CC BY
45
УДК 678.073
E. E. Mastalygina, N. N. Kolesnikova, S. G. Karpova, A. A. Popov, V. F. Shkodich, A. M. Kochnev
DSC AND EPR-SPECTROSCOPY INVESTIGATION OF POLYPROPYLENE / LOW-DENSITY
POLYETHYLENE BLENDS
Keywords: Polypropylene; Polyethylene; Blends; Structure; Differential scanning calorimetry; Electron paramagnetic resonance
spectroscopy.
Thermal and morphological study of blends based on isotactic polypropylene (iPP) and low-density polyethylene (LDPE) in a wide range of compositions were investigated by differential scanning calorimetry and electron paramagnetic resonance spectroscopy (paramagnetic probe method). The partial compatibility in the amorphous regions of iPP and LDPE providing the interface layer formation was observed for the blends containing 30-95 wt% of iPP. There was a plasticizing effect of LDPE on iPP, increasing the segmental mobility of the macromolecules chains in its amorphous phase. If the content of iPP in the blend was less than 30 wt%, the non-equilibrium molecular structure of the iPP/LDPE composition with a more rigid interface layer was formed.
Ключевые слова: Полипропилен; Полиэтилен; Смеси; Структура; Дифференциальная сканирующая калориметрия; Спектроскопия электронного парамагнитного резонанса.
В работе были исследованы теплофизические параметры морфология смесей на основе изотактического полипропилена (иПП) и полиэтилена низкой плотности (ПЭНП) в широком диапазоне составов методами дифференциальной сканирующей калориметрии и электронного парамагнитного резонанса (метод парамагнитного зонда). Была обнаружена частичная совместимость в аморфных областях иПП и ПЭНП с формированием межфазной области для смесей, содержащих 30-95 мас.% иПП. Был показан пластифицирующий эффект ПЭНП на иПП, который обеспечивал увеличение сегментарной подвижности цепей макромолекул в аморфной фазе. При содержании иПП в смеси менее 30 мас.% формировалась неравновесная молекулярная структура с образованием более жесткой межфазной области.
Introduction
Polyolefins are the most important and most widely used synthetic polymers; their annual production exceeds 130 million metric tons [1]. An enormous scale of industrial production and a wide variety of application of polyolefins, particularly polyethylene and polypropylene, cause the importance of developing new materials based on polyolefins. The combination of polyolefins by blending allows varying their properties and producing materials having the appropriated characteristics, avoiding an expensive stage of synthesis [2].
The range of polyolefins can be significantly extended by compounding polyethylene (PE) and polypropylene (PP) for a variety of ratios. The addition of small amounts of PE to PP increases its frost resistance [3], impact strength [4] and oxidation resistance [5]. The addition of PP to PE, in its turn, allows increasing mechanical strength and rigidity of the composition [6] [7], as well as resistance to high temperatures [8].
Polyethylene and polypropylene are commonly used in industries such as agriculture, construction, and domestic uses [9, 10], so both polymers could bring possible environmental problem after their usages. They are very stable and hence remain inert to degradation and deterioration leading to their accumulation in the environment.
The recycling of plastic wastes could partly solve environmental problems. Plastic recovery starts with the separating of the different types of plastics [11]. The structural similarity and close density values of PP and PE cause the technical difficulties and economic irrationality of separating these polymers in the recycling process [12]. So the resulting product of plastics separation contains both PE and PP and can be used only for
low quality recycled materials [13]. The PP/PE blend developing expands the opportunities for improving properties of recycled polyolefin mix. Thus, the recycling is a stimulating factor for creation of PP/PE blends.
The second direction in solving the environmental problems of polyolefin wastes is the production of composite materials with rapid degradation in natural conditions [14-16]. It is considered that biodegradation is more suitable route for waste disposal rather than recycling [17]. There are works devoted to creation of the biodegradable compositions based on PP/PE blends with natural biodegradable fillers such as wood flour [18], flax fibers [19], rape straw [20] and others. So, creating of biodegradable composites based on polypropylene/polyethylene blend matrix is also a currently important research trend.
Creation of materials with definite properties based on PP/PE blends, including biocomposites with natural filler, requires studying morphology PP/PE blend matrix. Morphology of polymer blends depends on many factors, including the molecular structure, the melt flow index, the ratio of components in the blend, and the presence of additives improving compatibility [21]. Generally, the creation of compositions of these polymers implies the formation of the dispersed system, in which a smaller phase is dispersed in the matrix of a bigger phase. The size of dispersed domains of the smaller phase increases with addition of the dispersed polymer, as well as with deterioration of blending, more continuous annealing or slower cooling of the material [21]. Phase inversion occurs in a wide range of polymer concentrations, approximately 30-70 wt%, due to the high viscosity of PP/PE blends and a practical impossibility of achieving the phase equilibrium [22]. Both polymer phases are continuous in this concentration range.
A large number of studies were devoted to the research of PP/PE blends. Nevertheless, there are still discussions on the formation of the structure and compatibility of these polymers in the blend. First of all, it is connected with the fact that the morphology of the blend strongly depends on the type of PP and PE which are compounded. According to market studies, three types of PE that conquer the plastic market in sold volume are high density polyethylene (HDPE) and low density polyethylene (LDPE) [9]. Among polypropylenes the isotac-tic polypropylene (iPP) is the most widespread type in domestic uses. But most studies available in literature are devoted to the research of the blends of isotactic polypropylene (iPP) with high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE).
Some researchers believe that, despite the incompatibility of PP and PE, both of these polymers can influence the processes of crystallization and the structure formation of the other component. The study [23] has shown the decrease in crystallinity and heat of fusion upon the incorporation of PP into HDPE is due to the fact that the formation of crystallites in the blend was affected by the presence of PP. Some researchers have proven not only the mutual influence of PP and PE on the structure and properties of the blend, but also the partial compatibility of these polymers at particular ratios of the components. Galeski A., Bartczak Z., Martuscelli E., Pracella M. reported in their study [24] about the partial compatibility of iPP and HDPE in the melt for the blends with the content of iPP less than 10 wt% and more than 60 wt%. Moreover, they argued that there was a mutual influence of iPP and HDPE on the composition structure at these ratios of the components. Investigating the nucleation process in iPP/HDPE blends the authors determined that the heterogeneous nucleation was characteristic for the blend compositions with a low content of iPP. Jose S. et al. [7] stated that the crystallization temperatures of polymers do not depend on the composition of the blend, while the degree of crystallinity of both polymers decreases with the increase of the amount of the other component, which also indicates the mutual influence of the blend components on each other.
In the studies, devoted to the blends of LLDPE and iPP, the researchers also pointed to the partial compatibility of LLDPE and iPP. It was attributed to the structural similarity between the blend components, particularly the low degree of branching. Jun Li and Shanks R. [25] found the reduction in the growth rate of spherulites during the process of iPP crystallization for the blend of iPP/LLDPE = 20/80. This result was explained by the decrease in the supercooling temperature of iPP indicating the partial solubility of iPP in LLDPE.
Low-density polyethylene (LDPE) is used as widely as HDPE and LLDPE; however, iPP/LDPE blend compositions have given less attention by researchers. In addition, most studies investigate only three or four iPP/LDPE blends, and the results of some studies differ with each other, which makes it difficult to obtain complete information. For example, Ujhelyiova A. et al. [26] reported the partial iPP and LDPE compatibility at the temperature above the melting point of the components and at low LDPE content (less 5-10 wt%). Moreover, the authors of the study [26] judged about it by reduction of
the total melting enthalpy of the blend. Dong L. et al. [27] showed the different range of compositions characterized by iPP solubility in LDPE - less 15 wt% of iPP in the blend. Besides, the structural changes in the iPP/LDPE blends at low content of iPP were explained not only by iPP solubility in LDPE, but also by the fact that the concentration of iPP was too small for its proper crystallization. Murin J. et al. [28] and Gorrasi G. et al. [29] by contrast, argued in favor of the complete incompatibility of iPP and LDPE; however, they pointed to the possibility of the mutual influence of these polymers on the composition properties.
To date, PP/PE blends are already widely used in industry for manufacturing ropes, nets, packaging materials, as well as engineering plastics [8, 30, 31] Determination of the effects of the iPP/LDPE blend composition on the structure and properties will allow creating composite materials with required characteristics. This makes it possible to predict the behaviour of these materials during the operation and to define possible directions of their use, which, in turn, will expand fields of application of material based on polyolefin blends.
Experimental
Materials
The blends under investigation consisted of iso-tactic polypropylene (iPP, TM 01030 Caplen from Gazpromneft-MNPZ, OJSC, Russia) and low-density polyethylene (LDPE, TM 15803-020 from Neftekhimsevilen, OJSC, Russia) with melt flow index of 1.2 ± 0.1 g/10min and 1.65 ± 0.1 g/10min (at 190 °C and 2.16 kg load), respectively, were used in this study. Close values of melt flow indexes provided the possibility of qualitative compounding of the polymers with the formation of the homogeneous mixture [2]. The average molecular weight and molecular-weight distribution for iPP were Mw = 2.1 x 105, Mn = 4.6 x 104, Mw/Mn = 4.6 and for LDPE were Mw = 1.0 x 105, Mn = 1.5 x 104, Mw/Mn = 6.7 (1,2,4-trichlorobenzene, 140 °C, Waters 150C GPC). The iPP content in the blends was 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 100 wt%. A wide range of compositions allowed to determine accurately the changes in the properties in the case of varying the blend composition.
Processing
All the blends used for this study were prepared by the Plasticorder PLD-651 (Brabender, GmbH & Co, Germany) in an argon atmosphere (State Standard GOST 10157-79) at a temperature of 190 °C and a rotor rotational speed of 30 rev min-1. A weighted amount of iPP was placed in the mixing chamber, 2 minutes later LDPE sample weight was added. Mixing of polymers was performed for 5 min. The inert argon atmosphere was used to reduce the oxidation of polymers. After cooling, the obtained material was ground in the knife mill RM-120 (Vibrotechnik, LLC, Russia). The isotropic films were obtained by pressing the ground material in the hydraulic hand press PRG-10 (VNIR, LLC, Russia) at a temperature of 190 °C and a pressure of 7.8 MPa (80 kgf cm-2) on a cellophane substrate, followed by quenching in water at 20 °C. The thickness of the films was 130 ± 10 ^m.
Methods
Differential scanning calorimetry
The behaviour of polymers during melting and crystallization was investigated by differential scanning calorimetry (DSC) using a differential scanning microcalorimeter DSM-10M (Institute for Biological Instrumentation, Russia). The scanning speed was 8 °C min-1, sample weight was 10 ± 0.1 mg. The temperature scale was scaled by indium (melting temperature Tm = 156.6 °C, specific heat of fusion AH = 28.44 J g-1).
The temperatures of melting and crystallization, Tm and Tcr, were determined by the endothermic maximum of the melting peak and the exothermic maximum of the crystallization peak on DSC thermograms, respectively. To obtain cooling curves, the samples of iPP/LDPE blends were heated up to 200 °C, stored at this temperature for 5 minutes, then cooled at a speed of 8 °C min-1 to 40 °C. The enthalpy of fusion of the samples (AH,) was calculated from the area of the melting peak limited by the baseline. To calculate the degree of crystallinity /(PP) and /(PE). the Equation (1) was used
[32]: x= —X 100 (1), where AHj is the specific heat
of melting calculated per the content of polymer i (iPP or LDPE) in the blend; AHo(PP) = 147 J g-1 - specific heat of melting of a completely crystalline PP [33], AHo(PE) = 295 J g-1 - specific heat of melting of a completely crystalline PE [34]. Each value of the parameters AHi, Tm, Tc was obtained by averaging of five measurements.
Hydrostatic weighing method
The density of the samples was measured by the method of hydrostatic weighing using an analytical balance KERN ALT 220-4M (KERN & SOHN GmbH, Germany). The test temperature was 25 °C. The sample density of the iPP/LDPE blends was determined based on experimental weighting data by the Equation (3): = where A™ is a density of the
sample; pf = 0.8070 g cm-3 is a density of the working fluid (ethyl alcohol of 95 wt%); ma and mf are sample masses in the air and in the working fluid. The density of the amorphous regions of iPP and LDPE was calculated on the basis of the reference values of the density of completely crystalline PP and PE - 0.936 g cm-3 and 0.999 g cm-3 [35], relatively.
Electron paramagnetic resonance spectroscopy
The structure of amorphous regions of the samples was investigated using electron paramagnetic resonance spectroscopy (paramagnetic probe method) [36] by the ESP spectrometer (Institute of Chemical Physics, Russia). The stable nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-1) was used as a paramagnetic probe. The radical was introduced into the films from its vapor at 30 °C. The molecular mobility in the initial polymers, blend compositions and in the interface layer, as well as the radical concentration in these materials were defined. The rotational mobility of the probe was determined by the correlation time tc. To calculate the values of tc the Equation (2) was used [37]:
= l r -j , ^ I ::, where I+ and L
are intensities of the first and the third peaks in the EPR spectrum; AH+ is a half-width of the EPR spectrum component located in the weak field.
The amount of the radical absorbed by the amorphous phase of the sample was evaluated by the area of the EPR spectrum normalized to the fraction of the amorphous phase in this sample [36]. The proportion of the amorphous phase was calculated proceeding from the degree of crystallinity calculated based on the DSC data.
Results and Discussion
According to the data available in literature, PP and PE in the blend are crystallized separately and form crystalline lattices typical of these polymers [3]. Moreover, both polymers can have a mutual effect on the process of crystallization and the morphology of the blend.
The processes of melting and crystallization of the iPP/LDPE blend samples of different compositions were investigated by the DSC method. DSC cooling curves of the samples are shown in Figure 1. The two endothermic peaks in the temperature range corresponding to the melting of the individual polymers are observed for all the thermograms at the heating of iPP/LDPE blend samples.
Fig. 1 - DSC cooling curves of LDPE (a), iPP (i) and iPP/LDPE blends with the iPP content of 5 (b), 10 (c), 20 (d), 30 (e), 40 (f), 60 (g), 80 (h) wt%
The melting temperatures Tm of iPP and LDPE in the blends and their degrees of crystallinity were determined from the obtained experimental data. The results are shown in Table 1. The melting temperatures of iPP and LDPE do not significantly depend on the composition of the blend. The decrease in Tm of the minor component by 1-2 °C in relation to the homopolymer is observed at the low content of iPP or LDPE in the blend, which indicates a reduction in the perfection of the formed crystallites.
Table 1 - Melting temperatures (Tm), crystallization temperatures (Tcr) and degree of crystallinity (X of LDPE and iPP in the blends of different compositions
PP, Melting Crystallization Degree of
wt temperature,°C temperature, °C crystallinity, %
% (±0.3°С) (±0.3°C) (±2%)
iPP LDPE iPP LDPE iPP LDPE
0 - 106.6 - 94.0 - 23.8
5 161.0 106.6 91.4 56.5 23.7
10 162.4 107.0 91.4 55.9 23.1
20 162.8 106.8 91.5 59.9 21.5
30 163.2 107.0 91.5 58.2 22.8
40 162.9 106.9 111.3 91.8 54.0 22.2
50 162.8 106.7 113.0 91.6 44.0 18.6
60 163.3 106.9 113.9 91.5 51.3 21.6
70 163.0 106.5 113.8 91.9 53.6 21.5
80 163.2 106.0 113.9 92.5 52.6 21.1
90 163,1 105.0 113.3 92.5 53.6 24.7
95 163.5 - 113.5 92.6 53.6 24.5
100 163.1 - 114.3 - 63.3 -
The analysis of the DSC curves and the dependence of crystalline phase on the cooling temperature of the iPP/LDPE blend samples allow to observe the abnormalities in the process of crystallization for the blends containing 5-30 wt% of iPP. There are two crystallization peaks corresponding to the temperature areas of crystallization of individual polymers on the cooling thermograms for the blends when the iPP content is equal to 40 wt% or more, while only one crystallization peak is observed in the thermograms for the blends with the PP content of 5-30 wt%. Upon the second heating of the studied samples with the iPP content of 5, 10, 20 and 30 wt%, two peaks corresponding to the melting peaks for homopolymers LDPE and iPP are observed on the melting thermograms. The crystallization abnormalities of iPP described above are also at other cooling rates of the samples, namely, 2, 4 and 16 °C min-1. In other words, despite the absence of the peak in the temperature range of crystallization of the individual iPP, both polymers in this mixture form crystalline phases.
It should be noted, that the crystallization peak of iPP shifts to lower temperatures area gradually, starting from the blend containing 50 wt% of iPP. For example, rcr(PP) in the blend of iPP/LDPE = 60/40 is 113.9 °C, iPP/LDPE = 50/50 - 113.0 °C, iPP/LDPE = 40/60 -111.3 °C. Further, the iPP crystallization peak apparently moves to the area of lower temperatures and overlaps the crystallization peak of LDPE.
The iPP concentration in the blend melts of these compositions is low, so the compactness of iPP segments is not sufficient for the formation of crystallization nuclei. Only immediately after the onset of crystallization of LDPE, the latter forces iPP out of its phase, and the local concentration of iPP macro chains in the melt increases, allowing iPP to crystallize. Moreover, LDPE crystallites apparently can act as additional crystallization nuclei for iPP, as evidenced by the appearance of a low-temperature diffusion region on the cooling curves for the blends of these compositions. The possibility of heterogeneous iPP nucleation at its low content in the iPP/HDPE blend was described in the study [24]. Furthermore, the mentioned above abnormalities may be associated with a partial compatibility of iPP and LDPE
in a melt at a low iPP content in the blend (less than 30 wt%), as shown in the studies [23, 24].
It is noteworthy that the crystallization peak of LDPE in all the blend compositions shifts by 1.5-2.5 °C to the area of lower temperatures relatively to the crystallization peak of homopolymer LDPE (Table 1). The regularities of the crystallization processes for both components in the blends described above indicate a mutual influence of polymers on the composition.
The crystallinity of LDPE /(PE) for different blend compositions slightly varies with the maximum for the individual LDPE - 23.8%. At the same time, the changes in the crystallinity of iPP /(PP) are extreme (Table 1). Thus, LDPE has a greater impact on the crystallinity of iPP.
The individual iPP is characterized by the maximum degree of crystallinity of 63.3 %. Even the addition of a small amount of LDPE (5-10 wt%) to iPP reduces the degree of crystallinity of iPP by 10 %, which is associated with the difficulties of iPP crystallization in the presence of LDPE.
When the content of iPP in the blend is from 30 to 70 wt%, the phase inversion takes place, which affects the crystallization process and the properties of the crystalline phase. The composition with the equal content of components is characterized by the minimum degree of crystallinity of both polymers. When the content of iPP in the blend is 50 wt%, the decrease in /(PP) by ~ 20 % and /(PE) by ~ 5 % relatively to homopolymers occurs. In this composition the interpenetrating polymer networks form. The resulting steric hindrances make the crystallization of both polymers complicated.
When the content of iPP in the blend is 40 wt% and less, iPP is a dispersed phase in the LDPE matrix. The crystallization of iPP for these compositions occurs in the same temperature range as for LDPE, the number of defective crystallites of iPP, including those crystallized on the LDPE nuclei, increases, which leads to an increase in /(PP) (55.9-59.9 %).
The study of the amorphous phase of iPP and LDPE, as well as of the blended compositions of these polymers, was conducted using paramagnetic probe. The correlation time tc of the rotational motion of a stable nitroxyl radical in the iPP/LDPE blends, as well as the radical concentration in the amorphous phase of the samples and the interface region were defined.
The concentrations of the radical are high in individual LDPE and in the blend with the iPP content of 10 wt% after 7-hour saturation with the radical, while in individual iPP and in the iPP/LDPE blends of the other compositions the concentrations of the radical are low. With the increase in the saturation time in LDPE and in the blend of iPP/LDPE = 10/90, the radical concentration varies slightly (about 1.5 times), reaching the limit value on the 3 d or 4th day. In contrast, in the other blend compositions and in iPP the concentration increases approximately by 15-20 times, reaching saturation after 25 days. Moreover, the saturation time of the samples increases with the increase in the content of iPP. The observed retardation of the diffusion process in iPP is associated with its lower molecular mobility (25 x 10-10 s), as compared with LDPE (4 x 10-10 s).
However, after saturation is reached, the radical concentration is higher with the increase of the iPP content in the blend (Fig. 2, curve f). Obviously, this can be attributed to the lower density of the iPP amorphous regions, as compared with LDPE, and, hence, to the larger free fluctuation volume, where the radical can be adsorbed.
■ 23
40 60 80 PP content {wt%}
100
Fig. 2 - The dependence of the radical concentration in the amorphous phase of the samples on the iPP content in iPP/LDPE blends at 0.3 (a), 1.7 (b), 5.5 (c), 7.1 (d), 11.9 (e), 25 (f) days of saturation
The observed regularities can be explained from the viewpoint of non-homogeneity of the iPP amorphous phase, while the amorphous phase of LDPE has a homogeneous structure. At the initial stage of the storage of the samples in radical vapors, it is absorbed in the most loose regions of the iPP amorphous phase, as evidenced by the low correlation time (Figure 3, curve a). In time the growing amount of radical penetrates into denser structures of the iPP amorphous phase, and, as a result, the correlation time gradually increases (from 6.9 x 10-10 to 25 x 10-10 s). The saturation is reached on the 25th day of radical diffusion in the samples, after that the correlation time does not change.
Curve f on the Figure 3 shows the dependence of the correlation time of the radical rotation on the blend composition after the radical saturation. As mentioned above, the rotational correlation time of the radical probe in iPP is higher than in LDPE. First of all, it is associated with the peculiarities of the spatial organization of polymers. The optimum chain conformation of iPP is a 3/1 helix, while the LDPE chain has a planar zigzag conformation, which causes a more free rotation of the macrochain segments [3]. Therefore, despite a higher density, LDPE has a lower rigidity of segments than iPP.
Blends of iPP/LDPE show the total deviation of the mentioned above dependence on the additivity towards LDPE. Curve f (Figure 3) can be divided into 3 regions depending on the amount of the deviation from additivity: I - from 0 to 30 wt%; II - 30 to 70 wt% and III - from 70 to 100 wt% of iPP in the iPP/LDPE blend.
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20 40 60 PP content (wt%)
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Fig. 3 - The dependence of the correlation time of the radical rotation on the iPP content in iPP/LDPE blends at 0.3 (a), 1.7 (b), 5.5 (c), 7.1 (d), 11.9 (e), 25 (f) days of saturation
The non-additive dependence tc on the composition (Figure 3, curve f) indirectly indicates a partial compatibility of iPP and LDPE with formation of the interface layer. The studies of Jose S. et al. [7], Zhou X. et al. [37] pointed out the possibility of forming the interface layer when polyolefins are compounded in a certain ratio, which indicated a partial compatibility of polymers. Different values of deviation from additivity in this case can be explained by changing in the density and the segmental mobility in the interface region due to the varying of the blend composition.
The dependence of the radical rotational correlation time (curve a) and the amount of the absorbed radical (curve b) in the interface region on the blend composition are shown in Figure 4. Curve a (Figure 4) shows that the interface layer in the region I are characterized by a lower segmental mobility (7-9 x 10-10 s) as compared with those in regions II and III (3.5-4 x 10-10 s). When the content of iPP in the blend is lower than 30 wt% (region I), it forms discrete domains in the LDPE matrix. After saturation the radical easily penetrates into the amorphous phase of LDPE, but its penetration into iPP is likely to be hindered by a sufficiently rigid interface layer between the iPP and LDPE amorphous regions. This is also evidenced by a low concentration of the radical in the interface regions of the blends of these compositions (Figure 4, curve b) and the same value of the tc of the radical as in the individual LDPE (~4.5 x 10-10 s) (Figure 3).
This is most likely to be associated with the abnormalities in the crystallization process of the blends of these compositions, which results in formation of the nonequilibrium structure of the material with a large number of rigid strained tie chains. It should be noted that the study [5] shows a significant increase in the rate of ozone oxidation of iPP/LDPE blends with the iPP content of 10-30 wt%, which also indicates the formation of non-equilibrium structures.
Fig. 4 - The dependence of the correlation time of the radical rotation (a) and the radical concentration (b) in the interphase layer on the iPP content in iPP/LDPE blends
When the content of iPP in the blend is 30 to 70 wt% (region II), the samples are characterized by the interpenetrating networks structure, the rigidity of interface layer is reduced, and more favorable conditions for the penetration of the radical in iPP exist. Therefore, an increase in tc with the increase of the iPP content in the mixture (from 10 to 5.5 x 10-10 s) is observed in this region (Figure 3, curve f).
When the content of iPP is 50 wt%, the minimum value of tc and the maximum concentration of the radical in the interface regions are observed in the blend as compared with other blend compositions (Figure 4). This indicates the formation of a less dense border layer, which is apparently associated with interpenetration of the segments of iPP and LDPE macrochains and the increase in the share of the interface region. It could also be the reason for the observed reduction in iPP and LDPE crystallinity.
On the region III (Figure 3, curve f) the material is the iPP matrix with distributed LDPE domains. However, the interface layer is characterized by a high segmental mobility (Figure 4, curve a), so the radical is able to easily penetrate into LDPE. The value of tc on this region (Figure 3, curve f) increases from 10 x 10-10 s (iPP/LDPE = 70/30) to 25 x 10-10 s (iPP). The negative deviation from the additivity law is, obviously, associated with the possibility of LDPE to modify iPP, leading to the increase of the segmental mobility in its amorphous regions and the reduction of the degree of crystallinity as compared with homopolymer of iPP. In other words, small additions of LDPE act as a plasticizer for iPP, as pointed out in the papers [4, 37].
Conclusion
The analysis of the results obtained by DSC and EPR spectroscopy methods has made it possible to follow the changes in the structure and properties of the
crystalline and amorphous phases of iPP and LDPE blends in the wide range of compositions. The remarkable mutual influence of both components on the formation of the morphology of the composition was observed.
Despite the fact that iPP and LDPE are incompatible polymers, their partial compatibility can occur in the least ordered amorphous regions with the formation of an interface layer.
Three ranges of compositions differing in structure and properties have been identified. The blends with a low content of iPP (5-30 wt%) in the form of a discrete phase have a non-equilibrium structure with a more rigid interface layer. The abnormal iPP crystallization process in the temperature range of LDPE crystallization causes it.
The blends containing from 30 to 70 wt% of iPP have structure of interpenetrating polymer networks, in which both polymers form a continuous phase. In this range of compositions, LDPE and iPP are partially compatible in the amorphous phase with formation of a developed interface layer.
In the blends with a high content of iPP (70-95 wt%), iPP forms a continuous phase with dispersed LDPE domains. Moreover, LDPE has a plasticizing effect on iPP, increasing the segmental mobility of the macromolecules chains in its amorphous phase. A similar plasticizing effect of LDPE is observed for the range of compositions where interpenetrating polymer networks (30-70 wt% of iPP) are formed.
Acknowledgments The authors are grateful to the Center of shared scientific equipment of Emanuel Institute of Biochemical Physics of Russian Academy of Sciences for the equipment provision.
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© E. E. Mastalygina - Senior Scientific Researcher, Laboratory of Advanced Composite Materials and Technologies, Department of Chemistry and Physics, Plekhanov Russian University of Economics; Post-graduate Student, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, elena.mastalygina@gmail.com; N. N. Kolesnikova - Candidate of Chemical sciences, Senior scientific researcher, Laboratory of Physical Chemistry of Synthetical and Natural Polymers, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, kolesnikova@sky.chph.ras.ru; S. G. Karpova - Candidate of Physical and Mathematical Sciences, Senior Scientific Researcher, Laboratory of Physical Chemistry of Synthetical and Natural Polymers, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, karpova@sky.chph.ras.ru; A. A. Popov - Doctor of Chemical Sciences, Professor, Head of Laboratory, Laboratory of Physical Chemistry of Synthetical and Natural Polymers, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, popov@sky.chph.ras.ru; V. F. Shkodich - docent of Rubber Technology Department of KNRTU; A. M. Kochnev - professor, Head of Rubber Technology Department of KNRTU.
© Е. Е. Масталыгина - ст. науч. сотр. лаб. перспективных композиционных материалов и технологий каф. химии и физики, Российский экономический университет им. Г.В. Плеханова, асп. Институт биохимической физики им. Н.М. Эмануэля» РАН, mastalygina@gmail.com; Н. Н. Колесникова - канд. хим. наук, ст. науч. сотр. лаб. физико-химии композиций синтетических и природных полимеров, Институт биохимической физики им. Н.М. Эмануэля» РАН, kolesnikova@sky.chph.ras.ru; С. Г. Карпова - канд. физ.-мат. наук, ст. науч. сотр. той же лаборатории, karpova@sky.chph.ras.ru; А. А. Попов - д-р хим. наук. проф.. зав. каф. химии и физики, Российский экономический университет им. Г.В. Плеханова, popov@sky.chph.ras.ru; В. Ф. Шкодич - канд. техн. анук, доц. каф. технологии синтетического каучука КНИТУ; А. М. Кочнев - д-р пед наук, проф. зав. каф. технологии синтетического каучука КНИТУ.
