CHAPTER 3 Experimental Results
3.2.6 DSC analysis on nonisothermal crystallization
The nonisothermal crystallization behaviors of the nanocomposites were studied by DSC, cooling the samples from 410 to 50oC at constant cooling rates of 2.5, 5, 10, 15, 20, 25, and 30oC/min. As shown in Figs. 3.33 to 3.37, the crystallization initiation, peak, and finishing
temperatures, Tci, Tcp, and Tcf, shift to lower temperatures, for both the PEEK and nanoparticle-filled PEEK, as the cooling rate increases. The faster the cooling rate, the more supercooling is required to initiate the crystallization of the PEEK chain segments, since the motion speed of the PEEK chain segments could not catch up the cooling rate [137].
Following the cooling step, the subsequently heating step of the PEEK nanocomposites shows no significant change on the melting points, Tm, of both the filled and unfilled specimens in the DSC diagrams, as shown in Figs. 3.38 to 3.42. The melting temperatures are mostly scattered within 338 ± 2oC, in the typical range of 330-385oC for the PEEK resin [162, 163]. As for the addition of nano particles on the crystallization of PEEK, there are several factors involved; some of them are counteracting each other making the net effect obscure sometimes. For example, in terms of heterogeneous nucleation of PEEK on the nano particle interfaces, the crystallization initiation and peak temperature might increase. However, the obstacle effect from the nano particles on the PEEK mobility and crystallization would lower the crystallization temperatures. Tables 3.7 to 3.9 summarize the data on Tm, Tci, Tcp, Tcf, and the crystallization enthalpy, Hc, for the pure PEEK and nanocomposites.
It is suggested [138,139] that the inclusion of inorganic fillers would lower the crystallization temperature of the resulting nanocomposites under non-isothermal crystallization process. There are two major effects acting simultaneously when the inorganic particles filled polymer undergoes crystallization. One is the decrease in mobility of the chain segments, and the other is the heterogeneous nucleation. Lowering in molecular mobility would play a reverse effect on the perfect crystallite and in turn lower the Tcp and Tm, as a consequence. However, the heterogeneous nucleation would accelerate the deposition of polymer molecules and in turn increase the Tcp and Tm. It should be noted that, while with a lower Tcp for a polymer based materials during solidification, the Tm during subsequent heating would be lower in consequence. This is because the crystalline spherulites formed at
a lower temperature tend to be smaller and possess more defects, leading to a lower Tm in subsequent heating. It follows that the trends for Tcp and Tm tend to be parallel.
As show in Fig. 3.43, the Tcp temperatures of the nanocomposites are found to be all lower than that of the neat PEEK. Overall, the role of decreased molecular mobility seems to be more dominant. It is expected that the nanoparticles in the PEEK matrix could more or less hinder the motion of the polymer chain segments, and in turn impart the smaller and more defects spherulites to the resulting nanocomposite, as compared with the homogeneous crystallization of pristine PEEK. As shown in Figs. 3.43(b), the 0.8 vol% alumina filled PEEK nanocomposite reveals the lowest Tcp. It appears that a very small amount (0.8 vol%) of nanoparticles would result in the greatest reduction in polymer chain mobility and thus in the greatest lowering in Tcp. With increasing nanoparticle amount, the heterogeneous nucleation effect would gradually evolve, providing more sites for nucleation and accelerating the deposition of polymer molecules; both in turn increasing Tcp. Hence, as shown in Fig. 3.43, the Tcp temperatures for all the nanocomposites would again shift to higher temperatures as the filler contents increase. However, even at 10 wt% or ~5 vol%, the Tcp temperatures for all the nanocomposites are still slightly lower than that of the pure PEEK.
Another factor affecting the Tcp of the nanocomposite could be the thermal conductivity of the inorganic fillers. This factor has not been carefully considered before. The thermal conductivities of the PEEK, silica, and alumina at room temperature are reported to be 0.2, 1, and 30 Wm-1K-1, respectively [59]. It is obvious that the thermal conductivities of the ceramic fillers are higher than that of the PEEK polymer, and in turn the thermal conductivity of the PEEK polymer would be enhanced when the inorganic filler was incorporated. With the higher thermal conductivity for the PEEK based nanocomposites, the temperature of the
PEEK polymer could reach the set temperature earlier during DSC cooling. Accordingly, the PEEK nanocomposite would crystallize at a higher crystallization temperature when the filler contents are sufficiently high. That could also be one of the reasons why Tcp of the nanocomposites show increasing trend with increasing nanoparticle content. This might also the reason that the alumina-filled PEEK composite, even with a lowest Tcp at 0.8 vol%, exhibits greater increasing tendency in Tcp at higher filler contents.
Therefore, the joint effects from the lowering of the PEEK molecule mobility, the enhancement of heterogeneous nucleation at higher filler contents, and the increases in thermal conductivities of the nanocomposites might be concurrently responsible for the lower extremas in Tcp at the filler content of 2.5 wt%, as shown in Fig. 3.43.
As expected, the addition of inorganic filler could result in more defects in crystallites when the nanoparticle-filled PEEK underwent crystallization during cooling stage, and the crystallization defects would lower the melting temperature Tm of the resulting PEEK nanocomposite. In addition, the spherulites in smaller size or of more defects could be molten at lower melting temperature Tm, as compared with the homogeneous crystallization in the neat PEEK. On the other hand, the nanoparticles would offer the sites for heterogeneous nucleation [139]. Hence, the higher nanoparticle content might impart the higher Tcp, Tm, and a higher degree of crystallinity to the resulting nanocomposites when the filler content increases. However, as shown in Tables 3.7 to 3.9 and Figs. 3.43 to 3.44, as the filler content increases, there is a decreasing trend for both Tcp and Tm, as compared with those of the neat PEEK, due to mainly the hindrance in molecular mobility. The heterogeneous nucleation effect was shadowed somewhat. It is well known that the polymer molecules could rearrange and re-crystallize at the heating stage. Hence, the maximum decrements for the Tm of the PEEK nanocomposites (in Fig. 3.44, about -8oC) would be
smaller than those of the Tcp (in Fig. 3.43, about -30oC).
In literature, it was proposed [164] that the inclusion of nano-sized zinc oxide filler (2 wt% in amount and 40 nm in size) into the isotactic polypropylene would increase the Tcp by about 3oC and the heat of crystallization. This crystallization study [164] was conducted isothermally. It was concluded that the enhancement in Tcp could be attributed to the increase in specific surface area provided by zinc oxide nanoparticles, where the polymer chain segments would deposit on and crystallize.
Nevertheless, in the sense of non-isothermal crystallization, Kim et al. [139] suggested that the inclusion of the nano-sized silica into the poly(ethylene 2,6-naphthalate), PEN, would lower the Tcp by about 4 to 9oC as the filler content increases from 0.3 to 0.9 wt% at a cooling rate of 10 oC/min. However, the Tm was found to only slightly increase by 0.5 to 0.9oC. The increase in filler content would lower the Tcp of the PEN nanocomposite. As expected, the decrease in Tcp might be attributed to the less mobility of the PEEK chain segments when the nano-sized silica was introduced. The current study appears to follow along the line of this paper, with the effect in lowering polymer mobility being stronger than the heterogeneous nucleation effect.
It is possible to investigate the effects of cooling rate and filler content (wt% or vol% in Table 2.1) on the overall crystallization time, tc, of the nanoparticles filled PEEK polymer, as shown in Figs. 3.45 and 3.46, respectively. The overall crystallization time can be defined as follows [139]:
c cf ci
c R
T T
t −
= , (3.6)
where Rc is cooling rate. As shown in Fig. 3.45, as expected, the increase in cooling rate would significantly lower the overall crystallization for both the pristine PEEK and the nanoparticles filled PEEK composites. At the same cooling rate, it is shown that the smaller size of 15 nm silica nanoparticles would contribute the more crystallization time to the PEEK polymer, as compared with that of the 30 nm silica, when the filler contents were increased from nil to 10 wt%, Fig. 3.46(a). The same trend could be also seen in Fig. 3.46(b), which the filler content is expressed in terms of volume fraction.
From the DSC curves, the absolute crystallinity fraction Xc at different cooling rates can be estimated by relating to the heat of fusion of an infinitely thick PEEK crystal,
∆
Hfo, as [162],W 100 H X H
polymer o
f c
c = ×
∆
∆ , (3.7)
where
∆
Hfo is ~130 J/g [163] and Wpolymer is the weight fraction of polymer matrix. As shown in Tables 3.7 to 3.9 and Fig. 3.47, it is obvious that a slower cooling rate would result in a slightly higher crystallinity value, as a result of more sufficient time for crystallization.Furthermore, the inclusion of nanoparticles, irrespective of silica or alumina, would result in slightly lower crystallinity fractions of the resulting PEEK composites on the basis of same cooling rate, as compared with the pristine PEEK. However, irrespective of the silica or the alumina filled into the PEEK polymer, the resulting nanocomposites with a nanoparticle content of 5 wt% show an extrema in crystallinity: the maximum Xc values for the prestine PEEK, silica-filled PEEK (15 and 30 nm SiO2), and alumina-filled PEEK (30 nm Al2O3) at a cooling rate of 2.5oC are 39.1, 39.6, 39.3, and 39.8, respectively. As a result, the inclusion of nanoparticles is found no significant enhancement on the crystalllinity of the resulting nanocomposite. On the contrary, the inclusion of nanoparticles into PEEK matrix could more
or less lower the crystallinity of the resulting PEEK nanocomposite at low cooling rates. The more the content of the nanoparticles in PEEK, the lower the crystallinity of the PEEK segments would be, as shown in Tables 3.7 to 3.9 and Fig. 3.48.
As stated early, the filler in polymer matrix do affect the molecular mobility when the molecules start to crystallize. As a consequence, the crystallinity of the polymer would decrease with the filler content up to the contents level of 2-5 wt%, as shown in Fig. 3.48(a).
The same trend could be also found in Fig. 3.48(b) when the filler content is presented in terms of volume fraction. It seems that the smaller nanoparticles would result in a higher crystallinity.
The DSC results, coupled with the XRD patterns, suggest that there has been minimum chemical interaction between the PEEK polymer and ceramic nanoparticles occurred at the forming temperature of 400oC. But the crystallization temperature and of the crystallinity fraction Xc of the PEEK matrix would be affected by the amount of nanoparticles, with the melting temperature Tm of PEEK matrix unchanged.