Polyolefins based crystalline block copolymers: Ordered nanostructures from control of crystallization
Graphical abstract
Introduction
Block copolymers (BCP) consist of chemically distinct polymer chains covalently linked to form a single molecule. Owing to their mutual repulsion, dissimilar blocks tend to segregate into different domains, the spatial extent of the domains being limited by the constraint imposed by the chemical connectivity of the blocks [[1], [2], [3], [4], [5]]. The system tends to minimize the area at the unfavorable interface between the two incompatible blocks to lower the interfacial energy via formation of the intermaterial dividing surfaces (IMDS). Therefore, the driving force for microphase separation in amorphous block copolymers is the incompatibility between the blocks that are chemically linked, but the formed structure is a result of the competition between phase separation and chemical connectivity of the different blocks [[1], [2], [3], [4], [5]].
From an entropic standpoint, the molecules prefer random coil shapes but the blocks are stretched away from the IMDS to avoid unfavorable contacts. As a result of these competing effects, self-organized periodic microstructures emerge on the nanoscopic length scale. The ordered microstructures have periods on the order of the polymer chain dimensions and different microphase-separated morphologies can form depending on the inherent block incompatibility (characterized by χ, the Flory-Huggins segmental interaction parameter, which is generally found to be inversely proportional to the temperature), the total degree of polymerization (Nt), the volume fraction of the components and the chain architecture as well as the persistence lengths of the respective blocks [6]. For high values of the product χNt the block copolymer is strongly segregated and well organized microdomain structures result [[6], [7], [8]]. For low values of the product χNt (low block incompatibility and/or low molecular weight and/or at high temperature) the block copolymer presents a homogeneous disordered phase [[6], [7], [8]].
In the simplest case of non-crystalline flexible coil AB diblock copolymers, the composition of the AB diblock (i.e. the volume fraction fA of block A) controls the geometry of the microdomain structure [1,2,[6], [7], [8], [9]]. As shown in Fig. 1 [9], for high compositional asymmetry, the minority component forms spheres arranged on a body-centered cubic lattice and, then, hexagonally packed cylinders with slight increase of the volume fraction of the short block. For nearly symmetric diblocks fA = 0.5 a lamellar morphology forms characterized by alternating layers of the two blocks. For moderate compositional asymmetries, a complex bicontinuous morphology, known as the double gyroid phase, has been observed in which the minority blocks form domains consisting of two interweaving threefold coordinated networks [1,2,[6], [7], [8], [9]]. The equilibrium morphologies of ABC triblock terpolymers are more diverse than those of AB diblock copolymers [10]. In ABC triblock terpolymers, there exist two composition variables and three interaction parameters, which make their phase behaviors much more complicated than AB diblocks having only one composition variable and one interaction parameter [2]. More extensive information regarding morphologies of complex BCP systems including ABC triblocks and star copolymers can be found in the literature [11,12].
More complex phenomena are involved in semicrystalline block copolymers containing one or more crystallizable blocks. First of all the final morphology depends on the competition between two thermodynamic transitions, phase separation between incompatible blocks and crystallization of one or more blocks and different microstructures may develop depending on which of the two processes has a stronger driving force. Moreover, the final structure is also affected by the succession of phase transformations upon cooling from the melt and depends on which transition occurs first, phase separation or crystallization. Complex phenomena such as, crystallization confined in preformed microdomains, breaking out of the structure formed in the melt by the subsequent crystallization, phase separation driven by crystallization or crystallization of one block confined within the crystalline domains previously formed, define the final morphology and properties of crystalline block copolymers.
In this paper, the morphologies and crystallization properties of crystalline BCPs composed of one or more crystallizable blocks of polyolefins are reviewed. In particular, the equilibrium morphologies expected in crystalline-amorphous diblock copolymers, the effects of confinement on crystallization mode and kinetics and methods of formation of ordered nanostructures through orientation of the crystal phase are described.
Section snippets
Crystalline block copolymers
Crystallizable block copolymers have been mainly studied in the past for their possible application as replacements for glassy/rubbery BCPs as thermoplastic elastomers due to their improved mechanical properties as well as better thermal stability [[13], [14], [15], [16], [17], [18], [19]]. Recently, it has been demonstrated that the presence of a crystallizable component can be exploited for controlling the final morphology through the control of crystallization and orientation of the crystals
Ordered nanostructure from crystalline BCPs
The ability of BCPs to form spontaneously periodic nanostructures (Fig. 1) with periodicity and dimensions of nanodomains in the range from 5 to 100 nm, has attracted increasing interest for many applications that require generation of nanometer scale patters. In fact, self-assembly is emerging as elegant bottom-up method for fabricating nanostructured materials [[95], [96], [97], [98], [99]]. This perspective has heightened an already growing interest in the use of self-assembly of materials
Crystalline block copolymers from stereoregular polyolefins and linear polyethylene
Crystalline BCPs containing blocks based on stereoregular polyolefins (for instance, isotactic or syndiotactic polypropylene) have been less studied due to the difficulty of the synthesis in performing living polymerization with high stereochemical control. In recent years BCPs containing crystallizable stereoregular polyolefins and linear PE have been synthesized thanks to the development of metal-based coordination polymerization methods able to ensure a high stereochemical control, as in
Conclusions
The morphologies and crystallization properties of crystalline block copolymers composed of one or more crystallizable blocks of polyolefins are reviewed. In particular, the equilibrium morphologies expected in crystalline-amorphous diblock copolymers and the different modes of crystallization, such as, breakout, templated crystallization, confined crystallization and pass through crystallization are described. The effects of confinement on crystallization mode and crystallization kinetics and
CRediT authorship contribution statement
Claudio De Rosa: Conceptualization, Writing - original draft, Writing - review & editing. Rocco Di Girolamo: Investigation, Data curation, Methodology. Anna Malafronte: Investigation, Methodology. Miriam Scoti: Investigation, Methodology. Giovanni Talarico: Methodology. Finizia Auriemma: Investigation. Odda Ruiz de Ballesteros: Investigation, Methodology.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Claudio De Rosa is full professor of Macromolecular Chemistry at the University of Naples Federico II (Italy). He received the Master degree in Chemistry in 1983 and the Ph.D. in Chemistry in 1989 from the University of Naples. His research activity comprises the study of the crystal structure of polymers and the interplay between physical properties and the molecular structure of polymers. He is author of more than 300 papers.
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Claudio De Rosa is full professor of Macromolecular Chemistry at the University of Naples Federico II (Italy). He received the Master degree in Chemistry in 1983 and the Ph.D. in Chemistry in 1989 from the University of Naples. His research activity comprises the study of the crystal structure of polymers and the interplay between physical properties and the molecular structure of polymers. He is author of more than 300 papers.
Rocco Di Girolamo is assistant professor of Industrial Chemistry at the University of Naples Federico II (Italy). He graduated in Industrial Chemistry at the University of Naples in 2007 and got the Ph.D in Chemistry in 2010 from the same University. His main research interests are: synthesis and characterization of block copolymers and study of the relationships between the molecular structure and the physical properties of crystalline polymers.
Anna Malafronte is researcher of Industrial Chemistry at the University of Naples Federico II (Italy). She graduated in Chemical Sciences at the University of Naples in 2011 and she got the Ph.D in Chemical Sciences in 2015 from the same University. Her main research interests are the study of periodic nanostructures formed through self-assembly of block-copolymers and study of the relationships between the molecular structure and the physical properties of crystalline polymers.
Miriam Scoti is researcher of Industrial Chemistry at the University of Naples Federico II (Italy). She received the Master Degree in Chemical Sciences in 2014 and the Ph.D in Chemical Sciences in 2018 from the University of Naples. Her research interest is the study of the structure and morphology of crystalline polymers synthesized with organometallic catalysts and of the relationships between the molecular architecture induced by the catalysts and the crystalline structure and physical properties of polymers.
Giovanni Talarico is professor of Industrial Chemistry at the University Federico II of Naples (Italy). He received the Doctor degree in Chemistry in 1994 and Ph.D. in Chemistry in 1997 from the University of Naples. His main scientific interests are catalytic reactions promoted by transition metal from experimental and computational point of view.
Finizia Auriemma is full professor of Macromolecular Chemistry at the University of Naples Federico II (Italy). She received the Doctor degree in Chemistry in 1984 and the Ph.D. in Chemistry in 1989 from the University of Naples. Her research activity comprises the study of the crystal structure of polymers and of the disorder in polymer crystals, aimed at finding relationships between physical properties and the structures of polymers. She is author of about 200 papers.
Odda Ruiz de Ballesteros is professor of Industrial Chemistry at the University Federico II of Naples (Italy). She received the Doctor degree in Industrial Chemistry in 1993 and Ph.D. in Chemistry in 1996 from the University of Naples. Her scientific activity is mainly devoted to the study of the crystal structure, physical properties and relationships between molecular structure and mechanical properties of polymers.