Thermorheologically Complex Self-Seeded Melts ... - ACS Publications

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Thermorheologically Complex Self-Seeded Melts of Propylene− Ethylene Copolymers Leire Sangroniz,† Dario Cavallo,*,§ Antxon Santamaria,*,† Alejandro J. Müller,†,‡ and Rufina G. Alamo∥ †

POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018, Donostia-San Sebastián, Spain ‡ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain § Department of Chemistry and Industrial Chemistry, University of Genova, Genova, Italy ∥ Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St., Tallahassee, Florida 32310-6046, United States S Supporting Information *

ABSTRACT: A novel approach to study self-nucleation in semicrystalline polymers is presented. Self-nucleation, i.e., the peculiar increase of recrystallization kinetics associated with a range of melting temperatures above or below the melting point, is investigated in parallel with differential scanning calorimetry (DSC) and rheological measurements, for a series of metallocene propene/ethylene random copolymers of varying comonomer content. DSC experiments revealed the exact temperature region where self-nucleation effects are detected in the various samples, while with dynamic viscoelastic results the obedience to the time−temperature superposition principle (TTS) is tested, for both self-nucleated and homogeneous melts. Self-nucleated melts do not obey to the TTS principle, contrary to fully isotropic copolymer melts. Such rheological thermocomplexity constitutes the first physical experimental evidence of the presence of melt heterogeneities, which act as self-nuclei when the melt is cooled and recrystallizes. The degree of thermorheological complexity of the different copolymers is quantified and correlated with the original crystalline content of the copolymer.

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INTRODUCTION Self-nucleation (SN) is a unique primary nucleation mechanism active in semicrystalline polymers. SN, or self-seeding, was first recognized by Blundell et al.1 in 1966 and employed to produce single crystals with similar sizes crystallized from solution. They partially dissolved previously crystallized single crystals to provide seeds upon which polymer molecules could nucleate by replicating the crystalline structure of the self-seed via an epitaxial mechanism. Moreover, the manifestation of SN in increasing crystallization temperature on cooling from the melt and the experimental thermal protocols to identify regions of self-nucleated melts were described in detail by Fillon et al., while analyzing SN of melt-crystallized isotactic polypropylene (iPP).2 The procedure to study SN by differential scanning calorimetry and the SN behavior of a large number of polymers and copolymers has been recently reviewed by Müller and coworkers.3 The thermal protocol first creates a standard crystalline state, and then the sample is heated to a temperature above the observed melting point, where self-nuclei may reside within the polymer melt. The presence of self-nuclei is indirectly observed by a large increase in nucleation density, and accordingly a much faster crystallization rate, which is © XXXX American Chemical Society

proportional to the number of seeds left in the melt. When at high temperatures the melt is free of self-nuclei, the nucleation density and crystallization rates are low and unchanged. These two melt temperature regions have been termed domain II and domain I, respectively.2,3 Small crystal fragments can also act as self-nuclei when surviving from partial melting experiments; moreover, the unmolten crystal population is capable of annealing.2,3This region of partial melting is identified as domain III. It has been extensively demonstrated in the literature that self-nuclei can also be produced by completely melting the existing polymeric crystals at temperatures well above the observed melting point.3−10 In those cases, self-nuclei may be constituted by regions of the melt where chain conformations still retain some memory of the molecular conformations they had inside the crystalline regions of the sample. Often unduly long annealing times are needed to produce an isotropic melt where chains can finally adopt their random coil conformations. Received: November 4, 2016 Revised: December 26, 2016

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DOI: 10.1021/acs.macromol.6b02392 Macromolecules XXXX, XXX, XXX−XXX

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stability and their kinetics of dissolution. It is known that ethylene, 1-butene, and longer 1-alkene comonomers are defects restricting iPP crystallization to degrees proportional to the partitioning of the counit between crystalline and noncrystalline regions.18−24 Since the crystallization of random propylene copolymers is also sequence selective, on the one hand, one expects a behavior for SN akin to the strong SN behavior observed in random ethylene copolymers. On the other hand, the entanglement molecular weight of the iPP chain is about 4 times higher than for polyethylene;25,26 hence, the number of entanglements per iPP chain is much lower than for a polyethylene chain of the same length. The expectation from the latter leads to fewer restrictions to dissolve any remaining seeds in the melt and consequently a weaker melt memory for iPP copolymers than that observed for polyethylene-based copolymers. Despite the role of entanglements, chain dynamics and viscosity can be intuitively recognized to play an important role in self-nucleation; surprisingly rheological tools have been largely neglected in the study of this phenomenon. In this work, we choose a series of single-site metallocene propylene− ethylene copolymers with the same molar mass and varying in a wide range of ethylene content. The effect of decreasing iPP sequence length on the demarcation between homogeneous and heterogeneous melt states is first extracted from standard DSC measurements. Subsequently, melt viscoelasticity is studied with increasing temperature, from self-nucleated to fully homogeneous melts, and the thermorheological complexity of self-nucleated melts is highlighted, as opposed to the high-temperature region of homogeneous melt behavior.

Such crystalline memory effects enhance nucleation upon cooling from nonisotropic melts.3−5,9,10 In recent works it has been found that random copolymers of ethylene display memory of crystallization even at temperatures ∼40 °C above their equilibrium melting points. This unusual strong melt memory of copolymers is in sharp contrast with the behavior of linear polyethylene fractions that, independent of molar mass, display a region of SN at temperatures well below the equilibrium melting temperature.4 The copolymer’s strong memory is associated with the process of sequence partitioning during crystallization. Because branches longer than methyl are excluded from the crystal, the crystallization of random copolymers evolves through a process of sequence length selection by which long ethylene sequences are selected first, and other shorter sequences of suitable length will need to diffuse through the entangled melt to the crystal front in order to propagate lamellar crystallites. The path of selecting and dragging ethylene sequences to build copolymer crystallites generates a complex topology of knots, loops, ties, and other entanglements in the intercrystalline regions, especially at high levels of transformation, which is responsible for the unusual strong melt memory observed in random ethylene copolymers. It is reasonable to assume that when the crystallites of ethylene copolymers melt, clusters from the initial crystalline ethylene sequences remain in close proximity because segmental melt diffusion to randomize all sequences is hampered by branches and the constrained intercrystalline topology. We can hypothesize that these clusters are effective self-nuclei and only fully dissolve into a homogeneous melt at temperatures well above the equilibrium melting point.4,5 Segmented thermoplastic polyurethanes are also examples of systems with sequence selective crystallization and have also shown relatively strong melt memory.11 The experimental evidence is consistent with a kinetic nature of self-nuclei.3−13 Even when cooling from the same self-seeded melt, the increase of crystallization temperature depends on the initial level of crystallinity or on how the standard state is prepared.4,6,11,14 Furthermore, the strength of melt memory depends on chain length.4 On the other hand, self-nucleation of homopolymers, observed few degrees above the nominal melting temperature, has been shown to arise from remnants of crystals, which underwent thickening during the heating process. For example, Reiter et al.15 reported a high correlation between the orientation of small recrystallized seeds and that of the original “parent” single crystal, testifying the remaining of a certain degree of crystallographic order. Self-nucleation has also been explained within a thermodynamic context. On the basis of the multistep path to polymer crystallization postulated by Strobl,16 Muthukumar has considered the heterogeneous melt as a hypothetic metastable intermediate melt state in the path between the melt and the crystalline state and has derived a general expression for the nucleation rate using a standard reaction kinetics scheme and accounting for the transitions between the crystalline, heterogeneous, and homogeneous states.17 The strong melt memory of random copolymers was also observed in Monte Carlo simulations carried out by Wu and co-workers and was interpreted as a state with weak liquid−liquid phase separation.14 Contrasting the characteristics of self-nucleated melts of propylene 1-alkene copolymers by their SN effect on crystallization with the features of ethylene based copolymers is of interest, especially in consideration of the self-seeds

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EXPERIMENTAL SECTION

Materials. The i-PP homopolymer (i-PP) and the propene/ ethylene (PE) copolymers under investigation are experimental samples synthesized with a metallocene catalyst and kindly provided by ExxonMobil. Detailed characterization of the molecular features and of the crystallization behavior of the samples was previously reported.18−22 The content of comonomer and total defects (including regio- and stereoerrors) of the copolymers is summarized in Table 1,

Table 1. Characteristics of the Employed Propene/Ethylene Copolymers sample

comonomer (mol %)

total defects (mol %)

Mw × 10−3 (g/mol)

Mw/Mn

Tom (°C)

iPP PE1.8 PE2.8 PE3.4 PE5.8 PE8.7 PE10.1

0 0.8 1.7 2.2 4.6 7.5 7.5

1.17 1.8 2.8 3.4 5.8 8.7 10.1

217 233 221 215 251 188 217

1.90 1.98 1.81 1.75 2.12 1.71 1.80

186.1 185.2 184.1 183.5 180.7 177.2 174.2

together with GPC-derived molecular weight and the estimation of the equilibrium melting temperature obtained in previous works.18−21 We note that propylene−ethylene copolymers synthesized with single-site metallocene catalysts are characterized by narrow interchain comonomer composition distribution, which eliminates complications in the crystallization behavior that may result from broad comonomer distributions, such as those present in most copolymers synthesized with the Ziegler−Natta catalyst.5,27 For differential scanning calorimetry measurements, the samples in the form of pellets were compression molded at 210 °C in a Carver press to obtain films with thickness between 0.15 and 0.2 mm. Similarly, for rheological investigation the samples were compression B


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Macromolecules molded at 180 °C in a Collin P 200 E obtaining sheets with a thickness of 1 mm. Differential Scanning Calorimetry. Self-nucleation experiments were performed by means of differential scanning calorimetry, employing a PerkinElmer DSC 7 calibrated with high purity indium and operating under a nitrogen flow. Flat disks, weighing between 3 and 4 mg, were punched out from the pressed films and placed into aluminum pans. At first, the crystallization behavior of the polymers was characterized by a “standard” heating−cooling−heating thermal ramp. The samples were heated from 30 to 210 °C, annealed in the melt for 5 min, cooled down to 30 °C again, and finally remelted by bringing them to 210 °C. All heating and cooling ramps were performed at a rate of 10 °C/min. Subsequently, i-PP homopolymer and PE copolymers were submitted to the self-nucleation procedure originally proposed by Fillon et al.2 and described in the following: (1) Erasure of any previous thermal history by holding the sample in the melt at 210 °C for 5 min. (2) Creation of a crystalline “standard state” by cooling from 210 to 30 °C at a rate of 10 °C/min. (3) Complete/partial melting of the sample by heating the sample at 10 °C/min from 30 °C to a selected temperature (TS) and subsequent annealing for 5 min. (4) Crystallization of the annealed samples by cooling from TS to 30 °C at a rate of 10 °C/min. (5) Subsequent melting of the recrystallized sample by heating from 30 to 210 °C at a rate of 10 °C/min. The TS was varied with steps of 2 °C, and its range of variation was refined for each sample in order to determine the boundaries between the selfnucleation Domains proposed by Fillon et al.2 Lowering the selfnucleation temperature (TS), we first encounter the melt annealing temperature at which the memory effect on crystallization is first detected (domain I/domain II boundary) and then the one at which the unmolten crystals at TS can anneal during the self-nucleation stage (domain II/domain III boundary).2,3 Rheological Measurements. The rheological measurements were performed using an ARG-2 rheometer (TA Instruments) with parallel plates geometry (diameter 12 mm) and under a nitrogen atmosphere. Frequency sweeps were performed in the range of 0.40−628.3 rad/s at different temperatures in the linear viscoelastic regime. The time required for a frequency sweep was 5 min. Each measurement was repeated at least two times observing a good repeatability. The procedure employed in the rheometer was essentially adapted from the self-nucleation DSC procedure and consists of the following steps: (1) Erasure of previous thermal history by holding the sample in the melt at high enough temperatures (typically Tm + 30 °C) for 4 min. (2) Creation of a crystalline “standard state” by cooling the sample at a rate of 10 °C/min to 50 °C. The sample is held at 50 °C during 10 min to allow the material to completely crystallize. (3) Heating of the crystallized sample at 10 °C/min from 50 °C to a selected temperature (TS) and subsequent annealing for 5 min at TS.

Figure 1. DSC cooling (a) and heating (b) curves measured with a scanning rate of 10 °C/min for the analyzed P/E copolymers. The total defect content is indicated on the curves.

ymer is approximately 150 °C, while it is about 50 °C lower for the PE10.1 sample. We notice that the decrease of the experimentally determined melting points is much larger than the drop in the equilibrium melting points that one can estimate (Table 1), indicating that kinetics effects determine crystal stability in this series of propene/ethylene copolymers. An example of the outcome of self-nucleation experiments is provided in Figure 2 for the propene/ethylene copolymers with a total defect content of 10.1 mol %. The DSC curves acquired during cooling from the indicated TS and during the subsequent heating scans are presented in Figures 2a and 2b, respectively. In reference to the crystallization and melting behavior we can classify the self-nucleation temperatures into three different domains, as defined by Fillon et al.2 In Figure 2, the DSC curves are drawn in different colors in order to better identify the three self-nucleation domains:3 red for domain I, blue for domain II, and green for domain III. The memory effect of previous crystalline morphology is totally erased for TS temperatures equal to or larger than 150 °C. Under these conditions, the melt is in domain I or melting domain and the peak crystallization temperature is close to 61 °C, a value equal to that obtained for a TS temperature of 210 °C. Figure 2 shows only two curves in domain I, corresponding to TS = 210 and 150 °C as examples. For temperatures lower than or equal to 140 °C but higher than 108 °C, the sample is in domain II or within the selfnucleation domain. Therefore, the crystallization temperature of PE10.1 increases over 15 °C with respect to domain I crystallization temperature as the nucleation density is greatly enhanced by the self-nuclei produced in domain II.2,3 The appearance of a small melting peak in the heating traces of Figure 2b for TS values equal to 108 °C identifies the boundary of domain III, where the unmolten crystals at the selfnucleation temperature undergo annealing and therefore increase their melting point.2,3 We note that domain II for

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RESULTS AND DISCUSSION Calorimetric Measurements. The effect of ethylene comonomer on the crystallization and melting behavior of iPP can be deduced from Figure 1, which shows the standard DSC cooling and heating runs at a rate of 10 °C/min for the various samples. With increasing the content of total chain defects from slightly above 1 mol %, for the homopolymer, to around 10 mol %, for the P/E random copolymers, the crystallization peak temperature (TC) drops substantially, from about 107 to 61 °C (see Figure 1a). This large shift of crystallization temperature is due to the effect of the defects in reducing the crystallizable sequence length of the copolymer and the corresponding large reduction in crystallization kinetics observed in this series of P/ E copolymers.18−22 The hindrance to the crystallization process brought by ethylene counits randomly placed along the i-PP chains is also manifested in the melting point depression (Figure 1b). The peak melting temperature of the homopolC


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corresponds to i-PP homopolymer and the lowest to the P/E copolymer with 10.1 mol % of defects. As such, a decrease of 10 °C in the self-nucleation temperature produces an increase of roughly 15 and 4 °C in the crystallization temperature of i-PP homopolymer and of PE10.1, respectively. The different response of the crystallization temperature to the variation of TS could be tentatively explained by considering that with increasing the defect content, the overall crystallization and crystal growth rates are greatly reduced;20−22 thus, in order to achieve a meaningful increase in the overall crystallization kinetics (i.e., in the crystallization peak temperature), a much larger number of self-nuclei are required, which can only be obtained by lowering the self-nucleation temperature further and further. It is convenient to plot the crystallization temperature vs selfnucleation temperature values superposed on the “standard” DSC melting curve of the copolymers, in order to understand the location of the different domains with respect to the melting of the original crystals. An example is given in Figure 4, for three different materials: i-PP homopolymer, PE3.4, and PE10.1. Starting from the low temperature, we can notice that

Figure 2. Example of DSC cooling (a) and heating (b) scans after selfnucleation at the indicated temperatures for the sample PE10.1. The color of the curves indicates the self-nucleation domain (see text).

PE10.1 is over 30 °C wide, i.e., extremely large in comparison to the domain II width for Ziegler−Natta i-PP homopolymers reported in the literature, i.e., about 3−4 °C.2,10 This aspect will be discussed in more detail below. Figure 3 summarizes the effect of self-nucleation temperature on the crystallization temperature of the analyzed samples. A

Figure 3. Crystallization temperature as a function of self-nucleation temperature (TS) for the different P/E copolymers.

distinct increase of crystallization temperature with decreasing TS is detected for all propene/ethylene copolymers examined here. TC starts from a plateau value at high self-nucleation temperatures (domain I) and increases to a new plateau level at low TS values in domain III. The absolute value of the two plateaus depends on the total defect content of the copolymer: both values are depressed by the increase in the concentration of defects along the chains. The magnitude of the increase in crystallization temperature between domain I and III is about 15 °C, similar for all the considered propene/ethylene copolymers, although it is slightly higher (i.e., 21 °C) for the i-PP homopolymer. On the other hand, the slope of the crystallization temperature vs self-nucleation temperature curve in domain II varies largely among the samples. The steepest slope in Figure 3

Figure 4. Self-nucleation domains for i-PP homopolymer (a), PE3.4 (b), and PE10.1 (c) superposed to the DSC melting trace. Crystallization temperatures are plotted on the right y-axis as a function of TS (reported on the x-axis). The colors of the melting curve indicate the different self-nucleation domains. Red = domain I, blue = domain II, green = domain III. D


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approximately linearly with the thermal stability of the “standard state” crystals of the copolymers (Tm,end). However, the slope of this linear dependence is much lower than one; therefore, as already grasped from Figure 4, the lower the copolymer crystal’s melting point, the higher is the “overheating” required to erase the memory effect. In other words, the distance between the domain II/domain I boundary transition line and the bisector of Figure 5a increases with decreasing Tm,end. On the other hand, domain III is found to start just few degrees below the end temperature of the melting endotherm in Figure 5. Indeed, the domain III/domain II transition line as a function of Tm,end is basically parallel to the bisector and with a downward shift of about 5 °C on the y-axis. The width of domain II thus increases with decreasing the copolymer melting point or, equivalently, with increasing the total defect content in the chain. It is worth noticing that in the investigated propene/ethylene copolymers the memory effect disappears at temperatures that are always well below the estimated equilibrium melting point (about 30 °C lower). This behavior is different to what Alamo et al.4,5 recently observed in model ethylene/1-butene copolymers, where for counit compositions G′) to the rubbery zone (G′ > G″). Thermorheological complexity noticed in the flow region, at low G* values and high δ values, typically stands for well-defined two phase systems like block copolymers36 or filled polymers, such as polymer nanocomposites.41,42 Along the same line, we have shown that this is also observed for “partially molten” polymers, which contain crystallites.45 Clearly, this situation is analogous to a self-nucleated polymer in domain III. In fact, Figure 10 shows the phase angle/complex modulus curves of the PE10.1 sample for temperatures in between domains II and III. When the temperature falls within the selfnucleation and annealing domain (i.e., domain III), thermorheological complexity is clearly detected in the flow region, i.e.,

Figure 10. MSVP plots for PE10.1 sample at temperatures in domain II and domain III according to DSC measurements. G


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Macromolecules morphological observation of recrystallized samples,13,15 or on rheological measurements as proposed in this work. We underline that the existence of crystalline remnants which act as self-nuclei15 is also possible, in particular for homopolymers self-nucleated just slightly above the nominal melting point. However, for the random copolymers investigated in the present article, we tend to exclude this hypothesis. In fact, the rheological signature of a melt which certainly contains residual crystals (i.e., at temperatures in domain III) is very different from that of a self-seeded melt (Figure 10). Furthermore, in the propene/ethylene copolymers with the highest amount of comonomer, the memory effect exists at temperatures up to 30 °C above the end point of the observed melting endotherm. In our view, the survival of parts of polymer crystals to these high temperatures is difficult to justify, since it would imply an enormous thickening of the lamellae during heating at TSN. In the random copolymers studied here such thickening process seems highly improbable, given that the crystallite thickness is usually “set” by the limited length of the crystallizable sequences. Finally, we note that the two different views, i.e., crystal remnants and residual clustering of chain segments previously in the crystalline state, may not differ largely if the crystal remnants are considered highly defective. The approach based on MSBP plots provides a convenient method to quantitatively assess differences in the degree of thermorehological complexity among the various samples. This can be done by considering the variation of G* (measured at δ = 45°) with the reciprocal of the absolute temperature. It is observed that the natural logarithm of G* is linear with the inverse temperature. The slope of this line is indicated as S and can be considered a parameter related to the degree of thermorheological complexity of the system. An example of the above-described linear relationship is shown in Figure 11 for two of the copolymers, measured in domain II. Obviously, in domain I (not shown) the slope S is equal to 0, being G* at δ = 45° independent from temperature.

Figure 12. S parameter (slope of the lines in the ln G* vs 1/T plot) and degree of crystallinity (measured by DSC) of the investigated sample, as a function of the total concentration of defects.

relation between the level of crystallinity and melt memory in ethylene copolymers.6 It is also in agreement with the hypothesis we put forward, regarding the origin of thermorheological complexity in self-nucleated iPP and copolymer melts: as the degree of crystallinity decreases, the ordered regions will be less abundant and less extra ties will be retained upon melting the sample in domain II. Thus, the thermorheological complexity of less crystalline sample will be less severe, showing lower S values.

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CONCLUSIONS Despite being known for five decades, self-nucleation in semicrystalline polymers still needs to be fully elucidated. We have carried out a parallel calorimetric and rheological investigation of the phenomenon for a series of single-site metallocene propylene−ethylene copolymers with similar molar mass and varying in a wide range of ethylene content. DSC self-nucleation experiments allow the determination of the self-nucleation domains and how they are affected by the composition of the copolymers. The stability of self-nuclei (domain I/domain II boundary) is directly related to the melting temperature of the copolymer crystals. However, the higher the comonomer content, the larger the difference between the end-point of the melting endotherm, and the temperature at which self-nucleation effects disappear. Dynamic viscoelastic measurements were carried out on both self-nucleated and fully isotropic melts. No difference in the relaxation times measured for the two melt states could be observed. On the other hand, self-nucleated melts revealed a thermorheologically complex behavior, defying the application of the time−temperature superposition principle. Melts annealed at temperatures within domain I are instead thermorheologically simple. The singular frequency dependence of thermorheological complexity in domain II suggests that the melt heterogeneities which constitute the self-nuclei are not crystal fragments undetected by conventional techniques, as expected given the large “overheating” at which melt memory is observed (up to 30 °C above the crystal’s melting temperature). Most likely, the rheologically detected heterogeneities in the melt are related to chain clusters in the regions of previous crystalline order, persisting in the melt due to interchain interactions between isotactic sequences that have partitioned during the formation of the standard crystalline state. A novel procedure to evaluate the degree of thermorheological complexity is proposed, and it is demonstrated that more crystalline materials indeed show higher complexity.

Figure 11. Natural logarithm of G* at a phase angle of 45° as a function of reciprocal temperature for PE1.8 and PE8.7 measured in domain II.

From Figure 11 it is seen that the slope S is higher for the polymer with lower ethylene content. This is an observed general trend, as can be noticed in Figure 12, where the S values for the copolymers are plotted against the total defect content. Interestingly, an analogous trend is found when considering the copolymer crystallinity, which decreases by more than a factor 2 with increasing defect content from 1.17 to 10.1 mol % (see Figure 12). Therefore, it can be deduced that the thermorheological complexity (measured by the parameter S) decreases with the decrease in crystallinity, i.e., with the increase in comonomer content. This result is in agreement with the H


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(8) Arandia, I.; Mugica, A.; Zubitur, M.; Arbe, A.; Liu, G.; Wang, D.; Mincheva, R.; Dubois, P.; Müller, A. J. How Composition Dtermines the Properties of Isodimorphic Poly(butylene succinate-ran.butylene azelate) Random Biobased Copolymers: From Single to Double Crystalline Random Copolymers. Macromolecules 2015, 48, 43−57. (9) Hamad, F. G.; Colby, R. H.; Milner, S. T. Lifetime of FlowInduced Precursors in Isotactic Polypropylene. Macromolecules 2015, 48, 7286−7299. (10) Lorenzo, A. T.; Arnal, M. L.; Sanchez, J. J.; Müller, A. J. Effect of annealing time on the self-nucleation behaviour of semicrystalline polymers. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1738−1750. ́ (11) Fernández-dArlas, B.; Balko, J.; Baumann, R. P.; Pöselt, E.; Dabbous, R.; Eling, B.; Thurn-Albrecht, T.; Müller, A. J. Tailoring the Morphology and Melting Points of Segmented Thermoplastic Polyurethanes by Self-Nucleation. Macromolecules 2016, 49, 7952. (12) Luo, C.; Sommer, J. U. Frozen Topology: Entanglements Control Nucleation and Crystallization in Polymers. Phys. Rev. Lett. 2014, 112, 195702. (13) Mamun, A.; Umemoto, S.; Okui, N.; Ishihara, N. Self-Seeding Effect on Primary Nucleation of Isotactic Polystyrene. Macromolecules 2007, 40, 6296−6303. (14) Gao, H.; Vadlamudi, M.; Alamo, R. G.; Hu, W. Monte Carlo Simulations of the Strong Memory Effect of Crystallization above the Equilibrium Melting Point of Random Copolymers. Macromolecules 2013, 46, 6498−6506. (15) Xu, J.; Ma, Y.; Hu, W.; Rehahn, M.; Reiter, G. Cloning polymer single crystals through self-seeding. Nat. Mater. 2009, 8, 348−353. (16) Strobl, G. A thermodynamic multiphase scheme treating polymer crystallization and melting. Eur. Phys. J. E: Soft Matter Biol. Phys. 2005, 18, 295−309. (17) Muthukumar, M. Communication: Theory of melt-memory in polymer crystallization. J. Chem. Phys. 2016, 145, 031105. (18) Alamo, R. G.; VanderHart, D. L.; Nyden, M. R.; Mandelkern, L. Morphological Partitioning of Ethylene Defects in Random PropyleneEthylene Copolymers. Macromolecules 2000, 33, 6094−6105 (b). (19) Nyden, M. R.; VanderHart, D. L.; Alamo, R. G. The Conformational Structures of Defect- Containing Chains in the Crystalline Regions of Isotactic Polypropylenes. Comput. Theor. Polym. Sci. 2001, 11, 175−189. (20) Jeon, K.; Chiari, Y. L.; Alamo, R. G. Maximum Rate of Crystallization and Morphology of Random Propylene Ethylene Copolymers as a Function of Concentration of Comonomer. Macromolecules 2008, 41, 95−108. (21) Alamo, R. G.; Ghosal, A.; Chatterjee, J.; Thompson, K. L. Linear Growth Rates of Random Propylene Ethylene Copolymers. The Changeover from γ Dominated Growth to Mix (α+γ) Polymorphic Growth. Polymer 2005, 46, 8774−8789. (22) Jeon, K.; Palza, H.; Quijada, R.; Alamo, R. G. Effect of Comonomer Type on the Crystallization Kinetics of Random Isotactic Propylene 1-Alkene Copolymers. Polymer 2009, 50, 832−844. (23) Ruiz-Orta, C.; Alamo, R. G. Morphological and Kinetic Comonomer Partitioning in Random Propylene 1-Butene Copolymers. Polymer 2012, 53, 810−822. (24) Hu, W.; Mathot, V.; Alamo, R. G.; Gao, H.; Chen, X. Crystallization of Statistical Copolymers. Adv. Polym. Sci. 2016, 276, 1. (25) Eckstein, A.; Suhm, J.; Friedrich, C.; Maier, R. D.; Sassmannshausen, J.; Bochmann, M.; Mulhaupt, R. Determination of Plateau Moduli and Entanglement Molecular Weights of Isotactic, Syndiotactic, and Atactic Polypropylenes Synthesized with Metallocene Catalysts. Macromolecules 1998, 31, 1335−1340. (26) Fetters, L. J.; Lohse, D. J.; Colby, R. H. Chain Dimension and Entanglement Spacings. In Physical Properties of Polymers Handbook; Mark, J. Ed.; Springer-Verlag: New York, 2007; Chapter 25. (27) Ren, M.; Chen, X.; Sang, Y.; Alamo, R. G. Effect of Heterogeneous Short Chain Branching Distribution on Acceleration or Retardation of the Rate of Crystallization from Melts of Ethylene Copolymers Synthesized with Ziegler-Natta Catalysts. Macromol. Symp. 2015, 356, 131−141.

The approach described in this work offers a direct mean of measuring the peculiar rheological heterogeneity caused by selfnucleation in a polymer melt and its correlation with nucleation ability upon recrystallization. This rheological method can be applied to materials with varying molecular features (e.g., molecular weight, branching content, etc.) and different thermal protocols (e.g., different crystalline standard states, annealing time at TS, etc.) to gain new insights into the detailed nature of self-nuclei.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02392. Figure S1: crystallization temperatures of PE2.8 annealed at 145 °C for different times (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.C.). *E-mail: [email protected] (A.S.). ORCID

Dario Cavallo: 0000-0002-3274-7067 Alejandro J. Müller: 0000-0001-7009-7715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.G.A. acknowledges support by the USA National Science Foundation, Grant DMR 1607786. The UPV/EHU team wacknowledges funding from the following projects: Mineco MAT2014-53437-C2-P and Basque Government IT586-13. L.S. gratefully acknowledges a FPU thesis grant from the Spanish Government.

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REFERENCES

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