Morphology and structure of poly(p-phenylene terephthalamide) crystallized from dilute organic solution

The high-temperature crystallization of poly(p-phenylene terephthalamide) (PPTA) from dilute organic solutions was achieved through the introduction of a non-solvent, or precipitating agent, at the desired crystallization temperature. The morphology and crystal structure were examined for crystals produced from PPTA polymer with two different molecular weights (M_w = 46000 and 3430 g mol⁻¹), using transmission electron microscopy. For the high-molecular-weight polymer, ribbon-like crystals were produced, while the low-molecular-weight polymer yielded small needles or platelets. In both cases, electron diffraction showed that the Northolt allomorph was obtained. For the high-molecular-weight polymer, the molecular axes were parallel to the ribbon axes in a chain-extended type structure. A hypothesis for the orientation of the low-molecular-weight PPTA in the small platelets, is also given.     

Polymer interface changes in electrophoretic deposition

A Monte Carlo computer simulation model for the electrophoretic deposition of polymer chains on a discrete lattice is used to study the polymer density profile, interface growth, and its dependence on field, temperature, and molecular weight. The interface width (W) decreases WE^−1/2 on increasing the field (E). Width (W) depends non-monotonically on the temperature (T): a power-law decay is followed by a power-law increase on raising the temperature. Monotonic decay of the interface width with the molecular weight is possibly a stretched exponential. Conformation and dynamics of a tracer chain is used to probe its characteristics in interface to bulk region. The root mean square (rms) displacement of the center of mass of the tracer chain shows an ultra-slow motion, Rt^ν (ν0.1–0.01 at E=0.1–1.0) as the driven chain moves deeper from interface to bulk. Longitudinal compression of the radius of gyration (R_g) of the chain increases with the field; transverse components (R_gx, R_gy) are larger than the longitudinal component (R_gz). The transverse component (R_gx(y)) becomes oscillatory due to periodic squeezing at high fields as the field competes with the polymer barriers.     

Characterization of polymer conformation and morphology through small-angle neutron scattering—A literature review

Small-angle neutron scattering, SANS, stands forth as one of the most important of the new tools for evaluating polymer chain conformation and morphology. This paper reviews the SANS literature through 1982, with a few early 1983 references added. The theory of SANS is outlined and compared to light scattering. SANS values of polymer molecular weights and radii of gyration obtained in the bulk state were found to be in agreement with values obtained from dilute solutions by light scattering. In each case, deuterated fractions of polymer were inserted into the hydrogeneous matrix, or vice versa, to provide contrast. Several new research areas are then discussed, including unidirectional stretching of elastomers, stress-relaxation, polymer-polymer miscibility, crystallization from the melt compared with crystallization from dilute solutions, nonclassical aggregation during bulk polymerization of thermoset systems, morphology of polymer blends, block copolymers and ionomers, the core-shell structure of latexes and polymer blocks, and grafts as surfactants in emulsions and latexes. Much new and sometimes unexpected information is being provided by the SANS research now in progress.     

High-pressure crystallization and melting of polyethylene in n-pentane

The fluid–solid phase separation and crystallization in relatively dilute solutions of polyethylene in n-pentane at high pressures were studied using time- and angle-resolved laser light scattering techniques. Majority of the experiments were carried out by cooling (crystallization) or heating (melting) while holding the pressure constant at selected pressures in the range from 10 to 54 MPa. Crystallizations were also carried out via two other pathways: (1) cooling without pressure adjustment and (2) first crossing the L–L phase boundary via pressure reduction at constant temperature followed by cooling. Crystallization and melting transitions were assessed from the variations of the transmitted light intensity or the scattered light intensity (averaged over all angles) with temperature. Kinetics of phase separation during both the crystallization and the melting were followed by the time evolution of the angular distribution of the scattered light intensities. The kinetics were typical of nucleation and growth processes. From the time evolution of the light scattering patterns the mean particle radii and their evolution were also followed. The polymer particles that form upon crystallization were collected and characterized using a field emission scanning electron microscope (FESEM) and differential scanning calorimetry (DSC). The results show that the PE crystals that form from these high-pressure conditions display a plate-like morphology, which tend to aggregate into ellipsoid-shape structures. DSC studies show that the PE crystals show higher crystallinity compared to the original polyethylene sample and display multiple melting peaks during the first heating scan, which however collapse to a single melting transition peak after the initial heating scan. The crystals that formed from the experiments in which L–L phase boundary was crossed first were found to display two distinct particle groups. These were attributed to crystals that form from polymer-rich and polymer-lean phases that form when L–L phase boundary is crossed.     

超高分子质量聚丙烯冻胶纤维的结晶性能研究

采用冻胶纺丝法制备超高分子质量聚丙烯纤维（UHMWPP）,并在不同的拉伸温度下对其进行拉伸。研究了在相同的拉伸倍率下,拉伸温度对UHMWPP纤维结晶性能、热性能的影响。结果表明：UHMWPP纤维结晶晶体间存在很多间隙;纤维的结晶性能和耐热性能都随拉伸温度的提高而提高。     

Crystallization of dilute polyethylene solutions: influence of molecular weight

The crystallization kinetics of linear polyethylene over the molecular weight range 14 000–1 200 000 was studied in dilute solutions of α-chloronaphthalene by dilatometric techniques. The experimental results could be quantitatively described by the Avrami and Göler-Sachs theories for a significant part of the transformation. The influence of molecular weight on the time necessary for 10% of crystallinity to develop was also analysed and the obtained curves follow the same pattern as in the bulk. The temperature coefficient of crystallization was studied by using the two-dimensional nucleation theory and it was found that the basic interfacial free energy is about four times lower than the value reported previously for the pure polymer.     

Chain conformation of polystyrene in bulk by small-angle X-ray scattering

Small-angle x-ray scattering from solid solutions of different molecular weight fractions of partially brominated polystyrene in polystyrene has been studied. The results of this study indicate that polymer chains in bulk have a radius of gyration proportional to the square root of molecular weight, which is consistent with the random coil model. However, these radii are approximately 20% larger than the unperturbed radii deduced by light scattering in dilute solutions.     

Effect of molecular weight on spherulite growth rate of poly(ethylene terephthalate) via real-time small angle light scattering

The effect of molecular weight on the spherulite growth rate of poly(ethylene terephthalate) (PET) has been determined using a real-time small angle light scattering (RTSALS) device equipped with a two-dimensional position-sensitive detector. This detector, the design of which was previously reported, incorporates a CCD camera, a personal computer, and an imaging board as the major hardware components. The device performs real-time analysis of the light scattering pattern and calculates the average spherulite radius as a function of time during the crystallization experiment. Growth rate data were obtained for PET having number-average molecular weights of 18,000, 24,700, and 33,100. Samples were crystallized isothermally following a temperature jump from the glass at room temperature to the desired crystallization temperature, which ranged from 130 to 200°C. Both the temperature and molecular weight dependence were found to be well described by the Hoffman equation. The temperature dependence agrees well with that found by previous workers, but the molecular weight dependence is somewhat different.     

Crystal growth kinetics and the lateral habits of polyethylene crystals

The lateral growth habits of lamellar crystals of polyethylene grown from solution can be typified in terms of the aspect ratio, r, which is the ratio of the dimension of the crystals in the b-axis direction to that in the a-axis direction. The aspect ratio depends on crystallization temperature, undercooling, solvent, polymer concentration and molecular weight. At steady state growth, r can be expressed in terms of the ratio of growth rates normal to the {110} and (200) faces. Expressing the growth rates in terms of the kinetic theory of polymer crystal growth yields an expression which is used to analyse experimental results on the effect of temperature and concentration on the lateral growth habits of crystals grown from xylene. Using as two adjustable parameters the ratio of end surface free energies for the two growth surfaces and the ratio of the lateral surface free energies to fit the rversusΔT data permits the determination of these ratios with high sensitivity. The actual values obtained are dependent upon concentration, the assumed growth regime, and, most importantly, ?, the parameter in the growth rate equations apportioning the bulk free energy change to the forward and backward steps in the stem deposition process.     

Structure and sorption properties of hypercrosslinked polystyrenes and magnetic nanocomposite materials based on them

Some special habits of dilute solution-grown single crystals were introduced by means of atomic force microscopy (AFM) and transmission electron microscopy (TEM) equipped with electron diffraction (ED). Well-defined block copolymers of poly(ethylene glycol)-block-polystyrene (PEG-b-PS) having predetermined molecular weight and narrow molecular weight distribution (1.06–1.08) were synthesized by atom transfer radical polymerization (ATRP). The single crystals of PEG-b-PS copolymers were grown in a mixed solvent comprised of chlorobenzene/octane and separately in amyl acetate, both using a self-seeding technique. We were capable to develop various growth fronts that resulted from different growth rates through infinitesimal temperature fluctuations. Elevated crystallization temperatures and a higher ratio of molecular weight of PS to that of PEG intensified the curvatures. The corresponding overall, crystalline, and amorphous thickness of a given single crystal, either in square and truncated shapes or with concave and convex lateral sides, had high consistency with each other.     

Molecular packing and crystalline morphologies of biodegradable poly(alkylene dicarboxylate)s derived from 1,6-hexanediol

The crystalline structures of two aliphatic polyesters derived from 1,6-hexanediol and suberic or dodecanodioic acid are studied by means of X-ray and electron diffraction and real space electron microscopy. Chain-folded lamellar crystals have been obtained by isothermal crystallization of dilute n-hexanol or n-heptanol solutions and their crystalline habit have been studied. A regular folding habit is deduced from polyethylene decoration techniques. An orthorhombic unit cell containing four chain segments with a planar zig-zag conformation is determined for the two studied polyesters. A P2₁ab space group symmetry can be assumed taking into account the detected systematic absences and the symmetry of diffraction patterns. In addition, evidence of a second non-orthorhombic structure is found for the suberic acid derivative. The orthorhombic crystal packing of these polymers is characterized by a setting angle close to 45°, showing a similarity with polyethylene. Refinement of the X-ray fiber diffraction pattern of polyester 6-12 allows to determine the c-chain axis shift between the molecular chains, being observed an alignment of the CH₂–O methylene groups with the CH₂–O oxygen atoms of neighbouring chains. Crystallization from the melt renders spherulites with a negative birefringence and a fibrillar texture, whereas banded morphologies constituted by multilayered single crystals can be obtained by evaporation of xylene solutions. Electron diffraction patterns of ultrathin samples allow the spherulitic radial growth direction to be correlated with the crystalline structure.     

Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper.Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C.We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature.Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60-105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.     

Studies of Conformational Changes and Intermolecular Interactions in Dilute Solutions of Partially Hydrolyzed Polyacrylamide by Optical Mixing Spectroscopy

Abstract

Moderately concentrated aqueous solutions of certain high molecular weight polyelectrolytes exhibit complex molecular relaxation modes in the range of 1 to 10⁵ sec^?1 which confer useful commercial or biological properties. Although many different kinds of molecular relaxation phenomena have been proposed as causing the various unique properties of such solutions, the relative importance of the various modes remains unanswered mainly because of the lack of any accurate experimental means of studying these events. We describe a new laser light scattering technique which provides such a probe. We have used this method to study molecular dynamics, especially molecular transport processes, in dilute aqueous solutions of a partially ionized high molecular weight polyacrylamide. Two kinds of diffusion mode of the polymer are observed. This paper reports studies of the effect of ionic strength, polymer concentration and temperature on the slow diffusion mode.     

Poly(oxymethylene) crystals grown in the polymerization of trioxane initiated with perchloric acid in nitrobenzene

Summary Nascent crystalline poly(oxymethylene) (POM) was synthesized by cationic polymerization of trioxane with anhydrous perchloric acid in nitrobenzene solution. The high polarity of the solvent enabled to obtain high molecular weight polymer. Also the yield obtained indicates that the polymerization reached the expected thermodynamic ceiling-equilibrium. The participation of free oxy carbenium ions from growing POM-chains are relevant on the mecha nism of polymerization and crystallization. After an induction period, hexagonal single crystals of POM are formed whose macro conformation correspond to extended chain crystals. Thermal da ta is in agreement with this morphology, typical of a simultaneous polymerization and crystallization. The spiral growth of the crystals proceeds by a catalyzed crystal growth process.     

Effect of the nucleation mechanism on complex poly(L‐lactide) nonisothermal crystallization process. Part 1: Thermal and structural characterization

The effect of the holding temperature and time in the melt state of poly(L ‐lactide) (PLLA) samples on the nonisothermal melt crystallization process and on the structure have been investigated by means of DSC, polarized optical microscopy and wide angle X‐ray scattering. As standard starting material, single crystals grown from dilute solution were used. In the mild melting condition, the survived athermal nuclei favor high temperature polymer crystallization, while the more severe treatment leads the PLLA to crystallize at higher supercooling with a sporadic nucleation. At the intermediate melting temperature a distinct double nucleation mechanism was observed while at the lower nuclei concentration, a double crystallization rate was also found. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Enthalpic and entropic origins of nucleation barriers during polymer crystallization: the Hoffman-Lauritzen theory and beyond

In the search for a greater understanding of polymer crystallization, numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century. There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth, i.e. molecular dynamic/scattering and structural/morphological. Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process, structural/morphological analysis of polymer crystals reveal information recorded during this process. Namely, the final structure and morphology of polymer crystals have atomic, stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process. It is commonly understood that polymer crystallization, from the thermodynamic perspective, is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals. This process is controlled by a free energy barrier. A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization. The Hoffman-Lauritzen (HL) theory, which was put forward more than 40 years ago, was one of the first analytical theories to illustrate how polymers crystallize. Since then, modifications to the HL theory and suggestions for new approaches have been reported, but the core physical picture of the HL theory has largely remained intact. This article consists of four major parts: (1) we will compare the crystallization of small molecules and long chain molecules, and the relationship between crystallization and crystal habits. The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question: what is the nucleation barrier? Specifically, we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results. This barrier, from our perspective, consists of selection processes taking place in different length- and time-scales (4) finally, there is a brief discussion on what issues remain, in particular, in the areas of undercooled liquid structures and the initial stages of crystallization.     

Use of solution crystallization analysis by laser light scattering for studying the solution crystallization of various Ziegler–Natta‐catalysed polypropylenes

Solution crystallization analysis by laser light scattering (SCALLS) involves the observation of the scattering of diode mercury laser lamp light after it passes through a polymer solution. An increase in turbidity occurs when the hot polymer solution is cooled and the polymer starts to crystallize out of solution. This causes a decrease in the amount of laser light that can pass through the solution and an increase in the amount of scattered light. The reverse of this process leads to the turbidity decreasing with an increase in temperature. According to this concept, it is possible to follow the solution crystallization of various polypropylenes under controlled cooling. In this study, SCALLS was able to differentiate between different isotactic and syndiotactic polypropylenes with similar chemical structures, but different tacticity and molecular weights. Furthermore, SCALLS provided good crystallization information that is similar to that from crystallization analysis fractionation and temperature rising elution fractionation. In addition, SCALLS can be used as a quantitative tool for the measurement of weight fractions during dissolution. © 2014 Society of Chemical Industry     

Low shear viscosity and crystallization in dilute solutions of polymers at high pressures: Falling body viscometry of high molecular weight polyethylene solutions

A specially designed falling body viscometer has been used to obtain low shear viscosities of dilute solutions of high molecular weight polyethylene in decalin and for detecting phase transitions at high pressures. The method provides valuable rheological data at pressures that correspond to those I lubrication and some extrusion processes. In addition, the method enables detection of phase transitions, such as crystallization, that result in detectable changes in rheology. The influence of pressure as high as 0.6 GPa ( ～ 80.000 psi) is illustrated here through a study of crystallization, with verification from light scattering and calorimetry.     

Thermal stability of polyacrylamide solutions: effect of residual impurities in the molecular-weight-degradation process upon heating

Summary We have investigated the effect of temperature on the chemical (hydrolysis) and thermal (molecular weight) stability of purified polyacrylamide solution.From viscosity and light scattering data we show that no polymer degradation due to oxygen contamination results from heating polymer solutions although higher temperature is responsible for hydrolysis of amide groups.     

Rheooptical investigations of shear-induced aggregation of polymers in dilute solutions

The phenomenon of shear-induced aggregation has been investigated in dilute solutions of high molar mass polystyrene standards in phthalic acid esters by light scattering and simultaneous rheometrical measurements. An optical-rheological correlation is found which can easily be understood by simple molecular kinetic models. Systematic examination of the polymer solutions, varying temperature, concentration, and molar mass, shows a dependence of the aggregation on these parameters which fits thermodynamic models. Received: 3 November 1997/Revised version: 16 December 1997/Accepted: 18 December 1997