Tailoring the Formation Rate of the Mesophase in Random Propylene

Article pubs.acs.org/Macromolecules

Tailoring the Formation Rate of the Mesophase in Random Propylene-co-1-pentene Copolymers Ernesto Pérez,* José M. Gómez-Elvira, Rosario Benavente, and María L. Cerrada Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain ABSTRACT: The mesophase structuring has been studied by fast scanning calorimetry in a series of random propylene-co-1-pentene copolymers, up to 7.9 mol % of 1-pentene units. It was found that the formation rate of the mesophase can be easily tailored in a wide range covering 2 orders of magnitude, in such a way that the rates involved in the copolymer with highest comonomer content can be also analyzed by conventional techniques, namely DSC and real-time X-ray diffraction employing synchrotron radiation. The advantage from the standpoint of easiness on the study of mesophase structuring is, therefore, well evident. The penalty to pay is the decrease on the overall degree of order attained in the copolymers.

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fold conformation,14−19 have been reported, and a new trigonal form has been recently described7,20−24 in the case of copolymers of iPP with high contents of 1-hexene or 1pentene as comonomers. As usual in polymer crystallization, both thermodynamic and kinetic considerations are of capital importance for the obtainment of a certain modification, and it is usual to observe a competition between the different polymorphs.23,25 In addition to those crystal modifications, fast quenching of iPP leads to a phase of intermediate or mesomorphic order.14,17,18,26−32 This phase is metastable and, on heating, undergoes a transformation into the thermodynamically more stable α-form.33−35 Many investigations have been devoted to the mesophase of iPP, owing to the fundamental and practical relevance of this phase with macroscopic properties (density, optical, and mechanical properties) laying between those of the amorphous state and of the monoclinic α-structure.36−39 The main prerequisite for the mesophase formation is the absence of crystallization. This can be attained by cooling from the melt at rates higher than around 100 °C/s for iPP homopolymer, since the competition between formation of crystalline and mesomorphic order is favorable to the mesophase at cooling rates higher than that mentioned value.40,41 Consequently, the study of the mesophase formation under real-time conditions requires, first, the use of experimental setups able to generate (and control) those very high cooling rates and, second, detection systems fast enough to record the structure evolution. At present, the technique of fast scanning calorimetry (FSC) is probably the best choice,42,43 allowing heating rates even above 40000 °C/s and cooling up to 4000 °C/s. It takes advantage of the use of very fast amplifiers together with rather reduced sample sizes, below micrograms.

INTRODUCTION Structure development (or suppression) under very high cooling rates is a rather interesting subject for several reasons. First, the solidification process of polymers in typical processing conditions usually involves those very high cooling rates, and considering that the final properties may depend strongly on the details of the generated structure, it will be useful to reproduce those conditions and analyze the final structure attained and its influence on the properties. A second interest is related to the fact that under such circumstances, far away from equilibrium conditions, metastable phases are often obtained. In this context, the polymer crystallization has been considered as a multistage process via an intermediate mesomorphic phase.1−3 Foundations for this scheme are based on the Ostwald’s rule of stages, formulated more than 100 years ago.4 This empirical rule states that a phase transformation will proceed through metastable states, whenever they exist, and it has been recently invoked in the crystallization of different materials5 and specifically for polymer crystallization1,2,6 by combining both phase stability and kinetic considerations. Finally, we have entered the era of nanoscience and nanotechnology, with the associated reduction in the size of the materials and devices. And if we deal with nanosizes in polymers (or materials in general), much higher cooling rates can be attained, since the time for temperature equilibration will be reduced in parallel with the size. Among polymers, isotactic poly(propylene)s, iPPs, are of special interest. The use of metallocene catalysts makes possible the synthesis of random copolymers with α-olefins presenting a homogeneous distribution of comonomer and a low polydispersity.7,8 In these copolymers, the final properties can be easily tailored by controlling both the general microstructure and the crystallization conditions.7,9−13 Moreover, iPP exhibits an amazing polymorphism, depending on microstructural features, crystallization conditions and other factors like the use of specific nucleants. Thus, three different polymorphic modifications, α, β, and γ, all sharing a 3© 2012 American Chemical Society

Received: June 22, 2012 Revised: July 26, 2012 Published: August 7, 2012 6481

dx.doi.org/10.1021/ma3012834 | Macromolecules 2012, 45, 6481−6490

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Table 1. Characterization of the Different Samplesa

Several investigations have been devoted41,43−46 to the mesophase formation of iPP under dynamic or isothermal conditions by using FSC. Typically, the mesophase peak appears at temperatures between 40 and 20 °C when cooling above 100 °C/s, and its isothermal formation at around room temperature goes to completion in fractions of a second. Eventually, at cooling rates exceeding 1000 °C/s, the formation of the mesophase is also prevented and i-PP solidifies in a fully amorphous state. The FSC technique is able to detect the thermal transitions under those very high scanning rates, but the phases involved cannot be unambiguously determined. For this purpose, spectroscopy or diffraction techniques are appropriate. Unfortunately, these are not yet implemented enough to accomplish those very high scanning and detection rates. So far, for instance, real-time diffraction experiments on the iPP mesophase formation have been reported47 by collecting 20 frames/s and cooling at nominal rates up to 200 °C/s, although the instantaneous cooling rate at temperatures around the mesophase formation is 1 order of magnitude smaller. Nevertheless, that work deals with the mesophase development in two propene/ethylene copolymers, where the formation rates are considerably smaller than those in iPP homopolymer. Other studies have been reported on the mesophase formation in copolymers of iPP with ethylene, 1-butene, 1hexene, or 1-octene,47−52 all of them restricted to just one or two comonomer contents. Anyway, a general conclusion from these works is that the critical cooling rate required to obtain a semimesomorphic sample steadily decreases on increasing the concentration of counits, although it seems that there are important differences depending on the type of counit.52 To our knowledge, there are no such studies dealing with copolymers with 1-pentene. The aim of the present study is to determine the window of cooling rates for observing the mesophase in copolymers of iPP with 1-pentene. For that, a relatively broad range of compositions (up to 7.9 mol % 1-pentene) is analyzed by means of FSC. As will be shown, the cooling rates for mesophase formation can be easily tailored, and interestingly, those rates for the copolymer with highest 1-pentene content are low enough to allow a partial real-time structural investigation by means of synchrotron X-ray diffraction and with conventional DSC.

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sample

mol % 1-pentene

Mw

Mw/Mn

Tm (°C)

ΔHm (J/g)

iPP cPPe2 cPPe4 cPPe6 cPPe8

0 1.9 4.1 5.8 7.9

326 100 248 600 150 900 144 000 126 100

2.1 2.1 2.0 2.0 2.0

153.8 135.5 122.6 106.6 93.0

103 92 76 64 52

Melting temperatures, Tm, and enthalpies of melting, ΔHm, obtained by conventional DSC after cooling from the melt at 20 °C/min.

a

The thermal properties of the different specimens were first analyzed in a Perkin-Elmer DSC-7 calorimeter connected to a cooling system (Intracooler 2P from Perkin-Elmer) and calibrated with different standards (indium for enthalpy as well as zinc, indium, and ndodecene for temperature). The sample weight ranged from 3 to 5 mg. A scanning rate of 20 °C/min (0.333 °C/s) was used. The values obtained for the melting temperatures, Tm, and for the enthalpies of melting, ΔHm, are also shown in Table 1. Although there is a continuous and important decrease of molecular weight with increasing comonomer content, those molecular weights are still high enough, and then, no significant influence on the thermal parameters is expected. A commercial power-compensation differential scanning chip calorimeter Flash DSC 1 from Mettler-Toledo was used for the FSC analysis. Comprehensive details of this equipment can be found elsewhere.53 By means of a two-stage intracooler device (Huber TC100), temperatures as low as around −100 °C can be reached. The samples were typically prepared in a first stage as films of 10−20 μm with a microtome. From those films, and with the aid of a microscope attached to the equipment, a small peace is cut with a scalpel and placed in the middle of one of the two sensors of the calorimeter chip (previously calibrated with the parameters from factory and conditioned afterward). Ideally, samples below around 50 ng are required in order to attain the maximum rates of the device (4000 °C/ s on cooling and 40000 °C/s on heating) without excessive problems of overheating. As will be shown, however, the scanning rates involved in the copolymers are decreasing progressively as the comonomer content increases, so that the sample weight can be increased accordingly, with the subsequent advantage of a greater sensitivity for the smaller cooling rates. The sample weight is estimated from the apparent enthalpy of melting obtained by FSC after cooling from the melt at the same rate than in conventional DSC (20 °C/min = 0.333 °C/s) and comparing with the actual enthalpy shown in Table 1. Moreover, the peak melting temperatures in Table 1 have been also considered for the FSC tests in what the initial temperatures for the cooling experiments were set to a value around 50−60 °C above those melting temperatures. Thus, for instance, the FSC cooling experiments on the iPP sample were registered after melting up to 210 °C, and that temperature was 150 °C in the case of copolymer cPPe8. Synchrotron experiments at wide angle were performed on the Non-Crystalline Diffraction Station of the Spanish Collaborative Research Group (CRG) beamline BM16-CRG at the ESRF (Grenoble, France). The X-ray beam was monochromatized at the Selenium K-edge energy (λ = 0.0978 nm). A MarCCD 165 detector located at around 25 cm from the polymer sample was used. The calibration of the spacings was made with a sample of silver behenate (giving a well-defined diffraction at a spacing of 5.838 nm and several orders). A Linkam THMS600 stage, provided with a liquid-nitrogen cooling device, was used for controlling the temperature. With this setup, cooling rates up to 80 °C/min can be reached. The acquisition time was set to 5 s, but another 5 s is needed to refresh the detector, so that the total time between diffractograms was 10 s. A cPPe8 film sample of around 5 × 5 × 0.1 mm (length, width, and thickness) was used for the synchrotron study. The sample was cooled from the melt (130 °C) to −20 °C at different cooling rates (20, 40, 60, and 80 °C/min) and subsequently molten at 16 °C/min up to 130 °C.

EXPERIMENTAL SECTION

The copolymerizations of propylene and 1-pentene were carried out in a 250 mL stainless steel autoclave at −5 °C in toluene by using racdimethylsilylbis(1-indenyl)zirconium dichloride/MAO as the catalyst/ cocatalyst system ([Al]/[Zr] = 3648). The homopolymer iPP was also prepared under the same conditions. The molecular weights were obtained by gel permeation chromatography in a Waters 150 CV-plus system equipped with an optical differential refractometer (model 150 C). The comonomer and defect contents were determined by 13C NMR spectroscopy. The copolymer samples are designated as cPPe followed by the closest integer value of the mol % 1-pentene content in the copolymer. The corresponding molecular characteristics are shown in Table 1. For the homopolymer, the stereo defects content, obtained from the mrrm pentad (the mrrr pentad is not detectable), takes the value of 1.23 mol %, while the regio defects (propylene 2,1 additions) amount to 1.18 mol %. The content of stereo and regio errors in the copolymers is expected to be similar to that found in the homopolymer. For the 1-pentene comonomer contents analyzed in this work (below 8 mol %) the new trigonal form has not been detected under any crystallization conditions. 6482


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RESULTS AND DISCUSSION Calorimetric Study (FSC and DSC). Preliminary information on the copolymers was obtained from conventional DSC. The corresponding heating curves for the different samples, after cooling from the melt at 20 °C/min (0.333 °C/s), are shown in Figure 1. The values of peak melting temperatures

ization to sample weight and scanning rate, increases as the sample weight decreases. For clarity of the presentation, therefore, the cooling curves have been cut when that curvature appears. Moreover, they have been baseline adjusted and shifted vertically. The behavior of the iPP homopolymer sample is rather similar to that mentioned before in other reports; i.e., for cooling rates below around 70 °C/s a single exotherm is observed, corresponding to the formation of the monoclinic crystals, and when the cooling rate exceeds around 100 °C/s, the mesophase exothermic peak begins to be observed at lower temperatures. Furthermore, at cooling rates above 1500 °C/s, no exotherm is observed and a fully amorphous phase is obtained. The behavior of the copolymers is rather analogous, excepting the fact that the cooling rates for the different events are progressively decreased as the comonomer content increases. For instance, in copolymer cPPe2 the mesophase exotherm is already clearly observed at 20 °C/s, while the cooling rate for a fully amorphous sample is reduced to 500 °C/s. Finally, for copolymer cPPe8, the cooling curve at 1 °C/s shows already the mesophase peak, while at 20 °C/s a totally amorphous sample is obtained. As mentioned above, this reduction of the cooling rates allows one to increase the sample weight, so that the sensitivity is proportionally higher. Thus, the estimated weight for the cPPe8 sample used here was 520 ng, compared to the only 13 ng for iPP homopolymer. The cooling rates for those two “transition” regions, i.e., for the beginning of the observation of the mesophase exotherm and for obtaining a fully amorphous sample, have been determined for the various samples (around 40 different cooling rates have been tested for each sample, but only the most representative curves are displayed in Figure 2). The results are shown in Figure 3a, in the so-called continuous cooling curve (CCC) diagrams,47,54,55 widely used in metallurgy for the characterization of quenching of steels. In this figure, the line formed by the full squares represents the limit of

Figure 1. Conventional DSC heating curves for the different samples. The curves have been normalized to scanning rate (20 °C/min) and sample weight and shifted vertically for clarity of the presentation.

and enthalpies of melting are collected in Table 1. As usual, both parameters are steadily decreasing as the comonomer content increases. These enthalpies of melting have been used to estimate the sample weight in the FSC experiments, as commented above. The different samples were, then, analyzed by FSC in a wide range of cooling rates. The cooling curves corresponding to four of the samples and at selected cooling rates are displayed in Figure 2. Inherent to the rather small sample weights used in FSC is the fact that at relatively low cooling rates the curves are rather noisy, and they show a continuously increased curvature in the low temperature side, thus preventing the correct analysis of the glass transition. Evidently, the curvature, after normal-

Figure 3. CCC diagrams for the 1-pentene copolymers, representing the cooling rates for: (a) full squares: beginning of observing the mesophase exotherm; full triangles: only mesophase exotherm seen; full circles: totally amorphous samples (evidently, all the ordered phases are accompanied by a certain amount of amorphous component). (b) The top diagram, open circles, represents the heating rates necessary to maintain the sample totally amorphous on melting, after cooling at a rate of at least that defined by the full circles.

Figure 2. Selected FSC cooling exotherms for the indicated samples and cooling rates. The curves have been normalized to cooling rate and sample weight and baseline adjusted and shifted vertically for clarity of the presentation. 6483


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The most interesting conclusion from the present results is that the formation of the mesophase (and of the α-phase at high undercoolings) will be much easier to study in these copolymers. In fact, the rates involved in copolymer cPPe8 are practically inside the range of conventional techniques, as will be shown below. A final boundary line, not so closely connected with phase diagrams, is the one constructed with the heating rates for avoiding reorganizations in the different phases once formed. Thus, a fully amorphous sample, even in the case of iPP homopolymer, is relatively easy to be obtained using FSC techniques, but to avoid any sign of cold crystallization in the subsequent melting is much more difficult. This is well evident in the melting curves shown in Figure 4. These curves were

mesophase observation, i.e., at lower cooling rates, the mesophase is not observed, and only the α-phase is obtained. Eventually, if the cooling rate is low enough, a certain proportion of γ phase will be also generated. Thus, rather significant proportions of γ crystals are expected56 in metallocenic iPPs for cooling rates lower than around 60 °C/min (1 °C/s). On the other hand, the line of full circles in Figure 3a represents the boundary above which fully amorphous samples are obtained. Again, a rather marked decrease of cooling rates with increasing comonomer content is observed, with a difference of about 2 orders of magnitude between iPP and cPPe8, similarly to the behavior of the line for observing the mesophase. The diagram can be supplemented with another boundary region, slightly more diffuse than the two previous ones. That boundary is the limit of cooling rates where the α-phase exotherm is not observed anymore, and the mesophase is the only ordered structure. This boundary is rather well-defined in the copolymers with higher comonomer contents, but for iPP homopolymer it occurs that when the mesophase is beginning to predominate over the α-phase, the cooling rate is high enough to prevent the formation of any ordered structure, and only the amorphous component is obtained. This behavior differs somewhat from that described in other reports.44 The reason can be attributed, most probably, to the metallocenic nature of the present iPP sample, with a more homogeneous distribution of defects along the different chains and with a significantly lower melting temperature. Several important conclusions can be deduced from the CCC diagram in Figure 3a. First, the three boundary lines are rather parallel, especially for high comonomer contents. It seems, therefore, that the comonomeric units are affecting in a rather similar way to the formation rate of both the mesophase and the monoclinic crystals (and to the total amorphous sample). Preliminary isothermal experiments at high undercoolings are confirming this aspect, since the formation rates of the mesophase and of the α-phase are decreasing in a similar way as the comonomer content increases. The second aspect from Figure 3a is that the mesophase formation rate can be easily tailored by introducing the appropriate comonomer content, and with 7.9 mol % of 1pentene units, the rates are decreased by 2 orders of magnitude in relation to iPP homopolymer. This effect of the comonomer content was already pointed out in iPP copolymers with ethylene or 1-butene,47−51 and more recently with 1-hexene and 1-octene,52 but those studies were not so much systematic, since only one or two comonomer contents were studied. It seems, however, that in the cases of ethylene or 1-butene as counits the decrease of the mesophase formation rates is considerably smaller: no more that 1 order of magnitude for comonomer contents comparable to the ones in this study.47,51 The relative probability for the comonomeric units of being incorporated into the crystalline (or mesomorphic) entities may be of capital importance for the decrease of those rates of formation, what is expected to be in parallel to the corresponding reduction on the melting temperatures. Thus, the melting point depression of the copolymers with 1-butene is considerably smaller than the one for the ethylene copolymers,13,47,57−60 this one being also much smaller than in the case of the 1-pentene copolymers here studied (see Table 1).

Figure 4. Normalized melting curves (heating rate of 3000 °C/s) for copolymers cPPe2 (upper frame) and cPPe6 (lower frame) after cooling from the melt at the indicated cooling rates (in °C/s). The curves have been vertically shifted for clarity.

registered at a heating rate of 3000 °C/s after cooling at different rates. In the case of copolymer cPPe2, the curve after cooling at 4000 °C/s exhibits a clear cold “crystallization” on melting, indicating that although a totally amorphous sample was obtained on cooling (as deduced from Figure 2), the heating rate of 3000 °C/s is not fast enough to prevent structuration of the sample on heating. On the contrary, such heating rate is sufficiently high in the case of copolymer cPPe6, and in agreement with the results in Figure 3a, the sample keeps totally amorphous when the cooling rate has been higher than 50 °C/s. In this context, several additional FSC experiments have been performed on all the samples here studied by cooling initially from the melt at a rate well above the solid circles in Figure 3a (in order to ensure completely amorphous samples on cooling) and subsequently melting the sample at different heating rates. Some of the results are presented in Figure 5, for copolymers cPPe2 and cPPe6, after cooling from the melt at 1000 °C/s, a rate high enough to prevent any ordering on cooling; i.e., initially all the samples are totally amorphous. It can be observed, however, a clear cold crystallization if the heating rate is not high enough, and again the critical heating rate for avoiding any structuration depends very much on the copolymer content. Thus, it can be seen in Figure 5 that a heating rate of 2000 °C/s keeps the sample totally amorphous 6484


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heating is not a problem if the total neat enthalpy just above the glass transition is considered, since that cold crystallization will be exactly canceled by its subsequent melting. The variation of the total enthalpy with the cooling rate for the different samples is displayed in the upper frame of Figure 6. The data represent the values deduced from the subsequent

Figure 5. Normalized melting curves (at the indicated heating rates, in °C/s) for copolymers cPPe2 (upper frame) and cPPe6 (lower frame) after cooling from the melt at 1000 °C/s. The curves have been vertically shifted for clarity.

for copolymer cPPe6, while even 10 000 °C/s is generating a small but appreciable structuration on heating for copolymer cPPe2. From these experiments, the critical heating rate to maintain the sample in the amorphous state was deduced. The variation of such rate as a function of the comonomer content is displayed in Figure 3b. Now, the decrease observed is smaller than before, in the cooling experiments, since it amounts to somewhat more than 1 order of magnitude. Specifically, a decrease by a factor of 20 is found on passing from iPP to cPPe8, while for the cooling rates in Figure 3a, it is on the order of 80−100. Thus, it seems that the structuration of these copolymers from the quenched state is much less affected by the comonomeric units than the one from the melt. It is concluded, therefore, that a fully amorphous sample is obtained for the present iPP homopolymer, as shown above, when cooling from the melt at rates of at least 1500 °C/s, but to keep it amorphous on melting, a heating rate of at least 30 000 °C/s is needed. These so high rates can be reached by FSC, but the sample weight has to be kept below around 40 ng to avoid excessive overheating effects. Moreover, such high rates may preclude the observation of different melting peaks when they appear not far away in temperature. The advantage of using these copolymers from the standpoint of easiness on the study of mesophase structuring is, therefore, well evident. The price to pay is the decrease on the overall degree of order attained in the copolymers. An idea of this disadvantage is deduced from the data in Table 1 about the enthalpy of melting (in this case related to the α-crystals melting, but expected to be similar for the mesophase), which decreases to about one-half from iPP to cPPe8. Anyway, if the copolymers are chosen for the study of the iPP mesophase formation, it is mandatory to ascertain that the phenomenology of the process is similar in all cases. A rather good idea of this being fulfilled can be deduced from Figure 3a, since the different lines are more or less parallel. A more complete scrutiny can be performed by analyzing the variation of the enthalpy with the cooling rate. This can be done either from the cooling or from the subsequent heating curves. In this last case, evidently, the occurrence of cold crystallization on

Figure 6. Variation of the total enthalpy (upper frame) and relative enthalpy (lower frame, see text for the procedure) with the cooling rate for the different samples (the big solid points define the boundary lines of Figure 3a). The data represent the values deduced from the subsequent melting experiments, which coincide rather well with the cooling ones (the cooling values for cPPe4 are plotted for comparison in the upper frame).

melting experiments, which are a much better choice for the smaller cooling rates, since the corresponding cooling curves in such cases are either very noisy or even the cooling exotherms cannot be discerned at all. Anyway, for sufficiently high cooling rates, the melting and cooling values coincide rather well. This can be deduced from the two data sets (enthalpies from melting or from cooling curves) for copolymer cPPe4, both plotted in the upper frame of Figure 6. From those values, it seems to follow that the enthalpy variation is rather similar in shape for the different samples, excepting, evidently, the enthalpy depression. This reduction can be artificially suppressed if an appropriate relative enthalpy is used. For instance, the big solid points in Figure 6 define the boundary lines as in Figure 3a. Probably the best choice for a relative enthalpy is to “normalize” to the enthalpy values corresponding to the beginning of observation of mesophase (the values for the solid squares). In such case, the relative enthalpy values shown in the lower part of Figure 6 are obtained. Now the similitude among the different samples is more evident. One step forward is to apply the usual time-other magnitude superposition approach. In fact, the x-axis scale in Figure 6 is just a kind of crystallization or residence time, allowing, or not, the development of the different structures. If the values for the copolymers are shifted in order to superimpose them with those for iPP homopolymer, the results shown in Figure 7 are obtained, for both the absolute and the relative enthalpy. They represent, therefore, a kind of time−composition superposition. Several interesting aspects can be deduced from this figure. First, the data for all the samples can be superimposed fairly well with the appropriate shifting factors. Only a certain 6485


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Some selected exotherms on cooling from the melt at different rates are shown in Figure 8b compared with the curves

Figure 7. Variation for the different samples of the total enthalpy (upper frame) and of the relative enthalpy (lower frame, see text for the procedure) with the cooling rate shifted by a factor to superimpose the values of the copolymers with those of iPP homopolymer. The shifting factors are 3.4, 9, 27, and 57 for cPPe2, cPPe4, cPPe6, and cPPe8, respectively.

Figure 8. Comparison between FSC (a) and DSC (b) normalized cooling curves at the indicated rates for copolymer cPPe8. Note that now the units are °C/min, as usual in conventional DSC. The samples weights are 520 ng for FSC and 4.34 mg for DSC.

discrepancy seems to occur in the region where the mesophase begins to be observed. It is rather small and almost inside the experimental error (an idea of the magnitude of this error can be deduced from both Figures 6 and 7, from the comparison with the results at the same cooling rate, which, in certain cases, have been repeated on purpose). Anyway, this can be related to the fact mentioned in the discussion of the results in Figure 3a, when it was found that the mesophase of iPP cannot be obtained without α-phase in this particular metallocenic sample. The second aspect from Figure 7 is related to the enthalpy depression in the copolymers. From the results in Table 1, it is deduced that the enthalpy for copolymer cPPe8 is just one-half of that for iPP. This important reduction, however, arises from two factors. The first one is the evident disruption of the crystallizable isotactic propylene sequences due to the comonomer units, which is general in all random copolymers with non-cocrystallizable units. But there is also a second factor, readily deduced from Figure 7, which arises from the considerably slower crystallization rates in the copolymers. Thus, an increase of around 30% of the crystallinity might be obtained if copolymer cPPe8 is crystallized at a rate relatively similar to that of iPP, as clearly anticipated in the lower part of Figure 7. Considering the value of 57 for the shifting factor of cPPe8 in this figure, that cooling rate should be as low as 0.35 °C/min (but this rate will introduce the additional complication of obtaining, most probably, a big amount of γphase). As commented above, the rates involved in copolymer cPPe8 are practically inside the range of conventional techniques (excepting, of course, the heating rate for avoiding any ordering in the sample). Consequently, several additional studies have been performed on this copolymer. The first one was the analysis of the mesophase formation by conventional DSC. The equipment used for this analysis allows cooling under control at a maximum rate of 50 °C/min (0.833 °C/s), almost in the limit deduced from Figure 3a for observing the mesophase.

from FSC (note that now the units are °C/min, as usual in conventional DSC). The curves from the two techniques are rather similar, with the evident much better signal-to-noise ratio in the DSC experiments. Focusing the attention on the common cooling rate of 40 °C/min, it appears that the mesophase peak is already barely observed in the DSC curve, but not in the FSC one. This may be just a problem of appreciation limit in the FSC curve, with much more noise. On the other hand, the main exotherm, corresponding to the crystallization of the α-form, appears centered at around 47 °C in the FSC curve and at 39 °C in the DSC experiment. One of the most probable reasons for this difference is the expected higher surface nucleation effects in the much smaller FSC sample: 520 ng compared with the 4.34 mg in the conventional DSC sample. Real-Time X-ray Diffraction Study Employing Synchrotron Radiation. In order to ascertain the nature of the phases involved, additional real-time diffraction experiments, employing synchrotron radiation, have been performed on copolymer cPPe8. In this case, the temperature controller device allows cooling at rates up to 80 °C/min (1.33 °C/s), which is, in principle, inside the region for observing the mesophase (yet with some proportion of α-crystals), as deduced from Figures 3a and 8. The diffractograms of a cPPe8 sample on cooling from the melt at 80 °C/min are shown in Figure 9a (although the initial temperature is 130 °C, only the profiles below 100 °C are depicted in the figure, since at high temperatures only the amorphous halo is obtained). It can be observed that when the temperature decreases to around 50 °C, some diffraction peaks characteristic of the α-form begin to be detected, and specifically those at s values of 1.57 and 1.86 nm−1, assigned to the [110] and [040] diffraction planes of that form. At lower temperatures, the diffraction centered at 2.39 nm−1 grows considerably, while the former diffraction peaks have reached 6486


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increases, while the peak at 2.39 nm−1 remains approximately with a constant intensity. The interpretation is that the α-phase content is decreasing with increasing cooling rate, simultaneously to the growth of the mesophase content, thus confirming the nature of the two exotherms in the FSC (and DSC) experiments. And as expected from the CCC diagram in Figure 3a, a cooling rate of 80 °C/min (1.33 °C/s) is not enough to get the mesophase without α-form. The pure mesophase of copolymer cPPe8 was obtained from a film (with a thickness of about 0.1 mm) by efficiently quenching from the melt into ice water (sample Qice). The diffractogram corresponding to this specimen is shown in Figure 10b. It can be observed that now only the mesophase is achieved, with no indication of α-reflections. Since the totally amorphous (molten) profiles are obtained at the beginning of those cooling experiments, it is relatively easy to determine the “crystallinity” evolution (more properly the content of ordered structures: both mesophase and α-form). This determination is not straightforward, since the amorphous profile has to be appropriately shifted to account for the temperature differences. The temperature coefficient of the amorphous halo has been deduced from Figure 11, where the

Figure 9. Synchrotron X-ray diffractograms, as a function of temperature, for copolymer cPPe8 corresponding to samples cooled from the melt at 80 °C/min (a) and at 20 °C/min (b). All the diffractograms are plotted in the first case, but only one every three is shown for the lower cooling rate, for clarity of the presentation.

already an asymptotic intensity. The problem with this diffraction is that it may arise both from the α-form (diffractions [111], [−131], and [041]) or from the mesophase, assigned to the repeating period within the 31 helix of an isotactic sequence of the macromolecule.18,29,43 Similar experiments have been carried out at lower cooling rates, namely at 60, 40, and 20 °C/min. The results for this last rate are presented in Figure 9b, showing now diffractograms typical of the α-form. The final diffractograms, acquired at −20 °C, from the experiments at different cooling rates are shown in Figure 10a. It can be clearly observed that the intensity of the diffractions from the α-phase decreases very much as the cooling rate Figure 11. Temperature dependence of the spacing corresponding to the maximum of the amorphous (molten) profile on cooling cPPe8 from the melt at 20 °C/min.

variation with temperature of the spacing of its maximum is shown. Evidently, the deviation of the common straight line of the results below 60 °C indicates the beginning of crystallization. A temperature coefficient of (0.000 29 ± 0.000 01) nm/°C is deduced from the slope of the straight line. This coefficient has been used for the crystallinity assessments at the different temperatures, by shifting the amorphous profile by the appropriate amount (and scaling it to account for the specific noncrystalline content). Accordingly, the pure “crystalline” profiles for the final diffractograms after cooling at different rates have been determined with this procedure, these patterns being plotted in Figure 10c. After the cooling experiments aforementioned, the subsequent melting runs were immediately registered (in all cases at a rate of 16 °C/min). The results for two of the experiments are presented in Figure 12. The upper frame shows the diffractograms for the melting run after cooling at 80 °C/min (quoted as f16c80). The melting−recrystallization of the mesophase into the α-modification is well evident at around 60 °C, previous to the total melting at around 100 °C. On the

Figure 10. Synchrotron X-ray diffractograms for copolymer cPPe8 corresponding to (a) final diffractograms, at −20 °C, after cooling from the melt at the indicated rates; (b) room-temperature diffractogram after efficiently quenching into ice water (Qice), and (c) pure “crystalline” profiles for the diffractograms in (a). 6487


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consistent with the higher amounts of mesophase obtained when that cooling rate increases, as already deduced from the pure “crystalline” profiles in Figure 10c. The next step is, evidently, to determine the proportions of mesophase and α-modification in all those diffractograms. This is not an easy task, considering the overlapping of the diffractions of those two ordered structures, as indicated above. Moreover, the location of the different diffraction peaks changes with temperature. An approach to the problem can be made by postulating a couple of hypotheses. The first one is to consider that the cooling experiment at 20 °C/min does not produce any mesophase at all. This is a fairly reasonable assumption because of the absence of the mesophase peak in both the FSC and conventional DSC experiments in Figure 8. Consequently, the top pure crystalline profile in Figure 10c can be ascribed solely to α-crystals. And in that profile, the peak at s = 1.86 nm−1 (diffraction plane [040]) represents the best compromise of high intensity and isolation from the mesophase diffractions. This particular peak involves around 13% of the total crystal intensity. The second hypothesis is to assume that the relative proportion of that diffraction peak over the total α-form crystallinity keeps constant, irrespectively of the total crystallinity or of the perfection of the α-crystals formed at different cooling rates. This is also fairly reasonable when comparing results for the same polymer sample, as in the present case (although it may change with the comonomer content). With these two hypotheses, the relative area of the [040] diffraction peak has been determined in all former results of total crystallinity, and that area rescaled by the factor 100/13, thus obtaining a degree of crystallinity of only the α-phase. The results are plotted as circles in Figure 13. The self-consistency of the method used and the validity of the two assumptions can be checked from this figure. First, the total crystallinity and the α-one coincide rather well for the cooling at 20 °C/min and for its subsequent melting. It seems that the percentage of area under the [040] peak over the total crystal area keeps fairly constant with temperature and total crystallinity. And more importantly, the total crystallinity and the α-one coincide again in the melting runs after the melting−recrystallization of the mesophase, i.e., above around 80 °C. The mesophase “crystallinity” can be straightforwardly determined from those results just by subtracting the αcrystallinity from the total one. The corresponding values for all the different experiments are plotted in the lower frame of Figure 14. Several interesting features can be highlighted from these results. Focusing the attention on the cooling results, it can be observed that the α-phase is generated first, mainly at around 50−40 °C, while the mesophase ordering appears at lower temperatures, mostly in the temperature interval from 30 to 10 °C, in rather good agreement with the results in Figure 8 for the calorimetric analysis, thus confirming the assignment of the high-temperature peak to the formation of α-crystals and of the low-temperature one to the mesophase ordering. The results for the heating runs are even more interesting. Thus, the melting−recrystallization of the mesophase is perfectly observed, either from the evolution of the mesophase crystallinity, which disappears in the interval from around 60 to 80 °C, or from the growing of the α-crystallinity, in the same temperature range. At higher temperatures, the melting of the α-crystals occurs, with the apparent contradiction of experiment

Figure 12. Synchrotron X-ray diffractograms, as a function of temperature, for copolymer cPPe8 corresponding to the heating runs of the samples cooled from the melt at 80 °C/min (upper frame) and at 20 °C/min (lower frame). The heating rate was 16 °C/min. Only one every five diffractograms are plotted, for clarity of the presentation.

contrary, the experiment after cooling at 20 °C/min (f16c20) only shows the melting of the α-crystals. All those diffractograms (for the cooling at different rates and for the subsequent melting runs) have been analyzed by determining, first, the total “crystallinity”, following the procedure mentioned above for obtaining the profiles in Figure 10c. The corresponding results are plotted as squares in Figure 13. The most interesting feature of the evolution of the total

Figure 13. Variation of the total crystallinity (squares) and of the αcrystallinity (circles) as a function of temperature for the different cooling (full symbols) and subsequent melting runs (open symbols) for copolymer cPPe8.

crystallinity is found on the melting experiments, where the smooth tendency of decreasing crystallinity with increasing temperature is broken at around 60 °C, associated with the melting−recrystallization of the mesophase. This tendency is not observed for the sample cooled at 20 °C/min, but it is getting more pronounced as the cooling rate rises, which is 6488


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CONCLUSIONS The formation rate of the mesophase in random propylene-co1-pentene copolymers can be easily tailored in a wide range covering 2 orders of magnitude, in such a way that the rates involved in copolymer cPPe8 are inside the range of conventional techniques. The advantage from the standpoint of easiness on the study of mesophase structuring is, consequently, well noticeable. The overall degree of order attained in these copolymers is, however, significantly reduced and decreases to about one-half from iPP to cPPe8, although part of that enthalpy could be recovered by cooling at smaller rates. The phenomenology of the mesophase formation appears to be rather similar among the different samples, and the variations with the cooling rate can be superimposed by shifting this variable by an appropriate amount for each copolymer. A kind of time−composition superposition is, therefore, fulfilled. The mesophase in copolymer cPPe8 seems to appear at lower cooling rates both in the conventional DSC experiments and in the real-time synchrotron diffraction analysis when comparing with the FSC results. The reason may be associated with higher surface nucleation effects in the much smaller FSC sample or with differences in the thermal equilibration rate across the whole specimen.

Figure 14. Variation of the α-crystallinity (upper frame) and of the mesophase “crystallinity” (lower frame) as a function of temperature for the different cooling (full symbols) and subsequent melting runs (open symbols) for copolymer cPPe8. Note that, by hypothesis, the mesophase content is zero in the sample cooled at 20 °C/min (c20) and in the subsequent melting (f16c20), so that they are not plotted in the lower frame.

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f16c80 melting at temperatures around 3 °C higher than f16c20. Evidently, this is not unexpected, since when cooling at 20 °C/min the α-crystals were formed at around 50 °C, while for the cooling at 80 °C/min, the majority of the α-crystals have been obtained from the recrystallization of the mesophase, on heating, at around 70 °C. A summary of the degrees of “crystallinity” (total, α, and meso) is shown in Table 2, as a function of the cooling rate, for the final diffractograms at −20 °C.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of MICINN (Project MAT2010-19883) is gratefully acknowledged. The synchrotron work was also supported by MICINN through specific grants for the access to the CRG beamline BM16 of the ESRF. The inestimable help of all the beamline personnel is also acknowledged.

fcWAXD total

α

meso

20 40 60 80

0.33 0.30 0.28 0.26

0.33 0.19 0.09 0.04

0 0.11 0.19 0.22

AUTHOR INFORMATION

Corresponding Author

Table 2. “Crystallinity” Degrees Deduced from the RealTime Diffraction Experiments for cPPe8 Cooled from the Melt at Different Rates cooling rate (°C/min)

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REFERENCES

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It is evident that the majority of the ordered structures (around 85%) corresponds to the mesophase for a cooling rate of 80 °C/min. This stands in contrast with the calorimetric results in Figures 2 and 8, where it is deduced that cooling rates of 2.5 °C/s (150 °C/min) or higher are needed to get more mesophase than α-form in copolymer cPPe8. The most plausible explanation for this behavior is, evidently, the expected higher surface nucleation effects in the much smaller FSC sample, as commented above. A second reason may be just the difference in thermal equilibration rate, especially when considering that the synchrotron experiments at high cooling rates where conducted practically at the limit of the temperature control system. More systematic studies are being planned in order to get more insight into this aspect. 6489


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Tailoring the Formation Rate of the Mesophase in Random Propylene

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