An official website of the United States government
Toughness is a critically important trait for commercial semicrystalline plastics, ultimately motivating decades of research into the structure-property relationships relating to their mechanical behavior. Within this thesis, the crystalline and amorphous morphologies in semicrystalline polymers were modulated through the synthesis of novel random and block copolymers, with emphasis placed on studying their ductility and lamellar fragmentation mechanism. Hydrogenated polynorbornene-containing copolymers were used as the model system here because of their ease of synthesis and model properties. In the first section of this work, the brittle-ductile transition of hydrogenated polynorbornene was analyzed, where bulk crystallinity was varied through random copolymerization of norbornene with hexylnorbornene. The tie molecule fraction at the brittle-ductile transition was observed to be a strong function of initial crystal thickness, and we conclude that tie molecules are lost during lamellar fragmentation; the fraction lost is inversely related to initial crystal thickness. In the second section of this work, the morphologies of the slow-cooled random copolymers were analyzed. Unexpectedly, these specimens featured two separate types of lamellar stack. We ultimately explained this behavior through analysis of their maximum crystallizable sequence length distribution. For the low molecular-weight copolymers analyzed, a large fraction of chains is unable to crystallize with the primary stack, and these chains ultimately crystallize into a separate lamellar stack at a substantially lower temperature. The mechanical properties of these materials were then analyzed, where the secondary stacks were determined to govern bulk ductility in the highlighted case. In the final section of this work, block copolymers consisting of long crystalline segments and short amorphous segments were analyzed. These short amorphous segments were found to promote ductility, manifesting as a drop in the polymers’ tie molecule content at the brittle-ductile transition. Additionally, the effects of amorphous-layer segmental dynamics on the lamellar fragmentation mechanism were investigated. Novel triblock copolymers were synthesized with the sequence of short rubbery and glassy blocks alternated. Positioning the glassy block at the end of the chain led to a ~25% increase in the yield stress compared to the triblock copolymer with the rubbery endblock.
more »
« less
This content will become publicly available on January 13, 2027
Re-entry with Rate and Temperature of Brittle–Ductile Transition in Glassy Polymers
This study investigates brittle-to-ductile transition (BDT) and re-entry phenomena in glassy polymers, against the background provided by the conventional Ludwik-Davidenkov-Wittman-Orowan (LDWO) hypothesis and available phenomenological theory for BDT (J. Chem. Phys. 141, 094905 (2014)).Through systematic experiments on bisphenol A polycarbonate (bpA-PC), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), we explore effects of polymer structure (e.g., crosslinking), temperature, strain rate, and sample geometry on mechanical characteristics of these polymers. We found adequate crosslinking to rend PMMA ductile at room temperature. We discovered a non-monotonic relationship regarding temperature and rate effects on polymer ductility: a ductile polymer can turn brittle at higher temperatures; a polymer is ductile in fast extension and becomes brittle in slow extension. Moreover, a specimen is ductile when it is shorter, but a long specimen can be brittle. These findings significantly expanded and improved our theoretical understanding of polymer ductility in glassy state.
more »
« less
- Award ID(s):
- 2210184
- PAR ID:
- 10691749
- Publisher / Repository:
- ACS Publications
- Date Published:
- Journal Name:
- Macromolecules
- Volume:
- 59
- Issue:
- 1
- ISSN:
- 0024-9297
- Page Range / eLocation ID:
- 279 to 287
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Toughness is a critically important trait for commercial semicrystalline plastics, ultimately motivating decades of research into the structure-property relationships relating to their mechanical behavior. Within this thesis, the crystalline and amorphous morphologies in semicrystalline polymers were modulated through the synthesis of novel random and block copolymers, with emphasis placed on studying their ductility and lamellar fragmentation mechanism. Hydrogenated polynorbornene-containing copolymers were used as the model system here because of their ease of synthesis and model properties. In the first section of this work, the brittle-ductile transition of hydrogenated polynorbornene was analyzed, where bulk crystallinity was varied through random copolymerization of norbornene with hexylnorbornene. The tie molecule fraction at the brittle-ductile transition was observed to be a strong function of initial crystal thickness, and we conclude that tie molecules are lost during lamellar fragmentation; the fraction lost is inversely related to initial crystal thickness. In the second section of this work, the morphologies of the slow-cooled random copolymers were analyzed. Unexpectedly, these specimens featured two separate types of lamellar stack. We ultimately explained this behavior through analysis of their maximum crystallizable sequence length distribution. For the low molecular-weight copolymers analyzed, a large fraction of chains is unable to crystallize with the primary stack, and these chains ultimately crystallize into a separate lamellar stack at a substantially lower temperature. The mechanical properties of these materials were then analyzed, where the secondary stacks were determined to govern bulk ductility in the highlighted case. In the final section of this work, block copolymers consisting of long crystalline segments and short amorphous segments were analyzed. These short amorphous segments were found to promote ductility, manifesting as a drop in the polymers’ tie molecule content at the brittle-ductile transition. Additionally, the effects of amorphous-layer segmental dynamics on the lamellar fragmentation mechanism were investigated. Novel triblock copolymers were synthesized with the sequence of short rubbery and glassy blocks alternated. Positioning the glassy block at the end of the chain led to a ~25% increase in the yield stress compared to the triblock copolymer with the rubbery endblock.more » « less
-
Based on spatially-temporally resolved polarized optical microscopy (str-POM) measurements, we studied the fracture behavior of ductile and brittle glassy polymers as well as highly crosslinked rubbers to draw the following conclusions: (1) there is no tip plasticity below a threshold load in ductile plastics such as polyethylene terephthalate. (2) In ductile polymer glasses, before tip yielding at a common tip stress, the remote load scales with notch length a as a1/2, in agreement with the Inglis solution. (3) A finite stress saturation zone is observed in elastomers at loading levels even well below fatigue threshold due to significant crack tip blunting. (4) When the thickness is small enough for the plane stress condition to prevail at crack tip, in double-edge notch tension (DENT) for both ductile glassy polymers and rubbers that is characterized by ligament length l, nominal strain in the ligament is defined by elig = X/l, where X is tensile displacement; tensile force F increases linearly with X independent of l; tip stress increases linearly with the far-field slig (Belig). By demonstrating stress concentration at the crack tip in DENT in elastic materials and characterizing crack propagation in ductile polymers, the present study fills the missing gap in our understanding of fracture behavior in a wider range of polymeric materials. The acquired knowledge may be useful to guide specific design for packaging materials.more » « less
-
Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a “brittle-to-ductile” transition (BDT) region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a BDT in marble during deformation experiments in the laboratory is accompanied by systematic increase in the frequency of acoustic emissions suggesting a profound change in the mean size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle–ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At higher pressures, signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole pressure range, in agreement with postmortem microstructural observations. Our measurements provide unique, real-time data of microscale dynamics over a broad range of pressures (10 to 200 MPa) and can inform micromechanical models for semi-brittle deformation.more » « less
-
The effect of composition and morphology on mechanochemical activation in nanostructured block copolymers was investigated in a series of poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) (PMMA-b-PnBA-b-PMMA) triblock copolymers containing a force-responsive spiropyran unit in the center of the rubbery PnBA midblock. Triblock copolymers with identical PnBA midblocks and varying lengths of PMMA end-blocks were synthesized from a spiropyran-containing macroinitiatior via atom transfer radical polymerization, yielding polymers with volume fractions of PMMA ranging from 0.21 to 0.50. Characterization by transmission electron microscopy revealed that the polymers self-assembled into spherical and cylindrical nanostructures. Simultaneous tensile tests and optical measurements revealed that mechanochemical activation is strongly correlated to the chemical composition and morphologies of the triblock copolymers. As the glassy (PMMA) block content is increased, the overall activation increases, and the onset of activation occurs at lower strain but higher stress, which agrees with predictions from our previous computational work. These results suggest that the self-assembly of nanostructured morphologies can play an important role in controlling mechanochemical activation in polymeric materials and provide insights into how polymer composition and morphology impact molecular-scale force distributions.more » « less
