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Influence of molecular weight on rheological, thermal, and mechanical properties of PEEK.

INTRODUCTION

An important category of melt-processable engineering polymers based on primarily aromatic backbone structures provide outstanding chemical, mechanical, and thermal properties demanded by high-tech industries. Poly(ether-ether-ketone) (PEEK) is an example of such polymers with three aromatic structures alternated with one ketone group and two ether groups on its molecular backbone in a single repeating unit. This repeating unit can be represented in Fig. 1.

[FIGURE 1 OMITTED]

PEEK has generated much interest and abundant applications in many advanced industries, such as electronics, automotives, health care, oil-well, marine, and aircraft, because of its exceptional chemical and radiation resistance, high mechanical strength, high wear and abrasion resistance, and high heat distortion temperature.

A semicrystalline polymer, such as PEEK, is amorphous in its melt state but forms a crystalline structure during the solidification process. The type and shape of the crystalline structure is highly dependent on its crystallization kinetics and processing conditions, such as cooling rate, melt pressure, and flow history. There is a large body of literature on PEEK morphology, thermal and crystallization behavior, and processing (1-7). Unlike many other polymers, such as polyamides, PEEK does not exhibit a polymorphism phenomenon owing to its distinct molecular structure. PEEK, however, does have both the glass-rubber ([alpha]) relaxation and the subglass ([beta]) relaxation (8). A unique feature reported for PEEK is its double melting peak behavior, wherein a low-temperature melting endotherm occurs about 10[degrees]C above its annealing temperature, and a high-temperature endotherm exists close to its widely documented melting temperature of 343[degrees]C (9), (10).

As PEEK has a significantly smaller number (up to several orders of magnitude) of repeat units than a commodity polyolefin resin, a slight change in molecular weight can significantly alter the rheological, thermal, mechanical, and thermomechanical properties of PEEK. The effect of molecular weight of PEEK on its crystallization kinetics and rheology has received little attention in the literature, even though it is essential to understanding its processing behavior and design characteristics for critical applications. A systematic study of these effects is crucial to optimizing PEEK conversion processes, both primary and secondary, and, ultimately, to the design of high-performance PEEK composites. In addition, a study of the effect of molecular weight on PEEK properties has great implications for many other high-performance polymers with primarily aromatic backbone structures.

In this article, differential scanning calorimetric (DSC) analysis, dynamic mechanical thermal analysis (DMTA), and tensile and impact tests were performed to characterize the thermal, thermomechanical, and mechanical behavior of different grades of PEEK. The rheological studies were conducted using a capillary rheometer in steady shear mode and a parallel plate rheometer both in steady state mode and in dynamic mode. The purpose of using these different techniques is to ensure the characterization over a wide range of shear rates representative of most polymer conversion processes. The aforementioned techniques cover a range of 5 decades of shear rates. The goal of this study is to provide a comparison of rheological, thermal, mechanical, and structural behavior of PEEK with different molecular weights.

EXPERIMENTAL

Materials and Sample Preparation

Three different grades of PEEK were used in this study. The molecular weights of these different grades of PEEK were measured as 23,000, 27,000, and 37,000 g/mol, respectively, by size exclusion chromatography using polystyrene reference standards. These molecular weights correspond to 80, 94, 129 repeat units of ether-ether-ketone group, and these three different grades of materials are designated as PEEK80, PEEK94, and PEEK129, respectively. No significant difference in poly-dispersity for the molecular weight distribution of these PEEK resins has been found or reported.

The three different grades of PEEK were processed as received (no additives were added) using a twin screw extruder with the same extrusion conditions. After drying for 6 h at 150[degrees]C, the pellets were molded using a 110 ton all-electric injection molding machine with the same molding conditions. Samples for mechanical testing, DMTA, and parallel plate rheological measurements were molded in a single shot.

DMTA tests were performed on rectangular bars measuring 45 mm by 5 mm by 2 mm using a TA Instruments RSA3. The samples were tested in three-point bending mode over a temperature range of 90-345[degrees]C, using a frequency of 5 rad/s. For each grade of PEEK, the strain was selected to correspond to the linear viscoelastic region as determined from dynamic strain sweep experiments.

DSC tests were conducted using a Perkin Elmer DSC-7. For the thermal behavior and crystallization kinetics study, specimens cut from extruded pellets underwent isothermal and nonisothermal scans. The weights of DSC specimens were between 8 mg and 10 mg. Each specimen was heated to 380[degrees]C at 10[degrees]C/min and stabilized for 6 min before isothermal and nonisothermal scans to eliminate its thermal and stress history. All the DSC experiments were carried out under nitrogen atmosphere. The isothermal crystallization tests were conducted at specific temperatures in the corresponding crystallization temperature ranges of the different grades of PEEK as discussed in the following. The nonisothermal crystallization tests were implemented specifically at the different cooling rates of 2.5, 5, 10, 20, and 40[degrees]C/min.

Dynamic and steady shear parallel plate rheological measurements were performed using a TA Instruments advanced rheometric expansion system (ARES) rheometer. Before testing, injection-molded circular disks (d = 25 mm, h = 1.5 mm) were vacuum dried at 110[degrees]C for 8 h. Samples were tested at temperatures ranging from 348 to 420[degrees]C, although only a portion of the data is reported here. Dynamic strain sweeps were performed to determine the linear viscoelastic region and the critical strain for each PEEK. Dynamic frequency sweeps were performed over a frequency range of 0.1-100 rad/s using strains in the linear viscoelastic region. Steady rate sweeps were performed over a shear rate range of 0.01-1 [s.sup.-1]. All parallel plate rheological measurements were conducted under a nitrogen atmosphere to minimize degradation. The stress relaxation experiments were also conducted for the different grades of PEEK using parallel plate rheometry. For comparative purposes, all stress relaxation experiments were performed with a step strain of 10% at 380[degrees]C under nitrogen atmosphere.

Steady shear rheological measurements were performed using a Rosand RH10 twin-bore capillary rheometer. Before testing, extruded pellets were dried at 150[degrees]C for 6 h. Samples were tested at 380[degrees]C at apparent shear rates ranging from 25 to 800 [s.sup.-1]. Two different die lengths were used to correct for entrance and exit effects.

Tensile tests were conducted using an Instron[R] Model-5582 instrument in accordance with the ASTM standard D638. Impact tests were performed using a Ceast[R] Resil Impactor Junior instrument in accordance with the ASTM standard D256. A minimum of eight specimens were used for each test for each grade of PEEK.

RESULTS AND DISCUSSION

Rheological Properties of PEEK With Different Molecular Weights

Rheological behavior is fundamental to any conversion process involving melt flow and solidification. The rate and magnitude of displacement of polymer melt and solid is a determinant of the properties of the end product. The techniques mentioned above were used to characterize PEEK rheological behavior over a broad range of shear rates. The data from thosea techniques have been superposed to form one master curve for each grade of PEEK. Such an overlay is possible by an empirical relationship called the Cox-Merz rule. The Cox-Merz rule states the shear rate dependence of the steady state viscosity [eta] is equal to the frequency dependence of the complex viscosity [eta]*:

[eta]([gamma]) = |[eta]*([omega])|; with [gamma] = [omega] (1)

Figure 2 depicts the shear viscosity data for PEEK80, PEEK94, and PEEK129 at 380[degrees]C over a shear rate range of 5 decades (from 0.01 to 1000 [s.sup.-1]). For the data presented in Fig. 2, the parallel plate rheometrical data, in steady state mode from 0.01 to 1 [s.sup.-1] and in dynamic mode from 0.1 to 100 rad/s, are presented as open and solid circles, respectively, and those from the capillary rheometer, from 50 to 1000 [s.sup.-1] is represented by open diamond symbols. With the Cox-Merz rule approximation, the rheological data from the dynamic frequency sweep tests in the frequency range of 0.1-100 [s.sup.-1] was exchanged into the data in the shear rate range of 0.1-100 [s.sup.-1] in Fig. 2. The choice of instruments for the above shear rate ranges was based on the instrument resolution and limitations.

[FIGURE 2 OMITTED]

As shown in Fig. 2, two distinct melt shear thinning regions, one in a high shear rate region and another in a very low shear rate region, were observed for each grade of PEEK. These two shear-thinning regions correspond to the shear rates observed during polymer processing or conversion process, that is, high shear flow and low strain relaxation, respectively. This double shear-thinning behavior is analogous to that of liquid crystalline polymers and distinguishes PEEK from many other polymers. The shear-thinning at the low frequency region can be assigned to the molecular relaxation at the crystalline interfaces and is related to the double melt peak phenomenon of PEEK. More detailed work on this aspect will be reported separately.

Parallel Plate Rheometry. In the dynamic frequency sweep experiments, the critical strains at 380[degrees]C for PEEK80, PEEK94, and PEEK129 are found to be 35, 30, and 25%, respectively. As mentioned above, the data collected from the dynamic frequency sweep tests are functions of shear frequency. In general, the dynamic frequency sweep data overlap very well with the steady rate sweep data. Based on the results in Fig. 2, it seems that the CoxMerz rule is valid for the interrelations between two different shear functions for all grades of PEEK. However, it should be noted that with the increase in molecular weight, the merging point between the steady rate sweep tests and the dynamic frequency sweep tests shifts to the higher shear rate position. From Fig. 2, it can be seen that the shear viscosity of PEEK increases, as expected, with its molecular weight. The difference in shear viscosity for the different grades of PEEK is more pronounced in the low shear rate region. Both PEEK80 and PEEK94 exhibit a plateau region in the shear rate range of 1-10 [s.sup.-1] in between two shear thinning domains. PEEK129 does not have a distinct plateau in the entire shear rate region, although the two different shear thinning zones exist.

From the dynamic frequency sweep tests, both the storage modulus G' and the loss modulus G" data for PEEK80, PEEK94, and PEEK129 can be plotted as in Fig. 3. Within the entire shear rate region of interest, all these grades of PEEK exhibit typical polymeric viscous behavior, where G" is higher than the corresponding G', and both moduli decrease with the decrease in shear rate or frequency. The loss modulus G" increases with the increase in molecular weight. This can be correlated to the better toughness and other relevant mechanical properties as presented in the following. For the storage modulus G', both PEEK80 and PEEK94 have a plateau region at low shear rates where G' of PEEK80 is also close to that of PEEK94. Unlike PEEK80 and PEEK94, PEEK129 possesses a G' and G" crossover point at the frequency of around 100 [s.sup.-1]. Because of instrument limitations, the crossover frequencies and moduli for both PEEK80 and PEEK94 were not experimentally attainable, even with the use of time-temperature-superposition and a dynamic frequency sweep test at the temperature of 348[degrees]C (only 5[degrees]C higher than the nominal melting temperature for PEEK). It is possible that PEEK80 has an even higher crossover frequency and modulus than PEEK94 which in turn has a higher crossover frequency and modulus than PEEK129. For polymeric materials, a higher crossover frequency usually indicates a lower molecular weight, and a higher crossover modulus indicates a narrower molecular weight distribution. Based on the crossover frequency and modulus data, PEEK129 may have a broader molecular weight distribution than PEEK80 and PEEK94.

[FIGURE 3 OMITTED]

Capillary Rheometry. As shown in Fig. 2, all three different grades of PEEK exhibit the shear thinning behavior at shear rates tested by capillary rheometry. As verified by parallel plate experiments, shear viscosity increases with the increase in molecular weight. In the high shear rate region, the differences in shear viscosity of the different grades of PEEK become smaller. In or above this region, PEEK melt processing is most suitable. From Fig. 2, it can also be seen that especially for PEEK94 resin, the shear viscosity data from the capillary rheometer agreed well with those from the parallel plate rheometer. Nevertheless, the capillary rheometry provided higher viscosity data than the parallel plate rheometry for both PEEK80 and PEEK129 resins.

Stress Relaxation. The linear viscoelasticity of PEEK can also be studied with stress relaxation tests or the stress relaxation spectrum in the time domain, where the linear dependence of stress relaxation on strain can be achieved. In these tests, the relaxation modulus G(t) is defined as the material relaxing stress [tau](t) divided by the step strain [[gamma].sub.0] utilized as shown in Eq. 2 [11]. Preferably, a relaxation spectrum H([lambda]) as a continuous function of relaxation time X as in Eq. 3 should be found in the relaxation analysis.

G(t) = [tau](t)/[[gamma].sub.0] (2)

G(s) = [[infinity].[integral] 0][H([lambda])/[lambda]][e.sup.[-s/[lambda]]]d[lambda] (3)

where s = t - t', s is the time variable, and t' is the past time variable running from the infinite past -[infinity] to the present time t during the relaxation deformation.

A practical and alternative way to circumvent the difficulty of finding a continuous relaxation spectrum form is to use a discrete set of relaxation times [[lambda].sub.k] and a set of relaxation modulus weighting constants [G.sub.k] to approximate and describe the relaxation spectrum. Therein, a sum of exponentially fading functions in time as shown in Eq. 4 can be used for the curve fitting and for the spectrum regeneration.

G(t) = [N.summation over (k=1)][G.sub.k]exp(-t/[[lambda].sub.k]) (4)

The experimental data of the different grades of PEEK were reduced to determine the [G.sub.k] values that minimize the least square of the deviation of the approximating function from the experimental data, using a TA Instruments transformation software with a linear regression program. The numerical results are presented in Table 1, Figs. 4 and 5, respectively.
TABLE 1. Stress relaxation spectrum data for PEEK with different
molecular weights.

                  PEEK80                            PEEK94

[[lambda].sub.k] (s)  [G.sub.k]   [[lambda].sub.k] (s)  [G.sub.k] (Pa)
                         (Pa)

9.75E - 03            4.64E + 04       1.08E - 02         4.16E + 04
3.46E - 02            1.07E + 03       4.08E - 02         1.87E + 03
1.23E - 01            6.24E + 01       1.54E - 01         1.13E + 02
4.37E - 01            3.64E + 01       5.84E - 01         4.17E + 01
1.55E + 00            2.10E + 01       2.21E + 00         2.67E + 01
5.51E + 00            1.13E + 01       8.38E + 00         1.84E + 01
1.96E + 01            6.35E - 01       3.17E + 01         3.67E + 00
6.95E + 01            3.54E - 02       1.20E + 02         1.49E - 01
2.47E + 02            4.96E - 03       4.55E + 02         1.60E - 02
8.77E + 02            9.94E - 05       1.72E + 03         4.49E - 04

                 PEEK 129

[[lambda].sub.k] (s)  [G.sub.k] (Pa)
8.40E - 03              6.38E + 04
3.18E - 02              3.58E + 04
1.20E - 01              9.69E + 03
4.56E - 01              2.55E + 03
1.73E + 00              7.07E + 02
6.54E + 00              1.32E + 02
2.47E + 01              5.88E + 01
9.37E + 01              2.87E + 01
3.55E + 02              1.06E + 00
1.34E + 03              9.67E - 03


[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Figure 4 compares the experimental data with the relaxation curve fitting data from a series of relaxation times [[lambda].sub.k] and weighting constants [G.sub.k] listed in Table 1. As illustrated in Fig. 4, the relaxation time increases with the increase in molecular weight. Comparatively speaking, PEEK129 has much longer relaxation time than PEEK80 and PEEK94. After a short time initial start, PEEK129 shows a relatively linear behavior analogous to a rubber-type relaxation behavior. Unlike PEEK129, PEEK80 and PEEK94 have at least two different relaxation regions. The relaxation moduli of PEEK80 and PEEK90 decrease sharply within the first 0.1 sec followed by a slow relaxation or flow process. Understanding and appropriate control of the PEEK relaxation process has great implications for several melt processes such as the cooling phase in injection molding and in post treatment (e.g., annealing) of PEEK materials.

As shown in Fig. 4, there is some discrepancy between the fitting data for the relaxation spectrum and the experimental data for PEEK80 at short relaxation times. It is often difficult to obtain the very accurate data for the relaxation time t < 0.1s by stress relaxation methods due to the instrument response limits. The experimental data points at long relaxation times scatter considerably owing to the limited transducer resolution for measuring the decaying stresses. Therefore, the experimental data in the middle part of each PEEK relaxation curve are most reliable and meaningful. However, this does not necessarily disqualify the qualitative insight into the PEEK relaxation behavior over its entire relaxation spectrum.

As shown in Table 1 and Fig. 5, 10 sets of relaxation times and weighting constants were used to construct the relaxation spectrum for each grade of PEEK. This represents one of numerous choices for each grade of PEEK. Regardless of the exact parameters used, as a general trend, at a comparable relaxation time level [[lambda].sub.k], the relaxation modulus weighting constant value [G.sub.k] increases with the increase in molecular weight. Choosing a different number of relaxation spectrum fitting parameters may produce different values for [[lambda].sub.k] and [G.sub.k], especially at very low and very high relaxation time regions, if the influence of instrument resolution on the experimental data in those regions is considered. In the relaxation region of interest including two transition regions, the weighting constant [G.sub.k] increases with the decrease in relaxation time [[lambda].sub.k], that is, the contribution from the large weighting constant [G.sub.k] and the short relaxation time [[lambda].sub.k] may account for a large portion of the entire PEEK relaxation process.

Dynamic Mechanical Thermal Analysis. As shown in Fig. 6, the storage moduli of different grades of PEEK are comparable in the low temperature region, where PEEK129 has an only slightly smaller flexural modulus than PEEK94 and PEEK80. In the glassy region and in the rubbery phase past the glass transition region, the storage modulus decreases greatly with the increase in molecular weight. At temperatures above 180[degrees]C, PEEK80 has the highest storage modulus among the three grades of PEEK, and the storage modulus of PEEK129 drops more dramatically than those of PEEK80 and PEEK94. This result conflicts with the general expectation of an increase in storage modulus with the increase in molecular weight. This result can be associated with the lower crystallinity and a possibly broader molecular weight distribution of PEEK129. With the decrease in molecular weight, the degree of crystallinity of PEEK increases, and the storage modulus increases because of the enhancement of crystalline structures.

[FIGURE 6 OMITTED]

Thermal Properties of PEEK With Different Molecular Weights

Crystallinity. PEEK is a semicrystalline thermoplastic material with a reported glass transition temperature ([T.sub.g]) of 143[degrees]C and a melting temperature ([T.sub.m]) of 343[degrees]C. However, variation in molecular weight may alter the melting behavior of PEEK, which could greatly influence the processing window and product morphological features of PEEK and its composites.

In this study, the peak melting temperatures of 341.9, 340.7, and 336.9[degrees]C were identified with the DSC scans for PEEK80, PEEK94, and PEEK129, respectively. The level of crystallinity of PEEK with different molecular weights is calculated based on the ratio of [DELTA][H.sub.m]/[DELTA][H.sub.m.sup.o], where [DELTA][H.sub.m] is the heat of fusion determined by integrating the sample heating scan peak at the heating rate of 10[degrees]C/min, and [DELTA]/[H.sub.m.sup.o], the heat of fusion for the completely crystalline PEEK, is taken as 130 J/g (12). The crystallinity of PEEK with different molecular weights is calculated as 39.5% (PEEK80), 39.1% (PEEK94), and 27.1% (PEEK129). Here, it can be seen that the crystallinity of PEEK decreases with the increase in molecular weight, possibly because of the different percentages of the semirigid molecular structure of PEEK acting as reinforcement. Owing to the semirigid structural nature of PEEK molecules, the higher molecular weight or the longer polymer chain, the more difficulty of molecular reorganization from the entanglements to the crystalline structures during cooling. Therefore, the lower crystallinity is expected for the higher molecular weight of PEEK. It should be mentioned that throughout this study all processing and test conditions are kept the same for the different grades of PEEK.

Isothermal Crystallization Kinetics Analysis. For the isothermal crystallization analysis, the Avrami equation has been widely used and expressed in the following form (13):

X(t) = 1 - exp[-[Kt.sup.n]] (5)

where X(t) is the relative degree of crystallinity at time t, n is the Avrami exponent, and K is the isothermal crystallization rate parameter. These parameters reflect the mechanism of nucleation and growth.

Usually, the crystallization rate [R.sub.c] is used to describe the crystallization process and determined by the reciprocal of crystallization half-time [t.sub.1/2], the time when the crystallization reaches 50% completion.

[R.sub.c] = 1/[t.sub.[1/2]] = [(K/1n2).sup.[1/n]] (6)

Similar to many other polymers, the PEEK isothermal crystallization exothermal curve becomes flatter with the increase in crystallization temperature [T.sub.c], and the total crystallization time is lengthened with the increase in [T.sub.c]. Each grade of PEEK has an isothermal crystallization temperature range in between its melting temperature and its glass transition temperature. As shown in Table 2, with the increase in molecular weight, the effective isothermal crystallization temperature at which the crystallization process could occur shifts dramatically to the lower temperature region. This can be attributed to the easier molecular chain movement of lower molecular weights or shorter molecular chains.
TABLE 2. Isothermal crystallization constants of PEEK with different
molecular weights.

                                       n

Crystallization temperature  PEEK80  PEEK94  PEEK 129
        ([degrees]C)

300                                            2.27
302                                            2.52
304                                            2.63
306                                            2.56
308                                            2.64
310                                   2.69     2.76
312                                   2.81     3.02
314                                   2.72     2.84
316                           2.93    2.62     2.74
318                           2.86    2.64
320                           2.95    2.77
322                           2.84    2.61
324                           2.87


Based on Eq. 5, the values of the crystallization constants n and K can be directly derived from the relative degree of crystallinity versus crystallization time curve. The crystallization constants for all grades of PEEK are summarized in Table 2. Empirically, the Avrami exponent n = 2 usually reflects the development of a disk-like morphology, whereas n = 3 suggests a simultaneous nucleation and spherulitic growth of the crystallization process. The results in Table 2 indicate that the n values of three PEEK grades are all between 2 and 3 but very close to 3, suggesting the primarily spherulitic crystalline structures.

Figure 7 compares the crystallization rates of PEEK80, PEEK94, and PEEK129, at different isothermal crystallization temperatures. From Fig. 7, it can also be seen that the isothermal crystallization temperature drops to the lower temperature region with the increase in molecular weight. For each specific grade, the higher crystallization rate is associated with the lower isothermal crystallization temperature owing to a larger super-cooling effect. Therefore, at a certain designated temperature, the crystallization rate of PEEK129 is lower compared with its counterparts. Comparatively speaking, if the same crystallization rate needs to be achieved, PEEK129 needs to be crystallized at a much lower temperature than PEEK80 and PEEK94.

[FIGURE 7 OMITTED]

Nonisothermal Crystallization Kinetics Analysis. The nonisothermal crystallization analysis is similar to that for the isothermal crystallization process but more suitable to melt processes, such as injection molding and compression molding. The nonisothermal crystallization exothermic curves of PEEK80, PEEK94, and PEEK129 at various cooling rates are shown in Fig. 8. It can be seen that [T.sub.peak], the curve peak temperature, shifts to a lower temperature region with the increase in cooling rate for each material. However, the [T.sub.peak] values of PEEK129 curves are always lower than those of PEEK80 and PEEK94. At the same crystallization rate, the crystallization peak temperature of PEEK94 is between those of PEEK80 and PEEK129. This also suggests that the molecular weight has a direct and large influence on the crystallization temperature.

[FIGURE 8 OMITTED]

The Ozawa equation is usually used to characterize the nonisothermal crystallization process (14):

X(t) = 1 - exp[[-Z.sub.t][t.sup.n]] (7)

where [Z.sub.t] is the rate constant of the nonisothermal crystallization process. Both [Z.sub.t] and n are functions of the cooling rate [PHI]. Their values can be determined from the slopes and the intercepts of the fitting lines for the plot of log{- 1n[l - X(t)]} versus logt. The kinetic parameters of the nonisothermal crystallization process for all three materials are summarized in Table 3.
TABLE 3. Nonisothermal crystallization constants of PEEK with different
molecular weights.

                                         n

Cooling rate ([degrees]C/min)  PEEK80  PEEK94  PEEK 129

2.5                             3.20    3.26     2.92
5                               3.16    3.26     2.94
10                              3.38    3.23     3.16
20                              3.37    3.18     3.06
40                              3.12    3.18     3.02


The value of the index n from nonisothermal crystallization processes can also be used to provide insight into the kinetics of crystalline nucleation and growth. For all three grades of PEEK at the different cooling rates studied, the exponent n is around 3.0 to 3.4, which suggests a simultaneous nucleation and spheru-litic crystal growth process in a nonisothermal crystallization process.

From the nonisothermal crystallization analysis, the crystallization peak temperature of nonisothermal crystallization processes of PEEK80, PEEK94, and PEEK129 can be compared in Fig. 9. It can be seen that with the same cooling rate, the crystallization temperature of PEEK129 is much lower than those of its counterparts. This also means that with the same crystallization temperature, the cooling rate of PEEK80 and PEEK94 can be much greater than that of PEEK129.

[FIGURE 9 OMITTED]

Crystallization Activation Energy. The Arrhenius equation (15) is applicable to thermally activated, isothermal, and homogeneous crystallization.

[K.sup.[1/n]] = [k.sub.0] exp (-[[DELTA]E/[RT.sub.c]]) (8)

where [k.sub.0] is a temperature-independent pre-exponential factor and R is the gas constant. The crystallization activation energy, [DELTA]E, for the isothermal melt crystallization of PEEK can be determined from the slope of the plot of (1n K)/n versus 1/[T.sub.c]. Through the data fitting, the values of the isothermal crystallization activation energy were determined to be -565, -553, and -408 kJ/mol for PEEK80, PEEK94, and PEEK 129, respectively. The negative sign of the data indicates that transforming the polymeric melt into the crystalline state involves a release of energy. Among all three grades of PEEK, PEEK1.29 has the smallest magnitude of crystallization activation energy, suggesting the slowest reorganization motion of the polymer chain segments.

By taking into account the influence of the various cooling rates, [PHI], in the nonisothermal crystallization process, the activation energy can be determined using the Kissinger method as follows (16).

[[d[1n([PHI]/[T.sub.p.sup.2])]]/[d(1/[T.sub.p])]] = - [[DELTA]E/R] (9)

where [T.sub.p] is the nonisothermal crystallization peak temperature. Through linear regression of the plotted data for ln([[PHI]/[T.sub.p.sup.2]]) versus (1/[T.sub.p]) as shown in Fig. 10, the slope of the line for each material can be determined. Thereafter, the values of the nonisothermal crystallization activation energies were found to be -594, -580, and -492 kJ/mol for PEEK80, PEEK94, and PEEK129, respectively. This agrees well with the discussion above for isothermal crystallization processes. These data are also summarized in Table 4.

[FIGURE 10 OMITTED]
TABLE 4. Comparison of crystallization activation energy of PEEK with
different molecular weights.

kJ/mol         PEEK80  PEEK94  PEEK 129

Isothermal      -565    -553     -408
Nonisothermal   -594    -580     -492


Mechanical Properties of PEEK With Different Molecular Weights

Mechanical properties of the three grades of PEEK are summarized in Figs. 11 through 14. As shown in Fig. 11, PEEK80, PEEK94, and PEEK129 all have a tensile yield region, which indicates that they are generally ductile materials. With an increase in molecular weight, the tensile strain of PEEK increases. Among all three grades of PEEK, only PEEK129 shows the strain hardening phenomenon with a longer yielding time or a larger yielding region, whereas both PEEK80 and PEEK94 are more brittle than PEEK129.

[FIGURE 11 OMITTED]

Figure 12 compares the tensile strength at yield and the tensile strength at break of all three grades of PEEK. As the molecular weight of PEEK increases, the tensile strength at yield decreases, but the tensile strength at break increases. The tensile strength at break of PEEK80 and PEEK94 was significantly lower than the tensile strength at yield of PEEK80 and PEEK94 and that of PEEK129. The tensile strength at break of PEEK129 is very close to its tensile strength at yield. As shown in Fig. 13, the tensile modulus of PEEK decreases slightly, but the tensile strain of PEEK increases greatly with an increase in molecular weight. Comparing PEEK80 with PEEK129, while the number of repeat units reduces about 38%, the tensile strain value decreases more than 56%.

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The modulus of toughness also called the specific volume energy absorption to fracture, which is an indication of the toughness of a material, was calculated based on the area integration of the stress-strain curve from the tensile tests. The modulus of toughness and the impact strength of all three grades of PEEK are compared in Fig. 14. The modulus of toughness and the impact strength of PEEK also increase with the increase in molecular weight. In general, the modulus of toughness and the impact strength follow the same trend as the molecular weight of PEEK changes.

[FIGURE 14 OMITTED]

Although all properties originate from different molecular characteristics of polymers, processing conditions could play a significant role in the end-product properties of semicrystalline PEEK polymers, which is not covered in this study. Ogale and McCullough reported earlier that the modulus is a function of the degree of crystallinity for a given grade of PEEK (17). Based on the results reported above, the variation of mechanical properties of different grades of PEEK can be partially attributed to their difference in the degree of crystallinity.

CONCLUSIONS

Molecular weight significantly influences PEEK thermal behavior, such as melting temperature, crystallization behavior, and crystallinity, which warrant consideration during PEEK processing. PEEK80 has the highest melting temperature, the highest crystallinity, and the highest magnitude of crystallization activation energy. PEEK80, PEEK94, and PEEK129 all tend to form spherical crystalline structures. PEEK129 has lower isothermal and noni-sothermal crystallization temperatures than PEEK80 and PEEK94. The three grades of PEEK also have significantly different rheological behavior. PEEK129 has significantly higher shear viscosity and significantly longer relaxation time than PEEK80 and PEEK94. PEEK80, PEEK94, and PEEK129 all exhibit the characteristic double shear thinning behavior, where shear thinning behavior occurs in both the low shear rate and high shear rate regions. The Cox-Merz rule works well in rheological data interpretation for PEEK with different molecular weights. The merging point between the steady rate sweep and the dynamic frequency sweep data for the Cox-Merz rule varies depending on the PEEK molecular weight. The three grades of PEEK also have significantly different mechanical properties. The tensile strength at break, the tensile strain at break, the modulus of toughness, and the impact strength of PEEK all increase significantly with molecular weight. However, the tensile modulus and the tensile strength at yield decrease slightly with an increase in molecular weight. The influence of PEEK molecular weight should be taken into consideration when developing products or processes using PEEK.

ACKNOWLEDGMENTS

The authors acknowledge the assistance from Ms. Erin Adkins and Ms. Anna Anderson in proofreading and editing the manuscript.

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Correspondence to: Sanjiv Bhatt; e-mail: sanjiv_bhatt@entegris.com

Contract grant sponsor: Entegris, Inc.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2010 Society of Plastics Engineers

Mingjun Yuan, Jeffrey A. Galloway, Richard J. Hoffman, Sanjiv Bhatt

TEGO Polymers, Entegris, Inc., 101 Peavey Road, Chaska, MN 55318

DOI 10.1002/pen.21785
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Author:Yuan, Mingjun; Galloway, Jeffrey A.; Hoffman, Richard J.; Bhatt, Sanjiv
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2011
Words:5722
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