Dominant Lamellae Research Articles

Dynamic electrical conductivity induced by isothermal crystallization in aluminum hydroxide filled silicone rubber

In this paper, taking aluminum hydroxide (ATH) filled silicone rubber as a model composite, we report the “n-shape” dynamic electrical conductivity during isothermal crystallization in nonconductive particles filled insulating polymers. During the initial crystallization, rapid growth of dominant lamellae blocks the transport path of impurity ions, contributing to the remarkable reduction of conductivity. Addition of ATH fillers could reduce the silicone rubber content and randomness of crystals formation, lessening the amplitude of decreased conductivity. Afterward, the silicone rubber composites further shrink due to the development of subsidiary lamellae. It results in low activation energy of ion migration and thereby enhances conductivity, whose amplitude is positively related to the crystalline phase. Finally, continuous growth of rigid amorphous fraction with reduced chain mobility impedes the ions transport, decreasing the conductivity again. The growth of rigid amorphous fraction is accelerated in silicone rubber with more ATH fillers because of more constraint of ATH on molecular chains. Our findings afford a simple means to manipulate insulation performance of nonconductive particles filled semi-crystalline polymers and provide a guideline for their use in real operation.

Adaptive properties of human cementum and cementum dentin junction with age

ObjectivesThe objective of this study was to evaluate age related changes in physical (structure/mechanical properties) and chemical (elemental/inorganic mineral content) properties of cementum layers interfacing dentin. MethodsHuman mandibular molars (N=43) were collected and sorted by age (younger=19–39, middle=40–60, older=61–81 years). The structures of primary and secondary cementum (PC, SC) types were evaluated using light and atomic force microscopy (AFM) techniques. Chemical composition of cementum layers were characterized through gravimetric analysis by estimating ash weight and concentrations of Ca, Mn, and Zn trace elements in the analytes through inductively coupled plasma mass spectroscopy. The hardness of PC and SC was determined using microindentation and site-specific reduced elastic modulus properties were determined using nanoindentation techniques. ResultsPC contained fibrous 1–3µm wide hygroscopic radial PDL-inserts. SC illustrated PC-like structure adjacent to a multilayered architecture composing of regions that contained mineral dominant lamellae. The width of the cementum dentin junction (CDJ) decreased as measured from the cementum enamel junction (CEJ) to the tooth apex (49–21µm), and significantly decreased with age (44–23µm; p<0.05). The inorganic ratio defined as the ratio of post-burn to pre-burn weight increased with age within primary cementum (PC) and secondary cementum (SC). Cementum showed an increase in hardness with age (PC (0.40–0.46GPa), SC (0.37–0.43GPa)), while dentin showed a decreasing trend (coronal dentin (0.70–0.72GPa); apical dentin (0.63–0.73GPa)). SignificanceThe observed physicochemical changes are indicative of increased mineralization of cementum and CDJ over time. Changes in tissue properties of teeth can alter overall tooth biomechanics and in turn the entire bone–tooth complex including the periodontal ligament. This study provides baseline information about the changes in physicochemical properties of cementum with age, which can be identified as adaptive in nature.

Melting temperature evolution of non-reorganized crystals. Poly(3-hydroxybutyrate)

In the present study the correlation between the melting behaviour of poly(3-hydroxybutyrate) (PHB) original, non-reorganized crystals and the crystallinity increase during isothermal crystallization is presented and discussed. Since the reorganization processes modify the melting curve of original crystals, it is necessary to prevent and hinder all the processes that influence and increase the lamellar thickness. PHB exhibits melting/recrystallization on heating, the occurring of lamellar thickening in the solid state being excluded. The first step of the study was the identification of the scanning rate which inhibits PHB recrystallization at sufficiently high T c . For the extrapolated onset and peak temperatures of the main melting endotherm, which is connected to fusion of dominant lamellae, a double dependence on the crystallization time was found. The crystallization time at which T onset and T peak change their trends was found to correspond to the spherulite impingement time, so that the two different dependencies were put in relation with primary and secondary crystallizations respectively. The increase of both T onset and T peak at high crystallization times after spherulite impingement was considered an effect due to crystal superheating and an indication of a stabilization process of the crystalline phase. Such stabilization, which produces an increase of the melting temperature, is probably connected with the volume filling that occurs after spherulite impingement.

Crystallization of poly(l‐lactid acid) monitored by dielectric relaxation spectroscopy and atomic force microscopy

An investigation was carried out on the crystallization process of poly(l-lactid acid) by dielectric relaxation spectroscopy and atomic force microscopy. Experimental results were generated by dielectric relaxation spectroscopy over a wide range of frequency and temperature in both the wholly amorphous state and during crystallization. The variation of the average relaxation time was studied during crystallization at 80 degrees C and the temperature dependence of this relaxation time for wholly amorphous and crystallized samples was analysed. This behaviour was modelled by Havriliak-Negami and Vogel-Fulcher equations. The sensitivity of the segmental dynamics to the degree of crystallinity was analysed, taking into account the relaxing segments and the thickness of the amorphous layer between lamellae. The morphologies obtained during crystallization processes at 80, 130 and 150 degrees C were monitored by atomic force microscopy at both the lamellar level and by analysing the multilayered superstructures formed. Hedrites, intermediate structures between single lamellar crystals and mature spherulites, were found to appear at the highest temperatures, whereas no evidence of hedrites was found at 80 degrees C, the spherulites seemed to be constructed from a framework of individual dominant lamellae that splay apart and branch. Complementary to the atomic force microscopy study, the evolution of the obtained morphologies was also followed by optical microscopy. Supporting evidence about the thermal behaviour of the polymers was obtained with differential scanning calorimetry.

Differential initial fold surface ordering: a new morphological indicator

Linear polyethylene crystallized isothermally on linear nuclei from blends with low density, branched polyethylene, cellulates at low concentrations with individual lead lamellae, typically ∼1 μm or so apart, advancing at a constant, faster rate than intervening lamellae whose growth is slowed by accumulated segregants. In these circumstances it is found that while initially all lamellae form with poor fold packing in {001} surfaces, only the faster-growing, lead lamellae continue to add molecules in this manner and eventually twist to relieve surface stress. For the majority, remaining lamellae growth slows and they soon adopt and retain {201} surfaces with their preferred fold packing. This not only illustrates how different lamellar components of texture can attain different habits with associated properties according to their respective growth rates, though with dominant lamellae determining the overall morphology, but in doing so, provides a new means to explore the intricacies of macromolecular self-organization.

Conductivity relaxation in the PEO–salt polymer electrolytes

The ionic conductivity of polyethylene oxide (PEO)-based electrolytes is complicated due to the coexistence of crystalline and amorphous phase below melting point of PEO complexes. The two-phase characteristics are greatly dependent upon thermal history, exhibiting variety of spherulitic morphology and crystallinity. Further complicacy comes from slow crystallization kinetics of the spherulites. We found that the ionic conductivity of PEO n LiClO 4 polymer electrolytes under isothermal conditions, after quenching from high-temperature phase, drops significantly for roughly first 10 h and then decreases very slowly thereafter. The conductivity relaxation observed can be assigned to be a consequence of the slow recrystallization kinetics of PEO. It corresponds to a gradual, slow secondary crystallization of PEO and PEO–salt complexes corresponding to thickening of spherulitic aggregates, possibly through a development of subsidiary lamellae which fill in the space between the dominant lamellae crystals. Hence, large inconsistencies in the conductivity values reported in many papers, varying more than three orders of magnitude, are rather obvious, originated from non-equilibrium nature and slow recrystallization kinetics of semicrystalline state.

In Situ Wide- and Small-Angle X-ray Scattering Study of Melting Kinetics of Isotactic Poly(propylene)

We describe a comparative study of long time isothermal melting kinetics of Ziegler−Natta catalyzed, and metallocene catalyzed, isotactic poly(propylenes). The role of the transverse lamellae in stabilizing the crystalline structures against melting is presented in light of the different molecular chain architecture and lamellar structure of these poly(propylenes). Both poly(propylenes) were first isothermally crystallized at 152 °C. Real time in-situ wide- and small-angle X-ray scattering (WAXS and SAXS) were used to study isothermal melting kinetics in a temperature region between melting of transverse and dominant (radial) lamellae, i.e., when most transverse lamellae are molten. Changes in WAXS crystalline peak height, SAXS invariant, and Bragg peak height with time all confirm that Ziegler−Natta iPP had slower melting kinetics than metallocene iPP. Melting kinetics is, apparently, unique to poly(propylene) as a consequence of a favorable nearly orthogonal epitaxial crystallization. What is observed is the instability of the dominant thicker lamellae due to the removal of the epitaxy. Both Ziegler−Natta and metallocene catalyzed polymers show a prolonged, gradual decay of crystallinity with time. Hence, melting kinetics is a general phenomenon observed in poly(propylenes) that crystallize preferentially in the alpha (monoclinic) modification. The crystallization conditions under which these iPPs exhibit melting kinetics are when crystallization occurs at relatively low undercooling, when the crystallites are not subject to reorganization.

Nonlinear time-dependent response of isotactic polypropylene

Tensile creep tests, tensile relaxation tests and a tensile test with a constant strain rate are performed on injection-molded isotactic polypropylene at room temperature. A constitutive model is derived for the time-dependent behavior of semicrystalline polymers. A polymer is treated as an equivalent network of chains bridged by permanent junctions. The network is modeled as an ensemble of passive mesoregions (with affine junctions) and active mesodomains (where junctions slide with respect to their reference positions with various rates). The distribution of activation energies for sliding of junctions in active mesoregions is described by a random energy model. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. It is demonstrated that the concentration of active mesodomains monotonically grows with strain, whereas the average activation energy for sliding of junctions and the standard deviation of energies suffer substantial down jumps in the vicinity of the yield point. With reference to the concept of dual population of lamellae, these changes in material parameters are attributed to transition from breakage of subsidiary (thin) lamellae in the subyield region to fragmentation of dominant (thick) lamellae in the postyield region of deformations.

New Contexts for Polymer Crystallization

The study of polyethylene grown as row structures from linear nuclei has brought new contexts to polymer crystallization. First, there is a division between slower and faster growth. For slower growth, fold surfaces are initially ordered, as {201} facets, whereas for normal, faster growth, they form as {001} and then reorganize behind the growth front as they separate, thicken, and form S-profiled dominant lamellae in banded growth. Surface free energies for growth will thus be higher than those for melting. Second, faster and slower growth correspond to the time for molecules to order at the growth front, being, respectively, less than or greater than the time for radial advance. This time increases for longer and more-branched molecules. Third, the lamellar thickness at the growth front increases logarithmically with elapsed time, with the first formed lamellae being thinnest and melting first; in less congested conditions, lamellae must also thicken behind the growth front. Fourth, for linear polymer and slower growth, the radial advance increases from one constant rate to another once the lamellar separation exceeds the molecular length and internuclear interference is lost. For a branched polymer, segregation is an additional factor producing nonlinear growth and kinetic discontinuities when cellulation sets in and again when its pattern changes at the onset of banding. Fifth, banding is confined to faster growth for linear polyethylene but also occurs for slower growth in linear-low-density polymers, when initially ordered fold surfaces become stressed as isothermal thickening brings excluded branches deeper into fold surface regions.

ON MORPHOLOGY AND MULTIPLE MELTING IN POLYPROPYLENE

A series of polypropylenes, including two modern high crystallinity materials (HCPP) were subjected to a stepwise crystallization procedure as a guide to their properties. The results were complicated by the development of double melting endotherms at the highest crystallization temperature. Both HCPP, when grown at the temperature of 145°C, give a double melting peak, but the proportions of the two peaks varied according to the density of nucleation. By partially melting a specimen between the two peaks, it was possible to assign the higher peak to radial dominant lamellae and the lower peak to mostly tangential subsidiary lamellae filling the space in between. Uniformly cross-hatched lamellae at the center of spherulites melt along with the lower melting population in the outer regions of the spherulites. Even if differences in crystallization temperature are eliminated, the properties of nucleated as opposed to nonnucleated PP may therefore be influenced by differences between the center and periphery of spherulites, with “central” properties much more in evidence in nucleated material. The development of these double endotherms is related to morphological constraint, rather than molecular fractionation. Their form is strongly influenced by cross-hatching, but the phenomenon is still found at 160°C where cross-hatching does not form.

On permanent cilia and segregation in the crystallization of binary blends of monodisperse n-alkanes

The isothermal crystallization kinetics and morphology have been investigated for a series of dilute binary blends using six monodisperse n-alkanes as guest in C162H326 as host. Two patterns of behaviour were observed. Guest molecules shorter than the host segregate as a separate population causing growth rates to become both reduced and non-linear. Morphologies are then noticeably less spherulitic than the host with less divergence between adjacent dominant lamellae but exhibiting no additional splaying at zero supercooling. By contrast, those blends with an n-alkane longer than the host co-crystallize (producing permanent cilia of controlled length) with a constant, but reduced, isothermal lamellar growth rate. Textures are now more spherulitic than the host, with additional splaying of an amount directly proportional to the number of permanent cilia and increasing with their length. The intercepts and slopes of plots of splaying data against supercooling are consistently related to permanent cilia plus inclined packing of initially rough lamellar surfaces and transient ciliation, respectively. The underlying causes of spherulitic growth for long molecules are thereby further confirmed and clarified.

Morphology and crystallization kinetics of dilute binary blends of two monodisperse n‐alkanes with a length ratio of two

AbstractThe crystallization kinetics and morphologies of dilute binary blends of the monodisperse alkane n‐C122H246 in n‐C246H494 and vice versa have been investigated. With a molecular length ratio close to two, this pair of n‐alkanes does not produce permanent cilia when once‐folded C246H494 molecules cocrystallize with extended chains of C122H246. In this condition, the supplementary splaying of adjacent dominant lamellae and the consequently more spherulitic textures, which are present in previous blends for which a longer guest molecule gives permanent cilia, are absent, although other features of blend crystallization remain. Specifically, the isothermal radial growth rate is constant for cocrystallizing blends, although less than for their pure hosts, but becomes nonlinear with cellulation when C122H246 forms a segregated population within extended‐chain C246H494. Increased nucleation in the blends give smaller scale textures than for the host materials, but the presence of a second component reduces splaying and thereby disfavors spherulitic growth. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2874–2887, 2001

Spherulitic Morphology of Isotactic Polypropylene and its Melting Behavior

The crystallization and lamellar morphology of isotactic polypropylene (ipp) have been investigated by thermal analysis and electron microscopy. When ipp samples are crystallized above 132°C, no β-phase was detected and the samples consisted of pure α-spherulites and those showing double melting endotherms, the occurrence of double-peak shapes was attributed to the melting and reorganizing of the radial dominant lamellae, while melting of tangential cross-hatched lamellae belongs to the broad tail endotherm which leads the lower melting endotherm peak.

On interfering nuclei and their novel kinetics during the crystallization of polyethylene from the melt

At high temperatures, linear polyethylene crystallizes from the quiescent melt as lamellae with inclined, ∼{201}, fold surfaces. Accordingly, when grown on linear nuclei to form row structures, the first lamellae to form do so epitaxially in two orientations symmetrically inclined to the axis of the row. With increasing radial distance, dominant lamellae widen, separate and become normal to the row albeit still with inclined fold surfaces. Isothermal growth rates are constant in both conditions but that for the later one is 3–4 times faster than the initial one. It is suggested that faster growth is a consequence of diminishing interference between secondary nuclei on neighbouring lamellae growing under Regime I conditions. The concept of interfering nuclei, competing for the same molecule, also provides a convincing explanation for previously reported kinetic effects in polyethylene and long n-alkanes as well as the presence of faster-growing dominant lamellae in the crystallization of such linear molecules.

Spherulitic Growth and Cellulation in Dilute Blends of Monodisperse Long n-Alkanes

Dilute blends of two guest monodisperse n-alkanes one, n-C 122 H 246 , shorter and one, n-C 246 H 494 , longer than the pure host, n-C 162 H 326 , have been investigated regarding the kinetics and morphology of both spherulitic growth and cellulation to clarify the related phenomena in polymers. All samples were crystallized isothermally between 118 and 124 °C. For the pure material this gave the optical textures expected of chain-extended crystallization, i.e., objects which may display circular envelopes but differ from classic spherulites in that they have domains of uniform crystallographic orientation. With increasing crystallization temperature these objects become larger and coarser to a limit when they are no more than aggregates of single crystals. Addition of 5% or 10% of a second component to n-C 162 H 326 lowers growth rates and drastically alters morphologies. When the guest is longer than the host, there is cocrystallization producing permanent cilia and finer-textured, radially symmetric spherulites. When it is shorter, it segregates with a decelerating growth rate akin to observations of cellulation in branched polyethylenes. The resulting solids have disrupted crystallographic ordering and textures with randomly oriented lamellae. In all cases, adjacent dominant lamellae splay apart by angles which increase linearly with supercooling. The rate of increase is less for blends than for the pure host and the magnitudes increase with guest concentration being greatest when there is both permanent and transient ciliation. Positive intercepts at zero supercooling may indicate a supplementary contribution from surface roughness of the embryonic lamella. The combined observations provide strong confirmation of the importance of ciliation in promoting spherulitic growth of long molecules, independent of segregation and cellulation.

A study of the morphologies and growth kinetics of three monodisperse n-alkanes: C122H246, C162H326 and C246H494

An optical and electron microscopic study has been made of three monodisperse n-alkanes crystallising as extended-chain lamellae. All showed similar morphologies at the same supercooling and the same progressive changes of texture with crystallization temperature. This is, with increasing supercooling, from a morphology composed essentially of individual lamellae arranged in parallel stacks and radiating from common nuclei through coarse, somewhat branched microstructures whose parallel stacks diverge, to finer, more-branched pseudo-spherulitic textures. Quenched, once-folded, C246 shows a random spherulitic texture with a dominant/subsidiary microstructure as for polymeric systems. The kinetics of extended-chain growth increase linearly with supercooling, as predicted by Hoffman, except for a local dip at the onset of lamellar branching. This new phenomenon occurs within one quantized state of lamellar thickness in contrast to ‘self-poisoning’ when once-folded forms give way to extended-chain crystallization. When dominant lamellae are no longer parallel but diverge, their mutual splaying angles increase with supercooling for all the three n-alkanes studied. The values are less, for the same supercooling, for the longer homologues with their thicker lamellae. This behaviour is as expected of transient ciliation due to the excess of molecular length over that of the secondary nucleus and thereby reinforces previous evidence demonstrating the responsibility of ciliation for spherulitic development in polymers.

DSC and TEM investigations on multiple melting phenomena in isotactic polystyrene

The multiple melting behavior and morphologies of isotactic polystyrene (iPS) isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The combination of thermal analysis and morphological results indicates that two lamellar populations are responsible for the so-called double melting behavior in iPS. The low-temperature melting peak is attributed to the melting of less perfect (thinner or defect containing) subsidiary lamellae formed in the framework of the dominant (thicker or more perfect crystalline) lamellae upon isothermal crystallization. The high-temperature one is mainly due to the melting of the dominant thicker lamellae, and to some less extent, the melting of a recrystallized population coming from the melted defect lamellae during the heating process in DSC.

From microns to nanometers: Morphology development in semicrystalline polymers by scanning force microscopy

In studies of spherulitic crystallization in polymers, many questions pertaining to the mechanism of the crystallization process have remained unanswered. A currently accepted view describes the development of spherulites from a framework of individual dominant lamellae that splay apart and branch (e.g., by a dislocation mechanism). This model, in addition, assumes that the space between the dominant lamellae is filled by subsidiary lamellae. In the center of a spherulitic entity, there is a hedritic core that occupies a relative volume that depends on the level of maturity of the spherulitic development. Typical hedritic views consist of lamellae that have a flat stack, or sheaflike, splaying appearance depending on the angle and depth of observation with respect to the central stack of lamellae. Visualization of the three-dimensional appearance of these hedrites by atomic force microscopy (AFM) and the observation of their growth in situ are the subjects of this article. First, we describe AFM studies of hedrites observed at etched surfaces of β-isotactic polypropylene (β-IPP). Evidence sup-porting splaying, branching via dislocations, and development of curved lamellae is presented. In the second part, we describe real-time hot-stage AFM in situ observations of the hedritic growth in poly(ethylene oxide) (PEO) and poly-(ε-caprolactone) (PCL).

On Spherulitic Growth and Cellulation in Polymers. A Unified Context

The new, unified, context for crystallization of linear macromolecules from the melt, established by recent work, is expounded. The morphology of spherulites shows that they form because of a short-range force operative at the branch points of dominant lamellae causing them to diverge at non-crystallographic angles ∼20°. Work on monodisperse n-alkanes has confirmed the identification of this short-range force with the pressure from (transient) cilia during growth. Accordingly, spherulites, like chainfolding, are a direct consequence of molecular length. It is suggested that the tendency to form coarse spherulites even for extended-chain growth at lower temperatures may result from the increasing difference between the lengths of nucleus and molecule. Crystallization on linear nuclei has been used to maximize the concentration of segregants at the growth front and to demonstrate cellulation in undoped polymers for the first time. The behavior of branched polyethylenes differs from the uniform growth of the linear polymer in coarsening and developing a corrugated growth front all the while slowing continuously towards an asymptotic steady state; differences of detail may be useful in distinguishing polymers of different catalytic origin and branch content. Spherulitic growth is also non-linear for these polymers but is always faster than for rows. When there is sufficient segregation spherulites themselves cellulate, increasingly so for higher branch content. Cellulation is thus an uncommon and secondary process which may be superposed on regular spherulitic growth beyond a certain distance. Cell dimensions do not scale with the diffusion length; in so doing the phenomenon displays new physics.

On the self-organization of linear macromolecules

The new unified context for crystallization of linear macromolecules from the melt, established by recent work, is expounded. The morphology of spherulites shows that they form because of a short-range force operative at the branch points of dominant lamellae that causes them to diverge at noncrystallographic angles of about 20°. Work on monodisperse n-alkanes has confirmed the identification of this short-range force with the pressure from dynamic cilia during growth. Accordingly, spherulites, like chain folding, are a direct consequence of molecular length. It is suggested that the tendency to form coarse spherulites even for extended-chain growth at lower temperatures may result from the increasing difference between the lengths of nucleus and molecule. Crystallization on linear nuclei has been used to maximize the concentration of segregants at the growth front and to demonstrate cellulation in undoped polymers for the first time. The behavior of branched polyethylenes differs from the uniform growth of the linear polymer in coarsening and developing protuberances at the growth front, all the while slowing continuously toward an asymptotic steady state; differences of detail may be useful in distinguishing polymers of different catalytic origin and branch content. Spherulitie growth is also nonlinear for these polymers, but is always faster than for rows. When there is sufficient segregation, spherulites themselves cellulate, increasingly so for higher branch content. Cellulation is thus an uncommon and secondary process may be superposed on regular spherulitie growth beyond a certain distance. Cell dimensions do not scale with the diffusion length; in so doing, the phenomenon displays new physics.