Rufina G. Alamo

The crystallization behavior of random copolymers of ethylene

Abstract The crystallization behavior of random ethylene—1-alkene copolymers is reviewed. Major attention is focused on fractions and samples that have well-defined and narrow molecular mass and composition distributions. Emphasis is given to the analysis of thermodynamic properties, the basic elements of phase structure, the purity of the crystalline phase and the supermolecular structure. The molar mass, co-unit content and chemical type are treated as independent variables in the analysis of a large amount of experimental data. As is expected from theoretical considerations, the thermodynamic properties are very sensitive to the sequence distribution of the co-units in the chain. It is found that, except for directly bonded methyl groups, the crystallite structure remains pure, irrespective of the chemical nature of the co-unit. Thermodynamic properties and the major elements of phase structure, with the exception of the interfacial structure, are independent of the nature of the side group for groups larger than methyl. The analyses of a large amount of experimental data makes it clear that the molar mass and co-unit content need to be treated as independent variables.

The degree of crystallinity of monoclinic isotactic poly(propylene)

The degree of crystallinity of a set of monoclinic (alpha) isotactic poly(propylenes), prepared by a metallocene-type catalyst, were determined at room temperature. Three different methods were used: density, enthalpy of fusion, and wide-angle X-ray scattering, and the results compared. The relation between the heat of fusion and the specific volume of these poly(propylenes) was found to be nonlinear, thus precluding any linear extrapolation to obtain the heat of fusion of the pure crystal (ΔHu). The value of ΔHu obtained from depression of the melting temperature by diluents is used. Based on the unit cell density of monoclinic crystals formed from a low defected fraction, the density obtained crystallinity levels were found to be between 0.l5–0.25 higher than those calculated from the heat of fusion. This relatively large difference holds for the isothermally crystallized and quenched isotactic poly(propylenes), and reflects the contribution of the interphase to the density determined crystallinity, which does not contribute to the heat of fusion. Paralleling results found in other systems, the crystallinity levels obtained from wide-angle X-ray scattering agree with those obtained from density, indicating a significant contribution of the partially ordered phase to the total diffraction. Emphasis is given on the need to account for the large differences in the crystallinities of poly(propylene) measured by different techniques when evaluating the dependence of properties on this quantity.

Carbon nanotubes on a spider silk scaffold

Understanding the compatibility between spider silk and conducting materials is essential to advance the use of spider silk in electronic applications. Spider silk is tough, but becomes soft when exposed to water. Here we report a strong affinity of amine-functionalised multi-walled carbon nanotubes for spider silk, with coating assisted by a water and mechanical shear method. The nanotubes adhere uniformly and bond to the silk fibre surface to produce tough, custom-shaped, flexible and electrically conducting fibres after drying and contraction. The conductivity of coated silk fibres is reversibly sensitive to strain and humidity, leading to proof-of-concept sensor and actuator demonstrations.

A morphological study of a highly structurally regular isotactic poly(propylene) fraction

Abstract Studies of the thermal properties and lamellar morphology of a highly structurally regular fraction of a Ziegler–Natta type isotactic poly(propylene) have been carried out. This fraction has an isotacticity content of mmmm=0.995 and a molar fraction of defects of 0.001. It is thus, among the most structurally regular isotactic poly(propylene) samples whose properties are reported. Differential scanning calorimetry as well as electron and optical microscopy were used to characterize the specimens. The fraction was crystallized from the melt over a very wide range of crystallization temperatures ( 135° C ≤T c ≤167° C ). Monoclinic, α type crystals were formed over the whole crystallization range. The formation of cross-hatching, or lamellae branching, was also observed over the complete interval of crystallization temperatures. The formation of the tangential ‘daughter’ lamellae at temperatures greater than 160°C can be attributed to the high structural regularity of the fraction studied. Relatively low crystallization temperatures (130°C to 150°C) show extended regions of woven lamellae having similar thicknesses with occasional groups of parallel long radiating lamellae. A morphology of rather thick long radiating lamellae and thin, transverse lamellae is formed at temperatures ≥160°C. The angle between the daughter and mother lamellae of approximately 100° is in agreement with crystallographic predictions. The two endotherms that are observed by differential scanning calorimetry can be identified with the melting of the two distinct lamellae populations. It is consistent with the optical microscopy observations where a change in the sign of the birefringence is observed on the melting of the daughter lamellae. When formed at relatively high temperatures ( T c >160° C ) the mother lamellae subsequently melt at temperatures >180°C

The crystallization of blends of different types of polyethylene: The role of crystallization conditions

Abstract Cocrystallization in blends of linear and branched polyethylenes has been studied under both isothermal and slow-cooling crystallization conditions. Before the more common, polydisperse-type polyethylenes were studied and the results analysed, model systems were investigated in detail. The components used in the model binary blends were molecular weight fractions of linear polyethylene, hydrogenated poly(butadiene) as a model for the ethylene-1-alkene copolymers, and a three-arm star hydrogenated poly(butadiene) as a model for the long chain branched polyethylenes. It was found that a key factor in governing the extent of cocrystallization in these blends is the closeness of crystallization rates of each of the components. The extent of the cocrystallization thus diminishes with increasing concentration of the linear component in the blend. It is found that copolymer composition and molecular structure also have a strong influence on cocrystallization. The amount of cocrystallization is favoured at the lowest isothermal crystallization temperatures and is maximized under quenching conditions. The general features that influence cocrystallization, which have evolved from this study, are discussed.

Crystallization of polyformals: 1. Crystallization kinetics of poly(1,3-dioxolane)

Abstract Poly(1,3-dioxolane) fractions ranging in molecular weight from 8800 to 120 000 have been isothemally crystallized in the temperature range 25–41°C. From the dilatometric isotherms, the Avrami exponent is an integral number, 3, and is independent of temperature and molecular weight. The level of crystallinity is dependent on molecular weight and there is a change from ∼55% for the highest molecular weight fraction to ∼80% for the lowest molecular weight fraction. The overall crystallization rate temperature coefficient was studied using two dimensional nucleation theory and it was found that the interfacial free energies do not change with molecular weight. However, the usual plots are nonlinear in the whole range of crystallization temperatures. For the high crystallization temperatures the slope is about twice the low crystallization temperature slope, this change being related to a morphological transition.

Some aspects of the crystallization of ethylene copolymers

Abstract This report presents a continuation of studies from these laboratories that have been concerned with the crystallization behavior of ethylene copolymers. More specifically a comparison is made between the thermodynamic properties of ethylene–norbornene copolymers with those of the 1-alkene copolymers. In addition a detailed comparison is made of the thermodynamic, tensile, morphological and structural properties and the overall crystallization kinetics between the ethylene–1-alkene copolymers that do or do not contain small concentrations of long-chain branches. Except for the crystallization kinetics, the crystallization properties have been found to be essentially the same for both types of copolymers. A correlation has also been developed between the lamellar and supermolecular structures, the chemical nature of the comonomer and its concentration. Supermolecular structures range from spherulitic to micellar, depending on both comonomer type and concentration. Analysis of the overall crystallization kinetics demonstrates that the interfacial free energies that govern nucleation are independent of the lamellar and superstructures that evolve.

Chain‐length dependence of interlayer interaction in crystalline n‐alkanes from Raman longitudinal acoustic mode measurements

An analysis of the observed frequencies of the Raman‐active longitudinal acoustic mode (LAM) bands of room‐temperature crystalline n‐alkanes in the chain‐length range C33 to C246 indicates that, as the chains become longer, there is a significant decrease in interlayer interaction, this interaction being measured by the vibrational coupling between the ends of chains in adjoining layers. This conclusion is based mainly on LAM measurements on n‐alkanes longer than 100 carbons that have recently become available. In the present analysis, the n‐alkane crystals are modeled as collinear monatomic chains having end‐to‐end interactions to simulate interlayer interaction. One intrachain force constant (F) and one interchain coupling force constant (f ) were evaluated from the observed frequencies of the LAM‐1 and LAM‐3 bands for each of seven n‐alkanes: C48, C62, C70, C72, C94, C150, and C192. As expected, the values found for F are essentially independent of chain length. However, the values of f were found to d...

The role of crystallinity in the crystallographic texture evolution of polyethylenes during tensile deformation

Abstract The crystallographic texture evolution of rapidly crystallized high-density polyethylene (HDPE) and ethylene-1-octene copolymer prepared with a Ziegler–Natta (LLDPE) catalyst, was studied during tensile deformation using wide-angle X-ray diffraction WAXD. The popLA software suite, a methodology based on spherical harmonics for texture analysis, was utilized to produce recalculated pole figures and orientation distribution function plots from the raw data. An important aspect of this work has been the in situ measurement of texture during tensile deformation and the subsequent measurement of texture in the relaxed samples. The difference in molecular structure of the polyethylenes had a strong effect in the initial texture as well as in the rate of texture evolution during deformation. Texture evolves slowly in the HDPE while a fast drastic change in texture is observed in LLDPE after yield. At higher strains the texture of LLDPE was basically unchanged revealing not only a strong (100)[001] ‘c-axis’ texture component, but also other weaker texture components such as (010)[001], (1×0)[010], (011)[100] and (201)[102]. Furthermore, while relaxation after unloading mitigated or eliminated two of the preferred texture components in HDPE strengthening the (001) component aligned along the extension axis, it was observed that the texture of LLDPE did not undergo any significant changes after relaxation. At large strains (e>1.0), microfibers formed in both HDPE and LLDPE. A lamellar substructure that differs between the HDPE and LLDPE is observed by AFM images of drawn materials. The role of possible slip mechanisms and stress relaxation in the evolving crystallographic texture of these polyethylenes is discussed.

Measurement of the 13C spin–lattice relaxation time of the non-crystalline regions of semicrystalline polymers by a cp MAS-based method

Abstract The non-crystalline 13C spin–lattice relaxation times of atactic and isotactic polypropylene and those of an ethylene–1-octene copolymer of low crystallinity have been measured by classical inversion and saturation recovery methods as well as by a cp MAS-based pulse sequence. The latter is a saturation recovery-type sequence that involves cross-polarization. It samples preferentially the soft non-crystalline regions of semicrystalline polymers. The method is found to be useful in determining T1C of the amorphous regions of semicrystalline iPP at room temperature. It is found that the atactic PP molecule and the non-crystalline iPP regions have the same average segmental relaxation rate. The T1C of some of the carbons investigated was