Nonisothermal crystallization and melting behavior of β‐nucleated isotactic polypropylene and polyamide 66 blends

A highly novel nano‐CaCO₃ supported β‐nucleating agent was employed to prepare β‐nucleated isotactic polypropylene (iPP) blend with polyamide (PA) 66, β‐nucleated iPP/PA66 blend, as well as its compatibilized version with maleic anhydride grafted PP (PP‐g‐MA), maleic anhydride grafted polyethylene‐octene (POE‐g‐MA), and polyethylene‐vinyl acetate (EVA‐g‐MA), respectively. Nonisothermal crystallization behavior and melting characteristics of β‐nucleated iPP and its blends were investigated by differential scanning calorimeter and wide angle X‐ray diffraction. Experimental results indicated that the crystallization temperature (T) of PP shifts to high temperature in the non‐nucleated PP/PA66 blends because of the α‐nucleating effect of PA66. T of PP and the β‐crystal content (K_β) in β‐nucleated iPP/PA66 blends not only depended on the PA66 content, but also on the compatibilizer type. Addition of PP‐g‐MA and POE‐g‐MA into β‐nucleated iPP/PA66 blends increased the β‐crystal content; however, EVA‐g‐MA is not benefit for the formation of β‐crystal in the compatibilized β‐nucleated iPP/PA66 blend. It can be relative to the different interfacial interactions between PP and compatibilizers. The nonisothermal crystallization kinetics of PP in the blends was evaluated by Mo's method. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Isothermal crystallization and melting behaviors of nano TiO2‐modified polypropylene/polyamide 6 blends

Titanium dioxide (TiO₂) nanoparticles were functionalized with toluene‐2,4‐diisocyanate and then polypropylene/polyamide 6/(PP/PA6) blends containing functionalized‐TiO₂ were prepared using a twin screw extruder. Isothermal crystallization and melting behavior of the as‐prepared composites were investigated using differential scanning calorimetry and wide‐angle X‐ray diffraction. Isothermal crystallization analysis shows that the TiO₂ nanoparticles have two effects on PP/PA6 blends, i.e., it can favor the improvement of crystallization ability and decrease the crystallization rate of PP/PA6 blends. The improvement of crystallization ability is superior over decreasement of crystallization rate of PA6 chains caused by TiO₂, therefore PA6 in PP/PA6/TiO₂ nanocomposites have higher crystallization rate than that of PA6 in pure PP/PA6 blends, which indicated TiO₂ nanoparticles favored the crystallization of PA6. The TiO₂ nanoparticles show no effects on the equilibrium melting temperature (T) values of PP phase but decreases the T values of PA6 phase. In addition, the TiO₂ nanoparticles did not change the crystalline polymorph of PP/PA6 blends basically; however, favored the formation of β‐PP. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Isothermal crystallization and spherulite structure of partially miscible polypropylene–linear low-density polyethylene blends

Polypropylene (PP) was blended with a linear low-density polyethylene (LLDPE, containing 5% hexene comonomer) over a composition range of 10–90% of PP. The crystallization and morphology of the PP–LLDPE blends were studied by differential scanning calorimetry (DSC), polarized optical microscopy with a hot stage (HSOM), and scanning electron microscopy (SEM). In particular, the isothermal crystallization of PP in molten LLDPE was investigated. It was observed that the crystallization and melting behavior of PP and LLDPE changed in the blends, indicating that there was some degree of miscibility between the PP and the LLDPE. A depression of the equilibrium melting temperature (T) of PP in the blends with no more than 15% of PP confirmed that PP was miscible with LLDPE at and below 15% of PP. In addition, a drastic decrease in T from the 25% PP blend to the 20% blend led us to conclude that the miscible behavior between PP and LLDPE became favorable at a PP concentration of 20%. The optical microscopic images showed that, in the blends with 10 and 15% of PP, the PP crystallized as open-armed diffuse spherulites, similar to those in the miscible blends. In contrast, the PP crystallized in a phase-separated matrix or droplets with more than 25% of PP, when obvious phase separation occurred. The SEM image revealed that the PP lamella was able to penetrate the PP and LLDPE phase boundary and grow in the LLDPE phase. The above results displayed that the PP dissolved in the LLDPE, and, particularly, when the PP concentration was below 20%, the dissolution was substantial. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 628–639, 2001     

Influence of composition,crystallization conditions and melt phase structure on solid morphology,kinetics of crystallization and thermal behavior of binary polymer/polymer blends

Results of an investigation on the morphology, the crystallization and the thermal behavior of several binary crystallizable blends are reported. The composition, molecular mass and crystallization conditions strongly influence the crystallization and the thermal behavior as well as the overall morphology of crystallizable binary blends. Quantities such as nucleation density (N), radial growth rate (G) of spherulites, overall rate of crystallization (K), and equilibrium melting temperature (T_m) are strongly dependent upon composition, crystallization conditions, and molecular mass of components. The type of dependence is to be related to the physical state of the melt, which, at the crystallization temperature, is in equilibrium with or coexists with the developing solid phase. In the ease of compatible blends such as poly(ethylene oxide)/poly(methyl methacrylate) the depression observed for G and T_m is mainly to be attributed to the diluent effect of the non-crystallizable component. For such a blend it is found that, after crystallization, the non-crystallizable component is trapped in intralamellar regions increasing the distance between adjacent lamellae. Depression of G, in the case of incompatible blends such as isotactic polypropylene/rubbers is mainly accounted for by rejection and deformation of rubber drops. The coexistence during crystallization of different processes such as molecular fractionation and segregation, preferential inclusion or dissolution of molecules with lower molecular mass and/or high degree of steric disorder of the crystallizable component in the phase rich in non-crystallizable component and vice versa may explain some minima observed in the plots of T and T^m, vs. composition in the case of blends semicompatible in the melt. It was found that the addition of a second non-crystallizable component causes drastic variations on some morphological and structural quantities of the semicrystalline matrix (isotactic polypropylene or nylon 6) such as the shape, dimensions, and regularity of spherulites and interspherulite boundary regions and lamella and interlamella thickness. In some cases the formation of new boundary lines connecting occluded particles are also observed. Such phenomena may have great importance on crack propagation and on impact behavior as well as on the tensile mechanical properties of binary blends characterized by a semicrystalline polymer component with a relatively high T^g and a rubber-like component with a lower T^g.     

Thermal analysis of polyamide blends as obtained by reactive melt-extrusion: Influence of blend composition on crystallization and melting behavior

Polyamide blends composed of PA 46 and PA 6I were melt-mixed using a double-screw extruder, and the obtained compounds were subsequently processed by injection molding. The blend materials obtained were found to have undergone transamidation processes during the extrusion and injection-molding operations which alter the crystallization and melting behavior. The variation in thermal behavior is strongly dependent on the change in the average sequence length of the crystallizable component in the copolymer formed; it is affected by the compounding and processing conditions and by the blend composition. The influence of every melt-processing step on the crystallization and melting behavior of the blends was investigated by thermal analysis using extruded, injection-molded, and solution-prepared blends. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 573–580, 1997     

Crystallization,melting behavior,and wettability of poly(ε‐caprolactone) and poly(ε‐caprolactone)/ poly(N‐vinylpyrrolidone) blends

Both wettability and crystallizability control poly(ε‐caprolactone)'s (PCL) further applications as biomaterial. The wettability is an important property that is governed by both chemical composition and surface structure. In this study, we prepared the PCL/poly(N‐vinylpyrrolidone) (PVP) blends via successive in situ polymerization steps aiming for improving the wettability and decreasing crystallizability of PCL. The isothermal crystallization of PCL/PVP at different PVP concentrations was carried out. The equilibrium melting point (T), crystallization rate, and the melting behavior after isothermal crystallization were investigated using differential scanning calorimetry (DSC). The Avrami equation was used to fit the isothermal crystallization. The DSC results showed that PVP had restraining effect on the crystallizability of PCL, and the crystallization rate of PCL decreased clearly with the increase of PVP content in the blends. The X‐ray diffraction analysis (WAXD) results agreed with that. Water absorptivity and contact angle tests showed that the hydrophilic properties were improved with the increasing content of PVP in blends. The coefficient for the water diffusion into PCL/PVP blends showed to be non‐Fickian in character. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Microstructure and crystallization of melt‐mixed poly(ethylene terephthalate)/poly(ethylene isophthalate) blends

Physical blends of poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI), abbreviated PET/PEI (80/20) blends, and of PET and a random poly(ethylene terephthalate‐co‐isophthalate) copolymer containing 40% ethylene isophthalate (PET₆₀I₄₀), abbreviated PET/PET₆₀I₄₀ (50/50) blends, were melt‐mixed at 270°C for different reactive blending times to give a series of copolymers containing 20 mol % of ethylene isophthalic units with different degrees of randomness. ¹³C‐NMR spectroscopy precisely determined the microstructure of the blends. The thermal and mechanical properties of the blends were evaluated by DSC and tensile assays, and the obtained results were compared with those obtained for PET and a statistically random PETI copolymer with the same composition. The microstructure of the blends gradually changed from a physical blend into a block copolymer, and finally into a random copolymer with the advance of transreaction time. The melting temperature and enthalpy of the blends decreased with the progress of melt‐mixing. Isothermal crystallization studies carried out on molten samples revealed the same trend for the crystallization rate. The effect of reaction time on crystallizability was more pronounced in the case of the PET/PET₆₀I₄₀ (50/50) blends. The Young's modulus of the melt‐mixed blends was comparable to that of PET, whereas the maximum tensile stress decreased with respect to that of PET. All blend samples showed a noticeable brittleness. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3076–3086, 2003     

Melting and crystallization behavior of PA1010/TPU blends

PA1010/TPU blends were prepared by melt blending. The melting, crystallization behavior, and isothermal crystallization kinetics were investigated using differential scanning calorimetry (DSC). The results showed that the DSC thermograms of blend samples exhibit double melting peaks. With increasing the TPU content, the position of the double melting peaks shifted to a lower temperature, and the total heat of fusion decreased. With increasing the heating rates, the position of the lower melting peak shifted to a higher temperature, while the position of the higher melting peak shifted to a lower temperature; however, the total heat of fusion remained almost constant. With prolonging the annealing time and increasing the crystallization temperature, the position of the lower melting peak shifted to a higher temperature, while the position of the higher melting peak almost did not change; however, the total heat of fusion increased. The addition of TPU could promote the crystallization of PA1010 but not affect the crystallization mechanism. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 839–844, 2004     

Reactive blends of polyamide 6 with polyester elastomer using coupling agents

In situ compatibilized melt blends of polyamide 6 (PA‐6) with polyester elastomer (PEL) were prepared in a corotating twin‐screw extruder using two types of coupling agent (CA): diglycidyl ether of bisphenol A (DGEBA) and 1,4‐phenylene bis(2‐oxazoline) (PBO). The notched impact strength of PA‐6 and PA‐6/PEL blends increased with the addition of coupling agent, especially DGEBA, and the maximum impact toughening of the blend was obtained with 0.6 mol % DGEBA, the composition of minimum domain size observed from SEM. Viscosities of the untreated blends increased over those of the base resins at low frequencies. Viscosities of both the base resins and the blends increased with the addition of CA, and the effect was much more pronounced with DGEBA, especially for PA‐6 and PA‐6–rich blends. The crystallization temperature (T_c) of PEL increased over 10°C, whereas the T_c of PA‐6 decreased by 2–3°C in the blends. With the addition of coupling agents, the crystallization melting temperature (T_m) and T_c of PA‐6 decreased by up to 5°C with DGEBA, implying that the crystallization of PA‐6 is disturbed by the in situ formed PA‐6–CA–PEL or PA‐6–CA–PA‐6 type copolymer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3966–3973, 2004     

Curie transition and piezoelectricity of the blends of a ferroelectric VDF/TrFE copolymer and PMMA

The Curie transition, even though the conformational change at the Curie transition primarily arises from intermolecular interaction, is highly dependent on the crystallization conditions. A slower cooling rate from the melt during paraelectric crystallization lowers T_c, increases the portion of F_β at the expense of F_α reduction, and produces a more unstable ferroelectric phase. T is rarely dependent upon the amount of PMMA, but T is increased with the PMMA content. PMMA has a favorable action in forming a more stable ferroelectric phase in the P(VDF/TrFE)/PMMA blend and elevating the Curie transition point because of the all-trans sequence conformation of PMMA and a specific intermolecular interaction with P(VDF/TrFE) in the melt state. However, PMMA reduces the total amount of the crystalline phase, the electric response, and the piezoelectricity. © 1993 John Wiley & Sons, Inc.     

Isothermal crystallization behavior and melting characteristics of injection sample of nucleated polypropylene

The isothermal crystallization behavior and melting characteristics of pure polypropylene (PP) and PPs nucleated with a phosphate nucleating agent (A) and a sorbitol derivative (D) have been studied by differential scanning calorimetry (DSC). Compared with pure PP, nucleated PPs show a shorter half‐times of crystallization. Dependence of crystallization rate of nucleated PP on the crystallization temperature is stronger than that of pure PP at the higher crystallization temperature, whereas the opposite results are obtained at the lower crystallization temperature. Addition of nucleating agent shifts the temperature at the deviation from the baseline of DSC melting curve, T, and the temperature at the completion of melting, T, to higher temperatures, indicating that nucleated PPs exhibit a higher perfection of PP crystals. A shoulder peak in the high temperature range of melting peak of nucleated PP and a wider low temperature region in the melting peak of pure PP are observed. Obviously, PP and nucleated PPs form different distribution of crystal perfection in the isothermal crystallization process. According to the half‐time of crystallization, nucleating agent A is more effective than D. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2547–2553, 2000     

Spherulitic crystallization behavior of linear low‐density polyethylene

In this study the crystallization behavior of linear low‐density polyethylenes (LLDPEs) (ethylene‐α‐olefin copolymers) was studied by polarized light microscopy. A modified Hoffman‐Lauritzen (MHL) expression is proposed whereby the equilibrium melting temperature, T (T), is replaced with the melting temperature of the crystal stem is replaced with the maximum possible stem length, T. It successfully describes the crystalline spherulitic growth kinetics for both homogeneous and heterogeneous LLDPEs. In addition to regimes III and II, another regime (IM) was found in the high crystallization temperature range. Linear growth behavior of crystalline spherulites was observed in regime III, and nonlinear growth behavior was found in regimes II and IM. The basal surface free energy can be estimated from the short chain branching polydispersity (SCBP) for LLDPEs with excluded comonomers. Polym. Eng. Sci. 45:74–83, 2005. © 2004 Society of Plastics Engineers.     

Strain-induced crystallization,Part III: Theory

A nucleation theory for strain-induced crystallization is formulated to explain and to predict the effects of molecular strain on crystallization kinetics and crystallite size. Unlike any current theories that have based their formulations on some assumed extended-chain line nuclei or folded-chain crystals, the present theory avoids all assumptions concerning the crystal morphology. It is based on experimental findings which indicate limited crystal growth in the strain direction, following a reciprocal dependence of crystal thickness on supercooling ΔT. (ΔT = T, ? T, where the equilibrium melting temperature, T, is a variable dependent on degree of molecular strain prior to strain-induced crystallization.) It is predicted that the logarithm of the nucleation rate, N^o, is dependent on (T)²/T(ΔT) or T/T(ΔT), and that the critical nucleus thickness l^*o is shown to be proportional to T/ΔT. In addition, expressions are also presented, including examples, to show the dependence of N^o, l^*o and T^o_m on degree of molecular strain, ?, or melt entropy reduction, Δs′. Our analysis predicts that, on comparing a polyethylene crystallized in the presence of strain to one crystallized in the absence of strain at 130°C, an increase in “coil” dimension of less than about 50 percent can bring about a 10⁴ fold increase in heterogeneous nucleation rate, a 30–40 percent reduction in critical nucleus thickness and a 10°C increase in equilibrium melting temperature. These results will be discussed and compared with available experimental evidence.     

Influence of the method of mixing on homogeneity and crystallization kinetics of blends of linear and branched polyethylene

The influence of mixing method—solution and melt mixing—on the homogeneity and crystallization kinetics of a series of blends of single‐site materials of linear polyethylene and ethyl‐branched polyethylene was studied by differential scanning calorimetry. Data obtained for heats of melting and crystallization, melting and crystallization peak temperatures, and melting and crystallization temperature profiles were essentially the same for the samples obtained by the two mixing methods. The results obtained can be interpreted as indicating that melt mixing is capable of producing homogeneous melts of these relatively low molar mass polymers, given that solution mixing is considered to give perfectly homogeneous blends. The heat associated with the high temperature melting peak after crystallization at 125°C of the blend samples, obtained by the two preparation methods, was higher than that of the linear polyethylene included in the blends, suggesting that a part of the branched polyethylene crystallized at 125°C. The unblended branched polyethylene showed no crystallization at 125°C. Samples obtained by powder mixing showed independent crystallization and melting of the linear and branched polyethylene components. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1730–1736, 2004     

The effect of poly(vinyl alcohol) hydrolysis on the properties of its blends with nylon 6

An investigation of the properties of the blends of nylon 6 (PA6) and poly(vinyl alcohol) (PVA) with varying degrees of hydrolysis was conducted. A near disappearance of the characteristics of the α‐form PVA crystals, crystallization exotherms, and hydrogen‐bonded hydroxyl groups and the tensile properties originally associated with the PVA molecules of PA6_xPVA PA6_xPVA, and PA6_xPVA specimens was observed as the PVA contents of the specimens became equal to or less than their corresponding ‘‘critical’’ values at 16.7 wt%, 33.3 wt%, and 50 wt%, respectively. These results support the idea that PVA molecules are miscible with PA6 molecules to some extent at the molecular level as the PVA contents of the blends become equal to or less than their corresponding critical PVA contents. In this article, we have proposed the possible reasons accounting for these properties of the PA6_xPVA_y series specimens with a varying degree of hydrolysis. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Influence of interchange reactions on the crystallization and melting behavior of nylon 6,6 blended with other nylons

The thermal behavior of blends of nylon 6,6, with an amorphous polyamide, Trogamid-T, and a semicrystalline polyamide, nylon 6,12, was studied. The blends were prepared both by solution blending and by melt blending, using a Maxwell extruder and a twin screw extruder. The concentration of the blends ranged from 75% to 95% by weight of nylon 6,6. Annealing the blend samples in the molten state in a differential scanning calorimeter (DSC) produced changes in the melting and crystallization behavior. This was attributed to transamidation reactions occurring between the blend components, leading to the formation of in-situ block copolymers. The length of the blocks decreased with annealing time, as suggested by reduced melting (T_m) and crystallization temperatures (T_c) and heat of fusion values. The changes in thermal behavior were dependent on the blending method, additive concentration, presence or absence of a catalyst, melt annealing time, and the extent of melt mixing. The extent of reaction, measured by the depression in equilibrium melting temperature, was linear with respect to the annealing time. The Trogamid-T containing blends appeared to be “nearly miscible” while those with nylon 6,12 were initially immiscible. The glass transition temperature (T_g) vs. the composition curve of the nylon 6,6/Trogamid-T blends showed a positive deviation from linear additivity, with the single T_g decreasing as a function of annealing time in the melt.     

Effects of maleated syndiotactic polystyrene on the morphology,mechanical properties,and crystallization behavior of syndiotactic polystyrene/polyamide 6 blends

The compatibilization of syndiotactic polystyrene (sPS)/polyamide 6 (PA‐6) blends with maleic anhydride grafted syndiotactic polystyrene (sPS‐g‐MA) as a reactive compatibilizer was investigated. The sPS/PA‐6 blends were in situ compatibilized by a reaction between the maleic anhydride (MA) of sPS‐g‐MA and the amine end group of PA‐6. The occurrence of the chemical reaction was substantiated by the disappearance of a characteristic MA peak from the Fourier transform infrared spectrum. Morphology observations showed that the size of the dispersed PA‐6 domains was significantly reduced and that the interfacial adhesion was much improved by the addition of sPS‐g‐MA. As a result of reactive compatibilization, the impact strengths of the sPS/PA‐6 blends increased with an increase in the sPS‐g‐MA content. The crystallization behaviors of the blends were affected by the compatibilization effect of sPS‐g‐MA. A single melting peak of sPS in the noncompatibilized blend was gradually split into two peaks as the amount of the compatibilizer increased. A single crystallization peak of PA‐6 in the noncompatibilized blend became two peaks with the addition of 3 wt % sPS‐g‐MA. The new peak was a result of the fractionation crystallization. As the amount of sPS‐g‐MA increased, the intensity of the new peak increased, and the original peak nearly disappeared. Finally, the crystallization peak of PA‐6 disappeared with 20 wt % sPS‐g‐MA in the blend. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2502–2506, 2003     

Melting behavior of poly(trimethylene terephthalate)

In this study, the melting behavior of isothermally crystallized polytri‐ methylene terephthalate (PTT) was investigated. Multiple melting behaviors in DSC heating trace were found because two populations of lamellar stacks were formed during primary crystallization and the recrystallization at heating process, respectively. This fact could be also confirmed from the result of optical microscopy observation. The Hoffman–Weeks equation was applied to obtain equilibrium melting temperature (T). The T value of PTT is about 525 K, which is 10 K higher than that reported. Combining the enthalpy of fusion from the DSC result and the degree of crystallinity from WAXD result, the value of the equilibrium‐melting enthalpy ΔH was deduced to be approximately 28.8 kJ mol^?1. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2426–2433, 2002     

Study on the thermal stability of heterogeneous nucleation effect of polypropylene nucleated by different nucleating agents

The thermal stability of the heterogeneous nucleation effect of polypropylene (PP) nucleated with an organic phosphate (A) and two kinds of sorbitol derivatives (B and D) was investigated by DSC multiscanning. For pure PP, the peak temperature of crystallization (T) was little changed with an increasing number of DSC scans, indicating that nucleation of PP is thermally stable. For the PP nucleated with an organic phosphate (PPA), the temperatures at the onset of crystallization (T) and at the completion of crystallization (T); the peak temperature of crystallization (T) and melting (T); and the heat of crystallization (ΔH_c) and fusion (ΔH_m) of PP are higher than those of pure PP and were little influenced with an increasing number of DSC scans. For PP nucleated with the sorbitol derivatives (PPB and PPD), the T, T, T, and T decreased with an increasing the number of scans. These results indicated that the thermal stability of heterogeneous nucleation effect of the nucleating agent A is higher than that of nucleating agents B and D. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1643–1650, 2002     

Isothermal crystallization of metallocene‐based propylene/α‐olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene‐1 copolymers and two propylene/octene‐1 copolymers prepared with metallocene catalyst were investigated. It is found that γ‐modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (T_c) the crystallization rate constant changes more rapidly with T_c and the crystallization half‐time substantially increases. Double melting peaks were also observed at high T_c, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005