Changes of Crystal Structure and Morphology during Two-Way
Shape-Memory Cycles in Cross-Linked Linear and Short-Chain
Igor Kolesov,*,† Oleksandr Dolynchuk,† Dieter Jehnichen,‡ Uta Reuter,‡ Manfred Stamm,‡,§ and Hans-Joachim Radusch† †Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany ‡Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, D-01069 Dresden, Germany §Technische Universitaẗ Dresden, 01062 Dresden, Germany
ABSTRACT: The present work comprehensively describes the formation of crystal structure and morphology of crosslinked linear and short-chain branched polyethylenes during their nonisothermal crystallization under constant mechanical load. The crystalline phase of linear as well as medium branched polyethylenes with about 30 CH3/1000Cformed as a result of the crystallization process under load and accompanied by an anomalous elongationis represented by lamellae oriented perpendicular to the stretch direction with tilted folded chains. In contrast, highly branched polyethylene with about 60 CH3/1000C processed under similar conditions contains only small crystallites, whose c-axis is oriented parallel to the applied force. Experimentally determined crystallinity, type of crystalline structure as well as size and orientation of the crystallites were compared with theoretical predictions got by modeling the two-way shape-memory (SM) behavior. Qualitative and quantitative characteristics of the two-way SM effect and the experimental curves of temperature dependent strain as well as the features of generated crystalline structures are in good agreement with the theory. ■ INTRODUCTION
Already more than 60 years ago Flory1,2 and Mandelkern3,4 had predicted and observed the anomalous elongation of a polymer network accompanying its crystallization from the oriented “molten” state. Such an anisotropic state of “molten” polymer network, e.g., of cross-linked polyethylene, characterized by a marked orientation of the molecular chains, can be caused by the application of sufficiently high uniaxial extension.3,4 The surprising macroscopic elongation of a predeformed polymer network taking place during its crystallization under load5 (at constant force) was denoted as “anomalous” due to discrepancy between this phenomenon and the simultaneously observed logical increase of the storage modulus E′ as shown below.
Recently, Mather and co-workers have reported on the socalled two-way shape-memory effect (SME) in polycyclooctene/trans-polyoctenamer (PCO/TOR) cross-linked by dicumyle peroxide as well as in a chemical/physical double-network on the basis of poly(ε-caprolactone) (PCL) and polyhedral oligosilesquioxane (POSS), respectively.6,7 This dual two-way
SME is revealed as the considerable increment and decrement of strain in the course of nonisothermal crystallization and subsequent melting under load, respectively.6,7 In addition to the dual two-way SME, a two-way triple-shape memory effect, which is accompanied by the appearance of two strain steps during both nonisothermal crystallization and melting under load, has been detected and described by Lendlein and coworkers for copolymer networks prepared by cross-linking of two star-shaped precursors of poly(ω-pentadecalactone) and
PCL having different melting and crystallization temperatures.8
More recently, Pandini et al. have investigated the dual two-way
SME of cross-linked PCL.9,10
Both the anomalous stress-induced elongation of a polymer network initiated by the nonisothermal crystallization during cooling under load and the expected contraction of a crosslinked sample during heating under the same load triggered by melting of the oriented crystalline phase are the physical background of the two-way SME, which was observed and described in aforementioned works.5−10 The performance of
SME in cross-linked crystallizable polymers strongly depends on the properties of the covalent polymer network and the crystalline structure, which has to be generated in the specimen during isometric programming or shape-memory (SM) nonisothermal creep. In particular, melting and crystallization temperatures (Tm and Tc) serve as the SM switching temperatures, whereas cross-link density and crystallinity of network are responsible for the magnitude of elastic forces produced by loading and for the ability to store these elastic forces, respectively.5−11
Received: January 16, 2015
Revised: June 19, 2015
Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.5b00097
Macromolecules XXXX, XXX, XXX−XXX
In contrast to the irreversible one-way SME, the invertible two-way SME can be reproduced as long as a sample is loaded and the temperature change is sufficient to cause the subsequent crystallization and melting of a sample. Although some specific investigation has been performed in that field already, there is still a lack in experimental study of the physical background of the two-way SME.
A novel theoretical approach, which is able to explain and describe the two-way SME in cross-linked crystallizable polymers, has been derived recently by Dolynchuk et al.12,13
The proposed theory was developed on the basis of a threeelement mechanical model taking into consideration the viscoelastic deformation of entangled slipping macromolecules and crystallization/melting of a covalent polymer network as two basic mechanisms involved in SM performance.12,13 It is postulated that crystallizing/melting of the covalent network plays a key role in the two-way SME and is responsible for the anomalous stress-induced elongation of a sample during nonisothermal crystallization at cooling. On the basis, partly, of the theory of stress-induced crystallization of polymer networks under isometric and isothermal conditions proposed by Gaylord,14,15 a thermodynamic description of the behavior of covalent networks has been performed, which allows calculating the free energy change of the network deformed under constant load (force) and cooled down below the crystallization temperature at a constant cooling rate, i.e., under nonisometric and nonisothermal conditions, respectively. The theoretical approach enables the prediction of crystalline structure and orientation of the crystals in case of different number of chain links. It has been determined that the anomalous elongation is possible when the orientation of crystal chains formed at cooling is parallel to the direction of external load or makes a relatively small angle with it. The conclusions of the developed theory were successfully confirmed by means of fitting the temperature dependent strain in the course of two-way SME in high-density polyethylene (HDPE) experimentally obtained for the first time.12,13 However, the theoretical predictions about the type of crystalline structure, crystal thickness, and crystal orientation are still needed to be experimentally verified.