Crystallization behavior, crystal transformation, and morphology of polypropylene/polybutene-1 blends
Introduction
The properties of polymer blends, whether amorphous or semicrystalline, depend, in general, on the method of mixing the components and their compatibility (or miscibility) as well as on the components' individual properties. The crystallization characteristics of polyblends in which both components are crystallizable are rather complicated. The constituents can either cocrystallize or form two separate crystalline phases. Moreover, in these semicrystalline polyblends, they can form a single or multiphase amorphous structure depending on the miscibility of the constituents. In the crystallization behavior of some pairs, the crystallinity of the crystallizable component was found to increase upon addition of lower Tg polymers, such as polycaprolactone (PCL) to polycarbonate (a compatible pair) [1], and ethylene–propylene rubber (EPR) to polypropylene (an incompatible pair) [2], [3]. On the contrary, the crystallization rate of a crystallizable polymer was found to be retarded by the addition of higher Tg polymers if these two polymers are compatible and the Tg of their blend is higher than that of this crystallizable polymer [4], [5], [6], [7], [8], [9]. The resulting crystallinity of the lower Tg crystallizable component in the blend is equal to or slightly lower than that of its pure state as found in many polyblends, such as polycaprolactone/polyvinyl chloride (PCL/PVC) [10], [11], polyvinylidene fluoride/polymethyl methacrylate (PVF2/PMMA) [12], [13], polyvinylidene fluoride/polyethyl methacrylate (PVF2/PEMA) [14], [15], polybutylene terephthalate/polyethylene terephthalate (PBT/PET) [16], PCL/saran [17], polylactones/PVC [18]. Although, in the blend of PVF2/polyvinyl acetate [19], PVF2 having a lower Tg develops a slightly higher crystallinity in the blend than in the pure state in a cyclic thermal scan, no explanation was given. When a blend of two crystallizable polymers, having different characteristic crystallization temperatures, is cooled from the melt, each component could act, if miscible, as a polymeric diluent for the other crystallizing polymer. The lower melting polymer crystallizes in the presence of an already crystallized phase, which may act as a nucleating agent for the crystallization.
Polypropylene (PP) and polybutene-1 (PB-1) are both crystallizable polymers. PP has a higher melting temperature than PB-1. Some properties of PP/PB-1 blends have been studied and reported [20], [21], [22]. Siegmann [20], [21] has studied the mutual influence of these two polymers, regarding their crystallization process from molten blends, their interaction in the amorphous phase, and the resulting tensile mechanical properties. The crystallization was followed by a differential scanning calorimeter (DSC), showing two separate PP and PB-1 crystallization processes that are both affected by the presence of the other component. The crystallization temperature of PP is significantly affected only in PB-1-rich blends whereas that of PB-1 is affected in the whole composition range. The PP crystalline phase, acting as a nucleating agent, increases the PB-1 crystallization temperature whereas the PP amorphous phase, acting as a high viscosity polymeric diluent, reduces the PB-1 crystallization temperature. The interaction between the two polymers in the amorphous phase was studied by applying dynamic mechanical analysis, and a single glass transition was observed, suggesting that the blends were compatible. Lee and Chen [22] have found that, contrary to the cases mentioned previously, PB-1, whose Tg is lower than that of PP, unexpectedly develops significantly higher crystallinity in the blends than that in the pure state under the same conditions, not only in the cyclic thermal scan but also in isothermal crystallization at temperatures higher than 90°.
PB-1 and PP are both polymorphic crystalline polymers. PB-1 exhibits four crystal modifications including I, II, III, and I′ depending on the formation conditions [23], [24]. Among these forms, form I crystals, twined hexagonal with a 31 helix [25], are the most stable and are usually obtained from the transition from the unstable form II, tetragonal with an 113 helix [26]. The unstable form II crystals can be obtained by cooling the melted PB-1 and slowly transform into stable form I crystals on aging at room temperature [27], [28], [29], [30], [31], [32]. This phase transformation is accelerated by the application of stress or strain on a form II sample [32], [33], [34], [35]. Forms III, orthorhombic with a 41 helix [27], [36], and I′, untwined hexagonal with a 31 helix [36], can be obtained by crystallization of PB-1 solution or polymerization of butene-1. Form I is similar to form I′ in morphology and X-ray diffraction patterns, but is different from form I′ in melting temperature [24], [25], [36]. The melting temperature of crystal form I is in the range 120–135°C whereas form I′ is in the range 90–100°C. PP exhibits three crystal forms including α, β, and γ [37]. Form α crystals are the most stable and are mostly obtained by cooling of melted PP. Form β crystals can be obtained by crystallization in the range 100–125°C [37]. Form γ crystals can be obtained by crystallization under high pressures [37]. Lattice types, molecular conformations, X-ray diffraction angles, and melting points of crystal forms α and β of PP [37], [38], [39] and crystal forms I, I′, II, and III of PB-1 [24], [25], [26], [27], [36], [40], [41], [42] are listed in Table 1.
Since both PB-1 and PP are crystallizable and have polymorphic morphologies, the effects of blend composition on morphologies and crystal transformations of the constituent polymers in the blends are complicated and have not been reported in the literature. In this work, we intend to investigate these complicated crystallization characteristics of PB-1/PP blends prepared by a different method, that is, the solution–precipitation method, using X-ray diffractometry, DSC and polarized optical microscopy (POM).
Section snippets
Sample preparation
Polypropylene [Profax PC366-MFR3, Mw=220,000, Mw/Mn=7.2, melt flow at 230°C with a load 2160 g (ASTM D1238)] used was obtained from Taiwan Polypropylene Co. Polybutene-1 [Duraflex 0200, Mw=428,000, Mw/Mn=9.5, melt flow measured under the same conditions as for the PP] was supplied by Shell Chemical Co. Infrared (IR) spectroscopic identification shows that both polymers are highly isotactic. The PB-1/PP blends were prepared by the solution–precipitation method.
Characterization of initial samples
Fig. 1 shows X-ray diffraction patterns for polypropylene, polybutene-1, and their blends with various compositions prepared by the solution–precipitation method. According to the diffraction angles listed in Table 1, Fig. 1a and e demonstrates that the PP precipitate from the xylene solution gives only crystal form α; however, the PB-1 precipitate does not clearly show any of the four crystal forms. PB-1 in the precipitates of the binary blends exhibits only crystal form I′ or I as
Conclusions
The crystalline/crystalline PP/PB-1 blends, prepared by the solution–precipitation method, are investigated for their crystallization behavior, crystal transformation and morphologies. The crystallization rate of PP is found to decrease with increasing PB-1 content in a cooling or isothermal crystallization process. The effects of PB-1 content on the Avrami constants of PP crystallization, however, are found to be insignificant. The crystallization rate and spherulite size of PB-1 can both be
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2022, PolymerCitation Excerpt :Blending of PB with iPP has aroused continuing interest and is particularly attractive because it offers a simple, effective and low-cost solution for industrial process, and the crystalline/crystalline blend exhibits complicated phase and crystallization behavior for academic research [36]. Previous studies on PB/iPP blends have revealed that the iPP in the blends can not only accelerate the form II-to-form I transition, but also lead to direct formation of form I′ modification upon crystallization from the molten state [35–37]. Shieh et al. [35] showed that PB in PB/iPP blends can crystallize into form I′ and form II at high iPP contents, however at low iPP contents only form II is generated, but its transformation to form I is facilitated by the iPP component.