Hi folks,
In the previous post, we have seen about sublimation science, which happens in primary drying. Now lets go for establishing the conditions for sublimation.
It is well known that primary drying shall be done below the critical temperatures.The critical temperature is nothing but the Collapse temperature. Determination of the critical collapse temperature of a product is an important step in establishing and optimizing a freeze drying process. This critical temperature determines the maximum temperature that the product can withstand during primary drying without it melting or collapsing.
The critical temperature depends on the nature of the formulation.
Crystalline products have a well defined “eutectic” freezing/melting point that is its collapse temperature.
Amorphous products have a corresponding “glass transition” temperature and they are much more difficult to freeze dry. The collapse temperature of amorphous products is typically a few degrees warmer than its glass transition temperature.
Usually, the critical temperatures are identified by thermal analysis of the formulation. Currently, major techniques that are using in Differential scanning Calorimetry (DSC) & Freeze Dry Microscopy (FDM).
In the current post, we will have a look at utilization of Differential scanning Calorimetry (DSC) to identify the critical temperatures.Before going to know how Tg is determined by Differential scanning Calorimetry, it makes sense to know basics of it.
Differential scanning calorimetry (DSC) is one of the popular and most used thermo-analytical techniques. Thermo= Heat, Analysis= detailed examination. Means thermo-analytical techniques like DSC, are analytical techniques that examine the properties of materials which change with temperature.
A typical Differential scanning calorimeter set up in a laboratory is provided below.
Before going to know about DSC thermograms and their interpretation, lets go through some key terminology which helps to understand the DSC curves.
How the sample is loaded in DSC:
Please refer the link provided https://youtu.be/aTWVCfRlMX8
How the result from a Differential scanning calorimeter represented:
Differential Scanning Calorimetry (DSC) is a method involving the measurement the difference of heat flow between a tested sample and a reference sample which is generated by the temperature control system.
The result of calorimetric measurements is a DSC curve shown as the temperature/time dependence on the heat flux (per time unit) and is provided below.
After sample was loaded and scanning ( Heating or cooling at a controlled rate) was done and no transformations/reactions occurs in the sample then, a 'baseline' forms and is represented below.
In the above example plot of heat flow versus temperature for a material that does not undergo any changes during the heating.
Any material when subjected to heat i.e., heat is given to the material or taken away from the material, then those changes are captured in DSC curve as represented below.
An endo- and exothermic peaks are recorded on these curves, which result from the temperature differences between a tested sample and a reference sample, showing negative or positive deviations from the baseline.
Now lets have a look on various changes in materials which are either endothermic or exothermic.
Endothermic events:
Melting
Sublimation
Desolvation chemical reactions
Exothermic events:
Crystallization
Decomposition chemical reactions
Now, guess what kind of change a glass transition is???????
Actually glass transition is a shift in the baseline rather than an event that slightly endothermic in nature.
Glass transition is physical state conversion of glassy material to a rubbery material.
Glass transition temperature is the temperature, below which the conversion of glassy material to a rubbery material.
At this point the mechanical properties of the material change from those of an elastic material to those of a brittle one due to changes in chain mobility.
A typical example of a heat flow versus temperature plot at a glass transition temperature is shown below.
The glass transition results in a kink in the heat versus temperature plot due to the change in heat capacity (A).
In a plot of heat flow versus temperature it is a gradual transition that occurs over a range of temperatures (B).
The glass transition temperature is taken to be the middle of the sloped region. The temperature in the middle of the inclined region is taken as the Tg.
The heat capacity Cp of material is usually higher above Tg.
The value of the glass transition temperature depends on the strain rate and cooling or heating rate, so there cannot be an exact value for Tg.
In the previous post, we have seen about sublimation science, which happens in primary drying. Now lets go for establishing the conditions for sublimation.
It is well known that primary drying shall be done below the critical temperatures.The critical temperature is nothing but the Collapse temperature. Determination of the critical collapse temperature of a product is an important step in establishing and optimizing a freeze drying process. This critical temperature determines the maximum temperature that the product can withstand during primary drying without it melting or collapsing.
The critical temperature depends on the nature of the formulation.
Crystalline products have a well defined “eutectic” freezing/melting point that is its collapse temperature.
Amorphous products have a corresponding “glass transition” temperature and they are much more difficult to freeze dry. The collapse temperature of amorphous products is typically a few degrees warmer than its glass transition temperature.
A eutectic is an intimate physical mixture of two or more
crystalline solids that melts as single pure compound. So at Eutectic temperature, is the temperature where the mixture melts. When the solids melt and ice get sublimed, the melted solutes will remain and that is called collapse or melt back.
In Amorphous system, glass transition temperature corresponds to a
change in the viscosity of solution from a viscous liquid to a glass or an
essentially solid solution of solute in water.
So, Eutectic temperature is the critical temperature for crystalline systems. For Amorphous systems, critical temperature is few degrees warmer than glass transition temperature.
In the current post, we will have a look at utilization of Differential scanning Calorimetry (DSC) to identify the critical temperatures.Before going to know how Tg is determined by Differential scanning Calorimetry, it makes sense to know basics of it.
DIFFERENTIAL SCANNING
CALORIMETRY (DSC)
Differential scanning calorimetry (DSC) is one of the popular and most used thermo-analytical techniques. Thermo= Heat, Analysis= detailed examination. Means thermo-analytical techniques like DSC, are analytical techniques that examine the properties of materials which change with temperature.
Differential scanning calorimetry (DSC) measures enthalpy changes
in samples due to changes in their physical and chemical properties as a
function of temperature or time.
Enthalpy is defined to be the sum of the internal energy E plus
the product of the pressure p and volume V. Using the
symbol H for the enthalpy:
H = E + p * V
( Enthalpy is similar to energy, but not the same. When a
substance grows or shrinks, energy is used up or released. Enthalpy accounts
for this energy. Because of this, scientists often calculate the change in
enthalpy, rather than the change in energy.)
DSC is a technique in which the difference in the amount of heat
required to increase the temperature of a sample and reference are measured as
function of temperature. Both the sample and reference are maintained at nearly
the same temperature throughout the experiment.
Principle of DSC:
Differential scanning calorimetry (DSC)
measures the difference between the heat flows from the sample and reference
sides of a sensor as a function of temperature or time.
When a sample undergoes a physical transformation such
as a phase transition, more or less heat will need to flow to it than to the
reference (typically an empty sample pan) to maintain both at the same temperature.
Whether more of less heat must flow to the sample depends on whether the
process is exothermic or endothermic.
For example as a solid sample melts to a liquid it will require more heat flowing to the sample to
increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as
it undergoes the endothermic phase transition from solid to liquid.
If a sample undergoes exothermic processes such as crystallization, less heat is required to raise the sample temperature by observing the difference in
heat flow between the sample and reference, DSC is able to measure the amount
of heat absorbs or release during such transition.
A typical Differential scanning calorimeter set up in a laboratory is provided below.
The various kind of sample pans/ crucibles for using in DSC are provided below.
The Differential scanning calorimeter and its componets shown in a block diagram
Before going to know about DSC thermograms and their interpretation, lets go through some key terminology which helps to understand the DSC curves.
Heat flow:
The heat flow is the amount of heat supplied per unit time. Heat flow = heat(q) / time (t) = q / t
This is the parameter that is plotted on Y-axis against temperature on X-axis of result of DSC which is represented in DSC curve i.e., thermogram.
This is the parameter that is plotted on Y-axis against temperature on X-axis of result of DSC which is represented in DSC curve i.e., thermogram.
Endothermic Event:
It is a thermal event of a material where energy is
absorbed by the material, i.e. melting.
Exothermic Event :
It is a thermal event of a material where energy is
expelled by the material, i.e. crystallization.
Glass Transition (Tg):
It is an endothermic event, a change in heat capacity that
is depicted by a shift in the baseline. It is considered the
softening point of the material or the melting of the
amorphous regions of a semi-crystalline material.
Scanning:
Heating or cooling at a controlled rate.
How the sample is loaded in DSC:
Please refer the link provided https://youtu.be/aTWVCfRlMX8
How the result from a Differential scanning calorimeter represented:
Differential Scanning Calorimetry (DSC) is a method involving the measurement the difference of heat flow between a tested sample and a reference sample which is generated by the temperature control system.
The result of calorimetric measurements is a DSC curve shown as the temperature/time dependence on the heat flux (per time unit) and is provided below.
After sample was loaded and scanning ( Heating or cooling at a controlled rate) was done and no transformations/reactions occurs in the sample then, a 'baseline' forms and is represented below.
In the above example plot of heat flow versus temperature for a material that does not undergo any changes during the heating.
Any material when subjected to heat i.e., heat is given to the material or taken away from the material, then those changes are captured in DSC curve as represented below.
An endo- and exothermic peaks are recorded on these curves, which result from the temperature differences between a tested sample and a reference sample, showing negative or positive deviations from the baseline.
Now lets have a look on various changes in materials which are either endothermic or exothermic.
Endothermic events:
Melting
Sublimation
Desolvation chemical reactions
Exothermic events:
Crystallization
Decomposition chemical reactions
Now, guess what kind of change a glass transition is???????
Actually glass transition is a shift in the baseline rather than an event that slightly endothermic in nature.
Glass Transition:
Glass transition is physical state conversion of glassy material to a rubbery material.
Glass transition temperature is the temperature, below which the conversion of glassy material to a rubbery material.
At this point the mechanical properties of the material change from those of an elastic material to those of a brittle one due to changes in chain mobility.
A typical example of a heat flow versus temperature plot at a glass transition temperature is shown below.
The glass transition results in a kink in the heat versus temperature plot due to the change in heat capacity (A).
In a plot of heat flow versus temperature it is a gradual transition that occurs over a range of temperatures (B).
The glass transition temperature is taken to be the middle of the sloped region. The temperature in the middle of the inclined region is taken as the Tg.
The heat capacity of the material is different before and after the glass transition temperature.
The heat capacity Cp of material is usually higher above Tg.
The value of the glass transition temperature depends on the strain rate and cooling or heating rate, so there cannot be an exact value for Tg.
When a certain amount of heat is transferred to the sample, its
temperature increases by a certain amount, and the amount of heat it takes to
get a certain temperature increase is called the heat capacity. (The heat capacity (Cp) of a system is the amount of heat needed to raise its temperature 1◦C. )
Glass Transitions characterized by change in heat capacity (no heat absorbed or evolved).
* Transition from a disordered solid to a liquid
* Appears as a step (endothermic direction) in the DSC curve
* Gradual enthalpy change may occur, producing an endothermic peak superimposed on the glass
transition.
(The glass transition (Tg) has been called the “melting of
amorphous material” and as unscientific as that is, it's an adequate
description. Amphorus material such as glass has no organization in the solid
state – it is random. This gives it the transparency that glass has, among
other properties. As you warm it up, its heat capacity increases. At some point
you have enough energy in the material that it can be mobile. This requires a fair amount of energy compared to
the baseline increase, although much
less energy than the melting point does. This energy normally appears as a step
change in the instrument baseline – pointing up in heat flow instruments and
down in heat flux.)
For better understanding, few DSC curves showing glass transition temperature from Literature are provided below.
It is important to note that the transition does not occur suddenly at one unique temperature but rather over a range of temperatures.
That's about determination of glass transition through DSC.
Till then, Bye...Bye.....
Your's
Teja Ponduri
