Fibrillar crystals of isotactic polystyrene. I. Morphological aspects

Fibrillar crystals have been prepared by the crystallization of isotactic polystyrene from stirred solutions in 1,3,5-trimethylben-zene and cyclohexanol. Similar microfibrils have been prepared in the nascent state by the polymerization of styrene in 1,3,5-trimethyl-benzene with a heterogeneous Ziegler-Natta catalyst. The microfibrils varied in width between 140 and 250 Å and were characterized by periodic lamellar overgrowths which were believed to give rise to a discrete X-ray reflection having a d-spacing of 90 A. Thermal analysis suggested that high growth temperatures favored greater crystal perfection. During crystallization from stirred cyclohexanol solutions, a fractionation of high molecular weight chains occurred which was more efficient at higher crystallization temperatures. In comparison to crystallizations under quiescent conditions, a marked facility of the crystallization process was observed in the formation of stirrer-crystallized and Ziegler-Natta polystyrene. The ease of crystallization of the shear-regenerated and nascent microfibrils has been related to a reduction in the free energy of nucleation. Mechanisms have been proposed to account for a favored nucleation process.     

ON FINE STRUCTURE OF NASCENT UHMWPE REACTOR POWDERS

Reactor powder of a number of commercial ultra-high molecular weight polyethylenes synthesized on heterogeneous Ziegler–Natta catalyst in a conventional slurry process have been investigated with the help of scanning electron microscopy (SEM), wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. The SEM study reveals a complicated structure of nascent particles consisting of a small (0.5–1.0 μm) spongy-like fibrillar spheres. It is suggested that the elementary morphological units are fibrils formed by the small defective crystallites with the dominating crystalline distortions caused by microstrains. The comparison of longitudinal crystallite sizes derived from WAXS data with the straight chain segment length calculated from Raman shift frequency and the length of extended segments computed from the true melting interval measured by DSC allows to conclude that there are a large number of extended tie chains bridging the neighboring crystallites, which is not in agreement with a lamellar model. The extended chains passing through the noncrystalline regions stabilize a structure and provides its thermal stability. A possible tilting of molecules in the crystals, the location of defects and the distribution of crystallite sizes are discussed.     

Molecular scale structure formation by folding

A conception of a structure formation suitable for nano-technology is proposed, which is programmable and suitable for mass production-like lithography. This conception utilizes the controlled folding of chains like the scan-lines of television. Its possibility and property were studied theoretically using the modeled chains consist of beads. By adopting the interaction among the beads which can distinguish the kind of the partner by its polarity and is chiral to break the chiral symmetry of the folded state, the special chains which have the unique ground states could be designed. In these ground states, the chains are folded like the scan-lines of television. The thermodynamic properties of the suggested chains were studied by the Monte Carlo simulations and the suggested chains showed the phase-transition-like behavior which is distinct compared to both the random chains and the chain that has only the non-specific attraction. The size dependence and the effects of adding the non-specific attraction and modifying the border of the folded conformation were also studied.     

Mechanism of necking as revealed by ultrasonic cavitation of polyethylene single crystals

Ultrasonic cavitation of polyethylene single crystals and single-crystal aggregates in the form of cakes results in lamella fragmentation and necking involving the transformation of lamellar crystals into fibrillar crystals between 20 to 400 Å in diameter. The smaller fibrils (～20–30 Å) have a very smooth appearance, whereas the larger ones (～100–400 Å) contain a beady structure about 100 Å periodically spaced along the fibrils. The smoother microfibrils are suggested to contain extended chains that are formed by unfolding of molecules directly from the chain-folded lamellae as well as from the folded-chain crystals contained within the beady fibrils. The presence of the chain-folded crystals within the larger beady fibrils is shown in numerous instances to be due to incorporation of mosaic crystalline blocks originally present, but weakly connected to one another, in the lamellar single crystals. The necking process is deduced by observation to involve primarily a mechanical shearing of mosaic crystalline blocks along the c-axis plus a rotation into the fibril direction. Observation of extreme resistance of lamellae in the overgrowth regions to cavitation damage suggests the presence of tie molecules and/or interpenetrating cilia between these lamellae. The suggestion finds strong support from additional studies carried out on lamellae that have been tied together at the folds by cross-linking with γ rays.     

The Effect of Orientation Relaxation on Polymer Melt Crystallization Studied by Monte Carlo Simulations

We use dynamic Monte Carlo simulations to study the athermal relaxation of bulk extended chains and the isothermal crystallization in intermediately relaxed melts. It is found that the memory of chain orientations in the melt can significantly enhance the crystallization rates. The crystal orientation and lamellar thickness essentially depend on the orientational relaxation. Moreover, there is a transition of the nucleation mechanism during the isothermal crystallization from the intermediately relaxed melts. These results explain the mechanism of the self-nucleation by orientation and suggest that in flow-induced polymer crystallization, the orientational relaxation of chains decides the crystal orientation.     

A Rule Governing Crystallization of Configurationally Directional Polymers: The Crystal Structure of the α and β Forms of Polypivalolactone

A “Rule” is proposed for incorporation of polymer chains having directional configuration, e.g. A‐B‐C‐A‐B‐C, into a crystal. Crystallization into a lamella morphology, as in slow crystallization from the melt, will incorporate antiparallel sequences (↑↓↑↓↑↓). Formation of a fiber by drawing the lamellar morphology must produce a different crystal structure containing parallel directional sequences. The drawn fiber must be polymorphic with a disordered aggregation of antiparallel and parallel crystal polymorphs. An example of this rule is found in the crystal structure of polypivalolactone. The melt crystallized α form is monoclinic, P2₁/c with a=9.05Å, b (fiber axis)=5.97Å, c=11.69Å, β=121.4° and consists of planar antiparallel sequences. The molecular conformation is a folded zig‐zag arrangement. On drawing a fiber, a disordered second phase of parallel plus antiparallel sequences is created. The chain conformation is a slightly distorted extended zigzag. The crystal structure of the directionally disordered β form is metrically monoclinic, with a=5.95Å, b=10.32Å, c (fiber axis)=4.94Å, β=101.3°. Examples of several classes of crystalline polymers demonstrating this Rule are presented.     

Mechanical denaturation of high polymers in solutions XXIX. Stress-induced crystallization of poly(vinyl alcohol) from its aqueous solution under steady-state flow

The crystallization of poly(vinyl alcohol), derived from its aqueous solution was carried out under a steady-state flow. The effect of the rate of stirring on the crystallization and details of several phenomena observed in stirred solutions during crystallization are presented. Moreover, the structure of crystallized poly(vinyl alcohol) was studied with use of an electron microscope. Evidence is presented for a crystal morphology that includes smooth fibrils and the absence of lamellar overgrowth, i.e., no shish kebabs.     

Mechanism of spherulitic crystallization as deduced from observations of partially crystallized glassy polystyrene

Thin films of isotactic polystyrene partially crystallized from the glassy state were studied in detail by means of transmission electron microscopy and electron diffraction. Initial nucleation and growth stages of spherulitic fibrils (or lamellae) were illustrated clearly by using novel techniques, such as Au decoration, and novel specimens such as thin films containing holes.

Spherulitic nucleation begins with the crystallization of a liquid-crystal-like nodule or a group of these nodules merging to form a spherulitic center. Fibrils or lamellae grow and fan out from the nucleus by additional incorporation of maturing nodules. Proliferation of fibrils is essentially a space-filling process through the crystallization of uncrystallized nodules or nodules that were left behind by growing fibrils which had initiated earlier. The deduced mechanism of spherulitic crystallization leads directly to the formation of interlamellar links between neighboring fibrils. However, no extended-chain-type interlamellar links were revealed by Au decoration.

The application of the mechanism of spherulitic crystallization from the glass to that from the melt is also suggested; it is based primarily on recent studies which show remarkable similarities between structures existing in the glassy and the melt states prior to crystallization.     

THE HIGH-PRESSURE CRYSTALLIZATION AND ANNEALING BEHAVIORS OF POLYETHYLENE TEREPHTHALATE OLIGOMER

Polyethylene terephthalate (PET) oligomer samples crystallized and annealed at high pressure were investigated with differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The results showed that better crystals were obtained through high-pressure crystallization from the melt than annealing under the same conditions. The difference of the effects of crystallization and annealing on the morphology of crystals reduced with the increase of crystallization time. The melting temperature was determined by the lamellar thickness when it was shorter than the length of the molecular chains, while the main factor governing the melting temperature changed from lamellar thickness to density of chain-end defects when the lamellar thickness was much longer than the molecular length. PET oligomer extended-chain crystals with thickness up 100 μm were obtained.     

Morphological Study of hydroxypropyl cellulose films prepared from thermotropic melt under shear

A strain-induced crystallization behavior of hydroxypropyl cellulose (HPC) from the thermotropic liquid-crystalline state is described based on morphological observations by electron microscopy. It is shown that originally round-shaped particles behave as a structure unit in formation of a variety of supermolecular architectures of HPC films prepared from the thermotropic melt under shear. In an oriented HPC film obtained under weak shear, many round particles are elongated and aligned in the direction of shear (SD), but with their bodies bent to some degree. As deformation increases further, fibrillation occurs on the surface of the elongated particles, and then the resulting fibrils are arranged in a zigzag fashion along the SD to form a banded structure. In some cases, a pleated arrangement of fibrils is noticeable between bands. The structural transformation mechanism of thermotropic HPC under shear is discussed in detail on the basis of the morphological evidence.     

高压下尼龙1010-单壁碳纳米管复合材料的结晶行为

 采用XKY-6×1200MN型六面顶压机,在不同温度、压力条件下处理30 min后制备了尼龙1010(PA1010)-单壁碳纳米管（SWCNT）复合材料的高压结晶样品,通过X射线衍射（XRD）、差热分析仪（DSC）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）,研究了高压处理样品的结晶行为、结构变化及形貌特征。结果表明：在1.0~2.5 GPa压力下,属于高压熔体结晶;在3.0和4.5 GPa压力下属于高压退火处理;高压结晶或高压退火均有助于聚合物片层晶体的增厚,并且高压熔体结晶的增厚效果优于高压退火处理。XRD结果表明,PA1010的三斜晶型在高压处理后保持不变,高压熔体结晶或高压退火都可以使(100)晶面和(010)晶面间距减小,即高压处理致使聚合物分子链紧密堆积。DSC结果表明：在高压熔体结晶过程中,升高压力和温度可以得到片层厚度较大的PA1010晶体;在2.0 GPa、350 ℃下获得的高压结晶样品的熔点和结晶度最高,分别达到208.5 ℃和64.6%。SEM和TEM结果表明：与常压结晶样品相比,高压结晶样品内部出现c轴厚度超过150 μm的大尺寸晶体;SWCNT与PA1010基体之间形成相互穿插的网络结构,刚性的SWCNT作为高压成核剂促进PA1010晶体生长和增厚。     

Mechanisms for regularization of chain folds during annealing of polyhexamethylene adipamide

The regular fold content of polyhexamethylene adipamide depends on the initial crystallization conditions, the degree of orientation, and the annealing time and temperature. At low annealing temperatures, the regular fold content increases linearly with crystallinity and arises from lamellar crystallization of isolated amorphous or interlamellar regions. At intermediate annealing temperatures, the increase in regular fold content greatly exceeds the crystallinity increase. The excessive increase in folds in this temperature range arises from regularization of loose loops and melting and recrystallization from extended to more folded type of crystals. At the higher annealing temperature, the crystallinity shows an increase relative to the fold content, and this implies that the increase in fold period occurs at the expense of folding.     

Isothermal lamellar thickening in blends of linear and low-density polyethylene

Polyethylene blends were studied by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Binary blends of commercial linear polyethylene (LPE) with two low-density polyethylenes (LDPEs) of melt indexes, about 20 and about 0.27 g/10 minutes, were investigated. The blends, with 10% and 50% LPE contents, and the pure LPE were isothermally crystallized at 124°C for up to 48 h under solid-liquid phase segregation conditions. Double melting endotherms were obtained for the blends. Results show that, despite differences in crystallization kinetics between both types of blends, the same depression in the LPE melting temperature and approximately the same LPE crystal thicknesses were found for the blend compositions. In addition, the extent of occurrence of lamellar thickening in LPE during crystallization is a function of its content in the blend.     

Small-angle neutron scattering study of aggregate structures of multi-headed pyridinium surfactants in aqueous solution

The aggregate structures of a set of novel single-chain surfactants bearing one, two and three pyridinium headgroups have been studied using small-angle neutron scattering (SANS). It is found that the nature of aggregate structures of these cationic surfactants depend on the number of headgroups present in the surfactants. The single-headed pyridinium surfactant forms the lamellar structure, whereas surfactants with double and triple headgroups form micelles in water. The aggregates shrink in size with increase in the number of headgroups in the surfactants. The aggregation number (N) continually decreases and the fractional charge (α) increases with more number of headgroups on the surfactants. The semimajor axis (a) and semiminor axis (b = c) of the micelle also decrease with the increase in the number of headgroups in the surfactants. This indicates that hydrocarbon chains in such micelles prepared from multiheaded surfactants adopt bent conformation and no longer stay in extended conformation.     

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Z_c), and half-time of crystallization (t_1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the crystallization of polyethylene. Moreover, an HDPE sample of both high molecular weight (M_w) and molecular weight distribution (MWD) had a different crystallization rate dependence from the other samples at various corresponding cooling rates. The crystallization activation energy for nonisothermal crystallization of different PEs was determined using the Kissinger method and showed that the presence of LCB as well as high M_w can increase the crystallization activation energy of polyethylene.     

The role of the amorphous fraction for the equilibrium shape of polymer single crystals

The equilibrium state of polymer single crystals is considered by explicitly taking into account the amorphous fraction formed by loops and tails of the chains using a statistical model introduced by Muthukumar (Philos. Trans. R. Soc. London, Ser. A 361, 539 (2003)). We show that under realistic conditions below the equilibrium melting temperature, tight loops and close re-entries are favored, and that the amorphous fraction can be mapped into an excess surface free energy. The model is extended to many-chain crystals where it is shown that the lamellar thickness increases with the number of chains in the crystal and extended-chain conformations are thermodynamically favored if the number of chains in the crystal is sufficiently large. The number of chains necessary to form an extended-chain crystal in thermodynamic equilibrium scales with the square of the degree of polymerization of the chains. We discuss the temperature behavior of the equilibrium crystal thickness in the under-cooled state.     

Effect of annealing on the structure and morphology of nylon 6 fibers

Changes in the structure of nylon 6 fibers annealed in dry and wet atmospheres were studied by small-and wide-angle x-ray diffraction. In the presence of water or saturated steam, fibers can be annealed to the same strucutral state at temperatures 70°C lower than in dry atmosphere. This is due to the enhanced mobility of the molecular segments in the amorphous region, a mechanism which is also known to lower the T_g by the same amount. Upon annealing under unconstrained conditions, lamellar spacing, crystallite size in the equatorial plane, crystalline as well as fiber density, and the chain-axis repeat increase with annealing-temperature; whereas crystalline orientation and the Van der Waals separation of the hydrogen-bonded sheets decrease. The monoclinic angle 8 remains constant at 66.7° (σ = 0.3°) and might depend on the starting fiber rather than on the treatment of the fiber. Most of these changes occur above a critical temperature of 170°C if dry, or 100°C if wet; rate of crystallization is also the highest under these conditions in nylon. The effect of these changes on such fiber properties as dyeing and the role of micro voids in dye diffusion and in dye uptake are discussed. Surface premelting and the accompanying changes in the surface structure of the lamellae, selective melting, and more importantly, the longitudinal motion of the nylon 6 chains and the resulting folding of interfibrillar extended amorphous chains are invoked to explain the shrinkage of the fiber, disorientation of the crystallites, increase in crystalline perfection, and the increase in lamellar spacing.     

Structure changes caused by strain annealing of nylon 6 fibers

Experiments are described in which nylon 6 fibers are annealed while subjected to a constant stretch (or slack). Subsequent mechanical and structural measurements are described and analyzed. A paracrystalline structure model is proposed in which folded chains, fully extended chains, partially folded, and partially extended chains coexist in the highly drawn high strength fiber. An explanation of structural changes occurring during the thermal-mechanical treatment is that the folded and partially folded chains are arranged randomly in staggered fashion in small units throughout the structure. During slack annealing, the chains become more folded and shrinkage occurs. Some of the chain refolding will be permanent and may act as new defect sites thereby reducing fracture stress. During annealing in the presence of comparatively high tensile stresses the folded chains are unfolded to some extent, but not completely, and the load-carrying chains in the structure are more uniformly loaded. At the higher applied strains during annealing, chains are broken and may snap back into folds. The tension annealing increases the overall chain orientation, the strained segment uniformity, and the number of load-bearing chains. These factors may contribute to an increased fracture stress.     

TIME-RESOLVED SAXS/WAXS STUDY OF PHASE BEHAVIOR AND CRYSTALLIZATION IN POLYMER BLENDS

Real-time SAXS and WAXS patterns have been simultaneously obtained during isothermal melt-crystallization of blends of low-molecular-weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA). The analysis of results shows that the originally homogeneous, single-phase polymer blend separates into two phases. The PMMA molecules diffuse from the blend and form completely segregated regions while PEO starts to crystallize. The first and dominating effect at the beginning of crystallization is the formation of unstable lamellae of nonintegrally folded chains (NIF). The real-time crystallinity and density of the PEO crystalline phase in absolute units were obtained from the time-resolved SAXS/WAXS results. The structure development proceeds in two steps. A very fast evolution of PEO crystals from the melt starts to crystallize in disordered NIF lamellae with thick amorphous interlayers and with a lower density of crystalline phase. The steep growth of crystallinity and crystalline density mean quick thickening of crystalline part of lamellae and improvement of their crystalline structure. In the second step, the structure of the crystalline phase gradually improves and crystallinity grows very slowly. The recrystallization of NIF lamellae into extended chain lamellae (EC) and lamellae with once folded chains (1F) proceeds during both stages of crystallization.     

Nonisothermal Crystallization of the Polypropylene/Organosilica Nanocomposite

Abstract

Nonisothermal crystallization of the neat isotactic polypropylene homopolymer (PP‐0) and of the nanocomposite containing 4.68 wt.% of organosilica (PP‐4.68) was studied in the standard differential scanning calorimetry (DSC) mode during constant‐rate cooling from the melt state. Analysis of the nucleation parameters derived from cooling rate dependencies of the temperatures for the onset of crystallization exotherms suggested a slight increase of the nucleation barrier for lamellar crystallization of PP within a confined space between neighboring nanoparticles of an infinite cluster of the nanocomposite, concomitant to stronger restrictions to transport of PP segments across the melt/lamellar crystal interface. The overall crystallization rate data for PP‐4.68 were consistent with the assumption of two separate contributions from the initial (unconstrained) and the subsequent (constrained) nucleation mechanisms, respectively. The obtained results were considered as evidence for a coexistence in an undercooled PP melt of the nanocomposite of initial nucleation sites characteristic for the neat PP‐0, and the basically different nucleation sites (presumably, PP chains anchored by both ends to the surfaces of two adjacent nanoparticles).