Healing of Thermoplastic Polymers at an Interface under Nonisothermal Conditions

Bousmina M., Qiu H., Grmela M., Klemberg-Sapieha J.E.

Diffusion at the polymer/polymer interface was probed by small-amplitude oscillatory shear measurements carried out on polystyrene (PS)/polystyrene (PS) sandwich-like assembly as a function of the time of welding in the molten state. It was found that the dynamic complex shear modulus G*(t) at a fixed frequency increases with the time of contact in two time regimes. First G*(t) increases proportionally to t1/2 and then a second regime takes place where G*(t) increases proportionally to t1/4. At longer times, G*(t) tends asymptotically toward G* of pure polystyrene. The results were interpreted in terms of reptation theory and the time of transition between the two scaling law regimes was found to be in agreement with the time needed for the transition from the Rouse mode to the reptation mode. Special attention was given to the initial state of the polymer surfaces before contact by performing experiments on (i) freshly prepared surfaces, (ii) presheared samples, (iii) fractured samples, and (iv) corona-t...

Butler C.A., Mccullough R.L., Pitchumani R., Gillespie J.W.

A number of mechanisms have been proposed in the literature as contributors to the strength development at the polymer-polymer interface during fusion bonding of thermoplastic composites. Of these, healing and intimate contact emerge as fundamental mechanisms governing bonding. Intimate contact refers to the development of the amount of surface area that is physically contacted at the interface at any time, and healing describes the migration of polymer chains across the interface in intimate contact. This work provides a new theoretical development of a coupled bonding model that accounts for variability in initiation time for healing due to growth in the area in intimate contact. The generalized coupled bonding model is valid for any set of processing conditions and reduces to the proper controlling mechanism as dictated by the process. Analysis revealed a key dimensionless group, Q, that captures the coupled nature of the mechanisms governing fusion bonding. By evaluating Q, which is a function of material and process parameters, one can determine the relative contributions of each mechanism. Experimental validation of the coupled model using two different processes, tow placement and resistance welding, is also presented. An evaluation of Q for the tow-placement process indicates that both mechanisms are controlling. For this case, the coupled model demonstrates better strength predictions than the conventional healing model alone. In contrast, the resistance welding process is shown to be intimate-contact controlled, in which case the coupled model reduces to a more simplified model. The ability to rigorously determine the controlling mechanisms is of critical importance to accurately model the strength development during fusion bonding processes.

Pitchumani R., Ranganathan S., Don R.C., Gillespie J.W., Lamontia M.A.

The thermoplastic automated tow-placement process offers the potential for cost-effective fabrication of composite parts via consolidation in situ , thus avoiding the costly autoclave consolidation. The degree of interfacial bonding between tow layers and the void content in the composite laminate directly affect the mechanical properties and performance of the products. Theoretical models for the physical phenomena governing interfacial bonding and void dynamics (growth and consolidation) during the process are presented, which constitute the core of a numerical process simulator. Simulation-based parametric studies are reported for the case of an AS-4/PEEK composite to illustrate the effects of several process conditions and placement head configurations on the resulting degree of bonding and final void content. The analysis provides valuable insight towards optimal process and placement head design.

Bastien L.J., Gillespie J.W.

A study to investigate the influence of processing on the fusion bonding of graphite (AS4) poly(etheretherketone) (PEEK) thermoplastic composites (BASF commingled PEEK/graphite NCS woven fabric) using a polyetherimide (PEI) film at the interface is presented. Fundamental to all fusion bonding processes is the intermolecular diffusion between surfaces in intimate contact. A model based on the healing theory of amorphous polymers has been proposed to predict strength and toughness as a function of non-isothermal process history. This model considers two different microscopic failure mechanisms of a healed interface. For the first time, using non-isothermal data and proper data reduction procedures, it is possible to differentiate between these two mechanisms, which are otherwise indistinguishable from isothermal data. Temperature dependent reptation times representative of the kinetics of chain diffusion in the polymer have been evaluated for both mechanisms over a large range of process temperatures using fracture tests conducted on lap shear specimens manufactured using a hot press. Three alternate and independent techniques to estimate the reptation time in PEI indicate that the model based on the average interpenetration distance is most representative of the physical system. Lap shear strength predictions based on this formulation have been generated for various non-isothermal conditions measured in the hot press and are within 20% of the experimental data. The model was used to show that in isothermal processes, maximum strength and toughness can be achieved in less than 1 s for temperatures exceeding 290°C. Application of the model to a highly non-isothermal technique such as resistance welding using amorphous film technology is also presented. Model predictions show that asymptotic strength may be achieved in relatively short process times with appropriate welding conditions.

Xu P., Li Q., Wang C., Li L., Tan D., Wu H.

Chakraborty D., Sockalingam S., Kodagali K., Sutton M.A., Kattil S.R.

Pibulchinda P., Barocio E., Evans C.V., Pipes R.B.

Kirchhoff J.G., Heathman N.T., Yap T., Koirala P., Hudson T.B., Tehrani M.

Nesheim O.S., Eikevåg S.W., Steinert M., Elverum C.W.

El Arwadi O., Raut A., Meyer J.L., Polycarpou A., Naraghi M.

Mitchell K., Shackleford A., Bandala E., Zhang C., Chai G., Jin Y.

Abstract Utilizing material extrusion three-dimensional printing methods, particularly fused filament fabrication (FFF), allows for the creation of complex architectures. Nevertheless, FFF-fabricated structures often suffer from inadequate mechanical properties and elevated surface roughness. In this study, we developed an embedded FFF (e-FFF) approach to produce thermoplastic products with enhanced mechanical characteristics and improved surface quality. This approach was achieved through the development of a thermostable yield-stress fluid made from fumed silica particles and sunflower oil. By tuning the rheological properties of the support bath, thermoplastic filaments were effectively supported in a molten state throughout printing. Biocompatible and biodegradable polycaprolactone (PCL) was selected as the exemplary thermoplastic polymer in this work. Filaments, single-layer sheets, and tensile test samples were printed to fine-tune the printing parameters, assess surface morphology, and certify mechanical properties of structures printed by e-FFF. To demonstrate the potential biomedical applications of the approach, an orbital implant model was designed by numerical simulation for evaluating the mechanical integrity. Then, the orbital implant was printed and measured to confirm the effectiveness of the proposed e-FFF approach. Lastly, cells were successfully incubated on the PCL implant, which was affixed to a mock orbital fracture to confirm that patient-specific orbital implants could be fabricated.

Benié K., Cherouat A., Barrière T., Placet V.

Olhan S., Antil B., Behera B.K.

JIN F., LIU L., ZHU X., XIE Z., CHEN W.

Pigeonneau F., Xu D., Agassant J.-.

Dong N., Luan C., Yao X., Ding Z., Ji Y., Niu C., Zheng Y., Xu Y., Fu J.

Jiang J., Wang Z., Wang L., Tian F., Li Y., Zhai W.

Héri-Szuchács A., Kovács J.G.

Le Mouellic P., Boyard N., Bailleul J., Lefevre N., Gaudry T., Veille J.