Isothermal crystallization kinetics of polypropylene latex–based nanocomposites with organo‐modified clay

The effect of organo‐modified clay (Cloisite 93A) on the crystal structure and isothermal crystallization behavior of isotactic polypropylene (iPP) in iPP/clay nanocomposites prepared by latex technology was investigated by wide angle X‐ray diffraction, differential scanning calorimetry and polarized optical microscopy. The X‐ray diffraction results indicated that the higher clay loading promotes the formation of the β‐phase crystallites, as evidenced by the appearance of a new peak corresponding to the (300) reflection of β‐iPP. Analysis of the isothermal crystallization showed that the PP nanocomposite (1% C93A) exhibited higher crystallization rates than the neat PP. The unfilled iPP matrix and nanocomposites clearly shows double melting behavior; the shape of the melting transition progressively changes toward single melting with increasing crystallization temperature. The fold surface free energy (σ_e) of polymer chains in the nanocomposites was lower than that in the PP latex (PPL). It should be reasonable to treat C93A as a good nucleating agent for the crystallization of PPL, which plays a determinant effect on the reduction in σ_e during the isothermal crystallization of the nanocomposites. The activation energy, ΔE_a, decreased with the incorporation of clay nanoparticles into the matrix, which in turn indicates that the nucleation process is facilitated by the presence of clay. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1927–1938, 2010     

Poly(butylene terephthalate)/clay nanocomposite compatibilized with poly(ethylene‐co‐glycidyl methacrylate). II. Nonisothermal crystallization

Melting behaviors and nonisothermal crystallization of poly(butylene terephthalate)/poly(ethylene‐co‐glycidyl methacrylate) (PBT/PEGMA), PBT/commercial modified montmorillonite clays (PBT/Clay), and PBT/exfoliated silicates (PBT/PEGMA/Clay) nanocomposites were studied by wide‐angle X‐ray diffraction and differential scanning calorimeter. PEGMA is used as a compatibilizer. For both isothermally and nonisothermally crystallized samples, PEGMA facilitates the recrystallization of PBT during the heating scans, and leads to a less degree of perfection of the crystals. However, the clay hinders the recrystallization growth during heating scans, and increases perfection of the crystals. Nonisothermal crystallization kinetics was described by kinetic models and undercooling was taken into account. The PEGMA would lead to an increase of the blend viscosity, rendering the chains less mobile and lower the crystallizability of PBT in PBT/PEGMA. The well‐dispersed exfoliated silicates in PBT/PEGMA/Clay cause a large number of nuclei to precede crystallization. The fold surface free energy (σ_e) and activation energy also supported the interpretation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 564–576, 2008     

Isothermal crystallization studies of poly(butylene terephthalate) composites

Different crystallization kinetic models (Avrami and Tobin) have been applied to study the crystallization kinetics of virgin poly(butylene terephthalate) (PBT) and filled PBT systems under isothermal experimental conditions. The experimental data have been analyzed with a nonlinear, multivariable regression program. The kinetic parameters for the isothermal crystallization have been determined. The analysis results indicate that both models satisfactorily represent the isothermal crystallization kinetics. PBT crystallizes most slowly. The presence of nanoclays or nanofibers, added as fillers, enhances the crystallization rate of PBT composites. An analysis of the kinetic data with the Avrami and Tobin models has shown little change in the crystallization exponent compared with that of virgin PBT. The crystallization rate constant decreases with a rise in the temperature for the two models. This trend has been observed for similar polyester systems reported in the literature. The dispersion of the clay layers in the PBT nanocomposites has been characterized with wide‐angle X‐ray diffraction and transmission electron microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1344–1353, 2007     

Characterization of PBT/POSS nanocomposites prepared by in situ polymerization of cyclic poly(butylene terephthalate) initiated by functionalized POSS

Novel poly(butylene terephthalate) (PBT)/polyhedral oligomeric silsesquioxane (POSS) nanocomposites were synthesized by ring‐opening polymerization of cyclic poly(butylene terephthalate) initiated by functionalized POSS with various feed ratios. The impact of POSS incorporation on melting and crystallization behaviors of PBT/POSS nanocomposites was investigated by means of X‐ray diffraction and differential scanning calorimetry. It was found that the novel organic–inorganic association result in the significant alterations in the melting and crystallization behavior of PBT. Thermal studies confirmed that the incorporation of POSS can enhance the thermal stability of the polymers, and the copolymer glass transition temperature increased with the increasing of POSS macromonomer content. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1853–1859, 2010     

Preparation and characterization of PP/clay nanocomposites based on modified polypropylene and clay

X‐ray diffraction and differential scanning calorimeter (DSC) methods have been used to investigate the crystallization behavior and crystalline structure of hexamethylenediamine (HMDA)‐modified maleic‐anhydride‐grafted polypropylene/clay (PP‐g‐MA/clay) nanocomposites. These nanocomposites have been prepared by using HMDA to graft the PP‐g‐MA (designated as PP‐g‐HMA) and then mixing the PP‐g‐HMA polymer in hot xylene solution, with the organically modified montmorillonite. Both X‐ray diffraction data and transmission electron microscopy images of PP‐g‐HMA/clay nanocomposites indicate that most of the swellable silicate layers are exfoliated and randomly dispersed into PP‐g‐HMA matrix. DSC isothermal results revealed that introducing 5 wt % of clay into the PP‐g‐HMA structure causes strongly heterogeneous nucleation, which induced a change of the crystal growth process from a three‐dimensional crystal growth to a two‐dimensional spherulitic growth. Mechanical properties of PP‐g‐HMA/clay nanocomposites performed by dynamic mechanical analysis show significant improvements in the storage modulus when compared to neat PP‐g‐HMA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3242–3254, 2005     

Multiple melting behavior of poly(lactic acid) filled with modified carbon black

Two melting peaks are generally observed in a heating scan for isothermally crystallized poly(lactic acid) (PLA)/carbon black (CB) and PLA/modified carbon black (MCB) composites. To investigate the origin of the above double melting behavior, the melting behavior after isothermal crystallization was analyzed with differential scanning calorimetry, wide‐angle X‐ray diffraction, and small angel X‐ray scattering techniques. The double melting of the crystallized samples can be explained by the model of two populations of lamellae, the double peaks of low and high temperatures are contributed to the melting of the small lamellae produced by secondary crystallization and that of the major crystals formed in the primary crystallization process, respectively. Spherulitic growth rates of the neat PLA and PLA/MCB composite were analyzed and the occurrence of a regime transition was demonstrated. For the PLA, a clear regime transition was observed at around 125 °C. For the PLA/MCB, it occurred at 130 °C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1971–1980, 2009     

Study of clay nanocomposites of the biodegradable polyhexamethylene succinate. Application of isoconversional analysis to nonisothermal crystallization

Nanocomposites of organomodified montmorillonites and the biodegradable polyester derived from hexanediol and succinic acid were prepared by the solution‐casting method using chloroform as solvent. Samples were studied by means of X‐ray diffraction and transmission electron microscopy. Intercalated structures differentiated by the stacking mode between silicate layers were observed. The highest variability in interlayer spacing was found when C30B organoclay was added. In this case, hydroxyl groups of the modifier could interact with polar carbonyl groups of the polyester. Thermal stability and crystallization behavior under both isothermal and nonisothermal conditions were evaluated. The overall crystallization rate of the intercalated nanocomposites was higher than that of the neat polyester due to a significant increase in their nucleation density, which compensated for their lower crystal growth rate. Isoconversional analysis was used to determine effective activation energies and to estimate nucleation and transport energy parameters from nonisothermal hot crystallization experiments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2234–2248, 2008     

Influence of low multi‐walled carbon nanotubes loadings on the crystallization behavior of biodegradable poly(butylene succinate) nanocomposites

Biodegradable poly(butylene succinate) (PBSU)/carboxyl‐functionalized multi‐walled carbon nanotubes (f‐MWNTs) nanocomposites were prepared via solution casting method at low f‐MWNTs loadings of 0.5 and 1 wt%, respectively, in this work. Scanning and transmission electron microscopic observations reveal a fine dispersion of f‐MWNTs throughout the PBSU matrix. Non‐isothermal melt crystallization at different cooling rates, isothermal melt crystallization at different crystallization temperatures, spherulitic morphology, and crystal structure of neat PBSU and its nanocomposites were investigated with various techniques in detail. The addition of f‐MWNTs is found to enhance the crystallization of PBSU, apparently in the nanocomposites during both nonisothermal and isothermal melt crystallization, due to the heterogeneous nucleation effect; however, the crystallization mechanism and crystal structure of PBSU remain almost unchanged. Effect of the presence of f‐MWNTs and their loadings on the thermodynamic driving force for nucleation and nucleation activity of PBSU was evaluated quantitatively through two methods. Moreover, it is found that incorporating with 1 wt% f‐MWNTs significantly improves the storage modulus of PBSU in the nanocomposites by about 147% at room temperature as compared with that of neat PBSU. Copyright © 2010 John Wiley & Sons, Ltd.     

Isothermal crystallization kinetics of poly(3‐hydroxybutyrate)/layered double hydroxide nanocomposites

Poly(3‐hydroxybutyrate) (PHB)/layered double hydroxides (LDHs) nanocomposites were prepared by mixing PHB and poly(ethylene glycol) phosphonates (PEOPAs)‐modified LDH (PMLDH) in chloroform solution. Both X‐ray diffraction data and TEM micrographs of PHB/PMLDH nanocomposites indicate that the PMLDHs are randomly dispersed and exfoliated into the PHB matrix. In this study, the effect of PMLDH on the isothermal crystallization behavior of PHB was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. Isothermal crystallization results of PHB/PMLDH nanocomposites show that the addition of 2 wt % PMLDH into PHB induced more heterogeneous nucleation in the crystallization significantly increasing the crystallization rate and reducing their activation energy. By adding more PMLDH into the PHB probably causes more steric hindrance of the diffusion of PHB, reducing the transportation ability of polymer chains during crystallization, thus increasing the activation energy. The correlation among crystallization kinetics, melting behavior and crystalline structure of PHB/PMLDH nanocomposites can also be discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3337–3347, 2006     

Nonisothermal crystallization behavior of syndiotactic polystyrene/montmorillonite nanocomposites

X‐ray diffraction methods and differential scanning calorimetry were used to investigate the crystalline structure and crystallization kinetics of syndiotactic polystyrene (sPS)/clay nanocomposites. X‐ray diffraction data showed the presence of polymorphism in sPS/montmorillonite (MMT) nanocomposites, which was strongly dependent on the processing conditions (premelting temperature and cooling rate) of the sPS/MMT nanocomposites and on the content of MMT in the sPS/MMT nanocomposites. The α‐crystalline form could be transformed into β‐crystalline forms at higher premelting temperatures. The nonisothermal melt‐crystallization kinetics and melting behavior of the sPS/MMT nanocomposites were also studied at various cooling rates. The correlation of the crystallization kinetics, melting behavior, and crystalline structure of the sPS/MMT nanocomposites was examined. The results indicated that the addition of a small amount of MMT to sPS caused a change in the mechanism of nucleation and the crystal growth of the sPS crystallite. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 560–570, 2003     

High‐density polyethylene layered silicate nanocomposites: Effect of mechanical stretching on the crystal‐to‐crystal transformations,morphology, and extent of exfoliation

High‐density polyethylene/clay nanocomposites were elongated until breakage to investigate the effect of the mechanical stretching on the crystal‐to‐crystal transformations and their morphology. Crystalline transformations of the polymer matrix were studied via Fourier transform infrared spectroscopy, differential scanning calorimetry, and X‐ray diffraction measurements. It was concluded that the stress‐induced crystal‐to‐crystal transformations from orthorhombic structures to monoclinic and pseudohexagonal structures as well as the back‐transformation during relaxation were hindered by the presence of the clay. X‐ray diffraction studies on stretched samples showed that the mechanical stretching led from an intercalated structure to an almost exfoliated structure. These findings agreed with scanning electron micrographs, in which the beneficial effect of stretching on the exfoliation of the clay was evident. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 914–924, 2006     

Miscibility and crystallization behavior of PBT/epoxy blends

The miscibility and the isothermal crystallization kinetics for PBT/Epoxy blends have been studied by using differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The Avrami exponents n were obtained for PBT/Epoxy blends. An addition of small amount of epoxy resin (3%) leads to an increase in the number of effective nuclei, thus resulting in an increase in crystallization rate and a stronger trend of instantaneous three‐dimensional growth. For isothermal crystallization, crystallization parameter analysis showed that epoxy particles could act as effective nucleating agents, accelerating the crystallization of PBT component in the PBT/Epoxy blends. The Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT/Epoxy 97/3 had lower nucleation constant K_g than 100/0, 93/7, and 90/10 PBT/Epoxy blends. Analysis of the crystallization data of PBT/Epoxy blends showed that crystallization occurs in regime II. The fold surface free energy, σ_e = 101.7–58.0 × 10^?3 J/m², and work of chain folding, q = 5.79–3.30 kcal/mol, were determined. The equilibrium melting point depressions of PBT/Epoxy blends were observed and the Flory–Huggins interaction parameters were obtained. It indicated that these blends were thermodynamically miscible in the melt. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1320–1330, 2006     

Nucleation ability of multiwall carbon nanotubes in polypropylene composites

The nonisothermal crystallization of multiwall carbon nanotube (MWNT)/isotactic polypropylene (iPP) nanocomposites was investigated. The results derived from the differential scanning calorimetry curves (onset temperature, melting point, supercooling, peak temperature, half‐time of crystallization, and enthalpy of crystallization) were compared with those of neat iPP. The data were also processed according to Ozawa's theory and Dobreva's approach. These results and X‐ray diffraction data showed that the MWNTs acted as α‐nucleating agents in iPP. Accordingly, MWNT/iPP was significantly different from neat iPP: A fibrillar morphology was observed instead of the usual spherulites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 520–527, 2003     

Nonisothermal cold crystallization behavior and kinetics of polylactide/clay nanocomposites

The nonisothermal cold crystallization behavior of intercalated polylactide (PLA)/clay nanocomposites (PLACNs) was studied using differential scanning calorimetry, polarized optical microscope, X‐ray diffractometer, dynamic mechanical thermal analysis, and Fourier transform infrared spectrometer. The results show that both the cold crystallization temperature (T_cc) and melting point (T_m) of PLA matrix decreases monotonously with increasing of clay loadings, accompanied by the decreasing degree of crystallinity (X_c%) at the low heating rates (≤5 °C/min). However, the X_c% of PLACNs presents a remarkable increase at the high heating rate of 10 °C/min in contrast to that of neat PLA. The crystallization kinetics was then analyzed by the Avrami, Jezioney, Ozawa, Mo, Kissinger and Lauritzen–Hoffman kinetic models. It can be concluded that at the low heating rate, the cold crystallization of both the neat PLA and nanocomposites proceeds by regime III kinetics. The nucleation effect of clay promote the crystallization to some extent, while the impeding effect of clay results in the decrease of crystallization rate with increasing of clay loadings. At the high heating rate of 10 °C/min, crystallization proceeds mainly by regime II kinetics. Thus, the formation of much more incomplete crystals in the PLACNs with high clay loadings due to the dominant multiple nucleations mechanism in regime II, may have primary contribution to the lower crystallization kinetics, also as a result to the higher degree of crystallinity and lower melting point in contrast to that of neat PLA. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1100–1113, 2007     

Isothermal and nonisothermal crystallization kinetics of nylon 6/functionalized multi‐walled carbon nanotube composites

The well dispersion of functionalized multi‐walled carbon nanotube (f‐MWCNT) in nylon 6 matrix was prepared by solution mixing techniques. The isothermal and nonisothermal crystallization kinetics of nylon 6 and nylon 6/f‐MWCNT nanocomposites were studied by differential scanning calorimetry (DSC), X‐ray diffraction and polarized optical microscopy analysis. DSC isothermal results revealed that the activation energy of nylon 6 extensively decreased by adding 1 wt % f‐MWCNT into nylon 6, suggesting that the addition of small amount of f‐MWCNT probably induces the heterogeneous nucleation. Nevertheless, the addition of more f‐MWCNT into nylon 6 matrix reduced the transportation ability of polymer chains during crystallization process and thus increased the activation energy. The nonisothermal crystallization of nylon 6/f‐MWCNT nanocomposites was also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 158–169, 2008     

Poly(vinylidene fluoride)/clay nanocomposites prepared by melt intercalation: Crystallization and dynamic mechanical behavior studies

The preparation and properties of poly(vinylidene fluoride) (PVDF)/clay nanocomposites are reported for the first time. PVDF/clay nanocomposites were prepared by melt intercalation with organophilic clay. The composites were characterized with X‐ray diffraction, differential scanning calorimetry, and dynamic mechanical analysis. X‐ray diffraction results indicated intercalation of the polymer into the interlayer spacing. PVDF in the nanocomposites crystallized in the β form. Differential scanning calorimetry nonisothermal curves showed an increase in the melting and crystallization temperatures along with a decrease in crystallinity, as evidenced by the melting and crystallization peaks. Isothermal crystallization studies showed an enhanced rate of crystallization with the addition of clay, as evidenced by a reduction in the crystallization time. Dynamic mechanical analysis indicated significant improvements in the storage modulus over a temperature range of ?100 to 150 °C. The tan δ peak signifying the glass‐transition temperature of PVDF shifted to higher temperatures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1682–1689, 2002     

Thermomechanical properties and crystallization behavior of layered double hydroxide/poly(ethylene terephthalate) nanocomposites prepared by in‐situ polymerization

Thermomechanical properties and crystallization behavior of poly(ethylene terephthalate) (PET) nanocomposites containing layered double hydroxide (LDH) were investigated. To enhance the compatibility between PET matrix and LDH, dimethyl 5‐sulfoisophthalate (DMSI) anion intercalated LDH (LDH‐DMSI) was synthesized by coprecipitation method, and its structure was confirmed by Fourier transform infrared (FTIR) spectrometer and X‐ray diffraction (XRD) measurements. Then, PET nanocomposites with LDH‐DMSI content of 0, 0.5, 1.0, and 2.0 wt% were prepared by in‐situ polymerization. The dispersion morphologies were observed by transmission electron microscopy (TEM) and XRD, showing that LDH‐DMSI was exfoliated in PET matrix. Thermal and mechanical properties, such as thermal stability, tensile modulus, and tensile yield strength of nanocomposites, were enhanced by exfoliated LDH‐DMSI nanolayers. However, elongation at break was drastically decreased with LDH loading owing to the increased stiffness and microvoids. The effect of exfoliated nanolayers, which acted as a nucleating agent confirmed by differential scanning calorimeter (DSC), on the microstructural parameters during isothermal crystallization, was analyzed by synchrotron small‐angle X‐ray scattering (SAXS). It is believed that nanocomposites could be crystallized more easily owing to the increased nucleation sites, which lead to the decrease of average amorphous region size and the long period with the increase of LDH‐DMSI content. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 28–40, 2007     

Effects of divinylbenzene‐maleic anhydride copolymer hollow microspheres on crystallization behaviors,mechanical properties and heat resistance of poly(l‐lactide acid)

Divinylbenzene‐maleic anhydride copolymer hollow microspheres (DMs) were used as novel organic nucleating agents to promote crystallization of poly(l‐lactide acid) (PLLA). The effects of these DMs on crystal behaviors of the PLLA were investigated by differential scanning calorimeter (DSC), polarizing optical microscopy (POM), and wide angle X‐ray diffraction (WAXD). Both isothermal and non‐isothermal processes in DSC demonstrated that the DMs significantly altered the crystal behaviors of PLLA as both crystallization velocity and degree of crystallinity increased with increasing DM loadings from 0 to 3%. Our POM results also indicated that as nucleating agents, the DMs promoted nucleating densities and decreased spherulitic sizes. In addition, WAXD suggeted that the addition of DMs did not induce new types of crystals. Finally, our results showed that the ductility of the PLLA was enhanced by a small amount of DMs during the PLLA crystallization process since 0.5% DMs added to the PLLA resulted in 1.4‐fold increase in the elongation at break in comparision with the neat PLLA.     

Melt blending of ethylene‐vinyl alcohol copolymer/clay nanocomposites: Effect of the clay type and processing conditions

Ethylene‐vinyl alcohol copolymer (EVOH)/clay nanocomposites were prepared via dynamic melt blending. The effect of the processing parameters on blends containing two clay types in different amounts was examined. The blends were characterized with a Brabender plastograph and capillary rheometer, differential scanning calorimetry, dynamic mechanical thermal analysis (DMTA), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). XRD showed advanced EVOH intercalation within the galleries, whereas TEM images indicated exfoliation, thereby complementing the XRD data. A dilution process with EVOH and clay treatment in an ultrasonic bath before melt blending did not add to the intercalation level. Different trends were observed for the EVOHs containing two different clay treatments, one claimed to be treated for EVOH and the other for amine‐cured epoxy. They reflected the differences in the amounts of the strongly interacting polymer for the two nanocomposites. Thermal analysis showed that the melting temperature, crystallization temperature, and heat of fusion of the EVOH matrix sharply decreased with both increasing clay content and processing times. Significantly higher viscosity levels were obtained for the blends in comparison with those of the neat polymer. The DMTA spectra showed higher glass‐transition temperatures for the nanocomposites in comparison with those of the neat EVOH. However, at high clay loadings, the glass‐transition temperature remained constant, presumably because of an adverse plasticizing effect of the low moleculared mass onium ions treating the clays. The storage modulus improved when clay treated for EVOH was used, and it deteriorated when amine‐cured epoxy clay was incorporated, except for the sonicated clay. TGA results showed significant improvements in the blends' thermal stability in comparison with that of the neat EVOH, which, according to TEM, was greater for the intercalated structures rather than for exfoliated ones. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1741–1753, 2002     

Effect of the premelting temperature and sample thickness on the polymorphic behavior of syndiotactic polystyrene/clay nanocomposites

X‐ray diffraction methods and differential scanning calorimetry thermal analysis have been used to investigate the structural changes of syndiotactic polystyrene (sPS)/clay nanocomposites. sPS/clay nanocomposites have been prepared by the mixing of sPS polymer solutions with organically modified montmorillonite. X‐ray diffraction data and differential scanning calorimetry results indicate that the dominating crystal forms and their relative fractions in sPS and sPS/clay nanocomposites are different for various premelting temperatures (T_max's). Higher T_max's favor the formation of the thermodynamically more stable β‐crystalline form, and its relative fraction has been obtained from the X‐ray diffraction data in the range of 11.5–13°. The intensity of the X‐ray diffraction data in the range of 11.5–13° decreases as the thickness of sPS/clay nanocomposites decreases from 150 to 20 μm. At the same time, the intensity of the X‐ray data in the range of 6–7° becomes sharper as the thickness of sPS/clay nanocomposites decreases. The calculation ratio based on the peak intensities at 6.2 and 6.8° for sPS/clay nanocomposites of equal thickness and crystallinity in the pure β and α forms has also been determined in this study. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1730–1738, 2003