Broadline n.m.r. studies of ultra-high modulus polyethylenes

Broadline nuclear magnetic resonance (n.m.r.) measurements have been made for a range of isotropic and drawn high density polyethylene materials with different molecular weight distributions and three well defined thermal treatments during preparation. For the drawn materials the dependence of the n.m.r. spectra on the orientation of the static magnetic field with respect to the draw direction has been studied with respect to both line shape and second moment variation. The spectra in general show three component lines with distinctly different line widths. The broad component shows a high degree of molecular orientation in the drawn samples and can be satisfactorily assigned to the crystalline regions of the polymer. It is proposed that the intermediate component corresponds to high molecular weight molecules which interconnect the crystalline regions and are sufficiently constrained to allow motion about the chain axis only. This component shows an increasing degree of molecular orientation and decreases in intensity with increasing draw ratio. Finally, there is an isotropic narrow component, which is attributed to the mobile fraction. From its intensity in samples of different molecular weight this fraction can be associated with low molecular weight material and the ends of molecules which are rejected from the crystalline regions or the rigid fraction in a drawn sample.     

Effect of heat‐sealing temperature on the properties of OPP/CPP heat seals. Part II. Crystallinity and thermomechanical properties

The crystalline structure of the heat‐sealed part of oriented polypropylene (OPP) and cast polypropylene (CPP) films was investigated by differential scanning calorimetry, Fourier transform infrared spectroscopy, and thermal mechanical analysis (TMA). The relationship between the crystalline structure and the mechanical properties was analyzed. It was found that the high total crystallinity of both OPP and CPP gave rise to good mechanical properties and that the orientation of the crystalline structure in the OPP film also was an important factor. The optimum condition for heat sealing was the temperature at which total crystallinity was highest while still retaining the crystalline orientation of OPP. The assessment of crystalline orientation by TMA is an innovation initiated by the authors. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 513–519, 2006     

Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films

The development of crystalline structure and orientation during uniaxial stretching of cast amorphous linear and branched lactic acid films were investigated in the rubbery temperature ranges that spans between glass transition temperature and cold crystallization temperature. This material exhibited almost ideal stress‐strain behavior in the temperature range 65–80°C. Because of its strain crystallizability, films with uniform thickness can be obtained at high deformation levels as a result of self‐leveling. Branching was found to retard this self‐leveling through its slightly detrimental effect on the strain hardening. Upon stretching the material undergoes rapid orientation in the amorphous state and beyond a critical level very sharp and highly oriented β crystalline form chains with ?3/1 helix. If the temperature is at or below Tg, with additional stretching, the films were found to revert to a highly oriented amorphous state through the destruction of the crystalline domains. At higher temperatures, further stretching results in continuation of improvement in crystalline order.     

Gas-filled polymers. I. Void morphology of polyethylene

Morphologic studies of gas-filled polyethylene show a characteristic void structure, with an interior region containing distinct gas bubbles surrounded by a surface layer of void-free polymer. The voids in the bubbled region frequently show an elongated shape with the long dimension oriented parallel to the surface of the specimens. The gas–polymer interface within individual voids is composed of fibrils of the polymer extending into the interior of the void. Studies of the annealing temperatures required to obtain gas bubbles in the material and of the melting range of the ungasified polymer indicate that melting of the crystalline component of polyethylene is required for void formation.     

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.     

The preparation of oriented morphologies of the thermotropic aromatic copolyester of poly(ethylene terephthalate) and 80 mole percent p-acetoxybenzoic acid

Highly oriented morphologies of poly(ethylene terephthalate) and 80 mole percent p-acetoxybenzoic acid were prepared by shearing in an optical plate-plate rheometer and by melt fiber drawing in a temperature range at which the copolyester was either in the semicrystalline state or liquid crystalline phase. The optical, thermal, and mechanical properties of the oriented films and drawn fibers depended significantly on the processing temperature.     

The skin-core morphology and structure–property relationships in injection-molded polypropylene

Tensile bars of an isotactic propylene homopolymer and an ethylene–propylene copolymer were prepared by injection molding under a variety of melt temperatures and injection pressures. The effects of these processing variables on morphology and crystalline orientation were studied using optical microscopy and x-ray diffraction. Microscopy of microtomed thin sections of the tensile bars revealed the presence of three distinct crystalline zones, namely, a highly oriented nonspherulitic skin, a row or shear-nucleated spherulitic intermediate layer, and a typically spherulitic core. The thickness of the oriented skin layer is a function of the polymer melt temperature and varies inversely with temperature. The thickness of the intermediate layer varies with injection pressure, but in a complex manner. Preferred crystallite orientation in the skin and intermediate layers exerts profound effects on mechanical properties. Tensile yield strength, impact strength, and shrinkage increase with increasing combined thickness of the two oriented outer layers.     

Microfibrillar composites based on polyamide/polyethylene blends. 1. Structure investigations in oriented and isotropic polyamide 6

The present paper discloses the structural changes caused by heating of polyamide 6 (PA6) samples with different thermal and mechanical histories in the 30-240 °C range. Wide and small-angle X-ray scattering (WAXS and SAXS) of synchrotron radiation, as well as solid-state nuclear magnetic resonance spectroscopy (NMR) measurements are performed. The NMR spectra show that in both isotropic and oriented samples there is a co-existence of α and γ-PA6 crystalline forms. Deconvolution of the WAXS patterns is performed to follow the temperature dependence of the unit cell parameters of the α and γ-forms and also of the equatorial (ECI) and total crystallinity indexes (CI), evaluating the contributions of the two crystalline phases. Estimates for the long spacing and for the average thicknesses of the crystalline (l_c) and amorphous (l_a) phases within the lamellae are calculated as a function of the heat treatment employing analysis of the linear correlation function calculated from the SAXS patterns. The X-ray results allowed the conclusion that upon heat treatment up to 160-200 °C, intensive transitions between the PA6 crystalline forms take place, whereby the content of the initial major crystalline phase decreases and that of the initial minor one increases reaching almost 1:1. Close to 200 °C a general trend toward increasing the content of the α-form is registered. The influence of annealing and quenching after melting on the PA6 crystalline structure is also studied.     

Orientational Phenomena in Poly[Bis-(2,2,3,3,3-Pentafluoropropoxy)-Phosphazene] Mesophase

Abstract

The thermotropic behavior and morphology of poly[bis-(2,2,3,3,3-pentafluoropropoxy)phosphazene] (PPPh) have been investigated by various methods such as: differential scanning calorimetry, scanning electron microscopy and X-ray diffraction. It was shown that a poiymer is able to exist in a four phases. A wide-angle X-ray temperature analysis indicates a monoclinic crystalline form for the first flow temperature) phase, the orthorhombic crystalline modification for the second one and the two-dimensional pseudohexagonal mesophase lattice for the mesomorphic state followed by the isotropic melt. Transition from the crystalline into the mesophase state in the range 85–100°C is observed. PPPh macromolecules in the mesomorphic state are found to be oriented in the extrusion direction.     

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。     

Critical crystallization temperature for the occurrence of epitaxy between high-density polyethylene and isotactic polypropylene

The crystallization behavior of high-density polyethylene (HDPE) on highly oriented isotactic polypropylene (iPP) at elevated temperatures (e.g., from 125 to 128°C), was studied using transmission electron microscopy and electron diffraction. The results show that epitaxial crystallization of HDPE on the highly oriented iPP substrates occurs only in a thin layer which is in direct contact with the iPP substrate, when the HDPE is crystallized from the melt on the oriented iPP substrates at 125°C. The critical layer thickness of the epitaxially crystallized HDPE is not more than 30 nm when the HDPE is isothermally crystallized on the oriented iPP substrates at 125°C. When the crystallization temperature is above 125°C, the HDPE crystallizes in the form of crystalline aggregates and a few individual crystalline lamellae. But both the crystalline aggregates and the individual crystalline lamellae have no epitaxial orientation relationship with the iPP substrate. This means that there exists a critical crystallization temperature for the occurrence of epitaxial crystallization of HDPE on the melt-drawn oriented iPP substrates (i.e., 125°C). © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2029–2034, 1997     

Structure of amorphous polyethylene from n.m.r. line shape analysis and MAR-n.m.r.

Based on results of proton nuclear magnetic resonance measurements with magic angle rotation (MAR n.m.r.), the line of amorphous polyethylene in conventionally measured proton n.m.r. spectra is described as a convolution of a basic narrow line-shape function S(v) with an orientation-dependent dipolar broadening function G(v). With this approach it is possible to describle to describe the broadline n.m.r. spectrum of polyethylene as a superposition of the crystalline component and of a single amorphous phase. The fit of experimental and computed spectra, and the parameters obtained by this type of line-shape analysis are discussed for a series of polyethylene samples of different crystallinity.     

Microstructural effects on hot drawing syndiotactic styrene P‐methyl styrene copolymer

Crystalline syndiotactic styrene/p‐methyl styrene copolymer (SPMS) has been oriented by tensile drawing at various temperatures between the glass transition and crystalline melting point. The microstructural changes resulting from drawing have been studied using differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). WIth increasing draw temperature, both melting temperature and crystalline dimensions of the oriented samples increase. The heat of fusion increases with increasing draw temperature up to ～200°C. It also increases with draw ratio and draw rate, while the crystalline width increases only with draw ratio. THe amorphous fraction shows a clear glass transition, the temperature of which (T_g) increases with draw ratio. However, T_g decreases somewhat with increasing draw temperature. This is interpreted in terms of the stretching of the randomly coiled amorphous phase molecules.     

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.     

Crystallization of oriented isotactic polypropylene (iPP) in the presence of in situ poly(ethylene terephthalate) (PET) microfibrils

The present article reports the nonisothermal crystallization process and morphological evolution of oriented iPP melt with and without in situ poly(ethylene terephthalate) (PET) microfibrils. The bars of neat iPP and PET/iPP microfibrillar blend were fabricated by shear controlled orientation injection molding (SCORIM), which exhibit the oriented crystalline structure (shish-kebab), especially in the skin layer. The skin layer was annealed at just above its melting temperature (175 °C) for a relatively short duration (5 min) to preserve a certain level of oriented iPP molecules. It was found that the existence of ordered clusters (i.e. oriented iPP molecular aggregates) leads to the primary nucleation at higher onset crystallization temperature, and formation of the fibril-like crystalline morphology. However, the overall crystallization rate decreases as a result that the relatively high crystallization temperature restrains the secondary nucleation. With the existence of PET microfibrils, the heterogeneous nucleation distinctly occurs in the unoriented iPP melt and results in the increase of crystallization peak temperature and overall crystallization rate, for the first time, we observed that the onset crystallization temperature has been enhanced further with addition of PET microfibrils in the oriented iPP melt, indicating the synergistic effect of row nucleation and heterogeneous nucleation under quiescent condition.     

Determination of the molecular orientation in the polymeric liquid crystalline systems by low frequency Raman spectroscopy

Anisotropic polymer composites with fixed, oriented liquid crystalline organisation prepared by in situ photopolymerisation of acrylic or methacrylic acids in several cellulose derivatives were investigated by low frequency Raman scattering. The results were interpreted basing on a model of non-continuous structure of polymer glasses. The Raman investigations have shown that the macromolecules of the non-mesogenic (acrylic) component are oriented in the anisotropic composites, and that the orientation of the polyacrylic chains is more pronounced in the composites with higher concentration of the liquid crystalline cellulose derivative. It was found, that the interactions between the components play a crucial role in the formation of the composites (template-like photopolymerisation) and they determine their supramolecular structure. The liquid crystalline cellulose derivatives able to form hydrogen bonds play a role of specific cross-linking agents, while the cellulose derivatives with aliphatic side chains work as plasticisers.     

Properties of poly(ethylene terephthalate) prepared by high-pressure extrusion in a capillary die

The procedure of Southern and Porter has been used to prepare poly(ethylene terephthalate) segments by high-pressure extrusion in an Instron capillary rheometer at temperatures from 245° to 265°C. X-Ray measurements of crystalline orientation along the axis of long-growth segments showed that segment properties were controlled by the time-dependent crystallization of polymer melt in the rheometer reservoir. During the initial stage of extrusion, a highly oriented, translucent segment was generated by flow-induced crystallization. However, formation of the translucent morphology eventually stopped, and thereafter a poorly oriented, opaque segment was generated by solid-state extrusion. From wide- and small-angle x-ray scattering, differential scanning calorimetry, and density measurements, it was determined that the most perfect morphologies were prepared at an extrusion temperature of 265°C. In addition to having the highest degree of crystalline orientation, translucent segments extruded at 265°C have a peak melting point of 267°C and a crystallinity value of 62%. Hot-stage optical microscopy showed that the translucent segments contained axially aligned fibrous crystals whose birefringence persisted to 290°C. The exceptional thermal stability of the segments was corroborated by the results of shrinkage tests at temperatures near the melting point; even after 1 hr at 260°C, the shrinkage did not exceed 7%. The accumulated evidence suggests that the translucent segments contain an extended-chain component.     

Defining the physical structure and properties in novel monofilaments with potential for use as absorbable surgical sutures based on a lactide containing block terpolymer

Monofilaments of a block terpolymer of l-lactide, ?-caprolactone and glycolide have been melt spun for potential use as absorbable surgical sutures. As-spun fibres of the terpolymers produced by melt spinning were elastic, amorphous and isotropic. A two-stage process involving hot drawing was employed to enhance their mechanical properties. WAXS and SAXS results coupled with DSC demonstrated that hot drawing leads to an orientated amorphous matrix containing small highly aligned crystals. Hot drawing was carried out at a range of temperatures using the highest possible draw rate commensurate with maintaining continuity of the fibre. A novel WAXS analysis based on a spherical harmonic analysis allowed a separation of the scattering into three components: oriented crystalline, oriented amorphous, and an isotropic amorphous. There is a steady increase in the fraction of oriented crystalline material with increasing hot draw temperature, although the level of crystallinity is ultimately limited by the statistical nature of the terpolymer. The material shows highly promising potential properties for use as a monofilament suture.     

The influence of anisotropic phase structure on the properties of crystalline polymers

By combining quantitative molecular, microscopic and macroscopic information from an oriented crystalline polymer, it becomes possible to unify, predict, and explain not only processing behavior but such important material properties as failure, shrinkage, modulus, yielding, melting, and storage and loss moduli. Among the advantages gained by this approach are the ability to: (a) identify the particular phase of the two-phase system which controls a given property-(b) correlate internal structure quantitatively with a large number of seemingly different types of properties; (c) identify quantitative behavioral rules which are generally valid for Very different crystalline polymers; (d) clarify component roles such that new techniques and processes result; and (e) predict the properties of a crystalline polymer for structural states not previously tested. Using two dissimilar crystalline polymers, isotactic polypropylene and poly(ethylene terephthalate), as examples, the general validity and unifying power of the structural approaches is demonstrated.     

A comparative study of structure–property relationships in highly oriented thermoplastic and thermotropic polyesters with different chemical structures

Structure and properties of commercially available fully oriented thermoplastic and thermotropic polyester fibers have been investigated using optical birefringence, infrared spectroscopy, wide‐angle X‐ray diffraction and tensile testing methods. The effect of the replacement of p‐phenylene ring in poly(ethylene terephthalate) (PET) with stiffer and bulkier naphthalene ring in Poly(ethylene 2,6‐naphthalate) (PEN) structure to result in an enhanced birefringence and tensile modulus values is shown. There exists a similar case with the replacement of linear flexible ethylene units in PET and PEN fibers with fully aromatic rigid rings in thermotropic polyesters. Infrared spectroscopy is used in the determination of crystallinity values through the estimation of trans conformer contents in the crystalline phase. The analysis of results obtained from infrared spectroscopy data of highly oriented PET and PEN fibers suggests that trans conformers in the crystalline phase are more highly oriented than gauche conformers in the amorphous phase. Analysis of X‐ray diffraction traces and infrared spectra shows the presence of polymorphic structure consisting of α‐ and β‐phase structures in the fully oriented PEN fiber. The results suggest that the trans conformers in the β‐phase is more highly oriented than the α‐phase. X‐ray analysis of Vectran® MK fiber suggests a lateral organization arising from high temperature modification of poly(p‐oxybenzoate) structure, whereas the structure of Vectran® HS fiber contains regions adopting lateral chain packing similar to the room temperature modification of poly(p‐oxybenzoate). Both fibers are shown by X‐ray diffraction and infrared analyses to consist of predominantly oriented noncrystalline (63–64%) structure together with smaller proportion of oriented crystalline (22–24%) and unoriented noncrystalline (12–15%) structures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 142–160, 2006