Search

Polymers of higher olefin, obtained by Ziegler-type polymerization, have been used in some critical fields, e.g., as the membrane for extracorporeal membrane oxygenation (ECMO), which plays an important role in the treatment of patients with severe COVID-19. The polymer obtained by a single-site catalyst, e.g., metallocene catalysts, demonstrated a higher performance. The homo- and co-polymerization of allyltrimethylisilane (ATMS) and 4-methyl-1-pentene (4M1P) were conducted using syndiospecific (cat 1) and isospecific (cat 2) metallocene catalysts. Cat 1 showed low conversions and provided a polymer with a higher molecular weight, while cat 2 behaved oppositely. 13C-NMR spectra certified the stereotacticity of the resultant polymer, and the resonance of the carbon atom of CH2 (αα’) between the two tertiary carbon atoms of the ATMS and 4M1P units were observed. This could be the evidence of the formation of a true copolymer. The crystallization of the polymer was explored using a differential scanning calorimeter (DSC) and wide angle X-ray diffraction (WAXD). All homopolymers and some of the copolymers showed high melting temperatures and low melting enthalpies. The WAXD patterns of the syndiotactic polymer and isotactic homopolymer or the ATMS-rich copolymer were consistent with the reported literature, but the isotactic 4M1P-rich copolymer provided the crystal form I, which is unusual for a 4M1P polymer without any pretreatment.

The crystallization of polyethylene (PE) blends is a highly complex process, owing to the significant differences in crystallizability of the various PE components and the varying PE sequence distributions resulting from short- or long-chain branching. In this study, we examined both the resins and their blends through crystallization analysis fractionation (CRYSTAF) to understand the PE sequence distribution and differential scanning calorimetry (DSC) to investigate the non-isothermal crystallization behavior of the bulk materials. Small-angle X-ray scattering (SAXS) was utilized to study the crystal packing structure. The results showed that the PE molecules in the blends crystallize at different rates during cooling, resulting in a complicated crystallization behavior characterized by nucleation, co-crystallization, and fractionation. We compared these behaviors to those of reference immiscible blends and found that the extent of the differences is related to the disparity in crystallizability between components. Furthermore, the lamellar packing of the blends is closely associated with their crystallization behaviors, and the crystalline structure varies significantly depending on the components’ compositions. Specifically, the lamellar packing of the HDPE/LLDPE and HDPE/LDPE blends is similar to that of the HDPE component owing to its strong crystallizability, while the lamellar packing of the LLDPE/LDPE blend is approximately an average of the two neat components.

Herein, we study comparatively melt-memory effects on recrystallization of two sets of linear low-density polyethylenes (LLDPE) synthesized with Ziegler–Natta (ZN-LLDPE) and with metallocene cataly...

Haze (H) is not an accumulative property based on the addition of internal haze (H i) and surface haze (H s) parameters, though the relation (H s = H − H i) is widely accepted and used nowadays in research and development of plastic films. To evaluate the correct relation, the analytical relation between the total haze (H) and the haze of a single film (h i) in the film superposition model was firstly concluded, which is 1 − H is equal to the product of 1 − h i of every single film. This relation was validated by the experimental results using thin films of different kinds of polymers with different hazes. Based on this relation, the expression of surface haze was concluded. When the two surfaces of a film were the same, the haze of the single surface H s ¯ = 1 − ( 1 − H ) / ( 1 − H i ) . $\overline {{H_{\rm{s}}}} = 1 - \sqrt {(1 - H)/(1 - {H_{\rm{i}}})} .$ The mathematical relation between haze and thickness was also obtained.

A new polymer grafting method, which is a solid-phase reaction but its grafting temperature is higher than the polymer melt point, is reported in this paper. This solid grafting reaction at high temperature can be achieved by using microwave which selectively heats the substance that can absorb microwave. Maleic acid-grafted polypropylene (PP-g-MA) was prepared by this new method. It is well known that polypropylene (PP) is transparent to microwave while maleic anhydride (MAH) can be heated by microwave. Under microwave irradiation, MAH dispersed in the micropores of PP spherical powder can absorb microwave to generate high temperature and cause PP nearby and MAH itself to generate free radicals. MAH-grafted PP (PP-g-MAH) can be prepared without using an initiator. The new method has many advantages that traditional methods do not have. For example, the prepared graft polymer has no initiator and MAH residue and thus is odorless; the prepared graft polymer also has excellent mechanical properties because ...

The miscibility, crystalline structure and morphology, as well as, properties of Polyamide 6 (PA6)/polyvinylidene fluoride (PVDF) blends (with up to 50 wt% of PVDF) were studied. Small Angle X-ray Scattering study above the melting point of PVDF shows that the blends have very limited miscibility with approximately 5 wt% of PVDF inclusion in PA6 amorphous lamellar region. SAXS results also indicate a certain degree of PA6 inclusion into PVDF region though quantitative evaluation is unavailable with the limited blends studied in this work. The fine PVDF dispersion in the PA6 blends with up to 50 wt% of PVDF with an average size about 150 nm is supposed to be the result of the partial miscibility. The partial miscibility has great effects on the morphology and properties of the blends. Crystallization of both components are suppressed in the blends, especially on the PVDF component, which shows a significantly lowered crystallization temperature (up to 30 °C) during a cooling and a dramatic drop in crystallinity. Morphological studies together with Small Angle Light Scattering (SALS) show that spherulites, consisting of PA6 skeleton with dispersed PVDF domains (∼150 nm) engulfed among PA6 fibrils, are formed in the blends. The PA6 based spherulites become more defective with the increase in PVDF content, resulting in short/broken fibrils and undetectable PA6 spherulites for the blends with 30–50 wt% of PVDF at the temperature over the melting point of PVDF, say 185 °C. The blends show a significantly improved toughness compared with neat PA6, due to the partial miscibility which leads to the fine dispersion of PVDF domains in PA6 spherulites.

The crystallization, crystalline structure, morphology and properties of poly (butylene succinate-co-butylene terephthalate) random copolymer (PBST) with 50 mol% of PBT, a very promising biodegradable polymer, was studied with a focus on understanding the structure property relationship. PBT spherulites are formed with an average diameter of ~11 μm, also implying a good crystal packing structure at lamellar level. The modulus and hardness of the copolymer are mainly contributed by the PBT crystals. It is known that no PBS crystal structure can be detected by X-ray diffraction; however, based on our FTIR and DSC studies, immature PBS crystals can be formed slowly (in days and months) at room temperature under no strain or quickly when stretched. These PBS aggregates contribute significantly to the mechanical properties of the copolymer, such as, mechanical strength, a good elongation to break and a permanent set at 80%. This biodegradable copolymer with a melting point (132 °C) close to HDPE, a modulus about 59 MPa, an elongation to break at 1300% and a shore D hardness of 42 can have many potential industrial applications.

Recrystallization behavior and the final crystallite orientation of biaxially oriented polyethylene (BOPE) film from different melted states have been studied by using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. WAXD result shows that two kinds of crystallites (1 and 2) with different orientations are aligned around the transverse direction (TD) of the film plane. When BOPE film is annealed in the temperature range of 110~125 °C, the residual crystallites 1 are still aligned along the TD, while the residual crystallites 2 are gradually rotated and aligned along the machine direction (MD). Recrystallization of BOPE film from the partially melted states is enhanced and the maximum crystallization peak shifts to higher temperature by 7 °C. The newly formed crystallites after recrystallization are aligned along the TD and MD, respectively. When annealing temperature attains to 128 °C, a trace amount of residual crystallites 1 persists along the TD. The newly formed crystallites after recrystallization are also aligned the TD. When the storing temperature range is in 130–150 °C, the melt memory effect occurs in BOPE film, and it influences crystallization kinetics and the crystallite orientation.

Synthesis of transparent polypropylene (PP) copolymers with low migration of soluble contents is of great significance due to their extensive use in food and medicine industries. We present here a systematical study on the relationship between the microstructure of PP copolymers and their soluble properties using a solvent gradient fractionation (SGF) method. Two sets of poly(propylene-co-1-butene) (PPBu) and poly(propylene-co-ethylene) (PPEt) samples with comparable comonomer content and molecular weight were first fractionated and characterized by 13C NMR, GPC, DSC, and WAXD. It was found that PPBu contained much less room temperature soluble fraction than PPEt because of the incorporation of 1-butene into higher molecular weight fraction than that of ethylene. The melting temperature of PPBu was found higher than that of PPEt with similar comonomer content because of the isomorphism phenomenon demonstrated by the expansion of the unit cell. The findings in this study suggest that tuning the two structure-related factors, namely, comonomer distribution homogeneity and isomorphism phenomenon, plays a key role to maintain low soluble contents of transparent PP copolymers.

Summary: The micro-structure evolution of isotactic polypropylene-1-butene (iPPBu) and polypropylene-ethylene (iPPEt) random copolymers with 4 mol% of 1-butene and ethylene was respectively investigated by differential scanning calorimetry (DSC) and in-situ small angle X-ray scattering (SAXS) techniques during heating process. The signal of melting enthalpy of iPPBu disappears a little earlier than that of iPPEt, which keeps consistent with the decay trend of scattering intensity during the late period of melting process. However, the SAXS data further show that the crystal thickness of iPPBu is a little larger than that of iPPEt during heating process. It is suggested that the melting behaviors of such copolymers depend on not only the lamellar thickness but also the crystal stability.

Summary: A series of polyethylene (PE) blends consisting of a high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with a butene-chain branch density of 77/1000 carbon was prepared at different concentrations. The LLDPE only crystallized below 50 °C, therefore, above 80 °C and below the melting temperature of HDPE, only HDPE crystallized in the PE blends. A specifically designed multi-step experimental procedure based on thermal analysis technique was utilized to monitor the liquid–liquid phase separation (LLPS) of this set of PE blends. The main step was first to quench the system from the homogeneous temperatures and isothermally anneal them at a prescribed temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from LLPS, and then cool the system at constant rate to record the non-isothermal crystallization. The crystallization peak temperature (Tp) was used to character the crystallization rate. Because LLPS results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the LLPS for the system indicated by increased Tp. The result showed that the LLPS boundary of the blend measured by this method was close to that obtained by phase contrast optical microscopy method. Therefore, we considered that the thermal analysis technique based on the non-isothermal crystallization could be effective to investigate the LLPS of PE blends.

The miscibility and the isothermal crystallization kinetics for PBT/Epoxy blends have been studied by using differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The Avrami exponents n were obtained for PBT/Epoxy blends. An addition of small amount of epoxy resin (3%) leads to an increase in the number of effective nuclei, thus resulting in an increase in crystallization rate and a stronger trend of instantaneous three-dimensional growth. For isothermal crystallization, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of PBT component in the PBT/Epoxy blends. The Lauritzen-Hoffman equation for DSC isothermal crystallization data revealed that PBT/Epoxy 97/3 had lower nucleation constant K, than 100/0, 93/7, and 90/10 PBT/Epoxy blends. Analysis of the crystallization data of PBT/Epoxy blends showed that crystallization occurs in regime II. The fold surface free energy, sigma(e) = 101.7-58.0 x 10(-3) J/m(2), and work of chain folding, q = 5.79-3.30 kcal/mol, were determined. The equilibrium melting point depressions of PBT/Epoxy blends were observed and the Flory-Huggins interaction parameters were obtained.

The effects of drawing conditions on the orientation and crystallinity of poly(ethylene terephthalate) (PET) fibers were investigated by using optical birefringence, sonic velocity, and wide-angle X-ray diffraction measurements, respectively. The preferred condition for preparation of uniaxially oriented amorphous PET fibers was suggested. The crystallization behavior of oriented PET fibers under relaxed and fixed length conditions was investigated by using differential scanning calorimetry (DSC). The multi-overlapping peaks were observed in the non-isothermal DSC curves of oriented PET fibers under relaxed condition. The kinetics of non-isothermal crystallization of oriented PET fibers under relaxed condition was analyzed by using an equation which takes the multi-crystallization processes into account. The kinetic parameters of every process were obtained and the crystallization mechanism was discussed. The crystallization behavior under fixed length condition differs from that under relaxed condition.

Wide-angle X-ray diffraction (WAXD) was used to investigate the crystal transition of nylon-12,12 under annealing and drawing. The triclinic α-form could be obtained by crystallizing from a melted state or by annealing the γ-form at high temperature (above 150°C). The crystal structure of the α-form annealed at 90 °C didn't change with time except for the perfection of crystals and an increase in the degree of crystallinity. The pseudo-hexagonal γ-form could be produced by crystallizing from the melted state at low temperature (90 °C) or by drawing at 90 and 160 °C. This is the first time a Brill transition has been observed under drawing conditions, instead of under the traditional conditions of continuous heating and cooling. Experimental results also confirm that drawing inducement is preferable to produce the γ-form and plays an important role in determining the crystal structure; there is a competition between drawing inducement and thermal inducement.

The crystallization kinetics of biodegradable poly(butylene succinate-co-adipate) (PBS/A) copolyester was investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The Avrami and Ozawa equations were used to analyze the isothermal and nonisothermal crystallization kinetics, respectively. By using wide-angle X-ray diffraction (WAXD), PBS/A was identified to have the same crystal structure with that of PBS. The spherulitic growth rates of PBS/A measured in isothermal conditions are very well comparable with those measured by nonisothermal procedures (cooling rates ranged from 0.5 to 15 C/min). The kinetic data were examined with the Hoffman-Lauritzen nucleation theory. The observed spherulites of PBS/A with different shapes and textures strongly depend on the crystallization temperatures.

The crystallization behavior of nylon 1212, irradiated at 60Co γ-rays (50 kGy), was studied by a rheometer, polarized optical microscopy (POM), and differential scanning calorimeter (DSC). The results showed that irradiated nylon 1212 samples exhibited abnormal crystallization behavior during the crystallization process: The Avrami exponent n was calculated and was found to be in the range from 2.06–2.41 for isothermal crystallization, and from 2.67–4.91 for nonisothermal crystallization; the spherulite morphology also changed largely by polarized optical microscopy (POM); the crystallization activation energy ΔE for isothermal and nonisothermal crystallization process of irradiated nylon 1212 are determined to be 57.4 kJ/mol and 78.65 kJ/mol, respectively, which are lower than that of nonirradiated nylon 1212. At the same time, a new method by a combination of the Avrami and Ozawa equations was successfully applied to analyze the noncrystallization process of irradiated nylon 1212. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2326–2333, 2005

Differential scanning calorimetry was used to investigate the isothermal crystallization, subsequent melting behavior, and nonisothermal crystallization of syndiotactic 1,2-polybutadiene (st-1,2-PB) produced with an iron-based catalyst system. The isothermal crystallization of two fractions was analyzed according to the Avrami equation. The morphology of the crystallite was observed with polarized optical microscopy. Double melting peaks were observed for the samples isothermally crystallized at 125-155 °C. The low-temperature melting peak, which appeared approximately 5 °C above the crystallization temperature, was attributed to the melting of imperfect crystals formed by the less stereoregular fraction. The high-temperature melting peak was associated with the melting of perfect crystals formed by the stereoregular fraction. With the Hoffman-Weeks approach, the value of the equilibrium melting temperature was derived. During the nonisothermal crystallization, the Ozawa method was limited in obtaining the kinetic parameters of st-1,2-PB. A new method that combined the Ozawa method and the Avrami method was employed to analyze the nonisothermal crystallization of st-1,2-PB. The activation energies of crystallization under nonisothermal conditions were calculated.

The isothermal and nonisothermal crystallization kinetics of partially melted nylon-1212 was investigated with differential scanning calorimetry. Because of partial melting, the pre-existing crystals changed the crystallization mechanism and had a strong effect on the crystallization process. The Avrami exponent and interfacial free energy of the chain-folded surface of partially melted nylon-1212 were higher than those of completely melted nylon-1212. The work of chain folding was determined to be 5.9 kcal/mol. The activation energy of the isothermal crystallization process was determined to be 399.1 kJ/mol, far higher than that of complete melting. The crystallization rate coefficient and Jeziorny analysis indicated that the ability of nonisothermal crystallization for partially melted nylon-1212 was enhanced. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3222–3230, 2005

The isothermal and non-isothermal melt-crystallization kinetics of nylon 1212 were investigated by differential scanning calorimetry. Primary and secondary crystallization behaviors were analysed based on different approaches. The results obtained suggested that primary crystallization under isothermal conditions involves three-dimensional spherulite growth initiated by athermal nucleation, while under non-isothermal conditions, the mechanism of primary crystallization is more complex. Secondary crystallization displays a lower-dimensional crystal growth, both in the isothermal and non-isothermal processes. The crystallite morphology of nylon 1212, isothermally crystallized at various temperatures, was observed by polarized optical microscopy. The activation energies of crystallization under isothermal and non-isothermal conditions were also calculated based on different approaches.

The results obtained for poly(butylene succinate) (PBS) after Co-60 gamma-ray irradiation, studied by wide-angle X-ray diffraction (WAXD), differential scanning calorimeter (DSC) and polarizing optical microscopy (POM), revealed that the degree of crystallinity, melting temperature and enthalpy decreased with increasing irradiation dose, but that the crystal structure of PBS did not vary when compared to non-irradiated PBS. By using Scherrer equation, small changes occurred in the crystal sizes of L-020, L-110 and L-111. The spherulitic morphology of PBS was strongly dependent on irradiation dose and changed significantly at higher irradiation dosages. The crystallization kinetics of PBS indicated that the Avrami exponent (n) for irradiated PBS was reduced to 2.3, when compared to non-irradiated PBS (3.3).

The differential scanning calorimeter heating curves of uniaxially oriented poly(ethylene terephthalate) (PET) fibers with three peaks were analyzed by using a newly proposed equation. The diffusion-controlled crystallization theory is suitable for describing cold crystallization of uniaxially oriented PET fibers. A crystallization model was proposed based on the kinetic parameters obtained. The model embraces the three sub-processes of crystallization corresponding to different growth geometries. The first sub-process corresponds to the nucleation of ordered molecular segments or the radial growth of preformed nucleus, resulting in the shorter bundle-like entities. The second sub-process corresponds to further growth of the bundle-like crystallites along chain direction, resulting in the longer bundle-like entities. The third sub-process corresponds to the three-dimensional growth of crystallites relating to the random segments, resulting in the spherical entities.

The isothermal and non-isothermal crystallization processes of nylon 1212 were investigated by polarized optical microscopy. The crystal growth rates of nylon 1212 measured in isothermal conditions at temperatures ranged from 182 to 132 degreesC are well comparable with those measured by non-isothermal procedures (cooling rates ranged from 0.5 to 11 degreesC/min). The kinetic data were examined with the Hoffman-Lauritzen nucleation theory on the basis of the obtained values of the thermodynamic parameters of nylon 1212. The classical regime I --> II and regime II --> III transitions occur at the temperatures of 179 and 159 degreesC, respectively. The crystal growth parameters were calculated with (100) plane assumed to be the growth plane. The regime I --> II --> III transition is accompanied by a morphological transition from elliptical-shaped structure to banded spherulite and then non-banded spherulite. The development of morphology during isothermal and non-isothermal processes shows a good agreement.

The non-isothermal equation was extended to describe non-isothermal cold crystallization kinetics of oriented polymers. The validity of the equation was examined by using a DSC crystallization curve of oriented poly(ethylene terephthalate) (PET) fibers with a constant heating rate. The double cold crystallization peaks appeared in the DSC curve. The relative degrees of crystallinity at different temperatures were analyzed by using the equation. The results show that the value of the Avrami exponent near to 1 at lower temperatures implies the bundle-like crystal growth geometry and the value of the Avrami exponent near to, 2, at higher temperatures implies the higher dimension crystal growth geometry. The first crystallization process crystallizes at faster rate than that of the isotropic sample, while the second process crystallizes at slower rate than that of isotropic sample. If a simple single process model was used, the value of the Avrami exponent, 0.77, was obtained. The result shows the simple single process model cannot describe the processes of crystallization of oriented PET fibers satisfactorily.

In this paper, melt blends of poly(propylene carbonate) (PPC) with poly(butylene succinate) (PBS) were characterized by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), tensile testing, wide-angle X-ray diffraction (WAXD), polarized optical microscopy and thermogravimetric analysis (TGA). The results indicated that the glass transition temperature of PPC in the 90/10 PPC/PBS blend was decreased by about 11 K comparing with that of pure PPC. The presence of 10% PBS was partially miscible with PPC. The 90/10 PPC/PBS blend had better impact and tensile strength than those of the other PPC/PBS blends. The glass transition temperature of PPC in the 80/20, 70/30, and 60/40 PPC/PBS blends was improved by about 4.9 K, 4.2 K, and 13 K comparing with that of pure PPC, respectively; which indicated the immiscibility between PPC and PBS. The DSC results indicated that the crystallization of PBS became more difficult when the PPC content increased. The matrix of PPC hindered the crystallization process of PBS. While the content of PBS was above 20%, significant crystallization-induced phase separation was observed by polarized optical microscopy. It was found from the WAXD analysis that the crystal structure of PBS did not change, and the degree of crystallinity increased with increasing PBS content in the PPC/PBS blends.