Stretch-induced bidirectional polymorphic transformation of crystals in poly(butylene adipate)

Stretch-induced structural changes of poly(butylene adipate) at various temperatures were systematically investigated by X-ray diffraction method. It was found that tensile deformation could lead to both occurrence of α to β and β to α crystal transition via solid–solid process. Experimental results shows that either α or β form can be adopted as the stable phase for PBA crystal depending on the competition between tensile strain and temperature. Higher temperature favors β to α crystal transformation, while larger strain would result in α to β crystal transformation. Due to the conversion of crystal stability during stretching, a unique “β to α to β” transformation phenomenon of PBA appears at appropriate temperature for original β spline. This study provides evidence that both temperature and tensile strain can alter the relative stability of polymorphic crystals of semi-crystalline polymers.     

Crystallization behavior of 1,3‐dipalmitoyl‐2‐oleoyl‐glycerol and 1‐palmitoyl‐2,3‐dioleoyl‐glycerol

A model margarine was stored under a temperature fluctuation cycle of 5—20 °C until granular crystals were observed. Using information obtained from the granular crystals, the crystallization behaviors of major triacylglycerols of palm oil, 1,3‐dipalmitoyl‐2‐oleoyl‐glycerol (POP), 1‐palmitoyl‐2,3‐dioleoyl‐glycerol (POO), and their mixtures were then investigated. It was shown that in the model margarine, the POP content in the granular crystals was higher than in their surrounding materials, and the X‐ray diffraction pattern of the granular crystals revealed that they were the most stable polymorph, β. 99% pure POP, POO, and their mixtures were then stored under the above‐mentioned temperature cycle. POP was found to form the unstable polymorph, α, when cooled rapidly from the melt. Within 24 hours transformation into the γ polymorph and then into the β polymorph was observed. POO was shown to transform into the β' polymorph from α. When POP and POO were mixed, the β polymorph did not emerge, instead it was shown that POP and POO were both agglomerated in the mixtures, giving rise to the formation of granular crystals.     

Synthesis and crystallization behavior of nylon 12 14. II: Chain‐folded lamellar crystals and crystalline transformation behavior of nylon 12 14

Chain‐folded lamellar crystals of nylon 12 14 have been grown from a dilute 1,4‐butanediol solution with the “self‐seeding” technique. The morphology and structure of nylon 12 14 lamellar crystals were studied by both transmission electron microscopy (TEM) and wide‐angle X‐ray diffraction (WAXD). Two kinds of electron diffraction patterns were detected when different areas were selected for diffraction, which indicates that the α crystal phase and the β crystal phase coexist for nylon 12 14 under the present crystallization conditions. The WAXD diffractograms of the crystal mats confirm the results obtained from electron diffraction (ED). In addition, the changes of the crystal structure as a function of temperature for melt‐crystallized and dilute solution‐crystallized nylon 12 14 were monitored by variable‐temperature WAXD and variable‐temperature infrared spectroscopy (IR). It was found that the melt‐crystallized sample undergoes a Brill transformation at 80°C–90°C, but no Brill temperature can be observed for the dilute solution‐crystallized nylon 12 14.     

Process analysis of phase transformation of α to β-form crystal of syndiotactic polystyrene investigated in supercritical CO2

Mechanism of solid phase transformation of α to β form crystal of syndiotactic polystyrene (sPS) was investigated in supercritical CO₂. The phase transformation occurred in the original pure α and mixed (α+β) form sPS in supercritical CO₂ was traced as a function of temperature and pressure by means of wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC). At appropriate temperature and pressure, sPS underwent solid phase transitions from α to β form. Higher temperature or higher pressure favored this transformation. Compared to the original pure α form sample, the original β form crystal in the mixed (α+β) form sample acted as the nucleus of β form crystal, so that reduced the transition temperature and pressure.     

Friction‐induced electroactive β polymorph of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) has been widely used in electric devices due to electroactive β polymorph. In this article, we probe the formation of β phase under friction by spectroscopy and thermal analysis. During continuous friction, entire sliding of PVDF is identified with two regimes, i.e., running‐in and steady‐state. At initial running‐in period, friction surfaces are dominated by plastic strain, which leads to striking formation of β phase from α polymorph (α→β). Subsequently, melting‐flow domains almost cover friction surfaces at steady‐state. Thus, formation of β crystal is correspondingly induced by shear crystallization. Nevertheless, β‐crystal content at steady‐state is lower than that at running‐in. With sliding proceeding, moreover, β‐crystal content exhibits a gradually decreasing tendency, attributed to rising surface temperature. Besides, the friction‐induced β phase is further confirmed by evaluation of wear debris. Overall, friction plays a crucial role as to the formation of β phase. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46395.     

Oil-binding capacity of plastic fats: Effects of intermediate melting point TAG

The oil-binding capacity (OBC) of fat crystals was investigated as a function of intermediate melting point TAG (IMP-TAG) and stearin composition. Samples were prepared by melting 14% hard fraction (palm-canola stearin and IMP-TAG blends) in 86% liquid oil (olive, canola, safflower, and triolein) and crystallizing the mixture under fast and slow cooling conditions. Under fast cooling conditions, the OBC increased as the IMP-TAG/stearin ratio increased. The OBC is the grams of bound oil (determined by centrifugation) divided by the grams of solid fat (determined by pulse NMR). The maximum OBC was observed at 14% IMP-TAG and 0% stearin. In constrast, under slow cooling conditions, the 14% IMP-TAG and 0% stearin sample did not form crystals, and only free oil was present. The OBC for each liquid oil was lower under slow cooling conditions than under fast cooling conditions when compared at the same IMP-TAG/ stearin ratio. Solid fat content and RP-HPLC analyses indicated that IMP-TAG were retained in the crystal structure when processed under fast cooling conditions. RP-HPLC analyses also revealed that TAG with two saturated FA were retained in the crystal structure and that the monosaturated TAG were not. It was concluded that the TAG composition and cooling conditions played an important role in determining the OBC.     

Crystallization from the melt of α and β forms of syndiotactic polystyrene

The crystallization of trans-planar α and β forms of syndiotactic polystyrene is studied through X-ray diffraction and DSC analyses of melt-crystallized samples. The factors controlling the crystallization of the two forms are analyzed. Pure α and β forms of syndiotactic polystyrene can be easily obtained setting the maximum temperature at which the melt is heated and the permanence time of the melt at this temperature. The crystallization of the α and β forms does not depend on the crystallization temperature, at least in the range of accessible crystallization temperatures, between 240 and 270 °C, but only depends on the presence of the ‘memory’ of the α form in the melt. The most important factors are, indeed, the crystalline form of the starting material used in the melt crystallization experiments and the maximum temperature of the melt. Relevant recrystallization phenomena, occurring during the melting of the samples crystallized from the melt at low crystallization temperatures, are responsible for the complex melting behavior of the α and β forms. The recrystallization involves only lamellar thickening of the crystals of the same form (α or β) and not structural transformation.     

Hot drawing of partially miscible blends of polycarbonate and poly(butylene terephthalate)

Samples of PC-PBT blends over the entire composition range were drawn at 160°C to high extensions, 2.1–5.8, to study the mechanical reinforcement and the molecular structure development upon deformation. Elastic modulus E' increases with extension ratio for all compositions and temperatures. Blends with 25 and 40 wt% of PC show higher E' at low temperature than pure PBT blends do. Crystallinity increases with extension ratio and is relatively smaller with increasing PC content. The influence of the reversible α to β crystal form transformation was also studied. The second moment of the orientation function f for both crystal forms increases to high values > 0.9 at relatively low extensions. f decreases with PC content for α crystals but decreases for β crystals. The α fraction is high for PBT and decreases with PC content and extension ratio in the blends. Strain recovery experiments show that the α to β transformation is also elastic in nature at high extension ratios and that the reinforcing effect in high PBT content blends is not due to the α/β ratio. © 1996 John Wiley & Sons, Inc.     

Effect of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator and crystallization temperature on the production of β crystal form in isotactic polypropylene

The influence of the composition ratio of pimelic acid/calcium stearate bicomponent nucleator on the β crystal form content of isotactic polypropylene (iPP) had been studied at the crystallization temperature of 120°C and duration of 30 min. It was found that the β crystal form content increased continuously with increasing amount of calcium stearate at the constant amount of 0.15% pimelic acid. High β crystal form content polypropylene could be produced when the amount of calcium stearate was greater than 0.30% (the mass composition ratio of pimelic acid/calcium stearate was less than 1/2, the mole ratio was less than 1.89/1). It was shown that pimelic acid and calcium stearate could react to produce a high effective β nucleator (calcium pimelate) “in situ” during the melt‐mixing of iPP and the bicomponent nucleator. The influence of crystallization temperatures (100–140°C) on the β crystal form content of iPP had also been studied at the constant composition ratio of 0.15% pimelic acid/0.5% calcium stearate (the calcium pimelate produced in situ was 0.16%, which was calculated from stoichiometry). It was found that the β crystal form content increased continuously with increasing crystallization temperature and it maximized at 130°C. β Crystal form content decreased sharply at the crystallization temperature of 140°C. It was shown that β → α modification transformed between 130 and 140°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The effects of melt vibration blending on the subsequent crystallization and melting behavior of polypropylene/ultra high molecular weight polyethylene

Pure isotactic polypropylene (iPP) and 90/10 wt iPP/ultra high molecular weight polyethylene (UHMWPE) blends, prepared by a novel vibration internal mixer reformed from a coventional internal mixer via parallel superposition of an oscillatory shear on a steady shear, were investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction. After plasticating pure iPP in the vibration field, the number of β form crystals of iPP was increased. The β form exhibited a single DSC melting peak different from that of the bulk α form crystals of iPP. After mixing of UHMWPE with iPP, the melting point of the UHMWPE component shifted to a lower temperature. For blends mixed at the higher‐frequency and/or larger‐amplitude vibrations, the melting point of the UHMWPE component was further gently lowered while the bulk melting point of the iPP component was slightly increased. The crystallization peaks of the two components overlapped into one single peak, and the total crystallinity became higher, together with a larger amount of the β iPP. These results showed that the two components influenced each other in blending. Hence, the resultant morphology affected the subsequent crystallization and melting behaviors. Additionally, vibration in mixing possibly affected the conformation of some polypropylene chains to favor the subsequent packing in the β form.     

Influence of α′‐/α‐crystal polymorphism on properties of poly(l‐lactic acid)

Poly(l ‐lactic acid) (PLLA) is a biodegradable and biocompatible thermoplastic polyester produced from renewable sources, widely used for biomedical devices, in food packaging and in agriculture. It is a semicrystalline polymer, and as such its properties are strongly affected by the developed semicrystalline morphology. As a function of the crystallization temperature, PLLA can form different crystal modifications, namely α′‐crystals below about 120 °C and α‐crystals at higher temperatures. The α′ modification is therefore of special importance as it may be the preferred polymorph developing at processing‐relevant conditions. It is a metastable modification which typically transforms into the more stable α‐crystals on annealing at elevated temperature. The structure, kinetics of formation and thermodynamics of α′‐ and α‐crystals of PLLA are reviewed in this contribution, together with the effect of α′‐/α‐crystal polymorphism on the properties of PLLA. © 2018 Society of Chemical Industry     

Flow patterns and oil—water dispersion in a 0.38 m diameter oscillatory baffled column

We report our experimental flow visualization observations of flow patterns and experimental oil‐water dispersion measurements in an oscillatory baffled column (OBC) of an internal diameter 380 mm. Both types of experiments were carried out covering an identical range of oscillation frequencies, amplitudes, orifice diameters and baffle spacings. The flow visualization observations show that eddy mixing has been achieved in the pilot OBC and the intensity of which is largely dependent on the operational and geometrical parameters tested, which is similar to that in a smaller scale OBC. The scale‐up correlation was found to be linear. The oil‐water dispersion measurements show that the degree of the dispersion depends significantly on the oscillation frequency and amplitude with an increase in either leading to an increase in dispersion. The effect of the orifice diameter on the oil—water dispersion is also evident, but the effect of the baffle spacing is much weaker. Based on the experimental data we have established a correlation relating the degree of oil—water dispersion to the power input to the system. We have also compared the power requirement to achieve a complete dispersion in the pilot OBC with that in a bench scale OBC of 50 mm diameter and found that the energy dissipation is more economical in the large scale application.     

Structural studies of electrospun nylon 6 fibers from solution and melt

Nylon 6 (N6) fibers have been fabricated via two different electrospinning schemes, from solution of N6 and formic acid at room temperature as well as from N6 melt at elevated temperature. The crystal structures of electrospun N6 fibers from solution and melt, and the annealing effect on the structures were studied by using various techniques. Combined analysis of the differential scanning calorimetry (DSC) at various heating rates, temperature-dependent X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy indicates that N6 fibers from melt predominantly exhibit the meta-stable γ-crystalline forms and low molecular orientation, while solution electrospun fibers from slowly evaporating solvent show both α- and γ-form crystals and higher degree of molecular orientation. At high annealing temperature, the meta-stable γ-crystals in melt electrospun fibers easily transform into thermodynamically stable α-form crystals, while crystals in solution electrospun fibers exhibit higher thermal stability. Nonisothermal modeling and in-situ measurements of jet temperature indicate that rapid quenching due to enhanced heat transfer by electrohydrodynamically driven air flow near the jet is responsible for the less stable γ-crystals and lower degree of molecular orientation in melt electrospun fibers.     

An Investigation into Parameters Affecting Crystal Purity of Urea in a Stirred Tank and an Oscillatory Baffled Crystallizer

Confidential industrial trials showed that higher purity crystals were produced in an oscillatory baffled crystallizer (OBC) compared to a stirred tank crystallizer (STC) when operating at similar conditions. This trend is verified in this work using urea as the test compound that is different from that was employed in industrial tests. We have observed that a higher supersaturation level at nucleation coupled with a lower nucleation temperature was measured in the STC at all investigated conditions compared to that in the OBC. This led to a higher nucleation rate in the STC, consequently producing smaller crystals. Crystal size distributions and imaging analyses suggest that these smaller crystals were more likely to form agglomerates in the STC, trapping either mother liquor or impurity or both and leading to the lower purity observed when compared to that in the OBC.     

Finite strain response, microstructural evolution and β→α phase transformation of crystalline isotactic polypropylene

An experimental study of the finite strain response of annealed α and β crystalline isotactic polypropylene (iPP) was conducted over a range of temperatures (25, 75, 110 and 135 °C) using uniaxial compression tests. Uniaxial compression results indicate nearly identical macroscopic stress vs. strain behavior for α-iPP and for β-iPP to true strains in excess of −1.1 at room temperature despite the different initial morphologies. At larger compressive strains (>1.2), β-iPP shows more rapid strain hardening. The orientation of crystalline planes during straining differs at room temperature from that at high temperature, indicating a change of slip mechanisms as temperature increases. In addition, strain-induced crystallization occurred at the highest temperature examined in α-iPP. A continuous transformation of β crystals to α crystals with inelastic deformation at room temperature was observed and it was facilitated at higher deformation temperatures. Scanning electron microscopy (SEM) observations of deformed β-iPP provide strong evidence that the transformation is achieved via a solid-to-solid mechanism despite the different helical hands in α and β crystal structures. Molecular simulations were used to investigate a conformational defect in the 3₁ helical chains of β-iPP, characterized by a 120° helical jump. The propagation of this conformational defect along molecular chains provides the reversal of helical hand required by the solid-to-solid transformation. The β→α phase transformation in iPP is proposed to be accomplished via a solid transformation that includes slip along β(110) and β(120) planes during shear of the crystal lattice.     

Comparison of chain structure and morphology of an olefinic blocky copolymer and a Ziegler–Natta‐based ethylene random copolymer

The microstructure and morphology of an olefinic blocky copolymer (OBC) and an ethylene‐hexene copolymer prepared by conventional Ziegler‐Natta catalysis (ZNEH) are compared. It is found that these two samples have similar melting temperatures, but the overall comonomer content in OBC is slightly higher. The crystallization temperature and crystallinity of OBC are markedly lower than those of ZNEH. A successive self‐nucleation annealing experiment reveals that OBC has a more uniform distribution of crystal thickness, indicating a more homogeneous composition distribution in its hard blocks. Small‐angle X‐ray scattering (SAXS) results show that the long period of OBC hardly changes with temperature in the low‐temperature range, whereas that of ZNEH increases gradually with temperature due to melting of the less perfect crystals. The average lamellar thickness of crystals is larger for OBC than for ZNEH, but the thickness of the thickest crystals is comparable in the two. The SAXS profiles were analyzed using a one‐dimensional correlation function. The result reveals that the partially ordered interphases in OBC are mainly located at the interface of the crystalline and amorphous phases. In contrast, the interface of the crystalline and amorphous phases in ZNEH is quite sharp and it is inferred that the partially ordered interphases are distributed in the bulk as separated domains. Scattered tiny crystals are formed in ZNEH, but OBC exhibits a macroscopic morphology of large spherulites. It is also observed that more amorphous phases are rejected outside of the lamellar stacks and spherulites in OBC. © 2012 Society of Chemical Industry     

Formation of ring-banded spherulites of α and β modifications in Poly(butylene adipate)

The morphology and crystalline structure of banded spherulites of poly(butylene adipate) (PBA) were investigated by polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM) and scanning electronic microscopy (SEM). It was found that after purification and fractionation, the obtained PBA fractions with different molecular weight formed ring-banded spherulites at different temperature ranges. Pure α and β form of PBA can form regular ring-banded spherulites. AFM and SEM observations of the thin film revealed that the alternative ridges and valleys along the radial direction consisted of edge-on and flat-on lamellae, respectively, indicating the ring-bands in PBA spherulites are the consequence of lamellar twisting. In addition, sequential growth of α and β form in one PBA spherulite at the same temperature is reported and interpreted by competition of primary nucleation and radial growth of the two crystalline modifications. At a certain temperature range, the α form PBA has larger primary nucleation rate but lower radial growth rate than the β form PBA, leading to formation of spherulites consisted of α form at center and β form at the outer region. But finally, a layer of α form ringless region appears at the outside of the β form ring-banded region before impingement of the neighboring spherulites. These results suggest that besides crystallization temperature, molecular weight has considerable effect on the formation of crystal modification of PBA and the corresponding banded spherulites.     

Thermal Expansion of BaO · 2B<Subscript>2</Subscript>O<Subscript>3</Subscript> in the Vitreous and Crystalline States: I. Thermal Expansion and Density of Barium Diborate Prepared from Monolithic Glass of the Same Composition

The thermal expansion and density of vitreous and polycrystalline barium diborates prepared by crystallization of monolithic glasses of stoichiometric composition are investigated in the temperature range 20–700°C. The glass crystallizes in the form of the α and β modifications depending on the heat treatment conditions. The α phase nucleates on the surface of the monolithic sample and grows inward the sample in the form of textured layers. The β phase nucleates in the melt bulk and grows in the form of spherulites. Textured polycrystalline barium diborate in the form of the α phase possesses a pronounced anisotropy of thermal expansion with respect to the direction of crystalline-layer growth. This is associated with the negative expansion along one of the crystallographic axes. The effective temperature coefficients of thermal expansion in the temperature range 20–600°C increase in the order: α phase-glass-β phase. Upon heating above 690°C, the β phase transforms partially into the α phase. This is accompanied by a considerable increase in the volume. The densities of the glass and polycrystalline samples in the temperature range 20–700°C increase in the order: glass-α phase–β phase.     

Effect of poly(vinylidene fluoride) polymorphic structures on the crystallization behavior of poly(butylene succinate)

The detail information of both α and β form poly(vinylidene fluoride) (PVDF) crystal effect on the crystallization behavior of poly(butylene succinate) (PBS) were systematically studied. The results show that β form PVDF can obviously improve the melt‐crystallization temperature of PBS during the nonisothermal crystallization process. Both crystallization time span and spherulitic size of PBS decrease with the increasing amount of β form PVDF, which enhances the primary nucleation of PBS. But α form PVDF shows no nucleating effect on PBS crystallization, exhibiting as almost unchanged T_c values for α form PVDF‐blended PBS samples. The intrinsic mechanism for the nucleating effect of β form PVDF on PBS was proposed to be the epitaxial crystallization. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40991.     

Isothermal crystallization of metallocene‐based polypropylenes with different isotacticity and regioregularity

Three polypropylenes with various isotacticities and regioregularities were prepared with metallocene catalysts at different polymerization temperatures. WAXD results showed that the relative content of γ crystals in polypropylenes increased as isotacticity decreased. It was found that the γ crystal overcame the α crystal and became predominant in polypropylene of low isotacticity and high regioerrors. More γ crystals were also formed at higher crystallization temperatures. The isothermal crystallization kinetics of α and γ crystals were compared and the metastability of these two crystal phases was interpreted in terms of the crystallization kinetics. It was observed that the α crystal has a faster crystallization rate than that of the γ crystal and, thus, higher stability at low temperature. By contrast, the γ crystal tends to have a faster crystallization rate and becomes more stable at high temperature. The metallocene‐based polypropylenes with different isotacticities have similar Avrami exponents. As the content of the γ crystal increases, double melting peaks become more evident. Equilibrium melting temperatures were derived from Hoffman–Weeks analysis and very close equilibrium melting temperatures were obtained, 185.5 and 184.0°C, for two metallocene‐based polypropylenes containing major α crystals and one of 182.5°C for the polypropylene with predominant γ crystals. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3215–3221, 2003