Structural changes in isothermal crystallization process of polyoxymethylene investigated by time-resolved FTIR, SAXS and WAXS measurements

Structural evolution in the isothermal crystallization process of polyoxymethylene from the molten state has been investigated by carrying out the time-resolved measurements of infrared spectra and synchrotron small angle X-ray scattering (SAXS) and wide angle X-ray scattering. In case of isothermal crystallization at 130 °C, for example, the infrared bands intrinsic of folded chain crystal (FCC) morphology appeared at first, and then the bands of extended chain crystal (ECC) morphology were detected with time delay of ca. 150 s. In the SAXS experiment at 130 °C, the lamellar stacking structure of the long period of ca. 15 nm was observed at first, which changed rapidly to ca. 12 nm in a short time. The SAXS peak with the long period of ca. 6 nm started to appear with a time delay of ca. 150 s after the initial lamellae appeared and coexisted with the initially-observed 12 nm peak. Judging from the timing to detect these characteristic infrared and SAXS signals, a good correspondence was found to exist between the stacked lamellar structure of 12 nm long period and FCC morphology and between the structure of 6 nm long period and ECC morphology. The quantitative analysis was made for the SAXS data on the basis of the lamellar insertion model combined with the paracrystalline theory of the second-kind of disorder. The following structural evolution was deduced from all these results. Immediately after the temperature jump from the melt to 130 °C, the stacked lamellar structure of FCC morphology was generated at first. New lamellae were formed from the amorphous region in between the originally-existing lamellae about 150 s later, where the random chain segments bridging the adjacent lamellae were extended to form the taut tie chains, giving infrared bands of ECC morphology. This inserted lamellar structure of 6 nm long period coexisted at a population of ca. 6% with the initially-formed lamellar stacking structure of 12 nm long period. When the experiment was made at 150 °C, only the formation of stacked lamellar structure of FCC morphology was observed and the insertion of new lamella did not occur.     

Structural changes in non-isothermal crystallization process of melt-cooled polyoxymethylene. [I] Detection of infrared bands characteristic of folded and extended chain crystal morphologies and extraction of a lamellar stacking model

Structural change in the crystallization process of polyoxymethylene (POM) cooled from the molten state has been investigated by the measurements of infrared spectra and small-angle (SAXS) and wide-angle X-ray scatterings (WAXS). When the melt was cooled slowly, the infrared bands characteristic of a folded chain crystal (FCC) were observed to appear around 156 °C. Below 140 °C, the infrared bands intrinsic of an extended chain crystal (ECC) were detected to increase in intensity. In the SAXS measurement, the peak (L₁) corresponding to a stacked lamellar structure with the long period of ca. 14 nm was found to grow in parallel to the growth of infrared FCC bands. In the temperature region of the observation of infrared ECC bands, the new peak (L₂) of long period of ca. 7 nm was found to appear and the intensity exchange occurred between the L₁ and L₂ peaks, that is, with decreasing temperature the L₂ peak increased the intensity and its height became comparable to the L₁ peak height. By combining all these experimental data, a model to illustrate the formation process of lamellar stacking structure has been presented. After the appearance of stacked lamellar structure of 14 nm long period from the melt, new lamellae are created in between the already existing lamellae and the long period changes to the half value, 7 nm. Some of molecular chain stems in a lamella are speculated to pass through the adjacent lamellae to form a bundle of fully extended taut tie chains, which are considered to be observed as the infrared bands characteristic of ECC morphology. Although the POM samples used in this experiment may contain small amount of low-molecular-weight macrocyclic component, it was not plausible judging from the various experimental data to assign the secondarily observed 7 nm SAXS peak to the repeating period originating from the stacked structure of macrocyclic compounds.     

Structural correlation between crystal lattice and lamellar morphology in the ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers as revealed by the simultaneous measurements of wide-angle and small-angle X-ray scatterings

Intimate correlation has been detected between the crystal structure change and the morphological change of stacked lamellae in the ferroelectric phase transition of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers on the basis of simultaneous measurement of wide-angle (WAXS) and small-angle X-ray scattering (SAXS) patterns in the heating process. The VDF content of copolymers investigated was 65 and 73 mol%. For both of the copolymers, a cold-drawn sample showed the WAXS pattern of the regular low-temperature phase and the four-points SAXS pattern corresponding to the stacking structure of lamellae tilted by about 30° from the draw axis. When the sample was heated above the Curie transition point T_c, the low-temperature phase changed to the conformationally-disordered paraelectric high-temperature phase and the stacked lamellae reoriented into the direction closer to the draw axis. It was also found that the thickness and long period of the lamellae increased remarkably above T_c. These large changes in lamellar stacking mode occurred in parallel with the trans-to-gauche conformational change of the molecular chains in the crystal lattice. In this way, the morphological change was found to relate intimately with the crystal structural change, both of which are considered to be caused commonly as a result of the trans-gauche conformational change and the translational motion of the thermally-activated molecular chains along the chain axis.     

The role of interlamellar chain entanglement in deformation-induced structure changes during uniaxial stretching of isotactic polypropylene

In-situ small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) were carried out to investigate the deformation-induced structure changes of isotactic polypropylene (iPP) films during uniaxial stretching at varying temperatures (room temperature, 60 °C and 160 °C). From the WAXD data, mass fractions of amorphous, mesomorphic and crystal phases were estimated. Results indicate that at room temperature, the dominant structure change is the transformation of folded-chain crystal lamellae (monoclinic α-form) to oriented mesomorphic phase; while at high temperatures (>60 °C); the dominant change is the transformation of amorphous phase to oriented folded-chain crystal lamellae. This behavior may be explained by the relative strength between the interlamellar entangled network of amorphous chains, which probably directly influence the tie chain distribution, and the surrounding crystal lamellae. It appears that during stretching at low temperatures, the interlamellar entanglement network is strong and can cause lamellar fragmentation, resulting in the formation of oriented mesomorphic phase. In contrast, during stretching at high temperatures, the chain disentanglement process dominates, resulting in the relaxation of restrained tie chains and the formation of more folded-chain lamellae.     

Supramolecular structure of the solid-state complexes of polyacrylamide and dodecylbenzenesulfonic acid

The solid-state complexes of a flexible polymer, polyacrylamide (PAA), and an amphiphilic surfactant, dodecylbenzenesulfonic acid (DBSA), have been investigated. Complexation between PAA and DBSA occurred via proton transfer from DBSA to the carbonyl group in PAA, giving rise to a “supramolecular comb-like polymer” with ionic bonding. The mesomorphic phase in the complexes was identified by the birefringent patterns under polarized optical microscopy. Wide angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) revealed that the complexes microphase separated into a lamellar morphology consisting of alternating polar and nonpolar layers with the long period of ca. 3 nm. As the composition x (the average number of DBSA molecules bound with a PAA repeating unit) ≥ 0.7, the SAXS profiles were characterized by a major scattering peak associated with the flat lamellar structure. Multiple scattering peaks were observed at lower degree of complexation (x ≤ 0.6), which were ascribed to the formation of undulated lamellae that organized into a macrolattice with the diffraction patterns observable by SAXS. Preliminary assignments of the lattice planes suggested that the lobes of the undulated lamellae organized into body centered cubic (bcc) or simple cubic (sc) types of unit cell. The glass transition temperature of the polar layers in PAA(DBSA) complexes increased with increasing degree of complexation owing to the stiffening of polymer chains. Complexation with DBSA also enhanced the thermal stability of PAA, where the thermal decomposition temperature can be raised by as much as 35 °C.     

Lamellar morphologies and crystal stability of syndiotactic polystyrene in α-crystalline form

Syndiotactic polystyrene (sPS) samples melt-crystallized into neat α″ hexagonal modifications were prepared at various temperatures thoroughly for the extensive morphological studies. Lamellar morphologies of the as-prepared sPS samples were investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Absence of a discernible scattering peak was found for SAXS conducted at room temperature, resulting from a negligible difference in the electron density between the lamellar and amorphous layers. To enhance the scattering contrast and strength, SAXS was carried out at 180 °C to obtain more reliable morphological parameters. Due to the broad thickness distribution of morphological features as revealed from the TEM observations, a pronounced variation is found for the long periods derived from the Bragg's law, one-dimensional correlation function, and interface distribution function of the SAXS data. In addition, relatively irregular packing of lamellar stacks with short lateral dimensions was detected in the as-prepared α″-form sPS, leading to the absence of spherulitic birefringence under polarized optical microscopy. Based on the interface distribution function analysis of the SAXS intensity profiles, the lamellar thicknesses were estimated. Using the Gibbs-Thomson relation, the ratio of fold surface free energy (σ_e) to the fusion enthalpy for α″-form sPS was successfully deduced to be ca. 0.057 nm, which is lower than that of β′-form sPS, ca. 0.12 nm. On this basis, a comparison of critical lamellar thicknesses for α″- and β′-form sPS at various crystallization temperatures is provided and the crystal stability associated with the lamellar thickness is discussed as well.     

Structural modifications in stretch-induced crystallization in PVDF films as measured by small-angle X-ray scattering

The nanostructure of stretched and nonstretched PVDF samples was studied by small-angle X-ray scattering (SAXS). The crystallinity of the samples was determined by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC), and crystalline phases by Fourier transform infrared spectroscopy (FTIR). The nanostructure can be described by a lamellar stacking of crystalline and amorphous layers, with a fairly well defined long period D and a diffuse-boundary in the interface between the crystalline and amorphous phases. The crystallinity of the stretched sample was found to be greater than that of the nonstretched sample. The long period D and the thicknesses of the crystalline lamellae T_c were found to be greater in the stretched sample than those in the nonstretched sample. The thickness of the diffuse-boundary was evaluated as being ∼ 1.4 nm in the nonstretched sample and 1.1 nm in the stretched sample. It was concluded that the growth of the thickness of the crystalline layer induced by the stretching process (stretch-induced crystallization) occurs partially at expense of the diffuse boundary and also by the coarsening of the structure with the stretching process, because of the diminution in the surface area to volume ratio observed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dual lamellar crystal structure in poly(vinylidene fluoride)/acrylic rubber blends and its biaxial orientation behavior

The miscibility for melt-mixed poly(vinylidene fluoride) (PVDF)/acrylic rubber (ACM) blends and the crystal morphology of PVDF in the blends were investigated over the whole composition ranges by dynamic mechanical analysis (DMA), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). DMA measurements revealed that PVDF is miscible with ACM in ACM-rich system, and partially miscible in PVDF-rich system. Two kinds of PVDF lamellar structures with different long periods were detected by SAXS and TEM for the partially miscible blends. In the miscible system, only one kind of crystal lamellae with enlarged long period is found. The two kinds of lamellar structures in the blend show different orientation behavior during the uniaxial stretching to result in a biaxial orientation. The lamellae with short long period are oriented vertical to the stretching direction, while those with large long period were found to be oriented parallel to the stretching direction.     

Formation of lamellar structure by competition in crystallization of both components for crystalline-crystalline block copolymers

Crystallization and structure formation of poly(ethylene oxide)-poly(?-caprolactone) block copolymers (PEG-PCL) in which the melting temperatures of the components are close to each other were elucidated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The diblock copolymers with 33, 46 and 59 wt% of PEG composition formed ordinary single spherulites similar to those of PCL homopolymers, while concentric double-circled spherulites appeared for the PCL-PEG-PCL triblock copolymer with 66 wt% PEG composition as observed previously. For the diblock copolymers, despite of the ordinary appearance of the single spherulites, the DSC thermograms and the WAXD patterns indicated the crystallization of PEG as well as PCL. The time-resolved SAXS profiles for the diblock copolymers showed that long spacings of the crystal lamellae decreased stepwise in the crystallization process. Synthesizing these results for the single spherulites, it was concluded that PCL crystallized first followed by the crystallization of PEG with preservation of the PCL crystal lamellar structure. This means that PEG must crystallize within confined space between the formerly formed PCL crystal lamellae. Such confined crystallization of PEG caused the suppressed melting temperature, crystallinity and crystallization rate especially in the smaller PEG compositions. In the melting process of the diblock copolymers, it was observed that the PEG component first melted with a stepwise increase in the long spacing.     

Radiation effects and re-crystallization mechanism of syndiotactic polystyrene with β′-crystalline form

The double melting behavior of syndiotactic polystyrene (sPS) with β′-form crystallites was systematically investigated by several analytical techniques, including differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), as well as wide-angle and small-angle X-ray scattering (WAXD, SAXS). For preventing the possible chain re-organization during intermediate melting, a high-energy electron beam (e-beam) radiation was carried out on the melt-crystallized samples to chemically cross-link the amorphous chains between lamellar crystals. The WAXD intensity profiles of the irradiated sPS samples revealed that no crystal transformation took place, and the crystallinity fraction remained unchanged for a received dose up to 2.4 MGy. As the received dose was increased, however, the high melting temperature peak was gradually diminished and finally disappeared after 1.8 MGy e-beam radiation, suggesting that the double melting phenomenon was mainly attributed to the melting/re-crystallization/re-melting behavior. The re-crystallization mechanism of sPS samples was studied using DSC and PLM to reveal the effects of heating rate and annealing temperature on the Avrami exponent and re-crystallization rate constant. In addition, the lamellar morphologies of the re-crystallized samples were also investigated by means of SAXS and TEM. With increasing heating rate or annealing temperature, the derived Avrami exponent was slightly decreased from 1.4 to 1.1; in comparison, the re-crystallization rate showed a shallow maximum at a rate of 10 °C/min, but it became evidently reduced at high annealing temperatures. Based on the morphological observations, we proposed that the re-crystallization of β^′-form sPS crystals involved with the presence of broad lamellar thickness distribution as well as abundant irregular loose folding chains on the lamellar surfaces, which became tightened and crystallized into the un-melted lamellae when the neighboring thinner lamellae trapped in-between were melted. Thus, the high melting temperature is dependent on the average thickness of lamellae consisting of the un-melted lamellae developed initially and thickened ones associated with re-crystallization.     

Acceleration or retardation to crystallization if liquid-liquid phase separation occurs: Studies on a polyolefin blend by SAXS/WAXD, DSC and TEM

Blends of statistical copolymers containing ethylene/hexene (PEH) and ethylene/butene (PEB) exhibited the behavior of upper critical solution temperature (UCST). The interplay between the early and intermediate stage liquid-liquid phase separation (LLPS) and crystallization of the PEH/PEB 50/50 blend was studied by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Samples were treated by two different quench procedures: in single quench, the sample was directly quenched from 160 °C to isothermal crystallization temperature of 114 °C; while in double quench, the sample was firstly quenched to 130 °C for 20 min annealing, where LLPS occurred, and then to 114 °C. It was found that in the early stage of crystallization, the integrated values of Iq² and crystallinity, X_c, in the double quench procedure were consistently higher than those in the single quench procedure, which could be attributed to accelerated nucleation induced by enhanced concentration fluctuations and interfacial tension. In the late stage of crystallization, some morphological parameters were found to crossover and then reverse, which could be explained by retardation of lamellar growth due to phase separation formed during the double quench procedure. This phenomenon was also confirmed by DSC measurements in blends of different compositions at varying isothermal crystallization temperatures. The crystal lamellar thickness determined by SAXS showed a good agreement with TEM observation. Results indicated that the early stage LLPS in the PEH/PEB blend prior to crystallization indeed dictated the resulting lamellar structures, including the average size of lamellar stack and the stack distribution. There seemed to be little variation of lamellar thickness and long period between the two quenching procedures (i.e., single quench versus double quench).     

Small-angle X-ray scattering studies on melting and recrystallization behaviors of poly(oxyethylene) crystallites in poly(d,l-lactide)/poly(oxyethylene) blends

Direct determination of the discrete distribution for crystalline lamellar thickness has been performed for poly(d,l-lactic acid)/poly(oxyethylene) (PDLLA/PEG) blends by conducting small-angle X-ray scattering (SAXS) measurements using synchrotron radiation. The PDLLA used was an random (racemic) copolymer of bio-based poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) with the lactide monomer ratio of l:d = 50:50. It is known that PLA is miscible with PEG in the amorphous state. In the current paper, we report comprehensive results on structural analyses of PDLLA/PEG blends in the course of heating and cooling process using SAXS to elucidate the change in the thickness distribution of the lamellae. As a consequence, it was found that the distribution of the lamellar thickness moves toward the larger value (in other words, lamellar thickening) as temperature approaches the melting point. Typically, the thickness distribution was dispersed in the range of 10–20 nm at room temperature and it changed toward 40 nm in the vicinity of the melting temperature. To the best of our knowledge, this is the first report of direct determination of the discrete distribution for the crystalline lamellar thickness and their in-situ changes in the course of the lamellar thickening process. As a result, the lamellar thickening was found to occur at much lower temperature for the blend samples with 10% and 20% of PDLLA contents as compared to the PEG 100% sample. This phenomenon can be ascribed to the melting point depression owing to the miscibility between PEG and PDLLA. Thereby, thinner lamellae were melted and thicker ones appeared at much lower temperature for the blends than for the PEG 100% sample. As for the average repeating distance (long period) of the lamellar stacks, an abrupt increase similar to the critical divergence was observed (from 25 nm to 50 nm) in the heating process. Not only for the melting behavior but also in the course of recrystallization, change in the lamellar-thickness distribution was uncovered, which shows strong hysteresis depending on what temperature the sample was cooled down from.     

Structural evolution of tensile deformed high-density polyethylene at elevated temperatures: Scanning synchrotron small- and wide-angle X-ray scattering studies

The structural evolution of an ice-quenched high-density polyethylene (HDPE) subjected to uniaxial tensile deformation at elevated temperatures was examined as a function of the imposed strains by means of combined synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The data show that when stretching an isotropic sample with the spherulitic structure, intralamellar slipping of crystalline blocks was activated at small deformations, followed by a stress-induced fragmentation and recrystallization process yielding lamellar crystallites with their normal parallel to the stretching direction. Stretching of an isothermally crystallized HDPE sample at 120 °C exhibited changes of the SAXS diagram with strain similar to that observed for quenched HDPE elongated at room temperature, implying that the thermal stability of the crystal blocks composing the lamellae is only dependent on the crystallization temperature. The strain at a characteristic transition point associated with the first indication for the occurrence of a fibrillar structure remains essentially constant in spite of the large changes in drawing temperature and crystalline thickness. In addition, WAXS experiments were used to probe the texture changes accompanying the uniaxial elongation and yield the relationship between the orientational order parameters associated with the crystallites and the amorphous chain segments, and the imposed strain. The results support the existence of intralamellar slip processes from the very beginning of tensile deformation.     

Finite size effects in multilayered polymer systems: Development of PET lamellae under physical confinement

The glass transition temperature and the crystallization behaviour of poly(ethylene terephthalate) PET ultra-thin layers (a few tens of nm) within multilayered PET/polycarbonate (PC) coextruded films are investigated as a function of layer thickness by means of calorimetric measurements. Results are discussed in terms of reduced thickness and interface effects. The appearance and evolution of lamellar orientation upon isothermal crystallization of ultra-thin PET layers from the glassy state are explored based on real time small-angle X-ray scattering (SAXS) studies. Analysis of the SAXS measurements reveals that finite size effects hamper the crystallization process. However, the final lamellar structure is similar in both, the nanolayered PET and the bulk material. Results suggest that not only lamellar insertion but also some lamellar thickening contribute to the development of the final lamellar structure. Room temperature SAXS and wide-angle X-ray diffraction (WAXS) measurements indicate that two lamellar populations develop: edge-on lamellae are proposed to appear close to the interphases while flat-on lamellae, arising as a consequence of confinement, should be preferentially located in the layers core.     

Inhibition of crystalline structure development in a reactive polycarbonate-poly(butyleneterephthalate) blend

Summary The crystallization behaviour of PBT and a reactive 50/50 PBT-PC blend were studied using synchrotron SAXS/DSC and TEM. The scattering data showed PBT crystallization to be inhibited in the blend due to PBT-PC transesterification, resulting in a progressive reduction in both melting and recrystallization temperatures and the degree of crystallinity developed. Analysis of the SAXS data using a one-dimensional correlation function showed PBT to exhibit an ordered lamellar morphology, whereas in the blend stacking of lamellae appeared to be inhibited. TEM of PBT-PC blends confirmed that PBT lamellae were randomly-oriented. Received: 17 September 2001/Revised version: 21 February 2002/ Accepted: 25 February 2002     

Liquid-liquid phase separation and crystallization behavior of poly(ethylene terephthalate)/poly(ether imide) blend

The liquid-liquid (L-L) phase separation and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blend were investigated with optical microscopy, light scattering, and small angle X-ray scattering (SAXS). The thermal analysis showed that the concentration fluctuation between separated phases was controllable by changing the time spent for demixing before crystallization. The L-L phase-separated specimens at 130 °C for various time periods were subjected to a temperature-jump of 180 °C for the isothermal crystallization and then effects of L-L phase separation on crystallization were investigated. The crystal growth rate decreased with increasing L-L phase-separated time (t_s). The slow crystallization for a long t_s implied that the growth path of crystals was highly distorted by the rearrangement of the spinodal domains associated with coarsening. The characteristic morphological parameters at the lamellar level were determined by the correlation function analysis on the SAXS data. The blend had a larger amorphous layer thickness than the pure PET, indicating that PEI molecules in the PET-rich phase were incorporated into the interlamellar regions during crystallization.     

A SAXS–WAXD study on the mesomorphic-α transition of isotactic polypropylene

The smectic-α transition of iPP was studied by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). WAXD and SAXS patterns were taken at different temperatures during the transition. Two different approaches were taken in the analysis of SAXS data: the correlation function and a fitting method based on theoretical distribution models. Up to 80°C just smectic phase was observed, whereas beyond that temperature α lamellae appeared and the two populations of lamellae were found to coexist in the sample. The new α phase population stemmed from within the preexistent smectic stacks, according to a lamellar insertion model. α lamellae thickened to a larger extent than the smectic phase, that just underwent thermal expansion. Results were consistent with a mechanism of transformation involving a rearrangement of the chains without a melting-recrystallization process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

3D Hierarchical orientation in polymer-clay nanocomposite films

Organically modified clay was used as reinforcement for HDPE using maleated polyethylene (PEMA) as a compatibilizer. The effect of compatibilizer concentration on the orientation of various structural features in the polymer-layered silicate nanocomposite (PLSN) system was studied using two-dimensional (2D) small angle X-ray scattering (SAXS) and 2D wide-angle X-ray scattering (WAXS). The dispersion (repeat period) and three-dimensional (3D) orientations of six different structural features were easily identified:

(a)

clay clusters/tactoids (0.12 μm),

(b)

modified clay (002) (24-31 Å),

(c)

unmodified clay (002) (13 Å),

(d)

clay (110) and (020) planes normal to (b) and (c),

(e)

polymer crystalline lamellae (001) (190-260 Å), and

(f)

polymer unit cell (110) and (200) planes.

A 3D study of the relative orientation of this hierarchical morphology was carried out by measuring three scattering projections for each sample. Quantitative data on the orientation of these structural units in the nanocomposite film is determined through calculation of the major axis direction cosines and through a ternary, direction-cosine plot. Surprisingly, it is the unmodified clay which shows the most intimate relationship with the polymer crystalline lamellae in terms of orientation. Association between clay and polymer lamellae may be related to an observed increase in lamellar thickness in the composite films. Orientation relationships also reveal that the modified clay is associated with large-scale tactoid structures.     

Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper.Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C.We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature.Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60-105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.