Branching and re-orientation of lamellar crystals in non-banded poly(butene-1) spherulites

The formation mechanism of non-banded polymer spherulites has been examined experimentally for isotactic poly(butene-1) grown from the melt by optical, atomic force, and transmission electron microscopies associated with quenching and chemical etching. At the growth front of the spherulites, the maximum width of lamellar crystals, λ_m, showed a square-root dependence on the growth rate. The dependence suggests an instability-driven branching. In terms of the correlation of lamellar orientation in the spherulites, an auto-correlation function has been determined from the image taken by polarizing optical microscopy. The correlation showed an exponential decay along the radial direction, and the correlation length was in proportion to λ_m. Those experimental evidences suggest that the structure is formed by the coupling of the branching instability and the random re-orientation of lamellar crystals on the occasion of branching in the non-banded spherulites of poly(butene-1).     

Modéalisation de la copolyméarisation éathylégne α-oléafine par catalyse ziegler—natta

A copolymerization model of ethylene and a α-olefin describing the monomer uptake and the quality of the polymer produced, namely M_n, M_w, M_z, the distribution of molecular weight, short branching index and double bonds, is presented. This model assumes the existence of several types of active sites (2 or 3) and do not involve any limiting diffusion. It is applied to ethylene and butene-1 bulk copolymerization at high pressure and high temperature. Parameters are identified with CSTR experiments.     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.     

Transition of spherulite morphology in a crystalline/crystalline binary blend of biodegradable microbial polyesters

Crystalline/crystalline binary blend films of microbial polyesters composed of poly[(R)-3-hydroxybutyrate-co-(R)-3hydroxyhexanoate] (P(3HB-co-3HH)) and poly[(R)-3-hydroxybutyrate] (P(3HB)) that exhibit a morphological change are prepared by solvent casting. Differential scanning calorimetry measurements indicate that P(3HB-co-3HH) and P(3HB) are miscible for all blend ratios because a single glass transition temperature is observed. Polarization optical microscopy is used to investigate the transition of spherulite morphology and measure the radial growth rate of spherulites in the blend films. P(3HB-co-3HH) and P(3HB) contain positive spherulites, whereas in the binary blends, spherulite morphology changes from positive to negative. This change is related to the different growth rates of P(3HB-co-3HH) and P(3HB) lamellar crystals. Partial enzymatic degradation of the film surfaces reveals that the lamellar crystals of negative spherulites are oriented both perpendicular and parallel to the radial direction of spherulites. A new growth mechanism for spherulites in crystalline/crystalline blends is constructed from the results obtained for the blend films.     

Crystallization and morphology of metallocene polyethylenes with well‐controlled molecular weight and branching content

The crystallization and morphology of some metallocene polyethylenes with well‐controlled molecular weight and branching content were investigated by DSC, WAXD, PLM and SALS. The banded spherulites observed in linear PE are not seen in crystallization of branched PEs. The small spherulites with small lamellae or fringed micelle crystals are formed when branching content is higher, as suggested by PLM and SALS. The expansion of the unit cell was observed by WAXD as the molecular weight and branching content increased. At even higher branching content (more than 7 mol%), a shrinkage of the unit cell was seen, probably due to a change of crystal morphology from lamellar‐like crystals to fringed micelle‐like crystals. Crystallization temperature, melting point and crystallinity are greatly decreased for branched PEs compared with linear PEs. The equilibrium melting temperature cannot be determined via the Hoffman–Weeks approach for branched PEs since T_m is always 5–6 °C higher than T_c and there is no intercept with the T_m = T_c line. Our results show a predominant role of branches in the crystallization of polyethylene. © 2003 Society of Chemical Industry     

Synthesis by self-condensing AGET ATRP and solution properties of arborescent poly(sodium 2-acrylamido-2-methyl-N-propane sulfonate)

Randomly branched (arborescent) poly(sodium 2-acrylamido-2-methyl-N-propanesulfonates) (NaPAMPS) were synthesized via self-condensing vinyl polymerization using activators generated by electron transfer atom transfer radical polymerization (AGET ATRP). The controlled self-condensing AGET ATRP of NaAMPS was realized in the presence of 2-(2-bromopropionyloxy)ethyl acrylate (BPEA) as a branching monomer (inimer) in water/pyridine (35-50% of Py) mixed solvents. The content of BPEA in the reaction feed was varied from 10 to 30 wt% allowing the synthesis of NaPAMPS with different degree of branching. SEC determined molecular weight of the prepared NaPAMPS was M_w = 94 000-120 000 g/mol, and the accompanying polydispersity index PDI ranged from 1.84 to 2.47. The definite evidence of highly branched structure of NaPAMPS was provided by the dependence of radius of gyration R_g on weight-average molecular weight M_w with characteristic slope a = 0.38-0.42, and by small-angle X-ray scattering (SAXS) analysis. Molecular parameters, conformation and dynamics of the branched NaPAMPS in dilute salt-free solutions and in the presence of a salt were elucidated by static and dynamic light scattering and SAXS.     

An unusual branching in single crystals of isotactic poly(4-methyl-1-pentene)

Single crystals of low M_w poly(4-methyl-1-pentene) (P4MP1) in its form I display an unusual streaking of their diffraction pattern. The crystals also frequently give rise to composite diffraction patterns made of two patterns rotated by 37°. The streaking indicates a structural disorder, namely a shift of nearby layers along the a or b axis by one quarter of the unit-cell edge. The daughter crystals, rotated by 37°, are produced on the edges of the parent crystals via an epitaxial growth that is a direct consequence of the structural disorder.     

Polymer spherulites: A critical review

Polymeric and non-polymeric materials often crystallize as spherulites when crystallized from viscous melts or solutions at large undercooling. The essential component of a spherulite is fibrillar crystals that grow in predominantly radial directions and branch irregularly. We review the growth, branching and twisting of crystals in the light of theoretical and experimental advances of the last decade, while maintaining an appreciation for historical context.The crucial role of self-generated fields ahead of the crystal–melt interface is developed. Pressure gradients from volume contraction have been treated, as well as impurity gradients ahead of a growing crystal; fibril width W is predicted and found to be proportional to δ^1/2, where the diffusion length δ = D/G, the quotient of diffusivity and growth rate, conveys the extent of the field gradient. Ribbon-like spherulite radii grow at a constant rate under diffusion-coupled interface control.Non-crystallographic branching is required to maintain the volume occupied by fibrillar crystals as the spherulite radius increases. Topological giant screw dislocations and induced nucleation at cilia tethered to crystals are observed mechanisms leading to branching normal to the wide dimension of lamellar crystals; but the relative importance of each of these is not yet established. Repetitive tip splitting by kinetic interface instability has been suggested as a branching mechanism in the wide dimension of lamellar crystals.Larger molecular mass reduces the spherulite growth rate, more so at low undercoolings, for reasons that remain unresolved. Miscible diluents often profoundly reduce G by lowering both thermodynamic driving force and local transport dynamics that govern the secondary nucleation rate. Spherulite blend morphology is linked to the competition between radial growth rate G and diffusivity D of the diluent, expressed as the diffusion length δ.Polymer crystals in which chain helices all have the same sense show banded spherulites, as do crystals in which the chain axes are not perpendicular to the basal surfaces. Recent analyses with optical birefringence and X-ray micro-diffraction support the presence of helicoidally twisted ribbons, although other structural arrangements have sometimes been revealed by microscopy. Assessments of twist directions in spherulites of chiral polymers point to unbalanced basal surface stress as the source of twisting, although a general mechanical analysis is lacking. Another twisting model employs regular arrays of isochiral giant screw dislocations; results are mixed for this model.     

Microbeam X-ray diffraction of non-banded polymer spherulites of it-polystyrene and it-poly(butene-1)

The orientation of lamellar crystals in non-banded spherulites of it-polystyrene and it-poly(butene-1) was investigated by microbeam X-ray diffraction. The two-dimensional intensity map of diffraction enables us to examine the local orientation of lamellar crystallites in the non-banded spherulites. The obtained results indicated the re-orientation of crystallites in non-banded spherulites and confirmed our previous observation on the anisotropic birefringence of a group of crystal stacks by polarizing optical microscopy.     

Crystallization kinetics and morphology of polymer blends of poly(tetramethyl-p-silphenylene siloxane) fractions

The crystallization behaviour of polymer blends or mixtures of the same system has been studied over a wide range of molecular weight and crystallization temperatures. Blends were made by mixing fractionated polymer samples. The spherulitic growth in these mixtures is dependent upon the number-average molecular weight of the system at the shorter chain lengths, but then becomes insensitive to molecular weight values when about 10⁵ to 10⁶ are reached. The growth rate kinetics of mixtures can be described by a kinetic model used for fractionated poly(tetramethyl-p-silphenylene siloxane) (TMPS) polymers. The crystal surface energies deduced from these rate data are molecular weight dependent as are the pre-exponential and transport factors in the rate equation. These parameters are explained in terms of the crystallite morphology. Mixtures (as well as fractions themselves) of all polymer fractions ranging from the monomer to the highest molecular weight (10⁶ approximately) have similar morphological features and form negatively birefringent spherulites. Although molecular weight segregation appears to play an important role in crystallization at comparatively small undercoolings, its influence seems to be minimal at large undercoolings (close to or below the growth rate maxima). Very low molecular weight additives significantly affect the overall crystallization kinetics. Compared to the undiluted sample, mixtures so formed have lower observed melting points and glass transition temperatures. Rates of crystallization are generally facilitated by the diluent with the peak in the growth rate being displaced to lower temperatures. The growth rates for diluted over the undiluted polymer at similar undercoolings are usually larger. At high molecular weights the log of the spherulitic growth rate varies as $\text{M}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}{}_{\mathrm{n}}$, over a considerable range, but at low molecular weight values the rate depends more strongly on M_n approaching a limit of M^?1.2_n as the monomeric state is approached.     

Polystyrene-equivalent molecular weight versus true molecular weight in size-exclusion chromatography

The evaluation of the size-exclusion chromatography (SEC) concentration elution curves by means of a calibration dependence obtained in a given SEC set for a polymer different from the polymer to be analyzed results in an error in the determination of both molecular weight and molecular-weight distribution (MWD). The problem is analyzed assuming the validity of the universal-calibration concept. The differences between the true and apparent values of molecular weight, MWD and M_w/M_n depend on and are expressed in terms of the parameters of the Mark-Houwink-Kuhn-Sakurada equation, describing the molecular-weight dependence of intrinsic viscosity, for the polymer to be analyzed and the polymer used for calibration. The differences in molecular weight and the M_w/M_n ratio are typically tens of percent and, in extreme cases, can amount up to a factor of three for molecular weight and a factor of two for the M_w/M_n ratio.     

Rigid backbone polymers

Summary It is shown that the inverse velocity of a fallingball in, or _m the apparent minimum viscosity of, anisotropic solutions of poly(n-hexylisocyanate) in toluene, are molecular weight dependent. The molecular weight dependence follows two linear branches. Below M_w=42600 the apparent minimum viscosity is very weakly dependent on M_w. Above M_w=42600 the apparent minimum viscosity of the anisotropic solution is strongly dependent on the molecular weight.     

Characterization of poly(1,4-phenyleneterephthalamide) in concentrated sulphuric acid. 2: Determination of molecular weight distributions

By combining static and dynamic properties (M_w, A₂, k_dR_g and R_h) of poly(1,4-phenyleneterephthalamide), PPTA (commercially known as Kevlar), with a detailed analysis of measured time correlation functions at different scattering angles in dilute solution, we have been able to estimate the molecular weight dependence of the radius of gyration, R_g(M), the persistence length ? ($\text{≈ 290}\text{A}\text{?}$), and the molecular weight distribution (M_z:M_w:M_n ≈ 6.2:1.8:1) using an unfractionated PPTA sample (M_w = 4.3 × 10⁴ g/mole). Laplace inversion of the time correlation function was accomplished independently by means of two different algorithms: the singular value decomposition technique with discrete multi-exponentials to approximate the normalized characteristic linewidth distribution function G(г) and the method of regularization whereby a linearized smoothing operator was used. The non-intrusive laser light scattering technique permits us to characterize, for the first time, the molecular weight distribution of PPTA which has been difficult to perform by means of other more established methods, such as size exclusion chromatography, because of the corrosive nature of solvents used in preparing PPTA solutions.     

Molecular weight distribution of commercial PVC

The molecular weight distribution (MWD) of commercial suspension grade poly(vinyl chloride) (PVC) resins with K values from 50 to 93 and mass grade PVC resins with K values from 58 to 68 has been determined by size exclusion chromatography (SEC), using literature Mark‐Houwink coefficients. The MWD is characterized by the number average molecular weight (M_n), the weight average molecular weight (M_w) and the polydispersity (M_w/M_n). Our results for M_w are consistent with recently published data, but we find different results for M_n and consequently for M_w/M_n. The polydispersity of PVC increases with increasing K value. This effect can be explained by two mechanisms. The first mechanism is a reduced terminating reaction rate between two growing polymer chains (disproportionation) at higher molecular weight owing to the reduced mobility of the polymer chains. The second mechanism is long‐chain branching of molecules with high molecular weight which lets the molecules grow at two ends. For two examples graphs of the measured MWD are compared with the theoretically expected MWD.     

Synthesis of tailored nanogels by means of two-stage irradiation

A new method allowing to synthesize polymeric nanogels of independently chosen weight-average molecular weight (M_w) and dimensions in an additive-free system consisting only of linear macromolecules and water is described and tested. This approach, based on generation of free radicals using ionizing radiation in oxygen-free polymer solutions, consists of two steps. In the first step, semi-concentrated polymer solution is irradiated at a moderate dose rate, so that in the stationary state the average number of radicals per chain is <1. This promotes intermolecular cross-linking with increase in M_w and coil dimensions. When the desired M_w is reached, in the second step intra-molecular recombination is induced by pulse irradiation of dilute solution (with the average number of radicals per chain >1 after each pulse), leading to a decrease in nanogel diameter while keeping M_w nearly constant. Experimental data on a model polymer – poly(N-vinylpyrrolidone) – confirm that the proposed method allows synthesizing nanogels of desired properties (independently chosen molecular weight and radius of gyration) in a pure polymer-water system, eliminating the use of monomers, cross-linking agents, or other auxiliary substances.     

The reactive modification of polyethylene. I: The effect of low initiator concentrations on molecular properties

Commercial samples of high density, linear low density, and low density polyethylene were modified by injection of low concentrations of free radical initiator during extrusion. Molecular properties monitored included molecular weight distribution, degree of unsaturation, and branching. When the polyethylene teed to this reactive extrusion process had similar values of M_w, but varying polydispersity, degree of branching and degree of unsaturation, the magnitude of the change in molecular weight distribution was found to be in the following order: HDPE 1 > LDPE2 > LLDPE. In general, terminal vinyls enhanced molecular weight increase, and branching promoted degradation. However, for a second high density polyethylene sample with M_w = 154,000 (rather than the previous sample's M_w of 85,600), the change in molecular weight distribution was small and located at the lower molecular weight end. This work provided data for the kinetic model development detailed in Part II.     

Observation of banded spherulites in pure poly(l-lactide) and its miscible blends with amorphous polymers

Banded spherulites of pure poly(l-lactide) (PLLA) were observed via the ‘crystallization after annealing’ procedure, while only common spherulites were obtained via the ‘direct isothermal crystallization’ procedure. Wide angle X-ray diffraction revealed that the two types of spherulites had the same crystal lattice of α-modification. Atomic force microscopy demonstrated that the alternative negative and positive birefringent bands resulted from the alternative edge-on and flat-on lamellar orientations in the spherulites. Furthermore, the effect of thermal history on the spherulitic morphology was investigated in details. The PLLA samples melted for longer time or those with lower melting point were more likely to form banded spherulites. The possibility that the change of molecular weight was a determining factor of banding was excluded by the results on differently prepared samples with the same molecular weight. Therefore, we conclude that it was complete melting of the crystalline residues that favored formation of PLLA banded spherulites. Blending of PLLA with atactic poly(d,l-lactide) or poly[(R,S)-3-hydroxybutyrate], led to reduced band spacing. Effect of blending on the chain mobility, spherulite growth kinetics, supercooling and lamellar surface energy was quantitatively studied, which suggests that the blending-reduced band spacing cannot be attributed to the above factors. Therefore, there are other blending-relevant factors leading to the reduced band spacing.     

Crystallization studies of isotactic polystyrene

The kinetics of lamellar crystallization in thin films of isotactic polystyrene have been determined using transmission electron microscopy. The morphological changes accompanying crystallization have also been investigated as a function of solvent, supercooling and strain prior to crystallization. Crystallization temperatures have been attained by both cooling from the melt and warming from the glass. Similar growth rates were obtained in both cases. The nucleation density of spherulites is difficult to control when warming from the glass but does depend on the solvent used in preparing the thin film. The rate of lamellar growth follows a ‘bell’ shaped curve versus crystallization temperature and the kinetics were analysed using the secondary nucleation theory of Hoffman and Lauritzen. The end surface free energy, δ_e, of the lamellar crystals was determined using the variation of lamellar thickness with supercooling.     

Synthesis and characterization of amphiphilic graft copolymer poly(ethylene oxide)-graft-poly(methyl acrylate)

A novel well-defined amphiphilic graft copolymer of poly(ethylene oxide) as main chain and poly(methyl acrylate) as graft chains is successfully prepared by combination of anionic copolymerization with atom transfer radical polymerization (ATRP). The glycidol is protected by ethyl vinyl ether first, then obtained 2,3-epoxypropyl-1-ethoxyethyl ether (EPEE) is copolymerized with EO by initiation of mixture of diphenylmethyl potassium and triethylene glycol to give the well-defined poly(EO-co-EPEE), the latter is deprotected in the acidic conditions, then the recovered copolymer [(poly(EO-co-Gly)] with multi-pending hydroxyls is esterified with 2-bromoisobutyryl bromide to produce the ATRP macroinitiator with multi-pending activated bromides [poly(EO-co-Gly)_(ATRP)] to initiate the polymerization of methyl acrylate (MA). The object products and intermediates are characterized by NMR, MALDI-TOF-MS, FT-IR, and SEC in detail. In solution polymerization, the molecular weight distribution of the graft copolymers is rather narrow (M_w/M_n < 1.2), and the linear dependence of Ln [M₀]/[M] on time demonstrates that the MA polymerization is well controlled.