Strain Dependent Evolution Of Structure And Stress In Propylene-based Elastomer During Stress Relaxation

The propylene-based elastomers with a high content of ethylene co-units always exhibit excellent elastomeric properties,which arise from homogeneously dispersed small crystallites and a large amount of amorphous chains in the bulk.Propylene-based copolymers are also semicrystalline polymers having the same crystalline forms as isotactic polypropylene(iPP)and have been applied ubiquitously in industry and our daily lives such as wire skin,impact modifier,membranes and so on because of their excellent elasticity and the specific crystallization behavior.To explore the service performance of propylene-based elastomers,the stress relaxation and the structural evolution behavior of random propylene-ethylene copolymers with various contents of ethylene co-units,which were pre-stretched at various strains at 30?,were investigated by means of in situ wide angle X-ray diffraction(WAXD)techniques and a specific data processing method-"Halo method" to decompose the scattering pattern into oriented and unoriented parts.Main results of the thesis are as follows:1.The stress relaxation behaviors of propylene-based elastomer of about 12 mol%of ethylene co-units with pure ? form(iPPcoE12-?)were investigated at 30? at various pre-stretched strains.The trend of stress decay was similar at different prestretched strains while the process of structural evolution was different.The stress decreased rapidly at the beginning of relaxation process and then showed a slow decay followed by a nearly constant value independent of pre-stretched strains.This decrease in stress during relaxation was accompanied by an obvious increase in the orientational degree and the crystallinity.However,the processes of increasing the crystallinity depended strongly on the imposed strain.At small strain(?H?0.17),besides the transition of some oriented amorphous phase into oriented crystals,a fraction of unoriented amorphous phase transformed into unoriented crystals.At medium strains(?H?0.76 and 1.28),a fraction of unoriented amorphous phase and unoriented crystals transformed into oriented crystals.A very small fraction of unoriented amorphous phase transformed into unoriented mesomorphic phase.At large strains(?H = 1.63 and 1.85),a fraction of oriented amorphous phase transformed to oriented crystals.The different network with varied fraction of amorphous and crystalline phases accounted for the distinct transformation processes at different pre-stretched strains.Especially at medium strains,the "appropriately stretched" high elastic network was formed so that higher portion of stress remained after the relaxation,which means the fraction of elastic stress ?e/?0 was higher.Furthermore,it was found that crystallization during stress relaxation provides an alternative route for lowering the macroscopic stress by reducing the effective number of network chain segments in unit volume.2.The stress relaxation behaviors of a propylene-based elastomer of about 12 mol%of ethylene co-units with pure ? form(iPPcoE12-?),which was pre-stretched at various strains at 30?,were investigated.The decay process of the stress during the relaxation was similar with that of iPPcoE12-?.And an obvious increase in the orientational degree and the crystallinity that occurs in iPPcoE12-? also appeared for iPPcoE12-?.The processes of increasing the crystallinity also depended on the imposed strain.Due to the unique "cross-?"arrangement in ? form,iPPcoE12-? exhibited a better elasticity and structural stability before stretching to a critical strain of fragmentation of y crystalline blocks.At small strain(?H=0.27),the fraction of elastic stress ?e/?0 of iPPcoE12-? was higher than that of iPPcoE 12-? and at medium strains(?H?0.71 and 1.27)the increased fraction of oriented parts was much less than that of iPPcoE12-?.By contrast,iPPcoE12-? was more elastic than iPPcoE12-? at large strain(?H=1.95).In addition,it was found that the y to a transition occurred during the relaxation,while crystallites with ? form were stable until orienting and the unoriented crystallites with y form were more difficult to transform than the oriented one.3.The stress relaxation behaviors of propylene-based elastomers of about 6 mol%of ethylene co-units with a form(iPPcoE6-?),about 16 mol%of ethylene co-units with a and ? form(iPPcoE 16-??)and about 6 mol%and 16 mol%of ethylene co-units with pure y form(iPPcoE6-y and iPPcoE16-y),which were pre-stretched at various strains at 30?,were investigated and compared with iPPcoE12-? and iPPcoE12-?.The decay processes of the stress during the relaxation of all samples,that stress decreased rapidly at the beginning of relaxation process and then showed a slow decay followed by a nearly constant value,were similar.And an obvious increase in the orientational degree and the crystallinity also appeared during the relaxation of all samples except for iPPcoE6-? pre-stretched at small strain(?H?0.15)with no change of the orientational degree and the crystallinity.The processes of structural evolution were essentially similar for iPPcoE6-? and iPPcoE12-? pre-stretched at similar strains,which were different from that of iPPcoE16-ay with ? and y form.The processes of structural evolution of iPPcoE6-y and iPPcoE12-? pre-stretched at medium strains showed features of crystallites with ? form that are unoriented and oriented y form are transformed to oriented ? form.While due to less fraction of crystallites with ? form,the phase transformation of ? into ? did not occur during the relaxation of iPPcoE16-?,which was similar-with samples with pure ? form like iPPcoE6-? and iPPcoE12-?.Various contents of ethylene co-units influenced the crystallinity of elastomers and had a further impact on the increasement and the decrement of fractions of various phases during the relaxation.And the dependencies of the fraction of elastic stress ?e/?0 of all samples pre-stretched at various on the content of ethylene co-units were discussed.It was similar that the ?e/?0 firstly increased and reached a higher value at medium pre-stretched strain,then decreased with increasing pre-stretched strains of iPPcoE6-?and iPPcoE12-?.For iPPcoE 16-???iPPcoE6-??iPPcoE12-y and iPPcoE16-y,the?e/?0 decreased with increasing strains for all these samples.Although there were similar trends of the ?e/?0 going with the change of pre-stretched strains,the ?e/?0 was higher of a sample with higher content of ethylene con-units and lower crystallinity.