High-Pressure Crystallization of iPP Nucleated with 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol

1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) is highly effective in nucleation of the α- form of isotactic polypropylene (iPP). However, its role in high-pressure crystallization of iPP, facilitating the formation of the γ- polymorph, has not been explored. The present paper focuses on the influence of DMDBS on nucleation of high-pressure crystallization of iPP. iPP with 0.2–1.0 wt.% of the DMDBS was crystallized under elevated pressure, up to 300 MPa, in various thermal conditions, and then analyzed by PLM, WAXD, SEM, and DSC. During cooling, crystallization temperatures (Tc) were determined. It was found that under high-pressure DMDBS nucleated crystallization of iPP in the orthorhombic γ- form. As a consequence, Tc and the γ- form content increased for the nucleated iPP, while the size of polycrystalline aggregates decreased, although the effects depended on DMDBS content. The significant increase of Tc and the decrease of grain size under high pressure of 200–300 MPa required higher content of DMDBS than the nucleation of the α-form under lower pressure, possibly due to the effect of pressure on crystallization of DMDBS itself, which is a prerequisite for its nucleating activity.


Introduction
Isotactic polypropylene (iPP) crystallizes in different crystallographic forms. Under atmospheric pressure (P atm ), iPP crystallizes usually in the monoclinic α-form. Crystallization of iPP in the trigonal β-form requires special nucleating agents [1] or special crystallization conditions, for instance zone solidification [2]. Copolymers of propylene with more than 10% of hexene or pentene comonomers, owing to the presence of their longer side chains, crystallize in the trigonal form termed as delta (δ) [3]. Moreover, in stereodefective iPP, the orthorhombic ε form was discovered, nucleated on the α-form lamellae [4]. The mesomorphic form occurs at very large undercooling.
This study focuses on the iPP crystallization in the orthorhombic γ-form. The γ-form lamellae are built of bilayers formed by parallel helices [5,6], whose axes in adjacent bilayers are inclined at approximately 80 • to each other [5][6][7] and tilted at approximately 40 • to the lamellar normal. The γ-phase was observed in iPP of very low molar mass [8][9][10], and also in propylene copolymers with a small content of 1-olefine co-units [11][12][13][14][15][16][17], as well as in metallocene iPP of high molar mass with stereo-or regio-chain irregularities [18][19][20][21][22]. That is explained as an enforcement of crystallization of relatively short isotactic sequences in the extended form by chain defects [20]. The requirement for chain-folding is reduced by the tilt of helix axes with respect to the lamellae normal, thus the factors impeding the chain folding facilitate crystallization of iPP in the γ-modification [21]. The effect of stereoand regio-defects is similar [23] because the effective average length of isotactic sequences that predominantly the γ-lamellae were nucleated through epitaxy on α-lamellae, which formed first on nucleant grains [43].
Our study focuses on high-pressure crystallization of iPP homopolymer nucleated with DMDBS. Samples of iPP nucleated with 0.2-1.0 wt.% of DMDBS were crystallized under various conditions. The effect of the nucleant on nonisothermal crystallization temperature was determined and the structure of crystallized iPP was analyzed by wide angle X-ray diffraction (WAXD), polarized light microscopy (PLM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The results evidenced the efficient nucleation of crystallization of iPP in the γ-form by DMDBS under high P. The ability of DMDBS to nucleate high-pressure crystallization of iPP resulted in an increase of crystallization temperature and a dramatic decrease of sizes of γ-polycrystalline aggregates. However, to increase crystallization temperature and decrease grain size under 200-300 MPa, larger concentration of the nucleant was necessary than that to obtain such effects through the nucleation of the α-phase under P atm .

Materials
iPP 3250 MR1 (PP) provided by Atofina (now Arkema, Colombes, France) with M n of 42 kg/mol, M w of 213 kg/mol, and isotacticity index of 0.97, was used in the study. Melt flow index, determined according to ISO 1133 under 230 • C/2.16 kg, was equal to 25 g/10 min. The same polymer was used in our previous studies [41][42][43]. DMDBS Millad 3988i from Milliken Chemical (Spartanburg, SC, USA) was used to nucleate PP. The chemical structure of DMDBS is shown in Figure S1 in SM (Supplementary Material). PP was mixed with Millad 3988i in a Brabender batch mixer at 195 • C, at 60 rpm for 10 min. Neat PP, without the nucleant, was processed in the same way. The nucleated PP samples with 0.2, 0.4, 0.6 and 1.0 wt.% of Millad 3988i are referred to through this paper as PP/M02, PP/M04, PP/M06, and PP/M1, respectively.

High Pressure Crystallization
High-pressure crystallization was carried out in a custom-built cell, which was described previously [30,40,46]. Specimens in the cell were compressed by an Instron tensile testing machine (Instron Corp., High Wycombe, UK) along the cell axis, at a cross-head speed of 2 mm/min. The hydrostatic P and temperature inside the cell were controlled with an accuracy of ±0.5 MPa and 1 • C, respectively.
To ensure good thermal contacts inside the cell, low P of 1.3 MPa was applied at first. Next, the specimens were heated to 250 • C and held at this temperature for 5 min, then P was increased to 50-300 MPa, and the specimens were cooled under this P to 50 • C (protocol I). Experiments under low P of 1.3 MPa were also carried out for comparison. In addition, selected specimens were cooled from 250 to 200 • C, P was increased to 200 or 300 MPa and the specimens were kept under these conditions for 1 and 4 h, or 15 min, respectively, and next cooled to 50 • C (protocol II). When the temperature of 50 • C was reached, P was released. The crystallized samples were analyzed ex-situ by WAXD, PLM, SEM and DSC.
To monitor the nonisothermal crystallization under high P, the cross-head displacement was recorded during cooling and differentiated with respect to time. In each case, the time dependence of the displacement rate exhibited a peak due to specimen volume change during crystallization. Simultaneous temperature measurement during cooling allowed to determine the temperature, at which the maximum of volume change rate occurred, which was taken as the crystallization temperature (T c ), as described previously [42]. Moreover, temperature measurements during cooling showed that the cooling from 200 • C to 50 • C, where crystallization could be expected, was nearly linear with an average rate of 8 • C/min.

Characterization
To characterize their melting behavior, specimens were heated at 10 and 30 • C/min from room temperature to 250 • C. To confirm the nucleating efficiency of DMBDS, the specimens were nonisothermally crystallized. After 5 min annealing at 250 • C, they were cooled to room temperature at 10 • C/min. All experiments were conducted in DSC 2920 TA Instruments (New Castle, DE, USA).
To determine content of the crystallographic forms and crystallinity degree (X c ), WAXD experiments were conducted using Panalytical Xpert' PRO (Malvern Panalytical Ltd., Malvern, UK) diffraction system operating at 40 kV and 30 mA in the reflection mode, with CuKα radiation. The diffractograms were collected in the 2Θ range of 7-70 • and then deconvoluted using WAXSFIT program [47]; exemplary deconvolution is shown in Figure S2 in SM. The contents of the γ-and α-phases in the crystalline phase, K γ and K α , were calculated according to the equation proposed by Turner-Jones et al. [48]: where I denotes the integral intensity of the respective peak. The crystallinity degree, X c , was also deduced from WAXD, with the amorphous halos also being determined by the deconvolution.
To have an insight into the structure of crystallized materials, 10 µm thick sections were microtomed and examined by PLM using a microscope (PZO, Warsaw, Poland) with a video camera. The exposed surfaces of selected samples were etched according to the procedure developed by Olley et al. [49] and used also by others [30,43]. Etched, washed, and dried specimens were sputtered with gold and studied with SEM JEOL JSM-5500LV (Tokyo, Japan) operating in high vacuum mode at an accelerating voltage of 10 kV. Figure 1 shows DSC cooling thermograms and PLM micrographs of thin sections of neat and nucleated PP crystallized in DSC. Crystallization exotherms of neat and nucleated PP exhibited maxima, at T c of 116 • C and between 128-132 • C, respectively. PLM micrographs of neat PP show well-developed spherulites whereas in those of nucleated PP polycrystalline aggregates are poorly discernible because of their small size. The increase of T c and strong decrease of grain size confirmed nucleating activity of DMBDS. It is worth noting that T c of PP/M02, at 128 • C, was lower and grain size was larger than those of PP with higher DMDBS content.     Figure 2 shows T c measured for neat and nucleated PP during nonisothermal crystallization (protocol I) under various P. It was not possible to determine precisely cross-head displacement under 1.3 MPa, hence T c values determined from DSC cooling thermograms are shown instead. It appears that T c of all materials increased with increasing P, as can be expected, and as was observed by us previously [42], due to the increase of T m 0 . For example, according to Mezghani and Phillips [29], and Angelloz et al. [50], under 200 MPa   Figure 2 shows Tc measured for neat and nucleated PP during nonisothermal c tallization (protocol I) under various P. It was not possible to determine precisely cr head displacement under 1.3 MPa, hence Tc values determined from DSC cooling ther grams are shown instead. It appears that Tc of all materials increased with increasin as can be expected, and as was observed by us previously [42], due to the increase of For example, according to Mezghani and Phillips [29], and Angelloz et al. [50], under MPa Tm 0 of the γ-form is at 241 °C and at 232 °C, respectively, whereas linear extrapola of their results allows to predict the rise of Tm 0 by 27 and 19 °C, respectively, with P creasing to 300 MPa   X-ray diffractograms of neat and nucleated PP crystallized under various P are collected in Figure 3. The diffractograms of PP/M06 are not shown because they were very similar to those of PP/M04 and PP/M1. It is seen that under 1.3 MPa, only the α-phase crystallized in neat PP, whereas in nucleated PP the predominant α-phase was accompanied by small fractions of the γ-phase, as reflected in weak (117) γ peak. Under higher P either αand γ-phases or pure γ-phase crystallized. The increase of P resulted in an increased γ-content in all materials, evidenced by the buildup of (117) γ peak, accompanied by the decrease of (130) α peak typical of the α-form.  nied by small fractions of the γ-phase, as reflected in weak (117)γ peak. Under higher P either α-and γ-phases or pure γ-phase crystallized. The increase of P resulted in an increased γ-content in all materials, evidenced by the buildup of (117)γ peak, accompanied by the decrease of (130)α peak typical of the α-form.     In all the materials, the γ-content enlarged with increasing P because of broadening of the temperature range of the γ-phase formation. PP nucleated with 0.4-1.0 wt.% of DMDBS exhibited the highest γ-contents. Under 200 and 300 MPa these materials crystallized in the pure γ-form. The same applies for PP/M02 crystallized under 300 MPa, whereas under 200 MPa a trace of the α-phase also formed in this material. In neat PP, the γ-content was smaller, at 200 MPa K γ was approximately 0.9, and even at 300 MPa it remained below 1.0. Obviously, DMDBS enhanced crystallization of PP in the γ-form, although stronger at 0.4-1.0 wt.% content. It is worth noting that there were no large differences in X c values, although crystallinity of nucleated PP increased slightly with increasing P.

Crystallization and Structure
PLM micrographs of thin sections of neat and nucleated PP crystallized during cooling under elevated P are shown in Figure 5. Polycrystalline aggregates are well visible in micrographs of neat PP crystallized in the entire P range, being slightly smaller at 300 MPa. According to the WAXD results the pure α-, αand γ-, or nearly pure γformed in neat PP, depending on crystallization P. In PP/M04, the aggregates are not discernible because of their small sizes, regardless of the crystallization P and crystallographic form, which changed from predominant α-form under P atm to predominant or pure γ-form under 100-300 MPa. The details of the morphology are better seen in SEM micrograph in Figure 6 showing PP/M04 permanganate etched after crystallization under 300 MPa. In the micrograph, small γ-polycrystalline aggregates are discernible, with fans composed of lamellae protruding from nucleation sites. PLM micrographs of PP nucleated with 0.6 and 1.0 wt.% of DMDBS (not shown) were similar to those of PP/M04. The PLM and SEM micrographs evidence that 0.4-1.0 wt.% of DMDBS efficiently nucleated crystallization in PP in the entire P range studied, either in the αor the γ-form, depending on crystallization P.  According to [38], in iPP with 0.2-1.0 wt.% of DMDBS upon cooling under Patm the nucleant crystallizes prior to the polymer, and provides active nucleation sites for iPP crystals. This is also evidenced by the DSC and PLM results shown in Figure 1. We hypothesize that at 0.2 wt.% concentration, during cooling under P above 50 MP, the temperature of DMDBS crystallization decreased below that of PP. As a result the intense nucleation of PP leading to the formation of its small grains occurred when large PP spherulites, nucleated as in neat PP, were already growing. However, only at 100 MPa the γcontent was significantly smaller than in PP nucleated with larger amounts of DMDBS. Thus, although at 0.2 wt.% content the temperature of nucleating activity of DMDBS under 200-300 MPa was lower than the temperature of usual heterogeneous nucleation in PP studied, it was sufficiently high to initiate crystallization in remaining melt in the γdomain, resulting in the pure or almost pure γ-form.
The additional crystallization experiments, conducted according to protocol II, confirmed the above described observations. Figure S3 in SM compares X-ray diffractograms whereas Figure 7 shows  In PP/M02 crystallized under 50 MPa polycrystalline aggregates are not discernible on PLM micrograph in Figure 5, due to very fine-grain structure. In this sample, the crystalline phase was composed of nearly equal parts of the αand γ-phases, similarly to PP with higher concentrations of DMDBS and crystallized under the same P. Moreover, T c of PP/M02 under 50 MPa was also similar to those of the other nucleated PP samples. Obviously, the nucleant was efficient under 50 MPa at 0.2 wt.% concentration. PLM micrograph of PP/M02 crystallized under 100 MPa shows few larger spherulites embedded in the fine-grain structure, and at 200 MPa the larger aggregates are even more numerous. At 300 MPa the structure is composed of well seen spherulites, with fine grains between them. More details of the structure formed under 300 MPa is shown in Figure 6 in SEM micrograph of PP/M02 etched after the crystallization. Although the presence of the fine grains evidenced nucleating activity of DMDBS, the large spherulites were obviously nucleated first, at higher temperature than the fine grains. Most possibly they were nucleated on the same nucleating heterogeneities and at the same temperature as spherulites in neat PP, without the nucleating agent. Taking into consideration the structure and also T c values of PP/M02, only slightly higher, by 2-4 • C, than those of neat PP under 100-300 MPa, one can conclude that in this P range DMDBS nucleated the crystallization of PP in PP/M02 at the temperature lower than the temperature, at which heterogeneous nucleation occurred in neat PP. Moreover, the delay increased with increasing P, as can be judged from the increasing number and size of the large spherulites seen in PP/M02 in Figure 5.
According to [38], in iPP with 0.2-1.0 wt.% of DMDBS upon cooling under P atm the nucleant crystallizes prior to the polymer, and provides active nucleation sites for iPP crystals. This is also evidenced by the DSC and PLM results shown in Figure 1. We hypothesize that at 0.2 wt.% concentration, during cooling under P above 50 MP, the temperature of DMDBS crystallization decreased below that of PP. As a result the intense nucleation of PP leading to the formation of its small grains occurred when large PP spherulites, nucleated as in neat PP, were already growing. However, only at 100 MPa the γ-content was significantly smaller than in PP nucleated with larger amounts of DMDBS. Thus, although at 0.2 wt.% content the temperature of nucleating activity of DMDBS under 200-300 MPa was lower than the temperature of usual heterogeneous nucleation in PP studied, it was sufficiently high to initiate crystallization in remaining melt in the γ-domain, resulting in the pure or almost pure γ-form.
The additional crystallization experiments, conducted according to protocol II, confirmed the above described observations. Figure S3 in SM compares X-ray diffractograms whereas Figure 7 shows K γ and K α , and also PLM micrographs of thin sections of neat PP and PP/M02 crystallized according to protocol II are shown in Figure 8. In neat PP, rather large polycrystalline aggregates, which obviously started to grow at 200 °C, are accompanied by smaller ones, which grew during cooling, as described previously [41]. Obviously, the crystallization was not completed at 200 °C and continued during cooling. In turn, in PP/M02, larger polycrystalline aggregates, although smaller than in neat PP, are embedded in fine grain structure. This evidences that during cooling very intense nucleation occurred on DMDBS, and the crystallization was quickly completed by the growth of numerous and small DMDBS nucleated grains. Moreover, large spherulites differed in sizes, especially in samples kept for 4 h under 200 MPa and for 15 min under 300 MPa, which indicates that some of them could be nucleated during cooling before the occurrence of nucleation on DMDBS. These observations corroborate the conclusion on the decrease of temperature of DMDBS nucleating activity in PP/M02 under high P. PLM micrographs of thin sections of neat PP and PP/M02 crystallized according to protocol II are shown in Figure 8. In neat PP, rather large polycrystalline aggregates, which obviously started to grow at 200 • C, are accompanied by smaller ones, which grew during cooling, as described previously [41]. Obviously, the crystallization was not completed at 200 • C and continued during cooling. In turn, in PP/M02, larger polycrystalline aggregates, although smaller than in neat PP, are embedded in fine grain structure. This evidences that during cooling very intense nucleation occurred on DMDBS, and the crystallization was quickly completed by the growth of numerous and small DMDBS nucleated grains. Moreover, large spherulites differed in sizes, especially in samples kept for 4 h under 200 MPa and for 15 min under 300 MPa, which indicates that some of them could be nucleated during cooling before the occurrence of nucleation on DMDBS. These observations corroborate the conclusion on the decrease of temperature of DMDBS nucleating activity in PP/M02 under high P. evidences that during cooling very intense nucleation occurred on DMDBS, and the crystallization was quickly completed by the growth of numerous and small DMDBS nucleated grains. Moreover, large spherulites differed in sizes, especially in samples kept for 4 h under 200 MPa and for 15 min under 300 MPa, which indicates that some of them could be nucleated during cooling before the occurrence of nucleation on DMDBS. These observations corroborate the conclusion on the decrease of temperature of DMDBS nucleating activity in PP/M02 under high P.

Melting
DSC heating thermograms of nonisothermally crystallized neat and DMDBS nucleated PP are collected in Figure 9. The thermograms show the evolution from the melting

Melting
DSC heating thermograms of nonisothermally crystallized neat and DMDBS nucleated PP are collected in Figure 9. The thermograms show the evolution from the melting peaks of the α-phase to those of the γ-phase. The melting peak temperatures (T m ) of the pure or predominant α-phase formed under 1.3 MPa were located at 164-166 • C and 164-167 • C during heating at 10 and 30 • C/min, respectively.   T m values of neat PP crystallized under elevated P during heating at 10 • C/min were similar, 162-164 • C, although, with increasing P the melting peaks broadened. During heating at 30 • C/min T m of PP, which was crystallized under 200 and 300 MPa, decreased to 160 and 159 • C, respectively, indicating that the reorganization of the crystalline phase, predominantly in the γ-form, occurred during slower heating, as already reported by us in [42].
PP/M04 and PP/M1 crystallized under 50 MPa, during heating at 10 • C/min exhibited double-melting behavior, with T m values near 158 and 165 • C. The double-melting behavior usually results from melting of different crystallographic forms or population of crystals differing in thickness or perfection, or from recrystallization phenomena occurring during heating. The melting temperature of the γ-phase is lower than that of the α-phase, although it can increase, for instance, due to isothermal thickening of lamellae at crystallization temperature [29]. In addition, according to the discussion of Mezghani and Phillips [29] during heating at 10 • C/min the melting of the γ-form rather occurs than its transformation to the α-form. At 100 MPa, the low-temperature melting peaks of PP/M04 and PP/M1 developed into the main peaks and their T m values decreased to 156 • C, whereas the high-temperature peaks diminished. At 200 and 300 MPa, the hightemperature peaks vanished and only low-temperature ones remained with T m shifted to 153-154 • C. These changes correspond to the increase of the γ-content in these materials with increasing crystallization P. Thus, the low-and high-temperature peaks observed at 50 and 100 MPa can be attributed to the melting of the γand α-form, respectively, although the reorganization in the crystalline phase cannot be entirely excluded because during faster heating only single broad melting peaks appeared, with T m values at 162-164 • C. It is worth mentioning that at 10 • C/min only single melting peaks were observed for neat PP crystallized under 50 and 100 MPa. However, in PP the γ-content was smaller and the γ-phase crystallized at higher undercooling that in PP/M1 and PP/M04, and its reorganization during heating could affect its melting behavior.
During heating of PP/M04 and PP/M1, which were crystallized under 200 and 300 MPa, only single peaks were observed, with T m values at 153-156 • C and 156-160 • C at 10 and 30 • C/min, respectively. Unlike in neat PP, the T m values did not increase with decreasing heating rate, most possible due to better stability of the γ-form, which crystallized at lower undercooling.
The melting behavior of PP/M02 crystalized under 50 MPa was similar to that of PP/M04 and PP/M1, although the high-temperature peak during heating at 10 • C/min was slightly higher, possibly due to a somewhat larger α-content. However, at 100 MPa the low-temperature peak did not develop into the main melting peak, and its T m decreased to 153 • C, most possibly because of the smaller content of the γ-phase, its lower T c , and reorganization phenomena in the crystalline phase. At 30 • C/min only broad hightemperature peaks remained, with T m of 162 • C, however, with a small low-temperature shoulder. In turn, the thermograms of PP/M02 crystallized under 200 and 300 MPa were similar to those of neat PP, in agreement with similar T c values of these materials during cooling.
DSC heating thermograms of PP and PP/M02 crystallized under 200 and 300 MPa according to protocol II, plotted in Figure 10, show broad melting peaks of predominant or pure γ-phase, with T m of 155-165 • C and 157-163 • C at 10 and 30 • C/min, respectively. The melting behavior reflects crystallization conditions of these materials. According to PLM results, in all these materials the crystallization started at 200 • C and was accomplished during cooling. Moreover, in PP/M02 during cooling strong nucleation on DMDBS occurred. Some thermograms recorded at 10 • C/min, shown in Figure 10

Conclusions
The influence of DMDBS on nucleation of high-pressure crystallization of iPP was studied. PP with 0.2-1.0 wt.% of the DMDBS was crystallized under elevated P, up to 300 MPa, in various thermal conditions. The obtained results evidence the ability of DMDBS to nucleate crystallization of PP under high P in the γ-form. In the entire P range studied, after nonisothermal crystallization (protocol I) the γ-content in the nucleated PP exceeded that in the neat PP, although it was smaller in PP/M02 than in the other nucleated materials. In the entire P range at 0.4-1.0 wt.% nucleant content Tc was significantly higher and a fine-grain structure formed. In PP/M02, the marked increase of Tc and the fine-grain structure was observed at 50 MPa. At 100 MPa Tc increased only slightly and a few relatively large spherulites were observed in this material embedded in the fine-grain structure. With increasing P the number of large spherulites increased and they became predominant at 300 MPa, although still surrounded by fine grains. Moreover, under 200 and 300 MPa Tc of PP/M02 was nearly the same as that of neat PP. Hence, it was concluded that at 0.4-1.0 wt.% concentration of DMDBS, the nucleation on it occurred at elevated temperature, whereas at 0.2 wt.% concentration DMDBS nucleated PP crystals at temperature lower than that of usual heterogeneous nucleation in this polymer. Nevertheless, the nucleation on DMDBS occurred in PP/M02 at sufficiently high temperature to promote the crystallization in the γ-domain. Hence, the γ-content increased in that P range, in which crystallization in the neat PP was not accomplished in γ-domain and continued in the α-form. The decrease of temperature of DMDBS activity at its 0.2 wt.% concentration under high P was corroborated by the results of additional studies, on crystallization of PP and PP/M02 under 200 or 300 MPa, which began in isothermal conditions at 200 °C,

Conclusions
The influence of DMDBS on nucleation of high-pressure crystallization of iPP was studied. PP with 0.2-1.0 wt.% of the DMDBS was crystallized under elevated P, up to 300 MPa, in various thermal conditions. The obtained results evidence the ability of DMDBS to nucleate crystallization of PP under high P in the γ-form. In the entire P range studied, after nonisothermal crystallization (protocol I) the γ-content in the nucleated PP exceeded that in the neat PP, although it was smaller in PP/M02 than in the other nucleated materials. In the entire P range at 0.4-1.0 wt.% nucleant content T c was significantly higher and a finegrain structure formed. In PP/M02, the marked increase of T c and the fine-grain structure was observed at 50 MPa. At 100 MPa T c increased only slightly and a few relatively large spherulites were observed in this material embedded in the fine-grain structure. With increasing P the number of large spherulites increased and they became predominant at 300 MPa, although still surrounded by fine grains. Moreover, under 200 and 300 MPa T c of PP/M02 was nearly the same as that of neat PP. Hence, it was concluded that at 0.4-1.0 wt.% concentration of DMDBS, the nucleation on it occurred at elevated temperature, whereas at 0.2 wt.% concentration DMDBS nucleated PP crystals at temperature lower than that of usual heterogeneous nucleation in this polymer. Nevertheless, the nucleation on DMDBS occurred in PP/M02 at sufficiently high temperature to promote the crystallization in the γ-domain. Hence, the γ-content increased in that P range, in which crystallization in the neat PP was not accomplished in γ-domain and continued in the α-form. The decrease of temperature of DMDBS activity at its 0.2 wt.% concentration under high P was corroborated by the results of additional studies, on crystallization of PP and PP/M02 under 200 or 300 MPa, which began in isothermal conditions at 200 • C, and was completed during cooling. In PP/M02, strong nucleation occurred on DMDBS during cooling. As a result, large spherulites nucleated at 200 • C were surrounded by fine grains nucleated on DMDBS.
The melting behavior of nonisothermally crystallized PP/M04 and PP/M1 was similar, and differed from that of PP. It reflected their high γ-content increasing with increasing crystallization P. Unlike for PP, no decrease of T m with increasing heating rate was observed, evidencing better stability of the γ-phase due to its crystallization at lower undercooling. The melting of PP/M02 crystallized under 50 and 100 MPa bore some similarities to that of PP/M04 and PP/M1. In turn, the melting behavior of PP/M02 crystallized under 200 and 300 MPa resembled that of neat PP, in agreement with nearly the same T c values of these materials. DSC heating thermograms of PP and PP/M02, which were crystallized under 200 and 300 MPa initially at 200 • C and then during cooling, reflected their complex crystallization conditions. Shoulders were observed on some thermograms recorded at 10 • C/min, which were most possibly related to the fraction of crystals formed during cooling and to reorganization phenomena in the crystalline phase, as the high-temperature parts of the peaks decreased and the shoulders disappeared during faster heating.
The obtained results show that DMDBS efficiently nucleates crystallization of PP in the γ-form under high P. When added in the appropriate amount, DMDBS increases T c and the γ-content, and simultaneously reduces the grain size in the course of nonisothermal crystallization during cooling under high P. This indicates the possibility to use it to increase efficiently the γ-content or even to obtain the pure γ-PP during industrial processing like injection molding.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.