Compared structure and morphology of nylon-12 and 10-polyurethane lamellar crystals

A comparative study of the structure and morphology of nylon-12 and 10-polyurethane (10-PUR) lamellar crystals, was carried out. Lamellar crystals were obtained by isothermal crystallization from diluted solution. Electron diffraction of lamellae combined with WAXS data recorded from crystal sediments indicated that nylon-12 crystallized in either α-form or γ-form according to the solvent chosen for crystallization. The α-form was the crystal modification predominant in doubly oriented films of nylon-12 prepared by epitaxial crystallization. On the contrary, 10-PUR invariably crystallized in α-form regardless crystallization conditions. The α-form of nylon-12 and 10-PUR shared the same crystal structure with hydrogen-bonded sheets made of antiparallel chains and stacked with progressive shifting along both b and c directions. Lamellar crystals of nylon-12 in γ-form and 10-PUR in α-form displayed similar morphological features but only the former appeared to be sensitive to temperature. Upon heating the nylon-12 crystals near to melting, the real-time WAXS analysis evidenced the occurrence of a partial γ-to-α crystal transition, and in situ AFM observations revealed the appearing of more or less regular ridges on the crystal surface. None of these changes were observed in 10-PUR crystals when subjected to similar treatment.     

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman-Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.     

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.     

聚乳酸在剪切作用下的结晶行为研究进展

综述了剪切作用对聚乳酸（PLA）结晶动力学、晶体形态和晶型的影响;可以看出剪切对PLA的结晶动力学有显著的促进作用,如缩短结晶诱导时间,提高晶核生长速率并最终提高PLA的结晶度;施加剪切作用时,PLA得到的晶体主要是点晶、柱晶甚至串晶,说明剪切对PLA晶体形态有显著的改善作用;剪切也能促使PLA进行α′-α相转变,即对其晶体结构的完善产生积极影响。     

The crystallization characterization of bulk syndiotactic polystyrene sample: immediate evidence from IR spectroscopy

The crystallization characterization of bulk syndiothactic polystyrene (s-PS) sample is thoroughly studied using the Fourier transform infrared spectroscopy (FTIR). The WAXD is further used to identify the s-PS crystal formation to confirm the specific absorbance in FTIR spectra. Both melt and cold-crystallization behavior are quantitatively determined using FTIR spectra ranging from 870 to 820 cm⁻¹ at 264 °C. Fitting curves to IR spectra provides direct evidence of bulk s-PS crystallization behavior in quantification. The melt-crystallization process yields the β-form only; while the cold-crystallization process yields both the α and the β-form crystal in bulk s-PS sample. The β-form crystal is generated from the phase-transformation of the α-form crystal by cold-crystallization process, the α-form crystal is the initial phase. The activity energy of the α-form formation is lower that that of the β-form, suggesting that the α-form crystal is kinetically favorable while the β-form crystal is thermodynamically favorable.     

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.     

Structure formation in stretched sheets of poly(butylene terephthalate)

Crystalline and amorphous sheets of poly(butylene terephthalate) (PBT) were drawn in the temperature range of 20–150°C. The molecular orientation and the relative amount of α- and β-form crystals in the stretched sheets were studied by wide-angle X-ray diffraction (WAXD) and density measurements. When crystalline PBT sheets are drawn at lower temperatures, α-form crystals are partially transformed into β-form crystals. Both α- and β-form crystals are formed by drawing amorphous PBT sheets. The relative amount of α- and β-form crystals is much more sensitive to drawing temperature than to draw ratio. The α-form crystallinity is higher at higher drawing temperature and increases slightly with increasing draw ratio. The second moments of orientation functions of α- and β-form crystals increase with increasing draw ratio, and the increase of the orientation function is suppressed at higher draw ratio. The orientation function of α-form crystals is higher than that of β-form crystals in a same sample.     

Structure and properties of high-speed melt-spun filaments of poly(butylene terephthalate)

Filaments of poly(butylene terephthalate) were prepared by melt spinning with take-up velocities in the range 1000–5600 m/min. Two polymers with different molecular weights were used (intrinsic viscosities of 0.75 and 1.0 dL/g). The filaments were characterized using measurements of density, birefringence, shrinkage, thermal properties (differential scanning calorimetry), crystal size, crystalline orientation and phases present (wide angle X-ray diffraction), and tensile mechanical properties. Filaments spun from the 0.75 IV polymer with a mass throughput of 6 g/min at 1000 m/min have essentially amorphous structures, while higher take-up velocities result in α-form crystals or, at the highest take-up velocity, a mixture of α-form and β-form crystals. Only α-form crystals were detected in the higher IV polymer. Crystal size varied with crystallographic direction but generally increased as take-up velocity increased. At the lowest take-up velocities the filaments increased in length during thermal shrinkage measurements. With increasing take-up velocity the shrinkage became positive and continued to increase until reaching a maximum in the range of the highest sprinning speeds. This behavior correlates with the variation of the orientation factors of the amorphous phase. A plateau was observed in stress versus strain curves corresponding to strain-induced transformation from α-form to β-form crystals. The length of this plateau increased with increase of take-up velocity and the α-form crystal content in the sample. Both morphology and physical properties varied with polymer molecular weight and melt spinning conditions.     

Binary and ternary nanocomposites based on PVDF,PMMA, and PVDF/PMMA blends: polymorphism,thermal, and rheological properties

Binary and ternary nanocomposites based on poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA), and PVDF/PMMA blends were successfully prepared through a melt-mixing process, using a commercial organoclay (15A) as the nanofiller. The 15A was more finely dispersed (intercalated/partially exfoliated) within the PVDF matrix compared with the PMMA matrix. A higher PVDF content in the ternary composite essentially led to a superior degree of 15A dispersion. The 15A addition induced the polar β-form PVDF crystals, whereas the presence of PMMA in ternary composites reduced the efficiency in promoting β-form formation by 15A. Adding 15A also enhanced the nucleation of PVDF, but the enhancement was inferior while PMMA was present. Conversely, the crystal growth of PVDF was retarded with the existence of 15A, and the PVDF/15A binary composite exhibited the greatest retardation. The equilibrium melting temperature (T_m°) of PVDF in the neat state and in the blends increased after 15A addition. The PVDF/15A binary composite possessed an evidently higher β-form T_m° than the α-form T_m° of neat PVDF (~20.1 °C rise). Similar effects on the individual components, 15A declined the thermal stability of PVDF but increased that of PMMA in the ternary composites. Rheological property measurements revealed that the ternary composites performed in-between that of individual PVDF/15A and PMMA/15A binary composites. A percolation of 15A (pseudo)network structure was developed in the composites, and a more elastic behavior was observed with increasing PVDF content in the composites.     

等规聚丙烯与液晶成核剂的共混改性研究

以单丙烯酸酯液晶单体（RLC）为成核剂,通过共混反应法对等规聚丙烯（iPP）进行改性,制备含β晶型的聚丙烯产品（β-iPP）。首先介绍了β-iPP的制备工艺,然后通过偏光显微镜、广角X射线衍射对纯iPP、iPP/RLC共混物的球晶结构进行了分析;最后通过X射线衍射、差示扫描量热分析等测试方法研究共混物的结晶结构、结晶行为和热性能。结果表明,液晶成核剂RLC能够诱导iPP生成β晶型;制备β-iPP的最佳工艺条件是RLC含量为0.5 %（质量分数,下同）,结晶温度为110 ℃;β晶型相比于α晶型处于热力学亚稳态,在升温过程中,会发生β晶向α晶的转变,但较高的升温速率会抑制这一转变。     

The influence of stem conformation on the crystallization of isotactic polypropylene

The influence of stem conformation on the crystallization of i-PP is studied by growing α-form lamellae in melts of β-form lamellae at different temperatures. The melting of β-form lamellae and the crystallization of α-form lamellae is observed in situ at the interface of α- and β-form spherulites by AFM. The growth rate of α-form lamellae in the melt of β-form lamellae is much lower than that in the isotropic melt due to the stem conformation barrier, which originates from the difference in the α and β unit cell packing models.     

Anomalous molecular orientation of isotactic polypropylene sheet containing N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide

Small amount of N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide as a β-form nucleating agent is dissolved beyond 280 °C in a molten isotactic polypropylene (iPP) and appears as needle crystals around at 240 °C during cooling procedure. Further, iPP molecules crystallize on the surface of the needle crystals, in which c-axis of the β-form iPP crystals grows perpendicular to the long axis of the needle crystals. Under flow field at extrusion processing, the needle crystals orient to the flow direction prior to the crystallization of iPP. As a result, c-axis of the β-form iPP crystals orients perpendicular to the applied flow direction with a small amount of α-form iPP. Moreover, the vertical molecular orientation of the extruded sheet sample is responsible for unique mechanical anisotropy; the fracture occurs along the transversal direction.     

Synthesis and characterization of novel random copolymers based on PBN: Influence of thiodiethylene naphthalate co-units on its polymorphic behaviour

Poly(butylene/thiodiethylene naphthalate) copolymers (PBN-PTDEN) were synthesized in bulk according to the usual polycondensation procedure and examined by NMR, GPC, TGA, DSC and XRD techniques. At room temperature they appeared as semicrystalline materials; the copolymerization caused a lowering in the T_g value, a decrement of T_m and of the crystallization rate. Pure α- or β′-form was obtained at low and high TDEN unit content, respectively; crystalline form transition never occurred in the solid state, analogously to PBN. After cooling from the melt, the pure α-form was always evidenced in PBN-PTDEN10, whereas the pure β′ crystal phase develops in the copolymers containing 30 and 40 mol% TDEN units, independently on the cooling rate. In the case of PBN-PTDEN20 a pure α- or β′-form was obtained at low and high cooling rate, respectively.     

X-ray studies of multiple melting behavior of poly(butylene-2,6-naphthalate)

Multiple melting behavior of poly(butylene-2,6-naphthalate) (PBN) was studied with X-ray analysis and differential scanning calorimetry (DSC). Double endothermic peaks L and H attributed to the α-form crystal modification, a small peak attributed to the β-form crystal modification, and a new shoulder peak S at a lower temperature of peak H appeared in the DSC melting curves. Wide-angle X-ray diffraction patterns of the samples isothermally crystallized at 200 and 220 °C were obtained at a heating rate of 1 K min⁻¹, successively. In this heating process, change of crystal structure and increase of quantity of the β-form crystallites could not be observed up to the final melting. With increasing temperature, the diffraction intensity decreased gradually and then increased distinctly before a steep decrease due to the final melting. The X-ray analysis clearly proved the melt-recrystallization during heating. The β-form crystal modification was formed during slow heating process in the high temperature region.     

Crystal transformation from the α- to the β-form upon tensile drawing of poly(l-lactic acid)

A film of poly(l-lactic acid) (PLLA) consisting of highly oriented α crystals was uniaxially drawn by tensile force. The effects of the draw ratio (DR), draw temperature (T_d), and draw stress on the crystal/crystal transformation from the α- to the β-form crystals were studied. At the initial stage of drawing, the highly oriented α crystals of the starting film transformed into a broader orientation distribution, and significant crystal disorder was introduced. Upon further drawing, the α crystals steadily transformed into β crystals with increasing the DR. For the drawing at a constant T_d, the crystal transformation proceeded more efficiently at a higher draw rate and, hence, at a higher draw stress. Furthermore, for the drawing at a constant draw rate, the transformation proceeded with DR most efficiently for the tensile draw at a T_d around 140 °C, although the draw stress increased with decreasing the T_d. The present result combined with the previous finding in the drawing of PLLA by solid-state extrusion [Macromolecules 36 (2003) 3601] suggests that there is a T_d of around 140 °C at which the crystal transformation proceeds most efficiently with DR, suggesting that there are two factors that have opposite effects on the efficiency of the crystal transformation with increasing the T_d. However, as a result of the combined effects of the T_d and DR on the crystal transformation and the ductility increase with the T_d, an oriented film consisting predominantly of β crystals was obtained by tensile drawing at a T_d in the range of 140-170 °C to the highest DR achieved at each T_d.     

Temperature dependence of polymorphism in electrospun nanofibres of PA6 and PA6/clay nanocomposite

Polymorphism found in nanofibres of polyamide 6 (PA6) and PA6/clay nanocomposite (PA6-NC), prepared by an electrospinning process, was studied by transmission electron microscopy (TEM) and variable-temperature wide angle X-ray scattering (WAXS), and compared with the polymorphic changes occurring in the pre-electrospun bulk materials. TEM results, concerning morphology and dispersion of the nanoclays, reveal that the produced electrospun nanofibres have an average diameter of 50 nm, and nanoclays are much more uniformly dispersed in the electrospun PA6-NC fibres than in the pristine PA6-NC. According to WAXS measurements, both types of electrospun nanofibres predominantly consist of γ-form crystals of PA6. Upon heating, from room temperature to the melting point, a number of successive transitions are observed for both systems, namely, crystalline γ to α′, α′ to α and α to the “amorphous” δ-form due to breakage of hydrogen bonds. On subsequent cooling, it has been observed, for the first time, that the development of crystalline forms for both systems is quite different from each other. The molten electrospun pure PA6 fibres first crystallize in the high temperature α′-form, and then they show the room temperature α-form. For these nanofibres, during a temperature cycle of heating and cooling, the initial γ-form crystals completely turn into the α-form crystals as in bulk PA6. In contrast, for the electrospun nanofibres of the PA6-NC, the γ-form crystals are preserved after completing a thermal cycle down to room temperature. The present findings on the evolution of polymorphism in the electrospun nanofibres of both systems provide useful information regarding their use as reinforcing elements in polymer composites.     

Effects of polyoxyethylene nonylphenol on dynamic mechanical properties and crystallization of polypropylene

Polypropylene (PP) was blended with polyoxyethylene nonylphenol (PN) in a twin-screw extruder and injection moulded. The dynamic mechanical properties of PP/PN blends were characterized by dynamic mechanical analyser (DMA). The glass transition temperature (T_g) of PP showed a slight decrease with incorporation of PN. Differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarized optical microscopy (POM) were employed to investigate the effects of PN on crystallization of PP. In a study of nonisothermal crystallization of PP and PP/PN blends, crystallization parameter analysis showed the addition of PN reduced the peak temperature of crystallization. β-form crystals of PP coexisted with α-form crystals in PP/PN blends, and oriented on the surface layer of injection moulded bar as revealed by WAXD. The degree of orientation was determined using Hermans orientation function. The thermal stability of β-form crystals was evaluated using high temperature WAXD and POM.     

Effect of impeller type and position in a batch cooling crystallizer on the growth of borax decahydrate crystals

The effects of impeller type and position in a batch cooling crystallizer on the crystal growth kinetic as well as the shape, agglomeration and crystal size distribution of disodium tetraborate decahydrate (borax) were investigated in detail. Three types of impellers applied in this work generated completely different fluid flow patterns in the crystallizer allowing investigation of the influence of the liquid flow on the kinetic parameters as well. The ratio of impeller off-bottom clearance, C, to the liquid height, H, was varied from 0.1 to 0.5 for all impeller types used. Results indicate that the impeller type and off-bottom clearance influence the overall crystal growth rate and the crystal size distribution of the final product. It was also found that agglomeration of crystals produced was less pronounced as the radial fluid flow in the crystallizer became more dominant. In that system borax crystals tend to be smaller, but more regularly shaped.