Mesophase Formation in Random Propylene-co-1 ... - ACS Publications

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Mesophase Formation in Random Propylene-co-1-octene Copolymers Javier Arranz-Andrés,† Rosa Parrilla,‡ María L. Cerrada,† and Ernesto Pérez*,† †

Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain Centro Tecnológico Repsol, Móstoles, 28935 Madrid, Spain

‡

S Supporting Information *

ABSTRACT: The development of the mesophase has been studied by either fast scanning calorimetry or conventional DSC in a series of random propylene-co-1octene copolymers. The results show that the mesophase formation rate can be easily tailored in a very wide range of magnitudes, covering almost 4 orders. That decrease is similar to that found previously in propylene-co-1-pentene copolymers at the same wt % of counits. On the contrary, the pertinent unit for the melting point depression (and for the crystallization temperatures) is found to be the mol % comonomer content. Consequently, the “kinetic” parameters in the ordering of iPP copolymers can be tailored somewhat independently of the transition temperatures just by changing the size of the comonomeric units. Additional X-ray diffraction experiments, employing either conventional or synchrotron radiation, were also carried out on these 1-octene copolymers in order to determine the precise nature of the different transitions.

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INTRODUCTION One of the topics in polymer science that is attracting a great interest is the obtainment (or inhibition) of ordered structures when cooling at high rates (more than about 102−103 °C/s). The significance of this topic implies both important technological issues and scientific aspects. For example, interesting metastable phases may be formed under those nonequilibrium conditions, and the crystallization of polymers is envisaged by some authors as a multistage path with intermediate metastable structures,1−3 so that stability and kinetic aspects are both considered.1,2,4 Structure development is of special significance in isotactic poly(propylene), iPP. This polymer shows an interesting polymorphism, which is reported to depend primarily on microstructural aspects and on thermal history but also on other different features, like employing appropriate nucleating agents.5−16 Regarding the thermal history, both thermodynamic and kinetic aspects are very important for obtaining a certain polymorph, being usually the result of a specific competition between the various possible crystalline forms.15−17 Interestingly, fast quenching of iPP (cooling at rates higher than about 100 °C/s) leads to a mesomorphic phase5,8,9,18−25 which is metastable, so that it transforms into the α crystals25−28 (with higher thermodynamical stability) when heating at usual rates of DSC. Several articles were dedicated to the mesomorphic phase in iPP, which has been reported to display some properties intermediate between the ones of amorphous specimens and those of the α modification.29−32 The nonexistence of crystallization is necessary for obtaining such mesophase, so that, in the case of the homopolymer, it is required to cool from the molten state at rates over 100 °C/s. The reason is that the © 2013 American Chemical Society

competition existing between crystal order and mesophase formation is unfavorable for the mesomorphic phase, and crystalline structures are obtained when the cooling rate is lower than that value.25,33,34 Therefore, two requirements are needed for studying, under conditions of real time, the formation of that mesophase: (a) experimental setups able to permit and control those extreme rates and (b) detector systems being sufficiently fast for registering the different ordering processes. In this respect, fast scanning calorimetry (FSC) is a technique recently implemented25,35,36 and fulfilling those two requirements. Thus, it is able to reach cooling rates of 4000 °C/s and heating in excess of 40 000 °C/s by using very small sample weights (below micrograms) and rapid amplifiers. Rates higher than 106 °C/s are even reported for the so-called ultrafast scanning calorimetry35 (UFSC). Several investigations have been devoted25,34,36−39 to the study of the mesomorphic phase in iPP by means of FSC, using both dynamic and isothermal experiments. It was found that the mesophase is formed with an associated characteristic exotherm appearing in the interval of 20−40 °C when cooling at rates higher than approximately 100 °C/s and that some tenths of second is enough for its complete isothermal development near ambient temperature. Moreover, the mesophase formation is prevented when cooling at rates over 1000 °C/s, so that completely amorphous iPP samples are obtained under such conditions. Received: July 22, 2013 Revised: October 21, 2013 Published: October 31, 2013 8557

dx.doi.org/10.1021/ma401539u | Macromolecules 2013, 46, 8557−8568

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Although the different transitions can be observed by FSC when using such high rates, it is not possible to establish unambiguously the nature of the corresponding transitions. Therefore, alternative spectroscopy or X-ray diffraction experiments are needed for such purpose. But the present technologies do not allow yet achieving those extreme rates for such experiments, nor are the detector systems fast enough. A very good alternative for studying the mesomorphic development is the analysis of iPP copolymers. It has been shown25 in several copolymers of propylene-co-1-pentene that the mesomorphic phase formation can be tailored rather simply in a broad range of cooling rates, in an interval of magnitude covering around 2 orders. Consequently, the copolymer presenting the highest counit content was also studied by using conventional techniques such as DSC and variabletemperature X-ray diffraction in a synchrotron source. This is a well-evident advantage, since mesomorphic development can be therefore easily studied. There is however a price to pay in such a case, since higher comonomer contents are inevitably accompanied by smaller degrees of ordering. Nevertheless, it was concluded that the rate of mesomorphic ordering seems to be phenomenologically very similar for the different copolymers (and the homopolymer) studied, since a certain superposition of a kind time−composition was obtained.25 The preparation of copolymers of iPP and different 1-olefins is not only of academic interest but also they offer the opportunity to increment the number of possible applications of iPP, and its processability can be improved as well,40,41 considering that by selecting the microstructure and the appropriate crystallization conditions the ultimate properties may be tailored for specific demands.11,42−46 There are other reports dealing with the mesomorphic development in iPP copolymers with different 1-olefins.40,47−52 All these studies are, however, limited to only one or two compositions. In any case, a common finding reached in those works is that when the comonomer concentration increases, the rate of cooling necessary for obtaining mesomorphic order decreases accordingly, with significant variations among the different counits.40,44b The purpose of this work is to gain a deeper knowledge into those possible differences arising from the comonomer type. Thus, the mesophase formation has been investigated in random iPP copolymers with 1-octene, with a wide range of contents. The results show an easy tailoring of the conditions for mesomorphic development, so that FSC techniques are mandatory for the lower comonomer contents, while conventional ones (such as DSC or X-ray diffraction) are employed for the copolymer of highest 1-octene content. Furthermore, the results were compared with the ones found in 1-pentene copolymers, showing both rather interesting differences and similarities.

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Table 1. Molecular Parameters and Thermal Properties of the 1-Octene Copolymer Samplesa sample

mol % 1octene

wt % 1octene

Mw (g/mol)

Tm (°C)

ΔHm (J/g)

cPO2 cPO6 cPO9

2.2 5.6 8.9

5.7 13.7 20.7

142 000 135 000 118 000

125 107 77

74 54 30

Temperature, Tm, and enthalpy, ΔHm, of melting determined by DSC when heating at 20 °C/min after cooling also at such rate. a

corresponding melting temperature of the copolymers and 1 MPa of pressure, applied during 4 min. Two well different conditions were chosen for the crystallization of the samples: cooling rapidly (about 80−90 °C/min) by using refrigerating water (treatment named as Q) and a second one with the samples allowed to cool slowly to ambient temperature (rate of about 0.5−1 °C/min, treatment named as S). A conventional DSC calorimeter25 (PerkinElmer DSC-7) was first used for analyzing the thermal behavior of the copolymers, with sample weights ranging from 4 to 8 mg. Table 1 shows the temperatures and enthalpies obtained for the three copolymers when melting at 20 °C/min after cooling also at such rate. The FSC study has been carried out in a commercial calorimeter (Mettler-Toledo, Flash DSC 1). A detailed description of this calorimeter, and of the sample preparation as well, can be found in other reports.25,54 As mentioned there, a sample weight below about 50 ng is recommended for attaining the maximum specifications of the equipment: heating at 40 000 °C/s and cooling at 4000 °C/s. The results shown in Table 1 for the DSC melting enthalpies were considered for estimating the weight of the samples in the FSC experiments, by comparison of such enthalpies with the apparent values found by FSC when cooling at an equal rate. The values determined by this method were 60 ng for cPO2 and 20 ng for cPO6. The FSC runs were programmed by initially melting the sample at 180 °C for cPO2 and 160 °C for cPO6. Copolymer cPO9 was not studied by FSC due to the very low crystallization rates involved. Following the “tradition”, the rates in conventional DSC are expressed in °C/min, while those in FSC are in °C/s. We keep this tradition in some respect throughout the article, although for comparison purposes the chosen units are always °C/s. The conventional X-ray diffractograms have been taken in a Bruker diffractometer (D8 Advance) with a PSD detector and using Cu Kα (λ = 0.1542 nm) radiation. The details about the optics (monochromators, mirrors, slits, etc.) and operating modes can de found in a previous publication.16 Real-time synchrotron experiments have been performed on beamline BM11-NCD at ALBA (Cerdanyola del Vallés, Barcelona, Spain) at a fixed wavelength of 0.1283 nm (9.6617 keV). An ADSC 210 detector (with a pixel size of 102.4 μm), placed approximately at 200 mm from the position of the sample, was used. The temperature control unit was a Linkam hot stage, connected to a cooling system of liquid nitrogen. Typically, the diffractograms were acquired every 12 s (9 s of acquisition and 3 s of waiting time needed to refresh the detector). The calibration of spacings was obtained by means of a high-crystallinity iPP specimen ([040] diffraction at 1.910 nm−1) and of a silver behenate sample (5.838 nm spacing in its first-order reflection). The program FIT2D (ESRF, Dr. Hammersley) was used to convert the initial 2D X-ray pictures into 1D diffractograms. Additionally, the normalization of these diffractograms to the intensity of the direct beam was performed as well as the background subtraction of the sample control unit. The final diffractograms are represented against the inverse scattering vector, s = 1/d = 2 sin θ/λ . A cPO6 Q film sample of around 5 × 5 × 0.1 mm was employed in the synchrotron analysis. This specimen was initially molten at 10 °C/ min up to 140 °C, then cooled at 30 °C/min, and subsequently molten at 10 °C/min.

EXPERIMENTAL SECTION

The three copolymers of propylene with 1-octene have been obtained by using a metallocene catalyst activated by MAO, as described elsewhere,44b,53 in solutions of toluene. The details for the GPC equipment used for determining the molecular weight of the samples and for the NMR spectrometer for comonomer content determinations have been given before.25 The different samples were named by cPO and the integer content in 1-octene (expressed as molar percentage). Table 1 shows those molecular parameters for the three 1-octene copolymers. Film samples of the different copolymers have been prepared in a Collin press with the following conditions: 30 °C above the 8558


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RESULTS AND DISCUSSION Calorimetric Studies (FSC and DSC). As mentioned above, conventional DSC experiments were performed in order to obtain initial information on these 1-octene copolymers. The heating curves for specimens cooled from the isotropic melt at 20 °C/min are presented in the Supporting Information. Table 1 shows the results deduced from those curves, namely melting temperature and enthalpy. It can be observed that these values decrease continuously with the increase of counits. As commented before, those enthalpies were used to get an estimation of the weights of the specimens employed for FSC studies. It is significant to observe that the curve obtained for copolymer cPO9 presents an evident cold crystallization on heating, so that the rate of formation of ordered entities in this sample is very low, since the crystallization is not completed when cooling the sample at 20 °C/min. The three copolymers were also studied by cooling at rates in appropriate broad ranges. Depending on the magnitude of these rates for a particular copolymer, the technique for that study has been FSC or DSC. The corresponding cooling curves have been plotted in Figures 1 and 2. The first important

Figure 2. Conventional DSC cooling exotherms for copolymers cPO6 (a) and cPO9 (b) at the indicated rates, after normalization to sample weight and cooling rate. For better clarity in the plots, a vertical shift was applied to these curves.

The inherent fact to the very little weight of the specimens in these FSC studies is the obtainment of very noisy curves when the cooling rate is low. In addition, they present well-evident curvatures (which increase for decreasing cooling rates) at low temperatures, in the region of the glass transition, so that this transition cannot be correctly analyzed. Consequently, the curves in Figure 1 for those low cooling rates have been cut in the low-temperature side. Moreover, and for clarity of the plot, the curves have been baseline adjusted and presented with a vertical shift. The behavior of the other two copolymers, cPO6 and cPO9, is rather similar, with the obvious exception of a progressive decrease of the rates involved in the different processes as the content in counits increases. Thus, Figure 2 shows that the cooling rates for the beginning of mesophase appearance are 15 °C/min (0.25 °C/s) for cPO6 and 1.5 °C/min (0.025 °C/s) for copolymer cPO9; i.e., they fall in both cases in the range of cooling rates typical of conventional DSC. For obtaining fully amorphous samples, it is deduced from Figure 2 that a rate of 60 °C/min (1 °C/s) is needed for cPO9. This rate is precisely the maximum one that is possible to be attained in our calorimeter for the temperatures involved in mesophase development. As a consequence, the corresponding analysis is incomplete for copolymer cPO6, since for that maximum cooling rate we are yet relatively far away of obtaining a fully amorphous sample. Consequently, the DSC analysis for cPO6 has been complemented by FSC. It was shown in a previous report25 that the reduction of the cooling rates for higher comonomer compositions allows one increasing the weight of the FSC sample, in order to get a superior sensitivity. However, that

Figure 1. Plot of FSC curves for copolymer cPO2 at the selected cooling rates indicated and after normalization to sample weight and cooling rate. For better clarity, the baseline has been adjusted and the curves shifted vertically.

finding derived from the inspection of these figures is that the mesophase formation rate decreases rather appreciably with increasing counit concentration, so that the rates involved for copolymer cPO6 (with 5.6 mol % 1-octene content) are already in the range of conventional DSC. This finding is different to that observed previously on 1-pentene copolymers,25 where 7.9 mol % comonomeric units was needed to observe by DSC just the beginning of mesophase development. The rates involved in the case of sample cPO2 are yet appropriate for FSC. Thus, Figure 1 shows that when cooling at a rate below about 5 °C/s, only one exotherm peak is obtained, which is assigned to the development of α crystals. On increasing the rate, at values about 7 °C/s, a second exotherm is detected in the lower temperature side, attributed to the appearing of mesomorphic order. Finally, if the rate is high enough (in excess of 200 °C/s), a completely amorphous sample is attained since no exotherms are visible. 8559


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concentration in counits is well evident, amounting to almost 4 orders of magnitude, considerably higher than that found previously for 1-pentene copolymers.25 Moreover, the DSC results seem to indicate that somewhat smaller rates are needed, as previously noticed for a 1-pentene copolymer.25 It is important to consider that the γ form may be additionally obtained for very low cooling rates. In particular, very important amounts of this γ modification are formed57 in iPPs synthesized by metallocenic catalysts when cooled at rates below about 60 °C/min. And for these 1-octene copolymers, γ crystals are also observed at very low cooling rates (see below). Moreover, another boundary line (the one defined by circles) is depicted in that Figure 3a, representing the minimum rates of cooling for obtaining a completely amorphous state. This line shows also an important dependence with the cooling rate, decreasing rather appreciably (in a magnitude of more than 3 orders) as the content in 1-octene counits increases. Such decrease, on passing from iPP to cPO9, is rather similar to that found in the line representing the rates for the observation of mesomorphic ordering. A third boundary line can be defined from the cooling curves, representing the limiting rates for the mesophase alone, without the peak corresponding to the monoclinic crystals. This boundary line is easily obtained for high counit concentrations, but it is rather diffuse or even not observed for the lower comonomer compositions and specifically in the case of the homopolymer.25 The reason is that for the increasingly high cooling rates needed to avoid the development of the monoclinic modification, the mesomorphic order is also prevented and a completely amorphous state is attained. This finding is somehow different of that reported by other authors for iPP homopolymer.37 A possible explanation for this difference may be found in the fact that the present homopolymer specimen was synthesized with a metallocene catalyst, so that the various defects are expected to be more homogeneously distributed along the macromolecular chains. This is also clearly reflected on a much lower temperature of melting. Moreover, the results for cPO2 (see Figure 1) also indicate that the completely amorphous state is attained before the mesophase appears without a certain proportion of monoclinic phase. Another significant aspect is derived from the observation of the CCC diagrams of Figure 3a. It is related to the fact that the mentioned boundary lines appear to be parallel, what is particularly evident in the case of the higher contents. The conclusion from this behavior is that the 1-octene counits seem to be influencing quite similarly the rates for obtaining either the mesomorphic entities or the α modification (and also the completely amorphous state). The same conclusion was deduced previously on 1-pentene copolymers.25 But the most interesting conclusion is that copolymers are much better systems for studying the mesomorphic ordering (and the crystallization of the monoclinic modification at very high undercooling). Thus, since the cooling rates needed in sample cPO9 are rather low, conventional experiments can be used for the analysis of structural ordering in that copolymer. Moreover, many aspects of the mesomorphic development in cPO6 occur also at low enough rates, so that such kind of experiments can be also applied for the corresponding study. Therefore, it appears rather easy to tailor the rates for the mesomorphic development, even at high undercoolings. The

report also showed that even for rather high sample weights (around 500 ng) the cooling curves below around 1 °C/s are too noisy to be analyzed. This precludes the study of mesophase development for cPO6 from the cooling FSC curves. Therefore, the strategy chosen here has been the contrary: the sample weight has been reduced, and it has been attempted to indirectly deduce such formation from the melting curves, as it will be shown below. At this point, the rates involved in the two “transition” regions (the cooling rate when the mesomorphic peak is first observed and that for the obtainment of a completely amorphous polymer) were determined in the case of the different copolymers (see below for determining the rate for fully amorphous cPO6). These values, plotted in Figure 3,

Figure 3. Representation of CCC diagrams as a function of the 1octene content showing the rates needed for: (a) On cooling: square symbols, mesomorphic peak first observed; triangles, mesomorphic exotherm alone; circles, completely amorphous state; open symbols, DSC results; full symbols, FSC results (obviously, some proportion of amorphous polymer is present in any ordered phase). (b) On heating: the diagram in the top frame (full diamonds) shows the FSC rate needed for avoiding cold “crystallization” on heating, provided that the cooling rate has been higher than that of the circles in the lower frame.

represent a kind of continuous cooling curve (CCC) diagram,47,55,56 commonly employed in metallurgy when characterizing quenched steel. The results for iPP homopolymer have been taken from a previous report.25 In this figure, the open symbols refer to DSC results, while the full ones were determined from FSC. The line of square symbols in Figure 3a indicates the limiting rates of mesophase observation; i.e., the mesophase will not be observed at lower cooling rates, and then only monoclinic crystals are formed. The enormous reduction of those rates for increased 8560


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content is increased, 3000 °C/s is a rate suitable for sample cPO6, as deduced from the results of the lower plot of Figure 4, where no cold “crystallization” is detected. Such a boundary line has been determined from the FSC melting curves shown in Figure 5 for copolymers cPO2 and

only requirement is to have the suitable concentration of counits. The most reasonable explanation for those two conclusions could be found on kinetic considerations for the formation of ordered structures. As comonomer content increases, the length of crystallizable segments will be progressively diminished if exclusion of the counits is supposed, and thus, those segments will be segregated before they reach the growing crystalline entities, resulting, therefore, on lower crystallization rates (and lower crystallization temperatures). A rather analogous slowing down seems to be deduced for the mesophase development. Another boundary line has been plotted in Figure 3b, representing now the heating rates needed to avoid cold crystallization on samples cooled from the melt at rates high enough to lead to a completely amorphous state. It is not difficult to obtain this state by FSC even for the homopolymer, but it is not so easy to prevent cold crystallization in the second melting of the sample, as it is very apparent from the inspection of the results in Figure 4, obtained after cooling from the

Figure 5. Plots of the normalized heating curves corresponding to samples cPO2 (upper plot) and cPO6 (lower plot) obtained when cooled at 1000 °C/s and subsequently heated at the rates indicated. For better clarity of the plots, a vertical shift was applied to these curves.

cPO6. These curves were obtained after an initial cooling at 1000 °C/s, which, according to the results in Figure 3a, is a rate sufficiently high for obtaining the completely amorphous state in both copolymers. After such constant initial cooling, the melting runs were subsequently registered at variable rates, as indicated in Figure 5. The boundary line will be therefore defined by the rate above which no cold “crystallization” exotherm is observed. From Figure 5, the corresponding rates for that event are found to be about 7000 °C/s for copolymer cPO2 and about 700 °C/s for cPO6. Again, a very important effect of the comonomer composition is deduced, and as always, the rates are clearly diminished as the 1-octene content increases. This aspect is well evident when inspecting the final results for this boundary line, represented in Figure 3b, together with the one for the homopolymer. A next step in the study of ordering processes in these samples is a quantification of both the enthalpies and crystallization temperatures involved. This is carried out by deconvolution and integration of the cooling exotherms shown in Figures 1 and 2. This is not an easy task, owing mainly to two facts: first, the overlapping of the two exotherms (monoclinic crystallization and mesophase formation) and, second, both temperature location and shape of these exotherms are changing with the cooling rate. Anyway, this

Figure 4. Plots of the normalized heating curves corresponding to samples cPO2 (upper plot) and cPO6 (lower plot) obtained when cooled at the rates indicated and subsequently heated at 3000 °C/s. For better clarity in the plots, a vertical shift was applied to these curves.

molten state with variable rates and subsequently heating the sample at 3000 °C/s. Thus, for copolymer cPO2, a well-evident cold crystallization exotherm is observed in the heating curve corresponding to a previous cooling from the melt at 200 °C/s. And according to the results in Figure 1, the sample is initially amorphous after such cooling rate. The conclusion is therefore that 3000 °C/s is not a rate sufficiently high to prevent a cold “crystallization” in cPO2. However, and owing to the general reduction of the rate of all events when the comonomer 8561


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7 or 10 °C/s, it seems rather evident that 3000 °C/s is sufficient to avoid the reorganization of the preexisting mesomorphic entities, since only a low-temperature melting peak, attributed to the true mesophase melting, is observed. Therefore, the curves will express, eventually, the true relative composition of mesophase and monoclinic crystals. A single melting peak (corresponding to the mesophase) is obtained when cooling above around 7 °C/s (before attaining a fully amorphous sample, obtained at around 20 °C/s). In the other extreme, a single endothermic peak is found after cooling at rates below around 0.5 °C/s (monoclinic phase), with two components after intermediate cooling rates. The rough deconvolution of the two components (now even more difficult, due to stronger overlapping) leads to the results shown in the frame at the middle in Figure 6. Comparing with the DSC results, it seems that the mesophase is first observed at slightly higher cooling rates by FSC. Evidently, this fact can be derived only from resolution problems of these FSC curves, with a very important overlapping of the two contributions. Nevertheless, a similar conclusion was reached previously25 for 1-pentene copolymer samples, so that it may be a general finding. It is not surprising that the greater surface nucleation effectiveness expected for FSC specimens, with a very small sample weight, will delay somewhat the mesophase formation, so that slightly higher cooling rates may be needed. Additionally, it was previously reported25 for 1-pentene copolymers that the overall enthalpy dependence with the cooling rate is very analogous for the various copolymers (as in this case), except, obviously, the decrease of such enthalpy with the content in counits. It has been also shown that by “normalizing” the data to a certain value the decrease is overcome, and the enthalpy variation is then rather similar in both shape and relative amount. The suggested25 procedure for such purpose is the “normalization” of the results to a value of 100 for the enthalpy related to the cooling rate at which the mesomorphic exotherm is first observed. This procedure has been successfully applied to the present samples, obtaining the normalized relative enthalpies presented in Figure 7. It can be observed that the similarities and differences are now rather evident between the three copolymers. Thus, the characteristic features mentioned above, namely the increase of the maximum relative content of the mesophase, the widening of the gap between the total enthalpy and the one arising from the monoclinic crystals, and the differences between FSC and DSC data for cPO6, are now better observed. Of particular interest is the relative content of mesophase, which increases very much as the comonomer content does: from 28% for cPO2 to 83% for cPO9. Evidently, these values are connected with the fact that the gap between the total enthalpy variation and that of the α crystals is increasing rather appreciably with comonomer content. The relevant finding is that the enthalpy arising from the α crystals is decaying at relatively much lower cooling rates than that of the total enthalpy, for increasing content of 1-octene counits. Therefore, a much higher proportion of mesophase is obtained. This is even true in the absolute, non-normalized, values (see Figure 6) where the absolute maximum contents of mesophase amount to 17.5, 24.5, and 32.7 J/g for cPO2, cPO6, and cPO9, respectively (the DSC value for cPO6 is around 29 J/g). The conclusion from these results is that higher amounts of mesophase are obtained as the comonomer content increases, and the reason may rely on a destabilization of the α crystals higher than that of the mesophase.

analysis has been performed for the enthalpies and crystallization temperatures of the constituent exothermic peaks. The variation with the cooling rate of the total enthalpy of crystallization, of the mesophase (low-temperature exotherm component), and of the monoclinic α phase (high-temperature component) is displayed in Figure 6 for the three copolymers.

Figure 6. Plots of total (squares), meso (circles), and α crystals enthalpy (triangles) as a function of cooling rate, corresponding to the indicated copolymers. Full symbols refer to FSC data, and open ones refer to conventional DSC results.

Besides the commented decrease of both the total enthalpy and of the cooling rates involved as the 1-octene content is increased, another important distinctive feature is derived from this figure: the maximum relative content of the mesophase increases rather evidently with the 1-octene content, and, in parallel to that, the gap between the total enthalpy and the one arising from the monoclinic crystals is also increasing. A further comment on this issue is made below. Regarding the findings for copolymer cPO6, the DSC results are incomplete, as mentioned above, since the maximum cooling rate of the calorimeter is not enough to obtain a completely amorphous sample. And no valuable information can be deduced from the FSC results on cooling, since the rather low cooling rates involved (below 1 °C/s) lead to cooling curves too noisy. Nevertheless, certain indirect information is inferred from the subsequent melting curves (see Figure 4). These curves have been obtained at 3000 °C/s, a rate that is sufficiently high to avoid noticeable restructuration when heating, as deduced from Figure 5. Strictly speaking, the results in this figure indicate that by heating at around 700 °C/s the cold crystallization is avoided upon heating in sample CPO6, but this may be a process with a different rate than that for the absence of reorganization of pre-existing crystals (or mesophase entities). However, and focusing the attention to the melting curves of the lower plot in Figure 4 after cooling at 8562


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change very much among the various copolymers, besides the great differences in the cooling rates involved. The continuous blue lines in that figure represent the absolute minimum values (peak temperatures) in the cooling exotherm, arising either from the mesophase peak or from the α crystals one. In principle, a “break” is expected to occur at around 50% of each component. However, considering that the mesophase exotherm is found to have a width which is 3−4 times narrower than that for the monoclinic phase peak, actually that break occurs at only around 20−25% of mesophase. The two extremes of that break (the one lying on the mesophase common line and that in the crystal line) can be taken as representative for a certain sample, and they have been plotted in Figure 9 as a function of comonomer content,

Figure 7. Plot of the dependence of normalized total (squares), meso (circles), and α enthalpy (triangles) as a function of cooling rate corresponding to the three 1-octene copolymers. These results are normalized to 100 for the total enthalpy obtained when the mesophase is first observed. Full symbols refer to FSC data, and open ones refer to conventional DSC results.

As a final aspect from Figure 7, if these normalized total enthalpies are shifted by an appropriate factor, they can be superimposed fairly well. This superposition involves certain time−composition issues, similarly to the previous findings on 1-pentene copolymers.25 The second important information deduced after deconvolution of the cooling exotherms shown in Figures 1 and 2 is related to the crystallization temperatures. The corresponding results are presented in Figure 8, indicating that they do not

Figure 9. Variation of the “crystallization” temperatures (see text) of the α crystals (squares) and that of the mesophase (circles) as a function of comonomer content corresponding to the indicated copolymer types.

together with the values determined from results reported previously25 for 1-pentene copolymers (cPPes), plus a copolymer with 2.4 mol % 1-octene from other report.40 In that figure, the crystallization temperatures of the α crystals for the different sets of samples seem to follow a common line, with a continuous decrease as the comonomer content increases. On the contrary, the mesophase crystallization temperatures (for that “reference” cooling rate of the mentioned “break”) appear to be more constant with the comonomer content, although now the scattering of the data is much higher. One of the reasons for this scattering may be that the mesophase exotherm may depend on the size (and geometry) of the sample much more than the crystallization peak. Those results (the decrease of the monoclinic crystallization temperatures and the rough practical constancy of the formation temperatures of the mesophase) reinforce the idea anticipated above that the higher amounts of mesophase obtained as the comonomer content increases are due to a destabilization of the α crystals higher than that of the mesophase.

Figure 8. Dependence of the “crystallization” temperatures of α crystals (squares) and of the mesophase (circles) as a function of cooling rate corresponding to the three 1-octene copolymers. Full symbols refer to FSC data, and open ones refer to conventional DSC results. The continuous blue lines represent the absolute minimum values in the cooling exotherm, arising either from the mesophase peak or from the α crystals one. 8563


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Another aspect from Figure 9 is that rather similar crystallization temperatures of the monoclinic phase are deduced for the two types of copolymers, 1-pentene and 1octene, when expressed as a function of the mol % of comonomeric units. On the contrary, as it was mentioned before, the decrease observed in Figure 3 for the cooling rates of the different events is considerably higher than that found in copolymers with 1-pentene. Anyway, we have plotted in Figure 10 the variation of different parameters as a function of the mol % of comonomeric units for both types of copolymers.

Figure 11. Variation of the cooling rates for different boundary regions (see Figure 3) and of the melting temperatures and enthalpies of melting with the wt % comonomer content for the copolymers with 1octene (cPOs) and 1-pentene (cPPes).

Established theories of polymer crystallization in the case of copolymers58 predict a lowering of crystallization temperatures irrespectively of whether the comonomeric units are included or excluded from the crystal. And the fact that the decrease is similar for 1-pentene and 1-octene copolymers is most probably due to one of the following possibilities: either the excess free energy of the defects formed by the incorporation of the comonomeric units in the crystallites of iPP is similar in both copolymers, or they are equally excluded from such lattice. Since the first possibility appears rather unlikely, considering the big differences in size between the propyl defects from 1pentene and the hexyl branches from 1-octene, the second possibility is more probable. Anyway, a close examination of the melting points variation in Figure 10 seems to indicate that the melting point depression for the 1-pentene copolymers is slightly smaller than that for 1-octene copolymers, although the difference is practically inside the experimental error. In this context, the kinetics of mesophase development and isothermal crystallization has been analyzed in different iPP copolymers as a function of temperature by FSC in a recent study.52 It is observed in that work that for 1-hexene and 1octene copolymers the decrease found in the crystallization rate is considerably higher than that involved in the formation of the mesophase. Such behavior was explained by suggesting that those kinds of comonomers can be partially trapped in the mesomorphic entities, especially when having into account that those entities are formed very rapidly. Evidently, more experiments are needed in order to ascertain this statement, but the present results in Figure 9 seem to indicate a destabilization of the α crystals higher than that of the mesophase as the comonomer content increases.

Figure 10. Dependence of the cooling rates for different boundary regions (see Figure 3) and of the melting temperatures and enthalpies of melting with the mol % comonomer content for the copolymers with 1-octene (cPOs) and 1-pentene (cPPes).

It is deduced from Figure 10 that only the variation in the actual DSC melting temperatures seems to follow a rather similar trend in both cases (either for our results or for the ones of other authors13b,45), while for all the other parameters (the rates for avoiding cold crystallization, for observing the mesophase or for obtaining fully amorphous sample, and also the final enthalpy after crystallizing at 20 °C/min) the decrease observed in 1-octene copolymers is much higher than that found in 1-pentene copolymers. Figure 11 shows, however, the dependence expressed as a function of the wt % of comonomeric units. All the variations are now rather similar for the two kinds of copolymers, except, obviously, the one for the actual DSC melting temperatures. We can extract, therefore, the following conclusions from Figures 9−11. All the crystallization rates as well as the total degree of order (as expressed by the final enthalpy of crystallization/melting) are dependent on the wt % comonomer content, meaning that those variables are correlated to the volume of the lateral substituents. On the contrary, the melting (Figure 11) and the crystallization (Figure 9) temperatures depend on the mol % content. 8564


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X-ray Diffraction Studies by Means of Conventional or Synchrotron Radiation. Further X-ray diffraction experiments, employing either conventional or synchrotron radiation, were also carried out on these 1-octene copolymers, in order to determine the precise nature of the different transitions. Figure 12 shows the conventional X-ray diagrams of films from the different copolymers either slowly (S samples) or

Figure 13. Room-temperature X-ray diagrams of two of the 1-octene copolymers crystallized from the molten state at various rates (indicated in °C/min). The Q and S samples of Figure 12 are also included for comparison.

Figure 12. Room-temperature X-ray diffractograms of the different copolymers, slowly (S) or rapidly (Q) crystallized from the melt.

appropriate shift in order to consider the temperature coefficient of the amorphous halo. These patterns can be obtained from the corresponding synchrotron experiments acquired under real time conditions at variable temperatures (see below). Once those amorphous diffractograms at ambient temperature are known, the “crystallinity” of the sample is easily determined just by scaling those profiles by a factor related to the actual amount of amorphous phase in the specimen. The procedure is shown in Figure 14 for specimen cPO2-S, where an amorphous component amounting to 46% is deduced, and the pure crystalline component is indicative of a mixture of α and γ modifications. Similarly, the pure ordered phases of the different specimens in Figures 12 and 13 have been obtained. These pure profiles are presented in Figure 15, and the deduced values of total “crystallinity” are given in Table 2. The next step is to calculate the proportion of the different ordered phases when a mixture of them is obtained. This is straightforward when α and γ modifications are present, just by considering the areas of the diffraction peaks [130] at around 18.5°, from the α form, and the [117], at around 20.1°, belonging to the γ modification. More difficult is to calculate the amounts of mesomorphic phase and α crystals in the X-ray diagrams, owing to the fact that those two phases show some characteristic diffraction peaks with an intense overlap. An approach to the problem has been made25 by considering that the diffraction plane [040] of the α form, at around 16.7°, is an optimal choice for calculations since it is rather intense and it appears well isolated from the diffraction peaks arising from the mesomorphic phase. That [040] diffraction was found to

rapidly (Q samples) cooled from the melt. Besides the expected decrease of crystallinity as comonomer content increases, it can be observed for the S samples (cooling rate around 0.5−1 °C/ min) that different proportions of α and γ crystals are obtained. This proportion of orthorhombic γ phase decreases for increasing comonomer contents, as it can be deduced from the intensity reduction in the [117] diffraction appearing at around 20.1°, so that a rather small quantity of the γ polymorph is observed for sample cPO9-S. For the case of the thermal treatment Q (characterized by cooling at around 80 °C/min) that γ phase is not observed at all. Moreover, the diffractogram for sample cPO6-Q indicates a major proportion of mesophase, and the one for cPO9-Q is almost entirely semimesomorphic, with very low proportion of α modification. On the contrary, specimen cPO2-Q shows only this α monoclinic phase (besides the amorphous component). Additionally, specimens of copolymers cPO6 and cPO9 (the ones with rates for obtaining the mesomorphic phase in the “conventional” range) have been crystallized from the molten state at various rates, selected according to the results in Figure 2, in order to obtain likely different amounts of mesophase. The corresponding diffractograms are shown in Figure 13, indicating a rather perfect agreement with the DSC results, with a clearly higher proportion of mesophase as the cooling rate increases. From these diffractograms it is fairly straightforward to calculate the corresponding degrees of “crystallinity” (i.e., the proportion of the different ordered phases: mesomorphic, α form, or, eventually, γ modification) if the totally amorphous profiles (at room temperature) are known. It has been shown16,25 that those profiles can be obtained from experiments at high temperature (molten profiles) after an 8565


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Table 2. “Crystallinity” Degrees Corresponding to the Different Samples Deduced from X-ray Diffraction Experiments

Figure 14. X-ray diffractogram, at room temperature, for specimen cPO2-S, its amorphous component (see text), and the corresponding pure crystalline phase.

sample

total ordered phase

γ phase

α phase

mesophase

cPO2-S cPO2-Q cPO6-S cPO6-c10 cPO6-c30 cPO6-c40 cPO6-c60 cPO6-Q cPO9-S cPO9-c1 cPO9-c2 cPO9-c2.5 cPO9-c5 cPO9-c10 cPO9-Q

0.54 0.44 0.40 0.35 0.31 0.29 0.28 0.28 0.30 0.27 0.25 0.24 0.22 0.20 0.22

0.39 0 0.28 0 0 0 0 0 0.07 0 0 0 0 0 0

0.15 0.44 0.12 0.35 0.11 0.07 0.05 0.05 0.23 0.18 0.09 0.06 0.02 0.01 0.02

0 0 0 0 0.20 0.22 0.23 0.23 0 0.09 0.16 0.18 0.20 0.19 0.20

reasonable estimation of the crystal content of the monoclinic phase alone is obtained. Evidently, the difference up to the total “crystallinity” represents the proportion of mesophase. With these premises, the relative proportions of the different ordered phases are those shown in Table 2, corresponding to the three copolymers with different thermal history. These results clearly show that by slow crystallization a certain amount of γ modification is obtained for the three copolymers, although it disappears when the region of mesophase formation is approaching. Moreover, the agreement between these findings and those shown in Figures 2 and 6 is rather satisfactory, although it is true that the proportion of mesophase is higher in these X-ray results. This may not be a real discrepancy, owing to the fact that the specimens for the X-ray experiments have been annealed at ambient temperature during several hours, before the acquisition of diffractograms. And, considering the results in Figures 2 and 9, the annealing at ambient temperature is expected to improve the amount of mesophase. As a final checking, some of these diffraction experiments were reproduced by using synchrotron radiation under real time conditions (the molten profiles have been used, as mentioned above, for determining the amorphous component at room temperature). Figure 16 shows representative examples of these variable-temperature experiments. The upper frame corresponds to the initial melting of specimen cPO6-Q, which, according to Table 2, exhibits an initial total “crystallinity” of 0.28 (0.05 corresponding to α form, and the rest, 0.23, to mesophase). It can be observed that between around 45 and 75 °C (diffractograms in blue) the mesophase is melting and recrystallizing into the α modification, and later on this form melts at about 120 °C (red diffractogram). The cooling from the molten state, at 30 °C/min, is represented in the middle frame of Figure 16. Now, the α form crystallization is observed first, at around 60 °C, and the mesomorphic phase is growing considerably when the temperature is below around 40 °C. This is in very good agreement with the findings deduced from Figure 2. Finally, the subsequent melting diagrams are shown in the lower part of Figure 16. The profiles and behavior are rather coincident with the initial melting process, except for the slightly lower total “crystallinity” (the initial sample stayed at

Figure 15. Pure ordered phases for the diffractograms in Figures 12 and 13 for copolymers cPO6 and cPO9 crystallized from the molten state at various rates (indicated in °C/min). The Q and S samples (see text) are also included.

include about 13% in relation to the whole crystalline area of the monoclinic modification, and more importantly, it keeps approximately invariable with both the overall crystal content and with possible differences in crystallite size arising from crystallization under different conditions. The procedure is therefore the following: (a) in the pure ordered profiles the areas of those [040] peaks are determined; (b) these areas are then rescaled with a factor of 100/13. With this procedure, a 8566


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CONCLUSIONS The results indicate that the formation rate of the mesophase in random copolymers of propylene-co-1-octene, up to 8.9 mol % content in 1-octene, can be tailored in a very broad range of magnitude, covering almost 4 orders. Thus, the rates implicated for the copolymers with higher contents are low enough to be investigated by means of conventional techniques, such as DSC or X-ray diffraction. The relative contents of the different phases and the transition temperatures have been analyzed from the crystallization exotherms in either FSC or DSC curves, registered at variable cooling rates in the appropriate ranges for observing the development of the mesomorphic phase. In principle, the decrease of the mesophase formation rate in these copolymers appears to be considerably higher than that found previously in propylene-co-1-pentene copolymers, for the same comonomer content expressed as mol %. However, if the wt % of comonomeric units is chosen for comparison, then that decrease follows a similar trend in both kinds of copolymers. On the contrary, the pertinent unit for the lowering of melting points (and for the crystallization temperature of the α phase) is found to be the mol % comonomer content. It may be concluded, therefore, that the “kinetic” parameters in the formation of ordered structures in iPP copolymers can be tailored somewhat independently of the transition temperatures just by changing the size of the comonomeric units. The nature of the phases involved has been ascertained by additional diffraction experiments, employing both conventional and synchrotron radiation.

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Figure 16. Plots of X-ray diffraction patterns in synchrotron experiments at variable temperature (heating rate of 10 °C/min), corresponding to cPO6 for the initial heating run of a Q sample (upper frame), the cooling from the molten state at 30 °C/min (middle frame), and the second melting (lower frame). For better clarity of the plots, not all diagrams have been represented.

ASSOCIATED CONTENT

S Supporting Information *

DSC heating curves corresponding to the three 1-octene copolymers (heating rate 20 °C/min) after cooling from the molten state at the same rate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

ambient temperature for long time and that crystallized at 30 °C/min has been immediately molten). These profiles can be analyzed, as before, for extracting the information about the amount of the different phases involved and its corresponding variation with temperature. The results for this particular sample are very similar to the ones previously reported25 for a copolymer with 7.9 mol % 1-pentene counits. Then, as a final consideration, it is remarkable to compare the behavior of these two samples: copolymer cPO6 (5.6 mol %, 13.7 wt % 1-octene) and the one reported previously:25 cPPe8 (7.9 mol %, 12.5 wt % 1-pentene). From the findings in Figure 11, it is deduced that the cooling rates involved are rather similar (slightly higher in cPPe8, due to its somewhat lower weight % of counits), and also the final enthalpies attained are similar (52−54 J/g). However, the melting temperatures are significantly higher for cPO6 (107 and 93 °C, respectively, for cPO6 and cPPe8), since this parameter is related to the mol % of counits. It may be concluded, therefore, that the “kinetic” parameters in the formation of ordered structures in iPP copolymers can be tailored somewhat independently of the transition temperatures just by changing the size of the comonomeric units.

*E-mail: [email protected] (E.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of MICINN (Project MAT2010-19883). The synchrotron experiments were performed at beamline BL11-NCD at ALBA Synchrotron Light Facility with the collaboration of ALBA staff.

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REFERENCES

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