Crystallization behavior of a propylene-1-butene random copolymer in its α and γ modifications

A random propylene-based copolymer containing 1.0 mol% 1-butene as co-unit, synthesized with Ziegler-Natta catalyst and then fractionated to make the sample having a uniform in molecular microstructure, was investigated by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXD), and atomic force microscopy (AFM). In the DSC curves, one can see clearly the endothermic peaks corresponding to the melting of α-iPP crystals and a group of broad endothermic peaks associated to the melting of the γ-iPP crystals. Wide-angle X-ray diffraction results indicate that both the α and γ modifications can be formed in the copolymer in a wide temperature range. The γ fraction increases first with increasing the crystallization temperature at the expense of its α component, which has been explained according to crystalline structures of iPP in its α and γ forms, and then decreases with increasing crystallization temperature as the crystallization of iPP in its γ phase has been suppressed at high temperatures. The γ-iPP content in the copolymer reaches maximum at the temperature of 130 °C. The in situ X-ray diffraction characterization on the isothermal crystallization process at 130 °C indicates that, as long as the γ-iPP can be detected, it takes always ca. 25% of the overall crystallinity. This leads to the conclusion that α- and γ-iPP crystals grow simultaneously during the crystallization process. The fact that the α and γ phases cannot be distinguished by morphological observation leads to the conclusion that they may intermix within one spherulite.     

Structural evolution in microbial polyesters

The crystallization behavior of microbially synthesized poly(3-hydroxybutyrate) (PHB) and its copolymers [P(HB-co-HHx)] containing 2.5, 3.4, and 12 mol % 3-hydroxyhexanoate (HHx) comonomer and the melting of the resultant crystals were studied in detail using time-resolved small-angle X-ray scattering and differential scanning calorimetry. The polyesters were found to undergo primary crystallization as well as secondary crystallization. In the primary crystallization, the thicknesses of the lamellar crystals were sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization at a given temperature. The thickness of the lamellar crystals in the PHB homopolymer was always larger than that of the amorphous layers. In the copolymers, by contrast, the randomly distributed HHx comonomer units were found to be excluded from the lamellar crystals into the amorphous regions during the isothermal crystallization process. This interrupted the crystallization of the copolymer chains, resulting in the formation of lamellar crystals with thicknesses smaller than those of the amorphous layers. The lamellar crystals in the copolymers had lower electron densities compared to those formed in the PHB homopolymer. On the other hand, secondary crystallization favorably occurred during the later stage of isothermal crystallization in competition with the continuous primary crystallization, forming secondary crystals in amorphous regions, in particular in the amorphous layers between the primarily formed lamellar crystal stacks. Compared to the primarily formed lamellar crystals, the secondary crystals had short-range-ordered structures of smaller size, a broader size distribution, and a lower electron density.     

The mechanism of α-γ transition of poly- (vinylidene fluoride) in the miscible blends

The simultaneous DSC-FTIR was used for the observation of crystallization and melting of poly(vinylidene fluoride) (PVDF) and its blends with poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA). The isothermal crystallization was carried out under the condition of both α-form and γ-form crystallized competitively. The crystal growth rate of α -form and γ -form were evaluated from the absorbance changes at 795 cm^-1 (α -form, CH₂ rocking) and 810 cm^-1 (γ -form, CH₂ rocking) obtained by the DSC-FTIR. The crystal growth rate of γ -form decreased at the same crystallization temperature in the order of PVDF/syn-PMMA, PVDF/PEMA and PVDF/at-PMMA, which was corresponding to the order of interaction parameter. The mechanism of α -g transition of PVDF in the miscible blends with at-PMMA, syn-PMMA and PEMA was evaluated from the relationship between the decrease of α -form and the increase of γ -form. The critical crystallization temperature, at which the transformation from α -form to γ -form proceeded only in the solid state, shifted to higher temperature side in the order of interaction parameter. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystallization and melting behavior of low molar weight PEO–PPO–PEO triblock copolymers

The crystallization behavior of a series of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymers (Pluronics) was investigated using time-resolved small-angle X-ray scattering (SAXS), thermal analysis, and polarized optical microscopy. For comparison, a PEO homopolymer, PEO3K, was also included. Time-resolved SAXS during the crystallization of PEO3K shows a typical “two-step” process, i.e., in the initial stage, a metastable crystal with nonintegral folding (NIF) structure forms first, then, it transforms into integral folding (IF) structures, the IF(0) and the IF(1). In contrast with PEO3K, the PEO–PPO–PEO triblock copolymers show a “one-step” crystallization process, i.e., the PEO blocks crystallize directly into the final state and do not change with time. In thermal analysis, only one major solid–melt transition is observed during isothermal crystallization and subsequent melting for triblock copolymers. In the full temperature range, a linear crystal growth is observed. The crystal growth rates monotonously decrease with crystallization temperatures. Notches or breaks due to the NIF–IF transition as clearly seen for PEO3K cannot be recognized for Pluronics. Based on these results, we conclude that the crystallization of PEO–PPO–PEO triblock copolymers follows a “one-step” process; no metastable structure serving as an intermediate state is formed during the crystallization process within the time scale of the current experiments (<120 min).     

Synthesis and Characterization of Ethylene/Propylene Copolymers in the Whole Composition Range

Summary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC-MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples.     

CRYSTALLIZATION KINETICS OF ETHYLENE-PROPYLENE COPOLYMERS PREPARED BY LIVING COORDINATION POLYMERIZATION

In this paper,crystallization kinetics of a series of ethylene-propylene copolymers prepared by living polymerization coordination catalyzed by a fluorinated bis(phenoxyimine)Ti catalyst(FI-EP copolymers)was studied,and was compared with that of ethylene-propylene copolymers prepared by a conventional Ziegler-Natta catalyst(ZN-EP copolymers).It is found that,the Avrami exponent and the crystallization rate constant of the FI-EP and ZN-EP copolymer show similar dependence on crystallization temperature,bu...     

Crystallization and melting of narrow composition distribution polyethylenes: Investigation and implications of crystal thickening

The crystal structure produced during the isothermal crystallization of polyethylene (PE) copolymers with a broad range of comonomer concentrations was determined by the measurement of the melting endotherms directly after crystallization. PE copolymers with higher concentrations of short‐chain branches (≥10 branches per 1000 total carbon atoms) exhibited strong resistance to crystal thickening during isothermal crystallization. Negligible thickening, estimated to be only about 0.1 nm in 10 min of isothermal crystallization, was observed. The side‐chain branches apparently acted as limiting points of chain incorporation into the crystals, which exhibited great resistance to the modification of their position, that is, crystal thickening. Even with long periods (up to 8 h) of isothermal storage, crystal thickening was very small or negligible, about 0.3 nm. The crystal thickness was calculated from differential scanning calorimetry data. The behavior of copolymers with lower branching concentrations and the unbranched PE homopolymer was quite different from that of the copolymers with higher branching. Polymers with low or no branching exhibited the initial crystallization of a thinner crystal population, which thickened substantially with increasing time. The thickening observed for these lower or unbranched polymers was an order of magnitude larger, that is, 1.6–2.0 nm in 10 min of isothermal crystallization. Copolymers with higher concentrations of branching had relatively short sequence lengths of ethylene units between branch points, and this resulted in strong control over the crystal thickness by the branch points and great resistance to crystal thickening, even with long times of isothermal crystallization. Copolymers with low concentrations of branching had relatively long sequence lengths of ethylene units between branch points, and this resulted in little control over the crystal thickness by the branch points and rapid crystal thickening upon isothermal crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 235–246, 2003     

Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization

The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only α-form crystal was formed during SIC process and no transition from α1- to α2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into γ-form crystal having a low melting temperature whereas the long sequences crystallized into α-form crystal having high melting temperature.     

Copolymerization of propylene with higher α‐olefins in the presence of the syndiospecific catalyst i‐Pr(Cp)(9‐Flu)ZrCl2/MAO

The catalyst system i‐Pr(Cp)(9‐Flu)ZrCl₂/methylaluminoxane was used for the synthesis of random syndiotactic copolymers of propylene with 1‐hexene, 1‐dodecene, and 1‐octadecene as comonomers. An investigation of the microstructure by ¹³C NMR spectroscopy revealed that the stereoregularity of the copolymers decreased because of an increase in skipped insertions in the presence of the higher 1‐olefin. The melting temperature of the copolymers, as measured by differential scanning calorimetry (DSC), decreased linearly with increasing comonomer content independently of the comonomer nature. During the DSC heating cycle, an exothermic peak indicating a crystallization process was observed. The decrease in the crystallization temperature with higher 1‐olefin content, measured by crystallization analysis fractionation, indicated a small but significant dependence on the nature of the comonomer. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 128–140, 2002     

Crystallization and enzymatic degradation of novel poly(ε-caprolactone-co-propylene succinate) copolymers

A series of novel poly(ε-caprolactone-co-propylene succinate) P(CL-co-PSu) copolymers having low propylene succinate content and high molecular weight were synthesized following a combinatory scheme of ring opening and polycondensation reactions, in an attempt to obtain copolymers of sufficient performance and increased biodegradation rates. Enzymatic hydrolysis of the copolymers was studied in the presence of mixture of Rhizopus delemar and Pseudomonas cepacia lipases. Much higher hydrolysis rates, comparing to neat PCL, were proved by both mass loss measurements and scanning electron microscopy (SEM) observations of the degraded film surfaces. Thermodynamics of cocrystallization and wide-angle X-ray diffraction (WAXD) patterns were investigated to estimate the extent of comonomer cocrystallization. Results of the study showed that comonomer inclusion may hold, though the molar fraction of the comonomer in the PCL crystals is lower than in the bulk. This means that not only the observed decrease of the degree of crystallinity from about 48% for PCL to about 29% for the P(CL-co-PSu) 75/25 favours enzymatic hydrolysis, but also the enrichment of the amorphous phase in the fast degrading propylene succinate units plays its role. The non-isothermal crystallization rates of the copolymers, like the melting points, decreased substantially when the propylene succinate content exceeded 8 mol%. The activation energy of crystallization was calculated using the isoconversional method of Friedman, over the whole range of crystallization temperatures. An increase was found in the activation energy with increasing the comonomer content in the copolymers also proving the reduced symmetry along the copolymer chains due to the presence of comonomer units.     

Propylene Copolymers with Odd Carbon Number Olefins

Preliminary investigation on propylene copolymers with odd carbon number olefin are reviewed. Additional experimental data presents propylene copolymers with 1-heptene and 1-nonene having higher impact strength and lower tensile strength values than copolymers of propylene and 1-pentene.Thermoanalysis shows that the melting temperatures of the different copolymers decreases with increasing comonomer content. Slight changes were observed between the different propylene/1-heptene copolymer melting and propylene/1-pentene while the 1-nonene copolymers show broadening of the melting curves as the comonomer content increases. It was highlighted that the source of novelty for these polymers is the comonomer type and content.     

Crystallization of novel poly(ε-caprolactone)-<Emphasis Type="Italic">block</Emphasis>-poly(propylene adipate) copolymers

Poly(ε-caprolactone)-block-poly(propylene adipate) (PCL-block-PPAd) copolymers were prepared using a combination of polycondensation and ring opening polymerization of ε-CL. ¹H-NMR and ¹³C-NMR spectroscopy showed that the prepared copolymers were block. Also, the copolymer composition was calculated from NMR spectra and was found similar to the feeding ratio. The copolymers formed PCL crystals as was proved by WAXD. The crystallization rates and degree of crystallinity, measured from DSC crystallization experiments, decreased with PPAd content. The equilibrium melting points of PCL were estimated applying the Hoffmann–Weeks method and the observed melting point depression was analyzed using the Nishi–Wang equation which showed that there is some miscibility of the copolymer segments. Isothermal crystallization experiments after self-nucleation were performed to distinguish the nucleation and crystal growth stages during isothermal crystallization. The secondary nucleation theory was then used and the obtained data for crystallization rates, estimated from the inverse of the crystallization half-times, were analyzed. The resulting values for nucleation constant K _g, and also for the surface free energies and work of chain folding, increased with PPAd content due to topological restrictions.     

Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. II. Influence on crystallization and melting behavior

In this work, crystallization and melting behavior of metallocene ethylene/α‐olefin copolymers were investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results indicated that the crystallization and melting temperatures for all the samples were directly related to the long ethylene sequences instead of the average sequence length (ASL), whereas the crystallization enthalpy and crystallinity were directly related to ASL, that is, both parameters decreased with a decreasing ASL. Multiple melting peaks were analyzed by thermal analysis. Three phenomena contributed to the multiple melting behaviors after isothermal crystallization, that is, the melting of crystals formed during quenching, the melting‐recrystallization process, and the coexistence of different crystal morphologies. Two types of crystal morphologies could coexist in samples having a high comonomer content after isothermal crystallization. They were the chain‐folded lamellae formed by long ethylene sequences and the bundlelike crystals formed by short ethylene sequences. The coexistence phenomenon was further proved by the AFM morphological observation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 822–830, 2002     

Isothermal and nonisothermal crystallization kinetics of novel biobased poly(ethylene succinate-<Emphasis Type="Italic">co</Emphasis>-ethylene sebacate) copolymers from the amorphous state

In this work, the isothermal and nonisothermal crystallization kinetics of three novel biobased poly(ethylene succinate-co-ethylene sebacate) (PESSe) copolymers was systematically investigated with differential scanning calorimetry under different crystallization conditions from the amorphous state. For the isothermal cold crystallization kinetics study, the Avrami equation could well describe the crystallization process of PESSe at various crystallization temperatures. All three PESSe copolymers crystallized through the same crystallization mechanism; moreover, the overall isothermal cold crystallization rate of PESSe decreased with increasing ethylene sebacate (ESe) comonomer content. The nonisothermal cold crystallization kinetics of PESSe was also studied at different heating rates. With increasing ESe content or heating rate, the nonisothermal cold crystallization exotherm of PESSe copolymers shifted to high temperature range. Both the crystallization rate parameter and crystallization rate coefficient of PESSe copolymers decreased with increasing ESe content, indicating that PESSe copolymer with higher ESe content crystallized more slowly than that with lower ESe content. The Ozawa equation was used to analyze the nonisothermal cold crystallization kinetics of PESSe copolymers, which was found to fit the crystallization process very well.     

Thermal properties and phase transitions in blends of Nylon-6 with silk fibroin

The thermal and structural properties of binary blends of Nylon-6 (N6) and a chemically related biopolymer, Bombyx mori silk fibroin (SF), are reported in this work. Homopolymers and blends, in composition ratios of N6/SF ranging from 95/05 to 70/30, were investigated by thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and wide angle X-ray scattering (WAXS). Silk fibroin typically degrades at temperatures just above 210°C, which occurs within the melting endotherm of N6. In TG studies, the measured mass remaining was slightly greater than expected, indicating the blends had improved thermal stability. No beta sheet crystals of SF were detected by FTIR analysis of the Amide I region. Strong interaction between N6 and SF chains was observed, possibly as a result of formation of hydrogen bonds between N6 and SF chains. DSC analysis showed that the addition of SF to N6 caused a decrease in the crystallization temperature, the melting temperature of the lowest melting crystals and the crystallinity of N6. Furthermore, the α-crystallographic phase dominates and the γ-crystallographic phase was not observed in N6/SF blends, in contrast to the homopolymer N6, which contains both phases. We suggest that the addition of SF might result in changes of the chain extension of N6, which lead to the appearance of α-rather than γ-phase crystals.     

A study of the synthesis and characterization of ethylene/dicyclopentadiene copolymers using a metallocene catalyst

Copolymers of ethylene/dicyclopentadiene were produced using a Me₂Si(Ind)₂ZrCl₂/methylaluminoxane catalyst system. The melting and crystallization points of the freshly prepared copolymers steadily decreased with increasing comonomer concentration. This was attributed to increased comonomer concentration in the polymer. When the comonomer incorporation, as measured by ¹³C NMR, is plotted against the comonomer concentration in the reactor, a plateau appears at concentrations higher than 0.12 mol/L. At concentrations greater than 0.12 mol/L time dependant crosslinking begins to be observed in the copolymers after exposure to air for several months. This crosslinking is also apparent in the thermosetting behavior of the copolymers when they are allowed sufficient time to crosslink. Copolymers with lower comonomer concentrations possess melting enthalpies even after several weeks, suggesting that there is a threshold concentration of 0.12 mol/L for the crosslinking process. Tensile tests of thermoplastic samples showed that incorporation of ca. 5 mol% of comonomer into the polyethylene main chain results in a semi-elastomeric material which possesses high strain recovery and whose strain hardening is similar to that observed for the homopolymer.     

Thermal behavior of isotactic polypropylene in different content of β-nucleating agent

The effects of non-isothermal and isothermal crystallization on the formation of α- and β-phase in isotactic polypropylene (iPP) with different content of β-nucleating agent are investigated by differential scanning calorimetry (DSC). On non-isothermal crystallization, the content of β-phase and regularity of its crystals are depended on both cooling rate and the content of β-nucleating agent. The faster cooling rate is, the lower of melting peak temperature (T_mp) and crystallization peak temperature (T_cp) of α- and β-phase are. The enthalpy of fusion (∆H) of β-phase increases with cooling rate in a certain range for the sample with 0.1 wt% β-nucleating agent (G₁) and decreases for that with 0.3 wt% β-nucleating agent (G₃). On isothermal crystallization, the enthalpy of fusion of β-phase in G₁ is higher than in G₃ which is related to the efficiency of nucleation in different concentration of nucleating center in two samples.     

乙烯含量对抗冲丙烯共聚物等温结晶行为的影响

利用DSC研究了乙烯含量不同的4种抗冲丙烯共聚物的等温结晶动力学.结果表明4种样品在考察的温度范围内(126~130℃)的等温结晶动力学完全符合Avrami方程,并得到了不同结晶温度下Avrami方程的结晶动力学参数k,n和t1/2,随着样品中乙烯含量的增加,Avrami指数(n)随温度变化不大,样品的结晶过程可能属于三维扩散控制的成核增长,4种样品的结晶活化能ΔE在279.5~343.1 kJ/mol范围内,且随乙烯含量增加,结晶活化能增大,充分说明样品中的乙烯含量是影响其结晶活化能的主要因素.结晶分级分析测试结果显示,随着乙烯含量的增加,聚丙烯均聚物部分链结构规整性提高,结构规整、可结晶的长序列含量在减少,可见乙烯含量的变化规律直接决定上述参数的变化规律.     

Spectroscopy of irradiated vinyl chloride–propylene copolymers

Spectra of vinyl chloride—propylene copolymers irradiated at low temperature in vacuum have been obtained at selected temperatures in the range 130–335°K. Copolymers and PVC homopolymer pass through identical intermediate states of dehydrochlorination in which alkyl, allyl, and polyenyl radicals are observed. Substantial spectral differences between copolymers and PVC appearing in the final states of dehydrochlorination after warming above room temperature are consistent with shorter average polyene lengths in the copolymers. This probably results from termination of polyene growth by propylene comonomer. Spectral differences at long wavelength between copolymers with varying amounts of propylene are minor compared to the basic changes between copolymer and homopolymer.     

Crystallization and melting behavior of isotactic polypropylene nucleated with individual and compound nucleating agents

In this study, α-phase nucleating agent (NA) 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS), β-phase rare earth NA (WBG), and their compound NAs were introduced into isotactic polypropylene (iPP) matrix, respectively. Crystallization kinetics and subsequent melting behavior of the nucleated iPPs were comparatively studied by differential scanning calorimetry (DSC) under both isothermal and nonisothermal conditions. For the isothermal crystallization process, it is found that the Avrami model successfully described the crystallization kinetics. The active energy of nonisothermal crystallization of iPP was determined by the Kissinger method and showed that the addition of nucleating agents increased the activation energy. Melting behavior and crystalline structure of the nucleated iPPs are dependent on the nature of NAs and crystallization conditions. Higher proportion of β-phase can be obtained at higher content of β-nucleating agent and lower crystallization temperature or lower cooling rate.