6 RESULTS AND DISCUSSION
6.7 S YNDIOTACTIC P OLYPROPYLENE /C ARBON B LACK N ANOCOMPOSITES
6.7.2 Crystallization and Melting Behavior of the sPP/CB Nanocomposites
6.7.2.1 Melting Temperature and CrystallinityThe melting temperatures of the sPP/CB nanocomposites were in the range of 139 to 141°C and, therefore, in the same region as for the neat polymer (see Tab. 8 at the end of this chapter). Also the crystallinity of 28 %, calculated with a value of 164 J/g (see section 6.1.3.1), was comparable to that of the pure polymer for all sPP/CB nanocomposites.
6.7.2.2 Crystallization Temperatures
The crystallization temperatures of the sPP/CB nanocomposites shown in Fig. 79 and Tab. 8 were elevated by 4 to 6 °C with respect to those of the pure polymer. A linear increase of the crystallization temperature with rising filler content was detected in the range of filler loadings studied.
90 92 94 96 98 100 102 104
0 0.2 0.4 0.6 0.8 1 1.2
Filler Content [weight-%]
Crystllization Temperature [°C]
sPP/CB pure sPP
Fig. 79: Influence of the filler content on the crystallization temperature of the sPP/CB nanocomposites.
6.7.2.3 Half-time of Crystallization
The influence of the isothermal crystallization temperature and the filler content on the half-time of crystallization of the sPP/CB nanocomposites is shown in Fig. 80 and Tab. 8.
Naturally, the crystallization half-time increased with increasing isothermal crystallization temperature because the crystallization proceeded more slowly. Although no significant differences in the half-time of crystallization were observed regarding the different filler
loadings, it should be noted that the crystallization proceeded faster in the presence of carbon black than in the pure polymer. The time that it took 50 % of the material to crystallize was reduced roughly by one-half upon addition of carbon black for all crystallization temperatures investigated.
0 5 10 15 20 25
114 116 118 120 122 124 126
Crystallization Temperature [°C]
Hlalf-time of Crystallization [min] KW334_0.1%
KW333_0.6%
KW335_1%
pure sPP
Fig. 80: Influence of the isothermal crystallization temperature on the half-time of crystallization of the sPP/CB nanocomposites.
6.7.2.4 Avrami Analysis
An Avrami analysis of the data obtained from isothermal crystallization experiments was conducted also for the sPP/CB nanocomposites. The rate constant of crystallization of the sPP/CB nanocomposites is shown in Fig. 81 (see also Tab. 8). It is clearly seen that the rate of crystallization decreased with increasing isothermal crystallization temperature. This was expected and observed for all sPP/CB samples investigated. The incorporation of CB also increased the rate of crystallization for all samples tested. Up to a filler content of 0.6 %, the rate constant of crystallization increased with increasing content. The value was four times that of the pure polymer at an isothermal crystallization temperature of 120 °C for a filler loading of 0.6 %, for example. The rate of crystallization slowed down somewhat for higher loadings. This could be due to a hindered mobility of the polymer chains due to the presence of the fillers.
118
120
122 pure sPP
0.1 %
0.6 % 0.000 1 %
0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010
Rate Constant of Crystallization [min-n ]
Isothermal Crystallization Temperature [°C]
Filler Content [weight-%]
Fig. 81: Influence of the isothermal crystallization temperature and the filler content on the rate constant of crystallization of the sPP/CB nanocomposites.
6.7.3 Thermal Stability of the sPP/CB Nanocomposites
0 0.1
0.6 1.0
Onset
Inflection Point End 300
320 340 360 380 400 420 440
Temperature [°C]
Filler Content [weight-%]
Fig. 82: Mass-loss of the sPP/CB nanocomposites in dependence of the temperature as determined by TGA.
The degradation behavior of the sPP/CB nanocomposites was investigated using TGA at a heating rate of 5 °C. The results for different filler loadings in comparison with the results for the pure sPP are shown in Fig. 73 and Tab. 8.
Low filler contents seemed to stabilize the polymeric matrix with regard to the degradation temperature. This was evident for the low filler content of only 0.1 % of carbon black. The onset of crystallization was raised by 6 °C and the inflection point by 20 °C as compared to the neat polymer, whereas, the end of degradation was reached at roughly the same temperature. Similar results were obtained for a filler content of 0.6 %. In contrast to this, the high filler content of 1 % led to lowered degradation temperatures. Not only onset but also inflection point and end of degradation took place at lower temperatures than in the pure polymer.
6.7.4 Electrical Conductivity of the sPP/CB Nanocomposites
The conductivity of the samples with different filler contents was evaluated using two-point measurements as described in the experimental section. None of the films investigated showed any conductivity.
6.7.5 Tensile Properties of the sPP/CB Nanocomposites
The tensile properties of the sPP/CB nanocomposites were determined in a similar manner as for the other nanocomposites. The influence of the filler content on the yield strength is depicted in Fig. 83 (see also Tab. 8). It shows that the presence of carbon black only has a minor impact on the yield strength of the polymer which was raised by 1.5 MPa on average upon addition of carbon black.
15 16 17 18 19 20 21
0 0.2 0.4 0.6 0.8 1 1.2
Filler Content [%]
Yield Strength [MPa]
sPP/CB pure sPP
Fig. 83: Influence of the filler content on the yield strength of the sPP/CB nanocomposites.
The elastic modulus exhibited a high degree of scattering and is not shown here. In comparison to the neat polymer, the elongation at break remained approximately unchanged for the range of filler contents investigated.
Tab. 8: Properties of some sPP/CB nanocomposites.
Filler Content [wt.-%] 0 0.1 0.5 1
Activity
[kgPol/(molZr⋅h⋅molMon/l)] 4,300 4,800 4,500 4,700
Melting Temperature [°C] 140 141 139 139
Crystallization
Temperature [°C] 96 100 101 101
Rate Constant of
Crystallization [⋅10-4 min-n] 7.5 6.4 48 37
Half-time of
Crystallization [min] 14.6 7.6 5.1 6.7
Degradation Temperature
(Tmax) [°C] 412 421 412 390
Tensile Strength [MPa] 17.5 18.8 18.4 19.1
aRate constant of crystallization and half-time of crystallization were determined from isothermal DSC measurements at 122 °C.