Control of molecular orientation

I. Amorphous polymers . The mechanical performance of a glassy amorphous polymer is strongly dependent upon molecular orientation. The pattern of molecular orientation is governed by the kinematics (and temperature) of mechanical forming operations. Three types of controllable orientation are: (a) uniaxial, (b) biaxial, and (c) “crossed.” The optimum pattern of orientation in a part is one which is appropriate for the mechanical stresses encountered in service. For a fiber subjected to tensile and bending loads, uniaxial orientation is appropriate. A shell structure, subjected to multiaxial stresses, requires either biaxial or crossed orientation for maximum performance. As a rule, the maximum achievable multidirectional strength in such a structure is less than the maximum strength of a uniaxially oriented fiber. II. Crystalline polymers . Oriented crystalline polymer structures can be created in two distinct ways. An isotropic polycrystalline polymer can be deformed below the melting point, with extensive reorganization of the crystal morphology, or an oriented amorphous melt can undergo crystallization to yield oriented crystalline polymer. Performance of an oriented semicrystalline polymer depends upon orientation of the amorphous portion as well as orientation of the crystallites. As with amorphous polymers, orientation can be uniaxial, biaxial, or crossed. “Orientation” usually denotes c-axis orientation only, but drawing followed by rolling can result in double orientation—orientation of a-axis, b-axis, and c-axis.     

The specification of orientation and its development in polymer processing

A critical review of the specification of orientation and its development in polymer-processing operations is presented. Orientation may in general be specified by orientation distribution functions, but is most conveniently expressed in terms of orientation factors which are second moments of the distribution. The Hermans orientation factor represents polymer-chain orientation for systems with fiber symmetry (uniaxial orientation) and the Hermans-Stein orientation factors express uniaxial orientation for each of the crystallographic axes of crystalline polymers. Biaxial orientation is, however, developed in tubular film extrusion, blowmolding and, indeed, all processing operations other than fiber formation. Orientation factors developed previously by the authors express biaxial orientation in terms of the angles between the machine and transverse directions and the polymer chain axis or crystallographic axes. In flowing polymer melts, the Rheo-Optical Law, which relates birefringence and stress, represents a relationship between polymer-chain orientation and stress. In vitrified polymeric glasses (e.g. polystyrene), the orientation factors are related linearly to the stress field at vitrification. This has been shown experimentally for melt spinning and tubular film extrusion. The results of studies of blowmolding and injection molding are consistent with this. The crystalline orientation factors have also been found to be determined by the stress field at solidification in melt spinning and tubular film extrusion.     

Strain-induced crystallization kinetics of vinylidene chloride/vinyl chloride (VDC/VC) copolymers. Part III. Comparison of the effect of biaxial and uniaxial strain

Specimens of amorphous VDC/VC tube taken immediately after the water quench bath on the blown film process were stretched uniaxially and biaxially using a T. M. Long film stretcher. The stress and temperature responses were monitored. The crystalline content as a function of time was calculated based on an energy balance and the observed temperature response. It was found that SIC kinetics shows a similar dependence on extension ratio (ER) for the uniaxial and biaxial cases. The crystallization rate increases rapidly when an extension ratio of 3.5 is first reached in either the machine direction (MD) or transverse direction (TD). However, significantly greater engineering and true stress values are reached for biaxial stretching than for uniaxial stretching. This is due to the greater molecular alignment in the plane of the specimen versus the thickness direction and to the lesser specimen thickness for the biaxial case. The copolymer studied was an 88/12 wt/wt VDC/VC copolymer with a weight average molecular weight of 100,000 containing liquid additives that function as processing aids.     

Structure development in processing of polypropylene films with additives

The crystallization behavior, modification of crystalline form, and orientation in polypropylene processed by blow film extrusion was studied as a function of processing parameters as well as different types of additives. The isothermal crystallization rate was greatly enhanced in the presence of certain additives, especially CaCO₃. The crystalline form was predominantly α type in both compression molded or blow extruded films. However, there was an unusually large intensity of the α₀₄₀ peak in the X-ray diffraction of the latter case films. The variation of the peak intensities and the increase of birefringence with increase of take-up speed has been explained on the basis of orientation induced by uniaxial stress in the machine direction. This orientation contained two components, namely the orientation of the b axis of the crystallites and the orientation of loosely bound polymer chains in the amorphous regions. © 1995 John Wiley & Sons, Inc.     

Biaxial deformation of polyamide‐6: Assessment of orientation by means of infrared trichroism

Infrared spectroscopy was used to study the evolution of structure in films of polyamide‐6 drawn on a Cellier tenter frame laboratory tester under conditions of simultaneous equibiaxial stretching and planar uniaxial stretching. The “tilted film” method was used to obtain trichroic spectra corresponding to the machine, transverse, and normal directions, as well the “structural factor” spectrum. From these it was possible to obtain information on the molecular orientation and the evolution of the crystalline structure. The starting films, prepared by melt casting from an extruder on a chilled roll, contained predominantly the mesomorphic β form. The structural factor spectra confirmed that strain‐induced transformation into the α form occurred upon drawing, and that the amount of α form increased with the extent of drawing. The trichroic spectra showed that the molecular orientation was localized mainly, but not exclusively, in the α form. Orientation functions could be determined for both the molecular chain axis and the normal to the hydrogen‐bonded sheets. For both the equibiaxial and planar uniaxial films, these sheets were found to be strongly oriented parallel to the plane of the film, with the degree of orientation increasing with overall draw ratio. For the biaxial samples, the molecular chain orientation was found to be equibiaxial, as expected. Mechanical test results indicated that the chains are evenly distributed in the film plane rather than showing a preference for the two orthogonal draw directions. For the planar uniaxial samples, the chain orientation was predominantly in the draw direction, but some degree of orientation in the transverse direction was also observed. The variation of orientation functions with draw ratio suggested that the α structure evolves in two stages, the first involving chain orientation in the draw direction and the second involving rotation of the sheets into the plane of the film.     

Influence of biaxial stretching mode on the crystalline texture in polylactic acid films

Structural evolution during simultaneous (SB) and sequential rubbery state biaxial stretching (SEQ) of polylactic acid (PLA) films from cast amorphous precursors was investigated. Simultaneous biaxial stretching always leads to films with in-plane isotropy and poor crystalline order. In the first stage of sequential biaxial stretching, oriented crystallization gradually develops while transverse isotropy is maintained. Application of transverse stretching to these films possessing semicrystalline structure gradually destroys the crystalline structure oriented in MD during this realignment while establishing a second population of oriented but poorly ordered crystallites in TD. This destruction is caused primarily by splaying action under transverse stretching as evidenced by the decrease of crystallite sizes in MD.     

Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE

Different types of polyethylene blown films (HDPE, LDPE, LLDPE) differ significantly in the ratio between machine and transverse direction tear resistance. In this paper, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) blown films at different draw-down ratios are studied, and the relation between crystalline structure and anisotropy of blown film properties is investigated. The crystalline morphology and orientation of HDPE, LDPE, LLDPE blown films were probed using microscopy and infrared trichroism. Significant differences in crystalline morphology were found: at medium DDR HDPE developed a row-nucleated type morphology without lamellar twisting, LDPE showed rod-like crystalline morphology and turned out to the row-nucleated structure with twisted lamellae at high draw-down ratio (DDR), while a spherulite-like superstructure was observed for LLDPEs at all processing conditions. They also showed quite different orientation characteristics corresponding to different morphologies. The morphologies and orientation structure for LDPE, LLDPE and HDPE are related to the stress applied (DDR) and their relaxations in the flow-induced crystallization process, which determine the amount of fibrillar nuclei available at the time of crystallization and therefore, the final crystalline morphology. These structure differences are shown to translate into different ratios of machine and transverse direction tear and tensile strengths.     

Mechanical Fabrication of Thermoplastic Polymers

Abstract

The physical properties and performance of fabricated thermoplastic items depend upon the molecular structure (mainly established during polymerization) and the spatial arrangement of the polymer molecules (established during fabrication). The spatial arrangements can be favorably influenced by controlled molecular orientation (uniaxial, biaxial and “crossed”) and by composites (fiber reinforcement, multi-layer films).     

Stress‐assisted formation of surface gratings on polymer films

The effect of biaxial‐residual–compressive surface stress on the surface modulation of solid films is analyzed. The results show that the biaxial compressive stress can create surface modulation with the orientation being 45° (135°) to the principal directions of the stresses. Using uniaxial tension, a complementary set of nonsymmetrical polymer ripple gratings is formed on the fracture surface of a polymer film that is sandwiched between two relatively glass slides. The effect of geometrical shape and boundary confinement is examined on the surface modulation. The orientation of the polymer ripple gratings is found to be 45° to the longitudinal direction of the films for long‐rectangular polymer thin films in accord with the analysis. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

An investigation of optical clarity and crystalline orientation in polyethylene tubular film

A study of the crystalline orientation, light transmission, and surface roughness of polyethylene tubular film prepared in our laboratories is presented. The present studies were primarily carried out on low-density (LDPE) and linear-low-density (LLDPE) polyethylene films. The optical properties of a few films of high-density polyethylene (HDPE) prepared for a previous study of morphology were characterized for comparison to the LDPE and LLDPE films. Wide angle X-ray diffraction and birefringence were used to characterize orientation. Both the LDPE and LLDPE films exhibited crystalline texture in which the b-axes tended to be perpendicular to the film surface and the a-axes had some tendency to align with the machine direction. The c-axes tended to be concentrated in the plane of the film with nearly equal biaxial orientation with respect to the machine and transverse directions. Little variation in the crystalline orientation was found with changes of process conditions in the range studied. Birefringence results indicate that the amorphous regions developed an orientation in which the chains tend to be normal to the film surface. The majority of light scattering from these films and a series of HDPE films was from the surface and not from the film interior. The transmission coefficient for the surface contribution was found to be a monotonic decreasing function of the standard deviation of the surface height obtained from surface profiles measured by profilometer. The surface asperites were largest for the HDPE and smallest for the LDPE samples. The intensity of both the surface and interior contributions to the scattering increased with increasing frostline height, i.e., a slower cooling rate. As draw-down ratio and blow-up ratio increase the scattering contribution from the film interior decreases but the contribution from the surface increases somewhat. These effects are discussed in terms of the changes in crystalline morphology and surface roughness produced by flow defects generated during extrusion.     

Structure-Properties Relationships for Bisphenol A Polycyanurate Network Modified with Polyoxytetramethylene Glycol

Abstract

This work deals with uniaxial orientation and shrinkage of two grades of well-characterized low-density polyethylene (LDPE), and a general-purpose polystyrene (PS). The LDPE grades differed in molecular-weight distribution and degree of long chain branching.

The polymer samples were stretched to different levels of deformation over a range of temperature, quenched, and subsequently annealed. The shrinkage and thermoelastic force during recovery were measured upon annealing. Molecular orientation resulting from the stretching process was characterized by birefringence measurements in the amorphous polymer and by X-ray diffraction in the semicrystalline polymer. Morphology changes in the latter were characterized by polarized optical and electron microscopy.

The relationship between the amount, rate and temperature of stretching, to the resulting orientation and morphology, and the shrinkage behavior were investigated.     

Process for the formation of biaxially oriented films of poly(p-phenylene terephthalamide) from liquid crystalline solutions

A new process for making equal biaxially oriented films from liquid crystalline solutions of poly(p-phenylene terephthalamide) (PPD-T) is described. The process involves extruding solutions of PPD-T/H₂SO₄ through an annular die and over an oil-coated mandrel into a coagulation bath. The films were studied using wide angle X-ray diffraction (WAXS) and scanning electron microscopy (SEM). Tensile stress–strain properties were obtained on samples cut at various directions in the plane of the film. Biaxially oriented films which possess equal properties in the various directions in the plane of the film were produced. Moduli of 2.3 × 10⁹ Pa and tensile strengths of 9.6 × 10⁷ Pa were obtained in the plane of the film. Films with unequal biaxial orientation were also produced. These tend to have higher modulus/tensile strength in the direction of major orientation, the machine direction (up to 8.3 × 10⁹ Pa/2.5 × 10⁸ Pa), but become brittle in the transverse direction.     

Biaxial orientation in HDPE films: comparison of infrared spectroscopy, X-ray pole figures and birefringence techniques

In this study, high-density polyethylene films (HDPE) were produced using different processes (film blowing and biaxial orientation) and processing conditions. The orientation of the films was characterized in terms of their biaxial crystalline, amorphous and global orientation factors using birefringence, Fourier transform infrared spectroscopy (FTIR) using a tilted incidence technique and X-ray pole figures. Evaluation of a simplified FTIR procedure without using the tilted method for the determination of crystalline orientation factors proposed in the literature is also evaluated and assessed. The results indicate that FTIR overestimate the crystalline orientation factors, particularly for the crystalline a-axis. Significant discrepancies are also observed for the b-axis orientation, which may be due to an overlap of the amorphous contribution and/or saturation of FTIR bands. Those differences are larger for films with low orientation, such as blown films. Amorphous phase orientation from FTIR depends on the band used and is not necessarily in agreement with that determined from combination of X-ray and birefringence.     

Structure-Property Relationships in a Polymer

Abstract

The way in which polymer molecular structure controls structure on higher levels of organization in a solid polymer is briefly reviewed, as is also the way in which structure, on all levels, controls physical properties. The “line of descent” from molecular structure to physical properties is then illustrated at length in the case of one particular polymer, cis-polyisoprene (natural rubber). It is shown how the crystalline-amorphous morphology in the solid is controlled both by the chemical microstructure of the polymer and by the physical conditions (temperature, time, strain) under which solidification occurs. By changing these “processing conditions” great changes can be effected in the morphology.

The mechanical properties of the solid are then examined as a function of morphology and shown to depend strongly on the various morphological parameters, such as the amount and orientation of the crystalline phase and the orientation and state (rubber or glass) of the amorphous phase.     

Plastic deformation and tearing in high density polyethylene blown films

Polymer films produced by tubular film blowing have a unique morphology that results from the large elongational flow in melt draw down and biaxial orientation due to bubble blow-up. Three high density polyethylene (HDPE) blown films were produced under similar processing conditions from resins which varied principally in molecular weight (MW) and molecular weight distribution (MWD). Scanning electron microscopy (SEM) showed that the lower MW and narrower MWD resin produced film which had a uniaxial orientation of stacked lamellar crystals. The higher MW (HMW) and broad MWD resins produced films consisting of a network of nearly orthotropically oriented lamellar stacks. Greater high molecular weight fraction (MW > 10⁶) in the resin resulted in more random orientation. The influence of these different structures on properties was studied by examining the plastic zone formation at crack tips and uniaxial tensile deformation with the SEM and comparing them to the macroscopic stress-strain behavior. A continuous deformation of the network structure was observed in the HMW films. Lamellar deformation occurred primarily in regions of stacks oriented parallel to the tensile axis. Macroscopic yield occurred at 6 to 10 percent strain via a shearing and opening the lamellar crystals. Irreversible deformation occurred from ?50 to 400 percent strain by transformation of the oriented lamellae to microfibrils. Eventually all the lamellar stacks in the network become aligned with the tensile axis. This process was found to improve the tear resistance in the crack propagation experiments. The lamellar stacks in the network orient perpendicular to the crack independent of crack propagation direction, insuring a more uniform transmission of stress and preventing local yielding. The tensile modulus, yield stress, and ultimate strength were highest in the film containing more high molecular weight polymer.     

Correlations between processing parameters,morphology, and properties of blown films of linear low‐density polyethylene/low‐density polyethylene blends. I. Crystalline biaxial orientation by IR and mechanical properties

The change of the processing parameters of a blown film operation alters the mechanical and optical properties of the films. This work studied the influence of some of these parameters on the properties of blown films made of blends of linear low‐density polyethylene (LLDPE) and LDPE. Correlations between the crystalline biaxial orientations of these films and the mechanical properties were found. The crystalline biaxial orientation was measured by IR following the Krishnaswamy approach. The a axis of the unit cell was oriented along the machine direction (MD) at all LDPE concentrations, and it was not affected by the blow‐up ratio (BUR). In contrast, the b axis changed its orientation from orthogonal to MD to along the transverse direction (TD), and it was affected by the BUR. Finally, the c axis changed its orientation from equiplanar between the MD and TD to along the thickness of the film, and it was influenced by the BUR. The decrease of the tensile mechanical properties along the MD with the increase in the amount of LDPE in the blends was attributed to the tilting of the c axis toward the film thickness. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3161–3167, 2006     

Ultra Thin Organic and Polymer Films

Abstract

Preparation of ultra thin organic and polymer films could be divided into two methods. One is the wet process such as Langmuir-Blodgett (LB), spreading, dipping and casting methods. The other is dry processing, such as vapor deposition, sputtering, chemical vapor deposition, plasma polymerization and vapor deposition polymerization methods. Of these methods, the LB method has been attractive for the last decade to prepare a monolayer film, however, the vapor deposition method has also attracted attention for the preparation of well organized ultra thin films. It is important to investigate a molecular assembly in terms of the extent of aggregation, the crystalline regularity and their orientation in the thin film, since they are closely related to the physical, electrical and functional properties. This review focuses on an evaluation of the structure, molecular assembly and orientation in the thin films prepared by the LB and vapor deposition methods.     

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.     

双轴取向聚乙烯薄膜晶体取向的二维X光衍射分析

聚乙烯膜的晶体取向决定着薄膜的多种力学性能和热力学性能。因此,针对薄膜晶体取向的表征显得非常重要,尤其是具有双轴取向的聚乙烯薄膜。通过二维广角衍射研究了单轴和双轴高取向聚乙烯薄膜的取向度。建立表征取向度的三种方法,包括Herman取向分析法、局部积分法和环向积分法。结果发现,上述方法均适用于所有的双轴取向的高分子薄膜,包括非晶态高分子薄膜。Herman取向分析法可以通过取向因子定量计算简单取向材料取向度;局部积分的方法能分析出衍射较弱方向晶体信息,更适用于取向度较复杂的样品;环向积分法能更直观地分析薄膜的取向特点、取向强度。     

Crystal structures and orientation development in tubular film extrusion of syndiotactic polypropylene and isotactic polypropylene

The differences in behavior of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) in tubular film extrusion are qualitatively described. The crystalline form and orientation in the films were characterized using wide‐angle X‐ray diffraction (WAXD) patterns, pole‐figure analysis and birefringence. The sPP films had the crystalline form of the disordered Form I and the a‐crystallographic axis was found to be preferentially oriented in the film normal direction (ND) under the conditions of biaxial stresses. High transverse orientations were developed in the sPP films. In the iPP films, the monoclinic crystalline form was found and the b‐crystallographic axis was preferentially oriented in the ND. The birefringence of the films showed trends very similar to the crystalline orientations characterized by WAXD in both iPP and sPP films.