Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part I: Orientation mechanism and structural hierarchy

The structure which results from solid state extrusion using biaxial orientation is analyzed for oriented polypropylene. Structural changes on the spherulitic, lamellar, and macromolecular level during orientation are investigated using optical microscopy (OM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS). The results show that polypropylene spherulites undergo stepwise biaxial affine deformation and deform homogeneously into a disc-like morphology. During this spherulitic flattening process, lamellar rotation into the planar direction occurs prior to lamellar break-up at a baxial draw ratio of about 1.5. On the macromolecular level, the crystalline c-axis orients in the plane concurrently with the lamellar break-up, while the crystalline b*-axis gradually orients normal to the plane. Amorphous chains are also oriented preferentially in the plane of deformation. A hierarchical model is proposed to illustrate the nature of the orientation in the flattened spherulites.     

Microindentation technique as a tool for investigating the development of order in PET under uniaxial stress

The development of microhardness during annealing was used as a tool to follow the development of the liquid crystalline transient mesophase during the crystallization of uniaxially oriented polyethylene terephthalate (PET). 2D X-ray diffraction was used to characterize the different stages of the crystallization process. We were able to separate those stages by quenching into air. Microindentation hardness experiments were done in real time where samples were heated with their ends fixed on a specially developed stage and microhardness was measured simultaneously. The oriented samples examined exhibit a clear difference in behavior from isotropic samples that mainly lack the existence of such an ordered mesophase. The mesophase clearly has a reinforcement effect on the whole polymer matrix that leads to an increase in hardness value with annealing of the oriented PET films. Microindentation hardness is shown to be a versatile tool to detect the existence of the liquid crystalline transient mesophase. It is also efficient in comparing and explaining results obtained by wide angle X-ray scattering.     

热处理条件对聚丙烯流延基膜取向片晶结构及拉伸成孔性的影响

摘要：为了优化挤出流延单向拉伸法制备聚丙烯微孔膜的工艺,以温度、时间和基膜受到的张应力做为热处理条件变量,通过DSC、FTIR、SEM、电子材料试验机和透气率测试仪等方法表征流延基膜的取向片晶结构参数和微孔膜的孔结构,考察热处理条件对聚丙烯流延基膜取向片晶结构和拉伸成孔性的影响。研究表明,随热处理温度从120℃提高到145℃,基膜的取向片晶结构得到改善,硬弹性增强,拉伸成孔性能变好;随热处理时间延长,基膜的结晶度、片晶厚度及取向程度不断提高,取向片晶结构趋于完善,当热处理时间达到30min后,取向片晶结构的完善程度接近稳定;在热处理过程中对基膜施加适当张应力可以提高基膜的片晶取向程度和硬弹性,改善其拉伸成孔性。     

Synthesis of hybrid liquid crystalline block copolymers by combination of cationic or promoted cationic and free-radical polymerizations

Summary The synthesis of two new classes of liquid crystalline block copolymers (1–5) by a two-step combination of cationic and free-radical polymerizations is described. An azomacroinitiator was prepared first by cationic polymerization of tetrahydrofuran or promoted cationic polymerization of cyclohexene oxide, which was then used to initiate the free-radical polymerization of an acrylate monomer containing a variously substituted biphenyl mesogenic group. The semicrystalline and liquid crystalline blocks were essentially microphase-separated and gave rise to different thermotropic mesophases.     

The relationship between microstructure and toughness of biaxially oriented semicrystalline polyester films

The relationship between microstructure and toughness of biaxially stretched semicrystalline polyester films was investigated. Optically transparent films were prepared by simultaneous biaxial stretching of melt-cast sheets near the glass transition temperature. Copolyesters of polyethylene terephthalate (PET) with different compositions of two diols: ethylene glycol (EG) and cyclohexane dimethanol (CHDM), and stoichiometrically matched terephthalic acid were used to produce films with different degrees of crystallinity. In addition, the PET films with different crystalline morphologies were produced by constrained high temperature annealing of biaxially oriented films. The toughness, degree of crystallinity and crystalline morphology/molecular ordering were studied using mechanical testing, synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD) techniques, and differential scanning calorimetry (DSC). The results indicate that the toughness of a semicrystalline polymeric film is determined by the interconnectivity of the crystalline phase within the amorphous phase and is greatly influenced by the degree of crystallinity and the underlying crystalline morphology.     

Secondary crystallization and premelting endo‐ and exotherms in oriented polymers

Crystallization in polyamide 6 (nylon 6) fibers during annealing was studied in detail by following the changes that occurred in the neighborhood of crystalline relaxation temperatures, by using wide‐ and small‐angle X‐ray scattering and differential scanning calorimetry (DSC). Two distinct crystallization regimes were observed depending on whether annealing was carried out below or above onset of crystalline relaxation at ～190°C. In fibers annealed below 190°C, minor melting peaks were followed by exothermic transitions. These were attributed to ～1.5% (by weight) of microcrystals formed during annealing that melt and recrystallize during the DSC scan. These microcrystals are nucleated from unoriented amorphous chains between the lamellar stack within a fibril, and are shown to account for the observed increase in the crystalline orientation and decrease in permeability. Fibers annealed above 190°C did not show the exotherm and had significantly fewer microcrystals. The crystallization in this regime was attributed to the growth of existing lamellae, as evidenced by the increase in crystallite size, crystalline density, crystalline orientation, lamellar spacing, and lamellar intensity. The changes at annealing temperatures >190°C are accompanied by increased dyeability, indicative of more open amorphous regions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 447–454, 2006     

Solid-state complexes of poly(N-vinyl-pyrrolidone) and dodecyl benzene sulfonic acid: Self-assembled structure and thermal properties

Solid-state complexes of flexible polymers with long-tail surfactants (surf) exhibit mesomorphic phase and microphase-separated lamellar morphology consisting of alternating polymer and surf layers. The alkyl tails in the surf layers are highly stretched and closely packed due to the excluded volume effect. In this study, the self-assembled structure of a new polymer(surf) complex system based on complexation between poly(N-vinyl pyrrolidone) (PVP) and dodecyl benzene sulfonic acid (DBSA) was investigated by optical microscopy and small-angle X-ray scattering (SAXS). Both mesomorphic phases and microphase-separated lamellar morphology were observed in PVP(DBSA) complexes. The thicknesses of polymer and surf layers were determined from the one-dimensional correlation function. Both types of thickness are dependent on complex composition. The interplay between chain conformational entropy and interaction energies (repulsive and attractive) is suggested to control the self-organized structure in the complexes. Complexation with DBSA reduced the thermal stability of PVP, which was in contrast with poly(ethyleneimine)-DBSA complexes reported previously.     

退火过程中聚己内酯片晶结构的演变

利用一维相关函数研究了退火过程对聚己内酯(PCL)片晶形态与结构的影响。根据广角X射线散射(WAXS)数据计算了PCL的质量结晶度Wc,进而求得其体积结晶度Vc。对小角X射线散射(SAXS)谱线进行一维相关函数分析后得到了PCL的片晶长周期(LP)和无定形层厚度(La),而后求得过渡层厚度(E)。结果表明:PCL在较低温度(30℃)充分等温结晶后,延长等温结晶时间,其相对结晶度基本不变,晶体发生明显的等温增厚现象;晶体在升温过程中,片晶首先发生退火增厚,部分无定形区与过渡区转变为晶区的一部分,晶区厚度(Lc)增大,La与E略有减小;随着温度的进一步提高,较薄的片晶首先熔融,从晶区中溢出后进入无定形区或过渡区,故Lc随之降低,La与E逐渐增大;最后所有片晶全部转变为无定形状态。     

Structural characterization of Celgard® microporous membrane precursors: Melt-extruded polyethylene films

“Row nucleated lamellar” structures are formed when highly crystalline polymers are melt-extruded and recrystallized under high stress. Polyethylene (PE) and polypropylene (PP) films with row lamellar structures have been utilized to produce microporous membranes. Birefringence measurements of melt-extruded PE films show that improved film orientation can be achieved by annealing, extruding at higher speed, and using higher molecular weight polymers. Images from scanning tunneling, atomic force, and field emission scanning electron microscopy (STM, AFM, and FESEM) clearly show the lamellar structures in the melt-extruded PE and PP films. Microscopy results also show that surface lamellar textures are more pronounced with thicker lamellae and are better aligned along the extrusion direction after annealing. X-ray diffraction results show that the increase in film orientation can be attributed to increased lamellar perfection and orientation during annealing and also to better crystallite alignment along the machine direction with higher extrusion speed or with higher molecular weight. High-resolution capabilities of STM, AFM, and FESEM prove to be very effective tools in elucidating lamellar structures in polymeric membrane precursors and can be used as an aid in establishing structure–process–property relationships in making microporous membranes. © 1994 John Wiley & Sons, Inc.     

Effects of sodium benzoate on distributions of lamellar structures of sheared isotactic polypropylene: A study using small angle synchrotron X-ray scattering

Small-angle X-ray scattering (SAXS) studies were carried out to investigate effects of sodium benzoate (SB) on the shear-induced distributions of lamellar structures through the depth direction of injection-moulded isotactic polypropylene (iPP) plates, using the micro beam available from a synchrotron to gain good spatial resolution. It was found that in the presence of SB, the shish-kebab structure emerges over a broader range to include regions in which a random arrangement of crystalline lamellae is found without SB. The oriented crystalline lamellae are even found in positions that are very close to the core center. The SB-induced shish-kebab structure under low levels of shear flow is attributed to lower free energy barriers for the coil-to-stretch transition and shorter times to start the crystallization. The effect of SB on the lamellar dimensions is significant only in the core region, and only when the concentration of SB is sufficiently high. A down-U distribution of the long spacing is found, which is mainly ascribed to the variation of thickness of crystalline lamellae with the depth. The results show that a higher level of lamellar orientation is not necessarily accompanied by a higher degree of linear crystallinity.     

Synthesis and characterization of block copolythiophene with hexyl and triethylene glycol side chains

A well-defined diblock copolythiophene, poly(3-hexylthiophene)-b-poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3HT-b-P3TEGT) was successfully synthesized by a Grignard metathesis (GRIM) polymerization. The structure of the block copolymer was confirmed by size exclusion chromatography (SEC), ¹H NMR spectroscopy, differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) spectroscopy and fluorescence spectroscopy. The self-assembled structure of the block copolythiophene was investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and synchrotron grazing incidence X-ray scattering (GIXS). These analyses found that the block components in films form vertically oriented lamellar structure via phase separation. Furthermore, both the phases were found to consist of molecular multi-layers respectively, in which the layers stack in the out-of-plane of the film. The P3HT phase exhibited crystalline, which is originated from the π−π stacked thiophene backbones. In contrast, the poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3TEGT) phase revealed amorphous. Overall, the amphiphilic nature of P3HT-b-P3TEGT successfully demonstrated to lead to well-defined self-assembly structure in films.     

Polyether based side-chain liquid crystalline polymers: Anionic polymerization and phase structures

Side-chain liquid crystalline (SCLC) polyethers with different spacers are successfully synthesized via anionic polymerization for the first time. The molecular weights of the polymers can range up to ∼53 kg/mol with M_w/M_n <1.1. The precise chemical structures including end groups have been characterized by employing Matrix-Assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF) and the polymerization condition free of side reactions is further discussed. The phase behavior and structures of the polyethers have been investigated by combining various techniques including differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS) and polarized light microscopy (PLM). Two phase transformations are generally observed for each type of SCLC polyethers. Based on 2-dimensional (2D) X-ray patterns of oriented fiber and mechanically sheared samples, the low ordered liquid crystalline phase at high temperature is identified as a SmA phase for both SCLC polyethers. It is found that the polyether with a longer spacer exhibits a complex phase which is a mixture of a HexB phase and a frustrated HexB phase in analogy to smectic antiphase, whereas no such effect occurs for the polymer with a shorter spacer. Only HexB phase is observed for the low temperature phase of this polymer. The difference in phase structures is also revealed in the texture changes of PLM.     

The development of microstructure in oriented polyethylene terephthalate (PET) during annealing

This work concerns the changes in structural order, which occur when amorphous polyethylene terephthalate (PET) is crystallised by drawing and then subsequent annealing. Real time wide angle X-ray fibre diffraction is used to obtain information about the microstructural changes taking place during drawing and subsequent annealing. The diffraction patterns obtained proved the existence of a liquid crystalline transient mesophase prior to crystallisation. The development of both the mesophase and the crystalline structure are also studied using small angle X-ray scattering during annealing of uniaxially drawn samples held at constant strain. These experiments proved the absence of any microstructure associated with the mesophase and that significant microstructural changes take place only when crystallisation starts to occur.     

Molecular composites via in situ polymerization: Poly(phenylene terephthalamide)–nylon 3

All-polyamide molecular level composites composed of rigid rod and flexible coil polymers were prepared using an in situ polymerization process in which the anion of the rigid rod poly-(phenylene terephthalamide) (PPTA) was used as the initiator for the anionic polymerization of acrvlamide to form the nylon 3 matrix. The rigid aramid component then serves as the reinforcing agent. This polymerization resulted in both graft and homo-nylon 3 formation. Composite films prepared using in situ processing showed greatly improved strength and modulus over unmodified nylon 3 with no loss of flexibility. The composites showed aggregation and phase separation of PPTA fibrils at PPTA weight fractions of > 30% as indicated by wide angle X-ray scattering and electron microscopic analysis. The structure of the PPTA formed is that produced from swelled liquid crystalline solutions, indicating that the in situ process involves polymerization in the liquid crystalline state.     

Oriented polymer networks obtained by photopolymerization of liquid-crystalline monomers

In this paper a new class of oriented polymers is introduced: highly crosslinked polymer networks obtained by photopolymerization of oriented liquid crystalline (LC) monomers. Orientation of these monomers can be induced in the same manner as usual for conventional LCs applied in LCDs: with the aid of an electric or a magnetic field, or using surface orientation. Photopolymerization offers the advantage that a temperature can be chosen right within the LC temperature region. The resulting polymer networks show a high degree of ordering which is somewhat dependent on the polymerization temperature, as demonstrated by the amount of birefringence. This ordering is maintained over a wide temperature region, contrary to that in LC side-chain polymers. Apart from their optical properties, uniaxially ordered polymer networks show anisotropy also in other physical properties, such as the thermal expansion coefficient and modulus.     

Perpendicular oriented cylinders via directional coalescence of spheres embedded in block copolymer films induced by solvent annealing

Polystyrene-b-poly(methyl acrylate) (PS-b-PMA) block copolymer with PS volume fraction of 25.2 vol% was synthesized by atom transfer radical polymerization. Non-pretreated silicon wafers were used as the substrates to prepare perpendicular oriented PS cylinders in PMA matrix via solvent annealing which could induce the transformation of spheres to vertically oriented and hexagonally packed cylinders. The spherical microdomains were formed after the evaporation of solvents from the solutions of the block copolymer in selective solvents mixed from methanol, acetone and dichloromethane. The thickness of films could be as thick as 1000 nm, which were much thicker than usual cases and the cylinders came from the directional coalescence of the spheres, thus any pre-treatments of the substrates were not required for perpendicular orientation. The structures were characterized by small angle X-ray scattering (SAXS), transmission electron microscope (TEM), atom force microscopy (AFM) and grazing incidence small angle X-ray scattering (GISAXS).     

Phase behavior in liquid crystallization for diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components

Phase behavior in liquid crystallization was studied for a series of liquid crystalline (LC) diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components, poly(n-butyl acrylate) (PBA) and poly[11-(4′-cyanophenyl-4″-phenoxy)undecyl acrylate] (PLC), respectively, using a time-resolved small-angle X-ray scattering (SAXS) techniques, DSC and polarized optical microscopy (POM). The block copolymers used had three kinds of copolymer compositions, 44, 20 and 15 wt% of PLC compositions (BLC44, BLC20 and BLC15, respectively). BLC44 showed a smectic liquid crystalline structure. In the process of liquid crystallization for BLC44, the SAXS peak due to the microphase separation structure existing before liquid crystallization was changed continuously to be at a smaller angular side, and at almost the same time, a new peak appeared at a further smaller angular side and developed. The former peak disappeared with the development of liquid crystallization. The behavior of these SAXS peaks suggests that the microphase separation structure was changed discretely at the transition from isotropic to smectic and that two phases coexist in the early stage of the liquid crystallization. The coexistence of two peaks in the early stage of the liquid crystallization corresponded to the POM observation. In the isotropization process, coexistence of two phases was not observed. For BLC20 exhibiting a cylindrical structure in both isotropic and liquid crystalline states, the liquid crystalline structure was not smectic but probably nematic, and the spacing was changed continuously in liquid crystallization. No liquid crystallization was observed in SAXS, POM and DSC for BLC15. The orientation of smectic layers within lamellar domains was investigated using 2D-SAXS images. The smectic layer was aligned perpendicularly to the lamellar interface.     

Impact of different die draw ratio on crystalline and oriented properties of polypropylene cast films and annealed films

Four kinds of polypropylene (PP) cast films with different die draw ratios (DDR) were prepared. The impact of different DDR on the crystalline and oriented properties of PP cast films and annealed films was explored herein. Wide angle X-ray diffraction (WAXD) and fourier transform infrared (FTIR) methods were adopted to examine the orientation degree of crystalline and amorphous phases. Long period distance (L_p) of the crystalline structure was tested by small angle X-ray scattering (SAXS). Crystallization was determined by differential scanning calorimeter (DSC). The oriented and crystalline behaviors of the samples were carried out by the elastic recovery (ER) testing. Then, samples after being annealed were examined by the same methods. The influence of annealing process on the films’ structures and properties was explored. Besides, the final stretched microporous membranes manufactured via stretching the annealed films along machine direction were examined by scanning electronic microscope (SEM). No matter for cast films or for annealed films, it is found that the films’ orientation degree of crystalline and amorphous phases, as well as L_p and crystallinity are larger at higher DDR and relatively lower at lower DDR. When the DDR is overly high (DDR?=?170), both the oriented and crystalline properties will decline. Elastic recovery testing indicates that a film with better orientation of the crystalline and the amorphous phases as well as with higher crystallinity can be obtained at an appropriate DDR. SEM images show that stretched membranes with better microporous structure can be obtained when the precursor film is prepared at a proper DDR.     

Solvent annealing thin films of poly(isoprene-b-lactide)

The use of solvent annealing to control the microdomain orientation and long-range ordering in poly(isoprene-b-d,l-lactide) thin films was investigated using scanning force microscopy and grazing incidence small angle X-ray scattering (GISAXS). Benzene and chloroform were used as annealing solvents. Both were found to improve the long-range order in the films. Additionally, at high concentrations of chloroform in the film, a perpendicular orientation of cylinders was observed where the solvent was able to mediate interfacial interactions sufficiently to prevent a preferential segregation of one of the blocks to the surface. In situ GISAXS measurements made during solvent swelling and evaporation allowed an examination of the film morphology over a wide range of solvent concentrations providing a efficient route to optimize conditions for morphology control by solvent annealing.     

Processing–structure–property relationships in poly(ethylene terephthalate) blown film

An investigation was undertaken to establish processing–structure–property relationships in poly(ethylene terephthalate) (PET) blown film. For the study, a commercial grade of PET was used to fabricate the film specimens by means of a tubular film blowing process. In this process, the stretch temperature was accurately controlled by an oven. The annealing treatment of the oriented specimens involved clamping the sample in an aluminum frame and then putting the clamped sample in an oven, controlled at a temperature between the glass transition temperature (70°C) and the melting point (255°C) of PET, for a specified annealing period. The structure of the blown film samples was characterized by density, bulk birefringence, flat plate wide-angle X-ray scattering, and pole figure analysis. The processing variables, namely, takeup ratio, blowup ratio, and stretch temperature were found to significantly affect the bulk birefringence and density of the oriented PET blown film samples. It was found that both the bulk birefringence and density of the specimens increased upon annealing at an elevated temperature. Both the crystalline and amorphous orientation functions were calculated from the data of bulk birefringence, density, and the pole figure analysis. Compared to the amorphous orientation functions, the crystalline orientation functions were found to be relatively insensitive to the processing variables. It was concluded that equibiaxially oriented PET films can be produced via a tubular film blowing process by judiciously controlling the processing and annealing conditions. It has also been observed that the tensile stress-at-break of equibiaxially oriented PET film increases with decreasing stretch temperature and increasing annealing temperature.