Polymer Morphology and Deformation Behavior

Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ, USA 

Deformation behavior of amorphous polymers has been studied for years, and explained in terms of structure of polymers. Especially, great progress has been made by using the concept of network (strand) density to understand the deformation behavior (such as crazing and shear deformation) [1,2]. When the concept of a critical thickness of the polymer layer, below which a sample behaves in a ductile manner even for normally brittle polymers like polystyrene (PS), has been added to the network density concept, more comprehensive understanding has become possible [3-8] and it provides a great opportunity for developing ductility of otherwise brittle 

Finally, as an example showing significant influence of morphology on deformation behavior, block copolymers are chosen. Because of the microphase separation occurring between different blocks, block copolymers form various stmctures (morphologies), which in turn influence their deformation behavior. This is described in Section 18.4, in which only a small number of examples have been chosen to demonstrate the morphology-deformation relationships, because a vast amount of hterature and work cannot be covered. Morphology and characterization of block copolymers are also described in several chapters in this book. 

Polymer Morphology Principles, Characterization, and Processing, First Edition. Edited by Qipeng Guo. 2016 John Wiley Sons, Inc. Published 2016 by John Wiley Sons, Inc. 

---

H. J. Ludwig and R Eyerer, Influence of the processing conditions on morphology and deformation behavior of polyfbutylene terephthalate) (PBT), Polym. Eng. Sci., 28 143-146,1988. 

The existence of tie molecules and/or tie fibrils was initially demonstrated by Keith and Padden (19, 20) via electron microscopy. The relationship of these morphological structures to the mechanical strength and deformation behavior of polymers has been demonstrated by Peterlin (21) and by Becht, DeVries, and Kausch (22). Fracture during low strain rate tensile failure can be expected to occur along a path of mini-... 

In polymer blends, both the morphology and flow behavior depend on the deformation field. Under different flow conditions the blend may adopt different structures, hence behave as different materials. Note that in multiphase systems, the relationships between the steady state, dynamic and elongational viscosities (known for simple fluids) are not observed. Similarly, the time-temperature (t-D superposition principle that has been a cornerstone of viscoelastometry is not valid. 

It seems plausible to assume that the fracture deformation behavior presented here may be characteristic of all polymers that are readily doubly orientable and that have an easy slip plane. Also note that the V-shaped fracture tip is absent in the other orientation geometries (Figure 3A and 3C). The morphological details of the deformation seem to be affected somewhat by environmental conditions, but it is too early to make precise comments on this point. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

Block copolymers in selective solvents exhibit a remarkable capacity to self-assemble into a great variety of micellar structures. The final morphology depends on the molecular architecture, tlie block composition, and the affinity of the solvent for the different blocks. The solvophobic blocks constitute the core of the micelles, while the soluble blocks form a soft and deformable corona (Fig. Id). Because of this architecture, micelles are partially Impenetrable, just like colloids, but at the same time inherently soft and deformable like polymers. Most of their properties result from this subtle interplay between colloid-like and polymer-like features. In applications, micelles are used to solubilize in solvents otherwise insoluble compounds, to compatibilize polymer blends, to stabilize colloidal particles, and to control tire rheology of complex fluids in various formulations. A rich literature describes the phase behavior, the structure, the dynamics, and the applications of block-copolymer micelles both in aqueous and organic solvents [65-67],... 

For most blends, the morphology changes with the imposed strain. Thus, it is expected that the dynamic low strain data will not follow the pattern observed for the steady-state flow. One may formulate it more strongly in polymer blends, the material morphology and the flow behavior depend on the deformation field, thus under different flow conditions, different materials are being tested. Even if low strain dynamic data can be generalized using the t-T principle, those determined in steady state will not follow the pattern. 

Blends are classified as either thermodynamically miscible or immiscible, with the latter dominating. However, imposition of a flow affects the thermodynamic equilibrium and it may enhance the miscibiUty of immiscible blends or vice-versa - there is an interrelation between rheology and thermodynamics. Similarly, flow affects the degree of deformation of the dispersed phase, thus in immiscible blends there are other interrelations between rheology and morphology, which affect the blend performance. To the complexity of polymer alloys and blends (PAB) behavior one must add the incorporation of soUds, either in the form of filler and nanofiller particles or by simple factofblendingtwocomponents with widely differenttransitiontemperatures. 

Chang and co-workers (52,53) studied the large-scale deformation behavior and recovery behavior of semicrystalline ethylene-co-styrene polymers as a function of temperature, comonomer content, and crystallinity and compared them to the behavior of metallocene-produced ethylene/l-octene copolymers. Chen and co-workers (54) have provided an in-depth comparison of the morphological structure and properties of copolymers and confirmed that aspects of deformation that depended on crystallinity, such as yielding and cold drawing, were determined primarily by comonomer content for both sets of copolymers. 

---