Poly(phenylene sulfide) magnetic composites. II. crystallization, thermal, and viscoelastic properties.
Poly(phenylene sulfide) (PPS) is a high-performance semicrystalline polymer that has been utilized in a variety of market segments such as electrical, electronic, automotive, appliance, industrial, and chemical sectors due to its excellent thermal stability, chemical resistance, flame resistance, and electroinsulating property [1, 2]. The relative low glass transition temperature and impact strength, however, restricts its further applications for engineering uses. Several methods were developed to overcome those marginal properties. One approach is to blend PPS with other polymers to improve impact strength and high heat distortion temperature [3-13]. Another approach to increase the glass transition temperature and toughness is to manufacture the filled PPS composites. Hitherto many micro-scaled and nano-scaled filler, such as short glass fiber, silicon dioxide, expanded graphite, metal and its oxide/sulfide have been successfully compounded with PPS. Majority of those work focuses on the tribological and mechanical properties, crystallization, and conductivity behavior [13-19].
In the filled polymer systems, those composites containing magnetic particles have attracted great attention due to their potential applications, including high-capacity magnetic storage media, magnetic refrigeration at high temperature, color imaging, ferrofluids, bioprocessing, medical diagnosis, electromagnetic wave absorption, and so forth [20-24]. The ferrosoferric oxide ([Fe.sub.3][O.sub.4]), which shows a distinct paramagnetic behavior, has been incorporated successfully into many polymer matrices such as polypyrrole, polyaniline, and polyamides by in-situ polymerization [25-28]. The nice dispersion of the magnetite particles in those composites is attributed to the strong interactions such as coordinate bonding and hydrogen bonding between matrix and particles. It is well accepted that PPS and Nylon are two most commonly used polymer matrices for bonded metal oxide. Thus, to incorporate PPS with magnetite particles is an interesting work because the low moisture absorption characteristics, good corrosion resistance, light weight for metal replacement, and multiple component integration capability all make filled PPS magnetic composite favorable for many automotive and high temperature applications.
In our previous work , the PPS/[Fe.sub.3][O.sub.4] composites with various loading levels were prepared by melt compounding and, the physical properties were then studied in terms of linear rheology, conductivity and magnetic properties, aiming at relating those behaviors to the mesoscopic percolation structure of magnetite particles. The results show that the PPS/[Fe.sub.3][O.sub.4] composite presents a typical percolation behavior in both the rheological and electrical response. The rheological percolation threshold is lower than 20 vol%, while that of electrical percolation is higher than 25 vol%. The lower level of rheology percolation is due to the formation of transient network, in which the particles loadings is in the levels of "close but no contact." However, the physical percolation behavior has little influence on the magnetic properties, because the yielded magnetic interactions among filled particles in the magnetic field are far stronger than those nonmagnetic physic interactions resulting in the percolation. In this work, the thermal and viscoelastic properties as well as crystallization behavior and kinetics were studied, aiming at further exploring the mesoscopic structure of the magnetite particles and its influence on the physical properties of PPS composite.
PPS (sieved through 40 mesh/[in.sup.2], number average molecular weight of 2100 g/mol) used in this study was obtained from Deyang Sci & Tech Co., P. R. China. The ferrosoferric oxide ([Fe.sub.3][O.sub.4]) powder, which has an average diameter of about 1-3 [micro]m, was supplied by Shanghai Shanhai Co., P. R. China. Its density is about 5.18 g/[cm.spu.3], and the [Fe.sub.3][O.sub.4] content is higher than 98 wt%, which comprises ferrous oxide (FeO) of about 27-29 wt%.
PPS/[Fe.sub.3][O.sub.4] composites (PPSs, where s denotes the weight ratio of [Fe.sub.3][O.sub.4]) were prepared by direct melt compounding [Fe.sub.3][O.sub.4] powder with PPS in a HAAKE Polylab Rheometer (Thermo Electron Co., USA) at 290[degrees]C and 50 rpm for 8 min. The [Fe.sub.3][O.sub.4] loadings are 20, 30, 40, 50, 55, 60, and 70 wt%, respectively. The PPS and [Fe.sub.3][O.sub.4] were respectively dried at 100 and 80[degrees]C under vacuum for 12 h before using. For better comparison, the pure PPS sample was also processed in Rheometer to keep identical thermal history with that of composites. The sheet samples in thickness of about 1 mm for the subsequent measurement were prepared by compression molding at 300[degrees]C and 10 MPa.
The morphologies of the fractured surfaces of the samples were investigated using a PHILIPS XL-30ESEM scanning electron microscope with 20-kV accelerating voltage. The sheet samples were kept in liquid nitrogen and then brittle fractured. An SPI sputter coater was used to coat the fractured surfaces with gold for enhanced conductivity. The crystallization morphology of neat PPS and its composites film samples were studied using a polarized optical microscope (POM, LEICA BX51) equipped with a hot stage (Linklam LTM350). The same temperature ramps were used as in differential scanning calorimeter (DSC) testing.
Nonisothermal Crystallization Process
Nonisothermal crystallization was carried out on a NETZSCH DSC-204F1 differential scanning calorimeter. The samples about 5 mg in weight for DSC were cut from the film. In the nonisothermal crystallization process, the samples were melted at 300[degrees]C for 10 min to eliminate the previous thermal history, and then cooled at constant cooling rates of 5, 10, 20, and 40[degrees]C/min. The exothermal curves of heat flow as a function of temperature were recorded to analyze the nonisothermal crystallization process of the PPS and its composites. All the experiments were carried out under nitrogen.
Rheological measurements were carried out on a rheometer (HAAKE RS600, Thermo Electron Co., USA) equipped with a parallel plate geometry using 20-mm diameter plates. All measurements were performed with a 200 FRTN1 transducer with a lower resolution limit of 0.02 g cm. The samples about 1.0 mm in thickness were melted at 290[degrees]C for 5 min in the parallel plate fixture to eliminate residual thermal history, and then the small amplitude oscillatory shear (SAOS) measurements were carried out immediately at predetermined temperatures. In the linear viscoelastic measurements, the dynamic strain sweep measurements were carried out first to determine the linear region. Then, the dynamic frequency sweep measurements were carried out on those samples pre-sheared or not at the strain level of 1%. In the steady shear measurements, the stress and viscosity response to the shear rates (0.001-100 [s.sup.-1]) were recorded.
Dynamic Mechanic Thermal Analysis
The dynamic mechanical properties of the PPS and its composites were determined using a DMA-242C dynamic mechanical thermal analyzer (NETZSCH Co., USA). The testing was performed in three-point bending mode at a vibration frequency of 5 Hz in the temperature range from 20 to 200[degrees]C at a heating rate of 5[degrees]C/min in [N.sub.2] atmosphere.
TGA analyses were performed on a Netzsch Instruments STA409PC (Germany). Samples of about 10-12 mg were heated from room temperature to 800[degrees]C at a rate of 10[degrees]C/min under a nitrogen atmosphere. All TGA results are the average of a minimum of three determinations and the temperatures are reproducible to [+ or -] 1[degrees]C.
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSION
Dispersion of the Magnetic Particles
The uniform distribution of the [Fe.sub.3][O.sub.4] particles in the PPS matrix is shown in Fig. 1. Nearly no obvious aggregation of particles can be observed even at high loading levels of 60 wt% and, those dispersed particles present the average diameters of about 3 [micro]m, which is very close to that of neat [Fe.sub.3][O.sub.4] particles. The nice dispersion is due to the strong interactions between particles and PPS matrix, and the possible interactive mechanisms have been discussed in our previous work .
Viscoelastic Property of the PPS Magnetic Composites
Viscoelastic property is one of the most important physical properties for those multiphase polymer systems because it is highly dependent on the internal structure, and also on the filler dispersion and the interfacial interactions especially for those filled polymer composites [30, 31]. Figure 2 gives the dependence of the apparent viscosity ([eta]) on the shear rate ([dot.[gamma]]) for the neat PPS and its composites obtained from the steady shear sweep. At low shear rate region, it can be seen that the apparent viscosity increases with increasing [Fe.sub.3][O.sub.4] loadings remarkably, and even by more than four orders at high loading levels of 60 wt%. This is due to the reinforcement effect of the well dispersed filler. On the other hand, the linear viscoelastic region reduces with increasing [Fe.sub.3][O.sub.4] loadings and finally disappears at high loading levels, showing a significant shear thinning behavior. However, it is notable that at the high shear rate region, the apparent viscosities of the PPS40, PPS50, PPS55, and PPS60 samples are close to one another, while those of PPS10 and PPS20 samples are almost equal to that of neat PPS. It suggests that the loading level of 40 wt% (about 18 vol%) for [Fe.sub.3][O.sub.4] particles is a critical value influencing the viscoelastic response of the PPS magnetic composites, which is corresponding to the percolation threshold obtained by the SAOS measurements in our previous work .
The Han plots of G' ~ G"  obtained from SAOS measurements are shown in Fig. 3. The curves of the composites present particle loading dependence clearly. It confirms the presence of strong interactions between magnetic particles and PPS matrix. In addition, the reduced slope with increase in the particle loadings indicates that the composites become more heterogeneous . The slope also changes with the frequency, giving three distinct regions and, it is noteworthy that the inflection point where the slope is changed shifts to a higher frequency with increasing ferrite content. This indicates that much energy is necessary to change the degree of heterogeneity due to the increased physical association within the composite systems at a high loading level. Those strong interactions and/or physical association between magnetic particles and matrix will change the relaxation behavior of the PPS inevitably. The relaxation time ([lambda]) can be calculated as follows:
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[lambda] = G'/([eta]* x [[omega].sup.2]) (1)
The [eta]* is the complex viscosity and [omega] is the frequency. The ratio of the relaxation time of composites to that of neat PPS ([DELTA][lambda]) increases gradually with particle loadings and, finally to about six for the PPS60 sample at the low frequency (0.01 Hz). The results indicate that the role of particles to make polymer need longer time for the relaxation becomes stronger with increasing loading levels. In other words, the presence of magnetic particles restricts chain mobility of the PPS matrix.
On the other hand, the strong interactions also occur in among particles themselves, namely percolation. Han  believed that the rule, temperature independence of G' ~ G" for homogeneous polymer systems, could be also applied to the filled polymer composites. Figure 4a gives the Han plots at various temperatures for the percolated PPS60 sample. It is obvious that the curves deviate from the slope of 2 in low-frequency region due to the enhanced elastic responses of this percolated system. However, the curves at various temperatures can not well coincide both at the low and high frequencies. It indicates that the mesoscopic percolation structure of magnetic particles in the PPS matrix may change with temperature. From the curves of unitary normalized modulus shown in the inset graph, it can be seen that the composite shows a weak strain overshoot at the region of large amplitude before shear thinning (See the arrow), which can not be observed on neat PPS. It is well accepted that this strain overshoot is attributed to the association and/or interactions between those aligned polymer chains and filler [32-35]. Those physical association and/or interactions alter with temperature inevitably because the mobility and characteristic structure of the chains, as well as the shortest distances among particles, are temperature-dependent. As a result, the three-dimensional percolation networks of the magnetic particles also show the temperature dependence.
Van Gurp-Palmen plots are usually used to detect the rheological percolation . Figure 4b plots the corresponded phase angle ([delta]) as a function of the absolute value of the complex modulus (|G*|) for the PPS60 sample at various temperatures. A significant decrease in the phase angle at low modulus can be observed clearly with increasing temperature (See the arrow), suggesting an increase in the density of the network structure. The downshift of phase angle to the low-modulus region further confirms that the PPS chain mobility increases with increasing temperature, which results in an enhancement of collision and friction among particles because of the strong interactions between PPS chain and [Fe.sub.3][O.sub.4] particles. Therefore, the percolation threshold decreases with increase of temperature. Similar phenomenon has also been observed on some polymer/nanotubes systems [37-39]. It can be expected that the temperature dependence of rheological percolation is significant to the processing of this kind of filled PPS composites.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Moreover, the rheological percolation also presents high sensitivity to the quiescent shear. Figure 5 shows the effect of quiescent preshear on the SAOS responses of the PPS40 sample. It can be seen that phase angle of the presheared sample increases with increase of preshear rate remarkably. As the shear rate achieves up to 0.5 [s.sup.-1], the minimum value of phase angle is higher than 45[degrees], which is indicative of the viscous fluid responses within the composites. In this case, both the PPS chain and particles are oriented along the shear direction. As a result, the transient percolation network is broken down due to the sharply reduced particle-particle interactions, leading to a remarkable change of viscoelastic behavior from solid-like to a liquid one.
Thermal Behavior of the PPS Magnetic Composites
Figure 6a shows the effect of particle loadings on the dynamic storage modulus (E') obtained from the dynamic mechanical thermal analysis. Clearly, addition of magnetic particles leads to remarkable increase of E'. As the loadings achieve up to 55 wt%, the E' increases from 4 GPa to about 8 GPa, almost one time higher than that of neat PPS. As mentioned above, on the one hand, the magnetic particles give rise to the enhanced modulus themselves. On the other hand, the presence of the magnetic particles restricts the chain mobility of PPS matrix due to their strong interactions and physical association, which also results in higher modulus for the composites. This restriction of the segmental motion is confirmed by the evident increase of the glass transition temperature ([T.sub.g]), as can be seen in Fig. 6b.
Thermal stability is an important property for which the filler in composites plays an important role. The results from the thermal analysis of neat PPS and its composites are summarized in Fig. 7. A remarkable improvement in the thermal stability can be observed. The 5 wt% loss temperature ([T.sub.5 wt%]) increases with increasing [Fe.sub.3][O.sub.4] loadings from 499 to 517[degrees]C and, the stable residue for the composite corresponds well with the particle loadings. This improvement is attributed to three aspects: good matrix-particle interaction, nice thermal conductivity of the [Fe.sub.3][O.sub.4] particles, and also due to their barrier effect. As the loadings achieve up to 55 wt%, [T.sub.5 wt%] even increases by about 18[degrees]C in contrast to that of neat PPS. Accordingly, the presence of [Fe.sub.3][O.sub.4] particles can take up heat and hinder transport of degradation products more easily, leading to remarkable increase of thermal stability.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
Nonisothermal Crystallization Behavior and Kinetics
It is well accepted that the mechanical and physical properties of the crystalline polymers are governed by the supermolecular morphology, which in turn is controlled by the crystallization process. The crystallization behavior of the neat PPS has been studied extensively [40-48]. For the PPS magnetic composites, the nonisothermal crystallization and the followed melting curves are shown in Fig. 8a and b, respectively. The corresponded calorimetric parameters are listed in Table 1. [T.sub.c] and [T.sub.m] are the temperatures of crystallization and melting peak. [DELTA][H.sub.c] and [DELTA][H.sub.m] are the corresponded enthalpies normalized to unit mass of PPS matrix. [X.sub.c]%, the degree of crystallinity, is calculated from the melt enthalpy [DELTA][H.sub.m] by the following equation:
[X.sub.c] = [DELTA][H.sub.m]/[DELTA][H.sub.m.sup.0] (2)
where [DELTA][H.sub.m.sup.0] is the melt enthalpy of the ideal crystal, which was assumed to be 114 J/g according to Huo and Cebe  It is more or less surprising that [T.sub.c] decreases gradually with increasing loading levels of particles. This suggests that the [Fe.sub.3][O.sub.4] particle may not act as additional active substrates to promote crystallization of PPS matrix and, only exist in the composites as inert filler. In addition, the presence of this inert particle reduces the ability of PPS chain to be fully incorporated into growing crystalline lamella, leading to the formation of more defect ridden crystalline lamella and less ordered crystals. As a result, both the [T.sub.m] and [DELTA][H.sub.m] decrease with increase of particle loadings, as can be seen in Fig. 8b.
POM observation was then carried out to explore the effect of the [Fe.sub.3][O.sub.4] particle on the spherulites structure of PPS, as shown in Fig. 9. The grainy structure seen on the micrograph represents spherulites. However, it is difficult to directly observe the influence of the [Fe.sub.3][O.sub.4] particle on the crystallization of the matrix, because both the neat PPS and the composite show the small spherulites sizes after nonisothermal crystallization, which are out of the range for POM experiment. Therefore, the kinetics study is necessary to further investigate the crystallization behavior of the PPS magnetic composites. The relative degree of crystallinity ([X.sub.t]), as a function of crystallization temperature (T), is defined as
[X.sub.t] = [[integral].sub.[T.sub.0].sup.T] (d[H.sub.c]/dT)DT/[[integral].sub.[T.sub.0].sup.[T.sub.[infinity]]] (d[H.sub.c]/dT)dT (3)
where [T.sub.0] and [T.sub.[infinity]] represent the onset and the end of crystallization temperature, respectively. The typical relative crystallinity curves for the composites are shown in Fig. 10. The results of half crystallization time [t.sub.1/2] are listed in Table 2. From the onset time [t.sub.0], the induction time of the crystallization until start of crystallization can be estimated. The [t.sub.0] value for all composites samples is always higher than that of pure PPS (see the arrows in Fig. 10b), which further confirms that the [Fe.sub.3][O.sub.4] particle does not have heterogeneous nucleating effect on the crystallization of PPS matrix. On the contrary, the presence of inert [Fe.sub.3][O.sub.4] particle restricts the segmental motion of PPS chain, results in a sharp increase of melt viscosity, as already discussed in Fig. 2. The onset time [t.sub.0] and the half crystallization time [t.sub.1/2], as a result, increase with increasing particle loadings due to the reduced ability of chain motion. Therefore, the [Fe.sub.3][O.sub.4] particle plays the role of inhibitor to the crystallization of PPS matrix.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
In generally, the Avrami equation  can also be used to describe nonisothermal crystallization process:
X(t) = 1 - exp(-k[t.sup.n]) (4)
log[-ln(1 - X(t))] = nlogt + logk (5)
[FIGURE 11 OMITTED]
where X(t) is relative crystallinity at crystallization time, t, n is the Avrami exponent, k is the crystallization rate constant. In this case, however, the parameters n and k lost their physical meanings, because the temperature changes constantly during nonisothermal crystallization. Presuming that the nonisothermal crystallization process is composed of many infinitesimal isothermal one, Ozawa  described the crystallization kinetics by the following equation:
X(T) = 1 - exp(-K(T)/[[phi].sup.m]) (6)
ln[-ln(1 - X(T))] = ln K(T) - mln[phi] (7)
where X(T) is a heating function, m is the Ozawa exponent, [phi] is heating rate. Plots of ln[-ln(l - X(T))] vs. ln[phi] are shown in Fig. 11. Clearly, only some folded lines can be obtained for the composites. These changing slopes indicate that m is not constant with temperature during the primary crystallization process. Thus, the Ozawa method is not adequate to describe the crystallization kinetics of PPS magnetic composites. The reasons of this invalidity are probably due to the strong secondary crystallization of PPS [6, 48], and/or the dependence of lamellar thickness on crystallization temperature as well as the constant heating function over the entire crystallization process .
Mo and coworkers [51, 52] developed an approach to study nonisothermal crystallization of the polymeric systems. For the nonisothermal crystallization process, physical variables relating to the process are relative degree of crystallinity, [X.sub.t], heating rate, [phi], and crystallization temperature, T. At a given crystallinity [X.sub.t], the Avrami equations can be rearranged as follows:
ln[phi] = lnF(T) - [alpha]lnt (8)
where F(T) = [K(T)/[Z.sub.t]][.sup.1/m] refers to the value of heating rate, which must be chosen within unit crystallization time when the measured system amounts to a certain degree of crystallinity. [alpha] is the ratio (n/m) of Avrami exponent n to Ozawa exponent m. According to Eq. 8, at a given degree of crystallinity, plots ln[phi] vs. lnt yields clearly a nice linear relationship between ln[phi] and lnt both for the neat PPS and for the composites, as can be seen in Fig. 12. The nice linearity of those curves indicates that the Mo model provides a satisfactory description to the nonisothermal crystallization for the PPS magnetic composites. The kinetic parameters, F(T) and [alpha], which are determined from the intercept and slope of those lines, are listed in Table 2.
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
It can be seen in Table 2 that the value of [alpha] for neat PPS is about 1.25-1.28. This indicates that n is higher than m more or less, and hence their ratio, [alpha], can better evaluate the overall crystallization process, including secondary crystallization. In contrast to that of neat PPS, the high value of [alpha] for the composites indicates that the PPS matrix has stronger secondary crystallization in the presence of filled particles. This is due to the recrystallization of those more defect ridden crystalline lamella and less ordered crystals in the composites. It is also obvious that for a certain relative degree of crystallinity, the value of F(T) for the composites is higher than that for neat PPS, that is, the composites require higher heating rate to approach to the identical relative degree of crystallinity. In other words, the crystallization rate of the composites is lower than that of neat PPS at identical experimental conditions. Moreover, F(T) increases with increasing particle loadings, which further confirms that the presence of particles impedes crystallization of the PPS matrix. Hence, it can be speculated that the crystallization activation energy may also increase with further addition of the [Fe.sub.3][O.sub.4] particle.
Kissinger  has suggested a method to determine the activation energy for the transport of the macromolecular segments to the growing surface, [DELTA]E, by calculating the variation of [T.sub.p] with the heating rate [phi]:
[d[ln([phi]/[T.sub.p.sup.2])]]/[d(1/[T.sub.p])] = [-[DELTA]E]/R (9)
where R is the gas constant. The plots of ln([phi]/[T.sub.p.sup.2]) vs. 1/[T.sub.p] are shown in Fig. 13, and the value of [DELTA]E is listed in Table 2. As expected, [DELTA]E presents monotonous increase with increasing [Fe.sub.3][O.sub.4] loadings. It indicates that presence of particles does impede the transport of PPS chain segments to the growing surface to some extent due to the strong restriction and the high viscosity within the composites. Those results hence further confirm that the inert [Fe.sub.3][O.sub.4] particle only acts as crystallization inhibitor in the PPS magnetic composites.
In this study, the PPS/[Fe.sub.3][O.sub.4] composites (PPSs) were prepared by melt mixing for the crystallization, thermal, and viscoelastic measurements. The results show that the presence of the [Fe.sub.3][O.sub.4] particles has large influence on the crystallization, thermal, and viscoelastic properties of the composite system. The restriction of chain mobility increases the glass transition temperature, dynamic modulus, and viscosity of the composites remarkably. The thermal stability is also improved by the well-dispersed particles. The mesoscopic structure of the [Fe.sub.3][O.sub.4] particles, percolation network, is easily broken by the steady shear flow due to the sharply reduced particle-particle interactions. But the percolation threshold reduces with increase of temperature. Moreover, both the crystallization and melt temperature decrease with increasing particle loadings. The filled [Fe.sub.3][O.sub.4] particles are inert and only act as crystallization inhibitor in the PPS magnetic composites, reducing crystallization kinetics as a result.
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Defeng Wu, (1,2) Lanfeng Wu, (1,2) Fei Gao, (1) Ming Zhang, (1,2) Changhao Yan (1,2)
(1) School of Chemistry and Chemical Engineering, Yangzhou University, Jiangsu 225002, People's Republic of China
(2) Provincial Key Laboratory of Environmental Material and Engineering, Jiangsu 225002, People's Republic of China
Correspondence to: Defeng Wu; e-mail: email@example.com
Contract grant sponsor: Natural Science Foundation of Jiangsu Provincial Startup Program of Innovative Talent; contract grant number: BK2007559; contract grant sponsor: Foundation of Jiangsu Provincial Key Program of Physical Chemistry in Yangzhou University.
TABLE 1. Calorimetric data derived from the crystallization and melting process for neat PPS and its composites. [phi] [T.sub.c] [DELTA] [T.sub.m] Samples ([degrees]C/min) ([degrees]C) [H.sub.c] (J/g) ([degrees]C) PPS 2 253.2 49.0 288.0 5 246.9 49.5 286.0 10 239.5 48.8 284.1 20 232.5 46.9 284.9 PPS20 2 251.0 38.2 286.4 5 241.9 34.0 284.8 10 239.1 37.6 283.2 20 230.7 32.8 284.3 PPS40 2 249.9 25.4 286.9 5 243.3 26.3 284.2 10 237.6 26.9 283.8 20 230.1 24.7 281.9 PPS60 2 245.5 17.0 285.3 5 239.6 16.0 282.9 10 233.9 16.2 282.1 20 225.2 20.0 280.4 [phi] Samples ([degrees]C/min) [DELTA][H.sub.m] (J/g) [X.sub.c] (%) PPS 2 31.7 27.8 5 32.4 28.4 10 34.3 30.0 20 29.5 25.9 PPS20 2 30.5 26.8 5 24.7 21.7 10 24.7 21.7 20 24.4 21.4 PPS40 2 16.6 14.6 5 18.7 16.4 10 17.6 15.4 20 16.7 14.6 PPS60 2 9.6 8.4 5 9.8 8.6 10 9.6 8.4 20 10.9 9.6 TABLE 2. Parameters from the Mo and Kissinger analysis. [phi] ([degrees]C/ [t.sub.1/2] [DELTA]E [X.sub.1] Samples min) (min) (kJ/mol) (%) [alpha] F(T) PPS 2 2.58 249.54 20 1.25 8.32 5 1.58 40 1.27 10.55 10 1.25 60 1.26 12.72 20 0.68 80 1.28 15.21 PPS20 2 2.78 263.17 20 1.62 9.03 5 1.75 40 1.65 11.27 10 1.27 60 1.64 14.73 20 0.72 80 1.68 17.15 PPS40 2 3.03 263.48 20 1.63 9.25 5 1.95 40 1.60 11.25 10 1.28 60 1.56 13.40 20 0.70 80 1.59 17.34 PPS60 2 3.13 249.62 20 1.67 9.38 5 2.08 40 1.61 11.73 10 1.33 60 1.58 17.32 20 0.73 80 1.56 18.07
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|Author:||Wu, Defeng; Wu, Lanfeng; Gao, Fei; Zhang, Ming; Yan, Changhao|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2008|
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