Branching and re-orientation of lamellar crystals in non-banded poly(butene-1) spherulites

The formation mechanism of non-banded polymer spherulites has been examined experimentally for isotactic poly(butene-1) grown from the melt by optical, atomic force, and transmission electron microscopies associated with quenching and chemical etching. At the growth front of the spherulites, the maximum width of lamellar crystals, λ_m, showed a square-root dependence on the growth rate. The dependence suggests an instability-driven branching. In terms of the correlation of lamellar orientation in the spherulites, an auto-correlation function has been determined from the image taken by polarizing optical microscopy. The correlation showed an exponential decay along the radial direction, and the correlation length was in proportion to λ_m. Those experimental evidences suggest that the structure is formed by the coupling of the branching instability and the random re-orientation of lamellar crystals on the occasion of branching in the non-banded spherulites of poly(butene-1).     

Morphological study on three kinds of two-dimensional spherulites of poly(butylene terephthalate) (PBT)

Three kinds of thin film specimens, each of which containing one of the three types of two-dimensional poly(butylene terephthalate) (PBT) spherulites (i: usual-positive type, ii: usual-negative type, and iii: unusual type), were prepared and studied mainly by transmission electron microscopy. It was confirmed that all these kinds of spherulites are made up of only the α-modification crystals, and the growth directions for usual-positive type, usual-negative type, and unusual type are the [010]^∗, ^∗, and ^∗ directions in the reciprocal lattice, respectively. Furthermore, the arrangement of unit cell in each type of the spherulites was determined. The relationships between the arrangements of unit cell and the types of each resulting spherulite are discussed.     

Microbeam X-ray diffraction of non-banded polymer spherulites of it-polystyrene and it-poly(butene-1)

The orientation of lamellar crystals in non-banded spherulites of it-polystyrene and it-poly(butene-1) was investigated by microbeam X-ray diffraction. The two-dimensional intensity map of diffraction enables us to examine the local orientation of lamellar crystallites in the non-banded spherulites. The obtained results indicated the re-orientation of crystallites in non-banded spherulites and confirmed our previous observation on the anisotropic birefringence of a group of crystal stacks by polarizing optical microscopy.     

Spherulitic growth kinetics in miscible blends of poly(ether ether ketone) and poly(ether imide)

Growths of poly(ether ether ketone) (PEEK) spherulites from both pure melt and its miscible blends with poly(ether imide) (PEI) have been studied by polarized optical microscopy. The nucleation density of PEEK spherulites was depressed upon blending with PEI, which can be attributed to the reduction in degree of supercooling arising from equilibrium melting point depression. A modified Lauritzen-Hoffman (L-H) theory was adopted to analyze the growth kinetics. Regime III-II transition was observed with the transition temperature decreasing with increasing PEI composition. Assuming free rotations of the virtual bonds in PEEK molecule, the side surface free energy of 12.0 erg/cm² was calculated from the characteristic ratio. The fold surface free energy of 188 erg/cm² and work of chain folding of 12.3 kcal/mol were then obtained from the modified L-H analysis.     

Modelling of butene-1 dehydrogenation in a fixed bed reactor — bed and pellet profiles

A homogeneous model incorporating the Dumez & Froment kinetics has been proposed for the butene-1 dehydrogenation reaction over a commercial chromia-alumina catalyst. The model predicts bed and pellet concentration and temperature profiles. While the temperature profiles obtained suggest a temperature minimum in the reactor, the pellet remains nearly isothermal. Under the condition used in this study, a series type of coke deposition is obtained. A comparison between experimental results obtained using the neutron attenuation technique and theoretical coke profiles reveals excellent agreement and confirms the validity of the model and the kinetics employed.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.     

聚乙二醇增塑聚乳酸的等温结晶动力学研究

采用DSC方法对聚乙二醇(PEG)增塑聚乳酸的等温结晶动力学进行了研究。结果表明,PEG的加入提高了聚乳酸的结晶速度,在相同结晶温度下,随着PEG摩尔质量的增大,结晶速率增大,结晶活化能先减小后增大。不同摩尔质量PEG增塑PLA的结晶成核机理和生长方式一样。     

High birefringence of poly(trimethylene terephthalate) spherulite

Poly(trimethylene terephthalate) (PTT) spherulite shows interference color under polarized light microscope without a sensitive tint plate. The fact indicates that the retardation of PTT spherulite is high, while it was reported that the birefringence in PTT fiber is low. In this study, the reason why the high birefringence is observed in PTT spherulite was discussed. By small area X-ray diffraction measurement, it was confirmed that a-axis of unit cell of PTT crystal was parallel to the radial direction of the spherulite. Based on the result, we calculated the refractive indices of parallel to a-axis and the other orthogonal directions. It was clarified that the refractive index of a-axis is much lower than the others and the intrinsic birefringence for a-axis orientation is high. It is the reason why the PTT spherulite shows high and negative birefringence.     

高分子量聚醋酸乙烯酯的醇解研究

探讨了催化剂浓度、醇解温度和反应时间等对高分子量聚醋酸乙烯酯醇解的影响,得出高分子量聚醋酸乙烯酯醇解的适合条件。     

Molecular dynamics simulation studies of binary blend miscibility of poly(3-hydroxybutyrate) and poly(ethylene oxide)

By means of full atomistic molecular dynamics simulation, the solubility parameters for pure poly(3-hydroxybutyrate) and poly(ethylene oxide) are calculated and the results are in agreement with the literature values. Furthermore, in order to reveal the blend property, the volume-temperature curve of the PHB/PEO blend system (1:2 blends in terms of repeated units) is simulated by employing the united atom approximation to obtain the glass transition temperature. From the volume-temperature curve, the glass transition temperature is about 258 K, which is compared well with the experimental results. It should be pointed out that the two simulated solubility parameters are similar and there is only one glass transition of the blend system, these indicate that the studied blend system is miscible.     

Single- and double-ring spherulites in poly(nonamethylene terephthalate)

A new mechanism of formation of ring-banded spherulites was discovered, where three different types of spherulites were present in poly(nonamethylene terephthalate) (PNT) cast in thin film forms. The growth and morphological features in three ring-banded spherulites were compared and analyzed using polarized/non-polarized light microscopy, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) and element analysis by energy dispersive spectroscopy (EDS) on surfaces of spherulites. Three types of spherulites (labeled: Type-0, 1, and 2) are possible in PNT when crystallized at wide range of crystallization temperature (T_c); however, only two of the three types can co-exist in PNT at a given T_c. At lower T_c (<55 °C), within Regime-III, the spherulites in PNT are either of Type-0 (appearing ringless in POM) or Type-2 (double-band in POM). When crystallized at temperature of T_c = 55 °C or higher (Regime-II), the morphology is featured with Type-1 (single-ring-banded in POM) spherulites as the majority species that co-exist with Type-2 (double-ring-banded in POM) spherulites as imbedded minority. Origins and morphological differences of the single-band vs. double-band spherulites were compared. The double-band spherulites have much larger inter-ring spacing than do the single-band spherulites. By comparison, the rod-like lamellae in the double-banded spherulites (Type-2) are longer and thicker, and more ordered than those in the single-banded spherulites (Type-1).     

负载钛催化丁烯-1本体聚合研究

研究了负载钛催化丁烯-1本体聚合的聚合规律,考察了聚合条件对转化率和聚合物性能的影响。实验结果表明采用负载钛催化剂本体聚合的方法合成的聚丁烯-1是一种热塑性弹性体,本反应适合采用反应挤出工艺方法,较佳的聚合条件为:n(Ti)/n(Bt)=2×10-5,n(Al)/n(Ti)=200,t=40℃。     

Molecular dynamics simulation of oxygen diffusion in dry and water-containing poly(vinyl alcohol)

The kinetics and mechanisms of diffusion of oxygen and water in dry and water-containing amorphous syndiotactic poly(vinyl alcohol) were studied at 502 K and normal pressure by molecular dynamics simulation. Penetrant molecule trajectories were obtained in a system with 600 repeating units of poly(vinyl alcohol) and 0, 40 (2.6 wt%) and 80 (5.2 wt%) water molecules. Under dry conditions, oxygen molecules jumped in a cage-like fashion. The oxygen molecule diffused in a liquid-like fashion while water diffusion was cage-like in the system with 5.2 wt% water. The hydrogen bond lifetimes among the water molecules were significantly shorter than those formed between water and the polymer and between different polymer segments. The hydrogen bond lifetimes among all species were, within experimental error, unaffected by the content of water, even though the oxygen diffusivity increased exponentially and the water diffusivity increased to some extent with increasing water content. It seemed that the diffusivity was sensitive primarily to the decrease in concentration of polymer-polymer hydrogen bonds, which followed from the increase in water content. This finding was consonant with the analysis of the oxygen molecule motion relative to the nearest polymer backbone, which revealed that it jumped preferentially along the polymer chain and towards the backbone. This behavior was more pronounced when the dynamics were analyzed over longer distances (5 Å) and it was less pronounced in the water-rich systems. The simulations indicated that water clustering was absent and consequently that water was homogeneously distributed in the polymer systems.     

Crystallization temperature dependence of interference color and morphology in poly(trimethylene terephthalate) spherulite

The relationship between retardation and morphology in highly birefringent poly(trimethylene terephthalate) spherulites was studied from the viewpoint of crystallization temperature dependence. Both the retardation and the morphology relate with the degree of orientation of the molecular chains. Therefore, the degree of orientation of the crystal lamellae was estimated by image processing of transmission electron microscope (TEM) images of the spherulite. It was found that the degree of orientation changed remarkably between the non-banded and the banded morphology and the temperature dependence of the degree of orientation correlated with that of the retardation. Based on the image-processed TEM images, it was recognized that the crystal lamellae formed bundles in the banded spherulite, while few bundled lamellae were observed in the non-banded ones. It is suggested that the formation of bundled lamellae played the significant role for both the magnitude of retardation and determination of the morphology; i.e. whether to form banded spherulites or non-banded ones.     

In vitro hydrolytic degradation of poly(para-dioxanone) with high molecular weight

The in vitro hydrolytic degradation of high molecular weight poly (para-dioxanone) was studied by examining the changes of weight retention, water absorption, pH value, tensile strength, break elongation, thermal properties, and morphology of high molecular weight PPDO in phosphate buffered saline (PBS) (pH 7.44) at 37°C for 8 weeks. During the degradation, all samples’ weight retention decreased and water absorption increased significantly, whereas hydrolysis rate of PPDO bars varied with molecular weight. Compared with lower molecular weight samples, higher molecular weight PPDO samples exhibited higher hydrolysis rate. The samples’ glass transition temperature (Tg) decreased notably, while the degrees of crystallinity (Dc) increased. The samples almost totally lost their tensile strengths and breaking elongation after 4 weeks of degradation. The results suggested that the stability of PPDO in vitro hydrolytic degradation increased with the increase of molecular weight.     

Effects of crystallization behavior on morphological change in poly(trimethylene terephthalate) spherulites

In poly(trimethylene terephthalate) (PTT) spherulites during isothermal crystallization, the morphological changed from an axialite/or elliptical banded spherulite to banded spherulite and then non-banded spherulite with temperature decreasing were studied by following the lamellar growth behaviors. We report lamellar growth mechanism on varied crystallization temperature, which explicitly probes the link between microscopic structure and macroscopic morphology in the development of patterns. Fibrillation of the edge-on lamellae was observed on the surfaces of axialite and the convex bands of banded spherulite. Terrace-like lamellae were observed on the surface of the non-banded spherulite and the concave bands of banded-spherulite. In thin film crystallization, PTT banded spherulite exhibits a texture of alternate edge-on and flat-on lamellae, wavy-like surface and rhythmic growth. The deceleration of growth rate takes place in convex bands with a growth habit of fibrillation of the edge-on lamellae for emerging ridge surface. On the other hand, the acceleration of growth rate appears in concave bands with a growth habit of terrace-like lamellae for emerging valley surface. The alternating growth mechanism of the lamellae was considered to be related with the formation of spatiotemporal self-organization patterns far from equilibrium. In order to explain the rhythmic growth and periodic growth of the lamellae, we may conjecture that the emergence of PTT banded spherulite in thin film crystallization is associated with an oscillatory dynamics of the spherulite growth front driven by latent heat diffusion. We present some tentative ideas on the possibility of band-to-nonband (BNB) morphological transition, which might be analogous with the second order transition in non-equilibrium phase transition.     

Unperturbed dimensions and intrinsic viscosity-molecular weight relationship of copolyesters poly(ethylene terephthalate-co-ethylene isophthalate)

The relationship [η] = KM^a and unperturbed dimensions of the copolyesters poly(ethylene terephthalate-co-ethylene isophthalate) with 7.5% and 25% isophthalic structures in the main chain were determined. The experimental data proved the influence of the amount of isophthalic structures upon the values of K and a from the relation between intrinsic viscosity and molecular weight and on the values for unperturbed dimensions.     

Crystalline morphology of poly(dimethylsiloxane)

The crystalline morphology of poly(dimethylsiloxane) was studied using a scanning electron microscope equipped with a cold stage. Samples of two different molecular weights were used. In both cases, spherulitic morphology is seen, from −70 °C, with spherulites of about 100 μ in size. Small single crystals of about a micron in size are also seen, and these are attributed to the presence of cyclics.     

Power law of molecular weight dependence of lateral growth rate of isotactic polypropylene

Molecular weight (M) dependence of the lateral growth rate (V) of α form crystal of isotactic polypropylene (iPP) was studied. Reliable equilibrium melting temperature determined in our previous study was used for the analysis of supercooling dependence of V. A power law of M of V, , was obtained, where H is a small constant (H = 0.7). The small H, which is similar to that of the hexagonal phase of polyethylene (H = 0.7) in comparison with the value of H = 1.7 for the orthorhombic phase of polyethylene, confirmed our prediction of smaller H for “rod like” chain polymers because of easier chain sliding within the interface between the crystalline phase and the melt. Thus, the universality of the important role of topological nature in polymer crystallization was confirmed. Lateral surface free energy (σ) of the α form of iPP was obtained as σ ≅ 1.59 × 10⁻⁶ J/cm².     

Crystallization, and morphology of poly(trimethylene terephthalate)/poly(ethylene oxide terephthalate) segmented block copolymers

A new type of poly(ether-ester) based on poly(trimethylene terephthalate) as rigid segments and poly(ethylene oxide terephthalate) as soft segments was synthesized and its crystallization behavior and morphology were investigated. Differential Scanning Calorimetry revealed that the copolymer containing 57 wt% soft segments presented a low glass transition temperature (−46.4 °C) and a high melting temperature (201.8 °C), suggesting that it had the typical characteristic of thermoplastic elastomer. With increasing soft segment content from 35 to 57 wt%, the crystallization morphology transformed from banded spherulites to compact seaweed morphology at a certain film thickness, which was due to the change of surface tension and diffusivity caused by increasing the soft segment content. Moreover, with the decrease of film thickness from 15 to 2 μm, the crystallization morphology of the copolymer (57 wt% soft segment) changed from wheatear-like, compact seaweed to dendritic. Scanning Electron Microscopy revealed that some flower-like crystals presenting in the bulk, which had been surprisingly found in the poly(ether-ester) segmented block copolymers for the first time. Possible mechanism was discussed in the text.