An improved model for crystalline orientation of spherulitic polymers

A model is proposed to account for the orientation of crystals in spherulites upon deforming a polymer. The model assumes affine deformation of the spherulite accompanied by three processes of crystal reorientation within the spherulite. These are tilting of molecular chains with respect to the plane of the lamellae, characterized by a parameter K, twisting of the lamallae about the spherulite radii described by η, and rotation of the b crystal axis about the c axis characterized by a third parameter, P. Values of these parameters are fitted to measured orientation functions and compared with experimental measurements of the azimuthal variation of the intensities of the 110 and 200 crystal reflections from low-density polyethylene. Good agreement is found between the experimental and theoretically predicted intensity variation. Time-dependent experiments under relaxational and vibrational conditions are also treated by the theory.     

Crystallization of polydioxolan. II. Morphology and melting properties

Polydioxolan samples crystallized between 25 and 35°C present two optical phases when viewed on the polarizing microscope. These phases, termed central and external phases, form a spherulite. It is shown in this paper that the central phase of the two-phase spherulite melts at about 63°C, and is made of modification III crystals. The external portion of the two-phase spherulites melts at a lower temperature, around 59°C, and is composed of modification II crystals. Differential scanning calorimeter (DSC) melting curves, photomicrographs, and x-ray results are presented to prove these assertions.     

Solution crystallization of nylon 6

Electron microscopy and x-ray diffraction data have been obtained on nylon 6 which has been crystallized from solutions in 1,6-hexanediol and 1,2,6-hexanetriol. Lamellar single crystals and spherulites of the γ form are obtained by crystallization from 1,2,6-hexanetriol. The morphology of the single crystals is different from that obtained from glycerine solutions. The spherulites of the γ form are composed of larger lamellae. Sheaflike crystals of the α form are obtained from both solvents. α-form and γ-form crystals both grow from 1,2,6-hexanetriol at appropriate crystallization temperatures. α-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 56 to 66 Å. The melting behavior of the solution-grown crystals is examined and discussed. Effects of solvent on growth of the two crystalline forms from solution are investigated.     

Crystal orientation in a semicrystalline polymer in relation to deformation of polymer spherulites. IV. Crystal orientation mechanism of nylon-6 under uniaxial stretching

A model relating crystal orientation to the deformation of nylon-6 spherulites under uniaxial stretching is discussed in terms of the orientation distribution functions of reciprocal lattice vectors of crystal planes, such as the (002) and (200) planes. The distribution functions calculated from the model are compared with those obtained from x-ray diffraction experiments. It is found that the crystal a axis and, consequently, the direction of hydrogen bonds within the crystal (α modification) orient parallel to the lamellar axis in the undeformed state, and that the crystal orientation behavior of nylon-6 is much different from that of low-density polyethylene, being characterized by much smaller values of the reorientation parameters of crystallites within orienting lamellae. Moreover, small-angle light scattering for H_v and V_v polarization is also calculated on the basis of the spherulite deformation model by taking the nylon-6 crystal as having orthogonal–biaxial symmetry in optical anisotropy. It is concluded that the H_v scattering can be realized in terms of the proposed model for spherulite deformation by taking into account a considerable contribution of hydrogen bonds to the molecular polarizability, so as to make the polarizability along the crystal a axis larger than that along the b axis. In other words, this conclusion suggests positively birefringent spherulites in the nylon-6 samples studied.     

Studies on the β-crystalline form of isotactic polypropylene

A study has been made of the mechanical, thermal, and morpholigical characteristics of melt-crystallized isotactic polypropylene containing high levels of the β or pseudohexagonal crystalline form. Different levels of β-form crystallinity were produced in the polymer by blending in low levels of quinacridone dye nucleating agent. Microscopical studies of the crystallization process revealed that both α-form (monoclinic), and β-form spherulites nucleated on the dye particles, with α-spherulite growth commencing at a higher temperature. These observations were able to qualitatively explain the dependence of β-form level on both the nucleant concentration and its state of dispersion in the polymer. Improving the dispersion of the nucleant was found to reduce the level of β-form crystallinity if the nucleant concentration exceeded an optimum level. A new procedure for quantifying the volume fraction of β spherulites in a sample was developed which utilized the technique of selective solvent extraction. From volume-fraction, x-ray, and density data, the pure α and β crystal densities were obtained. Dynamic mechanical measurements-obtained on unoriented specimens containing varying levels of β-form crystallinity showed an increase in the magnitude of the damping in the post-T_g region with increasing β content. High levels of the β form lead to lower values of the modulus and yield stress, and higher values of the elongation at break and impact strength.     

Polymorphic forms of nylon 3

The crystal structure of nylon 3 was studied, and four crystalline modifications were observed. Modification I, as determined from the x-ray diffraction pattern of drawn fibers, is similar to the α crystal structure of nylon 6. The unit cell is monoclinic; a = 9.33 Å, b = 4.78 Å, (fiber identity period), c = 8.73 Å, and β = 60°. The theoretical density for nylon 3 with four monomeric units in the unit cell is 1.39 g/cm³, and the observed density is 1.33 g/cm³. The space group is P2₁. The nylon 3 chains are in the extended planar zigzag conformation. Although other odd-numbered nylon form triclinic or pseudohexagonal crystals when oriented, drawn nylon 3 crystals are monoclinic. In addition to modification I, modifications II, III, and IV were studied. Lattice spacings of modifications II and III are equal to those of modification I. However x-ray diffraction intensities are different. Infrared spectra of those forms indicate an extended planar zigzag conformation of the chains. Modification IV is thought to correspond to the so-called smectic hexagonal form. No γ crystals were found, and it appears that polyamide chains with short sequences of methylene groups cannot form crystals of this type.     

MULTIPLE MELTING AND CRYSTALLIZATION BEHAVIOR OF NYLON 1212

The wide-angle X-ray diffraction (WAXD) patterns of isothermally crystallized Nylon 1212 show that γ-form crystals form below 90℃ and the α-form crystals can exist above 140℃. In the temperature range of 90-140℃, the α-form and γ-form crystals coexist. Variable-temperature WAXD exhibits that the nylon 1212 γ-form does not show crystal transition on heating, while α-form isothermally crystallized at 160℃ exhibits Brill transition at a little higher than 180℃ on heating. The multiple melting behaviors of Nylon 1212 isothermally crystallized from melt come from a complex mechanism of different crystal structures, dual lamellar population and melting-recrystallization. In polarized optical microscope (POM) observations, Nylon 1212 isothermally crystallized at 175℃ shows the ringed banded spherulites. However, at temperatures below 160℃ the ringed banded image disappears, and cross-extinct spherulites are formed.     

Structural studies on the polymorphism of nylon 3

The structure and morphology of crystalline nylon 3 [poly(β-alanine)] have been studied by electron microscopy and x-ray diffraction. Two clearly defined forms are detected. Form I appears as spherulites made up of ribbonlike lamellae upon crystallization at high temperature from a solution in phenol–butanediol-1,4. They have monoclinic unit cell with a = 9.60 Å, c = 8.96 Å, and β = 122.5°. The hydrogen-bonded planes run parallel to the long dimension of the crystals. Form II is observed when the samples are prepared from formic acid solution at room temperature. A second type of spherulite with a microfibrillar structure is formed in this case. The isolated crystalline structures obtained from Form II appear to grow along the intersheet direction rather than along the hydrogen bond direction, which constitutes anomalous behaviour. Our results for this second form are consistent with an orthorhombic lattice with a = 9.56 Å and c = 7.56 Å. No clear information is obtained on the b dimension of the unit cell (chain axis) in either case. We assume a value of 4.78 Å, which corresponds to fully extended chains. The two forms coexist in films prepared from a formic acid–water solution as well as in samples recovered immediately after polymerization.     

Study of crystallization kinetics of trans-1,4-polyisoprene

The concentrations and the growth rates of high- and low-melting type spherulites of trans-1,4-polyisoprene were measured in the temperature range 39–49°C. It was shown that above about 40°C., the crystallization rate of trans-1,4-polyisoprene is determined primarily by the radial growth rate of high-melting form (HMF) spherulites, whereas the predominance of the low-melting form (LMF) crystals below 40°C. can be attributed to the high rate of formation of LMF primary nuclei at lower crystallization temperatures. Temperature-independent rate parameters were calculated from optical and dilatometric measurements and were found to be in good agreement. Both the change in nucleation habit and spherulite growth rate with temperature can be explained on the basis of a lower end surface free energy of LMF crystals of trans-1,4-polyisoprene compared to that of the HMF crystals.     

Shear-induced β-form crystallization in isotactic polypropylene

Extruded, injection-molded, unoriented crystallized specimens and capillary rheometer efflux strands of commercially stabilized polypropylene without nucleating agents were examined by optical microscopy and x-ray diffraction to determine the conditions for β-form crystallization as a function of the distance from the surface and of the shear rate at commercial processing conditions. Results demonstrate that at all “cooling conditions” ΔT = T_m ? T_b (defined as the melt temperature T_m minus the bath temperature T_b) effects of strain flow initiate nucleation of β-form crystals. The shear rate is demonstrated to be important for β-form crystallization. A critical average threshold value for the shear rate of approximately 3 × 10² sec^?1 has to be exceeded. The β modification is mostly connected with type-III spherulites and partly to row structures, and it is observed at processing conditions in oriented structures only.     

向列相液晶共聚物作为β晶成核剂对等规聚丙烯结晶结构与热性能的影响

采用接枝聚合的方法,合成了一种新型聚硅氧烷类向列相液晶共聚物(LCP-H4),然后将LCP-H4与PP在一定工艺条件下密炼共混,得到了一系列的共混样品,采用WAXD、POM与DSC等研究了LCP-H4作为成核剂对PP样品结晶结构、形态与热性能的影响.结果表明,具有独特"液晶"性能的LCP-H4为PP结晶提供了更多的带自由能的晶核与较多的活性点,起到了异相成核的作用,既提高了PP的结晶速度、结晶温度和结晶度,又减小了球晶的尺寸,同时也改变了PP的结晶结构、形态及热力学与动力学,诱导出了β晶.此外,随着增加LCP-H4的含量及结晶温度,对应PP试样的β晶含量(Kβ)呈现先增加后降低的趋势,当LCP-H4含量为0.9%,在128℃等温结晶1h,对应成核PP的Kβ最大,为54%.     

Crystal orientation in a semicrystalline polymer in relation to deformation of polymer spherulites. III. Small-angle light scattering

Small-angle polarized light scattering from a deformed three-dimensional spherulite is formulated on the basis of the deformation model proposed in Part II of this series. The intensity distribution of scattered light is discussed chiefly for the cross-polarization condition, the so-called H_v polarization, as a function of elongation of the spherulite. In the undeformed state, the scattered intensity distribution forms the typical fourleaf clover pattern, and the intensity decreases with increasing fraction of crystals oriented randomly (type R crystals) within the crystal lamellae of the spherulites. In a system composed of type R crystals and folded-chain crystals (type B crystals) within the lamellae, the four-leaf pattern moves to the horizontal zone near the equator with increasing elongation of the spherulite, and, simultaneously, extends to some extent to the vertical zone near the meridional direction as a parameter measuring the ease of lamellar untwisting increases. In a system composed, in addition to type R and type B crystals, of crystals transformed from type B to type C_a and type C_r due to tilting and unfolding of polymer chains, respectively, within the crystal lamellae an eight-leaf pattern appears, even at small elongation up to about 30%. Each lobe of the eight-leaf pattern undergoes a characteristic change with increasing elongation. In both systems, the scattered intensity increases with sharpening of orientation distribution of crystals within the crystal lamellae.     

全同立构聚丙烯的晶片形态

本文应用光学显微镜,扫描和透射电子显微镜从三种不同层次的结构水平上研究了α和β两种晶型的全同立构聚丙烯的球晶和晶片形态结构,特别是应用四氧化钌染色技术直接观察到两种不同晶型聚丙烯球晶中单独分离的晶片形态.结果表明,不同晶型聚丙烯球晶的形态是不同的,其所呈现的性质与其内部晶片结构的排列特征相对应.同时研究了两种晶型聚丙烯在熔体拉伸结晶条件下生成的晶片形态,倾向于相同的取向晶片结构.电子衍射数据证明了,β型聚丙烯在拉伸取向结晶时将转变为α晶型.     

Quantitative characterization of deformation in drawn polypropylene films

The submicroscopic morphology of uniaxially deformed isotactic polypropylene films has been examined by small-angle light scattering (SALS), electron microscopy, optical microscopy, small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction, birefringence, sonic modulus, and density methods. Several new interpretations and extensions of existing theories are developed and verified experimentally as follows. (1) The V_v SALS pattern is shown to be a new tool for the identification of the sign of the birefringence of spherulites too small to be seen in the optical microscope. The theoretical dependence of the V_v SALS pattern is developed and verified experimentally with patterns from isotactic polypropylene, polyethylene, Penton, nylon 6,6, poly(ethylene terephthalate), and nylon 6,10. (2) Intraspherulitic lamellar behavior during deformation can be identified from the SAXS pattern. This includes quantitative evaluation of the long spacing between lamellae and their average orientation. (3) The two-phase sonic modulus theory is valid over the wide range of deformations, crystallinities, processing temperatures, and molecular weights used in this study. The deformation of isotactic polypropylene films drawn at 110 and 135°C. has been characterized quantitatively in terms of an integrated picture of mass movement on all morphological levels: the molecular, the interlamellar, and the spherulitic. At both temperatures, the spherulites deform affinely with extension, whereas the deformation mechanisms within the spherulite depend on the location of the radii with respect to the applied load. During spherulite deformation, lamellar orientation and separation processes predominate, whereas at high extensions, fibrillation occurs and crystal cleavage processes predominate. The noncrystalline region orients throughout the draw region. At 135°C. non-orienting relaxation processes appear in the noncrystalline region which retard the rate of molecular orientation with extension.     

Analysis of Polymorphism and Dual Crystalline Morphologies in Poly(hexamethylene terephthalate)

Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.

WAXD diffractograms for PHT melt‐crystallized at 140 °C, revealing a single type of β‐crystal cell.     

高压结晶PET/PC共混体系中生长在伸直链晶体内部球晶形态的观察

自从Wundedich等报道聚乙烯（PE）在高压结晶时可以生成伸直链晶体以来,相继很多有关聚合物体系的高压结晶行为方面的研究已见报道．研究结果表明,聚合物在高压下经历相转变时可产生非常丰富的微观结构,而球晶及伸直链晶体是其中最常见的两种结晶形态．但是,所观察到的这两种聚合物晶体都是分别存在,且独立生长的．到目前为止,尚未见到关于高压下球晶可以在伸直链晶体内部存在的报道．     

Non‐Radial Growth of Helical Homopolymer Crystals: Breaking the Paradigm of the Polymer Spherulite Microstructure

Radial symmetry is essential for the conventional view of the polymer spherulite microstructure. Typically it is assumed that, in the course of the spherulite morphogenesis, the lamellar crystals grow radially. Using submicron X‐ray diffraction, it is shown that in banded spherulites of poly(propylene adipate) the crystals have the shape of a helix with flat‐on crystals winding around a virtual cylinder of about 6 µm in diameter. The helix angle of 30° implies that the crystal growth direction is tilted away from the spherulite radius by this angle. The implications of the helical crystal shape contradict the paradigm of the spherulitic microstructure. The radial growth rate of such spherulites does not correspond to the crystal growth rate, but to the propagation rate of the virtual cylinder the ribbons wind around.

     

Crystal orientation in a semicrystalline polymer in relation to deformation of polymer spherulites. II. Orientation distribution function of crystallites within crystal lamella as a function of lamellar orientation

A model relating crystal orientation in a semicrystalline polymer to the deformation of polymer spherulites is proposed. The distribution function for orientation of crystallites within crystal lamellae is assumed to be a function of lamellar orientation. In addition to the orientation of crystal lamellae in affine fashion, several parameters are introduced to characterize the untwisting of the crystal lamellae and the four different types of orientations of the crystallites within the crystal lamellae in the undeformed and deformed states of the spherulite. The model was tested by experiments in uniaxial stretching of a low-density polyethylene. The theoretical distributions of orientation of given reciprocal lattice vectors of the crystallites, such as the reciprocal lattice vectors of the (110) and (200) crystal planes, are compared with the results of x-ray diffraction experiments. It was found that the most important factors in fitting the model to experimental results are: (a) the fraction of crystallites having random orientation within lamella and, in turn, representing the degree of imperfection of the lamella in the undeformed state; (b) the ease of transition of crystal orientation within lamella from b-axis orientation parallel to the lamellar axis to two types of c-axis orientations (type C_a and type C_r) parallel to the stretching direction; and (c) the fraction of crystallites having orientation in type C_r (unfolding mechanism) rather than type C_a (rotation mechanism).     

Influence of nucleating agent on the formation and enzymatic degradation of poly(butylene adipate) polymorphic crystals

The effects of nucleating agent multimethyl-benzilidene sorbitol (TM6) on crystallization and morphology of poly(butylene adipate) (PBA) with polymorphic crystal structures were studied by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized optical micrographs (POM). In addition to the heterogeneous nucleation, TM6 changes the formation conditions of PBA polymorphic crystals. The addition of TM6 is favorable for the formation of PBA α-form crystals, resulting in the morphological changes from spherulites to interpenetrated fibrils. The influences of TM6 on enzymatic degradation of PBA were studied in terms of the morphological change and weight loss. The results indicate that the α-form crystals induced by TM6 show much slower degradation rate. This work provides an efficient method to control the polymorphic crystal structure and further to regulate the biodegradation rate of polymer materials through modulating the homogeneous and heterogeneous nucleation modes by adding nucleating agents.     

Small-angle Hv light scattering from deformed spherulites with orientational fluctuation of optical axes

Mathematical evaluation was done for small-angle light scattering from disordered spherulites under Hv polarization conditions. The calculation was carried out for a two-dimensional deformed spherulite whose major optical axes are oriented at 0 or 45° with respect to the radial direction. The calculated results were compared with the scattering patterns observed for polypropylene (PP) spherulites, whose optical axes are oriented parallel to the radial direction, and poly(butylene terephthalate) (PBT) spherulites, whose optical axes are oriented at 45° with respect to the radial direction. The degree of disorder for PBT was much larger than that for PP. By selecting a parameter associated with the degree of disorder of the optical axes with respect to the radial direction, the patterns calculated as a function of draw ratios were in good agreement with the observed patterns, which changed from four leaves to streaks extended in the horizontal direction. Through a series of observed and calculated patterns, it turns out that an increase in the disorder under the deformation process occurs drastically even for perfect spherulites in an undeformed state.