Double Yielding in Deformation of Semicrystalline Polymers

Abstract

studies ignored the crucial role of the amorphous phase in delivering required force and chain tension into the crystalline phase in order to disintegrate it, and mainly focused on modeling the stressstrain curves or characterize the changes in crystalline structure. [8-13] However, the right strategy to tackle this elusive problem, i.e., molecular mechanics of semicrystalline polymers, should be a bottom up approach that first understands melt rheology and molecular mechanics of pure glassy phase and in the next stage by incorporating crystalline phase into the disordered phase tries to perceive the effects of crystalline phase on the mechanical performance of the resultant composite system. In this regard, study of semicrystalline polymers with higher T g than room temperature such as polyesters; e.g., poly(l-lactic acid) (PLLA) and poly(ethylene terephthalate) (PET), could be more useful in providing suitable model polymers that are able to be quenched below T g and produce amorphous counterparts as control samples. Another benefit with these polymers is that we are able to easily study the mechanical behavior both below and above T g. Previous studies by Flory [14] and Peterlin [15,16] tried to understand the yielding of semicrystalline polymers by characterizing the changes of crystalline phase in presence of stress. More recent studies by Strobl [17,18] on semicrystalline polymers with high T g treated the crystalline phase as a force-transmitting skeleton. Unlike preceding studies, the objective of this paper is to understand the semicrystalline state as a composite structure of amorphous and crystalline phases by focusing on the imperative role of amorphous phase in connecting crystalline regions to one another and providing local stress exerted on crystalline regions to cause yielding to occur in the crystalline phase. The importance of chain networking in amorphous phase, and interaction of amorphous phase with crystalline phase in delivering mechanical input into crystalline regions are highlighted in our study. Based on the new framework, for the first time, to the best of authors′ knowledge, the brittle response of a semicrystalline polymer under compression deformation is predicted and experimentally demonstrated in this study. We demonstrate that the effective deformation of crystalline phase is impossible in absence of a robust amorphous phase. In other words, based on our results that is the amorphous phase, which its deformation generates Different semicrystalline polymers including poly(l-lactic acid), poly(ethylene terephthalate), syndiotactic polystyrene, and polyamide 12 are studied in terms of their mechanical response to uniaxial compression deformation. Apparent decoupling of yielding of amorphous and crystalline phases is identified as separate peaks in the stress-strain curve in the vicinity of the glass transition temperature. The same feature is also observed for the uniaxial extension of predrawn semicrystalline poly(ethylene terephthalate). It is indicated that in absence of a stron...

The investigation of the stress cracking behavior of PBT by acoustic emission

“…The presence of a plateau after the maximum stress in Figure 3 is due to the transformation from α to β form of the triclinic phase of PBT, caused by the deformation, 36 then a second yield point is noted. The presence of double yield points is not very rare, and was observed with some polymers like low density polyethylene, 37 polyethylene terephthalate 38 and polyamide 6. 39 In the case of PBT, the second yield correspond to the end of the α-β transformation 40 and the neck is formed from this crystal structure following the usual mechanism for this type of polymer.…”

Enhanced Mechanical Properties of Uniaxially Stretched Polylactide/Poly(ethylene oxide)-b-Poly(butylene oxide) Blend Films

“…For most processes, chain orientation occurs predominately in the direction of extrusion, referred to as the machine direction (MD), in order to maximize the throughput and/or downgauge sheet/film thickness. Previous studies − have shown that uniaxial stretching reduces the activation energy for plastic flow (shear yielding), leading to ductile deformation when mechanically tested parallel to the chain orientation direction. In contrast, mechanical properties in the perpendicular or transverse direction (TD) typically remain the same or diminish compared to the unoriented material. , Biaxial orientation/stretching can be employed to balance the properties in the MD and TD; however, such processing requires sophisticated equipment and a meticulous stretching procedure, increasing the cost of biaxially oriented films compared to uniaxial analogues. , Therefore, methods to produce uniaxially stretched tough films that have balanced properties in both the MD and TD are highly desirable.…”

Overaging with Stress in Polymer Glasses? Faster Segmental Dynamics despite Larger Yield Stress!

“…The consequence of this treatment is an increase in the strength and yield stress of alloys . It has been observed that application of stress during heat treatment encourages the precipitation process and results in larger increase in the yield stress. − Recently, it has been shown that yielding of high- T g semicrystalline polymers can occur with two yielding peaks, an initial yielding of the glassy phase followed by yielding of the crystalline phase . In light of this, the observed behavior in Kramer’s study is likely a glassy feature rather than some events in the crystalline phase and in this regard the governing physics would not be different than in polycarbonate , and PMMA.…”