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Differential Scanning Calorimetry _DSC_

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					            Differential Scanning Calorimetry (DSC)




Name: Edward Flood




Student number: 01092669




Lab partner: Gary Shortall




Date: 16.04.’05




                                                      1
                                 Abstract:

The purpose of this experiment was to use Differential Scanning
Calorimetry to find numerous thermal properties of a number of polymer
samples. The credibility of the machine to be used was tested using Indium
metal as a reference material. Indium was used because its melting
temperature is low enough to coincide with the temperature range in which
most of the polymer transitions that apply to this experiment occur. Also, it
has a well-defined melting point with an accepted melting temperature and
enthalpy of melting of 156oC and 28.5J/g respectively. The values obtained
using DSC were 156oC and 21.445J/g, which shows that the enthalpy
measurement is off by approximately 25% and the temperature
measurements are about right. The polymers analysed were as follows:

   1.   Polyethylene (high density) (HDPE)
   2.   Poly(Vinyl Chloride) high molecular weight (high MW PVC)
   3.   Poly(Vinyl Chloride) low molecular weight (low MW PVC)
   4.   Poly(Acrylic Acid) (PAA)
   5.   Poly(Methyl Methacrylate) (PMMA)
   6.   Polystyrene
   7.   Poly(Vinyl Alcohol) (PVA)

As well as melting points and enthalpies of melting, the glass transition
temperatures of the above polymers were found (where applicable) and
attempts were made to account for any deviations from ideality that
inevitably occurred.

                       Introduction and theory:

Differential Scanning Calorimetry is a technique used to measure thermal
properties of polymers based on the rate at which they absorb heat energy
compared to a reference material. The technique takes advantage of the
energy changes involved in the various phase transitions of certain polymer
molecules. This allows several properties of the material to be ascertained;
melting points, enthalpies of melting, crystallisation temperatures, glass
transition temperatures and degradation temperatures.

A heat flux differential scanning calorimeter will be used in the experiment.
This is one in which a sample is heated along with a reference material with


                                                                                2
a known specific heat. One of the criteria of this technique is that the sample
and reference material remain at the same temperature during heating. This
can be achieved by Setting the machine to heat both the sample and
reference material at a specific rate (In this experiment, the rate is set to
10oC per minute). This allows the heat flux or difference in energy input
between the sample and reference to be measured. Maintaining a constant
supply of heat to both materials would not (unless the materials have the
same heat capacity at all points, which is unlikely) maintain a minimal
temperature difference between them. Instead, a computer is connected to
the machine, and using the software and various signals from the
calorimeter, “decides” when to supply heat to either material. This
information is then dealt with by the computer software and presents it as a
graph of the energy changes versus the temperature. A standard DSC curve
for a particular polymer is shown in fig. 1.




Fig 1. A standard output for a polymer from a DSC machine.

From the diagram, it can be seen that a sudden upward jump in the curve
signifies an exothermic process. A sudden drop in heat flux indicates an
endothermic process.

It is possible to approximate the heat flow into the sample holder using the
following equation:

                                      K Tb  T  ……………..(1)
                                  dQ
                                  dT

T is the sample temperature
Tb: programmed block temperature


                                                                               3
K: thermal conductivity of the material

                               Tb  T0  qt ……………..(2)

T0 is the initial temperature and q is the programmed heating rate.

The heat capacity is defined as the amount of heat energy required to raise
the temperature of a body by 1K. For a substance with a constant heat
capacity:

                          Q  C P T  T0  ……………..(3)

It is possible to derive an equation from equations 2 and 3 that forms the
basis for the DSC experiment. This is as follows:

                                        CP
                               T  q      ……………..(4)
                                        K

ΔT is the difference in temperature between the reference material and the
sample.

The heat capacity is given as

                               C P  mc P ……………..(5)

Where cp is the specific heat (Amount of heat required to raise the
temperature of unit mass by 1K).

The enthalpy change for a given phase transition may be found by
integrating over the area in which the transition is seen to occur on the DSC
plot. This change may be described by the following integral:
                          Tf         Tf
                                         KT 
                     H   C P dt    q dT ……………..(6)
                                              
                          Ti         Ti      

Where the limits of integration Ti and Tf are the initial and final temperatures
over which the graph is integrated. Therefore it is the area under the curve
that gives information about the enthalpy changes involved in the various
transitions.


                                                                                4
Some transitions can be identified at certain temperatures, and the following
paragraphs include short descriptions of these transitions, the order in which
they occur, and their significance at a molecular level.

Glass transition temperature, TG:

This is the point at which, on heating, an amorphous polymer changes from
being hard, brittle and glass like to being a soft rubber like substance. In a
way, this is analogous to a percolation threshold. As the polymer is cooled,
molecules have less freedom and become bonded to each other. As the
percentage of bonded molecules increases, there comes a point at which
long-range connectivity occurs (the “percolation threshold”) and the
polymer reverts to a glass like phase again. This point may be identified as a
dip on a graph of heat flux versus increasing temperature, this is due to the
molecule having a greater degree of freedom and absorbing energy to
maintain the same rate of heating as the reference. It can then be concluded
that glass transition is an endothermic process. Unlike a transition from a
solid to a liquid, or liquid to gas, the energy change involved in glass
transition (a second order phase change) is quite small. Fully crystalline
polymers have no glass transition and their structure remains intact until the
melting point. The following table is a list of factors relating to polymers
and their effect on the glass transition temperature:

         Decreasing Tg              Increasing Tg
         Caused by:                 Caused by:
         Main chain flexibility     Main chain rigidity
         Flexible side chains       Bulky or rigid side chains
         Increase in Tacticity      Increased cohesive energy density
         Increased symmetry         Increased molecular weight
         Branching                  Increased polarity
         Addition of plasticisers   Cross linking
         Or diluents


Temperature of crystallisation, TC:

Continuing to heat a polymer past its Glass transition temperature eventually
leads to another transition. Before this point in the thermal analysis, the
molecules are arranged in a random fashion and are coiled around each other

                                                                             5
in an unfavourable manner. A transition occurs when molecules acquire
enough freedom to move into a more energetically stable phase, i.e. a
crystalline state. This would indicate that the phase following the glass
transition is metastable and when sufficient energy is supplied, its molecules
adapt a more stable (lower energy) arrangement. Because the molecules in a
crystalline solid have less freedom than those in a flexible rubbery one, the
transition between these two states is exothermic and may be seen as a brief
upward “jump” in the graph.

Temperature of melting, TM:

This is the point at which the (now crystalline) polymer molecules have
gained enough vibrational freedom to break free from the solid binding
forces and form a liquid. Due to the increased freedom of these molecules,
the DSC graph should take a sudden dip at this temperature to indicate the
endothermic nature of the process, which is a first order transition. The
melting point of any substance can be given by:

                                  H m
                           Tm         …………….….(7)
                                  S m
ΔHm is the melting temperature and ΔSm is the change of entropy involved
in the melting phase (entropy of fusion).

Degradation Temperature, TD:

The final transition on a DSC graph is the degradation temperature, TD. At
this point in the heating cycle, individual bonds between atoms start to break
as the vibrations become more and more fierce until eventually, individual
polymer molecules decompose into their components. Depending on the
nature of the substance under investigation, this process can be either
endothermic or exothermic

In this experiment, the thermal properties of seven polymers will be
investigated along with those of a reference material, which, in this case,
will be indium metal. These are as follows:


   1. Indium
   2. Polyethylene (high density) (HDPE)
   3. Poly(Vinyl Chloride) high molecular weight (high MW PVC)

                                                                              6
    4.   Poly(Vinyl Chloride) low molecular weight (low MW PVC)
    5.   Poly(Acrylic Acid) (PAA)
    6.   Poly(Methyl Methacrylate) (PMMA)
    7.   Polystyrene
    8.   Poly(Vinyl Alcohol) (PVA)

The transitions listed above will, where applicable, be observed and the
enthalpy changes involved in these processes will be measured by
integration of areas underneath the start and endpoints of any peaks or dips
on the DSC graphs. Computer software accompanying the differential
scanning calorimeter will enable the enthalpy changes to be measured
without the need for evaluating large numerical integrals by hand.

                                           Apparatus:

Shown in fig.2 is a diagram of the workings of a differential scanning
calorimeter.
Figure 2, the workings of a DSC machine:




The reference pan is to remain empty at all times. Signals from both pans
provide information to the computer. This allows it to regulate the
temperature and provide a constant heating rate. Not shown are the gas inlets
and outlets, using these, a chemically inert gas such as Argon or nitrogen
may be pumped through the machine in order to prevent the accumulation of
dust. The gas also serves to remove any volatile chemicals that may be
present.




                                                                               7
                                 Procedure:

Firstly, samples were prepared for analysis. In order to avoid confusion, this
was performed as the samples were required as opposed to preparing all
samples at once. Extreme care was taken not to contaminate samples or
sample pans with either other chemicals or matter from human skin. This
meant that all equipment had to be handled with tweezers. Samples were
prepared as follows:

The tare weight of the sample pan and lid were obtained. Sample pans with
flat undersides were selected so as to make the maximum contact possible
with the bottom of the calorimeter. Following this, between 5 and 10
milligrams of polymer were placed in the sample pan and the mass of the
filled sample pan and lid were found. This was the mass of polymer to be
used. Again using tweezers, the sample pan and lid were moved to the
crimping press whereupon crimping the lid sealed the pan. Another sample
pan was weighed along with its lid. This, however, was left empty and
sealed using the crimping device, forming the reference pan to be heated
along with the sample.

The first sample to be observed was the Indium reference material. This was
placed in the left hand side of the furnace in the calorimeter while the
reference pan was placed in the right hand position, where it was to remain
for the duration of the experiment.

Heating cycle for Indium:

Nitrogen atmosphere, with a gas flow rate of 10Cm3/min, heating rate of
10oC/min from 120oC to 180oC.

For the polymer samples, the heating cycle proceeded as follows:

The same gas flow rate was maintained, along with the heating rate of
10oC/min. The only change was that the range of temperature was modified
so that it ranged from 30oC to 250oC. Following this, the furnace was
allowed cool to room temperature before the next sample could be
introduced.
The resulting graphs are then analysed and information about onset, peak
and endpoint temperatures and changes in enthalpy for the various processes
can be obtained using the computer software.

                                                                             8
                             Results & analysis:

Shown in table 1 are the masses of each of the polymer samples taken along
with the mass of Indium. Also temperatures of glass transition,
crystallisation, melting and degradation are shown along with the enthalpy
changes associated with each of these.
Table 1:
                 Sample Name         Mass in mg    TG (oC) TM (oC)
                    Indium             12.7          n/a    156.9
                  Polyethylene          7.1          n/a   124.92oC
                 (high density)
             Poly(Vinyl Chloride)       9.2        85.385      n/a
                   High MW
             Poly(Vinyl Chloride)       9.5        81.252      n/a
                   Low MW
               Poly(Acrylic Acid)       9.7          --       130
           Poly(Methyl Methacrylate)   10.3        103.32      --
                  Polystyrene           9.9          --      71.556
              Poly(Vinyl Alcohol)       8.1          90      162.18

The resultant graphs are shown on the next few pages along with the
enthalpy changes and temperature range of each transition. The first of these
graphs is for the Indium reference material.
Graph 1:




                                                                             9
Mass of sample: 12.7mg
Enthalpy of melting: 21.4451.07J/g
Onset temperature: 156.9oC
Peak melting temperature: 158.41oC

Polyethylene:
Graph 2:




Mass of sample: 7.1mg
Enthalpy of melting: 118.35.92J/g
Onset temperature: 124.92oC
Peak melting temperature: 133.78oC

Poly(Acrylic Acid):

Graph 3




                                      10
Mass of polymer taken: 9.7mg
Tg:
Enthalpy of melting: 77.9593.9J/g
Onset temperature: 130oC
Peak melting temperature: 149.11oC

Poly(Methylmethacryalte):   Graph 4:




                                       11
Mass of polymer taken: 10.3mg
Tg: 103.32oC
Enthalpy of melting:-
Onset temperature:-
Peak melting temperature:-
Degradation onset: 191.4oC
Peak degradation temperature: 201.9oC
Enthalpy: -1.4977J/g

Poly(Vinyl Alcohol):
Graph 5:




Mass of polymer taken: 8.1mg
Tg: 90oC
Enthalpy of melting: 43.782.2J/g
Onset temperature: 162.18oC
Peak melting temperature: 222.15oC

Polystyrene:
Graph 6




                                        12
Mass of polymer taken: 9.9mg
Tg:
Enthalpy of melting: 8.26860.4J/g
Onset temperature: 71.556oC
Peak melting temperature: 73.84oC

High MW Poly(Vinyl Chloride): Graph 7:




                                         13
Mass of polymer taken: 9.2mg
Tg: 85.385oC
Enthalpy of melting:
Onset temperature:
Peak melting temperature

Low MW Poly(vinyl Chloride):
Graph 8:




Mass of polymer taken: 9.5mg
Tg: 81.252oC
Enthalpy of melting:
Onset temperature:
Peak melting temperature




                               14
                                  Discussion:

In this section, the results will be discussed and any discrepancies between
theory and experiment will be addressed:

Indium:

A well-defined transition is seen to occur between the temperatures of 156
and 160oC (centred on a peak of 158.41oC) with an enthalpy change for the
transition of 21.4451.07J/g. The actual melting point of Indium metal is
156.0oC and its enthalpy of fusion is 28.6J/g, so it can be concluded that the
transition on the graph is the melting of the sample.

Polyethylene:

Polyethylene is a simple straight chain polymer composed of repeating units
of the ethylene molecule. Its structure is shown below:




The output graph from the DSC machine showed one distinct transition with
an onset temperature of 124.92oC and a peak of 133.78oC with an associated
enthalpy change of 118.315.9J/g. By comparison, the melting point of
high-density polyethylene is 133.5oC. It can be seen that this is in quite close
agreement with accepted theoretical values. Because Polyethylene consists
mainly of saturated carbon bonds and due to the lack of anything other than
weak London interactions to hold its molecules together, polyethylene has
low values for both the glass transition and melting temperatures.

Polyacrylic Acid:

In this case, two distinct transitions may be seen. The first of these occurs at
58.82oC with a change in enthalpy of 6.94420.35J/g and it can be seen that
the transition occurs over a relatively narrow range of temperature. The
other transition, however, occurs over a much broader range from
approximately 130oC to 200oC with a peak temperature of 149.11oC and an



                                                                               15
enthalpy change of 77.9593.9J/g. The structure of Poly(Acrylic Acid) is
shown here:




The presence of the hydroxyl group as a substituent on the molecule should
ensure that PAA has a higher melting point (due to Hydrogen bonding) than
that of the more structurally simple polyethylene. This is indeed the case as
the melting of this substance occurs at approximately 150oC (this is the peak
value). However, the melting occurs over a large range of temperatures
which could be explained in a number of ways. The most probable reason
for this is that there were impurities present in the sample. Most likely a
foreign object was picked up during the weighing process as the analytical
balance may not have been cleaned beforehand. Also the sample may have
been contaminated in the handling process, by either the tweezers or
accidental contact with human skin. Another possible explanation could
apply if there was a large distribution of molar masses of the polymer
present in the sample. Unfortunately this cannot be verified so the blame for
this must lay at human error. Also, in theory, a glass transition should occur
in the region of 105oC, and in the graph, no such transition is seen, pointing
further at sample contamination.

Poly(Methyl Methacrylate):

Two transitions are evident from the graph; firstly an onset temperature of
103.3oC leads to a peak at 112.7oC and an enthalpy change of 2.7120.14J/g.
Secondly, an exothermic transition is seen to occur, beginning at 191.4oC,
running through a peak of 204.9oC with a small enthalpy change of
1.49770.075J/g. The structure of Poly(methylmethacrylate) (PMMA) is
shown below:




                                                                            16
The melting point of PMMA is taken from literature as 378K or 105oC,
indicating the first transition on the graph to be a melting point. The second
transition is much more interesting, however. Due to an exothermic peak at
such a high temperature it may be concluded that this is the degradation
temperature.

Poly(Vinyl Alcohol):

The most noticeable thing about the output from the sample of PVA is the
apparent presence of two peaks. The first peak may be due to impurities in
the material as there are no transitions in its immediate “neighbourhood”.
The accepted melting point of this polymer is approximately 230oC (fully
hydrolysed) and no other transitions are known occur in the area of 139 to
180oC. The second transition occurs from 205oC with a peak at 222.15oC
and an enthalpy change of 43.7882.2J/g, meaning this is the point at which
the sample melted. Also, according to references, a glass transition occurs at
approximately 85oC, this can be seen as a miniscule dip in the graph at
approximately 90oC. Just by looking at the structure of PVA, it becomes
clear why it has such a high boiling point. The molecule itself has minimal
branching and can make the most of any London interactions between
molecules. These forces of interaction don’t dominate, however. Hydrogen
bonding is by far the dominant interaction in this particular polymer and
goes some way into explaining its thermal behaviour.




Polystyrene:

Polystyrene is accepted to have a glass transition at approximately 373K or
100oC. A peak is seen to occur at approximately 78oC but the nature of this
transition conflicts with that of a glass transition. Because the graph is seen
to dip sharply and all but rise back up to its original “height”, the possibility
of glass transition at this point can safely be ruled out. Because Polystyrene
comes in a massive variety of molar masses, large variations in properties
are not uncommon (viscosity etc.), therefore, the only logical conclusion to
draw is that the sample of polystyrene used for this test has a very low


                                                                               17
molecular weight or is very highly branched. (The small range in
temperature over which melting occurs displays that chain lengths of this
sample of polystyrene are very similar). This would lead to the transition in
question being deemed a melting point. Structure of polystyrene:




Running the sample through a viscometer in an appropriate solvent could
verify a low molecular weight. The relative bulk of the phenyl group
attached to the polymer also contributes another variable to the material-the
tacticity of polystyrene can dictate whether it can pack into a more ordered
solid structure with more stable thermal properties. The more ordered the
Phenyl groups lie along the chain, the better able they are to pack in an
ordered fashion and this case is syndiotactic polystyrene, the structure of
which is shown below:




This is obviously more suited to packing than atactic polystyrene (formed by
random polymerisation), which has its phenyl groups randomly oriented.

PVC:

High molecular weight PVC differs from the low molecular weight variety
only in terms of chain length and both compounds have the same structure:




                                                                            18
The difference between the two forms is merely in the value of “n”. The
similarity, however, does not end there. The lower temperature regions of
the graphs in each case are strikingly similar in shape with glass transitions
occurring at, 81.252oC and 85.385oC for low and high molecular weight
respectively. This is quite a high value for a glass transition and can be
attributed to the Cl- group attached to the main Carbon chain taking a greater
share of the electrons in its bond with Carbon. This has the effect of setting
up an electric dipole that may interact with the other molecules via a dipole-
dipole interaction. A second transition is evident in the higher temperature
regions of the graph for low density PVC. Starting at 175.75oC with a peak
at 187.53oC and an endothermic enthalpy change of 4.950.25J/g, it has far
too small an energy to be a melting point and is possibly a degradation
temperature. No information was available as regards degradation
temperatures, however, so to err on the side of caution, this will be put down
to impurities present in the material. An increase in the value of n for a
straight chain polymer causes an increase in the length over which a given
molecule can interact with its nearest neighbours by the Van der Waals
force, thus increasing the temperature at which various transitions occur.

Conclusions:

Analysis of the polymers was preceded by the analysis of a material with a
well-defined melting point and enthalpy of melting (Indium). The result of
this measurement compared with the theoretical values as follows: Accepted
enthalpy of melting of Indium: 28.5J/g Value obtained through DSC:
21.4451.07J/g. Accepted melting point of Indium: 156.6oC. Value obtained
through DSC: 156.9 (onset). These results display two important points.
Temperature values for transitions are quite accurate and agree to within a
fraction of a percent. However, values given for enthalpy transitions can be
taken with a pinch of salt with our value being about 25% out from what can
be called acceptable. The following table serves to compare values obtained
in this experiment with universally accepted values:

         Material          TG (oC) TM (oC) Theory Tg Theory Tm
         Indum               n/a    156.9     --     156
         HDPE                n/a   124.92            130
         High MW PVC       85.385    n/a      85
         Low MW PVC        81.252    n/a      81
         PAA                  --     130     106
         PMMA              103.32     --     110

                                                                            19
            Polystyrene                --        71.556          104
            PVA                        90        162.18          85          200
Previous page: Table 2-comparison of values obtained from the experiment versus accepted values.

All errors in enthalpies were specified by the equipment manufacturer at
5% of measured values.

Additional excercises:

Percentage crystallinity of polyethylene sample:

                                        100%  293.61J / g
The percentage crystallinity is therefore:
                                    118.3
                                            100  40.2  2.1%
                                    293.61
Thermogram of polymer with the following transitions:
Endothermic transition at 80oC, Tg at 145oC, Tm of 235oC and a
decomposition temperature of 400oC
Is shown in figure 3:
Figure 3:




All other requested information was discussed earlier in this report.




                                                                                                   20
References:

www.fsref.com/Fatal/FEA00002.SHTML

www.pslc.ws/mactest/dsc.htm

ois.nist.gov/nistpubs/technipubs/ recent/search.cfm?dbibid=12927

irc.nrc-cnrc.gc.ca/ cbd/cbd154e.html
repository.matdl.org/retrieve/ 2961/polymer+final+research+article.pdf

www.chem.ox.ac.uk/.../ Lavender/lavender.html
www.erkol.com/eng/Characteristics.htm




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