded individually and on, between the upper and lower surfaces or planes and edges of the growth plates is shown schematically in Fig. 15.

Fig. 15: folding of the chain to produce single crystal polymers (scheme)

This type of structure oriented to the formation of crystals or participation of any individual polymer molecules quietly without interference or interposition of other molecules is apparently possible thanks to the great distances that exist to separate individual molecules in very dilute solutions ideal, fig. 16. Width – the distance of separation ensures the virtual elimination of chain entanglements. As Therefore, when a segment of a polymer molecule is attached to a thin edge of the crystal grows, it faces virtually no competition from other molecules far to the occupation of the nearby site adjacent network. There will be as little hindrance as the successive occupation immediately adjacent sites by the segments of the same molecule by a folding mechanism of the chain will continue until the entire molecule is extracted and arranged and oriented in the folds.

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

Structure of polymers in bulk

crystalline polymers obtained in the cooling of their products also melts electron microscope showing the lamella structure of crystallites and offer little direct evidence for the presence of the main amorphous regions. An idealized model sheet structure as in Fig. 17 (a) is probably far from the reality of things and can not be applicable to all types of polymers. Most other polymers of polyethylene (HDPE and LDPE) contain amorphous regions to the extent of 20-50% or even more distributed in the material with the crystalline domains. In the structural model for a real system, a provision must be made to accommodate the amorphous material. In a micelle, or fringed – crystallite model – fringed Fig. 17 (b), disorientation, fractions of amorphous material are interspersed between and placed randomly distributed crystallites. This model explains and reveals the characteristics morphological characteristics of materials such as rubber and some polymers of cellulose or other non-crystalline or semi-crystalline isotropic property pattern. For different types of polymer crystallinity intermediate orders, random mix of fringed micelle model and regularly stacked laminae model can represent overall structural model. These structural concepts concessions to the imperfections commonly found, such as interlaminar entanglements, molecular bonds of different sizes, sometimes irregular lengths and interconnection of the chains passing through different layers.

Fig. 17: Schematic representation of (a) stacking lamellar crystals ideal (discrete folded chains), (b) fringes – micelle model are randomly distributed amorphous and crystalline areas, and (c) the amorphous model interlaminar. (Courtesy: Tata McGraw-Hill, New Delhi)

A model consisting of stacks of sheets interspersed and connected by amorphous regions can be called interlaminar amorphous model, fig. 17 (c). This unique model provides the most useful approach for understanding the profile of the mechanical properties bulk polymer crystallized at moderate to high degree of crystallinity. The different degrees of ductility and cohesive character are direct consequences of the existence of interlaminar bonds. Something like stacks of bricks without clay or sand – cement mortar as the interlayer, piles of sheets (glass) without the existence of interlaminar tie molecules as obtained by slow cooling of a very dilute solution, it would be relatively fragile and brittle. Tie molecules reduce fragility and instill ductility and stability.

Spherulites

The most distinctive and prominent feature common thick crystallized (cooled Fusion) polymers is the development of spherulites, ie spherical crystals. A spherulite are characterized by a symmetrical structure construction – until it emerges as a result of the cooperative growth of the segments of the chain of crystallites oriented radially outward call The base or core in three dimensions, fig. 18. Polymers crystallized in bulk, in fact, not just a series of stacked plates separated by regions amorphous film units are closely arranged radially within the spherulites. The crystallization process through which the spherulites are formed the following sequential steps from nucleation. The nucleation process can be helped by the intentional addition of an alien substance known as nucleation agent. The nucleating agents with their presence reduce the size of the spherulites by increasing the number of cores. Large spherulites growth contributes to increased fragility.

Fig. 18: State of the spherulite growth for [polypropylene (a) and (b)] and (c) the schematic structure of a spherulite (radial growth and branching of plates with a magnified portion showing the folding chain perpendicular to the spherulitic radius.) (Courtesy: Tata McGraw-Hill, New Delhi)

What usually shows that the majority of polymers continue to densify slowly long after spherulite growth is complete. The post – crystallization primary densification occurs in both regions and regions intraspherulitic interspherulitic. Densification due to secondary crystallization slowly taking place after the primary spherulite growth process leads to thickening of the plates, because the chain segments are drawn from areas amorphous. One more result of secondary crystallization is the tendency towards increased fragility. The overall consequences on the mechanical and related properties of the polymer is recognized to be complex and highly dependent on many factors including the type and duration of cooling, annealing, cold – the development or stretch – cool.

Thermal Analysis

The thermal properties of polymers are conveniently studied by using techniques such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The DTA technique at general allows the detection of the thermal response and the effects

1. Figure 19: block diagram of an apparatus of Figure DTA. 20: A typical thermogram DTA indicating

thermal changes of a crystallizable polymer (schedule)

(Courtesy: Tata McGraw-Hill, New Delhi)

accompany chemical or physical changes in a material when heated or cooled on a scheduled basis through a transition zone, phase change, chemical transformation or decomposition. Allows the location and measurement of glass transition temperature, Tg, the temperature of crystallization (Tc), the lens) fusion (point (Tm), and temperatures of the thermal and oxidative degradation, crosslinking and other reactions. Figures 19 and 20 show, respectively, a block diagram of a computer DTA and schematic representation of a DTA thermogram.

In practice, the sample material and a thermally inert reference material placed in the respective DTA Cell holders are heated on a scheduled basis. Any physical or chemical change in the test substance at a specific temperature, which is the hallmark of the matter under consideration is usually associated with change heat leads to a noticeable difference in temperature (? T), between the test and reference materials held in the temperature of the oven.? T is recorded as a function of temperature, T. For any thermal change / transition in the sample? T remains almost (constant). In DTA, the correlation between? T unchanged and the energy changes defined in a transition or a transformation (reaction) is uncertain and unknown, so that the conversion of the endothermic or exothermic peak areas of the energy is also uncertain. However, the DTA technique is applicable to virtually all polymers and materials for 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 generally unsuitable for quantitative measurements of parameters such as heat capacity, heat fusion or the heat of crystallization (for crystallizable polymers) or change in specific heat associated with glass transition amorphous polymers; quantitative measures are, however, rapidly carried out using differential scanning calorimetry (DSC). In DSC, the sample and reference material are heated separately controlled units individually. The power or input power to the heaters are controlled and continuously adjusted as a consequence of thermal effect on the test sample so as to ensure the two identical temperatures. Differential power or thermal energy required to achieve this state of things are recorded against the programmed temperature of the system. To transition involving latent heat to the merger, the heat of transition (fusion) is determined by integrating the heat (a) a source of energy than the time interval covered by the transition question.

different polymers decompose in different temperature ranges the release of some volatile compounds, leaving some residues. thermogravimetric analysis (TGA) is a useful technique log analysis for weight loss or weight retained in a test sample as a function of temperature, which can then be used to understand the chemical nature of the polymer. Together with the analysis of volatiles released and the residue left behind, TGA provides information about thermal stability, and decomposition of the material in an inert atmosphere or in air or oxygen and moisture on the content and other volatile or plasticizer content, content ash and the degree of cure for cross-linked polymer. The test sample is placed in an oven while it remains suspended from one arm of a precision scale. The TGA thermograms obtained by recording the change in sample weight, as is done at a fixed temperature or heated dynamically scheduled basis. TGA thermograms of some selected polymers are shown in fig.21.

Fig. 21: TGA thermograms of some selected polymers

(Courtesy: Tata McGraw-Hill, New Delhi)

References

1. Ghosh, P., Science and Technology of Polymers – Plastics, Rubber, 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, Textbook of Polymer Science, 3rd ed., Wiley – Interscience, New York, 1984.
4. Schmidt, AX, and CA Marlies, Principles of polymer high – Theory and Practice, McGraw-Hill, New York, 1948.
5. Mandelkern, L. Crystallization of Polymers, McGraw-Hill, New York, 1964.
6. Wood, Los Angeles, Advances in the science of colloids, H. Mark and GS Whitby Eds., Wiley Interscience, New York 1946, vol. 2, pp. 57 95.
7. Bekkedahl, N. and LA Wood, Ind. Eng. Chem 23 (1941) 381.
8. Geil, PH, Polymer Single Crystals, Interscience, New York, 1963.

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.

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