Interlamellar Amorphous Regions Research Articles

How 4-(7-Octen-1-yl)-N,N-diphenylaniline co-units induce strong melt memory effects in isotactic polypropylene random copolymers

The random incorporation of comonomer units in isotactic polypropylene usually disrupts the chemical regularity of the macromolecular chain, thereby remarkably affecting the crystallization and melting processes. We study the influence of a novel comonomer 4-(7-Octen-1-yl)-N,N-diphenylaniline (ODPA), in the concentration range of 1.1–6.8 mol%, on the crystallization and self-nucleation behavior of isotactic polypropylene copolymers. Non-isothermal and isothermal differential scanning calorimetry experiments demonstrated that crystallization kinetics are significantly retarded upon comonomer incorporation as the ODPA co-units are excluded to the amorphous regions. Furthermore, ODPA comonomer incorporation leads to a sharp increase in nucleation density; thus, spherulitic superstructures become significantly smaller. Remarkably, the ODPA co-units induce a strong melt memory effect that can persist even above the equilibrium melting temperature for copolymers with 4.4–6.8 mol% ODPA. The width of Domain IIa (exclusive melt-memory domain), an indicator of the self-nucleation strength, starts to increase sharply once the content of ODPA exceeds 1.8 mol%. In contrast, the width of Domain IIb (self-seeding domain) remains constant. We attribute the self-nucleation effect in these ODPA copolymers to the aggregation of co-units in the interlamellar amorphous regions that hinder, upon melting, the motion of the chain sequences that were initially packed within the crystal lattice. In this way, to reach the isotropic melt state, the sample must be heated to much higher temperatures than its apparent or even equilibrium melting point, depending on the comonomer content. Consequently, nucleation upon cooling from the self-nucleated melt becomes easier and faster.

Barrier Properties of Semicrystalline Polylactide: The Role of the Density of the Amorphous Regions

The work explains the mechanism of the improvement of oxygen barrier performance of cold-crystalized polylactide (PLA) modified with low molecular weight compounds (triethyl citrate, TEC; glycerol, Gly; and polyethylene glycol, PEG). It was found that the incorporation of small amounts of modifier (0.5–1.5 wt %) into the PLA matrix greatly reduces the oxygen transport properties. On the other hand, a further increase in the modifier content in PLA blends led to a deterioration of these properties. Based on the thermal analysis, it was shown that the modifier present in the PLA matrix did not affect the degree of crystallinity. Positron annihilation lifetime spectroscopy (PALS) and density measurements were used to correlate the observed changes in the parameters of oxygen permeability with the molecular packing in the interlamellar amorphous region. Overall, it was found that the nanostructure of the amorphous phase, largely dependent on the content and compatibility of the modifier with the polymer matrix, is of key importance in the barrier properties of PLA/modifier systems.

Plasticization of poly(3‐hydroxybutyrate) with triethyl citrate: Thermal and mechanical properties, morphology, and kinetics of crystallization

AbstractPoly(3‐hydroxybutyrate) (PHB), is one important biopolymer and a promising alternative to petroleum‐based plastics. In this article, formulations of PHB and triethyl citrate (TEC) as plasticizer were prepared by melt extrusion. The effect of TEC on the mechanical, thermal, and morphological properties of PHB was investigated by tensile tests, impact resistance, dynamic‐mechanical analysis, differential scanning calorimetry, polarized optical microscopy, and small‐ and wide‐angle X‐ray scattering. TEC acted as an efficient plasticizer for PHB, imparting gradual changes in the properties as the mass fraction of TEC increased. A reduction in the elastic modulus, an increase in the intensity of β relaxation indicated a higher capacity of mechanical energy dissipation for the formulations containing higher mass fractions of TEC. TEC reduced its glass transition and melting temperatures, contributing to the increase of the processing window of the temperature and minimizing thermal degradation of PHB. TEC had a strong influence on the kinetics of crystallization, the morphology of the spherulites, and the crystalline structural parameters, such as long period, crystalline lamella, and interlamellar amorphous region thicknesses. Our study clarifies how the morphology of the PHB crystalline phase evolves in the presence of the plasticizer and with the time of crystallization.

An Effect of Water Presence on Surface Exfoliation of Polypropylene Film Initiated by Photodegradation

A polypropylene (PP) film photodegradation test was performed in water at 60 °C with a specific photocatalyst under visible light irradiation to identify its fragmentation mechanism. The photodegradation degree reached a maximum at the 144 h photodegradation time and showed the decreases at the 288 h. The surface being uneven was observed at the 288 h in the SEM photograph. These results showed that the surface exfoliation was provoked by the test. The spherulite structure contributed to the surface microcrack location and growth. The microcrack allowed entrance of water into the inner amorphous part. The water caused an internal stress by expanding itself due to radiant heat from the visible light irradiation and induced a microcraze in the interlamellar amorphous region. The microcraze developed into an internal microcrack, and surface microcracks finally coalesced together, producing a planar exfoliation. The major long side length of the exfoliation part was below 100 μm at both the 144 h and 288 h. The weight change ratio and rate of the photodegraded PP film showed multi-stages with the increase of photodegradation time. The increment of positive rate was small, showing that the exfoliation caused suppression of the autoxidation rate. The negative rate was due to the exfoliation behavior and was nonlinear to the photodegradation time.

Structure–Property Evolution of Poly(ethylene terephthalate) Fibers in Industrialized Process under Complex Coupling of Stress and Temperature Field

The structure–property evolution of commercial poly(ethylene terephthalate) (PET) fibers obtained from the different drawing and heat-setting stages in industrial processing was systematically investigated. Upon combination of thermal analysis (DSC and DMA) with crystallization and orientation (WAXD and SAXS), the variation of crystallization and microstructures mainly containing lamellar and microfibrillar crystals following the processes were discussed in connection with properties. Results indicate the significant tenacity increase of fiber in the drawing process is mainly attributed to the orientation development of the interlamellar amorphous region, the interfibrillar extension of amorphous molecular chains, and its entanglement with the lamellae. Accordingly, a decline of shrinkage can be seen as a fact of the coiling of amorphous molecular chains, the formation of rigid amorphous fraction, and the increase of crystallinity. Thus, a new four-phase model has been proposed to clarify the structure–pr...

Molecular, Crystalline, and Lamellar Length-Scale Changes in the Poly(l-lactide) (PLLA) during Cyclopentanone (CPO) Desorption in PLLA/CPO Cocrystals

Polymer–solvent complexes, poly(l-lactide) (PLLA) with cyclopentanone (CPO), were studied at multiple length scales using differential scanning calorimetry, small-angle neutron scattering, Fourier transform infrared spectroscopy, and temperature-dependent wide- and small-angle X-ray scattering. PLLA crystallizes in the e form when organic solvents such as CPO are incorporated into the crystal lattice at subambient temperatures. The transformation of this structure into the α form during solvent desorption and the accompanying changes in the lamellar structure were followed by various measurements on PLLA/CPO cocrystals. SANS data suggest that CPO is present stoichiometrically in the crystal lattice and as clusters in the interlamellar amorphous regions in the nominally dried samples. DSC thermogram showed a sharp endotherm during this e to α transition. X-ray fiber diagrams showed that the e form transforms to the α form over a temperature range (40–55 °C) as the solvent molecules are expelled from the cr...

Incorporation of plasticizers in sugarcane-based poly(3-hydroxybutyrate)(PHB): Changes in microstructure and properties through ageing and annealing

Poly(3-hydroxybutyrate)(PHB), a natural polymer produced by bacteria via crop-based manufacturing, presents interesting characteristics like thermoplastic behaviour and biodegradability. However, its peculiar fine lamellar structure is highly susceptible to an intrinsic ageing phenomenon that contributes to a remarkable brittleness and low toughness. It has been shown that an annealing treatment can be used to increase toughness and avoid ageing for some PHB-based materials. The incorporation of plasticizers can be another alternative to increase toughness in brittle polymers. In this work, three industrial plasticizers, tri(ethylene glycol) bis(2-ethylhexanoate)(TEG-EH), triethyl citrate (TEC) and tributyl citrate (TBC), were incorporated in PHB via melt-mixing and the effect of these additives through ageing and after annealing was investigated. TEC and TBC were also added to a reactive blend of PHB/PEMGMA. All the compositions studied had 10wt% of plasticizer. Samples were characterized by differential scanning calorimetry, small angle X-ray scattering, scanning electron microscopy, tensile and impact testing and dynamic mechanical analysis. The compositions with plasticizers presented poor mechanical performance even though they were less affected by ageing. Annealing the samples deteriorated even further their mechanical properties due to plasticizer migration out of the interlamellar amorphous regions.

Nanocellular and needle-like structures in poly(l-lactic acid) using spherulite templates and supercritical carbon dioxide

The random nanocells and the radial needle-like cells are formed in the interlamellar amorphous regions of spherulites in PLLA.

Dependence of conformational relaxation on nanoconfinement in semicrystalline poly(ethylene terephthalate)

Novel aspects concerning the micromechanical response of amorphous regions confined by crystals in nano-sized domains are illustrated for semicrystalline poly(ethylene terephthalate) (PET), on the basis of microindentation hardness and dynamical mechanical spectroscopy (DMS) analyses (10-3÷60 Hz frequency range for DMS). PET samples were crystallized from the glassy state at low and high temperatures (Tc=100°C and 160°C, respectively) and subsequently recrystallized some degrees above to gain information on the nature of the amorphous regions. DMS at 95°C reveals two segmental relaxation processes ascribed to the interlamellar amorphous regions (slow mode) and to the interstack amorphous pockets (APs) (fast mode) respectively, the latter being the object of the present study. It is shown that recrystallization changes the cooperativity within the APs. At low Tc, an increase in the cooperativity and the free energy barrier for readjustment is found after recrystallization. Room temperature indentation measurements reveal an enhancement of the average hardness value of the amorphous regions, Ha, on samples recrystallized a few degrees above Tc=100°C. The reduced size of the APs regions upon recrystallization is discussed as a relevant parameter giving rise to an enhanced segmental confinement and a parallel hardening. Results for the high Tc material reveal that, in this case, both the cooperativity (measured at 95°C) and Ha (measured at room temperature) decrease upon recrystallization. The influence of the coupling of the APs with the crystal walls, on the relaxation mode and in turn on the mechanical behavior of the material, is envisaged to be a possible mechanism underlying these observations.

Multiaxial deformation of polyethylene and polyethylene/clay nanocomposites: in situ synchrotron small angle and wide angle X-ray scattering study

A unique in situ multiaxial deformation device has been designed and built specifically for simultaneous synchrotron small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) measurements. SAXS and WAXS patterns of high-density polyethylene (HDPE) and HDPE/clay nanocomposites were measured in real time during in situ multiaxial deformation at room temperature and at 55 C. It was observed that the morphological evolution of polyethylene is affected by the existence of clay platelets as well as the deformation temperature and strain rate. Martensitic transformation of orthorhombic into monoclinic crystal phases was observed under strain in HDPE, which is delayed and hindered in the presence of clay nanoplatelets. From the SAXS measurements, it was observed that the thickness of the interlamellar amorphous region increased with increasing strain, which is due to elongation of the amorphous chains. The increase in amorphous layer thickness is slightly higher for the nanocomposites compared to the neat polymer.

Composition-dependent phase segregation and cocrystallization behaviors of blends of metallocene-catalyzed octene-LLDPE(D) and LDPE(H)

The morphological structures of slowly cooled blends of deuterated metallocene-catalyzed octene linear low-density polyethylene (O-mLLDPE(D)), and hydrogenous low-density polyethylene (LDPE(H)) were studied by using small angle neutron scattering in combination with complementary small angle X-ray scattering and differential scanning calorimetry. The phase segregation, which is more nanoscale than macroscale, and cocrystallization behaviors were found to vary with the blend composition. Phase-segregated O-mLLDPE(D) lamellae are predominantly formed in LDPE(H)-rich compositions. In contrast, few segregated O-mLLDPE(D) lamellae form in O-mLLDPE(D)-rich compositions, and instead O-mLLDPE(D) lamellar stacks are extensively cocrystallized with LDPE(H) mostly in the interlamellar amorphous region.

Investigation of the mobility in poly(vinylidene fluoride) ferroelectric films with different structures

The molecular mobility in two isotropic poly(vinylidene fluoride) samples crystallized in the α phase from the melt with different crystallinities and lattice perfections has been investigated by broadband dielectric spectroscopy. It has been revealed that the local-mobility parameters are insensitive to structural features. The average relaxation times of micro-Brownian motion in the disordered phase are found to be identical in the samples under consideration, which can be associated with the localization of the mobility in the interphase layer at the crystal-amorphous phase interface. The motion in the crystal (α c transition) has been described using the soliton mechanism of relaxation. It has been found that, as should be expected, the average relaxation time increases with an increase in the longitudinal crystal size; however, in this case, a strict quantitative correlation is absent. According to the small-angle X-ray scattering data, this is caused by different microstructures of interlamellar amorphous regions. It has been demonstrated that the samples with a higher degree of crystallinity are characterized by a larger difference in the electron densities of the crystalline and amorphous phases and a larger size of the amorphous layer. It has been assumed that an increase in the concentration of chemical defects in the interlamellar layers with a simultaneous increase in their length is responsible for the increased probability of attenuation of the solitary wave (conformational defect) in its passage between neighboring lamellar crystals.

The role of the amorphous phase in the re-crystallization process of cold-crystallized poly(ethylene terephthalate)

The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures T(c) = 100°C and T (c) = 160°C, i.e. in the middle of the alpha relaxation region and close to its upper bound, respectively, are analyzed. During heating from room temperature, a structural rearrangement of the stacks is always found at T (r) approximately T (c) + 20°C. This process is characterized by a decrease of the linear crystallinity, irrespective of T(c); on the other hand, the WAXS crystallinity never increases with T below T(c+30)°C. The lamellar thickness in the low-T(c) sample decreases significantly after the structural transition, whereas in the high-T(c) sample the lamellar thickness remains almost unchanged. In both, high- and low-T(c), the interlamellar thickness increases above T(r). Moreover, the high-T(c) sample shows a lower rate of decrease of the mechanical performance with increasing T as the threshold T(r) is crossed. This result is interpreted in terms of the formation of rigid amorphous domains where the chains are partially oriented. The presence of these domains would determine i) the stabilization of the crystalline lamellae from the thermodynamic point of view and ii) the increase of the elastic modulus of the amorphous interlamellar regions. This idea is discussed by resorting to a phase diagram. An estimation of the chemical-potential increase of the interlamellar amorphous regions, due to the enhancement of the structural constraints hindering segmental mobility, is offered. Finally, previous calculations developed within the framework of the Gaussian chain model (F.J. Baltá Calleja et al., Phys. Rev. B 75, 224201 (2007)) are used here to estimate the degree of chain orientation induced by the structural transition of the stacks.

Role of Amorphous Region on the Deformation Behavior of Crystalline Polymers

As a means of better understanding the role of amorphous chains on the deformation behavior of crystalline polymers, we prepared high density polyethylene (HDPE) having various chain densities by extracting the petroleum resin dissolved in the interlamellar amorphous region of HDPE/petroleum resin blends. The small angle X-ray scattering intensity increased by increasing the amount of petroleum resin extracted, suggesting that the density of the amorphous chain in the interlamellar amorphous region decreases with an increase in the extracted petroleum resin. The yield stress and Young’s modulus decreased while the drawing property was improved by decreasing the amorphous chain density in the interlamellar amorphous region. TEM observation of the stretched specimen revealed that the decrease in overall stress is ascribed to the enhancement of the lamellar separation associated with a large void formation and lamellar deflection due to the low number density of tie chains, which act as transmitters of the external force in the interlamellar amorphous region.

Improving oxygen barrier properties of poly(ethylene terephthalate) by incorporating isophthalate. II. Effect of crystallization

AbstractThe present study examined crystallization of poly(ethylene terephthalate) (PET) and a series of random and blocky copolymers in which up to 30% of the terephthalate was replaced with isophthalate. Isothermal crystallization kinetics and direct observation of the spherulitic morphology revealed that the blocky copolymers crystallized more rapidly than PET, at least in part, as the result of enhanced spherulite nucleation. The statistical copolymers with 10 and 20% isophthalate achieved almost the same level of crystallinity as that of the blocky copolymers. The statistical copolymers with 10% isophthalate crystallized almost as fast as PET, although the statistical copolymer with 20% isophthalate crystallized much more slowly. Crystallization substantially reduced the oxygen permeability. Analysis of oxygen‐transport parameters in terms of a two‐phase structural model that considered a dispersion of lower‐permeability spherulites in an amorphous matrix of higher permeability revealed that dedensification of the PET interlamellar amorphous regions was responsible for the unexpectedly high oxygen solubility of crystallized PET. In contrast, copolymerization with isophthalate prevented dedensification of the interlamellar amorphous regions. As a result, crystallization was more effective in reducing the oxygen permeability. It was speculated that segregation of kinked isophthalate units to the amorphous regions of the spherulite relieved constraint on the interlamellar amorphous chain segments. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1629–1642, 2005

Exploring the melting of a semirigid-chain polymer with temperature-resolved small-angle X-ray scattering.

The thermal behavior of semirigid semicrystalline polymers differs significantly from that of flexible-chain polymers. The origin of the differences is believed to lie in the higher energy expenditure associated with the formation of adjacent re-entry folds at the crystalline surface in the case of semirigid chains. The effect of constraints imposed by the interlamellar amorphous regions on the neighboring crystals was studied with temperature-resolved synchrotron radiation small-angle X-ray scattering (SAXS). The analysis of SAXS patterns with a generalized paracrystalline lamellar stack model indicates that melting of a semirigid-chain polymer is not a random process but that the crystals grown in the smallest amorphous gaps melt first. This suggests that the hitherto largely neglected geometrical confinement effects may play an important role in determining the thermodynamic stability of semirigid-chain polymer crystals.

Deformation of lamellar structures: Simultaneous small‐ and wide‐angle X‐ray scattering studies of polyamide‐6

AbstractStructural changes during tensile deformation in semicrystalline polymers are studied by the analysis of both small‐ and wide‐angle X‐ray scattering data from polyamide‐6 fibers. The strain in the lamellar spacing is about the same as or higher than the fiber strain, suggesting that fiber elongation occurs by the deformation of the lamellar stack rather than slippage of the fibrils, especially during initial stages of elongation. The modulus of the lamellar structure is approximately 5 GN/m2, and this is close to the fiber modulus, which is only 2–3% of the crystal modulus. Fiber modulus is, therefore, determined by the lamellar structure as much as by the interfibrillar oriented chain segments. The four‐point small‐angle X‐ray scattering pattern in the relaxed fiber transforms reversibly into a two‐point pattern under strain. The structures that correspond to these two patterns, the bistable states of the lamellae, coexist until fiber breakage. The structure that gives rise to the two‐point pattern determines the ultimate strength of the fiber. Despite the small crystalline strain in the fiber direction, it is possible to follow the almost fully reversible changes in the orientation, size, and unit cell of the lamellar crystals. It is proposed that the appearance of the two‐point pattern, the decrease in the lateral crystallite size, and the increase in the stack diameter are due to tilting of the lamellar surface caused by large‐scale reversible strain in the interlamellar amorphous regions. This tilt is accomplished by slippage of the hydrogen‐bonded sheets along the chain axis. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 691–705, 2002

A Spramolecular Star-Like Polymer

This study reports the creation of star-like chain architecture through complexation of a mono-amino terminated poly(ethylene oxide) (PEO-NH2) with a macrocyclic compound, 4-sulfonic calix[n]arene (n=4 and 8) (SCA-n). The complexes were prepared in aqueous solution to render proton transfer from the sulfonic acid groups in SCA-n to the amino groups in PEO-NH2, and thereby generated 4- and 8-arm star-like complexes with the PEO arms attached to the SCA-n cores via ionic bonding. Formation of the complexes was verified by titration, solubility test, and dilute solution viscometry. The intrinsic viscosities ([η]) of the complexes in toluene were 60% higher than that of neat PEO-NH2, showing that the star-like structure retained in nonpolar solvents. On the other hand, PEO arms dissociated from the cores in water, so that the complexes and neat PEO-NH2 displayed similar [η]. The star-like supramolecules showed a SAXS peak associated with the core-core correlation in the melt with the inter-core distance of 7.3 nm. The concentration fluctuation in the melt was completely destroyed upon the crystallization of PEO arms. In the semicrystalline state, the SCA-n cores were excluded from the crystal lattice and resided in the interlamellar amorphous regions. Measurements of the crystal thickness by SAXS indicated that the PEO chains in the crystals were once- and twice-folded in neat PEO-NH2 and the complexes crystallized at 40 °C, respectively.

Structure and morphology of PET/PEN blends

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) have been analyzed in terms of their crystal structures and morphologies. Wide angle X-ray diffraction (WAXS), small angle X-ray diffraction (SAXS), and density measurements were performed on blend samples crystallized quiescently and on samples strain induced crystallized. For blends quiescently crystallized isothermally, the crystallite dimensions calculated from the WAXS diffraction patterns via the Scherrer equation, along with the results from the SAXS, led to the conclusion that the blends crystallized independently. This means the major component of the blend crystallized, while the minor component did not crystallize and was forced into the interlamellar amorphous region. WAXS studies on the strain induced crystallized samples showed a change in the lattice parameters with composition, leading to the conclusion of co-crystallization for these blends. Density measurement showed a decrease in density with increasing PEN content indicating a reduction of the percent crystallinity of the blends as compared to the unblended homopolymers.

Study on the double cold crystallization peaks of poly(ethylene terephthalate) 3. The influence of the addition of calcium carbonate (CaCO 3)

The appearance of double cold crystallization peaks in differential scanning calorimetry (DSC) heat scan for poly(ethylene terephthalate) (PET) samples properly treated before analysis was considered to have relation to the co-existence of two kinds of amorphous regions: interlamellar amorphous regions and complete amorphous regions between spherulites. The change in peaks positions, height and shape with the pre-treatment history reflects the corresponding change in amorphous regions. In this work, the influence of an additive, calcium carbonate (CaCO 3) on the cold crystallization behavior of PET was further studied using DSC. DSC study showed that the addition of CaCO 3 affected the double cold crystallization peaks of PET greatly, indicating that CaCO 3 exerted an influence on two amorphous regions of PET during the pre-treatment stage because the cold crystallization peak originated from the crystallization of remainder amorphous regions after thermal treatment. Specially, results revealed that the interspherulitic amorphous region was strongly reduced during the early pre-treatment stage of PET samples, resulting from the efficient nucleation effect of CaCO 3. The observation further revealed that CaCO 3 affects the cold crystallization behavior of PET to different degree, depending on the method of pre-thermal treatment. The difference is related to the temperature dependence of nucleation and chain diffusion.