Crazing and Toughness in Diblock Copolymer-Modified Semicrystalline Poly(l-lactide)

Abstract

Sustainable semicrystalline poly­(l-lactide) (PLLA) was melt mixed with 5 wt % poly­(ethylene oxide)-b-poly­(butylene oxide) (PEO-PBO) diblock copolymer, resulting in blends that display an exceptional combination of properties. The blends were annealed at various temperatures, leading to different degrees of crystallinity. The addition of 5 wt % PEO-PBO produced finely dispersed liquid particles that caused a significant reduction in the time for crystallization after quenching from the melt, where T m = 166 °C. At 95 °C, the halftime for crystallization was t 1/2(95 °C) = t 1/2 o/7, while at 135 °C, t 1/2(135 °C) = t 1/2 o/5, where t 1/2 o is the time required to obtain 50% of the final extent of crystallization with pure PLLA. The block copolymer particles also enhanced the ductility of the blends by facilitating stress-induced cavitation and uniform crazing without impacting the modulus. Tensile toughness increased by 7–15 fold, scaling inversely with the degree of crystallinity. The deformation mechanism was investigated by small- and wide-angle X-ray scattering as a function of applied strain, revealing that the craze volume is dependent on crystallinity, while the crystal structure displayed minimal changes. Regardless of the extent of crystallinity, crazing was found to be the primary deformation mechanism, countering the ductile-to-brittle transition associated with the aging of PLLA. Adding 5 wt % PEO-PBO extends the strain at break from 4% for pure PLLA after 2 days to more than approximately 50% after 85 or more days of aging. These findings, along with the industrially relevant blend preparation method, reveal that PEO-PBO is a unique and potent additive that could expand the applications served by PLLA, promoting a more sustainable future.

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

“…Q 0 was also determined for BUλT films (Figure b), and found to be relatively constant at all λ values, indicating that crazing is the dominant deformation mechanism. These observations in the MD can be attributed to an increase in the craze stress (σ cz ) with increasing λ. σ cz of a stretched film is defined by σ normalc normalz = σ normalc normalz normali normals normalo + σ normalc normalo normaln normalf β where σ cz iso is the craze stress for isotropic films, σ conf is the conformational stress associated with the fixed stress after film stretching and then quenching, and β is a geometric constant smaller than 1. , The parameter σ conf is proportional to λ k , σ normalc normalo normaln normalf = G normale ( λ k 2 − 1 λ k ) where G e represents the elastic modulus associated with entanglements (see Table S3 and the Supporting Information regarding the detailed calculations and assumptions). A comparison between σ cz and σ Y for BUλM films is provided in Figure S17.…”

“…Turning to the blend films, BU1M/T is significantly tougher than NU1M/T, which is consistent with our previous work. , As λ increases from 1 to 8, ε B decreases monotonically from ∼200 to ∼35%, which is attributed to λ e being limited by λ max , similar to the NUλM films. It is worth noting that at the same λ, ε B and the overall tensile toughness of the BUλM films are always larger than those of the NUλM films, as shown in Figure c,d.…”

“…These differences were further quantified by calculating the scattering invariant, Q 0 , along the equatorial axis (across the azimuthal angles −15 to 15°) (see the Supporting Information). The magnitude of Q 0 is proportional to the craze volume ( V ) as described elsewhere. , As shown in Figure a, only BU2M exhibits a large Q 0 , which increases with strain, while the other stretched films (both neat and blend films) generated limited scattering intensity. Note: despite the weak equatorial pattern found in NU2M, NUλM films display similar Q 0 values due to normalization of the scattering intensity by specimen thickness.…”

“…Another effective method to toughen PLA is to blend it with certain diblock copolymers. In previous studies on molded bulk systems, poly­(ethylene oxide)- block -poly­(butylene oxide) (PEO–PBO) − was used as an effective additive to toughen PLA. The PBO block drives phase separation due to its incompatibility with PLA, while the presence of the PEO block promotes interfacial activity, leading to uniformly dispersed, sub-micron size, rubbery particles that are stable to static thermal annealing .…”

Mechanical Properties, Crystallization Behaviors and Phase Morphologies of PLA/GTR Blends by Reactive Compatibilization

“…Thereby, the enhancement in toughness is always the interesting point of the research and applications of PLA. Additionally, many strategies have been applied to improve PLA toughness by the copolymerization [ 10 , 11 ], plasticization [ 12 , 13 ], blending [ 9 , 14 ] and incorporation of the second composition [ 15 , 16 ]. Among various modification techniques, blending PLA with elastomeric polymer is considered to be the most effective way of improving the toughness, such as polycaprolactone (PCL) [ 17 , 18 ], polyhydroxyalkanoate (PHA) [ 19 , 20 ], poly(butylene adipate-co-terephtha-late) (PBAT) [ 11 , 21 ] and rubber [ 22 , 23 ].…”