The Roles of Blending and of Molecular Weight Distribution on Craze Initiation

Abstract

Craze initiation stress was measured in three-point bending isochronal creep tests on a series of entangled bimodal blends of polystyrenes of narrow dispersity, on three polystyrenes of broad dispersity, and on four blends of polystyrenes of broad dispersity. Crazing stress was found to increase rapidly with small additions of the higher molar mass component, quickly reaching a plateau. A simple model based on the weighted addition of the crazing stress contributions of the individual weight fractions obtained from an established piecewise linear crazing law was able to predict the crazing stress accurately in the bimodal blends using a power law exponent of 2.59 (90% CI [1.75–17.34]). In broad dispersity systems, in particular where short unentangled chains dilute the polymer, it was found necessary to modify the model using dynamic tube dilution theory. Dilution leads to a change in the entanglement length and hence in the molar mass at which transitions to disentanglement and chain scission crazing occur. With the improved model, crazing stress could be predicted even for the broad dispersity polymers with wide and bimodal distributions. This represents an opportunity for the molecular design of polymers by blending to achieve improved resistance to craze initiation.

Load-bearing entanglements in polymer glasses

“…4C ). This trend in entanglement density is consistent with the previous empirical scaling that suggested the chain length between entanglements, N e , increases for blends between long, entangled chains and short, unentangled chains: ( 45 , 46 ). However, this proposed scaling does not capture the full trend, as the rate of increase between load-bearing entanglements and blend volume fraction depends on the length of the short-chain components ( Fig.…”

Crazing Reveals an Entanglement Network in Glassy Ring Polymers

“…Without changing the interactions in the simulation model, the comparison of crazing in linear and ring polymer glasses evaluates the traditional models ,− of polymer crazing from a new perspective. Previous studies showed that the entanglement network of linear glassy polymers can be tuned by introducing short unentangled chains to dilute the polymers , or by reducing the thickness of a thin glassy polymer film . This work shows changing polymer topology from linear to ring is an alternative method to alter the entanglement network without changing the chemistry.…”

Mechanical Properties of Ultrathin Polymer Nanocomposites

“…The PDI of the PMMA matrix is broad, and small polymer chains can decrease the number of entanglements and lead to brittle failure in neat glassy polymers. 52,53 Furthermore, the annealing times in this study are shorter than ∼43 repetition times, and previous work has shown that at least 170 repetition times are necessary to remove the residual stresses within the film. 54 Residual stresses could change the measured stresses; however, the role of the water surface may have the greatest impact on isolating the nanoparticle loading and film thickness effects on the PMMA nanocomposites.…”