Primary nucleation of polyethylene: Embryogenesis from a semidilute solution

Using “realistic” molecular dynamics simulation extended up to 100 ns, we have investigated the evolution of cluster size, intrachain vs. interchain potential energies and pair correlations of polyethylene (PE) in a semidilute (ca. 28 wt%) 1,2,4-trichlorobenzene solution at 300 K. Results indicate that the embryonic development begins with the aggregation of trans-rich sequences of characteristic length l_o ≈ 2 nm, forming clusters of short stems. This is immediately followed by reorganization/thickening via intracluster axial translation and reeling-in of segments from the surrounding matrix in dynamic competition with neighboring embryos. Up to this stage, the embryonic clusters are loosely packed, retaining largely the conformer populations in the solution state but with gauche conformers enriched in the loose fold loops. After reaching a critical size with l^∗ ≈ 4 nm, the intracluster order starts to significantly improve via a “solidification” process with sigmoidal decreases of valence and nonbonding energies, while axial diffusion dramatically slows down and intracluster torsions become fully adjusted to trans conformation by annihilation of gauche conformers. In these “solidified” embryos, although molecular packing remains deviated from the orthorhombic structure (as reflected in significant differences in pair correlations) while reminiscent of the mesomorphic “rotator” or hexagonal phase, the decrease in potential energy is already significant (corresponding to about half of the heat of crystallization) as the intrachain valence contribution is fully realized.     

Structural formation and gelation behavior of cold-crystallized poly(trimethylene terephthalate)

Experimentally observed a +-type H_v and a circular-type V_v scattering patterns from small-angle light scattering (SALS) measurement indicated a random assembly of sheaf-like or rod-like superstructure for poly(trimethylene terephthalate) (PTT) under cold-crystallization. The long period and crystallite thickness of cold-crystallized PTT have been determined by small-angle X-ray scattering (SAXS). The crystallite thickness is very small, ca 2 nm, which is close to the c-axis (chain axis) of unit cell in PTT crystal, 18.64 Å. The intermolecular crystallization may be viewed as a behavior of physical gelation. The liquid-solid transition in PTT crystallization from glassy state through the gel point has been investigated by dynamic mechanical experiments. The power law relaxation modulus, E(t)=St^−Δ identifies the gel point, and small frequency window is sufficient to determine the relaxation exponent, Δ. The gel point occurs at the very early stage of crystallization, in which the relative degree of crystallinity is found to be about 2-3%. The Δ value is ca 0.52, which is independent of crystallization temperature.     

Phase Morphology and Thermal Characteristics of Binary Blends Based on PTT and PA12

Summary Phase morphology, miscibility and thermal properties of a binary blend, poly- (trimethylene terephthalate) (PTT) / polyamide-12 (PA12), were examined and found to be immiscible showing a phase-separated morphology. DSC cooling data of the blends indicates that the melt crystallinity of PTT phase increases, with respect to pure state, upon blending with PA12 while that of polyamide phase declines in the presence of PTT phase implying that polyester molecules reduces the crystallization ability of polyamide phase. Also, PA12 in that blend where it is minor phase shows a fractionated crystallization. A significant cold crystallization exotherm supposed to be due to the reorganization of polymeric chains was exhibited by PTT phase in DSC second heating thermograms which its degree was raised upon blending with PA12. DMA graphs reveal a single tan peak due to the fact that the glass transition of the respective constituents overlap and a single peak comes into view. Therefore, DMA can not be solely used to assess the miscibility in this system.     

Morphological features and crystallization behavior of the conductive composites of poly(trimethylene terephthalate)/graphene nanosheets

Poly(trimethylene terephthalate) (PTT) composites filled with well‐dispersed graphene nanosheets (GNSs) were prepared through a coagulation method. The effects of increased GNS concentration on variations in the structure and properties of the PTT matrix, such as its electrical conductivity, crystallization kinetics, melting behavior, and crystal morphology, were investigated. Several analytical techniques were used, including electrical conductivity measurement, differential scanning calorimetry, Fourier transform infrared spectroscopy, wide‐angle X‐ray diffraction, polarized light microscopy, transmission electron microscopy (TEM), and thermo‐gravimetric analysis (TGA). Electrical conductivity increased from 1.8 × 10^?17 S/cm for neat PTT to 0.33 ± 0.23 S/cm for PTT/GNS composites with 2.97 vol % GNS content. Percolation scaling laws were applied, and then threshold concentration and exponent were determined. In the case wherein liquid nitrogen was used to quench the melt, a mesomorphic phase was formed despite the extremely short crystallization time after adding high GNS contents. PTT crystallization rate increased with the gradual addition of GNSs. The enhanced crystallization kinetics was attributed to the high nucleation ability of GNSs to induce epitaxially grown lamellae on their surfaces, as revealed by TEM. PTT nuclei were randomly developed on the GNS surface to form the lamellae. However, crystallinity reached its maximum value near the electrical percolation threshold because the PTT chain mobility was confined after the GNS–GNS network formed. The growth of PTT banded spherulites in the bulk was still observed for composites with high GNS content, and TGA results revealed that the GNS‐filled PTT composites had excellent thermal stability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43419.     

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     

Pulsed magnetic field as a new technique to control the crystallization and orientation of poly(trimethylene terephthalate)

Magnetic influence on the crystallization and orientation of poly(trimethylene terephthalate) (PTT) were studied by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), and X-ray diffraction (XRD) during isothermal crystallization at 195 °C under pulsed magnet. The results showed that the crystallization properties of PTT are changed under the influence of pulsed magnetic field. It should be emphasized that the pulsed magnetic field can be applied as a new method to control the crystallization and orientation of polymers.     

Melting and crystallization behaviors of compatibilized poly(trimethylene terephthalate)/acrylonitrile–butadiene–styrene blends

The melting and crystallization behaviors of poly(trimethylene terephthalate) (PTT)/acrylonitrile–butadiene–styrene (ABS) blends were investigated with and without epoxy or styrene–butadiene–maleic anhydride copolymer (SBM) as a reactive compatibilizer. The existence of two separate composition-dependent glass-transition temperatures (T_g's) indicated that PTT was partially miscible with ABS over the entire composition range. The melting temperature of the PTT phase in the blends was also composition dependent and shifted to lower temperatures with increasing ABS content. Both the cold crystallization temperature and T_g of the PTT phase moved to higher temperatures in the presence of compatibilizers, which indicated their compatibilization effects on the blends. A crystallization exotherm of the PTT phase was noticed for all of the PTT/ABS blends. The crystallization behaviors were completely different at low and high ABS contents. When ABS was 0–50 wt %, the crystallization process of PTT shifted slightly to higher temperatures as the ABS content was increased. When ABS was 60 wt % or greater, PTT showed fractionated crystallization. The effects of both the epoxy and SBM compatibilizers on the crystallization of PTT were content dependent. At a lower contents of 1–3 wt % epoxy or 1 wt % SBM, the crystallization was retarded, whereas at a higher content of 5 wt %, the crystallization was accelerated. The crystallization kinetics were analyzed with a modified Avrami equation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Crystallization and melting behaviors of poly(trimethylene terephthalate)

The crystallization and melting behaviors of poly(trimethylene terephthalate) (PTT) have been studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and solid-state NMR. At certain crystallization temperatures (T_c) for a given time, the isothermally crystallized PTT exhibits two melting endotherms, which is similar to that of PET and PBT. At higher crystallization temperature (T_c = 210 °C), the low-temperature endotherm is related to the melting of the original crystals, while the high-temperature endotherm is associated with the melting of crystals recrystallized during the heating. The peak temperatures of these double-melting endotherms depend on crystallization temperature, crystallization time, and cooling rate from the melt as well as the subsequent heating rate. At a low cooling rate (0.2 °C/min) or a high heating rate (40 °C/min), these two endotherms tend to coalesce into a single endotherm, which is considered as complete melting without reorganization. WAXD results confirm that only one crystal structure exists in the PTT sample regardless of the crystallization conditions even with the appearance of double melting endotherms. The results of NMR reveal that the annealing treatment increases proton spin lattice relaxation time in the rotation frame, T_1ρ ^H, of the PTT. This phenomenon suggests that the mobility of the PTT molecules decreases after the annealing process. The equilibrium melting temperature (T _m ^o ) determined by the Hoffman-Weeks plot is 248 °C.     

Crystallization kinetics of poly(trimethylene terephthalate)

The bulk isothermal crystallization kinetics of poly(trimethylene terephthalate) (PTT) was studied using a differential scanning calorimeter. Avrami's theory was used to analyze the data. Based on crystallinity growth rate, Avrami rate constant, K, and crystallization half‐time, PTT's crystallization rate is between those of poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) when compared at the same degree of undercooling. PBT has the highest crystallization rate with K in the order of 10^?2 to 10^?1 min^?n. It is about an order of magnitude faster than PTT at 10^?3 to 10^?2 min^?n, which in turn is an order of magnitude faster than PET with K of 10^?4 to 10^?2 min^?n. Contrary to previous reports (PTT was not included in the study) that aromatic polyesters with odd numbers of methylene units were more difficult to crystallize than the even‐numbered polyesters, PTT did not fit in the prediction and did not follow the odd‐even effect.     

Isothermal‐crystallization kinetics and melting behavior of crystalline/crystalline blends of poly(trimethylene terephthalate) and poly(ethylene 2,6‐naphthalate)

The isothermal crystallization and crystal morphology of poly(trimethylene terephthalate) (PTT)/poly (ethylene 2,6‐naphthalate) (PEN) blends were investigated with differential scanning calorimetry and polarized optical microscopy. The commonly used Avrami equation was used to fit the primary stage of isothermal crystallization. The Avrami exponents were evaluated to be in the range of 3.0–3.3 for isothermal crystallization. The subsequent melting endotherms of the blends after isothermal crystallization showed multiple melting peaks. The crystallization activation energies of the blends with 20 or 40% PTT was ?48.3 and ?60.9 kJ/mol, respectively, as calculated by the Arrhenius formula for the isothermal‐crystallization processes. The Hoffman–Lauritzen theory was also employed to fit the process of isothermal crystallization, and the kinetic parameters of the blends with 20 or 40% PTT were determined to be 1.5 × 10⁵ and 1.8 × 10⁵ K², respectively. The spherulite morphology of the six binary blends formed at 190°C showed different sizes and perfect Maltese crosses when the PTT or PEN component was varied, suggesting that the greater the PTT content was, the larger or more perfect the crystallites were that formed in the binary blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3316–3325, 2007     

Characterization and crystallization behavior of poly(ethylene‐co‐trimethylene terephthalate) copolymers

A series of poly(ethylene‐co‐trimethylene terephthalate) (PETT) copolymers were prepared by polycondensation. The synthesized PETT are block copolymers and the content of poly(trimethylene terephthalate) (PTT) units incorporated into the copolymers are always larger than that fed in the polymerization. The nonisothermal crystallization at the different cooling rates was studied by means of differential scanning calorimetry. The copolymers develop the crystallization later and show the lower melting temperature than the corresponding enriched homopolymers. The modified Avrami analysis fit well the nonisothermal crystallization of these polymers. The overall rate of crystallization of PTT is fastest and that of PET is slowest, whereas the copolymers are between them at the same cooling rate. The minor PET units incorporated into PTT polymer chains reduce the crystallization of PTT segments, but the present minor PTT units in the PET chains seem to accelerate the crystallization of PET segments. The Avrami exponent nvaries in the range of 3 – 4, indicating that the nonisothermal crystallization follows the homogeneous nucleation and two‐ to three‐dimensional growth mechanism. Wide angle X‐ray diffraction analysis explains that the PET and PTT units do not cocrystallize and it is considered as the enriched polymer segments to crystallize during crystallization. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Block phosphorus-containing poly(trimethylene terephthalate) copolyester via solid-state polymerization: Reaction kinetics and sequential distribution

Phosphorus-containing flame-retardant monomer entitled 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10-phospha-phenanthrene-10-oxide (DDP) was first esterified with 1,3-propane diol, then incorporated in poly(trimethylene terephthalate) (PTT) chain via solid-state polymerization (SSP) at 200 °C. Reaction kinetics of incorporation was studied choosing 30 wt% DDP content as representative example. The reaction rate constant k^f was calculated to be 1.68 h^?1. The intrinsic viscosity increased with the increase of t_ssp, and decreased with the increase of DDP content. The sequence distribution of resulted copolyester was also analyzed with ¹H NMR. Results suggested that all of the samples possess block chemical structure, and the degree of randomness increased with the increase of transesterification. For the transesterification reaction occurs only in amorphous phase of the material, the copolyester exhibited greater tendency to form block constitution when increasing DDP content. DSC investigation further confirmed the non-random constitution of the resulted copolyester.     

Long terminal linear alkyl group as internal crystallization accelerating moiety of poly(l-lactide)

Poly(l-lactide) (PLLA) polymers having terminal n-alkyl groups with a wide variety of lengths (C₀–C₂₂) were synthesized by ring-opening polymerization of l-lactide in the presence of coinitiators of l-lactic acid (C₀), 1-hexanol (C₆), 1-dodecanol (C₁₂), and 1-docosanol (C₂₂) and their segmental mobility and non-isothermal and isothermal crystallization behavior were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffractometry (WAXD). Glass transition and cold crystallization temperatures of melt-quenched samples during heating decreased with an increase in the length of terminal n-alkyl groups. The enhanced PLLA segmental mobility and hydrophobic interaction-based accelerated PLLA nucleation by the presence of terminal long n-alkyl groups should have caused the accelerated non-isothermal and isothermal crystallization of PLLA segments traced by cold crystallization temperature during heating and by radial growth rate of spherulites, respectively. The crystallization accelerating effect became higher with the length of terminal n-alkyl groups. The effects of the length of terminal n-alkyl group on the crystalline growth mechanism of PLLA at the lowest crystallizable temperature was insignificant, whereas the effects of the length of terminal n-alkyl group on the nucleation mechanism of PLLA chains were significant and insignificant for PLLA having M_n of 6–7 × 10³ of 2 × 10⁴ g mol⁻¹, respectively. WAXD measurements revealed that the transition crystallization temperature at which crystalline modification changes from δ-form to α-form was affected by the length of terminal n-alkyl group for PLLA having M_n of 6–7 × 10³ g mol⁻¹, but was not altered by the length of terminal n-alkyl group for PLLA having M_n of 2 × 10⁴ g mol⁻¹.     

Melt crystallization of poly(ethylene terephthalate): Comparing addition of graphene vs. carbon nanotubes

Poly(ethylene terephthalate) (PET)-based nanocomposites with graphene or multi-wall carbon nanotubes (MWCNT) were prepared by melt mixing. Aspect ratio, A_f, and interparticle distance, λ, of graphene in the nanocomposites were obtained from melt rheology and transmission electron microscopy respectively. λ of PET/graphene nanocomposites was much smaller than λ in PET/MWCNT. For PET/graphene with highest A_f, λ became <1 μm at more than 0.5 wt% graphene. Non-isothermal crystallization behavior from the melt was investigated by differential scanning calorimetry. The crystallization temperatures suggest that the nucleation effect of graphene was stronger than that of MWCNT. The half crystallization time of PET/graphene became longer than PET/MWCNT with increasing graphene loading, suggesting that confinement by graphene suppressed the crystal growth rate. XRD analysis indicated that smaller crystals formed in PET/graphene than in PET/MWCNT. From Raman spectroscopy, the π–π interaction between PET and graphene was stronger than that between PET and MWCNT. This stronger interaction in PET/graphene appears to result in formation of crystals with higher perfection.     

Toughened poly(trimethylene terephthalate) by blending with a functionalized ethylene–propylene–diene copolymer

A series of blends of poly(trimethylene terephthalate) (PTT) with ethylene–propylene–diene copolymer grafted with maleic anhydride (EPDM‐g‐MA) were prepared in composition by weight 95/5, 90/10, 80/20, and 70/30. Their morphologies, crystallization behavior, and mechanical properties were investigated. Morphology observation shows the well‐dispersed domains of EPDM‐g‐MA in PTT matrix with weight‐average particle size from 0.98 to 3.64 μm when the EPDM‐g‐MA content increases from 5% to 30% (mass fraction) in the blends. The constancy of the crystallinity level indicates that the elastomeric phase does not disturb the crystallization process of PTT. The addition of rubbery EPDM‐g‐MA to PTT matrix increases the notched Izod impact strength, but impairs the tensile strength properties. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Liquid-liquid phase separation and crystallization behavior of poly(ethylene terephthalate)/poly(ether imide) blend

The liquid-liquid (L-L) phase separation and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blend were investigated with optical microscopy, light scattering, and small angle X-ray scattering (SAXS). The thermal analysis showed that the concentration fluctuation between separated phases was controllable by changing the time spent for demixing before crystallization. The L-L phase-separated specimens at 130 °C for various time periods were subjected to a temperature-jump of 180 °C for the isothermal crystallization and then effects of L-L phase separation on crystallization were investigated. The crystal growth rate decreased with increasing L-L phase-separated time (t_s). The slow crystallization for a long t_s implied that the growth path of crystals was highly distorted by the rearrangement of the spinodal domains associated with coarsening. The characteristic morphological parameters at the lamellar level were determined by the correlation function analysis on the SAXS data. The blend had a larger amorphous layer thickness than the pure PET, indicating that PEI molecules in the PET-rich phase were incorporated into the interlamellar regions during crystallization.     

Crystallization and spherulitic growth kinetics of poly(trimethylene terephthalate)/polycarbonate blends

The macroscopic and microscopic melt‐crystallization kinetics of poly(trimethylene terphthalate) (PTT)/polycarbonate (PC) blends have been measured by differential scanning calorimetry (DSC), and optical microscopy (OM). The results are analyzed in terms of the Avrami equation and the Hoffman–Lauritzen crystallization theory (HL model). Blending with PC did not change the crystallization mechanism of PTT, but reduced the crystallization rate compared with that of neat PTT at the same crystallization temperature. The crystallization rate decreased with increasing crystallization temperature. The spherulitic morphology of PTT was influenced apparently by the crystallization temperature and by the addition of PC. X‐ray diffraction shows no change in the unit cell dimension of PTT was observed after blending. Through the HL theory, the classical regime II→III transition was detected for the neat PTT and the blends. The nucleation parameter (K_g), the fold‐surface free energy (σ_e), and the work of chain folding (q) were calculated. Blending with PC decreased all the aforementioned parameters compared with those of neat PTT. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Spherulite morphology and crystallization behavior of poly(trimethylene terephthalate)/poly(ether imide) blends

The spherulite morphology and crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(ether imide) (PEI) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). Thermal analysis showed that PTT and PEI were miscible in the melt over the entire composition range. The addition of PEI depressed the overall crystallization rate of PTT and affected the texture of spherulites but did not alter the mechanism of crystal growth. When a 50/50 blend was melt-crystallized at 180 °C, the highly birefringent spherulite appeared at the early stage of crystallization (t < 20 min). After longer times, the spherulite of a second form was developed, which exhibited lower birefringence. The SALS results suggested that the observed birefringence change along the radial direction of the spherulite was mainly due to an increase in the orientation fluctuation of the growing crystals as the radius of spherulite increased. The lamellar morphological parameters were evaluated by a one-dimensional correlation function analysis. The amorphous layer thickness showed little dependence on the PEI concentration, indicating that the noncrystallizable PEI component resided primarily in the interfibrillar regions of the growing spherulites.     

Effects of crystalline and orientational memory phenomena on the isothermal bulk crystallization and subsequent melting behavior of poly(trimethylene terephthalate)

The effects of crystalline and orientational memory phenomena on the subsequent isothermal crystallization and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated by studying the effect of prior melt‐annealing temperature, T_f, on the subsequent isothermal crystallization kinetics, crystalline structure and subsequent melting behavior of neat and sheared PTT samples. On partial melting, choices of the T_f used to melt the samples played an important role in determining their bulk crystallization rates, in which the bulk crystallization rate parameters studied were all found to decrease monotonically with increasing T_f. The decrease in the values of these rate parameters with T_f continued up to a critical T_f value (ie ca 275 °C for neat PTT samples and ca 280 °C for PTT samples which were sheared at shear rates of 92.1 and 245.6 s^?1). Choices of the T_f used to melt neat PTT samples had no effect on the crystal structure formed. The subsequent melting behavior suggested that the T_f used to melt both neat and sheared samples had no effect on the peak positions of the melting endotherms observed and that the observed peak values of these endotherms for all sample types studied were almost identical. Copyright © 2004 Society of Chemical Industry     

Crystal structure identification of poly(trimethylene 2,6-naphthalate) β-form crystal by X-ray diffraction and molecular modeling

X-ray powder diffraction and molecular modeling are used to identify the crystal structure and chain conformation of poly(trimethylene 2,6-naphthalate) (PTN) β-form crystal. The unit cell of PTN β-form crystal was determined to be a triclinic with dimensions of α=100.85°, β=88.78° and γ=120.63°, and the space group of the crystal is identified as The observed crystal density of 1.37 g cm⁻³ and the determined dimensions of unit cell indicate that the unit cell contains one polymer chain with two repeating units. In the unit cell, each trimethylene unit in PTN backbone is in gauche/gauche conformation and neighboring naphthalene units are in face-to-face type arrangement, forming π-stacks that lead to the lowest energy of the unit cell.