INTERFACIAL CONTROL IN THERMOPLASTIC COMPOSITE: RHEOLOGICAL AND THERMOMECHANICAL CHARACTERIZATION AND ATOMISTIC MODELLING OF POLYAMIDE-6/BASALT FIBER/BIOCHAR COMPOSITES

Mohammad Mezbah Ul Hoque

Engineering thermoplastic composites with bio-derived, sustainable reinforcements requires understanding interfacial mechanisms spanning from individual chemical bonds to bulk composite performance. This dissertation develops a multi-scale framework, integrating melt rheology, thermomechanical characterization, response surface optimization, mechanical recycling investigation, and all-atom molecular dynamics simulation, to establish the governing interfacial mechanisms in polyamide 6 composites reinforced with hemp hurd-derived biochar and surface-treated basalt fiber, and to produce the quantitative model for any polyamide/biochar system. Pyrolysis temperature selects between two mechanistically distinct reinforcement pathways: low-temperature biochar (300 ℃, BC300) forms directed hydrogen bonds with PA6 amide groups through retained polar surface groups, governing strength; high-temperature biochar (500 ℃, BC500) nucleates PA6 crystallinity through a structured aromatized surface, governing stiffness and thermal resistance. Amino-silane functionalization of basalt fibers created a nitrogen-rich interphase interacting simultaneously to the PA6 matrix and BC300 oxygen acceptors, a dual-anchoring architecture not previously identified in fiber-reinforced polyamide composites. Van Gurp–Palmen melt rheology provided the first direct evidence of a percolated H-bonded filler network in the PA6/BC300 melt, while BC500 showed near-matrix behavior despite higher surface area. Response surface optimization of the ternary system revealed that competing reinforcement pathways produce opposing property gradients; a strength-priority goal formulation outperformed the conventional all-maximize approach by 32 MPa in tensile strength, demonstrating that goal assignment is more consequential than importance weighting in mechanistically competing systems. Mechanical recycling reduced matrix molecular weight by 27%. BC300 restored tensile and flexural strength to virgin equivalence through H-bond network formation without molecular weight recovery, while impact toughness, governed by bulk chain length, recovered only partially, establishing a property-class-specific recovery boundary that surface chemistry alone cannot overcome. All-atom molecular dynamics simulation resolved the parameter identifiability problem in calorimetric data. The ratio of simulated to experimental glass transition temperature (Tg) elevation measured the accessible surface fraction (facc = 36.8%), revealing that 63.2% of BC300's surface area resides inside micropores narrower than the PA6 chain radius of gyration. Combining facc with trajectory H-bond occupancy yielded the first atomistic bonding parameters for any polyamide/biochar system, θacc = 13.6%, recovering Tg elevations within 3.1% and establishing accessible surface area, not total surface area, as the controlling interfacial metric.

INTERFACIAL CONTROL IN THERMOPLASTIC COMPOSITE: RHEOLOGICAL AND THERMOMECHANICAL CHARACTERIZATION AND ATOMISTIC MODELLING OF POLYAMIDE-6/BASALT FIBER/BIOCHAR COMPOSITES

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