Normal Spherulites Research Articles

Morphology and growth rate of spherulite of cyclic poly(ε-caprolactone) having a triazole group at the closing point

To clarify the effect of triazole (TR) group at the closing point within the cyclic polymer chain on the crystal growth, we studied morphology and the growth rate of spherulite G of cyclic poly(ε-caprolactone) with a TR group at the closing point (C-TR-PCL) as a function of the degree of supercooling ΔT using polarizing optical microscopy. We prepared C-TR-PCLs synthesized by the ring-closing reaction from their linear precursors with Mw = 30000 and 50000, and cyclic PCL polymerized by ring expansion reaction (C-PCL) having no TR group with Mw = 26000, and linear PCL with/without a TR group at a chain end (L-TR-PCL-OH and L-PCL) with Mw = 26000 and 36000, where Mw was determined by GPC using linear polystyrene standards.We found that C-TR-PCL and L-TR-PCL-OH show banded spherulites at small ΔT and band-less normal spherulites with maltese cross at large ΔT. Whereas, C-PCL and L-PCL only showed band-less normal spherulites at any ΔT. This speculates that steric hindrance of TR group near the surface of crystals will become significant at small ΔT. G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. B of C-PCL and L-PCL is constant irrespective of TΔT, however, that of C-TR-PCL and L-TR-PCL-OH showed two values depending on TΔT. At the region of lower B, i.e., small ΔT, the banded spherulites could be observed. The change of B corresponds to the morphological change of the spherulites. Since the size of the crystalline region closely related to lamellar thickness becomes large at small ΔT, TR groups excluded from the crystals become to assemble near the surface of the crystals. This is the reason why B becomes low at small ΔT. In C-TR-PCL with higher Mw and L-TR-PCL-OH, the decrease of B with decreasing ΔT becomes small compared to that of C-TR-PCL with lower Mw due to relatively small effect of TR group.

Poly(L-lactide) crystallization topography directs MC3T3-E1 cells response

Biomaterial surface topography significantly influences cellular form and function. Using poly(L-lactic acid) films with normal spherulites, banded spherulites, and amorphous surfaces as model substrates, we conducted a systematic assessment of the role for polymer crystallization induced surface morphologies on cell growth and contact guidance. Microscopy and image analysis showed that the MC3T3-E1 cells spread out in a random fashion on the amorphous substrate. At 24 h post-seeding, MC3T3-E1 cells on both types of spherulite surfaces were elongated and aligned along the spherulite radius direction. For the banded spherulite surface with radial stripes and coupling annular grooves, the cell orientation and cell nuclear localization were related to the grooves structure. With increasing time, this orientation preference was weaker. These results demonstrate that the patterning of polymer crystallization structure provide important signals for guiding cells to exhibit characteristic orientation and morphology especially in the early stages of regeneration.

Crystallization behavior and crystalline structural changes of poly(glycolic acid) investigated via temperature-variable WAXD and FTIR analysis

Poly(glycolic acid) (PGA) is a unique biodegradable, aliphatic polyester having a higher melting point (Tm) and better mechanical properties and chemical resistance than its analogs. The unusual physical properties of PGA can be attributed to its crystalline structure, chain packing and interactions. Herein, we systematically investigated the crystalline structure, spherulitic morphology, and structural evolution of PGA in the crystallization and melting processes by time-resolved wide-angle X-ray diffraction (WAXD) and FTIR spectrometry. PGA exhibits fast crystallization and it completely crystallizes in the fast cooling process. The Tm, degree of crystallinity, and long period of PGA increase as the crystallization temperature increases. PGA forms unique hedrites but not normal spherulites during crystallization. Because of the thermal expansion of the crystal lattice, the a and b axes of PGA crystal cells increase linearly with temperature in the heating process. The band splitting in the FTIR spectrum of PGA is observed in the ν(CO) and δ(CH2) regions and the spectral splitting intensifies with cooling, indicating the existence of intermolecular dipolar interactions in PGA crystals. It is speculated that the C–H⋯OC H-bonds are formed in the crystalline phase of PGA, as indicated by the significant red shifts of the ν(CO) band during crystallization.

Crystallization Regimes and Spherulitic Morphology of Poly(trimethylene terephthalate/isophthalate) Random Copolyesters

The crystallization regimes and spherulitic morphology of poly (trimethylene terephthalate/isophthalate) (TI) random copolyesters were studied in this work. A hot stage polarizing microscope was used to observe morphology and measure growth rate of the spherulite of TI copolyesters. Lauritzen-Hoffman secondary nucleation theory was applied to investigate crystallization kinetics and regimes of TI copolyesters. Results show that the spherulitic growth rate decreased with an increase in isophthalic acid content of the copolyesters. The analysis of Lauritzen-Hoffman secondary nucleation theory shows that regime I, II, and III were observed in TI11 and TI21. Regime III was observed in TI29. The spherulitic morphology of TI11, TI21, and TI29 changed from elliptical-shaped spherulite to irregular spherulite and then to normal spherulite at the regime transitions.

Frustrated Crystallization in the Coupled Viscoelastic Phase Separation

The interplay between crystallization and phase separation has been intensively studied recently. In this study, we extended the research into a dynamically asymmetric blend composed of amorphous poly(methyl methacrylate) (PMMA) and crystalline poly(ethylene oxide) (PEO). The large dynamic asymmetry induces network stress in concentration growth. We find that crystallization is seriously frustrated when it couples with a simultaneous viscoelastic phase separation. In a single quench experiment, normal spherulites grew in a limited temperature range when crystallization was faster, while crystallization was frustrated at deep quenches when phase separation was faster. In a double quench experiment, crystallization was more difficult to occur after the prior phase separation at a higher temperature. The calorimetric results indicated that both melting temperatures and enthalpies of crystallization decreased in the coupled viscoelastic phase separation. We propose that it is the network stress in the concentration growth that leads to the frustration of crystallization.

Peptide‐poly(ε‐caprolactone) biohybrids by grafting‐from ring‐opening polymerization: Synthesis, aggregation, and crystalline properties

AbstractWell‐defined peptide‐poly(ε‐caprolactone) (Pep‐PCL) biohybrids were successfully synthesized by grafting‐from ring‐opening polymerization (ROP) of ε‐caprolactone (CL) using designed amine‐terminated sequence‐defined peptides as macroinitiators. MALDI‐TOF‐MS and 1H NMR analyses confirmed the successful attachment of peptide to the PCL chain. The gel permeation chromatography (GPC) measurement showed that the Pep‐PCL biohybrids with controllable molecular weights and low polydispersities (PDI <1.5) were obtained by this approach. The aggregation of Pep‐PCL hybrid molecules in THF solution resulted in the formation of micro/nanospheres as confirmed through FESEM, TEM, and DLS analyses. The circular dichroism study revealed that the secondary structure of peptide moiety was changed in the peptide‐PCL biohybrids. The crystallization and melting behavior of Pep‐PCL hybrids were somewhat changed compared with that of neat PCL of comparable molecular weight as revealed by DSC and XRD measurements. In Pep‐PCL biohybrids, extinction rings were observed in the PCL spherulites, in contrast with the normal spherulite morphology of the neat PCL. There was a substantial decrease (4–5 folds) in the spherulitic growth rate after the incorporation of peptide moiety at the end of PCL chain as measured by polarizing optical microscopy. Pseudomonas lipase catalyzed enzymatic degradation was studied for Pep‐PCL hybrids and neat PCL. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Morphology and nonisothermal crystallization of in situ microfibrillar poly(ethylene terephthalate)/polypropylene blend fabricated through slit‐extrusion, hot‐stretch quenching

AbstractThis study describes the morphology and nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ micro‐fiber‐reinforced blends (MRB) obtained via slit‐extrusion, hot‐stretching quenching. For comparison purposes, neat PP and PET/PP common blends are also included. Morphological observation indicated that the well‐defined microfibers are in situ generated by the slit‐extrusion, hot‐stretching quenching process. Neat iPP and PET/iPP common blends showed the normal spherulite morphology, whereas the PET/iPP microfibrillar blend had typical transcrystallites at 1 wt % PET concentration. The nonisothermal crystallization kinetics of three samples were investigated with differential scanning calorimetry (DSC). Applying the theories proposed by Jeziorny, Ozawa, and Liu to analyze the crystallization kinetics of neat PP and PET/PP common and microfibrillar blends, agreement was found between our experimental results and Liu's prediction. The increases of crystallization temperature and crystallization rate during the nonisothermal crystallization process indicated that PET in situ microfibers have significant nucleation ability for the crystallization of a PP matrix phase. The crystallization peaks in the DSC curves of the three materials examined widened and shifted to lower temperature when the cooling rate was increased. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 374–385, 2004

Spherulite morphology studies of N‐alkyl chitosan films cast from formic acid solutions

AbstractThe films of N‐ethyl chitosan were prepared via the solution‐casting technique with formic acid as a solvent. The solutions with different concentrations (35 and 40 wt %) were prepared previously from dilute solution (1 wt %) via evaporating process. The crystalline morphology of these films was investigated by means of polarized optical microscopy and scanning electron microscopy. Normal spherulites with a low growth rate formed in the casting films. The different morphologies of spherulite appeared in the films cast from the solutions with different concentrations. After further crystallizing for a few days, the spherulites were decorated by thousands of needlelike extended‐chain crystals, which had a typical size of ∼50 μm (length) × 2∼5 μm (width) × 1∼2 μm (height) in the central part of the spherulite, but a typical size of ∼5 μm (lenght)× 1∼2 μm (width) × 1∼2 μm (height) in the fringe part of the spherulite. The real concentration for crystallization was determined to be 65–82 wt%. Thus, the crystallization actually appeared in supersaturated solution. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2033–2038, 2003

Crystalline Morphology Developing from Cholesteric Mesophase in Cyanoethyl Chitosan Solutions

Two completely different crystals can grow from cyanoethyl chitosan/formic acid solutions in which the anisotropic liquid crystalline (LC) and isotropic phases coexisted. One was normal spherulites developing from isotropic region, and the other was cholesteric-like crystallites developing from mesophase region. The latter had similar optical characters as fingerprint texture. This implies that the crystal structure in solution cast film from cholesteric LC phase remained the main structure characters of cholesteric LC texture. A model is presented to explain the crystallization process from these liquid crystalline polymer solutions.

Crystallization from the melt of poly(lactide)s with different optical purities and their blends

AbstractPoly(lactide)s (PLA) with different optical purities (OP) were synthesized by copolymerization of D‐lactide and L‐lactide. Crystallization of D‐ and L‐lactide‐rich copolymers before and after 1 : 1 blending was studied from the melt over a wide range of annealing temperatures (Ta) using differential scanning calorimetry and polarizing microscopy. Melting temperature and crystallinity increased with an increase in OP for both the non‐blended and the 1 : 1 blended PLAs and they lost their crystallizability when their OP became smaller than 76%, suggesting that the critical isotactic sequence length of PLA for crystallization was approximately 15 isotactic lactate units. Normal spherulite formation was observed for both the non‐blended and the blended PLAs even when the OP of PLAs decreased to 76%, whereas the spherulitic size of the non‐blended PLAs was larger than that of the 1 : 1 blended PLAs when compared at the same OP and Ta. The highest Ta for crystallization increased with increasing OP and was higher for the 1 : 1 blended PLAs than for the non‐blended PLAs at a fixed OP, indicating that the crystallizable Ta range was extended to higher temperature by blending D‐ and L‐lactide‐rich PLAs. This is ascribed to stereocomplex (racemic crystallites) formation at high Ta from the 1 : 1 blended PLAs. Based on these results, the phase diagram for crystallization was depicted for the non‐blended and the 1 : 1 blended PLAs.

Electron microscopy study of the structure of normal and abnormal poly(butylene terephthalate) spherulites

AbstractNormal and abnormal spherulites of polybutylene terephthalate cast from solution in HFIP were investigated by electron microscopy (CTEM and STEM). Both spherulite types crystallize in the α form. The crystallite is biaxial with its greatest polarizability directions Z and Y oriented along the molecular chain and perpendicular to it in the plane of the terephthalate residue. In the normal spherulites, the [210]* direction is parallel to the radius direction, so that both Z and Y directions are oriented tangentially (negative spherulites). In the abnormal spherulites, the [111]* direction is parallel to the radius, and this explains the observed abnormal light scattering pattern.

Heat of formation and critical size for nuclei of liquid crystals of p-azoxyanisole and cholesteryl caprinate

The centers for these liquid crystals resemble those of normal spherulites, so the heat of formation and critical size have been deduced from formulas for the crystallization of ordinary substances. The measured heats of melting and interphase energies have been employed. The minium size of center observed in polarized light exceeds the critical size by more than an order of magnitude.

Studies of orientation phenomena in crystallizing polymers

AbstractUnusual orientation effects encountered during the formation of filaments of polyamides (nylon 6,6, 6,10, and 6 (polyhexamethylene adipamide, sebacamide, and polycaproamide)), polythene, and Terylene (polyethylene terephthalate) are described and discussed. The paper is divided into two parts: (I) extrusion, (II) drawing. (I) It was found that, under certain conditions, the extrusion process can produce a characteristic perpendicular orientation of the crystallites. This type of orientation can correspond to two types of structures in the extruded samples: (1) the row structure, which consists of aggregates of partially developed spherulites forming along flow lines; (2) a new structure, in which the crystalline fibrillar units (helices in our interpretation) grow in the extrusion direction only, instead of forming the normal spherulites. Both structures occur in all the polymers investigated, but their relative frequency depends on the material and on the conditions of extrusion. (II) It was found that polyamides have a perpendicular orientation in the first stages of elongation corresponding to a strong elastic distortion of the spherulites, and further, that rupture occurs within the crystalline region during the later stages of drawing. It was shown that, under certain conditions, drawing can consist in the gliding past of crystal aggregates which themselves possess perpendicular orientation. Attention is drawn to some unusually pronounced relaxation phenomena observed in polyamides and to the variety of crystallographic tilts, which can occur in Terylene. These experimental findings are discussed. In the course of this discussion the usefulness of the morphological approach in the treatment of these types of phenomena is clearly brought out.