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Thermal Analysis of Polymers: Strength of DSC By Muhammad Zafar Iqbal P.E. Physical Properties of Polymer
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Page 1: Ppp  Dsc 2 Thermal Analysis Application Of Dsc  Strength Of Dsc

Thermal Analysis of Polymers: Strength of DSC

Thermal Analysis of Polymers: Strength of DSC

By

Muhammad Zafar Iqbal

P.E. Physical Properties of Polymers

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SequenceSequence

• Temperature measurement

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Temperature MeasurementTemperature Measurement

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Previous History RemovalPrevious History Removal

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TgTg

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Glass Transition in PolymersGlass Transition in Polymers• The glass transition of polymers is observed by DSC as

a stepped increase in the heat capacity of the sample during heating due to an enhancement of molecular motion in the polymer. Measurement of Tg of polymers is an important practical application of TA.

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• The glass transition may be difficult to measure for various reasons. Tg of a partially crystalline polymer, such as polyethylene or polypropylene, is difficult to detect by DSC because enhancement of molecular motion in the amorphous regions is restricted by the crystalline regions. Restricted motion of the amorphous regions can be observed using other techniques such as DMA or NMR.

• In densely crosslinked polymers it is hard to observe the glass transition due to restriction of main-chain motion and the sample baseline step occurs over a broad temperature interval introducing a large error in the determination of Tg .

• Owing to widespread intramolecular and intermolecular hydrogen bonding it is difficult to measure Tg of many natural polymers in the dry state.

• In systems composed of several incompatible amorphous polymers it may be difficult to measure Tg of a minor component owing to its relatively low concentration.

• The phase diagrams of multi-component systems are frequently compiled from Tg data measured by DSC and the above difficulties may lead to inaccurate estimates of Tg .

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Enthalpy Relaxation of glassy polymersEnthalpy Relaxation of glassy polymers• Polymers in the glassy state below the glass transition temperature

are not in thermodynamic equilibrium and relax towards equilibrium with time.

• For this reason the experimentally measured enthalpy of glassy polymers decreases as a function of time if the sample is maintained below Tg.

• This phenomenon is called enthalpy relaxation and is monitored through the heat capacity change at the glass transition.

• In the presence of enthalpy relaxation, the mechanical, transport and other physical properties of the polymer vary as a function of temperature and time. Gas diffusion through polymer membranes can decrease by as much as two orders of magnitude owing to enthalpy relaxation at ambient temperature. The stress-strain curves of glassy polymers reveal more brittle behavior as enthalpy relaxation proceeds.

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Glass Transition in the presence of waterGlass Transition in the presence of water• Polymers having hydrophilic components such as

hydroxyl or amide groups form intermolecular bonds in the presence of water which strongly affect the characteristics of the glass transition.

• Main-chain motion is restricted owing to these intermolecular interactions and the glass transition temperature is higher than that of the hydrophilic polymer in the completely dry state or a similar hydrophobic polymer.

• In certain kinds of proteins and polysaccharides no glass transition or melting is observed until decomposition of the main chain occurs because intramolecular and intermolecular hydrogen bonds stabilize the high order structure of these polymers.

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• On the other hand, introducing a small amount of water to a hydrophilic polymer may disrupt the intermolecular bonds, thereby enhancing the main-chain motion. In this case Tg shifts to lower temperatures in the presence of water.

• Hydrophilic polymers stored under ambient conditions contain a certain amount of bound water.

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Molecular Re-arrangement during scanning

Molecular Re-arrangement during scanning

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Bound Water ContentsBound Water Contents• The behavior of water can be transformed in the presence of a

polymer, depending on the degree of chemical or physical association between the water and polymer phases.

• Water whose melting/crystallization temperature and enthalpy of melting/ crystallization are not significantly different from those of normal (bulk) water is called freezing water.

• Those water species exhibiting large differences in transition enthalpies and temperatures, or those for which no phase transition can be observed calorimetrically, are referred to as bound water.

• It is frequently impossible to observe crystallization exotherms or melting endotherms for water fractions very closely associated with the polymer matrix.

• These water species are called non-freezable.

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• Less closely associated water species do exhibit melting/crystallization peaks, but often considerable super-cooling is observed and the area of the peaks on both the heating and cooling cycles are significantly smaller than those of bulk water. These water fractions are referred to as freezing-bound water

• The sum of the freezing-bound and non-freezing water fractions is the bound water content.

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These different water species are illustrated in Figure 5.27, which presents the DSC Crystallization curves for water sorbed on poly(4-hydroxystyrene). The water content isgiven by the mass of water in the polymer divided by the dry mass of the sample, expressed in units of g/g. At the lowest watercontent no exothermic peak is observed. All of the water in the polymer at this waterconcentration is non-freezing water. At a higher water content a freezing exotherm is observed at 225 K whose area is considerable smaller than 333 J/g, which is the enthalpy of crystallization of bulk water. This peak is due to freezing-bound water in the system. At the highest water content a large exotherm is observed in the region of 273 K whose enthalpy of transition is close to that of bulk water. This exotherm is ascribed to the crystallization of the freezing water in the hydrated polymer.