Rheo-optical studies on the deformation mechanism of semicrystalline polymers. 1. Deformation mechanism and recoverability of deformation of tubular extruded polybutene-1 films

Rheo-optical studies (involving small-angle X-ray and light scattering, wide-angle X-ray diffraction, and birefringence studies) have been applied to polybutene-1 films prepared by tubular extrusion (the preparation involving crystallization from oriented melts) to investigate the fundamental orientation and deformation mechanisms of molecules (in the crystalline and amorphous regions) and crystalline supermolecular structure. It is shown that the molecular orientation behaviour can be described by deformation of the ‘row-nucleated’ sheaf-like crystalline superstructure and that the deformation of the interlamellar spacing is quite uniform, the microscopic strain being identical to the strain in bulk specimens. The latter is quite important in understanding apparent ‘springy’ or ‘hard-elastic’ properties of the films observed at temperatures above the glass transition of the films. In relation to the springy properties, recoverabilities of the molecular and supermolecular structures from large extensions have been also studied.     

Rheo-optical studies on the deformation mechanism of semicrystalline polymers. VIII. Dynamic x-ray diffraction of a high-density polyethylene film having row-structure of C-axis orientation

Dynamic x-ray diffraction is employed to scrutinize the structural origin of the α mechanical dispersion of a highdensity polyethylene having preferentially oriented rownucleated cylindritic texture around the machine direction (MD). Results indicate that the α₁ dispersion is associated with two kinds of orientation processes of crystallites: (1) the orientation process involving the rotation of crystallites around the crystal b-axis (lamellar axis) dominates in the MD specimen; (2) the other process accompanying the rotation of crystallites or lamellar segments around their crystal a-axis dominates in the TD (transverse direction) specimen. The complex apparent crystal lattice compliances show no frequency dependence in the real component and no appreciable value in the imaginary component, indicating that the α₁ process is definitely associated with the intercrystalline process, but not relavent to the intracrystal process. However, at elevated temperatures, the complex lattice compliances exhibit a remarkable frequency dependence, suggesting that the α₂ dispersion concomitant to intracrystalline nature takes place in this time scale. The α₂ dispersion was found to be more pronounced upon stretching along the lamellar axis (TD) than normal to it (MD).     

Rheo-optical studies on the deformation mechanism of semi-crystalline polymers. IX. Dynamic birefringence behaviour of a high-density polyethylene film having row nucleated crystalline texture of c-axis orientation

The dynamic birefringence behaviour of a high-density polyethylene film having a row-nucleated crystalline texture of cylindrites oriented along the machine direction of fabrication was investigated over frequency and temperature ranges covering the α mechanical dispersion of this material. The results are discussed in combination with the dynamic X-ray diffraction behaviour of the material so as to explore the structural origins of the α mechanical dispersion, not only for this particular material, but also for bulk-crystallized polyethylene having a spherulitic texture. Two deformation processes contribute to the α₁ mechanism, corresponding to the lower temperature relaxation process in the α mechanical dispersion; i.e. (i) a lamellar detwisting process involving the rotation of crystal grains within the crystal lamellae or of lamellar segments around the crystal b-axis or the lamellar axis, which predominates in the MD (machine direction) specimen, and (ii) lamellar shearing associated with the rotation of the crystal grains or the lamellar segments around the crystal a-axis, which is accentuated in the TD (transverse direction to fabrication) specimen. An additional deformation process, lamellar bending, is also observed in the MD specimen as being likely elastic in the dynamic response, and its contribution is found to be substantial, not to the α dispersion, but rather to the β dispersion of this material. The α₂ mechanism corresponding to the higher temperature relaxation process in the α mechanical dispersion is observed to be more pronounced in the TD than in the MD specimen. The apparent dynamic crystal lattice compliance shows a definite dispersion during activation of the α₂ mechanism, but a slight dispersion during activation the α₁ mechanism, suggesting that the α₂ mechanism must be related to an intracrystal relaxation process whereas the α₁ mechanism must be associated with an intercrystal relaxation process at their boundaries.     

Drying of semicrystalline polymers: mathematical modeling and experimental characterization of poly(vinyl alcohol) films

A mathematical model was developed to predict the drying mechanism of semicrystalline polymers involving multiple solvents. Since drying of semicrystalline polymers can be accompanied by changes in polymer degree of crystallinity, the model integrates crystallization kinetics and the Vrentas-Duda diffusion model to provide a better understanding of the mechanism. The model considers the effect of external conditions such as temperature, film shrinkage and diffusion and evaporation of multiple solvents during drying. Poly(vinyl alcohol) (PVA)/water/methanol was chosen as a test system. The drying kinetics of PVA films swollen in water and methanol were investigated using gravimetric techniques. The model predicts that higher temperatures, lower film thicknesses and lower methanol to water ratios increase the drying rate. The model predictions were compared with experimental data and showed good agreement.     

Multiple thermosetting of semicrystalline polymers: Polyamide 66 and poly(ethylene terephthalate) fibers

Polyamide 66 fibers were thermoset in a torsion-bending deformation at various temperatures up to 240 °C. Some of the fibers were heat-set at constant length prior to the deformation at presetting temperatures of 150 °C and 200 °C to vary the structural state of the starting material. Fractional recovery was measured after various combinations of temperature and time. It was found that heat setting of PA66 is dominated by time-dependent stress relaxation exhibiting time-temperature equivalence. Increased crystallinity, and/or other molecular rearrangements occurring during presetting, impose additional constraints on molecular mobility, which delay onset of the flow regime and increase the time constant of relaxation at a given temperature. The thermosetting characteristics of PA66 fibers are very similar to those of poly(ethylene terephthalate) fibers. For both polymers, superposing the curves of fractional recovery vs. setting time at different temperatures produce satisfactory master curves, without the need for vertical shifting of the data. Arrhenius plots yield approximate activation energies for the thermosetting flow process of 35-65 kcal/mol in PA66 and 95-115 kcal/mol in PET.     

Uniaxial-planar deformation of poly(ethylene terephthalate) films: 1. Characterization of the crystalline phase

Amorphous isotropic poly(ethylene terephthalate) films were deformed under uniaxial-planar symmetry conditions at constant engineering stress close to and above the glass transition temperature. The structure of the crystalline phase is characterized in terms of orientation, size and organization of the crystalline blocks by X-ray scattering techniques. The orientation of the crystallographic axes and the size of the crystalline blocks are well rescaled by the macroscopic draw ratio, which is a function of the drawing temperature and the applied stress. The large-scale organization of the crystalline phase is not determined by the same parameter but changes mainly with temperature.     

Local conformation controlled crystallization of isotactic poly(butene-1)

The slow spontaneous solid-solid transition from metastable form II to stable form I in typical isotactic polybutene-1 (iPB-1) homopolymer after melt crystallization may be unavoidable and it is challenging to obtain form I directly from bulk iPB-1 melt. In this work, by DSC and WAXS, form I was observed to directly crystallize from bulk iPB-1 while cooling to T far below the crystallization temperature T_c of form II. It is further proposed that the local formation of 3/1 helix conformation is the determining process of direct formation of form I and there exists a temperature window for 3/1 helix conformation to form at low T due to the competition between the entropy effect and the synergistic effect of enthalpy and the energy barrier. The key of polymorph selection of iPB-1 lies in the T-dependent formation of the crystal-favored helix conformations. The results could shed lights on developing charming industrial approaches to obtain the stable form I directly in bulk iPB-1 as well as on understandings to the physics behind polymer crystallization and polymorph selection processes.     

Thermal degradation of polymers. XV. Vacuum pyrolysis studies on poly(p-methoxystyrene) and poly(p-hydroxystyrene)

The monomers p-hydroxystyrene (p-HS) and p-methoxystyrene (p-MS) have been synthesized and polymerized using azobisisobutyronitrile as free-radical initiator. The polymerization behavior of p-HS is anomalous, and a mechanism is suggested to account for this phenomenon. Poly(p-hydroxystyrene) (PHS) and poly(p-methoxystyrene) (PMS) homopolymers have been subjected to vacuum pyrolysis at temperatures in the range of 300–500°C. The products of degradation have been identified and qualitatively and quantitatively analyzed and the degradation behavior of the two systems compared with polystyrene. PHS shows anomalous behavior resulting from the high reactivity of the p-HS monomer and the facility for transfer afforded by the proton of the hydroxyl substituent. Mechanisms to account for the degradation behavior of each system are discussed.     

Branching and re-orientation of lamellar crystals in non-banded poly(butene-1) spherulites

The formation mechanism of non-banded polymer spherulites has been examined experimentally for isotactic poly(butene-1) grown from the melt by optical, atomic force, and transmission electron microscopies associated with quenching and chemical etching. At the growth front of the spherulites, the maximum width of lamellar crystals, λ_m, showed a square-root dependence on the growth rate. The dependence suggests an instability-driven branching. In terms of the correlation of lamellar orientation in the spherulites, an auto-correlation function has been determined from the image taken by polarizing optical microscopy. The correlation showed an exponential decay along the radial direction, and the correlation length was in proportion to λ_m. Those experimental evidences suggest that the structure is formed by the coupling of the branching instability and the random re-orientation of lamellar crystals on the occasion of branching in the non-banded spherulites of poly(butene-1).     

Luminescence studies in poly(ethylene terephthalate) films at 77 K. Investigation of emission centres in polymers

Optical luminescence of amorphous bright poly(ethylene terephthalate) (PET) films and the changes in luminescence due to certain morphological changes have been studies at 77 K to identify the luminescent centres in this polymer. The emission spectrum consists of both fluorescence and phosphorescence with distinct band peaks at 304, 326, 365 nm corresponding to fluorescence and at 426, 453, 476 nm corresponding to phosphorescence. Comparison of the PET emission spectrum with that one from dimethyl terephthalate microcrystals indicates that the emitter in the polymer is most probably the basic terephthalate unit. Polarized absorption and emission studies in oriented and unoriented PET films also support this observation. Change in the relative intensity distribution on changing the crystallinity of the film suggests that each of the 304, 326 and 365 nm fluorescence bands arises from different environments of the emitting centre. On γ-irradiation, a large increase in emission intensity is observed. This is explained on the basis of formation of a radical which in turn causes a large inductive effect on the benzene rings.     

Spherulite nucleation in blends of isotactic polypropylene with isotactic poly(butene-1)

The phase morphology and the influence of composition on the primary nucleation of isotactic polypropylene in isotactic polypropylene/isotactic poly(butene-1) (iPP/iPB) blends were investigated by electron and light microscopy and small-angle light scattering. It was found that iPP and iPB are miscible but the thermal treatment induces partial phase separation of components and the formation of iPP-rich and iPB-rich phases. The complete phase separation needs high temperatures and/or a long time of melt annealing. In samples crystallized isothermally at low undercooling the heterogeneous primary nucleation in blends is depressed as compared to plain iPP. In blends the less active heterogeneities lose their activity because of an increase of the energy barrier for critical size nucleus formation due to phase separation of blend components during crystallization. For the same reason the rate of homogeneous nucleation in blends decreases, as observed in samples crystallized at very high undercooling. At very high undercooling iPP and iPB are able to crystallize with similar rates, which results in the formation of a fraction of iPB spherulites in addition to iPP spherulites. Consequently the number of spherulites in the blend is larger than that in plain iPP, in spite of the decrease in the homogeneous nucleation rate of iPP in the blend. © 1994 John Wiley & Sons, Inc.     

Comparative studies of electrochemically deposited poly(o-toluidine) and poly(m-toluidine) films

Under galvanostatic deposition conditions poly(o-toluidine) exhibits a higher rate of polymerization than poly(m-toluidine). This observation is supported by results obtained by different characterization techniques such as spectrophotometry, scanning electron microscopy and thermogravimetric analysis. The monomer concentration was found to be the predominant parameter in obtaining selectively a conducting salt phase in both cases. However, the morphology of these polymeric films does not reveal any particular relationship with monomer concentration; instead a mixed morphology, i.e. a combination of granules and fibres, is observed. Finally, the thermal stability of poly(m-toluidine) is lower than that of poly(o-toluidine) with a shift of 190°C in the final decomposition temperature.     

Thermal degradation of polymers. XVI. Thermal analysis studies on poly(p-methoxystyrene) and poly(p-hydroxystyrene) in air and nitrogen

Comparative thermal degradation studies have been made in air and nitrogen on polystyrene (PS), poly(p-methoxystyrene) (PMS), poly(p-hydroxystyrene) (PHS) and p-hydroxystyrene/styrene copolymers (p-HS/S) by thermal analytical methods (TG and DSC). The degradation behavior shown by these polymers in an oxidizing or inert atmosphere is different. PMS and PS, however, degrade in a similar manner in the same test atmosphere, suggesting that similar degradation mechanisms are operative for these systems. Polymers containing p-HS behave differently to PMS and PS in both atmospheres. Their behavior in air is discussed it terms of the antioxidant effect of the phenolic hydroxyl grouping and the related crosslinking reactions. In nitrogen, the anomalous behavior is discussed in terms of the observed carbonization reaction.     

Mechanisms of orientational and photoelastic birefringence generation of methacrylates for the design of zero–zero‐birefringence polymers

The intrinsic birefringence Δn⁰ and photoelastic coefficient C of poly(methyl methacrylate), poly(2,2,2‐trifluroethyl methacrylate), poly(phenyl methacrylate), and poly(2,2,3,3,3‐pentafluorophenyl methacrylate) were determined. We categorized these methacrylate polymers into four birefringence‐types, even though their molecular structures differed only by the substituents on the side chains. Based on the results of Δn⁰ and C, novel polymers that exhibit neither orientational nor photoelastic birefringence, i.e., zero–zero‐birefringence polymers, were designed and synthesized by quaternary copolymerization system. Furthermore, we confirmed that the mechanisms of orientational birefringence and photoelastic birefringence generation were different in these methacrylate polymers. The conformation of the repeat unit of the polymers was nearly constant during the generation of orientational birefringence. In contrast, the conformation of the repeat unit of the polymers changed during the generation of photoelastic birefringence in the glassy state. These findings demonstrated the reasonability of evaluating orientational and photoelastic birefringence separately, as well as the adequacy of the classification of polymers into four birefringence‐types. Given these results and the fact that zero–zero‐birefringence polymers could be prepared successfully by four‐birefringence type monomers, we demonstrated the reasonability of the method for designing the zero–zero‐birefringence polymers. POLYM. ENG. SCI., 55:1330–1338, 2015. © 2015 Society of Plastics Engineers     

Influence of additives on the crystallization kinetics of semicrystalline polymers. II: Selective polymer‐additive interaction in poly(vinylidene difluoride)—poly(methylmethacrylate) blends

The influence of pigments and mineral fillers on the crystallization kinetics of poly(vinylidene fluoride) (PVDF) in its blends with poly(methyl methacrylate) (PMMA) was studied by Differential Scanning Calorimetry and Optical Microscopy. The introduction of organic pigments or mineral fillers into PVDF results in a more or less significant increase in non‐isothermal crystallization temperatures, depending on the additives used. In PVDF, the pigment nucleation activity is quite different from that previously found in poly(butylene terephthalate) (PBT). Attractive interactions between PVDF and additive surfaces are particularly important parameters. The competition between pigments and talc lead to the observation that some pigments are able to inhibit the nucleating power of this mineral in PVDF, probably through specific attractive interactions. In PVDF‐PMMA blends, selective interactions between polymers and additives were observed, depending on their chemical structures. Highly chlorinated pigments present a strong attraction for PMMA and then lose an important part of their nucleating power towards PVDF. On the other hand, a pigment containing carbonyl groups becomes the most active nucleating agent, illustrating the presence of strong interactions between such compounds and the PVDF C‐H bonds.     

Property–composition dependence of isotactic poly(butene-1)-chlorinated polyethylene blends

Melt-mixed blends of isotactic poly(butene-1) (PB) with chlorinated polyethylene (containing 48 wt % Cl) (CPE) were studied in the complete composition range. Phase contrast, polarizing, and scanning electron microscopy revealed that the blend is heterogeneous. The results were confirmed by the dynamic mechanical technique and differential scanning calorimetry. The latter technique indicated also that CPE does not influence the crystallinity of PB. Tensile behavior of the blends was good, especially at low CPE contents. The results were analyzed using phenomenological mechanics models. From the correlation obtained one can conclude that the blends are mechanically compatible. Limiting oxygen index data were also determined, to characterize the flammability behavior of the blends in the complete composition range.     

Fatigue-fracture mechanism of slowly notched poly(ethylene terephthalate) polymers

Summary Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (N_f) of these samples increased with increasing temperatures until it reached the transition temperatures of PET polymers, and most of the increase in N_f is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and N_f of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.     

Determination of the orientation in poly(ethylene terephthalate) films by comparison of birefringence and X-ray diffraction

The interpretation of the birefringence of biaxially drawn poly(ethylene terephthalate) is rather difficult because of its complicated and non-unique relation to the orientation. Moreover, crystalline and amorphous parts of the material assume different degrees of orientation and have often different directions of the principal axes of orientation. These relations, including the influence of different texture components, are analyzed theoretically and estimated experimentally by comparison with the orientation parameters found by X-ray diffraction.     

Structural studies of poly(butyl acrylate) - poly(ethylene oxide) miktoarm star polymers

Structural behavior of miktoarm star polymers comprising poly(butyl acrylate) (PBA) and poly(ethylene oxide) (PEO) arms was studied by means of Differential Scanning Calorimetry (DSC), Wide Angle X-Ray Scattering (WAXS), Polarized Optical Microscopy (POM) and Fourier Transform Infrared Spectroscopy (FTIR) methods. The aim of this study was to correlate changes in the composition of the arms of the PBA/PEO miktoarm star polymers with their structures. As a consequence of increasing PBA content, the decrease in crystallinity of the studied PBA/PEO heteroarm star copolymers was observed. Regardless of the copolymer composition, fraction of oxyethylene units in the crystalline PEO phase was similar in all investigated systems. The POM images showed spherulitic morphology of the materials having low PBA content, while an increase in PBA arms fraction leads to the formation of less ordered structures. The analysis of FTIR vibrational spectrum indicates helical conformation of PEO chains in the crystalline phase. Isothermal crystallization studies carried out using the FTIR technique suggest the existence of isolated domains in the nanoscopic scale of investigated materials.