Influence of scale effect and injection speed on morphology and structure of the microinjection molded isotactic polypropylene parts

The morphology and microstructure as well as their forming mechanism of the parts in microinjection molding process are critical. In this work, the coupling effect of scale factor and injection speed on the morphology of the microparts was systematically investigated. Neat isotactic polypropylene parts with thicknesses of 1 mm, 200 μm, and 100 μm were molded at different injection speeds. Polarized light microscope and wide‐angle X‐ray diffraction were used to inspect the microstructures along the sample thickness. In this way, three kinds of typical morphology were observed in the parts, including typical skin‐core structure for the parts with the thickness of 1 mm, noncore shear layer structure for the parts with the thickness of 200 μm, and special skin‐core structure with large fraction of columnar crystal for the parts with the thickness of 100 μm. Most interestingly, it was intuitively and straightforward found that the wall slip occurs when the injection speed exceeds a certain value. Specifically, opposite morphological change trend can be obtained when the parts were molded at different levels of injection speeds. Based on these experimental observations, the formation mechanism was proposed to interpret the morphological evolution. Our work provides a new insight for better understanding the morphology evolution mechanism for microinjection molding parts.     

On the origin of the “core‐free” morphology in microinjection‐molded HDPE

This study investigates the morphology of a high‐density polyethylene processed with microinjection molding. Previous work pointed out that a “core‐free” morphology exists for a micropart (150‐μm thick), contrasting with the well‐known “skin‐core” morphology of a conventional part (1.5‐mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide‐angle X‐ray diffraction measurements using a microfocused beam highlight that highly oriented shish–kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow‐induced crystallization and thermal crystallization during processing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1470–1478, 2011     

Stratified morphology of a polypropylene/elastomer blend following channel flow

A thermoplastic olefin blend consisting of isotactic polypropylene (PP) and an ethylene‐butene copolymer (EBR) impact modifier (25 wt % EBR) was subjected to a short, high‐shear pulse within the flow channel of a pressure‐driven microextruder following low‐shear channel filling from a reservoir of the melt. The resulting morphology was examined by laser scanning confocal fluorescence microscopy (LSCFM), with contrast provided by a fluorescent tracer in the EBR minor phase. Shear experiments were performed under isothermal conditions with a known wall shear stress for a specified duration, providing a well‐defined thermal and flow history. Low‐shear channel filling produces small droplets across the central region of the channel and large droplets, consistent with steady‐state shear, in the regions near the channel walls. After cooling the molten blend to a crystallization temperature of 153 °C, a brief interval (5 s ～ 1/2000 of the quiescent crystallization time) of high shear (wall shear stress: 0.1 MPa) induces rapid, highly oriented crystallization and a stratified morphology. Ex situ LSCFM reveals a “skin” at the channel walls (～70 μm) in which greatly elongated fiberlike droplets, oriented along the flow direction, are embedded in highly oriented crystalline PP. Further from the walls but directly beside the skin layers are surprising zones in which EBR domains show no deformation or orientation. Several zones of intermediate deformation and orientation at an angle to the flow direction are located closer to the center of the channel. At the center of the channel, EBR droplets are spherical, as expected for channel flow. The various strata are explained by the interplay of droplet deformation, breakup, and coalescence with the shear‐induced crystallization kinetics of the matrix. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2842–2859, 2002     

Morphology evolution and crystalline structure of controlled‐rheology polypropylene in micro‐injection molding

In this paper, the microstructural evolution of controlled‐rheology polypropylene (CRPP) with different melt viscoelasticities was investigated by polarized optical microscopy, scanning electronic microscopy, differential scanning calorimeter, and wide‐angle X‐ray diffraction. It is found that a typical “skin‐core” structure formed in CRPP microparts and the thickness of oriented layer of CRPP microparts decreases notably with the addition of peroxide. The thickness of oriented layer and the distribution of different layers strongly depend on the melt flow properties and the corresponding relaxation time (λ). Furthermore, the mechanisms of the suppressed formation of oriented layers during the micro‐injection molding process are discussed mainly from the viewpoint of rheology and thermodynamics. It is revealed that the shear‐induced orientation is one of the key factors for the formation of oriented molecular structure (row nuclei). The final thickness of the oriented layer is the result of the competition between the orientation behavior and the disorientation behavior. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of processing parameters on the mechanical properties of polypropylene random copolymer

The mechanical properties of polypropylene random copolymer (PP-R) with different processing parameters were studied. Special attention is devoted to the investigation of the influence of masterbatch addition on the variation in the mechanical properties of injection moulded PP-R. Tensile, instrumented Charpy impact, Shore D hardness, differential scanning calorimeter (DSC) and Vicat softening temperature (VST) tests were conducted on the test samples containing different colour masterbatches varying from 0.5 to 10 wt%. The observed changes in the mechanical behaviour are explained by the type and level of masterbatch content. The natural UV weathering performance of the PP-R material was studied from the masterbatch type point of view. The effect of processing parameters on material performance was studied on samples which were directly obtained from extruded pipes and on injection moulded samples.

Finally, the effects of storage time on the polymer properties were investigated.     

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.     

Morphology and mechanical behavior of isotactic polypropylene with different stereo‐defect distribution in injection molding

Large amount of work has been published on the isotacticity–properties relationship of isotactic polypropylene (iPP). However, the stereo‐defect distribution dependence of morphology and mechanical properties of iPP injection molding samples is still not clear. In this study, two different isotactic polypropylene (iPP) resins (PP‐A and PP‐B) with similar average isotacticity but different stereo‐defect distribution were selected to investigate the morphology evolution and mechanical properties (tensile and notching) of their injection molding samples using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), 2D wide angle X‐ray diffraction (2D‐WAXD), and scanning electron microscope (SEM). The results of DMA showed that the molecular movement ability of PP‐A (with less uniform distribution of stereo‐defect) was stronger than that of PP‐B, meanwhile the analysis of DSC and SEM suggested that after injection molding, smaller spherullites, and crystals with higher perfection had formed in the specimens of PP‐A. The resulting of tensile properties of PP‐A were found to be better than that of PP‐B. The results of morphology evolution by SEM observation and 2D‐WAXD showed that PP‐A is more likely to occur interspherulite deformation and can disperse the tensile stress more efficiently, and therefore, its crystal structure can withstand a greater force when tensile stress is applied. On the other hand, PP‐B has larger spherulites and boundaries, and low perfection of lamellaes, and the intraspherulte deformation tend to take place. It is easier for the crystal of PP‐B to be broken up and reoriented along the tensile direction. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of stereo-defect distribution on the crystalline morphology and tensile behavior of isotactic polypropylene prepared by compression molding process

In this study, the aggregation morphology, tensile behavior, and morphology evolution during the tensile test of two isotactic polypropylene (iPP) samples with similar molecular weight and average isotacticity but different uniformities of stereo-defect distribution are investigated by differential scanning calorimetry (DSC), two-dimensional wide angle X-ray diffraction (2D-WAXD), and scanning electronic microscopy (SEM). The results revealed that the uniformity of stereo-defect distribution of iPP determines the crystalline structure and aggregation morphology, and further influences the tensile behavior and morphology evolution during the tensile test. For PP-A with less uniform stereo-defect distribution, its ability of crystallization is stronger compared with PP-B, resulting in smaller spherulite sizes, higher melting point and degree of crystallinity, and narrower distribution of lamellar thickness of the compression molding specimens. During the tensile test, mainly the inter-spherulite deformation takes place at the early stage for deformation, which further results in drastic deformation of lamellar and high degree of reorientation at the strain increases, exhibiting higher yield strength and elastic modulus, and lower elongation at break compared with PP-B; for PP-B with more uniform stereo-defect distribution, larger spherulite sizes, lower melting point and degree of crystallinity in its compression molding sample are observed. During the tensile test, intra-spherulite deformation mainly takes place, which can disperse the tensile stress more uniformly. As the strain increases, lower degree of crystalline destruction and reorientation of the crystallites take place. The yield strength and elastic modulus of PP-B is lower than PP-A, and its elongation at break is higher.     

Effect of long chain branching on the rheological behavior,crystallization and mechanical properties of polypropylene random copolymer

Long chain branched polypropylene random copolymers (LCB-PPRs) were prepared via reactive extrusion with the addition of dicumyl peroxide (DCP) and various amounts of 1,6-hexanediol diacrylate (HDDA) into PPR. Fourier transform infrared spectrometer (FTIR) was applied to confirm the existence of branching and investigate the grafting degree for the modified PPRs. Melt flow index (MFI) and oscillatory shear rheological properties including complex viscosity, storage modulus, loss tangent and the Cole-Cole plots were studied to differentiate the LCB-PPRs from linear PPR. Differential scanning calorimetry (DSC) and polarized light microscopy (PLM) were used to study the melting and crystallization behavior and the spherulite morphology, respectively. Qualitative and quantitative analyses of rheological curves demonstrated the existence of LCB. The effect of the LCB on crystalline morphology, crystallization behavior and molecular mobility, and, thereby, the mechanical properties were studied and analyzed. Due to the entanglements between molecular chains and the nucleating effect of LCB, LCB-PPRs showed higher crystallization temperature and crystallinity, higher crystallization rate, more uniformly dispersed and much smaller crystallite compared with virgin PPR, thus giving rise to significantly improve impact strength. Moreover, the LCB-PPRs exhibited the improved yield strength. The mobility of the molecular chain segments, as demonstrated by dynamic mechanical analysis (DMA), was improved for the modified PPRs, which also contributed to the improvement of their mechanical properties.     

Phase behavior and crystallization analysis in binary crystalline blends of syndiotactic polypropylene and ethylene—Propylene random copolymer

The effects of liquid–liquid (L–L) phase separation on the crystallization behavior of binary syndiotactic polypropylene (sPP) and ethylene–propylene random copolymer (PEP) mixtures are examined by phase‐contrast microscopy (PCM), differential scanning calorimetry (DSC), and cloud point measurements. The PCM experiments reveal that blends of sPP and PEP exhibit a lower critical solution temperature behavior in the melt. The L–L phase diagram, constructed in terms of temperature (T) and composition by cloud point measurements, follows the prediction of the Flory–Huggins theory with the interaction parameter between sPP and PEP [χ(T) = 0.01153 ? 4.5738/T (K)]. When the blends are melted within the two liquid‐phase (α and β) regions, because of the fact that each phase domain reaches the equilibrium concentration ? and ? as well as the phase volume fraction ν^α and ν^β, the crystallinity of each component obeys the equation X_C,I = ν^α X + ν^β X, I = PEP, sPP. Also, the equilibrium melting temperatures of both components remain constants, slightly lower than those of neat polymers. For the sPP/PEP blends crystallized from one homogeneous phase in the melt, we observe that the crystallizability of the major component is not greatly affected upon blending. However, the crystallization behavior of the minority component in the presence of the major component is strongly dependent on the crystallization temperature (T_c). When T_c is high, because the decreasing degree of the minority mobility is much greater than the increasing degree of the formed nuclei, the crystallizability of the minor component is depressed significantly. On the other hand, the promotion of the minority crystallizability in the intermediate regime of T_c is mainly because of the large increase of the heterogeneous nuclei upon blending with a major component. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2995–3005, 2004     

Effect of in situ poly(ethylene terephthalate) (PET) microfibrils on the morphological structure and crystallization behavior of isotactic polypropylene (iPP) under an intensive shear rate

The morphological structure and crystallization behavior of in situ poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) microparts prepared through micro‐injection molding are investigated using a polarized light microscope, differential scanning calorimeter, scanning electron microscope, and two‐dimensional wide‐angle X‐ray. Results indicate that both the shear effect and addition of PET fibers greatly influence the morphologies of the iPP matrix. Typical “skin‐core” and oriented crystalline structures (shish‐kebab) may simultaneously be observed in neat iPP and iPP/PET microparts. The presence of PET phases reveals significant nucleation ability for iPP crystallization. High concentrations of PET phases, especially long PET fibers, correspond to rapid crystallization of the iPP matrix. The occurrence of PET microfibrils decreases the content and size of β‐crystals; by contrast, the orientation degree of β‐crystals increases with increasing PET content in the microparts. This result suggests that the existence of the microfibrillar network can retain the ordered clusters and promote the development of oriented crystalline structures to some extent. Copyright © 2015 John Wiley & Sons, Ltd.     

Novel fluorosilicone thermoplastic vulcanizates prepared via core‐shell dynamic vulcanization: Effect of fluororubber/silicone rubber ratio on morphology,crystallization behavior,and mechanical properties

Recently, we have reported a novel core‐shell dynamic vulcanization method to prepare poly(vinylidene fluoride) (PVDF)/fluororubber (FKM)/silicone rubber (SR) thermoplastic vulcanizates (TPVs) with cross‐linked rubber core‐shell particles. However, the shell thickness on the properties has not been studied in detail. Herein, these PVDF‐based TPVs different FKM‐shell thickness were prepared by changing FKM/SR ratios. The effect of FKM‐shell/SR‐core ratio on morphology, crystallization, and mechanical properties of the ternary TPVs was studied. The results showed that the FKM shell had more positive effect on interfacial‐induced crystallization behavior than the SR core due to its better compatibility with PVDF. When the FKM/SR ratio was <1, FKM was not enough to encapsulate SR completely, which resulted in the formation of imperfect core‐shell structure. However, when the FKM/SR ratio was >1, perfect core‐shell structure was formed. Therefore, the mechanical properties improved with increasing FKM content; especially, a remarkable improvement was observed when FKM/SR ratio was >1. This study could provide more information for the design of TPVs with core‐shell structure.     

Effect of strain‐induced molecular ordering on mechanical performance and barrier properties of polylactide nanocomposites

Plastic deformation of polylactide has been known as a self‐reinforcement alternative to improve mechanical and barrier properties. In this study, the structural evolution was investigated during a hot‐drawing process, at different initial strain rates and temperatures above T_g of polylactide. The drawing process at T_g +10 °C, led to the formation of an intermediate molecular ordering, between the crystalline and amorphous phases. A lower fraction of this mesomorphic phase was found to develop with the addition of nanoparticles. An increase in the stretching temperature to T_g +30 °C, caused an improvement of the crystallization kinetics, compared to that of thermally activated crystallization. A strain hardening behavior was observed in the presence of mesophase during a stretching process of the hot‐drawn films at room temperature. Permeability was discerned to its basic components, diffusivity, and solubility coefficients. The matrix degradation influenced the permeability components. The diffusivity decreases in the presence of the impermeable matters. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1865–1876     

Effect of random copolymerization on growth transition and morphology change in polypropylene

The growth rate of an ethylene-propylene random copolymer with a 2.8% w/w ethylene content was studied in a similar manner to polypropylene. A growth regime transition associated with a birefringence change was observed at 130C, while the same phenomena appeared at 138C in isotactic polypropylene. In both polymers positive birefringence corresponds to Regime III, whereas negative birefringence of spherulites is associated with Regime II. The birefringence change is attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae at low temperature (Regime III) and preferentially radiating lamellae at higher temperature (Regime II). We confirm that such a morphological change can be interpreted using the concept of non-adjacent re-entry introduced in Hoffman's kinetic theory. Thus, quadritic morphology seems to have a partly kinetic origin. The shift of the transition temperature in the copolymer is due to the rejection of ethylene segments at the surface of crystalline lamellae of polypropylene.     

Effects of toughening propylene/ethylene graft copolymer on the crystallization behavior and mechanical properties of polypropylene random-copolymerized with a small amount of ethylene

Linear low-density polyethylene (LLDPE) was grafted onto the backbone chains of isotactic polypropylene (iPP) during reactive melt-extrusion to produce a novel toughening modifier, propylene/ethylene graft copolymer (PEGC), to improve the properties of iPP random(-copolymerized with a small amount of ethylene) (PPR). The crystallization behavior as well as the non-isothermal crystallization kinetics of the PEGC modified PPRs were investigated via differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). The fractured surface topography was characterized using scanning electron microscopy (SEM), and the mechanical properties through notched impact and tensile testing as well as dynamic mechanical thermal analysis (DMTA). The results show that, at a PEGC content of 8 wt%, notched impact strength of the PEGC modified PPR increased by 30.6% at low temperature (−25 °C). As regards crystalline morphology, the PEGC, as an effective heterogeneous nucleating agent, fostered nucleation of the PPR to elevate its crystallization temperature as well as rate of crystallization, thus refining the PPR (iPP) spherulites and improving the interfacial structure between iPP spherulites. The Jeziorny approach was unsatisfactory for simulation of the non-isothermal crystallization process of the PEGC modified PPRs; however, the Mo method described consistently the crystallization kinetics over the entire isothermal process.     

Stretching‐induced phase transition of the butene‐1/ethylene random copolymer: Orientation and kinetics

The stretching‐induced phase transition from tetragonal Form II to hexagonal Form I and the evolution of corresponding crystallite orientation were studied for the butene‐1/ethylene random copolymer with 1.5 mol % ethylene by using a combination of tensile test and in situ wide‐angle X‐ray diffraction. Three orientation pathways were distinguished for II‐I phase transition, including phase transition accomplishing within off‐axis oriented crystallites (Orientation Pathway 1), phase transition with simultaneous formation of highly oriented crystallites (Orientation Pathway 2), and phase transition occurring within the highly oriented crystallites already formed (Orientation Pathway 3). The kinetics of II‐I transition was correlated with the macroscopic mechanical response, which exhibits a strong dependence on orientation. In Orientation Pathway 1, the triggering of phase transition corresponds to the mechanical yielding. More interestingly, the kinetics of transition exhibits the identical dependence on stress. However, in Orientation Pathways 2 and 3, appearance of the highly oriented crystallites substantially alters transition kinetics, which is tentatively associated with the stress bearing by interstack tie chains. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 116–126     

Solvent effects on the morphology and electrical conductivity of a D‐A random copolymer with low band gap

D‐A copolymer systems have unique characteristics, such as low band gap and ambipolar nature, which are important to design electronic polymer devices. In this contribution, we synthesized and characterized a D‐A random copolymer containing bis‐3‐hexylthiophene‐benzothiadiazole as acceptor unit and 9,9‐dioctylfluorene as donor unit. We show that the polymeric film morphology depends of the Hansen solubility parameters, evaporation rate, and surface tension of the solvent. Chloroform, toluene, and 1,2,4‐trichlorobenzene (TCB) promote the formation of self‐assembled structures due to breath‐figure mechanism. In contrast, THF causes aggregation and phase separation that affect negatively the electrical conductivity of the copolymer film. Among the solvents analyzed, TCB is the one with the highest molecular interaction with the copolymer synthetized in this work. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1035–1044     

Crystalline structure and morphology of microphases in compatible mixtures of poly(tetrahydrofuran‐methyl methacrylate) diblock copolymer and polytetrahydrofuran

The crystalline structure and morphology of compatible mixtures of poly(tetrahydrofuran‐methyl methacrylate) diblock copolymers (PTHF‐b‐PMMA) with a polytetrahydrofuran homopolymer (PTHF) were studied with synchrotron X‐rays. Wide‐angle diffraction was used to study the crystalline structures in a confined lamellar region with a PTHF thickness ranging from 12.2 to 19.5 nm, and in a PTHF matrix with an interface distance between the PMMA cylinders ranging from 17 to 22 nm. As the above thickness values are around the long period (ca. 17 nm) of PTHF homopolymer under the crystallization condition used, the crystalline structure has been found to be very sensitive to the average thickness of the PTHF phase. The changes in the diffraction patterns with changing PTHF homopolymer content suggested a chain folding model in confined PTHF lamellae with the PTHF fiber axes being perpendicular to the thick PTHF lamella. In the case of hexagonally packed cylindrical PMMA microdomains with an interface distance ranging from 12 to 16 nm, the effects of PMMA cylinders on the crystallization morphology of PTHF in the PTHF matrix, and the effects of the PTHF crystallization on the hexagonally packed structure of PMMA cylinders were also studied. It is shown that only when the interdistance of two neighboring PMMA cylinders is comparable with the long period of the pure PTHF homopolymer, ordered PTHF stacks can be formed in the PTHF matrix. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 779–792, 1999