Orientation of LDPE Crystals from Microscale to Nanoscale via Microlayer or Nanolayer Coextrusion

The microlayer or nanolayer coextrusion of hundreds or thousands of alternating low density polyethylene(LDPE)/polystyrene(PS) microlayers or nanolayers were used to study the orientation of LDPE crystals in the confined quasi-two-dimensional or two-dimensional space. The clear and continuous layer structures from microscale to nanoscale can be found in SEM images. The morphology evolution of LDPE crystals in the confined microlayer or nanolayer can be varied from 3D spherulites, 2D spherulites, stacked edge-on lamellar, to single edge-on lamellar. Due to the orientation of the LDPE crystals, the tensile strength of the films increases obviously when the layer thickness reduces to nanoscale. The 2D small angle X-ray scattering(SAXS) patterns can reflect the average degree of orientation of LDPE in the confined layers. The stacking of LDPE lamellae is suppressed in interlamination and oppositely in parallel to the extrusion direction. The specific orientation function f can be calculated from the patterns. The infrared dichroism further confirms the mutation of the orientation of LDPE crystals from microscale to nanoscale in the confined space.     

Structure of ultrathin polyethylene layers in multilayer films

The crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high-density polyethylene and polystyrene. Transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and wide-angle x-ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row-nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row-nucleated morphology imparts highly anisotropic mechanical properties to the nanolayer polyethylene.     

Quantitative small-angle light-scattering studies of the melting and crystallization of LLDPE/LDPE blends

Small-angle light-scattering (SALS) patterns were obtained during melting and crystallization of blends of linear low-density polyethylene (LLDPE) with conventional low-density polyethylene (LDPE). Quantitative measurements of these SALS patterns using a two-dimensional optical multichannel analyzer apparatus (OMA2) indicate that the LLDPE which is miscible with the LDPE component in the molten state crystallizes first, forming volume-filling spherulites. The LDPE then crystallizes within the preformed spherulites. These findings are supported by optical microscopy studies showing that the blend samples were volume filled with one kind of the spherulites having a radius comparable to that of the pure LLDPE. The SALS intensity curve changes with composition of the blends in a manner that may be interpreted by considering the orientation of crystals within spherulites. It has been observed that the spherulites in the blend have more diffuse boundaries as the LDPE content increases. The lattice spacing and long spacings in blends were obtained by wide-angle and small-angle x-ray scattering, respectively. The SALS technique along with differential scanning calorimetry (DSC) is shown to be useful for determining the crystallization behavior of a crystallizable polymer blend system.     

Correlation of crack patterns and ring bands in spherulites of low molecular weight poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid)

Only a single type of circular circumferential crack is conventionally reported for poly(l-lactic acid) (PLLA). In this study, PLLA samples were found to exhibit as many as four crack types of different directions and patterns, which cannot be feasibly explained simply by the directional difference in coefficients of thermal expansion. Depending on crystallization temperature (T _c), PLLA crystallizes into ringless or ring-banded spherulites, whereas the crack patterns are dramatically different in these two types of spherulites. In ring-banded spherulites of PLLA crystallized at intermediate T _c, two uniquely different crack types are present: (1) twin circumferential cracks coinciding with the dark–bright and bright–dark boundary and (2) radial short-segmental voids coinciding on the bright bands in spherulites. The radial short-segmental cracks on the bright band of ring-banded spherulites may be caused by PLLA crystals of radial direction with various twisting that contract laterally upon cooling. Only circumferential cracks are present in PLLA crystallized into ringless spherulites, where concentric continuous circumferential cracks are present in the ringless spherulites at low T _c with finer lamellae, but discontinuous and irregular circumferential cracks are present in the ringless spherulites at high T _c with coarse lamellae. Although all cracks are triggered by cooling from T _c, all evidences indicate that the crack patterns and types are highly associated with the lamellar orientation, patterns, and coarseness in spherulites.     

Morphological development of melt crystallized poly(propylene oxide) by in situ AFM: formation of banded spherulites

The morphology of poly(propylene oxide) (PPO) crystals grown from the melt was investigated. The spherulites of the optically pure S polymers displayed a regular pattern of concentric rings as observed by polarizing optical microscopy, while the stereocopolymer developed irregularly banded, or non-banded spherulites depending on the degree of undercooling. The organization of the lamellar crystals within the spherulites was examined by means of atomic force microscopy (AFM). For all cases, the lamellar structures appeared to adopt an alternating flat or edge-on orientation. Examination of the morphology of single crystals in the melt of the stereocopolymer revealed truncated-lozenge crystals, which were elongated in shape. Results from crystallization kinetics, obtained by in situ AFM observations, showed that the elongated habit is related to differences in the growth rates of the {2 0 0} and {1 1 0} facets. Interestingly, the melt-grown RS-PPO crystals developed a curved asymmetrical three-dimensional shape. Based on these observations it can be proposed that the chiral nature of the chain is transmitted to higher structural levels of ordering in the crystal aggregates.     

Polymer Nanostructures by Forced Assembly: Process,Structure, and Properties

The process of micro- and nanolayer coextrusion of polymeric systems with good layer uniformity is described. Coextrusion through a series of layer multiplying die elements has enabled the production of films containing tens to thousands of layers with individual layer thicknesses from the micro- to the nanoscale. Improvements in layer uniformity are discussed through optimization of layer multiplier die design, selection of viscosity matched polymer systems, and incorporation of surface layer capabilities. Design of ‘uneven’ split layer multiplication dies has enabled the coextrusion of layered films with a wide variety of layer thickness distributions having up to a 10× difference in the individual film layer thicknesses. Coextrusion of layered polymer films with individual layer thicknesses down to the nanoscale has resulted in the production of novel systems with improved properties. Nanolayered polymer films were utilized to develop an all-plastic polymer laser, to fabricate gradient refractive index lenses, and to investigate gas barrier enhancement of crystalline polymer nanolayers confined to induce a high aspect ratio, in-plane, single-crystal-like lamellar structure.     

Lamellar single crystals of nylon‐10,10 grown from a dimethylformamide solution

A new solvent, dimethylformamide (DMF), and the traditional solvent, 1,4‐butanediol, were used to prepare single crystals of nylon‐10,10 from a dilute solution. The lamellae grown from DMF inhabited a more perfect structure and regular shape than those crystals crystallized from traditional solvents such as 1,4‐butanediol and glycerin. These thin and perfect lamellar crystals demonstrated patterns of variation in spacing different from those of melt‐crystallized spherulites on heating. Specifically, the two main spacings slightly separated rather than continuously approaching each other when the temperature was greater than 180 °C. This is a novel phenomenon observed in nylons. Nevertheless, the usual pattern of change in spacing was observed during the cooling process. These lamellar crystals showed more compact spacing of the (002) and (010/100) planes than spherulites at room temperature. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 729–735, 2001     

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. II. Morphology and crystallization kinetics by time resolved X‐ray scattering

The development of the morphology in poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) (PVDF/PHB) blends upon isothermal and anisothermal crystallization is investigated by time‐resolved small‐ and wide‐angle X‐ray scattering. The components are completely miscible in the melt but crystallize separately; they crystallize stepwise at different temperatures or sequentially with isothermal or anisothermal conditions, respectively. The PVDF crystallizes undisturbed whereas PHB crystallizes in a confined space that is determined by the existing supermolecular structure of the PVDF. The investigations reveal that composition inhomogeneities may initially develop in the remaining melt or in the amorphous phases of the PVDF upon crystallization of that component. The subsequent crystallization of the PHB depends on these heterogeneities and the supermolecular structure of PVDF (dendritically or globularly spherulitic). PHB may form separate spherulites that start to grow from the melt, or it may develop “interlocking spherulites” that start to grow from inside a PVDF spherulite. Occasionally, a large number of PVDF spherulites may be incorporated into PHB interlocking spherulites. The separate PHB spherulites may intrude into the PVDF spherulites upon further growth, which results in “interpenetrating spherulites.” Interlocking and interpenetrating are realized by the growth of separate lamellar stacks (“fibrils”) of the blend components. There is no interlamellar growth. The growth direction of the PHB fibrils follows that of the existing PVDF fibrils. Depending on the distribution of the PHB molecules on the interlamellar and interfibrillar PVDF regions, the lamellar arrangement of the PVDF may contract or expand upon PHB crystallization and the adjacent fibrils of the two components are linked or clearly separated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 974–985, 2004     

Effect of titania nanoparticles on the morphology of low density polyethylene

The role of TiO₂ nanoparticle surfaces in affecting the crystalline structure of low‐density polyethylene (LDPE) has been investigated by varying the nanoparticle surface from hydrophilic (as‐received) to less hydrophilic (dried) or more hydrophilic (polar silane treated). Differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WXRD) were used to determine the degree of crystallinity and crystalline structure. The impact of nanoparticle aggregates on the nanometer to micrometer organization of LDPE crystals was studied with atomic force microscopy (AFM) and small‐angle light scattering (SALS). This characterization showed that the presence of the TiO₂ nanoparticles, with the various different surface conditions investigated, did not alter the degree of LDPE crystallinity, the unit cell dimensions, the average lamellar thickness, or the average spherulite size. However, the nanoparticles did affect the internal arrangement of intraspherulitic crystalline aggregates by decreasing the relative optic axis orientation of these crystals, usually referred to as internal spherulite disorder. The LDPE filled with the nanoparticles treated with a polar silane (N‐(2‐aminoethyl) 3‐aminopropyl‐trimethoxysilane (AEAPS)) showed the highest internal spherulitic disorder and exhibited the most poorly developed spherulite structure. The combination of SALS with AFM has allowed a detailed characterization of the morphology of the semicrystalline polymer nanocomposites. Information on the internal organization of the spherulites, the size of the nanoparticle aggregates, and the location of the nanoparticle aggregates can be uniquely obtained when both techniques are used. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 488–497, 2005     

Highly oriented structure formed in a lamella-forming diblock copolymer with high molar mass

Highly oriented films were prepared simply by annealing a lamella-forming block copolymer, poly(ethylene oxide-b-styrene) (PEO-b-PS), with high molar mass under a pressure of 0.2 MPa. The oriented structures were characterized by small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The SAXS measurements showed that the lamellar layers of the block copolymer are highly oriented parallel in the film plane. The WAXD images showed that the c-axis of PEO crystals was oriented normal to the film plane. The Hermans-Stein orientation functions for the lamellar layer and the crystal axis are 0.954 and −0.466, respectively, and are close to the values of perfect orientation. It was considered that the highly oriented structure was formed by the combined effects of shear flow and self-organization of the block copolymer during annealing under stress. The high degree of orientation both for the lamellar layer and crystal planes also suggested that the crystallization in the confined domains results in a high degree of orientation of PEO crystals with respect to the lamellar interface of the block copolymer.     

Dual types of spherulites in poly(octamethylene terephthalate) confined in thin-film growth

Spherulite morphology and growth kinetics of poly(octamethylene terephthalate) (POT), cast on single-side glass or confined between two slides in thin-film forms, were characterized using polarized versus nonpolarized optical microscopy, scanning electron microscopy (SEM), and wide-angle X-ray (WAXD) analysis. POT can simultaneously display solely one type of spherulite or dual types of spherulites (double-ring-banded and ringless ones), depending on T c or T max imposed. Fractions of these two types depend on T c when quenched from a fixed T max = 160 degrees C. At lower T c's, POT exhibits higher crystallization rates leading to higher fractions of ringless spherulites; at higher T c's, POT exhibits lower crystallization rates leading to ring-banded spherulites. At intermediate to high T c's where the growth kinetics of POT could be monitored, the ring-band type dominates and the fraction of ringless spherulites is insignificantly small. Both ringless and ring-banded spherulites can be seen in regime III ( T c = 70-110 degrees C), with fractions of ringless type of spherulites decreasing with temperature. Thus, growth kinetics for POT was mainly focused on the regime of ring-banded spherulites. In regime III, the ring-band pattern is more orderly concentric with smaller inter-ring spacing (1-2 mum) for lower T c's but intermediately larger spacing (3-5 mum) for higher T c's. The orderly lamellar orientation in the ring-bands in contrast with the inter-ring valley region is discussed. In regime II (115 degrees C and above), the ring-band pattern is first distorted to highly zigzag irregularity at higher T c's and then eventually disappears at extremely high T c, with the lamellar crystals eventually turning dendritic with no rings. Apparently, the types of spherulites in polymers are more influenced by the growth rates as determined by T c and slightly less by T max, but not by the substrate surface nucleation.     

Space charge distribution and crystalline structure in polyethylene blended with EVOH

The space charge distribution in polyethylene samples under direct current (DC) electrical field was measured by pulsed electro-acoustic (PEA) method. It was found that by blending with 5 wt.% of poly(ethylene-co-vinyl alcohol) (EVOH) in low-density polyethylene (LDPE) the amount of accumulated space charges decreased and the field distribution of space charge improved. The differential scanning calorimetry (DSC) study showed that crystallization of LDPE/EVOH started at a higher temperature than LDPE. The results of wide-angle X-ray diffraction (WAXD) and small-angle light scattering (SALS) for LDPE/EVOH indicated that the crystal forms did not change, whereas the spherulites became smaller and imperfective. It can be seen from the results that EVOH played a role of nucleation during the crystallization of LDPE in the blend. The observation of scanning electron microscope (SEM) showed that the domains of EVOH were dispersed in LDPE as particles in diameter of 1 μm. The reduction of space charges in the blend sample can be explained as the results of the trapping of homo-charges at the interface and the dissipation of charges through LDPE matrix consisting of smaller spherulites.     

Local deformation in stretched polybutene-1 spherulites

The orientation of crystals in spherulites of modifications I and II of polybutene-1 was studied by micro-x-ray diffraction. Changes in such micro-x-ray diffraction patterns were measured in various regions of the spherulites as a function of strain and time. The relationship between micro-x-ray diffraction from parts of spherulites and macro-x-ray diffraction from entire spherulites is discussed. Changes are resolved into contributions arising from (1) deformation of spherulites with change in crystal orientation and (2) transformation of crystals from modification II to modification I.     

Polypropylene spherulite morphology and growth rate changes in blends with low-density polyethylene

Morphological studies of isotactic polypropylene/low-density polyethlene blends revealed that the shape, size, and orientation of LDPE occlusions remain undisturbed during the crystallization of IPP spherulites. On the other hand, LDPE occlusions introduce large changes in the internal structure of IPP spherulites. It was found that many new boundaries similar to those between spherulites are fromed when the crystallizing front passes the LDPE occlusion. Dead-ended boundaries with soft LDPE occlusions at the ends give rise to an improvement of the impact properties. It was shown that LDPE obstacles do not influence significantly the IPP spherulite growth rate in thin films or in the bulk.     

Deformation of elastomeric polyolefin spherulites

The ethylene‐octene block copolymers in this study consist of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks form lamellae that organize into space‐filling spherulites even when the fraction of crystallizable block is so low that the crystallinity is only 7%. These unusual spherulites are highly elastic and recover from strains as high as 300%. This new class of thermoplastic elastomers is fundamentally different from conventional elastomeric olefin copolymers that depend on isolated, fringed micellar‐like crystals to provide the junctions for the elastomeric network. The elastomeric block copolymers are shown to be unique in that a hierarchical organization of space‐filling lamellar spherulites provides the junctions for the elastomeric network. The deformation of the elastic spherulites is readily studied with small angle light scattering, wide angle X‐ray diffractograms, and atomic force microscopy. At strains in excess of 300%, the spherulites break up into a fibrillar structure following lamellar deformation processes that are similar to those established for high density ethylenic polymers. The crystalline transformation produces a stiffer elastomer that exhibits complete recovery on subsequent loadings. Similar experiments on elastomeric random ethylene‐octene copolymers where fringed micellar crystals provide the physical crosslinks that connect the rubbery, amorphous chain segments reveal significant differences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1313–1330, 2009     

Probing the use of small‐angle light scattering for characterizing structure of titanium dioxide/low‐density polyethylene nanocomposites

Small‐angle light scattering (SALS) measurements were used to study the structure of titanium dioxide (TiO₂)/low‐density polyethylene (LDPE) nanocomposites. The results showed that the scattering from LDPE crystalline structures and the scattering from TiO₂ nanoparticles can be resolved and separated. It is shown that the independent effects of crystallization conditions and the presence of nanoparticle aggregates on the spherulitic structure of the LDPE matrix can be determined by analyzing the scattering patterns using the methods proposed. From the SALS results, we conclude that the nanoparticle surface chemistry affects both nucleation of spherulites and their structure particularly under rapid cooling conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1084–1095, 2006     

The orientation parameter for energy transfer in restricted geometries including block copolymer interfaces: a Monte Carlo study

We describe Monte Carlo simulations of resonance energy transfer (RET) experiments for immobile donor (D) and acceptor (A) dyes confined to planar, cylindrical, and spherical restricted geometries. We compare values of the quantum efficiency (PhiET) evaluated through consideration of individual donor-acceptor pairs, with values calculated assuming a pre-averaged value of the orientation parameter /kappa/2 = 0.476 appropriate for infinite three dimensional (3D) space. For dyes confined to restricted geometries where the length scale of the confining dimension is less than or equal to the F?rster radius R0, the coupling of the orientation parameter and the donor-acceptor distance becomes noticeable. Values of Phi(ET) obtained by proper consideration of the orientation parameter are smaller than those calculated using /kappa/2 = 0.476. We use this Monte Carlo method to reanalyze the fluoresce decay measured from dye-labeled poly(isoprene-b-methyl methacrylate) diblock copolymer with lamellar structure,(1) from which the interface thickness for PI-PMMA lamella can be retrieved. We found the retrieved interface thickness is sensitive to the choice of dipole orientation. If all dipoles in the confined polymer interface have a random orientation, the value of interface thickness was found to be 0.9 +/- 0.2 nm through consideration of individual dipole orientations. Assumption of /kappa/2 = 0.476 in the FRET calculations leads to a larger value of interface thickness (1.3 +/- 0.2 nm) due to the neglect of the coupling between dipole orientation and D-A distance for the dyes confined to lamellar interfaces.     

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.     

Photodegradation of ethylene–octene copolymers with different octene contents

The influence of the octene content on the photodegradation behaviour of ethylene–octene copolymers (EOCs) was revealed by investigating the photooxidation of low density polyethylene (LDPE) and EOCs with different octene contents through a series of characterisation methods. LDPE was very sensitive to ultraviolet light and the photostabilities of EOCs decreased with increasing octene concentration. The photodegradation of all samples produced hydroxyl, carbonyl and vinyl groups. The ease of chain crosslinking and scission was increased as the octene content rose. Crosslinking predominated in late irradiation period of LDPE while chain scission was dominant in that of EOCs. Annealing and chain scission promoted the secondary crystallisation of the crystallisable chain segments. Chain scission enhanced the crystallisation ability of the irradiated EOCs while it decreased that of the weathered LDPE. The photostabilities of crystals could be ranked as follows: the chain-folded lamellar crystals > the bundled crystals > the fringed micellar crystals. The thermal stabilities and mechanical properties of samples decreased with increasing irradiation time and the decreasing extent was correlated with the comonomer content.     

原子力显微镜研究高分子超薄膜结晶机理及功能化调控

近十年来,随着功能高分子单晶(含单层或寡层片晶)工程及应用研究的不断深入,除了纳米尺度结晶形貌的表征以外,多功能原子力显微镜还被用于研究分子结构、结晶条件和后处理条件对功能高分子晶体性能(电、热、光、磁等)的影响,进一步还可采用扫描探针加工技术(机械刻蚀、电致刻蚀和热致刻蚀等)对其性能进行调控以构筑功能化聚集态结构和微图案.另一方面,超薄膜中单层或寡层片晶可为研究高分子结晶提供合适的模型体系,与原子力显微镜相结合,不但可以原位、实空间、高分辨地研究高分子的成核与生长过程(生长形态演变和生长动力学),还可以用于研究亚稳态折叠链片晶厚度和形态随热处理温度与时间的演化,从而加深对片晶内有序差异、片晶增厚与熔融行为和自诱导成核的认识.