)
Polypropylene Phase Transitions Transitions Studied by DSC John Doe Chem ------Abstract: Isotactic and Amorphous polypropylene were studied through Diferential Scanning Calorimeter DSC!" #hen analy$ing amorphous polypropylene% both &rst order and second order transition phases are obser'ed with an enthalpy o( crystalli$ation o( )*"++ J,g" This shows the sample was not ). pure" The glass transition temperature was (ound to be -)+"/01C" The degree o( crystallinity (or the amorphous sample was calculated (or a 'alue o( /"0*." The isotactic sample had a degree o( crystallinity at 2)"+. as well as an enthalpy o( crystalli$ation o( 03"*+ J,g" The entropy entropy o( the glass glass transition phase was calculated calculated throug through h a series o( integrals ranging (rom 4+1C% this 'alue was )"30 J,mol-5" The entropy o( crystalli$ation was "+ J,mol-5% this was calculated through utili$ation o( the &rst law o( thermodynamics" This e6periment demonstrated diferences between stereochemistry in molecules% which showed diferences in thermodynamic
+ INTRODUCTION:
Diferential Scanning Calorimetry DSC! is used to determine and compare the phase transitions o( both amorphous and isotactic polypropylene" +Diferential scanning calorimeters are a widely used thermoanalyitcal instruments due to their ease o( use% relati'ely (ast data collection times and ability to use small sample si$es" *DSC is used to measure thermal properties o( nanocomposite thermal-plastics widely used in the engineering o( polymers" Polymer engineering is important due to the demand on inno'ati'e ways to create plastic" 7sing DSC can gi'e thermodynamic properties% which help engineers produce plastics that can resist high or low temperatures" DSC wor8s by measuring the heat 9ow to or (rom a sample when chemical or physical changes occur during applied temperature increase" #hen there is no chemical or physical change total heat is calculated by: q p
C p ∆T
=
)!
Cp is independent o( temperature and T is the temperature change" I( there is any e6cess heat trans(erred in the system e;uation ) turns into: C p , ex
=
qex ∆ T
+!
+
The e6cess heat can be 'iewed% as the change in heat capacity
there(ore the DSC thermogram is essentially a chart o( C p%e6 's" T" #hen the system undergoes any chemical or physical changes a pea8 (orms
* on the graph" These pea8s can be integrated to &nd enthalpy o( transition" The integration is shown in <;uation *" T 2
∆ H p=∫ C p , ex dT
*!
T 1
T) and T+ represent the beginning and ending temperatures o( transition" There are two diferent types o( transition being studied in this lab" =irst order and second order transition" A steep pea8 in the high
temperature
demonstrates
range
represents
crystalli$ation"
Slow
&rst
order
rising
pea8s
transition% in
the
this lower
temperature range represent second order" It is hypothesi$ed be(ore lab that the amorphous polypropylene would only show second order transition" This is due to its tacitcity% which is related to the stereochemistry o( a molecule" ) Tacticity is a term used to describe the way pendent groups on a polymer chain are arranged on a polymer bac8bone" ) The tacticity o( a polymer is determined by what side o( the polymer chain the pendant groups are on" Amorphous Polypropylene is a long carbon chain where each center carbon is connected to a methyl group and hydrogen> these are situated in random order" This random order o( side groups (orms an amorphous solid that does not crystali$e" Isotactic Polypropylene has all methyl groups in the same position (rom the center carbon% which allows it to crystali$e" These phenomena?s are studied through the graphs produced by DSC" ) st order transition occurs when the molecule crystalli$es with an applied
2 heat
9ow%
thus
represent
Isotactic
Polypropylene"
Amorphous
Polypropylene was predicted to only (orm second order transition because it does not crystalli$e" A(ter obser'ing the diferent pea8s o( transition the entropy will be calculated through the integral shown in <;uation 2" T 2
∆ S =∫ T 1
C p T
dT
2!
The &rst law o( thermodynamics shown in e;uation 3 helps shape an understanding o( the system and is used (or calculations o( entropy when the @ibbs (ree energy is $ero" ∆G
=
EXPERIMENTAL:
∆ H T ∆ S −
3!
3 The pressure o( the nitrogen CAS +-*-B! (eed was set to + psi% e6ceeding this pressure would damage the instrument" +-)+2 was then opened on the computer monitor" The cell was set to room temperature with a sample purge 9ow rate o( 3m,min" Amorphous Polypropylene> Aldrich CAS B*--! was then placed in DSC + diferential scanning calorimeter" There are three Eermetic T$ero aluminum sample pans in which the polymer samples sat on (or the e6periment"
It was essential to use twee$ers when handling the
samples> this pre'ented any s8in oil interaction% which would afect
=igure *: Diagram o( <6periment Attained (rom http:,,pslc"ws,macrog,dsc"htm results" A simplistic diagram o( the system is shown below"
/
The sample and re(erence pan are both placed on top o( two thermocouples inside the cell" The computer was set to B"* mg (or PPAF3/% which was the sample being tested" A(ter all e6perimental constraints were con&rmed the e6periment started" DSC graphs were made (rom the +-)+2 program and integrated to &nd enthalpy o( transition" The Data (or the PPC was attained (rom other sources a(ter completion"
RESULTS:
=igure 2: PPAF32 DSC thermogram
0 =igure 2 represents the graph created (rom the DSC" It is obser'ed that both )st order and + nd order transitions ta8e place in the GsupposedH amorphous polypropylene"
An a'erage o( the heating and cooling
pea8s ga'e a glass transition temperature o( -)+"/01C" The &rst order crystalli$ation pea8 had an enthalpy o( )*"++ J,g with a melting temperature o( )*"/B1C" The degree o( cystallinity was reported at
=igure 3: PPCF32 DSC Thermogram /"0*." =igure 3 represents the thermogram (or Isotactic Polypropylene"
The enthalpy o( crystalli$ation o( isotactic polypropylene is 03"*+ J,g at a temperature o( )3)"/31C" The degree o( crystallinity is 2)"+." The
B graph is symmetrical showing no glass transition" The entropy o( crystalli$ation is calculated using the &rst law o( thermodynamics and has a 'alue o( "+)J,g-5" =igure / represents
=igure /:
This
&gure
represents
the
change
in
dependent
on
temperature"
DISCUSSION:
) =igure 2 demonstrated the GsupposedH pure amorphous polypropylene" A(ter witnessing the crystalli$ation pea8 at )*"/B1C% it is pro'ed that the sample used was not ). pure" The degree o( crystallinity was calculated using a re(erence enthalpy o( crystalli$ation as + J,g and a 'alue o( /"0*. was reported" Since the sample was not ). pure it could not dissol'e properly which caused a small portion crystalli$ed" In (uture i( this sample was used% belie'ing it was ). pure% there would be a signi&cant amount o( error when creating products" This is can cause (ailure to (uture reactions and their products and would lower the percent yield o( a target chemical" + The melting temperature o( amorphous polypropylene is reported to be )/-)/31C and a transition temperature o( -)1C% these 'alues were obtained (rom Sigma Aldrich" =igure 2 gi'es 'alues slightly of (rom literature 'alues" n a molecular le'el the bonds released at lower temperatures% a melting temperature is obser'ed at )3+"31C" This could be caused by inconsistency o( pressure during the e6periment" The pressure o( the cell could ha'e 9uctuated% since the melting temperature was reported lower then ideal% it is assumed that the pressure decreased" 2 This assumption is made due to the direct relationship between pressure and temperature% when pressure is decreased so does the melting point" Also the discrepancy could ha'e been caused by too rapid o( increase in temperature" I( temperature increases (aster then the DSC can read% the melting temperature will be recorded lower then ideal"
)) Isotactic polypropylene reports a higher melting temperature o( )0B1C% this 'alue is (ound in the chemical boo8 website" +Since this melting point is slightly higher then amorphous this pro'es that a consistent stereochemistry within a molecule gi'es a stronger bond between carbon and its corresponding group" 7sing this phenomena when comparing both Isotactic and Amorphous polypropylene% it shows the bonds are either closer together or attracted to each other more when stereochemistry
is
the
same
(or
all
carbons%
the
isotactic
polypropylene" The temperature range (or the pea8 o( crystalli$ation 'aried when comparing the amorphous to isotactic polypropylene" The wider the temperature range and ultimately wider the pea8 corresponds to more crystalli$ation enthalpy o( the molecule" In the amorphous sample% the crystalli$ation pea8 was much thinner than the isotactic sample" This agrees with the 'alues o( enthalpy o( each% the isotactic had a much higher enthalpy compared to amorphous" This shows the isotactic sample in'ol'ed a greater temperature range during the crystalli$ation physical change" The entropies o( both amorphous and isotactic sample difered signi&cantly"
)+ the reason (or the calculation o( the integral ranging (rom -+ to +1C" The sum o( those gi'es a 'alue o( )"30 J,g-5" As the temperature changes so does the disorder o( the system" The entropy o( the isotactic sample ga'e a 'alue o( "+) J,g-5% this is signi&cantly lower then the amorphous sample because temperature stays at a constant 'alue% the melting temperature" The &rst law o( thermodynamics can be applied and the total energy in the system is e;ual to the enthalpy o( crystalli$ation% which gi'es lower disorder thus lower entropy"
CONCLUSION:
Diferential scanning calorimeter was used to test both amorphous and isotactic polypropylene" The DSC thermogram showed that the amorphous polypropylene was not ). pure> this was e'aluated through both its predicted glass transition phase as well as its /"0*. crystalli$ation )st order pea8 with an enthalpy 'alue o( )*"++J,g" The temperature o( glass transition was an a'erage o( both hot and cold temperatures and was a 'alue o( -)+"/0 1C" Pure amorphous polypropylene would not e6hibit a ) st order pea8" The entropy o( the glass transition was calculated through a series o( integrals% which ga'e a 'alue o( )"30 J,g-5" The entropy o( crystalli$ation% obser'ed in the isotactic thermogram was a 'alue o( "+ J,g-5" The degree o( crystalli$ation was 2)"+ ." The 'ariances between both the isotactic
)* and amorphous sample are due to its stereochemistry and strength o( bonds" The more consistent a structure the stronger its bonds are" Diferences (rom ideal 'alues could ha'e occurred (rom rapid temperature increase or decrease in cell pressure" DSC pro'es to be a power(ul tool when studying thermodynamic properties o( isomers and helps compare and contrast small diferences within each compound" ACKNOWLEDGMENTS:
I would li8e to than8 Sean KcCrea% Adam Liels8i and Pro(essor K (or their help with con&guration and understanding o( this e6periment" REFERENCES:
Comparatively Speaking: Isotactic vs. )! Menic8% Anthony" Syndiotactic vs. Atactic in Polymers. +B" +! Kilosa'lNe'ic% LratolNub" Lab Packet for Experimental Physical Chemistry. +)*% B)!-B/!" *! Polymer Engineering Science. Ool 32"%pp ++B+-+* 2!