The effect of end groups of PEG on the crystallization behaviors of binary crystalline polymer blends PEG/PLLA

The effect of end groups (2OH, 1OH, 1CH₃ and 2CH₃) of poly(ethylene glycol) (PEG) on the miscibility and crystallization behaviors of binary crystalline blends of PEG/poly(l-lactic acid) (PLLA) were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). A single glass-transition temperature was observed in the DSC scanning trace of the blend with a weight ratio of 10/90. Besides, the equilibrium melting point of PLLA decreased with the increasing PEG. A negative Flory interaction parameter, χ₁₂, indicated that the PEG/PLLA blends were thermodynamically miscible. The spherulitic growth rate and isothermal crystallization rate of PEG or PLLA were influenced when the other component was added. This could cause by the change of glass transition temperature, T_g and equilibrium melting point, T⁰_m. The end groups of PEG influenced the miscibility and crystallization behaviors of PEG/PLLA blends. PLLA blended with PEG whose two end groups were CH₃ exhibited the greatest melting point depression, the most negative Flory interaction parameter, the least fold surface free energy, the lowest isothermal crystallization rate and spherulitic growth rate, which meant better miscibility. On the other hand, PLLA blended with PEG whose two end groups were OH exhibited the least melting point depression, the least negative Flory interaction parameter, the greatest fold surface free energy, the greatest isothermal crystallization rate and spherulitic growth rate.     

Crystallization of polylactide films: An atomic force microscopy study of the effects of temperature and blending

Surface crystallinity on films of poly(l-lactide), poly(l/d-lactide) and their blends with poly(d-lactide) was studied. The isothermal spherulitic growth rate and its dependence on temperature were studied using tapping mode atomic force microscopy and ex situ isothermal crystallization. Using this technique, it is possible to extend spherulitic growth rate measurements to the region of significantly higher supercooling where nucleation concentration makes the use of in situ hot stage optical microscopy impossible. It was confirmed that while a poly(l/d-lactide) copolymer exhibits the typical “bell” shaped crystallization rate–temperature dependence, poly(l-lactide) exhibits a nonsymmetrical behavior having two crystallization rate maxima at 105 °C and 130 °C. As expected, the spherulitic growth rate of poly(l-lactide) was significantly higher than that of poly(l/d-lactide). The different types of crystalline formations exhibited at the surface of polylactide films are shown and discussed. The crystalline long spacing of poly(l-lactide) was also measured directly using tapping mode AFM and was found to be 19 nm at 165–170 °C. At low supercooling, several different scenarios of individual crystal formation were observed: purely flat-on stacks, purely edge-on stacks and scenarios where edge-on crystals flip to flat-on crystals and vice versa, where flat-on crystals yield edge-on sprouts. The preferred direction of growth of lamellae of both poly(l-lactide) and poly(d-lactide) was found to be counter-clockwise relative to the free surface.Finally, the crystallization kinetics of blends of poly(l-lactide) and poly(l/d-lactide) with poly(d-lactide) were studied. In such blends a triclinic stereocomplex crystalline structure forms between chains of opposite chirality and a pseudo-orthorhombic α-crystal structure forms between chains of like chirality. The presence of the stereocomplex crystals affects both the nucleation and the growth of the α-crystals. In fact depending on the stereocomplex content and the crystallization temperature the α-crystallization can either be enhanced or be inhibited. Interestingly it was found that the presence of the stereocomplex had a much stronger effect on the α-crystallization of poly(l/d-lactide) than on the α-crystallization of poly(l-lactide).     

Isothermal crystallization kinetics of nylon 6, blends and copolymers using simultaneous small and wide-angle X-ray measurements

Crystallization behavior in a number of blends and copolymers of nylons (polyamides) was investigated using time-resolved X-ray scattering data obtained simultaneously in the small- and wide-angle regimes. The following samples studied were: nylon 6 homopolymers (N6) of different molecular weights, copolymers of N6 and nylon 6,6 (N6/66), and blends of these with an amorphous nylon (N6I/T), which is a 70:30 random copolymer of poly(hexamethylene isophthalamide) and poly(hexamethylene terephthalamide). Addition of comonomers and blending with the N6I/T reduces the crystallinity of N6. Isothermal crystallization data obtained at several temperatures showed the expected faster crystallization kinetics at higher degrees of supercooling. Comonomer units in the N6 backbone reduce the rate of crystallization. N6I/T affects the crystallization (lamellar growth) behavior of N6/N66: the rate is higher at temperatures above the T_g of N6I/T (120 °C) where the crystallization is nucleation-driven, and lower below the T_g of N6I/T where it is growth-driven. Lamellar spacing decreases with an increase in the degree of supercooling, and this decrease is smaller in the blends than in the homopolymer. Larger lamellar spacing in N6I/T blends is due not to the insertion of N6I/T segments into the interlamellar regions but to an increase in the lamellar thickness. Blending seems to change the morphology by affecting the crystallization behavior rather than by thermodynamic phase separation. Residual monomers, which act as plasticizers, dramatically reduce the crystallization rate, whereas shear or similar mechanical history of the resin considerably accelerate the crystallization rate.     

Time-resolved X-ray scattering and calorimetric studies on the crystallization behaviors of poly(ethylene terephthalate) (PET) and its copolymers containing isophthalate units

Time-resolved small-angle X-ray scattering (SAXS) measurements were carried out for PET and its copolymers undergoing isothermal crystallization. Wide-angle X-ray diffraction and differential scanning calorimetric measurements were also performed. Our data analysis of the SAXS results for PET and the copolymers clearly demonstrate that the one layer thickness l₁ (derived directly from the correlation functions of the measured SAXS profiles) is the lamellar crystal thickness d_c, not the amorphous layer thickness d_a. The observed d_c values are found to be always smaller than d_a, regardless of polymer composition. d_c is highly dependent on the crystallization temperature, showing that the degree of supercooling is the major factor determining the thickness of lamellar crystals. No thickening, however, occurs in isothermal crystallizations. The kinked isophthalate units in the copolymer are found to be mostly excluded from the lamellar crystals during the crystallization process, leading to an increase of the amorphous layer thickness. Moreover, the kinked, rigid nature of the isophthalate unit was found to restrict crystal growth along the chain axis of the copolymers and also to lower their crystallinity. Unlike d_c, d_a decreases with crystallization time, causing a reduction of the long period in the lamellar stack. This drop in d_a is interpreted in this paper by taking into account several factors that could influence crystallization behavior: the d_a distribution in the lamellar stacks and its variation with time, the number of lamellae in the lamellar stacks and their effect on the SAXS profile, and the relaxation of polymer chains in the amorphous layers.     

Physical properties, crystallization kinetics, and spherulitic growth of well-defined poly(ε-caprolactone)s with different arms

Both well defined star-shaped poly(ε-caprolactone) having four arms (4sPCL) and six arms (6sPCL) and linear PCL having one arm (LPCL) and two arms (2LPCL) were synthesized and then used for the investigation of physical properties, isothermal and nonisothermal crystallization kinetics, and spherulitic growth. The maximal melting point, the cold crystallization temperature, and the degree of crystallinity of these PCL polymers decrease with the increasing number of polymer arms, and they have similar crystalline structure. The isothermal crystallization rate constant (K) of these PCL polymers is in the order of K_2LPCL>K_LPCL>K_4sPCL>K_6sPCL. Notably, the K of linear PCL decreases with the increasing molecular weight of polymer while that of star-shaped PCL inversely increases. The variation trend of K over the number of polymer arms or the molecular weight of polymer is consistent with the analyses of both nonisothermal crystallization kinetics and the spherulitic growth rate. These results indicate that both the number of polymer arms and the molecular weight of polymer mainly controlled the isothermal and nonisothermal crystallization rate constants, the spherulitic growth rate, and the spherulitic morphology of these PCL polymers.     

Effect of morphology on the crack propagation of the semicrystalline polyester poly(1,4-dimethylene-trans-cyclohexyl suberate)

Crack propagation of the semicrystalline polymer poly(1,4-dimethylene-trans-cyclohexyl suberate) (MCS) was studied as a function of polymer morphology. MCS was characterized in terms of degree of crystallinity and crystal growth kinetics. Spherulitic band size and radius show similar temperature dependencies. The energy to propagate a crack was correlated with spheruliticr adius for a low-molecular-weight material (M_n = 24 500). Brittle fracture occurs in this material with little large-scale plastic deformation. What plastic deformation there is, however, correlated with spherulitic band orientation. A higher-molecular-weight sample (M_n = 38 000) shows plastic deformation over the entire temperature range studied. Energy to fracture agrees with a modified Griffith criterion in which the characteristic dimension is spherulitic radius. Annealing experiments show that energy to fracture is controlled by lamellar thickness, decreasing with increasing thickness. Fracture morphology shows little interspherulitic failure, with intraspherulitic failure (low-molecular-weight material) or plastic deformation (high-molecular-weight material) being the prevalent modes.     

Melting behavior and isothermal crystallization kinetics of PP/mLLDPE blends

The melting/crystallization behavior and isothermal crystallization kinetics of polypropylene (PP)/metallocene-catalyzed linear low density polyethylene (mLLDPE) blends were studied with differential scanning calorimetry (DSC). The results showed that PP and mLLDPE are partially miscible and interactions mainly exist between the mLLDPE chains and the PE segments in PP molecules. The isothermal crystallization kinetics of the blends was described with the Avrami equation. Values of the Avrami exponent indicated that crystallization nucleation of the blends is heterogeneous, the growth of spherulites is almost three-dimensional, and the crystallization mechanism of PP is not affected much by mLLDPE. The Avrami exponents of the blends are higher than that of pure PP, showing that the mLLDPE helps PP to form perfect spherulites. The crystallization rates of PP are decreased by mLLDPE because the crystallization temperature of PP was decreased by addition of mLLDPE and consequently the supercooling of the PP was correspondingly lower. The crystallization activation energy was estimated by the Friedman equation, and the result showed that the activation energy increased by a small degree by addition of mLLDPE, but changed little with increasing content of mLLDPE in the blends. The nucleation constant (K _g) was determined by the Hoffman–Lauritzen theory. Supported by the Science Foundation of Hebei University (2006Q13).     

Studies on nonisothermal crystallization of HDPE/POSS nanocomposites

The nonisothermal crystallization of HDPE/POSS nanocomposites (POSS content varying from 1 to 10 wt%) was studied using differential scanning calorimetry (DSC) technique. The Ozawa approach failed to describe the crystallization behaviour of nanocomposites, whereas the modified Avrami analysis could explain the behaviour of HDPE/POSS (90:10) nanocomposite only. The value of Avrami exponent n for HDPE/POSS (90:10) nanocomposite ranged from 2.5 to 2.9 and decreased with increasing cooling rate. It is postulated that the values of n close to 3 are caused by spherulitic crystal growth with heterogeneous nucleation while simultaneous occurrence of spherulitic and lamellar crystal growth with heterogeneous nucleation account for lower values of n at higher cooling rates. A novel kinetic model by Liu et al. was able to satisfactorily describe the crystallization behaviour of HDPE/POSS nanocomposites. Presence of POSS did not cause significant change in the activation energy for the transport of polymer segments to the growing crystal surface. POSS molecules exhibit nucleation activity only at 10 wt% loading in HDPE and are not effective nuclei at lower loadings.     

Crystallization studies of isotactic polystyrene

The kinetics of lamellar crystallization in thin films of isotactic polystyrene have been determined using transmission electron microscopy. The morphological changes accompanying crystallization have also been investigated as a function of solvent, supercooling and strain prior to crystallization. Crystallization temperatures have been attained by both cooling from the melt and warming from the glass. Similar growth rates were obtained in both cases. The nucleation density of spherulites is difficult to control when warming from the glass but does depend on the solvent used in preparing the thin film. The rate of lamellar growth follows a ‘bell’ shaped curve versus crystallization temperature and the kinetics were analysed using the secondary nucleation theory of Hoffman and Lauritzen. The end surface free energy, δ_e, of the lamellar crystals was determined using the variation of lamellar thickness with supercooling.     

Thermal properties and non‐isothermal crystallization behavior of poly(trimethylene terephthalate)/poly(lactic acid) blends

Thermal properties and non‐isothermal melt‐crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(lactic acid) (PLA) blends were investigated using differential scanning calorimetry and thermogravimetric analysis. The blends exhibit single and composition‐dependent glass transition temperature, cold crystallization temperature (T_cc) and melt crystallization peak temperature (T_mc) over the entire composition range, implying miscibility between the PLA and PTT components. The T_cc values of PTT/PLA blends increase, while the T_mc values decrease with increasing PLA content, suggesting that the cold crystallization and melt crystallization of PTT are retarded by the addition of PLA. The modified Avrami model is satisfactory in describing the non‐isothermal melt crystallization of the blends, whereas the Ozawa method is not applicable to the blends. The estimated Avrami exponent of the PTT/PLA blends ranges from 3.25 to 4.11, implying that the non‐isothermal crystallization follows a spherulitic‐like crystal growth combined with a complicated growth form. The PTT/PLA blends generally exhibit inferior crystallization rate and superior activation energy compared to pure PTT at the same cooling rate. The greater the PLA content in the PTT/PLA blends, the lower the crystallization rate and the higher the activation energy. Moreover, the introduction of PTT into PLA leads to an increase in the thermal stability behavior of the resulting PTT/PLA blends. Copyright © 2011 Society of Chemical Industry     

Observation of banded spherulites in pure poly(l-lactide) and its miscible blends with amorphous polymers

Banded spherulites of pure poly(l-lactide) (PLLA) were observed via the ‘crystallization after annealing’ procedure, while only common spherulites were obtained via the ‘direct isothermal crystallization’ procedure. Wide angle X-ray diffraction revealed that the two types of spherulites had the same crystal lattice of α-modification. Atomic force microscopy demonstrated that the alternative negative and positive birefringent bands resulted from the alternative edge-on and flat-on lamellar orientations in the spherulites. Furthermore, the effect of thermal history on the spherulitic morphology was investigated in details. The PLLA samples melted for longer time or those with lower melting point were more likely to form banded spherulites. The possibility that the change of molecular weight was a determining factor of banding was excluded by the results on differently prepared samples with the same molecular weight. Therefore, we conclude that it was complete melting of the crystalline residues that favored formation of PLLA banded spherulites. Blending of PLLA with atactic poly(d,l-lactide) or poly[(R,S)-3-hydroxybutyrate], led to reduced band spacing. Effect of blending on the chain mobility, spherulite growth kinetics, supercooling and lamellar surface energy was quantitatively studied, which suggests that the blending-reduced band spacing cannot be attributed to the above factors. Therefore, there are other blending-relevant factors leading to the reduced band spacing.     

Crystallization kinetics and morphology of poly(butylene succinate)/poly(vinyl phenol) blend

Biodegradable crystalline poly(butylene succinate) (PBSU) can form miscible polymer blends with amorphous poly(vinyl phenol) (PVPh). The isothermal crystallization kinetics and morphology of neat and blended PBSU with PVPh were studied by differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS) in this work. The overall isothermal crystallization kinetics of neat and blended PBSU was studied with DSC in the crystallization temperature range of 80-88 °C and analyzed by applying the Avrami equation. It was found that blending with PVPh did not change the crystallization mechanism of PBSU, but reduced the crystallization rate compared with that of neat PBSU at the same crystallization temperature. The crystallization rate decreased with increasing crystallization temperature, while the crystallization mechanism did not change for both neat and blended PBSU irrespective of the crystallization temperature. The spherulitic morphology and growth were observed with hot stage OM in a wide crystallization temperature range of 75-100 °C. The spherulitic morphology of PBSU was influenced apparently by the crystallization temperature and the addition of PVPh. The linear spherulitic growth rate was measured and analyzed by the secondary nucleation theory. Through the Lauritzen-Hoffman equation, some parameters of neat and blended PBSU were derived and compared with each other including the nucleation parameter (K_g), the lateral surface free energy (σ), the end-surface free energy (σ_e), and the work of chain folding (q). Blending with PVPh decreased all the aforementioned parameters compared with those of neat PBSU; however, the decrease extent was limited. WAXD result showed that the crystal structure of PBSU was not modified after blending with PVPh. SAXS result showed that the long period of blended PBSU increased, possibly indicating that the amorphous PVPh might reside mainly in the interlamellar region of PBSU.     

Unusual crystallization of polyethylene at melt/atomically flat interface: Lamellar thickening growth under normal pressure

The morphology of polyethylene (PE) crystallized at the melt/atomically flat substrate interface was studied using atomic force microscopy (AFM). Our attention is concentrated on isothermal crystallization of PE on HOPG and MoS₂ substrates at high temperatures up to 135 °C. By quenching after different times of crystallization, it was possible to “freeze” the lamellar morphology at various stages of its development at a given supercooling. After detachment of the PE sample from the substrate, individual lamellae (even 150 nm thick) and stacks of the edge-on lamellae after different stages of growth were observed. The similarity of the individual lamellae with those grown from the hexagonal phase under high pressure (characteristic tapered edges), allows to conclude that at the interface, even under normal pressure, the crystallization proceeds according to the mechanism of lamellar thickening growth.     

Micro-morphology of modified and nonmodified blends of polypropylene with linear low density polyethylene

Modified and nonmodified blends of linear low-density polyethylene (PE) and polypropylene (PP) form separated phases of crystalline PP and PE. They form spherulitic crystals in the core, but highly oriented nonspherulitic phases at the skin of injection molded test bars. The dimension of the spherulites decreases with increasing PE content within the blends. Crystallization behavior of both crystalline phases is influenced by the other phase. The crystallization temperature of PP is increased in the presence of the compatibilizer. Transmission electron micrographs of blends modified by poly(styrene-block-ethylene/butylene) (SEBS) and stained by OsO₄ showed co-continuous lamellar structure of the blends with a polypropylene phase containing the majority of the modifier. Smaller portions of the modifier can be found on the surface of the two olefinic phases as dispersed spheres, with an average diameter of 50–90 nm. The lamellar structure is independent of the spherulitic structure, and interpenetrates the spherulites. The conclusion of this study is that this block copolymer, while improving the physical properties of the blends, is not a true compatibilizer of the system according to the conventional terminology of physical chemistry.     

Crystallization and phase separation kinetics in blends of linear low-density polyethylene copolymers

We have systematically studied the crystallization and liquid-liquid phase separation (LLPS) kinetics in statistical copolymer blends of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB) using primarily optical microscopy. The PEH/PEB blends exhibit upper critical solution temperature (UCST) in the melt and crystallization temperature below the UCST. The time evolution of the characteristic morphology for both crystallization and LLPS is recorded for blends at various compositions and following a quench from initial homogenous melts at high temperature to various lower temperatures. The crystallization kinetics is measured as the linear growth rate of the super structural crystals, whereas the LLPS kinetics is measured as the linear growth rate of the characteristic length of the late-stage spinodal decomposition. The composition dependence crystallization kinetics, G, shows very different characteristics at low and high isothermal crystallization temperature. Below 116 °C, G decreases with increasing PEB content in the blend, implying primarily the composition effect on materials transport; whereas at above 116 °C, G shows a minimum at about the critical composition for LLPS, implying the influence of the LLPS. On the other hand, LLPS kinetics at 130 °C is relatively invariant at different compositions in the two-phase regime. The length scale at which domains are kinetically pinned, however, depends strongly on the composition. In a blend near critical composition, a kinetics crossover is shown to separate the crystallization dominant and phase separation dominant morphology as isothermal temperature increases.     

The effect of supercooling on crystallization of cocoa butter-vegetable oil blends

The solid fat content (SFC), Avrami index (n), crystallization rate (z), fractal dimension (D), and the pre-exponential term [log(γ)] were determined in blends of cocoa butter (CB) with canola oil or soybean oil crystallized at temperatures (T _Cr) between 9.5 and 13.5°C. The relationship of these parameters with the elasticity (G′) and yield stress (σ^*) values of the crystallized blends was investigated, considering the equilibrium melting temperature (T _M ^o) and the supercooling (i.e., T _Cr ^o−T _M ^o) present in the blends. In general, supercooling was higher in the CB/soybean oil blend [T _M ^o=65.8°C (±3.0°C)] than in the CB/canola oil blend [T _M ^o=33.7°C (±4.9°C)]. Therefore, under similar T _Cr values, higher SFC and z values (P<0.05) were obtained with the CB/soybean oil blend. However, independent of T _Cr TAG followed a spherulitic crystal growth mechanism in both blends. Supercooling calculated with melting temperatures from DSC thermograms explained the SFC and z behavior just within each blend. However, supercooling calculated with T _M ^o explained both the SFC and z behavior within each blend and between the blends. Thus, independent of the blend used, SFC described the behavior of G′_eq and σ^* and pointed out the presence of two supercooling regions. In the lower supercooling region, G′_eq and σ^* decreased as SFC increased between 20 and 23%. In this region, the crystal network structures were formed by a mixture of small β′ crystals and large β crystals. In contrast, in the higher supercooling region (24 to 27% SFC), G′_eq and σ^* had a direct relationship with SFC, and the crystal network structure was formed mainly by small β′ crystals. However, we could not find a particular relationship that described the overall behavior of G′_eq and σ^* as a function of D and independent of the system investigated.     

Time resolved study of shear-induced crystallization of poly(p-dioxanone) polymers under low-shear, nucleation-enhancing shear conditions by small angle light scattering and optical microscopy

Isothermal crystallization of the biodegradable homopolymer polyester poly(p-dioxanone) (PDS), and the p-dioxanone copolymer with glycolide PDS-copolymer were studied in situ and in real-time under quiescent or nucleation-enhancing shear conditions. It was found that the spherulitic growth rates remain unchanged with shear, while the nucleation density increases dramatically. Nucleation-enhancing shear conditions, which do not alter the general spherulitic morphology, consist of a short-duration step-shear. This is in contrast to isothermal crystallization under steady-shear conditions, where at low-shear rates, fibrillar crystalline structures form. At high crystallization temperatures, where under quiescent conditions crystal development requires several days, both PDS, and the PDS-copolymer can be made to crystallize in several hours by the imposition of a step-shear.     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate) copolyester containing 5 mol% trimethylene succinate

Copolyester was synthesized and characterized as having 94.4 mol% ethylene succinate units and 5.6 mol% trimethylene succinate units in a random sequence as revealed by NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics of this copolyester in the temperature range (T_c) from 30 to 80 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC) by varying the T_c, the heating rate and the crystallization time. DSC and TMDSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stabilities. A small exothermic curve between the main melting peaks gives a direct evidence of recrystallization. As the T_c increases, the contribution of recrystallization gradually decreases and finally disappears. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 108.3 °C. The kinetic analysis of the spherulitic growth rates indicated that a regime II → III transition occurred at ∼65 °C.     

Segregation morphology of poly(3-hydroxybutyrate)/poly(vinyl acetate) and poly(3-hydroxybutyrate-co-10% 3-hydroxyvalerate)/poly(vinyl acetate) blends as studied via small angle X-ray scattering

Segregation morphology of poly(3-hydroxybutyrate) (PHB)/poly(vinyl acetate) (PVAc) and poly(3-hydroxybutyrate-co-10% 3-hydroxyvalerate) (P(HB-co-10% HV)/PVAc blends crystallized at 70 °C have been investigated by means of small angle X-ray scattering (SAXS). Morphological parameters including the crystal thickness (l_c) and the amorphous layer thickness (l_a) were deduced from the one-dimensional correlation function (γ(z)). Blending with PVAc thickened the PHB crystals but not the P(HB-co-10% HV) crystals. On the basis of the composition variation of l_a, and the volume fraction of lamellar stacks (?_s) revealed that PHB/PVAc blends created the interlamellar segregation morphology when the weight fraction of PVAc (w_PVAc)≤0.2 and the interlamellar and interfibrillar segregation coexisted when w_PVAc>0.2, while P(HB-co-10% HV)/PVAc blends yielded the interfibrillar segregation morphology at all blend compositions. For both PHB/PVAc and P(HB-co-10% HV)/PVAc blends, the distance of PVAc segregation was promoted by increasing PVAc composition and the distance of PVAc segregation in P(HB-co-10% HV)/PVAc blends was greater than in PHB/PVAc at a given PVAc composition. The crystal growth rate played a key role in controlling the segregation of PVAc.     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007