Analysis of Materials Polymers by Thermal Methods:

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• Analysis of Materials (Polymers) by Thermal
  Methods
• DSC, TG/DTA
• Instructor Ioan I. Negulescu
• CHEM 4010
• Tuesday,
• October 29, 2002

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• Thermal Methods
• Thermal methods are based upon the measurement of
  the dynamic relationship between temperature and
  some property of the system such as mass and heat
  absorbed or evolved by/from it.

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• Differential Scanning Calorimetry,
• DSC
• Differential Thermal Analysis,
• DTA
• Thermogravimetry,
• TG
• are the most important thermal methods
• used in characterization of polymers.

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• The temperature increase, ?T, of a body which is
  heated is directly proportional to the amount of
  heat absorbed, ?Q, and inversely proportional to
  its mass, m, and its capacity C to store heat
• ?T ?Q/m C Eq. 1

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• Consider the temperature increase of two
  different samples of the same mass
• m1 m2
• for which the same amount of heat was given
• ?Q1 ?Q2
• If their heat capacities are different
• C1 ? C2
• they do not experience the same temperature
  increase, i.e.,
• ?T1 ? ?T2

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• A greater heat flow (dQ/dt, where t is time)
• will always flow into the sample whose heat
  capacity is higher, in order that the
  steady-state heating rate be maintained.
• The heat capacity at constant pressure (Cp)
• of a material is defined as the temperature
  increase of a unit of substance (mass) as a
  result of the supply of a unit of heat at
• constant pressure.

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• For the same substance, Cp is dependent upon its
  aggregation state, i.e., it is different for the
  liquid state as compared to loose gaseous or to
  more compact solid state.

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• A polymeric material has different heat
  capacities for amorphous or crystalline
  morphologies. For amorphous polymers, the heat
  capacity for the glassy state (i.e., below glass
  transition, Tg, where only vibrations of atomic
  groups occur) is different from that
  characterizing the leathery (short range
  diffusional motion, i.e., of chain segments),
  rubbery (retarded long-range motions), rubbery
  flow (slippage of long-range entanglements) or
  liquid state.

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• Figure 1. Temperature - molecular mass diagram
  for amorphous polymers (1) Glass transition
  (Tg) (2) Diffuse transition zone and (3)
  Thermal decomposition.

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• Figure 2. Temperature - Molecular Mass diagram
  for (semi) -crystalline polymers (1) Glass
  transition (Tg) (2) Melting point (Tm) (3)
  Diffuse transition zone and (4) Thermal
  decomposition.

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• For amorphous polymers, the glass-rubber
  transition temperature is of considerable
  importance technologically. It (Tg) determines
  the lower use limit of a rubber (e.g.,
  polydienes, Tg ? -50C) and the upper limit use
  of an amorphous thermoplastic material(e.g.,
  polystyrene, Tg ? 100 C).

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• In the case of (semicrystalline) linear polymers
  it is possible to identify a melting temperature
  (Tm). Above this temperature the polymer may be
  liquid, viscoelastic or rubbery according to its
  molecular mass, but below it, at least in the
  high molecular mass range, it will tend to be
  leathery and tough down to the glass transition
  temperature.

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POLYHYDROXYALKANOATES
R can be hydrogen or hydrocarbon chains of
up to around C13 in length, and x can range
from 1 to 3 or more. Varying x and R
effect hydrophobicity, Tg, Tm, and level of
crystallinity
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POLYHYDROXYALKANOATES
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• Detailed information on glass transition,
  crystallization, and melting, is therefore
  critically important in formation, processing and
  utilization of polymers.
• Differential thermal methods (DTA and DSC) have
  been widely applied to the study and
  characterization of polymeric materials.

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• In DTA the heat absorbed or emitted by a system
  is observed by measuring the temperature
  difference (?T) between that system (the sample)
  and an inert reference material (often alumina),
  as the temperature of both is increased at a
  constant rate (usually 5-10 ?C/min).

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• Figure 3. Schematic diagram of a typical DTA
  apparatus.

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• In DSC, the sample and the reference are also
  subjected to a continuously increasing or
  decreasing temperature.
• In the scanning operation the sample and the
  reference show different temperature independent
  heat capacities.
• Heat (dQ/dt) is added to the sample or to the
  reference as necessary to maintain the two
  identical temperatures.

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• Bottom (b) measured curve. ?m is the measured
  heat flux. ?bl is the heat flux corresponding to
  the base line and t is time.

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• The ordinate is usually represented by the
  heat flux (denominated as ? or dQ/dt) or by the
  variation of the heat capacity.
• The glass-to-rubber transition, or shortly
  the glass transition (Tg) is a phase change
  reminiscent of a thermodynamic second order
  transition (melting and crystallization being
  first order transitions) for which a plot of
  specific heat versus temperature shows a sudden
  jump.
• The first order transitions appear
• as peaks.

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• DSC curve of a polymeric sample (1), (3) and (5)
  are base lines (2) is glass-to-rubber
  transition, Tg (4) is the interpolated base
  line and (6) is the first order transition peak.

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• The glass transition region in cooling (a) and
  subsequent heating (b) mode showing some commonly
  used definitions of glass transition, Tg.

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Poly(Lactic Acid)

• --O-CH(CH3)-CO-n
• Two of the most attractive features of
• poly(lactic acid), PLA, polymers are
• they are easily synthesized from
• renewable resources (corn!)
• they are both hydro- and biodegradable

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Poly (Lactic Acid) Glass transition temperature,
Tg.

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• DSC traces for melting and crystallization
• of a polymer sample.

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• Melting of polyoxymethylene with superheating.

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• DSC analysis of a poly(ethylene terephthalate)
  sample quenched from the melt.

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DSC traces of Low Crystallinity PLA treated in
Water at 70?C and 100?C. The higher the
crystallinity achievedat 100 ?C, the higher and
the less defined the Tg

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• Melting of two semicrystalline HDPE samples.

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• Considering ?H 200 mJ/mg as the enthalpic
  change for the melting of a 100 crystalline HDPE
  sample, from DSC data of these two recyclable
  HDPE it can be found that
• the polymer derived from detergent bottles was
  (132/200)x100 66 crystalline
• the polymer used for milk bottles was
  (165/200)x100 82.5 crystalline.

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• Determination of the HDPE content in a blend with
  inorganic filler from DSC data.

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Polyhydroxylated Nylons Similarity of Nylon 6,6
and the poly hydroxylated counterpart
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DSC Thermal Transitions in Polyhydroxylated
Nylon 6,6
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• Thermogravimetric (TG) analysis is concerned with
  the change in weight of a material as its
  temperature changes. This indicates
• the temperature at which the material loses
  weight through evaporation or decomposition
• the temperature at which no weight loss takes
  place is revealed, which indicates stability of
  the material.

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• TG Measurement Principle of Seiko TG/DTA
  Thermobalance

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Thermal Degradation of Polyhydroxylated Nylon
6,6
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Poly (4-dodecyl-1-4-aza heptamethylene-D-glucarami
de). Thermal
decomposition.

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• Initial decomposition of linear polymers.
• Initial sample weight 10 mg. Heating rate
  5?C/min.

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• Thermogravimetric analysis of a polymeric blend
  containing HDPE and an inorganic filler
  (phosphogypsum)

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• Almost any measurement that can be done at
  different temperatures can be expanded into
  thermal analysis and any series of thermal
  analysis techniques can be combined with other
  non-thermal technique for valuable
  multiple-parameter information.

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• Coupled techniques, such as Thermogravimetry,
  Differential Thermal Analysis and Mass
  Spectrometry (TG-DTA-MS) or evolved gas analysis
  of polymers by coupled Thermogravimetry, Gas
  Chromatography, Fourier Transform Infrared and
  Mass Spectrometry (TG-GC-FTIR-MS) are just two
  examples often used in industrial laboratories.