## 2 Materials and Methods  
### Fibers  
Alkali-resistant glass (AR-glass) fibers, with diameter of 13.4 \(\pm\) 1.3 \(\upmu\)m, were made at the Leibniz Institut fur Polymerforschung Dresden, Germany (IPF), by using a continuous pilot plant spinning equipment. The application of a sizing during the glass spinning process is crucial for subsequent usability and further processing. However, it was found in previous experiments that single fibers isolated from spun-sized strands present regions where sizing is accumulated between filaments and form a non-homogeneous layer on the surface. Therefore, in micro-mechanical experiments, the dip-coating of each single fiber is a more suitable procedure, since it provides a significantly more homogeneous sizing or coating layer compared to fibers that were surface treated in the spinning. On the other hand, it must be mentioned that the high effort to dip-coat each single fiber is limited to such a fundamental work as provided in this paper. For dip-coating, one filament was separated from a strand of unsized AR-glass filaments. Then, one fiber end was fixed to a small plate and the free end was dipped into the silane solution using a constant velocity. Further modification of the fiber surface with additional polymeric coatings was conducted with the same method followed by a fiber drying at 140 \({}^{\circ}\)C over 4 h in an oven, as schematized in Figure 2. Table 1 reports the composition of the sizings and the coatings used. A water solution containing 1% of 3-Aminopropyltriethoxysilane (AMEO) was used in the 1 st treatment step for the coated fibers. However, the use of AMEO was found to be unsuitable for the scale up of the sizing process, since the amino moiety of the silane reacts with the carboxylated styrene butadiene used as film former in the sizing formulation. For this reason, the fibers used to investigate the interaction of the matrix with the sized fibers were modified with N-Propyltrimethoxysilane (PTMO) instead of AMEO. Both N-Propyltrimethoxysilane and 3-Aminopropyltriethoxysilane were purchased from EVONIK industries (Essen, Germany).  
Images of the fibers before and after modification were acquired with a Ultra plus (Carl Zeiss NTS, Germany) microscope in high vacuum and an accelerating voltage of 3 kV.  
Information about the morphology of the coated fibers were obtained by Atomic force microscopy conducted with a Dimension Icon (Bruker Corporation, Billerica, MA,  
Figure 2: Schematic representation of dispersion preparation and fiber surface treatment process.  
USA) equipped with a Tap300-G BudgetSensors (Innovative Solutions Bulgaria Ltd., Sofia, Bulgaria) cantilever made of monolithic silicon with a rotated pyramid shaped tip and a tip radius less than 10 nm. Roughness parameter in terms of arithmetic average roughness (R\({}_{a}\)), square roughness (R\({}_{a}\)), and maximum roughness (R\({}_{max}\)) were obtained recording topography images with dimension 3 \(\upmu\)m \(\times\) 3 \(\upmu\)m and 512 px \(\times\) 512 px in tapping mode. Results were reported as the average of at least 3 images for each fiber type.  
Effect of the sizing and coating on the fiber tensile strength was investigated by means of single fiber tensile test performed with a Favigraph testing device (Texchno, Monchengladbach, Germany) equipped with a 1 N load cell. The fiber diameter of each filament was determined from the linear density obtained with an integrated vibroscope and considering a density of glass fibers of 2.71 g/cm\({}^{3}\). The tensile strength was calculated as the fracture load divided by the fiber cross sectional area. The testing velocity was 10 mm/min at a gauge length of 20 mm.