Simulation of Gas-Assisted Injection Molding of High-Density Polyethylene: The Role of Rheological Properties and Physical Fields on the Crystalline Morphology

The shear and temperature fields in the filling process during gas penetration of gas-assisted injection molding (GAIM) of two kinds of high-density polyethylene (HDPE) with different molecular weights (M_w) were simulated numerically via Moldflow 6.1. The simulated results indicated that HDPE with higher M_w showed a relatively low shear rate in both the outer and inner zone of GAIM parts as compared to that with lower M_w, while the shear rate of the inner zone of the parts of the two materials were lower than that of the outer zone, which has a lower temperature and shorter time for crystallization. Considering the remarkable difference between their viscosities, a model of the shear thinning and recovering effects on crystallization during the GAIM process is proposed. Through the simulation, along with the analysis of their viscosities and rheological properties, the influence of molecular weight and shear and temperature fields on two special kinds of crystalline morphology (i.e., shish-kebabs and banded spherulites) during GAIM were interpreted.     

An In situ Experimental Study on Solidification Kinetics of Polyolefins: Effect of Cooling Conditions

The solidification kinetics of polyolefins (PO) under three cooling conditions were investigated using an in situ measurement of the temperature decay within the PO resins. The phase-change temperature range of high-density polyethylene (HDPE) was located between 110 and 120°C, and those of low-density polyethylene (LDPE) and polypropylene (PP) were 90–110°C and 100–120°C, respectively. The cooling rate of the liquid-state stage is larger than that of the crystallization stage, primarily owing to the release of the latent heat of crystallization as well as the reduced temperature difference between the sample and cooling medium; they jointly slow down the cooling rate to an extent. The time with respect to phase transformation and its lasting period have close relations to the materials' molecular characteristics (e.g., Mw, MWD, LCB, etc.). Three empirical equations were proposed, and found to be applicable for the cooling analysis of the PO molten materials at relatively low cooling rates prior to crystallization.     

Non-isothermal crystallization kinetics of alkyl-functionalized graphene oxide/high-density polyethylene nanocomposites

Dodecyl amine-functionalized graphene oxide (DA-GO) was obtained via an amidation reaction. The results of X-ray diffraction and Fourier-transform infrared spectroscopy verified that long alkyl chains of DA were successfully grafted on the GO sheets. Transmission electron microscope and scanning electron microscope techniques illustrated that homogeneously dispersed DA-GO/high-density polyethylene (HDPE) nanocomposites were obtained. The effects of DA-GO on the non-isothermal crystallization of HDPE were then investigated by differential scanning calorimetry (DSC) at various cooling rates (2, 5, 10, and 20?°C/min). Significant increase in the onset crystalline temperature (T_o) and the peak crystallization temperature (T_p) of HDPE incorporating DA-GO indicated the strong nucleating ability of DA-GO. The investigation of half-time crystallization time (t_1/2) demonstrated that crystallization rate of HDPE consisting of DA-GO is faster than that of pure HDPE at a given cooling rate. Ozawa, Avrami, and the combined Avrami–Ozawa methods (Mo) were used for analyzing experimental data. The Mo approach was successful in describing the non-isothermal crystallization process of DA-GO/HDPE nanocomposites. The results indicated that low DA-GO content accelerates the crystallization of HDPE, while higher content hinders the crystallization of HDPE.     

Nonisothermal Crystallization Kinetics of Miscible Blends of Polycaprolactone and Crosslinked Carboxylated Polyester Resin

The nonisothermal crystallization process of polycaprolactone (PCL)/crosslinked carboxylated polyester resin (CPER) blends has been investigated for different blend concentrations by differential scanning calorimetry (DSC). The DSC measurements were carried out under different cooling rates namely: 1, 3, 5, 10, and 20°C/min. Thermally induced crosslinking of CPER in the blends was accomplished using triglycidyl isocyanurate as a crosslinking agent at 200°C for 10 min. The cured PCL/CPER blends were transparent above the melting temperature of PCL and only one glass transition temperature, T_g, located in the temperature range between the two T_gs of the pure polymer components, was observed, indicating that PCL and crosslinked CPER are miscible over the entire range of concentration. The nonisothermal crystallization kinetics was analyzed based on different theoretical approaches, including modified Avrami, Ozawa, and combined Avrami–Ozawa methods. All of the different theoretical approaches successfully described the kinetic behavior of the nonisothermal crystallization process of PCL in the blends. In addition, the spherulitic growth rate was evaluated nonisothermally from the spherulitic morphologies at different temperatures using polarized optical microscope during cooling the molten sample. Only one master curve of temperature dependence of crystal growth rate could be constructed for PCL/CPER blends, regardless of different blend concentrations. Furthermore, the activation energy of nonisothermal crystallization process (ΔE_a) was calculated as a function of blend concentration based on the Kissinger equation. The value of ΔE_a was found to be concentration dependent, i.e., increasing from 83 kJ/mol for pure PCL to 115 and 119 kJ/mol for 75 and 50 wt% PCL, respectively. This finding suggested that CPER could significantly restrict the dynamics of the PCL chain segments, thereby inhibit the crystallization process and consequently elevate the ΔE_a.     

Co-crystallization of Blends of High-density Polyethylene with Linear Low-density Polyethylene: An Investigation with Successive Self-nucleation and Annealing (SSA) Technique

Two kinds of polyethylenes, high-density polyethylene (HDPE) with few chain branches and short-chain branched linear low-density polyethylene (LLDPE) with a relatively larger average molecular weight, were melt blended together in various mass ratios based on consideration of their practical applications. After identifying the good compatibility of the blends, their crystallization behaviors were studied by the successive self-nucleation and annealing (SSA) technique. The SSA analysis showed that not merely the number of melting fractions in the SSA curves changed with the blend composition, but also the content of the first two melting fractions at high temperature of SSA curves showed a positive deviation and a negative deviation with the blend composition, respectively. These phenomena, as well as the interesting appearance of a stepped increase of the lamellar thickness of each fraction with the highest temperature in each sample, indicated that co-crystallization occurred between HDPE and LLDPE. The results from wide-angle X-ray diffraction (WAXD) supported the conjecture obtained by the SSA analysis.     

Nonisothermal Crystallization Kinetics of Poly(ϵ-Caprolactone)/Montmorillonite Nanocomposites

Montmorillonites modified by hydroxyethylhexadecyldimethyl ammonium bromine were used to prepare poly(?-caprolactone) (PCL)/montmorillonite (MMT) nanocomposites by in situ ring-opening polymerization of ?-caprolactone. Wide-angle X-ray diffraction (WAXD) analysis illustrated that an exfoliated structure of PCL/MMT nanocomposite was obtained. The nonisothermal crystallization kinetics of poly(?-caprolactone) and PCL/MMT nanocomposite was investigated by differential scanning calorimetry (DSC) at various cooling rates. The values of half-time of crystallization (t_1/2) and crystallization rate constant (Z_c) showed that crystallization rate increased with the increase of cooling rates for both PCL and PCL/MMT nanocomposite; however, the crystallization rate of PCL/MMT nanocomposite was faster than that of PCL at a given cooling rate.     

Influence of High-Density Polyethylene and Nano-Calcium Carbonate of Various Content Ratios on the Crystallization of Polypropylene

The influence of high-density polyethylene (HDPE) and nano-CaCO₃ of various content ratios on the crystallization of polypropylene (PP) was investigated by differential scanning calorimetry, dynamic rheology, wide angle X-ray diffraction (WAXD), and Izod impact strength measurements. The results showed that HDPE and PP were phase separated in their blends and the additive CaCO₃ filler mainly dispersed in the PP phase, acting as a nucleation agent to promote the crystallization of PP. For the samples HDPE/ nano-CaCO₃ 30/0 and 25/5, the β crystals content was much higher than the other samples. The reason is that the viscosity difference between HDPE and PP led to a velocity difference, which could induce shear stress at the interfaces of HDPE and PP during injection molding. The intensive shear stress at their phase interfaces is advantageous for orientation of the chains, inducing the formation of β crystals. However, with the increment of CaCO₃ content, there were dual effects of CaCO₃ on the crystallization of PP: at low CaCO₃ content, it would hamper the orientation of PP chains, thus leading to a decrease of β crystals; at high CaCO₃ content, it would induce β crystals by itself.     

Nonisothermal Crystallization of Recycled Poly(Ethylene Terephthalate)/Poly(Ethylene Octene) Blends

Recycled poly(ethylene terephthalate) (r-PET) was blended with poly(ethylene octene) (POE) and glycidyl methacrylate grafted poly(ethylene octene) (mPOE). The nonisothermal crystallization behavior of r-PET, r-PET/POE, and r-PET/mPOE blends was investigated using differential scanning calorimetry (DSC). The crystallization peak temperatures (T _p ) of the r-PET/POE and r-PET/mPOE blends were higher than that of r-PET at various cooling rates. Furthermore, the half-time for crystallization (t _1/2 ) decreased in the r-PET/POE and r-PET/mPOE blends, implying the nucleating role of POE and mPOE. The mPOE had lower nucleation activity than POE because the in situ formed copolymer PET-g-POE in the PET/mPOE blend restricted the movement of PET chains. Non-isothermal crystallization kinetics analysis was carried out based on the modified Avrami equation, the Ozawa equation, and the Mo method. It was found that the Mo method provided a better fit for the experimental data for all samples. The effective energy barriers for nonisothermal crystallization of r-PET and its blends were determined by the Kissinger method.     

Study of the Nonisothermal Crystallization Kinetics and Melting Behaviors of Polypropylene Reinforced with Single-Walled Carbon Nanotubes Nanocomposite

Nonisothermal crystallization kinetics of polypropylene (PP) nanocomposite reinforced with 0.5 wt. % single-walled carbon nanotubes (SWNT) was characterized by differential scanning calorimetry at five different cooling and heating rates. The Avrami, Ozawa, and Seo-Kim kinetic models were used to describe the nonisothermal crystallization of the polymer and its nanocomposite. The addition of nano-filler, in general, improved the crystallization rate and increased the peak crystallization temperature of the nanocomposite as compared to PP. The results show that the Avrami and Seo-Kim models are suitable under different cooling rate conditions but that the Ozawa model is inappropriate for the nanocomposite. Equilibrium melting temperatures, derived from the linear Hoffman-Weeks equation, were shown to decrease in the nanocomposite. Additional analysis was performed based on the Thomson-Gibbs, Lauritzen-Hoffman, and Dobreva-Gutzowa theories, which were applied to take into account the lamellar thickness, nucleating agent, and nucleating activity of the nanocomposite in the nonisothermal melt crystallization process.     

EFFECT OF CROSS-LINKING OF HIGH-DENSITY POLYETHYLENE. I. ON SPHERULITIC STRUCTURES

Crystallization behavior and spherulitic structure of linear high-density polyethylene (HDPE), after being irradiated in its molten state by γ-rays, was investigated by small-angle laser scattering (SALS) and differential scanning calorimetry (DSC). Significant changes in the crystallization of HDPE during cooling in air before and after being irradiated in the melt were observed. A critical minimum average molar mass between cross-links (200 carbon-carbon bonds) for spherulite formation in such an irradiated HDPE network was obtained.     

Thermorheological Properties and Thermal Stability of Polyethylene/Wood Composites

The thermal stability, flame retardancy, thermorheological, and mechanical properties of polyethylene/wood flour (PE/WF) composites were characterized. By time–temperature superposition treatment, addition of WF was found to lead to a complexity in the thermorheological behaviors in low-density PE/wood composites. However, high-density PE/wood counterparts showed no obvious thermorheological complexity. The effects of WF and ammonium polyphosphate contents on the thermorheological behavior and thermal stability were also studied. The current work should be of practical significance for the optimization of wood/plastic composite) formulae, as well as for further investigations on correlations between processing and performance of polymer composites.     

Study on the Nonisothermal Melt Crystallization Kinetics of DBS/PBT Blends

The modified Avrami, Mo, and Kissinger models were applied to investigate the nonisothermal melt crystallization process of dibenzylidene sorbitol (DBS)/poly(butylene terephthalate) (PBT) blends by differential scanning colorimetry (DSC) measurements. The modified Avrami model can describe the nonisothermal melt crystallization processes of DBS/PBT blends fairly well. The cooling rates and the blend composition affect the crystallization of the blends according to Mo crystallization kinetics parameters. The Mo model shows that F(T) increases with increasing crystallinity, indicating that the needed cooling rate when it reached a certain crystallinity increased in unit time, the crystallization rate of DBS/PBT blends is faster than the crystallization rate of pure PBT, and the crystallization rate of the DBS/PBT blends with 0.5% DBS is fastest. The Kissinger model showed that the crystallization activation energy of DBS/PBT blends is lower than the activation energy of pure PBT; the crystallization activation energy of the DBS/PBT blends with 0.5% DBS is the lowest.     

Crystallization Kinetics of Nucleated Polypropylene with Organic Phosphates

The crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP with two organic phosphates, sodium salt (NA7) and triglyceride ester (NA8) of 2,2'-methylene-bis(4,6-di-tert-butylphenyl) phosphoric acid, were investigated by means of a differential scanning calorimeter under isothermal and nonisothermal conditions. During isothermal crystallization, a modified Avrami equation was used to describe the crystallization kinetics. Moreover, kinetics parameters, such as the Avrami exponent, n, the crystallization rate constant, k, and the half-time of crystallization, τ_1/2, are compared. The results showed that a dramatic decrease of the half-time of crystallization, as well as a significant increase of the overall crystallization rate, were observed in the presence of the organic phosphates. During nonisothermal crystallization, the primary crystallization was analyzed using the Ozawa model, leading to similar Avrami exponents for iPP and iPP/NA7, which means simultaneous nucleation with three-dimensional spherulitic growth. However, for iPP/NA8, the Avrami exponent in nonisothermal crystallization is evidently different from that in isothermal crystallization, which would indicate a different mechanism of crystal growth. Adding the nucleating agent to iPP makes the overall crystallization activation energy increase.     

CHANGES IN MELTING BEHAVIOR AND MORPHOLOGY ON ANNEALING OF LINEAR AND BRANCHED POLYETHYLENE

Changes in morphology and melting behavior of various types of commercial polyethylenes as a result of annealing were studied using differential scanning calorimetry, transmission electron microscopy, and density measurements. The range of polyethylenes whose densities varied between 0.96 and 0.90 g/cm³ included linear polyethylene (LPE), high-density polyethylene, a 1-octene copolymer traditional linear low density polyethylene, low density polyethylene, and a 1-octene copolymer prepared by Dow's INSITE constrained geometry catalysts technology. Two sets of samples were initially prepared by fast cooling and slow cooling from the melt. Despite an initial lower crystalline content and crystal thickness for the fast cooled (FC) branched polyethylene samples, a higher melting temperature than for the slow cooled (SC) samples was found using a 10°C/min heating rate. In concordance with a recent work, melting–recrystallization processes are held responsible for the anomalous behavior. The annealing treatment consisted of heating the two sets of samples at 1°C/min from room temperature to a temperature located at the start of the endotherm. The thermal treatment stabilizes the crystals through an increase in their thickness, which prevents melting–recrystallization processes from taking place on subsequent heating. A lower melting temperature after annealing was observed for the FC branched polyethylene samples. No such behavior was found for the SC samples and even for the FC LPE sample.     

Characteristics of polyethylenes at low temperature studied by positron annihilation

The positron annihilation lifetime method (PAL) has been applied to study the characteristics of polyethylenes (PEs) at low temperatures between 273 and 100 K. It has been found that the intensity (I ₃) of the long-lived component of positronium (Ps) showed an increase in non-annealed PE and annealed PE in vacuo. However, PAL in PEs annealed in an atmosphere containing oxygen did not show such an increase. It has been indicated that the latter effect is due to formation of carbonyl groups through oxidation during the annealing.     

Non-Isothermal Crystallization Kinetics of Polyamide 6/h-Boron Nitride Composites

Hexagonal boron nitride nanosheets were mixed with polyamide 6 to fabricate polymer-based composites by using a solution blending method. The nonisothermal crystallization behaviors of the as-prepared composites were investigated via differential scanning calorimetry. Results showed that the peak temperature of the exothermic crystallization curve was moved to lower temperature with increase in the cooling rate. At the same cooling rate, the peak temperature of the pure polyamide 6 was lower than those of the composites. Moreover, the crystallization rate increased gradually with increase in the cooling rate. In addition, at the cooling rates of 5 or 10°C min^?1, the crystallization rate of pure polyamide 6 was higher than those of all compositions of the composites. However, at the cooling rates of 20 or 40°C min^?1, the crystallization rate decreased first and then increased with increase in the fillers loading. The crystallization mechanism was between one-dimensional and two-dimensional during the crystallization process.     

A comparison of the dynamic mechanical relaxation behavior of linear low- and high-density polyethylenes

The mechanical relaxation behaviors of oriented samples of a linear low-density polyethylene (LLDPE) and a high-density polyethylene (HDPE) were studied using tensile dynamic mechanical measurements. The anisotropy and activation energies of the relaxations were determined for several different samples to investigate why the α-relaxation in HDPE has characteristics similar in some respects to both the α- and β-relaxations in LLDPE. It is concluded that the anisotropy of the α-relaxation in LLDPE is determined by c-shear, whereas in HDPE it relates to interlamellar shear. The activation energy measurements, however, show that the thermally activated process for the α-relaxation in both the LLDPE and HDPE is c-shear. It is proposed that, in HDPE, c-shear has to occur before there is enough mobility at the fold surface for interlamellar shear. It is concluded that the β-relaxation is not observed in HDPE because the interlamellar regions are too constrained to allow interlamellar shear without c-shear.     

Branching in anionic polyacrylonitriles

The morphological and hydrodynamic properties of three series of anionic polyacrylonitrile fractions (10⁵<M _w< 5 × 10⁶) have been studied in dilute dimethylformamide (DMF) solution. All the observed values of the molecular parameters (M _w, S ² _w A₂, [η], S) are consistent with a trifunctional randomly branched structure characterized by a decrease of branching density with molecular weight. The sample obtained in a strongly dipolar solvent (DMF, ?70°C, diphenylmethylsodium) shows a unimodal molecular weight distribution and higher branching densities (2.5 × 10^?2 to 10^?1); on the other hand, the sample obtained in a nonpolar solvent (toluene, ?70°C, n-butyllithium) shows a binodal molecular weight distribution and smaller branching densities (10^?4 to 1.4 × 10^?3). These characteristic branched structures may be explained by transfer to polymer as the main branching process, taking into account the polarity of the medium and its thermodynamic quality for the polymer.     

Crystallization Kinetics for the Split Dual Phase Model of the Amorphous State of Polymers

We analyze constant rate cooling and heating crystallization kinetics of PET samples by DSC. The samples have various degrees of disentanglement, obtained by Rheo-Fluidification. According to the interactive Split Dual Phase model, the amorphous state is made up of two coupled and interactive amorphous phases. These two phases have distinct viscoelastic and thermodynamic characteristics (T_g, free volume, G′ and G^″, etc.), which are determined by the potential energy of the conformers and by the state of entanglement of the macromolecular coils. Semi-crystalline polymers, such as polyethylene terephthalate (PET), are amorphous in the molten state and should have Dual Phase behavior. The phase duality should manifest itself during crystallization from the melt during cooling, or during cold crystallization while heating quenched samples. The purpose of this communication is to quantitatively describe the kinetics of crystallization of PET samples with a dual phase kinetics formulation, and determine the respective influence of molecular weight and degree of entanglement on the kinetics parameters, rate of crystallization and percentage of crystallinity.