María L. Arnal

Applications of Successive Self-Nucleation and Annealing (SSA) to Polymer Characterization

A new technique to thermally fractionate polymers using DSC has been recently developed in our laboratory. The applications of the novel successive self-nucleation and annealing (SSA) technique to characterize polyolefins with very dissimilar molecular structures are presented as well as the optimum conditions to thermally fractionate any suitable polymer sample with SSA. For ethylene/α-olefin copolymers, the SSA technique can give information on the distribution of short chain branching and lamellar thickness. In the case of functionalized polyolefins, detailed examinations of SSA results can help to establish possible insertion sites of grafted molecules. The application of the technique to characterize crosslinked polyethylene and crystallizable blocks within ABC triblock copolymers is also presented.

Nucleation and crystallization of isotactic poly(propylene) droplets in an immiscible polystyrene matrix

In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1-2 μm), the iPP crystallizes in a fractionated fashion as temperatures between 104 and 42°C. By applying a self-nucleation procedure, we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i.e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sulficient amount of a compatibilizer is used to obtain very small particle sizes and more homogenerous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exbibits an onset at 51°C and peaks at 46°C. Wide-angle X-ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α-phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity-free droplets that nucleate homogeneously at temperatures close to 51°C.

Miscibility of linear and branched polyethylene blends by thermal fractionation : use of the successive self-nucleation and annealing (SSA) technique

Abstract In the present work, the recently developed thermal fractionation technique successive self-nucleation and annealing (SSA) was applied to blends of branched and linear polyethylenes (PEs), as a method to evaluate the phenomena of miscibility and segregation in PE blends. The melting behavior of the systems, after a controlled cooling and after the application of SSA was compared. The DSC scans corresponding to cooling of blend samples from the melt or subsequent heating displayed an overlap of the exotherms and endotherms, respectively, of the blend components that could lead to interpretations of partial miscibility. These effects were interpreted as arising from: a nucleation effect of the crystals formed by the more linear PE on the crystals of branched PE, a dilution effect of the molten branched chains on the crystals formed by the more linear PE and finally possible partial miscibility effects. However, after SSA, even when the nucleation and dilution effects are still present for some of the fractions produced, the thermal fractionation procedure helps to distinguish them from co-crystallization effects. This is mainly achieved by observing how the number of thermal fractions generated by SSA in the blends varies with composition and by comparing the relative amounts of the thermal fractions produced by SSA. The results indicate that only those PE fractions that are similar in chemical structure, as regards to content and distribution of short chain branches, are probably miscible in the melt and can oppose molecular segregation during SSA and therefore produce stable co-crystals in the solid state. The application of SSA to the study of PE blends can be a quick and convenient tool for making comparisons and ascertain miscibility of different types of PE blends.

Fractionated crystallisation of polyethylene and ethylene/α-olefin copolymers dispersed in immiscible polystyrene matrices

In this work, several PS/HDPE (high density polyethylene), PS/LLDPE (linear low density polyethylene) and PS/ULDPE (ultra low density polyethylene) blends were prepared in a composition range were atactic polystyrene (PS) was always the matrix component. All the types of polyethylenes employed, when dispersed into droplets, exhibited fractionated crystallisation exotherms in the temperature range between 67 and 70 °C. These low crystallisation temperatures are probably closer to the homogeneous nucleation temperature of linear polyethylene than any previously reported value, since they occur at higher supercoolings. The higher values of crystallisation temperatures previously reported could be explained by the crystallisation from heterogeneous nuclei of relatively low nucleation efficiency or by a weak nucleation capacity of the droplets interface. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i. e., those normally active at low supercoolings in the bulk polymer) in every droplet. When polyethylene/α-olefin copolymers are finely dispersed in a PS matrix, a molecular segregation process can be induced during mixing facilitated by the heterogeneous distribution of short chain branches in the copolymers. The crystallisation of droplets that contained mostly these highly branched chains occurs at temperatures lower than 50 °C, thereby producing another low temperature exotherm which may be the lowest present in the blend and could be mistaken by the crystallisation of homogeneously nucleated crystals. We have shown that the self-nucleation technique can help to distinguish these low temperature exotherms from those originated by differences in nucleation behaviour; therefore, a plausible interpretation of all the possible fractionated crystallisation exotherms of ethylene/α-olefin copolymer droplets was made possible.

The evaluation of the state of dispersion in immiscible blends where the minor phase exhibits fractionated crystallization

SummaryDifferential Scanning Calorimetry (DSC) can provide a qualitative measure of the state of dispersion of an immiscible blend if the minor phase exhibits fractionated crystallization when dispersed into fine particles. The technique is only sensitive to the volume of the dispersed particle and not to its shape and can only be used when the exotherms of interest do not overlap with other thermal transitions present in the multicomponent system. Selfnucleation is a valuable tool to ascertain the presence of fractionated crystallization. The morphology induced by fractionated crystallization in immiscible blends could lead to enhanced plastic deformation during yielding of the matrix.

Crystallization in Block Copolymers with More than One Crystallizable Block

Recent results on the crystallization of block copolymers with more than one crystallizable block are reviewed. The effect that each block has on the nucle- ation, crystallization kinetics and location of thermal transitions of the other blocks has been considered in detail. Depending on the thermodynamic repulsion between the blocks, the initial melt morphology in weakly segregated double crystalline di- block copolymers can be sequentially transformed by the crystallization of the dif- ferent blocks. The crystallization kinetics of each block can be dramatically affected by the presence of the other, and by the crystallization temperature; the magni- tude of the effect is a function of thermodynamic repulsion. Also the morphology has been investigated and peculiar double crystalline spherulites with intercalated semi-crystalline lamellae of each component have been observed in weakly segre- gated diblock copolymers. In the case of ABC triblock copolymers with more than one crystallizable block, many interesting effects have been found; among them, self-nucleation, sequential or coincident crystallization, and fractionated crystalliza- tion can be mentioned. Additionally, the effect of the topological constrains due to the number of free ends has been studied. Factors like chemical structure, molec- ular weight, molecular architecture and number of crystallizable blocks provide a very large number of possibilities to tailor the morphology and properties of these interesting novel materials.

New comb-like poly(n-alkyl itaconate)s with crystalizable side chains

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n ¼ 12; 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state 13 C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices. q 2003 Elsevier Science Ltd. All rights reserved.

Nucleation and crystallization of PS-b-PEO-b-PCL triblock copolymers

We have recently prepared a series of Polystyrene-b-Poly(ethylene oxide)-b-Polycaprolactone (PS-b-PEO-b-PCL or SEOCL) triblock copolymers of varying compositions and molecular weights. These ABC triblock copolymers present the peculiarity that two of the three blocks are able to crystallize upon cooling from an already phase segregated melt. When either of the crystallizable blocks or both are a minor phase, a fractionated crystallization process develops. The confinement of crystallizable blocks in the nanoscopic scale enables the clear observation in some cases of exclusive crystallization from homogeneous nuclei of two components within the triblock copolymer. The homogeneous nature of the nucleation was deduced since the supercooling attained is the maximum possible before vitrification of the material takes place. The self-nucleation domains were also found to depend on the composition and molecular weight of the copolymers. The block copolymers exhibited a marked decrease in crystalline memory and when the crystallizable blocks constitute minor phases, the self-nucleation domain disappears. The reason behind this behavior is that only at lower self-nucleation temperatures the density of self-nuclei becomes high enough to include at least one crystal fragment per confined microdomain in view of their vast numbers (e.g., 10 16 /cm 3 ).

Synthesis and structure of random and block copoly(β,l-aspartate)s containing short and long alkyl side chains

Copoly(α-alkyl-β,l-aspartate)s containing n-octadecyl and n-butyl side groups at different ratios were prepared by anionic ring-opening polymerization of the corresponding optically pure (S)-4-alkoxycarbonyl-2-azetidinones. Random copolymers were obtained by polymerization of comonomer mixtures. Diblock copolymers were achieved by sequential copolymerization using the living poly(α-n-octadecyl-β,l-aspartate) block to initiate the polymerization of (s)-4-n-butoxycarbonyl-2-azetidinone as a second block. Composition and sequence distribution were characterized by NMR with the help of a model copoly(β,l-aspartate) made of α-n-dodecyl and α-benzyl β,l-aspartate units. All the copolymers were found to adopt the layered structure made of 13/4 helices described previously for comb-like poly(α-n-alkyl-β,l-aspartate) homopolymers. Copolymers containing at least 50% of n-octadecyl side groups have these groups crystallized in an interlayer microphase and aligned normal to the main helical chain. Melting of the paraffinic crystallites happened within the 40–60°C temperature range with formation of a liquid-crystal phase in which side chains are molten but retain the alignment of the low temperature phase. Different from what is known to happen to poly(α-n-octadecyl-β,l-aspartate), no indications on the occurrence of a second structural transition were observed at higher temperatures.

Synthesis and characterization of aPP/aPS graft copolymers

SummaryGraft copolymers of atactic polypropylene (aPP) and polystyrene (PS) were synthesized and characterized by 13C NMR analysis. The 13C NMR spectra of the grafts exhibited changes with respect to physical blends of identical compositions. The most important occurred in the meso sequences of the aPP blocks of the copolymer. These changes suggest that some grafting took place during the synthesis. SEM micrographs indicate greater interfacial interaction between phases in the copolymers than in the physical blends. Thermal Analysis showed that the Tg of the copolymers is higher than that of the PS homopolymers prepared under the same conditions. This could be the result of an apparent increased in molecular weight caused by the grafting of the aPS into the aPP chains. TGA results indicated that the thermal stability of the copolymer decreases as the aPP content in the copolymer increases.