Optical studies of the crystallization of trans 1,4-polybutadiene in stretched networks

The stress-induced crystallization of trans 1,4-polybutadiene was studied by observing changes in birefringence, stress, x-ray diffraction, and low angle light scattering during the course of crystallization. From these data, the degree of crystallinity was determined as a function of time, temperature, and elongation. Data were fitted to the Avrami equation, leading to an exponent of the order of unity. Light-scattering patterns suggested the simultaneous existence of two stretched forms: a rod-like structure oriented preferentially parallel to the stretching direction and a deformed spherulitelike growth with its greatest extension perpendicular to the stretching direction.     

Stress-induced crystallization of poly(ethylene terephthalate)

Quenched amorphous films of poly(ethylene terephthalate) (PET) are stretched at temperatures less than T_g; changes in density, wide-angle x-ray diffraction, and small-angle light scattering are observed. The density increase upon stretching is attributed to an increase in crystallinity accompanied by an increase in the intensity of somewhat diffuse wide-angle x-ray diffraction and of both V_V and H_V small-angle light scattering patterns. The formation of oriented rodlike superstructure may be discerned from small-angle light scattering. Annealing of these samples increases the crystallinity as measured from density and leads to an increase in the perfection of crystalline and supercrystalline structure as measured by wide-angle x-ray diffraction and small-angle light scattering. The rodlike morphology changes to form spherulitelike aggregates as observed by small-angle light scattering and light micrographs. A model is proposed to explain the observations. Studies are extended to stretching films of PET above their T_g and observing changes in birefringence, density, wide-angle x-ray diffraction and small-angle light scattering as a function of elongation and stretching temperature. The formation of defomed spherulitelike superstructure may be discèrned from light micrographs. Results are compared with those obtained upon stretching films below T_g.     

The use of synchrotron radiation for study of structural changes in polymers during phase transition and deformation

An experimental method using monochromatized synchrotron radiation for a wide-angle x-ray scattering study of polymers during phase transition and deformation is described. Results obtained on low-density and high-density polyethylenes and on polypropylene are given. Melting processes could be studied conveniently by making exposures during a temperature scan; a few seconds were found sufficient to obtain a diffraction spectrum. This way relatively fast crystallization processes and the structural change accompanying stress relaxation could be followed easily. The crystalline-amorphous ratio was found to decrease continuously much below the melting temperature measured dilatometrically and by DSC, implying that in the α_c-relaxation transition region of polyethylene and polypropylene a structural change takes place. At the initial state of crystallization the 040 reflection in polypropylene was found to be stronger than the 110 one, while after crystallization has been completed this ratio was inverted. Equatorial spectra made after rapid uniaxial stretching of polypropylene sheet were found to exhibit a slight time dependence demonstrating that orientation went on after stretching had been stopped.     

Pressure effects on the inhomogeneities of environmentally sensitive polymer gels

The effects of pressure at gel preparation on the inhomogeneities were investigated by small-angle neutron scattering. Poly(N-isopropylacrylamide) (PNIPA) gels, known as thermosensitive polymer gels undergoing volume transition, were prepared at various pressures ranging from 0.1 to 300 MPa. The scattering intensity increased with increasing P_prep, up to 200 MPa, then decreased by further increase in P_prep. The degree of inhomogeneities evaluated by the ratio of the static and dynamic correlators exhibited a similar behavior to that of the ensemble average light scattered intensity, <I>_E. The physical meaning of the gel inhomogeneities is discussed from the viewpoint of swelling thermodynamics. It will be shown that the increase in the swelling degree is due not to an increase in the miscibility of PNIPA with water but to the increase in inhomogeneities.     

Relation of initial supermolecular organization in crystalline polymer systems to the organization produced by stretching

An investigation of small-angle scattering produced by polarized light from stretched and annealed polymer systems containing spherulites yields information on the supermolecular transitions that occur during such treatment. A series of semicrystalline polymer systems (films, fibers) show that stretching leads to deformation of spherulites and subsequent transformation to an orientational supermolecular order. The size of the single element C₂ of the supermolecular order in the direction of stretching, determined from the distance between the layer lines of the scattering pattern, is related to the diameter D₀ of the initial spherulites by the relation C₂ = KD₀λ_s, where λ, is the draw ratio of the macrosystem and K is a parameter determining the deformability of the spherulites. For polyethylene at room temperature K is unity and for polychloroprene it is 1.2. Changes of C₂ after annealing and restretching of the systems also obey this ratio.     

Structure and deformation recovery of the thermoplastic polyurethane spherulite

We investigated the structure and deformation behavior of the thermoplastic polyurethane (TPU) spherulite by optical microscopy, tensile testing, Hv light scattering, and small angle X‐ray scattering. The TPU spherulite structure obtained by melt crystallization was coarse consisting of bundle‐like structure containing hard segment (HS) lamellar domain in which the HS domains were stacked and the HS chain direction was perpendicular to the longitudinal axis of the HS domain. By stretching, the spherulite was deformed to ellipsoidal one and the stacked HS lamellar domains were tilted in the stretching direction. The deformed spherulite and the tilted HS domain in the spherulite were recovered to the unstretched state by retraction. The recovery of the structure is ascribed to the characteristic spherulite structure consisting of rubbery soft segment matrix physically cross‐linked with the stacked HS domain. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1585–1594     

Crystallization kinetics of biaxially stretched natural rubber

The isothermal crystallization of natural rubber was investigated under biaxial stretching. A marked planar orientation of crystals occurs in such a way that the (010) plane orients parallel to the film plane. The rate of crystallization increases with the biaxial stretching ratio, and the Avrami exponent n decreases. At the highest extension, n becomes less than unity. The equilibrium degree of crystallinity decreases with the stretching ratio. The crystallinity was less than 10% for the highest extension. The melting behavior is similar to that in uniaxial stretching.     

Changes in light scattering from poly(ethylene terephthalate) and nylon 6 films upon stretching in relation to the deformation mechanism

Unoriented T-die flat films of nylon 6 and PET films annealed at 90°C were stretched in water at 80°C. Amorphous PET films were stretched in water at 65–75°C. Changes in the light scattering patterns from these samples upon stretching were investigated. One of the observed LS patterns from the stretched samples is the H_v eight-leaf pattern consisting of four lobes and streaks. In the nylon 6 and heat-treated PET showing this pattern, spherulitic patterns can be seen in polarization microscopy. The microscopic spherulitic superstructure may possibly be the factor responsible for producing the lobe-and-streak pattern. On the other hand, many microscopic eight-leaf patterns can be observed in amorphous unannealed PET showing the lobe-and-streak pattern. These microscopic patterns are due to retardation at stress concentrations around impurities and nuclei. The superstructure giving these microscopic patterns must be the origin of the lobe-and-streak pattern from unannealed PET. Another scattering pattern, the V_v cruciform pattern, was observed in both stretched nylon 6 and unannealed PET. This pattern is due to an orientation change across the slip lines observed under a polarizing microscope. It is noted (1) that the appearance of the slip lines in PET coincides with the occurrence of oriented crystallization on stretching, (2) that the lobe-and-streak pattern from PET in which orientation crystallization has taken place is fairly stable to heat treatment and does not disappear until just before melting, and (3) that the superstructures produced at low stretching seem to be deformed on further stretching, in accordance with affine deformation theory.     

Influence of crystallization temperature,molecular weight,and additives on the crystallization kinetics of poly(ethylene terephthalate)

The spherulite growth rate, the maximum spherulite radius, and the overall rate of crystallization of poly(ethylene terephthalate) (PETP) were measured by means of scattering and transmission of depolarized light. The influence of crystallization temperature, molecular weight, and additives on the above-mentioned quantities was investigated. An expression has been derived for the spherulite growth rate of PETP as a function of crystallization temperature and the number-average molecular weight for M?_n in the range of 19,000 to 39,000.     

Deformation of polybutene-1 spherulites

The deformation of fresh and aged polybutene-1 spherulitic samples has been investigated by microscopic observation, interferometry, studying macroscopic and spherulitic birefringence changes, and study of light-scattering patterns. The spherulite deformation is not affine, the microscopic deformation ratio being less than the macroscopic deformation ratio of the sample and greater in the equatorial regions of the spherulite than in the polar regions. The deviation from affine deformation is less for fresh spherulites than for the aged, where void formation occurs in the equatorial part of the spherulite. This gives rise to large scattering by this part of the spherulite and to form birefringence. The spherulite birefringence and its change with elongation is dependent upon the degree of aging of the sample. The spherulite birefringence is more negative for the aged sample. In the polar regions of the spherulite, this negative birefringence decreases and turns positive at higher elongations, characteristic of a reorientation of the crystals with their optic axes turning from being perpendicular to parallel to the spherulite radius. The spherulite birefringence in the equatorial direction becomes somewhat more negative on stretching a fresh sample but less negative on stretching an aged one. Spherulite distortion and orientation changes are apparent from the light-scattering patterns of films possessing small spherulites. The changes in V_v and H_v scattering patterns upon stretch are different for the fresh and aged samples. The V_v patterns of the fresh samples decrease in intensity with time after stretching a fresh sample with the H_v patterns do not.     

Crystallinity and crystalline phase orientation of poly(1,4‐cis‐isoprene) from Hevea brasiliensis and Taraxacum kok‐saghyz

Crystallization is studied for poly(isoprene‐1,4‐cis) from Hevea brasiliensis (natural rubber [NR]) and from taraxacum kok‐saghyz, mainly by collecting wide‐angle X‐ray diffraction patterns after processing and stretching. Although rubber samples before stretching are generally fully amorphous, crystallization can be induced in NR samples by processing at room temperature under moderate pressure. This phenomenon is possibly associated with nucleation by saturated fatty acid components. For rubber samples being fully amorphous in the undeformed state, strain‐induced crystallization occurs only at high strain ratios (α > 4), leading to high degrees of crystalline phase orientation (f_c > 0.9 for α = 5). Rubber samples presenting some crystallinity already in the unstretched state, on the contrary, reach much lower degrees of axial orientation, even for high strain ratios (f_c < 0.7 for α = 5). These differences in crystallinity and in crystalline phase orientations produce large differences in stress–strain behavior of the rubber. By room temperature processing, the considered NR samples can also develop an unreported disordered crystalline modification, with low intensity of 120 and 121 reflections. This disordered crystalline modification, which is also maintained after axial stretching procedures, can rationalized by a structural disorder along the b axis, possibly associated with statistical sequences of A⁺TA^? or A^?T A⁺ conformations for poly(isoprene‐1,4‐cis) chains. Copyright © 2016 John Wiley & Sons, Ltd.     

Morphological studies on polybutylene terephthalate

The morphology of poly(butylene terephthalate) (PBT) crystallized from the melt at various temperatures was studied by small-angle light scattering, polarizing microscopy, and wide-angle x-ray diffraction. Spherulites with a maltese cross at 45° to the polars formed at lower temperatures while spherulites having an apparently higher melting point with a maltese cross along the polars (0°–90°) formed at higher temperatures. The spherulite size and crystallinity increased with increasing crystallization temperature. The H_v scattering patterns arising from the spherulites formed at lower temperature showed intensity maxima at azimuthal angles of 0° and 90°, while those obtained at higher temperatures showed the more common 45° intensity maxima. Microtomed samples from molded PBT bars showed spherulites with a 45° maltese cross which changed to a 0°–90° maltese cross upon heating just prior to melting. The skin-core effect due to varying thermal histories in these molded bars was clearly observed. Solvent crystallized films contained positive 0°–90° spherulites. Some changes occurring upon uniaxial stretching of PBT films are also discussed.     

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.     

Morphology of a liquid-crystalline polyester film

Morphological studies are reported for a thermotropic liquid crystalline polyester. Small angle light scattering studies were carried out as a function of temperature using H_v and V_v polarization with photographic as well as photometric techniques. No scattering was observed from a thin film cast from a dilute solution of the polymer in a highly volatile solvent. When the film was heated, scattering of light was observed above the glass transition temperature of the polyester. The scattering was found to be azimuthally dependent with V_v intensities being much higher than the corresponding H_v intensities. The size of the morphological features responsible for SALS patterns were calculated and were found not to change significantly with temperature ranging from glass transition temperature to the solid-nematic transition temperature of the polyester. The WAXS pattern of solution cast polymer was representative of an amorphous structure. Solution cast films heat treated under various conditions (all above the T_g of the polymer) contained crystalline as well as amorphous structures. The maximum apparent crystallinity for annealed samples was of the order of 30%.     

Solvent‐induced crystallization in poly(ethylene terephthalate) during mass transport

Solvent transport in poly(ethylene terephthalate) (PET) and related phase transformation were investigated. The data of mass sorption were analyzed according to Harmon's model for Case I (Fickian), Case II (swelling), and anomalous transport. This transport process in PET is accompanied by the induced crystallization of the original amorphous state. The transformation was examined by wide‐angle X‐ray scattering, small‐angle X‐ray scattering, differential scanning calorimetry, and Fourier transform infrared spectroscopy. During this process, the matrix is under a strain state that causes different kinetic paths of crystallization as compared with that by thermal annealing. This state of strain assists the development of the solvent‐induced crystallization. The model regarding crystallization was proposed in terms of the study of long period L, the crystal thickness l_c, and the thickness of amorphous layer l_a obtained from the one‐dimensional correlation function and interface distribution function. Different kinetic paths were discovered for different crystallization processes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1444–1453, 2002     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Non-isothermal crystallization behaviors of poly(4-methyl-pentene-1)

Non-isothermal crystallization of isotactic poly(4-methyl-pentene-1) (P4MP1) is studied by differential scanning calorimeter (DSC), and kinetic parameters such as the Avrami exponent and the kinetic crystallization rate (Z _c) are determined. From the cooling and melting curves of P4MP1 at different cooling rates, the crystalline enthalpy increases with the increasing cooling rate, but the degree of crystalline by DSC measurement shows not much variation. Degree of crystalline of P4MP1 calculated by wide angle X-ray diffraction pattern shows the same tendency with crystalline enthalpy, indicating that re-crystallization occurs when samples heated above the second glass transition temperature of P4MP1. By Jeziorny analysis, n ₁ value suggests that mainly spherulites’ growth at 2.5 K min⁻¹ transforms into a mixture mode of three-dimensional and two-dimensional space extensions with further increasing cooling rate. In the secondary crystallization process, n ₂ values indicate that the secondary crystallization is mainly the two-dimensional extension of the lamellar crystals formed during the primary crystallization process. The rates of the crystallization, Z _c and t _1/2 both increase obviously with the increase of cooling rate, especially at the primary crystallization stage. By Mo’s method, higher cooling rate should be required in order to obtain a higher degree of crystallinity at unit crystallization time.     

Crystallite orientation in stretched polychloroprene networks. II

The orientation of crystallites grown isothermally in several drawn trans-polychloroprene networks is studied as a function of crystallization temperature t_x, degree of crystallinity ω, and elongation ratio α. The orientation distribution is particularly simple for this polymer since the crystallographic c axis (chain axis) orients preferentially along the stretching direction, while a and b are randomly arranged about c. Hence the parameter cos² χ_c adequately characterizes the distribution, where χ_c is the angle between the c axis and the fiber axis, and the average is taken over all crystallites. A treatment due to Krigbaum and Roe is utilized to obtain values of v (the number of statistical segments comprising the crystallization nucleus of critical size) through comparison of the average orientation of crystallites and amorphous statistical segments. The behavior observed falls into two categories. First, if the initial amorphous network is well oriented, 〈cos² χ_c〉 is independent of crystallinity during both crystallization and melting, and v varies with t_z (or the degree of supercooling) as predicted by nucleation theory. If different networks are to have the same crystallite orientation distribution, they must not only be crystallized at the same supercooling, but must also have the same distribution of amorphous segment orientations. Both the relative elongation and the network crosslink density affect the latter distribution. Next, we consider the second category. If the initial amorphous orientation is poor, 〈cos² χ_c〉 decreases linearly during crystallization and increases along approximately the same path during melting. Further, 〈cos² χ_c〉 for a given t_z yields v values which are too large. These two behaviors can be explained if, in the former case, nucleation involves the best oriented statistical segments of all network chains, while in the latter there is a selection according to the chain displacement vector orientation. Thus, if the amorphous orientation is poor, both the orientation and thermodynamic stability of the crystallites decreases with further crystallization. If this decreased stability is reflected in shorter fold lengths, the reversible variation of long period spacing with temperature reported earlier for an oriented polychloroprene network can also be explained as a preferential melting process.     

Isothermal crystallization of poly(vinylidene fluoride) in the presence of high static electric fields. II. Effect of crystallization temperature and electric field strength on the crystal phase content and morphology

The melt crystallization of poly(vinylidene fluoride) in a static electric field was studied for different fields strengths and undercooling conditions. The γ-phase nucleation process was examined directly by polarized optical microscopy and indirectly by small-angle light scattering. The crystal phase content was assessed by differential scanning calorimetry. It is shown that the γ-phase nucleation density and γ-phase content increase with field strength and that the higher the crystallization temperature, the larger the effect of the field. These experimental results confirm the predictions of the model of nucleation in an electric field that we published previously. It is also noted that the degree of crystallinity and the perfection of crystal orientation along the γ-phase spherulite radical direction decrease with field strength. The homogeneity of morphology resulting from the crystallization in the field is also examined by polarized optical microscopy on specimens microtomed across their thickness. When the crystallization is carried out under high field (E ≈ 0.1 MV/cm) and high temperature (T > 166°C) a nonuniform morphology results, characterized by a higher nucleation density at the positive electrode than at the negative electrode. Near the negative electrode very large disklike spherulites are seen to grow parallel to the substrate.