Influence of the Processes Parameters on the Properties of The ...

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Chapter 3. Analytical Methods and Designs <strong>of</strong> Experiments Differential scanning calorimetry (DSC) is a <strong>the</strong>rmoanalytical technique in which <strong>the</strong> difference in <strong>the</strong> amount <strong>of</strong> heat required to increase <strong>the</strong> temperature <strong>of</strong> a sample and reference is measured as a function <strong>of</strong> temperature. Both <strong>the</strong> sample and reference are maintained at nearly <strong>the</strong> same temperature throughout <strong>the</strong> experiment. Generally, <strong>the</strong> temperature program for a DSC analysis is designed such that <strong>the</strong> sample holder temperature increases linearly as a function <strong>of</strong> time. The technique was developed by Watson and O’neill [1962] and was introduced commercially at <strong>the</strong> 1963, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy [ISO - International Organization for Standardization, 1963]. Heat-flux DSC and power-compensated DSC are <strong>the</strong> two types <strong>of</strong> DSC that have been widely used. One <strong>of</strong> <strong>the</strong> big advantages <strong>of</strong> DSC is that samples are very easily encapsulated, usually with little or no preparation, ready to be placed in <strong>the</strong> DSC cell, so that measurements can be quickly and easily made [Gabbott, 2008]. The specific heat <strong>of</strong> a material changes slowly with temperature in a particular physical state, but alters discontinuously at a change <strong>of</strong> state. As well as increasing <strong>the</strong> sample temperature, <strong>the</strong> supply <strong>of</strong> <strong>the</strong>rmal energy may induce physical or chemical processes in <strong>the</strong> sample, e.g. melting or decomposition, accompanied by a change in enthalpy, <strong>the</strong> latent heat <strong>of</strong> fusion, heat <strong>of</strong> reaction etc. In a heat flux DSC, <strong>the</strong> sample material, enclosed in a pan and an empty reference pan are placed on a <strong>the</strong>rmoelectric disk surrounded by a furnace. The furnace is heated at a linear heating rate and <strong>the</strong> heat is transferred to <strong>the</strong> sample and reference pan through <strong>the</strong>rmoelectric disk (cf. Figure 3.1). The temperatures <strong>of</strong> <strong>the</strong> two <strong>the</strong>rmometers are compared, and <strong>the</strong> electrical power supplied to each heater adjusted, so that <strong>the</strong> temperatures <strong>of</strong> both <strong>the</strong> sample and <strong>the</strong> reference remain equal to <strong>the</strong> programmed temperature, i.e. any temperature difference which would result from a <strong>the</strong>rmal event in <strong>the</strong> sample is ‘zero’. The analogical signal, <strong>the</strong> rate <strong>of</strong> energy absorption by <strong>the</strong> sample (e.g. W/s), is proportional to <strong>the</strong> specific heat <strong>of</strong> <strong>the</strong> sample since <strong>the</strong> specific heat at any temperature determines <strong>the</strong> amount <strong>of</strong> <strong>the</strong>rmal energy necessary to change <strong>the</strong> sample temperature by a given amount. In o<strong>the</strong>r words, <strong>the</strong> measuring principle is to compare <strong>the</strong> rate <strong>of</strong> heat flow to <strong>the</strong> sample and to an inert material which are heated or cooled at <strong>the</strong> same rate. 1.2 First Order Transitions (A)-Apparatus (B)-Principle Figure 3.1: Differential scanning calorimetry. Changes in <strong>the</strong> sample, which are associated with absorption or evolution <strong>of</strong> heat, cause a change in <strong>the</strong> differential heat flow which is <strong>the</strong>n recorded as a peak. The area under <strong>the</strong> peak is directly proportional to <strong>the</strong> enthalpy change and its direction indicates whe<strong>the</strong>r <strong>the</strong> <strong>the</strong>rmal event is endo<strong>the</strong>rmic or exo<strong>the</strong>rmic. Analysis <strong>of</strong> a DSC <strong>the</strong>rmogram enables <strong>the</strong> determination <strong>of</strong> two important parameters: <strong>the</strong> transition temperature peak (taken at <strong>the</strong> maximum T m or at <strong>the</strong> onset T onset ), and <strong>the</strong> enthalpy <strong>of</strong> melting/crystallization (H m /H c ). The extrapolated onset temperature (T onset ) corresponding to <strong>the</strong> transition temperature at zero heating rate can be obtained by plotting peak temperatures as a function <strong>of</strong> heating rate [Ruegg et al., 1977]. - 62 -
Chapter 3. Analytical Methods and Designs <strong>of</strong> Experiments The T m value depends on <strong>the</strong> molecular weight <strong>of</strong> <strong>the</strong> polymer. So, lower grades will have lower melting points (cf. Figure 3.2). The crystallinity ratio <strong>of</strong> a polymer can also be found using DSC from <strong>the</strong> following equation: C = (H m −H c )/H m (3.1) where H m is <strong>the</strong> melting enthalpy, H c is <strong>the</strong> crystallization enthalpy and H m <strong>the</strong> melting enthalpy <strong>of</strong> <strong>the</strong> totally crystallised PLA sample. According to Fischer et al. [1973], H m = 93 J.g -1 but according to Miyata and Masuko [1998] H m = 135 J.g -1 . It can be found from <strong>the</strong> crystallization peak from <strong>the</strong> DSC graph since <strong>the</strong> heat <strong>of</strong> melting can be calculated from <strong>the</strong> area under an absorption peak as shown on Figure 3.2. Ahmed et al. [2008] and Wunderlich [2005] observed that both P L LA exhibits an endo<strong>the</strong>rmic melting peak during <strong>the</strong>rmal run. P L LA samples exhibit crystallinity. However, for low molecular mass (M n < 1500) crystallization occurs during negative scan and for higher molecular mass, it is observed during positive run. 1.3 Second Order Transition (A) (B) Figure 3.2: Thermograms <strong>of</strong> two P L LAs <strong>of</strong> different Mw. [Ahmed et al., 2008] The glass transition is a much more subtle transition than melting or evaporation: glass transition is a<strong>the</strong>rmic. In Figure 3.3, <strong>the</strong> change in heat capacity on going through <strong>the</strong> glass transition temperature is drawn for a typical semi-crystalline PLGA polymer. There is only a jump in <strong>the</strong> heat capacity in <strong>the</strong> range 20 to 190 °C. The increase in heat capacity, Cp, generally occurs over a temperature range <strong>of</strong> 5 to 20 K, and <strong>the</strong> jump is <strong>of</strong>ten 11 J.K -1 .mol -1 for mobile units in <strong>the</strong> liquid. This means that <strong>the</strong> decrease in heat capacity at <strong>the</strong> glass transition is 11 J.K -1 .mol -1 , for a monatomic liquid. Figure 3.3: Characteristic variation <strong>of</strong> glass transition in PLGA. - 63 -
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