Structural evolution of PVDF during storage or annealing

The effect of annealing PVDF at temperatures above T_g and below T_m was investigated by differential scanning calorimetry (DSC), thermostimulated current spectroscopy (TSC) and solid-state NMR. This study evidences a progressive structural evolution, taking place during such annealing. Its characteristics (kinetics and its temperature dependence, lack of reversibility at lower temperature over extended periods of time, double organization corresponding to double annealing with unmodified kinetics) point to a mechanism of secondary crystallization as described by Marand et al. In addition to the formation of extra crystalline (hence rigid) material, this phenomenon is believed to generate increasing conformational constraints in the residual amorphous material. Accordingly, a progressive reduction of the molecular mobility was demonstrated by NMR during annealing.     

Effect of cross-linker length on the thermal and volumetric properties of cross-linked epoxy networks: A molecular simulation study

Volumetric and thermal properties of cross-linked epoxy systems consisting of diglycidyl ether of bisphenol A (DGEBA) and poly(oxypropylene) (POP) diamines of four different lengths ranging from 3 to 68 units were investigated by molecular dynamics (MD) simulations. The cross-linked structures were built by using the simulated annealing polymerization approach. The density, coefficients of volume thermal expansion and glass transition temperature (T_g) of each of the four cross-linked epoxy systems were obtained from their volume–temperature behavior. The density obtained in the simulations agreed well with the experimental value, whereas the coefficients of volume thermal expansion were at least 30% lower than their corresponding experimental results. The predicted T_g values were higher than the experimental values due to the considerably faster cooling rates that are used in the simulations. It was observed that an increase in the chain length of the cross-linker POP-diamines led to a larger difference between the predicted and experimental values of T_g. Three different approaches were used to estimate the expected shift in the experimental T_g to higher values had these measurements been made at cooling rates comparable to those used in MD simulations. It is shown that, in general, the T_g values obtained in MD simulations are consistent with such shifted T_g values that account for the difference in the cooling rates, although no one particular shift approach worked well for all four epoxy systems studied.     

Dynamic mechanical properties of annealed sulfonated poly(styrene-b-[ethylene/butylene]-b-styrene) block copolymers

Solution cast films of lightly sulfonated styrene-b-[ethylene-co-butylene]-b-styrene, (sSEBS) block copolymers were annealed for various times at 120 °C and thermal transitions are evaluated using dynamic mechanical analysis. Increased annealing time and increase in degree of sulfonation increases T_g for the PS phase while T_g for the EB phase is practically unchanged, and in some cases, there is suggestion of a relaxation due to EB-PS inter-phases. Annealing has a minor effect on the rubbery plateau storage modulus. Thus, annealing primarily alters the PS block phase. EB-PS phase separation appears to be refined with increasing SO₃H content. The region of rubber elasticity extends to higher temperatures with increased degree of sulfonation. A high temperature dynamic mechanical transition that is tentatively attributed to disruption of SO₃H—rich sub-domains within the PS block domains shifts to higher temperature with annealing.     

Catalytic activity of graphite-based nanofillers on cure reaction of epoxy resins

The strong influence of graphite oxide (GO) nanofiller on the glass transition temperature (T_g) of epoxy resins, generally attributed to restricted molecular mobility of the epoxy matrix by the nanofiller or to the crosslinking of GO layers via the epoxy chains, is investigated. The study confirms that large increases of the glass transition temperature of the nanocomposite can be observed in presence of GO. However, similar T_g increases are observed, when the filler is a high-surface-area graphite (HSAG), lacking oxidized groups. Moreover, these T_g differences tend to disappear as a consequence of aging or thermal annealing. These results suggest that the observed T_g increases are mainly due to a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine), rather than to reaction of the epoxide groups with functional groups of GO. This hypothesis is supported by investigating the catalytic activity of graphite-based materials on reactions between analogous monofunctional epoxide and amine compounds.     

Synthesis and characterization of flexible polyester coatings for automotive pre-coated metal

An automotive pre-coated metal system has investigated to remove the wet coating process, such as pre-treatment, dip coating and spray coating for environmental regulations. However, automotive pre-coated metal sheets must have high flexibility and formability to overcome the harsh conditions such as cutting, press and stamping process. For these reasons, flexible polyester coatings were designed to control flexibility using polycarbonatediol. The characteristics, curing behavior and viscoelastic property of the resins were measured by FT-IR, GPC, rheometer, DSC, RPT and DMA. The flexibility was evaluated using a texture analyzer. A nano-scratch tester, equipped optical microscope can measure the scratch resistance of coatings and the scratched surface morphology.With increasing polycarbonatediol content, the final frequency and the storage modulus increased and the T_g of the coatings decreased. This is related to the high mobility of the chain segment in polycarbonatediol. In terms of the flexibility and scratch resistance, CP-3 had high elongation value and good scratch resistance resulting from optimized stamping, pressing and cutting process in automotive pre-coated metal system. Therefore, polycarbonatediol is a powerful factor affecting the flexibility and scratch resistance of polyester coatings.     

Mechanical and thermal properties of (acrylonitrile‐styrene‐acrylic)/(α‐methylstyrene‐acrylonitrile) binary blends

In this work, (acrylonitrile‐styrene‐acrylic)/(α‐methylstyrene‐acrylonitrile) copolymer (ASA/α‐MSAN) binary alloy was prepared with different composition ratios via melt blending. This work mainly focused on improving the heat resistance of ASA. According to the results of dynamic mechanical thermal analysis, the binary blends exhibited three glass transition temperatures (T_gs) and the shift of the T_gs indicated the partial miscibility of binary blends. This partial miscibility maintained the high T_g of α‐MSAN, which led to the outstanding heat resistance of binary blends. Furthermore, heat distortion temperature also showed that the heat resistance of binary blends was significantly enhanced with the addition of α‐MSAN. However, the introduction of this highly rigid polymer also brought with it the sharp decrease of the impact strength and elongation at break, which is reflected in the morphologies of the blend system obtained via scanning electron microscopy. In addition, the incorporation of α‐MSAN increased the tensile strength, flexural strength, and modulus. There were no new groups observed from Fourier‐transform infrared spectra, which means no strong specific intermolecular interactions existed between ASA and α‐MSAN. Moreover, the processibility of the blend system was obviously improved from the results of melt flow rate. J. VINYL ADDIT. TECHNOL., 22:156–162, 2016. © 2014 Society of Plastics Engineers     

Improvement of heat stability of poly(l-lactic acid) by radiation-induced crosslinking

Poly(l-lactic acid) (PLLA) has poor heat stability above its glass transition temperature (T_g∼60 °C). To improve its softing above T_g, PLLA was mixed with small amount of crosslinking agents and irradiated with various irradiation doses to introduce crosslinking between polymer chains. The most effective agent for radiation crosslinking was triallyl isocyanurate (TAIC). For melt-quenched PLLA, it was found that the most optimal conditions to introduce crosslinking were around 3% of TAIC and the irradiation dose of 50 kGy. The typically crosslinked PLLA showed very low crystallinity because of wide formation of molecular chain network that inhibited molecular motion for crystallization. Notable heat stability above T_g was given by annealing of PLLA samples. Enzymatic degradation of PLLA was retarded with introduction of crosslinks.     

Association of stereoregular poly(methyl methacrylates): 2. Formation of stereocomplex in bulk

Blends of isotactic and syndiotactic poly(methyl methacrylate) (i- and s-PMMA) are obtained by precipitation from chloroform and acetone solutions. By differential scanning calorimetry and dynamicmechanical measurements the formation of stereocomplexes from i- and s-PMMA in bulk is demonstrated. After annealing of the blends at 130–160°C a melting endotherm is detected and it is established by X-ray analysis that this endotherm is caused by formation of crystalline stereocomplex. The rate of complex formation is maximal at 140°C and the extent of complex formation is maximal at an isotactic-rich composition. It appears that the difference in solvent history can be removed by heating to 240°C. The subsequent S-shaped course of glass transition temperature, T_g, with composition is explained by the occurrence of some complex formation during cooling from 240°C. Asymmetry and shift of the dynamic-mechanical damping curves after annealing are also explained by the formation of complexes. A mechanism is proposed with helical isotactic chains acting as nuclei for a fringed micelle type of complex formation.     

Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Thermal behaviour of poly(lactic acid) in contact with compressed carbon dioxide

BACKGROUND: The thermal behaviour of poly(lactic acid) (PLA) in contact with compressed CO₂ was studied using high‐pressure differential scanning calorimetry. In particular, the effect of annealing below and above the glass transition temperature (T_g) on the glass transition, cold crystallization and melting temperatures was studied systematically as a function of annealing time and pressure. RESULTS: The effect of compressed CO₂ on the thermal properties of PLA is time dependent. Annealing below T_g decreases the temperature and enthalpy of cold crystallization. Similar, but more evident, behaviours are observed when annealing above T_g. Crystallization temperature and enthalpy during cooling decrease with increasing pressure, and the peak is narrower. CONCLUSION: Annealing PLA in the presence of compressed CO₂ accelerates cold crystallization, but retards crystallization during cooling. Copyright © 2009 Society of Chemical Industry     

Segmented copolymers of uniform tetra-amide units and poly(phenylene oxide): 1. Synthesis and thermal-mechanical properties

Copolymers of telechelic poly(2,6-dimethyl-1,4-phenylene ether) segments with terephthalic methyl ester endgroups (PPE-2T), 13 wt% crystallisable tetra-amide segments of uniform length (two-and-a-half repeating unit of nylon-6,T) and dodecanediol (C12) as an extender were made via a polycondensation reaction in the melt. The maximum reaction temperature was 280 °C. The PPE-2T/C12/T6T6T copolymers are semi-crystalline materials with a T_g around 170 °C, a melting temperature of 264-270 °C and a T_g/T_m ratio of above 0.8. The modulus is high up to the T_g, which is not achievable in a blend of PPE and polyamide. The most probable morphology is that of long crystalline nano-ribbons in the amorphous matrix. The materials are slightly transparent and have good solvent resistance, low water absorption and good processability.     

Quantitative study of the annealing of poly(vinyl chloride) near the glass transition

Poly(vinyl chloride) displays a normal DSC of DTA curve for the glass transition when quenched from above its T_g. However if cooled slowly or annealed near the glass transition temperature, a peak appears on the DSC or DTA curve at the T_g. In this paper quantitative studies of the time and temperature effects on the production of this endothermal peak during the annealing of PVC homopolymer and an acetate copolymer are presented. The phenomenon conforms to the Williams, Landell, and Ferry equation for the relaxation of polymer chains, the rate of the peak formation becoming negligible at more than 50°C below T_g. The energy difference between the quenched and annealed forms is small. For a PVC homopolymer annealed 2 hr at 68°C, which is T_g ?10°C, the difference is 0.25 cal/g. For a 13% acetate copolymer of PVC similarly annealed, the difference is 0.36 cal/g. The measured rates of the process give a calculated activation energy of 13–14 kcal/mole for PVC homopolymer and copolymer. This appearance of a peak on the T_g curve for a polymer when annealed near the glass temperature appears to be a general phenomenon.     

Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

Polystyrene freeze-dried from dilute solution: Tg depression and residual solvent effects

The calorimetric glass transition temperature, T_g, was measured for both linear and cyclic polystyrenes freeze-dried from dilute solutions of 0.10, 0.05, and 0.02% of polymer by weight in benzene. Upon freeze-drying, T_g was found to be depressed by 4-15 K depending on the sample, solvent concentration, and freezing conditions. Annealing under vacuum at moderate temperatures, from 40 to 140 °C and 0.05 Torr, resulted in the shift of T_g back towards its bulk value and was accompanied by a decrease in sample weight. The data is consistent with the observed weight loss being due to residual solvent. The amount of residual solvent is a strong function of the annealing temperature and the initial freeze-drying solution concentration; exposure to vacuum at temperatures far below T_g is generally insufficient for residual solvent removal.     

Annealing studies on a fully cured epoxy resin: Effect of thermal prehistory,and time and temperature of physical annealing

The dynamic mechanical behavior at about 1 Hz of a fully cured epoxy resin (maximum glass transition temperature, T_g∞, ca. 170°C) ahs been studied during and after isothermal annealing in terms of the influence of thermal prehistory, time of annealing, and temperature of annealing (T_a). Annealing temperatures ranged from T_g ? 15 to T_g ? 130°C. The rate of isothermal annealing was observed to decrease by a decade for each decade increase of annelaing time when the material was far from equilibrium. Annealing at high temperatures did not measurably affect the mateiral properties during cooling (for T ? T_a); similarly the effect of annealing at low temperatures was not measurale during heating (for T ? T_a).     

Polyether-metal salt complexes: 1. The elevation of the glass transition temperature of polyethers by metal salts

Various metal halides have been dispersed in high and low molecular weight, amorphous poly(propylene oxide), by solution blending techniques, to give single phase polymeric complexes which remained thermoplastic even at very high salt loadings. These complexes were amorphous and showed a single well-defined glass transition temperature (T_g) by differential thermal analysis with the T_g of the complex up to 140°C greater than the T_g of the parent polymer. The T_g elevation depended upon both the amount and the type of salt added and for a given salt the elevation followed a sigmoidal curve which levelled out at high salt concentrations. The T_g data have been interpreted in terms of a chelate ring model involving the co-ordination of two adjacent ether oxygen atoms in the polymer backbone to the salt. Using this model it was possible to consider the complex as a random copolymer consisting of complexed and uncomplexed monomer units. The contribution of crosslinking by metal salts to the elevation of T_g was assessed by studying poly(tetramethylene glycol)-zinc chloride complexes in which chelate formation is entropically unfavourable. Mechanical data are reported for ZnCl₂ complexes with high molecular weight poly(propylene oxide). The results indicate that ZnCl₂ increased the rubbery modulus and this has been interpreted in terms of ZnCl₂ forming a few, weak intermolecular crosslinks.     

Structure and property studies of bioabsorbable poly(glycolide-co-lactide) fiber during processing and in vitro degradation

Structure and properties of a bioabsorbable poly(glycolide-co-lactide) (PGA-co-PLA) fiber during several processing stages and the final in vitro degradation stage were investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis and mechanical property tests. In the orientation stage, an increase in the temperature of the first encountered orientation roll resulted in a lower level of crystallinity and larger crystallites. The temperature of the second encountered pre-annealing roll (PR) imposed a smaller effect on the structure. In the hot-stretching stage after fibers were braided, the maximum crystallinity was achieved at around 126 °C. Higher hot-stretching temperatures increased the crystal size, glass transition temperature (T_g) and tensile strength, but decreased the elongation at break and the heat shrinkage near T_g. In the post-annealing stage, it was found that crystallinity, T_g and tensile strength all increased significantly while the heat shrinkage near T_g sharply decreased after annealing. This suggests that the internal stress accumulated in the orientation and hot-stretching stages can be effectively reduced by post-annealing. During in vitro degradation, crystallinity was found to increase with time while the heat shrinkage near T_g and in the supercooling region (T_g<T<T_m) was greatly reduced. These results support the process of cleavage-induced crystallization.     

Mechanical properties of miscible polycarbonate-copolyester blends

Miscible blends of polycarbonate and the copolyester based on 1,4-cyclohexane-dimethanol and a mixture of terephthalic and isophthalic acids were melt processed into film and injection moulded into test bars. Wholly amorphous specimens of each type were mechanically tested directly after fabrication and after a variety of annealing conditions. As processed blends exhibited nearly additive responses versus blend composition for modulus, strength, elongation at failure, and notched lzod impact strength. Various annealing protocols caused maxima to appear in plots of modulus versus composition. Similar responses were observed for blend density, and good correlations were noted between density and modulus as both composition andhistory were varied. Sub-T_g annealing of injection moulded specimens was demonstrated to involve simultaneous relaxations of volume and molecular orientation. The individual effects of this combined process were separated by sequential super-T_g (or T_m) and sub-T_g annealing steps.     

Molecular interpretation of T∥, the rubbery-viscous ‘transition’ temperature of amorphous polymers

Many physical and mechanical properties of amorphous polymers show anomalous behaviour at a temperature above T_g, designated T_∥, the ‘liquid-liquid transition temperature’. It is suggested here that T_∥ can be considered to be the rubbery-viscous ‘transition’ temperature of amorphous polymers, analogous to T_g, the glassy-rubbery transition temperature. At T_∥, the viscous dissipation goes through a maximum due to a shift in response of the molecules to stress from a coiling-uncoiling process (elastic rubbery region) to slippage between the molecules (viscous region). It is argued here that this maximum is responsible for the occurrence of a maximum in log(decrement) traces obtained by torsional braid analysis (t.b.a.) above T_g.     

Nanodroplet formation and exclusive homogenously nucleated crystallization in confined electrospun immiscible polymer blend fibers of polystyrene and poly(ethylene oxide)

By utilizing electrospun blend fibers of polystyrene (PS) and poly(ethylene oxide) (PEO) with diameters in sub-microns, nanodroplets of the minor component (PEO) were obtained by annealing the blend fibers above the glass transition temperature (T_g) of the matrix polymer (PS), as a result of the Rayleigh-Plateau instability in the melt. However, direct thermal annealing of the PS/PEO blend fibers led to poor Rayleigh breakup of the PEO fibers in the PS matrix, and fractionated crystallization with both homogeneous and heterogeneous nucleation was observed, probably due to a broad size distribution of PEO particles. On the contrary, after confining the PS/PEO blend fibers with a high T_g polymer, poly(4-tert-butyl styrene) (P4tBS, T_g ∼ 143 °C), well-defined Rayleigh breakup of the PEO fiber was achieved by annealing the P4tBS-coated PS/PEO blend fibers at 150 °C. Consequently, exclusive homogeneously nucleated PEO crystallization was observed at −20 °C. This report could provide a universal method to achieve nano-sized droplets for the study of nanoconfinement effect by utilizing electrospun immiscible polymer blend fibers without addition of any compabitilizers.