Temperature dependence of crack-propagation parameters and crack morphology of the semicrystalline polyester poly(1,4-dimethylene-rans-cyclohexyl suberate)

Temperature-dependent crack propagation in the semicrystalline polymer poly(1,4-dimethylene-trans-cyclohexyl suberate) was studied as a function of forming temperature (spherulitic morphology) and molecular weight. All experiments were conducted between the glass transition temperature and the crystal melting point. Higher forming temperature (larger spherulitic size) produced lower energy to propagate (G_p). For a given morphology (single forming temperature), increasing the propagation temperature decreased the propagation energy. An equation relating G_p to the microscopic viscosity of the amorphous polymer fraction was derived. Experimental data were fitted to the equation, and Arrhenius activation energies for microscopic flow were obtained. The results show a decrease in activation energy with increased forming temperature and molecular weight, and are discussed in terms of lamellar tie-molecule population. Fracture morphology is related to thermal parameters and chain entanglements. The results indicate that much of G_p is due to plastic deformation.     

Stress relaxation of poly(1,4-dimethylene-trans-cyclohexyl suberate)

Stress relaxation experiments were conducted on poly(1,4-dimethylene-trans-cyclohexyl suberate) (MCS) as a function of preparation condition and temperature. Deconvolution of the stress relaxations provides relaxation times, which can be plotted as a function of temperature to obtain an activation energy for the relaxation process. For an MCS sample of M_n=24.8 K, MWD=2, the activation energy varies from 12.7-5.0 kcal mol^?1 with forming temperatures varying from 45–90°C. These activation energies are associated with different populations of tie molecules between lamellae. We believe that these activation energies reflect the reorientation process in the amorphous segments of the polymer during stress relaxation.     

Miscibility, crystallization and melting behaviour, and semicrystalline morphology of binary blends of polycaprolactone with poly(hydroxy ether of bisphenol A)

Blends of poly(hydroxy ether of bisphenol A) (Phenoxy) with polycaprolactone (PCL) were prepared by the coprecipitation technique. The melt miscibility of the polymers was studied by optical microscopy, light transmission measurements and dynamic mechanical analysis. The crystallization kinetics of PCL in the miscible Phenoxy/PCL blends were studied using optical microscopy and the segregation behaviour of Phenoxy due to the crystallization of PCL was examined by means of optical microscopy and small-angle X-ray diffraction, while the melting behaviour of PCL in the blend was explored by differential scanning calorimetry. The polymers were found to be miscible over the entire composition and temperature range (up to 200°C), while Phenoxy is segregated interlamellarly as well as interfibrillarly and interspherulitically during the crystallization process of PCL.     

Crystallization kinetics and morphology of poly(ethylene suberate)

The crystallization kinetics and morphology of poly(ethylene suberate) (PESub) were studied in detail with differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction. The Avrami equation could describe the overall isothermal melt crystallization kinetics of PESub at different crystallization temperatures; moreover, the overall crystallization rate of PESub decreased with increasing crystallization temperature. The equilibrium melting point of PESub was determined to be 70.8°C. Ring‐banded spherulites and a crystallization regime II to III transition were found for PESub. The Tobin equation could describe the nonisothermal melt crystallization kinetics of PESub at different cooling rates, while the Ozawa equation failed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43086.     

Microscopic surface and bulk morphology of semicrystalline poly(dimethylsiloxane)–polyester copolymers

In this study, two series of semicrystalline poly(dimethylsiloxane) (PDMS)–polyester segmented copolymers with various PDMS contents were synthesized. One series was based on polybutylene adipate (PBA) as the polyester segment and the other was based on a polybutylene cyclohexanedicarboxylate ester (PBCH) segment. The copolymers were characterized using ¹H‐nuclear magnetic resonance, size exclusion chromatography, dynamic mechanical analyses, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The microscopic surface morphology and the microscopic bulk morphology were investigated using atomic force microscopy (AFM) and transmission electron microscopy, respectively. The effects of the polyester type and the PDMS content on the crystallinity degree as well as the copolymer surface and bulk morphology at room temperature were investigated for each series. DSC and WAXD results showed the ability of the copolymers to crystallize, to various degrees, depending on the polyester type and the PDMS content. The results showed that the PDMS content had a greater influence on the crystallinity degree in the PDMS‐s‐PBCH (cycloaliphatic) copolymer series than in the PDMS‐s‐PBA (aliphatic) copolymer series. In the copolymers with a low PDMS content, the AFM images showed spherulitic crystal morphology and evidence of PDMS nanodomains in between the crystal lamellae of the ester phase on the copolymer surface. A heterogeneous distribution of the PDMS domains was also observed for these copolymers in the bulk morphology as a result of this segregation between the polyester lamellae. All the copolymers, in both series, showed microphase separation as a result of the incompatibility between the PDMS segment and the polyester segment. Three types of surfaces and bulk morphologies were observed: spherical microdomains of PDMS in a matrix of polyester, bicontinuous double‐diamond type morphology, and spherical microdomains of polyester in a matrix of PDMS as the PDMS content increases. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Nucleation‐controlled dual semicrystalline morphology of polyamide 11

Crystallization of polyamide 11 at low supercooling of the melt proceeds via heterogeneous nucleation and spherulitic growth of lamellae, while at temperatures close to the glass transition homogeneous nucleation prevails, preventing spherulite formation and leading to formation of a large number of nanometer‐sized mesophase domains. It is shown that spherulitic and non‐spherulitic crystallization at low and high supercooling of the melt, respectively, can be enforced by tailoring the cooling conditions, causing a twofold semicrystalline morphology at ambient temperature. Analysis of non‐isothermal crystallization as a function of the cooling rate, using fast scanning chip calorimetry, reveals that in the case of polyamide 11 such twofold semicrystalline morphology is predicted when cooling at rates between about 20 and 200 K s^?1, since then two separate crystallization events are observed. The prediction has been confirmed by preparation of films crystallized during ballistic cooling at different rates which then were analyzed regarding their structure using optical microscopy, X‐ray diffraction and calorimetry. The study is completed by discussion of implications of twofold non‐isothermal crystallization for structure evolution in polymer processing, as well as by providing information that such behavior is not only typical for polyamide 11 but also for isotactic polypropylene or poly(butylene terephthalate) as two further examples. © 2018 Society of Chemical Industry     

Miscibility, crystallization and melting behaviour, and morphology of binary blends of polycaprolactone with styrene-co-maleic anhydride copolymers

Blends of polycaprolactone (PCL) and random copolymers of styrene and maleic anhydride (SMA) with different maleic anhydride contents were prepared by the coprecipitation technique. The miscibility of both polymers in the melt and in the solid state was studied by means of optical microscopy, light transmission measurements and dynamic mechanical analysis. The crystallization behaviour of PCL in the miscible blends was examined using optical microscopy and the morphology of the semicrystalline PCL/SMA blends was investigated by means of small-angle X-ray diffraction measurements. Their melting behaviour was studied by differential scanning calorimetry. SMA containing 14 and 25 wt% MA was found to be miscible with PCL over the entire composition and temperature range (up to 200°C). SMA appears to segregate interlamellarly during the isothermal crystallization of PCL. The double melting behaviour of PCL in the blends was attributed to a secondary crystallization process and not to a partial melting-recrystallization-remelting process.     

Supramolecular morphology of two‐step,melt‐spun poly(lactic acid) fibers

Fibers of poly(lactic acid) produced by two‐step melt spinning have been studied. The morphology is elucidated with respect to the thermal and mechanical properties of fibers produced at cold‐draw ratios of 1–8. With atomic force microscopy and small‐angle X‐ray scattering, a fibrillar morphology is found, with microfibril diameters ranging from 30 to 60 nm. Shrinkage properties indicate that, with increasing draw ratio, the fibers undergo a transition from class 2 to class 1 within the classification proposed by Keller. A supramolecular model for the morphology of the fibers is presented that entails a highly oriented skin with a core consisting of microfibrils. The orientation of the crystalline blocks within the microfibrils is similar to what has been reported for nylon fibers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2828–2838, 2002     

Influence of the morphology on the fire behavior of a polycarbonate/poly(butylene terephthalate) blend

Polycarbonate/Poly(butylene terephthalate) (PC/PBT) blends are used in various industrial sectors, particularly in the cable industry. In this work, the fire behavior of PC/PBT blends was studied for the entire range of blend composition to investigate the relation between fire properties and blend morphology. The morphology of the binary blends used presents a phase inversion point for 25–30 wt % PBT. Various tests have been performed to characterize the fire behavior [limiting oxygen index (LOI), epiradiator test, cone calorimeter, and pyrolysis combustion flow calorimeter (PCFC)]. A change in fire behavior has been observed when the PBT content increases from 20 to 30 wt %, corresponding to the phase inversion, from a continuous rich-PC phase to a continuous rich-PBT phase. Consequently, it can be suggested that the control of the morphology of binary polymer blends is crucial to improve their fire properties. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Toughening semicrystalline poly(lactic acid) by morphology alteration

The incorporation of the triblock copolymer (PDLA-PEG-PDLA) into Poly(l-Lactic Acid) (PLLA) has produced a semicrystalline polymer of substantial modulus and strength. These improvements in mechanical properties could potentially increase the utility of this biomass-based polymer. The blended samples have a continuous amorphous phase with crystalline regions being the discontinuous portion. Micro-Raman spectroscopy revealed that a stereocomplex involving PDLA and PLLA chains exists in the crystalline region. It can be concluded that the poly(ethylene glycol), the flexible midblock component, is necessarily dispersed in these crystalline regions. Both morphological features can contribute to the improvement in mechanical properties. Therefore, the successful toughening of PLA may be achieved due to several mechanisms working synergistically.     

The influence of temperature and absorbed water on the fatigue crack propagation in nylon-6,6

The effect of absorbed water on the fatigue crack propagation (FCP) of nylon-6,6 was investigated over a range of test temperatures and is correlated with dynamic mechanical properties. Both the storage modulus, a measure of specimen stiffness, and the loss compliance, a measure of energy dissipation and hysteretic heating, influence FCP response. At a given temperature, fatigue resistance is greatest for a given water content corresponding to an optimum combination of storage modulus, E′, and loss compliance, D″. The use of an empirical shift parameter to normalize the temperature dependence of the FCP behaviour of nylon with various water contents is discussed.     

Overload effect on the fatigue crack propagation of PC/ABS alloy

The effect of single overload within an otherwise constant amplitude loading sequence on the fatigue crack propagation (FCP) behavior of the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length of the specimen. Optical and scanning electron microscopes are used to observe the features of crack surface and the process of crack tip deformation. The overload waveform has slight effect, while the overload ratio has great effect on the crack growth retardation. A small crack increment is produced during overloading. The crack growth rate reduces quickly, and then increases gradually until it reaches the steady crack growth rate level when the loading recovers to normal constant amplitude fatigue loads. Porous or dimple features govern the fatigue crack surfaces.     

Effect of plasticizer on the crystallization behavior of poly(lactic acid)

In this article, the spherulitic morphology and growth rate of the neat and plasticized poly(lactic acid) (PLA) with triphenyl phosphate (TPP) were compared and analyzed by polarizing optical microscopy with hot stage at a temperature range of 100?142°C. The spherulitic morphology of the neat PLA underwent a series of changes such as the typical Maltese Cross at less than 132°C, the disappearance of the Maltese Cross at 133°C, the irregular and distorted spherulites at higher than 134 and 142°C, respectively. For plasticized PLA, the spherulitic morphology exhibited the same changes as neat PLA, but these changes were shifted to lower temperature when compared with neat PLA. In the case of the spherulitic growth, neat PLA had the maximum value of 0.28 μm/s at 132°C, and plasticized PLA had higher values than that of neat PLA. Further analysis based on the Lauritzen–Hoffman theory was presented and results showed that the values of nucleation parameter K_g increased with TPP content. The crystallization behavior of PLA was analyzed by differential scanning calorimetry and wide‐angle X‐ray diffraction. The results showed that the degree of crystallinity of plasticized PLA markedly increased when compared with neat PLA sharply with the incorporation of plasticizer. The crystallization kinetics for the neat and plasticized PLA under isothermal crystallization at 114°C was described by the Avrami equation and the Avrami exponent is close to 2, implying that the crystallization mechanism did not change. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Swelling and elasticity of ionic poly(N-isopropylacrylamide) gels immersed in the melt of poly(ethylene glycol) chains

The equilibrium network concentration ν₂ and the elastic modulus G of a series of poly(N-isopropylacrylamide) (PNIPA) gels immersed in the melt of poly(ethylene glycol) (PEG) chains were investigated. 2-Acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was used as the ionic comonomer in the gel synthesis. The molecular weight of PEG was varied between 200 and 1000 g/mol. It was found that the decrease in ν₂ with increasing charge density of the gels is considerable weaker than expected with the assumption of Gaussian statistics. This deviation is interpreted as the ion pair and multiplet formation from the mobile counter ions of AMPS units of the gel in the presence of PEG chains. It was shown both theoretically and experimentally that the PNIPA gel undergoes a swelling-deswelling transition in PEG, if the chain length of PEG exceeds the critical value y_cr=N^1/2, where N is the network chain length calculated from the elastic moduli of gels immersed in PEG melt.     

Study on morphology of ternary polymer blends. II. Effect of composition

The composition effect on morphology of polypropylene/ethylene–propylene–diene terpolymer/polyethylene (PP/EPDM/PE) and polypropylene/ethylene–propylene–diene terpolymer/polystyrene (PP/EPDM/PS) ternary blends has been investigated. In all of the blends, polypropylene as the major phase was blended with two minor phases, that is, EPDM and PE or PS. From morphological studies using the SEM technique a core–shell morphology for PP/EPDM/PE and separated dispersed morphology for PP/EPDM/PS were observed. These results were found to be in agreement with the theoretical predictions. The composition of components affected only the size of dispersed phases and had no appreciable effect on the type of morphology. The size of each dispersed phase, whether it forms core or shell or disperses separately in matrix, can be related directly to its composition in the blend. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1138–1146, 2001     

Optimization of poly(1,4-cyclohexylidene cyclohexane-1,4-dicarboxylate) (PCCD) preparation for increased crystallinity

A key factor, which affects the crystallization temperature on cooling (T_c) of PCCD is the cis/trans isomer ratio of the cyclohexyl diester in the polymer. Isomerization of pure dimethyl-trans-1,4-cyclohexanedicarboxylate can occur during the polymerization, and the T_c on cooling decreases along with the trans content. The isomerization reaction is enhanced with temperature, time and catalyst amount, and these variables should be minimized to prepare PCCD polymers with high T_c. However, these same variables also control the molecular weight growth of the polymer, and so a compromise between the best conditions for high T_c and those for high M_w must be made. A set of optimized conditions were obtained leading to PCCD polymers with M_w of 75,000-80,000 and T_c on cooling of 164-167 °C. Solid state polymerization was used to prepare high molecular weight PCCD with a high level of crystallinity (T_c on cooling ∼193 °C). It was also shown that adding small amounts of supplementary diols facilitates PCCD preparation by ensuring that high molecular weight PCCD polymers will be obtained even when the stoichiometry of monomer feed is off by >3%, i.e. conditions which would otherwise lead to low M_w. Finally, less crystalline PCCD's have been prepared via either incorporation of diethylene glycol or increasing the cis-diester amount in the polymer.     

Effect of molecular weight distribution on fatigue crack propagation in poly(methyl methacrylate)

It is well established that both molecular weight (M) and its distribution (MD) affect many polymer properties such as mechanical behavior. Thus studies have shown that fatigue life is enhanced by increases in M. Research here has shown that with notched specimens fatigue crack propagation (FCP) rates are dramatically decreased by increasing M, even when the M is high enough that the static fraeture energy has essentially reached its asymptotic limit. In this study, specimens of poly(methyl methacrylate) containing either high- or low-M tails were prepared and characterized. The earlier finding that FCP rates are inversely related to average M was confirmed, but specific effects of M distribution were observed. At constant Mⁿ, a low-M tail had little effect on FCP resistance, while a high-M tail improved FCP resistance of polymers whose average M was too low for effective entanglements. Thus with high-M tails, it was possible to test specimens whose average M's were too low to permit machining. It is proposed that the effects noted are due to relative stabilization or destabilization of crazes ahead of the crack.     

Effect of low‐frequency melt vibration on HDPE morphology

BACKGROUND: A low‐frequency vibration‐assisted injection‐molding (VAIM) device was developed to explore the morphology of high‐density polyethylene (HDPE) injection moldings. Scanning electron microscopy, wide‐angle X‐ray diffraction and differential scanning calorimetry were used to characterize structure‐property relationships of final products prepared under different VAIM processing conditions (vibration frequency and vibration pressure amplitude) with conventional injection molding for comparison. RESULTS: It was found that increasing the vibration frequency at constant vibration pressure amplitude was beneficial for obtaining ‘shish‐kebab’ structures in the core region of VAIM specimens, and increasing the vibration pressure amplitude at constant vibration frequency was a prerequisite for achieving HDPE specimens with large‐scale lamellas, more pronounced orientation and high crystallinity. CONCLUSION: Both preferred orientation lamellas and increased crystallinity allow one to obtain strong injection moldings with the application of the melt vibration technique. Copyright © 2009 Society of Chemical Industry     

Melt processing,mechanical, and fatigue crack propagation properties of reactively compatibilized blends of polyamide 6 and acrylonitrile–butadiene–styrene copolymer

Within a IUPAC study, melt processing, mechanical, and fatigue crack growth properties of blends of polyamide 6 (PA 6) and poly(acrylonitrile–butadiene–styrene) (ABS) were investigated. We focused on the influence of reactive compatibilization on blend properties using a styrene–acrylonitrile–maleic anhydride random terpolymer (SANMA). Two series of PA 6/ABS blends with 30 wt % PA 6 and 70 wt % PA 6, respectively, were prepared with varying amounts of SANMA. Our experiments revealed that the morphology of the matrix (PA 6 or ABS) strongly affects the blend properties. The viscosity of PA 6/ABS blends monotonically increases with SANMA concentration because of the formation of high‐molecular weight graft copolymers. The extrudate swell of the blends was much larger than that of neat PA 6 and ABS and decreased with increasing SANMA concentrations at a constant extrusion pressure. This observation can be explained by the effect of the capillary number. The fracture resistance of these blends, including specific work to break and impact strength, is lower than that of PA 6 or ABS alone, but increases with SANMA concentration. This effect is most strongly pronounced for blends with 70 wt % PA 6. Fatigue crack growth experiments showed that the addition of 1–2 wt % SANMA enhances the resistance against crack propagation for ABS‐based blends. The correlation between blend composition, morphology and processing/end‐use properties of reactively compatibilized PA 6/ABS blends is discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of molecular architecture on two‐step,melt‐spun poly(lactic acid) fibers

Fibers of poly(lactic acid) (PLA) produced by two‐step melt spinning have been studied. The PLA resins used contain a 96:04 ratio of L:D stereochemical centers; however, one of the materials is branched by a peroxide treatment. The thermal, mechanical, and morphological properties of the fibers are compared for the two different molecular architectures. In the branched material, at least some of the branches exceed the entanglement molecular weight. The branched material is accordingly characterized by greater shear and extensional viscosity than the linear material. Fiber properties are highly influenced by the draw ratio; both branched and linear materials reach a plateau of about 35% crystallinity. The branched polymer reaches the plateau at a lower draw ratio, and this is indicative of faster crystallization kinetics. Both materials shrink in boiling water, and the amount of shrinkage decreases with increasing draw ratio. At an intermediate draw ratio of 6, the branched material is characterized by significantly larger shrinkage. With small‐angle X‐ray scattering and atomic force microscopy, the morphology is found to be fibrillar. Microfibril diameters range from approximately 20 to 30 nm and are almost identical for the two molecular architectures studied. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2839–2846, 2002