CC BY
78
ХИМИЯ
UDC 678.741:678.7-13
A. V. Khvatov, Yu. K. Lukanina, N. N. Kolesnikova, A. A. Popov, G. E. Zaikov, Kh. S. Abzaldinov
EFFECT OF CRYSTALLIZATION RATE ON THE CRYSTALLINE STRUCTURE
OF POLYPROPYLENE AND ITS COPOLYMERS
Keywords: polypropylene, melting, crystal structure, DSC, microscopy.
Melting behavior and the crystal structure of polypropylene and its copolymers of different chemical structure depending on the crystallization rate were studied by Differential Scanning Calorimetry (DSC) and Microscopy. The experimental results indicate that both crystallization rate and chemical nature and regularity arrangement of co-monomer unites in the polypropylene main chain exhibit influence on the crystalline structure of polypropylene.
Ключевые слова: полипропилен, плавление, кристаллическая структура, ДСК, микроскопия.
Методами дифференциальной сканирующей калориметрии (ДСК) и микроскопии исследованы характер плавления и кристаллическая структура полипропилена и его сополимеров различной химической структуры в зависимости от скорости кристаллизации. Экспериментальные результаты показывают, что скорость кристаллизации, химическая природа и равномерность расположения звеньев сомономера в основной цепи полипропилена оказывают влияние на кристаллическую структуру полипропилена.
1 Aims and background
Polypropylene and its copolymers are the most important commodity thermoplastic polymers which are found in a wide variety of applications due to its excellent strength, toughness, high chemical resistance and high melting point. Polypropylene versatility along with its low cost and technological potentials has rendered it as one of the most used materials worldwide [1-4].
Isotactic homopolypropylene (PP) is a crystalline polymer whose properties are dependent on the degree of crystallinity and it exhibits three crystalline forms: the monoclinic a-phase, the hexagonal b-phase, and the orthorhombic c-phase [5-8]. When PP is formed, its crystal formation changes according to the heat treatment temperature and the conditions of cooling process. These changes create differences in strength, heat resistance and pressure bonding properties. Accordingly, it interesting and important to find out how to manage the structure of the polymer by varying the crystallization conditions during processing.
Propylene-based block copolymer (bPP) - the chain of molecules of propylene gap in the chain of eth-ylene copolymer. Propylene block copolymers produced in the form of homogeneous color, of the granules with high impact strength (at low temperatures), and high flexibility; increased long-term thermal stability; resistance to oxidative degradation during production and processing of polypropylene as well as the operation of the product out of it. Owing to the crystalline structure of the polypropylene block copolymer is a thermoplastic structural sufficiently economical polymer [9-10].
Propylene-based random copolymers (rPP) are made by copolymerizing propylene and small amounts of ethylene (usually up to 7 wt.%) and their structure is similar to isotactic polypropylene, but the regular repeating of propylene units along the macromolecular chains is randomly disrupted by the presence of the comonomer ones. The presence of ethylene units reduces the melting point and crystallinity by introducing
irregularities into the main polymeric chain. The advantages of this class of polymers are improved transparency, relative softness, lower sealing temperature, and moderate low-temperature impact strength due to the lowered glass-transition temperature [11-14].
Maleic anhydride grafted PP (mPP) is the most common functional adhesion promoter. It was proved to be effective functional molecule for the reactive compatibilization [6, 15, 16].
It is generally known that the properties of semi-crystalline polymers depend strongly on the size and shape of their supramolecular structure [1, 16, 17]. The purpose of this paper is to present new results concerning the melting behavior and the crystal structure of polypropylene and its copolymers in depends on the crystallization rates, chemical nature of co-monomer unites and regularity of co-monomer units arrangement in the polypropylene main chain.
2 Experimental
2.1 Materials and sample preparations
All polymers used in this study are commercial available. PP - Caplen 01030, melt index 1,2 g/10min; rPP - SEETEC R3400, content of ethylene groups - 3 wt%, melt index 8 g/10min; bPP - SEETEC M1400, content of ethylene groups - 9 wt%, melt index 8 g/10min, mPP - Polybond 3002, content of polar groups - 0,6 wt%, melt index 12 g/10min.
2.2 DSC measurements
DSC measurements were made on DSC thermal system Microcalorimeter DSM-10ma. Its temperature scale was calibrated from the melting characteristics of indium. The experiments were conducted in non-isothermally mode. The sample was first heated up to 200oC and maintained at this temperature for 10 min in order to erase any previous morphological history which the sample might be carrying. The sample then non-isothermally crystallized when it was cooled down to room temperature at different cooling rates (1, 2, 4, 8,
16, 32, 64 oC/min). It was subsequently heated at a heating rate of 8oC/min. The sample then repeatedly non-isothermally crystallized with the same cooling rates to permit structure microphotographs. The samples were approx. 7 mg. All curves were normalized to the unit weight of the sample.
The percent crystallinity, Xc, of the samples is calculated by:
Xc = AHm / [Wpp'AHma (1)
where, AHm and AHm0 are melting enthalpy of sample and 100% crystalline PP (147 J/g [12]), respectively and WPP is weight fraction of PP in the samples.
2.3 Microscopy
The Axio Imager microscopes Z2m (Collective use center "New materials and technologies", IBCP
RAS) with Transmitted-light differential interference contrast (TL DIC) was used to obtained microphotographs.
3 Results and Discussion
The influence of crystallization rate on melting behavior of PP and copolymers were investigated. The melting endotherms of PP and its copolymers after various crystallization rates are presented in Fig. 1. Henceforth, for all investigated polymers, the peak at the lower temperature is called peak-1, and at the higher temperature is called peak-2. The values of melting temperature (Tm) and crystallinity (Xc) are listed in Table 1.
o
T3
s
w
130 140 150 160 170 180 ToC
130 140 150 160 170 180 ToC
o -d a
w
130 140 150 160 170 180 ToC
6
after cryst.rate: 64 oC/min
O T3
s
w
110 120 130 140 150 160
ToC
c d
Fig. 1 - DSC thermograms of samples: a - PP, b - mPP, c - bPP, d - rPP after crystallization with different rates
a
Fig. 1a shows the corresponding DSC thermographs of the samples PP recorded after cooling with different rates. From Figure 1a, it can be seen that melting behavior depends remarkably on the cooling rate. At lower crystallization rates < 8o/min, the general feature of the DSC curves is the single melting peak which localized mainly at a temperature about 163oC. With increase of cooling rate one melting peak transformed into double melting peaks. At crystallization rate 8 o/min on the DSC melting curves appears shoulder peak at about 165oC, which grows into a complete peak-2 at the crystallization temperature 32 oC/min and 64 oC/min, whereas the position of the peak-1 displaced at the location of lower temperature 161oC. Such dependence is explained by the formation of more advanced and stable crystal structures at low cooling rate, whereas with increasing cooling rate increases supercooling and a large amount of defective crystals exposed to reorganization and recrystallization during heating process.
In Fig. 1b, mPP samples shows melting endo-therms after different crystallization rates. As well as pure PP, the DSC curve for mPP shows single melting peak under low cooling rate. It should be noted that appearance and increases of shoulder peak for such copol-ymer observed at higher cooling rates - 16 oC/min. Another distinguishing feature of mPP melting after different crystallization rates is significant decrease of melting temperature of peak-1 (from 165oC after cooling rate 1oC/min to 160oC after cooling rate 64 oC/min).
Table 1 - Thermal parameters of polymers samples during melting process after different crystallization rates
Sample Cryst. rate (° C/min) Tm (°C) Xc (%)
64 161, 166 50
32 161, 166 49
16 160, 165 46
PP 8 161 47
4 162 49
2 163 54
1 163 58
Sample Cryst. rate (° C/min) Tm (°C) Xc (%)
64 160, 166 62
32 160, 165 60
16 163, 166 59
bPP 8 163 60
4 163 61
2 164 63
1 165 63
Sample Cryst. rate (° C/min) Tm (°C) Xc (%)
64 160, 166 63
32 160, 166 62
16 161, 166 49
mPP 8 162 54
4 163 53
2 164 55
1 165 63
Sample Cryst. rate (° C/min) Tm (°C) Xc (%)
64 140, 147 31
32 140, 147 29
16 141, 148 29
rPP 8 142, 147 29
4 143, 147 25
2 144 25
1 145 26
Fig. 1c gives the heating DSC thermographs for the bPP samples prepared also at various cooling rates. Curves have the similar character described above by maintaining the main polypropylene chain under the block introduction of ethylene units. This later transition from a single peak to a double peak will explain in terms of polypropylene chemical structure change by the introduction of functional groups. Since the regular introduction of functional groups decreases mobility of the system and increases the viscosity of the system, at low cooling rates this leads to the rapid formation of crystallization centers around them to form more perfect crystals than in pure PP. This displayed the higher melting temperatures of mPP and bPP. In the transition to higher cooling rates of up to 16 o/min, at which the viscosity increases, the rearrangement of molecules copol-ymers is difficult, which in turn reduced the rate of crystallization, resulting in the formation of qualitatively less perfect package. The melting temperature Tm decreases on 5oC. A further increase of the material crystallization rate contributes to recrystallization and formation of amid bulk imperfect crystals a high-melting crystalline structure.
The statistical introduction of ethylene units into the main polypropylene chain affects greatly the crystalline structure of rPP in Fig. 1d. Firstly, a decrease in the melting temperature of the polymer matrix of about 20 °C is observed. Secondary, the peak-2 as a shoulder peak appears at crystallization rate 4 oC/min. For the propylene-based random copolymer the ethylene-monomers are often considered as a defect points in polypropylene matrix. This induces structure heterogeneity in the long chains of polypropylene namely short propylene sequences leading to a decrease in crystallizable sequence length. Even small co-monomer content (3%), as in our case results in short crystalline sequence.
From the Table 1 one can observe the relationship between the crystallization rate and crystalline of polymers samples. Comparing the crystalline degree of the samples at a rate of crystallization (for example 1o/min) one can note that the regular introduction of the co-monomer units of any chemical nature (mPP and bPP) leads to an increase in the polypropylene crystalline degree by 5%. Such incorporated co-units are not defect points, its lead to the facilitating folding crystalline structure. Due to described above statistical distribution of ethylene co-units - defect points in the pro-pylene random copolymer rPP crystallinity decrease on 30%. On the other hand, crystalline of PP, mPP and bPP characterized by decrease with increase crystallization rate, followed by crystallization increase at higher rates (32oC/min and 64 oC/min). Samples of rPP characterized by the constancy of crystalline degree at low cooling rates and its growth of 4-5% even at a cooling rate of 8
o/min. These dependences are as well as formation of the shoulder and peak-2 on the DSC thermograms are explainable in terms of reorganization and recrystalliza-tion of the polypropylene supramolecular structure.
Described above crystalline structures of PP, mPP, bPP and rPP obtained after different crystallization rates were visualized by microscopy. Figure 2 shows microphotographs of PP, mPP, bPP and rPP obtained after different cooling rates. For all polymer samples in the train of crystallization the line radial bundles emanating from a single point - polypropylene crystalline structure are formed. The crystalline sizes of the samples vary widely depending on the polymer crystallization rate and its chemical structure. At higher cooling rates and structural inhomogeneous of the propylene long chains in random copolymer formed submi-croscopic crystal structures - finely-grain-like structure. Crystallization at a lower cooling rates leads to the formed crystals in diameter of about 100 microns. It is necessary to note that the size and shape of the supramolecular structures have great influence on the mechanical properties of the polymer. Samples with finely-grain-like structure have high strength and have good elastic properties. Samples with large crystals destroyed fragile. Elasticity loss of crystalline polymers is manifested in the appearance of cracks and breaks at the crystals interface. The increase of its size leads to the fragility increased and decrease of strength.
4 Conclusions
The melting behavior of four polymers: polypropylene, propylene-based block copolymer, propyl-ene-based random copolymer and maleic anhydride grafted polypropylene, was studied with DSC and Microscopy. Obtained results indicate that both chemical structure of polypropylene chain and crystallization rate show great influence on the crystalline structure of polypropylene. It was found that the regular introduction of the co-monomer units of any chemical nature (mPP and bPP) and higher crystallization rates leads to the formation of large crystals. On the other hand, structural inhomogeneous of polypropylene chain and increase of crystallization rates promote to the formation of polymer finely-grain-like structure.
50 pin
50 M»',
1a
2a
50 Mm.
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3a
4a
1b
2b
t50 urn
50 urn
3b
4b
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1c
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3c
4c
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Fig. 2 - Microscope photographs of 1-PP, 2- mPP, 3 - bPP, 4 - rPP obtained after different crystallization rates: a - 64oC/min, b - 16oC/min, c -4oC/min, d - 1 oC/min
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© A. V. Khvatov - Ph.D., Researcher, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, Moscow, Russia, hvatovanatoliy@gmail.com, Yu. K. Lukanina - Ph.D., Researcher, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, Moscow, Russia, N. N. Kolesnikova - Ph.D., Researcher, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, Moscow, Russia, A. A. Popov - Doctor of Chemistry, Full Professor, Head of Laboratory, Emanuel Institute of Biochemical Physics of Russian Academy of Sciences, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, Kh. S. Abzaldinov - Ph.D., Associate Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© А. В. Хватов - кандидат химических наук, научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, hvatovanatoliy@gmail.com, Ю.К. Луканина - кандидат химических наук, научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Н. Н. Колесникова - кандидат химических наук, научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, А. А. Попов - доктор химических наук, профессор, заведующий лабораторией, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Г. Е. Заи-ков - доктор химических наук, профессор, кафедра Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, Х. С. Абзальдинов - кандидат химических наук, доцент, кафедра Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия,
