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Isothermal crystallization kinetics of poly(phenylene sulfide)/TLCP composites.

INTRODUCTION

Blending thermotropic liquid crystalline polymers (TLCP) with thermpoplastics permits the design of new high-performance composite materials with high strength and stiffness. During the composite processing, the mesogenic moieties of TLCPs promote high degree of molecular alignment, resulting in the formation of fibrillar structure in the molten state. However, the long relaxation time allows the orientation of the molecular chains to be frozen in the solid state, leading to the in situ reinforcement of thermoplastics (1). This microstructure development has a strong influence on the overall crystallization kinetics of thermoplastic/TLCP composites and mostly depend on the compatibility between the TLCP dispersed phase and thermoplastic in the molten and solid states. However, conflicting results were reported in the literature concerning the effects of TLCP on the crystallizability of crystalline thermoplastic in thermoplastic/TLCP composites (2), (3). It is reported that in some cases, the TLCPs act as nucleating sites for the growth of spherulites during crystallization process of thermoplastics (4-7), whereas on the other hand, some reports indicated that the TLCPs depress the crystallization rate (3), (8), (9). Even though, apart from producing in situ fibril-microstructure composite and improving the composites processability, the TLCPs are still considered to be competitive with other fibrillar materials where the potential for accelerating the rate of crystallization is an essential requirement. An increased crystallization rate enhances the productivity and thus the current trend is to optimize the processing conditions for the shorter cycle times and high cooling rate (10). The subject is comprehensively reviewed by Tjong (11).

In isothermal crystallization of polymer composites, it is assumed that the nuclei appear randomly in space, in supercooled melt, and subsequently grow at a constant rate in three dimensions. When the crystallization occurs below the melting point of crystallizable matrix polymer in composite; the crytsallizable component, diffuses toward the crystal growth front. At such a situation the polymer transport, in fact the resulting morphological formations are kinetically controlled by thermal history and the composition. The majority of the theories proposed for this process are based on the Avrami treatment (12-14). Although the Avrami approach has many disadvantages, to large extent the n and k parameters can be used to interpret the nucleation mechanism and crystallization rate of the polymer, respectively. Avrami equation describing kinetics of isothermal transformation is valid and the effects of secondary crystallization are negligible. However, otherwise also the overall crystallization rates are not as simple to be interpreted as spherulitic radial growth due to the combination of nucleation and growth processes. In fact, the crystal growth process is controlled by the secondary nucleation. In this context, the kinetic theory of polymer crystallization by Hoffman et al. (15) allows to calculate the temperature dependence of the linear growth rate of the theoretically defined crystallization regimes incorporating the initial recognition of the importance of chain folding.

Poly(p-phenylene sulfide) (PPS), a semicrystalline polymer belongs to high-performance engineering thermoplastics class. Because of its chemical structure, composed of phenyl groups linked by a sulfur atom, it has excellent thermal, mechanical, and chemical properties. PPS has been widely used for applications in the aerospace, automotive, electric, and electronic industries. It can be reinforced with various fillers and fibers, thereby extending its property spectrum, suitable used in making injection molded components of complex shapes for engineering applications (16). Blending with TLCP, PPS produces a high-performance composite, with varying degree of overall crystallinity depending on blending conditions, and thereby modifying the composite morphology (17), (18). Also the TLCP might form fibril morphology depending on the compatibility with PPS and its concentration in composite (19).

The crystallization behavior of neat PPS (20-28), of PPS filled with solid fillers (29-36), and PPS blended with different thermoplastic polymers (37-41) has been investigated extensively under isothermal and nonisothermal conditions. Similarly, during the last few years, a number of papers have been appeared in the literature, dealing with the effect of TLCPs on both isothermal and nonisothermal crystallization of PPS (5), (42-50) Minkova et al. (5) reported isothermal crystallization behavior of blend of PPS with the aromatic copoly(ester imide) Vectra B950 (Hoechst-celanese, USA). They reported that the Vectra B950 strongly accelerates the isothermal crystallization, as a result of an increased nucleation density whereas no reduction of PPS degree of crystallinity was observed. These PPS/Vectra B950 blends were shown to be practically biphasic and immiscible. In another study, on the isothermal and nonisothermal crystallization of blends of PPS and TLCP (HX4000-Du Pont), Gabellini and Bretas (49) observed increase in the overall crystallization rate and the dimensionality of the PPS crystalline morphology due to heterogeneous nucleation.

Previous studies of crystallization behavior of PPS/Vectra A950 are very scarce. Budgell and Day (50) studied the crystallization kinetics of PPS (Fortran and Ryton) blended with Vectra A950 (TLCP Hoechst). They found that Vectra A950 retards the Fortran crystallization but has very little effect on the Ryton PPS. These PPS/Vectra A950 blends are reported immiscible. No calculation of lateral ([sigma]) and fold surface interfacial free energies ([[sigma].sub.e]) are reported to correlate with these findings. Gopalkumar et al. (48) made an investigation of the morphology, mechanical, and thermal properties of uncompatibilized PPS/Vectra A950 blends. The reported thermal data indicate that in uncompatibilized PPS/TLCP blends the crystallization rate of PPS increases, as seen from a decrease in the supercooling ([DELTA][T.sub.c] = [T.sub.m] - [T.sub.c]) of PPS in blends decreased significantly when compared with pure PPS.

With the purpose of establishing the basis for the structure-physical property relationship of the melt blended thermoplastic/TLCP blends in this first part of the study with a large effort, we thoroughly investigated the isothermal crystallization kinetics of a composite system containing poly(phenylene sulfide)(PPS), a high-performance engineering thermoplastic and a liquid crystalline polymer--Vectra A950. In this study, we report the crystallization kinetics of PPS in PPS/TLCP composite as a function of crystallization temperature and composite composition and analyzed by the Avrami theory. Thermal analysis by differential scanning calorimeter (DSC) is employed to study the melting and crystallization behavior of PPS. The spherulitic growth rate is calculated using isothermal crystallization DSC data. The molecular thermodynamic parameters which control the crystalline growth of PPS in PPS/TLCP composites have been discussed. In this work, attention is also focused on the isothermal crystallization processes of these composites from the melt in order to analyze the effect of the concentration of TLCP on the morphological changes particularly on the growth of PPS crystals. For the first time in this article, the influence of TLCP blending on crystalline regime transition temperatures of PPS proposed by Hoffmann et al. will be reported. The results should be particularly interesting for the comprehension of crystallization behavior of PPS in PPS/TLCP composites to achieve the tailor-made properties.

EXPERIMENTAL

Materials

Poly (phenylene sulfide) (PPS) (d = 1.360) used in this investigation was procured (Lot. No. 08115EZ) from M/S Aldrich Chemical Company, Milwaukee, Wl. The melting temperature ([T.sub.m]) observed from DSC of PPS is 557 K. The TLCP used here is a Vectra A950 (TLCP), a commercial product, supplied by Polyplastic, Japan. This TLCP is a semicrystalline random aromatic copolyester of 4-hydoxy benzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with a monomer ratio of 73/27. The melting point ([T.sub.m]) of the as-received TLCP granules is 557 K. The TLCP forms a nematic melt above this temperature.

Composite Preparation

PPS and TLCP were dried at 373 K for 4 h in air circulating oven and stored in a dry environment before melt blending. A wide range of PPS/TLCP compositions 100/00, 90/10, 80/20, 70/30, 60/40, and 50/50 were prepared by melt compounding in a Haake (Rheomix 600) internal mixer at the co-screw temperature of 583 K with rotor speed 50 rpm. Blending was carried out for 5 min until the torque was stabilized. The composites were recovered in an air and allowed to solidify at room temperature.

Differential Scanning Calorimetry Measurements

A DSC (Perkin-Elmer, DSC-7) operating on a UNIX platform was used to record isothermal crystallization exotherms and subsequent melting endotherms of neat PPS and PPS/TLCP composites. The DSC was calibrated using an Indium standard (melting temperature [T.sub.m] = 429.4 K and enthalpy of fusion = 28.884 X [10.sup.-3] J [kg.sup.-1] to ensure accuracy of the data obtained. The sample mass was kept constant (9.000 [+ or -] 0.001 mg of PPS/TLCP) throughout the study for high reproducibility and used only once. Additional empty aluminium pan lids were placed in reference compartment as means of improving the thermal response. All the thermograms were obtained under nitrogen atmosphere to prevent thermal degradation.

For isothermal crystallization studies, each sample was heated from 323 K at a heating rate of 10 K [min.sup.-1] to a melt annealing temperature, 583 K. The sample was then held for 5 min to ensure complete melting and to eliminate residual anisotropy. Subsequently, each sample was rapidly cooled at the rate of 160 K [min.sup.-1] to the isothermal crystallization temperature ([T.sub.c]) of interest 523, 525, 528, 531, and 533 K. The sample was held at desired isothermal temperature for the completion of the crystallization process till no change in the heat flow as a function of time was further observed. It is then possible to monitor the resulting crystallization of sample as a function of time. The isothermal crystallization exotherms and subsequent melting endotherms at the heating rate of 10 K [min.sup.-1] were recorded. The weight fraction of crystalline material at a specific time, i.e., the ratio of the area under the isotherm at time t to the total area was calculated for each isothermal crystallization using DSC 7 kinetic software. The kinetics of isothermal crystallization process was carried out by directly fitting the experimental data to the macrokinetic Avrami model.

Optical Microscopy

The morphology development of spherulites, which appears as bright areas under polarized light in the dark background of neat PPS and PPS/TLCP composites was observed under cross polarizers using a Leica Laborlux 12 Pol S polar light microscope. The photomicrographs were taken with a Canon Powershot S 50 digital camera.

Analysis of the morphology was performed closely to the same conditions as experienced by samples during crystallization in the DSC experiments. As the nucleation density of PPS was very high, extra thin films of neat PPS and PPS/TLCP composites were prepared by melt pressing at 583 K between a glass slide and a cover slip and holding it at this temperature for 5 min to ensure complete melting. The microslide was then transferred to the hot stage held at the desired isothermal crystallization temperature. The system allowed the microslide to reach desired temperature in 15-20 sec, giving an average cooling rate 160 K [min.sup.-1], similar to the same as used with DSC. The sizes of the growing spherulites were determined by plotting their radius as a function of time and then calculating the slope of the best fit straight line.

Scanning Electron Microscopy

To observe phase behavior of PPS and PPS/TLCP composites, the morphology features of the cryogenically fractured surfaces of the PPS/TLCP composites were investigated using JEOL JSM 6380 LA analytical scanning electron microscope (SEM), applying 20 kV accelerating voltage. The samples were prepared by dipping the samples in liquid nitrogen for 5 min and then breaking. An SPI sputter coater (JEOL JFC-1600 auto fine coater) was used to coat these fractured surfaces with gold for enhanced conductivity.

RESULTS AND DISCUSSION

Miscibility of PPS/TLCP Composites

The examination of glass transition temperature ([T.sub.g]) values of PPS in composites (Table 1) suggests that blending TLCP has a little effect on temperatures. The ([T.sub.g]) values in general slightly decrease from 366 K for pure PPS to 361 K for PPS in PPS/TLCP 60/40 composites These results indicate that TLCP is not miscible in present case with PPS but acts as a diluent (plasticizer) in composites. Further in support of this immiscibility of two components in PPS/TLCP composites, the morphology study of these composites was also carried out employing SEM. The fractures of the PPS/TLCP composites (90/10, 70/30, and 50/50 compositions) presented in Fig. 1 clearly indicates the immiscibility of two components in composites. In all the composite samples, well-segregated spherical TLCP-particles of the minor component phase are observed. The boundaries of the TLCP-particles are well defined and dispersed in the PPS-matrix, with some observed spherical voids due to detachment of the TLCP-particles during the cryo-fracture process. This also indicates the weak adhesion at interface between TLCP-domain and PPS-matrix. It is also observed that domain size increases (from 6 /[micro] in 90/10 to 11 [micro]m in 70/30 and then to 23, [micro]m in 50/50 composition) with increasing its concentration in composites. Such an observed macrophase separation supports to immiscibility and may steer the high interfacial tension occurring between PPS and TLCP during melt mixing process.

[FIGURE 1 OMITTED]
TABLE 1. Melting behavior of PPS/TLCP composites.

 Melting peak
 temperature
 ([T.sub.m]) K

PPS/TLCP [T.sub.g] (K) [T.sub.c] (K) I II

 100/00 366 523 534 556
 525 534 558
 528 536 560
 531 539 559
 533 543 560
 90/10 358 523 534 556
 525 533 557
 528 536 558
 531 539 560
 533 544 560
 80/20 363 523 534 548
 525 533 557
 528 539 557
 531 539 559
 533 544 559
 70/30 357 523 536 554
 525 534 557
 528 540 557
 531 539 559
 533 544 559
 60/40 361 523 534 548
 525 533 557
 528 539 557
 531 539 559
 533 544 559
 50/50 363 523 536 554
 525 534 557
 528 540 557
 531 539 559
 533 544 559

 Heat of fusion Heat of
 [DELTA]H crystallization [x.sub.c]
 [DELTA]H X [[DELTA]H.sub.c] X ([[DELTA]
 [10.sup.-3] [10.sup.-3] H.sub.c]
PPS/TLCP (J. [kg.sup.-1] (J [kg.sup.-1]) /[[DELTA]H.sub.l])

 I II

 100/00 4 31 18 0.22
 2 38 37 0.46
 3 38 40 0.50
 3 37 36 0.45
 4 27 21 0.26
 90/10 3 31 21 0.26
 3 35 33 0.41
 3 33 34 0.42
 3 33 13 0.42
 3 23 21 0.16
 80/20 2 18 14 0.17
 2 23 29 0.36
 2 16 30 0.37
 2 21 26 0.33
 3 11 11 0.14
 70/30 2 12 14 0.18
 2 19 24 0.30
 2 11 26 0.32
 2 17 24 0.30
 2 10 10 0.13
 60/40 2 18 10 0.12
 2 23 22 0.27
 2 16 7 0.09
 2 21 17 0.21
 3 11 6 0.08
 50/50 2 12 6 0.07
 2 19 17 0.21
 2 11 5 0.06
 2 17 15 0.19
 2 10 6 0.07


Melting Behavior

PPS displays multiple melting peaks when scanned in DSC. The studies of Chung and Cebe (51) conversely indicated that the multiple melting behaviors and the dependence of melting temperature are a function of degree of crystal perfection, which depends on the degree of under cooling during crystallization. They attributed this behavior to the function of a broad distribution of crystals and thus the multiple peaks are due to the melting of two types of crystal populations formed during the entire crystallization process at [T.sub.c].

In this investigation, after completion of isothermal crystallization at [T.sub.c] = 523, 525, 528, 531, and 533 K, the samples were subjected to heating directly from [T.sub.c] to 573 K with a heating rate of 10 K [min.sup.-1]. The melting temperatures [T.sub.m] were obtained from peak temperatures of endo-therms. Figure 2 show the DSC heating curves of PPS and PPS/TLCP blends composed of a dominant upper main melting peak and a significant but broad lower premelting peak, after completion of isothermal crystallization at various [T.sub.c]. The data on the melting behavior of the blends are summarized in Table 1. From Fig. 2, it can be noted that in the main melting region of PPS, these two endothermic peaks are observed in the range of 528-552 K ([T.sub.m](I))and 541-564 K ([T.sub.m](II)). The positions of ([T.sub.m](I)) and ([T.sub.m](II)) are relatively constant for particular [T.sub.c] even with increasing TLCP loading in composites. The first melting peak ([T.sub.m](I)) is almost an appreciable endotherm by about 281 K above the crystallization temperatures. The intensity is relatively small and further decreases with decreasing PPS contents in the blends. The second melting peak ([T.sub.m](II)) shows sharp dependence on the [T.sub.c], being associated with the fusion of crystals grown at [T.sub.c]. Both the area and the position of this second endotherm increase with [T.sub.c]. However, the intensity of ([T.sub.m](II)) decreases with increasing TLCP content.

[FIGURE 2 OMITTED]

Depending upon the degree of undercooling [T.sub.c], the bimodal melting behavior of PPS in PPS/TLCP blends in the present case is attributed to the existence of the preformed imperfect PPS crystal populations because of rapid nucleation from the state of low mobility formed by reorganization and then by remelting during DSC scans (51). The first [T.sub.m](I) endotherm can then be due to the melting of the imperfect infilling lamellae (out of secondary crystallization) between the more perfect crystal structures, existing before DSC scanning. The second main melting endotherm [T.sub.m](II) can be assigned to the melting of the comparatively more perfect primary lamellae structures formed by a primary crystallization, which partly melted and recrystallized during DSC scanning. On increasing the crystallization temperatures [T.sub.c] from 523 to 533 K, the onset as well as completion temperatures of [T.sub.m](I) and [T.sub.m](II) endotherms marginally shift toward higher temperatures. The experimental data show that PPS crystals formed over present range (523-533 K) of crystallization temperature are subject to dominating melting process of reorganization and then remelting. Thus the crystal structure and overall morphology in melting PPS/TLCP blends are still in the reorganization domain. This process, however, involves in crystal perfection and crystal thickening.

The melting temperature of a perfect crystal formed by infinite molecular weight chains, understood as an equilibrium melting temperature ([T.sub.m.sup.0]), is determined by the Hoffmann-Weeks relationship (52). A general method of determining ([T.sub.m.sup.0]) involves the construction of a Hoffmann-Weeks plot. To provide a more reliable database in this work, the experimentally determined extrapolated onset melting temperature for each blend composition was used for the determination of ([T.sub.m.sup.0]). The advantage of using onset temperature rather than melting peak temperature is well reflected by the better values of average correlative coefficient of plots (53), (54). Assuming chain folding during crystallization, the dependence of the apparent melting temperature, [T.sub.m], on the crystallization temperature, [T.sub.c], is given by the following Hoffmann-Weeks equation:

[T.sub.m] = [T.sub.m.sup.0] (1 - 1/[beta]) + [T.sub.c]/[beta] (1)

where [T.sub.m] is the experimental onset melting temperature, [T.sub.c] is the crystallization temperature and [beta] is a factor that depends on the final lamellar thickness, which in fact describes the growth of lamellar thickness during crystallization. It is assumed that [beta] = l/l* where l and l* are the thicknesses of fully grown mature crystallite and of the critical crystalline nucleus, respectively. As the crystallization temperature range chosen for present experiments is narrow, [beta] is taken as a constant equal to or greater than one (55). The [T.sub.m] vs. [T.sub.c] representative plot for pure PPS and PPS/TLCP 60/40 composition are shown in Fig. 3. [T.sub.m.sup.0] is obtained from the intersect point of [T.sub.m] = [T.sub.c] line with the extrapolated [T.sub.m] vs. [T.sub.c] line .The [T.sub.m.sup.0] and [beta] values for all compositions are reported in Table 2. The extrapolated [T.sub.m.sup.0] value for pure PPS in present case is 579 K, which is similar to those previously reported (24). As far as PPS/TLCP composites are concerned, the [T.sub.m.sup.0] values were found increasing in the range of 580-587 K in comparison to [T.sub.m.sup.0] value of pure PPS but not vary regularly with composition. This indicates that the morphological factor [beta] is not constant in the present [T.sub.c] range and lamellar thickening is increasing with crystallization temperature in the composite samples.

[FIGURE 3 OMITTED]
TABLE 2. Extrapolated equilibrium temperature [T.sub.m.sup.0] (K) values
from H-W plots of PPS and PPS/TLCP composites.

PPS/TLCP composite Extrapolated equilibrium temperature [beta]
 [T.sub.m.sup.0](K)

100/00 579 1.84
 90/10 580 1.81
 80/20 582 1.76
 70/30 579 1.83
 60/40 586 1.63
 50/50 578 1.84


Degree of Crystallinity

Table 1 also summarizes the dependence of isothermal heat of crystallization ([DELTA][H.sub.c], equal to the area under the crystallization exotherm) on isothermal temperature as well as on TLCP content in composites. The degree of crystallinity ([alpha].sub.c], the mass fraction crystallinity) has been calculated ([DELTA][H.sub.c]/[DELTA][H.sub.f]) from the enthalpy of fusion normalized to the PPS content in compositions, assuming that the contribution of the TLCP phase is negligible and the heat of crystallization is equal to the heat of fusion for 100% crystallization PPS. The heat of fusion ([DELTA][H.sub.f]) for 100% crystalline PPS, 80 X [10.sub.-3] J [kg.sub.-1] was extrapolated from the data of Brady (56). In general, for particular isothermal temperature, the heats of fusion ([DELTA][H.sub.f]) and hence the degree of crystallinity steadily decrease with increasing TLCP content. However, for particular PPS/TLCP blend composition, it increases with increasing isothermal crystallization temperature from 523 to 528 K and then further decreases with increase of temperature ([T.sub.c]) to 531 and 533 K. The observed decrease in degree of crystallinity ([alpha]) of PPS with increasing TLCP content is a consequence of decreasing PPS concentration and thus the active number of nuclei for crystallization. Moreover, a comparatively large size TLCP domain with increasing concentration (see Fig. 1) reduces the PPS chain mobility during crystallization process. However, for a given PPS/TLCP composite, degree of crystallinity increases with increasing ([T.sub.c]); indicating a predominance of temperature induced PPS chain mobility over nucleation. From these observations, it may be concluded that the [alpha] of the PPS in PPS/TLCP composites depends on the competitive compromise between the nucleation and chain mobility.

Morphology and Spherulite Growth Rate

To investigate the effect of TLCP component on the growth processes and the superstructure of the PPS crystals in the PPS/TLCP composites, the films of neat PPS and PPS/TLCP composites were isothermally meltcrystallized at 523 and 533 K. The isothermal crystal growths in samples were followed by POM. The optical micrographs of isothermally crystalline neat PPS samples are presented in Fig. 4A. From the analysis of the micrographs, it is observed that the crystalline morphology of PPS is always spherulitic, exhibiting four-leaf-claver pattern. The spherulites with the fiber-like textures oriented along the radius of circle are being observed with well-defined Maltese cross suggesting a high order of both tangential and radical lamellae. Further, with increasing the crystallization time, the nuclei density increases indicating the thermal nucleation during isothermal crystallization and ultimately resulting in the impingement of spherulites into each other due to the space confine.

[FIGURE 4A OMITTED]

Figure 4B shows the optical micrographs of PPS/TLCP composites, crystallized at 523 and 533 K temperatures. It is observed from these figures that the number of nuclei slightly increased with decreasing crystallization temperature. Also, as the concentration of TLCP increases from 10 to 20 and than 30 wt% in composites, the numbers of spherulites increases but with broken boundaries, reduced size, and diffuse Maltese cross. For these samples with higher TLCP content, some TLCP may also locate between the lamellar ribbons in the spherulities, which would be responsible for an imperfect shperulite appearance (Fig. 4B). These morphological changes support the fact that primary heterogeneous (athermal) nucleation of PPS is facilitated in the presence of TLCP. Consequently, the spherulitic nucleation density of composites is increased predominantly as manifested by the reduction in spherulitic size. However, further loading of 40 wt% TLCP, the spherulite size promptly decreases to too small sizes and thus resulting in the dense impingement with each other.

[FIGURE 4B OMITTED]

The PPS spherulite radius as a function of crystallization time at two temperatures (523 and 533 K) of crystallization for 100/00, 90/10, 80/20, and 70/30 PPS/TLCP composites were measured. For all the samples, a linear increase in the spherulitic radius with increasing time was observed. The values of growth rate G were obtained from the slopes (results not sh wn here). Figure 5 represents the composition dependence of spherulitic growth rate G of PPS/TLCP at two temperatures of crystallization (523 and 533 K). The G values of neat PPS are comparable with those observed by others (28), (57). However, a large effect of the TLCP blending on the spherulitic growth rate of PPS in composites has been observed. A significant decrease in the growth rate of PPS in the composite system was observed over that of neat PPS, larger variations being observed at lower crystallization temperature. However, very little differences in growth rates were observed with increasing TLCP content in composites. Similar depression of the spherulite growth rate of the crystallizable component has been found in the case of polycaprolactane/polyvinyl chloride (58) and poly(tetramethylene terephthalate)/TLCP crystalline/crystalline polymer blends (3). Thus as a result, at high temperature of crystallization (533 K) the diluent effect dominates resulting in low viscosity and high chain mobility at the growth front. As a consequence, PPS lamellar crystals with a less regular fold surface are formed when the PPS/TLCP composites are allowed to crystallize isothermally at high temperature, resulting in decrease of the crystal nucleation rate. Similar trend in many polymer systems has been reported (3), (59-61).

[FIGURE 5 OMITTED]

Overall Crystallization Kinetics

In the present case, overall crystallization process is studied by employing DSC in isothermal conditions, by measuring the heat involved during the crystallization as a function of time, based on the assumption that the evolution of crystallinity is linearly proportional to heat released during the course of crystallization. In such a case, the relative crystallinity as a function of time X(t) can be obtained from the crystallization isotherm as the area of isotherm accumulated as of time t divided by the total exotherm area according to the following equation:

X(t) = [[[integral].sub.0.sup.t](d[H.sub.c]/dt)dt]/[[[integral].sub.0.sup.[infinity]](d[H.sub.c]/dt)dt] (2)

where t and [infinity] are the elapsed time during the course of crystallization and the end of the crystallization process, respectively, and d[H.sub.c] is the enthalpy of crystallization during time interval dt.

The experimental results for the heat flow versus time during the isothermal crystallization processes of neat PPS and PPS/TLCP composites at the different crystallization temperatures [T.sub.c] were obtained. Figure 6 depict the representative isothermal crystallization exotherms for the PPS and different PPS/TLCP compositions at different crystallization temperatures (523-533 K). It is observed that the crystallization exothermic peak shifts toward larger time scale. The width of the peak increases with increasing TLCP content for higher crystallization temperatures, 531 and 533 K. For lower crystallization temperatures 523-531 K, the exothermic peaks shifts to lower time scale but again with increasing width, with increasing TLCP content. This indicates that crystallization temperature is an important parameter determining the overall crystallization time.

[FIGURE 6 OMITTED]

Based on the change of heat flow with time, the temporal dependent of relative crystallinity of PPS and PPS/TLCP composites at different [T.sub.c]S are obtained according to Eq. 2 and using representative Fig. 7, which show the typical representative crystallization isotherms of PPS and PPS/TLCP composites crystallized at (a) 523, (b) 528, and (c) 533 K. Figure 8 shows the typical plots of reduced crystallinity versus time for different [T.sub.c] ranging from 523 to 533 K. It can be seen that the overall development of reduced crystallinity displays characteristic sigmoidal shape with time. It is possible to observe a remarkable variation of crystallization rate, depending on the crystallization temperature as well as on TLCP contents in composites. Further, it is observed from Fig. 8 that the slope of the isotherms for particular composite composition decreases with increasing crystallization temperature [T.sub.c], indicating progressively slower crystallization rate for PPS in PPS/TLCP composites. Thus evidently, within the temperature range studied, the time to reach the ultimate crystallinity increased with increasing crystallization temperature [T.sub.c]. The other kinetic parameters, [t.sub.0.5] (time to attain the [X.sub.c](t) = 0.5, [X.sub.c]([t.sub.max]) (reduced crystallinity at [t.sub.max]) were calculated from the Fig. 8 and similar results at other [T.sub.c] are presented in Table 3.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]
TABLE 3. Induction time ([t.sub.i]), half-time of crystallization
([t.sub.0.5]), maximum time for crystallization ([t.sub.max])
and relative crystallinity at time X([t.sub.max]) for isothermal
crystallization of PPS and PPS/TLCP composites.

PPS/TLCP [T.sub.c](K) [t.sub.i](sec) [t.sub.0.5](sec)
composites

100/00 523 8 24
 525 9 23
 528 22 49
 531 28 61
 533 58 110
 90/10 523 6 18
 525 10 24
 528 22 45
 531 36 70
 533 42 84
 80/20 523 12 30
 525 13 26
 528 20 38
 531 30 58
 533 60 136
 70/30 523 7 25
 525 14 26
 528 21 39
 531 28 60
 533 50 140
 60/40 523 8 25
 525 13 31
 528 14 41
 531 28 56
 533 55 135
 50/50 523 9 26
 525 10 22
 528 24 53
 531 18 59
 533 52 138

PPS/TLCP [t.sub.max](sec) X ([t.sub.max])
composites

100/00 21 40
 22 44
 46 43
 59 47
 112 51
 90/10 22 62
 21 42
 44 47
 68 48
 81 45
 80/20 30 49
 27 57
 36 45
 56 45
 131 46
 70/30 27 54
 27 54
 37 46
 56 44
 132 45
 60/40 23 44
 29 46
 38 44
 53 46
 124 44
 50/50 24 51
 22 49
 54 44
 49 44
 124 42


The overall rate of macroscopic development of crystallinity in neat PPS and each PPS/TLCP composite is further analyzed in terms of the classical theory of Avrami for phase transformation kinetics. Avrami model (12-14) is based on the geometric extension of crystalline spherulitic structures with constant density inside. It is observed that the crystallization process in polymers is highly time dependent and influenced by the perfection process inside the spherulites. However, the crystallization behavior is usual to distinguish the linear stage for the primary crystallization from the nonlinear stage for the secondary crystallization.

In the Avrami model, the quantitatively development of relative crystallinity as a function of time, [X.sub.c](t), is related to the crystallization time, t, according to the equation:

[X.sub.[epsilon]](t) = 1 - exp(-K[t.sup.n]) (3)

where [X.sub.c](t) represents the volume fraction of transformed or crystallized material after time, t. K is the overall crystallization rate constant containing contributions from both nucleation and growth rate and n is the Avrami exponent, which depends on the nucleation and growth mechanism of the crystals for a particular crystallization condition.

To deal conveniently with the thermal data operation, the Eq. 1 can be rewritten as the double logarithmic form, as follows:

log[ - ln(1 - [X.sub.c](t))] = n log t + log K (4)

A plot of double logarithm of the amorphous content, log[-ln(l-[X.sub.c](t))] as a function of the logarithm of time, log t, using Eq. 4, known as a classical Avrami plot, allows the calculation of n and K from the slope and intercept of the best-fit straight lines, respectively.

Avrami exponent n and the rate constant K can also be calculated by the use of the crystallization half time [t.sub.0.5]. The crystallization half time, [t.sub.0.5], is defined as the time at which the normalized crystalline content is 0.5. The advantage in this approach is that the analysis is based on all the isothermal data and not just a single point. Taking the logarithm of Eq. l at t = [t.sub.0.5], one obtains

K = log 2/[t.sub.0.5.sup.n]. (5)

Figure 9 illustrates the classical Avrami plot of Eq. 4 for neat PPS and PPS/TLCP composites. It can be seen that the experiment data particularly at low conversions closely agree with the Avrami equation and is due to primary crystallization. Here, it is understood that the primary crystallization consists of the outward clear growth of lamellar stacks. The observed nonlinear behavior at high conversions is due to the occurrence of the secondary crystallization which is believed to be caused by the spherulite impingement in the later stage of crystallization process and may well overlap the primary crystallization by filling in the spherulites of interstics. In this work, we focus only on primary crystallization (62). Table 4 lists the Avrami exponents of neat PPS and PPS/TLCP composites at various [T.sub.c]s.

[FIGURE 9 OMITTED]
TABLE 4. Avrami parameters and activation energy of PPS and PPS/TLCP
composites as function of crystallization temperature.

PPS/TLCP [T.sub.c] (K) n K(t) [K(t).sub.0.5]
composition

100/00 523 2.10 4.50 4.75
 525 2.52 7.96 7.77
 528 3.11 1.50 1.30
 531 2.61 0.66 0.66
 533 2.73 0.13 0.13
 90/10 523 2.30 8.83 11.05
 525 2.50 6.98 6.68
 528 3.01 1.59 1.65
 531 2.80 0.47 0.45
 533 2.40 0.29 0.31
 80/20 523 2.59 4.02 4.17
 525 3.37 9.89 11.61
 528 2.92 2.74 2.63
 531 2.62 0.71 0.76
 533 2.64 0.08 0.08
 70/30 523 2.06 3.94 4.21
 525 3.30 9.70 10.95
 528 3.01 2.71 2.53
 531 2.44 0.52 0.69
 533 2.38 0.09 0.09
 60/40 523 2.21 4.79 4.80
 525 2.55 3.97 3.73
 528 2.35 1.76 1.70
 531 2.61 0.94 0.83
 533 2.44 0.10 0.10
 50/50 523 2.20 4.02 4.36
 525 2.52 8.67 8.69
 528 2.33 0.69 0.93
 531 2.51 1.09 0.72
 533 2.69 0.08 0.07

PPS/TLCP Correlation [DELTA]E X [10.sup.-3]
composition coefficient (Fig. 9) (KJ [mol.sup.-1])

100/00 0.9992 108
 0.9976
 0.9992
 0.9974
 0.9992
 90/10 0.9992 76
 0.9972
 0.9982
 0.995
 0.9991
 80/20 0.9994 10
 0.995
 0.9977
 0.9954
 0.9985
 70/30 0.9961 75
 0.9979
 0.9982
 0.9969
 0.9995
 60/40 0.9991 61
 0.9992
 0.9976
 0.9972
 0.9982
 50/50 0.9994 40
 0.9911
 0.9983
 0.9931
 0.9923


An Avrami exponent n with value close to three is attributed to three-dimensional crystal growth (spherical structure) resulting from instantaneous athermal nucleation process. On the other hand, an n value between two and three represents non three-dimensional truncated spherical structures resulting from instantaneous nucleation, controlled by diffusion process. The nonintegral n values indicate the presence of the combination of thermal and athermal mixed nucleation and mechanisms (63). In present case for pure PPS, it can be observed that the exponent n is found to range from 2.10 to 3.11 (Table 4) when the crystallization temperature increases from 523 to 525 K and from 525 to 528 K. This indicates to gradual growth of two-dimensional morphology to a spherical three-dimensional morphology with a combination of thermal and athermal nucleation. This is well understood, based on the fact that as the crystallization temperature decreases from 528 to 525 K and from 525 to 523 K, the athermal nucleation density in PPS increases (64) resulting in enhanced instantaneous nucleation mechanism, causing the Avrami exponent n to decrease. At such larger undercooling ([DELTA]]T = [T.sub.m.sup.0]--[T.sub.c]), the spherulitic truncation from impingement ultimately prevents the developing of fully grown three-dimensional morphology. Further with increasing crystallization temperatures to 531 and 533 K, the Avrami plots show a deviation from the linear trend, indicating the presence of secondary crystallization, consecutively occurring with primary crystallization for PPS. These two plots are characterized by slightly lower n values, around 2.61 and 2.73, respectively, and then the ones found for lower crystallization temperatures at 528 K. The lower values may be attributed to comparatively imperfect three-dimensional crystal growths, resulting out of early spherulite impingement in the later stage of crystallization process. Also, because of the overall low polymer chain mobility at lower crystallization temperatures, the diffusions of crystallizable polymer segments become difficult and selective. Only the nearest neighbors of polymer segments could associate with each other for intermolecular crystallization. In such cases, the presence of intermediate sheaf-like structures in PPS is possible as reported by Lopez and Wilkes (24).

The values of the Avrami exponents corresponding to PPS/TLCP blends crystallized from the melt at different crystallization temperatures [T.sub.c]s are presented in Table 4. In present case, we found that the blending of TLCP did change n values. This indicates that the addition of TLCP into PPS does affect the nucleation process followed by crystal growth. The analysis of crystallization kinetic parameters, n and K, of blends indicate that the values of these parameters vary with respect to both, the crystallization temperature as well as composite composition.

In the present case, the n values as a function of wt % TLCP, two different behaviors are observed, one at low [T.sub.c] at values 523 and 525 K and another at higher [T.sub.c] temperatures at 528, 531, and 533 K. At 523 K, the n values (n = 2.30-2.59) are increasing for 90/10 and 80/20 compositions: afterward it decreases with the increase of TLCP content (n = 2.20 for 50/50 composition), which is in both cases comparatively still little higher than pure PPS (n = 2.10). In the case of 525 K crystallization temperature, n values are increasing with increasing TLCP content in blends, up to a concentration of 30 wt % TLCP (2.80, 3.37, and 3.30). However, with further increasing wt % TLCP it reaches to same value of PPS (2.52). For both the [T.sub.c]s, the maximum value of n representing higher dimensionality is observed for 80/20 blend composition. Further as the [T.sub.c] temperature increases to 531 and 533 K, compared with pure PPS, n values decrease with increase in TLCP content except for 90/10 composition (n = 2.80 for [T.sub.c] = 531 K) and are in the range of 2.62-2.40. The maximum n values (2.92 and 2.64) are again obtained at a low TLCP concentration ([less than or equal to]wt% 20%) in blends.

The analysis clearly indicates that in general the introduction of TLCP into the PPS notably causes heterogeneous growth process from crystal growth to a combination of two-dimensional and three-dimensional spherulitic growths particularly depending on the crystallization temperatures. The nonintegral values and particularly values of n lower than 3 support crystal branching and/or two-stage crystal growth and/or mixed growth and nucleation mechanisms (62). The results suggest that introducing low concentrations of 10-20 wt% of TLCP into PPS induces the uniform heterogeneous nucleation. On the other hand, higher TLCP loading induced more steric hindrance resulting in reduced PPS chain transportation ability during crystallization process, it seems, which is minimum at 525-528 K crystallization temperatures. Moreover, the calculated values of n (Table 4) for PPS/TLCP composites are in general consistent with a spherulitic growth of PPS initiated by athermal nucleation. This is well supported when combined with the observations under POM (Fig. 4B), where the PPS still shows spherulitic structures, however, depending on the TLCP content in composites.

The TLCP content in the PPS/TLCP composites can influence the crystallization rate parameter by providing nucleating sites and reduce the melt viscosity of PPS. As aforementioned, in present case the lowering of the [T.sub.g] of PPS (Table 1) indicates that the TLCP content in these composites may be acting as a diluent in the crystallization of PPS. Thus with lower TLCP content when the nucleation effect dominates, the result expected is a higher crystallization rate. However, the increased TLCP content will result in a lower crystallization rate as now the diluent effect becomes dominant. The data in present case reflect the results of the competition between these two effects.

The values of [t.sub.0.5] and K(T) of PPS and PPS/TLCP blends determined from Eq. 4 are listed in Tables 3 and 4, respectively. It can be deduced from the Table 4 that K(T) is affected by crystallization temperatures as well as by the blend compositions. It is observed that in the 90/10 composition, the PPS crystallization is faster than in pure PPS at 523 K crystallization temperature, which slows down on further addition of TLCP in blends. The value of [t.sub.0.5] is smaller than that of pure PPS. With further increase in the crystallization temperatures to 525 and 528 K, the rate is now faster for blends with increasing TLCP content upto 30 wt%. However, [t.sub.0.5] values remain almost unchanged for these blends but slightly higher than pure PPS at 525 K crystallization temperature, whereas at 528 K, compared with pure PPS the [t.sub.0.5] shows lower time values at all blend compositions except for 50/50 composition. This faster crystallization can be attributed to the increased nucleation density due to presence of TLCP on the crystallization of PPS. However, when TLCP content is increased more than 30 wt%, the K(T) as well as [t.sub.0.5] variations are irregular. This may be due to the competitive affect of nucleation effect of TLCP and the temperature induced morphological restrictions caused by TLCP itself on the diffusion of crystallizing PPS macromolecules. It should be noted that the K(T) contains contribution from both nucleation and growth rates. At a high crystallization temperature, 531 K, the K(T) slightly increases with marginally improvement in [t.sub.0.5] value for all the blend compositions except for 90/10 composition. However, with further increase in crystallization temperature to 533 K the K(T) remains almost unchanged with very high [t.sub.0.5] values compared with pure PPS for all the compositions, again except for 90/10 composition. Such a lower crystallization rate at high crystallization temperatures can be attributed to the lower nucleation density and reduced melt viscosity, ultimately resulting in slower spherulitic growth (62), which is well consistent with hypothesis that the crystallization kinetics in the particularly higher [T.sub.c] range is dominated by the thermodynamic driving forces of crystallization.

Activation Energy of Crystallization

Assuming crystallization process of the neat PPS and PPS/TLCP composites is thermally activated the crystallization rate parameter (K) can be approximately described by the following Arrehenius form of equation (65).

[K.sup.1/n] = [K.sub.0] exp( - [DELTA]E/[RT.sub.c] (6)

1/n in K = ln([K.sub.0] - [DELTA]E/[RT.sub.c] (7)

where [K.sub.0] is the temperature independent pre-exponential factor, [DELTA]E is the crystallization activation energy which consists of both, the transport molecular segments across the phase boundary to the crystallization surface and the free energy required for the formation of the critical size crystal nuclei at crystallization temperature, [T.sub.c]. R is the universal gas constant. The crystallization activation energy [DELTA]E, was determined by the slope coefficient of Arrehenius plots of 1/n In K vs. 1/[T.sub.c] in Eq. 7 (plots are not shown here). The value of [DELTA]E is considered negative heat quantity as it is energy released when the polymer melt transformed into crystalline state. In this study, the [DELTA]E values for neat PPS and PPS/TLCP in primary crystallization are reported in Table 4. The results indicated that activation energy is remarkably dependent on the content of the TLCP in composites, which ultimately comparatively governs the overall nucleation density and the TLCP total droplet external surface area in composites. Although the increasing TLCP content may increase the nucleation density, however, the overall effective external surface area decreases with increasing TLCP content due to larger droplets morphology (see Fig. 1). In present case, the crystallization activation energy decreases upto 30 wt% of TLCP content which, however, increases for 40 and 50 wt% of TLCP content in composites. It seems that initially TLCP droplets increase the mobility of the molecular chains of PPS to crystallize and accelerate crystallization rates. Also the large external surface of TLCP droplets contributes to enhanced nuclear density. Both contribute in overall decreasing activation energy. Further with increasing TLCP content (> 30 wt%), it increases due to now restricted mobility of PPS chains as well as total lower available droplet surface area. However, still the value is lower compared with neat PPS. This clearly indicates that the addition of higher content of TLCP induces ultimately more heterogeneous nucleation density which dominates over the effect of reduced polymer chain transportation ability.

Regime Kinetics and Chain Folding Mechanism

Thermodynamic parameters concerning the crystallization process can be determined from the calorimetric kinetic data obtained in isothermal conditions. In bulk crystallization, the crystal growth is strictly a process controlled by the secondary nucleation and thus the overall isokinetic crystallization rates are not as simple to be interpreted as only spherulitic radial growth, which is in fact a combination of nucleation and growth phenomena (15). According to Lauritzen-Hoffman (L-H) model of kinetic theory of polymer crystallization, the temperature dependence of the linear growth rate (G) of a chain-folded polymer crystal is given by the following biexponential relationship (15), (17):

G = [G.sub.0] exp ([-U*]/[R([T.sub.c] - [T.sub.[infinity]]])) exp (-[k.sub.g]/f[T.sub.c][DELTA]T). (8)

In L-H relation, the first term in parenthesis represents a contribution due to polymeric segments, whereas the second term represents the thermodynamic driving force. In Eq. 8, [G.sub.0] is a pre-exponential factor that includes all temperature-independent terms, U* is the activation energy (WLF energy term) for the transport of crystallizable segments at liquid--solid interface and is universal constant typically taken as 6.280 J [mol.sup.-1], [T.sub.[infinity]] the hypothetical temperature (WLF-temperature) below which all motion associated with viscous flow or reptation ceases, and usually assumed to be equal to ([T.sub.g] - 30)K, the term [florin] = 2[T.sub.c]/([T.sub.m.sup.0] + [T.sub.c]), a correlation factor that is closed to unity at high temperatures and introduced to account for the temperature dependence change of enthalpy of fusion of the perfect crystal ([DELTA][H.sub.f]). The factor [K.sub.g] understood as a nucleating constant is important as it contains the variable n that reflects the regime behavior. The second exponential term in L-H relation is a highly dependent function of crystallization temperature [T.sub.c] and undercooling [DELTA]T (where [DELTA]T = [T.sub.m.sup.0] - [T.sub.c]) and is measured from the thermodynamic melting point [T.sub.m.sup.0] of samples. The factor [K.sub.g], a (secondary) nucleation constant that controls crystal growth and reflects the regime behavior, and thus contains contributions from the surface free energies of the lamellar crystals, is expressed by,

[k.sub.g] = m[b.sub.0][sigma][[sigma].sub.e][T.sub.m.sup.0]/kd[DELTA][H.sub.f.sup.0] (9)

where [b.sub.0] is the thickness of a single molecular layer (stem) in the crystal and generally taken to be the prependicular separations of two adjacent fold planes. In PPS, the chain folding for nucleation takes place along the (020) crystal plane and the [b.sub.0] value has been taken as the perpendicular separation of growth crystal plane. [sigma] and [[sigma].sub.e] are the free energy of formation per unit area of the lateral (side surface) and folding surfaces, respectively, d is the density of perfect crystalline phase (1.35 X [10.sup.3] kg [m.sup.-3] for fully crystalline PPS) (65), (66), and k is the Boltzmann constant. According to the L-H (15), (67), the value of m in Eq. 9 depends on the crystallization regime.

Instead of the spherulitic radial growth, however, the use of the overall growth rate is supported by the literature (68), (69). Thus, assuming that the three-dimensional crystal growth is linear with crystallization time; the overall kinetic constant K can be expressed as:

K = (4[pi]/3) [G.sup.3]N (10)

where N is the nucleation density.

By combining Eqs. 6 and 8, the following relationship can be obtained:

1/3lnK = [C.sub.0] - U*/R([T.sub.c] - [T.sub.[infinity]]) - [k.sub.g]/[florin][T.sub.c][DELTA]T). (11)

where [C.sub.0] = In [G.sub.0 - 1/3 In (3/4[pi]N).

Next the regime analysis of the L-H model was performed to treat the growth rate data. For the kinetic analysis of crystallization rate, the transport term in Eq. 9 can be considered constant as the investigated [T.sub.c] range (523-533 K) is very narrow. Using U* = 1.400 kcal [mol.sup.-1], [T.sub.[infinity]] = ([T.sub.g] - 30) K, where [T.sub.g] = 367 K, and f = 2[T.sub.c]/([T.sub.m.sup.0] + [T.sub.c]), where [T.sub.m.sup.0] values derived using linear H-W extrapolation, [DELTA] [H.sub.m.sup.0] = 80 X [10.sup.-3] J [kg.sup.-1], and the other values of [t.sub.0.5] and n from Avrami analysis. The values of kinetic parameters, calculated from the growth rate data are listed in Table 5. In the present case, the Eq. 11 is used with In 2/[t.sub.0.5.sup.n] in place of K, as both of these two crystallization rate parameters are well related to the primary nucleation rate and there after crystal growth rate (15). Figure 10 shows plots of 1/3 In K + [U*/R([T - [T.sub.[infinity]) vs. 1/f[T.sub.c] [DELTA]T for pure PPS and PPS/TLCP composites, which provide the values of [K.sub.g] (slope) for regime II (discussed latter in this section characterized by m = 2) and [G.sub.0] (intercept) which describe the nucleation constants and absolute linear growth rates respectively. In fact, the nature of the transport of chain segments and its effect on the growth front are embodied in the [G.sub.0], which measure the effect on the growth rate of reptational diffusion as the molecular chain, is drawn onto the crystal substrate by the force of crystallization. The optimal fit in Fig. 10 is reflected in the correlation coefficients which are close to 1, except for the 70/30 PPS/TLCP composite. The [G.sub.0] and [K.sub.g] values thus obtained are presented in Table 6. It can be observed that the presence of TLCP in PPS lead to composition dependent [K.sub.g] and [G.sub.0] values. These data indicate that PPS/TLCP composites have significant values of nucleation constant [K.sub.g] and [G.sub.0] depending on the TLCP content in composites. The values [G.sub.0] of 90/10 composite is higher compared with pure PPS, suggesting that the introduction of low content of 10 wt% TLCP into PPS mainly plasticize the PPS and therefore resulting in faster nucleation chain motion, leading to higher rate constant [G.sub.0]. However, it is necessary to point out that the morphology of the PPS/TLCP composites indicated the formation of larger TLCP droplets with further increase in TLCP content (70), causing more gradual restriction of polymer transportation which is expected to lower the [G.sub.0] and [K.sub.g] values for 20 and 30 wt% of TLCP. On the other hand, the results indicate that, the addition of more TLCP, also induces the effective heterogeneous nucleation of PPS which probably now dominates over low transportation ability of PPS--molecular chains. As a consequence, the resultant values of [G.sub.0] and [K.sub.g] of composites increase with increasing TLCP contents to 40 and 50 wt%.

[FIGURE 10 OMITTED]
TABLE 5. Values of kinetic parameters calculated from Lauritzen-Hoffman
analysis for isothermal crystallization of PPS and PPS/TLCP composites
(U* = 1.400 kcal [mol.sup.-1]).

PPS/TLCP [T.sub.m.sup.0] [T.sub.c] (K) (1/3) In K =
 (K) (1/3) In (In 2/
 [t.sub.0.5.sup.n])

 100/00 578 523 -2.4104
 525 -2.7560
 528 -4.1567
 531 -3.6986
 533 -4.3996
 90/10 580 523 -2.4345
 525 -3.0884
 528 -4.0557
 531 -4.0874
 533 -3.6668
 80/20 582 523 -3.0585
 525 -3.7821
 528 -3.6628
 531 -3.6683
 533 -4.4453
 70/30 579 523 -2.3325
 525 -3.7061
 528 -3.7918
 531 -3.4522
 533 -4.0425
 60/40 586 523 -2.4977
 525 -3.0411
 528 -3.0311
 531 -3.6242
 533 -4.1118
 50/50 578 523 -2.5114
 525 -2.7186
 528 -3.2058
 531 -3.5337
 533 -4.5403

PPS/TLCP (1/3) In K + f = 2[T.sub.c]/ {1/([T.sub.c]
 U*/ ([T.sub.m.sup.0] ([T.sub.m.sup.0] -
 (R([T.sub.c]- + [T.sub.c]) [T.sub.c])
 [T.sub.[infinity]) f X [10.sup.5]

100/00 1.4413 0.9500 3.6592
 0.9918 0.9519 3.7753
 -0.4678 0.9548 3.9672
 -0.0668 0.9576 4.1842
 -0.8048 0.9595 4.3453
 90/10 1.4499 0.9483 3.5373
 0.9772 0.9502 3.6446
 -0.2526 0.9531 3.8215
 -0.4556 0.9559 4.0207
 -0.0720 0.9578 4.1679
 80/20 0.7295 0.9470 3.4513
 -0.0343 0.9489 3.5527
 0.0261 0.9518 3.7194
 -0.0364 0.9546 3.9065
 -0.8505 0.9565 4.0444
 70/30 1.4556 0.9496 3.6279
 0.0417 0.9515 3.7417
 -0.1029 0.9544 3.9297
 0.1796 0.9572 4.1420
 -0.4477 0.9591 4.2995
 60/40 1.2904 0.9432 3.2178
 0.7067 0.9451 3.3040
 0.6578 0.9479 3.4448
 0.0076 0.9508 3.6014
 -0.5170 0.9526 3.7160
 50/50 1.2766 0.9500 3.6592
 1.0291 0.9519 3.7753
 0.4831 0.9548 3.9672
 0.0981 0.9576 4.1842
 -0.9455 0.9595 4.3453

TABLE 6. Results of the Lauritzen--Hoffman analysis: value of nucleation
constant and parameters for isothermal crystallization of PPS and
PPS/TLCP composites in regime II ([sigma] = 16.8 x [10.sup.-3] J
[m.sup.-2]).

PPS/TLCP Nucleation [G.sub.0] Correlation
composites constant [10.sup.4]) (X coefficient
 ([k.sub.g]) [10.sup.4]) (Fig. 10)

100/00 303357 26.0 0.9888
 90/10 353926 92.0 0.9690
 80/20 266500 2.1 0.9999
 70/30 276395 7.5 0.8403
 60/40 361360 44.0 0.9860
 50/50 329265 68.0 0.9900

PPS/TLCP Fold surface Lateral surface free Work of
composites free energy energy [[sigma] chain folding
 [[sigma].sub.e] X [sigma].sub.e] X (q) (kcal
 [10.sup.-3] [10.sup.-6] ([J.sup.2] [mol.sup.-1])
 (J [m.sup.-2]) [m.sup.-4])

100/00 41.11 690.61 5.8
 90/10 47.80 802.95 6.8
 80/20 35.90 603.05 5.1
 70/30 37.42 628.68 5.3
 60/40 48.30 811.42 6.8
 50/50 44.62 749.59 6.3


At this point, it is important to mention here that the growth rates measured for PPS/TLCP by DSC and POM in this study do not agree absolutely. The discrepancies may be due to the fact that the overall crystallization data of polymer composites are much more complex involving primary and secondary crystallizations. Moreover, DSC is a macroscopic method in which the overall rate of the phenomenon is measured, whereas in POM, which is a rather microscopic method, the spherulite growth rate is measured directly that too in the constrained environment (68), (71). In the DSC data analysis, the entire secondary phenomenon that can occur during the overall crystallization are left out. Also, the assumption that the growth rate in the L-H equation can be replaced by the inverse half crystallization time has not been tested thoroughly. The validity of such a assumption influence the growth rate values, calculated from DSC data.

Most extensive set of melt crystallization growth data over the [T.sub.c] range from 383 to 553 K for pure PPS ([M.sub.w] = 15,000) has been reported by Lovinger et al. (22). They reported a regime II [right arrow] III transition centered at 481 K. They also predicted temperature of the regime I [right arrow] II transition between 524 and 536 K which is at the high temperature end of growth. Silverstre et al. (28) observed a possible regime II [right arrow] III break at 523 K for pure PPS ([M.sub.w] = 12,000). The higher value of regime transition temperature in latter case may be due to the difference in molecular mass of the PPS and the thermal treatments.

Figure 10 plots confirm that the present data is consistent with linear relationships over a present range of isothermal crystallization temperatures 523--533 K. No clear regime transition break is observed in this study. In this report, the isothermal crystallization data have been collected above 523 K, which lies at higher temperature end when compared with Lovinger's [T.sub.c] range (22). Additionally, the reasonable estimates of surface free energies and the work of chain folding for the PPS/TLCP composites indicate that regime II nucleation kinetics described by Lauritzen and Hoffman may be still followed at present crystallization temperatures in the PPS even after TLCP blending.

To further confirm the regime II crystallization kinetics, we employed Lauritzen treatment known as a Z-test (72). The isothermal crystallization of polymers is believed to proceed through the growth of lamellae which provide substrate for further growth. The test shows the dependence of lamellar growth rate (G) upon the mean length of the substrate (L) in the crystallographic register. The surface nucleation rate of new growth layers per unit length per unit time (i), the velocity with which the growth layer covers the substrate (g), and the thickness of the growth layer (b).

At high [T.sub.c]s (low [DELTA]T), the adsorbed molecules can spread rapidly along the width of the lamellar substrate prior to the next molecular nucleation event. This results in relatively smooth growth surface over the length, L, and [G.sub.i] = ibL where i is the nucleation rate. This temperature region is defined as regime I and m = 4 in Eq. 7. At lower [T.sub.c]s (medium [DELTA]T), the rate of nucleation is so high that adsorbed molecular strips have no place to spread laterally resulting in ultimately crystallization through accumulating nucleation events. Under such conditions (regime III) m is again equal to 4 ([G.sub.III] [varies] i). At intermediate temperatures, nucleation occurs at high rates compared the regime I so multiple nuclei form on the substrate at a rate, i, and spread slowly at a velocity g. In this case, during crystallization, the adjacent nucleus has to compete in spreading laterally on the crystal substrate ultimately resulting in multiple nucleation events commencing before previous ones have finishes. At molecular level the formed substrate surface is rough and uneven. Now the crystal growth is proportional to the square root, ([G.sub.II] = b[(2ig).sup.1/2]. In this regime II, m is now equal to 2.

In regime II crystallization, during the layer growth process, if the [bar.n] is the ensemble average of the number of nuclei for each completed growth layer then (1/[bar.n]) will be the fraction of nuclei initiating growth layers resulting in iL/[bar.n] number of growth layers per lamella per unit time, formed out of average number of nuclei, iL, per unit time. The growth rate (G) is now given by G = (biL/[bar.n]), Here,[bar.n] is a dimensionless number and depends on the finite possible numbers i, L, and g. Thus [bar.n] is a function of possible geometrical dimensionless arrangement, i[L.sup.2]/g. One define Z = i[L.sup.2]/4g (60), using the following approximated relations for the nucleation rate (i) and the velocity (g).

i = [beta]/[alpha] exp ([-4b[sigma][[sigma].sub.e]]/[kT[DELTA]f] - 2ab[[sigma].sub.e]/kT)) (12)

and

g = [alpha][beta] exp (-2ab[[sigma].sub.e]/kT)) (13)

The Lauritzen approximated Z-test equation for regime II is defined as

z [approximately equal to] [10.sup.3] [(L/2a).sup.2] exp (-X/T[DELTA]T) (14)

where for regime II, X = 2[K.sub.g] and Z [greater than or equal to] 1. The value of [k.sub.g] is determined from the slope of the plot shown in Fig. 10. With this approach, it follows that to be in regime II,

L [approximately equal to] 2a [[10.sup.-3] exp(2[k.sub.g]/[T.sub.c][DELTA]T)).sup.1/2]. (15)

The possible values of L in regime I and II for pure PPS and PPS/TLCP composites were calculated. It is generally observed that any one regime will produce reasonable values of L whereas other will give unrealistic values. In this case for regime I, L was determined to be very low values than that of regime II, ranging from 10.00 to 52.00 [Angstrom], which are unreasonable substrate lengths compared with latter. This appears to rule out crystallization in regime I. It is therefore possible to conclude that this secondary crystallization in pure PPS as well as PPS in PPS/TLCP composites growth occurs according to regime II kinetics where large numbers of surface nuclei form on the substrate with multiple nucleation acts. This occurs comparatively at faster rates and thus facilitates adjacent nuclei spreading laterally on the crystal substrate resulting in growth of chain folded PPS-crystallites following secondary crystallization process on the already existing crystalline substrate with the rate of secondary nucleation comparative with the rate of substrate completion rate. Thus, assuming that the PPS crystallization in PPS/TLCP composites occurs in regime II the calculated possible values of L in regime II are presented in Table 7. As the stringency of the Eq. 15 depends on the isothermal crystallization temperature it is observed that the L values for pure PPS as well as for particular PPS/TLCP composite increase with increasing [T.sub.c]. However for particular [T.sub.c] the trend in development in substrate length (L) in composites is complex which in comparison to pure PPS increases for 90/10 than which further reduces for 80/20 and 70/30 compositions. However, it increases for 60/40 and 50/50 PPS/TLCP composites. It is to be noted here that in regime II, the substrate of length L will suffer multiple nucleation and therefore be quite rough. Consequently, growth rates are now associated with both the parameters i and g [[G.sub.II] = [(i[b.sub.0]g).sup.1/2]. However, the critical factor in this regime is the niche separation between two neighboring nuclei. As the nucleation rate decreases with increasing [T.sub.c], the niche separation is continuously increased which now can accommodate number of stem width [a.sub.0]. This ultimately results in large substrate length.
TABLE 7. Maximum substrate length (L in [micro]m) required for
the crystallization to fall in regime II, using Lauritzen Z test.

 Isothermal crystallization
 temperature (K)

PPS/TLCP composition 523 525 528 531 533

100/00 2.08 2.97 5.97 10.40 17.00
 90/10 0.17 0.23 0.35 0.56 0.80
 80/20 0.33 0.44 0.69 1.13 4.88
 70/30 0.75 1.03 1.74 3.15 4.88
 60/40 1.24 1.65 2.64 4.47 6.55
 50/50 5.13 7.55 14.30 29.40 50.20


Once it is established that the crystallization occurs in regime II across a present range of [T.sub.c], the growth rate data is used to estimate [[sigma][sigma].sub.e]. The value of [sigma], the lateral surface energy, was calculated by the following Thomas-Stavely empirical relation (15):

[sigma] = [alpha]([DELTA][H.sub.f.sup.0])[([a.sub.0][b.sub.0]).sup.1/2] (16)

where d is a constant generally ranging from 0.1 to 0.3. As the value of [alpha] for organic molecules is closer to 0.30, a value of [alpha] = 0.25 has been chosen for estimation the lateral surface free energy ([sigma]) in present case. For calculations taking (020) growth substrate plane, the molecular width ([a.sub.0]) is 4.33 [angstrom] and the molecular thickness ([b.sub.0]) is 5.66 A. Here, Eq. 16 has yielded [sigma] = 1680 x [10.sup.-2] J [m.sup.-2]. Using experimentally observed [T.sub.m.sup.0] and [K.sub.g] values for neat PPS and PPS/TLCP composites listed in Tables 2 and 6, respectively, and the density of fully crystalline PPS equal to 1.35 x [10.sup.3] kg [m.sup.-3] (57), (58), the fold surface free energy values ([[sigma].sub.e]) can be obtained using Eq. 9. The calculated values of [[sigma].sub.e] are presented in Table 6. It can be seen that [[sigma].sub.e] found to be a function of TLCP content in composites.

In present case, for pure PPS, the value of [[sigma].sub.e] = 41.10 X [10.sup.-3] J [m.sup.-2] has been obtained, which is almost similar to the ones reported in the literature (73). The [[sigma].sub.e] value for PPS/TLCP composite with 10 wt% TLCP is 47.80 X [10.sup.-3] J [m.sup.-2] which is higher than that of pure PPS. With further increase of TLCP content to 20 and 30 wt% it is observed that there is a tendency of [[sigma].sub.e] to decrease (even much lower than that of pure PPS) which, however, shows a reverse trend as TLCP content increase to 40 and 50 wt% in composites. These results clearly indicate that the fold surface interfacial energies of PPS in composites are modified. This change can be understood in terms of the possible competitive compromise between the nucleation effect and the hindrance of TLCP particles on the mobilities of PPS chains. As [[sigma].sub.e] is strongly correlated with the work of chain folding, which is further understood in terms of bending of polymer chain back upon itself, the initial higher [[sigma].sub.e] value in composite indicates that there exist constraints on the mobility of the PPS chains in the interspherulitic regions due to the presence of TLCP. However the further increasing TLCP content in composites increases multiple nucleation which now dominates over restricted chain mobilities. Also, this increased nucleation density leads to more complex molecular architecture of PPS, ultimately resulting in formation of loops and the tie molecules and dangling chain ends, which contributes to hindrance in chain folding. Such morphology eventually decreases the end surface free energy ([[sigma].sub.e]) required for crystallization. As mentioned earlier, much higher loading of TLCP (> 30 wt%), now increases the size of TLCP particle domains as well as the number of domains, which could cause more effective restrictions on the mobility of PPS segments and slow down the PPS crystallization (72). At such a situation, we believe that to crystallize PPS-chain molecules in composites require overcoming a higher nucleation barrier during crystal growth.

The work of chain folding (q), which has been found to be the one parameter most closely correlated with molecular structure and contribution to its relative magnitude is from inherent stiffness of the chain itself (15). In fact, the large amount of the work of chain folding q may be attributed to the fact that few certain segments with fold may be in elevated intramolecular rotational energy levels. According to L-H (74), the work of chain folding permolecular fold can be obtained from:

[[sigma].sub.e] = [[sigma].sub.eo] + q/(2[a.sub.0][b.sub.0]) [approximately equal to][sigma] + q/(2[a.sub.0][b.sub.0]) (17)

where [[sigma].sub.eo] is the value equal to [[sigma].sub.e] when no work is required to bend the polymer chain back upon itself considering the conformational constraints on the fold imposed by the crystal structure. As a first approximation, it is assumed that the [[sigma].sub.eo] may be taken equal to the lateral surface interfacial energy [sigma]. Thus, it is expected that [[sigma].sub.eo] will be significantly less than q/(2[a.sub.0][b.sub.0]i) and ultimately may be set equal to zero. The Eq. 17, therefore, is usually written in the following form:

q = 2[a.sub.0][b.sub.0][[sigma].sub.e] (18)

For a polymer, [[sigma].sub.e] is considered to be inversely proportional to the chain area, and considering q/2 as a proportionality constant, and thus Eq. 18 allows to determine the q values in present case. The values of q are illustrated in Table 6. As can be seen for neat PPS, q is estimated to be 5.8 kcal [mol.sup.-1], which is some what lower than reported value for 6-7 kcal [mol.sup.-1] (22) and in present case, however, still indicates the moderately stiffer chains for PPS. Nevertheless, from the values of [[sigma].sub.e] and [sigma] from Table 6, it seems that the major contributions to [[sigma].sub.e] arise from the work of chain folding q, over the population of restricted folds, ultimately leading to orthorhombic PPS crystal state. In the case of PPS/TLCP composites, the changes in the magnitude of q (5.1-6.8 kcal [mol.sup.-1]) could be attributed to the overall TLCP concentrations dependent modified PPS chain mobility. It seems, therefore, that this difference between two, fold energies can be taken as evidence for the modification in structured folds of PPS in PPS/TLCP composites.

CONCLUSIONS

It is evident from this study that the crystallization kinetics in semicrystalline polymer/TLCP composites are a complex phenomena and undoubtedly are highly dependent on the composite composition and the chemical structures of polymeric components. Importantly, the results are very much dependent on the thermal history imposed on the systems. The crystallization rate or crystalline morphology does not show increasing or decreasing linear relationship with simply increasing TLCP content. The data reflect the results of competition between the effect of difference in the nucleation densities of the sized TLCP nematic domains or to an increase or decrease in the mobility of the polymer chains because of the TLCP phase (or both).

In this investigation, the contrasting effects of the presence of TLCP as a second immiscible component in PPS/ TLCP composites on the melting and the isothermal crystallization behavior of PPS were investigated. From the results, it can be concluded that the melting and the crystallization of PPS in PPS/TLCP composites are affected by addition of TLCP and observed to be highly sensitive both to the crystallization temperature as well as to the TLCP content in the composites. The observation of melting endotherms of the composites after isothermal crystallization at the particular crystallization temperature (in the temperature range of 523-533 K) exhibits double melting peaks of PPS in composites. These multiple endotherms are due to the TLCP induced melting of the different PPS crystal perfections and their recognizing during thermal scan. At larger undercooling (523-528 K) the PPS crystals are imperfect due to rapid nucleation out of additional contribution from TLCP-induced state of low mobility. At lower undercooling (531-533 K) both the endotherms marginally shift toward higher temperatures indicating PPS-crystals are now much perfect. However, crystal structure and morphology are still in the reorganization domain in the present temperature range. [T.sub.m.sup.0] values are found to increase with TLCP content in composites.

The most important conclusive observation for the kinetic data is the observed relationship between the crystallization temperature and the composite compositions. Avrami analysis showed that the addition of TLCP into PPS does affect the nucleation process through heterogeneous nucleation followed by a combination of two-dimensional and three-dimensional spherulitic crystal growths depending on the [T.sub.c]. The crystallization rate of PPS in PPS/TLCP composites decreases with increasing TLCP loading in composites. However, for a particular composite it increases with increasing [T.sub.c]. This suggest that in addition to the composite composition, the [T.sub.c] induced mobility is important factor governing the extend of modification of the PPS crystallization behavior in composites. The crystallization activation energy is initially reduced as the TLCP content increases upto 30 wt%, which further decreases with increasing TLCP content in composites. The result clearly indicates that TLCP behaves as a nucleating agent for the crystallization process for PPS.

The fold interfacial free energy and the regime kinetic analysis of the crystallization data obtained for PPS/TLCP composites show that the crystallization occurs in regime II on the large substrates, across the [T.sub.c] range covered in this study. As the fold surface free energy [[sigma][sigma].sub.e] is strongly correlated with the work of chain folding the results suggest that in general the higher TLCP loading is more of a hindrance to folding of PPS chains which is again affected by the [T.sub.c].

ACKNOWLEDGMENTS

The authors express their appreciation to Dr.(Mrs.) Jyoti P. Jog of NCL, Pune, India, for providing polarizing optical microscope facility.

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A. K. Kalkar, V. D. Deshpande, M. J. Kulkarni

Department of Physics, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India

Correspondence to: A.K. Kalkar; e-mail; kalkarak@udct.org

Contract grant sponsor: All India Council for Technical Education (AICTE), Govt. of India, New Delhi; contract grant number: NO.F. 8017/RDII/BOR/95/Rec.586/TAPTEC/1997.

DOI 10.1002/pen.21263

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Title Annotation:thermotropic liquid crystalline copolyester
Author:Kalkar, A.K.; Deshpande, V.D.; Kulkarni, M.J.
Publication:Polymer Engineering and Science
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Date:Feb 1, 2009
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