Formation of polymorphic structure and its influences on properties in uniaxially stretched polybutylene terephthalate films

The evolution of structure and properties in uniaxially stretched polybutylene terephthalate (PBT) films has been explored. The stretch temperature has pronounced influences on the development of PBT polymorphism; the mechanisms of its formation involve levels of both applied stress as well as chain relaxation during the stretch period. The stretch rate, on the other hand, tends to perfect the structure. PBT films produced under different conditions develop different levels of crystallinity and orientation in the individual α and β phases, in which annealing under tension induces a partial crystal transition from the β to α phase that leads to an increment of orientation in the α phase. The mechanical behavior of the films is related to the polymorphic structure as well as to the overall chain orientation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 412–423, 2000     

Influence of annealing treatments on the essential work of fracture of biaxially drawn poly(ethylene terephthalate)

The biaxial sequential stretching process of poly(ethylene terephthalate) produces films with a fibrillar microstructure in which fibrils are parallel to the transverse extrusion direction. The mechanical properties of such films are strongly anisotropic due to both the orientation of crystallites and the properties of the intrafibrillar and interfibrillar amorphous phases. The idea is to modulate the properties of the amorphous phase without altering the fibrillar structure by annealing treatments. The morphology (crystallinity and orientation of the crystalline phase) of the annealed films is characterized and their mechanical properties (tensile tests and essential work of fracture) are tested in the longitudinal direction (parallel to the micro fibrils) and in the transverse direction (perpendicular to the micro fibrils). The crystalline phase orientation is the key parameter governing modulus anisotropy. Concerning crack propagation, annealing treatments lead to opposite evolution of the specific essential work of fracture parameter (w_e) in the longitudinal and transverse directions. Thus, it is possible to erase fracture propagation anisotropy through an adequate annealing treatment. Copyright © 2012 Society of Chemical Industry     

Polymorphism and mechanical properties of syndiotactic polystyrene films

The dynamic-mechanical behaviour and the tensile moduli of unstretched and stretched semicrystalline s-PS films, presenting different polymorphic forms (α, γ, δ and clathrate) but similar crystallinity and orientation, have been compared. The main aim is to elucidate the possible influence of different crystalline phases, being largely different in chain conformation and density, on mechanical properties of s-PS semicrystalline samples. For unstretched films presenting a preferential perpendicular orientation of the chain axes, the highest elastic modulus is observed for films with the high density γ phase while for uniaxially oriented films the highest modulus is observed for films with the trans-planar α phase. As for the clathrate films, the guest molecules when only included into the crystalline clathrate phase, have no plasticizing effect.     

Effects of the biaxial orientation on the mechanical and optical properties and shrinkage of polyamide 6-66–montmorillonite–nanosilica nanocomposite films

Polyamide 6–66 (PA6-66)–montmorillonite (MMT)–nanosilica (NS) nanocomposite films were fabricated through a cast film process and then biaxially stretched on a laboratory stretcher. Uniaxial or biaxial stretching induced the elongated conformation of MMT and NS. The b axis of the α crystals and the amorphous phase were revealed to align along the machine direction (MD) after stretching, with the uniaxial orientation playing a more significant role. Furthermore, the crystallinity of PA6-66 stretching increased with the stretching ratio. Uniaxial stretching gave rise to a significantly enhanced tensile strength along the MD, whereas it slightly decreased the mechanical properties along the transverse direction (TD). In contrast, the films subjected to biaxial stretching exhibited more balanced mechanical properties. Uniaxial and biaxial stretching led to decreased transmittance and increased haze in the PA6-66–MMT–NS films; this could have been due to the elongated nanostructure of the two nanofillers, which inhibited the transmission and facilitated the scattering of visible light. The thermal shrinkage of the films increased with increasing stretching ratio, and the biaxially oriented films presented nearly equal shrinkage in the MD and TD. The addition of nanofillers decreased the shrinkage attributed to the mobility inhibition of the polymer chains during heating. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47504.     

Influence of extrusion,stretching and poling on the structural and piezoelectric properties of poly (vinylidene fluoride‐hexafluoropropylene) copolymer films

Three types of poly (vinylidene fluoride‐hexafluoropropylene) (PVDF–HFP) copolymer films were prepared by extrusion, stretching as well as simultaneously stretching and static electric field poling (SSSEP), respectively, and measured by the differential scanning calorimetric, wide angle X‐ray diffraction, fourier transformation infrared‐attenuated total reflection, and Dynamic mechanical analysis. The experimental results showed that the films prepared by stretching and SSSEP have higher crystallinity and β phase than by extrusion. SSSEP improved the chain orientation enormously both in crystalline and amorphous regions, resulting in the highest storage modulus. Because of the lower β phase content, the extruded films exhibited the lowest piezoelectric coefficient d₃₃. For the stretched and SSSEP films, although the β phase content was similar, the d₃₃ was distinct because of the different potential energy for the rotation of the dipoles. In addition, the SSSEP films gave the maximum d₃₃ (24 pC/N), higher than the other PVDF–HFP copolymer films that have been reported. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 858–862, 2007     

Morphology and physical properties of poly(butylene terephthalate)

Liquid nitrogen-quenched PBT samples produce much larger spherulites of an optic axis orientation different from the of the air-cooled samples. Optical and scanning electron microscopy show that glass fibers in the glass-reinforced PBT sample nucleate the growth of well-defined spherulites along the glass fiber axis. Fracture studies at temperatures below and above the T_g indicate, respectively, brittle and ductile interspherulite boundary fracture. From dynamic mechanical studies, three transitions designated by α (flow transition), β (T_g), and γ (secondary relaxation) are observed. The magnitudes of the β and γ transitions are larger for the more amorphous quenched sample than the air-cooled sample, suggesting their amorphous phase origin. Addition of glass fibers raises the dynamic modulus and flow temperature, but suppresses the γ transition without significantly affecting the melting and glass transition temperatures.     

A new approach for flexible PBAT/PLA/CaCO3 films into agriculture

The use of flexible films in agriculture has increased intensely in the last 15 years bringing benefits to producers. However, environmental impacts increased due to their incorrect post‐use disposal which leads the degradable films to emerge as an alternative. The production films of poly(butylene adipate‐co‐terephthalate) and poly(lactic acid) reinforced with calcium carbonate (CaCO₃) was studied focusing on producing lower cost materials and flexible films. Four different films (reinforcement compositions) were prepared by melt extrusion with 10 and 20 wt % of CaCO₃. Mechanical and thermal properties, crystallinity, water absorption, and soil degradation, were evaluated. The addition of reinforcement leads to improved compatibility between the polymers in the matrix, which usually presented phase segregation. The films showed better mechanical properties with the addition of CaCO₃. Highly orientated amorphous structures were obtained leading to low water absorption and low degradation in the simulated soil. This low degradation, suggests that the obtained films would be of interest in flexible mulch films manufacturing, particularly for Muridori plantation system, where long‐term plantations are needed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46660.     

Mechanical properties,stress‐relaxation,and orientation of double bubble biaxially oriented polyethylene films

Biaxially oriented linear low density polyethylene (LLDPE) films were produced using the double bubble process with different machine direction (MD) orientation levels and the same transverse direction (TD) blow‐up ratio. Their mechanical behavior was characterized in terms of the tensile strength and tear resistance. The viscoelastic behavior of oriented films was studied using dynamic‐mechanical thermal analysis (DMTA). The microstructure and orientation were characterized using microscopy, X‐ray diffraction pole figures, and birefringence. The results indicate that MD ultimate tensile strength increases and the TD one decreases with MD stretching ratio. Tear propagation resistance, in general, remained mainly constant in TD and decreased in MD, as the draw ratio was increased. The morphology analyses exhibit a typical biaxial lamellar structure for all samples with different lamellar dimensions. Orientation of c‐axis in crystalline phase, molecular chain in amorphous phase along MD increased with draw ratio. In most crystals, a‐axis was located in the normal direction (ND) and the b‐axis in the ND–TD plane. A good correlation was observed between c‐axis orientation factor and MD mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3545–3553, 2006     

Chirality amplification of syndiotactic polystyrene induced circular dichroism chiral film in δ form upon annealing

The induced circular dichroism (ICD) syndiotactic polystyrene (sPS) chiral films in sPS δ phase were prepared and the chirality of the ICD sPS films upon annealing at different temperature was investigated. The sPS δ crystals in the ICD films transform into γ and then into α crystals as those of achiral sPS δ crystals during annealing at different temperature. The CD signal intensity at λ of 200 nm remains unchanged during the δ to γ crystalline transformation, but enhanced tremendously during the γ to α crystalline transformation. The amplification CD signal attributes to the closer package of the phenol rings in sPS α phase than that in sPS δ and γ phases. The lamellae grown at a temperature above the γ to α transition follow the chiral aggregation structure, which also contribute to the amplification of the CD signal.     

Microconfinement effect on gas barrier and mechanical properties of multilayer rigid/soft thermoplastic polyurethane films

In this article, rigid/soft thermoplastic polyurethane (TPU) films were produced via layer‐multiplying co‐extrusion and the effect of confinement on morphology and gas barrier and mechanical properties is studied. The soft TPU, which is 52% hard‐segment, shows phase separation, while the rigid, 100% hard‐segment TPU exhibits amorphous structures. Even though the viscosity ratio of the two TPUs is over 10 and the elasticity ratio around 100, optical and atomic force microscopies show that a multilayer structure was successfully achieved. Then, the multilayer TPU films were uni‐axially stretched to different amounts of deformations, from 0% to 300%. DSC and WAXS results show that microconfinement occurs during orientation, which causes a significant reduction in oxygen permeability of multilayer TPU films, when stretched at 75%, by comparison to the mono and bi‐layer TPU. The dependence of gas barrier properties on temperature and deformation was also investigated, and a 100% improvement in elongation at break was found when compared to films of the rigid TPU. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41849.     

Effect of nano‐silica on the mechanical,thermal, and crystalline properties of poly(vinyl alcohol)/nano‐silica films

Poly(vinyl alcohol)/nano‐silica (PVA/nano‐SiO₂) films were prepared through extrusion blowing with the addition of water and glycerin as plasticizer. The characteristic properties of PVA/nano‐SiO₂ films were investigated by differential scanning calorimetry, dynamic mechanical analysis, Haake torque rheometry, and atomic force microscopy (AFM). The results showed that the mechanical properties of PVA/nano‐SiO₂ were improved dramatically. The tensile strength of the nanofilms increased from 62 MPa to 104 MPa with loading 0.3 wt % nano‐SiO₂ and the tear strength was improved from 222 KN/m to 580 KN/m. The crystallinity of the films loaded with 0.4 wt. % nano‐SiO₂ decreased from 32.2% to 21.0% and the AFM images indicated that the amorphous region of nanofilms increased with increasing nano‐SiO₂ content. The storage modulus and loss modulus increased to two and nearly three times with 0.3 wt % nano‐SiO₂ loading. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influence of annealing treatment on the structure and properties of poly(4‐methyl‐1‐pentene)‐based films and membranes

The poly(4‐methyl‐1‐pentene) casting films were prepared by melt extrusion and annealed below the melting temperature. The effect of annealing conditions on the structure and properties of casting films and stretched membranes was discussed. In this work, a new peak around annealing temperature, as shown in melting curves, revealed the increase in thickness of lamellar structure. Annealing treatment led to improvements of amorphous thickness and crystal orientation. And the thickness of crystal phase correlated with the logarithm of annealing time. The increase in annealing temperature or time led to the improvements of the hard elasticity of samples. Additionally, the larger porosity of stretched membranes was observed as the annealing time and temperature increased. An optimum annealing condition to prepare microporous membranes was 30 min, 200 °C. This work also discussed the importance of annealing treatment in the preparation of microporous membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46491.     

Study of crystal structure of (polyvinylidene fluoride/clay) nanocomposite films: Effect of process conditions and clay type

Three Polyvinylidene fluoride (PVDF) different in molecular structure were used to produce nanocomposities films by cast extrusion with a particular emphasis on maximizing the β crystal phase content. The PVDF/clay compounding followed by cast film production was carried out through melt extrusion using a twin screw extruder equipped with a slit die. X‐ray diffraction (XRD) results showed that clay melt intercalation is almost similar for all three PVDFs. The XRD results also revealed that nanocomposite films from PVDF with branched chain structure (PVDFB) generated the greatest amount of β phase. FTIR spectroscopy measurements confirmed the XRD results but also revealed that significant stretching of the melt films at the die or rapid cooling would adversely affect the formation of β phase. The amount of β phase obtained based on nanoclay compounding was compared with that obtained from conventional method: stretching of molded PVDF film with initial α phase. Stretching of PVDF film at 60°C yielded pure β phase that means complete transformation of α to β. From mechanical properties, tensile tests were carried out on PVDF nanocomposite films to evaluate mechanical strength. PVDF with low molecular weight exhibited a very low strain at break while branched PVDF and high molecular weight PVDF could sustain more strain. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

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.     

High temperature behaviour of the crystalline phases in unfilled and clay-filled nylon 6 fibers

Temperature-induced crystalline phase transitions in neat nylon 6 fibers as well as nylon 6/montmorillonite nanocomposite fibers have been studied by means of wide-angle X-ray scattering. Both types of melt spun fibers only consist of the γ crystalline phase that does not display any transition during heating up to the melt. In contrast, fibers drawn up to the maximum draw ratio at 140 °C display the single α phase with a high degree of chain orientation. During the temperature increase, the α phase undergoes a gradual structural disordering but preserves its monoclinic character up to melting. The structural evolution of the α form turned out sensitive to the thermal and mechanical treatment of the fibers. Annealing the unfilled drawn fibers at 150 °C prior to the WAXS experiment improves the thermal stability of the α form due to healing of the processing-induced crystalline defects. The montmorillonite-filled fibers display both the α and the γ crystals, which readily turn into α crystal form only upon drawing. Due to the matrix shearing between the MMT platelets, the H-bonded sheets display a higher thermal stability as compared with unfilled drawn fibers. Upon cooling from the melt, the first signs of crystallization are of γ form in the MMT-PA6 fibers, but the α form rapidly turns predominant. Crystallization kinetics considerations are put forward to account for this finding.     

The effect of synthesis methods on the mechanical properties of self‐crosslinkable poly(n‐butyl methacrylate‐co‐N‐methylolacrylamide) films

The effects of particle size and parent polymer characteristics on the mechanical properties, gel fraction, and swelling index of self‐crosslinkable poly(n‐butyl methacrylate‐co‐N‐methylolacrylamide) films made by two‐stage emulsion or microemulsion polymerization in the presence of variable amounts of the chain transfer agent, n‐butyl mercaptan, are reported here. In films prepared with latexes made by microemulsion polymerization, the crosslinking degree increased greatly on curing; by contrast, in those made by emulsion polymerization, the crosslinking degree practically did not increase after curing. Stress–strain tests of uncured and cured films indicate that microemulsion‐made films are tougher than the emulsion‐made films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Mechanical properties of nanocrystalline diamond/amorphous carbon composite films prepared by microwave plasma chemical vapour deposition

Nanocrystalline diamond films (NCD) have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures with varying methane content. They consist of diamond nanocrystallites with sizes of 3–5 nm embedded in an amorphous matrix with grain boundary widths of 1–1.5 nm. The CH₄ content in the gas phase has almost no influence on the microscopic structure but a strong effect on the macroscopic structure and morphology. The mechanical and tribological properties of these films have been investigated by nanoindentation, nano tribo tests, and nano scratch tests. The hardness of a 4-μm-thick film deposited with 17% methane was about 40 GPa, the indentation modulus 387 GPa, and the elastic recovery 75%. Ball-on-disk tests against an Al₂O₃ ball revealed, after initially higher values, a friction coefficient of ≤0.1. Tribo tests and scratch tests proved a strong adhesion and a protective effect on silicon substrates. Finally, the correlations between the macroscopic structure of the films and their mechanical and tribological properties are discussed.     

A dielectric study of the crystal and amorphous fractions in oriented polyethylenes

Polyethylenes have relaxation processes separately assignable to the crystal and amorphous components. The possibility thus presents itself for studying the interaction between the two phases by investigating the anisotropy of the separate relaxation processes induced by overall orientation of the material. Such a study was carried out here by the dielectric method on samples rendered dielectrically active by the introduction of a few carbonyl groups by oxidation. In addition to the usual advantage of ease of frequency variation, the dielectric method in this case has a very great advantage in interpretation resulting from the similarity of dielectric constants of the two phases. In specimens oriented by solid-state extrusion, it was found that the crystalline relaxation process (α) develops the expected anisotropy resulting from dipoles normal to C-axis alignment in the extrusion direction. In branched polyethylene (BPE) no accompanying anisotropy of the amorphous β and γ processes was observed. Since the extrusion was carried out well above the glass temperature of the amorphous component, this indicates that constraints from the crystal phase are not sufficient to prevent dissipation of amorphous orientation by segmental motion. However in linear polyethylene (LPE) (where there is no well-developed β process) some anisotropy of the γ process was observed. It is proposed that a higher degree of constraint of the amorphous component by the crystals in LPE results in suppression of the glass-rubber (β) relaxation and permits amorphous orientation to accompany crystal orientation.     

Ageing of poly(lactic acid) films plasticized with commercial polyadipates

BACKGROUND: Amorphous poly(lactic acid) (PLA) was plasticized with two polyadipates with different molar masses. Some physical properties were studied over time to evaluate the stability of these blends. The aim of this study was to improve PLA ductility and consider the feasibility of its use in flexible films for food packaging. RESULTS: The addition of polyadipates caused a decrease of the glass transition temperature (T_g) and an increase of PLA chain mobility. Samples with T_g values above the storage temperature suffered physical ageing with a reduction in free volume. All the unaged blends were mainly amorphous, but samples with T_g below the storage temperature developed crystallinity during ageing leading to phase separation. Ductile properties of films improved with plasticizer content immediately after blending, but there was a deterioration in such properties upon ageing due to matrix densification and crystallization of PLA chains. CONCLUSION: PLA can be efficiently plasticized by polyadipates and the results have shown that some of the prepared films remain flexible with no phase separation after 150 days. Copyright © 2009 Society of Chemical Industry     

Annealing effect on chain segment motion and charge detrapping in polyamide 610

The chain segment motion and charge detrapping in polyamide 610 films have been investigated by means of thermally stimulated depolarization current (TSDC) and wide‐angle X‐ray diffraction. There are three current peaks (namely α, ρ₁, and ρ₂ peaks) in the TSDC spectra above room temperature. α peak is attributed to a dipole relaxation by the motion of chain segments ρ₁ peak is caused by space charge trapped in amorphous phase and the interphase between crystalline and amorphous phases, and ρ₂ peak is brought about by space charge trapped in crystalline phase. By analyzing the characteristic parameters of these peaks, it is found that annealing induces the decrease of chain segment mobility and promotes the creation of structural traps in polyamide 610. The decrease of chain segment mobility in amorphous phase makes intensity of α peak weak and activation energy increscent. The higher the annealing temperature, the higher the degree of crystallinity, the more the charge carriers trapped in crystalline phase. So, the increase of degree of crystallinity makes intensity of ρ₂ peak strong and increases the stability of trapped charge in the crystalline phase. The increase of annealing temperature makes intensity of ρ₁ peak strong and decreases the stability of trapped charge in the amorphous phase and interphase. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011