A Study on Moisture-dependent Viscoelasticity of Bio-based Composites
Creators: Chen, Yu
Publisher: name: University of Alberta Library
Year: 2023

<p>Bio-based composites made from bio-renewable and biodegradable constituents have been proposed as alternatives to petroleum-based composites for a variety of applications. However, studies have shown that in many cases, these bio-based composites are hydrophilic, and their viscoelasticity can be affected by moisture absorbed from humid ambient environment. Thus, to address this problem, this thesis focuses on the understanding and describing the moisture-dependent viscoelasticity of these bio-based composites using analytical and experimental tools. </p>

<p>The mechanical properties of composites are closely related to the constituents’ assembly structure and their mechanical properties. Therefore, to model the moisture-dependent viscoelasticity of the bio-based composites, the moisture-dependent viscoelasticity of the constituents needs to be modeled first. Based on existing studies, the diffused water molecules within those hydrophilic polymers can cause both plasticization and anti-plasticization effect. However, the existing well recognized Reimschuessel model only considers the plasticization effect, leading to an underestimation of the material’s stiffness at intermediate moisture level. Therefore, a simple yet important modification was introduced to the Reimschuessel model which considered the anti-plasticization effect. Together with Burgers model as the constitutive relation, the proposed model was validated with the experimental results in the literature with reasonably good accuracy. </p>

<p>With the proposed modified Burgers-Reimschuessel model, an experimental methodology that can comprehensively evaluate the moisture-dependent viscoelasticity of hydrophilic polymers was proposed. The proposed methodology was used to evaluate the moisture-dependent viscoelasticity of polylactic acid (PLA), a commonly used bio-based thermoplastic polymer, and a significant anti-plasticization effect was discovered from the experimental results. The modified Burgers-Reimschuessel model was found to be able to predict the effect of moisture on the viscoelasticity of PLA and the results highlighted the importance of considering the moisture’s anti-plasticization effect. </p>

<p>A model was developed to predict the moisture-dependent viscoelasticity of homogeneously distributed and randomly oriented short fiber reinforced bio-based composites based on micromechanical framework. The development is divided into two steps. In the first step, a micromechanical model was developed by extending Halpin-Tsai-Pagano model to linear viscoelastic regime according to correspondence principle. The model was validated against the experimental results of regenerated cellulose fibers (RCF) reinforced PLA bio-based composites with different RCF concentrations. Good agreement was found, and it provides a solid foundation to the next step. In the second step, a two-layer model with two empirical parameters were introduced to describe the moisture distribution, fiber-matrix swelling mismatch and fiber/matrix interface debonding during the moisture diffusion process. In order to validate the model, sheet samples of 5.6 wt% RCF reinforced PLA bio-based composites were produced. The samples were conditioned under 98% RH to acquire equilibrium moisture content and tested under 24% RH. Results indicate that the developed model is able to describe the effect of non-equilibrium moisture diffusion on the creep compliance of this bio-based composites. </p>

<p>The model developed in this thesis can be used to predict the performance of products made by these bio-based composites. It can also provide guidance to the design and optimization of the products made by theses bio-based composites. </p>