Solvent induced polymorphism of quenched syndiotactic polypropylene in different liquids

The solvent induced crystallization phenomenon (SINC) was studied for syndiotactic polypropylene quenched from the melt at 0 °C and kept at this temperature a for long time. In these conditions a mesophase having the chains in trans-planar conformation was formed. The interaction polymer-solvent with liquids having different solubility parameters, derived by both the swelling and the weight uptake, considerably varies among the different liquids, showing a maximum corresponding to carbon tetrachloride ('=8.6). A smaller maximum was found for chloroform ('=9.3). These two maxima were attributed to interaction either with the amorphous phase or with the trans-planar mesophase. Infrared analysis showed that all the liquids induce a conformational transition from trans-planar to helix, and only a small residual fraction of chains in trans-planar conformation was detected for the samples immersed in the liquids and vacuum dried for many hours. The X-ray analysis showed that the quenched sample undergoes in the solvents a complex transformation, partially crystallizing into the helical form I and partially into the helical form II. All the liquids induced the same transformation, in spite of very different levels of interaction.     

Conformational variation and crystalline phase transformation of low syndiotactic polypropylene films in stretched and stress‐relaxed states

Low syndiotactic polypropylene (sPP; rrrr = 80%) films were isothermally crystallized at 0 °C (sample S₀) and 90 °C (sample S₉₀) for 65 h, respectively. Fourier transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction were used to characterize the structure transformation and orientation behavior of samples S₀ and S₉₀ at both stretched and stress‐relaxed states. It was found that stretching (λ = 0–700%) induces the transformation of the chain conformation from helical to trans‐planar form for both S₀ and S₉₀ films. The stretched S₀ and S₉₀ samples show well oriented trans‐planar chains as well as partially retained helices. Simultaneously, crystalline phase transformation occurs during the stretching and relaxing processes of the investigated sPP samples, i.e., stable form I crystals can be transformed into metastable form III or mesophase by stretching samples, and vice versa. For stretched S₀ sample, form III with trans‐planar conformation, which generally exists in highly stretched sPP, cannot be observed, even at higher strains. For sample S₉₀, however, stretching might induce the formation of both the form III crystals and mesophase with trans‐planar chains; releasing the tension, form III again gets converted into trans‐planar mesophase and form I crystals. In the stretched and stress‐relaxed states of samples S₀ and S₉₀, the difference of the delicate orientation behavior and relative content of chain conformation and crystalline form can be attributed to the different heat‐treating methods of the low syndiotacticity sPP. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2924–2936, 2005     

Structural and morphological changes during UV irradiation of the trans-planar form of syndiotactic polypropylene

Syndiotactic polypropylene films, obtained in the trans-planar mesophase, were exposed to accelerated weathering in a UV device at 45 °C for increasing times. A different series of films, characterized by the same structural organization, was annealed at 45 °C for the same times, to discern the thermal effects from those due to UV irradiation. The influence of UV irradiation and thermal treatments on the structural changes in terms of phase composition, crystallite sizes, and chain conformations was studied. Diffractometric and thermal analyses show that UV radiation causes a strong increase of helical crystallinity (189% with respect to the initial value), leaving the trans-planar mesophase unchanged. The mesophase domains have a higher rigidity and hinder the oxygen diffusion slowing down the photooxidative degradation.The chain scission in the amorphous phase produces a higher mobility, allowing crystallization. The crystallinity also increases in the thermally treated samples, but the increase is limited to the first 48 h and reaches about 89% of the initial value. Both in the thermally treated and UV irradiated samples the crystallite coherence length perpendicular to the 200 planes increases with time. The amorphous chains do crystallize at the beginning near the already formed crystals, thus increasing their dimensions. The carbonyl index shows an induction period of 48 h, before a rapid increase which apparently is inversely proportional to the percentage of mesomorphic and crystalline phases of the irradiated samples, indicating that the photooxidative reactions took place mainly in the amorphous region. A comparison with data already published on the photooxidative behavior of samples crystallized in form I shows that the presence of compact domains in trans-planar mesophase led to an increase in the induction period and to a greater stability of the material in the first 150 h of UV treatment.     

Phase transition of syndiotactic polypropylene in electrospun nanofibers during progressive heating

By means of high-temperature electrospinning process, syndiotactic polypropylene (sPP) nanofibers with an average diameter of 127 nm were obtained using a rotating disc as a collector. The aligned fibers were subjected to progressive heating for fiber melting. During heating, structural evolution of the sPP nanofibers was investigated in situ by means of two-dimensional wide-angle and small-angle X-ray scattering with synchrotron radiation sources. It was found that the as-spun fibers consist of the antichiral form I (9 %), mesophase (31 %), and amorphous phase (60 %), in the absence of isochiral form II. Upon heating, the mesophase started to melt and completely disappeared at 90 °C. The melting of the mesophase directly produced amorphous chains at 35–60 °C, and brought up the isochiral form II at low temperatures (60–70 °C), as well as the antichiral form I at high temperatures (70–110 °C). These events were in accordance with the DSC heating curve, which exhibited a small endotherm centered at 52 °C for the mesophase melting, followed by a shallow and broad exotherm associated with two phase-transition events, i.e., the crystal reorganization and the crystallization of supercooled liquid. The former is likely due to the solid–solid transition of meso→II phase as suggested by Lotz et al. (Macromolecules 31:9253, 1998), and the latter is relevant with crystallization of amorphous chains to develop the thermodynamic stable form I phase at high temperatures.     

Mechano‐reversible physical aging of elastic oriented syndiotactic polypropylene

Syndiotactic polypropylene (sPP), obtained at 0 °C in the trans‐planar mesophase, was drawn at room temperature up to λ = 6, and left at room temperature for 1 year with fixed or relaxed ends. Data analysis allowed the clarification of the structure of the crystalline phases and their transformations during the aging. In both oriented samples similar structural changes were observed, although they were due to different aging mechanisms. The physical aging led to the crystallization in both samples of an oriented helical form, due to a partial transformation of the mesophase and of the amorphous phase. In the oriented sample aged with fixed ends, a small fraction of the crystalline trans‐planar form III became stable even by releasing the tension after 1 year. This last sample did not undergo the large shrinkage, always observed by unloaded sPP after drawing, and therefore was no more elastic. Also, the sample aged with free ends for 1 year showed a reduced elasticity in terms of both dissipated energy and permanent set. However, after a new deformation up to λ = 6, the fiber recovered its previous elasticity. Indeed a mechano‐reversibility was apparent for the oriented elastic sample of sPP aged at room temperature with free ends, leading to a renewed elasticity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 599–606, 2008     

不同初始结构间规聚丙烯纤维熔融过程中的结晶转变行为

通过熔融纺丝及随后的热处理制备了具有不同初始结构的间规聚丙烯纤维(sPP).采用差示扫描量热仪(DSC)和变温广角X-射线衍射仪详细研究了sPP纤维在升温过程中的结构转变和熔融行为.结果表明,不同初始结构sPP纤维的晶型不同,卷绕纤维和退火处理纤维以Ⅰ型和Ⅱ型晶型为主,牵伸纤维介晶相占优;升高温度导致Ⅰ型和Ⅱ型两种晶型直接熔融,没有出现Ⅱ型向Ⅰ型的晶型转变;初始结构为介晶相的纤维在升温过程中部分介晶相直接转变为Ⅱ型晶型,还有一部分介晶相直接熔融,并在随后的升温过程中,形成Ⅰ型晶型.sPP纤维的多重熔融行为与其初始结构和纤维制备条件密切相关.     

Kinetics of the II-III phase transformation of polytetrafluoroethylene at high pressure

The rate of transformation from helical to all-trans-planar polytetrafluoroethylene (PTFE) at high pressures has been determined by monitoring the Raman spectra of PTFE following pressure jumps from the stability field of PTFE II to pressures between 9 and 14 kbar at temperatures between 0 and ?30°C. The transformation kinetics can be described by Avrami's equation for nucleation and growth kinetics with an exponent of 0.5, although observations at lower temperatures suggest that even smaller values of the exponent may be appropriate. At 10 kbar and 0°C, the specific rate constant is 0.51 min^?1/2. The energy and volume of activation are 11 kcal mole^?1 and ?7 cm³ mole^?1, respectively. The values of these parameters suggest that the transformation mechanism involves propagation of helix reversal planes along the several adjacent chains leaving all-trans material in their wakes.     

X-ray diffraction study of the smectic mesophase of some azobenzene-containing polyacrylates

Abstract

An X-ray study is performed on powder specimens and on stretched oriented fibres of two liquid crystalline polyacrylates containing the azobenzene mesogenic unit with either a 4′-n-pentyloxy (sample 1–4) or 4′-n-hexyloxy (sample 1–5) substituent. The X-ray diffraction patterns of both samples showed the presence of a bilayer smectic C mesophase with the mesogneic groups tilted by an angle β ～ 45° (1–4) or β ～ 38° (1–5) with respect to the layer normal. The electron density profile p(z) along the direction normal to the smectic layers was calculated by Fourier inversion and possible structural models of the smectic mesophase are discussed. A partial interdigitation of the terminal alkyloxy substituents appears to occur.     

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.     

Annealing induced variation of crystalline structure and mechanical properties of syndiotactic polypropylene fibers

<正>Syndiotactic polypropylene(sPP) as-spun fiber(sPP1) and drawn fiber(sPP2) were prepared by melt-spinning and melt-spinning/hot-drawing,respectively.The structure transition of the two fibers induced by annealing at different temperatures and the corresponding mechanical properties were subsequently investigated by the combination of Fourier transform infrared spectroscopy(FTIR),wide-angle X-ray diffraction(WAXD) and tensile testing.The results indicate that the chain conformation and crystal forms of the two sPP fibers are not obviously changed at low annealing temperature (40℃).With increasing the annealing temperature,the trans-planar conformation and mesophase in sPP1 and sPP2 fibers can be completely transformed to helical conformation and crystal form I under tension.Upon removing the tension,a small amount of mesophase and trans-planar conformation will be regained.The mechanical properties of the annealed fibers are manifestly dependent on their initial structure and the annealing temperature.     

Recognition of the syndiotactic polypropylene polymorphs via dynamic-mechanical analysis

Three syndiotactic polypropylene samples were crystallized under different conditions in order to obtain different polymorphs. A first sample was crystallized at high temperature, obtaining the helical form I; a second was crystallized from the melt at 0°C for many days obtaining the trans-planar mesophase; a third sample was obtained by solvent induced crystallization followed by annaeling of the trans-planar mesophase, leading to a mixture of both the helical forms I and II. In the dynamic-mechanical analysis the helical form I showed only one peak of tan δ corresponding to the amorphous glass transition. The other polymorphs also showed this transition centered at about the same temperature. Beside the peak corresponding to the Tg, the trans-planar mesophase was characterized by a peak appearing at 70°C, and the helical form II by a peak at 100°C. These peaks, unambiguously associated to transitions of the different forms, can be considered a distinctive evidence for the polymorphs obtained in different processing conditions.     

Elastic Behaviour of Oriented Syndiotactic Poly(propylene)

A syndiotactic poly(propylene) film (sPP), quenched from the melt at 0°C, was drawn at room temperature up to a draw ratio of λ = 6. The fibre was analysed under tension by X‐ray analysis and FTIR spectroscopy, showing the presence of the oriented crystalline form III described by Chatani, with the chains in trans‐planar conformation. Then it was unhooked, undergoing a large shrinkage, and reaching a length corresponding to a draw ratio of λ = 3.8. X‐ray and FTIR analysis showed that in the relaxed fibre, a conformational transition from the trans‐planar to the helical form occurred. The relaxed fibre was again submitted to increasing extensions up to λ = 6, step by step, and the elastic behaviour was analysed through the Mullins' curve and hysteresis cycles. It showed good elastic properties, which appear to be closely correlated to the conformational transition between helical and trans‐planar forms.     

Crystal and molecular structure of pyridazine-3-carboxylic acid hydrochloride and zinc(II) pyridazine-3-carboxylate tetrahydrate

Crystals of pyridazine-3-carboxylic acid hydrochloride contain almost planar molecular sheets in which the cations, composed of acid molecules each with a hydrogen atom attached to one of the ring-nitrogen atoms, interact with chloride anions via a network of weak hydrogen bonds. Van der Waals interactions between sheets are indicated by the intersheet spacing of 3.47?Å. The crystal structure of di(aqua-O)bis(trans-pyridazine-3-carboxylato-N,O)zinc(II) dihydrate is composed of monomeric molecules in which the zinc(II) ion at the center of symmetry is coordinated by two ligand molecules each via its N,O bonding moiety. The ligand molecules and the metal ion form a trans-planar configuration. Two water oxygen atoms, above and below the plane, complete a distorted octahedron. A network of weak hydrogen bonds holds the monomers together.     

Morphology of polylactic acid crystallized during annealing after uniaxial deformation

Uniaxial deformation of amorphous L -polylactic acid films was performed at two different temperatures at which thermal degradation was minimal, 70 °C or T_g + 10 and 90 °C or T_g + 30. Samples were annealed postdeformation for long times (either 15 or 45 min) to approach equilibrium conditions. Samples deformed and annealed at 70 °C showed low crystallinity and poor crystalline order or crystal size, as determined by wide-angle X-ray diffraction. At 90 °C, high crystallinity and order parameters were observed. In addition, once the oriented chains had crystallized at this temperature, nonoriented chains also underwent crystallization, and a small fraction of nonordered crystal phase was therefore observed after long annealing times. These observations are explained on the basis of different morphologies in samples drawn at the two temperatures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Extrusion,fiber formation,and characterization of thermotropic copolyesters

The flow behavior and the effect of the spinning conditions on the fiber properties and structure of poly(ethylene terephthalate) modified with 60 mol% p-hydroxybenzoic acid (PET/60PHB) were investigated. PET and its copolyesters with 28 and 80 mol% PHB were used as control samples. The melt of PET/60PHB at temperatures above 265°C exhibited extremely low viscosity and low flow activation energy. High birefringence, indicating the presence of a mesophase, was observed between 265 and 300°C on a hot-stage polarizing light microscope. The maximum tensile strength and initial modulus, 438 MPa and 37 GPa, respectively, were obtained at 275°C for a 0.69 IV polymer. The fiber strength and modulus were significantly lowered when extrusion was conducted at temperatures below 265°C. The fiber properties could also be improved when a high extrusion rate and/or a high draw down ratio was used. Scanning electron microscopy revealed that the fibers spun at temperatures above 265°C had a well-developed, highly oriented fibrillar structure. The fibers spun at lower temperatures, however, were poorly oriented and nonfibrillar in character. The high orientation and superior mechanical performance achieved at high temperatures were attributed to the presence of the nematic mesophase in the polymer melt.     

Structure and properties of the mesophase of syndiotactic polystyrene. VIII. Solvent sorption behavior of syndiotactic polystyrene/p‐chlorotoluene mesophase membranes

A crystalline δ form of a syndiotactic polystyrene (sPS) membrane was prepared from a solution of sPS (1 wt %) and p‐chlorotoluene (p‐CT) by a solution‐casting method. The mesophase (δ empty form) of sPS was obtained by the extraction of the guest solvent from the δ form of sPS by a stepwise solvent‐extraction method. The sPS/p‐CT mesophase membrane [p‐CT (A‐M)] was used for the sorption of 1 mol % p‐CT for different times and for the sorption of different concentrations of p‐CT, chlorobenzene (CB), p‐xylene (p‐X), toluene, and chloroform for 48 h. The presence of solvents in the sPS membrane was confirmed by IR analysis. A thermal study revealed that the sorption amount of 1 mol % p‐CT increased with increasing immersion time, and the sorption amounts of different solvents increased with increasing solvent concentration. Differential scanning calorimetry results showed that the desorption peak temperature increased as the amount of the solvent increased in the clathrated sPS membrane. Wide‐angle X‐ray diffraction results showed that 2θ at 8.25° was slightly shifted toward 8°, and there was no change in the peak position at 10° for p‐CT (A‐M), which was immersed in different solvents (1 mol %); however, the intensity of 2θ at 10° was not similar for all the samples. Among the solvents used for the sorption studies at 1 mol %, p‐CT (A‐M) could sorb more p‐CT and CB than p‐X, toluene, and chloroform. The solvent sorption isotherm was the Langmuir sorption mechanism. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3439–3446, 2004     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Synthesis and some properties of a polyamide containing trans-2,5-linked tetrahydropyran rings in the main chain

A novel polyamide containing trans-2,5-linked tetrahydropyran rings in the main chain, poly(trans-tetrahydropyran-2,5-diyliminocarbonyl) (trans-polyamide 6 ), was synthesized by the two-stage polycondensation of ethyl or 2,2,2-trifluoroethyl trans-5-aminotetrahydropyran-2-carboxylate ( 5b or 5c ). The polycondensation was carried out first in m-cresol at 180°C. The isolated polymer of a relatively low molecular weight was then allowed to react further in a solid state at different temperatures between 180 and 230°C under vacuum. The trans-polyamide 6 thus obtained was soluble only in protic solvents such as m-cresol, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-isopropanol, and trifluoroacetic acid. The viscosity numbers of 6 were up to 0.46 dL/g in m-cresol at 25°C. DSC and TGA analyses of 6 showed that the thermal decomposition occurred at about 350°C in nitrogen, whereas a rapid weight decrease due to the thermooxidative decomposition started at about 300°C in air. The moisture sorption isotherm was determined on coarsely ground samples of 6 at 25°C and compared with those for the structurally-related polyamides 2 and 4 . © 1993 John Wiley & Sons, Inc.     

Effects of thermal treatment on biaxially oriented poly (ethylene terephthalate). III. Creep behavior following various thermal histories

Elongational creep measurements were carried out on a biaxially oriented poly (ethylene terephthalate) (PET) film parallel to, orthogonal to, and at 45° to the principal optic axis. Measurements made after various thermal treatments which were intended to stabilize the physical state of the PET were shown to be ineffective. Samples were annealed at 140°C for 12 days and aged at 95°C for over 24 days before measurement without success. Thermal cycling between 41 and 91°C which was also employed to stabilize the mechanical response also failed. Significant deceleration of the creep rate caused by densification of amorphous regions of the samples during storage below the glass temperature T_g is illustrated. Because of physical aging below T_g and morphological changes occurring above T_g during the various thermal treatments and histories, time-scale shift factors were found to be not unique.     

Morphology and thermodynamic properties of solution-grown single crystals of trans-1,4-polybutadiene

The morphology and thermal behavior of trans-1,4-polybutadiene single crystals prepared from dilute solutions in n-heptane have been investigated by electron microscopy, wide-angle and low-angle x-ray diffraction, and differential scanning calorimetry. The thickness of the single crystals of trans-1,4-PBD is much greater, for the same undercooling, than that of other linear polymers. From the calorimetric study, convincing evidence of the presence, in the single crystals, of crystalline blocks of form 1 with two different thermodynamic stabilities is obtained and the relative amounts are calculated. From thermodynamic considerations we find a value of 75°C for the equilibrium transition temperature between forms 1 and 2, 139°C for the equilibrium melting temperature of form 2, and 31.2 erg/cm² for the fold-surface free energy of modification 2.