Crystallization behavior, crystal transformation, and morphology of polypropylene/polybutene-1 blends

Abstract

The effects of composition on crystallization behavior, crystal transformation, and morphology of crystalline/crystalline PP/PB-1 blends are complicated. The crystallization rate of PP decreases with increasing PB-1 content in the cooling or isothermal crystallization process. Avrami constants of the PP crystallization are, however, insignificantly dependent on the PB-1 content. The crystallization rate and spherulite size of PB-1 are both enhanced by PP in PB-1-rich blends. As prepared by the solution–precipitation method, pure PB-1 exhibits crystal forms I′ (or III) and II, whereas the PB-1 in the blends exhibits only crystal form I′; pure PP and PP in the blends both exhibit only crystal form α. Crystal transformation from form I′ to form II for PB-1 in the blends can occur upon dynamic heating and cooling, but the transformation is retarded with increasing PP content. The crystal form II of PB-1 can be transformed to form I upon aging at room temperature, and this transformation can be facilitated by the PP in PB-1-rich blends. The similar helix conformation, of crystal form α of PP and of crystal forms I′ and I of PB-1, is found to be a driving force in preventing crystal form I′ of PB-1 in PP-rich blends from transforming to form II upon dynamic heating and cooling, and in facilitating crystal transformation from form II to form I for PB-1 in PB-1-rich blends upon aging at room temperature.

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 T_g 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 T_g polymers if these two polymers are compatible and the T_g of their blend is higher than that of this crystallizable polymer [4], [5], [6], [7], [8], [9]. The resulting crystallinity of the lower T_g 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 T_g 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 T_g 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).