Tensile properties of random copolymers of propylene with ethylene and 1-butene: effect of crystallinity and crystal habit

The tensile modulus of elasticity and yield strength of semicrystalline random copolymers of propylene with different amount on ethylene or 1-butene co-units were analyzed as a function of the crystallinity and the crystal habit/shape. Samples were prepared by cooling the melt to ambient temperature, and subsequent annealing at elevated temperature. Variation of the cooling rate between 10⁻¹ and 10³ K s⁻¹ and of the temperature of annealing allowed preparation of semicrystalline specimens with either lamellar or non-lamellar crystals of different size, and with different crystallinity between about 30 and 70%. Young’s modulus and yield strength increase with increasing crystallinity and consistently are lower for samples containing nodular, that is, almost isometric, non-lamellar crystals of low aspect ratio. For samples of identical crystallinity and crystal habit, an only minor effect of presence of co-units in the crystalline and amorphous phases is observed.     

Mesophase formation in poly(propylene-ran-1-butene) by rapid cooling

The effect of random insertion of low amount of 1-butene of less than about 11 mol% into the isotactic polypropylene chain on structure formation at non-isothermal crystallization at different rate of cooling was investigated by X-ray scattering, density measurements, and atomic force and polarizing optical microscopy. Emphasis is put on the evaluation of the condition of crystallization for replacement of lamellar crystals by mesomorphic nodules on increasing the cooling rate/supercooling. In the polypropylene homopolymer, mesophase formation occurs on cooling at rates larger about 150–200 K s⁻¹, while in case of poly(propylene-ran-1-butene) mesophase formation is observed on cooling at a lower rate of about 100 K s⁻¹. It is suggested that the lowering of the critical rate of cooling for mesophase formation in poly(propylene-ran-1-butene) is due to a reduction of the maximum rate of formation of monoclinic/orthorhombic crystals at low supercooling, compared to the homopolymer. The data of the present study allowed the establishment of a non-equilibrium phase diagram which shows ranges of existence of phases as a function of the cooling rate on solidification the quiescent liquid and the concentration on 1-butene co-units.     

In situ X-ray analysis of mesophase formation in random copolymers of propylene and 1-butene

The solidification of random isotactic copolymers of propylene and 1-butene has been followed in real time by wide-angle X-ray scattering as a function of the rate of cooling the quiescent liquid. The experimental setup allowed simultaneous recording of cooling curves—sample temperature as a function of time—and X-ray patterns at high sampling rate of 20 Hz. This approach allowed establishing a correlation between cooling rate, temperature of crystallization/mesophase formation, and X-ray structure, which formerly has only been observed ex situ, after completion of structure formation during cooling and subsequent aging. It is quantitatively confirmed that addition of 1-butene co-units into the propylene chain allows mesophase formation on cooling the melt at distinctly lower rate than in case of the homopolymer. The experimental results are compiled into a continuous cooling transformation (CCT) diagram and compared with data obtained earlier on random copolymers of propylene with ethylene.     

Effect of structure on light transmission in isotactic polypropylene and random propylene-1-butene copolymers

The structure of semi-crystalline isotactic polypropylene and random isotactic copolymers of propylene and 1-butene at the nanometer and micrometer scales is controlled by the pathway of melt-crystallization, and the concentration of chain defects. Rapid cooling of the melt results in formation of mesomorphic nodules, being not organized in a higher order superstructure. Slow melt-crystallization, in contrast, allows formation of lamellae. These lamellae arrange within spherulites of a size which is decreased in copolymers. Analysis of the light transmission reveals a distinctly higher transparency in non-spherulitic preparations. Spherulitic samples exhibit a lower transparency, increasing with decreasing size of spherulites. Annealing of quenched preparations at elevated temperature leads to an increase of the crystallinity and of the dimensions of crystals, without affecting their habit, and their higher order organization. The transparency is only slightly decreased in these specimens. It can be demonstrated that samples of largely different transparency but identical crystallinity can be generated without the use of optical clarifiers/nucleation agents.     

Morphological and kinetic partitioning of comonomer in random propylene 1-butene copolymers

The partitioning of the 1-butene co-unit between crystalline and non-crystalline regions of random, homogeneous propylene 1-butene copolymers (PB) has been studied by WAXD, ¹³C NMR, and FTIR in a series of copolymers with a concentration of 1-butene ranging from 2 to ～ 20 mol%. A partial inclusion of the 1-butene co-unit in the crystallites is identified by the expansion of the unit cell, and quantified by extracting ¹³C NMR spectra of the crystalline regions. For slowly cooled copolymers, about 30% of the chain’s 1-butene co-units are incorporated into the crystallites. Analyses of FTIR absorbances associated with crystalline 1-butene provide additional quantitative information on the morphological partitioning of the co-unit and give evidence to support that the incorporation of the comonomer into the crystalline regions is controlled by crystallization kinetics. The presence of the comonomer in the crystalline region affects the observed vibration of the most sensitive iPP 3/1 regularity bands associated with the evolution of crystallites, i.e. 841 cm^?1 (12 isotactic units). The frequency of this band shifts toward higher values with increasing comonomer and with increasing undercooling, in support of an increasing concentration of entrapped crystalline 1-butene. The frequency shift is absent in copolymers with co-units that are excluded from the crystalline regions, such as the 1-octene comonomer.     

X-ray study of crystallization of random copolymers of propylene and 1-butene via a mesophase

Wide-angle X-ray scattering has been employed for evaluation of the crystal structure of cold-crystallized random copolymers of propylene with 1-butene. The crystallization procedure included the formation of a metastable mesophase at ambient temperature by quenching the quiescent melt, and the reorganization of the semimesomorphic structure into a semicrystalline structure on subsequent heating/annealing. Research was performed to gain information about incorporation/exclusion of 1-butene chain defects into the crystalline phase when formed via the intermediate step of mesophase formation. The X-ray data suggest that 1-butene co-units up to a concentration of at least 11 mol-% are incorporated into both the mesophase formed on quenching and crystals formed on subsequent heating/annealing according to their concentration in the chain. This observation is in agreement with results obtained on samples which were crystallized directly from the supercooled liquid state, indicating that incorporation of 1-butene co-units into the crystalline phase is not kinetically controlled. Furthermore, information about the α/γ polymorphism and the kinetics of the transition from the semimesomorphic into semicrystalline structure are provided.     

Tailoring the rigid amorphous fraction of isotactic polybutene-1 by ethylene chain defects

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structure of crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF). The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers and their accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasing concentration of co-units. A specific RAF was determined by normalization of the RAF to the crystal fraction. While in the butene-1 homopolymer a specific RAF of 20–30% is detected, it increases to more than 100% in copolymers with 5–10 mol% of ethylene co-units, being in accordance with the previously observed increase of the free energy of the crystal fold-surface due to copolymerization. It has also been shown that the specific RAF increases with decreasing temperature of crystallization, due to formation of a fold-surface of lower perfection, containing an increased number of chain segments traversing the crystalline-amorphous interface.     

Blends of propylene-ethylene and propylene-1-butene random copolymers: I. Morphology and structure

Samples of propylene-ethylene (EP) and propylene-(1-butene) (BP) random copolymers with various comonomer content (2-3.1 wt% ethylene, 9.9 wt% 1-butene), were melt-mixed in Brabender internal mixer at various compositions (25/75, 50/50, 75/25). Films of copolymers and blends, as well as of a homopolymer sample (iPP), obtained by compression moulding and with different thermal history were characterized by optical and scanning electron microscopy (OM, SEM), small-angle light scattering (SALS), small- and wide angle X-ray scattering (SAXS, WAXS) and differential scanning calorimetry (DSC). It was found that all copolymers and blends studied crystallized exclusively in monoclinic α-modification forming spherulitic structure in a very broad undercooling range. The average size of spherulites is smaller in the copolymer containing 1-butene as compared to those containing ethylene or to iPP homopolymer, due to enhanced heterogeneous nucleation in BP copolymer. SEM microscopic observations demonstrated that EP and BP copolymers were miscible at all examined compositions and form homogeneous blends. Structural and morphological analysis indicated that the comonomer units are incorporated into growing crystallites in both EP and BP copolymers, while the non-crystallizing material is rejected out of the crystallites. For small concentrations of comonomer some of non-crystallizing species are pushed ahead of the front of growing spherulite into interspherulitic regions. For higher comonomer concentration these species are mostly trapped in intraspherulitic regions. Melting behavior of copolymers reflects the incorporation of comonomer into crystalline phase: melting temperature and crystallinity degree decrease in copolymers and blends as compared to plain iPP.     

Formation and reorganization of the mesophase of random copolymers of propylene and 1-butene

Fast scanning chip calorimetry (FSC) has been employed to study the kinetics of formation of the mesophase of random copolymers of propylene and 1-butene from the glassy amorphous state and its reorganization on heating. The experiments performed consistently prove a distinct decrease of the rate of mesophase formation with increasing concentration of 1-butene chain defects. The time required for isothermal mesophase formation at 300 K is of the order of 0.1 s in case of the homopolymer, while it is prolonged by one order of magnitude to 1 s in the copolymer with 11 mol-% 1-butene. Similar, cold-ordering of amorphous structure on continuous heating at 1000 K s⁻¹ is only completed in the homopolymer while it is almost completely suppressed in the random copolymer containing about 11 mol-% 1-butene. The perfection of the mesophase and/or its reorganization into crystals is faster in the homopolymer than in copolymers containing 1-butene. The critical heating rate for complete inhibition of perfection and reorganization is reduced from about 40,000 K s⁻¹ in the homopolymer to about 10,000 K s⁻¹ in copolymers. The reduced rate of mesophase formation in random copolymers of propylene and 1-butene is attributed to the decrease of the thermodynamic driving force for the phase transformation.     

Kinetics of the melt – Form II phase transition in isotactic random butene-1/ethylene copolymers

The kinetics of formation of the Form II mesophase from the melt has been investigated as a function of the concentration of ethylene chain defects in isotactic random butene-1/ethylene copolymers, using standard and fast scanning chip calorimetry. Presence of ethylene co-units in the butene-1 chain leads to a distinct reduction of the melt – Form II phase transformation rate which has been quantified by evaluation of the critical cooling rate to suppress ordering, and by isothermal analysis of half-times of Form II mesophase formation. For the first time, the temperature-dependence of the rate of Form II mesophase formation has been evaluated for butene-1/ethylene random copolymers and the butene-1 homopolymer. This study needs to be considered as a complementary addendum to former work about the Form II to Form I polymorphic transformation in isotactic random butene-1/ethylene copolymers.     

Lamellar morphology of random metallocene propylene copolymers studied by atomic force microscopy

Four sets of propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene and 1-octene, and a total defect content up to ∼9 mol% (including co-unit and other defects), were studied after rapid and isothermal crystallization. Etched film surfaces and ultramicrotomed plaques were imaged so as to enhance contrast and minimize catalyst and co-catalyst residues. While increasing concentration of structural irregularities breaks down spherulitic habits, the formation of the gamma polymorph has a profound effect on the lamellar morphology. Lamellae grown in the radial axis of the spherulite and branches hereon are replaced in γ-rich copolymers with a dense array of short lamellae transverse or tilted to the main structural growth axis. This is the expected orientation for γ iPP branching from α seeds. Spherulites are formed in copolymers with non-crystallizable units (1-hexene and 1-octene) up to ∼3 mol% total defect content and were observed up to ∼6 mol% in those with partially crystallizable comonomers (ethylene and 1-butene). However, lamellae were observed in all the copolymers analyzed, even in the most defective ones, highlighting the important role of the gamma polymorph in propagating lamellar crystallites in poly(propylenes) with a high concentration of defects. Long periods measured from AFM and SAXS are comparatively analyzed.     

Unusual crystallization behavior of isotactic polypropylene and propene/1-alkene copolymers at large undercoolings

During the investigation of the crystallization of metallocene isotactic polypropylene and copolymers with low amount of 1-butene and 1-hexene at large undercoolings, an unexpected behavior has been found. Random copolymers crystallize faster than the homopolymer between 80 and 40 °C, while at high temperatures the overall crystallization rates follow the expected trend. On the basis of structural and morphological evidences we suggest that the overall structuring kinetics of the homopolymer is slowed down by the concomitant formation of mesophase and monoclinic structures. This effect is absent in the copolymers because the branched counits retard the development of mesophase.     

Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Solid-state reorganization,melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s⁻¹ suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.     

Direct analysis of annealing of nodular crystals in isotactic polypropylene by atomic force microscopy, and its correlation with calorimetric data

The process of isothermal annealing of nodular monoclinic crystals of isotactic polypropylene (iPP) was analyzed by atomic force microscopy (AFM) and temperature-modulated differential scanning calorimetry (TMDSC). Initially nodular and mesomorphic domains were obtained by controlled melt-crystallization at high cooling rate. Subsequent heating triggers transition from mesomorphic to monoclinic structure, and melting of unstable nodules. Annealing allows re-crystallization, which is recognized by enlargement of domains from initially about 20 nm to about 35 and 55 nm after annealing at 393 and 433 K, respectively. Furthermore, the re-crystallization process is connected with a slight change of the aspect ratio of crystals. The isothermal re-crystallization of the liquid is superimposed by aggregation of crystals, to yield blocky, and string-like objects. The direct analysis of structure on isothermal annealing by AFM is for the first time compared with the isothermal decrease of the apparent specific heat capacity, or change of enthalpy, monitored by TMDSC. The apparent specific heat capacity decreases during annealing with an identical non-linear time dependence as the directly observed growth of the crystal size. Analysis of the annealing processes at different temperatures yields proportionality between the increase of the crystal size and the reduction of the apparent specific heat capacity.     

Effect of comonomer partitioning on the kinetics of mesophase formation in random copolymers of propene and higher α-olefins

The effect of counit type on the kinetics of mesophase formation has been investigated by means of chip-calorimetry in propene/α-olefin random copolymers, containing counits which show large differences in their co-crystallization behavior with propene, i.e. 1-butene and 1-hexene. Non-isothermal crystallization experiments indicated that the minimum cooling rate at which mesophase formation is observed is directly related to the kinetic of α-phase crystallization, which is lower for the copolymer with the bulkier 1-hexene counit. Isothermal structuring was probed in a wide temperature range, revealing that a double bell-shaped curve is required to describe the temperature dependence of crystallization times of the two polymorphs. The ordering kinetics of the mesophase is the fastest in i-PP homopolymer and decreases with increasing comonomer bulkiness, analogous to what happens for the monoclinic phase. The results are discussed by considering the effect of comonomer on the driving force for mesophase formation, also at the light of new WAXD and density evidences, which prove different extents of inclusion of 1-butene and 1-hexene in the ordered phases.     

Formation of mesomorphic domains associated with dimer aggregates of phenyl rings in cold crystallization of poly(trimethylene terephthalate)

During the cold crystallization of poly(trimethylene terephthalate), PTT, multistage preordering activities associated with hierarchically structural evolution and origin of mesomorphic domains were investigated with time-resolved Fourier transform infrared (FTIR), photoluminescence (PL) and dielectric spectra (DS), and simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS). The observed fluorescent emission at 390 nm and an absorption band at 874 cm⁻¹ associated with the CH out-of-plane bending mode of the phenyl rings indicate that the ground-state dimers resulting from the phenyl rings of PTT chains via π–π interactions are formed in supercooled amorphous liquids. The time dependences of the increase of the fluorescent intensities, the wavenumber shift from 874 to 872 cm⁻¹, the conformational transformation of the trimethylene glycol units from trans to gauche, the dynamic α-to-α′ transition of the dielectric loss and the structural parameters reveal that the dimer aggregation accompanying a locally nematic-like orientation induced large phase-separated mesomorphic domains (over several tens of nanometers); nucleations of nanograin type (over several nm) subsequently formed within the mesomorphic domains after the conformational transformation form trans to gauche for the cold crystallization at 50 °C. Herein, we propose a preordering mechanism of cold crystallization in which the π–π interactions among the dimers serve as ordering bonding forces to drive intermolecular cooperativity before intramolecular changes of conformation, and the dimer aggregation triggers the phase-separated mesomorphic domains before the growth of crystalline nanograins (non-lamellar crystals). As distinct from the hierarchical structure of nanograins within mesomorphic domains, the formation of lamellar crystallites within spherulites without undergoing the dimer aggregation were observed form the melt crystallization at 200 °C.     

Multiscale Modeling of the Morphology and Properties of Segmented Silicone-Urea Copolymers

Abstract  

Molecular dynamics and mesoscale dynamics simulation techniques were used to investigate the effect of hydrogen bonding on the microphase separation, morphology and various physicochemical properties of segmented silicone-urea copolymers. Model silicone-urea copolymers investigated were based on the stoichiometric combinations of α,ω-aminopropyl terminated polydimethylsiloxane (PDMS) oligomers with number average molecular weights ranging from 700 to 15,000 g/mole and bis(4-isocyanatocyclohexyl)methane (HMDI). Urea hard segment contents of the copolymers, which were determined by the PDMS molecular weight, were in 1.7–34% by weight range. Since no chain extenders were used, urea hard segments in all copolymers were of uniform length. Simulation results clearly demonstrated the presence of very good microphase separation in all silicone-urea copolymers, even for the copolymer with 1.7% by weight hard segment content. Experimentally reported enhanced properties of these materials were shown to stem from strong hydrogen bond interactions which leads to the aggregation of urea hard segments and reinforcement of the PDMS.     

Structural changes in polycaproamide during thermal drawing of industrial fibres

It was shown that in going from the mesomorphic γ-modification of PA-6 to the stable α-form, formation of intermediate structures can play an important role. It was found that the formation of the polymorphous α-modification in conditions of heat treatment at 180°_C takes place due to the amorphous regions of the polymer. __________ Translated from Khimicheskie Volokna, No. 6, pp. 15–17, November–December, 2007.     

Polyisobutylene based thermoplastic elastomers: VI. Poly(α-methylstyrene-b-isobutylene-b-α-methylstyrene) triblock copolymers by coupling of living poly (α-methylstyrene-b-isobutylene) diblock copolymers

Summary Poly(α-methylstyrene-b-isobutylene-b-α-methylstyrene) (PαMeSt-PIB-PαMePSt) triblock copolymers have been prepared via coupling of living diblock copolymers in a one pot procedure. The PαMeSt-PIB diblock copolymers were synthesized by living sequential cationic polymerization in methylcyclohexane (MeChx)/methylchloride (MeCl) solvent mixtures at −80 °C using BCl₃ for αMeSt polymerization and TiCl₄ for IB polymerization as coinitiators. The crossover efficiency, however, was only ∼57 %, due to intermolecular alkylation (indanyl ring formation) after the addition of TiCl₄. By modifying the PαMeSt end with a short segment of p-chloro-α-methylstyrene before IB addition the crossover efficiency was increased to 90 %. Coupling of the living block copolymers with 2,2-bis[4-(1-phenylethenyl)phenyl]propane (BDPEP) was rapid and gave ∼ 90 % efficiency. Received: 13 July 2000/Accepted: 3 August 2000