The morphology of the aromatic copolyester of poly(ethylene terephthalate) and 80 mol % of p-acetoxybenzoic acid

The structure of the aromatic copolyester of poly(ethylene terephthalate) and 80 mol % of p-acetoxybenzoic acid was investigated by optical and electron microscopy, wide angle X-ray, and thermal analysis. The results of optical (LM) and electron microscopy (TEM and STEM) suggest the existence of ordered domains which appear as large lamellar thickness (～200 Å) as has been described earlier for single crystals of the pure hompolymer of p-hydroxybenzoic acid (PHBA). However, the characteristic reversible endotherm at 330–360°C for PHBA could not be obtained. In isolated cases, the lamellar blocks retained their general dimensions up to 535°C whereas the surrounding matrix had degraded severely. An irreversible endotherm was observed at 303°C which suggested the presence of a higher degree of order than expected for a random copolymer. The irreversible nature of the endotherm is consistent with a transition from a semicrystalline structure to a liquid crystalline state. X-ray analysis with as received samples and thermally treated specimen provided additional support for more ordered regions.     

Kinetics and extent of low-temperature crystallization of ethylene–vinyl acetate copolymers

The enthalpies of fusion of two types of EVA copolymers containing 9 and 16% of vinyl acetate, respectively, were investigated by DSC. After melting, the samples were cooled down and held at 10, 15, 20, and 25°C for different periods of time from 15 min to 2.5 months (only at 20°C). The enthalpy of fusion increased over the 2.5-month period for 9.3 and 11.3 J/g, respectively. There was a new small melting peak on the endotherm of the aged sample whose position and size depended on aging temperature and aging time. During 2.5 months, the peak shifted toward higher temperature for 8°C. The enthalpy of fusion and corresponding degree of crystallinity changed linearly with the logarithm of time, as is the case in high-temperature annealing or secondary crystallization at high temperatures. The rate and the extent of low-temperature crystallization of ethylene copolymers depend on the comonomer content, sequence length distribution, and temperature. © 1996 John Wiley & Sons, Inc.     

Effects of Heating,Tensile, and High Pressure Treatments on Aggregate Structure of a Low Density Polyethylene

The effects of heating, tensile, and high pressure treatments on the aggregate structure of a low density polyethylene (LDPE) are studied via modulated DSC (MDSC) and dynamic mechanical technique (DMA). The results indicate that the nonreversing heat flow can reflect the relative thermodynamic stability for crystalline structure of the polymer, and this makes nonreversing heat flow be a novel useful tool to qualitatively characterize the perfect degree of crystalline structures. During thermal treatment, it is found that there exists a low temperature melting endotherm at about 35°C. It presents an endothermal peak in nonreversing heat flow and a simultaneous glasslike transition in reversing heat flow, which behaves a feature of physical aging far above the glass transition temperature of the polymer. This suggests the existence of so-called “rigid amorphous fraction” (RAF). The structural changes of polyethylene induced by different treating methods are also investigated by DMA, and the relaxation characteristics of macromolecular chains are analyzed.     

Effects of processing and degree of fusion on the rheology of PVC

PVC foils made from polymer grades having K-values of 58 and 68 have been milled at different temperatures and shear conditions on a two-roll mill. Compression molded foils were also made from the softest grade. The resulting samples have been studied using DSC and rotational dynamic rheometry at temperatures below and above the processing temperatures. DSC confirmed the correlation between the processing temperature and the temperature T_b separating endotherm A and B. However, this correlation is only valid for high processing temperatures (≥150°C). Also, if the processing temperature is lowered at the end of the processing, T_b will be lower than the maximum processing temperature. In most cases the area of endotherm A increased with increasing processing temperature and shear. From measurements of tan(δ) we find that when measuring at a temperature below the lowest processing temperature, the higher the degree of fusion, the larger is the relative amount of the elastic energy stored. Measuring at a temperature above the highest processing temperatures yields the opposite result. In between there is a transition zone where polymer grades having different M?_w respond differently to changes in thermomechanical processing history. These and other observations are discussed in terms of the gel-destruction temperature introduced by Lyngaae-Jørgensen, the breakdown of the particulate structure, and the creation of a three-dimensional fusion network held together by ordered and crystalline regions.     

The Application of Thermal Analysis Methods to the Decomposition of Sodium Bicarbonate

Thermal analysis of sodium bicarbonate by DTA and DSC showed that the decomposition temperature of this salt is about 100±1°C. A decomposition temperature of 270°C given in the Handbooks seems to be an error. Adsorbed water is the cause of the small endotherm between 70 and 75°C.     

Effects of thermal history on isotactic polypropylene

The effect of past thermal history on the melting behavior of isotactic polypropylene is investigated in some detail. It is shown that a series of stepwise annealing treatments at steadily increasing temperatures will raise the final melting point and will result in a double endothermic peak if the final anneal temperature is at or close to 160°C. It is also shown that a series of stepwise annealing treatments at steadily decreasing temperature will lead to multiple DSC peaks. The number of such separate peaks is equal to or greater than the number of annealing steps. Even low-temperature anneals (100–130°C) affect the melting endotherm, while high-temperature anneals have a marked effect on both the degree of crystallinity of the sample and the final melting temperature. For a 3-min anneal, the highest degree of crystallinity is produced by an anneal temperature of 155°C. The highest melting temperature (～182°C) is produced by a 30-min, or longer, anneal at about 160°C. The implications of these results in terms of crystal thickening and perfection are discussed.     

Melting of ultrahigh molecular weight polyethylene

On heating in DSC, samples of UHMWPE show a single, fairly sharp, melting endotherm which may be increased to a peak temperature of 147°C and 77% crystallinity by annealing at elevated temperatures. An irreversible conversion of nascent to folded crystals, between 134 and 142°C, was observed by heating nascent UHMWPE powder in the calorimeter. In the presence of n-hexatriacontane, the melting endotherm of UHMWPE was depressed and broadened and the conversion of nascent to melt-crystallized polyethylene facilitated on heating. A melt-crystallized mixture of ordinary linear polyethylene (HDPE) and UHMWPE was not resolved on remelting. After annealing this mixture for 12 h at 130°C, HDPE was fractionated and the melting of UHMWPE was sharpened. Crystals of UHMWPE, prepared from dilute solution in xylene, show a single sharp melting endotherm and high crystallinity, but the melting peak is reduced in temperature compared to nascent crystallized powder.     

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.     

The effect of powder particle fusion on the mechanical properties of ultra-high molecular weight polyethylene

Rheo-optical and mechanical property studies with compression molded ultra-high molecular weight polyethylene specimens at different temperatures indicate that their mechanical performance is dependent on the degree of fusion of the powder particles during compression and can be enhanced by heating the polymer powder at temperatures above 220°C. Although the mechanical performance of the compression molded specimens can be improved further by solid-state drawing at a draw ratio 5, the anisotropic morphologies from molded specimen above 220°C have higher initial slope of stress to elongation, strength to break, and an outstanding elastic recovery in compreision to the compression molded specimens at 180°C.     

Synthesis of multi‐block copolymer and its compatibilization to the blends of poly(ether ether ketone) with thermotropic liquid crystalline polymer

A multiblock copolymer (BCP) containing amorphous poly(aryl ether ketone) (PAEK) and thermotropic liquid crystalline polymer (TLCP) segments was synthesized. The chemical structure and properties of BCP were characterized by fourier‐transform infrared spectrometer (FTIR), differential scanning calorimeter (DSC), gel permeation chromatograms (GPC), thermogravimetry analysis, polar light microscope (PLM), and solubility test respectively. BCP can dissolve in chloroform because of soluble PAEK block bonded with TLCP block, which was insoluble. The peak of the original PAEK oligomer was no more present in the GPC traces of the block copolymer. These facts indicated that polymer synthesized should be copolymers of the two components rather than blends. A single T_g at 138.1°C and broad melting endotherm at 315.7°C can be observed. The liquid crystalline texture of BCP showed uniformity in the view after heat treated for 10 min above its T_m under PLM. Ternary blends of poly(ether ether ketone) (PEEK)/TLCP/BCP were prepared by extrusion and characterized by DSC. DSC results showed that the crystallization temperature of PEEK phase in the blends shifted higher with the addition of TLCP. Wide angle X‐ray diffraction investigations indicated that the crystalline structure of PEEK was not disturbed by blending or compatibilizing. Scanning electron microscope and mechanical tests confirmed the compatibilizing effect of BCP. Reduction in dispersed phase TLCP size was observed when 2 phr by weight of compatibilizer was added to the blend. Measurement of the tensile properties showed increased elongation as well as improved modulus and strength to some extent. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Head to head polymers. XXV: Properties of head to head polyisobutylene

Head-to-head polyisobutylene, prepared by the Grignard coupling reaction of 2,2,3,3-tetramethyl-1,4-dibromobutane had molecular weight of 3,000 to 10,000 and was characterized by wide angle x-ray diffraction, optical microscopy and thermal behavior. Head-to-head polyisobutylene is crystalline, with a crystalline melting point of 187°C and a glass transition temperature of 87°C (measured by DSC at a scan rate of 20°/min.); these values compare to a glass transition temperature of head-to-tail polyisobutylene of similar molecular weight of ?61°C and a crystalline melting point of 5°C, which can only be observed when the sample was stretched. The maximum rate of degradation of head-to-head polyisobutylene (20°/min. programmed temperature increase) is at 315°C, 70°C lower than that of head-to-tail polyisobutylene.     

Effect of cellulose crystallinity on the progress of thermal oxidative degradation of paper

Dynamic differential scanning calorimetry (DSC) using paper samples of different compositions has evidenced varying degrees of endothermic activity prior to the exothermic, oxidative decomposition of cellulose under static air. Whereas cotton cellulose papers displayed a significant endothermic activity, wood‐pulp papers showed a much less pronounced effect or no activity at all. DSC measurements using microcrystalline cellulose submitted to different extents of milling indicated that the observed endotherm is related to the degree of crystallinity of cellulose. Crystallinity decrease is accompanied by a decrease in the area of the endothermic peak. Thermal disruption of cellulose crystalline domains is therefore believed to be the reason for the appearance of an endotherm in the thermograms of paper. The higher degree of crystallinity of cotton cellulose in comparison to wood‐pulp cellulose accounts for the more pronounced endothermic activity observed for cotton papers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 61–66, 2000     

Effect of Vibration Extrusion on Mechanical Properties and Structure of HDPE/OMMT Nanocomposites

A vibration technique was applied in extrusion molding of HDPE 6100M/OMMT nanocomposites. The results from the study suggest that samples obtained by vibration extrusion were strengthened effectively. The maximum increase percentage of tensile strength at 180°C and 200°C reached 25.14% and 21.43% respectively. It was found from microscopic structures measured by DSC, WAXD and SEM that the crystalline grains of polyethylene matrix became fine, that the orientation degree of crystalline increased and that crystallinity became perfect under the vibration field. Moreover, vibration can make nano-OMMT disperse more homogeneously in the HDPE matrix.     

Annealing-induced morphological changes in segmented elastomers

Thermal analysis has been used to study annealing-induced ordering in segmented elastomers. Twelve segmented elastomers were studied each having approximately 50% by wt hard segment content. Seven general classes of materials were examined including polyether and polyester polyurethanes, polyether polyurethane-urea, and polyether-polyester. Materials were slow cooled (?10°C min^?1) from the melt to an annealing temperature (?10°, 20°, 60°, 90° or 120°C) where they were annealed (16, 12, 8, 6 or 4 days, respectively). Annealing was followed by slow cooling (?10°C min^?1) to ?120°C after which a d.s.c. experiment was run. In general, annealing resulted in an endothermic peak at a temperature 20°–50°C above that of the temperature of annealing. This phenomenon was observed in both semicrystalline and amorphous materials. The closer the annealing endotherm was to a crystalline endotherm without exceeding it in temperature, the larger its size. Annealing endotherms resulted from hard or soft segment ordering. Only one annealing endotherm was observed for a given annealing history, even though in some materials hard and soft segments could exhibit annealing-induced morphological changes. Hard segment homopolymers were studied yielding results similar to the block polymers containing shorter sequences of the same material. This suggests that annealing-induced ordering is an intradomain phenomenon not associated with the interphase between domains, or necessarily dependent on the chain architecture of segmented elastomers.     

DSC and DMA studies on silane‐grafted and water‐crosslinked LDPE/LLDPE blends

Effects of silane grafting and water crosslinking reactions on crystallizations, melting behaviors, and dynamic mechanical properties of the LDPE/LLDPE blends are investigated using DSC and DMA. From DSC data, cocrystallization of LDPE and LLDPE does not occur, but cocrosslinking of these two polymers is evidenced at the experimental temperature of 100°C, a temperature lower than melting temperatures of both polymers. The water crosslinking reactions of the LLDPE‐rich blends enable development of a new phase having a melting endotherm in between that of LDPE and LLDPE. From the thermal fractionation data, interaction between LDPE and LLDPE is observed, and compatibilization of the blends can be achieved by the crosslinking reactions. From DMA data, the storage moduli of the blends are not found to be consistent with their degrees of crosslinking. The storage moduli of the blends are not simply determined by the degree of crosslinking but determined by very complicated but unclear factors. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1808–1816, 2001     

Crystallinity as a selection criterion for engineering properties of high density polyethylene

The Differential Scanning Calorimeter (DSC) was used to discriminate among 25 commercial high density polyethylenes (HDPE) on the basis of their degree of crystallinity and melting temperature. The area under the melting endotherm correlated directly with density and inversely with creep and thermal expansion measurements. Since high crystallinity was related to the design required properties of density, creep, and thermal expansion, DSC studies readily identified eight of the more promising polymers from the group of 25. The overall crystallization kinetics of polyethylenes with 75 percent crystallinity were analyzed by the Avrami and Fischer-Turnbull equations. Results indicate small disk-like spherulites (Avrami n = 2) following nucleation-controlled growth kinetics. These conclusions are in reasonable agreement with polarizing microscope observations. An equilibrium melting temperature between 141 and 142°C was estimated from Hoffman-Weeks plots. Processing thick parts from highly crystalline polyethylene is difficult because of the 14 percent volume change on crystallization. Higher degrees of crystallinity are associated with moderate molecular weight, so the viscosity range of these polyethylenes is not especially suited for processing by extrusion. These caveates necessitate tradeoffs between optimal design properties and processing requirements for HDPE parts.     

The thermal transition behavior of polyorganophosphazenes

The thermal transition behavior of poly[bis(trifluoroethoxyphosphazene)] (I) and two samples of poly[bis(p-chlorophenoxyphosphazene)] (II) have been studied as representative alkoxy- and aryloxy-substituted polyorganophosphazenes. Several of the polymers of this class are reported to exhibit two first-order transitions, denoted herein as T (1) for the transition from a crystalline to mesomorphic state and T_m for the true melt. Studies of these two polymers were undertaken to gain a further understanding of this behavior. Optical microscopy on a solution-cast film of I showed that the details of spherulitic morphology persist through T(1) = 90°C and remain undisturbed through the temperature interval up to T_m = 240°C. The study of II by x-ray diffraction reveals that two sharp lines are observed above T(1) = 165°C and that orientation is not randomized upon heating to temperatures as high as 238°C. Considerable improvement in the crystalline diffraction pattern results from the thermal treatment. A detailed examination was also made by differential scanning calorimetry (DSC) of the effects of cycling through T(1), annealing in the temperature interval between T(1) and T_m and for I, the influence of controlled crystallization from the melt. The results indicate that the organization in the mesomorphic state, as influenced by thermal history, has a profound affect on the peak position, area, and sharpness of the endotherm at T(1). For I, the apparent heat of fusion at T(1) is about ten times greater than at T_m, whereas for II, no DSC peak is observed at T_m = 365°C, suggesting that the ratio of the heats of fusion at T(1) and T_m is greater than 50. However, estimated volume changes at the two transitions are nearly equal. These results are compared with those of other polymers which exhibit an intermediate state of order and with molecular liquid crystals.     

A study on multiple melting of isotactic polypropylene

Multiple melting characteristics of a highly isotactic polypropylene (iPP) were studied by means of differential scanning calorimetry (DSC). Double melting characteristics were observed on melting iPP crystallized isothermally at temperatures ranging from 110 to 140°C. iPP crystallized below and above 125°C exhibited different double melting characteristics from each other. For iPP crystallized below 125°C, the single melting peak split into two peaks during slow DSC heating scans without changing the total crystallinity in the polymer. On the other hand, the double melting endotherm of iPP crystallized above 125°C seemed to come from two preexisting crystal fractions having different T_m. There existed an optimum annealing temperature range where the five-minute annealing of iPP raised T_m of the polymer significantly. The treatment also increased the crystallinity of iPP crystallized isothermally at 110°C by 12%.     

Solid‐state structure of thermally crystallized syndiotactic polystyrene

The solid‐state structure of syndiotactic polystyrene (s‐PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry (DSC), density, and X‐ray diffraction analysis. It was possible to prepare semicrystalline s‐PS containing either the pure α‐ or the pure β‐crystalline form by melt crystallizing s‐PS from 280 or 330°C. The measurements confirmed the low density of both crystalline forms, which in the case of α‐crystalline form was smaller and in the case of β‐crystalline form was only slightly larger than the density of the glassy amorphous s‐PS. An endeavor to introduce the crystalline phase in s‐PS through cold crystallization at constant temperature above the glass transition resulted in a complex ordered phase. This ordered phase, depending on the crystallization temperature, contained the planar chain mesomorphic phase and the α‐crystalline phase with a low degree of perfection (cold crystallization in the range 120–175°C) or a mixture of the α‐ and β‐crystalline forms with a high degree of perfection (cold crystallization in the range 210–260°C). The combination of DSC and X‐ray measurements enabled us to resolve the complex ordered structure in semicrystalline s‐PS after cold crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2705–2715, 2002