Effect of nucleating additives on crystallization of poly(m-xylylene adipamide).
The poly(m-xylylene adipamide) (MXD6) shows interesting properties, such as good barrier properties to dioxygen [1, 2] or a good miscibility with other polymers [3, 4] or a good transparency [5, 6] except that it presents a very low crystallization ability. Formulated with nucleating and/or reinforcing agents or blended with other polymers, this polyamide is used as a technical polymer in different applications. Its final properties are directly related to the crystalline morphology formed during processing. That is why the study of the crystalline morphology formation is of prior interest for industrial purposes as well as for scientists. Concerning the MXD6, many studies are missing to well understand and to predict how the material crystallizes with respect to the crystallization conditions and the polymer formulation.
The aromatic polyamide MXD6 was first synthesized in the early 1950s by Carlston and coworkers from m-xylylenediamine and adipic acid [7, 8]. Figure 1 shows its repeating unit, which corresponds to this of PA 66 with a benzene ring replacing the four methylene groups. Its crystalline unit cell was investigated by X-ray diffraction and infra red spectroscopy during the 1960s. A first model proposed by Yoda and Matsubara  was finally corrected by Ota et al. . The unit cell is triclinic with the following dimensions: a = 12.01 [Angstrom]; b = 4.83 [Angstrom]; c = 29.8 [Angstrom]; [alpha] = 75.0[degrees]; [beta] = 26.0[degrees]; [gamma] = 65.0[degrees].
The very low crystallization ability of MXD6 explains why this material is generally formulated with nucleating agents. This characteristic could also explain why there are only a few studies dealing with its crystallization properties. Single crystals of MXD6 were elaborated by slow cooling of a dilute glycerine solution  and crystallization induced by water sorption was investigated by Inoue . Except these two particular cases of MXD6 crystallization, measurements of the crystallization kinetics of MXD6 in static conditions were never fully investigated and predicted, using appropriate kinetics models. Indeed, several fundamental constants, such as the thermodynamic melting point [T.sub.m.sup.0] or both constants to describe the lamellae growth rate [K.sub.g] and [G.sub.0], are still lacking.
The present study deals with the measurement of the crystallization kinetics by complementary experimental techniques: calorimetry, optical microscopy and rheometry. These three methods were necessary to deeply observe the low crystallization kinetics of MXD6. The crystallization of the pure MXD6 is first studied to afterward highlight the role of the nucleating agents. Experimental study of the isothermal crystallization of both materials will allow the determination of the different constants ([T.sub.m.sup.0], [K.sub.g], and [G.sub.0]). Finally, two models are developed to fit the experimental crystallization kinetics according to the crystallization conditions (isothermal or constant cooling rate). They are based on the Avrami equation [13-15] coupled with the Hoffman-Lauritzen prediction of the growth rate .
[FIGURE 1 OMITTED]
MATERIALS AND METHODS
Two grades based on MXD6 were studied. The first one consisted in a virgin MXD6 labeled PA1 supplied by Solvay. The molecular weight of this initial MXD6 was about 15,000 g/mol. The second one, labeled PA2 was a nucleated formulation consisting in a blend of this MXD6 with 11% of PA 66 and 1% of talc. This blend was achieved, using a laboratory twin-screw extruder without any specific dispersion difficulty.
Crystallization experiments were performed in a Pyris Diamond DSC from Perkin Elmer. Slices of pellet (about 8 mg) were placed in an aluminum pan and were molten at 280[degrees]C during 4 min before isothermal and non isothermal experiments.
For isothermal crystallization experiments, sample was subjected to the following thermal cycle: the temperature was decreased as quick as possible to a temperature 10[degrees]C higher than the crystallization temperature and then to the desired crystallization at 10[degrees]C/min temperature. This "decreasing cooling rate" procedure was followed to ensure that the temperature did not go down to a temperature lower than the desired temperature before reaching it (possible undershoot due to the noncontrolled cooling) otherwise it could prematurely start the crystallization. With this procedure the cooling is slowed down and controlled just before the desired temperature avoiding this undershoot. Obviously, in this procedure, cautions must be taken to ensure that the crystallization does not start during the cooling, that is, the minimum valid crystallization temperature is such that the obtained crystallization time remains long enough compared with the cooling time. When the crystallization was finished, temperature was directly increased at 10[degrees]C/min to observe the corresponding melting peak. The melting point was determined as the maximum of the melting peak. Isothermal crystallizations were analyzed from 196 to 228[degrees]C, the temperature range depending on the material formulation. In the case of crystallization at constant cooling rate, temperature was decreased at the desired cooling rate down to 50[degrees]C and then increased at 10[degrees]C/min to record the corresponding heating run. Cooling rate was increased from 1 to 100[degrees]C/min.
Both isothermal and nonisothermal crystallization experiments were characterized by the variation of the heat flux. The relative crystallinity [alpha] was calculated by the following expression:
[alpha](t, T) = A(t, T)/[A.sub.total] (1)
where A(t, T) is the area under the crystallization peak up to the time t for isothermal measurements, or down to the Temperature T, for constant cooling rate experiments and [A.sub.total] is the total area under the crystallization peak. From isothermal crystallization kinetics, the half crystallization time [t.sub.1/2], which corresponds to the time where [alpha] = 0.5 can be deduced.
Optical Microscopy Experiments
Crystallization experiments were carried out in the shearing hot stage CSS 450 from Linkam Scientific Instruments UK  coupled with an optical microscope Orthoplan from Leitz. A pellet (about 20 mg) was placed in the hot stage. Gap was set to 75 [micro]m after the sample melting. The same melting conditions as for calorimetry experiments were used. As soon as the crystallization temperature was reached, data collection (pictures or transmitted light intensity) started. Each measurement was performed two or three times to obtain an average value.
Crystallization kinetics were obtained from the variation of the transmitted light intensity I(t,T). Even if the relation between the crystallization progress and the transmitted light is not trivial, the relative crystallinity [alpha] was simply estimated as follows:
[alpha](t, T) = [[I.sub.0] - I(t, T)]/[[I.sub.0] - [I.sub.[infinity]]] (2)
where [I.sub.0] is the initial transmitted light intensity and [I.sub.[infinity]] is the final transmitted light intensity.
Some isothermal crystallization experiments were also carried out in another hot stage FP-52 from Mettler. In this case, a very small piece of pellet was molten between two microscope glass slides. Thickness of sample was about 20 [micro]m. The hot stage was placed under the same optical microscope and the crystallization process was also measured from the transmitted light variation. Thermal calibration of both hot stages was performed to ensure their temperature correspondence.
A Rheometrics RMS 800 was used in the plate-plate geometry (diameter 25 mm) for material characterization by frequency sweep tests (from 100 to 0.1 rad/s at 250 and 260[degrees]C) and temperature sweep tests (from 275 to 165[degrees]C at 5[degrees]C/min and 1 rad/s). Gap was set to 500 [micro]m. The command strain was adapted to the crystallization state of the material: high values were needed in the case of very low viscosity material and low values were used when the material was crystallized. A strain sweep test was also performed at 230[degrees]C and 1 rad/s from 0.01 to 100% to determine the linear domain boundary. A decrease of the storage modulus G' was obtained for strains above 2%. Therefore, the command strain values were set to about 0.5-1.5%.
The cone-plate geometry (diameter 8 mm, cone angle 0.1 rad) was used to perform isothermal crystallization experiments at temperature ranged between 216 and 230[degrees]C. Two pellets were placed in the rheometer cell and were molten in the same melting conditions than for the two other experimental techniques. Frequency was set to 1 rad/s and strain was modified as a function of the sample crystallization state. At the very beginning of the crystallization experiment, the apparatus response was inaccurate because of the very low viscosity of the material. Nevertheless, the strain was set only to 1 or 2% because the application of a higher strain could affect the crystallization kinetics.
Crystallization kinetics were obtained from the evolution of the storage modulus G' and the loss modulus G" and a characteristic time of the crystallization was defined as the time of the interception of both moduli.
It should be mentioned that the reproducibility of these rheological measurements was slightly lower than for the other techniques (DSC and microscopy) because the temperature control system was not as efficient as the other ones. Nevertheless a reliable characteristic time was achieved (averaged from 3 to 5 experiments).
RESULTS AND DISCUSSION
Crystallization at Constant Cooling Rate
Crystallization at constant cooling rate was deeply analyzed by DSC only for PA2. Indeed, crystallization kinetics of PA1 were so slow that the crystallization peak was undetectable during the cooling even for the lowest cooling rates. Its crystallization appeared only during the heating step following the cooling.
Typical heat flux variation obtained for the crystallization and melting of PA2 at a constant cooling rate noted [phi] ([degrees]C/min) is shown in Fig. 2. Experiments on PA2 always showed a well defined crystallization peak. A secondary crystallization peak is located at higher temperature: about 220[degrees]C for the highest cooling rates. This peak is attributed to the PA 66 crystallization.
The glass transition is detected during both the cooling and heating steps. The obtained glass transition temperature [T.sub.g] is 85[degrees]C.
[FIGURE 2 OMITTED]
The melting peak of MXD6 is located at 236[degrees]C and slightly depends on the crystallization conditions. The measured melting enthalpy for the MXD6 peak was always around 40 J/g for PA2, that is, 45.5 J/g for MXD6 (accounting the MXD6 amount in PA2). Although the melting enthalpy of the MXD6 infinite crystal is not known, this value of 45.5 J/g can be compared, for example, with the known melting enthalpy of PA 66 crystal of 190 J/g . Therefore the crystallinity of MXD6 in PA2 can be estimated at 24%. This first melting peak is followed by a broad melting peak of PA 66 located at about 250[degrees]C.
The different crystallization temperatures [T.sub.c], corresponding to the maximum of the crystallization peak of MXD6, were collected as a function of the cooling rate. This crystallization temperature is shifted from 216.4 to 181.1[degrees]C when the cooling rate is increased from 1 to 100[degrees]C/min.
As already mentioned, PA1 shows low crystallization abilities. On the contrary, PA2 is able to crystallize even at very high cooling rates such as 100[degrees]C/min because its additives (PA 66 and nucleating agents) largely enhance the crystallization process of the MXD6.
Crystallization of PA1 and PA2 in isothermal conditions was studied by calorimetric, optical and rheological measurements.
Isothermal crystallization of PA1 was investigated by the three experimental methods but for different temperature ranges: between 214 and 230[degrees]C for optical and rheological experiments and between 197 and 206[degrees]C for calorimetry experiments.
Figure 3 shows the results obtained in isothermal crystallization of PA1 for optical and rheological experiments in terms of crystallization characteristic times as a function of the crystallization temperature. The characteristic time of the crystallization kinetics of PA1 increases from ~400 to 13,000 s when the crystallization temperature is increased from 214 to 230[degrees]C. As expected, the crystallization is slower for higher temperatures. The crystallization experiments at 230[degrees]C required 24 h of measurement in the optical system. Crystallization experiments performed in the rheometer were directly stopped after the crossover of the moduli G' and G". Both techniques show a good agreement on the whole temperature range, which confirms that the time of crossing of both moduli G' and G" can be considered as a characteristic time of the crystallization for PA1.
[FIGURE 3 OMITTED]
DSC measurements of isothermal crystallization of PA1 did not show results consistent with other experimental techniques. Indeed, the crystallization was so slow that, especially in its ending part, the DSC signal was indistinguishable from the baseline. Therefore relative crystallinity curves were not deduced from these results. Nevertheless, these experiments were useful for the thermodynamic melting point determination, which will be detailed in the next part.
Isothermal crystallization of PA2 was studied on a higher temperature range because of its highest crystallization ability. Crystallization temperature was varied from 218 to 234[degrees]C for optical experiments and only to 228[degrees]C for DSC, again because of the detection limit of the apparatus.
Figure 3 also shows the comparison between the half crystallization times of PA2 isothermal crystallization as a function of the temperature obtained from DSC and optical experiments. The half crystallization times increase from 215 to 8700 s when the temperature increases from 218 to 234[degrees]C and show slower crystallization kinetics at high temperatures. The longest experiments were stopped after 33 h. Even if the half crystallization times of PA2 are lower, the total crystallization times were as long as that of PA1 for the same crystallization temperature. Indeed the decrease of the transmitted light intensity during PA2 crystallization showed a very long trail attributed to a large part of secondary crystallization. This continuation of the crystallization process was not so noticeable in the DSC measurements because of the confusing between the signal and the baseline. The plot showed a good agreement between the optical and the calorimetry measurements in their common temperature range.
The rheometry study of the isothermal crystallization of PA2 did not allow the determination of a characteristic time. Indeed moduli G' and G" did not intercept during the crystallization process. G' was already higher than G" at the very beginning of the experiment. It seems that a kind of network appears in the material at higher temperature than the MXD6 crystallization temperature. This phenomenon, which is due to the presence of PA 66, will be discussed further.
As shown in Fig. 3, the correlation for the crystallization times obtained by different techniques is very obvious. However it may be somewhat surprising since calorimetry, rheology and light transmission are not expected to be sensitive in the same way to the presence of first crystals. Nevertheless, it can be mention that for each of the techniques the characteristic crystallization time is taken in a range where the crystallization rate is at the maximum. Therefore, if the characteristic crystallization times do not correspond to exactly the same conversion from one technique to the other, the difference between them is minimized. Obviously, the results would not have been so correlated if other characteristic time would have been taken as the time of the crystallization beginning for example. Moreover, it can be added that such a correlation is a a posteriori confirmation that, despite the differences of cooling procedures between the techniques prior to isothermal experiments, these procedures do not significantly influence the results.
Thermodynamic Melting Point
Thermodynamic melting point [T.sub.m.sup.0] was determined by the Hoffman-Weeks method . It corresponds to the intersection of the line of experimental melting points obtained after isothermal crystallization and the first diagonal ([T.sub.m] = [T.sub.c]). Generally, DSC results are used for this method. In the present work, calorimetry and optical measurements on PA1 and PA2 were gathered to broaden the temperature range. Melting of the different PA1 and PA2 samples crystallized in the Linkam hot stage were carried out at 10[degrees]C/min to follow the same conditions than that in calorimetry experiments. Melting point was determined as the temperature corresponding to the half variation of the light intensity.
Figure 4 shows the plot of the melting points as a function of crystallization temperature for PA1 and PA2 samples. Intersection of the line of the experimental melting points and the first diagonal is about 261[degrees]C. This value will be very useful in the determination of the parameters [K.sub.g] and [G.sub.0] of the growth rate model, which will be presented in the next paragraph.
Moreover, the used melting treatment of 280[degrees]C during 4 min prior to the crystallization experiments to erase the thermal history is afterwards validated.
[FIGURE 4 OMITTED]
Growth rate was easily studied on the PA1 material because of its low crystallization kinetics and its low nucleation density. On the other hand, the morphologies obtained for PA2 samples were so fine that it was impossible to distinguish the crystalline entities whatever the crystallization conditions.
Spherulite diameters of PA1 were measured as a function of crystallization time at different temperature from 216 to 228[degrees]C in the Mettler and the Linkam hot stages. Micrographs were taken with a fixed time step, which depended on the crystallization kinetics thus on the temperature. The lower the crystallization temperature was, the smaller were the spherulites and the higher was the measurement error. Several growing spherulites were chosen as the most spherical to be followed. Each experiment was made twice to minimize the experimental error. Most of the study was performed in the Mettler hot stage and some of the isothermal crystallizations were carried out in the Linkam shearing hot stage to verify the accuracy of the measurements.
The slope of the diameter variation as a function of time corresponds twice the growth rate G of the lamellae. Table 1 sums up the experimental values of the growth rate of PA1 spherulites as a function of the crystallization temperature.
From the Hoffman-Lauritzen theory, the growth rate is expressed by the following equation:
G = [G.sub.0] exp(-[[K.sub.g]/T[DELTA]T]) exp (-[U*/[R(T - [T.sub.[infinity]]])]) (3)
where [DELTA]T = [T.sub.m.sup.0] - T.
Using U* = 6270 J/mol and the experimental values of [T.sub.m.sup.0] and [T.sub.g] ([T.sub.[infinity]] = [T.sub.g] - 30[degrees]C), determination of both constants [K.sub.g] and [G.sub.0] is achievable thanks to the linearization of Eq. 3. Plot of In G + U*/R(T - [T.sub.[infinity]]) as a function of I/T[DELTA]T gives a slope of -[K.sub.g] and origin coordinate of In [G.sub.0]. From such a plot, the values of [K.sub.g] equal to 1.77 x [10.sup.5] [K.sup.2] and [G.sub.0] equal to 7.0 x [10.sup.-3] m/s were obtained. Figure 5 shows the comparison between the model of Hoffman-Lauritzen and the experimental values of growth rates of PA1 crystals as a function of the temperature. The experimental points are well predicted by the model. It should be noted that in the temperature range [219; 229[degrees]C], no regime transition was revealed. It is also important to notice that the nuclei number as a function of the temperature could not be evaluated from the optical microscope pictures because of the relative high nucleation density.
PA 66 Network
The rheometry study of the isothermal crystallization of PA2 showed curves of G' higher than those of G" whatever the crystallization time. It means that the material PA2 at 230[degrees]C presents a certain elastic characteristic of a gel behavior. However this gelation is not due to MXD6 crystallization because this one occurred after a quite long time at this temperature ([t.sub.1/2] equals to 3770 s). This gel behavior is likely due to the small amount of PA 66, which is able to quickly crystallize at 230[degrees]C. Hence, when the PA 66 crystallizes, it generates a network between crystalline parts dispersed in the matrix. Moreover, this behavior can be considered a an good indicator of the miscibility of the PA 66 and the MXD6. Indeed, if the PA 66 was not miscible, with such a low amount (11%), it would form dispersed droplets in the MXD6 matrix. Then, when the droplets would crystallize, the rheological behavior of PA2 would remain the behavior of a liquid, with only a small increase of the moduli due to a filler effect. Conversely, the observed G' increase of several decades when the PA 66 crystallizes shows that the PA 66 is able to create a percolating network, supporting the assumption. The behavior of this gel was then deeply investigated by several rheological experiments with the plate-plate cell.
[FIGURE 5 OMITTED]
First, frequency sweep tests, not presented in this article, were carried out from 0.1 to 100 rad/s on PA2 at 250 and 260[degrees]C. At 260[degrees]C, the curves of G' and G" versus the frequency typically described the behavior of a melt polymer. But, surprisingly, at 250[degrees]C they dramatically increased after about 120 s of measurements. These experiments showed that the gelation of the material PA2 could occur very quickly at a temperature of 250[degrees]C but seemed to be delayed at 260[degrees]C.
Thus time sweep tests were performed with a low strain (1-2%) at 250[degrees]C on PA2 samples to follow the progressive evolution of moduli G' and G". Both moduli intercepted after 120 s, which confirmed the previous increase of parameters during the frequency sweep tests at 250[degrees]C. Interception of G' and G" means that the network is formed in the PA2. As shown in Fig. 6, a particular experimental procedure was then applied. Indeed, after a few minutes duration of the first time sweep experiment, a higher strain of 100% was applied during 1 min. Immediately the gel behavior disappeared showing that the network is easily destroyed by a rather moderate strain. Then the strain was set to a low value of 1-2% again and the network could be rebuilt: both moduli increased and intercepted again. A new high strain was then applied during a short time and consequently, the network was destroyed. It then appeared again after several seconds of lower strain. Finally, temperature was decreased after 3000 s to 230[degrees]C and MXD6 could crystallize, G' being higher than G". Therefore, this network can be formed in PA2 as soon as the conditions are favorable, that is when the strain is not to high and when the temperature allows the crystallization of PA 66.
Finally, crystallizations at a constant cooling rate of PA1 and PA2 were recorded at 5[degrees]C/min thanks to the temperature sweep mode (frequency 1 rad/s and low strain of 1-2%). The data acquisition (variation of moduli G' and G") was continued until the transducer limit was attained. Figure 7 illustrates the progressive increasing of both moduli and their interception point during the crystallization of PA1 and PA2. For PA1, the crystallization process occurred at lower temperatures and the obtained temperature for the crossover of both moduli is about 204[degrees]C. For PA2, which crystallizes faster, the crystallization process occurs at higher temperatures. The G' and G" crossover temperature is about 245[degrees]C, which cannot correspond to the crystallization of MXD6 whose crystallization temperature is about 212[degrees]C when it is measured by calorimetry at 5[degrees]C/min in PA2. This high characteristic temperature could only be attributed to the crystallization of PA 66. As a conclusion, the small amount of PA 66 largely modifies the rheological properties of PA2 compared with PA1. During its crystallization process, PA 66 forms a kind of network, which rigidifies the material PA2. Due to the creation of the PA 66 network prior to the crystallization of MXD6, the time corresponding to the crossover of G' and G" cannot be considered as the MXD6 crystallization time since they already crossed each other during the PA 66 gelation.
[FIGURE 6 OMITTED]
Crystallization at a Constant Cooling Rate From DSC Measurements. The model based on the Ozawa theory  was applied on the relative crystallization curves without taking into account the small part of the curve corresponding to the crystallization of PA 66. Knowing that the constant cooling rate is noted [phi] (expressed in [degrees]C/s), the relative crystallinity [alpha](T) is described by the Eq. 4:
[FIGURE 7 OMITTED]
[alpha](T) = 1 - exp(-[k.sub.Oz](T)/[[phi].sup.n]). (4)
The conventional method for evaluating the function [k.sub.Oz.](T) is based on the linearization of Eq. 4 by plotting log (-In (1 - [alpha])) vs log [phi] for different temperatures. However, this method needs a large number of experiments at different cooling rates to provide, for each temperature, a reasonable number of points to describe the line of which the slope should equal to -n. Moreover the resulting value of n may vary from one temperature to another, contrary to the basic assumption of the Ozawa equation. Furthermore the dispersion generally obtained on both n and k(T) values make the predicting use of Eq. 4 difficult. Therefore, in the present work, an unusual procedure to fit Eq. 4 on the experimental results was used. This procedure can be summed up as follows: the exponent n was fixed to the physically meaning value of 3 because the microscopic observations on PA1 highlighted an instantaneous nucleation. Moreover a mathematical expression of [k.sub.Oz.](T) was introduced based on the consideration that the crystallization rate vanishes for temperature out of the range [[T.sub.g]; [T.sub.m.sup.0]] and it takes a bell shape between these temperatures. Consequently, the function [k.sub.Oz.](T) must be constant for temperature lower than [T.sub.g] and tend to zero for temperature higher than [T.sub.m.sup.0]. Between these limits, it should decrease monotonously. The following expression fulfills all these conditions:
ln [k.sub.Oz](T) = A + B/(1 + exp(C + DT)). (5)
Numerical calculation and optimization of the four parameters gave the following values: A = -1201.9; B = 1204.7; C = -18.897, and D = 0.0672 with T in [degrees]C. Figure 8 shows the comparison between the experimental and the predicted relative crystallization curves for the crystallization at constant cooling rate of PA2 obtained by DSC measurements. From this figure, it can be concluded that the model correctly predict the crystallization for cooling rates ranged from 1 to 100[degrees]C/min. Hence the imposed value of 3 for the Avrami exponent and the choice of the Eq. 5 are a posteriori validated. It should be pointed out in Fig. 8 that for each curve, the beginning and the end are not very well predicted by the model, obviously because it does not take into account both PA 66 crystallization at the beginning and secondary crystallization at the end.
Isothermal Crystallization. Classical use of the Avrami equation by linearizing Eq. 6 for the isothermal crystallization of PA2 obtained by calorimetry measurements is not detailed here. Nevertheless it is important to notice that the exponent n value was 2.9, which is extremely close to the theoretical value of 3.
[FIGURE 8 OMITTED]
[alpha](t) = 1 - exp(-[K.sub.Av.][.sub.dot.t.sup.n]). (6)
Hence the modeling was refined by introducing the suitable relation, corresponding to a value of n equal to 3, between the constant [K.sub.Av.] and the number of instantaneous nuclei and the growth rate G (Eq. 7).
[K.sub.Av.] = (4/3)[pi][N.sub.0][G.sup.3]. (7)
Moreover, the lamellae growth rate G was described, using the model of Hoffman-Lauritzen (Eq. 3). The nuclei number [N.sub.0], which could not be evaluated by optical observations, was expressed as a function of the supercooling [DELTA]T according to earlier works [21, 22] (Eq. 8).
ln [N.sub.0] = a[DELTA]T + b (8)
where [N.sub.0] is expressed in [m.sup.-3]
The previously obtained values of constant [K.sub.g], [G.sub.0], and [T.sub.m.sup.0] were used for PA2 even if they were determined from crystallization characterization of PA1 because these parameters are theoretically independent of the presence of nucleating agents. The numerical optimization of [N.sub.0] led to a = 0.2633 and b = 26.82 for PA2 and a = 0.0609 and b = 29.08 for PA1.
Figures 9 and 10 show the comparisons between experimental relative crystallinity curves and the calculated ones for PA1 and PA2, respectively. Moreover, for PA2 (Fig. 10), experimental curves obtained from both DSC measurements and optical microscopy are displayed. It can be added that the crystallization of the PA 66 in PA2 does not appear in these relative crystallinity curves because its crystallization is very fast compared with the crystallization of MXD6 for these crystallization temperatures. Thus, the PA 66 is already crystallized when the shown experiments begin. From these figures, it can be concluded that the presented model well describes the experimental results for both polymers since the relative crystallinity curves are correctly predict for all the temperature range.
[FIGURE 9 OMITTED]
However, it can be seen in Fig. 10 that the relative crystallinity curves of PA2 obtained from optical measurements display a slowdown from about 70% of transformation whereas both DSC and calculated curves does not. On the other hand for PA1, the curves obtained from optical experiments do not exhibit such a slowdown. We attribute this result to a larger secondary crystallization in PA2. Indeed, it was shown in previous works on PA66  that the presence of nucleating agents tends to enlarge the secondary crystallization part in the global crystallization curve, but this result was not really explained. Moreover, the used optical system apparently amplifies the indication of a secondary crystallization, namely the slowdown of the crystallization. Nevertheless, it should be recalled that the transmitted light intensity measured by this system is not directly proportional to the real relative crystallinity so that the large apparent secondary crystallization part may be overestimated. It seems that, for isothermal crystallization experiments, this optical signal still evolves in the secondary crystallization part whereas, for the corresponding experiments, the DSC signal becomes indistinguishable from the baseline leading to less noticeable secondary crystallization part. Furthermore, in the Avrami equation, no secondary crystallization is taken into account. Therefore, for PA2, the calculated and DSC curves are very close one to the other in Fig. 10 and, for PA I which presents certainly less secondary crystallization, the calculated and optical curves are relatively close in Fig. 9.
[FIGURE 10 OMITTED]
Nevertheless, apart these considerations about the secondary crystallization, the presented model for describing the isothermal crystallization kinetics appears reliable for both virgin and nucleated formulations in the investigated temperature range. Particularly, the assumption that the nucleating additives do not affect the lamellae growth rate is hence validated justifying the use of same parameters in the Hoffman-Lauritzen equation. Therefore, the introduction of these nucleating additives only affects the nuclei number in the crystallization kinetic model.
The present study shows the accelerating effect of the nucleating additives on the crystallization kinetics of polyamide MXD6 for a large temperature range by complementary techniques (calorimetry, optical system and rheometer). Use of the three experimental methods allowed the characterization of the crystallization properties of MXD6 with and without nucleating agents. The main points of this work are listed below:
Crystallization kinetics were measured in isothermal conditions and at a constant cooling rate from 1 to 100[degrees]C/min. Characteristic times of the crystallization were determined like the half crystallization time or the interception of moduli G' and G" both being consistent. The nucleating effect of PA 66 and talc was well highlighted by the results.
Isothermal crystallization at different temperatures allowed the determination of the thermodynamic melting point of MXD6 by the Hoffman-Weeks method. A value of 261[degrees]C was obtained. Moreover the measurement of the spherulites growth rate allowed the determination of both constant [K.sub.g] and [G.sub.0], which are essential in the Hoffman-Lauritzen model.
Crystallization experiments at constant cooling rates were well fitted by the Ozawa equation, introducing a characteristic function, which realistically predicts a cessation of the crystallization for temperature below [T.sub.g] and above [T.sub.m.sup.0].
Isothermal crystallization experiments, for pure and nucleated grades, were successfully depicted by a model based on the coupling of the classical Avrami and Hoffman-Lauritzen theories.
We would like to acknowledge gratefully the companies Legrand, Solvay and Moldflow for financial support, material supply and characterization. We would like to thank more particularly Peter Kennedy, Rong Zengh, Hugues Alglave, Vito Leo, Florentin Langouche, Michel Laplanche, Jean-Michel Rossignol, Joachim Correa, Gilles Regnier, Didier Delaunay, Delphine Dray, and Ronan Legoff for stimulating discussions.
1. H. Masahiro, Plast. Eng., 44, 27 (1988).
2. Y.S. Hu, V. Prattipati, S. Mehta, D.A. Schiraldi, A. Hiltner, and E. Baer, Polymer, 46, 2685 (2005).
3. M. Shibayama, K. Uenoyama, J.-I. Oura, and S. Nomura, Polymer, 36, 4811 (1995).
4. T. Takeda and D.R. Paul, Polymer, 32, 2771 (1991).
5. Y. Maruhashi and S. Iida, Polym. Eng. Sci., 41, 1987 (2001).
6. Y.S. Hu, V. Prattipati, A. Hiltner, E. Baer, and S. Mehta, Polymer, 46, 5202 (2005).
7. F.G. Lum, E.F. Carlston, and J.C. Butler, U.S. Patent 2.766,211 (1956).
8. E.F. Carlston and F.G. Lum, Ind. Eng. Chem. Res., 49, 1239 (1957).
9. N. Yoda and I. Matsubara, J. Polym. Sci. Part A: Gen. Pap., 2, 253 (1964).
10. T. Ota, M. Yamashita, O. Yoshizaki, and E. Nagai, J. Polym. Sci. Part A-2: Polym. Phys., 4, 959 (1966).
11. T. Ohta, Y. Fujiwara, Y. Toshimori, and K. Monobe, Polymer, 33, 1620 (1992).
12. K. Inoue, J. Polym. Sci. Polym. Phys. Ed., 23, 743 (1985).
13. M. Avrami, J. Chem. Phys., 7, 1103 (1939).
14. M. Avrami, J. Chem. Phys., 8, 224 (1940).
15. M. Avrami, J. Chem. Phys., 9, 177 (1941).
16. J.I. Lauritzen and J.D. Hoffman, J. Appl. Phys., 44, 4340 (1973).
17. C.K. Chai, N.M. Dixon, D.L. Gerrard, and W. Reed, Polymer, 36, 661 (1995).
18. D.W. Van Krevelen, Properties of Polymers, 3rd ed., Elsevier, Amsterdam (1990).
19. J.D. Hoffman and J.J. Weeks, J. Res. Natl. Bur. Stand. Sect. A, 66, 13 (1962).
20. T. Ozawa, Polymer, 12, 150 (1971).
21. C. Angelloz, R. Fulchiron, A. Douillard, B. Chabert, R. Fillit, A. Vautrin, and L. David, Macromolecules, 33, 4138 (2000).
22. E. Koscher and R. Fulchiron, Polymer, 43, 6931 (2002).
23. J.C. Won, R. Fulchiron, A. Douillard, B. Chabert, J. Varlet, and D. Chomier, Polym. Eng. Sci., 40, 2058 (2000).
Sophie Naudy, Rene Fulchiron
Universite de Lyon, Lyon, F-69003, France; universite Lyon 1, IMP/LMPB Laboratoire des Materiaux Polymeres et Biomateriaux, Bat ISTIL, 43 bd du 11 novembre, Villeurbanne, F-69622, France; CNRS, UMR5223, Ingenierie des Materiaux Polymeres, Villeurbanne, F-69621, France
Correspondence to: Rene Fulchiron; e-mail: firstname.lastname@example.org
Contract grant sponsors: Legrand; Solvay; and Moldflow
TABLE 1. Experimental values of the growth rate G of PA1 spherulites as a function of the crystallization temperature obtained by both hot stages Mettler and Linkam. G (m/s) T ([degrees]C) Mettler Linkam 228.6 1.50 x [10.sup.-9] -- 228 -- 2.3 x [10.sup.-9] 226.6 3.10 x [10.sup.-9] -- 226 -- 3.2 x [10.sup.-9] 224.6 4.20 x [10.sup.-9] -- 224 -- 4.8 x [10.sup.-9] 222.6 6.90 x [10.sup.-9] -- 222 -- 7.8 x [10.sup.-9] 220.6 1.01 x [10.sup.-8] -- 218.6 1.51 x [10.sup.-8] --
|Printer friendly Cite/link Email Feedback|
|Author:||Naudy, Sophie; Fulchiron, Rene|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 2007|
|Previous Article:||Solid-state structure and formation of organized molecular films of methacrylate copolymers containing fluorinated and hydrogenated side-chains.|
|Next Article:||Effect of uniaxial drawing of soy protein isolate biopolymer film on structure and mechanical properties.|