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  • 1
    Electronic Resource
    Electronic Resource
    Reversible Melting of Semi-crystalline Polymers Frequency dependence of the reversible melting enthalpy (1998)
    Springer
    Journal of thermal analysis and calorimetry 52 (1998), S. 293-313 
    Details
    ISSN: 1572-8943
    Keywords: crystallisation ; heat capacity ; MDSC ; melting ; polymers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Modulated DSC (MDSC) has been used to study the heat flow during melting and crystallisation of some semi-crystalline polymers i.e. different grades of polyethylene (LDPE, LLDPE and HDPE), and polypropylene (PP). The heat capacities measured by MDSC are compared with the hypothetical complex heat capacities of Schawe and it is shown that numerically they are equivalent; nevertheless, the concept of the complex heat capacity is problematic on a thermodynamic basis. A reversing heat flow (proportional to the experimental heat capacity of the material) was present at all conditions used for the study. In the melting zone of the polymers it depends on the modulation frequency and on the amplitude. Higher amplitude and frequency of modulation reduce the ratio of the reversing heat flow to the total heat flow, the latter is nearly independent on these parameters. The reversible component of the melting enthalpy of polymers depends on the modulation frequency, the modulation amplitude and the type of the polymer. It increases by increasing the branching in polyethylene. The existence of the reversible heat flow during the crystallisation and melting is contrary to the current hypotheses and theories of polymer crystallisation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010145506456
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  • 2
    Electronic Resource
    Electronic Resource
    Reversible Melting of Semi-Crystalline Polymers 2. Annealing Near to the Melting Point (1998)
    Springer
    Journal of thermal analysis and calorimetry 53 (1998), S. 493-508 
    Details
    ISSN: 1572-8943
    Keywords: crystallisation ; fringed micelle ; heat capacity ; MDSC ; melting ; semi-crystalline polymers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE, HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale, to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the same value as during the cooling section. This is not the case for LDPE and LLDPE. Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle is the same. The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron radiation during a heating cycle at temperatures below the melting peak. A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect to that of folded chain lamellae.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010145509633
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  • 3
    Electronic Resource
    Electronic Resource
    Modulated Differential Scanning Calorimetry (1997)
    Springer
    Journal of thermal analysis and calorimetry 50 (1997), S. 727-744 
    Details
    ISSN: 1572-8943
    Keywords: frequency dependence ; heat capacity ; MDSC ; thermal analyses
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The reproducibility and reliability of the TA Instruments Modulated Differential Scanning Calorimeter (MDSC) was tested over a range of conditions. The equipment base line was found to be fairly constant with a very small fluctuation (10 μW), which means a 0.1 % fluctuation on the scale of a normal polymer MDSC curve. The excellent stability of the base line and the reasonable reproducibility of the curves (5%) suggest that frequent calibration is not required. The heat capacities calculated from the modulated response to the variable temperature depend on the frequency for a given cell constant. The heat capacity cell constant is a unique function of the modulation frequency:k c =K c o p/(p−6.3) wherep is the time of the periodicity expressed in seconds and K c o is the heat capacity cell constant measured on a standard material and reduced to zero frequency. The cell constants depend on the flow rate of the helium according to:K(He)=K o(1.298−0.004424He+1.438·10−5 He 2) whereHe is the flow rate of helium in ml min−1 andK o represents a constant at 100 cm3 min−1. There is a strong dependence of cell constant on the flow rate ranges from 10 to 80 cm3 min−1, while above this rate (up to 135 ml min−1) the cell constant approaches a plateau.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01979203
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