Structural order in heat treated vinyl chloride polymers

Three vinyl chloride polymers were annealed at temperatures in the range 40–160°C for times varying from 0.5 to 5 h. Structural changes occurring in the polymers were examined by density measurement, differential thermal analysis and X-ray diffraction. It was shown that the three polymers varied in their original crystallinity. Attempts to totally remove this crystallinity, were unsuccessful; further crystallinity could be introduced by annealing above T_g Changes induced by annealing below T_g were not due to crystallization. Methods used to measure crystallinity in poly(vinyl chloride) (PVC) were assessed.     

Terpolymers. III. Isochronal viscoelastic parameters for terpolymers of vinyl stearate,vinyl acetate,and vinyl chloride

Isochronal viscoelastic parameters were collected for many of the copolymers, terpolymers, and diluent mixtures whose mechanical properties at ambient temperatures were reported in the preceding paper. In the polymeric systems, vinyl stearate, acting as the primary internal plasticizer, was introduced into terpolymers by displacing vinyl acetate from base copolymers of vinyl acetate and vinyl chloride, across the range of composition. In the diluent mixtures, poly(vinyl chloride) was plasticized by di-2-ethylhexyl phthalate across the range of compositions. For direct comparison with the mixtures, vinyl chloride was plasticized by copolymerization with vinyl stearate across the same range of compositions. Moduli for the co- and terpolymers reached the low values characteristic of soft materials at room temperature only through a short range of vinyl stearate composition. At higher internal plasticizer compositions, side-chain crystallization stiffened the samples and raised their moduli. In contrast, moduli for the mixtures decreased steadily with increase in diluent at ambient temperature. The effective use temperature ranges were narrow for the co- and terpolymers but broad for the mixtures. Curve broadening was similar for both types of systems, but reached a maximum at about 40 weight-% plasticizer for the diluent mixtures. The slopes of the glassy modulus with decreasing temperature at 50°C below T_g for the vinyl stearate copolymers were relatively large. However, moduli close to that of poly(vinyl chloride) were reached only near the temperature range associated with the γ-transition. Consequently, this behavior was attributed to motions of the side chains in the glassy matrix. Room temperature moduli, which could be obtained before the onset of melting, were correlated with the fractional side-chain crystallinity for polymers having a high vinyl stearate content. From this relation, the modulus for the hexagonal crystal form of the side-chain crystallites of poly(vinyl stearate) was estimated to be 1.2×10¹⁰ dynes/cm². Moduli for the glassy amorphous phase of this same polymer appeared to have one sixth of this value at 40°C below the glass transition. The glass transition temperature occurred about 10° below the inflection temperature at 10⁹ dynes/cm², as an average for all of the systems studied.     

Crystallization of itaconic anhydride grafted poly(lactic acid) during annealing

The effect of annealing poly(lactic acid) (PLA) and PLA grafted with itaconic anhydride (IA) at different temperatures was studied using differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS). For PLA, two crystal forms were obtained when annealed between 110 and 120 °C, transforming into only the α‐form at 130 °C while a mixture of α′ and α‐form were obtained in grafted PLA. Grafting increased the percentage crystallinity of PLA, but it was mostly unaffected by the degree of grafting. The rate of crystallization was strongly dependent on the degree of grafting; when annealed at 100 °C, the crystallinity increased from 27.7 to 43.1% while the crystallization halftime reduced from 10.7 to 4.4 min at the lowest degree of grafting. It was thought that the increase in crystallization rate was likely due heterogeneous nucleation in the presence of grafted chains. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44614.     

The influence of crystallinity on the beta-transition in poly(vinyl chloride)

The effect of changes in crystallinity on the β-transition in poly(vinyl chloride) was studied by dynamic mechanical measurements of loss tangent (tan δ) and storage modulus (E′) over the temperature range ?160° to 85°C. Four frequencies were covered, 3.5, 11, 35, and 110 Hz. The data presented demonstrate that crystallinity has a significant influence on the relaxation processes involved in the β-transition. The restrictions on segmental mobility imposed by crystallinity are most pronounced in the region between T_β and T_g, with T_β tending to shift to lower temperatures, T_g shifting to slightly higher temperatures, the magnitude of tan δ decreasing, and engineering tensile strength increasing. Our data also show that individual PVC resins have mechanical and physical properties which differ enough so that comparisons which are often made between them may be inconclusive. Moreover, the properties of one resin may be more sensitive to thermal treatment than those of another. We are continuing this study to establish correlations between the observed trends and other important mechanical properties such as impact strength, toughness, tensile, and compressive strength.     

Annealing and crystallization kinetics of poly(lactic acid) pieces obtained by additive manufacturing

In this study, we investigated the influence of isothermal treatment of poly(lactic acid) (PLA) 3D printed samples at different crystallization temperatures. In this case, we analyzed the effect of each crystallization temperature on spherulites formation in printed PLA, affecting the final mechanical properties of pieces. For such, the thermomechanical properties, morphological structure, and crystallization kinetics were analyzed before and after thermal treatment. The 3D printed samples were heat treated at 80°C, 90°C, 100°C, 110°C, and 119°C. With annealing, we observed an improvement in the mechanical PLA properties; however, the exothermic crystallization peak was different for the samples. Pieces before annealing were found to have a low crystallinity index (Ic) of 2%–7%, and the pieces after annealing presented a considerable Ic (27%–34%). Annealing temperatures of 100°C, 110°C, and 119°C produced the fastest crystallization kinetics, while annealing temperatures of 80°C and 90°C resulted in the lowest crystallization kinetics for complete crystallization. After annealing, improvement in the flexural strength (34%–47%) and Young's modulus (26%–51%) for all annealed pieces occurred. The appropriate condition was observed at 100°C, which was the onset temperature of crystallization, owing to the combination of the shorter time of crystallization with the increased mechanical properties.     

Properties of syndiotactic-rich poly(vinyl alcohol) thin film in water. V. Effect of initial temperature of heating on elongation

The effect of the initial temperature of heating on the elongation of syndiotactic-rich poly(vinyl alcohol) thin films was investigated in water under a load. The elongation ratios E_i after 4 h at fixed temperatures increased roughly with an increase in the initial temperature T_i and a decrease in the annealing temperature. E_i after 4 h was the smallest at T_i = 45°C for the films annealed at temperatures below 100°C. E_i was 6.8 at T_i = 60°C for the unannealed film and 1.12 at T_i = 70°C for the film annealed at 200°C. The elongation at break decreased and the temperature at break increased with an increase in annealing temperature, but those at the annealing temperature of 100°C were the smallest. The films annealed at 200°C did not break even at 98°C (boiling temperature) in water and the elongation ratio was 1.42–1.97 in the initial temperature range of 10–70°C. From these results, the relation between the elongation in water and the state of polymer chains in film was discussed.     

The crystallinity of poly(phenylene sulfide) and its effect on polymer properties

The crystallinity and crystallizability of poly(phenylene sulfide) have been examined by a number of common techniques. Several provided qualitative information, but only one, x-ray diffraction, was considered sufficiently reliable and reproducible to allow quantitative comparisons. Based on x-ray measurements, an approximate degree of crystallinity, termed crystallinity index (C_i), could be readily assigned. According to this method, virgin polymer possesses significant crystallinity (C_i ≈ 65%). Curing (crosslinking) the resin below its melting point did not change the crystallinity but did affect the crystallizability. Lightly cured resin suitable for molding and film extrusion was easily quenched from the melt to give amorphous polymer. The amorphous samples crystallized rapidly when heated to temperatures > 121°C (250°F). At mold temperatures below 93°C (200°F), moldings with very low surface crystallinity were produced. Annealing (204°C, 400°F) caused rapid crystallization of such moldings, and changes in crystallinity were correlated with observed changes in physical properties. The resin crystallizes so rapidly that these quenched moldings possessed a crystallinity gradient, the internal crystallinity being substantially greater. At high mold temperatures (121–204°C, 250–400°F), moldings very similar to fully annealed specimens were obtained.     

Crystallization kinetics of poly(vinyl alcohol)

The kinetics of crystallization of solvent-free poly(vinyl alcohol) were investigated by isothermal crystallization at temperatures of 142–192°C, for up to 18 min. The crystallization isotherms were analyzed by Avrami's theory. The magnitude of the exponent n was almost constant (0.67–0.71) in the range of 142–182°C, increasing to 1.53 for crystallization at 192°C. Based on thermodynamic analysis of the isotherms and the crystallite growth rate, there are strong indications that crystallization of PVA is one-dimensional. In the absence of water or other swelling agents, kinetic hindrances predominate due to the interactions of the hydroxyl groups. Therefore, the maximum attainable weight fraction of crystallized PVA is considerably lower than that of hydrated PVA samples. Additional parameters affecting the growth rate are discussed, including the degree of undercooling and the average chain length, as controlled by crosslinking.     

Effects of Annealing Temperature on the Structure and Capacitive Performance of Nanoscale Ti/IrO2–ZrO2 Electrodes

Electrodes consisting of coating of the Iridium oxide–Zirconium oxide (70%IrO₂–30%ZrO₂) binary oxide were formed on Ti substrates by thermal decomposition and annealing at 340°C–450°C. The effects of the annealing temperature on the structure, surface morphology, surface composition, and capacitive performance of the coatings were investigated using X‐ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy, X‐ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The XRD and TEM analyses showed that 360°C is greater than but very close to the crystallization temperature of the 70%IrO₂–30%ZrO₂ oxide coating. The 70%IrO₂–30%ZrO₂ oxide coatings annealed at this temperature consisted of an amorphous matrix containing a few IrO₂ nanocrystalline particles (diameter of 1–2 nm). The degree of crystallinity of the coatings was approximately 13.2%. EIS analysis showed that the electrode annealed at 360°C exhibited the highest specific capacitance, which was much higher than that of the electrode annealed at 340°C (which had a purely amorphous structure) as well as those of the electrodes annealed at 380°C and 400°C (which had higher degrees of crystallinity). On the basis of the obtained results, the following conclusion can be drawn: oxide coatings prepared at temperatures slightly higher than the crystallization temperature of the oxide and containing conductive nanocrystalline particles exhibit the best capacitive performance. We suggest that this phenomenon can be explained by the fact that the electronic conductivity of the coating is greatly improved by the presence of the homogeneously distributed conductive nanocrystalline particles in the amorphous matrix. Furthermore, the protonic conductivity and loose atomic configuration of the amorphous structure of the electrode are not adversely affected by the annealing treatment.     

Properties of blends of syndiotactic-rich and atactic poly(vinyl alcohols)

The crystallinity, solubility, degree of swelling, and hygroscopicity of the blends of syndiotactic-rich (st-PVA) and atactic poly(vinyl alcohols) (at-PVA) were studied. The crystallinity of the blends increased with increasing annealing temperature and that of the blends annealed at 200°C was larger than that of at-PVA. In the 200°C annealed blends containing st-PVA about 50%, the insoluble fraction in boiling water was larger than the fraction of st-PVA in each blend. The soluble fraction for the blend containing 75% st-PVA was 1.7%. The moisture regain of the blends was less than that of at-PVA.     

Effect of crystallinity on enthalpy recovery peaks and cold‐crystallization peaks in PET via TMDSC and DMA studies

Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110°C for various times. The peaks of enthalpy recovery and double cold‐crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature‐modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed ≤30 min but to decrease with annealing times for samples annealed ≥40 min. The nonreversing curves also found that the samples annealed short times (≤40 min) having low crystallinity exhibited double cold‐crystallization peaks (or a major peak with a shoulder) in the region of 108–130°C. For samples annealed long times (≥50 min), the cold‐crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold‐crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold‐crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold‐crystallization peaks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Non‐Isothermal Crystallization Kinetics of an Ethylene‐Vinyl‐Acetate. II. Time‐Temperature‐Crystallinity‐Superposition

Ethylene vinyl acetate (EVA) is a random copolymer of ethylene and varying amounts of vinyl acetate that interfere with poly‐ethylene chain packing reducing crystallinity, thus improving transparency and lowering the melting temperature to 40°C–60°C. The material viscoelastic properties in its working conditions may thus depend on the crystallinity degree. The crystallization process is here rheologically studied in non‐isothermal conditions and the frequency spectra are measured at different temperatures to investigate the viscoelasticity of EVA. Coupling the crystallization kinetics and the viscoelastic spectra at different temperatures, that is, at different degree of crystallinity, we here determine two independent shift factors, one for the time‐crystallinity shift, the other for the time‐temperature shift, so to propose a new time‐temperature‐crystallinity‐superposition to reconcile all the data on a single master curve. In this way, the experimentally observable frequency range has been widened significantly so to detect all the relaxation times of the material from the shortest to the largest ones. POLYM. ENG. SCI., 59:2550–2556, 2019. © 2019 Society of Plastics Engineers     

Poly(vinyl chloride)–polyol (AB)x block copolymers: Synthesis,characterization, and mechanical properties

Poly(vinyl chloride)–polyol (AB)_x block copolymers have been prepared by the condensation polymerization of low-molecular-weight hydroxy-terminated poly(vinyl chlorides) (PVC) and diisocyanate-capped polyester and polyether diols. The difunctional poly(vinyl chlorides) were synthesized by ozonization of commercial resin followed by metal hydride reduction. The (AB)_x block copolymers, which contained 3000 or 4300 molecular weight PVC block sizes and 1000–2000 molecular weight polyol segments, had a wide range of mechanical properties, depending on overall polymer structure. Tensile strengths ranged from 7.8 to 31.5 MPa, elongations from 125% to 610% and torsional stiffness temperatures (T_f) from 25°C to ?22°C.     

Tailoring the thermal and mechanical properties of injection-molded poly (lactic acid) parts through annealing

The effect of cold-crystallization on poly (lactic acid) (PLA) injection-molded parts was systemically investigated at different annealing temperatures (80/100/120°C) and annealing times (0.5/1/1.5/2 hr). The relative crystallinity (X_c) and crystal form (α' and α) of samples was investigated by differential scanning calorimetry (DSC) and wide X-ray angle diffraction (WAXD). The dependence of the thermal and mechanical performance on relative crystallinity and crystal form/morphology was discussed in detail. A linear relationship between the increment of heat distortion temperature (HDT) and that of X_c was found. The tensile strength, tensile modulus and storage modulus all increased with annealing time and annealing temperature, while the tensile toughness presented a different behavior. The elongation at break for specimens reached a maximum value of 16.9% after annealing at 80°C for 2 hr, which is a threefold improvement compared to PLA samples prepared without annealing. This work suggests that annealing is an effective method for tailoring the physical properties of PLA products.     

Effect of annealing on the structure and properties of polyvinylidene fluoride hollow fiber by melt‐spinning

Polyvinylidene fluoride hollow fibers were prepared by melt‐spinning technique under three spinning temperatures. The effects of annealing treatment on the structure and properties of hollow fiber were studied by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), tensile test, and scanning electron microscopy (SEM) measurements. DSC and WAXD results indicated that the annealing not only produced secondary crystallization but also perfected primary crystallization, and spinning and annealing temperature influenced the crystallinity of hollow fiber: the crystallinity decreased with the increase of spinning temperature; 140°C annealing increased the crystallinity, and hardly influenced the orientation of hollow fiber; above 150°C annealing increased the crystallinity as well, and furthermore had a comparative effect on the orientation. The tensile tests showed that the annealed samples, which did not present the obvious yield point, exhibited characteristics of hard elasticity, and all the hollow fiber had no neck phenomenon. Compared with the annealed sample, the precursor presented a clear yield point. In addition, the annealed samples had a higher break strength and initial modulus by contrast with the precursor, and the 140°C annealed sample showed the smallest break elongation. SEM demonstrated the micro‐fiber structure appeared in surface of drawn sample. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 935–941, 2007     

Structural order in oriented PVC mouldings

The structural changes occurring upon drawing and annealing of compression and injection mouldings of commercial poly(vinyl chloride) were studied by wide-angle X-ray diffraction. Low temperature drawing appears to lead to a reduction in 3-dimensional order and an increase in oriented 2-dimensional order. The degree of order of drawn and annealed PVC depends on draw ratio, annealing temperature and the restraint during annealing. The maximum in 2-dimensional order occurs on annealing at 110°C. Tensile yield stress is significantly increased by the drawing process and it was shown that the anisotropy of this mechanical property decreased upon annealing. This could not be explained by the reduction in amorphous orientation alone. Electron microscopy of the fracture surfaces shows a structure which appears to be related to the drawing and annealing process.     

Relationship between crystallization behavior, microstructure, and mechanical properties in a palm oil-based shortening

In this study, the effects of cooling rate, degree of supercooling, and storage time on the microstructure and rheological properties of a vegetable shortening composed of soybean and palm oils were examined. The solid fat content vs. temperature profile displayed two distinct regions: from 5 to 25°C, and from 25°C to the end of melt at 45–50°C. A peak melting temperature of 42.7°C was determined by DSC. Discontinuity in the crystallization induction time (determined by pulsed NMR) vs. temperature plot at 27°C also suggested the existence of two separate groups of crystallizing material. Isothermal crystallization kinetics were characterized using the Avrami and Fisher-Turnbull models. In using DSC and powder X-ray diffraction, the α polymorph formed upon fast cooling (>5°C/min), and the β′ form predominated at lower cooling rates (<1°C/min). An α to β′ transition took place upon storage. Fractal dimensions (D _f ) obtained by microscopy and image analysis showed no dependence on the degree of supercooling since D _f remained constant (∼1.89) at crystallization temperatures of 5, 22, and 27°C. Crystallization at 22°C at 1°C/min and 15°C/min yielded D _f values of 1.98 and 1.93, respectively. Differences in microstructure were observed, and changes in particle properties increased the parameter λ at higher degrees of supercooling.     

Rheological and mechanical properties of poly(vinyl chloride)/chlorinated poly(vinyl chloride) miscible blends

The properties of poly(vinyl chlorlde)/ehlorinated poly(vinyl chloride) (61.6 percent C1) blends, prepared by melt and solution blending, were measured by various tests. Based on the chlorinated poly(vinyl chloride) (CPVC) composition, percent chlorine, and mole percent CC1₂ groups, these blends were expected to show intermediate properties between miscible and immiscible systems. Indicative of miscible behavior were the single glass transition temperatures over the entire composition range for both melt and solution blended mixtures. A single phase was also indicated by transmission electron microscopy. However, the yield stress showed a minimum value less than either of the pure components in the 50 to 75 percent CPVC range, which is characteristic of two-phased systems. Specific volume, glass transition temperature, and heat distortion temperature were linear with binary composition. The storage modulus showed a small maximum, suggesting a weak interaction between the two miscible polymers. Heats of melting for the residual PVC crystallinity were also less than expected from linear additivity. At 160°C and 210°C, the logarithm of the complex viscosity was essentially linear with volume fraction of CPVC, except for a very slight decrease in the 50 to 75 percent CPVC range, which may have been a result of lower crystallinity. At 140°C, the complex viscosity of the CPVC was less than that of PVC owing to the higher crystallinity of the latter. The viscosities were similar at 160°C, but at 210°C, where most of the crystallites had melted, the complex viscosity of the CPVC was higher because of its higher glass transition temperature.     

Characterization of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and other novel polyethers

Diblock, triblock, and alternating block copolymers based on poly[3,3-bis(ethoxymethyl) oxetane] [poly(BEMO)] and a random copolymer center block poly(BMMO-co-THF) composed of poly[3,3-bis(methoxymethyl)oxetane] [poly(BMMO)], and poly(tetrahydrofuran) [poly(THF)] were synthesized and characterized with respect to molecular weight. Glass transition temperatures T_g and melting temperatures T_m were characterized via DSC, modulus–temperature, and dynamic mechanical spectroscopy (DMS). These polyethers had T_m between 70°C and 90°C, and T_g between ?55°C and ?30°C. The degree of crystallinity of poly(BEMO) was found to be 65% by X-ray powder diffraction. Tensile properties of the triblock copolymer, poly(BEMO-block-BMMO-co-THF-block-BEMO) were also studied. A yield point was found at 4.1 × 10⁷ dyn/cm² and 10% elongation and failure at 3.8 × 10⁷ dyn/cm² and 760 % elongation. Morphological features were examined by reflected light microscopy and the kinetics of crystallization were studied. Poly(BEMO) and its block copolymers were found to form spherulites of 2–10 μm in diameter. Crystallization was complete after 2–5 min.     

Thermal behaviour and annealing of poly(vinylidene fluoride)

In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF₂) annealed in a differential scanning calorimeter. PVF₂ annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (T_a) and the heating rate. The lower melting peak temperature increases as T_a increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF₂ not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF₂ is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF₂ is quenched from the melt into liquid nitrogen and subsequently annealed.