Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

Annealing induced microstructure and mechanical property changes of impact resistant polypropylene copolymer

The effects of annealing on microstructure and mechanical properties of an impact resistant polypropylene copolymer(IPC) were investigated. Different annealing temperatures ranging from 80 °C to 160 °C were selected. The phase reorganization of IPC during annealing process was studied through morphological characterization technologies, including scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The crystalline structure changes in the IPC sample, including the i PP matrix and PE component, were investigated using wide angle X-ray diffraction(WAXD) and differential scanning calorimetry(DSC). Dynamic mechanical analysis(DMA) was used to analyze the relaxation extent of IPC before and after annealing. The results showed that annealing induced phase reorganization in IPC and the degree of phase reorganization depended on annealing temperature. The annealed IPC samples exhibited largely increased crystallinity compared with the unannealed one. Intensified damping peak with increased molecular chain mobility was achieved for the annealed IPC samples. At an appropriate annealing temperature(140 °C), largely enhanced impact strength was achieved for the annealed IPC sample. The toughening mechanisms were analyzed based on the phase reorganization and relaxation behavior.     

Morphology of drawn syndiotactic polypropylene films

Morphological studies were conducted to investigate the drawability of melt-quenched (MQ) and slowly cooled (MSC) films of syndiotactic polypropylene (sPP) with a high syndiotactic pentad fraction. Transmission electron microscopy (TEM) using the ruthenium tetraoxide staining and ultrathin sectioning method revealed that amorphous chains as the matrix of MQ film played a role in drawability of the film by their alignment to machine direction (MD) and partial crystallizations into nanofibrils. On the other hand, the initial strain induced, rotations of clusters of long lamellar crystals as the major entity of MSC film accompanying breaks of long lamellae and formation of crazes and microvoids at the cluster boundary. Compared with a homogeneous thinning of MQ film by drawing, ca. 100 nm-thick layer slips along MD and parallel to the film surface took place in MSC film. This gave rise to the formation of V-shaped bent lamellar morphology and their further break into a smaller cluster of stacked lamellae which were aligned oblique by ca. 35° from MD. With elongation, some nanofibrils formed from chains generated by the partial breaks of lamellae are aligned perpendicular to the remained oblique lamellae and others parallel to MD in region where lamellar morphology almost disappeared. No chain slips in the nanofibrils can be related to a low elongation at break, i.e. a low ductility of sPP films. The lower elongation at break for MSC film than for MQ one can be interpreted by microvoids initially formed in the neck region and later moved to the fully drawn part, the microvoids initiating the break of the drawn film.     

Combined Effects of Stretching and Nanofillers on theCrystalline Structure and Mechanical Properties ofPolypropylene and Single-walled Carbon NanotubeComposite Fibers简

The combined effects of stretching and single-walled carbon nanotubes （SWCNTs） on crystalline structure and mechanical properties were systematically investigated in melt-spun polypropylene （PP） fibers prepared at two different draw ratios. The dispersion, alignment of the SWCNT bundles and interfacial crystalline structure in the composite fibers are significantly influenced by the stretching force during the melt spinning. The nanohybrid shish kebab （NHSK） superstructure where extended PP chains and aligned SWCNT bundle as hybrid shish and PP lamellae as kebab has been successfully obtained in the composite fibers prepared at the high draw ratio and the related formation mechanism is discussed based on the results of morphological observations and 2d-SAXS patterns. Large improvement in tensile strength and modulus has been realized at the high draw ratio due to the enhanced orientation and dispersion of SWCNT bundles as well as the formation of NHSK.     

Effect of annealing on microstructure and mechanical properties of polypropylene random copolymer

In this work, polypropylene random copolymer (PPR) was taken as an example to study the changes of mechanical properties related to its microstructure evolution. Firstly, the toughness and fracture morphology were analyzed by notched Izod impact test and scanning electron microscope. Annealing at relative lower temperatures (<100°C), mechanical properties are slightly enhanced, which should be pointed out that significant improvements have been observed when annealing at relative higher temperatures (>100°C). Secondly, the study was conducted from the conventional differential scanning calorimetry, wide angle X-ray diffraction, and small-angle X-ray scattering to analyses the changes in the crystalline and amorphous regions. Dynamic thermomechanical analysis was employed to explore the changes of molecular mobility in samples after annealing at different temperatures. Moreover, to find out the stress transfer between the crystalline regions and the amorphous regions, we did further analysis of the typical stress–strain curves and proposed the mechanism of microstructure evolution during annealing process. The results shown that amorphous rearranged and formed thinner lamellae when annealing at relative low temperature. While annealing at higher temperatures, the mobile and rigid amorphous regions rearranged into more perfect lamellae and the density of stress transmitters was increased significantly.     

Deformation temperature and lamellar thickness dependency of Form I to Form III phase transition in syndiotactic polypropylene during tensile stretching

Phase transition from form Ⅰ to form Ⅲ in syndiotactic polypropylene crystallized at different conditions during tensile deformation at different temperatures was investigated by using in situ synchrotron wide angle X-ray diffraction technique. In all cases, the occurrence of this phase transition was observed. The onset strain of this transition was found to be crystalline thickness decided by crystallization temperature and drawing temperature dependent. The effect of drawing temperature on this phase transition is understood by the changes in mechanical properties with temperature. Moreover, crystalline thickness dependency of the phase transition reveals that this form Ⅰ to from Ⅲ phase transition occurs first in those lamellae with their normal along the stretching direction which have not experienced stress induced melting and recrystallization.     

Influence of molecular characteristics on non-isothermal melt-crystallization kinetics of syndiotactic polypropylene

Non-isothermal melt-crystallization and subsequent melting behavior for six syndiotactic polypropylene (sPP) resins having different molecular characteristics were investigated by differential scanning calorimetry (DSC). For a given sPP resin, the crystallization exotherm became wider and shifted towards a lower temperature with increasing cooling rate. Among all of the sPP resins investigated, the crystallization exotherm of sPP#11 was found to locate at the highest temperature range, followed by that of sPP#14, sPP#10, sPP#13, sPP#12, and sPP#9, respectively. Based on the absolute temperature scale, sPP#11 showed the highest tendency to start crystallizing during a cooling scan. The ability of these resins to start crystallizing was found to be very similar when the difference in the equilibrium melting temperature of the resins was taken into account. The non-isothermal melt-crystallization kinetics of these sPP resins was well described by the Avrami, Urbanovici–Segal, Ozawa, and Ziabicki models. The subsequent melting behavior of these sPP resins exhibited either a single melting endotherm or double melting endotherms.     

Elasticity of syndiotactic polypropylene: Insights from temperature and time dependence

The origin of the unusual and puzzling elasticity of drawn sPP samples was investigated. The mechanism responsible of the elasticity was studied for drawn samples characterized by a very simple structural organization, where there are no involvements of crystallographic modifications with different chain conformation. The elastic behavior of the drawn samples, valued through the hysteresis cycles, was determined at different temperatures. At room temperature the samples show remarkable elastic properties, whereas decreasing the temperature the elastic behavior becomes worse and worse, disappearing at temperatures lower than 0 °C. The elasticity also disappears in drawn samples after a long aging under tension. Thermomechanical and structural investigations, as well as shrinkage as a function of temperature and aging at room temperature of the fixed drawn samples, support the idea that the elastic behavior of sPP can be explained by the model of the plastic deformation of semicrystalline polymers. The interpretation is based on the presence of “tie” molecules axially connecting the crystals in the oriented samples, whose extension, chain conformation and/or crystallization determine the retractive stress of the oriented sample, as well as other mechanical properties. We show that many experiments on the drawn samples, either fixed or relaxed, are strictly connected to the morphology of the drawn sample, derived by the transition between the lamellar initial and the fibrillar final structure. By applying the model we can answer the questions derived from the experimental facts not yet well clarified, giving a new insight into the interesting elasticity of sPP.     

On the crystallization behavior of cold-drawn syndiotactic polypropylene

The influence of the chain conformation on the crystallization behavior of cold-drawn syndiotactic polypropylene (sPP) has been investigated. The conformational and structural changes depending on drawing conditions and thermal treatments has been observed by x-ray diffraction, infra-red spectroscopy and modulated differential scanning calorimetry. A nucleation and crystal growth model is introduced, which explains the low crystallinity of cold-drawn sPP.     

Physical aging of syndiotactic polypropylene

Syndiotactic polypropylene (sPP) was quenched from the melt in an ice‐water bath, and changes in the structural organization, during the aging time, were followed by X‐rays, differential scanning calorimetry, and transport properties of dichloromethane at low activities. After 1 month, an increase of crystallinity from 19 to 26% was observed. In addition, the results of sorption and diffusion indicated a consistent increase of an intermediate phase not crystalline, yet impermeable to the vapors. The study of the mechanical properties showed that there is a remarkable increase of all the mechanical parameters with the aging time, and this effect was associated to the increase of the intermediate phase. The elastic modulus increased three times in the first hours of aging. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 173–180, 1999     

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.     

Influence of molecular characteristics on overall isothermal melt-crystallization behavior and equilibrium melting temperature of syndiotactic polypropylene

Overall isothermal melt-crystallization and subsequent melting behavior of metallocene-catalyzed syndiotactic polypropylene resins of various molecular weights were investigated using differential scanning calorimetry (DSC) technique. Two sets of molecular weight range were synthesized with two different metallocene catalyst systems. The kinetics of the overall isothermal melt-crystallization process was analyzed based on various macrokinetic models, i.e. the Avrami, Malkin and Urbanovici-Segal models. The effective activation energy describing the overall isothermal crystallization process over the crystallization temperature range studied was estimated based on an Arrhenius approximation of the obtained Avrami crystallization rate constants. The equilibrium melting temperature for each of these resins was estimated based on the linear and non-linear Hoffman-Weeks extrapolative methods.     

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.     

Vibrational spectroscopic investigation of stereoregularity effects on syndiotactic polypropylene structure and morphology

Vibrational spectroscopy is used to sensitively detect specific morphologies and microstructures present in metallocene-catalyzed syndiotactic polypropylenes (sPP). Six materials, ranging in racemic triad content from 26 to 96% rr, are studied. Changes in high-resolution infrared (IR) and Raman spectra of melt-slow-cooled films are observed as the degree of syndiotacticity varies. Three different types of peak behavior are observed: splitting, wavenumber shift, and change in peak intensity. An overall trend toward greater molecular order (e.g. ordered chain conformations, increased crystallinity) is observed as syndiotacticity increases. By combining results with supporting evidence from X-ray diffraction and IR linear dichroism experiments of highly syndiotactic sPP, new peak assignments are proposed for tacticity-sensitive vibrational bands. Some very interesting spectral behavior is observed for material of intermediate stereoregularity (49% rr). Previously unobserved peaks appear in X-ray diffractograms and IR absorbance spectra, suggesting the presence of an as-yet unidentified “transitional” structure—perhaps a disordered modification of crystalline Form I. This moderate level of syndiotacticty appears to be a critical point or threshold below which sPP chains are unable to adopt characteristic helical or planar zigzag conformations. Results from this work provide a more thorough understanding of stereochemical effects on vibrational spectra, which will be very useful in the interpretation of ongoing IR linear dichroism studies of newly available semi-syndiotactic (semi-sPP) materials.     

Influence of structure on the properties of polypropylene copolymers and terpolymers

Several Ziegler-Natta copolymers of iPP with ethylene or 1-butene, and terpolymers with both counits have been characterized, devoting special attention to the effect of composition and processing conditions on the crystal structure and final properties. DSC and X-ray diffraction were used to study the polymorphism of copolymers and terpolymers. Comonomer insertion interrupts the isotactic sequences, acting as a structural defect, and the formation of γ form is enhanced. Moreover, crystallinity decreases and crystal structure is modified. Comonomer type and concentration determine the extent of these modifications, resulting in important changes in macroscopic properties. In particular, the excellent optical properties of the analyzed terpolymers make them very attractive for applications such as transparent film or packaging.     

Crystallization of the mesomorphic form and control of the molecular structure for tailoring the mechanical properties of isotactic polypropylene

The combination of the control of the concentration of stereodefects in isotactic polypropylene using metallocene catalysts and the crystallization via the mesophase is a strategy to tailor the mechanical properties. Stiff materials, flexible materials, and thermoplastic elastomers can be produced depending only on the concentration of rr stereodefects. Modulus, ductility, and strength can be modulated through the crystallization of α and γ forms or of the mesophase. Different morphologies are observed depending on the stereoregularity and conditions of crystallization. Crystals of the mesomorphic form always exhibit a nodular morphology, accounting for the similar good deformability of all quenched samples, whatever the concentration of stereodefects. The mesophase transforms by thermal treatments into the α form preserving the nodular morphology, with increase of strength while maintaining the ductility typical of the mesophase. Annealing of the mesophase permits a precise adjustment of crystallinity and size of nodular crystals offering additional options to modify the mechanical properties. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 677–699     

Structure and properties of composites of highly crystalline cellulose with polypropylene: Effects of polypropylene molecular weight

Composite of highly crystalline fibrous cellulose (CE) and polypropylene (PP) of different molecular weights () was prepared via melting-mixing, maleic anhydride grafted polypropylene (MAPP) was used as a compatibilizer. And the effects of molecular weight of PP on the properties of the composites were investigated. Through the studying of mechanical properties, dynamic mechanical properties, melting and crystallization behaviors, thermo-oxidative properties, water absorption behaviors, and the morphology of the composites, it was found that PP with higher molecular weight revealed stronger interfacial interaction with cellulose in the composites. Compared with the lower molecular weight, the composites derived from higher molecular weight of PP exhibited stronger tensile strength at the same cellulose content.     

Thermal properties and crystalline structure of syndiotactic polystyrene‐based miscible blends

This work examined the effect of the pre‐melting temperature (T_max) on the thermal properties and crystalline structure of four miscible syndiotactic polystyrene (sPS)‐based blends containing 80 wt % sPS. The counterparts for sPS included a high‐molecular‐weight atactic polystyrene [aPS(H)], a medium‐molecular‐weight atactic polystyrene [aPS(M)], a low‐molecular‐weight atactic polystyrene [aPS(L)], and a low‐molecular‐weight poly(styrene‐co‐α‐methyl styrene) [P(S‐co‐αMS)]. According to differential scanning calorimetry measurements, upon nonisothermal melt crystallization, the crystallization of sPS shifted to lower temperatures in the blends, and the shift followed this order of counterpart addition: P(S‐co‐αMS) > aPS(L) > aPS(M) > aPS(H). The change in T_max (from 285 to 315 °C) influenced the crystallization of sPS in the blends to different degrees, depending on the counterpart's molecular weight and cooling rate. The change in T_max also affected the complex melting behaviors of pure sPS and an sPS/aPS(H) blend, but it affected those of the other blends to a lesser extent. Microscopy investigations demonstrated that changing T_max slightly affected the blends' crystalline morphology, but it apparently altered that of pure sPS. Furthermore, the X‐ray diffraction results revealed that the α‐form sPS crystal content in the blends generally decreased with an increase in T_max, and it decreased with a decrease in the cooling rate as well. The blends showed a lower α‐form content than pure sPS; a counterpart of a lower molecular weight more effectively reduced the formation of α‐form crystals. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2798–2810, 2006