Influence of the β‐crystalline phase on the mechanical properties of unfilled and calcium carbonate‐filled polypropylene: Ductile cracking and impact behavior

The β‐crystalline form of isotactic poly(propylene) (PP) has been long recognized to have a greater mechanical absorption capacity than the α‐crystalline form. This is of major importance for improving impact properties and crack resistance of injection‐molding parts. Unfilled PP samples together with calcium carbonate‐filled PP samples having various β/α‐phase ratios, with nearly constant morphological parameters, have been investigated from the standpoint of ductile crack propagation and impact behavior. The presence of the β‐crystalline phase turned out to improve both properties. The β spherulites are notably more prone to craze initiation than α spherulites that display a propensity for cracking. Subsequent crack propagation appears to be faster in the latter ones. The plastic zone ahead from the crack tip broadens, and the specific plastic energy increases with increasing β‐phase content. The lower elastic limit of the β phase is likely to promote the early crazing. However, the suspected higher density of tie molecules in β spherulites provides more numerous and stiffer microfibrils. The impact strength of PP is also improved by the presence of β crystals as a result of greater energy‐absorption capabilities. However, filled samples turned out insensitive to the β phase. A discussion is made about the origins of the β‐phase‐induced improvement of the mechanical properties. The possible role of the β → α transition is also explained. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 31–42, 2002     

向列相液晶共聚物作为β晶成核剂对等规聚丙烯结晶结构与热性能的影响

采用接枝聚合的方法,合成了一种新型聚硅氧烷类向列相液晶共聚物(LCP-H4),然后将LCP-H4与PP在一定工艺条件下密炼共混,得到了一系列的共混样品,采用WAXD、POM与DSC等研究了LCP-H4作为成核剂对PP样品结晶结构、形态与热性能的影响.结果表明,具有独特"液晶"性能的LCP-H4为PP结晶提供了更多的带自由能的晶核与较多的活性点,起到了异相成核的作用,既提高了PP的结晶速度、结晶温度和结晶度,又减小了球晶的尺寸,同时也改变了PP的结晶结构、形态及热力学与动力学,诱导出了β晶.此外,随着增加LCP-H4的含量及结晶温度,对应PP试样的β晶含量(Kβ)呈现先增加后降低的趋势,当LCP-H4含量为0.9%,在128℃等温结晶1h,对应成核PP的Kβ最大,为54%.     

Studies on the β-crystalline form of isotactic polypropylene

A study has been made of the mechanical, thermal, and morpholigical characteristics of melt-crystallized isotactic polypropylene containing high levels of the β or pseudohexagonal crystalline form. Different levels of β-form crystallinity were produced in the polymer by blending in low levels of quinacridone dye nucleating agent. Microscopical studies of the crystallization process revealed that both α-form (monoclinic), and β-form spherulites nucleated on the dye particles, with α-spherulite growth commencing at a higher temperature. These observations were able to qualitatively explain the dependence of β-form level on both the nucleant concentration and its state of dispersion in the polymer. Improving the dispersion of the nucleant was found to reduce the level of β-form crystallinity if the nucleant concentration exceeded an optimum level. A new procedure for quantifying the volume fraction of β spherulites in a sample was developed which utilized the technique of selective solvent extraction. From volume-fraction, x-ray, and density data, the pure α and β crystal densities were obtained. Dynamic mechanical measurements-obtained on unoriented specimens containing varying levels of β-form crystallinity showed an increase in the magnitude of the damping in the post-T_g region with increasing β content. High levels of the β form lead to lower values of the modulus and yield stress, and higher values of the elongation at break and impact strength.     

Study on the β to α transformation of polypropylene crystals in compatibilized blend of polypropylene/polyamide‐6

By using a commercial β‐nucleating agent (TMB‐5) for polypropylene (PP), it was observed that high β‐crystal content in a compatibilized blend of polypropylene/polyamide‐6 (labeled as Blend‐03 in this work) can be achieved for samples prepared by compression moulding. As β‐PP possesses more superior impact strength then α‐PP, and the β to α transformation is an important mechanism of energy absorption for β‐PP, it is of obvious interest to understand the possibilities of β to α transformation in β‐polypropylene/polyamide‐6 blends. Tensile tests were performed at temperatures of 20, 30, 40, and 50 °C, and the occurrence of β to α transformation was monitored by differential scanning calorimeter and wide angle X‐ray diffraction measurements. It was observed that the β to α transformation in Blend‐03 could only be activated at elevated tensile testing temperatures. This was related to the increase in tensile elongation at break with the increase in tensile testing temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2674–2681, 2007     

New technique of processing highly oriented poly(vinylidene fluoride) films exclusively in the β phase

Oriented β‐phase films were obtained by utilizing two different techniques: conventional uniaxial drawing at 80 °C of predominantly α‐phase films, and by drawing almost exclusively β‐phase films obtained by crystallization at 60 °C from dimethylformamide (DMF) solution with subsequent pressing. Wide angle X‐ray diffraction (WAXD) and pole figure plots showed that with the conventional drawing technique films oriented at a ratio (R) of 5 still contained about 20% of phase α, a crystallinity degree of 40% and β‐phase crystallographic c ‐axis orientation factor of 0.655. Drawing at 90 °C and with R = 4 of originally β‐phase films results in exclusively β‐phase films with crystallinity degree of 45% and orientation factor of 0.885. Crystalline phase, crystallinity degree, and crystallographic c‐axis orientation factor of both phases were also determined for α‐phase oriented films obtained by drawing α‐phase films at 140 °C. For films drawn at 140 °C the α to β phase transition drops to about 22%. Reduction in crystallinity degree with increasing R is more pronounced at draw temperature of 140 °C compared with 80 °C. Moreover, for both phases the c ‐axis orientation parallel to the draw direction is higher at draw temperature of 140 °C than at 80 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2793–2801, 2007     

Piezoelectric properties and molecular motion of poly(β-hydroxybutyrate) films

Piezoelectric, elastic, and dielectric properties of films of poly(β-hydroxybutyrate) (PHB), an optically active natural polymer, were measured as functions of frequency and temperature. In mechanical properties, three relaxation processes were observed at 10 Hz: the α dispersion at 130°C, the β dispersion at room temperature, and the γ dispersion at ?120°C. It was concluded from x-ray diffraction and the thermal expansion coefficient that the α dispersion can be ascribed to thermal molecular motions in the crystalline phase, that the β dispersion is the primary dispersion due to the glass transition, and that the γ dispersion is related to local molecular motion of the main chains in the amorphous phase. Piezoelectric relaxations were also observed in these relaxation regions. It is proposed that the high-temperature process is due to ionic dc conduction. The piezoelectric relaxation at room temperature is ascribed to the increase of piezoelectric activity in the oriented noncrystalline phase, in which the sign of the piezoelectric modulus is opposite to that in the oriented crystalline phase.     

Nucleation characteristics of the α/β compounded nucleating agents and their influences on crystallization behavior and mechanical properties of isotactic polypropylene

Nucleation characteristics of isotactic polypropylene (iPP) nucleated by the α/β compounded nucleating agents (NAs) were investigated by wide‐angle X‐ray scattering, differential scanning calorimetry and mechanical testing. The results showed that the nucleation effect of the α/β compounded NAs depends on not only nucleation efficiency (NE) of individual β and α NAs and their ratios but also the processing conditions, especially the cooling rates. The nucleating characteristics of the α/β compounded NAs can be illustrated by competitive nucleation. The NA with high NE played a leading role during iPP crystallization even at a low weight ratio and at different cooling rates. The stiffness and toughness of iPP can be simultaneously improved by using suitable compositions at the appropriate ratios. Finally, the nonisothermal crystallization kinetics of iPP nucleated with the α/β compounded NAs was described by Caze method and the crystallization activation energy of nucleated iPP was calculated by Kissinger equation. The result indicated that the crystal growth pattern of nucleated iPP was heterogeneous nucleation followed by three‐dimension spherical growth. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 653–665, 2010     

Effects of clay on polymorphism of polypropylene in polypropylene/clay nanocomposites

The effects of clay on polymorphism of polypropylene (PP) in PP/clay nanocomposites (PPCNs) under various thermomechanical conditions were studied. In extruded PP and PPCN pellet samples, only α-phase crystallites existed, as they were prepared by rapidly cooling the melt extrudates to room temperature. Under compression, β-phase crystallites can develop in neat PP under various thermal conditions, of which isothermal crystallizing at 120 °C gave the highest content of β-phase crystallites. In contrast, no β-phase crystallite was detected in the PPCN samples prepared under the same conditions. This indicated that clay significantly inhibits the formation of β-phase crystallites. The likely reason is that the presence of clay in PPCNs greatly sped up the crystallization process of the α phase, whereas it had an insignificant effect on the crystallization rates of the β phase. The results also showed that clay may slightly promote the formation of γ-phase PP crystallites in PPCNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1810–1816, 2004     

Plastic deformation behavior of β phase isotactic polypropylene in plane‐strain compression at elevated temperatures

Isotactic polypropylene (iPP) rich in β crystal modification was deformed by plane‐strain compression at T = 55–100 °C. The evolution of phase structure, morphology, and orientation were studied by DSC, X‐Ray, and SEM. The most important deformation mechanisms found were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands and the crystallographic slip systems, including the (110)[001]_β chain slip and (110)[ ]_β transverse slip. Shear within deformation bands leads to β→α solid state phase transformation in contrast to β→smectic transformation observed at room temperature. Newly formed α crystallites deform with an advancing strain by crystallographic slip mechanism, primarily the (010)[001]_α chain slip. As a result of deformation and phase transformation within deformation bands β lamellae are locally destroyed and fragmented into smaller crystals. Deformation to high strains, above e = 1, brings further heavy fragmentation of lamellae, followed by fast rotation of crystallites with chain axis towards the direction of flow FD. This process, together with still active crystallographic slip, leads to the final texture with molecular axis of both crystalline β and α phase oriented along FD. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 92–108, 2008     

In situ synchrotron X-ray scattering studies on the temperature dependence of oriented β-crystal growth in isotactic polypropylene

In this work, the growth dependence of oriented β-crystals in isotactic polypropylene (iPP) including their content, orientation and lamellar structures on the thermal treatment temperature (i.e. the final fusion temperature) is first investigated by in situ synchrotron X-ray diffraction/scattering. Interestingly, the dominance of oriented β-crystals is replaced by random α-crystals when the thermal treatment temperature in the range from 155 °C to 170 °C. This phenomenon is closely related to the nucleation efficiency of locally ordered domains of α-crystal and the template effect of residual α-crystals. Locally ordered domains of α-crystal are originated from the melting of β-crystals or (the partial melting of) α-crystals and β-α phase transformation. The orientation degree and lamellar structures of oriented β-crystals vary dramatically only when the thermal treatment temperature far exceeds the melting temperature of α-crystals. Comparably, the heat resistance of oriented β-crystals is stronger than the normal random ones.     

Molecular weight dependency of crystallization and melting behavior of β‐nucleated isotactic polypropylene

Compounds of isotactic polypropylene (iPP) and β‐nucleating agent were used to investigate the relationship between the development of β phase and molecular weight in iPP under quiescent crystallization conditions by using wide angle X‐ray diffraction and differential scanning calorimetry techniques. In all cases, the dependency of the formation of β phase in iPP on molecular weight of iPP at a defined crystallization temperature range was found. The iPP with high molecular weight possessed a wide range of crystallization temperature in inducing rich β phase. However, poor or even no β phase was obtained for the samples with low molecular weight in the same range. In addition, an upper critical crystallization temperature of producing dominant β phase was found at 125 °C. Beyond this temperature, a phenomenon of prevailing α phase became obvious. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1301–1308     

γ–α Solid–solid transition of isotactic polypropylene

X-ray diffraction patterns have been taken as a function of time and temperature on a sample of polypropylene held under high pressure (4.14 kbar) for 180 hr. at a temperature of 248°C. and subsequently cooled to room temperature. The molded sample initially crystallizes in the triclinic γ–phase but transforms to the γ–phase at elevated temperatures. The rate of conversion from γ to α is a function of time and temperature and tends to approach a constant value with increasing time. The nature of the thermal changes occurring in the sample was also studied by differential scanning calorimetry. It appears that at low scan speeds, there is a solid–solid transformation from the α-phase to the γ–phase, but at high scan speeds, the γ–phase melts without conversion to the α-phase.     

Polymorphic structure and physical properties of the polyester/ether poly(ethylene-1,2-diphenoxyethane-p,p′ -dicarboxylate)

X-ray diffraction studies of fibers of the polyester/ether poly(ethylene-1,2-diphenoxyethane-p,p′ -dicarboxylate) (PEET) produced by high-speed melt spinning show the existence of two polymorphic forms, designated α and β, in the solid state. The α form is obtained by annealing filaments melt spun at takeup speeds below 3000 m/min and is also found in samples crystallized from the melt and from dilute solutions. The α form has a monoclinic unit cell with dimensions a = 7.83, b = 10.33, c = 18.68 Å, and β = 83.1°. The equilibrium melting temperature and heat of fusion of the α form are 288.3°C and 19.1 cal/g, respectively. The β form predominates in highly oriented filaments obtained at takeup velocities above 6000 m/min. The unit cell is orthorhombic with dimensions a = 7.28, b = 5.65, and c = 18.64 Å. The β form does not transform to the α form on annealing.     

Structures cristallines et moléculaires des muscs macrocycliques. I. La cis-civettone et les variétés polymorphes α et β de sa 2,4-dinitrophénylhydrazone

Crystal and Molecular Structure of Macrocyclic Musks. I. cis-Civetone and polymorphous α- and β-forms of his 2,4-dinitrophenylhydrazone cis-Civetone (C₁₇H₃₀O) forms tetragonal plastic crystals, space groupe 14¹; a = 9.95(4), c = 32.79(1) Å; Z = 8. The plastic phase exists in a wide temperature range and 731 reflexions could be collected at 153 K. The highly disordered structure model was obtained by the use of direct methods. The molecules appear as ring-shaped diffuse electron-density distributions located in special position. Two polymorphous crystalline forms were isolated for the 2,4-dinitrophenyl-hydrazone of cis-civetone (DNPHCC). Both forms are triclinic, space group P1 . Z = 2 (α-Form: a = 6.279(5), b = 12.605(8), c = 15.253(10) Å, α = 105.49(7). β = 100.31 (6), γ = 91.23(7)°; β-Form: a = 7.950(2). b = 8.405 (2). c = 18.233(4) Å, α = 100.28(2), β = 92.29(3), γ = 94.18(2)°). The structures were solved by direct methods and refined to R = 0.11. Each polymorph is associated with a different quinquangular conformation of the macrocycle. In the crystals the intermolecular interactions between macrocycles and aromatic substituents are minimized, the DNPH group being oriented in a face-to-face arrangement across a centre of symmetry. Empirical force field calculations show that the overall intluence of the DNPH moiety on the attached cycle does not significantly modify its conformation with regard to that of the ketone itself.     

Shear-induced β-form crystallization in isotactic polypropylene

Extruded, injection-molded, unoriented crystallized specimens and capillary rheometer efflux strands of commercially stabilized polypropylene without nucleating agents were examined by optical microscopy and x-ray diffraction to determine the conditions for β-form crystallization as a function of the distance from the surface and of the shear rate at commercial processing conditions. Results demonstrate that at all “cooling conditions” ΔT = T_m ? T_b (defined as the melt temperature T_m minus the bath temperature T_b) effects of strain flow initiate nucleation of β-form crystals. The shear rate is demonstrated to be important for β-form crystallization. A critical average threshold value for the shear rate of approximately 3 × 10² sec^?1 has to be exceeded. The β modification is mostly connected with type-III spherulites and partly to row structures, and it is observed at processing conditions in oriented structures only.     

Polymorphism in copper pyrovanadate

Formation and stability temperatures were determined for the three polymorphs of copper pyrovanadate. The low-temperature β phase is formed at 500°C and is stable from room temperature to 610°C. The intermediate phase is stable within 610–705°C. The high-temperature γ phase is stable within 710–780°C. The rates of γ → α and α → β phase transitions upon cooling differ considerably. α-Cu₂V₂O₇ detected at room temperature upon cooling of a molten sample is metastable.     

Influence of molecular arrangement on the γ-ray-irradiation solid-state polymerization of 1-octadecyl vinyl ether with a characteristic polymorphism

The polymorphic behavior of 1-octadecyl vinyl ether was investigated by DSC and X-ray diffraction measurements under various temperatures. In DSC measurement of 1-octadecyl vinyl ether in the temperature range of −30 to approximately 50°C, four transition peaks were observed on heating, whereas three transition peaks appeared on cooling. The phase-transition behavior was investigated by the repeating scanning DSC measurements. It was concluded that this compound exhibited four crystalline modifications: α, sub α, β₀, and β₁. It was confirmed by the temperature-controlled X-ray diffraction measurement that these phase transitions are attributed to the change of crystal systems from hexagonal packing (α form) to a distorted orthorhombic (O⟂′) system (β₁ form) via orthorhombic (O⟂) (sub α form) and intermediate β₀ form, although the β₀ form has not yet been clarified. In the γ-ray-irradiation solid-state polymerization for these crystal forms of this compound, the polymerizability of the sub α form is higher than that of other forms, and that of the α form is lowest. The polymerizability demonstrated an unusual increase at a temperature of −83.6°C, probably because the cationic polymerization mechanism is dominant over that of the free radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3845–3853, 1999     

Effects of the oriented mesophase on the cold crystallization of syndiotactic polystyrene and its blend with poly(2,6‐dimethyl‐1,4‐phenylene oxide)

The effects of molecular orientation on the crystallization and polymorphic behaviors of syndiotactic polystyrene (sPS) and sPS/poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) blends were studied with wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry. The oriented amorphous films of sPS and sPS/PPO blends were crystallized under constraint at crystallization temperatures ranging from 140 to 240°C. The degree of crystallinity was lower in the cold‐crystallized oriented film than in the cold‐crystallized isotropic film. This was in contrast to the case of the cold crystallization of other polymers such as poly(ethylene terephthalate) and isotactic polystyrene, in which the molecular orientation induced crystallization and accelerated crystal growth. It was thought that the oriented mesophase was obtained in drawn films of sPS and that the crystallization of sPS was suppressed in that phase. The WAXD measurements showed that the crystal phase was more ordered in an sPS/PPO blend than in pure sPS under the same annealing conditions. The crystalline order recovered in the cold‐crystallized sPS/PPO blends in comparison with the cold‐crystallized pure sPS because of the decrease in the mesophase content. The crystal forms depended on the crystallization temperature, blend composition, and molecular orientation. Only the α′‐crystalline form was obtained in cold‐crystallized pure sPS, regardless of molecular orientation, whereas α′, α″, and β′ forms coexisted in the cold‐crystallized sPS/PPO blends prepared at higher crystallization temperatures (200–240°C). The β′‐form content was much lower in the oriented sPS/PPO blend than in the isotropic blend sample at the same temperature and composition. It was concluded that the oriented mesophase suppressed the crystallization of the stable β′ form more than that of the metastable α′ and α″ forms during the cold crystallization of sPS/PPO blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1665–1675, 2003     

Dielectric relaxation in ethylene–methacrylic acid copolymers and their salts

Dielectric relaxation data have been obtained for two ethylene–methacrylic acid copolymers (containing about 4 mole-% methacrylic acid units and about 8 mole-% methacrylic acid units, respectively) and the lithium, sodium, and calcium salts prepared by partial neutralization of the polyacids. The frequency range employed was from 50 Hz to 10 kHz and the temperature range was from ?130°C to 100°C. Attention is focused on three dielectric loss regions labeled β, β and α in order of increasing temperature. The β′ process (?10°C at 100 Hz in the salts only) correlates with a mechanical loss process previously reported and is attributed to microbrownian motion taking place in an amorphous hydrocarbon phase. The β′ process (20°C at 100 Hz) has also been observed mechanically and is attributed to the same mechanism as the β process. The higher temperature of this relaxation compared to the β relaxation is attributed to the presence of acid groups which form crosslinks composed of interchain hydrogen bonds. The α process (>50°C at 100 Hz in the salts only) correlates with dielectric and NMR data previously reported for a sodium salt and is assigned to motions within ionic domains formed by the clustering of salt groups.     

β Relaxations in polyethylenes and their anisotropy

Measurements have been made of the anisotropy of viscoelastic behavior in specially oriented sheets of low-density polyethylene. The results for the cold-drawn sheets show a β relaxation process of very characteristic anisotropy. The annealed sheets show two relaxations in this region of temperature. The lower relaxation (about ?10°C) is identified as an interlamellar shear process. The higher relaxation (about 70°C) has a very similar anisotropy to the β relaxation in cold-drawn samples. Isotropic sheets of low-density polyethylene have been also investigated. Two β relaxations are found in these materials.