Study On The Brittle-Ductile Transition Mechanism And Structure-Property Relationship Of Poly(Methyl Methacrylate) With Synchrotron Radiation

Thanks to the high modulus,hardness,excellent optical properties and processability,glassy polymers have been widely used in various fields of human life,such as optical display,aerospace,transportation,and driving.However,glassy polymers generally exhibit brittle failure in service due to their amorphous characteristics,which greatly limits their application field.With the change of molecular structure and external field parameters,glassy polymers will show the brittle to ductile transition (BDT) within a narrow range below the glass transition temperature.Since there is no convincing theoretical explanation for the structural and kinetic heterogeneity of glassy polymers,and the nature of the glass transition of polymers is still controversial,understanding the BDT mechanism is difficult.A universal multiscale BDT mechanism is still lacking.Based on the in-situ synchrotron radiation small angle X-ray scattering (SAXS)technique with high temporal resolution and spatial resolution,the structural evolution of samples during deformation can be effectively tracked.This approach has great advantages in studying the structural transition around the yield point,which is a critical stage in determining the macro toughness of samples.In this paper,the BDT phase diagram of poly (methyl methacrylate) (PMMA) films was systematically studied under uniaxial and biaxial melt pre-stretching conditions,a wide range of temperatures and loading rates based on the home-made uniaxial tensile device and biaxial tensile device.By utilizing the in-situ synchrotron radiation SAXS technique and infrared dichroism characterization,the mesoscopic deformation mechanism and microscopic stress response mechanism of PMMA were studied under multiple external fields,and the multi-scale BDT mechanism was discussed.Furthermore,the application of glassy polymers in the field of the optical display was further explored.This work will deepen the understanding of the multi-scale BDT mechanism of glassy polymers,and provide theoretical support for the optimization of processing parameters and the regulation of mechanical properties of glassy polymers.The main conclusions of this paper are summarized as follows:(1) The structure evolution of melt pre-stretched PMMA was investigated using in-situ SAXS.By semi-quantitative analysis of the evolution of craze size,craze fraction,and fibril fraction within the craze,it is revealed that the kinetic competition between crazing and shear banding is the key to the macroscopic toughness of glassy polymers.Without melt pre-stretching, the sample exhibits almost the same BDT behavior in different directions.The formed crazes are enabled to grow and propagate stably via shear deformation of the matrix.While for samples with melt pre-stretching,crazing is further promoted when stretching along the transverse direction (TD),showing destructive craze propagation and premature fracture.In contrast,shear banding dominates the deformation when stretching along the pre-stretching direction(MD),and the premature craze damage was consequently avoided and the toughness was enhanced.(2) By comparing the BDT behavior and molecular orientation between uniaxial width-Free Melt pre-Stretched (FMS) and width-Constrained Melt pre-Stretched (CMS)PMMA,it is found that CMS samples exhibit higher ductility than FMS samples along both the MD and TD.Combining the results from molecular orientation and stress relaxation,it is proposed that chain stretching and aligned chain density are responsible for the ductile deformation along the MD and TD,respectively.Compared to FMS samples,width constraint in CMS samples results in a stronger chain stretching along the MD and higher aligned chain density along the TD.As a consequence,CMS samples exhibit a lower activation energy barrier along both MD and TD than FMS samples.The chain tension transfer and chain activation were then promoted under external force and an enhanced toughness was exhibited.(3) The BDT phase diagram of PMMA under different temperatures,loading rates,and simultaneous biaxial melt pre-stretching conditions was constructed.Combined with the in-situ SAXS technique and molecular orientation characterization,the multiscale BDT mechanism of PMMA was further discussed.The results show that the toughness of PMMA can be enhanced by increasing temperature,decreasing loading rate,and melt pre-stretching.The temperature field stabilizes the craze by affecting the kinetics of shear banding initiation,while the loading rate regulates the kinetic competition between the crazing and shear banding by changing the craze initiation time.Based on the BDT phase diagram of simultaneous biaxial melt pre-stretching conditions,it was revealed that the chain orientation improves the toughness by increasing the brittle fracture strength,the load-bearing capacity was thus improved and the rapid failure was avoided.(4) By blending tricresyl phosphate (TCP) and bisphenoxy ethanol fluorene (BPEF)with PMMA and PC,the effects of TCP or BPEF content and melt pre-stretching treatment on the thermal and optical properties of the blended samples were studied.The results show that TCP (with positive intrinsic birefringence) has a birefringence enhancement effect on PC and compensation effect on PMMA,while BPEF (with negative birefringence) has a birefringence compensation effect on PC.Besides,the birefringence enhancement or compensation effect is directly correlated with the content of small molecules and melt stretching ratio.