Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Polymorphism and β-form structure of poly(octamethylene 2,6-naphthalate)

It was revealed that poly(octamethylene 2,6-naphthalate) (PON) existed in two different crystal structures, α- and β-form, depending on crystallization process: The α-form crystal was dominantly developed from the cold-crystallization, whereas the β-form was from the melt-crystallization. The apparent melting temperatures of α- and β-form crystals were characterized to be 175 and 183 °C, respectively. On the basis of X-ray diffraction and molecular modeling studies, the crystal structure of β-form, developed dominantly from the melt-crystallization, was identified to be triclinic with dimensions of a = 0.601 nm, b = 1.069 nm, c = 2.068 nm, α = 155.68°, β = 123.25°, γ = 52.85°, and with the space group of . The calculated crystal density was 1.243 g/cm³, supporting that one repeating unit of PON exists in a unit cell. The octamethylene units in the PON backbone take largely all-trans conformation in the β-form unit cell.     

Unique orientation textures formed in miscible blends of poly(vinylidene fluoride) and poly[(R)-3-hydroxybutyrate]

The oriented crystallization of poly[(R)-3-hydroxybutyrate] (PHB) in the miscible blends with poly(vinylidene fluoride) (PVDF) was investigated with various compositions. The PVDF/PHB blend films were prepared by solution casting and subsequent melt-quenching in ice water. Oriented films of the blends were prepared by uniaxially stretching the melt-quenched film at 0 °C in ice water using a hand-operated stretching apparatus. The oriented blend films were heat-treated at a fixed length in order to crystallize PHB in the oriented state. The crystal orientation and the lamellar textures of the obtained samples were studied with wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS), respectively. The SAXS measurements showed that a considerable amount of molecular chains of PHB are excluded from the lamellar stacks of PVDF and exist in the interfibrillar regions in the oriented films of the blends. The cold crystallization of PHB in the interfibrillar region results in the orientation of PHB crystals, and the type of crystal orientation depends upon the composition of the blends. For the PVDF/PHB=4/6-7/3 blends, the crystal a-axis of PHB is highly oriented parallel to the drawing direction and the crystal c-axis (molecular chain axis) in PHB crystals is perpendicular to the drawing direction, i.e. orthogonal to the chain axis of the crystals of PVDF. It is considered that the a-axis orientation is induced by the confinement of crystal growth in the interfibrillar nano-domains. For the PVDF/PHB=2/8-3/7 blends, however, the crystal c-axis of PHB is primarily oriented in the drawing direction, suggesting that the stressed molecular chains of PHB are crystallized with the molecular orientation retained.     

Higher-order structural analysis of bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanote] highly oriented films

Bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBH) highly oriented films were prepared by the combination of roll and uniaxial drawing processes. The change in the higher-order structure of PHBH films was investigated by wide-angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Extended films, which show superior mechanical properties and high ductility, have complicated structures. By roll extension, both deformed and undeformed spherulites co-exist, the former inclined to the direction perpendicular to the film surface. The latter were destroyed by further uniaxial extension. The tie-molecules between uniaxially oriented lamellae were extended and transformed to the β-form with a planar zig-zag conformation. Three kinds of structures, c-axis parallel to the uniaxial drawing direction, c-axis inclining to the normal vector of the film surface, and the β-form between lamellae, are intermingled in the film.     

Electrochemically and electrochromically stable polyimides bearing tert-butyl-blocked N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units

A new class of electrochemically active polyimides with di-tert-butyl-substituted N,N,N′,N′-tetraphenyl-1,4-phenylenediamine units was prepared from N,N-bis(4-aminophenyl)-N′,N′-bis(4-tert-butylphenyl)-1,4-phenylenediamine and various aromatic tetracarboxylic dianhydrides via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical or thermal cyclodehydration. Most of the polyimides are readily soluble in many organic solvents and can be solution-cast into tough and amorphous films. They had useful levels of thermal stability, with relatively high glass-transition temperatures (276-334 °C), 10% weight-loss temperatures in excess of 500 °C, and char yields at 800 °C in nitrogen higher than 60%. Cyclic voltammograms of the polyimide films cast on the indium-tin oxide (ITO)-coated glass substrate exhibited two reversible oxidation redox couples at 0.70-0.74 V and 1.05-1.08 V vs. Ag/AgCl in acetonitrile solution. The polyimide films revealed excellent stability of electrochromic characteristics, with a color change from colorless or pale yellowish neutral form to green and blue oxidized forms at applied potentials ranging from 0.0 to 1.3 V. These anodically coloring polymeric materials exhibited high optical contrast of percentage transmittance change (Δ%T) up to 44% at 413 nm and 43% at 890 nm for the green coloration, and 98% at 681 nm for the blue coloration. After over 50 cyclic switches, the polymer films still exhibited good redox and electrochromic stability.     

Anisotropic thermal expansion in polypropylene/poly(ethylene-co-octene) binary blends: influence of arrays of elastomer domains

In injection molded specimens consisting of isotactic polypropylene (iPP)/poly(ethylene-co-octene) (EOR) blends with different viscosity ratio of η(EOR)/η(iPP), the coefficient of linear thermal expansion (CLTE) was investigated by thermal mechanical analysis (TMA). It was found that the blend with a smaller viscosity ratio showed the larger anisotropy of CLTE depending upon the directions. TEM observations revealed that the shape of rubber domains varied from slabs, cylinders to ellipsoids in shape, by increasing η(EOR)/η(iPP). The crystal orientation analysis by WAXD have revealed that the blend with ‘slab’ EOR domains showed the orientation of the c-axis of iPP crystals was preferably oriented to FD (flow direction) and TD (transverse to FD), and that the b-axis was exclusively oriented to ND (thickness direction). The CLTE of each FD and TD was in good agreement with the rules-of-mixing for CLTE by introducing the effect of the arrays of the elastomer domains and the PP crystal orientation. On the other hand, the CLTE in ND showed massive discrepancy between the calculation and observation. It was found that the incorporation of the retraction effect could explain the discrepancy to some extent.     

Beta-sialon phosphor deposits fabricated by electrophoretic deposition (EPD) process in a magnetic field

Phosphor deposits of β-sialon:Eu²⁺ were prepared by electrophoretic deposition (EPD) process within a magnetic field. Under the action of the magnetic force, which was parallel to the direction of the electric field of the EPD, the β-sialon:Eu²⁺ crystals were aligned along the c-axis of the hexagonal cell unit to form an oriented deposit via the EPD fabrication. Higher orientation degree was obtained at longer depositing time (300 s) and stronger applied magnetic field (12 T). The oriented deposit aligned along the c-axis obtained higher relative deposit density than the randomly fabricated deposit. Due to the improved relative density, the oriented deposit prepared within the magnetic field possessed an enhanced external quantum efficiency (η_ex). Also, because of different relative densities of the deposits prepared within and without the magnetic field, they presented different chromaticity coordinates.     

Hydrogen-bond-assisted isotactic-specific radical polymerization of N-isopropylacrylamide with pyridine N-oxide

Radical polymerization of N-isopropylacrylamide (NIPAAm) in CHCl₃ at low temperatures in the presence of pyridine N-oxide (PNO) was investigated. An isotactic poly(NIPAAm) with meso diad content of 61% was successfully prepared at −60 °C in the presence of a two-fold amount of PNO. Thermodynamic analysis suggested that the isotactic-specificity was entropically induced, probably due to conformational fixation near the propagating chain-end through coordination by PNO.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.     

AFM study of crystallization and melting of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} in ultrathin films

Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M_n) of 5300 and 2100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T_c<44 °C, the monolayer crystals are dendrites. At T_c>48 °C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 °C≤T_c≤48 °C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T_c. At the same T_c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T_c, and their melting is mainly determined by the terrace thickness.     

Super porous organic-inorganic poly(N-isopropylacrylamide)-based hydrogel with a very fast temperature response

Porous organic-inorganic (O-I) hydrogels showing a very fast temperature response, including very fast reswelling were prepared: only 6 s are needed for 72% deswelling (gel collapse) as well as for 72% reswelling. Both deswelling and reswelling are practically complete in 14 s. The gels were prepared from N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (BAA) and tetramethoxysilane (TMOS) by simultaneous radical polymerization and hydrolytic polycondensation of TMOS. The syntheses were carried out at temperatures below the lower critical solution temperature (LCST) of poly(NIPA) in two steps: during the first stage the temperature was held at T = +15 °C and during the second the temperature was lowered below the freezing point of the reaction mixture, T = −18 °C. The ice crystals, which grew during the second stage, served as the pore-forming agent. The best samples were obtained if the second stage was started shortly before the gel point of the reaction mixture. The introduction of the inorganic phase (silica) is necessary for the ability of fast reswelling and also results in a strong improvement of the hydrogels' mechanical properties, while the maximum swelling degree remains nearly unaffected.     

Effects of crystallization temperature on the polymorphic behavior of syndiotactic polystyrene

The influence of crystallization temperature on formation of the α- and β-form crystals of syndiotactic polystyrene (sPS) was investigated by X-ray diffraction and non-isothermal differential scanning calorimetry analysis. For sPS samples without any thermal history, the crystallization temperature must be the intrinsic factor controlling the formation the α and β-form crystals. Being crystallized at different cooling rate from the melt, sPS forms the β-form crystal until the temperature cooled down to about 230 °C, and α-form crystal can only be obtained when the temperature was below about 230 °C.     

Effects of organic nucleating agents and zinc oxide nanoparticles on isotactic polypropylene crystallization

Organic nucleating agents and inorganic nanoparticles, as well as their hybrid composites, affect the crystallization temperature and morphology of the monoclinic α-form of isotactic polypropylene (iPP). Techniques such as differential scanning calorimetry, hot-stage optical microscopy with cross polars, wide angle X-ray diffraction, and transmission electron microscopy were employed. Nanoparticles of zinc oxide function as efficient supports for 1,3,5-benzene tricarboxylic-(N-2-methylcyclohexyl)triamine because the temperature at which the maximum rate of iPP crystallization occurs during 10 °C/min cooling from the molten state increases from 111 °C for the pure polymer to 125 °C at low concentrations of this hybrid nucleating agent. In the absence of zinc oxide, 0.06 wt% of this aliphatic triamine recrystallizes near 165 °C and increases the crystallization temperature of iPP by 7 °C, relative to the pure polymer. Fluorinated aromatic triamines, such as 1,3,5-benzene tricaboxylic-(N-4-fluorophenyl)triamine, are weak nucleating agents that reduce spherulite size in isotactic polypropylene but only increase the crystallization temperature marginally when the polymer is cooled from the molten state. Both micro- and nanoparticles of zinc oxide reduce spherulite size in isotactic polypropylene, but smaller spherulites are observed when the inorganic nanoparticles exhibit dimensions on the order of 40-150 nm relative to micron-size particles. In contrast, 0.06 wt% of the aliphatic triamine in iPP yields a distorted birefringent texture under cross polars that is not spherulitic. Non-spherulitic birefringent textures in iPP are also observed when the aliphatic triamine nucleating agent is coated onto micro- or nanoparticles of zinc oxide. This study demonstrates that the nonisothermal crystallization temperature of isotactic polypropylene increases by an additional 7 °C when an aliphatic triamine is distributed efficiently within the polymeric matrix by coating this nucleating agent onto zinc oxide nanoparticles.     

A highly active novel β-nucleating agent for isotactic polypropylene

A highly active novel β-nucleating agent for isotactic polypropylene (iPP), cadmium bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate (BCHE30), was found and its effects on the mechanical properties, the content of β-crystal, and crystallization behavior of iPP were investigated, respectively. The results show that the impact strength and crystallization peak temperature of nucleated iPP are greatly increased, while the spherulite size of nucleated iPP is dramatically decreased than that of pure iPP. The content of β-form of nucleated iPP (k_β value) can reach 87% with 0.1 wt% BCHE30. The Caze method was used to study the non-isothermal crystallization kinetics of nucleated iPP and the crystallization active energy was achieved by Kissinger method.     

Synthesis of sodium β″-alumina powder by sol-gel combustion

Magnesia stabilized sodium β″-alumina powder was prepared by sol-gel combustion using metal nitrates and citric acid. The thermal decomposition of the nitrate-citrate gels and the crystallization behavior of β″-alumina were studied. The nitrate-citrate gel exhibits self-propagating combustion behavior after ignition. β″-alumina crystallizes gradually from the sodium aluminum oxide intermediate phase to form the major phase at 1100 °C. The synthesized β″-alumina powder was formed into dense electrolyte tubes by slip casting and subsequent sintering. The non-conducting c-axis of the hexagonal β″-alumina grains tends to be oriented in the radial direction in the cast tubes.     

Influence of crystallization temperature on the morphologies of isotactic polypropylene single-polymer composite

The supermolecular structures of iPP fiber/matrix composites as a function of crystallization temperature were studied by means of optical microscopy. The results show that, even though partial melting of the iPP fibers is in favor of initiating the β-iPP crystal growth, the interfacial morphology of iPP single-polymer composites induced by its own fiber depends strongly on the crystallization temperature. It was found that transcrystalline structures of negative radial β_III-iPP or banded β_IV-iPP can be produced within the crystallization temperature range 105-137 °C, while transcrystallization zone of pure negative radial α_II-iPP crystals is observed at higher crystallization temperature, e.g. 141 °C. On the other hand, the surrounding iPP spherulites grown from the bulk are composed of α-iPP in the whole crystallization temperature range. However, the optical character of the spherulites is controlled by the thermal condition.     

Melting behavior of poly(l-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(l-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T_c) of 113 °C (= T_b) and formation of two crystal modifications for the ICSs obtained in the temperature range T_c ≤ T_b and T_c ≥ T_b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (α′-form) to the high-temperature one (α-form) occurred in a range 155-165 °C for the ICSs(T_c ≤ T_b), and that the crystal structure for the ICSs(T_c ≥ T_b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve.     

Temperature sensitive poly(N-t-butylacrylamide-co-acrylamide) hydrogels: synthesis and swelling behavior

A series of temperature sensitive hydrogels was prepared by free-radical crosslinking copolymerization of N-t-butylacrylamide (TBA) and acrylamide in methanol. N,N′-methylenebis(acrylamide) was used as the crosslinker. It was shown that the swelling behavior of the hydrogels can be controlled by changing the amount of TBA units in the network chains. Hydrogels immersed in dimethylsulfoxide (DMSO)-water mixtures exhibited reentrant swelling behavior, in which the gels first deswell then reswell if the DMSO content of the solvent mixture is continuously increased. In water over the temperature range of 2-64 °C, hydrogels with less than 40[percnt] TBA by mole were in a swollen state while those with TBA contents higher than 60[percnt] were in a collapsed state. Hydrogels with 40-60[percnt] TBA exhibited swelling-deswelling transition in water depending on the temperature. The temperature interval for the deswelling transition of 60[percnt] TBA gel was found to be in the range from 10 to 28 °C, while for the 40[percnt] TBA gel, the deswelling started at about 20 °C and continued until the onset of the hydrolysis of the network chains at around 64 °C. It was shown that the Flory-Rehner theory of swelling equilibrium provides a satisfactory agreement to the experimental swelling data of the hydrogels, provided that the sensitive dependence of the χ parameter on both temperature and polymer concentration is taken into account.     

Temperature effects on ZnO electrodeposition

A thermochemical study of the temperature effects on the Zn-Cl-H₂O system by means of potential-pH, solubility and species repartition diagrams is presented with the view to better understand the effect of temperature on the deposition mechanism and composition of zinc oxide thin films. These calculations have been completed by film preparation at different temperatures between room temperature and 90 °C. Below 34 °C, we observe the absence of continuous film growth and surface passivation. The oxide nucleation and film growth start above 34 °C, whereas the optimum film transparency and crystallinity is obtained from 40 °C. Above, the main effect of the temperature is to raise the film texturation with the c-axis perpendicular to the substrate surface.     

Phase diagram of a cellulose solvent: N-methylmorpholine-N-oxide-water mixtures

The phase diagram of the N-methylmorpholine-N-oxide-H₂O mixtures from 0 to 100% has been determined. Three crystalline hydrates have been identified, the already known monohydrate, a dihydrate and a hydrate composed of 8 water molecules per NMMO. The melting temperature of the 8H₂O-NMMO hydrate is −47 °C with a melting enthalpy of about 80 J/g. The region between 25 and 55% of water does not show any crystallisation, but a glass transition around −60 to −100 °C.