Annealing experiments have been carried out just below the melting temperature of both polyethylene (LLDPE) and polypropylene
(PP) and their blends. The total melting enthalpy measured after the annealing cycle was greater by 10-15% with respect to
the value having been measured before it. During the annealing period the heat capacity decreases to a lower value within
the first 2-3 min. Heat capacities of PP (either in pure form or in the blends) measured during the heating cycle following
the annealing cycle have the same value as during the cooling section. The heat capacities of the LLDPE in the heating cycle
following the annealing were those of the preceding heating cycle. The total heat flows in the cooling section following the
annealing cycle were greater than those in another cooling cycle at the same temperatures indicating that the crystallisation
takes place during the cooling rather than during the annealing periods.
The presence of LLDPE decreases the crystallisation temperature of PP. The presence of SEBS in the blend results in a greater
crystallisation temperature than that of pure PP. The crystallisation temperature of LLDPE increases with increasing levels
Blends of poly(ether ketone) (PEK) with poly(terephthaloyl-imide) (a thermoplasticpolyimide, TPI) were studied by temperature-modulated DSC (TMDSC) and X-ray diffraction. Samples were prepared by compression moulding of the premixed materials at 400°C and quenched to prevent crystallisation.The amorphous blends showed a single glass transition but with a jump in the temperature value at 60 mass% of PEK, indicating limited miscibility of the system at both sides of the composition series in the quenched, glassy state. Two cold crystallisation peaks over the concentration range 30 to 70 mass% of PEK were observed, but only one for all other compositions. A single melting peak was observed in all systems.Blends crystallised from the glassy state showed eutectic behaviour with the presence of the crystals of both pure components. This is the first reported case of two semicrystalline polymers exhibiting eutectic co-crystallisation. The formation of eutectic crystals is proof of full miscibility of the two polymers in their liquid state, i.e. at a temperature of 400°C and above. Blends cooled from the melt at a cooling rate of 2 K min–1 showed a single glass transition and an extended melting range.Crystallisation during a second melting run generally starts at a different temperature then during the first run indicating chemical changes occurred in the molten state. This change was also verified by an exothermic peak above the melting temperature using TMDSC.
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:kc=Kcop/(p−6.3) wherep is the time of the periodicity expressed in seconds and Kco 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)=Ko(1.298−0.004424He+1.438·10−5He2) whereHe is the flow rate of helium in ml min−1 andKo 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.
Recycled poly(ethylene terephthalate) (R-PET) was chain extended with pyromellitic dianhydride (PMDA) in a commercial size
twin-screw reactive extrusion system. Temperature-modulated differential scanning calorimetry (TMDSC) was used to evaluate
the effect of the chain extension process on the thermal transitions and crystallinity of R-PET. Reactive extruded recycled
PET (RER-PET) samples were tested based on different PMDA concentration and reactive extrusion residence times. The glass
transition temperature (Tg) did not show a significant change as a function of PMDA addition or the extrusion residence time. Melting temperature (Tm) and crystallisation temperature (Tc) decreased with increasing PMDA concentration and with increasing extrusion residence time. RER-PET samples showed double
melting peaks, it is believed that different melting mechanism is the reason behind this phenomenon. The crystallinity of
RER-PET samples is lower than that of R-PET. RER-PET samples at constant PMDA concentration showed a decrease in crystallinity
with increasing extrusion residence time. Results suggest that the reactive extrusion process is more dependent on PMDA concentration
rather than reactive extrusion process residence time.
Different grades of linear low density polyethylenes (LLDPEs) have been quenched cooled step-wise and crystallised isothermally
at (a series of increasing) temperatures in a DSC (thermal fractionated samples). These samples have been investigated by
temperature modulated DSC (MDSC). The heat flow curves of the thermal fractionated materials were compared with those obtained
from samples crystallised at a relatively slow cooling rate of 2 K min-1(standard samples).
The melting enthalpy obtained from the total heat flow of the thermal fractionated samples was 0-10 J g-1higher than those of standard samples. The melting enthalpy obtained from the reversing heat flows was 13-31 J g-1lower in the thermal fractionated samples than in the standard samples. The ratio of the reversing melting enthalpy to the
total melting enthalpy increased with decreasing density of the PE. The melting temperature of the endotherms formed by the
step-wise cooling was 9 K higher than the crystallisation temperature.
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.
Polyethylene samples prepared by thermal fractionation (TF) were annealed in several consecutive cycles in a temperature modulated
DSC (TMDSC) at a temperatures one C below the peak temperatures, increased from cycle to cycle relative to these peaks. The
transition enthalpy of each cooling cycle was greater or equal to that of the preceding heating cycle. The total heat-flows
of each heating cycle corresponded to those of the samples in the reference state up until the vicinity of the annealing temperature.
During the annealing, the heat capacities decreased to a lower value over a one minute period. The thermal memory effect caused
by the thermal fractionation was eliminated by a small overheating of the material for a short time. The fast disappearance
of the thermal memory by a relatively very small degree of heating above their melting temperature denies a long range physical
separation of macromolecules by TF.
Differential scanning calorimetry has been used to study the heat flow during melting and crystallisation of a range of polypropylene
post-consumer waste (PP PCW) grades and blends. The heat flow curves and the heat capacity curves indicated that the PP PCW
grades and blends contained contaminants even after manual sorting and a cleaning process. The enthalpies of the PP PCW grades
were lower than that for the virgin grades, as a result of degradation. Small amounts of polymeric contaminants (up to 10%)
did not affect the enthalpies of PP PCW although other contaminants may have had some effect. The enthalpies of the PCW blends
could in general be predicted by a linear additive rule, which is of importance for recycling a variety of PP PCW products.
Cross-linked polymers have particular rheological responses during reprocessing, e. g. if the material is recycled, special
processing conditions are required. Other virgin polymers can be used as a blending component to enhance rheological properties.
Bi-layer film of EVA/LLDPE was produced on a blown film line and cross-linked by high-energy radiation. This film was ‘agglomerated’
then reprocessed in a twin-screw extruder with virgin LLDPE and blown into film. The miscibility of the blend components was
then studied using a TA Instruments temperature modulated differential scanning calorimeter (TMDSC).
It was found that the cross-linked EVA/LLDPE scrap and the LLDPE have a slight miscibility in the liquid state. A bigger portion
of LLDPE was miscible (dissolved) in EVA in low LLDPE blends. A positive deviation in the heat capacity of the LLDPE component
compared to the additivity rule indicated melting to be more reversible in the first heating cycle. This initial miscibility
was attributed to being induced by high shear during processing. A smaller positive deviation also occurred in the second
heating cycle. This was attributed to intrinsic miscibility.