5. Investigation of nanocomposites based on polylactide and MgAl/LDH
5.3 Crystallinity of the nanocomposites investigated by DSC and WAXS
Figure 5.5 compares the DSC curves of the as prepared samples (first heating) run for pure PLA, MgAl-1 wt% and MgAl-12 wt%. All measurements discussed here were carried at heating rate of 10 K/min. A glass transition region as well as crystallization and melting is observed for each concentration of the nanofiller. The glass transition temperature does not change significantly with the concentration of the nanofiller. This concerns also the width of the glass transition.
Interestingly, compared to the pure PLA the melting enthalpy of MgAl-1 wt% is much larger. Moreover, a pronounced cold crystallization peak is observed for MgAl-1 wt%.
This suggests that the LDH acts as a nucleation agent. A similar effect was also discussed in the literature. For instance Pilla et al. showed that the addition of recycled wood fibers leads to an increase of the crystallinity of PLA164. This will be discussed in more detail below.
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250 300 350 400 450 500
exo
PLA PLA1 PLA12
Glass Transition Cold
Crystalization
Heat Flow [a.u.]
T[K]
Melting
Figure 5.5. Comparison of DSC thermograms (as prepared samples, first heating, heating rate of 10 K/min) for pure PLA and selected nanocomposites: solid line - pure PLA, dashed line - MgAl-1 wt%, dashed dotted line – MgAl-12 wt%. The curves are shifted along the y-scale for sake of clearness.
Table 5.1 shows that for concentrations higher than 1 wt% the melting enthalpies (first heating) decrease with increasing concentration of LDH because the amount of polymer decreases with increasing content of LDH. From the melting enthalpy Hm the degree of crystallization can be calculated. In a first step, the enthalpy values have to be normalized to the amount of polymer. The degree of crystallization is given
100
1 C ) H / H 100 *
( LDH m m, %
where Hm,100% is the melting enthalpy of a complete crystalline PLA sample. The value of Hm,100%=93 J/g is taken from references 168, 165. In Figure 5.6, increases for CLDH=1 wt% compared to the bulk value. This means for a small amount of LDH the degree of crystallinity increases. This gives further evidence that the nanofiller acts as a nucleating agent and the crystallinity of the nanocomposites increases for low concentrations of LDH compared to the pure PLA. For higher values of CLDH the degree of crystallization decreases approximately linearly with increasing concentration of LDH.
The extrapolation of to zero value results in a critical concentration of LDH CCri of ca.
21 wt%. From the DSC measurements (first heating run) it is therefore concluded that for concentrations higher than this value, the cold crystallization of PLA is completely suppressed at 21 wt% LDH for the as prepared samples. This behavior is similar to that observed for nanocomposites based on olefins and LDH.
63 Absolute values of the crystallinity can be also calculated with the wide angle X-ray scattering pattern (WAXS). For that analysis the q range from 1.5 nm-1 to 40 nm-1 is chosen and 60 counts were subtracted from the spectra as detector background. Moreover, the Kratky representation is used because q2I(q) is approximately proportional to the real number of scattered photons and more importantly to have equal condition in the whole q range. The integral
40 1
5 1 1
nm 2 nm .
total
total q I (q)dq
A is the total scattered intensity due to both the amorphous and crystalline regions. From the q2Itotal(q) curves the amorphous contribution is subtracted by hand yielding to q2Icrys(q) due to the scattering of the crystalline regions including possible mesomorphic contributions166,167. The integral
40 1 5 1 1
nm 2 nm .
crys crys q I (q)dq
A is
the total scattered intensity due to the crystalline regions. The degree of crystallization is then given by Acrys/Atotal. In Figure 5.6 the degree of crystallization estimated from the WAXS data are also included to compare with the DSC data. The dependence of versus the concentration of LDH resembles estimated from the WAXS measurements is similar to the dependence extracted from the DSC (see Fig. 5.6) was also the absolute values agree in the frame of the experimental error. For pure PLA a crystallization degree about 10 % is obtained which is in agreement with literature data (see for instance [168]). Both data sets can be described by the same linear regression leading to a similar critical concentration of ca. 21 wt% LDH were the cold crystallization of PLA is completely suppressed. The agreement of the degree of crystallization estimated from the WAXS investigation and the DSC data for the first heating run is expected because in both cases the samples have the same thermal history (as prepared samples).
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0 5 10 15 20 25
0 10 20 30 40 50 60
0 5 10 15 20 25
0 20 40 60
=0
[%]
CLDH [wt%]
[%]
CLDH [wt%]
Figure 5.6. Degree of crystallization vs. concentration of LDH squares – DSC measurements (first heating run), circles – WAXS data. The solid line is a linear regression using both data sets. The inset compares the concentration dependence of the degree of crystallization estimated from DSC measurements: squares – first heating run, stares – second heating run. The lines are linear regressions to the corresponding data.
The inset of Figure 5.6 compares the degree of crystallization estimated from the first and the second heating run of the DSC measurements versus the concentration of LDH.
For the second heating the degree of crystallization measured by DSC is essentially lower than the values obtained by the first heating run. Moreover, in the case of the second heating run the dependence of on the concentration is stronger than for the first one which gives rise to a lower value for the critical concentration of ca. 14 wt% where becomes zero (see inset Figure 5.6). This is different to the behavior found for polypropylene and polyethylene filled with LDH. Compared to polypropylene and polyethylene the crystallization rate of PLA is essentially lower. In the case of the first heating run, the samples were cooled down slowly in the press. For the second heating run, the samples were crystallized inside the DSC pan by cold crystallization at a heating rate of 10 K/min. Within the experimental error the different behavior observed for the different thermal histories can be understood by assuming that the main influence of the nanofiller on the cold crystallization behavior is a reduction of the crystallization rate. A detailed DSC study where the cooling rate is varied in a broad range is in progress to investigate this further.
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